Microstructural investigation of epitaxial aluminum films grown by molecular beam epitaxy

High-quality epitaxial Al thin films with thickness in nano-to atomic-scale were grown on Si, sapphire, and GaAs substrates by molecular beam epitaxy. Microstructure of Al films as well as the interfacial bonding between Al and substrates were carefully investigated. Structural analyses from x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) shows that the Al (111) films are epitaxially grown on Si, sapphire, and GaAs substrates. Atomic-resolution STEM imaging reveals smooth and sharp interfaces without any interlayer between Al films and substrate; and the interfacial bonding are determined as Al–Si, Al–O, and Al–Ga for Al grown on Si, sapphire, and GaAs, respectively. Atomic force microscopy shows very smooth surfaces of Al films with roughness of 0.16–0.75 nm. Secondary ion mass spectrometry (SIMS) reveals the chemical composition of the deposited films, highlighting the presence of minor amounts of trapped hydroxide content.