The electronic effects of H atoms at interstitial sites in ZnO were investigated by high-resolution electron energy loss spectroscopy. A reversible doping was achieved by exposing single crystalline (00•¯1)-oriented ZnO substrates to atomic hydrogen. At low temperatures, interstitial H atoms form shallow donor states. At sufficiently high temperatures, the electrons were excited into the conduction band. Electron energy loss spectroscopy was used to demonstrate the presence of plasmons resulting from this finite density of charge carriers in the conduction band. Above 100K, a strong, plasmon-induced broadening of the quasi-elastic peak in the high-resolution electron energy loss spectroscopic data was observed. The analysis of the temperature dependence yielded a donor level ionization energy of 25meV.

Ionization Energies of Shallow Donor States in ZnO Created by Reversible Formation and Depletion of H Interstitials. Qiu, H., Meyer, B., Wang, Y., Wöll, C.: Physical Review Letters, 2008, 101[23], 236401