Fabrication and characterisation of gold contacts on CdZnTe radiation detectors.

Spectroscopic X-ray imaging is an emerging technology with applications in the fields of medical imaging, security, science and industrial analysis. The wide bandgap semiconductor cadmium zinc telluride (CdZnTe) is the leading sensor material for this technology. Refinements in the growth process have delivered consistent improvements in both yield and quality of single crystal CdZnTe. The fabrication of metal contacts required for electronic readout is now considered to be the limiting factor for small pixel CdZnTe detector performance. This includes preparation of the CdZnTe surface, deposition of the metal contacts and pixellation. The current work is concerned with understanding and improving the detector fabrication process. A range of complementary characterisation techniques have been used to analyse the metal-semiconductor interface formed by electroless and sputter deposition of gold contacts onto CdZnTe. The characterisation included focused ion beam (FIB) cross section imaging, chemical analysis with X-ray photoelectron spectroscopy (XPS) and electronic analysis with current-voltage (IV) measurements. The electroless deposition was found to produce a complicated interface consisting of a surface gold layer on top of a mixed interface of gold, tellurium oxide and cadmium chloride. The effective barrier height of this contact was measured to be 0.78 ± 0.04 eV under positive bias and 0.83 ± 0.02 eV under negative bias. The interface of sputter deposited gold contacts was simple in comparison, with a sharp interface between the gold and CdZnTe and a barrier height of 0.64 ± 0.02 eV under positive bias and 0.78 ± 0.02 eV under negative bias. An optimised detector fabrication process has been developed. This process involves forming gold contacts on mechanically polished-only CdZnTe by electroless deposition. Pixellation is achieved with a positive photoresist prior to gold deposition. A small pixel (250 μm pitch) CdZnTe detector fabricated in this manner and bonded to the HEXITEC ASIC has produced a single pixel FWHM energy resolution at 59.54 keV of 560 eV, with a modal FWHM resolution of 1.5 keV across the full 74 × 74 pixel array