• Mccluskey, Matthew

Abstract

<jats:title>Abstract</jats:title><jats:p>Zinc oxide (ZnO) has shown great promise as a wide-bandgap semiconductor with a range of optical, electronic, and mechanical applications. The presence of compensating donors, however, is a major roadblock to achieving p-type conductivity. Recent first-principles calculations and experimental studies have shown that hydrogen acts as a shallow donor in ZnO, in contrast to hydrogen's usual role as a passivating impurity. Given the omnipresence of hydrogen during growth and processing, it is important to determine the structure and stability of hydrogen donors in ZnO.</jats:p><jats:p>To address these issues, we performed vibrational spectroscopy on bulk, single-crystal ZnO samples annealed in hydrogen (H<jats:sub>2</jats:sub>) or deuterium (D<jats:sub>2</jats:sub>) gas. Using infrared (IR) spectroscopy, we observed O-H and O-D stretch modes at 3326.3 cm<jats:sup>-1</jats:sup> and 2470.3 cm<jats:sup>-1</jats:sup> respectively, at a sample temperature of 10 K. These frequencies indicate that hydrogen forms a bond with a host oxygen atom, consistent with either an antibonding or bond-centered model. In the antibonding configuration, hydrogen attaches to a host oxygen and points away from the Zn-O bond. In the bond-centered configuration, hydrogen sits between the Zn and O. To discriminate between these two models, we measured the shift of the stretch-mode frequency as a function of hydrostatic pressure. By comparing with first-principles calculations, we conclude that the antibonding model is the correct one.</jats:p><jats:p>Surprisingly, we found that the O-H complex is unstable at room temperature. After a few weeks, the peak intensity decreases substantially. It is possible that the hydrogen forms H<jats:sub>2</jats:sub> molecules, which have essentially no IR signature. Electrical measurements show a corresponding decrease in electron concentration, which is consistent with the formation of neutral H<jats:sub>2</jats:sub> molecules. The correlation between the electrical and spectroscopic measurements, however, is not perfect. We therefore speculate that there may be a second “hidden” hydrogen donor. One candidate for such a donor is a hydrogen-decorated oxygen vacancy.</jats:p>

Topics

• impedance spectroscopy
• Oxygen
• zinc
• semiconductor
• Hydrogen
• vacancy
• vibrational spectroscopy