Influence of the Hydrogen Concentration on H Bonding in Zinc Oxide

Abstract

Despite of the recent developments the properties of hydrogen in ZnO are not fully understood. While in some ZnO single crystals the amount of H detected by gas effusion experiments is similar in concentration to the number of free electrons, a large number of samples exhibits much higher H concentrations. Raman backscattering experiments reveal that a significant amount of H is accommodated at sites that do not give rise to enhanced conductivity; a large amount of H is bound to carbon and nitrogen impurities forming C-H^X and N-H complexes. Depending on the method of the sample preparation H concentrations as high as 3.0×10²¹ cm⁻³ have been observed. To gain further insight into the properties of H in ZnO data obtained from H effusion measurements of single crystal and sputter deposited ZnO have been analyzed to deduce the hydrogen binding energy as a function of the H chemical potential. In samples with a low H content six peaks were observed in the H density-of-states distribution. With increasing H concentration the peaks broaden. In addition, an increase of the H concentration results in a pronounced increase of the average H binding energy. This observation indicates that the properties of H expand well beyond the formation of donors and the neutralization of impurities and deep defects.