J.J. Harris

Oscillations in the surface structure of Sn-doped GaAs during growth by MBE

Abstract The presence of predeposited or surface-accumulated Sn during MBE growth modifies the reconstruction of the (001)GaAs surface, as observed by RHEED, at coverages as low as 0.025 monolayer. If growth is initiated on such a surface, oscillation in the intensity modulation of some of the RHEED streaks occurs, with a period equal to the monolayer deposition time of the GaAs. This oscillation decays away at a rate determined by the substrate temperature and Ga flux.

Kinetic limitations to surface segregation during MBE growth of III–V compounds: Sn in GaAs

The incorporation of Sn as a dopant in GaAs has been studied in the temperature range of 500°–650° C, over a wide range of Ga and As fluxes, the latter being incident as either As4 or As2 molecules. The results are explained in terms of a surface segregation model in which the behaviour at high growth temperatures (above ∼600 °C) approaches thermal equilibrium, but growth at lower temperatures involves a kinetic limitation to the segregation process.

Silicon doping of MBE-grown GaAs films

Two concentration ranges of silicon doping in MBE-grown GaAs films have been investigated in some detail. In lightly doped films, with a free-electron concentration of ≈1016cm−3, low-temperature photoluminescence spectra have been analysed to develop a model to account for spectral features previously attributed to Ge and Si acceptor levels.In heavily doped films, a maximum free-electron concentration of ≈7×1018 cm−3 has been obtained, which is only rather weakly dependent on growth conditions and the nature of the arsenic species (As2 or As4). Transmission electron microscopy has shown that no significant precipitation effects occur when higher Si fluxes are used but there is evidence for autocompensation. The maximum PL intensity (300 K) is found at a lower free electron concentration then with Sn-doped films, and is more sharply peaked, but there is no evidence for an anomalous Moss-Burstein shift.

The Aharonov-Bohm effect in electrostatically defined heterojunction rings

Micrometer-sized loops of two-dimensional electron gas have been made on GaAs-AlGaAs heterostructures by electrostatic confinement. A split gate is used to define the loop, allowing the width of the conducting channels to be varied by changing the gate voltage. The magnetoresistance has been measured at low temperatures (T<100 mK) and shows strong Aharonov-Bohm oscillations with amplitudes of up to 7% of the total resistance in the narrowest devices. The oscillations are strong out to B approximately=0.5 T and then die out as B increases to approximately=1 T, with a possible dependence on the channel width. Magnetic depopulation of the ID-sub-bands is also seen.

Scattering mechanisms in (Al, Ga)As/GaAs 2DEG structures

Abstract We have prepared a large number of high mobility two-dimensional electron gas (2DEG) structures, with undoped spacer thicknesses ranging from 9 to 3200A. For samples with 400A of (Al, Ga)As Si-doped at 1.3×10 18 cm −3 , there is a peak in the 4K mobility at spacers of 400–800A, with a maximum value of 2×10 6 cm 2 V −1 s −1 . Increasing the thickness of the doped (Al, Ga)As to 500A produced an increase in mobility to 3×10 6 cm 2 V −1 s −1 for a 400A space sample. We have compared these results with published analyses of scattering processes in 2DEG structures, and conclude that a combination of ionised impurity and acoustic phonon scattering gives a qualitative explanation of the behaviour, but that the experimental mobility values are generally higher than those predicted theoretically.

The morphology and electrical properties of heteroepitaxial InAs prepared by MBE

An investigation of the growth of heteroepitaxial InAs by MBE is reported. The surface morphology and electrical properties are shown to be critically dependent on growth parameters and the conditions necessary to obtain good material quality are deduced. Analysis of the thickness dependence of the electrical properties of undoped, and Si- or Te-doped InAs shows that interfacial effects contribute to the measured properties. Material remote from the interfacial region compares favourably with VPE- and bulkgrown InAs.

Odd and even fractionally quantized states in GaAs-GaAlAs heterojunctions

Fractional quantization at values ν = pq has been observed in two ultra high mobility heterojunctions. For the N = 0 Landau level the set of fractions corresponding to q = 3, 5 and 7 has been observed for both spin states, while for the N = 1 Landau level even fractions are observed at 14, 12 and 34. Both the temperature and electron concentration dependence of the resistivity minima at fractional occupancies have been studied.

Conductivity of a two-dimensional electron gas in a Si/SiGe heterostructure near the metal-insulator transition: Role of the short- and long-range scattering potential

We report the observation of a metal-insulator transition (MIT) in a two-dimensional electron gas (2DEG) in a Si/SiGe heterostructure at zero magnetic field. On going through the MIT we observe the corresponding evolution of the magnetic field induced transition between the insulating phase and the quantum Hall (QH) liquid state in the QH regime. Similar to the previous reports for a GaAs sample, we find that the critical magnetic field needed to produce the transition becomes zero at the critical electron density corresponding to the zero field MIT. The temperature dependence of the conductivity in a metalliclike state at zero field is compared with the theory of the interaction corrections at intermediate and ballistic regimes

Cyclotron resonance and screening effects in GaAs-GaAlAs heterojunctions

{k}_{B}T\ensuremath{\tau}/\ensuremath{\Elzxh}g~1.

The quantized Hall effect in pulsed magnetic fields

The theory yields a good fit for the linear part of the curve. However, the slope of that part of