Electronic defect

The main scattering processes limiting the thermal conductivity are phonon-phonon (which is absent in the harmonic approximation), phonon defect, electron-phonon, electron impurity or point defects and more rare electron-electron. For both heat carriers, the thermal resistivity contributions due to the various scattering processes are additive. For... 

The description theoretical study of defects frequently refers to some computation of defect electronic structure i.e., a solution of the Schrodin-ger equation (Pantelides, 1978 Bachelet, 1986). The goal of such calculations is normally to complement or guide the corresponding experimental study so that the defect is either properly identified or otherwise better understood. Frequently, the experimental study suffices to identify the basic structure of the defect this is particularly true when the system is EPR (electron paramagnetic resonance) active. However, if the computational method properly simulates the defect, we are provided with a wealth of additional information that can be used to reveal some of the more basic and general features of many-electron defect systems and defect reactions. 

Reactions involving the creation, destruction, and elimination of defects can appear mysterious. In such cases it is useful to break the reaction down into hypothetical steps that can be represented by partial equations, rather akin to the half-reactions used to simplify redox reactions in chemistry. The complete defect formation equation is found by adding the partial equations together. The mles described above can be interpreted more flexibly in these partial equations but must be rigorously obeyed in the final equation. Finally, it is necessary to mention that a defect formation equation can often be written in terms of just structural (i.e., ionic) defects such as interstitials and vacancies or in terms of just electronic defects, electrons, and holes. Which of these alternatives is preferred will depend upon the physical properties of the solid. An insulator such as MgO is likely to utilize structural defects to compensate for the changes taking place, whereas a semiconducting transition-metal oxide with several easily accessible valence states is likely to prefer electronic compensation. 

In order to use a consistent terminology in the following chapters it is emphasized that the term hole will always refer to a defect electron, while an etched feature in the electrode will be designated as a pore if its depth exceeds its width or otherwise as an etch pit. All values of current density refer to the initial surface of the electrode exposed to the electrolyte, which is for example defined by the O-ring of the set-up. 

The donor electron level, cd, which may be derived in the same way that the orbital electron level in atoms is derived, is usually located close to the conduction band edge level, ec, in the band gap (ec - Ed = 0.041 eV for P in Si). Similarly, the acceptor level, Ea, is located close to the valence band edge level, ev, in the band gap (ea - Ev = 0.057 eV for B in Si). Figure 2-15 shows the energy diagram for donor and acceptor levels in semiconductors. The localized electron levels dose to the band edge may be called shallow levels, while the localized electron levels away from the band edges, assodated for instance with lattice defects, are called deep levels. Since the donor and acceptor levels are localized at impurity atoms and lattice defects, electrons and holes captured in these levels are not allowed to move in the crystal unless they are freed from these initial levels into the conduction and valence bands. 

Point defects, electrons, and holes as chemical species... 

Thus, lattice defects such as point defects and carriers (electrons and holes) in semiconductors and insulators can be treated as chemical species, and the mass action law can be applied to the concentration equilibrium among these species. Without detailed calculations based on statistical thermodynamics, the mass action law gives us an important result about the equilibrium concentration of lattice defects, electrons, and holes (see Section 1.4.5). 

It has been shown in Section 1.3.7 that in semiconductors or insulators the lattice defects and electronic defects (electrons and holes), derived from non-stoichiometry, can be regarded as chemical species, and that the creation of non-stoichiometry can be treated as a chemical reaction to which the law of mass action can be applied. This method was demonstrated for Nii O, Zr Cai Oiand Cuz- O in Sections 1.4.5, 1.4.6, and 1.4.9, as typical examples. We shall now introduce a general method based on the above-mentioned principle after Kroger, and then discuss the impurity effect on the electrical properties of PbS as an example. This method is very useful in investigating the relation between non-stoichiometry and electrical properties of semiconductive compounds. 

Self-Luminescence. The action of UV light or ionizing radiation on pure alkali-metal halide crystals causes intense luminescence particularly at low temperature. The emission spectrum is characteristic for each individual compound. This fluorescence is comparable with the recombination luminescence which occurs upon capture of an electron by a VK center (defect electron). 

The hypothesis can be tested if the catalytic activity of a metal can be modified by a controlled shift of the Fermi level of the support. With semiconducting supports such a shift is readily achieved by doping additions of cations of higher charge than that of the matrix cations produces quasi-free electrons and/or removes defect electrons and raises the Fermi level addition of lower charged cations has the opposite effect. This calls for investigation of metal catalysts on doped semiconductors as supports. 

The importance of the defect electron in surface processes on ZnO is also demonstrated in electrochemical investigations 81). 

According to an increase of (EF -Ey) in the depletion layer in NiO, the activation energy on NiO is raised by the influence of Ag support. The activation energy on ZnO on Ag support is decreased according to the decrease of (Ec - EF) in the accumulation layer in ZnO. The electron work function of Co304 is smaller than that of Ag, so that even in the dark, electrons flow into the silver, generating an accumulation layer of defect electrons. (EF - Ev) is decreased and so is the activation energy. 

The promotion of an electron or defect electron to the conduction band or valence band, respectively, is only a part of the whole reaction. This excitation is identical with the destruction of a bond. In the band model only that part of the bond destruction is described which is connected with electron movement the shift of the cores from the energy valleys is not taken into account. The activation energies of the conductivity and of the chemical reaction are proportional but not identical. 

Many radiation-induced redox reactions involve the removal of an electron from 02 which then becomes 0 . Such O states represent defect electrons or positive holes in an 02 sublattice. Having an unpaired spin they are paramagnetic. Electron paramagnetic resonance (EPR) spectroscopy is the... 

As a result oxygen ion vacancies V and excess electrons e are formed [xxiv]. On the other hand, by incorporation of oxygen into the lattice, defect electrons (holes) are generated... 

For the concentration of defect electrons (holes) the opposite sign (+1/4 or +1/6) was measured. 

Fig. 3.17 Schematic representation of some photophysical and photochemical processes in and on a semiconductor (SC) particle (for example Ti02). bg- Band gap energy VB valence band CB conduction band h electron hole ( defect electron ) in the valence band e photoelectron in the conduction band LT lattice trap ST surface trap A ds, Dads chemical species adsorbed on the surface of the SC particle with A being an electron acceptor and D an electron donor. Formation of an electron-hole pair (exciton) by irradiation SC-i-hv ecb + hvb (modified according to Serpone, 1996 and Bottcher 1991).

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