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Sample records for hexagonal gan thin

  1. Cubic and hexagonal GaN nanoparticles synthesized at low temperature

    NASA Astrophysics Data System (ADS)

    Qaeed, M. A.; Ibrahim, K.; Saron, K. M. A.; Salhin, A.

    2013-12-01

    This study involves a simple and low cost chemical method for the synthesis of Gallium Nitride (GaN) nanoparticles at low temperature. Structural and optical characterizations were carried out using various techniques in order to investigate the properties of the nanoparticles. The Field-Emission Scanning Electron Microscope (FESEM) images showed that the nanoparticles consist of cubic and hexagonal shapes, indicating crystallized structural quality of the GaN nanoparticles. The average size of the nanoparticles was found to be 51 nm. The X-ray Diffraction (XRD) and Raman analysis further confirmed the hexagonal and cubic phases of GaN nanoparticles. The room temperature photoluminescence deduced h-GaN energy gaps of 2.95, 3.12 and 3.13 eV.

  2. Time-resolved photoluminescence study of excitons in hexagonal GaN layers grown on sapphire

    NASA Astrophysics Data System (ADS)

    Pau, S.; Liu, Z. X.; Kuhl, J.; Ringling, J.; Grahn, H. T.; Khan, M. A.; Sun, C. J.; Ambacher, O.; Stutzmann, M.

    1998-03-01

    We performed time-resolved and continuous wave photoluminescence on two samples of hexagonal GaN, one with free exciton emission and the other without. For the sample with free exciton emission, very different decay dynamics are observed between the front and backside emission. We find that the strain caused by the lattice mismatch between the sapphire substrate and the GaN film has a large influence on the population decay of the sample with free exciton emission and a minor influence on the decay properties of the sample dominated by bound exciton emission. A polariton picture is used to describe the observed behavior.

  3. Identification of optical transitions in cubic and hexagonal GaN by spatially resolved cathodoluminescence

    NASA Astrophysics Data System (ADS)

    Menniger, J.; Jahn, U.; Brandt, O.; Yang, H.; Ploog, K.

    1996-01-01

    The hexagonal and cubic phases of GaN are characterized by spatially resolved cathodoluminescence (CL) spectra from micrometer-size single crystals with either hexagonal or cubic habits grown by plasma-assisted molecular-beam epitaxy. At 5 K, distinct narrow excitonic lines are found at 3.472 and 3.272 eV for the hexagonal and cubic phase, yielding energy gaps of 3.500 and 3.300 eV, respectively. Detailed temperature- and intensity-dependent CL measurements on cubic GaN crystals enable us to clearly identify the exciton (free: 3.272 eV, bound: 3.263 eV) and the donor-acceptor pair (3.150 eV) transition. Moreover, we determine the donor-band and acceptor-band transition energy for this phase. In addition, phonon replicas of the exciton line and of the donor-acceptor pair transition are observed at 3.185 and 3.064 eV, respectively.

  4. Photoluminescence of gallium ion irradiated hexagonal and cubic GaN quantum dots

    NASA Astrophysics Data System (ADS)

    Rothfuchs, Charlotte; Kukharchyk, Nadezhda; Koppe, Tristan; Semond, Fabrice; Blumenthal, Sarah; Becker, Hans-Werner; As, Donat J.; Hofsäss, Hans C.; Wieck, Andreas D.; Ludwig, Arne

    2016-09-01

    We report on ion implantation into GaN QDs and investigate their radiation hardness. The experimental study is carried out by photoluminescence (PL) measurements on molecular beam epitaxy-grown GaN quantum dots after ion implantation. Both quantum dots grown in the hexagonal (H) and the cubic (C) crystal structure were subjected to gallium ions with an energy of 400 kV (H) and 75 kV (C) with fluences ranging from 5 ×1010 cm-2 to 1 ×1014 cm-2 (H) and to 1 ×1015 cm-2 (C), respectively. Low-temperature PL measurements reveal a PL quenching for which a quantitative model as a function of the ion fluence is developed. A high degradation resistance is concluded. A non-radiative trap with one main activation energy is found for all QD structures by temperature-dependent PL measurements. Further analysis of fluence-dependent PL energy shifts shows ion-induced intermixing and strain effects. Particular for the hexagonal quantum dots, a strong influence of the quantum confined Stark effect is present.

  5. Cathodoluminescence study of luminescence centers in hexagonal and cubic phase GaN hetero-integrated on Si(100)

    NASA Astrophysics Data System (ADS)

    Liu, R.; Bayram, C.

    2016-07-01

    Hexagonal and cubic GaN—integrated on on-axis Si(100) substrate by metalorganic chemical vapor deposition via selective epitaxy and hexagonal-to-cubic-phase transition, respectively—are studied by temperature- and injection-intensity-dependent cathodoluminescence to explore the origins of their respective luminescence centers. In hexagonal (cubic) GaN integrated on Si, we identify at room temperature the near band edge luminescence at 3.43 eV (3.22 eV), and a defect peak at 2.21 eV (2.72 eV). At low temperature, we report additional hexagonal (cubic) GaN bound exciton transition at 3.49 eV (3.28 eV), and a donor-to-acceptor transition at 3.31 eV (3.18 eV and 2.95 eV). In cubic GaN, two defect-related acceptor energies are identified as 110 and 360 meV. For hexagonal (cubic) GaN (using Debye Temperature ( β ) of 600 K), Varshni coefficients of α = 7.37 ± 0.13 × 10 - 4 ( 6.83 ± 0.22 × 10 - 4 ) eV / K and E 0 = 3.51 ± 0.01 ( 3.31 ± 0.01 ) eV are extracted. Hexagonal and cubic GaN integrated on CMOS compatible on-axis Si(100) are shown to be promising materials for next generation devices.

  6. 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.

  7. 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.

  8. Properties of a hole trap in n-type hexagonal GaN

    NASA Astrophysics Data System (ADS)

    Muret, P.; Philippe, A.; Monroy, E.; Muñoz, E.; Beaumont, B.; Omnès, F.; Gibart, P.

    2002-03-01

    Minority carrier transient spectroscopy is performed in Schottky diodes fabricated on hexagonal n-type GaN grown by metalorganic chemical vapor deposition, either doped with two concentrations of Si or unintentionally doped. Capacitance transients are measured after a light pulse sent through the semitransparent contact which generates electron-hole pairs in the depletion zone. They display the characteristic sign of hole emission. The same deep level is detected in all the samples, independent of the doping level and doping species, with a concentration of some 1015 cm-3, even in the sample prepared by epitaxial lateral overgrowth. The ionization energy and capture cross section deduced from Fourier Transform transient spectroscopy are respectively 0.81±0.03 eV and 2×10-14 cm2. Such a capture cross section for holes indicates an attractive potential and hence a negatively charged center before the hole capture. Hole emission is suppressed by electron-hole recombination when a sufficiently long majority carrier pulse is applied after the light pulse. A single recombination time constant is measured and an electron capture cross section near 10-21 cm2, independent of temperature, is deduced. These facts demonstrate that this deep center is a point defect, still negatively charged after a hole has been captured, since it repels electrons, and hence it is a deep acceptor. All these properties fit very well the theoretical predictions previously published about the isolated gallium vacancy in n-type GaN.

  9. 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.

  10. 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

  11. An investigation of sol-gel spin coating growth of wurtzite GaN thin film on 6H-SiC substrate

    NASA Astrophysics Data System (ADS)

    Fong, C. Y.; Ng, S. S.; Yam, F. K.; Hassan, H. Abu; Hassan, Z.

    2015-03-01

    In this study, wurtzite gallium nitride (GaN) thin film was directly grown on hexagonal silicon carbide (6H-SiC) substrate without buffer layer using sol-gel spin coating method followed by annealing and nitridation process. The entire growth process was investigated in-depth. The results revealed that the conversion of GaN thin film proceeds through an intermediate of amorphous gallium(I) sub-oxide (Ga2O). In this case, the amorphous Ga2O was converted into GaN thin film after being nitridated at 950 °C under ammonia ambient. The intermediate of amorphous Ga2O can only be identified through infrared reflectance measurements.

  12. Periodic Radiation Patterns and Circulating Direction of Lasing Light by Quasi Whispering Gallery Mode in Hexagonal GaN Microdisk

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

    We have experimentally elucidated the periodic radiation patterns and circulating direction of the lasing light generated by the quasi-whispering-gallery mode (QWGM) in a hexagonal GaN microdisk. The radiated lasing light from the microdisk is highly directional, with the high intensities of the obtained radiated lasing light having a periodic spacing of 120° in the planar direction. The results show that the QWGM-generated lasing light circulates in a single direction in the microdisk, namely, either clockwise or counter-clockwise.

  13. Nanocrystalline-graphene-tailored hexagonal boron nitride thin films.

    PubMed

    Lee, Kang Hyuck; Shin, Hyeon-Jin; Kumar, Brijesh; Kim, Han Sol; Lee, Jinyeong; Bhatia, Ravi; Kim, Sang-Hyeob; Lee, In-Yeal; Lee, Hyo Sug; Kim, Gil-Ho; Yoo, Ji-Beom; Choi, Jae-Young; Kim, Sang-Woo

    2014-10-20

    Unintentionally formed nanocrystalline graphene (nc-G) can act as a useful seed for the large-area synthesis of a hexagonal boron nitride (h-BN) thin film with an atomically flat surface that is comparable to that of exfoliated single-crystal h-BN. A wafer-scale dielectric h-BN thin film was successfully synthesized on a bare sapphire substrate by assistance of nc-G, which prevented structural deformations in a chemical vapor deposition process. The growth mechanism of this nc-G-tailored h-BN thin film was systematically analyzed. This approach provides a novel method for preparing high-quality two-dimensional materials on a large surface. PMID:25204810

  14. 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].

  15. 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

    2015-04-01

    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. PMID:25968805

  16. Growth behavior of hexagonal GaN on Si(100) and Si(111) substrates prepared by pulsed laser deposition

    NASA Astrophysics Data System (ADS)

    Wang, Wei-Kai; Jiang, Ming-Chien

    2016-09-01

    In this study, we investigated the microstructure and optical properties of hexagonal GaN (h-GaN) films grown by high-temperature pulsed laser deposition (PLD) on Si(100) and Si(111) substrates. The growth mechanism, crystallization, and surface morphology of h-GaN deposition on both Si(100) and Si(111) substrates were monitored by transmission electron microscopy (TEM) and scanning electron microscopy at various times in the growth process. Our results indicated that the h-GaN grown on Si(111) has better crystalline structure and optical properties than that on Si(100) owing to the smaller mismatch of the orientations of the Si(111) substrate and h-GaN film. On the Si(100) substrate, the growth principles of PLD and N2 plasma nitridation are the main contributions to the conversion of the cubic GaN into h-GaN. Moreover, no significant Ga–Si meltback etching was observed on the GaN/Si surface with the PLD operation temperature of 1000 °C. The TEM images also revealed that an abrupt GaN/Si interface can be obtained because of the suppression of substrate–film interfacial reactions in PLD.

  17. Time-resolved photoluminescence study of m-plane GaN thin films

    NASA Astrophysics Data System (ADS)

    Pan, Ji-Hong; Jang, Der-Jun; Quadir, Shaham; Lo, Ikai

    2014-03-01

    The optical properties and the carrier relaxation of GaN thin films were studied by time-resolved photoluminescence apparatus. The m-plane GaN thin films were grown on GaN buffer layer and γ-LiAlO2 substrates by molecular beam epitaxy with variation of N/Ga ratio. We found that the PL associated with defect is prominent for large N/Ga ratio due to the increasing of stacking faults. The intensity of PL perpendicular to the GaN [0001] direction is more intensive than that of PL parallel to the perpendicular to the GaN [0001] direction. The PL decay times exhibit dependence on the direction of the PL polarizations.

  18. 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. PMID:26285135

  19. 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

  20. A Raman Study of the Origin of Oxygen Defects in Hexagonal Manganite Thin Films

    NASA Astrophysics Data System (ADS)

    Chen, Xiang-Bai; Hien Nguyen Thi, Minh; Yang, In-Sang; Lee, Daesu; Noh, Tae-Won

    2012-12-01

    Oxygen defects are usually unavoidable when synthesizing oxide thin films. We study the origin of the oxygen defects in hexagonal manganite HoMnO3 epitaxial thin films through Raman scattering spectroscopy. Our results show that the oxygen defects in hexagonal HoMnO3 thin films have distinct effects on different phonon modes and on magnon scattering. Our analyses indicate that the oxygen defects in hexagonal HoMnO3 thin films mainly originate from the basal O3 and/or O4 oxygen vacancies. Furthermore, our analyses of oxygen defects predict that the Mn 3d orbitals would be more strongly hybridized with the apical O1 and/or O2 2p orbitals than the basal O3 and/or O4 2p orbitals. This prediction is consistent with our resonant Raman scattering study and earlier first-principle calculations of the electronic structures of hexagonal manganites.

  1. The study of in situ scanning tunnelling microscope characterization on GaN thin film grown by plasma assisted molecular beam epitaxy

    SciTech Connect

    Yang, R.; Krzyzewski, T.; Jones, T.

    2013-03-18

    The epitaxial growth of GaN by Plasma Assisted Molecular Beam Epitaxy was investigated by Scanning Tunnelling Microscope (STM). The GaN film was grown on initial GaN (0001) and monitored by in situ Reflection High Energy Electron Diffraction and STM during the growth. The STM characterization was carried out on different sub-films with increased thickness. The growth of GaN was achieved in 3D mode, and the hexagonal edge of GaN layers and growth gradient were observed. The final GaN was of Ga polarity and kept as (0001) orientation, without excess Ga adlayers or droplets formed on the surface.

  2. Preparation and properties of inverse perovskite Mn3GaN thin films and heterostructures

    NASA Astrophysics Data System (ADS)

    Tashiro, H.; Suzuki, R.; Miyawaki, T.; Ueda, K.; Asano, H.

    2013-08-01

    Thin films and heterostructures of Mn3GaN with an inverse perovskite structure were grown epitaxially on SrTiO3 (001) and (La0.18Sr0.82)(Al0.59Ta0.41)O3 (001) (LSAT) substrates by ion beam sputtering, and their structural and electrical properties have been investigated. Mn3GaN epitaxial thin films showed metallic behavior of temperature-dependent resistivity with a small maximum at 290-340 K. The maximum resistivity could be attributed to the magnetic transition from antiferromagnetism to paramagnetism. It has been found that epitaxial heterostructures formed by ferroelectric Ba0.7Sr0.3TiO3 and Mn3GaN layers exhibit a large magnetocapacitance effect of more than 2000% in an applied magnetic filed of 1.5 T.

  3. Schottky barrier formation at the Au to rare earth doped GaN thin film interface

    NASA Astrophysics Data System (ADS)

    McHale, S. R.; McClory, J. W.; Petrosky, J. C.; Wu, J.; Rivera, A.; Palai, R.; Losovyj, Ya. B.; Dowben, P. A.

    2011-09-01

    The Schottky barriers formed at the interface between gold and various rare earth doped GaN thin films (RE = Yb, Er, Gd) were investigated in situ using synchrotron photoemission spectroscopy. The resultant Schottky barrier heights were measured as 1.68 ± 0.1 eV (Yb:GaN), 1.64 ± 0.1 eV (Er:GaN), and 1.33 ± 0.1 eV (Gd:GaN). We find compelling evidence that thin layers of gold do not wet and uniformly cover the GaN surface, even with rare earth doping of the GaN. Furthermore, the trend of the Schottky barrier heights follows the trend of the rare earth metal work function.

  4. An ultra-thin compliant sapphire membrane for the growth of less strained, less defective GaN

    NASA Astrophysics Data System (ADS)

    Moon, Daeyoung; Jang, Jeonghwan; Choi, Daehan; Shin, In-Su; Lee, Donghyun; Bae, Dukkyu; Park, Yongjo; Yoon, Euijoon

    2016-05-01

    An ultra-thin (26 nm) sapphire (Al2O3) membrane was used as a compliant substrate for the growth of high quality GaN. The density of misfit dislocations per unit length at the interface between the GaN layer and the sapphire membrane was reduced by 28% compared to GaN on the conventional sapphire substrate. Threading dislocation density in GaN on the sapphire membrane was measured to be 2.4×108/cm2, which is lower than that for GaN on the conventional sapphire substrate (3.2×108/cm2). XRD and micro-Raman results verifed that the residual stress in GaN on the sapphire membrane was as low as 0.02 GPa due to stress absorption by the ultra-thin compliant sapphire membrane.

  5. Temperature dependent dielectric function and the E{sub 0} critical points of hexagonal GaN from 30 to 690 K

    SciTech Connect

    Kim, Tae Jung Hwang, Soon Yong; Byun, Jun Seok; Barange, Nilesh S.; Park, Han Gyeol; Dong Kim, Young

    2014-02-15

    The complex dielectric function ε and the E{sub 0} excitonic and band-edge critical-point structures of hexagonal GaN are reported for temperatures from 30 to 690 K and energies from 0.74 to 6.42 eV, obtained by rotating-compensator spectroscopic ellipsometry on a 1.9 μm thick GaN film deposited on a c-plane (0001) sapphire substrate by molecular beam epitaxy. Direct inversion and B-splines in a multilayer-structure calculation were used to extract the optical properties of the film from the measured pseudodielectric function 〈ε〉. At low temperature sharp E{sub 0} excitonic and critical-point interband transitions are separately observed. Their temperature dependences were determined by fitting the data to the empirical Varshni relation and the phenomenological expression that contains the Bose-Einstein statistical factor.

  6. 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.

  7. 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.

  8. Resonant photoemission of rare earth doped GaN thin films

    NASA Astrophysics Data System (ADS)

    McHale, S. R.; McClory, J. W.; Petrosky, J. C.; Wu, J.; Palai, R.; Losovyj, Ya. B.; Dowben, P. A.

    2011-10-01

    The 4d → 4f Fano resonances for various rare earth doped GaN thin films (RE = Gd, Er, Yb) were investigated using synchrotron photoemission spectroscopy. The resonant photoemission Fano profiles show that the major Gd and Er rare earth 4f weight is at about 5-6 eV below the valence band maximum, similar to the 4f weights in the valence band of many other rare earth doped semiconductors. For Yb, there is very little resonant enhancement of the valence band of Yb doped GaN, consistent with a largely 4f14 occupancy.

  9. Structural and magnetic impact of Cr+-implantation into GaN thin film

    NASA Astrophysics Data System (ADS)

    Husnain, G.; Shu-De, Yao; Ahmad, Ishaq; Rafique, H. M.

    2012-06-01

    Thin films of GaN with thickness of 2 μm were synthesized on sapphire. Cr+ ions were implanted into GaN with150 keV energy at a fluence of 3 × 1015 cm-2. The annealing of the samples was carried out for a short time using rapid thermal annealing (RTA). Structural properties of the implanted samples were undertaken by XRD and Rutherford backscattering. The annealed samples demonstrated lattice recovery and damages caused by implantation. The structural properties were also studied by High-resolution X-ray Diffraction (HRXRD). Magnetic measurements of the samples were performed by Alternating Gradient Magnetometer (AGM) at room temperature and by SQUID in the range of 5-380 K. The SQUID results showed ferromagnetic behavior at T = 5 K and above 380 K for Cr+-implanted GaN.

