Study On The Single Crystal Growth,Process And Properties Of β-Ga₂O₃

Semiconducting technology plays an irreplaceable role in modern information society.With the development of technology,the demand for high-performance electronic devices and deep UV optoelectronic devices with high voltage,low loss,high temperature and radiation resistance are becoming more and more urgent.At present,silicon based power devices have gradually reached their theoretical limit.With the development of third-generation wide-bandgap semiconductor materials and devices represented by SiC and GaN,the devices performance have gradually approached or surpassed that of silicon based power devices.However,because of the difficulty in large crystal growth and high cost of the bulk SiC and GaN,the developments of their industrial application are seriously restricted.Therefore,new type wide-bandgap semiconductors with good properties and low cost still remain to be explored.The bandgap of β-Ga₂O₃ is as large as 4.7 eV,as a new type of ultra-wide bandgap semiconductor,it has the advantages of larger bandgap,higher breakdown field,bigger Baliga FOM,shorter absorption edge and lower cost,compared to the third-generation semiconductors.Therefore,β-Ga₂O₃ may become a preferred material for high voltage,high power,low loss power devices and deep UV optoelectronic devices.In recent years,the basic research and industrialization of β-Ga₂O₃ crystal and its devices show a significant acceleration.The size of single crystals is expanding,the quality of epitaxial films is improving,and the performance of devices is constantly refreshing.In 2015 and 2017,the first and the second "International Workshop on Gallium Oxide and Related Materials" were held in Japan and Italy,respectively.The United States,Japan,Germany and other countries have also set up related research projects,which fully embodies the important research value and good development prospects of β-Ga₂O₃.However,as a new semiconductor material,the research of β-Ga₂O₃ is still in the early stage.The crystal quality,basic physical properties,physical properties optimization,devices performance optimization and applications are still need be systematically studied.The acquisition of high quality β-Ga₂O₃ single crystals is the precondition for the devices fabrication.In this thesis,we focus on the crystal growth of high quality β-Ga₂O₃ to meet the requirements of different devices.The crystal growth methods and equipment modification,crystal growth process optimization,characterization of crystal quality,crystal wafer processing and devices performance optimization are systematically studied.At the same time,the electrical properties optimization by elements doping,energy band engineering are explored.Furthermore,we also explored the new applications of transition metal ion doped β-Ga₂O₃ crystal in laser field.The main contents and results in this thesis are as follows:Ⅰ.Crystal growth and process optimization of β-Ga₂O₃ single crystalThe growth of β-Ga₂O₃ single crystals were studied by Czochralski and EFG method.The advantages and disadvantages of the two methods were compared.Later,high quality β-Ga₂O₃ single crystals were grown by EFG method.According the technical features of EFG method,we focused on the aspects of thermal field control at the top of Ir die,volatilization and decomposition inhibition of gallium oxide,seed crystal optimizing,shoulder control and growth atmosphere etc.to optimize the crystal growth process.The high quality β-Ga₂O₃ single crystal was obtained by the improved EFG crystal growth furnace.There was no twins,grains and boundary in the as grown crystal,which was demonstrated by X-ray Laue back scattering and high resolution X-ray Φ scan.The FWHM of（100）plane rocking curve was less than 50 arcsec,which proved that crystal quality is very good.