Temperature activated conductivity of Ge2Sb2Te5: Connection to the variation of Fermi level and implications on resistance drift

The non-Arrhenius behaviour of Ge₂Sb₂Te₅ conductivity is attributed to the non-linear temperature dependence of the Fermi level, assuming extended state conduction by free holes. The temperature-dependent Fermi level is shown to alter values of activation energy and prefactor for conductivity so that their exact determination becomes impossible using the conductivity data alone. However, if one assumes the temperature dependence of the Fermi level to be a parabolic function. Then, two of three model parameters can be retrieved; to obtain the third parameter one has to make an assumption about the value of prefactor for conductivity. Applying the proposed analysis scheme to Ge₂Sb₂Te₅, reasonable values of Fermi level position are obtained and the change of Fermi level as a result of resistance drift phenomenon is demonstrated.