Effect of compressive and tensile stresses in GaMnAs layers on their magnetic properties

The effect of elastic stresses (compressive, tensile) on the magnetic properties of epitaxial GaMnAs layers prepared by laser deposition of solid-state targets in a gas atmosphere on different buffer sublayers (In _x Ga_{1 − x} As and In _x Ga_{1 − x} P) and substrates (GaAs, InP) has been investigated. It has been established from the investigations of magnetic-field dependences of the Hall resistance that all layers exhibit ferromagnetic properties with the Curie temperature ∼50 K. It has been shown that, in the case of tensile stresses in GaMnAs layers (In _x Ga_{1 − x} As and In _x Ga_{1 − x} P buffers and InP substrate), the anomalous Hall effect shape demonstrates a predominant orientation of the easy-magnetization axis in the growth direction, unlike the GaMnAs layers prepared on a GaAs substrate (with compressive stresses), which demonstrate the predominance of the component of the magnetization vector in the layer plane.     

Effect of biaxial strain on the band gap of wurtzite AlxGa1?xN

First-principles calculations are applied to investigate the effect of biaxial strain on the band gap of wurtzite Al _x Ga_1−x N. The band gap and band gap bowing parameter increase with compressive strain and decrease with tensile strain. The strain-induced changes in the band gap of Al _x Ga_1−x N are linear in the strain range of about −1% to 1% while the linearity is invalid out of the range. The linear coefficient B(x), characterizing the relationship between the band gap and the biaxial stress, with a quadratic form is obtained. The value of the band gap bowing parameter decreases from 1.0 eV for −2% strain to 0.91 eV for unstrained and to 0.67 eV for 2% strain.     

Band offset calculations applied to III–V nitride quantum well device engineering

Band offset calculations for zinc-blende pseudomorphically strained Al_1−xGa_xN/Al_1−yGa_yN and In_xGa_1−xN/In_yGa_1−yN interfaces have been performed on the basis of the model solid theory combined with ab initio calculations. From the results obtained, we have calculated, separately, the valence and conduction band discontinuities of In_xGa_1−xN/GaN and GaN/Al_1−xGa_xN as a function of the indium and gallium contents respectively. Using the latter results, we have extended our study to simulate band discontinuities for strained Ga_1−xIn_xN/relaxed Al_1−yGa_yN heterointerfaces. Information derived from this investigation will be useful for the design of lattice mismatched heterostructures in modeling optoelectronic devices emitting at ultraviolet to near infrared wavelengths.     

Momentum conservation in the luminescence of modulation-doped Ga x In1−x As−Al y In1−y As quantum wells

Summary Thek-conserving selection rule in the electron-hole recombination is investigated by intensity-dependent photoluminescence measurements inn-type modulation-doped Ga _x In_1−x As−Al _y In_1−y As single quantum wells intentionally doped with Be acceptors in the well centre. Thek-non-conserving recombination process involves electrons with momentum up to the Fermi edge and holes localized on the Be acceptors. The transition from a one-component electron plasma to a two-component electron-hole plasma is studied by comparing the experimental results with theoretical line shape models. The density-dependent band gap renormalization is determined for the one-component and the two-component electron-hole plasma. The obtained results are found to agree well with recent theoretical calculations.     

Lpe growth method possibilities in A3B5 semiconductors preparation

Pre-diffusion limited technique of liquid phase epitaxy is used for preparation of quaternary solid solution Ga _x In_1−x As _y P_1−y and binary InP. Surface morphology of the layers prepared at this condition has been compared with those prepared by conventional LPE. Quaternary strained layers with composition near to Ga_0.21In_0.79As_0.75P_0.25 were grown with perpendicular lattice mismatch up to 1.6% in compression, on the InP substrates of (100) orientation. The used epitaxial technique has been modified for perspective attempt to heal the growth interfaces. Modification was tested by InP growth. Presented at the 6th Joint Seminar “Development of Materials Science in Research and Education”, Karlštejn, Czech Republic, 17–19 September 1996.     

