Study On The Structure,electronic Energy Band And Thermal Transport In Two-dimensional GaN

With wide direct band gap,excellent electronic mobility,high disruptive strength and thermal conductivity,III-V group semiconductor gallium nitride(GaN)has been widely used in the fabrication of optoelectronic devices and high-power devices.Motivated by the exploration of two-dimensional materials such as graphene,Mo S2,silicene etc.,two-dimensional(2D)GaN has also attracted much attention.It has been suggested that 2D GaN has even wider electronic band gap and excellent optical properties which make it an ideal candidate for electronic material.Former researches mainly focus on 2D GaN truncated from polar c-plane.The research on non-polar m-plane 2D GaN is still lack.As a non-layer 2D material,the highly exposed dangling bonds on the surface provide lots of active sites for chemical modification,further increasing the possibility by surface engineering.Based on this,we carry out the following works:By performing first principles calculation,we systematically study the geometric structure and electronic band of m-plane 2D GaN.The results indicate that monolayer m-plane 2D GaN has a planar hexagonal honeycomb structure.Multilayer m-plane 2D GaN is formed by stacking buckled hexagonal monolayer GaN.Monolayer GaN has an indirect bandgap.With the increasing of layer number,there will be an indirect to direct band transition at 12L,while the bandgap will decrease from 1-3L and then increase from 3L to14L.We further investigate the effect of hydrogenation to m-plane GaN.For GaN monolayer,we consider three types of hydrogenation,all of which change the structure from planar to buckled.All the three hydrogenation types will enlarge the band gap,but only type?would change the band type from indirect to direct.For bilayer and trilayer m-planes,hydrogenation will change the states of the surface atoms,tune the band type from indirect to direct and enlarge the bandgap.By performing first principle calculation and solving phonon Boltzmann equation,we investigate the effect of type?hydrogenation(Gaand N arranged in different side of the monolayer)on thermal conductivity of GaN monolayer.The results show that hydrogenation will slightly increase the thermal conductivity of GaN monolayer from 70.32Wm^-1K^-1 to76.23 Wm^-1K^-1.While little effect on the thermal conductivity,hydrogenation will significantly influence the phonon transport in GaN monolayer.The contribution to thermal conductivity of ZA mode decrease from 46.94 Wm^-1K^-1 to 26.89 Wm^-1K^-1,while the contribution of TA mode increases from 17.10 Wm^-1K^-1 to 22.99 Wm^-1K^-1 and the contribution of LA increases from 4.4 Wm^-1K^-1 to 23.95 Wm^-1K^-1.Further calculations indicate that the decrease of ZA mode contribution mainly come from the decrease of group velocity and phonon life time at low frequency.The increase of TA and LA mode contribution mainly come from the increase of phonon life time which is caused by the lower anharmonicity.