Optimization of off-oriented Ge substrates for MOVPE-grown GaAs solar cells

GaAs/Ge heterostructures having abrupt interfaces were grown on 2degrees, 6degrees, and 9degrees off-cut Ge substrates and investigated by cross-sectional high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy, photoluminescence spectroscopy and electrochemical capacitance voltage (ECV) profiler. The GaAs films were grown on off-oriented Ge substrates with growth temperature in the range of 600-700degreesC, growth rate of 3-12 mum/hr and a V/III ratio of 29-88. The lattice indexing of HRTEM exhibits an excellent lattice line matching between GaAs and Ge substrate. The PL spectra from GaAs layer on 6degrees off-cut Ge substrate shows the higher excitonic peak compared with 2degrees and 9degrees off-cut Ge substrates. In addition, the luminescence intensity from the GaAs solar cell grown on 6degrees off-cut is higher than on 9degrees off-cut Ge substrates and signifies the potential use of 6degrees off-cut Ge substrate in the GaAs solar cells industry. The ECV profiling shows an abrupt film/substrate interface as well as between various layers of the solar cell structures.

Kinetic decomposition of Ni2SiO4 in oxygen potential gradients

Nickel orthosilicate (Ni2SiO4) has been found to decompose into its component binary oxides in oxygen potential gradients at 1373 K. Nickel oxide was formed at the high oxygen potential boundary, while silica was detected at the low oxygen potential side. Significant porosity and fissures were observed near the Ni2SiO4/SiO2 interface and the SiO2 layer. The critical oxygen partial pressure ratio required for decomposition varied from 1.63 to 2.15 as the oxygen pressures were altered from 1.01 ⊠ 105 to 2.7X 10−4 Pa, well above the dissociation pressure of Ni2SiO4. Platinum markers placed at the boundaries of the Ni2SiO4 sample indicated growth of NiO at the higher oxygen potential boundary, without any apparent transport of material to the low oxygen potential side. However, significant movement of the bulk Ni2SiO4 crystal with respect to the marker was not observed. The decomposition of the silicate occurs due to the unequal rates of transport of Ni and Si. The critical oxygen partial pressure ratio required for decomposition is related both to the thermodynamic stability of Ni2SiO4 with respect to component oxides and the ratio of diffusivities of nickel and silicon. Kinetic decomposition of multicomponent oxides, first discovered by Schmalzried, Laqua, and co-workers [H. Schmalzried, W. Laqua, and P. L. Lin, Z. Natur Forsch. Teil A 34, 192 (1979); H. Schmalzried and W. Laqua, Oxid. Met. 15, 339 (1981); W. Laqua and H. Schmalzried, Chemical Metallurgy—A Tribute to Carl Wagner (Metallurgical Society of the AIME, New York, 1981), p. 29] has important consequences for their use at high temperatures and in geochemistry.

Structural, optical and nanomechanical properties of (1 1 1) oriented nanocrystalline ZnTe thin films

Structural, optical and nanomechanical properties of nanocrystalline Zinc Telluride (ZnTe) films of thickness upto 10 microns deposited at room temperature on borosilicate glass substrates are reported. X-ray diffraction patterns reveal that the films were preferentially oriented along the (1 1 1) direction. The maximum refractive index of the films was 2.74 at a wavelength of 2000 nm. The optical band gap showed strong thickness dependence. The average film hardness and Young's modulus obtained from load-displacement curves and analyzed by Oliver-Pharr method were 4 and 70 GPa respectively. Hardness of (1 1 1) oriented ZnTe thin films exhibited almost 5 times higher value than bulk. The studies show clearly that the hardness increases with decreasing indentation size, for indents between 30 and 300 nm in depth indicating the existence of indentation size effect. The coefficient of friction for these films as obtained from the nanoscratch test was ~0.4.

Structural, optical and nanomechanical properties of (1 1 1) oriented nanocrystalline ZnTe thin films

Structural, optical and nanomechanical properties of nanocrystalline Zinc Telluride (ZnTe) films of thickness upto 10 microns deposited at room temperature on borosilicate glass substrates are reported. X-ray diffraction patterns reveal that the films were preferentially oriented along the (1 1 1) direction. The maximum refractive index of the films was 2.74 at a wavelength of 2000 nm. The optical band gap showed strong thickness dependence. The average film hardness and Young's modulus obtained from load-displacement curves and analyzed by Oliver-Pharr method were 4 and 70 GPa respectively. Hardness of (1 1 1) oriented ZnTe thin films exhibited almost 5 times higher value than bulk. The studies show clearly that the hardness increases with decreasing indentation size, for indents between 30 and 300 nm in depth indicating the existence of indentation size effect. The coefficient of friction for these films as obtained from the nanoscratch test was ~0.4.

