Preparation And Research Of Ga₂O₃ Single Crystal Films And Devices Based On β Phase And α Phase

Semiconductor gallium oxide(Ga2O3)has ultra-wide band gap about 4.8 eV and theoretical breakdown field as high as 8 MV/cm,and thus it has great potential in high breakdown field and high power electronic devices.Moreover,the ultraviolet absorption edge of Ga2O3 is about 258 nm,locating in the solar-blind ultraviolet region,thereby it has great potential in solar-blind photodetection.Ga2O3 can crystalize lin many phases,and the monoclinic β-phase is the thermal stablest one,as well as the most widely studied one.β-Ga2O3 films can be obtained by mechanical exfoliation or epitaxial growth.The crystal quality of mechanically exfoliated β-Ga2O3 single crystal thin films is as high as that of its mother bulk single crystal,but at present,the fieldeffect transistor(FET)devices based on them still have some problems,such as relatively high interface defect density and poor device stability,especially for the bottom-gate FETs with the channel transferred to the gate dielectric layer.For epitaxial β-Ga2O3 films,the quality of βGa2O3 films grown by the low-cost pulsed laser deposition(PLD)method is relatively poor,especially for those grown on low-cost substrates such as sapphire,and the performances of the related devices still need to be improved.Compared with β-Ga2O3,thermodynamic metastable phase α-Ga2O3 has better lattice matching with the corundum structure sapphire substrate,wider band gap(5.2～5.3 eV),higher breakdown field(10 MV/cm),and deeper UV absorption edge(234～239 nm).Therefore,it is expected that higher quality epitaxial α-Ga2O3 crystalline films can be obtained on low-cost sapphire heterogeneous substrates,and the device based on them are expected to achieve higher breakdown field and detect shorter wavelength solar-blind ultraviolet light.However,the preparation techniques to grow α-Ga2O3 films are not mature at present,and the researches on the devices based on α-Ga2O3 are just starting.To solve the above problems,this thesis work systematically studied the high-performance top-gate FETs with low defect state density based on the mechanical exfoliated,β-Ga2O3 single crystal thin films,PLD epitaxial β-Ga2O3 thin films and the high-performance photodetectors based on them,PLD epitaxial α-Ga2O3 thin films and the photodetectors based on them.This thesis work has made the following innovative achievements:1.Preparation and properties of high-performance top-gate β-Ga2O3 FETTop gate FETs with 50 nm Al2O3 or 200 nm SiO2 gate dielectric were prepared based on mechanical exfoliated β-Ga2O3 single crystal thin films.Compared with bottom gate FETs with the same channel layer,lower channel defect state density was achieved.The hysteresis of 200 nm SiO2 top gate FET decreased from 22.5 V to 0.9 V.The top-gate FET solves the problems of the high defect density and poor stability of bottom-gate FETs,and improves the mobility,threshold voltage,hysteresis,subthreshold swing,and temperature stability,etc,and provides theoretical and technical guidances for the fabrication of high-power FETs and highperformance photodetectors.2.PLD epitaxial high quality β-Ga2O3 thin films the high-performance photodetectors based on themPLD epitaxial(-201)single orientated β-Ga2O3 thin films were prepared on the c-plane sapphire substrate.The effects of in situ annealing temperature and time on the crystal quality,surface morphology,oxygen vacancy concentration,and photoelectric properties of the βGa2O3 films were systematically investigated.Based on the β-Ga2O3 films annealed at 1000℃for 30 min in situ,the MSM photodetector achieves an extremely high responsivity of 2185 A/W,an high external quantum efficiency of 1.1 × 106%,and a high-light-dark current ratio of 107 under the 254-nm solar blind ultraviolet irradiation.3.PLD epitaxial α-Ga2O3 thin films the photodetectors based on themPLD epitaxial single orientated metastable α-Ga2O3 thin films were prepared on the mface sapphire substrate.The effects of growth temperature and oxygen partial pressure on the crystal quality,surface morphology and grain size of the α-Ga2O3 films were systematically studied.The high-performance MSM photodetector was developed based on the optimized αGa2O3 single crystal thin films,and achieves high response of 36.7 A/W and high external quantum efficiency of 1.79×104%even with the 254-nm light source(the optimum wavelength of light source should be～235 nm).It provides a new and effective scheme for realizing single orientated α-Ga2O3 thin films by PLD and high performance α-Ga2O3 thin-film photodetectors.