Role of diatomic hydrogen in the electronic structure of ZnO

Abstract

According to the traditional point of view, interstitial H in ZnO is a shallow donor. We report that the feature can be inhibited by another interstitial H. Although there is a chemical bonding between the two H atoms, they cannot necessarily be considered as a ${\text{H}}_{2}$ molecule: one of the H atoms is tightly bonded into the nearest O and forms the O-H complex, the other H atom forms a chemical bond with the nearest Zn and these bonds are stronger than the H-H bond. The defect molecule can be called a ${\text{H}}_{2}^{*}$ complex and it forms a deep energy level in the band gap. At Fermi levels closer to the conduction-band minimum, the complex is found to be more stable than the two distant H atoms. We discuss the role of ${\text{H}}_{2}^{*}$ in electronic structure, optical, and electrical properties of ZnO. In particular, formation of the ${\text{H}}_{2}^{*}$ complex provides a natural limitation for the $n$-type doping of ZnO by H.