Schottky and Ohmic Contacts on Non-Polar Cubic GaN Epilayers

In this work we focus on the fabrication of ohmic contacts and of Schottky barrier devices (SBD) on non-polar cubic GaN epilayers grown by molecular beam epitaxy (MBE). A Ti/Al/Ni/Au metallization was used for ohmic contacts and the contact resistance was measured by transmission line measurements (TLM). Ni, Pd, Ag and NiSi Schottky barrier devices 300 µm in diameter were fabricated by thermal evaporation using contact lithography on cubic GaN epilayers. The current-voltage (I-V) and the capacity-voltage (C-V) characteristics were studied at room temperature in detail. A clear rectifying behavior was measured in all SBDs. In the Ni and Ag SBDs an abnormal large leakage current under reverse bias was observed. Isochronal thermal annealing of these Ni and Ag based SBDs at 200°C in air improved the reverse characteristics by up to three orders of magnitude. This is in contrast to the Pd contacts, where the as grown contact showed already good performance and thermal annealing had nearly no influence on the I-V characteristics. For all SBDs the magnitude of the reverse current is generally larger than that expected due to thermionic emission and an exponential increase of the reverse current is observed with increasing reverse voltage. In-depth analysis of the I-V characteristic showed that a thin surface barrier is formed at the metal semiconductor interface and that crystal defects like dislocations may be the reasons for the discrepancy between experimental data and thermionic emission theory.