The electrical characteristics of metal-insulator-semiconductor (MIS) capacitors with Al/Al2O3/ZnO structures were investigated by capacitance-voltage (C-V) measurements. The ZnO films were prepared by plasma-enhanced atomic layer deposition as a common semiconducting layer. The insulators of Al2O3 were composed of a thin protection layer (PL), a first gate insulator, directly deposited on ZnO, and a main gate insulator by changing the oxygen sources (water vapor, in situ generated O-2 plasma, and ozone) during atomic layer deposition (ALD). When the PL was prepared by water, good electrical behaviors in C-V measurements such as no voltage hysteresis and normal flatband voltage (V-FB) were observed for the capacitors, regardless of the ALD condition for the deposition of the main gate insulator. For the MIS containing a PL deposited with O-2 plasma or ozone, the Al2O3/ZnO interfaces were markedly degraded, in which large hystereses and positive shifts of V-FB were observed. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3421680] All rights reserved.