Structure And Optoelectronic Properties Of Low-dimensional Wide Band ?A-nitrides:A First-principles Investigation

Wide band ?A-nitrides represented by GaN show high thermal conductivity,stable chemical properties,high breakdown electric field and directly tunable band gap,etc.,and are widely used in power electronics and optoelectronic devices.With the rapid development of science and technology,the function and performance of devices need to be further improved,therefore,the quality and performance of materials are put forward higher requirements.The rapid rise of low-dimensional materials has provided a new direction for the development of wide-band ?A-nitrides materials and devices.Compared with bulk materials,low-dimensional structures have the following advantages:Quantum-confined effect enriches the physical properties of materials;ultra-thin material thickness greatly reduces the self-absorption and scattering of photons inside the material,which is beneficial to light extraction;material flexibility is enhanced,which facilitates the preparation of wearable devices,etc.At present,except for hexagonal boron nitride h-BN,other researches on low-dimensional wide band ?A-nitrides are still in the initial stage,and there is a lack of understanding of key issues such as material microstructure,structure-property relationship,and defect properties.Therefore,in this thesis,the atomic structure and optoelectronic properties of low-dimensional wide-band ?A-nitrides materials are investigated at the theoretical level using first-principles calculations.The structure and optoelectronic properties of point defects in monolayer hexagonal aluminum nitride(h-AlN);the structure and carrier transport behavior of low-dimensional B_xAl_1-xN materials;and the structural evolution and band structure modulation of low-dimensional AlN and GaN are explored in depth,and the following results are obtained:Single-photon emission based on point defects in monolayer h-AlN:Point defects are inevitably introduced during material growth,and they have significant effects on the optoelectronic properties of the material.However,the study of defects in low-dimensional wide-band group ?A-nitrides is still rarely reported.We have systematically investigated the structure and optoelectronic properties of point defects in monolayer h-AlN,including vacancies,anti-sites and impurity defects,by using hybrid density functional theory calculations.The calculation results show that the formation of intrinsic point defects in the material is more difficult than impurity defects,while all intrinsic point defects and their stable charged state transition levels are in deep levels.In addition,we calculate the photoelectric properties of point defects and find that the charged Al vacancy defect V_Al^- with-1 charge and the N anti-defect N_Al⁺ with+1 charge can achieve single photon emission with zero phonon line(ZPL)corresponding to 0.77 and 1.40 eV,respectively.In addition,the charged complex defect C_AlV_N⁺ consisting of N vacancy and C anti-defect can also achieve single-photon emission.Structural and transport properties of low-dimensional B_xAl_1-xN:B_xAl_1-xN ternary compounds are considered as ideal materials for the preparation of deep-UV optoelectronic devices.However,the poor quality of the material severely limits its application in optoelectronic devices,especially the growth of high B-component B_xAl_1-xN is very difficult.The B component of B_xAl_1-xN films grown by MOCVD method is usually less than 22%,but the physical mechanism is still unclear.To address the above problems,we combined particle swarm optimization and density functional theory to predict the ground state structures of B_xAl_1-xN with different B components(x=0.25,0.50,0.75)by high-throughput calculations.The calculation results show that the B_xAl_1-xN material gradually changes from the wurtzite structure to the hexagonal structure with the gradual increase of the B component,which is due to the fact that the B atoms maintain the planar triple coordination configuration in the crystal,while the Al atoms still maintain the tetrahedron configuration in the wurtzite structure.We also found an unreported B_0.5Al_0.5N ten-membered ring structure with an indirect band gap of 3.52 eV.We investigated the carrier transport behaviour of B_xAl_1-xN using the Wannier interpolation method.It is found that the electron-hole mobility is highly anisotropic.In addition,the electron mobility decreases rapidly with the increase of B component,indicating that B_xAl_1-xN can effectively block the electron diffusion.Structural evolution and energy band modulation of low-dimensional AlN and GaN:Unlike the van der Waals stacking mode of most two-dimensional materials,group ?A-nitrides are connected by covalent bonds.Therefore,as the material thickness decreases,it exhibits a different structural and property change pattern from other 2D materials.We have investigated the changes of lattice structure and electronic properties of AlN and GaN with varying material thickness by first-principles calculations.By surface energy calculations,we found that AlN and GaN exhibit structural phase transition from wurtzite structure?haeckelite(4|8)structure?hexagonal structure as the number of atomic layers decreases.Haeckelite(4|8)structure can exist stably in the following thicknesses:28?n>4(AlN),18?n>2(GaN).The low-dimensional group ?A-nitrides with haeckelite(4|8)and hexagonal structures have zero internal electrostatic field,and the 4|8 structure has good optical properties similar to those of wurtzite structure.In addition,applying uniaxial/biaxial stress to haeckelite(4|8)AlN can adjust the position of the material valence band maximum to achieve the transition from indirect band gap to direct band gap.