  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.

Materials Science