Electronic Raman spectra of shallow acceptors in semi-insulating GaAs

Abstract

We report the first well-resolved, sharp-line electronic Raman (ER) spectra of shallow acceptors in GaAs. The material-dependent circumstances of these observations are particularly interesting. The spectra, taken with cw, neodymium-doped yttrium-aluminum-garnet (Nd:YAG) laser radiation at \ensuremath{\sim}15 K, were observed only in undoped, semi-insulating bulk GaAs, grown by the liquid-encapsulated Czochralski technique. The residual, shallow acceptors present in such material are completely compensated by the double donor, mid-band-gap defects EL2. The latter, in their metastable form, provide the essential means whereby a nonequilibrium population of holes can be generated and maintained on the acceptors. This situation is exploited to obtain a detailed analysis of the ER spectra of the shallow acceptors in this technologically important material, including the identification of the residual acceptors (C and Zn), the estimates of their concentration (\ensuremath{\ll}${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$), the testing of their adherence to the parity selection rule, and their response to internal strains and electric fields. A comparison of ER spectra is made with p-type, semiconducting bulk GaAs, where only broad, unresolved spectra are obtained, presumably because of much greater acceptor concentrations (${>10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$).