Semiconductors polarity

The discussion of Eq. (4.5.7) has shown that the potential of a polarized semiconductor electrode at sufficiently high electrolyte concentrations is... 

In polar semiconductors, carrier-mediated generation occurs in the form of transient depletion field screening (TDFS) depicted in Fig. 2.5 [30]. The driving force in (2.1) can be expressed by the sum of the Raman term and the nonlinear longitudinal polarization [10] ... 

In photoexcited polar semiconductors, the coherent LO phonons couple with photocarriers to form coherent LO phonon-plasmon coupled (LOPC) modes, which exhibit fundamentally different properties from those of bare phonons. Huber and coworkers revealed the ultrafast transition of an optical... 

On polar semiconductors the dissolution may also involve electrons from the conduction band, leading to the production of soluble anions. For example, under accumulation conditions the dissolution of n-type CdS takes place according to the reaction scheme ... 

Devreese, J. T. (1972) Polarons in Ionic Crystals and Polar Semiconductors. North-Holland, Amsterdam. 

In covalently bonded non-polar semiconductors the higher levels of the valence band are formed by electrons that are shared between neighbouring atoms and which have ground state energy levels similar to those in isolated atoms. In silicon, for instance, each silicon atom has four sp3 electrons which it shares with four similar atoms at the comers of a surrounding tetrahedron. As a result each silicon atom has, effectively, an outer shell of eight electrons. The... 

This scattering process is due to the interaction of electrons with the electric field induced by the lattice vibration polarization (polar longitudinal-optical phonons) occurring in polar semiconductors with partially ionic bonding. According to Devlin [55], the optical Hall mobility can be calculated by... 

Chadi and Cohen (1975) for the polar semiconductors as well as for silicon and germanium. 

Most properties of solids represent the response of the solid to some external electromagnetic field or mechanical force. The response of the electronic states to electromagnetic fields is so directly related to electronic structure that it is appropriate to discuss it first. We are interested in absorption and reflection of light as well as in the ordinary dielectric and diamagnetic properties of solids. When static or slowly varying electric fields act on a polar semiconductor, the two types of atoms in the semiconductor will move with respect to each other, giving a lattice... 

The last form is obtained by using the expressions for the interatomic matrix elements from the Solid State Table. For polar semiconductors the splitting becomes 2(Vl+ with the polar energy- m contrast to the hybrid polar... 

Eq. (5-17) gives negative values for polarities less than 0.45, whereas experimental values are positive for homopolar as well as for polar semiconductors. Substitution of values into Eq. (5-17) leads to a value for GaAs of 7 x 10 electrostatic units. The experimental value of Yablonovitch et al. (1972) is 970 X 10 This large discrepancy reflects the extreme sensitivity of these higher-order susceptibilities to the details of the calculation, rellected by the high powers with which parameters such as the band gap enter the calculations. Indeed, because of these, the property tends to be dominated by states near the band edges and associated with the of Fig. 4-3, a fact noted earlier by Van Vechten et al. (1970). 

The energy bands of Si, Ge, and Sn and the energy bands of the polar semiconductors isoclectronic with Ge and Sn, as calculated by Chclikowsky and Cohen (1976b) by means of the Empirical Pseudopotential Method. Mctallicity increases, material by material, downward polarity increases from left to right. 

Cohesive energy per bond, in eV (multiplying by 92.2 gives the value in kilocalories per mole for the polar semiconductors). 

The same effect can be seen in the zig-zag chain of Fig.. 3-11. It is remarkable that we can compute the angular force constant in that model exactly, as well as in the Bond Orbital Approximation (see Problems 8-1 and 8-2). The results turn out to be identical for the homopolar semiconductors, but for polar semiconductors, the exact solution has a, replaced by . Sokel has shown that the result is not so simple for the tetrahedral solid, but turns out quantitatively to be very close to an dependence. We will also find an ot dependence when we treat tetrahedral solids in terms of the chemical grip in Section I9-F. This suggests the approximation to the full calculation,... 

Use these matrix elements to obtain the shift in bond energy to second order in 0. Use this to correct the value of C, obtained in Problem 8-1 for errors arising from the Bond Orbital Approximation. (Notice that = 20.) The finding that there arc only corrections for polar semiconductors carries over to the tetrahedral case. 

LEED patterns do not tell the nature of the reconstruction, only its symmetry. Thus a scries of workers have speculated a variety of patterns of distortion or missing atoms. It has not been possible to choose between these experimentally (except possibly in the case of (110) surfaces of polar semiconductors, to be discussed), so the field has been left in an unsatisfactory state. 

We can also expect reconstruction on the surfaces of polar semiconductors. In particular, the distortions on the (110) cleavage plane may be expected to be of the same form as on the (110) surfaces of homopolar semiconductors, as shown in Fig. 10-5, with the nonmetallic atom displaced outward since its hybrid is doubly occupied the metallic atom is displaced inward with its purely p-like hybrid unoccupied. This is the distortion proposed by MacRac and Gobcli (1966) for essentially the same reasons described here. This appears to have been confirmed by recent analysis of LEED data (Lubinsky, Duke, Lee, and Mark, 1976). 

We indicated that a planar (III) surface of a polar semiconductor would have high energy because one face would be purely metallic and the other purely... 

This mcclianism is not so efrcctivc in polar semiconductors. The conversion of empty hybrids to doubly occupied hybrids on a GaAs surface would require the double occupation of a gallium hybrid, which is unfavorable because of the polar energy. Indeed, recent experiments (Chye, Babalola, Sukegawa, and Spicer, 1975) indicate that the I crmi level is not pinned on surfaces of GaP at the vacuum. Nonetheless, Schottky barriers can arise at GaP- metal interfaces. Metal-induced surface states" have been proposed as a mechanism (discussed in Section 18-1 ) but the barriers could well arise simply from incorporation of metal atoms in the semiconductor or vice versa. 

The principal trends among the polar semiconductors can be described as an increase in pholothreshold with increasing polarity and with decreasing metal-licity. It may be more precise to say that the principal determinant of the photo-threshold is the p-stale energy of the nonmctallic atom, with a secondary influence from the p-slatc energy of the metallic atom. 

In order to treat polar semiconductors, we must make some assumption as to how the odd part of the pseudopotential, K, varies with distortion. The assumption that it is independent of shear, which was used in the LCAO theory, gives a polar value different from the homopolar value by a factor 2/( 2 + V Y = ae, or values for the isoelectronic series of Ge, GaAs, and ZnSe of 0.87,0.81, and 0.74, respectively, compared to the experimental values of 0.80, 0.65, and 0.32. The trend is right, though it is not quantitatively very accurate. To estimate a, we used the empty-core polarities from Table 18-2. The agreement is better if LCAO values are used but not significantly so. 

In detail the doping process of the polymers is more complicated than described above, because oxidation or reduction of the backbone severely distorts the chain (Heeger et al., 1988). Again, in analogy to the behavior of carriers in polar semiconductors, these distortions, together with their charge, are referred to as polarons. A chemist may regard them simply as radical cations or radical anions on the chain. However, an important... 

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