Measurement and control of ion-doping-induced defects in cadmium telluride films

Homoepitaxial p-type CdTe films were grown by coevaporation of CdTe and phosphorus in vacuum, where the phosphorus vapor was ionized and accelerated toward the substrate. Hole densities up to 2×10¹⁷ cm ^-3 were obtained using an ion energy of 60 eV. Effects of residual ion damage were observed using cross-sectional transmission electron microscopy, etch-pit density, and minority-carrier diffusion length measurements. This ion damage is dependent on both the ion dose and the ion energy. Reducing the ion energy below 60 eV results in lower doping densities, but using electron irradiation and Cd overpressure during deposition makes it possible to achieve equivalent doping levels for 20 eV ions while reducing the ion damage. At an ion energy of 20 eV, using electron irradiation of the growing film, and a 0.2% overpressure of Cd, films with hole density of 1×10¹⁷ cm ^-3 and diffusion length of 0.35 μm were obtained. Photovoltaic behavior of the films deposited in different conditions was tested by fabricating n-CdS/p-CdTe heterojunctions.