Investigation of forward and reverse current conduction in GaN films by conductive atomic force microscopy

We have used conductive atomic force microscopy (C-AFM) to investigate the forward and reverse bias current conduction of homo- and heteroepitaxial GaN-based films grown by molecular beam epitaxy. In the case of homoepitaxy, C-AFM shows enhanced current conduction at the centers of ∼30% of spiral hillocks, which are associated with screw dislocations. Local current-voltage spectra taken by C-AFM on and off such hillocks indicate Frenkel-Poole and field emission mechanisms, respectively, for low current levels in forward conduction. In the case of heteroepitaxial GaN films grown on sapphire, the correlation between conduction pathways and topography is more complex. We do observe, however, that films with more rectifying nominal Schottky behavior (less reverse leakage current) produce forward and reverse bias C-AFM images with strong asymmetry