Transparent conducting oxides (TCOs) have recently attracted considerable attention due to their potential in making "invisible electronic circuits". Among different TCOs, ZnO is probably one of the most promising candidates due to its desirable optical and electrical properties. In the transistor, metal and metal-dielectric in general are used as the gate on the active channel layer to control the flow of carriers. In order to obtain high mobility, the interface between the gate and ZnO must be well controlled so that the unwanted surface recombination can be minimized. Although studies of metal/ZnO have been well documented, only a few are seen on metal/dielectric/ZnO. Here, we report a systematic study on AlOx/ZnO and MgOx/ZnO interfaces for the potential use on transistor. From previous optical measurements, both the photoluminescence (PL) and cathodoluminescence (CL) show that the cap layers improves the emission characteristics of ZnO by enhancing the band-edge emission while at the same time reducing the unwanted deep-level emissions [K.C. Hui, H.C. Ong, P.F. Lee, J.Y. Dai, Appl. Phys. Lett. 86, 152116 (2005)]. On the contrary, in this study, we find post-thermal annealing of AlOx/ZnO and MgOx/ZnO gradually degrades the emission as well as charge transport properties, which indicates defects evolve upon annealing. By using secondary ion mass spectroscopy (SIMS), we elucidate out-diffusion of Zn into the oxide layer is the root cause of degrading the optical and electrical properties of oxide/ZnO.