Nanocrystalline silicon films prepared from silane plasma in RF-PECVD, using helium dilution without hydrogen: structural and optical characterization

The effect of RF power on the nanocrystallization of a Si:H network has been studied by PECVD at a substrate temperature of 200 °C and a gas pressure of 0.5 Torr, using silane as the source gas and helium as diluent, without using hydrogen. Optical characterization of the films has been done by UV–vis spectroscopy. Structural characterization has been performed by infrared absorption, x-ray diffraction, micro-Raman studies and electron microscopy by HRTEM and FESEM. In general, a structural transformation from the amorphous to nanocrystalline phase accomplished by metastable helium atoms in the plasma has been identified at a low RF power of 80 W. With an increase in the applied RF power up to 150 W, systematic improvement in crystallinity has been shown as depicted by increased crystalline volume fraction (∼77%), smaller grain size (∼7 nm) reduced bonded hydrogen content (∼8 at.%), enhanced polymerization in the network and gradual widening in the optical gap (∼1.86 eV) obtained at a high deposition rate (107 Å min⁻¹), using 1 sccm of silane as the source gas and helium as the only diluent. He dilution of the SiH₄ plasma is a well-proven approach to increase the growth rate. However, a nanocrystalline network with high crystalline volume fraction and well-aligned crystallographic lattice distribution, attained in Si:H from a low-power RF plasma at a growth temperature as low as 200 °C, and that obtained from purely He dilution, without using H₂, is being reported for the first time. The striking feature comprises that nc-Si:H films of increasing crystalline volume fraction, reduced bonded hydrogen content and wider optical gap are produced with simultaneously increasing deposition rates, which deserves extensive technological impact.