R.W. Martin

Growth of InAs by MOVPE: A comparative study using arsine, tertiarybutylarsine and phenylarsine

Abstract The growth of bulk heteroepitaxial layers of InAs on GaAs substrates (and in some cases on InP substrates) by atmospheric pressure MOVPE is described. The indium source used was trimethylindium and we present a comparative study of the use of arsine, tertiarybutylarsine (TBAs) or phenylarsine (PhAs). The quality of the epitaxial layers was established from electrical and morphology measurements and showed a marked improvement with TBAs as a result of improved pyrolysis at lower temperatures. The 77 K mobility was found to increase from about 11,000 cm2/V·s for samples grown from arsine, to nearly 30,000 cm2/V·s for those grown from tertiarybutylarsine. The use of tertiarybutylarsine allows low growth temperatures close to those used in MBE to be used in MOVPE. PhAs was found to pyrolyse at a temperature very similar to arsine and grown layers were very similar to those obtained from arsine.

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.

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.

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.

Piezoelectric control of doping and band structure in the crossed gap system GaSb/InAs

We describe the use of strain induced piezoelectric fields to modify the doping and band profiles of GaSb/InAs heterostructures in both the semiconducting and semimetallic regimes. The piezoelectric fields lead to substantial absorption enhancement as well as significant carrier density increases in [111] oriented samples over their [001] equivalents. Strain relaxation is shown to reduce the strength of the piezoelectric field at large layer thicknesses, giving similar carrier densities in both [001] and [111] structures, indicating that the band offsets are probably independent of crystal orientation. Hole cyclotron resonance masses have been observed for the first time in “intrinsic” superlattices and are in the range 0.08-0.20m0. The high hole masses and crossed sets of Landau levels from the electrons and holes result in strong pinning of the Fermi energy in the semimetallic superlattices which we find leads to an interchange of oscillatory behaviour of the Hall and longitudinal resistivity components, as compared to a unipolar system.

The design of quantum-confined Stark effect modulators for integration with 1.5 mu m lasers

Various factors affecting the design of an optical modulator based on the quantum-confined Stark effect and suitable for integration with a 1.55 mu m InGaAs(P) laser are considered. Multiple quantum well (MQW) structures comprising various combinations of InP, InGaAs, InGaAsP and InGaAlAs materials are considered both theoretically and experimentally. Calculations of Stark shifts and overlap integrals, as functions of applied electric field are used to assess the potential for modulator performance. A quaternary/quaternary system is shown to give the best performance at the wavelength of interest and this has been demonstrated using MOVPE-grown structures. The calculations of Stark shifts and wavefunction localization have also been employed to assess the optimum layer thicknesses in the MQW and are compared with photocurrent data.

Optimization of the growth by MOVPE of strained GaSb/InAs double heterojunctions and superlattices on [111] GaAs substrates

Abstract We report here on the optimization of the growth by atmospheric pressure MOVPE of double heterojunctions and superlattices of GaSb/InAs on both [111]A and [111]B GaAs substrates. The layers were not intentionally doped. Various indium precursors were used in this study to compare their effectiveness. Adduct purified TMIn, EDMIn, and a mixture of TMIn and TEIn were used. It was found that both the EDMIn and the mixture behaved essentially the same as TMIn in the growth of these structures. Various V/III ratios were tried for the InAs layer to establish whether different conditions were needed for the [111]A and [111]B cases in the double heterojunction structure. The superlattices were grown using a 2:1 ratio of GaSb to InAs on top of a thick (4 μm) buffer layer of GaSb. Various different switching and pause sequences were investigated at the interfaces to deliberately produce “InSb-like” interfaces which are expected from previous work on the double heterojunctions to give superior results. Both the semiconducting and semimetallic regions were studied. The superlattices were characterized by absorption methods when the band gap is positive in the semiconducting samples, and by magnetotransport studies in the semimetallic regime where the band gap is negative. The influence of the piezoelectric field on the optical properties in the infrared region of the thin layer superlattices (up to about 80 A total thickness) is marked by a large increase in the measured absorption for the [111] samples compared to [100]. In the semimetallic regime, the structures showed close to intrinsic behaviour with electron: hole concentration ratios of 3:2. The [111]A structures showed large mobility enhancement over the equivalent [100] analogues.

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.

Valence band spin splitting in strained In0.18Ga0.82As/GaAs quantum wells

Magneto-transport at low temperature has been employed to study a series of p-type In0.18Ga0.82As/GaAs quantum wells with carrier concentrations in the range (1.5-4.3)*1011 cm-2. Tilting the field from the growth direction does not change the relative magnitudes of the spin-split conductivity minima. This is a remarkable effect, and confirms the recent results of Martin et al. (see Phys. Rev. B, vol.42, p.9237, 1990) on an In0.15Ga0.85Sb/GaSb quantum well. Strain decoupling of the mod MJ mod =3/2 and mod MJ mod =1/2 states projects the spin onto the growth direction. The authors find here that this effect occurs for the nu =3, 4 and nu =5, 6 and nu =7, 8 spin-split conductivity minima. The Shubnikov-de Haas oscillations in perpendicular field have been compared to the Landau levels calculated with an eight-band k.p model and this determines an upper limit to the value of kappa l, the spin-magnetic-field coupling parameter.

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.