Characterization of transport and charge collection in high resistive CdTe by photoelectric measurements

Systematic study of photoelectric properties of high resistivity CdTe (undoped and O, In doped) in the configuration, when the sample surface is illuminated in direction parallel to the applied electric field strength to create experimental conditions similar to those used by measurements of detection properties of X- and gamma ray detectors. Detailed investigation of photoconductivity in dependence of illumination intensity and applied voltage was performed at above bandgap energy 1.55eV. It was observed, that the transport in the material is generally non-linear in respect to both the illumination intensity and the applied voltage. Drift-diffusion equations for photogenerated electron and hole transport and the Poisson equation were solved numerically using a model including one near mid gap energy level. It was found out, that this level in detector-grade samples acts as a weak hole trap and has moderate concentration ~10¹⁴ cm ^-3. Simulations of electric field in dependence of illumination intensity show formation of dead layer extending up to 100 mum for the flux 10¹⁴cm^-2