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Results of studies of conductivity activation energy and its evolution during the aging process are presented for the common phase change material Ge2Sb2Te5. The evolution of temperature dependence of conductivity during quasi-isothermal annealing are interpreted in the framework of simple Arrhenius model as the change of two corresponding parameters: activation energy and pre-exponential factor. It is shown, that the change in resistivity of the Ge2Sb2Te5 during aging is attributed to the simultaneous change of both parameters of the Arrhenius equation, that is, activation energy and pre-exponential factor. Temperature dependencies of those parameters are shown as well, and the Meyer-Neldel rule for conductivity of Ge2Sb2Te5 is tested on the basis of the obtained data.

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.

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Bismuth doped Ge2Sb2Te5 thin films were prepared by magnetron sputtering of Ge2Sb2Te5 target with following ion implantation of Bi. Thin film compositions and elemental distribution across the film thicknesses were determined. An amorphous state of the as-deposited undoped and Bi-doped Ge2Sb2Te5 thin films was confirmed by X-ray diffraction. Temperature dependencies of the resistivity for Bi-doped Ge2Sb2Te5 thin films were measured. Two resistivity drops due to the crystallization and transformation from the cubic to hexagonal structure were determined. Decreases of the onset temperatures for the first and second transitions from 169.3 to 120.3 °C and from 188.0 to 153.0 °C with the increase of Bi concentration were found. It was shown, that the resistivity of the amorphous films sufficiently decreases with Bi concentration, while the variation of resistivity for the crystallized state is much smaller.

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