STRUCTURAL STUDY OF POROUS SILICON

The X-ray absorption near-edge structure of porous silicon with effici ent photoluminescence (PL) produced by anodizing Si wafers in an I-IF- based etch shows the existence of a feature between those assigned to Si-Si and Si-O bonding, which we recently suggested results from the p resence of SI-OH. The enhancement of PL intensity was also shown to be dependent on the relative numbers of Si-Si, Si-O and Si-OH bonds. Por ous silicon with good visible PL has been produced using (i) photo-ass isted etching in HF and (ii) NH4F:HF-based anodization, and the local structure of these materials is presented. Extended X-ray absorption f ine structure( EXAFS) provides evidence for the presence of similar to 3 degrees configurational disorder above the thermal contribution in Si-Si-Si bond angles and for Si-O-Si bridges. i.e. for internal oxygen . In some samples which give strong visible FL, transmission electron microscopy shows that the majority of features on a scale of 5-10 nm h ave diffraction patterns indicating an amorphous structure. Informatio n on the structure has been obtained as a function of depth from the r eflected EXAFS