Theoretical Model of Phosphorus Incorporation in Silica in Modified Chemical Vapor Deposition

Abstract

Based on experiment, a theoretical model is developed of phosphorus incorporation in modified chemical vapor deposition. In the glass formation stage of this process, gas‐phase reaction of silicon tetrachloride and phosphorus oxychloride generates submicrometer particles of phosphorus‐doped silica. These particles deposit in a thin porous soot layer which is then viscously sintered. Equations for thermal and dopant transport through the soot voids and particles during sintering predict that gradients in each layer are due to inhibited diffusion of dopant through the gaseous voids of the soot, not to gas‐phase gradients. Thermal gradients create variations in gas and solid diffusion and dopant solubility which enhance concentration gradients. These results apply to viscous sintering of glasses produced by flame hydrolysis and sol‐gel processes.