Oxygen vacancies centers

As discussed in Sec. 3.1.3 the presence of defects and impurities can promote loss in piezoelectric crystals. Defects are divided into two main groups, point and extended defects. The point defects include the aluminum-related centers as well as oxygen-vacancy centers. Aluminum ions can easily substitute for silicon in quartz however, charge compensation is required. Aluminum has a - -3 charge whereas the silicon valence is +4. An additional positive charge is required that can be supplied by H-I-, Li- -, Na- -, or holes at interstitial sites in the crystal lattice. Iron-related defects are also possible since iron is also a trivalent (-1-3) ion. 

Oxygen-vacancy centers, which are also called E centers, are also possible. These centers have been extensively studied in quartz crystals and can take several forms. These point defects have not yet been... 

Zinc oxide is a thoroughly studied typical semiconductor of n-type with the width of forbidden band of 3.2 eV, dielectric constant being 10. Centers responsible for the dope electric conductivity in ZnO are provided by interstitial Zn atoms as well as by oxygen vacancies whose total concentration vary within limits 10 - 10 cm. Electron mobility in monocrystals of ZnO at ambient temperature amounts to 200 cm -s". The depth of donor levels corresponding to interstitial Zn and oxygen vacancies under the bottom of conductivity band is several hundredth of electron volt [18]. 

A number of the well-known y-induced centers in n-type Si are also neutralized by atomic hydrogen (Pearton, 1982). The A-center (oxygen-vacancy complex, Ec-0.18eV and divacancy level (Ec-0.23eV) are passivated, while the E-center (phosphorus-vacancy complex, Ec -0.44 eV) is thermally removed at relatively low temperatures and its susceptibility to hydrogenation could not be determined. Point defects... 

The Fs+ color center in the surface of an MgO crystal is an oxygen vacancy plus ... 

Investigations of the generation of super base sites on alkaline earth metal oxides by doping with alkali metals (246,247,253) led to the inference that when zero-valent alkali metals react with a metal oxide surface, the electron donated by the alkali metal to the oxide lattice resides in a defect site, such as an oxygen vacancy, generating a one-electron donor site (F center) (254,255) (Scheme 40). 

The unit cell of pyrochlore can be considered as eight CaF2-type cells stacked as octants of a cube. There are two types of cells shown in Figure 6.21. They differ in the position of the oxygen vacancy and the relative positions of atoms A and B. Only two of the eight octants are shown to make it easier to visualize the relative positions of the three types of atoms. The octant on the lower left is type I and the other one is type II. There are four of each type, and they are not in adjacent cells. Atoms A and B are in ccp layers (a face-centered cubic structure) with oxygens in T layers. The type I cube has A ions located on face... 

A new view on this problem arose after the diamagnetic centers with two-coordinate silicon atom were observed at the surface of mechanically activated silica [70]. The subsequent studies of the optical properties of these centers [71] have shown that they are close to those centers that were observed earlier in the bulk of quartz glass but referred to the oxygen vacancies. It has become evident that, apart from the combination reaction of two radicals (=Si-0)3Si and (=Si-0)3Si resulting in the oxygen vacancy, an alternative reaction of their disproportionation with the formation of a center containing two-coordinate silicon and the restoration of one of the siloxane bonds is possible ... 

Oxygen deficiency in pure quartz glasses is ordinarily associated with the presence of the two types of diamagnetic centers in glass oxygen vacancies (=Si-0)3Si-Si(0-Si=)3 and SCs (=Si-0)2Si . The enthalpy of the equilibrium process ... 

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