Study On Heterogeneous Wide Band-gap Semiconductor Ultraviolet Photodetectors Based On Self-depleting Effect

Ultraviolet?UV?photodetectors with high performance have been widely applied in the fields of military,environment,medical and so on.With the development of semiconductor materials and device fabrication technology,wide band-gap semiconductor UV photodetectors have become a research hotspot in the field of UV detection.Wide band-gap materials do not absorb visible light,which effectively overcomes the shortcoming of narrow band-gap semiconductor UV photodetectors based on Si and GaAs that need to work with filters.And there are many kinds of wide band-gap materials,especially including some oxide materials,such as ZnO,TiO₂,Ga₂O₃,NiO and so on.These materials have stable properties,various preparation methods and low cost,possessing important research significance.However,some inherent drawbacks of wide band-gap materials,such as low carrier mobility,short exciton lifetime,and a large number of trap and defect states in oxide materials,lead to inadequate device performance.With the development of science and technology,requirements for performance parameters of UV photodetectors are constantly increasing in various application fields,such as higher light current,lower dark current and faster response-recovery speed.Among them,it is very important to reduce the dark current of devices,which contributes to the improved signal-to-noise ratio and detectivity,as well as lowered light intensity detection,resulting in wider application prospects for devices.To suppress the dark current of device,the depleting effect has been proved as a reasonable and effective method.In this work,photoconductive heterogeneous composites and devices were fabricated.The dark current and some other performance of devices were optimized by incorporating majority carrier self-depleting effect.The following research works were carried out:At first,TiO₂/NiO heterogeneous composite films UV photodetectors based on dark self-depleting effect were designed and fabricated.The TiO₂ film was fabricated by sol-gel method.NiO film and Ni/Au alloy electrodes of device were prepared simultaneously by a unique oxidation method.Compared with devices based on single material,the dark current and noise of composite films devices were significantly reduced.In dark,composite films are almost completely depleted due to the self-depleting effect generated by the PN heterostructure,thus the majority carrier density in the bulk of composite films are effectively decreased,and the device shows a higher resistance state.At the bias of 6 V,the dark current was only 0.033 nA,which is two orders of magnitude lower than that of devices with single film.Under UV illumination,the built-in electric field near the interface of heterostructure is balanced due to the dissociation and accumulation of photogenerated charge carrier,and the self-depleting effect is offset.This moment,the composite films returns to the high conductivity state and the devices have sufficient light response,ultimately obtaining a higher detectivity of 1.56×10¹⁴ Jones.The optimization process of device performance and the detailed analysis of self-depleting effect are given in Chapter 2 of this thesis.In Chapter 3,the research of UV photodetectors based on ZnO were carried out.Because of the high exciton binding energy and a large number of surface defects of ZnO,the responsivity of ZnO devices is insufficient and the dark current is high.Here,the self-depleting effect in localized heterostructures was incorporated to improve the device performance.Firstly,the annealing-free N-type ZnO nanoparticles were prepared by solution method,and then different P-type materials were doped into ZnO films respectively,uniformly distributing in the mixed films and forming localized heterostructures.In dark,the electron density in ZnO is effectively decreased due to the formation of majority carrier depletion region generated by the heterostructure,which reduces the electron transport capacity of ZnO film,thus decreasing the dark current of device.Under UV illumination,the excitons generated in localized heterostructure are dissociated faster and more effectively by the built-in electric field,which reduces the excitons direct recombination probability in ZnO,thus minimizing the charge loss.At a certain light intensity,the self-depleting effect can be completely offset,and the heterostructure devices exhibit higher responsivity.In Chapter 4,the N-type TiO₂ nanowires array was firstly fabricated on FTO glass substrate by hydrothermal method.Then,the P-type organic wide band-gap material N,N?-Di?2-naphthyl-N,N?-diphenyl?-1,1?-biphenyl-4,4?-diamine?NPB?was filled into the gaps of nanowires array by static deposition and dynamic solvent cleaning,and the heterogeneous one-dimensional photoconductive devices based on the organic/inorganic hybrid were fabricated.For most one-dimensional wide band-gap oxide materials,their special nanostructures are conducive to the more efficient carrier transport,which is beneficial for the higher responsivity and faster response/recovery speed of fabricated devices.However,because of a large number of defects in one-dimensional oxide materials,the majority carrier density in these materials is large,which leads to a high dark current of device.Here,the TiO₂/NPB composite heterostructure was constructed,which effectively reduces the free electron density by self-depleting effect in dark,leading to the decreased dark current.Under UV illumination,the excitons generated in the TiO₂/NPB heterostructure will be dissociated under the built-in electric field,and then more photogenerated electrons will flow to TiO₂,while the built-in electric field is balanced,and the self-depleting effect is offset,thus ensuring that the TiO₂ nanowires have high photoconductivity.Ultimately,the performance of devices in dark and under illumination is effectively improved.In this thesis,a variety of composite photoconductive UV photodetectors were fabricated and studied.Some performance in dark and under UV illumination including dark current,responsivity,and detectivity were significantly improved by forming heterostructures to generate majority carrier self-depleting effect and other mechanisms.This thesis provides valuable references for material selection,structure design and working mechanism analysis of wide band-gap semiconductor UV photodetectors.