Structural and optical properties of nanocrystalline silicon thin films grown by 150MHz VHF-PECVD

Nanocrystalline silicon thin film is a promising material potentially used in the optoelectronic field due to its improved and unique properties. In this work, nanocrystalline silicon thin films were grown by using a 150MHz VHF-PECVD to study the effect of deposition times, substrate temperatures and RF powers on their structural and optical properties. The thicknesses of the films were found to be in the range of 100 nm to 300 nm. Surface analysis from FESEM and AFM showed the existence of grain-like morphology which was later determined by EDX as silicon grains. The average grain diameter given by AFM analysis was around 50 nm. Surface roughness was found to be strongly dependent on the grain diameter where larger grain sizes showed a rougher surface. In average, surface rms roughness was 1.00 nm. Analysis from Raman showed that the films comprised of two phases, namely amorphous and nanocrystalline as revealed by a peak at 510 cm-1 with pronounced shoulder on lower frequency. The presence of nanocrystalline silicon was evident from the red-shift of peak frequency from those of pure crystalline silicon at 520 cm-1. The average grain size as obtained from Raman was around 3 nm. Optical energy band gap, Egopt deduced from Tauc’s plot and energy band gap, Eg obtained from PL were found to be higher than 1.12 eV within the range of 1.66 – 2.51 eV. All analysis showed that the properties of nc-Si were size dependent and followed the quantum confinement effect

Studies of the self-assembled growth mechanism on nanocrystalline silicon nanodots

Nanocrystalline silicon (nc-Si) nanodots have been grown on corning glass (7059) substrate using a self-assembly VHF-PECVD method under the following experimental conditions: Fixed deposition temperatures of 300/400 °C, deposition times 30/60 s, plasma power of 10 W, silane gas flow rate of 10 sccm, as well as deposition pressure of 10-2 torr. It is predicted that the onset of nucleation began immediately after the deposition and start to appear clearly after 20-60 s during which growth mechanisms occur. Essentially, the nanodots were formed onto the substrate in dome-like shapes by virtue of equilibrium surface energies, yLS, yLN and yNS. The associated liquid/solid nucleation mechanism was then simulated and related parameters were obtained: Free energy change per unit volume ?Gv ~-104 Jmol-1; Surface energies per unit area, ?LN = 1.44 Jm-2, ?NS = 19 - 60 Jm-2 and ?LS = 0.74 Jm-2; Critical energies ?G* ~10-15 J; Critical radii r* = 16 - 48 nm. These results were experimentally verified, in particular for selected critical radius r* less than 50 nm. Other measurements were also carried out: PL analysis gave bandgap energies ~ 1.8-2.4 eV, whilst Raman spectra revealed the coexistence of nc-Si and amorphous Si. It is strongly suggested that, the nc-Si nanodot grown on glass substrate fulfills the Volmer-Weber growth mode with a minor modification

A review on effect of plasma power density and gas flow rate on structural properties of nanocrystalline silicon

Effects of plasma power density and gas density on structural properties of nanocrystalline silicon grown by Plasma Enhanced Chemical Vapor Deposition (PECVD)are discussed in this paper. It has been found that both gas flow rates and plasma power density have an opposite effect on the film’s crystallinity. It was observed that higher plasma power density tend to increase the degree of crystallinity, while higher gas flow rates appeared to decrease its curves. It was also observed that microstructural defects were lower for samples with lower crystallinity