First-Principles Analysis of Indirect-to-Direct Band Gap Transition of Ge under Tensile Strain

Abstract

Ge is a promising material not only for its high carrier mobility but also for its unique optical property. The optical transition properties of Ge under six types of tensile strain, which was hardly investigated using a conventional semi-empirical method, have been analyzed using first-principles calculation. It has been found that the direct band gap of Ge shrinks faster than the indirect band gap under five types of tensile strain. In particular, Ge becomes a direct-transition semiconductor under 2% (001) in-plane biaxial tensile strain. Therefore, the use of Ge and strain technology is an excellent approach to the construction of Si-compatible photonic devices and to the monolithic integration of such devices with Si electronics. Furthermore, it has been predicted that direct-transition tensile-strained Ge can be constructed using only Si and Ge by a finite-element method.