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

23 pages, 8 figures

References in corpus (7)

• Restoring the density-gradient expansion for exchange in solids and surfaces
• Generalized gradient approximation for solids and their surfaces
• Direct Bandgap Emission from Hexagonal Ge and SiGe Alloys
• Local modified Becke-Johnson exchange-correlation potential for interfaces, surfaces, and two-dimensional materials
• Efficient strain-induced light emission in lonsdaleite germanium
• An efficient direct band-gap transition in germanium by three-dimensional strain
• Giant optical oscillator strengths in perturbed hexagonal germanium