Characterisation of GaN–In x Ga1−x N quantum-well lasers

An efficient analytical model to calculate the optical gain of the quantum-well laser of the GaN–In _x Ga_1?x N material system is used in this paper. Based on the anisotropic effective mass theories, empirical formulas delineating the relations between optical gain, emission wavelength, well width and material compositions are obtained for such a quantum-well lasers.     

Surface effect on the reverse current of Al x Ga1−x Sbp−n structures

The effect of an invertedp-region along the free surface ofn-Al _x Ga_1−x Sb on the reverse current ofp−n structures from the given solid solution is analyzed. Expressions which describe “collection” of the inverted layer current on the cylindrical surface of ann-region are discussed. The contribution of the near-surface and bulk components to the reverse current ofp−n structures with a semi-infiniten-region is estimated. For structures with a two-layern-region of finite thickness we have calculated the dependence of the near-surface current on the voltage across thep−n structure, the thickness of then-region, and its composition and doping level. We have compared the calculated current-voltage characteristics with experiment using a Al_0.15Ga_0.85Sbp−n structure as an example. Tomsk State University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 42, No. 1, pp. 34–40, January, 1999.     

Cathodoluminescence from epitaxial layers of p-Alx Ga1−xSb

The cathodoluminescence (CL) of doped and undoped layers of p-Al_xGa_1–xsb(0x0.8) grown by liquid-phase epitaxy has been investigated. Changing the composition of the solid solution resulted in emission bands having maxima between 0.77 and 1.58 eV at 77°K and between 0.72 and 1.55 eV at 300°K. It is shown that the following factors play a role in determining the luminescence intensity as a function of x: 1) the transfer of electrons to the L minimum in the conduction band; 2) a decrease in the hole concentration in the layers; 3) a decrease in the concentration of nonradiative recombination centers. The CL spectrum at 77°K contains a longwavelength band whose behavior is attributed to recombination through a level similar to the level known to be present in GaSb and believed to be caused by an intrinsic defect. The ionization energy of this level increases as x increases.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 40–46, November, 1976.     

A simple model for optimization design of high performance In1−x−y Ga y Al x As strained MQW DFB lasers

It is well known that complex rate equations and the couple wave equation have to be solved by the method of iteration in the simulation of multi-quantum well (MQW) distributed feedback Bragg (DFB) lasers, and a long CPU time is needed. In this paper, from the oscillation condition of lasers, we propose a simple and fast model for optimization of In_1–x–y Ga _y Al _x As strained MQW DFB lasers. The well number and the cavity length of 1.55 m wavelength In_1–x–y Ga _y Al _x As MQW DFB lasers are optimized using the model. As a result, the simple model gives almost the same results as the complex one, but 90% CPU time can be saved. In addition, a low threshold, high maximum operating temperature of 550–560 K, and high relaxation oscillation frequency of over 30 GHz MQW DFB laser is presented.     

Pressure-induced non-linear optical properties in a wurtzite GaN/Al x Ga1− x N strained quantum dot

The effects of hydrostatic pressure on the exciton ground-state binding energy and the interband emission energy in a GaN/Al _x Ga_{1??? x} N quantum dot are investigated. The effects of strain and the internal field due to spontaneous and piezo-electric polarizations are included in the Hamiltonian. Numerical calculations are performed using variational procedure within the framework of single-band effective-mass approximation. The dependence of non-linear optical processes on the dot sizes is brought out in the influence of pressure. Pressure-induced optical properties are obtained using the compact density matrix approach. The effects of hydrostatic pressure on the linear, third-order non-linear optical absorption coefficients and the refractive index changes of the exciton as a function of photon energy are calculated. Our results show that the effects of pressure and the geometrical confinement have great influence on the optical properties of GaN/Al _x Ga_{1??? x} N dot.     

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 .     

Low temperature photoluminescence properties of p- and n-type Al x Ga1−x As withx>0.42

The photoluminescence (PL) spectra of n- and p-type Al _x Ga_1−x As (x>0.42) grown by metalorganic vapor phase epitaxy (MOVPE) and liquid phase epitaxy show typically a broad PL band (BB) centered about 300meV below the near band-gap PL lines. In the MOVPE grown samples the BB is composed out of four lines. The BB intensity increases with the doping level and dominates the spectrum at concentrations > 10¹⁷ cm⁻³. The temperature dependence of the BB intensity shows two distinct maxima at ≈ 19K and ≈ 80K. Hydrogenation of MOVPE grown samples at 170°C reveal an effective passivation of the shallow acceptors and the centers associated with the BB line. On the contrary hydrogenation at temperatures >250°C leads to an increase in the BB intensity which is related with the evaporation of As from the surface and the generation of V_As in the layers at these temperatures. Our results suggest that the BB is related with isoelectronic centers formed when V_As and C_As or Si_As associate to form next nearest neighbor ion pairs. The PL is then due to a recombination of excitons in analogy to the case of (Zn, O) isoelectronic complexes in GaP. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998. This work was supported by the DFG under the contract AZ 436 TSE 113/22/0 in the framework of the cooperation between the DFG and the Academy of Sciences of the Czech Republic.     

