The role of surface reactions in monosilane pyrolysis

Abstract

Monosilane pyrolysis has been studied by measuring the rate of silicon deposition from SiH4–Ar mixtures in a flow reactor at temperatures between 400–500 °C. By varying gas residence time, SiH4 partial pressure and total pressure, the process could be separated into experimental regimes dominated by either homogeneous or heterogeneous reactions. In the surface regime at low total pressures and short residence times, silicon deposition is due to SiH4 alone, follows Langmuir–Hinshelwood kinetics, and saturates at high SiH4 partial pressures. The rate-determining step is most likely hydrogen desorption, and the overall activation energy is 37.5 kcal/mol. Film deposition in the homogeneous limit at high total pressures and long residence times is dominated by Si2H6 and higher silane products of the gas phase decomposition. The ratio of surface to gas phase decomposition rate has been obtained from the surface rate data and RRKM calculations. The contribution to the initial pyrolysis rate of direct (SiH4 only) surface channels at 430 °C, e.g., is >50% below 10 Torr, but becomes negligibly small for pressures exceeding 100 Torr. In experiments which follow the time evolution of the pyrolysis, Si deposition rates increase as the homogeneous reaction accelerates into the Purnell–Walsh ‘‘later time,’’ and film growth is soon superseded by homogeneous nucleation of silicon powder.