Study of hot wire chemical vapor deposition of amorphous hydrogenated silicon using optical diagnostics

This thesis work develops a suitable in situ diagnostic technique—vacuum ultraviolet (VUV) single photon ionization (SPI) laser spectroscopy—to probe multiple gas phase reactive species simultaneously and to investigate fundamental aspects of the reaction mechanisms in the hot wire chemical vapor deposition (HWCVD) process. The ultimate purpose of this work is to optimize the film quality and growth rate in the deposition of amorphous hydrogenated silicon (a-Si:H).; Gas phase species resulting from decomposition of silane precursor on a hot tungsten surface were studied by VUV-SPI. Si, SiH₃ and Si ₂H₆ are the species detected. Si is identified as the major radical formed directly from the hot filament, while the production of SiH ₃ and Si₂H₆ are found to arise from secondary reactions caused by H produced from the filament. The gas phase species were studied as a function filament temperature and filament material. Kinetic modeling, together with the experimental results, indicates that the Si production at different filament temperatures falls into a surface reaction limited regime at low filament temperatures and gas transport and adsorption limited regime at high filament temperatures. Studies on different refractive metal filaments, including W, Re, Mo and Ta, all show the Si radical as the major species produced in the surface reaction limited regime, but with different apparent activation energies, indicating different reaction kinetics. Modeling of the surface reaction kinetics suggests that the different behavior among different filament materials is due to accumulation of silicon surface layers on W and Re, but not on Mo and Ta. The different filament behaviors also provide direct evidence that the decomposition of silane on the hot wire surface is a catalytic reaction rather than a thermal process.; The developed VUV-SPI technique has also been combined with a surface optical diagnostic method, multiple internal reflection infrared spectroscopy, to study the relationship between the gas phase species and the film growth on a substrate. The film growth rate is found to follow the Si production rate for all four filament metals studied, indicating that Si is the major film growth precursor under these low pressure, non-collision experimental conditions.