Study On The First Principles Of Electron States Of Graphene-gallium Nitride Heterostroms

The discovery of graphene has broken the traditional concept that two dimensional(2D)materials cannot exist stably under normal temperature and pressure,thereby creating a new era of two-dimensional materials.Owing to unique structural characteristics and novel physical properties,2D materials have huge application potentials in many fields such as electronics,optoelectronics and energy.The monolayer with a single atom thickness is the limit for the feature scale of the material and nearly all the atoms are located at the surface.In practical applications,the abnormal interface effects could dominate the physical and chemical of 2D materials.Based on the characteristics of the layered structure,the different two-dimensional materials are stacked together to form a heterogeneous stack through the weak van der Waals force.The quantum coupling between layers can induce new series of characteristics and functions,which has an extremely important significance for integration of various optoelectronic devices.Subsequently,it is the prerequisite for design and applications of 2D materials based devices to clarify the interface effects and regulation mechanism of the electronic characteristics.In this thesis,the effects of interlayer separation,strain and doping on the electronic and electrical properties of graphene/GaN heterojunctions were systematically studied by first-principle calculations.The main results can be obtained from above investigations.(1)The dependence of the electronic state and contact characteristics of the graphene/GaN heterojunction on the interlayer spacing and vacancy defects was studied.It is found that the electronic structure is simple superposition when GaN is stacked on graphene,indicating that the interlayer interaction is too weak.By fixing the graphene layer and moving the GaN layer up and down to change the interlayer distance between them.The interlayer interaction can be adjusted by the interlayer distance.With increase of the interlayer distance,the graphene/GaN heterojunction changes from ohmic contact to p-type Schottky contact.By random removal of one Ga atom or N atom in the GaN layer to build an interface defect,the effects of the structural defects on the electronic state and magnetic properties of the graphene/GaN heterojunction were investigated.In the presence of Ga vacancies,the system has 1.65 μB,while N vacancies cannot induce the magnetic properties of the system and the contact barrier types caused by different atomic vacancies are different as well.(2)Strain engineering was used to systematically study the electronic states and contact characteristics of graphene/GaN heterojunctions,and the relationship between the change of electronic states and the strain was clarified.By changing the lattice constant a and keeping the lattice constant b unchanged,a uniaxial strain was applied to the graphene/GaN heterojunction In addition,by changing the lattice constants a and b at the same time,and the amount of change is synchronized to achieve the biaxial loading strain for the graphene/GaN heterojunction Moreover,by increasing the lattice constant a,while reducing the lattice constant b to achieve the loading mixture strain of the graphene/GaN heterojunction,the effect of strain on the heterojunction eletronic states and contact characteristics was explored.The amount of graphene layer charge transfer to GaN increases with increasing the loading uniaxial or biaxial compressive strain,thus causing the GaN electronic state to shift to a lower energy level and the formation of p-type Schottky contact with graphene.With increase of uniaxial or biaxial tensile strain,the contact type of GaN and graphene changes from p-type contact/ohmic contact/p-type contact.As the loading mixture strain increases,the type of contact between GaN and graphene changes from p-type contact to n-type contact,but it is almost independent of the type of loading mixture strain.(3)The effect of doping on the electronic state and contact characteristics of graphene/GaN heterojunction was discussed.Graphene/GaN heterojunction is doped n-type by replacing Ga atom with C atom or O atom instead of N atom,while B atom substitutes Ga atom or N atom for p-type doped.By n/p-type doping of GaN,the overall graphene/GaN heterojunction electronic structure shifts to a low/high energy state,the bottom of the conduction band of GaN and the graphene Dirac cone are below/above the Fermi level,forming Ohmic contact.Though it can regulate the electronic state and contact characteristics of the doped graphene/GaN heterojunction by changing the layer spacing to some extent,there is not too obvious effect.(4)By replacing B(O)atoms with Ga(N)atoms and p-type(n-type)doped graphene/GaN heterojunction loading uniaxial,biaxial and mixture strains,the interlayer charge transfer and the relative positional changes of the band edges were comprehensively investigated.The influence of the dual effects of doping and strain on the electronic state and contact characteristics of the graphene/GaN heterojunction was analyzed.With the change of loading uniaxial strain,BGa-doped graphene/GaN heterojunction transforms into ohmic contact,whereas the change of n-type/p-type/ohmic contact occurs with the change of loading biaxial strain.While the ON-doped graphene/GaN heterojunction always exhibits ohmic contact with the strain loading.Moreover,the loading uniaxial/biaxial compressive strain remarkably affects the interactions between the defective graphene/GaN heterojunction layers,but the increased uniaxial/biaxial tensile strain significantly decreases the GaN band gap width.The loading mixture strain causes the p-type/n-type contact transition of the graphene/GaN heterojunction.