Sub-THz thermally activated-electrical conductivity of CdS thin films

The electrical conductivity of a CdS thin film, controlled by grain structures is essential to enhance its photoconductivity to be able to be fit as a window material in CdS/CdTe heterojunction solar cell. In order to characterize a thin film, electromagnetically, we employed an open cavity resonator with a sub-millimeter VNA (Vector Network Analyzer). Our technique is capable of measuring complex dielectric permittivity, $\tilde{\epsilon}$, of a photovoltaic film as thin as 0.1 $\mu$m. We measured the real part of the complex dielectric permittivity, $\epsilon_{re}$, and electrical conductivity, $\sigma_{re}$ (derived from the imaginary part, $\epsilon_{im}$), of unannealed and annealed CdS films with thicknesses $\sim$ 0.15 $\mu$m on $\sim$ 3 mm thick-borosillicate glass substrates, at room temperature. We obtain the (thermally activated) electrical conductivity between 100 and 312 GHz, which is less in annealed samples than in unannealed one by $\sim$ 2 orders of magnitude. Contrary to our expectations, the carrier concentrations extracted from these data by fitting a Drude model, are $\sim$ 10$^{16}$ cm$^{-3}$ (unannealed) and $\sim$ 10$^{14}$ cm$^{-3}$ (annealed). We investigate the connection between grain size and carrier concentration.