C.T. Foxon

Photoluminescence of MBE grown wurtzite Be-doped GaN

We report new lines in the photoluminescence (PL) spectrum of lightly Be-doped GaN. The low-temperature PL spectrum of the lightly doped sample is dominated by a transition at 3.385 eV with first and second LO phonon replicas. Power-resolved PL measurements showed that the peak at 3.385 eV narrowed in width and shifted to higher energies with increasing excitation intensity. Thus the transition is attributed to donor-to-acceptor recombination, involving a Be acceptor of optical ionization energy of between 90 and 100 meV. This is much shallower than the acceptor level of 250 meV induced by Mg doping. Increasing the doping, however, resulted in a quenching of the band-edge luminescence and the appearance of a broad transition centred around 2.4 eV which we assign to a complex involving Be. Undulations on the peak were consistent with interference effects. On increasing the doping level even further all luminescence was quenched.

Investigation of Au Schottky contacts on GaN grown by molecular beam epitaxy

Gold Schottky diodes have been fabricated, for the first time, on n-type GaN grown by molecular beam epitaxy. These diodes have been studied by I - V, C - V and deep-level transient spectroscopy. The conduction process inside the diodes is shown to be by thermionic field emission. The barrier height of 1.1 eV compares well with Au Schottky diodes manufactured from n-GaN grown by metalorganic chemical vapour deposition. However, there is evidence to suggest that the grown-in deep levels are different for the two growth techniques.

High-temperature processing of GaN: The influence of the annealing ambient on strain in GaN

Using micro-Raman scattering we have investigated the influence of the annealing ambient on the high-temperature processing of GaN. Compressive strain is found in GaN layers after high-temperature processing in oxygen-containing atmospheres. This strain is significantly enhanced by the addition of water vapor to the annealing ambient, suggesting the enhanced inclusion of oxygen into GaN. Characteristic photoluminescence lines appear at 3.355 and 3.406 eV after annealing in oxygen in the presence of water vapor. No strain is introduced by high-temperature processing in nitrogen ambient, even at temperatures close to the thermal decomposition temperature and in the presence of water vapor.

FRACTIONAL QUANTUM HALL EFFECT MEASUREMENTS AT ZERO G FACTOR

Fractional quantum Hall effect energy gaps have been measured as a function of Zeeman energy. The gap at ν = 1/3 decreases as the g factor is reduced by hydrostatic pressure. This behavior is similar to that at ν = 1 and shows that the excitations are spinlike. At small Zeeman energy, the excitation is consistent with the reversal of 3 spins and may be interpreted as a small composite Skyrmion. At 20 kbar, where g has changed sign, the 1/3 gap appears to increase again.

Microstructural characterisation of GaN(As) films grown on (001) GaP by molecular beam epitaxy

Abstract Samples of GaN(As) (001) GaP grown by molecular beam epitaxy and studied using transmission electron microscopy, have been found to exhibit increasing amounts of zincblende GaN within the deposited films with increasing As flux and independent of the N flux used. All of the samples grown at 620°C contained phases of GaAs, wurtzite GaN and zincblende GaN, while there was no evidence for the formation of the ternary alloy Ga(As,N). Zincblende GaN was found to form immediately in the presence of an As flux, while wurtzite GaN formed in the absence of As. A sample grown at 700°C under the highest As flux exhibited single crystal zincblende GaN. Differential cross-diffusion of both N and As with P was observed, and was associated with the formation of V-shaped pits in the substrate.

Phonon absorption at the magnetoroton minimum in the fractional quantum Hall effect.

We have made the first phonon absorption measurements in the fractional quantum Hall regime. Experiments have been conducted on two samples which have similar electron densities but greatly differing mobilities. The energy gaps as measured by activation studies of the longitudinal resistance differ by a factor of two. Phonon absorption measurements give almost identical values for the energy gap demonstrating that the gap measured in this way is rather insensitive to disorder. The value of this gap is in agreement with the activation gap measured in the high mobility sample. Values obtained at

Mechanisms of magnesium incorporation into GaN layers grown by molecular beam epitaxy

nu=2/3

Nano‐Fabrication of GaN Pillars Using Focused Ion Beam Etching

are in good agreement with theory.

The quantized Hall effect in pulsed magnetic fields

Abstract We report on an investigation by Auger Electron Spectroscopy (AES) of Mg-doped GaN films grown by Molecular Beam Epitaxy (MBE). This study shows that there is a finite concentration gradient in a region close to the surface (10–80xa0nm) in which the Mg concentration decreases rapidly from a few percent of a monolayer to a bulk value of about (2–3)×10 19 xa0cm −3 for heavily doped samples. Such behaviour can be understood in terms of a kinetically limited migration of the dopant to the surface during the growth.

MBE growth and characterization of magnesium-doped gallium nitride

Focused ion beam etching was used for the nano-fabrication of GaN. GaN pillars as small as 20 to 30 nm in diameter were fabricated. Using secondary electron imaging, the quality of the GaN pillars was investigated as function of the gallium ion beam current used for the etching. Gallium ion beam currents as low as 1 to 4 pA are needed to fabricate nanometer-size GaN structures.