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Sample records for hexagonal gan thin

  1. The Formation and Characterization of GaN Hexagonal Pyramids

    NASA Astrophysics Data System (ADS)

    Zhang, Shi-Ying; Xiu, Xiang-Qian; Lin, Zeng-Qin; Hua, Xue-Mei; Xie, Zi-Li; Zhang, Rong; Zheng, You-Dou

    2013-05-01

    GaN with hexagonal pyramids is fabricated using the photo-assisted electroless chemical etching method. Defective areas of the GaN substrate are selectively etched in a mixed solution of KOH and K2S2O8 under ultraviolet illumination, producing submicron-sized pyramids. Hexagonal pyramids on the etched GaN with well-defined {101¯1¯} facets and very sharp tips are formed. High-resolution x-ray diffraction shows that etched GaN with pyramids has a higher crystal quality, and micro-Raman spectra reveal a tensile stress relaxation in GaN with pyramids compared with normal GaN. The cathodoluminescence intensity of GaN after etching is significantly increased by three times, which is attributed to the reduction in the internal reflection, high-quality GaN with pyramids and the Bragg effect.

  2. Self-assembled GaN hexagonal micropyramid and microdisk

    SciTech Connect

    Lo Ikai; Hsieh, C.-H.; Hsu, Y.-C.; Pang, W.-Y.; Chou, M.-C.

    2009-02-09

    The self-assembled GaN hexagonal micropyramid and microdisk were grown on LiAlO{sub 2} by plasma-assisted molecular-beam epitaxy. It was found that the (0001) disk was established with the capture of N atoms by most-outside Ga atoms as the (1x1) surface was constructing, while the pyramid was obtained due to the missing of most-outside N atoms. The intensity of cathode luminescence excited from the microdisk was one order of amplitude greater than that from M-plane GaN.

  3. Optical properties and temperature dependence of the interband transitions of cubic and hexagonal GaN

    NASA Astrophysics Data System (ADS)

    Logothetidis, S.; Petalas, J.; Cardona, M.; Moustakas, T. D.

    1994-12-01

    The optical properties of cubic and hexagonal GaN thin films, grown by electron-cyclotron resonance microwave plasma-assisted molecular-beam epitaxy on silicon and sapphire substrates, respectively, have been studied at photon energies up to 25 eV with conventional and synchrotron-radiation spectroscopic ellipsometry. The fundamental gaps of the two polytypes are located at different energies, namely at 3.25 and 3.43 eV for cubic and hexagonal GaN. Analysis of the dielectric function of the two phases in the region 4.5-9.5 eV with appropriate models yields the energy location and broadening of the observed critical points. These critical points are assigned to specific points in the zinc-blende and wurtzite Brillouin zones, respectively, making use of the latest published band-structure studies and a comparison is made between the corresponding results for GaN, GaAs, and GaP. Measurements in the temperature range from 80 to 650 K provide the temperature dependence of these parameters. The features observed in the reflectivity spectra of hexagonal GaN are discussed in relation to other works. Kramers-Kronig analysis of the reflectivity between 0 and 33 eV of the hexagonal polytype verifies the existence of a broad feature centered at 14 eV. Finally, average properties, such as the effective ir dielectric constant and the effective number of valence electrons per atom are calculated for the two polytypes and compared to GaAs and GaP.

  4. Uniform GaN thin films grown on (100) silicon by remote plasma atomic layer deposition

    NASA Astrophysics Data System (ADS)

    Shih, Huan-Yu; Lin, Ming-Chih; Chen, Liang-Yih; Chen, Miin-Jang

    2015-01-01

    The growth of uniform gallium nitride (GaN) thin films was reported on (100) Si substrate by remote plasma atomic layer deposition (RP-ALD) using triethylgallium (TEG) and NH3 as the precursors. The self-limiting growth of GaN was manifested by the saturation of the deposition rate with the doses of TEG and NH3. The increase in the growth temperature leads to the rise of nitrogen content and improved crystallinity of GaN thin films, from amorphous at a low deposition temperature of 200 °C to polycrystalline hexagonal structures at a high growth temperature of 500 °C. No melting-back etching was observed at the GaN/Si interface. The excellent uniformity and almost atomic flat surface of the GaN thin films also infer the surface control mode of the GaN thin films grown by the RP-ALD technique. The GaN thin films grown by RP-ALD will be further applied in the light-emitting diodes and high electron mobility transistors on (100) Si substrate.

  5. Hexagonal pyramid shaped light-emitting diodes based on ZnO and GaN direct wafer bonding

    NASA Astrophysics Data System (ADS)

    Murai, Akihiko; Thompson, Daniel B.; Masui, Hisashi; Fellows, Natalie; Mishra, Umesh K.; Nakamura, Shuji; DenBaars, Steven P.

    2006-10-01

    The authors report on hexagonal pyramid shaped light-emitting diode (LED) based on ZnO and GaN wafer bonding. After direct wafer bonding of an n-type ZnO substrate to a III-nitride LED wafer, O-plane ZnO was selectively etched to form an electrode having a truncated hexagonal pyramid shape. This wafer bonded LED chip was evaluated with optical output power as a function of forward current and was 2.2 times higher than a conventional-type LED chip having thin Ni (5nm)/Au (10nm) p-type electrode at forward current condition of 20mA.

  6. Facets formation mechanism of GaN hexagonal pyramids on dot-patterns via selective MOVPE

    SciTech Connect

    Hiramatsu, Kazumasa; Kitamura, Shota; Sawaki, Nobuhiko

    1996-11-01

    Three-dimensional GaN pyramids have been successfully obtained on dot-patterned GaN(0001)/sapphire substrates by using the selective MOVPE technique. The dot-pattern is a hexagon arranged with a 5{micro}m width and a 10{micro}m spacing. The GaN structure comprises a hexagonal pyramid covered with six {l_brace}1{bar 1}01{r_brace} pyramidal facets on the side or a frustum of a hexagonal pyramid having a (0001) facet on the top. The facet formation mechanism has been investigated by observing the facet structure with the growth time. The {l_brace}1{bar 1}01{r_brace} facets are very stable during the growth. The (0001) facet growth is dominant at the initial growth but almost stops at a certain growth time and then the facet structure is maintained. The appearance of the self-limited (0001) facet is attributed to the balance of flux between incoming Ga atoms from the vapor phase to the (0001) surface and outgoing Ga atoms from the (0001) surface to the {l_brace}1{bar 1}01{r_brace} surface via migration. The longer the diffusion length of the Ga atoms on the (0001) surface is, the more the surface migration is enhanced, resulting in the appearance of the wider (0001) facet on the top.

  7. Multi-color broadband visible light source via GaN hexagonal annular structure

    PubMed Central

    Ko, Young-Ho; Song, Jie; Leung, Benjamin; Han, Jung; Cho, Yong-Hoon

    2014-01-01

    Multi-color and broadband visible emission was realized thorough the hexagonal annular structure of GaN. The annular structure fabricated by selective-area growth emitted purple, blue and green color-emission from the multi-facets. The hexagonal annular structure provided various sidewalls of {101} and {112} semi-polar facets, and (0001) polar facet. From the cathodoluminescence study, the (0001) plane had the longest wavelength of 525 nm, and the {101} facet of 440 nm peak wavelength had longer wavelength emission than the {112} of 412 nm peak wavelength. The origin of longer wavelength emission of {101} was mostly due to high In-composition, as well as slightly larger well thickness, which means that {101} facet has higher In-incorporation efficiency. Various In-composition of each facet provided multi-color and broadband emission with the international commission on illumination (CIE) of (0.22, 0.45) and high emission efficiency. The hexagonal annular structure becomes building blocks for highly efficient broadband visible lighting sources. PMID:24981889

  8. Some of structural and morphological optimization of GaN thin film on Si(100) substrate grown by RF sputter

    NASA Astrophysics Data System (ADS)

    Mantarcı, Asim; Kundakçı, Mutlu

    2017-04-01

    One of important material of III-nitrides can be said to be GaN with direct-wide band gap ( 3.4 eV) and many industrial devices such as solar cell, LED has been based on GaN thin film. In this research, we elaborately investigated growth of GaN thin film on Si(100) substrate by RF sputter technique and characterization of the film. We have successfully grown GaN thin film on Si substrate with hexagonal structure which has been confirmed by analysis of X-ray measurements. Also, we obtained structural properties of GaN film by (XRD) X-ray Diffraction measurements depending on different Argon, nitrogen and RF power values. During experiment, the value from 25sccm to 100sccm Argon gas value, the value from 0sccm to 4sccm Nitrogen gas value and from 50 watt to 125 watt RF power value has been applied. Among these values, we determined the best film in terms of crystalline structure of film. From AFM results, we attained and analyzed average roughness (Ra), maximum peak height (Rp), and maximum depth (Rv), average absolute slope of the profile (Δa)(°) of the fılms successfully. The film having the lowest roughness (Ra) has been achieved depending on different Argon, nitrogen and RF power values. Atomic Force Microscopy results confirmed that some of the films have homogeneous and uniform structure without any holes and crack; but others has voids referring impurities coming from growth process. To sum up, not only growing GaN thin film on Si substrate has been investigated, but also some of structural and morphological parameters' optimization has been studied, analyzed and the best film was determined in view of varied Argon, nitrogen and RF power values. For future direction, optimization of GaN thin film in detail can enable us to fabricate high quality film; therefore it will helps to improving device technology.

  9. Synthesis and characterization of GaN thin films deposited on different substrates using a low-cost electrochemical deposition technique

    SciTech Connect

    Al-Heuseen, K.; Hashim, M. R.

    2012-09-06

    Gallium nitride GaN thin films were deposited on three different substrates; Si (111), Si (100) and ITO coated glass using electrochemical deposition technique at 20 Degree-Sign C. A mixture of gallium nitrate, ammonium nitrate was used as electrolyte. The deposited films were investigated at room temperature by a series of material characterization techniques, namely; scanning electron microscopy (SEM), EDX and X-ray diffraction (XRD). SEM images and EDX results indicated that the growth of GaN films varies according to the substrates. XRD analyses showed the presence of hexagonal wurtzite and cubic zinc blende GaN phases with the crystallite size around 18-29 nm.

  10. Morphology Control of Hot-Wall MOCVD Selective Area Grown Hexagonal GaN Pyramids

    NASA Astrophysics Data System (ADS)

    Lundskog, Anders; Forsberg, Urban; Holtz, Per Olof; Janzen, Erik

    2012-11-01

    Morphological variations of gallium polar (0001)-oriented hexagonal GaN pyramids grown by hot wall metal organic chemical vapor deposition under various growth conditions are investigated. The stability of the semipolar {1 (1) over bar 02} and nonpolar {1 (1) over bar 00} facets is particularly discussed. The presence of the {1 (1) over bar 02} facets near the apex of the pyramid was found to be controllable by tuning the absolute flow rate of ammonia during the growth Vertical nonpolar {1 (1) over bar 00} facets appeared in gallium rich conditions, which automatically were created when the growth time was prolonged beyond pyramid completion. The result was attributed to a gallium passivation of the {1 (1) over bar 00} surface.

  11. Raman mapping of hexagonal hillocks in N-polar GaN grown on c-plane sapphire

    NASA Astrophysics Data System (ADS)

    Jiang, Teng; Lin, Zhiyu; Zhang, Jincheng; Xu, Shengrui; Huang, Jun; Niu, Mutong; Gao, Xiaodong; Guo, Lixin; Hao, Yue

    2017-04-01

    A large amount of huge hexagonal hillocks were observed on the surface of N-polar GaN film grown on c-plane sapphire substrate by MOCVD. The distribution of residual stress and dislocation density in a typical hexagonal hillock was investigated by the mapping measurement of Micro-Raman and Cathodoluminescence (CL) spectroscopy. It is found that the residual stress at the top region of the hillock is much smaller than that of the sidewall region and the region around the hillock. Meanwhile, the CL images confirmed that the dislocation density around the hexagonal hillock is higher than the top region of the hillock. The bending and annihilation of the dislocations during the growth of the hexagonal hillock result in the relaxation of residual stress which should be responsible for the spatial variation of dislocation density and residual stress.

  12. Hexagonal boron nitride thin film thermal neutron detectors with high energy resolution of the reaction products

    NASA Astrophysics Data System (ADS)

    Doan, T. C.; Majety, S.; Grenadier, S.; Li, J.; Lin, J. Y.; Jiang, H. X.

    2015-05-01

    Hexagonal boron nitride (h-BN) is highly promising for solid-state thermal neutron detector applications due to its many outstanding physical properties, especially its very large thermal neutron capture cross-section (~3840 barns for 10B), which is several orders of magnitude larger than those of most other isotopes. The focus of the present work is to carry out studies on h-BN thin film and detector properties to lay the foundation for the development of a direct-conversion solid-state thermal neutron detector with high sensitivity. The measured carrier mobility-lifetime (μτ) product of h-BN thin films grown on sapphire substrates is 2.83×10-7 cm2/V for electrons and holes, which is comparable to the value of about 10-7 cm2/V for GaN thin films grown on sapphire. Detectors based on h-BN thin films were fabricated and the nuclear reaction product pulse height spectra were measured. Under a bias of 20 V, very narrow individual peaks corresponding to the reaction product energies of α and Li particles as well as the sum peaks have been clearly resolved in the pulse height spectrum for the first time by a B-based direct-conversion semiconductor neutron detector. Our results indicate that h-BN thin film detectors possess unique advantages including small size, low weight, portability, low voltage operation and high energy resolution of specific reaction products.

  13. Liquid phase deposition synthesis of hexagonal molybdenum trioxide thin films

    SciTech Connect

    Deki, Shigehito; Beleke, Alexis Bienvenu; Kotani, Yuki; Mizuhata, Minoru

    2009-09-15

    Hexagonal molybdenum trioxide thin films with good crystallinity and high purity have been fabricated by the liquid phase deposition (LPD) technique using molybdic acid (H{sub 2}MoO{sub 4}) dissolved in 2.82% hydrofluoric acid (HF) and H{sub 3}BO{sub 3} as precursors. The crystal was found to belong to a hexagonal hydrate system MoO{sub 3}.nH{sub 2}O (napprox0.56). The unit cell lattice parameters are a=10.651 A, c=3.725 A and V=365.997 A{sup 3}. Scanning electron microscope (SEM) images of the as-deposited samples showed well-shaped hexagonal rods nuclei that grew and where the amount increased with increase in reaction time. X-ray photon electron spectroscopy (XPS) spectra showed a Gaussian shape of the doublet of Mo 3d core level, indicating the presence of Mo{sup 6+} oxidation state in the deposited films. The deposited films exhibited an electrochromic behavior by lithium intercalation and deintercalation, which resulted in coloration and bleaching of the film. Upon dehydration at about 450 deg. C, the hexagonal MoO{sub 3}.nH{sub 2}O was transformed into the thermodynamically stable orthorhombic phase. - Abstract: SEM photograph of typical h-MoO{sub 3}.nH{sub 2}O thin film nuclei obtained after 36 h at 40 deg. C by the LPD method. Display Omitted

  14. ''Cube-on-hexagon'' orientation relationship for Fe on GaN(0001): The missing link in bcc/hcp epitaxy

    SciTech Connect

    Gao Cunxu; Brandt, Oliver; Laehnemann, Jonas; Jahn, Uwe; Jenichen, Bernd; Schoenherr, Hans-Peter; Erwin, Steven C.

    2010-09-15

    We investigate, experimentally and theoretically, the epitaxy of body-centered-cubic Fe on hexagonal GaN. For growth on the Ga-polar GaN(0001) surface we find the well-known Pitsch-Schrader orientation relationship between Fe and GaN. On the N-polar GaN(0001) surface we observe coexistence between the familiar Burgers orientation and a new orientation in which the Fe(001) plane is parallel to GaN(0001). This 'cube-on-hexagon' orientation constitutes the high-symmetry link required for constructing a symmetry diagram for bcc/hcp systems in which all orientation relationships are connected by simple rotations.

  15. Selective area growth of high-density GaN nanowire arrays on Si(111) using thin AlN seeding layers

    NASA Astrophysics Data System (ADS)

    Wu, C. H.; Lee, P. Y.; Chen, K. Y.; Tseng, Y. T.; Wang, Y. L.; Cheng, K. Y.

    2016-11-01

    Selective area growth (SAG) of high-density (2.5×109 cm-2) GaN nanowires (NWs) on Si(111) substrate by plasma-assisted molecular beam epitaxy is presented. The effects of morphology and thickness of the AlN seeding layer on the quality of SAG GaN NWs are investigated. A thin AlN seeding layer of 30 nm thick with a surface roughness of less than 0.5 nm is suitable for high quality SAG GaN NWs growth. High-density AlN nanopedestal arrays used as seeds for SAG GaN NWs are fabricated from thin AlN seeding layers using soft nanoimprint lithography. By adjusting the growth temperature and Ga/N flux ratio, hexagonal shaped SAG GaN NWs are realized. The quality of SAG GaN NWs is evaluated by low temperature photoluminescence (PL) measurements. Three major groups of PL peaks at 3.47, 3.45, and 3.41 eV are identified. The peak at 3.471 eV is related to the neutral donor-bound exciton emission, and the 3.41 eV broadband emission is attributed to stacking faults or structural defects. The 3.45 eV peak is identified as the emission due to exciton recombination at polar inversion domain boundaries of NWs.

  16. Improving the quality of GaN crystals by using graphene or hexagonal boron nitride nanosheets substrate.

    PubMed

    Zhang, Lei; Li, Xianlei; Shao, Yongliang; Yu, Jiaoxian; Wu, Yongzhong; Hao, Xiaopeng; Yin, Zhengmao; Dai, Yuanbin; Tian, Yuan; Huo, Qin; Shen, Yinan; Hua, Zhen; Zhang, Baoguo

    2015-03-04

    The progress in nitrides technology is widely believed to be limited and hampered by the lack of high-quality gallium nitride wafers. Though various epitaxial techniques like epitaxial lateral overgrowth and its derivatives have been used to reduce defect density, there is still plenty of room for the improvement of gallium nitride crystal. Here, we report graphene or hexagonal boron nitride nanosheets can be used to improve the quality of GaN crystal using hydride vapor phase epitaxy methods. These nanosheets were directly deposited on the substrate that is used for the epitaxial growth of GaN crystal. Systematic characterizations of the as-obtained crystal show that quality of GaN crystal is greatly improved. The fabricated light-emitting diodes using the as-obtained GaN crystals emit strong electroluminescence under room illumination. This simple yet effective technique is believed to be applicable in metal-organic chemical vapor deposition systems and will find wide applications on other crystal growth.

  17. MOCVD of BN and GaN thin films on silicon: new attempt of GaN growth with BN buffer layer

    NASA Astrophysics Data System (ADS)

    Boo, Jin-Hyo; Rohr, Carsten; Ho, Wilson

    1998-06-01

    Highly oriented polycrystalline h-BN thin films were deposited on silicon substrates in the temperature range of 600-900°C from the single molecular precursor of borane-triethylamine complex, (C 2H 5) 3N : BH 3, by supersonic jet assisted chemical vapor deposition. Hydrogen was used as carrier gas, and additional nitrogen was supplied by either ammonia through a nozzle or nitrogen via a remote microwave plasma. Hexagonal GaN films were also grown on Si(1 0 0) with h-BN buffer layers at temperatures between 550 and 750°C with dual supersonic molecular beam sources. Triethylgallium, (C 2H 5) 3Ga, and ammonia, NH 3, were used as precursors. Hydrogen was used as seeding gas for the precursors, providing a wide range of possible kinetic energies for the supersonic beams. The h-BN buffer layers and the GaN films were characterized in situ by Auger electron spectroscopy (AES), and ex situ by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and optical transmission. This is the first report of growing the h-BN films on silicon substrates from the single source precursor of borane-triethylamine complex and new attempts of GaN film growth on silicon with BN buffer layer.

  18. Demonstration of flexible thin film transistors with GaN channels

    NASA Astrophysics Data System (ADS)

    Bolat, S.; Sisman, Z.; Okyay, A. K.

    2016-12-01

    We report on the thin film transistors (TFTs) with Gallium Nitride (GaN) channels directly fabricated on flexible substrates. GaN thin films are grown by hollow cathode plasma assisted atomic layer deposition (HCPA-ALD) at 200 °C. TFTs exhibit 103 on-to-off current ratios and are shown to exhibit proper transistor saturation behavior in their output characteristics. Gate bias stress tests reveal that flexible GaN TFTs have extremely stable electrical characteristics. Overall fabrication thermal budget is below 200 °C, the lowest reported for the GaN based transistors so far.

  19. Compression-induced phase transition of GaN bulk from wurtzite phase to five-fold coordination hexagonal phase

    NASA Astrophysics Data System (ADS)

    Qian, Yu; Shang, Fulin; Wan, Qiang; Yan, Yabin

    2017-09-01

    The phase transformation of GaN bulk from the Wurtzite phase (WZ) to the hexagonal phase (HX) is studied by molecular dynamics simulation. The mechanical response and atomic structural evolution of transition are analyzed in detail. In addition, the loading rate effect on the phase transition is determined, that is, the phase transition ratio declines with a decrease of the strain rate. The WZ GaN bulk completely transforms into the HX phase in the case of compression at an ultrahigh strain rate. However, at a relatively slower strain rate, the HX phase of GaN partly nucleates and the untransformed regions are proved to be elastic deformed regions. Combined with an energy analysis, two atomic movement modes are recognized as the inducements for the phase transition and formation of elastic deformed regions. The first mode, which is responsible for the formation of elastic deformed regions, is an atomic sliding motion along the c {0001} planes. The second mode is a radial stretching atomic motion. The radial stretching motion, which requires a lot of energy, induces the WZ-HX phase transition. Moreover, the phase transition is affected drastically by the rise of temperature.

  20. Terahertz study of m-plane GaN thin fims

    NASA Astrophysics Data System (ADS)

    Quadir, Shaham; Jang, Der-Jun; Lin, Ching-Liang; Lo, Ikai

    2014-03-01

    We investigate the optical properties of m-plane GaN thin films using the terahertz time domain spectroscopy. The m-plane GaN thin films were grown on γ-LiAlO2 substrates with buffer layers of low temperature grown GaN. The thin films were illuminated with terahertz radiation generated by a LT-GaAs antenna and the transmitted signal was detected by a ZnTe crystal. The polarization of the terahertz wave was chosen to be either parallel or perpendicular to the GaN [0001] direction. We compared the transmitted signals of the m-plane GaN thin films to that of the LAO substrate. The samples as well as the LAO substrate exhibited polarization dependence of absorption in terahertz spectrum. The carrier densities and the mobilities were derived from the transmittance of the THz wave using extended Drude model. We found, in all samples, both the carrier densities and mobilities along the GaN [0001] direction were smaller than those along the GaN [1120] direction due to the stripe formation along the GaN [1120].

  1. Polar properties of a hexagonally bonded GaN sheet under biaxial compression

    NASA Astrophysics Data System (ADS)

    Gao, Yanlin; Yayama, Tomoe; Okada, Susumu

    2016-09-01

    Using the density functional theory, we study the geometric and electronic structures of a GaN sheet possessing a honeycomb network. The sheet preserves the planar conformation under an equilibrium lattice constant of 3.2 Å, and has a semiconducting electronic structure with an indirect band gap of 2.28 eV. The biaxial compressive strain causes structural buckling, leading to polarization normal to the atomic layer. An external electric field normal to the layer also induces structural buckling with a height proportional to the field strength. The polarity of the buckled GaN sheet is tunable by attaching H atoms on Ga and N atoms.

  2. Photoelectrochemical water splitting on nanoporous GaN thin films for energy conversion under visible light

    NASA Astrophysics Data System (ADS)

    Cao, Dezhong; Xiao, Hongdi; Fang, Jiacheng; Liu, Jianqiang; Gao, Qingxue; Liu, Xiangdong; Ma, Jin

    2017-01-01

    Nanoporous (NP) GaN thin films, which were fabricated by an electrochemical etching method at different voltages, were used as photoelectrodes during photoelectrochemical (PEC) water splitting in 1 M oxalic acid solution. Upon illumination at a power density of 100 mW cm-2 (AM 1.5), water splitting is observed in NP GaN thin films, presumably resulting from the valence band edge which is more positive than the redox potential of the oxidizing species. In comparison with NP GaN film fabricated at 8 V, NP GaN obtained at 18 V shows nearly twofold enhancement in photocurrent with the maximum photo-to-hydrogen conversion efficiency of 1.05% at ~0 V (versus Ag/AgCl). This enhancement could be explained with (i) the increase of surface area and surface states, and (ii) the decrease of resistances and carrier concentration in the NP GaN thin films. High stability of the NP GaN thin films during the PEC water splitting further confirms that the NP GaN thin film could be applied to the design of efficient solar cells and solar fuel devices.

  3. White emission from non-planar InGaN/GaN MQW LEDs grown on GaN template with truncated hexagonal pyramids.

    PubMed

    Lee, Ming-Lun; Yeh, Yu-Hsiang; Tu, Shang-Ju; Chen, P C; Lai, Wei-Chih; Sheu, Jinn-Kong

    2015-04-06

    Non-planar InGaN/GaN multiple quantum well (MQW) structures are grown on a GaN template with truncated hexagonal pyramids (THPs) featuring c-plane and r-plane surfaces. The THP array is formed by the regrowth of the GaN layer on a selective-area Si-implanted GaN template. Transmission electron microscopy shows that the InGaN/GaN epitaxial layers regrown on the THPs exhibit different growth rates and indium compositions of the InGaN layer between the c-plane and r-plane surfaces. Consequently, InGaN/GaN MQW light-emitting diodes grown on the GaN THP array emit multiple wavelengths approaching near white light.

  4. Systematic Study on the Self-Assembled Hexagonal Au Voids, Nano-Clusters and Nanoparticles on GaN (0001)

    PubMed Central

    Pandey, Puran; Sui, Mao; Li, Ming-Yu; Zhang, Quanzhen; Kim, Eun-Soo; Lee, Jihoon

    2015-01-01

    Au nano-clusters and nanoparticles (NPs) have been widely utilized in various electronic, optoelectronic, and bio-medical applications due to their great potentials. The size, density and configuration of Au NPs play a vital role in the performance of these devices. In this paper, we present a systematic study on the self-assembled hexagonal Au voids, nano-clusters and NPs fabricated on GaN (0001) by the variation of annealing temperature and deposition amount. At relatively low annealing temperatures between 400 and 600°C, the fabrication of hexagonal shaped Au voids and Au nano-clusters are observed and discussed based on the diffusion limited aggregation model. The size and density of voids and nano-clusters can systematically be controlled. The self-assembled Au NPs are fabricated at comparatively high temperatures from 650 to 800°C based on the Volmer-Weber growth model and also the size and density can be tuned accordingly. The results are symmetrically analyzed and discussed in conjunction with the diffusion theory and thermodynamics by utilizing AFM and SEM images, EDS maps and spectra, FFT power spectra, cross-sectional line-profiles and size and density plots. PMID:26285135

  5. Systematic Study on the Self-Assembled Hexagonal Au Voids, Nano-Clusters and Nanoparticles on GaN (0001).

    PubMed

    Pandey, Puran; Sui, Mao; Li, Ming-Yu; Zhang, Quanzhen; Kim, Eun-Soo; Lee, Jihoon

    2015-01-01

    Au nano-clusters and nanoparticles (NPs) have been widely utilized in various electronic, optoelectronic, and bio-medical applications due to their great potentials. The size, density and configuration of Au NPs play a vital role in the performance of these devices. In this paper, we present a systematic study on the self-assembled hexagonal Au voids, nano-clusters and NPs fabricated on GaN (0001) by the variation of annealing temperature and deposition amount. At relatively low annealing temperatures between 400 and 600°C, the fabrication of hexagonal shaped Au voids and Au nano-clusters are observed and discussed based on the diffusion limited aggregation model. The size and density of voids and nano-clusters can systematically be controlled. The self-assembled Au NPs are fabricated at comparatively high temperatures from 650 to 800°C based on the Volmer-Weber growth model and also the size and density can be tuned accordingly. The results are symmetrically analyzed and discussed in conjunction with the diffusion theory and thermodynamics by utilizing AFM and SEM images, EDS maps and spectra, FFT power spectra, cross-sectional line-profiles and size and density plots.

  6. Air-bridged lateral epitaxial overgrowth of GaN thin films

    NASA Astrophysics Data System (ADS)

    Kidoguchi, Isao; Ishibashi, Akihiko; Sugahara, Gaku; Ban, Yuzaburoh

    2000-06-01

    A promising technique of selective lateral epitaxy, namely air-bridged lateral epitaxial overgrowth, is demonstrated in order to reduce the wing tilt as well as the threading dislocation density in GaN thin films. A seed GaN layer was etched to make ridge-stripe along <11¯00>GaN direction and a GaN material was regrown from the exposed (0001) top facet of the ridged GaN seed structures, whose sidewalls and etched bottoms were covered with silicon nitride mask, using low-pressure metalorganic vapor phase epitaxy. The density of dislocations in the wing region was reduced to be <107cm-2, which was at least two orders of magnitude lower than that of underlying GaN. The magnitude of the wing tilt was determined to be 0.08° by x-ray diffraction (XRD) measurements, which was smaller than other lateral epitaxial overgrown GaN thin films. The full width at half maximum of XRD for the wing region was 138 arc sec, indicating high uniformity of c-axis orientation.

  7. Nanometer-Thick Single-Crystal Hexagonal Gd2 O3 on GaN for Advanced Complementary Metal-Oxide-Semiconductor Technology.

    PubMed

    Chang, Wen Hsin; Lee, Chih Hsun; Chang, Yao Chung; Chang, Pen; Huang, Mao Lin; Lee, Yi Jun; Hsu, Chia-Hung; Hong, J Minghuang; Tsai, Chiung Chi; Kwo, J Raynien; Hong, Minghwei

    2009-12-28

    Hexagonal-phase single-crystal Gd2 O3 is deposited on GaN in a molecular beam epitaxy system. The dielectric constant is about twice that of its cubic counterpart when deposited on InGaAs or Si. The capacitive effective thickness of 0.5 nm in hexagonal Gd2 O3 is perhaps the lowest on GaN-metal-oxide-semiconductor devices. The heterostructure is thermo dynamically stable at high temperatures and exhibits low interfacial densities of states after high-temperature annealing. Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  8. Dual-wavelength GaN-based LEDs grown on truncated hexagonal pyramids formed by selective-area regrowth on Si-implanted GaN templates.

    PubMed

    Lee, Ming-Lun; Yeh, Yu-Hsiang; Tu, Shang-Ju; Chen, P C; Wu, Ming-Jui; Lai, Wei-Chih; Sheu, Jinn-Kong

    2013-09-09

    GaN-based blue light-emitting diodes (LEDs) with micro truncated hexagonal pyramid (THP) array were grown on selective-area Si-implanted GaN (SIG) templates. The GaN epitaxial layer regrown on the SIG templates exhibited selective growth and subsequent lateral growth to form the THP array. The observed selective-area growth was attributed to the different crystal structures between the Si-implanted and implantation-free regions. Consequently, LEDs grown on the GaN THP array emitted broad electroluminescence spectra with multiple peaks. Spatially resolved cathodoluminescence revealed that the broad spectra originated from different areas within each THP. Transmission electron microscopy showed the GaN-based epitaxial layers, including InGaN/GaN multi-quantum wells regrown at different growth rates (or with different In content in the InGaN wells) between the semi-polar and c-face planes of each THP.

  9. Terahertz response of GaN thin films.

    PubMed

    Tsai, Tsong-Ru; Chen, Shi-Jie; Chang, Chih-Fu; Hsu, Sheng-Hsien; Lin, Tai-Yuan; Chi, Cheng-Chung

    2006-05-29

    The indices of refraction, extinction constants and complex conductivities of the GaN film for frequencies ranging from 0.2 to 2.5 THz are obtained using THz time-domain spectroscopy. The results correspond well with the Kohlrausch stretched exponential model. Using the Kohlrausch model fit not only provides the mobility of the free carriers in the GaN film, but also estimates the relaxation time distribution function and average relaxation time.

  10. Substrate impact on the low-temperature growth of GaN thin films by plasma-assisted atomic layer deposition

    SciTech Connect

    Kizir, Seda; Haider, Ali; Biyikli, Necmi

    2016-07-15

    Gallium nitride (GaN) thin films were grown on Si (100), Si (111), and c-plane sapphire substrates at 200 °C via hollow-cathode plasma-assisted atomic layer deposition (HCPA-ALD) using GaEt{sub 3} and N{sub 2}/H{sub 2} plasma as group-III and V precursors, respectively. The main aim of the study was to investigate the impact of substrate on the material properties of low-temperature ALD-grown GaN layers. Structural, chemical, and optical characterizations were carried out in order to evaluate and compare film quality of GaN on different substrates. X-ray reflectivity measurements showed film density values of 5.70, 5.74, and 5.54 g/cm{sup 3} for GaN grown on Si (100), Si (111), and sapphire, respectively. Grazing incidence x-ray diffraction measurements exhibited hexagonal wurtzite structure in all HCPA-ALD grown GaN samples. However, dominant diffraction peak for GaN films grown on Si and sapphire substrates were detected differently as (002) and (103), respectively. X-ray diffraction gonio scans measured from GaN grown on c-plane sapphire primarily showed (002) orientation. All samples exhibited similar refractive index values (∼2.17 at 632 nm) with 2–3 at. % of oxygen impurity existing within the bulk of the films. The grain size was calculated as ∼9–10 nm for GaN grown on Si (100) and Si (111) samples while it was ∼5 nm for GaN/sapphire sample. Root-mean-square surface roughness values found as 0.68, 0.76, and 1.83 nm for GaN deposited on Si (100), Si (111), and sapphire, respectively. Another significant difference observed between the samples was the film growth per cycle: GaN/sapphire sample showed a considerable higher thickness value when compared with GaN/Si samples, which might be attributed to a possibly more-efficient nitridation and faster nucleation of sapphire surface.

  11. Fabrication and Characterization Of Volatile Organic Compound Gas Sensor Based GaN Thin Film

    NASA Astrophysics Data System (ADS)

    Rusdiana, D.; Aminudin, A.

    2017-02-01

    Gas sensor using GaN film has been proposed on silicon substrate by sol gel spin coating technique with parameters growth temperature of 850°C, spinner rate of 1000 rpm, Ga2O3 molarity of 1.33 M and N sources derived from nitrogen gas at a flow rate of 100 sccm. From the results of testing the electrical properties of the gas environment type volatile organic compound, especially hydrogen gas turns electric resistance GaN semiconductor thin films declined sharply from 1.5 x 10-2 to 7 x 10-3 Ohm.cm. The GaN thin film not only exhibited good sensitivity to hydrogen gas but also showed good linearity in the characteristic of the sensitivity to hydrogen gas concentration.

  12. Growth and characterization of GaN thin films on Si(111) substrates using SiC intermediate layer

    SciTech Connect

    Lim, K.Y.; Lee, K.J.; Park, C.I.; Kim, K.C.; Choi, S.C.; Lee, W.H.; Suh, E.K.; Yang, G.M.; Nahm, K.S.

    2000-07-01

    GaN films have been grown atop Si-terminated 3C-SiC intermediate layer on Si(111) substrates using low pressure metalorganic chemical vapor deposition (LP-MOCVD). The SiC intermediate layer was grown by chemical vapor deposition (CVD) using tetramethylsilane (TMS) as the single source precursor. The Si terminated SiC surface was obtained by immediately flow of SiH{sub 4} gas after growth of SiC film. LP-MOCVD growth of GaN on 3C-SiC/Si(111) was carried out with trimethylgallium (TMG) and NH{sub 3}. Single crystalline hexagonal GaN layers can be grown on Si terminated SiC intermediate layer using an AlN or GaN buffer layer. Compared with GaN layers grown using a GaN buffer layer, the crystal qualities of GaN films with AlN buffer layers are extremely improved. The GaN films were characterized by x-ray diffraction (XRD), photoluminescence (PL) and scanning electron microscopy (SEM). Full width at half maximum (FWHM) of double crystal x-ray diffraction (DCXD) rocking curve for GaN (0002) on 3C-SiC/Si(111) was 890 arcsec. PL near band edge emission peak position and FWHM at room temperature are 3.38 eV and 79.35 meV, respectively.

  13. Refractive index of erbium doped GaN thin films

    SciTech Connect

    Alajlouni, S.; Sun, Z. Y.; Li, J.; Lin, J. Y.; Jiang, H. X.; Zavada, J. M.

    2014-08-25

    GaN is an excellent host for erbium (Er) to provide optical emission in the technologically important as well as eye-safe 1540 nm wavelength window. Er doped GaN (GaN:Er) epilayers were synthesized on c-plane sapphire substrates using metal organic chemical vapor deposition. By employing a pulsed growth scheme, the crystalline quality of GaN:Er epilayers was significantly improved over those obtained by conventional growth method of continuous flow of reaction precursors. X-ray diffraction rocking curve linewidths of less than 300 arc sec were achieved for the GaN (0002) diffraction peak, which is comparable to the typical results of undoped high quality GaN epilayers and represents a major improvement over previously reported results for GaN:Er. Spectroscopic ellipsometry was used to determine the refractive index of the GaN:Er epilayers in the 1540 nm wavelength window and a linear dependence on Er concentration was found. The observed refractive index increase with Er incorporation and the improved crystalline quality of the GaN:Er epilayers indicate that low loss GaN:Er optical waveguiding structures are feasible.

  14. Physical Vapor Deposition of Hexagonal and Tetragonal CuIn5Se8 Thin Films

    NASA Astrophysics Data System (ADS)

    Kohara, Naoki; Nishiawaki, Shiro; Negami, Takayuki; Wada, Takahiro

    2000-11-01

    Hexagonal and tetragonal CuIn5Se8 thin films have been successfully prepared by physical vapor deposition. A hexagonal CuIn5Se8 thin film was obtained via the reaction of (Cu+Se) and (In+Se) layers. This hexagonal film had a layered structure with a lattice spacing of approximately 16 Å, corresponding to fivefold close-packed stacking of Se. With an alternative deposition process using (CuInSe2) and (In2Se3) layers, a tetragonal CuIn5Se8 thin film was obtained. The tetragonal film had a columnar structure with a lattice spacing of 3.3 Å, corresponding to the cubic close-packed stacking of Se. As evident from examination of the Cu2Se-In2Se3 pseudo-binary system phase diagram, hexagonal CuIn5Se8 is a stable phase and tetragonal CuIn5Se8 is metastable. The growth mechanism of the hexagonal and tetragonal phase CuIn5Se8 thin films is discussed from a crystallographic point of view.

  15. Injection Laser Using Rare Earth Doped GaN Thin Films for Visible and Infrared Applications

    DTIC Science & Technology

    2010-05-01

    was firstly synthesized in 1937[3, 4]. In 1959 small GaN crystals was synthesized by hot gallium metals in ammonia gas ambient condition.[5] GaN...energy of 3.39eV and electron concentration above 1019cm-3. In 1971, Manasevit[7] utilized metal -organic chemical vapor deposition (MOCVD) to grow...quality of GaN thin films were obtained in 1986 by Amano and Akasaki[9] with the optimized “two-step growth” method in metal -organic vapor phase

  16. Improved crystalline properties of laser molecular beam epitaxy grown SrTiO{sub 3} by rutile TiO{sub 2} layer on hexagonal GaN

    SciTech Connect

    Luo, W. B.; Zhu, J.; Chen, H.; Wang, X. P.; Zhang, Y.; Li, Y. R.

    2009-11-15

    Epitaxial SrTiO{sub 3} films were fabricated by laser molecular beam epitaxy on bare and TiO{sub 2} buffered GaN(0002), respectively. The whole deposition processes were in situ monitored by reflection high energy electron diffraction (RHEED). X-ray diffraction (XRD) was carried out to study the growth orientation and crystalline quality of STO films. The interfacial characters and epitaxial relationships were also investigated by high revolution transition electron microscope and selected area electron diffraction (SAED). According to the RHEED observation, the lowest epitaxy temperature of STO on TiO{sub 2} buffered GaN was decreased compared with the direct deposited one. The epitaxial relationship was (111)[110]STO//(0002)[1120]GaN in both cases as confirmed by RHEED, XRD, and SAED. The full width at half maximum of omega-scan and PHI-scan of STO on TiO{sub 2} buffered GaN was reduced compared with that deposited on bare GaN, indicating that epitaxial quality of STO film is improved by inserting TiO{sub 2} layer. In summary, the lattice mismatch was reduced by inserting rutile TiO{sub 2}. As a result, the crystalline temperature was reduced and enhanced epitaxial quality of STO thin film was obtained.

  17. Hexagonal photonic crystal waveguide based on barium titanate thin films

    NASA Astrophysics Data System (ADS)

    Li, Jianheng; Liu, Zhifu; Wessels, Bruce W.; Tu, Yongming; Ho, Seng-Tiong; Joshi-Imre, Alexandra; Ocola, Leonidas E.

    2011-03-01

    The simulation, fabrication and measurement of nonlinear photonic crystals (PhCs) with hexagonal symmetry in epitaxial BaTiO3 were investigated. The optical transmission properties of a PhC were simulated by a 2-D finite-difference time domain (FDTD) method. A complete bandgap exists for both the TE and TM optical modes. The fabricated PhC has a well-defined stop band over the spectral region of 1525 to 1575 nm. A microcavity structure was also fabricated by incorporation of a line defect in the PhC. Transmission of the microcavity structure over the spectral region from 1456 to 1584nm shows a well-defined 5 nm wide window at 1495nm. Simulations indicate that the phase velocity matched PhC microcavity device of 0.5 mm long can potentially serve as modulator with a 3 dB bandwidth of 4 THz.

  18. Visualizing weak ferromagnetic domains in multiferroic hexagonal ferrite thin film

    NASA Astrophysics Data System (ADS)

    Wang, Wenbo; Mundy, Julia A.; Brooks, Charles M.; Moyer, Jarrett A.; Holtz, Megan E.; Muller, David A.; Schlom, Darrell G.; Wu, Weida

    2017-04-01

    We report cryogenic magnetic force microscopy (MFM) studies of a 200-nm-thick hexagonal (h ) LuFeO3 film grown by molecular-beam epitaxy on a (111)-oriented yttria-stabilized cubic-zirconia substrate. Labyrinthlike domains ˜1.8 μ m in size were observed after zero-field cooling below the Néel temperature, TN≈147 K, where weak ferromagnetic order (P 63 cm) with a canted moment of MS≈0.02 μB/f .u . exists. At 6 K, MFM images of the magnetization reversal process reveal a typical domain behavior of a pinning-dominated hard magnet. The pinning strength is substantially reduced at elevated temperature. The temperature dependence of the domain contrast demonstrates that our MFM is able to detect the domain contrast of magnets with tiny magnetic moments (˜0.002 μB/f .u . ). An upper limit of the linear magnetoelectric coefficient of h -LuFeO3 (αz z<6 ps /m ) is estimated by magnetoelectric force microscopy measurements.

  19. Synthesis of Hexagonal FeMnP Thin Films from a Single-Source Molecular Precursor.

    PubMed

    Leitner, Andrew P; Schipper, Desmond E; Chen, Jing-Han; Colson, Adam C; Rusakova, Irene; Kumar Rai, Binod; Morosan, Emilia; Whitmire, Kenton Herbert

    2017-03-08

    The first heterobimetallic phosphide thin film containing iron, manganese, and phosphorus derived from the single-source precursor FeMn(CO)8(μ-PH2) has been prepared using a home-built metal-organic chemical vapor deposition apparatus. The thin film contains the same ratio of iron, manganese, and phosphorus as the initial precursor. The film becomes oxidized when deposited on a quartz substrate whereas the film deposited on an alumina substrate provides a more homogeneous product. Powder X-ray diffraction confirms the formation of metastable, hexagonal FeMnP phase that was previously only observed at temperatures above 1200 ºC. Selected area electron diffraction on single crystals isolated from the films were indexed to the hexagonal phase. The effective moment of the films (µeff = 3.68µB) matches the previously reported theoretical value for the metastable hexagonal phase whereas the more stable orthorhombic phase is known to be antiferromagnetic. These results not only demonstrate the successful synthesis of a bimetallic, ternary thin film from a single-source precursor, but also the first low temperature approach to the hexagonal phase of FeMnP.

  20. The importance of structural inhomogeneity in GaN thin films

    NASA Astrophysics Data System (ADS)

    Liliental-Weber, Z.; Reis, Roberto dos; Weyher, Jan L.; Staszczak, Grzegorz; Jakieła, Rafał

    2016-12-01

    This paper describes two types of MOCVD-grown n-type GaN layers (Samples A and B) with similar carrier concentration but behaved differently under galvanic photo-etching. In order to understand this behavior, Transmission Electron Microscopy (TEM) for cross-section and plan-view samples, Secondary Ion Mass Spectroscopy (SIMS) and photoluminescence (PL) techniques were applied. SIMS studies showed that Si, C and O are approximately at the same concentration in both samples, but Sample B also contained Fe and Mg. Both GaN samples were grown on sapphire substrate with Ga growth polarity, which was confirmed by Convergent Beam Electron Diffraction (CBED). Despite a smaller layer thickness in Sample B, the density of edge dislocations is almost one order of magnitude lower than in Sample A. In addition, planar defects formed in this sample in the transition area between the undoped buffer and Si doped layers resulted in a substantial decrease in the density of screw dislocations at the sample surface. These planar defects most probably gave rise to the PL lines observed at 3.42 eV and 3.32 eV. The new PL lines that only appeared in Sample B might be related to Mg impurities found in this sample. There were no detectable gettering of these impurities at dislocations using different diffraction conditions. However, Fe rich platelets were found only in Sample B due to the presence of Fe as well as hexagonal features, similar to defects reported earlier in highly Mg-doped GaN. These structural and chemical non-uniformities between the two GaN samples can explain their different etching behaviors. This paper demonstrates that samples with similar carrier concentrations do not necessarily ensure similar structural and optical properties and that additional material characterization are needed to ensure that devices built on such samples have similar performance.

  1. Photoluminescence Observation of GaN Thin Films Treated by Inductively-Coupled Plasmas

    NASA Astrophysics Data System (ADS)

    Nakamura, Keiji; Itoh, Noriyoshi; Nakano, Yoshitaka; Sugai, Hideo

    2011-10-01

    This paper reports observations of photoluminescence from plasma-treated GaN thin films. A 10 mTorr Ar ICP was used, and irradiation of 313 nm ultraviolet (UV) light from Hg-Xe light source induced the photoluminescence of the GaN film. In both in-situ and ex-situ observations, significant yellow luminescence was observed visually, and the ex-situ observed luminescence ranges in a wavelength of 500-800 nm corresponding to defect-states-related transition. The measurements also revealed that the luminescence also contains UV emission at a wavelength of ~365 nm attributed to transition related to near band edges. In order to examine effects of the plasma on the luminescence, the ex-situ observation was made as a function of the plasma treatment time. As the treatment time increased, both the UV and the luminescence intensity decreased, and the decrease in the emission became significant when the 313 nm UV light was irradiated onto the plasma-exposed GaN surface. These results suggested that plasma-induced defect formation leads to the luminescence degradation, and that the photoluminescence observation will be useful for damage monitoring of the GaN surface. This work is partly supported by the 2nd stage Knowledge Cluster Initiative and Grant-in-Aid for Scientific Research (C) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

  2. Effective Hamiltonian for surface states of topological insulator thin films with hexagonal warping

    SciTech Connect

    Siu, Zhuo Bin; Jalil, Mansoor B. A.; Tan, Seng Ghee

    2016-05-15

    The effective Hamiltonian of the surface states on semi-infinite slabs of the topological insulators (TI) Bi{sub 2}Te{sub 3} and Bi{sub 2}Se{sub 3} require the addition of a cubic momentum hexagonal warping term on top of the usual Dirac fermion Hamiltonian in order to reproduce the experimentally measured constant energy contours at intermediate values of Fermi energy. In this work, we derive the effective Hamiltonian for the surface states of a Bi{sub 2}Se{sub 3} thin film incorporating the corresponding hexagonal warping terms. We then calculate the dispersion relation of the effective Hamiltonian and show that the hexagonal warping leads distorts the equal energy contours from the circular cross sections of the Dirac cones.

  3. Far-infrared transmission in GaN, AlN, and AlGaN thin films grown by molecular beam epitaxy

    SciTech Connect

    Ibanez, J.; Hernandez, S.; Alarcon-Llado, E.; Cusco, R.; Artus, L.; Novikov, S. V.; Foxon, C. T.; Calleja, E.

    2008-08-01

    We present a far-infrared transmission study on group-III nitride thin films. Cubic GaN and AlN layers and c-oriented wurtzite GaN, AlN, and Al{sub x}Ga{sub 1-x}N (x<0.3) layers were grown by molecular beam epitaxy on GaAs and Si(111) substrates, respectively. The Berreman effect allows us to observe simultaneously the transverse optic and the longitudinal optic phonons of both the cubic and the hexagonal films as transmission minima in the infrared spectra acquired with obliquely incident radiation. We discuss our results in terms of the relevant electromagnetic theory of infrared transmission in cubic and wurtzite thin films. We compare the infrared results with visible Raman-scattering measurements. In the case of films with low scattering volumes and/or low Raman efficiencies and also when the Raman signal of the substrate material obscures the weaker peaks from the nitride films, we find that the Berreman technique is particularly useful to complement Raman spectroscopy.

  4. Heteroepitaxial VO2 thin films on GaN: Structure and metal-insulator transition characteristics

    NASA Astrophysics Data System (ADS)

    Zhou, You; Ramanathan, Shriram

    2012-10-01

    Monolithic integration of correlated oxide and nitride semiconductors may open up new opportunities in solid-state electronics and opto-electronics that combine desirable functional properties of both classes of materials. Here, we report on epitaxial growth and phase transition-related electrical properties of vanadium dioxide (VO2) thin films on GaN epitaxial layers on c-sapphire. The epitaxial relation is determined to be (010)vo2‖(0001)GaN‖(0001)A12O3 and [100]vo2‖[1¯21¯0]A12O3 from x-ray diffraction. VO2 heteroepitaxial growth and lattice mismatch are analyzed by comparing the GaN basal plane (0001) with the almost close packed corrugated oxygen plane in vanadium dioxide and an experimental stereographic projection describing the orientation relationship is established. X-ray photoelectron spectroscopy suggests a slightly oxygen rich composition at the surface, while Raman scattering measurements suggests that the quality of GaN layer is not significantly degraded by the high-temperature deposition of VO2. Electrical characterization of VO2 films on GaN indicates that the resistance changes by about four orders of magnitude upon heating, similar to epitaxial VO2 films grown directly on c-sapphire. It is shown that the metal-insulator transition could also be voltage-triggered at room temperature and the transition threshold voltage scaling variation with temperature is analyzed in the framework of a current-driven Joule heating model. The ability to synthesize high quality correlated oxide films on GaN with sharp phase transition could enable new directions in semiconductor-photonic integrated devices.

  5. Large-roll growth of 25-inch hexagonal BN monolayer film for self-release buffer layer of free-standing GaN wafer

    NASA Astrophysics Data System (ADS)

    Wu, Chenping; Soomro, Abdul Majid; Sun, Feipeng; Wang, Huachun; Huang, Youyang; Wu, Jiejun; Liu, Chuan; Yang, Xiaodong; Gao, Na; Chen, Xiaohong; Kang, Junyong; Cai, Duanjun

    2016-10-01

    Hexagonal boron nitride (h-BN) is known as promising 2D material with a wide band-gap (~6 eV). However, the growth size of h-BN film is strongly limited by the size of reaction chamber. Here, we demonstrate the large-roll synthesis of monolayer and controllable sub-monolayer h-BN film on wound Cu foil by low pressure chemical vapor deposition (LPCVD) method. By winding the Cu foil substrate into mainspring shape supported by a multi-prong quartz fork, the reactor size limit could be overcome by extending the substrate area to a continuous 2D curl of plane inward. An extremely large-size monolayer h-BN film has been achieved over 25 inches in a 1.2” tube. The optical band gap of h-BN monolayer was determined to be 6.0 eV. The h-BN film was uniformly transferred onto 2” GaN or 4” Si wafer surfaces as a release buffer layer. By HVPE method, overgrowth of thick GaN wafer over 200 μm has been achieved free of residual strain, which could provide high quality homo-epitaxial substrate.

  6. Large-roll growth of 25-inch hexagonal BN monolayer film for self-release buffer layer of free-standing GaN wafer.

    PubMed

    Wu, Chenping; Soomro, Abdul Majid; Sun, Feipeng; Wang, Huachun; Huang, Youyang; Wu, Jiejun; Liu, Chuan; Yang, Xiaodong; Gao, Na; Chen, Xiaohong; Kang, Junyong; Cai, Duanjun

    2016-10-19

    Hexagonal boron nitride (h-BN) is known as promising 2D material with a wide band-gap (~6 eV). However, the growth size of h-BN film is strongly limited by the size of reaction chamber. Here, we demonstrate the large-roll synthesis of monolayer and controllable sub-monolayer h-BN film on wound Cu foil by low pressure chemical vapor deposition (LPCVD) method. By winding the Cu foil substrate into mainspring shape supported by a multi-prong quartz fork, the reactor size limit could be overcome by extending the substrate area to a continuous 2D curl of plane inward. An extremely large-size monolayer h-BN film has been achieved over 25 inches in a 1.2" tube. The optical band gap of h-BN monolayer was determined to be 6.0 eV. The h-BN film was uniformly transferred onto 2" GaN or 4" Si wafer surfaces as a release buffer layer. By HVPE method, overgrowth of thick GaN wafer over 200 μm has been achieved free of residual strain, which could provide high quality homo-epitaxial substrate.

  7. Large-roll growth of 25-inch hexagonal BN monolayer film for self-release buffer layer of free-standing GaN wafer

    PubMed Central

    Wu, Chenping; Soomro, Abdul Majid; Sun, Feipeng; Wang, Huachun; Huang, Youyang; Wu, Jiejun; Liu, Chuan; Yang, Xiaodong; Gao, Na; Chen, Xiaohong; Kang, Junyong; Cai, Duanjun

    2016-01-01

    Hexagonal boron nitride (h-BN) is known as promising 2D material with a wide band-gap (~6 eV). However, the growth size of h-BN film is strongly limited by the size of reaction chamber. Here, we demonstrate the large-roll synthesis of monolayer and controllable sub-monolayer h-BN film on wound Cu foil by low pressure chemical vapor deposition (LPCVD) method. By winding the Cu foil substrate into mainspring shape supported by a multi-prong quartz fork, the reactor size limit could be overcome by extending the substrate area to a continuous 2D curl of plane inward. An extremely large-size monolayer h-BN film has been achieved over 25 inches in a 1.2” tube. The optical band gap of h-BN monolayer was determined to be 6.0 eV. The h-BN film was uniformly transferred onto 2” GaN or 4” Si wafer surfaces as a release buffer layer. By HVPE method, overgrowth of thick GaN wafer over 200 μm has been achieved free of residual strain, which could provide high quality homo-epitaxial substrate. PMID:27756906

  8. Strong second harmonic generation in SiC, ZnO, GaN two-dimensional hexagonal crystals from first-principles many-body calculations.

    PubMed

    Attaccalite, C; Nguer, A; Cannuccia, E; Grüning, M

    2015-04-14

    The second harmonic generation (SHG) intensity spectrum of SiC, ZnO, GaN two-dimensional hexagonal crystals is calculated by using a real-time first-principles approach based on Green's function theory [Attaccalite et al., Phys. Rev. B: Condens. Matter Mater. Phys. 2013 88, 235113]. This approach allows one to go beyond the independent particle description used in standard first-principles nonlinear optics calculations by including quasiparticle corrections (by means of the GW approximation), crystal local field effects and excitonic effects. Our results show that the SHG spectra obtained using the latter approach differ significantly from their independent particle counterparts. In particular they show strong excitonic resonances at which the SHG intensity is about two times stronger than within the independent particle approximation. All the systems studied (whose stabilities have been predicted theoretically) are transparent and at the same time exhibit a remarkable SHG intensity in the range of frequencies at which Ti:sapphire and Nd:YAG lasers operate; thus they can be of interest for nanoscale nonlinear frequency conversion devices. Specifically the SHG intensity at 800 nm (1.55 eV) ranges from about 40-80 pm V(-1) in ZnO and GaN to 0.6 nm V(-1) in SiC. The latter value in particular is 1 order of magnitude larger than values in standard nonlinear crystals.

  9. Synthesis and ionic liquid gating of hexagonal WO3 thin films

    NASA Astrophysics Data System (ADS)

    Wu, Phillip M.; Ishii, Satoshi; Tanabe, Kenji; Munakata, Ko; Hammond, R. H.; Tokiwa, Kazuyasu; Geballe, T. H.; Beasley, M. R.

    2015-01-01

    Via thin film deposition techniques, the meta-stable in bulk crystal hexagonal phase of tungsten oxide (hex-WO3) is stabilized as a thin film. The hex-WO3 structure is potentially promising for numerous applications and is related to the structure for superconducting compounds found in WO3. Utilizing ionic liquid gating, carriers were electrostatically induced in the films and an insulator-to-metal transition is observed. These results show that ionic liquid gating is a viable technique to alter the electrical transport properties of WO3.

  10. Synthesis and ionic liquid gating of hexagonal WO{sub 3} thin films

    SciTech Connect

    Wu, Phillip M. E-mail: beasley@stanford.edu; Munakata, Ko; Hammond, R. H.; Geballe, T. H.; Beasley, M. R. E-mail: beasley@stanford.edu; Ishii, Satoshi; Tanabe, Kenji; Tokiwa, Kazuyasu

    2015-01-26

    Via thin film deposition techniques, the meta-stable in bulk crystal hexagonal phase of tungsten oxide (hex-WO{sub 3}) is stabilized as a thin film. The hex-WO{sub 3} structure is potentially promising for numerous applications and is related to the structure for superconducting compounds found in WO{sub 3}. Utilizing ionic liquid gating, carriers were electrostatically induced in the films and an insulator-to-metal transition is observed. These results show that ionic liquid gating is a viable technique to alter the electrical transport properties of WO{sub 3}.

  11. Low temperature thin film transistors with hollow cathode plasma-assisted atomic layer deposition based GaN channels

    SciTech Connect

    Bolat, S. E-mail: aokyay@ee.bilkent.edu.tr; Tekcan, B.; Ozgit-Akgun, C.; Biyikli, N.; Okyay, A. K. E-mail: aokyay@ee.bilkent.edu.tr

    2014-06-16

    We report GaN thin film transistors (TFT) with a thermal budget below 250 °C. GaN thin films are grown at 200 °C by hollow cathode plasma-assisted atomic layer deposition (HCPA-ALD). HCPA-ALD-based GaN thin films are found to have a polycrystalline wurtzite structure with an average crystallite size of 9.3 nm. TFTs with bottom gate configuration are fabricated with HCPA-ALD grown GaN channel layers. Fabricated TFTs exhibit n-type field effect characteristics. N-channel GaN TFTs demonstrated on-to-off ratios (I{sub ON}/I{sub OFF}) of 10{sup 3} and sub-threshold swing of 3.3 V/decade. The entire TFT device fabrication process temperature is below 250 °C, which is the lowest process temperature reported for GaN based transistors, so far.

  12. Minimal Topographic Patterns for Guiding Hexagonal Arrays of Cylindrical Microdomains in Block Copolymer Thin Films

    NASA Astrophysics Data System (ADS)

    Choi, Jaewon; Li, Yinyong; Gunkel, Ilja; Sun, Zhiwei; Liu, Feng; Carter, Kenneth; Russell, Thomas

    Topographically patterned substrates have been widely studied to control the lateral order of block copolymer (BCP) microdomains in thin films. However, most studies have been focused on deep topographic patterns, where a confinement depth is comparable to or larger than domain spacing of BCP, limiting the grain size of BCP due to the confinement width. Also, the investigation of BCP microdomains using grazing incidence small angle X-ray scattering (GISAXS) is limited because the scattering peaks from BCP are generally hidden by the strong scattering peaks from the deep topographic pattern. Here, we present the use of minimal topographic patterns for guiding hexagonal arrays of cylindrical microdomains of poly(styrene-b-ethylene oxide) (PS-b-PEO) thin films. Since the confinement depth of the minimal pattern is much smaller than domain spacing of BCP, this enables cylindrical microdomains to overcome the confined width, generating macroscopic ordered hexagonal arrays. In the GISAXS experiment, we confirmed that the (10) plane of hexagonal arrays was parallel to the direction of the trench by rotating the sample stage and changing the incident angle of X-ray.

  13. Epitaxial Growth and Electronic Properties of Large Hexagonal Graphene Domains on Cu(111) Thin Film

    NASA Astrophysics Data System (ADS)

    Ago, Hiroki; Kawahara, Kenji; Ogawa, Yui; Tanoue, Shota; Bissett, Mark A.; Tsuji, Masaharu; Sakaguchi, Hidetsugu; Koch, Roland J.; Fromm, Felix; Seyller, Thomas; Komatsu, Katsuyoshi; Tsukagoshi, Kazuhito

    2013-07-01

    Large hexagonal single-crystalline domains of single-layer graphene are epitaxially grown by ambient-pressure chemical vapor deposition over a thin Cu(111) film deposited on c-plane sapphire. The hexagonal graphene domains with a maximum size of 100 µm are oriented in the same direction due to the epitaxial growth. Reflecting high crystallinity, a clear band structure with the Dirac cone is observed by angle-resolved photoelectron spectroscopy (ARPES), and a high carrier mobility exceeding 4,000 cm2 V-1 s-1 is obtained on SiO2/Si at room temperature. Our epitaxial approach combined with large domain growth is expected to contribute to future electronic applications.

  14. Realization of compressively strained GaN films grown on Si(110) substrates by inserting a thin AlN/GaN superlattice interlayer

    SciTech Connect

    Shen, X. Q.; Takahashi, T.; Kawashima, H.; Ide, T.; Shimizu, M.

    2012-07-16

    We investigate the strain properties of GaN films grown by plasma-assisted molecular beam epitaxy on Si(110) substrates. It is found that the strain of the GaN film can be converted from a tensile to a compressive state simply by inserting a thin AlN/GaN superlattice structure (SLs) within the GaN film. The GaN layers seperated by the SLs can have different strain states, which indicates that the SLs plays a key role in the strain modulation during the growth and the cooling down processes. Using this simple technique, we grow a crack-free GaN film exceeding 2-{mu}m-thick. The realization of the compressively strained GaN film makes it possible to grow thick GaN films without crack generation on Si substrates for optic and electronic device applications.

  15. Oriented Y-type hexagonal ferrite thin films prepared by chemical solution deposition

    NASA Astrophysics Data System (ADS)

    Buršík, J.; Kužel, R.; Knížek, K.; Drbohlav, I.

    2013-07-01

    Thin films of Ba2Zn2Fe12O22 (Y) hexaferrite were prepared through the chemical solution deposition method on SrTiO3(1 1 1) (ST) single crystal substrates using epitaxial SrFe12O19 (M) hexaferrite thin layer as a seed template layer. The process of crystallization was mainly investigated by means of X-ray diffraction and atomic force microscopy. A detailed inspection revealed that growth of seed layer starts through the break-up of initially continuous film into isolated grains with expressive shape anisotropy and hexagonal habit. The vital parameters of the seed layer, i.e. thickness, substrate coverage, crystallization conditions and temperature ramp were optimized with the aim to obtain epitaxially crystallized Y phase. X-ray diffraction Pole figure measurements and Φ scans reveal perfect parallel in-plane alignment of SrTiO3 substrate and both hexaferrite phases.

  16. A Rapid Method for Deposition of Sn-Doped GaN Thin Films on Glass and Polyethylene Terephthalate Substrates

    NASA Astrophysics Data System (ADS)

    Pat, Suat; Özen, Soner; Korkmaz, Şadan

    2017-09-01

    We report the influence of Sn doping on microstructure, surface, and optical properties of GaN thin films deposited on glass and polyethylene terephthalate (PET) substrate. Sn-doped GaN thin films have been deposited by thermionic vacuum arc (TVA) at low temperature. TVA is a rapid deposition technology for thin film growth. Surface and optical properties of the thin films were presented. Grain size, height distribution, roughness values were determined. Grain sizes were calculated as 20 nm and 13 nm for glass and PET substrates, respectively. Nano crystalline forms were shown by field emission scanning electron microscopy. Optical band gap values were determined by optical methods and photoluminescence measurement. The optical band gap values of Sn doped GaN on glass and PET were determined to be approximately ˜3.40 eV and ˜3.47 eV, respectively. As a result, TVA is a rapid and low temperature deposition technology for the Sn doped GaN deposited on glass and PET substrate.

  17. Oriented Y-type hexagonal ferrite thin films prepared by chemical solution deposition

    SciTech Connect

    Buršík, J.; Kužel, R.; Knížek, K.; Drbohlav, I.

    2013-07-15

    Thin films of Ba{sub 2}Zn{sub 2}Fe{sub 12}O{sub 22} (Y) hexaferrite were prepared through the chemical solution deposition method on SrTiO{sub 3}(1 1 1) (ST) single crystal substrates using epitaxial SrFe{sub 12}O{sub 19} (M) hexaferrite thin layer as a seed template layer. The process of crystallization was mainly investigated by means of X-ray diffraction and atomic force microscopy. A detailed inspection revealed that growth of seed layer starts through the break-up of initially continuous film into isolated grains with expressive shape anisotropy and hexagonal habit. The vital parameters of the seed layer, i.e. thickness, substrate coverage, crystallization conditions and temperature ramp were optimized with the aim to obtain epitaxially crystallized Y phase. X-ray diffraction Pole figure measurements and Φ scans reveal perfect parallel in-plane alignment of SrTiO{sub 3} substrate and both hexaferrite phases. - Graphical abstract: XRD pole figure and AFM patterns of Ba{sub 2}Zn{sub 2}Fe{sub 12}O{sub 22} thin film epitaxially grown on SrTiO{sub 3}(1 1 1) single crystal using seeding layer templating. - Highlights: • Single phase Y-type hexagonal ferrite thin films were prepared by CSD method. • Seed M layer breaks into isolated single crystal islands and serves as a template. • Large seed grains grow by consuming the grains within the bulk of recoated film. • We explained the observed orientation relation of epitaxial domains. • Epitaxial growth on SrTiO{sub 3}(1 1 1) with relation (0 0 1){sub M,Y}//(1 1 1){sub ST}+[1 0 0]{sub M,Y}//[2 −1 −1]{sub ST}.

  18. Synthesis and Luminescent Properties of GaN and GaN-Mn Blue Nanocrystalline Thin-film Phosphor for FED

    SciTech Connect

    Bondar, V D; Felter, T E; Hunt, C E; Kucharsky, I Y; Chakhovskoi, A G

    2003-04-09

    The technologies of fabrication of thin film phosphors based on gallium nitride using rf-magnetron sputtering are developed and structural properties of films are studied. Luminescence and electron spin resonance (ESR) spectra of GaN and GaN-Mn thin films have been studied. The correlation between cathodoluminescence intensity and conductivity of GaN films has been found. The nature of emission centers in GaN and GaN-Mn thin films is discussed as well as mechanism of luminescence in these films is proposed.

  19. Heteroepitaxial VO{sub 2} thin films on GaN: Structure and metal-insulator transition characteristics

    SciTech Connect

    Zhou You; Ramanathan, Shriram

    2012-10-01

    Monolithic integration of correlated oxide and nitride semiconductors may open up new opportunities in solid-state electronics and opto-electronics that combine desirable functional properties of both classes of materials. Here, we report on epitaxial growth and phase transition-related electrical properties of vanadium dioxide (VO{sub 2}) thin films on GaN epitaxial layers on c-sapphire. The epitaxial relation is determined to be (010){sub vo{sub 2}} parallel (0001){sub GaN} parallel (0001){sub A1{sub 2O{sub 3}}} and [100]{sub vo{sub 2}} parallel [1210]{sub GaN} parallel [0110]{sub A1{sub 2O{sub 3}}} from x-ray diffraction. VO{sub 2} heteroepitaxial growth and lattice mismatch are analyzed by comparing the GaN basal plane (0001) with the almost close packed corrugated oxygen plane in vanadium dioxide and an experimental stereographic projection describing the orientation relationship is established. X-ray photoelectron spectroscopy suggests a slightly oxygen rich composition at the surface, while Raman scattering measurements suggests that the quality of GaN layer is not significantly degraded by the high-temperature deposition of VO{sub 2}. Electrical characterization of VO{sub 2} films on GaN indicates that the resistance changes by about four orders of magnitude upon heating, similar to epitaxial VO{sub 2} films grown directly on c-sapphire. It is shown that the metal-insulator transition could also be voltage-triggered at room temperature and the transition threshold voltage scaling variation with temperature is analyzed in the framework of a current-driven Joule heating model. The ability to synthesize high quality correlated oxide films on GaN with sharp phase transition could enable new directions in semiconductor-photonic integrated devices.

  20. Study of different type of dislocations in GaN thin films

    NASA Astrophysics Data System (ADS)

    Yu, L. P.; Shi, J. Y.; Wang, Y. Z.; Zhang, H.

    2004-08-01

    High-resolution X-ray diffraction was used to analyze the type of dislocations in GaN epitaxial thin films. Rocking curves of five planes were investigated, (0 0 0 2) , (1 0 1¯ 3) , (1 0 1¯ 2) , (1 0 1¯ 1) , and (2 0 2¯ 1) , respectively. Pseudo-Voigt function was used to simulate the rocking-curve of every plane. Every extension of the rocking-curve was regarded as the effect of the interaction of the twist and tilt fractions of the dislocations. From the result, it is found that carrier mobility is more sensitive to substrate normal tilt dislocation than to in-plane twist and the interaction of the twist and tilt fractions also affect the carrier mobility.

  1. MOCVD of ZnO thin films for potential use as compliant layers for GaN on Si

    NASA Astrophysics Data System (ADS)

    Black, Kate; Jones, Anthony C.; Chalker, Paul R.; Gaskell, Jeffrey M.; Murray, Robert T.; Joyce, Tim B.; Rushworth, Simon A.

