We confirm the recent prediction that interstitial protium may act as a shallow donor in zinc oxide, by direct spectroscopic observation of its muonium counterpart. On implantation into ZnO, positive muons--chemically analogous to protons in this context--form paramagnetic centers below about 40 K. The muon-electron contact hyperfine interaction, as well as the temperature and activation energy for ionization, imply a shallow level. Similar results for the cadmium chalcogenides suggest that such shallow donor states are generic to the II-VI compounds. The donor level depths should serve as a guide for the electrical activity of interstitial hydrogen.

The nature of the electron states associated with hydrogen in InN has been inferred by studying the behavior of positive muons, which mimic protons when implanted into semiconductors. The muons capture electrons below 60 K, forming paramagnetic centers with a binding energy of about 12 meV. Together with an exceedingly small muon-electron hyperfine constant indicative of a highly delocalized electron wave function, the results confirm the recently predicted shallow-donor properties of hydrogen in InN. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1539547͔The role of hydrogen has been crucial to the development of nitride semiconductors for device applications and remains a key issue in material deposition and doping. 1 Whether present inadvertently or added deliberately, the effect of hydrogen on electronic properties is nearly always significant. Most widely known is its propensity to pair with and passivate other impurities and defects, making isolated defect centers formed by interstitial hydrogen exceedingly difficult to study spectroscopically. Muon implantation is a convenient way of simulating the presence of hydrogen and has proven to be a valuable means of identifying the crystallographic sites and electronic structures of hydrogen defect centers in semiconductors. We refer here specifically to positive muons which, when implanted into nonmetals at low temperatures, can pick up and retain electrons to form muonium, Muϭ͓ ϩ e Ϫ ͔, effectively a light isotope of hydrogen. Despite the large isotopic mass ratio, m Mu /m H ϭ1/9, muonium mimics hydrogen rather faithfully in its chemical and electrical interactions with the lattice. 2,3 The recent discovery 4 -6 that muons implanted into several semiconductors of the II-VI family form shallow donor states was surprising in that investigations conducted over three decades had invariably found muonium to create states lying deep in the semiconducting gap. These deep states are known for Si, Ge, many III-Vs and indeed some II-VI compounds, e.g., ZnS and ZnSe. Experiments on muonium in ZnO 5 were made in direct response to theoretical work 7 which predicted that hydrogen should form shallow centers in this material. This result has recently been explicitly confirmed for hydrogen by ENDOR ͑electron-nuclear double resonance spectroscopy͒. 8

Muonium, and by analogy hydrogen, is shown to form a shallow-donor state in In 2 O 3 and SnO 2 . The paramagnetic charge state is stable below ϳ50 K in In 2 O 3 and ϳ30 K in SnO 2 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.