Direct-band-gap absorption in germanium under pressure

The optical absorption of Ge near the direct band gap $E_0$ (${\Gamma }_{8v}^{+}$→${\Gamma }_{7c}^{\mathrm{-}}$) has been investigated at pressures up to 9 GPa with use of thin samples of 3 and 11 μm thickness. We find a linear blue shift of the gap energy (${\mathrm{dE}}_0$/dP=0.121 eV/GPa) and an indication for a small increase of the corresponding spin-orbit splitting ${\Delta }_0$ by 2.4 meV/GPa. The pressure dependence of the band gap $E_0$ predicted by recent ab initio band-structure calculations is in good agreement with the present experimental results. The strength of the absorption at the direct edge is found to increase by a factor of 3 in the pressure range from 0 to 9 GPa.