Growth Of ε-Ga₂O₃ Film By Pulsed Laser Deposition And Research On Its Applications

Gallium oxide（Ga₂O₃）,as an emerging ultra-wide band semiconductor material,has drawn much attention due to its ultra-wide band and high theoretical critical breakdown electric field.The Baliga’s figure of merit value of Ga2O3 is 3000 times higher than that of Si,8 times higher than that of Si C,and 4 times higher than that of Ga N.As a result,gallium oxide electronic devices feature low on-state resistance and high breakdown voltage.In addition,the absorption cutoff wavelenngth of gallium oxide is located at 250nm and the UV transmittance is over 80%.Therefore,Ga₂O₃ is a natural material for solar-blind UV detector.The short response wavelength and fast response speed of gallium oxide optoelectronic devices make Ga₂O₃ widely used in defense and aerospace fields,such as radar,jamming,countermeasures,missile detection and wireless communication.In recent years,gallium oxide is becoming a hot research topic in semiconductor materials and devices.Ga₂O₃ possesses five polyforms,namelyα,β,γ,δ,andε（κ）.The current feasibility of fabricating large size and high qualityβ-Ga2O3 single crystals makes it attractive for homoepitaxial thin films and device applications.However,the anisotropy of monoclinicβ-Ga₂O₃ in terms of thermal,optical and electrical properties as well as the shortcomings ofβ-Ga2O3 bulk material susceptible to cleavage have led to the extended Ga2O3 study to other polyforms.In particular,orthorhombicε-Ga₂O₃ has attracted attention due to its high crystal symmetry,good match with Al₂O₃ substrate,good thermodynamic stability,and the only Ga₂O₃ polyform with spontaneous polarization.However,the synthesis,doping and application of orthorhombicε-Ga2O3 are still in the embryonic stage.Based on the above research background,this paper synthesized Sn-assisted orthorhombicε-Ga₂O₃ andε-(Al_xGa_1-x)₂O₃ thin films on C-plane sapphire substrate,and Sn-assisted orthorhombic n-typeε-Ga₂O₃:Zr thin films on C-plane Ga N substrate by pulsed laser deposition（PLD）.In this paper,the crystal structure,surface morphology,composition and optical（or electrical）properties of thin films were characterized to optimize deposition conditions ofε-Ga2O3,ε-（Al_xGa1-x）₂O3 and n-typeε-Ga2O3 films.Theε-Ga2O3 andε-（Alx Ga1-x）₂O3 based solar blind UV detectors,as well asε-Ga₂O₃-based Shotky diodes and p-Ga N/n-Ga₂O₃ heterojunction diodes are fabricated to verify the possibility ofε-Ga2O3 in photodetectors and power devices.The main research contents and results of this paper are as follows:1.The effects of PLD growth parameters（oxygen partial pressure and deposition temperature）on the quality of Sn-assisted orthorhombicε-Ga₂O₃ films grown on C-plane sapphire substrate are obtained and solar blind photodetectors are fabricated.Tin acts as a surfactant in the orthorhombicε-Ga₂O₃ film deposited by PLD（no free electrons are generated）to produce a certain amount of Sn layer on the film surface to stabilize the formation ofε-phase.The results show that the growth window of orthorhombicε-Ga2O3 is the oxygen pressure below 0.011mbar and the deposition temperature between 520℃and630℃.Therefore,when the deposition temperature is 570℃and the oxygen partial pressure is 0.011mbar,theε-Ga₂O₃ film possesses the largest grain size,the least grain boundary,and the best quality.The performance of solar blind photo detectors were compared based onε-Ga₂O₃ films grown at 520℃,570℃and 630℃.The MSM（Metal Semiconductor-Metal）ssolar blind photo detector grown at 570℃has a photo-to-dark current ratio of 2.9×10⁴,a responsiveness of 266A/W,and a detection of 3.18×10¹⁴Jones,showing good photoelectric response.Therefore,ε-Ga₂O₃ thin film has good DUV detection performance,which is expected to achieve high sensitivity and fast response of solar blind UV detector.2.The effects of PLD growth parameters（oxygen pressure,deposition temperature and trace Al components）on orthorhombicε-(Al_xGa_1-x)₂O₃ thin