Modeling of Ga1−xInxAs1−y−zNySbz/GaAs quantum well properties for near-infrared lasers

The aim of this work is to model the properties of GaInAsNSb/GaAs compressively strained structures. Indeed, Ga_1?xIn_xAs_1?y?zN_ySb_z has been found to be a potentially superior material to GaInAsN for long wavelength laser dedicated to optical fiber communications. Furthermore, this material can be grown on GaAs substrate while having a bandgap smaller than that of GaInNAs. The influence of nitrogen and antimony on the bandgap and the transition energy is explored. Also, the effect of these two elements on the optical gain and threshold current density is investigated. For example, a structure composed of one 7.5 nm thick quantum well of material with In=30%, N=3.5%, Sb=1% composition exhibits a threshold current density of 339.8 A/cm² and an emission wavelength of 1.5365 μm (at T=300 K). It can be shown that increasing the concentration of indium to 35% with a concentration of nitrogen and antimony, of 2.5% and 1%, respectively, results in a decrease of the threshold current density down to 253.7 A/cm² for a two well structure. Same structure incorporating five wells shows a threshold current density as low as 221.4 A/cm² for T=300 K, which agrees well with the reported experimental results.     

Room-Temperature Quantum Cascade Laser: ZnO/Zn1−x Mg x O Versus GaN/Al x Ga1−x N

A ZnO/Zn_1?x Mg _x O-based quantum cascade laser (QCL) is proposed as a candidate for generation of THz radiation at room temperature. The structural and material properties, field dependence of the THz lasing frequency, and generated power are reported for a resonant phonon ZnO/Zn_0.95Mg_0.05O QCL emitting at 5.27 THz. The theoretical results are compared with those from GaN/Al _x Ga_1?x N QCLs of similar geometry. Higher calculated optical output powers [ $ {P}_{\rm{ZnMgO}} $  = 2.89 mW (nonpolar) at 5.27 THz and 2.75 mW (polar) at 4.93 THz] are obtained with the ZnO/Zn_0.95Mg_0.05O structure as compared with GaN/Al_0.05Ga_0.95N QCLs [ $ {P}_{\rm{AlGaN}} $  = 2.37 mW (nonpolar) at 4.67 THz and 2.29 mW (polar) at 4.52 THz]. Furthermore, a higher wall-plug efficiency (WPE) is obtained for ZnO/ZnMgO QCLs [24.61% (nonpolar) and 23.12% (polar)] when compared with GaN/AlGaN structures [14.11% (nonpolar) and 13.87% (polar)]. These results show that ZnO/ZnMgO material is optimally suited for THz QCLs.     

Fundamentals of Ge1−xSnx and SiyGe1−x-ySnx RPCVD epitaxy

We have studied epitaxial growth of Ge_1−xSn_x and Si_yGe_1−x-ySn_x materials in 200 mm and 300 mm industrial CVD reactors using industry standard precursors. The growth kinetics of undoped GeSn were firstly studied via varying growth parameters including growth temperatures, GeH₄and SnCl₄precursor flows, which indicated that the material growth is highly dependent on surface kinetic limitations involving the SnCl₄reaction pathway. Secondly, the growth kinetics of doped layer growth by varying the growth temperatures and the PH₃and B₂H₆dopants flows were investigated. It was shown that B₂H₆had the effect of increasing the growth rate and decreasing the Sn incorporation whereas PH₃had no effect on the growth rate but increased the Sn incorporation. Thirdly, the SiGeSn growth kinetics using SiH₄, GeH₄, and SnCl₄as precursors were discussed, which revealed that the careful control of the growth rate was required to produce compositionally homogenous SiGeSn alloy. Moreover, the material and optical characterizations have been conducted to examine the material quality. Finally, the GeSn quantum well structure was grown to exhibit the precise control of the growth parameters.     

