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

A systematic study of photoelectric properties of high resistivity CdTe (undoped and Cl, In doped) has been undertaken in the configuration, where the sample surface is illuminated in a direction parallel to the applied electric field to create experimental conditions similar to those used by measurements of detection properties of X-ray and gamma-ray detectors. A detailed investigation of photoconductivity was performed at above bandgap energy 1.55 eV in dependence of the illumination intensity and the applied voltage. It was observed, that the transport in the material is generally nonlinear 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 midgap energy level. It was found, that this level in detector-grade samples acts as a weak hole trap and has a moderate concentration ∼10¹⁴ cm^-3. Simulations of electric field in dependence of illumination intensity show the formation of a dead layer extending up to 100 μm for a flux of 10¹⁴ cm^-2 s^-1.