Deposition phase diagrams for Si1−xGex:H from real time spectroscopic ellipsometry

Phase diagrams have been developed to describe rf plasma-enhanced chemical vapor deposition (PECVD) of hydrogenated silicon–germanium alloys (Si1−xGex:H) on crystalline Si substrates held at 200 °C. For a series of three such overlapping diagrams established at different flow ratios G = [GeH4]/{[SiH4] + [GeH4]}, the boundary describing the thickness at which the amorphous-to-(mixed-phase microcrystalline) transition occurs [designated a → (a + μc)] shifts rapidly to higher H2-dilution flow ratio R = [H2]/{[SiH4] + [GeH4]} with increasing G. This demonstrates that Ge incorporation strongly suppresses microcrystallite nucleation from the amorphous phase. For each individual phase diagram of this series, the thickness at which an amorphous regime roughening transition occurs [designated a → a] increases with increasing R right up to the a → (a + μc) boundary. Under wide ranges of deposition conditions, however, the maximum a → a transition thickness decreases rapidly with increasing G for the series of phase diagrams with different ratios G. This decrease is weakest for cathodic PECVD of Si1−xGex:H with a self-bias near −20 V, suggesting precursor surface diffusion enhancement under such conditions.