P. C. Klipstein

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.

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.

Quantitative analysis on the effects of AlAs X states on Γ resonance in a GaAs∕AlAs double barrier structure under elevated hydrostatic pressures

We have investigated Γ-symmetry resonant tunneling in a GaAs∕AlAs “double barrier structure” with 20 A thick AlAs barriers as a function of hydrostatic pressure. A quantitative analysis based on transfer matrix and self-consistent Schrodinger-Poisson calculations is performed to understand the effects of charge accumulation in each layer on the resonance bias and the resonance peak/valley currents, yielding the value of the longitudinal mass (mz*) at the band edge of 20 A thick AlAs, mz*=(0.35±0.05)m0.

XX–XY interface band mixing in GaAs/AlAs heterostructures

Abstract The in-plane dispersion of the XX,Y states in a GaAs/AlAs “double-barrier structure” is measured by varying the angle of an in-plane magnetic field. When XX,Y (1) states in the emitter AlAs layer tunnel into collector X X,Y (m>1) states, a characteristic dumbbell shape is observed for the bias shift of the resonant tunneling peak versus magnetic field angle, with the major axis along [1 1 0] or [ 1 1 0] . This corresponds to an elliptical constant energy surface in the collector AlAs layer which is rotated by 45° with respect to the bulk Fermi surface. We explain the new symmetry by XX–XY interface band mixing which is closely analogous to the widely studied Γ–XZ mixing. Our results provide new insight into the microscopic origin of both types of mixing.

Enhancement of 2D 2D Tunneling by Γ–XZ Mixing in GaAs/AlAs Resonant Tunneling Structures at High Pressure

We have investigated the 2D 2D resonant tunneling in coupled X-band double quantum well structures, which can be achieved in GaAs/AlAs heterostructures by using high hydrostatic pressure. Until recently, there has been clear observation of 2D 2D resonant tunneling between confined transverse XX,Y states but not between longitudinal XZ states. In this paper, we demonstrate the existence of such resonant tunneling in samples with very thin well and barrier layers. The existence of detectable 2D 2D resonant tunneling between XZ states, even in a structure with a barrier thickness of 40 A, is striking. However, by modelling the transport in terms of quantum beats between symmetric and anti-symmetric double well states, we show that Γ–XZ mixing can produce enhancements of up to ≈102.

Identification of zone boundary and interface phonon recombinations in photoluminescence from type-II GaAs/AlAs short-period superlattices

Abstract We present the first identification of the direct participation of the GaAs interface (IF) mode in the phonon-assisted recombinations from type-II GaAs/AlAs short-period superlattices (SPSL). This is achieved by utilizing a novel first-order resonant Raman process associated with the type-II Xz–Γ band gap to enable a direct comparison, in the same sample, of zone boundary phonons with the photoluminescence (PL) phonon satellite energies. We present PL measurements on two complementary SPSLs to identify the reversal in the parity of the GaAs IF branch, where coupling is only allowed to the IF(+) branch. We also identify weak satellites, not previously reported, between the IF(+) and LA(X) satellites.

Growth and application of group III-antimonides by MOVPE

There are a number of important considerations in common in the growth of InP/InGaAs and InAs/GaSb superlattices. These include: the uniformity of the periodicity within the superlattice, the type of interfaces and their arrangement within the superlattice and the abruptness of those interfaces. InAs/GaSb superlattices are a small sub-set of antimonide based semiconductors which are of interest to those studying solid state physics and designing devices with infrared applications. This paper will highlight the growth, by MOVPE, of some InAs/GaSb superlattices, and compare this with published results from the growth of InGaAs/InP superlattices, particularly with respect to the issues mentioned above.