RE plasma deposition of amorphous silicon-germanium alloys: evidence for a chemisorption-based growth process

Amorphous silicon-germanium alloys (a-Si_1-xGe_x :H,F) have been deposited by glow-discharge decomposition of silicon tetrafluoride (SiF₄) and germane (GeH₄) in mixture with hydrogen. The effect of total RF power and deposition temperature on the characteristics of the gas phase and the grown material has been evaluated. Also, the effect of the dopant addition has been studied by adding PH₃ and B₂H₆ to the feeding mixture. Gas-phase characterization has been performed with optical emission spectroscopy for the analysis of the emitting species, mass spectrometry for the analysis of the stable species, and Langmuir electrical probes for the evaluation of electron density and temperature. The deposition rate has been measured in situ by laser interferometry. The deposited films have been analyzed with infrared and visible-range absorption spectroscopy and with X-ray microanalysis for the measurement of the chemical composition and optical properties. A kinetic analysis of the results points to a deposition model in which dissociative chemisorption of the gaseous reactants (SiF₄, GeH₄) plays a relevant role. There is also strong evidence that the chemisorption process is of the anionic type for SiF₄ and of the cationic type for GeH₄