    2008-03-01

    This paper explores the use of nanostructured zinc oxide (ZnO) films as a compliant buffer layer for the growth of gallium nitride (GaN) on silicon substrates. Thin films of ZnO have been deposited on silicon (1 1 1) substrates by liquid injection metalorganic chemical vapour deposition (MOCVD) using dimethyl zinc-tetrahydrofuran adduct and oxygen. The use of the adduct complex avoids pre-reaction between the dialkyl zinc complex and oxygen which has been observed elsewhere. ZnO films deposited by this method were stoichiometric and of high purity, with no detectable carbon contamination. Films were deposited over a temperature range 350-550 °C, and exhibited a nanowire-like morphology. Subsequent deposition of GaN layers grown by molecular beam epitaxy (MBE) on the ZnO film resulted in the transformation of the nanowires to gallium oxide, accompanied by virtually complete removal of zinc from the layer. A heteroepitaxially oriented ( c-axis) GaN/gallium oxide/silicon structure was produced after the nitride deposition which consisted of characteristic columnar GaN with the GaN[0 0 0 1]||Si [1 1 1]. Selective area electron diffraction of the by-product oxide interlayer showed a polycrystalline-like behaviour that gave rise to a random azimuthal distribution of the GaN grains.

  2. Growth of AlN and GaN thin films via OMVPE and gas source MBE and their characterization

    NASA Astrophysics Data System (ADS)

    Davis, Robert F.; Weeks, T. W.; Bremser, M. D.; Tanaka, S.; Kern, R. S.; Sitar, Z.; Ailey, K. S.; Perry, W. G.; Wang, C.

    1997-02-01

    Thin films of AlN and GaN are deposited primarily via the common forms of organometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE). Sapphire is the most common substrate; however, a host of materials have been used with varying degrees of success. Both growth techniques have been employed by the authors to grow AlN and GaN primarily on 6H-SiC(0001). The mismatch in atomic layer stacking sequences along the growth direction produces inversion domain boundaries in the AlN at the SiC steps; this sequence problem may discourage the nucleation of GaN. Films of AlN and GaN grown by MBE at 650°C are textured; monocrystalline films are achieved at 1050°C by this technique and OMVPE. Donor and acceptor doping of GaN has been achieved via MBE without post growth annealing. Acceptor doping in CVD material requires annealing to displace the H from the Mg and eventually remove it from the material. High brightness light emitting diodes are commercially available; however, numerous concerns regarding metal and nitrogen sources, heteroepitaxial nucleation, the role of buffer layers, surface migration rates as a function of temperature, substantial defect densities and their effect on film and device properties, ohmic and rectifying contacts, wet and dry etching and suitable gate and field insulators must and are being addressed.

  3. First-principle-based full-dispersion Monte Carlo simulation of the anisotropic phonon transport in the wurtzite GaN thin film

    SciTech Connect

    Wu, Ruikang; Hu, Run E-mail: luoxb@hust.edu.cn; Luo, Xiaobing E-mail: luoxb@hust.edu.cn

    2016-04-14

    In this study, we developed a first-principle-based full-dispersion Monte Carlo simulation method to study the anisotropic phonon transport in wurtzite GaN thin film. The input data of thermal properties in MC simulations were calculated based on the first-principle method. The anisotropy of thermal conductivity in bulk wurtzite GaN is found to be strengthened by isotopic scatterings and reduced temperature, and the anisotropy reaches 40.08% for natural bulk GaN at 100 K. With the GaN thin film thickness decreasing, the anisotropy of the out-of-plane thermal conductivity is heavily reduced due to both the ballistic transport and the less importance of the low-frequency phonons with anisotropic group velocities. On the contrary, it is observed that the in-plane thermal conductivity anisotropy of the GaN thin film is strengthened by reducing the film thickness. And the anisotropy reaches 35.63% when the natural GaN thin film thickness reduces to 50 nm at 300 K with the degree of specularity being zero. The anisotropy is also improved by increasing the surface roughness of the GaN thin film.

  4. Vertically self-ordered orientation of nanocrystalline hexagonal boron nitride thin films for enhanced thermal characteristics.

    PubMed

    Cometto, Olivier; Sun, Bo; Tsang, Siu Hon; Huang, Xi; Koh, Yee Kan; Teo, Edwin Hang Tong

    2015-12-07

    Vertically self-ordered hexagonal boron nitride (ordered h-BN) is a highly ordered turbostratic BN (t-BN) material similar to hexagonal BN, with its planar structure perpendicularly oriented to the substrate. The ordered h-BN thin films were grown using a High Power Impulse Magnetron Sputtering (HiPIMS) system with a lanthanum hexaboride (LaB6) target reactively sputtered in nitrogen gas. The best vertical alignment was obtained at room temperature, with a grounded bias and a HiPIMS peak power density of 60 W cm(-2). Even though the film contains up to 7.5 at% lanthanum, it retains its highly insulative properties and it was observed that an increase in compressive stress is correlated to an increase in film ordering quality. Importantly, the thermal conductivity of vertically ordered h-BN is considerably high at 5.1 W m(-1) K(-1). The favourable thermal conductivity coupled with the dielectric properties of this novel material and the low temperature growth could outperform SiO2 in high power density electronic applications.

  5. Investigation of hexagonal boron nitride as an atomically thin corrosion passivation coating in aqueous solution.

    PubMed

    Zhang, Jing; Yang, Yingchao; Lou, Jun

    2016-09-09

    Hexagonal boron nitride (h-BN) atomic layers were utilized as a passivation coating in this study. A large-area continuous h-BN thin film was grown on nickel foil using a chemical vapor deposition method and then transferred onto sputtered copper as a corrosion passivation coating. The corrosion passivation performance in a Na2SO4 solution of bare and coated copper was investigated by electrochemical methods including cyclic voltammetry (CV), Tafel polarization and electrochemical impedance spectroscopy (EIS). CV and Tafel analysis indicate that the h-BN coating could effectively suppress the anodic dissolution of copper. The EIS fitting result suggests that defects are the dominant leakage source on h-BN films, and improved anti-corrosion performances could be achieved by further passivating these defects.

  6. Suppression of Cross Contamination in Multi-Layer Thin Film Prepared by Using Rotating Hexagonal Sputtering Cathode.

    PubMed

    Park, Se Yeon; Choi, Bum Ho; Lee, Jong Ho

    2015-01-01

    In this study, single- and multi-layered thin films were prepared on a glass substrate using a newly developed rotating hexagonal sputtering cathode in a single chamber. The rotatinghexagonal sputtering cathode can install up to six different sputtering targets or six single targets in a cathode. Using the rotating hexagonal cathode, we prepared a single-layered AZO film and a multi-layer film to evaluate the performance of hexagonal gun. Cross-contamination, which is often observed in multi-layer thin film preparation, was suppressed to nearly zero by controlling process parameters and revising hardware. Energy-saving effects of five-layered glass were also verified by measuring the temperature.

  7. Growth, Nitrogen Vacancy Reduction and Solid Solution Formation in Cubic GaN Thin Films and the Subsequent Fabrication of Superlattice Structures Using AIN and InN

    DTIC Science & Technology

    1992-12-01

    AD-A258 804 Final Technical Report Ii Growth, Nitrogen Vacancy Reduction and Solid Solution Formation in Cubic GaN Thin Films and the Subsequent...Technical 6/1/86-12/31/92 4. TITLE AND SUBTITLE Growth, Nitrogen Vacancy Reduction and 5. FUNDING NUMBERS Solid Solution Formation in Cubic GaN Thin...According to the structural and chemical analyses, there is no reason to believe that a homogeneous solid solution close to this composition had

  8. Photoluminescence study of wurtzite Si-doped GaN thin films

    NASA Astrophysics Data System (ADS)

    Soltani, Mohammed; Carlone, Cosmo; Charbonneau, N. Sylvain; Khanna, Shyam M.

    1998-10-01

    The photoluminescence (PL) temperature dependence of wurtzite n-type GaN thin films grown on (0001) sapphire substrates by Magnetron sputter epitaxy is reported. Samples were non-intentionally doped, lightly and highly Si-doped. The PL of non-intentionally doped samples consist of the near band edge emission and a broad yellow band (YB) near 2.2 eV. This yellow emission is equally present in spectra of all Si-doped samples. The bound exciton (D0-X) at 3.488 eV and (A0-X) at 3.456 eV are present only in the lightly Si-doped samples. The evolution of the energy position of the (D0-X) is the same as the band gap temperature variation, but the (A0-X) transition is anormally independent of the temperature in the range studied here. In both Si-doped GaN samples a peak at 3.318 eV and transitions between 3.36 and 3.39 eV are observed. The temperature dependence of the latter shows a fine structure composed of four peaks at 3.364 eV, 3.368 eV, 3.375 eV and 3.383 eV. They are tentatively attributed to the superposition of two donor-acceptor and band-acceptor transitions. This interpretation implies the presence of two donors (D1,D2) and two acceptors (A1,A2). From the energy position of the band-acceptor and the energy gap of GaN at 20 K, an acceptor ionization energy of 120 and 135 meV respectively is obtained. Assuming 10 meV for a Coulomb interaction energy of the ionized donor-acceptor pairs, a donor ionization energy of 14 and 18 meV respectively is obtained from the energy difference between the donor-acceptor and the band-acceptor positions. An activation energy of 10.8 meV is deduced from the temperature dependence of the YB. The shallow donor (about 10 meV) contributes to the mechanism of the YB.

  9. Preparation and characterization of one-dimensional GaN nanorods with Tb intermediate layer

    SciTech Connect

    Shi, Feng; Xue, Chengshan

    2012-12-15

    Graphical abstract: Display Omitted Highlights: ► GaN nanorods have been prepared on Si substrates by magnetron sputtering. ► GaN nanorods are single crystal with hexagonal wurtzite structure. ► GaN nanorods are high-quality crystalline after ammoniating at 950 °C for 15 min. ► Ammoniating temperatures and times affect the growth of GaN nanorods significantly. -- Abstract: GaN nanorods have been successfully prepared on Si(1 1 1) substrates by magnetron sputtering through ammoniating Ga{sub 2}O{sub 3}/Tb thin films. X-ray diffraction (XRD), X-ray photoelectron spectroscope (XPS), FT-IR spectrophotometer, scanning electron microscope (SEM), high-resolution transmission electron microscope (HRTEM), and photoluminescence (PL) spectroscopy were used to characterize the microstructures, morphologies compositions and optical properties of the GaN samples. The results demonstrate that the nanorods are single crystal GaN with hexagonal wurtzite structure and high-quality crystalline after ammoniating at 950 °C for 15 min, which have the size of 100–150 nm in diameter. Ammoniating temperatures and times affect the growth of GaN nanorods significantly. The growth procedure mainly follows the Tb catalyst-assisted VLS mechanism.

  10. Structural and chemical characterization of terbia thin films grown on hexagonally close packed metal substrates

    NASA Astrophysics Data System (ADS)

    Cartas, William

    Rare earth oxides (REOs) exhibit favorable catalytic performance for a diverse set of chemical transformations, including both partial and complete oxidation reactions. I will discuss our efforts to develop thin film systems of terbia for model surface science investigations of a REO that is effectively reducible, and which is thus expected to promote complete oxidation chemistry of adsorbed species. The growth of terbia on Cu(111) is shown to produce a complex surface that exhibits multiple phases of the oxide as well as exposed substrate. Growing the film on Pt(111) results in more uniform, single phase, and closed film. We used low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) to characterize the structural properties of terbia thin films grown on Pt(111) in ultrahigh vacuum (UHV) using physical vapor deposition. We find that the REO grows as a high quality Tb2O 3(111) film, and adopts oxygen-deficient fluorite structures wherein the metal cations form a hexagonal lattice in registry with the Pt(111) substrate, while oxygen vacancies are randomly distributed within the film. The Tb 2O3(111) films are thermally stable when heated to 1000 K in UHV. LEED and STM show that a fraction of the Tb2O3 forms hexagonal islands when first deposited, and further depositions typically result in three dimensional growth of the film. The Tb2O3 (111) / Pt(111) system produces a coincidence structure, seen very clearly in LEED images. We have also found that Tb2O3(111) films can be oxidized in UHV by exposure to plasma-generated atomic oxygen beams. The oxidized films have an estimated TbO2 stoichiometry and decompose to Tb2O3 during heating, with O2 desorption starting at about 500 K. Terbia films oxidized at 90 K show a weakly bound state of oxygen that is likely chemisorbed. Temperature programmed reaction spectroscopy (TPRS) studies using methanol show that increased oxygen in the film does not modify the chemical selectivity of the film; however

  11. Light Trapping Enhancement in a Thin Film with 2D Conformal Periodic Hexagonal Arrays

    NASA Astrophysics Data System (ADS)

    Yang, Xi; Zhou, Suqiong; Wang, Dan; He, Jian; Zhou, Jun; Li, Xiaofeng; Gao, Pingqi; Ye, Jichun

    2015-07-01

    Applying a periodic light trapping array is an effective method to improve the optical properties in thin-film solar cells. In this work, we experimentally and theoretically investigate the light trapping properties of two-dimensional periodic hexagonal arrays in the framework of a conformal amorphous silicon film. Compared with the planar reference, the double-sided conformal periodic structures with all feature periodicities of sub-wavelength (300 nm), mid-wavelength (640 nm), and infrared wavelength (2300 nm) show significant broadband absorption enhancements under wide angles. The films with an optimum periodicity of 300 nm exhibit outstanding antireflection and excellent trade-off between light scattering performance and parasitic absorption loss. The average absorption of the optimum structure with a thickness of 160 nm is 64.8 %, which is much larger than the planar counterpart of 38.5 %. The methodology applied in this work can be generalized to rational design of other types of high-performance thin-film photovoltaic devices based on a broad range of materials.

  12. Influence of different aspect ratios on the structural and electrical properties of GaN thin films grown on nanoscale-patterned sapphire substrates

    NASA Astrophysics Data System (ADS)

    Lee, Fang-Wei; Ke, Wen-Cheng; Cheng, Chun-Hong; Liao, Bo-Wei; Chen, Wei-Kuo

    2016-07-01

    This study presents GaN thin films grown on nanoscale-patterned sapphire substrates (NPSSs) with different aspect ratios (ARs) using a homemade metal-organic chemical vapor deposition system. The anodic aluminum oxide (AAO) technique is used to prepare the dry etching mask. The cross-sectional view of the scanning electron microscope image shows that voids exist between the interface of the GaN thin film and the high-AR (i.e. ∼2) NPSS. In contrast, patterns on the low-AR (∼0.7) NPSS are filled full of GaN. The formation of voids on the high-AR NPSS is believed to be due to the enhancement of the lateral growth in the initial growth stage, and the quick-merging GaN thin film blocks the precursors from continuing to supply the bottom of the pattern. The atomic force microscopy images of GaN on bare sapphire show a layer-by-layer surface morphology, which becomes a step-flow surface morphology for GaN on a high-AR NPSS. The edge-type threading dislocation density can be reduced from 7.1 × 108 cm-2 for GaN on bare sapphire to 4.9 × 108 cm-2 for GaN on a high-AR NPSS. In addition, the carrier mobility increases from 85 cm2/Vs for GaN on bare sapphire to 199 cm2/Vs for GaN on a high-AR NPSS. However, the increased screw-type threading dislocation density for GaN on a low-AR NPSS is due to the competition of lateral growth on the flat-top patterns and vertical growth on the bottom of the patterns that causes the material quality of the GaN thin film to degenerate. Thus, the experimental results indicate that the AR of the particular patterning of a NPSS plays a crucial role in achieving GaN thin film with a high crystalline quality.

  13. Growth and characterization of GaN thin film on Si substrate by thermionic vacuum arc (TVA)

    NASA Astrophysics Data System (ADS)

    Kundakçı, Mutlu; Mantarcı, Asim; Erdoğan, Erman

    2017-01-01

    Gallium nitride (GaN) is an attractive material with a wide-direct band gap (3.4 eV) and is one of the significant III-nitride materials, with many advantageous device applications such as high electron mobility transistors, lasers, sensors, LEDs, detectors, and solar cells, and has found applications in optoelectronic devices. GaN could also be useful for industrial research in the future. Chemical vapor deposition (CVD), molecular beam epitaxy (MBE), sputter, and pulsed laser deposition (PLD) are some of the methods used to fabricate GaN thin film. In this research, a GaN thin film grown on a silicon substrate using the thermionic vacuum arc (TVA) technique has been extensively studied. Fast deposition, short production time, homogeneity, and uniform nanostructure with low roughness can be seen as some of the merits of this method. The growth of the GaN was conducted at an operating pressure of 1× {{10}-6} \\text{Torr} , a plasma current 0.6 \\text{A} and for a very short period of time of 40 s. For the characterization process, scanning electron microscopy (SEM) was conducted to determine the structure and surface morphology of the material. Energy dispersive x-ray spectroscopy (EDX) was used to comprehend the elemental analysis characterization of the film. X-ray diffraction (XRD) was used to analyze the structure of the film. Raman measurements were taken to investigate the phonon modes of the material. The morphological properties of the material were analyzed in detail by atomic force microscopy (AFM).

  14. Spin Seebeck effect in Y-type hexagonal ferrite thin films

    NASA Astrophysics Data System (ADS)

    Hirschner, J.; Maryško, M.; Hejtmánek, J.; Uhrecký, R.; Soroka, M.; Buršík, J.; Anadón, A.; Aguirre, M. H.; Knížek, K.

    2017-08-01

    The longitudinal spin Seebeck effect (SSE) has been investigated using Pt/ferrite bilayers employing two Y-hexagonal ferrites Ba2Zn2Fe12O22 (Zn2Y) and Ba2Co2Fe12O22 (Co2Y) deposited by a spin-coating method on SrTiO3(111 ) substrates. The prepared hexagonal ferrites are highly oriented with c axes perpendicular to the substrate plane. The room-temperature magnetic moments of both ferrimagnetic ferrites amount to similar values and, most importantly, both have easy magnetization normal to the c axis. Despite their similar magnetic response the notable SSE signal is only observed for Zn2Y whereas the SSE signal of Co2Y is below the experimental noise level. A plausible explanation for this surprising discrepancy is magnetic disorder induced by cobalt cations, the random distribution of which in the Co2Y ferrite structure might critically limit the spin-wave propagation. This results in suppression of the SSE signal in Co2Y, while the Zn2Y with nonmagnetic substituent exhibits significant SSE signal. The temperature dependence of SSE for Zn2Y was measured over the 30 -300 -K range and quantitatively analyzed considering the heat flow through the Pt/Zn2Y bilayer and thermal gradient across the Zn2Y thin layer as the most relevant parameters. Using this approach the normalized SSE smoothly increases with lowering temperature, which correlates to increasing magnon propagation length and magnetization with decreasing temperature.

  15. Thin film growth and Zn doping of h-MoO3 hexagonal rods by hydrothermal technique

    NASA Astrophysics Data System (ADS)

    Mousavi-Zadeh, Seyyed Hamid; Rahmani, Mohammad Bagher

    2016-12-01

    In this research, rod-like undoped and Zn doped h-MoO3 thin films were grown on top of MoO3 seed layers, using hydrothermal technique without adding any surfactant. Seed layers of MoO3 were coated on top of glass substrates using spray pyrolysis technique. Structural, morphological and optical properties of thin films were examined. XRD pattern analysis showed that the seed layer has orthorhombic crystal structure. Also, it confirms the formation of hexagonal structure for thin films grown by hydrothermal. FESEM images show the formation of long, well-shaped hexagonal rod-likes. UV-Vis spectroscopy reveals band gap increasing from 3.2 eV to 3.54 eV, by increasing Zn.

  16. Biosensing operations based on whispering-gallery-mode optical cavities in single 1.0-µm diameter hexagonal GaN microdisks grown by radio-frequency plasma-assisted molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Kouno, Tetsuya; Sakai, Masaru; Kishino, Katsumi; Hara, Kazuhiko

    2016-05-01

    Biosensing operations based on a whispering-gallery-mode optical cavity in a single hexagonal GaN microdisk of approximately 1.0 µm diameter were demonstrated here. The sharp resonant peak in the photoluminescence spectrum obtained from the microdisk in aqueous sucrose solution redshifts with a change in sucrose concentration. The results indicate that an extremely small microdisk could be used as an optical transducer for sensing sugar, namely, as a biosensor. Furthermore, we investigate the relationship between the diameter of the microdisk and the sensitivity of the biosensor.

  17. Effect of GaN interlayer on polarity control of epitaxial ZnO thin films grown by molecular beam epitaxy

    SciTech Connect

    Wang, X. Q.; Sun, H. P.; Pan, X. Q.

    2010-10-11

    Epitaxial ZnO thin films were grown on nitrided (0001) sapphire substrates with an intervening GaN layer by rf-plasma-assisted molecular beam epitaxy. It was found that polarity of the ZnO epilayer could be controlled by modifying the GaN interlayer. ZnO grown on a distorted 3-nm-thick GaN interlayer has Zn-polarity while ZnO on a 20-nm-thick GaN interlayer with a high structural quality has O-polarity. High resolution transmission electron microscopy analysis indicates that the polarity of ZnO epilayer is controlled by the atomic structure of the interface between the ZnO buffer layer and the intervening GaN layer.

  18. Cathodoluminescence and Cross-sectional Transmission Electron Microscopy Studies for Deformation Behaviors of GaN Thin Films Under Berkovich Nanoindentation

    PubMed Central

    2008-01-01

    In this study, details of Berkovich nanoindentation-induced mechanical deformation mechanisms of metal-organic chemical-vapor deposition-derived GaN thin films have been systematic investigated with the aid of the cathodoluminescence (CL) and the cross-sectional transmission electron microscopy (XTEM) techniques. The multiple “pop-in” events were observed in the load-displacement (P–h) curve and appeared to occur randomly by increasing the indentation load. These instabilities are attributed to the dislocation nucleation and propagation. The CL images of nanoindentation show very well-defined rosette structures with the hexagonal system and, clearly display the distribution of deformation-induced extended defects/dislocations which affect CL emission. By using focused ion beam milling to accurately position the cross-section of an indented area, XTEM results demonstrate that the major plastic deformation is taking place through the propagation of dislocations. The present observations are in support to the massive dislocations activities occurring underneath the indenter during the loading cycle. No evidence of either phase transformation or formation of micro-cracking was observed by means of scanning electron microscopy and XTEM observations. We also discuss how these features correlate with Berkovich nanoindentation produced defects/dislocations structures.

  19. Enhanced Field Emission from a Carbon Nanotube Array Coated with a Hexagonal Boron Nitride Thin Film.

    PubMed

    Yang, Xiaoxia; Li, Zhenjun; He, Feng; Liu, Mingju; Bai, Bing; Liu, Wei; Qiu, Xiaohui; Zhou, Hang; Li, Chi; Dai, Qing

    2015-08-12

    A high-quality field emission electron source made of a highly ordered array of carbon nanotubes (CNTs) coated with a thin film of hexagonal boron nitride (h-BN) is fabricated using a simple and scalable method. This method offers the benefit of reproducibility, as well as the simplicity, safety, and low cost inherent in using B(2)O(3) as the boron precursor. Results measured using h-BN-coated CNT arrays are compared with uncoated control arrays. The optimal thickness of the h-BN film is found to be 3 nm. As a result of the incorporation of h-BN, the turn-on field is found to decrease from 4.11 to 1.36 V μm(-1), which can be explained by the significantly lower emission barrier that is achieved due to the negative electron affinity of h-BN. Meanwhile, the total emission current is observed to increase from 1.6 to 3.7 mA, due to a mechanism that limits the self-current of any individual emitting tip. This phenomenon also leads to improved emission stability and uniformity. In addition, the lifetime of the arrays is improved as well. The h-BN-coated CNT array-based field emitters proposed in this work may open new paths for the development of future high-performance vacuum electronic devices. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  20. Magnetic properties of epitaxial hexagonal HoFeO3 thin films

    NASA Astrophysics Data System (ADS)

    Wang, Xiao; Xiao, Zhuyun; Xu, Xiaoshan; Wang, Wenbin; Keavney, David; Liu, Yaohua; Cheng, X. M.

    2014-03-01

    Multiferroic materials exhibit multiple ferroic orders simultaneously and thus have great potential applications in information technology, sensing and actuation. Epitaxial hexagonal HoFeO3 (h-HFO) films are very promising candidates as multiferroic materials with room temperature ferromagnetism, because magnetic Ho3+ ions are expected to have stronger exchange interactions with Fe3+ ions than the well-studied h-LuFeO3 films. We report study of magnetic properties of epitaxial h-HFO thin films deposited using laser molecular beam epitaxy on Yttria-stabilized zirconia (YSZ) substrates. X-ray diffraction measurements confirmed the epitaxial registry and six-fold symmetry of the film. Temperature dependence of magnetization of the film measured by a Quantum Design SQUID magnetometer shows dominating paramagnetic characteristic. Element specific x-ray magnetic circular dichroism measurements performed at beamline 4-ID-C of the Advanced Photon Source show a ferromagnetic ordering of Fe and an exchange coupling between Ho3+ and Fe3+ ions. Work at BMC is supported by NSF Career award (DMR 1053854). Work at ANL is supported by US-DOE, Office of Science, BES (No. DE-AC02-06CH11357).

  1. Electronic and optical device applications of hollow cathode plasma assisted atomic layer deposition based GaN thin films

    SciTech Connect

    Bolat, Sami Tekcan, Burak; Ozgit-Akgun, Cagla; Biyikli, Necmi; Okyay, Ali Kemal

    2015-01-15

    Electronic and optoelectronic devices, namely, thin film transistors (TFTs) and metal–semiconductor–metal (MSM) photodetectors, based on GaN films grown by hollow cathode plasma-assisted atomic layer deposition (PA-ALD) are demonstrated. Resistivity of GaN thin films and metal-GaN contact resistance are investigated as a function of annealing temperature. Effect of the plasma gas and postmetallization annealing on the performances of the TFTs as well as the effect of the annealing on the performance of MSM photodetectors are studied. Dark current to voltage and responsivity behavior of MSM devices are investigated as well. TFTs with the N{sub 2}/H{sub 2} PA-ALD based GaN channels are observed to have improved stability and transfer characteristics with respect to NH{sub 3} PA-ALD based transistors. Dark current of the MSM photodetectors is suppressed strongly after high-temperature annealing in N{sub 2}:H{sub 2} ambient.

  2. Growth, Nitrogen Vacancy Reduction and Solid Solution Formation in Cubic GaN Thin Films and the Subsequent Fabrication of Superlattice Structures Using AlN and InN

    DTIC Science & Technology

    1991-06-01

    p PTIC (AD-A238 521 Semi-Annual Letter Report Growth, Nitrogen Vacancy Reduction and Solid Solution Formation in Cubic GaN Thin Films and the...SUBTITLE Growth, Nitrogen Vacancy Reduction and S. FUNDING NUMBERS solid Solution Formation in Cubic GaN Thin Films and the R&T:s40000lsrqO5 Subsequent

  3. Epitaxial stabilization of artificial hexagonal GdMnO3 thin films and their magnetic properties

    NASA Astrophysics Data System (ADS)

    Lee, D.; Lee, J.-H.; Murugavel, P.; Jang, S. Y.; Noh, T. W.; Jo, Y.; Jung, M.-H.; Ko, Y.-D.; Chung, J.-S.

    2007-04-01

    The authors investigated the role of oxygen partial pressure on the epitaxial growth of an artificial hexagonal GdMnO3 phase, which should exist in an orthorhombic structure in bulk. The hexagonal GdMnO3 film showed diverse, but obvious, magnetic phase transitions with highly enhanced ferromagnetic properties. Its remnant magnetization at 4.2K is higher than those of other hexagonal RMnO3 (R =Ho, Er, and Yb) compounds, and the Curie temperature increases by around 25K. The results demonstrate that the epitaxial stabilization technique is a promising method for fabricating an artificial material with enhanced magnetic properties.

  4. Structural, optical, and magnetic properties of highly-resistive Sm-implanted GaN thin films

    SciTech Connect

    Lo, Fang-Yuh Huang, Cheng-De; Chou, Kai-Chieh; Guo, Jhong-Yu; Liu, Hsiang-Lin; Chia, Chi-Ta; Ney, Verena; Ney, Andreas; Shvarkov, Stepan; Reuter, Dirk; Wieck, Andreas D.; Pezzagna, Sébastien; Chern, Ming-Yau; Massies, Jean

    2014-07-28

    Samarium ions of 200 keV in energy were implanted into highly-resistive molecular-beam-epitaxy grown GaN thin films with a focused-ion-beam implanter at room temperature. The implantation doses range from 1 × 10{sup 14} to 1 × 10{sup 16 }cm{sup −2}. Structural properties studied by x-ray diffraction and Raman-scattering spectroscopy revealed Sm incorporation into GaN matrix without secondary phase. The optical measurements showed that the band gap and optical constants changed very slightly by the implantation. Photoluminescence measurements showed emission spectra similar to p-type GaN for all samples. Magnetic investigations with a superconducting quantum interference device identified magnetic ordering for Sm dose of and above 1 × 10{sup 15 }cm{sup −2} before thermal annealing, while ferromagnetism was only observed after thermal annealing from the sample with highest Sm dose. The long-range magnetic ordering can be attributed to interaction of Sm ions through the implantation-induced Ga vacancy.

  5. Elimination of surface band bending on N-polar InN with thin GaN capping

    SciTech Connect

    Kuzmík, J. Haščík, Š.; Kučera, M.; Kúdela, R.; Dobročka, E.; Adikimenakis, A.; Mičušík, M.; Gregor, M.; Plecenik, A.; Georgakilas, A.

    2015-11-09

    0.5–1 μm thick InN (0001) films grown by molecular-beam epitaxy with N- or In-polarity are investigated for the presence of native oxide, surface energy band bending, and effects introduced by 2 to 4 monolayers of GaN capping. Ex situ angle-resolved x-ray photo-electron spectroscopy is used to construct near-surface (GaN)/InN energy profiles, which is combined with deconvolution of In3d signal to trace the presence of InN native oxide for different types of polarity and capping. Downwards surface energy band bending was observed on bare samples with native oxide, regardless of the polarity. It was found that the In-polar InN surface is most readily oxidized, however, with only slightly less band bending if compared with the N-polar sample. On the other hand, InN surface oxidation was effectively mitigated by GaN capping. Still, as confirmed by ultra-violet photo-electron spectroscopy and by energy band diagram calculations, thin GaN cap layer may provide negative piezoelectric polarization charge at the GaN/InN hetero-interface of the N-polar sample, in addition to the passivation effect. These effects raised the band diagram up by about 0.65 eV, reaching a flat-band profile.

  6. Thermal conductivity of ultra-thin chemical vapor deposited hexagonal boron nitride films

    NASA Astrophysics Data System (ADS)

    Alam, M. T.; Bresnehan, M. S.; Robinson, J. A.; Haque, M. A.

    2014-01-01

    Thermal conductivity of freestanding 10 nm and 20 nm thick chemical vapor deposited hexagonal boron nitride films was measured using both steady state and transient techniques. The measured value for both thicknesses, about 100 ± 10 W m-1 K-1, is lower than the bulk basal plane value (390 W m-1 K-1) due to the imperfections in the specimen microstructure. Impressively, this value is still 100 times higher than conventional dielectrics. Considering scalability and ease of integration, hexagonal boron nitride grown over large area is an excellent candidate for thermal management in two dimensional materials-based nanoelectronics.

  7. Bi2Te3 thin hexagonal nanoplatelets: Synthesis and its characterization studies

    NASA Astrophysics Data System (ADS)

    Vinoth, S.; Balaganapathi, T.; KaniAmuthan, B.; Arun, T.; Muthuselvam, I. Panneer; Chou, Fang-Cheng; Thilakan, P.

    2017-08-01

    Solvothermal synthesis and optimization of pure Bismuth telluride (Bi2Te3) hexagonal nanoplatelets was carried out from Bismuth Oxide (Bi2O3) and Tellurium dioxide (TeO2). XRD measurements revealed a sensitive change in crystallization behaviour in correlation with variation in Te/Bi stoichiometry identified through the exchange in intensities between (10 10 ̅) and (110) peaks. Further, Energy Dispersive X-ray (EDAX) analysis revealed the variation in Te/Bi ratio with respect to autoclave temperature. Field emission scanning electron Microscope (FESEM) and the high resolution transmission electron Microscope (HRTEM) studies show the complete growth of hexagonal nanoplatelets at 200 °C. Confocal Micro-Raman measurements revealed the occurrence of symmetry breaking in the synthesized hexagonal nanoplatelets. The electrical conductivity and the activation energy were recorded as 6.01×10-3 S/m and 0.042 eV respectively. Highest maximum absolute value of Seebeck coefficient of -355 μV/K was obtained for the hexagonal nanoplatelets.

  8. Growth, Nitrogen Vacancy Reduction and Solid Solution Formation in Cubic GaN Thin Films and the Subsequent Fabrication of Superlattice Structures Using AIN and InN

    DTIC Science & Technology

    1992-02-01

    IK AD-A248 058 - - H Final Technical ReportI I Growth, Nitrogen Vacancy Reduction and Solid Solution Formation in Cubic GaN Thin Films and the...structural and chemical analyses, there is no reason to believe that a homogeneous solid solution close to this composition had formed. Moreover

  9. Spin exchange interactions in hexagonal manganites RMnO3 (R = Tb, Dy, Ho, Er) epitaxial thin films

    NASA Astrophysics Data System (ADS)

    Chen, Xiang-Bai; Thi Minh Hien, Nguyen; Lee, D.; Jang, S.-Y.; Noh, T. W.; Yang, In-Sang

    2011-08-01

    We present the results of an optical method of quantitatively estimating the spin exchange interactions in hexagonal manganites RMnO3 (R = Tb, Dy, Ho, Er) epitaxial thin films. The two in-plane (a-b plane) spin exchange integrals J1 (intratrimer Mn-Mn interaction) and J2 (intertrimer Mn-Mn interaction) are deduced from the magnon scattering peak wavenumbers. We found that J2 decreases systematically when the R ionic radius increases, while J1 is nearly independent of R ionic radius, contrary to the expectation in single crystals. We show that the R dependence of J1 could be understood in terms of the stress in the thin films. Our result indicates that the stress has stronger effect on the atomic displacement of the intratrimer Mn-Mn distance than the intertrimer Mn-Mn distance.

  10. Nanoscale control of Si nanoparticles within a 2D hexagonal array embedded in SiO2 thin films

    NASA Astrophysics Data System (ADS)

    Castro, Celia; BenAssayag, Gérard; Pecassou, Béatrice; Andreozzi, Andrea; Seguini, Gabriele; Perego, Michele; Schamm-Chardon, Sylvie

    2017-01-01

    In this work, we investigate the ability to control Si nanoparticles (NPs) spatially arranged in a hexagonal network of 20 nm wide nanovolumes at controlled depth within SiO2 thin films. To achieve this goal an unconventional lithographic technique was implemented based on a bottom-up approach, that is fully compatible with the existing semiconductor technology. The method combines ultra-low energy ion beam synthesis with nanostructured block-copolymer thin films that are self-assembled on the SiO2 substrates to form a nanoporous template with hexagonally packed pores. A systematic analytical investigation using time of flight-secondary ion mass spectroscopy and low-loss energy filtered transmission electron microscopy demonstrates that by adjusting few fabrication parameters, it is possible to narrow the size distribution of the NPs and to control the number of NPs per nanovolume. Experimental results are critically discussed on the basis of literature data, providing a description of the mechanism involved in the formation of Si NPs.

  11. Structural and magnetic properties of ultra-thin Fe films on metal-organic chemical vapour deposited GaN(0001)

    NASA Astrophysics Data System (ADS)

    Kim, Jun-Young; Ionescu, Adrian; Mansell, Rhodri; Farrer, Ian; Oehler, Fabrice; Kinane, Christy J.; Cooper, Joshaniel F. K.; Steinke, Nina-Juliane; Langridge, Sean; Stankiewicz, Romuald; Humphreys, Colin J.; Cowburn, Russell P.; Holmes, Stuart N.; Barnes, Crispin H. W.

    2017-01-01

    Structural and magnetic properties of 1-10 nm thick Fe films deposited on GaN(0001) were investigated. In-situ reflecting high energy electron diffraction images indicated a α-Fe(110)/GaN(0001) growth of the 3D Volmer-Weber type. The α-Fe(110) X-ray diffraction peak showed a 1° full-width at half-maximum, indicating ≈20 nm grain sizes. A significant reduction in Fe atomic moment from its bulk value was observed for films thinner than 4 nm. Both GaN/Fe interface roughness and Fe film coercivity increased with Fe thickness, indicating a possible deterioration of Fe crystalline quality. Magnetic anisotropy was mainly uniaxial for all films while hexagonal anisotropies appeared for thicknesses higher than 3.7 nm.

  12. Thermal conductivity of ultra-thin chemical vapor deposited hexagonal boron nitride films

    SciTech Connect

    Alam, M. T.; Haque, M. A.; Bresnehan, M. S.; Robinson, J. A.

    2014-01-06

    Thermal conductivity of freestanding 10 nm and 20 nm thick chemical vapor deposited hexagonal boron nitride films was measured using both steady state and transient techniques. The measured value for both thicknesses, about 100 ± 10 W m{sup −1} K{sup −1}, is lower than the bulk basal plane value (390 W m{sup −1} K{sup −1}) due to the imperfections in the specimen microstructure. Impressively, this value is still 100 times higher than conventional dielectrics. Considering scalability and ease of integration, hexagonal boron nitride grown over large area is an excellent candidate for thermal management in two dimensional materials-based nanoelectronics.

  13. Preparation of ultra-thin hexagonal boron nitride nanoplates for cancer cell imaging and neurotransmitter sensing.

    PubMed

    Nurunnabi, Md; Nafiujjaman, Md; Lee, Sang-Joon; Park, In-Kyu; Huh, Kang Moo; Lee, Yong-Kyu

    2016-04-26

    A facile and convenient process was optimized for preparing water-soluble hydroxyl-functionalized hexagonal boron nitride (hBN-OH) from hBN. The hBN-OH (2-3 nm thickness) contains ∼40% oxygen and exhibits blue emission with a quantum yield of approximately 36%. The hBN-OH could be used for imaging cells and for the in vitro detection of biomolecules through electrochemical analysis.

  14. Plasmonic effects for light concentration in organic photovoltaic thin films induced by hexagonal periodic metallic nanospheres

    NASA Astrophysics Data System (ADS)

    Zhu, Jinfeng; Xue, Mei; Shen, Huajun; Wu, Zhe; Kim, Seongku; Ho, Jyh-Jier; Hassani-Afshar, Aram; Zeng, Baoqing; Wang, Kang L.

    2011-04-01

    We present a plasmonic nanostructure design by embedding a layer of hexagonal periodic metallic nanospheres between the active layer and transparent anode for bulk heterojunction organic solar cells. The hybrid structure shows broadband optical absorption enhancement from localized surface plasmon resonance with a weak dependence on polarization of incident light. We also theoretically study the optimization of the design to enhance the absorption up to 1.90 times for a typical hybrid active layer based on a low band gap material.

  15. Low-threshold GaN thin-film random laser through the weak scattering feedback

    NASA Astrophysics Data System (ADS)

    Zhu, Hai; Chen, Anqi; Wu, Yanyan; Ji, Xu; He, Yiting; Qiu, Zhiren; Tang, Zikang; Yu, Siufung

    2017-02-01

    Room temperature random lasing is demonstrated from a GaN epitaxy film with defect pits that result from growth imperfection. The optical coherence feedback is attributed to the formation of closed-loop paths of light through the scattering effect of the defect pits, which can avoid the difficulty of fabricating an artificial cavity. The random lasing action was also investigated through near and far-field patterns that imaged onto the CCD camera. In addition, the angle distribution of the laser beam was illustrated by use of an angle-resolved spectrometer. The lasing threshold, based on the weak scattering diffusive mode of GaN, is about one order of magnitude lower than that strong scattering random laser (RL). Hence, the results in this paper represent a low-cost technique to realize GaN-based laser diodes without the fabrication difficulty of cavity facets that result from the hardness of the sapphire substrate.

  16. Growth of Hexagonal Columnar Nanograin Structured SiC Thin Films on Silicon Substrates with Graphene–Graphitic Carbon Nanoflakes Templates from Solid Carbon Sources

    PubMed Central

    Liu, Xingfang; Sun, Guosheng; Liu, Bin; Yan, Guoguo; Guan, Min; Zhang, Yang; Zhang, Feng; Chen, Yu; Dong, Lin; Zheng, Liu; Liu, Shengbei; Tian, Lixin; Wang, Lei; Zhao, Wanshun; Zeng, Yiping

    2013-01-01

    We report a new method for growing hexagonal columnar nanograin structured silicon carbide (SiC) thin films on silicon substrates by using graphene–graphitic carbon nanoflakes (GGNs) templates from solid carbon sources. The growth was carried out in a conventional low pressure chemical vapor deposition system (LPCVD). The GGNs are small plates with lateral sizes of around 100 nm and overlap each other, and are made up of nanosized multilayer graphene and graphitic carbon matrix (GCM). Long and straight SiC nanograins with hexagonal shapes, and with lateral sizes of around 200–400 nm are synthesized on the GGNs, which form compact SiC thin films. PMID:28809227

  17. Electric-field-induced lamellar to hexagonally perforated lamellar transition in diblock copolymer thin films: kinetic pathways.

    PubMed

    Mukherjee, Arnab; Ankit, Kumar; Reiter, Andreas; Selzer, Michael; Nestler, Britta

    2016-09-14

    Symmetric block-copolymers, hitherto, are well known to evolve into parallel, perpendicular and mixed lamellar morphologies under the concomitant influence of an electric field and substrate affinity. In the present work, we show that an additional imposed confinement can effectuate a novel parallel lamellar to hexagonally perforated lamellar (HPL) transition in monolayer and bilayer films. Three dimensional numerical studies are performed using the Ohta-Kawasaki functional, complemented with an exact solution of Maxwell's equation. HPL is shown to stabilize at large substrate affinity in a narrow region of the phase diagram between parallel and perpendicular lamellar transitions in ultra-thin films. Additionally, we also identify perforated lamellae as intermediate structures during parallel-to-perpendicular lamellar transition. A systematic analysis using Minkowski functionals yields deeper insights into the associated kinetic pathways.

  18. Broadband nanophotonic waveguides and resonators based on epitaxial GaN thin films

    SciTech Connect

    Bruch, Alexander W.; Xiong, Chi; Leung, Benjamin; Poot, Menno; Han, Jung; Tang, Hong X.

    2015-10-05

    We demonstrate broadband, low loss optical waveguiding in single crystalline GaN grown epitaxially on c-plane sapphire wafers through a buffered metal-organic chemical vapor phase deposition process. High Q optical microring resonators are realized in near infrared, infrared, and near visible regimes with intrinsic quality factors exceeding 50 000 at all the wavelengths we studied. TEM analysis of etched waveguide reveals growth and etch-induced defects. Reduction of these defects through improved material and device processing could lead to even lower optical losses and enable a wideband photonic platform based on GaN-on-sapphire material system.

  19. Growth, Nitrogen Vacancy Reduction and Solid Solution Formation in Cubic GaN Thin Films and the Subsequent Fabrication of Superlattice Structures Using AlN and InN

    DTIC Science & Technology

    1992-06-01

    AD-A253 331 Semiannual Report Growth, Nitrogen Vacancy Reduction and Solid Solution Formation in Cubic GaN Thin Films and the Subsequent Fabrication...Growth, Nitrogen Vacancy Reduction and Solid Solution Formation in Cubic GaN Thin Films and the Subsequent Fabrication of Superlattice Structures Using...34 substrates, such as using a graded AlxGal-xN solid solution as a buffer layer. E. Conclusion We have shown that in the use of our modified gas source MBE

  20. Marine corrosion protective coatings of hexagonal boron nitride thin films on stainless steel.

    PubMed

    Husain, Esam; Narayanan, Tharangattu N; Taha-Tijerina, Jose Jaime; Vinod, Soumya; Vajtai, Robert; Ajayan, Pulickel M

    2013-05-22

    Recently, two-dimensional, layered materials such as graphene and hexagonal boron nitride (h-BN) have been identified as interesting materials for a range of applications. Here, we demonstrate the corrosion prevention applications of h-BN in marine coatings. The performance of h-BN/polymer hybrid coatings, applied on stainless steel, were evaluated using electrochemical techniques in simulated seawater media [marine media]. h-BN/polymer coating shows an efficient corrosion protection with a low corrosion current density of 5.14 × 10(-8) A/cm(2) and corrosion rate of 1.19 × 10(-3) mm/year and it is attributed to the hydrofobic, inert and dielectric nature of boron nitride. The results indicated that the stainless steel with coatings exhibited improved corrosion resistance. Electrochemical impedance spectroscopy and potentiodynamic analysis were used to propose a mechanism for the increased corrosion resistance of h-BN coatings.

  1. Anisotropic charge carrier transport in free-standing hexagonal boron nitride thin films

    NASA Astrophysics Data System (ADS)

    Dahal, Rajendra; Ahmed, Kawser; Woei Wu, Jia; Weltz, Adam; Jian-Qiang Lu, James; Danon, Yaron; Bhat, Ishwara B.

    2016-06-01

    The in-plane and out-of-plane mobility-lifetime products of electrons and holes in free-standing hexagonal boron nitride (hBN) films are extracted from current-voltage characteristics of metal-hBN-metal structures measured under external excitations. The in-plane mobility-lifetime products for electrons and holes are ˜2.8 × 10-5 and ˜4.85 × 10-6 cm2/V, measured from lateral carrier collection, whereas the out-of-plane mobility-lifetime products for electrons and holes are ˜5.8 × 10-8 and ˜6.1 × 10-9 cm2/V, measured from vertical carrier collection, respectively. The mobility-lifetime product is a few orders of magnitude higher along the plane than along the out of plane in hBN films.

  2. Synthesis of atomically thin hexagonal boron nitride films on nickel foils by molecular beam epitaxy

    SciTech Connect

    Nakhaie, S.; Wofford, J. M.; Schumann, T.; Jahn, U.; Ramsteiner, M.; Hanke, M.; Lopes, J. M. J. Riechert, H.

    2015-05-25

    Hexagonal boron nitride (h-BN) is a layered two-dimensional material with properties that make it promising as a dielectric in various applications. We report the growth of h-BN films on Ni foils from elemental B and N using molecular beam epitaxy. The presence of crystalline h-BN over the entire substrate is confirmed by Raman spectroscopy. Atomic force microscopy is used to examine the morphology and continuity of the synthesized films. A scanning electron microscopy study of films obtained using shorter depositions offers insight into the nucleation and growth behavior of h-BN on the Ni substrate. The morphology of h-BN was found to evolve from dendritic, star-shaped islands to larger, smooth triangular ones with increasing growth temperature.

  3. Analysis of reactor geometry and diluent gas flow effects on the metalorganic vapor phase epitaxy of AIN and GaN thin films on α(6H)-SiC substrates

    NASA Astrophysics Data System (ADS)

    Hanser, A. D.; Wolden, C. A.; Perry, W. G.; Zheleva, T.; Carlson, E. P.; Banks, A. D.; Therrien, R. J.; Davis, R. F.

    1998-04-01

    The influence of diluent gas on the metalorganic vapor phase epitaxy of AlN and GaN thin films has been investigated. A computational fluid dynamics model using the finite element method was employed to improve film uniformity and to analyze transport phenomena. The properties of AlN and GaN thin films grown on α(6H)-SiC(0001) substrates in H2 and N2 diluent gas environments were evaluated. Thin films of AlN grown in H2 and N2 had root mean square (rms) roughness values of 1.5 and 1.8 nm, respectively. The surface and defect microstructures of the GaN thin films, observed by scanning and transmission electron microscopy, respectively, were very similar for both diluents. Low temperature (12K) photoluminescence measurements of GaN films grown in N2 had peak intensities and full widths at half maximum equal to or better than those films grown in H2. A room temperature Hall mobility of 275 cm2/V·s was measured on 1 µm thick, Si-doped, n-type (1×1017 cm-3) GaN films grown in N2. Acceptor-type behavior of Mg-doped GaN films deposited in N2 was repeatably obtained without post-growth annealing, in contrast to similar films grown in H2. The GaN growth rates were ˜30% higher when H2 was used as the diluent. The measured differences in the growth rates of AlN and GaN films in H2 and N2 was attributed to the different transport properties of these mixtures, and agreed well with the computer model predictions. Nitrogen is shown to be a feasible alternative diluent to hydrogen for the growth of AlN and GaN thin films.

  4. Atomically Thin Layers of Graphene and Hexagonal Boron Nitride Made by Solvent Exfoliation of Their Phosphoric Acid Intercalation Compounds.

    PubMed

    Kovtyukhova, Nina I; Perea-López, Nestor; Terrones, Mauricio; Mallouk, Thomas E

    2017-07-25

    The development of scalable and reliable techniques for the production of the atomically thin layers of graphene and hexagonal boron nitride (h-BN) in bulk quantities could make these materials a powerful platform for devices and composites that impact a wide variety of technologies (Nature 2012, 490, 192-200). To date a number of practical exfoliation methods have been reported that are based on sonicating or stirring powdered graphite or h-BN in common solvents. However, the products of these experiments consist mainly of few-layer sheets and contain only a small fraction of monolayers. A possible reason for this is that splitting the crystals into monolayers starts from solvent intercalation, which must overcome the substantial interlayer cohesive energy (120-720 mJ/m(2)) of the van der Waals solids. Here we show that the yield of the atomically thin layers can be increased to near unity when stage-1 intercalation compounds of phosphoric acid are used as starting materials. The exfoliation to predominantly monolayers was achieved by stirring them in medium polarity organic solvents that can form hydrogen bonds. The exfoliation process does not disrupt the sp(2) π-system of graphene and is gentle enough to allow the preparation of graphene and h-BN monolayers that are tens of microns in their lateral dimensions.

  5. Mechanisms of ion-induced GaN thin layer splitting

    NASA Astrophysics Data System (ADS)

    Moutanabbir, O.; Chabal, Y. J.; Chicoine, M.; Christiansen, S.; Krause-Rehberg, R.; Schiettekatte, F.; Scholz, R.; Seitz, O.; Senz, S.; Süßkraut, F.; Gösele, U.

    2009-05-01

    The underlying physics and the role of H-defect interaction in H ion-induced splitting of GaN were investigated by transmission electron microscopy, high resolution X-ray diffraction, positron annihilation spectroscopy, ion channeling, elastic recoil detection, and infrared spectroscopy. A high concentration of void-like nanoscopic structures, nanobubbles, is detected immediately after implantation. Positron annihilation measurements demonstrate that the detected structures are vacancy clusters. FTIR data show that H-defect vibrational spectrum peaks at 3141 cm-1 mode attributed to VGa-H4. A large fraction of H was found to be trapped in higher frequency modes which we associate tentatively to N-H stretch modes in the internal surfaces of nanobubbles. These nanobubbles persist during annealing up to 450 °C. An increase of the strain is observed in this temperature range. This strain relaxes partially above 450 °C following the formation of the platelets which are embryos of the microcracks.

  6. Isotropic thin PTCDA films on GaN(0 0 0 1)

    NASA Astrophysics Data System (ADS)

    Ahrens, Ch; Flege, J. I.; Jaye, C.; Fischer, D. A.; Schmidt, Th; Falta, J.

    2016-11-01

    The growth of 3, 4, 9, 10-perylene tetracarboxylic dianhydride (PTCDA) on the Ga-polar GaN(0 0 0 1) surface has been studied by x-ray photoelectron spectroscopy (XPS), spot profile analysis low-energy electron diffraction (SPA-LEED), near edge x-ray absorption fine structure (NEXAFS), and scanning tunneling microscopy (STM). The stoichiometric ratios derived from XPS indicate that the molecules remain intact upon adsorption on the surface. Furthermore, no chemical shifts can be observed in the C 1s and O 1s core levels with progressing deposition of PTCDA, suggesting none or only weak interactions between the molecules and the substrate. NEXAFS data indicate the PTCDA molecules being oriented with their molecular plane parallel to the surface. High-resolution STM shows PTCDA islands of irregular shape on the sub-micron scale, and together with corresponding SPA-LEED data reveals a lateral ordering of the molecules that is compatible with the presence of (1 0 2) oriented PTCDA nano-crystals. SPA-LEED moreover clearly shows the presence of homogeneously distributed rotational domains of two-dimensionally isotropic PTCDA.

  7. Aligned Growth of Millimeter-Size Hexagonal Boron Nitride Single-Crystal Domains on Epitaxial Nickel Thin Film.

    PubMed

    Meng, Junhua; Zhang, Xingwang; Wang, Ye; Yin, Zhigang; Liu, Heng; Xia, Jing; Wang, Haolin; You, Jingbi; Jin, Peng; Wang, Denggui; Meng, Xiang-Min

    2017-03-07

    Atomically thin hexagonal boron nitride (h-BN) is gaining significant attention for many applications such as a dielectric layer or substrate for graphene-based devices. For these applications, synthesis of high-quality and large-area h-BN layers with few defects is strongly desirable. In this work, the aligned growth of millimeter-size single-crystal h-BN domains on epitaxial Ni (111)/sapphire substrates by ion beam sputtering deposition is demonstrated. Under the optimized growth conditions, single-crystal h-BN domains up to 0.6 mm in edge length are obtained, the largest reported to date. The formation of large-size h-BN domains results mainly from the reduced Ni-grain boundaries and the improved crystallinity of Ni film. Furthermore, the h-BN domains show well-aligned orientation and excellent dielectric properties. In addition, the sapphire substrates can be repeatedly used with almost no limit. This work provides an effective approach for synthesizing large-scale high-quality h-BN layers for electronic applications.

  8. The interface analysis of GaN grown on 0° off 6H-SiC with an ultra-thin buffer layer

    NASA Astrophysics Data System (ADS)

    Sun, Zheng; Ohta, Akio; Miyazaki, Seiichi; Nagamatsu, Kentaro; Lee, Hojun; Olsson, Marc; Ye, Zheng; Deki, Manato; Honda, Yoshio; Amano, Hiroshi

    2016-01-01

    Previously, we reported a growth method by metalorganic vapor phase epitaxy using a single two-dimensional growth step, resulting in 1.2-µm crack-free GaN directly grown on 6H-SiC substrate. The introduction of Al-treatment prior to the standard GaN growth step resulted in improved surface wetting of gallium on the SiC substrate. Transmission electron microscope and energy dispersive spectrometer analysis of the epitaxial interface to the SiC determined that an ultra-thin AlGaN interlayer had formed measuring around 2-3 nm. We expect our growth technique can be applied to the fabrication of GaN/SiC high frequency and high power devices.

  9. Evaluation of the interface of thin GaN layers on c- and m-plane ZnO substrates by Rutherford backscattering

    SciTech Connect

    Izawa, Y.; Oga, T.; Ida, T.; Kuriyama, K.; Hashimoto, A.; Kotake, H.; Kamijoh, T.

    2011-07-11

    Lattice distortion at the interfaces between thin GaN layers with {approx}400 nm in thickness and ZnO substrates with non-polar m-plane (10-10) and polar c-plane (0001) is studied using Rutherford backscattering/ion channeling techniques. The interface between GaN/m-plane ZnO is aligned clearly to m-axis, indicating no lattice distortion, while between GaN/c-plane ZnO causes the lattice distortion in the GaN layer due to the piezoelectric field. The range of distortion exceeds {approx}90 nm from the interface of GaN/c-plane ZnO. These results are confirmed by x-ray diffraction and reflection high energy electron diffraction studies.

  10. Structural development of gold and silver nanoparticles within hexagonally ordered spherical micellar diblock copolymer thin films

    NASA Astrophysics Data System (ADS)

    Chen, Chia-Min; Huang, Yi-Jiun; Wei, Kung-Hwa

    2014-05-01

    The spatial arrangement of metal nanoparticle (NP) arrays in block copolymers has many potential applications in OFET-type memory devices. In this study, we adopted a trapping approach in which we used a monolayer thin film of polystyrene-block-poly(4-vinylpyridine) (PS56k-b-P4VP8k)--a highly asymmetric diblock copolymer having a spherical micelle morphology--to incorporate various amounts of one-phase-synthesized dodecanethiol-passivated silver (DT-Ag) NPs and a fixed amount of ligand-exchanged pyridine-coated gold (Py-Au) NPs into the polystyrene (PS) and poly(4-vinylpyridine) (P4VP) blocks, respectively. We characterized the packing of these metal NPs in the two blocks of the nanostructured diblock copolymer using reciprocal-space synchrotron grazing incidence small-angle X-ray scattering (GISAXS) as well as atomic force microscopy (AFM) and transmission electron microscopy (TEM) in the real space. The packing of the Ag NPs in the PS block was dependent on their content, which we tuned by varying the concentrations in the composite solution at a constant rate of spin-coating. The two-dimensional hierarchical arrangement of Ag and Au NPs within the BCP thin films was enhanced after addition of the Py-Au NPs into the P4VP block and after spin-coating a thinner film from a low concentration solution (0.1 wt%), due to the DT-Ag NPs accumulating around the Py-Au/P4VP cores; this two-dimensional hierarchical arrangement decreased at a critical DT-Ag NP weight ratio (c) of 0.8 when incorporating the Py-Au NPs into the P4VP domains through spin-coating at higher solution concentration (0.5 wt%), where the DT-Ag NPs realigned by rotating 20° along the z axis in the real space, due to oversaturation of the DT-Ag NPs within the PS domains.The spatial arrangement of metal nanoparticle (NP) arrays in block copolymers has many potential applications in OFET-type memory devices. In this study, we adopted a trapping approach in which we used a monolayer thin film of

  11. Growth, Nitrogen Vacancy Reduction and Solid Solution Formation in Cubic GaN Thin Films and the Subsequent Fabrication of Superlattice Structures Using AlN and InN

    DTIC Science & Technology

    1990-07-01

    that the BGaN film remained predominantly single crystal, but shows both a normal cubic [110] pattern and a second hexagonal pattern [0110]. By contrast...27 Transmission electron microscopy (iEM) was used (Hitachi H-800) to more closely examine the microstructural evolution of the BN/ BGaN /GaN epitaxial...area diffraction (SAD) also showed the BGaN layer to be a mixture of cubic and wurtzitic phases. This layer was heavily faulted. The latter phenomenon

  12. Hexagon solar power panel

    DOEpatents

    Rubin, Irwin

    1978-01-01

    A solar energy panel comprises a support upon which silicon cells are arrayed. The cells are wafer thin and of two geometrical types, both of the same area and electrical rating, namely hexagon cells and hourglass cells. The hourglass cells are composites of half hexagons. A near perfect nesting relationship of the cells achieves a high density packing whereby optimum energy production per panel area is achieved.

  13. Hexagon solar power panel

    NASA Technical Reports Server (NTRS)

    Rubin, I. (Inventor)

    1978-01-01

    A solar energy panel support is described upon which silicon cells are arrayed. The cells are wafer thin and of two geometrical types, both of the same area and electrical rating, namely hexagon cells and hourglass cells. The hourglass cells are composites of half hexagons. A near perfect nesting relationship of the cells achieves a high density packing whereby optimum energy production per panel area is achieved.

  14. The interplay of blocking properties with charge and potential redistribution in thin carbon-doped GaN on n-doped GaN layers

    NASA Astrophysics Data System (ADS)

    Koller, Christian; Pobegen, Gregor; Ostermaier, Clemens; Huber, Martin; Pogany, Dionyz

    2017-07-01

    In carbon-doped GaN (GaN:C) buffers used in a GaN-on-Si technology, the buffer is embedded in between transition and channel layers. This makes the analysis of buffer properties difficult due to e.g., carrier injection from or potential drop at these adjacent layers. Here, we analyze capacitance- and current-voltage characteristics of 200-300 nm thick GaN:C ([C] = 1019 cm-3) layers which are embedded between a top metal electrode and bottom n-doped GaN (n-GaN). Such structures allow a better potential control in GaN:C and thus determination of the band diagram quantitatively. The accumulation of negative charge (concentration up to 6 × 1017 cm-3) with bias is observed in GaN:C at both polarities. For biases Vappl < +1.7 V at the top electrode, negative charges accumulate in GaN:C near to its interface with n-GaN so that GaN:C exhibits no potential drop and blocks leakage current. For Vappl > +1.7 V, accumulated negative charges in GaN:C raise an energy barrier of ˜1.1 eV for electron injection from n-GaN to GaN:C. This causes a potential drop in GaN:C leading to a significant leakage current increase. The Fermi level pinning in GaN:C at a commonly referred acceptor at EV + 0.7(±0.2) eV is extracted only from electrostatic considerations. The occupancy change of carbon acceptors is attributed to trapping processes where the dislocation-related conductive paths are supposed to be involved in carrier transport from the top metal electrode to the carbon defect.

  15. High-voltage thin-film GaN LEDs fabricated on ceramic substrates: the alleviated droop effect at 670 W/cm(2).

    PubMed

    Tsai, M L; Liao, J H; Yeh, J H; Hsu, T C; Hon, S J; Chung, T Y; Lai, K Y

    2013-11-04

    High-voltage thin-film GaN LEDs with the emission wavelength of 455 nm were fabricated on ceramic substrates (230 W/m · K). The high-voltage operation was achieved by three cascaded sub-LEDs with dielectric passivation and metal bridges conformally deposited on the side walls. Under the driving power of 670 W/cm(2), the high-voltage LEDs exhibit much alleviated efficiency droop and the operative temperature below 80 °C. The excellent performances were attributed to the improved current spreading within each sub-LED and the superior heat sinking of the ceramic substrate.

  16. Growth, nitrogen vacancy reduction and solid solution formation in cubic GaN thin films and the subsequent fabrication of superlattice structures using AIN and InN

    NASA Astrophysics Data System (ADS)

    Davis, Robert F.

    1992-02-01

    An atomic layer epitaxy deposition system configured for the growth of thin films of the III-V nitrides of Al, Ga and In has been designed, constructed and commissioned. The system allows the introduction of up to 16 gases without mixing. Self-terminating growth of crystalline GaN films has been achieved on single crystal wafers of (0001) alpha(6H)-SiC. Results of analyses via Auger spectroscopy, electron microscopy and electron diffraction are described. Deposition of AlN and GaN via gas-source MBE was also continued during this period. The principal emphasis concerned the initial stages of growth of both compounds on the substrates of (00001) alpha(6H)-SiC and (0001) sapphire, as determined using X-ray photoelectron spectroscopy. An initial layer of silicon nitride formed on the surface of SiC prior to the deposition of either nitride. The deposition of GaN on sapphire followed the Stranski-Krastanov mode of nucleation and growth, while on SiC, characteristics of three-dimensional growth were evident. By contrast, AlN grew initially in a layer-by-layer mode. Deposition of GaN on vicinal (100) Beta-SiC during UV irradiation resulted in the formation of a new 4H polytype of this material. Deposition of BN via gas-source MBE on Cu(110) resulted in nanocrystalline cBN; films grown on (111) Cu resulted in h-BN (graphitic phase). Similar studies using Si(100) substrates also resulted in the occurrence of cBN. The occurrence of the cubic polytype was enhanced while that of h-BN was discouraged with the use of the UV light at 400-500 C.

  17. Role of an ultra-thin AlN/GaN superlattice interlayer on the strain engineering of GaN films grown on Si(110) and Si(111) substrates by plasma-assisted molecular beam epitaxy

    SciTech Connect

    Shen, X. Q.; Takahashi, T.; Matsuhata, H.; Ide, T.; Shimizu, M.; Rong, X.; Chen, G.; Wang, X. Q.; Shen, B.

    2013-12-02

    We investigate the role of an ultra-thin AlN/GaN superlattice interlayer (SL-IL) on the strain engineering of the GaN films grown on Si(110) and Si(111) substrates by plasma-assisted molecular beam epitaxy. It is found that micro-cracks limitted only at the SL-IL position are naturally generated. These micro-cracks play an important role in relaxing the tensile strain caused by the difference of the coefficient of thermal expansion between GaN and Si and keeping the residual strain in the crack-free GaN epilayers resulted from the SL-IL during the growth. The mechanism understanding of the strain modulation by the SL-IL in the GaN epilayers grown on Si substrates makes it possible to design new heterostructures of III-nitrides for optic and electronic device applications.

  18. Fabrication of well-aligned ZnO nanorods by hydrothermal process using GaN epitaxial layer

    NASA Astrophysics Data System (ADS)

    Jang, Jae-Min; Yi, Sung-Hak; Choi, Seung-Kyu; Kim, Jung-A.; Jung, Woo-Gwang

    2007-02-01

    One dimensional (1-D) ZnO nanorod structure of hexagonal shape was fabricated on epitaxial GaN layer by hydrothermal method. The growth of GaN epitaxial layer was carried out in a two-flow horizontal MOCVD reactor maintained at a pressure of 200 torr. Firstly, a 25 nm thick GaN buffer layer was grown at 520 °C. Then 2~3μm thick GaN epilayer was deposited at 1070 °C. Trimethylgallium (TMG) and NH 3 were used as Ga and N source, and H II gas was used as carrier gas. After the deposition of GaN epilayer thin-film, single crystalline ZnO nanorod was fabricated in aqueous solution. XRD and FE-SEM results showed ZnO nanorod arrays were oriented highly along the (002) plane. The ZnO nanorod was analyzed to have good quality crystallization by FE-TEM. The SAED pattern has shown that ZnO nanorod was grown in the direction along (002)-plane. Photoluminescence (PL) has shown that the GaN-ZnO hetero-structure has shown ultra-violet lasing action at room temperature. Narrow and strong ultra-violet peak was observed in comparison with PL result from epitaxial GaN layer. The analysis results have proved that aqueous solution growth method developed in the present work can be a good application for optical electronic device.

  19. Self-organization of dislocation-free, high-density, vertically aligned GaN nanocolumns involving InGaN quantum wells on graphene/SiO2 covered with a thin AlN buffer layer.

    PubMed

    Hayashi, Hiroaki; Konno, Yuta; Kishino, Katsumi

    2016-02-05

    We demonstrated the self-organization of high-density GaN nanocolumns on multilayer graphene (MLG)/SiO2 covered with a thin AlN buffer layer by RF-plasma-assisted molecular beam epitaxy. MLG/SiO2 substrates were prepared by the transfer of CVD graphene onto thermally oxidized SiO2/Si [100] substrates. Employing the MLG with an AlN buffer layer enabled the self-organization of high-density and vertically aligned nanocolumns. Transmission electron microscopy observation revealed that no threading dislocations, stacking faults, or twinning defects were included in the self-organized nanocolumns. The photoluminescence (PL) peak intensities of the self-organized GaN nanocolumns were 2.0-2.6 times higher than those of a GaN substrate grown by hydride vapor phase epitaxy. Moreover, no yellow luminescence or ZB-phase GaN emission was observed from the nanocolumns. An InGaN/GaN MQW and p-type GaN were integrated into GaN nanocolumns grown on MLG, displaying a single-peak PL emission at a wavelength of 533 nm. Thus, high-density nitride p-i-n nanocolumns were fabricated on SiO2/Si using the transferred MLG interlayer, indicating the possibility of developing visible nanocolumn LEDs on graphene/SiO2.

  20. Heteroepitaxial growth and surface structure of L1{sub 0}-MnGa(111) ultra-thin films on GaN(0001)

    SciTech Connect

    Mandru, Andrada-Oana; Wang, Kangkang; Cooper, Kevin; Ingram, David C.; Smith, Arthur R.; Garcia Diaz, Reyes; Takeuchi, Noboru; Haider, Muhammad

    2013-10-14

    L1{sub 0}-structured MnGa(111) ultra-thin films were heteroepitaxially grown on GaN(0001) under lightly Mn-rich conditions using molecular beam epitaxy. Room-temperature scanning tunneling microscopy (STM) investigations reveal smooth terraces and angular step edges, with the surface structure consisting primarily of a 2 × 2 reconstruction along with small patches of 1 × 2. Theoretical calculations were carried out using density functional theory, and the simulated STM images were calculated using the Tersoff-Hamman approximation, revealing that a stoichiometric 1 × 2 and a Mn-rich 2 × 2 surface structure give the best agreement with the observed experimental images.

  1. Characterization of hexagonal ɛ-Ga1.8Sn0.2O3 thin films for solar-blind ultraviolet applications

    NASA Astrophysics Data System (ADS)

    Zhao, Xiaolong; Zhi, Yusong; Cui, Wei; Guo, Daoyou; Wu, Zhenping; Li, Peigang; Li, Linghong; Tang, Weihua

    2016-12-01

    Ga1.8Sn0.2O3 thin films were deposited on c-plane Al2O3 (0001) substrates by laser molecular beam epitaxy technology. Well crystallized (002) oriented ɛ-phase Ga1.8Sn0.2O3 thin films were obtained at the substrate temperature above 750 °C and the oxygen partial pressure more than 5 × 10-3 Pa. The band-gap slightly shrinks with Sn4+ ions incorporated into Ga3+ sites, showing an excellent solar-blind ultraviolet (UV) characteristic. The conductivity of hexagonal ɛ-Ga1.8Sn0.2O3 films is very low in the dark, and permitting the design and fabrication of solar-blind photodetector. The photodetector exhibits obvious photo-response under 254 nm UV light irradiation, and it increases in photocurrent with both the rise of applied bias and optical input power. The results suggest that ɛ-Ga1.8Sn0.2O3 thin film is a promising candidate for using in solar-blind photodetectors.

