Study On Crystal Growth,Defects And Properties Of Cylindrical β-Ga₂O₃ By EFG Method

With the rapid development of energy,information,national defense,rail transit,electric vehicles and other fields,higher performance of power devices are needed.Electronic devices with high voltage,low loss and high power become the future development trend.β-Ga₂O₃ crystal is a new wide band-gap semiconductor material.Compared with SiC and GaN,β-Ga₂O₃ crystal has larger band gap,higher critical breakdown field strength and higher Baliga value,indicating β-Ga₂O₃-based device has better voltage resistance and lower conduction resistance.It has become one of the preferred materials for ultra-high voltage power devices and deep ultraviolet optoelectronic devices.Japan,USA,Germany,South Korea,and other developed countries have achived great progress in the growth of β-Ga₂O₃ crystals and fabrications of devices based on β-Ga₂O₃ since 2012.In domestic,thrice"Cross-strait Ga₂O₃ Symposium" were successfully held in Shanghai,Nanning and Hefei in 2017,2019 and 2021,respectively.The systematical researcher from crystal growth to device fabrication has been formed.Single crystal and substrate are the foundation of the whole β-Ga₂O₃ industry in the future.At present,β-Ga₂O₃ crystal is in the critical stage of rapid breakthrough and industrialization.While,there are a series of basic scientific problems to be solved urgently still.In this thesis,to obtain cylindrical β-Ga₂O₃ crystals and high-performance device development,optimizationc of crystal growth process,control of crystal quality,characterization of physical and chemical properties,suppression of crystal defects and device applications and so on were studied systematically.The main research results are as follows:Ⅰ.Study on growth process of cylindrical β-Ga₂O₃ crystals by EFG methodBased on the EFG furnace independently developed by the research group,in domestic,unintentionally doped and Sn-doped cylindrical β-Ga₂O₃ crystals with high quality were grown by the introduction of the cylindrical die,the optimization of pulling rate,the control of die height and design of after heater.The problems in crystal growth,including spiral growth,shoulder with polycrystalline,second neck and asymmetric shoulder were effectively solved.The experimental results showed that the optimal pulling rate,the die height,the die diameter and the after heater height.The FWHM in the rocking curve of high resolution X-ray diffraction was as low as 59.4 arcsec-69.3 arcsec.The diffraction spots of Laue were clear,symmetrical and consistent.The lattice stripes of high resolution TEM were highly ordered.The above measurements fully indicated that the crystal quality is high.By comparing with the shape characteristics of cylindrical β-Ga₂O₃ crystals grown by CZ and combining with theoretical simulation methods,the theoretical morphology of cylindrical β-Ga₂O₃ crystals grown by EFG was creatively proposed,which provides an important reference for the later actual crystal growth.Based on high quality β-Ga₂O₃ crystals,the spectral characteristics of unintentionally doped and Sn doped cylindrical β-Ga₂O₃ crystals were compared in detail by Raman spectra,transmittance spectra,X-ray photoelectron spectroscopy,CL and EPR.Ⅱ.Study on the β-Ga₂O₃ color centerFocusing on the coloring problem of unintentionally doped β-Ga₂O₃ crystal,the systematical study on the coloring reason and elimination mechanism of unintentionally doped β-Ga₂O₃ crystal was carried out.GDMS,SIMS and EPMA techniques were used to measure the types,concentrations and distributions of different elements.The results showed that impurity ions were not the root cause of β-Ga₂O₃ crystal coloring.By single crystal XRD test,the lattice parameters of blue and colorless regions are a=12.2291（19）,b=3.0390（5）,c=5.8080（11）,β=103.847°（7）and a=12.2387（17）,b=3.0417（3）,c=5.8120（8）,103.850℃（5）,respectively.The crystal lattice in the blue region had a certain degree of contraction;The high resolution XRD rocking curves of blue and colorless regions are 98.3 arcsec and 56.3 arcsec,respectively.The defect density is 4.8 × 104 cm-2 and 2.2 × 104 cm-2,respectively,which proves that the crystal quality of colorless region is higher.Raman tests