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.

Muonium, and by analogy hydrogen, is shown to form a shallow-donor state in ${\text{In}}_2{\text{O}}_3$ and ${\text{SnO}}_2$. The paramagnetic charge state is stable below $\sim 50\text{K}$ in ${\text{In}}_2{\text{O}}_3$ and $\sim 30\text{K}$ in ${\text{SnO}}_2$ 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.

Experimental data on shallow donor muonium states in the $\mathrm{II}\mathrm{-}\mathrm{VI}$ semiconductor compounds CdS, CdSe, CdTe, and ZnO are presented. These are characterized by very weak hyperfine interactions amounting to approximately ${10}^{-4}$ of the free-atom value, and by donor levels with binding energies comparable to those calculated on the hydrogenic model of the shallow centers. The data are discussed in terms of a model generalizing the knowledge of muonium and hydrogen states in tetrahedrally bonded semiconductors. Within this model the shallow muonium state is attributed to muonium bound to the anion of the $\mathrm{II}\mathrm{-}\mathrm{VI}$ compound and located at the antibonding site or close to it in the bond direction.

A new type of muonium defect center has been observed in undoped CdS below 20 K. The hyperfine interaction amounts only to approximately ${10}^{-4\mathrm{}}$ of the vacuum value, and is shown to have axial symmetry along the Cd-S bond direction. Results suggest that the muon is close to the sulfur antibonding site and the paramagnetic electron density is distributed over a large volume. In contrast to the behavior in other semiconductors, muonium forms a shallow center in CdS. By implication, analog isolated hydrogen impurity atoms act as electrically active shallow-level defect centers in CdS.

Muonium, a positive muon and an electron, is often used as an experimentally accessible substitute for hydrogen in materials research. In semiconductors and insulators, a large amount of information on the hydrogen behavior is deduced from this analogy; however, it is seldom demonstrated that this procedure is justified. We show here, via a comparison of the hyperfine interactions, that in ${\mathrm{TiO}}_2$ muonium and hydrogen form the same configuration with the same basic electronic structure. A detailed description of the bonding characteristics of the muon to the ${\mathrm{Ti}}^{3+}$ polaron is presented. The special role of muon motion within the so-called oxygen channel in the rutile structure, which occurs at a lower temperature than for hydrogen, is emphasized.