Batio ₃ /gan Heterostructure Study On Electronic Structure And Characteristics Of Two - Dimensional Electron Gas

This thesis mainly studies BaTiO3-GaN heterostructure consisting of classic ferroelectric and semiconductor materials with the help of first principle calculation software CASTEP and charge control model. Theoretical exploration about its electronic properties and two-dimension electron gas(2DEG)can provide a useful reference for future fabrications of ferroelectric-semiconductor-based nano-sized devices.Firstly, this thesis discusses in detail surface structures of BaTiO3(001),(111), zinc-blende GaN(001),(110)and wurtzite GaN(0001),(0001-)from two aspects such as band structure, density of states.TiO2-terminated surface of BaTiO3(001)is less stable due to dangling bonds of Ti atoms and its induced surface states. The band gap of BaO-terminated surface is 1.761 eV after relaxation, which approaches to that of its bulk, 1.804 eV. However, the band gap of TiO2-terminated surface is only 0.745 eV, which is much smaller than that of its bulk. BaO3-terminated surface has a good stability, the band gap of which has little change after relaxation. Several surfaces of GaN are found quite different due to the differences of phase structures, atomic terminations and polarization directions. Both two different terminated surfaces of zinc-blende GaN(001)and wurtzite GaN(0001-)surface have metallic properties. However, both zinc-blende GaN ( 110 ) surface and wurtzite GaN ( 0001 ) surface remain semiconductive. In addition, wurtzite GaN(0001)surface has similar stratiform structure of graphite after relaxation.Secondly, the electronic properties of BaTiO3/GaN heterostructure from its interfacial atomic structures are discussed. As far as the heterojunction is concerned, which is composed of BaTiO3(001)TiO2-terminated surface and 3C-GaN(001)Ga-terminated surface. It is found cubic structure for BaTiO3 epilayer becomes tetragonal ferroelectric phase structure as a result of tensile strain. At the interface, Ga-O bonds are almost not formed. Both Ti atom and Ga atom can't form bonds. As to the heterostructure which consists of BaTiO3(111)Ti-terminated surface and wurtzite GaN(0001)N-terminated surface, BaTiO3 epilayer form smaller compressible strain, but it doesn't happen to distort. The heterojunction has a large pseudo band gap of 2.23 eV in the conduction band. Ti-N bonds are formed due to their mutual weak hybridization. Both of two heterostructure have metallic characteristic.Thirdly, based on first-principle, the method is found in which several physical parameters of the heterostructure under the strain or polarization. On the basis of the method, as to the heterostructure which consists of BaTiO3(111)surface and wurtzite GaN(0001)surface, the effective mass mn* of BaTiO3 reduces 42% under the substrate GaN dual strain. However, the effective mass mn* of GaN on BaTiO3 reduces 10%. As far as BaTiO3(111)/2H-GaN(0001)is concerned, its valence band offset and conduction band offset are about 0.77 eV and 0.47 eV respectively. The obtained parameters of BaTiO3/GaN heterojunction provide a necessary condition for prediction of its properties.Finally, based on obtained material parameters of heterostructure by first-principle, combining the relationship of ferroelectric polarization and applied electric field and making use of charge control model principles, the intercoupling model of both ferroelectric and semiconductor is found. Two-dimension electron gas(2DEG) of three typical ferroelectric-semiconductor heterojunctions is studied, such as FE/GaN,FE/AlGaN/GaN and AlGaN/GaN/FE. Results show that FE/GaN forms enhancement devices much easier than AlGaN/GaN and large ferroelectric polarization can induce high dense two-dimension carrier gas(2DCG).In the FE/AlGaN/GaN, positive ferroelectric polarization can make 2DEG density of GaN channel improve. There are dual channels in GaN layer of AlGaN/GaN/FE heterostructure so that carrier density improves more than twice. Besides the electron density, the electron mobility would further improve due to the tensile stress in GaN channel. This strained semiconductor on ferroelectric (SSOF) structure may be promising for high speed power devices.