IRRAS dielectrics

Optimization of a PM-IRRAS experiment on a dielectric substrate covered with a monolayer having a thickness d can be made using the behavior of the difference PM-IRRAS signal vs. the angle of incidence AS = S(d)-S(0). 

In order to calculate A.I/ I) from the measured PM IRRAS spectra, one has to determine functions J2 and Jq in an independent experiment. A reliable method to measure the PEM response functions was described by Buffeteau et al. [69]. Below we describe a similar method that we adapted with minor changes to use for electrochemical systems [81]. The spectroelectrochemical cell is replaced by the dielectric total external reflection mirror (a Cap2 equilateral prism can be used for this purpose). The second polarizer is inserted just after the PEM and set to admit p-polarized light (identical setting to that of the first polarizer). The PEM is turned off and the reference spectrum is acquired. This spectrum gives the intensity of the p-polarized light Ip (cal), which passes through the whole optical bench. 

The IRRAS method can be used to obtain information about ultrathin films not only at metals but also on semiconductor and dielectric (including liquid) substrates. This class of problem is applicable to many areas, including thin-fihn optics, electronic and electroluminescent devices [27] (Chapter 5), sensors and transducers [28], flotation technology [29] (Section 7.4.4), and biomedical problems [30, 31]. Although the sensitivity is much lower than when metallic substrates are used, the waiving of the metal selection rule allows both s- and /7-polarized spectra to be measured and thus a more thorough investigation of molecular orientation within the layer. 

By making the buffer layer optically thin (< 70 nm thick) but chemically thick, the absorption due to perpendicular modes of ultrathin films on dielectrics can be measured in the whole IR range with surface sensitivity above that in the transparent IRRAS but below that in metallic IRRAS. 

When both bordering media are transparent, one can apply transmission spectroscopy in polarized radiation (Section 2.1) or, when there is a difference in the refractive indices of these media, the ATR method and IRRAS. For each type of solid-solid interface, except for the metal-metal interface, one can study the layers in the contact zone by IRRAS or ATR in the transparent spectral range of one of the media in the system. To choose the technique with which to investigate dielectric (semiconductor)-liquid, dielectric (semiconductor)-semiconductor, and dielectric-dielectric interfaces, several factors must be considered, including the region of transparency of the media under study and the relationship between their refractive indices. If the medium with the largest refractive index is the most transparent, one should use the ATR method otherwise IRRAS is more appropriate. 

Fig. 3.10). The main observation from Fig. 3.10 is that as e decreases in the series Al, Cr, and Ti, the band near i>ro shifts to lower frequencies [in agreement with (Eq. 3.23)], whereas the band near vlo is unaffected (the latter conclusion also follows from Fig. 3.9). Similar results were obtained for IRRAS of LiF [3] and CdS [43] films on metallic and dielectric substrates. 

It is of interest to elucidate how the absorption of a substrate affects the band shape in p-polarized IRRAS of ultratbin films on transparent substrate. A representative example is the IRRAS spectra of a 1-nm hypothetical organic layer whose dielectric function is characterized by 5 = 0.001, y = 10 cm", Vo = 2800 cm", and eoo = 1-7 (Fig. 3.28). The simulations are for refractive index ( 3 < 2) and for substrates with a high refractive index ( 3 = 2.0-4.0), the absorption bands are oppositely directed while in the intermediate case the band is distorted assuming a derivative-like shape. 

Figure 3.28. Band shape in p-polarized IRRAS spectra of hypothetical organic layer 1 nm thick as function of refractive index ns of transparent substrate (indicated in figure). Dielectric function of film was specified by S = 0.001, > = 10 cm", vo = 2800 cm", and Eoo = ,

The band distortions are explained by the redistribution of contributions of the vertical and lateral components of the mode in the p-polarized spectrum as either the real or imaginary part of the refractive index of the substrate is changed. In spite of this, however, the SSR for dielectrics (Sections 3.3.2 and 3.11.4) is independent of the optical properties of the substrate. The phenomenon outlined above can also be considered from the viewpoint of geometric optics, rather than invoking the complex origin of the absorption bands in the p-polarized spectra (Section 3.2). The intensity of the radiation reflected from the film-substrate system can be represented as the sum of the intensities of the radiation reflected from the front fihn-snbstrate interface, 7i, and the radiation multiply reflected in and emerging from the film, I (see Fig. 1.12). Clearly, in IRRAS, the... 

