Low-temperature photoluminescence study of thin epitaxial GaAs films on Ge substrates

Abstract

Thin epitaxial GaAs films, with thickness varying from 140to1000nm and different Si doping levels, were grown at 650°C by organometallic vapor phase epitaxy on Ge substrates and analyzed by low-temperature photoluminescence (PL) spectroscopy. All spectra of thin GaAs on Ge show two different structures, one narrow band-to-band (B2B) structure at an energy of ∼1.5eV and a broad inner-band-gap (IB) structure at an energy of ∼1.1eV. Small strain in the thin GaAs films causes the B2B structure to be separated into a light-hole and a heavy-hole peak. At 2.5K the good structural quality of the thin GaAs films on Ge can be observed from the narrow excitonic peaks. Peak widths of less than 1meV are measured. GaAs films with thickness smaller than 200nm show B2B PL spectra with characteristics of an n-type doping level of approximately 1018at.∕cm3. This is caused by heavy Ge diffusion from the substrate into the GaAs at the heterointerface between the two materials. The IB structure observed in all films consists of two Gaussian peaks with energies of 1.04 and 1.17eV. These deep trapping states arise from Ge-based complexes formed within the GaAs at the Ge–GaAs heterointerface, due to strong diffusion of Ge atoms into the GaAs. Because of similarities with Si-based complexes, the peak at 1.04eV was identified to be due to a GeGa–GeAs complex, whereas the peak at 1.17eV was attributed to the GeGa-VGa complex. The intensity of the IB structure decreases strongly as the GaAs film thickness is increased. PL intensity of undoped GaAs films containing antiphase domains (APDs) is four orders of magnitude lower than for similar films without APDs. This reduction in intensity is due to the electrically active Ga–Ga and As–As bonds at the boundaries between the different APDs. When the Si doping level is increased, the PL intensity of the APD-containing films is increased again as well. A film containing APDs with a Si doping level of ∼1018at.∕cm3 has only a factor 10 reduced intensity. We tentatively explain this observation by Si or Ge clustering at antiphase boundaries, which eliminates the effects of the Ga–Ga and As–As bonds. This assumption is confirmed by the fact that, at 77K, the ratio between the intensity of the IB peak at 1.17eV to the intensity of the peak at 1.04eV is smaller than 1.4 for all films containing APDs, whereas it is larger than 1.4 for all films without APDs. This shows stronger clustering of Si or Ge in the material with APDs. For future electronic applications, Ge diffusion into the GaAs will have to be reduced. PL analysis will be a rapid tool for studying the Ge diffusion into the GaAs thin films.