  2. Fabrication of WS2/GaN p-n Junction by Wafer-Scale WS2 Thin Film Transfer

    PubMed Central

    Yu, Yang; Fong, Patrick W. K.; Wang, Shifeng; Surya, Charles

    2016-01-01

    High quality wafer-scale free-standing WS2 grown by van der Waals rheotaxy (vdWR) using Ni as a texture promoting layer is reported. The microstructure of vdWR grown WS2 was significantly modified from mixture of crystallites with their c-axes both parallel to (type I) and perpendicular to (type II) the substrate to large type II crystallites. Wafer-scale transfer of vdWR grown WS2 onto different substrates by an etching-free technique was demonstrated for the first time that utilized the hydrophobic property of WS2 and hydrophilic property of sapphire. Our results show that vdWR is a reliable technique to obtain type-II textured crystallites in WS2, which is the key factor for the wafer-scale etching-free transfer. The transferred films were found to be free of observable wrinkles, cracks, or polymer residues. High quality p-n junctions fabricated by room-temperature transfer of the p-type WS2 onto an n-type GaN was demonstrated with a small leakage current density of 29.6 μA/cm2 at −1 V which shows superior performances compared to the directly grown WS2/GaN heterojunctions. PMID:27897210

  3. Fabrication of WS2/GaN p-n Junction by Wafer-Scale WS2 Thin Film Transfer

    NASA Astrophysics Data System (ADS)

    Yu, Yang; Fong, Patrick W. K.; Wang, Shifeng; Surya, Charles

    2016-11-01

    High quality wafer-scale free-standing WS2 grown by van der Waals rheotaxy (vdWR) using Ni as a texture promoting layer is reported. The microstructure of vdWR grown WS2 was significantly modified from mixture of crystallites with their c-axes both parallel to (type I) and perpendicular to (type II) the substrate to large type II crystallites. Wafer-scale transfer of vdWR grown WS2 onto different substrates by an etching-free technique was demonstrated for the first time that utilized the hydrophobic property of WS2 and hydrophilic property of sapphire. Our results show that vdWR is a reliable technique to obtain type-II textured crystallites in WS2, which is the key factor for the wafer-scale etching-free transfer. The transferred films were found to be free of observable wrinkles, cracks, or polymer residues. High quality p-n junctions fabricated by room-temperature transfer of the p-type WS2 onto an n-type GaN was demonstrated with a small leakage current density of 29.6 μA/cm2 at ‑1 V which shows superior performances compared to the directly grown WS2/GaN heterojunctions.

  4. Hexagonal quartz resonator

    DOEpatents

    Peters, Roswell D. M.

    1982-01-01

    A generally flat, relatively thin AT-cut piezoelectric resonator element structured to minimize the force-frequency effect when mounted and energized in a housing. The resonator is in the form of an equilateral hexagon with the X crystallographic axis of the crystal passing through one set of opposing corners with mounting being effected at an adjacent set of corners respectively .+-.60.degree. away from the X axis which thereby results in a substantially zero frequency shift of the operating frequency.

  5. Issues and examples regarding growth of AlN, GaN and Al{sub x}Ga{sub 1{minus}x}N thin films via OMVPE and gas source MBE

    SciTech Connect

    Davis, R.F.; Weeks, T.W. Jr.; Bremser, M.D.; Tanaka, S.; Kern, R.S.; Sitar, Z.; Ailey, K.S.; Perry, W.G.; Wang, C.

    1996-11-01

    Organometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE) are the most common methods for the growth of thin films of AlN and GaN. Sapphire is the most common substrate; however, a host of materials have been used with varying degrees of success. Both growth techniques have been employed by the authors to grow AlN, GaN and Al{sub x}Ga{sub 1{minus}x}N thin films primarily on 6H-SiC(0001). The mismatch in atomic layer stacking sequences along the growth direction produces double positioning boundaries in AlN and the alloys at the SiC steps; the sequence problem appears to discourage the two-dimensional nucleation of GaN. Films of these materials grown by MBE at 650 C are textured; monocrystalline films are achieved between 850 C (pure GaN) and 1,050 C (pure AlN) by this technique and OMVPE. Donor and acceptor doping of GaN has been achieved via MBE without post growth annealing. Acceptor doping in CVD material requires annealing to displace the H from the Mg and eventually remove it from the material. High brightness light emitting diodes are commercially available; however, numerous concerns regarding metal and nitrogen sources, heteroepitaxial nucleation, the role of buffer layers, surface migration rates as a function of temperature, substantial defect densities and their effect on film and device properties, ohmic and rectifying contacts, wet and dry etching and suitable gate and field insulators must and are being addressed. Selected issues surrounding the growth of these materials with particular examples drawn from the authors` research are presented herein.

  6. Temperature dependence of the point defect properties of GaN thin films studied by terahertz time-domain spectroscopy

    NASA Astrophysics Data System (ADS)

    Fang, HeNan; Zhang, Rong; Liu, Bin; Li, YeCao; Fu, DeYi; Li, Yi; Xie, ZiLi; Zhuang, Zhe; Zheng, YouDou; Wu, JingBo; Jin, BiaoBing; Chen, Jian; Wu, PeiHeng

    2013-11-01

    The dielectric functions of GaN for the temperature and frequency ranges of 10-300 K and 0.3-1 THz are obtained using terahertz time-domain spectroscopy. It is found that there are oscillations of the dielectric functions at various temperatures. Physically, the oscillation behavior is attributed to the resonance states of the point defects in the material. Furthermore, the dielectric functions are well fitted by the combination of the simple Drude model together with the classical damped oscillator model. According to the values of the fitting parameters, the concentration and electron lifetime of the point defects for various temperatures are determined, and the temperature dependences of them are in accordance with the previously reported result. Therefore, terahertz time-domain spectroscopy can be considered as a promising technique for investigating the relevant characteristics of the point defects in semiconductor materials.

  7. High-yield thin GaN LED using metal bonding and laser lift-off technology

    NASA Astrophysics Data System (ADS)

    Horng, Ray-Hua; Chen, Ching-Ho; Kao, Wei-Cheng; Wuu, Dong-Sing

    2012-10-01

    Gold-indium metal bonding method was used in this study to increase the product yield of vertical light emitting diodes (LEDs) during laser lift-off (LLO) process. The vertical GaN LED transferred onto Si substrate presented good electrical and optical properties due to the existence of high reflective mirror and texture surface. The chip size and dominant wavelength for vertical type LED are 40×40 mil2 and 450 nm. The optimal conditions of temperature and pressure for 2-inch wafer bonding are set of 200oC and 100 kg/inch2, respectively. The products yield of light output power, forward voltage and leakage current are 96 %, 96.4% and 61.2%, respectively. After aging test, the characteristics decay of light output power, forward voltage and leakage current are less than 4%. Summarization of optical and electrical properties, the total yield of these LEDs products is about 60 %.

  8. Dislocation annihilation in regrown GaN on nanoporous GaN template with optimization of buffer layer growth

    NASA Astrophysics Data System (ADS)

    Soh, C. B.; Hartono, H.; Chow, S. Y.; Chua, S. J.; Fitzgerald, E. A.

    2007-01-01

    Nanoporous GaN template has been fabricated by electrochemical etching to give hexagonal pits with nanoscale pores of size 20-50nm in the underlying grains. The effect of GaN buffer layer grown at various temperatures from 650to1015°C on these as-fabricated nanopores templates is investigated by transmission electron microscopy. The buffer layer grown at the optimized temperature of 850°C partially fill up the pores and voids with annihilation of threading dislocations, serving as an excellent template for high-quality GaN growth. This phenomenon is, however, not observed for the samples grown with other temperature buffer layers. Micro-Raman measurements show significant strain relaxation and improvement in the crystal quality of the overgrown GaN layer on nanoporous GaN template as compared to overgrown on conventional GaN template.

  9. Analysis of light extraction efficiency enhancement for thin-film-flip-chip InGaN quantum wells light-emitting diodes with GaN micro-domes.

    PubMed

    Zhao, Peng; Zhao, Hongping

    2012-09-10

    The enhancement of light extraction efficiency for thin-film flip-chip (TFFC) InGaN quantum wells (QWs) light-emitting diodes (LEDs) with GaN micro-domes on n-GaN layer was studied. The light extraction efficiency of TFFC InGaN QWs LEDs with GaN micro-domes were calculated and compared to that of the conventional TFFC InGaN QWs LEDs with flat surface. The three dimensional finite difference time domain (3D-FDTD) method was used to calculate the light extraction efficiency for the InGaN QWs LEDs emitting at 460nm and 550 nm, respectively. The effects of the GaN micro-dome feature size and the p-GaN layer thickness on the light extraction efficiency were studied systematically. Studies indicate that the p-GaN layer thickness is critical for optimizing the TFFC LED light extraction efficiency. Significant enhancement of the light extraction efficiency (2.5-2.7 times for λ(peak) = 460nm and 2.7-2.8 times for λ(peak) = 550nm) is achievable from TFFC InGaN QWs LEDs with optimized GaN micro-dome diameter and height.

  10. Crystallization from amorphous structure to hexagonal quantum dots induced by an electron beam on CdTe thin films

    NASA Astrophysics Data System (ADS)

    Becerril, M.; Zelaya-Angel, O.; Medina-Torres, A. C.; Aguilar-Hernández, J. R.; Ramírez-Bon, R.; Espinoza-Beltran, F. J.

    2009-02-01

    Amorphous cadmium-telluride films were prepared by rf sputtering on Corning 7059 glass substrates at room temperature. The deposition time was 10 and 12 h with a thickness of 400 and 480 (±40 nm), respectively. As-prepared films were amorphous according to X-ray diffraction (XRD) patterns, but a win-fit-software analysis of the main XRD broad band suggests a wurtzite structure at short range. Transmission electron microscopy (TEM) at 200 keV produces crystallization of the amorphous CdTe. The TEM-electron beam induces the formation of CdTe quantum dots with the wurtzite hexagonal structure (the metastable structure of CdTe) and with ˜6 nm of average grain size. As effect of a probable distortion of the CdTe crystalline lattice, the unit cell volume (UCV) shrinks to about 30% with respect to the bulk-UCV of CdTe. Besides, the energy band gap increases as expected, according to literature data on quantum confinement.

  11. Bending stability of GaN grown on a metallic flexible substrate by plasma-assisted molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Rodríguez, A. G.; Chávez-Veloz, S. G.; Compeán-García, V. D.; López-Luna, E.; Vidal, M. A.

    2017-08-01

    GaN thin films were grown on flexible metallic substrates by molecular beam epitaxy. MgO buffer layers were deposited by spin coating on Ni-Mo-Cr (Hastelloy C-276) alloy tapes that were used as substrates. The structural characterization of the GaN/MgO/hastelloy samples was performed by x-ray diffraction and Raman spectroscopy. The obtained nanometric films have the stable hexagonal phase (α-GaN) with an average crystallite size of 18 nm. The long and short range order of GaN decrease when the structure is bent. The most significant variations in the structural properties occur between 100 and 250 bending cycles.

  12. Self-Assembly of Octapod-Shaped Colloidal Nanocrystals into a Hexagonal Ballerina Network Embedded in a Thin Polymer Film

    PubMed Central

    2014-01-01

    Nanoparticles with unconventional shapes may exhibit different types of assembly architectures that depend critically on the environmental conditions under which they are formed. Here, we demonstrate how the presence of polymer (polymethyl methacrylate, PMMA) molecules in a solution, in which CdSe(core)/CdS(pods) octapods are initially dispersed, affects the octapod-polymer organization upon solvent evaporation. We show that a fast drop-drying process can induce a remarkable two-dimensional (2D) self-assembly of octapods at the polymer/air interface. In the resulting structure, each octapod is oriented like a “ballerina”, that is, only one pod sticks out of the polymer film and is perpendicular to the polymer–air interface, while the opposite pod (with respect to the octapod’s center) is fully immersed in the film and points toward the substrate, like a ballerina performing a grand battement. In some areas, a hexagonal-like pattern is formed by the ballerinas in which the six nonvertical pods, which are all embedded in the film, maintain a pod–pod parallel configuration with respect to neighboring particles. We hypothesize that the mechanism responsible for such a self-assembly is based on a fast adsorption of the octapods from bulk solution to the droplet/air interface during the early stages of solvent evaporation. At this interface, the octapods maintain enough rotational freedom to organize mutually in a pod–pod parallel configuration between neighboring octapods. As the solvent evaporates, the octapods form a ballerina-rich octapod-polymer composite in which the octapods are in close contact with the substrate. Finally, we found that the resulting octapod-polymer composite is less hydrophilic than the polymer-only film. PMID:24447264

  13. Self-assembly of octapod-shaped colloidal nanocrystals into a hexagonal ballerina network embedded in a thin polymer film.

    PubMed

    Arciniegas, Milena P; Kim, Mee R; De Graaf, Joost; Brescia, Rosaria; Marras, Sergio; Miszta, Karol; Dijkstra, Marjolein; van Roij, René; Manna, Liberato

    2014-02-12

    Nanoparticles with unconventional shapes may exhibit different types of assembly architectures that depend critically on the environmental conditions under which they are formed. Here, we demonstrate how the presence of polymer (polymethyl methacrylate, PMMA) molecules in a solution, in which CdSe(core)/CdS(pods) octapods are initially dispersed, affects the octapod-polymer organization upon solvent evaporation. We show that a fast drop-drying process can induce a remarkable two-dimensional (2D) self-assembly of octapods at the polymer/air interface. In the resulting structure, each octapod is oriented like a "ballerina", that is, only one pod sticks out of the polymer film and is perpendicular to the polymer-air interface, while the opposite pod (with respect to the octapod's center) is fully immersed in the film and points toward the substrate, like a ballerina performing a grand battement. In some areas, a hexagonal-like pattern is formed by the ballerinas in which the six nonvertical pods, which are all embedded in the film, maintain a pod-pod parallel configuration with respect to neighboring particles. We hypothesize that the mechanism responsible for such a self-assembly is based on a fast adsorption of the octapods from bulk solution to the droplet/air interface during the early stages of solvent evaporation. At this interface, the octapods maintain enough rotational freedom to organize mutually in a pod-pod parallel configuration between neighboring octapods. As the solvent evaporates, the octapods form a ballerina-rich octapod-polymer composite in which the octapods are in close contact with the substrate. Finally, we found that the resulting octapod-polymer composite is less hydrophilic than the polymer-only film.

  14. Optical and structural characterization of GaN thin films at different N to Ga flux ratios

    SciTech Connect

    El-Naggar, Ahmed M.

    2011-01-15

    GaN films were grown on Si(111) substrates under various beam equivalent pressure (BEP) ratios by plasma-assisted molecular beam epitaxy. The optical properties for the grown samples were studied over a wide spectral range from 200 to 3300 nm using the reflectance spectrum only. It was found that increasing the N/Ga BEP ratio from 17.9 to 46.1 increases the refractive index (n) from 2.05 to 2.38 at wavelength 630 nm (for example), while the optical energy gap (E{sub g}) were found to be in the range between 3.325 to 3.35 eV with no specific trend. The structural properties for the grown films were studied through two types of rocking curve measurements; normal rocking curve ({omega}-scan) and triple axis rocking curve ({omega}/2{theta}-scan). It was found that with decreasing the N/Ga ratio from 46.1 to 17.9 the full width at half maximum decreases from 0.62 deg. to 0.58 deg. for {omega}-scan and from 0.022 deg. to 0.021 deg. for {omega}/2{theta}-scan. Thus, our results showed a clear correlation between the optical-structural parameters and the BEP ratios of N and Ga.

  15. GaN nanorods coated with pure BN

    NASA Astrophysics Data System (ADS)

    Han, Wei-Qiang; Zettl, A.

    2002-12-01

    We report a method to efficiently synthesize gallium nitride (GaN) nanorods coated with insulating boron nitride (BN) layers. The GaN core is crystalline (with either a cubic zincblende or hexagonal wurtzite structure) and has diameters ranging from 10 to 85 nm and lengths up to 60 μm. The outer encapsulating BN shells with typical thicknesses less than 5 nm extend fully over, and adhere well to, the entire nanorod surface.

  16. Optical properties of GaN pyramids

    SciTech Connect

    Zeng, K.C.; Lin, J.Y.; Jiang, H.X.; Yang, W.

    1999-03-01

    Picosecond time-resolved photoluminescence (PL) spectroscopy has been used to investigate the optical properties of GaN pyramids overgrown on hexagonal-patterned GaN(0001) epilayers on sapphire and silicon substrates with AlN buffer layers. We found that: (i) the release of the biaxial compressive strain in GaN pyramids on GaN/AlN/sapphire substrate led to a 7 meV redshift of the spectral peak position with respect to the strained GaN epilayer grown under identical conditions; (ii) in the GaN pyramids on GaN/AlN/sapphire substrate, strong band edge transitions with much narrower linewidths than those in the GaN epilayer have been observed, indicating the improved crystalline quality of the overgrown pyramids; (iii) PL spectra taken from different parts of the pyramids revealed that the top of the pyramid had the highest crystalline quality; and (iv) the presence of strong band-to-impurity transitions in the pyramids were primarily due to the incorporation of the oxygen and silicon impurities from the SiO{sub 2} mask. {copyright} {ital 1999 American Institute of Physics.}

  17. Bulk GaN Ion Cleaving

    NASA Astrophysics Data System (ADS)

    Moutanabbir, O.; Gösele, U.

    2010-05-01

    Bulk or freestanding GaN is a key material in various devices other than the blue laser diodes. However, the high cost of bulk GaN wafers severely limits the large scale exploitation of these potential technologies. In this paper, we discuss some engineering issues involved in the application of the ion-cut process to split a thin layer from 2-inch freestanding GaN. This process combines the implantation of light ions and wafer bonding and can possibly be used to reduce the cost of the fabrication of GaN-based devices by allowing the transfer of several bulk quality thin layers from the same donor wafer. To achieve this multi-layer transfer several conditions must be fulfilled. Here issues related to bulk GaN surface irregularities and wafer bowing are discussed. We also describe a method to circumvent most of these problems and achieve high quality bonding.

  18. Nanolabyrinthine ZrAlN thin films by self-organization of interwoven single-crystal cubic and hexagonal phases

    NASA Astrophysics Data System (ADS)

    Ghafoor, Naureen; Johnson, Lars J. S.; Klenov, Dmitri O.; Demeulemeester, Jelly; Desjardins, Patrick; Petrov, Ivan; Hultman, Lars; Odén, Magnus

    2013-08-01

    Self-organization on the nanometer scale is a trend in materials research. Thermodynamic driving forces may, for example, yield chessboard patterns in metal alloys [Y. Ni and A. G. Khachaturyan, Nature Mater. 8, 410-414 (2009)], 10.1038/nmat2431 or nitrides [P. H. Mayrhofer, A. Hörling, L. Karlsson, J. Sjölén, T. Larsson, and C. Mitterer, Appl. Phys. Lett. 83, 2049 (2003)], 10.1063/1.1608464 during spinodal decomposition. Here, we explore the ZrN-AlN system, which has one of the largest positive enthalpies of mixing among the transition metal aluminum nitrides [D. Holec, R. Rachbauer, L. Chen, L. Wang, D. Luefa, and P. H. Mayrhofer, Surf. Coat. Technol. 206, 1698-1704 (2011), 10.1016/j.surfcoat.2011.09.019; B. Alling, A. Karimi, and I. Abrikosov, Surf. Coat. Technol. 203, 883-886 (2008)], 10.1016/j.surfcoat.2008.08.027. Surprisingly, a highly regular superhard (36 GPa) two-dimensional nanolabyrinthine structure of two intergrown single crystal phases evolves during magnetron sputter thin film synthesis of Zr0.64Al0.36N/MgO(001). The self-organization is surface driven and the synergistic result of kinetic limitations, where the enthalpy reduction balances both investments in interfacial and elastic energies.

  19. Hexagonal quartz resonator

    DOEpatents

    Peters, R.D.M.

    1982-11-02

    A generally flat, relatively thin AT-cut piezoelectric resonator element structured to minimize the force-frequency effect when mounted and energized in a housing. The resonator is in the form of an equilateral hexagon with the X crystallographic axis of the crystal passing through one set of opposing corners with mounting being effected at an adjacent set of corners respectively [+-]60[degree] away from the X axis which thereby results in a substantially zero frequency shift of the operating frequency. 3 figs.

  20. Wafer-Size and Single-Crystal MoSe2 Atomically Thin Films Grown on GaN Substrate for Light Emission and Harvesting.

    PubMed

    Chen, Zuxin; Liu, Huiqiang; Chen, Xuechen; Chu, Guang; Chu, Sheng; Zhang, Hang

    2016-08-10

    Two-dimensional (2D) atomic-layered semiconductors are important for next-generation electronics and optoelectronics. Here, we designed the growth of an MoSe2 atomic layer on a lattice-matched GaN semiconductor substrate. The results demonstrated that the MoSe2 films were less than three atomic layers thick and were single crystalline of MoSe2 over the entire GaN substrate. The ultrathin MoSe2/GaN heterojunction diode demonstrated ∼850 nm light emission and could also be used in photovoltaic applications.

  1. Separation of thick HVPE-GaN films from GaN templates using nanoporous GaN layers

    NASA Astrophysics Data System (ADS)

    Dong, Zengyin; Yang, Ruixia; Zhang, Song; Wang, Zaien; Chen, Jianli; Li, Xun

    2017-10-01

    In this work, we have succeeded in growing an approximate 2-inch self-separated thick GaN wafer by hydride vapor phase epitaxy with an introduction of a sacrificial layer of nanoporous GaN. Such nanoporous GaN layer is invented by using the HVPE growth of thin GaN layer on the spin-coating silica nanosphere layer followed by a hydrofluoric acid etching to the silica nanosphere layer. It has been found that the nanoporous GaN layer, enabling a reduction of stickiness between thick GaN films and the substrates, plays a significant role in the self-separation of thick GaN films during the cooling process. However, the thickness of the nanoporous GaN layer is another key issue to achieve good quality self-separated GaN thick films. In our study, we suggest that the nanoporous GaN layer with a thickness of approximately 150-240 nm can best serve as the sacrificial layer in self-separation process. Raman spectroscopy also indicates the self-separated thick GaN films by using the proposed approach are virtually strain-free.

  2. Growth, nitrogen vacancy reduction, and solid solution formation in cubic GaN thin films and the subsequent fabrication of superlattice structures using AlN and InN

    NASA Astrophysics Data System (ADS)

    Davis, Robert F.; Ailey-Trent, K. S.; Kester, Daniel; Paisley, Michael J.; Perry, Bill

    1992-06-01

    Undoped GaN films have been deposited by gas-source MBE having essentially intrinsic electrical character. Acceptor-type behavior has been achieved with Mg doping. The electrical properties of these latter films were resistivity = 0.5 omega-cm, Hall mobility (holes = 10 sq cm/V-s and carrier concentration = 1(10)(exp 18) cu cm. Photo-assisted gas-source MBE growth of stoichiometric GaN was also achieved using a 500 W Hg lamp. Illumination and Ga cell temperature altered the texture of the polycrystalline GaN in unusual ways, changing the growth habit from (0001) is parallel to (100) to (0001) is parallel to (111) and back again. Thin films of cubic-BN (c-BN) were also deposited on various substrates via both gas-source MBE and electron beam MBE. The use of Si(100) substrates, the latter technique, and the characterization tools of RHEED, XPS, LEED, SEM, FTIR, and HRTEM resulted in the achievement of an initial amorphous BN layer followed by a layer of turbostratic BN and subsequently by a layer of cubic BN. Cubic BN films were also deposited on polycrystalline diamond films grown via CVD on Si(100). The effect of the bombarding species was examined. Finally, the plans for both a systematic investigation of the ion implantation and contact development and related characterization of AlN and GaN with n- and p-type dopants and the construction and employment of a UV luminescence facility is discussed.

  3. Magnetic resonance studies of the Mg acceptor in thick free-standing and thin-film GaN

    NASA Astrophysics Data System (ADS)

    Zvanut, Mary Ellen

    Mg, the only effective p-type dopant for the nitrides, substitutes for Ga and forms an acceptor with a defect level of about 0.16 eV. The magnetic resonance of such a center should be highly anisotropic, yet early work employing both optically detected magnetic resonance (ODMR) and electron paramagnetic resonance (EPR) spectroscopies revealed a defect with a nearly isotropic g-tensor. The results were attributed to crystal fields caused by compensation and/or strain typical of the heteroepitaxially grown films. The theory was supported by observation of the expected highly anisotropic ODMR signature in homoepitaxially grown films in which dislocation-induced non-uniform strain and compensation are reduced. The talk will review EPR measurements of thin films and describe new work which takes advantage of the recently available thick free-standing GaN:Mg substrates grown by hydride vapor phase epitaxy (HVPE) and high nitrogen pressure solution growth (HNPS). Interestingly, the films and HVPE substrates exhibit characteristically different types of EPR signals, and no EPR response could be induced in the HNPS substrates, with or without illumination. In the heteroepitaxial films, a curious angular dependent line-shape is observed in addition to the nearly isotropic g-tensor characteristic of the Mg-related acceptor. On the other hand, the free-standing HVPE crystals reveal a clear signature of a highly anisotropic shallow acceptor center. Comparison with SIMS measurements implies a direct relation to the Mg impurity, and frequency-dependent EPR studies demonstrate the influence of the anisotropic crystal fields. Overall, the measurements of the thick free-standing crystals show that the Mg acceptor is strongly affected by the local environment. The ODMR was performed by Evan Glaser, NRL and the free-standing Mg-doped HVPE crystals were grown by Jacob Leach, Kyma Tech. The work at UAB is supported by NSF Grant No. DMR-1308446.

  4. ZrB2 thin films deposited on GaN(0001) by magnetron sputtering from a ZrB2 target

    NASA Astrophysics Data System (ADS)

    Tengdelius, Lina; Lu, Jun; Forsberg, Urban; Li, Xun; Hultman, Lars; Janzén, Erik; Högberg, Hans

    2016-11-01

    ZrB2 films were deposited on 900 °C-preheated or non-preheated GaN(0001) surfaces by direct current magnetron sputtering from a compound target. Analytical transmission electron microscopy and scanning transmission electron microscopy with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy revealed a 0001 fiber textured ZrB2 film growth following the formation of a 2 nm thick amorphous BN layer onto the GaN(0001) at a substrate temperature of 900 °C. The amorphous BN layer remains when the substrate temperature is lowered to 500 °C or when the preheating step is removed from the process and results in the growth of polycrystalline ZrB2 films. The ZrB2 growth phenomena on GaN(0001) is compared to on 4H-SiC(0001), Si(111), and Al2O3(0001) substrates, which yield epitaxial film growth. The decomposition of the GaN surface during vacuum processing during BN interfacial layer formation is found to impede epitaxial growth of ZrB2.

  5. Investigation of different mechanisms of GaN growth induced on AlN and GaN nucleation layers

    SciTech Connect

    Tasco, V.; Campa, A.; Tarantini, I.; Passaseo, A.; Gonzalez-Posada, F.; Munoz, E.; Redondo-Cubero, A.; Lorenz, K.; Franco, N.

    2009-03-15

    The evolution of GaN growth on AlN and GaN nucleation layers is compared through morphological and structural analyses, including ion beam analysis. By using AlN nucleation layer grown at high temperature, improved crystalline quality is exhibited by 300 nm thin GaN epilayers. GaN (002) x-ray rocking curve as narrow as 168 arc sec and atomic-step surface morphology characterize such a thin GaN film on AlN. Defects are strongly confined into the first 50 nm of growth, whereas a fast laterally coherent growth is observed when increasing thickness, as an effect of high temperature AlN surface morphology and Ga adatom dynamics over this template.

  6. Crystal field analysis of rare-earth ions energy levels in GaN

    NASA Astrophysics Data System (ADS)

    Stachowicz, M.; Kozanecki, A.; Ma, C.-G.; Brik, M. G.; Lin, J. Y.; Jiang, Hx; Zavada, J. M.

    2014-11-01

    Much effort has been put to achieve optoelectronic devices based on Er doped GaN, operating on the intra-4f-shell transitions of erbium. The key issue for good understanding of energy transfer mechanisms to Er and its luminescence properties is the position of Er3+ ions in the crystalline lattice of GaN. After doping, Er3+ ions are assumed to be placed in substitutional position for Ga3+ in GaN. Although Ga is positioned in high symmetry, tetrahedral [ErN4]9 - cluster, deviations from this after doping are impossible to avoid because of a large difference in ionic radii of Ga3+ (47 pm) and Er3+ (89 pm). In this work we report on crystal field analysis of Er ion energy levels in cubic and hexagonal GaN. It is shown that local symmetry of Er in cubic GaN is D2, whereas calculations reveal that in hexagonal GaN local symmetry is C3V. Some trends in crystal field parameters of trivalent lanthanides in hexagonal GaN are discussed.

  7. Improvement in optical and structural properties of ZnO thin film through hexagonal nanopillar formation to improve the efficiency of a Si-ZnO heterojunction solar cell

    NASA Astrophysics Data System (ADS)

    Maity, S.; Bhunia, C. T.; Sahu, P. P.

    2016-05-01

    We propose to use ZnO thin film with hexagonal nanopillars deposited on Si substrate to enhance the efficiency of a solar cell. It has been treated chemically and thermally and various crystal orientations have been obtained. X-ray diffraction of ZnO thin film shows relatively high intensity peak at 34.3° angle (0 0 2) compared to other orientations. Photoluminescence measurements also confirm a narrow full width at half maximum peak at 3.3 eV, which is more than that obtained for as-grown (broad emission peak around 3.0 eV). The alignment of nanorod structure made by adding a dopant of 0.15 mole fraction of magnesium increases both photon collection and electron collection efficiency. As a result, the solar cell efficiency is enhanced from 10% to 20%.

  8. GaN-Si direct wafer bonding at room temperature for thin GaN device transfer after epitaxial lift off

    NASA Astrophysics Data System (ADS)

    Mu, Fengwen; Morino, Yuki; Jerchel, Kathleen; Fujino, Masahisa; Suga, Tadatomo

    2017-09-01

    Room temperature GaN-Si direct wafer bonding was done by surface activated bonding (SAB). At first, a feasibility study using GaN template has been done. Then, crystal-face dependence of the bonding results for freestanding GaN substrate has been investigated between Ga-face and N-face. The results of Ga-face to Si bonding are better than that of N-face to Si bonding such as higher bonding energy and larger bonded area. This difference should be caused by their different surface roughnesses after chemical-mechanical polishing (CMP). Besides, both of the structure and composition of the two kinds of interfaces were investigated to understand the bonding mechanisms. The phenomenon of Ga-enrichment during surface activation and Ga-diffusion into Si at room temperature for both Ga-face bonding and N-face bonding has been confirmed.

  9. Structures and stability of polar GaN thin films on ScAlMgO4 substrate: An ab initio-based study

    NASA Astrophysics Data System (ADS)

    Nakane, Harunobu; Akiyama, Toru; Nakamura, Kohji; Ito, Tomonori

    2017-06-01

    The structures and stability of polar GaN/ScAlMgO4(0001) interfaces are investigated by performing density-functional calculations. On the basis of the calculated interface energies, we find characteristic features of atomic arrangements depending on the polarity of interface. The interface with Ga-adatom is stabilized over the wide range of Ga chemical potential for Ga-polar GaN, while the interface with N-adatom is always stable for N-polar GaN. Furthermore, the interface resulting in Ga-polar films is found to be more stable than that in N-polar films on ScAlMgO4(0001) substrate. The stability of polar GaN/ScAlMgO4 interfaces is interpreted in terms of the formation of stable bonds and charge neutrality at the interface.

  10. Substrate temperature influence on the properties of GaN thin films grown by hollow-cathode plasma-assisted atomic layer deposition

    SciTech Connect

    Alevli, Mustafa Gungor, Neşe; Haider, Ali; Kizir, Seda; Leghari, Shahid A.; Biyikli, Necmi

    2016-01-15

    Gallium nitride films were grown by hollow cathode plasma-assisted atomic layer deposition using triethylgallium and N{sub 2}/H{sub 2} plasma. An optimized recipe for GaN film was developed, and the effect of substrate temperature was studied in both self-limiting growth window and thermal decomposition-limited growth region. With increased substrate temperature, film crystallinity improved, and the optical band edge decreased from 3.60 to 3.52 eV. The refractive index and reflectivity in Reststrahlen band increased with the substrate temperature. Compressive strain is observed for both samples, and the surface roughness is observed to increase with the substrate temperature. Despite these temperature dependent material properties, the chemical composition, E{sub 1}(TO), phonon position, and crystalline phases present in the GaN film were relatively independent from growth temperature.

  11. Conductivity based on selective etch for GaN devices and applications thereof

    DOEpatents

    Zhang, Yu; Sun, Qian; Han, Jung

    2015-12-08

    This invention relates to methods of generating NP gallium nitride (GaN) across large areas (>1 cm.sup.2) with controlled pore diameters, pore density, and porosity. Also disclosed are methods of generating novel optoelectronic devices based on porous GaN. Additionally a layer transfer scheme to separate and create free-standing crystalline GaN thin layers is disclosed that enables a new device manufacturing paradigm involving substrate recycling. Other disclosed embodiments of this invention relate to fabrication of GaN based nanocrystals and the use of NP GaN electrodes for electrolysis, water splitting, or photosynthetic process applications.

  12. Hexagons of the Heart

    ERIC Educational Resources Information Center

    Burkhauser, Beth; Porter, Dave

    2010-01-01

    This article discusses the international interdependence Hexagon Project for Haiti which invites students, ages five through eighteen, to create an image within a hexagonal template and respond to big questions surrounding a global culture of interdependence. The hexagon is a visual metaphor for interdependence, with its potential to infinitely…

  13. Hexagons of the Heart

    ERIC Educational Resources Information Center

    Burkhauser, Beth; Porter, Dave

    2010-01-01

    This article discusses the international interdependence Hexagon Project for Haiti which invites students, ages five through eighteen, to create an image within a hexagonal template and respond to big questions surrounding a global culture of interdependence. The hexagon is a visual metaphor for interdependence, with its potential to infinitely…

  14. Interfacial reaction between metal-insulator transition material NbO2 thin film and wide band gap semiconductor GaN

    NASA Astrophysics Data System (ADS)

    Posadas, Agham; Kvit, Alexander; Demkov, Alexander

    Materials that undergo a metal-insulator transition (MIT) are potentially useful for a wide variety of applications including electronic and opto-electronic switches, memristors, sensors, and coatings. In most such materials, the MIT is driven by temperature. In one such material, NbO2, the MIT mechanism is primarily of the Peierls-type, in which the dimerization of the Nb atoms without electron correlation causes the transition from metallic to semiconducting. We describe our initial work at combining NbO2 and GaN in epitaxial form, which could be potentially useful in resistive switching devices operating at very high temperatures. We grow NbO2 films on GaN(0001)/Si(111) substrates using reactive molecular beam epitaxy from a metal evaporation source and molecular oxygen. X-ray diffraction shows that the films are found to grow with a single out of plane orientation but with three symmetry-related orientation domains in the plane. In situ x-ray photoelectron spectroscopy confirms that the phase pure NbO2 is formed but that a chemical reaction occurs between the GaN and NbO2 during the growth forming a polycrystalline interfacial layer. We perform STEM-EELS analysis of the film and the interface to further elucidate their chemical and structural properties.

  15. Implementing Room-Temperature Multiferroism by Exploiting Hexagonal-Orthorhombic Morphotropic Phase Coexistence in LuFeO3 Thin Films.

    PubMed

    Song, Seungwoo; Han, Hyeon; Jang, Hyun Myung; Kim, Young Tae; Lee, Nam-Suk; Park, Chan Gyung; Kim, Jeong Rae; Noh, Tae Won; Scott, James F

    2016-09-01

    Room-temperature multiferroism in LuFeO3 (LFO) films is demonstrated by exploiting the orthorhombic-hexagonal (o-h) morphotrophic phase coexistence. The LFO film further reveals a magnetoelectric coupling effect that is not shown in single-phase (h- or o-) LFO. The observed multiferroism is attributed to the combination of sufficient polarization from h-LFO and net magnetization from o-LFO.

  16. Plasticity and optical properties of GaN under highly localized nanoindentation stress fields

    NASA Astrophysics Data System (ADS)

    Caldas, P. G.; Silva, E. M.; Prioli, R.; Huang, J. Y.; Juday, R.; Fischer, A. M.; Ponce, F. A.

    2017-03-01

    Nanoscale plasticity has been studied on (0001) GaN thin films, using tips with very small radius of curvature. Cross-section transmission electron microscopy images of the nanoindentations indicate that the primary slip systems are the pyramidal {1 1 ¯ 01 } ⟨11 2 ¯ 3 ⟩ and {11 2 ¯ 2 } ⟨11 2 ¯ 3 ⟩ , followed by the basal {0002 } ⟨11 2 ¯ 0 ⟩ . Incipient plasticity was observed to be initiated by metastable atomic-scale slip events that occur as the crystal conforms to the shape of the tip. Large volumetric material displacements along the {1 1 ¯ 01 } ⟨11 2 ¯ 3 ⟩ and {11 2 ¯ 2 } ⟨11 2 ¯ 3 ⟩ slip systems were observed at an average shear stress of 11 GPa. Hexagonal shaped nanoindentation impressions following the symmetry of GaN were observed, with material pile-up in the ⟨11 2 ¯ 0 ⟩ directions. Spatially resolved cathodoluminescence images were used to correlate the microstructure with the optical properties. A large number of non-radiative defects were observed directly below the indentation. Regions under tensile stress extending from the nanoindentation along ⟨11 2 ¯ 0 ⟩ directions were associated with the {0002 } ⟨11 2 ¯ 0 ⟩ slip.

  17. Transmission electron microscopy of electrospun GaN nanofibers

    NASA Astrophysics Data System (ADS)

    Robles-García, Joshua L.; Meléndez, Anamaris; Yates, Douglas; Santiago-Avilés, Jorge J.; Ramos, Idalia; Campo, Eva M.

    2011-06-01

    We have reported earlier progress in producing polycrystalline wurtzite-polymorph and photo-conductive GaN nanofibers by electrospinning. This paper shows grain stacking during heat treatment and suggests the need to understand nucleation and grain growth following electrospinning. Transmission Electron Microscopy (TEM) analysis of GaN shows brittle fibers, grain stacking, and unfinished grain nucleation. X-Ray Diffraction analysis confirmed dominant hexagonal 101-wurtzite preferential overall orientation and the incipient grains are of high crystalline quality as seen by high resolution TEM.

  18. Understanding the pyramidal growth of GaN

    SciTech Connect

    Rouviere, J.L.; Arlery, M.; Bourret, A.

    1996-11-01

    By a combination of conventional, HREM and CBED TEM experiments the authors have studied wurtzite GaN layers grown by Metal-Organic Chemical Vapor Deposition (MOCVD) on (0001)Al{sub 2}O{sub 3}. They experimentally determine the structure of the macroscopic hexagonal pyramids that are visible at the surface of the layers when no optimized buffer is introduced. These pyramids look like hexagonal volcanoes with one hexagonal microscopic chimney (up to 75 nm wide) at their core. The crystal inside the chimney is a pure GaN crystal with a polarity opposed to the one of the neighboring material: the GaN layers grown on (0001)Al{sub 2}O{sub 3} are everywhere Ga-terminated except in the chimneys where they are N-terminated. Some of the N-terminated chimneys grow faster and form macroscopic hexagonal pyramids. Chimneys bounded by Inversion Domains Boundaries (IDBs) originate from steps at the surface of the substrate and may be suppressed by an adapted buffer layer.

  19. GaN nanowire arrays by a patterned metal-assisted chemical etching

    NASA Astrophysics Data System (ADS)

    Wang, K. C.; Yuan, G. D.; Wu, R. W.; Lu, H. X.; Liu, Z. Q.; Wei, T. B.; Wang, J. X.; Li, J. M.; Zhang, W. J.

    2016-04-01

    We developed an one-step and two-step metal-assisted chemical etching method to produce self-organized GaN nanowire arrays. In one-step approach, GaN nanowire arrays are synthesized uniformly on GaN thin film surface. However, in a two-step etching processes, GaN nanowires are formed only in metal uncovered regions, and GaN regions with metal-covering show nano-porous sidewalls. We propose that nanowires and porous nanostructures are tuned by sufficient and limited etch rate, respectively. PL spectra shows a red-shift of band edge emission in GaN nanostructures. The formation mechanism of nanowires was illustrated by two separated electrochemical reactions occur simultaneously. The function of metals and UV light was illustrated by the scheme of potential relationship between energy bands in Si, GaN and standard hydrogen electrode potential of solution and metals.

  20. GaN membrane metal-semiconductor-metal ultraviolet photodetector.

    PubMed

    Müller, A; Konstantinidis, G; Dragoman, M; Neculoiu, D; Kostopoulos, A; Androulidaki, M; Kayambaki, M; Vasilache, D

    2008-04-01

    GaN is a wide-bandgap semiconductor with still unexplored capabilities for ultraviolet detection. To exploit GaN properties better for ultraviolet detection, a metal-semiconductor-metal-type photodetector structure was designed and manufactured on a 2.2 microm thin GaN membrane fabricated by micromachining techniques. As a result, a very low dark current (30 pA at 3 V) and a maximum responsivity of 14 mA/W at a wavelength of 370 nm were obtained.

  1. Optical and field emission properties of layer-structure GaN nanowires

    SciTech Connect

    Cui, Zhen; Li, Enling; Shi, Wei; Ma, Deming

    2014-08-15

    Highlights: • The layer-structure GaN nanowires with hexagonal-shaped cross-sections are produced via a process based on the CVD method. • The diameter of the layer-structure GaN nanowire gradually decreases from ∼500 nm to ∼200 nm along the wire axis. • The layer-structure GaN nanowire film possesses good field emission property. - Abstract: A layer-structure gallium nitride (GaN) nanowires, grown on Pt-coated n-type Si (1 1 1) substrate, have been synthesized using chemical vapor deposition (CVD). The results show: (1) SEM indicates that the geometry structure is layer-structure. HRTEM indicates that GaN nanowire’s preferential growth direction is along [0 0 1] direction. (2) The room temperature PL emission spectrum of the layer-structure GaN nanowires has a peak at 375 nm, which proves that GaN nanowires have potential application in light-emitting nano-devices. (3) Field-emission measurements show that the layer-structure GaN nanowires film has a low turn-on field of 4.39 V/μm (at room temperature), which is sufficient for electron emission devices, field emission displays and vacuum nano-electronic devices. The growth mechanism for GaN nanowires has also been discussed briefly.

  2. Abnormal selective area growth of irregularly-shaped GaN structures on the apex of GaN pyramids and its application for wide spectral emission

    NASA Astrophysics Data System (ADS)

    Yu, Yeon Su; Lee, Jun Hyeong; Ahn, Hyung Soo; Yang, Min

    2014-12-01

    We report on the growth and the characterization of three-dimensional randomly-shaped InGaN/GaN structures selectively grown on the apex of GaN pyramids for the purpose of enlarging the emission spectral range. We found that the variations in the shape and the size of the three-dimensional GaN structures depend on the growth temperature and the surface area for selective growth under intentional turbulence in the gas stream. The selectively grown GaN structures grown at 1020 °C have irregular shape, while the samples grown at 1100 °C have rather uniform hexagonal pyramidal shapes. Irregularly shaped GaN structures were also obtained on the apex of GaN pyramids when the SiO2 mask was removed to 1/10 of the total height of the underlying GaN pyramid. When only 1/5 of the SiO2 mask was removed, however, the selectively grown GaN structures had similar hexagonal pyramidal shapes resembling those of the underlying GaN pyramids. The CL (Cathodoluminescence) spectra of the InGaN layers grown on the randomly shaped GaN structures showed a wide emission spectral range from 388 to 433 nm due to the non-uniform thickness and spatially inhomogeneous indium composition of the InGaN layers. This new selective growth method might have great potential for applications of non-phosphor white light emitting diodes (LEDs) with optimized growth conditions for InGaN active layers of high indium composition and with optimum process for fabrication of electrodes for electrical injection.

  3. An investigation of thin Zr films on 6H-SiC(0001) and GaN(0001) surfaces by XPS, LEED, and STM

    NASA Astrophysics Data System (ADS)

    Idczak, K.; Mazur, P.; Zuber, S.; Markowski, L.

    2016-04-01

    In this work, the results of the growth of zirconium films deposited under the ultrahigh vacuum at room temperature on the 6H-SiC(0001) and GaN(0001) surfaces were studied. Observed changes in the chemical composition, bonding environment, and surface reconstruction, and the effects of high-temperature annealing of the film are presented and discussed as well. In the performed experiment, the X-ray photoelectron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy were used. The results show that for both investigated substrates, the grown films have eminently rich and varied compositions. Besides the metallic zirconium, there are also zirconium oxides, zirconium carbides, or zirconium nitrides. The growth process proceeds according to the Volmer-Weber mode. Moreover, the zirconium-semiconductor interface does not form typical Schottky contact, but some paths with a quasi-ohmic conduction character can be observed.

  4. The process and mechanism of the GaN nanoparticles formed by nitridation of β-Ga2O3 crystal

    NASA Astrophysics Data System (ADS)

    Cui, Xiao-Jun; Wang, Liang-Ling

    2017-04-01

    The process of conversion from β-Ga2O3 single crystal to gallium nitride (GaN) in an atmosphere of NH3 by chemical vapor deposition is investigated. The surface morphology and microstructure of the GaN nanoparticles are observed by scanning electron microscope, which indicates that the growth of GaN is via the Volmer-Weber mechanism. The β-Ga2O3 is firstly evaporated at high temperature to form the porous layer, followed by the surface-defect induced GaN nucleation formation. The crystalline structure and epitaxial relationship of the GaN nanoparticles are investigated by X-ray diffraction (XRD) via ω-2𝜃, showing GaN (0002) and (0004) diffraction peaks in the XRD spectra. It is concluded that the polycrystalline GaN film with hexagonal structure has a strong c-axis preferential orientation.

  5. GaN membrane MSM ultraviolet photodetectors

    NASA Astrophysics Data System (ADS)

    Muller, A.; Konstantinidis, G.; Kostopoulos, A.; Dragoman, M.; Neculoiu, D.; Androulidaki, M.; Kayambaki, M.; Vasilache, D.; Buiculescu, C.; Petrini, I.

    2006-12-01

    GaN exhibits unique physical properties, which make this material very attractive for wide range of applications and among them ultraviolet detection. For the first time a MSM type UV photodetector structure was manufactured on a 2.2 μm. thick GaN membrane obtained using micromachining techniques. The low unintentionally doped GaN layer structure was grown by MOCVD on high resistivity (ρ>10kΩcm) <111> oriented silicon wafers, 500μm thick. The epitaxially grown layers include a thin AlN layer in order to reduce the stress in the GaN layer and avoid cracking. Conventional contact lithography, e-gun Ni/Au (10nm /200nm) evaporation and lift-off techniques were used to define the interdigitated Schottky metalization on the top of the wafer. Ten digits with a width of 1μm and a length of 100μm were defined for each electrode. The distance between the digits was also 1μm. After the backside lapping of the wafer to a thickness of approximately 150μm, a 400nm thick Al layer was patterned and deposited on the backside, to be used as mask for the selective reactive ion etching of silicon. The backside mask, for the membrane formation, was patterned using double side alignment techniques and silicon was etched down to the 2.2μm thin GaN layer using SF 6 plasma. A very low dark current (30ρA at 3V) was obtained. Optical responsivity measurements were performed at 1.5V. A maximum responsivity of 18mA/W was obtained at a wavelength of 370nm. This value is very good and can be further improved using transparent contacts for the interdigitated structure.

  6. GaN HEMTs

    NASA Astrophysics Data System (ADS)

    Anderson, Jonathan W.; Lee, Kyoung-Keun; Piner, Edwin L.

    2012-03-01

    Gallium nitride (GaN) has enormous potential for applications in high electron mobility transistors (HEMTs) used in RF and power devices. Intrinsic device properties such as high electron mobility, high breakdown voltage, very high current density, electron confinement in a narrow channel, and high electron velocity in the 2-dimensional electron gas of the HEMT structure are due in large part to the wide band gap of this novel semiconductor material system. This presentation discusses the properties of GaN that make it superior to other semiconductor materials, and outlines the research that will be undertaken in a new program at Texas State University to advance GaN HEMT technology. This program's aim is to further innovate the exceptional performance of GaN through improved material growth processes and epitaxial structure design.

  7. Dislocation filtering in GaN nanostructures.

    PubMed

    Colby, Robert; Liang, Zhiwen; Wildeson, Isaac H; Ewoldt, David A; Sands, Timothy D; García, R Edwin; Stach, Eric A

    2010-05-12

    Dislocation filtering in GaN by selective area growth through a nanoporous template is examined both by transmission electron microscopy and numerical modeling. These nanorods grow epitaxially from the (0001)-oriented GaN underlayer through the approximately 100 nm thick template and naturally terminate with hexagonal pyramid-shaped caps. It is demonstrated that for a certain window of geometric parameters a threading dislocation growing within a GaN nanorod is likely to be excluded by the strong image forces of the nearby free surfaces. Approximately 3000 nanorods were examined in cross-section, including growth through 50 and 80 nm diameter pores. The very few threading dislocations not filtered by the template turn toward a free surface within the nanorod, exiting less than 50 nm past the base of the template. The potential active region for light-emitting diode devices based on these nanorods would have been entirely free of threading dislocations for all samples examined. A greater than 2 orders of magnitude reduction in threading dislocation density can be surmised from a data set of this size. A finite element-based implementation of the eigenstrain model was employed to corroborate the experimentally observed data and examine a larger range of potential nanorod geometries, providing a simple map of the different regimes of dislocation filtering for this class of GaN nanorods. These results indicate that nanostructured semiconductor materials are effective at eliminating deleterious extended defects, as necessary to enhance the optoelectronic performance and device lifetimes compared to conventional planar heterostructures.

  8. Nanoheteroepitaxy of GaN on AlN/Si(111) nanorods fabricated by nanosphere lithography

    NASA Astrophysics Data System (ADS)

    Lee, Donghyun; Shin, In-Su; Jin, Lu; Kim, Donghyun; Park, Yongjo; Yoon, Euijoon

    2016-06-01

    Nanoheteroepitaxy (NHE) of GaN on an AlN/Si(111) nanorod structure was investigated by metal-organic chemical vapor deposition. Silica nanosphere lithography was employed to fabricate a periodic hexagonal nanorod array with a narrow gap of 30 nm between the nanorods. We were successful in obtaining a fully coalesced GaN film on the AlN/Si(111) nanorod structure. Transmission electron microscopy revealed that threading dislocation (TD) bending and termination by stacking faults occurred near the interface between GaN and the AlN/Si(111) nanorods, resulting in the reduction of TD density for the NHE GaN layer. The full width at half-maximum of the X-ray rocking curve for (102) plane of the NHE GaN was found to decrease down to 728 arcsec from 1005 arcsec for the GaN layer on a planar AlN/Si(111) substrate, indicating that the crystalline quality of the NHE GaN was improved. Also, micro-Raman measurement showed that tensile stress in the NHE GaN layer was reduced significantly as much as 70% by introducing air voids between the nanorods.

  9. Thickness measurement of semiconductor thin films by energy dispersive X-ray fluorescence benchtop instrumentation: Application to GaN epilayers grown by molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Queralt, I.; Ibañez, J.; Marguí, E.; Pujol, J.

    2010-07-01

    The importance of thin films in modern high technology products, such as semiconductors, requires fast and non-destructive analysis. A methodology to determine the thickness of single layers with benchtop energy dispersive X-ray fluorescence (EDXRF) instrumentation is described and tested following analytical validation criteria. The experimental work was carried out on gallium nitride thin films epitaxially grown on sapphire substrate. The results of samples with layers in the range from 400 to 1000 nm exhibit a good correlation with the layer thickness determined by optical reflectance. Spectral data obtained using thin layered samples indicate the possibility to precisely evaluate layer thickness from 5 nm, with a low relative standard deviation (RSD < 2%) of the results. In view of the limits of optical reflectance for very thin layer determination, EDXRF analysis offers the potential for the thickness determination of such kind of samples.

  10. Validity of Vegard’s rule for Al1-xInxN (0.08  <  x  <  0.28) thin films grown on GaN templates

    NASA Astrophysics Data System (ADS)

    Magalhães, S.; Franco, N.; Watson, I. M.; Martin, R. W.; O'Donnell, K. P.; Schenk, H. P. D.; Tang, F.; Sadler, T. C.; Kappers, M. J.; Oliver, R. A.; Monteiro, T.; Martin, T. L.; Bagot, P. A. J.; Moody, M. P.; Alves, E.; Lorenz, K.

    2017-05-01

    In this work, comparative x-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) measurements allow a comprehensive characterization of Al1-xInxN thin films grown on GaN. Within the limits of experimental accuracy, and in the compositional range 0.08  <  x  <  0.28, the lattice parameters of the alloys generally obey Vegard’s rule, varying linearly with the InN fraction. Results are also consistent with the small deviation from linear behaviour suggested by Darakchieva et al (2008 Appl. Phys. Lett. 93 261908). However, unintentional incorporation of Ga, revealed by atom probe tomography (APT) at levels below the detection limit for RBS, may also affect the lattice parameters. Furthermore, in certain samples the compositions determined by XRD and RBS differ significantly. This fact, which was interpreted in earlier publications as an indication of a deviation from Vegard’s rule, may rather be ascribed to the influence of defects or impurities on the lattice parameters of the alloy. The wide-ranging set of Al1-xInxN films studied allowed furthermore a detailed investigation of the composition leading to lattice-matching of Al1-xInxN/GaN bilayers.

  11. Influence of vicinal sapphire substrate on the properties of N-polar GaN films grown by metal-organic chemical vapor deposition

    SciTech Connect

    Lin, Zhiyu; Zhang, Jincheng Xu, Shengrui; Chen, Zhibin; Yang, Shuangyong; Tian, Kun; Hao, Yue; Su, Xujun; Shi, Xuefang

    2014-08-25

    The influence of vicinal sapphire substrates on the growth of N-polar GaN films by metal-organic chemical vapor deposition is investigated. Smooth GaN films without hexagonal surface feature are obtained on vicinal substrate. Transmission electron microscope results reveal that basal-plane stacking faults are formed in GaN on vicinal substrate, leading to a reduction in threading dislocation density. Furthermore, it has been found that there is a weaker yellow luminescence in GaN on vicinal substrate than that on (0001) substrate, which might be explained by the different trends of the carbon impurity incorporation.

  12. Ammonothermal synthesis of GaN using Ba(NH2)2 as mineralizer

    NASA Astrophysics Data System (ADS)

    Hertrampf, J.; Alt, N. S. A.; Schlücker, E.; Knetzger, M.; Meissner, E.; Niewa, R.

    2016-12-01

    It is demonstrated that hexagonal GaN can be obtained under ammonothermal conditions (125 MPa and 723 K) using Ba(NH2)2 as mineralizer. The hexagonal wurtzite-type GaN crystallites are several μm in diameter, as examined by scanning electron microscopy. This is to our knowledge the first successful ammonothermal GaN synthesis using an alkaline-earth metal as mineralizer. Ba[Ga(NH2)4]2 was identified as intermediate species in the ammonothermal synthesis process. The formation of h-GaN using Sr(NH2)2 as mineralizer was indicated only at higher temperatures above 1000 K.

  13. Measurement of the electrostatic edge effect in wurtzite GaN nanowires

    SciTech Connect

    Henning, Alex; Rosenwaks, Yossi; Klein, Benjamin; Bertness, Kris A.; Blanchard, Paul T.; Sanford, Norman A.

    2014-11-24

    The electrostatic effect of the hexagonal corner on the electronic structure in wurtzite GaN nanowires (NWs) was directly measured using Kelvin probe force microscopy (KPFM). By correlating electrostatic simulations with the measured potential difference between the nanowire face and the hexagonal vertices, the surface state concentration and band bending of GaN NWs were estimated. The surface band bending is important for an efficient design of high electron mobility transistors and for opto-electronic devices based on GaN NWs. This methodology provides a way to extract NW parameters without making assumptions concerning the electron affinity. We are taking advantage of electrostatic modeling and the high precision that KPFM offers to circumvent a major source of uncertainty in determining the surface band bending.

  14. Synthesis, morphology and optical properties of GaN and AlGaN semiconductor nanostructures

    SciTech Connect

    Kuppulingam, B. Singh, Shubra Baskar, K.

    2014-04-24

    Hexagonal Gallium Nitride (GaN) and Aluminum Gallium Nitride (AlGaN) nanoparticles were synthesized by sol-gel method using Ethylene Diamine Tetra Acetic acid (EDTA) complex route. Powder X-ray diffraction (PXRD) analysis confirms the hexagonal wurtzite structure of GaN and Al{sub 0.25}Ga{sub 0.75}N nanoparticles. Surface morphology and elemental analysis were carried out by Scanning Electron Microscope (SEM) and Energy Dispersive X-ray spectroscopy (EDX). The room temperature Photoluminescence (PL) study shows the near band edge emission for GaN at 3.35 eV and at 3.59 eV for AlGaN nanoparticles. The Aluminum (Al) composition of 20% has been obtained from PL emission around 345 nm.

  15. High Cubic-Phase Purity InN on MgO (001) Using Cubic-Phase GaN as a Buffer Layer

    SciTech Connect

    Sanorpim, S.; Kuntharin, S.; Parinyataramas, J.; Yaguchi, H.; Iwahashi, Y.; Orihara, M.; Hijikata, Y.; Yoshida, S.

    2011-12-23

    High cubic-phase purity InN films were grown on MgO (001) substrates by molecular beam epitaxy with a cubic-phase GaN buffer layer. The cubic phase purity of the InN grown layers has been analyzed by high resolution X-ray diffraction, {mu}-Raman scattering and transmission electron microscopy. It is evidenced that the hexagonal-phase content in the InN overlayer much depends on hexagonal-phase content in the cubic-phase GaN buffer layer and increases with increasing the hexagonal-phase GaN content. From Raman scattering measurements, in addition, the InN layer with lowest hexagonal component (6%), only Raman characteristics of cubic TO{sub InN} and LO{sub InN} modes were observed, indicating a formation of a small amount of stacking faults, which does not affect on vibrational property.

  16. Role of sapphire nitridation temperature on GaN growth by plasma assisted molecular beam epitaxy: Part II. Interplay between chemistry and structure of layers

    NASA Astrophysics Data System (ADS)

    Losurdo, Maria; Capezzuto, Pio; Bruno, Giovanni; Namkoong, Gon; Doolittle, W. Alan; Brown, April S.

    2002-02-01

    The effect of sapphire nitridation temperature on the chemistry and microstructure of the sapphire substrate/GaN interface, nucleation layer, and of the GaN epilayers grown by rf plasma assisted molecular beam epitaxy is investigated. It is found that a sapphire nitridation temperature as low as 200 °C improves the structural and optical quality of GaN epilayers. This result can be explained by the chemistry of the sapphire nitridation process, which is discussed in the framework of a model considering the competitive formation of AlN and oxynitride (NO). In particular, at 200 °C, NO desorbs from the sapphire surface, yielding an homogeneous 6 Å AlN layer upon N2 plasma nitridation. This low temperature AlN template favors the nucleation of hexagonal GaN nuclei which coalesce completely resulting in a hexagonal GaN buffer layer that homogeneously covers the sapphire substrate. This condition promotes the growth of a high quality GaN epilayer. In contrast, high nitridation temperatures result in a mixed AlN/NO nitrided sapphire surface which induce a perturbed and more defected interface with the occurrence of cubic crystallites in the GaN buffer. A sapphire surface with random GaN islands is found upon annealing of the GaN buffer and this condition results in a low-quality GaN epilayer.

  17. Hexagonal ferrites for millimeter wave applications

    NASA Astrophysics Data System (ADS)

    Polk, Donald E.; Hathaway, Kristl B.

    1993-01-01

    A review of the work accomplished on this contract is presented. A review of the physics of hexagonal ferrite materials and the effective linewidth concept and the detailed overall research plan are contained in the original proposal document. The focus of the program was on the effective linewidth in millimeter wave materials, including planar hexagonal ferrite Y-type materials, uniaxial M-type materials, and thin ferromagnetic transition metal and alloy films. The key idea in the original proposal was that the ferromagnetic resonance (FMR) linewidth in hexagonal ferrites is dominated by inhomogeneous and two-magnon scattering losses and that off-resonance measurements of the effective linewidth would (1) show that the FMR losses do not represent the intrinsic losses, and (2) that the intrinsic losses are significantly lower. This basic idea was verified. Results were obtained on the off-resonance far-field effective linewidth in planar Zn-Y hexagonal ferrite single crystal platelets, single crystal spheres of Ba- and Sr-hexaferrite materials, and permalloy thin films. Three papers on these results were published.

  18. Synthesis and optical properties of GaN micro/nanocone bundles on copper wafers

    SciTech Connect

    Zhou Qingtao; Chen Yiqing Su Yong; Jia Chong; Pen, Bo; Yin Song; Li Sen; Kong Weihai

    2008-08-04

    GaN micro/nanocone bundles with controllable size and density have been synthesized on Cu or Cu{sub 95}In{sub 5} alloy wafers using a modified thermal-evaporation process. The size and density of GaN cone bundles could be efficiently controlled by adjusting growth temperatures and the components of metal substrates. The structure and morphology of the as-synthesized GaN cones were characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The as-synthesized micro/nanocones are single crystals with a hexagonal wurtzite structure, growing along the [101-bar1] direction. The growth process follows a vapor-liquid-solid mechanism. The dependence of photoluminescence property on the size and density of GaN cone bundles at room temperature was also investigated.

  19. Why Hexagonal Basalt Columns?

    PubMed

    Hofmann, Martin; Anderssohn, Robert; Bahr, Hans-Achim; Weiß, Hans-Jürgen; Nellesen, Jens

    2015-10-09

    Basalt columns with their preferably hexagonal cross sections are a fascinating example of pattern formation by crack propagation. Junctions of three propagating crack faces rearrange such that the initial right angles between them tend to approach 120°, which enables the cracks to form a pattern of regular hexagons. To promote understanding of the path on which the ideal configuration can be reached, two periodically repeatable models are presented here involving linear elastic fracture mechanics and applying the principle of maximum energy release rate. They describe the evolution of the crack pattern as a transition from rectangular start configuration to the hexagonal pattern. This is done analytically and by means of three-dimensional finite element simulation. The latter technique reproduces the curved crack path involved in this transition.

  20. Synthesis of p-type GaN nanowires.

    PubMed

    Kim, Sung Wook; Park, Youn Ho; Kim, Ilsoo; Park, Tae-Eon; Kwon, Byoung Wook; Choi, Won Kook; Choi, Heon-Jin

    2013-09-21

    GaN has been utilized in optoelectronics for two decades. However, p-type doping still remains crucial for realization of high performance GaN optoelectronics. Though Mg has been used as a p-dopant, its efficiency is low due to the formation of Mg-H complexes and/or structural defects in the course of doping. As a potential alternative p-type dopant, Cu has been recognized as an acceptor impurity for GaN. Herein, we report the fabrication of Cu-doped GaN nanowires (Cu:GaN NWs) and their p-type characteristics. The NWs were grown vertically via a vapor-liquid-solid (VLS) mechanism using a Au/Ni catalyst. Electrical characterization using a nanowire-field effect transistor (NW-FET) showed that the NWs exhibited n-type characteristics. However, with further annealing, the NWs showed p-type characteristics. A homo-junction structure (consisting of annealed Cu:GaN NW/n-type GaN thin film) exhibited p-n junction characteristics. A hybrid organic light emitting diode (OLED) employing the annealed Cu:GaN NWs as a hole injection layer (HIL) also demonstrated current injected luminescence. These results suggest that Cu can be used as a p-type dopant for GaN NWs.

  1. Hexagonal Ferrites for Millimeter Wave Applications

    DTIC Science & Technology

    1993-01-07

    single crystal platelets, single crystal spheres of Ba- and Sr- hexaferrite materials, and permalloy thin films. Three papers on these results have been...effective linewidth in planar Zn-Y hexagonal ferrite single crystal platelets, single crystal spheres of Ba- and Sr- hexaferrite materials, and...basic thesis of the original proposal - that the measured linewidth in single crystal hexaferrites (1) may contain significant contributions related to

  2. Synthesis and characterization of hexagonal ferrite Sr1.8Sm0.2Co2Ni1.50Fe10.50O22/PST thin films for high frequency application

    NASA Astrophysics Data System (ADS)

    Ali, Irshad; Islam, M. U.; Ashiq, Muhammad Naeem; Asif Iqbal, M.; Karamat, Nazia; Azhar Khan, M.; Sadiq, Imran; Ijaz, Sana; Shakir, Imran

    2015-11-01

    Y-type hexagonal ferrite (Sr1.8Sm0.2Co2Ni1.50 Fe10.50O22) was prepared by a normal microemulsion route. The ferrite/polymer composites thin films are formed at different ferrite ratios in pure polystyrene matrix. The X-ray diffraction analysis shows broad peak at low angles which is due to the PST and the peaks for Y-type ferrite are also observed in composite samples. The peaks become more intense and show less broadening with increasing concentration of ferrite which suggests that crystallinity is improved with the addition of ferrite. DC resistivity of the composites samples is lower than that of the pure PST and decreases by increasing ferrite filler into the polymer. This decrease of resistivity is mainly due to the addition of comparatively less resistive ferrite into the highly insulating polymer matrix of PST. The observed increase in the dielectric constant (permittivity) with increasing concentration ratio of ferrites is mainly due to the electron exchange between Fe2+↔Fe3++e- which consequently results in enhancement of electric polarization as well as dielectric constant. The existence of resonances peaks in the dielectric loss tangent spectra is due to the fact when the external applied frequency becomes equal to the jumping frequency of electrons between Fe2+ and Fe3+. The increasing behavior of the dielectric constant, dielectric loss and AC conductivity with increasing ferrite ratio in PST matrix proposes their versatile use in different technological applications especially for electromagnetic shielding.

  3. Epitaxial hexagonal materials on IBAD-textured substrates

    DOEpatents

    Matias, Vladimir; Yung, Christopher

    2017-08-15

    A multilayer structure including a hexagonal epitaxial layer, such as GaN or other group III-nitride (III-N) semiconductors, a <111> oriented textured layer, and a non-single crystal substrate, and methods for making the same. The textured layer has a crystalline alignment preferably formed by the ion-beam assisted deposition (IBAD) texturing process and can be biaxially aligned. The in-plane crystalline texture of the textured layer is sufficiently low to allow growth of high quality hexagonal material, but can still be significantly greater than the required in-plane crystalline texture of the hexagonal material. The IBAD process enables low-cost, large-area, flexible metal foil substrates to be used as potential alternatives to single-crystal sapphire and silicon for manufacture of electronic devices, enabling scaled-up roll-to-roll, sheet-to-sheet, or similar fabrication processes to be used. The user is able to choose a substrate for its mechanical and thermal properties, such as how well its coefficient of thermal expansion matches that of the hexagonal epitaxial layer, while choosing a textured layer that more closely lattice matches that layer.

  4. Dynamics of wet granular hexagons

    NASA Astrophysics Data System (ADS)

    Baur, Manuel; Huang, Kai

    2017-03-01

    The collective behavior of vibrated hexagonal disks confined in a monolayer is investigated experimentally. Due to the broken circular symmetry, hexagons prefer to rotate upon sufficiently strong driving. Due to the formation of liquid bridges, short-ranged cohesive interactions are introduced upon wetting. Consequently, a nonequilibrium stationary state with the rotating disks self-organized in a hexagonal structure arises. The bond length of the hexagonal structure is slightly smaller than the circumdiameter of a hexagon, indicating geometric frustration. This investigation provides an example where the collective behavior of granular matter is tuned by the shape of individual particles.

  5. Hexagonalization of correlation functions

    NASA Astrophysics Data System (ADS)

    Fleury, Thiago; Komatsu, Shota

    2017-01-01

    We propose a nonperturbative framework to study general correlation functions of single-trace operators in N = 4 supersymmetric Yang-Mills theory at large N . The basic strategy is to decompose them into fundamental building blocks called the hexagon form factors, which were introduced earlier to study structure constants using integrability. The decomposition is akin to a triangulation of a Riemann surface, and we thus call it hexagonalization. We propose a set of rules to glue the hexagons together based on symmetry, which naturally incorporate the dependence on the conformal and the R-symmetry cross ratios. Our method is conceptually different from the conventional operator product expansion and automatically takes into account multi-trace operators exchanged in OPE channels. To illustrate the idea in simple set-ups, we compute four-point functions of BPS operators of arbitrary lengths and correlation functions of one Konishi operator and three short BPS operators, all at one loop. In all cases, the results are in perfect agreement with the perturbative data. We also suggest that our method can be a useful tool to study conformal integrals, and show it explicitly for the case of ladder integrals.

  6. Morphological and microstructural stability of N-polar InAlN thin films grown on free-standing GaN substrates by molecular beam epitaxy

    SciTech Connect

    Hardy, Matthew T. Storm, David F.; Downey, Brian P.; Katzer, D. Scott; Meyer, David J.; McConkie, Thomas O.; Smith, David J.; Nepal, Neeraj

    2016-03-15

    The sensitivity of the surface morphology and microstructure of N-polar-oriented InAlN to variations in composition, temperature, and layer thickness for thin films grown by plasma-assisted molecular beam epitaxy (PAMBE) has been investigated. Lateral compositional inhomogeneity is present in N-rich InAlN films grown at low temperature, and phase segregation is exacerbated with increasing InN fraction. A smooth, step-flow surface morphology and elimination of compositional inhomogeneity can be achieved at a growth temperature 50 °C above the onset of In evaporation (650 °C). A GaN/AlN/GaN/200-nm InAlN heterostructure had a sheet charge density of 1.7 × 10{sup 13 }cm{sup −2} and no degradation in mobility (1760 cm{sup 2}/V s) relative to 15-nm-thick InAlN layers. Demonstration of thick-barrier high-electron-mobility transistors with good direct-current characteristics shows that device quality, thick InAlN layers can be successfully grown by PAMBE.

  7. Vibrational modes of GaN nanowires in the gigahertz range

    NASA Astrophysics Data System (ADS)

    Johnson, W. L.; Kim, S. A.; Geiss, R.; Flannery, C. M.; Bertness, K. A.; Heyliger, P. R.

    2012-12-01

    Brillouin-light-scattering measurements and finite-element modeling of vibrational spectra in the range of 5-40 GHz are presented for an array of monocrystalline GaN nanowires with hexagonal cross sections. Analysis of the spectra is substantially complicated by the presence of a distribution of nanowire diameters. The measurements and calculations reveal a variety of modes with simple flexural, higher-order flexural, approximately ‘plane-strain’, approximately longitudinal and torsional displacement patterns that are similar to the corresponding modes of isotropic cylinders. The largest peaks in the spectra with acoustic angular wavenumbers in the range of 4 to ˜15 μm-1 were determined to arise from modes with relatively large transverse displacements, consistent with inelastic light scattering arising predominantly from surface ripple. These dominant modes have finite frequencies in the limit of zero wavenumber, corresponding to transverse standing waves. At higher wavenumbers, the spectra provide evidence for increased scattering through elasto-optic coupling, especially with respect to the emergence of a peak from a mode analogous to the longitudinal guided modes of thin films. This manuscript is a contribution of the National Institute of Standards and Technology and is not subject to copyright in the United States.

