Combinatorial investigation of structural quality of Au/Ni contacts on GaN

A combinatorial library of Au/Ni metallizations on GaN was microstructurally characterized by X-ray diffraction (XRD), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). The array of single- and bi-layered metal elements of systematically varying thicknesses was deposited by electron-beam evaporation on a GaN/c-sapphire wafer. The elements with a single layer of Au on GaN had a fiber texture with 〈1 1 1〉 preferred growth orientation. TEM revealed a 2nm thick amorphous contamination layer between the Au and GaN, which prevented the gold from being epitaxial. By contrast, nickel in both the single-layered Ni and bi-layered Au/Ni elements formed epitaxially on the GaN with a (1 1 1) _fcc //(0 0 0 1) _hex , 〈1 1 0〉 _fcc //〈1 1 2̄ 0〉 _hex orientation relation, as observed by TEM and EBSD. The Ni layer formed two types of domains related by a 60° rotation about 〈1 1 1〉 _fcc , which were replicated by the Au over-layer in the Au/Ni structures. The improved structural quality of the bi-layered Au/Ni as compared to the single-layered Au was due to the removal of native contamination from the GaN surface during the initial step of Ni deposition; this promoted epitaxial growth of both metal layers. However, as the nickel interlayer thickness was increased above 5 nm, the Au/Ni structural quality decreased, as measured by increased deviations from the (1 1 1) _fcc //(0 0 0 1) _hex orientation relation.