Nature of the dead layer in CdS and its effect on exciton reflectance spectra

Experimental spectra of exciton reflectance in CdS at {~} 2^∘{}K are measured for different conditions of the surface. The results are explained in terms of optical interference across a "dead layer" which can be of either intrinsic or extrinsic origin. In the former case, a one-to-one correspondence between the dead-layer depth and the orbital size of each hydrogenic line is established. The Hopfield and Thomas additional boundary conditions with spatial dispersion are shown to be quite adequate at explaining the observed features. The general significance of the surface interference effect in the reflectance behavior of Wannier excitons is demonstrated by consideration of results known for other semiconductors.