Electronic band structure of isotopically pure germanium: Modulated transmission and reflectivity study

The piezomodulated and photomodulated reflectivity spectra of monoisotopic Ge (70Ge, Ge73, Ge74, Ge76) and natural Ge measured at T=6 K show clear signatures of the E0 and the E00 transitions; their energies increase with increasing isotopic mass. From a comparison of the phonon-assisted indirect transition energies obtained from electromodulated and wavelength-modulated transmission and photoluminescence, the isotopic mass dependence of both the indirect gap and the relevant zone-boundary phonons are deduced. The isotopic-mass dependence of the energy gaps is interpreted in the context of the electron-phonon interaction and the volume expansion of the crystal lattice.