Research On Ga₂O₃ Based Flexible Solar-blind UV Detectors And Ozone Sensors

As a new emerging wide bandgap semiconductor,gallium oxide(Ga₂O₃)has the advantages of ultra-wide direct band gap of 4.5?5.0 e V at room temperature,high absorption coefficient,good stability,simple preparation,and low cost,making it a natural deep ultraviolet(UV)detecting material,which can be potentially applied in a wide range of fields such as space communication,ultraviolet detection,biomedicine,and gas monitoring.Besides,as the deveploment of flexible optoelectronics,the demand for high-performance flexible photodetectors(PDs)is growing.However,present research on Ga₂O₃ UV PDs and gas sensors demonstrates that many parameters are urgently to be optimized.Specially,due to the strict temperature limits(usually(27)200°C)on the flexible polymer substrates,it is hardly to grow high-quality Ga₂O₃ films(which often require high-temperature growth above 800°C),thus restraining the device performance.In fact,achieving high photoresponsivity and high signal-to-noise ratio simultaneously is always big challenge for Ga₂O₃ materials and devices,where oxygen vacancies(V_O)play a key role.Meanwhile,the abundant V_Odefetcs in Ga₂O₃ material make it sensitive to the ozone gas.Combined with its outstanding UV detection behavior,it is expected to develop flexible Ga₂O₃-based ozone gas sensors working at room temperature.In present thesis,the research of defect engineering of Ga₂O₃ materials has been carried out through in-situ doping and post-annealing.Flexible UV PDs with high responsivity and high light-to-dark current ratio have been achived.Portable and inexpensive ozone gas sensors have been investigated with unique surface modification,which can work at room temperature.The main research achievements are as follows:(1)A flexible transparent?-Ga₂O₃ solar-blind UV PD with high performance is fabricated on mica substrate.A laminated amorphous Ga₂O₃/Ga/amorphous Ga₂O₃structure is thermally annealed at 1050?,forming a?-Ga₂O₃ film incorporated with Ga nanospheres.The PD based on the nanocomposite film shows a peak response at250 nm,an extremely low dark current of 0.6 p A at 10 V bias,a very high light-to-dark current ratio of 3×10⁶,and a fast recovery speed less than 50 ms.Robust flexibility is demonstrated in the bending tests with a radius of 8 mm and corresponding 10000 times fatigue test.The insertion of gallium interlayer and high-temperature post annealing are proposed to contribute to a better stoichiometry and lattice order of the?-Ga₂O₃ thin film,evidenced by the pronounced Raman peaks related with Ga _I(O_I)₂ and Ga _IO₄ vibration modes in?-phase Ga₂O₃.Compared with the room-temperature fabricated flexible amorphous Ga₂O₃ PD,the flexible?-Ga₂O₃PD on mica exhibits improved solar-blind UV photoresponse characteristics.(2)In-situ hydrogen doping to passivate defects in amorphous Ga₂O₃ on flexible PEN substrate is developed,where hydrogen flow is precisely controlled the during the magnetron sputtering process.A series of amorphous Ga₂O₃ films with different hydrogen content were prepared and confirmed by the combination of Neutron reflection and X-ray reflection measurements.The best device performs an ultra-high light-to-dark current ratio of more than 10⁷ and an ultra-high responsivity of 3.04×10⁴A/W,resulting in the capability to detect weak UV light with a power density of<10n W/cm².The flexible bending tests,10,000 fatigue tests,and stability tracking tests for more than 3 months suggest that the device owns good flexibility,robustness and stability,indicating great advantages and potential in practical applications.(3)By using In Ga Zn O(IGZO)decorated amorphous Ga₂O₃ thin films as the sensitive layer,an ozone gas sensor working at room temperature with the assistance of UV light has been realized.The gas sensing tests demonstrate that the sensing capability will improve and then degrade as the surface morphology of IGZO develops from dispersed particles to a continuous film.Finite difference time domain(FDTD)simulation results prove that IGZO nanoparticles can remarkably increase the surface density of photo-generated electrons,which clearly manifests a strong dependence on IGZO particle size and contributes to the boosted responsiveness.However,once IGZO particles coalesce into a thin film,the ozone sensor's responsivity starts to decrease even though the number of photo-generated carriers still increases.The smaller specific surface area of IGZO thin film is believed responsible for this phenomenon.The proposed ozone gas sensors own the merits of low cost,room-temperature detection,easy integration and mass production,which significantly expands the application fields of amorphous Ga₂O₃ thin film.In summary,flexible and low-cost Ga₂O₃ UV PDs and ozone gas sensors have been investigated with high performance,expanding the application area of Ga₂O₃materials even under various extreme environments.