Surfactant assisted ion beam erosion of GaN and AlN surfaces

We investigate the evolution of the surface morphology of AlN and GaN surfaces during ion beam erosion. AlN and GaN films grown on sapphire substrates were irradiated at room temperature with few keV Xe ions at normal ion incidence with ion fluences up to 4.1017 cm-2. The surface morphology was analyzed using atomic force microscopy and scanning electron microscopy. For GaN selective sputtering causes segregation of Ga on the surface, resulting in a dense coverage with small spherical Ga droplets. With increasing ion fluence the droplet size increases. The Ga coverage was quantitatively analyzed with SEM and Rutherford backscattering. For AlN, the rms roughness of the surface decreases with increasing ion fluence from initially 8 nm to about 3 nm at 4.1017 cm-2. No ripple or dot patterns were observed. Sputtering of AlN with simultaneous co-deposition of Ga, results in the formation of randomly distributed small Ga droplets and Al0.8Ga0.2 islands of few nm in size on the AlN surface. The rms roughness of such a surface is only about 1.3 nm. The density of droplets and islands decreases with increasing ion fluence. In the intermediate regions the AlN surface appears rather flat with rms roughness well below 1 nm.