  8. Effects of Ga:N addition on the electrical performance of zinc tin oxide thin film transistor by solution-processing.

    PubMed

    Ahn, Byung Du; Jeon, Hye Ji; Park, Jin-Seong

    2014-06-25

    This paper addressed the effect of gallium nitrate hydrate addition on thin film transistor (TFT) performance and positive bias stability of amorphous zinc tin oxide (ZTO) TFTs by solution processing, Further, the mechanisms responsible for chemical properties and electronic band structure are explored. A broad exothermic peak accompanied by weight loss appeared in the range from about 350 to 570 °C for the ZTO solution; the thermal reaction of the Ga-ZTO:N solution was completed at 520 °C. This is because the gallium nitrate hydrate precursor promoted the decomposition and dehydroxylation reaction for Zn(CH3COO)2·2H2O and/or SnCl2·2H2O precursors. The concentrations of carbon and chloride in gallium nitrate hydrate added ZTO films annealed at 400 °C have a lower value (C 0.65, Cl 0.65 at. %) compared with those of ZTO films (C 3.15, Cl 0.82 at. %). Absorption bands at 416, 1550, and 1350 cm(-1) for GaZTO:N films indicated the presence of ZnGa2O4, N-H, and N═O groups by Fourier transform infrared spectroscopy measurement, respectively. As a result, an inverted staggered Ga-ZTO:N TFT exhibited a mobility of 4.84 cm(2) V(-1) s(-1) in the saturation region, a subthreshold swing of 0.35 V/decade, and a threshold gate voltage (Vth) of 0.04 V. In addition, the instability of Vth values of the ZTO TFTs under positive bias stress conditions was suppressed by adding Ga and N from 13.6 to 3.17 V, which caused a reduction in the oxygen-related defects located near the conduction band.

  9. Pulsed laser annealing of Be-implanted GaN

    SciTech Connect

    Wang, H.T.; Tan, L.S.; Chor, E.F.

    2005-11-01

    Postimplantation thermal processing of Be in molecular-beam-epitaxy-grown GaN by rapid thermal annealing (RTA) and pulsed laser annealing (PLA) was investigated. It has been found that the activation of Be dopants and the repair of implantation-induced defects in GaN films cannot be achieved efficiently by conventional RTA alone. On the other hand, good dopant activation and surface morphology and quality were obtained when the Be-implanted GaN film was annealed by PLA with a 248 nm KrF excimer laser. However, observations of off-resonant micro-Raman and high-resolution x-ray-diffraction spectra indicated that crystal defects and strain resulting from Be implantation were still existent after PLA, which probably degraded the carrier mobility and limited the activation efficiency to some extent. This can be attributed to the shallow penetration depth of the 248 nm laser in GaN, which only repaired the crystal defects in a thin near-surface layer, while the deeper defects were not annealed out well. This situation was significantly improved when the Be-implanted GaN was subjected to a combined process of PLA followed by RTA, which produced good activation of the dopants, good surface morphology, and repaired bulk and surface defects well.

  10. Pulsed laser annealing of Be-implanted GaN

    NASA Astrophysics Data System (ADS)

    Wang, H. T.; Tan, L. S.; Chor, E. F.

    2005-11-01

    Postimplantation thermal processing of Be in molecular-beam-epitaxy-grown GaN by rapid thermal annealing (RTA) and pulsed laser annealing (PLA) was investigated. It has been found that the activation of Be dopants and the repair of implantation-induced defects in GaN films cannot be achieved efficiently by conventional RTA alone. On the other hand, good dopant activation and surface morphology and quality were obtained when the Be-implanted GaN film was annealed by PLA with a 248 nm KrF excimer laser. However, observations of off-resonant micro-Raman and high-resolution x-ray-diffraction spectra indicated that crystal defects and strain resulting from Be implantation were still existent after PLA, which probably degraded the carrier mobility and limited the activation efficiency to some extent. This can be attributed to the shallow penetration depth of the 248 nm laser in GaN, which only repaired the crystal defects in a thin near-surface layer, while the deeper defects were not annealed out well. This situation was significantly improved when the Be-implanted GaN was subjected to a combined process of PLA followed by RTA, which produced good activation of the dopants, good surface morphology, and repaired bulk and surface defects well.

  11. Step-flow growth mode instability of N-polar GaN under N-excess

    SciTech Connect

    Chèze, C.

    2013-08-12

    GaN layers were grown on N-polar GaN substrates by plasma-assisted molecular beam epitaxy under different III/V ratios. Ga-rich conditions assure step-flow growth with atomically flat surface covered by doubly-bunched steps, as for Ga-polar GaN. Growth under N-excess however leads to an unstable step-flow morphology. Particularly, for substrates slightly miscut towards <1010>, interlacing fingers are covered by atomic steps pinned on both sides by small hexagonal pits. In contrast, a three-dimensional island morphology is observed on the Ga-polar equivalent sample. We attribute this result to lower diffusion barriers on N-polar compared to Ga-polar GaN under N-rich conditions.

  12. Hexagonal Mirror Array

    NASA Technical Reports Server (NTRS)

    1999-01-01

    NASA's Space Optics Manufacturing Technology Center has been working to expand our view of the universe via sophisticated new telescopes. The Optics Center's goal is to develop low-cost, advanced space optics technologies for the NASA program in the 21st century, including the long-term goal of imaging Earth-like planets in distant solar systems. A segmented array of mirrors was designed by the Space Optics Manufacturing Technology Center for solar the concentrator test stand at the Marshall Space Flight Center (MSFC) for powering solar thermal propulsion engines. Each hexagon mirror has a spherical surface to approximate a parabolic concentrator when combined into the entire 18-foot diameter array. The aluminum mirrors were polished with a diamond turning machine, that creates a glass-like reflective finish on metal. The precision fabrication machinery at the Space Optics Manufacturing Technology Center at MSFC can polish specialized optical elements to a world class quality of smoothness. This image shows optics physicist, Vince Huegele, examining one of the 144-segment hexagonal mirrors of the 18-foot diameter array at the MSFC solar concentrator test stand.

  13. Hexagonal Mirror Array

    NASA Technical Reports Server (NTRS)

    1999-01-01

    NASA's Space Optics Manufacturing Technology Center has been working to expand our view of the universe via sophisticated new telescopes. The Optics Center's goal is to develop low-cost, advanced space optics technologies for the NASA program in the 21st century, including the long-term goal of imaging Earth-like planets in distant solar systems. A segmented array of mirrors was designed by the Space Optics Manufacturing Technology Center for the solar concentrator test stand at the Marshall Space Flight Center (MSFC) for powering solar thermal propulsion engines. Each hexagon mirror has a spherical surface to approximate a parabolic concentrator when combined into the entire 18-foot diameter array. The aluminum mirrors were polished with a diamond turning machine that creates a glass-like reflective finish on metal. The precision fabrication machinery at the Space Optics Manufacturing Technology Center at MSFC can polish specialized optical elements to a world class quality of smoothness. This image shows optics physicist, Vince Huegele, examining one of the 144-segment hexagonal mirrors of the 18-foot diameter array at the MSFC solar concentrator test stand.

  14. Hexagonal Mirror Array

    NASA Technical Reports Server (NTRS)

    1999-01-01

    NASA's Space Optics Manufacturing Technology Center has been working to expand our view of the universe via sophisticated new telescopes. The Optics Center's goal is to develop low-cost, advanced space optics technologies for the NASA program in the 21st century, including the long-term goal of imaging Earth-like planets in distant solar systems. A segmented array of mirrors was designed by the Space Optics Manufacturing Technology Center for solar the concentrator test stand at the Marshall Space Flight Center (MSFC) for powering solar thermal propulsion engines. Each hexagon mirror has a spherical surface to approximate a parabolic concentrator when combined into the entire 18-foot diameter array. The aluminum mirrors were polished with a diamond turning machine, that creates a glass-like reflective finish on metal. The precision fabrication machinery at the Space Optics Manufacturing Technology Center at MSFC can polish specialized optical elements to a world class quality of smoothness. This image shows optics physicist, Vince Huegele, examining one of the 144-segment hexagonal mirrors of the 18-foot diameter array at the MSFC solar concentrator test stand.

  15. Hexagonal Mirror Array

    NASA Technical Reports Server (NTRS)

    1999-01-01

    NASA's Space Optics Manufacturing Technology Center has been working to expand our view of the universe via sophisticated new telescopes. The Optics Center's goal is to develop low-cost, advanced space optics technologies for the NASA program in the 21st century, including the long-term goal of imaging Earth-like planets in distant solar systems. A segmented array of mirrors was designed by the Space Optics Manufacturing Technology Center for the solar concentrator test stand at the Marshall Space Flight Center (MSFC) for powering solar thermal propulsion engines. Each hexagon mirror has a spherical surface to approximate a parabolic concentrator when combined into the entire 18-foot diameter array. The aluminum mirrors were polished with a diamond turning machine that creates a glass-like reflective finish on metal. The precision fabrication machinery at the Space Optics Manufacturing Technology Center at MSFC can polish specialized optical elements to a world class quality of smoothness. This image shows optics physicist, Vince Huegele, examining one of the 144-segment hexagonal mirrors of the 18-foot diameter array at the MSFC solar concentrator test stand.

  16. Optical properties of self assembled GaN polarity inversion domain boundary

    SciTech Connect

    Liu, M.-C.; Cheng, Y.-J.; Chang, J.-R.; Chang, C.-Y.; Hsu, S.-C.

    2011-07-11

    We report the fabrication of GaN lateral polarity inversion heterostructure with self assembled crystalline inversion domain boundaries (IDBs). The sample was fabricated by two step molecular-beam epitaxy (MBE) with microlithography patterning in between to define IDBs. Despite the use of circular pattern, hexagonal crystalline IDBs were self assembled from the circular pattern during the second MBE growth. Both cathodoluminescent (CL) and photoluminescent (PL) measurements show a significant enhanced emission at IDBs and in particular at hexagonal corners. The ability to fabricate self assembled crystalline IDBs and its enhanced emission property can be useful in optoelectronic applications.

  17. Pyramidal defects in Mg-doped GaN in light of strain-energy minimization

    NASA Astrophysics Data System (ADS)

    Lee, Dong Nyung

    2011-12-01

    The planar segregation gives rise to stress and strain fields which are approximated by a uniaxial character in a displacement controlled system. In this condition, the elastic strain energy is proportional to Young's modulus. Young's modulus of GaN is minimized when the directions normal to a conical segregation surface make about 48° with the c-axis of hexagonal GaN, which is close to the angle 47.3° between the c-axis and the directions normal to the {112¯3} planes. This implies that the formation of pyramidal defects in magnesium-doped GaN can be a compromise between minimization of the elastic strain energy due to segregation of magnesium and the planar segregation.

  18. Patterned growth of aligned ZnO nanowire arrays on sapphire and GaN layers

    NASA Astrophysics Data System (ADS)

    Fan, H. J.; Fleischer, F.; Lee, W.; Nielsch, K.; Scholz, R.; Zacharias, M.; Gösele, U.; Dadgar, A.; Krost, A.

    2004-07-01

    Patterned growth of vertically aligned ZnO nanowire arrays on the micrometer and nanometer scale on sapphire and GaN epilayers is reported. In order to control the position and distribution density of the ZnO nanowires, Au seeding nanodots are defined, as regular arrays, with the assistance of deposition shadow masks. Electron micrographs reveal that the wires are single crystals having wire axes along the hexagonal c-axes. The epitaxial growth of ZnO nanowires on sapphire and GaN films on Si substrates was further verified by cross sectional electron microscopy investigations. Compared to the sapphire case, the perfect epitaxial growth on a GaN film on a Si substrate is believed to be more suitable for potential electronic device applications of ZnO nanowire arrays.

  19. Cross Slip of Dislocation Loops in GaN Under Shear

    DTIC Science & Technology

    2014-03-01

    systems unique to hexagonal close-packed ( hcp ) and wurtzite crystals. Therefore, it is important to understand cross slip of dislo- cations in GaN to...dislocations on different planes for hcp metals [24]. Table 2 The drag coefficients as functions of slip plane for screw (Bs) and edge (Be...plane. The mobility values are qualitatively con- sistent with earlier reports of dislocation motion in hcp - based structures. Staroselsky and Anand’s

  20. Superlattice-like stacking fault array in ion-irradiated GaN

    SciTech Connect

    Ishimaru, Dr. Manabu; Usov, Igor Olegovich; Zhang, Yanwen; Weber, William J

    2012-01-01

    Controlling defects in crystalline solids is of technological importance for realizing desirable materials properties. Irradiation with energetic particles is useful for designing the spatial distribution and concentration of defects in materials. Here, we performed ion irradiation into hexagonal GaN with the wurtzite structure and demonstrated the spontaneous formation of superlattice-like stacking fault arrays. It was found that the modulation period can be controlled by irradiation conditions and post-irradiation heat treatments.

  1. Superlattice-Like Stacking Fault Array in Ion-Irradiated GaN

    SciTech Connect

    Ishimaru, Manabu; Usov, Igor O.; Zhang, Yanwen; Weber, William J.

    2012-01-01

    Controlling defects in crystalline solids is of technological importance for realizing desirable material properties. Irradiation with energetic particles is useful for designing the spatial distribution and concentration of defects in materials. Here, we performed ion irradiation into hexagonal GaN with the wurtzite structure and demonstrated the spontaneous formation of superlattice-like stacking fault arrays. It was found that the modulation period can be controlled by varying the irradiation conditions and subsequent thermal treatments.

  2. Nucleation conditions for catalyst-free GaN nanowires

    NASA Astrophysics Data System (ADS)

    Bertness, K. A.; Roshko, A.; Mansfield, L. M.; Harvey, T. E.; Sanford, N. A.

    2007-03-01

    We have examined the initial steps for catalyst-free growth of GaN nanowires by molecular beam epitaxy (MBE) on Si (1 1 1) substrates using AlN buffer layers. These wires form spontaneously under high N-to-Ga ratios for a growth temperature range of about 810-830 °C. Field emission scanning electron microscopy (FESEM) shows that part of the GaN forms a "matrix layer" that also grows with the [0 0 0 1] direction perpendicular to the substrate surface. This layer contains small, dense hexagonal pits in which the nanowires nucleate. Using both FESEM and atomic force microscopy (AFM), we identify the pit facets as {1 0 1¯ 2} planes. The nucleation studies show that the use of an AlN buffer layer is essential to the regular formation of the nanowires and matrix layers under our growth conditions. Our typical AlN buffer layer is 40-50 nm thick. We conclude that the nucleation mechanism for nanowires includes formation of nanocolumns in the AlN buffer layer. The propagation of the nanowires in GaN growth appears to be driven by differences in growth rates among crystallographic planes under N-rich conditions.

  3. Pit assisted oxygen chemisorption on GaN surfaces.

    PubMed

    Mishra, Monu; Krishna T C, Shibin; Aggarwal, Neha; Kaur, Mandeep; Singh, Sandeep; Gupta, Govind

    2015-06-21

    A comprehensive analysis of oxygen chemisorption on epitaxial gallium nitride (GaN) films grown at different substrate temperatures via RF-molecular beam epitaxy was carried out. Photoemission (XPS and UPS) measurements were performed to investigate the nature of the surface oxide and corresponding changes in the electronic structure. It was observed that the growth of GaN films at lower temperatures leads to a lower amount of surface oxide and vice versa was observed for a higher temperature growth. The XPS core level (CL) and valence band maximum (VBM) positions shifted towards higher binding energies (BE) with oxide coverage and revealed a downward band bending. XPS valence band spectra were de-convoluted to understand the nature of the hybridization states. UPS analysis divulged higher values of electronic affinity and ionization energy for GaN films grown at a higher substrate temperature. The surface morphology and pit structure were probed via microscopic measurements (FESEM and AFM). FESEM and AFM analysis revealed that the film surface was covered with hexagonal pits, which played a significant role in oxygen chemisorption. The favourable energetics of the pits offered an ideal site for oxygen adsorption. Pit density and pit depth were observed to be important parameters that governed the surface oxide coverage. The contribution of surface oxide was increased with an increase in average pit density as well as pit depth.

  4. Zn-dopant dependent defect evolution in GaN nanowires

    NASA Astrophysics Data System (ADS)

    Yang, Bing; Liu, Baodan; Wang, Yujia; Zhuang, Hao; Liu, Qingyun; Yuan, Fang; Jiang, Xin

    2015-10-01

    Zn doped GaN nanowires with different doping levels (0, <1 at%, and 3-5 at%) have been synthesized through a chemical vapor deposition (CVD) process. The effect of Zn doping on the defect evolution, including stacking fault, dislocation, twin boundary and phase boundary, has been systematically investigated by transmission electron microscopy and first-principles calculations. Undoped GaN nanowires show a hexagonal wurtzite (WZ) structure with good crystallinity. Several kinds of twin boundaries, including (101&cmb.macr;3), (101&cmb.macr;1) and (202&cmb.macr;1), as well as Type I stacking faults (...ABABC&cmb.b.line;BCB...), are observed in the nanowires. The increasing Zn doping level (<1 at%) induces the formation of screw dislocations featuring a predominant screw component along the radial direction of the GaN nanowires. At high Zn doping level (3-5 at%), meta-stable cubic zinc blende (ZB) domains are generated in the WZ GaN nanowires. The WZ/ZB phase boundary (...ABABAC&cmb.b.line;BA...) can be identified as Type II stacking faults. The density of stacking faults (both Type I and Type II) increases with increasing the Zn doping levels, which in turn leads to a rough-surface morphology in the GaN nanowires. First-principles calculations reveal that Zn doping will reduce the formation energy of both Type I and Type II stacking faults, favoring their nucleation in GaN nanowires. An understanding of the effect of Zn doping on the defect evolution provides an important method to control the microstructure and the electrical properties of p-type GaN nanowires.Zn doped GaN nanowires with different doping levels (0, <1 at%, and 3-5 at%) have been synthesized through a chemical vapor deposition (CVD) process. The effect of Zn doping on the defect evolution, including stacking fault, dislocation, twin boundary and phase boundary, has been systematically investigated by transmission electron microscopy and first-principles calculations. Undoped GaN nanowires show a

  5. Raman study of magnetic phase transitions of hexagonal manganites

    NASA Astrophysics Data System (ADS)

    Nam, Ji-Yeon; Hien, Nguyen T. M.; Huyen, Nguyen T.; Han, Kiok; Chen, Xiang-Bai; Cheong, S. W.; Lee, D.; Noh, T. W.; Sung, N. H.; Cho, B. K.; Yang, In-Sang

    2014-03-01

    Results of Raman studies of magnetic phase transitions of hexagonal LuMnO3 single crystal and HoMnO3 thin films are compared directly with the results of magnetic measurements. Our results show that the temperature dependent Raman study of magnon scattering provides a simple and accurate method for investigating magnetic phase transitions, especially in HoMnO3 thin films. In single crystal, our optical method provides results as good as magnetization measurements.

  6. InGaN quantum dot formation mechanism on hexagonal GaN/InGaN/GaN pyramids.

    PubMed

    Lundskog, A; Palisaitis, J; Hsu, C W; Eriksson, M; Karlsson, K F; Hultman, L; Persson, P O Å; Forsberg, U; Holtz, P O; Janzén, E

    2012-08-03

    Growing InGaN quantum dots (QDs) at the apex of hexagonal GaN pyramids is an elegant approach to achieve a deterministic positioning of QDs. Despite similar synthesis procedures by metal organic chemical vapor deposition, the optical properties of the QDs reported in the literature vary drastically. The QDs tend to exhibit either narrow or broad emission lines in the micro-photoluminescence spectra. By coupled microstructural and optical investigations, the QDs giving rise to narrow emission lines were concluded to nucleate in association with a (0001) facet at the apex of the GaN pyramid.

  7. Growth, Nitrogen Vacancy Reduction and Solid Solution Formation in Cubic GaN Thin Films and Subsequent Fabrication of Superlattice Structures Using AlN and InN

    DTIC Science & Technology

    1990-12-01

    Laboratories, Inc. 3. Structural Analysis Reflection high-energy electron diffraction performed during growth indicated that the BGaN film remained...was used (Hitachi H-800) to more closely examine the microstructural evolution of the BN/ BGaN /GaN epitaxial films. Cross-section TEM specimens were... BGaN layer to be a mixture of cubic and wurtzitic phases. This layer was heavily faulted. The latter phenomenon is to be expected given the high

  8. Formation of long-lived resonances in hexagonal cavities by strong coupling of superscar modes

    NASA Astrophysics Data System (ADS)

    Song, Qinghai; Ge, Li; Wiersig, Jan; Cao, Hui

    2013-08-01

    The recent progresses in single crystalline wide bandgap hexagonal disk have stimulated intense research attention on pursuing ultraviolet (UV) laser diodes with low thresholds. While whispering-gallery modes based UV lasers have been successfully obtained in GaN, ZnO nanorods, and nanopillars, the reported thresholds are still very high, due to the low-quality (Q) factors of the hexagonal resonances. Here we demonstrate resonances whose Q factors can be more than two orders of magnitude higher than the hexagonal modes, promising the reduction of the energy consumption. The key to our finding is the avoided resonance crossing between superscar states along two sets of nearly degenerated triangle orbits, which leads to the formation of hexagram modes. The mode couplings suppress the field distributions at the corners and the deviations from triangle orbits simultaneously and therefore enhance the Q factors significantly.

  9. Surface state of GaN after rapid-thermal-annealing using AlN cap-layer

    NASA Astrophysics Data System (ADS)

    El-Zammar, G.; Khalfaoui, W.; Oheix, T.; Yvon, A.; Collard, E.; Cayrel, F.; Alquier, D.

    2015-11-01

    Critical issues need to be overcome to produce high performance Schottky diodes on gallium nitride (GaN). To activate dopant, high temperature thermal treatments are required but damage GaN surface where hexagonal pits appear and prevent any device processing. In this paper, we investigated the efficiency of cap-layers on GaN during thermal treatments to avoid degradation. Aluminum nitride (AlN) and silicon oxide (SiOx) were grown on GaN by direct current reactive magnetron sputtering and plasma-enhanced chemical vapor deposition, respectively. AlN growth parameters were studied to understand their effect on the grown layers and their protection efficiency. Focused ion beam was used to measure AlN layer thickness. Crystalline quality and exact composition were verified using X-ray diffraction and energy dispersive X-ray spectroscopy. Two types of rapid thermal annealing at high temperatures were investigated. Surface roughness and pits density were evaluated using atomic force microscopy and scanning electron microscopy. Cap-layers wet etching was processed in H3PO4 at 120 °C for AlN and in HF (10%) for SiOx. This work reveals effective protection of GaN during thermal treatments at temperatures as high as 1150 °C. Low surface roughness was obtained. Furthermore, no hexagonal pit was observed on the surface.

  10. Thermal evolution of microstructure in ion-irradiated GaN

    SciTech Connect

    Bae, In-Tae; Jiang, Weilin; Wang, Chong M.; Weber, William J.; Zhang, Yanwen

    2009-04-20

    The thermal evolution of the microstructure created by irradiation of a GaN single crystal with 2 MeV Au2+ ions at 150 K is characterized following annealing at 973 K using transmission electron microscopy. In the as-irradiated sample characterized at 300 K, Ga nanocrystals with the diamond structure, which is an unstable configuration for Ga, are directly observed together with nitrogen bubbles in the irradiation-induced amorphous layer. Upon thermal annealing, the thickness of the amorphous layer decreases by ~13.1 %, and nano-beam electron diffraction analysis indicates no evidence for residual Ga nanocrystals, but instead reveals a mixture of hexagonal and cubic GaN phases in the annealed sample. Nitrogen molecules, captured in the as-irradiated bubbles, appear to debond and react with the Ga nanocrystals during the thermal annealing to form crystalline GaN. In addition, electron energy loss spectroscopy measurements reveal an atomic volume change of 18.9 % for the as-irradiated amorphous layer relative to the virgin single crystal GaN. This relative swelling of the damaged layer reduces to 7.7 % after thermal annealing. Partial recrystallization and structural relaxation of the GaN amorphous state are believed responsible for the volume change.

  11. Effect of residual stress on the microstructure of GaN epitaxial films grown by pulsed laser deposition

    NASA Astrophysics Data System (ADS)

    Wang, Haiyan; Wang, Wenliang; Yang, Weijia; Zhu, Yunnong; Lin, Zhiting; Li, Guoqiang

    2016-04-01

    The stress-free GaN epitaxial films have been directly grown by pulsed laser deposition (PLD) at 850 °C, and the effect of different stress on the microstructure of as-grown GaN epitaxial films has been explored in detail. The as-grown stress-free GaN epitaxial films exhibit very smooth surface without any particles and grains, which is confirmed by the smallest surface root-mean-square roughness of 2.3 nm measured by atomic force microscopy. In addition, they also have relatively high crystalline quality, which is proved by the small full-width at half maximum values of GaN(0002) and GaN (10 1 bar 2) X-ray rocking curves as 0.27° and 0.68°, respectively. However, when the growth temperature is lower or higher than 850 °C, internal or thermal stress would be increased in as-grown GaN epitaxial films. To release the larger stress, a great number of dislocations are generated. Many irregular particulates, hexagonal GaN gains and pits are therefore produced on the films surface, and the crystalline quality is greatly reduced consequently. This work has demonstrated the direct growth of stress-free GaN epitaxial films with excellent surface morphology and high crystalline quality by PLD, and presented a comprehensive study on the origins and the effect of stress in GaN layer. It is instructional to achieve high-quality nitride films by PLD, and shows great potential and broad prospect for the further development of high-performance GaN-based devices.

  12. Depth dependence of defect density and stress in GaN grown on SiC

    SciTech Connect

    Faleev, N.; Temkin, H.; Ahmad, I.; Holtz, M.; Melnik, Yu.

    2005-12-15

    We report high resolution x-ray diffraction studies of the relaxation of elastic strain in GaN grown on SiC(0001). The GaN layers were grown with thickness ranging from 0.29 to 30 {mu}m. High level of residual elastic strain was found in thin (0.29 to 0.73 {mu}m thick) GaN layers. This correlates with low density of threading screw dislocations of 1-2x10{sup 7} cm{sup -2}, observed in a surface layer formed over a defective nucleation layer. Stress was found to be very close to what is expected from thermal expansion mismatch between the GaN and SiC. A model based on generation and diffusion of point defects accounts for these observations.

  13. Hail the Hexagon

    NASA Image and Video Library

    2017-05-08

    Saturn hexagonal polar jet stream is the shining feature of almost every view of the north polar region of Saturn. The region, in shadow for the first part of NASA's Cassini mission, now enjoys full sunlight, which enables Cassini scientists to directly image it in reflected light. Although the sunlight falling on the north pole of Saturn is enough to allow us to image and study the region, it does not provide much warmth. In addition to being low in the sky (just like summer at Earth's poles), the sun is nearly ten times as distant from Saturn as from Earth. This results in the sunlight being only about 1 percent as intense as at our planet. This view looks toward Saturn from about 31 degrees above the ring plane. The image was taken with the Cassini spacecraft wide-angle camera on Jan. 22, 2017 using a spectral filter which preferentially admits wavelengths of near-infrared light centered at 939 nanometers. The view was obtained at a distance of approximately 560,000 miles (900,000 kilometers) from Saturn. Image scale is 33 miles (54 kilometers) per pixel. https://photojournal.jpl.nasa.gov/catalog/PIA21327

  14. Preparation of Freestanding GaN Wafers by Hydride Vapor Phase Epitaxy with Void-Assisted Separation

    NASA Astrophysics Data System (ADS)

    Oshima, Yuichi; Eri, Takeshi; Shibata, Masatomo; Sunakawa, Haruo; Kobayashi, Kenji; Ichihashi, Toshinari; Usui, Akira

    2003-01-01

    We have developed a novel technique for preparing large-scale freestanding GaN wafers. Hydride vapor phase epitaxy (HVPE) growth of thick GaN layer was performed on a GaN template with a thin TiN film on the top. After the cooling process of the HVPE growth, the thick GaN layer was easily separated from the template by the assistance of many voids generated around the TiN film. As a result, a freestanding GaN wafer was obtained. The wafer obtained had a diameter of 45 mm, and a mirror-like surface. The-full-width-at-half-maximum (FWHM) of (0002) and (10\\bar{1}0) peaks in the X-ray rocking curve profile were 60 and 92 arcsec, respectively. The dislocation density was evaluated at 5× 106 cm-3 by etch pit density measurement.

  15. Ga vacancy induced ferromagnetism enhancement and electronic structures of RE-doped GaN

    NASA Astrophysics Data System (ADS)

    Zhong, Guohua; Zhang, Kang; He, Fan; Ma, Xuhang; Lu, Lanlan; Liu, Zhuang; Yang, Chunlei

    2012-09-01

    Because of their possible applications in spintronic and optoelectronic devices, GaN dilute magnetic semiconductors (DMSs) doped by rare-earth (RE) elements have attracted much attention since the high Curie temperature was obtained in RE-doped GaN DMSs and a colossal magnetic moment was observed in the Gd-doped GaN thin film. We have systemically studied the GaN DMSs doped by RE elements (La, Ce-Yb) using the full-potential linearized augmented plane wave method within the framework of density functional theory and adding the considerations of the electronic correlation and the spin-orbital coupling effects. We have studied the electronic structures of DMSs, especially for the contribution from f electrons. The origin of magnetism, magnetic interaction and the possible mechanism of the colossal magnetic moment were explored. We found that, for materials containing f electrons, electronic correlation was usually strong and the spin-orbital coupling was sometimes crucial in determining the magnetic ground state. It was found that GaN doped by La was non-magnetic. GaN doped by Ce, Nd, Pm, Eu, Gd, Tb and Tm are stabilized at antiferromagnetic phase, while GaN doped by other RE elements show strong ferromagnetism which is suitable materials for spintronic devices. Moreover, we have identified that the observed large enhancement of magnetic moment in GaN is mainly caused by Ga vacancies (3.0μB per Ga vacancy), instead of the spin polarization by magnetic ions or originating from N vacancies. Various defects, such as substitutional Mg for Ga, O for N under the RE doping were found to bring a reduction of ferromagnetism. In addition, intermediate bands were observed in some systems of GaN:RE and GaN with intrinsic defects, which possibly opens the potential application of RE-doped semiconductors in the third generation high efficiency photovoltaic devices.

  16. Enhancement in wafer bow of free-standing GaN substrates due to high-dose hydrogen implantation: implications for GaN layer transfer applications

    NASA Astrophysics Data System (ADS)

    Singh, R.; Radu, I.; Bruederl, G.; Eichler, C.; Haerle, V.; Gösele, U.; Christiansen, S. H.

    2007-04-01

    Two-inch free-standing GaN wafers were implanted by 100 keV H+2 ions with a dose of 1.3 × 1017 cm-2 at room temperature. The hydrogen implantation induced damage in GaN extends between 230 to 500 nm from the surface as measured by cross-sectional transmission electron microscopy (XTEM). The wafer bow of the free-standing GaN wafers was measured using a Tencor long range profilometer on a scan length of 48 mm before and after the hydrogen implantation. Before implantation the bow of two different free-standing GaN wafers (named A and B) with different thicknesses was 1.5 µm and 6 µm, respectively. Initially, both wafers were concave in shape. After implantation the bow changed to convex with a value of 36 µm for wafer A and a value of 32 µm for wafer B. High dose hydrogen implantation leads to an in-plane compressive stress in the top damaged layer of the GaN, which is responsible for the enhancement of wafer bow and change of bow direction. The high value of bow after implantation hinders the direct wafer bonding of the free-standing GaN wafers to sapphire or any other handle wafers. Tight bonding between hydrogen implanted GaN wafers and the handle wafers is a necessary requirement for the successful layer transfer of thin GaN layers onto other substrates based on wafer bonding and layer splitting (Smart-cut).

  17. An Explanation for Saturn's Hexagon

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2015-08-01

    For over three decades, weve been gathering observations of the mysterious hexagonal cloud pattern encircling Saturns north pole. Now, researchers believe they have a model that can better explain its formation.Fascinating GeometrySaturns northern Hexagon is a cloud band circling Saturns north pole at 78 N, first observed by the Voyager flybys in 198081. This remarkable pattern has now persisted for more than a Saturn year (29.5 Earth years).Eight frames demonstrating the motion within Saturns Hexagon. Click to watch the animation! The view is from a reference frame rotating with Saturn. [NASA/JPL-Caltech/SSI/Hampton University]Observations by Voyager and, more recently, Cassini have helped to identify many key characteristics of this bizarre structure. Two interesting things weve learned are:The Hexagon is associated with an eastward zonal jet moving at more than 200 mph.The cause of the Hexagon is believed to be a jet stream, similar to the ones that we experience on Earth. The path of the jet itself appears to follow the hexagons outline.The Hexagon rotates at roughly the same rate as Saturns overall rotation.While we observe individual storms and cloud patterns moving at different speeds within the Hexagon, the vertices of the Hexagon move at almost exactly the same rotational speed as that of Saturn itself.Attempts to model the formation of the Hexagon with a jet stream have yet to fully reproduce all of the observed features and behavior. But now, a team led by Ral Morales-Juberas of the New Mexico Institute of Mining and Technology believes they have created a model that better matches what we see.Simulating a Meandering JetThe team ran a series of simulations of an eastward, Gaussian-profile jet around Saturns pole. They introduced small perturbations to the jet and demonstrated that, as a result of the perturbations, the jet can meander into a hexagonal shape. With the initial conditions of the teams model, the meandering jet is able to settle into a

  18. Hexagonal cobalt carbide formed by carbon ion implantation

    NASA Astrophysics Data System (ADS)

    Liu, B. X.; Wang, J.; Fang, Z. Z.

    1991-05-01

    Thin films of ferromagnetic metals, i.e., bcc Fe, hcp Co, and fcc Ni, were subjected to 50-keV carbon ion implantation at room temperature. At the dose of 2.5×1017 ions/cm2, the formation of hexagonal Fe3C and Ni3C phases was confirmed by transmission electron microscopy selected area electron diffraction patterns; and more interestingly a similar pattern for Co was also observed for the first time. The phase was identified as hexagonal Co3C with a=2.685 Å and c=4.335 Å based on the spacings and intensities of the diffraction rings. The carbide formation was also confirmed by Auger electron spectra. The stoichiometry of the hexagonal structure may be extended in the range of Co3-2C as estimated from the experiments performed up to the dose of 9×1017 ions/cm2.

  19. Specific peptide for functionalization of GaN

    NASA Astrophysics Data System (ADS)

    Estephan, E.; Larroque, C.; Cloitre, T.; Cuisinier, F. J. G.; Gergely, C.

    2008-04-01

    Nanobiotechnology aims to exploit biomolecular recognition and self-assembly capabilities for integrating advanced materials into medicine and biology. However frequent problems are encountered at the interface of substrate-biological molecule, as the direct physical adsorption of biological molecules is dependent of unpredictable non-specific interactions with the surface, often causing their denaturation. Therefore, a proper functionalization of the substrate should avoid a loss of biological activity. In this work we address the functionalization of the semiconductor GaN (0001) for biosensing applications. The basic interest of using III-V class semiconductors is their good light emitting properties and a fair chemical stability that allows various applications of these materials. The technology chosen to elaborate GaN-specific peptides is the combinatorial phage-display method, a biological screening procedure based on affinity selection. An M13 bacteriophage library has been used to screen 10 10 different peptides against the GaN (0001) surface to finally isolate one specific peptide. The preferential attachment of the biotinylated selected peptide onto the GaN (0001), in close proximity to a surface of different chemical and structural composition has been demonstrated by fluorescence microscopy. Further physicochemical studies have been initiated to evaluate the semiconductor-peptide interface and understand the details in the specific recognition of peptides for semiconductor substrates. Fourier Transform Infrared spectroscopy in Attenuated Total Reflection mode (FTIR-ATR) has been employed to prove the presence of peptides on the surface. Our Atomic Force Microscopy (AFM) studies on the morphology of the GaN surface after functionalization revealed a total surface coverage by a very thin, homogeneous peptide layer. Due to its good biocompatibility, functionalized GaN devices might evolve in a new class of implantable biosensors for medical applications.

  20. Photoluminescence characterization of Mg implanted GaN

    SciTech Connect

    Ronning, C.; Hofsaess, H.; Stoetzler, A.; Deicher, M.; Carlson, E.P.; Hartlieb, P.J.; Gehrke, T.; Rajagopal, P.; Davis, R.F.

    2000-07-01

    Single crystalline (0001) gallium nitride layers, capped with a thin epitaxial aluminum nitride layer, were implanted with magnesium and subsequently annealed in vacuum to 1150--1300 C for 10--60 minutes. Photoluminescence (PL) measurements showed the typical donor acceptor pair (DAP) transition at 3.25 eV after annealing at high temperatures, which is related to optically active Mg acceptors in GaN. After annealing at 1300 C a high degree of optical activation of the implanted Mg atoms was reached in the case of low implantation doses. Electrical measurements, performed after removing the AlN-cap and the deposition of Pd/Au contacts, showed no p-type behavior of the GaN samples due to the compensation of the Mg acceptors with native n-type defects.

  1. Hexagonal tessellations in image algebra

    NASA Astrophysics Data System (ADS)

    Eberly, David H.; Wenzel, Dennis J.; Longbotham, Harold G.

    1990-11-01

    In image algebra '' the concept of a coordinate set X is general in that such a set is simply a subset of ndimensional Euclidean space . The standard applications in 2-dimensional image processing use coordinate sets which are rectangular arrays X 72 x ZZm. However some applications may require other geometries for the coordinate set. We look at three such related applications in the context of image algebra. The first application is the modeling of photoreceptors in primate retinas. These receptors are inhomogeneously distributed on the retina. The largest receptor density occurs in the center of the fovea and decreases radially outwards. One can construct a hexagonal tessellation of the retina such that each hexagon contains approximately the same number of receptors. The resulting tessellation called a sunflower heart2 consists of concentric rings of hexagons whose sizes increase as the radius of the ring increases. The second application is the modeling of the primary visual . The neurons are assumed to be uniformly distributed as a regular hexagonal lattice. Cortical neural image coding is modeled by a recursive convolution of the retinal neural image using a special set of filters. The third application involves analysis of a hexagonally-tessellated image where the pixel resolution is variable .

  2. Strain-induced step bunching in orientation-controlled GaN on Si

    NASA Astrophysics Data System (ADS)

    Narita, Tetsuo; Iguchi, Hiroko; Horibuchi, Kayo; Otake, Nobuyuki; Hoshi, Shinichi; Tomita, Kazuyoshi

    2016-05-01

    We report a technique for the fabrication of high-quality GaN-on-silicon (Si) substrates for use in various power applications. GaN epitaxial layers were generated on Si(111) vicinal faces that had been previously covered with a thin coating of Al2O3 to control the orientation of the AlN seed layers. We obtained orientation-controlled GaN layers and found a linear relationship between the GaN c-axis and Si[111] tilt angles. As a result, the threading dislocation density in the AlN seed layer was reduced and high-quality GaN layers were generated. The X-ray rocking curves for these layers exhibited full width at half maximum values of 390‧‧ and 550‧‧ for the (004) and (114) reflections, respectively. Significant step bunching was observed on a GaN(0001) vicinal face produced using this technique, attributed to strain-induced attractive interactions between steps. Thus, by controlling the strain near the surface layer, we achieved the step flow growth of GaN on Si.

  3. Preparation of GaN on GaAs (100) substrate by annealing and post-nitridation

    SciTech Connect

    Kuppulingam, B.; Baskar, K.

    2016-05-23

    Preparation of GaN on GaAs (100) substrate by annealing and post-nitridation process has been studied. X-ray diffraction analysis confirmed the formation β-Ga{sub 2}O{sub 3} phase on annealed substrate. Nitridation of annealed samples resulted in formation hexagonal structure of GaN. Morphological changes of annealed and nitridated samples were analyzed using optical microscopy, scanning electron microscopy, atomic force microscopy. Chemical composition and elemental impurities were analysed by energy dispersive x-ray diffraction. The room-temperature photoluminescence study of annealed samples shows broad emission from 300 nm to 500 nm related to of β-Ga{sub 2}O{sub 3}, whereas nitridated samples show near band edge emission at 370 nm (3.36 eV) which corresponds to GaN.

  4. Finite-difference time-domain analysis on light extraction in a GaN light-emitting diode by empirically capable dielectric nano-features

    NASA Astrophysics Data System (ADS)

    Park, ByeongChan; Noh, Heeso; Yu, Young Moon; Jang, Jae-Won

    2014-11-01

    Enhancement of light extraction in GaN light-emitting diode (LED) by addressing an array of nanomaterials is investigated by means of three dimensional (3D) finite-difference time-domain (FDTD) simulation experiments. The array of nanomaterials is placed on top of the GaN LED and is used as a light extraction layer. Depending on its empirically capable features, the refractive index of nanomaterials with perfectly spherical (particle) and hemispherical (plano-convex lens) shapes were decided as 1.47 [Polyethylene glycol (PEG)] and 2.13 [Zirconia (ZrO2)]. As a control experiment, a 3D FDTD simulation experiment of GaN LED with PEG film deposited on top is also carried out. Different light extraction profiles between subwavelength- and over-wavelength-scaled nanomaterials addressed GaN LEDs are observed in distributions of Poynting vector intensity of the light extraction layer-applied GaN LEDs. In addition, our results show that the dielectric effect on light extraction is more efficient in the light extraction layer with over-wavelength scaled features. In the case of a Zirconia particle array (ϕ = 500 nm) with hexagonal closed packed (hcp) structure on top of a GaN LED, light extraction along the normal axis of the LED surface is about six times larger than a GaN LED without the extraction layer.

  5. Nanostructure surface patterning of GaN thin films and application to AlGaN/AlN multiple quantum wells: A way towards light extraction efficiency enhancement of III-nitride based light emitting diodes

    SciTech Connect

    Guo, Wei Kirste, Ronny; Bryan, Zachary; Bryan, Isaac; Collazo, Ramón; Sitar, Zlatko; Gerhold, Michael

    2015-03-21

    Enhanced light extraction efficiency was demonstrated on nanostructure patterned GaN and AlGaN/AlN Multiple-Quantum-Well (MQW) structures using mass production techniques including natural lithography and interference lithography with feature size as small as 100 nm. Periodic nanostructures showed higher light extraction efficiency and modified emission profile compared to non-periodic structures based on integral reflection and angular-resolved transmission measurement. Light extraction mechanism of macroscopic and microscopic nanopatterning is discussed, and the advantage of using periodic nanostructure patterning is provided. An enhanced photoluminescence emission intensity was observed on nanostructure patterned AlGaN/AlN MQW compared to as-grown structure, demonstrating a large-scale and mass-producible pathway to higher light extraction efficiency in deep-ultra-violet light-emitting diodes.

  6. Nanostructure surface patterning of GaN thin films and application to AlGaN/AlN multiple quantum wells: A way towards light extraction efficiency enhancement of III-nitride based light emitting diodes

    NASA Astrophysics Data System (ADS)

    Guo, Wei; Kirste, Ronny; Bryan, Zachary; Bryan, Isaac; Gerhold, Michael; Collazo, Ramón; Sitar, Zlatko

    2015-03-01

    Enhanced light extraction efficiency was demonstrated on nanostructure patterned GaN and AlGaN/AlN Multiple-Quantum-Well (MQW) structures using mass production techniques including natural lithography and interference lithography with feature size as small as 100 nm. Periodic nanostructures showed higher light extraction efficiency and modified emission profile compared to non-periodic structures based on integral reflection and angular-resolved transmission measurement. Light extraction mechanism of macroscopic and microscopic nanopatterning is discussed, and the advantage of using periodic nanostructure patterning is provided. An enhanced photoluminescence emission intensity was observed on nanostructure patterned AlGaN/AlN MQW compared to as-grown structure, demonstrating a large-scale and mass-producible pathway to higher light extraction efficiency in deep-ultra-violet light-emitting diodes.

  7. Growth of β-Ga2O3 and GaN nanowires on GaN for photoelectrochemical hydrogen generation.

    PubMed

    Hwang, Jih-Shang; Liu, Tai-Yan; Chattopadhyay, Surjit; Hsu, Geng-Ming; Basilio, Antonio M; Chen, Han-Wei; Hsu, Yu-Kuei; Tu, Wen-Hsun; Lin, Yan-Gu; Chen, Kuei-Hsien; Li, Chien-Cheng; Wang, Sheng-Bo; Chen, Hsin-Yi; Chen, Li-Chyong

    2013-02-08

    Enhanced photoelectrochemical (PEC) performances of Ga(2)O(3) and GaN nanowires (NWs) grown in situ from GaN were demonstrated. The PEC conversion efficiencies of Ga(2)O(3) and GaN NWs have been shown to be 0.906% and 1.09% respectively, in contrast to their 0.581% GaN thin film counterpart under similar experimental conditions. A low crystallinity buffer layer between the grown NWs and the substrate was found to be detrimental to the PEC performance, but the layer can be avoided at suitable growth conditions. A band bending at the surface of the GaN NWs generates an electric field that drives the photogenerated electrons and holes away from each other, preventing recombination, and was found to be responsible for the enhanced PEC performance. The enhanced PEC efficiency of the Ga(2)O(3) NWs is aided by the optical absorption through a defect band centered 3.3 eV above the valence band of Ga(2)O(3). These findings are believed to have opened up possibilities for enabling visible absorption, either by tailoring ion doping into wide bandgap Ga(2)O(3) NWs, or by incorporation of indium to form InGaN NWs.

  8. Instability and Spontaneous Reconstruction of Few-Monolayer Thick GaN Graphitic Structures.

    PubMed

    Kolobov, A V; Fons, P; Tominaga, J; Hyot, B; André, B

    2016-08-10

    Two-dimensional (2D) semiconductors are a very hot topic in solid state science and technology. In addition to van der Waals solids that can be easily formed into 2D layers, it was argued that single layers of nominally 3D tetrahedrally bonded semiconductors, such as GaN or ZnO, also become flat in the monolayer limit; the planar structure was also proposed for few-layers of such materials. In this work, using first-principles calculations, we demonstrate that contrary to the existing consensus the graphitic structure of few-layer GaN is unstable and spontaneously reconstructs into a structure that remains hexagonal in plane but with covalent interlayer bonds that form alternating octagonal and square (8|4 Haeckelite) rings with pronounced in-plane anisotropy. Of special interest is the transformation of the band gap from indirect in planar GaN toward direct in the Haeckelite phase, making Haeckelite few-layer GaN an appealing material for flexible nano-optoelectronics.

  9. GaN Haeckelite Single-Layered Nanostructures: Monolayer and Nanotubes.

    PubMed

    Camacho-Mojica, Dulce C; López-Urías, Florentino

    2015-12-10

    Nowadays, III-V semiconductors are interesting candidate materials for the tailoring of two dimensional (2D) graphene-like structures. These new 2D materials have attracted profound interest opening the possibility to find semiconductor materials with unexplored properties. First-principles density functional theory calculations are performed in order to investigate the electronic properties of GaN planar and nanotube morphologies based on Haeckelite structures (containing octagonal and square membered rings). Optimized geometries, band-structures, phonon dispersion, binding energies, transmission electron microscopy images simulations, x-ray diffraction patterns, charge densities, and electronic band gaps are calculated. We demonstrated that GaN Haeckelite structures are stable exhibiting a semiconducting behavior with an indirect band gap. Furthermore, it was found that GaN Haeckelite nanotubes are semiconductor with a band gap nature (direct or indirect) that depends of the nanotube's chirality and diameter. In addition, it was demonstrated that surface passivation and the interaction with hydrazine, water, ammonia, and carbon monoxide molecules can change the band-gap nature. Our results are compared with the corresponding GaN hexagonal honeycomb structures.

  10. GaN Haeckelite Single-Layered Nanostructures: Monolayer and Nanotubes

    PubMed Central

    Camacho-Mojica, Dulce C.; López-Urías, Florentino

    2015-01-01

    Nowadays, III-V semiconductors are interesting candidate materials for the tailoring of two dimensional (2D) graphene-like structures. These new 2D materials have attracted profound interest opening the possibility to find semiconductor materials with unexplored properties. First-principles density functional theory calculations are performed in order to investigate the electronic properties of GaN planar and nanotube morphologies based on Haeckelite structures (containing octagonal and square membered rings). Optimized geometries, band-structures, phonon dispersion, binding energies, transmission electron microscopy images simulations, x-ray diffraction patterns, charge densities, and electronic band gaps are calculated. We demonstrated that GaN Haeckelite structures are stable exhibiting a semiconducting behavior with an indirect band gap. Furthermore, it was found that GaN Haeckelite nanotubes are semiconductor with a band gap nature (direct or indirect) that depends of the nanotube´s chirality and diameter. In addition, it was demonstrated that surface passivation and the interaction with hydrazine, water, ammonia, and carbon monoxide molecules can change the band-gap nature. Our results are compared with the corresponding GaN hexagonal honeycomb structures. PMID:26658148

  11. Structural performance of two aerobrake hexagonal heat shield panel concepts

    NASA Technical Reports Server (NTRS)

    Dorsey, John T.; Dyess, James W.

    1992-01-01

    Structural sizing and performance are presented for two structural concepts for an aerobrake hexagonal heat shield panel. One concept features a sandwich construction with an aluminum honeycomb core and thin quasi-isotropic graphite-epoxy face sheets. The other concept features a skin-rib isogrid construction with thin quasi-isotropic graphite-epoxy skins and graphite-epoxy ribs oriented at 0, +60, and -60 degs along the panel. Linear static, linear bifurcation buckling, and nonlinear static analyses were performed to compare the structural performance of the two panel concepts and assess their feasibility for a lunar transfer vehicle aerobrake application.

  12. Gradual tilting of crystallographic orientation and configuration of dislocations in GaN selectively grown by vapour phase epitaxy methods

    PubMed

    Kuwan; Tsukamoto; Taki; Horibuchi; Oki; Kawaguchi; Shibata; Sawaki; Hiramatsu

    2000-01-01

    Cross-sectional transmission electron microscope (TEM) observation was performed for selectively grown gallium nitride (GaN) in order to examine the dependence of GaN microstructure on the growth conditions. The GaN films were grown by hydride vapour phase epitaxy (HVPE) or metalorganic vapour phase epitaxy (MOVPE) on GaN covered with a patterned mask. Thin foil specimens for TEM observation were prepared with focused ion beam (FIB) machining apparatus. It was demonstrated that the c-axis of GaN grown over the terrace of the mask tilts towards the centre of the terrace when the GaN is grown in a carrier gas of N2. The wider terrace results in a larger tilting angle if other growth conditions are identical. The tilting is attributed to 'horizontal dislocations' (HDs) generated during the overgrowth of GaN on the mask terrace. The HDs in HVPE-GaN have a semi-loop shape and are tangled with one another, while those in MOVPE-GaN are straight and lined up to form low-angle grain boundaries.

  13. Effect of defects in oxide templates on Non-catalytic growth of GaN nanowires for high-efficiency light-emitting diodes

    NASA Astrophysics Data System (ADS)

    Hwang, Sung Won; Choi, Suk-Ho

    2016-04-01

    Two kinds of oxide templates, one with and one without undercuts, are employed to study the effect of defects in oxide templates on non-catalytic growth of GaN nanowires (NWs). GaN NWs abnormally grown from the templates containing undercuts exhibit two types of patterns: earlystage growth of premature NWs and abnormally-overgrown (~2 μm) NWs. GaN NWs grown on perfectly-symmetric template patterns are highly crystalline and have high aspect ratios (2 ~ 5), and their tops are shaped as pyramids with semipolar facets, clearly indicating hexagonal symmetry. The internal quantum efficiency of the well-grown NWs is 10% larger than that of the deformed NWs, as estimated by using photoluminescence. These results suggest that our technique is an effective approach for growing large-area-patterned, vertically-aligned, hexagonal GaN NWs without catalysts, in strong contrast to catalytic vapor-liquid-solid growth, and that good formation of the oxide templates is crucial for the growth of high-quality GaN NWs.

  14. MgCaO Dry Etching on GaN

    NASA Astrophysics Data System (ADS)

    Hlad, M.; Ren, F.

    2005-11-01

    MgCaO films grown by rf plasma-assisted Molecular Beam Epitaxy and capped with Sc2O3 are promising candidates as surface passivation layers and gate dielectrics on GaN-based high electron mobility transistors (HEMTs) and metal-oxide semiconductor HEMTs (MOS-HEMTs) respectively. Two different plasma chemistries were examined for etching these thin films on GaN. Inductively Coupled Plasmas of CH4/H2/Ar produced etch rates only in the range 20-70 å/min, comparable to the Ar sputter rates under the same conditions. Similarly slow MgCaO etch rates (˜100 å/min) were obtained with Cl2/Ar discharges under the same conditions, but GaN showed rates almost an order of magnitude higher. The MgCaO removal rates are limited by the low volatilities of the respective etch products. The CH4/H2/Ar plasma chemistry produced a selectivity of around 2 or etching the MgCaO with respect to GaN.

  15. Nucleation and Growth of GaN on GaAs (001) Substrates

    SciTech Connect

    Drummond, Timothy J.; Hafich, Michael J.; Heller, Edwin J.; Lee, Stephen R.; Liliental-Weber, Zuzanna; Ruvimov, Sergei; Sullivan, John P.

    1999-05-03

    The nucleation of GaN thin films on GaAs is investigated for growth at 620 "C. An rf plasma cell is used to generate chemically active nitrogen from N2. An arsenic flux is used in the first eight monolayer of nitride growth to enhance nucleation of the cubic phase. Subsequent growth does not require an As flux to preserve the cubic phase. The nucleation of smooth interfaces and GaN films with low stacking fault densities is dependent upon relative concentrations of active nitrogen species in the plasma and on the nitrogen to gallium flux ratio.

  16. Nonlithographic nanopatterning through anodic aluminum oxide template and selective growth of highly ordered GaN nanostructures

    NASA Astrophysics Data System (ADS)

    Wang, Y. D.; Zang, K. Y.; Chua, S. J.

    2006-09-01

    Ordered GaN nanostructures, i.e., nanopore and nanodot arrays, have been demonstrated by combining a nonlithographic nanopatterning technique and nanoscale selective epitaxial growth. Hexagonal-close-packed nanopore arrays were fabricated in GaN surfaces and SiO2 surfaces on GaN films by inductively coupled plasma etching using anodic aluminum oxide templates as etching masks. Selective area growth through nanopores in SiO2 by metal organic chemical vapor deposition results in ordered GaN nanodot arrays with an average dot diameter and height of 60 and 100nm, respectively. The diameter and density of the GaN nanopore arrays and nanodot arrays are controlled by that of the anodic aluminum oxide template, which can be tuned in a wide range by controlling the anodization conditions. Applying anodic aluminum oxide as an etching mask provides an effective nonlithographic and free of foreign catalysts method to fabricate ordered and dense nitride nanostructures for either bottom-up or top-down technique in the application of high efficiency nitride light emitting diodes.

  17. Hexagonal boron-nitride nanomesh magnets

    NASA Astrophysics Data System (ADS)

    Ohata, C.; Tagami, R.; Nakanishi, Y.; Iwaki, R.; Nomura, K.; Haruyama, J.

    2016-09-01

    The formation of magnetic and spintronic devices using two-dimensional (2D) atom-thin layers has attracted attention. Ferromagnetisms (FMs) arising from zigzag-type atomic structure of edges of 2D atom-thin materials have been experimentally observed in graphene nanoribbons, hydrogen (H)-terminated graphene nanomeshes (NMs), and few-layer oxygen (O)-terminated black phosphorus NMs. Herein, we report room-temperature edge FM in few-layer hexagonal boron-nitride (hBN) NMs. O-terminated hBNNMs annealed at 500 °C show the largest FM, while it completely disappears in H-terminated hBNNMs. When hBNNMs are annealed at other temperatures, amplitude of the FM significantly decreases. These are highly in contrast to the case of graphene NMs but similar to the cases of black phosphorus NM and suggest that the hybridization of the O atoms with B(N) dangling bonds of zigzag pore edges, formed at the 500 °C annealing, strongly contribute to this edge FM. Room-temperature FM realizable only by exposing hBNNMs into air opens the way for high-efficiency 2D flexible magnetic and spintronic devices without the use of rare magnetic elements.

  18. Si Complies with GaN to Overcome Thermal Mismatches for the Heteroepitaxy of Thick GaN on Si.

    PubMed

    Tanaka, Atsunori; Choi, Woojin; Chen, Renjie; Dayeh, Shadi A

    2017-08-21

    Heteroepitaxial growth of lattice mismatched materials has advanced through the epitaxy of thin coherently strained layers, the strain sharing in virtual and nanoscale substrates, and the growth of thick films with intermediate strain-relaxed buffer layers. However, the thermal mismatch is not completely resolved in highly mismatched systems such as in GaN-on-Si. Here, geometrical effects and surface faceting to dilate thermal stresses at the surface of selectively grown epitaxial GaN layers on Si are exploited. The growth of thick (19 µm), crack-free, and pure GaN layers on Si with the lowest threading dislocation density of 1.1 × 10(7) cm(-2) achieved to date in GaN-on-Si is demonstrated. With these advances, the first vertical GaN metal-insulator-semiconductor field-effect transistors on Si substrates with low leakage currents and high on/off ratios paving the way for a cost-effective high power device paradigm on an Si CMOS platform are demonstrated. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  19. AlGaN/GaN field effect transistors for power electronics—Effect of finite GaN layer thickness on thermal characteristics

    SciTech Connect

    Hodges, C. Anaya Calvo, J.; Kuball, M.; Stoffels, S.; Marcon, D.

    2013-11-11

    AlGaN/GaN heterostructure field effect transistors with a 150 nm thick GaN channel within stacked Al{sub x}Ga{sub 1−x}N layers were investigated using Raman thermography. By fitting a thermal simulation to the measured temperatures, the thermal conductivity of the GaN channel was determined to be 60 W m{sup −1} K{sup −1}, over 50% less than typical GaN epilayers, causing an increased peak channel temperature. This agrees with a nanoscale model. A low thermal conductivity AlGaN buffer means the GaN spreads heat; its properties are important for device thermal characteristics. When designing power devices with thin GaN layers, as well as electrical considerations, the reduced channel thermal conductivity must be considered.

  20. Nitrogen-Polar (0001¯) GaN Grown on c-Plane Sapphire with a High-Temperature AlN Buffer

    PubMed Central

    Song, Jie; Han, Jung

    2017-01-01

    We demonstrate growing nitrogen-polar (N-polar) GaN epilayer on c-plane sapphire using a thin AlN buffer layer by metalorganic chemical vapor deposition. We have studied the influence of the AlN buffer layer on the polarity, crystalline quality, and surface morphology of the GaN epilayer and found that the growth temperature of the AlN buffer layer played a critical role in the growth of the GaN epilayer. The low growth temperature of the AlN buffer results in gallium-polar GaN. Even a nitridation process has been conducted. High growth temperature for an AlN buffer layer is required to achieve pure N-polarity, high crystalline quality, and smooth surface morphology for a GaN epilayer. PMID:28772612

  1. Structural and optical properties of GaN and InGaN nanoparticles by chemical co-precipitation method

    SciTech Connect

    Gopalakrishnan, M.; Purushothaman, V.; Venkatesh, P. Sundara; Ramakrishnan, V.; Jeganathan, K.

    2012-11-15

    Highlights: ► First report on InGaN NPs by chemical co-precipitation method. ► There is no phase separation in InGaN NPs. ► Both NPs are suitable for optoelectronic devices in the visible region. ► First experimental observation of phonon mode at 272 cm{sup −1} for GaN NPs. ► First report on μ-Raman analysis for InGaN NPs. -- Abstract: A facile method for the synthesis of gallium nitride (GaN) and indium gallium nitride (InGaN) nanoparticles (NPs) has been reported by simple chemical co-precipitation method. The average diameters of the GaN and InGaN NPs were 12 nm and 38 nm respectively. GaN NPs show high crystalline quality with hexagonal structure while InGaN NPs exhibits some cubic inclusion by X-ray diffraction. Room-temperature photoluminescence analysis shows the near-band edge emission at 3.43 eV for GaN and a strong blue emission at 3.0 eV for In{sub 0.4}Ga{sub 0.6}N NPs. The E{sub 2}{sup H} phonon peaks from micro-Raman scattering at 567 cm{sup −1} for GaN and 564 cm{sup −1} for InGaN confirms the wurtzite nature of both the NPs. In addition, we have also assigned some other phonon modes of GaN associated with zone boundary K point of the Brillouin zone which is not experimentally observed for their bulk counterparts.

  2. Graphene on hexagonal boron nitride

    NASA Astrophysics Data System (ADS)

    Yankowitz, Matthew; Xue, Jiamin; LeRoy, B. J.

    2014-07-01

    The field of graphene research has developed rapidly since its first isolation by mechanical exfoliation in 2004. Due to the relativistic Dirac nature of its charge carriers, graphene is both a promising material for next-generation electronic devices and a convenient low-energy testbed for intrinsically high-energy physical phenomena. Both of these research branches require the facile fabrication of clean graphene devices so as not to obscure its intrinsic physical properties. Hexagonal boron nitride has emerged as a promising substrate for graphene devices as it is insulating, atomically flat and provides a clean charge environment for the graphene. Additionally, the interaction between graphene and boron nitride provides a path for the study of new physical phenomena not present in bare graphene devices. This review focuses on recent advancements in the study of graphene on hexagonal boron nitride devices from the perspective of scanning tunneling microscopy with highlights of some important results from electrical transport measurements.

  3. Coaxial metal-oxide-semiconductor (MOS) Au/Ga2O3/GaN nanowires.

    PubMed

    Hsieh, Chin-Hua; Chang, Mu-Tung; Chien, Yu-Jen; Chou, Li-Jen; Chen, Lih-Juann; Chen, Chii-Dong

    2008-10-01

    Coaxial metal-oxide-semiconductor (MOS) Au-Ga2O3-GaN heterostructure nanowires were successfully fabricated by an in situ two-step process. The Au-Ga2O3 core-shell nanowires were first synthesized by the reaction of Ga powder, a mediated Au thin layer, and a SiO2 substrate at 800 degrees C. Subsequently, these core-shell nanowires were nitridized in ambient ammonia to form a GaN coating layer at 600 degrees C. The GaN shell is a single crystal, an atomic flat interface between the oxide and semiconductor that ensures that the high quality of the MOS device is achieved. These novel 1D nitride-based MOS nanowires may have promise as building blocks to the future nitride-based vertical nanodevices.

  4. The pyroelectric coefficient of free standing GaN grown by HVPE

    NASA Astrophysics Data System (ADS)

    Jachalke, Sven; Hofmann, Patrick; Leibiger, Gunnar; Habel, Frank S.; Mehner, Erik; Leisegang, Tilmann; Meyer, Dirk C.; Mikolajick, Thomas

    2016-10-01

    The present study reports on the temperature dependent pyroelectric coefficient of free-standing and strain-free gallium nitride (GaN) grown by hydride vapor phase epitaxy (HVPE). The Sharp-Garn method is applied to extract the pyroelectric coefficient from the electrical current response of the crystals subjected to a sinusoidal temperature excitation in a range of 0 °C to 160 °C. To avoid compensation of the pyroelectric response by an internal conductivity, insulating GaN crystals were used by applying C, Mn, and Fe doping during HVPE growth. The different pyroelectric coefficients observed at room temperature due to the doping correlate well with the change of the lattice parameter c. The obtained data are compared to previously published theoretical and experimental values of thin film GaN and discussed in terms of a strained lattice.

  5. Epitaxial Garnets and Hexagonal Ferrites.

    DTIC Science & Technology

    1983-12-01

    Ferrites Lithium Ferrite Magnetostatic Wave Garnets Epitaxy Yttrium Iron Garnet Liquid Phase Epitaxy Hexagonal Ferrite Microwave Signal Processing...epitaxial ferrit ( materials for use in microwave and millirreter-wave signal processing devices. The major emphasis has been on multiple layer...overall objective of this research is to develop epitaxial single crystal ferrite films suitable for microwave and millimeter-wave signal processing at

  6. Epitaxial Garnets and Hexagonal Ferrites.

    DTIC Science & Technology

    1982-04-20

    Iron Garnet Liquid Phase Epitaxy Hexagonal Ferrite microwave Signal Processing Millimeter-Wave 20. ABSTRACT (Continue ani revee arde if necoeermy and...le.’uIfy by block rns.) e objective of this research is to develop new and improved epitauial ferrite materials for use in microwave and millimeter... ferrite films suitable for microwave and millimeter-wave signal processing at frequencies above 1 GHz. The specific tasks are: a. Analyze and develop

  7. GaN Device Processing

    SciTech Connect

    Pearton, S.J.; Ren, F.; Zolper, J.C.; Shul, R.J.

    1998-01-01

    Recent progress in the development of dry and wet etching techniques, implant doping and isolation, thermal processing, gate insulator technology and high reliability contacts is reviewed. Etch selectivities up to 10 for InN over AlN are possible in Inductively Coupled Plasmas using a Cl2/Ar chemistry, but in general selectivities for each binary nitride relative to each other are low ({lt} OR = 2) BECAUSE OF THE HIGH ION ENERGIES NEEDED TO INITIATE ETCHING. IMPROVED N-TYPE OHMIC CONTACT RESISTANCES ARE OBTAINED BY SELECTIVE AREA SI+ IMPLANTATION FOLLOWED BY VERY HIGH TEMPERATURE ({gt}1300 deg C) anneals in which the thermal budget is minimized and AlN encapsulation prevents GaN surface decomposition. Implant isolation is effective in GaN, AlGaN and AlInN, but marginal in InGaN. Candidate gate insulators for GaN include AlN, AlON and Ga(Gd)O(x), but interface state densities are still to high to realize state-of-the-art MIS devices.

  8. Strain engineering of atomic and electronic structures of few-monolayer-thick GaN

    NASA Astrophysics Data System (ADS)

    Kolobov, A. V.; Fons, P.; Saito, Y.; Tominaga, J.; Hyot, B.; André, B.

    2017-07-01

    Two-dimensional (2D) semiconductors possess the potential to ultimately minimize the size of devices and concomitantly drastically reduce the corresponding energy consumption. In addition, materials in their atomic-scale limit often possess properties different from their bulk counterparts paving the way to conceptually novel devices. While graphene and 2D transition-metal dichalcogenides remain the most studied materials, significant interest also exists in the fabrication of atomically thin structures from traditionally 3D semiconductors such as GaN. While in the monolayer limit GaN possesses a graphenelike structure and an indirect band gap, it was recently demonstrated that few-layer GaN acquires a Haeckelite structure in the direction of growth with an effectively direct gap. In this work, we demonstrate the possibility of strain engineering of the atomic and electronic structure of few-monolayer-thick GaN structures, which opens new avenues for their practical application in flexible nanoelectronics and nano-optoelectronics. Our simulations further suggest that due to the weak van der Waals-like interaction between a substrate and an overlayer, the use of a MoS2 substrate may be a promising route to fabricate few-monolayer Haeckelite GaN experimentally.

  9. The growth of heteroepitaxial CuInSe{sub 2} on free-standing N-polar GaN

    SciTech Connect

    Shih, Cheng-Hung; Lo, Ikai You, Shuo-Ting; Tsai, Cheng-Da; Tseng, Bae-Heng; Chen, Yun-Feng; Chen, Chiao-Hsin; Lee, Chuo-Han; Lee, Wei-I; Hsu, Gary Z. L.

    2014-12-15

    We report that chalcopyrite CuInSe{sub 2} thin films were grown on free-standing N-polar GaN (0001{sup -}) by molecular beam epitaxy. X-ray diffraction showed that the CuInSe{sub 2} thin film was grown in (112) orientation, and its peak of rocking curve with full width at half maximum of about 897.8 arc-sec indicated the epitaxial growth of CuInSe{sub 2} (112) film on N-polar GaN. Microstructure analysis of the CuInSe{sub 2 } showed that the large lattice mismatch (28.5%) between CuInSe{sub 2 } and GaN is accommodated by domain matching, and no interface reaction occurs between CuInSe{sub 2} and GaN. Our experimental results show that GaN is stable for the epitaxial growth of CuInSe{sub 2} thin film, which exhibits a promising potential for optoelectronic applications.

  10. GaN High Power Devices

    SciTech Connect

    PEARTON,S.J.; REN,F.; ZHANG,A.P.; DANG,G.; CAO,X.A.; LEE,K.P.; CHO,H.; GILA,B.P.; JOHNSON,J.W.; MONIER,C.; ABERNATHY,C.R.; HAN,JUNG; BACA,ALBERT G.; CHYI,J.-I.; LEE,C.-M.; NEE,T.-E.; CHUO,C.-C.; CHI,G.C.; CHU,S.N.G.

    2000-07-17

    A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers, GaN/AlGaN heterojunction bipolar transistors, GaN heterostructure and metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.

  11. Melting of hexagonal skyrmion states in chiral magnets

    NASA Astrophysics Data System (ADS)

    Ambrose, M. C.; Stamps, R. L.

    2013-05-01

    Skyrmions are spiral structures observed in thin films of certain magnetic materials (Uchida et al 2006 Science 311 359-61). Of the phases allowed by the crystalline symmetries of these materials (Yi et al 2009 Phys. Rev. B 80 054416), only the hexagonally packed phases (SCh) have been observed. Here the melting of the SCh phase is investigated using Monte Carlo simulations. In addition to the usual measure of skyrmion density, chiral charge, a morphological measure is considered. In doing so it is shown that the low-temperature reduction in chiral charge is associated with a change in skyrmion profiles rather than skyrmion destruction. At higher temperatures, the loss of six-fold symmetry is associated with the appearance of elongated skyrmions that disrupt the hexagonal packing.

