On the effect of substrate temperature on a-Si:H deposition using an expanding thermal plasma

Fast (7 nm/s) deposition of amorphous hydrogenated silicon with a midgap density of states less than 10¹⁶ cm^-3 and an Urbach energy of 50 meV has been achieved using a remote argon/hydrogen plasma. The plasma is generated in a dc thermal arc (0.5 bar, 5 kW) and expands into a low pressure chamber (20 Pa) thus creating a plasma jet with a typical flow velocity of 10³ m/s. Pure silane is injected into the jet immediately after the nozzle, in a typical flow mixture of Ar:H₂:SiH₄=55:10:10 scc/s. As the electron temperature in the recombining plasma is low (typ. 0.3 eV), silane radicals are thought to be produced mainly by hydrogen abstraction. Material quality in terms of refractive index, conductivity, microstructure parameter and optical bandgap was found to increase monotonously with substrate temperature, even up to 350 °C; for practically all low growth rate deposition schemes an optimum around 250°C is observed. It will be argued that this behavior is consistent with a simple kinetic model involving physisorption and hopping, growth on dangling bonds and thermal desorption of hydrogen.