L. R. Dawson

Dependence of critical layer thickness on strain for InxGa1−xAs/GaAs strained‐layer superlattices

Various InxGa1−xAs/GaAs strained‐layer superlattices have been characterized by Hall effect, ion beam channeling, and photoluminescence measurements in order to evaluate their crystalline quality. Structural characteristics (e.g., layer strains and thicknesses) were obtained from channeling or x‐ray diffraction studies. The structures had strains in the alloy layers of 0.5–2.7%. Critical layer thicknesses for degradation of sample quality are in good agreement with the theoretical expression proposed by J. W. Matthews and A. E. Blakeslee [J. Cryst. Growth 27, 118 (1974)]. Our results provide important information for design of strained‐layer devices.

Controversy of critical layer thickness for InGaAs/GaAs strained‐layer epitaxy

The critical layer thickness for InxGa1−xAs layers in InxGa1−xAs/GaAs single strained quantum wells (SSQW’s) and strained‐layer superlattices (SLS’s) are investigated. Photoluminescence microscopy (PLM) images and x‐ray rocking curves for two series of SSQW and SLS structures corresponding to many different layer thicknesses were obtained. We find that the PLM technique, which directly images dislocations and is sensitive to low dislocation densities, is much more suitable for determining the onset of dislocation creation. The x‐ray technique can detect lattice relaxation by dislocations but only at relatively high densities of dislocations. Using the former technique, we determine critical thicknesses of 190 A for SSQW’s and 250 A for SLS’s with x≊0.2. These results are near the theoretical predictions of J. W. Matthews, S. Mader, and T. B. Light [J. Appl. Phys. 41, 3800 (1970)] (150 and 300 A, respectively) and are much lower than results obtained by x‐ray or other techniques which sense lattice relaxation.

Critical layer thickness in In0.2Ga0.8As/GaAs single strained quantum well structures

We report accurate determination of the critical layer thickness (CLT) for single strained‐layer epitaxy in the InGaAs/GaAs system. Our samples were molecular beam epitaxially grown, selectively doped, single quantum well structures comprising a strained In0.2Ga0.8As layer imbedded in GaAs. We determined the CLT by two sensitive techniques: Hall‐effect measurements at 77 K and photoluminescence microscopy. Both techniques indicate a CLT of about 20 nm. This value is close to that determined previously (∼15 nm) for comparable strained‐layer superlattices, but considerably less than the value of ∼45 nm suggested by recent x‐ray rocking‐curve measurements. We show by a simple calculation that photoluminescence microscopy is more than two orders of magnitude more sensitive to dislocations than x‐ray diffraction. Our results re‐emphasize the necessity of using high‐sensitivity techniques for accurate determination of critical layer thicknesses.

Light-hole conduction in InGaAs/GaAs strained-layer superlattices

We report the first observation of light‐hole band carriers in In0.2Ga0.8As/GaAs strained‐layer superlattices by direct measurements of their effective mass (m*mo=0.14) using oscillatory magnetoresistance data. Preferential population of light‐hole states, due to splitting of the degenerate bulk valence bands by built‐in strain, allows this direct observation.

Midwave (4 μm) infrared lasers and light‐emitting diodes with biaxially compressed InAsSb active regions

Heterostructures with biaxially compressed, As‐rich InAsSb are being investigated as active regions for midwave infrared emitters. InAs1−xSbx/In1−xGaxAs (x≊0.1) strained‐layer sublattices (SLSs), nominally lattice matched to InAs, were grown using metalorganic chemical vapor deposition. An SLS light‐emitting diode was demonstrated which emitted at 3.6 μm with 0.06% efficiency at 77 K. Optically pumped laser emission at 3.9 μm was observed in a SLS/InPSb heterostructure. The laser had a maximum operating temperature of approximately 100 K.

Stability of strained quantum-well field-effect transistor structures

Conditions for stability of strained-layer structures and their implications for device fabrication are examined. Structures which have exhibited the best performance to date are found to be thermodynamically metastable (or at best marginally stable) structures, which will restrict the processing steps permissible in the integration of these devices to form complex circuits.<<ETX>>

HIGHLY REFLECTIVE, LONG WAVELENGTH ALASSB/GAASSB DISTRIBUTED BRAGG REFLECTOR GROWN BY MOLECULAR BEAM EPITAXY ON INP SUBSTRATES

Surface normal optoelectronic devices operating at long wavelengths (≳1.3 μm), require distributed Bragg reflectors (DBRs) with a practical number (≤50) of mirror layers. This requirement implies a large refractive index difference between the mirror layers, which is difficult to achieve in the traditionally used phosphide compounds. We demonstrate a highly reflective AlAsSb/GaAsSb DBR grown nominally lattice matched to an InP substrate by molecular beam epitaxy. Reflectivity measurements indicate a stop band centered at 1.74 μm with maximum reflectivity exceeding 98%, which is well fitted by our theoretical predictions. Atomic force microscopy and transmission electron microscopy indicate reasonable crystal quality with some defects due to an unintentional lattice mismatch to the substrate.

GaAs/(In,Ga)As, p‐channel, multiple strained quantum well field‐effect transistors with high transconductance and high peak saturated drain current

GaAs/In0.2 Ga0.8 As structures with two paralleled 10 nm quantum wells, modulation doped from the top, bottom, and middle with Be, have been fabricated into multiple strained quantum well field‐effect transistors (MQWFET’s) with 1×150 μm2 Ti/Au gates and examined both illuminated and in the dark at 300 and 77 K. Measurements on van der Pauw structures fabricated simultaneously with the transistors showed hole mobilities and sheet carrier densities to be 200, 3100, and 8040 cm2/V s, and 5.7×1012, 1.8×1012, and 1.5×1012 cm−2 , at 300, 77, and 4 K, respectively. Shubnikov–de Haas measurements made below 4 K verified the existence of a double‐channel two‐dimensional hole gas with a strain‐shifted light‐hole ground state in the quantum wells with an effective hole mass of 0.15 me . A representative p‐channel MQWFET showed well‐saturated common‐source output characteristics, both illuminated and unilluminated, at all measurement temperatures. Measured peak extrinsic transconductances and peak saturated drain cu...

Strain measurements by channeling angular scans

We demonstrate the first direct measurement of strain in InxGa1−xAs/GaAs strained‐layer superlattices by ion channeling angular scans. This technique permits quantitative measurement of the strain without requiring computer simulations of dechanneling. The measurements are generally applicable to the study of strain in heteroepitaxial layers.

Extended infrared response of InAsSb strained-layer superlattices

Strained‐layer superlattices of InAsSb were grown with low densities of dislocations and microcracks for optical characterization to determine the suitability of these structures for infrared photodetectors. Infrared transmission measurements revealed absorption throughout the 8–12 μm region and extended to longer wavelengths than predicted from consideration of the tensile strain‐induced band‐gap shift in a type‐I superlattice. We conclude that a type‐II superlattice occurs in the InAsSb system for alloy compositions >60% Sb.