Effect of substrate type on Ni self-assembly process

Ni self-assembly has been performed on GaN (0001), Si (111) and sapphire (0001) substrates. Scanning electron microscopy (SEM) images verify that the Si (111) substrate leads to failure of the Ni assembly due to Si–N interlayer formation; the GaN (0001) and sapphire (0001) substrates promote assembly of the Ni particles. This indicates that the GaN/sapphire (0001) substrates are fit for Ni self-assembly. For the Ni assembly process on GaN/sapphire (0001) substrates, three differences are observed from the x-ray diffraction (XRD) patterns: (i) Ni self-assembly on the sapphire (0001) needs a 900 °C annealing temperature, lower than that on the GaN (0001) at 1000 °C, and loses the Ni network structure stage; (ii) the Ni particle shape is spherical for the sapphire (0001) substrate, and truncated-cone for the GaN (0001) substrate; and (iii) a Ni–N interlayer forms between the Ni particles and the GaN (0001) substrate, but an interlayer does not appear for the sapphire (0001) substrate. All these differences are attributed to the interaction between the Ni and the GaN/sapphire (0001) substrates. A model is introduced to explain this mechanism. (paper)