Origin of the distinct diffusion behaviors of Cu and Ag in covalent and ionic semiconductors

Group IB elements Cu and Ag are important contact materials in semiconductor devices due to their low resistivity. However, they have shown puzzling diffusion behaviors in semiconductors: Cu diffuses much faster than Ag in covalent semiconductors like Si and GaAs, but Ag diffuses faster than Cu in more ionic II-VI semiconductors such as CdS and CdTe despite Ag has larger size than Cu. In this work, we reveal the underlying mechanisms of these different diffusion behavior by combining the first-principles calculations and group theory analysis. We identified the important roles of the crystal symmetry enforced s-d coupling as well as the Coulomb energy and strain energy in determining the diffusion behaviors of Cu and Ag in the covalent and ionic semiconductors. We show that the s-d coupling is absent in pure covalent semiconductors but increases with the ionicity of the zinc-blende semiconductors, and the coupling strength of Cu, owing to its higher d orbital energy, is much larger than Ag. In conjunction with the Coulomb interaction and strain energy, the s-d coupling is able to explain all the diffusion behaviors of Cu and Ag in covalent to ionic semiconductors. H.-X. Deng, J.-W. Luo, S.-S. Li, S.-H. Wei, Phys. Rev. Lett. 117, 165901 (2016).