Superstructured ordering in Al x Ga1 − x As and Ga x In1 − x P alloys

Epitaxial heterostructures produced on the basis of Al _x Ga_{1 ? x} As and Ga _x In_{1 ? x} P ternary alloys by metal-organic chemical vapor deposition are studied. The composition parameter x of the alloys was ～0.50. By X-ray diffraction studies, scanning electron microscopy, atomic force microscopy, and photoluminescence spectroscopy, it is shown that superstructured ordered phases with the stoichiometry composition III_{1 ? η}III_{1 + η}V₂ can be formed. As a consequence of this effect, not only does the cubic crystal symmetry change to the tetragonal type in the new compound, but also the optical properties become different from those of disordered alloy with the same composition.     

Relaxation of crystal lattice parameters and structural ordering in In x Ga1 − x As epitaxial alloys

Epitaxial In _x Ga_{1 − x} As/GaAs(100) heterostructures grown by the MOC-hydride method with a considerable lattice mismatch are studied by X-ray diffraction and scanning electron microscopy. The relaxation coefficient of the crystal lattice of the epitaxial alloy is calculated and the deformation energy is evaluated. It is shown that, at a concentration of the In atoms in metal sublattice close to x = 0.5, the superstructural phase formed on the surface of the epitaxial In _x Ga_{1 − x} As alloy is the InGaAs₂ compound with a layered tetragonal crystal lattice and ordered arrangement of the atoms of the metal sublattice in the growth plane of the epitaxial film.     

In As1–x Sb x heteroepitaxial structures on compositionally graded GaInSb and AlGaInSb buffer layers

Semiconductors - Unrelaxed InAs1–x Sb x (x = 0.43 and 0.38) alloy layers are produced by molecular-beam epitaxy on compositionally graded GaInSb and AlGaInSb buffer layers. The high quality...     

Determination of the valence-band offset and its temperature dependence in isotypic heterojunctions p-AlxGa1−xAs/p-AlyGa1−yAs from C-V measurements

The isotypic heterojunctions p-Al_xGa_1−x As/p-Al_yGa_1−y As, grown by MOVPE on n-GaAs substrates, have been investigated by the voltage-capacitance method at temperatures ranging from 300 to 100 K. To determine the valence-band offset ΔE _V and the built-in charge in the heterojunction, the Poisson equation was solved numerically on a nonuniform coordinate grid. The incomplete ionization of the acceptors and the different magnitude of the permittivity in different layers of the heterostructure were taken into account in the calculation. It was found that for a p-Al _0.2 a_0.8As/p-Al _0.5 Ga_0.5As heterojunction ΔE _V at room temperature is 39% (113 meV) of the total gap ΔE _g and decreases monotonically to 35% at T=120 K. Fiz. Tekh. Poluprovodn. 33, 940–944 (August 1999)     

Properties of epitaxial (Al x Ga1 − x As)1 − y C y alloys grown by MOCVD autoepitaxy

The growth of epitaxial Al _x Ga_{1 ? x} As:C alloys by metal-organic chemical vapor deposition (MOCVD) at low temperatures results in the formation of quaternary (Al _x Ga_{1 ? x} As)_{1 ? y} C _y alloys, in which carbon atoms can be concentrated at lattice defects in the epitaxial alloy with the formation of impurity nanoclusters.     

Microwave Synthesis and Characterization of the Series Co1−x Fe x Sb3 High Temperature Thermoelectric Materials

The use of microwave energy for materials processing has a major potential and real advantages over conventional heating such as (1) time and energy savings, (2) rapid heating rates (volumetric heating vs. conduction), (3) considerably reduced processing time and temperature, (4) fine microstructures and hence improved mechanical properties and better product performance, and (5) finally lower environmental impact. In this study, we investigated the use of microwave-assisted synthesis to synthesize a series of Co_1?x Fe _x Sb₃ using this novel approach, which gave high quality materials with little or no impurities in a fraction of the time needed for conventional synthesis. X-ray diffraction analysis was used to examine the structure and the lattice parameters of the samples, while scanning electron microscopy with energy dispersive x-ray spectroscopy was used to study the morphology of the compounds. The samples were sintered by spark plasma sintering, and the highest ZT of 0.33 was obtained for x = 0.2 at 700 K.     

Phase formation under the effect of spinodal decomposition in epitaxial alloys of Ga x In1 − x P/GaAs(100) heterostructures

The effect of instability of alloys of Ga _x In_{1 − x} P/GaAs(100) semiconductor epitaxial heterostructures in the composition region x ≈ 0.50 is studied by X-ray diffraction and electron microscopy. The possibility of emergence of modulated relaxation structures on the surface of a Ga _x In_{1 − x} P alloy is shown. This phenomenon is accompanied by the emergence of satellites of main X-ray reflections corresponding to a single-phase structure.     

