Optical properties and structure of si 1 − xc x: H alloys

Abstract

Measurements of the absorption constant by transmission and PDS spectroscopy, of the refractive index and of the reflectivity up to 10 eV of silicon carbon alloys prepared by plasma deposition from silane methane mixtures are reported for composition up to x = 0.87. Mass density measurements up to x = 0.4 show little variation of the total density of Si and C atoms. In this range of composition the alloys are very similar to amorphous silicon with steep absorption edges, shifting to higher energy, and an ε 2 peak which remains near 3.8 eV though decreasing in strength. It is concluded that these alloys are built by Si-Si bonds. Carbon contributes little to the formation of the lattice and is incorporated into the Si lattice as CH 3 and CH 2 groups. At higher carbon content the silicon based lattice is replaced by a new lattice which is not a tetrahedrally coordinated Si-C lattice. This transition is obvious by a dramatic change of the spectra in a narrow range of compositions. The absorption spectra develop an extremely shallow tail and the ge 2 peak near 3.8 eV disappears.