It is noteworthy here that some bands of anisotropic films on transparent substrates can vanish from the jp-polarized IRRAS [69,70] or polarization modulation (PM) IRRAS [71, 72] spectra. This phenomenon is a consequence of the SSR for dielectrics (see Section 3.11.4 for more detail). 

Layers on Metals. The bands in IRRAS are positive, independent of the angle of incidence, as in the absorption spectrum. The band shape is sensitive to the dielectric properties of the metal substrate and the angle of incidence The band shape becomes asymmetrical with decreasing optical conductivity of the metal and at grazing angles. 

This model explains why SEIRA is observed in both s- and p- polarized IRRAS [384] and ATR [391, 405] spectra and in normal-incidence transmission spectra [377] and why the enhancement is not uniformly spread over each metal island but occurs mainly on the lateral faces of the metal islands [378, 384, 385]. The quasi-static interpretation of the SEIRA also defines the material parameters necessary for excitation and observation of SPR (1) The resonance frequency determined from the general Mie condition must be as low as possible and (2) Ime((Ures) must be as small as possible. The maximum enhancement effect should be observed for the absorption bands near the Mie (resonance) frequency of the particle. As mentioned in Section 3.9.1, the resonance frequencies of metal particles lie in the visual or near-IR range. However, they can be shifted into the mid-IR range by (1) increasing the aspect ratio of the ellipsoids, (2) adding the support to an immersion medium, (3) coating the particles by a dielectric shell [24, 406], or (4) varying the optical properties of the support [24, 349, 350, 384]. As emphasized by Metiu [299], the surface enhancement effect is not restricted to metals but can also be observed for such semiconductors as SiC and InSb. 

It follows that to distinguish the contribution of the surface to the reflectivity, the dielectric function of the bulk substrate should be known. However, in the case of a transparent substrate at energies less than Eg, 3 = 0 and the reflectivity becomes dAs". For transitions with energies hv > Eg, the second term in Eq. (3.41) cannot in principle be neglected. However, it was found that B 0 at energies lower than 3.2 eV for Si, 2 eV for Ge, 2.8 eV for GaAs, and 3.5 eV for GaP. When 0, the IRRAS spectrum is related to both and s" (see also Section 6.5). 

The SSR for dielectrics allows one to obtain a quick, qualitative impression about molecular orientations in ultrathin films. For example (Fig. 3.90), the spectrum of a SAM, obtained by IRRAS at negative absorbance due to the methylene and methyl stretching modes. 

In the case of a nonmetal substrate, both Rs and Rp contain contributions from absorption by the film, and interpretation of the PM-IRRAS signal in the form of Eq. (4.6) is less straightforward. Moreover, a dielectric substrate has its own large, specific, and spectrally dependent PM-lRRAS signal [532], To eliminate this contribution, the PM-IRRAS of an ultrathin film at a dielectric substrate is normalized with the PM-IRRAS signal of the clean dielectric [532, 568, 569, 585] ... 

Besides the standard methods of SIMS, AES, and XPS for investigation of these ultrathin dielectric layers in Schottky-type solar cells, IRRAS has also been applied [5-8]. Ultrathin (2-10-nm) Si02 films in metal-oxide-silicon strnctnres have also been investigated by IRRAS [9-14]. 

Low-temperature silicon dioxide films prepared by evaporating silicon in an oxygen atmosphere onto Mo were studied [30], It was found that the stretching Si—O bond appears as a broad asymmetric peak centered at 1178 cm in IRRAS of the film prepared at 50°C. Upon heating, this peak gradually shifts to higher frequencies up to 1252 cm for a fihn annealed at 1200°C. The correlation of the electric parameters of MOS capacitors with the dielectric and IR properties of the thermal and low-temperature oxides, as well as with their stoichiometry, were investigated [31, 32]. 

T, quasielastic neutron scattering, dielectric relaxation measurements BML-IRRAS... 

V and d are the wavenumber and film thickness, respectively. The method-dependent term in the round bracket, containing the refractive index of the dielectric medium and the angle of incidence, ni and 9, is a factor of 1.75 higher for our ATR method than for IRRAS on Au with an incidence angle of 80°, due to the higher ni (ATR diamond with ni = 2.4 instead of IRRAS air with ni = 1.0). For samples with the same molecular structure, the imaginary part of the material-dependent complex function in the square bracket should be identical. As the close similarity of the IRRAS spectrum of a PEG-SAM reported by Herrwerth et al. [14] suggests a very similar structure, it appears reasonable to take it as a... 

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