Establishing an upper bound on contact resistivity of ohmic contacts to n-GaN nanowires**Contribution of the US government, not subject to copyright.

Abstract

Contact resistivity ρc is an important figure of merit in evaluating and improving the performance of electronic and optoelectronic devices. Due to the small size, unique morphology, and uncertain transport properties of semiconductor nanowires (NWs), measuring ρc of contacts to NWs can be particularly challenging. In this work, Si-doped n-GaN NWs were grown by molecular beam epitaxy. Four-contact structures with 20 nm Ti/200 nm Al contacts were fabricated on individual NWs by photolithography, and the contacts were annealed to achieve ohmic behavior. Two-point resistances R23 and four-point collinear resistances R23collinear were measured between the middle two contacts on each NW. These resistances were then modeled by taking into account the non-uniform distribution of current flow along the length of each contact. Contrary to the assumption that the resistance difference R23−R23collinear is equal to the total contact resistance Rc, the distributed-current-flow contact model shows that R23−R23collinear ≪ Rc when ρc is sufficiently small. Indeed, the measured R23−R23collinear was so small in these devices that it was within the measurement uncertainty, meaning that it was not possible to directly calculate ρc from these data. However, it was possible to calculate an upper bound on ρc for each device based on the largest possible value of R23−R23collinear. In addition, we took into account the large uncertainties in the NW transport properties by numerically maximizing ρc with respect to the uncertainty range of each measured and assumed parameter in the contact model. The resulting upper limits on ρc ranged from 4.2 × 10−6 to 7.6 × 10−6 Ω cm2, indicating that 20 nm Ti/200 nm Al is a good choice of ohmic contact for moderately-doped n-GaN NWs. The measurement and numerical analysis demonstrated here offer a general approach to modeling ohmic contact resistivity via NW four-point measurements.