Formation of low-resistance transparent Ni/Au ohmic contacts to a polarization field-induced p-InGaN/GaN superlattice

Low-resistance and transparent ohmic contacts are formed by using a strained p-In0.15Ga0.85N (3 nm thick) capping layer and oxidized Ni (5 nm)/Au (5 nm) scheme. Unlike as-deposited Ni/Au contacts to p-GaN (without the strained p-InGaN), as-deposited contacts to InGaN/GaN produce ohmic behaviour. Upon annealing, the Schottky barrier height (SBH) of the contacts to the InGaN/GaN is reduced down to 0.29 eV and so the samples yield a contact resistivity as low as 6.3 x 10(-6) Omega cm(2). In addition, the annealed contacts give a light transmittance of 86% at a wavelength of 460 nm. Possible ohmic formation mechanisms for the Ni/Au contacts to InGaN/GaN are described in terms of a reduction of SBH, an increase of surface carrier concentration and the formation of a NiO:Au structure.