Tetrafluorosilane-sensitized decomposition of germane by a pulsed carbon dioxide TEA laser

Abstract

The decomposition of GeH{sub 4}, sensitized by multiphoton absorption of unfocused infrared laser radiation by SiF{sub 4}, has been studied in the fluence range 0.2-0.9 J/cm{sup 2} and over a total pressure range of 1-70 Torr. Strong absorption of energy from a pulsed CO{sub 2} laser photon field (P40 line; 1027.4 cm{sup {minus}1}) and subsequent energy-transfer processes cause an increase in the temperature of the system. As a result of these processes, decomposition of GeH{sub 4} occurs leading to the formation of stoichiometric amounts of metallic germanium and molecular hydrogen. The effects of initial composition, pressure, fluence, and presence of foreign gases (He, H{sub 2}) on the yield of GeH{sub 4} decomposition were examined. A reaction model is proposed which assumes that molecules initially present in the absorption volume are heated by the laser pulse and subsequently decompose unimolecularly. Since the decomposition process is exothermic, it prompts a further increase in temperature and thus enhances the decomposition process. Both absorption and decomposition create a shock wave causing expansion of hot gas into cool surrounding volume. The latter process brings about cooling of the hot gas and quenches the decomposition process. Taking into account the above-mentioned effects the temperature profiles (Tmore » versus time and geometry of the reaction zone) were evaluated on the basis of statistical thermodynamics, assuming that, at any stage, the system attains thermal equilibrium. Yields of decomposition were then derived with k{sub uni} values calculated by the RRKM method. The proposed approach accounts satisfactorily for the experimentally observed dependencies, and we believe it can be generally applied to the description of photosensitized processes in the infrared region.« less