Effect of band gap engineering in anionic-doped TiO2 photocatalyst

NASA Astrophysics Data System (ADS)

Samsudin, Emy Marlina; Abd Hamid, Sharifah Bee

2017-01-01

A simple yet promising strategy to modify TiO2 band gap was achieved via dopants incorporation which influences the photo-responsiveness of the photocatalyst. The mesoporous TiO2 was successfully mono-doped and co-doped with nitrogen and fluorine dopants. The results indicate that band gap engineering does not necessarily requires oxygen substitution with nitrogen or/and fluorine, but from the formation of additional mid band and Ti3+ impurities states. The formation of oxygen vacancies as a result of modified color centres and Ti3+ ions facilitates solar light absorption and influences the transfer, migration and trapping of the photo-excited charge carriers. The synergy of dopants in co-doped TiO2 shows better optical properties relative to single N and F doped TiO2 with c.a 0.95 eV band gap reduction. Evidenced from XPS, the synergy between N and F in the co-doped TiO2 uplifts the valence band towards the conduction band. However, the photoluminescence data reveals poorer electrons and holes separation as compared to F-doped TiO2. This observation suggests that efficient solar light harvesting was achievable via N and F co-doping, but excessive defects could act as charge carriers trapping sites.

Structural studies and band gap tuning of Cr doped ZnO nanoparticles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Srinet, Gunjan, E-mail: gunjansrinet@gmail.com; Kumar, Ravindra, E-mail: gunjansrinet@gmail.com; Sajal, Vivek, E-mail: gunjansrinet@gmail.com

2014-04-24

Structural and optical properties of Cr doped ZnO nanoparticles prepared by the thermal decomposition method are presented. X-ray diffraction studies confirmed the substitution of Cr on Zn sites without changing the wurtzite structure of ZnO. Modified form of W-H equations was used to calculate various physical parameters and their variation with Cr doping is discussed. Significant red shift was observed in band gap, i.e., a band gap tuning is achieved by Cr doping which could eventually be useful for optoelectronic applications.

Determination of gap solution and critical temperature in doped graphene superconductivity

NASA Astrophysics Data System (ADS)

Xu, Chenmei; Yang, Yisong

2017-04-01

It is shown that the gap solution and critical transition temperature are significantly enhanced by doping in a recently developed BCS formalism for graphene superconductivity in such a way that positive gap and transition temperature both occur in arbitrary pairing coupling as far as doping is present. The analytic construction of the BCS gap and transition temperature offers highly effective globally convergent iterative methods for the computation of these quantities. A series of numerical examples are presented as illustrations which are in agreement with the theoretical and experimental results obtained in the physics literature and consolidate the analytic understanding achieved.

Doping and band gap control at poly(vinylidene fluoride)/graphene interface

NASA Astrophysics Data System (ADS)

Cai, Jia; Wang, Jian-Lu; Gao, Heng; Tian, Bobo; Gong, Shi-Jing; Duan, Chun-Gang; Chu, Jun-Hao

2018-05-01

Using the density-functional first-principles calculations, we investigate the electronic structures of poly(vinylidene fluoride) PVDF/graphene composite systems. The n- and p-doping of graphene can be flexibly switched by reversing the ferroelectric polarization of PVDF, without scarifying the intrinsic π-electron band dispersions of graphene that are usually undermined by chemical doping. The doping degree is also dependent on the thickness of PVDF layers, which will get saturated when PVDF is thick enough. In PVDF/bilayer graphene (BLG) heterostructure, the doping degree directly determines the local energy gap of the charged BLG. The sandwich structure of PVDF/BLG/PVDF can further enhance the local energy gap as well as keep the electric neutrality of BLG, which will be of great application potentials in graphene-based nanoelectronics.

Toward tunable band gap and tunable dirac point in bilayer graphene with molecular doping.

PubMed

Yu, Woo Jong; Liao, Lei; Chae, Sang Hoon; Lee, Young Hee; Duan, Xiangfeng

2011-11-09

The bilayer graphene has attracted considerable attention for potential applications in future electronics and optoelectronics because of the feasibility to tune its band gap with a vertical displacement field to break the inversion symmetry. Surface chemical doping in bilayer graphene can induce an additional offset voltage to fundamentally affect the vertical displacement field and the band gap opening in bilayer graphene. In this study, we investigate the effect of chemical molecular doping on band gap opening in bilayer graphene devices with single or dual gate modulation. Chemical doping with benzyl viologen molecules modulates the displacement field to allow the opening of a transport band gap and the increase of the on/off ratio in the bilayer graphene transistors. Additionally, Fermi energy level in the opened gap can be rationally controlled by the amount of molecular doping to obtain bilayer graphene transistors with tunable Dirac points, which can be readily configured into functional devices, such as complementary inverters.

Anisotropic selective etching between SiGe and Si

NASA Astrophysics Data System (ADS)

Ishii, Yohei; Scott-McCabe, Ritchie; Yu, Alex; Okuma, Kazumasa; Maeda, Kenji; Sebastian, Joseph; Manos, Jim

2018-06-01

In Si/SiGe dual-channel FinFETs, it is necessary to simultaneously control the etched amounts of SiGe and Si. However, the SiGe etch rate is higher than the Si etch rate in not only halogen plasmas but also physical sputtering. In this study, we found that hydrogen plasma selectively etches Si over SiGe. The result shows that the selectivity of Si over SiGe can be up to 38 with increasing Ge concentration in SiGe. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) results indicate that hydrogen selectively bonds with Si rather than with Ge in SiGe. During the etching, hydrogen-induced Si surface segregation is also observed. It is also observed that the difference in etched amount between SiGe and Si can be controlled from positive to negative values even in Si/SiGe dual-channel fin patterning while maintaining the vertical profiles. Furthermore, no plasma-induced lattice damage was observed by transmission electron microscopy for both Si and SiGe fin sidewalls.

Effect of solvents on optical band gap of silicon-doped graphene oxide

NASA Astrophysics Data System (ADS)

Tul Ain, Qura; Al-Modlej, Abeer; Alshammari, Abeer; Naeem Anjum, Muhammad

2018-03-01

The objective of this study was to determine the influence on the optical band gap when the same amount of silicon-doped graphene oxide was dissolved in three different solvents namely, distilled water, benzene, and dichloroethane. Ultraviolet-visible spectroscopy was used to analyse the optical properties of the solutions. Among all these solutions distilled water containing silicon-doped graphene oxide has the smallest optical band gap of 2.9 eV and is considered a semiconductor. Other solutions are not considered as semiconductors as they have optical band gaps greater than 4 eV. It was observed that there is an increase in the value of optical band gap of distilled water, benzene, and dichloroethane solutions indicating a rise in the insulating behaviour. In this experiment, graphene oxide was synthesised from graphite powder by modified Hummer’s method and was then doped with silicon. Synthesis and doping of graphene oxide were confirmed by various characterization techniques. Fourier transmission infrared spectroscopy was used for identification of surface functional groups. X-ray diffraction was carried out to confirm the formation of crystalline graphene oxide and silicon doped graphene oxide. In x-ray diffraction pattern, shifting of intensity peak from a 2θ value of 26.5° to 10° confirmed the synthesis of graphene oxide and various intensity peaks at different values of 2θ confirmed doping of graphene oxide with silicon. Scanning electron microscopy images indicated that graphene oxide sheets were decorated with spherical silicon nanoparticles. Energy dispersive x-ray spectroscopy showed that silicon doped graphene oxide powder contained 63.36% carbon, 34.05% oxygen, and 2.6% silicon.

Ion-Beam-Induced Atomic Mixing in Ge, Si, and SiGe, Studied by Means of Isotope Multilayer Structures

PubMed Central

Radek, Manuel; Liedke, Bartosz; Schmidt, Bernd; Voelskow, Matthias; Bischoff, Lothar; Lundsgaard Hansen, John; Nylandsted Larsen, Arne; Bougeard, Dominique; Böttger, Roman; Prucnal, Slawomir; Posselt, Matthias; Bracht, Hartmut

2017-01-01

Crystalline and preamorphized isotope multilayers are utilized to investigate the dependence of ion beam mixing in silicon (Si), germanium (Ge), and silicon germanium (SiGe) on the atomic structure of the sample, temperature, ion flux, and electrical doping by the implanted ions. The magnitude of mixing is determined by secondary ion mass spectrometry. Rutherford backscattering spectrometry in channeling geometry, Raman spectroscopy, and transmission electron microscopy provide information about the structural state after ion irradiation. Different temperature regimes with characteristic mixing properties are identified. A disparity in atomic mixing of Si and Ge becomes evident while SiGe shows an intermediate behavior. Overall, atomic mixing increases with temperature, and it is stronger in the amorphous than in the crystalline state. Ion-beam-induced mixing in Ge shows no dependence on doping by the implanted ions. In contrast, a doping effect is found in Si at higher temperature. Molecular dynamics simulations clearly show that ion beam mixing in Ge is mainly determined by the thermal spike mechanism. In the case of Si thermal spike, mixing prevails at low temperature whereas ion beam-induced enhanced self-diffusion dominates the atomic mixing at high temperature. The latter process is attributed to highly mobile Si di-interstitials formed under irradiation and during damage annealing. PMID:28773172

Ion-Beam-Induced Atomic Mixing in Ge, Si, and SiGe, Studied by Means of Isotope Multilayer Structures.

PubMed

Radek, Manuel; Liedke, Bartosz; Schmidt, Bernd; Voelskow, Matthias; Bischoff, Lothar; Hansen, John Lundsgaard; Larsen, Arne Nylandsted; Bougeard, Dominique; Böttger, Roman; Prucnal, Slawomir; Posselt, Matthias; Bracht, Hartmut

2017-07-17

Crystalline and preamorphized isotope multilayers are utilized to investigate the dependence of ion beam mixing in silicon (Si), germanium (Ge), and silicon germanium (SiGe) on the atomic structure of the sample, temperature, ion flux, and electrical doping by the implanted ions. The magnitude of mixing is determined by secondary ion mass spectrometry. Rutherford backscattering spectrometry in channeling geometry, Raman spectroscopy, and transmission electron microscopy provide information about the structural state after ion irradiation. Different temperature regimes with characteristic mixing properties are identified. A disparity in atomic mixing of Si and Ge becomes evident while SiGe shows an intermediate behavior. Overall, atomic mixing increases with temperature, and it is stronger in the amorphous than in the crystalline state. Ion-beam-induced mixing in Ge shows no dependence on doping by the implanted ions. In contrast, a doping effect is found in Si at higher temperature. Molecular dynamics simulations clearly show that ion beam mixing in Ge is mainly determined by the thermal spike mechanism. In the case of Si thermal spike, mixing prevails at low temperature whereas ion beam-induced enhanced self-diffusion dominates the atomic mixing at high temperature. The latter process is attributed to highly mobile Si di-interstitials formed under irradiation and during damage annealing.

Strain analysis of SiGe microbridges

NASA Astrophysics Data System (ADS)

Anthony, Ross; Gilbank, Ashley; Crowe, Iain; Knights, Andrew

2018-02-01

We present the analysis of UV (325 nm) Raman scattering spectra from silicon-germanium (SiGe) microbridges where the SiGe has been formed using the so-called "condensation technique". As opposed to the conventional condensation technique in which SiGe is grown epitaxially, we use high-dose ion implantation of Ge ions into SOI as a means to introduce the initial Ge profile. The subsequent oxidation both repairs implantation induced damage, and forms epitaxial Ge. Using Si-Si and Si-Ge optical phonon modes, as well as the ratio of integrated intensities for Ge-Ge and Si-Si, we can determine both the composition and strain of the material. We show that although the material is compressively strained following condensation, by fabricating microbridge structures we can create strain relaxed or tensile strained structures, with subsequent interest for photonic applications.

Band gap and conductivity variations of ZnO thin films by doping with Aluminium

NASA Astrophysics Data System (ADS)

Vattappalam, Sunil C.; Thomas, Deepu; T, Raju Mathew; Augustine, Simon; Mathew, Sunny

2015-02-01

Zinc Oxide thin films were prepared by Successive Ionic layer adsorption and reaction technique(SILAR). Aluminium was doped for different doping concentrations from 3 at.% to 12 at.% in steps of 3 at.%. Conductivity of the samples were taken at different temperatures. UV Spectrograph of the samples were taken and the band gap of each sample was found from the data. It was observed that as the doping concentration of Aluminium increases, the band gap of the samples decreases and concequently conductivity of the samples increases.

The Effect of Strain on the Base Resistance and Transit Time of Ungraded and Compositional-Graded SiGe HBTs

NASA Technical Reports Server (NTRS)

Rosenfeld, D.; Alterovitz, S. A.

1994-01-01

A theoretical study of the effects of the strain on the base properties of ungraded and compositional-graded n-p-n SiGe Heterojunction Bipolar Transistors (HBT) is presented. The dependencies of the transverse hole mobility and longitudinal electron mobility upon strain, composition and doping, are formulated using published Monte-Carlo data and, consequently, the base resistance and transit time are modeled and calculated. The results are compared to results obtained using common formulas that ignore these dependencies. The differences between the two sets of results are shown. The paper's conclusion is that for the design, analysis and optimization of high frequency SiGe HBTs the strain effects on the base properties cannot be ignored.

Improved electrical properties of n-type SiGe alloys

NASA Technical Reports Server (NTRS)

Scoville, A. N.; Bajgar, Clara; Vandersande, Jan; Fleurial, Jean-Pierre

1992-01-01

The effect of changes in the carrier concentration and mobility for heavily doped n-type SiGe on the electrical power factor has been investigated. It has been shown that power factors of 37-40 microV/cm-K-squared can be achieved with carrier concentrations of 2.0 - 2.5 x 10 exp 20/cu cm and mobilities of 38-40 sq cm/V-sec. Many samples with suitable carrier concentration do not have high mobilities and some rationale for this behavior is presented. Initial results are presented on fabrication of n-type samples from ultrafine powders. The emphasis in this work is to achieve thermal conductivity reductions by adding inert particles to scatter midfrequency phonons.

Widely tunable band gap in a multivalley semiconductor SnSe by potassium doping

NASA Astrophysics Data System (ADS)

Zhang, Kenan; Deng, Ke; Li, Jiaheng; Zhang, Haoxiong; Yao, Wei; Denlinger, Jonathan; Wu, Yang; Duan, Wenhui; Zhou, Shuyun

2018-05-01

SnSe, a group IV-VI monochalcogenide with layered crystal structure similar to black phosphorus, has recently attracted extensive interest due to its excellent thermoelectric properties and potential device applications. Experimental electronic structure of both the valence and conduction bands is critical for understanding the effects of hole versus electron doping on the thermoelectric properties, and to further reveal possible change of the band gap upon doping. Here, we report the multivalley valence bands with a large effective mass on semiconducting SnSe crystals and reveal single-valley conduction bands through electron doping to provide a complete picture of the thermoelectric physics. Moreover, by electron doping through potassium deposition, the band gap of SnSe can be widely tuned from 1.2 eV to 0.4 eV, providing new opportunities for tunable electronic and optoelectronic devices.

Thermoelectric properties of heavily GaP- and P-doped Si0.95Ge0.05

NASA Astrophysics Data System (ADS)

Yamashita, Osamu

2001-06-01

The Seebeck coefficient S, the electrical resistivity ρ and the thermal conductivity κ of Si0.95Ge0.05 samples doped with 0.4 at. % P and/or 0.5-2.0 mol % GaP, which were prepared by a conventional arc melting method, were measured as functions of GaP content and temperature T in the range from 323 to 1208 K. When multidoped with P and GaP, Ga tends to segregate more strongly with Ge to the grain boundaries than P, while when doped with GaP alone, both P and Ga segregate equally strongly with Ge. For multidoped samples, the S values at 323 K have a minimum at 1.0 mol % GaP and then increase with additional GaP, while the values of ρ and κ decrease monotonically with increasing GaP content. The optimum additional content of GaP that gives the largest thermoelectric figures of merit (ZT=S2T/κρ) for multidoped n-type Si0.95Ge0.05 samples was 1.5 mol %, which is slightly less than the 2.0 mol % of GaP added to Si0.8Ge0.2 alloy by hot pressing. The ZT value for multidoped Si0.95Ge0.05 with an optimum content of GaP increases linearly with temperature, and at 1073 K is 18% higher than that obtained previously for Si0.95Ge0.05 doped with only 0.4 at. % P. At 1173 K the ZT value is 1.16, which corresponds to 95% of that obtained previously at the corresponding temperature for Si0.8Ge0.2 alloy doped with 2.0 mol % GaP.

Universal doping evolution of the superconducting gap anisotropy in single crystals of electron-doped Ba(Fe1‑x Rh x )2As2 from London penetration depth measurements

NASA Astrophysics Data System (ADS)

Kim, Hyunsoo; Tanatar, M. A.; Martin, C.; Blomberg, E. C.; Ni, Ni; Bud’ko, S. L.; Canfield, P. C.; Prozorov, R.

2018-06-01

Doping evolution of the superconducting gap anisotropy was studied in single crystals of 4d-electron doped Ba(Fe1‑x Rh x )2As2 using tunnel diode resonator measurements of the temperature variation of the London penetration depth . Single crystals with doping levels representative of an underdoped regime x  =  0.039 ( K), close to optimal doping x  =  0.057 ( K) and overdoped x  =  0.079 ( K) and x  =  0.131( K) were studied. Superconducting energy gap anisotropy was characterized by the exponent, n, by fitting the data to the power-law, . The exponent n varies non-monotonically with x, increasing to a maximum n  =  2.5 for x  =  0.079 and rapidly decreasing towards overdoped compositions to 1.6 for x  =  0.131. This behavior is qualitatively similar to the doping evolution of the superconducting gap anisotropy in other iron pnictides, including hole-doped (Ba,K)Fe2As2 and 3d-electron-doped Ba(Fe,Co)2As2 superconductors, finding a full gap near optimal doping and strong anisotropy toward the ends of the superconducting dome in the T-x phase diagram. The normalized superfluid density in an optimally Rh-doped sample is almost identical to the temperature-dependence in the optimally doped Ba(Fe,Co)2As2 samples. Our study supports the universal superconducting gap variation with doping and pairing at least in iron based superconductors of the BaFe2As2 family.

Facile doping of anionic narrow-band-gap conjugated polyelectrolytes during dialysis.

PubMed

Mai, Cheng-Kang; Zhou, Huiqiong; Zhang, Yuan; Henson, Zachary B; Nguyen, Thuc-Quyen; Heeger, Alan J; Bazan, Guillermo C

2013-12-02

PCPDTBTSO3 K, an anionic, narrow-band-gap conjugated polyelectrolyte, was found to be doped after dialysis. The proposed doping mechanism involves protonation of the polymer backbone, followed by electron transfer from a neutral chain, to generate radical cations, which are stabilized by the pendant sulfonate anions. Formation of polarons is supported by spectroscopy and electrical-conductivity measurements. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Universal doping evolution of the superconducting gap anisotropy in single crystals of electron-doped Ba(Fe1-x Rh x )2As2 from London penetration depth measurements.

PubMed

Kim, Hyunsoo; Tanatar, M A; Martin, C; Blomberg, E C; Ni, Ni; Bud'ko, S L; Canfield, P C; Prozorov, R

2018-06-06

Doping evolution of the superconducting gap anisotropy was studied in single crystals of 4d-electron doped Ba(Fe 1-x Rh x ) 2 As 2 using tunnel diode resonator measurements of the temperature variation of the London penetration depth [Formula: see text]. Single crystals with doping levels representative of an underdoped regime x  =  0.039 ([Formula: see text] K), close to optimal doping x  =  0.057 ([Formula: see text] K) and overdoped x  =  0.079 ([Formula: see text] K) and x  =  0.131([Formula: see text] K) were studied. Superconducting energy gap anisotropy was characterized by the exponent, n, by fitting the data to the power-law, [Formula: see text]. The exponent n varies non-monotonically with x, increasing to a maximum n  =  2.5 for x  =  0.079 and rapidly decreasing towards overdoped compositions to 1.6 for x  =  0.131. This behavior is qualitatively similar to the doping evolution of the superconducting gap anisotropy in other iron pnictides, including hole-doped (Ba,K)Fe 2 As 2 and 3d-electron-doped Ba(Fe,Co) 2 As 2 superconductors, finding a full gap near optimal doping and strong anisotropy toward the ends of the superconducting dome in the T-x phase diagram. The normalized superfluid density in an optimally Rh-doped sample is almost identical to the temperature-dependence in the optimally doped Ba(Fe,Co) 2 As 2 samples. Our study supports the universal superconducting gap variation with doping and [Formula: see text] pairing at least in iron based superconductors of the BaFe 2 As 2 family.

Universal doping evolution of the superconducting gap anisotropy in single crystals of electron-doped Ba(Fe 1–xRh x) 2As 2 from London penetration depth measurements

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Hyunsoo; Tanatar, M. A.; Martin, C.

Doping evolution of the superconducting gap anisotropy was studied in single crystals of 4d-electron doped Ba(Fe 1–xRh x) 2As 2 using tunnel diode resonator measurements of the temperature variation of the London penetration depth Δλ( T). Single crystals with doping levels representative of an underdoped regime x = 0.039 ( T c = 15.5 K), close to optimal doping x = 0.057 ( T c = 24.4 K) and overdoped x = 0.079 ( T c = 21.5 K) and x = 0.131( T c = 4.9 K) were studied. Superconducting energy gap anisotropy was characterized by the exponent, n,more » by fitting the data to the power-law, Δλ = AT n. The exponent n varies non-monotonically with x, increasing to a maximum n = 2.5 for x = 0.079 and rapidly decreasing towards overdoped compositions to 1.6 for x = 0.131. This behavior is qualitatively similar to the doping evolution of the superconducting gap anisotropy in other iron pnictides, including hole-doped (Ba,K)Fe 2As 2 and 3d-electron-doped Ba(Fe,Co) 2As 2 superconductors, finding a full gap near optimal doping and strong anisotropy toward the ends of the superconducting dome in the T-x phase diagram. The normalized superfluid density in an optimally Rh-doped sample is almost identical to the temperature-dependence in the optimally doped Ba(Fe,Co) 2As 2 samples. In conclusion, our study supports the universal superconducting gap variation with doping and pairing at least in iron based superconductors of the BaFe 2As 2 family.« less

Universal doping evolution of the superconducting gap anisotropy in single crystals of electron-doped Ba(Fe 1–xRh x) 2As 2 from London penetration depth measurements

DOE PAGES

Kim, Hyunsoo; Tanatar, M. A.; Martin, C.; ...

2018-05-08

Doping evolution of the superconducting gap anisotropy was studied in single crystals of 4d-electron doped Ba(Fe 1–xRh x) 2As 2 using tunnel diode resonator measurements of the temperature variation of the London penetration depth Δλ( T). Single crystals with doping levels representative of an underdoped regime x = 0.039 ( T c = 15.5 K), close to optimal doping x = 0.057 ( T c = 24.4 K) and overdoped x = 0.079 ( T c = 21.5 K) and x = 0.131( T c = 4.9 K) were studied. Superconducting energy gap anisotropy was characterized by the exponent, n,more » by fitting the data to the power-law, Δλ = AT n. The exponent n varies non-monotonically with x, increasing to a maximum n = 2.5 for x = 0.079 and rapidly decreasing towards overdoped compositions to 1.6 for x = 0.131. This behavior is qualitatively similar to the doping evolution of the superconducting gap anisotropy in other iron pnictides, including hole-doped (Ba,K)Fe 2As 2 and 3d-electron-doped Ba(Fe,Co) 2As 2 superconductors, finding a full gap near optimal doping and strong anisotropy toward the ends of the superconducting dome in the T-x phase diagram. The normalized superfluid density in an optimally Rh-doped sample is almost identical to the temperature-dependence in the optimally doped Ba(Fe,Co) 2As 2 samples. In conclusion, our study supports the universal superconducting gap variation with doping and pairing at least in iron based superconductors of the BaFe 2As 2 family.« less

Self-assembled antireflection coatings for light trapping based on SiGe random metasurfaces

NASA Astrophysics Data System (ADS)

Bouabdellaoui, Mohammed; Checcucci, Simona; Wood, Thomas; Naffouti, Meher; Sena, Robert Paria; Liu, Kailang; Ruiz, Carmen M.; Duche, David; le Rouzo, Judikael; Escoubas, Ludovic; Berginc, Gerard; Bonod, Nicolas; Zazoui, Mimoun; Favre, Luc; Metayer, Leo; Ronda, Antoine; Berbezier, Isabelle; Grosso, David; Gurioli, Massimo; Abbarchi, Marco

2018-03-01

We demonstrate a simple self-assembly method based on solid state dewetting of ultrathin silicon films and germanium deposition for the fabrication of efficient antireflection coatings on silicon for light trapping. We fabricate SiGe islands with a high surface density, randomly positioned and broadly varied in size. This allows one to reduce the reflectance to low values in a broad spectral range (from 500 nm to 2500 nm) and a broad angle (up to 55°) and to trap within the wafer a large portion of the impinging light (˜40 % ) also below the band gap, where the Si substrate is nonabsorbing. Theoretical simulations agree with the experimental results, showing that the efficient light coupling into the substrate is mediated by Mie resonances formed within the SiGe islands. This lithography-free method can be implemented on arbitrarily thick or thin SiO2 layers and its duration only depends on the sample thickness and on the annealing temperature.

Ultra-low Thermal Conductivity in Si/Ge Hierarchical Superlattice Nanowire.

PubMed

Mu, Xin; Wang, Lili; Yang, Xueming; Zhang, Pu; To, Albert C; Luo, Tengfei

2015-11-16

Due to interfacial phonon scattering and nanoscale size effect, silicon/germanium (Si/Ge) superlattice nanowire (SNW) can have very low thermal conductivity, which is very attractive for thermoelectrics. In this paper, we demonstrate using molecular dynamics simulations that the already low thermal conductivity of Si/Ge SNW can be further reduced by introducing hierarchical structure to form Si/Ge hierarchical superlattice nanowire (H-SNW). The structural hierarchy introduces defects to disrupt the periodicity of regular SNW and scatters coherent phonons, which are the key contributors to thermal transport in regular SNW. Our simulation results show that periodically arranged defects in Si/Ge H-SNW lead to a ~38% reduction of the already low thermal conductivity of regular Si/Ge SNW. By randomizing the arrangement of defects and imposing additional surface complexities to enhance phonon scattering, further reduction in thermal conductivity can be achieved. Compared to pure Si nanowire, the thermal conductivity reduction of Si/Ge H-SNW can be as large as ~95%. It is concluded that the hierarchical structuring is an effective way of reducing thermal conductivity significantly in SNW, which can be a promising path for improving the efficiency of Si/Ge-based SNW thermoelectrics.

Ultra-low Thermal Conductivity in Si/Ge Hierarchical Superlattice Nanowire

PubMed Central

Mu, Xin; Wang, Lili; Yang, Xueming; Zhang, Pu; To, Albert C.; Luo, Tengfei

2015-01-01

Due to interfacial phonon scattering and nanoscale size effect, silicon/germanium (Si/Ge) superlattice nanowire (SNW) can have very low thermal conductivity, which is very attractive for thermoelectrics. In this paper, we demonstrate using molecular dynamics simulations that the already low thermal conductivity of Si/Ge SNW can be further reduced by introducing hierarchical structure to form Si/Ge hierarchical superlattice nanowire (H-SNW). The structural hierarchy introduces defects to disrupt the periodicity of regular SNW and scatters coherent phonons, which are the key contributors to thermal transport in regular SNW. Our simulation results show that periodically arranged defects in Si/Ge H-SNW lead to a ~38% reduction of the already low thermal conductivity of regular Si/Ge SNW. By randomizing the arrangement of defects and imposing additional surface complexities to enhance phonon scattering, further reduction in thermal conductivity can be achieved. Compared to pure Si nanowire, the thermal conductivity reduction of Si/Ge H-SNW can be as large as ~95%. It is concluded that the hierarchical structuring is an effective way of reducing thermal conductivity significantly in SNW, which can be a promising path for improving the efficiency of Si/Ge-based SNW thermoelectrics. PMID:26568511

Multiple doping of silicon-germanium alloys for thermoelectric applications

NASA Technical Reports Server (NTRS)

Fleurial, Jean-Pierre; Vining, Cronin B.; Borshchevsky, Alex

1989-01-01

It is shown that heavy doping of n-type Si/Ge alloys with phosphorus and arsenic (V-V doping interaction) by diffusion leads to a significant enhancement of their carrier concentration and possible improvement of the thermoelectric figure of merit. High carrier concentrations were achieved by arsenic doping alone, but for a same doping level higher carrier mobilities and lower resistivities are obtained through phosphorus doping. By combining the two dopants with the proper diffusion treatments, it was possible to optimize the different properties, obtaining high carrier concentration, good carrier mobility and low electrical resistivity. Similar experiments, using the III-V doping interaction, were conducted on boron-doped p-type samples and showed the possibility of overcompensating the samples by diffusing arsenic, in order to get n-type behavior.

Characterisation of diode-connected SiGe BiCMOS HBTs for space applications

NASA Astrophysics Data System (ADS)

Venter, Johan; Sinha, Saurabh; Lambrechts, Wynand

2016-02-01

Silicon-germanium (SiGe) bipolar complementary metal-oxide semiconductor (BiCMOS) transistors have vertical doping profiles reaching deeper into the substrate when compared to lateral CMOS transistors. Apart from benefiting from high-speed, high current gain and low-output resistance due to its vertical profile, BiCMOS technology is increasingly becoming a preferred technology for researchers to realise next-generation space-based optoelectronic applications. BiCMOS transistors have inherent radiation hardening, to an extent predictable cryogenic performance and monolithic integration potential. SiGe BiCMOS transistors and p-n junction diodes have been researched and used as a primary active component for over the last two decades. However, further research can be conducted with diode-connected heterojunction bipolar transistors (HBTs) operating at cryogenic temperatures. This work investigates these characteristics and models devices by adapting standard fabrication technology components. This work focuses on measurements of the current-voltage relationship (I-V curves) and capacitance-voltage relationships (C-V curves) of diode-connected HBTs. One configuration is proposed and measured, which is emitterbase shorted. The I-V curves are measured for various temperature points ranging from room temperature (300 K) to the temperature of liquid nitrogen (77 K). The measured datasets are used to extract a model of the formed diode operating at cryogenic temperatures and used as a standard library component in computer aided software designs. The advantage of having broad-range temperature models of SiGe transistors becomes apparent when considering implementation of application-specific integrated circuits and silicon-based infrared radiation photodetectors on a single wafer, thus shortening interconnects and lowering parasitic interference, decreasing the overall die size and improving on overall cost-effectiveness. Primary applications include space-based geothermal

The reliability studies of nano-engineered SiGe HBTs using Pelletron accelerator

NASA Astrophysics Data System (ADS)

Prakash, A. P. Gnana; Praveen, K. C.; Pushpa, N.; Cressler, John D.

2015-05-01

The effects of high energy ions on the electrical characteristics of silicon-germanium heterojunction bipolar transistors (SiGe HBTs) were studied in the total dose of ranging from 600 krad to 100 Mrad (Si). The two generations (50 GHz and 200 GHz) of SiGe HBTs were exposed to 50 MeV lithium, 75 MeV boron and 100 MeV oxygen ions. The electrical characteristics of SiGe HBTs were studied before and after irradiation. The SiGe HBTs were exposed to 60Co gamma radiation in the same total dose. The results are systematically compared in order to understand the interaction of ions and ionizing radiation with SiGe HBTs.

Doping induced carrier and band-gap modulation in bulk versus nano for topological insulators: A test case of Stibnite

NASA Astrophysics Data System (ADS)

Maji, Tuhin Kumar; Pal, Samir Kumar; Karmakar, Debjani

2018-04-01

We aim at comparing the electronic properties of topological insulator Sb2S3 in bulk and Nanorod using density-functional scheme and investigating the effects of Se-doping at chalcogen-site. While going from bulk to nano, there is a drastic change in the band gap due to surface-induced strain. However, the trend of band gap modulation with increased Se doping is more prominent in bulk. Interestingly, Se-doping introduces different type of carriers in bulk and nano.

Surface tension and density of Si-Ge melts

NASA Astrophysics Data System (ADS)

Ricci, Enrica; Amore, Stefano; Giuranno, Donatella; Novakovic, Rada; Tuissi, Ausonio; Sobczak, Natalia; Nowak, Rafal; Korpala, Bartłomiej; Bruzda, Grzegorz

2014-06-01

In this work, the surface tension and density of Si-Ge liquid alloys were determined by the pendant drop method. Over the range of measurements, both properties show a linear temperature dependence and a nonlinear concentration dependence. Indeed, the density decreases with increasing silicon content exhibiting positive deviation from ideality, while the surface tension increases and deviates negatively with respect to the ideal solution model. Taking into account the Si-Ge phase diagram, a simple lens type, the surface tension behavior of the Si-Ge liquid alloys was analyzed in the framework of the Quasi-Chemical Approximation for the Regular Solutions model. The new experimental results were compared with a few data available in the literature, obtained by the containerless method.

Thermoelectric properties of hot-pressed fine particulate powder SiGe alloys

NASA Technical Reports Server (NTRS)

Beaty, John S.; Rolfe, Jonathan; Vandersande, Jan

1991-01-01

A novel material system and its fabrication technique have been defined and applied to the production of SiGe thermoelectric material through the hot pressing of 50-100 A ultrafine particulates into 80/20 SiGe. Relative to conventionally processed SiGe, a reduction of thermal conductivity of up to 40 percent is achieved in conjunction with an enhancement of material figure-of-merit of the order of 10-15 percent.

Band-gap bowing and p-type doping of (Zn, Mg, Be)O wide-gap semiconductor alloys: a first-principles study

NASA Astrophysics Data System (ADS)

Shi, H.-L.; Duan, Y.

2008-12-01

Using a first-principles band-structure method and a special quasirandom structure (SQS) approach, we systematically calculate the band gap bowing parameters and p-type doping properties of (Zn, Mg, Be)O related random ternary and quaternary alloys. We show that the bowing parameters for ZnBeO and MgBeO alloys are large and dependent on composition. This is due to the size difference and chemical mismatch between Be and Zn(Mg) atoms. We also demonstrate that adding a small amount of Be into MgO reduces the band gap indicating that the bowing parameter is larger than the band-gap difference. We select an ideal N atom with lower p atomic energy level as dopant to perform p-type doping of ZnBeO and ZnMgBeO alloys. For N doped in ZnBeO alloy, we show that the acceptor transition energies become shallower as the number of the nearest neighbor Be atoms increases. This is thought to be because of the reduction of p- d repulsion. The NO acceptor transition energies are deep in the ZnMgBeO quaternary alloy lattice-matched to GaN substrate due to the lower valence band maximum. These decrease slightly as there are more nearest neighbor Mg atoms surrounding the N dopant. The important natural valence band alignment between ZnO, MgO, BeO, ZnBeO, and ZnMgBeO quaternary alloy is also investigated.

Cross-sectional aspect ratio modulated electronic properties in Si/Ge core/shell nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Nuo; Lu, Ning; Yao, Yong-Xin

2013-02-28

Electronic structures of (4, n) and (m, 4) (the NW has m layers parallel to the {1 1 1} facet and n layers parallel to {1 1 0}) Si/Ge core/shell nanowires (NWs) along the [1 1 2] direction with cross-sectional aspect ratio (m/n) from 0.36 to 2.25 are studied by first-principles calculations. An indirect to direct band gap transition is observed as m/n decreases, and the critical values of m/n and diameter for the transition are also estimated. The size of the band gap also depends on the aspect ratio. These results suggest that m/n plays an important role inmore » modulating the electronic properties of the NWs.« less

Density-functional theory molecular dynamics simulations of a-HfO2/a-SiO2/SiGe and a-HfO2/a-SiO2/Ge with a-SiO2 and a-SiO suboxide interfacial layers

NASA Astrophysics Data System (ADS)

Chagarov, Evgueni A.; Kavrik, Mahmut S.; Fang, Ziwei; Tsai, Wilman; Kummel, Andrew C.

2018-06-01

Comprehensive Density-Functional Theory (DFT) Molecular Dynamics (MD) simulations were performed to investigate interfaces between a-HfO2 and SiGe or Ge semiconductors with fully-stoichiometric a-SiO2 or sub-oxide SiO interlayers. The electronic structure of the selected stacks was calculated with a HSE06 hybrid functional. Simulations were performed before and after hydrogen passivation of residual interlayer defects. For the SiGe substrate with Ge termination prior to H passivation, the stacks with a-SiO suboxide interlayer (a-HfO2/a-SiO/SiGe) demonstrate superior electronic properties and wider band-gaps than the stacks with fully coordinated a-SiO2 interlayers (a-HfO2/a-SiO2/SiGe). After H passivation, most of the a-HfO2/a-SiO2/SiGe defects are passivated. To investigate effect of random placement of Si and Ge atoms additional simulations with a randomized SiGe slab were performed demonstrating improvement of electronic structure. For Ge substrates, before H passivation, the stacks with a SiO suboxide interlayer (a-HfO2/a-SiO/Ge) also demonstrate wider band-gaps than the stacks with fully coordinated a-SiO2 interlayers (a-HfO2/a-SiO2/Ge). However, even for a-HfO2/a-SiO/Ge, the Fermi level is shifted close to the conduction band edge (CBM) consistent with Fermi level pinning. Again, after H passivation, most of the a-HfO2/a-SiO2/Ge defects are passivated. The stacks with fully coordinated a-SiO2 interlayers have much stronger deformation and irregularity in the semiconductor (SiGe or Ge) upper layers leading to multiple under-coordinated atoms which create band-edge states and decrease the band-gap prior to H passivation.

Doping-dependent anisotropic superconducting gap in Na1-δ(Fe1-xCox)As from London penetration depth

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cho, Kyuil; Tanatar, Makariy A.; Spyrison, Nicholas

2012-07-30

The London penetration depth was measured in single crystals of self-doped Na1-δFeAs (from under doping to optimal doping, Tc from 14 to 27 K) and electron-doped Na(Fe1-xCox)As with x ranging from undoped, x=0, to overdoped, x=0.1. In all samples, the low-temperature variation of the penetration depth exhibits a power-law dependence, Δλ(T)=ATn, with the exponent that varies in a domelike fashion from n˜1.1 in the underdoped, reaching a maximum of n˜1.9 in the optimally doped, and decreasing again to n˜1.3 on the overdoped side. While the anisotropy of the gap structure follows a universal domelike evolution, the exponent at optimal doping,more » n˜1.9, is lower than in other charge-doped Fe-based superconductors (FeSCs). The full-temperature range superfluid density, ρs(T)=λ(0)/λ(T)2, at optimal doping is also distinctly different from other charge-doped FeSCs but is similar to isovalently substituted BaFe2(As1-xPx)2, believed to be a nodal pnictide at optimal doping. These results suggest that the superconducting gap in Na(Fe1-xCox)As is highly anisotropic even at optimal doping.« less

Low Temperature Rhombohedral Single Crystal SiGe Epitaxy on c-plane Sapphire

NASA Technical Reports Server (NTRS)

Duzik, Adam J.; Choi, Sang H.

2016-01-01

Current best practice in epitaxial growth of rhombohedral SiGe onto (0001) sapphire (Al2O3) substrate surfaces requires extreme conditions to grow a single crystal SiGe film. Previous models described the sapphire surface reconstruction as the overriding factor in rhombohedral epitaxy, requiring a high temperature Al-terminated surface for high quality films. Temperatures in the 850-1100 C range were thought to be necessary to get SiGe to form coherent atomic matching between the (111) SiGe plane and the (0001) sapphire surface. Such fabrication conditions are difficult and uneconomical, hindering widespread application. This work proposes an alternative model that considers the bulk sapphire structure and determines how the SiGe film nucleates and grows. Accounting for thermal expansion effects, calculations using this new model show that both pure Ge and SiGe can form single crystal films in the 450-550 C temperature range. Experimental results confirm these predictions, where x-ray diffraction and atomic force microscopy show the films fabricated at low temperature rival the high temperature films in crystallographic and surface quality. Finally, an explanation is provided for why films of comparable high quality can be produced in either temperature range.

Opening the band gap of graphene through silicon doping for the improved performance of graphene/GaAs heterojunction solar cells.

PubMed

Zhang, S J; Lin, S S; Li, X Q; Liu, X Y; Wu, H A; Xu, W L; Wang, P; Wu, Z Q; Zhong, H K; Xu, Z J

2016-01-07

Graphene has attracted increasing interest due to its remarkable properties. However, the zero band gap of monolayered graphene limits it's further electronic and optoelectronic applications. Herein, we have synthesized monolayered silicon-doped graphene (SiG) with large surface area using a chemical vapor deposition method. Raman and X-ray photoelectron spectroscopy measurements demonstrate that the silicon atoms are doped into graphene lattice at a doping level of 2.7-4.5 at%. Electrical measurements based on a field effect transistor indicate that the band gap of graphene has been opened via silicon doping without a clear degradation in carrier mobility, and the work function of SiG, deduced from ultraviolet photoelectron spectroscopy, was 0.13-0.25 eV larger than that of graphene. Moreover, when compared with the graphene/GaAs heterostructure, SiG/GaAs exhibits an enhanced performance. The performance of 3.4% silicon doped SiG/GaAs solar cell has been improved by 33.7% on average, which was attributed to the increased barrier height and improved interface quality. Our results suggest that silicon doping can effectively engineer the band gap of monolayered graphene and SiG has great potential in optoelectronic device applications.

Reduction in the Band Gap of Manganese-Doped Zinc Oxide: Role of the Oxidation State

NASA Astrophysics Data System (ADS)

Sharma, Sonia; Ramesh, Pranith; Swaminathan, P.

2015-12-01

Manganese-doped zinc oxide powders were synthesized by solid state reaction of the respective oxides. The high-temperature conditions were chosen such that multiple valence states of manganese were doped in the host zinc oxide lattice. Structural characterization was carried out to confirm the doping and to find the maximum amount of manganese that can be incorporated. Diffuse reflectance spectroscopy was used to measure the optical band gap of the doped sample and the lowering with respect to pure ZnO was attributed to the presence of higher oxidation states of manganese. The presence of these oxidation states was confirmed using x-ray photoelectron spectroscopy. The study shows that a solid state reaction is a viable route for synthesizing doped metal oxides with desired optical properties.

Single-Gap Superconductivity and Dome of Superfluid Density in Nb-Doped SrTiO 3

NASA Astrophysics Data System (ADS)

Thiemann, Markus; Beutel, Manfred H.; Dressel, Martin; Lee-Hone, Nicholas R.; Broun, David M.; Fillis-Tsirakis, Evangelos; Boschker, Hans; Mannhart, Jochen; Scheffler, Marc

2018-06-01

SrTiO3 exhibits a superconducting dome upon doping with Nb, with a maximum critical temperature Tc≈0.4 K . Using microwave stripline resonators at frequencies from 2 to 23 GHz and temperatures down to 0.02 K, we probe the low-energy optical response of superconducting SrTiO3 with a charge carrier concentration from 0.3 to 2.2 ×1020 cm-3 , covering the majority of the superconducting dome. We find single-gap electrodynamics even though several electronic bands are superconducting. This is explained by a single energy gap 2 Δ due to gap homogenization over the Fermi surface consistent with the low level of defect scattering in Nb-doped SrTiO3 . Furthermore, we determine Tc, 2 Δ , and the superfluid density as a function of charge carrier concentration, and all three quantities exhibit the characteristic dome shape.

The effect of gap fluctuations on interacting and non-interacting polarization for nano-superconducting grains in electron- and hole-doped cuprates

NASA Astrophysics Data System (ADS)

Afzali, R.; Alizadeh, A.

2017-12-01

The behavior of non-interacting and interacting polarization under influence of fluctuations of the superconducting gap with D-wave symmetry and under consideration of the gap dependence on nano- grain size is obtained in terms of the frequency, temperature and the size at zero and finite temperatures for rectangular cuprate nano-superconducting grains. By using Eliashberg equations and applying the relations of the fermionic dispersion for the hole-doped and electron-doped cuprates, we numerically compute the real part of size-dependent polarization for both types of cuprates. We show that the peak of real part of polarization moves to higher frequency by including the additional fluctuating part of gap (or the nano-size effect). Also, we obtain the temperatures for different frequencies, in which the effect of gap fluctuations fades. In the case of size-dependent gap, there is a critical frequency; for frequencies lower (higher) than the critical frequency, the nano-effect weakens (improves) the superconducting state. Moreover, it is concluded that the real part of polarization for hole- doped cuprates in terms of the grain size has more significant amount in comparison with electron-doped ones.

Opening the band gap of graphene through silicon doping for the improved performance of graphene/GaAs heterojunction solar cells

NASA Astrophysics Data System (ADS)

Zhang, S. J.; Lin, S. S.; Li, X. Q.; Liu, X. Y.; Wu, H. A.; Xu, W. L.; Wang, P.; Wu, Z. Q.; Zhong, H. K.; Xu, Z. J.

2015-12-01

Graphene has attracted increasing interest due to its remarkable properties. However, the zero band gap of monolayered graphene limits it's further electronic and optoelectronic applications. Herein, we have synthesized monolayered silicon-doped graphene (SiG) with large surface area using a chemical vapor deposition method. Raman and X-ray photoelectron spectroscopy measurements demonstrate that the silicon atoms are doped into graphene lattice at a doping level of 2.7-4.5 at%. Electrical measurements based on a field effect transistor indicate that the band gap of graphene has been opened via silicon doping without a clear degradation in carrier mobility, and the work function of SiG, deduced from ultraviolet photoelectron spectroscopy, was 0.13-0.25 eV larger than that of graphene. Moreover, when compared with the graphene/GaAs heterostructure, SiG/GaAs exhibits an enhanced performance. The performance of 3.4% silicon doped SiG/GaAs solar cell has been improved by 33.7% on average, which was attributed to the increased barrier height and improved interface quality. Our results suggest that silicon doping can effectively engineer the band gap of monolayered graphene and SiG has great potential in optoelectronic device applications.Graphene has attracted increasing interest due to its remarkable properties. However, the zero band gap of monolayered graphene limits it's further electronic and optoelectronic applications. Herein, we have synthesized monolayered silicon-doped graphene (SiG) with large surface area using a chemical vapor deposition method. Raman and X-ray photoelectron spectroscopy measurements demonstrate that the silicon atoms are doped into graphene lattice at a doping level of 2.7-4.5 at%. Electrical measurements based on a field effect transistor indicate that the band gap of graphene has been opened via silicon doping without a clear degradation in carrier mobility, and the work function of SiG, deduced from ultraviolet photoelectron

Recent Results on SEU Hardening of SiGe HBT Logic Circuits

NASA Technical Reports Server (NTRS)

Krithivasan, Ramkumar; Marshall, Paul W.; Nayeem, Mustayeen; Sutton, Akil K.; Kuo, Wei-Min Lance; Haugerud, Becca M.; Najafizadeh, Laieh; Cressler, John D.; Carts, martin A.; Marshall, Cheryl J.

2006-01-01

A viewgraph presentation on SEU tolerant SiGe HBT technology is shown. The topics include: 1) Introduction; 2) TID and SEU in SiGe Technology; 3) RHBD Techniques; 4) Experiment; 5) Heavy-Ion Data and Analysis; and 6) Summary.

Photocatalytic hydrogen generation enhanced by band gap narrowing and improved charge carrier mobility in AgTaO3 by compensated co-doping.

PubMed

Li, Min; Zhang, Junying; Dang, Wenqiang; Cushing, Scott K; Guo, Dong; Wu, Nianqiang; Yin, Penggang

2013-10-14

The correlation of the electronic band structure with the photocatalytic activity of AgTaO3 has been studied by simulation and experiments. Doping wide band gap oxide semiconductors usually introduces discrete mid-gap states, which extends the light absorption but has limited benefit for photocatalytic activity. Density functional theory (DFT) calculations show that compensated co-doping in AgTaO3 can overcome this problem by increasing the light absorption and simultaneously improving the charge carrier mobility. N/H and N/F co-doping can delocalize the discrete mid-gap states created by sole N doping in AgTaO3, which increases the band curvature and the electron-to-hole effective mass ratio. In particular, N/F co-doping creates a continuum of states that extend the valence band of AgTaO3. N/F co-doping thus improves the light absorption without creating the mid-gap states, maintaining the necessary redox potentials for water splitting and preventing from charge carrier trapping. The experimental results have confirmed that the N/F-codoped AgTaO3 exhibits a red-shift of the absorption edge in comparison with the undoped AgTaO3, leading to remarkable enhancement of photocatalytic activity toward hydrogen generation from water.

Interface Engineering for Atomic Layer Deposited Alumina Gate Dielectric on SiGe Substrates.

PubMed

Zhang, Liangliang; Guo, Yuzheng; Hassan, Vinayak Vishwanath; Tang, Kechao; Foad, Majeed A; Woicik, Joseph C; Pianetta, Piero; Robertson, John; McIntyre, Paul C

2016-07-27

Optimization of the interface between high-k dielectrics and SiGe substrates is a challenging topic due to the complexity arising from the coexistence of Si and Ge interfacial oxides. Defective high-k/SiGe interfaces limit future applications of SiGe as a channel material for electronic devices. In this paper, we identify the surface layer structure of as-received SiGe and Al2O3/SiGe structures based on soft and hard X-ray photoelectron spectroscopy. As-received SiGe substrates have native SiOx/GeOx surface layers, where the GeOx-rich layer is beneath a SiOx-rich surface. Silicon oxide regrows on the SiGe surface during Al2O3 atomic layer deposition, and both SiOx and GeOx regrow during forming gas anneal in the presence of a Pt gate metal. The resulting mixed SiOx-GeOx interface layer causes large interface trap densities (Dit) due to distorted Ge-O bonds across the interface. In contrast, we observe that oxygen-scavenging Al top gates decompose the underlying SiOx/GeOx, in a selective fashion, leaving an ultrathin SiOx interfacial layer that exhibits dramatically reduced Dit.

A simplified approach to the band gap correction of defect formation energies: Al, Ga, and In-doped ZnO

NASA Astrophysics Data System (ADS)

Saniz, R.; Xu, Y.; Matsubara, M.; Amini, M. N.; Dixit, H.; Lamoen, D.; Partoens, B.

2013-01-01

The calculation of defect levels in semiconductors within a density functional theory approach suffers greatly from the band gap problem. We propose a band gap correction scheme that is based on the separation of energy differences in electron addition and relaxation energies. We show that it can predict defect levels with a reasonable accuracy, particularly in the case of defects with conduction band character, and yet is simple and computationally economical. We apply this method to ZnO doped with group III elements (Al, Ga, In). As expected from experiment, the results indicate that Zn substitutional doping is preferred over interstitial doping in Al, Ga, and In-doped ZnO, under both zinc-rich and oxygen-rich conditions. Further, all three dopants act as shallow donors, with the +1 charge state having the most advantageous formation energy. Also, doping effects on the electronic structure of ZnO are sufficiently mild so as to affect little the fundamental band gap and lowest conduction bands dispersion, which secures their n-type transparent conducting behavior. A comparison with the extrapolation method based on LDA+U calculations and with the Heyd-Scuseria-Ernzerhof hybrid functional (HSE) shows the reliability of the proposed scheme in predicting the thermodynamic transition levels in shallow donor systems.

Band gap opening of bilayer graphene by F4-TCNQ molecular doping and externally applied electric field.

PubMed

Tian, Xiaoqing; Xu, Jianbin; Wang, Xiaomu

2010-09-09

The band gap opening of bilayer graphene with one side surface adsorption of F4-TCNQ is reported. F4-TCNQ doped bilayer graphene shows p-type semiconductor characteristics. With a F4-TCNQ concentration of 1.3 x 10(-10) mol/cm(2), the charge transfer between each F4-TCNQ molecule and graphene is 0.45e, and the built-in electric field, E(bi), between the graphene layers could reach 0.070 V/A. The charge transfer and band gap opening of the F4-TCNQ-doped graphene can be further modulated by an externally applied electric field (E(ext)). At 0.077 V/A, the gap opening at the Dirac point (K), DeltaE(K) = 306 meV, and the band gap, E(g) = 253 meV, are around 71% and 49% larger than those of the pristine bilayer under the same E(ext).

Proton Tolerance of SiGe Precision Voltage References for Extreme Temperature Range Electronics

NASA Astrophysics Data System (ADS)

Najafizadeh, Laleh; Bellini, Marco; Prakash, A. P. Gnana; Espinel, Gustavo A.; Cressler, John D.; Marshall, Paul W.; Marshall, Cheryl J.

2006-12-01

A comprehensive investigation of the effects of proton irradiation on the performance of SiGe BiCMOS precision voltage references intended for extreme environment operational conditions is presented. The voltage reference circuits were designed in two distinct SiGe BiCMOS technology platforms (first generation (50 GHz) and third generation (200 GHz)) in order to investigate the effect of technology scaling. The circuits were irradiated at both room temperature and at 77 K. Measurement results from the experiments indicate that the proton-induced changes in the SiGe bandgap references are minor, even down to cryogenic temperatures, clearly good news for the potential application of SiGe mixed-signal circuits in emerging extreme environments

Fully gapped superconductivity in In-doped topological crystalline insulator Pb 0.5Sn 0.5Te

DOE PAGES

Du, Guan; Gu, G. D.; Du, Zengyi; ...

2015-07-27

In this study, superconductors derived from topological insulators and topological crystalline insulators by chemical doping have long been considered to be candidates as topological superconductors. Pb 0.5Sn 0.5Te is a topological crystalline insulator with mirror symmetry protected surface states on (001)-, (011)-, and (111)-oriented surfaces. The superconductor (Pb 0.5Sn 0.5) 0.7In 0.3Te is produced by In doping in Pb 0.5Sn 0.5Te, and is thought to be a topological superconductor. Here we report scanning tunneling spectroscopy measurements of the superconducting state as well as the superconducting energy gap in (Pb 0.5Sn 0.5) 0.7In 0.3Te on a (001)-oriented surface. The spectrum canmore » be well fitted by an anisotropic s-wave gap function of Δ = 0.72 + 0.18cos4θ meV using Dynes model. The results show that the superconductor seems to be a fully gapped one without any in-gap states, in contradiction with the expectation of a topological superconductor.« less

Oxidation kinetics of Si and SiGe by dry rapid thermal oxidation, in-situ steam generation oxidation and dry furnace oxidation

NASA Astrophysics Data System (ADS)

Rozé, Fabien; Gourhant, Olivier; Blanquet, Elisabeth; Bertin, François; Juhel, Marc; Abbate, Francesco; Pribat, Clément; Duru, Romain

2017-06-01

The fabrication of ultrathin compressively strained SiGe-On-Insulator layers by the condensation technique is likely a key milestone towards low-power and high performances FD-SOI logic devices. However, the SiGe condensation technique still requires challenges to be solved for an optimized use in an industrial environment. SiGe oxidation kinetics, upon which the condensation technique is founded, has still not reached a consensus in spite of various studies which gave insights into the matter. This paper aims to bridge the gaps between these studies by covering various oxidation processes relevant to today's technological needs with a new and quantitative analysis methodology. We thus address oxidation kinetics of SiGe with three Ge concentrations (0%, 10%, and 30%) by means of dry rapid thermal oxidation, in-situ steam generation oxidation, and dry furnace oxidation. Oxide thicknesses in the 50 Å to 150 Å range grown with oxidation temperatures between 850 and 1100 °C were targeted. The present work shows first that for all investigated processes, oxidation follows a parabolic regime even for thin oxides, which indicates a diffusion-limited oxidation regime. We also observe that, for all investigated processes, the SiGe oxidation rate is systematically higher than that of Si. The amplitude of the variation of oxidation kinetics of SiGe with respect to Si is found to be strongly dependent on the process type. Second, a new quantitative analysis methodology of oxidation kinetics is introduced. This methodology allows us to highlight the dependence of oxidation kinetics on the Ge concentration at the oxidation interface, which is modulated by the pile-up mechanism. Our results show that the oxidation rate increases with the Ge concentration at the oxidation interface.

SiGe Integrated Circuit Developments for SQUID/TES Readout

NASA Astrophysics Data System (ADS)

Prêle, D.; Voisin, F.; Beillimaz, C.; Chen, S.; Piat, M.; Goldwurm, A.; Laurent, P.

2018-03-01

SiGe integrated circuits dedicated to the readout of superconducting bolometer arrays for astrophysics have been developed since more than 10 years at APC. Whether for Cosmic Microwave Background (CMB) observations with the QUBIC ground-based experiment (Aumont et al. in astro-ph.IM, 2016. arXiv:1609.04372) or for the Hot and Energetic Universe science theme with the X-IFU instrument on-board of the ATHENA space mission (Barret et al. in SPIE 9905, space telescopes & instrumentation 2016: UV to γ Ray, 2016. https://doi.org/10.1117/12.2232432), several kinds of Transition Edge Sensor (TES) (Irwin and Hilton, in ENSS (ed) Cryogenic particle detection, Springer, Berlin, 2005) arrays have been investigated. To readout such superconducting detector arrays, we use time or frequency domain multiplexers (TDM, FDM) (Prêle in JINST 10:C08015, 2016. https://doi.org/10.1088/1748-0221/10/08/C08015) with Superconducting QUantum Interference Devices (SQUID). In addition to the SQUID devices, low-noise biasing and amplification are needed. These last functions can be obtained by using BiCMOS SiGe technology in an Application Specific Integrated Circuit (ASIC). ASIC technology allows integration of highly optimised circuits specifically designed for a unique application. Moreover, we could reach very low-noise and wide band amplification using SiGe bipolar transistor either at room or cryogenic temperatures (Cressler in J Phys IV 04(C6):C6-101, 1994. https://doi.org/10.1051/jp4:1994616). This paper discusses the use of SiGe integrated circuits for SQUID/TES readout and gives an update of the last developments dedicated to the QUBIC telescope and to the X-IFU instrument. Both ASIC called SQmux128 and AwaXe are described showing the interest of such SiGe technology for SQUID multiplexer controls.

Energy-gap reduction in heavily doped silicon: Causes and consequences

NASA Astrophysics Data System (ADS)

Pantelides, Sokrates T.; Selloni, Annabella; Car, Roberto

1985-02-01

The authors review briefly the existing theoretical treatments of the various effects that contribute to the reduction of the energy gap in heavily doped Si, namely electron-electron and electron-impurity interactions and the effect of disorder in the impurity distribution. They then turn to the longstanding question why energy-gap reductions extracted from three different types of experiments have persistently produced values with substantial discrepancies, making it impossible to compare with theoretical values. First, they demonstrate that a meaningful comparison between theory and experiment can indeed be made if theoretical calculations are carried out for actual quantities that experiments measure, e.g. luminescence spectra, as recently done by Selloni and Pantelides. Then, they demonstrate that, independent of any theoretical calculations, the optical absorption spectra are fully consistent with the luminescence spectra and that the discrepancies in the energy-gap reductions extracted from the two sets of spectra are caused entirely by the curve-fitting procedures used in analyzing optical-absorption data. Finally, they show explicitly that, as already believed by many authors, energy-gap reductions extracted from electrical measurements on transistors do not correspond to true gap reductions. They identify two corrections that must be added to the values extracted from the electrical data in order to arrive at the true gap reductions and show that the resulting values are in good overall agreement with luminescence and absorption data. They, therefore, demonstrate that the observed reduction in emitter injection efficiency in bipolar transistors is not strictly due to a gap reduction, as generally believed, but to three very different effects.

Magnetic engineering in InSe/black-phosphorus heterostructure by transition-metal-atom Sc-Zn doping in the van der Waals gap

NASA Astrophysics Data System (ADS)

Ding, Yi-min; Shi, Jun-jie; Zhang, Min; Zhu, Yao-hui; Wu, Meng; Wang, Hui; Cen, Yu-lang; Guo, Wen-hui; Pan, Shu-hang

2018-07-01

Within the framework of the spin-polarized density-functional theory, we have studied the electronic and magnetic properties of InSe/black-phosphorus (BP) heterostructure doped with 3d transition-metal (TM) atoms from Sc to Zn. The calculated binding energies show that TM-atom doping in the van der Waals (vdW) gap of InSe/BP heterostructure is energetically favorable. Our results indicate that magnetic moments are induced in the Sc-, Ti-, V-, Cr-, Mn- and Co-doped InSe/BP heterostructures due to the existence of non-bonding 3d electrons. The Ni-, Cu- and Zn-doped InSe/BP heterostructures still show nonmagnetic semiconductor characteristics. Furthermore, in the Fe-doped InSe/BP heterostructure, the half-metal property is found and a high spin polarization of 100% at the Fermi level is achieved. The Cr-doped InSe/BP has the largest magnetic moment of 4.9 μB. The Sc-, Ti-, V-, Cr- and Mn-doped InSe/BP heterostructures exhibit antiferromagnetic ground state. Moreover, the Fe- and Co-doped systems display a weak ferromagnetic and paramagnetic coupling, respectively. Our studies demonstrate that the TM doping in the vdW gap of InSe/BP heterostructure is an effective way to modify its electronic and magnetic properties.

SiGe Based Low Temperature Electronics for Lunar Surface Applications

NASA Technical Reports Server (NTRS)

Mojarradi, Mohammad M.; Kolawa, Elizabeth; Blalock, Benjamin; Cressler, John

2012-01-01

The temperature at the permanently shadowed regions of the moon's surface is approximately -240 C. Other areas of the lunar surface experience temperatures that vary between 120 C and -180 C during the day and night respectively. To protect against the large temperature variations of the moon surface, traditional electronics used in lunar robotics systems are placed inside a thermally controlled housing which is bulky, consumes power and adds complexity to the integration and test. SiGe Based electronics have the capability to operate over wide temperature range like that of the lunar surface. Deploying low temperature SiGe electronics in a lander platform can minimize the need for the central thermal protection system and enable the development of a new generation of landers and mobility platforms with highly efficient distributed architecture. For the past five years a team consisting of NASA, university and industry researchers has been examining the low temperature and wide temperature characteristic of SiGe based transistors for developing electronics for wide temperature needs of NASA environments such as the Moon, Titan, Mars and Europa. This presentation reports on the status of the development of wide temperature SiGe based electronics for the landers and lunar surface mobility systems.

