Bipolaron and single-polaron contributions to a.c. conduction in r.f. sputtered SiO2 films

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Abstract
The a.c. conductivity of r.f. sputtered SiO2 films, measured in the frequency range 40–105 Hz and in the temperature range 77–569 K, is analysed. Below ∼ 320 K the conductivity is almost temperature independent and obeys an equation of the form [sgrave]1(ω) = Aω. This is interpreted in terms of Elliott's model where two electrons simultaneously hop between two oppositely charged sites, not randomly distributed but paired as first considered by Kastner, Adler and Fritzsche in the case of lone-pair semiconductors. The defect density is N ∼ 3 × 1024 m−3. The thermal energy W M required to take two electrons from doubly occupied localized states to the conduction band is estimated to be about 6·2 eV and is compared with the levels for defects proposed by Mott (1977), Greaves (1978) and Lucovsky (1980). For T ≳ 320 K it is found that with W = 0·28 eV and W H = 0·35 eV. It is proposed that neutral defects are created according to a reaction of the form D+ + D− + |U eff| → 2D° where U eff = -0·56 eV is the effective correlation energy for the inverse reaction, and that quantum mechanical tunnelling (QMT) of hole-like polarons takes place between D− and D° centres. The polaron hopping energy is W H = 0·35 eV and the distortion energy of D° centres is W p = 0·7 eV. Using the distortion energy of D+ centres proposed by Mott we derive a value of 2·7 eV for the true Hubbard energy at D− centres. Considering only our experimental results- independently of other information concerning D°, D+ and D− centres we derive, in addition to the preceding values given for N, W M, W H, W p, U eff, the position of the D− level, about 4·1 eV below the conduction band, and a thermal energy of about 2·8 eV required to take an electron from D° centre to the conduction band. Finally it is found that the distortion energy associated with D+ should be of the order of the Hubbard energy.

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