Electrical properties and oxygen sensing ability of Zn1−xCoxO epitaxial films

While Co-doped ZnO has been much studied from the viewpoint of dilute magnetic semiconductors, we offer a new perspective by shedding light on the oxygen sensing ability of Co-doped ZnO. Epitaxial Zn1−xCoxO (x = 0.0,0.05,0.1,0.15) thin films are grown on α-Al2O3 (0001) by pulsed laser deposition. We successfully optimize growth conditions of Zn1−xCoxO films to prevent Co clustering and also to obtain high crystallinity. As Co content (x) increases, the c-axis lattice constant of Zn1−xCoxO films linearly increases. We particularly pay attention to the effect of oxygen annealing and dynamic response under reduction gas (2% hydrogen in Ar) and oxygen gas environment in electrical properties of Zn1−xCoxO. It is shown that the resistance of Zn0.85Co0.15O changes more than two orders of magnitude reversibly as it is reduced and oxidized alternatively. Thus, the present work demonstrates a potential of Zn1−xCoxO as an oxygen sensor at high temperatures. Oxygen (oxygen vacancy and/or oxygen interstitial) and Co in the system appear to play a role cooperatively in determining the electrical properties of Zn1−xCoxO.