Optical band gap in a cholesteric elastomer doped by metallic nanospheres

NASA Astrophysics Data System (ADS)

Hernández, Julio C.; Reyes, J. Adrián

2017-12-01

We analyzed the optical band gaps for axially propagating electromagnetic waves throughout a metallic doped cholesteric elastomer. The composed medium is made of metallic nanospheres (silver) randomly dispersed in a cholesteric elastomer liquid crystal whose dielectric properties can be represented by a resonant effective uniaxial tensor. We found that the band gap properties of the periodic system greatly depend on the volume fraction of nanoparticles in the cholesteric elastomer. In particular, we observed a displacement of the reflection band for quite small fraction volumes whereas for larger values of this fraction there appears a secondary band in the higher frequency region. We also have calculated the transmittance and reflectance spectra for our system. These calculations verify the mentioned band structure and provide additional information about the polarization features of the radiation.

Bending and buckling of rolled-up SiGe /Si microtubes using nanorobotic manipulation

NASA Astrophysics Data System (ADS)

Zhang, Li; Dong, Lixin; Nelson, Bradley J.

2008-06-01

Mechanical properties of individual rolled-up SiGe /Si microtubes are investigated experimentally using nanorobotic manipulation. By applying bending loads, individual SiGe /Si microtubes demonstrate various deformation modes with increasing bending angle. Remarkably, the tested microtubes resist fracture even when bent back onto themselves (180° bending angle). Axial compression tests of microtubes with different turns are also performed. Among those tubes, 1.6-turn rolled-up SiGe /Si microtubes show typical Euler buckling behavior when the load is larger than a critical load, which can be estimated by the Euler formula for columns.

An investigation of the effects of radiation exposure on stability constraints in epitaxial SiGe strained layers

NASA Astrophysics Data System (ADS)

Chen, Tianbing; Sutton, Akil K.; Haugerud, Becca M.; Henderson, Walter; Gnana Prakash, A. P.; Cressler, John D.; Doolittle, Alan; Liu, Xuefeng; Joseph, Alvin; Marshall, Paul W.

2006-07-01

The thermodynamic stability of device-relevant epitaxial SiGe strained layers under proton irradiation is investigated using X-ray diffraction techniques, and compared with its stability constrain under high-temperature annealing. Irradiation with 63 MeV protons is found to introduce no significant microdefects into the SiGe thin films, regardless of starting stability condition of the SiGe film, and thus does not appear to be an issue for the use of SiGe HBT technology in emerging space systems. The strain relaxation of SiGe thin film under thermal annealing, however, is sensitive to the composition and thickness of the as-grown samples, as expected, with the subsequent lattice relaxation of the unstable samples occurring at a much higher rate than that of metastable samples.

SiGe layer thickness effect on the structural and optical properties of well-organized SiGe/SiO2 multilayers

NASA Astrophysics Data System (ADS)

Vieira, E. M. F.; Toudert, J.; Rolo, A. G.; Parisini, A.; Leitão, J. P.; Correia, M. R.; Franco, N.; Alves, E.; Chahboun, A.; Martín-Sánchez, J.; Serna, R.; Gomes, M. J. M.

2017-08-01

In this work, we report on the production of regular (SiGe/SiO2)20 multilayer structures by conventional RF-magnetron sputtering, at 350 °C. Transmission electron microscopy, scanning transmission electron microscopy, raman spectroscopy, and x-ray reflectometry measurements revealed that annealing at a temperature of 1000 °C leads to the formation of SiGe nanocrystals between SiO2 thin layers with good multilayer stability. Reducing the nominal SiGe layer thickness (t SiGe) from 3.5-2 nm results in a transition from continuous SiGe crystalline layer (t SiGe ˜ 3.5 nm) to layers consisting of isolated nanocrystals (t SiGe ˜ 2 nm). Namely, in the latter case, the presence of SiGe nanocrystals ˜3-8 nm in size, is observed. Spectroscopic ellipsometry was applied to determine the evolution of the onset in the effective optical absorption, as well as the dielectric function, in SiGe multilayers as a function of the SiGe thickness. A clear blue-shift in the optical absorption is observed for t SiGe ˜ 2 nm multilayer, as a consequence of the presence of isolated nanocrystals. Furthermore, the observed near infrared values of n = 2.8 and k = 1.5 are lower than those of bulk SiGe compounds, suggesting the presence of electronic confinement effects in the nanocrystals. The low temperature (70 K) photoluminescence measurements performed on annealed SiGe/SiO2 nanostructures show an emission band located between 0.7-0.9 eV associated with the development of interface states between the formed nanocrystals and surrounding amorphous matrix.

Quantification of local strain distributions in nanoscale strained SiGe FinFET structures

NASA Astrophysics Data System (ADS)

Mochizuki, Shogo; Murray, Conal E.; Madan, Anita; Pinto, Teresa; Wang, Yun-Yu; Li, Juntao; Weng, Weihao; Jagannathan, Hemanth; Imai, Yasuhiko; Kimura, Shigeru; Takeuchi, Shotaro; Sakai, Akira

2017-10-01

Strain within nanoscale strained SiGe FinFET structures has been investigated using a combination of X-ray diffraction and transmission electron microscopy-based nanobeam diffraction (NBD) techniques to reveal the evolution of the stress state within the FinFETs. Reciprocal space maps collected using high-resolution X-ray diffraction exhibited distinct features corresponding to the SiGe fin width, pitch, and lattice deformation and were analyzed to quantify the state of stress within the fins. Although the majority of the SiGe fin volume exhibited a uniaxial stress state due to elastic relaxation of the transverse in-plane stress, NBD measurements confirmed a small interaction region near the SOI interface that is mechanically constrained by the underlying substrate. We have quantitatively characterized the evolution of the fin stress state from biaxial to uniaxial as a function of fin aspect ratio and Ge fraction and confirmed that the fins obey elastic deformation based on a model that depends on the relative difference between the equilibrium Si and SiGe lattice constants and relative fraction of in-plane stress transverse to the SiGe fins. Spatially resolved, nanobeam X-ray diffraction measurements conducted near the SiGe fin edge indicate the presence of additional elastic relaxation from a uniaxial stress state to a fully relaxed state at the fin edge. Mapping of the lattice deformation within 500 nm of this fin edge by NBD revealed large gradients, particularly at the top corner of the fin. The values of the volume averaged lattice deformation obtained by nanoXRD and NBD are qualitatively consistent. Furthermore, the modulation of strain at the fin edge obtained by quantitative analysis of the nanoXRD results agrees with the lattice deformation profile obtained by NBD.

High Mobility Transport Layer Structures for Rhombohedral Si/Ge/SiGe Devices

NASA Technical Reports Server (NTRS)

Choi, Sang Hyouk (Inventor); Park, Yeonjoon (Inventor); King, Glen C. (Inventor); Kim, Hyun-Jung (Inventor); Lee, Kunik (Inventor)

2017-01-01

An electronic device includes a trigonal crystal substrate defining a (0001) C-plane. The substrate may comprise Sapphire or other suitable material. A plurality of rhombohedrally aligned SiGe (111)-oriented crystals are disposed on the (0001) C-plane of the crystal substrate. A first region of material is disposed on the rhombohedrally aligned SiGe layer. The first region comprises an intrinsic or doped Si, Ge, or SiGe layer. The first region can be layered between two secondary regions comprising n+doped SiGe or n+doped Ge, whereby the first region collects electrons from the two secondary regions.

Aluminum doping tunes band gap energy level as well as oxidative stress-mediated cytotoxicity of ZnO nanoparticles in MCF-7 cells

PubMed Central

Akhtar, Mohd Javed; Alhadlaq, Hisham A.; Alshamsan, Aws; Majeed Khan, M.A.; Ahamed, Maqusood

2015-01-01

We investigated whether Aluminum (Al) doping tunes band gap energy level as well as selective cytotoxicity of ZnO nanoparticles in human breast cancer cells (MCF-7). Pure and Al-doped ZnO nanoparticles were prepared by a simple sol-gel method. Characterization study confirmed the formation of single phase of AlxZn1-xO nanocrystals with the size range of 33–55 nm. Al-doping increased the band gap energy of ZnO nanoparticles (from 3.51 eV for pure to 3.87 eV for Al-doped ZnO). Al-doping also enhanced the cytotoxicity and oxidative stress response of ZnO nanoparticles in MCF-7 cells. The IC50 for undoped ZnO nanoparticles was 44 μg/ml while for the Al-doped ZnO counterparts was 31 μg/ml. Up-regulation of apoptotic genes (e.g. p53, bax/bcl2 ratio, caspase-3 & caspase-9) along with loss of mitochondrial membrane potential suggested that Al-doped ZnO nanoparticles induced apoptosis in MCF-7 cells through mitochondrial pathway. Importantly, Al-doping did not change the benign nature of ZnO nanoparticles towards normal cells suggesting that Al-doping improves the selective cytotoxicity of ZnO nanoparticles toward MCF-7 cells without affecting the normal cells. Our results indicated a novel approach through which the inherent selective cytotoxicity of ZnO nanoparticles against cancer cells can be further improved. PMID:26347142

Aluminum doping tunes band gap energy level as well as oxidative stress-mediated cytotoxicity of ZnO nanoparticles in MCF-7 cells

NASA Astrophysics Data System (ADS)

Akhtar, Mohd Javed; Alhadlaq, Hisham A.; Alshamsan, Aws; Majeed Khan, M. A.; Ahamed, Maqusood

2015-09-01

We investigated whether Aluminum (Al) doping tunes band gap energy level as well as selective cytotoxicity of ZnO nanoparticles in human breast cancer cells (MCF-7). Pure and Al-doped ZnO nanoparticles were prepared by a simple sol-gel method. Characterization study confirmed the formation of single phase of AlxZn1-xO nanocrystals with the size range of 33-55 nm. Al-doping increased the band gap energy of ZnO nanoparticles (from 3.51 eV for pure to 3.87 eV for Al-doped ZnO). Al-doping also enhanced the cytotoxicity and oxidative stress response of ZnO nanoparticles in MCF-7 cells. The IC50 for undoped ZnO nanoparticles was 44 μg/ml while for the Al-doped ZnO counterparts was 31 μg/ml. Up-regulation of apoptotic genes (e.g. p53, bax/bcl2 ratio, caspase-3 & caspase-9) along with loss of mitochondrial membrane potential suggested that Al-doped ZnO nanoparticles induced apoptosis in MCF-7 cells through mitochondrial pathway. Importantly, Al-doping did not change the benign nature of ZnO nanoparticles towards normal cells suggesting that Al-doping improves the selective cytotoxicity of ZnO nanoparticles toward MCF-7 cells without affecting the normal cells. Our results indicated a novel approach through which the inherent selective cytotoxicity of ZnO nanoparticles against cancer cells can be further improved.

Aluminum doping tunes band gap energy level as well as oxidative stress-mediated cytotoxicity of ZnO nanoparticles in MCF-7 cells.

PubMed

Akhtar, Mohd Javed; Alhadlaq, Hisham A; Alshamsan, Aws; Majeed Khan, M A; Ahamed, Maqusood

2015-09-08

We investigated whether Aluminum (Al) doping tunes band gap energy level as well as selective cytotoxicity of ZnO nanoparticles in human breast cancer cells (MCF-7). Pure and Al-doped ZnO nanoparticles were prepared by a simple sol-gel method. Characterization study confirmed the formation of single phase of Al(x)Zn(1-x)O nanocrystals with the size range of 33-55 nm. Al-doping increased the band gap energy of ZnO nanoparticles (from 3.51 eV for pure to 3.87 eV for Al-doped ZnO). Al-doping also enhanced the cytotoxicity and oxidative stress response of ZnO nanoparticles in MCF-7 cells. The IC50 for undoped ZnO nanoparticles was 44 μg/ml while for the Al-doped ZnO counterparts was 31 μg/ml. Up-regulation of apoptotic genes (e.g. p53, bax/bcl2 ratio, caspase-3 &caspase-9) along with loss of mitochondrial membrane potential suggested that Al-doped ZnO nanoparticles induced apoptosis in MCF-7 cells through mitochondrial pathway. Importantly, Al-doping did not change the benign nature of ZnO nanoparticles towards normal cells suggesting that Al-doping improves the selective cytotoxicity of ZnO nanoparticles toward MCF-7 cells without affecting the normal cells. Our results indicated a novel approach through which the inherent selective cytotoxicity of ZnO nanoparticles against cancer cells can be further improved.

Pleiotropic effect of sigE over-expression on cell morphology, photosynthesis and hydrogen production in Synechocystis sp. PCC 6803.

PubMed

Osanai, Takashi; Kuwahara, Ayuko; Iijima, Hiroko; Toyooka, Kiminori; Sato, Mayuko; Tanaka, Kan; Ikeuchi, Masahiko; Saito, Kazuki; Hirai, Masami Yokota

2013-11-01

Over-expression of sigE, a gene encoding an RNA polymerase sigma factor in the unicellular cyanobacterium Synechocystis sp. PCC 6803, is known to activate sugar catabolism and bioplastic production. In this study, we investigated the effects of sigE over-expression on cell morphology, photosynthesis and hydrogen production in this cyanobacterium. Transmission electron and scanning probe microscopic analyses revealed that sigE over-expression increased the cell size, possibly as a result of aberrant cell division. Over-expression of sigE reduced respiration and photosynthesis activities via changes in gene expression and chlorophyll fluorescence. Hydrogen production under micro-oxic conditions is enhanced in sigE over-expressing cells. Despite these pleiotropic phenotypes, the sigE over-expressing strain showed normal cell viability under both nitrogen-replete and nitrogen-depleted conditions. These results provide insights into the inter-relationship among metabolism, cell morphology, photosynthesis and hydrogen production in this unicellular cyanobacterium. © 2013 The Authors The Plant Journal © 2013 John Wiley & Sons Ltd.

High Temperature Stable Nanocrystalline SiGe Thermoelectric Material

NASA Technical Reports Server (NTRS)

Yang, Sherwin (Inventor); Matejczyk, Daniel Edward (Inventor); Determan, William (Inventor)

2013-01-01

A method of forming a nanocomposite thermoelectric material having microstructural stability at temperatures greater than 1000 C. The method includes creating nanocrystalline powder by cryomilling. The method is particularly useful in forming SiGe alloy powder.

Very low temperature (450 °C) selective epitaxial growth of heavily in situ boron-doped SiGe layers

NASA Astrophysics Data System (ADS)

Aubin, J.; Hartmann, J. M.; Veillerot, M.; Essa, Z.; Sermage, B.

2015-11-01

We have investigated the feasibility of selectively growing SiGe:B layers at 450 °C, 20 Torr in a 300 mm industrial reduced pressure chemical vapor deposition tool. A reduced H2 carrier gas mass-flow has been used in order to have acceptable growth rates at such a temperature, which is very low indeed. We have first of all studied on blanket Si wafers the in situ boron doping of SiGe with Si2H6, GeH4 and B2H6. A growth rate increase by a factor close to 7 together with a Ge concentration decrease from 53% down to 32% occurred as the diborane mass-flow increased. Very high B+ ion concentrations were obtained in layers that were single crystalline and smooth. Their concentration increased almost linearly with the B2H6 mass-flow, from 1.8 up to 8.3 × 1020 cm-3. The associated resistivity dropped from 0.43 down to 0.26 mΩ cm. We have then tested whether or not selectivity versus SiO2 could be achieved by adding various amounts of HCl to Si2H6 + GeH4 +B2H6. Single crystalline growth rates of intrinsic SiGe(:B) on Si were very similar to poly-crystalline growth rates on SiO2-covered substrates irrespective of the HCl flow. Straightforward selectivity was thus not feasible with a co-flow approach. As a consequence, a 450 °C deposition/etch (DE) process was evaluated. Growth occurred at 20 Torr with the above-mentioned chemistry, while the selective etch of poly-SiGe:B versus c-SiGe:B was conducted at 740 Torr with a medium HCl mass-flow (F(HCl)/F(H2) = 0.2) and a high H2 flow. A 2.2 etch selectivity was achieved while retaining single crystalline if slightly rough SiGe:B layers.

Cu doped diamond: Effect of charge state and defect aggregation on spin interactions in a 3d transition metal doped wide band-gap semiconductor

NASA Astrophysics Data System (ADS)

Benecha, E. M.; Lombardi, E. B.

2018-05-01

We present a first principles study of Cu in diamond using DFT+U electronic structure methods, by carefully considering the impact of co-doping, charge state, and Fermi level position on its stability, lattice location, spin states, and electronic properties. We show that the energetic stability and spin states of Cu are strongly dependent on the Fermi level position and the type of diamond co-doping, with Cu being energetically more favorable in n-type or p-type co-doped diamond compared to intrinsic diamond. Since Cu has been predicted to order magnetically in a number of other wide band-gap semiconductors, we have also evaluated this possibility for Cu doped diamond. We show that while Cu exhibits strong spin interactions at specific interatomic separations in diamond, a detailed consideration of the impact of Fermi level position and Cu aggregation precludes magnetic ordering, with Cu forming non-magnetic, antiferromagnetic, or paramagnetic clusters. These results have important implications in the understanding of the properties of transition metal dopants in diamond for device applications.

SiGe BiCMOS manufacturing platform for mmWave applications

NASA Astrophysics Data System (ADS)

Kar-Roy, Arjun; Howard, David; Preisler, Edward; Racanelli, Marco; Chaudhry, Samir; Blaschke, Volker

2010-10-01

TowerJazz offers high volume manufacturable commercial SiGe BiCMOS technology platforms to address the mmWave market. In this paper, first, the SiGe BiCMOS process technology platforms such as SBC18 and SBC13 are described. These manufacturing platforms integrate 200 GHz fT/fMAX SiGe NPN with deep trench isolation into 0.18μm and 0.13μm node CMOS processes along with high density 5.6fF/μm2 stacked MIM capacitors, high value polysilicon resistors, high-Q metal resistors, lateral PNP transistors, and triple well isolation using deep n-well for mixed-signal integration, and, multiple varactors and compact high-Q inductors for RF needs. Second, design enablement tools that maximize performance and lowers costs and time to market such as scalable PSP and HICUM models, statistical and Xsigma models, reliability modeling tools, process control model tools, inductor toolbox and transmission line models are described. Finally, demonstrations in silicon for mmWave applications in the areas of optical networking, mobile broadband, phased array radar, collision avoidance radar and W-band imaging are listed.

SiGe derivatization by spontaneous reduction of aryl diazonium salts

NASA Astrophysics Data System (ADS)

Girard, A.; Geneste, F.; Coulon, N.; Cardinaud, C.; Mohammed-Brahim, T.

2013-10-01

Germanium semiconductors have interesting properties for FET-based biosensor applications since they possess high surface roughness allowing the immobilization of a high amount of receptors on a small surface area. Since SiGe combined low cost of Si and intrinsic properties of Ge with high mobility carriers, we focused the study on this particularly interesting material. The comparison of the efficiency of a functionalization process involving the spontaneous reduction of diazonium salts is studied on Si(1 0 0), SiGe and Ge semiconductors. XPS analysis of the functionalized surfaces reveals the presence of a covalent grafted layer on all the substrates that was confirmed by AFM. Interestingly, the modified Ge derivatives have still higher surface roughness after derivatization. To support the estimated thickness by XPS, a step measurement of the organic layers is done by AFM or by profilometer technique after a O2 plasma etching of the functionalized layer. This original method is well-adapted to measure the thickness of thin organic films on rough substrates such as germanium. The analyses show a higher chemical grafting on SiGe substrates compared with Si and Ge semiconductors.

Si /SiGe n-type resonant tunneling diodes fabricated using in situ hydrogen cleaning

NASA Astrophysics Data System (ADS)

Suet, Z.; Paul, D. J.; Zhang, J.; Turner, S. G.

2007-05-01

In situ hydrogen cleaning to reduce the surface segregation of n-type dopants in SiGe epitaxy has been used to fabricate Si /SiGe resonant tunneling diodes in a joint gas source chemical vapor deposition and molecular beam epitaxial system. Diodes fabricated without the in situ clean demonstrate linear current-voltage characteristics, while a 15min hydrogen clean produces negative differential resistance with peak-to-valley current ratios up to 2.2 and peak current densities of 5.0A/cm2 at 30K. Analysis of the valley current and the band structure of the devices suggest methods for increasing the operating temperature of Si /SiGe resonant tunneling diodes as required for applications.

Mid-Gap States and Normal vs Inverted Bonding in Luminescent Cu+- and Ag+-Doped CdSe Nanocrystals.

PubMed

Nelson, Heidi D; Hinterding, Stijn O M; Fainblat, Rachel; Creutz, Sidney E; Li, Xiaosong; Gamelin, Daniel R

2017-05-10

Mid-gap luminescence in copper (Cu + )-doped semiconductor nanocrystals (NCs) involves recombination of delocalized conduction-band electrons with copper-localized holes. Silver (Ag + )-doped semiconductor NCs show similar mid-gap luminescence at slightly (∼0.3 eV) higher energy, suggesting a similar luminescence mechanism, but this suggestion appears inconsistent with the large difference between Ag + and Cu + ionization energies (∼1.5 eV), which should make hole trapping by Ag + highly unfavorable. Here, Ag + -doped CdSe NCs (Ag + :CdSe) are studied using time-resolved variable-temperature photoluminescence (PL) spectroscopy, magnetic circularly polarized luminescence (MCPL) spectroscopy, and time-dependent density functional theory (TD-DFT) to address this apparent paradox. In addition to confirming that Ag + :CdSe and Cu + :CdSe NCs display similar broad PL with large Stokes shifts, we demonstrate that both also show very similar temperature-dependent PL lifetimes and magneto-luminescence. Electronic-structure calculations further predict that both dopants generate similar localized mid-gap states. Despite these strong similarities, we conclude that these materials possess significantly different electronic structures. Specifically, whereas photogenerated holes in Cu + :CdSe NCs localize primarily in Cu(3d) orbitals, formally oxidizing Cu + to Cu 2+ , in Ag + :CdSe NCs they localize primarily in 4p orbitals of the four neighboring Se 2- ligands, and Ag + is not oxidized. This difference reflects a shift from "normal" to "inverted" bonding going from Cu + to Ag + . The spectroscopic similarities are explained by the fact that, in both materials, photogenerated holes are localized primarily within covalent [MSe 4 ] dopant clusters (M = Ag + , Cu + ). These findings reconcile the similar spectroscopies of Ag + - and Cu + -doped semiconductor NCs with the vastly different ionization potentials of their Ag + and Cu + dopants.

Magnetism in Mn-nanowires and -clusters as δ-doped layers in group IV semiconductors (Si, Ge)

NASA Astrophysics Data System (ADS)

Simov, K. R.; Glans, P.-A.; Jenkins, C. A.; Liberati, M.; Reinke, P.

2018-01-01

Mn doping of group-IV semiconductors (Si/Ge) is achieved by embedding nanostructured Mn-layers in group-IV matrix. The Mn-nanostructures are monoatomic Mn-wires or Mn-clusters and capped with an amorphous Si or Ge layer. The precise fabrication of δ-doped Mn-layers is combined with element-specific detection of the magnetic signature with x-ray magnetic circular dichroism. The largest moment (2.5 μB/Mn) is measured for Mn-wires with ionic bonding character and a-Ge overlayer cap; a-Si capping reduces the moment due to variations of bonding in agreement with theoretical predictions. The moments in δ-doped layers dominated by clusters is quenched with an antiferromagnetic component from Mn-Mn bonding.

Thermoelectric properties of hot pressed p-type SiGe alloys

NASA Technical Reports Server (NTRS)

Bajgar, Clara; Masters, Richard; Scoville, Nancy; Vandersande, Jan

1991-01-01

This paper presents the results of measurements of electrical resistivity, Seebeck coefficient, thermal conductivity, as well as Hall carrier concentration, and mobility, for hot pressed SiGe 80 at. pct Si-20 at. pct Ge (SiGe) thermoelectric materials containing 0.24-3.0 at. pct boron. The carrier concentration was varied by annealing and quenching at different high temperatures. Figure-of-merit, Z, was found to be 0.60 +/- 0.03 x 10 exp -3/K over a carrier concentration range from 1.8- 3.5 x 10 exp -20/cu cm. This result is very encouraging from a production standpoint, since the dopant concentration is not critical.

Novel band gap-tunable K-Na co-doped graphitic carbon nitride prepared by molten salt method

NASA Astrophysics Data System (ADS)

Zhao, Jiannan; Ma, Lin; Wang, Haoying; Zhao, Yanfeng; Zhang, Jian; Hu, Shaozheng

2015-03-01

Novel band gap-tunable K-Na co-doped graphitic carbon nitride was prepared by molten salt method using melamine, KCl, and NaCl as precursor. X-ray diffraction (XRD), N2 adsorption, Scanning electron microscope (SEM), UV-vis spectroscopy, Photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS) were used to characterize the prepared catalysts. The CB and VB potentials of graphitic carbon nitride could be tuned from -1.09 and +1.55 eV to -0.29 and +2.25 eV by controlling the weight ratio of eutectic salts to melamine. Besides, ions doping inhibited the crystal growth of graphitic carbon nitride, enhanced the surface area, and increased the separation rate of photogenerated electrons and holes. The visible-light-driven Rhodamine B (RhB) photodegradation and mineralization performances were significantly improved after K-Na co-doping.

Formation of nanocrystalline SiGe in Polycrystalline-Ge/Si thin film without any metal induced crystallization

NASA Astrophysics Data System (ADS)

Tah, Twisha; Singh, Ch. Kishan; Madapu, K. K.; Polaki, S. R.; Ilango, S.; David, C.; Dash, S.; Panigrahi, B. K.

2017-05-01

The formation of nanocrystalline SiGe without the aid of metal induced crystallization is reported. Re-crystallization of the as-deposited poly-Ge film (deposited at 450 °C) leads to development of regions with depleted Ge concentration upon annealing at 500 °C. Clusters with crystalline facet containing both nanocrystalline SiGe and crystalline Ge phase starts appearing at 600 °C. The structural phase characteristics were investigated by X-ray diffraction (XRD) and Raman spectroscopy. The stoichiometry of the SiGe phase was estimated from the positions of the Raman spectral peaks.

Electrical characterisation of SiGe heterojunction bipolar transistors and Si pseudo-HBTS

NASA Astrophysics Data System (ADS)

De Barros, O.; Le Tron, B.; Woods, R. C.; Giroult-Matlakowski, G.; Vincent, G.; Brémond, G.

1996-08-01

This paper reports an electrical characterisation of the emitter-base junction of Si pseudo-HBTs and SiGe HBTs fabricated in a CMOS compatible single polysilicon self-aligned process. From the reverse characteristics it appears that the definition of the emitter-base junction by plasma etching induces peripheral defects that increase the base current of the transistors. Deep level transient spectroscopy measurements show a deep level in the case of SiGe base, whose spatial origin is not fully determinate up to now.

Practical Doping Principles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zunger, A.

2003-05-01

'Theoretical investigations of doping of several wide-gap materials suggest a number of rather general, practical"doping principles" that may help guide experimental strategies of overcoming doping bottlenecks. This paper will be published as a journal article in the future.

Tuning near-gap electronic structure, interface charge transfer and visible light response of hybrid doped graphene and Ag3PO4 composite: Dopant effects

PubMed Central

He, Chao-Ni; Huang, Wei-Qing; Xu, Liang; Yang, Yin-Cai; Zhou, Bing-Xin; Huang, Gui-Fang; Peng, P.; Liu, Wu-Ming

2016-01-01

The enhanced photocatalytic performance of doped graphene (GR)/semiconductor nanocomposites have recently been widely observed, but an understanding of the underlying mechanisms behind it is still out of reach. As a model system to study the dopant effects, we investigate the electronic structures and optical properties of doped GR/Ag3PO4 nanocomposites using the first-principles calculations, demonstrating that the band gap, near-gap electronic structure and interface charge transfer of the doped GR/Ag3PO4(100) composite can be tuned by the dopants. Interestingly, the doping atom and C atoms bonded to dopant become active sites for photocatalysis because they are positively or negatively charged due to the charge redistribution caused by interaction. The dopants can enhance the visible light absorption and photoinduced electron transfer. We propose that the N atom may be one of the most appropriate dopants for the GR/Ag3PO4 photocatalyst. This work can rationalize the available experimental results about N-doped GR-semiconductor composites, and enriches our understanding on the dopant effects in the doped GR-based composites for developing high-performance photocatalysts. PMID:26923338

Electrochemical capacitance voltage measurements in highly doped silicon and silicon-germanium alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sermage, B.; Essa, Z.; Taleb, N.

2016-04-21

The electrochemical capacitance voltage technique has been used on highly boron doped SiGe and Si layers. Although the boron concentration is constant over the space charge depth, the 1/C{sup 2} versus voltage curves are not linear. They indeed present a negative curvature. This can be explained by the existence of deep acceptors which ionise under a high electric field (large inverse voltage) and not at a low inverse voltage. The measured doping concentration in the electrochemical capacitance voltage increases strongly as the inverse voltage increases. Thanks to a comparison with the boron concentration measured by secondary ions mass spectrometry, wemore » show that the relevant doping concentrations in device layers are obtained for small inverse voltage in agreement with the existence of deep acceptors. At the large inverse voltage, the measured doping can be more than twice larger than the boron concentration measured with a secondary ion mass spectroscopy.« less

Isoelectronic co-doping

DOEpatents

Mascarenhas, Angelo

2004-11-09

Isoelectronic co-doping of semiconductor compounds and alloys with deep acceptors and deep donors is used to decrease bandgap, to increase concentration of the dopant constituents in the resulting alloys, and to increase carrier mobilities lifetimes. Group III-V compounds and alloys, such as GaAs and GaP, are isoelectronically co-doped with, for example, N and Bi, to customize solar cells, thermal voltaic cells, light emitting diodes, photodetectors, and lasers on GaP, InP, GaAs, Ge, and Si substrates. Isoelectronically co-doped Group II-VI compounds and alloys are also included.

Thermally induced effect on sub-band gap absorption in Ag doped CdSe thin films

NASA Astrophysics Data System (ADS)

Kaur, Jagdish; Sharma, Kriti; Bharti, Shivani; Tripathi, S. K.

2015-05-01

Thin films of Ag doped CdSe have been prepared by thermal evaporation using inert gas condensation (IGC) method taking Argon as inert gas. The prepared thin films are annealed at 363 K for one hour. The sub-band gap absorption spectra in the as deposited and annealed thin films have been studied using constant photocurrent method (CPM). The absorption coefficient in the sub-band gap region is described by an Urbach tail in both as deposited and annealed thin films. The value of Urbach energy and number density of trap states have been calculated from the absorption coefficient in the sub-band gap region which have been found to increase after annealing treatment indicating increase in disorderness in the lattice. The energy distribution of the occupied density of states below Fermi level has also been studied using derivative procedure of absorption coefficient.

The Effect of High N-DOPED Anatase TiO2 on the Band Gap Narrowing and Redshift by First-Principles

NASA Astrophysics Data System (ADS)

Hou, Qingyu; Jin, Yongjun; Ying, Chun; Zhao, Erjun; Zhang, Yue; Dong, Hongying

2012-10-01

Anatase TiO2 supercells were studied by first-principles, in which one was undoped and another three were high N-doping. Partial densities of states, band structure, population and absorption spectrum were calculated. The calculated results indicated that in the condition of TiO2-xNx (x = 0.0625, 0.125, 0.25), the higher the doping concentration is, the shorter will be the lattice parameters parallel to the direction of c-axis. The strength of covalent bond significantly varied. The formation energy increases at first, and then decreases. The doping models become less stable as N-doping concentration increases. Meanwhile, the narrower the band gap is, the more significant will be the redshift, which is in agreement with the experimental results.

Superconducting gap structure in the electron doped BiS2-based superconductor

NASA Astrophysics Data System (ADS)

Bhattacharyya, A.; Adroja, D. T.; Hillier, A. D.; Jha, R.; Awana, V. P. S.; Strydom, A. M.

2017-07-01

The influence of electron doping on semimetallic SrFBiS2 has been investigated by means of resistivity, zero and transverse  -  field (ZF/TF) muon spin relaxation/rotation (μSR) experiments. SrFBiS2 is semimetallic in its normal state and small amounts of La doping results in bulk superconductivity at 2.8 K, at ambient pressure. The temperature dependence of the superfluid density as determined by TF-μSR can be best modelled by an isotropic s  -  wave type superconducting gap. We have estimated the magnetic penetration depth {λL}(0)=1087 nm, superconducting carrier density {{n}s}=3.7× {{10}26} carriers m-3 and effective-mass enhancement m *  =  1.558 m e. Additionally, there is no clear sign of the occurrence of spontaneous internal magnetic fields below {{T}\\mathbf{c}} , which implies that the superconducting state in this material can not be categorized by the broken time-reversal symmetry which is in agreement with the previous theoretical prediction.

Formation mechanisms of nano and microcones by laser radiation on surfaces of Si, Ge, and SiGe crystals

PubMed Central

2013-01-01

In this work we study the mechanisms of laser radiation interaction with elementary semiconductors such as Si and Ge and their solid solution SiGe. As a result of this investigation, the mechanisms of nanocones and microcones formation on a surface of semiconductor were proposed. We have shown the possibility to control the size and the shape of cones both by the laser. The main reason for the formation of nanocones is the mechanical compressive stresses due to the atoms’ redistribution caused by the gradient of temperature induced by strongly absorbed laser radiation. According to our investigation, the nanocone formation mechanism in semiconductors is characterized by two stages. The first stage is characterized by formation of a p-n junction for elementary semiconductors or of a Ge/Si heterojunction for SiGe solid solution. The generation and redistribution of intrinsic point defects in elementary semiconductors and Ge atoms concentration on the irradiated surface of SiGe solid solution in temperature gradient field take place at this stage due to the thermogradient effect which is caused by strongly absorbed laser radiation. The second stage is characterized by formation of nanocones due to mechanical plastic deformation of the compressed Ge layer on Si. Moreover, a new 1D-graded band gap structure in elementary semiconductors due to quantum confinement effect was formed. For the formation of microcones Ni/Si structure was used. The mechanism of the formation of microcones is characterized by two stages as well. The first stage is the melting of Ni film after irradiation by laser beam and formation of Ni islands due to surface tension force. The second step is the melting of Ni and subsequent manifestations of Marangoni effect with the growth of microcones. PMID:23735193

Secondary growth mechanism of SiGe islands deposited on a mixed-phase microcrystalline Si by ion beam co-sputtering.

PubMed

Ke, S Y; Yang, J; Qiu, F; Wang, Z Q; Wang, C; Yang, Y

2015-11-06

We discuss the SiGe island co-sputtering deposition on a microcrystalline silicon (μc-Si) buffer layer and the secondary island growth based on this pre-SiGe island layer. The growth phenomenon of SiGe islands on crystalline silicon (c-Si) is also investigated for comparison. The pre-SiGe layer grown on μc-Si exhibits a mixed-phase structure, including SiGe islands and amorphous SiGe (a-SiGe) alloy, while the layer deposited on c-Si shows a single-phase island structure. The preferential growth and Ostwald ripening growth are shown to be the secondary growth mechanism of SiGe islands on μc-Si and c-Si, respectively. This difference may result from the effect of amorphous phase Si (AP-Si) in μc-Si on the island growth. In addition, the Si-Ge intermixing behavior of the secondary-grown islands on μc-Si is interpreted by constructing the model of lateral atomic migration, while this behavior on c-Si is ascribed to traditional uphill atomic diffusion. It is found that the aspect ratios of the preferential-grown super islands are higher than those of the Ostwald-ripening ones. The lower lateral growth rate of super islands due to the lower surface energy of AP-Si on the μc-Si buffer layer for the non-wetting of Ge at 700 °C and the stronger Si-Ge intermixing effect at 730 °C may be responsible for this aspect ratio difference.

Intersubband linear and nonlinear optical response of the delta-doped SiGe quantum well

NASA Astrophysics Data System (ADS)

Duque, C. A.; Akimov, V.; Demediuk, R.; Belykh, V.; Tiutiunnyk, A.; Morales, A. L.; Restrepo, R. L.; Mora-Ramos, M. E.; Fomina, O.; Tulupenko, V.

2015-11-01

The degree of ionization, controlled by external fields, of delta-doped layers inside the quantum wells can affect their energy structure, therefore delta-doped QWs can be used to engineer different kinds of tunable THz optical devices on intersubband transitions. Here it is calculated and analyzed the linear and nonlinear (Kerr-type) optical response, including absorption coefficient and refractive index change of 20 nm-wide Si0.8Ge0.2/Si/Si0.8Ge0.2 QW structures n-delta-doped either at the center or at the edge of the well under different temperatures. The conduction subband energy structure was found self-consistently, including the calculation of the impurity binding energy. Our results show that the degree of ionization of the impurity layer as well as the heterostructure symmetry has a strong influence on optical properties of the structures in THz region.

Effect of n-type doping level on direct band gap electroluminescence intensity for asymmetric metal/Ge/metal diodes

NASA Astrophysics Data System (ADS)

Maekura, T.; Tanaka, K.; Motoyama, C.; Yoneda, R.; Yamamoto, K.; Nakashima, H.; Wang, D.

2017-10-01

The direct band gap electroluminescence (EL) intensity was investigated for asymmetric metal/Ge/metal diodes fabricated on n-type Ge with doping levels in the range of 4.0 × 1013-3.1 × 1018 cm-3. Up to a doping level of 1016 cm-3 order, commercially available (100) n-Ge substrates were used. To obtain a doping level higher than 1017 cm-3 order, which is commercially unavailable, n+-Ge/p-Ge structures were fabricated by Sb doping on p-type (100) Ge substrates with an in-diffusion at 600 °C followed by a push-diffusion at 700 °C-850 °C. The EL intensity was increased with increasing doping level up to 1.0 × 1018 cm-3. After that, it was decreased with a further increase in n-type doping level. This EL intensity decrease is explained by the decreased number of holes in the active region. One reason is the difficulty in hole injection through the PtGe/n-Ge contact due to the occurring of tunneling electron current. Another reason is the loss of holes caused by both the small thickness of n+-Ge layer and the existence of n+p junction.

Si/Ge elatform for lasers, amplifiers, and nonlinear optical devices based on the Raman Effect

NASA Astrophysics Data System (ADS)

Claps, Ricardo; Dimitropoulos, Dimitrios; Raghunathan, Varun; Fathpour, Sasan; Jalali, Bahram; Jusserand, Bernard

2007-02-01

The use of a silicon-germanium platform for the development of optically active devices will be discussed in this paper, from the perspective of Raman and Brillouin scattering phenomena. Silicon-Germanium is becoming a prevalent technology for the development of high speed CMOS transistors, with advances in several key parameters as high carrier mobility, low cost, and reduced manufacturing logistics. Traditionally, Si-Ge structures have been used in the optoelectronics arena as photodetectors, due to the enhanced absorption of Ge in the telecommunications band. Recent developments in Raman-based nonlinearities for devices based on a silicon-on-insulator platform have shed light on the possibility of using these effects in Si-Ge architectures. Lasing and amplification have been demonstrated using a SiGe alloy structure, and Brillouin/Raman activity from acoustic phonon modes in SiGe superlattices has been predicted. Moreover, new Raman-active branches and inhomogeneously broadened spectra result from optical phonon modes, offering new perspectives for optical device applications. The possibilities for an electrically-pumped Raman laser will be outlined, and the potential for design and development of silicon-based, Tera-Hertz wave emitters and/or receivers.

High-responsivity vertical-illumination Si/Ge uni-traveling-carrier photodiodes based on silicon-on-insulator substrate.

PubMed

Li, Chong; Xue, ChunLai; Liu, Zhi; Cong, Hui; Cheng, Buwen; Hu, Zonghai; Guo, Xia; Liu, Wuming

2016-06-09

Si/Ge uni-traveling carrier photodiodes exhibit higher output current when space-charge effect is overcome and the thermal effects is suppressed. High current is beneficial for increasing the dynamic range of various microwave photonic systems and simplifying high-bit-rate digital receivers in many applications. From the point of view of packaging, detectors with vertical-illumination configuration can be easily handled by pick-and-place tools and are a popular choice for making photo-receiver modules. However, vertical-illumination Si/Ge uni-traveling carrier (UTC) devices suffer from inter-constraint between high speed and high responsivity. Here, we report a high responsivity vertical-illumination Si/Ge UTC photodiode based on a silicon-on-insulator substrate. When the transmission of the monolayer anti-reflection coating was maximum, the maximum absorption efficiency of the devices was 1.45 times greater than the silicon substrate owing to constructive interference. The Si/Ge UTC photodiode had a dominant responsivity at 1550 nm of 0.18 A/W, a 50% improvement even with a 25% thinner Ge absorption layer.

High-responsivity vertical-illumination Si/Ge uni-traveling-carrier photodiodes based on silicon-on-insulator substrate

PubMed Central

Li, Chong; Xue, ChunLai; Liu, Zhi; Cong, Hui; Cheng, Buwen; Hu, Zonghai; Guo, Xia; Liu, Wuming

2016-01-01

Si/Ge uni-traveling carrier photodiodes exhibit higher output current when space-charge effect is overcome and the thermal effects is suppressed. High current is beneficial for increasing the dynamic range of various microwave photonic systems and simplifying high-bit-rate digital receivers in many applications. From the point of view of packaging, detectors with vertical-illumination configuration can be easily handled by pick-and-place tools and are a popular choice for making photo-receiver modules. However, vertical-illumination Si/Ge uni-traveling carrier (UTC) devices suffer from inter-constraint between high speed and high responsivity. Here, we report a high responsivity vertical-illumination Si/Ge UTC photodiode based on a silicon-on-insulator substrate. When the transmission of the monolayer anti-reflection coating was maximum, the maximum absorption efficiency of the devices was 1.45 times greater than the silicon substrate owing to constructive interference. The Si/Ge UTC photodiode had a dominant responsivity at 1550 nm of 0.18 A/W, a 50% improvement even with a 25% thinner Ge absorption layer. PMID:27279426

Doping - dependent anisotropy of the superconducting gap in underdoped pnictide superconductors

NASA Astrophysics Data System (ADS)

Prozorov, Ruslan

2012-02-01

The in-plane London penetration depth, δλ(T), was studied in single crystals of Ba1-xKxFe2As2 (``Ba122") and Ca10(Pt3As8)[(Fe1-xPtx)2As2]5 (``10-3-8"). Whereas in Ba122 magnetism and superconductivity coexist in the underdoped regime, the 10-3-8 compound exhibits a clear separation of two order parameters. By comparing the results obtained in these two systems, we could study general features of the superconducting gap structure as function of doping in the underdoped regime. Similar to all other pnictides, the low-temperature variation of London penetration depth exhibits a power-law behavior, δλ(T)= AT^n, in both systems. Moving towards the underdoped edge of the superconducting dome, the exponent n decreases well below scattering - limited value of n=2 and, at the same time, the pre-factor A increases. Both trends indicate an increasing anisotropy of the superconducting gap in more underdoped compounds. These and previous results suggest that the development of the superconducting gap anisotropy towards the underdoped edge of the superconducting dome is an intrinsic property of iron pnictides, similar to the known tendency on the overdoped side where magnetism and superconductivity do not interfere.[4pt] In collboration with M.A. Tanatar, H. Kim, The Ames Laboratory; Bing Shen, Hai-Hu Wen, Nanjing University; and N. Ni, R.J. Cava, Princeton University.

Heavy Ion Microbeam and Broadbeam Transients in SiGe HBTs

NASA Technical Reports Server (NTRS)

Pellish, Jonathan A.; Reed, Robert A.; McMorrow, Dale; Vizkelethy, Gyorgy; Dodd, Paul E.; Ferlet-Cavrois, Veronique; Baggio, Jacques; Paillet, Philippe; Duhamel, Olivier; Phillips, Stanley D.;

Degradation of Si/Ge core/shell nanowire heterostructures during lithiation and delithiation at 0.8 and 20 A g-1.

PubMed

Kim, Dongheun; Li, Nan; Sheehan, Chris J; Yoo, Jinkyoung

2018-04-26

Si/Ge core/shell nanowire heterostructures have been expected to provide high energy and power densities for lithium ion battery anodes due to the large capacity of Si and the high electrical and ionic conductivities of Ge. Although the battery anode performances of Si/Ge core/shell nanowire heterostructures have been characterized, the degradation of Si/Ge core/shell nanowire heterostructures has not been thoroughly investigated. Here we report the compositional and structural changes of the Si/Ge core/shell nanowire heterostructure over cycling of lithiation and delithiation at different charging rates. The Si/Ge core/shell nanowire heterostructure holds the core and shell structure at a charging rate of 0.8 A g-1 up to 50 cycles. On the other hand, compositional intermixing and loss of Si occur at a charging rate of 20 A g-1 within 50 cycles. The operation condition-dependent degradation provides a new aspect of materials research for the development of high performance lithium ion battery anodes with a long cycle life.

Effect of Synthesis Procedure on Thermoelectric Property of SiGe Alloy

NASA Astrophysics Data System (ADS)

Li, Jing; Han, Jun; Jiang, Tao; Luo, Lili; Xiang, Yongchun

2018-05-01

SiGe thermoelectric material has been synthesized by ball milling combined with hot pressing (HP) or spark plasma sintering (SPS). Effects of ball milling time, powder to ball weight ratio and sintering method on microstructure and thermoelectric properties of SiGe are studied. The results show that longer ball milling time leads to decreased density and worse electrical properties. In the sintering process, SPS results in much larger density and better electrical properties than HP. The Si0.795Ge0.2B0.005 sample prepared by 2 h ball milling combined with SPS obtains a maximum power factor of 3.0 mW m-1 K-2 at 860 K and ZT of 0.95 at 1000 K.

Significant reduction of thermal conductivity in Si/Ge core-shell nanowires.

PubMed

Hu, Ming; Giapis, Konstantinos P; Goicochea, Javier V; Zhang, Xiaoliang; Poulikakos, Dimos

2011-02-09

We report on the effect of germanium (Ge) coatings on the thermal transport properties of silicon (Si) nanowires using nonequilibrium molecular dynamics simulations. Our results show that a simple deposition of a Ge shell of only 1 to 2 unit cells in thickness on a single crystalline Si nanowire can lead to a dramatic 75% decrease in thermal conductivity at room temperature compared to an uncoated Si nanowire. By analyzing the vibrational density states of phonons and the participation ratio of each specific mode, we demonstrate that the reduction in the thermal conductivity of Si/Ge core-shell nanowire stems from the depression and localization of long-wavelength phonon modes at the Si/Ge interface and of high frequency nonpropagating diffusive modes.

Theoretical study of room temperature ferromagnetism and band gap energy of pure and ion doped In2O3 nanoparticles

NASA Astrophysics Data System (ADS)

Apostolov, A. T.; Apostolova, I. N.; Wesselinowa, J. M.

2018-06-01

Using the s-d microscopic model including the electron-phonon interaction and the Green's function theory we have considered the origin of room temperature ferromagnetism (RTFM) in pure and ion doped In2O3 nanoparticles (NPs). The magnetization M increases with decreasing particle size. M of Fe, Tb and Mn doped In2O3 NPs is investigated, which increases, decreases and has a maximum, respectively, with increasing doping concentration. The RTFM is due to surface oxygen vacancies and different ionic radius of the dopants compared to that of the host ions. This differences lead to different strains which changes the exchange interaction constants. We have calculated the dependence of the band gap energy on the particle size in In2O3 NPs and the Fe concentration of Fe doped In2O3 NPs. The results are in good qualitative agreement with the experimental data.

Role of Fe doping in tuning the band gap of TiO2 for photo-oxidation induced cytotoxicity paradigm

PubMed Central

George, Saji; Pokhrel, Suman; Ji, Zhaoxia; Henderson, Bryana L.; Xia, Tian; Li, LinJiang; Zink, Jeffrey I.; Nel, André E.; Mädler, Lutz

2014-01-01

UV-Light induced electron-hole (e−/h+) pair generation and free radical production in TiO2 based nanoparticles is a major conceptual paradigm for biological injury. However, to date, this hypothesis has been difficult to experimentally verify due to the high energy of UV light that is intrinsically highly toxic to biological systems. Here, a versatile flame spray pyrolysis (FSP) synthetic process has been exploited to synthesize a library of iron doped (0–10 at wt%) TiO2 nanoparticles. These particles have been tested for photoactivation-mediated cytotoxicity using near-visible light exposure. The reduction in TiO2 band gap energy with incremental levels of Fe loading maintained the nanoparticle crystalline structure in spite of homogeneous Fe distribution (demonstrated by XRD, HRTEM, SAED, EFTEM, and EELS). Photochemical studies showed that band gap energy was reciprocally tuned proportional to the Fe content. The photo-oxidation capability of Fe-doped TiO2 was found to increase during near-visible light exposure. Use of a macrophage cell line to evaluate cytotoxic and ROS production showed increased oxidant injury and cell death in parallel with a decrease in band gap energy. These findings demonstrate the importance of band gap energy in the phototoxic response of the cell to TiO2 nanoparticles and reflect the potential of this material to generate adverse effects in humans and the environment during high intensity light exposure. PMID:21678906

Band-gap narrowing and magnetic behavior of Ni-doped Ba(Ti0.875Ce0.125)O3 thin films

NASA Astrophysics Data System (ADS)

Zhou, Wenliang; Deng, Hongmei; Yu, Lu; Yang, Pingxiong; Chu, Junhao

2015-11-01

Band-gap narrowing and magnetic effects have been observed in a Ni-doped Ba(Ti0.875Ce0.125)O3 (BTC) thin film. Structural characterizations and microstructural analysis show that the as-prepared Ba(Ti0.75Ce0.125Ni0.125)O3-δ (BTCN) thin film exhibits a cubic perovskite structure with an average grain size of 25 nm. The Ce doping at the Ti-site results in an increasing perovskite volume to favour an O-vacancy-stabilized Ni2+ substitution. Raman spectroscopy, however, shows the cubic symmetry of crystalline structures is locally lowered by the presence of dopants, significantly deviating from the ideal Pm3m space group. Moreover, BTCN presents a narrowed band-gap, much smaller than that of BaTiO3 and BTC, due to new states of both the highest occupied molecular orbital and the lowest unoccupied molecular orbital in an electronic structure with the presence of Ni. Also, magnetic enhancement driven by co-doping has been confirmed in the films, which mainly stems from the exchange interaction of Ni2+ ions via an electron trapped in a bridging oxygen vacancy. These findings may open an avenue to discover and design optimal perovskite compounds for solar-energy devices and information storage.

High-Power, High-Frequency Si-Based (SiGe) Transistors Developed

NASA Technical Reports Server (NTRS)

Ponchak, George E.

2002-01-01

Future NASA, DOD, and commercial products will require electronic circuits that have greater functionality and versatility but occupy less space and cost less money to build and integrate than current products. System on a Chip (SOAC), a single semiconductor substrate containing circuits that perform many functions or containing an entire system, is widely recognized as the best technology for achieving low-cost, small-sized systems. Thus, a circuit technology is required that can gather, process, store, and transmit data or communications. Since silicon-integrated circuits are already used for data processing and storage and the infrastructure that supports silicon circuit fabrication is very large, it is sensible to develop communication circuits on silicon so that all the system functions can be integrated onto a single wafer. Until recently, silicon integrated circuits did not function well at the frequencies required for wireless or microwave communications, but with the introduction of small amounts of germanium into the silicon to make silicon-germanium (SiGe) transistors, silicon-based communication circuits are possible. Although microwavefrequency SiGe circuits have been demonstrated, there has been difficulty in obtaining the high power from their transistors that is required for the amplifiers of a transmitter, and many researchers have thought that this could not be done. The NASA Glenn Research Center and collaborators at the University of Michigan have developed SiGe transistors and amplifiers with state-of-the-art output power at microwave frequencies from 8 to 20 GHz. These transistors are fabricated using standard silicon processing and may be integrated with CMOS integrated circuits on a single chip. A scanning electron microscope image of a typical SiGe heterojunction bipolar transistor is shown in the preceding photomicrograph. This transistor achieved a record output power of 550 mW and an associated power-added efficiency of 33 percent at 8

270GHz SiGe BiCMOS manufacturing process platform for mmWave applications

NASA Astrophysics Data System (ADS)

Kar-Roy, Arjun; Preisler, Edward J.; Talor, George; Yan, Zhixin; Booth, Roger; Zheng, Jie; Chaudhry, Samir; Howard, David; Racanelli, Marco

2011-11-01

TowerJazz has been offering the high volume commercial SiGe BiCMOS process technology platform, SBC18, for more than a decade. In this paper, we describe the TowerJazz SBC18H3 SiGe BiCMOS process which integrates a production ready 240GHz FT / 270 GHz FMAX SiGe HBT on a 1.8V/3.3V dual gate oxide CMOS process in the SBC18 technology platform. The high-speed NPNs in SBC18H3 process have demonstrated NFMIN of ~2dB at 40GHz, a BVceo of 1.6V and a dc current gain of 1200. This state-of-the-art process also comes with P-I-N diodes with high isolation and low insertion losses, Schottky diodes capable of exceeding cut-off frequencies of 1THz, high density stacked MIM capacitors, MOS and high performance junction varactors characterized up to 50GHz, thick upper metal layers for inductors, and various resistors such as low value and high value unsilicided poly resistors, metal and nwell resistors. Applications of the SBC18H3 platform for millimeter-wave products for automotive radars, phased array radars and Wband imaging are presented.

Strain-induced formation of fourfold symmetric SiGe quantum dot molecules.

PubMed

Zinovyev, V A; Dvurechenskii, A V; Kuchinskaya, P A; Armbrister, V A

2013-12-27

The strain field distribution at the surface of a multilayer structure with disklike SiGe nanomounds formed by heteroepitaxy is exploited to arrange the symmetric quantum dot molecules typically consisting of four elongated quantum dots ordered along the [010] and [100] directions. The morphological transition from fourfold quantum dot molecules to continuous fortresslike quantum rings with an increasing amount of deposited Ge is revealed. We examine key mechanisms underlying the formation of lateral quantum dot molecules by using scanning tunneling microscopy and numerical calculations of the strain energy distribution on the top of disklike SiGe nanomounds. Experimental data are well described by a simple thermodynamic model based on the accurate evaluation of the strain dependent part of the surface chemical potential. The spatial arrangement of quantum dots inside molecules is attributed to the effect of elastic property anisotropy.

Crystal Lattice Controlled SiGe Thermoelectric Materials with High Figure of Merit

NASA Technical Reports Server (NTRS)

Kim, Hyun-Jung; Park, Yeonjoon; King, Glen C.; Lee, Kunik; Choi, Sang H.

2010-01-01

Direct energy conversion between thermal and electrical energy, based on thermoelectric (TE) effect, has the potential to recover waste heat and convert it to provide clean electric power. The energy conversion efficiency is related to the thermoelectric figure of merit ZT expressed as ZT=S(exp 2)(sigma)T/Kappa, T is temperature, S is the Seebeck coefficient, sigma is conductance and Kappa is thermal conductivity. For a lower thermal conductivity Kappa and high power factor (S(exp 2)(sigma)), our current strategy is the development of rhombohedrally strained single crystalline SiGe materials that are highly [111]-oriented twinned. The development of a SiGe "twin lattice structure (TLS)" plays a key role in phonon scattering. The TLS increases the electrical conductivity and decreases thermal conductivity due to phonon scattering at stacking faults generated from the 60 X rotated primary twin structure. To develop high performance materials, the substrate temperature, chamber working pressure, and DC sputtering power are controlled for the aligned growth production of SiGe layer and TLS on a c-plane sapphire. Additionally, a new elevated temperature thermoelectric characterization system, that measures the thermal diffusivity and Seebeck effect nondestructively, was developed. The material properties were characterized at various temperatures and optimized process conditions were experimentally determined. The present paper encompasses the technical discussions toward the development of thermoelectric materials and the measurement techniques.

Design and development of SiGe based near-infrared photodetectors

NASA Astrophysics Data System (ADS)

Zeller, John W.; Puri, Yash R.; Sood, Ashok K.; McMahon, Shane; Efsthadiatis, Harry; Haldar, Pradeep; Dhar, Nibir K.

2014-10-01

Near-infrared (NIR) sensors operating at room temperatures are critical for a variety of commercial and military applications including detecting mortar fire and muzzle flashes. SiGe technology offers a low-cost alternative to conventional IR sensor technologies such as InGaAs, InSb, and HgCdTe for developing NIR micro-sensors that will not require any cooling and can operate with high bandwidths and comparatively low dark currents. Since Ge has a larger thermal expansion coefficient than Si, tensile strain may be incorporated into detector devices during the growth process, enabling an extended operating wavelength range above 1600 nm. SiGe based pin photodetectors have advantages of high stability, low noise, and high responsivity compared to metal-semiconductor-metal (MSM) devices. We have developed a process flow and are fabricating SiGe detector devices on 12" (300 mm) silicon wafers in order to take advantage of high throughput, large-area leading-edge silicon based CMOS technology that provides small feature sizes with associated device cost/density scaling advantages. The fabrication of the detector devices is facilitated by a two-step growth process incorporating initial low temperature growth of Ge/SiGe to form a thin strain-relaxed layer, followed by high temperature growth to deposit a thicker absorbing film, and subsequent high temperature anneal. This growth process is designed to effectively reduce dark current and enhance detector performance by reducing the number of defects and threading dislocations which form recombination centers during the growth process. Various characterization techniques have been employed to determine the properties of the epitaxially deposited Ge/SiGe layers, and the corresponding results are discussed.

Elemental boron-doped p(+)-SiGe layers grown by molecular beam epitaxy for infrared detector applications

NASA Technical Reports Server (NTRS)

Lin, T. L.; George, T.; Jones, E. W.; Ksendzov, A.; Huberman, M. L.

1992-01-01

SiGe/Si heterojunction internal photoemission (HIP) detectors have been fabricated utilizing molecular beam epitaxy of p(+)-SiGe layers on p(-)-Si substrates. Elemental boron from a high-temperature effusion cell was used as the dopant source during MBE growth, and high doping concentrations have been achieved. Strong infrared absorption, mainly by free-carrier absorption, was observed for the degenerately doped SiGe layers. The use of elemental boron as the dopant source allows a low MBE growth temperature, resulting in improved crystalline quality and smooth surface morphology of the Si(0.7)Ge(0.3) layers. Nearly ideal thermionic emission dark current characteristics have been obtained. Photoresponse of the HIP detectors in the long-wavelength infrared regime has been demonstrated.

Evidence for the formation of SiGe nanoparticles in Ge-implanted Si 3N 4

DOE PAGES

Mirzaei, S.; Kremer, F.; Feng, R.; ...

2017-03-14

SiGe nanoparticles were formed in an amorphous Si 3N 4 matrix by Ge + ion implantation and thermal annealing. The size of the nanoparticles was determined by transmission electron microscopy and their atomic structure by x-ray absorption spectroscopy. Nanoparticles were observed for excess Ge concentrations in the range from 9 to 12 at. % after annealing at temperatures in the range from 700 to 900 °C. The average nanoparticle size increased with excess Ge concentration and annealing temperature and varied from an average diameter of 1.8±0.2 nm for the lowest concentration and annealing temperature to 3.2±0.5 nm for the highestmore » concentration and annealing temperature. Our study demonstrates that the structural properties of embedded SiGe nanoparticles in amorphous Si 3N 4 are sensitive to the implantation and post implantation conditions. Furthermore, we demonstrate that ion implantation is a novel pathway to fabricate and control the SiGe nanoparticle structure and potentially useful for future optoelectronic device applications.« less

Characterization of electrical properties in axial Si-Ge nanowire heterojunctions using off-axis electron holography and atom-probe tomography

DOE PAGES

Gan, Zhaofeng; Perea, Daniel E.; Yoo, Jinkyoung; ...

2016-09-13

Doped Si-Ge nanowire (NW) heterojunctions were grown using the vapor-liquid-solid method with AuGa and Au catalyst particles. Transmission electron microscopy and off-axis electron holography (EH) were used to characterize the nanostructure and to measure the electrostatic potential profile across the junction resulting from electrically active dopants, while atom-probe tomography (APT) was used to determine the Si, Ge and total (active and inactive) dopant concentration profiles. A comparison of the measured potential profile with simulations indicated that Ga dopants unintentionally introduced during AuGa catalyst growth were electronically inactive despite APT results that showed considerable amounts of Ga in the Si region.more » 10% P in Ge and 100% B in Si were estimated to be activated, which was corroborated by in situ electron-holography biasing experiments. This combination of EH, APT, in situ biasing and simulations allows a better knowledge and understanding of the electrically active dopant distributions in NWs.« less

On the Application of Inverse-Mode SiGe HBTs in RF Receivers for the Mitigation of Single-Event Transients

NASA Astrophysics Data System (ADS)

Song, Ickhyun; Cho, Moon-Kyu; Oakley, Michael A.; Ildefonso, Adrian; Ju, Inchan; Buchner, Stephen P.; McMorrow, Dale; Paki, Pauline; Cressler, John. D.

2017-05-01

Best practice in mitigation strategies for single-event transients (SETs) in radio-frequency (RF) receiver modules is investigated using a variety of integrated receivers utilizing inverse-mode silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs). The receivers were designed and implemented in a 130-nm SiGe BiCMOS technology platform. In general, RF switches, low-noise amplifiers (LNAs), and downconversion mixers utilizing inverse-mode SiGe HBTs exhibit less susceptibility to SETs than conventional RF designs, in terms of transient peaks and duration, at the cost of RF performance. Under normal RF operation, the SET-hardened switch is mainly effective in peak reduction, while the LNA and the mixer exhibit reductions in transient peaks as well as transient duration.

The Electronic and Optical Properties of Au Doped Single-Layer Phosphorene

NASA Astrophysics Data System (ADS)

Zhu, Ziqing; Chen, Changpeng; Liu, Jiayi; Han, Lu

2018-01-01

The electronic properties and optical properties of single and double Au-doped phosphorene have been comparatively investigated using the first-principles plane-wave pseudopotential method based on density functional theory. The decrease from direct band gap 0.78 eV to indirect band gap 0.22 and 0.11 eV are observed in the single and double Au-doped phosphorene, respectively. The red shifts of absorbing edge occur in both doped systems, which consequently enhance the absorbing of infrared light in phosphorene. Band gap engineering can, therefore, be used to directly tune the optical absorption of phosphorene system by substitutional Au doping.

