Optical spectroscopy of muon/hydrogen defects in 6H-SiC

Positive muons can be implanted into silicon carbide (SiC), where they model the isolated hydrogen defect in the negative, neutral, or positive charge states and act as either an acceptor or a donor with midgap energy levels [Lichti et al., Phys. Rev. B 70, 165204 (2004); Lichti et al., Phys. Rev. Lett. 101, 136403 (2008)]. The charge states evolve after implantation depending on the temperature and material doping. We have measured optically induced effects on muons implanted in 6H-SiC using a pulsed, tunable laser [Yokoyama et al., Rev. Sci. Instrum. 87, 125111 (2016)]. In n-type 6H-SiC at 85 K and 40 K, with a laser pulse of energy below the bandgap, we observe photoionization of the doubly occupied level (Mu-) to the neutral defect Mu0 and also ionization of Mu0 to Mu+. Varying the timing of the laser pulse relative to muon arrival confirms that the laser interacts directly with the muons in a stable or metastable state. There is no evidence of any interaction when the laser pulse is timed to arrive before the muons, so either few free carriers are generated by absorption at other dopant sites or the excess carriers have a very short lifetime (\u3c0 and Mu- charge states, with the muon or hydrogen acting as a deep compensating impurity. The technique can be applied to many other semiconductors where the muon has been observed to be electrically active, modeling hydrogen

The first 25 years of semiconductor muonics at ISIS, modelling the electrical activity of hydrogen in inorganic semiconductors and high-κ dielectrics

Early muonium studies provided the very first atomistic pictures of interstitial hydrogen in semiconductors. By the time ISIS muons came on line, the main crystallographic sites, and the electronic structures for the neutral centres, were established in archetypal materials such as Si and GaAs. The results were quite unanticipated, and raised awareness of this deceptively simple defect system. This paper marks contributions to the subject made using ISIS muon beams, in the first 25 years of their operation since 1987. By this time, hydrogen was understood to be a significant and unavoidable impurity in all electronic grade material, and attention was turning to the interaction with charge carriers, revealing an equally unanticipated interplay of site and charge state. In particular, muonium spectroscopy now provides a model for hydrogen in dozens of materials where hydrogen itself is difficult or impossible to study directly, and is able to predict its effect on the electronic properties of new materials, such as those envisaged for optoeletronic or dielectric applications. Donor, acceptor and so-called pinning levels are known in a good many of these materials, revealing intriguing systematics and providing severe tests and challenges to current theory. Progress and prospects are summarized in this report, addressing the obvious questions such as 'why, how and what next?

Modelling isolated hydrogen impurity in Lu2O3 with muonium spectroscopy

We identified in this experiment two muon configurations in Lu2O3, the oxygen-bound (O−Mu+) ground state and a metastable (energy barrier 0.7(3) eV) atom-like excited state. These configurations are partially not formed immediately after implantation but somewhat delayed due to the requirement of a lattice rearrangement around the muon. These rearrangement processes occur on a timescale of ns to μs and are thus observable in μSR experiments. A special role plays a fairly long-lived (ns to μs) transition state as an intermediate step in the reaction process

Shallow donor state of hydrogen in In2O3 and SnO2: Implications for conductivity in transparent conducting oxides

Muonium, and by analogy hydrogen, is shown to form a shallow-donor state in In2 O3 and SnO2. The paramagnetic charge state is stable below ∼50 K in In2 O3 and ∼30 K in SnO2 which, coupled with its extremely small effective hyperfine splitting in both cases, allows its identification as the shallow-donor state. This has important implications for the controversial issue of the origins of conductivity in transparent conducting oxides. © 2009 The American Physical Society

Barrier model in muon implantation and application to Lu2O3

In implantation experiments, the implanted particle is shot with a certain energy into the material and comes to rest at a site which may not correspond to the final position. The rearrangements of the surrounding atoms to accommodate the particle, i.e., the reaction with the host atoms may require some time and lead to delayed formation of the final states. In the case of the implantation of positive muons, this rearrangement process can be followed on a timescale of nanoseconds to microseconds. A delay is expected if an energy barrier inhibits the prompt reaction. We note that the barrier height may change during the rearrangement of the lattice, thus giving rise to a two-dimensional potential profile for the conversion process. The barrier model describes the reaction path of the muon in analogy to the passage over a mountain with a saddle point. The passing over the saddle point corresponds to the lowest energy trajectory. As an example, we discuss the application of the barrier model to solid Lu2O