Anionic and Hidden Hydrogen in ZnO

First-principles calculations are performed to study energetics and kinetics of hydrogen in ZnO, in particular, the H- anion and the H-2 molecule on the interstitial site and in the oxygen vacancy. We show that the H-2 molecule kinetically trapped in the oxygen vacancy, rather than interstitial H-2, can explain a variety of experimental observations on hidden hydrogen in ZnO. The accumulation of shallow donors, especially the substitutional H, near the ZnO surface is important to the formation of hidden hydrogen in the ZnO bulk and can also lead to persistent photoconductivity.