Alternative Sigma Factors SigF, SigE, and SigG Are Essential for Sporulation in Clostridium botulinum ATCC 3502

PubMed Central

Kirk, David G.; Zhang, Zhen; Korkeala, Hannu

2014-01-01

Clostridium botulinum produces heat-resistant endospores that may germinate and outgrow into neurotoxic cultures in foods. Sporulation is regulated by the transcription factor Spo0A and the alternative sigma factors SigF, SigE, SigG, and SigK in most spore formers studied to date. We constructed mutants of sigF, sigE, and sigG in C. botulinum ATCC 3502 and used quantitative reverse transcriptase PCR and electron microscopy to assess their expression of the sporulation pathway on transcriptional and morphological levels. In all three mutants the expression of spo0A was disrupted. The sigF and sigE mutants failed to induce sigG and sigK beyond exponential-phase levels and halted sporulation during asymmetric cell division. In the sigG mutant, peak transcription of sigE was delayed and sigK levels remained lower than that in the parent strain. The sigG mutant forespore was engulfed by the mother cell and possessed a spore coat but no peptidoglycan cortex. The findings suggest that SigF and SigE of C. botulinum ATCC 3502 are essential for early sporulation and late-stage induction of sigK, whereas SigG is essential for spore cortex formation but not for coat formation, as opposed to previous observations in B. subtilis sigG mutants. Our findings add to a growing body of evidence that regulation of sporulation in C. botulinum ATCC 3502, and among the clostridia, differs from the B. subtilis model. PMID:24928875

Diluted magnetic semiconductors with narrow band gaps

NASA Astrophysics Data System (ADS)

Gu, Bo; Maekawa, Sadamichi

2016-10-01

We propose a method to realize diluted magnetic semiconductors (DMSs) with p - and n -type carriers by choosing host semiconductors with a narrow band gap. By employing a combination of the density function theory and quantum Monte Carlo simulation, we demonstrate such semiconductors using Mn-doped BaZn2As2 , which has a band gap of 0.2 eV. In addition, we found a nontoxic DMS Mn-doped BaZn2Sb2 , of which the Curie temperature Tc is predicted to be higher than that of Mn-doped BaZn2As2 , the Tc of which was up to 230 K in a recent experiment.

DFT study of Al doped armchair SWCNTs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dhiman, Shobhna, E-mail: s-dhiman@hotmail.com; Rani, Anita; Kumar, Ranjan

2016-05-23

Electronic properties of endohedrally doped armchair single-walled carbon nanotubes (SWCNTs) with a chain of six Al atoms have been studied using ab-initio density functional theory. We investigate the binding energy/atom, ionization potential, electron Affinity and Homo-Lumo gap of doped armchair SWNTs from (4,4) to (6,6) with two ends open. BE/dopant atom and ionization potential is maximum for (6, 6) doped armchair carbon nanotube; suggest that it is more stable than (4, 4) and (5, 5) doped tubes. HOMO - LUMO gap of Al doped arm chair carbon nanotubes decreases linearly with the increase in diameter of the tube. This showsmore » that confinement induce a strong effect on electronic properties of doped tubes. These combined systems can be used for future nano electronics. The ab–initio calculations were performed with SIESTA code using generalized gradient approximation (GGA).« less

The reduction of critical H implantation dose for ion cut by incorporating B-doped SiGe/Si superlattice into Si substrate

NASA Astrophysics Data System (ADS)

Xue, Zhongying; Chen, Da; Jia, Pengfei; Wei, Xing; Di, Zengfeng; Zhang, Miao

2016-11-01

An approach to achieve Si or SiGe film exfoliation with as low as 3 × 1016/cm2 H implantation dose was investigated. Two intrinsic Si0.75Ge0.25/Si samples, merged with B-doped Si0.75Ge0.25 layer and B-doped Si0.75Ge0.25/Si superlattice (SL) layer respectively, were used to study the formation of crack after 3 × 1016/cm2 H implantation and annealing. For the sample into which B doped Si0.75Ge0.25 layer is incorporated, only few discrete cracks are observed along both sides of the B doped Si0.75Ge0.25 layer; on the contrary, a continuous (100) oriented crack is formed in the B-doped Si0.75Ge0.25/Si SL layer, which means ion cut can be achieved using this material with 3 × 1016/cm2 H implantation. As the SIMS profiles confirm that hydrogen tends to be trapped at B-doped SiGe/Si interface, the formation of continuous crack in SL layer can be ascribed to the more efficient hydrogen trapping by the multiple B-doped SiGe/Si interfaces.

Magneto-transport Properties Using Top-Gated Hall Bars of Epitaxial Heterostructures on Single-Crystal SiGe Nanomembranes

NASA Astrophysics Data System (ADS)

Jacobson, R. B.; Li, Yize; Foote, Ryan; Cui, Xiaorui; Savage, Donald; Sookchoo, Pornsatit; Eriksson, Mark; Lagally, Max

2014-03-01

A high-quality 2-dimensional electron gas (2DEG) is crucial for quantum electronics and spintronics. Grown heterostructures on SiGe nanomembranes (NMs) show promise to create these 2DEG structures because they have reduced strain inhomogeneities and mosaic tilt. We investigate charge transport properties of these SiGe NMs/heterostructures over a range of temperatures and compare them with results from heterostructures grown on compositionally graded SiGe substrates. Measurements are done by creating Hall bars with top gates on the samples. From the magneto-transport data, low-carrier-density mobility values are calculated. Initial results on the grown heterostructures give a typical curve for mobility versus carrier density, but extraction of the zero-carrier-density mobility is dependent on the curve-fitting technique. Sponsored by United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the U.S. Government.

Thermal conductivity engineering of bulk and one-dimensional Si-Ge nanoarchitectures

PubMed Central

Kandemir, Ali; Ozden, Ayberk; Cagin, Tahir; Sevik, Cem

2017-01-01

Various theoretical and experimental methods are utilized to investigate the thermal conductivity of nanostructured materials; this is a critical parameter to increase performance of thermoelectric devices. Among these methods, equilibrium molecular dynamics (EMD) is an accurate technique to predict lattice thermal conductivity. In this study, by means of systematic EMD simulations, thermal conductivity of bulk Si-Ge structures (pristine, alloy and superlattice) and their nanostructured one dimensional forms with square and circular cross-section geometries (asymmetric and symmetric) are calculated for different crystallographic directions. A comprehensive temperature analysis is evaluated for selected structures as well. The results show that one-dimensional structures are superior candidates in terms of their low lattice thermal conductivity and thermal conductivity tunability by nanostructuring, such as by diameter modulation, interface roughness, periodicity and number of interfaces. We find that thermal conductivity decreases with smaller diameters or cross section areas. Furthermore, interface roughness decreases thermal conductivity with a profound impact. Moreover, we predicted that there is a specific periodicity that gives minimum thermal conductivity in symmetric superlattice structures. The decreasing thermal conductivity is due to the reducing phonon movement in the system due to the effect of the number of interfaces that determine regimes of ballistic and wave transport phenomena. In some nanostructures, such as nanowire superlattices, thermal conductivity of the Si/Ge system can be reduced to nearly twice that of an amorphous silicon thermal conductivity. Additionally, it is found that one crystal orientation, <100>, is better than the <111> crystal orientation in one-dimensional and bulk SiGe systems. Our results clearly point out the importance of lattice thermal conductivity engineering in bulk and nanostructures to produce high

Atomistic simulations of thermal transport in Si and SiGe based materials: From bulk to nanostructures

NASA Astrophysics Data System (ADS)

Savic, Ivana; Mingo, Natalio; Donadio, Davide; Galli, Giulia

2010-03-01

It has been recently proposed that Si and SiGe based nanostructured materials may exhibit low thermal conductivity and overall promising properties for thermoelectric applications. Hence there is a considerable interest in developing accurate theoretical and computational methods which can help interpret recent measurements, identify the physical origin of the reduced thermal conductivity, as well as shed light on the interplay between disorder and nanostructuring in determining a high figure of merit. In this work, we investigate the capability of an atomistic Green's function method [1] to describe phonon transport in several types of Si and SiGe based systems: amorphous Si, SiGe alloys, planar and nanodot Si/SiGe multilayers. We compare our results with experimental data [2,3], and with the findings of molecular dynamics simulations and calculations based on the Boltzmann transport equation. [1] I. Savic, N. Mingo, and D. A. Stewart, Phys. Rev. Lett. 101, 165502 (2008). [2] S.-M. Lee, D. G. Cahill, and R. Venkatasubramanian, Appl. Phys. Lett. 70, 2957 (1997). [3] G. Pernot et al., submitted.

Raman and photoluminescence spectroscopy of SiGe layer evolution on Si(100) induced by dewetting

NASA Astrophysics Data System (ADS)

Shklyaev, A. A.; Volodin, V. A.; Stoffel, M.; Rinnert, H.; Vergnat, M.

2018-01-01

High temperature annealing of thick (40-100 nm) Ge layers deposited on Si(100) at ˜400 °C leads to the formation of continuous films prior to their transformation into porous-like films due to dewetting. The evolution of Si-Ge composition, lattice strain, and surface morphology caused by dewetting is analyzed using scanning electron microscopy, Raman, and photoluminescence (PL) spectroscopies. The Raman data reveal that the transformation from the continuous to porous film proceeds through strong Si-Ge interdiffusion, reducing the Ge content from 60% to about 20%, and changing the stress from compressive to tensile. We expect that Ge atoms migrate into the Si substrate occupying interstitial sites and providing thereby the compensation of the lattice mismatch. Annealing generates only one type of radiative recombination centers in SiGe resulting in a PL peak located at about 0.7 and 0.8 eV for continuous and porous film areas, respectively. Since annealing leads to the propagation of threading dislocations through the SiGe/Si interface, we can tentatively associate the observed PL peak to the well-known dislocation-related D1 band.

Electrical and optical properties of Si-doped Ga2O3

NASA Astrophysics Data System (ADS)

Li, Yin; Yang, Chuanghua; Wu, Liyuan; Zhang, Ru

2017-05-01

The charge densities, band structure, density of states, dielectric functions of Si-doped β-Ga2O3 have been investigated based on the density functional theory (DFT) within the hybrid functional HSE06. The heavy doping makes conduction band split out more bands and further influences the band structure. It decreases the band gap and changes from a direct gap to an indirect gap. After doping, the top of the valence bands is mainly composed by the O-2p states, Si-3p states and Ga-4p states and the bottom of the conduction bands is almost formed by the Si-3s, Si-3p and Ga-4s orbits. The anisotropic optical properties have been investigated by means of the complex dielectric function. After the heavy Si doping, the position of absorption band edges did not change much. The slope of the absorption curve descends and indicates that the absorption became more slow for Si-doped β-Ga2O3 than undoped one due to the indirect gap of Si-doped β-Ga2O3.

The effects of hot carrier and swift heavy ion irradiation on electrical characteristics of advanced 200 GHz SiGe HBTs

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vinayakprasanna, N. H.; Praveen, K. C.; Prakash, A. P. Gnana, E-mail: gnanaprakash@physics.uni-mysore.ac.in

The 200 GHz SiGe HBTs were irradiated with 80 MeV Carbon ions up to a total dose of 100 Mrad to understand the degradation in electrical characteristics. The degradation in the electrical characteristics of SiGe HBTs was also studied by mixed mode electrical stress up to 10,000 s. The electrical characteristics were measured before and after every total dose and after fixed stress time. The normalized peak h{sub FE} of the stressed and irradiated SiGe HBTs are compared to estimate the equivalent stress time for a particular total dose. These correlations are drawn for the first time and the resultsmore » will establish a systematic relation between stress time and total dose.« less

High electron doping to a wide band gap semiconductor 12CaO•7Al2O3 thin film

NASA Astrophysics Data System (ADS)

Miyakawa, Masashi; Hirano, Masahiro; Kamiya, Toshio; Hosono, Hideo

2007-04-01

High-density electrons (˜1.9×1021cm-3) were doped into a polycrystalline film of a wide band gap (˜7eV) semiconductor 12CaO•7Al2O3 (C12A7) by an in situ postdeposition reduction treatment using an oxygen-deficient C12A7 overlayer. The resultant film exhibits metallic conduction with a Hall mobility of ˜2.5cm2V-1s-1 and a conductivity of ˜800Scm-1. Optical analyses indicate that most of the doped electrons behave as free carriers with an effective mass of 0.82me and the estimated in-grain mobility is 5.2cm2V-1s-1, which agrees reasonably with the value obtained for high-quality single crystals.

Reduced thermal conductivity of nanotwinned random layer structures: a promising nanostructuring towards efficient Si and Si/Ge thermoelectric materials

NASA Astrophysics Data System (ADS)

Samaraweera, Nalaka; Chan, Kin L.; Mithraratne, Kumar

2018-05-01

Si and Si/Ge based nanostructures of reduced lattice thermal conductivity are widely attractive for developing efficient thermoelectric materials. In this study, we demonstrate the reduced thermal conductivity of Si nanotwinned random layer (NTRL) structures over corresponding superlattice and twin-free counterparts. The participation ratio analysis of vibrational modes shows that a possible cause of thermal conductivity reduction is phonon localization due to the random arrangement of twin boundaries. Via non-equilibrium molecular dynamic simulations, it is shown that ~23 and ~27% reductions over superlattice counterparts and ~55 and 53% over twin-free counterparts can be attained for the structures of total lengths of 90 and 170 nm, respectively. Furthermore, a random twin boundary distribution is applied for Si/Ge random layer structures seeking further reduction of thermal conductivity. A significant reduction in thermal conductivity of Si/Ge structures exceeding the thermal insulating performance of the corresponding amorphous Si structure by ~31% for a total length of 90 nm can be achieved. This reduction is as high as ~98% compared to the twin-free Si counterpart. It is demonstrated that application of randomly organised nanoscale twin boundaries is a promising nanostructuring strategy towards developing efficient Si and Si/Ge based thermoelectric materials in the future.

Degradation of Si/Ge core/shell nanowire heterostructures during lithiation and delithiation at 0.8 and 20 A g −1

DOE PAGES

Kim, Dongheun; Li, Nan; Sheehan, Chris J.; ...

2018-01-01

The charging rate-dependent degradation of lithium ion battery anodes based on Si/Ge core/shell nanowire heterostructure was observed. Si/Ge core/shell structure was preserved at the charging rate of 0.8 A g −1 . On the other hand, compositional intermixing and loss of Si occurs at the charging rate of 20 A g −1 .

Degradation of Si/Ge core/shell nanowire heterostructures during lithiation and delithiation at 0.8 and 20 A g −1

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Dongheun; Li, Nan; Sheehan, Chris J.

The charging rate-dependent degradation of lithium ion battery anodes based on Si/Ge core/shell nanowire heterostructure was observed. Si/Ge core/shell structure was preserved at the charging rate of 0.8 A g −1 . On the other hand, compositional intermixing and loss of Si occurs at the charging rate of 20 A g −1 .

Highly conducting and wide band gap phosphorous doped nc-Si–QD/a-SiC films as n-type window layers for solar cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kar, Debjit; Das, Debajyoti, E-mail: erdd@iacs.res.in

2016-05-23

Nano-crystalline silicon quantum dots (Si-QDs) embedded in the phosphorous doped amorphous silicon carbide (a-SiC) matrix has been successfully prepared at a low temperature (300 °C) by inductively coupled plasma assisted chemical vapor deposition (ICP-CVD) system from (SiH{sub 4} + CH{sub 4})-plasma with PH{sub 3} as the doping gas. The effect of PH{sub 3} flow rate on structural, optical and electrical properties of the films has been studied. Phosphorous doped nc-Si–QD/a-SiC films with high optical band gap (>1.9 eV) and superior conductivity (~10{sup −2} S cm{sup −1}) are obtained, which could be appropriately used as n-type window layers for nc-Si solarmore » cells in n-i-p configuration.« less

Anisotropy of Seebeck coefficient in Si/Ge composite quantum dots

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hsin, Cheng-Lun, E-mail: clhsin@ee.ncu.edu.tw; Tsai, Yue-Yun; Lee, Sheng-Wei

2016-08-22

In this report, Si{sub 5}Ge{sub 5} alloy and Si/Ge composite quantum dots (CQDs) layers were grown on Si substrates. Seebeck coefficient (S) of Si and Ge wafers, as well as these two samples, were patterned and measured from 60 to 180 °C in [110] and [010] directions. For Si, Ge, and Si{sub 5}Ge{sub 5}, the S of each is a constant in this temperature range. However, the S of the CQDs at 60–80 °C is anomalous and much higher than the others. The behavior of the voltage difference is linear to the temperature difference even as large as 50 °C, except for CQDsmore » at 60–80 °C. This result indicates that a narrow distribution of carriers energy with a sharp change in density of state near Fermi-level and selective carrier scattering in the miniband at Si/Ge interface make the discrepancy of charge transport enhanced. The CQDs can be a good candidate for temperature sensing and thermoelectric applications due to their high S and low thermal conductivity near room temperature.« less

Magnetic behavior of Si-Ge bond in SixGe4-x nano-clusters

NASA Astrophysics Data System (ADS)

Nahali, Masoud; Mehri, Ali

2018-06-01

The structure of SixGe4-x nano-clusters were optimized by MPW1B95 level of theory using MG3S and SDB-aug-cc-PVTZ basis set. The agreement of the calculated ionization and dissociation energies with experimental values validates the reported structures of nano-clusters and justifies the use of hybrid meta density functional method. Since the Si-Si bond is stronger than Si-Ge and Ge-Ge bonds, the Si-Si, Si-Ge, and Ge-Ge diagonal bonds determine the precedence of the stability in these nano-clusters. The hybrid meta density functional calculations were carried out to investigate the adsorption of CO on all possible SixGe4-x nano-clusters. It was found that the silicon atom generally makes a stronger bond with CO than germanium and thereby preferentially affects the shape of structures having higher multiplicity. In Si-Ge structures with higher spin more than 95% of spins accumulate on positions with less bonds to other atoms of the cluster. Through CO adsorption on these clusters bridge structures are made that behave as spin bridge which conduct the spin from the nano-cluster surface to the adsorbate atoms. A better understanding of bridged structures was achieved upon introducing the 'spin bridge' concept. Based on exhaustive spin density analysis, it was found that the reason for the extra negative charge on oxygen in the bridged structures is the relocation of spin from the surface through the bridge.

Observation of dx2-y-Like Superconducting Gap in an Electron-Doped High-Temperature Superconductor

NASA Astrophysics Data System (ADS)

Sato, T.; Kamiyama, T.; Takahashi, T.; Kurahashi, K.; Yamada, K.

2001-02-01

High-resolution angle-resolved photoemission spectroscopy of the electron-doped high-temperature superconductor Nd2-xCexCuO4 (x = 0.15, transition temperature Tc = 22 K) has found the quasiparticle signature as well as the anisotropic dx2-y-like superconducting gap. The spectral line shape at the superconducting state shows a strong anisotropic nature of the many-body interaction. The result suggests that the electron-hole symmetry is present in the high-temperature superconductors.

Thermal conductivity engineering of bulk and one-dimensional Si-Ge nanoarchitectures.

PubMed

Kandemir, Ali; Ozden, Ayberk; Cagin, Tahir; Sevik, Cem

2017-01-01

Various theoretical and experimental methods are utilized to investigate the thermal conductivity of nanostructured materials; this is a critical parameter to increase performance of thermoelectric devices. Among these methods, equilibrium molecular dynamics (EMD) is an accurate technique to predict lattice thermal conductivity. In this study, by means of systematic EMD simulations, thermal conductivity of bulk Si-Ge structures (pristine, alloy and superlattice) and their nanostructured one dimensional forms with square and circular cross-section geometries (asymmetric and symmetric) are calculated for different crystallographic directions. A comprehensive temperature analysis is evaluated for selected structures as well. The results show that one-dimensional structures are superior candidates in terms of their low lattice thermal conductivity and thermal conductivity tunability by nanostructuring, such as by diameter modulation, interface roughness, periodicity and number of interfaces. We find that thermal conductivity decreases with smaller diameters or cross section areas. Furthermore, interface roughness decreases thermal conductivity with a profound impact. Moreover, we predicted that there is a specific periodicity that gives minimum thermal conductivity in symmetric superlattice structures. The decreasing thermal conductivity is due to the reducing phonon movement in the system due to the effect of the number of interfaces that determine regimes of ballistic and wave transport phenomena. In some nanostructures, such as nanowire superlattices, thermal conductivity of the Si/Ge system can be reduced to nearly twice that of an amorphous silicon thermal conductivity. Additionally, it is found that one crystal orientation, [Formula: see text]100[Formula: see text], is better than the [Formula: see text]111[Formula: see text] crystal orientation in one-dimensional and bulk SiGe systems. Our results clearly point out the importance of lattice thermal conductivity

Influencing factors on the size uniformity of self-assembled SiGe quantum rings grown by molecular beam epitaxy.

PubMed

Cui, J; Lv, Y; Yang, X J; Fan, Y L; Zhong, Z; Jiang, Z M

2011-03-25

The size uniformity of self-assembled SiGe quantum rings, which are formed by capping SiGe quantum dots with a thin Si layer, is found to be greatly influenced by the growth temperature and the areal density of SiGe quantum dots. Higher growth temperature benefits the size uniformity of quantum dots, but results in low Ge concentration as well as asymmetric Ge distribution in the dots, which induces the subsequently formed quantum rings to be asymmetric in shape or even broken somewhere in the ridge of rings. Low growth temperature degrades the size uniformity of quantum dots, and thus that of quantum rings. A high areal density results in the expansion and coalescence of neighboring quantum dots to form a chain, rather than quantum rings. Uniform quantum rings with a size dispersion of 4.6% and an areal density of 7.8×10(8) cm(-2) are obtained at the optimized growth temperature of 640°C.

A Theoretical Simulation of the Radiation Responses of Si, Ge, and Si/Ge Superlattice to Low-Energy Irradiation.

PubMed

Jiang, Ming; Xiao, Haiyan; Peng, Shuming; Yang, Guixia; Liu, Zijiang; Qiao, Liang; Zu, Xiaotao

2018-05-02

In this study, the low-energy radiation responses of Si, Ge, and Si/Ge superlattice are investigated by an ab initio molecular dynamics method and the origins of their different radiation behaviors are explored. It is found that the radiation resistance of the Ge atoms that are around the interface of Si/Ge superlattice is comparable to bulk Ge, whereas the Si atoms around the interface are more difficult to be displaced than the bulk Si, showing enhanced radiation tolerance as compared with the bulk Si. The mechanisms for defect generation in the bulk and superlattice structures show somewhat different character, and the associated defects in the superlattice are more complex. Defect formation and migration calculations show that in the superlattice structure, the point defects are more difficult to form and the vacancies are less mobile. The enhanced radiation tolerance of the Si/Ge superlattice will benefit for its applications as electronic and optoelectronic devices under radiation environment.

The Alternative Sigma Factors SigE and SigB Are Involved in Tolerance and Persistence to Antitubercular Drugs

PubMed Central

Pisu, Davide; Provvedi, Roberta; Espinosa, Dulce Mata; Payan, Jorge Barrios; Boldrin, Francesca; Palù, Giorgio; Hernandez-Pando, Rogelio

2017-01-01

ABSTRACT The emergence and spread of drug-resistant Mycobacterium tuberculosis strains possibly threaten our ability to treat this disease in the future. Even though two new antitubercular drugs have recently been introduced, there is still the need to design new molecules whose mechanisms of action could reduce the length of treatment. We show that two alternative sigma factors of M. tuberculosis (SigE and SigB) have a major role in determining the level of basal resistance to several drugs and the amount of persisters surviving long-duration drug treatment. We also demonstrate that ethambutol, a bacteriostatic drug, is highly bactericidal for M. tuberculosis mutants missing either SigE or SigB. We suggest that molecules able to interfere with the activity of SigE or SigB not only could reduce M. tuberculosis virulence in vivo but also could boost the effect of other drugs by increasing the sensitivity of the organism and reducing the number of persisters able to escape killing. PMID:28993339

Growth of strained Si/relaxed SiGe heterostructures on Si(110) substrates using solid-source molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Arimoto, Keisuke; Nakazawa, Hiroki; Mitsui, Shohei; Utsuyama, Naoto; Yamanaka, Junji; Hara, Kosuke O.; Usami, Noritaka; Nakagawa, Kiyokazu

2017-11-01

A strained Si/relaxed SiGe heterostructure grown on Si(110) substrate is attractive as a platform for high-hole-mobility Si-based electronic devices. To improve the electrical property, a smoother surface is desirable. In this study, we investigated surface morphology and microstructural aspects of strained Si/relaxed SiGe/Si(110) heterostructures grown by solid-source (SS) molecular beam epitaxy (MBE). It was revealed that SSMBE provides a way to grow strained Si/relaxed SiGe heterostructures with smooth surfaces. In addition, it was found that the strain in the SiGe layer of the SSMBE-grown sample is highly anisotropic whereas that of the GSMBE-grown sample is almost biaxially relaxed. Along with the surface morphology, the symmetry in degree of strain relaxation has implications for the electrical property. Results of a calculation shows that anisotropic strain is preferable for device application since it confines holes solely in the strained Si layer where hole mobility is enhanced.

A Theoretical Simulation of the Radiation Responses of Si, Ge, and Si/Ge Superlattice to Low-Energy Irradiation

NASA Astrophysics Data System (ADS)

Jiang, Ming; Xiao, Haiyan; Peng, Shuming; Yang, Guixia; Liu, Zijiang; Qiao, Liang; Zu, Xiaotao

2018-05-01

In this study, the low-energy radiation responses of Si, Ge, and Si/Ge superlattice are investigated by an ab initio molecular dynamics method and the origins of their different radiation behaviors are explored. It is found that the radiation resistance of the Ge atoms that are around the interface of Si/Ge superlattice is comparable to bulk Ge, whereas the Si atoms around the interface are more difficult to be displaced than the bulk Si, showing enhanced radiation tolerance as compared with the bulk Si. The mechanisms for defect generation in the bulk and superlattice structures show somewhat different character, and the associated defects in the superlattice are more complex. Defect formation and migration calculations show that in the superlattice structure, the point defects are more difficult to form and the vacancies are less mobile. The enhanced radiation tolerance of the Si/Ge superlattice will benefit for its applications as electronic and optoelectronic devices under radiation environment.

Phase separation in SiGe nanocrystals embedded in SiO{sub 2} matrix during high temperature annealing

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mogaddam, N. A. P.; Turan, R.; Alagoz, A. S.

2008-12-15

SiGe nanocrystals have been formed in SiO{sub 2} matrix by cosputtering Si, Ge, and SiO{sub 2} independently on Si substrate. Effects of the annealing time and temperature on structural and compositional properties are studied by transmission electron microscopy, x-ray diffraction (XRD), and Raman spectroscopy measurements. It is observed that Ge-rich Si{sub (1-x)}Ge{sub x} nanocrystals do not hold their compositional uniformity when annealed at high temperatures for enough long time. A segregation process leading to separation of Ge and Si atoms from each other takes place. This process has been evidenced by a double peak formation in the XRD and Ramanmore » spectra. We attributed this phase separation to the differences in atomic size, surface energy, and surface diffusion disparity between Si and Ge atoms leading to the formation of nonhomogenous structure consist of a Si-rich SiGe core covered by a Ge-rich SiGe shell. This experimental observation is consistent with the result of reported theoretical and simulation methods.« less

SiGe quantum wells for uncooled long wavelength infra-red radiation (LWIR) sensors

NASA Astrophysics Data System (ADS)

Wissmar, S. G. E.; Radamsson, H. H.; Yamamoto, Y.; Tillack, B.; Vieider, C.; Andersson, J. Y.

2008-03-01

We demonstrate a novel single-crystalline high-performance thermistor material based on SiGe quantum well heterostructures. The SiGe/Si quantum wells are grown epitaxially on standard Si [001] substrates. Holes are used as charge carriers utilizing the discontinuities in the valence band structure. By optimizing design parameters such as the barrier height (by variation of the germanium content) and the fermi level Ef (by variation of the quantum well width and doping level) of the material, the layer structure can be tailored. Then a very high temperature coefficient of resistivity (TCR) can be obtained which is superior to the previous reported conventional thin film materials such as vanadium oxide and amorphous silicon. In addition, the high quality crystalline material promises very low 1/f-noise characteristics promoting an outstanding signal to noise ratio as well as well defined and uniform material properties. High-resolution X-ray diffraction was applied to characterize the thickness and Ge content of QWs. The results show sharp oscillations indicating an almost ideal super lattice with negligible relaxation and low defect density. The impact of growth temperature on the thermistor material properties was characterized by analyzing how the resulting strain primarily affects the performance of the TCR and 1/f noise. Results illustrate a value of 3.3 %/K for TCR with a low 1/f noise.

On the interplay between phonon-boundary scattering and phonon-point-defect scattering in SiGe thin films

NASA Astrophysics Data System (ADS)

Iskandar, A.; Abou-Khalil, A.; Kazan, M.; Kassem, W.; Volz, S.

2015-03-01

This paper provides theoretical understanding of the interplay between the scattering of phonons by the boundaries and point-defects in SiGe thin films. It also provides a tool for the design of SiGe-based high-efficiency thermoelectric devices. The contributions of the alloy composition, grain size, and film thickness to the phonon scattering rate are described by a model for the thermal conductivity based on the single-mode relaxation time approximation. The exact Boltzmann equation including spatial dependence of phonon distribution function is solved to yield an expression for the rate at which phonons scatter by the thin film boundaries in the presence of the other phonon scattering mechanisms. The rates at which phonons scatter via normal and resistive three-phonon processes are calculated by using perturbation theories with taking into account dispersion of confined acoustic phonons in a two dimensional structure. The vibrational parameters of the model are deduced from the dispersion of confined acoustic phonons as functions of temperature and crystallographic direction. The accuracy of the model is demonstrated with reference to recent experimental investigations regarding the thermal conductivity of single-crystal and polycrystalline SiGe films. The paper describes the strength of each of the phonon scattering mechanisms in the full temperature range. Furthermore, it predicts the alloy composition and film thickness that lead to minimum thermal conductivity in a single-crystal SiGe film, and the alloy composition and grain size that lead to minimum thermal conductivity in a polycrystalline SiGe film.

Semiconducting behavior of substitutionally doped bilayer graphene

NASA Astrophysics Data System (ADS)

Mousavi, Hamze; Khodadadi, Jabbar; Grabowski, Marek

2018-02-01

In the framework of the Green's functions approach, random tight-binding model and using the coherent potential approximation, electronic characteristics of the bilayer graphene are investigated by exploring various forms of substitutional doping of a single or both layers of the system by either boron and (or) nitrogen atoms. The results for displacement of the Fermi level resemble the behavior of acceptor or donor doping in a conventional semiconductor, dependent on the impurity type and concentration. The particular pattern of doping of just one layer with one impurity type is most efficient for opening a gap within the energy bands which could be tuned directly by impurity concentration. Doping both layers at the same time, each with one impurity type, leads to an anomaly whereby the gap decreases with increasing impurity concentration.

Towards rhombohedral SiGe epitaxy on 150mm c-plane sapphire substrates

NASA Astrophysics Data System (ADS)

Duzik, Adam J.; Park, Yeonjoon; Choi, Sang H.

2015-04-01

Previous work demonstrated for the first time the ability to epitaxially grow uniform single crystal diamond cubic SiGe (111) films on trigonal sapphire (0001) substrates. While SiGe (111) forms two possible crystallographic twins on sapphire (0001), films consisting primarily of one twin were produced on up to 99.95% of the total wafer area. This permits new bandgap engineering possibilities and improved group IV based devices that can exploit the higher carrier mobility in Ge compared to Si. Models are proposed on the epitaxy of such dissimilar crystal structures based on the energetic favorability of crystallographic twins and surface reconstructions. This new method permits Ge (111) on sapphire (0001) epitaxy, rendering Ge an economically feasible replacement for Si in some applications, including higher efficiency Si/Ge/Si quantum well solar cells. Epitaxial SiGe films on sapphire showed a 280% increase in electron mobility and a 500% increase in hole mobility over single crystal Si. Moreover, Ge possesses a wider bandgap for solar spectrum conversion than Si, while the transparent sapphire substrate permits an inverted device structure, increasing the total efficiency to an estimated 30-40%, much higher than traditional Si solar cells. Hall Effect mobility measurements of the Ge layer in the Si/Ge/Si quantum well structure were performed to demonstrate the advantage in carrier mobility over a pure Si solar cell. Another application comes in the use of microelectromechanical devices technology, where high-resistivity Si is currently used as a substrate. Sapphire is a more resistive substrate and offers better performance via lower parasitic capacitance and higher film carrier mobility over the current Si-based technology.

Phosphorus doped graphene by inductively coupled plasma and triphenylphosphine treatments

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shin, Dong-Wook, E-mail: shindong37@skku.edu; Kim, Tae Sung; Yoo, Ji-Beom, E-mail: jbyoo@skku.edu

Highlights: • Substitution doping is a promising method for opening the energy band gap of graphene. • Substitution doping with phosphorus in the graphene lattice has numerous advantage such as high band gap, low formation energy, and high net charge density compared to nitrogen. • V{sub dirac} of Inductively coupled plasma (ICP) and triphenylphosphine (TPP) treated graphene was −57 V, which provided clear evidence of n-type doping. • Substitutional doping of graphene with phosphorus is verified by the XPS spectra of P 2p core level and EELS mapping of phosphorus. • The chemical bonding between P and graphene is verymore » stable for a long time in air (2 months). - Abstract: Graphene is considered a host material for various applications in next-generation electronic devices. However, despite its excellent properties, one of the most important issues to be solved as an electronic material is the creation of an energy band gap. Substitution doping is a promising method for opening the energy band gap of graphene. Herein, we demonstrate the substitutional doping of graphene with phosphorus using inductively coupled plasma (ICP) and triphenylphosphine (TPP) treatments. The electrical transfer characteristics of the phosphorus doped graphene field effect transistor (GFET) have a V{sub dirac} of ∼ − 54 V. The chemical bonding between P and C was clearly observed in XPS spectra, and uniform distribution of phosphorus within graphene domains was confirmed by EELS mapping. The capability for substitutional doping of graphene with phosphorus can significantly promote the development of graphene based electronic devices.« less

Combined wet and dry cleaning of SiGe(001)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Park, Sang Wook; Kaufman-Osborn, Tobin; Kim, Hyonwoong

Combined wet and dry cleaning via hydrofluoric acid (HF) and atomic hydrogen on Si{sub 0.6}Ge{sub 0.4}(001) surface was studied at the atomic level using ultrahigh vacuum scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and x-ray photoelectron spectroscopy to understand the chemical transformations of the surface. Aqueous HF removes native oxide, but residual carbon and oxygen are still observed on Si{sub 0.6}Ge{sub 0.4}(001) due to hydrocarbon contamination from post HF exposure to ambient. The oxygen contamination can be eliminated by shielding the sample from ambient via covering the sample in the HF cleaning solution until the sample is introduced tomore » the vacuum chamber or by transferring the sample in an inert environment; however, both processes still leave carbon contaminant. Dry in-situ atomic hydrogen cleaning above 330 °C removes the carbon contamination on the surface consistent with a thermally activated atomic hydrogen reaction with surface hydrocarbon. A postdeposition anneal at 550 °C induces formation of an atomically flat and ordered SiGe surface observed by STM. STS verifies that the wet and dry cleaned surface has an unpinned Fermi level with no states between the conduction and valence band edge comparable to sputter cleaned SiGe surfaces.« less

Band gap narrowing in nitrogen-doped La2Ti2O7 predicted by density-functional theory calculations.

PubMed

Zhang, Junying; Dang, Wenqiang; Ao, Zhimin; Cushing, Scott K; Wu, Nianqiang

2015-04-14

In order to reveal the origin of enhanced photocatalytic activity of N-doped La2Ti2O7 in both the visible light and ultraviolet light regions, its electronic structure has been studied using spin-polarized conventional density functional theory (DFT) and the Heyd-Scuseria-Ernzerhof (HSE06) hybrid approach. The results show that the deep localized states are formed in the forbidden band when nitrogen solely substitutes for oxygen. Introducing the interstitial Ti atom into the N-doped La2Ti2O7 photocatalyst still causes the formation of a localized energy state. Two nitrogen substitutions co-exist stably with one oxygen vacancy, creating a continuum energy band just above the valence band maximum. The formation of a continuum band instead of mid-gap states can extend the light absorption to the visible light region without increasing the charge recombination, explaining the enhanced visible light performance without deteriorating the ultraviolet light photocatalytic activity.

Effect of Boron Doping on High-Resolution X-Ray Diffraction Metrology

NASA Astrophysics Data System (ADS)

Faheem, M.; Zhang, Y.; Dai, X.

2018-03-01

The effect of boron (B) doping on high-resolution X-ray diffraction (HXRD) metrology has been investigated. Twelve samples of Si1-xGex films were epitaxially grown on Si (100) substrates with different thicknesses, germanium (Ge) concentrations and with/without B dopants. Secondary ion mass spectroscopy (SIMS) and HXRD were employed for measurements of B doping, Ge concentration, strain, and thickness of the layers. The SIMS results show the absence of B in two samples while the rest of the samples have B doping in the range of 8.40 × 1018-8.7 × 1020 atoms/cm3 with Ge concentration of 13.3-55.2 at.%. The HXRD measurements indicate the layers thickness of 7.07-108.13 nm along with Ge concentration of 12.82-49.09 at.%. The difference in the Ge concentration measured by SIMS and HXRD was found to deend on B doping. For the undoped samples, the difference is 0.5 at.% and increases with B doping but with no linear proportionality. The difference in the Ge concentration was 7.11 at.% for the highly B-doped (8.7 × 1020 atoms/cm3) sample. The B doping influences the Si1-xGex structure, causing a change in the lattice parameter and producing tensile strains shifting Si1-xGex peaks towards Si (100) substrate peaks in the HXRD diffraction patterns. As a result, Vegard's law is no longer effective and makes a high impact on the HXRD measurement. The comparison between symmetric (004) and asymmetric (+113, +224) reciprocal space mappings (RSM) showed a slight difference in Ge concentration between the undoped and lower B-doped samples. However, there is a change of 0.21 at.% observed for the highly doped Si1-xGex samples. RSM's (+113) demonstrate the small SiGe peak broadening as B doping increases, which indicates a minor crystal distortion.

The Electronic Properties of O-Doped Pure and Sulfur Vacancy-Defect Monolayer WS₂: A First-Principles Study.

PubMed

Wang, Weidong; Bai, Liwen; Yang, Chenguang; Fan, Kangqi; Xie, Yong; Li, Minglin

2018-01-31

Based on the density functional theory (DFT), the electronic properties of O-doped pure and sulfur vacancy-defect monolayer WS₂ are investigated by using the first-principles method. For the O-doped pure monolayer WS₂, four sizes (2 × 2 × 1, 3 × 3 × 1, 4 × 4 × 1 and 5 × 5 × 1) of supercell are discussed to probe the effects of O doping concentration on the electronic structure. For the 2 × 2 × 1 supercell with 12.5% O doping concentration, the band gap of O-doped pure WS₂ is reduced by 8.9% displaying an indirect band gap. The band gaps in 3 × 3 × 1 and 4 × 4 × 1 supercells are both opened to some extent, respectively, for 5.55% and 3.13% O doping concentrations, while the band gap in 5 × 5 × 1 supercell with 2.0% O doping concentration is quite close to that of the pure monolayer WS₂. Then, two typical point defects, including sulfur single-vacancy (V S ) and sulfur divacancy (V 2S ), are introduced to probe the influences of O doping on the electronic properties of WS₂ monolayers. The observations from DFT calculations show that O doping can broaden the band gap of monolayer WS₂ with V S defect to a certain degree, but weaken the band gap of monolayer WS₂ with V 2S defect. Doping O element into either pure or sulfur vacancy-defect monolayer WS₂ cannot change their band gaps significantly, however, it still can be regarded as a potential method to slightly tune the electronic properties of monolayer WS₂.

Delayed plastic relaxation limit in SiGe islands grown by Ge diffusion from a local source

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vanacore, G. M.; Zani, M.; Tagliaferri, A., E-mail: alberto.tagliaferri@polimi.it

2015-03-14

The hetero-epitaxial strain relaxation in nano-scale systems plays a fundamental role in shaping their properties. Here, the elastic and plastic relaxation of self-assembled SiGe islands grown by surface-thermal-diffusion from a local Ge solid source on Si(100) are studied by atomic force and transmission electron microscopies, enabling the simultaneous investigation of the strain relaxation in different dynamical regimes. Islands grown by this technique remain dislocation-free and preserve a structural coherence with the substrate for a base width as large as 350 nm. The results indicate that a delay of the plastic relaxation is promoted by an enhanced Si-Ge intermixing, induced by themore » surface-thermal-diffusion, which takes place already in the SiGe overlayer before the formation of a critical nucleus. The local entropy of mixing dominates, leading the system toward a thermodynamic equilibrium, where non-dislocated, shallow islands with a low residual stress are energetically stable. These findings elucidate the role of the interface dynamics in modulating the lattice distortion at the nano-scale, and highlight the potential use of our growth strategy to create composition and strain-controlled nano-structures for new-generation devices.« less

Significant thermal conductivity reduction of silicon nanowire forests through discrete surface doping of germanium

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pan, Ying; Hong, Guo; Raja, Shyamprasad N.

2015-03-02

Silicon nanowires (SiNWs) are promising materials for the realization of highly-efficient and cost effective thermoelectric devices. Reduction of the thermal conductivity of such materials is a necessary and viable pathway to achieve sufficiently high thermoelectric efficiencies, which are inversely proportional to the thermal conductivity. In this article, vertically aligned forests of SiNW and germanium (Ge)-doped SiNW with diameters around 100 nm have been fabricated, and their thermal conductivity has been measured. The results show that discrete surface doping of Ge on SiNW arrays can lead to 23% reduction in thermal conductivity at room temperature compared to uncoated SiNWs. Such reduction canmore » be further enhanced to 44% following a thermal annealing step. By analyzing the binding energy changes of Ge-3d and Si-2p using X-ray photoelectron spectroscopy, we demonstrate that surface doped Ge interacts strongly with Si, enhancing phonon scattering at the Si-Ge interface as has also been shown in non-equilibrium molecular dynamics studies of single nanowires. Overall, our results suggest a viable pathway to improve the energy conversion efficiency of nanowire-forest thermoelectric nanomaterials.« less

Nanogrids and Beehive-Like Nanostructures Formed by Plasma Etching the Self-Organized SiGe Islands

NASA Astrophysics Data System (ADS)

Chang, Yuan-Ming; Jian, Sheng-Rui; Juang, Jenh-Yih

2010-09-01

A lithography-free method for fabricating the nanogrids and quasi-beehive nanostructures on Si substrates is developed. It combines sequential treatments of thermal annealing with reactive ion etching (RIE) on SiGe thin films grown on (100)-Si substrates. The SiGe thin films deposited by ultrahigh vacuum chemical vapor deposition form self-assembled nanoislands via the strain-induced surface roughening (Asaro-Tiller-Grinfeld instability) during thermal annealing, which, in turn, serve as patterned sacrifice regions for subsequent RIE process carried out for fabricating nanogrids and beehive-like nanostructures on Si substrates. The scanning electron microscopy and atomic force microscopy observations confirmed that the resultant pattern of the obtained structures can be manipulated by tuning the treatment conditions, suggesting an interesting alternative route of producing self-organized nanostructures.

Synthesis of Antimony Doped Amorphous Carbon Films

NASA Astrophysics Data System (ADS)

Okuyama, H.; Takashima, M.; Akasaka, H.; Ohtake, N.

2013-06-01

We report the effects of antimony (Sb) doping on the electrical and optical properties of amorphous carbon (a-C:H) films grown on silicon and copper substrates by magnetron sputtering deposition. For film deposition, the mixture targets fabricated from carbon and antimony powders was used. The atomic concentration of carbon, hydrogen, and antimony, in the film deposited from the 1.0 mol% Sb containing target were 81, 17, 2 at.%, respectively. These elements were homogeneously distributed in the film. On the structural effect, the average continuous sp2 carbon bonding networks decreased with Sb concentration increasing, and defects in the films were increased with the Sb incorporation because atomic radius of Sb atoms is twice larger size than that of carbon. The optical gap and the electrical resistivity were carried out before and after the Sb doping. The results show that optical gap dropped from 3.15 to 3.04 eV corresponding to non-doping to Sb-doping conditions, respectively. The electrical resistivity reduced from 10.5 to 1.0 MΩm by the Sb doping. These results suggest the doping level was newly formed in the forbidden band.

Influence of the ``second gap'' on the optical absorption of transparent conducting oxides

NASA Astrophysics Data System (ADS)

Ha, Viet-Anh; Waroquiers, David; Rignanese, Gian-Marco; Hautier, Geoffroy

Transparent conducting oxides (TCOs) are critical to many technologies (e.g., thin-film solar cells, flat-panel displays or organic light-emitting diodes). TCOs are heavily doped (n or p-type) oxides that satisfy many design criteria such as high transparency to visible light (i.e., a band gap > 3 eV), high concentration and mobility of carriers (leading to high conductivity), ... In such (highly doped) systems, optical transitions from the conduction band minimum to higher energy bands in n-type or from lower energy bands to the valence band maximum in p-type are possible and can degrade transparency. In fact, it has been claimed that a high energy (> 3eV) for any of these transitions made possible by doping, commonly referred as a high ``second gap'', is a necessary design criterion for high performance TCOs. Here, we study the influence of this second gap on the transparency of doped TCOs by using ab initio calculations within the random phase approximation (RPA) for several well-known p-type and n-type TCOs. Our work highlights how the second gap affects the transparency of doped TCOs, shining light on more accurate design criteria for high performance TCOs.

Optoelectronic Properties of X-Doped (X = O, S, Te) Photovoltaic CSe with Puckered Structure.

PubMed

Zhang, Qiang; Xin, Tianyuan; Lu, Xiaoke; Wang, Yuexia

2018-03-16

We exploited novel two-dimensional (2D) carbon selenide (CSe) with a structure analogous to phosphorene, and probed its electronics and optoelectronics. Calculating phonon spectra using the density functional perturbation theory (DFPT) method indicated that 2D CSe possesses dynamic stability, which made it possible to tune and equip CSe with outstanding properties by way of X-doping (X = O, S, Te), i.e., X substituting Se atoms. Then systematic investigation on the structural, electronic, and optical properties of pristine and X-doped monolayer CSe was carried out using the density functional theory (DFT) method. It was found that the bonding feature of C-X is intimately associated with the electronegativity and radius of the doping atoms, which leads to diverse electronic and optical properties for doping different group VI elements. All the systems possess direct gaps, except for O-doping. Substituting O for Se atoms in monolayer CSe brings about a transition from a direct Γ-Γ band gap to an indirect Γ-Y band gap. Moreover, the value of the band gap decreases with increased doping concentration and radius of doping atoms. A red shift in absorption spectra occurs toward the visible range of radiation after doping, and the red-shift phenomenon becomes more obvious with increased radius and concentration of doping atoms. The results can be useful for filtering doping atoms according to their radius or electronegativity in order to tailor optical spectra efficiently.

Optoelectronic Properties of X-Doped (X = O, S, Te) Photovoltaic CSe with Puckered Structure

PubMed Central

Zhang, Qiang; Xin, Tianyuan; Lu, Xiaoke; Wang, Yuexia

2018-01-01

We exploited novel two-dimensional (2D) carbon selenide (CSe) with a structure analogous to phosphorene, and probed its electronics and optoelectronics. Calculating phonon spectra using the density functional perturbation theory (DFPT) method indicated that 2D CSe possesses dynamic stability, which made it possible to tune and equip CSe with outstanding properties by way of X-doping (X = O, S, Te), i.e., X substituting Se atoms. Then systematic investigation on the structural, electronic, and optical properties of pristine and X-doped monolayer CSe was carried out using the density functional theory (DFT) method. It was found that the bonding feature of C-X is intimately associated with the electronegativity and radius of the doping atoms, which leads to diverse electronic and optical properties for doping different group VI elements. All the systems possess direct gaps, except for O-doping. Substituting O for Se atoms in monolayer CSe brings about a transition from a direct Γ-Γ band gap to an indirect Γ-Y band gap. Moreover, the value of the band gap decreases with increased doping concentration and radius of doping atoms. A red shift in absorption spectra occurs toward the visible range of radiation after doping, and the red-shift phenomenon becomes more obvious with increased radius and concentration of doping atoms. The results can be useful for filtering doping atoms according to their radius or electronegativity in order to tailor optical spectra efficiently. PMID:29547504

Bulk superconducting phase with a full energy gap in the doped topological insulator Cu(x)Bi₂Se₃.

PubMed

Kriener, M; Segawa, Kouji; Ren, Zhi; Sasaki, Satoshi; Ando, Yoichi

2011-03-25

The superconductivity recently found in the doped topological insulator Cu(x)Bi₂Se₃ offers a great opportunity to search for a topological superconductor. We have successfully prepared a single-crystal sample with a large shielding fraction and measured the specific-heat anomaly associated with the superconductivity. The temperature dependence of the specific heat suggests a fully gapped, strong-coupling superconducting state, but the BCS theory is not in full agreement with the data, which hints at a possible unconventional pairing in Cu(x)Bi₂Se₃. Also, the evaluated effective mass of 2.6m(e) (m(e) is the free electron mass) points to a large mass enhancement in this material.

Visible sub-band gap photoelectron emission from nitrogen doped and undoped polycrystalline diamond films

NASA Astrophysics Data System (ADS)

Elfimchev, S.; Chandran, M.; Akhvlediani, R.; Hoffman, A.

2017-07-01

In this study the origin of visible sub-band gap photoelectron emission (PEE) from polycrystalline diamond films is investigated. The PEE yields as a function of temperature were studied in the wavelengths range of 360-520 nm. Based on the comparison of electron emission yields from diamond films deposited on silicon and molybdenum substrates, with different thicknesses and nitrogen doping levels, we suggested that photoelectrons are generated from nitrogen related centers in diamond. Our results show that diamond film thickness and substrate material have no significant influence on the PEE yield. We found that nanocrystalline diamond films have low electron emission yields, compared to microcrystalline diamond, due to the presence of high amount of defects in the former, which trap excited electrons before escaping into the vacuum. However, the low PEE yield of nanocrystalline diamond films was found to increase with temperature. The phenomenon was explained by the trap assisted photon enhanced thermionic emission (ta-PETE) model. According to the ta-PETE model, photoelectrons are trapped by shallow traps, followed by thermal excitation at elevated temperatures and escape into the vacuum. Activation energies of trap levels were estimated for undoped nanocrystalline, undoped microcrystalline and N-doped diamond films using the Richardson-Dushman equation, which gives 0.13, 0.39 and 0.04 eV, respectively. Such low activation energy of trap levels makes the ta-PETE process very effective at elevated temperatures.

Magnetic, electronic, optical, and photocatalytic properties of nonmetal- and halogen-doped anatase TiO2 nanotubes

NASA Astrophysics Data System (ADS)

Fadlallah, M. M.

2017-05-01

The structure stability, magnetic, electronic, optical, and photocatalytic properties of nonmetal (B, C, N, P, and S), and halogen (F, Cl, Br, and I)-doped anatase TiO2 nanotubes (TNTs) have been investigated using spin polarized density functional theory. The N- and F-doped TNTs are the most stable among other doped TNTs. It is found that the magnetic moment of doped TNT is the difference between the number of the valence electrons of the dopant and host anion. All dopants decrease the band gap of TNT. The decrease in the band gap of nonmetal (C, N, P, and S)-doped TNTs, in particular N and P, is larger than that of halogen-doped TNTs due to the created states of the nonmetal dopant in the band gap. There is a good agreement between the calculation results and the experimental observations. Even though C-, N-, and P-doped TNTs have the lowest band gap, they cannot be used as a photocatalysis for water splitting. The B-, S-, and I-doped TiO2 nanotubes are of great potential as candidates for water splitting in the visible light range.

Insight into the Role of Size Modulation on Tuning the Band Gap and Photocatalytic Performance of Semiconducting Nitrogen-Doped Graphene.

PubMed

Yang, Mei-Ling; Zhang, Nan; Lu, Kang-Qiang; Xu, Yi-Jun

2017-04-04

Considerable attention has been focused on transforming graphene (GR) into semiconducting GR by diverse strategies, which can perform as one type of promising photocatalyst toward various photoredox reactions. Herein, we report a facile alkali-assisted hydrothermal method for simultaneous tailoring of the lateral size of GR and nitrogen (N) doping into the GR matrix, by which small-sized N-doped GR (S-NGR) can be obtained. For comparison, large-sized N-doped GR (L-NGR) has also been achieved through the same hydrothermal treatment except for the addition of alkali. The photocatalytic activity test shows that S-NGR exhibits much higher activity than L-NGR toward the degradation of organic pollutants under visible-light irradiation. Structure-photoactivity correlation analysis and characterization suggest that the underlying origin for the significantly enhanced visible-light photoactivity of S-NGR in comparison with L-NGR can be assigned to the lateral size decrease in the NGR sheet, which is able to tune the band gap of semiconducting NGR, to facilitate the separation and transfer of photogenerated charge carriers, and to improve the adsorption capacity of NGR toward the reactant. It is expected that this work will cast new light on the judicious utilization of semiconducting GR with controlled size modulation and heteroatom doping to tune its physicochemical properties, thereby advancing further developments in the rational design of more efficient semiconducting GR materials for diverse applications in photocatalysis.

Universal behavior of surface-dangling bonds in hydrogen-terminated Si, Ge, and Si/Ge nanowires.

NASA Astrophysics Data System (ADS)

Nunes, Ricardo; Kagimura, Ricardo; Chacham, Hélio

2007-03-01

We report an ab initio study of the electronic properties of surface dangling bond (SDB) states in hydrogen-terminated Si, Ge, and Si/Ge nanowires with diameters between 1 and 2 nm. We find that the charge transition levels ɛ(+/-) of SDB states are deep in the bandgap for Si wires, and shallow (near the valence band edge) for Ge wires. In both Si and Ge wires, the SDB states are localized. We also find that the SDB ɛ(+/-) levels behave as a ``universal" energy reference level among Si, Ge, and Si/Ge wires within a precision of 0.1 eV. By computing the average bewteen the electron affinity and ionization energy in the atomi limit of several atoms from the III, IV and V columns, we conjecture that the universality is a periodic-table atomic property.

The influence of different alkaline earth oxides on the structural and optical properties of undoped, Ce-doped, Sm-doped, and Sm/Ce co-doped lithium alumino-phosphate glasses

NASA Astrophysics Data System (ADS)

Othman, H. A.; Arzumanyan, G. M.; Möncke, D.

2016-12-01

Undoped, singly Sm doped, Ce doped, and Sm/Ce co-doped lithium alumino-phosphate glasses with different alkaline earth modifiers were prepared by melt quenching. The structure of the prepared glasses was investigated by FT-IR and Raman, as well as by optical spectroscopy. The effect of the optical basicity of the host glass matrix on the added active dopants was studied, as was the effect doping had on the phosphate structural units. The optical edge shifts toward higher wavelengths with an increase in the optical basicity due to the increased polarizability of the glass matrix, but also with increasing CeO2 concentration as a result of Ce3+/Ce4+ inter valence charge transfer (IV-CT) absorption. The optical band gap for direct and indirect allowed transitions was calculated for the undoped glasses. The glass sample containing Mg2+ modifier ions is found to have the highest value (4.16 eV) for the optical band gap while Ba2+ has the lowest value (3.61 eV). The change in the optical band gap arises from the structural changes and the overall polarizability (optical basicity). Refractive index, molar refractivity Rm and molar polarizability αm values increase with increasing optical basicity of the glasses. The characteristic absorption peaks of Sm3+ were also investigated. For Sm/Ce co-doped glasses, especially at high concentration of CeO2, the absorption of Ce3+ hinders the high energy absorption of Sm3+ and this effect becomes more obvious with increasing optical basicity.

A Comparative Study of Heavy Ion and Proton Induced Bit Error Sensitivity and Complex Burst Error Modes in Commercially Available High Speed SiGe BiCMOS

NASA Technical Reports Server (NTRS)

Marshall, Paul; Carts, Marty; Campbell, Art; Reed, Robert; Ladbury, Ray; Seidleck, Christina; Currie, Steve; Riggs, Pam; Fritz, Karl; Randall, Barb

2004-01-01

A viewgraph presentation that reviews recent SiGe bit error test data for different commercially available high speed SiGe BiCMOS chips that were subjected to various levels of heavy ion and proton radiation. Results for the tested chips at different operating speeds are displayed in line graphs.

Doping effect on monolayer MoS2 for visible light dye degradation - A DFT study

NASA Astrophysics Data System (ADS)

Cheriyan, Silpa; Balamurgan, D.; Sriram, S.

2018-04-01

The electronic and optical properties of, Nitrogen (N), Cobalt (Co), and Co-N co-doped monolayers of MoS2 has been studied by using density functional theory (DFT) for visible light photocatalytic activity. From the calculations, it has been observed that the band gap of monolayer MoS2 has been reduced while doping. However, the band gaps of pristine and N doped MoS2 monolayers only falls in the visible region while for Co and Co-N co-doped systems, the band gap shifted to IR region. The optical calculation also confirms the results. The formation energy values of the doped system reaveal that MoS2 monolayer drops its stability while doping. To evaluate the photocatalytic response, band edge potentials of pristine and N-MoS2 are calculated, and the observed results show that compared to N-doped MoS2 monolayer, pure MoS2 is highly suitable for visible light photocatalytic dye degradation.

Effects on the magnetic and optical properties of Co-doped ZnO at different electronic states

NASA Astrophysics Data System (ADS)

Huo, Qingyu; Xu, Zhenchao; Qu, Linfeng

2017-12-01

Both blue and red shifts in the absorption spectrum of Co-doped ZnO have been reported at a similar concentration range of doped Co. Moreover, the sources of magnetism of Co-doped ZnO are controversial. To solve these problems, the geometry optimization and energy of different Co-doped ZnO systems were calculated at the states of electron spin polarization and nonspin polarization by adopting plane-wave ultra-soft pseudopotential technology based on density function theory. At the state of electron nonspin polarization, the total energies increased as the concentration of Co-doped increased. The doped systems also became unstable. The formation energies increased and doping became difficult. Furthermore, the band gaps widened and the absorption spectrum exhibited a blue shift. The band gaps were corrected by local-density approximation + U at the state of electron spin polarization. The magnetic moments of the doped systems weakened as the concentration of doped Co increased. The magnetic moments were derived from the coupling effects of sp-d. The band gaps narrowed and the absorption spectrum exhibited a red shift. The inconsistencies of the band gaps and absorption spectrum at the states of electron spin polarization and nonspin polarization were first discovered in this research, and the sources of Co-doped ZnO magnetism were also reinterpreted.

Visible and infrared emission from Si/Ge nanowires synthesized by metal-assisted wet etching.

PubMed

Irrera, Alessia; Artoni, Pietro; Fioravanti, Valeria; Franzò, Giorgia; Fazio, Barbara; Musumeci, Paolo; Boninelli, Simona; Impellizzeri, Giuliana; Terrasi, Antonio; Priolo, Francesco; Iacona, Fabio

2014-02-12

Multi-quantum well Si/Ge nanowires (NWs) were realized by combining molecular beam epitaxy deposition and metal-assisted wet etching, which is a low-cost technique for the synthesis of extremely dense (about 1011 cm-2) arrays of NWs with a high and controllable aspect ratio. In particular, we prepared ultrathin Si/Ge NWs having a mean diameter of about 8 nm and lengths spanning from 1.0 to 2.7 μm. NW diameter is compatible with the occurrence of quantum confinement effects and, accordingly, we observed light emission assignable to the presence of Si and Ge nanostructures. We performed a detailed study of the photoluminescence properties of the NWs, with particular attention to the excitation and de-excitation properties as a function of the temperature and of the excitation photon flux, evaluating the excitation cross section and investigating the presence of non-radiative phenomena. PACS: 61.46.Km; 78.55.-m; 78.67.Lt.

Visible and infrared emission from Si/Ge nanowires synthesized by metal-assisted wet etching

PubMed Central

2014-01-01

Abstract Multi-quantum well Si/Ge nanowires (NWs) were realized by combining molecular beam epitaxy deposition and metal-assisted wet etching, which is a low-cost technique for the synthesis of extremely dense (about 1011 cm−2) arrays of NWs with a high and controllable aspect ratio. In particular, we prepared ultrathin Si/Ge NWs having a mean diameter of about 8 nm and lengths spanning from 1.0 to 2.7 μm. NW diameter is compatible with the occurrence of quantum confinement effects and, accordingly, we observed light emission assignable to the presence of Si and Ge nanostructures. We performed a detailed study of the photoluminescence properties of the NWs, with particular attention to the excitation and de-excitation properties as a function of the temperature and of the excitation photon flux, evaluating the excitation cross section and investigating the presence of non-radiative phenomena. PACS 61.46.Km; 78.55.-m; 78.67.Lt PMID:24521284

Heavy Ion Microbeam- and Broadbeam-Induced Transients in SiGe HBTs

NASA Technical Reports Server (NTRS)

Pellish, Jonathan A.; Reed, Robert A.; McMorrow, Dale; Vizkelethy, Gyorgy; Ferlet-Cavrois, Veronique; Baggio, Jacques; Duhamel, Olivier; Moen, Kurt A.; Phillips, Stanley D.; Diestelhorst, Ryan M.;

Ferromagnetism in two-dimensional hole-doped SnO

NASA Astrophysics Data System (ADS)

Houssa, M.; Iordanidou, K.; Pourtois, G.; Afanas'ev, V. V.; Stesmans, A.

2018-05-01

Hole-doped monolayer SnO has been recently predicted to be a ferromagnetic material, for a hole density typically above 5x1013/cm2. The possibility to induce a hole-doped stable ferromagnetic order in this two-dimensional material, either by intrinsic or extrinsic defects, is theoretically studied, using first-principles simulations. Sn vacancies and Sn vacancy-hydrogen complexes are predicted to be shallow acceptors, with relatively low formation energies in SnO monolayers grown under O-rich conditions. These defects produce spin-polarized gap states near the valence band-edge, potentially stabilizing the ferromagnetic order in 2D SnO. Hole-doping resulting from substitutional doping is also investigated. Among the considered possible dopants, As, substituting O, is predicted to produce shallow spin-polarized gap states near the valence band edge, also potentially resulting in a stable ferromagnetic order in SnO monolayers.