Electronic properties of multinary alloys

Summary A model version of the coherent-potential approximation (CPA) is outlined to evaluate the fundamental energy gapsΓ _15v -Γ _1c ′,Γ _15v -L _1c ,Γ _15v -X _1c of a multinary solid solution. A comparison of theoretical results with the transition energy values obtained from direct interpolation of the corresponding values of ternary alloys is presented for some composition of the solution Al_1−x−y Ga _x In _y As at fixed lattice constants (matching the lattice constants of InP and AlAs). Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

High-temperature operation of AlGaInAs/InP lasers1994

We discuss the design of uncooled lasers which minimizes the change in both threshold current and slope efficiency over the temperature range from–40 to +85°C [1]. To prevent carrier overflow under high-temperature operation, the electron confinement energy is increased by using the Al _x Ga _y In_1–x–y As/InP material system [1] instead of the conventional Ga _x In_1–x As _y P_1–y /InP material system. Experimentally, we have investigated strained quantum well lasers with three different barrier layers and confirmed that the static and dynamical performance of the lasers with insufficient carrier confinement degrades severely under high-temperature operation [2]. With an optimized barrier layer, the Al _x Ga _y In_1–x–y As/InP strained quantum well lasers show superior hightemperature performance, such as a small drop of 0.3 dB in slope efficiency when the heat sink temperature changes from 25 to 100°C [3], a maximum CW operation temperature of 185°C [4], a thermally-limited 3-dB bandwidth of 13.9 GHz at 85°C [2], and a mean-time-to-failure of 33 years at 100°C and 10 mW output power [5].     

Energy band gaps for the GaxIn1−xAsyP1−y alloys lattice matched to different substrates

A pseudopotential formalism within the virtual crystal approximation in which the effects of composition disorder are involved is applied to the Ga_xIn_1−xAs_yP_1−y quaternary alloys in conditions of lattice matching to GaAs, InP and ZnSe substrates so as to predict their energy band gaps. Very good agreement is obtained between the calculated values and the available experimental data for the alloy lattice matched to InP and GaAs. The alloy is found to be a direct-gap semiconductor for all y compositions whatever the lattice matching to the substrates of interest. The (Γ→Γ) band-gap ranges and the ionicity character are found to depend considerably on the particular lattice-matched substrates suggesting therefore that, for an appropriate choice of y and the substrate, Ga_xIn_1−xAs_yP_1−y could provide more diverse opportunities to obtain desired band gaps, which opens up the possibility of discovering new electronic devices with special features and properties.     

The band structures of BSb and B<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sb alloys

The band structures of BSb and B _x Ga_1−x Sb alloys are studied using first-principles calculations in the generalized gradient approximation. By SQS-8 supercells to model a random alloy, the direct transition energy-gap (Γ _15v−Γ _1c) bowing of 3.0 eV is obtained for B _x Ga_1−x Sb alloys in x = 0–50%, in x = 0–11% the energy-gap is the band-gap and increases by 7 meV/%B with boron composition increasing; by SQS-16 supercells the bowing parameter is about 1.9 eV in x = 0–12.5%. The formation enthalpies of mixing, ΔH, are calculated for B _x Ga_1−x As and B _x Ga_1−x Sb alloys. A comparison of enthalpies indicates that B _x Ga_1−x Sb films with boron composition of 7% may be possible. Supported by the National Natural Science Foundation of China (Grant No. 10774031) and the Guangdong Provincial Natural Science Foundation (Grant No. 07001790)     

Comparison of the band alignment of strained and strain-compensated GaInNAs QWs on GaAs and InP substrates

We present a comparison of the band alignment of the Ga_1−xIn_xN_yAs_1−y active layers on GaAs and InP substrates in the case of conventionally strained and strain-compensated quantum wells. Our calculated results present that the band alignment of the tensile-strained Ga_1−xIn_xN_yAs_1−y quantum wells on InP substrates is better than than that of the compressively strained Ga_1−xIn_xN_yAs_1−y quantum wells on GaAs substrates and both substrates provide deeper conduction wells. Therefore, tensile-strained Ga_1−xIn_xN_yAs_1−y quantum wells with In concentrations of x0.53 on InP substrates can be used safely from the band alignment point of view when TM polarisation is required. Our calculated results also confirm that strain compensation can be used to balance the strain in the well material and it improves especially the band alignment of dilute nitride Ga_1−xIn_xN_yAs_1−y active layers on GaAs substrates. Our calculations enlighten the intrinsic superiority of N-based lasers and offer the conventionally strained and strain-compensated Ga_1−xIn_xN_yAs_1−y laser system on GaAs and InP substrates as ideal candidates for high temperature operation.     

Effect of In and Al interdiffusion on electron states and light absorption in InxGa1? x As/AlyGa1? y As quantum dots

A new method of theoretical investigation of the interdiffusion effect on electron states in quantum dots is proposed. The main point of the method is the replacement of the “veritable” confining potential formed due to the diffusion by a model potential, for which the Schr?dinger equation solutions and the energy spectrum are known. In the framework of the proposed method we calculate the positions of edges of the conduction and heavy hole bands and the absorption coefficient of interband transitions depending on the diffusion length in spherical In_xGa_1−x As/Al_yGa_1−y As quantum dots, using the Wood-Saxon potential as a model one. Original Russian Text ? V.N. Mughnetsyan, A.A. Kirakosyan, 2007, published in Izvestiya NAN Armenii, Fizika, 2007, Vol. 42, No. 2, pp. 83–91.     

