Ohmic contacts properties of Ni/Ag metallization scheme on p-type GaN

In the present work, we report on the characteristics of nickel(Ni)/silver(Ag) bi-layer contacts on p-type GaN. High quality doped GaN layers were grown on silicon (1 1 1) substrates using high temperature grown AlN (about 200 nm) as buffer layer by radio frequency nitrogen plasma-assisted molecular beam epitaxy (PAMBE). Doping was done using high purity Mg as p-type dopant. In this work, a metal Ag layer was selected as the capping layer due to its reflective characteristics at visible wavelengths. The structural and electrical stability of the contacts at various annealing temperatures (400–700 °C) were investigated, as thermally stable metal semiconductor contacts are essential for high quality devices. Changes in the surface morphology of the contacts on annealing were examined using scanning electron microscopy (SEM). Specific contact resistivity was determined using transmission line method (TLM) and current–voltage (I–V) measurements. The low contact resistance is attributed to the reduction of the native oxide by Ni diffusion along with the formation of Ni and Ag related-gallide phases at the p-GaN surface region. We deduce that high Mg doping may lead to the creation of a large number of deep level defect in p-GaN, leading to the reduction of the depletion layer width in the p-GaN near the interface and an increase in the probability of thermionic filed emission.