Effect of growth conditions on incorporation of Si into Ga and As sublattices of GaAs during molecular-beam epitaxy

The effect of dopant concentration and growth-surface crystallographic orientation on the incorporation of Si into Ga and As sublattices was investigated during GaAs molecular-beam epitaxy. The epitaxial layers (epilayers) were grown on GaAs substrates with (100), 2°(100), 4°(100), and 8°(100) orientations at a temperature of 520°C and with (111)A, 2°(111)A, 2°(111)A, 5°(111)A, 6°(111)A, and 8°(111)A (where A = Ga) orientations at a temperature of 480°C. The Sidopant concentration was varied within 10¹⁷–10¹⁹ cm^?3. Through electrical and photoluminescent methods of investigation, the Si impurity was found to occur at the sites of both GaAs-layer sublattices not only as simple donors and acceptors (Si_Ga and Si_As), but also as Si_Ga-Si_As, Si_Ga-V_Ga, and Si_As-V_As complexes. The concentration of Si impurity in various forms depends on the doping level of the layers and on the growth-surface orientation. Amphoteric properties of Si manifest themselves more prominently on the (111)A face than on the (100) one. It is shown that impurity defects form at the stage of layer crystallization and depend on the growth-surface structure.     

Structural and photoluminescence properties of low-temperature GaAs grown on GaAs(100) and GaAs(111)A substrates

Undoped, uniformly Si-doped, and δ-Si-doped GaAs layers grown by molecular-beam epitaxy on (100)- and (111)A-oriented GaAs substrates at a temperature of 230°C are studied. The As₄ pressure is varied. The surface roughness of the sample is established by atomic-force microscopy; the crystal quality, by X-ray diffraction measurements; and the energy levels of different defects, by photoluminescence spectroscopy at a temperature of 79 K. It is shown that the crystal structure is more imperfect in the case of GaAs(111)A substrates. The effect of the As₄ flux during growth on the structure of low-temperature GaAs grown on different types of substrates is shown as well.     

Photoreflectance Spectroscopy Study of LT-GaAs Layers Grown on Si and GaAs Substrates

The mechanical strains and densities of surface charge states in GaAs layers grown by low-temperature (LT) molecular-beam epitaxy on Si(100) and GaAs(100) substrates are investigated by photoreflectance spectroscopy. Lines corresponding to the fundamental transition (E _g ) and the transition between the conduction band and spin-orbit-split valence subband (E _g + Δ _SO ) in GaAs are observed in the photoreflectance spectra of Si/LT-GaAs structures at 1.37 and 1.82 eV, respectively. They are shifted to lower and higher energies, respectively, relative to the corresponding lines in GaAs/LT-GaAs structures. Comparing the spectra of the Si/LT-GaAs and GaAs/LT-GaAs structures, it is possible to estimate mechanical strains in LT-GaAs layers grown on Si (by analyzing the spectral-line shifts) and the density of charge-carrier states at the GaAs/Si heterointerface (by analyzing the period of Franz–Keldysh oscillations).     

Piezospectroscopic study of the emission band of <Emphasis Type="Italic">n</Emphasis>-GaAs:S peaked at about 1.2 eV

The effect of uniaxial pressure up to 8 kbar applied along the crystallographic directions [111] and [001] on the spectra and polarization of a broad photoluminescence band of n-GaAs:S peaked at 1.2 eV, which is related to the trapping of electrons by V_GaS_As complexes, was studied. The dependences of the polarization of the band emission on the photon energy and temperature indicate that the initial trigonal symmetry of the V_GaS_As complex is reduced and can be monoclinic with a 011 plane of symmetry. In this case, the distortions of a complex are not reoriented and the axis of its emitting optical dipole deviates at nearly the same angle from the 〈111〉 and 〈001〉 directions lying in the plane of symmetry. Selection of the components of the photoluminescence band, split due to the uniaxial pressure, based on the piezospectroscopic behavior of the emission from anisotropic centers, confirms the model proposed for the V_GaS_As complex and shows that the rotator contribution to the emission of this complex is small. Comparison of the results obtained with the data for a similar complex, V_GaTe_As, indicate a significant change in the properties of vacancy-shallow-level donor complexes when a donor of Group VI is replaced by a different donor of the same group.     

