Refractive index of Ga1−xAlxAs

A semi-empirical method for calculating the room temperature refractive index of Ga_1?xAl_xAs at energies below the direct band edge is presented. This quantity is important in the design of GaAs heterostructure lasers as well as other wave-guiding devices using these materials. The calculated values compare favorably with recent data. The method is shown to be useful for the Ga_1?xAs_xP system as well.     

Defects in as-grown and annealed AlxGa1−xAs single crystals

Photoluminescence studies were made on Sn-, Te- and Ge-doped Al_xGa_1-xAs single crystals before and after annealing at 800°C for 21 h in evacuated and sealed quartz ampoules. The thin p-type surface layer, which was present on all nondegenerate n-Al_xGa_1-x As crystals after annealing, is attributed to the introduction of Si from the inner wall of the ampoule to form shallow Si_As and V_As-Si_As acceptor defects. Sn_As, V_Ga-Te (or) V_Al-Te), Ge_As and V_As-Ge_As defects were observed in Sn-, Te- and Ge-doped Al_x Ga_1-xAs after annealing. The defects were identified by plotting their characteristic recombination radiation energies versus Al composition x and comparing with previously established assignments in GaAs (x = 0). Two prominent bands, located at 1.53 and 1.46 eV independent of Al composition x, were detected in annealed (p) Al_xGa_1-xAs: Ge. The exact nature of the defects giving rise to these bands is not known.     

First principles investigation of optical properties of zinc-blende AlxGa1−xAs1−yNy materials

We have performed a first-principle Full Potential Linearized Augmented Plane Waves calculation within the local density approximation (LDA) to the zinc-blende Al_xGa_1−xAs_1−yN_y to predict its optical properties as a function of N and Al mole fractions. The accurate calculations of electronic properties such as band structures and optical properties like refractive index, reflectivity and absorption coefficient of Al_xGa_1−xAs and Al_xGa_1−xAs_1−yN_y with x≤0.375 and y up to 4% are presented. Al_xGa_1−xAs on GaAs have a lattice mismatch less than 0.16% and the lattice constant of Al_xGa_1−xAs has a derivation parameter of 0.0113±0.0024. The band gap energies are calculated by LDA and the band anticrossing model using a matrix element of C_MN=2.32 and a N level of E_N=(1.625+0.069x) eV. The results show that Al_xGa_1−xAs can be very useful as a barrier layer in separate confinement heterostructure lasers and indicate that the best choice of x and y Al_xGa_1−xAs_1−yN_y could be an alternative to Al_xGa_1−xAs when utilized as active layers in quantum well lasers and high-efficiency solar cell structures.     

Structural and electronic properties of zincblende phase of Tl x Ga1−x As y P1−y quaternary alloys: First-principles study

Using the first-principles band-structure method, we have calculated the structural and electronic properties of zincblende TlAs, TlP, GaAs and GaP compounds and their new semiconductor Tl _x Ga_1?x As _y P_1?y quaternary alloys. Structural properties of these semiconductors are obtained with the Perdew and Wang local-density approximation. The lattice constants of Tl _x Ga_1?x As, Tl _x Ga_1?x P ternary and Tl _x Ga_1?x As _y P_1?y quaternary alloys were composed by Vegard’s law. Our investigation on the effect of the doping (Thallium and Arsenic) on lattice constants and band gap shows a non-linear dependence for Tl _x Ga_1?x As _y P_1?y quaternary alloys. The band gap of Tl _x Ga_1?x As _y P_1?y , E _g (x, y) concerned by the compositions x and y. To our awareness, there is no theoretical survey on Tl _x Ga_1?x As _y P_1?y quaternary alloys and needs experimental verification.     

Diamagnetic shift of exciton energy levels in GaAs-Ga1−xAlxAs quantum wells

The results of direct measurements of the diamagnetic shift of axciton levels in narrow quantum wells of a thickness varying between 25 and 150 Å are reported. A perturbation type approach is used to calculate the diamagnetic shift of 1s exciton levels in quantum well structures of Ga_1−xAl_xAs-GaAs-Ga_1−xAl_xAs. The calculations are applicable in the weak field range for which the Coulomb energy dominates over the magnetic one. The experimental results are in satisfactory agreement with the theory throughout the entire well thicknesses range.     

Vibrational spectra of one-dimensional mass-disordered quaternary alloys

The spectral density of vibrational states for one-dimensional mass-disordered quaternary alloys A_yB_1?yC_1?xD_x is evaluated using the negative eigenvalue theorem. Spectra for masses corresponding to Al_yGa_1?yAs_1?xP_x are presented.     

