Hydrogen complexes in Zn deficient ZnO

This work presents a study of H complexes in Zn deficient ZnO by density functional calculations. It is found that the location of two H atoms at the Zn vacancy (V-Zn) is energetically more preferable than the distant H and V-Zn. Each of the two H atoms saturates one dangling O bond by forming an O-H bond. The resulting defect can be considered to be a 2(O-H)-V-Zn complex. Since there are four O dangling bonds at the V-Zn, the site can host four H atoms. It is shown that V-Zn with four H atoms is energetically more preferable than distant H and V-Zn. Depending on the number of H atoms at the V-Zn, the influence of the H-V-Zn system on the electronic structure of Zn deficient ZnO can be qualitatively different. V-Zn without a H is a double acceptor and V-Zn containing one H, i.e., 1(O-H)-V-Zn complex, is a single acceptor. In ZnO with 2(O-H)-V-Zn complexes the Fermi level is located at the top of the valence band. A donor level is formed by the 3(O-H)-V-Zn and 4(O-H)-V-Zn complexes. Two of the H atoms in 2(O-H)-V-Zn, 3(O-H)-V-Zn, and 4(O-H)-V-Zn complexes do not contribute to free carrier concentration and will cause discrepancy in the free electron and H concentrations. The influence of the complexes on the electrical and optical properties of ZnO is discussed.