(622d) High Pressure Chemical Vapor Deposition In Extreme Aspect Ratio Microcapillaries

Semiconductor micro and nanowires have recently been made within the capillary spaces of highly-ordered microstructured optical fibers by the chemical vapor deposition process. This approach has led to the formation of the highest aspect-ratio micro and nanowires yet produced (on the order of 10⁵). We consider chemical vapor deposition of silicon under high pressure flow of a silane/helium mixture within extreme aspect-ratio microcapillaries. Experimental observations show that the thickness of the deposited silicon layer varies greatly with axial position within the capillary. The growth rate of silicon near the entrance of the capillary is found to be dramatically higher than that downstream, leading to eventual blockage of the capillary cross-section in the entrance region, and termination of the reaction downstream of the blockage. The observed non-uniformity in film growth is clearly undesirable and detrimental to potential applications of the product. Assuming silane decomposition to be a first-order reaction, we use a compressible flow model to examine the reasons for the observed non-uniformity in the thickness of the deposited semiconductor film. Predicted values of the volumetric flow rate based on the compressible flow model are comparable to the experimentally measured values, and the film growth rate is predicted to decrease significantly along the microcapillary as both silane density and temperature decrease with distance from the inlet. However, the predicted film growth profile is qualitatively different from the experimentally measured profiles which show a local maximum in film thickness a short distance downstream of the microcapillary entrance. Rapid film growth near the inlet of the microcapillary would lead to the formation of a converging-diverging structure that could cause a qualitative change in the nature of flow from subsonic to supersonic. This could give rise to the development of a local hot spot of reaction just downstream of the constriction.