PHYSICAL-PROPERTIES OF AMORPHOUS GE20SB25-XBIXSE55 THIN-FILMS

The influence of substitution of Sb atoms by Bi atoms on the electrical and optical properties of thin films of the Ge20Sb25-xBixSe55 [0 less-than-or-equal-to x less-than-or-equal-to 15] system are reported. Results of dc conductivity and thermoelectric power measurements between 150 and 450 K show that the Ge-Sb-Se system is chemically modified by addition of large concentrations of Bi atoms (between x = 5 and x = 10 at.%). A transition from p-type for Sb-doped to n-type for Bi-doped films and a decrease of resistivity is observed. The absorption edge shifts to shorter wavelength, thereby decreasing the optical band gap of the system. Compositional dependences of electrical conductivity, thermoelectric power, and the appearance of n-type conduction are discussed from the stand point of chemical bonds formed in the films and related to the defect states produced due to incorporation of Bi atoms in high concentrations. The coexistence of band and hopping conduction is proposed. The ac conductivity in 0.1-10.0 kHz frequency and 150-450 K temperature range was found to obey a power law-sigma(omega, T) = A-omega-s. The results were interpreted in terms of Elliott's theory, which assumes correlated barrier hopping (CBH) between the charged defect centres. It was found that computed results from the CBH model and experimental one are in qualitative agreement for the present materials.