Electrical properties of Cu/Pd2Si Schottky contacts to AlGaN/GaN-on-Si HEMT heterostructures

In this study, we analyze gold-free Cu/Pd2Si Schottky contacts to AlGaN/GaN-on-Si HEMT (high electron mobility transistor) heterostructures. The silicide layers are fabricated through magnetron sputtering deposition using a stoichiometric Pd2Si target and are subjected to post-gate annealing at different temperatures. After annealing at 500 °C the leakage currents of the planar diodes are reduced below 3.2 mA/cm2. The microstructures of the diodes are analyzed through transmission electron microscopy (TEM). The post-gate annealing increases lateral dimensions of silicide grains. The electrical properties of the contacts are determined by performing temperature-dependent current-voltage (I-V-T) measurements. The forward current exhibits a specific voltage dependence, which is interpreted based on the two-diode model, reflecting the properties of the Pd2Si/heterostructure interface along with those of the heterostructure itself. The model is fitted to extract the parameters of the Schottky contacts. The barrier height of 0.91 eV and ideality factor n = 1.5 are obtained after post-gate annealing at 500 °C. The effects of post-gate annealing on the forward conduction and leakage mechanisms are then discussed. A simple method is proposed to estimate the polarization charge density, which is required to calculate the electric fields in the heterostructure in the reverse bias. It is observed that post-gate annealing affects the I-V characteristics, likely by reducing the density of the unintentional donors and trap states, leading to an increase in the apparent barrier height along with a reduction in the density of defects contributing to the leakage current.