Residual donor contamination in MOCVD, MOMBE and MBE GaAs studied by far-infrared spectroscopy

Abstract

Far-infrared photothermal ionisation spectroscopy is used to investigate the nature of the donors present in high-purity epitaxial films of GaAs grown by metal-organic chemical vapour deposition (MOCVD), metal-organic gas source molecular beam epitaxy (MOMBE) and solid source molecular beam epitaxy (MBE). Except for one lower mobility sample, the MOCVD samples showed the donor line X1 to be dominant with >50% relative intensity. The higher mobility samples showed the presence of two relatively closely spaced components to X1. Hydrostatic pressures up to 20 kbar were used to increase the chemical shifts and to vary the absorption strengths of the lines. By this means, and by the simultaneous observation of the same components on the 1s-2p and 1s-3p transitions, it was possible to show that these components were not artefacts of over-absorption but arose from two different donor species of very similar chemical shifts. Consequently, any identification of donor X1 as being solely due to silicon must be treated with caution. Germanium and sulphur accounted for the remaining donor contamination in the MOCVD material, with germanium usually being present at higher levels than sulphur. In contrast, earlier far-infrared and photoluminescence data with samples of different origin have shown germanium to be dominant in MOCVD material. MOMBE growth produced very similar proportions of X1, germanium and sulphur, indicating that the gas sources are responsible for the contamination. The MBE samples grown in reactors of different manufacture showed levels of sulphur donors comparable with the background carbon acceptors as well as traces of germanium, in addition to the silicon deliberately introduced. The results for the highest mobility MBE sample with deliberate silicon doping suggest that the lower energy component of the 'X1' doublet is due to silicon. The most likely candidate for the higher energy component is selenium. An even higher mobility MBE sample which was n-type without deliberate silicon doping showed the higher energy component attributed to selenium.