Nanocrystalline Si : H thin films grown at room temperature with plasma-enhanced chemical vapour deposition at a very high pressure

Hydrogenated nanocrystalline silicon (nc-Si : H) films were grown using the plasma-enhanced chemical vapour deposition method, with the plasma operated under a gas pressure of up to 30 mbar, i.e. on the verge of plasma instability. The heating effect in determining the crystallite size and crystalline volume fraction can be excluded as the maximum substrate temperature was below 80 °C at the end of growth. Crystallite initiation occurs in the plasma sheath, and Si nanocrystallites appear only when the gas pressure is above 10 mbar. With the pressure increasing to 30 mbar, the volume fraction of the nanocrystalline phase increases steadily, and the crystallite size changes from ∼7.8 to ∼4.5 nm. The optical bandgap of the deposits varies between 2.30 eV (at 15 mbar) and 2.03 eV (at 30 mbar), which can be explained in terms of the reduced hydrogen content and confinement effect. This work opens up the possibility of growing nc-Si : H films with well-controlled crystallite features on substrates held at near room temperature.