Non-parabolicity in the lowest conduction band of CdS

Pulse measurements on the CdS/electrolyte system are used to induce and study space-charge layers at the CdS surface. Such measurements show that the free-electron-like lowest conduction band in hexagonal CdS is highly non-parabolic. A simple model for the structure of this band, which assumes parabolicity up to 0.125 eV above the band edge and a linear dependence of the energy on the wavevector at higher energies, accounts well for the experimental results. The density of states function that emerges from these results is compatible with reported theoretical calculations. In addition, the measurements indicate that surface states are practically absent at the CdS surface in contact with the electrolyte. They also shed light on the process of charge leakage across the CdS/electrolyte interface, leakage that occurs mostly when strong accumulation layers are induced.