Valence band offset in HgTe/Hg1−xCdxTe superlattices

The valence band offset (Λ) between HgTe and CdTe has been determined by means of an optical investigation of (112)B oriented HgTe/Hg_1−xCd_xTe superlattices. Based on the fact that the difference in energy between the first heavy hole and the first light hole subband is to a good approximation due primarily to Λ, it has been shown that Λ=580±40 meV at 5K. In addition Λ has a significant temperature dependence with a linear coefficient of −0.34±0.02 meV/K, i.e., Λ is 480±40 meV at room temperature.     

Parabolic negative magnetoresistance in p-Ge/Ge1−x Six heterostructures

Quantum corrections for the conductivity due to the weak localization (WL) and the disorder-modified electron-electron interaction (EEI) are investigated for the high-mobility multilayer p-Ge/Ge_1−x Se_x heterostructures at T=(0.1–20) K in magnetic field B up to 1.5 T. Negative magnetoresistance with logarithmic dependence on T and linear in B ² is observed for B⩾0.1 T. Such a behavior is attributed to the connection between the classical cyclotron motion and the EEI effect. The Hartree part of the interaction constant is estimated (F _σ =0.44) and the WL and EEI contributions to the total quantum correction Δσ at B=0 are separated (Δσ _WL≈0.3Δσ; Δσ ^ee ≈0.7Δ σ). Fiz. Tekh. Poluprovodn. 33, 1073–1075 (September 1999) This article was published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Vacancies in Hg1−xCdxTe

Measurements have been performed of the carrier concentrations in vacancy-doped Hg_1−xCd_xTe with x=0.22, 0.29, 0.45, and 0.5. Anneals to establish the carrier concentrations were performed on both the mercury- and tellurium-rich sides of the phase field. When these results were added to earlier data for x=0.2 and 0.4, and assuming that all vacancies are doubly ionized, then vacancy concentrations for all values of x and anneal temperature can be represented by simple equations. On the mercury side of the phase field, the vacancy concentrations varied as 2.50×10²³(1−x) exp[−1.00/kT] for low concentrations, and as 3.97×10⁷(1−x)^1/3n _i ^2/3 exp[−0.33/kT] for high concentrations, where n_i is the intrinsic carrier concentration. On the tellurium rich side, the vacancy concentrations varied as 2.81 × 10²²(1−x) exp[−0.65/kT] for low concentrations and as 1.92×10⁷(1−x)^1/3n _i ^2/3 exp[−0.22/kT] for high concentrations.     

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)     

Magnetotransport in a semimetal channel in p-Ga1−x InxAsySb1−y /p-InAs heterostructures with various compositions of the solid solution

Magnetotransport properties of an electron channel at the heteroboundary in type II separated p-Ga_1?x In_xAs_ySb_1?y /p-InAs heterostructures grown by LPE (x=0.09–0.22) were studied in the temperature range of 77–300 K. It is shown that an electron channel, which is formed at the heteroboundary and has high mobility μ=(3–5)×10⁴ cm² V^?1 s^?1, exists throughout the whole composition range. The band diagram of the heterostructures under study is discussed, and some parameters of the electron channel are evaluated. It is found that the electron channel with high mobility persists up to room temperature. Type II GaInAsSb/p-InAs heterostructures can find application in new Hall sensor devices with an electron channel at the heteroboundary.     

Optical band gap of Cd1−x MnxTe and Zn1−x MnxTe semiconductors

The dependence of the optical band gap for Zn_1?x Mn_xTe and Cd_1?x Mn_xTe semiconductor compounds was investigated by the methods of cathodoluminescence and optical reflection. It was found that, for Zn_1?x Mn_xTe compounds in the region x?0.2, the band gap is additionally broadened by a magnitude of about 0.08 eV, which is related to the high density of interstitial-type defects in single crystals. For x?0.3, the probability of the existence of these defects decreases substantially, which is related to the distortion of tetrahedra of the crystal lattice of Zn_1?x Mn_xTe by Mn atoms, which are incorporated into each tetrahedron.     

