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

Abstract

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.