Ⅱ.Physical properties characterization of β-Ga₂O₃The anisotropic physical properties of β-Ga₂O₃ were systematically characterized with high quality single crystal,which would provide basic parameters for the applications.The hardness values of β-Ga₂O₃ single crystal along a*,b and c*directions were 6.39,5.87 and 6.82;with the refractive index at 632.8 nm being 1.9027,1.9405,1.9304.The thermal conductivities of the crystal along a*,b,c*directions decreased from 14.9,27.9 and 17.9 W·m-1·K-1 at room temperature to 6.0,11.3,7.4 W·m·K-1 at 500℃.Through data fitting under the exponential coordinate system,we could find that the thermal transport of β-Ga₂O₃ single crystal is mainly depend on the phonons.The UV cut-off edge of（100）,（010）,（001）planes were measured to be 262.2 nm,270.8 nm,262.2 nm and the optical bandgap were fitted to be 4.70 eV,4.55 eV,4.70 eV respectively.The polarization transmission spectrum of crystals with different polarization direction were measured and the anisotropic of UV cutoff edges was analyzed.Among them,the cutoff edges of E//a,E//b and E//c were 270 nm,261.8 nm and 276.1 nm which explains the phenomenon that there was a platform in the UV transmission spectrum of（100）plane.The maximum phonon energy of β-Ga₂O₃ was found to be 767,8 cm-1 through the Raman spectra.The carrier concentration,carrier mobility and electrical resistivity of the as grown crystal were measured to be 3.92×1016 cm-3,107 cm2·V-1·s-1,1.49 Ω·cm by Hall effect measurement.Through the GDMS measurement,we found that the source of donor in unintentional doped β-Ga₂O₃ crystal was the Si impurity from the raw material.III.Single crystal growth and physical properties characterization ofβ-（AlxGa1-x）2O3The band engineering was carried out to enrich the material system of β-Ga₂O₃.β-（AlxGa1-x）2O3（named β-AlGaO）single crystals with larger bandgap and Si:β-AlGaO single crystal were obtained by EFG method.The segregation coefficient of A1 in β-（AlxGa1-x）2O3 was measured to be 1.01.The thermal diffusion coefficients of β-AlGaO（Al=2.22 at.%）depended on the temperature were tested.At room temperature,the thermal diffusivity coefficients was 6.00 mm2·s-1 and decreased to 1.53 mm2·s-1 at 500℃.The thermal transport ability decreased obviously compared with pure β-Ga₂O₃ crystal.The relationship of UV transmittance spectra,Raman spectra,bandgap changes with the concentration of Al in β-AlGaO was analyzed.The anisotropy of UV transmittance spectra and optical bandgap of β-AlGaO crystal were studied.When the concentration of A1 was 4.01 at.%,the optical bandgap of（100）plane was 4.9 eV.The optical bandgap of E//c and E//b were 4.64 eV and 4.91 eV,respectively.The carrier concentration,carrier mobility,electrical resistivity were measured to be 4.7×1018 cm-3,13.4 cm2·V-1·s-1 and 0.099 Ω·cm by Hall effect measurement.The conductivity of the β-AlGaO crystal was effectively enhanced by doing Si ions.IV.Research on the processing of β-Ga₂O₃ and surface modificationThe problems of cracking and side damage in the crystal process were solved by optimizing the cutting technology.Mechanical exfoliation was used to obtain substrates and the problem of crystal cracking in traditional grinding and polishing process was avoid by method avoided.Ultra-smooth substrates were obtained with the surface roughness less than 0.1 nm.Hundreds of pieces substrate were provided for many research institutes and the development of β-Ga₂O₃ thin films epitaxy and related devices were promoted.The influence of different atmospheres on the properties of β-Ga₂O₃ and β-AlGaO substrate was studied through systematic atmosphere annealing,and the mechanism of surface resistivity change was preliminarily analyzed.The effect of annealing in oxygen or hydrogen atmospheres on infrared transmission rate and the carrier concentration is negligible to high resistivity pure β-Ga₂O₃.The infrared transmission rate of β-Ga₂O₃ with high carrier concentration could be effectively improved by annealing in oxygen at 1150℃.At the same time,The C-V test proved that the carrier concentration of crystal surface is greatly reduced,even become semi-insulating.However,there is negligible carrier concentration changes in the interior part of the wafer.Combined with the analysis of the surface and interior impurities concentrations of the wafer,we believed that the main reason for the decrease of carrier concentration is that the donor impurities lost the ability of providing free electrons after annealing in oxygen.The effect of oxygen annealing on the reduction of carrier concentration is irreversible.The carrier concentration can not be restored to the original level by annealing in inert atmosphere and reducing atmosphere.In addition,we found that the surface roughness of the wafer could be effectively reduced by annealing in oxygen at appropriate temperature.V.