First Principles Of Two-Dimensional Aluminum Nitride Nanosheets

In the past decades,two-dimensional?2D?materials represented by graphene have shown many attractive electronic,optical,mechanical and thermal properties.Especially its excellent electronic and optical properties,two-dimensional materials are expected to replace the traditional photodetector,meeting the growing demand for high-frequency communications and new biomedical imaging.III-V wide bandgap semiconductor materials have great application prospects due to their excellent optoelectronic properties,high temperature resistance,good material mechanical properties and corrosion resistance,and are expected to become the next generation of new two-dimensional materials.Among them,aluminum nitride is a typical representative of group III-V compounds.In this paper,the first-principles calculation method based on density functional theory is used to explore the intrinsic defects and doped electronic structures and optical properties of two-dimensional hexagonal aluminum nitride?h-AlN?nanosheets.The main processes are as follows:1.The electronic and optical properties of intrinsic h-AlN nanosheets and ones with four intrinsic defects(Al vacancy(V_Al),N vacancy?V_N?,N substitution for Al(N_Al)and Al substitution for N?Al _N?were studied by using the first-principles method based on density functional theory.The calculation model uses a 4 x 4 x 1 nanosheet structure.By comparing the defect formation,it can be found that the formation of vacancy defects can be generally lower than that of the anti-type defects in the case of nitrogen-rich or aluminum-rich,indicating that the inversion defects are not easily present in the aluminum nitride nanosheets.The intrinsic h-AlN nanosheet has a band gap width of 2.93 eV and an Al-N bond length of 1.977?,which is an indirect bandgap semiconductor material.The composition of the ionic bond is shown around the Al and N atoms,which together form the covalent bond.The band gap of h-aln nanosheets under V_All is narrowed,while the Fermi level entering the conduction band under V_N makes h-aln nanosheets exhibit n-type properties.Under the inversion defect,the narrowing of the band gap leads to the tendency of the conduction band and the valence to be connected,and the nanosheet exhibits semi-metallic properties.Obtained from optical properties,defects can lead to different degrees of increase of the static dielectric constant.The imaginary figure shows that V_N and Al _N defects can also cause electronic transitions in the low energy region.In general,the defects have a great influence on the low energy region of the nanosheet.2.In order to study the effect of doping h-AlN nanosheets,the first-principles calculation method based on density functional theory was used to calculate the electronic structure and optical properties of h-AlN nanosheets with three doping types?Be substituted Al,S substituted N and be-s co-doping?.It was found that doping changes the band gap of h-AlN nanosheet,but still shows the semiconductor properties.Be doping has little effect on the structure of h-AlN nanosheets,while S-doping and Be-S co-doping change the structure of AlN nanosheets,resulting in the bending movement of S atoms and Be atoms in the Z-axis direction.Compared with Be single doping,Be-S co-doping has the advantages of increasing the concentration of acceptor atoms and reducing the localization characteristics.At the same time,the S atom in the Be-S co-doping plays the role of activating the acceptor impurity Be atom,making the acceptor level move towards the lower energy direction.The doped AlN nanosheets are still indirect band gaps semiconductor and the band width is narrowed.At the same time,the S-atom dopng h-AlN nanosheet shows semi-metallic properties,and peaks appear in the low energy region of the complex dielectric function and absorption spectrum,and has certain absorption effect on visible light such as red light within a specific range,which may be helpful for the production of red light detection device.The co-doping AlN nanosheet has a wider response range to light,and its peak value decreases and moves towards the high-energy region,and the absorption intensity of high-frequency light increases.