Direct bandgap silicon through strain engineering of type-VIII silicon clathrate Si₄₆: A first-principles study

In this work, we suggest a way to achieve a direct bandgap silicon clathrate by means of first-principles calculations. Effects of biaxial-strain on physical properties of type-VIII silicon clathrate Si-46 are investigated. For that purpose, biaxial strain tensors (-4% and < 010 > in-plane directions, while the out-of-plane < 001 > axis is strain-free. Under normal conditions, type-VIII Si-46 is indirect bandgap semiconductor with magnitude of about 1.152eV (GGA-PBE). For sake of comparison, the band structure was as well calculated by means of the hybrid PBE0 functional, in that case its allure and nature were not affected, whereas the bandgap's magnitude was increased to 2.62eV (Hybrid PBE0). It was observed that a tensile strain of +4% and above will induce a bandgap alteration from indirect to direct where both the conduction band minimum CBM and the valence band maximum VBM are located at the same point within the Gamma(0,0,0)-H(1/2,1/2,-1/2) symmetry segment (Delta). Optical properties revealed better spectrums for the +4% strained material with direct bandgap as the optical light absorption was increased by about 12%. These findings play in favor for this material as a candidate for future "all-Si" photonic and photovoltaic devices.