Hybrid density functional theory simulation of sodium impurity and impurity–vacancy defect complexes in germanium: perspectives of defect engineering for activation of shallow donors

Abstract

At present, germanium (Ge) as a high-mobility material is considered as a possible replacement for Si in microelectronics. The main obstacle in the method is the large vacancy concentration obtained after the implantation of shallow donors. This prevents the formation of n+ regions and has a negative impact on device performance. One way to eliminate the detrimental effects of such defects is through the codoping approach. In this study, the behavior of Na impurities in germanium together with the formation of defect complexes with vacancies, divacancies and shallow donors (P and As) were investigated using hybrid density functional theory calculations. The site preference, diffusion barriers, charge states and binding energies of various complexes, as well as the activation of donor–vacancy complexes with both F and Na codoping were investigated. For this purpose, two extreme cases were considered. Firstly, we calculate the concentrations of substitutional and interstitial Na together with monovacancies in the case of quasi-equilibrium conditions realized for doping from melt. Another extreme case is Na codoping during ion implantation. Our calculations show that Na passivates vacancies, divacancies and donor–vacancy complexes (E-centers) with a large energy gain, which prevents rapid donor diffusion similar to the case of doping with fluorine. We have shown that the passivation of E-centers with both F and Na impurities leads to the back transformation of such complexes into shallow donors. This means that codoping with Na may neutralize the harmful effects of vacancies and is a perspective for defect engineering.