Characterisation of surface treated CdZnTe and thin film CdTe based devices.

Semiconductor materials have a vast range of applications varying from basic electronic products to astronomy and their semiconducting properties can be altered through the growth of binary or ternary compound materials. CdZnTe and CdTe are prominent materials in radiation detector and photovoltaic solar cell applications. For radiation detectors, in the fabrication process, surface preparation (chemical polishing and passivation) and contact deposition are key to the detector performance. This thesis investigates the effect of these two processing steps on CdZnTe detectors through varying the passivation procedures and gold contact configurations. The surface composition, layer thickness and non-uniformity resulting from the passivation treatments have been investigated using X-ray Photoelectron Spectroscopy (XPS), Scanning Transmission Electron Microscopy (STEM), Energy Dispersive X-ray spectroscopy (EDX) and other materials characterisation techniques. The device electrical and spectroscopic responses were measured using the I-V characteristics and alpha spectroscopy respectively. Passivation using 30 % H2O2 and 5% NaClO treatments develops a very thin oxide layer of up to ~2 nm, while NH4F/H2O2 and KOH+NH4F/H2O2 treatments yield oxide layers of varying thickness (30 – 142 nm) and metal oxides comprising of Te2/Te3, CdO and ZnO. Devices were fabricated in metal-semiconductor (MS) and metal-insulator-semiconductor (MIS) configurations. The MIS configuration improves the mobility-lifetime product, partial charge collection and leakage current of a CdZnTe device. The MIS device barrier heights were calculated to be 0.83 ± 0.02 eV and 0.86 ± 0.02 eV for very thin and thick oxide layers respectively. For ultra-thin (0.5 µm CdTe layer in) CdTe/CdZnS solar cells, XPS and X-ray diffraction (XRD) were employed to study the effect of varying the CdCl2 processing step. Increasing the degree of CdCl2 activation and annealing treatment was found to increase sulphur diffusion into the CdTe layer (up to a concentration of ~ 2 at.%). Cell performance measurements showed that the increase in S concentration is directly related to the open-circuit voltage (Voc), and increasing the degree of CdCl2 treatment gives higher Voc values