Effect of Group-V Impurities on the Electronic Properties of Germanium Detectors: An Insight from First-Principles Calculations

TL;DR

This study uses first-principles calculations to analyze how group-V impurities and vacancies in germanium affect charge trapping, which impacts the performance of high-purity germanium detectors in particle detection applications.

Contribution

It provides new insights into the formation and electronic effects of group-V impurities and vacancy complexes in germanium, highlighting their role in charge trapping.

Findings

P, As, Sb impurities form shallow traps near the conduction band.

N impurities create deep trap states, unlike P, As, Sb.

Vacancy-impurity complexes contribute to charge trapping.

Abstract

The outstanding properties of high-purity germanium (HPGe) detectors, such as excellent energy resolution, high energy sensitivity, and a low background-to-signal ratio, make them essential and ideal candidates for detecting particle signatures in nuclear processes such as neutrino-less double beta decay. However, the presence of defects and impurities in HPGe crystals can lead to charge trapping, which affects carrier mobility and results in significant energy resolution degradation. In this work, we employ density functional theory with a hybrid functional to study the energetics of possible point defects in Ge. Our findings indicate that n-type group-V impurities, such as phosphorus (P), arsenic (As), and antimony (Sb), form more readily in Ge compared to nitrogen (N), Ge vacancies, and Ge interstitials. Unlike N dopants, which yield deep trap states, P, As, and Sb create shallow traps close to the conduction band edge of Ge. Furthermore, we predict that n-type defects can condense into defect complexes with Ge vacancies. These vacancy-impurity complexes form deep traps in Ge, similar to Ge vacancies, suggesting that both vacancies and vacancy-impurity complexes contribute to charge trapping in these detectors, thereby diminishing their performance.