Preparation and conduction mechanism of n-type semiconducting chalcogenide glasses chemically modified by bismuth

Abstract

Two serieses of Bi containing glasses Ge 20Bi xSe 80−x (x≦13) and Ge 20Bi xSe 70−xTe 10 (x≦11) in at.% were prepared, and their resistivity, thermoelectric power, and optical absorption coefficient were measured. The resistivity at 25°C of Ge 20Bi xSe 80−x glasses decreased by about four orders of magnitude between x=9 and 10, and remained constant for x≧10 ; accompanied with this abrupt decrease, the conduction type changed from p to n in the thermoelectric power measurement. In Ge 20Bi xSe 70−xTe 10 glasses the conduction type changed from p to n at practically the same value of x as in Ge 20Bi xSe 80−x glasses; the resisitivity was nearly the same for both serieses of glasses with x≧10 , while for x<10 Te containing glasses showed lower resistivities by about four orders of magnitude. Optical absorption measurements revealed that the band gap decreased with the incorporation of a small amount of Bi, while remained nearly constant for the further increase in Bi content, indicating that the conduction type was controlled independently of the band gap. These results were discussed on the basis of chemical bonds present in the glasses, and the shift of the Fermi energy toward the conduction band was related to the formation of negatively charged Bi and positively charged dangling bonds.