Defects zinc vacancy

In recent years, there has been considerable effort to derive defect formation enthalpies of intrinsic defects in ZnO [108-110,113-117]. An example is shown in Fig. 1.13 [115]. Horizontal curves belong to neutral defects, curves with positive or negative slopes to charged donors or acceptors, respectively. The donor with the lowest formation enthalpy is the oxygen vacancy Vo, the acceptor with the lowest formation enthalpy the zinc vacancy Vzn-... 

The electron concentration in donor-doped TCOs becomes compensated with increasing oxygen partial pressure. The nature of the compensating defect thereby depends on the material. As mentioned earlier, compensation of n-type doping in ZnO occurs by introduction of zinc vacancies. In contrast, compensation in 1 03 is accomplished by oxygen interstitials [117], Their importance in Sn-doped 1 03 has been already pointed out by Frank... 

Self-diffusion in materials occurs by repeated occupation of defects. Depending on the defects involved one can distinguish between (1) vacancy, (2) interstial, and (3) interstitialcy mechanisms [107], As an example, different diffusion paths for oxygen interstitials are illustrated in Fig. 1.16 [129]. For a detailed description of diffusion paths for oxygen vacancies, zinc vacancies and zinc interstitials the reader is also referred to literature [129,130]. 

Also in other semiconductors such donor-acceptor pair emissions have been found. A well-known example is ZnS Cu,Al where Alzn is the donor and Cuzo the acceptor. This material is used as the green-emitting phosphor in color television tubes. The blue emission of ZnS Al is due to recombination in an associate consisting of a zinc vacancy (acceptor) and Alzn (donor). Since these centres occur as coupled defects, their distance is restricted to one value only (this is sometimes called a molecular center). Due to strong electron-lattice coupling the emissions from ZnS consist of broad bands. 

The formation energies of native defects in ZnO have been calculated by several groups of theorists and the results generally agree [86,88-91). The results for oxygen and zinc vacancies, interstitials, and antisites in ZnO are shown in Figure 3.17 for the... 

One feature of oxides is drat, like all substances, they contain point defects which are most usually found on the cation lattice as interstitial ions, vacancies or ions with a higher charge than dre bulk of the cations, refened to as positive holes because their effect of oxygen partial pressure on dre electrical conductivity is dre opposite of that on free electron conductivity. The interstitial ions are usually considered to have a lower valency than the normal lattice ions, e.g. Zn+ interstitial ions in the zinc oxide ZnO structure. 

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... 

In this equation, the zinc is lost as atoms, not ions, and two electrons have to be donated to the Zn2+ for this to be possible, leaving two holes in the crystal. These holes can sit at the vacancy to create a neutral defect, VZn ... 

At low concentrations, defect clusters can be arranged at random, mimicking point defects but on a larger scale. This seems to be the case in zinc oxide, ZnO, doped with phosphorus, P. The favored defects appear to be phosphorus substituted for Zn, P n, and vacancies on zinc sites, Vzn. These defects are not isolated but preferentially form clusters consisting of (Pzn + 2V n). 

Sodium chloride structure crystals have all octahedral sites filled, and so cation diffusion will be dependent upon vacancies on octahedral sites. In the zinc blende (sphalerite) structure, adopted by ZnS, for example, half of the tetrahedral sites are empty, as are all of the octahedral sites, so that self-diffusion can take place without the intervention of a population of defects. 

However, calculations show this to be true only over a limited range of conditions. Although the calculations differ in detail, it is now fairly certain that under zinc-rich conditions oxygen vacancies are the main defect ... 

While zinc interstitials are possible, the formation energy for these defects is higher than that of oxygen vacancies. As in the case of NiO, continuing theoretical studies are needed to clarify the location of holes and electrons in these phases. 

Class (ii) structures with close-packed S. In these structures metal atoms occupy tetrahedral and/or octahedral holes in a c.p. assembly of S atoms. With the exception of one form of AI2S3 which crystallizes with the corundum structure these are not typically M2X3 structures but are defect structures, that is, MX structures (zinc-blende, wurtzite NiAs, or NaCl) from which one-third of the M atoms are missing. In some structures the arrangement of the vacancies is random and in others regular. 

We normally define the energy level of electrons in a solid in terms of the Fermi level, eF, which is essentially equivalent to the electrochemical potential of electrons in the solid. In the case of metals, the Fermi level is equal to the highest occupied level of electrons in the partially filled frontier band. In the case of semiconductors of covalent and ionic solids, by contrast, the Fermi level is situated within the band gap where no electron levels are available except for localized ones. A semiconductor is either n-type or p-type, depending on its impurities and lattice defects. For n-type semiconductors, the Fermi level is located close to the conduction band edge, while it is located close to the valence band edge for p-type semiconductors. For examples, a zinc oxide containing indium as donor impurities is an n-type semiconductor, and a nickel oxide containing nickel ion vacancies, which accept electrons, makes a p-type semiconductor. In semiconductors, impurities and lattice defects that donate electrons introduce freely mobile electrons in the conduction band, and those that accept electrons leave mobile holes (electron vacancies) in the valence band. Both the conduction band electrons and the valence band holes contribute to electronic conduction in semiconductors. 

Structure compounds characterized by structures which are related to zinc blende or wurtzite as substitution and/or vacancy variants (for details, see Parth, 1990). As example for a normal adamantane structure we shall present in Rgure 1 the zinc blende structure and, as example for a defect adamantane structure, the Cdlr Se4 structure, a vacancy and substitution variant of zinc blende. 

Defect exarriDles V Iron vacancy in e.g. FejO Vp Oxygen vacancy in a metal oxide Zn" Zinc interstitial in e.g. ZnO Al( Al substitutional dopant in e.g. SrTIOj Defect reaction reouirements 1. Conservation of mass 2. Conservation of lattice site stoichiometry 3. Conservation of charge... 

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