Defect charge state

Fig. 1.13. Top Variation of defect formation enthalpies with Fermi level under zinc- (left) and oxygen-rich (right) conditions as obtained from GGA+U calculations. The gray shaded area indicates the difference between the calculated and the experimental band gap. The numbers in the plot indicate the defect charge state parallel lines imply equal charge states Bottom Transition levels in the band gap calculated within GGA (a), GGA+U (b) and using an extrapolation formula described in [115]. The dark gray shaded areas indicate error bars. Copyright (2006) by the American Physical Society...

Point defects and complexes exliibit metastability when more than one configuration can be realized in a given charge state. For example, neutral interstitial hydrogen is metastable in many semiconductors one configuration has H at a relaxed bond-centred site, bound to the crystal, and the other has H atomic-like at the tetrahedral interstitial site. 

Thus, sensor effect deals with the change of various electrophysical characteristics of semiconductor adsorbent when detected particles occur on its surface irrespective of the mechanism of their creation. This happens because the surface chemical compounds obtained as a result of chemisorption are substantially stable and capable on numerous occasions of exchanging charge with the volume bands of adsorbent or directly interact with electrically active defects of a semiconductor, which leads to direct change in concentration of free carriers and, in several cases, the charge state of the surface. 

Another type of absorption is also possible, i.e., exciton absorption which enriches the crystal in free excitons if the latter annihilate then on the lattice defects, causing a change in the charged state of the defects and leading to the appearance of free carriers in the crystal. In this case photoconduction arises as a secondary effect. 

It should be noted that excitons can annihilate on surface defects as well, in particular on chemisorbed particles participating in the reaction. This involves a change in the charged state of these particles and, as a result, the chemisorption capacity of the surface with respect to these particles and the rate of the reaction in which these particles participate are also changed. This case requires a special investigation since the quantities p and involved in the theory are of a different form (8) than in the case of the electronic mechanism of light absorption to which our attention was restricted in the present article. 

Defect levels typically contain both valence- and conduction-band character. If the relative position of these bands is inaccurate, the position of the defect level will also be uncertain. We thus see that, at this point in time, none of the theoretical methods is able to make accurate predictions for positions of defect levels in the band gap. However, it should be noted that, while the absolute position of defect levels is uncertain, their relative motion induced by displacements of the impurity or by changes in the charge state is quite reliable. These observations generally allow the derivation of reliable qualitative conclusions about defect levels, such as deep... 

Most impurities can occur in different charge states we will see that H in Si can occur as H+, H°, or H. Which charge state is preferred depends on the position of the Fermi level, with which the defect can exchange electrons. Relative formation energies as a function of Fermi level position can be calculated and tell us which charge state will be preferred in material of a certain doping type. Section V will discuss charge states in detail. 

Thus far, I have mainly discussed neutral impurities. From the treatment of the electronic states, however, it should be clear that occupation of the defect level with exactly one electron is by no means required. In principle, zero, one, or two electrons can be accommodated. To alter the charge state, electrons are taken from or removed to a reservoir the Fermi level determines the energy of electrons in this reservoir. In a self-consistent calculation, the position of the defect levels in the band structure changes as a function of charge state. For H in Si, it was found that with H fixed at a particular site, the defect level shifted only by 0.1 eV as a function of charge state (Van de Walle et al., 1989). 

It is clear that the diffusion of H through semiconductors is a very complicated issue not only does the motion itself involve complex interactions between the impurity and the lattice, but the calculation of a diffusion coefficient requires the inclusion of different charge states plus the interaction of H with itself (molecule formation) and with other defects and impurities in the crystal. Chapter 10 of this volume discusses this problem in more detail. 

Zinc oxide is normally an w-type semiconductor with a narrow stoichiometry range. For many years it was believed that this electronic behavior was due to the presence of Zn (Zn+) interstitials, but it is now apparent that the defect structure of this simple oxide is more complicated. The main point defects that can be considered to exist are vacancies, V0 and VZn, interstitials, Oj and Zn, and antisite defects, 0Zn and Zno-Each of these can show various charge states and can occupy several different... 

However, if the molecules of 5 had R alkyl chains longer than Me, the steric hindrance prevented 100% substitution and IR examinations indicated a 50% less derivatization. Moreover, XPS analysis showed that the surface is partly modified by substitution of hydrogen by halogen . In the case of 5 with X = I and to some extent X = Br, the formation of X radicals (besides 12) in a secondary reaction was reported . They participate in reactions analogous to equations 21 and 22b, but with X instead of 12, and attach to the Si surface improving the electronic passivation of the surface at defect sites, sterically inaccessible to 12. A possibility that surface dangling bonds may also appear in the charged states was discussed as well . 

Multiple-Charge-State Vacancy Model. On the basis of the previous discussion, diffusion depends upon the concentration of point defects, such as vacancies or self-interstitials, in the crystal. Therefore, diffusion coefficients can be manipulated by raising or lowering the concentration of point defects. 

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