Photoelectronic effects in cadmium selenide
Donor and acceptor type defect and Impurity centres in CdSe single crystals have been Investigated using photoconductivity and space-charge capacitance techniques, such as photocapacltance (PHCAF), Infrared quenching of PHCAP, PHCAP transients, deep level transient spectroscopy (DLTS) and optical DLTS. Highly resistive ( ρ> 10(^6) ῼ cm) photoconductive samples and medium resistivity (1 - 1000) n cm material for Schottky barrier formation have been prepared from the very conducting ( ~ 10(^-2) ῼ cm) as-grown crystals, either by annealing in selenium vapour or by copper doping. Samples annealed in selenium vapour had an acceptor level 0.62-0.64eV above the valence band, with a hole capture cross-section of 1010(^-14) cm(^2) which indicated that it is the main sensitising centre for photoconductivity in CdSe. There was also a well defined donor level ~ 0.12eV below the conduction band and an acceptor level ~0.22eV above the valence band. Similar measurements on CdSe crystals intentionally doped with copper revealed that the copper centre lies ~l.0eV above the valence band and has capture cross-sections of ~4.10(-12) cm(^2) and 7.10(^-18)cm(^2) for holes and electrons respectively, clearly demonstrating that copper behaves as a sensitising centre in CdSe, as it does in most of the II-VI compounds. Oxygen played an important role in controlling the electrical characteristics of the Schottky devices associated with the conversion of the surface structure of CdSe from a hexagonal to cubic phase. This phenomenon can be reproduced by mechanically polishing a hexagonal crystal. The cubic surface layers produced in this way have much higher resistivities than the underlying hexagonal base material and they give rise to photoconductlve effects. Measurements on such surfaces revealed an additional acceptor level ~ 0.38eV above the valence band. The barrier height of the CdSe-Au Schottky contact has proved to be dependent on the work function of the metal, the thickness and nature of Interfacial layers, and the amount of charge stored in the Interface states.