  10. Deep levels and persistent photoconductivity in GaN thin films

    SciTech Connect

    Qiu, C.H.; Pankove, J.I.

    1997-04-01

    Photocurrent decay in GaN thin films was studied in the time span from a few seconds to several days. The persistent photoconductivity (PPC) behavior was observed not only in Mg-doped {ital p}-type GaN films but also in undoped {ital n}-type GaN films. The photoconductivity spectra and the photocurrent response time were measured using a weak probe light at several times after the samples had been kept in the dark. During the relaxation, the photocurrent due to the subband-gap probe light decreased more than the photocurrent due to the UV probe light. It is suggested that metastable centers at 1.1, 1.40, and 2.04 eV above the valence band edge are responsible for the PPC behavior in Mg-doped GaN, and that Ga vacancy is the candidate for PPC effect in {ital n}-type GaN. {copyright} {ital 1997 American Institute of Physics.}

  11. 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.

  12. Growth of atomically thin hexagonal boron nitride films by diffusion through a metal film and precipitation

    NASA Astrophysics Data System (ADS)

    Suzuki, Satoru; Molto Pallares, Roger; Hibino, Hiroki

    2012-09-01

    Atomically thin hexagonal boron nitride films were grown on both the top and bottom surfaces of a polycrystalline Co or Ni film by annealing a Co (Ni)/amorphous boron nitride/SiO2 structure in vacuum. This method of growing hexagonal boron nitride is much simpler than other methods, such as thermal chemical vapour deposition. B and N atoms diffuse through the metal film, although N is almost completely insoluble in both Co and Ni, and precipitation occurs at the topmost surface. The mass transport is considered to be caused by grain boundary diffusion.

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

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

    The effective Hamiltonian of the surface states on semi-infinite slabs of the topological insulators (TI) Bi2Te3 and Bi2Se3 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 Bi2Se3 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.

  14. 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.

  15. Morphological and optical comparison of the Si doped GaN thin film deposited onto the transparent substrates

    NASA Astrophysics Data System (ADS)

    Özen, Soner; Şenay, Volkan; Pat, Suat; Korkmaz, Şadan

    2016-04-01

    The aim of this paper is to expand the body of knowledge about the silicon doped gallium nitride thin films deposited on different substrates. The physical properties of the Si doped GaN thin films deposited on the glass and polyethylene terephthalate substrates by thermionic vacuum arc which is plasma production technique were investigated. Thermionic vacuum arc method is a method of producing pure material plasma. The Si doped GaN thin films were analyzed using the following methods and the devices: atomic force microscopy, x-ray diffraction device, spectroscopic ellipsometer and energy dispersive x-ray spectroscopy detector. The produced Si doped GaN thin films are in the (113) orientation. The thicknesses and refractive index were determined by using Cauchy dispersion model. Surface morphologies of produced thin films are homogenous and low roughness. Our analysis showed that the thermionic vacuum arc method present important advantages for optical and industrial applications.

  16. 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.

  17. Optical transmission through hexagonal arrays of subwavelength holes in thin metal films.

    PubMed

    Ctistis, G; Patoka, P; Wang, X; Kempa, K; Giersig, M

    2007-09-01

    We have studied the light transmission through hexagonal arrays of subwavelength holes in thin gold and aluminum films, varying the film thickness between 20 and 120 nm while the hole diameter as well as the interhole distance have been kept constant at approximately 300 and approximately 500 nm, respectively. The films were characterized by means of UV-vis spectroscopy and scanning near-field optical microscopy (SNOM). PMID:17715985

  18. Electron Spin Resonance in GaN Thin Film Doped with Fe

    NASA Astrophysics Data System (ADS)

    Kashiwagi, Takanari; Sonoda, Saki; Yashiro, Haruhiko; Ishihara, Yujiro; Usui, Akira; Akasaka, Youichi; Hagiwara, Masayuki

    2007-02-01

    High-quality and high-resistivity semiconducting substrates are needed to fabricate high-frequency devices such as high-mobility transistors based on gallium nitride (GaN). A GaN thin film doped with Fe ions becomes one of such high-resistivity substrates. To obtain microscopic information on the Fe ions in the GaN:Fe film, we have performed electron spin resonance (ESR) measurements using a conventional X-band apparatus and home made Q-band equipment. The observed ESR signals were analyzed with a spin Hamiltonian given by considering the local symmetry of the Ga site (C3v) and assuming that the Fe3+ ions (S=5/2) are substituted for Ga3+ ions. As a result, the angular dependence of the resonance fields and the temperature dependence of the signal intensities are reproduced very well by the calculations. Consequently, we confirmed that the Fe3+ ions occupy some of the Ga sites in the GaN thin film.

  19. 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}.

  20. 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.

  1. Domain wall conductivity in semiconducting hexagonal ferroelectric TbMnO3 thin films.

    PubMed

    Kim, D J; Connell, J G; Seo, S S A; Gruverman, A

    2016-04-15

    Although enhanced conductivity of ferroelectric domain boundaries has been found in BiFeO3 and Pb(Zr,Ti)O3 films as well as hexagonal rare-earth manganite single crystals, the mechanism of the domain wall conductivity is still under debate. Using conductive atomic force microscopy, we observe enhanced conductance at the electrically-neutral domain walls in semiconducting hexagonal ferroelectric TbMnO3 thin films where the structure and polarization direction are strongly constrained along the c-axis. This result indicates that domain wall conductivity in ferroelectric rare-earth manganites is not limited to charged domain walls. We show that the observed conductivity in the TbMnO3 films is governed by a single conduction mechanism, namely, the back-to-back Schottky diodes tuned by the segregation of defects. PMID:26933770

  2. Domain wall conductivity in semiconducting hexagonal ferroelectric TbMnO3 thin films

    NASA Astrophysics Data System (ADS)

    Kim, D. J.; Connell, J. G.; Seo, S. S. A.; Gruverman, A.

    2016-04-01

    Although enhanced conductivity of ferroelectric domain boundaries has been found in BiFeO3 and Pb(Zr,Ti)O3 films as well as hexagonal rare-earth manganite single crystals, the mechanism of the domain wall conductivity is still under debate. Using conductive atomic force microscopy, we observe enhanced conductance at the electrically-neutral domain walls in semiconducting hexagonal ferroelectric TbMnO3 thin films where the structure and polarization direction are strongly constrained along the c-axis. This result indicates that domain wall conductivity in ferroelectric rare-earth manganites is not limited to charged domain walls. We show that the observed conductivity in the TbMnO3 films is governed by a single conduction mechanism, namely, the back-to-back Schottky diodes tuned by the segregation of defects.

  3. 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.

  4. 3D hexagonal (R-3m) mesostructured nanocrystalline titania thin films : synthesis and characterization.

    SciTech Connect

    Choi, S. Y.; Lee, B.; Carew, D. B.; Mamak, M; Peiris, F. C.; Speakman, S.; Chopra, N.; Ozin, G. A.; X-Ray Science Division; Univ. of Toronto; ORNL; Xerox Research Centre of Canada

    2006-01-01

    A straightforward and reproducible synthesis of crack-free large-area thin films of 3D hexagonal (R-3m) mesostructured nanocrystalline titania (meso-nc-TiO{sub 2}) using a Pluronic triblock copolymer (P123)/1-butanol templating system is described. The characterization of the films is achieved using a combination of electron microscopy (high-resolution scanning electron microscopy and scanning transmission electron microscopy), grazing-incidence small-angle X-ray scattering, in situ high-temperature X-ray diffraction, and variable-angle spectroscopic ellipsometry. The mesostructure of the obtained films is found to be based upon a 3D periodic array of large elliptically shaped cages with diameters around 20 nm interconnected by windows of about 5 nm in size. The mesopores of the film calcined at 300 C are very highly ordered, and the titania framework of the film has a crystallinity of 40 % being composed of 5.8 nm sized anatase crystallites. The film displays high thermal stability in that the collapse of the pore architecture is incomplete even at 600 C. The accessible surface area of 3D hexagonal meso-nc-TiO{sub 2} estimated by the absorption of methylene blue is nearly twice as large as that of 2D hexagonal meso-nc-TiO{sub 2} at the same annealing temperature.

  5. 3D HEXAGONAL (R-3M) MESOSTRUCTURED NANOCRYSTALLINE TITANIA THIN FILMS: SYNTHESIS AND CHARACTERIZATION

    SciTech Connect

    Choi, S Y; Lee, B; Carew, D B; Peiris, F C; Mamak, M; Speakman, Scott A; Chopra, N; Ozin, G A

    2006-01-01

    A straightforward and reproducible synthesis of crack-free large-area thin films of 3D hexagonal (R-3m) mesostructured nanocrystalline titania (meso-nc-TiO{sub 2}) using a Pluronic triblock copolymer (P123)/1-butanol templating system is described. The characterization of the films is achieved using a combination of electron microscopy (high-resolution scanning electron microscopy and scanning transmission electron microscopy), grazing-incidence small-angle X-ray scattering, in situ high-temperature X-ray diffraction, and variable-angle spectroscopic ellipsometry. The mesostructure of the obtained films is found to be based upon a 3D periodic array of large elliptically shaped cages with diameters around 20 nm interconnected by windows of about 5 nm in size. The mesopores of the film calcined at 300 C are very highly ordered, and the titania framework of the film has a crystallinity of 40 % being composed of 5.8 nm sized anatase crystallites. The film displays high thermal stability in that the collapse of the pore architecture is incomplete even at 600 C. The accessible surface area of 3D hexagonal meso-nc-TiO{sub 2} estimated by the absorption of methylene blue is nearly twice as large as that of 2D hexagonal meso-nc-TiO{sub 2} at the same annealing temperature.

  6. X-ray Magnetic circular dichroism study of hexagonal YbFeO3 thin films

    NASA Astrophysics Data System (ADS)

    Wang, Xiao; Sinha, Kishan; Xu, Xiaoshan; Liu, Yaohua; Keavney, David; Cheng, X. M.

    Multiferroic materials exhibit multiple ferroic orders simultaneously and thus have potential applications in information technology, sensing, and actuation. Hexagonal YbFeO3 is a promising candidate for a multiferroic material with room temperature ferromagnetism because of the expected enhanced Fe moment and higher transition temperature due to the exchange interaction between magnetic Yb and Fe ions. Here we report an x-ray magnetic circular dichroism (XMCD) study of (0001) Hexagonal YbFO3 thin films deposited on (111) yttria-stabilized zirconia substrates via pulsed laser deposition. XMCD spectra for the Fe L2,3 edges and Yb M5 edge were measured with the magnetic field applied parallel to the x-ray propagation direction and 20 degree away from the film normal at beamline 4ID-C of the APS at ANL. Field dependence of the XMCD spectra show that Fe and Yb each has a ferromagnetic ordering at around 6.7 K but with opposite orientations in between. The saturation magnetic moment for Fe is determined by the sum rules to be 0.07 μB / Fe cation at around 6.7 K, about 4 times larger than that in Hexagonal LuFeO3.

  7. 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.

  8. 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}.

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

    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.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

  10. Optical resistance of GaN and InGaN thin films

    NASA Astrophysics Data System (ADS)

    Å čiuka, Mindaugas; Dmukauskas, Mantas; Grinys, Tomas; Melninkaitis, Andrius

    2012-11-01

    Group III nitrides are wide band-gap semiconductors which are commonly used in high power and high frequency electronics and optoelectronics. A rapid development of GaN/InGaN devices is in progress however many technological improvements are still demanded. One of them is a convenient formation of electrical contacts attached to appropriate layers. Currently a selective etching step of GaN and InGaN layers is performed by using quite expensive methods such as plasma, chemical-lithographic or electron beam exposure. However, very little research has been done towards investigation of an alternative selective laser etching possibility. Therefore in this work we study optical resistance and damage morphology of thin film GaN and InxGa1-xN layers grown on sapphire substrates in the femtosecond regime. Laser induced damage threshold (LIDT) tests were carried out in both S-on-1 and 1-on-1 regimes by exposing samples from front (deposited) and rear (substrate) sides. For optical resistance testing a femtosecond Yb:KGW laser combined with harmonic generator covering near IR spectrum to visible and UV was used. Experimental results of optical resistance dependence on band-gap in InxGa1-xN layers with different indium concentration (X up to 22%) are presented. Also detailed morphology study for different laser wavelengths is performed and discussed.

  11. 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.

  12. Effect of perturbative hexagonal warping on quantum capacitance in ultra-thin topological insulators

    NASA Astrophysics Data System (ADS)

    Menon, Anirudha; Chowdhury, Debashree; Basu, Banasri

    2016-04-01

    Ultra-thin 3D topological insulators provide a stage to study the surface physics of such materials by minimizing the bulk contribution. Further, the experimentally verified snowflake like structure of the Fermi surface leads to a hexagonal warping term, and this shows it to be a perturbation in the presence of a magnetic field. We find that there are corrections to both energy dispersion and eigenstates which in turn alter the density of states in the presence of a magnetic field. Both the quantum capacitance and the Hall coefficient are evaluated analytically and it is shown here that we recover their established forms along with small corrections which preserve the object of treating hexagonal warping perturbatively. In our approach, the established Hall conductivity expression develops several minute correction terms and thus its behavior remains largely unaffected due to warping. The zero-temperature quantum capacitance exhibits Shubnikov-de Haas oscillations with reduced frequencies, with a lowered average capacitance with increased warping of the Fermi surface, while maintaining the usual amplitudes.

  13. 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

    2015-12-01

    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. PMID:26510890

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

    NASA Astrophysics Data System (ADS)

    Wu, Ruikang; Hu, Run; Luo, Xiaobing

    2016-04-01

    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.

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

    NASA Astrophysics Data System (ADS)

    Zhang, Jing; Yang, Yingchao; Lou, Jun

    2016-09-01

    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.

  16. Hexagonal nanosized molybdenum diselenide thin film deposited at 333 K by chemical method

    NASA Astrophysics Data System (ADS)

    Sathe, D. J.; Chate, P. A.

    2015-10-01

    Molybdenum diselenide thin films have been deposited on to stainless steel and glass substrates by the chemical process, using ammonium molybdate, sodium selenosulphite as a precursor sources and citric acid was used as a complexing agent. The structural and optical properties of the deposited films have been studied using X-ray diffraction and optical absorption techniques, respectively. XRD studies reveal that the films are polycrystalline with hexagonal crystal structure. Optical absorption study shows the presence of direct transition with band gap energy 1.51 eV. EDAX analysis shows that the films are nearly stoichiometry of Mo: Se: 1:2. The configuration of fabricated cell is n-MoSe2 | NaI (2 M) + I2 (1 M) | C (graphite) yielded a conversion efficiency of 1.08%.

  17. Optical and magnetic properties of hexagonal arrays of subwavelength holes in optically thin cobalt films.

    PubMed

    Ctistis, G; Papaioannou, E; Patoka, P; Gutek, J; Fumagalli, P; Giersig, M

    2009-01-01

    In this study, we present our experimental results on the optical, magnetic, as well as magneto-optic properties of hexagonal arrays of subwavelength holes in optically thin cobalt films. Different meshes were used with hole diameters ranging between 220 and 330 nm while the interhole distance has been kept constant at 470 nm. The hole pattern modifies completely the magnetic behavior of the cobalt films; it gives rise to an increase of the coercive field of the in-plane magnetization with increasing hole diameter and to the appearance of out-of-plane magnetization components. Magneto-optic measurements show a spectacular magneto-optic response at wavelengths where surface plasmon-polaritons are supported by the structure as deduced in optical measurements. The experiments demonstrate the ability to artificially control the magnetic and thus the magneto-optic properties in hole array structures. PMID:19072720

  18. 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.

  19. 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.

  20. 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.

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

    PubMed

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

    2015-12-01

    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. PMID:26153124

  2. 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.

  3. Dynamic scaling of the growth process of GaN thin films deposited on sapphire substrates by HVPE

    NASA Astrophysics Data System (ADS)

    Lu, Dianqing; Zhang, Rong; Yu, Huiqiang; Xiu, Xiangqian; Li, Xuefei; Gu, Shulin; Shen, Bo; Shi, Yi; Zheng, Youdou

    2004-06-01

    The growth front evolution of gallium nitride (GaN) thin films deposited on sapphire substrates by HVPE has been studied with atomic force microscope (AFM). The dynamic scaling characteristics are observed during the deposition process. After numerical correlation analysis, the roughness exponent α=0.75 and the growth exponent β=0.59 are obtained by using self-affine model. The roughening mechanisms of GaN thin films has been simply investigated, and the results indicate that one or more roughening mechanisms are present in the growth process in addition to stochastic roughening and diffusion effect. The computed values of the correlations in the input noise are very close to our results.

  4. 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.

  5. Unusual M2-mediated metal-insulator transition in epitaxial VO2 thin films on GaN substrates

    NASA Astrophysics Data System (ADS)

    Yang, Hyoung Woo; Inn Sohn, Jung; Yang, Jae Hoon; Jang, Jae Eun; Cha, Seung Nam; Kim, Jongmin; Kang, Dae Joon

    2015-01-01

    We report on the epitaxial growth of vanadium dioxide (\\text{VO}2) thin films on (0001) GaN substrates using a radio frequency magnetron sputtering method and discuss their unusual M2-mediated metal-insulator transition (MIT) properties. We found that large lattice misfits between the \\text{VO}2 film and the GaN substrate could favor the stabilization of the intermediate insulating \\text{M}2 phase, which is known to be observed only in either doped or uniaxially strained samples. We demonstrated that the MIT in \\text{VO}2 films on GaN substrates could be mediated via a monoclinic \\text{M}2 phase during the transition from a monoclinic \\text{M}1 to a rutile R phase. This was confirmed by temperature-dependent Raman studies that exhibited both an evident upshift of a high-frequency phonon mode (ω\\text{V-O}) from 618 \\text{cm}-1 (\\text{M}1) to 645 \\text{cm}-1 (\\text{M}2) and a distinct peak splitting of a low-frequency phonon mode (ω\\text{V-V}) at 221 \\text{cm}-1 (\\text{M}2) for increasing temperatures. Moreover, a resistance change of four orders of magnitude was observed for \\text{VO}2 thin films on GaN substrates, being indicative of the high quality of \\text{VO}2 thin films. This study may offer great opportunities not only to improve the understanding of M2-mediated MIT behavior in \\text{VO}2 thin films, but also to realize novel electronic and optoelectronic devices.

  6. Vertically Oriented Growth of GaN Nanorods on Si Using Graphene as an Atomically Thin Buffer Layer.

    PubMed

    Heilmann, Martin; Munshi, A Mazid; Sarau, George; Göbelt, Manuela; Tessarek, Christian; Fauske, Vidar T; van Helvoort, Antonius T J; Yang, Jianfeng; Latzel, Michael; Hoffmann, Björn; Conibeer, Gavin; Weman, Helge; Christiansen, Silke

    2016-06-01

    The monolithic integration of wurtzite GaN on Si via metal-organic vapor phase epitaxy is strongly hampered by lattice and thermal mismatch as well as meltback etching. This study presents single-layer graphene as an atomically thin buffer layer for c-axis-oriented growth of vertically aligned GaN nanorods mediated by nanometer-sized AlGaN nucleation islands. Nanostructures of similar morphology are demonstrated on graphene-covered Si(111) as well as Si(100). High crystal and optical quality of the nanorods are evidenced through scanning transmission electron microscopy, micro-Raman, and cathodoluminescence measurements supported by finite-difference time-domain simulations. Current-voltage characteristics revealed high vertical conduction of the as-grown GaN nanorods through the Si substrates. These findings are substantial to advance the integration of GaN-based devices on any substrates of choice that sustains the GaN growth temperatures, thereby permitting novel designs of GaN-based heterojunction device concepts. PMID:27124605

  7. 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.

