Influence of deposition pressure on structural, optical and electrical properties of nc-Si:H films deposited by HW-CVD

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited using HW-CVD technique at various deposition pressures. Characterisation of these films from Raman spectroscopy revealed that nc-Si:H thin films consist of a mixture of two phases, crystalline phase and amorphous phase containing small Si crystals embedded therein. We observed increase in crystallinity in the films with increase in deposition pressure whereas the size of Si nanocrystals was found ∼2 nm over the entire range of deposition pressure studied. The FTIR spectroscopic analysis showed that with increasing deposition pressure the predominant hydrogen bonding in the films shifts from, Si–H to Si–H2 and (Si–H2)n complexes and the hydrogen content in the films was found in the range 6.2–9.3 at% over the entire range of deposition pressure studied. The photo and dark conductivities results also indicate that the films deposited with increasing deposition pressure get structurally modified. It has been found that the optical energy gap range was between 1.72 and 2.1 eV with static refractive index between 2.85 and 3.24. From the present study it has been concluded that the deposition pressure is a key process parameter to induce the crystallinity in the Si:H thin films using HW-CVD.

► Deposition pressure is a key process parameter in HW-CVD to induce the crystallinity in the films.
► The nc-Si:H films synthesised using HW-CVD consist of two phases i.e. crystalline phase and other is amorphous phase with Si nanocrystals embedded in it.
► Volume fraction of the crystallites in the film increases whereas its size remain almost constant (∼2 nm) over the entire range of deposition pressure studied.
► HW-CVD deposited nc-Si:H films are under high compressive stress like PE-CVD films.
► Hydrogen content films were found to be <10 at% but the band gap remains as high as 2.0 eV or even more.