Electronic and optical properties of body-centered-tetragonal Si and Ge

We present a first-principles calculation of the quasiparticle and optical excitation spectra of recently predicted silicon and germanium polytypes in the body-centered-tetragonal (bct) structure. The quasiparticle spectra calculated within the $GW$ approximation predict that both silicon and germanium in the bct structure are small band gap materials with indirect gaps of 0.86 and 0.38 eV, respectively. The optical spectra is evaluated by solving the Bethe-Salpeter equation taking into account electron-hole interactions. We then make comparison to the cubic phases of Si and Ge which suggest the possible utility of the silicon bct phase in photovoltaic applications.