Fatigue and its effect on the piezopotential properties of gallium nitride nanowires.

The gallium nitride (GaN) nanowires (NWs) in piezotronic applications are usually under cyclic loading, which thus may inevitably suffer the mechanical fatigue. In this paper, the fatigue behaviours of defective GaN NWs are investigated by using molecular dynamics (MD) simulations. Our results show no significant changes in the molecular structures of GaN NWs until their final failure during the fatigue process. The final fracture occurring in the GaN NWs under fatigue loading is triggered by the crack that unusually initiates from the NW surface. The GaN NW with a smaller defect concentration or under the fatigue load with a smaller amplitude is found to possess a longer fatigue life. In addition, the ultimate fatigue strain of GaN NWs can be significantly increased by reducing the defect concentration of NWs. The material parameters including elastic constants, piezoelectric coefficients, and dielectric constants of GaN NWs in the fatigue test are evaluated through MD simulations, all of which are found to keep almost unchanged during the fatigue process. These material parameters together with the band gaps of GaN NWs extracted from first-principles calculations are employed in finite element calculations to investigate the piezopotential properties of GaN NWs under fatigue loading. No significant changes are found in the piezopotential properties of GaN NWs during the fatigue process, which indicates the long-term dynamic reliability of GaN NWs in piezotronic applications.