“…There is strong evidence that interstitial hydrogen, H i , plays a role of donor [ 6 , 8 ], while hydrogen molecule, H 2 , has been shown to be electrically inert in ZnO [ 9 , 10 ]. However, recent investigations strongly suggest that hydrogen impurity in ZnO material may be involved in a number of complexes with native point defects, such as V Zn ·nH, Zn i ·V O ·H, and others [ 2 , 3 , 4 , 5 , 11 , 12 , 13 , 14 ]. Some of these complexes introduce shallow donor and acceptor levels that affect the resulting ZnO conductivity [ 4 , 10 , 15 , 16 , 17 , 18 ].…”