Research On Amorphous Ga₂O₃-Based Optoelectronic Device Physics

As a new emerging ultra-wide bandgap oxide semiconductor material,amorphous gallium oxide（a-Ga₂O₃）owns a direct bandgap as high as 4.9～5.2 e V,right falling in the solar-blind UV spectrum region（200～280 nm）.The corresponding absorption coefficient larger than 10⁵ cm^-1 enables it an ideal candidate for solar-blind UV photodetection.More significantly,the high relative dielectric constant（～15）imparts dielectric strength to a-Ga2O3,which provides the potential application capability in electronic technologies.In this paper,a-Ga₂O₃ thin films were synthesized by radio-frequency magnetron sputtering technique at room temperature.A delicate control of the oxygen gas flow during the sputtering process was adopted to effectively modulate the oxygen vacancy（V_O）defect content.Three kinds of optoelectronic devices were thereafter designed and constructed for different purposes:an optoelectronic synapse with ultra-low energy consumption,a solar-blind UV detector with ultra-high detectivity and fast refresh rate,a non-volatile optoelectronic memory with low operate voltage,multi-state storage capability and robust adaptability.These three devices all take advantage of the regulation of a-Ga₂O₃ in optoelectronic properties by V_O,which fully demonstrates the wide application of a-Ga₂O₃ in optoelectronic devices.Some crucial issues regarding the defect science and device physics were explored with integrated optical and electrical technique,and further utilized to achieve the desirable device performance.The main research contents are as follows:（1）Based on the urgent demand of low power consumption in brain-inspired neuromorphic computing systems,design and construction of low-power a-Ga₂O₃optoelectronic synapse device are carried out in this paper.Metal-semiconductor-metal（MSM）two-terminal planar devices were first fabricated using V_O-rich a-Ga₂O₃ thin film and Au electrodes.Moreover,the basic synaptic functions such as short-term plasticity（STP）,paired pulse facilitation（PPF）and long-term plasticity（LTP）have been successfully simulated due to the V_O-induced persistent photoconductivity（PPC）effect.In addition,an image preprocessing function to suppress noise has also been successfully developed.Low dark current and high responsivity of a-Ga₂O₃ result in a total energy consumption of 136 f J,which basically meets the biological synapse（1～100 f J）levels.Further,through a combination of Kelvin probe force microscope（KPFM）characterization,temperature-dependent photoresponse measurements,polarity-dependent I-V tests and TCAD simulations,we found that the unique low-energy synaptic performance of the a-Ga₂O₃ based optoelectronic synapse comes from two crucial factors,i.e.,the higher deionization barrier of V_O and enhanced migration ability of V_O²⁺under the synergistic effect of optical and electric fields.Meanwhile,the a-Ga₂O₃ material also demonstrates an intriguing potential in low-power optoelectronic synapses.（2）Due to the well-known‘double-edged sword’role of V_O defect,the current solar-blind UV detectors can hardly meet the application requirements of high detectivity,high UV-Vis suppression ratio and fast refresh ability at the same time.Based on the in-depth understanding of the dynamic characteristics of V_O obtained in the optoelectronic synapse studies,we made a full use of both defect control and device design,and constructed a two-terminal planar a-Ga₂O₃ solar-blind UV detector based on a dual-active layer（V_O-rich/V_O-poor Ga₂O₃）structure.The design of the double-layer structure overcomes the respective disadvantages of the two single-layer structures and makes better use of their respective advantages.By optimizing various process parameters of films and device preparation,we finally obtained an ultra-high detectivity of 2.5×10¹⁸ Jones,which indicates that the detector has good weak-light detection capability.In addition,since the V_O-poor Ga₂O₃ layer hardly absorbs visible light,the detector also achieves an ultra-high UV-Vis rejection ratio of 10⁸.More importantly,based on the device working principle,we also developed a simple and effective working mode.That is,using a square wave signal source will alternately provide a new good Schottky contact barrier between Au/V_O-poor Ga₂O₃,which effectively eliminates the persistent photoconductivity（PPC）phenomenon commonly found in oxide semiconductors,enabling fast refresh rates.This unique experimental method can be hopefully extended to other oxide systems to achieve high-performance photodetectors with high detectivity and fast fresh rate.（3）Most of the ever-reported optoelectronic memory work illustrate some typical problems such as high operating voltage（>10 V）and poor adaptability.In this paper,a new non-volatile optoelectronic memory was firstly proposed by introducing a photosensitive dielectric（PSD）layer in the conventional cell structure of flash memory.In the new floating-gate architecture,we replaced the original tunneling dielectric layer and insulating dielectric layer with a conventional insulating dielectric layer and a-Ga₂O₃ PSD layer,respectively.Based on this unique design,the writing of electrons requires the application of a light pulse and a small negative gate voltage pulse（the maximum operating voltage used in this article is only-4.5 V）at the same time,while the erasing of electrons only requires the application of a light pulse.In addition,the new memory also demonstrated four implementations of multi-state storage based on this structure,and successfully implemented 76 different state storages.In particular,since this design does not depend on the selection and tailoring of channel materials,the structure can be transplanted to any transistor based on different channel materials to adapt to different application scenarios in principle.This work provides a new development direction for optoelectronic storage with low operating voltage and multi-state storage capability.