Preparation And Performance Study Of Ultraviolet Detector Based On ZnO Nanorods

High performance ultraviolet(UV)detector has a wide application prospect in environmental monitoring,clinical medicine,space exploration,military security and so on.In recent years,UV sensors based on ZnO nanorods(NRs)have attracted much attention because of their high sensitivity(S),high responsivity(R)and fast response,and become a research hotspot at home and abroad.In this paper,a metal-semiconductor-metal(MSM)UV detector was prepared based on ZnO NRs grown by hydrothermal method as a photosensitive layer.Based on experiments and theories,the effects of hydrothermal parameters and annealing treatment,such as seed-layer annealing,hydrothermal time,precursor concentration,anhydrous ethanol additive,on the crystal properties and UV sensing characteristics of ZnO NRs were systematically investigated.By further optimizing the technological scheme and conditions,the UV detector with high performance was prepared.The main work and results of this paper are as follows:(1)The effect of preparation method of ZnO NRs photosensitive layer on the performance of UV detector was investigated.The ZnO NRs sensing layer was prepared on Si O₂/Si substrate by transfer method,direct growth method and ZnO seed layer growth method,respectively.The characteristics of the sensor show that the UV sensor prepared by transfer method has low sensitivity,unstable current and persistent photoconductance.The device with NRs directly grown on bare substrate still has a good response when the voltage is only 0.2 m V,and the static power is only 6×10^-12 W,which shows the potential of this sensor for ultra-low power application.The ZnO NRs grown on the seed layer show a c-axis preferred orientation because of the match with the lattice of the substrate seed layer.The prepared sensor shows better ultraviolet response characteristics,with S and R up to 1.07×10⁴ and 4.94×10⁴ A/W,respectively,while the dark current is only 254 n A.(2)The effects of annealing of ZnO seed layer at different temperatures on the properties and UV response of ZnO NRs crystals have been systematically studied.The results show that the annealing treatment of the seed layer of ZnO film at appropriate temperature is helpful to improve the adhesion ability of ZnO NRs to the substrate,and obtain the appropriate crystallization quality and density of ZnO NRs.Compared with 300?,400?and 500?,the device prepared by seed layer annealing at 200?showed relatively excellent sensing characteristics,with a S of 2.9×10⁴ and a signal-to-noise ratio(SNR)of 89.25 d B.(3)The dependence of the morphology,structure,optical and electrical properties of ZnO NRs on the hydrothermal process parameters has been systematically studied.Prolonging the hydrothermal time is beneficial to the improvement of crystal quality,but excessive hydrothermal time can inhibit the aspect ratio.The results show that the UV detector prepared by hydrothermal treatment for 4 h shows better performance,with the S of 2.47×10³,R and detectivity(^*)of 5.51×10³ A/W and 1.97×10¹² Jones,respectively.The diameter and length of ZnO NRs increased with the increase of the growth solution concentration,because the high concentration of precursor solution provided more Zn²⁺and continued to diffuse to the ZnO core to promote the growth of NRs.Excess OH^-was introduced into the precursor solution by low temperature decomposition of anhydrous ethanol catalyzed by Zn²⁺to adjust the p H value of the growth environment.The results show that the transient response characteristics of the device are significantly improved with the increase of ethanol dosage.When the volume ratio of ethanol is 15%,the response time is 35.6 s and the recovery time is as low as 3.9 s.(4)The effects of annealing temperature and time on the crystal properties and UV response of ZnO NRs in air were investigated.The post-annealing treatment of ZnO NRs showed that the morphology of ZnO NRs did not change significantly,but the dark current of the device was effectively inhibited.This phenomenon is explained as the compensating effect of annealing on the donor-like defects and the inhibiting effect on the band-tail states,which leads to the decrease of electron concentration in the crystal,thus effectively suppressing the dark current of the device.At the same time,the compensating effect on the interstitial zinc(Zn_i)donor-like states weakes the photoresponse current of the detector.The S and SNR of the UV sensor annealed at 400?for 180 min are as high as 4.4×10⁵ and 112.9 d B,respectively,and the^*reaches 26.97 A/W.Meanwhile,the best response time and recovery time are 9.23 s and 0.48 s,respectively.