Metamorphic GayIn1−yP/Ga1−xInxAs tandem solar cells for space and for terrestrial concentrator applications at C > 1000 suns

The use of Ga_1−xIn_xAs instead of GaAs as a bottom solar cell in a Ga_yIn_1−yP/Ga_1−xIn_xAs tandem structure increases the flexibility of choosing the optimum bandgap combination of materials for a multijunction solar cell. Higher theoretical efficiencies are calculated and different cell concepts are suggested for space and terrestrial concentrator applications. Various Ga_yIn_1−yP/Ga_1−xIn_xAs material combinations have been investigated for the first time and efficiencies up to 24·1% (AM0) and 27·0% (AM1·5 direct) have been reached under one-sun conditions. An efficiency of 30·0–31·3% was measured for a Ga_0·35In_0·65P/Ga_0·83In_0·17As tandem concentrator cell with prismatic cover at 300 suns. The top and bottom cell layers of this structure are grown lattice-matched to each other, but a large mismatch is introduced at the interface to the GaAs substrate. This cell structure is well suited for the use in next-generation terrestrial concentrators working at high concentration ratios. For the first time a cell efficiency up to 29–30% has been measured at concentration levels up to 1300 suns. A small prototype concentrator with Fresnel lenses and four tandem solar cells working at C = 120 has been constructed, with an outdoor efficiency of 23%. Copyright © 2001 John Wiley & Sons, Ltd.     

Conduction- and Valence-Band Energies in Bulk InAs1−x Sb x and Type II InAs1−x Sb x /InAs Strained-Layer Superlattices

The energy gaps were studied in two types of structures: unrelaxed bulk InAs_1?x Sb _x layers with x = 0.2 to 0.46 grown on metamorphic buffers and type II InAs_1?x Sb _x /InAs strained-layer superlattices (SLS) with x = 0.225 to 0.296 in the temperature range from T = 13 K to 300 K. All structures were grown on GaSb substrates. The longest wavelength of photoluminescence (PL) at low temperatures was observed from bulk InAs_0.56Sb_0.44 with a peak at 10.3 μm and full-width at half-maximum (FWHM) of 11 meV. The PL data for the bulk InAs_1?x Sb _x materials of various compositions imply an energy gap bowing parameter of 0.87 eV. A low-temperature PL peak at 9.1 μm with FWHM of 13 meV was observed for InAs_0.704Sb_0.296/InAs SLS. The PL spectrum of InAs_0.775Sb_0.225/InAs SLS under pulsed excitation revealed a second peak associated with recombination of electrons in the three-dimensional (3D) continuum with holes in the InAs_0.775Sb_0.225. This experiment determined the conduction-band offset in the InAs_0.775Sb_0.225/InAs SLS. The energies of the conduction and valence bands in unstrained InAs_1?x Sb _x and their bowing with respect to the Sb composition are discussed.     

MOVPE grown Ga1−xInxAs solar cells for GaInP/GaInAs tandem applications

Lattice-mismatched Ga_1−xIn_xAs solar cells with an absorption edge between 900 and 1150 nm have been grown on GaAs substrates. Different graded Ga_1−xIn_xAs buffer layers and solar cell structures were evaluated to achieve a good electrical performance of the device. External quantum efficiencies comparable to our best GaAs solar cells were measured. The best 1 cm² cell with a bandgap energy of 1.18 eV has an efficiency of 22.6% at AM1.5g and a short circuit current density of 36.4 mA/cm². To our knowledge, this is the highest reported efficiency for a Ga_0.83In_0.17As solar cell.     

