Transition between direct gap and indirect gap in two dimensional hydrogenated honeycomb $$\mathrm{Si}_{x}\mathrm{Ge}_{1-x}$$ alloys

Abstract

AbstractUsing fi rst-principles calculations, we have explored the structural and electronic properties of fully hydrogenated honeycomb \(\mathrm{Si}_{x}\mathrm{Ge}_{1-x}\) alloys. Finite band gaps are opened by hydrogenation for x in the whole range from 0 to 1, while their nature and values can be tuned by x. When x is ≥ 0.7, the gap turns into indirect (from Γ to M). For all the computed compositions, the two kinds of energy differences between valence band and conduction band, Γ – Γ and Γ – M, are described well by two polynomial functions of x. The smaller of the two functions gives a good prediction for the overall band gap at any x. The two curves cross at x = 0.7, leading to the change of band gap type. At PBE level, the values of band gap for different x spread from 1.09 to 2.29 eV. These fi ndings give a new route to tune the electronic properties of these materials and may have potential applications in nanoscale optoelectronics.KeywordsHydrogenated Si x Ge 1 – x alloysVirtual crystal approximationBand gap engineeringFirst-principles calculation