DEPENDENCE OF THE LATTICE-PARAMETERS AND THE ENERGY-GAP OF ZINCBLENDE-TYPE SEMICONDUCTORS ON ISOTOPIC MASSES

The dependence of the Eo direct gap of Ge, GaAs, and ZnSe on isotopic masses at low temperatures has been investigated. Contributions of the variation of the lattice parameter to the gap shift of the binary com pounds have been evaluated by using a volume-dependent lattice dynamic s, while local empirical pseudopotential techniques have been employed to calculate gap shifts due to electron-phonon interaction. The depen dence of these terms on the lattice-dynamical model and on the q-->O e xtrapolation of the pseudopotential form factors has been investigated . The contributions of the optical and acoustical modes to the isotopi c shift are analyzed. The results are compared to previous experimenta l data, in the case of germanium, and to low-temperature reflectance m easurements performed as part of this work on GaAs samples with differ ent isotopic gallium composition. Particular attention has been paid t o the differences in the effects of changing either the cation or the anion masses. The temperature dependence of the Eo gap of ZnSe has als o been calculated, and reasonable agreement with experiment has been f ound.