Observation of superstructure in high-quality pseudomorphic films of NiAl grown on GaAs

Interest in the epitaxial nickel aluminum (NiAl) metal/gallium arsenide (GaAs) semiconductor material system is driven by the potential for its use in novel electrical and optical devices. Molecular beam epitaxy was used to grow thick, epitaxial, pseudomorphic layers of NiAl on GaAs substrates. In order to extend the critical thickness of the metal film, the NiAl layer in these samples was intentionally grown in the nickel-rich region of the equilibrium NiAl phase. The high quality of these films is evidenced by a narrow rocking curve diffraction peak and the observation of finite size oscillations in the double crystal X-ray diffraction from the NiAl layer. Plan view and cross-sectional transmission electron microscopy (TEM) show extra diffraction spots and corresponding superstructures in the image. An 8 × surface reconstruction is observed by in situ reflection high-energy electron diffraction (RHEED). These structures require the layer-by-layer growth mode, are temperature and composition dependent, and appear to be related. A model for Ni anti-site ordering is proposed that explains the observed diffraction spots and the pseudomorphic growth to extended thicknesses.