Structural investigation of GaN nanograins nucleated on a sapphire substrate

The structural characteristics of GaN layers nucleated by Ga-assisted nitridation on an Al ₂O ₃ (0001) substrate were investigated by using atomic force microscopy (AFM) and synchrotron X-ray scattering. The nitridation was carried out by exposing an sapphire substrate in a quartz reactor to NH ₃ and Ga vapor carried by Na. The AFM images of the surface morphology for the samples showed that the mean height and width of the GaN grains formed on the substrate had a strong dependence on the growth temperature. Particularly, for the sample grown at 600°C with a mean width of 50 ± 5 nm, the specular reflectivity obtained through the (002) Bragg peak revealed that the nucleated GaN layers consisted of strained and fully relaxed layers. A detailed analysis of the X-ray scattering data showed that the 4.5 ± 0.3-nm-thick GaN layers were compressively strained and highly coherent with the substrate, but that approximately 17-nm-thick fully relaxed layers were relatively disordered in orientation. From the combined results of AFM and X-ray rocking curve measurements, we could conclude that fully relaxed nanograins were grown on top of highly strained GaN layers, not directly on top of the sapphire substrate.