Local coordination of Ga impurity in hydrogenated amorphous germanium studied by extended x-ray absorption fine-structure spectroscopy

The local structure of Ga-doped a-Ge:H films has been investigated by extended x-ray absorption fine-structure (EXAFS) fluorescence for impurity concentrations ranging from 1.5 X 10(18) atoms cm(-3) to 4.5 X 10(20) atoms cm(-3). The mean-coordination number of Ga atoms changes from around 4 (1.5 X 10(18)-1.5 X 10(19) cm(-3)) to below 3 (1.5 X 10(20)- 4.5 X 10(20) cm(-3)) with rising concentration. The change from fourfold to threefold coordination occurs in a rather narrow impurity concentration range. The variance of the distance distribution function decreases with increasing Ga content, suggesting that well-ordered sites are present at high-impurity concentration. From EXAFS phase analysis the first Ga-Ge shell distance has been found to be 0.03 Angstrom larger in the amorphous network than in Ga-doped crystalline Ge. (C) 1999 American Institute of Physics. [S0003-6951(99)05102-5].