Alloy-disorder scattering of the interacting electron gas in quantum wells and heterostructures of Al x Ga1 − x As

The question whether alloy disorder is screened or unscreened is of fundamental importance. Therefore, we calculate the mobility of the interacting two-dimensional electron gas as realized in Al _x Ga_{1 ? x} As quantum wells and heterostructures in the presence of alloy-disorder scattering. For the screening we use the randomphase approximation and we include many-body effects due to exchange and correlation. We propose to determine the alloy disorder potential V _AD from mobility measurements. If we use V _AD = 1.04 eV we can explain recent experimental results obtained for quantum wells and heterostructures with ultrahigh mobility. From the anomalous linear temperature dependence of the mobility measured in heterostructures, we conclude that the alloy disorder is screened. More experiments are needed to confirm the screening of the alloy disorder and we propose some measurements.     

Vibrational spectra of CuAl x Ga1−x S2 solid solutions

Investigations of the IR reflection and Raman spectra in polarized light are carried out on single crystals of the ternary compounds CuAlS₂ and CuGaS₂ and solid solutions based on them grown by the method of chemical transport reactions. The frequencies of longitudinal and transverse phonons are determined, the IR intensities and damping factors of phonons (ε₀, ε_∞) are calculated, and their concentration dependences are constructed. It is established that optical vibrations in the solid solutions manifest a mixed behavior. Belarusian State University of Information Science and Radioelectronics 6 P. Brovka Str., Minsk, 220072, Belarus; e-mail: kafchim@gw.bsuir.unibel.by. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 6, pp. 800–803, November–December, 1999.     

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.     

Photoluminescence of Al x Ga1−x As/GaAs quantum well heterostructures grown by molecular beam epitaxy

Nominally undoped Al_xGa_1–xAs grown by molecular beam epitaxy from As₄ species at elevated substrate temperatures of 670°C exhibits well-resolved excitonic fine structure in the low-temperature photoluminescence spectra, if the effective As-to-(Al+Ga) flux ratio on the growth surface is kept within a rather narrow range of clearly As-stabilized conditions. In contrast to previous results on Al_xGa_1–xAs of composition 0.15not to shift in energy by changing the excitation intensity. This implies a simple freeelectron carbon-acceptor recombination mechanism for the line without any participation of a donor. In Al_xGa_1–xAs of composition close to the direct-to-indirect cross-over point, two distinct LO-phonons separated by 34 and 48 meV from the (D ⁰,C ⁰) peak position at x=0.43 were observed which were before only detectable by Raman scattering experiments. The intensity of the carbon-impurity related luminescence lines in bulk-type Al_xGa_1–xAs and GaAs layers was found to be strongly reduced, as compared to the excitonic recombination lines, if the respective active layer was covered by a very thin confinement layer of either GaAs on top of Al_xGa_1–xAs or vice versa grown in the same growth cycle.     

Electron–hole superlattices in GaAs/Al x Ga1− x As multiple quantum wells

A GaAs/Al _x Ga_{1? x} As semiconductor structure is proposed, which is predicted to superconduct at T _c?≈?2?K. Formation of an alternating sequence of electron- and hole-populated quantum wells (an electron–hole superlattice) in a modulation-doped GaAs/Al _x Ga_{1? x} As superlattice is considered. This superlattice may be analogous to the layered electronic structure of high-T _c superconductors. In the structures of interest, the mean spacing between nearest electron (or hole) wells is the same as the mean distance between the electrons (or holes) in any given well. This geometrical relationship mimics a prominent property of optimally doped high-T _c superconductors. Band bending by built-in electric fields from ionized donors and acceptors induces electron and heavy-hole bound states in alternate GaAs quantum wells. A proposed superlattice structure meeting this criterion for superconductivity is studied by self-consistent numerical simulation.     

