D.M. Symons

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.

“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.

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.

Optical and magnetotransport properties of semimetallic InAs/(In,Ga)Sb superlattices

Abstract Semimetallic superlattices and double heterojunctions of the system InAs / In x Ga 1− x Sb have been studied by cyclotron resonance and magnetotransport in the magnetic field range 10–100 T. The cyclotron resonance is used to show that there is a magnetic field induced transition from semimetallic to semiconducting behaviour, at which almost all the carriers in the system disappear. The magnetotransport shows that large oscillatory Hall voltages occur in the quantised regime, and these are strongly temperature dependent. The field positions of the Hall resistivity peaks are related to the crossing of the electron and hole Landau levels. Hydrostatic pressure is used to achieve a controlled decrease of the band overlap, and to show that the Hall peaks and minima move to zero field as the zero overlap condition is approached. Both the cyclotron resonance and the magnetotransport show that the magnitude of the band overlap is orientation. dependent, being ≈ 60 meV larger for (1 1 1) oriented structures. This is attributed to the presence of a large interface dipole for this orientation.

Pressure investigation of the quantum Hall effect in intrinsic semimetallic InAs/(Ga,In)Sb heterostructures

Abstract We have performed magnetotransport measurements on InAs/(Ga,In)Sb heterostructures under hydrostatic pressure. The system has a cross gap band alignment which leads to the formation of co-existent 2D electron-hole gases. The amount of overlap and thus the electron and hole concentrations can be tuned by the application of hydrostatic pressure. The samples studied here have nearly equal electron and hole concentrations and show large oscillatory quantum Hall features. Measurements of the band overlap Δ and its rate of change with pressure d(Δ) dP provides evidence that both the growth direction and also the composition of the interface layer play an important role in determining the band line-up.

Piezoelectric effects in superlattices

Magnetotransport and optical measurements have been carried out on (001) and piezoelectrically active (111) InGaSb/GaSb and InAs/GaSb structures. In accordance with theory, these comparative studies reveal enhanced carrier density and mobility for growth in the (111) direction. The observed Stark shifts of up to 100 meV, in the (111) oriented InGaSb/GaSb structures are well accounted for by the estimated built-in piezoelectric field, of the order of 1*105 V cm-1. The absorption coefficient of (111) oriented InAs/GaSb superlattices shows significant enhancement.

Selective area epitaxy of InGaAs/InGaAsP quantum wells studied by magnetotransport

Selective area epitaxy can have a marked effect on the growth of quantum well structures by metal-organic vapour phase epitaxy. An understanding of the resulting modifications to the layer thicknesses and compositions is important for the fabrication of integrated devices. In this paper we present magnetotransport measurements from modulation-doped InGaAs/InGaAsP multi-quantum wells grown on InP substrates patterned with silica masks. The observed increase in carrier density with decreasing width of the conducting channel is related to changes induced by the selective area growth. A self-consistent solution of the Schrodinger and Poisson equations is used to calculate the modifications to the band profile. The occupation of two subbands in one of the structures allows the subband separation to be extracted from the magnetoresistance data, leading to an estimate of the well thickness over three times its nominal value.

Cyclotron and intersubband resonance studies in [001] and piezoelectric [111] InAs/(Ga,In)Sb superlattices

Abstract Cyclotron resonance and inter-subband excitation experiments have been performed on superlattices of the semimetallic, strained type II system InAs/(Ga,In)Sb. We observe simultaneously one cyclotron resonance from the 2D electrons located in the InAs and two resonances from the 2D holes in the (Ga,In)Sb layers. The two hole resonances originate from the M J = ¦ 3 2 > states. We have found a strong crystallographic anisotropy in the hole masses between structures grown along [001] and [111]A orientations attributed to the strain decoupling of the heavy and light hole levels in the GrmGa 1- x In x Sb valence band. In addition, we demonstrate for the first time that intersubband resonances can be directly seen for the electrons at large tilt angles.

Interface composition dependence of the band offset in InAs/GaSb

Abstract We have performed 4 K magnetotransport measurements on intrinsic InAs/GaSb multi quantum wells (MQWs) under hydrostatic pressure. Through careful configuration of the growth we are able to produce samples that have differing interface monolayers (either InSb or GaAs). Analysing the data to calculate the band overlap (Δ), we find that InSb-like samples have an overlap 30 meV larger than GaAs-like in good agreement with recent theoretical predictions.

Magnetotransport studies of GaSb/InAs crossed gap heterostructures in high magnetic fields

Abstract Magnetotransport measurements of MOVPE-grown GaSb/InAs heterostructures in magnetic fields up to 50 T have demonstrated novel transport effects for a system with coexisting electrons and holes. The Hall resistivity, ϱxy, has large features and strong minima, while the longitudinal magnetoresistance, ϱxx, shows plateau-like features. This is in sharp contrast with the behaviour of two-dimensional systems containing only one carrier type, where ϱxx oscillates and ϱxy shows quantised plateaus. The samples measured are close to the intrinsic limit ( n e n h ⋍ 1.5 ) , and compensated quantum Hall plateaus are seen, with ϱxy approaching zero for fields at which there is the same integral number of filled Landau levels of electrons as of holes.