Studies of hydrogen in semiconductors using the positive muon as a proton analogue

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Abstract
Implanted in a material, the positive muon mimics the proton in its chemical and elastic interactions. In non-metals it can pick up an electron to form muoniurn , Mu = [μ + e - ], which behaves as a light isotope of hydrogen. This may be observed with remarkable sensitivity by the μSR techniques (muon rotation, relaxation and resonance), whereas hydrogen, when present only as a trace impurity, may be difficult or impossible to detect by conventional spectroscopies. Much of the structural information on isolated hydrogen defect centres in semiconductors has in fact come from μSR studies of their muonium counterparts: particularly important has been the discovery of metastability and identification of the bond-centred state. Current studies are of both fundamental and technological interest and concern the interplay of site and charge state, the trapping and detrapping of carriers at these deep level centres, and the nature of interstitial diffusion. Extension to the modelling of hydrogen passivation in doped material, the development of optical and IR spectroscopy and of thin-film or surface studies, all appear timely and promising.