Two-dimensional electrons at the n-GaAs/Al x Ga1−x As heterointerface under uniaxial compression

The transport characteristics and quantum oscillations of magnetoresistance have been studied in n-(001)GaAs/Al _x Ga_1?x As heterostructures at liquid helium temperatures and uniaxial compressions up to 3.5 kbar. Under such loading conditions, the density of two-dimensional (2D) electrons at the heterointerface is determined primarily by a piezoelectric field induced in the [001] direction. The screening of this field leads to an increase in the 2D electron density at the heterointerface under compression along the [110] axis and to a decrease in this density, under compression along the $[1\bar 10]$ axis. The formation and subsequent redistribution of a compensating charge at the heterointerface are impeded, which leads to the development of relaxation processes in the stressed system at sufficiently high pressures. The pressure dependences of the anisotropic mobility and the effective cyclotron mass of 2D electrons show that their energy spectrum remains isotropic and the dispersion obeys a parabolic law in the pressure range studied.     

Theory of the electronic structure of Ga1-yInyNxAs1−x and related alloys

Abstract

We present a quantitative k.P Hamiltonian which describes analytically the composition dependence of the energy gap, interband momentum matrix element, band edge effective masses and conduction band dispersion of GaN_XAs_1?x alloys for low N concentrations (x < ～ 0.05). The model has been confirmed using an sp³s? tight-binding Hamiltonian whose results agree well both with experiment and with previous pseudopotential calculations. The model should be of wide use to guide the future development of this material system and its applications.     

Miscibility of In x Ga1−x As alloys: a study using atomistic simulations

Atomistic simulations are used in combination with the two potential energy functions, namely, the Valence Force Field (VFF) model and the Tersoff model, to study the solution thermodynamics of In _x Ga_1?x As alloy. The simulation data, in the form of a T ? x diagram, is contrasted with the results obtained by using the Ho and Stringfellow approach. It is observed that for the VFF model, the upper critical solution temperature obtained from simulation data is approximately 850 K, which is higher than the 729 K predicted by the Ho and Stringfellow treatment. The composition range for which the two-phase heterogeneous region exists is wider than that predicted by the Ho and Stringfellow approach. The Tersoff model predicts a complex miscibility diagram, where the 850 K temperature corresponds to the approximate ‘eutectic’ temperature. Further improvement of model predictions may be made possible by investigation of temperature and composition dependent interaction parameter in a modified regular solution theory, and investigation of non-random, non-ideal solution models in the Ho and Stringfellow treatment, development of temperature dependent VFF model parameters and adjustment of Tersoff model parameters to account for longer range interactions which exist at temperatures above 850 K. The miscibility diagram constructed using the Tersoff model simulation data can be used to provide information on the phase stability and equilibrium Indium content at any given temperature for the crystalline solid solution.     

High-Moment-Low-Moment Description of Magnetovolume Effects in Y(Mn x Al 1− x ) 2 and Y x Sc 1− x Mn 2

Based on the assumption of a high-moment-low-moment instability of the Mn atom, we construct a simple spin model with coupled magnetic and spatial degrees of freedom to describe the Laves phase systems Y(Mn x Al 1 m x ) 2 and Y x Sc 1 m x Mn 2 . Monte Carlo simulations of this model qualitatively reproduce anomalies observed in these materials like a discontinuous giant volume change and anomalous thermal expansion behavior.     

Spin-wave resonance in [Co x Ni1 − x ]N and [Co x P1 − x ]N gradient films

Gradient films of ferromagnetic 3d metals with prescribed magnetic potential profile along the film thickness are obtained. It is found that the spin-wave resonance spectrum in these films is characterized by anomalous dependences of resonance fields of spin-wave modes H _r on the mode number: H _r(n) ～ n, H _r(n) ～ n ^2/3.     

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.     

Mössbauer study of amorphous (Fe1−x Ga x )84B16

Using the⁵⁷Fe Mössbauer effect the influence of the Ga content in amorphous (Fe_1?x Ga _x )₈₄B₁₆ on the average hyperfine fields \(\bar H\) and isomer shift has been studied. For the sample (Fe_0.98Ga_0.02)₈₄B₁₆ the \(\bar H\) , as well as the recoilless fraction,f _a were measured as functions of temperature ranging from 12 K to 300 K. The experimental results show a linear correlation between Inf _a and δ, and well as between δ andx. In the temperature range \(\bar H(T)\) can be described by the Brillouin function and the second-order Doppler shift is appreciable. The characteristic temperature for such an amorphous alloy is 372 K. the effective vibrating massM _eff=79 a.u.     

Laser-induced chemical etching of ceramic PbTi1−xZrxO3

The laser-induced etching of ceramic PbTi_1–xZr_xO₃ in a hydrogen atmosphere and in air has been investigated. Visible Ar⁺ and Kr⁺ laser radiation was employed in most of the experiments. In H₂ atmosphere, regular patterning of the ceramic is possible. Average etch rates reach up to about 250 m/s.