The infrared-absorption spectra of silicon dioxide have been studied for many years and most of the peaks have been identified. During the investigation of silicon dioxide deposited by the liquid-phase-deposition technique, an interesting phenomenon was observed. It was found that the intensity ratio between the side lobe (1200 cm−1) and main peak (1090 cm−1) varies and depends on both the concentration of boric acid and the dilution ratio of the growth solution. Measurement of the refractive index shows that the material with larger absorption at 1200 cm−1 has a smaller index and thus more porous structure; therefore, the peak at 1200 cm−1 is suggested to arise from porous oxide, i.e., Si—O—Si, in a large void.
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Selective deposition was observed in the growth of silicon dioxide using liquid-phase deposition process. By adjusting the growth parameters, the key mechanism leading to the observed selective deposition was identified. It is found that within the same period of time, a thicker oxide film is grown when the native oxide on the silicon substrate surface is not removed. This difference in oxide growth is attributed to different surface conditions which affect the initial growth rather than the subsequent deposition rate. It is proposed that the chemical reaction that replaces Si-H surface bonds to St-OH bonds as well as the formation of some kinds of intermediate product are the rate limiting processes for initial growth. On the other hand, changing the supersaturation level of the hydrofluosilicic acid results in different deposition rates although the substrate surfaces are the same. Combining these results, a critical concentration of hydrofluoric acid which depends on the substrate surface conditions is determined. As long as the concentration of hydrofluoric acid in the solution is smaller than this critical value, the oxide deposition occurs. Otherwise, no oxide can be grown.
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