Characterization and modeling of thermally-induced doping contaminants in high-purity germanium

High purity germanium (HPGe) is the key material for gamma ray detectors production. Its high purity level (≤2 10−4 ppb of doping impurity) has to be preserved in the bulk during the processes needed to form the detector junctions. With the goal of improving the device performance and expanding the application fields, in this paper many alternative doping processes are evaluated, in order to verify their effect on the purity of the material. In more detail, we investigated the electrical activation of contaminating doping defects or impurities inside the bulk HPGe, induced by both conventional and non-conventional surface doping processes, such as boron ion implantation, phosphorus and gallium diffusion from spin-on doping sources, antimony equilibrium diffusion from a remote sputtered source and laser thermal annealing (LTA) of sputtered antimony. Doping defects, thermally-activated during high temperature annealing, were characterized through electrical measurements. A phenomenological model describing the contamination process was developed and used to analyze the diffusion parameters and possible process thermal windows. It resulted that out-of-equilibrium doping processes confined to the HPGe surface have higher possibilities to be successfully employed for the formation of thin contacts, maintaining the pristine purity of the bulk material. Among them, LTA turned out to be the most promising.