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

The piezomodulated and photomodulated reflectivity spectra of monoisot opic 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; the ir energies increase with increasing isotopic mass. From a comparison of the phonon-assisted indirect transition energies obtained from elec tromodulated and wavelength-modulated transmission and photoluminescen ce, the isotopic mass dependence of both the indirect gap and the rele vant zone-boundary phonons are deduced. The isotopic-mass dependence o f the energy gaps are interpreted in the context of the electron-phono n interaction and the volume expansion of the crystal lattice.