  12. Incorporation of Mg in Free-Standing HVPE GaN Substrates

    NASA Astrophysics Data System (ADS)

    Zvanut, M. E.; Dashdorj, J.; Freitas, J. A.; Glaser, E. R.; Willoughby, W. R.; Leach, J. H.; Udwary, K.

    2016-06-01

    Mg, the only effective p-type dopant for nitrides, is well studied in thin films due to the important role of the impurity in light-emitting diodes and high-power electronics. However, there are few reports of Mg in thick free-standing GaN substrates. Here, we demonstrate successful incorporation of Mg into GaN grown by hydride vapor-phase epitaxy (HVPE) using metallic Mg as the doping source. The concentration of Mg obtained from four separate growth runs ranged between 1016 cm-3 and 1019 cm-3. Raman spectroscopy and x-ray diffraction revealed that Mg did not induce stress or perturb the crystalline quality of the HVPE GaN substrates. Photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopies were performed to investigate the types of point defects in the crystals. The near-band-edge excitonic and shallow donor-shallow acceptor radiative recombination processes involving shallow Mg acceptors were prominent in the PL spectrum of a sample doped to 3 × 1018 cm-3, while the EPR signal was also thought to represent a shallow Mg acceptor. Detection of this signal reflects minimization of nonuniform strain obtained in the thick free-standing HVPE GaN compared with heteroepitaxial thin films.

  13. Bulk ammonothermal GaN

    NASA Astrophysics Data System (ADS)

    Dwiliński, R.; Doradziński, R.; Garczyński, J.; Sierzputowski, L. P.; Puchalski, A.; Kanbara, Y.; Yagi, K.; Minakuchi, H.; Hayashi, H.

    2009-05-01

    In this work, results of structural characterization of high-quality ammonothermal GaN are presented. Besides expected low dislocation density (being of the order of 10 3 cm -2) the most interesting feature seems perfect flatness of the crystal lattice of studied crystals. Regardless the size of crystals, lattice curvature radius exceeds 100 m, whereas better crystals reveal radius of several hundred meters and the best above 1000 m. Excellent crystallinity manifests in very narrow X-ray diffraction peaks of full-width at half-maximum (FWHM) values about 16 arcsec.

  14. Epitaxial growth of GaAs and GaN by gas source molecular beam epitaxy using organic group V compounds

    NASA Astrophysics Data System (ADS)

    Okumura, H.; Yoshida, S.; Misawa, S.; Sakuma, E.

    1992-05-01

    GaAs and GaN epilayers were grown on GaAs substrates by gas source molecular beam epitaxy technique using triethylarsine (TEAs) and diethylarsine (DEAsH) as As sources, and dimethylhydrazine (DMHy) as an N source. It was found that GaAs grows layer by layer even when organic arsine molecular sources are used. Cubic GaN was found to grow epitaxially on sufficiently nitrided surfaces of GaAs (001) substrates, in contrast with the growth of hexagonal GaN on GaAs (111) surfaces. It was also found that nitridation of GaAs surfaces does not occur when DEAsH and DMHy beams are supplied onto the GaAs substrates, simultaneously. Thus, GaN/GaAs multilayers were obtained only by intermittent supply of a DEAsH beam.

  15. Influence of growth temperature and temperature ramps on deep level defect incorporation in m-plane GaN

    SciTech Connect

    Armstrong, A. M.; Kelchner, K.; Nakamura, S.; DenBaars, S. P.; Speck, J. S.

    2013-12-02

    The dependence of deep level defect incorporation in m-plane GaN films grown by metal-organic chemical vapor deposition on bulk m-plane GaN substrates as a function of growth temperature (T{sub g}) and T{sub g} ramping method was investigated using deep level optical spectroscopy. Understanding the influence of T{sub g} on GaN deep level incorporation is important for InGaN/GaN multi-quantum well (MQW) light emitting diodes (LEDs) and laser diodes (LDs) because GaN quantum barrier (QB) layers are grown much colder than thin film GaN to accommodate InGaN QW growth. Deep level spectra of low T{sub g} (800 °C) GaN films grown under QB conditions were compared to deep level spectra of high T{sub g} (1150 °C) GaN. Reducing T{sub g}, increased the defect density significantly (>50×) through introduction of emergent deep level defects at 2.09 eV and 2.9 eV below the conduction band minimum. However, optimizing growth conditions during the temperature ramp when transitioning from high to low T{sub g} substantially reduced the density of these emergent deep levels by approximately 40%. The results suggest that it is important to consider the potential for non-radiative recombination in QBs of LED or LD active regions, and tailoring the transition from high T{sub g} GaN growth to active layer growth can mitigate such non-radiative channels.

  16. Nature of exciton transitions in hexagonal boron nitride

    SciTech Connect

    Li, J.; Cao, X. K.; Lin, J. Y.; Jiang, H. X.; Hoffman, T. B.; Edgar, J. H.

    2016-03-21

    In contrast to other III-nitride semiconductors GaN and AlN, the intrinsic (or free) exciton transition in hexagonal boron nitride (h-BN) consists of rather complex fine spectral features (resolved into six sharp emission peaks) and the origin of which is still unclear. Here, the free exciton transition (FX) in h-BN bulk crystals synthesized by a solution method at atmospheric pressure has been probed by deep UV time-resolved photoluminescence (PL) spectroscopy. Based on the separations between the energy peak positions of the FX emission lines, the identical PL decay kinetics among different FX emission lines, and the known phonon modes in h-BN, we suggest that there is only one principal emission line corresponding to the direct intrinsic FX transition in h-BN, whereas all other fine features are a result of phonon-assisted transitions. The identified phonon modes are all associated with the center of the Brillouin zone. Our results offer a simple picture for the understanding of the fundamental exciton transitions in h-BN.

  17. Nature of exciton transitions in hexagonal boron nitride

    NASA Astrophysics Data System (ADS)

    Li, J.; Cao, X. K.; Hoffman, T. B.; Edgar, J. H.; Lin, J. Y.; Jiang, H. X.

    2016-03-01

    In contrast to other III-nitride semiconductors GaN and AlN, the intrinsic (or free) exciton transition in hexagonal boron nitride (h-BN) consists of rather complex fine spectral features (resolved into six sharp emission peaks) and the origin of which is still unclear. Here, the free exciton transition (FX) in h-BN bulk crystals synthesized by a solution method at atmospheric pressure has been probed by deep UV time-resolved photoluminescence (PL) spectroscopy. Based on the separations between the energy peak positions of the FX emission lines, the identical PL decay kinetics among different FX emission lines, and the known phonon modes in h-BN, we suggest that there is only one principal emission line corresponding to the direct intrinsic FX transition in h-BN, whereas all other fine features are a result of phonon-assisted transitions. The identified phonon modes are all associated with the center of the Brillouin zone. Our results offer a simple picture for the understanding of the fundamental exciton transitions in h-BN.

  18. GaN nanowire arrays with nonpolar sidewalls for vertically integrated field-effect transistors.

    PubMed

    Yu, Feng; Yao, Shengbo; Römer, Friedhard; Witzigmann, Bernd; Schimpke, Tilman; Strassburg, Martin; Bakin, Andrey; Schumacher, Hans Werner; Peiner, Erwin; Wasisto, Hutomo Suryo; Waag, Andreas

    2017-03-03

    Vertically aligned gallium nitride (GaN) nanowire (NW) arrays have attracted a lot of attention because of their potential for novel devices in the fields of optoelectronics and nanoelectronics. In this work, GaN NW arrays have been designed and fabricated by combining suitable nanomachining processes including dry and wet etching. After inductively coupled plasma dry reactive ion etching, the GaN NWs are subsequently treated in wet chemical etching using AZ400K developer (i.e., with an activation energy of 0.69 ± 0.02 eV and a Cr mask) to form hexagonal and smooth a-plane sidewalls. Etching experiments using potassium hydroxide (KOH) water solution reveal that the sidewall orientation preference depends on etchant concentration. A model concerning surface bonding configuration on crystallography facets has been proposed to understand the anisotropic wet etching mechanism. Finally, NW array-based vertical field-effect transistors with wrap-gated structure have been fabricated. A device composed of 99 NWs exhibits enhancement mode operation with a threshold voltage of 1.5 V, a superior electrostatic control, and a high current output of >10 mA, which prevail potential applications in next-generation power switches and high-temperature digital circuits.

  19. GaN nanowire arrays with nonpolar sidewalls for vertically integrated field-effect transistors

    NASA Astrophysics Data System (ADS)

    Yu, Feng; Yao, Shengbo; Römer, Friedhard; Witzigmann, Bernd; Schimpke, Tilman; Strassburg, Martin; Bakin, Andrey; Schumacher, Hans Werner; Peiner, Erwin; Suryo Wasisto, Hutomo; Waag, Andreas

    2017-03-01

    Vertically aligned gallium nitride (GaN) nanowire (NW) arrays have attracted a lot of attention because of their potential for novel devices in the fields of optoelectronics and nanoelectronics. In this work, GaN NW arrays have been designed and fabricated by combining suitable nanomachining processes including dry and wet etching. After inductively coupled plasma dry reactive ion etching, the GaN NWs are subsequently treated in wet chemical etching using AZ400K developer (i.e., with an activation energy of 0.69 ± 0.02 eV and a Cr mask) to form hexagonal and smooth a-plane sidewalls. Etching experiments using potassium hydroxide (KOH) water solution reveal that the sidewall orientation preference depends on etchant concentration. A model concerning surface bonding configuration on crystallography facets has been proposed to understand the anisotropic wet etching mechanism. Finally, NW array-based vertical field-effect transistors with wrap-gated structure have been fabricated. A device composed of 99 NWs exhibits enhancement mode operation with a threshold voltage of 1.5 V, a superior electrostatic control, and a high current output of >10 mA, which prevail potential applications in next-generation power switches and high-temperature digital circuits.

  20. Multicycle rapid thermal annealing optimization of Mg-implanted GaN: Evolution of surface, optical, and structural properties

    NASA Astrophysics Data System (ADS)

    Greenlee, Jordan D.; Feigelson, Boris N.; Anderson, Travis J.; Tadjer, Marko J.; Hite, Jennifer K.; Mastro, Michael A.; Eddy, Charles R.; Hobart, Karl D.; Kub, Francis J.

    2014-08-01

    The first step of a multi-cycle rapid thermal annealing process was systematically studied. The surface, structure, and optical properties of Mg implanted GaN thin films annealed at temperatures ranging from 900 to 1200 °C were investigated by Raman spectroscopy, photoluminescence, UV-visible spectroscopy, atomic force microscopy, and Nomarski microscopy. The GaN thin films are capped with two layers of in-situ metal organic chemical vapor deposition -grown AlN and annealed in 24 bar of N2 overpressure to avoid GaN decomposition. The crystal quality of the GaN improves with increasing annealing temperature as confirmed by UV-visible spectroscopy and the full widths at half maximums of the E2 and A1 (LO) Raman modes. The crystal quality of films annealed above 1100 °C exceeds the quality of the as-grown films. At 1200 °C, Mg is optically activated, which is determined by photoluminescence measurements. However, at 1200 °C, the GaN begins to decompose as evidenced by pit formation on the surface of the samples. Therefore, it was determined that the optimal temperature for the first step in a multi-cycle rapid thermal anneal process should be conducted at 1150 °C due to crystal quality and surface morphology considerations.

  1. Multicycle rapid thermal annealing optimization of Mg-implanted GaN: Evolution of surface, optical, and structural properties

    SciTech Connect

    Greenlee, Jordan D.; Feigelson, Boris N.; Anderson, Travis J.; Hite, Jennifer K.; Mastro, Michael A.; Eddy, Charles R.; Hobart, Karl D.; Kub, Francis J.; Tadjer, Marko J.

    2014-08-14

    The first step of a multi-cycle rapid thermal annealing process was systematically studied. The surface, structure, and optical properties of Mg implanted GaN thin films annealed at temperatures ranging from 900 to 1200 °C were investigated by Raman spectroscopy, photoluminescence, UV-visible spectroscopy, atomic force microscopy, and Nomarski microscopy. The GaN thin films are capped with two layers of in-situ metal organic chemical vapor deposition -grown AlN and annealed in 24 bar of N{sub 2} overpressure to avoid GaN decomposition. The crystal quality of the GaN improves with increasing annealing temperature as confirmed by UV-visible spectroscopy and the full widths at half maximums of the E{sub 2} and A{sub 1} (LO) Raman modes. The crystal quality of films annealed above 1100 °C exceeds the quality of the as-grown films. At 1200 °C, Mg is optically activated, which is determined by photoluminescence measurements. However, at 1200 °C, the GaN begins to decompose as evidenced by pit formation on the surface of the samples. Therefore, it was determined that the optimal temperature for the first step in a multi-cycle rapid thermal anneal process should be conducted at 1150 °C due to crystal quality and surface morphology considerations.

  2. Corrosion resistance of monolayer hexagonal boron nitride on copper

    PubMed Central

    Mahvash, F.; Eissa, S.; Bordjiba, T.; Tavares, A. C.; Szkopek, T.; Siaj, M.

    2017-01-01

    Hexagonal boron nitride (hBN) is a layered material with high thermal and chemical stability ideal for ultrathin corrosion resistant coatings. Here, we report the corrosion resistance of Cu with hBN grown by chemical vapor deposition (CVD). Cyclic voltammetry measurements reveal that hBN layers inhibit Cu corrosion and oxygen reduction. We find that CVD grown hBN reduces the Cu corrosion rate by one order of magnitude compared to bare Cu, suggesting that this ultrathin layer can be employed as an atomically thin corrosion-inhibition coating. PMID:28191822

  3. Corrosion resistance of monolayer hexagonal boron nitride on copper.

    PubMed

    Mahvash, F; Eissa, S; Bordjiba, T; Tavares, A C; Szkopek, T; Siaj, M

    2017-02-13

    Hexagonal boron nitride (hBN) is a layered material with high thermal and chemical stability ideal for ultrathin corrosion resistant coatings. Here, we report the corrosion resistance of Cu with hBN grown by chemical vapor deposition (CVD). Cyclic voltammetry measurements reveal that hBN layers inhibit Cu corrosion and oxygen reduction. We find that CVD grown hBN reduces the Cu corrosion rate by one order of magnitude compared to bare Cu, suggesting that this ultrathin layer can be employed as an atomically thin corrosion-inhibition coating.

  4. Corrosion resistance of monolayer hexagonal boron nitride on copper

    NASA Astrophysics Data System (ADS)

    Mahvash, F.; Eissa, S.; Bordjiba, T.; Tavares, A. C.; Szkopek, T.; Siaj, M.

    2017-02-01

    Hexagonal boron nitride (hBN) is a layered material with high thermal and chemical stability ideal for ultrathin corrosion resistant coatings. Here, we report the corrosion resistance of Cu with hBN grown by chemical vapor deposition (CVD). Cyclic voltammetry measurements reveal that hBN layers inhibit Cu corrosion and oxygen reduction. We find that CVD grown hBN reduces the Cu corrosion rate by one order of magnitude compared to bare Cu, suggesting that this ultrathin layer can be employed as an atomically thin corrosion-inhibition coating.

  5. Epitaxy of highly ordered organic semiconductor crystallite networks supported by hexagonal boron nitride

    PubMed Central

    Matković, Aleksandar; Genser, Jakob; Lüftner, Daniel; Kratzer, Markus; Gajić, Radoš; Puschnig, Peter; Teichert, Christian

    2016-01-01

    This study focuses on hexagonal boron nitride as an ultra-thin van der Waals dielectric substrate for the epitaxial growth of highly ordered crystalline networks of the organic semiconductor parahexaphenyl. Atomic force microscopy based morphology analysis combined with density functional theory simulations reveal their epitaxial relation. As a consequence, needle-like crystallites of parahexaphenyl grow with their long axes oriented five degrees off the hexagonal boron nitride zigzag directions. In addition, by tuning the deposition temperature and the thickness of hexagonal boron nitride, ordered networks of needle-like crystallites as long as several tens of micrometers can be obtained. A deeper understanding of the organic crystallites growth and ordering at ultra-thin van der Waals dielectric substrates will lead to grain boundary-free organic field effect devices, limited only by the intrinsic properties of the organic semiconductors. PMID:27929042

  6. Controlling morphology and optical properties of self-catalyzed, mask-free GaN rods and nanorods by metal-organic vapor phase epitaxy

    NASA Astrophysics Data System (ADS)

    Tessarek, C.; Bashouti, M.; Heilmann, M.; Dieker, C.; Knoke, I.; Spiecker, E.; Christiansen, S.

    2013-10-01

    A simple self-catalyzed and mask-free approach will be presented to grow GaN rods and nanorods based on the metal-organic vapor phase epitaxy technique. The growth parameter dependent adjustment of the morphology of the structures will be discussed. Rods and nanorods with diameters reaching from a few μm down to 100 nm, heights up to 48 μm, and densities up to 8ṡ107 cm-2 are all vertically aligned with respect to the sample surface and exhibiting a hexagonal shape with smooth sidewall facets. Optical properties of GaN nanorods were determined using cathodoluminescence. It will be shown that the optical properties can be improved just by reducing the Ga precursor flow. Furthermore, for regular hexagonal shaped rods and nanorods, whispering gallery modes with quality factors up to 500 were observed by cathodoluminescence pointing out high morphological quality of the structures. Structural investigations using transmission electron microscopy show that larger GaN nanorods (diameter > 500 nm) contain threading dislocations in the bottom part and vertical inversion domain boundaries, which separate a Ga-polar core from a N-polar shell. In contrast, small GaN nanorods (˜200 nm) are largely free of such extended defects. Finally, evidence for a self-catalyzed, Ga-induced vapor-liquid-solid growth will be discussed.

  7. XPS investigation of ion beam induced conversion of GaAs(0 0 1) surface into GaN overlayer

    NASA Astrophysics Data System (ADS)

    Kumar, Praveen; Kumar, Mahesh; Govind; Mehta, B. R.; Shivaprasad, S. M.

    2009-10-01

    For the advance of GaN based optoelectronic devices, one of the major barriers has been the high defect density in GaN thin films, due to lattice parameter and thermal expansion incompatibility with conventional substrates. Of late, efforts are focused in fine tuning epitaxial growth and in search for a low temperature method of forming low defect GaN with zincblende structure, by a method compatible to the molecular beam epitaxy process. In principle, to grow zincblende GaN the substrate should have four-fold symmetry and thus zincblende GaN has been prepared on several substrates including Si, 3C-SiC, GaP, MgO, and on GaAs(0 0 1). The iso-structure and a common shared element make the epitaxial growth of GaN on GaAs(0 0 1) feasible and useful. In this study ion-induced conversion of GaAs(0 0 1) surface into GaN at room temperature is optimized. At the outset a Ga-rich surface is formed by Ar + ion bombardment. Nitrogen ion bombardment of the Ga-rich GaAs surface is performed by using 2-4 keV energy and fluence ranging from 3 × 10 13 ions/cm 2 to 1 × 10 18 ions/cm 2. Formation of surface GaN is manifested as chemical shift. In situ core level and true secondary electron emission spectra by X-ray photoelectron spectroscopy are monitored to observe the chemical and electronic property changes. Using XPS line shape analysis by deconvolution into chemical state, we report that 3 keV N 2+ ions and 7.2 × 10 17 ions/cm 2 are the optimal energy and fluence, respectively, for the nitridation of GaAs(0 0 1) surface at room temperature. The measurement of electron emission of the interface shows the dependence of work function to the chemical composition of the interface. Depth profile study by using Ar + ion sputtering, shows that a stoichiometric GaN of 1 nm thickness forms on the surface. This, room temperature and molecular beam epitaxy compatible, method of forming GaN temperature can serve as an excellent template for growing low defect GaN epitaxial overlayers.

  8. Microstructural evolution in H ion induced splitting of freestanding GaN

    NASA Astrophysics Data System (ADS)

    Moutanabbir, O.; Scholz, R.; Senz, S.; Gösele, U.; Chicoine, M.; Schiettekatte, F.; Süßkraut, F.; Krause-Rehberg, R.

    2008-07-01

    We investigated the microstructural transformations during hydrogen ion-induced splitting of GaN thin layers. Cross-sectional transmission electron microscopy and positron annihilation spectroscopy data show that the implanted region is decorated with a high density of 1-2nm bubbles resulting from vacancy clustering during implantation. These nanobubbles persist up to 450°C. Ion channeling data show a strong dechanneling enhancement in this temperature range tentatively attributed to strain-induced lattice distortion. The dechanneling level decreases following the formation of plateletlike structures at 475°C. Extended internal surfaces develop around 550°C leading to the exfoliation of GaN thin layer.

  9. Optical properties of C-doped bulk GaN wafers grown by halide vapor phase epitaxy

    NASA Astrophysics Data System (ADS)

    Khromov, S.; Hemmingsson, C.; Monemar, B.; Hultman, L.; Pozina, G.

    2014-12-01

    Freestanding bulk C-doped GaN wafers grown by halide vapor phase epitaxy are studied by optical spectroscopy and electron microscopy. Significant changes of the near band gap (NBG) emission as well as an enhancement of yellow luminescence have been found with increasing C doping from 5 × 1016 cm-3 to 6 × 1017 cm-3. Cathodoluminescence mapping reveals hexagonal domain structures (pits) with high oxygen concentrations formed during the growth. NBG emission within the pits even at high C concentration is dominated by a rather broad line at ˜3.47 eV typical for n-type GaN. In the area without pits, quenching of the donor bound exciton (DBE) spectrum at moderate C doping levels of 1-2 × 1017 cm-3 is observed along with the appearance of two acceptor bound exciton lines typical for Mg-doped GaN. The DBE ionization due to local electric fields in compensated GaN may explain the transformation of the NBG emission.

  10. Growth of GaN epitaxial films on polycrystalline diamond by metal-organic vapor phase epitaxy

    NASA Astrophysics Data System (ADS)

    Jiang, Quanzhong; Allsopp, Duncan W. E.; Bowen, Chris R.

    2017-04-01

    Heat extraction is often essential in ensuring efficient performance of semiconductor devices and requires minimising the thermal resistance between the functional semiconductor layers and any heat sink. This paper reports the epitaxial growth of N-polar GaN films on polycrystalline diamond substrates of high thermal conductivity with metal-organic vapor phase epitaxy, by using a Si x C layer formed during deposition of polycrystalline diamond on a silicon substrate. The Si x C layer acts to provide the necessary structure ordering information for the formation of a single crystal GaN film at the wafer scale. It is shown that a three-dimensional island (3D) growth process removes hexagonal defects that are induced by the non-single crystal nature of the Si x C layer. It is also shown that intensive 3D growth and the introduction of a convex curvature of the substrate can be deployed to reduce tensile stress in the GaN epitaxy to enable the growth of a crack-free layer up to a thickness of 1.1µm. The twist and tilt can be as low as 0.65° and 0.39° respectively, values broadly comparable with GaN grown on Si substrates with a similar structure.

  11. Scalable preparation and characterization of GaN nanopowders with high crystallinity by soluble salts-assisted route

    NASA Astrophysics Data System (ADS)

    Lv, Yingying; Yu, Leshu; Ai, Wenwen; Li, Chungen

    2014-11-01

    By using Na3PO4 as a dispersant, soluble salt-assisted route has been further developed to prepare high-crystalline GaN nanoparticles powder on a large scale through the direct nitridation of Ga-Na3PO4 mixture at 750-950 °C and followed by washing with water. The systematical characterizations including XRD, Raman, IR, TEM, XPS, and PL spectrum showed that the as-prepared nanopowders were composed of nanoparticles in diameters of 8-18 nm, hexagonal phase, pure GaN, and had a broad UV centered at 388 nm and blue emissions band centered at around 547 nm. Because of the utilization of the simple reaction between metallic Ga and NH3, the preparation of pure GaN nanopowders becomes very easy, economical, and scalable, suggesting broad application in optoelectronic device material. The interesting results indicate the wide range of soluble salt-assisted route for promising industrial production of GaN nanopowders.

  12. Study of InN epitaxial films and nanorods grown on GaN template by RF-MOMBE

    PubMed Central

    2012-01-01

    This paper reports on high-quality InN materials prepared on a GaN template using radio-frequency metalorganic molecular beam epitaxy. We also discuss the structural and electro-optical properties of InN nanorods/films. The X-ray diffraction peaks of InN(0002) and InN(0004) were identified from their spectra, indicating that the (0001)-oriented hexagonal InN was epitaxially grown on the GaN template. Scanning electron microscopic images of the surface morphology revealed a two-dimensional growth at a rate of approximately 0.85 μm/h. Cross-sectional transmission electron microscopy images identified a sharp InN/GaN interface and a clear epitaxial orientation relationship of [0001]InN // [0001]GaN and ( 2¯110)InN // ( 2¯110)GaN. The optical properties of wurtzite InN nanorods were determined according to the photoluminescence, revealing a band gap of 0.77 eV. PMID:22908859

  13. Spectroscopic ellipsometry studies of GaN films deposited by reactive rf sputtering of GaAs target

    SciTech Connect

    Biswas, A.; Bhattacharyya, D.; Sahoo, N. K.; Yadav, Brajesh S.; Major, S. S.; Srinivasa, R. S.

    2008-04-15

    GaN films have been deposited by reactive rf sputtering of GaAs target in 100% nitrogen ambient on quartz substrates at different substrate temperatures ranging from room temperature to 700 deg. C. A series of films, from arsenic-rich amorphous to nearly arsenic-free polycrystalline hexagonal GaN, has been obtained. The films have been characterized by phase modulated spectroscopic ellipsometry to obtain the optical parameters, viz., fundamental band gap, refractive index, and extinction coefficient, and to understand their dependence on composition and microstructure. A generalized optical dispersion model has been used to carry out the ellipsometric analysis for amorphous and polycrystalline GaN films and the variation of the optical parameters of the films has been studied as a function of substrate temperature. The refractive index values of polycrystalline films with preferred orientation of crystallites are slightly higher (2.2) compared to those for amorphous and randomly oriented films. The dominantly amorphous GaN film shows a band gap of 3.47 eV, which decreases to 3.37 eV for the strongly c-axis oriented polycrystalline film due to the reduction in amorphous phase content with increase in substrate temperature.

  14. Low-temperature GaN growth on silicon substrates by single gas-source epitaxy and photo-excitation

    SciTech Connect

    Trivedi, R.A.; Tolle, J.; Chizmeshya, A.V.G.; Roucka, R.; Ritter, Cole; Kouvetakis, J.; Tsong, I.S.T.

    2005-08-15

    We report a unique low-temperature growth method for epitaxial GaN on Si(111) substrates via a ZrB{sub 2}(0001) buffer layer. The method utilizes the decomposition of a single gas-source precursor (D{sub 2}GaN{sub 3}){sub 3} on the substrate surface to form GaN. The film growth process is further promoted by irradiation of ultraviolet light to enhance the growth rate and ordering of the film. The best epitaxial film quality is achieved at a growth temperature of 550 deg. C with a growth rate of 3 nm/min. The films exhibit intense photoluminescence emission at 10 K with a single peak at 3.48 eV, indicative of band-edge emission for a single-phase hexagonal GaN film. The growth process achieved in this study is compatible with low Si processing temperatures and also enables direct epitaxy of GaN on ZrB{sub 2} in contrast to conventional metalorganic chemical vapor deposition based approaches.

  15. Optical properties of C-doped bulk GaN wafers grown by halide vapor phase epitaxy

    SciTech Connect

    Khromov, S.; Hemmingsson, C.; Monemar, B.; Hultman, L.; Pozina, G.

    2014-12-14

    Freestanding bulk C-doped GaN wafers grown by halide vapor phase epitaxy are studied by optical spectroscopy and electron microscopy. Significant changes of the near band gap (NBG) emission as well as an enhancement of yellow luminescence have been found with increasing C doping from 5 × 10{sup 16} cm{sup −3} to 6 × 10{sup 17} cm{sup −3}. Cathodoluminescence mapping reveals hexagonal domain structures (pits) with high oxygen concentrations formed during the growth. NBG emission within the pits even at high C concentration is dominated by a rather broad line at ∼3.47 eV typical for n-type GaN. In the area without pits, quenching of the donor bound exciton (DBE) spectrum at moderate C doping levels of 1–2 × 10{sup 17} cm{sup −3} is observed along with the appearance of two acceptor bound exciton lines typical for Mg-doped GaN. The DBE ionization due to local electric fields in compensated GaN may explain the transformation of the NBG emission.

  16. Robust Visible and Infrared Light Emitting Devices Using Rare-Earth-Doped GaN

    DTIC Science & Technology

    2006-05-31

    is encapsulated within thin barium titanate (BTO) dielectric layers for enhanced charge trapping at phosphor-dielectric interface. A high density...XRD). Fig. 3 illustrates the XRD spectra for the 4 15 min IGE and 60 min MBE GaN samples . The inserts of Fig. 3 show SEM microphotographs of both... samples grown using various 3 20 min IGE and 60 min MBE on Si substrates. Above bandgap PL was measured at room temperature under 325nm HeCd laser

  17. X-ray diffuse scattering from threading dislocations in epitaxial GaN layers

    NASA Astrophysics Data System (ADS)

    Barchuk, M.; Holý, V.; Miljević, B.; Krause, B.; Baumbach, T.; Hertkorn, J.; Scholz, F.

    2010-08-01

    In this article, we combine diffuse x-ray scattering with a Monte Carlo simulation method for the determination of the dislocation density in thin heteroepitaxial layers. As a model, we consider GaN epitaxial layers containing threading dislocations perpendicular to the surface. The densities of particular types of threading dislocations following from the comparison of measured and simulated distributions of diffusely scattered x-ray intensity are compared with the dislocation densities determined by etching. A good agreement was found.

  18. Patterned growth of InGaN/GaN quantum wells on freestanding GaN grating by molecular beam epitaxy.

    PubMed

    Wang, Yongjin; Hu, Fangren; Hane, Kazuhiro

    2011-02-04

    We report here the epitaxial growth of InGaN/GaN quantum wells on freestanding GaN gratings by molecular beam epitaxy (MBE). Various GaN gratings are defined by electron beam lithography and realized on GaN-on-silicon substrate by fast atom beam etching. Silicon substrate beneath GaN grating region is removed from the backside to form freestanding GaN gratings, and the patterned growth is subsequently performed on the prepared GaN template by MBE. The selective growth takes place with the assistance of nanoscale GaN gratings and depends on the grating period P and the grating width W. Importantly, coalescences between two side facets are realized to generate epitaxial gratings with triangular section. Thin epitaxial gratings produce the promising photoluminescence performance. This work provides a feasible way for further GaN-based integrated optics devices by a combination of GaN micromachining and epitaxial growth on a GaN-on-silicon substrate.PACS81.05.Ea; 81.65.Cf; 81.15.Hi.

  19. Electrodeposited Silver Nanoparticles Patterned Hexagonally for SERS

    SciTech Connect

    Gu, Geun Hoi; Lee, Sue Yeone; Suh, Jung Sang

    2010-08-06

    We have fabricated hexagonally patterned silver nanoparticles for surface-enhanced Raman scattering (SERS) by electrodepositing silver on the surface of an aluminum plate prepared by completely removing the oxide from anodic aluminum oxide (AAO) templates. Even after completely removing the oxide, well-ordered hexagonal patterns, similar to the shape of graphene, remained on the surface of the aluminum plate. The borders of the hexagonal pattern protruded up to form sorts of nano-mountains at both the sides and apexes of the hexagon, with the apexes protruding even more significantly than the sides. The aluminum plate prepared by completely removing the oxide has been used in the preparation of SERS substrates by sputter-coating of gold or silver on it. Instead of sputter-coating, here we have electro-deposited silver on the aluminum plate. When silver was electro-deposited on the plate, silver nanoparticles were made along the hexagonal margins.

  20. High efficiency GaN LEDs with submicron-scale 2Dperiodic structures directly fabricated by laser interference ablation

    NASA Astrophysics Data System (ADS)

    Chen, Yuanyuan; Yuan, Dajun; Yang, Muchuan; Wang, Deli; Sun, Xiaohan

    2017-05-01

    We report the investigation of directly ablating submicron-scale 2D periodic structure method on the p-layer of blue GaN light-emitting diode (LED) by laser interference. Hexagonal lattice structures on the p-layer surface of GaN LED are fabricated by three beam laser interference and the air hole radius can be changed by adjusting the laser fluence. The structure with a period of 400 nm, hole radius of 180 nm, and depth of 78 nm is patterned with the laser fluence of 215 mJ/cm2. Experimental results coincide well with the simulation, and reveal that the patterned LED get a maximum enhancement of 55.7% in light output power compared to flat LED.

  1. Magnetic tunnel junctions with monolayer hexagonal boron nitride tunnel barriers

    SciTech Connect

    Piquemal-Banci, M.; Galceran, R.; Bouzehouane, K.; Anane, A.; Petroff, F.; Fert, A.; Dlubak, B.; Seneor, P.; Caneva, S.; Martin, M.-B.; Weatherup, R. S.; Kidambi, P. R.; Robertson, J.; Hofmann, S.; Xavier, S.

    2016-03-07

    We report on the integration of atomically thin 2D insulating hexagonal boron nitride (h-BN) tunnel barriers into Co/h-BN/Fe magnetic tunnel junctions (MTJs). The h-BN monolayer is directly grown by chemical vapor deposition on Fe. The Conductive Tip Atomic Force Microscopy (CT-AFM) measurements reveal the homogeneity of the tunnel behavior of our h-BN layers. As expected for tunneling, the resistance depends exponentially on the number of h-BN layers. The h-BN monolayer properties are also characterized through integration into complete MTJ devices. A Tunnel Magnetoresistance of up to 6% is observed for a MTJ based on a single atomically thin h-BN layer.

  2. Atomic layer deposition of GaN at low temperatures

    SciTech Connect

    Ozgit, Cagla; Donmez, Inci; Alevli, Mustafa; Biyikli, Necmi

    2012-01-15

    The authors report on the self-limiting growth of GaN thin films at low temperatures. Films were deposited on Si substrates by plasma-enhanced atomic layer deposition using trimethylgallium (TMG) and ammonia (NH{sub 3}) as the group-III and -V precursors, respectively. GaN deposition rate saturated at 185 deg. C for NH{sub 3} doses starting from 90 s. Atomic layer deposition temperature window was observed from 185 to {approx}385 deg. C. Deposition rate, which is constant at {approx}0.51 A/cycle within the temperature range of 250 - 350 deg. C, increased slightly as the temperature decreased to 185 deg. C. In the bulk film, concentrations of Ga, N, and O were constant at {approx}36.6, {approx}43.9, and {approx}19.5 at. %, respectively. C was detected only at the surface and no C impurities were found in the bulk film. High oxygen concentration in films was attributed to the oxygen impurities present in group-V precursor. High-resolution transmission electron microscopy studies revealed a microstructure consisting of small crystallites dispersed in an amorphous matrix.

  3. Characterization of GaN nanowires grown on PSi, PZnO and PGaN on Si (111) substrates by thermal evaporation

    SciTech Connect

    Shekari, Leila; Hassan, Haslan Abu; Thahab, Sabah M.; Hassan, Zainuriah

    2012-06-20

    In this research, we used an easy and inexpensive method to synthesize highly crystalline GaN nanowires (NWs); on different substrates such as porous silicon (PSi), porous zinc oxide (PZnO) and porous gallium nitride (PGaN) on Si (111) wafer by thermal evaporation using commercial GaN powder without any catalyst. Micro structural studies by scanning electron microscopy and transmission electron microscope measurements reveal the role of different substrates in the morphology, nucleation and alignment of the GaN nanowires. The degree of alignment of the synthesized nanowires does not depend on the lattice mismatch between wires and their substrates. Further structural and optical characterizations were performed using high resolution X-ray diffraction and energy-dispersive X-ray spectroscopy. Results indicate that the nanowires are of single-crystal hexagonal GaN. The quality and density of grown GaN nanowires for different substrates are highly dependent on the lattice mismatch between the nanowires and their substrates and also on the size of the porosity of the substrates. Nanowires grown on PGaN have the best quality and highest density as compared to nanowires on other substrates. By using three kinds of porous substrates, we are able to study the increase in the alignment and density of the nanowires.

  4. Influence of High Nitrogen Flux on Crystal Quality of Plasma-Assisted MBE Grown GaN Layers Using Raman Spectroscopy: Part-II

    SciTech Connect

    Asghar, M.; Hussain, I.; Islah u din; Saleemi, F.

    2007-05-09

    We have investigated lattice properties of plasma assisted MBE grown hexagonal GaN layers at varying nitrogen and gallium fluxes using Raman spectroscopy. Room temperature Raman spectra of Ga-rich layers and stoichiometric GaN are similar showing excitation modes at 434 cm-1, 567 cm-1 and 729 cm-1 identified as residual laser line, E{sub 2}{sup H} and A1(LO) mode, respectively. Similarity of Ga-rich and stoichiometric GaN layers is interpreted as the indication of comparable crystal quality of both GaN layers. In contrast, Raman scattering associated with N-rich GaN samples mere exhibit a broad band of excitations in the range of 250-650cm-1 leaving out A1(LO) mode. This typical observation along with intensity distribution of the peaks, is correlated with rough surface, bad crystal quality and high concentration of defects. Based on atomic displacement scheme, the broad band is identified as Ga- vacancies.

  5. Effect of nitridation on the growth of GaN on ZrB{sub 2}(0001)/Si(111) by molecular-beam epitaxy

    SciTech Connect

    Wang, Z.-T.; Yamada-Takamura, Y.; Fujikawa, Y.; Sakurai, T.; Xue, Q. K.; Tolle, J.; Kouvetakis, J.; Tsong, I. S. T.

    2006-08-01

    The effect of nitridation on the epitaxial growth of GaN on lattice-matched ZrB{sub 2}(0001) films prepared ex situ and in situ was studied using an ultrahigh-vacuum molecular-beam epitaxy (MBE)-scanning probe microscopy system. The growth of GaN was carried out by rf-plasma-assisted MBE, and epitaxy of wurtzite GaN was observed on both ex situ and in situ prepared ZrB{sub 2} samples. The polarity was found to be consistently N-polar regardless of the samples, based on the observation of a series of N-polar Ga-rich reconstructions: (3x3) (6x6), and c(6x12). The nitridation of ZrB{sub 2} film was conducted by exposing it to active nitrogen and well-ordered hexagonal-BN (h-BN) formation was observed when the annealing temperature was above 900 deg.C. The partially formed BN layer affected neither the epitaxy nor the polarity of GaN, but when the surface was fully covered with well-ordered h-BN, GaN growth did not occur.

  6. Effect of nitridation on the growth of GaN on ZrB2(0001)/Si (111) by molecular-beam epitaxy

    NASA Astrophysics Data System (ADS)

    Wang, Zhi-Tao; Yamada-Takamura, Y.; Fujikawa, Y.; Sakurai, T.; Xue, Q. K.; Tolle, J.; Kouvetakis, J.; Tsong, I. S. T.

    2006-08-01

    The effect of nitridation on the epitaxial growth of GaN on lattice-matched ZrB2(0001) films prepared ex situ and in situ was studied using an ultrahigh-vacuum molecular-beam epitaxy (MBE)-scanning probe microscopy system. The growth of GaN was carried out by rf-plasma-assisted MBE, and epitaxy of wurtzite GaN was observed on both ex situ and in situ prepared ZrB2 samples. The polarity was found to be consistently N-polar regardless of the samples, based on the observation of a series of N-polar Ga-rich reconstructions: (3×3), (6×6), and c (6×12). The nitridation of ZrB2 film was conducted by exposing it to active nitrogen and well-ordered hexagonal-BN (h-BN) formation was observed when the annealing temperature was above 900°C. The partially formed BN layer affected neither the epitaxy nor the polarity of GaN, but when the surface was fully covered with well-ordered h-BN, GaN growth did not occur.

  7. Nanoair-bridged lateral overgrowth of GaN on ordered nanoporous GaN template

    NASA Astrophysics Data System (ADS)

    Wang, Y. D.; Zang, K. Y.; Chua, S. J.; Tripathy, S.; Chen, P.; Fonstad, C. G.

    2005-12-01

    We report the growth of high-quality GaN epilayers on an ordered nanoporous GaN template by metalorganic chemical vapor deposition. The nanopores in GaN template were created by inductively coupled plasma etching using anodic aluminum oxide film as an etch mask. The average pore diameter and interpore distance is about 65 and 110nm, respectively. Subsequent overgrowth of GaN first begins at the GaN crystallite surface between the pores, and then air-bridge-mediated lateral overgrowth leads to the formation of the continuous layer. Microphotoluminescence and micro-Raman measurements show improved optical properties and significant strain relaxation in the overgrown layer when compared to GaN layer of same thickness simultaneously grown on sapphire without any template. Similar to conventional epitaxial lateral overgrown GaN, such overgrown GaN on a nanopatterned surface would also serve as a template for the growth of ultraviolet-visible light-emitting III-nitride devices.

  8. Influence of a low-temperature GaN cap layer on the electron concentration in AlGaN/GaN heterostructure

    NASA Astrophysics Data System (ADS)

    Andreev, A. A.; Vavilova, E. A.; Ezubchenko, I. S.; Zanaveskin, M. L.; Maiboroda, I. O.

    2017-08-01

    The influence of low-temperature passivating GaN cap layers on the electrophysical parameters of a 2D electron gas (2DEG) in heterostructure high-electron mobility transistors has been studied. It has been found that thin GaN layers deposited in situ at 550°C do not exhibit polar properties and do not change the carrier concentration in the 2DEG. However, GaN layers deposited at 830°C decrease the carrier concentration in the 2DEG, which is in agreement with theoretical calculations. Using the reflected high-energy electron diffraction technique, it has been established that this effect may be associated with different structures and morphologies of GaN layers deposited at different temperatures.

  9. Carbon-assisted chemical vapor deposition of hexagonal boron nitride

    NASA Astrophysics Data System (ADS)

    Ismach, Ariel; Chou, Harry; Mende, Patrick; Dolocan, Andrei; Addou, Rafik; Aloni, Shaul; Wallace, Robert; Feenstra, Randall; Ruoff, Rodney S.; Colombo, Luigi

    2017-06-01

    We show that in a low-pressure chemical vapor deposition (CVD) system, the residual oxygen and/or air play a crucial role in the mechanism of the growth of hexagonal boron nitride (h-BN) films on Ni foil ‘enclosures’. Hexagonal-BN films grow on the Ni foil surface via the formation of an intermediate boric-oxide (BO x ) phase followed by a thermal reduction of the BO x by a carbon source (either amorphous carbon powder or methane), leading to the formation of single- and bi-layer h-BN. Low energy electron microscopy (LEEM) and diffraction (LEED) were used to map the number of layers over large areas; Raman spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS), x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were used to characterize the structure and physical quality of the ultra-thin h-BN film. The growth procedure reported here leads to a better understanding and control of the synthesis of ultra-thin h-BN films.

  10. Bronze-mean hexagonal quasicrystal.

    PubMed

    Dotera, Tomonari; Bekku, Shinichi; Ziherl, Primož

    2017-10-01

    The most striking feature of conventional quasicrystals is their non-traditional symmetry characterized by icosahedral, dodecagonal, decagonal or octagonal axes. The symmetry and the aperiodicity of these materials stem from an irrational ratio of two or more length scales controlling their structure, the best-known examples being the Penrose and the Ammann-Beenker tiling as two-dimensional models related to the golden and the silver mean, respectively. Surprisingly, no other metallic-mean tilings have been discovered so far. Here we propose a self-similar bronze-mean hexagonal pattern, which may be viewed as a projection of a higher-dimensional periodic lattice with a Koch-like snowflake projection window. We use numerical simulations to demonstrate that a disordered variant of this quasicrystal can be materialized in soft polymeric colloidal particles with a core-shell architecture. Moreover, by varying the geometry of the pattern we generate a continuous sequence of structures, which provide an alternative interpretation of quasicrystalline approximants observed in several metal-silicon alloys.

  11. Bronze-mean hexagonal quasicrystal

    NASA Astrophysics Data System (ADS)

    Dotera, Tomonari; Bekku, Shinichi; Ziherl, Primož

    2017-10-01

    The most striking feature of conventional quasicrystals is their non-traditional symmetry characterized by icosahedral, dodecagonal, decagonal or octagonal axes. The symmetry and the aperiodicity of these materials stem from an irrational ratio of two or more length scales controlling their structure, the best-known examples being the Penrose and the Ammann-Beenker tiling as two-dimensional models related to the golden and the silver mean, respectively. Surprisingly, no other metallic-mean tilings have been discovered so far. Here we propose a self-similar bronze-mean hexagonal pattern, which may be viewed as a projection of a higher-dimensional periodic lattice with a Koch-like snowflake projection window. We use numerical simulations to demonstrate that a disordered variant of this quasicrystal can be materialized in soft polymeric colloidal particles with a core-shell architecture. Moreover, by varying the geometry of the pattern we generate a continuous sequence of structures, which provide an alternative interpretation of quasicrystalline approximants observed in several metal-silicon alloys.

  12. Microstructure of heteroepitaxial GaN grown on mesa-patterned 4H-SiC substrates

    NASA Astrophysics Data System (ADS)

    Bassim, N. D.; Twigg, M. E.; Eddy, C. R.; Henry, R. L.; Holm, R. T.; Culbertson, J. C.; Stahlbush, R. E.; Neudeck, P. G.; Trunek, A. J.; Powell, J. A.

    2004-06-01

    Cross-sectional transmission electron microscopy and atomic force microscopy have been used to study the microstructure of a thin heteroeptiaxial GaN film grown on (0001) 4H-SiC mesa surfaces with and without atomic scale steps. Analysis of a mesa that was completely free of atomic-scale surface steps prior to III -N film deposition showed that these GaN layers had a wide variation in island height (1-3μm ) and included the presence of pit-like defects on the film surface. This sample had a low dislocation density (5×108/cm2) as compared to conventionally grown samples on unpatterned (0001) on-axis 4H-SiC (2×109/cm2), coupled with a 3-5 times increase in grain size. A comparison of a GaN film on the step-free 4H-SiC mesa region with a GaN film on a stepped 4H-SiC mesa region on the same substrate showed that the presence of surface steps reduced the overall grain size of the film from 7-10μm to a grain size of about 2-3μm. Since the GaN films grow via a Volmer-Weber mechanism, a decrease in the number of heterogeneous nucleation sites may allow the growth of large GaN islands before coalescence, thus reducing the number of threading dislocations. These results are promising for the further development of unique, low-dislocation density active regions for GaN device structures on 4H-SiC.

  13. Spontaneous emission enhancement in micropatterned GaN

    NASA Astrophysics Data System (ADS)

    Niehus, M.; Sanguino, P.; Monteiro, T.; Soares, M. J.; Schwarz, R.

    2004-10-01

    With two interfering pulses from the fourth harmonic of a Nd-YAG laser we burnt a periodic lattice structure into the surface of GaN thin films. The lattice period of this permanent grating could be controlled between less than one and several tens of microns. Above the decomposition threshold, nitrogen evades from the sample surface, and the residual metallic gallium accumulates in the form of tiny droplets at the surfaces. The patterned structure shows structural similarities with microcavities. The question arises if the residual metallic gallium may act as a partially reflecting mirror. To test this hypothesis, we studied the steady-state and transient photoluminescence through the modulation of light emerging from the ubiquitous broad "yellow" photoluminescence band. The microlattice is evidenced by energy-equidistant spontaneous emission enhancement peaks in the steady-state photoluminescence spectra. We suggest that the partial reflection due to the residual metallic gallium leads to the observed enhancement effect.

  14. Rode's iterative calculation of surface optical phonon scattering limited electron mobility in N-polar GaN devices

    SciTech Connect

    Ghosh, Krishnendu Singisetti, Uttam

    2015-02-14

    N-polar GaN channel mobility is important for high frequency device applications. Here, we report theoretical calculations on the surface optical (SO) phonon scattering rate of two-dimensional electron gas (2DEG) in N-polar GaN quantum well channels with high-k dielectrics. Rode's iterative calculation is used to predict the scattering rate and mobility. Coupling of the GaN plasmon modes with the SO modes is taken into account and dynamic screening is employed under linear polarization response. The effect of SO phonons on 2DEG mobility was found to be small at >5 nm channel thickness. However, the SO mobility in 3 nm N-polar GaN channels with HfO{sub 2} and ZrO{sub 2} high-k dielectrics is low and limits the total mobility. The SO scattering for SiN dielectric on GaN was found to be negligible due to its high SO phonon energy. Using Al{sub 2}O{sub 3}, the SO phonon scattering does not affect mobility significantly only except the case when the channel is too thin with a low 2DEG density.

  15. Ultrahigh Si+ implant activation efficiency in GaN using a high-temperature rapid thermal process system

    NASA Astrophysics Data System (ADS)

    Cao, X. A.; Abernathy, C. R.; Singh, R. K.; Pearton, S. J.; Fu, M.; Sarvepalli, V.; Sekhar, J. A.; Zolper, J. C.; Rieger, D. J.; Han, J.; Drummond, T. J.; Shul, R. J.; Wilson, R. G.

    1998-07-01

    Si+ implant activation efficiencies above 90%, even at doses of 5×1015cm-2, have been achieved in GaN by rapid thermal processing at 1400-1500 °C for 10 s. The annealing system utilizes molybdenum intermetallic heating elements capable of operation up to 1900 °C, producing high heating and cooling rates (up to 100 °C s-1). Unencapsulated GaN shows severe surface pitting at 1300 °C and complete loss of the film by evaporation at 1400 °C. Dissociation of nitrogen from the surface is found to occur with an approximate activation energy of 3.8 eV for GaN (compared to 4.4 eV for AlN and 3.4 eV for InN). Encapsulation with either rf magnetron reactively sputtered or metal organic molecular beam epitaxy-grown AlN thin films provides protection against GaN surface degradation up to 1400 °C, where peak electron concentrations of ˜5×1020cm-3 can be achieved in Si-implanted GaN. Secondary ion mass spectrometry profiling showed little measurable redistribution of Si, suggesting DSi⩽10-13 cm2 s-1 at 1400 °C. The implant activation efficiency decreases at higher temperatures, which may result from SiGa to SiN site switching and resultant self-compensation.

  16. New approaches for calculating absolute surface energies of wurtzite (0001)/(000 1 ¯ ): A study of ZnO and GaN

    NASA Astrophysics Data System (ADS)

    Zhang, Jingzhao; Zhang, Yiou; Tse, Kinfai; Deng, Bei; Xu, Hu; Zhu, Junyi

    2016-05-01

    The accurate absolute surface energies of (0001)/(000 1 ¯ ) surfaces of wurtzite structures are crucial in determining the thin film growth mode of important energy materials. However, the surface energies still remain to be solved due to the intrinsic difficulty of calculating the dangling bond energy of asymmetrically bonded surface atoms. In this study, we used a pseudo-hydrogen passivation method to estimate the dangling bond energy and calculate the polar surfaces of ZnO and GaN. The calculations were based on the pseudo chemical potentials obtained from a set of tetrahedral clusters or simple pseudo-molecules, using density functional theory approaches. The surface energies of (0001)/(000 1 ¯ ) surfaces of wurtzite ZnO and GaN that we obtained showed relatively high self-consistencies. A wedge structure calculation with a new bottom surface passivation scheme of group-I and group-VII elements was also proposed and performed to show converged absolute surface energy of wurtzite ZnO polar surfaces, and these results were also compared with the above method. The calculated results generally show that the surface energies of GaN are higher than those of ZnO, suggesting that ZnO tends to wet the GaN substrate, while GaN is unlikely to wet ZnO. Therefore, it will be challenging to grow high quality GaN thin films on ZnO substrates; however, high quality ZnO thin film on GaN substrate would be possible. These calculations and comparisons may provide important insights into crystal growth of the above materials, thereby leading to significant performance enhancements in semiconductor devices.

  17. Triclinic deformation of InGaN layers grown on vicinal surface of GaN (00.1) substrates

    NASA Astrophysics Data System (ADS)

    Krysko, M.; Domagala, J. Z.; Czernecki, R.; Leszczynski, M.

    2013-09-01

    We report on a triclinic unit cell deformation of fully strained InGaN layers grown on vicinal GaN (00.1) substrates. The samples were examined using the high-resolution X-ray diffraction (HR XRD) using a set of asymmetrical reflections and one symmetrical reflection of 00.2. The substrate miscut induced triclinic deformation of the layer unit cells, breaking the hexagonal symmetry. The experimental results are compared with predictions of the theory of elasticity. We formulate equations for unit cell parameters of layers grown on substrates cut in any direction, based on the equations given by Romanov et al. [J. Appl. Phys. 100, 023522 (2006)]. Additionally, the paper provides a recipe of the XRD measurements necessary to establish unit cell parameters (useful for composition determination of ternary compounds) of the hexagonal mismatched layers grown on off-axis substrates.

  18. Mushroom structure of GaN template for epitaxial growth of GaN

    NASA Astrophysics Data System (ADS)

    Lee, Sung Bo; Kwon, Tae-Wan; Park, Jungwon; Jin Choi, Won; Sung Park, Hae

    2012-07-01

    In the present study, we show the formation of mushroom morphology produced by a ramp anneal of a low-temperature GaN buffer layer. Structural analysis by transmission electron microscopy indicates that the cap of the mushroom has the stable wurtzitic GaN structure, whereas the stem possesses the metastable zinc-blende structure. With the air gap introduced between the substrate and the cap of the mushroom structure, threading dislocations propagate along its stem. The formation of the mushroom morphology is suggested to result from the nucleation of wurtzitic GaN on the surface of the low-temperature buffer layer during the ramp anneal, followed by mass transport of GaN from the buffer layer, which remains zinc-blende during the anneal, to the surface, because wurtzitic GaN has the lower structure energy than zinc-blende GaN. This study extends limits of the conventional use of the buffer layer, laying the foundation for the development of low-cost recipes for achieving GaN templates with a low density of threading dislocations.

  19. Ultra-thin semiconductor membrane nanotechnology based on surface charge lithography

    NASA Astrophysics Data System (ADS)

    Tiginyanu, Ion; Popa, Veaceslav; Stevens-Kalceff, Marion A.

    2011-05-01

    We show that by subjecting GaN epilayers on sapphire substrates to low-energy/low-dose ion treatment with subsequent photoelectrochemical etching it is possible to fabricate ultra-thin GaN membranes in the form of nano-roof hanging over networks of whiskers representing threading dislocations. The suspended membranes prove to be transparent to both UV-radiation and keV-energy electrons, their architecture being dependent upon the stirring conditions of the electrolyte during electrochemical etching. The obtained results are indicative of electrical conductivity, flexibility and excellent mechanical stability of ultra-thin GaN membranes characterized by prevailing yellow cathodoluminescence.

  20. New hexagonal structure for silicon atoms

    NASA Astrophysics Data System (ADS)

    Naji, S.; Belhaj, A.; Labrim, H.; Benyoussef, A.; El Kenz, A.

    2012-11-01

    Motivated by recent experimental and theoretical works on silicene and its derived materials and based on the exceptional Lie algebra G2 we propose a new hexagonal symmetry producing the (√3 × √3)R30° superstructure for silicon atoms. The principal hexagonal unit cell contains twelve atoms instead of the usual structure involving only six ones and it is associated with the G2 root system. In this silicon atom configuration appears two hexagons of unequal side length at angle 30°. This atomic structure can be tessellated to exhibit two superstructures (1 × 1) and (√3 × √3)R30° on the same atomic sheet. To test this double hexagonal structure, we perform a numerical study using Ab-initio calculations based on FPLO9.00-34 code. We observe that the usual silicon electronic properties and the lattice parameters of planar geometry are modified. In particular, the corresponding material becomes a conductor rather than zero gaped semi-conductor arising in single hexagonal structure. Although the calculation is done for silicon atoms, we expect that this structure could be adapted to all two dimensional materials having a single hexagonal flat geometry.

  1. Ultra-high implant activation efficiency in GaN using novel high temperature RTP system

    SciTech Connect

    Cao, X.A.; Abernathy, C.R.; Singh, R.K.

    1998-04-01

    Si{sup +} implant activation efficiencies above 90%, even at doses of 5 {times} 10{sup 15} cm{sup {minus}2}, have been achieved in GaN by RTP at 1,400--1,500 C for 10 secs. The annealing system utilizes with MoSi{sub 2} heating elements capable of operation up to 1,900 C, producing high heating and cooling rates (up to 100 C{center_dot}s{sup {minus}1}). Unencapsulated GaN show severe surface pitting at 1,300 C, and complete loss of the film by evaporation at 1,400 C. Dissociation of nitrogen from the surface is found to occur with an approximate activation energy of 3.8 eV for GaN (compared to 4.4 eV for AlN and 3.4 eV for InN). Encapsulation with either rf-magnetron reactively sputtered or MOMBE-grown AlN thin films provide protection against GaN surface degradation up to 1,400 C, where peak electron concentrations of {approximately} 5 {times} 10{sup 20} cm{sup {minus}3} can be achieved in Si-implanted GaN. SIMS profiling showed little measurable redistribution of Si, suggesting D{sub Si} {le} 10{sup {minus}13} cm{sup 2}{center_dot}s{sup {minus}1} at 1,400 C . The implant activation efficiency decreases at higher temperatures, which may result from Si{sub Ga} to Si{sub N} site switching and resultant self-compensation.

  2. Theoretical study of gallium nitride molecules, GaN2 and GaN4.

    PubMed

    Tzeli, Demeter; Theodorakopoulos, Giannoula; Petsalakis, Ioannis D

    2008-09-18

    The electronic and geometric structures of gallium dinitride GaN 2, and gallium tetranitride molecules, GaN 4, were systematically studied by employing density functional theory and perturbation theory (MP2, MP4) in conjunction with the aug-cc-pVTZ basis set. In addition, for the ground-state of GaN 4( (2)B 1) a density functional theory study was carried out combining different functionals with different basis sets. A total of 7 minima have been identified for GaN 2, while 37 structures were identified for GaN 4 corresponding to minima, transition states, and saddle points. We report geometries and dissociation energies for all the above structures as well as potential energy profiles, potential energy surfaces and bonding mechanisms for some low-lying electronic states of GaN 4. The dissociation energy of the ground-state GaN 2 ( X (2)Pi) is 1.1 kcal/mol with respect to Ga( (2)P) + N 2( X (1)Sigma g (+)). The ground-state and the first two excited minima of GaN 4 are of (2)B 1( C 2 v ), (2)A 1( C 2 v , five member ring), and (4)Sigma g (-)( D infinityh ) symmetry, respectively. The dissociation energy ( D e) of the ground-state of GaN 4, X (2)B 1, with respect to Ga( (2)P) + 2 N 2( X (1)Sigma g (+)), is 2.4 kcal/mol, whereas the D e of (4)Sigma g (-) with respect to Ga( (4)P) + 2 N 2( X (1)Sigma g (+)) is 17.6 kcal/mol.

  3. GaN on Silicon Substrate with AlN Buffer Layer for UV Photodiode

    NASA Astrophysics Data System (ADS)

    Chuah, L. S.; Thahab, S. M.; Hassan, Z.

    Nitrogen plasma-assisted molecular beam epitaxy (PAMBE) deposited GaN thin films on (111) n-type silicon substrate with different thickness AlN buffer layers are investigated and distinguished by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and Raman scattering. The thickness of AlN buffer layer ranged from 200 nm to 300 nm. Besides that, the electrical characteristics of the GaN thin film for ultraviolet detecting utilizations are studied by calculating the photo current/dark current ratio on a metal-semiconductor-metal (MSM) photodiode with and without the illumination of Hg-lamp source. The devices have been tested over room temperature (RT). The photocurrent analysis, together with the study of Schottky barrier height (SBH) development, ascertain that the principal mechanism of photo transport is thermionic emission. The photocurrent value is rigorously dependent on Schottky barrier height. The GaN/AlN(200 nm)/n-Si MSM photodiode produces the highest photo/dark current ratio for the lowest strain that consists of the GaN film grown on the AlN (200 nm) buffer layer.

  4. Polarity in GaN and ZnO: Theory, measurement, growth, and devices

    NASA Astrophysics Data System (ADS)

    Zúñiga-Pérez, Jesús; Consonni, Vincent; Lymperakis, Liverios; Kong, Xiang; Trampert, Achim; Fernández-Garrido, Sergio; Brandt, Oliver; Renevier, Hubert; Keller, Stacia; Hestroffer, Karine; Wagner, Markus R.; Reparaz, Juan Sebastián; Akyol, Fatih; Rajan, Siddharth; Rennesson, Stéphanie; Palacios, Tomás; Feuillet, Guy

    2016-12-01

    The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade.

  5. Epitaxial integration of (0001) BiFeO{sub 3} with (0001) GaN

    SciTech Connect

    Tian, W.; Vaithyanathan, V.; Schlom, D. G.; Zhan, Q.; Yang, S. Y.; Chu, Y. H.; Ramesh, R.

    2007-04-23

    Epitaxial growth of (0001)-oriented BiFeO{sub 3} thin films on the (0001) surface of GaN has been realized using intervening epitaxial (111) SrTiO{sub 3}/(100) TiO{sub 2} buffer layers. The epitaxial BiFeO{sub 3} thin films have two in-plane orientations: [1120]BiFeO{sub 3}(parallel sign)[1120]GaN plus a twin variant related by a 180 deg. in-plane rotation. BiFeO{sub 3} shows an out-of-plane remanent polarization of {approx}90 {mu}C/cm{sup 2}, which is comparable to the remanent polarization of BiFeO{sub 3} prepared on (111) SrTiO{sub 3} single crystal substrates. The orientation of BiFeO{sub 3} realized on GaN provides the maximal out-of-plane polarization of BiFeO{sub 3}, which is equivalent to a surface charge of 5x10{sup 14} electrons/cm{sup 2}.

  6. High-luminous efficacy white light-emitting diodes with thin-film flip-chip technology and surface roughening scheme

    NASA Astrophysics Data System (ADS)

    Hu, Xiao-Long; Zhang, Jing; Wang, Hong; Zhang, Xi-Chun

    2016-11-01

    High-luminous efficacy white light-emitting diodes (LEDs) were realized by using GaN-based thin-film (TF) flip-chip (FC) LEDs with phosphor-silicone encapsulation. The TFFC-LEDs were fabricated by electrode isolation, FC configuration, copper electroplating, and laser lift-off (LLO) techniques. During the fabrication process, the high-defect undoped GaN layer was eliminated by inductively coupled plasma (ICP) etching to lower the absorption loss. Then, the exposed N-face n-GaN surface formed after the ICP etching was systematically studied through control of the temperature, time and concentration of the KOH solution to acquire hexagonal cones with high extraction efficiency. It is found that the external quantum efficiency was improved by a maximum value of 169% for the TFFC-LEDs with optimized surface hexagonal cones compared to TFFC-LEDs with flat surfaces. To further improve the output power, the chip size and n-contact via holes of the TFFC-LEDs were increased. A maximum luminous efficacy of 139 lm W-1 was realized for white LEDs (5700 K, 350 mA, 2.98 V) using these TFFC-LEDs with phosphor-silicone encapsulation. In addition, these white LEDs also have a lower junction temperature of 87 °C even at 700 mA. These results indicate that the proposed TFFC-LEDs are promising for use in automotive and solid-state lighting applications.

  7. Insulating gallium oxide layer produced by thermal oxidation of gallium-polar GaN: Insulating gallium oxide layer produced by thermal oxidation of gallium-polar GaN

    SciTech Connect

    Hossain, T.; Wei, D.; Nepal, N.; Garces, N. Y.; Hite, J. K.; Meyer, H. M.; Eddy, C. R.; Baker, Troy; Mayo, Ashley; Schmitt, Jason; Edgar, J. H.

    2014-02-24

    We report the benefits of dry oxidation of n -GaN for the fabrication of metal-oxide-semiconductor structures. GaN thin films grown on sapphire by MOCVD were thermally oxidized for 30, 45 and 60 minutes in a pure oxygen atmosphere at 850 °C to produce thin, smooth GaOx layers. Moreover, the GaN sample oxidized for 30 minutes had the best properties. Its surface roughness (0.595 nm) as measured by atomic force microscopy (AFM) was the lowest. Capacitance-voltage measurements showed it had the best saturation in accumulation region and the sharpest transition from accumulation to depletion regions. Under gate voltage sweep, capacitance-voltage hysteresis was completely absent. The interface trap density was minimum (Dit = 2.75×1010 cm–2eV–1) for sample oxidized for 30 mins. These results demonstrate a high quality GaOx layer is beneficial for GaN MOSFETs.

  8. Growth of self-standing GaN substrates

    NASA Astrophysics Data System (ADS)

    Lee, Hyun-Jae; Fujii, Katsushi; Goto, Takenari; Kim, Chinkyo; Chang, Jiho; Hong, Soon-Ku; Cho, Meoungwhan; Yao, Takafumi

    2010-03-01

    Large-sized and high-quality free standing GaN are required with the development of GaN-based devices. We have developed new techniques to reduce the price of GaN substrates. In this paper, we introduce a simple fabrication way of freestanding GaN substrate using hydride vapor phase epitaxy (HVPE). An evaporable buffer layer was applied for the fabrication of 2inch freestanding GaN to separate from a sapphire substrate, in other words, a freestanding GaN was fabricated only by HVPE (one-stop process) without any process.

  9. Resonantly enhanced selective photochemical etching of GaN

    NASA Astrophysics Data System (ADS)

    Trichas, E.; Kayambaki, M.; Iliopoulos, E.; Pelekanos, N. T.; Savvidis, P. G.

    2009-04-01

    Wavelength dependent photochemical etching of GaN films reveals a strong resonant enhancement of the photocurrent at the GaN gap, in close agreement with the excitonic absorption profile of GaN. The corresponding etching rate of GaN strongly correlates with the measured photocurrent. No photocurrent, nor etching is observed for AlGaN films under same excitation conditions. The method could pave the way to the development of truly selective etching of GaN on AlGaN for the fabrication of nitride based optoelectronic devices.

  10. Surfactant assisted growth of MgO films on GaN

    SciTech Connect

    Paisley, E. A.; Shelton, T. C.; Collazo, R.; Sitar, Z.; Maria, J.-P.; Christen, H. M.; Biegalski, M. D.; Mita, S.

    2012-08-27

    Thin epitaxial films of <111> oriented MgO on [0001]-oriented GaN were grown by molecular beam epitaxy and pulsed laser deposition using the assistance of a vapor phase surfactant. In both cases, surfactant incorporation enabled layer-by-layer growth and a smooth terminal surface by stabilizing the {l_brace}111{r_brace} rocksalt facet. Metal-insulator-semiconductor capacitor structures were fabricated on n-type GaN. A comparison of leakage current density for conventional and surfactant-assisted growth reveals a nearly 100 Multiplication-Sign reduction in leakage current density for the surfactant-assisted samples. These data verify numerous predictions regarding the role of H-termination in regulating the habit of rocksalt crystals.

  11. High-quality GaN nanowires grown on Si and porous silicon by thermal evaporation

    NASA Astrophysics Data System (ADS)

    Shekari, L.; Ramizy, A.; Omar, K.; Hassan, H. Abu; Hassan, Z.

    2012-12-01

    Nanowires (NWs) of GaN thin films were prepared on as-grown Si (1 1 1) and porous silicon (PS) substrates using thermal evaporation method. The film growth produced high-quality wurtzite GaN NWs. The size, morphology, and nanostructures of the crystals were investigated through scanning electron microscopy, high-resolution X-ray diffraction and photoluminescence spectroscopy. The NWs grown on porous silicon were thinner, longer and denser compared with those on as-grown Si. The energy band gap of the NWs grown on PS was larger than that of NWs on as-grown Si. This is due to the greater quantum confinement effects of the crystalline structure of the NWs grown on PS.

  12. Photoelectrochemical liftoff of LEDs grown on freestanding c-plane GaN substrates.

    PubMed

    Hwang, David; Yonkee, Benjamin P; Addin, Burhan Saif; Farrell, Robert M; Nakamura, Shuji; Speck, James S; DenBaars, Steven

    2016-10-03

    We demonstrate a thin-film flip-chip (TFFC) process for LEDs grown on freestanding c-plane GaN substrates. LEDs are transferred from a bulk GaN substrate to a sapphire submount via a photoelectrochemical (PEC) undercut etch. This PEC liftoff method allows for substrate reuse and exposes the N-face of the LEDs for additional roughening. The LEDs emitted at a wavelength of 432 nm with a turn on voltage of ~3 V. Etching the LEDs in heated KOH after transferring them to a sapphire submount increased the peak external quantum efficiency (EQE) by 42.5% from 9.9% (unintentionally roughened) to 14.1% (intentionally roughened).

  13. Multilayer porous structures of HVPE and MOCVD grown GaN for photonic applications

    NASA Astrophysics Data System (ADS)

    Braniste, T.; Ciers, Joachim; Monaico, Ed.; Martin, D.; Carlin, J.-F.; Ursaki, V. V.; Sergentu, V. V.; Tiginyanu, I. M.; Grandjean, N.

    2017-02-01

    In this paper we report on a comparative study of electrochemical processes for the preparation of multilayer porous structures in hydride vapor phase epitaxy (HVPE) and metal organic chemical vapor phase deposition (MOCVD) grown GaN. It was found that in HVPE-grown GaN, multilayer porous structures are obtained due to self-organization processes leading to a fine modulation of doping during the crystal growth. However, these processes are not totally under control. Multilayer porous structures with a controlled design have been produced by optimizing the technological process of electrochemical etching in MOCVD-grown samples, consisting of five pairs of thin layers with alternating-doping profiles. The samples have been characterized by SEM imaging, photoluminescence spectroscopy, and micro-reflectivity measurements, accompanied by transfer matrix analysis and simulations by a method developed for the calculation of optical reflection spectra. We demonstrate the applicability of the produced structures for the design of Bragg reflectors.

  14. Germanium doping of GaN by metalorganic chemical vapor deposition for polarization screening applications

    NASA Astrophysics Data System (ADS)

    Young, N. G.; Farrell, R. M.; Iza, M.; Nakamura, S.; DenBaars, S. P.; Weisbuch, C.; Speck, J. S.

