Raman scattering evaluation of lattice damage and electrical activity in Be-implanted GaAs

Abstract

Systematic Raman scattering experiments were carried out on [001] GaAs implanted with 70-keV Be+ ions both after implantation for samples subjected to fluences spanning the 1013–1015 cm−2 range and after successive isochronal thermal annealing steps performed between 200 and 900 °C for a selected series of Be doses. The implantation-induced damages were observed via the modifications with increasing fluence, of the first-order Raman line parameters (mainly for the LO peak) classified in terms of mode energy softening and linewidth increase. These modifications were quantitatively accounted for within the spatial coherence-length reduction formalism, which makes it possible to evaluate the mean size of unperturbed regions in as-implanted GaAs. Upon successive thermal treatments, the implanted layers were found to recover their crystalline perfection at a temperature as low as 500 °C. Above this temperature the Be electrical activation is evidenced by the sudden and drastic modification of only the ‘‘longitudinal’’ response, which is interpreted as due to the coupling between the LO phonon and an overdamped plasmon. The detailed analysis of the coupled-mode behavior is presented in the case of p-doped GaAs. From the Raman line-shape analysis, including a near-surface free-carrier depleted zone, we have obtained information about the electrical properties of the layer in terms of carrier density and mobility. It is further shown that in the case of strong damping, the ratio ω2p/Γp, where ωp and Γp are, respectively, the plasmon frequency and damping, is a pertinent quantity since it is proportional to the electrical conductivity. All the reported Raman conclusions agree well with results found in the literature that were obtained from direct electrical techniques. Possible mechanisms involved in the electrical activation of Be in GaAs are briefly discussed.