In this paper we present the results of two new experiments (1) Infrared Reflectivity (IR) measurements of the thickness of the modified layers following the proton implantation in n-type GaAs layers at various implant temperatures and (2) a detailed study of the thermal stability of n-type GaAs layers isolated by proton implantation. It is found that the threshold dose (minimum implant dose required for a maximum sheet resistivity) for a device is a key parameter to obtain good thermal stability. The optical properties also depend on the implantation temperature and dose.