Nucleation mechanism for epitaxial growth of aluminum films on sapphire substrates by molecular beam epitaxy

Aluminum (Al) epitaxial films with various thicknesses are grown on sapphire substrates by molecular beam epitaxy (MBE). The nucleation evolution of surface morphology and structural property during the growth of Al epitaxial films on sapphire substrates are investigated in detail. It is found that the 10 nm-thick Al epitaxial films grown on the sapphire substrates show the full-width at half-maximum (FWHM) for Al(111) of 0.35° and the root-mean square (RMS) surface roughness of 2.4 nm. When the thickness increases, the surface initially starts to roughen and then becomes smoother. At the same time, the crystal quality of the Al epitaxial films becomes better thanks to the annihilation of dislocations. As the thickness of Al epitaxial films reaches 800 nm, the FWHM for Al(111) is 0.04° and the RMS surface roughness is 0.14 nm, indicating the high crystal quality and flat surface morphology of Al epitaxial films. The corresponding nucleation mechanism of Al epitaxial films grown on sapphire substrates is hence proposed. This work is of great significance for the fabrication of Al-based devices.