| The faulty surfaces of GaN,which contain line defects or steps,are constructed respectively,combining with the related experimental results,and also TiO2adsorption on them. The total energy of the various models are simulated with the software of Materials Studio using first principles method.The density of states, band structure, surface energy, adsorption energy and electron density are also gained.Firstly, the Ga-terminated GaN (0001) and N-terminated GaN (0001) surfaces containing different line defects are researched and compared with the corresponding clean surfaces. The results show that the impact on the relaxation of top-layer atoms is larger than atoms below caused by the two kinds of defects,and the line defects of [1120] direction make the relaxation of the surface atoms larger than the [1010] direction.Column atoms of each top layer paralleling to the direction of respective defects is the same case of relaxation. The study also found that the line defects of Ga-terminated surfaces increase the width of the band gap, and N-terminated surfaces reduced.For Ga-terminated surfaces,the line defects of [1120] direction indicate an overlap between the valence band and conduction band,and the surfaces change to semi-metal.The electrons in the valence band will be able to transition to the conduction band for a small perturbation, which, in certain extent, increases the conductivity.The faulty surfaces are more stable than clean surfaces.The line defects of [1010] direction are more conducive to the stability of the surface than the [1120] direction.The N-terminated surfaces containing defect is relatively stable in rich Gallium,and the Ga-terminated in rich nitrogen.Secondly,the G-N monolayer steps of0.259A is studied.The final structure shows that the bonds between N atoms and adjacent Ga atoms of step edge of N-terminated surfaces with [1120] line defect is broken.The dangling bonds of N atoms combine with the below bare N atoms by strong covalent bonds.Through the analysis of surface energy, the steps-surfaces of [1120] direction of Ga-terminated are the most stable structure,agreeing with the Ga-N steps of this direction which experiment observed.The steps of each surface all reduce the width of band gap.Finally, the TiO2absorption on faulty surfaces of GaN is concerned.For Ga-terminated surfaces,the model containing [1120] line defects and one of the O atoms located on the top of line defects promote the absorption of TiO2.the minimum of adsorption energy is-11.258eV.The adsorption is also associated with the initial position of TiO2. |
