Structure and properties of amorphous silicon-metal alloys: I. The Si1-xNix system

Structural and optical measurements have been made on a series of amorphous thin films of Si1-xNix with 0 < x < 0.75, prepared by RF sputtering. We pr esent the results of structural studies using extended x-ray absorption fin e structure (EXAFS) and small angle x-ray scattering (SAXS) together with o ptical measurements of the band gap. The EXAFS analysis gave interatomic distances, partial coordination numbers and Debye-Waller factors for the nearest neighbours of the two atom types. The Ni environment was found to he independent of composition for x < 0.5. A small variation in the mean Si-Si distance was also found in this compos ition range. The SAXS results show phase segregation, on the 30 Angstrom sc ale, for this composition range. We interpret the structural data in terms of a two-phase model, amorphous Si plus amorphous NiSi, for x < 0.5 and a s ingle-phase model for x > 0.5. The optical data show that a metal-insulator transition occurs at about 10- 15% Ni content. We suggest that this transition should be interpreted in te rms of a percolation theory involving regions of conducting amorphous NiSi in an amorphous Si matrix.