The effect of impurities on hydrogen bonding site and local vibrational frequency in ZnO

For isovalent impurities such as Be, Mg, Ca, Sr, and Cd and group-I element such as Na in ZnO, first-principles local-density-functional calculations show that the interstitial position of H depends on the type of impurities, either occupying a bond center (BC) site or an antibonding (AB) site adjacent to the impurity atom. The AB site is more favorable in the vicinity of Na, Ca, Sr, and Cd, while the stable position is the BC site in the case of Be. We find that both electronegativity and atomic size play a role in switching the H interstitial position between the BC and AB sites. Previous studies have suggested that two infrared lines observed at 3611 and 3326 cm(-1) result from hydrogen atoms positioned at BC and AB sites, respectively. The results for the H bonding sites and defect concentrations suggest that Ca is the most probable impurity as the origin of the infrared line at 3326 cm(-1). However, for impurities around which H is positioned at the AB site, the calculated local vibrational frequencies are found to be similar to within 30 cm(-1), making it difficult to determine the specific impurity responsible for the 3326 cm(-1) line.