Electronic and structural properties of zincblende Al<Subscript>x</Subscript>Ga<Subscript>1-x</Subscript>N

Numerical calculations based on first-principles are applied to study the electronic and structural properties of zincblende Al_xGa_1-xN. The results show that the lattice constant has a very small deviation from the linear Vegard’s law. The direct and indirect bowing parameters of 0.295±0.034 eV and -0.125±0.060 eV are obtained, respectively, and there is a direct-indirect crossover near x=0.692. Besides, the bulk moduli and their pressure derivatives are monotonically increased with an increase of the aluminum composition x. The deviation parameter of the bulk modulus of -5.32±1.60 GPa is obtained. PACS 71.15.-m; 71.15.Nc; 71.55.Eq; 71.20.Nr; 42.70.Qs     

Elasticity and polarization of Ga<Subscript>x</Subscript>Al<Subscript>1-x</Subscript>N alloys subjected to uniaxial and biaxial compression

Elasticity and polarization of Ga_xAl_1-xN alloys subjected to uniaxial and homogeneous biaxial compression are calculated using first-principles methods. The uniaxial compression along the c-axis reduces Young’s modulus along the c-axis, and enhances bulk modulus and total polarization, whereas the biaxial compression in the plane perpendicular to the c-axis enhances bulk and Young’s moduli. It is also found that when the in-plane biaxial compression is applied by constraining the a-axis lattice constant to that of AlN, the bulk and Young’s moduli dramatically increase with increasing Ga concentration, and the total polarization could be suppressed, even annihilated, and finally enhanced by controlling Ga concentration.     

Electron mobility for a model Al<Subscript>x</Subscript> Ga<Subscript>1-x</Subscript> As/GaAs heterojunction under pressure

A variational method and a memory function approach are adopted to investigate the electron mobility parallel to the interface for a model Al_xGa_1-xAs/GaAs heterojunction and its pressure effect by considering optical phonon modes (including both of the bulk longitudinal optical (LO) in the channel side and interface optical (IO) phonons). The influence of a realistic interface heterojunction potential with a finite barrier and conduction band bending are taken into account. The properties of electron mobility versus Al concentration, electronic density and pressure are given and discussed, respectively. The results show that the electron mobility increases with Al concentration and electronic density, whereas decreases with pressure from 0 to 40 kbar obviously. The Al concentration dependent and the electron density dependent contributions to the electron mobility from the scattering of IO phonons under pressure becomes more obvious. The variation of electron mobility with the Al concentration and electron density are dominated by the properties of IO and LO phonons, respectively. The effect of IO phonon modes can not be neglected especially for higher pressure and electronic density.     

Sacrificial etching of Al<Subscript>x</Subscript>Ga<Subscript>1-x</Subscript>As for III–V MEMS surface micromachining

A study of Al_xGa_1-xAs as a sacrificial film for surface micromachining is presented. Al_xGa_1-xAs etch rate and selectivity are measured over a range of aluminum mole fractions and HF etchant concentrations during the release of structural features up to 500 μm in width. The etch process is found to be diffusion limited, with an inverse power law relationship between etch depth and etch rate. Excellent selectivity greater than 10⁵ is achieved between sacrificial AlAs and structural GaAs, even for long etches up to 250 μm in length. Compared with previous studies of Al_xGa_1-xAs etching for epitaxial liftoff processing, measured etch rates for surface micromachining are approximately an order of magnitude lower, primarily due to the longer effective etch lengths required. However, unlike epitaxial liftoff, Al_xGa_1-xAs surface micromachining is compatible with higher HF concentrations which can provide comparable overall etch rates, with important implications for AlGaAs MEMS fabrication. PACS 81.05.Ea; 85.85.+j     

Structural properties of zinc-blende Ga<Subscript>x</Subscript>In<Subscript>1-x</Subscript>N: ab initio calculations

We present a theoretical study of the structural properties, namely lattice constant, bulk modulus and its pressure derivative of zinc-blende Ga_xIn_1-xN. The calculations are performed using first-principles calculations in the framework of the density-functional-theory within the local density approximation under the virtual crystal approximation. The computed values are in good agreement with the available experimental data. The composition dependence of the studied quantities is examined. Besides, the deviation of the alloy lattice constant from Vegard's law is evaluated.     

