Vertical Schottky Contacts to Bulk GaN Single Crystals and Current Transport Mechanisms: A Review

Wide band gap III-nitride materials have gained considerable attention as promising semiconductor materials for light-emitting photonic diodes and high-frequency/power devices. However, the material quality of GaN grown on foreign substrates has hindered the achievement of theoretically superior properties of GaN in commercialized real devices. GaN epilayers grown on GaN substrates (GaN-on-GaN) offer a lower dislocation density and thus a higher epitaxial quality of GaN. However, the critical issues of metal contacts to bulk GaN, such as barrier height, leakage currents, and interfacial states, can limit the performance of bulk GaN-based devices. In this review, Schottky metal contacts to bulk GaN are described, and their electrical characterization is discussed in terms of the barrier formation and current transport mechanisms, namely: (1) Schottky barrier inhomogeneity, (2) Schottky barrier modulation by an ultrathin interlayer, (3) leakage current transport, (4) dislocation-related current transport, and (5) deep level defects. However, the dislocation density in GaN-on-GaN remains high. Therefore, further research for clarifying the correlation between current conduction and point/extended defects is necessary to enhance the performance and reliability of bulk GaN-based devices.