We have studied the optical properties of GaAs partially amorphized by As+-ion implantation. The model used is based on the two-phase mixture Bruggeman effective-medium approximation in which the partially amorphized layer is assumed as a physical mixture of fully amorphous GaAs and crystalline GaAs. The dielectric functions of the crystalline components deduced from this study differ appreciably from that of the bulk crystalline GaAs, especially in the vicinity of the sharp critical point (CP) features. We find that the change in the optical properties of the crystalline component is due to the increase in the broadening parameters of the CPs caused by implantation-induced damage. The amorphous volume fraction f(a) is also found to be simply expressed as f(a) = ([As+]/A)alpha, where [As+] is the ion fluence, A (= 8.5 X 10(13) cm-2) is an amorphization-threshold fluence (i.e., the minimum fluence required to form fully amorphous layer), and alpha( = 0.79) is an amorphization-rate factor of the incident ions.