Structural and Thermoelectric Properties of Bi1−x Sb x Nanoalloys Prepared by Mechanical Alloying

Bi_1?x Sb _x nanoparticles were prepared by mechanical alloying and compacted using different techniques. The influence of the composition as well as the pressing conditions on the thermoelectric performance was investigated. A strong dependence of the thermoelectric properties on the composition was found, which deviates from the behavior of single crystals. The results indicate a significant change in the band structure of the material induced by the reduced size. The influence of the pressing conditions on the thermoelectric properties also showed composition dependence. The results show that the compacting method has to be chosen carefully.     

Width of the excitonic absorption line in Al x Ga1 − x as alloys

Experimental data on the width of the absorption line corresponding to the exciton ground state in high-quality samples of Al _x Ga_{1 ? x} As (x = 0.15) quasi-binary alloy, obtained by Seisyan et al. (2005), are analyzed. The line corresponding to the 1s state is divided into separate components taking into account strain-induced splitting; Lorentzian- and Gaussian-shape contributions to the absorption curve; and overlap with the continuum absorption band, which broadens with increasing temperature. Analysis of the integrated absorption, relying on the fact that the temperature dependence of the absorption is typical of exciton polaritons in media possessing spatial dispersion, was carried out with the following parameters: critical value of the dissipation damping of the exciton Γ_c = 0.32 meV and maximum value of the absorption coefficient K _max = 89.5 eV/cm. This analysis makes it possible to determine the homogeneous component of the line broadening, which, up to the critical temperature T _c = 155 K does not exceed 0.2 meV. It is found that the “natural” width of the 1s-exciton line does not exceed 2.6 meV at T = 1.7 K, which is in agreement with theoretical estimates. At T = 1.7–60 K, the inhomogeneous broadening of the exciton peak related to the exciton scattering and localization by fluctuations of the alloy composition exceeds contributions related to phonons and ionized impurities by more than an order of magnitude and is dominant, but does not affect the integrated absorption by excitonic polaritons.     

Features of defect formation during the growth of double heterostructures for injection lasers based on Al x Ga1 − x As y Sb1 − y /GaSb materials

A lack of lattice defects and, in particular, a lack of dislocations in the active layer in complex multilayer heteroepitaxial systems is the basic condition for the efficient and reliable operation of optoelectronic microdevices. Minimum elastic stresses in multilayer heteroepitaxial systems and their lack in the active layer at that elevated temperature that occurs in an efficiently operating electronic device is the second necessary condition for its long-term operation.     

Photoelectrical and electrical properties of polycrystalline CdxHg1−x Te layers on GaAs substrates

Temperature dependences of electrical conductivity, concentration, and mobility of electrons, as well as photoconductivity spectra and conductivity-illumination characteristics of Cd_0.8Hg_0.2Te polycrystalline layers grown on GaAs substrates are studied. The features of charge transport and photoconductivity of Cd_xHg_1?x Te/CdTe/GaAs structures are discussed. It is established that a high photoconductivity at a temperature of 300 K and a jump in conductivity-illumination characteristics at high levels of excitation are caused by the influence of electrically active grain boundaries, which produce the potential barriers for the drift and recombination of charge carriers. It is shown within the framework of the semiconductor barrier model with a random potential relief pattern that, for high levels of excitation by the radiation pulses of ruby or neodymium lasers, the height of potential barriers at the grain boundaries lowers due to screening by nonequilibrium carriers.     

Effect of the material of injecting contacts on the CVCs of Pb1 − x Sn x Te:In films

The flowing of the injection current in Pb_{1 ? x} Sn _x Te:In structures (x ≥ 0.3) with various metal contacts at the temperature of helium is considered. The current-voltage characteristics (CVCs) of the structures in the dark and when they are illuminated from the blackbody model are given. It is found that the CVCs depend on the materials of the metal contact. The localized state density distribution over the forbidden band in structures with various contacts is given and the influence of the contact region and localized states on the CVC is discussed. The problems of constructing photodetectors with optimum threshold characteristics are discussed.     

Dispersion of the refractive index of epitaxial Pb1 ? xEuxTe (0 ≤ x ≤ 1) alloy layers below the absorption edge

The transmittance spectra of epitaxial Pb_{1 − x} Eu _x Te (0 ≤x ≤ 0.1) alloy layers are exploited to study the dispersion of their refractive index in the spectral range from 650 to 8000 cm⁻¹ (below the absorption edge). The refractive index and the position of the absorption edge as functions of the composition parameter of the alloys are determined at two temperatures, 80 and 295 K. The refractive index is calculated in the context of the classic wave concepts of propagation of electromagnetic radiation. The experimentally determined dispersion dependences are described by the empiric Sellmeier expression of the second order. From analysis of the transmittance of the layers, it follows that the band gap of the epitaxial Pb_{1 − x} Eu _x Te alloys increases with increasing temperature at x < 0.5 and decreases at x > 0.5.     

