M. Lakrimi

Growth of InAs/GaSb strained layer superlattices. I

Abstract InAs/GaSb strained layer superlattices (SLSs) have been grown by metalorganic vapour phase epitaxy (MOVPE) at atmospheric pressure. Whilst long period SLSs have been successfully grown by this technique, the growth of short period structures is adversely affected by step-bunching. By growing the SLSs faster and cooler, good periodicity was achieved as measured by Raman spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction (XRD) in SLSs with bilayer (GaSb + InAs) thicknesses as thin as 50 A. We have also detected the InSb-like and GaAs-like interface modes from room temperature Raman measurements for the first time in MOVPE grown samples. The most promising samples have been assessed by FIR photoconductivity at 4.2 K and show bandgaps (dependent on the bilayer thickness) between 5 and 20 μm.

Growth of InAsGaSb strained layer superlattices by MOVPE III. Use of UV absorption to monitor alkyl stability in the reactor

InAs/GaSb strained layer superlattices have been grown by atmospheric pressure MOVPE and the growth conditions optimised by observing, in real time, the in-situ UV absorption of the alkyls in the growth chamber. The Raman scattering of folded longitudinal acoustic phonons in the superlattices has been used as a probe of the periodicity of the superlattice. Atomic force microscopy has also been used to give information about the final surface morphology and RMS roughness of the superlattices. By combining all three techniques, optimum conditions have been found for the growth of short period InAs/GaSb superlattices. These have been used to sandwich a long period superlattice designed for transport measurements. The use of the short period superlattices eliminated additional conducting layers at each end of the semimetallic superlattice and produced structures where the hole and electron densities are equal. Such structures exhibit a dramatic new quantum transport effect where the Hall resistance goes to zero at high pressures and low temperatures.

GaSb/InAs heterojunctions grown by MOVPE: Effect of gas switching sequences on interface quality

Abstract The GaSb/InAs interface can be grown in two quite different ways either with In and Sb atoms forming the interface “InSb-like” or Ga and As atoms forming the interface “GaAs-like”. This is a result of both the Group III and Group V atoms changing at the interface. Different interfaces have been achieved in GaSb/InAs heterojunctions grown by atmospheric MOVPE using different gas switching sequences and the consequent changes in the electrical behaviour have been assessed using low field magnetotransport measurements. The results range from very poor (“GaAs-like”) to excellent (a particular “InSb-like”) interface. A further comparison is made to a previously used growth sequence for these structures. The effect of pauses during the interface sequence has also been investigated.

Growth of InAsGaSb strained layer superlattices. II

Abstract InAs GaSb strained layer superlattices (SLSs) have been grown by metalorganic vapour phase epitaxy (MOVPE) at atmospheric pressure. Initially the interfaces of the SLS have been biased towards pairs of GaAs or InSb using three different gas switching sequences. Room temperature Raman optical modes show that growing the interfaces using an ALE (atomic layer epitaxy) like switching sequence gives interfaces of very high quality probably near the optimum, which is a monolayer, Growing with other switching sequences leads to one of the interfaces being non-uniform. By growing samples with alternating (InSb,GaAs or GaAs,InSb) pairs of interfaces it is possible to unambiguously assign this non-uniformity to one of the two possible interfaces for the first time. Furthermore, the influence of the band overlap on interface type has been studied using optimised SLSs in the semimetallic regime.

High magnetic field studies of the crossed-gap superlattice system InAs/GaSb

Abstract A variety of optical and electrical studies are described for superlattices and heterostructures based on the materials system InAs/GaSb. The crossed-band-gap alignment of this system leads to a semimetal to semiconductor transition as a function of either superlattice period, magnetic field or pressure. Cyclotron resonance is studied for both electrons and holes, and the electron resonance is observed in the magnetic field range where the field induced band crossing occurs. Studies of the pressure dependence of the band offset show that both (1 1 1)A and (1 0 0) oriented structures have a pressure coefficient of 10.7 meV/kbar, but the band crossing at zero pressure is larger for the (1 1 1)A case. Compensated quantum Hall plateaux are observed at high magnetic fields and low temperatures, and large oscillatory features are observed in the Hall voltage under a range of conditions. In very high fields we have observed the zero-resistance Hall plateaux occurring due to total compensation of the electron and hole states.

Improved photoluminescence from electrochemically passivated GaSb

A new class of insulating and passivating layers on gallium antimonide has been prepared by means of an electrochemical process. In previous work we used this new process of fabrication of passivating and insulating layers for gating devices made from GaSb/InAs/GaSb nanostructures (Chen Y et al 1994 Superlatt. Microstruct. 15 41 and Chen Y 1995 PhD Thesis Hertford College, Oxford, UK). In this publication we describe the effects of the electrochemical process leading to an improvement of the photoluminescence (PL) after the growth of the passivating layer on GaSb. The PL measurements on , and GaSb substrates and on GaSb epilayers grown by MOVPE on GaAs indicate significant improvement of the PL intensity even after 12 months. Similar results have been observed on InGaSb/GaSb superlattice structures.

Metal-insulator oscillations in a two-dimensional electron-hole system

The electrical transport properties of a bipolar InAs/GaSb system have been studied in a magnetic field. The resistivity oscillates between insulating and metallic behavior while the quantum Hall effect shows a digital character oscillating from 0 to 1 conductance quantum e(2)/h. The insulating behavior is attributed to the formation of a total energy gap in the system. A novel looped edge state picture is proposed associated with the appearance of a voltage between Hall probes which is symmetric on magnetic field reversal.

“Intrinsic” quantum Hall effect in InAs/Ga1−xInxSb crossed gap heterostructures in high magnetic fields

Abstract We report a study of the quantum Hall effect and Shubnikov-dc Haas oscillations in semimctallic type II heterostructures of the strained layer system InAs/Ga1−xInxSb which are almost intrinsic. In high magnetic fields up to 50 T, ρxy has large peaks, demonstrating the high degree of charge compensation in the system. Between these peaks, intrinsic quantum Hall minima arc observed, where ρxy approaches zero.

Magnetotransport of piezoelectric [111] oriented strained quantum wells

We report the first observation of magnetotransport in piezoelectrically active heterostructures. Well‐resolved quantum Hall plateaus and magnetoresistance minima are observed for two‐dimensional hole gases confined in [111] oriented strained‐layer Ga1−xInxSb/GaSb quantum wells with a piezoelectric field, of order 1×105 V/cm. We discuss the enhanced carrier densities induced by the in‐built field and the differences observed between [111]A and [111]B orientations. Comparisons are made with simultaneously grown [001] structures. Stark energy shifts observed in photoluminescence are well accounted for by the estimated electric field.

Orientation and pressure dependence of the band overlap in InAs/GaSb structures

The band overlap at the InAs-GaSb interface has been measured using electron and hole densities deduced from magnetotransport measurements, combined with self-consistent energy level calculations, for structures with both (001) and (111)A orientations. This band crossing is found to be 140 meV for the (001) orientation but increases to 200 meV for (111)A. The difference is attributed to the presence of an interface dipole for (111)A growth. In addition, the band overlap decreases, with applied hydrostatic pressure, at a higher rate for the (111)A orientation.