Top: Schematic diagram of the experimental geometry. Bottom: Muon distribution (blue solid line) and photon flux (red broken line) as a function of depth measured from the surface on which muons are incident. The muon distribution has been calculated using a Monte Carlo simulation package based on GEANT4. Credit: K Yokoyama
Knowledge of the kinetics of excess charge carriers (electrons and holes) in semiconductors is crucial when determining the performance of electronic devices such as solar cells or computer chips. The excess carrier recombination lifetime is a key figure of merit in photovoltaics applications that governs the efficiency of solar cells. However, traditional lifetime spectroscopy techniques currently used for characterisation only provide a bulk average for the material. The method described in this study, using photoexcited muon spin spectroscopy (photo-µSR), allows the carrier lifetime to be probed at a specific position within a wafer, giving a better picture of carrier dynamics.
When a positively charged anti-muon is implanted into a semiconducting material and localised in its crystalline lattice, it captures an electron to form muonium, a hydrogen-like species that can undergo spin and carrier exchange interactions with free electrons and holes. If there are more charge carriers present, then more of this exchange can occur, and the faster the measured muon spin relaxation. By combining this muon spin spectroscopy with optical carrier injection, the dynamics of the charge carriers can be investigated.
In this study, the researchers used the photo-μSR pump-probe technique on HiFi to measure the temperature dependence of the lifetime and mobility of the charge carriers inside germanium. Their results are in agreement with those using other methods, proving that photo-μSR can successfully be used to measure these kinetics. Based on the fact that muonium is a common state of implanted muons in a wide range of semiconductor materials, the method could contribute to the characterization of emerging functional materials, such as perovskite-structured compounds and wide-bandgap semiconductors.
Related publication: “Muon probes of temperature-dependent charge carrier kinetics in semiconductors” Appl. Phys. Lett. 115, 112101 (2019)