​Key to their success is understanding the behaviour of electrons and holes, and controlling their dynamics. How long these electron-hole pairs live impacts their performance in solar cells, and their lifetime is also used as an indicator of cleanliness in chip manufacturing. These “bulk" lifetimes represent purity and intrinsic quality of the wafer. However traditional measures of carrier lifetime, such as photoconductance decay, are strongly influenced by the surface condition – without proper cleaning, wafer surfaces are normally full of impurities. The surface recombination is extremely fast and therefore limits the measurement accuracies. A group of scientists have been using the muon facility at ISIS to test a new technique for measuring excess carrier lifetime that allows them to overcome this difficulty and directly probe the bulk carrier lifetime.

The research at ISIS used photoexcited muon spin spectroscopy, where the standard μSR spectroscopy was performed whilst optically injecting excess carriers with monochromatic laser light. The major advantage of using muons is that they are implanted deep in the bulk material, where the surface recombination can be negligible.​

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​(b) Schematic diagram of the experimental geometry and timing diagram of laser and muon pulse. Pulse duration (FWHM) of the laser and muon pulse are $\approx 16$ and $\approx 70\mathrm{ns}$ respectively. (c) $\mu \mathrm{SR}$ time spectra for light OFF (black squares) and ON (red circles, $\Delta n=4.7\times {10}^{13}{\mathrm{cm}}^{-3}$). $5\times {10}^6$events are averaged for each spectrum. Fit parameters are $A\left(0\right)=14.44\left(3\right)\%$, ${\lambda }^{\prime }=0.068\left(2\right)\mu s^{-1}$ for light OFF, and $\lambda =0.94\left(2\right)\mu s^{-1}$ for light ON. (d) $\lambda$ as a function of $\Delta n$. The fit gives $(\alpha ,\beta [\mu s^{-1}],\Delta n_0\left[{\mathrm{cm}}^{-3}\right])=\left[0.68\right(4),1.46(4),8.9\times {10}^{13}]$. (e) Carrier decay curve. The fit gives $\Delta n\left(0\right)=9.4\left(4\right)\times {10}^{13}{\mathrm{cm}}^{-3}$ and $\tau =11.1\left(9\right)\mu s$. More information can be found in: “Photoexcited Muon Spin Spectroscopy: A New Method for Measuring Excess Carrier Lifetime in Bulk Silicon", Phy​s Rev Lett 119, no. 22 (2017): 226601.​​

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When muons are implanted in a semiconductor material they form Muonium (Mu), a hydrogen-like atom made of a positive muon and a negative electron. When the light injects carriers, the Mu finds itself in a sea of electrons and holes, which then interact with Mu and cause muon spin relaxation. The research ​found that this relaxation rate could be a useful yardstick of the excess carrier density – though this seems rather trivial: the more interaction with carriers should lead to the faster relaxation. However this detailed study on relaxation relative to excess carrier density enabled them not only to observe the carrier recombination dynamics but also to investigate the microscopic interaction between Mu and carriers.

This was a collaborative work between Queen Mary University of London and ISIS. Koji Yokoyama, one of the team members who is now appointed as an instrument scientist in ISIS, says, “Based on this successful experiment the next step is to apply this technique to other semiconductor systems including both traditional and novel semiconductors, such as new perovskite structured compounds."

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Further information

“Photoexcited Muon Spin Spectroscopy: A New Method for Measuring Excess Carrier Lifetime in Bulk Silicon", Phy​s Rev Lett 119, no. 22 (2017): 226601.​

More on the HiFi​ muon spectrometer here. ​

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