    2016-12-01

    We demonstrate n-type doping of GaN with Ge by MOCVD at high concentrations that are necessary to fully screen the polarization fields in c-plane InGaN/GaN quantum wells. Hall measurements show linear Ge incorporation with dopant flow rate and carrier concentrations exceeding 1×1020 cm-3. GaN:Ge layers exhibit excellent electron mobility, high conductivity, and contact resistivity comparable to the best unannealed contacts to Si-doped GaN. However, the surface morphology begins to degrade with Ge concentrations above 1×1019 cm-3, resulting in severe step bunching and a network of plateaus and trenches, even in layers as thin as 10 nm.

  15. Self-assembled growth and structural analysis of inclined GaN nanorods on nanoimprinted m-sapphire using catalyst-free metal-organic chemical vapor deposition

    NASA Astrophysics Data System (ADS)

    Lee, Kyuseung; Chae, Sooryong; Jang, Jongjin; Min, Daehong; Kim, Jaehwan; Nam, Okhyun

    2016-04-01

    In this study, self-assembled inclined (1-10-3)-oriented GaN nanorods (NRs) were grown on nanoimprinted (10-10) m-sapphire substrates using catalyst-free metal-organic chemical vapor deposition. According to X-ray phi-scans, the inclined GaN NRs were tilted at an angle of ˜57.5° to the [10-10]sapp direction. Specifically, the GaN NRs grew in a single inclined direction to the [11-20]sapp. Uni-directionally inclined NRs were formed through the one-sided (10-11)-faceted growth of the interfacial a-GaN plane layer. It was confirmed that a thin layer of a-GaN was formed on r-facet nanogrooves of the m-sapphire substrate by nitridation. The interfacial a-GaN nucleation affected both the inclined angle and the growth direction of the inclined GaN NRs. Using X-ray diffraction and selective area electron diffraction, the epitaxial relationship between the inclined (1-10-3) GaN NRs and interfacial a-GaN layer on m-sapphire substrates was systematically investigated. Moreover, the inclined GaN NRs were observed to be mostly free of stacking fault-related defects using high-resolution transmission electron microscopy.

  16. Self-assembled growth and structural analysis of inclined GaN nanorods on nanoimprinted m-sapphire using catalyst-free metal-organic chemical vapor deposition

    SciTech Connect

    Lee, Kyuseung; Chae, Sooryong; Jang, Jongjin; Min, Daehong; Kim, Jaehwan; Nam, Okhyun

    2016-04-15

    In this study, self-assembled inclined (1-10-3)-oriented GaN nanorods (NRs) were grown on nanoimprinted (10-10) m-sapphire substrates using catalyst-free metal-organic chemical vapor deposition. According to X-ray phi-scans, the inclined GaN NRs were tilted at an angle of ∼57.5° to the [10-10]{sub sapp} direction. Specifically, the GaN NRs grew in a single inclined direction to the [11-20]{sub sapp}. Uni-directionally inclined NRs were formed through the one-sided (10-11)-faceted growth of the interfacial a-GaN plane layer. It was confirmed that a thin layer of a-GaN was formed on r-facet nanogrooves of the m-sapphire substrate by nitridation. The interfacial a-GaN nucleation affected both the inclined angle and the growth direction of the inclined GaN NRs. Using X-ray diffraction and selective area electron diffraction, the epitaxial relationship between the inclined (1-10-3) GaN NRs and interfacial a-GaN layer on m-sapphire substrates was systematically investigated. Moreover, the inclined GaN NRs were observed to be mostly free of stacking fault-related defects using high-resolution transmission electron microscopy.

  17. Heteroepitaxial growth of GaN on atomically flat LiTaO 3 (0 0 0 1) using low-temperature AIN buffer layers

    NASA Astrophysics Data System (ADS)

    Tsuchiya, Y.; Kobayashi, A.; Ohta, J.; Fujioka, H.; Oshima, M.

    2006-07-01

    We have grown GaN films on atomically-flat LiTaO 3 substrates by using pulsed laser deposition (PLD), and we then investigated the effect of the use of low-temperature AlN (LT-AlN) buffer layers on the structural properties of GaN. The full-width at half-maximum (FWHM) values for the crystal orientation distribution of the GaN films in the tilt directions were reduced from 0.48° to 0.17°, and those in the twist directions were reduced from 0.40° to 0.17° by the incorporation of AlN buffer layers grown at 580 °C. The surface morphology of GaN has also been improved by the insertion of LT-AlN buffer layers. X-ray reflectivity measurements have revealed that the interfacial layer thickness between LT-AlN and LiTaO 3 is as thin as 1.7 nm, and that the increase in the interfacial layer thickness caused by annealing at up to 700 °C is quite small. These results indicate that the PLD growth of GaN on atomically flat substrates using LT-AlN buffer layers is quite promising for achieving GaN on LiTaO 3.

  18. Interfacial chemistry and valence band offset between GaN and Al{sub 2}O{sub 3} studied by X-ray photoelectron spectroscopy

    SciTech Connect

    Duan, T. L.; Ang, D. S.; Pan, J. S.

    2013-05-20

    The interface region between Ga-face n-type GaN and Al{sub 2}O{sub 3} dielectric (achieved via atomic-layer deposition or ALD) is investigated by X-ray photoelectron spectroscopy (XPS). An increase in the Ga-O to Ga-N bond intensity ratio following Al{sub 2}O{sub 3} deposition implies that the growth of an interfacial gallium sub-oxide (GaO{sub x}) layer occurred during the ALD process. This finding may be ascribed to GaN oxidation, which may still happen following the reduction of a thin native GaO{sub x} by trimethylaluminum (TMA) in the initial TMA-only cycles. The valence band offset between GaN and Al{sub 2}O{sub 3}, obtained using both core-level and valence band spectra, is found to vary with the thickness of the deposited Al{sub 2}O{sub 3}. This observation may be explained by an upward energy band bending at the GaN surface (due to the spontaneous polarization induced negative bound charge on the Ga-face GaN) and the intrinsic limitation of the XPS method for band offset determination.

  19. Effect of ZnO seed layer on the morphology and optical properties of ZnO nanorods grown on GaN buffer layers

    SciTech Connect

    Nandi, R. Mohan, S. Major, S. S.; Srinivasa, R. S.

    2014-04-24

    ZnO nanorods were grown by chemical bath deposition on sputtered, polycrystalline GaN buffer layers with and without ZnO seed layer. Scanning electron microscopy and X-ray diffraction show that the ZnO nanorods on GaN buffer layers are not vertically well aligned. Photoluminescence spectrum of ZnO nanorods grown on GaN buffer layer, however exhibits a much stronger near-band-edge emission and negligible defect emission, compared to the nanorods grown on ZnO buffer layer. These features are attributed to gallium incorporation at the ZnO-GaN interface. The introduction of a thin (25 nm) ZnO seed layer on GaN buffer layer significantly improves the morphology and vertical alignment of ZnO-NRs without sacrificing the high optical quality of ZnO nanorods on GaN buffer layer. The presence of a thick (200 nm) ZnO seed layer completely masks the effect of the underlying GaN buffer layer on the morphology and optical properties of nanorods.

  20. Microstructural properties of Eu-doped GaN luminescent powders

    NASA Astrophysics Data System (ADS)

    Contreras, O.; Srinivasan, S.; Ponce, F. A.; Hirata, G. A.; Ramos, F.; McKittrick, J.

    2002-09-01

    GaN powders doped with europium have been prepared using Eu and Ga nitrates and N2H4 as reactants. The resulting particles have dimensions ranging from 0.5 to 1.0 mum. The crystalline structure was studied by transmission electron microscopy, and it consisted of single crystals with a hexagonal (wurtzite) structure containing small cubic domains (zinc blende) and a high density of stacking faults, all aligned along the 0001 and <111> directions, respectively. Cathodoluminescence measurements show strong light emission in the red region. This luminescence corresponds to transitions of Eu with the strongest emission in the 611 nm line, which is associated to the Eu3+ 4f transition from 5D0 to 7F2. These results demonstrate the feasibility of GaN:RE powders for luminescent applications.

  1. Photonic band gaps of wurtzite GaN and AlN photonic crystals at short wavelengths

    NASA Astrophysics Data System (ADS)

    Melo, E. G.; Alayo, M. I.

    2015-04-01

    Group III-nitride materials such as GaN and AlN have attracted a great attention in researches on photonic devices that operate at short light wavelengths. The large band gaps of these materials turn them suitable for nanophotonic devices that operate in light ranges from visible to deep ultraviolet. The physical properties of wurtzite GaN and AlN such as their second and third order nonlinear susceptibilities, and their thermal and piezoelectric coefficients, also make them excellent candidates for integrate photonic devices with electronics, microelectromechanics, microfluidics and general sensing applications. Using a plane wave expansion method (PWE) the photonic band gap maps of 36 different two-dimensional photonic crystal lattices in wurtzite GaN and AlN were obtained and analyzed. The wavelength dependence and the effects of the material anisotropy on the position of the photonic band gaps are also discussed. The results show regions with slow group velocity at the edges of a complete photonic band gap in the M-K direction of the triangular lattices with circular, hexagonal, and rhombic air holes. Was also found a very interesting disposition of the photonic band gaps in the lattices composed of rhombic air holes.

  2. Pattern Coarsening in a Two Dimensional Hexagonal System

    NASA Astrophysics Data System (ADS)

    Chaikin, Paul

    2008-03-01

    We have been studying the ordering, annealing, coarsening and alignment of two dimensional periodically ordered structures in thin films of diblock copolymers*. Coarsening by dislocation and disclination annihilation is clearly observed in AFM studies of monolayer films of cylindrical patterns with a time dependence given by t^α, with α about 1/4. However in hexagonal structures the mechanism is less well defined and appears to involve the collapse of small grains entrained in the grain boundaries of larger domains. Remarkably the exponent of α about 1/4 remains. We also report on shear aligned samples and samples quenched in a gradient after alignment. * Harrison C, Angelescu DE, Trawick M, Cheng ZD, Huse DA, Chaikin PM, Vega DA, Sebastian JM, Register RA, Adamson DH, EUROPHYSICS LETTERS 67 800-806 (2004)

  3. A Computational Study of the Growth of Hexagonal Ice

    NASA Astrophysics Data System (ADS)

    Fulford, Maxwell; Salvalaglio, Matteo; Parrinello, Michele; Molteni, Carla

    Hexagonal ice (Ih) has two distinct crystallographic surfaces; a basal and prism surface. At low vapour pressures, Ih forms thin plates and elongated prisms, depending on the temperature. The macroscopic shape depends on the relative rate of growth of the basal and prism surfaces. The aim of our research is to estimate the relative rate of growth of the two surfaces for a range of temperatures and ultimately predict the shape of Ih, using computer simulations. Our simulations show the well-know phenomenon that the surface of ice lowers its interfacial free energy by forming a stable quasi-liquid layer (QLL). The QLL mediates crystal growth and has a thickness which varies with temperature and crystallographic surface. We use a combination of Molecular Dynamics and Metadynamics to study how the interfacial structure at the ice/quasi-liquid and quasi-liquid/vapour interfaces influence the adsorption potential, surface transport properties and growth shape..

  4. Spin transport in fully hexagonal boron nitride encapsulated graphene

    NASA Astrophysics Data System (ADS)

    Gurram, M.; Omar, S.; Zihlmann, S.; Makk, P.; Schönenberger, C.; van Wees, B. J.

    2016-03-01

    We study fully hexagonal boron nitride (hBN) encapsulated graphene spin valve devices at room temperature. The device consists of a graphene channel encapsulated between two crystalline hBN flakes: thick-hBN flake as a bottom gate dielectric substrate which masks the charge impurities from SiO2/Si substrate and single-layer thin-hBN flake as a tunnel barrier. Full encapsulation prevents the graphene from coming in contact with any polymer/chemical during the lithography and thus gives homogeneous charge and spin transport properties across different regions of the encapsulated graphene. Further, even with the multiple electrodes in-between the injection and the detection electrodes which are in conductivity mismatch regime, we observe spin transport over 12.5 -μ m -long distance under the thin-hBN encapsulated graphene channel, demonstrating the clean interface and the pinhole-free nature of the thin hBN as an efficient tunnel barrier.

  5. Strong coupling of ferroelectricity and magnetism in the hexagonal ferrites

    NASA Astrophysics Data System (ADS)

    Das, Hena

    2013-03-01

    During the last decade one of the most extensively studied class of multiferroics has been the hexagonal rare-earth manganites RMnO3 where R=Dy-Lu, Y, Sc. These compounds exhibit antiferromagnetic (AFM) order with a Néel temperature TN ~ 100 K . In addition, they are improper ferroelectrics (TC > 1200 K) driven a by zone-tripling structural distortion associated with a buckling of the R-planes and a rotation of the oxygen trigonal bipyramids. The improper nature of the transition is responsible for the fascinating, topologically protected trimer-domains. Even though magnetism and ferroelectricity in these materials are not intrinsically coupled, there is a non-trivial interaction between the structural and magnetic domain walls. In contrast to the manganites, the ground state structure of the rare-earth ferrites RFeO3 is the orthorhombic perovskite. Recently, however, thin films of RFeO3 have been epitaxially stabilized in the hexagonal rare-earth manganite structure. This development has triggered several new studies of these hexagonal ferrite systems. Similar to manganites, ferrites exhibit ferroelectricity above room temperature and crystallize in P63cm polar structure but conflicting results have been reported as to the origin of ferroelectricity in these materials. Unlike the manganites, recent neutron diffraction measurements suggest a considerably high AFM ordering temperature, TN = 440 K. Additionally there is an indication of a second temperature, TwFM ~ 100 K , at which weak ferromagnetism has been observed. In this work my collaborators (Alex Wysocki and Craig J. Fennie) and I address the nature of ferroelectricity and magnetic order in the RFeO3 systems from first-principles. We elucidate the origin of ferroelectricity in the rare-earth ferrites and provide many useful insights into their magnetic behavior, which we will show is fundamentally different than that observed in the manganites. Combining first-principles calculations with a detailed

  6. Kinetic Monte Carlo simulations of GaN homoepitaxy on c- and m-plane surfaces.

    PubMed

    Xu, Dongwei; Zapol, Peter; Stephenson, G Brian; Thompson, Carol

    2017-04-14

    The surface orientation can have profound effects on the atomic-scale processes of crystal growth and is essential to such technologies as GaN-based light-emitting diodes and high-power electronics. We investigate the dependence of homoepitaxial growth mechanisms on the surface orientation of a hexagonal crystal using kinetic Monte Carlo simulations. To model GaN metal-organic vapor phase epitaxy, in which N species are supplied in excess, only Ga atoms on a hexagonal close-packed (HCP) lattice are considered. The results are thus potentially applicable to any HCP material. Growth behaviors on c-plane (0001) and m-plane (011¯0) surfaces are compared. We present a reciprocal space analysis of the surface morphology, which allows extraction of growth mode boundaries and direct comparison with surface X-ray diffraction experiments. For each orientation, we map the boundaries between 3-dimensional, layer-by-layer, and step flow growth modes as a function of temperature and growth rate. Two models for surface diffusion are used, which produce different effective Ehrlich-Schwoebel step-edge barriers and different adatom diffusion anisotropies on m-plane surfaces. Simulation results in agreement with observed GaN island morphologies and growth mode boundaries are obtained. These indicate that anisotropy of step edge energy, rather than adatom diffusion, is responsible for the elongated islands observed on m-plane surfaces. Island nucleation spacing obeys a power-law dependence on growth rate, with exponents of -0.24 and -0.29 for the m- and c-plane, respectively.

  7. Kinetic Monte Carlo simulations of GaN homoepitaxy on c- and m-plane surfaces

    NASA Astrophysics Data System (ADS)

    Xu, Dongwei; Zapol, Peter; Stephenson, G. Brian; Thompson, Carol

    2017-04-01

    The surface orientation can have profound effects on the atomic-scale processes of crystal growth and is essential to such technologies as GaN-based light-emitting diodes and high-power electronics. We investigate the dependence of homoepitaxial growth mechanisms on the surface orientation of a hexagonal crystal using kinetic Monte Carlo simulations. To model GaN metal-organic vapor phase epitaxy, in which N species are supplied in excess, only Ga atoms on a hexagonal close-packed (HCP) lattice are considered. The results are thus potentially applicable to any HCP material. Growth behaviors on c-plane (0001) and m-plane (01 1 ¯ 0 ) surfaces are compared. We present a reciprocal space analysis of the surface morphology, which allows extraction of growth mode boundaries and direct comparison with surface X-ray diffraction experiments. For each orientation, we map the boundaries between 3-dimensional, layer-by-layer, and step flow growth modes as a function of temperature and growth rate. Two models for surface diffusion are used, which produce different effective Ehrlich-Schwoebel step-edge barriers and different adatom diffusion anisotropies on m-plane surfaces. Simulation results in agreement with observed GaN island morphologies and growth mode boundaries are obtained. These indicate that anisotropy of step edge energy, rather than adatom diffusion, is responsible for the elongated islands observed on m-plane surfaces. Island nucleation spacing obeys a power-law dependence on growth rate, with exponents of -0.24 and -0.29 for the m- and c-plane, respectively.

  8. Kinetic Monte Carlo simulations of GaN homoepitaxy on c- and m-plane surfaces

    DOE PAGES

    Xu, Dongwei; Zapol, Peter; Stephenson, G. Brian; ...

    2017-04-12

    The surface orientation can have profound effects on the atomic-scale processes of crystal growth and is essential to such technologies as GaN-based light-emitting diodes and high-power electronics. We investigate the dependence of homoepitaxial growth mechanisms on the surface orientation of a hexagonal crystal using kinetic Monte Carlo simulations. To model GaN metal-organic vapor phase epitaxy, in which N species are supplied in excess, only Ga atoms on a hexagonal close-packed (HCP) lattice are considered. The results are thus potentially applicable to any HCP material. Growth behaviors on c-plane (0001) and m-plane (011¯0) surfaces are compared. We present a reciprocal spacemore » analysis of the surface morphology, which allows extraction of growth mode boundaries and direct comparison with surface X-ray diffraction experiments. For each orientation, we map the boundaries between 3-dimensional, layer-by-layer, and step flow growth modes as a function of temperature and growth rate. Two models for surface diffusion are used, which produce different effective Ehrlich-Schwoebel step-edge barriers and different adatom diffusion anisotropies on m-plane surfaces. Simulation results in agreement with observed GaN island morphologies and growth mode boundaries are obtained. These indicate that anisotropy of step edge energy, rather than adatom diffusion, is responsible for the elongated islands observed on m-plane surfaces. As a result, island nucleation spacing obeys a power-law dependence on growth rate, with exponents of –0.24 and –0.29 for the m- and c-plane, respectively.« less

  9. Fundamental Bulk/Surface Structure Photoactivity Relationships of Supported (Rh2-yCryO3)/GaN Photocatalysts

    SciTech Connect

    Phivilay, Somphonh; Roberts, Charles; Puretzky, Alexander A; Domen, Kazunari Domen; Wachs, Israel

    2013-01-01

    ABSTRACT. The supported (Rh2-yCryO3)/GaN photocatalyst was examined as a model nitride photocatalyst system to assist in the development of fundamental structure photoactivity relationships for UV activated water splitting. Surface characterization of the outermost surface layers by High Sensitivity-LEIS and High Resolution-XPS revealed for the first time that the GaN support consists of a GaOx outermost surface layer and a thin film of GaOxNy in the surface region. HR-XPS also demonstrates that the supported (Rh2-yCryO3) mixed oxide nanoparticles (NPs) exclusively consist of Cr+3 and Rh+3 cations and are surface enriched for the supported (Rh2-yCryO3)/GaN photocatalyst. Bulk analysis by Raman and UV-vis spectroscopy show that the bulk molecular and electronic structures, respectively, of the GaN support are not perturbed by the deposition of the (Rh2-yCryO3) mixed oxide NPs. The function of the GaN bulk lattice is to generate photoexcited electrons/holes, with the electrons harnessed by the surface Rh+3 sites for evolution of H2 and the holes trapped at the Ga oxide/oxynitride surface sites for splitting of water and evolving O2. These new structure-photoactivity relationships for supported (Rh2-yCryO3)/GaN also extend to the best performing visible light activated supported (Rh2-yCryO3)/(Ga1-xZnx)(N1-xOx) photocatalyst.

  10. Self-Healing Thermal Annealing: Surface Morphological Restructuring Control of GaN Nanorods

    SciTech Connect

    Conroy, Michele; Li, Haoning; Zubialevich, Vitaly Z.; Kusch, Gunnar; Schmidt, Michael; Collins, Timothy; Glynn, Colm; Martin, Robert W.; O’Dwyer, Colm; Morris, Michael D.; Holmes, Justin D.; Parbrook, Peter J.

    2016-12-07

    With advances in nanolithography and dry etching, top-down methods of nanostructuring have become a widely used tool for improving the efficiency of optoelectronics. These nano dimensions can offer various benefits to the device performance in terms of light extraction and efficiency, but often at the expense of emission color quality. Broadening of the target emission peak and unwanted yellow luminescence are characteristic defect-related effects due to the ion beam etching damage, particularly for III–N based materials. In this article we focus on GaN based nanorods, showing that through thermal annealing the surface roughness and deformities of the crystal structure can be “self-healed”. Correlative electron microscopy and atomic force microscopy show the change from spherical nanorods to faceted hexagonal structures, revealing the temperature-dependent surface morphology faceting evolution. The faceted nanorods were shown to be strain- and defect-free by cathodoluminescence hyperspectral imaging, micro-Raman, and transmission electron microscopy (TEM). In-situ TEM thermal annealing experiments allowed for real time observation of dislocation movements and surface restructuring observed in ex-situ annealing TEM sampling. This thermal annealing investigation gives new insight into the redistribution path of GaN material and dislocation movement post growth, allowing for improved understanding and in turn advances in optoelectronic device processing of compound semiconductors.

  11. Thermally induced microstrain broadening in hexagonal zinc

    SciTech Connect

    Lawson, Andrew C; Valdez, James A; Roberts, Joyce A; Leineweber, Andreas; Mittemeijer, E J; Kreher, W

    2008-01-01

    Neutron powder-diffraction experiments on polycrystalline hexagonal zinc show considerable temperature-dependent line broadening. Whereas as-received zinc at 300 K exhibits narrow reflections, during cooling to a minimum temperature of 10K considerable line-broadening appears, which largely disappears again during reheating. The line broadening may be ascribed to microstrains induced by thermal microstresses due to the anisotropy of the thermal expansion (shrinkage) of hexagonal zinc. Differences between the thermal microstrains and theoretical predictions considering elastic deformation of the grains can be explained by plastic deformation and surface effects.

  12. Disclinations in square and hexagonal patterns.

    PubMed

    Golovin, A A; Nepomnyashchy, A A

    2003-05-01

    We report the observation of defects with fractional topological charges (disclinations) in square and hexagonal patterns as numerical solutions of several generic equations describing many pattern-forming systems: Swift-Hohenberg equation, damped Kuramoto-Sivashinsky equation, as well as nonlinear evolution equations describing large-scale Rayleigh-Benard and Marangoni convection in systems with thermally nearly insulated boundaries. It is found that disclinations in square and hexagonal patterns can be stable when nucleated from special initial conditions. The structure of the disclinations is analyzed by means of generalized Cross-Newell equations.

  13. Optimization of GaN thin films via MOCVD

    NASA Technical Reports Server (NTRS)

    Dickens, Corey; Wilson, Sylvia L.

    1995-01-01

    A unique characteristic of every semiconductor is the amount of energy required to break an electron bond in the lowest band of allowed states, the valence band. The energy necessary to set an electron free and allow it to conduct in the material is termed the energy gap (Eg). Semiconductors with wide bandgap energies have been shown to possess properties for high power, high temperature, radiation resistance damage, and short wavelength optoelectronic applications. Gallium nitride, which has a wide gap of 3.39 eV, is a material that has demonstrated these characteristics. Various growth conditions are being investigated for quality gallium nitride heteroepitaxy growth via the technique of low pressure metal organic chemical vapor deposition (MOCVD) that can be used for device development.

  14. Optimization of GaN thin films via MOCVD

    NASA Technical Reports Server (NTRS)

    Dickens, Corey; Wilson, Sylvia L.

    1995-01-01

    A unique characteristic of every semiconductor is the amount of energy required to break an electron bond in the lowest band of allowed states, the valence band. The energy necessary to set an electron free and allow it to conduct in the material is termed the energy gap (Eg). Semiconductors with wide bandgap energies have been shown to possess properties for high power, high temperature, radiation resistance damage, and short wavelength optoelectronic applications. Gallium nitride, which has a wide gap of 3.39 eV, is a material that has demonstrated these characteristics. Various growth conditions are being investigated for quality gallium nitride heteroepitaxy growth via the technique of low pressure metal organic chemical vapor deposition (MOCVD) that can be used for device development.

  15. Cubic and hexagonal gallium nitride/aluminum nitride superlattice structures for near-infrared detector applications

    NASA Astrophysics Data System (ADS)

    Decuir, Eric Anthony, Jr.

    Intersubband transitions and photoconductivity were observed for the first time in metastable GaN/AlN cubic superlattice structures. Further studies of intersubband transitions and photoconductivity in the well-studied stable hexagonal system were also presented herein. The main motivation prompting this study is linked to the polar nature of the hexagonal crystal structure system grown on c-plane sapphire substrates, which gives rise to the presence of large polarization fields (up to 10 MV/cm). Since these fields have been shown to complicate the design of intersubband transitions, a study of the nonpolar cubic phase, lacking these large built-in fields due to its symmetric nature, is presented in an effort to reduce the complexity of future device design. Structural characterization of GaN/AlN superlattices was performed using X-ray Diffraction, Atomic Force Microscopy, and Transmission Electron Microscopy. Optical and electrical characterization of intersubband absorption and photoconductivity in cubic and hexagonal GaN/AlN superlattices were recorded respectively using Fourier-transform spectroscopy in the temperature range of 77K--300K. An Electrochemical capacitance voltage technique was used to measure and confirm the three dimensional carrier concentrations in doped GaN layers since the presence of free carriers in the conduction is required for intersubband transitions. Reactive ion etching and e-beam evaporation was employed in the fabrication of mesa device structures with ohmic contacts. Theoretical calculations of the bound state energy levels in quantum well structures were made using a transfer matrix technique and then compared with experimental results.

  16. Size-induced changes in optical and X-ray photoelectron spectra of GaN nanoparticles deposited at lower substrate temperature.

    PubMed

    Mann, A K; Varandani, D; Mehta, B R; Malhotra, L K; Shivaprasad, S M

    2005-11-01

    This study reports the synthesis of GaN nanoparticles having hexagonal structure by a simple technique of activated reactive evaporation with substrates kept at comparatively lower temperatures than usually reported. By varying the substrate temperature from 30 degrees C to 350 degrees C, it is possible to vary nanoparticle sizes from 5-30 nm. X-ray diffraction and X-ray photoelectron spectroscopy analysis confirm the formation of GaN on quartz and silicon substrates at room temperature. The observed size dependent shift in energy position, large increase in full width at half maximum value of Ga 3d and N 1s X-ray photoelectron spectroscopy peaks and blue shift in the optical absorption edge are related to nanoparticle character.

  17. An orthogonal oriented quadrature hexagonal image pyramid

    NASA Technical Reports Server (NTRS)

    Watson, Andrew B.; Ahumada, Albert J., Jr.

    1987-01-01

    An image pyramid has been developed with basis functions that are orthogonal, self-similar, and localized in space, spatial frequency, orientation, and phase. The pyramid operates on a hexagonal sample lattice. The set of seven basis functions consist of three even high-pass kernels, three odd high-pass kernels, and one low-pass kernel. The three even kernels are identified when rotated by 60 or 120 deg, and likewise for the odd. The seven basis functions occupy a point and a hexagon of six nearest neighbors on a hexagonal sample lattice. At the lowest level of the pyramid, the input lattice is the image sample lattice. At each higher level, the input lattice is provided by the low-pass coefficients computed at the previous level. At each level, the output is subsampled in such a way as to yield a new hexagonal lattice with a spacing sq rt 7 larger than the previous level, so that the number of coefficients is reduced by a factor of 7 at each level. The relationship between this image code and the processing architecture of the primate visual cortex is discussed.

  18. A composite nodal finite element for hexagons

    SciTech Connect

    Hennart, J.P.; Mund, E.H. |; Valle, E. Del

    1997-10-01

    A nodal algorithm for the solution of the multigroup diffusion equations in hexagonal arrays is analyzed. Basically, the method consists of dividing each hexagon into four quarters and mapping the hexagon quarters onto squares. The resulting boundary value problem on a quadrangular domain is solved in primal weak formulation. Nodal finite element methods like the Raviart-Thomas RTk schemes provide accurate analytical expansions of the solution in the hexagons. Transverse integration cannot be performed on the equations in the quadrangular domain as simply as it is usually done on squares because these equations have essentially variable coefficients. However, by considering an auxiliary problem with constant coefficients (on the same quadrangular domain) and by using a preconditioning approach, transverse integration can be performed as for rectangular geometry. A description of the algorithm is given for a one-group diffusion equation. Numerical results are presented for a simple model problem with a known analytical solution and for k{sub eff} evaluations of some benchmark problems proposed in the literature. For the analytical problem, the results indicate that the theoretical convergence orders of RTk schemes (k = 0,1) are obtained, yielding accurate solutions at the expense of a few preconditioning iterations.

  19. Intrinsic ferromagnetism in hexagonal boron nitride nanosheets

    SciTech Connect

    Si, M. S.; Gao, Daqiang E-mail: xueds@lzu.edu.cn; Yang, Dezheng; Peng, Yong; Zhang, Z. Y.; Xue, Desheng E-mail: xueds@lzu.edu.cn; Liu, Yushen; Deng, Xiaohui; Zhang, G. P.

    2014-05-28

    Understanding the mechanism of ferromagnetism in hexagonal boron nitride nanosheets, which possess only s and p electrons in comparison with normal ferromagnets based on localized d or f electrons, is a current challenge. In this work, we report an experimental finding that the ferromagnetic coupling is an intrinsic property of hexagonal boron nitride nanosheets, which has never been reported before. Moreover, we further confirm it from ab initio calculations. We show that the measured ferromagnetism should be attributed to the localized π states at edges, where the electron-electron interaction plays the role in this ferromagnetic ordering. More importantly, we demonstrate such edge-induced ferromagnetism causes a high Curie temperature well above room temperature. Our systematical work, including experimental measurements and theoretical confirmation, proves that such unusual room temperature ferromagnetism in hexagonal boron nitride nanosheets is edge-dependent, similar to widely reported graphene-based materials. It is believed that this work will open new perspectives for hexagonal boron nitride spintronic devices.

  20. Intrinsic ferromagnetism in hexagonal boron nitride nanosheets.

    PubMed

    Si, M S; Gao, Daqiang; Yang, Dezheng; Peng, Yong; Zhang, Z Y; Xue, Desheng; Liu, Yushen; Deng, Xiaohui; Zhang, G P

    2014-05-28

    Understanding the mechanism of ferromagnetism in hexagonal boron nitride nanosheets, which possess only s and p electrons in comparison with normal ferromagnets based on localized d or f electrons, is a current challenge. In this work, we report an experimental finding that the ferromagnetic coupling is an intrinsic property of hexagonal boron nitride nanosheets, which has never been reported before. Moreover, we further confirm it from ab initio calculations. We show that the measured ferromagnetism should be attributed to the localized π states at edges, where the electron-electron interaction plays the role in this ferromagnetic ordering. More importantly, we demonstrate such edge-induced ferromagnetism causes a high Curie temperature well above room temperature. Our systematical work, including experimental measurements and theoretical confirmation, proves that such unusual room temperature ferromagnetism in hexagonal boron nitride nanosheets is edge-dependent, similar to widely reported graphene-based materials. It is believed that this work will open new perspectives for hexagonal boron nitride spintronic devices.

  1. Intrinsic ferromagnetism in hexagonal boron nitride nanosheets

    NASA Astrophysics Data System (ADS)

    Si, M. S.; Gao, Daqiang; Yang, Dezheng; Peng, Yong; Zhang, Z. Y.; Xue, Desheng; Liu, Yushen; Deng, Xiaohui; Zhang, G. P.

    2014-05-01

    Understanding the mechanism of ferromagnetism in hexagonal boron nitride nanosheets, which possess only s and p electrons in comparison with normal ferromagnets based on localized d or f electrons, is a current challenge. In this work, we report an experimental finding that the ferromagnetic coupling is an intrinsic property of hexagonal boron nitride nanosheets, which has never been reported before. Moreover, we further confirm it from ab initio calculations. We show that the measured ferromagnetism should be attributed to the localized π states at edges, where the electron-electron interaction plays the role in this ferromagnetic ordering. More importantly, we demonstrate such edge-induced ferromagnetism causes a high Curie temperature well above room temperature. Our systematical work, including experimental measurements and theoretical confirmation, proves that such unusual room temperature ferromagnetism in hexagonal boron nitride nanosheets is edge-dependent, similar to widely reported graphene-based materials. It is believed that this work will open new perspectives for hexagonal boron nitride spintronic devices.

  2. Powdered Hexagonal Boron Nitride Reducing Nanoscale Wear

    NASA Astrophysics Data System (ADS)

    Chkhartishvili, L.; Matcharashvili, T.; Esiava, R.; Tsagareishvili, O.; Gabunia, D.; Margiev, B.; Gachechiladze, A.

    2013-05-01

    A morphology model is suggested for nano-powdered hexagonal boron nitride that can serve as an effective solid additive to liquid lubricants. It allows to estimate the specific surface, that is a hard-to-measure parameter, based on average size of powder particles. The model can be used also to control nanoscale wear processes.

  3. Hexagonal and Pentagonal Fractal Multiband Antennas

    NASA Technical Reports Server (NTRS)

    Tang, Philip W.; Wahid, Parveen

    2005-01-01

    Multiband dipole antennas based on hexagonal and pentagonal fractals have been analyzed by computational simulations and functionally demonstrated in experiments on prototypes. These antennas are capable of multiband or wide-band operation because they are subdivided into progressively smaller substructures that resonate at progressively higher frequencies by virtue of their smaller dimensions. The novelty of the present antennas lies in their specific hexagonal and pentagonal fractal configurations and the resonant frequencies associated with them. These antennas are potentially applicable to a variety of multiband and wide-band commercial wireless-communication products operating at different frequencies, including personal digital assistants, cellular telephones, pagers, satellite radios, Global Positioning System receivers, and products that combine two or more of the aforementioned functions. Perhaps the best-known prior multiband antenna based on fractal geometry is the Sierpinski triangle antenna (also known as the Sierpinski gasket), shown in the top part of the figure. In this antenna, the scale length at each iteration of the fractal is half the scale length of the preceding iteration, yielding successive resonant frequencies related by a ratio of about 2. The middle and bottom parts of the figure depict the first three iterations of the hexagonal and pentagonal fractals along with typical dipole-antenna configuration based on the second iteration. Successive resonant frequencies of the hexagonal fractal antenna have been found to be related by a ratio of about 3, and those of the pentagonal fractal antenna by a ratio of about 2.59.

  4. Dry etching of MgCaO gate dielectric and passivation layers on GaN

    NASA Astrophysics Data System (ADS)

    Hlad, M.; Voss, L.; Gila, B. P.; Abernathy, C. R.; Pearton, S. J.; Ren, F.

    2006-09-01

    MgCaO films grown by rf plasma-assisted molecular beam epitaxy and capped with Sc 2O 3 are promising candidates as surface passivation layers and gate dielectrics on GaN-based high electron mobility transistors (HEMTs) and metal-oxide semiconductor HEMTs (MOS-HEMTs), respectively. Two different plasma chemistries were examined for etching these thin films on GaN. Inductively coupled plasmas of CH 4/H 2/Ar produced etch rates only in the range 20-70 Å/min, comparable to the Ar sputter rates under the same conditions. Similarly slow MgCaO etch rates (˜100 Å/min) were obtained with Cl 2/Ar discharges under the same conditions, but GaN showed rates almost an order of magnitude higher. The MgCaO removal rates are limited by the low volatilities of the respective etch products. The CH 4/H 2/Ar plasma chemistry produced a selectivity of around 2 for etching the MgCaO with respect to GaN.

  5. Anisotropy of effective electron masses in highly doped nonpolar GaN

    SciTech Connect

    Feneberg, Martin Lange, Karsten; Lidig, Christian; Wieneke, Matthias; Witte, Hartmut; Bläsing, Jürgen; Dadgar, Armin; Krost, Alois; Goldhahn, Rüdiger

    2013-12-02

    The anisotropic effective electron masses in wurtzite GaN are determined by generalized infrared spectroscopic ellipsometry. Nonpolar (112{sup ¯}0) oriented thin films allow accessing both effective masses, m{sub ⊥}{sup *} and m{sub ∥}{sup *}, by determining the screened plasma frequencies. A n-type doping range up to 1.7 × 10{sup 20} cm{sup −3} is investigated. The effective mass ratio m{sub ⊥}{sup *}/m{sub ∥}{sup *} is obtained with highest accuracy and is found to be 1.11 independent on electron concentration up to 1.2 × 10{sup 20} cm{sup −3}. For higher electron concentrations, the conduction band non-parabolicity is mirrored in changes. Absolute values for effective electron masses depend on additional input of carrier concentrations determined by Hall effect measurements. We obtain m{sub ⊥}{sup *}=(0.239±0.004)m{sub 0} and m{sub ∥}{sup *}=(0.216±0.003)m{sub 0} for the parabolic range of the GaN conduction band. Our data are indication of a parabolic GaN conduction band up to an energy of approximately 400 meV above the conduction band minimum.

  6. Radaitive decay engineering in GaN Quantum Dots for biomedical application

    NASA Astrophysics Data System (ADS)

    Neogi, Arup; Basu Neogi, Purnima; Morkoc, Hadis

    2003-03-01

    Thin metallic films containing nanoscale surface features result in giant enhancement of linear and nonlinear optical responses. These enhancements are associated with excitation of surface plasmon (SP), collective electromagnetic modes whose characteristics are strongly dependent on the geometric structure of the metallic component of the medium and can be further enhanced via the directional emission from a semiconductor microcavity. The SP energy of Ag (3 eV) is modified ( 2.92 eV) by the GaN dielectric constant with bandgap at 3.4 eV [1] and is the ideal candidate for the resonantly modifying optical responses at metal-semiconductor interface. The spontaneous emission rate in strained GaN quantum dots grown using molecular beam epitaxy can be enhanced and the photoluminescence (PL) is reduced by nearly five times in the presence of a continuous Ag film due to resonant silver-surface plasmon interaction. The PL can also be enhanced using directional coupling through silver nano-shells fabricated by spin coating a silver halide solution over the GaN QDs having a 2 nm AlN cap layer. We also propose the use of radaitive decay engineering effects in wide-bandgap Ag-GaN semiconductor QD system for biomedical application. [1] A. Neogi, et al.; Phys. Rev., 66, 153305 (2002).

  7. Optical characteristics of nanocrystalline Al{sub x}Ga{sub 1−x}N thin films deposited by hollow cathode plasma-assisted atomic layer deposition

    SciTech Connect

    Goldenberg, Eda; Ozgit-Akgun, Cagla; Biyikli, Necmi; Kemal Okyay, Ali

    2014-05-15

    Gallium nitride (GaN), aluminum nitride (AlN), and Al{sub x}Ga{sub 1−x}N films have been deposited by hollow cathode plasma-assisted atomic layer deposition at 200 °C on c-plane sapphire and Si substrates. The dependence of film structure, absorption edge, and refractive index on postdeposition annealing were examined by x-ray diffraction, spectrophotometry, and spectroscopic ellipsometry measurements, respectively. Well-adhered, uniform, and polycrystalline wurtzite (hexagonal) GaN, AlN, and Al{sub x}Ga{sub 1−x}N films were prepared at low deposition temperature. As revealed by the x-ray diffraction analyses, crystallite sizes of the films were between 11.7 and 25.2 nm. The crystallite size of as-deposited GaN film increased from 11.7 to 12.1 and 14.4 nm when the annealing duration increased from 30 min to 2 h (800 °C). For all films, the average optical transmission was ∼85% in the visible (VIS) and near infrared spectrum. The refractive indices of AlN and Al{sub x}Ga{sub 1−x}N were lower compared to GaN thin films. The refractive index of as-deposited films decreased from 2.33 to 2.02 (λ = 550 nm) with the increased Al content x (0 ≤ x ≤ 1), while the extinction coefficients (k) were approximately zero in the VIS spectrum (>400 nm). Postdeposition annealing at 900 °C for 2 h considerably lowered the refractive index value of GaN films (2.33–1.92), indicating a significant phase change. The optical bandgap of as-deposited GaN film was found to be 3.95 eV, and it decreased to 3.90 eV for films annealed at 800 °C for 30 min and 2 h. On the other hand, this value increased to 4.1 eV for GaN films annealed at 900 °C for 2 h. This might be caused by Ga{sub 2}O{sub 3} formation and following phase change. The optical bandgap value of as-deposited Al{sub x}Ga{sub 1−x}N films decreased from 5.75 to 5.25 eV when the x values decreased from 1 to 0.68. Furthermore, postdeposition annealing did not

  8. Two-dimensional X-ray diffraction and transmission electron microscopy study on the effect of magnetron sputtering atmosphere on GaN/SiC interface and gallium nitride thin film crystal structure

    SciTech Connect

    Shen, Huaxiang; Zhu, Guo-Zhen; Botton, Gianluigi A.; Kitai, Adrian

    2015-03-21

    The growth mechanisms of high quality GaN thin films on 6H-SiC by sputtering were investigated by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). The XRD θ-2θ scans show that high quality (0002) oriented GaN was deposited on 6H-SiC by reactive magnetron sputtering. Pole figures obtained by 2D-XRD clarify that GaN thin films are dominated by (0002) oriented wurtzite GaN and (111) oriented zinc-blende GaN. A thin amorphous silicon oxide layer on SiC surfaces observed by STEM plays a critical role in terms of the orientation information transfer from the substrate to the GaN epilayer. The addition of H{sub 2} into Ar and/or N{sub 2} during sputtering can reduce the thickness of the amorphous layer. Moreover, adding 5% H{sub 2} into Ar can facilitate a phase transformation from amorphous to crystalline in the silicon oxide layer and eliminate the unwanted (33{sup ¯}02) orientation in the GaN thin film. Fiber texture GaN thin films can be grown by adding 10% H{sub 2} into N{sub 2} due to the complex reaction between H{sub 2} and N{sub 2}.

  9. Gallium Nitride (GaN) High Power Electronics (FY11)

    DTIC Science & Technology

    2012-01-01

    Gallium Nitride (GaN) High Power Electronics (FY11) by Kenneth A. Jones, Randy P. Tompkins, Michael A. Derenge, Kevin W. Kirchner, Iskander...Army Research Laboratory Adelphi, MD 20783-1197 ARL-TR-5903 January 2012 Gallium Nitride (GaN) High Power Electronics (FY11) Kenneth A...DSI 3. DATES COVERED (From - To) 4. TITLE AND SUBTITLE Gallium Nitride (GaN) High Power Electronics (FY11) 5a. CONTRACT NUMBER 5b. GRANT

  10. Enhanced photoluminescence from photonic crystal-coated GaN LED wafers

    NASA Astrophysics Data System (ADS)

    Rahman, F.; Khokhar, A. Z.

    2011-06-01

    This paper describes results of studies on photoluminescence from blue-emitting GaN LED wafers coated with a layer of synthetic opal photonic crystals. Commercial LED wafer material was used and samples were coated with thin films consisting of several layers of stacked spherical polystyrene balls. Various optical measurements were performed on these samples while they were excited with a 405 nm laser beam. Diffraction pattern due to the photonic crystal structure, showing the underlying six-fold symmetry, was recorded. The spectrum and angle-resolved intensity of photoluminescence were measured to understand the coupling of LED light with the grown photonic crystal structure on top of the wafer.

  11. Size-dependent photoconductivity and dark conductivity of m-axial GaN nanowires with small critical diameter

    NASA Astrophysics Data System (ADS)

    Chen, Hsin-Yi; Chen, Reui-San; Chang, Fu-Chieh; Chen, Li-Chyong; Chen, Kuei-Hsien; Yang, Ying-Jay

    2009-10-01

    The size effects on both the photoconductivity and dark conductivity have been observed in m-axial GaN nanowires grown by chemical vapor deposition (CVD). For these nanowires with diameters at 50-130 nm, the products of carrier lifetime (τ) and mobility (μ) derived from the photocurrent measurements are typically at (2-8)×10-1 cm2/V, which are over two orders of magnitude higher than the maximal reported values [τμ=(1-5)×10-4 cm2/V] for their thin film counterparts. A significant decrease of τμ value at diameter below the critical values (dcrt) at 30-40 nm is observed. Similar size dependence is also found from the dark conductivity study. The temperature-dependent measurements further indicate two different thermal activation mechanisms in GaN nanowires with sizes above and below the dcrt. These results suggest a surface-dominant transport property in GaN nanowires both in dark and under light illumination due to the presence of surface depletion and band bending. Probable reasons leading to the smaller dcrt of the CVD-grown m-axial GaN nanowires, compared to the c-axial ones grown by molecular beam epitaxy are discussed as well.

  12. Reduction of crack density in ammonothermal bulk GaN growth

    NASA Astrophysics Data System (ADS)

    Letts, Edward; Key, Daryl; Hashimoto, Tadao

    2016-12-01

    The growth of high quality GaN by the ammonothermal method is appealing due to the potential to scale and achieve very high crystal quality. Several applications could benefit from the supply of very high quality GaN such as high power light emitting diodes, laser diodes, and high power electronics. Despite steady advancement by the few groups developing ammonothermal growth technology, high quality ammonothermal GaN wafers have yet be manufactured in great quantities. This paper reviews the current progress of ammonothermal growth at SixPoint Materials. Growths were performed at T<600 °C and P<300 MPa on GaN seed crystals produced by hydride vapor phase epitaxy (HVPE). For thin boules, <1 mm growth thickness, no cracking is observed. Historically however, SixPoint Materials' ammonothermal growth on HVPE seeds eventually experiences a curvature flip giving extremely high radius of curvature at a critical thickness. As the growth continues the radius of curvature degrades and cracking is observed. Since IWBNSVIII, SixPoint Materials has improved the crack free area for 5 mm thick boules from 5 to 80 mm2 to the complete seed area. This result is repeatable in multiple reactors. Careful selection of the HVPE seeds led to the greatest reduction in cracking. Seed selection combined with an additional technique has allowed boules to be grown crack free. X-ray diffraction was carried out on an ammonothermally grown boule at 90 points along a 44 mm line providing a mean (002) and (201) full width half max (FWHM) reflection of 29 and 35″ respectively using a beam spot of 0.3 mm x 0.3 mm and an open detector. The radius of curvature is typically between 3 and 20 m across the sample. Dislocation densities are routinely low 105 cm-2 .

  13. Highly mismatched GaN1-x Sb x alloys: synthesis, structure and electronic properties

    NASA Astrophysics Data System (ADS)

    Yu, K. M.; Sarney, W. L.; Novikov, S. V.; Segercrantz, N.; Ting, M.; Shaw, M.; Svensson, S. P.; Martin, R. W.; Walukiewicz, W.; Foxon, C. T.

    2016-08-01

    Highly mismatched alloys (HMAs) is a class of semiconductor alloys whose constituents are distinctly different in terms of size, ionicity and/or electronegativity. Electronic properties of the alloys deviate significantly from an interpolation scheme based on small deviations from the virtual crystal approximation. Most of the HMAs were only studied in a dilute composition limit. Recent advances in understanding of the semiconductor synthesis processes allowed growth of thin films of HMAs under non-equilibrium conditions. Thus reducing the growth temperature allowed synthesis of group III-N-V HMAs over almost the entire composition range. This paper focuses on the GaN x Sb1-x HMA which has been suggested as a potential material for solar water dissociation devices. Here we review our recent work on the synthesis, structural and optical characterization of GaN1-x Sb x HMA. Theoretical modeling studies on its electronic structure based on the band anticrossing (BAC) model are also reviewed. In particular we discuss the effects of growth temperature, Ga flux and Sb flux on the incorporation of Sb, film microstructure and optical properties of the alloys. Results obtained from two separate MBE growths are directly compared. Our work demonstrates that a large range of direct bandgap energies from 3.4 eV to below 1.0 eV can be achieved for this alloy grown at low temperature. We show that the electronic band structure of GaN1-x Sb x HMA over the entire composition range is well described by a modified BAC model which includes the dependence of the host matrix band edges as well as the BAC model coupling parameters on composition. We emphasize that the modified BAC model of the electronic band structure developed for the full composition of GaN x Sb1-x is general and is applicable to any HMA.

  14. Stress analysis of transferred thin-GaN LED by Au-Si wafer bonding

    NASA Astrophysics Data System (ADS)

    Hsu, S. C.; Liu, C. Y.

    2005-09-01

    Nowadays, the high power GaN-based LED has attracted serious attention for the lighting application. One of key issues for high power GaN-base LED to achieve sufficient lighting efficiency over the traditional light sources, such as, white incandescent and halogen light bulb is the efficiency of heat dissipation. Typically, GaN epi-layer is grown on sapphire substrates. The poor thermal conductivity of sapphire substrate has been identified to be the main limitation for the application of high power GaN LED. To improve the heat dissipation and lighting efficiency, we report a thin GaN structure by using Au-Si wafer bonding and Laser lift-off (LLO) technique. The GaN wafer was first deposited with a Au bonding layer and bonded onto a good thermal conduction substrate, i.e., heavy-doped Si. Then, 248nm KrF excimer Laser was used to strip the original sapphire substrate. To assure a successful GaN epi-layer transferring, Raman spectrum on the transferred GaN layer was performed and the result shows no quality change in the transferred GaN layer. In this work, we also fabricated the vertical LED devices on the transferred GaN epi-layer. Therefore, L-I-V result was obtained which will be presented in this talk. Moreover, we will discuss the effects and advantages of Au-Si bonding on the efficiency of lighting.

  15. Phonon assignments in GaN bulk

    NASA Astrophysics Data System (ADS)

    Kunert, H. W.

    2004-07-01

    The measured phonon-density of states of bulk GaN by time-of-flight neutron spectroscopy has been recently reported by Nipko et al. [CITE]. The authors have also calculated the true partial and total DOS as well as the phonon dispersion curves along major symmetry directions in the Brillouin zone. However, the group-theoretical phonon assignments have not been provided. Based on calculated symmetry allowed modes spanned by displacement representation and on the derived connectivity relations along the major directions in the Brillouin zone we have assigned Nipko's phonon dispersion curves to irreducible representations (species) of the C^46v (P63mc) space group of GaN.

  16. Theoretical Investigation of GaN

    NASA Astrophysics Data System (ADS)

    Achour, H.; Louhibi-Fasla, S.; Mana, F.

    Using the Full Potential Linear Muffin Tin Orbitals (FPLMTO) method, the structural properties of GaN were studied. Different crystal structures were considered: NaCl, CsCl, wurtzite, zincblende, β-tin, Cinnabar and NiAs structures. The wurtzite is the calculated ground state structure. Results are given for lattice parameters, bulk modulus and its first derivative in the different cases. The phase transitions for GaN were also investigated. The results are compared with the available theoretical and experimental data. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependences of the volume, the bulk modulus, the variation of the thermal expansion α, as well as the heat capacity Cv were successfully obtained in the whole range from 0 to 30 GPa and temperature range from 0 to 1000 K.

  17. Amphoteric arsenic in GaN

    SciTech Connect

    Wahl, U.; Correia, J. G.; Araujo, J. P.; Rita, E.; Soares, J. C.

    2007-04-30

    The authors have determined the lattice location of implanted arsenic in GaN by means of conversion electron emission channeling from radioactive {sup 73}As. They give direct evidence that As is an amphoteric impurity, thus settling the long-standing question as to whether it prefers cation or anion sites in GaN. The amphoteric character of As and the fact that As{sub Ga} 'antisites' are not minority defects provide additional aspects to be taken into account for an explanantion of the so-called miscibility gap in ternary GaAs{sub 1-x}N{sub x} compounds, which cannot be grown with a single phase for values of x in the range of 0.1

  18. Chain hexagonal cacti with the extremal eccentric distance sum.

    PubMed

    Qu, Hui; Yu, Guihai

    2014-01-01

    Eccentric distance sum (EDS), which can predict biological and physical properties, is a topological index based on the eccentricity of a graph. In this paper we characterize the chain hexagonal cactus with the minimal and the maximal eccentric distance sum among all chain hexagonal cacti of length n, respectively. Moreover, we present exact formulas for EDS of two types of hexagonal cacti.

  19. Chain Hexagonal Cacti with the Extremal Eccentric Distance Sum

    PubMed Central

    Qu, Hui

    2014-01-01

    Eccentric distance sum (EDS), which can predict biological and physical properties, is a topological index based on the eccentricity of a graph. In this paper we characterize the chain hexagonal cactus with the minimal and the maximal eccentric distance sum among all chain hexagonal cacti of length n, respectively. Moreover, we present exact formulas for EDS of two types of hexagonal cacti. PMID:24741365

  20. Enhanced catalyst-free nucleation of GaN nanowires on amorphous Al{sub 2}O{sub 3} by plasma-assisted molecular beam epitaxy

    SciTech Connect

    Sobanska, Marta Klosek, Kamil; Borysiuk, Jolanta; Kret, Slawomir; Tchutchulasvili, Giorgi; Gieraltowska, Sylwia; Zytkiewicz, Zbigniew R.

    2014-01-28

    We report on plasma-assisted molecular beam epitaxial growth of GaN nanowires (NWs) on Si(111) substrates with a thin amorphous Al{sub 2}O{sub 3} buffer layer deposited by atomic layer deposition. Comparison of nucleation kinetics shows that presence of amorphous Al{sub 2}O{sub 3} buffer significantly enhances spontaneous nucleation of GaN NWs. Slower nucleation was observed on partially amorphous silicon nitride films. No growth of NWs was found on sapphire substrate under the same growth conditions which we explain by a low density of defects on monocrystalline substrate surface where NWs may nucleate. Our finding shows that tuning of substrate microstructure is an efficient tool to control rate of self-induced nucleation of GaN NWs.

  1. TEM characterization of GaN nanowires

    SciTech Connect

    Liliental-Weber, Zuzanna; Gao, Y.H.; Bando, Y.

    2002-02-21

    Transmission electron microscopy was applied to study GaN nanowires grown on carbon nanotube surfaces by chemical reaction between Ga{sub 2}O and NH{sub 3} gas in a conventional furnace. These wires grew in two crystallographic directions, <2{und 11}0> and <01{und 1}0> (fast growth directions of GaN), in the form of whiskers covered by small elongated GaN platelets. The morphology of these platelets is similar to that observed during the growth of single crystals from a Ga melt at high temperatures under high nitrogen pressure. It is thought that growth of nanowires in two different crystallographic directions and the arrangement of the platelets to the central whisker may be influenced by the presence of Ga{sub 2}O{sub 3} (based on the observation of the energy dispersive x-ray spectra), the interplanar spacings in the wire, and the presence of defects on the interface between the central part of the nanowire and the platelets surrounding it.

  2. Carbon-rich hexagonal (BN)C alloys

    SciTech Connect

    Uddin, M. R.; Li, J.; Lin, J. Y.; Jiang, H. X.

    2015-06-07

    Thin films of hexagonal boron nitride carbon, h-(BN){sub 1−x}(C{sub 2}){sub x}, alloys in the C-rich side have been synthesized by metal-organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates. X-ray diffraction measurements confirmed single hexagonal phase of h-(BN){sub 1−x}(C{sub 2}){sub x} epilayers. Electrical transport and Raman spectroscopy measurements results revealed evidences that homogenous h-(BN){sub 1−x}(C{sub 2}){sub x} alloys with x ≥ 95% can be synthesized by MOCVD at a growth temperature of 1300 °C. The variable temperature Hall-effect measurements suggested that a bandgap opening of about 93 meV with respect to graphite has been obtained for h-(BN){sub 1−x}(C{sub 2}){sub x} with x = 0.95, which is consistent with the expected value deduced from the alloy dependence of the energy gap of homogenous h-(BN){sub 1−x}(C{sub 2}){sub x} alloys. Atomic composition results obtained from x-ray photoelectron spectroscopy measurements revealed that the carrier type in C-rich h-(BN){sub 1−x}(C{sub 2}){sub x} alloys is controlled by the stoichiometry ratio between the B and N via changing the V/III ratio during the growth. The demonstration of bandgap opening and conductivity control in C-rich h-(BN){sub 1−x}(C{sub 2}){sub x} alloys provide feasibilities for realizing technologically significant devices including infrared (IR) emitters and detectors active from near to far IR and multi-spectral IR emitters and detectors.

  3. In situ study of the endotaxial growth of hexagonal CoSi{sub 2} nanoplatelets in Si(001)

    SciTech Connect

    Silva Costa, Daniel da; Kellermann, Guinther; Huck-Iriart, Cristián; Giovanetti, Lisandro J.; Requejo, Félix G.

    2015-11-30

    This investigation aims at studying–by in situ grazing-incidence small-angle x-ray scattering–the process of growth of hexagonal CoSi{sub 2} nanoplatelets endotaxially buried in a Si(001) wafer. The early formation of spherical Co nanoparticles with bimodal size distribution in the deposited silica thin film during a pretreatment at 500 °C and their subsequent growth at 700 °C were also characterized. Isothermal annealing at 700 °C promotes a drastic reduction in the number of the smallest Co nanoparticles and a continuous decrease in their volume fraction in the silica thin film. At the same time, Co atoms diffuse across the SiO{sub 2}/Si(001) interface into the silicon wafer, react with Si, and build up thin hexagonal CoSi{sub 2} nanoplatelets, all of them with their main surfaces parallel to Si(111) crystallographic planes. The observed progressive growths in thickness and lateral size of the hexagonal CoSi{sub 2} nanoplatelets occur at the expense of the dissolution of the small Co nanoparticles that are formed during the pretreatment at 500 °C and become unstable at the annealing temperature (700 °C). The kinetics of growth of the volume fraction of hexagonal platelets is well described by the classical Avrami equation.

  4. Multilayer hexagonal silicon forming in slit nanopore.

    PubMed

    He, Yezeng; Li, Hui; Sui, Yanwei; Qi, Jiqiu; Wang, Yanqing; Chen, Zheng; Dong, Jichen; Li, Xiongying

    2015-10-05

    The solidification of two-dimensional liquid silicon confined to a slit nanopore has been studied using molecular dynamics simulations. The results clearly show that the system undergoes an obvious transition from liquid to multilayer hexagonal film with the decrease of temperature, accompanied by dramatic change in potential energy, atomic volume, coordination number and lateral radial distribution function. During the cooling process, some hexagonal islands randomly appear in the liquid first, then grow up to grain nuclei, and finally connect together to form a complete polycrystalline film. Moreover, it is found that the quenching rate and slit size are of vital importance to the freezing structure of silicon film. The results also indicate that the slit nanopore induces the layering of liquid silicon, which further induces the slit size dependent solidification behavior of silicon film with different electrical properties.

  5. Discrete breathers in hexagonal dusty plasma lattices

    SciTech Connect

    Koukouloyannis, V.; Kourakis, I.

    2009-08-15

    The occurrence of single-site or multisite localized vibrational modes, also called discrete breathers, in two-dimensional hexagonal dusty plasma lattices is investigated. The system is described by a Klein-Gordon hexagonal lattice characterized by a negative coupling parameter epsilon in account of its inverse dispersive behavior. A theoretical analysis is performed in order to establish the possibility of existence of single as well as three-site discrete breathers in such systems. The study is complemented by a numerical investigation based on experimentally provided potential forms. This investigation shows that a dusty plasma lattice can support single-site discrete breathers, while three-site in phase breathers could exist if specific conditions, about the intergrain interaction strength, would hold. On the other hand, out of phase and vortex three-site breathers cannot be supported since they are highly unstable.

  6. Synthesis of silicon carbide hexagonal nanoprisms

    NASA Astrophysics Data System (ADS)

    Wu, R. B.; Yang, G. Y.; Pan, Y.; Chen, J. J.

    2007-02-01

    SiC hexagonal nanoprisms have been prepared by a reaction of multiwall carbon nanotubes and Si vapor in an Astro furnace at 1450 °C for 3 h. The polytype, morphology, crystal structure of the nanoprisms were studied by X-ray powder diffraction, scanning electron microscopy and high resolution transmission electron microscopy, showing their hexagonal nanoprism shapes with a 3C-SiC single crystal structure with a diameter of about 100 nm and 2 μm in length. The photoluminescence spectrum of the nanoprisms exhibits a significant blue-shift relative to bulk 3C-SiC and other nanostructured SiC. The possible growth mechanism that controls the nanostructure formation is also analysed.

  7. Growth kinetics of AlN and GaN films grown by molecular beam epitaxy on R-plane sapphire substrates

    SciTech Connect

    Chandrasekaran, R.; Moustakas, T. D.; Ozcan, A. S.; Ludwig, K. F.; Zhou, L.; Smith, David J.

    2010-08-15

    This paper reports the growth by molecular beam epitaxy of AlN and GaN thin films on R-plane sapphire substrates. Contrary to previous findings that GaN grows with its (1120) A-plane parallel to the (1102) R-plane of sapphire, our results indicate that the crystallographic orientation of the III-nitride films is strongly dependent on the kinetic conditions of growth for the GaN or AlN buffer layers. Thus, group III-rich conditions for growth of either GaN or AlN buffers result in nitride films having (1120) planes parallel to the sapphire surface, and basal-plane stacking faults parallel to the growth direction. The growth of these buffers under N-rich conditions instead leads to nitride films with (1126) planes parallel to the sapphire surface, with inclined c-plane stacking faults that often terminate threading dislocations. Moreover, electron microscope observations indicate that slight miscut ({approx}0.5 deg. ) of the R-plane sapphire substrate almost completely suppresses the formation of twinning defects in the (1126) GaN films.

  8. Influence of post-deposition annealing on interfacial properties between GaN and ZrO{sub 2} grown by atomic layer deposition

    SciTech Connect

    Ye, Gang; Wang, Hong Arulkumaran, Subramaniam; Ng, Geok Ing; Li, Yang; Ang, Kian Siong; Geok Ng, Serene Lay; Ji, Rong; Liu, Zhi Hong

    2014-10-13

    Influence of post-deposition annealing on interfacial properties related to the formation/annihilation of interfacial GaO{sub x} layer of ZrO{sub 2} grown by atomic layer deposition (ALD) on GaN is studied. ZrO{sub 2} films were annealed in N{sub 2} atmospheres in temperature range of 300 °C to 700 °C and analyzed by X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy. It has been found that Ga-O bond to Ga-N bond area ratio decreases in the samples annealed at temperatures lower than 500 °C, which could be attributed to the thinning of GaO{sub x} layer associated with low surface defect states due to “clean up” effect of ALD-ZrO{sub 2} on GaN. However, further increase in annealing temperature results in deterioration of interface quality, which is evidenced by increase in Ga-O bond to Ga-N bond area ratio and the reduction of Ga-N binding energy.

  9. The reconstructed edges of the hexagonal BN.

    PubMed

    Zhao, Ruiqi; Gao, Junfeng; Liu, Zhongfan; Ding, Feng

    2015-06-07

    As an important two-dimensional material which shows exceptional mechanical and chemical stability, superior electronic properties, along with broad applications, the hexagonal-BN (h-BN) has drawn great attention recently. Here we report a systematic study on the structural stability, electronic and magnetic properties of various h-BN edges, including both bare and hydrogen-terminated ones. It is found that along the armchair (AC) direction, the pristine edge is the most stable one because of the formation of a triple B≡N bond, while, along the zigzag (ZZ) directions, the reconstructed ones, ZZB + N and ZZN57 are more stable. The pristine edges are more stable in bare BN in most cases if saturated with hydrogen. By applying the theory of Wulff construction, we predicted that an unpassivated BN domain prefers the hexagonal shape enclosed with bare AC edges i.e., AC-Ns, AC, AC-Bs if the feedstock varies from N-rich to B-rich. However, the evolution from ZZN edged triangular domain, to hexagonal domain enclosed with AC edges, and ZZB edged triangle may occur if the edges are terminated by hydrogen atoms. Further calculation shows that these edges present rich type-dependent properties and thus are important for various applications. This theoretical study showed that controlling the morphologies of BN domains and BN edges is crucial for various applications.

  10. Correlation between magnon and magnetic symmetries of hexagonal RMnO3 (R = Er, Ho, Lu)

    NASA Astrophysics Data System (ADS)

    Nguyen, Thi Minh Hien; Nguyen, Thi Huyen; Chen, Xiang-Bai; Park, Yeonju; Jung, Young Mee; Lee, D.; Noh, T. W.; Cheong, Sang-Wook; Yang, In-Sang

    2016-11-01

    The correlation between the magnon scattering and the magnetic symmetries of hexagonal RMnO3 (R = Er, Ho) thin films and LuMnO3 single crystal was studied through the 2D Correlation Spectroscopy (2D COS) and Perturbation-Correlation Moving Window 2D (PCMW2D) Correlation Spectroscopy which were performed on the temperature-dependent Raman spectra of RMnO3 (R = Er, Ho, Lu). From the Raman spectra, we observed much stronger intensity and more asymmetrical magnon peak in LuMnO3 single crystal than in ErMnO3 and HoMnO3 thin films. While the ratio between magnon and phonon's linewidth of LuMnO3 and HoMnO3 display an anomalous behavior, that ratio of ErMnO3 is almost stable. The result from PCMW2D also supports these results. In addition, our 2D COS analysis showed that there are more overlap peaks in broad four-spin flipping magnon peak in LuMnO3 than that in ErMnO3 and HoMnO3. The differences of hexagonal RMnO3 (R = Er, Ho, Lu) in magnon scattering are very similar to the actual differences of the magnetic symmetries of these compounds. Therefore, we suggest that the magnon scattering of hexagonal RMnO3 is strongly correlated with the magnetic symmetries of these materials.

  11. P-type doping of GaN

    SciTech Connect

    Wong, Raechelle Kimberly

    2000-04-01

    After implantation of As, As + Be, and As + Ga into GaN and annealing for short durations at temperatures as high as 1500 C, the GaN films remained highly resistive. It was apparent from c-RBS studies that although implantation damage did not create an amorphous layer in the GaN film, annealing at 1500 C did not provide enough energy to completely recover the radiation damage. Disorder recovered significantly after annealing at temperatures up to 1500 C, but not completely. From SIMS analysis, oxygen contamination in the AIN capping layer causes oxygen diffusion into the GaN film above 1400 C. The sapphire substrate (A1203) also decomposed and oxygen penetrated into the backside of the GaN layer above 1400 C. To prevent donor-like oxygen impurities from the capping layer and the substrate from contaminating the GaN film and compensating acceptors, post-implantation annealing should be done at temperatures below 1500 C. Oxygen in the cap could be reduced by growing the AIN cap on the GaN layer after the GaN growth run or by depositing the AIN layer in a ultra high vacuum (UHV) system post-growth to minimize residual oxygen and water contamination. With longer annealing times at 1400 C or at higher temperatures with a higher quality AIN, the implantation drainage may fully recover.

  12. GaN Electronics For High Power, High Temperature Applications

    SciTech Connect

    PEARTON,S.J.; REN,F.; ZHANG,A.P.; DANG,G.; CAO,X.A.; LEE,K.P.; CHO,H.; GILA,B.P.; JOHNSON,J.W.; MONIER,C.; ABERNATHY,C.R.; HAN,JUNG; BACA,ALBERT G.; CHYI,J.-I.; LEE,C.-M.; NEE,T.-E.; CHUO,C.-C.; CHU,S.N.G.

    2000-06-12

    A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers. GaN/AlGaN heterojunction bipolar transistors and GaN metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.

  13. Ultrahigh Si{sup +} implant activation efficiency in GaN using a high-temperature rapid thermal process system

    SciTech Connect

    Cao, X.A.; Abernathy, C.R.; Singh, R.K.; Pearton, S.J.; Fu, M.; Sarvepalli, V.; Sekhar, J.A.; Zolper, J.C.; Rieger, D.J.; Han, J.; Drummond, T.J.; Shul, R.J.; Wilson, R.G.

