Schottky mechanism

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The results of several studies were interpreted by the Poole-Erenkel mechanism of field-assisted release of electrons from traps in the bulk of the oxide. In other studies, the Schottky mechanism of electron flow controlled by a thermionic emission over a field-lowered barrier at the counter electrode oxide interface was used to explain the conduction process. Some results suggested a space charge-limited conduction mechanism operates. The general lack of agreement between the results of various studies has been summari2ed (57). 

In some ionic crystals (primarily in halides of the alkali metals), there are vacancies in both the cationic and anionic positions (called Schottky defects—see Fig. 2.16). During transport, the ions (mostly of one sort) are shifted from a stable position to a neighbouring hole. The Schottky mechanism characterizes transport in important solid electrolytes such as Nernst mass (Zr02 doped with Y203 or with CaO). Thus, in the presence of 10 mol.% CaO, 5 per cent of the oxygen atoms in the lattice are replaced by vacancies. The presence of impurities also leads to the formation of Schottky defects. Most substances contain Frenkel and Schottky defects simultaneously, both influencing ion transport. 

Ikonopisov284 has conducted a systematic study of breakdown mechanisms in growing anodic oxides. He has enumerated factors significantly affecting the breakdown (nature of the anodized metal, electrolyte composition and resistivity) as well as those of less importance (current density, surface topography, temperature, etc.). By assuming a mechanism of avalanche multiplication of electrons injected into the oxide by the Schottky mechanism, Ikonopisov has correctly predicted the dependence of Ub on electrolyte resistivity and other breakdown features. 

In the case of stoichiometric oxides, the equilibrium constant in the case of the Frenkel and Schottky mechanisms is given by Equations 5.60 and 5.61, which are equivalent to Equations 8.1 and 8.2. 

It follows (Table II) that the experimental values are in good agreement with the theoretical assumptions of the Schottky mechanism. However, this cannot be treated as a definite solution, if only because of the fact that in some cases 3S = 3P-F (32,33), and in simple models values of the coefficient ffer by a factor of 2 only, which is insufficient for unequivocal differentiation between the two mechanisms. 

In view of the above, conductivity measurements were conducted in asymmetric systems Au-polymer-Si for polystyrene and polysilazane, and Au-polymer-In for polysiloxane. The difference in barrier height between Au-polymer and Si-polymer estimated on the basis of measurements of the Au-Si barrier is ca. 0.5 eV (M) which, in the case of the conductivity limited by the electrodes, should produce a difference in the intensity of the currents of opposite polarizations equal to about 8 orders of magnitude. The difference in work function of Au and In, on the other hand, is ca. 1 eV so, on the assumption of the Schottky mechanism of conductivity, the difference in the intensity of opposite polarizations should amount to 17 orders of magnitude ( ). As can be seen in Fig. 4 in the case of an asymmetric polysilazane sample there is a difference in the intensity of the currents although this difference does take the expected course, it is several times smaller than expected, and is thus virtually negligible. A similar result was obtained for the polystyrene sample, while in the case of the asymmetric system based on polysiloxane there was no difference in the intensity of the opposite-biassed fields over the entire range of fields used - up to 3 x 10 V/m. It can thus be assumed that the conductivity in the films under study is dominated by the Poole-Frenkel volume generation independent of the contact effects. Such were also the conclusions of the workers who studied the conductivity in polystyrene (29) and polysiloxane (21). 

This leads to a reconsideration of the significance of the Schottky mechanism. The results of current density studies in samples supplied with asymmetric electrodes mentioned above did not confirm its importance unequivocally, but assuming the existence of surface states one has to take it into account. In fact, Mizutani at al. (58) have shown that the barrier bights at the metal-polymer contacts as estimeted by photoinjection for two differen metals may differ only by 0.07 eV, while theoretical predictions are 1.1 eV. This disappearance of the barrier bights difference is related to the surface states. Similar effects may be expected in the case of polysilazane films. Measurements of thermal activation energies in the system Au-polysilazane-Si for opposite biased fields have shown a small but distinct difference for polarization of +50 V Ea = 0.980 ... 

The Poole-Frenkel effect is the high-field assisted thermally-activated escape from traps. The derivation of an expression for the rate of escape from traps is similar to the derivation of the Schottky equation. Because the charge on the trap is fixed in position, the attractive force is proportional to l/x instead of to l/(2x). This gives an apparent difference of 2 in the slope of the plot of log / vs. E as compared with the Schottky mechanism. 

