Insulator surface phonon

For insulating surfaces, the friction p can be only due to phonon emission into the substrate, but on metal surfaces damping to vibration may result from both phononic and electronic excitations so that p= %/+ pp. The damping coefficient is assumed to be in the form of a diagonal matrix. 

Phonon surface bands of some insulators and semiconductors are given in Figs. 5.2-56-5.2-58. Surface phonon energies of alkali halide crytals are summarized in Table 5.2-23. Since insulators and semiconductors have in general more than one atom per unit cell, they display both acoustical and optical branches. Surface Debye temperatures of some semiconductors are given in Table 5.2-22. 

Defect-mediated sputtering model. This model developed to explain emissions from insulating surfaces (Si02, LiF, NaCl, etc.) asserts that the defects are introduced in the form of self-trapped excitons and/or holes in response to the valence band excitation induced on ion neutralization/impact. Electron-phonon conpling then results in desorption. 

In the 2- 20 eV range, surface excitations related to the dielectric response function and free carrier density of the surface are observed. Clean-surface loss spectra can include features due to surface phonons, surface plasmons, interband transitions and surface optical phonons in ionic insulators. The probe depth for these phenomena in HREELS is about 10 nm. Transitions between surface states can also be observed in the loss spectrum. Some examples are given in the section related to hydrogen adsorption and surface states below. 

As in the case of metals and semi-conductors, there exist specific surface excitations in insulating oxides. Three types of surface phonon modes may be distinguished the Rayleigh mode, the Fuchs and Kliewer modes and the microscopic surface modes. The first two modes have a long penetration length into the crystal. They are located below the bulk acoustic branches and in the optical modes, respectively. The latter are generally found in the gap of the bulk phonon spectrum. 

The data for surface phonon dispersion determined either experimentally or theoretically for adsorbed covered systems is reported and compared with the surface phonon dispersion of the corresponding bare system. The data is organised according to the electrical properties of the material firstly metals, secondly elemental semiconductors and insulators, and finally compound semiconductors, oxides and salts. The reported systems are collected in Table I. 

Both HREELS and RAIRS have been applied extensively of the study of adsorbates on metal surfaces. The extension of the techniques to semiconducting or insulating oxide surfaces is hampered by a number of problems. The result is that until even relatively recently [9] there were only a couple of examples of RAIRS studies on oxides, and these were confined to polycrystalline systems. Most early HREELS studies were concerned with the characterisation of the intrinsic phonon modes of metal oxide surfaces. This contrasts strongly with the extensive literature concerning the vibrational characterisation of adsorbates and intermediates on powdered oxide surfaces that have been obtained by transmission or diffuse reflection IR techniques. 

A review of the applications of the pseudopotential method and total energy techniques to the electronic and structural properties of solids is presented. With this approach, it has recently become possible to determine with accuracy crystal structures, lattice constants, bulk moduli, shear moduli, cohesive energies, phonon spectra, solid-solid phase transformations, and other static and dynamical properties of solids. The only inputs to these calculations, which are performed either with plane wave or LCAO bases, are the atomic numbers and masses of the constituent atoms. Calculations have also been carried out to study the atomic and electronic structure of surfaces, chemisorption systems, and interfaces. Results for several selected systems including the covalent semiconductors and insulators and the transition metals are discussed. The review is not exhaustive but focuses on specific prototype systems to illustrate recent progress. 

Complete dispersion curves along symmetry directions in the Brillouin zone are obtained from calculated force constants. Calculations of enharmonic terms and phonon-phonon interaction matrix elements are also presented. In Sec. IIIC, results for solid-solid phase transitions are presented. The stability of group IV covalent materials under pressure is discussed. Also presented is a calculation on the temperature- and pressure-induced crystal phase transitions in Be. In Sec. IV, we discuss the application of pseudopotential calculations to surface studies. Silicon and diamond surfaces will be used as the prototypes for the covalent semiconductor and insulator cases while surfaces of niobium and palladium will serve as representatives of the transition metal cases. In Sec. V, the validity of the local density approximation is examined. The results of a nonlocal density functional calculation for Si and... 

Phonons once propagating in a crystal system undergo various other scattering interactions. Such scattering events, which cause a change in the phonon wavevector or phase, occur at crystal boundaries and as a result of interactions with lattice imperfections or with conduction electrons. It is possible experimentally to limit interactions with surfaces and electrons the latter by concentrating on insulators and semiconductors tjith low carrier concentrations. 

It has been reported that the permittivity of a material decreases with decreasing film thickness. As a result, the capacitance increase is not as large as expected based on the inverse relation between C and t. The reason is the existence of dielectric dead layers at the dielectric s surface and interfaces, or interfacial layers between the dielectric and its neighboring materials, which in both cases are characterized by a much lower permittivity. As the dead layer is in series with the dielectric, the effective permittivity is reduced as well. In the case of MOS devices, a notorious interfacial reaction is the formation of silicates between the dielectric and silicon, hence the importance of the thermodynamic stability of the dielectric on Si. In the case of MIM (metal-insulator-metal) devices, alloy formation or in-diffusion of the electrode is a possibility, for example, in the case of Pt. However, also in case no secondary phases are formed, permittivity can be thickness dependent. Possible reasons include breaking of the lattice periodicity or the presence of ion vacancies at the interface, which disturb or inhibit the soft phonon mode and other intrinsic effects [6]. It was shown that electrodes with a shorter electronic screening length, for example, Pt or Au, lead to a smaller dead layer effect compared with, for instance, SrRuOs electrodes [13]. 

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