Optimization of Amorphous Germanium Electrical Contacts and Surface Coatings on High Purity Germanium Radiation Detectors

Semiconductor detector fabrication technologies developed decades ago are widely employed today to produce gamma-ray detectors from large volume, single crystals of high purity Ge (HPGe). Most all of these detectors are used exclusively for spectroscopy measurements and are of simple designs with only two impurity based electrical contacts produced with B implantation and Li diffusion. Though these technologies work well for the simple spectroscopy detectors, the Li contact in particular is thick and lacks room temperature stability in a manner that makes it inappropriate for many of the more complex detectors needed for gamma-ray imaging and particle tracking applications. Thin films of amorphous semiconductors such as sputter deposited amorphous Ge (a-Ge) are the basis for an alternative electrical contact that is easy to fabricate, thin, and can be finely segmented. The a-Ge also functions well as a passivation coating on the HPGe surfaces not covered by the electrical contacts. The properties of the a-Ge affect the performance of the resultant detectors, and these properties substantially depend on and are controllable through the sputter deposition process parameters. The subject of this paper is this interconnection of fabrication process parameters, a-Ge properties, and detector performance. The properties of a-Ge thin film electrical resistance, a-Ge contact electron injection, and room temperature storage stability were evaluated as a function of the sputter process parameters of sputter gas pressure and sputter gas H2 composition. Two different sputter deposition systems were used to produce a-Ge resistors and HPGe detectors with a-Ge electrical contacts. These samples were electrically characterized as a function of temperature. A summary of this study and discussion of the relevance of the findings to the optimization of detector performance are given in this paper.