Insulators, and Semiconductors

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. 

Crystal Bulk d (K) Face Surface o (K) Pilpb II Technique 

Finally, it should be mentioned that neither the ideal metal model nor the damped metal model are able to explain why the actual reflectivity of aluminum is lower than the calculated one (iC = 1) at frequencies lower than cop. Also, these simple models do not reproduce features such as the reflectivity dip observed around 1.5 eV. In order to account for these aspects, and then to have a better understanding of real metals, the band structure must be taken into account. This will be discussed at the end of this chapter, in Section 4.8. 

Unlike metals, semiconductors and insulators have bound valence electrons. This aspect gives rise to inter band transitions. The objective of this and the next section is 

The absorption spectrum involving the valence band to first empty (conduction) band transitions is usually called the fundamental absorption spectrum. For many crystals, this spectrum lies within the optical range. 

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Dielectric constants of metals, semiconductors and insulators can be detennined from ellipsometry measurements [38, 39]. Since the dielectric constant can vary depending on the way in which a fihn is grown, the measurement of accurate film thicknesses relies on having accurate values of the dielectric constant. One connnon procedure for detennining dielectric constants is by using a Kramers-Kronig analysis of spectroscopic reflectance data [39]. This method suffers from the series-tennination error as well as the difficulty of making corrections for the presence of overlayer contaminants. The ellipsometry method is for the most part free of both these sources of error and thus yields the most accurate values to date [39]. 

Fiorentini V and Baldereschi A 1995 Dielectric scaling of the self-energy scissor operator in semiconductors and insulators Phys. Rev. B 51 17 196... 

Flybertsen M S and Louie S G 1985 First-principles theory of quasiparticles Calculation of band gaps in semiconductors and insulators Phys. Rev. Lett. 55 1418... 

Louie S G 1987 Theory of quasiparticle energies and excitation spectra of semiconductors and insulators Eleotronio Band Struoture and Its Applioations (Leoture Notes in Physios vol 283) ed M Youssouf (Berlin Springer)... 

Semiconductors. The basic material employed in LEDs is the semiconductor, a soHd which possesses a conductivity intermediate between that of a conductor and an insulator. Unlike conductors, semiconductors and insulators possess an energy gap, E, between two energy bands, the... 

Eor metals, S generally varies between about 0 and 40 ]lV/K low conductivity semiconductors and insulators have values of S up to 1000 ]lV/K and higher. 

Sample requirements STM—solid oonduaois and semiconductors, conduaive coating required for insulators SFM— solid conduaors, semiconductors and insulators... 

Single-crystal samples of conductors best other solid samples are suitable, including polycrystalline metals, polymeric materials, semiconductors, and insulators, ultrahigh vacuum compatible typically > 5 mm diameter, 1-3 mm thick... 

Phonon modes of metals and films, semiconductors, and insulators... 

CVD in Electronic Applications Semiconductors 347 2.1 Conductors, Semiconductors, and Insulators... 

The muonium centers observed in the curpous halides (see Table II) are unusual in several respects compared with Mu in other semiconductors and insulators. Figure 12 shows the reduced hyperfine parameters for Mu in semiconductors and ionic insulators plotted as a function of the ionicity (Philips, 1970). The positive correlation is especially apparent for compounds composed of elements on the same row of the periodic table where the lattice constants and valence orbitals are similar (see solid points in Fig. 12). Note however that the Mu hyperfine parameters in cuprous halides lie well below the line and in fact are smaller than in any other semiconductor or insulator (Kiefl et al., 1986b). The reason for this unusual behaviour is still uncertain but may be related to other unusual properties of the cuprous halides. For example the upper valence band is believed... 

Fig. 12. The reduced hyperfine parameter tor Mu in semiconductors and insulators plotted as a function of ionicity (Philips, 1970). Compounds to the left of the critical ionicity are tetrahedrally coordinated whereas those to the right are octahedrally coordinated. From Kiefl et al. (1986b).

D.E. Aspnes and N. Bottka, Electric-Field Effects on the Dielectric Function of Semiconductors and Insulators... 

With metals, semiconductors, and insulators as possible electrode materials, and solutions, molten salts, and solid electrolytes as ionic conductors, there is a fair number of different classes of electrochemical interfaces. However, not all of these are equally important The majority of contemporary electrochemical investigations is carried out at metal-solution or at semiconductor-solution interfaces. We shall focus on these two cases, and consider some of the others briefly. 

Figure 29. For the latter case, it is considered that the semiconducting re-electron systems are separated by insulating hydrocarbon spacers, resulting in alternate thin layers of organic semiconductor and insulator in these monolayer assemblies. The direct current - voltage (I - V) characteristics were mea- sured for the multilayers H2Pc(R)8 and Cu-Pc(R)8 in directions perpendicular and parallel to the film plane. In both cases, the linear I - V relationships of Ohm s law were observed at low electric field and obtained DC conductivities are summarized in Table 3. The normal conductivity (ajJ were ca. 10 13 S cm-1, while the lateral ones p//) were 3.4 x 10-7 and 9.9 x 10 7 S cm 1 for films of the metal-free and copper Pc derivatives, respectively. The former (ojJ tended to decrease slightly with increase of Figure 29. Schematical illustration of the substituent alkyl chain length,...

The processible organic conductors, semiconductors, and insulators (not discussed in this chapter but well known historically for saturated polymers with sp3 electronic configuration) form fundamental material set for device applications. In the following sections, we discuss how to construct a PLED with such material set. 

We shall briefly discuss the electrical properties of the metal oxides. Thermal conductivity, electrical conductivity, the Seebeck effect, and the Hall effect are some of the electron transport properties of solids that characterize the nature of the charge carriers. On the basis of electrical properties, the solid materials may be classified into metals, semiconductors, and insulators as shown in Figure 2.1. The range of electronic structures of oxides is very wide and hence they can be classified into two categories, nontransition metal oxides and transition metal oxides. In nontransition metal oxides, the cation valence orbitals are of s or p type, whereas the cation valence orbitals are of d type in transition metal oxides. A useful starting point in describing the structures of the metal oxides is the ionic model.5 Ionic crystals are formed between highly electropositive... 

This same equation is, of course, also used to rationalise the general electronic behaviour of metals, semiconductors and insulators. The quantitative application of Eqn (2.1) is handicapped for ionic conductors by the great difficulty in obtaining independent estimates of c,- and u,-. Hall effect measurements can be used with electronic conductors to provide a means of separating c, and u,- but the Hall voltages associated with ionic conduction are at the nanovolt level and are generally too small to measure with any confidence. Furthermore, the validity of Hall measurements on hopping conductors is in doubt. 

Table 6.1. Values of fi. h for selected organic semiconductors and insulators as active components ofOFETs...

A large fraction of the material science research, and an important chapter of solid state physics are concerned with interfaces between solids, or between a solid and a two dimensional layer. Solid state electronics is based on metal-semiconductor and insulator-semiconductor junctions, but the recent developments bring the interface problem to an even bigger importance since band gap engineering is based on the stacking of quasi two dimensional semiconductor layers (quantum wells, one dimensional channels for charge transport). 

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