Phonons and fundamental gap in ZnSe: Effects of the isotopic composition

We have investigated the effects of isotopic composition on the phonon freq uencies and linewidths as well as the fundamental gap of ZnSe on a series o f samples made from the stable isotopes Zn-64, Zn-68, Se-76, and Se-80. Bes ides four nearly isotopically pure samples, we have measured natural ZnSe, which is highly isotopically disordered. In addition, we have investigated samples with monoisotopic cation or anion sublattices for which the anion o r cation sublattice, respectively, consists of an artificial mixture of iso topes. Using Raman spectroscopy, we find a TO(Gamma) linewidth of similar t o 0.4 cm(-1) for nominally isotopically pure samples at low temperatures, a s compared to 1.75(1) cm(-1) in natural ZnSe. In samples with only cation o r anion disorder, the linewidths amount to 1.16(6) cm(-1) ((ZnSe)-Zn-dis-Se -80) and 0.84(3) cm(-1) ((ZnSe)-Zn-68-Se-dis). This strong isotope-disorder -induced broadening of the TO(Gamma) phonon can be attributed to a large on e-phonon density of states in this energy range. With linear optical spectr oscopy (luminescence), we find, also at low temperature, that the fundament al gap of ZnSe increases by 214(30)mu eV/amu when increasing the Zn mass. I t increases by 216(30)mu eV/amu when increasing the Se mass. A two-harmonic -oscillator model for the temperature dependence of the gap yields values f or its mass dependence that are in qualitative agreement with this observat ion. [S0163-1829(99)09803-3].