Effect of Power on Crystallinity and Opto-Electronic Properties of Silicon Thin Films Grown Using VHF PECVD Process

Abstract

The silicon thin films have been deposited using VHF (60 MHz) plasma enhanced chemical vapor deposition (PECVD) process. The influence of the power (10–50 W), used during the deposition, on the silicon thin films properties was studied. The structural transition (different crystalline fraction) along with their variable grain distribution within amorphous silicon matrix has been discussed with the help of optical, electrical and structural measurements. Raman characterization along with photoluminescence (PL), UV-VIS spectroscopy, micro-ellipsometry spectroscopy and Field emission scanning electron microscopy (FESEM) have been performed to recognize the transitions in deposited silicon films including the microstructure mainly related with crystallite fraction and optical constants. FESEM was used to investigate the characteristics and variation in the microstructure. From Raman results, the structural transition from amorphous to micro/nanocrystalline growth occur as indicated by narrow band at 515 cm−1 observed at 10 W applied power deposited film. From these results, it was found that with increasing power, the crystallinity increases and the films are highly crystalline (Xc > 50%) in nature above 20 W applied power. The contribution of nano-sized crystallites increases the crystalline volume fraction from 32 to 84%. The peak observed nearly at 2.0 eV in photoluminescence emission spectra (PL) confirms the presence of nanocrystallites in the deposited films which is well correlated with the band gap values observed from UV-VIS spectroscopy. However, red shift in PL peak energy and decrease in PL intensity observed and are explained in terms of grain boundaries. The shift in refractive index (n) and extinction coefficient (k) were also observed at different power variation. The investigation of spectroscopic ellipsometry result explains the microstructure of deposited film with changes in crystallite sizes and their volume fractions.