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.     

Effect of lattice constant on band-gap energy and optimization and stabilization of high-temperature In x Ga1?x N quantum-dot lasers

We analyze the effect of the lattice constant on the band-gap energy of In _x Ga_1?x N and optimize the structure of the device with a separate-confinement heterostructure. To vary the lattice constants, we change the In molar fraction, which permits us to investigate a wide range of the band gap of the active material employed in diode lasers. In _x Ga_1?x N is a promising active material for high-performance 1.55???m quantum-dot lasers due to its excellent band-gap-energy stability with respect to temperature variations. The band gap of In _x Ga_1?x N decreases from 3.4 to 0.7?eV, and the necessary band gap can be achieved by changing the lattice parameters depending on the device application. It has been found that In_0.86Ga_0.14N can be a promising material for emitting light at a wavelength of 1.55???m.     

Chromium mask for plasma-chemical etching of Al x Ga1 − x N layers

The influence of the mask material used for formation of GaN mesa-structures using plasma-chemical etching in a BCl₃ : Ar : N₂ atmosphere is studied. It is shown that a bilayer SiO₂/Cr mask in which the thickness of the chromium layer is six to seven times smaller than a desired etch depth provides effective protection of the surface and allows the formation of structures with a flat surface for an etch depth of up to 2.5 μm. When the mask is produced by conventional lithography, the slope of the side walls is no more than 10° and decreases when liftoff lithography is applied.     

LaAg x In1−x

The structures of LaAg _x In_1–x alloys withx=0.75, 0.89 are determined by neutron diffraction on powder samples. The space group isI4/mmm (D _4h ^/17 ). The lattice constants splitting, the order parameter and the mean square vibrational amplitudes of the atoms are given in the temperature range from 20 KT300 K.     

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.     

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.     

Structural and electronic properties of GaN x As1−x alloys

The structural and electronic properties of cubic GaN _x As_1−x with N-concentration varying between 0.0 and 1.0 with step of 0.25 were investigated using the full potential–linearized augmented plane wave (FP-LAPW) method. We have used the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange and correlation potential. In addition the Engel-Vosko generalized gradient approximation (EVGGA) was used for the band-structure calculations. The structural properties of the binary and ternary alloys were investigated. The electronic band structure, total and partial density of states as well as the electron charge density were determined for both the binary and their related ternary alloys. The energy gap of the alloys decreases when we move from x=0.0 to 0.25; then it increases by a factor of about 1.8 when we move from 0.25 to 0.5, 0.75 and 1.0 using EVGGA. For both LDA and GGA moving from x=0.0 to 0.25 causes the band gap to close, showing the metallic nature of the GaN_0.25As_0.75 alloy. When the composition of N moves through x=0.25, 0.5, 0.75 and 1, the band gap increases.     

Effect of coherency strain on the deformation of In x Ga1− x As superlattices under nanoindentation and bending

It has been shown elsewhere that the room temperature yield pressure of In _x Ga_{1? x} As superlattices measured by nanoindentation, decreases from a high value as the volume averaged strain modulation is increased, while at 500°C under uniaxial compression or tension the yield stress increases from a low value with increasing strain modulation. We have used cross-sectional transmission electron microscopy to examine the deformation mechanisms in these two loading regimes. At room temperature both twinning and dislocation flow was found with the proportion of twinning decreasing with increasing strain modulation. The coherency strain of the superlattice is retained in a twin but partially relaxed by dislocation flow. The strain energy released by the loss of coherency assists dislocation flow and weakens the superlattice. Twins are only nucleated when a critical elastic shear of about 7° is achieved at the surface. The plastic zone dimensions under the indent are finite at the yield point, with a width and depth of approximately 1.3?µm and 1.1?µm respectively. Under uniaxial compression and tension at 500°C the superlattices deform by dislocation flow along {111} planes. The most highly strained samples also partially relax through the formation of misfit dislocations.     

