Hydrogen a relevant shallow donor in Zinc Oxide

Electron paramagnetic resonance and Hall measurements show consistently the presence of two donors ( $D1$ and $D2$) in state-of-the-art, nominally undoped ZnO single crystals. Using electron nuclear double resonance it is found that $D1$ shows hyperfine interaction with more than 50 shells of surrounding ${}^{67}\mathrm{Zn}$ nuclei, proving that it is a shallow, effective-mass-like donor. In addition $D1$ exhibits a single interaction with a H nucleus ( ${a}_{H}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.4\phantom{\rule{0ex}{0ex}}\mathrm{MHz}$), thus H is the defining element. It is in agreement with the prediction of Van de Walle [Phys. Rev. Lett. 85, 1012 (2000)] that H acts as a donor in ZnO. The concentration of $D1$ is $6\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0ex}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ emphasizing its relevance for carrier statistics and applications.