Conductive atomic force microscopy study of MBE GaN films

Abstract

We have used the techniques of atomic force microscopy (AFM) and conductive AFM (C-AFM) to investigate the morphology and localized current conduction of GaN films grown by molecular beam epitaxy (MBE) on metal organic chemical vapor deposition (MOCVD) templates. The most common type of surface morphology consists of undulating spiral "hillocks" terminated by small pits. A low density of holes are interspersed between these hillocks with typical diameters of ~150 nm and densities on the order of 10⁸ cm^-2. For C-AFM measurements, a Pt-coated AFM tip was brought into contact with the GaN surface to form a microscopic Schottky contact. In reverse bias, C-AFM shows localized current leakage at the centers of approximately 10% of spiral hillocks, which are presumably associated with screw dislocations. Shifts in forward-bias turn-on voltages and changes in the conduction mechanism are observed in these defect regions. Local <i>I-V</i> curves indicate a Frenkel-Poole mechanism for forward conduction on defect regions.