Mechanism Of Defects On Ultraviolet Photoresponse Of Gallium Oxide Thin Film

Solar-blind ultraviolet detecting is a promising technique with low background noise,high detectivity and high sensitivity since it is difficult for the solar radiations with the wavelengthes below 280 nm to reach the Earth surface through the atmospheric.Hence,this technique has attracted a lot of attentions in the recent years.Moreover,without the employment of any optical filters,the solar-blind ultraviolet photodetectors based on wide-band-gap materials are more attractive.In contrast to Al Ga N,An MgO and so on,?-Ga₂O₃ has a band-gap of ⁴.9 Ev,which is naturally suitable for the solar-blind photodetection and makes the complicatedalloying process avoidable.In this thesis,molecular beam epitaxy was used for fabricating ?-Ga₂O₃ thin films grown on c-plane sapphire substrates,and some other auxiliary methods,including substrate pretreatment,fluorine doping,and comparative study on the gallium oxide film prepared by magnetron sputtering,were employed to tune the density of defects.Various methods for characterizing material properties were used,the photodetection characteristivs of solar-blind ultraviolet photodetectors were also measured.As a result,the impacting mechanism of defects on the solar-blind ultraviolet photosensitive properties of gallium-oxide thin-film was studied.In this paper,the works are divided into different parts:?1?The vaccum-annealing pretreatment on substrates was adopted under different temperatures,and the effect on solar-blind ultraviolet photosensitive properties of ?-Ga₂O₃ film was investigated.It was found that the high-temperature vacuum-annealing pretreatment on substrates can effectively reduce the density of defects in ?-Ga₂O₃ film and improve the conductivity of the material.This results in the significantly optimization on photocurrent,responsivity and detectivity.However,it also leads to an increase of dark current.In addition,the reduction of deep-level defects may cause the decrease of recombination centers,and thus prolong the decay time.?2?The fluorine doping was adopted to the thin films with different durations,and the effect on solar-blind ultraviolet photosensitive properties of ?-Ga₂O₃ film was investigated.It was found that the appropriate extent of fluorine doping can effectivel y reduce the density of oxygen vacancies in ?-Ga₂O₃ film so as to improve the various properties of the devices.However,excess fluorine doping may introduce some new types of defects,for example,fluorine interstitials,which will result in the deterioration on the solar-blind-ultraviolet photosensitive properties of ?-Ga₂O₃ thin-film.?3?The magnetron sputtering was used for preparing amorphous gallium-oxide films with higher density of defects,and both material characteristics and solar-blind-ultraviolet photosensitive properties were comparative studied between amorphous and nanocrystalline gallium-oxide films.It was found that the amorphous gallium-oxide films prepared by magnetron sputtering have different kinds of defects,which affect the properties of the solar-blind-ultraviolet photodetectors.For example,the defect-induced internal gain mechanism and extrinsic excitation lead to high responsivity and detectivity.Meanwhile,the abundant recombination centers,which was caused by the deep-level defects,can shorten the decay time of the devices.However,a large number of surface defects would make the barrier of metal-semiconductor contact easier to tunnel and accordingly cause an increase in dark current.In conclusion,different kinds of defects were detected in the amorphous gallium-oxide films,but they don't cause a negative impact to the device performance of solar-blind ultraviolet photodetector at all.In the contrary,some of the defects may even make the device perform better in certern respects.