Misfit strain induced tweed-twin transformation on composition modulation Zn1−xMgxSxSe1−y layers and the quality control of the ZnSe buffer/GaAs interface

[100] composition modulation as well as [101] and $$1$$ tweed strain contrast were observed in 0.72 μm thick Zn_1?xMg_xS Se_1?y epitaxial films grown on ZnSe buffer layers. The lattice distortion induced tweed strain contrast disappears in relaxed Zn_1?xMg_xS Se_1?y layers of thicknesses above ～ 0.8—1 μm even though the [100] composition modulation remains. Instead, the formation of microtwins takes place to relieve the strain in the distorted lattice of the quaternary films. The Zn_1?xMg_xS_ySe_1?y layers were obtained by growing a ZnSe buffer layer on Asstabilized GaAs substrates with Zn treatment of the substrate prior to the growth of the film. The samples with film thickness of ～0.72 μm were of very high quality with a defect density of less than 5 x lO⁴/cm². Some samples showed rough ZnSe/ GaAs interfaces and a high density of Frank partial dislocations originating at the ZnSe/GaAs interface. The interface roughness is believed to result from an As-rich GaAs surface after the oxide desorption.     

Long-wavelength photodiodes based on Ga1−x InxAsySb1−y wit composition near the miscibility boundary

The possibility of using liquid-phase epitaxy to obtain Ga_1−x In_xAs_ySb_1−y solid solutions isoperiodic with GaSb near the miscibility boundary is investigated. The effect of crystallographic orientation of the substrate on the composition of the solid solutions grown in this way is examined, and the indium concentration is observed to grow from 0.215 to 0.238 in the Ga_1−x In_xAs_ySb_1−y solid phase in the series of substrate orientations (100), (111)A, (111)B. A change in the composition of the solid solution leads to a shift of the long-wavelength edge of the spectral distribution of the photosensitivity. The use of a GaSb (111)B substrate made it possible, without lowering the epitaxy temperature, to increase the indium content in the solid phase to 23.8% and to create long-wavelength photodiodes with spectral photosensitivity threshold λ _th=2.55 μm. The primary characteristics of such photodiodes are described, along with aspects of their fabrication. The proposed fabrication technique shows potential for building optoelectronic devices (lasers, LED’s, photodiodes) based on Ga_1−x In_xAs_ySb_1−y solid solutions with red boundary as high as 2.7 μm. Fiz. Tekh. Poluprovodn. 33, 249–253 (February 1999)     

Au−Ni−Sn intermetallic phase relationships in eutectic Pb−Sn solder formed on Ni/Au metallization

Recent work has shown that a Au−Ni−Sn ternary compound with a nominal composition of Au_0.5Ni_0.5Sn₄ redeposits and grows at the interface between eutectic Pb−Sn solder and Ni/Au metallization during aging at 150°C. The present work verifies the existence of the Au_0.5Ni_0.5Sn₄ phase by examining the Sn-rich corner of the Au−Ni−Sn ternary phase diagram. The reconfiguration mechanism of the AuSn₄ from the bulk solder is also discussed, with detailed observations of the Au_0.5Ni_0.5Sn₄ microstructure. The results show that the Ni solubility limit in the AuSn₄ phase is approximately 12 at.% at 150°C and thus, the Au_0.5Ni_0.5Sn₄ phase is a ternary AuSn₄-based compound with high Ni solubility. Due to the slight solubility and the fast diffusion of Au in the eutectic Pb−Sn at 150°C, the AuSn₄ intermetallics in the bulk solder can reconfigure to form a Au_xNi_1−xSn₄ compound at the interface where Ni is available. The Au_xNi_1−xSn₄ compound layer consists of nanocrystals arranged in a larger grainlike morphology. It appears that the inherent lattice strain of the Au_xNi_1−xSn₄ compound and the volume change due to its formation results in a nanocrystalline microstructure.     