Microscopic indium distribution and electron localization in zinc blende InGaN alloys and InGaN/GaN strained quantum wells

In order to give an atomic level understanding of the light emission mechanism and seek In distribution patterns closely related to the elusive electron localization centers, we optimize the crystal structure of zinc blende In _x Ga_1−x N (0≤x≤1) alloys with different In distributions and investigate their electronic structures using first-principles calculations. Our results show that In _x Ga_1−x N forms a random alloy, in which several-atom In–N clusters and In–N chains can exist stably with a high concentration due to their small formation energy. These In–N clusters and chains form more easily in zinc blende structure than in wurtzite structure. The band gap of zinc blende In _x Ga_1−x N alloys insensitively depends on the In distribution. Moreover, we find that both small In–N clusters and straight In–N chains with three or more In atoms, acting as radiative recombination centers, highly localize the electrons of the valence band maximum state and dominate the light emission of Ga-rich In _x Ga_1−x N alloys. The strains of In _x Ga_1−x N layers can enhance the electron localization in In _x Ga_1−x N/GaN strained quantum wells. Our results are in good agreement with experiments and other calculations.     

Highly strained InxGa(1−x)As−InyAl(1−y)As (x>0.8,y<0.3) layers for short wavelength QWIP and QCL structures grown by MBE

Highly strained quantum cascade laser (QCL) and quantum well infrared photodetector (QWIPs) structures based on In_xGa_(1−x)As−In_yAl_(1−y)As (x>0.8,y<0.3) layers have been grown by molecular beam epitaxy. Conditions of exact stoichiometric growth were used at a temperature of 420°C to produce structures that are suitable for both emission and detection in the 2–5 μm mid-infrared regime. High structural integrity, as assessed by double crystal X-ray diffraction, room temperature photoluminescence and electrical characteristics were observed. Strong room temperature intersubband absorption in highly tensile strained and strain-compensated In_0.84Ga_0.16As/AlAs/In_0.52Al_0.48As double barrier quantum wells grown on InP substrates is demonstrated. Γ–Γ intersubband transitions have been observed across a wide range of the mid-infrared spectrum (2–7 μm) in three structures of differing In_0.84Ga_0.16As well width (30, 45, and 80 Å). We demonstrate short-wavelength IR, intersubband operation in both detection and emission for application in QC and QWIP structures. By pushing the InGaAs–InAlAs system to its ultimate limit, we have obtained the highest band offsets that are theoretically possible in this system both for the Γ–Γ bands and the Γ–X bands, thereby opening up the way for both high power and high efficiency coupled with short-wavelength operation at room temperature. The versatility of this material system and technique in covering a wide range of the infrared spectrum is thus demonstrated.     

Numerical optimization of In-mole fractions and layer thicknesses in AlxGa1−xN/AlN/GaN high electron mobility transistors with InGaN back barriers

The effects of the In-mole fraction (x) of an In_xGa_1−xN back barrier layer and the thicknesses of different layers in pseudomorphic Al_yGa_1−yN/AlN/GaN/In_xGa_1−xN/GaN heterostructures on band structures and carrier densities were investigated with the help of one-dimensional self-consistent solutions of non-linear Schrödinger-Poisson equations. Strain relaxation limits were also calculated for the investigated Al_yGa_1−yN barrier layer and In_xGa_1−xN back barriers. From an experimental point of view, two different optimized structures are suggested, and the possible effects on carrier density and mobility are discussed.     

Influence of stoichiometry on the luminescent properties of InAs quantum dots grown on a heterostructure

We report photoluminescence (PL) spectra of InP/In_xGa_1-xAs/InAs/InP dot-in-a-well structures grown by MOVPE, with different compositions of the ternary layer. Measurements with atomic force microscopy showed that the largest quantum dot (QD) height is obtained when the InAs QDs are grown on the In_xGa_1-xAs layer with a mismatch of 1000 ppm, and the height decreases as the mismatch departs from this value. PL spectra of the QDs showed an asymmetric band, which involves transitions between dot energy levels and can be deconvoluted into two peaks. The highest energy PL peak of this band was observed for the sample with the QDs grown on top of the lattice-matched In_xGa_1-xAs layer and it shifted to lower energies for strained samples as the degree of mismatch increased. Theoretical calculations of the energy levels of the entire structure were used to interpret the obtained PL spectra and determine the possible detection tunability range.     

