Semiconductors lattice defects

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. 

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),... 

In this case, the number of zinc ions in interstitial positions and the number of free electrons will be decreased by an increase in the partial pressure of oxygen. These disorder reactions result in a dependence of the electrical conductivity on the oxygen pressure. This effect is a well known phenomenon in the field of semiconductors (1). Complicated relations, however, will occur at lower temperatures, at which no equilibrium can be attained between the gas phase and the lattice defects in the whole... 

Volkenshtein (19) and Schwab (20) suggested that the active centers of adsorption were actually lattice defects in the semiconductor. Schwab felt that the concentration of free or quasi-free electrons was an important factor in metal catalysts. 

The rate of flow of electrons from such a charged particle depends on the availability of an accessible site for this transfer. Although it is known that lattice defects provide such sites and that conduction band electrons can trickle down through solid dislocation levels reduction sites for electron accumulation are usually provided by metallization of the semiconductor particle. This can be achieved through photo-platinization or by a number of vapor transfer techniques and the principles relevant to hydrogen evolution on such platinized surfaces have been delineated by Heller The existence of such sites will thus control whether single or multiple electron transfer events can actually take place under steady state illumination. 

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). 

The studies on Cu2 aO mentioned above concluded that CujO is a metal-deficient p-type semiconductor with cation vacancies. It was not established, however, which kinds of defects (Vcu, Vcu) were dominant and what the effect of Q (interstitial oxygen) was on non-stoichiometry. To clarify these points, Peterson and Wiley measured the diffusion coefficient, D, of Cu in Cu2 O, by use of "Cu as a tracer over the temperature range 700-1153 °C and for oxygen partial pressures, greater than 10 atm. It has been widely accepted that lattice defects play an important role in the diffusion of atoms or ions. Accordingly it can be expected that the measurement of D gives important information on the lattice defects. 

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. 

Semiconductor particles typically contain a high density of lattice defect sites (Te, Th), most of which are concentrated at the particle surface. The nature of these defect sites depends strongly on the constituent material of the particle and the method of particle synthesis [61]. Thus, charge carriers photogenerated in accordance with equation (9.1) may subsequently either recombine directly, re-emitting the absorbed energy as heat (A) or light (hv),... 

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-... 

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. 

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