Chemical Modification of Surface Steps on SI(111) Vicinal Wafers: a Bonding Model for Phase Changes in Second Harmonic G
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Z. JING, G. LUCOVSKY AND J. L. WHITTEN Department of Physics, Box 8202, North Carolina State University, Raleigh, NC, 27695. Department of Chemistry, Box 8201, North Carolina State University, Raleigh, NC, 27695. ABSTRACT There have been several studies of second harmonic generation (SHG) from chemicallymodified vicinal Si(111) wafers. The SH fields contain one-fold (0) and three-fold (30) symmetry contributions, which originate respectively from the terrace and surface step atoms. The phase of these contributions are different for native oxides, and are a function of the frequency of the incident radiation, w. To identify the origin of these different phases for the terrace and step SH fields, we use a classical anharmonic oscillator model based on two assumptions: (a) a significant fraction of Si atoms at the steps have dangling bonds when oxides are formed below -850 C, and (b) these step atoms are linked to atoms at the bottom of the steps by O-Si-O groups following annealing at > 900 0C. 1. INTRODUCTION Recently, there have been several studies of second harmonic generation (SHG) from chemically-modified vicinal Si(lll) surfaces[I-7]. The polarized SH signal from the Si surfaces and Si-SiO2 interfaces has been observed to be anisotropic; i.e., it varies as the sample is rotated about its surface normal. This angular anisotropy of the SH field depends on the symmetry of the surface. For example, the SH field from the Si(111) surface exhibits a three-fold rotational symmetry. The introduction of steps by using vicinal wafers adds a one-fold symmetry to the SH field, so that the rotational anisotropy of the SH field from vicinal Si(111) surfaces displays a superposition of three-fold and one-fold contributions. On an ideal Si(111) surface, each surface atom possesses a three-fold symmetry. On a vicinal Si(111) surface, however, atoms on the step edges with bond projections in the [11ý] direction display a two-fold symmetry axis, while atoms on the terrace maintain their three-fold symmetry. The symmetry properties and dependence of the relative contributions of one-fold and three-fold SH signals on the step density (or, equivalently the offset angle) confirm that the three-fold and one-fold SH signals originate from the terraces and steps, respectively[1-7]. 287 Mat. Res. Soc. Symp. Proc. Vol. 318. 01994 Materials Research Society
Generally, the intensity at the frequency 2w can be written as 2 Iab( w) =
Ia0 exp(iz 0 )
+ a1 exp(i$l)cos(41) + a 3 exp(iD3 )cos(3'I) 12
where a 0, a1 and a3 are amplitude factors proportional to the second-order nonlinear susceptibility, X'jkl, for the isotropic, C factors[5-7].
and C3v SH components, and -0' q1' and -3 are phase
In this representation, Y is the angle between the SH field vector and the [112]
direction. The subscript, ab, denotes the relative polarization of the incident w field, and the reflected SH 2w field. The three-fold (3,k) and one-fold (F) symmetry contributions were found to have different phases for Si surfaces terminated by native oxides and this ph
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