Dielectric spectroscopy of a-Se and some a-Se : As alloys

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Abstract
Dielectric activity is observed in a‐Se and in some a‐Se : As alloy films in the neighborhood of their respective glass transition temperatures. When measurements are made at fixed frequency in the temperature scan mode, enthalpy relaxation and volume hysteresis effects are readily identified. Relaxation behavior conforms to the phenomenological WLF equations and for a:Se C1=10.4 and C2=20.5 K. The free volume fraction is 0.042. Arsenic addition to a‐Se significantly broadens the relaxation distribution function and increases the relaxation oscillator strength. The effective incremental dipolar contribution per As atom is 0.6 D despite the very small Pauling ionicity of the Se : As bond. The latter is consistent with the formation of a polar pyramidal complex at each incorporated As site. Sub‐Tg relaxation processes are not observed in a‐Se. Dielectric activity in homopolar a‐Se could reflect contributions from polar chemical impurities, deformation induced dipoles, or dipoles associated with native charged defect pairs. It is the latter two which are the most likely candidates. In both cases estimates of the unit dipole strength and density will vary depending on calculational models.