SHORT-RANGE ORDER IN AMORPHOUS (GE(1-X)M(X))SE-2 (M=AS,SB AND BI)

X-ray diffraction and Raman scattering measurements have been carried out on amorphous (Ge(1-x)M(x))Se-2 (M = As, Sb and Bi). The results indicate that pyramidal structural units, MSe(3/2), in which an M atom is covalently surrounded by three Se atoms, are incorporated into the tetrahedral GeSe4/2 network matrix, although the Bi-Se distance in the BiSe3/2 unit is somewhat longer than the covalent one. It is concluded that the occurrence of a p-n transition in the Bi glasses results from the inherent n-type nature of the BiSe3/2 unit.