| Due to the wide bandgap,high breakdown electric field strength and high saturation electron drift velocity,the third-generation semiconductor materials are suitable for hightemperature,high-frequency,radiation-resistant and high-power electronic devices.The representative GaN and SiC have been used in many fields such as semiconductor lighting,laser,radar and electronic power devices.However,with the continuous exploration and development of cutting-edge technologies such as 5G communications,new energy vehicles and aerospace,semiconductor materials are developing towards wider bandgap,higher breakdown field and lower energy consumption.In this context,gallium oxide(Ga2O3)has become one of the current research hotspots.Among the five isomers of Ga2O3 materials,monoclinic β-Ga2O3 has received much attention and research.β-Ga2O3 has a bandgap of about 4.9 eV,a theoretical breakdown electric field of up to 8 MV·cm-1,and has good UV transparency and excellent physicochemical stability.Therefore,it is considered to have great application potential in the fields of electronic power devices and solar blind UV detection.The preparation of high-quality single crystal epitaxial films and the effective control of their electrical properties are of great significance for the fabrication of β-Ga2O3-based highperformance devices.The preparation of single crystal β-Ga2O3 films can be done homo-or hetero-epitaxial methods.However,due to the low thermal conductivity of β-Ga2O3,the power devices prepared by using homoepitaxial films have the problem of poor heat dissipation performance,and the current single crystal β-Ga2O3 wafers are expensive.In contrast,heteroepitaxial growth of β-Ga2O3 films can effectively solve the heat dissipation problem of the device on the one hand,and can greatly reduce the cost on the other hand.In addition,the currently commonly used n-type doping elements Sn and Si have the problems that the ionic radius is quite different from that of Ga,the damage to β-Ga2O3 lattice is relatively large,and the doping effect is not ideal.Therefore,it is very necessary to find suitable substrate materials for β-Ga2O3 heteroepitaxy and new effective doping elements to improve the crystalline quality and doping efficiency of films.In this subject,we selected the metal organic chemical vapor deposition(MOCVD)film preparation process,and used SrTiO3 and KTaO3 single crystal wafers as substrates.First,the intrinsic β-Ga2O3 films were successfully heteroepitaxially grown,and the properties of the films and the effects of the preparation conditions were studied.Secondly,tantalum(Ta)dopedβ-Ga2O3 epitaxial films were prepared on the basis of the above research work,and the control effects of Ta doping on the electrical properties of the films were studied.Finally,Ta-doped βGa2O3-based solar-blind UV detectors were prepared on SrTiO3 substrates,and the effects of Ta doping concentration on the performance of the detectors were investigated.The main research contents of this paper are as follows:1.Ga2O3 films were deposited on SrTiO3(100)substrates by using MOCVD method.The films prepared at the substrate temperature of 650℃ were amorphous.Post-deposition annealing of the samples in atmosphere were performed at 900,1000 and 1100℃,respectively.High temperature annealing transformed the films into β-phase Ga2O3 films,among which the 1000℃-annealed film had the best crystalline quality and the composition was close to the stoichiometric Ga2O3.Further tructural analyses revealed that the film annealed at 1000℃ was a single crystal β-Ga2O3 epitaxial film.The out-of-plane and in-plane epitaxial relationships between the film and substrate were determined as β-Ga2O3(100)∥ SrTiO3(100)with βGa2O3[001]∥ SrTiO3<011>.The average transmittances of the samples annealed at 900,1000 and 1100℃ in the visible light range were 73.4%,74.1%and 69.8%,respectively.2.Ga2O3 films were prepared by MOCVD method on KTaO3(100)substrates at 750,800 and 850℃,respectively.The films grown at 850℃ were annealed in atmosphere at 900 and 1000℃,respectively.Structural analyses confirmed that the as-deposited film at 850℃ wasβ-Ga2O3 with high crystalline quality,and the crystalline quality did not change significantly after annealing at higher temperature.The epitaxial relationships between the film and substrate were β-Ga2O3(100)∥ KTaO3(100)with βGa2O3[001]∥ KTaO3<011>,and the microstructure at the growth interface was observed by HRTEM.In addition,the resistivity of the β-Ga2O3 films grown at 850℃ increased obviously after being annealed in atmosphere.The average transmittance in the visible range of the sample prepared at 850℃ was 73.6%,and the optical band gap of the film was about 4.79 eV.3.