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.     

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.     

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.     

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.     

Raman scattering study of Al/Ga interdiffusion in ion-implanted and annealed GaAsGa1−xAlxAs superlattices

The Al/Ga interdiffusion after annealing has been characterized by Raman scattering on a GaAsGa_1−xAl_xAs superlattice where different densities of damage had been initially induced by implantation of electrically inactive isoelectronic elements, ³¹P⁺, in order to eliminate the impurity charge associated effects. To probe the mixing beyond the damaged zone of the superlattice, complementary Auger and SIMS experiments have implantation induced defects proceeds from the examination of the whole set of results.     

Anisotropy of Two-Dimensional Electron and Hole Mobilities in (0 0 1) GaAs/Al x Ga 1− x As Heterostructures Under Uniaxial Stress

Transport properties of two-dimensional electron and hole gas at (0 0 1) GaAs/Al x Ga 1 m x As heterointerface in [1 1 0] and [1 m 1 0] directions have been investigated for the first time under in plane uniaxial compression up to 5 kbar. Resistivity, Shubnikov-de Haas oscillations and Hall effect were measured and carrier densities and mobilities were determined. The in-plane uniaxial compression significantly modifies the energy spectrum of p-GaAs/Al x Ga 1 m x As that leads to strong anisotropy of hole mobility under compression. In the case of the n-type heterostructure uniaxial compression causes only change of the carrier concentration and the corresponding small change of the initial mobility anisotropy, determined by anisotropic surface roughness scattering.     

Materials and device properties of pseudomorphic In x Ga1−x As/Al0.3Ga0.7As/GaAs high electron mobility transistors (0<x<0.5)

Modulation doped Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures for device application have been grown using molecular beam epitaxy. Initially the critical layer thickness for InAs mole fractions up to 0.5 was investigated. For InAs mole fractions up to 0.35 good agreement with theoretical considerations was observed. For higher InAs mole fractions disagreement occurred due to a strong decrease of the critical layer thickness. The carrier concentration for Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures with a constant In _x Ga_1–x As quantum well width was investigated as a function of InAs mole fraction. If the In _x Ga_1–x As quantum well width is grown at the critical layer thickness the maximum carrier concentration is obtained for an InAs mole fraction of 0.37. A considerable higher carrier concentration in comparison to single-sided -doped structures was obtained for the structures with -doping on both sides of the In _x Ga_1–x As quantum well. Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures with InAs mole fractions in the range 0–0.35 were fabricated for device application. For the presented field effect transistors best device performance was obtained for InAs mole fractions in the range 0.25–0.3. For the field effect transistors with an InAs mole fraction of 0.25 and a gate length of 0.15 m a f _T of 115 GHz was measured.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

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.     

Valence-band structure and optical absorption inp-type GaAs–Al0.3Ga0.7As multi-quantum-well infrared photodetectors under an electric field

Hole structure of a GaAs–Al_0.3Ga_0.7Asp-type multiple quantum well (MQW) subjected to an electric field parallel to the growth axis is studied using the envelope-function approximation and taking into account the valence subband mixing. The system considered in this work consists of five GaAs wells and six thick Al_0.3Ga_0.7As barriers. The valence subband structure and the optical-absorption coefficient are calculated as functions of the electric-field strength for various doping levels. The subband structure is shown to be nonparabolic and anisotropic in the plane of the layers with a four-fold symmetry. The spin splitting due to the lack of specular symmetry of quantum wells is a growing function of the electric-field strength. The calculated optical absorption is in good agreement with the experimental spectra.     

Magnetic properties of GaAs/δ〈Mn〉/GaAs/In x Ga1 − x As/GaAs quantum wells

The field and temperature dependences of the magnetization of GaAs/δ〈Mn〉/GaAs/In _x Ga_{1 ? x} As/GaAs quantum wells with the δ〈Mn〉 layer separated from the well by a 3-nm GaAs spacer have been studied in the temperature range of 3–300 K in a magnetic field up to 6 T. An external magnetic-field-induced phase transition to a ferromagnetic state with a magnetization hysteresis loop shifted from a zero magnetic field has been found to occur at a temperature below 40 K. A theoretical model is proposed that implies the coexistence of ferromagnetically and antiferromagnetically ordered regions within the GaAs layers.     

Ferromagnetic transition in GaAs/Mn/GaAs/In x Ga1 − x As/GaAs structures with a two-dimensional hole gas

The transport properties of GaAs/Mn/GaAs/In _x Ga_{1 ? x} As/GaAs structures with a layer that is separated from the quantum well and contains Mn impurities in the concentration range 4–10 at % corresponding to the reentrant metal-insulator transition observed in the bulk GaMnAs material [17] have been investigated. The hole mobility in the objects under investigation is more than two orders of magnitude higher than the known values for the GaMnAs semiconductor and GaMnAs-based magnetic heterostructures. This makes it possible to observe Shubnikov-de Haas oscillations, which confirm a two-dimensional character of the hole energy spectrum. The calculated Curie temperature for heterostructures with indirect exchange interaction through a two-dimensional hole channel is in good agreement with the position of the maximum (at 25–40 K) in the temperature dependences of the electrical resistance of the channel. This suggests that two-dimensional holes play an important role in ferromagnetic ordering of the Mn layer under these conditions. The observations of a negative spin-dependent magnetoresistance and an anomalous Hall effect, whose magnitude correlates well with the results of theoretical calculations for two-dimensional ferromagnetic systems based on III-Mn-V, also indicate a significant role of the two-dimensional channel in ferromagnetic ordering.     

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.     

High-Density Effects on the Exciton Systems and Their Dynamical Behavior in Al x Ga 1− x As/AlAs Quantum Wires

In Al x Ga 1 m x As/AlAs quantum wire (QWR) structures, the lower lying indirect exciton (IE) photoluminescence (PL) peak shows remarkable blue-shift under intense light-excitation contrary to the higher lying direct exciton (DE) PL band with very small blue-shift, although the two kinds of exciton states consist of the common hole state. In time-resolved PL spectra in the type-II QWR of x = 0.4, the DE PL band appears at an earlier stage without peak-shift and the excitons relax to the IE state making the IE PL peak dominant with time evolution. The blue-shift of the IE peak in a quasi equilibrium after the relaxation seriously depends on the excitation density. The origin of the blue-shift is explained in terms of many-body effects including band-bending effect due to the electric field induced by spatially separated electrons and holes in the QWR structures.     

Finite confining effect on electron–phonon interaction in GaAs/Ga1−xAlxAs single heterostructures

The form factors of the electron–phonon interaction for GaAs/Ga_1−xAl_xAs single heterostructures have been evaluated using a finite height barrier. The calculations are performed within the extreme quantum limit approximation, assuming for the envelope electronic wavefunction a modified Fang–Howard wavefunction. Both types of long-wave phonons, longitudinal optical and interface phonons, are considered. It is found that the effect of the finite height is to reduce the strength of the electron–phonon interaction.     

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.     

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.     

Effect of Al and Ga interdiffusion on the electronic states in GaAs/Ga<Subscript>1−x</Subscript>Al<Subscript>x</Subscript>As semiconductor superlattice

The effect of interdiffusion of Al and Ga atoms on the confining potential and band structure of a three-dimensional superlattice, composed of initially spherical GaAs/Ga_1?xAl_xAs quantum dots, is investigated in the framework of the modified Wood-Saxon potential model. It is shown that the interdiffusion leads to the disappearance of the quantum dots’ spherical symmetry and to the broadening of the superlattice energy minibands.     

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.