Mechanisms behind slow photoresponse character of Pulsed Electron Deposited ZnO thin films

Abstract

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.

Disappearance of rapid photoresponse in ultraviolet illumination of Ag–Sb–S films

“…In the phototransients of ZnO films and nanowires, the surface adsorption of H 2 O and O 2 molecules plays a crucial role [15][16][17][18][19][20][21][23][24][25]. In this regard, we show in figure 4 that the electrical conduction in the Ag-Sb-S films was affected significantly by the surface condition.…”

“…Similar slow photoresponses are known to occur for thin films and nanowires of ZnO [15][16][17][18][19][20][21]. The slow transients in the latter are attributed to the modifications in the transport of photocarriers due to environment effects caused by the surface of the films and nanowires and/or the bulk effects associated with crystalline defects [22][23][24][25]. That is, the charge transport in separating and uniting the photo-generated electrons and holes is affected by the electric fields induced by the attachment of water and/or oxygen molecules at the surface as well as by the trapping of the holes at deep-level defects [26].…”

The Fast and One-Step Growth of ZnO Nanorods on Cellulose Nanofibers for Highly Sensitive Photosensors

“…The slow transition time in ZnO nanostructures can be attributed to the surface defects related to electron-trapping regions. It is also reported that O 2 and H 2 O molecules present in the environment can be trapped on the surface of n-type materials like ZnO to produce electron-trapping centers to slow down the transition time [ 44 ]. In the work reported herein the effect of H 2 O could also be amplified by the ability of CNF to scavenge H 2 O molecules from the environment.…”

Facet‐Driven Vapor Phase Growth of Directional Tin‐Doped CdS Nanowires on Sapphire Surface for Photodetectors

“…More importantly, the trapping centers created during the doping process cause the reduction in photoresponse speed. [ 63 ] Typically, the device performance is highly affected by the total internal reflection in the hexagonal cross‐sectional tin‐doped CdS NWs. Numerous other factors, such as intrinsic material properties, defect tolerance, the influence of the ambient environment, and contact quality, all have an impact on the efficacy of PD devices.…”