  8. Electrical current flow at conductive nanowires formed in GaN thin films by a dislocation template technique

    NASA Astrophysics Data System (ADS)

    Amma, Shin-ichi; Tokumoto, Yuki; Edagawa, Keiichi; Shibata, Naoya; Mizoguchi, Teruyasu; Yamamoto, Takahisa; Ikuhara, Yuichi

    2010-05-01

    Conductive nanowires were fabricated in GaN thin film by selectively doping of Al along threading dislocations. Electrical current flow localized at the nanowires was directly measured by a contact mode atomic force microscope. The current flow at the nanowires was considered to be Frenkel-Poole emission mode, suggesting the existence of the deep acceptor level along the nanowires as a possible cause of the current flow. The results obtained in this study show the possibility for fabricating nanowires using pipe-diffusion at dislocations in solid thin films.

  9. 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.

  10. Numerical analysis on the origin of thickness unevenness and formation of pits at GaN thin film grown by HVPE

    NASA Astrophysics Data System (ADS)

    Han, Xue-Feng; Lee, Jae-Hak; Lee, Yoo-Jin; Song, Jae-Ho; Yi, Kyung-Woo

    2016-09-01

    In this study, we propose a 3D model for analyzing the fluid flow, mass fractions of reacting gases, GaN deposition thickness distribution and V/III ratio distribution at the GaN deposition surface in the multi-susceptor HVPE equipment. The GaN thin film is grown in the multi-susceptor HVPE equipment at 1213 K and 1 bar. The deposition thickness distribution from the calculation has been compared with the experimental results. Moreover, the standard deviations of deposition thickness of the films achieved from calculations and experiments have been compared. Besides, in the calculation results, we found that the V/III ratio at the GaN deposition surface increased from the center to the periphery and from low susceptor to high susceptor. Our calculation results have also been verified by 3D measuring laser microscope observation of the surface morphology of the GaN thin film. In according with the calculation results, the density of the pits also decreases from the center to the periphery as well as from low susceptor to high susceptor, demonstrating that the pit density at the surface of the GaN thin films could be reduced when the V/III ratio is increased.

  11. Optical spectroscopic study on new magnetoelectric hexagonal REMnO3 (RE=Gd, Tb, Dy, and Ho) thin films

    NASA Astrophysics Data System (ADS)

    Choi, Woo Seok; Seo, Sung Seok A.; Lee, Jung Hyuk; Lee, Daesu; Noh, Tae Won; Lee, Yunsang

    2007-03-01

    Recently, magnetoelectric effects in various oxides have been attracting lots of attentions and are being extensively investigated due to their intriguing couplings between the magnetic and electric order parameters. Here we report optical spectroscopic investigations on new hexagonal REMnO3 (RE = Gd, Tb, Dy, and Ho) thin films, which are fabricated by epi-stabilization technique [1]. From the in-plane optical conductivity spectra of the hexagonal REMnO3, we observe a dramatic increase of the optical transition related to Mn 3d a1g energy level, as the ionic radius of the R ion increases. The optical transition at 1.64 eV for DyMnO3 shifts to 1.67 and 1.81 for TbMnO3 and GdMnO3 respectively. For natural hexagonal REMnO3 (RE = Y, Er, Lu, and Sc) with smaller ionic sizes, the same optical transitions occur at ˜1.6 eV. The large peak shift in new hexagonal REMnO3 is understood by local flattening of Mn-O bipyramid, which will enhance the crystal field energy of a1g, as the RE ionic size increases. [1] J. H. Lee et al., Adv. Mat., to be published (2006).

  12. 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.

  13. 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.

  14. Suppression of thermally activated carrier transport in atomically thin MoS2 on crystalline hexagonal boron nitride substrates

    NASA Astrophysics Data System (ADS)

    Chan, Mei Yin; Komatsu, Katsuyoshi; Li, Song-Lin; Xu, Yong; Darmawan, Peter; Kuramochi, Hiromi; Nakaharai, Shu; Aparecido-Ferreira, Alex; Watanabe, Kenji; Taniguchi, Takashi; Tsukagoshi, Kazuhito

    2013-09-01

    We present the temperature-dependent carrier mobility of atomically thin MoS2 field-effect transistors on crystalline hexagonal boron nitride (h-BN) and SiO2 substrates. Our results reveal distinct weak temperature dependence of the MoS2 devices on h-BN substrates. The room temperature mobility enhancement and reduced interface trap density of the single and bilayer MoS2 devices on h-BN substrates further indicate that reducing substrate traps is crucial for enhancing the mobility in atomically thin MoS2 devices.We present the temperature-dependent carrier mobility of atomically thin MoS2 field-effect transistors on crystalline hexagonal boron nitride (h-BN) and SiO2 substrates. Our results reveal distinct weak temperature dependence of the MoS2 devices on h-BN substrates. The room temperature mobility enhancement and reduced interface trap density of the single and bilayer MoS2 devices on h-BN substrates further indicate that reducing substrate traps is crucial for enhancing the mobility in atomically thin MoS2 devices. Electronic supplementary information (ESI) available. See DOI: 10.1039/c3nr03220e

  15. Effect of annealing on M-plane GaN thin films grown by PAMBE on tilt-cut LAO substrate

    NASA Astrophysics Data System (ADS)

    Lin, Yu-Chiao; Lo, Ikai; Wang, Ying-Chieh; Tsai, Cheng-Da; Yang, Chen-Chi; You, Shuo-Ting; Chou, Ming-Chi; Department of Materials and Optoelectronic Science Collaboration

    2014-03-01

    The non-polar GaN thin film is a potential candidate for high-efficient photoelectric devices. In this work, we analyzed the characteristics of M-plane GaN thin films which were grown on tilt-cut LiAlO2 (LAO) substrate by plasma-assisted molecular beam epitaxy (PAMBE). A series of samples were grown with different N/Ga flux ratios. The crystal structure and optical property of GaN samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and photoluminescence (PL) measurements. The peak of 32.2o in the XRD measurement showed the [1100] oriented (M-plane) for the GaN samples. To improve the crystal quality, we performed the thermal treatment by rapid thermal annealing (RTA) system on these samples and analyzed the crystal structure, surface morphology and optical property of the samples after thermal treatment. The effect of annealing on the M-plane GaN thin films was under investigation. This project is supported by National Science council of Taiwan(101-2112-M-110-006-MY3).

  16. Polarization-dependent x-ray absorption spectroscopy of hexagonal and orthorhombic TbMnO3 thin films

    NASA Astrophysics Data System (ADS)

    Wu, K. H.; Gou, I. C.; Luo, C. W.; Uen, T. M.; Lin, J.-Y.; Juang, J. Y.; Kobayashi, T.; Chen, C. K.; Lee, J. M.; Chen, J. M.

    2010-01-01

    Pure phase TbMnO3 manganite thin films with hexagonal (h-TMO) and orthorhombic (o- TMO) crystal structures were prepared by pulsed laser deposition. The distinctive orientation alignments between film and substrate obtained here have allowed us to perform the x-ray absorption near edge spectroscopy (XANES) measurements with the electric field applied along the three major crystallographic directions. The XANES results, as expected, display significantly different spectral features for the h-TMO and o-TMO films. In addition, the XANES spectra also exhibit strong polarization dependence at O K and Mn L edges for both samples.

  17. 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.

  18. A new system for synthesis of high quality nonpolar GaN thin films.

    PubMed

    Li, Guoqiang; Shih, Shao-Ju; Fu, Zhengyi

    2010-02-28

    High quality nonpolar m-plane GaN films were successfully grown on LiGaO(2) (100) substrates for the first time. This m-plane GaN/LiGaO(2) (100) system opens a new approach for realizing highly-efficient nitride devices. PMID:20449251

  19. Planar Millimeter Wave Notch Filters Based on Magnetostatic Wave Resonance in Barium Hexagonal Ferrite Thin Films

    NASA Astrophysics Data System (ADS)

    Lu, Lei; Song, Young-Yeal; Bevivino, Joshua; Wu, Mingzhong

    2010-10-01

    There is a critical need for planar millimeter (mm) wave devices. To meet this need, one important strategy is in the use of high-anisotropy hexagonal ferrite films. The high internal anisotropy field for the hexagonal ferrites can be used to realize low-loss devices in the 30-100 GHz regime without the need for high external magnetic fields. Previous work has demonstrated the use of M-type barium hexagonal ferrite (BaM) films and ferromagnetic resonance therein to make mm-wave notch filters. This presentation reports on a new mm-wave notch filter that uses magnetostatic wave (MSW) resonance in BaM films. The device consists of a BaM film strip positioned on the top of a coplanar waveguide (CPW), with the strip's length along the CPW signal line. The BaM strip was grown by pulsed laser deposition and had uniaxial anisotropy along the strip's length. The device showed a band-stop filtering response centered at 53 GHz in absence of external fields. One can increase this frequency with nonzero external fields. A reduction in the strip's width resulted in an enhancement in peak absorption. This filtering response resulted from MSW resonance across the BaM strip's width. The MSW modes were excited by CPW-produced non-uniform alternating magnetic fields.

  20. Characterization of M-plane GaN thin films grown on misoriented γ-LiAlO2 (100) substrates

    NASA Astrophysics Data System (ADS)

    Lin, Yu-Chiao; Lo, Ikai; Wang, Ying-Chieh; Yang, Chen-Chi; Hu, Chia-Hsuan; Chou, Mitch M. C.; Schaadt, D. M.

    2016-09-01

    M-plane GaN thin films were grown on 11° misoriented γ-LiAlO2 substrates without peeling off or cracking by plasma-assisted molecular beam epitaxy. Because of anisotropic growth kinetics, which leads to an anisotropic compressive in-plane strain in the M-plane GaN films, the surface presents a rough morphology with worse crystal quality. The crystal quality of sample was optimally improved, XRD rocking curve FWHM of which is about 900 arcsec, by raising growth temperature to 800 °C with proper Ga/N flux ratio. As the crystal quality was improved, the polarization ratio decreased from the unity (less than 0.8) which could be attributed to the effect of exciton localization due to the partial increased in-plane strain.

  1. Strain distribution of thin InN epilayers grown on (0001) GaN templates by molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Delimitis, A.; Komninou, Ph.; Dimitrakopulos, G. P.; Kehagias, Th.; Kioseoglou, J.; Karakostas, Th.; Nouet, G.

    2007-02-01

    A structural characterization of thin InN films is performed to determine the post-growth strain distribution, using electron microscopy techniques. A 60° misfit dislocation network at the InN /GaN interface effectively accommodates the lattice mismatch. The InN in-plane lattice parameter, which remained practically constant throughout the epilayer thickness, was precisely determined by electron diffraction analysis, and cross-section and plan-view lattice images. Image analysis using the geometric phase and projection methods revealed a uniform distribution of the residual tensile strain along the growth and lateral directions. The in-plane strain is primarily attributed to InN island coalescence during the initial stages of growth.

  2. Improved growth of GaN layers on ultra thin silicon nitride/Si (1 1 1) by RF-MBE

    SciTech Connect

    Kumar, Mahesh; Roul, Basanta; Bhat, Thirumaleshwara N.; Rajpalke, Mohana K.; Misra, P.; Kukreja, L.M.; Sinha, Neeraj; Kalghatgi, A.T.; Krupanidhi, S.B.

    2010-11-15

    High-quality GaN epilayers were grown on Si (1 1 1) substrates by molecular beam epitaxy using a new growth process sequence which involved a substrate nitridation at low temperatures, annealing at high temperatures, followed by nitridation at high temperatures, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. The material quality of the GaN films was also investigated as a function of nitridation time and temperature. Crystallinity and surface roughness of GaN was found to improve when the Si substrate was treated under the new growth process sequence. Micro-Raman and photoluminescence (PL) measurement results indicate that the GaN film grown by the new process sequence has less tensile stress and optically good. The surface and interface structures of an ultra thin silicon nitride film grown on the Si surface are investigated by core-level photoelectron spectroscopy and it clearly indicates that the quality of silicon nitride notably affects the properties of GaN growth.

  3. Structural, magnetic, and transport properties of sputtered hexagonal MnNiGa thin films

    SciTech Connect

    Li, Yueqing; Liu, E. K.; Wu, G. H.; Wang, Wenhong; Liu, Zhongyuan

    2014-12-14

    We report on a systematical study of the structure, magnetism, and magnetotransport behavior of the hexagonal MnNiGa films deposited on thermally oxidized Si (001) substrates by magnetron sputtering. X-ray diffractions reveal that all the films deposited at different temperatures crystallized in hexagonal Ni{sub 2}In-type structure (space group P6{sub 3}/mmc). Scanning electron microscopy observations show that the surface morphology of the films varies with deposition temperature, and energy dispersive spectroscopy analysis shows compositions of the films remain nearly unchanged, independent of the deposition temperature. Magnetic measurements indicate that all films are ferromagnetic and exhibit a magnetic anisotropy behavior. The magnetoresistance (MR) exhibits a negative temperature- and field-dependent behavior. The possible origin of the negative MR is discussed. Furthermore, we found that the Hall effect is dominated by an anomalous Hall effect (AHE) only due to skew scattering independent of the deposition temperature of films. Moreover, the anomalous Hall resistivity presents a non-monotonously temperature-dependent behavior.

  4. Hot pressing of Cr{sub 2}O{sub 3} powder with thin hexagonal plate particles

    SciTech Connect

    Komeda, T.; Fukumoto, Y.; Yoshinaka, M.; Hirota, K.; Yamaguchi, O.

    1996-08-01

    Chromium oxide (Cr{sub 2}O{sub 3}) powders with thin hexagonal plate particles have been obtained at 1,200 C to 1,400 C from an amorphous material prepared by adding hydrazine monohydrate to the aqueous solution of chromium nitrate. Tablets molded by wet-uniaxial-pressing were hot-pressed for 2 h at 1,500 C and 30 MPa. The dense sintered Cr{sub 2}O{sub 3} ceramics (>98% of theoretical) obtained had crystal-orientation, in which a <001> axis was parallel to the pressing direction. They exhibited an anisotropy in electrical conductivity, s; the values measured at 700 C in the perpendicular and parallel directions for the <001> axis were 5.5 and 0.58 S-m{sup {minus}1}, respectively. Vickers hardness, HV, and fracture toughness, KIC, were 24.1 GPa and 4.6 MPa{center_dot}m{sup 1/2}, respectively.

  5. Orientational relationship between cubic boron nitride and hexagonal boron nitride in a thin film synthesized by ion plating

    NASA Astrophysics Data System (ADS)

    Zhou, Wei-Lie; Ikuhara, Yuichi; Suzuki, Tetsuya

    1995-12-01

    Cubic boron nitride (c-BN) thin films synthesized by the ion-plating method were examined by high-resolution electron microscopy. It was found that the {0002} planes of hexagonal boron nitride (h-BN) at the boundaries of c-BN grains preferred to nucleate almost parallel to {111} planes of c-BN. Cross-sectional observation in the initial stage of growth showed that the c-BN can grow on top of the prismatic planes and the {0001} basal planes of h-BN, keeping the parallelism of the (111)c-BN to (0001)h-BN. A few degrees deviation (˜4°) between h-BN {0002} planes and c-BN {111} planes was frequently found in the film. The nucleation mechanism of c-BN was discussed analogous to that of diamond on graphite.

  6. 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.

  7. 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.

  8. 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.

  9. 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. PMID:23618222

  10. Homo- and hetero-epitaxial growth of hexagonal and cubic MgxZn1-x O alloy thin films by pulsed laser deposition technique

    NASA Astrophysics Data System (ADS)

    Hullavarad, S. S.; Hullavarad, N. V.; Pugel, D. E.; Dhar, S.; Takeuchi, I.; Venkatesan, T.; Vispute, R. D.

    2007-08-01

    In this work, we describe the homo- and hetero-epitaxial growth of hexagonal and cubic MgxZn1-xO thin films on lattice matched substrates of c-Al2O3, ZnO, MgO and SrTiO3. The crystalline quality, composition and epitaxial nature of the alloy films are obtained by x-ray diffraction and Rutherford backscattering spectroscopy (RBS) techniques. The RBS channeling yields are in the range 3-8% for homoepitaxial and hetero-epitaxial thin films. The metal-semiconductor-metal and ultraviolet detectors were fabricated on hexagonal and cubic MgxZn1-xO thin films and the leakage current and UV-visible rejection ratio are correlated with the epitaxial relationship between the film and substrates.

  11. Two coexisting mechanisms of dislocation reduction in an AlGaN layer grown using a thin GaN interlayer

    SciTech Connect

    Bai, J.; Wang, T.; Parbrook, P. J.; Wang, Q.; Lee, K. B.; Cullis, A. G.

    2007-09-24

    A significant dislocation reduction is achieved in an AlGaN layer grown on an AlN buffer by introducing a thin GaN interlayer. The mechanisms for the dislocation reduction are explored by transmission electron microscopy, energy-dispersive x-ray spectroscopy, atomic force microscopy, and micro-Raman spectroscopy. The GaN interlayer grown on the AlN takes the form of platelets. The mechanisms of dislocation reduction in the platelet area and the area between the platelets are different. In the GaN platelets, due to the large misfit strain, the threading dislocations (TDs) in the AlN layer migrate into the interface and annihilate with each other. However, the GaN between the platelets is highly strained so that a higher density of TDs from AlN is incorporated into the upper layer. The coalescing of the platelets induced by the AlGaN growth makes the TDs in the areas between the platelets assemble and annihilate, resulting in additional dislocation reduction.

  12. The annealing effects of V-doped GaN thin films grown by MOCVD

    NASA Astrophysics Data System (ADS)

    Souissi, M.; Bouzidi, M.; El Jani, B.

    2012-02-01

    We have investigated the annealing effect of V-doped GaN (GaN:V) epitaxial layers grown on sapphire by metal organic chemical vapor deposition (MOCVD). The film was annealed at a temperature of 1075 °C for 30 min in N 2 ambient after growth. The structural, surface morphology and optical properties of GaN:V films were studied by high resolution X-ray diffraction (HRXRD), atomic force microscope (AFM) and photoluminescence (PL). The results show that the annealing makes for the destruction in the crystal quality and surface morphology. After thermal annealing, the photoluminescence (PL) measurement showed a reduction of the blue luminescence (BL) band observed in GaN:V at room temperature (RT). The phenomenon is attributed to vanadium diffusion or to the V-related complex dissociation. Near-band-edge (NBE) peak exhibited a red shift after 1075 °C anneal. This is due to the decrease in the level of strain. In the infrared region, we observed the emergence of the line 0.93 eV accompanied by a decrease in the intensity of the 0.82 eV emission. Their possible origins are discussed.

  13. 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.

  14. 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.