Dislocation theory as a portion of the subject of solid-state physics is somewhat beyond the scope of this book, but it is desirable to examine the subject briefly in terms of its implications in surface chemistry. Perhaps the most elementary type of defect is that of an extra or interstitial atom—Frenkel defect [110]—or a missing atom or vacancy—Schottky defect [111]. Such point defects play an important role in the treatment of diffusion and electrical conductivities in solids and the solubility of a salt in the host lattice of another or different valence type [112]. Point defects have a thermodynamic basis for their existence in terms of the energy and entropy of their formation, the situation is similar to the formation of isolated holes and erratic atoms on a surface. Dislocations, on the other hand, may be viewed as an organized concentration of point defects they are lattice defects and play an important role in the mechanism of the plastic deformation of solids. Lattice defects or dislocations are not thermodynamic in the sense of the point defects their formation is intimately connected with the mechanism of nucleation and crystal growth (see Section IX-4), and they constitute an important source of surface imperfection. 

In pure and stoichiometric compounds, intrinsic defects are formed for energetic reasons. Intrinsic ionic conduction, or creation of thermal vacancies by Frenkel, ie, vacancy plus interstitial lattice defects, or by Schottky, cation and anion vacancies, mechanisms can be expressed in terms of an equilibrium constant and, therefore, as a free energy for the formation of defects, If the ion is to jump into a normally occupied lattice site, a term for... 

A polymer layer al a contact can enhance current How by serving as a transport layer. The transport layer could have an increased carrier mobility or a reduced Schottky barrier. For example, consider an electron-only device made from the two-polymer-layer structure in the top panel of Figure 11-13 but using an electron contact on the left with a 0.5 eV injection barrier and a hole contact on the right with a 1.2 cV injection barrier. For this case the electron current is contact limited and thermionic emission is the dominant injection mechanism for a bias less than about 20 V. The electron density near the electron injecting contact is therefore given by... 

Crystals with Frenkel or Schottky defects are reasonably ion-conducting only at rather high temperatures. On the other hand, there exist several crystals (sometimes called soft framework crystals ), which show surprisingly high ionic conductivities even at the room or slightly elevated temperatures. This effect was revealed by G. Bruni in 1913 two well known examples are Agl and Cul. For instance, the ar-modification of Agl (stable above 146°C, sometimes denoted also as y-modification ) exhibits at this temperature an Ag+ conductivity (t+ = 1) comparable to that of a 0.1m aqueous solution. (The solid-state Ag+ conductivity of a-Agl at the melting point is actually higher than that of the melt.) This unusual behaviour can hardly be explained by the above-discussed defect mechanism. It has been anticipated that the conductivity of ar-Agl and similar crystals is described... 

Much of the discussion of this subsection has been based on the behavior of hydrogenated diodes annealed under reverse bias. Annealing under forward bias has also been studied, though less extensively, and some of the observations have suggested the possibility of a new type of thermal breakup of BH complexes, namely BH + e— B + H° (Tavendale et al., 1985, 1986a). These authors reported breakup of BH in a few hours at 300 K under forward bias, both in Schottky diodes and in n+-p junctions. However, in a similar experiment with an n+-p junction, Johnson (1986) found a slight buildup of BH under forward-bias anneal. Available details of the various experiments are too sketchy to allow useful speculation on the reasons for the different outcomes or possible mechanisms for accelerated breakup. 

Despite the fact that not all details of the photographic process are completely understood, the overall mechanism for the production of the latent image is well known. Silver chloride, AgBr, crystallizes with the sodium chloride structure. While Schottky defects are the major structural point defect type present in most crystals with this structure, it is found that the silver halides, including AgBr, favor Frenkel defects (Fig. 2.5). 

The same analysis can be applied to more complex situations. Suppose that cation vacancy diffusion is the predominant migration mechanism, in a sodium chloride structure crystal, of formula MX, which contains Schottky defects as the major type of intrinsic defects. The relevant defect concentration [ii] is [Eq. (2.11)]... 

The authors ascribed the high activity of Au/thoria to a Schottky junction effect between the metal and oxide leading to an enhancement of active OH groups associated with oxygen deficiencies. The authors did observe formation of formate, carbonate, and bicarbonate species, but are still elucidating which, if any, of the species are important for the mechanism. 

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