Influences of thermal annealing and humidity exposure on surface structure of (100) single-crystal MgO substrate
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Influences of thermal annealing and humidity exposure on surface structure of (100) single-crystal MgO substrate M. P. Delplancke-Ogletree,a) M. Ye, and R. Winand Department of Metallurgy and Electrochemistry, Faculty of Applied Sciences, CP165, Universit´e Libre de Bruxelles, Avenue F.D. Roosevelt 50, B-1050 Brussels, Belgium
J. F. de Marneffe and R. Deltour Laboratory of Solid State Physics, CP233, Faculty of Sciences, Universit´e Libre de Bruxelles, Boulevard du Triomphe, B-1050 Brussels, Belgium (Received 21 September 1998; accepted 31 December 1998)
We studied the influence of thermal annealing on the surface structure of (100) singlecrystal MgO substrates by atomic force microscopy (AFM). By annealing MgO substrates at various temperatures for 4 h in flowing oxygen, we showed that the surface reconstruction could be explained by considering surface diffusion, surface evaporation, and condensation. At an annealing temperature of 1473 K, a stepped structure was formed with screw dislocations acting as step sources. The influence of humidity on the surface morphology of MgO substrates was also studied by exposing them to a constant humidity of 40 and 80% for different times. After an exposure time of 1.5 h in 80% humidity, the substrate surface was already covered by reaction products. For the 40% humidity, the corresponding time is 10 h. The major reaction product was identified as Mg(OH)2 by x-ray diffraction.
I. INTRODUCTION
MgO (100) single crystals have been used extensively for studies of thin film formation and epitaxial growth. Due to a good match in lattice constants, similarities of crystal symmetry, and a low chemical reactivity with many elements or compounds, MgO has been selected for the preparation of oriented metallic thin films,1 ferroelectric oxides,2 and high temperature superconducting films.3,4 It is well known that the quality of the substrate surface strongly affects the crystallinity and material properties of the grown films. Surface morphology (i.e., step structure) plays an important role during the nucleation stage by offering preferred nucleation sites and so influencing the microstructure of the films. Kabasawa et al.5 and Awaji et al.6 showed that YBa2 Cu3 O72d films grown on defective MgO substrate surfaces were not suitable for high electric field applications. The importance of MgO surface quality is greater for very thin films. Holt et al.7 investigated the modification induced by gaseous or liquid water on MgO surface morphology and confirmed that the degradation of YBa2 Cu3 O72d thin film properties is related to chemical modification of the MgO substrates.8 Therefore, it is essential to prepare high quality substrate materials to grow thin epitaxial films. It is also critical to monitor the effect
a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 14, No. 5, May 1999
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of atmospheric exposure with time so MgO crystals can be use
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