  Zinc Oxide (ZnO) semiconductor is ideal candidates for ultra-violet (UV)
photodetector due to its promising optoelectronic properties. Photodetectors
based on ZnO nanostructures show very high photoconductivity under UV light,
but they are plagued by slow photo-response time as slow as several tens of
hours, even more. Most of the studies claimed that atmospheric adsorbates such
as water and oxygen create charge traps states on the surface and remarkably
increase both the photoconductivity and response time, but there are also
limited studies that claiming the defect induced states acting as hole trap
centers responsible for these problems. However, the underlying physical
mechanism is still unclear. Here we study the effects of both adsorbates and
defect-related states on the photo-response character of Pulsed Electron
Deposited ZnO thin films. In order to distinguish between these two mechanisms,
we have compared the time-dependent photo-response measurements of bare-ZnO and
SiO2 encapsulated-ZnO thin film samples taken under UV light and high vacuum.
We show that the dominant mechanism of photo-response in ZnO is the
adsorption/desorption of oxygen and water molecules even when the measurement
is performed in high vacuum. When the samples are encapsulated by a thin SiO2
layer, the adsorption/desorption rates can significantly improve, and the
effects of these molecules partially removed.
