Evaporated and Recrystallized CdS Layers

Abstract

Heat treatments of evaporated CdS layers in nitrogen containing HCl and traces of oxygen, and providing a transport of CdS and copper are reported. Recrystallization of areas up to several mm² are observed. At 25°C, the treated layers show mobilities of 140 to 230 cm²/V·sec, photoconductivities of 10⁻³ to 2×10⁻¹Ω⁻¹cm⁻¹ at 750 ft‐c (2600°K white light) with light‐to‐dark‐current ratios of 10⁸–10⁹ and response time (decay) of 300 μsec to 1.2 msec at 100 ft‐c. The level distribution and capture cross section for electrons is investigated using spectral distribution, light intensity, and temperature dependence of photoconductivity, thermally stimulated current and response time analyses. Levels at 0.23, 0.43, 0.67, 1.05, and 2.05 eV are observed and the latter three attributed to Cu centers. Compared to other layers and single crystals, these layers show a density of 12 cm⁻³ of levels attributed to sulfur vacancies in the range between 0.3 and 0.65 eV and a nondetectable amount of intrinsic defects acting as quenching centers at 0.9 and 1.35 eV. This is explained by a Cu‐enhanced recrystallization in a CdS‐supplying atmosphere at temperatures (620° to 650°C) below the temperatures otherwise used for crystal growth, and thereby efficient annealing of intrinsic defects.