Nonlinear Optical Study of Si Epitaxy
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NONLINEAR OPTICAL STUDY OF Si EPITAXY
T. F. Heinz, M. M. T. Loy, and S. S. Iyer IBM T.J. Watson Research Center, Yorktown Heights, NY 10598
ABSTRACT The applicability of the nonlinear optical technique of surface second-harmonic generation to in-situ studies of epitaxial and non-epitaxial crystal growth of centrosymmetric materials is demonstrated. In measurements of the deposition of atomic Si on Si(1 11)-7x7 surfaces, the (anisotropic) second-harmonic response is seen to be sensitive to the ordering of fractional monolayers of adatoms. For deposition on a substrate held at room temperature, the second-harmonic data are consistent with the formation of a disordered adlayer on top of the original reconstructed surface. The results of real-time measurements of the thermal annealing of disordered Si adlayers of monolayer thickness are also presented.
INTRODUCTION In the past few years, the nonlinear optical process of second-harmonic generation (SHG) has been demonstrated to be a useful probe of surfaces and interfaces of centrosymmetric media [1]. Among optical techniques for the study of surface properties, SHG is unique in its intrinsic sensitivity on the monolayer level. This feature, arising from the basic selection rules for a second- order nonlinear effect, has led to the application of the method in a variety of studies of adsorbed species and of adsorbate-covered surfaces and interfaces. Recent work has also demonstrated the utility of the SHG technique as a tool for examining the symmetry and order of well-characterized crystal surfaces [2-41. Precise measurements have been performed on the symmetry of the Si(l 1 )-2xl and Si(111 )-7x7 surfaces, as well as on the thermally driven surface phase transformation between these two structures. The method has permitted surface processes such as the ioninduced disordering [3] and oxidation [3,5] of silicon to be followed in real time. In this paper, we consider some applications of the SHG technique to problems of silicon deposition and growth on clean Si( 111 )-7x7 surfaces. The surface SHG process is seen to provide a means of assessing the ordering in adlayers of monolayer and submonolayer thickness. This behavior is exemplified by real-time monitoring of the deposition of atomic Si on Si( 111 )-7x7 under epitaxial and non-epitaxial growth conditions. In an effort to understand some of the basic processes in epitaxial growth, we have also used the SHG probe to follow the thermal annealing of disordered Si adlayers of monolayer thickness created by room-temperature deposition. Before turning to a detailed account of these issues, we first discuss some general aspects of the SHG process for ordered surfaces and describe the salient features of the experimental procedure employed in these investigations.
Mat. Res. Soc. Symp. Proc. Vol. 75. ý 1987 Materials Research Society
698
SHG FROM CRYSTALLINE SURFACES The SH response of a material is generally described by the nonlinear susceptibility tensor X(2) relating the induced polarization at the harmonic frequen
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