Thermal annealing effect on electrical and structural properties of Tungsten Carbide Schottky contacts on AlGaN/GaN heterostructures

Tungsten carbide (WC) contacts have been investigated as an original 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 degrees C. The Schottky barrier height (phi(B)) at the WC/AlGaN interface, extracted from the forward current-voltage characteristics of the diode, decreased from 0.82-0.85 eV in the as-deposited and 400 degrees C annealed sample, to 0.56 eV after annealing at 800 degrees C. This large reduction of phi(B)was accompanied by a corresponding increase of the reverse leakage current. Transmission electron microscopy coupled with electron energy loss spectroscopy analyses revealed the presence of oxygen (O) uniformly distributed in the WC layer, both in the as-deposited and 400 degrees 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 degrees 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 degrees 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.