Influence of strain on built-in dipole moment in asymmetric In x Ga1−x As quantum dot molecules

Strain effects on a built-in electron-hole dipole moment are investigated in asymmetric In _x Ga_1?x As coupled quantum dots. We compute electron-hole separation as a function of alloy compositions for both electron and hole resonance cases. It is noted that the inclusion of strain enhances the built-in dipole moments and that the inverted electron-hole alignment is found for electron and hole resonances. Furthermore, the reversal of dipole moments gives rise to different asymmetric Stark shifts in each transition spectrum.     

Strained In x Ga1−x As/GaAs multiple quantum wells grown by MOVPE

Luminescence properties of strained In _x Ga_1−x As/GaAs multiple quantum wells of different thickness and In content, prepared by metal organic vapour phase epitaxy were studied. The influence of the quantum well material composition on the shape of luminescence spectra was investigated. The experimental results were fitted by the Model Solid Theory. This fit was improved by the use of adjustedQ parameter. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998. This work was supported by Grant Agency of Czech Republic under grants numbers 202/98/0074, 102/99/0414 and Grant Agency of Academy of Sciences No. A 10110807/1998.     

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.     

Segregation in In x Ga1− x As/GaAs Stranski–Krastanow layers grown by metal–organic chemical vapour deposition

Using quantitative high-resolution transmission electron microscopy we studied the chemical morphology of wetting layers in In _x Ga_{1? x} As/GaAs quantum dot structures which were optimized for applications to optical devices operating around 1.3?µm. The samples are grown by low-pressure metal–organic chemical vapour deposition on GaAs substrates. The In concentration profiles of the wetting layers are evaluated with the composition evaluation by lattice fringe analysis method. The profiles reveal a clear signature of segregation. A fit of the profiles with the Muraki et al. model for segregation reveals a segregation efficiency R?=?0.65?±?0.05 at the growth temperature of 550°C, which is significantly lower than segregation efficiencies observed in samples grown by molecular beam epitaxy at similar temperatures.     

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.     

Bandgap evolution of GaN1−x As x in the whole composition range

The bandgap evolution of GaN_1?x As _x in the whole composition range is investigated and a model describing its bandgap energy is developed. It is found that the bandgap evolution is due to two factors. One is the interaction between the impurity band and the Γ conduction band or the Γ valence band of the host materials. The other one is the intraband coupling within the conduction band and separately within the valence band. The former is dominant in the As-rich GaNAs and the N-rich GaNAs. The latter plays an important part in the N-rich range and the moderate composition range.     

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.     

Martensitic transformation in La1−x Yb x Ag1−y In y

The structural phase transformation of La_1–x Yb _x Ag_1–y In _y has been studied on single crystals by low temperature Laue-technique. The martensitic transformation in this pseudobinary intermetallic alloy has to be characterized as a weak orthorhtombic distortion of a single I centered unit cell (c/a1.04;a/b1.006) and a collective slipping or twinning of these cells that gives a fixed orientation between the remaining cubic room temperature structure and the martensitic phase. Above room temperature exists an order-disorder transformation from the CsCl-B2 structure to an at room temperature metastable W-A2 structure. There is no dramatic change in the physical properties of this alloy by substituting La by Yb, so we may approximate our results to LaAg_1–y In _y .This paper is dedicated to Prof. Dr. S. Methfessel on the occasion of his 60^th birthday     

Pressure induced phase transitions in Ga1 − x In x P

We have theoretically investigated the effect of pressure on the structural stability of GaP?:?InP mixed system. The three-body-potential (TBP) model has been used. The TBP model consists of long-range as well as short-range interactions; the long-range part includes the modified Coulomb force as well as a three-body term; the short-range part in TBP defines the van der Waals and overlap repulsive interactions. We observe a pressure-induced structural phase transformation from ZnS (B3) to NaCl (B1) type phase in Ga _1?x In _x P. Our calculated transition pressures for the initial GaP and final InP compound semiconductors are in good agreement with other reported data.