AC Conductivity and dielectric anomaly in annealed EuSe

The complex permittivity of annealed n-type EuSe is measured in the temperature range T = 4.2 to 295 K for frequencies 103 to 2.4 × 1010 Hz. Using further the results of the crystallographic characterization and the dc conductivity σdc, Hall effect, and optical transmittance measurements one finds that shallow donors associated with Se vacancies and a decrease in their concentration due to annealing can explain the T and annealing dependences of σdc and σac. At T = 295 K where σdc is high, no frequency dispersion of σac is found even at microwave frequencies. When σdc decreases drastically at low temperatures a strong frequency dependence of σac is seen. It follows the relation for ac impurity hopping conduction proposed by Pollak: σac ⋐ ωs with s < 1 at low frequencies and 1 < s ≦ 2 at high frequencies. A maximum hopping frequency τ−1 = 4.0 × 109 s−1 and an average hopping length R = 9 Å at low temperatures is estimated. The annealing dependence of the dielectric constant at low temperatures can be explained by the polarizability αd of neutral Se vacancies. From the measured αd = 3.2 × 106 Å3 the carrier effective mass m* = 0.28 is estimated.