β-Ga₂O₃ and β-AlGaO based solar-blind photodetector and SBDThe ultimate goal of new material is to be applied in semiconductor devices.Based on the high quality β-Ga₂O₃ and β-AlGaO single crystal wafers,we have carried out the study of UV detector and Schottky diode.In the respect of UV detection,the selective response to the "solar blind" band is realized based on the simple MSM structure.The detector response wavelength band was 220 nm-280 nm with the maximum responsivity at 250 nm.The response time of Tr was 4.4 s,and the attenuation time of Td was 0.14 s.In order to optimize the device performances,we fabricated a new MSM detector on the Ti³⁺:β-Ga₂O₃ with high electrical resistivity.The device response time and decay time were 0.6 s and 0.12 s,respectively.The performance of the "solar blind" detector was improved obviously and was also comparable or even better than the reported MSM type detectors.In the respect of Schottky diode,we optimized the performance of Pt/β-Ga₂O₃ by controlling carrier concentration of the crystal.The Schottky barrier height（Φb）and on resistance（Ron）of the device based on the high electrical resistance substrate（carrier concentration=2.4×1014 cm-3）were 1.39 eV and 12.5 mΩ·cm2,respectively.The performances of device based on low electrical resistance substrate（carrier concentration=2×1017 cm-3）were promoted.The Rop was reduced to 2.9 mΩ·cm2 and the current density at 2 V was increased from 56 A/cm2 to 421 A/cm2.The performance of the device was at the advanced level in the world.A new type Schottky diode of Pt//β-AlGaO was designed and its performance was preliminarily studied.The ideal factor of the device was 1.1,the Schottky barrier height was 1.35 eV,the threshold voltage was 1.18 V,the on resistance was only 2.05 mΩ·cm2,and the current density at 2 V reached to 400 A/cm2 and the performance was improved obviously.The Ron was smaller than the reported devices based on β-Ga₂O₃ or β-AlGaO.The excellent performances of the devices fully demonstrated the potential application of β-Ga₂O₃ and β-AlGaO in power devices,and also proved that the quality of our single crystal substrates was very high.VI.Crystal growth of transition metal ions doped β-Ga₂O₃ and new application explorationThe optical and electrical properties were optimized by doping the transition metal ions of Cr³⁺,Cr4+,Co²⁺and Ti³⁺,meanwhile,the applications of β-Ga₂O₃ were expanded to solid-state laser area.The crystal of Cr³⁺:β-Ga₂O₃ and Cr4+:β-Ga₂O₃ were grown by EFG method and the optical properties of there two crystals were compared.The passive modulation of 1064 nm laser was realized with the Cr4+:β-Ga₂O₃ as a saturable absorber.The laser output of 50 mW,repetition rate of 421.5 kHz and pulse width of 235.2 ns were achieved.Theβ-Ga₂O₃ crystal was successfully demonstrated as a laser element for the first time.The crystal of Co²⁺:β-Ga₂O₃ was obtained for the first time.By co-doping with Si4+ions,the problem of crystal quality reduction caused by different valence doping was solved.The characteristic broadband absorption peaks of Co²⁺ ions were found at 1173 nm and 1588 nm with the Co²⁺:β-Ga₂O₃ crystal.The passive modulation of 1341 nm laser was realized by using Co²⁺:β-Ga₂O₃ crystal as a saturable absorber with the highest output power of 35 mW,repetition rate of 181 kHz,pulse width of 280 ns.The electrical resistivity of Co²⁺:β-Ga₂O₃ at room temperature was measured to be 1.4×1010Ω·cm by the equipment of COREMA-VT and could be used as a semi-insulating substrate.Bulk single crystals of Ti³⁺:β-Ga₂O₃ were grown by EFG method,and the thermal properties,anisotropic optical properties were characterized in detail.The Cp of Ti³⁺:β-Ga₂O₃ was measured to be 0.483 J·g-1·K-1（20℃）,the thermal diffusion coefficient of b axis was 8.427 mm2/s and the thermal conductivity was 24.7 W·m-1·K-1（25.5℃）.The thermal conductivity has obvious advantages compared with the other common laser crystals.The anisotropy of the absorption and emission cross section was obvious.There was a broadband absorption peak caused by the typical 2T2→2E transition of Ti³⁺.The maximum absorption cross section of E//c could be 3.58×10-20 cm2 at 518 nn.The fluorescence was typical of Ti³⁺ transition 2E→2T2 and covers the wavelength range between 650 and 900 nm with the maximum emission intensity at 730 nm and the FWHM was about 140 nm.It was worth noting that the fluorescence lifetime of Ti³⁺:β-Ga₂O₃ was as large as 176±5μs,which was 50 times longer than that of Ti³⁺:Al2O3.Therefore,it was expected to become an ultrafast laser gain medium with excellent comprehensive properties.