Abstract
The transition from an indirect to a direct gap semiconductor in unstrained as well as compressively and tensile strained alloys is investigated as a function of the Sn content 0 ≤ ≤ 1 by means of both a very accurate supercell approach and the more approximate virtual crystal approximation (VCA). In the local density approximation we calculate the bowing parameter of the lattice constant of unstrained alloys. Provided that pseudopotentials suitable for the VCA are used, the random supercell and VCA approaches yield consistent bowing parameters for the lattice constant of −0.21 and −0.28 Å, respectively, in the entire Sn concentration range. The band structures and energy gaps are calculated using the modified Becke-Johnson potential, which, for Ge, yields a one-electron band gap in very good agreement with experimental data. The crossover from an indirect to a direct gap semiconducting alloy is determined at about 4.5% Sn in unstrained . When is grown commensurately and thus strained on Ge(100), a transition to a direct gap is also observed but at Sn concentrations of about 10%. We finally predict the direct and indirect band gaps as a function of the in-plane lattice constant and Sn concentration for alloys grown on (100) substrates.
- Received 20 December 2013
- Revised 6 March 2014
DOI:https://doi.org/10.1103/PhysRevB.89.165201
©2014 American Physical Society