STM Studies of Ultra-Thin Overlayers of Cu on SrTiO 3 (001) Surfaces
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YONG LIANG, D. L. CARROLL, DAWN A. BONNELL Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104
ABSTRACT Scanning tunneling microscopy has been used to study the initial stage of Cu growth on SrTiO3(001) surfaces. Our STM results show that Cu grows initially into 3-D island-like structures. Two different morphologies were observed. In some areas loosely bound individual Cu islands appear preferentially attached to step edges. The islands were frequently relocated by the STM tip during the scanning as a result of tip-surface interaction. Other areas have higher densities of Cu islands that exhibit a narrow size distribution with an average diameter of 70 A and are homogeneously distributed on those areas. The driving force for the morphological evolution of Cu on the SrTiO3(001) surface is discussed in terms of surface free energy and interfacial interaction of metal on transition metal oxide surfaces.
INTRODUCTION For the past 20 years, systems with strong metal-support interactions have been of intense interest. Metal-oxides such as, TiO2, SiO2, NiO, MgO, and A1203 have been studied extensively as supports for highly reactive transition metal films[I-9]. Since the reactivity ofthe metal overlayer with oxygen from the substrate determines degree of charge transfer, structural factors such as oxygen coordination and surface termination may play important roles in wetting phenomena. The copper-oxygen interaction is known to be of importance to the catalytic properties of supported Cu. Further, Cu-O interaction and charge transfer of dorbital electron in a perovskite based structure is thought to play a significant role in high temperature superconductivity. The metalization of SrTiO3 with Cu provides an interesting system for the study of Cu-O interactions. This support differs from those previously studied
in that it is a ternary compound that has a complex surface structure[ 10,11]. Wetting phenomena of Cu overlayers on metal-oxide supports have been investigated by several other workers[ 1,7,9]. In the case of TiO2, general thermodynamic arguments have
been useful in explaining the overall behavior of the film[1-4, 12]. A comparison of the free energies of the film, interface and substrate surface provides qualitative insight into the evolution of specific film morphologies. In this work, bulk thermodynamics have been applied 653 Mat. Res. Soc. Symp. Proc. Vol. 318. ©1994 Materials Research Society
to the formation of Cu islands on SrTiO3 in the initial stages of film growth. Island growth was observed by scanning tunneling microscopy. The topography and mobility of Cu on the surface suggest a small metal-surface interaction which is consistent with observed wetting
behavior of Cu on other oxides.
EXPERIMENTAL METHODS Our experiments were carried out in ultra high vacuum (UHV) with base pressure of 2x10- 10 Torr. The size of the single crystal SrTiO 3 sample was 10mm x 5mm x 1mm. The surface was polished with alumina paste down to 0.05 ;LM and was oriented wit
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