Influence of Morphology on Current–Voltage Behavior of GaN Nanowires

We demonstrate the effect that the different morphologies of molecular beam epitaxy-grown GaN nanowires (NWs) can have upon current-voltage (I-V) behavior. Two aspects of NW morphology were investigated. The first aspect was the NW diameter, d
_NW
. For single-crystal Si-doped GaN NW devices with d
_NW
<; 120 nm, I-V curves were nonlinear. In contrast, single-crystal Si-doped NWs from the same growth run with d
_NW
> 120 nm consistently showed ohmic I-V behavior. This discrepancy is likely the result of the comparatively larger surface depletion in thin NWs, which contributes to 1) an increased contact barrier, and 2) a barrier resulting from an axial band offset between the portion of the NW directly beneath the contact and the portion extending from the contact. The second aspect of NW morphology that we investigated was NW coalescence, which occurs when neighboring NWs fuse together during growth. I-V measurements of undoped coalesced NWs showed that these structures can have a free carrier concentration that is significantly higher than the background carrier concentration that is present in single-crystal (noncoalesced), undoped NWs.