Transition between direct gap and indirect gap in two dimensional hydrogenated honeycomb Si _x Ge_1−x alloys

Abstract

Using first-principles calculations, we have explored the structural and electronic properties of fully hydrogenated honeycomb Si _x Ge_1−x H 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 band gap is direct (from Γ to Γ). And 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 findings give a new route to tune the electronic properties of these materials and may have potential applications in nanoscale optoelectronics.