  15. On-wafer millimeter wave notch filter based on barium hexagonal ferrite thin films on platinum

    NASA Astrophysics Data System (ADS)

    Harward, Ian Roylance

    In this work, the growth of BaM and Al doped Ba M thin films on Pt templates, layered on a Si wafer, is demonstrated using a newly developed metallo-organic decomposition (MOD) process. It is shown that the BaM films are polycrystalline, with preferred perpendicular c-axis grain orientation. The magnetic properties such as anisotropy field, saturation magnetization, and remnant magnetization are studied as a function of temperature and film composition, and are shown to be correlated to the film microstructure. It is shown that these films exhibit high remnant magnetization, a property not measured in BaM single crystals, meaning a biasing magnet may not be necessary for millimeter wave device applications. Ferromagnetic resonance (FMR) studies were performed on the ferrite films using the tool developed at UCCS for the study of high frequency magnetic materials, the broadband FMR (BFMR) system. The instrument is described in great detail, and the FMR studies on BaM show that the MOD-grown films exhibit narrow FMR linewidths, on the order of 150 Oe, and are therefore of sufficient quality for use in mm wave devices. Finally, notch filters using the Pt/BaM are demonstrated. The filters are based on a microstrip design, where the Pt serves as the ground plane and the BaM is part of the dielectric. The Ba M absorbs signals at the ferromagnetic resonance frequency, which takes place in the mm wave range. The filters described were based on pure BaM, but Al doped BaM could easily be used to increase the operating frequency of the device. The operating frequency of these devices is also tunable using an externally applied magnetic field.

  16. 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.

  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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    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. 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.

  19. The 310 340 nm ultraviolet light emitting diodes grown using a thin GaN interlayer on a high temperature AlN buffer

    NASA Astrophysics Data System (ADS)

    Wang, T.; Lee, K. B.; Bai, J.; Parbrook, P. J.; Ranalli, F.; Wang, Q.; Airey, R. J.; Cullis, A. G.; Zhang, H. X.; Massoubre, D.; Gong, Z.; Watson, I. M.; Gu, E.; Dawson, M. D.

    2008-05-01

    Previously, we reported that a thin GaN interlayer approach has been developed for growth of 340 nm ultraviolet light emitting diodes (UV-LEDs) with significantly improved performance. In this paper, more recent results on the further development of UV-LEDs with shorter wavelengths are reported, and the limitation of the wavelength of the UV-LEDs that can be pushed to, while retaining high device performance using the approach has been investigated. Transmission electron microscopy and device-performance data, including electrical and optical characteristics, indicated that the thin GaN interlayer approach can be effectively employed for growth of UV-LEDs to an emission wavelength approaching at least 300 nm. The approach should be taken into account in growth of UV-LEDs on sapphire substrates, as it provides a simple but effective growth method to achieve UV-LEDs with high performance. This paper also reports that a micro-LED array using the UV-LED wafer has been successfully fabricated, offering versatile micro-structured UV light sources for a wide range of applications.

  20. 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

    2016-02-01

    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. PMID:26674458

  1. 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

    NASA Astrophysics Data System (ADS)

    Hayashi, Hiroaki; Konno, Yuta; Kishino, Katsumi

    2016-02-01

    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.

  2. Transition from amorphous boron carbide to hexagonal boron carbon nitride thin films induced by nitrogen ion assistance

    NASA Astrophysics Data System (ADS)

    Gago, R.; Jiménez, I.; Agulló-Rueda, F.; Albella, J. M.; Czigány, Zs.; Hultman, L.

    2002-11-01

    Boron carbon nitride films (BCN) were grown by B4C evaporation under concurrent N2 ion beam assistance. The films were characterized by x-ray absorption near-edge spectroscopy, infrared and Raman spectroscopies, and high-resolution transmission electron microscopy. The bonding structure and film composition correlate with the momentum transfer per incoming atom during deposition. As the momentum transfer is increased, the film structure evolves from an amorphous boron carbide network towards a hexagonal ternary compound (h-BCN) with standing basal planes. The growth of h-BCN takes place for momentum transfer in the window between 80 and 250 (eV×amu)1/2. The characteristic vibrational features of the h-BCN compounds have also been studied. Finally, the solubility limit of carbon in the hexagonal BN structure, under the working conditions of this article, is found to be ˜15 at. %.

  3. Properties of TiO{sub 2}-based transparent conducting oxide thin films on GaN(0001) surfaces

    SciTech Connect

    Kasai, J.; Nakao, S.; Yamada, N.; Hitosugi, T.; Moriyama, M.; Goshonoo, K.; Hoang, N. L. H.; Hasegawa, T.

    2010-03-15

    Anatase Nb-doped TiO{sub 2} transparent conducting oxide has been formed on GaN(0001) surfaces using a sputtering method. Amorphous films deposited at room temperature were annealed at a substrate temperature of 500 deg. C in vacuum to form single-phase anatase films. Films with a thickness of 170 nm exhibited a resistivity of 8x10{sup -4} {Omega} cm with absorptance less than 5% at a wavelength of 460 nm. Furthermore, the refractive index of the Nb-doped TiO{sub 2} was well matched to that of GaN. These findings indicate that Nb-doped TiO{sub 2} is a promising material for use as transparent electrodes in GaN-based light emitting diodes (LEDs), particularly since reflection at the electrode/GaN boundary can be suppressed, enhancing the external quantum efficiency of blue LEDs.

  4. Effects of thickness on optical characteristics and strain distribution of thin-film GaN light-emitting diodes transferred to Si substrates

    NASA Astrophysics Data System (ADS)

    Li, Heng; Shi, Yang-Da; Feng, Meixin; Sun, Qian; Lu, Tien-Chang

    2016-04-01

    We investigated the effect of device thickness on the internal quantum efficiency (IQE) of thin-film GaN light-emitting diodes (LEDs), which were grown on Si substrates and transferred to other Si substrates with reduced film thickness. It was confirmed by Raman spectroscopy and photoluminescence measurement that the compressive strain is released and the quantum-confined Stark effect (QCSE) is suppressed after reducing the thickness. The best IQE of 62.9% was reached with a large suppression of the band tilting by QCSE, from 7.9 meV in the original structure to 2.4 meV in the thinnest sample, and this value can compete with that of GaN-based LEDs grown on a sapphire substrate.

  5. 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.

  6. Preparation of thin hexagonal highly-ordered anodic aluminum oxide (AAO) template onto silicon substrate and growth ZnO nanorod arrays by electrodeposition

    NASA Astrophysics Data System (ADS)

    Chahrour, Khaled M.; Ahmed, Naser M.; Hashim, M. R.; Elfadill, Nezar G.; Qaeed, M. A.; Bououdina, M.

    2014-12-01

    In this study, anodic aluminum oxide (AAO) templates of Aluminum thin films onto Ti-coated silicon substrates were prepared for growth of nanostructure materials. Hexagonally highly ordered thin AAO templates were fabricated under controllable conditions by using a two-step anodization. The obtained thin AAO templates were approximately 70 nm in pore diameter and 250 nm in length with 110 nm interpore distances within an area of 3 cm2. The difference between first and second anodization was investigated in details by in situ monitoring of current-time curve. A bottom barrier layer of the AAO templates was removed during dropping the voltage in the last period of the anodization process followed by a wet etching using phosphoric acid (5 wt%) for several minutes at ambient temperature. As an application, Zn nanorod arrays embedded in anodic alumina (AAO) template were fabricated by electrodeposition. Oxygen was used to oxidize the electrodeposited Zn nanorods in the AAO template at 700 °C. The morphology, structure and photoluminescence properties of ZnO/AAO assembly were analyzed using Field-emission scanning electron microscope (FESEM), Energy dispersive X-ray spectroscopy (EDX), Atomic force microscope (AFM), X-ray diffraction (XRD) and photoluminescence (PL).

  7. 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.

  8. Biaxial Strain in the Hexagonal Plane of MnAs Thin Films: The Key to Stabilize Ferromagnetism to Higher Temperature

    NASA Astrophysics Data System (ADS)

    Garcia, V.; Sidis, Y.; Marangolo, M.; Vidal, F.; Eddrief, M.; Bourges, P.; Maccherozzi, F.; Ott, F.; Panaccione, G.; Etgens, V. H.

    2007-09-01

    The α-β magnetostructural phase transition in MnAs/GaAs(111) epilayers is investigated by elastic neutron scattering. The in-plane parameter of MnAs remains almost constant with temperature from 100 to 420 K, following the thermal evolution of the GaAs substrate. This induces a temperature dependent biaxial strain that is responsible for an α-β phase coexistence and, more importantly, for the stabilization of the ferromagnetic α phase at a higher temperature than in the bulk. We explain the premature appearance of the β phase at 275 K and the persistence of the ferromagnetic α phase up to 350 K with thermodynamical arguments based on the MnAs phase diagram. It results that the biaxial strain in the hexagonal plane is the key parameter to extend the ferromagnetic phase well over room temperature.

  9. Biaxial strain in the hexagonal plane of MnAs thin films: the key to stabilize ferromagnetism to higher temperature.

    PubMed

    Garcia, V; Sidis, Y; Marangolo, M; Vidal, F; Eddrief, M; Bourges, P; Maccherozzi, F; Ott, F; Panaccione, G; Etgens, V H

    2007-09-14

    The alpha-beta magnetostructural phase transition in MnAs/GaAs(111) epilayers is investigated by elastic neutron scattering. The in-plane parameter of MnAs remains almost constant with temperature from 100 to 420 K, following the thermal evolution of the GaAs substrate. This induces a temperature dependent biaxial strain that is responsible for an alpha-beta phase coexistence and, more importantly, for the stabilization of the ferromagnetic alpha phase at a higher temperature than in the bulk. We explain the premature appearance of the beta phase at 275 K and the persistence of the ferromagnetic alpha phase up to 350 K with thermodynamical arguments based on the MnAs phase diagram. It results that the biaxial strain in the hexagonal plane is the key parameter to extend the ferromagnetic phase well over room temperature. PMID:17930469

  10. 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.

  11. Discrete-variational Hartree-Fock-Slater calculation of polarized B K-emission band from hexagonal boron nitride thin film

    NASA Astrophysics Data System (ADS)

    Hidenori Kohzuki; Takahiro Kaneyoshi; Muneyuki Motoyama; Yoshiyuki Kowada; Yasuji Muramatsu; Jun Kawai; Fumikazu Kanamaru

    1997-03-01

    The B K X-ray emission spectrum of h-BN can be approximately reproduced by the DV-Xα calculation, using the two-dimensional model cluster constructed of a mono-layer of the planar hexagonal network. In order to verify the polarized B K X-ray emission spectrum of h-BN, the contributions of the σ, π and π ∗ subbands to its spectral feature were estimated at different take-off angles for the c axis of an h-BN crystal. The high-energy satellite band increases its intensity, remarkably, due to the contribution of the π ∗ subband with increase in take-off angle, whereas the low-energy satellite band decreases its intensity due to the contribution of the σ subband. The measured B K X-ray emission spectrum of the ion-plated h-BN thin film is in good agreement with the calculated one at a take-off angle of 90°.

  12. On-wafer magnetically tunable millimeter wave notch filter using M-phase Ba hexagonal ferrite/Pt thin films on Si

    NASA Astrophysics Data System (ADS)

    Harward, I.; Camley, R. E.; Celinski, Z.

    2014-10-01

    A prototype of a fully integrated on-wafer, magnetically tunable band-stop filter operating at millimeter wave frequencies is demonstrated on a Si substrate. In contrast to earlier studies, the filter uses a very thin barium hexagonal ferrite film incorporated into the dielectric layer of a microstrip transmission line to filter the signal. The zero-field operational frequency is about 34 GHz, increasing linearly with the strength of a static, perpendicularly applied magnetic field at a rate of about 2.7 GHz/kOe. Experimentally, high signal attenuation (33-67 dB/cm) at the resonance frequency and insertion losses as low as 4.5 dB were simultaneously observed, while the 3 dB device bandwidths were generally below 1 GHz. Our calculations are in quantitative agreement with the experimental results. We also find an important result that the thickness and conductivity of the Pt ground plane plays a key role in insertion losses, indicating directions for further improvements.

  13. 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.

  14. 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. PMID:27409977

  15. Determination via luminescence spectroscopy and x-ray diffraction of the strain and composition of GaN and Al(x)Ga(1-x)N thin films grown on 6H-SiC(0001) substrates

    NASA Astrophysics Data System (ADS)

    Perry, William George

    1997-12-01

    This dissertation describes the luminescence and x-ray diffraction characterization of GaN and AlxGa1-xN thin films that were deposited on 6H-SiC(0001) substrates. These materials have applications for optoelectronic devices that are operational in the UV to green regions of the spectrum and for high-temperature, high-frequency and high-power microelectronic devices. The primary tools used in this research were photoluminescence and cathodoluminescence spectroscopies and high-resolution x-ray diffraction. Biaxial strains resulting from the mismatches in thermal expansion coefficients and lattice parameters in GaN films grown on AlN buffer layers previously deposited on vicinal and on-axis 6H-SiC(0001) substrates were measured using photoluminescence. A linear relationship between the bound exciton energy (EBX) and the biaxial strain along the c-axis direction was observed. A marked variation in the biaxial strain in GaN films deposited on off- and on-axis SiC was determined. It was attributed to the difference in the density and nature of the microstructural defects that originate at the steps on the SiC surface. The strain in the GaN films was either in tension or compression; whereas, only tensile strains were reported in all previous studies using SiC wafers. This indicated that the lattice mismatch strain in the former films was not fully relieved by defect formation. This result was confirmed by the observation via HRTEM of a 0.9% residual compressive strain at the GaN/AlN interface. Cathodoluminescence was used to determine the optical spectra in AlxGa1-xN films over the entire composition range of x. A bowing parameter of b = 1.65 eV for the bound exciton peak was observed. This bound exciton peak became more localized as the Al mole fraction increased. This was attributed to the increase in the ionization energy (ED) of the donor to which the exciton was bound. The donor-acceptor pair (DAP) band and the so-called 'yellow' emission band that are commonly

  16. 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.

  17. Growth of GaN micro/nanolaser arrays by chemical vapor deposition

    NASA Astrophysics Data System (ADS)

    Liu, Haitao; Zhang, Hanlu; Dong, Lin; Zhang, Yingjiu; Pan, Caofeng

    2016-09-01

    Optically pumped ultraviolet lasing at room temperature based on GaN microwire arrays with Fabry–Perot cavities is demonstrated. GaN microwires have been grown perpendicularly on c-GaN/sapphire substrates through simple catalyst-free chemical vapor deposition. The GaN microwires are [0001] oriented single-crystal structures with hexagonal cross sections, each with a diameter of ∼1 μm and a length of ∼15 μm. A possible growth mechanism of the vertical GaN microwire arrays is proposed. Furthermore, we report room-temperature lasing in optically pumped GaN microwire arrays based on the Fabry–Perot cavity. Photoluminescence spectra exhibit lasing typically at 372 nm with an excitation threshold of 410 kW cm‑2. The result indicates that these aligned GaN microwire arrays may offer promising prospects for ultraviolet-emitting micro/nanodevices.

  18. Enhancing the field emission properties of Se-doped GaN nanowires

    NASA Astrophysics Data System (ADS)

    Li, Enling; Wu, Guishuang; Cui, Zhen; Ma, Deming; Shi, Wei; Wang, Xiaolin

    2016-07-01

    Pure and Se-doped GaN nanowires (NWs) are synthesized on Pt-coated Si(111) substrates via chemical vapor deposition. The GaN NWs exhibit a uniform density with an average diameter of 20–120 nm. The structure of the NWs is wurtzite hexagonal, and the growth direction is along [0001]. Field emission measurements show that the Se-doped GaN NWs possess a low turn-on field (2.9 V μm‑1) compared with the pure GaN NWs (7.0 V μm‑1). In addition, density functional theory calculations indicate that the donor states near the Fermi level are mainly formed through the hybridization between Se 4p and N 2p orbitals and that the Fermi level move towards the vacuum level. Consequently, the work functions of Se-doped GaN NWs are lower than those of pure GaN NWs.

  19. Growth of GaN micro/nanolaser arrays by chemical vapor deposition.

    PubMed

    Liu, Haitao; Zhang, Hanlu; Dong, Lin; Zhang, Yingjiu; Pan, Caofeng

    2016-09-01

    Optically pumped ultraviolet lasing at room temperature based on GaN microwire arrays with Fabry-Perot cavities is demonstrated. GaN microwires have been grown perpendicularly on c-GaN/sapphire substrates through simple catalyst-free chemical vapor deposition. The GaN microwires are [0001] oriented single-crystal structures with hexagonal cross sections, each with a diameter of ∼1 μm and a length of ∼15 μm. A possible growth mechanism of the vertical GaN microwire arrays is proposed. Furthermore, we report room-temperature lasing in optically pumped GaN microwire arrays based on the Fabry-Perot cavity. Photoluminescence spectra exhibit lasing typically at 372 nm with an excitation threshold of 410 kW cm(-2). The result indicates that these aligned GaN microwire arrays may offer promising prospects for ultraviolet-emitting micro/nanodevices. PMID:27454350

  20. 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…

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

    SciTech Connect

    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.

  2. Structural and chemical analysis of pulsed laser deposited Mg xZn 1- xO hexagonal ( x = 0.15, 0.28) and cubic ( x = 0.85) thin films

    NASA Astrophysics Data System (ADS)

    Hullavarad, S. S.; Hullavarad, N. V.; Pugel, D. E.; Dhar, S.; Venkatesan, T.; Vispute, R. D.

    2008-02-01

    Hexagonal and cubic Mg xZn 1- xO thin films corresponding to optical band gaps of 3.52 eV, 4 eV and 6.42 eV for x = 0.15, 0.28 and 0.85 compositions were grown by pulsed laser deposition technique. The crystalline quality of the films was investigated by X-ray diffraction-rocking curve measurements and indicated a high degree of crystallinity with narrow FWHM's of 0.21°-0.59°. Rutherford back scattering-channeling spectroscopy provides channeling yields of 7-14% indicating the good crystalline quality of the thin films. X-Ray photoelectron spectroscopy measurements clearly indicated different level of oxidation states of Mg and Zn.

  3. 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%.

  4. 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.

  5. 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.

  6. 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.

  7. Planar millimeter wave band-stop filters based on the excitation of confined magnetostatic waves in barium hexagonal ferrite thin film strips

    NASA Astrophysics Data System (ADS)

    Lu, Lei; Song, Young-Yeal; Bevivino, Joshua; Wu, Mingzhong

    2011-05-01

    A planar millimeter wave band-stop filter based on confined magnetostatic wave (MSW) excitations in an M-type barium hexagonal ferrite (BaM) film strip was demonstrated. The device consists of a BaM film strip on the top of a coplanar waveguide with the strip length along the signal line. For zero magnetic fields, the device shows a band-stop filtering response at 53 GHz. This response originates from the excitation of confined MSW modes across the BaM strip width. The filter operation frequency is tunable with low fields. This tuning relies on the change in the MSW dispersion with field.

  8. 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.

  9. 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.

