Three-dimensional quantum-size effect in chemically deposited cadmium selenide films

Optical band gaps, $E_g$, up to 0.5 eV higher than in single-crystal samples, are observed for chemically deposited films of CdSe and explained in terms of a quantum-size effect, whereby the electrons are localized in individual crystallites. The increase in $E_g$ depends strongly on deposition temperature, with the greatest increase obtained at the lowest temperature. Annealing at temperatures above the deposition temperature causes a decrease in $E_g$; this decrease is stronger at higher annealing temperature. Structural studies of the as-deposited layers showed them to be composed of microcrystalline, cubic CdSe, and electron microscopy resolved them into individual crystallites of typically 40–80-Å diameter, depending on deposition temperature. This is the first example reported of a three-dimensional quantum-size effect in a film.