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Photoexcited muon spin spectroscopy (photo-$\mu$SR) was used to measure excess charge carrier lifetimes in passivated silicon wafers. Optically generated excess carriers interact with muonium centres via carrier exchange interaction and induce relaxation in the $\mu$SR time spectrum. The photo-$\mu$SR technique utilises this additional relaxation rate as a measure of the excess carrier density, which in turn enables us to measure carrier lifetime spectra by controlling delays between a muon and laser pulse $[1]$. In addition, the depth-resolved measurement can characterise carrier kinetics at specific depths within a Si wafer and enables us to separate bulk and surface recombination rates $[2]$. Based on these developments, we recently applied the technique to passivated Si samples with extremely long effective lifetimes ($>$1 ms) and observed that prolonged muon irradiation resulted in significant degradation of a measured lifetime $[3]$. Follow-up characterisation measurements, including deep-level transient spectroscopy, strongly suggested that beam damage generated defect-related recombination centres in bulk. Our results demonstrate an extremely rare case in $\mu$SR applications, where beam damage to crystalline lattice was clearly detected by virtue of high-lifetime Si wafers and, in turn, low native defect densities.
$[1]$ K. Yokoyama, et al. Phys. Rev. Lett. 119, 226601 (2017); Appl. Phys. Lett. 115, 112101 (2019).
$[2]$ K. Yokoyama, et al. Appl. Phys. Lett. 118, 252105 (2021).
$[3]$ J. D. Murphy, et al. submitted to Journal of Applied Physics.
