Research On Gallium Oxide Based Photodetectors

In recent years,Ga₂O₃ as a new wide bandgap oxide semiconductor material has been attracting great attention of researchers.Besides a direct bandgap up to 4.5⁵ eV and a high absorption coefficient,Ga₂O₃ also owns many advantages such as good stability,easy preparation,and high radiation resistance etc.,which makes it an ideal candidate for solar-blind ultraviolet?SB-UV?and high-energy radiation detection.Due to the significantly low background noise and high sensitive detection ability,SB-UV detectors can be widely used in space communications,biomedicine,missile guidance,and ozone monitoring etc.It has always been a researh focus in the photodetection field.On the othe hand,high-energy radiation detection has similarly extensive and important applications in medical imaging,safety detection in public places,X-ray space communication,and material microstructure analysis etc.At present,Ga₂O₃-based photodetectors?PDs?are still in the laboratory research stage,and it has a long way to go before its commercialization.There are two key scientific issues that need to be solved for Ga₂O₃ PD:one is to enhance the photoresponsivity and the other is to speed up the photoresponse speed.In addition,the rapid development of flexible and transparent electronics has greatly promoted the research of flexible transparent PDs.Flexible devices require more stringent process temperatures?<200°C?.However,current growth techniques for Ga₂O₃ materials?such as molecular beam epitaxy,pulsed laser deposition,and metalorganic chemical vapor deposition?usually require high temperature and high vacuum conditions,which is not only costly in large-scale industrial productions,but also impossible to achieve flexible device applications,thus seriously hindering its research progress in the field of flexible PDs.Based on the above problems,this thesis focused on improving the device's performance from the aspects of increasing the photoresponsivity and promoting the photoresponse speed.Through the design of device structure and material's properties,the device performance has been greatly enhanced.On this basis,high-performance flexible Ga₂O₃ PDs with excellent mechanical robustness were fabricated using the low-temperature?LT?process.Firstly,in order to improve the responsivity of Ga₂O₃ SB-UV PD,we adopted the surface plasmon?SP?resonance effect which is widely used in various optoelectronic research.Lots of literatures indicate that the energy of SP supported by metal Ga and Al nanoarrays may fall into the SB-UV region.Therefore,we studied the important influence of the resonant coupling between these two metals and Ga₂O₃ on the PDs'performances.PD decorated with Ga particles was fabricated based on Ga/Ga₂O₃nanocomposite film,in which Ga nanospheres were embedded in Ga₂O₃ matrix with a three-dimensional contact mode.Compared with the pure Ga₂O₃ PD,the responsivity of SP coupled Ga/Ga₂O₃ PD can be enhanced by up to 220 times through adjusting the size of Ga nanospheres.The Al particle modified device adopted usual surface-point contact configuration,and the photocurrent was increased by one order of magnitude.Thus,it demonstrates that SP effect is an effective method to improve the photoresponse performance of Ga₂O₃ SB-UV PDs.Secondly,we developed an amorphous Ga₂O₃?a-Ga₂O₃?SB-UV PD with ultrafast response speed and systematically studied the main mechanism of the device's performance improvement by using photoelectric performance testing and X-ray photoelectron spectroscopy?XPS?characterization.Extensive scientific research proves that persistent photoconductivity phenomenon in oxide semiconductors is related to oxygen vacancy?V_O?defect in the samples.Based on this premise,we developed a method of controlling the flow of oxygen.Through the micro-adjustment of the oxygen flow during the sputtering process,a series of a-Ga₂O₃ films with different V_O concentrations were obtained.The photoresponse performance of the corresponding devices indicates that the V_O concentration plays a crucial role in determining the detection properties.By increasing the oxygen flux from 0 to 0.14 sccm,the dark current at 10V droppes drastically from 1.97×10^-7 A to 5.71×10^-13 A,the responsivity decreases from 91.88 A/W to 0.19 A/W,and the response speed increases from 1.5 s to 19.1?s.Combining XPS valence band spectra,dark current fitting and analysis of ITO/Ga₂O₃ energy band structure,we further reveal the intrinsic mechanism,that is,increasing the sputtering oxygen flux not only reduces the V_O concentration in the film,but also increases the contact barrier height of the device,which greatly improves the photoresponse speed.Then,based on the LT process of a-Ga₂O₃ PD and the oxygen flux tuning method,we successfully prepared a fast-response flexible Ga₂O₃ SB-UV PD on polymer substrate.The photoelectric properties of the flexible devices are basically the same as those of the rigid devices,which proves that the fabrication technology of a-Ga₂O₃ PD has good process compatibility with flexible substrate.In addition,there is no obvious degradation of the performances when flexible device is under bending and fatigue tests,demonstrating the a-Ga₂O₃ PD has good mechanical robustness.Lastly,we explored the application prospects of a-Ga₂O₃ material in flexible X-ray detectors.By systematically studying the influence of sputtering oxygen partial pressure on the X-ray response characteristics,we deeply analyzed its intrinsic mechanism,combined with theoretical analysis,and gave the corresponding physical model.The flexible a-Ga₂O₃ X-ray detector in this thesis not only has excellent high pressure and radiation resistance,but also exhibits good mechanical properties.In summary,in view of Ga₂O₃ SB-UV PDs,we improved both of the devices'responsivity and response speed,and we sucessfully developed flexible a-Ga₂O₃ PDs with good mechanical properties through LT process.In addition,we also fabricated a-Ga₂O₃ flexible X-ray detectors,proves the high radiation resistance of a-Ga₂O₃ and opens up a new way for the development of flexible X-ray detectors.