Dielectric properties of c-axis oriented Zn1-xMgxO thin films grown by multimagnetron sputtering

Zn1−xMgxO (x = 0.3) thin films have been fabricated on Pt/TiO2/SiO2/Si substrates using multimagnetron sputtering technique. The films with wurtzite structure showed a (002) preferred orientation. Ferroelectricity in Zn1−xMgxO films was established from the temperature dependent dielectric constant and the polarization hysteresis loop. The temperature dependent study of dielectric constant at different frequencies exhibited a dielectric anomaly at 110 °C. The resistivity versus temperature characteristics showed an anomalous increase in the vicinity of the dielectric transition temperature. The Zn1−xMgxO thin films exhibit well-defined polarization hysteresis loop, with a remanent polarization of 0.2 μC/cm2 and coercive field of 8 kV/cm at room temperature.

Dielectric properties of (110) oriented PbZrO3 and La-modified PbZrO3 thin films grown by sol-gel process on Pt(111)/Ti/SiO2/Si substrate

Highly (110) preferred orientated antiferroelectric PbZrO3 (PZ) and La-modified PZ thin films have been fabricated on Pt/Ti/SiO2/Si substrates using sol-gel process. Dielectric properties, electric field induced ferroelectric polarization, and the temperature dependence of the dielectric response have been explored as a function of composition. The Tc has been observed to decrease by ∼ 17 °C per 1 mol % of La doping. Double hysteresis loops were seen with zero remnant polarization and with coercive fields in between 176 and 193 kV/cm at 80 °C for antiferroelectric to ferroelectric phase transformation. These slim loops have been explained by the high orientation of the films along the polar direction of the antiparallel dipoles of a tetragonal primitive cell and by the strong electrostatic interaction between La ions and oxygen ions in an ABO3 perovskite unit cell. High quality films exhibited very low loss factor less than 0.015 at room temperature and pure PZ; 1 and 2 mol % La doped PZs have shown the room temperature dielectric constant of 135, 219, and 142 at the frequency of 10 kHz. The passive layer effects in these films have been explained by Curie constants and Curie temperatures. The ac conductivity and the corresponding Arrhenius plots have been shown and explained in terms of doping effect and electrode resistance.

Digital Simulation of Field-Oriented Control of Induction Motor

This paper describes the simulation of a control scheme using the principle of field orientation for the control of a voltage source inverter-fed induction motor. The control principle is explained, followed by an algorithm to simulate various components of the system in the digital computer. The dynamic response of the system for the load disturbance and set-point variations have been studied. Also, the results of the simulation showing the behavior of field coordinates for such disturbances are given.

Microprocessor-Based Field-Oriented Control of A CSI-Fed Induction Motor Drive

This paper describes the method of field orientation of the stator current vector with respect to the stator, mutual, and rotor flux vectors, for the control of an induction motor fed from a current source inverter (CSI). A control scheme using this principle is described for orienting the stator current with respect to the rotor flux, as this gives natural decoupling between the current coordinates. A dedicated micro-computer system developed for implementing this scheme has been described. The experimental results are also presented.

Nanomechanical and optical properties of highly a-axis oriented AlN films

This paper reports optical and nanomechanical properties of predominantly a-axis oriented AlN thin films. These films were deposited by reactive DC magnetron sputtering technique at an optimal target to substrate distance of 180 mm. X-ray rocking curve (FWHM = 52 arcsec) studies confirmed the preferred orientation. Spectroscopic ellipsometry revealed a refractive index of 1.93 at a wavelength of 546 nm. The hardness and elastic modulus of these films were 17 and 190 GPa, respectively, which are much higher than those reported earlier can be useful for piezoelectric films in bulk acoustic wave resonators. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.4772204]

``On-the-fly'' kinetics of enzymatic racemization using deuterium NMR in DNA-based chiral oriented media

We report the in situ and real-time monitoring of the interconversion of L- and D-alanine-d(3) by alanine racemase from Bacillus stearothermophilus directly observed by H-2 NMR spectroscopy in anisotropic phase. The enantiomers are distinguished by the difference of their H-2 quadrupolar splittings in a chiral liquid crystal containing short DNA fragments. The proof-of-principle, the reliability, and the robustness of this new method is demonstrated by the determination of the turnover rates of the enzyme using the Michaelis Menten model.

Micropillar and macropillar compression responses of magnesium single crystals oriented for single slip or extension twinning

Uniaxial compression experiments were conducted on two magnesium (Mg) single crystals whose crystallographic orientations facilitate the deformation either by basal slip or by extension twinning. Specimen size effects were examined by conducting experiments on mu m- and mm-sized samples. A marked specimen size effect was noticed, with micropillars exhibiting significantly higher flow stress than bulk samples. Further, it is observed that the twin nucleation stress exerts strong size dependence, with micropillars requiring substantially higher stress than the bulk samples. The flow curves obtained on the bulk samples are smooth whereas those obtained from micropillars exhibit intermittent and precipitous stress drops. Electron backscattered diffraction and microstructural analyses of the deformed samples reveal that the plastic deformation in basal slip oriented crystals occurs only by slip while twin oriented crystals deform by both slip and twinning modes. The twin oriented crystals exhibit a higher strain hardening during plastic deformation when compared to the single slip oriented crystals. The strain hardening rate, theta, of twin oriented crystals is considerably greater in micropillars compared to the bulk single crystals, suggesting the prevalence of different work hardening mechanisms at these different sample sizes. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.