Defects vacancy-like

Positron annihilation lifetime measurements have been performed on float-zone Si at high teipperatures and show that vacancy-like defects are formed (20). 

Irradiation creates in semiconductors defects with deep levels that act as non-radiative recombination centers. However, an existence of stable point defects created by atomic displacements at RT inside the In(Ga)As QDs or ultra-thin QWs has never been proven. As primary defects (vacancies and interstitial atoms) are mobile at RT in GaAs [4-6] and, certainly, in InAs, it is very likely that they are captured at the interfaces (cf [7]). The defects raise the free energy of the crystal, so... 

The results were consistent with a diffusion mechanism that was mediated by vacancy-like defects in the amorphous ceramics. 

In a real system, the positron exists in different states. It may annihilate either with valence or conduction electrons of the bulk. These processes give rise to a bulk annihilation rate Ab. It may also be trapped in various defect states Dj where the electron density is smaller than in the bulk, i.e. a single vacancy, a cluster of vacancies, dislocations, impurities etc. Each defect state will be characterized by an annihilation rate Dj- In a vacancy-like defect the trapped-positron lifetime is increased compared to free positrons aimihilating in the bulk, as the electron density is locally reduced. Each defect state leads to a different lifetime Tdj = 1/Ap,. 

A consequence of the trapping of electronic defects is that materials become insulating at low temperatures. Defect association can also occur between oxygen vacancies and acceptor dopants, as it was observed in Y-doped zirconia, where Vq are trapped by defects at low temperature, which led to a drastic decrease in oxide ion conductivity [6-8]. Similarly, protonic defects are likely to form associations with acceptor dopants, which hence would limit protonic conduction in metal oxides [9]. 

Impurity atoms can form solid solutions in ceramic materials much as they do in metals. Solid solutions of both substitutional and interstitial types are possible. For an interstitial, the ionic radius of the impurity must be relatively small in comparison to the anion. Because there are both anions and cations, a substitutional impurity substitutes for the host ion to which it is most similar in an electrical sense If the impurity atom normally forms a cation in a ceramic material, it most probably will substitute for a host cation. For example, in sodium chloride, impurity Ca and ions would most likely substitute for Na and Cl ions, respectively. Schematic representations for cation and anion substitutional as well as interstitial impurities are shown in Figure 12.21. To achieve any appreciable sohd solubility of substituting impmity atoms, the ionic size and charge must be very nearly the same as those of one of the host ions. For an impurity ion having a charge different from that of the host ion for which it substitutes, the crystal must compensate for this difference in charge so that electroneutrality is maintained with the solid. One way this is accomplished is by the formation of lattice defects—vacancies or interstitials of both ion types, as discussed previously. 

Thus, we see that the concentration of ionized vacancies increases with the square root of F gas pressure (and therefore with concentration of F). In general, the effect of foreign atoms will be to increase the concentration of native defects of opposite charge and to decrease the concentration of native defects of like charge. 

The defects generated in ion—soHd interactions influence the kinetic processes that occur both inside and outside the cascade volume. At times long after the cascade lifetime (t > 10 s), the remaining vacancy—interstitial pairs can contribute to atomic diffusion processes. This process, commonly called radiation enhanced diffusion (RED), can be described by rate equations and an analytical approach (27). Within the cascade itself, under conditions of high defect densities, local energy depositions exceed 1 eV/atom and local kinetic processes can be described on the basis of ahquid-like diffusion formalism (28,29). 

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. 

Fig. 10.4. Ball bearings can be used to simulate how atoms are packed together in solids. Our photograph shows a ball-bearing model set up to show what the grain boundaries look like in a polycrystalline material. The model also shows up another type of defect - the vacancy - which is caused by a missing atom.

A number of selenium and tellurium compounds of the presently discussed metals show a quite different behavior from the Fe-S system. Iron and selenium form two compounds FeSe with a broad stoichiometry range and FeSe2 with a much narrower composition field. Below 400 the non-stoichiometric Fei xSe exists by creation of iron vacancies and can have compositions lying between FeySes and Fe3Se4. At low temperatures there exist two phases an a (PbO type) and a f) (NiAs type) phase. The crystal sUiicture of the diselenide, FeSe2, is an orthorhombic, C18 (marcasite) type. In the Fe-Te system, the defect NiAs structure is found at a composition close to FeTei.s, as about one-third of the Fe atoms are missing. At compositions around FeTe the behavior is complex, and the f)-phase has the PbO structure (like FeSe) but with additional metal atoms (i.e., FeuTe). 

On the right are the t5rpes of point defects that could occur for the same sized atoms in the lattice. That is, given an array of atoms in a three dimensional lattice, only these two types of lattice point defects could occur where the size of the atoms are the same. The term vacancy is self-explanatory but self-interstitial means that one atom has slipped into a space between the rows of atoms (ions). In a lattice where the atoms are all of the same size, such behavior is energetically very difficult unless a severe disruption of the lattice occurs (usually a "line-defect" results. This behavior is quite common in certain types of homogeneous solids. In a like manner, if the metal-atom were to have become misplaced in the lattice cuid were to have occupied one of the interstitial... 

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