Heterojunction band discontinuities of quaternary semiconductor alloys

A simple sp³ hybrid-orbital model for predicting valence-band discontinuities is proposed and applied to quaternaries. The effects of anion and cation disorder are taken into account explicitly within the coherent-potential approximation. Results for In_1?xGa_xP_1?yAs_y, In_1?xGa_xAs_1?ySb_y, Ga_1?xAl_xAs_1?ySb_y and In_1?xAl_xP_1?ySb_y lattice-matched to some binaries are shown. The broadening of valence-band-top levels due to alloy scattering is found negligible.     

X-ray diffraction studies of heterostructures based on solid solutions Al x Ga1 − x As y P1 − y : Si

The growth of MOCVD-hydride epitaxial heterostructures based on ternary solid solutions Al _x Ga_1?x As heavily doped with phosphorus and silicon has been studied using high-resolution X-ray diffraction and X-ray microanalysis. The prepared epitaxial films are five-component solid solutions (As _x Ga_1?x As _y P_{1 ? y} )_{1 ? z} Si _z .     

A new experimental method for the determination of the one phonon density of states in GaAs

The one phonon density of states in GaAs is obtained from the measured disorder-activated-first-order Raman-scattering in Ga_1?xAl_xAs_x alloys. The agreement with several theoretical determinations is excellent. We discuss the requirements that, both the isoelectronic substitution and the experimental conditions must fullfil in order to apply this method to other III–V compounds.     

Raman scattering determination of folded acoustical phonon dispersion curves in large period GaAs/GaAlAs superlattices

We report the first experimental determination of the dispersion curves of folded acoustical modes in large period GaAs/Ga_1?xAl_xAs superlattices, obtained by scanning the incident wavelength in a Raman scattering experiment. Moreover, the resonant scattering evidences disorder activated Raman lines similar to those already reported in Ga_1?xAl_xAs single layers. These results imply the coexistence in Ga_1?xAl_xAs of dispersive virtual crystal acoustical modes with non dispersive disorder induced contributions. These features are explained on the basis of the Coherent Potential Approximation.     

Thermodynamics of type A1−xBxC1−yDy III–V quaternary solid solutions

An improved theory based on the pair approximation is given for the thermodynamic properties of type A_1?xB_xC_1?yD_yIII–V quaternary solid solutions. The theory takes into account the quasi-chemical nature of the nearest neighbor pair distribution, which has been neglected or inadequately treated in previous work. With a suitable thermodynamic treatment, a quasi-chemical equilibrium relation for the nearest neighbor pair distribution is derived. Numerical calculations using available thermodynamic data show that the distribution will deviate appreciably from random mixing values in the In_1?xGa_xP_1?yAs_y and Al_1?xGa_xAs_1?ySb_y systems due to a short-range clustering effect of nearest neighbor pairs with strong bond energy, whereas the deviation will be small in the Al_1?xGa_xP_1?yAS_y system. The expressions for the chemical potentials and the activity coefficients of the binary compound components are given. These are readily applicable to phase diagram calculations.     

Exciton in wurtzite GaN/AlxGa1−xN coupled quantum dots

Based on effective-mass approximation, we present a three-dimensional study of the exciton in GaN/Al_xGa_1−xN vertically coupled quantum dots (QDs) by a variational approach. The strong built-in electric field due to the piezoelectricity and spontaneous polarization is considered. The relationship between exciton states and structural parameters of wurtzite GaN/Al_xGa_1−xN coupled QDs is studied in detail. Our numerical results show that the strong built-in electric field in the GaN/Al_xGa_1−xN strained coupled QDs leads to a marked reduction of the effective band gap of GaN QDs. The exciton binding energy, the QD transition energy and the electron-hole recombination rate are reduced if barrier thickness L^AlGaN is increased. The sizes of QDs have a significant influence on the exciton state and interband optical transitions in coupled QDs.     

Principal energy band gaps of the quaternary alloy AlxGa1−xSbyAs1−y

The correlated function expansion (CFE) interpolation procedure was presented to efficiently estimate principal energy band gaps and lattice constants of the quaternary alloy Al_xGa_1−xSb_yAs_1−y over the entire composition variable space. The lattice matching conditions between x and y for the alloy Al_xGa_1−xSb_yAs_1−y substrated to InAs and GaSb were obtained by optimizing the alloy lattice constant to that of the substrates. The corresponding principal band gaps (E(Γ), E(L), and E(X)) were also calculated along the lattice matching condition on each substrate (InAs and GaSb).     

Tenseur de mobilite d'un super-reseau en presence de desordre d'interface

Mobility tensor, limited by island-like interface disorder, in type-I superlattices, is studied within a simple model (Kronig-Penney and plane waves). Computed results are presented, for GaAs-Ga_1?xAl_xAs superlattices, versus variations of temperature, superlattice period and defect parameters such as localisation, width and thickness. This interface scattering process has a significant influence on near equilibrium transport at low temperature.     

