Reflection anisotropy spectroscopy

Principle of RAS, shown here in the case of a sample with dimers on a cubic (100) surface. If the alignment of the dimers is rotated by 90 °, the sign of the signal changes (case a, green to case b, red).

The Reflexionsanisotropiespektroskopie (RAS) is an optical spectroscopy similar to the ellipsometry , by which the anisotropy of solids and their surfaces is examined. Here, linearly polarized light is radiated onto a surface with almost perpendicular incidence and its reflectance is measured:

${\displaystyle RAS=2{\frac {r_{\alpha }-r_{\beta }}{r_{\alpha }+r_{\beta }}},r\in \mathbb {C} }$

${\displaystyle r_{\alpha }}$and denote the complex reflectances with respect to two orthogonal axes. ${\displaystyle r_{\beta }}$

If there is optical anisotropy in the measured sample, the reflected light is elliptically polarized. This polarization is then analyzed as a function of the photon energy . If the anisotropy, which can arise, for example, from dimers on the surface, is rotated by 90 °, this means a change in the sign of the signal (see Figure 1).

The method was introduced in 1985 to investigate the optical properties of the cubic semiconductors silicon and germanium . Because of its independence from ultra-high vacuum conditions, the application of the method was extended to the in situ monitoring of the epitaxial growth of semiconductors. If the crystal volume is optically isotropic, RAS becomes very surface sensitive and can also be used, for example, to examine adsorbates on surfaces. However, the signals are usually very complex and require theoretical modeling in order to be able to assign the individual spectral features to specific electronic transitions or surface reconstructions.

Individual evidence

1. ↑ ^{a b} M. M. May, H.-J. Lewerenz, T. Hannappel: Optical in situ Study of InP (100) Surface Chemistry: Dissociative Adsorption of Water and Oxygen . In: Journal of Physical Chemistry C . 118, No. 33, 2014, p. 19032. doi : 10.1021 / jp502955m .
2. ^ ^{A b} P. Weightman, DS Martin, RJ Cole, T. Farrell: Reflection anisotropy spectroscopy . In: Reports on Progress in Physics . 68, No. 6, 2005, p. 1251. bibcode : 2005RPPh ... 68.1251W . doi : 10.1088 / 0034-4885 / 68/6 / R01 .
3. ↑ DE Aspnes, AA Studna: Anisotropies in the Above-bandgap Optical Spectra of Cubic Semiconductors . In: Physical Review Letters . 54, No. 17, 1985, p. 1956. doi : 10.1103 / PhysRevLett.54.1956 .
4. ↑ W. Richter, J.-T. Zettler: Real-time analysis of III - V-semiconductor epitaxial growth . In: Applied Surface Science . 100-101, 1996, p. 465. doi : 10.1016 / 0169-4332 (96) 00321-2 .