Forward and reverse current transport mechanisms in tungsten carbide Schottky contacts on AlGaN/GaN heterostructures

In this paper, the forward and reverse current transport mechanisms in as-deposited and 400 °C annealed tungsten carbide (WC) Schottky contacts on AlGaN/GaN heterostructures have been studied. In particular, under forward bias, the WC/AlGaN Schottky contacts exhibited a deviation from the ideal thermionic emission model due to the occurrence of a tunneling component of the current. From the temperature dependence of the ideality factor, a characteristic tunneling energy E00 in the range of 33-36 meV has been estimated. On the other hand, two different transport mechanisms have been identified under reverse bias. At low reverse bias (VR < 2 V), Poole-Frenkel emission rules the current transport, with an emission barrier ϕt = 0.68 eV in the as-deposited contact, which increases up to ϕt = 0.79 eV upon annealing at 400 °C. This behavior has been correlated with the improvement of the metal/AlGaN electronic properties. At higher reverse bias (VR > 2 V), the leakage current is dominated by a thermally activated process with an activation energy (0.27 eV) that is independent of the Schottky contact fabrication process. In this case, the temperature dependence of the leakage could be well described by a two-dimensional variable range hopping conduction associated with the presence of surface defects in the material.