Electronic structure changes during the on-surface synthesis of nitrogen-doped chevron-shaped graphene nanoribbons

NASA Astrophysics Data System (ADS)

Maaß, Friedrich; Utecht, Manuel; Stremlau, Stephan; Gille, Marie; Schwarz, Jutta; Hecht, Stefan; Klamroth, Tillmann; Tegeder, Petra

2017-07-01

Utilizing suitable precursor molecules, a thermally activated and surface-assisted synthesis results in the formation of defect-free graphene nanoribbons (GNRs), which exhibit electronic properties that are not present in extended graphene. Most importantly, they have a band gap in the order of a few electron volts, depending on the nanoribbon width. In this study, we investigate the electronic structure changes during the formation of GNRs, nitrogen-doped (singly and doubly N-doped) as well as non-N-doped chevron-shaped CGNRs on Au(111). Thus we determine the optical gaps of the precursor molecules, the intermediate nonaromatic polymers, and finally the aromatic GNRs, using high-resolution electron energy loss spectroscopy and density functional theory calculations. As expected, we find no influence of N-doping on the size of the optical gaps. The gap of the precursor molecules is around 4.5 eV. Polymerization leads to a reduction of the gap to a value of 3.2 eV due to elongation and thus enhanced delocalization. The CGNRs exhibit a band gap of 2.8 eV, thus the gap is further reduced in the nanoribbons, since they exhibit an extended delocalized π -electron system.

The role of SiGe buffer in growth and relaxation of Ge on free-standing Si(001) nano-pillars.

PubMed

Zaumseil, P; Kozlowski, G; Schubert, M A; Yamamoto, Y; Bauer, J; Schülli, T U; Tillack, B; Schroeder, T

2012-09-07

We study the growth and relaxation processes of Ge nano-clusters selectively grown by chemical vapor deposition on free-standing 90 nm wide Si(001) nano-pillars with a thin Si(0.23)Ge(0.77) buffer layer. We found that the dome-shaped SiGe layer with a height of about 28 nm as well as the Ge dot deposited on top of it partially relaxes, mainly by elastic lattice bending. The Si nano-pillar shows a clear compliance behavior-an elastic response of the substrate on the growing film-with the tensile strained top part of the pillar. Additional annealing at 800 °C leads to the generation of misfit dislocation and reduces the compliance effect significantly. This example demonstrates that despite the compressive strain generated due to the surrounding SiO(2) growth mask it is possible to realize an overall tensile strain in the Si nano-pillar and following a compliant substrate effect by using a SiGe buffer layer. We further show that the SiGe buffer is able to improve the structural quality of the Ge nano-dot.

Improvement in thermoelectric power factor of mechanically alloyed p-type SiGe by incorporation of TiB{sub 2}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ahmad, Sajid, E-mail: sajidahmadiitkgp@gmail.com; Dubey, K.; Bhattacharya, Shovit

2016-05-23

Nearly 60% of the world’s useful energy is wasted as heat and recovering a fraction of this waste heat by converting it as useful electrical power is an important area of research{sup [1]}. Thermoelectric power generators (TEG) are solid state devices which converts heat into electricity. TEG consists of n and p-type thermoelements connected electrically in series and thermally in parallel{sup [2]}. Silicon germanium (SiGe) alloy is one of the conventional high temperature thermoelectric materials and is being used in radio-isotopes based thermoelectric power generators for deep space exploration programs.Temperature (T) dependence of thermoelectric (TE) properties of p-type SiGe andmore » p-type SiGe-x wt.%TiB{sub 2} (x=6,8,10%) nanocomposite materials has been studied with in the temperature range of 300 K to 1100 K. It is observed that there is an improvement in the power factor (α{sup 2}/ρ) of SiGe alloy on addition of TiB{sub 2} upto 8 wt.% that is mainly due to increase in the Seebeck coefficient (α) and electrical conductivity (σ) of the alloy.« less

Cryogenic ultra-low-noise SiGe transistor amplifier.

PubMed

Ivanov, B I; Trgala, M; Grajcar, M; Il'ichev, E; Meyer, H-G

2011-10-01

An ultra-low-noise one-stage SiGe heterojunction bipolar transistor amplifier was designed for cryogenic temperatures and a frequency range of 10 kHz-100 MHz. A noise temperature T(N) ≈ 1.4 K was measured at an ambient temperature of 4.2 K at frequencies between 100 kHz and 100 MHz for a source resistance of ~50 Ω. The voltage gain of the amplifier was 25 dB at a power consumption of 720 μW. The input voltage noise spectral density of the amplifier is about 35 pV/√Hz. The low noise resistance and power consumption makes the amplifier suitable for readout of resistively shunted DC SQUID magnetometers and amplifiers.

Novel mid-infrared silicon/germanium detector concepts

NASA Astrophysics Data System (ADS)

Presting, Hartmut; Konle, Johannes; Hepp, Markus; Kibbel, Horst; Thonke, Klaus; Sauer, Rolf; Corbin, Elizabeth A.; Jaros, Milan

2000-10-01

Highly p-doped silicon/silicon-germanium (Si/SiGe) quantum well (QW) structures are grown by molecular beam epitaxy on double-sided polished (100)Si substrates for mid-IR (3 to 5 micrometers and 8 to 12 micrometers ) detection. The samples are characterized by secondary ion mass spectroscopy, x-ray diffraction, and absorption measurements. Single mesa detectors are fabricated as well as large-area focal plane arrays with 256 X 256 pixels using standard Si integrated processing techniques. The detectors, based on heterointernal photo-emission (HIP) of photogenerated holes from a heavily p-doped (p++ approximately 5 X 1020 cm-3) SiGe QW into an undoped silicon layer, operate at 77 K. Various novel designs of the SiGe HIP's such as Ge- and B-grading, double- and multi-wells, are realized; in addition, thin doping setback layers between the highly doped well and the undoped Si layer are introduced. The temperature dependence of dark currents and photocurrents are measured up to 225 K. In general, we observe broad photoresponse curves with peak external quantum efficiencies, up to (eta) ext approximately 0.5% at 77 K and 4(mu) , detectivities up to 8 X 1011 cm(root)Hz/W are obtained. We demonstrate that by varying the thickness, Ge content, and doping level of the single- and the multi-QWs of SiGe HIP detectors, the photoresponse peak and the cutoff of the spectrum can be tuned over a wide wavelength range. The epitaxial versatility of the Si/SiGe system enables a tailoring of the photoresponse spectrum which demonstrates the advantages of the SiGe system in comparison over commercially used silicide detectors.

Characterization of rodlike structures in Si-Ge-GaP alloys

NASA Astrophysics Data System (ADS)

Srikant, V.; Jesser, W. A.; Rosi, F. D.

1996-07-01

High-temperature microstructure of Si-Ge alloys containing 10-15 mole % GaP were studied. Quenching the 80/20 Si-Ge alloy (80 at. % Si) from above 1125 °C and the 50/50 Si-Ge alloy (50 at. % Si) from above 1025 °C resulted in a duplex microstructure. The two-phase regions consisted of a regular array of rodlike structures (GaP) in a Si-Ge matrix whereas the monophase regions were pure Si-Ge. These rodlike structures were found to lie along the [001] direction and result in {002} spots in a [100] electron diffraction pattern. The ``rods'' were about 35 and 45 nm in diameter in the case of the 80/20 and 50/50 alloy, respectively. These structures are not stable on annealing and do not form when the solidification rate is decreased.

Optical, structural and thermal properties of bismuth nitrate doped polycarbonate composite

NASA Astrophysics Data System (ADS)

Mirji, Rajeshwari; Lobo, Blaise

2018-04-01

Bismuth nitrate (Bi(NO3)3) doped polycarbonate (PC) films were prepared by solution casting method, in the doping range varying from 0.1 wt% to 5 wt %. The prepared samples were characterized using UV-Visible spectroscopy, X-Ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC). Optical band gap was calculated by analyzing the UV-Visible spectra of pure as well as doped PC. Optical band gap is found to decrease from 4.38 eV to 4.33 eV as the Bi(NO3)3 content within PC increases. XRD patterns showed an increase in the degree of crystallinity of Bi(NO3)3 doped PC, especially at 3.5 wt% and 5 wt%. DSC study showed an increase in the degradation temperature, as the doping level is increased from 0 wt% up to 0.3 wt%. A decrease in Tg is observed as the doping level of these samples increases from 0 wt% up to 5 wt%.

First-principles study on silicon atom doped monolayer graphene

NASA Astrophysics Data System (ADS)

Rafique, Muhammad; Shuai, Yong; Hussain, Nayyar

2018-01-01

This paper illustrates the structural, electronic and optical properties of individual silicon (Si) atom-doped single layer graphene using density functional theory method. Si atom forms tight bonding with graphene layer. The effect of doping has been investigated by varying the concentration of Si atoms from 3.125% to 9.37% (i.e. From one to three Si atoms in 4 × 4 pure graphene supercell containing 32 carbon atoms), respectively. Electronic structure, partial density of states (PDOS) and optical properties of pure and Si atom-doped graphene sheet were calculated using VASP (Vienna ab-initio Simulation Package). The calculated results for pure graphene sheet were then compared with Si atom doped graphene. It is revealed that upon Si doping in graphene, a finite band gap appears at the high symmetric K-point, thereby making graphene a direct band gap semiconductor. Moreover, the band gap value is directly proportional to the concentration of impurity Si atoms present in graphene lattice. Upon analyzing the optical properties of Si atom-doped graphene structures, it is found that, there is significant change in the refractive index of the graphene after Si atom substitution in graphene. In addition, the overall absorption spectrum of graphene is decreased after Si atom doping. Although a significant red shift in absorption is found to occur towards visible range of radiation when Si atom is substituted in its lattice. The reflectivity of graphene improves in low energy region after Si atom substitution in graphene. These results can be useful for tuning the electronic structure and to manipulate the optical properties of graphene layer in the visible region.

Magnetic disorder and gap symmetry in the optimally electron-doped Sr(Fe ,Co ) 2As2 superconductor

NASA Astrophysics Data System (ADS)

Harnagea, Luminita; Mani, Giri; Kumar, Rohit; Singh, Surjeet

2018-02-01

We study magnetic pair breaking due to Mn impurities in the optimally electron-doped superconductor Sr (Fe0.88Co0.12)2As2 . We found that the as-grown Sr (Fe0.88-yCo0.12Mny) 2As2 single crystals exhibit a Tc suppression rate of ˜30 mK/μ Ω cm . This rate is slow but in good agreement with the previous reports on various magnetic/nonmagnetic impurities doped in other structurally analogous iron-based superconductors. The slow Tc suppression rate for magnetic impurities is often cited as an evidence for the nonvalidity of the s++-wave symmetry, which should have suppressed Tc in accordance with the Abrikosov-Gor'kov theory. Here, we show that the crystallographic defects are the main source of pair breaking in the as-grown crystals. Once these defects are healed by a low-temperature annealing, the true Tc suppression rate due to Mn impurities is revealed. We thus estimate the actual Tc suppression rate due to Mn alone to be ≥325 mK / μ Ω cm , and that due to the nonmagnetic crystallographic defects to be nearly 35 mK/μ Ω cm . These findings can be reconciled with the fully gapped s+--wave symmetry provided the interband scattering is rather weak. On the other hand, the s++-wave symmetry, which is resilient to the nonmagnetic defects and fragile against the magnetic impurities, can be a possible pairing symmetry in the optimally electron-doped SrFe2As2 . The crucial information that we provide here is that the magnetic pair breaking in these superconductors is not as weak as is generally believed.

SiGe epitaxial memory for neuromorphic computing with reproducible high performance based on engineered dislocations

NASA Astrophysics Data System (ADS)

Choi, Shinhyun; Tan, Scott H.; Li, Zefan; Kim, Yunjo; Choi, Chanyeol; Chen, Pai-Yu; Yeon, Hanwool; Yu, Shimeng; Kim, Jeehwan

2018-01-01

Although several types of architecture combining memory cells and transistors have been used to demonstrate artificial synaptic arrays, they usually present limited scalability and high power consumption. Transistor-free analog switching devices may overcome these limitations, yet the typical switching process they rely on—formation of filaments in an amorphous medium—is not easily controlled and hence hampers the spatial and temporal reproducibility of the performance. Here, we demonstrate analog resistive switching devices that possess desired characteristics for neuromorphic computing networks with minimal performance variations using a single-crystalline SiGe layer epitaxially grown on Si as a switching medium. Such epitaxial random access memories utilize threading dislocations in SiGe to confine metal filaments in a defined, one-dimensional channel. This confinement results in drastically enhanced switching uniformity and long retention/high endurance with a high analog on/off ratio. Simulations using the MNIST handwritten recognition data set prove that epitaxial random access memories can operate with an online learning accuracy of 95.1%.

Superconductivity, pseudo-gap, and stripe correlations in high-Tc cuprates

NASA Astrophysics Data System (ADS)

Zhang, Zailan; Denis, Sylvain; Lebert, Blair W.; Bertran, Francois; Le Fèvre, Patrick; Taleb-Ibrahimi, Amina; Castellan, John-Paul; Bolloc'h, David Le; Jacques, Vincent L. R.; Sidis, Yvan; Baptiste, Benoît; Decorse, Claudia; Berthet, Patrick; Perfetti, Luca; d'Astuto, Matteo

2018-05-01

Under-doped La-214 cuprates show a charge- and spin-modulation known as "stripes" [1]. These stripe modulations are (quasi)-static close to 1/8 hole doping where superconductivity is suppressed. The pseudo-gap phase of other cuprate compounds recently also revealed charge modulation, but interpreted rather as a charge density wave (CDW) [2-4], that possibly competes with superconductivity. In this context, to better understand the interplay between the stripe phase and the superconductivity, we use angle-resolved photoemission spectroscopy to study the electronic band structure and gap in La-214 cuprates near 1/8 doping (La2-x-yNdySrxCuO4 (x = 0.12; y = 0.0 & 0.4)) and compare with the previous results in the same system [5] and La1.86Ba0.14CuO4 [6]. Our data shows a loss of spectral intensity towards the end of the Fermi arcs, that is possibly due to a strong renormalisation, as already pointed out elsewhere [6], with a noisy but still measurable gap. On the nodal direction no gap is observed within our statistics, but a sizeable decrease in intensity with temperature. Moreover, we do not see any shadow band, but our Fermi surface can be well modelled with a single electron band calculation in the tight binding approximation, even very close to the 1/8 doping La2-x-yNdySrxCuO4 with and without Nd substitution.

Electronic properties of B and Al doped graphane: A hybrid density functional study

NASA Astrophysics Data System (ADS)

Mapasha, R. E.; Igumbor, E.; Andriambelaza, N. F.; Chetty, N.

2018-04-01

Using a hybrid density functional theory approach parametrized by Heyd, Scuseria and Ernzerhof (HSE06 hybrid functional), we study the energetics, structural and electronic properties of a graphane monolayer substitutionally doped with the B (BCH) and Al (AlCH) atoms. The BCH defect can be integrated within a graphane monolayer at a relative low formation energy, without major structural distortions and symmetry breaking. The AlCH defect relaxes outward of the monolayer and breaks the symmetry. The density of states plots indicate that BCH doped graphane monolayer is a wide band gap semiconductor, whereas the AlCH defect introduces the spin dependent mid gap states at the vicinity of the Fermi level, revealing a metallic character with the pronounced magnetic features. We further examine the response of the Al dependent spin states on the multiple charge states doping. We find that the defect formation energy, structural and electronic properties can be altered via charge state modulation. The +1 charge doping opens an energy band gap of 1.75 eV. This value corresponds to the wavelength in the visible spectrum, suggesting an ideal material for solar cell absorbers. Our study fine tunes the graphane band gap through the foreign atom doping as well as via defect charge state modulation.

Evaluation of COTS SiGe, SOI, and Mixed Signal Electronic Parts for Extreme Temperature Use in NASA Missions

NASA Technical Reports Server (NTRS)

Patterson, Richard L.; Hammoud, Ahmad

2010-01-01

The NASA Electronic Parts and Packaging (NEPP) Program sponsors a task at the NASA Glenn Research Center titled "Reliability of SiGe, SOI, and Advanced Mixed Signal Devices for Cryogenic Space Missions." In this task COTS parts and flight-like are evaluated by determining their performance under extreme temperatures and thermal cycling. The results from the evaluations are published on the NEPP website and at professional conferences in order to disseminate information to mission planners and system designers. This presentation discusses the task and the 2010 highlights and technical results. Topics include extreme temperature operation of SiGe and SOI devices, all-silicon oscillators, a floating gate voltage reference, a MEMS oscillator, extreme temperature resistors and capacitors, and a high temperature silicon operational amplifier.

Immunogenicity and protection induced by a Mycobacterium tuberculosis sigE mutant in a BALB/c mouse model of progressive pulmonary tuberculosis.

PubMed

Hernandez Pando, Rogelio; Aguilar, Leon Diana; Smith, Issar; Manganelli, Riccardo

2010-07-01

Tuberculosis is still one of the main challenges to human global health, leading to about two million deaths every year. One of the reasons for its success is the lack of efficacy of the widely used vaccine Mycobacterium bovis BCG. In this article, we analyze the potential use of an attenuated mutant of Mycobacterium tuberculosis H37Rv lacking the sigma factor sigma(E) as a live vaccine. We have demonstrated that BALB/c mice infected by the intratracheal route with this mutant strain showed significantly higher survival rates and less tissue damage than animals infected with the parental or complemented mutant strain. Although animals infected with the sigE mutant had low bacillary loads, their lungs showed significantly higher production of the protective factors gamma interferon (IFN-gamma), tumor necrosis factor alpha (TNF-alpha), inducible nitric oxide synthase (iNOS), and beta-defensins than those of animals infected with the parental or complemented mutant strain. Moreover, we demonstrate that the sigE mutant, when inoculated subcutaneously, was more attenuated than BCG in immunodeficient nude mice, thus representing a good candidate for a novel attenuated live vaccine strain. Finally, when we used the sigE mutant as a subcutaneous vaccine, it was able to induce a higher level of protection than did BCG with both H37Rv and a highly virulent strain of M. tuberculosis (Beijing code 9501000). Taken together, our findings suggest that the sigE mutant is a very promising strain for the development of a new vaccine against tuberculosis.

A self-ordered, body-centered tetragonal superlattice of SiGe nanodot growth by reduced pressure CVD

NASA Astrophysics Data System (ADS)

Yamamoto, Yuji; Zaumseil, Peter; Capellini, Giovanni; Schubert, Markus Andreas; Hesse, Anne; Albani, Marco; Bergamaschini, Roberto; Montalenti, Francesco; Schroeder, Thomas; Tillack, Bernd

2017-12-01

Self-ordered three-dimensional body-centered tetragonal (BCT) SiGe nanodot structures are fabricated by depositing SiGe/Si superlattice layer stacks using reduced pressure chemical vapor deposition. For high enough Ge content in the island (>30%) and deposition temperature of the Si spacer layers (T > 700 °C), we observe the formation of an ordered array with islands arranged in staggered position in adjacent layers. The in plane periodicity of the islands can be selected by a suitable choice of the annealing temperature before the Si spacer layer growth and of the SiGe dot volume, while only a weak influence of the Ge concentration is observed. Phase-field simulations are used to clarify the driving force determining the observed BCT ordering, shedding light on the competition between heteroepitaxial strain and surface-energy minimization in the presence of a non-negligible surface roughness.

Study of morphology effects on magnetic interactions and band gap variations for 3d late transition metal bi-doped ZnO nanostructures by hybrid DFT calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Datta, Soumendu, E-mail: soumendu@bose.res.in; Baral, Sayan; Mookerjee, Abhijit

2015-08-28

Using density functional theory (DFT) based electronic structure calculations, the effects of morphology of semiconducting nanostructures on the magnetic interaction between two magnetic dopant atoms as well as a possibility of tuning band gaps have been studied in the case of the bi-doped (ZnO){sub 24} nanostructures with the impurity dopant atoms of the 3d late transition metals—Mn, Fe, Co, Ni, and Cu. To explore the morphology effect, three different structures of the host (ZnO){sub 24} nano-system, having different degrees of spatial confinement, have been considered: a two dimensional nanosheet, a one dimensional nanotube, and a finite cage-shaped nanocluster. The presentmore » study employs hybrid density functional theory to accurately describe the electronic structure of all the systems. It is shown here that the magnetic coupling between the two dopant atoms remains mostly anti-ferromagnetic in the course of changing the morphology from the sheet geometry to the cage-shaped geometry of the host systems, except for the case of energetically most stable bi-Mn doping, which shows a transition from ferromagnetic to anti-ferromagnetic coupling with decreasing aspect ratio of the host system. The effect of the shape change, however, has a significant effect on the overall band gap variations of both the pristine as well as all the bi-doped systems, irrespective of the nature of the dopant atoms and provides a means for easy tunability of their optoelectronic properties.« less

Study of morphology effects on magnetic interactions and band gap variations for 3d late transition metal bi-doped ZnO nanostructures by hybrid DFT calculations

NASA Astrophysics Data System (ADS)

Datta, Soumendu; Kaphle, Gopi Chandra; Baral, Sayan; Mookerjee, Abhijit

2015-08-01

Using density functional theory (DFT) based electronic structure calculations, the effects of morphology of semiconducting nanostructures on the magnetic interaction between two magnetic dopant atoms as well as a possibility of tuning band gaps have been studied in the case of the bi-doped (ZnO)24 nanostructures with the impurity dopant atoms of the 3d late transition metals—Mn, Fe, Co, Ni, and Cu. To explore the morphology effect, three different structures of the host (ZnO)24 nano-system, having different degrees of spatial confinement, have been considered: a two dimensional nanosheet, a one dimensional nanotube, and a finite cage-shaped nanocluster. The present study employs hybrid density functional theory to accurately describe the electronic structure of all the systems. It is shown here that the magnetic coupling between the two dopant atoms remains mostly anti-ferromagnetic in the course of changing the morphology from the sheet geometry to the cage-shaped geometry of the host systems, except for the case of energetically most stable bi-Mn doping, which shows a transition from ferromagnetic to anti-ferromagnetic coupling with decreasing aspect ratio of the host system. The effect of the shape change, however, has a significant effect on the overall band gap variations of both the pristine as well as all the bi-doped systems, irrespective of the nature of the dopant atoms and provides a means for easy tunability of their optoelectronic properties.

Doping-evolution of the superconducting gap in single crystals of (Ca 1-x La x ) 10 (Pt 3 As 8 )(Fe 2 As 2 ) 5 superconductor from London penetration depth measurements

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cho, K.; Tanatar, M. A.; Ni, N.

2014-09-19

The doping-evolution of the superconducting gap structure in iron-based superconductor (Ca 1-xLax)10(Pt3As8)(Fe2As2)5(x = 0.04, 0.06, 0.09, 0.11, and 0.18) was probed by high-resolution measurements of the London penetration depth, λ(T). The samples spanned compositions from underdoped to slightly overdoped with superconducting critical temperatures, Tc, from 12.7 K (x = 0.04) through (optimal) 23.3 K (x = 0.11) to 21.9 K (x = 0.18). The low-temperature variation (up to 0.3 Tc ) of λ(T) was analysed using a power-law function, Δλ = ATn. For compositions close to the optimal doping, (x = 0.09, 0.11, and 0.18), characterized by Tc > 20K,more » Δλ(T) shows a tendency to saturation, indicative of a full gap on the Fermi surface. Fitting over the lowest temperature range (T < 0.1 Tc) gives n = 2.6. This value is well outside the range 1 ≤ n ≤ 2 expected for the line-nodal superconductor. The exponent n decreased to n ~ 2 in the two most underdoped compositions x = 0.04 (Tc = 12.7 K) and 0.06 (Tc = 18.2 K), implying the development of a notable gap anisotropy revealed by the enhanced influence of pair-breaking scattering. This decrease is accompanied by a significant increase of the total variation of the penetration depth Δλ in a fixed temperature interval (e.g., Tmin - 0.3Tc). Both the decrease of the exponent and the increase of the absolute value of Δλ in the underdoped regime are similar to the observations in other charge-doped iron-based superconductors, such as doped BaFe2As2 and NaFeAs, suggesting a universal behavior in iron-based superconductors.« less

Electronic theoretical study on the influence of torsional deformation on the electronic structure and optical properties of BN-doped graphene

NASA Astrophysics Data System (ADS)

Fan, Dazhi; Liu, Guili; Wei, Lin

2018-06-01

Based on the density functional theory, the effect of torsional deformation on the electronic structure and optical properties of boron nitride (BN)-doped graphene is studied by using the first-principles calculations. The band structure calculations show that the intrinsic graphene is a semi-metallic material with zero band gap and the torsional deformation has a large effect on its band gap, opening its band gap and turning it from the semi-metal to the medium band gap semiconductor. The doping of BN in graphene makes its band gap open and becomes a medium band gap semiconductor. When it is subjected to a torsional effect, it is found to have a weak influence on its band gap. In other words, the doping of BN makes the changes of the band gap of graphene no longer sensitive to torsional deformation. Optical properties show that the doping of BN leads to a significant decrease in the light absorption coefficient and reflectivity of the graphene at the characteristic peak and that of BN-doped graphene system is also weakened by torsional deformation at the characteristic peak. In the absorption spectrum, the absorption peaks of the doping system of the torsion angle of 2-20∘ are redshifted compared with that of the BN-doped system (the torsion angle is 0∘). In the reflection spectrum, the two reflection peaks are all redshifted relative to that of the BN-doped system (the torsion angle is 0∘) and when the torsion angle exceeds 12∘, the size relationship between the two peaks is interchanged. The results of this paper are of guiding significance for the study of graphene-based nanotube devices in terms of deformation.

Variation in band gap energy and electrical analysis of double doped cobalt ferrite

NASA Astrophysics Data System (ADS)

Parveen, Azra; Agrawal, Shraddha; Azam, Ameer

2018-05-01

The Ca and Cr doped cobalt ferrite nanoparticles (Co0.9Ca0.1) (Fe0.8 Cr0.2)2O4 were synthesized by microwave gel combustion method. Microstructural studies were carried out by XRD and SEM. Structural studies suggest that the crystal system remains spinal even with the doping of calcium and chromium. The SEM image shows the spherical morphology of surface of the sample. Optical properties of Ca and Cr doped cobalt ferrite were studied by UV-visible technique in the range of 400-600 nm. The electrical conductivity of pure and doped cobalt ferrite were studied as a function of frequency and were explained on the basis of electron hopping.

Ultra-low temperature (≤300 °C) growth of Ge-rich SiGe by solid-liquid-coexisting annealing of a-GeSn/c-Si structures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sadoh, Taizoh, E-mail: sadoh@ed.kyushu-u.ac.jp; Chikita, Hironori; Miyao, Masanobu

2015-09-07

Ultra-low temperature (≤300 °C) growth of Ge-rich SiGe on Si substrates is strongly desired to realize advanced electronic and optical devices, which can be merged onto Si large-scale integrated circuits (LSI). To achieve this, annealing characteristics of a-GeSn/c-Si structures are investigated under wide ranges of the initial Sn concentrations (0%–26%) and annealing conditions (300–1000 °C, 1 s–48 h). Epitaxial growth triggered by SiGe mixing is observed after annealing, where the annealing temperatures necessary for epitaxial growth significantly decrease with increasing initial Sn concentration and/or annealing time. As a result, Ge-rich (∼80%) SiGe layers with Sn concentrations of ∼2% are realized by ultra-low temperature annealingmore » (300 °C, 48 h) for a sample with the initial Sn concentration of 26%. The annealing temperature (300 °C) is in the solid-liquid coexisting temperature region of the phase diagram for Ge-Sn system. From detailed analysis of crystallization characteristics and composition profiles in grown layers, it is suggested that SiGe mixing is generated by a liquid-phase reaction even at ultra-low temperatures far below the melting temperature of a-GeSn. This ultra-low-temperature growth technique of Ge-rich SiGe on Si substrates is expected to be useful to realize next-generation LSI, where various multi-functional devices are integrated on Si substrates.« less

Optical properties of pure and PbSe doped TiS2 nanodiscs

NASA Astrophysics Data System (ADS)

Parvaz, M.; Islamuddin; Khan, Zishan H.

2018-06-01

Titanium disulfide, being one of the popular transition-metal dichalcogenide (TMD) materials, shows wonderful properties owing to tunable optical band gap. Pure and PbSe doped titanium disulfide nanodiscs have been synthesized by solid-state reaction method. FESEM, TEM and Raman images confirm the synthesis of nanodiscs. XRD spectra suggest the polycrystalline structure of as-prepared as well as PbSe doped TiS2 nanodiscs. PL spectra of the as-synthesized nanodiscs has been studied in the wavelength range of (300–550 nm), at room temperature. The position of the peak shifts towards the lower wavelength (blue shift) and intensity of the PL increases after the doping of PbSe, which may be due to a broadening of the optical band gap. UV–vis spectra has been used to calculate optical band gap of pure and PbSe doped titanium disulfide nanodiscs. The calculated value are found to be 1.93 eV and 2.03 eV respectively. Various optical constants such as n and k have been calculated. The value of extinction coefficient (k) of pure and doped titanium disulfide increases while the value of the refractive index (n) decreases with increase in photon energy.

Electronic properties of BN-doped bilayer graphene and graphyne in the presence of electric field

NASA Astrophysics Data System (ADS)

Majidi, R.; Karami, A. R.

2013-11-01

In the present paper, we have used density functional theory to study electronic properties of bilayer graphene and graphyne doped with B and N impurities in the presence of electric field. It has been demonstrated that a band gap is opened in the band structures of the bilayer graphene and graphyne by B and N doping. We have also investigated influence of electric field on the electronic properties of BN-doped bilayer graphene and graphyne. It is found that the band gaps induced by B and N impurities are increased by applying electric field. Our results reveal that doping with B and N, and applying electric field are an effective method to open and control a band gap which is useful to design carbon-based next-generation electronic devices.

Spectral evolution with doping of an antiferromagnetic Mott state

NASA Astrophysics Data System (ADS)

Wu, Huan-Kuang; Lee, Ting-Kuo

2017-01-01

Since the discovery of half-filled cuprate to be a Mott insulator, the excitation spectra above the chemical potential for the unoccupied states has attracted much research attention. There were many theoretical works using different numerical techniques to study this problem, but many have reached different conclusions. One of the reasons is the lack of very detailed high-resolution experimental results for the theories to be compared with. Recently, the scanning tunneling spectroscopy [P. Cai et al., Nat. Phys. 12, 1047 (2016), 10.1038/nphys3840; C. Ye et al., Nat. Commun. 4, 1365 (2013), 10.1038/ncomms2369] on lightly doped Mott insulator with an antiferromagnetic order found the presence of in-gap states with energy of order half an eV above the chemical potential. The measured spectral properties with doping are not quite consistent with earlier theoretical works. Although the experiment has disorder and localization effect, but for the energy scale we will study here, a model without disorder is sufficed to illustrate the underlying physics. We perform a diagonalization method on top of the variational Monte Carlo calculation to study the evolution of antiferromagnetic Mott state with doped hole concentration in the Hubbard model. Our results found in-gap states that behave similarly with ones reported by STS. These in-gap states acquire a substantial amount of dynamical spectral weight transferred from the upper Hubbard band. The in-gap states move toward chemical potential with increasing spectral weight as doping increases. Our result also provides information about the energy scale of these in-gap states in relation with the Coulomb coupling strength U .

Thermoelectric properties of doped BaHfO{sub 3}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dixit, Chandra Kr., E-mail: ckparadise@gmail.com, E-mail: sharmarameshfgiet@gmail.com; Bhamu, K. C.; Sharma, Ramesh, E-mail: ckparadise@gmail.com, E-mail: sharmarameshfgiet@gmail.com

2016-05-06

We have studied the structural stability, electronic structure, optical properties and thermoelectric properties of doped BaHfO{sub 3} by full potential linearized augmented plane wave (FP-LAPW) method. The electronic structure of BaHfO{sub 3} doped with Sr shows enhances the indirect band gaps of 3.53 eV, 3.58 eV. The charge density plots show strong ionic bonding in Ba-Hf, and ionic and covalent bonding between Hf and O. Calculations of the optical spectra, viz., the dielectric function, refractive index and extinction coefficient are performed for the energy range are calculated and analyzed. Thermoelectric properties of semi conducting are also reported first time. Themore » doped BaHfO{sub 3} is approximately wide band gap semiconductor with the large p-type Seebeck coefficient. The power factor of BaHfO{sub 3} is increased with Sr doping, decreases because of low electrical resistivity and thermal conductivity.« less

A Quasi-Classical Model of the Hubbard Gap in Lightly Compensated Semiconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Poklonski, N. A.; Vyrko, S. A.; Kovalev, A. I.

2016-03-15

A quasi-classical method for calculating the narrowing of the Hubbard gap between the A{sup 0} and A{sup +} acceptor bands in a hole semiconductor or the D{sup 0} and D{sup –} donor bands in an electron semiconductor is suggested. This narrowing gives rise to the phenomenon of a semiconductor transition from the insulator to metal state with an increase in doping level. The major (doping) impurity can be in one of three charge states (–1, 0, or +1), while the compensating impurity can be in states (+1) or (–1). The impurity distribution over the crystal is assumed to be randommore » and the width of Hubbard bands (levels), to be much smaller than the gap between them. It is shown that narrowing of the Hubbard gap is due to the formation of electrically neutral acceptor (donor) states of the quasicontinuous band of allowed energies for holes (electrons) from excited states. This quasicontinuous band merges with the top of the valence band (v band) for acceptors or with the bottom of the conduction band (c band) for donors. In other words, the top of the v band for a p-type semiconductor or the bottom of the c band for an n-type semiconductor is shifted into the band gap. The value of this shift is determined by the maximum radius of the Bohr orbit of the excited state of an electrically neutral major impurity atom, which is no larger than half the average distance between nearest impurity atoms. As a result of the increasing dopant concentration, the both Hubbard energy levels become shallower and the gap between them narrows. Analytical formulas are derived to describe the thermally activated hopping transition of holes (electrons) between Hubbard bands. The calculated gap narrowing with increasing doping level, which manifests itself in a reduction in the activation energy ε{sub 2} is consistent with available experimental data for lightly compensated p-Si crystals doped with boron and n-Ge crystals doped with antimony.« less

Disappearance of nodal gap across the insulator-superconductor transition in a copper-oxide superconductor.

PubMed

Peng, Yingying; Meng, Jianqiao; Mou, Daixiang; He, Junfeng; Zhao, Lin; Wu, Yue; Liu, Guodong; Dong, Xiaoli; He, Shaolong; Zhang, Jun; Wang, Xiaoyang; Peng, Qinjun; Wang, Zhimin; Zhang, Shenjin; Yang, Feng; Chen, Chuangtian; Xu, Zuyan; Lee, T K; Zhou, X J

2013-01-01

The parent compound of the copper-oxide high-temperature superconductors is a Mott insulator. Superconductivity is realized by doping an appropriate amount of charge carriers. How a Mott insulator transforms into a superconductor is crucial in understanding the unusual physical properties of high-temperature superconductors and the superconductivity mechanism. Here we report high-resolution angle-resolved photoemission measurement on heavily underdoped Bi₂Sr₂-xLaxCuO(₆+δ) system. The electronic structure of the lightly doped samples exhibit a number of characteristics: existence of an energy gap along the nodal direction, d-wave-like anisotropic energy gap along the underlying Fermi surface, and coexistence of a coherence peak and a broad hump in the photoemission spectra. Our results reveal a clear insulator-superconductor transition at a critical doping level of ~0.10 where the nodal energy gap approaches zero, the three-dimensional antiferromagnetic order disappears, and superconductivity starts to emerge. These observations clearly signal a close connection between the nodal gap, antiferromagnetism and superconductivity.

Impurity Resonant States p-type Doping in Wide-Band-Gap Nitrides

NASA Astrophysics Data System (ADS)

Liu, Zhiqiang; Yi, Xiaoyan; Yu, Zhiguo; Yuan, Gongdong; Liu, Yang; Wang, Junxi; Li, Jinmin; Lu, Na; Ferguson, Ian; Zhang, Yong

2016-01-01

In this work, a new strategy for achieving efficient p-type doping in high bandgap nitride semiconductors to overcome the fundamental issue of high activation energy has been proposed and investigated theoretically, and demonstrated experimentally. Specifically, in an AlxGa1-xN/GaN superlattice structure, by modulation doping of Mg in the AlxGa1-xN barriers, high concentration of holes are generated throughout the material. A hole concentration as high as 1.1 × 1018 cm-3 has been achieved, which is about one order of magnitude higher than that typically achievable by direct doping GaN. Results from first-principle calculations indicate that the coupling and hybridization between Mg 2p impurity and the host N 2p orbitals are main reasons for the generation of resonant states in the GaN wells, which further results in the high hole concentration. We expect this approach to be equally applicable for other high bandgap materials where efficient p-type doing is difficult. Furthermore, a two-carrier-species Hall-effect model is proposed to delineate and discriminate the characteristics of the bulk and 2D hole, which usually coexist in superlattice-like doping systems. The model reported here can also be used to explain the abnormal freeze-in effect observed in many previous reports.

Impurity Resonant States p-type Doping in Wide-Band-Gap Nitrides.

PubMed

Liu, Zhiqiang; Yi, Xiaoyan; Yu, Zhiguo; Yuan, Gongdong; Liu, Yang; Wang, Junxi; Li, Jinmin; Lu, Na; Ferguson, Ian; Zhang, Yong

2016-01-18

In this work, a new strategy for achieving efficient p-type doping in high bandgap nitride semiconductors to overcome the fundamental issue of high activation energy has been proposed and investigated theoretically, and demonstrated experimentally. Specifically, in an AlxGa(1-x)N/GaN superlattice structure, by modulation doping of Mg in the AlxGa(1-x)N barriers, high concentration of holes are generated throughout the material. A hole concentration as high as 1.1 × 10(18) cm(-3) has been achieved, which is about one order of magnitude higher than that typically achievable by direct doping GaN. Results from first-principle calculations indicate that the coupling and hybridization between Mg 2p impurity and the host N 2p orbitals are main reasons for the generation of resonant states in the GaN wells, which further results in the high hole concentration. We expect this approach to be equally applicable for other high bandgap materials where efficient p-type doing is difficult. Furthermore, a two-carrier-species Hall-effect model is proposed to delineate and discriminate the characteristics of the bulk and 2D hole, which usually coexist in superlattice-like doping systems. The model reported here can also be used to explain the abnormal freeze-in effect observed in many previous reports.

Impurity Resonant States p-type Doping in Wide-Band-Gap Nitrides

PubMed Central

Liu, Zhiqiang; Yi, Xiaoyan; Yu, Zhiguo; Yuan, Gongdong; Liu, Yang; Wang, Junxi; Li, Jinmin; Lu, Na; Ferguson, Ian; Zhang, Yong

2016-01-01

In this work, a new strategy for achieving efficient p-type doping in high bandgap nitride semiconductors to overcome the fundamental issue of high activation energy has been proposed and investigated theoretically, and demonstrated experimentally. Specifically, in an AlxGa1−xN/GaN superlattice structure, by modulation doping of Mg in the AlxGa1−xN barriers, high concentration of holes are generated throughout the material. A hole concentration as high as 1.1 × 1018 cm−3 has been achieved, which is about one order of magnitude higher than that typically achievable by direct doping GaN. Results from first-principle calculations indicate that the coupling and hybridization between Mg 2p impurity and the host N 2p orbitals are main reasons for the generation of resonant states in the GaN wells, which further results in the high hole concentration. We expect this approach to be equally applicable for other high bandgap materials where efficient p-type doing is difficult. Furthermore, a two-carrier-species Hall-effect model is proposed to delineate and discriminate the characteristics of the bulk and 2D hole, which usually coexist in superlattice-like doping systems. The model reported here can also be used to explain the abnormal freeze-in effect observed in many previous reports. PMID:26777294

Simultaneous Magnetic and Charge Doping of Topological Insulators with Carbon

NASA Astrophysics Data System (ADS)

Shen, Lei; Zeng, Minggang; Lu, Yunhao; Yang, Ming; Feng, Yuan Ping

2013-12-01

A two-step doping process, magnetic followed by charge or vice versa, is required to produce massive topological surface states (TSS) in topological insulators for many physics and device applications. Here, we demonstrate simultaneous magnetic and hole doping achieved with a single dopant, carbon, in Bi2Se3 by first-principles calculations. Carbon substitution for Se (CSe) results in an opening of a sizable surface Dirac gap (up to 82 meV), while the Fermi level remains inside the bulk gap and close to the Dirac point at moderate doping concentrations. The strong localization of 2p states of CSe favors spontaneous spin polarization via a p-p interaction and formation of ordered magnetic moments mediated by surface states. Meanwhile, holes are introduced into the system by CSe. This dual function of carbon doping suggests a simple way to realize insulating massive TSS.

First Principles Electronic Structure of Mn doped GaAs, GaP, and GaN Semiconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schulthess, Thomas C; Temmerman, Walter M; Szotek, Zdzislawa

We present first-principles electronic structure calculations of Mn doped III-V semiconductors based on the local spin-density approximation (LSDA) as well as the self-interaction corrected local spin density method (SIC-LSD). We find that it is crucial to use a self-interaction free approach to properly describe the electronic ground state. The SIC-LSD calculations predict the proper electronic ground state configuration for Mn in GaAs, GaP, and GaN. Excellent quantitative agreement with experiment is found for magnetic moment and p-d exchange in (GaMn)As. These results allow us to validate commonly used models for magnetic semiconductors. Furthermore, we discuss the delicate problem of extractingmore » binding energies of localized levels from density functional theory calculations. We propose three approaches to take into account final state effects to estimate the binding energies of the Mn-d levels in GaAs. We find good agreement between computed values and estimates from photoemisison experiments.« less

Electronic structure modifications and band gap narrowing in Zn0.95V0.05O

NASA Astrophysics Data System (ADS)

Ahad, Abdul; Majid, S. S.; Rahman, F.; Shukla, D. K.; Phase, D. M.

2018-04-01

We present here, structural, optical and electronic structure studies on Zn0.95V0.05O, synthesized using solid state method. Rietveld refinement of x-ray diffraction pattern indicates no considerable change in the lattice of doped ZnO. The band gap of doped sample, as calculated by Kubelka-Munk transformed reflectance spectra, has been found reduced compared to pure ZnO. Considerable changes in absorbance in UV-Vis range is observed in doped sample. V doping induced decrease in band gap is supported by x-ray absorption spectroscopy measurements. It is experimentally confirmed that conduction band edge in Zn0.95V0.05O has shifted towards Fermi level than in pure ZnO.

Controlling the optical properties of polyaniline doped by boric acid particles by changing their doping agent and initiator concentration

NASA Astrophysics Data System (ADS)

Cabuk, Mehmet; Gündüz, Bayram

2017-12-01

In this study, polyaniline doped by boric acid (PAni:BA) conducting polymers were chemically synthesized by oxidative polymerization method using (NH4)2S208 (APS) as initiator. Pani:BA conducting polymers were synthesized by using two different APS/aniline molar ratios as 1:1 and 2:1. Their results were compared with PAni doped by HCl (PAni) conducting polymer. Structural properties of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were characterized by using FTIR, SEM, TGA, particle size and apparent density measurements. Effects of doping agents and initiator concentrations on optical properties were investigated in detail. The optoelectronic parameters such as absorption band edge, molar extinction coefficient, direct allowed band gap, refractive index, optical conductance and electrical conductance of the PAni, PAni:BA (1:1) and PAni:BA (2:1) were determined. The absorption band edge and direct allowed band gap of PAni were decreased with doping BA and increasing APS ratio. Also, the refractive index values of the materials were calculated from experimental results and compared with obtained results from Moss, Ravindra, Herve-Vandamme, Reddy and Kumar-Singh relations.

Study of structural, electronic and optical properties of tungsten doped bismuth oxychloride by DFT calculations.

PubMed

Yang, Wenjuan; Wen, Yanwei; Chen, Rong; Zeng, Dawen; Shan, Bin

2014-10-21

First-principle calculations have been carried out to investigate structural stabilities, electronic structures and optical properties of tungsten doped bismuth oxychloride (BiOCl). The structures of substitutional and interstitial tungsten, and in the form of WO6-ligand-doped BiOCl are examined. The substitutional and interstitial tungsten doping leads to discrete midgap states within the forbidden band gap, which has an adverse effect on the photocatalytic properties. On the other hand, the WO6-ligand-doped BiOCl structure induces a continuum of hybridized states in the forbidden gap, which favors transport of electrons and holes and could result in enhancement of visible light activity. In addition, the band gap of WO6-BiOCl decreases by 0.25 eV with valence band maximum (VBM) shifting upwards compared to that of pure BiOCl. By calculating optical absorption spectra of pure BiOCl and WO6-ligand-doped BiOCl structure, it is found that the absorption peak of the WO6-ligand-doped BiOCl structure has a red shift towards visible light compared with that of pure BiOCl, which agrees well with experimental observations. These results reveal the tungsten doped BiOCl system as a promising material in photocatalytic decomposition of organics and water splitting under sunlight irradiation.

Electronic, optical and photocatalytic behavior of Mn, N doped and co-doped TiO{sub 2}: Experiment and simulation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhao, Ya Fei; Li, Can, E-mail: canli1983@gmail.com; Lu, Song

2016-03-15

The crystal phase structure, surface morphology, chemical states and optical properties of Mn, N mono-doped and co-doped TiO{sub 2} nanoparticles were investigated by X-ray powder diffractometry, Raman spectra, scanning electron microscopy, X-ray photoelectron spectroscopy and UV–vis diffuse reflectance spectroscopy. Meanwhile, geometry structures, formation energies, electronic and optical properties of all systems have been also analyzed by density functional theory. The results showed that the band gap values and the carrier mobility in the valence band, conduction band and impurity levels have a synergetic influence on the visible-light absorption and photocatalytic activity of the doped TiO{sub 2}. The number and themore » carrier mobility of impurity level jointly influence the photocatalytic activity of catalyst under visible-light. Especially, the photocatalytic activity of Mn-2N co-doped TiO{sub 2} beyond three-fold than that of pure TiO{sub 2} under visible-light. - Graphical abstract: The ILs formed by N-2p orbital in N single doped specimen lie above the VB, while the ILs formed by Mn-3d orbital in Mn single doped specimen appear below the CB. However, a large amount of ILs formed by N-2p orbital and Mn-3d orbital in N and Mn codoped specimens. The band gap values and the carrier mobility in the valence band, conduction band and impurity levels have a synergetic influence on the visible-light absorption and photocatalytic activity of the doped TiO{sub 2}. The number and the carrier mobility of impurity level jointly influence the photocatalytic activity of catalyst under visible-light.« less

Exceptional cracking behavior in H-implanted Si/B-doped Si0.70Ge0.30/Si heterostructures

NASA Astrophysics Data System (ADS)

Chen, Da; Wang, Dadi; Chang, Yongwei; Li, Ya; Ding, Rui; Li, Jiurong; Chen, Xiao; Wang, Gang; Guo, Qinglei

2018-01-01

The cracking behavior in H-implanted Si/B-doped Si0.70Ge0.30/Si structures after thermal annealing was investigated. The crack formation position is found to closely correlate with the thickness of the buried Si0.70Ge0.30 layer. For H-implanted Si containing a buried 3-nm-thick B-doped Si0.70Ge0.30 layer, localized continuous cracking occurs at the interfaces on both sides of the Si0.70Ge0.30 interlayer. Once the thickness of the buried Si0.70Ge0.30 layer increases to 15 and 70 nm, however, a continuous sharp crack is individually observed along the interface between the Si substrate and the B-doped Si0.70Ge0.30 interlayer. We attribute this exceptional cracking behavior to the existence of shear stress on both sides of the buried Si0.70Ge0.30 layer and the subsequent trapping of hydrogen, which leads to a crack in a well-controlled manner. This work may pave the way for high-quality Si or SiGe membrane transfer in a feasible manner, thus expediting its potential applications to ultrathin silicon-on-insulator (SOI) or silicon-germanium-on-insulator (SGOI) production.

A microwave cryogenic low-noise amplifier based on sige heterostructures

NASA Astrophysics Data System (ADS)

Ivanov, B. I.; Grajcar, M.; Novikov, I. L.; Vostretsov, A. G.; Il'ichev, E.

2016-04-01

A low-noise cryogenic amplifier for the measurement of weak microwave signals at sub-Kelvin temperatures is constructed. The amplifier has five stages based on SiGe bipolar heterostructure transistors and has a gain factor of 35 dB in the frequency band from 100 MHz to 4 GHz at an operating temperature of 800 mK. The parameters of a superconducting quantum bit measured with this amplifier in the ultralow-power mode are presented as an application example. The amplitude-frequency response of the "supercon-ducting qubit-coplanar cavity" structure is demonstrated. The ground state of the qubit is characterized in the quasi-dispersive measurement mode.

Doping dependence of charge order in electron-doped cuprate superconductors

NASA Astrophysics Data System (ADS)

Mou, Yingping; Feng, Shiping

2017-12-01

In the recent studies of the unconventional physics in cuprate superconductors, one of the central issues is the interplay between charge order and superconductivity. Here the mechanism of the charge-order formation in the electron-doped cuprate superconductors is investigated based on the t-J model. The experimentally observed momentum dependence of the electron quasiparticle scattering rate is qualitatively reproduced, where the scattering rate is highly anisotropic in momentum space, and is intriguingly related to the charge-order gap. Although the scattering strength appears to be weakest at the hot spots, the scattering in the antinodal region is stronger than that in the nodal region, which leads to the original electron Fermi surface is broken up into the Fermi pockets and their coexistence with the Fermi arcs located around the nodal region. In particular, this electron Fermi surface instability drives the charge-order correlation, with the charge-order wave vector that matches well with the wave vector connecting the hot spots, as the charge-order correlation in the hole-doped counterparts. However, in a striking contrast to the hole-doped case, the charge-order wave vector in the electron-doped side increases in magnitude with the electron doping. The theory also shows the existence of a quantitative link between the single-electron fermiology and the collective response of the electron density.

Evolution of dielectric function of Al-doped ZnO thin films with thermal annealing: effect of band gap expansion and free-electron absorption.

PubMed

Li, X D; Chen, T P; Liu, Y; Leong, K C

2014-09-22

Evolution of dielectric function of Al-doped ZnO (AZO) thin films with annealing temperature is observed. It is shown that the evolution is due to the changes in both the band gap and the free-electron absorption as a result of the change of free-electron concentration of the AZO thin films. The change of the electron concentration could be attributed to the activation of Al dopant and the creation/annihilation of the donor-like defects like oxygen vacancy in the thin films caused by annealing.

The investigation of Ce doped ZnO crystal: The electronic, optical and magnetic properties

NASA Astrophysics Data System (ADS)

Wen, Jun-Qing; Zhang, Jian-Min; Qiu, Ze-Gang; Yang, Xu; Li, Zhi-Qin

2018-04-01

The electronic, optical and magnetic properties of Ce doped ZnO crystal have been studied by using first principles method. The research of formation energies show that Ce doped ZnO is energetically stable, and the formation energies reduce from 6.25% to 12.5% for Ce molar percentage. The energy band is still direct band gap after Ce doped, and band gap increases with the increase of Cesbnd Ce distance. The Fermi level moves upward into conduction band and the DOS moves to lower energy with the increase of Ce concentration, which showing the properties of n-type semiconductor. The calculated optical properties imply that Ce doped causes a red-shift of absorption peaks, and enhances the absorption of the visible light. The transition from ferromagnetic to antiferromagnetic has been found in Ce doped ZnO.

Effect of potassium doping on electronic structure and thermoelectric properties of topological crystalline insulator

DOE Office of Scientific and Technical Information (OSTI.GOV)

Roychowdhury, Subhajit; Biswas, Kanishka, E-mail: kanishka@jncasr.ac.in; Sandhya Shenoy, U.

2016-05-09

Topological crystalline insulator (TCI), Pb{sub 0.6}Sn{sub 0.4}Te, exhibits metallic surface states protected by crystal mirror symmetry with negligibly small band gap. Enhancement of its thermoelectric performances needs tuning of its electronic structure particularly through engineering of its band gap. While physical perturbations tune the electronic structure of TCI by breaking of the crystal mirror symmetry, chemical means such as doping have been more attractive recently as they result in better thermoelectric performance in TCIs. Here, we demonstrate that K doping in TCI, Pb{sub 0.6}Sn{sub 0.4}Te, breaks the crystal mirror symmetry locally and widens electronic band gap, which is confirmed bymore » direct electronic absorption spectroscopy and electronic structure calculations. K doping in Pb{sub 0.6}Sn{sub 0.4}Te increases p-type carrier concentration and suppresses the bipolar conduction via widening a band gap, which collectively boosts the thermoelectric figure of merit (ZT) to 1 at 708 K.« less

Structural and optical properties of ITO and Cu doped ITO thin films

NASA Astrophysics Data System (ADS)

Chakraborty, Deepannita; Kaleemulla, S.; Rao, N. Madhusudhana; Subbaravamma, K.; Rao, G. Venugopal

2018-04-01

(In0.95Sn0.05)2O3 and (In0.90Cu0.05Sn0.05)2O3 thin films were coated onto glass substrate by electron beam evaporation technique. The structural and optical properties of ITO and Cu doped ITO thin films have been studied by X-ray diffractometer (XRD) and UV-Vis-NIR spectrophotometer. The crystallite size obtained for ITO and Cu doped ITO thin films was in the range of 24 nm to 22 nm. The optical band gap of 4 eV for ITO thin film sample has been observed. The optical band gap decreases to 3.85 eV by doping Cu in ITO.

Effect on Electron Structure and Magneto-Optic Property of Heavy W-Doped Anatase TiO2.

PubMed

Hou, Qingyu; Zhao, Chunwang; Guo, Shaoqiang; Mao, Fei; Zhang, Yue

2015-01-01

The spin or nonspin state of electrons in W-doped anatase TiO2 is very difficult to judge experimentally because of characterization method limitations. Hence, the effect on the microscopic mechanism underlying the visible-light effect of W-doped anatase TiO2 through the consideration of electronic spin or no-spin states is still unknown. To solve this problem, we establish supercell models of W-doped anatase TiO2 at different concentrations, followed by geometry optimization and energy calculation based on the first-principle planewave norm conserving pseudo-potential method of the density functional theory. Calculation results showed that under the condition of nonspin the doping concentration of W becomes heavier, the formation energy becomes greater, and doping becomes more difficult. Meanwhile, the total energy increases, the covalent weakens and ionic bonds strengthens, the stability of the W-doped anatase TiO2 decreases, the band gap increases, and the blue-shift becomes more significant with the increase of W doping concentration. However, under the condition of spin, after the band gap correction by the GGA+U method, it is found that the semimetal diluted magnetic semiconductors can be formed by heavy W-doped anatase TiO2. Especially, a conduction electron polarizability of as high as near 100% has been found for the first time in high concentration W-doped anatase TiO2. It will be able to be a promising new type of dilute magnetic semiconductor. And the heavy W-doped anatase TiO2 make the band gap becomes narrower and absorption spectrum red-shift.

Infrared Optical Absorption in Low-spin Fe2+-doped SrTiO3

DOE Office of Scientific and Technical Information (OSTI.GOV)

Comes, Ryan B.; Kaspar, Tiffany C.; Heald, Steve M.

2016-01-06

Band gap engineering in SrTiO3 and related titanate perovskites has long been explored due to the intriguing properties of the materials for photocatalysis and photovoltaic applications. A popular approach in the materials chemistry community is to substitutionally dope aliovalent transition metal ions onto the B site in the lattice to alter the valence band. However, in such a scheme there is limited control over the dopant valence, and compensating defects often form. Here we demonstrate a novel technique to controllably synthesize Fe2+- and Fe3+-doped SrTiO3 thin films without formation of compensating defects by co-doping with La3+ ions on the Amore » site. We stabilize Fe2+-doped films by doping with two La ions for every Fe dopant, and find that the Fe ions exhibit a low-spin electronic configuration, producing optical transitions in the near infrared regime and degenerate doping. The novel electronic states observed here offer a new avenue for band gap engineering in perovskites for photocatalytic and photovoltaic applications.« less

Influence of nitrogen-doping concentration on the electronic structure of CuAlO2 by first-principles studies

NASA Astrophysics Data System (ADS)

Liu, Wei-wei; Chen, Hong-xia; Liu, Cheng-lin; Wang, Rong

2017-02-01

Effect of N doping concentration on the electronic structure of N-doped CuAlO2 was investigated by density functional theory based on generalized-gradient approximation plus orbital potential. Lattice parameters a and c both increase with increasing N-doping concentration. Formation energies increase with increasing N doping concentration and all N-doped CuAlO2 were structurally stable. The calculated band gaps for N-doped CuAlO2 narrowed compared to pure CuAlO2, which was attributed to the stronger hybridization between Cu-3d and N-2p states and the downward shift of Cu-3p states in conduction bands. The higher the N-doping concentration is, the narrower the band gap. N-doped CuAlO2 shows a typical p-type semiconductor. The band structure changed from indirect to direct after N doping which will benefit the application of the CuAlO2 materials in optoelectronic and electronic devices.

Specific heat and Nernst effect of electron-doped cuprate superconductors

NASA Astrophysics Data System (ADS)

Balci, Hamza

This thesis consists of two separate studies on Pr2- xCexCuO4 (PCCO), a member of the electron-doped high temperature cuprate superconductor family: specific heat and the Nernst effect. We measured the specific heat of PCCO single crystals in order to probe the symmetry of the superconducting order parameter, to study the effect of oxygen reduction (annealing) on bulk properties of the crystals, and to determine proper ties like the condensation energy and the thermodynamic critical field. The order parameter symmetry has been established to be d-wave in the hole-doped cuprates. Experiments performed on electron-doped cuprates show conflicting results. Different experiments suggest s-wave symmetry, d-wave symmetry, or a transition from d-wave to s-wave symmetry with increasing cerium doping. However, most of these experiments are surface sensitive experiments. Specific heat, as a bulk method of probing the gap symmetry is essential in order to convincingly determine the gap symmetry. Our data proposes a way to reconcile all these conflicting results regarding the gap symmetry. In addition, prior specific heat measurements attempting to determine thermodynamic properties like the condensation energy were not successful due to inefficient methods of data analysis or poor sample quality. With improvements on sample quality and data analysis, we reliably determined these properties. The second part of this thesis is a study of the Nernst effect in PCCO thin films with different cerium dopings. We probed the superconducting fluctuations, studied transport phenomena in the normal state, and accurately measured H c2 by using the Nernst effect. After the discovery of the anomalous Nernst effect in the normal state of the hole-doped cuprates, many alternative explanations have been proposed. Vortex-like excitations above Tc, superconducting fluctuations, AFM fluctuations, and preformed Cooper pairs are some of these proposals. The electron-doped cuprates, due to their

Energy band gap and optical transition of metal ion modified double crossover DNA lattices.

PubMed

Dugasani, Sreekantha Reddy; Ha, Taewoo; Gnapareddy, Bramaramba; Choi, Kyujin; Lee, Junwye; Kim, Byeonghoon; Kim, Jae Hoon; Park, Sung Ha

2014-10-22

We report on the energy band gap and optical transition of a series of divalent metal ion (Cu(2+), Ni(2+), Zn(2+), and Co(2+)) modified DNA (M-DNA) double crossover (DX) lattices fabricated on fused silica by the substrate-assisted growth (SAG) method. We demonstrate how the degree of coverage of the DX lattices is influenced by the DX monomer concentration and also analyze the band gaps of the M-DNA lattices. The energy band gap of the M-DNA, between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), ranges from 4.67 to 4.98 eV as judged by optical transitions. Relative to the band gap of a pristine DNA molecule (4.69 eV), the band gap of the M-DNA lattices increases with metal ion doping up to a critical concentration and then decreases with further doping. Interestingly, except for the case of Ni(2+), the onset of the second absorption band shifts to a lower energy until a critical concentration and then shifts to a higher energy with further increasing the metal ion concentration, which is consistent with the evolution of electrical transport characteristics. Our results show that controllable metal ion doping is an effective method to tune the band gap energy of DNA-based nanostructures.

Flexible Low-power SiGe HBT Amplifier Circuits for Fast Single-shot Spin Readout

NASA Astrophysics Data System (ADS)

England, Troy; Lilly, Michael; Curry, Matthew; Carr, Stephen; Carroll, Malcolm

Fast, low-power quantum state readout is one of many challenges facing quantum information processing. Single electron transistors (SETs) are potentially fast, sensitive detectors for performing spin readout of electrons bound to Si:P donors. From a circuit perspective, however, their output impedance and nonlinear conductance are ill suited to drive the parasitic capacitance of coaxial conductors used in cryogenic environments, necessitating a cryogenic amplification stage. We will introduce two new amplifier topologies that provide excellent gain versus power tradeoffs using silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs). The AC HBT allows in-situ adjustment of power dissipation during an experiment and can provide gain in the millikelvin temperature regime while dissipating less than 500 nW. The AC Current Amplifier maximizes gain at nearly 800 A/A. We will also show results of using these amplifiers with SETs at 4 K. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy under Contract No. DE-AC04-94AL85000. Flexible Low-power SiGe HBT Amplifier Circuits for Fast Single-shot Spin Readout.

Optical and electronic properties of SO2 molecule adsorbed on Si-doped (8, 0) boron nitride nanotube

NASA Astrophysics Data System (ADS)

Guo, Shuang-Shuang; Wei, Xiu-Mei; Zhang, Jian-Min; Zhu, Gang-Qiang; Guo, Wan-Jin

2016-09-01

The study of the optical properties of pristine BNNT, Si-doped BNNTs and SO2 molecule adsorption on Si-doped BNNTs is that, to our knowledge, few relevant research have ever been found. In this paper, the adsorption behaviors of Sulfur dioxide (SO2) molecule on Si-doped Boron nitride nanotubes (BNNTs) are investigated applying the first-principles calculations. The main contribution of this paper is that the foremost investigation for the optical properties of the pristine BNNT, Si-doped BNNTs and SO2 adsorption on Si-doped BNNTs. Additionally, the electronic properties and the structural properties are also presented. In our calculations of optical properties, the dielectric constant, the refractive index and the absorption coefficient are obtained. Comparing the pristine BNNT, our results indicate that, the blue-shifts (in the main peaks of the dielectric constant of SiB -BNNT and SO2-SiB -BNNT), and the red-shifts (in the main peaks of the refractive index of SiN -BNNT and SO2-SiN -BNNT) are appeared. Under these conditions, Si-doped BNNT and Si-doped BNNT with SO2 adsorption, the gaps are reduced both for the speculated optical band gaps and the electronic structure band gaps.

