Interpretation of current transport properties at Ni/n–GaN Schottky interfaces

Current transport properties at Ni/n–GaN Schottky interfaces formed on oxide-etched or thermally oxidized surfaces are studied by current–voltage–temperature (I–V–T) and capacitance–voltage measurements. The results support existence of surface patches with low Schottky barrier height (SBH), which cause a leakage current of the Schottky structures. Based on “surface patch” model, the fraction of the total patch area of 10−4–10−5 and the SBH lowering of 0.4 eV within patches are deduced for the oxide-etched Ni/n–GaN metalorganic chemical vapor deposition (MOCVD) samples. The influence of surface patches was found much larger for the present Ni/n–GaN molecular beam epitaxy (MBE) samples. The inclusion of N-polarity domains in the Ga-polarity layer seems to be main origin of the patches for the MBE-grown samples. Ni/n–GaN Schottky samples formed on dry-oxidized or wet-oxidized MOCVD layers represented reduction of the effective SBH with negligible increase of the n value, indicating that an intermediary native oxide between Ni and n-GaN degrades the SBH. I–V–T characteristics showed that the leakage current due to surface patches is comparable with that of the oxide-etched sample. Concerning thermal stability of the bulk GaN (MOCVD), the impurity concentration in the layer increased after wet oxidation above 500 °C, although little change was observed after wet oxidation at temperatures below 400 °C. Finally, a postfabrication annealing in nitrogen at 400 °C led to increase of the effective SBH even for the oxidized Ni/n–GaN Schottky samples.