Common phonon spectrum

The technique of Raman scattering (RS) to study vibrational spectra in the numerous polytypes of SiC will be described. An explanation of the various notations used to describe the stacking sequences in these polytypes will then be given. Section C discusses the various optical phonons studied by RS and the concept of a common phonon spectrum for all polytypes will be introduced. Raman studies are also used to assess crystalline structure and quality of epitaxial layers of SiC on Si and SiC substrates. Section D outlines several other excitations of interest, e.g. polaritons, plasmons, and electronic RS, as well as impurity and defect recognition in irradiated and ion implanted material. 

RS studies to other polytypes and verified the concept of a common phonon spectrum (within 2%) for all polytypes [13]. They also were able to construct the phonon dispersion curves for SiC from their first-order RS data from various polytypes (except 3C-SiC, since this polytype is isotropic), namely 4H, 6H, 15R and 21R. The phonon dispersion curves show discontinuities in the LZ representation (observed as doublet peaks in the Raman spectra), which result from the dependence of the force constant on the types of layers (hexagonal or cubic) in the polytype structure [17]. Discontinuities in the phonon dispersion curves are the main effect of layered structures (for a detailed discussion of mini-gaps , see [14] p.98). Recently Nakashima et al [18] used Raman measurements from 8H and 27R polytypes to fill out the phonon dispersion curve reported by Feldman et al [16]. 

Alloying additions are commonly made to known superconducting materials in order to raise 7. By suitable solid solution alloying, the electron concentration and thus the Fermi level (and density of states) may be altered to such an extent so as to result in an increase in 7. At the same time the phonon spectrum is altered. For atomically ordered phases disorder and volume changes may be partly responsible for the changes in the phonon spectrum or density of states. These and other alloying effects are interrelated and difficult at the present time to explicitly separate from one another. 

It is possible to model the vibronic bands in some detail. This has been done, for example, by Liu et al. (2004) forthe 6d-5f emission spectrum of Pa4+ in Cs2ZrCl6, which is analogous to the emission spectrum of Ce3+. However, most of the simulations discussed in this chapter approximate the vibronic band shape with Gaussian bands. The energy level calculations yield zero-phonon line positions, and Gaussian bands are superimposed on the zero-phonon fines in order to reproduce the observed spectra. Peaks of the Gaussian band are offset from the zero phonon fine by a constant. Peak offset and band widths, which are mostly host-dependent, may be determined from examination of the lowest 5d level of the Ce3+ spectrum, as they will not vary much for different ions in the same host. It is also common to make the standard... 

The structural disorder formalism has been mostly utilized to discuss electronic transport in organic solids [29,38] (cf. Sec. 4.6), and only a few works show its applicability to interpret optical spectra [62,67], and, recently, quantum efficiency of organic LEDs [68]. The absorption spectrum of an organic material with impurities disorder, local electric fields, or strong exciton-phonon coupling exhibits an exponential tail, commonly referred to as the Urbach tail [69,70]. Such a spectrum can often be decomposed into broad bands featuring... 

The experimental techniques most commonly used to measure the phonon distributions are IR absorption, Raman scattering and neutron scattering. The IR and Raman spectra of crystalline silicon reflect the selection rules for optical transitions and are very different from the phonon density of states. The momentum selection rules are relaxed in the amorphous material so that all the phonons contribute to the spectrum. 

The spectra obtained for ice Ih, LDA and HDA, using the TFXA spectrometer at 10K [53] is shown in Fig. 11. Ice Ih is the most common and readily obtainable phase of ice which has now been well studied [14,15,48,49]. Its spectrum has a very simple structure, the translational modes below 40 meV are well separated from the librational modes (or hindered rotations) in the energy region between 65-125 meV (very few system shows similar behaviour and this is due to the large mass difference between O and H). The observed acoustic phonon peak is at 7 meV. The two sharp peaks at 28 and 37 meV are the optic-phonon bands and have an unusual triangular-shape. In contrast, only a single feature appears in the IR spectrum, at 27 meV, and the Raman spectrum has an additional shoulder at 36 meV (see Fig. 10). 

There is only one known acceptor in diamond, responsible for the p-type conductivity of the lib diamonds. For some time, it was assumed that this acceptor was aluminium [49], but it has been suggested [43] and finally shown conclusively [38] that boron was indeed responsible for the p-type conductivity and the spectroscopic properties of type lib blue diamonds. Natural lib diamonds had been identified ca. 1954 (see Sect. 2.11), and synthetic lib diamonds were obtained at the beginning of the 1960s [80]. Boron is commonly introduced as a dopant in synthetic diamonds and its ionization energy ) is 370 meV [177]. The discrete acceptor spectrum of B extends approximately 70 meV below ) and is superimposed on the two- and three-phonon spectra of Cdiam- Boron acceptor absorption lines are observed at 305, 347 and 363 meV ( 2780, 2800, and 2930 cm 1) at RT, giving phonon-assisted transitions near 464 and 504meV (see [140], and references therein). 

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