Bound donor-acceptor edge luminescence line shapes in CdS

The principal contribution to the linewidth of the bound donor-acceptor luminescence spectra is shown to be due to coupling with the 5.5 meV TO phonon, with electron-phonon strength parameter equal to that of the 38.5 meV LO phonon. By including the Coulomb pair interactions, and in addition a Lorentzian-shaped Coulomb contribution from all other charged defects, excellent agreement with the lineshapes observed in the time-resolved mode is obtained over the entire range of delay times utilized (10 micros – 100 ms) for bulk and platelet samples of various defect concentrations. This permits relating the energy of the observed luminescence line maximum to the pair Coulomb energy corresponding to that delay, resulting in an improved determination of W_o, the donor-acceptor radiative transition rate for complete donor-acceptor wavefunction overlap. The effect of a particular type of heat treatment upon W_o is also presented.