Spacer layer thickness fluctuation scattering in a modulation-doped Al_xGa_1-xAs/GaAs/Al_xGa_1-xAs quantum well

We theoretically study the influence of spacer layer thickness fluctuation(SLTF) on the mobility of a twodimensional electron gas(2DEG) in the modulation-doped Al x Ga 1 x As/GaAs/Al x Ga 1 x As quantum well.The dependence of the mobility limited by SLTF scattering on spacer layer thickness and donor density are obtained.The results show that SLTF scattering is an important scattering mechanism for the quantum well structure with a thick well layer.     

Prediction of Γ-L and L-X crossovers in Ga<Subscript>x</Subscript>Al<Subscript>1-x</Subscript>Sb alloys by empirical pseudopotential method

Using empirical pseudopotential method Γ-L crossover is found for the Ga_0.74Al_0.26Sb. The conduction band minimum is observed to switch at the (0.87, 0, 0) point for Ga_0.51Al_0.49Sb which shifts to the X point for Ga_0.21Al_0.79Sb and remains at X leading finally to indirect band gap in AlSb. Band structure calculations for a large number of alloys are performed and bowing parameters b_X and b_L are proposed for the E_X and E_L respectively. Our findings may serve as directive to select the materials in a range of composition to examine the bowing parameters and thereby effective mass experimentally for the Ga_xAl_1-xSb alloys.     

Rashba effect in Ga<Subscript>x</Subscript>In<Subscript>1-x</Subscript>As/InP quantum wire structures

An overview is given on the Rashba effect in Ga_xIn_1-xAs/InP quantum wires. First, the effect of Rashba spin–orbit coupling on the energy level spectrum of quantum wires with different shapes of the confining potential is theoretically investigated. The wave functions as well as the spin densities in the quantum wire are analyzed for different magnetic fields. It is found that, owing to the additional geometrical confinement, a modification of the characteristic beating pattern in the magnetoresistance can be expected. The theoretical findings are compared to measurements on two different types of wires: First, single wires and, second, sets of parallel wires. A characteristic beating pattern in the Shubnikov–de Haas oscillations is observed for wires with an effective width down to approximately 400 nm. The beating pattern is significantly better resolved for the samples with sets of parallel wires, owing to the effective suppression of conductance fluctuations. A comparison with theoretical simulations confirms that the strength of the Rashba effect is basically not affected by the geometrical confinement of the wires. However, for wires with a very small effective width the strong carrier confinement leads to a suppression of the characteristic beating pattern in the Shubnikov–de Haas oscillations. PACS 71.70.Ej; 73.63.-b; 71.70.Di     

High temperature dependence of the density of two-dimensional electron gas in Al<Subscript>0.18</Subscript>Ga<Subscript>0.82</Subscript>N/GaN heterostructures

Temperature dependence of the density of two-dimensional electron gas (2DEG) in Al_0.18Ga_0.82N/GaN heterostructures has been investigated by means of high temperature Hall measurements ranging from room temperature to 500 °C. It is found that the 2DEG density decreases with increasing temperature in the range from room temperature to 250 °C, and then increases with the temperature above 250 °C. It is thought that the decrease of the 2DEG density from room temperature to 250 °C is caused by the reduction of the conduction band offset at high temperatures. The increase of measured 2DEG density at higher temperatures is attributed to the background electron concentration in the GaN layer. Theoretical calculation of the 2DEG density in Al_0.18Ga_0.82N/GaN heterostructures at various temperatures is consistent with the experimental results using the multilayer Hall effect model. PACS 73.40.Kp; 73.61.Ey     

Effect of interdiffusion of In and Al atoms on excitonic states in In<Subscript>x</Subscript>Ga<Subscript>1−x</Subscript>As/Al<Subscript>y</Subscript>Ga<Subscript>1−y</Subscript>As quantum dots

The effect of interdiffusion of aluminum and indium atoms on the exciton emission energy and binding energy in In_xGa_1?xAs/Al_yGa_1?yAs quantum dots is studied. It is shown that the emission energy increases monotonically with increasing diffusion length, while the binding energy has a maximum.     

