Elementary Excitations II: Pair and Collective Excitations
Optical spectroscopies are emerging as particularly promising tools to probe surfaces, since they allow for in situ, non-destructive and real-time monitoring under challenging conditions as may be encountered, for instance, during epitaxial growth. For ep
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6.1 Probing Surfaces by Excitations 6.1.1 Optical Spectroscopies Optical spectroscopies are emerging as particularly promising tools to probe surfaces, since they allow for in situ, non-destructive and real-time monitoring under challenging conditions as may be encountered, for instance, during epitaxial growth. For epitaxial growth by means of chemical reactions, such as, e.g., metal-organic chemical vapor deposition (MOCVD), optical spectroscopies provide the only possibility for such monitoring. Other advantages are that the material damage and contamination associated with charged particle beams are avoided. Insulators can be studied without the problem of charging effects, and buried interfaces are accessible owing to the large penetration depth of the electromagnetic radiation. Optical techniques offer micron lateral spatial resolution and femtosecond temporal resolution. However, since light penetration and wavelength are much larger than surface thicknesses (a few ˚ A), such techniques are actually poorly sensitive to surfaces. Some ‘tricks’ have to be employed in order to increase their surface sensitivity. The experimental progress in the characterization of surfaces using light has been summarized in a couple of excellent reviews and monographs [6.1–6.5]. Theoretical considerations can be found in review articles by R. Del Sole [6.6,6.7]. The probing depth of light in a solid, even in the spectral range of highest absorption, is of the order of 10–500 nm. For a characteristic surface layer of 0.5 nm thickness, the relative surface contribution to the total optical signal only amounts to 10−1 –10−3 . Several approaches have been developed to improve the surface sensitivity. The basic idea is to measure difference signals which enhance the surface contribution with respect to that of the bulk. Four techniques are commonly used. One is surface differential reflectance (SDR) spectroscopy. It is based on measuring the difference in reflectance due to chemical modification of the surface, for example, often the adsorption of oxygen or hydrogen. The percentage difference is related to the surface structure. However, to what extent it is related to the clean or to the chemisorbed surface and whether or not it is sensitive to the spectrum of surface states and/or to the atomic structure of the surface, is in general difficult to determine [6.8]. Figure 6.1 shows
F. Bechstedt, Principles of Surface Physics © Springer-Verlag Berlin Heidelberg 2003
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6. Elementary Excitations II: Pair and Collective Excitations
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Fig. 6.1. Differential reflectance spectra of a single-domain Si(111)2×1 surface for light polarized along the x [¯ 211] (open circles) and y [0¯ 11] (dots) directions. From [6.9].
a spectrum that documents the breakthrough of SDR spectroscopy because of the use of polarized light on Si(111) samples with single-domain 2×1 reconstruction [6.9]. An oxidized surface is used as a reference. After oxygen chemisorp
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