Size-dependent persistent photocurrent and surface band bending inm-axial GaN nanowires

The size-dependent persistent photocurrent (PPC), which refers to a photocurrent persisting for a long time after the excitation light source is terminated, has been investigated in m-axial GaN nanowires (NWs) with diameters of 20--130 nm. These NWs possess polar side surfaces and thus exhibit strong surface band bending (SBB). With different diameters, a different rise time in photoconductivity (PC) upon excitation light illumination and a different decay time in the PPC are observed; the latter is attributed to a long carrier lifetime caused by a frustrated recombination process. The intensity (I)-dependent photocurrent--gain ({Γ}) measurement displays a {Γ}-I dependence that follows a power-law relationship with fitting indices of {~}0.85--0.89, indicating that the long carrier lifetime--induced PPC of GaN NWs is caused by an SBB effect instead of a bulk trap effect. In addition, size-dependent decay times reveal two regimes for the different sizes of NWs. The decay time of the NW above critical diameter (dcrt, 30--40 nm) is found to be {~}13 000 s, while the smaller NW (dcrt) is 800 s. Herein, we propose that the surface-induced effective barrier height for different sizes of GaN NWs is the dominant factor that explains the apparent size dependence. The temperature-dependent decay time measurements determine an effective barrier height of 226 meV for a 65-nm NW, whereas the 20-nm NW has an effective barrier height of 32 meV, confirming that SBB effects of different sizes are responsible for the size-dependent PPC in m-axial GaN NWs.