ZnO Nanostructure Based UV Photodetectors

Ultraviolet photodetectors(UV PDs)have shown great prospects in the fields of optical communication,image sensors and light switch,etc.Zinc oxide(ZnO),a kind of wide bandgap semiconductors,has great potential in high-performance UV PDs owing to its unique properties such as large bandgap of 3.4 e V,high exciton binding energy of 60 me V,environmental friendly and easy to be fabricated.However,the low carrier concentration and fast recombination of electrons and holes of intrinsic ZnO impede its practical application as UV PDs.In this dissertation,several strategies have been developed to enhance the performance of ZnO-based UV PDs,including design and construction of ZnO-based pn junction and heterostucture,and decoration with Au nanoparticle.The main contents are as follows:(1)The core-shell homogeneous pn junction of p-ZnO:Na/n-ZnO:Al was successfully prepared.The influences of different Na doping concentration on the properties of ZnO nanorods were studied.The results show that the hole concentration of ZnO reaches up to 1.0×10¹⁷cm^-3with Na dopping concentration of 5.8 at%based on Hall measurement,low-temperature photoluminescence and electroluminescence analysis.UV PDs based on p-ZnO:Na/n-ZnO:Al homojunction exhibits fast UV sensing ability with rise and fall times of less than 0.08 s.(2)ZnO/Zn S core-shell nanostructures have been successfully prepared on glass substrate.The effects of Zn S shell layer on the optoelectronic properties of ZnO/ZnS heterostructures were systematically investigated.It was found that the near band edge emission(NBE)of the ZnO/Zn S heterostructures was enhanced with the increase of the sulfidation time,while the defect related emission of the ZnO/Zn S heterostructures was restrained.The best UV PDs based on ZnO/Zn S heterostructures were obtained with the sulfidation time of 3 h.The on/off ratio,responsibility,detectivity and response time of self-powered ZnO/Zn S-3h PDs are 44.7,0.76 m A/W,1.85×10⁸Jones and 3.9/0.08 s,respectively.Furthermore,the PDs exhibit excellent environmental stability and a high UV selectivity with a UV-Vis rejection ratio of 109.(3)ZnO/Ti₃C₂T_xhybrid structure was fabricated by simple dipping method on micro glass fibers,showing significantly enhanced photodetect ability.The responsivity of the ZnO/Ti₃C₂T_xPD is 5.05 A/W,which is 360 times of that of ZnO PD(0.014 A/W;@5 V;365 nm;0.28 m W/cm²).Furthermore,the PDs exhibit outstanding performance as UV light photoswitch and image sensor.(4)Self-powered ZnO/Au/Al₂O₃PDs with enhanced UV sensing performance were successfully fabricated.The effects of Au nanoparticles on optical and photodetect ability of ZnO/Au/Al₂O₃nanorods were studied in detail.The results showed that the ZnO nanorods exhibited enhanced UV emission and reduced visible emission after the Au decoration.The best UV sensing performance was achieved from the ZnO/Au/Al₂O₃-based PD,of which the size of the Au nanoparticles and the thickness of the Al₂O₃were 3–4 nm and 6 nm,respectively.The on/off ratio,UV sensing responsibility,and detectivity were 805,6.8 m A/W and 1.7×10⁹Jones,respectively,which were 8.5,8.6 and 8.5 times to that of ZnO/Al₂O₃-based PD without Au nanoparticle decoration(@light intensity of 0.6 m W/cm²).(5)Self-powered UV PDs based on ZnO/CuI/Au heterojunction were successfully fabricated.An ultra-high on/off ratio of 4250 and fast rise/decay times of 0.41/0.08 s have been demonstrated from the ZnO/CuI/Au PD.Moreover,the PD exhibits a high responsivity of 61.5 m A/W and detectivity of 1.7×10¹⁰Jones.The device displays long-term cyclic stability with 98.9%retention after 1000 cycles continuous test and superior environmental stability with 97.1%retention tested after 6 months.The synergistic effects of plasmonic Au nanoparticles and ZnO/CuI pn heterostructure play important roles in the enhancement of the photodetect ability.