Electrical properties of tellurium clusters on the void sublattice of an opal crystal: The important role played by the Te-SiO2 interface

The temperature dependences of electrical resistivity and of the Hall effect of nanocluster tellurium crystals obtained by filling the voids in a dielectric (opal) matrix with a melt of pure and doped Te were studied. The Hall hole concentration p _eff was found to increase anomalously (by more than two orders of magnitude) in a sample prepared from pure Te and cooled to helium temperatures. At T=1.45 K, the hole concentration in this sample was p _eff?6×10¹⁷ cm^?3. At the same time, the Hall effect in this sample was observed to reverse sign at T?200 K from positive for T<200 K to negative at higher temperatures. This implies a low impurity concentration (N _A is less than at least 10¹⁵ cm^?3). A nanocluster crystal of doped Te does not exhibit this anomaly; here, we have p _eff?6×10¹⁷ cm^?3 throughout the temperature region covered, as in the original Te. These features are assigned to the formation of a two-dimensional conducting accumulation layer near the Te-amorphous SiO₂ (the opal material) interface at low temperatures; such a layer determines the low-temperature properties of nanocluster crystals prepared from pure Te. Actually, we obtained a model of a three-dimensional structure formed from a two-dimensional film.