OBSERVATION OF SUPERSTRUCTURE IN HIGH-QUALITY PSEUDOMORPHIC FILMS OF NIAL GROWN ON GAAS

Interest in the epitaxial nickel aluminum (NiAl) metal/gallium arsenid e (GaAs) semiconductor material system is driven by the potential for its use in novel electrical and optical devices. Molecular beam epitax y was used to grow thick, epitaxial, pseudomorphic layers of NiAl on G aAs 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 an d the observation of finite size oscillations in the double crystal X- ray diffraction from the NiAl layer. Plan view and cross-sectional tra nsmission electron microscopy (TEM) show extra diffraction spots and c orresponding superstructures in the image. An 8 x surface reconstructi on 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 mod el for Ni anti-site ordering is proposed that explains the observed di ffraction spots and the pseudomorphic growth to extended thicknesses.