D Korakakis

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.

Photoluminescence spectroscopy on annealed molecular beam epitaxy grown GaN

Photoluminescence (PL) spectroscopy has been used to investigate the effect that annealing temperature and ambient has on annealed molecular beam epitaxy grown GaN. Significant differences induced by the different annealing conditions occur in the PL spectra in the 3.424 eV region as well as the deep level band (2.0–3.0 eV). Power resolved measurements indicate that the 3.424 eV emission is a donor–acceptor pair transition. In the deep level region peaks are observed in all spectra at 2.3 and 2.6 eV. This suggests that the 2.3 and 2.6 eV peaks are related and a model is proposed to explain this luminescence.

Nano‐Fabrication of GaN Pillars Using Focused Ion Beam Etching

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.

An investigation into the origin of the 3.424 eV peak in the low-temperature photoluminescence of GaN grown by molecular beam epitaxy

GaN films grown by RF plasma source molecular beam epitaxy (MBE) were investigated by low-temperature photoluminescence (PL) and secondary ion mass spectroscopy (SIMS). A peak that is generally accepted as being the donor bound exciton was observed in all spectra. The energy of the transition varied from sample to sample, but was always in the range of 3.466-3.473 eV. In some of the samples, an additional peak was observed. The energy of the transition also varied and was in the range of 3.417-3.428 eV. An attempt was made to correlate the presence of this peak to the oxygen and silicon content of the samples using SIMS. We conclude that, contrary to some previous reports, the peak seen in the 4K PL spectra of GaN in the region of 3.424 eV cannot always be attributed to oxygen in the film. We also find no correlation between the line and the Si or C concentrations. The position of the DoX peak was also found to be independent of the Si and C concentrations.

Interpretation of the Temperature‐Dependent Transport Properties of GaN/Sapphire Films Grown by MBE and MOCVD

Electron transport in Si-doped and unintentionally-doped GaN films grown on sapphire by MOCVD and MBE has been analysed assuming that a parallel conducting channel, via an impurity band, is present. No dependence on growth method or dopant type was observed, but other trends were apparent: a) the activation energy for the impurity band fell with increased doping; b) the temperature of the minimum in the Hall carrier density versus temperature curves increased with doping, but did not depend strongly on the absolute value of mobility; c) the ratio of the mobility in the GaN conduction band to that in the impurity band also showed systematic behaviour, possibly arising from structure-related scattering processes. An STM study of the surface characteristics of some of these samples suggests that potential variations associated with particular structural features may be important in influencing the electrical properties.

Ga-metal inclusions in GaN grown on sapphire

We have shown that for GaN films grown directly on sapphire substrates using plasma-assisted molecular beam epitaxy, the application of a localised mechanical stress on the films can produce metallic droplets on the surface. By using X-ray photoelectron spectroscopy measurements we have demonstrated that the droplets consist of Ga, which can be removed by etching in HCl. This problem can be avoided using nitridation of the surface combined with a low-temperature buffer layer, or by using alternative substrates such as GaAs or Si.

RHEED Studies of Group III-Nitrides Grown by MBE

We report on the mechanisms giving rise to surface reconstruction for homo-epitaxial GaN grown by molecular beam epitaxy on (0001-) bulk GaN substrates. We have studied the surface reconstruction observed during growth and investigated the effects of supplying Ga and active nitrogen separately to the GaN surface. During growth we can observe a (2 × 2) reconstruction on (0001-) GaN under appropriate conditions. We have observed a (2 × 2) reconstruction on the GaN(0001-) substrates induced by supplying Ga to that surface. We have shown that the (2 × 2) surface reconstruction is stable in the presence of an active nitrogen flux at high temperature, but it disappears on cooling the GaN sample below ≈400 °C. Finally, we propose two possible growth models for GaN which can explain our RHEED data.