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 67Zn nuclei, proving that it is a shallow, effective-mass-like donor. In addition D1 exhibits a single interaction with a H nucleus ( a(H) = 1.