Oxygen vacancies, intrinsic defect

The defects of the matrix play an important role on luminescent performances in these materials. Taking into consideration the preparation process of these compounds with the solid-state reaction of mixtures of BaC03, H3BO3, and NH4H2PO4 at different molar ratio, non-equal evaporation during the sintering process of these powders is inevitable and thus results in the formation of intrinsic defects, such as cation and oxygen vacancies. Positional disorder of B and Vacant B (Vb)" have been reported in SrBPOs crystals on the basis of... 

In comparison to the research in n-type oxide semiconductors, little work has been done on the development of p-type TCOs. The effective p-type doping in TCOs is often compensated due to their intrinsic oxide structural tolerance to oxygen vacancies and metal interstitials. Recently, significant developments have been reported about ZnO, CuA102, and Cu2Sr02 as true p-type oxide semiconductors. The ZnO exhibits unipolarity or asymmetry in its ability to be doped n-type or p-type. ZnO is naturally an n-type oxide semiconductor because of a deviation from stoichiometry due to the presence of intrinsic defects such as Zn interstitials and oxygen vacancies. A p-type ZnO, doped with As or N as a shallow acceptor and codoped with Ga or Zn as a donor, has been recently reported. However, the origin of the p-type conductivity and the effect of structural defects on n-type to p-type conversion in ZnO films are not completely understood. 

In the fluorides, chlorides and oxides of the Group-A main-group metals and the transition metals zirconium and hafnium, aliovalent cation substitutions are generally charge-compensated by the introduction of native defects (e.g. an oxygen vacancy in Zr, ,Ca 02 x) because the intrinsic is large however, in some oxides neutral oxygen or water may... 

Fig. 2. (a) Sketch of the relations between defect concentrations and partial pressure (Brouwer diagram) of a pure oxide MO In regime II the intrinsic Schottky disorder determines the concentration, whereas in I and III non-stoichiometry prevails, (b) Dependence of the hole and electron concentration on the frozen-in oxygen vacancy concentration in a negatively (acceptor) doped oxide. 

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-... 

Creation of intrinsic donors by lattice defects (for instance oxygen vacancies or metal atoms on interstitial lattice sites) or... 

Creation and control of intrinsic donor levels by defects such as oxygen vacancies or Zn atoms on interstial sites. Undoped ZnO fdms with resistivities of less than 5x10 Qcm have been prepared by magnetron sputtering without adding impurities but were found to be unstable at T > 150... 

Among the various intrinsic defects at the MgO surface, the oxygen vacancy has received the strongest attention. O vacancies can be created by thermal treatments of hydroxylated MgO surfaces. The treatment conditions of the sample, such as the temperature of calcination and the extent of surface dehydration, significantly affect the morphology and defectivity of the oxide... 

Fig. 2 illustrates (he room-temperature photoluminescence (PL) spectra recorded from the as-prepared ZnO colloidal solution and the ZnO nanostructure formed after deposition of the colloid on the silicon substrate. An UV band at 385 nm was detected from all ZnO products. In addition, a broad orange-red photoluminescence band centered at around 620 nm could be also observed in some materials. The UV photoluminescence peak at 385 nm is well known to be related to the exciton emission, ihe mechanism of visible emission is suggested mainly due to the present of various point defects, either extrinsic or intrinsic, which can easily form recombination centers. Photoluminescence measurements show that the deposited ZnO nanostructures have the stronger UV emission than the ZnO nanoparticles in the colloidal solutions. The better UV emission characteristic of deposited ZnO is suggested to be due to the lower defect density and oxygen vacancies in ZnO nanocrystals in the first case. Similar results have also been reported previously [8]. In addition, the aqueous surrounding can change the surface states of ZnO nanocrystals. It is well known that surface states may... 

In additimi to foreign cation and anion dopants, native point defects are also usually present in the material. Examples are vacancies, interstitials, or substituents. These defects can have a similar influence on the optical, electrical, and catalytic properties as dopants. They are formed by intrinsic defect-chemical reactions, or by a change in the lattice stoichiometry due to exchange of, e.g., oxygen with the gas phase. Since virtually every defect affects the performance of the material in some way, the ability to understand and predict the relatiruiship between dopants and defect concentrations is of paramoimt importance for designing efficient photoelectrodes. 

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