Research On Amorphous Gallium Oxide Based Phototransistor And Imaging System

Deep ultraviolet(DUV)detection has demonstrated essential applications in missile warning and guidance,secure space communications,optical imaging,flame detection,and ozone hole monitoring due to its unique merits of low background noise and high sensitivity.In present commercial technology,the vacuum photomultiplier tube has been greatly limited due to its large size and low sensitivity;the Si-based photodiode needs to add multiple filters to eliminate the influence of non-ultraviolet light absorption caused by its small bandgap,which suffer from the high cost and difficulty of miniaturization.In recent years,DUV photodetectors(PDs)based on wide-bandgap semiconductors have been attracting increasing attention due to the advantages of direct photoresponse in DUV region,small size and simple operation.Among the ever-reported materials,amorphous gallium oxide(a-Ga₂O₃)is a desirable candidate due to its wide bandgap(4.5-5.0 eV),good chemical stability,high radiation hardness,low cost,and capability of large-area preparation at low temperature.However,the current a-Ga₂O₃ DUV PDs encounter some crucial problems in the material properties and device structures.Firstly,the deep-level oxygen vacancy(V_O)defects in a-Ga₂O₃seriously affect the photoelectric performance;Secondly,the most widely studied a-Ga₂O₃ metal-semiconductor-metal(MSM)structure DUV PDs cannot obtain high photoresponsivity and photoresponse speed simultaneously.Besides,as an essential application for DUV detection,there is no research about the a-Ga₂O₃ DUV array imaging system in the true sense.Based on above problems,this thesis mainly focuses on two major targets.Firstly,high-performance gate-controlled a-Ga₂O₃ DUV PDs have been fabricated by optimizing the device structure and regulating the oxygen flux during the material growth.Secondly,active-matrix(AM)a-Ga₂O₃ DUV imaging system driven by field-effect diodes(FEDs)has been fabricated through process design and improvement.In order to suppress the common long-time persistent photoconductivity(PPC)in two-terminal MSM PDs,three-terminal gate-controlled phototransistors has been fabricated.The main difficulty in fabricating three-terminal devices with multilayer is the patterning of the channel.Here,a novel method for stably etching a-Ga₂O₃ by using tetramethylammonium hydroxide(TMAH)solution has been developed with advantages of high selectivity,good pattern conformability with flat surface and positive trapezoid shape which is beneficial to the subsequent deposition of thin-film materials.Based on the etching method,a-Ga₂O₃ phototransistors with common-gate TFT structure on Si O₂/Si substrate have been fabricated.The devices with patterned channel layer exhibit typical n-type transistor's behavior,while the devices without patterned channel layer have a large gate leakage current,indicating the importance of a-Ga₂O₃ patterning.To further optimize the device's performance,the bottom-gate staggered phototransistors using high-k Al₂O₃ as gate insulator have been fabricated on quartz substrate with interdigitated source/drain electrodes.The devices exhibit excellent electrical and optical properties,including a high on/off of?10⁷,a high light-to-dark ratio of 5×10⁷,a high responsivity of 5.67×10³ A W^-1,and a high detectivity of1.87×10¹⁵ Jones.More importantly,the PPC phenomenon is effectively suppressed by applying a 20 V gate bias pulse,and the current decay time is greatly reduced from hundreds of seconds to 5 ms.It can be seen that the gate-controlled a-Ga₂O₃ DUV PDs achieve both high responsivity and high response speed simultaneously,which promotes the practical process of the a-Ga₂O₃ DUV detection.For the above phototransistors,to ensure sufficient carrier concentration,no O₂gas was introduced during the sputtering process of a-Ga₂O₃ channel layer,leading to plenty of V_O defects in the film,which causes the photoresponse in the near-visible spectrum region and hence deteriorates the UV/visible rejection ratio.Besides,the above phototransistors demonstrate a significant positive shift of the threshold voltage during the continuous gate voltage sweeping,which is not conducive to the device's stability.Therefore,an IGZO/a-Ga₂O₃ dual-active-layer(DAL)phototransistors has been proposed,where the IGZO ultra-thin insertion layer is supposed to effectively passivate the interface defects between the insulator and channel layer.Moreover,an appropriate amount of O₂ can be adopted during the sputtering of the top a-Ga₂O₃ layer.Thanks to the fewer Vo defects in the top a-Ga₂O₃ layer,the device shows more pronounced DUV photoresponse,and the UV/visible rejection ratio is significantly improved to 125.The devices exhibit not only stable and excellent TFT characteristics but also high responsibility,high photoresponse speed and high UV/visible rejection ratio,which greatly surpasses the device performance of a-Ga₂O₃ phototransistors with single active layer.Finally,the AM a-Ga₂O₃ DUV imaging system driven by FEDs has been designed and fabricated,where pixels with serially connected a-Ga₂O₃ PDs and FEDs have been realized on the quartz substrate through micro-fabrication design and optimization.Readout circuits and test systems have been constructed.By solving the problem of electrical crosstalk in passive matrix(PM)imaging systems,the integration and imaging contrast of the imaging system has been improved.This system demonstrates the actual application of two-dimensional DUV array imaging based on a-Ga₂O₃,providing a practical method for its potential development in this area.