Cd-vacancy and Cd-interstitial complexes in Si and Ge

TL;DR

This study uses advanced ab initio methods to analyze how cadmium impurities interact with vacancies and interstitials in silicon and germanium, revealing stable split-vacancy complexes and providing insights into measured electric field gradients.

Contribution

It demonstrates that Cd-vacancy complexes in Si and Ge are unstable as substitutional impurities and instead form split-vacancy complexes, clarifying their local geometries and EFGs.

Findings

Cd-vacancy complexes relax to split-vacancy structures

Small EFGs explain measured values in Ge and Si

Cd-selfinterstitial complexes form symmetrical split configurations

Abstract

The electrical field gradient (EFG), measured e.g. in perturbed angular correlation (PAC) experiments, gives particularly useful information about the interaction of probe atoms like 111In / 111Cd with other defects. The interpretation of the EFG is, however, a difficult task. This paper aims at understanding the interaction of Cd impurities with vacancies and interstitials in Si and Ge, which represents a controversial issue. We apply two complementary ab initio methods in the framework of density functional theory (DFT), (i) the all electron Korringa-Kohn-Rostoker (KKR) Greenfunction method and (ii) the Pseudopotential-Plane-Wave (PPW) method, to search for the correct local geometry. Surprisingly we find that both in Si and Ge the substitutional Cd-vacancy complex is unstable and relaxes to a split-vacancy complex with the Cd on the bond-center site. This complex has a very small EFG, allowing a unique assignment of the small measured EFGs of 54MHz in Ge and 28MHz in Si. Also, for the Cd-selfinterstitial complex we obtain a highly symmetrical split configuration with large EFGs, being in reasonable agreement with experiments.