Far infrared studies of radiation damaged and annealed GaAs and GaP

Abstract

Far infrared absorption spectra (20-700cm-1) have been recorded using either a modified Grubb-Parsons/NPL cube interferometer with a maximum resolution of 0.1cm-1, or a Bruker 113V FTIR interferometer with a maximum resolution of 0.03cm-1, with samples mounted on a wheel inside a liquid helium cryostat close to the bolometer. Nominally pure GaAs and GaP samples and GaAs containing Al, B, C and Si impurities, were studied. The GaAs samples had either been irradiated with fast neutrons or 2MeV electrons with a range of doses varying from the minimum required to produce detectable changes in the infrared spectrum , up to a maximum corresponding to saturation of the damage produced. The only GaP sample to be studied was nominally pure and was irradiated with a dose of 2MeV electrons. All the samples were subsequently annealed in 30°C stages up to about 550°C at which temperature the recovery from the effects of theradiation damage was essentially complete and in many cases free carrier absorption had returned.The damage produced in the samples caused a relaxation of the selection rules, which normally forbid certain absorption processes, so that pronounced structure wasdetected which corresponded to features in the one-phonon density of states of the pure crystal. On annealing thesamples, the damage and hence this structure, was progressively removed. In addition, in GaAs irradiated with fast neutrons, new lines at 45cm 119cm-1 and328cm-1 were observed which first grew in strength withincreasing anneal temperature and then disappeared at higher temperatures. These lines were present in all samples given a sufficiently large dose of neutrons and hence were attributed to in-band resonances arising from intrinsic defect centres. However, in GaAs irradiated with 2MeV electrons only the feature at 45cm-l was observed, showing that the features at 119cm-l and 328cm-1 are produced only by neutron irradiation. In electron irradiated and annealed GaP a resonant mode at 149cm-1 was observed together with four gap mode lines which were all attributed to intrinsic radiation damage centres. In order to identify the defect centres responsible, these results were compared with a) the observations of local modes in doped samples which had undergone similar treatment and for which site switching of impurity atoms was observed during the annealing process and b) EPR data for similar annealing studies on the Asga antisite defect in irradiated GaAs.High resolution studies of neutron irradiated GaAs doped with carbon showed that the previously reported C(l) lines at 577.5cm-1 and 606cm-l show structure consistent with the centre responsible for these lines being Cas bonded to an interstitial As atom, surrounded by four Ga atoms.