R.J. Warburton

GaSb heterostructures grown by MOVPE

GaSb based heterostructures have been grown with a varying degree of strain in the GaSb layer as a result of the lattice mismatch. The GaSb/Ga1−xAlxSb system has ⩽0.65% mismatch making it potentially attractive for epitaxial growth. However, both the crystallinity and electrical quality of MOVPE grown Ga1−xAlxSb were found to be limited by carbon contamination from the TMAl starting material. Quantum wells (QWs) of GaSb/Ga1−xInxSb, with ≈ 1.2% mismatch for x=0.2, were successfully grown with abrupt interfaces and few dislocations. Shubnikov-De Hass oscillations in the transverse magnetoresistance (ϱxx) and the associated quantum Hall effect indicated that a two-dimensional (2D) hole gas was present in these structures. Unusually, the strongest oscillations were seen for occupancy of an odd number of (spin split) Landau levels (ν=1,3,5,…, etc.) Finally, QWs have been grown in the highly strained GaSb/GaAs system (7% lattice mismatch). TEM micrographs showed the critical thickness for 2D epitaxial growth to be about 15 A. Photoluminescence spectra and photoconductivity both show a transition at ≈ 1.27 eV, arising from the effect of the strain on the GaSb energy gap.

GaAs/GaSb strained‐layer heterostructures deposited by metalorganic vapor phase epitaxy

The growth of strained GaSb/GaAs quantum wells has been attempted for the first time (7% lattice mismatch), with the antimonide layers being constrained to take on the GaAs lattice parameter in the interface plane. The critical thickness for pseudomorphic growth of the strained layer was about 15 A, with further growth resulting in islands of GaSb crystallites over the wafer surface. Photoluminescence spectra and photoconductivity from both single and double wells showed a strong signal at approximately 1.3 eV, identified as a Γ point transition. This was not consistent with band structure calculations for a GaSb/GaAs well, suggesting an error in the estimation of the band offsets and/or As incorporation in the strained layer.

GaSb/GaInSb quantum wells grown by metalorganic vapor phase epitaxy

Single and multiple quantum wells of GaSb/GaInSb were grown by metalorganic vapor phase epitaxy. X‐ray diffraction on an 80 A single well confirmed the Ga1−xInxSb composition to be x=0.15, for which the lattice mismatch is ≊1.0%. Photoluminescence and photoconductivity from this sample both showed a signal due to carriers in the well, the position of which was in good agreement with the calculated band diagram. Shubnikov–de Haas oscillations in the transverse magnetoresistance (ρxx) of a four‐period multiquantum well, and the associated quantum Hall effect, indicated that a two‐dimensional hole gas was present in one of the wells. Unusually, the strongest oscillations were seen for occupancy of an odd number of (spin split) Landau levels (ν=1,3,5,...,etc.) This sample also showed luminescence peaks at 738 and 755 meV which were attributed to recombination in the wells.

Strain reconstruction of the valence band in Ga1 − xInxSb/GaSb quantum wells

Abstract We report measurements of the dispersion relation for holes in the M j = 3 2 strain split valence band in Ga − xInxSb/GaSb quantum wells. The strain decoupling of the M j = 3 2 and M j = 1 2 levels gives the M j = 3 2 ∼ 0.07 m0) for in-plane motion. The band is found to be highly non-parabolic, and the change in mass has been modelled directly using a full Luttinger-Kohn Hamiltonian calculation. Good agreement with experiment is achieved. Inter-band magneto-absorption experiments agreed with the low hole masses observed in cyclotron resonance and extended the study to higher energies in both the conduction and valence band.

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.

Cyclotron resonance in InAs/GaSb heterostructures

Cyclotron resonance measurements are reported for both electrons and holes in type II InAs/GaSb superlattices and double heterostructures (DHETS). Superlattice (multi quantum well) samples of InAs/GaSb grown by MOVPE have sufficiently high hole gas mobilities and densities to allow the first measurements of hole cyclotron resonance in this system. The measured hole masses are approximately 0.1me and 0.2me, indicating a large reduction over the bulk values due to the decoupling of the valence band by strain and confinement. This is in good agreement with eight-band k . p theory reported here, and previous calculations. The electron masses in both superlattices and DHETS are found to be strongly influenced by non-parabolicity and the carrier concentration, leading to considerable increases over the band edge values. Similar values are found for both electron and hole masses in structures grown along both (001) and (111)A directions.

“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.

An optically detected cyclotron resonance study of bulk GaAs

We have measured optically detected cyclotron resonance using far-infrared radiation on an exceptionally pure sample of GaAs in fields up to 15.5 T. This relatively new experimental technique is shown to offer high resolution of free and donor impurity-bound electron transitions without the reproducibility problems of photoconductivity. The data confirm the existence of metastable donor states and provide a detailed picture of chemical shifts. The optically detected cyclotron resonance signal represents an interaction between the donor bound electron states which are influenced by the far-infrared radiation and the donor bound exciton states which are responsible for the photoluminescence. Attenuation of the luminescence intensity under far-infrared illumination is primarily the result of a photothermal effect. At high fields, there is indication of an interaction between the electron and excitonic energy levels.

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.