Vacancy ionization

In order to exclude the influence of gaseous phase at this stage, it is essential to take into consideration a simple example (e.g., silicon) as a semiconducting material with vacancies. Assuming that possible defects of the crystal structure are vacancies and electron defects, the processes of intrinsic electronic disordering, vacancy formation, and vacancy ionization can be written, respectively, as ... 

We note that this value cannot, in any case, correspond to conduction by vacancy ionization produced electrons. 

Table 7.2 with ez = 2). Far from thermodynamic equilibrium conditions, we neglect the amount of cation vacancies (ionized particles) at the internal interface with respect to the external interface ... 

Table 11.2. Enthalpy of formation of oxygen vacancies ionized twice...

As mentioned in Section 1.3, point defects in a crystal may ionize. For example, in the hypothetical compound MX, when X atoms are removed to form X vacancies, the electrons which were associated with the X ion in the crystal (or with the bond in covalent compounds) remain behind. At low temperatures these electrons are associated with the vacancy, but at high temperatures they become dissociated from the vacancy (ionize) and are free to move through the crystal. The equation for ionization of an X vacancy is written... 

Electrons trapped at the vacancy can become partially or fully ionized, leading to weak n-ty e electronic conduction in an electric field. Again, the conductivity is low. 

In some cases, a valence electron can be completely ionized, resulting in vacancies in both the core and valence levels. In those cases, weak peaks referred to as shake-off satellites are also observed at binding energies a few electron volts higher than the photoelectron peak. Such cases are, however, not very common. 

In case when the major input into the change of concentration of free carriers is provided by double-ionized oxygen vacancies which is valid at high temperatures the concentration of conductivity electrons is [e] w 2[Vq ], which, recalling expressions (1.121), (1.124) and (1.125) brings us to formula... 

Finally, the function ct-Pq can be obtained for the case of practically full ionization of superstoichiometric metal atoms or oxygen vacancies as well. This means that the expression is being obtained when condition [Pg.126]

Metals are immune to radiation damage by ionization. This is also a consequence of the free electron structure. Fast charged particles and ionizing rays can knock off electrons from the atoms they encounter. In metals, the positive vacancies so formed are immediately filled up by the electron gas, leaving no sign of damage apart from a small amount of heat. 

The transitions involved in Auger emission are illustrated in Fig. 10. The primary process is the ionization of an inner shell by bombardment with electrons. The vacancy is then filled by an electron from an outer shell, and the energy released can either appear as an X-ray quantum, or... 

It is known that hydrogen incorporated into Si subsequently exposed to ionizing radiation inhibits the formation of induced secondary point defect (Pearton and Tavendale, 1982a). For example, in both Si and Ge a number of electron or y irradiation induced defect states appear to be vacancy-related, and exposure of the Si or Ge to a hydrogen plasma (or implantation of hydrogen into the sample) prior to irradiation induces a degree of... 

These diffuse through the lithium iodide via cation vacancies that form as part of the intrinsic Schottky defects in the crystals, to reach the iodine in the cathode (Fig. 2.3b). The electrons lost by the lithium metal on ionizing traverse the external circuit and arrive at the interface between the cathode and the electrolyte. Here they react with the iodine and the incoming Li+ ions to form more lithium iodide. 

The same sort of considerations will apply to vacancies. For instance, an anion vacancy may give rise to a set of shallow donor levels just below the lower edge of the conduction band. If the vacancy is created by removing a neutral nonmetal atom from the crystal, the electrons that were on the anion are transferred to the conduction band to produce an n-type semiconductor. The energies of neutral and ionized vacancies are slightly different. 

Fig. 3-11. Energy for decomposing ionization of compound AB to form gaseous ions A(giD) and via electron-hole pair formation and via cation-anion vacancy pair formation r = reaction coordinate of decomposing ionization e, s semiconductor band gap . vmb) = cation-anion vacancy pair formation energy (Va- Vb-) Lab = decomposing ionization energy of compound AB.

The decomposing ionization of soUd compound AB may also take place through the formation of a cation-anion vacancy pair as follows ... 

The decomposing ionization will take place preferentially by way ofthe electron-hole pair formation, if the formation energy of the electron-hole pair, e, is smaller than the formation energy of the cation-emion vacancy pair, Hv(ab>, and vice versa. In general, compound semiconductors, in which the band gap is small (e,< Jfv(AB>), will prefer the formation of electron-hole pairs whereas, compound insulators such as sodium chloride, in which the band gap is great (e(>Hv(AB>), will prefer the formation of cation-anion vacancy pairs [Fumi-Tosi, 1964]. 

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