Calculation of optical band gaps of a-Si:H thin films by ellipsometry and UV-Vis spectrophotometry

NASA Astrophysics Data System (ADS)

Qiu, Yijiao; Li, Wei; Wu, Maoyang; Fu, Junwei; Jiang, Yadong

2010-10-01

Hydrogenated amorphous silicon (a-Si:H) thin films doped with Phosphorus (P) and Nitrogen (N) were deposited by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD). The optical band gaps of the thin films obtained through either changing the gas pressure (P-doped only) or adulterating nitrogen concentration (with fixed P content) were investigated by means of Ellipsometric and Ultraviolet-Visible (UV-Vis) spectroscopy, respectively. Tauc formula was used in calculating the optical band gaps of the thin films in both methods. The results show that Ellipsometry and UV-Vis spectrophotometry can be applied in the research of the optical properties of a-Si:H thin films experimentally. Both methods reflect the variation law of the optical band gaps caused by CVD process parameters, i.e., the optical band gap of the a-Si:H thin films is increased with the rise of the gas pressure or the nitrogen concentration respectively. The difference in optical band gaps of the doped a-Si:H thin films calculated by Ellipsometry or UV-Vis spectrophotometry are not so great that they both can be used to measure the optical band gaps of the thin films in practical applications.

Investigation of the stability of Co-doped apatite ionic conductors in NH 3

NASA Astrophysics Data System (ADS)

Headspith, D. A.; Orera, A.; Slater, P. R.; Young, N. A.; Francesconi, M. G.

2010-12-01

Hydrogen powered solid oxide fuel cells (SOFCs) are of enormous interest as devices for the efficient and clean production of electrical energy. However, a number of problems linked to hydrogen production, storage and transportation are slowing down the larger scale use of SOFCs. Identifying alternative fuel sources to act as intermediate during the transition to the full use of hydrogen is, therefore, of importance. One excellent alternative is ammonia, which is produced on a large scale, is relatively cheap and has the infrastructure for storage and transportation already in place. However, considering that SOFCs operate at temperatures higher than 500 °C, a potential problem is the interaction of gaseous ammonia with the materials in the cathode, anode and solid electrolyte. In this paper, we extend earlier work on high temperature reactions of apatite electrolytes with NH 3 to the transition metal (Co) doped systems, La 9.67Si 5CoO 26 and La 10(Si/Ge) 5CoO 26.5. A combination of PXRD, TGA and XAFS spectroscopy data showed a better structural stability for the silicate systems. Apatite silicates and germanates not containing transition metals tend to substitute nitride anions for their interstitial oxide anions, when reacted with NH 3 at high temperature and, consequentially, lower the interstitial oxide content. In La 9.67Si 5CoO 26 and La 10(Si/Ge) 5CoO 26.5 reduction of Co occurs as a competing process, favouring lower levels of nitride-oxide substitution.

Growth and characterization of gold catalyzed SiGe nanowires and alternative metal-catalyzed Si nanowires

PubMed Central

2011-01-01

The growth of semiconductor (SC) nanowires (NW) by CVD using Au-catalyzed VLS process has been widely studied over the past few years. Among others SC, it is possible to grow pure Si or SiGe NW thanks to these techniques. Nevertheless, Au could deteriorate the electric properties of SC and the use of other metal catalysts will be mandatory if NW are to be designed for innovating electronic. First, this article's focus will be on SiGe NW's growth using Au catalyst. The authors managed to grow SiGe NW between 350 and 400°C. Ge concentration (x) in Si1-xGex NW has been successfully varied by modifying the gas flow ratio: R = GeH4/(SiH4 + GeH4). Characterization (by Raman spectroscopy and XRD) revealed concentrations varying from 0.2 to 0.46 on NW grown at 375°C, with R varying from 0.05 to 0.15. Second, the results of Si NW growths by CVD using alternatives catalysts such as platinum-, palladium- and nickel-silicides are presented. This study, carried out on a LPCVD furnace, aimed at defining Si NW growth conditions when using such catalysts. Since the growth temperatures investigated are lower than the eutectic temperatures of these Si-metal alloys, VSS growth is expected and observed. Different temperatures and HCl flow rates have been tested with the aim of minimizing 2D growth which induces an important tapering of the NW. Finally, mechanical characterization of single NW has been carried out using an AFM method developed at the LTM. It consists in measuring the deflection of an AFM tip while performing approach-retract curves at various positions along the length of a cantilevered NW. This approach allows the measurement of as-grown single NW's Young modulus and spring constant, and alleviates uncertainties inherent in single point measurement. PMID:21711709

Efficient acetone sensor based on Ni-doped ZnO nanostructures prepared by spray pyrolysis technique

NASA Astrophysics Data System (ADS)

Darunkar, Swapnil S.; Acharya, Smita A.

2018-05-01

Ni-doped ZnO thin film was prepared by home-built spray pyrolysis unit for the detection of acetone at 300°C. Scanning electron microscopic (SEM) images of as-developed thin film of undoped ZnO exhibits large quantity of spherical, non-agglomerated particles with uniform size while in Ni-doped ZnO, particles are quite non-uniform in nature. The particle size estimated by using image J are obtained to be around 20-200 nm. Ni-doping effect on band gaps are determined by UV-vis optical spectroscopy and band gap of Ni-doped ZnO is found to be 3.046 eV. Nickel doping exceptionally enhances the sensing response of ZnO as compared to undoped ZnO system. The major role of the Ni-doping is to create more active sites for chemisorbed oxygen on the surface of sensor and correspondingly, to improve the sensing response. The 6 at.% of Ni-doped ZnO exhibits the highest response (92%) for 100 ppm acetone at 300 °C.

Scanning tunneling spectroscopic (STS) studies of magnetically doped MBE-grown topological insulators (TIs)

NASA Astrophysics Data System (ADS)

Chu, Hao; Teague, Marcus; Chen, Chien-Chang; Woodward, Nicholas; Yeh, Nai-Chang; Kou, Xufeng; He, Liang; Lang, Murong; Wang, Kang; Caltech Collaboration; UCLA Collaboration

2013-03-01

We conduct STS studies on MBE-grown heterostructures of non-magnetic TI (Bi2Se3) with a range of thicknesses (d = 1, 3, 5, 7 quintuple layers, QL) on top of 7-QL magnetically doped TI (Cr-doped Bi2Se3) . For d = 1 and 3-QL, a spatially homogeneous magnetism-induced surface gap (as large as about 150 meV for d = 1-QL) is observed at 77 K, whereas gapless Dirac spectra are found for d = 5 and 7-QL, suggesting that the effective magnetic length for Cr-doped Bi2Se3 is approximately 4 ~ 5-QL. These findings are further corroborated by ARPES and bulk electrical transport measurements. The magnetism-induced surface gap differs from those found in pure Bi2Se3 and (Bi0.5Sb0.5)2 Te3 films of thicknesses smaller than 6-QL, because the latter are due to overlaps of wave functions between the surface and interface layers, which lead to Rashba-like spin-orbit splitting and spin-preserving quasiparticle interference wave-vectors. In contrast, STS studies of TIs with magnetism-induced surface gap do not yield any quasiparticle interferences for energies within the bulk Bi2Se3 gap. Finally, comparative STS studies of pure and magnetically doped TIs in high magnetic fields will be discussed. This work was supported by DARPA.

Structural, dynamic, and vibrational properties during heat transfer in Si/Ge superlattices: A Car-Parrinello molecular dynamics study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ji, Pengfei; Zhang, Yuwen, E-mail: zhangyu@missouri.edu; Yang, Mo

The structural, dynamic, and vibrational properties during heat transfer process in Si/Ge superlattices are studied by analyzing the trajectories generated by the ab initio Car-Parrinello molecular dynamics simulation. The radial distribution functions and mean square displacements are calculated and further discussions are made to explain and probe the structural changes relating to the heat transfer phenomenon. Furthermore, the vibrational density of states of the two layers (Si/Ge) are computed and plotted to analyze the contributions of phonons with different frequencies to the heat conduction. Coherent heat conduction of the low frequency phonons is found and their contributions to facilitate heatmore » transfer are confirmed. The Car-Parrinello molecular dynamics simulation outputs in the work show reasonable thermophysical results of the thermal energy transport process and shed light on the potential applications of treating the heat transfer in the superlattices of semiconductor materials from a quantum mechanical molecular dynamics simulation perspective.« less

Structural, dynamic, and vibrational properties during heat transfer in Si/Ge superlattices: A Car-Parrinello molecular dynamics study

NASA Astrophysics Data System (ADS)

Ji, Pengfei; Zhang, Yuwen; Yang, Mo

2013-12-01

The structural, dynamic, and vibrational properties during heat transfer process in Si/Ge superlattices are studied by analyzing the trajectories generated by the ab initio Car-Parrinello molecular dynamics simulation. The radial distribution functions and mean square displacements are calculated and further discussions are made to explain and probe the structural changes relating to the heat transfer phenomenon. Furthermore, the vibrational density of states of the two layers (Si/Ge) are computed and plotted to analyze the contributions of phonons with different frequencies to the heat conduction. Coherent heat conduction of the low frequency phonons is found and their contributions to facilitate heat transfer are confirmed. The Car-Parrinello molecular dynamics simulation outputs in the work show reasonable thermophysical results of the thermal energy transport process and shed light on the potential applications of treating the heat transfer in the superlattices of semiconductor materials from a quantum mechanical molecular dynamics simulation perspective.

Effect on Electron Structure and Magneto-Optic Property of Heavy W-Doped Anatase TiO2

PubMed Central

Hou, Qingyu; Zhao, Chunwang; Guo, Shaoqiang; Mao, Fei; Zhang, Yue

2015-01-01

The spin or nonspin state of electrons in W-doped anatase TiO2 is very difficult to judge experimentally because of characterization method limitations. Hence, the effect on the microscopic mechanism underlying the visible-light effect of W-doped anatase TiO2 through the consideration of electronic spin or no-spin states is still unknown. To solve this problem, we establish supercell models of W-doped anatase TiO2 at different concentrations, followed by geometry optimization and energy calculation based on the first-principle planewave norm conserving pseudo-potential method of the density functional theory. Calculation results showed that under the condition of nonspin the doping concentration of W becomes heavier, the formation energy becomes greater, and doping becomes more difficult. Meanwhile, the total energy increases, the covalent weakens and ionic bonds strengthens, the stability of the W-doped anatase TiO2 decreases, the band gap increases, and the blue-shift becomes more significant with the increase of W doping concentration. However, under the condition of spin, after the band gap correction by the GGA+U method, it is found that the semimetal diluted magnetic semiconductors can be formed by heavy W-doped anatase TiO2. Especially, a conduction electron polarizability of as high as near 100% has been found for the first time in high concentration W-doped anatase TiO2. It will be able to be a promising new type of dilute magnetic semiconductor. And the heavy W-doped anatase TiO2 make the band gap becomes narrower and absorption spectrum red-shift. PMID:25955308

Multivalent Mn-doped TiO2 thin films

NASA Astrophysics Data System (ADS)

Lin, C. Y. W.; Channei, D.; Koshy, P.; Nakaruk, A.; Sorrell, C. C.

2012-07-01

Thin films of TiO2 doped with Mn were deposited on F-doped SnO2-coated glass using spin coating. The concentration of the dopant was in the range 0-7 wt% Mn (metal basis). The films were examined in terms of the structural, chemical, and optical properties. Glancing angle X-ray diffraction data show that the films consisted of the anatase polymorph of TiO2, without any contaminant phases. The X-ray photoelectron spectroscopy data indicate the presence of Mn3+ and Mn4+ in the doped films as well as atomic disorder and associated structural distortion. Ultraviolet-visible spectrophotometry data show that the optical indirect band gap of the films decreased significantly with increasing manganese doping, from 3.32 eV for the undoped composition to 2.90 eV for that doped with 7 wt% Mn.

Insight into doping efficiency of organic semiconductors from the analysis of the density of states in n-doped C60 and ZnPc

NASA Astrophysics Data System (ADS)

Gaul, Christopher; Hutsch, Sebastian; Schwarze, Martin; Schellhammer, Karl Sebastian; Bussolotti, Fabio; Kera, Satoshi; Cuniberti, Gianaurelio; Leo, Karl; Ortmann, Frank

2018-05-01

Doping plays a crucial role in semiconductor physics, with n-doping being controlled by the ionization energy of the impurity relative to the conduction band edge. In organic semiconductors, efficient doping is dominated by various effects that are currently not well understood. Here, we simulate and experimentally measure, with direct and inverse photoemission spectroscopy, the density of states and the Fermi level position of the prototypical materials C60 and zinc phthalocyanine n-doped with highly efficient benzimidazoline radicals (2-Cyc-DMBI). We study the role of doping-induced gap states, and, in particular, of the difference Δ1 between the electron affinity of the undoped material and the ionization potential of its doped counterpart. We show that this parameter is critical for the generation of free carriers and influences the conductivity of the doped films. Tuning of Δ1 may provide alternative strategies to optimize the electronic properties of organic semiconductors.

Synthesis and characterization of single-phase Mn-doped ZnO

NASA Astrophysics Data System (ADS)

Chattopadhyay, S.; Dutta, S.; Banerjee, A.; Jana, D.; Bandyopadhyay, S.; Chattopadhyay, S.; Sarkar, A.

2009-05-01

Different samples of Zn 1-xMn xO series have been prepared using conventional solid-state sintering method. We identified up to what extent doping will enable us to synthesize single-phase polycrystalline Mn-doped ZnO sample, which is one of the prerequisites for dilute magnetic semiconductor, and we have analyzed its some other physical aspects. In synthesizing the samples, proportion of Mn varies from 1 to 5 at%. However, the milling time varied (6, 12, 24, 48 and 96 h) only for 2 at% Mn-doped samples while for other samples (1, 3, 4 and 5 at% Mn doped) the milling time has been fixed to 96 h. Room-temperature X-ray diffraction (XRD) data reveal that all of the prepared samples up to 3 at% of Mn doping exhibit wurtzite-type structure, and no segregation of Mn and/or its oxides has been found. The 4 at% Mn-doped samples show a weak peak of ZnMn 2O 4 apart from the other usual peaks of ZnO and the intensity of this impurity peak has been further increased for 5 at% of Mn doping. So beyond 3 at% doping, single-phase behavior is destroyed. Band gap for all the 2 at% Mn-doped samples has been estimated to be between 3.21 and 3.19 eV and the reason for this low band gap values has been explained through the grain boundary trapping model. The room-temperature resistivity measurement shows an increase of resistivity up to 48 h of milling and with further milling it saturates. The defect state of these samples has been investigated using the positron annihilation lifetime (PAL) spectroscopy technique. Here all the relevant lifetime parameters of positron i.e. free annihilation ( τ1) at defect site ( τ2) and average ( τav) increases with milling time.

Realizing luminescent downshifting in ZnO thin films by Ce doping with enhancement of photocatalytic activity

NASA Astrophysics Data System (ADS)

Narayanan, Nripasree; Deepak, N. K.

2018-04-01

ZnO thin films doped with Ce at different concentration were deposited on glass substrates by spray pyrolysis technique. XRD analysis revealed the phase purity and polycrystalline nature of the films with hexagonal wurtzite geometry and the composition analysis confirmed the incorporation of Ce in the ZnO lattice in the case of doped films. Crystalline quality and optical transmittance diminished while electrical conductivity enhanced with Ce doping. Ce doping resulted in a red-shift of optical energy gap due to the downshift of the conduction band minimum after merging with Ce related impurity bands formed below the conduction band in the forbidden gap. In the room temperature photoluminescence spectra, UV emission intensity of the doped films decreased while the intensity of the visible emission band increased drastically implying the degradation in crystallinity as well as the incorporation of defect levels capable of luminescence downshifting. Ce doping showed improvement in photocatalytic efficiency by effectively trapping the free carriers and then transferring for dye degradation. Thus Ce doped ZnO thin films are capable of acting as luminescent downshifters as well as efficient photocatalysts.

Heavy Ion Microbeam- and Broadbeam-Induced Current Transients in SiGe HBTs

NASA Technical Reports Server (NTRS)

Pellish, Jonathan A.; Reed, R. A.; McMorrow, D.; Vizkelethy, G.; Ferlet-Cavrois, V.; Baggio, J.; Duhamel, O.; Moen, K. A.; Phillips, S. D.; Diestelhorst, R. M.;

Si/SiGe heterointerfaces in one-, two-, and three-dimensional nanostructures: their impact on SiGe light emission

NASA Astrophysics Data System (ADS)

Lockwood, David; Wu, Xiaohua; Baribeau, Jean-Marc; Mala, Selina; Wang, Xialou; Tsybeskov, Leonid

2016-03-01

Fast optical interconnects together with an associated light emitter that are both compatible with conventional Si-based complementary metal-oxide- semiconductor (CMOS) integrated circuit technology is an unavoidable requirement for the next-generation microprocessors and computers. Self-assembled Si/Si1-xGex nanostructures, which can emit light at wavelengths within the important optical communication wavelength range of 1.3 - 1.55 μm, are already compatible with standard CMOS practices. However, the expected long carrier radiative lifetimes observed to date in Si and Si/Si1-xGex nanostructures have prevented the attainment of efficient light-emitting devices including the desired lasers. Thus, the engineering of Si/Si1-xGex heterostructures having a controlled composition and sharp interfaces is crucial for producing the requisite fast and efficient photoluminescence (PL) at energies in the range 0.8-0.9 eV. In this paper we assess how the nature of the interfaces between SiGe nanostructures and Si in heterostructures strongly affects carrier mobility and recombination for physical confinement in three dimensions (corresponding to the case of quantum dots), two dimensions (corresponding to quantum wires), and one dimension (corresponding to quantum wells). The interface sharpness is influenced by many factors such as growth conditions, strain, and thermal processing, which in practice can make it difficult to attain the ideal structures required. This is certainly the case for nanostructure confinement in one dimension. However, we demonstrate that axial Si/Ge nanowire (NW) heterojunctions (HJs) with a Si/Ge NW diameter in the range 50 - 120 nm produce a clear PL signal associated with band-to-band electron-hole recombination at the NW HJ that is attributed to a specific interfacial SiGe alloy composition. For three-dimensional confinement, the experiments outlined here show that two quite different Si1-xGex nanostructures incorporated into a Si0.6Ge0.4 wavy

Ferromagnetism in doped or undoped spintronics nanomaterials

NASA Astrophysics Data System (ADS)

Qiang, You

2010-10-01

Much interest has been sparked by the discovery of ferromagnetism in a range of oxide doped and undoped semiconductors. The development of ferromagnetic oxide semiconductor materials with giant magnetoresistance (GMR) offers many advantages in spintronics devices for future miniaturization of computers. Among them, TM-doped ZnO is an extensively studied n-type wide-band-gap (3.36 eV) semiconductor with a tremendous interest as future mini-computer, blue light emitting, and solar cells. In this talk, Co-doped ZnO and Co-doped Cu2O semiconductor nanoclusters are successfully synthesized by a third generation sputtering-gas-aggregation cluster technique. The Co-doped nanoclusters are ferromagnetic with Curie temperature above room temperature. Both of Co-doped nanoclusters show positive magnetoresistance (PMR) at low temperature, but the amplitude of the PMRs shows an anomalous difference. For similar Co doping concentration at 5 K, PMR is greater than 800% for Co-doped ZnO but only 5% for Co-doped Cu2O nanoclusters. Giant PMR in Co-doped ZnO which is attributed to large Zeeman splitting effect has a linear dependence on applied magnetic field with very high sensitivity, which makes it convenient for the future spintronics applications. The small PMR in Co-doped Cu2O is related to its vanishing density of states at Fermi level. Undoped Zn/ZnO core-shell nanoparticle gives high ferromagnetic properties above room temperature due to the defect induced magnetization at the interface.

Structural and optical properties of pure and copper doped zinc oxide nanoparticles

NASA Astrophysics Data System (ADS)

Sajjad, Muhammad; Ullah, Inam; Khan, M. I.; Khan, Jamshid; Khan, M. Yaqoob; Qureshi, Muhammad Tauseef

2018-06-01

Pure and copper-doped zinc oxide nanoparticles (NPs) have been synthesized via chemical co-precipitation method where hydrazine is used as reducing agent and aqueous extract of Euphorbia milii plant as capping agent. Main objectives of the reported work are to investigate the effect of copper doping on crystal structure of ZnO nanoparticles; to study the effect of copper doping on optical band gap of ZnO nanoparticles and photoluminescence (PL) study of pure and copper-doped ZnO nanoparticles. To achieve the aforementioned objectives, XRD and SEM tests were performed for the identification and confirmation of crystal structure and morphology of the prepared samples. From XRD data the average grain size for pure ZnO was observed to be 24.62 nm which was first decreased to 18.95 nm for 5 wt% Cu-doped sample and then it was found to increase up to 37.80 nm as the Cu doping was increased to 7 wt%. Optical band gap of pure and Cu-doped ZnO nanoparticles was calculated from diffuse reflectance spectroscopy (DRS) spectra and was found to decrease from 3.13 eV to 2.94 eV as the amount of Cu increases up to 7 wt%. In photoluminescence study, PL technique was used and enhanced visible spectrum was observed. For further characterization FT-IR and EDX tests were also carried out.

Superconducting gap evolution in overdoped BaFe₂(As 1-xP x)₂ single crystals through nanocalorimetry

DOE PAGES

Campanini, D.; Diao, Z.; Fang, L.; ...

2015-06-18

We report on specific heat measurements on clean overdoped BaFe₂(As 1-xP x)₂ single crystals performed with a high resolution membrane-based nanocalorimeter. A nonzero residual electronic specific heat coefficient at zero temperature γr=C/T| T→0 is seen for all doping compositions, indicating a considerable fraction of the Fermi surface ungapped or having very deep minima. The remaining superconducting electronic specific heat is analyzed through a two-band s-wave α model in order to investigate the gap structure. Close to optimal doping we detect a single zero-temperature gap of Δ₀~5.3 me V, corresponding to Δ₀/k BT c ~ 2.2. Increasing the phosphorus concentration x,more » the main gap reduces till a value of Δ₀ ~ 1.9 meV for x = 0.55 and a second weaker gap becomes evident. From the magnetic field effect on γ r, all samples however show similar behavior [γ r(H) - γ r (H = 0)∝ H n, with n between 0.6 and 0.7]. This indicates that, despite a considerable redistribution of the gap weights, the total degree of gap anisotropy does not change drastically with doping.« less

Strain-mediated electronic properties of pristine and Mn-doped GaN monolayers

NASA Astrophysics Data System (ADS)

Sharma, Venus; Srivastava, Sunita

2018-04-01

Graphene-like two-dimensional (2D) monolayer structures GaN has gained enormous amount of interest due to high thermal stability and inherent energy band gap for practical applications. First principles calculations are performed to investigate the electronic structure and strain-mediated electronic properties of pristine and Mn-doped GaN monolayer. Binding energy of Mn dopant at various adsorption site is found to be nearly same indicating these sites to be equally favorable for adsorption of foreign atom. Depending on the adsorption site, GaN monolayer can act as p-type or n-type magnetic semiconductor. The tensile strength of both pristine and doped GaN monolayer (∼24 GPa) at ultimate tensile strain of 34% is comparable with the tensile strength of graphene. The in-plane biaxial strain modulate the energy band gap of both pristine and doped-monolayer from direct to indirect gap semiconductor and finally retendered theme into metal at critical value of applied strain. These characteristics make GaN monolayer to be potential candidate for the future applications in tunable optoelectronics.

Electron doping a kagome spin liquid

DOE PAGES

Kelly, Z. A.; Gallagher, M. J.; McQueen, T. M.

2016-10-13

Herbertsmithite, ZnCu 3(OH) 6Cl 2, is a two-dimensional kagome lattice realization of a spin liquid, with evidence for fractionalized excitations and a gapped ground state. Such a quantum spin liquid has been proposed to underlie high-temperature superconductivity and is predicted to produce a wealth of new states, including a Dirac metal at 1/3 electron doping. Here, we report the topochemical synthesis of electron-doped ZnLi xCu 3(OH) 6Cl 2 from x=0 to x=1.8 (3/5 per Cu 2+). Contrary to expectations, no metallicity or superconductivity is induced. Instead, we find a systematic suppression of magnetic behavior across the phase diagram. Lastly, ourmore » results demonstrate that significant theoretical work is needed to understand and predict the role of doping in magnetically frustrated narrow band insulators, particularly the interplay between local structural disorder and tendency toward electron localization, and pave the way for future studies of doped spin liquids.« less

Influence of axial tensile strain on the electronic and structural properties as well as NO gas sensitivity and reactivity of C-doped SW-BNNTs

NASA Astrophysics Data System (ADS)

Roohi, Hossein; Maleki, Layla

2017-11-01

The insulating character of BNNTs strongly imposes a great restriction on their applicability in nano-electronic devices. Therefore, it is desirable to find the practical routes for reducing the H-L gap. In this work, we demonstrate that the structural and electronic properties of the C-doped SW-BNNT can be significantly engineered and tuned by applying the axial tensile strain. Defect formation energies, cohesive energies, dipole moments, NBO charges, and global reactivity descriptors for un-doped SW-BNNT and C1-3-doped SW-BNNTs are calculated upon the axial strain. The B3LYP/6-31 +G(d) calculated H-L gap for five C-doped SW-BNNTs are expected to be smaller than that of un-doped SW-BNNT. At 10% axial tensile strain, CB NT is a suitable conductance with a 1.947 eV H-L gap. The decrease in the H-L gap for 2C-doped CN,B (-0.839 eV) NT within 15% strain is greater than 1C- and 3C-doped SW-BNNTs. In the second part of this work, reactivity and the sensitivity of strained C1-2-doped SW-BNNTs toward NO gas were evaluated at M06-2X/6-31 ++G(d,p) level of theory. Optimized structures, molecular graphs, adsorption energies (AE), dispersion corrected AEs, H-L gap, NBO charges, charge transfer values, density of states and electrostatic potentials were calculated. The strained C1-doped SW-BNNTs showed an increased ability for the sensitivity and adsorption of NO molecule, as compared with unstrained doped SW-BNNT. In general, the CN,B NTs have practically less potential for the adsorption of NO molecule than CB and CN ones.

First principles study of CuAlO2 doping with S

NASA Astrophysics Data System (ADS)

Gao, Haigen; Zhou, Jian; Lu, Minghui

2010-07-01

We study the electronic properties of CuAlO2 doped with S by the first principles calculations and find that the band gap of CuAlO2 is reduced after the doping. At the same time, the effective masses are also reduced and the density of states could cross the Fermi level. These results show that the conductivity of CuAlO2 could be enhanced by doping the impurities of S, which needs to be further studied.

Origin of band gap bowing in dilute GaAs1-xNx and GaP1-xNx alloys: A real-space view

NASA Astrophysics Data System (ADS)

Virkkala, Ville; Havu, Ville; Tuomisto, Filip; Puska, Martti J.

2013-07-01

The origin of the band gap bowing in dilute nitrogen doped gallium based III-V semiconductors is largely debated. In this paper we show the dilute GaAs1-xNx and GaP1-xNx as representative examples that the nitrogen-induced states close to the conduction band minimum propagate along the zigzag chains on the {110} planes. Thereby states originating from different N atoms interact with each other resulting in broadening of the nitrogen-induced states which narrows the band gap. Our modeling based on ab initio theoretical calculations explains the experimentally observed N concentration dependent band gap narrowing both qualitatively and quantitatively.

First-principles study on the magnetic and electronic properties of Al or P doped armchair silicene nanoribbons

NASA Astrophysics Data System (ADS)

Zhang, Xiaojiao; Zhang, Dan; Xie, Fang; Zheng, Xialian; Wang, Haiyan; Long, Mengqiu

2017-07-01

Using the first-principles calculations, we investigate the geometric structure, electronic and magnetic properties of armchair silicene nanoribbons (ASiNRs) doped with aluminum (Al) or phosphorus (P) atoms. Total energy analysis shows that both Al and P atoms are preferentially doping at the edge site of ASiNRs. And the magnetism can be found in both Al and P doped systems. For Al doped ASiNRs, we find that the magnetic moment and band gap are dependent on the ribbon width. While for P doped ASiNRs, the magnetic moment always keeps 1μB and is independent of the ribbon width, meanwhile the band gap oscillates with a period of three with the ribbon width increasing. Our results present a new avenue for band engineering of SiNRs and benefit for the designing of silicone-based nano-spin-devices in nanoelectronics.

Measurement of steady-state minority-carrier transport parameters in heavily doped n-type silicon

NASA Technical Reports Server (NTRS)

Del Alamo, Jesus A.; Swanson, Richard M.

1987-01-01

The relevant hole transport and recombination parameters in heavily doped n-type silicon under steady state are the hole diffusion length and the product of the hole diffusion coefficient times the hole equilibrium concentration. These parameters have measured in phosphorus-doped silicon grown by epitaxy throughout nearly two orders of magnitude of doping level. Both parameters are found to be strong functions of donor concentration. The equilibrium hole concentration can be deduced from the measurement. A rigid shrinkage of the forbidden gap appears as the dominant heavy doping mechanism in phosphorus-doped silicon.

First-principles study of nitrogen-doped CuAlO2

NASA Astrophysics Data System (ADS)

Xu, Ying; Ao, Zhi Min; Yuan, Ding Wang

2012-08-01

The electronic structure and formation energies of N-doped CuAlO2 are studied using first-principles calculations. It is found that, when a N atom is doped into CuAlO2, the N atom prefers to substitute an O atom rather than to occupy an interstitial site of the Cu layer. The NO acts as a shallow accepter while the Ni acts as a deep accepter. The results of the electronic structure show that the N-doping doesn't alter the band gap of CuAlO2 for the both cases. In the substitutional case, the N impurity states occur at the top of valance band maximum (VBM), which provides holes and increases the p-type conductivity. However, in the interstitial case, the N impurity states occur in the middle of the band gap, which are more localized and this indicates that it is not good for p-type conductivity.

Negative Photoconductance in Heavily Doped Si Nanowire Field-Effect Transistors.

PubMed

Baek, Eunhye; Rim, Taiuk; Schütt, Julian; Baek, Chang-Ki; Kim, Kihyun; Baraban, Larysa; Cuniberti, Gianaurelio

2017-11-08

We report the first observation of negative photoconductance (NPC) in n- and p-doped Si nanowire field-effect transistors (FETs) and demonstrate the strong influence of doping concentrations on the nonconventional optical switching of the devices. Furthermore, we show that the NPC of Si nanowire FETs is dependent on the wavelength of visible light due to the phonon-assisted excitation to multiple conduction bands with different band gap energies that would be a distinct optoelectronic property of indirect band gap semiconductor. We attribute the main driving force of NPC in Si nanowire FETs to the photogenerated hot electrons trapping by dopants ions and interfacial states. Finally, comparing back- and top-gate modulation, we derive the mechanisms of the transition between negative and positive photoconductance regimes in nanowire devices. The transition is decided by the competition between the light-induced interfacial trapping and the recombination of mobile carriers, which is dependent on the light intensity and the doping concentration.

Interface Energetics and Chemical Doping of Organic Electronic Materials

NASA Astrophysics Data System (ADS)

Kahn, Antoine

2014-03-01

The energetics of organic semiconductors and their interfaces are central to the performance of organic thin film devices. The relative positions of charge transport states across the many interfaces of multi-layer OLEDs, OPV cells and OFETs determine in great part the efficiency and lifetime of these devices. New experiments are presented here, that look in detail at the position of these transport states and associated gap states and electronic traps that tail into the energy gap of organic molecular (e.g. pentacene) or polymer (P3HT, PBDTTT-C) semiconductors, and which directly affect carrier mobility in these materials. Disorder, sometime caused by simple exposure to an inert gas, impurities and defects are at the origin of these electronic gap states. Recent efforts in chemical doping in organic semiconductors aimed at mitigating the impact of electronic gap states are described. An overview of the reducing or oxidizing power of several n- and p-type dopants for vacuum- or solution-processed films, and their effect on the electronic structure and conductivity of both vacuum- and solution-processed organic semiconductor films is given. Finally, the filling (compensation) of active gap states via doping is investigated on the electron-transport materials C60 and P(NDI2OD-T2) , and the hole-transport polymer PBDTTT-C.

Electron Doping a Kagome Spin Liquid

NASA Astrophysics Data System (ADS)

Kelly, Zachary; Gallagher, Miranda; McQueen, Tyrel

In 1987, Anderson proposed that charge doping a material with the resonating valance bond (RVB) state would yield a superconducting state. Ever since, there has been a search for these RVB containing spin liquid materials and their charge doped counterparts. Studies on the most promising spin liquid candidate, Herbertsmithite, ZnCu3(OH)6Cl2, a two dimensional kagomé lattice, show evidence of fractionalized excitations and a gapped ground state. In this work, we report the synthesis and characterization of a newly synthesized electron doped spin liquid, ZnLixCu3(OH)6Cl2 from x = 0 to x = 1.8 (3 / 5 th per Cu2+). Despite heavy doping, the series remains insulating and the magnetism is systematically suppressed. We have done extensive structural studies of the doped series to determine the effect of the intercalated atoms on the structure, and whether these structural differences induce strong localization effects that suppress the metallic and superconducting states. Other doped spin liquid candidates are also being explored to understand if this localization is system dependent or systemic to all doped spin liquid systems. NSF, Division of Materials Research (DMR), Solid State Chemistry (SSMC), CAREER Grant under Award No. DMR- 1253562, Institute for Quantum Matter under Grant No.DE-FG02- 08ER46544, and the David and Lucile Packard Foundation.

Thermoelectric Properties of Silicon Germanium: An In-depth Study to the Reduction of Lattice Thermal Conductivity

NASA Astrophysics Data System (ADS)

Thompson, Daniel Ross

properties of the resulting materials were investigated. Based on the densities, x-ray diffraction patterns, derived lattice constants, and Vegard's law it will be shown that the SE SPS method does successfully alloy multiple compositions of undoped SiGe. The third and most important study demonstrated that SiGe alloyed using the SE SPS synthesis technique can be successfully doped to a n and p type thermoelectric (TE) material. This required an investigation of all of the TE transport properties of these materials. A significant investigation and commentary will be provided for the lattice thermal conductivity of SiGe. The need for this investigation arises from the difference in synthesis processes between the traditional MA and the novel SE SPS techniques. The MA powder is already alloyed into micron sized powders that are consolidated by the HP for an extended time (>1 hour), which allows for grain growth. The SE SPS method relies on diffusion being promoted by the electric field assisted sintering technique and occurs over a very short period of time (<30 minutes). Therefore it can not be assumed that grain growth is not affected by the time dependent processes of sintering and diffusion with the SE SPS process. As will be discussed grain size plays a role in the lattice thermal conductivity of SiGe. It is surprising and physically interesting that the MA+HP standards and the SE SPS samples have lattice thermal conductivities that indicate the dominant scattering mechanism is the same. The physical insight provided by the fourth study is made possible by the existence of the new SE SPS synthesis method for SiGe. The MA method is optimized by the addition of GaP to the n-type SiGe materials during processing. The explanation for this optimization is a subject of debate within the community. Although, a staunch conclusion can not be made due to the need for more samples and carrier concentration data, this initial study does indicate that one physical explanation within

Theoretical studies on anisotropic electrical conductivity of trans-polyacetylene doped with n-type dopants

NASA Astrophysics Data System (ADS)

Wang, Cunguo; Wang, Rongshun

2000-12-01

Based on energy band theory of solid states, extended Hückel molecular orbital methods (EHMO/CO) were used to calculate the two-dimensional (2D) energy band structures of highly oriented trans-polyacetylene (PA) undoped and doped with n-type dopant (Li, Na, K). The band gaps ( Eg) of undoped PA in directions parallel and perpendicular to the oriented direction were 1.195 and 3.040 eV, respectively. When PA was doped with n-type dopant, the corresponding band gaps Eg1 and Eg2 decreased significantly. Based on the calculated results, we could successfully account for the changes of electrical anisotropy of PA from the undoped state to the doped form. The conductivity anisotropy ratio σ1/ σ2 decreased when PA was doped with n-type dopant, because the PA chains and the dopant showed a strong interchain coupling. It was the interchain coupling that acted as a bridge between two neighboring chains, and made the charge-carrier transport easier between the interchains. The theoretical results for undoped and doped PA are in good agreement with the experiment.

Theory of nitrogen doping of carbon nanoribbons: Edge effects

DOE PAGES

Jiang, Jie; Turnbull, Joseph; Lu, Wenchang; ...

2012-01-01

Nitrogen doping of a carbon nanoribbon is profoundly affected by its one-dimensional character, symmetry, and interaction with edge states. Using state-of-the-art ab initio calculations, including hybrid exact-exchange density functional theory, we find that, for N-doped zigzag ribbons, the electronic properties are strongly dependent upon sublattice effects due to the non-equivalence of the two sublattices. For armchair ribbons, N-doping effects are different depending upon the ribbon family: for families 2 and 0, the N-induced levels are in the conduction band, while for family 1 the N levels are in the gap. In zigzag nanoribbons, nitrogen close to the edge is amore » deep center, while in armchair nanoribbons its behavior is close to an effective-mass-like donor with the ionization energy dependent on the value of the band gap. In chiral nanoribbons, we find strong dependence of the impurity level and formation energy upon the edge position of the dopant, while such site-specificity is not manifested in the magnitude of the magnetization.« less

Band gap modulation in magnetically doped low-defect thin films of (Bi1-xSbx)2 Te3 with minimized bulk carrier concentration

NASA Astrophysics Data System (ADS)

Maximenko, Yulia; Scipioni, Kane; Wang, Zhenyu; Katmis, Ferhat; Steiner, Charles; Weis, Adam; van Harlingen, Dale; Madhavan, Vidya

Topological insulators Bi2Te3 and Sb2Te3 are promising materials for electronics, but both are naturally prone to vacancies and anti-site defects that move the Fermi energy onto the bulk bands. Fabricating (Bi1-xSbx)2 Te3 (BST) with the tuned x minimizes point defects and unmasks topological surface states by reducing bulk carriers. BST thin films have shown topological surface states and quantum anomalous Hall effect. However, different studies reported variable Sb:Bi ratios used to grow an undoped BST film. Here, we develop a reliable way to grow defect-free subnanometer-flat BST thin films having the Fermi energy tuned to the Dirac point. High-resolution scanning tunneling microscopy (STM) and Landau level spectroscopy prove the importance of crystallinity and surface roughness-not only Sb:Bi ratio-for the final bulk carrier concentration. The BST thin films were doped with Cr and studied with STM with atomic resolution. Counterintuitively, Cr density is anticorrelated with the local band gap due to Cr's antiferromagnetic order. We analyze the correlations and report the relevant band gap values. Predictably, high external magnetic field compromises antiferromagnetic order, and the local band gap increases. US DOE DE-SC0014335; Moore Found. GBMF4860; F. Seitz MRL.

Real-space visualization of remnant Mott gap and magnon excitations.

PubMed

Wang, Y; Jia, C J; Moritz, B; Devereaux, T P

2014-04-18

We demonstrate the ability to visualize real-space dynamics of charge gap and magnon excitations in the Mott phase of the single-band Hubbard model and the remnants of these excitations with hole or electron doping. At short times, the character of magnetic and charge excitations is maintained even for large doping away from the Mott and antiferromagnetic phases. Doping influences both the real-space patterns and long timescales of these excitations with a clear carrier asymmetry attributable to particle-hole symmetry breaking in the underlying model. Further, a rapidly oscillating charge-density-wave-like pattern weakens, but persists as a visible demonstration of a subleading instability at half-filling which remains upon doping. The results offer an approach to analyzing the behavior of systems where momentum space is either inaccessible or poorly defined.

Observation of Phase-Filling Singularities in the Optical Dielectric Function of Highly Doped n-Type Ge.

PubMed

Xu, Chi; Fernando, Nalin S; Zollner, Stefan; Kouvetakis, John; Menéndez, José

2017-06-30

Phase-filling singularities in the optical response function of highly doped (>10^{19}  cm^{-3}) germanium are theoretically predicted and experimentally confirmed using spectroscopic ellipsometry. Contrary to direct-gap semiconductors, which display the well-known Burstein-Moss phenomenology upon doping, the critical point in the joint density of electronic states associated with the partially filled conduction band in n-Ge corresponds to the so-called E_{1} and E_{1}+Δ_{1} transitions, which are two-dimensional in character. As a result of this reduced dimensionality, there is no edge shift induced by Pauli blocking. Instead, one observes the "original" critical point (shifted only by band gap renormalization) and an additional feature associated with the level occupation discontinuity at the Fermi level. The experimental observation of this feature is made possible by the recent development of low-temperature, in situ doping techniques that allow the fabrication of highly doped films with exceptionally flat doping profiles.

Field-Theoretical Studies of a doped Mott Insulator

NASA Astrophysics Data System (ADS)

Juricic, Vladimir

2006-06-01

In this thesis, the magnetic and the transport properties of La(2-x)Sr(x)CuO(4) in the undoped and lightly doped regime are investigated. In Chapter 2, we consider the role of the Dzyaloshinskii-Moriya (DM) and the pseudodipolar (XY) interactions in determining the magnetic properties of the undoped material, La(2)CuO(4), motivated by recent experiments, which show a complete anisotropy in the magnetic susceptibility response. We start with the microscopic spin model, which, besides the Heisenberg superexchange interaction, contains the anisotropic DM and the XY interactions. We map this microscopic model into a corresponding field theory, which turns out to be a generalized nonlinear sigma model. The effect of the anisotropies is to introduce gaps for the spin excitations, which are responsible for the ground-state properties of the material. When a magnetic field is applied, the DM anisotropy leads to an unexpected linear coupling of the staggered magnetization to the magnetic field, which is responsible for a completely anisotropic magnetic susceptibility, in agreement with experiments. In Chapter 3, we investigate the effect of the DM and the XY anisotropies on the magnetism when Sr doping is introduced in La(2)CuO(4). Our starting point is the nonlinear sigma model, which includes these anisotropies, and also the dopant holes, represented via an effective dipole field which couples to the background magnetization current. In the antiferromagnetic phase, x<2%, the dipole-magnetization current coupling leads to a decrease of the spin gaps, in good agreement with recent experiments. The DM gap gives rise to the stability of the antiferromagnetic state up to the doping level x=2%, at which the dipole field acquires a nonzero expectation value, causing a change in the magnetic ground state of the system. Beyond this doping concentration, the spins rearrange to form an incommensurate helicoidal state, which gives rise to two incommensurate peaks in the spin

Electronic structure of silver doped As2S3

NASA Astrophysics Data System (ADS)

Kaur, Veerpal; Khatta, Swati; Tripathi, S. K.; Prakash, S.

2018-04-01

We have studied the band structure, density of states and partial density of states for pure arsenic trisulfide (As2S3) and silver (Ag) doped arsenic trisulfide (As2S3) using DFT based GGA approach. It is observed that with the introduction of silver in As2S3, some extra states are observed in the gap region hence modifying the semiconducting gap in As2S3. These extra states in the gap region are due to 4d-states of silver.

Electrical properties of Er-doped CdS thin films

NASA Astrophysics Data System (ADS)

Dávila-Pintle, J. A.; Lozada-Morales, R.; Palomino-Merino, M. R.; Rivera-Márquez, J. A.; Portillo-Moreno, O.; Zelaya-Angel, O.

2007-01-01

Cadmium sulfide thin films were prepared by chemical bath on glass substrates at 80°C. CdS was Er-doped during the growth process by adding water-diluted Er(NO3)33•H2O to the CdS aqueous growing solution. The relative volume of the doping solution was varied in order to obtain different doping levels. The crystalline structure of CdS:Er films was cubic zinc blende for all the doped layers prepared. The (111) interplanar distance has an irregular variation with the Er doping level. Consequently, the band gap energy (Eg) firstly increases and afterward diminishes becoming, at last, approximately constant at around Eg=2.37eV. For higher doping levels, in the as-grown films, dark electrical conductivity (σ ) values reach 1.8×10-2Ω-1cm-1 at room temperature. The logarithm of σ vs 1/kT plot, where k is Boltzmann's constant and T the absolute temperature, indicates an effective doping of CdS as a result of the Er introduction into the lattice of the material. Hall effect measurements reveal a n-type doping with 2.8×1019cm-3 as maximum carrier density.

Investigations of optical and thermoelectric response of direct band gap Ca3XO (X = Si, Ge) anti-perovskites stabilized in cubic and orthorhombic phases

NASA Astrophysics Data System (ADS)

Mahmood, Q.; Ashraf, A.; Hassan, M.

2018-02-01

We predict the phase dependent electronic properties for elaborating the optical and thermoelectric behaviors of both cubic (Pm-3m) and orthorhombic (Pbnm) Ca3XO (X = Si, Ge) antiperovskites using first-principles density functional theory (DFT) computations. The mBJ functional is employed for computing the most accurate electronic characteristics. A direct band gap semiconducting nature has been found appearing due to hybridization between O and Si/Ge p-states. The calculated band gaps lying in the infrared energy region suggest that the studied anti-perovskites can absorb visible and ultraviolet energy revealing potential optoelectronics device applications. Moreover, the important thermoelectric parameters are computed for illustrating the potential thermoelectric applications. Hence, the studied anti-perovskites can simultaneously exhibit various flexible material properties, which reveal their worth for the devices demonstrating versatile characteristics.

Heavy doping effects in high efficiency silicon solar cells

NASA Technical Reports Server (NTRS)

Lindholm, F. A.

1984-01-01

Several of the key parameters describing the heavily doped regions of silicon solar cells are examined. The experimentally determined energy gap narrowing and minority carrier diffusivity and mobility are key factors in the investigation.

Composite films of highly ordered Si nanowires embedded in SiGe0.3 for thermoelectric applications

NASA Astrophysics Data System (ADS)

Kikuchi, Akiou; Yao, Akifumi; Mori, Isamu; Ono, Takahito; Samukawa, Seiji

2017-10-01

We fabricated a high-density array of silicon nanowires (SiNWs) with a diameter of 10 nm embedded in silicon germanium (SiGe0.3) to give a composite thin film for thermoelectric device applications. The SiNW array was first fabricated by bio-template mask and neutral beam etching techniques. The SiNW array was then embedded in SiGe0.3 by thermal chemical vapor deposition. The cross-plane thermal conductivity of the SiNW-SiGe0.3 composite film with a thickness of 100 nm was 3.5 ± 0.3 W/mK in the temperature range of 300-350 K. Moreover, the temperature dependences of the in-plane electrical conductivity and in-plane Seebeck coefficient of the SiNW-SiGe0.3 composite were evaluated. The fabricated SiNW-SiGe0.3 composite film displayed a maximum power factor of 1 × 103 W/m K2 (a Seebeck coefficient of 4.8 × 103 μV/K and an electrical conductivity of 4.4 × 103 S/m) at 873 K. The present high-density SiNW array structure represents a new route to realize practical thermoelectric devices using mature Si processes without any rare metals.

Co-doped phosphorene: Enhanced sensitivity of CO gas sensing

NASA Astrophysics Data System (ADS)

Lei, S. Y.; Luan, S.; Yu, H.

2018-03-01

First-principle calculation was carried out to systematically investigate carbon monoxide (CO) adsorption on pristine and cobalt (Co)-doped phosphorenes (Co-bP). Whether or not CO is adsorped, pristine phosphorene is a direct-band-gap semiconductor. However, the bandgap of Co-bP experiences direct-to-indirect transition after CO molecule adsorption, which will affect optical absorption considerably, implying that Co doping can enhance the sensitivity of phosphorene as a CO gas sensor. Moreover, Co doping can improve an adsorption energy of CO to 1.31 eV, as compared with pristine phosphorene (0.12 eV), also indicating that Co-bP is energetically favorable for CO gas sensing.

Transition-Metal Substitution Doping in Synthetic Atomically Thin Semiconductors

DOE PAGES

Gao, Jian; Kim, Young Duck; Liang, Liangbo; ...

2016-09-20

Semiconductor impurity doping has enabled an entire generation of technology. The emergence of alternative semiconductor material systems, such as transition metal dichalcogenides (TMDCs), requires the development of scalable doping strategies. We report an unprecedented one-pot synthesis for transition-metal substitution in large-area, synthetic monolayer TMDCs. Electron microscopy, optical and electronic transport characterization and ab initio calculations indicate that our doping strategy preserves the attractive qualities of TMDC monolayers, including semiconducting transport and strong direct-gap luminescence. These results are expected to encourage exploration of transition-metal substitution in two-dimensional systems, potentially enabling next-generation optoelectronic technology in the atomically-thin regime.

Gas cluster ion beam assisted NiPt germano-silicide formation on SiGe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ozcan, Ahmet S., E-mail: asozcan@us.ibm.com; Lavoie, Christian; Jordan-Sweet, Jean

We report the formation of very uniform and smooth Ni(Pt)Si on epitaxially grown SiGe using Si gas cluster ion beam treatment after metal-rich silicide formation. The gas cluster ion implantation process was optimized to infuse Si into the metal-rich silicide layer and lowered the NiSi nucleation temperature significantly according to in situ X-ray diffraction measurements. This novel method which leads to more uniform films can also be used to control silicide depth in ultra-shallow junctions, especially for high Ge containing devices, where silicidation is problematic as it leads to much rougher interfaces.

Bright photoluminescence from ordered arrays of SiGe nanowires grown on Si(111)

PubMed Central

Rowell, N L; Benkouider, A; Ronda, A; Favre, L; Berbezier, I

2014-01-01

Summary We report on the optical properties of SiGe nanowires (NWs) grown by molecular beam epitaxy (MBE) in ordered arrays on SiO2/Si(111) substrates. The production method employs Au catalysts with self-limited sizes deposited in SiO2-free sites opened-up in the substrate by focused ion beam patterning for the preferential nucleation and growth of these well-organized NWs. The NWs thus produced have a diameter of 200 nm, a length of 200 nm, and a Ge concentration x = 0.15. Their photoluminescence (PL) spectra were measured at low temperatures (from 6 to 25 K) with excitation at 405 and 458 nm. There are four major features in the energy range of interest (980–1120 meV) at energies of 1040.7, 1082.8, 1092.5, and 1098.5 meV, which are assigned to the NW-transverse optic (TO) Si–Si mode, NW-transverse acoustic (TA), Si–substrate–TO and NW-no-phonon (NP) lines, respectively. From these results the NW TA and TO phonon energies are found to be 15.7 and 57.8 meV, respectively, which agree very well with the values expected for bulk Si1− xGex with x = 0.15, while the measured NW NP energy of 1099 meV would indicate a bulk-like Ge concentration of x = 0.14. Both of these concentrations values, as determined from PL, are in agreement with the target value. The NWs are too large in diameter for a quantum confinement induced energy shift in the band gap. Nevertheless, NW PL is readily observed, indicating that efficient carrier recombination is occurring within the NWs. PMID:25671145

Photocatalytic decomposition of Rhodamine B on uranium-doped mesoporous titanium dioxide

DOE PAGES

Liu, Yi; Becker, Blake; Burdine, Brandon; ...

2017-04-13

Mesoporous uranium-doped TiO 2 anatase materials were studied to determine the influence of U-doping on the photocatalytic properties for Rhodamine B (RhB) degradation. The physico-chemical properties of the samples were characterized and the results of X-ray diffraction, transmission electron microscopy, and Raman spectroscopy demonstrate homogeneous incorporation of uranium into the anatase lattice. X-ray photoelectron spectroscopy of the doped anatase confirmed the dominance of the U 4+ species and an increasing proportion of U 6+ species as the uranium doping was increased. The absorption thresholds of the uranium-doped anatase extended into the visible light region. A synergistic effect of the bandmore » gap energy and oxidation state of the dopant contribute to an enhanced photocatalytic capability for RhB degradation by U-doped TiO 2.« less

Photocatalytic decomposition of Rhodamine B on uranium-doped mesoporous titanium dioxide

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liu, Yi; Becker, Blake; Burdine, Brandon

Mesoporous uranium-doped TiO 2 anatase materials were studied to determine the influence of U-doping on the photocatalytic properties for Rhodamine B (RhB) degradation. The physico-chemical properties of the samples were characterized and the results of X-ray diffraction, transmission electron microscopy, and Raman spectroscopy demonstrate homogeneous incorporation of uranium into the anatase lattice. X-ray photoelectron spectroscopy of the doped anatase confirmed the dominance of the U 4+ species and an increasing proportion of U 6+ species as the uranium doping was increased. The absorption thresholds of the uranium-doped anatase extended into the visible light region. A synergistic effect of the bandmore » gap energy and oxidation state of the dopant contribute to an enhanced photocatalytic capability for RhB degradation by U-doped TiO 2.« less

p-type doping by platinum diffusion in low phosphorus doped silicon

NASA Astrophysics Data System (ADS)

Ventura, L.; Pichaud, B.; Vervisch, W.; Lanois, F.

2003-07-01

In this work we show that the cooling rate following a platinum diffusion strongly influences the electrical conductivity in weakly phosphorus doped silicon. Diffusions were performed at the temperature of 910 °C in the range of 8 32 hours in 0.6, 30, and 60 Ωrm cm phosphorus doped silicon samples. Spreading resistance profile analyses clearly show an n-type to p-type conversion under the surface when samples are cooled slowly. On the other hand, a compensation of the phosphorus donors can only be observed when samples are quenched. One Pt related acceptor deep level at 0.43 eV from the valence band is assumed to be at the origin of the type conversion mechanism. Its concentration increases by lowering the applied cooling rate. A complex formation with fast species such as interstitial Pt atoms or intrinsic point defects is expected. In 0.6 Ωrm cm phosphorus doped silicon, no acceptor deep level in the lower band gap is detected by DLTS measurement. This removes the opportunity of a pairing between phosphorus and platinum and suggests the possibility of a Fermi level controlled complex formation.

A detailed study on Sn4+ doped ZnO for enhanced photocatalytic degradation

NASA Astrophysics Data System (ADS)

Beura, Rosalin; Pachaiappan, R.; Thangadurai, P.

2018-03-01

The samples of Sn4+ doped (1, 5, 10, 15, 20 & 30%) ZnO nanostructures were synthesized by a low temperature hydrothermal method. Structural analysis by XRD and Raman spectroscopy showed the hexagonal wurtzite phase of ZnO and the formation of a secondary phase Zn2SnO4 beyond 10% doping of Sn4+. Microstructural analysis by TEM also confirmed the wurtzite ZnO with rod as well as particle like structure. Presence of various functional groups (sbnd OH, sbnd CH, Znsbnd O) were confirmed by FTIR. Optical properties were studied by UV-vis absorption, photoluminescence emission spectroscopies and lifetime measurement. Band gap of the undoped and Sn4+ doped ZnO were analyzed by Tauc plot and it was observed that the band gap of the materials had slightly decreased from 3.2 to 3.16 eV and again increased to 3.23 eV with respect to the increase in the doping concentration from 1 to 30%. A significant change was also noticed in the photoluminescence emission properties of ZnO i.e. increase in the intensity of NBE emission and decrease in DLE, on subject to Sn4+ doping. Average PL lifetime had increased from 29.45 ns for ZnO to 30.62 ns upon 1% Sn ion doping in ZnO. Electrical properties studied by solid state impedance spectroscopy showed that the conductivity had increased by one order of magnitude (from 7.48×10-8 to 2.21×10-7 S/cm) on Sn4+ doping. Photocatalytic experiments were performed on methyl orange (MO) as a model industrial dye under UV light irradiation for different irradiation times. The optimum Sn4+ content in order to achieve highest photocatalytic activity was found to be 1% Sn 4+ doping. The enhancement was achieved due to a decrease in the band gap favoring the generation of electron-hole pairs and the enhanced PL life time that delays the recombination of these charge carrier formation. The third reason was that the increased electrical conductivity that indicated the faster charge transfer in this material to enhance the photocatalytic activity. The Sn

Mobility and Device Applications of Heavily Doped Silicon and Strained SILICON(1-X) Germanium(x) Layers

NASA Astrophysics Data System (ADS)

Carns, Timothy Keith

discovery of mobility and conductivity enhancement in coupled delta-doped layers is highlighted in Chapter 5. Finally, the third part of this work discusses the implementation of boron delta -doped layers in Si homojunction bipolar transistors and FETs. Chapter 6 includes the fabrication of the first coupled delta-doped base layer Si BJT, the first p-type Si delta-doped layer MESFET, the first coupled delta -doped layer FET, and the first SiGe delta -FET.

First-principles research on the optical and electrical properties and mechanisms of In-doped ZnO

NASA Astrophysics Data System (ADS)

Hou, Qingyu; Xi, Dongmin; Li, Wenling; Jia, Xiaofang; Xu, Zhenchao

2018-05-01

The absorption spectra and conductivity of In-doped ZnO still exhibit differences. To resolve this contradiction, the ZnO supercell models with different In doping amounts and the Zn0.9375In0.0625(Zni)0.0625O supercell model were both constructed. When the geometrical structure of all the models was optimized, the GGA + U and GGA used to calculate the energy. In the range of In doping used in this study, the formation energy of In-doped ZnO under Zn-rich conditions is lower than that under O-rich conditions, thereby implying a more stability of In-doped ZnO under Zn-rich than that under O-rich. With the increased In doping content, the volume and the formation energy of the doped system increase, the doped systems become unstable, and doping becomes difficult. Furthermore, the band gaps are narrowed, and the red shift of absorption spectrum is enhanced. In the In doping amount ranging within 0.01389-0.05556, the electron effective mass decreases first and subsequently increases, and the electron concentration increases. The mobility and conductivity also increase first and subsequently decrease. These results are in accordance with the experimental results. The volume of Zn0.9375In0.0625(Zni)0.0625O with the coexistence of In replacing Zn and interstitial Zn is large. The band gap is widened and the absorption spectrum is blue-shifted in the UV region.

Thin film nano-photocatalyts with low band gap energy for gas phase degradation of p-xylene: TiO2 doped Cr, UiO66-NH2 and LaBO3 (B  =  Fe, Mn, and Co)

NASA Astrophysics Data System (ADS)

Loc Luu, Cam; Thuy Van Nguyen, Thi; Nguyen, Tri; Nguyen, Phung Anh; Hoang, Tien Cuong; Ha, Cam Anh

2018-03-01

By dip-coating technique the thin films of nano-photocatalysts TiO2, Cr-doped TiO2, LaBO3 perovskites (B  =  Fe, Mn, and Co) prepared by sol-gel method, and UiO66-NH2 prepared by a solvothermal were obtained and employed for gas phase degradation of p-xylene. Physicochemical characteristics of the catalysts were examined by the methods of BET, SEM, TEM, XRD, FT-IR, TGA, Raman and UV-vis spectroscopies. The thickness of film was determined by a Veeco-American Dektek 6M instrument. The activity of catalysts was evaluated in deep photooxidation of p-xylene in a microflow reactor at room temperature with the radiation sources of a UV (λ  =  365 nm) and LED lamps (λ  =  400-510 nm). The obtained results showed that TiO2 and TiO2 doped Cr thin films was featured by an anatase phase with nanoparticles of 10-100 nm. Doping TiO2 with 0.1%mol Cr2O3 led to reduce band gap energy from 3.01 down to 1.99 eV and extend the spectrum of photon absorption to the visible region (λ  =  622 nm). LaBO3 perovkite thin films were also featured by a crystal phase with average particle nanosize of 8-40 nm, a BET surface area of 17.6-32.7 m2 g-1 and band gap energy of 1.87-2.20 eV. UiO66-NH2 was obtained in the ball shape of 100-200 nm, a BET surface area of 576 m2 g-1 and a band gap energy of 2.83 eV. The low band gap energy nano-photocatalysts based on Cr-doped TiO2 and LaBO3 perovskites exhibited highly stable and active for photo-degradation of p-xylene in the gas phase under radiation of UV-vis light. Perovskite LaFeO3 and Cr-TiO2 thin films were the best photocatalysts with a decomposition yield being reached up to 1.70 g p-xylene/g cat.

Effect of doping on electronic properties of HgSe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nag, Abhinav, E-mail: abhinavn76@gmail.com; Sastri, O. S. K. S., E-mail: sastri.osks@gmail.com; Kumar, Jagdish, E-mail: jagdishphysicist@gmail.com

2016-05-23

First principle study of electronic properties of pure and doped HgSe have been performed using all electron Full Potential Linearized Augmented Plane Wave (FP-LAPW) method using ELK code. The electronic exchange and co-relations are considered using Generalized Gradient Approach (GGA). Lattice parameter, Density of States (DOS) and Band structure calculations have been performed. The total energy curve (Energy vs Lattice parameter), DOS and band structure calculations are in good agreement with the experimental values and those obtained using other DFT codes. The doped material is studied within the Virtual Crystal Approximation (VCA) with doping levels of 10% to 25% ofmore » electrons (hole) per unit cell. Results predict zero band gap in undopedHgSe and bands meet at Fermi level near the symmetry point Γ. For doped HgSe, we found that by electron (hole) doping, the point where conduction and valence bands meet can be shifted below (above) the fermi level.« less

Stabilization of Fermi level via electronic excitation in Sn doped CdO thin films

NASA Astrophysics Data System (ADS)

Das, Arkaprava; Singh, Fouran

2018-04-01

Pure and Sn doped CdO sol-gel derived thin films were deposited on corning glass substrate and further irradiated by swift heavy ion (SHI) (Ag and O) with fluence upto 3×1013 ions/cm2. The observed tensile stress from X-ray diffraction pattern at higher fluence for Ag ions can be corroborated to the imbrications of cylindrical tracks due to multiple impacts. The anomalous band gap enhancement after irradiation may be attributed to the consolidated effect of Burstein-Moss shift (BMS) and impurity induced virtual gap states (ViGs). At higher excitation density as Fermi stabilization level (EFS) tends to coincide with charge neutrality level (CNL), band gap enhancement saturates as further creation of additional defects inside the lattice becomes unsustainable. Raman spectroscopy divulges an intensity enhancement of 478 cm-1 LO phonon mode with Sn doping and irradiation induces further asymmetric peak broadening due to damage and disordering inside the lattice. However for 3% Sn doped thin film irradiated with Ag ions having 3×1013 fluence shows a drastic change in structural properties and reduction in band gap which might be attributed to the generation of localized energy levels between conduction and valance band due to high density of defects.

Sonocatalytic degradation of humic acid by N-doped TiO2 nano-particle in aqueous solution.

PubMed

Kamani, Hossein; Nasseri, Simin; Khoobi, Mehdi; Nabizadeh Nodehi, Ramin; Mahvi, Amir Hossein

2016-01-01

Un-doped and N-doped TiO2 nano-particles with different nitrogen contents were successfully synthesized by a simple sol-gel method, and were characterized by X-ray diffraction, field emission scanning electron microscopy, Energy dispersive X-ray analysis and UV-visible diffuse reflectance spectra techniques. Then enhancement of sonocatalytic degradation of humic acid by un-doped and N-doped TiO2 nano-particles in aqueous environment was investigated. The effects of various parameters such as initial concentration of humic acid, N-doping, and the degradation kinetics were investigated. The results of characterization techniques affirmed that the synthesis of un-doped and N-doped TiO2 nano-particles was successful. Degradation of humic acid by using different nano-particles obeyed the first-order kinetic. Among various nano-particles, N-doped TiO2 with molar doping ratio of 6 % and band gap of 2.92 eV, exhibited the highest sonocatalytic degradation with an apparent-first-order rate constant of 1.56 × 10(-2) min(-1). The high degradation rate was associated with the lower band gap energy and well-formed anatase phase. The addition of nano-catalysts could enhance the degradation efficiency of humic acid as well as N-doped TiO2 with a molar ratio of 6 %N/Ti was found the best nano-catalyst among the investigated catalysts. The sonocatalytic degradation with nitrogen doped semiconductors could be a suitable oxidation process for removal of refractory pollutants such as humic acid from aqueous solution.

