The present disclosure provides an improved method for forming a thin semiconductor alloy layer on top of a semiconductor layer. The proposed method relies on an implantation of appropriate impurity species before performing deposition of the semiconductor alloy film. The implanted species cause the semiconductor alloy layer to be less unstable to wet and dry etches performed on the device surface after deposition. Thus, the thickness uniformity of the semiconductor alloy film may be substantially increased if the film is deposited after performing the implantation. On the other hand, some implanted impurities have been found to decrease the growth rate of the semiconductor alloy layer. Thus, by selectively implanting appropriate impurities in predetermined portions of a wafer, a single deposition step may be used in order to form a semiconductor alloy layer with a thickness which may be locally adjusted at will.