Electronic transport properties of amorphous Sb$_2$Te$_3$ and Ge$_2$Sb$_2$Te$_5$ films

The electrical conductivity, Seebeck coefficient, and Hall coefficient of amorphous Sb$_2$Te$_3$ and Ge$_2$Sb$_2$Te$_5$ films have been measured as functions of temperature from room temperature down to as low as 200~K. The electrical conductivities manifest an Arrhenius behavior with a larger pre-exponential factor. In Sb$_2$Te$_3$ the energy characterizing the p-type Seebeck coefficient's temperature dependence, about 0.10~eV, is considerably smaller than the activation energy of the electrical conductivity, about 0.28~eV. In addition, the heat-of-transport constant of the Seebeck coefficient is much larger than that of conventional semiconductors. The Hall mobility is low (near 0.1~cm$^2$/V-sec at room temperature), anomalously signed (n-type), and increases with rising temperature with an activation energy of about 0.05~eV. These results are consistent with the charge carriers being hole-like small polarons that move by thermally assisted hopping. Ge$_2 $Sb$_2$Te$_5$ also has low mobility (0.7~cm$^2$/V-sec) and a high conductivity activation energy (0.41~eV), but Seebeck data is indicative of multi-band transport.