Wavelength Tunable Ultraviolet Photodetector Based On Ga₂O₃/GaN Heterojunction

Ultraviolet photodetectors are widely used in the fields of local confidential communication,missile warning and tracking,flame detection and ozone detection.The band-gap of Ga₂O₃ material is about 4.5-5.3 eV（234-275 nm）,which is an ideal material for solar-blind UV detection.At the same time.Ga₂O₃ also has other excellent properties,such as:high stability,high temperature and radiation resistance,large breakdown electric field（～8MV/cm）and so on.Therefore,in recent years,Ga₂O₃ materials have attracted much attention in the fields of solar blind detection and power electronic devices.In addition to the excellent properties of Ga₂O₃,Ga₂O₃/GaN heterojunction also attract much attention due to its special band structure.For example,the bandgap of the Ga₂O₃/GaN heterojunction covers the solar-blind-near-UV band and can be used as broadband ultraviolet detection material.During my PhD period,ultraviolet photodetectors were investigated based on Ga₂O₃ materials and Ga₂O₃/GaN heterojunction materials.The main research work and novelties are listed as follows:Ga₂O₃/GaN heterojunction films were fabricated by high temperature thermal oxidation of GaN films and the MSM ultraviolet detector was prepared based on the Ga₂O₃/GaN heterojunction films.The detector exhibits wavelength tunable detectivity at different bias voltages.At low bias,the detector exhibits solar-blind band detection.This is mainly due to that at low bias,the depletion layer is mainly located in the Ga₂O₃ film,and only the photogenerated carriers of the Ga₂O₃ film layer can be effectively collected,so the device behaves as solar-blind band detector.As the bias increases,the depletion layer becomes larger and enters the GaN film,and the photogenerated carriers in both the Ga₂O₃ and GaN can be collected,so the device shows as Broad-UV band detector.When the bias increases further,the ultra-narrow band response due to the GaN free exciton was first discovered based on the Ga₂O₃/GaN heterojunction UV detector at room temperature.Experimental results show that with the increase of bias voltage,there is an obvious ultra-narrow response peak in the photocurrent spectrum.which is located at 363 nm and its half-height width is only～4 nm.Temperaturedependent PL.room temperature PLE and light absorption spectrum were adopted to investigate GaN and Ga₂O₃/GaN heterojunction.the ultra-narrow band response peaks originate from free excitons of GaN.Meanwhile,after high-temperature thermal oxidation,the exciton binding energy of GaN was increased,which is not easily thermal ionized at room temperature.However,at large bias voltage,free excitons in GaN are ionized into free carriers under high electric field.Meanwhile,due to the high valence band offset（1.4 eV）of the Ga₂O₃/GaN heterojunction,the holes are blocked at the Ga₂O₃/GaN heterojunction interface,so the device has a high internal gain.At 28 V bias,the responsivity of ultra-narrow band（363 nm）reaches to 2580 A/W.Further,Ga₂O₃/GaN micro-nanorods arrays with the core-shell structure were prepared by high-temperature thermal oxidation using Polystyrene Spheres（PS）as a mask.The MSM-type UV photodetectors were fabricated by using the formed Ga₂O₃/GaN core-shell micro-nanorod arrays.The responsivity of the device was improved significantly in comparison with the detector based on the planar Ga₂O₃/GaN heterojunction film.At 7 V bias,the PD based on Ga₂O₃/GaN nano-rods exhibits broad UV band response,the responsivity in the 280-370 nm band exceeded 1.5 × 104 A/W.and the peak responsivity of 2.65 × 104 A/W at 363 nm was achieved.We believe that the significant increase in responsivity is mainly due to the large specific surface area of the Ga₂O₃/GaN micro-nanopillar array,which prevents the collection of holes in both Ga₂O₃ and GaN,resulting in large internal gain.Specifically,the larger specific surface area is conducive to the reaction of O₂ in the air with the electrons on the surface of Ga₂O₃,resulting in a band upward bending near the surface of Ga₂O₃.Under the UV-C irradiation,the photogenerated holes in the Ga₂O₃ migrate to the surface of Ga₂O₃,so that the photogenerated holes in the Ga₂O₃ are difficult to be collected by electrode.When irradiated by UV-A light,the photogenerated holes in the GaN will converge on the Ga₂O₃/GaN heterojunction interface due to the capacitance effect.Holes in these areas are also difficult to collect with the electrodes.These mechanisms significantly improve the internal gain of device.In general,the detector based on Ga₂O₃/GaN micronanorods arrays can achieve weak light detection at low bias voltage.In order to improve the detection performance of UV-C,Rh nanoparticles was modified on the surface of the device.Our results showed that the dark current of the device decreased from 6.5 × 10-10 A to 3.1×10^-11 A after devices being modified with Rh nanoparticles.Meanwhile,the responsivity of the device in the solar blind wavelength range（220-280 nm）increased,and its peak responsivity increased from 1 A/W to 2.76 A/W.However,when the wavelength was larger than 290 nm,the responsivity was suppressed.Therefore,the device has a higher rejection ratio between UV-A and UV-B,which improves the device UV-band selectivity.In addition,the response speed（T_r/T_d）of the device has decreased from 3.1 8 ms/3.66 ms to 1.76 ms/0.8 ms.Further research showed that the local surface plasmon effect caused by the coupling of Rh nanoparticle and UV-C is the reason for the increased responsivity of the device.Meanwhile,the localized Schottky barrier was formed between the Rh nanoparticles and the Ga₂O₃ film,thus a depletion layer on the surface of the Ga₂O₃ films was formed,leading to a decreased device dark current.These results show that the utilization of localized surface plasmon effects of Rh nanoparticles is an alternative method to improve the performance of Ga₂O₃/GaN solar-blind detectors.