Berreman effect

The spectra from strong oscillators have special features which are different from those from metallic and dielectric substrates. Different structures in tanf and A are observed on a metallic substrate, dependent on the thickness of the film (Fig. 4.65). For very thin films up to approximately 100 nm the Berreman effect is found near the position of n = k and n < 1 with a shift to higher wavenumbers in relation to the oscillator frequency. This effect decreases with increasing thickness (d > approx. 100 nm) and is replaced by excitation of a surface wave at the boundary of the dielectric film and metal. The oscillator frequency (TO mode) can now also be observed. On metallic substrates for thin films (d < approx. 2 pm) only the 2-component of the electric field is relevant. With thin films on a dielectric substrate the oscillator frequency and the Berreman effect are always observed simultaneously, because in these circumstances all three components of the electric field are possible (Fig. 4.66). 

Fig. 4.65. Different spectral features of tanf for a strong model oscillator at 1000 cm" on a metal substrate. The TO mode (1000 cm" ), Berreman effect (1050 cm" ), and excitation ofa surface wave (1090 cm" ) are seen for different 1150 thicknesses - 1, 5, 10, 50,100, 500, and 1000 nm.

It is usually possible to investigate very thin films (up to the subnanometer range) by use of infrared wavelengths, which are much greater than the thickness of the film (a factor of 10000) because of the interference optics of the strong oscillator (Berreman effect). 

Figure 6.4-7 Berreman effect observed with a ca. 22 nm thick layer of SiOi on Si substrate.

An outstanding gain in sensitivity, allowing one to identify and quantify surface layers of some nanometers thickness, can be observed for strong oscillators due to the Berreman effect. 

RAIRS is used for characterization of adsorbates on metal surfaces, their transformations and kinetics (for example CO on metallic surfaces), catalytic reactions, characterisation of semiconductor structures, high-temperature oxidation of metals (use of so-called Berreman effect), electrode/electrolyte interface, Langmuir-Blodgett and other ultrathin organic films. 

DEPENDENCE OF TRANSMISSION, ATR, AND IRRAS SPECTRA OF ULTRATHIN FILMS ON POLARIZATION (BERREMAN EFFECT)... 

As shown in Chapter 2, to optimize the contrast in the IR spectrum of an ultrathin film, it is necessary in many cases to use nonnormal angles of incidence and p-polarized radiation, which creates specific difficulties in the interpretation of the spectrum. The problems stem from the appearance of additional bands in the spectra of samples that are small relative to the wavelength these bands are due to the surface charges resulting from the polarization of the samples. The dependence of the transverse vibrational frequency of a polar crystal on the crystal size, called the size effect, was discovered by Frohlich [2]. A convincing explanation of this effect in the IR spectra of thin films was presented in 1963 by Berreman [3] while studying the transmission of 325-348-nm LiF layers. Consequently, this size effect in the IR spectra of ultrathin films became known as the Berreman effect by Harbecke et al. [4]. 

In this section the manifestation of the Berreman effect in experimental work will be illustrated by some typical examples and interpreted qualitatively. In the following section this effect will be considered from the viewpoint of Maxwell s... 

Identification of Berreman Effect in IR Spectra of Ultrathin Films... 

Infrared spectroscopy of ultrathin oxide films on Si exploits the Berreman effect, arising when p-polarized radiation is incident at an inclined angle (Sections 3.1... 

Berreman effect, which is the essential difference between spectra of ultrathin films and spectra of the absorption index of the film material. Several studies of dielectric films on metals [281-287] have illustrated this concept. In spite of this, a number of attempts have been made to assign the high-frequency bands to various structures in the dielectric [288-291] or, which is also inconsistent with optical theory (Section 3.2), to the LO film mode [292]. 

Electromagnetic wave propagating along a chain of oppositely charged ions can excite a transverse optical (TO) mode. To excite a longitudinal optical (LO) mode, the k-vector must make an angle with the chain in order to project a component of its E-vector along the chain (Berreman effect). 

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