Size dependent surface band bending in GaN nanowires*

Group III-nitride nanowires (NWs) are recently emerged as a potential candidate for the single nanostructure high-performance optoelectronic devices because of the unidirectional conduction of charge carriers and absence of extended defects. However, surface states play important role in nanostructure-based device performance. The role of surface states and Fermi-level pinning is still unclear for III-nitride semiconductors. Therefore, the present study intends to investigate the effect of surface states on the band bending of the unintentionally doped n-GaN NW using Kelvin probe force microscopy (KPFM) with high spatial resolution.
The KPFM measurements were carried out at high vacuum (~10^-7 mbar) to avoid the effect of surface adsorbents. Single-pass imaging was adopted for the simultaneous measurement of topography, and the contact potential difference (CPD). Several measurements were carried out for the NWs with different diameter and the corresponding surface band bending were calculated. The surface band bending value was found to depend on the diameter of the NWs. Diameter dependant surface band banding of the NWs is attributed to the variation in the density of surface states in the NWs.