STM/STS study of the superconducting gap in SmFeAsO1-xFx

NASA Astrophysics Data System (ADS)

Kawashima, Yuki; Ichimura, Koichi; Katono, Kazuhiro; Kurosawa, Tohru; Oda, Migaku; Tanda, Satoshi; Kamihara, Yoichi; Hosono, Hideo

2015-02-01

We report an electron tunneling study of SmFeAsO1-xFx in the low doping region (x=0, 0.045, 0.046, 0.069) by low temperature UHV-STM/STS. Superconducting gaps are observed for each superconducting sample x=0.045 (Tc=12.9 K), x=0.046 (Tc=32.9 K) and x=0.069 (Tc=46.9 K). We obtained corresponding superconducting gap size of ΔSC = 9.5 ± 0.5 meV, 9.75±0.25 meV and 11±1 meV. While Tc increases, ΔSC is kept the same. This suggests that the effective attractive interaction is the same and that there is some mechanism that suppresses the superconductivity in the low doping region. On the other hand, similar gap structures were found in a non-superconducting sample with x=0 at 7.8 K. The obtained gap size was ΔN = 8.5 ± 1.5 meV, which is almost the same as the superconducting gap in the superconducting samples (x=0.045, 0.046, 0.069).

Effect of ZnSe and CdSe nanoparticles on the fluorescence and optical band gap of Sm3+ doped lead borate glasses

NASA Astrophysics Data System (ADS)

Fatokun, Stephen O.

For the first part of this work, we prepared a series of Sm-doped lead borate (PbO-B2O3) glasses containing zinc selenide (ZnSe) and cadmium selenide (CdSe) nanoparticles (NPs) and studied the Sm 3+ fluorescence by varying the glass composition and size of the NPs. We have chosen these heavy metal oxide glasses to incorporate Sm3+ ions because they have large glass forming region, high refractive index, and good physical and thermal stability. Lead borate glasses with the following compositions xPbO:(96.5-x)B2O 3:0.5Sm2O3:3ZnSe/CdSe, x=36.5 and 56.5 mol%) are prepared using the melt-quenching method. Transmission electron microscopy characterization was done to confirm both nucleation and growth of the NPs for different annealing times. Fluorescence spectra of these samples are obtained with the excitation wavelengths at 403 and 477nm. Three fluorescence transitions are observed at 563 nm, 598 nm and 646 nm. The transition at 646 nm is a electric dipole (ED) transition that strongly depends on the covalency of the Sm-O bond and the asymmetry of the crystal field at the Sm3+ site. The 646 nm/598 nm fluorescence intensity ratio has been studied for different annealing times and PbO concentration for both ZnSe and CdSe samples. Longer annealing times tend to make the crystal field at the Sm3+ site more symmetric in nature for these glasses. The presence of CdSe NPs is seen to produce the greatest influence on the fluorescence intensity ratio. This is believed to be due to the larger size of the CdSe nanoparticles and its stronger influence on Sm3+ ions. The second part of this work was dedicated to the understanding of the optical band gap of samarium doped lead borate glasses with and without ZnSe/CdSe NPs. Optical absorption spectra for all these glass samples show their absorption edge in the ultraviolet region. Detailed analysis of the absorption edge was carried out using the Mott-Davis model and the optical band gap and the width of the tail in the band gap

Enhanced direct-gap light emission from Si-capped n+-Ge epitaxial layers on Si after post-growth rapid cyclic annealing: impact of non-radiative interface recombination toward Ge/Si double heterostructure lasers.

PubMed

Higashitarumizu, Naoki; Ishikawa, Yasuhiko

2017-09-04

Enhanced direct-gap light emission is reported for Si-capped n + -Ge layers on Si after post-growth rapid cyclic annealing (RCA), and impact of non-radiative recombination (NRR) at the Ge/Si interface is discussed toward Ge/Si double heterostructure (DH) lasers. P-doped n + -Ge layer (1 × 10 19 cm -3 , 400 nm) is grown on Si by ultra-high vacuum chemical vapor deposition, followed by a growth of Si capping layer (5 nm) to form a Si/Ge/Si DH structure. Post-growth RCA to eliminate defects in Ge is performed in N 2 at temperatures between 900°C and 780°C, where the annealing time is minimized to be 5 s in each RCA cycle to prevent an out-diffusion of P dopants from the Ge surface. Direct-gap photoluminescence (PL) intensity at 1.6 µm increases with the RCA cycles up to 40, although the threading dislocation density in Ge is not reduced after 3 cycles in the present condition. The PL enhancement is ascribed to the suppression of NRR at the Ge/Si interface, where an intermixed SiGe alloy is formed. For Ge/Si DH lasers, NRR at the Ge/Si interface is found to have a significant impact on the threshold current density Jth. In order to achieve Jth on the order of 1 kA/cm 2 , similar to III-V lasers, the interface recombination velocity S is required below 10 3 cm/s in spite of S as large as 10 5 cm/s at the ordinary defect-rich Ge/Si interface.

Zinc oxide wide band gap semiconductor for optoelectronic devices

NASA Astrophysics Data System (ADS)

Choopun, Supab

The main objective of this dissertation is to study the key aspects of ZnO-based materials for fabrication of wide band gap optoelectronic devices. ZnO has received attention due to its direct band gap, alloying and doping capabilities. It has similar properties to that of GaN, a material system that has become very important for the fabrication of blue light emitting diodes, laser diodes, detectors, etc. In this study, ZnO and related materials were grown in thin film form on c-plane sapphire substrates by pulsed laser deposition and then, these films were mainly studied in terms of their structural, optical and electrical properties. The studied key aspects include growth and optimization of device quality ZnO films, band gap tailoring of ZnO films by alloying, fabrication of quantum well structures, and impurity doping for n-type and p-type ZnO films. The growth and optimization of ZnO films have been studied as a function of substrate temperature and oxygen background pressure. By tuning the growth temperature and oxygen pressure during the initial and final stages of growth, it was possible to control desirable surface, interface chemistry structure, crystalline quality, and optoelectronic properties of the films while maintaining high quality epitaxy. Band gap tailoring has been studied by alloying of ZnO with MgO. MgZnO alloy films exhibit two phases, hexagonal and cubic, depending on the Mg concentration in the MgZnO lattice. The band gap energy of MgZnO alloys can be varied in a wide range from 3.3 eV to 4.0 eV for hexagonal structured films and 4.0 to 7.6 eV for cubic structured films. Studies both n-type as well as p-type doping and activation in ZnO films are reported. It was found that In-doped ZnO films with high optical transparency and high electrical conductivity can be grown at temperature below 300°C. P-type ZnO films have been studied by using a cationic-codoping method. Weak p-type conductivity in ZnO films was obtained from Cu and Al codoping

Visible light photocatalytic antibacterial activity of Ni-doped and N-doped TiO2 on Staphylococcus aureus and Escherichia coli bacteria.

PubMed

Ananpattarachai, Jirapat; Boonto, Yuphada; Kajitvichyanukul, Puangrat

2016-03-01

The Ni-doped and N-doped TiO2 nanoparticles were investigated for their antibacterial activities on Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) bacteria. Their morphological features and characteristics such as particle size, surface area, and visible light absorbing capacity were compared and discussed. Scanning electron microscopy, X-ray diffraction, and UV-visible spectrophotometry were used to characterize both materials. The inactivation of E. coli (as an example of Gram-negative bacteria) and S. aureus (as an example of Gram-positive bacteria) with Ni-doped and N-doped TiO2 was investigated in the absence and presence of visible light. Antibacterial activity tests were conducted using undoped, Ni-doped, and N-doped TiO2. The N-doped TiO2 nanoparticles show higher antibacterial activity than Ni-doped TiO2. The band gap narrowing of N-doped TiO2 can induce more visible light absorption and leads to the superb antibacterial properties of this material. The complete inactivation time for E. coli at an initial cell concentration of 2.7 × 10(4) CFU/mL was 420 min which is longer than the 360 min required for S. aureus inactivation. The rate of inactivation of S. aureus using the doped TiO2 nanoparticles in the presence of visible light is greater than that of E. coli. The median lethal dose (LD50) values of S. aureus and E. coli by antibacterial activity under an 18-W visible light intensity were 80 and 350 mg/ml for N-doped TiO2, respectively.

Doping Induced Structural Stability and Electronic Properties of GaN Nanotubes

PubMed Central

Khan, Mohammad Irfan; Tyagi, Neha; Swaroop Khare, Purnima

2014-01-01

The present paper discusses the effect of manganese doping on the structural stability and electronic band gap of chiral (2, 1), armchair (3, 3), and zigzag ((6, 0) and (10, 0)) single walled GaN nanotube by using density functional theory based Atomistix Toolkit (ATK) Virtual NanoLab (VNL). The structural stability has been analyzed in terms of minimum ground state total energy, binding, and formation energy. As an effect of Mn doping (1–4 atoms), all the GaN nanotubes taken into consideration show semiconducting to metallic transition first and after certain level of Mn doping changes its trend. PMID:24707225

Structural and optical characteristics of GaAs films grown on Si/Ge substrates

NASA Astrophysics Data System (ADS)

Rykov, A. V.; Dorokhin, M. V.; Vergeles, P. S.; Baidus, N. V.; Kovalskiy, V. A.; Yakimov, E. B.; Soltanovich, O. A.

2018-03-01

A GaAs/AlAs heterostructure and a GaAs film grown on Si/Ge substrates have been fabricated and studied. A Ge buffer on a silicon substrate was fabricated using the MBE process. A3B5 films were grown by MOCVD at low pressures. Photoluminescence spectroscopy was used to define the optical quality of A3B5 films. Structural properties were investigated using the electron beam induced current method. It was established that despite a rather high density of dislocations on the epitaxial layers, the detected photoluminescence radiation of layers indicates the acceptable crystalline quality of the top GaAs layer.

Engineering Topological Surface State of Cr-doped Bi2Se3 under external electric field

NASA Astrophysics Data System (ADS)

Zhang, Jian-Min; Lian, Ruqian; Yang, Yanmin; Xu, Guigui; Zhong, Kehua; Huang, Zhigao

2017-03-01

External electric field control of topological surface states (SSs) is significant for the next generation of condensed matter research and topological quantum devices. Here, we present a first-principles study of the SSs in the magnetic topological insulator (MTI) Cr-doped Bi2Se3 under external electric field. The charge transfer, electric potential, band structure and magnetism of the pure and Cr doped Bi2Se3 film have been investigated. It is found that the competition between charge transfer and spin-orbit coupling (SOC) will lead to an electrically tunable band gap in Bi2Se3 film under external electric field. As Cr atom doped, the charge transfer of Bi2Se3 film under external electric field obviously decreases. Remarkably, the band gap of Cr doped Bi2Se3 film can be greatly engineered by the external electric field due to its special band structure. Furthermore, magnetic coupling of Cr-doped Bi2Se3 could be even mediated via the control of electric field. It is demonstrated that external electric field plays an important role on the electronic and magnetic properties of Cr-doped Bi2Se3 film. Our results may promote the development of electronic and spintronic applications of magnetic topological insulator.

Ultrasound assisted synthesis of iron doped TiO2 catalyst.

PubMed

Ambati, Rohini; Gogate, Parag R

2018-01-01

The present work deals with synthesis of Fe (III) doped TiO 2 catalyst using the ultrasound assisted approach and conventional sol-gel approach with an objective of establishing the process intensification benefits. Effect of operating parameters such as Fe doping, type of solvent, solvent to precursor ratio and initial temperature has been investigated to get the best catalyst with minimum particle size. Comparison of the catalysts obtained using the conventional and ultrasound assisted approach under the optimized conditions has been performed using the characterization techniques like DLS, XRD, BET, SEM, EDS, TEM, FTIR and UV-Vis band gap analysis. It was established that catalyst synthesized by ultrasound assisted approach under optimized conditions of 0.4mol% doping, irradiation time of 60min, propan-2-ol as the solvent with the solvent to precursor ratio as 10 and initial temperature of 30°C was the best one with minimum particle size as 99nm and surface area as 49.41m 2 /g. SEM analysis, XRD analysis as well as the TEM analysis also confirmed the superiority of the catalyst obtained using ultrasound assisted approach as compared to the conventional approach. EDS analysis also confirmed the presence of 4.05mol% of Fe element in the sample of 0.4mol% iron doped TiO 2 . UV-Vis band gap results showed the reduction in band gap from 3.2eV to 2.9eV. Photocatalytic experiments performed to check the activity also confirmed that ultrasonically synthesized Fe doped TiO 2 catalyst resulted in a higher degradation of Acid Blue 80 as 38% while the conventionally synthesized catalyst resulted in a degradation of 31.1%. Overall, the work has clearly established importance of ultrasound in giving better catalyst characteristics as well as activity for degradation of the Acid Blue 80 dye. Copyright © 2017 Elsevier B.V. All rights reserved.

The Effects of Graphene Stacking on the Performance of Methane Sensor: A First-Principles Study on the Adsorption, Band Gap and Doping of Graphene

PubMed Central

Yang, Daoguo; Zhang, Guoqi; Chen, Liangbiao; Liu, Dongjing; Cai, Miao; Fan, Xuejun

2018-01-01

The effects of graphene stacking are investigated by comparing the results of methane adsorption energy, electronic performance, and the doping feasibility of five dopants (i.e., B, N, Al, Si, and P) via first-principles theory. Both zigzag and armchair graphenes are considered. It is found that the zigzag graphene with Bernal stacking has the largest adsorption energy on methane, while the armchair graphene with Order stacking is opposite. In addition, both the Order and Bernal stacked graphenes possess a positive linear relationship between adsorption energy and layer number. Furthermore, they always have larger adsorption energy in zigzag graphene. For electronic properties, the results show that the stacking effects on band gap are significant, but it does not cause big changes to band structure and density of states. In the comparison of distance, the average interlamellar spacing of the Order stacked graphene is the largest. Moreover, the adsorption effect is the result of the interactions between graphene and methane combined with the change of graphene’s structure. Lastly, the armchair graphene with Order stacking possesses the lowest formation energy in these five dopants. It could be the best choice for doping to improve the methane adsorption. PMID:29389860

The Effects of Graphene Stacking on the Performance of Methane Sensor: A First-Principles Study on the Adsorption, Band Gap and Doping of Graphene.

PubMed

Yang, Ning; Yang, Daoguo; Zhang, Guoqi; Chen, Liangbiao; Liu, Dongjing; Cai, Miao; Fan, Xuejun

2018-02-01

The effects of graphene stacking are investigated by comparing the results of methane adsorption energy, electronic performance, and the doping feasibility of five dopants (i.e., B, N, Al, Si, and P) via first-principles theory. Both zigzag and armchair graphenes are considered. It is found that the zigzag graphene with Bernal stacking has the largest adsorption energy on methane, while the armchair graphene with Order stacking is opposite. In addition, both the Order and Bernal stacked graphenes possess a positive linear relationship between adsorption energy and layer number. Furthermore, they always have larger adsorption energy in zigzag graphene. For electronic properties, the results show that the stacking effects on band gap are significant, but it does not cause big changes to band structure and density of states. In the comparison of distance, the average interlamellar spacing of the Order stacked graphene is the largest. Moreover, the adsorption effect is the result of the interactions between graphene and methane combined with the change of graphene's structure. Lastly, the armchair graphene with Order stacking possesses the lowest formation energy in these five dopants. It could be the best choice for doping to improve the methane adsorption.

Effect of doping (C or N) and co-doping (C+N) on the photoactive properties of magnetron sputtered titania coatings for the application of solar water-splitting.

PubMed

Rahman, M; Dang, B H Q; McDonnell, K; MacElroy, J M D; Dowling, D P

2012-06-01

The photocatalytic splitting of water into hydrogen and oxygen using a photoelectrochemical (PEC) cell containing titanium dioxide (TiO2) photoanode is a potentially renewable source of chemical fuels. However, the size of the band gap (-3.2 eV) of the TiO2 photocatalyst leads to its relatively low photoactivity toward visible light in a PEC cell. The development of materials with smaller band gaps of approximately 2.4 eV is therefore necessary to operate PEC cells efficiently. This study investigates the effect of dopant (C or N) and co-dopant (C+N) on the physical, structural and photoactivity of TiO2 nano thick coating. TiO2 nano-thick coatings were deposited using a closed field DC reactive magnetron sputtering technique, from titanium target in argon plasma with trace addition of oxygen. In order to study the influence of doping such as C, N and C+N inclusions in the TiO2 coatings, trace levels of CO2 or N2 or CO2+N2 gas were introduced into the deposition chamber respectively. The properties of the deposited nano-coatings were determined using Spectroscopic Ellipsometry, SEM, AFM, Optical profilometry, XPS, Raman, X-ray diffraction UV-Vis spectroscopy and tri-electrode potentiostat measurements. Coating growth rate, structure, surface morphology and roughness were found to be significantly influenced by the types and amount of doping. Substitutional type of doping in all doped sample were confirmed by XPS. UV-vis measurement confirmed that doping (especially for C doped sample) facilitate photoactivity of sputtered deposited titania coating toward visible light by reducing bandgap. The photocurrent density (indirect indication of water splitting performance) of the C-doped photoanode was approximately 26% higher in comparison with un-doped photoanode. However, coating doped with nitrogen (N or N+C) does not exhibit good performance in the photoelectrochemical cell due to their higher charge recombination properties.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kao, Kuo-Hsing; Meyer, Kristin De; Department of Electrical Engineering, KU Leuven, Leuven

Band-to-band tunneling parameters of strained indirect bandgap materials are not well-known, hampering the reliability of performance predictions of tunneling devices based on these materials. The nonlocal band-to-band tunneling model for compressively strained SiGe is calibrated based on a comparison of strained SiGe p-i-n tunneling diode measurements and doping-profile-based diode simulations. Dopant and Ge profiles of the diodes are determined by secondary ion mass spectrometry and capacitance-voltage measurements. Theoretical parameters of the band-to-band tunneling model are calculated based on strain-dependent properties such as bandgap, phonon energy, deformation-potential-based electron-phonon coupling, and hole effective masses of strained SiGe. The latter is determined withmore » a 6-band k·p model. The calibration indicates an underestimation of the theoretical electron-phonon coupling with nearly an order of magnitude. Prospects of compressively strained SiGe tunneling transistors are made by simulations with the calibrated model.« less

Tailoring the electronic structure of β-Ga2O3 by non-metal doping from hybrid density functional theory calculations.

PubMed

Guo, Weiyan; Guo, Yating; Dong, Hao; Zhou, Xin

2015-02-28

A systematic study using density functional theory has been performed for β-Ga2O3 doped with non-metal elements X (X = C, N, F, Si, P, S, Cl, Se, Br, and I) to evaluate the effect of doping on the band edges and photocatalytic activity of β-Ga2O3. The utilization of a more reliable hybrid density functional, as prescribed by Heyd, Scuseria and Ernzerhof, is found to be effective in predicting the band gap of β-Ga2O3 (4.5 eV), in agreement with the experimental result (4.59 eV). Based on the relaxed structures of X-doped systems, the defect formation energies and the plots of density of states have been calculated to analyze the band edges, the band gap states and the preferred doping sites. Our results show that the doping is energetically favored under Ga-rich growth conditions with respect to O-rich growth conditions. It is easier to replace the threefold coordinated O atom with non-metal elements compared to the fourfold coordinated O atom. X-doped systems (X = C, Si, P) show no change in the band gap, with the presence of discrete midgap states, which have adverse effect on the photocatalytic properties. The photocatalytic redox ability can be improved to a certain extent by doping with N, S, Cl, Se, Br, and I. The band alignments for Se-doped and I-doped β-Ga2O3 are well positioned for the feasibility of both photo-oxidation and photo-reduction of water, which are promising photocatalysts for water splitting in the visible region.

Recent Advances in Doping of Molybdenum Disulfide: Industrial Applications and Future Prospects.

PubMed

Pham, Viet Phuong; Yeom, Geun Young

2016-11-01

Owing to their excellent physical properties, atomically thin layers of molybdenum disulfide (MoS 2 ) have recently attracted much attention due to their nonzero-gap property, exceptionally high electrical conductivity, good thermal stability, and excellent mechanical strength, etc. MoS 2 -based devices exhibit great potential for applications in optoelectronics and energy harvesting. Here, a comprehensive review of various doping strategies is presented, including wet doping and dry doping of atomically crystalline MoS 2 thin layers, and the progress made so far for their doping-based prospective applications is also discussed. Finally, several significant research issues for the prospects of doped-MoS 2 in industry, as a guide for 2D material community, are also provided. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Structural and optical properties of Mg doped ZnS quantum dots and biological applications

NASA Astrophysics Data System (ADS)

Ashokkumar, M.; Boopathyraja, A.

2018-01-01

Zn1-xMgxS (x = 0, 0.2 and 0.4) quantum dots (QDs) were prepared by co-precipitation method. The Mg dopant did not modify the cubic blende structure of ZnS QDs. The Mg related secondary phase was not detected even for 40% of Mg doping. The size mismatch between host Zn ion and dopant Mg ion created distortion around the dopant. The creation of distortion centres produced small changes in the lattice parameters and diffraction peak position. All the QDs showed small sulfur deficiency and the deficiency level were increased by Mg doping. Band gap of the QD was decreased due to the dominated quantum confinement effect over compositional effect at initial doping of Mg. But at higher doping the band gap was increased due to compositional effect, since there was no change in average crystallite size. The prepared QDs had three emission bands in the UV and Visible regions corresponding to near band edge emission and defect related emissions. The electron transport reaction chain which forms free radicals was broken by sulfur vacancy trap sites. Therefore, the ZnS QDs had better antioxidant activity and the antioxidant behaviour was enhanced by Mg doping. The enhanced UV absorption and emission of 20% of Mg doped ZnS QDs let to maximize the zone of inhibition against E. Coli bacterial strain.

Spin polarized first principles study of Mn doped gallium nitride monolayer nanosheet

NASA Astrophysics Data System (ADS)

Sharma, Venus; Kaur, Sumandeep; Srivastava, Sunita; Kumar, Tankeshwar

2017-05-01

The structural, electronic and magnetic properties of gallium nitride nanosheet (GaNs) doped with Mn atoms have been studied using spin polarized density functional theory. The binding energy per atom, Energy Band gap, Fermi energy, magnetic moment, electric dipole moment have been found. The doped nanosheet is found to be more stable than pure GaN monolayer nanosheet. Adsorption of Mn atom has been done at four different sites on GaNs which affects the fermi level position. It is found that depending on the doping site, Mn can behave both like p-type semiconductor and also as n-type semiconductor. Also, it is ascertained that Mn doped GaNs (GaNs-Mn) exhibits ferromagnetic behavior.

Investigation of the structural, optical and electrical transport properties of n-doped CdSe thin films

NASA Astrophysics Data System (ADS)

Ali, H. M.; Abd El-Ghanny, H. A.

2008-04-01

Thin films of (CdSe)90(In2O3)10, (CdSe)90(SnO2)10 and (CdSe)90(ZnO)10 have been grown on glass substrates by the electron beam evaporation technique. It has been found that undoped and Sn or In doped CdSe films have two direct transitions corresponding to the energy gaps Eg and Eg+Δ due to spin-orbit splitting of the valence band. The electrical resistivity for n-doped CdSe thin films as a function of light exposure time has been studied. The influence of doping on the structural, optical and electrical characteristics of In doped CdSe films has been investigated in detail. The lattice parameters, grain size and dislocation were determined from x-ray diffraction patterns. The optical transmittance and band gap of these films were determined using a double beam spectrophotometer. The DC conductivity of the films was measured in vacuum using a two-probe technique.

Investigation of electronic structures and optical properties of β -Si3N4 doped with IV A elements: A first-principles simulation

NASA Astrophysics Data System (ADS)

Lu, Xuefeng; Gao, Xu; Ren, Junqiang; Li, Cuixia; Guo, Xin; Wei, Yupeng; La, Peiqing

2018-04-01

Based on first-principles simulations with the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) functional, we studied the electronic structures and optical properties of hexagonal silicon nitride (β-Si3N4) doped with IV A elements, C, Ge, Sn and Pb. It was found that the Ge-doped system is characterized by a more stable structure with a lower formation energy of 2.584 eV compared with those of the C-, Sn- and Pb-doped systems of 3.877 eV, 5.249 eV and 7.672 eV, respectively. The band gap (EG) of the Pb-doped system was the lowest at 1.6 eV, displaying semiconducting characteristics. Additionally, there was a transition from a direct band gap to an indirect band gap in the C-doped system. Charge difference density analysis showed that the covalent property of the C-N bonds was enhanced by expansion of the electron-free region and the larger Mulliken population values of 0.71 and 0.86. Furthermore, lower absorption and reflectivity peaks at 11.30 eV were observed for the C-doped system, demonstrating its broader potential for application in photoelectric and microelectronic devices.

Doping with Graphitic Nitrogen Triggers Ferromagnetism in Graphene

PubMed Central

2017-01-01

Nitrogen doping opens possibilities for tailoring the electronic properties and band gap of graphene toward its applications, e.g., in spintronics and optoelectronics. One major obstacle is development of magnetically active N-doped graphene with spin-polarized conductive behavior. However, the effect of nitrogen on the magnetic properties of graphene has so far only been addressed theoretically, and triggering of magnetism through N-doping has not yet been proved experimentally, except for systems containing a high amount of oxygen and thus decreased conductivity. Here, we report the first example of ferromagnetic graphene achieved by controlled doping with graphitic, pyridinic, and chemisorbed nitrogen. The magnetic properties were found to depend strongly on both the nitrogen concentration and type of structural N-motifs generated in the host lattice. Graphenes doped below 5 at. % of nitrogen were nonmagnetic; however, once doped at 5.1 at. % of nitrogen, N-doped graphene exhibited transition to a ferromagnetic state at ∼69 K and displayed a saturation magnetization reaching 1.09 emu/g. Theoretical calculations were used to elucidate the effects of individual chemical forms of nitrogen on magnetic properties. Results showed that magnetic effects were triggered by graphitic nitrogen, whereas pyridinic and chemisorbed nitrogen contributed much less to the overall ferromagnetic ground state. Calculations further proved the existence of exchange coupling among the paramagnetic centers mediated by the conduction electrons. PMID:28110530

The effect of n- and p-type doping on coherent phonons in GaN.

PubMed

Ishioka, Kunie; Kato, Keiko; Ohashi, Naoki; Haneda, Hajime; Kitajima, Masahiro; Petek, Hrvoje

2013-05-22

The effect of doping on the carrier-phonon interaction in wurtzite GaN is investigated by pump-probe reflectivity measurements using 3.1 eV light in near resonance with the fundamental band gap of 3.39 eV. Coherent modulations of the reflectivity due to the E2 and A1(LO) modes, as well as the 2A1(LO) overtone are observed. Doping of acceptor and donor atoms enhances the dephasing of the polar A1(LO) phonon via coupling with plasmons, with the effect of donors being stronger. Doping also enhances the relative amplitude of the coherent A1(LO) phonon with respect to that of the high-frequency E2 phonon, though it does not affect the relative intensity in Raman spectroscopic measurements. We attribute this enhanced coherent amplitude to the transient depletion field screening (TDFS) excitation mechanism, which, in addition to impulsive stimulated Raman scattering (ISRS), contributes to the generation of coherent polar phonons even for sub-band gap excitation. Because the TDFS mechanism requires photoexcitation of carriers, we argue that the interband transition is made possible at a surface with photon energies below the bulk band gap through the Franz-Keldysh effect.

MSM-Metal Semiconductor Metal Photo-detector Using Black Silicon Germanium (SiGe) for Extended Wavelength Near Infrared Detection

DTIC Science & Technology

2012-09-01

MSM) photodectors fabricated using black silicon-germanium on silicon substrate (Si1–xGex//Si) for I-V, optical response, external quantum ...material for Si for many applications in low-power and high-speed semiconductor device technologies (4, 5). It is a promising material for quantum well ...MSM-Metal Semiconductor Metal Photo-detector Using Black Silicon Germanium (SiGe) for Extended Wavelength Near Infrared Detection by Fred

Ab initio study of phonon dispersion and thermodynamic properties of pure and doped pyrites

NASA Astrophysics Data System (ADS)

Musari, Abolore A.; Joubert, Daniel P.; Olowofela, Joseph A.; Akinwale, Adio T.; Adebayo, Gboyega A.

2017-12-01

Pyrites (FeS2) are solid minerals that are found abundantly in Nigeria and are easy to prepare in laboratories. In this work, FeS2 is studied extensively in its pure state as well as when iron is substitutionally doped with zinc and calcium at concentrations of 0, 0.25, 0.5, 0.75 and 1. Using density functional theory, the eectronic, dynamic and thermodynamic properties were calculated. The results revealed that the lattice parameters and bulk modulus increases with increasing concentration and the obtained values are in agreement with available experimental and theoretical values. Though pyrite, when doped with zinc, obeys Vegard's law, doping with calcium revealed pronounced deviation from this law. The calculated band structures showed that FeS2 has an indirect band gap whose size decreases after introducing zinc while doping with calcium increases the band gap. The phonon dispersion of the end members FeS2 and ZnS2 indicate that the systems are dynamically stable while CaS2 is dynamically unstate. Also, the thermodynamic properties of the pure and doped pyrites were calculated and the ranges of temperature at which the lattice and electronic degrees of freedom contribute to the specific heat capacity are presented.

Hydrothermal preparation of copper doped NaTaO3 nanoparticles and study on the photocatalytic mechanism.

PubMed

Liu, Yulu; Su, Yiguo; Han, Hui; Wang, Xiaojing

2013-02-01

Effects of copper cations doping into wide band gap semiconductor photocatalysts of tantalate on morphology, visible light response, and photocatalytic performance were studied. A series of Cu-doped NaTaO3 catalysts were prepared by hydrothermal method. XRD and XPS results suggested that copper were successfully doped into the NaTaO3 nanocrystal in the Cu2+ state. TEM studies showed the formation of the cube shape nanoparticles of NaTaO3 as well as Cu-doped NaTaO3. UV-Vis diffuse reflectance spectra clearly indicated the red-shift in the series of copper doped NaTaO3 catalysts, resulting in a decrease in the band gap of NaTaO3. The trend of red shift was increased with an increase of copper doping concentration, whereas the photo-degradation methylene blue (MB) is not improved by the doping of copper ions. The simulation of energy band structure by density functional theory unfolded that the substitution of Ta5+ ions by Cu2+ ions results in forming an intermediate band (IB) upper the top of the valence band (VB), which is mainly attributed to the state of Cu 3d. The intermediate band is responsible for the red-shift caused by the doping of Cu ions. Meanwhile Cu species can become the recombination centers of photoinduced electrons and holes. Thus, the quickly recombination of e(-)h(+) pairs is one of the most significant factors which deteriorate the photoactivity of Cu-doped NaTaO3.

Nonradiative recombination centers in GaAs:N δ-doped superlattice revealed by two-wavelength-excited photoluminescence

NASA Astrophysics Data System (ADS)

Dulal Haque, Md.; Kamata, Norihiko; Fukuda, Takeshi; Honda, Zentaro; Yagi, Shuhei; Yaguchi, Hiroyuki; Okada, Yoshitaka

2018-04-01

We use two-wavelength-excited photoluminescence (PL) to investigate nonradiative recombination (NRR) centers in GaAs:N δ-doped superlattice (SL) structures grown by molecular beam epitaxy. The change in photoluminescence (PL) intensity due to the superposition of below-gap excitation at energies of 0.75, 0.80, 0.92, and 0.95 eV and above-gap excitation at energies of 1.69 or 1.45 eV into the GaAs conduction band and the E- band implies the presence of NRR centers inside the GaAs:N δ-doped SL and/or GaAs layers. The change in PL intensity as a function of the photon number density of below-gap excitation is examined for both bands, which enables us to determine the distribution of NRR centers inside the GaAs:N δ-doped SL and GaAs layers. We propose recombination models to explain the experimental results. Defect-related parameters that give a qualitative insight into the samples are investigated systematically by fitting the rate equations to the experimental data.

Universal heat conduction in Ce 1-xYb xCoIn 5: Evidence for robust nodal d-wave superconducting gap

DOE PAGES

Xu, Y.; Petrovic, C.; Dong, J. K.; ...

2016-02-01

In the heavy-fermion superconductor Ce 1-xYb xCoIn 5, Yb doping was reported to cause a possible change from nodal d-wave superconductivity to a fully gapped d-wave molecular superfluid of composite pairs near x ≈ 0.07 (nominal value x nom = 0.2). Here we present systematic thermal conductivity measurements on Ce 1-xYb xCoIn 5 (x = 0.013, 0.084, and 0.163) single crystals. The observed finite residual linear term κ 0/T is insensitive to Yb doping, verifying the universal heat conduction of the nodal d-wave superconducting gap in Ce 1-xYb xCoIn 5. Similar universal heat conduction is also observed in the CeCo(Inmore » 1–yCd y) 5 system. Furthermore, these results reveal a robust nodal d-wave gap in CeCoIn 5 upon Yb or Cd doping.« less

Stabilization and enhanced energy gap by Mg doping in ɛ-phase Ga2O3 thin films

NASA Astrophysics Data System (ADS)

Bi, Xiaoyu; Wu, Zhenping; Huang, Yuanqi; Tang, Weihua

2018-02-01

Mg-doped Ga2O3 thin films with different doping concentrations were deposited on sapphire substrates using laser molecular beam epitaxy (L-MBE) technique. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and ultraviolet-visible (UV-vis) absorption spectrum were used to characterize the crystal structure and optical properties of the as-grown films. Compared to pure Ga2O3 thin film, the Mg-doped thin films have transformed from the most stable β-phase into ɛ-phase. The absorption edge shifted to about 205 nm and the optical bandgap increased to ˜ 6 eV. These properties reveal that Mg-doped Ga2O3 films may have potential applications in the field of deep ultraviolet optoelectronic devices, such as deep ultraviolet photodetectors, short wavelength light emitting devices and so on.

Cu doping concentration effect on the physical properties of CdS thin films obtained by the CBD technique

NASA Astrophysics Data System (ADS)

Albor Aguilera, M. L.; Flores Márquez, J. M.; Remolina Millan, A.; Matsumoto Kuwabara, Y.; González Trujillo, M. A.; Hernández Vásquez, C.; Aguilar Hernandez, J. R.; Hernández Pérez, M. A.; Courel-Piedrahita, M.; Madeira, H. T. Yee

2017-08-01

Cu(In, Ga)Se2 (CIGS) and Cu2ZnSnS4 (CZTS) semiconductors are direct band gap materials; when these types of material are used in solar cells, they provide efficiencies of 22.1% and 12.6%, respectively. Most traditional fabrication methods involve expensive vacuum processes including co-evaporation and sputtering techniques, where films and doping are conducted separately. On the other hand, the chemical bath deposition (CBD) technique allows an in situ process. Cu-doped CdS thin films working as a buffer layer on solar cells provide good performing devices and they may be deposited by low cost techniques such as chemical methods. In this work, Cu-doped CdS thin films were deposited using the CBD technique on SnO2:F (FTO) substrates. The elemental analysis and mapping reconstruction were conducted by EDXS. Morphological, optical and electrical properties were studied, and they revealed that Cu doping modified the CdS structure, band-gap value and the electrical properties. Cu-doped CdS films show high resistivity compared to the non-doped CdS. The appropriate parameters of Cu-doped CdS films were determined to obtain an adequate window or buffer layer on CIGS and CZTS photovoltaic solar cells.

Pseudogap and electronic structure of electron-doped Sr2IrO4

NASA Astrophysics Data System (ADS)

Moutenet, Alice; Georges, Antoine; Ferrero, Michel

2018-04-01

We present a theoretical investigation of the effects of correlations on the electronic structure of the Mott insulator Sr2IrO4 upon electron doping. A rapid collapse of the Mott gap upon doping is found, and the electronic structure displays a strong momentum-space differentiation at low doping level: The Fermi surface consists of pockets centered around (π /2 ,π /2 ) , while a pseudogap opens near (π ,0 ) . Its physical origin is shown to be related to short-range spin correlations. The pseudogap closes upon increasing doping, but a differentiated regime characterized by a modulation of the spectral intensity along the Fermi surface persists to higher doping levels. These results, obtained within the cellular dynamical mean-field-theory framework, are discussed in comparison to recent photoemission experiments and an overall good agreement is found.

Microscopic effects of Dy doping in the topological insulator Bi2Te3

NASA Astrophysics Data System (ADS)

Duffy, L. B.; Steinke, N.-J.; Krieger, J. A.; Figueroa, A. I.; Kummer, K.; Lancaster, T.; Giblin, S. R.; Pratt, F. L.; Blundell, S. J.; Prokscha, T.; Suter, A.; Langridge, S.; Strocov, V. N.; Salman, Z.; van der Laan, G.; Hesjedal, T.

2018-05-01

Magnetic doping with transition metal ions is the most widely used approach to break time-reversal symmetry in a topological insulator (TI)—a prerequisite for unlocking the TI's exotic potential. Recently, we reported the doping of Bi2Te3 thin films with rare-earth ions, which, owing to their large magnetic moments, promise commensurately large magnetic gap openings in the topological surface states. However, only when doping with Dy has a sizable gap been observed in angle-resolved photoemission spectroscopy, which persists up to room temperature. Although disorder alone could be ruled out as a cause of the topological phase transition, a fundamental understanding of the magnetic and electronic properties of Dy-doped Bi2Te3 remained elusive. Here, we present an x-ray magnetic circular dichroism, polarized neutron reflectometry, muon-spin rotation, and resonant photoemission study of the microscopic magnetic and electronic properties. We find that the films are not simply paramagnetic but that instead the observed behavior can be well explained by the assumption of slowly fluctuating, inhomogeneous, magnetic patches with increasing volume fraction as the temperature decreases. At liquid helium temperatures, a large effective magnetization can be easily introduced by the application of moderate magnetic fields, implying that this material is very suitable for proximity coupling to an underlying ferromagnetic insulator or in a heterostructure with transition-metal-doped layers. However, the introduction of some charge carriers by the Dy dopants cannot be excluded at least in these highly doped samples. Nevertheless, we find that the magnetic order is not mediated via the conduction channel in these samples and therefore magnetic order and carrier concentration are expected to be independently controllable. This is not generally the case for transition-metal-doped topological insulators, and Dy doping should thus allow for improved TI quantum devices.

Spectroscopic and fiber optic ethanol sensing properties Gd doped ZnO nanoparticles.

PubMed

Noel, J L; Udayabhaskar, R; Renganathan, B; Muthu Mariappan, S; Sastikumar, D; Karthikeyan, B

2014-11-11

We report the structural, optical and gas sensing properties of prepared pure and Gd doped ZnO nanoparticles through solgel method at moderate temperature. Structural studies are carried out by X-ray diffraction method confirms hexagonal wurtzite structure and doping induced changes in lattice parameters is observed. Optical absorption spectral studies shows red shift in the absorption peak corresponds to band-gap from 3.42 eV to 3.05 eV and broad absorption in the visible range after Gd doping is observed. Scanning electron microscopic studies shows increase in particle size where the particle diameters increase from few nm to micrometers after Gd doping. The clad modified ethanol fiber-optic sensor studies for ethanol sensing exhibits best sensitivity for the 3% Gd doped ZnO nanoparticles and the sensitivity get lowered incase of higher percentage of Gd doped ZnO sample. Copyright © 2014 Elsevier B.V. All rights reserved.

Structural and Visible-Near Infrared Optical Properties of Cr-Doped TiO2 for Colored Cool Pigments

NASA Astrophysics Data System (ADS)

Yuan, Le; Weng, Xiaolong; Zhou, Ming; Zhang, Qingyong; Deng, Longjiang

2017-11-01

Chromium-doped TiO2 pigments were synthesized via a solid-state reaction method and studied with X-ray diffraction, SEM, XPS, and UV-VIS-NIR reflectance spectroscopy. The incorporation of Cr3+ accelerates the transition from the anatase phase to the rutile phase and compresses the crystal lattice. Moreover, the particle morphology, energy gap, and reflectance spectrum of Cr-doped TiO2 pigments is affected by the crystal structure and doping concentration. For the rutile samples, some of the Cr3+ ions are oxidized to Cr4+ after sintering at a high temperature, which leads to a strong near-infrared absorption band due to the 3A2 → 3 T1 electric dipole-allowed transitions of Cr4+. And the decrease of the band gap causes an obvious redshift of the optical absorption edges as the doping concentration increases. Thus, the VIS and near-infrared average reflectance of the rutile Ti1 - x Cr x O2 sample decrease by 60.2 and 58%, respectively, when the Cr content increases to x = 0.0375. Meanwhile, the color changes to black brown. However, for the anatase Ti1 - x Cr x O2 pigments, only the VIS reflection spectrum is inhibited by forming some characteristic visible light absorption peaks of Cr3+. The morphology, band gap, and NIR reflectance are not significantly affected. Finally, a Cr-doped anatase TiO2 pigment with a brownish-yellow color and 90% near-infrared reflectance can be obtained.

Effects of shape, size, and pyrene doping on electronic properties of graphene nanoflakes.

PubMed

Kuamit, Thanawit; Ratanasak, Manussada; Rungnim, Chompoonut; Parasuk, Vudhichai

2017-11-25

Effects of size, shape, and pyrene doping on electronic properties of graphene nanoflakes (GNFs) were theoretically investigated using density functional theory method with PBE, B3PW91, and M06-2X functionals and cc-pVDZ basis set. Two shapes of zigzag GNFs, hexagonal (HGN) and rhomboidal (RGN), were considered. The energy band gap of GNF depends on shape and decreases with size. The HGN has larger band gap energy (1.23-3.96 eV) than the RGN (0.13-2.12 eV). The doping of pyrene and pyrene derivatives on both HGN and RGN was also studied. The adsorption energy of pyrene and pyrene derivatives on GNF does not depend on the shape of GNFs with energies between 21 and 27 kcal mol -1 . The substituent on pyrene enhances the binding to GNF but the strength does not depend on electron withdrawing or donating capability. The doping by pyrene and pyrene derivatives also shifts the HOMO and LUMO energies of GNFs. Both positive (destabilizing) and negative (stabilizing) shifts on HOMO and LUMO of GNFs were seen. The direction and magnitude of the shift do not follow the electron withdrawing and donating capability of pyrene substituents. However, only a slight shift was observed for doped RGN. A shift of 0.19 eV was noticed for HOMO of HGN doped with 1-aminopyrene (pyNH 2 ) and of 0.04 eV for LUMO of HGN doped with 1-pyrenecarboxylic acid (pyCOOH). Graphical Abstract HOMO and LUMO Energies of pyrene/pyrene derivatives doped Graphene Nanoflakes.

Mixed-signal 0.18μm CMOS and SiGe BiCMOS foundry technologies for ROIC applications

NASA Astrophysics Data System (ADS)

Kar-Roy, Arjun; Howard, David; Racanelli, Marco; Scott, Mike; Hurwitz, Paul; Zwingman, Robert; Chaudhry, Samir; Jordan, Scott

2010-10-01

Today's readout integrated-circuits (ROICs) require a high level of integration of high performance analog and low power digital logic. TowerJazz offers a commercial 0.18μm CMOS technology platform for mixed-signal, RF, and high performance analog applications which can be used for ROIC applications. The commercial CA18HD dual gate oxide 1.8V/3.3V and CA18HA dual gate oxide 1.8V/5V RF/mixed signal processes, consisting of six layers of metallization, have high density stacked linear MIM capacitors, high-value resistors, triple-well isolation and thick top aluminum metal. The CA18HA process also has scalable drain extended LDMOS devices, up to 40V Vds, for high-voltage sensor applications, and high-performance bipolars for low noise requirements in ROICs. Also discussed are the available features of the commercial SBC18 SiGe BiCMOS platform with SiGe NPNs operating up to 200/200GHz (fT/fMAX frequencies in manufacturing and demonstrated to 270 GHz fT, for reduced noise and integrated RF capabilities which could be used in ROICs. Implementation of these technologies in a thick film SOI process for integrated RF switch and power management and the availability of high fT vertical PNPs to enable complementary BiCMOS (CBiCMOS), for RF enabled ROICs, are also described in this paper.

A Physics-Based Engineering Methodology for Calculating Soft Error Rates of Bulk CMOS and SiGe Heterojunction Bipolar Transistor Integrated Circuits

NASA Astrophysics Data System (ADS)

Fulkerson, David E.

2010-02-01

This paper describes a new methodology for characterizing the electrical behavior and soft error rate (SER) of CMOS and SiGe HBT integrated circuits that are struck by ions. A typical engineering design problem is to calculate the SER of a critical path that commonly includes several circuits such as an input buffer, several logic gates, logic storage, clock tree circuitry, and an output buffer. Using multiple 3D TCAD simulations to solve this problem is too costly and time-consuming for general engineering use. The new and simple methodology handles the problem with ease by simple SPICE simulations. The methodology accurately predicts the measured threshold linear energy transfer (LET) of a bulk CMOS SRAM. It solves for circuit currents and voltage spikes that are close to those predicted by expensive 3D TCAD simulations. It accurately predicts the measured event cross-section vs. LET curve of an experimental SiGe HBT flip-flop. The experimental cross section vs. frequency behavior and other subtle effects are also accurately predicted.

Computational Studies of Magnetically Doped Semiconductor Nanoclusters

NASA Astrophysics Data System (ADS)

Gutsev, Lavrenty Gennady

Spin-polarized unrestricted density functional theory is used to calculate the molecular properties of magnetic semiconductor quantum dots doped with 3d-metal atoms. We calculate total energies of the low spin antiferromagnetically coupled states using a spin-flipping algorithm leading to the broken-symmetry states. Given the novel nature of the materials studied, we simulate experimental observables such as hyperfine couplings, ionization/ energies, electron affinities, first and second order polarizabilities, band gaps and exchange coupling constants. Specifically, we begin our investigation with pure clusters of (CdSe )16 and demonstrate the dependence of molecular observables on geometrical structures. We also show that the many isomers of this cluster are energetically quite closely spaced, and thus it would be necessary to employ a battery of tests to experimentally distinguish them. Next, we discuss Mn-doping into the cage (CdSe)9 cluster as well as the zinc-blende stacking type cluster (CdSe)36. We show that the local exchange coupling mechanism is ligand-mediated superexchange and simulate the isotropic hyperfine constants. Finally, we discuss a novel study where (CdSe)9 is doped with Mn or Fe up to a full replacement of all the Cd's and discuss the transition points for the magnetic behavior and specifically the greatly differing band-gap shifts. We also outline an unexpected pattern in the polarizability of the material as metals are added and compare our results with the results from theoretical studies of the bulk material.

First principles study of crystal Si-doped Ge2Sb2Te5

NASA Astrophysics Data System (ADS)

Yan, Beibei; Yang, Fei; Chen, Tian; Wang, Minglei; Chang, Hong; Ke, Daoming; Dai, Yuehua

2017-02-01

Ge2Sb2Te5 (GST) and Si-doped GST with hexagonal structure were investigated by means of First-principles calcucations. We performed many kinds of doping types and studied the electronic properties of Si-doped GST with various Si concentrations. The theoretical calculations show that the lowest formation energy appeared when Si atoms substitute the Sb atoms (SiSb). With the increasing of Si concentrations from 10% to 30%, the impurity states arise around the Fermi level and the band gap of the SiSb structure broadens. Meanwhile, the doping supercell has the most favorable structure when the doping concentration keeps in 20%. The Si-doped GST exhibits p-type metallic characteristics more distinctly owing to the Fermi level moves toward the valence band. The Te p, d-orbitals electrons have greater impact on electronic properties than that of Te s-orbitals.

Ultrasensitive molecular sensor using N-doped graphene through enhanced Raman scattering

PubMed Central

Feng, Simin; dos Santos, Maria Cristina; Carvalho, Bruno R.; Lv, Ruitao; Li, Qing; Fujisawa, Kazunori; Elías, Ana Laura; Lei, Yu; Perea-López, Nestor; Endo, Morinobu; Pan, Minghu; Pimenta, Marcos A.; Terrones, Mauricio

2016-01-01

As a novel and efficient surface analysis technique, graphene-enhanced Raman scattering (GERS) has attracted increasing research attention in recent years. In particular, chemically doped graphene exhibits improved GERS effects when compared with pristine graphene for certain dyes, and it can be used to efficiently detect trace amounts of molecules. However, the GERS mechanism remains an open question. We present a comprehensive study on the GERS effect of pristine graphene and nitrogen-doped graphene. By controlling nitrogen doping, the Fermi level (EF) of graphene shifts, and if this shift aligns with the lowest unoccupied molecular orbital (LUMO) of a molecule, charge transfer is enhanced, thus significantly amplifying the molecule’s vibrational Raman modes. We confirmed these findings using different organic fluorescent molecules: rhodamine B, crystal violet, and methylene blue. The Raman signals from these dye molecules can be detected even for concentrations as low as 10−11 M, thus providing outstanding molecular sensing capabilities. To explain our results, these nitrogen-doped graphene-molecule systems were modeled using dispersion-corrected density functional theory. Furthermore, we demonstrated that it is possible to determine the gaps between the highest occupied and the lowest unoccupied molecular orbitals (HOMO-LUMO) of different molecules when different laser excitations are used. Our simulated Raman spectra of the molecules also suggest that the measured Raman shifts come from the dyes that have an extra electron. This work demonstrates that nitrogen-doped graphene has enormous potential as a substrate when detecting low concentrations of molecules and could also allow for an effective identification of their HOMO-LUMO gaps. PMID:27532043

Effect of Mo and Ti doping concentration on the structural and optical properties of ZnS nanoparticles

NASA Astrophysics Data System (ADS)

Naz, Hina; Ali, Rai Nauman; Zhu, Xingqun; Xiang, Bin

2018-06-01

In this paper, we report the effect of single phase Mo and Ti doping concentration on the structural and optical properties of the ZnS nanoparticles. The structural and optical properties of the as-synthesized samples have been examined by x-ray diffraction, transmission electron microscopy (TEM), UV-visible near infrared absorption spectroscopy and x-ray photoelectron spectroscopy. TEM characterizations reveal a variation in the doped ZnS nanoparticle size distribution by utilizing different dopants of Mo and Ti. In absorption spectra, a clear red shift of 14 nm is observed with increasing Mo concentration as compared to pure ZnS nanoparticles, while by increasing Ti doping concentration, blue shift of 14 nm is obtained. Moreover, it demonstrates that the value of energy band gap decreases from 4.03 eV to 3.89 eV in case of Mo doping. However, the value of energy band gap have shown a remarkable increase from 4.11 eV to 4.27 eV with increasing Ti doping concentration. Our results provide a new pathway to understand the effect of Mo and Ti doping concentrations on the structural and optical properties of ZnS nanoparticles as it could be the key to tune the properties for future optoelectronic devices.

Sonochemical synthesis and photocatalytic property of zinc oxide nanoparticles doped with magnesium(II)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lu, Xianyong, E-mail: xylu@buaa.edu.cn; Liu, Zhaoyue; Zhu, Ying

2011-10-15

Highlights: {yields} Mg-doped ZnO nanoparticles were synthesized by sonochemical strategy. {yields} Mg-doped ZnO nanoparticles present good photocatalytic properties. {yields} The change of band gap contributes to their high efficiency in photocatalyst. -- Abstract: Mg-doped ZnO nanoparticles were successfully synthesized by sonochemical method. The products were characterized by scan electron microscopy (SEM) and X-ray powder diffraction (XRD). SEM images revealed that ZnO doped with Mg(II) nanoparticles and ZnO nanoparticles synthesized by the same strategy all had spherical topography. XRD patterns showed that the doped nanoparticles had the same crystals structures as the pure ZnO nanoparticles. The Mg-doped ZnO nanoparticles had largermore » lattice volume than the un-doped nanoparticles. X-ray photoelectron spectroscopy (XPS) not only demonstrated the moral ratio of Mg and Zn element on the surface of nanoparticles, but their valence in nanoparticles as well. The Mg-doped ZnO nanoparticles presented good properties in photocatalyst compared with pure ZnO nanoparticles.« less

Power SiGe Heterojunction Bipolar Transistors (HBTs) Fabricated by Fully Self-Aligned Double Mesa Technology

NASA Technical Reports Server (NTRS)

Lu, Liang-Hung; Mohammadi, Saeed; Ma, Zhen-Qiang; Ponchak, George E.; Alterovitz, Samuel A.; Strohm, Karl M.; Luy, Johann-Friedrich; Downey, Alan (Technical Monitor)

2001-01-01

Multifinger SiGe HBTs have been fabricated using a novel fully self-aligned double-mesa technology. With the novel process technology, a common-emitter 2x2x30 sq micrometer device exhibits high maximum oscillating frequency (f(sub max)) and cut-off frequency (f(sub T)) of 78 and 37 GHz, respectively. In class-A operation, a multifinger device with l0x2x30 sq micrometer emitter is expected to provide an output power of 25.6 dBm with a gain of 10 dB and a maximum power added efficiency (PAE) of 30.33% at 8 GHz.

Synchrotron-Radiation Photoemission Study of Electronic Structures of a Cs-Doped Rubrene Surface

NASA Astrophysics Data System (ADS)

Cheng, Chiu-Ping; Lu, Meng-Han; Chu, Yu-Ya; Pi, Tun-Wen

Using synchrotron-radiation photoemission spectroscopy, we have studied the electronic structure of a cesium-doped rubrene thin film. The addition of cesium atoms causes the movement of the valence-band spectra and the change in line shapes at different concentration that can be separated into four different stages. In the first stage, the cesium atoms continuously diffuse into the substrate, and the Fermi level moves in the energy gap as a result of an electron transferred from the cesium to the rubrene. The second stage, in which the shifts of the spectra are interrupted, is characterized by the introduction of two in-gap states. When increasing doping of cesium into the third stage, the spectra move again; whereas, the line shapes maintain at the stoichiometric ratio of one. In the fourth stage, new in-gap states appear, which are the highest occupied molecular orbital (HOMO) and HOMO+1 states of (rubrene)2- anion.

Raman spectra boron doped amorphous carbon thin film deposited by bias assisted-CVD

NASA Astrophysics Data System (ADS)

Ishak, A.; Fadzilah, A. N.; Dayana, K.; Saurdi, I.; Malek, M. F.; Nurbaya, Z.; Shafura, A. K.; Rusop, M.

2018-05-01

Boron doped amorphous carbon thin film carbon was deposited at 200°C-350°C by bias assisted-CVD using palm oil as a precursor material. The structural boron doped amorphous carbon films were discussed by Raman analysis through the evolution of D and G bands. The spectral evolution observed showed the increase of upward shift of D and G peaks as substrate deposition temperatures increased. These structural changes were further correlated with optical gap and the results obtained are discussed and compared. The estimated optical band gap is found to be 1.9 to 2.05 eV and conductivity is to be in the range of 10-5 Scm-1 to 10-4 Scm-1. The decrease of optical band gap is associated to conductivity increased which change the characteristic parameters of Raman spectra including the position of G peak, full width at half maximum of G peak, and ID/IG.

Low-temperature charged impurity scattering-limited conductivity in relatively high doped bilayer graphene

NASA Astrophysics Data System (ADS)

Hu, Bo

2015-08-01

Based on semiclassical Boltzamnn transport theory in random phase approximation, we develop a theoretical model to investigate low-temperature carrier transport properties in relatively high doped bilayer graphene. In the presence of both electron-hole puddles and band gap induced by charged impurities, we calculate low-temperature charged impurity scattering-limited conductivity in relatively high doped bilayer graphene. Our calculated conductivity results are in excellent agreement with published experimental data in all compensated gate voltage regime of study by using potential fluctuation parameter as only one free fitting parameter, indicating that both electron-hole puddles and band gap induced by charged impurities play an important role in carrier transport. More importantly, we also find that the conductivity not only depends strongly on the total charged impurity density, but also on the top layer charged impurity density, which is different from that obtained by neglecting the opening of band gap, especially for bilayer graphene with high top layer charged impurity density.

Superconductivity, pairing symmetry, and disorder in the doped topological insulator Sn1 -xInxTe for x ≥0.10

NASA Astrophysics Data System (ADS)

Smylie, M. P.; Claus, H.; Kwok, W.-K.; Louden, E. R.; Eskildsen, M. R.; Sefat, A. S.; Zhong, R. D.; Schneeloch, J.; Gu, G. D.; Bokari, E.; Niraula, P. M.; Kayani, A.; Dewhurst, C. D.; Snezhko, A.; Welp, U.

2018-01-01

The temperature dependence of the London penetration depth Δ λ (T ) in the superconducting doped topological crystalline insulator Sn1 -xInxTe was measured down to 450 mK for two different doping levels, x ≈0.45 (optimally doped) and x ≈0.10 (underdoped), bookending the range of cubic phase in the compound. The results indicate no deviation from fully gapped BCS-like behavior, eliminating several candidate unconventional gap structures. Critical field values below 1 K and other superconducting parameters are also presented. The introduction of disorder by repeated particle irradiation with 5 MeV protons does not enhance Tc, indicating that ferroelectric interactions do not compete with superconductivity.

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

NASA Astrophysics Data System (ADS)

Pat, Suat; Özen, Soner; Korkmaz, Şadan

2018-01-01

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

Transparent nanostructured Fe-doped TiO2 thin films prepared by ultrasonic assisted spray pyrolysis technique

NASA Astrophysics Data System (ADS)

Rasoulnezhad, Hossein; Hosseinzadeh, Ghader; Ghasemian, Naser; Hosseinzadeh, Reza; Homayoun Keihan, Amir

2018-05-01

Nanostructured TiO2 and Fe-doped TiO2 thin films with high transparency were deposited on glass substrate through ultrasonic-assisted spray pyrolysis technique and were used in the visible light photocatalytic degradation of MB dye. The resulting thin films were characterized by scanning electron microscopy (SEM), Raman spectroscopy, photoluminescence spectroscopy, x-ray diffraction (XRD), and UV-visible absorption spectroscopy techniques. Based on Raman spectroscopy results, both of the TiO2 and Fe-doped TiO2 films have anatase crystal structure, however, because of the insertion of Fe in the structure of TiO2 some point defects and oxygen vacancies are formed in the Fe-doped TiO2 thin film. Presence of Fe in the structure of TiO2 decreases the band gap energy of TiO2 and also reduces the electron–hole recombination rate. Decreasing of the electron–hole recombination rate and band gap energy result in the enhancement of the visible light photocatalytic activity of the Fe-doped TiO2 thin film.

Impurity-Band Model for GaP1-xNx

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fluegel, B.; Zhang, Y.; Geisz, J. F.

2005-11-01

Low-temperature absorption studies on free-standing GaP1-xNx films provide direct experimental evidence that the host conduction-band minimum (CBM) near X1C does not plunge downward with increased nitrogen doping, contrary to what has been suggested recently; rather, it remains stationary for x up to 0.1%. This fact, combined with the results of earlier studies of the CBM at ..GAMMA.. and conduction-band edge near L, confirms that the giant bandgap lowering observed in GaP1-xNx results from a CBM that evolves purely from nitrogen impurity bands.

Scanning Tunneling Microscopy/Spectroscopy study on Optimally Potassium Doped Single Crystal BaFe2 As 2

NASA Astrophysics Data System (ADS)

Ma, Jihua; Li, Ang; Zhang, Chenglin; Dai, Pengcheng; Pan, Shuheng

2011-03-01

The iron pnictide parent compound material can be brought into superconducting state by chemical doping. It is worthwhile to study and compare the hole- and electron-doped iron pnictides. Among the well-known family of AEFe 2 As 2 (AE=Ca, Sr, Ba), the scanning tunneling microscopy/spectroscopy study on hole-doped samples is insufficient. In this talk we will present high resolution STM/STS results on (001) surface of the optimally doped single crystal Ba 0.6 K0.4 Fe 2 As 2 (Tc ~ 37 K). With the data we will discuss the spatial variation of the superconducting energy gap.

Doped Sc2C(OH)2 MXene: new type s-pd band inversion topological insulator.

PubMed

Balcı, Erdem; Akkuş, Ünal Özden; Berber, Savas

2018-04-18

The electronic structures of Si and Ge substitutionally doped Sc 2 C(OH) 2 MXene monolayers are investigated in density functional theory. The doped systems exhibit band inversion, and are found to be topological invariants in Z 2 theory. The inclusion of spin orbit coupling results in band gap openings. Our results point out that the Si and Ge doped Sc 2 C(OH) 2 MXene monolayers are topological insulators. The band inversion is observed to have a new mechanism that involves s and pd states.

Doped Sc2C(OH)2 MXene: new type s-pd band inversion topological insulator

NASA Astrophysics Data System (ADS)

Balcı, Erdem; Özden Akkuş, Ünal; Berber, Savas

2018-04-01

The electronic structures of Si and Ge substitutionally doped Sc2C(OH)2 MXene monolayers are investigated in density functional theory. The doped systems exhibit band inversion, and are found to be topological invariants in Z 2 theory. The inclusion of spin orbit coupling results in band gap openings. Our results point out that the Si and Ge doped Sc2C(OH)2 MXene monolayers are topological insulators. The band inversion is observed to have a new mechanism that involves s and pd states.

Structure, reactivity and electronic properties of Mn doped Ni13 clusters

NASA Astrophysics Data System (ADS)

Banerjee, Radhashyam; Datta, Soumendu; Mookerjee, Abhijit

2013-06-01

In this work we have studied the structural and magnetic properties of Ni13 cluster mono- and bi-doped with Mn atoms. We have noted their tendency of being reactive toward the H2 molecule. We have found unusually enhanced stability in the mono-doped cluster (i.e. of the Ni12Mn) and the diminished stability of the corresponding chemisorbed cluster, Ni12MnH2. Our analysis of the stability and HOMO-LUMO gap explains this unusual behavior. Interestingly, we have also seen the quenching in the net magnetic moment upon H2 absorption in the doped NiMnm alloy clusters. This has been reported earlier for smaller Nin clusters [1].