films withε-Ga₂O₃ buffer layer grown on C-plane sapphire substrate were studied and solar blind photo detector were prepared.With oxygen pressure increasing（0.006-0.03mbar）,the Al incorporation inε-(Al_xGa_1-x)₂O₃ films decreases,the thickness increases,and the optical bandgap decreases.The solar blind photodetectors based onε-（Alx Ga1-x）₂O3 films at different oxygen partial pressures were prepared.With the increment of oxygen partial pressures,the external quantum efficiency and cutoff wavelength ofε-（Alx Ga1-x）₂O3 photodetectors increased.Considering the non-stoichiometry and photodetector performance ofε-（Alx Ga1-x）₂O3 film,the optimal oxygen pressure ofε-（Alx Ga1-x）₂O3 film was selected as 0.011mbar.With the growth temperature increasing from 530℃to 610℃,the Al component ofε-(Al_xGa_1-x)₂O₃increases,the film thickness decreases,and the optical band gap increases from 5.11e V to5.18e V,but the crystal quality decreases.The solar blind photo detectors prepared onε-(Al_xGa_1-x)₂O₃ thin film with deposition temperature of 570℃and oxygen pressure of0.011mbar has a photo-to-dark current ratio of 103,a responsiveness of 1A/W and an external quantum efficiency of 1×10¹² Jones at a bias of 20V.With the increment of Al component（0at%-6at%）in the target material,the surface roughness,film thickness and band gap are decreased,but the oxygen vacancy density is increased,and the photo-to-dark current ratio,responsivity and detectivety ofε-(Al_xGa_1-x)₂O₃-based solar blind UV detector are reduced by two orders of magnitude.3.The effects of PLD growth parameters（deposition temperature and oxygen pressure）,on n-typeε-Ga2O3 film grown on C-plane Ga N substrates were studied.Schottky diodes were fabricated based onε-Ga2O3 film.The result presents that the growth window of orthorhombic n-typeε-Ga₂O₃ at deposition temperature 570℃and oxygen pressure0.011mbar is very narrow.Therefore,n-type orthorhombicε-Ga₂O₃ film grown at 570℃and0.012 mbar has the least defects.The Hall resistivity,mobility and carrier concentration of the n-type orthorhombicε-Ga₂O₃ film are 108Ω·cm,10.3 cm²/V·s and 5.5×10¹⁵cm^-3,respectively,proving that Zr acts as an effective donor impurity in n-type orthorhombicε-Ga₂O₃ film by Sn-assisted PLD.The effect of Zr doping inε-Ga₂O₃ films on the structure,surface morphology and composition were analyzed.The peak position of Zr dopedε-Ga2O3is moved to a lower angle than that of undopedε-Ga₂O₃,corresponding to a larger interplanar spacing and lower surface roughness.A lateral Schottky diode was fabricated onε-Ga2O3film and forward and breakdown characteristics were studied.The current on/off ratio of diode is around 10⁶～10⁷,and the breakdown voltage is 392 V,which verifies thatε-Ga₂O₃has potential in power device applications.4.The effects of PLD growth parameters（oxygen pressure and number of deposition pulses）on the growth of n-typeε-Ga₂O₃ thin films withε-Ga₂O₃ buffer layer and controlled carrier concentration on C-plane Ga N substrates are investigated,and p-Ga N/n-Ga₂O₃heterojunction diodes are fabricated onε-Ga₂O₃ thin films.The results show that the growth window of orthorhombicε-Ga2O3:Zr is expanded by growingε-Ga2O3 buffer layer on Ga N substrate.The growth conditions are oxygen pressure between 0.005-0.03 mbar and laser pulses between 3000-8000.Both the oxygen pressure and the number of laser pulses affect the Zr component in n-typeε-Ga2O3 thin films.The optimal deposition conditions ofε-Ga2O3:Zr thin films are as follows:0.012 mbar oxygen pressure and 7000 laser pulses,where theε-Ga2O3:Zr films have a minimum resistivity 2.4Ω·cm,a carrier concentration of1.6×10¹⁷ cm^-3,and the mobility of 17.4 cm²V^-1s^-1.In this paper,the n-typeε-Ga₂O₃ thin film was deposited on p-Ga N substrate and p-Ga N/n-Ga₂O₃ heterojunction diodes were fabricated.The rectifying performance of the p-n heterojunction diode is 10⁴-10⁵,and the reverse breakdown voltage is 954V,which means thatε-Ga₂O₃ has potential in power device applications.