  10. Influences of growth parameters on the film formation of hexagonal boron nitride thin films grown on sapphire substrates by low-pressure chemical vapor deposition

    NASA Astrophysics Data System (ADS)

    Umehara, Naoki; Masuda, Atsushi; Shimizu, Takaki; Kuwahara, Iori; Kouno, Tetsuya; Kominami, Hiroko; Hara, Kazuhiko

    2016-05-01

    Hexagonal boron nitride (h-BN) films were grown on c-plane sapphire substrates by low-pressure chemical vapor deposition with BCl3 and NH3 as the boron and nitrogen sources, respectively, and the influences of growth parameters on the film quality were investigated for samples with a thickness of about 1 µm. The dependence of X-ray diffraction on the growth temperature (T g) indicated that the crystalline quality is most improved in the sample grown at 1200 °C, in which the epitaxial relationship of {100}h-BN ∥ {110}sapphire and {001}h-BN ∥ {001}sapphire was confirmed. This condition enhanced lateral growth, resulting in the formation of grains with flat top surfaces. The T g dependence was discussed in relation to the amorphous AlN formed on the substrate surface and the reaction between BCl3 and NH3 in the vapor phase. The correlation between the structural and luminescent properties, which was found from the T g dependence of CL, was also discussed.

  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. 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.

  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. 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.

  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. Growth of high quality GaN layer on carbon nanotube-graphene network structure as intermediate layer

    NASA Astrophysics Data System (ADS)

    Seo, Taeo Hoon; Park, Ah Hyun; Park, Sungchan; Kim, Myung Jong; Suh, Eun-Kyung

    2015-03-01

    In general, high-quality GaN layers are synthesized on low-temperature (LT) GaN buffer layer on a single crystal sapphire substrate. However, large differences in fundamental properties such as lattice constants and thermal expansion coefficients between GaN layer and sapphire substrate generate high density of threading dislocation (TD) that leads to deterioration of optical and structural properties. Graphene has been attracting much attention due to its excellent physical properties However, direct epitaxial growth of GaN film onto graphene layer on substrates is not easily accessible due to the lack of chemical reactivity on graphene which consisted of C-C bond of sp2 hexagonally arranged carbon atoms with no dangling bonds. In this work, an intermediate layer for the GaN growth on sapphire substrate was constructed by inserting carbon nanotubes and graphene hybrid structure (CGH) Optical and structural properties of GaN layer grown on CGH were compared with those of GaN layer directly grown on sapphire CNTs act as nucleation sites and play a crucial role in the growth of single crystal high-quality GaN on graphene layer. Also, graphene film acts as a mask for epitaxial lateral overgrowth of GaN layer, which can effectively reduce TD density. A grant from the Korea Institute of Science and Technology (KIST) institutional program.

  17. Doping of Si into GaN nanowires and optical properties of resulting composites.

    PubMed

    Xu, Congkang; Chung, Sangyong; Kim, Misuk; Kim, Dong Eon; Chon, Bonghwan; Hong, Sangsu; Joo, Taiha

    2005-04-01

    Doping of Si into GaN nanowires has been successfully attained via thermal evaporation in the presence of a suitable gas atmosphere. Analysis indicated that the Si-doped GaN nanowire is a single crystal with a hexagonal wurtzite structure, containing 2.2 atom % of Si. The broadening and the shift of Raman peak to lower frequency are observed, which may be attributed to surface disorder and various strengths of the stress. The band-gap emission (358 nm) of Si-doped GaN nanowires relative to that (370 nm) of GaN nanowires has an apparent blue shift (approximately 12 nm), which can be ascribed to doping impurity Si. PMID:16004115

  18. Why Hexagonal Basalt Columns?

    PubMed

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

    2015-10-01

    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. PMID:26550724

  19. Synthesis of GaN nanocrystallites by pulsed laser ablation in pure nitrogen background gases

    NASA Astrophysics Data System (ADS)

    Yoshida, Takehito; Kakumoto, Soichiro; Sugimura, Akira; Umezu, Ikurou

    2011-09-01

    GaN is a promising material not only for electronic devices but also for photocatalysts. Synthesis of GaN nanocrystal is a key issue to improve performance for these applications. In the present study, GaN nanocrystallites have been synthesized by pulsed laser ablation (PLA), where safe and inactive pure N2 gases were used as reactive background gases. The third harmonics beam of a Q-switched Nd:YAG laser (355 nm, 10 mJ/pulse, 4 J/(cm2 pulse)) was used to ablate a sintered high purity GaN target. The deposition substrates were not heated. It was clarified that the formed GaN nanoparticles contained a hexagonal system with the wurtzite structure. The diameter of the nanocrystallites was about 10 nm, and showed only little dependence on the background gas pressure, while the porosity of the assembly of nanocrystallites and content of GaN nanocrystallites in the assembly increased with background gas pressure. Highly porous nanometer-sized GaN film obtained at higher gas pressure is considered to be candidate structures for the photocatalysts.

  20. Dissociation of Al2O3(0001) substrates and the roles of silicon and oxygen in n-type GaN thin solid films grown by gas-source molecular beam epitaxy

    NASA Astrophysics Data System (ADS)

    Van Nostrand, J. E.; Solomon, J.; Saxler, A.; Xie, Q.-H.; Reynolds, D. C.; Look, D. C.

    2000-06-01

    Unintentionally doped and silicon doped GaN films prepared by molecular beam epitaxy using ammonia are investigated. Hall, secondary ion mass spectroscopy (SIMS), photoluminescence, and x-ray data are utilized for analysis of sources of autodoping of GaN epitaxial films in an effort to identify whether the n-type background electron concentration is of impurity origin or native defect origin. We identify and quantify an anomalous relationship between the Si doping concentration and free carrier concentration and mobility using temperature dependent Hall measurements on a series of 2.0-μm-thick GaN(0001) films grown on sapphire with various Si doping concentrations. SIMS is used to identify oxygen as the origin of the excess free carriers in lightly doped and undoped GaN films. Further, the source of the oxygen is positively identified to be dissociation of the sapphire substrate at the nitride-sapphire interface. Dissociation of SiC at the nitride-carbide interface is also observed. Finally, SIMS is again utilized to show how Si doping can be utilized to suppress the diffusion of the oxygen into the GaN layer from the sapphire substrate. The mechanism of suppression is believed to be formation of a Si-O bond and a greatly reduced diffusion coefficient of the subsequent Si-O complex in GaN.

  1. 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.

  2. Wurtzite-type faceted single-crystalline GaN nanotubes

    NASA Astrophysics Data System (ADS)

    Liu, Baodan; Bando, Yoshio; Tang, Chengchun; Shen, Guozhen; Golberg, Dmitri; Xu, Fangfang

    2006-02-01

    We report on the direct fabrication of single-crystalline wurtzite-type hexagonal GaN nanotubes via a newly designed, controllable, and reproducible chemical thermal-evaporation process. The nanotubes are single crystalline, have one end closed, an average outer diameter of ˜300nm, an inner diameter of ˜100nm, and a wall thickness of ˜100nm. The structure and morphology of the tubes are characterized using a scanning electron microscope and a transmission electron microscope. The cathodoluminescence of individual nanotubes is also investigated. The growth mechanism, formation kinetics, and crystallography of GaN nanotubes are finally discussed.

  3. Strong atomic ordering in Gd-doped GaN

    SciTech Connect

    Ishimaru, Manabu; Higashi, Kotaro; Hasegawa, Shigehiko; Asahi, Hajime; Sato, Kazuhisa; Konno, Toyohiko J.

    2012-09-03

    Gd-doped GaN (Ga{sub 1-x}Gd{sub x}N) thin films were grown on a GaN(001) template by radio frequency plasma-assisted molecular beam epitaxy and characterized by means of x-ray diffraction (XRD) and transmission electron microscopy (TEM). Three samples with a different Gd composition were prepared in this study: x = 0.02, 0.05, and 0.08. XRD and TEM results revealed that the low Gd concentration GaN possesses the wurtzite structure. On the other hand, it was found that an ordered phase with a quadruple-periodicity along the [001] direction in the wurtzite structure is formed throughout the film with x = 0.08. We proposed the atomistic model for the superlattice structure observed here.

  4. 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.

  5. Enhancing the field emission properties of Se-doped GaN nanowires.

    PubMed

    Li, Enling; Wu, Guishuang; Cui, Zhen; Ma, Deming; Shi, Wei; Wang, Xiaolin

    2016-07-01

    Pure and Se-doped GaN nanowires (NWs) are synthesized on Pt-coated Si(111) substrates via chemical vapor deposition. The GaN NWs exhibit a uniform density with an average diameter of 20-120 nm. The structure of the NWs is wurtzite hexagonal, and the growth direction is along [0001]. Field emission measurements show that the Se-doped GaN NWs possess a low turn-on field (2.9 V μm(-1)) compared with the pure GaN NWs (7.0 V μm(-1)). In addition, density functional theory calculations indicate that the donor states near the Fermi level are mainly formed through the hybridization between Se 4p and N 2p orbitals and that the Fermi level move towards the vacuum level. Consequently, the work functions of Se-doped GaN NWs are lower than those of pure GaN NWs. PMID:27197556

  6. Behavior of aluminum adsorption and incorporation at GaN(0001) surface: First-principles study

    SciTech Connect

    Qin, Zhenzhen; Xiong, Zhihua Wan, Qixin; Qin, Guangzhao

    2013-11-21

    First-principles calculations are performed to study the energetics and atomic structures of aluminum adsorption and incorporation at clean and Ga-bilayer GaN(0001) surfaces. We find the favorable adsorption site changes from T4 to T1 as Al coverage increased to 1 monolayer on the clean GaN(0001) surface, and a two-dimensional hexagonal structure of Al overlayer appears. It is interesting the Al atoms both prefer to concentrate in one deeper Ga layer of clean and Ga-bilayer GaN(0001) surface, respectively, while different structures could be achieved in above surfaces. For the case of clean GaN(0001) surface, corresponding to N-rich and moderately Ga-rich conditions, a highly regular superlattice structure composed of wurtzite GaN and AlN becomes favorable. For the case of Ga-bilayer GaN(0001) surface, corresponding to extremely Ga-rich conditions, the Ga bilayer is found to be sustained stable in Al incorporating process, leading to an incommensurate structure directly. Furthermore, our calculations provide an explanation for the spontaneous formation of ordered structure and incommensurate structure observed in growing AlGaN films. The calculated results are attractive for further development of growth techniques and excellent AlGaN/GaN heterostructure electronic devices.

  7. Leakage mechanism in GaN and AlGaN Schottky interfaces

    NASA Astrophysics Data System (ADS)

    Hashizume, Tamotsu; Kotani, Junji; Hasegawa, Hideki

    2004-06-01

    Based on detailed temperature-dependent current-voltage (I-V-T) measurements the mechanism of leakage currents through GaN and AlGaN Schottky interfaces is discussed. The experiments were compared to calculations based on thin surface barrier model in which the effects of surface defects were taken into account. Our simulation method reproduced the experimental I-V-T characteristics of the GaN and AlGaN Schottky diodes, and gave excellent fitting results to the reported Schottky I-V curves in GaN for both forward and reverse biases at different temperatures. The present results indicate that the barrier thinning caused by unintentional surface-defect donors enhances the tunneling transport processes, leading to large leakage currents through GaN and AlGaN Schottky interfaces.

  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. PMID:24892383

  9. 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.

  10. Cross-Disciplinary Physics and Related Areas of Science and Technology: Fabrication of Mn-Doped GaN Nanobars

    NASA Astrophysics Data System (ADS)

    Xue, Cheng-Shan; Liu, Wen-Jun; Shi, Feng; Zhuang, Hui-Zhao; Guo, Yong-Fu; Cao, Yu-Ping; Sun, Hai-Bo

    2010-03-01

    We report a new method for large-scale production of GaMnN nanobars, by ammoniating Ga2O3 films doped with Mn under flowing ammonia atmosphere at 1000° C. The Mn-doped GaN sword-like nanobars are a single-crystal hexagonal structure, containing Mn up to 5.43 atom%. Thickness is about 100 nm and with a width of 200-400 nm. The nanobars are characterized by x-ray diffraction, scanning electron microscopy, x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and photoluminescence. The GaN nanobars show two emission bands with a well-defined PL peak at 388 nm and 409 nm respectively. The large distinct redshift (409 nm) are comparable to pure GaN(370 nm) at room temperature. The red-shift photoluminescence is due to Mn doping. The growth mechanism of crystalline GaN nanobars is discussed briefly.

  11. Structural and antireflective characteristics of catalyst-free GaN nanostructures on GaN/sapphire template for solar cell applications

    NASA Astrophysics Data System (ADS)

    Park, C. Y.; Lim, J. M.; Yu, J. S.; Lee, Y. T.

    2010-04-01

    We report the structural and antireflective characteristics of catalyst-free GaN nanostructures on GaN/sapphire template for solar cell applications. The formation of GaN nanostructure depends on the growth temperature. At 530 °C, the nucleation of the initial seed due to the enhanced strain leads to the hexagonal closely packed nanorods. As the growth temperature decreases, the depression of atomic Ga transport changes the films into nanocolumns, and then the nanorods. The catalyst-free GaN nanorods have wurtzite structure and good single crystalline quality. The GaN nanorods exhibit a remarkably low reflectance over a wide-angle broadband spectrum, enhancing the antireflective property of GaN surface.

  12. Femtosecond dynamics of exciton bleaching in bulk GaN at room temperature

    NASA Astrophysics Data System (ADS)

    Huang, Yin-Chieh; Chern, Gia-Wei; Lin, Kung-Hsuan; Liang, Jian-Chin; Sun, Chi-Kuang; Hsu, Chia-Chen; Keller, Stacia; DenBaars, Steven P.

    2002-07-01

    Femtosecond transient transmission pump-probe technique was used to investigate exciton dynamics in a nominally undoped GaN thin film at room temperature. An exciton ionization time of 100-250 femtoseconds was observed by the time-resolved pump-probe measurement. A comparison experiment with pre-excited free carriers also confirmed the observation of the exciton ionization process in bulk GaN.

  13. Vertical architecture for enhancement mode power transistors based on GaN nanowires

    NASA Astrophysics Data System (ADS)

    Yu, F.; Rümmler, D.; Hartmann, J.; Caccamo, L.; Schimpke, T.; Strassburg, M.; Gad, A. E.; Bakin, A.; Wehmann, H.-H.; Witzigmann, B.; Wasisto, H. S.; Waag, A.

    2016-05-01

    The demonstration of vertical GaN wrap-around gated field-effect transistors using GaN nanowires is reported. The nanowires with smooth a-plane sidewalls have hexagonal geometry made by top-down etching. A 7-nanowire transistor exhibits enhancement mode operation with threshold voltage of 1.2 V, on/off current ratio as high as 108, and subthreshold slope as small as 68 mV/dec. Although there is space charge limited current behavior at small source-drain voltages (Vds), the drain current (Id) and transconductance (gm) reach up to 314 mA/mm and 125 mS/mm, respectively, when normalized with hexagonal nanowire circumference. The measured breakdown voltage is around 140 V. This vertical approach provides a way to next-generation GaN-based power devices.

  14. 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. PMID:25991084

  15. 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.

  16. 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.

  17. Synthesis of p-type GaN nanowires

    NASA Astrophysics Data System (ADS)

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

    2013-08-01

    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.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

  18. Nanoscale lateral epitaxial overgrowth of GaN on Si (111)

    SciTech Connect

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

    2005-11-07

    We demonstrate that GaN can selectively grow by metalorganic chemical vapor deposition into the pores and laterally over the nanoscale patterned SiO{sub 2} mask on a template of GaN/AlN/Si. The nanoporous SiO{sub 2} on GaN surface with pore diameter of approximately 65 nm and pore spacing of 110 nm was created by inductively coupled plasma etching using anodic aluminum oxide template as a mask. Cross-section transmission electron microscopy shows that the threading-dislocation density was largely reduced in this nanoepitaxial lateral overgrowth region. Dislocations parallel to the interface are the dominant type of dislocations in the overgrown layer of GaN. A large number of the threading dislocations were filtered by the nanoscale mask, which leads to the dramatic reduction of the threading dislocations during the growth within the nano-openings. More importantly, due to the nanoscale size of the mask area, the very fast coalescence and subsequent lateral overgrowth of GaN force the threading dislocations to bend to the basal plane within the first 50 nm of the film thickness. The structure of overgrown GaN is a truncated hexagonal pyramid which is covered with six {l_brace}1101{r_brace} side facets and (0001) top surface depending on the growth conditions.

  19. 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.

  20. Synthesis, microstructure, and cathodoluminescence of [0001]-oriented GaN nanorods grown on conductive graphite substrate.

    PubMed

    Yuan, Fang; Liu, Baodan; Wang, Zaien; Yang, Bing; Yin, Yao; Dierre, Benjamin; Sekiguchi, Takashi; Zhang, Guifeng; Jiang, Xin

    2013-11-27

    One-dimensional GaN nanorods with corrugated morphology have been synthesized on graphite substrate without the assistance of any metal catalyst through a feasible thermal evaporation process. The morphologies and microstructures of GaN nanorods were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The results from HRTEM analysis indicate that the GaN nanorods are well-crystallized and exhibit a preferential orientation along the [0001] direction with Ga(3+)-terminated (101̅1) and N(3-)-terminated (101̅1̅) as side facets, finally leading to the corrugated morphology surface. The stabilization of the electrostatic surface energy of {101̅1} polar surface in a wurtzite-type hexagonal structure plays a key role in the formation of GaN nanorods with corrugated morphology. Room-temperature cathodoluminescence (CL) measurements show a near-band-edge emission (NBE) in the ultraviolet range and a broad deep level emission (DLE) in the visible range. The crystallography and the optical emissions of GaN nanorods are discussed. PMID:24164686

  1. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2005-12-01

    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 0.29to30μm. High level of residual elastic strain was found in thin (0.29to0.73μm thick) GaN layers. This correlates with low density of threading screw dislocations of 1-2×107cm-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.

  3. Rare earth 4f hybridization with the GaN valence band

    NASA Astrophysics Data System (ADS)

    Wang, Lu; Mei, Wai-Ning; McHale, S. R.; McClory, J. W.; Petrosky, J. C.; Wu, J.; Palai, R.; Losovyj, Y. B.; Dowben, P. A.

    2012-11-01

    The placement of the Gd, Er and Yb 4f states within the GaN valence band has been explored by both experiment and theory. The 4d-4f photoemission resonances for various rare-earth(RE)-doped GaN thin films (RE = Gd, Er, Yb) provide an accurate depiction of the occupied 4f state placement within the GaN. The resonant photoemission show that the major Er and Gd RE 4f weight is at about 5-6 eV below the valence band maximum, similar to the 4f weights in the valence band of many other RE-doped semiconductors. For Yb, there is a very little resonant enhancement of the valence band of Yb-doped GaN, consistent with a large 4f14-δ occupancy. The placement of the RE 4f levels is in qualitative agreement with theoretical expectations.

  4. The rare earth 4 f hybridization with the GaN valence band

    NASA Astrophysics Data System (ADS)

    Wang, Lu; Mei, Wai-Ning; McHale, Steve; McClory, John; Petrosky, James; Wu, J.; Palai, Ratnakar; Losovyj, Yaroslav; Dowben, Peter

    2013-03-01

    The placement of the Gd, Er, and Yb 4f states within the GaN valence band has been explored by both experiment and theory. The 4 d - 4 f photoemission resonances for various rare earth doped GaN thin films (RE = Gd, Er, Yb) provide an accurate depiction of the occupied 4 f state placement within the GaN. The resonant photoemission show that the major Er and Gd rare earth 4 f weight is at about 5-6 eV below the valence band maximum, similar to the 4 f weights in the valence band of many other rare earth doped semiconductors. For Yb, there is very little resonant enhancement of the valence band of Yb doped GaN, consistent with a largely 4f 14 - δ occupancy. The placement of the rare earth 4 f levels is in qualitative agreement with theoretical expectations.