The Stark shift of the hole states in separate InAs/GaAs quantum dots grown on (100) and (311)A GaAs substrates

Deep-level transient spectroscopy is used to study charge-carrier emission from the states of separate quantum dots in InAs/GaAs p-n heterostructures grown on (100)-and (311)A-oriented GaAs substrates in relation to the reverse-bias voltage U. It is established that the structures under consideration exhibit different bias-voltage dependences of the Stark shift for the energy levels of the quantum-dot states on the value of U.     

Effective electron mass in heavily doped GaAs in the ordering of impurity complexes

Spectra of edge photoluminescence (PL) at 300 K have been studied in a set of Czochralski-grown Te-doped GaAs single crystals with a free carrier density of n₀=10¹⁷–10¹⁹ cm^?3. The carrier density dependences of the chemical potential and band gap narrowing are obtained by analyzing the PL spectral line profiles. The dependence of the effective mass of electrons at the bottom of the conduction band on their density, m ₀ ^* (n₀), is calculated. It is shown that the nonmonotonic m ₀ ^* (n₀) dependence correlates with data on electron scattering in the material under study and results from the ordering of impurity complexes.     

Photoluminescence of electron-hole plasma in semi-insulating undoped GaAs

The dependence of the photoluminescence spectrum of electron-hole plasma in semi-insulating undoped GaAs on the concentration of the background carbon impurity N_C(3×10¹⁵ cm^?3≤N_C≤4×10¹⁶ cm^?3) is studied at 77 K. It is established that the density of the electron-hole plasma, which is equal to n _e?h ≈1.1×10¹⁶ cm^?3 in crystals with the lowest impurity concentration at an excitation intensity of 6×10²² photons/(cm² s), decreases considerably as the value of N_C increases in the range mentioned above. A decrease in the density of the electron-hole plasma with increasing N_C is attributed to the effect of fluctuations in the carbon concentration N_C, which give rise to a nonuniform distribution of interacting charge carriers and to localization of holes in the tails of the density of states of the valence band.     

On the growth,structure, and surface morphology of epitaxial CdTe films

The structure and surface morphology of epitaxial CdTe films grown on glassy substrates with and without compensation with an additional Te vapor source during growth are studied. The optimal conditions of the production of structurally perfect epitaxial films with a pure smooth surface with no inclusions of another phase (T _so = 1000–1100 K, T _su = 570–670 K) are determined. It is established that, on glassy substrates, the epitaxial films grow via the (111) plane of the face-centered cubic (fcc) lattice with the parameter a = 6.481 Å. By varying the temperature of the main and compensating sources, CdTe films with n- and p-type conductivity are produced.     

CdHgTe heterostructures for new-generation IR photodetectors operating at elevated temperatures

The parameters of multilayer Cd_xHg_1–xTe heterostructures for photodetectors operating at wavelengths of up to 5 μm, grown by molecular-beam epitaxy (MBE) on silicon substrates, are studied. The passivating properties of thin CdTe layers on the surface of these structures are analyzed by measuring the C–V characteristics. The temperature dependences of the minority carrier lifetime in the photoabsorption layer after growth and thermal annealing are investigated. Samples of p ⁺–n-type photodiodes are fabricated by the implantation of arsenic ions into n-type layers, doped with In to a concentration of (1–5) × 10¹⁵ cm^–3. The temperature dependences of the reverse currents are measured at several bias voltages; these currents turn out to be almost two orders of magnitude lower than those for n ⁺–p-type diodes.     

Stimulated radiation of optically pumped Cd <Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript> Te-Based heterostructures at room temperature

The experimental observation of stimulated radiation of optically pumped Cd _x Hg_{1 ? x} Te-based heterostructures in the wavelength range of 1.4–4.5 μm is reported. In the experiments, graded-gap Cd _x Hg_{1 ? x} Te samples grown on GaAs and Si substrates by molecular beam epitaxy were used. Superluminescence of such structures was observed at 77–300 K under the pulsed pumping of the samples by a Nd:YAG laser at a wavelength of 1.064 μm. At room temperature, stimulated radiation was observed at wavelengths of 1.4–1.7 μm. The obtained experimental data are the first results on the observation of stimulated radiation from graded-gap Cd _x Hg_{1 ? x} Te structures on Si and GaAs substrates at these wavelengths at room temperature.     

