Atmoic-Scale Control of Oxide Substrate Surface/Termination and Novel Heteroepitaxial Growth
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Atmoic-scale Control of Oxide Substrate Surface/Termination a nd Novel Hetero epita xial Gro wth Mamoru Yoshimoto, Keisuke Mizuno and Takafumi Miyahara Materials & Structures Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori, Yokohama, 226-8503, Japan ( [email protected]) ABSTRACT Atomic-scale surface/termination of single-crystal oxide substrates were examined by coaxial impact collision ion scattering spectroscopy (CAICISS) and atomic force microscopy (AFM). CAICISS enabled us to determine the terminating atomic species and their arrangements of single crystal oxide substrates and epitaxial oxide films. Through thermal-annealing of the single crystal oxide substrates, atomically flat terrace and stepped structures were developed on the surface. The molecular layer-by-layer growth was verified by in situ monitoring of reflection high energy electron diffraction (RHEED) intensity oscillation. The atomic-scale substrate engineering made it possible to attain the novel heteroepitaxial growth such as step-decoration epitaxy resulting in the nanowire structures and diamond epitaxy on the ultrasmooth sapphire substrate. The diamond films could be grown epitaxially only on the atomically flat sapphire substrates by pulsed laser ablation of graphite. The novel application of the ultrasmooth sapphire substrate to the AFM observation stage for DNA molecules was also presented. INTRODUCTION Recently, heteroepitaxial growth of oxide thin films has been attracting much attention from the viewpoint of development of electronically active devices utilizing the superior properties such as high-Tc superconductivity, ferroelectricity, ferromagnetism, and giant magnetoresistance. Novel oxide-based devices are expected to be developed via combination of such specific functions [1]. In order to open new oxide electronics, we have proposed an advanced oxide epitaxy technology based on the nanoscale substrateengineering as well as on the atomic layer control of oxide films by laser MBE, i.e. pulsed laser deposition (PLD) in ultrahigh vacuum (UHV) [2]. The epitaxial growth manner is influenced by atomic-scale surface morphology and terminating atomic planes as well as lattice mismatching. The mirror-polished commercial substrates have the nano-scale irregular corrugation on the surface, leading to the appearance of heterogeneous termination with the different crystallographic planes. For example, perovskite oxides with the formula of ABO3 such as SrTiO3 have two possible terminating atomic planes on (100) atomically flat surfaces; AO layer (A-site plane) and BO2 layer (B-site plane), so that the commercial SrTiO3(100) substrates are thought to have the heterogeneous termination with the mixture of these atomic planes as well as those different from the (100) plane . The homogeneous terminations of substrate surfaces are considered to be essential for attaining the perfect epitaxy and interface [3,4]. Coaxial impact collision ion scattering spectroscopy (CAICISS) is a powerful tool to identify both atomic species
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