First-principles Study On Electronic Structures And Related Properties Of GaN-based Alloys And Two-dimensional GaN

As a third-generation semiconductor material,GaN has wide application prospects in the lighting display,blue-green lasers,high-temperature detectors and high-power RF switching equipment,due to its excellent physical and chemical properties,such as wide band gap,high thermal conductivity,high electron saturation rate,high carrier mobility,and favorable chemical stability(almost no corrosion by any acid),etc.In recent years,the study of graphene as a carbon-based spintronic material has been highly valued and developed.However,graphene is a zero gap half-metal,which limits its practical application in nanoelectronic devices.Compared with graphene,currently fabricated GaN monolayer with band gap is expected to be more suitable for preparing nano-spin electronic devices.However,the indirect bandgap of GaN monolayer will limit its application in optoelectronics.Therefore,it is very important to study the regulation of bandgap and non-magnetic element doping for the regulation of electron structure and magnetism of GaN monolayer.In this thesis,firstly,we study luminescent mechanism of GaN-based alloys and prove the source of GaN-based alloys luminescence.Secondly,the effects of doping and strain on the electronic structure and related properties of two-dimensional GaN are calculated,which provides theoretical guidance for the development of spintronic devices and optoelectronic devices,and has important practical application value.This thesis is divided into five chapters.The first chapter mainly introduces the research status and research significance of GaN materials.The theoretical framework of density functional theory and the VASP software package used in this thesis are presented in the second chapter.In the third chapter,the electronic structure and luminescence mechanism of GaN-based alloys are calculated.In the fourth chapter,the electron structure and magnetism in GaN monolayer doped with nonmagnetic atoms and vacancy defect is studied.In the last chapter,the effects of strain on the electron structure and migration rate of two-dimensional GaN are studied.The main research contents and conclusions are as follows:(1)The microscopic mechanism of GaN-based alloys luminescence is studied.Inorder to ensure the reliability of the calculation,we calculate the electronic structures of the intrinsic GaN to optimize the structure by the AM05 XC function and revise the underestimation of the band gap by LDA-1/2 method.The results show that both short In-N-chain and small In-N clusters are localized electrons at the valence band maximum(VBM)states;the localization of the valence electrons in the small In–N clusters at the VBM becomes much stronger than that in the short In–N chains if the small cluster and the short chain coexist in the InGaN alloy.On the contrary,in addition to the small Al-N clusters and short Al-N-chains,other structures are localized electrons at the VBM states.The microscopic arrangement of In(Al)atoms in the alloys not only causes the electron localization and strongly influences the light emission,but also influences its band gap and bowing parameter.(2)The electronic structural and magnetic properties of the vacancy defects and group1 A and group 2A nonmagnetic-element doped GaN monolayers are investigated systematically.The calculation results show that GaN monolayer itself is not magnetic,and Ga vacancy defect and non-magnetic metal element X doping GaN monolayer induce magnetic properties.The three nearest-neighbor N atoms of the dopant X atom are the main contributors to the magnetic properties of the non-magnetic metal-doped GaN monolayer.Moreover,By study the magnetic properties of double X(X = Li,Na,K,Be,Mg,Ca)atoms doped systems,we find that possess a ferromagnetic ground state.(3)The structure,electronic properties and effects on charge carrier mobility of uniaxial,biaxially strained GaN monolayer.The structure and electronic properties in GaN monolayer in single-biaxial strain were calculated,and the effect of strain on the effective mass and carrier mobility was calculated.The results show that in the strain range of-10%to 10%,the structure of GaN monolayer is not damaged,that is,still within the elastic deformation range.When the strain is in the range of-3% to 10%,GaN monolayer still maintains the indirect band gap characteristics.When the compressive strain is more than4%,the band gap of the GaN monolayer is changed from the indirect band gap to the direct band gap,and exhibits unable bandgap under strain.Uniaxial and biaxial strains can also change the carrier mobility.When the compressive strain is more than 4%,the hole effective mass is significantly reduced,and significantly higher hole mobility.In the strain range of-10% to 10%,electron mobility decreases with the increase of compressive strain.