Electromodulation spectroscopy of In0.53Ga0.47As/In0.53Ga0.23Al0.24As quantum wells

In_0.53Ga_0.47As/In_0.53Ga_0.23Al_0.24As quantum wells (QWs) of various widths have been grown by molecular beam epitaxy on the InP substrate and investigated by electromodulation spectroscopy, i.e. photoreflectance (PR) and contactless electroreflectance (CER). The optical transitions related to the QW barrier and the QW ground and excited states have been clearly observed in PR and CER spectra. The experimental QW transition energies have been compared with theoretical predictions based on an effective mass formalism model. A good agreement between experimental data and theoretical calculations has been observed when the conduction band offset for the In_0.53Ga_0.47As/In_0.53Ga_0.23Al_0.24As interface equals 60%. In addition, it has been concluded that the conduction band offset for the In_0.53Ga_0.23Al_0.24As/InP interface is close to zero. The obtained results show that InGa(Al)As alloys are very promising materials in the band gap engineering for structures grown on InP substrate.     

Strain accumulation in InAs/In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>As quantum dots

The effect of strain accumulation in the InAs/In _x Ga_1−x As quantum dots (QDs) system was studied in this work. We found strain in the In _x Ga_1−x As layer with accumulation in the QD layer. This effect resulted in a dramatic reduction of growth-mode transition thickness of the QD layer. For InAs/In_0.25Ga_0.75As QDs, critical thickness is measured to be as low as 1.08 ML. The experimental results in this work highlight the importance of strain accumulation in the design and fabrication of QD-based devices with metamorphic buffer layer involved.     

Size quatized levels of the valence band and the optical gain in strained <Emphasis Type="Italic">p</Emphasis>-AlGaAs/GaAsP/<Emphasis Type="Italic">n</Emphasis>-AlGaAs structures under uniaxial compression

The band structure, size quatized levels, and wave functions in the conduction and valence bands of strained n-Al _x Ga_{1 − x} As/GaAs _y P_{1 − y} /p-Al _x Ga_{1 − x} As (y = 0.84) heterostructures are calculated numerically upon a uniaxial compression along the [110] direction. The calculation indicates a sublinear increase of the effective optical gap in the GaAs_0.84P_0.16 quantum well, strong mixing of states of light and heavy holes, and merging of the corresponding ground states in the quantum well of the valence band under a pressure of 4.5–5 kbar. The calculation of matrix elements of the electron-photon interaction operator for a system of possible interband transitions permits one to determine the optical gain for the TE and TM modes. The increase in this coefficient by two to fourfold under uniaxial compression agrees with the previously published experimental data on the increase of the electroluminescence intensity.     

Different temperature dependence of carrier transport properties between AlxGa1-xN/InyGa1-yN/GaN and AlxGa1-xN/GaN heterostructures

The temperature dependence of carrier transport properties of AlxGa1-xN/InyGa1-yN/GaN and AlxGa1-xN/GaN heterostructures has been investigated.It is shown that the Hall mobility in Al0.25Ga0.75N/In0.03Ga0.97N/GaN heterostructures is higher than that in Al0.25Ga0.75N/GaN heterostructures at temperatures above 500 K,even the mobility in the former is much lower than that in the latter at 300 K.More importantly,the electron sheet density in Al0.25Ga0.75N/In0.03Ga0.97N/GaN heterostructures decreases slightly,whereas the electron sheet density in Al0.25Ga0.75N/GaN heterostructures gradually increases with increasing temperature above 500 K.It is believed that an electron depletion layer is formed due to the negative polarization charges at the InyGa1-yN/GaN heterointerface induced by the compressive strain in the InyGa1-yN channel,which e-ectively suppresses the parallel conductivity originating from the thermal excitation in the underlying GaN layer at high temperatures.     

First principles study on electronic and optical properties of Al<Subscript>x</Subscript>Ga<Subscript>1−x</Subscript>N and In<Subscript>y</Subscript>Ga<Subscript>1−y</Subscript>N

The band structure, density of states of Al_xGa_1?xN and In_yGa_1?yN was performed by the first-principles method within the local density approximation. The calculated energy gaps of the AlN, Al_0.5Ga_0.5N, GaN, In_0.5Ga_0.5N and InN were 5.48, 4.23, 3.137, 1.274 and 0.504 eV, which were in agreement with the experimental result. The dielectric functions, absorption coefficient and loss function were calculated based on Kramers–Kronig relations. Further more, the relationships between electronic structure and optical properties were investigated theoretically. For Al_xGa_1?xN and In_yGa_1?yN materials, the micromechanism of the optical properties were explained.