The role of cation vacancy in compensation of II-VI compounds by fast diffusors - Example of Cu in CdS

Defects responsible for compensation of CdS doped with fast diffusers were studied using perturbed angular correlation spectroscopy (PAC). Cu was evaporated on CdS containing In-111 probe atoms, and annealed at room temperature (RT). This resulted in gradual increase of pairing between substitutional In-111 atoms and native cadmium vacancies, V-Cd, up to 100% in cca 7 days period. The model is presented which explains these results as well as electrical compensation of CdS by Cu: The RT in-diffusion of Cu donors into CdS provokes simultaneous in-diffusion of cadmium vacancies, which are double accepters. Hence Cu compensates CdS crystal indirectly, by bringing into the crystal new V-Cd, that pair with existing donors causing their electrical compensation. Model explains consistently experimental results on Cu doped CdS and also reconciles two widely accepted but apparently contradictory assumptions: one, that Cu in CdS is a fast diffuser which moves as an interstitial, and hence is a donor, and the other, that Cu compensates donors in CdS and hence effectively acts as an acceptor.