Properties of a radiation-induced charge multiplication region in epitaxial silicon diodes

TL;DR

This study investigates charge multiplication in irradiated epitaxial silicon diodes, demonstrating its potential to improve charge collection in high-radiation environments like future Super-LHC detectors.

Contribution

It provides detailed characterization of radiation-induced charge multiplication in epitaxial silicon diodes, including effects of material, thickness, annealing, and temperature, highlighting operational challenges and stability.

Findings

Charge multiplication occurs near the p+ implant after irradiation.

Charge collection efficiency is stable and proportional to deposited charge.

Micro discharges and noise are challenges but CM improves signal-to-noise ratio.

Abstract

Charge multiplication (CM) in p$^+$n epitaxial silicon pad diodes of 75, 100 and 150 $\upmu$m thickness at high voltages after proton irradiation with 1 MeV neutron equivalent fluences in the order of $10^{16}$ cm$^{-2}$ was studied as an option to overcome the strong trapping of charge carriers in the innermost tracking region of future Super-LHC detectors. Charge collection efficiency (CCE) measurements using the Transient Current Technique (TCT) with radiation of different penetration (670, 830, 1060 nm laser light and $\alpha$-particles with optional absorbers) were used to locate the CM region close to the p$^+$-implantation. The dependence of CM on material, thickness of the epitaxial layer, annealing and temperature was studied. The collected charge in the CM regime was found to be proportional to the deposited charge, uniform over the diode area and stable over a period of several days. Randomly occurring micro discharges at high voltages turned out to be the largest challenge for operation of the diodes in the CM regime. Although at high voltages an increase of the TCT baseline noise was observed, the signal-to-noise ratio was found to improve due to CM for laser light. Possible effects on the charge spectra measured with laser light due to statistical fluctuations in the CM process were not observed. In contrast, the relative width of the spectra increased in the case of $\alpha$-particles, probably due to varying charge deposited in the CM region.