High pressure structural phase transition and elastic properties of Ga<Subscript>1-x</Subscript>In<Subscript>x</Subscript>As semiconducting compounds

The present paper addresses the high-pressure phase transformation and mechanical properties of Ga_1-xIn_xAs (x = 0.25, 0.5 and 0.75) by formulating an effective interionic interaction potential. This potential consists of the long-range Coulomb and charge transfer caused by the deformation of the electron shells of the overlapping ions and the Hafemeister and Flygare type short-range overlap repulsion extended upto the second neighbor ions and the van der Waals (vdW) interaction. The estimated values of phase transition pressure and the vast volume discontinuity in pressure-volume (PV) phase diagram indicate the structural phase transition from zinc blende (B3) to rock salt (B1). The equation of state curves plotted between V (P)/ V (0) and pressure are for both the zincblende (B3) and rocksalt (B1) structures. Further, the variations of the second and third order elastic constants with pressure have followed a systematic trend, which are almost identical to those exhibited by the observed data measured for other compounds of this family.     

Solar-blind MSM-photodetectors based on Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>N heterostructures

Solar-blind MSM photodetectors based on the AlGaN heterostructures have been fabricated and investigated. The influence of material properties on device parameters is discussed. Effect of different buffer layers on the detector performances has been examined. Detectors exhibit low dark currents and high sensitivity within the range of 250–290 nm. Effect of optical excitation energy on GaN-based MSM-detector performance is analyzed and discussed. At high excitation level the detector speed of response is limited by the field screening caused by the space-charge of the holes. The impulse response of GaN-based MSM-detector is compared favorably with GaAs MSM-device.     

Energy spectra of exciton states in disk-shaped GaAs-Ga<Subscript>1-x</Subscript>Al<Subscript>x</Subscript>As quantum dots under growth-direction magnetic fields

A theoretical study, within the effective-mass approximation, of the effects of applied magnetic fields on excitons in disk-shaped GaAs-Ga_1-xAl_xAs quantum dots is presented. Magnetic fields are applied in the growth direction of the semiconductor heterostructure. The parity of the excitonic envelope function related to the simultaneous exchange of z_e→-z_e and z_h→-z_h is a good quantum number and the wave function, both the odd and even parity, can be expanded as combination of products of the quantum well electron and hole function that preserves the parity with appropriate Gaussian functions. We have simultaneously obtained the energies of the excitonic ground and excited states and discuss the behavior of these energies as a function of the magnetic field.     

Anisotropic piezoeffect in microelectromechanical systems based on epitaxial Al<Subscript>0.5</Subscript>Ga<Subscript>0.5</Subscript>As/AlAs heterostructures

A microelectromechanical system is created that has the form of a cantilever-fitted microbar with a cross-sectional area of several square micrometers. The system is formed by applying epitaxial AlGaAs layers on the GaAs(001) surface and selective chemical etching of the AlAs layer lying under the bar. Two micro-cantilevers that are made on the same GaAs(001) wafer and directed along the [110] and [1$ \bar 1 $ \bar 1 0] orthogonal diagonal axes are studied. The static and dynamic characteristics of the systems are studied by white light optical interferometry. The deflection of the bars as a function of the applied voltage is measured in the static mode. An opposite shift of orthogonal microcantilevers on which the same voltage is applied is considered as direct evidence of the efficiency of a piezoeffect-based microengine. The calculated parameters of the micro-electromechanical system, the sensitivity and eigenfrequency, are in good agreement with the measurements.     

Influence of different precursor surface layers on Cu(In<Subscript>1-x</Subscript>Ga<Subscript>x</Subscript>)Se<Subscript>2</Subscript> thin film solar cells

The properties of Cu(In_1-xGa_x)Se₂ (CIGS) thin films obtained by selenization of the precursors with different surface layers have been studied, and photovoltaic devices based on the absorbers were measured and analyzed. The devices constructed by the absorbers obtained by selenization of the precursors with CuGa-rich surface layers are improved, compared with those with In-rich surface layers. Through XRD, SEM, SIMS, illuminated J–V, QE and Raman spectra measurements, it was found that the increased Ga contents within the surface region of films and the graded Ga distribution can be realized in the selenized thin films fabricated by the precursors with the CuGa-rich surface layer. Consequently, the performances of the photovoltaic devices based on these thin films are further improved. PACS 61.72.Ss; 87.64.Jt; 68.60.Bs; 81.15.Cd; 84.60.Jt     

Electronic structure of paramagnetic In<Subscript>1-x</Subscript>Mn<Subscript>x</Subscript> As nanowires

