Band lineup at hexagonal Si$_x$Ge$_{1-x}$/Si$_y$Ge$_{1-y}$ alloy interfaces

TL;DR

This study investigates the electronic band profiles at hexagonal SiGe alloy heterojunctions using advanced computational methods, revealing how strain and composition influence band alignment and heterojunction type for optoelectronic applications.

Contribution

It provides a comprehensive analysis of band offsets and the effects of strain in hexagonal SiGe alloys, offering insights for designing better heterostructures.

Findings

Ge-rich alloys exhibit type-I direct band gaps

Si-rich alloys are type-I with indirect band gaps

Strain reduces the type-I character at interfaces

Abstract

The natural and true band profiles at heterojunctions formed by hexagonal Si$_x$Ge$_{1-x}$ alloys are investigated by a variety of methods: density functional theory for atomic geometries, approximate quasiparticle treatments for electronic structures, different band edge alignment procedures, and construction of various hexagonal unit cells to model alloys and heterojunctions. We demonstrate that the natural band offsets are rather unaffected by the choice to align the vacuum level or the branch point energy, as well as by the use of a hybrid or the Tran-Blaha functional. At interfaces between Ge-rich alloys we observe a type-I heterocharacter with direct band gaps, while Si-rich junctions are type-I but with an indirect band gap. The true band lineups at pseudomorphically grown heterostructures are strongly influenced by the generated biaxial strain of opposite sign in the two adjacent alloys. Our calculations show that the type-I character of the interface is reduced by strain. To prepare alloy heterojunctions suitable for active optoelectronic applications, we discuss how to decrease the compressive biaxial strain at Ge-rich alloys.