Effect of thermal annealing on the Ir/Ag contact to p-type GaN

In the present work, ohmic contacts of iridium(Ir)/silver(Ag) bi-layer contacts on p-type GaN have been fabricated. A metal Ag thin layer was selected as the capping layer due to its surface plasmon coupling and reflective characteristics at visible light emissions. The ohmic contacts were annealed at various temperatures to ascertain their thermal stability. The structural and electrical characteristics of the Ir/Ag contacts at 400 °C-700 °C were investigated, as thermally stable metal-semiconductor junction contacts are essential for high quality ohmic contact devices. The changes in the top surface morphology of the contacts on annealing were observed using scanning electron microscopy (SEM). Ohmic behavior and specific contact resistivity were determined using transmission line method (TLM). Current-voltage (I-V) measurements were carried out to verify the ohmic nature of the annealed contacts. All the as-deposited Ir/Ag contacts revealed non-linear I-V behaviors. However, the contacts become ohmic as a function of anneal temperature. The annealing induced improvement of the I-V behaviors of the Ir/Ag contacts could be related to the minimization of agglomeration of intermetallics and formation of intra-agglomerate voids. The Ir/Ag interlayer effectively prevents the void formation at the Ag-GaN interfaces. The remarkable contact resistance is attributed to the reduction of the oxide by Ir diffusion along with the formation of metal gallide phases at the p-type GaN surface region. The improved ohmic behaviors could be because of the inhomogeneous barrier formation at the contact scheme interface properties related to the breaking up of the IrO2 thin layer. The electronic transport theory at the Ir/Ag and semiconductor interface with inhomogeneous Schottky barriers shows that the existence of the IrO2 and IrO2/GaN could lead to higher built in electric field at contact schemes of GaN interface.