In-situ Radiation Damage Study of Silicon Carbide Detectors Subjected to Clinical Proton Beams

TL;DR

This work addresses radiation damage in 4H-SiC detectors exposed to clinical proton beams, focusing on how irradiation reduces free carriers via donor removal. The authors perform two 8-hour proton-irradiation campaigns at MedAustron using SiC PiN diodes from CNM and onsemi, with in-situ and ex-situ IV and CV measurements enabling tracking of the evolving effective doping $N_{ ext{eff}}$. Donor removal rates are quantified as $N_{ ext{eff}}( abla) = N_0 - g abla$, yielding $g$ values in the range $4.2$ to $6.4$ cm$^{-1}$, and extrapolations suggest full compensation near $8.8 imes 10^{13}$ p+/cm$^2$ for certain samples. The results provide a quantitative basis for predicting the operational lifetime of radiation-hard detectors, including 4H-SiC LGADs, and highlight the value and trade-offs of in-situ irradiation studies at ion-therapy centers for controlled, low-fluence damage assessment.

Abstract

Silicon carbide (SiC) planar PiN diodes from two different manufacturers were irradiated with 252.7 MeV protons from a medical synchrotron. Over the course of two 8h irradiation shifts, the samples were exposed to increasing fluences ranging from 1.4e+11 to 3.5e+13 p+/cm^2. Electrical characterizations, including IV and CV measurements, were performed both before and after irradiation using probe stations, and for selected samples even in-situ between fluence steps directly at the irradiation facility. The results show a gradual compensation of the effective epitaxial doping concentration with each incremental fluence step, observed as a reduction in capacitance before full depletion and confirmed by the extracted effective doping concentration. From these measurements, linear donor removal rates are determined for all sample groups, with values ranging from 4.2/cm to 6.4/cm. These findings provide a quantitative basis for understanding radiation-induced charge carrier removal in 4H-SiC devices and are relevant for predicting the performance and lifetime of future radiation-hard detector technologies, including 4H-SiC LGADs.

Figures (5)

• Figure 1: Left: CNM SiC-diode mounted on a ceramic board with bond-wires connecting the top-side to a separate connector. SMA cables are connected to a switchbox during the in-situ proton irradiation. Diagram of the in-situ setup in the middle. Right: Gel-pak with SiC-detector samples mounted in front of the beam nozzle. The beam's isocenter is located at the crossing point of all three alignment lasers.
• Figure 2: Forward I-V in-situ measurement results of two planar CNM PiN samples from different wafers (a,b) and probe station I-V measurements on irradiated onsemi samples (c). The epitaxial doping concentration in the W4 sample (a) is higher, and hardly any change in forward current is visible throughout the applied fluence range. With lower epitaxial doping, we can observe that the exponential increase in forward current is delayed to higher bias voltages with each irradiation step due to the growth of an electric field barrier near the electrodes.
• Figure 3: CV-measurements of all samples zoomed on lower voltages. In both the in-situ (a,b) and traditional irradiation campaign (c), samples show a gradual reduction in the measured capacitance before full depletion.
• Figure 4: Doping profiles calculated from CV-measurements for the CNM W4 sample (a) and onsemi PiN samples (b). CV-data in (a) was smoothed using a low-pass filter before the calculation of the doping concentration, as the measured data was noisier in the in-situ measurement.
• Figure 5: Donor removal rate fitted for all samples, and discrepancy between chosen common depletion widths shown as uncertainty as shaded areas (one/three standard-deviations). Due to the high epi-doping of the CNM W4 sample, full compensation was not achieved, and a linear fluence projection until full dopant compensation according to the line fit is shown on the right hand side.