Surface differential reflectivity

Rg. 5.2-M Surface differential reflectivity (SDR) spectra for Si(l 11)2 X1, obtained with two polarizations with the electric vector parallel [011],filledcircles) and perpendicular ([211], open circles) to the chains of the r-bonded chain model. For the transitions shown by arrows in Fig. 5.2-36 hv < leV) the change in reflectivity can be observed only with the electric vector parallel to the chains. The spectrum for hv > 1 eV is associated to transitions along FJ in the SBZ. The change of anisotropy at approximately 1.2 eV is conform to the sum rule / Cxtodto = / eytodo) [2.70]... 

Fig.5.2-W Surface differential reflectivity versus energy for Ge(l 11)2x1 with polarization parallel and perpendicular to the chains of the jr-bonded model. The anisotropy for liv < 1 eV is smaller than for Si( 111 )2 x 1. The same considerations done in the caption of Fig. 5.2-41 hold also here [2.72]...

Whereas SE measures the ratio of reflection coefficients for different polarizations, various reflection difference techniques probe relative differences in reflectivity. Among these techniques one distinguishes surface differential reflectivity (SDR), surface photoabsorption (SPA) and reflection anisotropy spectroscopy (RAS). 

Problem 5.2. A thin film of thickness d Ais deposited onto a semiconductor surface. A light beam strikes the film from the vacuum side perpendicular to its surface. In fhe framework of the three-phase model derive an expression for the surface differential reflectivity at energies below the forbidden energy gap in terms of the film absorption coefficient. 

Figure 4. Differential spectra of CO chemisorbed on alumina-supported Ni particles both before and after heating to 425 K. Very little surface hydrocarbon is seen to form on the Ni particles. This lack of surface hydrocarbon reflects the selectivity of such catalysts for methanation over Fisher-Tropsch synthesis.

As they differentiate into populations with differing functions, B and T cells acquire molecules on their surfaces that reflect their specializations. It is possible to produce homogenous antibodies of a single specificity, termed monoclonal antibodies, which can recognize such surface markers. When laboratories from all over the world compared the monoclonal antibodies they had raised, it was found that clusters of monoclonal antibodies were recognizing the same molecule on the surface of the lymphocyte. Each surface molecule so defined was referred to as a CD molecule (Table 2), where CD refers to a cluster determinant. 

In fact, with the very recent addition of differential reflection spectroscopy (DRS) to the suite of applicable technologies, as described in Chapter 15, we now have the possibility of sensing trace quantities of explosives where they are most often found in the environment, adsorbed to solid surfaces. Technologies that can, like DRS, locate these traces in situ offer a very different way to approach the problem. There have been several recent attempts to do this in situ detection from some distance away. To date the DRS seems the most successful. It has demonstrated detection at a range of a few meters. 

Hummel, R. E. Differential reflectance spectroscopy in analysis of surfaces, in R. A. Meyers, Ed. Encyclopedia of Analytical Chemistry, Wiley, Chichester, 2000, p. 9047-9071. 

The structures were grown in an ultra high vacuum (UHV) chamber VARIAN with a base pressure of 2-10 °Torr equipped with differential reflectance spectroscopy (DRS) [3] for a study of optical properties of the samples. Samples were cut from n-type 0.3 D cm Si(l 11) substrates. The silicon was cleaned by flashes at 1250 °C (7 times). Surface purity was controlled by AES. RDE was carried out at 500 °C, 550 °C, and 600 °C. The Cr deposition rate was about 0.04 nm/min controlled by a quartz sensor. An additional annealing during 2 min at 700 °C was done for all samples before the growth of silicon epitaxial cap layer. 

UV-VISIBLE reflectance spectroscopy is used to investigate the optical properties of metal surfaces and its change with electrode potential, to detect surface states at the metal-electrolyte interface. Differential reflectance spectroscopy gives information on surface reactions or adsorbate formation. 

The double modulation of the inddent beam and the mathematical treatment of the detected signal allow a differential reflectivity AR/R) surface signal to be obtained in a single step and with the whole dynamic range of the detection. 

Growth experiments were carried out in two ultra high vacuum (UHV) cambers with sublimation sources of Si, Fe and Cr and quartz sensors of film thickness. Optical properties of the samples were studied in UHV chamber VARIAN (210 10Torr) equipped with differential reflectance spectroscopy (DRS) facilities. The samples surface was studied in the second UHV chamber (1 -10 9 Torr) equipped with LEED optics. Si(100) and Si(l 11) wafers were used as substrates for different series of the growth experiments. For the growth of silicide islands, metal films of 0.01-1.0 nm were deposited onto silicon surface. Silicon overgrowth with the deposition rate of 3-4 nm/min was carried out by molecular beam epitaxy (MBE) at 600-800 °C for different substrates. The samples were then analyzed in situ by LEED and ex situ by HRTEM and by... 

The reflectivity is defined as the ratio of the intensities of the reflected and incident beams and should be differentiated from the reflectance which is the ratio of the amplitudes of the incident and reflected waves. The reflectance in general is a complex number because there is usually a change in phase of a wave on reflection whereas reflectivity is a real number varying from zero to unity. The specular reflection can provide information on the composition distribution normal to the surface. The reflectivity is a function of both the angle of incidence of the beam to the surface and the refractive index changes of the substrate. The reflectivity is a function of the length scale of interactions of... 

In a reflectance experiment, incident light reflected from a sample at a surface / 2 is measured and compared to light reflected from a bare surface, Ri. The differential reflectance, ARIRj, where A/ is i> is the difference in reflectance between a clean surface and that on which the analyte is placed. In a spectroelectrochemical experiment, it may indicate the difference in reflectance in the absence and presence of applied potential. Since this is a differential value, it is frequently very small and therefore it is important to limit background contributions to the signal. 

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