Electron paramagnetic resonance and Hall measurements showed consistently the presence of two donors (D1, D2) in state-of-the-art, nominally undoped ZnO single crystals. Using electron nuclear double resonance it was found that D1 showed hyperfine interaction with more than 50 shells of surrounding 67Zn nuclei, proving that it was a shallow, effective-mass-like donor. In addition D1 exhibited a single interaction with a H nucleus (aH = 1.4MHz), thus H was the defining element. It was in agreement with the prediction of Van de Walle that H acts as a donor in ZnO. The concentration of D1 was 6 x 1016/cm3 emphasizing its relevance for carrier statistics and applications.

Hydrogen - a Relevant Shallow Donor in Zinc Oxide. Hofmann, D.M., Hofstaetter, A., Leiter, F., Zhou, H., Henecker, F., Meyer, B.K., Orlinskii, S.B., Schmidt, J., Baranov, P.G.: Physical Review Letters, 2002, 88[4], 045504