Zinc interstitials

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]. 

Erhart and Albe also calculated zinc diffusion in ZnO [130]. The results are displayed in Fig. 1.18 together with a comparison to experimental data. Depending on chemical potential and Fermi level position either zinc vacancy or zinc interstitial diffusion can dominate. In the case of n-type material, where the Fermi level is close to the conduction band, zinc diffusion is mostly accomplished via the vacancy mechanism. 

Assuming that only the doubly charged zinc interstitials (or oxygen vacancies) contribute to the excess electrons leads to the following oxygen partial pressure dependence of the conductivity ... 

Von Baumbach and Wagner [4] argued that the zinc interstitial is more probable because of the smaller ionic radius of the Zn++ ion (74 pm) compared with the oxygen ion (138 pm). What could not be decided for decades was the question whether the oxygen vacancy or the zinc interstitial constitutes the donor [5], see Sect. 2.1.1.1. 

The concentration of ionized zinc interstitials in the zinc oxide crystal lattice thereby reflect the polarization by the oxygen reduction. 

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... 

This can be converted into a form where the number of zinc interstitials is a function of the oxygen partial pressure of the environment... 

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]. 

There are also charged versions of this process. For example, the zinc interstitial can leave with a H-1 charge, leaving behind a vacancy with a — 1 charge Znzn Zn( -I- Vz . 

We begin by writing balanced equations and corresponding equilibrium constants that describe the oxygen and zinc interstitials in the ZnO film. The oxygen is assumed to dissodatively adsorb on the surface and be incorporated into an interstitial, as follows ... 

Thus, for constant temperature and oxygen pressure, a decrease in hole concentration translates into increased [0, ]. We deduce that the growth rate decreases when the oxygen interstitial concentration increases. Similarly, Reaction (E9.23C) shows that the zinc interstitial concentration decreases as the electron concentration increases. So the growth rate decreases when the zinc interstitial concentration decreases. 

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. 

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