ELECTRONIC BAND-STRUCTURE OF ISOTOPICALLY PURE GERMANIUM - MODULATED TRANSMISSION AND REFLECTIVITY STUDY

The piezomodulated and photomodulated reflectivity spectra of monoisotopic Ge (Ge-70, Ge-73, Ge-74, Ge-76) and natural Ge measured at T = 6 K show clear signatures of the E0 and the E0 + DELTA0 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 are interpreted in the context of the electron-phonon interaction and the volume expansion of the crystal lattice.