IN-SITU SPECTROSCOPIC ELLIPSOMETRIC INVESTIGATION OF VACUUM ANNEALED AND OXIDIZED POROUS SILICON LAYERS

Vacuum annealed and oxidized porous silicon layers (PSL) were investig ated by in situ spectroscopic ellipsometry (SE). The nominal porosity of the layers was between 60 and 77% and the nominal thickness was 500 nm. The annealing was performed by direct ohmic heating (R.T. < 450 d egrees C) in 5 x 10(-10) Torr vacuum. The oxidation was performed in t wo steps, the first step at 5 x 10(-5) Torr, the second at 10 Torr. Tw o optically different types of silicon compounds, a bulk-type silicon (c-Si) and a fine-grain polycrystalline silicon with enhanced absorpti on due to extensive grain-boundary regions (p-Si) were mixed with void s in the appropriate ratio to fit the spectra of as-prepared PSL. For the annealed PSL amorphous silicon (a-Si) was needed in conjunction wi th p-Si. The oxidized PSL could be fitted with a reduced a-Si content. We can interpret the annealing effect as a depassivation process of t he inner surfaces of the PSL (a-Si fraction). At the same time, oxidat ion leads to a repassivation process. (C) 1998 Elsevier Science S.A.