(809g) Formation of Extreme Aspect-Ratio Semiconductor Waveguides Via High-Pressure Chemical Deposition in Microcapillaries

Extreme aspect-ratio semiconductor waveguides can be fabricated by high-pressure chemical deposition within microcapillary templates. In this process, a high-pressure (~35 MPa) precursor comprised of a mixture of silane and helium flows within a microcapillary template with elevated wall temperature. Silane decomposes at the capillary wall to deposit a silicon film. To obtain high quality waveguides, complete filling of the microcapillary template is desirable. However, complete filling of the plugged central channel over lengths of centimeters or more is challenging in view of its extreme aspect ratio. Experimental observations show that the thickness of the deposited silicon layer varies greatly with axial position within the microcapillary. The process to completely fill the microcapillary consists of three phases. In phase I, there is little or no constriction of the pore, and semiconductor deposition has little impact on gas flow. In phase II, the narrowing of the microcapillary cross-section results in a strong coupling between flow and temperature-dependence of the rate of deposition. In phase III, the microcapillary is largely or completely closed, and an axial concentration gradient arises within the microcapillary as silica selectively filters silane from helium and hydrogen, due to its permeability to the latter components at high temperature.  We report experimental observations of film growth within microcapillaries, and compare our experimental measurements with model predictions to examine the observed non-uniformity in the thickness of the deposited semiconductor film.