Study of cobalt effect on structural and optical properties of Dy doped ZnO nanoparticles

NASA Astrophysics Data System (ADS)

Kumar, Pawan; Pandey, Praveen C.

2018-05-01

The present study has been carried out to investigate the effect of Co doping on structural and optical properties of Dy doped ZnO nanoparticles. We have prepared pure Zinc oxide, Dy (1%) doped ZnO and Dy (1%) doped ZnO co-doped with Co(2%) with the help of simple sol-gel combustion method. The structural analysis carried out using X-ray diffraction spectra (XRD) indicates substitution of Dy and Co at Zn site of ZnO crystal structure and hexagonal crystal structure without any secondary phase formation in all the samples. The surface morphology was analyzed by transmission electron microscopy (TEM). Absorption study indicates that Dy doping causes a small shift in band edge, while Co co-doping results significant change is absorption edge as well as introduce defect level absorption in the visible region. The band gap of samples decreases due to Dy and Co doping, which can be attributed to defect level formation below the conduction band in the system.

Synthesis and Characterization of Molybdenum Doped ZnO Thin Films by SILAR Deposition Method

NASA Astrophysics Data System (ADS)

Radha, R.; Sakthivelu, A.; Pradhabhan, D.

2016-08-01

Molybdenum (Mo) doped zinc oxide (ZnO) thin films were deposited on the glass substrate by Successive Ionic Layer Adsorption and Reaction (SILAR) deposition method. The effect of Mo dopant concentration of 5, 6.6 and 10 mol% on the structural, morphological, optical and electrical properties of n-type Mo doped ZnO films was studied. The X-ray diffraction (XRD) results confirmed that the Mo doped ZnO thin films were polycrystalline with wurtzite structure. The field emission scanning electron microscopy (FESEM) studies shows that the surface morphology of the films changes with Mo doping. A blue shift of the optical band gap was observed in the optical studies. Effect of Mo dopant concentration on electrical conductivity was studied and it shows comparatively high electrical conductivity at 10 mol% of Mo doping concentration.

Moving Beyond 3D Hetero-Integration and Towards Monolithic Integration of Phase-Change RF Switches with SiGe BiCMOS

DTIC Science & Technology

2016-03-31

Corporation, Linthicum, Maryland *Corresponding author: Pavel.Borodulin@ngc.com Abstract: A chip -scale, highly-reconfigurable transmitter and...the technology has been used in a chip -scale, reconfigurable receiver demonstration and ongoing efforts to increase the level of performance and...circuit (RF-FPGA). It consists of a heterogeneous assembly of a SiGe BiCMOS chip with multiple 3D-integrated, low-loss, phase-change switch chiplets

Impact of dislocation densities on n+/p and p+/n junction GaAs diodes and solar cells on SiGe virtual substrates

NASA Astrophysics Data System (ADS)

Andre, C. L.; Wilt, D. M.; Pitera, A. J.; Lee, M. L.; Fitzgerald, E. A.; Ringel, S. A.

2005-07-01

Recent experimental measurements have shown that in GaAs with elevated threading dislocation densities (TDDs) the electron lifetime is much lower than the hole lifetime [C. L. Andre, J. J. Boeckl, D. M. Wilt, A. J. Pitera, M. L. Lee, E. A. Fitzgerald, B. M. Keyes, and S. A. Ringel, Appl. Phys. Lett. 84, 3884 (2004)]. This lower electron lifetime suggests an increase in depletion region recombination and thus in the reverse saturation current (J0 for an n+/p diode compared with a p+/n diode at a given TDD. To confirm this, GaAs diodes of both polarities were grown on compositionally graded Ge /Si1-xGex/Si (SiGe) substrates with a TDD of 1×106cm-2. It is shown that the ratio of measured J0 values is consistent with the inverse ratio of the expected lifetimes. Using a TDD-dependent lifetime in solar cell current-voltage models we found that the Voc, for a given short-circuit current, also exhibits a poorer TDD tolerance for GaAs n+/p solar cells compared with GaAs p+/n solar cells. Experimentally, the open-circuit voltage (Voc) for the n+/p GaAs solar cell grown on a SiGe substrate with a TDD of ˜1×106cm-2 was ˜880mV which was significantly lower than the ˜980mV measured for a p+/n GaAs solar cell grown on SiGe at the same TDD and was consistent with the solar cell modeling results reported in this paper. We conclude that p+/n polarity GaAs junctions demonstrate superior dislocation tolerance than n+/p configured GaAs junctions, which is important for optimization of lattice-mismatched III-V devices.

Photo-induced intersubband absorption in {Si}/{SiGe} quantum wells

NASA Astrophysics Data System (ADS)

Boucaud, P.; Gao, L.; Visocekas, F.; Moussa, Z.; Lourtioz, J.-M.; Julien, F. H.; Sagnes, I.; Campidelli, Y.; Badoz, P.-A.; Vagos, P.

1995-12-01

We have investigated photo-induced intersubband absorption in the valence band of {Si}/{SiGe} quantum wells. Carriers are optically generated in the quantum wells using an argon ion laser. The resulting infrared absorption is probed with a step-scan Fourier transform infrared spectrometer. The photo-induced infrared absorption in SiGe quantum wells is dominated by two contributions: the free carrier absorption, which is similar to bulk absorption in a uniformly doped SiGe layer, and the valence subband absorption in the quantum wells. Both p- and s-polarized intersubband absorptions are measured. We have observed that the photo-induced intersubband absorption in doped samples is shifted to lower energy as compared to direct intersubband absorption. This absorption process is attributed to carriers away from the Brillouin zone center. We show that the photo-induced technique is appropriate to study valence band mixing effects and their influence on intersubband absorption.

Superconductivity, pairing symmetry, and disorder in the doped topological insulator Sn 1 - x In x Te for x ≥ 0.10

DOE PAGES

Smylie, M. P.; Claus, H.; Kwok, W. -K.; ...

2018-01-19

The temperature dependence of the London penetration depth Δλ(T) in the superconducting doped topological crystalline insulator Sn 1-xIn x Te was measured down to 450 mK for two different doping levels, x ≈ 0.45 (optimally doped) and x ≈ 0.10 (underdoped), bookending the range of cubic phase in the compound. The results indicate no deviation from fully gapped BCS-like behavior, eliminating several candidate unconventional gap structures. Critical field values below 1 K and other superconducting parameters are also presented. The introduction of disorder by repeated particle irradiation with 5 MeV protons does not enhance T c, indicating that ferroelectric interactionsmore » do not compete with superconductivity.« less

Superconductivity, pairing symmetry, and disorder in the doped topological insulator Sn 1 - x In x Te for x ≥ 0.10

DOE Office of Scientific and Technical Information (OSTI.GOV)

Smylie, M. P.; Claus, H.; Kwok, W. -K.

The temperature dependence of the London penetration depth Δλ(T) in the superconducting doped topological crystalline insulator Sn 1-xIn x Te was measured down to 450 mK for two different doping levels, x ≈ 0.45 (optimally doped) and x ≈ 0.10 (underdoped), bookending the range of cubic phase in the compound. The results indicate no deviation from fully gapped BCS-like behavior, eliminating several candidate unconventional gap structures. Critical field values below 1 K and other superconducting parameters are also presented. The introduction of disorder by repeated particle irradiation with 5 MeV protons does not enhance T c, indicating that ferroelectric interactionsmore » do not compete with superconductivity.« less

Effects of B site doping on electronic structures of InNbO4 based on hybrid density functional calculations

NASA Astrophysics Data System (ADS)

Lu, M. F.; Zhou, C. P.; Li, Q. Q.; Zhang, C. L.; Shi, H. F.

2018-01-01

In order to improve the photocatalytic activity under visible-light irradiation, we adopted first principle calculations based on density functional theory (DFT) to calculate the electronic structures of B site transition metal element doped InNbO4. The results indicated that the complete hybridization of Nb 4d states and some Ti 3d states contributed to the new conduction band of Ti doped InNbO4, barely changing the position of band edge. For Cr doping, some localized Cr 3d states were introduced into the band gap. Nonetheless, the potential of localized levels was too positive to cause visible-light reaction. When it came to Cu doping, the band gap was almost same with that of InNbO4 as well as some localized Cu 3d states appeared above the top of VB. The introduction of localized energy levels benefited electrons to migrate from valence band (VB) to conduction band (CB) by absorbing lower energy photons, realizing visible-light response.

Doping Evolution of Magnetic Order and Magnetic Excitations in (Sr1 -xLax)3Ir2O7

NASA Astrophysics Data System (ADS)

Lu, Xingye; McNally, D. E.; Moretti Sala, M.; Terzic, J.; Upton, M. H.; Casa, D.; Ingold, G.; Cao, G.; Schmitt, T.

2017-01-01

We use resonant elastic and inelastic x-ray scattering at the Ir-L3 edge to study the doping-dependent magnetic order, magnetic excitations, and spin-orbit excitons in the electron-doped bilayer iridate (Sr1 -xLax )3Ir2 O7 (0 ≤x ≤0.065 ). With increasing doping x , the three-dimensional long range antiferromagnetic order is gradually suppressed and evolves into a three-dimensional short range order across the insulator-to-metal transition from x =0 to 0.05, followed by a transition to two-dimensional short range order between x =0.05 and 0.065. Because of the interactions between the Jeff=1/2 pseudospins and the emergent itinerant electrons, magnetic excitations undergo damping, anisotropic softening, and gap collapse, accompanied by weakly doping-dependent spin-orbit excitons. Therefore, we conclude that electron doping suppresses the magnetic anisotropy and interlayer couplings and drives (Sr1 -xLax )3Ir2 O7 into a correlated metallic state with two-dimensional short range antiferromagnetic order. Strong antiferromagnetic fluctuations of the Jeff=1/2 moments persist deep in this correlated metallic state, with the magnon gap strongly suppressed.

Performance analysis of SiGe double-gate N-MOSFET

NASA Astrophysics Data System (ADS)

Singh, A.; Kapoor, D.; Sharma, R.

2017-04-01

The major purpose of this paper is to find an alternative configuration that not only minimizes the limitations of single-gate (SG) MOSFETs but also provides the better replacement for future technology. In this paper, the electrical characteristics of SiGe double-gate N-MOSFET are demonstrated and compared with electrical characteristics of Si double-gate N-MOSFET. Furthermore, in this paper the electrical characteristics of Si double-gate N-MOSFET are demonstrated and compared with electrical characteristics of Si single-gate N-MOSFET. The simulations are carried out for the device at different operational voltages using Cogenda Visual TCAD tool. Moreover, we have designed its structure and studied both {I}{{d}}{-}{V}{{g}} characteristics for different voltages namely 0.05, 0.1, 0.5, 0.8, 1 and 1.5 V and {I}{{d}}{-}{V}{{d}} characteristics for different voltages namely 0.1, 0.5, 1 and 1.5 V at work functions 4.5, 4.6 and 4.8 eV for this structure. The performance parameters investigated in this paper are threshold voltage, DIBL, subthreshold slope, GIDL, volume inversion and MMCR.

Enhancement in ferroelectric, pyroelectric and photoluminescence properties in dye doped TGS crystals

NASA Astrophysics Data System (ADS)

Sinha, Nidhi; Goel, Neeti; Singh, B. K.; Gupta, M. K.; Kumar, Binay

2012-06-01

Pure and dye doped (0.1 and 0.2 mol%) Triglycine Sulfate (TGS) single crystals were grown by slow evaporation technique. A pyramidal coloring pattern, along with XRD and FT-IR studies confirmed the dye doping. Decrease in dielectric constant and increase in Curie temperature (Tc) were observed with increasing doping concentration. Low absorption cut off (231 nm) and high optical transparency (>90%) resulting in large band gap was observed in UV-VIS studies. In addition, strong hyper-luminescent emission bands at 350 and 375 nm were observed in which the relative intensity were found to be reversed as a result of doping. In P-E hysteresis loop studies, a higher curie temperature and an improved and more uniform figure of merit over a large region of the ferroelectric phase were observed. The improved dielectric, optical and ferroelectric/pyroelectric properties make the dye doped TGS crystals better candidate for various opto- and piezo-electronics applications.

Plasmonic nanohole arrays on Si-Ge heterostructures: an approach for integrated biosensors

NASA Astrophysics Data System (ADS)

Augel, L.; Fischer, I. A.; Dunbar, L. A.; Bechler, S.; Berrier, A.; Etezadi, D.; Hornung, F.; Kostecki, K.; Ozdemir, C. I.; Soler, M.; Altug, H.; Schulze, J.

2016-03-01

Nanohole array surface plasmon resonance (SPR) sensors offer a promising platform for high-throughput label-free biosensing. Integrating nanohole arrays with group-IV semiconductor photodetectors could enable low-cost and disposable biosensors compatible to Si-based complementary metal oxide semiconductor (CMOS) technology that can be combined with integrated circuitry for continuous monitoring of biosamples and fast sensor data processing. Such an integrated biosensor could be realized by structuring a nanohole array in the contact metal layer of a photodetector. We used Fouriertransform infrared spectroscopy to investigate nanohole arrays in a 100 nm Al film deposited on top of a vertical Si-Ge photodiode structure grown by molecular beam epitaxy (MBE). We find that the presence of a protein bilayer, constitute of protein AG and Immunoglobulin G (IgG), leads to a wavelength-dependent absorptance enhancement of ~ 8 %.

Rietveld-refinement and optical study of the Fe doped ZnO thin film by RF magnetron sputtering

NASA Astrophysics Data System (ADS)

Kumar, Arun; Dhiman, Pooja; Singh, M.

2017-05-01

Fe Doped ZnO Dilute Magnetic Semiconductor thin film prepared by RF magnetron sputtering on glass substrate and Influence of 3% Fe-doping on structural and Optical properties has been studied. The Rietveld-refinement analysis shows that Fe doping has a significant effect on crystalline structure, grain size and strain in the thin film. Two dimensional and three-dimensional atom probe tomography of the thin film shows that Fe ions are randomly distributed which is supported by Xray Diffraction (XRD). Fe-doping is found to effectively modify the band gap energy up to 3.5 eV.

Superconductivity, Pairing Symmetry, and Disorder in the Doped Topological Insulator Sn 1-xIn xTe for x >= 0.10.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Smylie, M. P.; Claus, H.; Kwok, W. -K.

The temperature dependence of the London penetration depth Delta lambda(T) in the superconducting doped topological crystalline insulator Sn1-xInxTe was measured down to 450 mK for two different doping levels, x approximate to 0.45 (optimally doped) and x approximate to 0.10 (underdoped), bookending the range of cubic phase in the compound. The results indicate no deviation from fully gapped BCS-like behavior, eliminating several candidate unconventional gap structures. Critical field values below 1 K and other superconducting parameters are also presented. The introduction of disorder by repeated particle irradiation with 5 MeV protons does not enhance T-c, indicating that ferroelectric interactions domore » not compete with superconductivity.« less

Structural properties and optical characterization of flower-like Mg doped NiO

DOE Office of Scientific and Technical Information (OSTI.GOV)

Allaedini, Ghazaleh, E-mail: jiny-ghazaleh@yahoo.com; Tasirin, Siti Masrinda; Aminayi, Payam

In this study, un-doped and Mg doped NiO nanoparticles have been synthesized through a simple sol-gel method. To investigate the effect of Mg-doping on the structure of NiO, the obtained nanoparticles were characterized using scanning electron microscopy (SEM). Flower/star like morphology was clearly observed in the SEM micrographs. The BET (Brunauer-Emmett-Teller) nitrogen absorption isotherm exhibits high specific surface area (∼37 m{sup 2} /g) for the Mg doped NiO nanoparticles. X-Ray diffraction (XRD) of the prepared Mg-NiO nanoparticles showed a face-centered cubic (f.c.c) structure, and the average particle size was estimated to be 32 nm using Scherrer’s formula. Energy Dispersive X-Ray (EDX)more » confirms that the NiO particles are successfully doped with Mg. Photoluminescence (PL) and UV-Vis optical absorption characteristics of the prepared nanoparticles have also been investigated in this study. The PL emission response showed a blue shift when NiO was doped with Mg, which is indicative of interstitial oxygen. The UV-Vis results demonstrate a band gap increase as NiO nanoparticles are doped with Mg.« less

Influence of Fe doping on the structural, optical and acetone sensing properties of sprayed ZnO thin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Prajapati, C.S.; Kushwaha, Ajay; Sahay, P.P., E-mail: dr_ppsahay@rediffmail.com

2013-07-15

Graphical abstract: All the films are found to be polycrystalline ZnO possessing hexagonal wurtzite structure. The intensities of all the peaks are diminished strongly in the Fe-doped films, indicating their lower crystallinity as compared to the undoped ZnO film. The average crystallite size decreases from 35.21 nm (undoped sample) to 15.43 nm (1 at% Fe-doped sample). - Highlights: • Fe-doped ZnO films show smaller crystallinity with crystallite size: 15–26 nm. • Optical band gap in ZnO films decreases on Fe doping. • Fe-doped films exhibit the normal dispersion for the wavelength range 450–600 nm. • PL spectra of the Fe-dopedmore » films show quenching of the broad green-orange emission. • Acetone response of the Fe-doped films increases considerably at 300 °C. - Abstract: The ZnO thin films (undoped and Fe-doped) deposited by chemical spray pyrolysis technique have been analyzed by X-ray powder diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Results show that all the films possess hexagonal wurtzite structure of zinc oxide having crystallite sizes in the range 15–36 nm. On 1 at% Fe doping, the surface roughness of the film increases which favors the adsorption of atmospheric oxygen on the film surface and thereby increase in the gas response. Optical studies reveal that the band gap decreases due to creation of some defect energy states below the conduction band edge, arising out of the lattice disorder in the doped films. The refractive index of the films decreases on Fe doping and follows the Cauchy relation of normal dispersion. Among all the films examined, the 1 at% Fe-doped film exhibits the maximum response (∼72%) at 300 °C for 100 ppm concentration of acetone in air.« less

A phenomenological model for systematization and prediction of doping limits in II{endash}VI and I{endash}III{endash}VI{sub 2} compounds

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, S.B.; Wei, S.; Zunger, A.

1998-03-01

Semiconductors differ widely in their ability to be doped. As their band gap increases, it is usually possible to dope them either n or p type, but not both. This asymmetry is documented here, and explained phenomenologically in terms of the {open_quotes}doping pinning rule.{close_quotes} {copyright} {ital 1998 American Institute of Physics.}

An impurity intermediate band due to Pb doping induced promising thermoelectric performance of Ca5In2Sb6.

PubMed

Feng, Zhenzhen; Wang, Yuanxu; Yan, Yuli; Zhang, Guangbiao; Yang, Jueming; Zhang, Jihua; Wang, Chao

2015-06-21

Band engineering is one of the effective approaches for designing ideal thermoelectric materials. Introducing an intermediate band in the band gap of semiconducting thermoelectric compounds may largely increase the carrier concentration and improve the electrical conductivity of these compounds. We test this hypothesis by Pb doping in Zintl Ca5In2Sb6. In the current work, we have systematically investigated the electronic structure and thermoelectric performances of substitutional doping with Pb on In sites at a doping level of 5% (0.2 e per cell) for Ca5In2Sb6 by using density functional theory combined with semi-classical Boltzmann theory. It is found that in contrast to Zn doping, Pb doping introduces a partially filled intermediate band in the band gap of Ca5In2Sb6, which originates from the Pb s states by weakly hybridizing with the Sb p states. Such an intermediate band dramatically increases the electrical conductivity of Ca5In2Sb6 and has little detrimental effect on its Seebeck coefficient, which may increase its thermoelectric figure of merit, ZT. Interestingly, a maximum ZT value of 2.46 may be achieved at 900 K for crystalline Pb-doped Ca5In2Sb6 when the carrier concentration is optimized. Therefore, Pb-doped Ca5In2Sb6 may be a promising thermoelectric material.

Fingerprints of spin-orbital polarons and of their disorder in the photoemission spectra of doped Mott insulators with orbital degeneracy

NASA Astrophysics Data System (ADS)

Avella, Adolfo; Oleś, Andrzej M.; Horsch, Peter

2018-04-01

We explore the effects of disordered charged defects on the electronic excitations observed in the photoemission spectra of doped transition metal oxides in the Mott insulating regime by the example of the R1 -xCaxVO3 perovskites, where R = La, ⋯, Lu. A fundamental characteristic of these vanadium d2 compounds with partly filled t2 g valence orbitals is the persistence of spin and orbital order up to high doping, in contrast to the loss of magnetic order in high-Tc cuprates at low defect concentration. We study the disordered electronic structure of such doped Mott-Hubbard insulators within the unrestricted Hartree-Fock approximation and, as a result, manage to explain the spectral features that occur in photoemission and inverse photoemission. In particular, (i) the atomic multiplet excitations in the inverse photoemission spectra and the various defect-related states and satellites are qualitatively well reproduced, (ii) a robust Mott gap survives up to large doping, and (iii) we show that the defect states inside the Mott gap develop a soft gap at the Fermi energy. The soft defect-states gap, which separates the highest occupied from the lowest unoccupied states, can be characterized by a shape and a scale parameter extracted from a Weibull statistical sampling of the density of states near the chemical potential. These parameters provide a criterion and a comprehensive schematization for the insulator-metal transition in disordered systems. Our results provide clear indications that doped holes are bound to charged defects and form small spin-orbital polarons whose internal kinetic energy is responsible for the opening of the soft defect-states gap. We show that this kinetic gap survives disorder fluctuations of defects and is amplified by the long-range electron-electron interactions, whereas we observe a Coulomb singularity in the atomic limit. The small size of spin-orbital polarons is inferred by an analysis of the inverse participation ratio and by

Thermal conductivity of nanocrystalline SiGe alloys using molecular dynamics simulations

NASA Astrophysics Data System (ADS)

Abs da Cruz, Carolina; Katcho, Nebil A.; Mingo, Natalio; Veiga, Roberto G. A.

2013-10-01

We have studied the effect of nanocrystalline microstructure on the thermal conductivity of SiGe alloys using molecular dynamics simulations. Nanograins are modeled using both the coincidence site lattice and the Voronoi tessellation methods, and the thermal conductivity is computed using the Green-Kubo formalism. We analyze the dependence of the thermal conductivity with temperature, grain size L, and misorientation angle. We find a power dependence of L1/4 of the thermal conductivity with the grain size, instead of the linear dependence shown by non-alloyed nanograined systems. This dependence can be derived analytically underlines the important role that disorder scattering plays even when the grains are of the order of a few nm. This is in contrast to non-alloyed systems, where phonon transport is governed mainly by the boundary scattering. The temperature dependence is weak, in agreement with experimental measurements. The effect of angle misorientation is also small, which stresses the main role played by the disorder scattering.

Sonochemically synthesized iron-doped zinc oxide nanoparticles: Influence of precursor composition on characteristics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Roy, Anirban; Maitra, Saikat; Ghosh, Sobhan

Highlights: • Sonochemical synthesis of iron-doped zinc oxide nanoparticles. • Green synthesis without alkali at room temperature. • Characterization by UV–vis spectroscopy, FESEM, XRD and EDX. • Influence of precursor composition on characteristics. • Composition and characteristics are correlated. - Abstract: Iron-doped zinc oxide nanoparticles have been synthesized sonochemically from aqueous acetyl acetonate precursors of different proportions. Synthesized nanoparticles were characterized with UV–vis spectroscopy, X-ray diffraction and microscopy. Influences of precursor mixture on the characteristics have been examined and modeled. Linear correlations have been proposed between dopant dosing, extent of doping and band gap energy. Experimental data corroborated with themore » proposed models.« less

Ferromagnetic properties of Mn-doped HfS2 monolayer under strain

NASA Astrophysics Data System (ADS)

Ma, Xu; Zhao, Xu; Wu, Ninghua; Xin, Qianqian; Liu, Xiaomeng; Wang, Tianxing; Wei, Shuyi

2017-12-01

Using the first-principles calculations, we investigated electronic and magnetic properties of Mn-doped HfS2 monolayer for 4% and 8% Mn concentration. We study the strain tuning of electronic and magnetic properties of 4% Mn-doped HfS2 monolayer firstly. Our results show that the Mn-doped HfS2 monolayer is magnetic nanomaterial without strain. It keeps this character until the compressive strain comes to -8%, and the magnetism disappear with lager compressive strain. With the increasing tensile strain, the doped system transforms from semiconductor to half-metallic when the tensile strain is equivalent to or greater than 5%. The largest half-metallic gap is 1.307 eV at 5% tensile strain and the magnetic moment always keeps about 3μB, which indicates that Mn-doped HfS2 monolayer can be a candidate for superior half-metallic namomaterial. Furthermore, we find two Mn dopants couple ferromagnetically via antiferromagnetic (AFM) p-d exchange interaction at the environment of 8% concentration. It keeps the properties of magnetic semiconductor under two Mn-doped configurations with different Mn-Mn separations. Our studies predict Mn-doped HfS2 monolayer under strain to be candidates for dilute magnetic semiconductors.

Valence-band-edge shift due to doping in p + GaAs

NASA Astrophysics Data System (ADS)

Silberman, J. A.; de Lyon, T. J.; Woodall, J. M.

1991-05-01

Accurate knowledge of the shifts in valence- and conduction-band edges due to heavy doping effects is crucial in modeling GaAs device structures that utilize heavily doped layers. X-ray photoemission spectroscopy was used to deduce the shift in the valence-band-edge induced by carbon (p type) doping to a carrier density of 1×1020 cm-3 based on a determination of the bulk binding energy of the Ga and As core levels in this material. Analysis of the data indicates that the shift of the valence-band maximum into the gap and the penetration of the Fermi level into the valence bands exactly compensate at this degenerate carrier concentration, to give ΔEv =0.12±0.05 eV.

Boron, bismuth co-doping of gallium arsenide and other compounds for photonic and heterojunction bipolar transistor devices

DOEpatents

Mascarenhas, Angelo

2015-07-07

Isoelectronic co-doping of semiconductor compounds and alloys with acceptors and deep donors is sued to decrease bandgap, to increase concentration of the dopant constituents in the resulting alloys, and to increase carrier mobilities lifetimes. For example, Group III-V compounds and alloys, such as GaAs and GaP, are isoelectronically co-doped with, for example, B and Bi, to customize solar cells, and other semiconductor devices. Isoelectronically co-doped Group II-VI compounds and alloys are also included.

Defect-mediated magnetism of transition metal doped zinc oxide thin films

NASA Astrophysics Data System (ADS)

Roberts, Bradley Kirk

Magnetism in transition metal doped wide band-gap materials is of interest to further the fundamental science of materials and future spintronics applications. Large inter-dopant separations require mediation of ferromagnetism by some method; carrier-mediated mechanisms are typically applicable to dilute magnetic semiconductors with low Curie temperatures. Dilute magnetic oxides, commonly with poor conductivity and TC above room temperature, cannot be described within this theory. Recent experiment and theory developments suggest that ferromagnetic exchange in these materials can be mediated by defects. This research includes experimental results justifying and developing this approach. Thin films of Cr doped ZnO (band gap ˜3.3 eV) were deposited with several processing variations to enhance the effects of either 0-dimensional (vacancy, hydrogen-related defect) or two-dimensional defects (surface/interface) and thereby affect magnetism and conductivity. We observe surface magnetism in dielectric thin films of oxygen-saturated ZnO:Cr with spontaneous magnetic moment and conductance dropping approximately exponentially with increasing thickness. Uniform defect concentrations would not result in such magnetic ordering behavior indicating that magnetism is mediated either by surface defects or differing concentrations of point defects near the surface. Polarized neutron reflectivity profiling confirms a magnetically active region of ˜8 nm at the film surface. Hydrogen is notoriously present as a defect and carrier dopant in ZnO, and artificial introduction of hydrogen in dielectric ZnO:Cr films results in varying electronic and magnetic behavior. Free carriers introduced with hydrogen doping are not spin-polarized requiring an alternative explanation for ferromagnetism. We find from positron annihilation spectroscopy measurements that hydrogen doping increases the concentration of an altered VZn-related defect (a preliminary interpretation) throughout the film, which

Microstructural and optical properties of Mn doped NiO nanostructures synthesized via sol-gel method

NASA Astrophysics Data System (ADS)

Shah, Shamim H.; Khan, Wasi; Naseem, Swaleha; Husain, Shahid; Nadeem, M.

2018-04-01

Undoped and Mn(0, 5%, 10% and 15%) doped NiO nanostructures were synthesized by sol-gel method. Structure, morphology and optical properties were investigated through XRD, FTIR, SEM/EDS and UV-visible absorption spectroscopy techniques. XRD data analysis reveals the single phase nature with cubic crystal symmetry of the samples and the average crystallite size decreases with the doping of Mn ions upto 10%. FTIR spectra further confirmed the purity and composition of the synthesized samples. The non-spherical shape of the nanostructures was observed from SEM micrographs and gain size of the nanostructures reduces with Mn doping in NiO, whereas agglomeration increases in doped sample. Optical band gap was estimated using Tauc'srelation and found to increase on incorporation of Mn upto 10% in host lattice and then decreases for further doping.

Improving surface-enhanced Raman scattering properties of TiO(2) nanoparticles by metal Co doping.

PubMed

Yang, Libin; Qin, Xiaoyu; Gong, Mengdi; Jiang, Xin; Yang, Ming; Li, Xiuling; Li, Guangzhi

2014-04-05

In this paper, pure and different amount Co ions doped TiO2 nanoparticles were synthesized by a sol-hydrothermal method and were served as SERS-active substrate. The effect of metal Co doping on SERS properties of TiO2 nanoparticles was mostly investigated. The results indicate that abundant metal doping energy levels can be formed in the energy gap of TiO2 by an appropriate amount Co ions doping, which can promote the charge transfer from TiO2 to molecule, and subsequently enhance SERS signal of adsorbed molecule on TiO2 substrate, and improve remarkably SERS properties of TiO2 nanoparticles. Copyright © 2013 Elsevier B.V. All rights reserved.

Dual doped monolayer and bilayer graphene: The case of 4p and 2p elements

NASA Astrophysics Data System (ADS)

Denis, Pablo A.; Iribarne, Federico

2016-08-01

4p/2p dual-doped monolayer and bilayer graphene were studied via first principle calculations. Generally, dopants prefer to be agglomerated. A second dopant significantly reduces formation energies. Thus, partially reduced graphene oxide would favor substitutional doping by facilitating the introduction of the 4p dopants. Dual-doping can tune the band gap from 0.1 to 0.8 eV. For bilayer graphene, large atomic radii elements (Gallium and Germanium) form interlayer bonds with the undoped sheet. For some dual-doped graphenes, interlayer GaC and GeC bonds were formed, increasing the chemical reactivity of the undoped layer and affecting its electronic structure, with metallic or semiconducting characters observed.

Partially filled intermediate band of Cr-doped GaN films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sonoda, S.

2012-05-14

We investigated the band structure of sputtered Cr-doped GaN (GaCrN) films using optical absorption, photoelectron yield spectroscopy, and charge transport measurements. It was found that an additional energy band is formed in the intrinsic band gap of GaN upon Cr doping, and that charge carriers in the material move in the inserted band. Prototype solar cells showed enhanced short circuit current and open circuit voltage in the n-GaN/GaCrN/p-GaN structure compared to the GaCrN/p-GaN structure, which validates the proposed concept of an intermediate-band solar cell.

Multi-wavelength laser from dye-doped cholesteric polymer films.

PubMed

Huang, Yuhua; Wu, Shin-Tson

2010-12-20

A multi-wavelength laser is demonstrated using a dye-doped cholesteric polymer film whose reflection bandwidth is broadened with several oscillations. Due to the abrupt change of the density of state between oscillation peak and valley, each oscillation functions as a photonic band gap for generating a laser wavelength under the excitation of a pumping laser. As a result, a multiple wavelength laser is generated. Results indicate that the dye-doped cholesteric liquid crystal polymer film is a good candidate for fabricating broadband lasers such as white light lasers. Potential applications include experimental testing of laser materials, identification markers, information displays, and inertial confinement laser fusion.

Effect of Mo-doping concentration on the physical behaviour of sprayed ZnO layers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Reddy, T. Sreenivasulu; Reddy, M. Vasudeva; Reddy, K. T. Ramakrishna, E-mail: ktrkreddy@gmail.com

2015-06-24

Mo-doped zinc oxide layers (MZO) have been prepared on cleaned glass substrates by chemical spray pyrolysis technique by varying Mo-doping concentration in the range, 0 – 5 at. %. The X-ray diffraction studies revealed that all the as prepared layers were polycrystalline in nature and exhibited wurtzite structure. The layers prepared with lower Mo-doping concentration (<2 at. %) were preferably oriented along the (100) plane, whereas in the case of higher Mo-doping concentration (>2 at. %), the films showed the (002) plane as the dominant peak. The optical analysis indicated that all the layers had an average optical transmittance ofmore » 80% in the visible region and the evaluated band gap varied in the range, 3.28 - 3.50 eV.« less

Superparamagnetic behavior of Fe-doped SnO2 nanoparticles

NASA Astrophysics Data System (ADS)

Hachisu, M.; Onuma, K.; Kondo, T.; Miike, K.; Miyasaka, T.; Mori, K.; Ichiyanagi, Y.

2014-02-01

SnO2 is an n-type semiconductor with a wide band gap of 3.62 eV, and SnO2 nanoparticles doped with magnetic ions are expected to realized new diluted magnetic semiconductors (DMSs). Realizing ferromagnetism at room temperature is important for spintronics device applications, and it is interesting that the magnetic properties of these DMS systems can be varied significantly by modifying the preparation methods or conditions. In this study, the magnetic properties of Fe-doped (3% and 5%) SnO2 nanoparticles, prepared using our novel chemical preparation method and encapsulated in amorphous SiO2, were investigated. The particle size (1.8-16.9 nm) and crystal phase were controlled by the annealing temperature. X-ray diffraction confirmed a rutile SnO2 single-phase structure for samples annealed at 1073-1373 K, and the composition was confirmed using X-ray fluorescence analysis. SQUID magnetometer measurements revealed superparamagnetic behavior of the 5%-Fe-doped sample at room temperature, although SnO2 is known to be diamagnetic. Magnetization curves at 5 K indicated that the 3%-Fe-doped has a larger magnetization than that of the 5%-Fe-doped sample. We conclude that the magnetization of the 5%-Fe-doped sample decreased at 5 K due to the superexchange interaction between the antiferromagnetic coupling in the nanoparticle system.

Analysis of photonic band gap in novel piezoelectric photonic crystal

NASA Astrophysics Data System (ADS)

Malar Kodi, A.; Doni Pon, V.; Joseph Wilson, K. S.

2018-03-01

The transmission properties of one-dimensional novel photonic crystal having silver-doped novel piezoelectric superlattice and air as the two constituent layers have been investigated by means of transfer matrix method. By changing the appropriate thickness of the layers and filling factor of nanocomposite system, the variation in the photonic band gap can be studied. It is found that the photonic band gap increases with the filling factor of the metal nanocomposite and with the thickness of the layer. These structures possess unique characteristics enabling one to operate as optical waveguides, selective filters, optical switches, integrated piezoelectric microactuators, etc.

Study on electrical structure and magneto-optical properties of W-doped ZnO

NASA Astrophysics Data System (ADS)

Li, Yong; Hou, Qingyu; Zhao, Chunwang; Xu, Zhenchao

2018-04-01

For W-doping amounts ranging from 0.0417 to 0.0833, experimental UV-visible absorption spectra blue shift and red shift results have been reported in the literatures. However, there is few literature reported research on magnetic mechanism. To solve this problem, this study investigates the disagreement about blue shift and red shift results and research on magnetic mechanism. The band structures, density of states, absorption spectra and magnetism have been investigated using first-principles planewave ultrasoft pseudopotential method based on the density functional theory. The calculated results showed that increased W-doping amounts first increase the volumes, and then reduce the volumes, decrease the formation energies, and stabilize the doped system. The band gaps become narrower and the absorption spectrum exhibits a significant red shift in UV and visible light emission. Moreover, the covalent bond vertical to c-axis strengthens, and the ionic bond parallel to c-axis weakens. Increased W-doping amounts decrease the magnetism of doped system. The magnetism of doped system originates from the electron exchange among W-5d, O-2p and Zn-3d orbitals of the W-doped ZnO. In W double-doped system, the ferromagnetic Curie temperature can be above room temperature when the doped system has a longer W-W distance.

Discrete impurity band from surface danging bonds in nitrogen and phosphorus doped SiC nanowires

NASA Astrophysics Data System (ADS)

Li, Yan-Jing; Li, Shu-Long; Gong, Pei; Li, Ya-Lin; Cao, Mao-Sheng; Fang, Xiao-Yong

2018-04-01

The electronic structure and optical properties of the nitrogen and phosphorus doped silicon carbide nanowires (SiCNWs) are investigated using first-principle calculations based on density functional theory. The results show doping can change the type of the band gap and improve the conductivity. However, the doped SiCNWs form a discrete impurity levels at the Fermi energy, and the dispersion degree decreases with the diameter increasing. In order to reveal the root of this phenomenon, we hydrogenated the doped SiCNWs, found that the surface dangling bonds were saturated, and the discrete impurity levels are degeneracy, which indicates that the discrete impurity band of the doped SiCNWs is derived from the dangling bonds. The surface passivation can degenerate the impurity levels. Therefore, both doping and surface passivation can better improve the photoelectric properties of the SiCNWs. The result can provide additional candidates in producing nano-optoelectronic devices.

Partially suppressed shot noise in hopping conduction: observation in SiGe quantum wells

PubMed

Kuznetsov; Mendez; Zuo; Snider; Croke

2000-07-10

We have observed shot noise in the hopping conduction of two-dimensional carriers confined in a p-type SiGe quantum well at a temperature of 4 K. Moreover, shot noise is suppressed relative to its "classical" value 2eI by an amount that depends on the length of the sample and the carrier density. We have found a suppression factor to the classical value of about one-half for a 2 &mgr;m long sample, and of one-fifth for a 5 &mgr;m sample. In each case, the factor decreased slightly as the density increased toward the insulator-metal transition. We explain these results in terms of the characteristic length ( approximately 1 &mgr;m in our case) of the inherent inhomogeneity of hopping transport, obtained from percolation theory.

PdO doping tunes band-gap energy levels as well as oxidative stress responses to a Co₃O₄ p-type semiconductor in cells and the lung.

PubMed

Zhang, Haiyuan; Pokhrel, Suman; Ji, Zhaoxia; Meng, Huan; Wang, Xiang; Lin, Sijie; Chang, Chong Hyun; Li, Linjiang; Li, Ruibin; Sun, Bingbing; Wang, Meiying; Liao, Yu-Pei; Liu, Rong; Xia, Tian; Mädler, Lutz; Nel, André E

2014-04-30

We demonstrate through PdO doping that creation of heterojunctions on Co3O4 nanoparticles can quantitatively adjust band-gap and Fermi energy levels to study the impact of metal oxide nanoparticle semiconductor properties on cellular redox homeostasis and hazard potential. Flame spray pyrolysis (FSP) was used to synthesize a nanoparticle library in which the gradual increase in the PdO content (0-8.9%) allowed electron transfer from Co3O4 to PdO to align Fermi energy levels across the heterojunctions. This alignment was accompanied by free hole accumulation at the Co3O4 interface and production of hydroxyl radicals. Interestingly, there was no concomitant superoxide generation, which could reflect the hole dominance of a p-type semiconductor. Although the electron flux across the heterojunctions induced upward band bending, the E(c) levels of the doped particles showed energy overlap with the biological redox potential (BRP). This allows electron capture from the redox couples that maintain the BRP from -4.12 to -4.84 eV, causing disruption of cellular redox homeostasis and induction of oxidative stress. PdO/Co3O4 nanoparticles showed significant increases in cytotoxicity at 25, 50, 100, and 200 μg/mL, which was enhanced incrementally by PdO doping in BEAS-2B and RAW 264.7 cells. Oxidative stress presented as a tiered cellular response involving superoxide generation, glutathione depletion, cytokine production, and cytotoxicity in epithelial and macrophage cell lines. A progressive series of acute pro-inflammatory effects could also be seen in the lungs of animals exposed to incremental PdO-doped particles. All considered, generation of a combinatorial PdO/Co3O4 nanoparticle library with incremental heterojunction density allowed us to demonstrate the integrated role of E(v), E(c), and E(f) levels in the generation of oxidant injury and inflammation by the p-type semiconductor, Co3O4.

An investigation of the Nb doping effect on structural, morphological, electrical and optical properties of spray deposited F doped SnO2 films

NASA Astrophysics Data System (ADS)

Turgut, G.; Keskenler, E. F.; Aydın, S.; Yılmaz, M.; Doǧan, S.; Düzgün, B.

2013-03-01

F and Nb + F co-doped SnO2 thin films were deposited on glass substrates by the spray pyrolysis method. The microstructural, morphological, electrical and optical properties of the 10 wt% F doped SnO2 (FTO) thin films were investigated specifically for niobium (Nb) doping in the range of 0-4 at.% with 1 at.% steps. As shown by the x-ray diffraction patterns, the films exhibited a tetragonal cassiterite structure with (200) preferential orientation. It was observed that grain sizes of the films for (200) and (301) peaks depended on the Nb doping concentration and varied in the range of 25.11-32.19 and 100.6-183.7 nm, respectively. The scanning electron microscope (SEM) micrographs showed that the FTO films were made of small pyramidal grains, while doubly doped films were made of small pyramidal grains and big polyhedron grains. From electrical studies, although 1 at.% Nb doped FTO films have the lowest sheet resistance and resistivity values, the highest figure-of-merit and optical band gap values obtained for FTO films were 16.2 × 10-2 Ω-1 and 4.21 eV, respectively. Also, infrared reflectivity values of the films were in the range of 97.39-98.98%. These results strongly suggest that these films are an attractive candidate for various optoelectronic applications and for photothermal conversion of solar energy.

Superconductivity, pairing symmetry, and disorder in the doped topological insulator Sn 1 - x In x Te for x ≥ 0.10

DOE PAGES

Smylie, M. P.; Claus, H.; Kwok, W. -K.; ...

2018-01-19

In this paper, the temperature dependence of the London penetration depth Δλ(T) in the superconducting doped topological crystalline insulator Sn 1-xIn xTe was measured down to 450 mK for two different doping levels, x ≈ 0.45 (optimally doped) and x ≈ 0.10 (underdoped), bookending the range of cubic phase in the compound. The results indicate no deviation from fully gapped BCS-like behavior, eliminating several candidate unconventional gap structures. Critical field values below 1 K and other superconducting parameters are also presented. Finally, the introduction of disorder by repeated particle irradiation with 5 MeV protons does not enhance T c, indicatingmore » that ferroelectric interactions do not compete with superconductivity.« less

Superconductivity, pairing symmetry, and disorder in the doped topological insulator Sn 1 - x In x Te for x ≥ 0.10

DOE Office of Scientific and Technical Information (OSTI.GOV)

Smylie, M. P.; Claus, H.; Kwok, W. -K.

In this paper, the temperature dependence of the London penetration depth Δλ(T) in the superconducting doped topological crystalline insulator Sn 1-xIn xTe was measured down to 450 mK for two different doping levels, x ≈ 0.45 (optimally doped) and x ≈ 0.10 (underdoped), bookending the range of cubic phase in the compound. The results indicate no deviation from fully gapped BCS-like behavior, eliminating several candidate unconventional gap structures. Critical field values below 1 K and other superconducting parameters are also presented. Finally, the introduction of disorder by repeated particle irradiation with 5 MeV protons does not enhance T c, indicatingmore » that ferroelectric interactions do not compete with superconductivity.« less

Quantitative analysis of the Ge concentration in a SiGe quantum well: comparison of low-energy RBS and SIMS measurements.

PubMed

Krecar, D; Rosner, M; Draxler, M; Bauer, P; Hutter, H

2006-01-01

The germanium concentration and the position and thickness of the quantum well in molecular beam epitaxy (MBE)-grown SiGe were quantitatively analyzed via low-energy Rutherford backscattering (RBS) and secondary ion mass spectrometry (SIMS). In these samples, the concentrations of Si and Ge were assumed to be constant, except for the quantum well, where the germanium concentration was lower. The thickness of the analyzed quantum well was about 12 nm and it was situated at a depth of about 60 nm below the surface. A dip showed up in the RBS spectra due to the lower germanium concentration in the quantum well, and this was evaluated. Good depth resolution was required in order to obtain quantitative results, and this was obtained by choosing a primary energy of 500 keV and a tilt angle of 51 degrees with respect to the surface normal. Quantitative information was deduced from the raw data by comparing it with SIMNRA simulated spectra. The SIMS measurements were performed with oxygen primary ions. Given the response function of the SIMS instrument (the SIMS depth profile of the germanium delta (delta) layer), and using the forward convolution (point-to-point convolution) model, it is possible to determine the germanium concentration and the thickness of the analyzed quantum well from the raw SIMS data. The aim of this work was to compare the results obtained via RBS and SIMS and to show their potential for use in the semiconductor and microelectronics industry. The detection of trace elements (here the doping element antimony) that could not be evaluated with RBS in low-energy mode is also demonstrated using SIMS instead.

Structural, optical, and electrical properties of Ni-doped ZnO nanorod arrays prepared via sonicated sol-gel immersion method

NASA Astrophysics Data System (ADS)

Ismail, A. S.; Mamat, M. H.; Malek, M. F.; Saidi, S. A.; Yusoff, M. M.; Mohamed, R.; Sin, N. D. Md; Suriani, A. B.; Rusop, M.

2018-05-01

Nickel (Ni)-doped zinc oxide (ZnO) nanorod array films were synthesised using sonicated sol-gel immersion method. The FESEM images showed that the Ni-doped ZnO nanorod arrays possess hexagonal shape with average diameter about 120 nm and thickness about 1.10 µm. The Ni-doped ZnO nanorod arrays possess better transmittance properties with 3.27 eV of optical band gap energy and 40 meV of urbach energy. The current-voltage (I-V) measurement indicated that the conductivity of ZnO film slightly improved with Ni-doping. The doped film displayed good humidity sensing performance with sensitivity of 1.21.

Structural, Optical, and Electronic Characterization of Fe-Doped Alumina Nanoparticles

NASA Astrophysics Data System (ADS)

Heiba, Zein K.; Mohamed, Mohamed Bakr; Wahba, Adel Maher; Imam, N. G.

2018-01-01

The effects of iron doping on the structural, optical, and electronic properties of doped alumina have been studied. Single-phase iron-doped alumina Al2- x Fe x O3 ( x = 0.00 to 0.30) nanoparticles were synthesized via citrate-precursor method. Formation of single-phase hexagonal corundum structure with no other separate phases was demonstrated by x-ray diffraction (XRD) analysis and Fourier-transform infrared spectroscopy. The effects of iron doping on the α-Al2O3 structural parameters, viz. atomic coordinates, lattice parameters, crystallite size, and microstrain, were estimated from XRD data by applying the Rietveld profile fitting method. Transmission electron microscopy further confirmed the nanosize nature of the prepared samples with size ranging from 12 nm to 83 nm. The electronic band structure was investigated using density functional theory calculations to explain the decrease in the energy gap of Al2- x Fe x O3 as the amount of Fe was increased. The colored emission peaks in the visible region (blue, red, violet) of the electromagnetic spectrum obtained for the Fe-doped α-Al2O3 nanoparticles suggest their potential application as ceramic nanopigments.

Band Gap Engineering of Titania Systems Purposed for Photocatalytic Activity

NASA Astrophysics Data System (ADS)

Thurston, Cameron

Ab initio computer aided design drastically increases candidate population for highly specified material discovery and selection. These simulations, carried out through a first-principles computational approach, accurately extrapolate material properties and behavior. Titanium Dioxide (TiO2 ) is one such material that stands to gain a great deal from the use of these simulations. In its anatase form, titania (TiO2 ) has been found to exhibit a band gap nearing 3.2 eV. If titania is to become a viable alternative to other contemporary photoactive materials exhibiting band gaps better suited for the solar spectrum, then the band gap must be subsequently reduced. To lower the energy needed for electronic excitation, both transition metals and non-metals have been extensively researched and are currently viable candidates for the continued reduction of titania's band gap. The introduction of multicomponent atomic doping introduces new energy bands which tend to both reduce the band gap and recombination loss. Ta-N, Nb-N, V-N, Cr-N, Mo-N, and W-N substitutions were studied in titania and subsequent energy and band gap calculations show a favorable band gap reduction in the case of passivated systems.

Electronic excitations in electron-doped cuprate superconductors

NASA Astrophysics Data System (ADS)

Unger, P.; Fulde, P.

1995-04-01

We calculate the electronic single-particle spectrum of an electron-doped cuprate superconductor such as Nd2-xCexCuO4-y. The dynamics of holes in the Cu-O planes is described by the extended Hubbard or Emery model. We consider the system at half-filling (one hole per unit cell, nh=1) and in the case of electron doping where the ground state is paramagnetic. The projection technique of Mori and Zwanzig is applied to derive the equations of motion for the Green's functions of Cu and O holes. These equations are solved self-consistently as in a previous calculation, where we considered the case of hole doping. At half-filling the system exhibits a charge-transfer gap bounded by Zhang-Rice singlet states and the upper Hubbard band. Upon electron doping the upper Hubbard band crosses the Fermi level and the system becomes metallic. With increasing electron doping the singlet band loses intensity and finally vanishes for nh=0. The corresponding spectral weight is transferred to the upper Hubbard band, which becomes a usual tight-binding band for zero hole concentration. The shape of the flat band crossing the Fermi level fits well to angle-resolved photoemission spectra of Nd2-xCexCuO4-y for x=0.15 and 0.22. Furthermore, our findings are in excellent agreement with exact diagonalization studies of 2×2 CuO2 cluster with periodic boundary conditions.

Direct optical band gap measurement in polycrystalline semiconductors: A critical look at the Tauc method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dolgonos, Alex; Mason, Thomas O.; Poeppelmeier, Kenneth R., E-mail: krp@northwestern.edu

2016-08-15

The direct optical band gap of semiconductors is traditionally measured by extrapolating the linear region of the square of the absorption curve to the x-axis, and a variation of this method, developed by Tauc, has also been widely used. The application of the Tauc method to crystalline materials is rooted in misconception–and traditional linear extrapolation methods are inappropriate for use on degenerate semiconductors, where the occupation of conduction band energy states cannot be ignored. A new method is proposed for extracting a direct optical band gap from absorption spectra of degenerately-doped bulk semiconductors. This method was applied to pseudo-absorption spectramore » of Sn-doped In{sub 2}O{sub 3} (ITO)—converted from diffuse-reflectance measurements on bulk specimens. The results of this analysis were corroborated by room-temperature photoluminescence excitation measurements, which yielded values of optical band gap and Burstein–Moss shift that are consistent with previous studies on In{sub 2}O{sub 3} single crystals and thin films. - Highlights: • The Tauc method of band gap measurement is re-evaluated for crystalline materials. • Graphical method proposed for extracting optical band gaps from absorption spectra. • The proposed method incorporates an energy broadening term for energy transitions. • Values for ITO were self-consistent between two different measurement methods.« less

Laser annealed in-situ P-doped Ge for on-chip laser source applications (Conference Presentation)

NASA Astrophysics Data System (ADS)

Srinivasan, Ashwyn; Pantouvaki, Marianna; Shimura, Yosuke; Porret, Clement; Van Deun, Rik; Loo, Roger; Van Thourhout, Dries; Van Campenhout, Joris

2016-05-01

Realization of a monolithically integrated on-chip laser source remains the holy-grail of Silicon Photonics. Germanium (Ge) is a promising semiconductor for lasing applications when highly doped with Phosphorous (P) and or alloyed with Sn [1, 2]. P doping makes Ge a pseudo-direct band gap material and the emitted wavelengths are compatible with fiber-optic communication applications. However, in-situ P doping with Ge2H6 precursor allows a maximum active P concentration of 6×1019 cm-3 [3]. Even with such active P levels, n++ Ge is still an indirect band gap material and could result in very high threshold current densities. In this work, we demonstrate P-doped Ge layers with active n-type doping beyond 1020 cm-3, grown using Ge2H6 and PH3 and subsequently laser annealed, targeting power-efficient on-chip laser sources. The use of Ge2H6 precursors during the growth of P-doped Ge increases the active P concentration level to a record fully activated concentration of 1.3×1020 cm-3 when laser annealed with a fluence of 1.2 J/cm2. The material stack consisted of 200 nm thick P-doped Ge grown on an annealed 1 µm Ge buffer on Si. Ge:P epitaxy was performed with PH3 and Ge2H6 at 320oC. Low temperature growth enable Ge:P epitaxy far from thermodynamic equilibrium, resulting in an enhanced incorporation of P atoms [3]. At such high active P concentration, the n++ Ge layer is expected to be a pseudo-direct band gap material. The photoluminescence (PL) intensities for layers with highest active P concentration show an enhancement of 18× when compared to undoped Ge grown on Si as shown in Fig. 1 and Fig. 2. The layers were optically pumped with a 640 nm laser and an incident intensity of 410 mW/cm2. The PL was measured with a NIR spectrometer with a Hamamatsu R5509-72 NIR photomultiplier tube detector whose detectivity drops at 1620 nm. Due to high active P concentration, we expect band gap narrowing phenomena to push the PL peak to wavelengths beyond the detection limit

Enhanced photoelectrochemical and optical performance of ZnO films tuned by Cr doping

NASA Astrophysics Data System (ADS)

Salem, M.; Akir, S.; Massoudi, I.; Litaiem, Y.; Gaidi, M.; Khirouni, K.

2017-04-01

In this paper, pure and Cr-doped nanostructured Zinc oxide thin films were synthesized by simple and low cost co-precipitation and spin-coating method with Cr concentration varying between 0.5 and 5 at.%. Crystalline structure of the prepared films was investigated by X-ray diffraction (XRD) and Raman spectroscopy techniques. XRD analysis indicated that the films were indexed as the hexagonal phase of wurtzite-type structure and demonstrated a decrease in the crystallite size with increasing Cr doping content. Cr doping revealed a significant effect on the optical measurements such as transmission and photoluminescence properties. The optical measurements indicated that Cr doping decreases the optical band gap and it has been shifted from 3.41 eV for pure ZnO film to 3.31 eV for 5 at.% Cr-doped one. The photoelectrochemical (PEC) sensing characteristics of Cr-doped ZnO layers were investigated. Amongst all photo-anodes with different Cr dopant concentration, the 2 at.% Cr incorporated ZnO films exhibited fast response and higher photoconduction sensitivity.

Effects of Si-doping on magnetic properties of Ga1-xCrxN

NASA Astrophysics Data System (ADS)

Zhou, Zhongpo; Yang, Zongxian; Liu, Chang

2015-01-01

Ga1-xCrxN thin films with and without the Si doping have been prepared by molecular beam epitaxy. The samples have been investigated by X-ray diffraction, X-ray photoemission spectroscopy, photoluminescence, optical absorption spectra and magnetic measurements. It has been confirmed that for the undoped samples Cr in GaN is predominantly trivalent when substituting for Ga and that the Cr 3d state appears within the band gap of GaN. In Si doped specimens the upward shifts of the chemical potential are observed, and the electrons supplied by the Si doping are trapped at Cr sites forming Cr2+. As a result, the Si doping effects show an increase of the Curie temperature, and a reduction of the saturation magnetization in the Ga1-xCrxN:Si samples. The significant effect on the ferromagnetism with Si doping in Ga1-xCrxN is explained by the percolation theory of bound magnetic polarons.

Micromachined single-level nonplanar polycrystalline SiGe thermal microemitters for infrared dynamic scene projection

NASA Astrophysics Data System (ADS)

Malyutenko, V. K.; Malyutenko, O. Yu.; Leonov, V.; Van Hoof, C.

2009-05-01

The technology for self-supported membraneless polycrystalline SiGe thermal microemitters, their design, and performance are presented. The 128-element arrays with a fill factor of 88% and a 2.5-μm-thick resonant cavity have been grown by low-pressure chemical vapor deposition and fabricated using surface micromachining technology. The 200-nm-thick 60×60 μm2 emitting pixels enforced with a U-shape profile pattern demonstrate a thermal time constant of 2-7 ms and an apparent temperature of 700 K in the 3-5 and 8-12 μm atmospheric transparency windows. The application of the devices to the infrared dynamic scene simulation and their benefit over conventional planar membrane-supported emitters are discussed.

Gap state charge induced spin-dependent negative differential resistance in tunnel junctions

NASA Astrophysics Data System (ADS)

Jiang, Jun; Zhang, X.-G.; Han, X. F.

2016-04-01

We propose and demonstrate through first-principles calculation a new spin-dependent negative differential resistance (NDR) mechanism in magnetic tunnel junctions (MTJ) with cubic cation disordered crystals (CCDC) AlO x or Mg1-x Al x O as barrier materials. The CCDC is a class of insulators whose band gap can be changed by cation doping. The gap becomes arched in an ultrathin layer due to the space charge formed from metal-induced gap states. With an appropriate combination of an arched gap and a bias voltage, NDR can be produced in either spin channel. This mechanism is applicable to 2D and 3D ultrathin junctions with a sufficiently small band gap that forms a large space charge. It provides a new way of controlling the spin-dependent transport in spintronic devices by an electric field. A generalized Simmons formula for tunneling current through junction with an arched gap is derived to show the general conditions under which ultrathin junctions may exhibit NDR.

Electrochemically synthesis and optoelectronic properties of Pb- and Zn-doped nanostructured SnSe films

NASA Astrophysics Data System (ADS)

Jamali-Sheini, Farid; Cheraghizade, Mohsen; Yousefi, Ramin

2018-06-01

In this study, electrodeposition technique was applied to deposit un-, lead (Pb), and zinc (Zn)-doped SnSe films. X-ray diffraction (XRD) patterns of the films showed a polycrystalline SnSe phase with orthorhombic crystalline lattice. SEM images revealed ball-shaped, rod-shaped, and wire-shaped morphologies for SnSe films. Moreover, optical measurements indicated incorporation of dopant in the crystalline lattice of films by varying the optical energy band gap. Electrical characterization of Pb- and Zn-doped SnSe films showed their p-type nature. Finally, the solar cell device fabricated using the Zn-doped SnSe films reveal a higher efficiency because of their higher carrier concentration.

Ultrasonic synthesis of In-doped SnS nanoparticles and their physical properties

NASA Astrophysics Data System (ADS)

Jamali-Sheini, Farid; Cheraghizade, Mohsen; Yousefi, Ramin

2018-05-01

Indium (In)-doped Tin (II) Sulfide (SnS) nanoparticles (NPs) were synthesized by an ultra-sonication method and their optical, electrical, dielectric and photocatalytic properties were investigated. XRD patterns of the obtained NPs indicated formation of orthorhombic polycrystalline SnS. Field emission scanning electron microscopy exhibited flower-like NPs with particle sizes below 100 nm for both SnS and In-doped SnS samples. Optical analysis showed a decrease in energy band gap of SnS NPs upon In doping. In addition, electrical results demonstrated p-type nature of the synthesized SnS NPs and enhanced electrical conductivity of the NPs due to increased tin vacancy. Dielectric experiments on SnS NPs suggested an electronic polarizations effect to be responsible for changing dielectric properties of the particles, in terms of frequency. Finally, photocatalytic experiments revealed that high degradation power can be obtained using In-doped SnS NPs.

Formation of Sol Gel Dried Droplets of Carbon Doped Titanium Dioxide (TiO2) at Low Temperature via Electrospraying

NASA Astrophysics Data System (ADS)

Halimi, S. U.; Hashib, S. Abd; Abu Bakar, N. F.; Ismail, S. N.; Nazli Naim, M.; Rahman, N. Abd; Krishnan, J.

2018-05-01

The high band gap energy of TiO2 and inconsistency in particles size has imposed a significant drawback on TiO2 applications. Dried droplets of carbon-doped TiO2 fine particles were produced by using electrospraying technique. The C-doped TiO2 particles were prepared by hydrolysis of titanium isopropoxide with the addition of carbon precursor followed by electrospraying the suspension in stable Taylor cone-jet mode. Coulomb fission of charged droplets from the electrospraying technique successfully transformed dispersed liquid C-doped TiO2 particles into solid. The deposited C-doped TiO2 droplets were collected on aluminium substrates placed at working distances of 10 to 20 cm from the tip of the electrospray needle. The collected C-doped TiO2 droplets were characterized by using FESEM, UV-Vis, FTIR and XRD. By increasing the working distance, the average droplets size of the deposited C-doped TiO2 was reduced from ±163.2 nm to ±147.56 nm. UV-Vis analysis showed a strong absorption in the visible-light region and about 93 nm red shift of the onset spectrum for C-doped TiO2. The red shift indicates an increase in photocatalytic efficiency by reducing the TiO2 band gap energy from 3.0 eV to 2.46 eV and shifting its activity to the visible-light region. FTIR analysis indicated the presence of Ti-C and C-O chemical bonding in the C-doped TiO2.

The structural, electronic and optical properties of Nd doped ZnO using first-principles calculations

NASA Astrophysics Data System (ADS)

Wen, Jun-Qing; Zhang, Jian-Min; Chen, Guo-Xiang; Wu, Hua; Yang, Xu

2018-04-01

The density functional theory calculations using general gradient approximation (GGA) applying Perdew-Burke-Ernzerhof (PBE) as correlation functional have been systematically performed to research the formation energy, the electronic structures, band structures, total and partial DOS, and optical properties of Nd doping ZnO with the content from 6.25% to 12.5%. The formation energies are negative for both models, which show that two structures are energetically stable. Nd doping ZnO crystal is found to be a direct band gap semiconductor and Fermi level shifts upward into conduction band, which show the properties of n-type semiconductor. Band structures are more compact after Nd doping ZnO, implying that Nd doping induces the strong interaction between different atoms. Nd doping ZnO crystal presents occupied states at near Fermi level, which mainly comes from the Nd 4f orbital. The calculated optical properties imply that Nd doping causes a red-shift of absorption peaks, and enhances the absorption of the visible light.

Doping induced c-axis oriented growth of transparent ZnO thin film

NASA Astrophysics Data System (ADS)

Mistry, Bhaumik V.; Joshi, U. S.