The electronic structure, spin splitting energies, and g factors of paramagnetic In_1-xMn_xAs nanowires under magnetic and electric fields are investigated theoretically including the sp-d exchange interaction between the carriers and the magnetic ion. We find that the effective g factor changes dramatically with the magnetic field. The spin splitting due to the sp-d exchange interaction counteracts the Zeeman spin splitting. The effective g factor can be tuned to zero by the external magnetic field. There is also spin splitting under an electric field due to the Rashba spin-orbit coupling which is a relativistic effect. The spin-degenerated bands split at nonzero k_z (k_z is the wave vector in the wire direction), and the spin-splitting bands cross at k_z = 0, whose k_z-positive part and negative part are symmetrical. A proper magnetic field makes the k_z-positive part and negative part of the bands asymmetrical, and the bands cross at nonzero k_z. In the absence of magnetic field, the electron Rashba coefficient increases almost linearly with the electric field, while the hole Rashba coefficient increases at first and then decreases as the electric field increases. The hole Rashba coefficient can be tuned to zero by the electric field.     

Scaling and self-similarity in Pb<Subscript>1-x</Subscript>Fe<Subscript>x</Subscript>S nanoparticle films

This paper addresses the issues of scaling and self-similarity in typical nanoparticle films. The role played by microscopic processes contributing to growth on these issues is probed. While we perform this investigation for a specific system viz., Pb_1-xFe_xS nanoparticle films for clarity of the procedures, the analysis is general and can be applied to a variety of systems obtained using different deposition techniques.     

Structural and optical characterization of polycrystalline CdSe<Subscript>x</Subscript>Te<Subscript>1-x</Subscript> thin films

Solid solutions of CdSe_xTe_1-x (0.7x1) were synthesized by vacuum fusion of stoichiometric amounts of CdSe and CdTe constituents in a silica tube. X-ray and electron microscope diffractometry techniques revealed that the CdSe_xTe_1-x thin films were polycrystalline with a hexagonal structure. The variation of lattice constants with composition was found to obey Vegards law. The compositional dependence of the optical constants, the refractive index n and the absorption index k, of the films was determined in the spectral range of 400–2000 nm. The dispersion of the refractive index of the films could be described using the Wemple–DiDomenico single oscillator model. Changes of the dispersion parameters were also studied as a function of the mole fraction x. A plot representing ²=f(h) showed that the CdSe_xTe_1-x thin films of different compositions have two direct transitions corresponding to the energy gaps E_g and E_g+. The variation in either E_g or E_g+ with x indicates that this system belongs to the amalgamation type. The variation follows a quadratic dependence and the bowing parameters were found to be 0.4 and 0.5 eV, respectively. PACS 78.20.-e; 81.15.-z     

Non-critical phase-matched second harmonic generation in Gd<Subscript>x</Subscript>Y<Subscript>1-x</Subscript>COB

We have characterized non-critical phase-matching (NCPM) for both Type I and Type II second harmonic generation (SHG) in y-cut Gd_xY_1-xCOB using a nanosecond optical parametric oscillator (OPO). The variation of the NCPM wavelength with temperature was investigated for different values of the compositional parameter x. Efficient SHG of 1064 nm was achieved by choosing the suitable compositional parameter x=0.28 and by tuning the temperature of the crystal to 52 °C. Using a 25-mm-long Gd_0.28Y_0.72COB crystal, conversion efficiencies of 41 and 43% were obtained respectively from a mode-locked Nd:YAG and a Q-switched Nd:YAG laser. PACS 42.25.Lc; 42.65.Ky; 42.70.Mp; 42.79.Nv     

Electrical switching studies on As<Subscript>40</Subscript>Te<Subscript>60-x</Subscript>Se<Subscript>x</Subscript> and As<Subscript>35</Subscript>Te<Subscript>65-x</Subscript>Se<Subscript>x</Subscript> glasses

The I–V characteristics of bulk As₄₀Te_60-xSe_x and As₃₅Te_65-xSe_x glasses have been studied with a current sweep of 0–18 mA-0, over a wide range of compositions (4≤x≤22). All the glasses studied showed a threshold electrical switching behaviour. The number of switching cycles withstood by the samples has been found to depend on the ON-state current. It is seen that the switching voltages increase with increase in selenium content. Further, the switching voltages are found to be almost independent of the thickness of the sample (d), in the range 0.18–0.3 mm. Also, the switching voltages and the number of switching cycles withstood by the samples are found to decrease with temperature. Received: 6 November 2002 / Accepted: 8 November 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. Fax: +91-80/360-0135, E-mail: sasokan@isu.iisc.ernet.in