Wurtzite silicon as a potential absorber in photovoltaics: Tailoring the optical absorption by applying strain

We present ab initio calculations of the electronic structure and the optical properties of wurtzite Si (Si-IV). We find an indirect band gap of 0.95 eV $({\mathrm{\ensuremath{\Gamma}}}_{5}\ensuremath{\rightarrow}{M}_{1})$ and an optically forbidden direct gap of 1.63 eV $({\mathrm{\ensuremath{\Gamma}}}_{5}\ensuremath{\rightarrow}{\mathrm{\ensuremath{\Gamma}}}_{10})$, which is due to a backfolding of the ${L}_{1}$ state of Si in the diamond structure (Si-I). Optical absorption spectra including excitonic and local-field effects are calculated. Further, the effects of hydrostatic pressure, uniaxial strain, and biaxial strain on the absorption properties are investigated. Biaxial tensile strains enhance the optical absorption of Si-IV in the spectral range which is relevant for photovoltaic applications. High biaxial tensile strains $(>4%)$ even transform Si-IV into a direct semiconductor.