Electrical Properties of Graphene Contacts to AlGaN/GaN Heterostructures

Abstract

A nanoscale electrical characterization of graphene (Gr) contacts to AlxGa1-xN/GaN heterostructures has been carried out using conductive atomic force microscopy. The impact of the AlGaN microstructure on the current transport at Gr/AlGaN interface was evaluated considering two Al0.25Ga0.75N/GaN heterostructures with very different quality in terms of surface roughness and defectivity, i.e. a uniform and defect-free sample and a sample with a high density of V-defects, that locally cause a reduction of the AlGaN thickness. Rectifying contacts were found on the bare (Gr-free) AlGaN surfaces of both samples, but with a more inhomogeneous and lower Schottky barrier height (ΦB≈0.6 eV) in the presence of V-defects with respect to the case of the uniform AlGaN (ΦB≈0.9 eV). Very different electrical behaviour was observed for Gr on the two AlGaN samples, i.e. a low barrier height Schottky contact (ΦB≈0.4 eV) for the uniform AlGaN and an Ohmic contact for the defective AlGaN. Both Schottky and ohmic Gr/AlGaN contacts exhibit an excellent lateral uniformity, that can be ascribed to an averaging effect of the Gr electrode over the AlGaN interfacial inhomogeneities.