Barrier Inhomogeneity of Ni Schottky Contacts to Bulk GaN

Gallium nitride (GaN) is a promising candidate for high-power and high-frequency devices. To date, the lack of large area bulk GaN materials of reasonable cost and quality has limited the technology almost completely to lateral devices. However, vertical structures are attractive to obtain a higher current density and a reduced device size. In this work, the electrical behavior of a Ni/Au Schottky barrier on bulk GaN material is studied, using vertical Schottky diodes. The forward current-voltage characteristics of the diodes reveal a temperature dependence of both the ideality factor (n) and of the Schottky barrier height (phi(B)). The ideal value of the barrier of 1.72eV extrapolated at n=1 is in agreement with the results obtained by capacitance-voltage measurements. A nanoscale electrical analysis performed by conductive atomic force microscopy (C-AFM) allow to visualize the barrier height inhomogeneity and to correlate the current distribution to the surface morphology of the material. The barrier inhomogeneity explains the temperature behavior of ideality factor and barrier height determined by the macroscopic diodes. Preliminary structural analyses carried out by transmission electron microscopy (TEM) of the metal semiconductor interface revealed a typically flat Au/Ni bilayer structure, the Ni layer being epitaxial to GaN, with some mosaicity.