STRAIN-INDUCED SPLITTING OF THE HEAVY-HOLE-EXCITON AND LIGHT-HOLE-EXCITON ENERGIES IN NAI THIN-FILMS

Optical absorption spectra and x-ray-diffraction patterns of NaI thin films grown by vacuum evaporation onto alkali-halide substrates (NaCl, KCl, RbCl, and KBr) have been studied. The (Gamma(8)(-),Gamma(6)(+))- exciton energy in the films depends strongly on the materials of subst rates and on the values of lattice mismatches between the NaI films an d substrates. The NaI films in NaI/KBr and NaI/RbCl with small positiv e lattice mismatches (+1.9% and +1.7%) show splittings of the exciton- absorption peak, which are caused by the biaxial strains arising from the heteroepitaxial growth: The strains remove the degeneracy of the h eavy-hole and light-hole energies at the Gamma(8)(-) state. The critic al thicknesses of the NaI films, at which the strains begin to relax, are estimated to be 50 Angstrom for NaI/KCl and 100 Angstrom for NaI/K Br and NaI/RbCl. We discuss these results in terms of the concept of t he epitaxial growth of the films. [S0163-1829(98)10503-9].