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The polarization in materials can be due to several mechanisms: [1-7]
          i) Electronic polarization.
          ii) Ionic (molecular) polarization.
          iii) Atomic polarization.
          iv) Orientational (dipolar) polarization.
          v) Spontaneous polarization
          vi) Hopping polarization.
          vii) Space charge (interfacial) polarization.
          viii) Polarization created by hopping of charge carries between localized sites.

The mechanisms of electronic, ionic and atomic polarizations and depolarizations are much faster (<10^-12 sec) than the others. This deformational polarization process is also referred to as the resonance process since it involves vibrating modes. The time required for orientational, hopping, or space charge polarization and depolarization is very long and varies in a wide range, depending on the dielectric systems; such polarization processes are sometimes referred to as relaxation processes because they involve a relaxation time. A relaxation phenomenon occurs when restoring action tends to bring the excited system back to its original equilibrium state. Depending on the time scale, the fast polarizations together can be considered as an instantaneous polarization P₀. On the other hand, the polarization is to be finite at longer times, P(t →∞) = P_s, because an infinite amount of electrical energy cannot be stored in a dielectric (or a capacitor). [8] Figure 4555a shows the approximate time required for polarizations and their frequency dependence. At DRAM data rate (in the order of 10⁹ Hz), we have both electronic polarization and ionic polarization responding.

(a)

(b)

A dielectric material consisting of polyatomic molecules usually has: [9]
          i) Electronic polarization.
          ii) Ionic polarization.
          iii) In some cases, orientational polarization when it is subjected to an electric field.

Electric polarization may be associated with mobile and trapped charges. [9] This polarization is normally referred to as space charge polarization, P_d. This occurs mainly in amorphous or polycrystalline solids or in materials consisting of traps. The charge carriers (electrons, holes, or ions), which may be injected from electrical contacts, may be trapped in the bulk or at the interfaces, or may be impeded to be discharged or replaced at the electrical contacts. In this case, space charges will be formed, and then the field distribution will be distorted, and thus, the average dielectric constant will be affected.

The electronic polarization is obtained by the displacement of electrons in an atom relative to the deformation of the electron shell about a nucleus by the application of external electric field. Electronic polarization is extremely fast. Even when the frequency of the applied voltage is very high in the optical range (~10¹⁵ Hz), electronic polarization occurs during every cycle of the applied voltage.

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