Defects zinc interstitial

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. 

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]. 

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... 

The opto-electrical property of the ZnO/Pt IPMC was characterized using photoluminescence (PL). In order to understand the PL quenching phenomenon, measurements of the PL spectrum as a function of the potential were carried out with potential variation of 0-2.0 V. Fig 3.14 (a) shows the variation of PL spectra of the ZnO/PT IPMC recorded at the room temperature using an excitation wavelength of 280 nm. The spectra of the sample displays a broad emission band with some vibronic structure from 350 to 500 nm and the maximum emission wavelength is Xmax = 468 nm. The blue emission is believed to originate from intrinsic defects, particularly interstitial zinc [Fang et al. (2004)]. The maximum PL intensity is observed... 

N-type nature of ZnO is due to the sensitiveness of ZnO lattice constants to the presence of extended defects (planar dislocations/threading) and structural point defects (interstitials and vacancies) that are commonly found in ZnO resulting in a non-stoichiometric compound. The excess zinc atoms in Zni+dO have the tendency to act as donor interstitials that give its natural N-type conductivity. In ionic form, the excess zinc tends to occupy special Zn interstitial sites with Miller index (1/3, 2/3, 0.875) as shown in fig. 12. These special sites offer passage routes for zinc interstitials to easily migrate within the wurtzite structure [109]. 

On the other hand, the presence of Li increases the growth rate. Equation (E9.23F) shows that the presence of Li increases the hole concentration, decreasing the electron concentration. From Equation (E9.23C), we conclude that [Znt] increases. We deduce that the growth rate increases when the zinc interstitial concentration increases. Additionally, Equation (E9.23G) shows that the oxygen interstitial concentration decreases. Since the growth rate goes in proportion to the zinc defect concentration, and behaves oppositely to the oxygen interstitial concentration, we conclude that transport of Zn across the ZnO film is the mechanism by which the oxidation reaction proceeds. 

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. 

Another source of departure from stoichiometry occurs when cations are reduced, as for example in tire reduction of zinc oxide to yield an oxygen-defective oxide. The zinc atoms which are formed in tlris process dissolve in the lattice, Zn+ ions entering interstitial sites and the coiTesponding number of electrons being released from these dissolved atoms in much the same manner as was found when phosphorus was dissolved in the Group IV semiconductors. The Kroger-Viirk representation of dris reduction is... 

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... 

Oxidation of zinc to zinc oxide is another example whose kinetics have been interpreted in terms of the Wagner model (Wagner Grunewald, 1938). At 670 K, the reaction has been found to be independent of oxygen pressure between 0.02 and 1 atm. ZnO is a n-type semiconductor, having a stoichiometric excess of zinc accommodated as interstitials the defect equilibrium could be represented as... 

Let us consider again the surface of a zinc oxide crystal but suppose now that the underlying crystal is nonstoichiometric and contains excess zinc atoms in interstitial lattice positions. A fraction of these defects is ionized and their valence electrons move quasifreely through the crystal. A molecule such as nitrous oxide, adsorbed on the surface, can make use... 

The existence of the latter has been recognized for many years, since Wagner (20) applied his thermodynamic theory of defect oxides to the system zinc oxide-oxygen (21). According to this scheme, at sufficiently high temperature an equihbrium sets in between zinc oxide and oxygen in the gas phase, whereby excess zinc (Zn ) can be accommodated in interstitial positions of the lattice ... 

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... 

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... 

An exciton bound to a shallow neutral donor of interstitial zinc (Fig 1 a) and of interstitial lithium (Fig. lb) is presented, for example, in our spectra. In some instances the radiative recombination of an exciton bound to a neutral defect may not lead to the ground state of the respective defect but to an excited state of the carrier at this occupied center (2 - electron transition). In a hydrogenic model we can calculate an ionization energy of the neutral donor state of interstitial zinc to 0.05 eV and of interstitial lithium to 0.033 eV. 

The charges on the defects are measured relative to the normal site occupation and are indicated by the superscripts ( ) and ( ) for positive and negative charges, respectively, e.g., Zn represents a divalent zinc ion on an interstitial site and carries a charge of +2 relative to the normal, unoccupied, site. 

According to the equilibria established for this defect structure, the concentration gradient of interstitial zinc ions across the scale will depend also upon the oxygen partial pressure of the atmosphere, i.e., for divalent interstitials formed according to Equation (4.6),... 

In both cases, increasing the oxygen partial pressure reduces the concentration of defects. Thus, the difference in concentration of interstitial zinc ions can be represented as in Equation (4.10),... 

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