Surface Reconstructions of Strained Epitaxial CoSi 2 /Si(100) Layers Studied by Scanning Tunneling Microscopy.
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SURFACE RECONSTRUCTIONS OF STRAINED EPITAXIAL CoSi 2/Si(100) LAYERS STUDIED BY SCANNING TUNNELING MICROSCOPY.
R. STALDER, C. SCHWARZ, H. SIRRINGHAUS and H. VON KANEL Laboratorium fuir Festkt5rperphysik, Eidgen~ssische Technische Hochschule Zurich, 8093 Zdrich, Switzerland
ABSTRACT Epitaxial single-domain CoSi 2(100) layers were grown on Si(100) by use of a template technique. In-situ scanning tunneling microscopy (STM) and reflection high energy electron diffraction (RHEED) were used for a detailed surface study. The ('J2x'12)R45 reconstruction of the Co-rich "C-surface" and the (342x412)R45 as well as a newly discovered (,42x42)R45 of the Si-rich "S-surface" were resolved in real space and are discussed in detail. The transition from the C- to the S-surface above 500 *C is related to a (2x2) reconstruction. INTRODUCTION Epitaxial CoSi 2 films on Si are an excellent model system for fundamental studies, e.g. of Schottky barrier formation, requiring perfect control of the atomic structure at an interface. According to a band structure calculation of Mattheiss and Hamann 11 the (100)-oriented CoSi 2/Si interface has favorable matching conditions for the transmission of electrons as required in applications such as the metal base transistor [2]. While the growth on Si( I l 1) wats widely studied and is now well under control [3 - 7], the situation is more complicated in Ohw case of (100)-substrates where various epitaxial orientations were observed 18 - 101. Rcccutlvy. single-oriented high-quality (100) layers were realized by the use of an optimized growill procedure including a Co template [11] or by codepositing Co and Si at Co-rich ratios at a temperature of 500 'C [12, 13]. In this paper a study of the reconstructions of high-quality single-domain CoSi 2/Si(100) strained layers by in-situ scanning tunneling microscopy (STM) is presented. GROWTH METHOD AND EXPERIMENTAL SETUP The substrates used in this study were n-doped 3-inch Si(100) wafers with a resistivity of 15 - 800 Q2cm and oriented to within 0.3 0 of the nominal axis. The preparation method included thermal desorption of the oxide at 840 °C under a weak Si-flux. Afterwards a Si-buffer layer of 1000 A thickness was grown by molecular beam epitaxy (MBE) at 660 0C at a deposition rate of 1 A/sec. Silicide growth was performed by a template technique [11] consisting of the deposition of 1.6 A - 2 A of pure Co followed by the codeposition of 5 - 7 A of silicide with a stoichiometry of CoSit. 6 - CoSit. 8 [12, 13] near RT. Epitaxial layers with various thicknesses up to 105 A were grown onto the annealed templates by further RT codeposition and annealing cycles. Depending on the surface stoichiometry we have observed different reconstructions as discussed in detail below. CoSi 2-C SURFACE AND CoSi 2 -S SURFACE, SURFACE STEPS According to the CaF2 structure of CoSi 2, adjacent (100) lattice planes (with a distance of a'/4 A) are alternately occupied with Co and Si, respectively. Each Co atom has a cubic environment of eight nearest Si neighbours whereas every
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