Band parameters of CdS and CdSe single crystals determined from optical exciton spectra

An investigation is made of the energy spectra of A and B excitons in CdS by measuring the transmission and reflectivity of very thin highly perfect crystals at 1.8 K. The excited states of both series follow hydrogen-like series in a good approximation yielding different Rydberg's R0sup(A) = (28 +- 0.4) and R0sup(B) = (31.3 +- 0.4) MeV, whereas the ground states are shifted in opposite direction with respect to the series, deltaE1sup(A) = -2.2, deltaE1sup(B) = +0.5 MeV, resulting in exciton binding energies Esub(ex)sup(b,A) = 30.2 and Esub(ex)sup(b, b0 6 30.8 MeV, which are different from the corresponding Rydberg energies. This effect could not be attributed to a single perturbation acting on the ground state in different ways. Moreover, oscillator strengths are determined. A discussion is given in terms of the quasi-cubic model. For CdSe, the exciton-LO-phonon interaction is investigated using a line-shape analysis of the absorption lines at different temperatures. Analogous to other II-VI compounds, at higher temperatures (T > 77 K) this interaction is the dominate one for the exciton absorption spectra. (author)