Defects and acceptor removal in 60Co {\gamma}-irradiated p-type silicon

TL;DR

This study investigates how gamma irradiation causes defects and reduces active boron dopants in p-type silicon detectors, affecting their performance in high-radiation environments like the HL-LHC.

Contribution

It provides a detailed defect characterization using TSC and DLTS techniques, linking microscopic defect formation to macroscopic device degradation in irradiated silicon.

Findings

DLTS identifies di-vacancy defects correlating with TSC signals.

BiOi defect concentration is about half of the change in effective carrier concentration.

Boron deactivation is mainly due to BiOi defect formation.

Abstract

Boron-doped silicon detectors used in high radiation environments like the future HL-LHC show a degradation in device performance due to the radiation induced deactivation of the active boron dopant. This effect, known as the so-called Acceptor Removal Effect (ARE), depends on particle type, particle energy and radiation dose and is usually explained by the formation of boroninterstitial - oxygen-interstitial (BiOi) defects that induce a donor-type defect level in the upper part of the Si band gap. Here we present defect characterization studies using Thermally Stimulated Current technique (TSC) and Deep Level Transient Spectroscopy (DLTS) on a set of epitaxially grown p-type silicon diodes of different resistivity, irradiated with 60Co {\gamma}-rays. We used the defect parameters (activation energy, charge carrier capture cross sections and defect concentration) obtained from DLTS experiments for modeling the corresponding TSC spectra, and subsequently compared those with the experimental TSC results. This approach shows that the di-vacancy which is well characterized by DLTS correlates with the so-far unspecified charge emission signal of the X-defect that partially overlaps with the BiOi peak in TSC spectra. Additionally, in order to evaluate the impact of BiOi defect formation on the macroscopic properties of the device, we compared the BiOi defect concentration with the change in the effective carrier concentration Neff obtained from C-V measurements. It shows that the variations in Neff are about twice the changes in the BiOi concentration, which is in perfect consistency with the assumption of boron deactivation by the formation of the BiOi donor in irradiated p-type Si.