β-Ga2O3 films were grown on SrTiO3(100)substrates at different growth rates,and the substrate temperature was 850℃.The effects of growth rate on surface morphology,crystalline quality,chemical composition and optical properties of β-Ga2O3 films were investigated.When the thickness of the β-Ga2O3 films was the same,the surface roughness of the films increased from 6.96 nm to 8.41 nm with the growth rate increasing from 2.1 nm·min-1 to 10.1 nm·min-1,while the crystalline quality of the films decreased gradually.The β-Ga2O3 films exhibited good single crystal characteristics when the growth rate was less than 4.7 nm·min-1.Structural analyses identified that the epitaxial relationships between the film and the substrate were βGa2O3(100)∥ SrTiO3(100)with β-Ga2O3[001]∥ SrTiO3<011>,which was the same as that of the epitaxial β-Ga2O3 film obtained by annealing on SrTiO3 substrates.However,the film directly grown at 850 ℃ had higher crystalline quality.The increased growth rate resulted in more oxygen vacancies in the prepared β-Ga2O3 films,but the band gap and the average transmittance in the visible region did not change significantly.Moreover,the refractive index of the film in the wavelength range of 450-800 nm was calculated to be 2.25-2.02 according to the optical transmittance spectrum,which decreased with the increase of wavelength.4.According to the research results of the preparation conditions of intrinsic β-Ga2O3 heteroepitaxial films,β-Ga2O3 films with Ta doping concentrations of 0,0.2,0.6,1.0 and 1.4 at.%were prepared on SrTiO3(100)substrates by MOCVD equipment.XPS measurement indicated that Ta mainly existed in the β-Ga2O3 lattice as+5 valence state,and the measured doping concentrations were close to the experimental set values.All the prepared Ta-doped films were high quality β-Ga2O3 single crystal films.The crystalline quality decreased slightly with the increase of doping concetration,but the epitaxial relationships did not change with the doping of Ta element.The electrical properties of β-Ga2O3 films could be controlled effectively by Ta doping.The carrier concentration of the films gradually increased with the increase of doping concentration and tended to be saturated.The film with 0.2 at.%Ta doping concentration presented a carrier Hall mobility of 11.9 cm2·V-1·s-1,which gradually decreased as the Ta doping concertation increased.The introduction of a small amount of Ta made the resistivity of the films decreases rapidly,and reached the lowest value of 68.8 Ω·cm when the concentration was 1.0 at.%.Further increasing the doping concentration would make the resistivity start to rise.The doped β-Ga2O3 films could still maintain hight transparency,and the average transmittance in the visible region exceeded 95%.5.β-Ga2O3 epitaxial films with different Ta doping concentrations were prepared on KTaO3(100)substrates,and the effects of Ta doping concentration on the structure,morphology,electrical and optical properties of the films were studied.XRD results showed that the epitaxial relationships of the films were not changed by Ta doping.With the increase of doping concentration,the crystalline quality of the films only deteriorated slightly,but the surface roughness of the films gradually decreased.The band gap of the 1.4 at.%Ta-doped film was about 4.94 eV,and the films with different concentrations all had high optical transmittance.Hall effect measurement at room temperature determined that with the increase of Ta doping concentration,the carrier concentration of the films gradually increased from 3.85×1016 to 3.97×1018 cm-3,while the Hall mobility gradually decreased.The resistivity of the films first decreased and then increased,and reached the lowest value of 14.8 Ω·cm when the Ta doping concentration was 1.0 at.%.Compared with films grown on SrTiO3 substrates,the electrical properties of β-Ga2O3:Ta films on KTaO3 substrates had similar trends with Ta doping concentration,but the films had higher carrier concentration and lower mobility,and the corresponding resistivity was lower.6.Based on the β-Ga2O3:Ta films with doping concetrations of 0.5,1.0 and 1.5 at.%prepared on SrTiO3(100)substrates,solar-blind ultraviolet(UV)detectors with a metalsemiconductor-metal(MSM)structure were fabricated.The effects of Ta doping concentration on the UV light response of the devices were investigated,and the wavelengths of the UV light source were 254 and 222 nm,respectively.The research results showed that the prepared detectors exhibited response characteristics to both wavelengths of the UV light,and the responsivity was higher under 222 nm illumination.As the Ta doping concentration increased,the responsivity of the detectors increased significantly.For the 1.5 at.%Ta-doped UV detector,the responsivity under 222 nm UV light at 8 V bias reached 8.23 A·W-1,and the light-dark current ratio is close to 103.Moreover,the rise time(Tr1/Tr2)and decay time(Td1/Td2)of its response to continuous switching 222 nm UV light at 1 V bias were as short as 0.37 s/0.38 s and 0.41 s/0.85 s,respectively,which showed high response speed and good repeatability for the detection of UV light. |
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