Tunable local and global piezopotential properties of graded InGaN nanowires

Owing to the continuously tunable physical properties, composition-graded wurtzite nanowires (NWs) recently become an attractive class of materials with potential applications in novel nanodevices. In this paper, the piezopotential properties of graded InGaN NWs are studied through multiscale modeling. Our atomic-scale simulations show that the structural and material parameters of graded InGaN NWs are dependent on the NW position, which are well described by the analytic expressions based on the Vegard's law. The non-uniform distribution of structural and material parameters triggers the flexoelectric effect in graded InGaN NWs that can enhance the polarization components in NWs. The modified electromechanical theory incorporating the flexoelectric effect and analytic expressions of the position-dependent structural and material parameters is employed in the finite element calculations of piezopotential. The piezopotential in graded InGaN NWs is found to be asymmetric to NW center, which is different to the symmetric piezopotential distribution observed in binary wurtzite NWs. Both the local piezopotential and the global piezopotential difference of graded InGaN NWs are enhanced due to the flexoelectric effect, which becomes more significant as the concentration of donors grows. In addition, the piezopotential distribution in graded InGaN NWs can be significantly affected by the length of graded InGaN region, while it almost has no influence on the piezopotential difference. However, both the local piezopotential and the global piezopotential difference of graded InGaN NWs can be greatly enhanced by increasing the maximum In composition.