First-principle Study Of Low-dimensional Gallium Oxide

Compared with the third-generation semiconductors,the fourth-generation ultra-wide band gap semiconductor materials gallium oxide(Ga₂O₃)has many advantages such as larger band gap,higher breakdown field strength,larger Baliga quality factor,and lower growth cost,which make it be the preferred material for high-voltage,high-power,low-loss power devices and deep ultraviolet optoelectronic devices.In recent years,many researchers continue to study and improve the performance of gallium oxide materials and devices based it.It was found that reducing the dimensions of gallium oxide materials could increase its surface to volume ratio and quantum confinement effect,and promoted its potentials in new flexible devices,photoelectric detectors,field effect transistors,etc.At present,the international research on low-dimensional gallium oxide is still immature,and the theoretical and experimental research is in its infancy,and there is a lack of guiding mechanism and system.Based on this research status,the first-principles calculation method was adopted to research on the crystal structure,electronic properties,transport properties and optical properties of low-dimensional gallium oxide materials in this paper.The performances of low-dimensional gallium oxide were further adjusted through strain engineering,electric field,functionalization,heterojunction,etc.,in order to summarize the characteristics of low-dimensional gallium oxide materials and provide reference for subsequent research on low-dimensional wide-gap semiconductor materials.The main contents and results of this paper were as follows:First,the minimum exfoliation energy along the(100)plane of the bulk?-Ga₂O₃ was about2.01 J/m²,so the 2D Ga₂O₃ film could be obtained by mechanical exfoliation.Hydrogen atoms were used to functionalize the dangling bonds on the surface of the 2D Ga₂O₃material,then a 2D Ga₂O₃ structure with thermodynamic and lattice dynamics stability could be obtained.The first-principles calculation methods were adopted to study and predict the electrical,transport,optical,mechanical and thermal properties of the two-dimensional(2D)Ga₂O₃.Compared with the bulk?-Ga₂O₃,the hydrogenated 2D Ga₂O₃ presented the obvious quantum effect,had a larger indirect band gap and higher deep ultraviolet light absorption.2D Ga₂O₃ maintained an indirect band gap semiconductor,the band gaps of monolayer and bilayer Ga₂O₃ were 3.27 e V and 2.22 e V,respectively.The room temperature electron mobility of monolayer and bilayer Ga₂O₃were calculated to be 2684.93 cm²V^-1s^-1 and 24485.47 cm²V^-1s^-1,respectively.The elastic properties continued reduce,but its compressive anisotropic properties were enhanced with the decrease of the 2D Ga₂O₃ thickness.Besides,the thermal conductivity,average sound velocity and heat capacity increased with the decrease of the 2D Ga₂O₃ thickness.The Debye temperature decreased with the decrease of the 2D Ga₂O₃ thickness.The thermal conductivity and heat capacity of monolayer Ga₂O₃ were enhanced to 0.49W·cm^-1·K^-1 and27.58 J·mol^-1·K^-1,respectively.Then,uniaxial strain and electric field were applied to the 2D Ga₂O₃ to theoretically study its band structure,transport properties and optical properties.The physical mechanism of various properties changes were analyzed from detailed orbital contribution.When uniaxial strain was applied,the band gap of 2D Ga₂O₃ could be effectively adjusted.An indirect-to-direct band gap transition occurred under uniaxial tension due to the difference of?bonding at the valence bands.The 2D Ga₂O₃ band gap exhibited a parabolic variation character from compression to tension,and related to the direction of uniaxial strain.The effective electron masses m_e(?-Y)*and m_e(?-X)*of 2D Ga₂O₃ exhibited obvious anisotropy.The maximum electron mobility of monolayer Ga₂O₃ along the?-X and?-Y directions was 48368.14 cm²V^-1s^-1 and 4468.33 cm²V^-1s^-1,respectively.The maximum electron mobility of bilayer Ga₂O₃ along the?-X and?-Y directions was 32655.21 cm²V^-1s^-1 and53537.33 cm²V^-1s^-1,respectively.Uniaxial strain would cause changes in the real part of the complex dielectric function and the absorption edge of the 2D Ga₂O₃.The applied electric field could also adjust the band gap of 2D Ga₂O₃,and the band gap would be reduced to zero when the electric field was large enough.Next,halogen(Cl/F)atoms were chosen to functionalize the dangling bonds on the surface of 2D Ga₂O₃ for the performance modulation.The structural stability and optoelectronic properties of 2D Ga₂O₃ tuned by halogen functionalization were investigated and compared with the hydrogenation.The physical mechanism was analyzed based on the orbital contribution and the bonding type.It was found that the Ga-Cl/F bonding is stronger than the Ga-O bonding.Therefore,whether it was fully-saturated halogen functionalization or half-saturated halogen functionalization 2D Ga₂O₃,the indirect-to-direct band gap transition could be realized and the stability was not damaged at the same time.It was found that the Cl atom promoted the transfer of electrons to the Ga-O bond,thereby reducing the band gap of the 2D Ga₂O₃ and achieving p-type conductivity.The F atoms formed the?*anti-bonding,which led to enlarge the band gap of the 2D Ga₂O₃ and introduce impurity levels.The charge transfer direction caused by Cl and F was opposite.The effective mass of electrons/holes and the transport properties of the 2D Ga₂O₃ could be adjusted by the halogen atoms.The monolayer Cl Ga₂O₃H and Cl Ga₂O₃Cl had the highest hole and electron mobility 3913.52 cm²V^-1s^-1 and 5177.71 cm²V^-1s^-1,respectively.The bilayer Clbi H and Clbi Cl had the highest hole and electron mobility6934.67 cm²V^-1s^-1 and 495.76 cm²V^-1s^-1,respectively.Halogen functionalization increased the absorption of 2D Ga₂O₃ in the visible and infrared regions.Finally,a stable 2D Ga₂O₃ and Mo O₃ heterojunction based on the bulk?-Ga₂O₃ and?-Mo O₃ crystal structure was constructed.By introducing interface functionalization or vacancy defects,the electrical properties of different heterojunction structures were compared and analyzed.The band gaps of Ga₂O₃ and Mo O₃ in the 2D Ga₂O₃/Mo O₃heterojunction were both smaller than the band gap of their isolated interface material.Besides,both Ga₂O₃ and Mo O₃ maintained the indirect band gap semiconductor characteristics.The band gap of Ga₂O₃/Mo O₃ heterojunction was approximately the superposition of the band gap of bilayer Ga₂O₃ and bilayer Mo O₃,which exhibited the characteristics of type-II heterojunction.O functionalization on Ga₂O₃ surface,O vacancy in Mo O₃,and Mo vacancy in Mo O₃ were adopted to modify the heterojunction characteristics.The adjustment of the band gaps of Ga₂O₃ and Mo O₃,and the carrier effective mass of Mo O₃ were realized.Besides,there were defect energy levels in the band gap of the heterojunction.After modulation,the carrier transfer efficiency was all reduced,but the Mo vacancy heterojunction had the largest charge transfer at the interface of 0.41e V.The O functionalization and the O vacancy heterojunctions exhibited the type-II heterojunction characteristics,while Mo vacancy heterojunction exhibited the type-I heterojunction characteristics.In all of these heterojunctions,Mo O₃ gained electrons and Ga₂O₃ lost electrons.