Raman-scattering studies of silicon-implanted gallium arsenide: The role of amorphicity

Abstract

A series of Raman-scattering experiments were carried out on GaAs implanted with Si+ and with SiF+3 ions, both before and after annealing, for samples subjected to fluences spanning a wide range. The implantation-induced amorphization of the damage layer was cleanly observed via the evolution, with increasing fluence, of the broad three-band continuum of amorphous GaAs which extends from near zero up to 300 cm−1. Annealing recovers the simple line spectrum of crystalline GaAs, but with a changed longitudinal-optical/transverse-optical intensity ratio which indicates a departure from epitaxial regrowth. Three lines observed near 400 cm−1 in heavily implanted samples were identified with silicon vibrational local modes. The effect of annealing on these local-mode lines is not an intensity increase but is instead a line narrowing which reveals an annealing-induced sharpening of the distribution of local settings sampled by the substitutional silicons. In particular, a line (at 381 cm−1) assigned to the Si-at-a-Ga-site donor impurity is clearly seen before annealing, even though annealing is needed to transform the highly resistive implanted material into semiconducting n-type GaAs. We propose that the primary role of annealing in the ‘‘electrical activation’’ of implanted semiconductors is not to shift the impurity atoms into substitutional donor or acceptor sites (they already occupy such sites), but is instead to recover the high carrier mobility of the crystalline form. ‘‘Healing’’ and ‘‘activation’’ thus correspond to the same process, the elimination of amorphicity.