  5. 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.

  6. Screw dislocations in GaN

    SciTech Connect

    Liliental-Weber, Zuzanna; Jasinski, Jacek B.; Washburn, Jack; O'Keefe, Michael A.

    2002-02-15

    GaN has received much attention over the past few years because of several new applications, including light emitting diodes, blue laser diodes and high-power microwave transistors. One of the biggest problems is a high density of structural defects, mostly dislocations, due to a lack of a suitable lattice-matched substrate since bulk GaN is difficult to grow in large sizes. Transmission Electron Microscopy (TEM) has been applied to study defects in plan-view and cross-sections on samples prepared by conventional techniques such as mechanical thinning and precision ion milling. The density of dislocations close to the sample surface of a 1 mm-thick HVPE sample was in the range of 3x109 cm-2. All three types of dislocations were present in these samples, and almost 50 percent were screw dislocations. Our studies suggest that the core structure of screw dislocations in the same material might differ when the material is grown by different methods.

  7. Room-Temperature Multiferroic Hexagonal LuFeO3

    SciTech Connect

    Cheng, Xuemei; Balke, Nina; Chi, Miaofang; Gai, Zheng; Keavney, David; Lee, Ho Nyung; Shen, Jian; Snijders, Paul C; Wang, Wenbin; Ward, Thomas Z; Xu, Xiaoshan; Yi, Jieyu; Zhu, Leyi; Christen, Hans M; Zhao, Jun

    2013-01-01

    We observed the coexistence of ferroelectricity and weak ferromagnetism at room temperature in the hexagonal phase of LuFeO3 stabilized by epitaxial thin film growth. While the ferroelectricity in hexagonal LuFeO3 can be understood as arising from its polar structure, the observation of weak ferromagnetism at room temperature is remarkable considering the frustrated triangular spin structure. An explanation of the room temperature weak ferromagnetism is proposed in terms of a subtle lattice distortion revealed by the structural characterization. The combination of ferroelectricity and weak ferromagnetism in epitaxial films at room temperature offers great potential for the application of this novel multiferroic material in next generation devices.

  8. Metalorganic chemical vapor deposition growth of high-mobility AlGaN/AlN/GaN heterostructures on GaN templates and native GaN substrates

    SciTech Connect

    Chen, Jr-Tai Hsu, Chih-Wei; Forsberg, Urban; Janzén, Erik

    2015-02-28

    Severe surface decomposition of semi-insulating (SI) GaN templates occurred in high-temperature H{sub 2} atmosphere prior to epitaxial growth in a metalorganic chemical vapor deposition system. A two-step heating process with a surface stabilization technique was developed to preserve the GaN template surface. Utilizing the optimized heating process, a high two-dimensional electron gas mobility ∼2000 cm{sup 2}/V·s was obtained in a thin AlGaN/AlN/GaN heterostructure with an only 100-nm-thick GaN spacer layer homoepitaxially grown on the GaN template. This technique was also demonstrated viable for native GaN substrates to stabilize the surface facilitating two-dimensional growth of GaN layers. Very high residual silicon and oxygen concentrations were found up to ∼1 × 10{sup 20 }cm{sup −3} at the interface between the GaN epilayer and the native GaN substrate. Capacitance-voltage measurements confirmed that the residual carbon doping controlled by growth conditions of the GaN epilayer can be used to successfully compensate the donor-like impurities. State-of-the-art structural properties of a high-mobility AlGaN/AlN/GaN heterostructure was then realized on a 1 × 1 cm{sup 2} SI native GaN substrate; the full width at half maximum of the X-ray rocking curves of the GaN (002) and (102) peaks are only 21 and 14 arc sec, respectively. The surface morphology of the heterostructure shows uniform parallel bilayer steps, and no morphological defects were noticeable over the entire epi-wafer.

  9. Metalorganic chemical vapor deposition growth of high-mobility AlGaN/AlN/GaN heterostructures on GaN templates and native GaN substrates

    NASA Astrophysics Data System (ADS)

    Chen-Tai, Jr.; Hsu, Chih-Wei; Forsberg, Urban; Janzén, Erik

    2015-02-01

    Severe surface decomposition of semi-insulating (SI) GaN templates occurred in high-temperature H2 atmosphere prior to epitaxial growth in a metalorganic chemical vapor deposition system. A two-step heating process with a surface stabilization technique was developed to preserve the GaN template surface. Utilizing the optimized heating process, a high two-dimensional electron gas mobility ˜2000 cm2/V.s was obtained in a thin AlGaN/AlN/GaN heterostructure with an only 100-nm-thick GaN spacer layer homoepitaxially grown on the GaN template. This technique was also demonstrated viable for native GaN substrates to stabilize the surface facilitating two-dimensional growth of GaN layers. Very high residual silicon and oxygen concentrations were found up to ˜1 × 1020 cm-3 at the interface between the GaN epilayer and the native GaN substrate. Capacitance-voltage measurements confirmed that the residual carbon doping controlled by growth conditions of the GaN epilayer can be used to successfully compensate the donor-like impurities. State-of-the-art structural properties of a high-mobility AlGaN/AlN/GaN heterostructure was then realized on a 1 × 1 cm2 SI native GaN substrate; the full width at half maximum of the X-ray rocking curves of the GaN (002) and (102) peaks are only 21 and 14 arc sec, respectively. The surface morphology of the heterostructure shows uniform parallel bilayer steps, and no morphological defects were noticeable over the entire epi-wafer.

  10. Strain-free GaN thick films grown on single crystalline ZnO buffer layer with in situ lift-off technique

    SciTech Connect

    Lee, S. W.; Minegishi, T.; Lee, W. H.; Goto, H.; Lee, H. J.; Lee, S. H.; Lee, Hyo-Jong; Ha, J. S.; Goto, T.; Hanada, T.; Cho, M. W.; Yao, T.

    2007-02-05

    Strain-free freestanding GaN layers were prepared by in situ lift-off process using a ZnO buffer as a sacrificing layer. Thin Zn-polar ZnO layers were deposited on c-plane sapphire substrates, which was followed by the growth of Ga-polar GaN layers both by molecular beam epitaxy (MBE). The MBE-grown GaN layer acted as a protecting layer against decomposition of the ZnO layer and as a seeding layer for GaN growth. The ZnO layer was completely in situ etched off during growth of thick GaN layers at low temperature by hydride vapor phase epitaxy. Hence freestanding GaN layers were obtained for the consecutive growth of high-temperature GaN thick layers. The lattice constants of freestanding GaN agree with those of strain-free GaN bulk. Extensive microphotoluminescence study indicates that strain-free states extend throughout the high-temperature grown GaN layers.

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

    SciTech Connect

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

    2008-07-21

    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 1-2 nm bubbles resulting from vacancy clustering during implantation. These nanobubbles persist up to 450 deg. 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 deg. C. Extended internal surfaces develop around 550 deg. C leading to the exfoliation of GaN thin layer.

  12. 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

  13. Stress reduction in epitaxial GaN films on Si using cubic SiC as intermediate layers

    NASA Astrophysics Data System (ADS)

    Komiyama, Jun; Abe, Yoshihisa; Suzuki, Shunichi; Nakanishi, Hideo

    2006-08-01

    Stress in the epitaxial films of GaN on Si is reduced by using SiC as intermediate layers. The crystalline films of cubic SiC (0-1μm), thin AlN (50nm), and GaN (1-3μm) were prepared on 3in. (1 1 1) Si substrates—stacked in the order of GaN /AlN/SiC/Si—by metalorganic vapor-phase epitaxy. It is revealed by Raman spectroscopy that the tensile stress in GaN is reduced to half (reduction of about 300MPa) for GaN on Si with SiC intermediate layers compared with GaN on Si without SiC intermediate layers. Because of stress reduction, crack-free GaN on Si with a thickness of 2μm was obtained by using SiC intermediate layers. Cracking was minimized even on thicker GaN on Si (3μm thick) with SiC intermediate layers. The SiC intermediate layers are promising for the realization of nitride based electronic devices on Si.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    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 (Tg) and Tg ramping method was investigated using deep level optical spectroscopy. Understanding the influence of Tg 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 Tg (800 °C) GaN films grown under QB conditions were compared to deep level spectra of high Tg (1150 °C) GaN. Reducing Tg, 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 Tg 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 Tg GaN growth to active layer growth can mitigate such non-radiative channels.

  15. Effect of GaAs substrate orientation on the growth kinetic of GaN layer grown by MOVPE

    NASA Astrophysics Data System (ADS)

    Laifi, J.; Chaaben, N.; Bouazizi, H.; Fourati, N.; Zerrouki, C.; El Gmili, Y.; Bchetnia, A.; Salvestrini, J. P.; El Jani, B.

    2016-06-01

    We have investigated the kinetic growth of low temperature GaN nucleation layers (LT-GaN) grown on GaAs substrates with different crystalline orientations. GaN nucleation layers were grown by metal organic vapor phase epitaxy (MOVPE) in a temperature range of 500-600 °C on oriented (001), (113), (112) and (111) GaAs substrates. The growth was in-situ monitored by laser reflectometry (LR). Using an optical model, including time-dependent surface roughness and growth rate profiles, simulations were performed to best approach the experimental reflectivity curves. Results are discussed and correlated with ex-situ analyses, such as atomic force microscopy (AFM) and UV-visible reflectance (SR). We show that the GaN nucleation layers growth results the formation of GaN islands whose density and size vary greatly with both growth temperature and substrate orientation. Arrhenius plots of the growth rate for each substrate give values of activation energy varying from 0.20 eV for the (001) orientation to 0.35 eV for the (113) orientation. Using cathodoluminescence (CL), we also show that high temperature (800-900 °C) GaN layers grown on top of the low temperature (550 °C) GaN nucleation layers, grown themselves on the GaAs substrates with different orientations, exhibit cubic or hexagonal phase depending on both growth temperature and substrate orientation.

  16. Effect of growth temperature on defects in epitaxial GaN film grown by plasma assisted molecular beam epitaxy

    SciTech Connect

    Kushvaha, S. S. Pal, P.; Shukla, A. K.; Joshi, Amish G.; Gupta, Govind; Kumar, M.; Singh, S.; Gupta, Bipin K.; Haranath, D.

    2014-02-15

    We report the effect of growth temperature on defect states of GaN epitaxial layers grown on 3.5 μm thick GaN epi-layer on sapphire (0001) substrates using plasma assisted molecular beam epitaxy. The GaN samples grown at three different substrate temperatures at 730, 740 and 750 °C were characterized using atomic force microscopy and photoluminescence spectroscopy. The atomic force microscopy images of these samples show the presence of small surface and large hexagonal pits on the GaN film surfaces. The surface defect density of high temperature grown sample is smaller (4.0 × 10{sup 8} cm{sup −2} at 750 °C) than that of the low temperature grown sample (1.1 × 10{sup 9} cm{sup −2} at 730 °C). A correlation between growth temperature and concentration of deep centre defect states from photoluminescence spectra is also presented. The GaN film grown at 750 °C exhibits the lowest defect concentration which confirms that the growth temperature strongly influences the surface morphology and affects the optical properties of the GaN epitaxial films.

  17. On the perfect hexagonal packing of rods

    NASA Astrophysics Data System (ADS)

    Starostin, E. L.

    2006-04-01

    In most cases the hexagonal packing of fibrous structures or rods extremizes the energy of interaction between strands. If the strands are not straight, then it is still possible to form a perfect hexatic bundle. Conditions under which the perfect hexagonal packing of curved tubular structures may exist are formulated. Particular attention is given to closed or cycled arrangements of the rods like in the DNA toroids and spools. The closure or return constraints of the bundle result in an allowable group of automorphisms of the cross-sectional hexagonal lattice. The structure of this group is explored. Examples of open helical-like and closed toroidal-like bundles are presented. An expression for the elastic energy of a perfectly packed bundle of thin elastic rods is derived. The energy accounts for both the bending and torsional stiffnesses of the rods. It is shown that equilibria of the bundle correspond to solutions of a variational problem formulated for the curve representing the axis of the bundle. The functional involves a function of the squared curvature under the constraints on the total torsion and the length. The Euler-Lagrange equations are obtained in terms of curvature and torsion and due to the existence of the first integrals the problem is reduced to the quadrature. The three-dimensional shape of the bundle may be readily reconstructed by integration of the Ilyukhin-type equations in special cylindrical coordinates. The results are of universal nature and are applicable to various fibrous structures, in particular, to intramolecular liquid crystals formed by DNA condensed in toroids or packed inside the viral capsids. International Workshop on Biopolymers: Thermodynamics, Kinetics and Mechanics of DNA, RNA and Proteins, 30.05.2005-3.06.2005, The Abdus Salam International Centre for Theoretical Physics (ICTP), Trieste, Italy.

  18. Ba4GaN3O

    PubMed Central

    Hashimoto, Takayuki; Yamane, Hisanori

    2014-01-01

    Red transparant platelet-shaped single crystals of tetra­barium gallium trinitride oxide, Ba4GaN3O, were synthesized by the Na flux method. The crystal structure is isotypic with Sr4GaN3O, containing isolated triangular [GaN3]6− anionic groups. O2− atoms are inserted between the slabs of [Ba4GaN3]2+, in which the [GaN3]6− groups are surrounded by Ba2+ atoms. PMID:24940188

  19. 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.

  20. The hydride vapor phase epitaxy of GaN on silicon covered by nanostructures

    NASA Astrophysics Data System (ADS)

    Jahn, U.; Musolino, M.; Lähnemann, J.; Dogan, P.; Fernández Garrido, S.; Wang, J. F.; Xu, K.; Cai, D.; Bian, L. F.; Gong, X. J.; Yang, H.

    2016-06-01

    GaN several tens of μm thick has been deposited on a silicon substrate using a two-step hydride vapor phase epitaxy (HVPE) process. The substrates were covered by AlN layers and GaN nanostructures grown by plasma-assisted molecular-beam epitaxy. During the first low-temperature (low-T) HVPE step, stacking faults (SF) form, which show distinct luminescence lines and stripe-like features in the cathodoluminescence images of the cross-section of the layers. These cathodoluminescence features provide an insight into the growth process. During a second high-temperature (high-T) step, the SFs disappear, and the luminescence of this part of the GaN layer is dominated by the donor-bound exciton. For templates consisting of both a thin AlN buffer and GaN nanostructures, the incorporation of silicon into the GaN grown by HVPE is not observed. Moreover, the growth mode of the (high-T) HVPE step depends on the specific structure of the AlN/GaN template, where in the first case, epitaxy is dominated by the formation of slowly growing facets, while in the second case, epitaxy proceeds directly along the c-axis. For templates without GaN nanostructures, cathodoluminescence spectra excited close to the Si/GaN interface show a broadening toward higher energies, indicating the incorporation of silicon at a high dopant level.

  1. Vapor-liquid-solid growth of GaN nanowires by reactive sputtering of GaAs

    NASA Astrophysics Data System (ADS)

    Mohanta, P.; Chaturvedi, P.; Major, S. S.; Srinivasa, R. S.

    2013-02-01

    Uniformly distributed nanosized Au-Ga alloy particles were formed on ultrathin Au coated quartz substrate by sputtering of GaAs with argon at 700 °C. Subsequent deposition of GaN by reactive sputtering of GaAs in 100 % nitrogen results in the growth of GaN nanowires. X-ray diffraction analysis confirmed the formation of hexagonal GaN. Field emission gun scanning electron microscopy studies show that the nanowires are of average length 400±50 nm and average diameter 40±5 nm. The presence of spherical Au-Ga nanoparticles of diameter ˜ 50 nm at the top of the nanowires suggests that the growth takes place by vapor-liquid-solid mechanism.

  2. Effect of double superlattice interlayers on growth of thick GaN epilayers on Si(110) substrates by metalorganic chemical vapor deposition

    NASA Astrophysics Data System (ADS)

    Shen, Xu-Qiang; Takahashi, Tokio; Ide, Toshihide; Shimizu, Mitsuaki

    2016-05-01

    The effect of double thin AlN/GaN superlattice interlayers (SL ILs) on the growth of thick GaN epilayers by metalorganic chemical vapor deposition (MOCVD) on Si(110) substrates is investigated. It is found that the GaN middle layer (GaN layer between the two SL ILs) can affect the strain state of the GaN epilayer. By comparison with the case of a single SL IL, it is shown that the double SL ILs can have a stronger compressive effect on the GaN epilayer grown on it, which results in lower residual tensile strain in the GaN film after the growth. By optimizing the GaN middle layer thickness, a 4-µm-thick crack-free GaN epilayer is successfully achieved. By this simple technique, it is expected that high-quality crack-free thick GaN can be grown on Si substrates for optical and electronic device applications.

  3. 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. PMID:27387659

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

    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.

  5. 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

  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

    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.

  7. 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.

  8. 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.

  9. Germanium-catalyzed growth of single-crystal GaN nanowires

    NASA Astrophysics Data System (ADS)

    Saleem, Umar; Wang, Hong; Peyrot, David; Olivier, Aurélien; Zhang, Jun; Coquet, Philippe; Ng, Serene Lay Geok

    2016-04-01

    We report the use of Germanium (Ge) as catalyst for Gallium Nitride (GaN) nanowires growth. High-yield growth has been achieved with Ge nanoparticles obtained by dewetting a thin layer of Ge on a Si (100) substrate. The nanowires are long and grow straight with very little curvature. The GaN nanowires are single-crystalline and show a Wurtzite structure growing along the [0001] axis. The growth follows a metal-free Vapor-Liquid-Solid (VLS) mechanism, further allowing a CMOS technology compatibility. The synthesis of nanowires has been done using an industrial Low Pressure Chemical Vapor Deposition (LPCVD) system.

  10. Growth and process modeling studies of nickel-catalyzed metalorganic chemical vapor deposition of GaN nanowires

    NASA Astrophysics Data System (ADS)

    Burke, Robert A.; Lamborn, Daniel R.; Weng, Xiaojun; Redwing, Joan M.

    2009-06-01

    A combination of experimental and computational fluid dynamics-based reactor modeling studies were utilized to study the effects of process conditions on GaN nanowire growth by metalorganic chemical vapor deposition (MOCVD) in an isothermal tube reactor. The GaN nanowires were synthesized on (0 0 0 1) sapphire substrates using nickel thin films as a catalyst. GaN nanowire growth was observed over a furnace temperature range of 800-900 °C at V/III ratios ranging from 33 to 67 and was found to be strongly dependent on the position of the substrate relative to the group III inlet tube. The modeling studies revealed that nanowire growth consistently occurred in a region in the reactor where the GaN thin-film deposition rate was reduced and the gas phase consisted primarily of intermediate species produced by the reaction and decomposition of trimethylgallium-ammonia adduct compounds. The GaN nanowires exhibited a predominant [1 1 2¯ 0] growth direction. Photoluminescence measurements revealed an increase in the GaN near-band edge emission intensity and a reduction in the deep-level yellow luminescence with increasing growth temperature and V/III ratio.

