Structure and properties of amorphous silicon-metal alloys: II. The Si1-xTix system

Structural and optical measurements have been made on a series of amorphous thin films of Si1-xTix with 0 < x < 0.6, prepared by RF sputtering. We pre sent the results of structural studies using extended x-ray absorption fine 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 Ti environment is found to be independent of composition for x < 0.4, with only the coordination numbers varying with composition at higher Ti co ntents. A small variation in the Si-Si interatomic distance was also found for x < 0.4. Our samples show a metal-insulator transition (MIT) at about 6 at.% Ti. The SAXS results show that some phase segregation, on the 30 Angstrom scale, o ccurs for low-Ti samples. We therefore suggest that the MIT should be inter preted in terms of a percolation theory involving regions of conducting amo rphous Si2Ti. We interpret the structural data in terms of such a phase seg regated model, finding good agreement with the results of fits to experimen t.