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 Zn-67 nuclei, proving that it is a shallow, effective-mass-like donor. In addition D 1 exhibits a single interaction with a H nucleus (a(H) = 1.4 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 x 10(16) cm(-3) emphasizing its relevance for carrier statistics and applications.