ISOTOPE DEPENDENCE OF THE LOWEST DIRECT ENERGY-GAP IN CRYSTALLINE GERMANIUM

Using optical absorption measurements we establish that the lowest dir ect energy gap changes with atomic mass number m at the rate dE/dm = 0 .49 +/- 0.03 meV at 2 K, decreasing to 0.11 +/- 0.03 meV at 300 K, in both isotopically pure and isotopically disordered germanium. These va lues are broadly consistent with a simple prediction from the measured temperature dependence of the gap and the known dependence of the lat tice on the isotope. We confirm, from optical absorption data, that th e lowest indirect energy gap of germanium increases at the rate dE/dm = 0.36 +/- 0.01 meV, in agreement with recent photoluminescence data.