The results of an experimental study of the effect of nonstoichiometric defects in oxygen and the Bi:Si ratio, as well as Cr ions, on the photoconductivity of Bi12SiO20 crystals are presented. It is shown that varying the annealing conditions allows one to modify the spectral distribution and quantitative characteristics of photoconductivity substantially.
Frequency-temperature characteristics of dielectric permittivity and loss factor of Bi 12 SiO 20 crystals doped by Cr and Mn ions have been carried out in audio frequency range in temperature interval 290 to 800 K. Dielectric anomalies of relaxation (Bi 12 SiO 20 :Cr) and resonance (Bi 12 SiO 20 :Mn) types and Cole-Cole dependences close to linear ones have been observed. The results are discussed on the basis of "screened hopping" model. Photorefractive crystals Bi 12 MO 20 (BMO, where M = Si, Ge, Ti) possess high dielectric permittivity (ε = 30 to 160 at T ≈ 300 K [1-4]) which is sensitive to concentration of intrinsic defects [2-4]. Particularly the increase of ε is supposed due to the increase of Bi 2 O 3 content [4].Near 4.2 K in BMO crystals anomalies of frequency-temperature characteristics ε(ν, T ) and tan δ(ν, T ), typical for "virtual" ferroelectrics are observed [4][5][6]. Anomalies are caused by relaxors with small activation energies E a ∼ = 0.05 eV [5]. It seems that the same relaxors in the form of the "dipole" impurities in Bi 12 GeO 20 (BGO) crystals are responsible for the relaxation anomalies of ε(ν, T ) and tan δ(ν, T ) in audio frequencies range at T = 20 to 40 K. It is noted correlation between the "strength of relaxation" (concentration of relaxors) and intensity of jellow coloration of BGO crystals [6].Discontinuities of ε(ν, T ) and tan δ(ν, T ) typical for the relaxation polarization also take place near the room temperature [7]. Mutually correlating anomalies of internal friction Q −1 (T ) and ε(T ) at high temperatures T = 500 to 700 K (ν = 10 3 Hz), which depend on type of impurity (Al or Cr) and content of oxygen vacancies are described. It is supposed that these 67
We suggest applying a method of thermally activated spectroscopy to the problem of impurity optical absorption. The method consists in measuring the temperature dependence of optical absorption in a wide-gap semiconductor crystal and analyzing the temperature derivative of this dependence. The above technique allows for determining the energies of thermal and optical activations and the strength of electron-phonon interaction for the impurity centres.
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