Optical properties of thermally oxidized silicon

The effect of furnace grown SiO₂ layers on the optical properties of p- on p+ (100) Si substrates are investigated. The real part, n, of the complex refractive index n = n + ik is calculated for radiation measured in the infra-red (IR) region between 3000 and 8000 cm^-1 where the extinction coefficient, k, is negligible. The expression for n is obtained using the Fresnel coefficients for a three medium air-oxide-Si model. Strain in the silicon, which affects n, and caused by the stress in the SiO₂ layer, increases with oxide thickness. X-ray diffraction (XRD) was used to measure the strain in Si for oxides layers ranging from native to 5124angstrom. The data showed a monotonically increasing normal compressive strain, ε^N (up to 0.47%) with oxide thickness, however, the corresponding change in n due to strain was not well defined. The effect of strain on the direct optical gap, E_d, at 3.46 eV when determined from results of other investigators by electroreflectance, suggests an average shift in E_d of about 25 meV.