Research On Photoelectric Properties Of Amorphous Oxide Semiconductor Active Matrix

The solar-blind ultraviolet photodetectors（SBUPDs）have a lot of natural advantages such as low spatial background noise and strong ability to resist environmental interference.Therefore,the large-area solar-blind ultraviolet imaging system plays a very important role in applications like Face Recognition,Intelligent Vehicle Glass,Real-time Imaging,and further Virtual Reality（VR）and Augmented Reality（AR）and so on.Although there are many researches about imaging,the large-area real-time solar-blind ultraviolet imaging system is still absent.If the devices are vertically integrated,it’s inevitable to use insulating layer,which hasn’t been realized with no damages on photosensitivity of the PDs,between different functional areas.Therefore,this work is mainly about plane integration,meaning photosensitive sensors and switchers are electrically interconnected in the same plane.Meanwhile,this work’s diodes are gate-drain shorted of the thin film transistors（TFTs）as the switching devices,which simplifies the complexity of the interconnection lines and increases the density of integration.The core functional materials are amorphous gallium oxide（a-Ga₂O₃）and amorphous indium gallium zinc oxide（a-IGZO）.High-performance SBUPDs are fabricated by using the heterojunction between the two materials;field effect diodes（FEDs）operating at low work voltage are fabricated by using a-IGZO homojunctions,and active matrices are fabricated respectively.Firstly,the a-IGZO/a-Ga₂O₃ dual-active-layer solar-blind UV phototransistors,with bottom-gate interdigital structure,are successfully formed.The carriers,provided by low-resistance a-IGZO,generate a conductive channel at the interface between gate insulator and active channel layer and at the same time passivate the defect states here,which greatly improves the device stability.Eventually,the phototransistor with a switch ratio of～10⁸ and a subthreshold swing（SS）of 0.36 V/dec is obtained.After testing the further manufactured（2×3）active matrix,it is found that there are still interferences between different pixels.The performance of the switcher was tested under the condition that the photodetector is fully opened.It’s obvious that it does not have switching characteristics,so it could not achieve the expected function.After post-annealing treatment,the reverse current（I_R）of the FED can be reduced by 10⁴ orders of magnitude at 200℃/30 min in the atmosphere.Combined with the simulation results of TCAD,there’s a close relationship between the drain current value（I_D）of TFT at V_GS=0 V and I_R of the corresponding FED.Because the electron well at a-IGZO/a-Ga₂O₃ interface in the heterojunction provides a large number of electrons,which not only stabilizing the device performance,but also making I_D at V_GS=0 V too high.Therefore,the channel layer material is changed to a-IGZO/a-IGZO to convert the depletion device into an enhanced device,and finally the I_R of the FED is reduced from 10^-6 to 10^-8.Meantime,the enhancement-mode provides the FED to operate under ultralow voltage.Combined with post-annealing treatment,the properties of TFT and FED are optimized to some extent,I_Rof FED is further reduced to 10^-11.However,the problem of instability of the device has not been solved.When a（2×2）active matrix is obtained,the instability will be amplified through the whole.The device requires higher operating voltage to drive the active matrix normally.The instability of a-IGZO/a-IGZO ultraviolet photodetector is caused by the trap-states at the interface between gate oxide layer and channel layer generated during magnetron sputtering process.From the perspective of film growth,the density of trap-states caused by sputtering damage can be reduced by reducing the growth time of low resistance layer;from the perspective of structural parameters,reducing the W/L ratio of the device can lower the probability of carrier captured in the defect state.Unfortunately,the stability of the device has not been significantly improved through the two methods.Based on the reset effect of gate pulse voltage on TFT,introducing squarewave signal into the testing process of FED can eliminate the instability during continuous measurement.However,it will return to the initial state after minutes,which limits the practical application of squarewave signal.