Magnetic ordering effects in heavily doped GaAs:Fe crystals

The exchange coupling of Fe centers in GaAs crystals is studied by electron spin resonance (ESR). Transitions to a superparamagnetic state and to an impurity ferromagnetism domain are analyzed. A study of a system of single-domain magnetically ordered regions in GaAs:Fe with the transition to a ferromagnetic state occurring at the temperature T_C1 = 460 K is described. It is shown that impurity ferromagnetism with a transition temperature T_C2 of 60 K in a disordered system of Fe centers randomly distributed among superparamagnetic regions exists in GaAs:Fe.     

Structural transformations and optical properties of As<Subscript>2</Subscript>S<Subscript>3</Subscript> chalcogenide glasses

The effects of melt temperature T _i and quenching rate V _i on the structure and optical properties of As₂S₃ glasses is studied. It is found that the glass band gap increases with T _i and V _i , whereas a decrease is observed in the glass density, refractive index (from 2.71 to 2.48), and two-photon absorption coefficient (from 0.37 to 0.15 cm/MW), which is accompanied by an increase in the optical-breakdown damage threshold.     

Thermal transport of charge and polarization of 1.2-eV luminescence broad band in uniaxially strained <Emphasis Type="Italic">n</Emphasis>-GaAs:Te

The effect of temperature on the pressure-induced polarization of the photoluminescence broad band related to V_GaTe_As complexes in GaAs:Te is analyzed. It is shown that, within a certain temperature range, the pressure-induced difference in the activation energy of thermal emission of holes for dissimilarly oriented complexes leads to an abrupt increase in the integral polarization of radiation caused by recombination of free electrons with holes localized at these complexes compared to polarization at lower temperatures. In the samples subjected to 10-kbar pressure applied along the [111] axis, the described behavior of polarization was observed in the temperature range from 140 to 190 K. The results obtained validate the previously suggested model of a set of defects that give rise to the luminescence band under study. Fitting results of calculations to experimental data enabled us to correct the values of a number of the parameters of defects and to estimate a pressure-induced change in the activation energy of the thermal emission of holes from the defects with different orientation.     

Transition from the type-II broken-gap heterojunction to the staggered one in the GaInAsSb/InAs(GaSb) system

Conditions for the transition from the staggered heterojunction to the type-II broken-gap one were considered for isolated Ga_1?x In_xAs_ySb_1?y /InAs(GaSb) heterostructures in relation to the quaternary alloy composition. Energy-band diagrams of such heterojunctions were estimated and energy band offsets Δ at the heterointerface were determined. It was experimentally found that the type-II broken-gap heterojunction in the Ga_1?x In_xAs_ySb_1?y /p-InAs structure is observed in the entire range of composition parameters under study, 0.03 < x < 0.23, and becomes staggered in the range 0.3 < x < 1. In p-Ga_1?x In_xAs_ySb_1?y /p-GaSb heterostructures with the indium content 0.85 < x < 0.92 in the solid phase, the p-type conductivity is observed, which is indicative of the staggered heterojunction. At x > 0.92, the contribution of electrons of the semimetal channel at the heterointerface to the total conductivity was observed, as well as the transition from the staggered heterojunction to the type-II broken-gap one.     

Carrier Transport Mechanism and Band Offsets at the Interface of ZnS/<Emphasis Type="Italic">n</Emphasis>-Si (111) Heterojunctions Fabricated by Vacuum Thermal Evaporation

The electrical properties and band offset of ZnS/n-Si(111) heterojunctions with and without annealing were analyzed. The result showed that the rectifying characteristics of ZnS/n-Si(111) heterojunctions became better and the leakage current increased after annealing. This phenomenon is mostly due to the volatilization of S atoms of ZnS films and leads to defect levels appearing at the interface of the ZnS/n-Si(111) hetrojunctions. The valence band offset (ΔE _V) of the ZnS/n-Si(111) heterojunctions can be calculated to be ?0.7 ± 0.15 eV by means of photoelectron spectroscopy, indicating that the band offsets of ZnS/n-Si(111) heterojunctions show a type-II band alignment.     