  11. Comparative Raman and HRTEM study of nanostructured GaN nucleation layers and device layers on sapphire (0001).

    PubMed

    Pant, P; Narayan, J; Wushuer, A; Manghnani, M H

    2008-11-01

    Raman spectroscopy in conjunction with high-resolution transmission electron microscopy (HRTEM) has been used to study structural characteristics and strain distribution of the nanostructured GaN nucleation layer (NL) and the GaN device layer on (0001) sapphire substrates used for light-emitting diodes and lasers. Raman peaks corresponding to the cubic and the hexagonal phase of GaN are observed in the Raman spectrum from 15 nm and 45 nm NLs. A comparison of the peak intensities for the cubic and hexagonal phases of GaN in the NLs suggests that the cubic phase is dominant in the 15 nm NL and the hexagonal phase in the 45 nm NL. An increase in the density of stacking faults in the metastable cubic GaN (c-GaN) phase with increasing growth time lowers the system energy as well as locally converts c-GaN phase into hexagonal GaN (h-GaN). It also explains the observation of the more intense peaks of h-GaN in the 45 nm NL compared to c-GaN peaks. For the sample wherein an h-GaN device layer was grown at higher temperatures on the NL, narrow Raman peaks corresponding to only h-GaN were observed, confirming the high-quality of the films. The peak shift of the E2(H)(LO) mode of h-GaN in the NLs and the h-GaN film suggests the presence of a tensile stress in the NL which is attributed to defects such as stacking faults and twins, and a compressive stress in high-temperature grown h-GaN film which is attributed to the thermal-expansion mismatch between the film and the substrate. The peak shifts of the substrate also reveal that during the low temperature growth of the NL the substrate is under a compressive stress which is attributed to defects in the NL and during the high temperature growth of the device layer, there is a tensile strain in the substrate as expected from differences in coefficients of thermal expansion of the film and the substrate during the cooling cycle. PMID:19198336

  12. Non-polar a-plane ZnO films grown on r-Al2O3 substrates using GaN buffer layers

    NASA Astrophysics Data System (ADS)

    Xu, C. X.; Chen, W.; Pan, X. H.; Chen, S. S.; Ye, Z. Z.; Huang, J. Y.

    2016-09-01

    In this work, GaN buffer layer has been used to grow non-polar a-plane ZnO films by laser-assisted and plasma-assisted molecular beam epitaxy. The thickness of GaN buffer layer ranges from ∼3 to 12 nm. The GaN buffer thickness effect on the properties of a-plane ZnO thin films is carefully investigated. The results show that the surface morphology, crystal quality and optical properties of a-plane ZnO films are strongly correlated with the thickness of GaN buffer layer. It was found that with 6 nm GaN buffer layer, a-plane ZnO films display the best crystal quality with X-ray diffraction rocking curve full-width at half-maximum of only 161 arcsec for the (101) reflection.

  13. 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.

  14. Hexagonal diamonds in meteorites: implications.

    PubMed

    Hanneman, R E; Strong, H M; Bundy, F P

    1967-02-24

    A new polymorph of carbon, hexagonal diamond, has been discovered in the Canyon Diablo and Goalpara meteorites. This phase had been synthesized recently under specific high-pressure conditions in the laboratory. Our results: provide strong evidence that diamonds found in these meteorites were produced by intense shock pressures acting on crystalline graphite inclusions present within the meteorite before impact, rather than by disintegration of larger, statically grown diamonds, as some theories propose. PMID:17830485

  15. 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. PMID:18778107

  16. Effect of Capping on Electrical and Optical Properties of GaN Layers Grown by HVPE

    NASA Astrophysics Data System (ADS)

    Reshchikov, M. A.; Usikov, A.; Helava, H.; Makarov, Yu.; Puzyk, M. V.; Papchenko, B. P.

    2016-04-01

    Gallium nitride, grown by hydride vapor phase epitaxy and capped with a thin AlGaN layer, was studied by photoluminescence (PL) methods. The concentration of free electrons in GaN was found from the time-resolved PL data, and the concentrations of point defects were estimated from the steady-state PL measurements. The intensity of PL from GaN decreases moderately after capping it with Si-doped AlGaN, and it decreases dramatically after capping with Mg-doped AlGaN. At the same time, the concentration of free electrons and the concentrations of main radiative defects in GaN are not affected by the AlGaN capping. We demonstrate that PL is a powerful tool for nondestructive characterization of semiconductor layers buried under overlying device structures.

  17. Low temperature inorganic chemical vapor deposition of heteroepitaxial GaN

    NASA Astrophysics Data System (ADS)

    McMurran, Jeffrey; Todd, M.; Kouvetakis, J.; Smith, David J.

    1996-07-01

    We have developed a highly efficient method of growing thin oriented films of GaN on basal plane sapphire and (100) Si substrates using an exclusively inorganic single-source precursor free of carbon and hydrogen. Cross sectional transmission electron microscopy of the highly conformal films revealed columnar material growth on Si and heteroepitaxial columnar growth of crystalline GaN on sapphire. Rutherford backscattering spectroscopy (RBS) of layers grown at 700 °C confirmed stoichiometric GaN. Auger and RBS oxygen and carbon resonance profiles indicated that the films were pure and highly homogeneous. With respect to current chemical vapor deposition processes for GaN growth, our approach offers a number of potentially important improvements. These include high growth rates of 5-350 nm/min, low deposition temperature of 650-700 °C, nearly ideal Ga-N stoichiometry, elimination of the highly inefficient use of toxic ammonia, and a carbon-hydrogen free growth environment that could prove to be beneficial to p-doping processes.

  18. Structural and optical studies of GaN pn-junction with AlN buffer layer grown on Si (111) by RF plasma enhanced MBE

    NASA Astrophysics Data System (ADS)

    Yusoff, Mohd Zaki Mohd; Hassan, Zainuriah; Woei, Chin Che; Hassan, Haslan Abu; Abdullah, Mat Johar

    2012-06-01

    GaN pn-junction grown on silicon substrates have been the focus in a number of recent reports and further effort is still necessary to improve its crystalline quality for practical applications. GaN has the high n-type background carrier concentration resulting from native defects commonly thought to be nitrogen vacancies. In this work, we present the growth of pn-junction of GaN on Si (111) substrate using RF plasma-enhanced molecular beam epitaxy (MBE). Both of the layers show uniformity with an average thickness of 0.709 μm and 0.095 μm for GaN and AlN layers, respectively. The XRD spectra indicate that no sign of cubic phase of GaN are found, so it is confirmed that the sample possessed hexagonal structure. It was found that all the allowed Raman optical phonon modes of GaN, i.e. the E2 (low), E1 (high) and A1 (LO) are clearly visible.

  19. Structural and optical studies of GaN pn-junction with AlN buffer layer grown on Si (111) by RF plasma enhanced MBE

    SciTech Connect

    Yusoff, Mohd Zaki Mohd; Hassan, Zainuriah; Woei, Chin Che; Hassan, Haslan Abu; Abdullah, Mat Johar

    2012-06-29

    GaN pn-junction grown on silicon substrates have been the focus in a number of recent reports and further effort is still necessary to improve its crystalline quality for practical applications. GaN has the high n-type background carrier concentration resulting from native defects commonly thought to be nitrogen vacancies. In this work, we present the growth of pn-junction of GaN on Si (111) substrate using RF plasma-enhanced molecular beam epitaxy (MBE). Both of the layers show uniformity with an average thickness of 0.709 {mu}m and 0.095 {mu}m for GaN and AlN layers, respectively. The XRD spectra indicate that no sign of cubic phase of GaN are found, so it is confirmed that the sample possessed hexagonal structure. It was found that all the allowed Raman optical phonon modes of GaN, i.e. the E2 (low), E1 (high) and A1 (LO) are clearly visible.

  20. 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.

  1. 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.

  2. 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.

  3. Bandgap engineering of GaN nanowires

    NASA Astrophysics Data System (ADS)

    Ming, Bang-Ming; Wang, Ru-Zhi; Yam, Chi-Yung; Xu, Li-Chun; Lau, Woon-Ming; Yan, Hui

    2016-05-01

    Bandgap engineering has been a powerful technique for manipulating the electronic and optical properties of semiconductors. In this work, a systematic investigation of the electronic properties of [0001] GaN nanowires was carried out using the density functional based tight-binding method (DFTB). We studied the effects of geometric structure and uniaxial strain on the electronic properties of GaN nanowires with diameters ranging from 0.8 to 10 nm. Our results show that the band gap of GaN nanowires depends linearly on both the surface to volume ratio (S/V) and tensile strain. The band gap of GaN nanowires increases linearly with S/V, while it decreases linearly with increasing tensile strain. These linear relationships provide an effect way in designing GaN nanowires for their applications in novel nano-devices.

  4. Distinguishing cubic and hexagonal phases within InGaN/GaN microstructures using electron energy loss spectroscopy

    PubMed Central

    CHERNS, D; ALBERT, S.; BENGOECHEA‐ENCABO, A.; ANGEL SANCHEZ, M.; CALLEJA, E.; SCHIMPKE, T.; STRASSBURG, M.

    2015-01-01

    Summary 3D InGaN/GaN microstructures grown by metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) have been extensively studied using a range of electron microscopy techniques. The growth of material by MBE has led to the growth of cubic GaN material. The changes in these crystal phases has been investigated by Electron Energy Loss Spectroscopy, where the variations in the fine structure of the N K‐edge shows a clear difference allowing the mapping of the phases to take place. GaN layers grown for light emitting devices sometimes have cubic inclusions in the normally hexagonal wurtzite structures, which can influence the device electronic properties. Differences in the fine structure of the N K‐edge between cubic and hexagonal material in electron energy loss spectra are used to map cubic and hexagonal regions in a GaN/InGaN microcolumnar device. The method of mapping is explained, and the factors limiting spatial resolution are discussed. PMID:26366483

  5. Distinguishing cubic and hexagonal phases within InGaN/GaN microstructures using electron energy loss spectroscopy.

    PubMed

    Griffiths, I J; Cherns, D; Albert, S; Bengoechea-Encabo, A; Angel Sanchez, M; Calleja, E; Schimpke, T; Strassburg, M

    2016-05-01

    3D InGaN/GaN microstructures grown by metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) have been extensively studied using a range of electron microscopy techniques. The growth of material by MBE has led to the growth of cubic GaN material. The changes in these crystal phases has been investigated by Electron Energy Loss Spectroscopy, where the variations in the fine structure of the N K-edge shows a clear difference allowing the mapping of the phases to take place. GaN layers grown for light emitting devices sometimes have cubic inclusions in the normally hexagonal wurtzite structures, which can influence the device electronic properties. Differences in the fine structure of the N K-edge between cubic and hexagonal material in electron energy loss spectra are used to map cubic and hexagonal regions in a GaN/InGaN microcolumnar device. The method of mapping is explained, and the factors limiting spatial resolution are discussed. PMID:26366483

  6. 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.

  7. 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.

  8. Vertical current-flow enhancement via fabrication of GaN nanorod p-n junction diode on graphene

    NASA Astrophysics Data System (ADS)

    Ryu, Sung Ryong; Ram, S. D. Gopal; Lee, Seung Joo; Cho, Hak-dong; Lee, Sejoon; Kang, Tae Won; Kwon, Sangwoo; Yang, Woochul; Shin, Sunhye; Woo, Yongdeuk

    2015-08-01

    Mg doped GaN nanorods were grown on undoped n-type GaN nanorods uniaxial on monolayer graphene by hydride vapor phase epitaxy (HVPE) method. The monolayer graphene used as the bottom electrode and a substrate as well provides good electrical contact, acts as a current spreading layer, well suitable for the growth of hexagonal GaN nanorod. In addition it has a work function suitable to that of n-GaN. The formed p-n nanorods show a Schottky behavior with a turn on voltage of 3 V. Using graphene as the substrate, the resistance of the nanorod is reduced by 700 times when compared with the case without using graphene as the current spreading layer. The low resistance of graphene acts in parallel with the resistance of the GaN buffer layer, and reduces the resistance drastically. The formed p-n junction in a single GaN nanorod is visualized by Kelvin Force Probe Microscopy (KPFM) to have distinctively contrast p and n regions. The measured contact potential difference of p-and n-region has a difference of 103 mV which well confirms the formed regions are electronically different. Low temperature photoluminescence (PL) spectra give evidence of dopant related acceptor bound emission at 3.2 eV different from 3.4 eV of undoped GaN. The crystalline structure, compositional purity is confirmed by X-ray diffraction (XRD), Transmission and Scanning electron microcopies (SEM), (TEM), Energy dispersive analysis by X-ray (EDAX) and X-ray photoelectron spectroscopy (XPS) as well.

  9. Optimization of inductively coupled plasma deep etching of GaN and etching damage analysis

    NASA Astrophysics Data System (ADS)

    Qiu, Rongfu; Lu, Hai; Chen, Dunjun; Zhang, Rong; Zheng, Youdou

    2011-01-01

    Inductively coupled plasma (ICP) etching of GaN with an etching depth up to 4 μm is systemically studied by varying ICP power, RF power and chamber pressure, respectively, which results in etch rates ranging from ∼370 nm/min to 900 nm/min. The surface morphology and damages of the etched surface are characterized by optical microscope, scanning electron microscope, atomic force microscopy, cathodoluminescence mapping and photoluminescence (PL) spectroscopy. Sub-micrometer-scale hexagonal pits and pillars originating from part of the structural defects within the original GaN layer are observed on the etched surface. The density of these surface features varies with etching conditions. Considerable reduction of PL band-edge emission from the etched GaN surface indicates that high-density non-radiative recombination centers are created by ICP etching. The density of these non-radiative recombination centers is found largely dependent on the degree of physical bombardments, which is a strong function of the RF power applied. Finally, a low-surface-damage etch recipe with high ICP power, low RF power, high chamber pressure is suggested.

  10. Onset of hexagons in surface-tension-driven Benard convection

    NASA Technical Reports Server (NTRS)

    Schatz, Michael F.; Vanhook, Stephen J.; Swift, John B.; Mccormick, William D.; Swinney, Harry L.

    1994-01-01

    High resolution laboratory experiments with large aspect ratio are being conducted for thin fluid layers heated from below and bounded from above by a free surface. The fluid depths are chosen sufficiently small (less than 0.06 cm) so that surface tension is the dominant driving mechanisms; the Rayleigh number is less than 5 for the results reported here. Shadowgraph visualization reveals that the primary instability leading to hexagons is slightly hysteretic (approximately 1 percent). Preliminary measurements of the convection amplitude using infrared imaging are also presented.

  11. 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.

  12. 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.

  13. 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.

  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. 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.

  16. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-06-01

    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.

  18. 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.

  19. 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.

  20. Energy dependence of electron inelastic mean free paths in bulk GaN crystals

    NASA Astrophysics Data System (ADS)

    Krawczyk, M.; Zommer, L.; Jablonski, A.; Grzegory, I.; Bockowski, M.

    2004-09-01

    Recent advances in fabrication and commercialization of high-brightness blue and green light-emitting devices based on gallium nitride have renewed intense research of its basic properties. Since information on electron transport processes in GaN is scarce, their systematic studies are highly desirable. The electron inelastic mean free path (IMFP) is a crucial parameter for quantitative interpretation of surface electron spectra. The energy dependence of IMFP for bulk GaN crystals with different surface concentrations of their constituents was obtained from elastic peak electron spectroscopy (EPES) with use of the Ni standard in the energy range 200-2000 eV. The measured IMFPs were compared with the values predicted by the TPP-2M and G-1 formulae. A reasonable agreement was found between the measured IMFPs in bulk GaN with an ideal stoichiometric surface composition and the corresponding calculated IMFPs. Compared with the bulk IMFPs, experimental IMFPs valid for the GaN sample with a thin surface layer enriched in ˜70 at.% Ga are only slightly smaller by 5-10%, depending on the electron energy.

  1. Coaxial InGaN epitaxy around GaN micro-tubes: Tracing the signs

    NASA Astrophysics Data System (ADS)

    Fikry, M.; Ren, Z.; Madel, M.; Tischer, I.; Thonke, K.; Scholz, F.

    2013-05-01

    This work focuses on investigations of the luminescence properties of coaxial InGaN layers grown around single GaN micro and sub-micron tubes on top of GaN micro-pyramids. The tube structure was formed after the controlled desorption of ZnO nano-pillar templates during the coaxial GaN epitaxy. A thin layer near the area around the inner diameter of the micro-tube is believed to be heavily doped with Zn impurities leading to an intense and broad photoluminescence (PL) peak centered around 2.85 eV that quenches the luminescence from coaxial InGaN quantum wells (QWs). When the thickness of the GaN tube wall before the QW growth was doubled, a clear indication of In incorporation in low temperature PL was observed via an intense peak around 3.1 eV. Moreover, as the temperature of the QW growth was changed from 830 °C to 780 °C, a shift of the peak corresponding to an increase in In incorporation from 3.5% to 7.5% was noticed.

  2. Reduced stability of copper interconnects due to wrinkles and steps on hexagonal boron nitride substrates

    NASA Astrophysics Data System (ADS)

    Gao, Jian; Chow, Philippe K.; Thomas, Abhay V.; Lu, Toh-Ming; Borca-Tasciuc, Theodorian; Koratkar, Nikhil

    2014-09-01

    There is great scientific and technological interest in the use of chemical-vapor-deposition grown hexagonal boron nitride dielectric substrates for microelectronics applications. This interest stems from its superior heat spreading capability compared to silicon dioxide as well as the lack of surface dangling bonds or charge traps in hexagonal boron nitride which results in superior performance for graphene based electronics devices. However, surface heterogeneities, such as wrinkles or steps, are ubiquitous in such devices due to the fabrication and processing of chemical vapor deposition grown hexagonal boron nitride. In this study, we characterize the effect of such surface heterogeneities on the stability of copper interconnects used in microelectronics devices. Based on the theoretical thermo-physical properties of the constituent thin film layers, our simulations predict that copper interconnects deposited on hexagonal boron nitride can withstand ˜1.9 times more power than on a silicon dioxide substrate, due to its superior in-plane thermal conductivity. However, our electrical measurements reveal that copper wires melt and fail at consistently lower current densities on hexagonal boron nitride than on silicon dioxide. This was verified by testing in air as well as under vacuum. Scanning electron microscopy and atomic force microscopy characterization of the hexagonal boron nitride surface indicates that this contradictory result is due to nanoscale surface non-uniformities (i.e., wrinkles and steps) which are omnipresent in chemical-vapor-deposition grown and transferred hexagonal boron nitride films. Our results highlight the critical need for improved processing methods before large-scale microelectronics applications of chemical vapor deposition grown hexagonal boron nitride can be realized.