2018-04-01

c-Axis oriented In doped ZnO (IZO) transparent conducting thin films were optimized on glass substrate using sol gel spin coating method. The Indium content in ZnO was varied systematically and the structural parameters were studied. Along with the crystallographic properties, the optoelectronic and electrical properties of IZO thin films were investigated in detail. The IZO thin films revealed hexagonal wurtzite structure. It was found that In doping in ZnO promotes the c-axis oriented growth of the thin films deposited on amorphous substrate. The particle size of the IZO films were increase as doping content increases from 2% to 5%. The 2% In doped ZnO film show electrical resistivity of 0.11 Ω cm, which is far better than the reported value for ZnO thin film. Better than 75% average optical transmission was estimated in the wavelength range from 400-800 nm. Systematic variartions in the electron concentration and band gap was observed with increasing In doping. Note worthy finding is that, with suitable amount of In doping improves not only transparency and conductivity but also improves the preferred orientation of the oxide thin film.

High temperature ferromagnetism in Ni doped ZnO nanoparticles: Milling time dependence

NASA Astrophysics Data System (ADS)

Pal, Bappaditya; Giri, P. K.; Sarkar, D.

2014-04-01

We report on the room temperature ferromagnetism (RT FM) in the Zn1-xNixO (x = 0, 0.03, and 0.05) nanoparticles (NPs) synthesized by a ball milling technique. X-ray diffraction analysis confirms the single crystalline ZnO wurtzite structure with presence of small intensity secondary phase related peak which disappear with increasing milling time for Ni doped samples. HRTEM lattice images show that the doped NPs are single crystalline with a dspacing of 2.44 Å. Energy-dispersive X-ray spectroscopy analysis confirms the presence of Ni ions in the ZnO matrix. Magnetic measurement (RT) exhibits the hysteresis loop with saturation magnetization (Ms) of 1.6-2.56 (emu/g) and coercive field (Hc) of 296-322 Oe. M-T measurement shows a Curie temperature of the order of 325°C for 3% Ni doped sample. Micro -Raman studies show doping/disorder induced additional modes at ˜510, 547, 572 cm-1 in addition to 437 cm-1 peak of pure ZnO. UV-Vis absorption spectra illustrate band gap shift due to doping. Alteration of Ms value with the variation of doping concentration and milling time has been studied and discussed.

Optical properties of rhodamine 6G-doped TiO2 sol-gel films

NASA Astrophysics Data System (ADS)

Tomás, S. A.; Stolik, S.; Palomino, R.; Lozada, R.; Persson, C.; Ahuja, R.; Pepe, I.; Ferreira da Silva, A.

2005-06-01

The optical properties of titania (TiO2) thin films prepared by the sol-gel process and doped with rhodamine 6G were studied by Photoacoustic Spectroscopy. Rhodamine 6G-doping was achieved by adding 0.01%, 0.02%, 0.05% y 0.1% mol rhodamine to a solution that contained titanium isopropoxide as precursor. Two absorption regions were distinguished in the absorption spectrum of a typical rhodamine 6G-doped TiO2 film. A shift of these bands occured as a function of rhodamine 6G-doping concentration. In addition, the optical absorption and band gap energy for rutile-phase TiO2 films were calculated employing the full-potential linearized augmented plane wave method. A comparison of these calculations with experimental data of TiO2 films prepared by sol-gel at room temperature was performed.

Doping of TiO 2 Polymorphs for Altered Optical and Photocatalytic Properties

DOE PAGES

Nie, Xiliang; Zhuo, Shuping; Maeng, Gloria; ...

2009-01-01

Tmore » his paper reviews recent investigations of the influence of dopants on the optical properties of iO 2 polymorphs. he common undoped polymorphs of iO 2 are discussed and compared. he results of recent doping efforts are tabulated, and discussed in the context of doping by elements of the same chemical group. Dopant effects on the band gap and photocatalytic activity are interpreted with reference to a simple qualitative picture of the iO 2 electronic structure, which is supported with first-principles calculations.« less

Tunable photonic crystals with partial bandgaps from blue phase colloidal crystals and dielectric-doped blue phases.

PubMed

Stimulak, Mitja; Ravnik, Miha

2014-09-07

Blue phase colloidal crystals and dielectric nanoparticle/polymer doped blue phases are demonstrated to combine multiple components with different symmetries in one photonic material, creating a photonic crystal with variable and micro-controllable photonic band structure. In this composite photonic material, one contribution to the band structure is determined by the 3D periodic birefringent orientational profile of the blue phases, whereas the second contribution emerges from the regular array of the colloidal particles or from the dielectric/nanoparticle-doped defect network. Using the planewave expansion method, optical photonic bands of the blue phase I and II colloidal crystals and related nanoparticle/polymer doped blue phases are calculated, and then compared to blue phases with no particles and to face-centred-cubic and body-centred-cubic colloidal crystals in isotropic background. We find opening of local band gaps at particular points of Brillouin zone for blue phase colloidal crystals, where there were none in blue phases without particles or dopants. Particle size and filling fraction of the blue phase defect network are demonstrated as parameters that can directly tune the optical bands and local band gaps. In the blue phase I colloidal crystal with an additionally doped defect network, interestingly, we find an indirect total band gap (with the exception of one point) at the entire edge of SC irreducible zone. Finally, this work demonstrates the role of combining multiple - by symmetry - differently organised components in one photonic crystal material, which offers a novel approach towards tunable soft matter photonic materials.

First-principles study of the heavy metal atoms X (X=Au, Hg, Tl or Pb) doped monolayer WS2

NASA Astrophysics Data System (ADS)

Xie, Ling-Yun; Zhang, Jian-Min

2017-12-01

The heavy metal atoms X (X = Au, Hg, Tl or Pb) doped monolayer WS2 systems have been studied by using the spin-polarized first-principles calculations. Although pure monolayer WS2 system is a nonmagnetic semiconductor with a direct band gap of 1.820 eV, the Au and Hg atoms doped monolayer WS2 systems change to half-metal (HM) ferromagnets with the total magnetic moments 0.697 and 1.776 μB as well as the smaller spin-down gaps 0.605 and 0.527 eV, respectively, while the Tl and Pb atoms doped monolayer WS2 systems change to magnetic metal with the total magnetic moment 0.584 μB and a nonmagnetic metal. From the minimization of the formation energy, we find that it is easy to incorporate these heavy metal atoms into monolayer WS2 system under S-rich condition, especially for the Au doped monolayer WS2 system not only easily to be formed but also a HM ferromagnet, and thus the best candidate used in the spintronic devices.

Magnetic hard gap due to bound magnetic polarons in the localized regime.

PubMed

Rimal, Gaurab; Tang, Jinke

2017-02-08

We investigate the low temperature electron transport properties of manganese doped lead sulfide films. The system shows variable range hopping at low temperatures that crosses over into an activation regime at even lower temperatures. This crossover is destroyed by an applied magnetic field which suggests a magnetic origin of the hard gap, associated with bound magnetic polarons. Even though the gap forms around the superconducting transition temperature of lead, we do not find evidence of this being due to insulator-superconductor transition. Comparison with undoped PbS films, which do not show the activated transport behavior, suggests that bound magnetic polarons create the hard gap in the system that can be closed by magnetic fields.

Fluorine-doped NiO nanostructures: Structural, morphological and spectroscopic studies

NASA Astrophysics Data System (ADS)

Singh, Kulwinder; Kumar, Manjeet; Singh, Dilpreet; Singh, Manjinder; Singh, Paviter; Singh, Bikramjeet; Kaur, Gurpreet; Bala, Rajni; Thakur, Anup; Kumar, Akshay

2018-05-01

Nanostructured NiO has been prepared by co-precipitation method. In this study, the effect of fluorine doping (1, 3 and 5 wt. %) on the structural, morphological as well as optical properties of NiO nanostructures has been studied. X-ray diffraction (XRD) has employed for studying the structural properties. Cubic crystal structure of NiO was confirmed by the XRD analysis. Crystallite size increased with increase in doping concentration. Nelson-Riley factor (NRF) analysis indicated the presence of defect states in the synthesized samples. Field emission scanning electron microscopy showed the spherical morphology of the synthesized samples and also revealed that the particle size varied with dopant content. The optical properties were studied using UV-Visible Spectroscopy. The results indicated that the band gap energy of the synthesized nanostructures decreased with increase in doping concentration upto 3% but increased as the doping concentration was further raised to 5%. This can be ascribed to the defect states variations in the synthesized samples. The results suggested that the synthesized nanostructures are promising candidate for optoelectronic as well as gas sensing applications.

Nernst effect in electron-doped Pr2-xCexCuO4

NASA Astrophysics Data System (ADS)

Balci, Hamza; Hill, C. P.; Qazilbash, M. M.; Greene, R. L.

2003-08-01

The Nernst effect of Pr2-xCexCuO4 (x=0.13, 0.15, and 0.17) has been measured on thin film samples between 5 120 K and 0 14 T. In comparison to recent measurements on hole-doped cuprates that showed an anomalously large Nernst effect above the resistive Tc and Hc2 [Z. A. Xu et al., Nature (London) 406, 486 (2000); Yayu Wang et al., Phys. Rev. B 64, 224519 (2001); Yayu Wang et al., Phys. Rev. Lett. 88, 257003 (2002); C. Caplan et al., ibid 88, 056601 (2002)], we find a normal Nernst effect above Tc and Hc2 for all dopings. The lack of an anomalous Nernst effect in the electron-doped compounds supports the models that explain this effect in terms of amplitude and phase fluctuations in the hole-doped cuprates. In addition, the Hc2(T) determined from the Nernst effect shows a conventional behavior for all dopings. The energy gap determined from Hc2(0) decreases as the system goes from underdoping to overdoping in agreement with the recent tunneling experiments.

Role of Zn doping in oxidative stress mediated cytotoxicity of TiO2 nanoparticles in human breast cancer MCF-7 cells

NASA Astrophysics Data System (ADS)

Ahamed, Maqusood; Khan, M. A. Majeed; Akhtar, Mohd Javed; Alhadlaq, Hisham A.; Alshamsan, Aws

2016-07-01

We investigated the effect of Zn-doping on structural and optical properties as well as cellular response of TiO2 nanoparticles (NPs) in human breast cancer MCF-7 cells. A library of Zn-doped (1-10 at wt%) TiO2 NPs was prepared. Characterization data indicated that dopant Zn was incorporated into the lattice of host TiO2. The average particle size of TiO2 NPs was decreases (38 to 28 nm) while the band gap energy was increases (3.35 eV-3.85 eV) with increasing the amount of Zn-doping. Cellular data demonstrated that Zn-doped TiO2 NPs induced cytotoxicity (cell viability reduction, membrane damage and cell cycle arrest) and oxidative stress (reactive oxygen species generation & glutathione depletion) in MCF-7 cells and toxic intensity was increases with increasing the concentration of Zn-doping. Molecular data revealed that Zn-doped TiO2 NPs induced the down-regulation of super oxide dismutase gene while the up-regulation of heme oxygenase-1 gene in MCF-7 cells. Cytotoxicity induced by Zn-doped TiO2 NPs was efficiently prevented by N-acetyl-cysteine suggesting that oxidative stress might be the primarily cause of toxicity. In conclusion, our data indicated that Zn-doping decreases the particle size and increases the band gap energy as well the oxidative stress-mediated toxicity of TiO2 NPs in MCF-7 cells.

Role of Zn doping in oxidative stress mediated cytotoxicity of TiO2 nanoparticles in human breast cancer MCF-7 cells

PubMed Central

Ahamed, Maqusood; Khan, M. A. Majeed; Akhtar, Mohd Javed; Alhadlaq, Hisham A.; Alshamsan, Aws

2016-01-01

We investigated the effect of Zn-doping on structural and optical properties as well as cellular response of TiO2 nanoparticles (NPs) in human breast cancer MCF-7 cells. A library of Zn-doped (1–10 at wt%) TiO2 NPs was prepared. Characterization data indicated that dopant Zn was incorporated into the lattice of host TiO2. The average particle size of TiO2 NPs was decreases (38 to 28 nm) while the band gap energy was increases (3.35 eV–3.85 eV) with increasing the amount of Zn-doping. Cellular data demonstrated that Zn-doped TiO2 NPs induced cytotoxicity (cell viability reduction, membrane damage and cell cycle arrest) and oxidative stress (reactive oxygen species generation & glutathione depletion) in MCF-7 cells and toxic intensity was increases with increasing the concentration of Zn-doping. Molecular data revealed that Zn-doped TiO2 NPs induced the down-regulation of super oxide dismutase gene while the up-regulation of heme oxygenase-1 gene in MCF-7 cells. Cytotoxicity induced by Zn-doped TiO2 NPs was efficiently prevented by N-acetyl-cysteine suggesting that oxidative stress might be the primarily cause of toxicity. In conclusion, our data indicated that Zn-doping decreases the particle size and increases the band gap energy as well the oxidative stress-mediated toxicity of TiO2 NPs in MCF-7 cells. PMID:27444578

Modification of band gaps and optoelectronic properties of binary calcium chalcogenides by means of doping of magnesium atom(s) in rock-salt phase- a first principle based theoretical initiative

NASA Astrophysics Data System (ADS)

Debnath, Bimal; Sarkar, Utpal; Debbarma, Manish; Bhattacharjee, Rahul; Chattopadhyaya, Surya

2018-02-01

The band gaps and optoelectronic properties of binary calcium chalcogenide semiconductors have been modified theoretically by doping magnesium atom(s) into their respective rock-salt unit cells at some specific concentrations x = 0.0, 0.25, 0.50, 0.75 and 1.0 and confirmed such modifications by studying their structural, electronic and optical properties using DFT based FP-LAPW approach. The WC-GGA functional is used to calculate structural properties, while mBJ, B3LYP and WC-GGA are used for calculating electronic and optical properties. The concentration dependences of lattice parameter, bulk modulus and fundamental band gap for each alloy system exhibit nonlinearity. The atomic and orbital origin of different electronic states in the band structure of each compound are explored from its density of states (DOS). The microscopic origin of band gap bowing for each of the alloy systems is explored in terms of volume deformation, charge exchange and structural relaxation. The chemical bonds between the constituent atoms in each compound are found as ionic in nature. Optical properties of each specimen are calculated from its computed spectra of dielectric function, refractive index, extinction coefficient, normal incidence reflectivity, optical conductivity, optical absorption and energy loss function. Several calculated results have been compared with available experimental and other theoretical data.

Structural and optical properties of chromium doped zinc oxide nanoparticles synthesized by sol-gel method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Naqvi, Syed Mohd. Adnan, E-mail: adiaks2004@yahoo.co.in; Irshad, Kashif, E-mail: alig.kashif@gmail.com; Soleimani, Hassan, E-mail: hassan.soleimani@petronas.com.my, E-mail: noorhana-yahya@petronas.com.my

2014-10-24

Nanosized Cr-doped ZnO nano particles were synthesized by facile sol-gel auto combustion method. The structural and optical properties of Cr-doped ZnO nanoparticles have been investigated by XRD and UV-Vis spectroscopy at room temperature for 0% to 8% concentration. X-ray diffraction analysis reveals that the Cr-doped ZnO crystallizes in a single phase polycrystalline nature with wurtzite lattice. With every % of doping, the peaks are shifting scarcely and doping of Cr is possible up to 7%. After that, the last peak vanishes, that signifies its structure is transmuted from 8% doping. The average crystallite size decreases with increase in Cr concentrationmore » (i.e. 28.9 nm for 0% to 25.8 nm for 8%). The UV-Vis spectra of the nanoparticles betoken an incrementation in the band gap energy from 3.401, 3.415, 3.431, 3.437,3.453, 3.514,3.521, 3.530 and 3.538 eV respectively, for 0,1, 2, 3, 4, 5, 6, 7 and 8 % doping concentration.« less

Low temperature solution synthesis of silicon, germanium and Si-Ge axial heterostructures in nanorod and nanowire form.

PubMed

Flynn, G; Stokes, K; Ryan, K M

2018-05-31

Herein, we report the formation of silicon, germanium and more complex Si-SixGe1-x and Si-Ge axial 1D heterostructures, at low temperatures in solution. These nanorods/nanowires are grown using phenylated compounds of silicon and germanium as reagents, with precursor decomposition achieved at substantially reduced temperatures (200 °C for single crystal nanostructures and 300 °C for heterostructures), through the addition of a reducing agent. This low energy route for the production of these functional nanostructures as a wet chemical in high yield is attractive to meet the processing needs for next generation photovoltaics, batteries and electronics.

Band gap engineering of BC2N for nanoelectronic applications

NASA Astrophysics Data System (ADS)

Lim, Wei Hong; Hamzah, Afiq; Ahmadi, Mohammad Taghi; Ismail, Razali

2017-12-01

The BC2N as an example of boron-carbon-nitride (BCN), has the analogous structure as the graphene and boron nitride. It is predicted to have controllable electronic properties. Therefore, the analytical study on the engineer-able band gap of the BC2N is carried out based on the schematic structure of BC2N. The Nearest Neighbour Tight Binding (NNTB) model is employed with the dispersion relation and the density of state (DOS) as the main band gap analysing parameter. The results show that the hopping integrals having the significant effect on the band gap, band structure and DOS of BC2N nanowire (BC2NNW) need to be taken into consideration. The presented model indicates consistent trends with the published computational results around the Dirac points with the extracted band gap of 0.12 eV. Also, it is distinguished that wide energy gap of boron nitride (BN) is successfully narrowed by this carbon doped material which assures the application of BC2N on the nanoelectronics and optoelectronics in the near future.

SiGe:C Heterojunction Bipolar Transistors: From Materials Research to Chip Fabrication

NASA Astrophysics Data System (ADS)

Ruecker, H.; Heinemann, B.; Knoll, D.; Ehwald, K.-E.

Incorporation of substitutional carbon ( ~10^20 cm^-3) into the SiGe region of a heterojunction bipolar transistor (HBT) strongly reduces boron diffusion during device processing. We describe the physical mechanism behind the suppression of B diffusion in C-rich Si and SiGe, and explain how the increased thermal stability of doping profiles in SiGe:C HBTs can be used to improve device performance. Manufacturability of SiGe:C HBTs with transit frequencies of 100 GHz and maximum oscillation frequencies of 130 GHz is demonstrated in a BiCMOS technology capable of fabricating integrated circuits for radio frequencies with high yield.

Hole polarons and p -type doping in boron nitride polymorphs

NASA Astrophysics Data System (ADS)

Weston, L.; Wickramaratne, D.; Van de Walle, C. G.

2017-09-01

Boron nitride polymorphs hold great promise for integration into electronic and optoelectronic devices requiring ultrawide band gaps. We use first-principles calculations to examine the prospects for p -type doping of hexagonal (h -BN ), wurtzite (w z -BN ), and cubic (c -BN ) boron nitride. Group-IV elements (C, Si) substituting on the N site result in a deep acceptor, as the atomic levels of the impurity species lie above the BN valence-band maximum. On the other hand, group-II elements (Be, Mg) substituting on the B site do not give impurity states in the band gap; however, these dopants lead to the formation of small hole polarons. The tendency for polaron formation is far more pronounced in h -BN compared to w z -BN or c -BN . Despite forming small hole polarons, Be acceptors enable p -type doping, with ionization energies of 0.31 eV for w z -BN and 0.24 eV for c -BN ; these values are comparable to the Mg ionization energy in GaN.

Superconducting properties of under- and over-doped BaxK1‑xBiO3 perovskite oxide

NASA Astrophysics Data System (ADS)

Szczȩśniak, D.; Kaczmarek, A. Z.; Szczȩśniak, R.; Turchuk, S. V.; Zhao, H.; Drzazga, E. A.

2018-06-01

In this study, we investigate the thermodynamic properties of the BaxK1‑xBiO3 (BKBO) superconductor in the under- (x = 0.5) and over-doped (x = 0.7) regime, within the framework of the Migdal-Eliashberg formalism. The analysis is conducted to verify that the electron-phonon pairing mechanism is responsible for the induction of the superconducting phase in the mentioned compound. In particular, we show that BKBO is characterized by the relatively high critical value of the Coulomb pseudopotential, which changes with doping level and does not follow the Morel-Anderson model. In what follows, the corresponding superconducting band gap size and related dimensionless ratio are estimated to increase with the doping, in agreement with the experimental predictions. Moreover, the effective mass of electrons is found to take on high values in the entire doping and temperature region. Finally, the characteristic dimensionless ratios for the superconducting band gap, the critical magnetic field and the specific heat for the superconducting state are predicted to exceed the limits set within the Bardeen-Cooper-Schrieffer theory, suggesting pivotal role of the strong-coupling and retardation effects in the analyzed compound. Presented results supplement our previous investigations and account for the strong-coupling phonon-mediated character of the superconducting phase in BKBO at any doping level.

Electronic, magnetic and transport properties of transition metal-doped holely C2N-h2D nanoribbons

NASA Astrophysics Data System (ADS)

He, Jing-Jing; Guo, Yan-Dong; Yan, Xiao-Hong; Zeng, Hong-Li

2018-01-01

A novel layered two-dimensional graphene-like material C2N-h2D with evenly distributed holes and nitrogen atoms has been synthesized via a bottom-up wet-chemical reaction [Nat. Commun. 6, 6486 (2015)]. The presence of holes provides a ground for further functionalization by doping. By performing a first-principles study, we have doped transition metals at the center of the holes of C2N-h2D nanoribbons and explored their doping effects on electronic, magnetic and transport properties. It is found that the doping can essentially regulate the electronic properties of C2N-h2D nanoribbons. The metallic zigzag ribbon is tuned into a semiconductor for Mn, Fe and Co-doped cases, but half-metal for Ni-doping. This transition is derived from the peculiar band morphology which has a big band gap between the edge state and the higher band, so when the energy of the edge state is reduced by the impurity state, the band gap falls too and crosses the Fermi level. In contrast, the pristine semiconducting armchair C2N-h2D nanoribbon is changed into metallic. Different from the zigzag case, its physical mechanism originates from the hybridization of 3 d orbitals of transition metal atoms and the p orbitals of carbon and nitrogen atoms which introduces several resonant peaks at the Fermi level in the density of states. Furthermore, the magnetic moments of all doped materials are enhanced compared to the pristine structures but decrease as the atomic number of the transition metal atom increases. And the spin polarization of armchair C2N-h2D nanoribbon is increased, while that of the zigzag structure is decreased except the Ni-doped one which is completely spin-polarized suggesting great prospects in the future of spintronics and nanoelectronics.

Structural and optical properties of Ag-doped copper oxide thin films on polyethylene napthalate substrate prepared by low temperature microwave annealing

NASA Astrophysics Data System (ADS)

Das, Sayantan; Alford, T. L.

2013-06-01

Silver doped cupric oxide thin films are prepared on polyethylene naphthalate (flexible polymer) substrates. Thin films Ag-doped CuO are deposited on the substrate by co-sputtering followed by microwave assisted oxidation of the metal films. The low temperature tolerance of the polymer substrates led to the search for innovative low temperature processing techniques. Cupric oxide is a p-type semiconductor with an indirect band gap and is used as selective absorption layer solar cells. X-ray diffraction identifies the CuO phases. Rutherford backscattering spectrometry measurements confirm the stoichiometry of each copper oxide formed. The surface morphology is determined by atomic force microscopy. The microstructural properties such as crystallite size and the microstrain for (-111) and (111) planes are calculated and discussed. Incorporation of Ag led to the lowering of band gap in CuO. Consequently, it is determined that Ag addition has a strong effect on the structural, morphological, surface, and optical properties of CuO grown on flexible substrates by microwave annealing. Tauc's plot is used to determine the optical band gap of CuO and Ag doped CuO films. The values of the indirect and direct band gap for CuO are found to be 2.02 eV and 3.19 eV, respectively.

Using a double-doping strategy to improve physical properties of nanostructured CdO films

NASA Astrophysics Data System (ADS)

Aydin, R.; Sahin, B.

2018-06-01

In this present study nanostructured dually doped samples of Cd1‑x‑yMgxMyO (M: Sn, Pb, Bi) are synthesized by SILAR method. The effects of the mono and dual doping on the structural, morphological and optoelectronic characteristics of CdO nanoparticles are examined. The SEM images verify that deposited CdO films are nano-sized. Also the SEM computations demonstrated that the morphological surface structures of the films were influenced from the Mg mono doping and (Mg, Sn), (Mg, Pb) and (Mg, Bi) dual doping. The XRD designs specified that all the CdO samples have polycrystalline structure exhibiting cubic crystal form with dominant peaks of (111) and (220). The results display that Mg and (Mg, Sn), (Mg, Pb) and (Mg, Bi) ions were successfully doped into CdO film matrix. The UV spectroscopy results show that the optical energy band gap of the CdO films, ranging from 2.21 to 2.66 eV, altered with the dopant materials.

Measurements of undoped accumulation-mode SiGe quantum dot devices

NASA Astrophysics Data System (ADS)

Eng, Kevin; Borselli, Mathew; Holabird, Kevin; Milosavljevic, Ivan; Schmitz, Adele; Deelman, Peter; Huang, Biqin; Sokolich, Marko; Warren, Leslie; Hazard, Thomas; Kiselev, Andrey; Ross, Richard; Gyure, Mark; Hunter, Andrew

2012-02-01

We report transport measurements of undoped single-well accumulation-mode SiGe quantum dot devices with an integrated dot charge sensor. The device is designed so that individual forward-biased circular gates have dominant control of dot charge occupancy, and separate intervening gates have dominant control of tunnel rates and exchange coupling. We have demonstrated controlled loading of the first electron in single and double quantum dots. We used magneto-spectroscopy to measure singlet-triplet splittings in our quantum dots: values are typically ˜0.1 meV. Tunnel rates of single electrons to the baths can be controlled from less than 1 Hz to greater than 10 MHz. We are able to control the (0,2) to (1,1) coupling in a double quantum dot from under-coupled (tc < kT˜ 5μeV) to over-coupled (tc ˜ 0.1 meV) with a bias control of one exchange gate. Sponsored by the United States Department of Defense. Approved for Public Release, Distribution Unlimited. The views expressed are those of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government.

Structural and electrical investigations of MBE-grown SiGe nanoislands

NASA Astrophysics Data System (ADS)

Şeker, İsa; Karatutlu, Ali; Gürbüz, Osman; Yanık, Serhat; Bakış, Yakup; Karakız, Mehmet

2018-01-01

SiGe nanoislands were grown by Molecular Beam Epitaxy (MBE) method on Si (100) substrates with comparative growth parameters such as annealing temperature, top Ge content and layer-by-layer annealing (LBLA). XRD and Raman data suggest that annealing temperature, top Ge content and layer-by-layer annealing (LBLA) can overall give a control not only over the amorphous content but also over yielding the strained Ge layer formation in addition to mostly Ge crystallites. Depending on the layer design and growth conditions, size of the crystallites was observed to be changed. Four Point Probe (FPP) Method via Semiconductor Analyzer shows that 100 °C rise in annealing temperature of the samples with Si0.25Ge0.75 top layers caused rougher islands with vacancies which further resulted in the formation of laterally higher resistive thin film sheets. However, vertically performed I-AFM analysis produced higher I-V values which suggest that the vertical and horizantal conductance mechanisms appear to be different. Ge top-layered samples gained greater crystalline structure and better surface conductivity where LBLA resulted in the formation of Ge nucleation and tight 2D stacking resulting in enhanced current values.

Si/Ge double-layered nanotube array as a lithium ion battery anode.

PubMed

Song, Taeseup; Cheng, Huanyu; Choi, Heechae; Lee, Jin-Hyon; Han, Hyungkyu; Lee, Dong Hyun; Yoo, Dong Su; Kwon, Moon-Seok; Choi, Jae-Man; Doo, Seok Gwang; Chang, Hyuk; Xiao, Jianliang; Huang, Yonggang; Park, Won Il; Chung, Yong-Chae; Kim, Hansu; Rogers, John A; Paik, Ungyu

2012-01-24

Problems related to tremendous volume changes associated with cycling and the low electron conductivity and ion diffusivity of Si represent major obstacles to its use in high-capacity anodes for lithium ion batteries. We have developed a group IVA based nanotube heterostructure array, consisting of a high-capacity Si inner layer and a highly conductive Ge outer layer, to yield both favorable mechanics and kinetics in battery applications. This type of Si/Ge double-layered nanotube array electrode exhibits improved electrochemical performances over the analogous homogeneous Si system, including stable capacity retention (85% after 50 cycles) and doubled capacity at a 3C rate. These results stem from reduced maximum hoop strain in the nanotubes, supported by theoretical mechanics modeling, and lowered activation energy barrier for Li diffusion. This electrode technology creates opportunities in the development of group IVA nanotube heterostructures for next generation lithium ion batteries. © 2011 American Chemical Society

Madelung and Hubbard interactions in polaron band model of doped organic semiconductors

PubMed Central

Png, Rui-Qi; Ang, Mervin C.Y.; Teo, Meng-How; Choo, Kim-Kian; Tang, Cindy Guanyu; Belaineh, Dagmawi; Chua, Lay-Lay; Ho, Peter K.H.

2016-01-01

The standard polaron band model of doped organic semiconductors predicts that density-of-states shift into the π–π* gap to give a partially filled polaron band that pins the Fermi level. This picture neglects both Madelung and Hubbard interactions. Here we show using ultrahigh workfunction hole-doped model triarylamine–fluorene copolymers that Hubbard interaction strongly splits the singly-occupied molecular orbital from its empty counterpart, while Madelung (Coulomb) interactions with counter-anions and other carriers markedly shift energies of the frontier orbitals. These interactions lower the singly-occupied molecular orbital band below the valence band edge and give rise to an empty low-lying counterpart band. The Fermi level, and hence workfunction, is determined by conjunction of the bottom edge of this empty band and the top edge of the valence band. Calculations are consistent with the observed Fermi-level downshift with counter-anion size and the observed dependence of workfunction on doping level in the strongly doped regime. PMID:27582355

A D-band Active Imager in a SiGe HBT Technology

NASA Astrophysics Data System (ADS)

Yoon, Daekeun; Song, Kiryong; Kim, Jungsoo; Kaynak, Mehmet; Tillack, Bernd; Rieh, Jae-Sung

2015-04-01

In this paper, an amplifier and a detector operating near 140 GHz have been developed and integrated together with an on-chip antenna for an integrated active imager based on a 0.13-μm SiGe HBT technology. The 5-stage differential common-emitter (CE) amplifier shows a peak gain of 14 dB and noise figure (NF) down to 10 dB around 140 GHz with a DC power dissipation of 18 mW. The common-base (CB) differential detector exhibits a peak responsivity of 52.5 kV/W and a noise equivalent power (NEP) of 3.3 pW/Hz1/2. For the integrated imager, a peak responsivity of 1,740 kV/W and a minimum NEP of 80 fW/Hz1/2 were achieved with a DC power dissipation of 18 mW. With the fabricated active imager with on-chip antenna, which occupies an area of 2,200 × 600 μm2 including the antenna and bonding pads, images of various objects were successfully acquired.

Optical spectroscopy of rare earth ion-doped TiO2 nanophosphors.

PubMed

Chen, Xueyuan; Luo, Wenqin

2010-03-01

Trivalent rare-earth (RE3+) ion-doped TiO2 nanophosphors belong to one kind of novel optical materials and have attracted increasing attention. The luminescence properties of different RE3+ ions in various TiO2 nanomaterials have been reviewed. Much attention is paid to our recent progresses on the luminescence properties of RE3+ (RE = Eu, Er, Sm, Nd) ions in anatase TiO2 nanoparticles prepared by a sol-gel-solvothermal method. Using Eu3+ as a sensitive optical probe, three significantly different luminescence centers of Eu3+ in TiO2 nanoparticles were detected by means of site-selective spectroscopy at 10 K. Based on the crystal-field (CF) splitting of Eu3+ at each site, C2v and D2 symmetries were proposed for Eu3+ incorporated at two lattice sites. A structural model for the formation of multiple sites was proposed based on the optical behaviors of Eu3+ at different sites. Similar multi-site luminescence was observed in Sm(3+)- or Nd(3+)-doped TiO2 nanoparticles. In Eu(3+)-doped TiO2 nanoparticles, only weak energy transfer from the TiO2 host to the Eu3+ ions was observed at 10 K due to the mismatch of energy between the TiO2 band-gap and the Eu3+ excited states. On the contrary, efficient host-sensitized luminescences were realized in Sm(3+)- or Nd(3+)-doped anatase TiO2 nanoparticles due to the match of energy between TiO2 band-gap and the Sm3+ and Nd3+ excited states. The excitation spectra of both Sm(3+)- and Nd(3+)-doped samples exhibit a dominant broad peak centered at approximately 340 nm, which is associated with the band-gap of TiO2, indicating that sensitized emission is much more efficient than direct excitation of the Sm3+ and Nd3+ ions. Single lattice site emission of Er3+ in TiO2 nanocrystals can be achieved by modifying the experimental conditions. Upon excitation by a Ti: sapphire laser at 978 nm, intense green upconverted luminescence was observed. The characteristic emission of Er3+ ions was obtained both in the ultraviolet-visible (UV-vis) and

Fe-N co-doped SiO2@TiO2 yolk-shell hollow nanospheres with enhanced visible light photocatalytic degradation

NASA Astrophysics Data System (ADS)

Wan, Hengcheng; Yao, Weitang; Zhu, Wenkun; Tang, Yi; Ge, Huilin; Shi, Xiaozhong; Duan, Tao

2018-06-01

SiO2@TiO2 yolk@shell hollow nanospheres (STNSs) is considered as an outstanding photocatalyst due to its tunable structure and composition. Based on this point, we present an unprecedentedly excellent photocatalytic property of STNSs toward tannic acid via a Fe-N co-doped strategy. Their morphologies, compositions, structure and properties are characterized. The Fe-N co-doped STNSs formed good hollow yolk@shell structure. The results show that the energy gap of the composites can be downgraded to 2.82 eV (pure TiO2 = 3.2 eV). Photocatalytic degradation of tannic acid (TA, 30 mg L-1) under visible light (380 nm < λ < 780 nm) irradiation is used to evaluate the photocatalytic activity of the composites. Compared with pure TiO2 nanospheres, non-doped STNSs and N-doped STNSs, the Fe-N co-doped STNSs exhibits the highest activity, which can degrade 99.5% TA into CO2 and H2O in 80 min. The probable degradation mechanism of the composites is simultaneously proposed, the band gap of STNSs becomes narrow by co-doping Fe-N, so that the TiO2 shell can stimulate electrons under visible light exposure, generate the ions of radOH and radO2- with a strong oxidizing property. Therefore this approach works is much desired for radioactive organic wastewater photocatalytic degradation.

Suppression of the impurity-induced local magnetism by the opening of a spin pseudogap in Ni-doped Sr2CuO3

NASA Astrophysics Data System (ADS)

Utz, Yannic; Hammerath, Franziska; Nishimoto, Satoshi; Hess, Christian; Beesetty, Neela Sekhar; Saint-Martin, Romuald; Revcolevschi, Alexandre; Büchner, Bernd; Grafe, Hans-Joachim

2015-08-01

The S =1 /2 antiferromagnetic Heisenberg spin chain compound Sr2CuO3 doped with 1 % and 2 % of Ni impurities has been studied by means of 63Cu nuclear magnetic resonance. A strong decrease of the spin-lattice relaxation rate T1-1 at low temperatures points toward a spin gap, while a stretching exponent λ <1 and a frequency dependence of T1-1 indicate that this spin gap varies spatially and should rather be characterized as a spin pseudogap. The magnitude of the spin pseudogap scales with doping level. Our results therefore evidence the finite-size character of this phenomenon. Moreover, an unusual narrowing of the low-temperature NMR lines reveals the suppression of the impurity-induced staggered paramagnetic response with increasing doping level.

Synthesis and characterization of cobalt doped nickel oxide thin films by spray pyrolysis method

NASA Astrophysics Data System (ADS)

Sathisha, D.; Naik, K. Gopalakrishna

2018-05-01

Cobalt (Co) doped nickel oxide (NiO) thin films were deposited on glass substrates at a temperature of about 400 °C by spray pyrolysis method. The effect of Co doping concentration on structural, optical and compositional properties of NiO thin films was investigated. X-ray diffraction result shows that the deposited thin films are polycrystalline in nature. Surface morphologies of the deposited thin films were observed by FESEM and AFM. EDS spectra showed the incorporation of Co dopants in NiO thin films. Optical properties of the grown thin films were characterized by UV-visible spectroscopy. It was found that the optical band gap energy and transmittance of the films decrease with increasing Co doping concentration.

Enhanced ferromagnetic properties in Nd and Gd co-doped BiFeO3 ceramics

NASA Astrophysics Data System (ADS)

Jena, A. K.; Chelvane, J. Arout; Mohanty, J.

2018-05-01

Structural, optical and magnetic properties of Nd3+ and Gd3+ doped BiFeO3 were studied. X-ray diffraction studies confirmed that all the co-doped Bi1-x-yNdxGdyFeO3 samples are polycrystalline in nature crystallizing in rhombohedral type structure (Space group: R3c). In addition to this presence of residual phases like Bi2Fe4O9, Bi25FeO40 were also observed. Raman spectra confirms the structural distortion in co-doped ceramics. Band gap of samples decrease from 2.08eV to 1.95eV with increase in Gd concentration. Room temperature magnetization measurement indicated enhancement of magnetic properties with increase in Gd concentration.

Study on silver doped and undoped ZnO thin films working as capacitive sensor

NASA Astrophysics Data System (ADS)

Kiran, S.; Kumar, N. Santhosh; Kumar, S. K. Naveen

2013-06-01

Nanomaterials have been found to exhibit interesting properties like good conductivity, piezoelectricity, high band gap etc. among those metal oxide family, Zinc Oxide has become a material of interest among scientific community. In this paper, we present a method of fabricating capacitive sensors, in which Silver doped ZnO and pure ZnO nanoparticles act as active layer. For the synthesis of the nanoparticle, we followed biosynthesis method and wet chemical method for Ag and Ag doped ZnO nanoparticles respectively. Characterization has been done for both the particles. The XRD pattern taken for the Ag Doped ZnO nanoparticles confirmed the average size of the particles to be 15nm. AFM image of the sample is taken by doping on Silicon wafer. Also we have presented the results of CV characteristics and IV characteristics of the capacitive sensor.

Modification of graphene electronic properties via controllable gas-phase doping with copper chloride

NASA Astrophysics Data System (ADS)

Rybin, Maxim G.; Islamova, Vera R.; Obraztsova, Ekaterina A.; Obraztsova, Elena D.

2018-01-01

Molecular doping is an efficient, non-destructive, and simple method for changing the electronic structure of materials. Here, we present a simple air ambient vapor deposition method for functionalization of pristine graphene with a strong electron acceptor: copper chloride. The doped graphene was characterized by Raman spectroscopy, UV-vis-NIR optical absorption spectroscopy, scanning electron microscopy, and electro-physical measurements performed using the 4-probe method. The effect of charge transfer from graphene to a dopant results in shifting the Fermi level in doped graphene. The change of the electronic structure of doped graphene was confirmed by the tangential Raman peak (G-peak) shift and by the appearance of the gap in the UV-vis-NIR spectrum after doping. Moreover, the charge transfer resulted in a substantial decrease in electrical sheet resistance depending on the doping level. At the highest concentration of copper chloride, a Fermi level shift into the valence band up to 0.64 eV and a decrease in the sheet resistance value by 2.36 times were observed (from 888 Ω/sq to 376 Ω/sq for a single graphene layer with 97% of transparency).

Band gap and mobility of epitaxial perovskite BaSn1 -xHfxO3 thin films

NASA Astrophysics Data System (ADS)

Shin, Juyeon; Lim, Jinyoung; Ha, Taewoo; Kim, Young Mo; Park, Chulkwon; Yu, Jaejun; Kim, Jae Hoon; Char, Kookrin

2018-02-01

A wide band-gap perovskite oxide BaSn O3 is attracting much attention due to its high electron mobility and oxygen stability. On the other hand, BaHf O3 was recently reported to be an effective high-k gate oxide. Here, we investigate the band gap and mobility of solid solutions of BaS n1 -xH fxO3 (x =0 -1 ) (BSHO) as a basis to build advanced perovskite oxide heterostructures. All the films were epitaxially grown on MgO substrates using pulsed laser deposition. Density functional theory calculations confirmed that Hf substitution does not create midgap states while increasing the band gap. From x-ray diffraction and optical transmittance measurements, the lattice constants and the band-gap values are significantly modified by Hf substitution. We also measured the transport properties of n -type La-doped BSHO films [(Ba ,La ) (Sn ,Hf ) O3 ] , investigating the feasibility of modulation doping in the BaSn O3/BSHO heterostructures. The Hall measurement data revealed that, as the Hf content increases, the activation rate of the La dopant decreases and the scattering rate of the electrons sharply increases. These properties of BSHO films may be useful for applications in various heterostructures based on the BaSn O3 system.

Disordered dimer state in electron-doped Sr 3 Ir 2 O 7

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hogan, Tom; Dally, Rebecca; Upton, Mary

2016-09-06

Spin excitations are explored in the electron-doped spin-orbit Mott insulator (Sr 1-xLa x) 3Ir 2O 7. As this bilayer square lattice system is doped into the metallic regime, long-range antiferromagnetism vanishes, yet a spectrum of gapped spin excitation remains. Excitation lifetimes are strongly damped with increasing carrier concentration, and the energy-integrated spectral weight becomes nearly momentum independent as static spin order is suppressed. Local magnetic moments, absent in the parent system, grow in metallic samples and approach values consistent with one J=12 impurity per electron doped. Our combined data suggest that the magnetic spectra of metallic (Sr 1-xLa x) 3Irmore » 2O 7 are best described by excitations out of a disordered dimer state.« less

First principles study of electronic and structural properties of single walled zigzag boron nitride nanotubes doped with the elements of group IV

NASA Astrophysics Data System (ADS)

Bahari, Ali; jalalinejad, Amir; Bagheri, Mosahhar; Amiri, Masoud

2017-11-01

In this paper, structural and electronic properties and stability of (10, 0) born nitride nanotube (BNNT) are considered within density functional theory by doping group IV elements of the periodic table. The HOMO-LUMO gap has been strongly modified and treated a dual manner by choosing B or N sites for dopant atoms. Formation energy calculation shows that B site doping is more stable than N site doping. Results also show that all dopants turn the pristine BNNT into a p-type semiconductor except for carbon-doped BNNT at B site.

Three-stage nucleation and growth of Ge self-assembled quantum dots grown on partially relaxed SiGe buffer layers

NASA Astrophysics Data System (ADS)

Kim, H. J.; Zhao, Z. M.; Xie, Y. H.

2003-11-01

Three-stage nucleation and growth of Ge self-assembled quantum dots (SAQDs) on a relaxed SiGe buffer layer has been studied. Plastic relaxation of the SiGe buffer layer is associated with a network of buried 60° dislocations leading to an undulating strain field. As a result, the surface possesses three different types of sites for the nucleation and growth of Ge SAQDs: over the intersection of two perpendicular buried dislocations, over a single dislocation line, and in the region beyond one diffusion length away from any dislocation. Ge SAQDs are observed to nucleate exclusively over the dislocation intersections first, followed by over single dislocation lines, and finally in the region far away from dislocations. By increasing the Ge coverage at a slow rate, the prenucleation stage at the various sites is observed. It appears that the varying strain field has a significant effect on both the diffusion of Ge adatoms before SAQD nucleation, as well as the shape evolution of the SAQDs after they form. Moreover, two distinctly different self-assembly mechanisms are observed at different sites. There exist denuded zones free of Ge SAQDs adjacent to dislocation lines. The width of the denuded zone can be used to make direct determination of the Ge adatom diffusion lengths. The partially relaxed substrate provides a useful experimental vehicle for the in-depth understanding of the formation mechanism of SAQDs grown epitaxially in the Stranski-Krastanov growth mode.

Influence of magnetism and correlation on the spectral properties of doped Mott insulators

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Yao; Moritz, Brian; Chen, Cheng-Chien

Unraveling the nature of the doping-induced transition between a Mott insulator and a weakly correlated metal is crucial to understanding novel emergent phases in strongly correlated materials. Here, for this purpose, we study the evolution of spectral properties upon doping Mott insulating states by utilizing the cluster perturbation theory on the Hubbard and t – J -like models. Specifically, a quasifree dispersion crossing the Fermi level develops with small doping, and it eventually evolves into the most dominant feature at high doping levels. Although this dispersion is related to the free-electron hopping, our study shows that this spectral feature is,more » in fact, influenced inherently by both electron-electron correlation and spin-exchange interaction: the correlation destroys coherence, while the coupling between spin and mobile charge restores it in the photoemission spectrum. Due to the persistent impact of correlations and spin physics, the onset of gaps or the high-energy anomaly in the spectral functions can be expected in doped Mott insulators.« less

Influence of magnetism and correlation on the spectral properties of doped Mott insulators

DOE PAGES

Wang, Yao; Moritz, Brian; Chen, Cheng-Chien; ...

2018-03-01

Unraveling the nature of the doping-induced transition between a Mott insulator and a weakly correlated metal is crucial to understanding novel emergent phases in strongly correlated materials. Here, for this purpose, we study the evolution of spectral properties upon doping Mott insulating states by utilizing the cluster perturbation theory on the Hubbard and t – J -like models. Specifically, a quasifree dispersion crossing the Fermi level develops with small doping, and it eventually evolves into the most dominant feature at high doping levels. Although this dispersion is related to the free-electron hopping, our study shows that this spectral feature is,more » in fact, influenced inherently by both electron-electron correlation and spin-exchange interaction: the correlation destroys coherence, while the coupling between spin and mobile charge restores it in the photoemission spectrum. Due to the persistent impact of correlations and spin physics, the onset of gaps or the high-energy anomaly in the spectral functions can be expected in doped Mott insulators.« less

Electronic and magnetic properties of Mn-doped WSe2 monolayer under strain

NASA Astrophysics Data System (ADS)

Xin, Qianqian; Zhao, Xu; Wang, Tianxing

2017-04-01

Electronic and magnetic properties of Mn-doped WSe2 monolyer subject to isotropic strain are investigated using the first-principles methods based on the density functional theory. Our results indicate that Mn-doped WSe2 monolayer is a magnetic semiconductor nanomaterial with strong spontaneous magnetism without strain and the total magnetic moment of Mn-doped system is 1.038μB. We applied strain to Mn-doped WSe2 monolayer from -10% to 10%. The doped system transforms from magnetic semiconductor to half-metallic material from -10% to -2% compressive strain and from 2% to 6% tensile strain. The largest half-metallic gap is 0.450 eV at -2% compressive strain. The doped system shows metal property from 7% to 10%. Its maximum magnetic moment comes to 1.181μB at 6% tensile strain. However, the magnetic moment of system decreases to zero sharply when tensile strain arrived at 7%. Strain changes the redistribution of charges and arises to the magnetic effect. The coupling between the 3d orbital of Mn atom, 5d orbital of W atom and 4p orbital of Se atom is analyzed to explain the strong strain effect on the magnetic properties. Our studies predict Mn-doped WSe2 monolayers under strain to be candidates for thin dilute magnetic semiconductors, which is important for application in semiconductor spintronics.

Anomalous antibacterial activity and dye degradation by selenium doped ZnO nanoparticles.

PubMed

Dutta, Raj Kumar; Nenavathu, Bhavani Prasad; Talukdar, Soumita

2014-02-01

Selenium doped ZnO nanoparticles synthesized by mechanochemical method were spherically shaped of size distribution of 10.2±3.4 nm measured by transmission electron microscopy. Diffused reflectance spectroscopy revealed increase in the band gap, ranging between 3.47 eV and 3.63 eV due to Se doping in ZnO nanoparticles. The antibacterial activity of pristine and Se doped ZnO nanoparticles was attributed to ROS (reactive oxygen species) generation in culture media confirmed by TBARS assay. Compared to complete inhibition of growth by 0.45 mg/mL of pristine ZnO nanoparticles, the batches of 0.45 mg/mL of selenium doped ZnO nanoparticles exhibited only 51% inhibition of growth of Escherichia coli. The reduced antibacterial activity of selenium doped ZnO nanoparticles was attributed to two opposing factors, e.g., ROS generation for inhibition of growth, countered by sustaining growth of E. coli due to availability of Se micronutrients in culture media, confirmed by inductively coupled plasma mass spectrometer measurement. Higher ROS generation by selenium doped ZnO nanoparticles was attributed to creation of oxygen vacancies, confirmed from green emission peak observed at 565 nm. The impact of higher ROS generation by selenium doped ZnO nanoparticles was evident from enhanced photocatalytic degradation of trypan blue dye, than pristine ZnO nanoparticles. Copyright © 2013 Elsevier B.V. All rights reserved.

Athlete support personnel and anti-doping: Knowledge, attitudes, and ethical stance.

PubMed

Mazanov, J; Backhouse, S; Connor, J; Hemphill, D; Quirk, F

2014-10-01

Athlete support personnel (ASP) failing to meet responsibilities under the World Anti-Doping Code risk sanction. It is unclear whether the poor knowledge of responsibilities seen in sports physicians and coaches applies to other ASP (e.g., administrators, chiropractors, family, nutritionists, physiotherapists, psychologists, and trainers). A purposive sample of Australian ASP (n = 292) responded to a survey on knowledge of anti-doping rules (35 true/false questions), ethical beliefs and practice, and attitudes toward performance enhancement. Some ASP declined to participate, claiming doping was irrelevant to their practice. Physicians were most knowledgeable (30.8/35), with family and trainers the least (26.0/35). ASP reported that improvements were needed to support anti-doping education (e.g., basis for anti-doping) and practice (e.g., rules). ASP also had a slightly negative attitude toward performance enhancement. Linear regression showed that being a sports physician, providing support at the elite level, and 15 years of experience influenced knowledge. The results confirm gaps in knowledge, suggesting that stronger engagement with ASP anti-doping education and practice is needed. Applying the principles of andragogy could help foster active engagement through emphasis on active inquiry, rather than passive reception of content. Future work on the context within which ASP experience anti-doping is needed, exploring acquisition and translation of knowledge into practice. © 2013 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.

PdO Doping Tunes Band-Gap Energy Levels as Well as Oxidative Stress Responses to a Co3O4p-Type Semiconductor in Cells and the Lung

PubMed Central

2014-01-01

We demonstrate through PdO doping that creation of heterojunctions on Co3O4 nanoparticles can quantitatively adjust band-gap and Fermi energy levels to study the impact of metal oxide nanoparticle semiconductor properties on cellular redox homeostasis and hazard potential. Flame spray pyrolysis (FSP) was used to synthesize a nanoparticle library in which the gradual increase in the PdO content (0–8.9%) allowed electron transfer from Co3O4 to PdO to align Fermi energy levels across the heterojunctions. This alignment was accompanied by free hole accumulation at the Co3O4 interface and production of hydroxyl radicals. Interestingly, there was no concomitant superoxide generation, which could reflect the hole dominance of a p-type semiconductor. Although the electron flux across the heterojunctions induced upward band bending, the Ec levels of the doped particles showed energy overlap with the biological redox potential (BRP). This allows electron capture from the redox couples that maintain the BRP from −4.12 to −4.84 eV, causing disruption of cellular redox homeostasis and induction of oxidative stress. PdO/Co3O4 nanoparticles showed significant increases in cytotoxicity at 25, 50, 100, and 200 μg/mL, which was enhanced incrementally by PdO doping in BEAS-2B and RAW 264.7 cells. Oxidative stress presented as a tiered cellular response involving superoxide generation, glutathione depletion, cytokine production, and cytotoxicity in epithelial and macrophage cell lines. A progressive series of acute pro-inflammatory effects could also be seen in the lungs of animals exposed to incremental PdO-doped particles. All considered, generation of a combinatorial PdO/Co3O4 nanoparticle library with incremental heterojunction density allowed us to demonstrate the integrated role of Ev, Ec, and Ef levels in the generation of oxidant injury and inflammation by the p-type semiconductor, Co3O4. PMID:24673286

Synthesis and characterization of Ce, Cu co-doped ZnS nanoparticles

NASA Astrophysics Data System (ADS)

Harish, G. S.; Sreedhara Reddy, P.

2015-09-01

Ce, Cu co-doped ZnS nanoparticles were prepared at room temperature using a chemical co-precipitation method. The prepared nanoparticles were characterized by X- ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive analysis of X-rays (EDAX), diffuse reflectance spectroscopy (DRS), photoluminescence (PL) and high resolution Raman spectroscopic techniques. Transmission electron microscopy (TEM) and X-ray diffraction studies showed that the diameter of the particles was around 2-3 nm. Broadened XRD peaks revealed the formation of nanoparticles with a face centered cubic (fcc) structure. DRS studies confirmed that the band gap increased with an increase in the dopant concentration. The Raman spectra of undoped and Ce, Cu ions co-doped ZnS nanoparticles showed longitudinal optical mode and transverse optical mode. Compared with the Raman modes (276 and 351 cm-1) of undoped ZnS nanoparticles, the Raman modes of Ce, Cu co- doped ZnS nanoparticles were slightly shifted towards lower frequency. PL spectra of the samples showed remarkable enhancement in the intensity upon doping.

Spray deposition of highly transparent fluorine doped cadmium oxide thin films

NASA Astrophysics Data System (ADS)

Deokate, R. J.; Pawar, S. M.; Moholkar, A. V.; Sawant, V. S.; Pawar, C. A.; Bhosale, C. H.; Rajpure, K. Y.

2008-01-01

The cadmium oxide (CdO) and F:CdO films have been deposited by spray pyrolysis method using cadmium acetate and ammonium fluoride as precursors for Cd and F ions, respectively. The effect of temperature and F doping on the structural, morphological, optical and Hall effect properties of sprayed CdO thin films was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), optical absorption and electrical measurement techniques. TGA and DTA studies, indicates the formation of CdO by decomposition of cadmium acetate after 250 °C. XRD patterns reveal that samples are polycrystalline with cubic structure and exhibits (2 0 0) preferential orientation. Considerable broading of (2 0 0) peak, simultaneous shifting of corresponding Bragg's angle have been observed with respect to F doping level. SEM and AFM show the heterogeneous distribution of cubical grains all over the substrate, which are randomly distributed. F doping shifts the optical gap along with the increase in the transparency of CdO films. The Hall effect measurement indicates that the resistivity and mobility decrease up to 4% F doping.

All-Optical, Photonic Band Gap Modulation in Azobenzene Liquid Crystal Doped Cholesterics (Preprint)

DTIC Science & Technology

2006-10-01

4348 5e. TASK NUMBER RG 6. AUTHOR(S) Uladzimir A . Hrozhyk, Svetlana V . Serak, and Nelson V . Tabiryan (Beam Engineering for Advanced Measurements...modulation in azobenzene liquid crystal doped cholesterics Uladzimir A . Hrozhyk, Svetlana v . Serak, Nelson V . Tabiryan Beam Engineeringfor Advanced...Tsutsumi, Science, 1995,268, 1873. [6] T. Ikeda, J Mat. Chern., 2003,13, 2037. [7] A . Urbas, J. Klosterman, V . Tondiglia, L. Natarajan, R

Radiation Performance of Commercial SiGe HBT BiCMOS-High Speed Operational Amplifiers

NASA Technical Reports Server (NTRS)

Chen, Dakai; Pellish, Jonathan; Phan, Anthony; Kim, Hak; Burns, Sam; Albarian, Rafi; Holcombe, Bruce; Little, Bradley; Salzman, James; LaBel, Kenneth

2010-01-01

We present results on heavy-ion and proton irradiations for commercial SiGe BiCMOS operational amplifiers: LTC6400-20 from Linear Technology and THS4304 from Texas Instruments. We found that the devices are susceptible to heavy-ion-induced SETs. The SET cross-sections increase with increasing operating frequency. The LTC6400 exhibits a LET(sub th) < 7.4 MeV·sq cm/mg for frequencies ranging from 10 to 1000 MHz. The THS4304 exhibits a LET(sub th) < 4.4 MeV·sq cm/mg at 200 MHz; the LET(sub th) decreases with increasing frequency. The significance of the SETs also increases with frequency. The SETs at 1000 MHz can erase several signal cycles. We al.so found that the LTC6400 is relatively robust against 198 and 54 MeV protons. We did not observe angular sensitivity from the proton irradiations.

Local symmetry breaking in SnO2 nanocrystals with cobalt doping and its effect on optical properties.

PubMed

Roy, S; Joshi, Amish G; Chatterjee, S; Ghosh, Anup K

2018-06-07

X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) have been used to study the structural and morphological characteristics of cobalt doped tin(iv) oxide (Sn1-xCoxO2; 0 ≤ x ≤ 0.04) nanocrystals synthesized by a chemical co-precipitation technique. Electronic structure analysis using X-ray photoemission spectroscopy (XPS) shows the formation of tin interstitials (Sni) and reduction of oxygen vacancies (VO) in the host lattice on Co doping and that the doped Co exists in mixed valence states of +2 and +3. Using XRD, the preferential position of the Sni and doped Co in the unit cell of the nanocrystals have been estimated. Rietveld refinement of XRD data shows that samples are of single phase and variation of lattice constants follows Vegard's law. XRD and TEM measurements show that the crystallite size of the nanocrystals decrease with increase in Co doping concentration. SAED patterns confirm the monocrystalline nature of the samples. The study of the lattice dynamics using Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy shows the existence of many disorder activated forbidden optical phonon modes, along with the corresponding classical modes, signifying Co induced local symmetry breaking in the nanocrystals. UV-Vis spectroscopy shows that the optical band gap has red shifted with increase in doping concentration. The study of Urbach energy confirms the increase in disorder in the nanocrystals with Co doping. Local symmetry breaking induced UV emission along with violet, blue and green luminescence has been observed from the PL study. The spectral contribution of UV emission decreases and green luminescence increases with increase in doping. Using PL, in conjunction with Raman spectroscopy, the type of oxygen vacancy induced in the nanocrystals on Co doping has been confirmed and the position of the defect levels in the forbidden zone (w.r.t. the optical band gap) has been studied.

Efficient Fabrication of Nanoporous Si and Si/Ge Enabled by a Heat Scavenger in Magnesiothermic Reactions

PubMed Central

Luo, Wei; Wang, Xingfeng; Meyers, Colin; Wannenmacher, Nick; Sirisaksoontorn, Weekit; Lerner, Michael M.; Ji, Xiulei

2013-01-01

Magnesiothermic reduction can directly convert SiO2 into Si nanostructures. Despite intense efforts, efficient fabrication of highly nanoporous silicon by Mg still remains a significant challenge due to the exothermic reaction nature. By employing table salt (NaCl) as a heat scavenger for the magnesiothermic reduction, we demonstrate an effective route to convert diatom (SiO2) and SiO2/GeO2 into nanoporous Si and Si/Ge composite, respectively. Fusion of NaCl during the reaction consumes a large amount of heat that otherwise collapses the nano-porosity of products and agglomerates silicon domains into large crystals. Our methodology is potentially competitive for a practical production of nanoporous Si-based materials. PMID:23860418

Efficient fabrication of nanoporous si and Si/Ge enabled by a heat scavenger in magnesiothermic reactions.

PubMed

Luo, Wei; Wang, Xingfeng; Meyers, Colin; Wannenmacher, Nick; Sirisaksoontorn, Weekit; Lerner, Michael M; Ji, Xiulei

2013-01-01

Magnesiothermic reduction can directly convert SiO2 into Si nanostructures. Despite intense efforts, efficient fabrication of highly nanoporous silicon by Mg still remains a significant challenge due to the exothermic reaction nature. By employing table salt (NaCl) as a heat scavenger for the magnesiothermic reduction, we demonstrate an effective route to convert diatom (SiO2) and SiO2/GeO2 into nanoporous Si and Si/Ge composite, respectively. Fusion of NaCl during the reaction consumes a large amount of heat that otherwise collapses the nano-porosity of products and agglomerates silicon domains into large crystals. Our methodology is potentially competitive for a practical production of nanoporous Si-based materials.

Direct visualization of atomically precise nitrogen-doped graphene nanoribbons

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Yi; Zhang, Yanfang; Li, Geng

2014-07-14

We have fabricated atomically precise nitrogen-doped chevron-type graphene nanoribbons by using the on-surface synthesis technique combined with the nitrogen substitution of the precursors. Scanning tunneling microscopy and spectroscopy indicate that the well-defined nanoribbons tend to align with the neighbors side-by-side with a band gap of 1.02 eV, which is in good agreement with the density functional theory calculation result. The influence of the high precursor coverage on the quality of the nanoribbons is also studied. We find that graphene nanoribbons with sufficient aspect ratios can only be fabricated at sub-monolayer precursor coverage. This work provides a way to construct atomically precisemore » nitrogen-doped graphene nanoribbons.« less

Synthesis, Optical and Photoluminescence Properties of Cu-Doped Zno Nano-Fibers Thin Films: Nonlinear Optics

NASA Astrophysics Data System (ADS)

Ganesh, V.; Salem, G. F.; Yahia, I. S.; Yakuphanoglu, F.

2018-03-01

Different concentrations of copper-doped zinc oxide thin films were coated on a glass substrate by sol-gel/spin-coating technique. The structural properties of pure and Cu-doped ZnO films were characterized by different techniques, i.e., atomic force microscopy (AFM), photoluminescence and UV-Vis-NIR spectroscopy. The AFM study revealed that pure and doped ZnO films are formed as nano-fibers with a granular structure. The photoluminescence spectra of these films showed a strong ultraviolet emission peak centered at 392 nm and a strong blue emission peak cantered at 450 nm. The optical band gap of the pure and copper-doped ZnO thin films calculated from optical transmission spectra (3.29-3.23 eV) were found to be increasing with increasing copper doping concentration. The refractive index dispersion curve of pure and Cu-doped ZnO film obeyed the single-oscillator model. The optical dispersion parameters such as E o , E d , and n_{∞}2 were calculated. Further, the nonlinear refractive index and nonlinear optical susceptibility were also calculated and interpreted.

Doping dependence of low-energy quasiparticle excitations in superconducting Bi2212.

PubMed

Ino, Akihiro; Anzai, Hiroaki; Arita, Masashi; Namatame, Hirofumi; Taniguchi, Masaki; Ishikado, Motoyuki; Fujita, Kazuhiro; Ishida, Shigeyuki; Uchida, Shinichi

2013-12-05

: The doping-dependent evolution of the d-wave superconducting state is studied from the perspective of the angle-resolved photoemission spectra of a high-Tc cuprate, Bi2Sr2CaCu2 O8+δ (Bi2212). The anisotropic evolution of the energy gap for Bogoliubov quasiparticles is parametrized by critical temperature and superfluid density. The renormalization of nodal quasiparticles is evaluated in terms of mass enhancement spectra. These quantities shed light on the strong coupling nature of electron pairing and the impact of forward elastic or inelastic scatterings. We suggest that the quasiparticle excitations in the superconducting cuprates are profoundly affected by doping-dependent screening.