INTERVALLEY SCATTERING POTENTIALS OF GE FROM DIRECT EXCITON ABSORPTION UNDER PRESSURE

We have measured the dependence on pressure of the low-temperature (10 K) direct exciton optical absorption of Ge up to 12.3 GPa. rhe sharp exciton peak at the direct energy gap (E0) of Ge is found to broaden significantly with increasing pressure. This effect, which is attributed to intervalley scattering via electron-phonon interaction, is most pronounced for pressures above approximately 0.6 GPa, where the X valley becomes the lowest conduction-band minimum. From the pressure-induced exciton broadening we determine the GAMMA to X point intervalley deformation-potential constant D(GAMMAX)=2.2(3) eV/angstrom and an upper bound of D(GAMMAL) = 4.5 eV/A for scattering from GAMMA to the L valleys. The deformation potential D(GAMMAX) of Ge is about 50% smaller compared to isoelectronic GaAs. This difference is attributed to the fact that interatomic matrix elements between s and d states of the GAMMA and X conduction-band minima as well as the d character of the X minimum are larger in GaAs.