On the temperature dependence of the electrical and optical properties of Cu2GeSe3

The Hall effect and electrical resistivity measurements on p-type Cu2GeSe3 crystals were measured in the temperature range from 80 to 300 K. The temperature variation of the hole concentration p from about 200 to 300 K is explained as due to the thermal activation of a shallow acceptor level with an ionization energy of around 50 meV. At low temperatures the impurity band conduction dominates the electrical transport processes. From the analysis of the p vs T data, the density-of-states hole effective mass is estimated to be of the same magnitude as the free electron mass. The temperature variation of the hole mobility in the valence band is analyzed by taking into account the scattering of charge carriers by ionized impurities and acoustic phonons. In the impurity band, the mobility is explained as due to thermally activated hopping transport. The optical absorption coefficient spectrum shows the presence of three absorption narrow bands below the fundamental gap. From the analysis of their temperature dependence, these bands are attributed as due to free–to–bound transitions related to intrinsic defect acceptor states. Activation energies of these states are estimated to be around 0.12, 0.24, and 0.30 eV. Tentative assignment of the nature and origin of these defect states were also made.