R.J. Nicholas

Growth of GaSb by MOVPE

High-quality homo- and hetero-epitaxial GaSb has been grown from TMGa and TMSb using atmospheric pressure metal-organic vapour-phase epitaxy (MOVPE). Unintentionally doped material was p-type. At 295 K it had a carrier concentration of around 3.0*1016 cm-3 and a corresponding Hall mobility in the range of 670 to 1000 cm2 V-1 s-1. Growth parameters were carefully correlated with surface morphology, electrical quality and photoluminescence data to establish the growth window for this material.

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.

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.

Cyclotron resonance of electrons in a narrow GaAs/(Ga,Al)As quantum well: Polaron effects and non-parabolicity

Cyclotron resonance of electrons in a GaAs/Ga0.64Al0.36As multi-quantum-well sample with 2.2 nm well width is reported, in magnetic fields up to 19 T applied perpendicular to the well planes. The resonances are observed by using optical pumping to populate the electron subband of the wells, which are nominally undoped. In addition, inter-band magneto-optical experiments were performed on the same sample, and the transitions are fitted using a calculation of the excitonic Landau level transitions: this gives a separate check on the electron band non-parabolicity. It is found that the band non-parabolicity of the electrons for motion perpendicular to the well plane (i.e. confinement) is much smaller than the band non-parabolicity due to motion in the well plane (i.e. cyclotron motion): the latter is close to the recently deduced values for bulk GaAs. The polaron contribution to the non-parabolicity is found to be almost negligibly enhanced over the predicted three-dimensional value.

Cyclotron resonance and screening effects in GaAs-GaAlAs heterojunctions

Abstract Cyclotron resonance absorption has been studied in several high quality GaAs-GaAlAs heterojunctions. The dependence of the effective mass upon frequency, electron concentration and temperature has been measured and gives a nonparabolicity that is some 20% smaller than that measured in bulk GaAs. This is contrary to simple theory which predicts an enhanced polaron contribution in two-dimensional systems, and we attribute this difference to strong screening of the electron-phonon interaction. The presence of a small magnetic field component parallel to the interface introduces a coupling at energies corresponding to the electric subband separations. These couplings have been studied as a function of electron concentration and the results interpreted in terms of changes in the depletion charge in the GaAs layer.

Persistent photoconductivity in Ga0.49In0.51P/GaAs heterojunctions

We have studied the persistent photoconductivity (PPC) effect in Ga0.49 In0.51 P/GaAs heterostructures. Through time‐ and temperature‐dependent Hall effect, we observe very small relaxation rates and the PPC remains observable at room temperature. Optical experiments show an optical energy threshold of 1.15 eV and an infrared quenching of the PPC. Thermal cycling of the samples strongly affects the PPC and the quenching temperature. The center responsible for the observed PPC, therefore, appears related to defects. Most of our observations are qualitatively understood in a large lattice relaxation DX‐like center approach. However, the origin of the high quenching temperature remains to be explained.

Pressure dependence of light-hole transport in strained InGaAs/GaAs

A single modulation p-doped InGaAs/GaAs quantum well, 90 A wide, was grown by MBE. The hole effective mass, mh∗, at the Fermi energy was measured to be 0.155 from cyclotron resonance experiments. The valence Landau level dispersion was calculated and, in contrast to unstrained quantum wells, showed an approximately linear dependence on magnetic field, in agreement with the cyclotron resonance data. Transport measurements as a function of temperature (4–300 K) and hydrostatic pressure (4–8 kbar) show for the first time that the valence subband structure of strained structures is sensitive to pressure, reflecting the low in-plane mass at the valence band maximum.

Experimental tests of fractional quantum hall effect theory

Abstract The predicted effect of disorder on the hierarchical model of the FQHE, the extension of the FQHE to the N =1 Landau level and the Laughlin scaling diagram are examined in a range of experiments. A new method of obtaining the ground state energy gaps by analysis of the widths of fractional resistivity minima is outlined and preliminary data are presented that point to a method for the experimental determination of quasi-particle charge.

Millimeter and submillimeter detection using Ga1−xAlxAs/GaAs heterostructures

We have shown that a Ga1−xAlxAs/GaAs heterostructure can be used as a sensitive tunable detector of mm-wave/sub-mm-wave radiation. The mechanism for detection requires the application of a magnetic field varying from approximately 0.2T at 94GHz (3.2mm wavelength) to 6.2T at 2500GHz (119μm wavelength). The responsivity and N.E.P. at 3.2mm have been roughly estimated at 200V/W and 5×10−11W/✓Hz respectively. The speed of such a detector could be several orders of magnitude greater than comparable InSb detectors.