Importance of layer distribution in Ni and Au based ohmic contacts to p-type GaN

In this work, we report on the importance of layer distribution after annealing to form a high quality ohmic contact on p-type GaN, using nickel (Ni) and gold (Au) thin layer association. Both the standard GaN/Ni/Au and its reverse, GaN/Au/Ni on p-type GaN were studied. The Au/Ni stack exhibits the most promising results in this study. While the standard GaN/Ni/Au contact exhibits a quasi-linear current-voltage (I-V) characteristic, its counterpart, GaN/Au/Ni, shows pure ohmic behavior, with a specific contact resistance (ρc) as low as 2.0 × 10−4 Ω.cm2 after rapid thermal annealing (RTA) at 500 °C for 5 min under air ambient, equivalent to the best literature results. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses demonstrate the incomplete inversion of layers during annealing leading to a GaN/Ni/Au/NiO stack that explains why GaN/Ni/Au contact shows inferior electrical performance. On the other hand, for the GaN/Au/Ni contact annealed in the same conditions, the excellent results can be attributed to both (i) the presence of the gold layer at the interface with GaN, allowing the formation of gallide solid solution (Ga-Au) and (ii) the formation of NiO directly contacted with the p-GaN. Those two mechanisms are known to lead to the formation of good ohmic contact on p-type GaN. These results demonstrate that although GaN/Ni/Au is a standard contact for p-GaN layers, the opposite stack (GaN/Au/Ni) gives the best Ohmic behavior. This is important for achieving the best performance of GaN power diodes or transistors including a p-gate structure.