Formation mechanism of Ohmic contacts on AlGaN∕GaN heterostructure: Electrical and microstructural characterizations

Abstract

The electrical characteristics and interfacial reactions of Ti∕Al∕Mo∕Au metallization on AlGaN∕GaN heterostructures at various annealing temperatures ranging from 400to950°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 Ti∕Al∕Mo∕Au on AlGaN∕GaN 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.