Prediction of indirect to direct band gap transition under tensile biaxial strain in type-I guest-free silicon clathrate Si46: A first-principles approach

We suggest a bandgap engineering approach of first-principles calculations on basis of density functional theory (DFT), to monitor the bandgap transition from indirect to direct for the type-I guest-free silicon clathrate (Si46). Hence, we have systematically investigated the effect of planar biaxial strain in either compressive or tensile directions on electronic and optical properties using both GGA-PBE and hybrid GGA-BLYP functionals as implemented in Castep and Dmol3 codes respectively. For unstrained Si46, electronic structure has revealed a semiconducting behavior with quasi-direct bandgap estimated at 1.364eV (GGA-PBE) and 1.618eV (GGA-BLYP). Thus, we predict that tensile biaxial strain above +2% can trigger the indirect to direct bandgap transition, which will be located at the X(1/2,0,0) high symmetry point in the first Brillouin zone. The bandgap magnitude increases with the decreasing of biaxial strain and reaches a maximum value of 1.406eV (GGA-PBE) and 1.641eV (GGA-BLYP) at +1% tensile strain, and starts to decrease toward smaller values when the strain is increased. In addition, some relevant optical properties such as the complex dielectric function and the absorption coefficient are computed at each step of biaxial strain. Under tensile strain of +4%, results show better optical properties, the general magnitude of the absorption spectrum is increased by 17.5% with a maximum magnitude of 2.7 × 104 in the visible range of the electromagnetic spectrum.