Structure and optical properties of heterostructures based on MOCVD (Al x Ga1 − x As1 − y P y )1 − z Si z alloys

Epitaxial heterostructures produced by MOCVD on the basis of Al _x Ga_{1 ? x} As ternary alloys with the composition parameter x ≈ 0.20–0.50 and doped to a high Si and P atomic content are studied. Using the high-resolution X-ray diffraction technique, scanning electron microscopy, X-ray microanalysis, Raman spectroscopy, and photoluminescence spectroscopy, it is shown that the epitaxial films grown by MOCVD are formed of five-component (Al _x Ga_{1 ? x} As_{1 ? y} P _y )_{1 ? z} Si _z alloys.     

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.     

Hole scattering mechanisms in Hg1−xCdxTe

In this paper, we analyze and discuss the roles of nine different scattering mechanisms—ionized impurity, polar and nonpolar optical, acoustic, dislocation, strain field, alloy disorder, neutral impurity, and piezoelectric—in limiting the hole mobilities in p-type Hg_1−xCd_xTe crystals. The analysis is based on obtaining a good fit between theory and experiment for the light and heavy hole drift mobilities by optimizing certain unknown (or at the most vaguely known) material parameters such as the heavy hole mobility effective mass, degree of compensation, and the dislocation and strain field scattering strengths. For theoretical calculations, we have adopted the relaxation time approach, keeping in view its inadequacy for the polar scattering. The energy dispersive hole relaxation times have been drawn from the published literature that take into account the p-symmetry of valence band wave functions. The temperature dependencies of multiple charge states of impurities and of Debye screening length have been taken into account through a numerical calculation for the Fermi energy. Mobility data for the present analysis have been selected from the HgCdTe literature to represent a wide range of material characteristics (x=0.2–0.4, p=3×10¹⁵–1×10¹⁷ cm⁻³ at 77K, μ_peak≅200-1000cm²V⁻¹s⁻¹). While analyzing the light hole mobility, the acoustic deformation and neutral impurity potentials were also treated as adjustable. We conclude that

Design of Dislocation-Compensated ZnS y Se1−y /GaAs (001) Heterostructures

The understanding of lattice relaxation and dislocation dynamics in lattice-mismatched semiconductors makes it possible to design metamorphic device structures utilizing the dislocation compensation mechanism for reduced defects, improved performance, and enhanced reliability. We have developed a dislocation dynamics model accounting for misfit–threading interactions and have applied it to ZnS _y Se_1?y /GaAs (001) heterostructures.1 Dislocation compensation involves the removal of threading dislocations associated with one sense of misfit dislocations by bending them over to create misfit dislocations of the opposite sense at an intentionally mismatched interface. Here we investigated the design of dislocation-compensated ZnS _y Se_1?y /GaAs (001) heterostructures and considered the sulfur mole fraction tolerances applicable to such structures. We considered two types of structures: type A involved a uniform-composition (ungraded) layer on top of a uniform-composition buffer, while type B involved a uniform-composition layer on a linearly graded buffer. For each structure type we studied the requirements on the thickness and compositional profile of the buffer layer to optimize the removal of mobile threading dislocations from the top uniform (device) layer as well as the allowed tolerance in compositional overshoot to achieve structures with low threading dislocation density. We show for both types of structure that (i) for given compositional overshoot at the buffer–device layer interface, there is an optimum buffer thickness which minimizes the dislocation density; and (ii) for given buffer thickness there is an optimum overshoot which minimizes the dislocation density.     

Shallow acceptors in strained Ge/Ge1−x Six heterostructures with quantum wells

The energies of localized acceptor states in quantum wells (strained Ge layers in Ge/Ge_1?x Si_x heterostructures) were analyzed theoretically in relation to the quantum well width and the impurity position in the well. The impurity absorption spectrum in the far IR range is calculated. Comparison of the results of the calculation with experimental photoconductivity spectra allows an estimation of the acceptor distribution in the quantum well to be made. In particular, it was concluded that acceptors may largely concentrate near the heterointerfaces. The absorption spectrum is calculated taking into account the resonance impurity states. This allows the features observed in the short-wavelength region of the spectrum to be interpreted as being due to transitions into the resonance energy levels “linked” to the upper size-quantization subbands.