Lattice defects insulators

Possible sites for the hydrogen anneal phenomena might be surface states, for example, on either side of the native oxide on top of the SiC surface. However, lattice defects and contamination in the surface layer of the epilayer may also be involved, as indicated in the 1/C curves. These may be on different sides of the thin insulator between the metal and the SiC. The thin insulator in these samples is probably a thin native oxide, SiO and oxidized Ta, Ta O, from the TaSi layer. 

Whereas in good-conducting doped or polymeric dyes ft-or -type conductivity can be explained without difficulty by analogy with inorganic semiconductors, the p- and -type photoconductivity in insulating (intrinsic) dye films cannot be explained in this manner. It is necessary to take into consideration the existence of defect states (lattice defects, dislocations, impurities etc.) distributed at different depths in the forbidden zone between valence and conduction band these defect states are able to trap electrons and holes, respectively, with different probability 10,11,88),... 

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. 

However, in the last two decades it has been shown experimentally [1,7, 8,12-14] and theoretically [15-18] that in many wide-gap insulators including alkali halides the primary mechanism of the Frenkel defect formation is subthreshold, i.e., lattice defects arise from the non-radiative decay of excitons whose formation energy is less than the forbidden gap of solids, typically 10 eV. These excitons are created easily by X-rays and UV light. Under ionic or electron beam irradiations the main portion of the incident particle... 

At the absolute zero point of temperature a typical intrinsic, ideal monocrystal of a semiconductor like germanium, is virtually an insulator. By "intrinsic" is meant a Ge crystal without any trace of admixture, neither intentionally added, nor inadvertently present. "Ideal" means without any lattice defects. The electrons are all bound to the Ge atoms and therefore immobile. When the temperature is raised, some electrons become free, they can move through the crystal and hence confer a certain conductivity on the crystal. In a semiconductor dK / dT > 0, this in contrast to metallic conductors, in which the electrons are always present and the randomization due to thermal motion opposes their directional displacement with increasing temperature. In electrolyte solutions dK / dT > 0 because the viscosity decreases with Increasing tempera-... 

CDW can bear an electric current while the system is insulating below Tp in the sense of the single particle transport. The current is carried by a CDW sliding in the lattice with no restoring force at T=0 if 2kp is incommensurate with the underlying reciprocal lattice. In real materials impurities or lattice defects interact with the CDW leading to various phenomena such as the nonlinear transport, a type of mode-locking etc. [63] However, we will leave these problems out of the scope of this article. 

The case of insulators and, more particularly, of porous solids (silica, alumina) that we used, is, insofar as the principles involved are concerned, very similar to the case of semiconductors, with regard to the creation and the influence of lattice defects. A very small number of free carriers are present in insulators, and, therefore, it seems that relatively small energy doses are able to appreciably modify their properties. However, the energy gap between valency and conduction bands is very large and the various phenomena are liable to be more intricate. It is probable that the lattice defects artificially created by irradiation exert a strong influence in both the trapping and carrier recombination phenomena later on, this topic will be discussed further. 

It has been seen above that through irradiation the properties of an insulator may be considerably modified, because of both the created lattice defects and the possible trapping of the excess carriers. In this case of considerable modification, important catalytic effects may be expected, as well qualitative as quantitative ones. They will be widely varied in character, depending upon the nature of the radiation and of the pre-existing impurities. These results are obtained even with very small doses of dissipated energy. 

For solids other than insulators activation through preliminary irradiation is dependent upon either the presence of pre-existing impurities or the creation of lattice defects for these sohds the difference between the catalytic effects produced by both modes of activation is small. 

These processes can be described by the concept of discrete energy levels in a crystal. In insulators and semiconductors, a forbidden band (band gap) exists sep-aratingthe valence and conduction bands (Figure 7.18). The presence of activators (lattice defects, impurity ions) that occupy discrete energy levels within the band gap are the important preconditions for cathodoluminescence (CL). Cathodoluminescence glow is thus caused by electronic transitions within the limits of the ion activator levels. 

Owing to the very high activation energy needed to move electrons or positive holes from one ion to another, semiconductors when in the stoichiometric condition have the low conductivity of insulators. The conductivity can, however, be increased by the addition of an excess of either the cationic or anionic constituent, which introduces lattice defects either as interstitial ions or as lattice vacancies. The introduction of foreign altervalent ions also increases or decreases the concentration of the lattice defects. Thus the introduction of 2-mol.% LiaO, in the presence of air, into NiO increases the conductivity about 10,000-fold 21). 

Fig. Ila-d. Radiative transitions in solids. Conductor a with incompletely filled conduction band b with band overlap c insulator or semiconductor depending on the width of the gap and d insulator with lattice defects. From Alonso M, Finn EJ. Fundamental University Physcs, vol Vlll. Copyright Addison-Wesley Publishing Company. Reprinted by permission...

Band structure details of insulators can be determined from their UV/VIS spectra. Defects in the crystal produce electronic levels within the gap between the conduction and the valence bands. Spectroscopic measurements at low temperature allow the investigation of the phonon structure of a crystal. Absorptions due to lattice or point defects can be used to describe the optical and electronic properties of the insulator. For example, Cr in AI2O3 crystals leads to an intense color change of the crystal. Many so-caUed color centers are based on lattice defects caused by intercalation of atoms in the crystal lattice. 

Wright, J.C., 1985, Laser Spectroscopy of Point Defect Equilibria in Insulators Transitions of Insulators to Superionic State, in Proc. Int. Conf. on Defects in Insulating Crystals, Cryst. Lattice Defects and Amorphous Mat., Vol. 12, ed. F. Liithi (Gordon and Breach, New York) pp. 505. 

Smyth, D.M., Defects and structural changes in perovskite systems from insulators to superconductors, Cryst. Lattice Defects Amorph. Mat., 1989, 18, 355-75. 

Lattice defects may act as preferential nucleation sites. For example, a-C films have a high density of defects that may act as nucleation sites for gold deposition. When depositing adatoms on electrically insulating substrates, charge sites on the svuface may act as preferential nucleation sites. Electron irradiation, UV radiation, and ion bombardment may be used to create charge sites. 

Generating nucleation sites on the surface e.g. lattice defects, charge sites on insulators, by ... 

In a sense, a superconductor is an insulator that has been doped (contains random defects in the metal oxide lattice). Some of the defects observed via neutron diffraction experiments include metal site substitutions or vacancies, and oxygen vacancies or interstituals (atomic locations between normal atom positions). Neutron diffraction experiments have been an indispensable tool for probing the presence of vacancies, substitutions, or interstituals because of the approximately equal scattering power of all atoms. 

At the beginning of the century, nobody knew that a small proportion of atoms in a crystal are routinely missing, even less that this was not a mailer of accident but of thermodynamic equilibrium. The recognition in the 1920s that such vacancies had to exist in equilibrium was due to a school of statistical thermodynamicians such as the Russian Frenkel and the Germans Jost, Wagner and Schollky. That, moreover, as we know now, is only one kind of point defect an atom removed for whatever reason from its lattice site can be inserted into a small gap in the crystal structure, and then it becomes an interstitial . Moreover, in insulating crystals a point defect is apt to be associated with a local excess or deficiency of electrons. 

This technique, first developed by Renninger using laboratory equipment, has been used to study the relation between deep level, electrically active EL2 defects and lattice perfection in semi-insulating GaAs. Ishikawa et al. performed plane wave topography with a separate monochromator-collimator in... 

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