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Physics Seminar

Muon probes of charge carrier kinetics in semiconductors

Speaker: Koji Yokoyama (ISIS Neutron and Muon Source)
Date: Wednesday 13 October 2021
Time: 15:00
Venue: N/3.23 and Zoom

Muons, as a bulk probe of materials, have been used to study the charge carrier kinetics in semiconductors. The photoexcited muon spin spectroscopy technique [1] can optically generate excess carriers in semiconductor wafers, while muons can measure the excess carrier density. A hydrogen-like muonium atom (Mu = µ+ + e-) interacts with the carriers, which in turn causes muon spin relaxation via µ+-e- hyperfine interaction. By having a pulse delay between muons and laser light, carrier recombination lifetime spectra can be obtained. We have so far applied the method to indirect gap semiconductors (silicon [2] and germanium [3]), and recently demonstrated a depth-dependent study in Si by changing the energy of the incoming muon beam [4]. Unlike other traditional lifetime spectroscopy techniques, depth-dependent lifetime spectra enable us to de-convolute the bulk and surface recombination properties. These advantages can be useful in measuring carrier lifetimes in silicon solar cells and eventually optimising their device performance. In this talk, I will start with a short introduction to the muon spin spectroscopy technique and finish with our most recent attempt in measuring the fast carrier recombination in direct gap semiconductors.

[1] K. Yokoyama, et al. Review of Scientific Instruments 87, 125111 (2016).
[2] K. Yokoyama, et al. Phys. Rev. Lett. 119, 226601 (2017).
[3] K. Yokoyama, et al. Appl. Phys. Lett. 115, 112101 (2019).
[4] K. Yokoyama, et al. Appl. Phys. Lett. 118, 252105 (2021).

Short bio: I started my career in science as a PhD student at the Department of Physics and Astronomy in University of California at Riverside in 2001. I was in the laser lab and worked with Prof. Harry Tom, who is an expert of nonlinear optics. I was first working on molecular dynamics of liquid water using a THz spectroscopy, but later in my PhD career, worked on semiconductor research using muons and laser light. This was when Prof. Ken Nagamine, the founder of the RIKEN-RAL Muon facility at ISIS, came to UCR as a faculty member and started a collaboration with Prof. Tom. The project focused on the Mu-carrier interaction in GaAs, which would significantly overlap with this talk (this means that I’ve been doing this sort of studies for nearly 15 years!). In 2007, for the first time, I came to the ISIS Pulsed Neutron and Muon Source for a muon experiment. After obtaining my PhD degree, I became a post-doc of the RIKEN-RAL Muon facility and worked on a project for generating low-energy muons using a vacuum-UV laser. After spending 3 years on the facility development, I really missed science research and became a post-doc of Queen Mary University of London. The ERC-funded project, which led by Prof. Alan Drew, installed a laser system on the HIFI instrument at ISIS for developing photoexcited muon spin spectroscopy. After the 5-year project, I became an instrument scientist at ISIS. I’m now in charge of EMU, which is one of the MuSR spectrometers at ISIS, but also support illumination experiments using the HIFI Laser system.