Resonant Raman scattering and atomic force microscopy of InGaAs/GaAs multilayer nanostructures with quantum dots

The transition from two-dimensional (2D) pseudomorphic growth to the three-dimensional (3D) (nanoisland) growth in In_xGa_1?xAs/GaAs multilayer structures grown by molecular-beam epitaxy was investigated by atomic force microscopy, photoluminescence, and Raman scattering. The nominal In content x in In_xGa_1?xAs was varied from 0.20 to 0.50. The thicknesses of the deposited In_xGa_1?xAs and GaAs layers were 14 and 70 monolayers, respectively. It is shown that, at these thicknesses, the 2D-3D transition occurs at x ≥ 0.27. It is ascertained that the formation of quantum dots (nanoislands) does not follow the classical Stranski-Krastanov mechanism but is significantly modified by the processes of vertical segregation of In atoms and interdiffusion of Ga atoms. As a result, the In_xGa_1?xAs layer can be modeled by a 2D layer with a low In content (x < 0.20), which undergoes a transition into a thin layer containing nanoislands enriched with In (x > 0.60). For multilayer In_xGa_1?xAs structures, lateral alignment of quantum dots into chains oriented along the \([\overline 1 10]\) direction can be implemented and the homogeneity of the sizes of quantum dots can be improved.     

High-efficiency dual-junction GaInP/GaAs tandem solar cells obtained by the method of MOCVD

Monolithic dual-junction GaInP/GaAs solar cells grown by the MOCVD method were studied. The conditions of the growth of ternary Ga _x In_1?x P and Al _x In_1?x P alloys lattice-matched to GaAs are optimized. Technology for fabrication of a tunneling diode with a high peak current density of 207 A/cm² on the basis of heavily doped n ⁺⁺-GaAs:Si and p ⁺⁺-AlGaAs:C layers is developed. Cascade GaInP/GaAs solar cells obtained as a result of relevant studies featuring a good efficiency of the solar-energy conversion both for space and terrestrial applications. The maximum value of the GaInP/GaAs solar-cell efficiency was 30.03% (at AM1.5D, 40 suns).     

Growth,structure, and properties of GaAs-based (GaAs)1–x–y (Ge2) x (ZnSe) y epitaxial films

The possibility of growing the (GaAs)_1–x–y (Ge₂) _x (ZnSe) _y alloy on GaAs substrates by the method of liquid-phase epitaxy from a tin solution–melt is shown. X-ray diffraction shows that the grown film is single-crystal with the (100) orientation and has the sphalerite structure. The crystal-lattice parameter of the film is a _f = 0.56697 nm. The features of the spectral dependence of the photosensitivity are caused by the formation of various complexes of charged components. It is established that the I–V characteristic of such structures is described by the exponential dependence I = I ₀exp(qV/ckT) at low voltages (no higher than 0.4 V) and by the power dependence J ~ V ^α, where the exponent α varies with increasing voltage at high voltages (V > 0.5 V). The results are treated within the framework of the theory of the drift mechanism of current transfer taking into account the possibility of the exchange of free carriers within the recombination complex.     

Type II broken-gap GaSb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>As<Emphasis Type="Italic">x</Emphasis>/InAs heterojunction (<Emphasis Type="Italic">x</Emphasis> < 0.15): Evolution of the band diagram for the ternary solid solution

Layers of the GaSb_{1 ? x} As _x alloy with arsenic content in the range x = 0.06–0.15 have been grown for the first time on InAs (100) substrates by metal-organic vapor-phase epitaxy. A new approach to the calculation of the band diagram of the GaSbAs alloy is suggested. It is demonstrated on the basis of magnetotransport measurements in p-GaSbAs/p-InAs heterostructures and with the method suggested by the authors for the calculation of band diagrams for alloys in the GaSbAs system that, in the composition range under study, the GaSbAs/InAs heterojunction is a type II broken-gap heterojunction.