Modification of the electronic structure and formation of an accumulation layer in ultrathin Ba/n-GaN and Ba/n-AlGaN interfaces

The electronic structure of the n-GaN(0001) and Al _x Ga_{1 ? x} N(0001) (x = 0.16, 0.42) surfaces and the Ba/n-GaN and Ba/AlGaN interfaces is subjected to in situ photoemission investigations in the submonolayer Ba coverage range. The photoemission spectra of the valence band and the spectra of the surface states and the core 3d level of Ga, the 2p level of Al, and the 4d and 5p levels of Ba are studied during synchrotron excitation in the photon energy range 50–400 eV. A spectrum of the surface states in Al _x Ga_{1 ? x} N (x = 0.16, 0.42) is found. The electronic structure of the surface and the near-surface region is found to undergo substantial changes during the formation of the Ba/n-GaN and Ba/AlGaN interfaces. The effect of narrowing the photoemission spectrum in the valence band region from 10 to 2 eV is detected, and surface eigenstates are suppressed. The Ba adsorption is found to induce the appearance of a new photoemission peak in the bandgap at the Fermi level in the Ba/n-GaN and Ba/n-Al_0.16Ga_0.84N interfaces. The nature of this peak is found to be related to the creation of an accumulation layer due to a change in the near-surface potential and enriching band bending. The energy parameters of the potential well of the accumulation layer are shown to be controlled by the Ba coverage.     

Theoretical research on optical properties and quantum efficiency of Ga1−xAlxN photocathodes

In order to design the optimal component structure of transmission-mode (t-mode) Ga_1−xAl_xN photocathode, the optical properties and quantum efficiency of Ga_1−xAl_xN photocathodes are simulated. Based on thin film principle, optical model of t-mode Ga_1−xAl_xN photocathodes is built. And the quantum efficiency formula is put forward. Results show that Ga_1−xAl_xN photocathodes can satisfy the need of detectors with “solar blind” property when the Al component is bigger than 0.375. There is an optimal thickness of Ga_1−xAl_xN layer to get highest quantum efficiency, and the optimal thickness is 0.3 μm. There is close relation between absorptivity and quantum efficiency, which is in good agreement with the “three-step” model. This work gives a reference for the experimental research on the Ga_1−xAl_xN photocathodes.     

Self-consistent calculations of the density of 2D electron gas in AlxGa1−xAsGaAs heterointerface as a function of the doping level in n-AlxGa1−xAs layer

Sheet density of two-dimensional electron gas at the Al_xGa_1?xAs heterointerface is calculated self-consistently as a function of doping level in n-Al_xGa_1?xAs layer by taking into account shallow donor level and the DX centers. The results show a good agreement with the experimental results reported by Ishikawa et al. Dingle's rule of the conduction band discontinuity does not agree with the present results but with 0.62 rule proposed recently by Watanabe et al.     

X-ray spectra and electronic band structure of nitrogen in Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N solid solid solutions

The electronic energy structure of the valence band and the x-ray absorption near edge structure (XANES) region of nitrogen in Al _x Ga_1?x N solid solutions and binary crystals of gallium nitride GaN and aluminum nitride AlN are calculated using the local coherent potential method and the cluster version of the muffin-tin approximation within the framework of the multiple scattering theory. It is demonstrated that the calculated electron densities of states correlate with the nitrogen K x-ray emission and nitrogen K x-ray absorption spectra. The electronic energy structure of the top of the valence band and the XANES region in Al _x Ga_1?x N solid solutions are compared with those in the binary crystals of the GaN and AlN nitrides, and an interpretation of their specific features is proposed. An analogy is drawn between the evolution of the electronic energy structure of the valence band and the XANES region in the alloys under investigation and the evolution of the electronic band structure in the Al _x B_1?x N and B _x Ga_1?x N alloys. General trends in the transformation of the structure and variations in properties of these alloys are discussed.     

Non-equivalent minima scattering in n-type Ga1−xAlxAs alloys from Monte Carlo methods

Using the known conduction band structure for Ga_1?xAl_xAs alloys, the electron drift mobilities in the Γ, L and X minima have been calculated as a function of pressure using Monte Carlo methods. These mobilities are found to decrease first, show a minimum at a pressure near the Γ-L minima crossover and then increase again with pressure due to strong non-equivalent intervalley scattering. The calculated Hall mobility also shows similar behaviour and the results are in qualitative agreement with the experimental observations reported earlier.     

Photoluminescence properties of heavily doped heterostructures based on (Al x Ga1 − x As)1 − y Si y solid solutions

It has been established that the photoluminescence spectra of heavily doped heterostructures based on Al _x Ga_{1 ? x} As)_{1 ? y} Si _y solid solutions exhibit quenching of the main exciton bands of Al _x Ga_{1 ? x} As ternary solid solutions and appearance of other maxima. The quenching of the main exciton bands can be associated both with the DX-center formation and with the change in the character of the band structure of (Al _x Ga_{1 ? x} As)_{1 ? y} Si _y quaternary solid solutions.