Quantitative comparisons of dissolved hydrogen density and the electrical and optical properties of ZnO

Abstract

Recent density functional theory calculations indicate that hydrogen is soluble in ZnO, effectively forming a shallow donor state. It has been suggested that these donors are responsible for the large increases in electron concentration seen in ZnO samples annealed at elevated temperatures in ${\mathrm{H}}_2$ gas. In order to make a quantitative connection between the amount of dissolved hydrogen and any observed changes in electrical properties, we have annealed single crystal ZnO samples from several sources in ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ gas at 750 °C and compared the observed changes in carrier concentration with nuclear reaction analysis and secondary ion mass spectrometry profiles of deuterium. We find that the amount of deuterium remaining in our gas-charged samples is ${\text{∼3.6–5.5×10}}^{17} {\mathrm{cm}}^{\text{−3}},$ substantially larger than the increase seen in conduction band electron densities at 350 K. Our modeling indicates that these gas treatments produce a hydrogen-related donor state at 0.036±0.004 eV below the conduction band minimum and also cause significant increases in the measured conduction band electron mobility. These mobility increases and other features of our data strongly suggest that a substantial fraction of the dissolved hydrogen forms complexes with (passivates) acceptor impurities. Analysis of optical and electrical data also indicates that, while hydrogen is present in significant amounts in as-received samples, it is not the dominant “native” donor. Several infrared absorption peaks are observed before and after ${\mathrm{H}}_2/{\mathrm{D}}_2$ exposure, which we identify with local O–H/D stretch modes. The most prominent of these has an asymmetric absorption peak at 3546 cm⁻¹ arising from a c-axis oriented defect dipole, which is postulated to be hydrogen located in a bond-centered position between oxygen and zinc atoms. Comparisons are made between our experimental results and density functional theory calculations.