Ge vacancies atGe∕Siinterfaces: Stress-enhanced pairing distortion

Abstract

We have performed first-principles total-energy electronic-structure calculations based on the density-functional theory to clarify energetics and electron states of the Ge vacancies in strained Ge layers on the Si(001) surface. We find that pairing distortion is a principal relaxation pattern around the vacancies. The pairing of the two atoms located on either (110) or $(1\overline{1}0)$ plane is remarkably enhanced due to compressed strain in the lateral plane. It is found that the enhanced pairing causes reduction of formation energies, disappearance of deep levels in the monovacancy, deep-level crossing in the divacancy, and arrangement of the trivacancy on the (110) or the $(1\overline{1}0)$ plane. We have also found that the vacancy at the very interface layer facing the Si substrate is energetically unfavorable due to the larger energy cost to generate Si dangling bonds compared with Ge dangling bonds upon removal of atoms.