Nonlinear temperature dependence of the refractive index of polycrystalline silicon films and the influence of microstructural disorder

The optical properties of thin films are known to be affected by microstructural inhomogeneities. In this paper we study, using ellipsometry, the influence of microstructural disorder on the refractive index at 632.8 nm and on its dependence on temperature in the range 300–700 K in the case of thin noncrystalline low-pressure chemical-vapor deposition silicon films. It is found that films with large amorphous component have a behavior distinct from that shown by polycrystalline films. In particular, both the refractive index n and the thermo-optic coefficient dn/dT at room temperature have larger values for amorphous films than for polysilicon films. Furthermore, the thermo-optic coefficient dn/dT is found to vanish at about 500 K in the case of highly noncrystalline films.