    1998-07-01

    Si{sup +} implant activation efficiencies above 90{percent}, even at doses of 5{times}10{sup 15}thinspcm{sup {minus}2}, have been achieved in GaN by rapid thermal processing at 1400{endash}1500thinsp{degree}C for 10 s. The annealing system utilizes molybdenum intermetallic heating elements capable of operation up to 1900thinsp{degree}C, producing high heating and cooling rates (up to 100thinsp{degree}Cthinsps{sup {minus}1}). Unencapsulated GaN shows severe surface pitting at 1300thinsp{degree}C and complete loss of the film by evaporation at 1400thinsp{degree}C. Dissociation of nitrogen from the surface is found to occur with an approximate activation energy of 3.8 eV for GaN (compared to 4.4 eV for AlN and 3.4 eV for InN). Encapsulation with either rf magnetron reactively sputtered or metal organic molecular beam epitaxy-grown AlN thin films provides protection against GaN surface degradation up to 1400thinsp{degree}C, where peak electron concentrations of {approximately}5{times}10{sup 20}thinspcm{sup {minus}3} can be achieved in Si-implanted GaN. Secondary ion mass spectrometry profiling showed little measurable redistribution of Si, suggesting D{sub Si}{le}10{sup {minus}13}thinspcm{sup 2}thinsps{sup {minus}1} at 1400thinsp{degree}C. The implant activation efficiency decreases at higher temperatures, which may result from Si{sub Ga} to Si{sub N} site switching and resultant self-compensation. {copyright} {ital 1998 American Institute of Physics.}

  14. Impact of defects on the electrical transport, optical properties and failure mechanisms of GaN nanowires.

    SciTech Connect

    Armstrong, Andrew M.; Aubry, Sylvie; Shaner, Eric Arthur; Siegal, Michael P.; Li, Qiming; Jones, Reese E.; Westover, Tyler; Wang, George T.; Zhou, Xiao Wang; Talin, Albert Alec; Bogart, Katherine Huderle Andersen; Harris, C. Thomas; Huang, Jian Yu

    2010-09-01

    We present the results of a three year LDRD project that focused on understanding the impact of defects on the electrical, optical and thermal properties of GaN-based nanowires (NWs). We describe the development and application of a host of experimental techniques to quantify and understand the physics of defects and thermal transport in GaN NWs. We also present the development of analytical models and computational studies of thermal conductivity in GaN NWs. Finally, we present an atomistic model for GaN NW electrical breakdown supported with experimental evidence. GaN-based nanowires are attractive for applications requiring compact, high-current density devices such as ultraviolet laser arrays. Understanding GaN nanowire failure at high-current density is crucial to developing nanowire (NW) devices. Nanowire device failure is likely more complex than thin film due to the prominence of surface effects and enhanced interaction among point defects. Understanding the impact of surfaces and point defects on nanowire thermal and electrical transport is the first step toward rational control and mitigation of device failure mechanisms. However, investigating defects in GaN NWs is extremely challenging because conventional defect spectroscopy techniques are unsuitable for wide-bandgap nanostructures. To understand NW breakdown, the influence of pre-existing and emergent defects during high current stress on NW properties will be investigated. Acute sensitivity of NW thermal conductivity to point-defect density is expected due to the lack of threading dislocation (TD) gettering sites, and enhanced phonon-surface scattering further inhibits thermal transport. Excess defect creation during Joule heating could further degrade thermal conductivity, producing a viscous cycle culminating in catastrophic breakdown. To investigate these issues, a unique combination of electron microscopy, scanning luminescence and photoconductivity implemented at the nanoscale will be used in

  15. Thermal conductivity of hexagonal Si and hexagonal Si nanowires from first-principles

    NASA Astrophysics Data System (ADS)

    Raya-Moreno, Martí; Aramberri, Hugo; Seijas-Bellido, Juan Antonio; Cartoixà, Xavier; Rurali, Riccardo

    2017-07-01

    We calculate the thermal conductivity, κ, of the recently synthesized hexagonal diamond (lonsdaleite) Si using first-principles calculations and solving the Boltzmann Transport Equation. We find values of κ which are around 40% lower than in the common cubic diamond polytype of Si. The trend is similar for [111] Si nanowires, with reductions of the thermal conductivity that are even larger than in the bulk in some diameter range. The Raman active modes are identified, and the role of mid-frequency optical phonons that arise as a consequence of the reduced symmetry of the hexagonal lattice is discussed. We also show briefly that popular classic potentials used in molecular dynamics might not be suited to describe hexagonal polytypes, discussing the case of the Tersoff potential.

  16. GaN photovoltaic leakage current and correlation to grain size

    SciTech Connect

    Matthews, K. D.; Chen, X.; Hao, D.; Schaff, W. J.; Eastman, L. F.

    2010-10-15

    GaN p-i-n solar PV structures grown by rf plasma assisted molecular beam epitaxy (MBE) produce high performance IV characteristics with a leakage current density of less than 1x10{sup -4} mA cm{sup -2} at 0.1 V forward bias and an on-resistance of 0.039 {Omega} cm{sup 2}. Leakage current measurements taken for different size diodes processed on the same sample containing the solar cells reveal that current density increases with diode area, indicating that leakage is not a large function of surface leakage along the mesa. Nonannealed Pt/Au Ohmic p-contacts produce a contact resistivity of 4.91x10{sup -4} {Omega} cm{sup -2} for thin Mg doped contact layers with sheet resistivity of 62196 {Omega}/{open_square}. Under concentrated sunlight the cells produce an open-circuit voltage of 2.5 V and short circuit currents as high as 30 mA cm{sup -2}. Multiple growths comprised the study and on each wafer the IV curves representing several diodes showed considerable variation in parasitic leakage current density at low voltages on some wafers and practically no variation on others. It appears that a smaller grain size within the GaN thin film accounts for higher levels of dark current.

  17. GaN photovoltaic leakage current and correlation to grain size

    NASA Astrophysics Data System (ADS)

    Matthews, K. D.; Chen, X.; Hao, D.; Schaff, W. J.; Eastman, L. F.

    2010-10-01

    GaN p-i-n solar PV structures grown by rf plasma assisted molecular beam epitaxy (MBE) produce high performance IV characteristics with a leakage current density of less than 1×10-4 mA cm-2 at 0.1 V forward bias and an on-resistance of 0.039 Ω cm2. Leakage current measurements taken for different size diodes processed on the same sample containing the solar cells reveal that current density increases with diode area, indicating that leakage is not a large function of surface leakage along the mesa. Nonannealed Pt/Au Ohmic p-contacts produce a contact resistivity of 4.91×10-4 Ω cm-2 for thin Mg doped contact layers with sheet resistivity of 62196 Ω/◻. Under concentrated sunlight the cells produce an open-circuit voltage of 2.5 V and short circuit currents as high as 30 mA cm-2. Multiple growths comprised the study and on each wafer the IV curves representing several diodes showed considerable variation in parasitic leakage current density at low voltages on some wafers and practically no variation on others. It appears that a smaller grain size within the GaN thin film accounts for higher levels of dark current.

  18. Atomic force microscopy studies of homoepitaxial GaN layers grown on GaN template by laser MBE

    SciTech Connect

    Choudhary, B. S.; Singh, A.; Tyagi, P. K.; Tanwar, S.; Kumar, M. Senthil; Kushvaha, S. S.

    2016-04-13

    We have grown homoepitaxial GaN films on metal organic chemical vapor deposition (MOCVD) grown 3.5 µm thick GaN on sapphire (0001) substrate (GaN template) using an ultra-high vacuum (UHV) laser assisted molecular beam epitaxy (LMBE) system. The GaN films were grown by laser ablating a polycrystalline solid GaN target in the presence of active r.f. nitrogen plasma. The influence of laser repetition rates (10-30 Hz) on the surface morphology of homoepitaxial GaN layers have been studied using atomic force microscopy. It was found that GaN layer grown at 10 Hz shows a smooth surface with uniform grain size compared to the rough surface with irregular shape grains obtained at 30 Hz. The variation of surface roughness of the homoepitaxial GaN layer with and without wet chemical etching has been also studied and it was observed that the roughness of the film decreased after wet etching due to the curved structure/rough surface.

  19. Growth and characterization of thin and thick gallium nitride

    NASA Astrophysics Data System (ADS)

    Mastro, Michael Anthony

    2001-07-01

    The III-nitrides have received considerable attention in recent years for applications that require a wide band gap semiconductor. Specifically, short wavelength light emitters are required for full color displays, laser printers, high-density information storage, and underwater communication. High-temperature and high-power devices are needed for a number of applications including avionics, automobile engines and future advanced power distribution systems. Unfortunately, III-nitride substrates are not available. This dissertation explores three alternative substrates (LiGaO2, LiAlO2, and Si) for the growth of thick and potentially freestanding GaN substrates. The requirement to grow a protective layer of GAN by metal organic chemical vapor deposition (MOCVD) and the need for high rate deposition by hydride vapor phase epitaxy (HVPE) motivated the development of a single deposition system capable of growing in both modes. The successful growth of high quality GaN on LiGaO2 by MOCVD was first demonstrated. Nitridation of the LiGaO2 substrate using NH3 prior to growth leads to the reconstruction of the substrate surface and to the formation of a thin layer of nitrided material having the same orientation as the substrate. It was found that the thick GaN layer grown by HVPE spontaneously separated from the underlying LiGaO2 substrate upon cooling if proper nitridation was performed. This then eliminates the need for substrate removal by HCl etching and gives a reusable template substrate. The related substrate LiAlO2 is also closely lattice matched to GaN. LiAlO2, however, is more stable, particularly in an HCl ambient making it suitable for thick HVPE growth of GaN. Finally, it was shown that epitaxial GaN could be fabricated by a low-temperature deposition sequence on silicon substrates. Measurements revealed that a thin compliant SiOx layer was an effective intermediate layer for the GaN film grown epitaxially on Si. The deposition temperature of 560°C is one of

  20. Epitaxial integration of the highly spin-polarized ferromagnetic semiconductor EuO with silicon and GaN.

    PubMed

    Schmehl, Andreas; Vaithyanathan, Venu; Herrnberger, Alexander; Thiel, Stefan; Richter, Christoph; Liberati, Marco; Heeg, Tassilo; Röckerath, Martin; Kourkoutis, Lena Fitting; Mühlbauer, Sebastian; Böni, Peter; Muller, David A; Barash, Yuri; Schubert, Jürgen; Idzerda, Yves; Mannhart, Jochen; Schlom, Darrell G

    2007-11-01

    Doped EuO is an attractive material for the fabrication of proof-of-concept spintronic devices. Yet for decades its use has been hindered by its instability in air and the difficulty of preparing and patterning high-quality thin films. Here, we establish EuO as the pre-eminent material for the direct integration of a carrier-concentration-matched half-metal with the long-spin-lifetime semiconductors silicon and GaN, using methods that transcend these difficulties. Andreev reflection measurements reveal that the spin polarization in doped epitaxial EuO films exceeds 90%, demonstrating that EuO is a half-metal even when highly doped. Furthermore, EuO is epitaxially integrated with silicon and GaN. These results demonstrate the high potential of EuO for spintronic devices.

  1. Method of growing GaN films with a low density of structural defects using an interlayer

    DOEpatents

    Bourret-Courchesne, Edith D.

    2003-01-01

    A dramatic reduction of the dislocation density in GaN was obtained by insertion of a single thin interlayer grown at an intermediate temperature (IT-IL) after the growth of an initial grown at high temperature. A description of the growth process is presented with characterization results aimed at understanding the mechanisms of reduction in dislocation density. A large percentage of the threading dislocations present in the first GaN epilayer are found to bend near the interlayer and do not propagate into the top layer which grows at higher temperature in a lateral growth mode. TEM studies show that the mechanisms of dislocation reduction are similar to those described for the epitaxial lateral overgrowth process, however a notable difference is the absence of coalescence boundaries.

  2. An atomistic mechanism study of GaN step-flow growth in vicinal m-plane orientations

    DOE PAGES

    Liu, Zhun; Wang, Ru-Zhi; Zapol, Peter

    2016-10-12

    We present elucidation of homoepitaxial growth mechanisms on vicinal non-polar surfaces of GaN that is highly important for gaining an understanding of and control thin film surface morphology and properties. Using first-principles calculations, we study the step-flow growth in m-plane GaN based on atomic row nucleation and kink propagation kinetics. Ga–N dimer adsorption onto the m-plane is energetically more favorable than that of Ga and N isolated adatoms. Therefore, we have treated the dimers as the dominant growth species attached to the step edges. By calculating the free energies of sequentially attached Ga–N dimers, we have elucidated that the a-stepmore » edge kink growth proceeds by parallel attachment rather than by across the step edge approach. We found a series of favorable configurations of kink propagation and calculated the free energy and nucleation barriers for kink evolution on five types of step edges (a, +c, -c, +a + c, and -a - c). By changing the chemical potential μGa and the excess chemical potential Δμ, the growth velocities at the five types of edges are controlled by the corresponding kink pair nucleation barrier E* in their free energy profiles. To explore the kink-flow growth instability observed at different Ga/N flux ratios, calculations of kink pairs on the incompact -c and +c-step edges are further performed to study their formation energies. Variations of these step edge morphologies with a tuned chemical environment are consistent with previous experimental observations, including stable diagonal ±a ± c-direction steps. In conclusion, our work provides a first-principles approach to explore step growth and surface morphology of the vicinal m-plane GaN, which is applicable to analyze and control the step-flow growth of other binary thin films.« less

  3. An atomistic mechanism study of GaN step-flow growth in vicinal m-plane orientations

    SciTech Connect

    Liu, Zhun; Wang, Ru-Zhi; Zapol, Peter

    2016-10-12

    We present elucidation of homoepitaxial growth mechanisms on vicinal non-polar surfaces of GaN that is highly important for gaining an understanding of and control thin film surface morphology and properties. Using first-principles calculations, we study the step-flow growth in m-plane GaN based on atomic row nucleation and kink propagation kinetics. Ga–N dimer adsorption onto the m-plane is energetically more favorable than that of Ga and N isolated adatoms. Therefore, we have treated the dimers as the dominant growth species attached to the step edges. By calculating the free energies of sequentially attached Ga–N dimers, we have elucidated that the a-step edge kink growth proceeds by parallel attachment rather than by across the step edge approach. We found a series of favorable configurations of kink propagation and calculated the free energy and nucleation barriers for kink evolution on five types of step edges (a, +c, -c, +a + c, and -a - c). By changing the chemical potential μGa and the excess chemical potential Δμ, the growth velocities at the five types of edges are controlled by the corresponding kink pair nucleation barrier E* in their free energy profiles. To explore the kink-flow growth instability observed at different Ga/N flux ratios, calculations of kink pairs on the incompact -c and +c-step edges are further performed to study their formation energies. Variations of these step edge morphologies with a tuned chemical environment are consistent with previous experimental observations, including stable diagonal ±a ± c-direction steps. In conclusion, our work provides a first-principles approach to explore step growth and surface morphology of the vicinal m-plane GaN, which is applicable to analyze and control the step-flow growth of other binary thin films.

  4. Solubilization of nutraceuticals into reverse hexagonal mesophases.

    PubMed

    Amar-Yuli, Idit; Aserin, Abraham; Garti, Nissim

    2008-08-21

    The solubilization of four bioactive molecules with different polarities, in three reverse hexagonal (HII) systems has been investigated. The three HII systems were a typical reverse hexagonal composed of glycerol monooleate (GMO)/tricaprylin/water and two fluid hexagonal systems containing either 2.75 wt % Transcutol or ethanol as a fourth component. The phase behavior of the liquid crystalline phases in the presence of ascorbic acid, ascorbyl palmitate, D-alpha-tocopherol and D-alpha-tocopherol acetate were determined by small-angle X-ray scattering (SAXS) and optical microscopy. Differential scanning calorimetry (DSC) and Fourier-transform infrared (FT-IR) techniques were utilized to follow modifications in the thermal behavior and in the vibrations of different functional groups upon solubilizing the bioactive molecules. The nature of each guest molecule (in both geometry and polarity) together with the different HII structures (typical and fluids) determined the corresponding phase behavior, swelling or structural transformations and its location in the HII structures. Ascorbic acid was found to act as a chaotropic guest molecule, localized in the water-rich core and at the interface. The AP was also a chaotropic guest molecule with its head located in the vicinity of the GMO headgroup while its tail embedded close to the surfactant tail. D-alpha-tocopherol and D-alpha-tocopherol acetate were incorporated between the GMO tails; however, the D-alpha-tocopherol was located closer to the interface. Once Transcutol or ethanol was present and upon guest molecule incorporation, partial migration was detected.

  5. Wargaming in Both Rectilinear and Hexagonal Spaces

    NASA Technical Reports Server (NTRS)

    Hoover, Alex

    2012-01-01

    There are two main approaches to managing wargame entity interactions (movement, line of sight, area of effect, etc) freespace and gridded In the freespace approach, the units exist as entities in a continuous volume of (usually) Cartesian 3D space. They move in any direction (based on interaction with "terrain" that occupies the same space) and interact with each other based on references and displacements from their position in that space. In the gridded approach, space is broken up into (usually regular) shaped pieces. Units are considered to occupy the entire volume of one of these pieces, movement, line of sight, and other interactions are based on the relationships among the spaces rather than the absolute positions of the units themselves. Both approaches have advantages and drawbacks. The general issue that this discussion has addressed is that there is no "perfect" approach to implementing a wargaming battlespace. Each of them (and this extends to others not discussed) has different sets of advantages and disadvantages. Nothing will change that basic nature of the various approaches, nor would it be desirable to do so. Along with the advantages, the challenges define the feel of the game and focus the thinking of the players on certain aspects and away from others. The proposed approach to combining square and hexagonal approaches, which we will call the rhombus interface, leverages rhombuses constructed from equilateral triangles into which the hexagon can be decomposed to bridge the gap between the approaches, maintain relative consistency between the two as much as possible, and provide most of the feel of the hexagonal approach.

  6. GaN: Defect and Device Issues

    SciTech Connect

    Pearton, S.J.; Ren, F.; Shul, R.J.; Zolper, J.C.

    1998-11-09

    The role of extended and point defects, and key impurities such as C, O and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.

  7. Yellow Luminescence Centers of GaN

    NASA Astrophysics Data System (ADS)

    Zhao, Guangyuan; Hubbard, Seth; Pavlidis, Dimitris

    2004-05-01

    The method for measuring Shockley-Read-Hall (SRH) lifetime of yellow centers of GaN was developed. The capture-section ratio (150) of hole to electron is extracted by comparing the experimental and theoretical results. A marked increase in the SRH lifetime (from 0.75 to 7.0 ns) with the increasing in Si doping density (from 1.5× 1017 to 8.8× 1018 cm-3) was observed, and it is attributed to some Si dopant substituting for the Ga vacancy. In addition, it is also found that the YL centers are an important factor limiting the performance of GaN-based devices.

  8. Method for exfoliation of hexagonal boron nitride

    NASA Technical Reports Server (NTRS)

    Lin, Yi (Inventor); Connell, John W. (Inventor)

    2012-01-01

    A new method is disclosed for the exfoliation of hexagonal boron nitride into mono- and few-layered nanosheets (or nanoplatelets, nanomesh, nanoribbons). The method does not necessarily require high temperature or vacuum, but uses commercially available h-BN powders (or those derived from these materials, bulk crystals) and only requires wet chemical processing. The method is facile, cost efficient, and scalable. The resultant exfoliated h-BN is dispersible in an organic solvent or water thus amenable for solution processing for unique microelectronic or composite applications.

  9. Thermoelectric properties of the 3C, 2H, 4H, and 6H polytypes of the wide-band-gap semiconductors SiC, GaN, and ZnO

    SciTech Connect

    Huang, Zheng; Lü, Tie-Yu; Wang, Hui-Qiong; Zheng, Jin-Cheng

    2015-09-15

    We have investigated the thermoelectric properties of the 3C, 2H, 4H, and 6H polytypes of the wide-band-gap(n-type) semiconductors SiC, GaN, and ZnO based on first-principles calculations and Boltzmann transport theory. Our results show that the thermoelectric performance increases from 3C to 6H, 4H, and 2H structures with an increase of hexagonality for SiC. However, for GaN and ZnO, their power factors show a very weak dependence on the polytype. Detailed analysis of the thermoelectric properties with respect to temperature and carrier concentration of 4H-SiC, 2H-GaN, and 2H-ZnO shows that the figure of merit of these three compounds increases with temperature, indicating the promising potential applications of these thermoelectric materials at high temperature. The significant difference of the polytype-dependent thermoelectric properties among SiC, GaN, and ZnO might be related to the competition between covalency and ionicity in these semiconductors. Our calculations may provide a new way to enhance the thermoelectric properties of wide-band-gap semiconductors through atomic structure design, especially hexagonality design for SiC.

  10. Preparation and optical spectroscopy of Eu{sup 3+}-doped GaN luminescent semiconductor from freeze-dried precursors

    SciTech Connect

    El-Himri, Abdelouahad; Perez-Coll, Domingo; Nun-tilde ez, Pedro . E-mail: pnunez@ull.es; Martin, Inocencio R.; Lavin, Victor; Rodriguez, Vicente D.

    2004-11-01

    Pure and 0.5% and 5mol% Eu{sup 3+} doped GaN nanoparticles have been prepared by ammonolysis of the corresponding freeze-dried precursors. A single hexagonal phase with the wurtzite structure was obtained as determined by X-ray Powder Diffraction. The crystallite size determined by XRD was lower than 10nm. From optical spectroscopy characterization, it is found that the Eu{sub 2}O{sub 3} formation is avoided by using nitrates as starting reagent. Fluorescence line narrowing spectra show excitation wavelength dependence, which is indicative that the Eu{sup 3+} ions are well dispersed in the prepared samples. The environment distribution occupied by the Eu{sup 3+} ions has been analyzed by crystal-field calculation and the results are compared with those for other materials.

  11. GaN power devices for automotive applications

    NASA Astrophysics Data System (ADS)

    Uesugi, T.; Kachi, Tetsu

    2013-03-01

    GaN is an attractive material for high performance power devices. Vertical GaN power devices are suitable for high current operation, on the other hand, lateral GaN power devices, namely GaN lateral HEMTs have both low on-resistance and low parasitic capacitance. In addition, the GaN lateral HEMTs can be fabricated on Si substrate. We can get low conduction loss and low switching loss devices with low cost. So the GaN lateral HEMTs are suitable for subsystems like an air conditioner and an electric power steering. Serious technical issues about GaN power devices are a normally-off operation, a current collapse, and a high quality gate insulator. Several normally-off operation techniques have been proposed but there is no decisive method. An NH3 surface treatment and a SiO2 passivation are useful to suppress the current collapse. An Al2O3 deposited by ALD is excellent for gate insulator in breakdown and it has enough TDDB reliability under room temperature and 150°C.

  12. Cathodoluminescence characterization of suspended GaN nanomembranes

    NASA Astrophysics Data System (ADS)

    Stevens-Kalceff, M. A.; Tiginyanu, I. M.; Popa, V.; Braniste, T.; Brenner, P.

    2013-07-01

    Continuous suspended ˜15 nm thick gallium nitride (GaN) nano-membranes have been investigated using cathodoluminescence microanalysis. The GaN nanomembranes are fabricated by focused ion beam (FIB) pre-treatment of GaN epilayer surfaces followed by photoelectrochemical (PEC) etching. CL microanalysis enables high sensitivity, nanoscale spatial resolution detection of impurities, and defects, and is associated with key features of the suspended GaN nano-membranes. CL spectra and images of the suspended nano-membranes reveal the broad emission band at ˜2.2 eV which is associated with deep acceptor states and the near edge emission at ˜3.4 eV which is associated with free exciton transitions at 295 K. The near edge emission can be resolved into two components, one associated with emission from the nanomembrane and the other associated with CL from underlying GaN transmitted through the nanomembrane. CL spectroscopy gives insight into the physical properties and optical quality of the suspended GaN nano-membranes. Blue shift of the CL near band edge emission indicates that the suspended GaN nanomembranes exhibit the combined effects of quantum confinement and strain.

  13. Exfoliation of Hexagonal Boron Nitride via Ferric Chloride Intercalation

    NASA Technical Reports Server (NTRS)

    Hung, Ching-cheh; Hurst, Janet; Santiago, Diana; Rogers, Richard B.

    2014-01-01

    Sodium fluoride (NaF) was used as an activation agent to successfully intercalate ferric chloride (FeCl3) into hexagonal boron nitride (hBN). This reaction caused the hBN mass to increase by approx.100 percent, the lattice parameter c to decrease from 6.6585 to between 6.6565 and 6.6569 ?, the x-ray diffraction (XRD) (002) peak to widen from 0.01deg to 0.05deg of the full width half maximum value, the Fourier transform infrared (FTIR) spectrum's broad band (1277/cm peak) to change shape, and new FTIR bands to emerge at 3700 to 2700 and 1600/cm. This indicates hBN's structural and chemical properties are significantly changed. The intercalated product was hygroscopic and interacted with moisture in the air to cause further structural and chemical changes (from XRD and FTIR). During a 24-h hold at room temperature in air with 100 percent relative humidity, the mass increased another 141 percent. The intercalated product, hydrated or not, can be heated to 750 C in air to cause exfoliation. Exfoliation becomes significant after two intercalation-air heating cycles, when 20-nm nanosheets are commonly found. Structural and chemical changes indicated by XRD and FTIR data were nearly reversed after the product was placed in hydrochloric acid (HCl), resulting in purified, exfoliated, thin hBN products.

  14. DNA Translocation through Hydrophilic Nanopore in Hexagonal Boron Nitride

    PubMed Central

    Zhou, Zhi; Hu, Ying; Wang, Hao; Xu, Zhi; Wang, Wenlong; Bai, Xuedong; Shan, Xinyan; Lu, Xinghua

    2013-01-01

    Ultra-thin solid-state nanopore with good wetting property is strongly desired to achieve high spatial resolution for DNA sequencing applications. Atomic thick hexagonal boron nitride (h-BN) layer provides a promising two-dimensional material for fabricating solid-state nanopores. Due to its good oxidation resistance, the hydrophilicity of h-BN nanopore device can be significantly improved by UV-Ozone treatment. The contact angle of a KCl-TE droplet on h-BN layer can be reduced from 57° to 26° after the treatment. Abundant DNA translocation events have been observed in such devices, and strong DNA-nanopore interaction has been revealed in pores smaller than 10 nm in diameter. The 1/f noise level is closely related to the area of suspended h-BN layer, and it is significantly reduced in smaller supporting window. The demonstrated performance in h-BN nanopore paves the way towards base discrimination in a single DNA molecule. PMID:24256703

  15. DNA Translocation through Hydrophilic Nanopore in Hexagonal Boron Nitride

    NASA Astrophysics Data System (ADS)

    Zhou, Zhi; Hu, Ying; Wang, Hao; Xu, Zhi; Wang, Wenlong; Bai, Xuedong; Shan, Xinyan; Lu, Xinghua

    2013-11-01

    Ultra-thin solid-state nanopore with good wetting property is strongly desired to achieve high spatial resolution for DNA sequencing applications. Atomic thick hexagonal boron nitride (h-BN) layer provides a promising two-dimensional material for fabricating solid-state nanopores. Due to its good oxidation resistance, the hydrophilicity of h-BN nanopore device can be significantly improved by UV-Ozone treatment. The contact angle of a KCl-TE droplet on h-BN layer can be reduced from 57° to 26° after the treatment. Abundant DNA translocation events have been observed in such devices, and strong DNA-nanopore interaction has been revealed in pores smaller than 10 nm in diameter. The 1/f noise level is closely related to the area of suspended h-BN layer, and it is significantly reduced in smaller supporting window. The demonstrated performance in h-BN nanopore paves the way towards base discrimination in a single DNA molecule.

  16. Nanocrystalline hexagonal diamond formed from glassy carbon

    NASA Astrophysics Data System (ADS)

    Shiell, Thomas. B.; McCulloch, Dougal G.; Bradby, Jodie E.; Haberl, Bianca; Boehler, Reinhard; McKenzie, David. R.

    2016-11-01

    Carbon exhibits a large number of allotropes and its phase behaviour is still subject to significant uncertainty and intensive research. The hexagonal form of diamond, also known as lonsdaleite, was discovered in the Canyon Diablo meteorite where its formation was attributed to the extreme conditions experienced during the impact. However, it has recently been claimed that lonsdaleite does not exist as a well-defined material but is instead defective cubic diamond formed under high pressure and high temperature conditions. Here we report the synthesis of almost pure lonsdaleite in a diamond anvil cell at 100 GPa and 400 °C. The nanocrystalline material was recovered at ambient and analysed using diffraction and high resolution electron microscopy. We propose that the transformation is the result of intense radial plastic flow under compression in the diamond anvil cell, which lowers the energy barrier by “locking in” favourable stackings of graphene sheets. This strain induced transformation of the graphitic planes of the precursor to hexagonal diamond is supported by first principles calculations of transformation pathways and explains why the new phase is found in an annular region. Our findings establish that high purity lonsdaleite is readily formed under strain and hence does not require meteoritic impacts.

  17. Nanocrystalline hexagonal diamond formed from glassy carbon.

    PubMed

    Shiell, Thomas B; McCulloch, Dougal G; Bradby, Jodie E; Haberl, Bianca; Boehler, Reinhard; McKenzie, David R

    2016-11-29

    Carbon exhibits a large number of allotropes and its phase behaviour is still subject to significant uncertainty and intensive research. The hexagonal form of diamond, also known as lonsdaleite, was discovered in the Canyon Diablo meteorite where its formation was attributed to the extreme conditions experienced during the impact. However, it has recently been claimed that lonsdaleite does not exist as a well-defined material but is instead defective cubic diamond formed under high pressure and high temperature conditions. Here we report the synthesis of almost pure lonsdaleite in a diamond anvil cell at 100 GPa and 400 °C. The nanocrystalline material was recovered at ambient and analysed using diffraction and high resolution electron microscopy. We propose that the transformation is the result of intense radial plastic flow under compression in the diamond anvil cell, which lowers the energy barrier by "locking in" favourable stackings of graphene sheets. This strain induced transformation of the graphitic planes of the precursor to hexagonal diamond is supported by first principles calculations of transformation pathways and explains why the new phase is found in an annular region. Our findings establish that high purity lonsdaleite is readily formed under strain and hence does not require meteoritic impacts.

  18. Nanocrystalline hexagonal diamond formed from glassy carbon

    SciTech Connect

    Shiell, Thomas. B.; McCulloch, Dougal G.; Bradby, Jodie E.; Haberl, Bianca; Boehler, Reinhard; McKenzie, David. R.

    2016-11-29

    Carbon exhibits a large number of allotropes and its phase behaviour is still subject to signifcant uncertainty and intensive research. The hexagonal form of diamond, also known as lonsdaleite, was discovered in the Canyon Diablo meteorite where its formation was attributed to the extreme conditions experienced during the impact. However, it has recently been claimed that lonsdaleite does not exist as a well-defned material but is instead defective cubic diamond formed under high pressure and high temperature conditions. Here we report the synthesis of almost pure lonsdaleite in a diamond anvil cell at 100GPa and 400 C. The nanocrystalline material was recovered at ambient and analysed using difraction and high resolution electron microscopy. We propose that the transformation is the result of intense radial plastic fow under compression in the diamond anvil cell, which lowers the energy barrier by locking in favourable stackings of graphene sheets. This strain induced transformation of the graphitic planes of the precursor to hexagonal diamond is supported by frst principles calculations of transformation pathways and explains why the new phase is found in an annular region. Furthermore, our findings establish that high purity lonsdaleite is readily formed under strain and hence does not require meteoritic impacts.

  19. Nanocrystalline hexagonal diamond formed from glassy carbon

    PubMed Central

    Shiell, Thomas. B.; McCulloch, Dougal G.; Bradby, Jodie E.; Haberl, Bianca; Boehler, Reinhard; McKenzie, David. R.

    2016-01-01

    Carbon exhibits a large number of allotropes and its phase behaviour is still subject to significant uncertainty and intensive research. The hexagonal form of diamond, also known as lonsdaleite, was discovered in the Canyon Diablo meteorite where its formation was attributed to the extreme conditions experienced during the impact. However, it has recently been claimed that lonsdaleite does not exist as a well-defined material but is instead defective cubic diamond formed under high pressure and high temperature conditions. Here we report the synthesis of almost pure lonsdaleite in a diamond anvil cell at 100 GPa and 400 °C. The nanocrystalline material was recovered at ambient and analysed using diffraction and high resolution electron microscopy. We propose that the transformation is the result of intense radial plastic flow under compression in the diamond anvil cell, which lowers the energy barrier by “locking in” favourable stackings of graphene sheets. This strain induced transformation of the graphitic planes of the precursor to hexagonal diamond is supported by first principles calculations of transformation pathways and explains why the new phase is found in an annular region. Our findings establish that high purity lonsdaleite is readily formed under strain and hence does not require meteoritic impacts. PMID:27897174

  20. Nanocrystalline hexagonal diamond formed from glassy carbon

    DOE PAGES

    Shiell, Thomas. B.; McCulloch, Dougal G.; Bradby, Jodie E.; ...

    2016-11-29

    Carbon exhibits a large number of allotropes and its phase behaviour is still subject to signifcant uncertainty and intensive research. The hexagonal form of diamond, also known as lonsdaleite, was discovered in the Canyon Diablo meteorite where its formation was attributed to the extreme conditions experienced during the impact. However, it has recently been claimed that lonsdaleite does not exist as a well-defned material but is instead defective cubic diamond formed under high pressure and high temperature conditions. Here we report the synthesis of almost pure lonsdaleite in a diamond anvil cell at 100GPa and 400 C. The nanocrystalline materialmore » was recovered at ambient and analysed using difraction and high resolution electron microscopy. We propose that the transformation is the result of intense radial plastic fow under compression in the diamond anvil cell, which lowers the energy barrier by locking in favourable stackings of graphene sheets. This strain induced transformation of the graphitic planes of the precursor to hexagonal diamond is supported by frst principles calculations of transformation pathways and explains why the new phase is found in an annular region. Furthermore, our findings establish that high purity lonsdaleite is readily formed under strain and hence does not require meteoritic impacts.« less

  1. Two-dimensional hexagonal semiconductors beyond graphene

    NASA Astrophysics Data System (ADS)

    Nguyen, Bich Ha; Hieu Nguyen, Van

    2016-12-01

    The rapid and successful development of the research on graphene and graphene-based nanostructures has been substantially enlarged to include many other two-dimensional hexagonal semiconductors (THS): phosphorene, silicene, germanene, hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMDCs) such as MoS2, MoSe2, WS2, WSe2 as well as the van der Waals heterostructures of various THSs (including graphene). The present article is a review of recent works on THSs beyond graphene and van der Waals heterostructures composed of different pairs of all THSs. One among the priorities of new THSs compared to graphene is the presence of a non-vanishing energy bandgap which opened up the ability to fabricate a large number of electronic, optoelectronic and photonic devices on the basis of these new materials and their van der Waals heterostructures. Moreover, a significant progress in the research on TMDCs was the discovery of valley degree of freedom. The results of research on valley degree of freedom and the development of a new technology based on valley degree of freedom-valleytronics are also presented. Thus the scientific contents of the basic research and practical applications os THSs are very rich and extremely promising.

  2. GaN for LED applications

    NASA Technical Reports Server (NTRS)

    Pankove, J. I.

    1973-01-01

    In order to improve the synthesis of GaN the effect of various growth and doping parameters has been studied. Although Be, Li, Mg, and Dy can be used to overcompensate native donors, the most interesting acceptor element is Zn. The emission spectrum and the luminescence efficiency depend on the growth temperature (below 800 C), on the partial pressure of the doping impurity, and on the duration of growth. Blue-green electroluminescence with a power efficiency of 0.1 percent and a brightness of 850 fL (at 0.6 mA and 22.5 V) was obtained. Some diodes allow the color of the emitted light to change by reversing the polarity of the bias. Continuous operation of a diode over a period of 5 months showed no evidence of degradation. The luminescence properties of ion-implanted GaN were studied. Delay effects were found in the electroluminescence of diodes, although, with a dc bias, a 70-MHz modulation was possible.

  3. Optical spectroscopy of cubic GaN in nanowires

    NASA Astrophysics Data System (ADS)

    Renard, J.; Tourbot, G.; Sam-Giao, D.; Bougerol, C.; Daudin, B.; Gayral, B.

    2010-08-01

    We show that highly homogeneous cubic GaN can be grown by plasma-assisted molecular beam epitaxy on wurtzite GaN nanowires. The line width of the donor bound exciton is below 3 meV and can reach 1.6 meV in the best parts of the studied sample. This allows to perform a detailed spectroscopy of cubic GaN, and, in particular, to determine the precise spectral positions of the donor bound exciton, the fundamental free exciton and the split-off exciton in a photoluminescence experiment.

  4. Development of GaN photocathodes for UV detectors

    NASA Astrophysics Data System (ADS)

    Siegmund, O.; Vallerga, J.; McPhate, J.; Malloy, J.; Tremsin, A.; Martin, A.; Ulmer, M.; Wessels, B.

    2006-11-01

    We have made substantial progress in the development of GaN photocathodes, including crystalline and polycrystalline GaN and InGaN coatings grown by chemical vapor deposition or molecular beam epitaxy on sapphire substrates. GaN and InGaN photocathodes have been developed with efficiencies up to 70% and cutoffs at ˜380 nm with low out of band response, and high stability and longevity. Samples have been processed and tested at ultra high vacuum to establish cathode process parameters, and some have been integrated into sealed tubes for long-term evaluation.

  5. Fabrication of GaN nanowire arrays by confined epitaxy

    NASA Astrophysics Data System (ADS)

    Wang, Xin; Sun, Xinyu; Fairchild, Michael; Hersee, Stephen D.

    2006-12-01

    The authors report the fabrication of GaN nanowire arrays inside a thick SiNx, selective growth mask that was patterned by interferometric lithography and dry etching. The GaN nanowires are molded by the apertures in the selective growth mask and the growth is epitaxial with respect to the underlying GaN layer. The precise location and diameter of each nanowire in the array are controlled by the growth mask patterning, and the resulting array has a long-range order that is compatible with photonic crystal applications. This process uses conventional metal organic precursors and does not require any additional metal catalysts.

  6. Stress-Induced Cubic-to-Hexagonal Phase Transformation in Perovskite Nanothin Films.

    PubMed

    Cao, Shi-Gu; Li, Yunsong; Wu, Hong-Hui; Wang, Jie; Huang, Baoling; Zhang, Tong-Yi

    2017-08-09

    The strong coupling between crystal structure and mechanical deformation can stabilize low-symmetry phases from high-symmetry phases or induce novel phase transformation in oxide thin films. Stress-induced structural phase transformation in oxide thin films has drawn more and more attention due to its significant influence on the functionalities of the materials. Here, we discovered experimentally a novel stress-induced cubic-to-hexagonal phase transformation in the perovskite nanothin films of barium titanate (BaTiO3) with a special thermomechanical treatment (TMT), where BaTiO3 nanothin films under various stresses are annealed at temperature of 575 °C. Both high-resolution transmission electron microscopy and Raman spectroscopy show a higher density of hexagonal phase in the perovskite thin film under higher tensile stress. Both X-ray photoelectron spectroscopy and electron energy loss spectroscopy does not detect any change in the valence state of Ti atoms, thereby excluding the mechanism of oxygen vacancy induced cubic-to-hexagonal (c-to-h) phase transformation. First-principles calculations show that the c-to-h phase transformation can be completed by lattice shear at elevated temperature, which is consistent with the experimental observation. The applied bending plus the residual tensile stress produces shear stress in the nanothin film. The thermal energy at the elevated temperature assists the shear stress to overcome the energy barriers during the c-to-h phase transformation. The stress-induced phase transformation in perovskite nanothin films with TMT provides materials scientists and engineers a novel approach to tailor nano/microstructures and properties of ferroelectric materials.

  7. Reconstruction of the polar interface between hexagonal LuFeO3 and intergrown Fe3O4 nanolayers

    PubMed Central

    Akbashev, A. R.; Roddatis, V. V.; Vasiliev, A. L.; Lopatin, S.; Amelichev, V. A.; Kaul, A. R.

    2012-01-01

    We report the observation of an unusual phase assembly behavior during the growth of hexagonal LuFeO3 thin films which resulted in the formation of epitaxial Fe3O4 nanolayers. The magnetite layers were up to 5 nm thick and grew under the conditions at which Fe2O3 is thermodynamically stable. These Fe3O4 nanolayers act as buffer layers promoting a highly epitaxial growth of the hexagonal LuFeO3 thin film up to 150 nm thick. Using scanning transmission electron microscopy, we show that the interface between (001) LuFeO3 and (111) Fe3O4 can be reconstructed in two ways depending on the sequence in which these compounds grow on each other. We suggest the polarity of the interface is the reason behind the observed interface reconstruction and epitaxial stabilization of magnetite. PMID:22993697

  8. Progress and Prospect of the Growth of Wide-Band-Gap Group III Nitrides: Development of the Growth Method for Single-Crystal Bulk GaN

    NASA Astrophysics Data System (ADS)

    Amano, Hiroshi

    2013-05-01

    Thin films of III-V compound semiconductors such as GaAs and InP can be grown on native substrates, whereas such growth was difficult for group III nitride semiconductors. Despite this drawback, scientists have gradually become able to use the functions of group III nitride semiconductors by growing their thin films on non-native substrates such as sapphire and Si substrates. With the continuously increasing demand for the conservation and generation of energy, bulk substrates of group III nitride semiconductors are highly expected to maximize their potential. In this report, I review the current status of the growth methods for bulk GaN single crystals used for substrates as well as summarize the characteristics of blue light-emitting diodes (LEDs), heterojunction field-effect transistors (HFETs), and photovoltaic cells on GaN substrates.

  9. Visible and near infrared emitting thin film electroluminescent gallium nitride doped with rare earths

    NASA Astrophysics Data System (ADS)

    Kim, Joo Han

    Visible and near-infrared (NIR) light-emitting thin-film electroluminescent gallium nitride (GaN) doped with rare earth (RE) elements was studied. The rare-earth-doped GaN thin films were prepared by radio frequency (RF) planar magnetron co-sputtering of separate targets consisting of a GaN compound target and a metallic rare earth target in a pure nitrogen atmosphere. The luminescence of rare-earth-doped GaN was shown to be a strong function of its structure and properties, and growth parameters affected the structure and properties of the GaN host films. A phase transition from the thermodynamically stable wurtzite to the metastable zinc-blende structure at room temperature in GaN host films was observed upon increasing the impact energy of the bombarding species, thereby increasing the compressive stress in the GaN film. The switch from wurtzite to zinc-blende GaN occurred at a compressive internal stress of ˜1 GPa. The internal compressive stress above this threshold value apparently stabilizes the zinc-blende GaN phase at room temperature. Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) showed that the impact by hyperthermal species yielded a highly condensed fibrous GaN microstructure with a smooth surface morphology due to annihilation of porosity by knock-on and bombardment-induced adatom mobility. X-ray diffraction and texture analyses showed that the GaN films deposited at a low pressure had a predominant cubic phase with a preferred crystallographic orientation of the [111] direction perpendicular to the surface of the film. X-ray rocking curve data revealed that cubic GaN films grown with a lower growth rate exhibited a more highly [111]-textured structure. Alternating-current thin-film electroluminescent (ACTFEL) devices were fabricated based on GaN doped with rare earth (RE) elements. Visible electroluminescent light emission peaks at 475 (blue), 530 (green), and 614 nm (red) were demonstrated at room temperature

  10. Synthesis of aligned symmetrical multifaceted monolayer hexagonal boron nitride single crystals on resolidified copper.

    PubMed

    Tay, Roland Yingjie; Park, Hyo Ju; Ryu, Gyeong Hee; Tan, Dunlin; Tsang, Siu Hon; Li, Hongling; Liu, Wenwen; Teo, Edwin Hang Tong; Lee, Zonghoon; Lifshitz, Yeshayahu; Ruoff, Rodney S

    2016-01-28

    Atomically smooth hexagonal boron nitride (h-BN) films are considered as a nearly ideal dielectric interface for two-dimensional (2D) heterostructure devices. Reported mono- to few-layer 2D h-BN films, however, are mostly small grain-sized, polycrystalline and randomly oriented. Here we report the growth of centimetre-sized atomically thin h-BN films composed of aligned domains on resolidified Cu. The films consist of monolayer single crystalline triangular and hexagonal domains with size of up to ∼10 μm. The domains converge to symmetrical multifaceted shapes such as "butterfly" and "6-apex-star" and exhibit ∼75% grain alignment for over millimetre distances as verified through transmission electron microscopy. Scanning electron microscopy images reveal that these domains are aligned for over centimetre distances. Defect lines are generated along the grain boundaries of mirroring h-BN domains due to the two different polarities (BN and NB) and edges with the same termination. The observed triangular domains with truncated edges and alternatively hexagonal domains are in accordance with Wulff shapes that have minimum edge energy. This work provides an extensive study on the aligned growth of h-BN single crystals over large distances and highlights the obstacles that are needed to be overcome for a 2D material with a binary configuration.

  11. Saturation Behavior of Eu ion emission in GaN

    NASA Astrophysics Data System (ADS)

    Hernandez, Natalie; Mitchell, Brandon; Fujiwara, Yasufumi; Dierolf, Volkmar

    Europium doped Gallium Nitride (GaN:Eu) has been recognized as a candidate for the red-emitting active layer in nitride-based light emitting diodes. To better comprehend the excitation energy transfer from the excited GaN host to the Eu ion, we performed an extensive analysis of GaN:Eu and GaN co-doped with Eu and other dopants (Silicon and Magnesium). We determined how various growth parameters manipulated site formations and measured the optical accessibility of the Eu ions within the GaN host and the excitation efficiency of the energy transfer between the host material and the Eu ions. Furthermore, we derived a model for the saturation behavior of the emission of Eu ions within GaN. Our results suggest that the saturation behavior is strongly influenced by different crystal growth specifications and co-dopants.

  12. Crystallography and elasticity of individual GaN nanotubes

    NASA Astrophysics Data System (ADS)

    Liu, Baodan; Bando, Yoshio; Wang, Mingsheng; Tang, Chengchun; Mitome, Masanori; Golberg, Dmitri

    2009-05-01

    High-purity, crystalline [001]-oriented GaN nanotubes with outer diameters of 200 nm or more, rough surfaces and irregular internal channels were synthesized under epitaxial growth on [001]-oriented sapphire substrates. Elastic property measurements on free-standing individual GaN nanotubes, using the in situ transmission electron microscopy (TEM) electromechanical resonance technique, pointed at an average Young's modulus E of 37 GPa and minimum quality factor of 320. These numbers are notably lower than those for previously reported GaN nanowires. The crystallography and chemistry of the GaN nanotubes were analyzed using TEM and energy dispersion x-ray spectroscopy (EDS). It is suggested that the lowered Young's modulus and quality factor of the nanotubes are mainly due to the surface roughness and defectiveness.

  13. Hexagonal boron nitride and water interaction parameters.

    PubMed

    Wu, Yanbin; Wagner, Lucas K; Aluru, Narayana R

    2016-04-28

    The study of hexagonal boron nitride (hBN) in microfluidic and nanofluidic applications at the atomic level requires accurate force field parameters to describe the water-hBN interaction. In this work, we begin with benchmark quality first principles quantum Monte Carlo calculations on the interaction energy between water and hBN, which are used to validate random phase approximation (RPA) calculations. We then proceed with RPA to derive force field parameters, which are used to simulate water contact angle on bulk hBN, attaining a value within the experimental uncertainties. This paper demonstrates that end-to-end multiscale modeling, starting at detailed many-body quantum mechanics and ending with macroscopic properties, with the approximations controlled along the way, is feasible for these systems.

  14. The hexagon hypothesis: Six disruptive scenarios.

    PubMed

    Burtles, Jim

    2015-01-01

    This paper aims to bring a simple but effective and comprehensive approach to the development, delivery and monitoring of business continuity solutions. To ensure that the arguments and principles apply across the board, the paper sticks to basic underlying concepts rather than sophisticated interpretations. First, the paper explores what exactly people are defending themselves against. Secondly, the paper looks at how defences should be set up. Disruptive events tend to unfold in phases, each of which invites a particular style of protection, ranging from risk management through to business continuity to insurance cover. Their impact upon any business operation will fall into one of six basic scenarios. The hexagon hypothesis suggests that everyone should be prepared to deal with each of these six disruptive scenarios and it provides them with a useful benchmark for business continuity.

  15. Hexagonal boron nitride and water interaction parameters

    NASA Astrophysics Data System (ADS)

    Wu, Yanbin; Wagner, Lucas K.; Aluru, Narayana R.

    2016-04-01

    The study of hexagonal boron nitride (hBN) in microfluidic and nanofluidic applications at the atomic level requires accurate force field parameters to describe the water-hBN interaction. In this work, we begin with benchmark quality first principles quantum Monte Carlo calculations on the interaction energy between water and hBN, which are used to validate random phase approximation (RPA) calculations. We then proceed with RPA to derive force field parameters, which are used to simulate water contact angle on bulk hBN, attaining a value within the experimental uncertainties. This paper demonstrates that end-to-end multiscale modeling, starting at detailed many-body quantum mechanics and ending with macroscopic properties, with the approximations controlled along the way, is feasible for these systems.

  16. Quantum emission from hexagonal boron nitride monolayers

    NASA Astrophysics Data System (ADS)

    Tran, Toan Trong; Bray, Kerem; Ford, Michael J.; Toth, Milos; Aharonovich, Igor

    2016-01-01

    Artificial atomic systems in solids are widely considered the leading physical system for a variety of quantum technologies, including quantum communications, computing and metrology. To date, however, room-temperature quantum emitters have only been observed in wide-bandgap semiconductors such as diamond and silicon carbide, nanocrystal quantum dots, and most recently in carbon nanotubes. Single-photon emission from two-dimensional materials has been reported, but only at cryogenic temperatures. Here, we demonstrate room-temperature, polarized and ultrabright single-photon emission from a colour centre in two-dimensional hexagonal boron nitride. Density functional theory calculations indicate that vacancy-related defects are a probable source of the emission. Our results demonstrate the unprecedented potential of van der Waals crystals for large-scale nanophotonics and quantum information processing.

  17. Quantum emission from hexagonal boron nitride monolayers

    NASA Astrophysics Data System (ADS)

    Aharonovich, Igor; Tran, Toantrong; Bray, Kerem; Ford, Michael J.; Toth, Milos; MTEE Collaboration

    Artificial atomic systems in solids are widely considered the leading physical system for a variety of quantum technologies, including quantum communications, computing and metrology. To date, however, room-temperature quantum emitters have only been observed in wide-bandgap semiconductors such as diamond and silicon carbide, nanocrystal quantum dots, and most recently in carbon nanotubes. Here, we demonstrate room-temperature, polarized single-photon emission from a colour centre in two-dimensional hexagonal boron nitride. The emitters emit at the red and the near infrared spectral range and exhibit narrowband ultra bright emission (~full width at half maximum of below 10 nm with more than three million counts/s). Density functional theory calculations indicate that vacancy-related defects are a probable source of the emission. Our results demonstrate the unprecedented potential of van der Waals crystals for large-scale nanophotonics and quantum information processing.

  18. Novel high frequency devices with graphene and GaN

    NASA Astrophysics Data System (ADS)

    Zhao, Pei

    This work focuses on exploring new materials and new device structures to develop novel devices that can operate at very high speed. In chapter 2, the high frequency performance limitations of graphene transistor with channel length less than 100 nm are explored. The simulated results predict that intrinsic cutoff frequency fT of graphene transistor can be close to 2 THz at 15 nm channel length. In chapter 3, we explored the possibility of developing a 2D materials based vertical tunneling device. An analytical model to calculate the channel potentials and current-voltage characteristics in a Symmetric tunneling Field-Effect-Transistor (SymFET) is presented. The symmetric resonant peak in SymFET is a good candidate for high-speed analog applications. Rest of the work focuses on Gallium Nitride (GaN), several novel device concepts based on GaN heterostructure have been proposed for high frequency and high power applications. In chapter 4, we compared the performance of GaN Schottky diodes on bulk GaN substrates and GaN-on-sapphire substrates. In addition, we also discussed the lateral GaN Schottky diode between metal/2DEGs. The advantage of lateral GaN Schottky diodes is the intrinsic cutoff frequency is in the THz range. In chapter 5, a GaN Heterostructure barrier diode (HBD) is designed using the polarization charge and band offset at the AlGaN/GaN heterojunction. The polarization charge at AlGaN/GaN interface behaves as a delta-doping which induces a barrier without any chemical doping. The IV characteristics can be explained by the barrier controlled thermionic emission current. GaN HBDs can be directly integrated with GaN HEMTs, and serve as frequency multipliers or mixers for RF applications. In chapter 6, a GaN based negative effective mass oscillator (NEMO) is proposed. The current in NEMO is estimated under the ballistic limits. Negative differential resistances (NDRs) can be observed with more than 50% of the injected electrons occupied the negative

  19. Structural domain walls in polar hexagonal manganites

    NASA Astrophysics Data System (ADS)

    Kumagai, Yu

    2014-03-01

    The domain structure in the multiferroic hexagonal manganites is currently intensely investigated, motivated by the observation of intriguing sixfold topological defects at their meeting points [Choi, T. et al,. Nature Mater. 9, 253 (2010).] and nanoscale electrical conductivity at the domain walls [Wu, W. et al., Phys. Rev. Lett. 108, 077203 (2012).; Meier, D. et al., Nature Mater. 11, 284 (2012).], as well as reports of coupling between ferroelectricity, magnetism and structural antiphase domains [Geng, Y. et al., Nano Lett. 12, 6055 (2012).]. The detailed structure of the domain walls, as well as the origin of such couplings, however, was previously not fully understood. In the present study, we have used first-principles density functional theory to calculate the structure and properties of the low-energy structural domain walls in the hexagonal manganites [Kumagai, Y. and Spaldin, N. A., Nature Commun. 4, 1540 (2013).]. We find that the lowest energy domain walls are atomically sharp, with {210}orientation, explaining the orientation of recently observed stripe domains and suggesting their topological protection [Chae, S. C. et al., Phys. Rev. Lett. 108, 167603 (2012).]. We also explain why ferroelectric domain walls are always simultaneously antiphase walls, propose a mechanism for ferroelectric switching through domain-wall motion, and suggest an atomistic structure for the cores of the sixfold topological defects. This work was supported by ETH Zurich, the European Research Council FP7 Advanced Grants program me (grant number 291151), the JSPS Postdoctoral Fellowships for Research Abroad, and the MEXT Elements Strategy Initiative to Form Core Research Center TIES.

  20. Formation of hexagonal 9R silicon polytype by ion implantation

    NASA Astrophysics Data System (ADS)

    Korolev, D. S.; Nikolskaya, A. A.; Krivulin, N. O.; Belov, A. I.; Mikhaylov, A. N.; Pavlov, D. A.; Tetelbaum, D. I.; Sobolev, N. A.; Kumar, M.

    2017-08-01

    Transmission electron-microscopy examination revealed the appearance of a hexagonal silicon (9R polytype) inclusions in the subsrface silicon layer upon ion implantation and subsequent heat treatment of the SiO2/Si structure. The formation of this hexagonal phase is stimulated by mechanical stresses arising in the heterophase system in the course of ion implantation.

  1. ARM MJO Investigation Experiment on Gan Island (AMIE-Gan) Science Plan

    SciTech Connect

    Long, CL; Del Genio, A; Deng, M; Fu, X; Gustafson, W; Houze, R; Jakob, C; Jensen, M; Johnson, R; Liu, X; Luke, E; May, P; McFarlane, S; Minnis, P; Schumacher, C; Vogelmann, A; Wang, Y; Webster, P; Xie, S; Zhang, C

    2011-04-11

    The overarching campaign, which includes the ARM Mobile Facility 2 (AMF2) deployment in conjunction with the Dynamics of the Madden-Julian Oscillation (DYNAMO) and the Cooperative Indian Ocean experiment on intraseasonal variability in the Year 2011 (CINDY2011) campaigns, is designed to test several current hypotheses regarding the mechanisms responsible for Madden-Julian Oscillation (MJO) initiation and propagation in the Indian Ocean area. The synergy between the proposed AMF2 deployment with DYNAMO/CINDY2011, and the corresponding funded experiment on Manus, combine for an overarching ARM MJO Investigation Experiment (AMIE) with two components: AMF2 on Gan Island in the Indian Ocean (AMIE-Gan), where the MJO initiates and starts its eastward propagation; and the ARM Manus site (AMIE-Manus), which is in the general area where the MJO usually starts to weaken in climate models. AMIE-Gan will provide measurements of particular interest to Atmospheric System Research (ASR) researchers relevant to improving the representation of MJO initiation in climate models. The framework of DYNAMO/CINDY2011 includes two proposed island-based sites and two ship-based locations forming a square pattern with sonde profiles and scanning precipitation and cloud radars at both island and ship sites. These data will be used to produce a Variational Analysis data set coinciding with the one produced for AMIE-Manus. The synergy between AMIE-Manus and AMIE-Gan will allow studies of the initiation, propagation, and evolution of the convective cloud population within the framework of the MJO. As with AMIE-Manus, AMIE-Gan/DYNAMO also includes a significant modeling component geared toward improving the representation of MJO initiation and propagation in climate and forecast models. This campaign involves the deployment of the second, marine-capable, AMF; all of the included measurement systems; and especially the scanning and vertically pointing radars. The campaign will include sonde

  2. High nitrogen pressure solution growth of GaN

    NASA Astrophysics Data System (ADS)

    Bockowski, Michal

    2014-10-01

    Results of GaN growth from gallium solution under high nitrogen pressure are presented. Basic of the high nitrogen pressure solution (HNPS) growth method is described. A new approach of seeded growth, multi-feed seed (MFS) configuration, is demonstrated. The use of two kinds of seeds: free-standing hydride vapor phase epitaxy GaN (HVPE-GaN) obtained from metal organic chemical vapor deposition (MOCVD)-GaN/sapphire templates and free-standing HVPE-GaN obtained from the ammonothermally grown GaN crystals, is shown. Depending on the seeds’ structural quality, the differences in the structural properties of pressure grown material are demonstrated and analyzed. The role and influence of impurities, like oxygen and magnesium, on GaN crystals grown from gallium solution in the MFS configuration is presented. The properties of differently doped GaN crystals are discussed. An application of the pressure grown GaN crystals as substrates for electronic and optoelectronic devices is reported.

  3. Fabrication and properties of nanoporous GaN films

    NASA Astrophysics Data System (ADS)

    Wang, Y. D.; Chua, S. J.; Sander, M. S.; Chen, P.; Tripathy, S.; Fonstad, C. G.

    2004-08-01

    Nanopore arrays with pore diameters of approximately 75nm were fabricated in GaN films by inductively coupled plasma etching using anodic aluminum oxide (AAO) films as etch masks. Nanoporous AAO films were formed on the GaN surface by evaporating an Al film onto a GaN epilayer and subsequently anodizing the aluminum. To minimize plasma-induced damage, the template was exposed to CF4-based plasma conditions. Scanning electron microscopy analysis shows that the diameter and the periodicity of the nanopores in the GaN were directly transferred from the original anodic alumina template. The pore diameter in the AAO film can be easily controlled by tuning the anodization conditions. Atomic force microscopy, photoluminescence, and micro-Raman techniques were employed to assess the etched GaN nanopore surface. This cost-effective, nonlithographic method to produce nano-patterned GaN templates is expected to be useful for growth and fabrication of nitride-based nanostructures and photonic band gap materials.

  4. Comparison of presumptive blood test kits including hexagon OBTI.

    PubMed

    Johnston, Emma; Ames, Carole E; Dagnall, Kathryn E; Foster, John; Daniel, Barbara E

    2008-05-01

    Four presumptive blood tests, Hexagon OBTI, Hemastix(R), Leucomalachite green (LMG), and Kastle-Meyer (KM) were compared for their sensitivity in the identification of dried bloodstains. Stains of varying blood dilutions were subjected to each presumptive test and the results compared. The Hexagon OBTI buffer volume was also reduced to ascertain whether this increased the sensitivity of the kit. The study found that Hemastix(R) was the most sensitive test for trace blood detection. Only with the reduced buffer volume was the Hexagon OBTI kit as sensitive as the LMG and KM tests. However, the Hexagon OBTI kit has the advantage of being a primate specific blood detection kit. This study also investigated whether the OBTI buffer within the kit could be utilized for DNA profiling after presumptive testing. The results show that DNA profiles can be obtained from the Hexagon OBTI kit buffer directly.

  5. Elastic and mechanical properties of hexagonal diamond under pressure

    NASA Astrophysics Data System (ADS)

    Güler, E.; Güler, M.

    2015-05-01

    Hexagonal diamond is the harder and stiffer alternative of traditional cubic diamond for today's technology. Although several theoretical attempts have been performed to understand the ground-state elastic properties of hexagonal diamond, little is known about the high-pressure elastic properties of this key material. Unlike previous theoretical methods, we report the application of second-generation reactive bond order potential for the first time to elaborate the pressure-dependent properties of hexagonal diamond in conjunction with geometry optimization calculations up to 500 GPa. Pressure dependency of density, five independent elastic constants, bulk, shear and Young moduli, Poisson ratio, elastic wave velocities, anisotropy parameter, Kleinman parameter, and stability conditions of hexagonal diamond were evaluated. Overall, considered properties of hexagonal diamond display evident increments under pressure, and their ground-state values are in reasonable agreement with available theoretical data.

  6. Epitaxial PbxZr1-xTiO3 on GaN

    NASA Astrophysics Data System (ADS)

    Paisley, E. A.; Craft, H. S.; Losego, M. D.; Lu, H.; Gruverman, A.; Collazo, R.; Sitar, Z.; Maria, J.-P.

    2013-02-01

    Epitaxial integration of PbxZr1-xTiO3 (PZT) (111) with GaN (0002) presents the possibility of polarity coupling across a functional-oxide/nitride heteropolar interface. This work describes the synthesis and characterization of such thin film heterostructures by magnetron sputtering, with specific attention given to process optimization. Using x-ray diffraction and electrical characterization, the growth of epitaxial PZT (˜250 nm) on GaN and PZT on MgO/GaN stacks was verified. A two-stage growth process was developed for epitaxial PZT with a deposition temperature of 300 °C and an ex-situ anneal at 650 °C, which was effective in mitigating interfacial reactions and promoting phase-pure perovskite growth. Electrical analysis of interdigital capacitors revealed a nonlinear and hysteretic dielectric response consistent with ferroelectric PZT. Piezoresponse force microscopy (PFM) characterization shows clear evidence of ferroelectric switching, and PFM hysteresis loop analysis shows minimal evidence for direct polarity coupling, but suggests that band offsets which accompany the oxide-nitride heterostructures influence switching.

  7. Surfactant assisted growth of MgO films on GaN

    SciTech Connect

    Paisley, Elisibeth A.; Shelton, T C; Mita, S; Gaddy, Brian E.; Irving, D L; Christen, Hans M; Sitar, Z; Biegalski, Michael D; Maria, Jon Paul

    2012-01-01

    Thin epitaxial films of <111> oriented MgO on [0001]-oriented GaN were grown by molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) using the assistance of a vapor phase surfactant. In both cases, surfactant incorporation enabled layer-by-layer growth and a smooth terminal surface due to stabilizing the {111} rocksalt facet. MBE growth of MgO in water terminates after several monolayers, and is attributed to saturation of surface active sites needed to facilitate the Mg oxidation reaction. MgO films prepared by PLD grow continuously, this occurs due to the presence of excited oxidizing species in the laser plasma eliminate the need for catalytic surface sites. Metal-insulator-semiconductor capacitor structures were fabricated on n-type GaN. A comparison of leakage current density for conventional and surfactant-assisted growth reveals a nearly two order of magnitude reduction in leakage current density for the smoother surfactant-assisted samples. Collectively, these data verify numerous predictions and calculations regarding the role of H-termination in regulating the habit of MgO crystals.

  8. Thermoelectric Properties of MOVPE Grown AlInN, Lattice-Matched to GaN

    NASA Astrophysics Data System (ADS)

    Zhang, Jing; Tong, Hua; Liu, Guangyu; Herbsommer, Juan; Huang, Gensheng; Tansu, Nelson

    2010-03-01

    In this work, we investigate experimentally the growth and thermoelectric properties, i.e., thermal conductivity, Seebeck coefficient, and electrical conductivity, of n-type wurtzite high quality AlxIn1-xN, grown on GaN template on sapphire substrate by MOVPE, in-plane lattice-matched to GaN. The thermal conductivity is measured by 3φ method differential technique for thin films. The thermal conductivity value of Al0.83In0.17N is measured as 5.7 W/(mK). The Seebeck coefficient is calculated as the ratio of measured voltage difference and temperature difference when a temperature gradient is created in the sample. The absolute Seebeck coefficient value of Al0.83In0.17N is measured as 6.2x10-4 V/K. The sheet resistivity of lattice-matched Al0.83In0.17N is measured using Van der Pauw scheme and the electric conductivity is acquired accordingly to be 2.9x10^4 /(φ.m). The Z*T value of Al0.83In0.17N obtained is above 0.2 at room temperature. The results indicate AlInN based alloys are good candidates for thermoelectric devices.

  9. Doped GaN nanowires on diamond: Structural properties and charge carrier distribution

    NASA Astrophysics Data System (ADS)

    Schuster, Fabian; Winnerl, Andrea; Weiszer, Saskia; Hetzl, Martin; Garrido, Jose A.; Stutzmann, Martin

    2015-01-01

    In this work, we present a detailed study on GaN nanowire doping, which is vital for device fabrication. The nanowires (NWs) are grown by means of molecular beam epitaxy on diamond (111) substrates. Dopant atoms are found to facilitate nucleation, thus an increasing NW density is observed for increasing dopant fluxes. While maintaining nanowire morphology, we demonstrate the incorporation of Si and Mg up to concentrations of 9 × 1020cm-3 and 1 × 1020cm-3 , respectively. The dopant concentration in the nanowire cores is determined by the thermodynamic solubility limit, whereas excess dopants are found to segregate to the nanowire surface. The strain state of the NWs is investigated by X-ray diffraction, which confirms a negligible strain compared to planar thin films. Doping-related emissions are identified in low-temperature photoluminescence spectroscopy and the temperature quenching yields ionization energies of Si donors and Mg acceptors of 17 meV and 167 meV, respectively. At room temperature, luminescence and absorption spectra are found to coincide and the sub-band gap absorption is suppressed in n-type NWs. The charge carrier distribution in doped GaN nanowires is simulated under consideration of surface states at the non-polar side facets. For doping concentrations below 1017cm-3 , the nanowires are depleted of charge carriers, whereas they become highly conductive above 1019cm-3 .

  10. Molecular beam epitaxy as a method for the growth of free-standing bulk zinc-blende GaN and AlGaN crystals

    NASA Astrophysics Data System (ADS)

    Novikov, S. V.; Staddon, C. R.; Foxon, C. T.; Luckert, F.; Edwards, P. R.; Martin, R. W.; Kent, A. J.

    2011-05-01

    We have studied the growth of zinc-blende GaN and AlxGa1-xN layers, structures and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for growth of very thin layers with monolayer control of their thickness. However, we have used the MBE technique for bulk crystal growth and have produced GaN layers up to 100 μm in thickness. Thick, undoped, cubic GaN films were grown on semi-insulating GaAs (0 0 1) substrates by a modified plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The resulting free-standing GaN wafers may be used as substrates for further epitaxy of cubic GaN-based structures and devices. We have demonstrated that the PA-MBE process, we had developed, also allows us to achieve free-standing zinc-blende AlxGa1-xN wafers.

  11. Role of the ganSPQAB Operon in Degradation of Galactan by Bacillus subtilis

    PubMed Central

    Watzlawick, Hildegard; Altenbuchner, Josef

    2016-01-01

    ABSTRACT Bacillus subtilis possesses different enzymes for the utilization of plant cell wall polysaccharides. This includes a gene cluster containing galactan degradation genes (ganA and ganB), two transporter component genes (ganQ and ganP), and the sugar-binding lipoprotein-encoding gene ganS (previously known as cycB). These genes form an operon that is regulated by GanR. The degradation of galactan by B. subtilis begins with the activity of extracellular GanB. GanB is an endo-β-1,4-galactanase and is a member of glycoside hydrolase (GH) family 53. This enzyme was active on high-molecular-weight arabinose-free galactan and mainly produced galactotetraose as well as galactotriose and galactobiose. These galacto-oligosaccharides may enter the cell via the GanQP transmembrane proteins of the galactan ABC transporter. The specificity of the galactan ABC transporter depends on the sugar-binding lipoprotein, GanS. Purified GanS was shown to bind galactotetraose and galactotriose using thermal shift assay. The energy for this transport is provided by MsmX, an ATP-binding protein. The transported galacto-oligosaccharides are further degraded by GanA. GanA is a β-galactosidase that belongs to GH family 42. The GanA enzyme was able to hydrolyze short-chain β-1,4-galacto-oligosaccharides as well as synthetic β-galactopyranosides into galactose. Thermal shift assay as well as electrophoretic mobility shift assay demonstrated that galactobiose is the inducer of the galactan operon regulated by GanR. DNase I footprinting revealed that the GanR protein binds to an operator overlapping the −35 box of the σA-type promoter of Pgan, which is located upstream of ganS. IMPORTANCE Bacillus subtilis is a Gram-positive soil bacterium that utilizes different types of carbohydrates, such as pectin, as carbon sources. So far, most of the pectin degradation systems and enzymes have been thoroughly studied in B. subtilis. Nevertheless, the B. subtilis utilization system of galactan

  12. Role of the ganSPQAB Operon in Degradation of Galactan by Bacillus subtilis.

    PubMed

    Watzlawick, Hildegard; Morabbi Heravi, Kambiz; Altenbuchner, Josef

    2016-10-15

    Bacillus subtilis possesses different enzymes for the utilization of plant cell wall polysaccharides. This includes a gene cluster containing galactan degradation genes (ganA and ganB), two transporter component genes (ganQ and ganP), and the sugar-binding lipoprotein-encoding gene ganS (previously known as cycB). These genes form an operon that is regulated by GanR. The degradation of galactan by B. subtilis begins with the activity of extracellular GanB. GanB is an endo-β-1,4-galactanase and is a member of glycoside hydrolase (GH) family 53. This enzyme was active on high-molecular-weight arabinose-free galactan and mainly produced galactotetraose as well as galactotriose and galactobiose. These galacto-oligosaccharides may enter the cell via the GanQP transmembrane proteins of the galactan ABC transporter. The specificity of the galactan ABC transporter depends on the sugar-binding lipoprotein, GanS. Purified GanS was shown to bind galactotetraose and galactotriose using thermal shift assay. The energy for this transport is provided by MsmX, an ATP-binding protein. The transported galacto-oligosaccharides are further degraded by GanA. GanA is a β-galactosidase that belongs to GH family 42. The GanA enzyme was able to hydrolyze short-chain β-1,4-galacto-oligosaccharides as well as synthetic β-galactopyranosides into galactose. Thermal shift assay as well as electrophoretic mobility shift assay demonstrated that galactobiose is the inducer of the galactan operon regulated by GanR. DNase I footprinting revealed that the GanR protein binds to an operator overlapping the -35 box of the σ(A)-type promoter of Pgan, which is located upstream of ganS IMPORTANCE: Bacillus subtilis is a Gram-positive soil bacterium that utilizes different types of carbohydrates, such as pectin, as carbon sources. So far, most of the pectin degradation systems and enzymes have been thoroughly studied in B. subtilis Nevertheless, the B. subtilis utilization system of galactan, which is

  13. DLTS study of n-type GaN grown by MOCVD on GaN substrates

    NASA Astrophysics Data System (ADS)

    Tokuda, Y.; Matsuoka, Y.; Ueda, H.; Ishiguro, O.; Soejima, N.; Kachi, T.

