Origin of Novel Diffusions of Cu and Ag in Semiconductors: The Case of CdTe

It is well known in experimental studies that Cu is usually a fast diffuser in semiconductors. In some semiconductors (e.g., CdTe), Ag is also a fast diffuser. The diffusion plays an important role in many applications when Cu (Ag) is employed to tune the semiconductor's electrical or optical properties. However, the origin of why Cu (Ag) shows different diffusion behavior compared to group-IA elements is still unclear. Using first-principles method, we compare the diffusion behaviors between Cu (Ag) and group-IA elements in CdTe, and find that the novel diffusion is due to the strong coupling between Cu (Ag) $d$ levels and unoccupied host $s$ levels. This coupling alters the stable doping site, diffusion pathway, and diffusion energy curve from those of group-IA elements, which have no active $d$ levels, thus making the Cu (Ag) diffusion faster in many semiconductors.