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

TL;DR

This paper investigates how various doping processes affect the purity of high-purity germanium used in gamma ray detectors, developing a model to analyze contamination and identifying laser thermal annealing as a promising method.

Contribution

It introduces a phenomenological model for contamination during doping processes and evaluates alternative doping techniques to preserve germanium purity.

Findings

Out-of-equilibrium surface doping processes better preserve bulk purity.

Laser thermal annealing is identified as the most promising doping method.

Contamination can be effectively modeled to optimize doping procedures.

Abstract

High purity Ge (HPGe) is the key material for gamma ray detector production. Its high purity level (< 2x10^(-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 B ion implantation, P and Ga diffusion from Spin-On Doping (SOD) sources, Sb equilibrium diffusion from a remote sputtered source and laser thermal annealing (LTA) of sputtered Sb. 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, laser thermal annealing turned out to be the most promising.