Chemical Composition and Ageing Effects

The compressive stress in PS decreases with annealing temperature and is reversed for temperatures above 350 °C, indicating that the PS is under tension [Su4], These changes are reversible if the hydrogen coverage is restored by a short HF dip however, at annealing temperatures above 500 °C the PS microstructure is irreversibly changed [Ha4], 

In micro PS a considerable number of silicon atoms are located at the inner surface, as shown in Table 6.1. Therefore, oxidation or absorption of hydrocarbons in the order of a monolayer will significantly change the chemical composition of the material. It is therefore essential for the comparison of chemical data if they have been obtained in situ, directly after preparation, or after longer storage peri- 

On storage in ambient air or immersion in H202 the Si-O-Si vibrational mode becomes detectable for electrochemically formed PS, without a significant change in the number of Si-Hx bonds, as shown in Fig. 6.15 [Th5]. This supports the assumption that oxygen penetrates the Si lattice and breaks the backbonds of the 

PS (left) and an oxidized sample (right), with the most important vibrational modes assigned as indicated. After [Th6], 

The absorption of water in the microporous silicon network was evaluated by exposing the porous samples to HzO and DzO vapor at low pressure (10 s Torr). FTIR spectral analyses revealed that water dissociates at the PS internal surface to SiH (SiD) and SiOH (SiOD). Upon annealing to 350°C, Si-O-Si is formed from the SiOH (SiOD) groups while H2 (D2) is desorbed [Gu3]. 

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