In   situ spectroscopic ellipsometry of porous silicon layers annealed under ultrahigh vacuum

In situ spectroscopic ellipsometry measurements in the 1.8–2.8 eV spectral range were performed on porous silicon (PS) layers annealed in an ultrahigh vacuum chamber up to 600 °C. It is shown that both the refractive index and the absorption coefficient increase after the thermal anneal and the effect is reversible when samples are returned to atmosphere and HF dipped. These changes in the optical properties of the material can be attributed to an intrinsic strain modification in the PS cellular structure induced by hydrogen desorption upon annealing.