Formation mechanism of Ohmic contacts on AlGaNGaN heterostructure: Electrical and microstructural characterizations

The electrical characteristics and interfacial reactions of TiAlMoAu metallization on AlGaNGaN heterostructures at various annealing temperatures ranging from 400 to 950 °C have been investigated in an effort to elucidate the Ohmic contact formation mechanism. A transition from Schottky to Ohmic electrical behavior occurred at ∼500 °C. This transition was engendered by the formation of a thin epitaxial TiN layer on the AlGaN. Binary and ternary intermetallic compound formations were observed to have formed as a result of intermixing and reactions among the metals. The Mo layer remained continuous until 700 °C, where it disintegrated into particles embedded in the metallic matrix. An optimal contact performance was obtained for TiAlMoAu on AlGaNGaN at 850 °C; this was correlated to TiN island formation along dislocations penetrating into the heterostructure beyond the plane of the two-dimensional electron gas. Overannealing degradation mechanism at 950 °C is also explained based on structural characterization.