IN-SITU INFRARED CHARACTERIZATION OF THE INTERFACIAL OXIDE DURING THEANODIC-DISSOLUTION OF A SILICON ELECTRODE IN FLUORIDE ELECTROLYTES

The interfacial oxides formed under steady-state anodic polarisation o f p-silicon in fluoride electrolytes have been studied using in situ i nfrared spectroscopy in the difference mode. The oxide is characterize d in the 900 to 1250 cm(-1) range by the vSiO TO and LO bands, and by other absorption bands related to oxide defects such as non-bridging o xygens. The shape and magnitude of all these bands strongly depend on formation potential as well as on electrolyte composition. Oxide thick ness can be estimated from the intensity of the TO absorption band. It is shown to increase with potential and to be little dependent on ele ctrolyte composition. On the other hand, obtaining reliable structural information requires to quantitatively account for both LO and TO ban ds. Specifically, the frequency splitting between LO and TO modes as w ell as a properly defined average frequency appear as the relevant par ameters. In the present case, the oxides grown at low current (i.e., f or low fluoride concentration and at potentials corresponding to the s econd electropolishing plateau) are those exhibiting a high SiO vibrat or density and a low defect concentration.