Study Of The UV Photoresponse Properties Of ZnO Nanorods Arrays

Ultraviolet(UV) detecting technology has been widely applicated in many fileds such as,aviation, military, civil and so on. The self-powered UV photodector can not only greatly reduce the size and weight of the system but also largely enhance the adaptability of the devices and this is very important to the circuits integrated with the nano-devices in the future.One-dimensional Zn O nanostructure is the ideal material for constructing self-powered photodecector due to a lot of advantages including the low reflectance, the high ability of photo collection and the directional transport route for charge carrier. Now, the photoresponse speed and the photosensitivity of the UV photodetector based one-dimensional Zn O nanostructure still need to be improved. In this study, we deposited Cu SCN film on the Zn O nanorods arrays(NRs) by different methods and investigated the UV photoresponse properties. Finally, a self-powered UV photodetector based Zn O NRs/Cu SCN heterojunction had been achieved. The main contents and results are as follows:(1) The surface states and defects play an important role on the UV photoresponse properties of Zn O NRs. We fabricated the wide band-gap n-type Zn O NRs with different surface-to-volume ratio by chemical bath deposition through changing the concentration of precursor solution. The defects of Zn O NRs can be reduced by vacuum annealing. We found that the increasd surface-to-volume ratio can lead to the increased photocurrent and the reduced defects can shorten the photoresponse recover time.(2) We deposited the wide band-gap p-type Cu SCN on Zn O NRs by successive ionic layer adsorption and reaction(SILAR) method to form the type-II p-n heterojunctions. The number of the SILAR cycles plays an important role in the UV photoresponse properties. As the number of the SILAR cycles increases, the amount of Cu SCN on the Zn O NRs increases and leads to the area of the localized heterojunction increasing. Compared to the pristine Zn O NRs, the dark current of the Zn O NRs deposited Cu SCN with 30 and 60 cycles decrease by two orders of magnitude while the photocurrent is almost no change. Under –3 V applied bias,the photosensitivity increases from 1.65 to 20.45 and 131.53 and the photoresponse speed decreases from 1819 s to 685 s and 331 s. As the number of SILAR cycles reaches 120,self-powered UV photoresponse property with fast response speed was observed. The deposited Cu SCN can not only restrict the oxygen species from directly reabsorbing onto the Zn O NRs surface but also provide more heterojunction interface area for the separation of the photogenerated carriers.(3) The Cu SCN film was deposited on the Zn O NRs by electrochemical deposition methid. Cu SCN exists on the surface of Zn O NRs with large trigonal pyramid shape under continous depositing. The large amount of gaps and voids in the Zn O NRs results in a large reverse leakage current and a slow photoresponse process of the devices. The UVphotoresponse properties can be been improved through changing the depositon time,temperature and the steps of cycle deposition. When the deposition temperature and the steps of cycle deposition were low, Cu SCN locally exists on the surface of Zn O NRs and can improve the rectifying characteristic and photosensitivity of the devices. The devices with Cu SCN depositing at room temperature show the self-powered UV response properties whereas the devices with Cu SCN depositing at 0 o C have not been observed this behavior.This self-powered UV response behavior can be attributed to the photovoltaic effect of Zn O NRs/Cu SCN p–n heterojunction. The Cu SCN layer on the Zn O NRs surfaces depositing at room temperature has single-crystalline structure. For the device with Cu SCN depositing at room temperature for three cycle depositing steps(i.e, 1800 s), Cu SCN can fill and cover Zn O NRs. The photocurrent can quickly reach 3.86×10-5 A at 0 V and the corresponding responsivity Rl is a relative high value of 1.57 A/W. This is mainly attributed to the large area of the heterojunctions. The internal electric field in the p-n junction allows rapid charge-carrier separation at the interface and prevents the recombination of photoexcited electrons in Zn O NRs and holes at the p–n junction.