    2006-10-01

    Electron traps in n-type GaN layers grown homoepitaxially by MOCVD on free-standing GaN substrates have been characterized using DLTS for vertical Schottky diodes. Two free-standing HVPE GaN substrates (A and B), obtained from two different sources, are used. The Si-doped GaN layers with the thickness of 5 μm are grown on an area of 0.9×0.9 cm 2 of substrate A and on an area of 1×1 cm 2 of substrate B. Two traps labeled B1 (Ec-0.23 eV) and B2 (Ec-0.58 eV) are observed with trap B2 dominant in GaN on both substrates. There exist no dislocation-related traps which have been previously observed in MOCVD GaN on sapphire. This might be correlated to the reduction in dislocation density due to the homoepitaxial growth. However, it is found that there is a large variation, more than an order of magnitude, in trap B2 concentration and that the B2 spatial distributions are different between the two substrates used.

  14. Growth of freestanding GaN using pillar-epitaxial lateral overgrowth from GaN nanocolumns

    NASA Astrophysics Data System (ADS)

    Bougrioua, Z.; Gibart, P.; Calleja, E.; Jahn, U.; Trampert, A.; Ristic, J.; Utrera, M.; Nataf, G.

    2007-12-01

    Dislocation-free and strain-free GaN nanopillars, grown on Si by molecular beam epitaxy, were used as nanoseeds for a new form of epitaxial lateral overgrowth (ELO) by metalorganic vapour phase epitaxy (MOVPE) until full coalescence. Such overgrown GaN films are almost relaxed and were used as templates for producing thick GaN layers by halide vapour phase epitaxy (HVPE). The final GaN film is easily separated from the starting Si substrate. This is henceforth a new technology to produce freestanding GaN. The GaN crystal quality was assessed by transmission electron microscopy (TEM), photo- and cathodoluminescence (PL, CL). It was seen that the pillar-ELO is produced from a limited number of nanopillars. Some dislocations and basal stacking faults are formed during the coalescence. However, those that propagate parallel to the substrate do not replicate in the top layer and it is expected that the thickened material present a reduced defect density.

  15. Saturn's Hexagon as Summer Solstice Approaches

    NASA Image and Video Library

    2017-05-24

    These natural color views from NASA's Cassini spacecraft compare the appearance of Saturn's north-polar region in June 2013 and April 2017. In both views, Saturn's polar hexagon dominates the scene. The comparison shows how clearly the color of the region changed in the interval between the two views, which represents the latter half of Saturn's northern hemisphere spring. In 2013, the entire interior of the hexagon appeared blue. By 2017, most of the hexagon's interior was covered in yellowish haze, and only the center of the polar vortex retained the blue color. The seasonal arrival of the sun's ultraviolet light triggers the formation of photochemical aerosols, leading to haze formation. The general yellowing of the polar region is believed to be caused by smog particles produced by increasing solar radiation shining on the polar region as Saturn approached the northern summer solstice on May 24, 2017. Scientists are considering several ideas to explain why the center of the polar vortex remains blue while the rest of the polar region has turned yellow. One idea is that, because the atmosphere in the vortex's interior is the last place in the northern hemisphere to be exposed to spring and summer sunlight, smog particles have not yet changed the color of the region. A second explanation hypothesizes that the polar vortex may have an internal circulation similar to hurricanes on Earth. If the Saturnian polar vortex indeed has an analogous structure to terrestrial hurricanes, the circulation should be downward in the eye of the vortex. The downward circulation should keep the atmosphere clear of the photochemical smog particles, and may explain the blue color. Images captured with Cassini's wide-angle camera using red, green and blue spectral filters were combined to create these natural-color views. The 2013 view (left in the combined view), was captured on June 25, 2013, when the spacecraft was about 430,000 miles (700,000 kilometers) away from Saturn. The

  16. Effect of periodic deflector embedded in InGaN /GaN light emitting diode

    NASA Astrophysics Data System (ADS)

    Kim, Hyung Gu; Na, Min Gyu; Kim, Hyun Kyu; Kim, Hee Yun; Ryu, Jae Hyoung; Cuong, Tran Viet; Hong, Chang-Hee

    2007-06-01

    This letter proposes a concept of InGaN /GaN light emitting diodes with periodic deflector embedded structure (PDE-LED). The PDE-LED was grown on a sapphire substrate with SiO2 hexagonal patterned mask using selective metal-organic chemical deposition. More than 200 artificial inverted polygonal pyramids (AIPPs), which included six R planes and six N planes deflectors with inclined angles of 57° and 61°, respectively, were formed and periodically distributed on masked area. These AIPP deflectors revealed a superior capability of enhancing light extraction efficiency mainly because the AIPP deflector structure could provide multiple chances for photons to escape from the LED sidewall as opposed to a rectangular conventional LED. Thus, the light output power of the PDE-LED was 1.51 times higher than that of a conventional LED at an injection current of 20mA, while forward bias voltage and leakage current were compatible to those of conventional LEDs.

  17. Epitaxial chemical vapour deposition growth of monolayer hexagonal boron nitride on a Cu(111)/sapphire substrate.

    PubMed

    Uchida, Yuki; Iwaizako, Tasuku; Mizuno, Seigi; Tsuji, Masaharu; Ago, Hiroki

    2017-03-22

    Hexagonal boron nitride (h-BN), an atomically thin insulating material, shows a large band gap, mechanical flexibility, and optical transparency. It can be stacked with other two-dimensional (2D) materials through van der Waals interactions to form layered heterostructures. These properties promise its application as an insulating layer of novel 2D electronic devices due to its atomically smooth surface with a large band gap. Herein, we demonstrated the ambient-pressure chemical vapour deposition (CVD) growth of high-quality, large-area monolayer h-BN on a Cu(111) thin film deposited on a c-plane sapphire using ammonia borane (BH3NH3) as the feedstock. Highly oriented triangular h-BN grains grow on Cu(111), which finally coalescence to cover the entire Cu surface. Low-energy electron diffraction (LEED) measurements indicated that the hexagonal lattice of the monolayer h-BN is well-oriented along the underlying Cu(111) lattice, thus implying the epitaxial growth of h-BN, which can be applied in various 2D electronic devices.

  18. Ultracold Quantum Gases in Hexagonal Optical Lattices

    NASA Astrophysics Data System (ADS)

    Sengstock, Klaus

    2010-03-01

    Hexagonal structures occur in a vast variety of systems, ranging from honeycombs of bees in life sciences to carbon nanotubes in material sciences. The latter, in particular its unfolded two-dimensional layer -- Graphene -- has rapidly grown to one of the most discussed topics in condensed-matter physics. Not only does it show proximity to various carbon-based materials but also exceptional properties owing to its unusual energy spectrum. In quantum optics, ultracold quantum gases confined in periodic light fields have shown to be very general and versatile instruments to mimic solid state systems. However, so far nearly all experiments were performed in cubic lattice geometries only. Here we report on the first experimental realization of ultracold quantum gases in a state-dependent, two-dimensional, Graphene-like optical lattice with hexagonal symmetry. The lattice is realized via a spin-dependent optical lattice structure with alternating σ^+ and σ^- -sites and thus constitutes a so called `magnetic'-lattice with `antiferromagnetic'-structure. Atoms with different spin orientation can be loaded to specific lattice sites or -- depending on the parameters -- to the whole lattice. As a consequence e.g. superpositions of a superfluid spin component with a different spin component in the Mott-insulating phase can be realized as well as spin-dependent transport properties, disorder etc. After preparing an antiferromagnetically ordered state we e.g. measure sustainable changes of the transport properties of the atoms. This manifests in a significant reduction of the tunneling as compared to a single-component system. We attribute this observation to a partial tunneling blockade for one spin component induced by population in another spin component localized at alternating lattice sites. Within a Gutzwiller-Ansatz we calculate the phase diagrams for the mixed spin-states and find very good agreement with our experimental results. Moreover, by state-resolved recording

  19. Microcrystalline hexagonal tungsten bronze. 2. Dehydration dynamics.

    PubMed

    Luca, Vittorio; Griffith, Christopher S; Hanna, John V

    2009-07-06

    Low-temperature (25-600 degrees C) thermal transformations have been studied for hydrothermally prepared, microcrystalline hexagonal tungsten bronze (HTB) phases A(x)WO(3+x/2).zH(2)O as a function of temperature, where A is an exchangeable cation (in this case Na(+) or Cs(+)) located in hexagonal structural tunnels. Thermal treatment of the as-prepared sodium- and cesium-exchanged phases in air were monitored using a conventional laboratory-based X-ray diffractometer, while thermal transformations in vacuum were studied using synchrotron X-ray and neutron diffraction. Concurrent thermogravimetric, diffuse reflectance infrared (DRIFT), and (23)Na and (133)Cs magic angle spinning (MAS) NMR spectroscopic studies have also been undertaken. For the cesium variant, cell volume contraction occurred from room temperature to about 350 degrees C, the regime in which water was "squeezed" out of tunnel sites. This was followed by a lattice expansion in the 350-600 degrees C temperature range. Over the entire temperature range, a net thermal contraction was observed, and this was the result of an anisotropic change in the cell dimensions which included a shortening of the A-O2 bond length. These changes explain why Cs(+) ions are locked into tunnel positions at temperatures as low as 400 degrees C, subsequently inducing a significant reduction in Cs(+) extractability under low pH (nitric acid) conditions. The changing Cs(+) speciation as detected by (133)Cs MAS NMR showed a condensation from multiple Cs sites, presumably associated with differing modes of Cs(+) hydration in the tunnels, to a single Cs(+) environment upon thermal transformation and water removal. While similar lattice contraction was observed for the as-prepared sodium variant, the smaller radius of Na(+) caused it to be relatively easily removed with acid in comparison to the Cs(+) variant. From (23)Na MAS NMR studies of the parent material, complex Na(+) speciation was observed with dehydrated and various

  20. Spectroscopic measurements of the surface stoichiometry of chemical vapor deposited GaN

    NASA Astrophysics Data System (ADS)

    Craft, H. S.; Rice, A. L.; Collazo, R.; Sitar, Z.; Maria, J.-P.

    2011-02-01

    We report on the surface stoichiometry of Ga-polar GaN films grown by metalorganic chemical vapor deposition as studied by x-ray photoelectron spectroscopy. GaN film surfaces are found to be Ga-rich, with Ga:N ratios ranging from 1.3:1 to 3.2:1. In vacuo ion-beam sputter/annealing studies show that these treatments drive the apparent Ga:N surface composition farther from unity, either through a decrease in surface contamination, oxidation of the surface, or both. Simple annealing experiments decrease the Ga:N ratio. The measured GaN ratio is correlated with the GaN growth time, suggesting that residual Ga precursor after growth interacts with the GaN surface as it cools.

  1. Viability and proliferation of endothelial cells upon exposure to GaN nanoparticles.

    PubMed

    Braniste, Tudor; Tiginyanu, Ion; Horvath, Tibor; Raevschi, Simion; Cebotari, Serghei; Lux, Marco; Haverich, Axel; Hilfiker, Andres

    2016-01-01

    Nanotechnology is a rapidly growing and promising field of interest in medicine; however, nanoparticle-cell interactions are not yet fully understood. The goal of this work was to examine the interaction between endothelial cells and gallium nitride (GaN) semiconductor nanoparticles. Cellular viability, adhesion, proliferation, and uptake of nanoparticles by endothelial cells were investigated. The effect of free GaN nanoparticles versus the effect of growing endothelial cells on GaN functionalized surfaces was examined. To functionalize surfaces with GaN, GaN nanoparticles were synthesized on a sacrificial layer of zinc oxide (ZnO) nanoparticles using hydride vapor phase epitaxy. The uptake of GaN nanoparticles by porcine endothelial cells was strongly dependent upon whether they were fixed to the substrate surface or free floating in the medium. The endothelial cells grown on surfaces functionalized with GaN nanoparticles demonstrated excellent adhesion and proliferation, suggesting good biocompatibility of the nanostructured GaN.

  2. Large-area monolayer hexagonal boron nitride on Pt foil.

    PubMed

    Park, Ji-Hoon; Park, Jin Cheol; Yun, Seok Joon; Kim, Hyun; Luong, Dinh Hoa; Kim, Soo Min; Choi, Soo Ho; Yang, Woochul; Kong, Jing; Kim, Ki Kang; Lee, Young Hee

    2014-08-26

    Hexagonal boron nitride (h-BN) has recently been in the spotlight due to its numerous applications including its being an ideal substrate for two-dimensional electronics, a tunneling material for vertical tunneling devices, and a growth template for heterostructures. However, to obtain a large area of h-BN film while maintaining uniform thickness is still challenging and has not been realized. Here, we report the systematical study of h-BN growth on Pt foil by using low pressure chemical vapor deposition with a borazine source. The monolayer h-BN film was obtained over the whole Pt foil (2 × 5 cm(2)) under <100 mTorr, where the size is limited only by the Pt foil size. A borazine source was catalytically decomposed on the Pt surface, leading to the self-limiting growth of the monolayer without the associating precipitation, which is very similar to the growth of graphene on Cu. The orientation of the h-BN domains was largely confined by the Pt domain, which is confirmed by polarizing optical microscopy (POM) assisted by the nematic liquid crystal (LC) film. The total pressure and orientation of the Pt lattice plane are crucial parameters for thickness control. At high pressure (∼0.5 Torr), thick film was grown on Pt (111), and in contrast, thin film was grown on Pt (001). Our advances in monolayer h-BN growth will play an important role to further develop a high quality h-BN film that can be used for vertical tunneling, optoelectronic devices and growth templates for a variety of heterostructures.

  3. Visualization of the hexagonal lattice in the erythrocyte membrane skeleton.

    PubMed

    Liu, S C; Derick, L H; Palek, J

    1987-03-01

    The isolated membrane skeleton of human erythrocytes was studied by high resolution negative staining electron microscopy. When the skeletal meshwork is spread onto a thin carbon film, clear images of a primarily hexagonal lattice of junctional F-actin complexes crosslinked by spectrin filaments are obtained. The regularly ordered network extends over the entire membrane skeleton. Some of the junctional complexes are arranged in the form of pentagons and septagons, approximately 3 and 8%, respectively. At least five forms of spectrin crosslinks are detected in the spread skeleton including a single spectrin tetramer linking two junctional complexes, three-armed Y-shaped spectrin molecules linking three junctional complexes, three-armed spectrin molecules connecting two junctional complexes with two arms bound to one complex and the third arm bound to the adjacent complex, double spectrin filaments linking two junctional complexes, and four-armed spectrin molecules linking two junctional complexes. Of these, the crosslinks of single spectrin tetramers and three-armed molecules are the most abundant and represent 84 and 11% of the total crosslinks, respectively. These observations are compatible with the presence of spectrin tetramers and oligomers in the erythrocyte membrane skeleton. Globular structures (9-12 nm in diameter) are attached to the majority of the spectrin tetramers or higher order oligomer-like molecules, approximately 80 nm from the distal ends of the spectrin tetramers. These globular structures are ankyrinor ankyrin/band 3-containing complexes, since they are absent when ankyrin and residual band 3 are extracted from the skeleton under hypertonic conditions.

  4. Effective elastic properties of two dimensional multiplanar hexagonal nanostructures

    NASA Astrophysics Data System (ADS)

    Mukhopadhyay, T.; Mahata, A.; Adhikari, S.; Asle Zaeem, M.

    2017-06-01

    A generalized analytical approach is presented to derive closed-form formulae for the elastic moduli of hexagonal multiplanar nano-structures. Hexagonal nano-structural forms are common for various materials. Four different classes of materials (single layer) from a structural point of view are proposed to demonstrate the validity and prospective application of the developed formulae. For example, graphene, an allotrope of carbon, consists of only carbon atoms to form a honeycomb like hexagonal lattice in a single plane, while hexagonal boron nitride (hBN) consists of boron and nitrogen atoms to form the hexagonal lattice in a single plane. Unlike graphene and hBN, there are plenty of other materials with hexagonal nano-structures that have the atoms placed in multiple planes such as stanene (consists of only Sn atoms) and molybdenum disulfide (consists of two different atoms: Mo and S). The physics based high-fidelity analytical model developed in this article are capable of obtaining the elastic properties in a computationally efficient manner for wide range of such materials with hexagonal nano-structures that are broadly classified in four classes from structural viewpoint. Results are provided for materials belonging to all the four classes, wherein a good agreement between the elastic moduli obtained using the proposed formulae and available scientific literature is observed.

  5. Phase transformation of molecular beam epitaxy-grown nanometer-thick Gd₂O₃ and Y₂O₃ on GaN.

    PubMed

    Chang, Wen-Hsin; Wu, Shao-Yun; Lee, Chih-Hsun; Lai, Te-Yang; Lee, Yi-Jun; Chang, Pen; Hsu, Chia-Hung; Huang, Tsung-Shiew; Kwo, J Raynien; Hong, Minghwei

    2013-02-01

    High quality nanometer-thick Gd₂O₃ and Y₂O₃ (rare-earth oxide, R₂O₃) films have been epitaxially grown on GaN (0001) substrate by molecular beam epitaxy (MBE). The R₂O₃ epi-layers exhibit remarkable thermal stability at 1100 °C, uniformity, and highly structural perfection. Structural investigation was carried out by in situ reflection high energy electron diffraction (RHEED) and ex-situ X-ray diffraction (XRD) with synchrotron radiation. In the initial stage of epitaxial growth, the R₂O₃ layers have a hexagonal phase with the epitaxial relationship of R₂O₃ (0001)(H)<1120>(H)//GaN(0001)(H)<1120>(H). With the increase in R₂O₃ film thickness, the structure of the R₂O₃ films changes from single domain hexagonal phase to monoclinic phase with six different rotational domains, following the R₂O₃ (201)(M)[020](M)//GaN(0001)(H)<1120>(H) orientational relationship. The structural details and fingerprints of hexagonal and monoclinic phase Gd₂O₃ films have also been examined by using electron energy loss spectroscopy (EELS). Approximate 3-4 nm is the critical thickness for the structural phase transition depending on the composing rare earth element.

  6. Mathematical Foundation for Plane Covering Using Hexagons

    NASA Technical Reports Server (NTRS)

    Johnson, Gordon G.

    1999-01-01

    This work is to indicate the development and mathematical underpinnings of the algorithms previously developed for covering the plane and the addressing of the elements of the covering. The algorithms are of interest in that they provides a simple systematic way of increasing or decreasing resolution, in the sense that if we have the covering in place and there is an image superimposed upon the covering, then we may view the image in a rough form or in a very detailed form with minimal effort. Such ability allows for quick searches of crude forms to determine a class in which to make a detailed search. In addition, the addressing algorithms provide an efficient way to process large data sets that have related subsets. The algorithms produced were based in part upon the work of D. Lucas "A Multiplication in N Space" which suggested a set of three vectors, any two of which would serve as a bases for the plane and also that the hexagon is the natural geometric object to be used in a covering with a suggested bases. The second portion is a refinement of the eyeball vision system, the globular viewer.

  7. Instability of vibrational modes in hexagonal lattice

    NASA Astrophysics Data System (ADS)

    Korznikova, Elena A.; Bachurin, Dmitry V.; Fomin, Sergey Yu.; Chetverikov, Alexander P.; Dmitriev, Sergey V.

    2017-02-01

    The phenomenon of modulational instability is investigated for all four delocalized short-wave vibrational modes recently found for the two-dimensional hexagonal lattice with the help of a group-theoretic approach. The polynomial pair potential with hard-type quartic nonlinearity ( β-FPU potential with β > 0) is used to describe interactions between atoms. As expected for the hard-type anharmonic interactions, for all four modes the frequency is found to increase with the amplitude. Frequency of the modes I and III bifurcates from the upper edge of the phonon spectrum, while that of the modes II and IV increases from inside the spectrum. It is also shown that the considered model supports spatially localized vibrational mode called discrete breather (DB) or intrinsic localized mode. DB frequency increases with the amplitude above the phonon spectrum. Two different scenarios of the mode decay were revealed. In the first scenario (for modes I and III), development of the modulational instability leads to a formation of long-lived DBs that radiate their energy slowly until thermal equilibrium is reached. In the second scenario (for modes II and IV) a transition to thermal oscillations of atoms is observed with no formation of DBs.

  8. Bootstrapping the Three-Loop Hexagon

    SciTech Connect

    Dixon, Lance J.; Drummond, James M.; Henn, Johannes M.; /Humboldt U., Berlin /Santa Barbara, KITP

    2011-11-08

    We consider the hexagonal Wilson loop dual to the six-point MHV amplitude in planar N = 4 super Yang-Mills theory. We apply constraints from the operator product expansion in the near-collinear limit to the symbol of the remainder function at three loops. Using these constraints, and assuming a natural ansatz for the symbol's entries, we determine the symbol up to just two undetermined constants. In the multi-Regge limit, both constants drop out from the symbol, enabling us to make a non-trivial confirmation of the BFKL prediction for the leading-log approximation. This result provides a strong consistency check of both our ansatz for the symbol and the duality between Wilson loops and MHV amplitudes. Furthermore, we predict the form of the full three-loop remainder function in the multi-Regge limit, beyond the leading-log approximation, up to a few constants representing terms not detected by the symbol. Our results confirm an all-loop prediction for the real part of the remainder function in multi-Regge 3 {yields} 3 scattering. In the multi-Regge limit, our result for the remainder function can be expressed entirely in terms of classical polylogarithms. For generic six-point kinematics other functions are required.

  9. Hyperbolic phonon polaritons in hexagonal boron nitride

    NASA Astrophysics Data System (ADS)

    Dai, Siyuan

    2015-03-01

    Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. While hyperbolic responses are normally achieved with metamaterials, hexagonal boron nitride (hBN) naturally possesses this property due to the anisotropic phonons in the mid-infrared. Using scattering-type scanning near-field optical microscopy, we studied polaritonic phenomena in hBN. We performed infrared nano-imaging of highly confined and low-loss hyperbolic phonon polaritons in hBN. The polariton wavelength was shown to be governed by the hBN thickness according to a linear law persisting down to few atomic layers [Science, 343, 1125-1129 (2014)]. Additionally, we carried out the modification of hyperbolic response in heterostructures comprised of a mononlayer graphene deposited on hBN. Electrostatic gating of the top graphene layer allows for the modification of wavelength and intensity of hyperbolic phonon polaritons in bulk hBN. The physics of the modification originates from the plasmon-phonon coupling in the hyperbolic medium. Furthermore, we demonstrated the ``hyperlens'' for subdiffractional imaging and focusing using a slab of hBN.

  10. Ab-initio study of hexagonal apatites

    NASA Astrophysics Data System (ADS)

    Calderin, Lazaro; Stott, Malcom J.

    2001-03-01

    A silicon stabilized mixture of calcium phosphate phases has been recognized as playing an important role in actively resorbable coatings and in ceramics as bone materials. The nature of this material is being investigated using a variety of techniques including a combination of crystallographic analysis of measured x-ray diffraction spectra, and ab initio quantum mechanics simulations. We have used all-electron, density functional based calculations to investigate a group of hexagonal apatites. The fully relaxed crystallographic structures of hydroxyapatite, and related apatites have been obtained. We will present the results and discuss the nature of the bonding in these materials. The x-ray diffraction pattern and the infra-red spectra have also been obtained and will be compared with experiment. Acknowledgments:This work is part of a collaboration with the Applied Ceramics group of M.Sayer, and with Millenium Biologix Inc. Support of the NSERC of Canada through the award of a Co-operative R & D grant to the collaboration is acknowledged.

  11. Pyramidal Image-Processing Code For Hexagonal Grid

    NASA Technical Reports Server (NTRS)

    Watson, Andrew B.; Ahumada, Albert J., Jr.

    1990-01-01

    Algorithm based on processing of information on intensities of picture elements arranged in regular hexagonal grid. Called "image pyramid" because image information at each processing level arranged in hexagonal grid having one-seventh number of picture elements of next lower processing level, each picture element derived from hexagonal set of seven nearest-neighbor picture elements in next lower level. At lowest level, fine-resolution of elements of original image. Designed to have some properties of image-coding scheme of primate visual cortex.

  12. Nylon flocked swab severely reduces Hexagon Obti sensibility.

    PubMed

    Frippiat, Christophe; De Roy, Gilbert; Fontaine, Louis-Marie; Dognaux, Sophie; Noel, Fabrice; Heudt, Laeticia; Lepot, Laurent

    2015-02-01

    Hexagon Obti immunological blood test and flocked swab are widely used in forensic laboratories. Nevertheless, up to now, no compatibility tests have been published between sampling with the ethylene oxide treated flocked swab and the Hexagon Obti blood detection strip. In this study, we investigated this compatibility. Our work shows that sampling with ethylene oxide treated flocked swab reduces by a factor of at least 100 the detection threshold of blood using the Hexagon Obti immunological test. Copyright © 2014 Elsevier Ireland Ltd. All rights reserved.

  13. Pyramidal Image-Processing Code For Hexagonal Grid

    NASA Technical Reports Server (NTRS)

    Watson, Andrew B.; Ahumada, Albert J., Jr.

    1990-01-01

    Algorithm based on processing of information on intensities of picture elements arranged in regular hexagonal grid. Called "image pyramid" because image information at each processing level arranged in hexagonal grid having one-seventh number of picture elements of next lower processing level, each picture element derived from hexagonal set of seven nearest-neighbor picture elements in next lower level. At lowest level, fine-resolution of elements of original image. Designed to have some properties of image-coding scheme of primate visual cortex.

  14. Calculation of refractive-index distribution of hexagonal GRIN lenses

    NASA Astrophysics Data System (ADS)

    Zhou, Zigang; Zhang, Ren; Chen, Kai

    2007-12-01

    The GRIN lens is widely used in optical communication and imaging systems. Its array can be used to design integrated optic imaging system, especially for hexagonal GRIN. In this paper, the analytic solution of refractive-index distribution of regularly hexagonal GRIN was obtained by separating variables and transforming coordinate. Having been simulated and compared, the correctness of this analytic solution was proved qualitatively and quantitatively. It has great benefit for further research of regular hexagonal GRIN lens and compound eye imaging system. Furthermore, a universal solution of the refractive-index distribution of a regular N-gon (N is even) lens was obtained by this method.

  15. Tailoring GaN semiconductor surfaces with biomolecules.

    PubMed

    Estephan, Elias; Larroque, Christian; Cuisinier, Frédéric J G; Bálint, Zoltán; Gergely, Csilla

    2008-07-24

    Functionalization of semiconductors constitutes a crucial step in using these materials for various electronic, photonic, biomedical, and sensing applications. Within the various possible approaches, selection of material-binding biomolecules from a random biological library, based on the natural recognition of proteins or peptides toward specific material, offers many advantages, most notably biocompatibility. Here we report on the selective functionalization of GaN, an important semiconductor that has found broad uses in the past decade due to its efficient electroluminescence and pronounced chemical stability. A 12-mer peptide ("GaN_probe") with specific recognition for GaN has evolved. The subtle interplay of mostly nonpolar hydrophobic and some polar amino acidic residues defines the high affinity adhesion properties of the peptide. The interaction forces between the peptide and GaN are quantified, and the hydrophobic domain of the GaN_probe is identified as primordial for the binding specificity. These nanosized binding blocks are further used for controlled placement of biotin-streptavidin complexes on the GaN surface. Thus, the controlled grow of a new, patterned inorganic-organic hybrid material is achieved. Tailoring of GaN by biological molecules can lead to a new class of nanostructured semiconductor-based devices.

  16. Native defects in GaN: a hybrid functional study

    NASA Astrophysics Data System (ADS)

    Diallo, Ibrahima Castillo; Demchenko, Denis

    Intrinsic defects play an important role in the performance of GaN-based devices. We present hybrid density functional calculations of the electronic and possible optical properties of interstitial N (Ni-Ni) , N antisite (NGa) , interstitial Ga (Gai) , Ga antisite (GaN) , Ga vacancy (VGa) , N vacancy (VN) and Ga-N divacancies (VGaVN) in GaN. Our results show that the vacancies display relatively low formation energies in certain samples, whereas antisites and interstitials are energetically less favorable. However, interstitials can be created by electron irradiation. For instance, in 2.5 MeV electron-irradiated GaN samples, a strong correlation between the frequently observed photoluminescence (PL) band centered around 0.85 eV accompanied with a rich phonon sideband of ~0.88 eV and the theoretical optical behavior of interstitial Ga is discussed. N vacancies are found to likely contribute to the experimentally obtained green luminescence band (GL2) peaking at 2.24 eV in high-resistivity undoped and Mg-doped GaN. National Science Foundation (DMR-1410125) and the Thomas F. and Kate Miller Jeffress Memorial Trust.

  17. Atomic layer deposition TiO2-Al2O3 stack: An improved gate dielectric on Ga-polar GaN metal oxide semiconductor capacitors

    DOE PAGES

    Wei, Daming; Edgar, James H.; Briggs, Dayrl P.; ...

    2014-10-15

    This research focuses on the benefits and properties of TiO2-Al2O3 nano-stack thin films deposited on Ga2O3/GaN by plasma-assisted atomic layer deposition (PA-ALD) for gate dielectric development. This combination of materials achieved a high dielectric constant, a low leakage current, and a low interface trap density. Correlations were sought between the films’ structure, composition, and electrical properties. The gate dielectrics were approximately 15 nm thick and contained 5.1 nm TiO2, 7.1 nm Al2O3 and 2 nm Ga2O3 as determined by spectroscopic ellipsometry. The interface carbon concentration, as measured by x-ray photoelectron spectroscopy (XPS) depth profile, was negligible for GaN pretreated bymore » thermal oxidation in O2 for 30 minutes at 850°C. The RMS roughness slightly increased after thermal oxidation and remained the same after ALD of the nano-stack, as determined by atomic force microscopy. The dielectric constant of TiO2-Al2O3 on Ga2O3/GaN was increased to 12.5 compared to that of pure Al2O3 (8~9) on GaN. In addition, the nano-stack's capacitance-voltage (C-V) hysteresis was small, with a total trap density of 8.74 × 1011 cm-2. The gate leakage current density (J=2.81× 10-8 A/cm2) was low at +1 V gate bias. These results demonstrate the promising potential of plasma ALD deposited TiO2/Al2O3 for serving as the gate oxide on Ga2O3/GaN based MOS devices.« less

  18. ALD TiO2-Al2O3 Stack: An Improved Gate Dielectrics on Ga-polar GaN MOSCAPs

    DOE PAGES

    Wei, Daming; Edgar, James H.; Briggs, Dayrl P.; ...

    2014-10-15

    This research focuses on the benefits and properties of TiO2-Al2O3 nano-stack thin films deposited on Ga2O3/GaN by plasma-assisted atomic layer deposition (PA-ALD) for gate dielectric development. This combination of materials achieved a high dielectric constant, a low leakage current, and a low interface trap density. Correlations were sought between the films’ structure, composition, and electrical properties. The gate dielectrics were approximately 15 nm thick and contained 5.1 nm TiO2, 7.1 nm Al2O3 and 2 nm Ga2O3 as determined by spectroscopic ellipsometry. The interface carbon concentration, as measured by x-ray photoelectron spectroscopy (XPS) depth profile, was negligible for GaN pretreated bymore » thermal oxidation in O2 for 30 minutes at 850°C. The RMS roughness slightly increased after thermal oxidation and remained the same after ALD of the nano-stack, as determined by atomic force microscopy. The dielectric constant of TiO2-Al2O3 on Ga2O3/GaN was increased to 12.5 compared to that of pure Al2O3 (8~9) on GaN. In addition, the nano-stack's capacitance-voltage (C-V) hysteresis was small, with a total trap density of 8.74 × 1011 cm-2. The gate leakage current density (J=2.81× 10-8 A/cm2) was low at +1 V gate bias. These results demonstrate the promising potential of plasma ALD deposited TiO2/Al2O3 for serving as the gate oxide on Ga2O3/GaN based MOS devices.« less

  19. 14. DETAIL OF ROOF SUPPORT BEAMS BRACED AGAINST HEXAGONAL WOODEN ...

    Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

    14. DETAIL OF ROOF SUPPORT BEAMS BRACED AGAINST HEXAGONAL WOODEN COMPRESSION RING AT TOP OF CENTRAL ROOF TRUSS. - Saratoga Gas Light Company, Gasholder No. 2, Niagara Mohawk Power Corporation Substation Facility, intersection of Excelsior & East Avenues, Saratoga Springs, NY

  20. Hydrogen bonding in the hexagonal ice surface.

    PubMed

    Barnett, Irene Li; Groenzin, Henning; Shultz, Mary Jane

    2011-06-16

    A recently developed technique in sum frequency generation spectroscopy, polarization angle null (or PAN-SFG), is applied to two orientations of the prism face of hexagonal ice. It is found that the vibrational modes of the surface are similar in different faces. As in the basal face, the prism face of ice contains five dominant resonances: 3096, 3146, 3205, 3253, and 3386 cm(-1). On the basal face, the reddest resonance occurs at 3098 cm(-1); within the bandwidth, the same as the prism face. On both the prism and basal faces, this mode contains a significant quadrupole component and is assigned to the bilayer stitching hydrogen bonds. The bluest of the resonances, 3386 cm(-1), occurs slightly blue-shifted at 3393 cm(-1) in the basal face. The prism face has two orientations: one with the optic or c axis in the input plane (the plane formed by the surface normal and the interrogating beam propagation) and one with the c axis perpendicular to the input plane. The 3386 cm(-1) mode has significant intensity only with the c axis in the input plane. On the basis of these orientation characteristics, the 3386 cm(-1) mode is assigned to double-donor molecules in either the top half bilayer or in the lower half bilayer. On the basis of frequency considerations, it is assigned to double-donor molecules in the top half bilayer. These are water molecules containing a nonbonded lone pair. In addition to identification of the components of the broad hydrogen-bonded region, PAN-SFG measures the tangential vs longitudinal content of the vibrational modes. In accord with previous suggestions, the lower frequency modes are predominantly tangential, whereas the higher frequency modes are mainly longitudinal. On the prism face, the 3386 cm(-1) mode is entirely longitudinal.

  1. Optoelectronic properties of hexagonal boron nitride epilayers

    NASA Astrophysics Data System (ADS)

    Cao, X. K.; Majety, S.; Li, J.; Lin, J. Y.; Jiang, H. X.

    2013-01-01

    This paper summarizes recent progress primarily achieved in authors' laboratory on synthesizing hexagonal boron nitride (hBN) epilayers by metal organic chemical vapor deposition (MCVD) and studies of their structural and optoelectronic properties. The structural and optical properties of hBN epilayers have been characterized by x-ray diffraction (XRD) and photoluminescence (PL) studies and compared to the better understood wurtzite AIN epilayers with a comparable energy bandgap. These MOCVD grown hBN epilayers exhibit highly efficient band-edge PL emission lines centered at around 5.5 eVat room temperature. The band-edge emission of hBN is two orders of magnitude higher than that of high quality AlN epilayers. Polarization-resolved PL spectroscopy revealed that hEN epilayers are predominantly a surface emission material, in which the band-edge emission with electric field perpendicular to the c-axis (Eemi⊥c) is about 1.7 times stronger than the component along the c-axis (Eemillc). This is in contrast to AIN, in which the band­ edge emission is known to be polarized along the c-axis, (Eemillc). Based on the graphene optical absorption concept, the estimated band-edge absorption coefficient of hBN is about 7x105 cm-1, which is more than 3 times higher than the value for AlN (~2x105 cm-1 . The hBN epilayer based photodetectors exhibit a sharp cut-off wavelength around 230 nm, which coincides with the band-edge PL emission peak and virtually no responses in the long wavelengths. The dielectric strength of hBN epilayers exceeds that of AlN and is greater than 4.5 MV/cm based on the measured result for an hBN epilayer released from the host sapphire substrate.

  2. Structural defects in bulk GaN

    NASA Astrophysics Data System (ADS)

    Liliental-Weber, Z.; dos Reis, R.; Mancuso, M.; Song, C. Y.; Grzegory, I.; Porowski, S.; Bockowski, M.

    2014-10-01

    Transmission Electron Microscopy (TEM) studies of undoped and Mg doped GaN layers grown on the HVPE substrates by High Nitrogen Pressure Solution (HNPS) with the multi-feed-seed (MFS) configuration are shown. The propagation of dislocations from the HVPE substrate to the layer is observed. Due to the interaction between these dislocations in the thick layers much lower density of these defects is observed in the upper part of the HNPS layers. Amorphous Ga precipitates with attached voids pointing toward the growth direction are observed in the undoped layer. This is similar to the presence of Ga precipitates in high-pressure platelets, however the shape of these precipitates is different. The Mg doped layers do not show Ga precipitates, but MgO rectangular precipitates are formed, decorating the dislocations. Results of TEM studies of HVPE layers grown on Ammonothermal substrates are also presented. These layers have superior crystal quality in comparison to the HNPS layers, as far as density of dislocation is concern. Occasionally some small inclusions can be found, but their chemical composition was not yet determined. It is expected that growth of the HNPS layers on these substrate will lead to large layer thickness obtained in a short time and with high crystal perfection needed in devices.

  3. Characterization of electrospun GaN nanofibers

    NASA Astrophysics Data System (ADS)

    Ramos, Idalia; Melendez, Anamaris; Morales, Kristle; Campo, Eva M.; Santiago-Aviles, Jorge J.

    2010-03-01

    Gallium Nitride shows characteristics pertinent to optoelectronics and gas sensing applications. Nanofibers have been produced using electrospinning and a precursor composed of Gallium (III) Nitrate Hydrate dissolved in Dimethyl-Acetamide and Cellulose Acetate in Acetone and DMA. The resulting nanofibers were sintered at 400C in nitrogen for one hour to decompose the polymer, the furnace atmosphere switched to ammonia and the fibers sintered for periods of 3, 5 and 7 hrs at 900C. They showed morphologies with unclear dependence on processing parameters. X-ray Diffraction revealed the evolution towards wurtzite phase through annealing. From line broadening we estimate a crystalline domain size of about 12 nm. Transmission Electron Microscopy suggests nucleation and growth of X-tallites while Fourier-Transform Infrared Spectroscopy and Ultraviolet-Visible Spectroscopy confirm the material evolution towards crystallinity and the production of wurtzite GaN nanofibers. I-V characteristics of single nanofibers show linearity with increments in conductivity for those fibers ammoniated during longer periods of time. Ongoing efforts aim at improving fabrication, sensing and photoluminescence characterization.

  4. Electrical contact to carbon nanotubes encapsulated in hexagonal boron nitride

    NASA Astrophysics Data System (ADS)

    Huang, Jhao-Wun; Pan, Cheng; Tran, Son; Taniguchi, Takashi; Bockrath, Marc; Lau, Jeanie

    2015-03-01

    Hexagonal boron nitride has been an excellent platform for low dimensional materials. We have fabricated ultra clean single-walled carbon nanotube(SWNT) devices encapsulated in hexagonal boron nitride by a dry transfer technique. Contacts to the SWNTs were made by reactive ion etching to expose the ends of SWNTs, followed by metal deposition. Ohmic contacts to SWNTs were achieved. We will discuss the quality of the contacts using different combinations of metals and present latest transport data.

  5. Structural and electronic properties of hexagonal yttrium trihydride

    SciTech Connect

    Wang, Y.; Chou, M.Y. )

    1995-03-15

    The structural and electronic properties of yttrium trihydride with metal atoms in the hexagonal-close-packed (hcp) structure are studied by the pseudopotential method within the local-density-functional approximation (LDA). It is found that the hydrogen positions within the metal lattice have a major role in determining these properties. Calculations confirmed that hexagonal YH[sub 3] with unusual wavelike hydrogen displacements (space group [ital D][sub 3[ital d

  6. Synthesis and Characterization of Hexagonal Boron Nitride (h- BN) Films

    DTIC Science & Technology

    2014-01-09

    Synthesis 1. Diborane- ammonia (B2H6-NH3- gases): Early results with these precursors were published in 2012. 5 Briefly, LPCVD growth of h-BN in a hot-wall...Approved for public release; distribution is unlimited. Synthesis and Characterization of Hexagonal Boron Nitride (h- BN) Films. The views, opinions and...1 ABSTRACT Number of Papers published in peer-reviewed journals: Synthesis and Characterization of Hexagonal Boron Nitride (h-BN) Films. Report Title

  7. The charge and spin transport properties in hexagonal silicene nanorings

    NASA Astrophysics Data System (ADS)

    Xu, N.; Zhang, H. Y.; Wu, X. Q.; Chen, Q.; Ding, J. W.

    2017-09-01

    Based on the tight-binding model, charge and spin transport properties of hexagonal silicene rings are investigated within the nonequilibrium Green’s function technique. The effects of external electric, magnetic and exchange fields are taken into account. The calculated results reveal that the hexagonal silicene nanorings act as a controllable spin filter. The near-perfect spin polarization can be achieved by adjusting the electric, magnetic and exchange fields. The calculated results offer new possibilities for silicene ring based spin nanodevices.

  8. Solar concentrating properties of truncated hexagonal, pyramidal and circular cones

    NASA Technical Reports Server (NTRS)

    Burkhard, D. G.; Strobel, G. L.; Shealy, D. L.

    1978-01-01

    The solar concentrating properties of specularly reflecting truncated pyramidal, hexagonal, and circular cones are evaluated. Pyramidal and hexagonal configurations are discussed with reference to the concentration factor as a function of half apex angle and the length of the side over the width, and to the irradiance distribution. Expressions are derived for the concentration factor and the irradiance at the base of a circular cone when the sunlight is incident normal to the aperture and for oblique incidence.

  9. Epitaxial Hexagonal Ferrites for Millimeter Wave Tunable Filters.

    DTIC Science & Technology

    1982-12-13

    anisotropy fields which, in effect, provide built-in biasing. The result is that ferrite components, similar to those used in microwave systems, can operate... method for growing hexagonal ferrites in the form of single crystal layers on non-magnetic, trAnsparmat subsrates - . The LPE method circumvents... method , single crystal hexagonal ferrites which are superior in quality to those grown by conventional methods . In order to have a more specific goal

  10. Hexagonal Organic Nanopillar Array from Melamine-Cyanuric Acid Complex

    PubMed Central

    Ji, Hai-Feng; Xu, Xiaohe

    2010-01-01

    We report a well-defined, organic, hexagonal nanopillar array on gold surface. The array was prepared from a cyanuric acid-melamine complex by mean of a sequential mixing on a gold surface. These nanopillars had uniform diameters of 200–400 nm and 1 µm in length. They were well facetted with hexagonal cross-sections. The nanopillars had a crystalline structure, and the pillars exhibited a layered texture in the longitudinal direction. PMID:20205460

  11. Activation of ion implanted Si in GaN using a dual AlN annealing cap

    NASA Astrophysics Data System (ADS)

    Hager, C. E.; Jones, K. A.; Derenge, M. A.; Zheleva, T. S.

    2009-02-01

    A dual annealing cap composed of a thin, low temperature metal-organic chemical vapor deposition (MOCVD) deposited AlN adhesion layer and a thicker, sputtered AlN film for added mechanical strength enabled us to anneal Si-implanted layers for 30 min at temperatures up to 1250 °C. At higher temperatures the cap was destroyed by the large partial pressure of the N2 from the GaN, which exceeds the yield strength of AlN. Electrical activations as high as 70% and electron mobilities comparable to those of in situ doped films were achieved. Compared to other methods, the surfaces are better protected using this cap because it adheres better than sputtered AlN, SiO2, or Si3N4; does not crack like MOCVD grown AlN films deposited at normal temperatures (˜1100 °C); and is stronger than thin MOCVD grown AlN films deposited at low temperatures (˜600 °C). Even though N does not escape, and in so doing, forms thermal etch pits, the surface of the annealed GaN is roughened by solid state diffusion with the surface roughness increasing with the annealing temperature.

  12. In situ monitoring and control of multicomponent gas-phase streams for growth of GaN via MOCVD

    NASA Astrophysics Data System (ADS)

    Johnson, M. C.; Poochinda, K.; Ricker, N. L.; Rogers, J. W., Jr.; Pearsall, T. P.

    A monitoring method based on ultraviolet absorption spectroscopy (UVAS) coupled with a multivariate analysis technique was developed to measure the partial pressure of several metalorganic reactant species and the hydride commonly used for III-V nitride growth. Trimethylaluminum, triethylgallium, dimethylethylamine alane and ammonia are all shown to absorb strongly in the UV with highly overlapping spectra. It was found that a single absorption spectrum can be decomposed to obtain accurate predictions of pure component partial pressures in a multicomponent stream. This method was used to monitor and control the gas-phase concentrations for the multicomponent mixture of triethylgallium and ammonia diluted in nitrogen, which is used to grow GaN thin films via MOCVD. There was a 5% average error for the predicted partial pressure of TEGa in the range 245-700 mTorr and a 2% average error for the predicted partial pressure of NH 3 in the range 15-35 Torr. A detection limit of 10 mTorr and 1 Torr were measured for TEGa and NH 3, respectively. For simple feedback control and using this method as a measurement device, settling times of 1 min for TEGa and 30 s for NH 3 to reach within 2% of the desired partial pressure have been obtained. Control tests included both setpoint tracking and gas-phase disturbance rejection. These settling times were found to be no greater than {1}/{40} of the total growth time for a commonly grown GaN thin film.

  13. Desorption induced formation of negative nanowires in GaN

    NASA Astrophysics Data System (ADS)

    Kim, Bong-Joong; Stach, Eric A.

    2011-06-01

    We report in-situ transmission electron microscopy studies of the formation of negative nanowires created by thermal decomposition of single crystal GaN. During annealing, vertical negative nanowires are formed in [0 0 0 1] by preferential dissociation of GaN along the {1 0 1¯ 0} prism planes, while lateral negative nanowires grow in close-packed <1 0 1¯ 0> by the self-catalytic solid-liquid-vapor (SLV) mechanism. Our quantitative measurements show that the growth rates of the laterally grown negative nanowires are independent of the wire diameter, indicating that the rate-limiting step is the decomposition of GaN on the surface of the Ga droplets that catalyze their creation. These nanoscale features offer controllable templates for the creation and integration of a broad range of nanoscale materials systems, with potential applications in nanoscale fluidics.

  14. Biosensors based on GaN nanoring optical cavities

    NASA Astrophysics Data System (ADS)

    Kouno, Tetsuya; Takeshima, Hoshi; Kishino, Katsumi; Sakai, Masaru; Hara, Kazuhiko

    2016-05-01

    Biosensors based on GaN nanoring optical cavities were demonstrated using room-temperature photoluminescence measurements. The outer diameter, height, and thickness of the GaN nanorings were approximately 750-800, 900, and 130-180 nm, respectively. The nanorings functioned as whispering-gallery-mode (WGM)-type optical cavities and exhibited sharp resonant peaks like lasing actions. The evanescent component of the WGM was strongly affected by the refractive index of the ambient environment, the type of liquid, and the sucrose concentration of the analyzed solution, resulting in shifts of the resonant wavelengths. The results indicate that the GaN nanorings can potentially be used in sugar sensors of the biosensors.

  15. GaN nanowire tips for nanoscale atomic force microscopy.

    PubMed

    Behzadirad, Mahmoud; Nami, Mohsen; Rishinaramagalam, Ashwin; Feezell, Daniel; Busani, Tito

    2017-04-07

    Imaging of high-aspect-ratio nanostructures with sharp edges and straight walls in nanoscale metrology by Atomic Force Microscopy (AFM) has been challenging due to the mechanical properties and conical geometry of the majority of available commercial tips. Here we report on the fabrication of GaN probes for nanoscale metrology of high-aspect-ratio structures to enhance the resolution of AFM imaging and improve the durability of AFM tips. GaN nanowires (NWs) were fabricated using bottom-up and top-down techniques and bonded to Si cantilevers to scan vertical trenches on Si substrates. Over several scans, the GaN probes demonstrated excellent durability while scanning uneven structures and showed resolution enhancements in topography images, independent of scan direction, compared to commercial Si tips.

  16. High-Sensitivity GaN Microchemical Sensors

    NASA Technical Reports Server (NTRS)

    Son, Kyung-ah; Yang, Baohua; Liao, Anna; Moon, Jeongsun; Prokopuk, Nicholas

    2009-01-01

    Systematic studies have been performed on the sensitivity of GaN HEMT (high electron mobility transistor) sensors using various gate electrode designs and operational parameters. The results here show that a higher sensitivity can be achieved with a larger W/L ratio (W = gate width, L = gate length) at a given D (D = source-drain distance), and multi-finger gate electrodes offer a higher sensitivity than a one-finger gate electrode. In terms of operating conditions, sensor sensitivity is strongly dependent on transconductance of the sensor. The highest sensitivity can be achieved at the gate voltage where the slope of the transconductance curve is the largest. This work provides critical information about how the gate electrode of a GaN HEMT, which has been identified as the most sensitive among GaN microsensors, needs to be designed, and what operation parameters should be used for high sensitivity detection.

  17. Transient atomic behavior and surface kinetics of GaN

    NASA Astrophysics Data System (ADS)

    Moseley, Michael; Billingsley, Daniel; Henderson, Walter; Trybus, Elaissa; Doolittle, W. Alan

    2009-07-01

    An in-depth model for the transient behavior of metal atoms adsorbed on the surface of GaN is developed. This model is developed by qualitatively analyzing transient reflection high energy electron diffraction (RHEED) signals, which were recorded for a variety of growth conditions of GaN grown by molecular-beam epitaxy (MBE) using metal-modulated epitaxy (MME). Details such as the initial desorption of a nitrogen adlayer and the formation of the Ga monolayer, bilayer, and droplets are monitored using RHEED and related to Ga flux and shutter cycles. The suggested model increases the understanding of the surface kinetics of GaN, provides an indirect method of monitoring the kinetic evolution of these surfaces, and introduces a novel method of in situ growth rate determination.

  18. Gallium kinetics on m-plane GaN

    NASA Astrophysics Data System (ADS)

    Lim, C. B.; Ajay, A.; Monroy, E.

    2017-07-01

    In this paper, we study the adsorption/desorption kinetics of gallium on (1-100) m-GaN during molecular-beam epitaxy on bulk substrates. We demonstrate the stabilization of a laterally contracted biatomic layer of Ga containing up to ≈2.5 times the (0001) surface atom density, on top of the GaN(1-100) surface. By assessing the surface morphology of m-GaN layers grown under different Ga/N flux ratios, we show that the presence of the Ga bilayer allows the growth of atomically smooth m-GaN without accumulation of Ga droplets and in conditions (substrate temperature and Ga/N ratio) that are compatible with the growth of c-GaN.

  19. Application of GaN for photoelectrolysis of water

    NASA Astrophysics Data System (ADS)

    Puzyk, M. V.; Usikov, A. S.; Kurin, S. Yu; Puzyk, A. M.; Fomichev, A. D.; Ermakov, I. A.; Kovalev, D. S.; Papchenko, B. P.; Helava, H.; Makarov, Yu N.

    2015-11-01

    GaN layers of n-type and p-type conductivity grown by HVPE on sapphire substrates were used as working electrodes for water electrolysis, photoelectrolysis and hydrogen gas generation. Specifically the water splitting process is discussed. Corrosion of the GaN materials is also considered. The hydrogen production rate under 365-nm UV LED irradiation of the GaN and external bias was 0.3 ml/(cm2*h) for an n-GaN photoanode (n∼8×1016 cm-3) in 1M Na2SO4 electrolyte and 1.2 ml/(cm2*h) for an n-GaN photoanode (n∼1×1017 cm-3) in 1M KOH electrolyte.

  20. Stability of Carbon Incorpoated Semipolar GaN(1101) Surface

    NASA Astrophysics Data System (ADS)

    Akiyama, Toru; Nakamura, Kohji; Ito, Tomonori

    2011-08-01

    The structural stability of carbon incorporated GaN(1101) surfaces is theoretically investigated by performing first-principles pseudopotential calculations. The calculated surface formation energies taking account of the metal organic vapor phase epitaxy conditions demonstrate that several carbon incorporated surfaces are stabilized depending on the growth conditions. Using surface phase diagrams, which are obtained by comparing the calculated adsorption energy with vapor-phase chemical potentials, we find that the semipolar surface forms NH2 and CH2 below ˜1660 K while the polar GaN(0001) surface with CH3 is stabilized below ˜1550 K. This difference could be one of possible explanations for p-type doping on the semipolar GaN(1101) surface.

  1. Desorption Induced Formation of Negative Nanowires in GaN

    SciTech Connect

    Stach, E.A.; Kim, B.-J.

    2011-06-01

    We report in-situ transmission electron microscopy studies of the formation of negative nanowires created by thermal decomposition of single crystal GaN. During annealing, vertical negative nanowires are formed in [0 0 0 1] by preferential dissociation of GaN along the 1 0 {bar 1} 0 prism planes, while lateral negative nanowires grow in close-packed 1 0 {bar 1} 0 by the self-catalytic solid-liquid-vapor (SLV) mechanism. Our quantitative measurements show that the growth rates of the laterally grown negative nanowires are independent of the wire diameter, indicating that the rate-limiting step is the decomposition of GaN on the surface of the Ga droplets that catalyze their creation. These nanoscale features offer controllable templates for the creation and integration of a broad range of nanoscale materials systems, with potential applications in nanoscale fluidics.

  2. Adsorption of Al on GaN(110) Surface

    NASA Astrophysics Data System (ADS)

    Xie, Jianjun; Ping, Jiang; Zhang, Kaiming; Xie, Xide

    The electronic properties of Al deposited on GaN(110) surface with different adsorption geometries have been studied by using the surface linear muffin tin orbital approach. The layer projected density of states for Al-covered GaN(110) surface is calculated and compared with that of the clean surface. The charge distribution before and after the adsorption of Al are investigated. It is found that the deposited Al atoms prefer to bond with the surface N atoms with some charge transferred from Al to the GaN substrate. Finally, the Al-Ga exchange mechanism is also studied and it is found that the adsorbed Al may replace the second layer Ga atom to form a more stable configuration.

  3. Gate stack engineering for GaN lateral power transistors

    NASA Astrophysics Data System (ADS)

    Yang, Shu; Liu, Shenghou; Liu, Cheng; Hua, Mengyuan; Chen, Kevin J.

    2016-02-01

    Developing optimal gate-stack technology is a key to enhancing the reliability and performance of GaN insulated-gate devices for high-voltage power switching applications. In this paper, we discuss current challenges and review our recent progresses in gate-stack technology development toward high-performance and high-reliability GaN power devices, including (1) interface engineering that creates a high-quality dielectric/III-nitride interface with low trap density; (2) barrier-layer engineering that enables optimal trade-off between performance and stability; (3) bulk quality and reliability enhancement of the gate dielectric. These gate-stack techniques in terms of new process development and device structure design are valuable to realize highly reliable and competitive GaN power devices.

  4. Mg doping and its effect on the semipolar GaN(1122) growth kinetics

    SciTech Connect

    Lahourcade, L.; Wirthmueller, A.; Monroy, E.; Chauvat, M. P.; Ruterana, P.; Laufer, A.; Eickhoff, M.

    2009-10-26

    We report the effect of Mg doping on the growth kinetics of semipolar GaN(1122) synthesized by plasma-assisted molecular-beam epitaxy. Mg tends to segregate on the surface, inhibiting the formation of the self-regulated Ga film which is used as a surfactant for the growth of undoped and Si-doped GaN(1122). We observe an enhancement of Mg incorporation in GaN(1122) compared to GaN(0001). Typical structural defects or polarity inversion domains found in Mg-doped GaN(0001) were not observed for the semipolar films investigated in the present study.

  5. A DFT study on NEA GaN photocathode with an ultrathin n-type Si-doped GaN cap layer

    NASA Astrophysics Data System (ADS)

    Xia, Sihao; Liu, Lei; Kong, Yike; Diao, Yu

    2016-10-01

    Due to the drawbacks of conventional negative electron affinity (NEA) GaN photocathodes activated by Cs or Cs/O, a new-type NEA GaN photocathodes with heterojunction surface dispense with Cs activation are proposed. This structure can be obtained through the coverage of an ultrathin n-type Si-doped GaN cap layer on the p-type Mg-doped GaN emission layer. The influences of the cap layer on the photocathode are calculated using DFT. This study indicates that the n-type cap layer can promote the photoemission characteristics of GaN photocathode and demonstrates the probability of the preparation of a NEA GaN photocathode with an n-type cap layer.

  6. Solar energy harnessing in hexagonally arranged Si nanowire arrays and effects of array symmetry on optical characteristics.

    PubMed

    Li, Junshuai; Yu, HongYu; Li, Yali

    2012-05-17

    Investigation of solar energy harvesting in hexagonally arranged Si nanowire (NW) arrays is performed through optimizing the structural parameters, such as array periodicity (P), Si NW diameter (D) and length (L). The results demonstrate that there exist wide P and D/P 'windows' for the Si NW arrays, locating around 600 nm and 0.833 (i.e., D=500 nm), respectively, for achieving enhanced light absorption compared to their thin film counterparts with the same thickness, but with much less materials consumption. Calculation of the ultimate efficiency (UE) indicates that the light trapping capability is not monotonically increased with L, and that UE vibration is found when L is >1000 nm. Comparison of the light absorption spectra for hexagonally and squarely arranged Si NW arrays demonstrates that these two most widely employed array symmetries in practice have little impact on the light trapping capability.

  7. Ultralow nonalloyed Ohmic contact resistance to self aligned N-polar GaN high electron mobility transistors by In(Ga)N regrowth

    NASA Astrophysics Data System (ADS)

    Dasgupta, Sansaptak; Nidhi, Brown, David F.; Wu, Feng; Keller, Stacia; Speck, James S.; Mishra, Umesh K.

    2010-04-01

    Ultralow Ohmic contact resistance and a self-aligned device structure are necessary to reduce the effect of parasitic elements and obtain higher ft and fmax in high electron mobility transistors (HEMTs). N-polar (0001¯) GaN HEMTs, offer a natural advantage over Ga-polar HEMTs, in terms of contact resistance since the contact is not made through a high band gap material [Al(Ga)N]. In this work, we extend the advantage by making use of polarization induced three-dimensional electron-gas through regrowth of graded InGaN and thin InN cap in the contact regions by plasma (molecular beam epitaxy), to obtain an ultralow Ohmic contact resistance of 27 Ω μm to a GaN 2DEG.

  8. Ultralow nonalloyed Ohmic contact resistance to self aligned N-polar GaN high electron mobility transistors by In(Ga)N regrowth

    SciTech Connect

    Dasgupta, Sansaptak; Nidhi,; Brown, David F.; Wu, Feng; Keller, Stacia; Speck, James S.; Mishra, Umesh K.

    2010-04-05

    Ultralow Ohmic contact resistance and a self-aligned device structure are necessary to reduce the effect of parasitic elements and obtain higher f{sub t} and f{sub max} in high electron mobility transistors (HEMTs). N-polar (0001) GaN HEMTs, offer a natural advantage over Ga-polar HEMTs, in terms of contact resistance since the contact is not made through a high band gap material [Al(Ga)N]. In this work, we extend the advantage by making use of polarization induced three-dimensional electron-gas through regrowth of graded InGaN and thin InN cap in the contact regions by plasma (molecular beam epitaxy), to obtain an ultralow Ohmic contact resistance of 27 OMEGA mum to a GaN 2DEG.

  9. Flexible resistive random access memory devices by using NiO x /GaN microdisk arrays fabricated on graphene films

    NASA Astrophysics Data System (ADS)

    Lee, Keundong; Park, Jong-woo; Tchoe, Youngbin; Yoon, Jiyoung; Chung, Kunook; Yoon, Hosang; Lee, Sangik; Yoon, Chansoo; Park, Bae Ho; Yi, Gyu-Chul

    2017-05-01

    We report flexible resistive random access memory (ReRAM) arrays fabricated by using NiO x /GaN microdisk arrays on graphene films. The ReRAM device was created from discrete GaN microdisk arrays grown on graphene films produced by chemical vapor deposition, followed by deposition of NiO x thin layers and Au metal contacts. The microdisk ReRAM arrays were transferred to flexible plastic substrates by a simple lift-off technique. The electrical and memory characteristics of the ReRAM devices were investigated under bending conditions. Resistive switching characteristics, including cumulative probability, endurance, and retention, were measured. After 1000 bending repetitions, no significant change in the device characteristics was observed. The flexible ReRAM devices, constructed by using only inorganic materials, operated reliably at temperatures as high as 180 °C.

  10. Light extraction efficiency enhancement for InGaN quantum wells light-emitting diodes with GaN micro-domes

    NASA Astrophysics Data System (ADS)

    Zhao, Peng; Han, Lu; Zhao, Hongping

    2013-03-01

    The enhancement of light extraction efficiency for thin-film-flip-chip (TFFC) InGaN QWs LEDs with GaN microdomes on n-GaN layer was studied. The three dimensional FDTD method was used to calculate the light extraction efficiency for the TFFC InGaN QWs LEDs emitting at visible spectral regime, as compared to that of the conventional TFFC InGaN QWs LEDs. The calculation indicates significant dependence of the p-GaN layer thickness on the light extraction efficiency. Significant enhancement of the light extraction efficiency (2.5-2.7 times for λpeak=460nm and 2.7- 2.8 times for λpeak=550nm) is achievable from LEDs with GaN micro-domes with optimized micro-dome diameter and height.

  11. Zero lattice mismatch and twin-free single crystalline ScN buffer layers for GaN growth on silicon

    SciTech Connect

    Lupina, L.; Zoellner, M. H.; Dietrich, B.; Capellini, G.; Niermann, T.; Lehmann, M.; Thapa, S. B.; Haeberlen, M.; Storck, P.; Schroeder, T.

    2015-11-16

    We report the growth of thin ScN layers deposited by plasma-assisted molecular beam epitaxy on Sc{sub 2}O{sub 3}/Y{sub 2}O{sub 3}/Si(111) substrates. Using x-ray diffraction, Raman spectroscopy, and transmission electron microscopy, we find that ScN films grown at 600 °C are single crystalline, twin-free with rock-salt crystal structure, and exhibit a direct optical band gap of 2.2 eV. A high degree of crystalline perfection and a very good lattice matching between ScN and GaN (misfit < 0.1%) makes the ScN/Sc{sub 2}O{sub 3}/Y{sub 2}O{sub 3} buffer system a very promising template for the growth of high quality GaN layers on silicon.

  12. Low-Angle-Incidence Microchannel Epitaxy of a-Plane GaN Grown by Ammonia-Based Metal-Organic Molecular Beam Epitaxy

    NASA Astrophysics Data System (ADS)

    Lin, Chia-Hung; Uchiyama, Shota; Maruyama, Takahiro; Naritsuka, Shigeya

    2012-04-01

    Low-angle-incidence microchannel epitaxy (LAIMCE) of a-plane GaN was performed using ammonia-based metal-organic molecular beam epitaxy to obtain wide and thin lateral overgrowth over a SiO2 mask. Trimethylgallium (TMG) was supplied perpendicular to the openings cut in the mask with a low incident angle of 5° relative to the substrate plane. The [NH3]/[TMG] ratio (R) dependence of GaN LAIMCE was optimized by varying R from 5 to 30. A wide lateral overgrowth of 3.7 µm with a dislocation density below the transmission electron microscope detection limit was obtained at R=15 for a thickness of 520 nm.

  13. Surface electronic structure of ZrB2 buffer layers for GaN growth on Si wafers

    NASA Astrophysics Data System (ADS)

    Yamada-Takamura, Yukiko; Bussolotti, Fabio; Fleurence, Antoine; Bera, Sambhunath; Friedlein, Rainer

    2010-08-01

    The electronic structure of epitaxial, predominantly single-crystalline thin films of zirconium diboride (ZrB2), a lattice-matching, conductive ceramic to GaN, grown on Si(111) was studied using angle-resolved ultraviolet photoelectron spectroscopy. The existence of Zr-derived surface states dispersing along the Γ¯-M¯ direction indicates a metallic character provided by a two-dimensional Zr-layer at the surface. Together with the measured work function, the results demonstrate that the surface electronic properties of such thin ZrB2(0001) buffer layers are comparable to those of the single crystals promising excellent conduction between nitride layers and the substrate in vertical light-emitting diodes on economic substrates.

  14. Critical issues for homoepitaxial GaN growth by molecular beam epitaxy on hydride vapor-phase epitaxy-grown GaN substrates

    NASA Astrophysics Data System (ADS)

    Storm, D. F.; Hardy, M. T.; Katzer, D. S.; Nepal, N.; Downey, B. P.; Meyer, D. J.; McConkie, Thomas O.; Zhou, Lin; Smith, David J.

    2016-12-01

    While the heteroepitaxial growth of gallium nitride-based materials and devices on substrates such as SiC, sapphire, and Si has been well-documented, the lack of a cost-effective source of bulk GaN crystals has hindered similar progress on homoepitaxy. Nevertheless, freestanding GaN wafers are becoming more widely available, and there is great interest in growing GaN films and devices on bulk GaN substrates, in order to take advantage of the greatly reduced density of threading dislocations, particularly for vertical devices. However, homoepitaxial GaN growth is far from a trivial task due to the reactivity and different chemical sensitivities of N-polar (000_1) and Ga-polar (0001) GaN surfaces, which can affect the microstructure and concentrations of impurities in homoepitaxial GaN layers. In order to achieve high quality, high purity homoepitaxial GaN, it is necessary to investigate the effect of the ex situ wet chemical clean, the use of in situ cleaning procedures, the sensitivity of the GaN surface to thermal decomposition, and the effect of growth temperature. We review the current understanding of these issues with a focus on homoepitaxial growth of GaN by molecular beam epitaxy (MBE) on c-plane surfaces of freestanding GaN substrates grown by hydride vapor phase epitaxy (HVPE), as HVPE-grown substrates are most widely available. We demonstrate methods for obtaining homoepitaxial GaN layers by plasma-assisted MBE in which no additional threading dislocations are generated from the regrowth interface and impurity concentrations are greatly reduced.

  15. Chemical mechanical polishing of freestanding GaN substrates

    NASA Astrophysics Data System (ADS)

    Huaiyue, Yan; Xiangqian, Xiu; Zhanhui, Liu; Rong, Zhang; Xuemei, Hua; Zili, Xie; Ping, Han; Yi, Shi; Youdou, Zheng

    2009-02-01

    Chemical mechanical polishing (CMP) has been used to produce smooth and scratch-free surfaces for GaN. In the aqueous solution of KOH, GaN is subjected to etching. At the same time, all surface irregularities, including etch pyramids, roughness after mechanical polishing and so on will be removed by a polishing pad. The experiments had been performed under the condition of different abrasive particle sizes of the polishing pad. Also the polishing results for different polishing times are analyzed, and chemical mechanical polishing resulted in an average root mean square (RMS) surface roughness of 0.565 nm, as measured by atomic force microscopy.

  16. Ferromagnetism in undoped One-dimensional GaN Nanowires

    SciTech Connect

    Jeganathan, K. E-mail: jagan@physics.bdu.ac.in; Purushothaman, V.; Debnath, R.; Arumugam, S.

    2014-05-15

    We report an intrinsic ferromagnetism in vertical aligned GaN nanowires (NW) fabricated by molecular beam epitaxy without any external catalyst. The magnetization saturates at ∼0.75 × emu/gm with the applied field of 3000 Oe for the NWs grown under the low-Gallium flux of 2.4 × 10{sup −8} mbar. Despite a drop in saturation magnetization, narrow hysteresis loop remains intact regardless of Gallium flux. Magnetization in vertical standing GaN NWs is consistent with the spectral analysis of low-temperature photoluminescence pertaining to Ga-vacancies associated structural defects at the nanoscale.

  17. Dislocation luminescence in GaN single crystals under nanoindentation

    PubMed Central

    2014-01-01

    This work presents an experimental study on the dislocation luminescence in GaN by nanoindentation, cathodoluminescence, and Raman. The dislocation luminescence peaking at 3.12 eV exhibits a series of special properties in the cathodoluminescence measurements, and it completely disappears after annealing at 500°C. Raman spectroscopy shows evidence for existence of vacancies in the indented region. A comprehensive investigation encompassing cathodoluminescence, Raman, and annealing experiments allow the assignment of dislocation luminescence to conduction-band-acceptor transition involving Ga vacancies. The nanoscale plasticity of GaN can be better understood by considering the dislocation luminescence mechanism. PMID:25593548

  18. Development of Partial-Charge Potential for GaN

    SciTech Connect

    Gao, Fei; Devanathan, Ram; Oda, Takuji; Weber, William J.

    2006-09-01

    Partial-charged potentials for GaN are systematically developed that describes a wide range of structural properties, where the reference data for fitting the potential parameters are taken from ab initial calculations or experiments. The present potential model provides a good fit to different structural geometries and high pressure phases of GaN. The high-pressure transition from wurtzite to rock-salt structure is correctly described yielding the phase transition pressure of about 55 GPa, and the calculated volume change at the transition is in good agreement with experimental data. The results are compared with those obtained by ab initio simulations.

  19. Redistribution of Implanted Dopants in GaN

    SciTech Connect

    Fu, M.; Gao, X.A.; Han, J.; Pearton, S.J.; Rieger, D.J.; Scarvepalli, V. Sekhar, J.A.; Shul, R.J.; Singh, R.K.; Wilson, R.G.; Zavada, J.M.; Zolper, J.C.

    1998-11-20

    Donor (S, Se and Te) and acceptor (Mg, Be and C) dopants have been implanted into GaN at doses of 3-5x1014 cm-2 and annealed at temperatures up to 1450 *C. No redistribution of any of the elements is detectable by Secondary Ion Mass Spectrometry, except for Be, which displays an apparent damage-assisted diffusion at 900 "C. At higher temperatures there is no further movement of the Be, suggesting that the point defect flux that assists motion at lower temperatures has been annealed. Effective diffusivities are <2X 1013 cm2.sec-1 at 1450 `C for each of the dopants in GaN.

  20. GaN surface states investigated by electrochemical studies

    NASA Astrophysics Data System (ADS)

    Winnerl, Andrea; Garrido, Jose A.; Stutzmann, Martin

    2017-03-01

    We present a systematic study of electrochemically active surface states on MOCVD-grown n-type GaN in aqueous electrolytes using cyclic voltammetry and impedance spectroscopy over a wide range of potentials and frequencies. In order to alter the surface states, the GaN samples are either etched or oxidized, and the influence of the surface treatment on the defect-mediated charge transfer to the electrolyte is investigated. Etching in HCl removes substoichiometric GaO x , and leads to a pronounced density of electrochemically active surface states. Oxidation effectively removes these surface states.