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p-GaN/u-GaN alternating-layer nanostructures are grown with molecular beam epitaxy to show a low p-type resistivity level of 0.04 Ohm-cm. The obtained low resistivity is due to the high hole mobility in the u-GaN layers, which serve as effective transport channels of holes diffused from the neighboring p-GaN layers. A model for estimating the current penetration behavior in a p-GaN/u-GaN alternating-layer structure when its I-V characteristics is to be measured is proposed. In this model, an exponential decay with a characteristic penetration depth is assumed for a layered structure of low effective conductivity. In a high-conductivity structure, this penetration depth is regarded as infinity such that the depth-dependent current distribution is uniform. By growing p-type structures with an upper portion of a layered structure of unknown effective conductivity and a lower portion of high conductivity, which can be a layered or a uniform structure, of different individual thicknesses, we can have a sequence of sheet conductance data for best-fitting with the proposed model to simultaneously obtain the characteristic penetration depth and effective conductivity of the upper layered structure. Simulation studies are performed to provide the results supporting the proposed model. From the simulation results, it is found that the key factor hindering the current penetration is the low conductivity and finite thickness of a sub-layer around the middle of a u-GaN layer, which is not covered by the hole diffusion range from the neighboring p-GaN layers.
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