Study On The Characteristic Of α-Ga₂O₃ Materials Based On First Principles And Mist-CVD

Ultrawide bandgap semiconductor materialα-Ga₂O₃has an ultra-wideband gap of 5.3eV,a breakdown voltage of 10 M/cm and a high baliga quality factor.Therefore,α-Ga₂O₃power device has lower on-resistance loss and higher power conversion efficiency,and its application in high-power power electronic devices,ultraviolet detectors and other fields has been widely studied.In recent years,because the Mist-CVD method successfully produced high qualityα-Ga₂O₃film with heteroepitaxy on sapphire,resulting in the rapid development ofα-Ga₂O₃power devices.In this paper,the material properties and growth ofα-Ga₂O₃are studied based on first principles calculations and the Mist-CVD method.The specific research content and conclusions of this paper are as follows:1.Based on density functional theory,this paper analyzes terminal atomic layer structure ofα-Al₂O₃andα-Ga₂O₃.The adsorption of hydroxyl and H atoms on the surface of theα-Al₂O₃andα-Ga₂O₃terminal atomic layer.The most stable surface structure ofα-Al₂O₃andα-Ga₂O₃terminal atomic layer is Ga（Al）-O-Ga（Al）.The termination layer shows that three hydroxyl groups and three H atoms cannot be adsorbed at the same time,which results in the polycrystalline layer in the film growth process.Then the molecular structure of gallium acetylacetonate and the process of its hydrogenated bonding with O atom to form Ga₂O₃were studied.2.α-Ga₂O₃film was grown on the heteroepitaxial of c-plane sapphire by a self-made fine-channel Mist-CVD method.The effects of substrate temperature,carrier gas flow rate,carrier gas oxygen content,fine channel structure on the optical band gap,surface morphology and structure of the film were characterized by X-ray diffraction（XRD）,atomic force microscopy（AFM）,and UV-visible spectrophotometer.The experimental and characterization results show that when the fine channel is flat and the height is 1.5 mm,α-Ga₂O₃samples have the best characterization results,with a half-width of 140 arcsec and a surface roughness of 1.21 nm which was prepared at 440℃and 5L/min in air.When the substrate temperature is reduced to 400℃,the surface morphology of the film shows nanowire stripes.However,when the substrate temperature increases by20℃,there is X-ray diffraction peak ofε-Ga₂O₃in the XRD profile of the thin films.When the carrier gas flow rate is in the range of 3～5 L/min,and the film mass increases with the increase of the carrier gas flow rate.When the carrier gas flow rate increases to 6L/min,a large amount of amorphous powder appears in the chamber due to the limitation of the equipment outlet velocity.At the same time,the quality of the sample prepared by nitrogen is slightly lower than that of the sample prepared by air,but its impact is far less than the substrate temperature and carrier gas flow rate,indicating that the O source in the preparation ofα-Ga₂O₃mainly comes from water molecules in gallium acetylacetone solution.3.Using the atomic layer deposition（ALD）technology,polycrystalline Al₂O₃thin films with a thickness of 2 nm were deposited on the surface ofα-Ga₂O₃andε-Ga₂O₃thin films using H₂O,O₃or O₂plasma as the oxygen source.The band-order structure diagram of polycrystalline Al₂O₃andα-,ε-Ga₂O₃grown by ALD technology were calculated by X-ray photoelectron spectroscopy（XPS）.Through above calculation,α-Ga₂O₃andε-Ga₂O₃can use low-cost ALD to calculate the growth medium layer.At the same time,O₃and O₂plasma as oxygen sources reduce the the interface OH defect and increase the conduction band order.4.The band structure ofα-Ga₂O₃was calculated using the PBE functional and the HSE hybrid functional.And its optical,electrical and mechanical properties are analyzed.The electrical and optical properties ofα-Ga₂O₃caused by strain along the c-axis are studied by combining the experimental analysis structure.The results show thatα-Ga₂O₃has high lattice symmetry and low optical and mechanical anisotropy.Its density of states distribution map is similar to that ofβ-Ga₂O₃,but its band gap value reaches 5.22 eV.The volume modulus value ofα-Ga₂O₃is 217.38 Gpa,which is about 60 Gpa higher thanβ-Ga₂O₃,indicating that the gallium oxide stability is lower than that ofβ-Ga₂O₃.Meanwhile,applying a tensile strain along the c-axis can change gallium oxide from an indirect bandgap semiconductor to a direct bandgap semiconductor,increasing its bandgap value.5.The defect formation energy of vacancy-defect,transposition defects,and gap defects ofα-Ga₂O₃was calculated by PBE functional.The results show that the O vacancy is easy to form under both Ga rich and O rich conditions,but its transition level belongs to the deep donor level,which also proves that the cause of the n-type conductivity ofα-Ga₂O₃under the intrinsic conditions is not the O vacancy.In the O-rich condition,the formation energy of Ga vacancy is negative at both the conduction and valence band bottoms,indicating the presence of large Ga vacancies in the case of n-type or p-type doping.Meanwhile,H gap defect belongs to the localization defect,which is prone to form OH complex defect together with Ga vacancy,which is consistent with the results of Mist-CVD mechanism analysis.The H-gap atoms lie to the three oxygen atoms of the Ga O₆,with an angle of about 90 to the c-axis.