  Tungsten carbide (WC) contacts have been investigated as a novel gold-free
Schottky metallization for AlGaN/GaN heterostructures. The evolution of the
electrical and structural/compositional properties of the WC/AlGaN contact has
been monitored as a function of the annealing temperature in the range from 400
to 800{\deg}C. The Schottky barrier height ($\Phi$B) at WC/AlGaN interface,
extracted from the forward current-voltage characteristics of the diode,
decreased from 0.8 eV in the as-deposited and 400{\deg}C annealed sample, to
0.56 eV after annealing at 800 {\deg}C. This large reduction of $\Phi$B was
accompanied by a corresponding increase of the reverse bias leakage current.
Transmission electron microscopy coupled to electron energy loss spectroscopy
analyses revealed the presence of oxygen (O) uniformly distributed in the WC
layer, both in the as-deposited and 400{\deg}C annealed sample. Conversely,
oxygen accumulation in a 2-3 nm thin W-O-C layer at the interface with AlGaN
was observed after the annealing at 800 {\deg}C, as well as the formation of
W2C grains within the film (confirmed by X-ray diffraction analyses). The
formation of this interfacial W-O-C layer is plausibly the main origin of the
decreased $\Phi$B and the increased leakage current in the 800{\deg}C annealed
Schottky diode, whereas the decreased O content inside the WC film can explain
the reduced resistivity of the metal layer. The results provide an assessment
of the processing conditions for the application of WC as Schottky contact for
AlGaN/GaN heterostructures.