Bulk moduli of PbSxSe1−x,PbSxTe1−x and PbSexTe1−x from a thermodynamical model compared to generalized gradient approximation approach

Very recently, first-principle technique of full-potential linearized augmented plane-wave method, by using for exchange-correlation potential the generalized gradient approximation (GGA), was employed for the study of the lead chalcogenide semiconductors’ alloys PbS_xSe_1−x, PbS_xTe_1−x and PbSe_xTe_1−x. These density functional calculations led to the determination of structural, electronic and optical properties, including the values of lattice constants and bulk moduli as a function of composition. Here, we investigate the latter properties, but by employing a thermodynamical model which has been suggested for the formation and migration of defects in solids including several recent applications in semiconductors. The following crucial difference emerges when comparing the present results with those deduced by density functional calculations: Among the alloys studied, GGA calculations identify that PbS_xTe_1−x exhibits the most evident non-linear variation of the bulk modulus versus the composition, while according to the thermodynamical model such an evident non-linear behavior – and maybe somewhat stronger – is also expected for PbSe_xTe_1−x. A tentative origin of this diversity is discussed.     

Growth and Characterization of In x Ga1−x N Multiple Quantum Wells Without Phase Separation

Efficient conversion of photon energy into electricity is a crucial step toward a sustainable solar-energy economy. Likewise, solid-state lighting devices are gaining prominence because of benefits such as reduced energy consumption and reduced toxicity. Among the various semiconductors investigated, In _x Ga_1–x N alloys or superlattices are fervently pursued because of their large range of bandgaps between 0.65 eV and 3.4 eV. This paper reports on the fabrication of multiple quantum wells on LiGaO₂ (001) substrates by plasma-assisted molecular beam epitaxy. Metal modulated epitaxy was utilized to prevent formation of metal droplets during the growth. Streaky patterns, seen in reflection high-energy electron diffraction, indicate two-dimensional growth throughout the device. Postdeposition characterization using scanning electron microscopy also showed smooth surfaces, while high-resolution x-ray diffraction and high-resolution transmission electron microscopy confirm the epitaxial nature of the overall quantum well structure.     

SIMS quantification of As and In in Hg1−xCdxTe materials of different x values

This paper presents a study of Secondary Ion Mass Spectrometry (SIMS) quantification of As and In in Hg_1–xCd_xTe materials. The SIMS results show that for Hg_1-xCd_xTe with x from 0.2 to 0.8, As and In quantification is independent of the x-values (0.2–0.8). The relative sensitivity factors (RSF) for In and As derived from the standard of one x value can be used to accurately quantify unknown samples of different x value(s). We also determined the dependence of sputtering rate vs. x values under oxygen beam bombardment.     

Temperature and hydrostatic pressure dependence of the electronic structure of AlxGa1−xAs alloys

The study of the electronic band structure of Al_xGa_1−-xAs alloy is calculated within the local empirical pseudo-potential method including the effective disorder potential into the virtual crystal approximation. Monotonic decreasing and increasing functions are obtained for the temperature and pressure dependent form factors, respectively. Some physical quantities as band gaps, bowing parameters, refractive indices, and dielectric constants of the considered alloy with different Al concentration are calculated under the effects of temperature and hydrostatic pressure. The obtained results have been found in good agreement with the experimental and published data.     

Ionization of the sulfur-related DX Center in In 1 − x Ga x P in an electric field

Using deep-level transient spectroscopy and capacitance-voltage measurements, the effect of electric field on ionization of the sulfur-related DX center is investigated for n-In_{1 ? x} Ga _x P layers (x = 0.65) grown by vapor-phase epitaxy. It is shown that, upon increasing the electric field strength from 1.3 × 10⁴ to 1.9 × 10⁵ V/cm, the activation energy of the center decreases from ～0.38 to ～0.26 eV, and the ionization process is governed by multiphonon tunneling. By the estimated tunneling time of the defect, it is concluded that the DX center corresponds to a model of large lattice relaxation.     

Spectral shift of photoluminescence bands of the (SiC)1−x (AlN)x epitaxial films due to laser annealing

The effect of laser annealing on the photoluminescence properties of (SiC)_1?x (AlN)_x epitaxial films was studied. It was proposed that annealing causes the displacement of the Al and N atoms from their lattice sites and the formation of Al_Si-N_C donor-acceptor pairs acting as the luminescence centers. According to this model, the increase in the annealing time is accompanied by the formation of donor-acceptor pairs with the shortest interatomic distances at the expense of associations of the distant defects and by a shift of the respective photoluminescence band to the high-energy spectral region.