Helium incorporation induced direct-gap silicides

Abstract

The search of direct-gap Si-based semiconductors is of great interest due to the potential application in many technologically relevant fields. This work examines the incorporation of He as a possible route to form a direct band gap in Si. Structure predictions and first-principles calculations show that He and Si, at high pressure, form four dynamically stable phases of Si₂He (oP36-Si₂He, tP9-Si₂He, mC18-Si₂He, and mC12-Si₂He). All phases adopt host–guest structures consisting of a channel-like Si host framework filled with He guest atoms. The Si frameworks in oP36-Si₂He, tP9-Si₂He, and mC12-Si₂He could be retained to ambient pressure after removal of He, forming three pure Si allotropes. Among them, oP36-Si₂He and mC12-Si₂He exhibit direct band gaps of 1.24 and 1.34 eV, respectively, close to the optimal value (~1.3 eV) for solar cell applications. Analysis shows that mC12-Si₂He with an electric dipole transition allowed band gap possesses higher absorption capacity than cubic diamond Si, which makes it to be a promising candidate material for thin-film solar cell.