showed that the peak positions of the two regions are not significantly shifted,but peak position at 772 cm-1 of blue region almost disappeared compared with the colorless region,which is believed to be related to the existence of vacancy defects in GaⅠO4 tetrahedron.The high intensity of CL blue emission in the blue region indicated that there are enough vacancy defects in the blue region.XPS test showed that there is a large amount of Ga+ in the blue region.The conductivity is higher,the carrier concentration is about 2.25 × 1018 cm-3,which is one order of Lagnitude higher than that in the colorless region（1.19×1017 cm-3）.The transmittance is lower in the UV-vision-NIR band.Comprehensive analysis showed that Ga3+ions combine with vacancy defects and transform into Ga+ions,resulting in the formation of negative charge center and color center in the crystal.From the perspective of crystal growth technology,the phenomenon of two different colors in the same β-Ga₂O₃ substrate should be attributed to the use of after heater.In addition,high temperature oxygen atmosphere annealing is an effective method to eliminate β-Ga₂O₃ color center.Ⅲ.The laser damage mechanism of β-Ga₂O₃ crystalIn view of the application of β-Ga₂O₃ crystal in ultrafast and tunable lasers,the laser damage mechanism of β-Ga₂O₃ crystal was analyzed and established from the perspective of microstructure.A Nd:YAG with a wavelength of 1064 nm（1.16 eV）was used.β-Ga₂O₃（100）substrate was irradiated by the Nd:YAG laser,and the damage threshold of β-Ga₂O₃（100）substrate is 716 MW/cm2.By FIB-SEM,TEM and SAED,it was found that the four typical crystal defects,including voids,amorphous,nanocrystalline and stacking faults appeared successively with the increase of damage depth.β-Ga₂O₃ crystal volatiled and decomposed to produce a lot of vacancy defects under laser irradiation,leading to the formation of stacking fault.With an increase in the laser irradiation time,vacancies in the crystal surface diffused and gathered into bigger voids.Thus,the laser damage of β-Ga₂O₃ crystal was established.Under the electron beam irradiation of TEM,the phenomenon of high density stacking faults relaxation in β-Ga₂O₃ crystal laser damage layer was observed.It is related to the thermal annealing effect caused by electron irradiation.The luminescence characteristics of stacking faults were revealed by CL spectra,and it was found that there was strong blue emission,which was related to high-density vacancy defects in the stacking,faults region.Ⅳ.Study on the properties and devices of β-Ga₂O₃（101）substrateThe mechanical,thermal,optical,electrical and etch properties of β-Ga₂O₃（101）substrate were studied in detial.The results showed that β-Ga₂O₃（101）substrate had obvious advantages in ohmic contact and fast etch,compared with the current β-Ga₂O₃（010）,（100）,（001）and（-201）substrates.β-Ga₂O₃ crystals were grown by EFG method,and（101）oriented substrates were obtained.The high resolution XRD rocking curve was as low as 90 arcsec,and the Laue diffraction spots were clear,symmetrical and consistent,which proved that the substrate quality was high.The unique structure of（101）surface atoms terminated with the Ga（GaⅠ and GaⅡ）planes or O（OⅠ,OⅡ and OⅢ）planes was found,and the corresponding surface dangling bond densities were 2.230 × 1015 cm-2 and 2.376 × 1015 cm-2,respectively.The physical properties of β-Ga₂O₃（101）substrate were systematically characterized.The thermal diffusion coefficient and thermal conductivity coefficient at room temperature were 5.96 mm2 s-1 and 15.4 w m-1 K-1,respectively.The Mohs hardness was about 6.77.The Raman spectrum selectively presents different Raman active modes with the change of polarization angle.The optical band gap is 4.76 eV,the valence band top is 3.62 eV,and the corresponding surface barrier height is 1.14 eV.C-V and I-V measurments showed that the electron concentration is 1.8 × 1018 cm-3 and the schottky barrier height is 1.07 eV.Under the wet chemical etching condition of KOH solution,the etching activation energy is determined to be 0.451 eV.The performance of（101）and（001）SBD and MSM devices was compared.It is found that the（101）plane rectification ratio is as high as 107±1 eV and the Ohmic contact is good.