ABSTRACTWe describe the effects of rapid thermal annealing on the photoluminescence (PL) and electrical properties of heteroepitaxial ZnSe grown by molecular beam epitaxy on GaAs, using either no cap or plasma-deposited SiO2, Si3N4, or diamond-like C caps, and annealing temperatures from 500 to 800°C. Capless anneals (in contact with GaAs) produce badly degraded PL properties, while capped anneals can prevent this degradation. We show that Si3N4 is significantly more effective in preventing Zn out-diffusion through t e cap than previously employed SiO2 films, as evidenced by less pronounced PL features related to the creation of Zn vacancies during the anneal. Implant damage tends to enhance the Zn vacancy formation. Rapid thermal annealing with Si3N4 caps is shown to optically activate shallow N acceptor implants.