The Meyer-Neldel rule in the conductivity of insulating germanium nitride and oxynitride films

Electrical conductivity, σ( T), of a-Ge ₃N _x (3.7 < x < 4.6) and quasi-stoichiometric a-Ge ₂O _yN _x films has been investigated as a function of the temperature. Layers present a highly insulating behavior with conductivity and thermal activation energy rapidly rising, increasing T from room temperature to 600 K. Depending on the film composition, the conductivity can vary up to six orders of magnitude. Data analysis of σ( T ) revealed a clear relation between the pre-exponential factor and the thermal activation energy, suggesting the validity of the Meyer-Neldel rule. For all investigated films, typical Meyer-Neldel parameters assume values in a narrow range: 10 ⁻⁸ < σ ₀₀ < 10 ⁻⁶( Ωcm) ⁻¹ and 41 < E _MN < 51 meV. To account for this behavior we have considered a model based on a statistical shift of the Fermi level upon the temperature raise. Our study is the first experimental evidence of this kind of electrical behavior in this class of materials.