  3. Gluing hexagons at three loops

    NASA Astrophysics Data System (ADS)

    Basso, Benjamin; Goncalves, Vasco; Komatsu, Shota; Vieira, Pedro

    2016-06-01

    We perform extensive three-loop tests of the hexagon bootstrap approach for structure constants in planar N = 4 SYM theory. We focus on correlators involving two BPS operators and one non-BPS operator in the so-called SL (2) sector. At three loops, such correlators receive wrapping corrections from mirror excitations flowing in either the adjacent or the opposing channel. Amusingly, we find that the first type of correction coincides exactly with the leading wrapping correction for the spectrum (divided by the one-loop anomalous dimension). We develop an efficient method for computing the second type of correction for operators with any spin. The results are in perfect agreement with the recently obtained three-loop perturbative data by Chicherin, Drummond, Heslop, Sokatchev [2] and by Eden [3]. We also derive the integrand for general multi-particle wrapping corrections, which turns out to take a remarkably simple form. As an application we estimate the loop order at which various new physical effects are expected to kick-in.

  4. Fermionic pentagons and NMHV hexagon

    NASA Astrophysics Data System (ADS)

    Belitsky, A. V.

    2015-05-01

    We analyze the near-collinear limit of the null polygonal hexagon super Wilson loop in the planar N = 4 super-Yang-Mills theory. We focus on its Grassmann components which are dual to next-to-maximal helicity-violating (NMHV) scattering amplitudes. The kinematics in question is studied within a framework of the operator product expansion that encodes propagation of excitations on the background of the color flux tube stretched between the sides of Wilson loop contour. While their dispersion relation is known to all orders in 't Hooft coupling from previous studies, we find their form factor couplings to the Wilson loop. This is done making use of a particular tessellation of the loop where pentagon transitions play a fundamental role. Being interested in NMHV amplitudes, the corresponding building blocks carry a nontrivial charge under the SU(4) R-symmetry group. Restricting the current consideration to twist-two accuracy, we analyze two-particle contributions with a fermion as one of the constituents in the pair. We demonstrate that these nonsinglet pentagons obey bootstrap equations that possess consistent solutions for any value of the coupling constant. To confirm the correctness of these predictions, we calculate their contribution to the super Wilson loop demonstrating agreement with recent results to four-loop order in 't Hooft coupling.

  5. 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.

  6. 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.

  7. Photoluminescence between 3.36 eV and 3.41 eV from GaN epitaxial layers

    SciTech Connect

    Seitz, R.; Gaspar, C.; Monteiro, T.; Pereira, E.; Poisson, M.A.; Beaumont, B.

    1999-07-01

    GaN, its alloys, QWs and MQWs have gained an important place among short-wavelength optical emitters and high temperature electronic devices. The performance of such devices is limited by the presence of native and impurity defects. The understanding of the optical properties of the basic material allows them to improve its quality and thus increase the performance of these materials. In non intentionally doped (nid) hexagonal good quality GaN layers grown on sapphire, 6H-SiC or Si, free exciton (FXC, FXB, FXA), donor bound exciton (DX), acceptor bound exciton (AX) and donor-acceptor pair (DAP) transitions have been reported by several authors. Besides these typical emissions, emission lines in the range 3.3--3.44 eV have been observed in nid and intentionally doped hexagonal GaN layers. However, the nature of these recombinations is not completely clarified. Some authors assigned them to a superposition of LO phonon assisted transitions of DX and FX, excitons bound to neutral donors with deeper donor levels, band to impurity transitions and/or free to bound emission involving oxygen, DAP transitions, shallow bound excitons of cubic phases, excitons bound to structural defects and Zn related recombinations. In this work the authors analyze the luminescence between 3.36 eV and 3.41 eV of nid hexagonal GaN samples grown on sapphire. They found sample dependent emission lines with no DAP behavior. From the data they are able to identify different kinds of recombination processes in the same spectral region.

  8. 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.

  9. Selective-area growth of GaN on non- and semi-polar bulk GaN substrates

    NASA Astrophysics Data System (ADS)

    Okada, Shunsuke; Miyake, Hideto; Hiramatsu, Kazumasa; Enatsu, Yuuki; Nagao, Satoru

    2014-01-01

    We carried out the selective-area growth of GaN and fabricated InGaN/GaN MQWs on non- and semi-polar bulk GaN substrates by MOVPE. The differences in the GaN structures and the In incorporation of InGaN/GaN MQWs grown on non- and semi-polar GaN substrates were investigated. In the case of selective-area growth, different GaN structures were obtained on (20\\bar{2}1) GaN, (20\\bar{2}\\bar{1}) GaN, and (10\\bar{1}0) GaN substrates. A repeating pattern of \\{ 1\\bar{1}01\\} and \\{ 1\\bar{1}0\\bar{1}\\} facets appeared on (20\\bar{2}1) GaN. Then, we fabricated InGaN/GaN MQWs on the facet structures on (20\\bar{2}1) GaN. The emission properties characterized by cathodoluminescence were different for \\{ 1\\bar{1}01\\} and \\{ 1\\bar{1}0\\bar{1}\\} facets. On the other hand, for InGaN/GaN MQWs on non- and semi-polar GaN substrates, steps along the a-axis were observed by AFM. In particular on (20\\bar{2}1) GaN, undulations and undulation bunching appeared. Photoluminescence characterization indicated that In incorporation increased with the off-angle from the m-plane and also depended on the polarity.

  10. Reconstructions of the GaN(1011) surfaces: Density functional theory calculations

    NASA Astrophysics Data System (ADS)

    Hyun, Jung-Min; Kim, Yong-Sung; Kim, Hanchul

    2012-02-01

    GaN has been extensively studied for its potential applicability in optoelectronics as well as in spintronics. The functional performance in such applications depends on the surface characteristics of thin films. Thin films of GaN are typically grown along the polar [0001] direction, but their light-emission efficiency is reduced due to the electron-hole separation. A strategy to remedy such an undesired effect is to grow films along nonpolar or semipolar directions. In this presentation, we will address the reconstructions of the Ga-terminated semipolar (1011) surface. We performed the density functional theory calculations using the generalized gradient approximation, the projector augmented wave potentials, and the repeated slabs. From the calculated energetics of various reconstructions, we found that there exist a few structural motifs of GaN(1011). They are short Ga chains and Ga vacancies. For instance, a 4 x 2 reconstruction with a Ga tetramer and surface Ga vacancies is stable in the N-rich condition, which is significantly different from the previous results [Akiyama et al, Jpn. J. Appl. Phys. 48, 100201 (2009)]. Our results would provide a comprehensive understanding on the Ga-terminated semipolar surfaces.

  11. Hydrothermal synthesis of hexagonal magnesium hydroxide nanoflakes

    SciTech Connect

    Wang, Qiang; Li, Chunhong; Guo, Ming; Sun, Lingna; Hu, Changwen

    2014-03-01

    Graphical abstract: Hexagonal Mg(OH){sub 2} nanoflakes were synthesized via hydrothermal method in the presence of PEG-20,000. Results show that PEG-20,000 plays an important role in the formation of this kind of nanostructure. The SAED patterns taken from the different positions on a single hexagonal Mg(OH){sub 2} nanoflake yielded different crystalline structures. The structure of the nanoflakes are polycrystalline and the probable formation mechanism of Mg(OH){sub 2} nanoflakes is discussed. - Highlights: • Hexagonal Mg(OH){sub 2} nanoflakes were synthesized via hydrothermal method. • PEG-20,000 plays an important role in the formation of hexagonal nanostructure. • Mg(OH){sub 2} nanoflakes show different crystalline structures at different positions. • The probable formation mechanism of hexagonal Mg(OH){sub 2} nanoflakes was reported. - Abstract: Hexagonal magnesium hydroxide (Mg(OH){sub 2}) nanoflakes were successfully synthesized via hydrothermal method in the presence of the surfactant polyethylene glycol 20,000 (PEG-20,000). Results show that PEG-20,000 plays an important role in the formation of this kind of nanostructure. The composition, morphologies and structure of the Mg(OH){sub 2} nanoflakes were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). The SAED patterns taken from the different positions on a single hexagonal Mg(OH){sub 2} nanoflake show different crystalline structures. The structure of the nanoflakes are polycrystalline and the probable formation mechanism of Mg(OH){sub 2} nanoflakes is discussed. Brunauer–Emmett–Teller (BET) analysis were performed to investigate the porous structure and surface area of the as-obtained nanoflakes.

  12. 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.

  13. MOVPE of AlN and GaN by using novel precursors

    NASA Astrophysics Data System (ADS)

    Ho, Kwok-Lun; Jensen, Klavs F.; Hwang, Jen-Wei; Gladfelter, Wayne L.; Evans, John F.

    1991-01-01

    The technique of low pressure MOVPE has been used to grow thin films of AlN and GaN on silicon and (0001) sapphire substrates using the single-source precursors diethylaluminum azide [Et 2AlN 3] 3 (DEAA) and diethygallium azide [Et 2GaN 3] 3 (DEGA). In-situ growth rate measurements have been performed for AlN growth on Si substrates. For comparison with conventional MOVPE growth, epitaxial films of Al xGa 1- xN have also been deposited from triethylgallium (TEG), triethylaluminum (TEAl) and NH 3 under similar conditions. The properties of the grown films are discussed in terms of precursor selection and growth conditions.

  14. 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.

  15. 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.

  16. Piezotronic Effect in Polarity-Controlled GaN Nanowires.

    PubMed

    Zhao, Zhenfu; Pu, Xiong; Han, Changbao; Du, Chunhua; Li, Linxuan; Jiang, Chunyan; Hu, Weiguo; Wang, Zhong Lin

    2015-08-25

    Using high-quality and polarity-controlled GaN nanowires (NWs), we studied the piezotronic effect in crystal orientation defined wurtzite structures. By applying a normal compressive force on c-plane GaN NWs with an atomic force microscopy tip, the Schottky barrier between the Pt tip and GaN can be effectively tuned by the piezotronic effect. In contrast, the normal compressive force cannot change the electron transport characteristics in m-plane GaN NWs whose piezoelectric polarization axis is turned in the transverse direction. This observation provided solid evidence for clarifying the difference between the piezotronic effect and the piezoresistive effect. We further demonstrated a high sensitivity of the m-plane GaN piezotronic transistor to collect the transverse force. The integration of c-plane GaN and m-plane GaN indicates an overall response to an external force in any direction. PMID:26256533

  17. Experimental evidence of homonuclear bonds in amorphous GaN

    SciTech Connect

    Ishimaru, Dr. Manabu; Zhang, Yanwen; Wang, Xuemei; Chu, Wei-Kan; Weber, William J

    2011-01-01

    Although GaN is an important semiconductor material, its amorphous structures are not well understood. Currently, theoretical atomistic structural models which contradict each other, are proposed for the chemical short-range order of amorphous GaN: one characterizes amorphous GaN networks as highly chemically ordered, consisting of heteronuclear Ga-N atomic bonds; and the other predicts the existence of a large number of homonuclear bonds within the first coordination shell. In the present study, we examine amorphous structures of GaN via radial distribution functions obtained by electron diffraction techniques. The experimental results demonstrate that amorphous GaN networks consist of heterononuclear Ga-N bonds, as well as homonuclear Ga-Ga and N-N bonds.

  18. Dependence of the stresses on grain orientations in hexagonal films

    NASA Astrophysics Data System (ADS)

    Zhang, Jian-Min; Zhang, Yan; Xu, Ke-Wei; Ji, Vincent

    2007-01-01

    A thin polycrystalline film attached tightly to a thick substrate of different thermal expansion coefficients will experience thermal stresses when the temperature is changed during device fabrication and in service. Calculations of these stresses in various ( h k l )-oriented grains relative to the film surface have been made for a polycrystalline film composed of the hexagonal metal Be, Cd, Co, Hf, Mg, Re, Ru, Sc, Ti, Y, Zr and Zn, respectively. For all these hexagonal films, the stresses σ1 and σ2 in plane of the film surface are equal only in (0 0 1)-oriented grains due to the highest six-fold rotation symmetry of the crystallographic Z-axis. Excepting σ1 of Be, Ru, Zr, Zn and σ2 of Cd, Zn, the maximum values of the film plane stresses σ1 and σ2 correspond to the (0 0 1)-oriented grains means that the significant reliability problems, such as, voiding, cracking, hillocking induced by the stresses may be taken place preferred in (0 0 1)-oriented grains.

  19. Blue-noise halftoning for hexagonal grids.

    PubMed

    Lau, Daniel L; Ulichney, Robert

    2006-05-01

    In this paper, we closely scrutinize the spatial and spectral properties of aperiodic halftoning schemes on rectangular and hexagonal sampling grids. Traditionally, hexagonal sampling grids have been shunned due to their inability to preserve the high-frequency components of blue-noise dither patterns at gray-levels near one-half, but as will be shown, only through the introduction of diagonal correlations between dots can even rectangular sampling grids preserve these frequencies. And by allowing the sampling grid to constrain the placement of dots, a particular algorithm may introduce visual artifacts just as disturbing as excess energy below the principal frequency. If, instead, the algorithm maintains radial symmetry by introducing a minimum degree of clustering, then that algorithm can maintain its grid defiance illusion fundamental to the spirit of the blue-noise model. As such, this paper shows that hexagonal grids are preferrable because they can support gray-levels near one-half with less required clustering of minority pixels and a higher principal frequency. Along with a thorough Fourier analysis of blue-noise dither patterns on both rectangular and hexagonal sampling grids, this paper also demonstrates the construction of a blue-noise dither array for hexagonal grids. PMID:16671307

  20. Micellar hexagonal phases in lyotropic liquid crystals

    NASA Astrophysics Data System (ADS)

    Amaral, L. Q.; Gulik, A.; Itri, R.; Mariani, P.

    1992-09-01

    The hexagonal cell parameter a of the system sodium dodecyl lauryl sulfate and water as a function of volume concentration cv in phase Hα shows the functional behavior expected for micelles of finite length: a~c-1/3v. The interpretation of x-ray data based on finite micelles leads to an alternative description of the hexagonal phase Hα: spherocylindrical micelles of constant radius with length that may grow along the range of the Hα phase. Results are compared with recent statistical-mechanical calculations for the isotropic I-Hα transition. The absence of diffraction in the direction perpendicular to the hexagonal plane is ascribed to polydispersity of micellar length, which also is a necessary condition for the occurrence of direct I-Hα transitions.

  1. Hexagonal phase ordering in strongly segregated copolymer films

    NASA Astrophysics Data System (ADS)

    Glasner, Karl

    2015-10-01

    Strongly segregated copolymer mixtures with uneven composition ratio can form hexagonally ordered thin films. A simplified model describing the size and position of micellelike clusters is derived, allowing for investigation of much larger domain sizes than in previous studies. Simulations of this model are performed to study the generation of large scale order and evolution of pattern defects. We find three temporal regimes exhibiting different scaling laws for orientational correlation length and defect number. In the early stage, topological defects are rapidly eliminated by pairwise annihilation. A slower intermediate stage is characterized by the migration of grain boundaries and the elimination of small grains. In the final stage, grain boundaries become pinned and the evolution halts. A scaling law for defect interaction is proposed which is consistent with the crossover between the first and second stages.

  2. 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.

  3. 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.

  4. Intervalley scattering in hexagonal boron nitride

    NASA Astrophysics Data System (ADS)

    Cassabois, G.; Valvin, P.; Gil, B.

    2016-01-01

    We report photoluminescence experiments bringing the evidence for intervalley scattering in bulk hexagonal boron nitride. From a quantitative analysis of the defect-related emission band, we demonstrate that transverse optical phonons at the K point of the Brillouin zone assist inter-K valley scattering, which becomes observable because stacking faults in bulk hexagonal boron nitride provide a density of final electronic states. Time-resolved experiments highlight the different recombination dynamics of the phonon replicas implying either virtual excitonic states or real electronic states in the structural defects.

  5. Hexagonal structure of baby Skyrmion lattices

    SciTech Connect

    Hen, Itay; Karliner, Marek

    2008-03-01

    We study the zero-temperature crystalline structure of baby Skyrmions by applying a full-field numerical minimization algorithm to baby Skyrmions placed inside different parallelogramic unit cells and imposing periodic boundary conditions. We find that within this setup, the minimal energy is obtained for the hexagonal lattice, and that in the resulting configuration the Skyrmion splits into quarter Skyrmions. In particular, we find that the energy in the hexagonal case is lower than the one obtained on the well-studied rectangular lattice, in which splitting into half Skyrmions is observed.

  6. Efficient electrochemical reaction in hexagonal WO 3 forests with a hierarchical nanostructure

    NASA Astrophysics Data System (ADS)

    Shibuya, Masachika; Miyauchi, Masahiro

    2009-04-01

    Nanotree-like hexagonal tungsten oxide (WO 3) arrays were grown on metal tungsten substrates by a facile hydrothermal method. The WO 3 nanotrees, composed of 'trunks' and 'branches', were single crystals oriented in the <0 0 1> direction. Nanotree thin films exhibited efficient electrochromism due to their large tunnels in the crystal and nano-channels between the nanotrees. Moreover, their coloration efficiency and reversibility were superior to polycrystalline WO 3 films.

  7. 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.

  8. 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.

  9. Large-area GaN n-core/p-shell arrays fabricated using top-down etching and selective epitaxial overgrowth

    NASA Astrophysics Data System (ADS)

    Krylyuk, Sergiy; Paramanik, Dipak; King, Matt; Motayed, Abhishek; Ha, Jong-Yoon; Bonevich, John E.; Talin, Alec; Davydov, Albert V.

    2012-12-01

    We present large-area, vertically aligned GaN n-core and p-shell structures on silicon substrates. The GaN pillars were formed by inductively coupled plasma etching of lithographically patterned n-GaN epitaxial layer. Mg-doped p-GaN shells were formed using selective overgrowth by halide vapor phase epitaxy. The diameter of the cores ranged from 250 nm to 10 μm with varying pitch. The p-shells formed truncated hexagonal pyramids with {11¯01} side-facets. Room-temperature photoluminescence and Raman scattering measurements indicate strain-relaxation in the etched pillars and shells. Cross-sectional transmission electron microscopy revealed dislocation bending by 90° at the core-shell interface and reduction in their density in the shells.

  10. Regularly patterned multi-section GaN nanorod arrays grown with a pulsed growth technique

    NASA Astrophysics Data System (ADS)

    Tu, Charng-Gan; Su, Chia-Ying; Liao, Che-Hao; Hsieh, Chieh; Yao, Yu-Feng; Chen, Hao-Tsung; Lin, Chun-Han; Weng, Chi-Ming; Kiang, Yean-Woei; Yang, C. C.

    2016-01-01

    The growth of regularly patterned multi-section GaN nanorod (NR) arrays based on a pulsed growth technique with metalorganic chemical vapor deposition is demonstrated. Such an NR with multiple sections of different cross-sectional sizes is formed by tapering a uniform cross section to another through stepwise decreasing of the Ga supply duration to reduce the size of the catalytic Ga droplet. Contrast line structures are observed in either a scanning electron microscopy or transmission electron microscopy image of an NR. Such a contrast line-marker corresponds to a thin Ga-rich layer formed at the beginning of GaN precipitation of a pulsed growth cycle and illustrates the boundary between two successive growth cycles in pulsed growth. By analyzing the geometry variation of the contrast line-markers, the morphology evolution in the growth of a multi-section NR, including a tapering process, can be traced. Such a morphology variation is controlled by the size of the catalytic Ga droplet and its coverage range on the slant facets at the top of an NR. The comparison of emission spectra between single-, two-, and three-section GaN NRs with sidewall InGaN/GaN quantum wells indicates that a multi-section NR can lead to a significantly broader sidewall emission spectrum.