Observation of Charge Enhancement in forward-biased neutron-irradiated 4H-SiC PiN Detectors in UV-TCT Measurements

TL;DR

This study investigates the charge collection behavior of irradiated 4H-SiC detectors under UV-TCT, revealing charge enhancement in forward bias related to injection profiles and laser parameters, with implications for detector characterization.

Contribution

It demonstrates the correlation between charge enhancement and injection conditions in irradiated 4H-SiC detectors, highlighting effects not previously fully understood.

Findings

Excessive charge collection efficiency (CCE) can surpass 100% in forward bias.

CCE depends on laser focus and injection rate, varying with irradiation fluence.

In reverse bias, CCE is independent of laser spot size.

Abstract

Due to the increased commercial availability, wide-bandgap semiconductors and their radiation hardness have recently received increased interest from the particle physics community. 4H-Silicon Carbide (SiC), especially, is an attractive candidate for future radiation-hard detectors which do not require cooling. This paper investigates the radiation hardness of 4H-SiC p-in-n detectors irradiated up to $5\times 10^{15}$ $\text{n}_{\text{eq.}}/\text{cm}^2$ using UV-TCT. The samples have been operated in reverse and forward bias, which is possible due to the heavily decreased forward current after irradiation. Previous studies have already hinted at an excessive charge collection in forward bias, even exceeding a charge collection efficiency (CCE) of 100%. In this work, the excessive CCE in forward bias was shown to correlate heavily with the spatial profile of injected charge. For a sufficiently focused laser, the CCE starts to increase at high forward bias and even surpasses 100% instead of saturating as it does for a defocused laser beam. In reverse bias, the CCE was found to be independent of the beam spot size. For samples irradiated to high fluences ($\geq 1\times 10^{15}$ $\text{n}_{\text{eq.}}/\text{cm}^2$) the excessive CCE in forward bias is smaller and negligible at the highest fluences. Additionally, the CCE was observed to correlate to the rate of charge injection (laser pulses per second), with a logarithmic increase of the collected charge if a threshold of injected carrier density is exceeded. The mechanism of these effects is still an ongoing topic of study, however, the observations already pose implications for the accurate experimental characterization of irradiated SiC detectors.