I suggest following publications:

1) D. T. Pierce and W. E. Spicer, Electronic structure of amorphous Si from photoemission and optical studies, Phys. Rev. B 5, 3017-3029 (1972)

2) Handbook of Optical Constants of Solids, Edward D. Palik, ed. Academic Press, Boston, 1985

Refractive Index Database is a good starting point:

https://refractiveindex.info/?shelf=main&book=Si&page=Vuye-20C

Thank you so much.