Effects of the distance between the inductance coil and substrates on the microstructure and optical properties of silicon films deposited by ICP-CVD

Inductively coupled plasma (ICP) system with the adjustable distance (d) between the inductance coil and substrates was designed to effectively utilize the spatial confinement of ICP discharge, and then control the gas-phase transport process. The effects of the distance on the microstructures and optical properties of silicon films were systematically investigated in our work. The investigation was conducted in the ICP-chemical vapor deposition process with a mixture of SiH4/H2 as the source gas at a low temperature of 240 °C. Characterization of the films with X-ray diffraction and Raman spectroscopy revealed that the crystallinity and crystallite size firstly increased and then decreased with increasing the distance. The maximum was reached at the distance of 5 cm. By SEM measurements, the film morphologies were shown to be consistent with the XRD and Raman analysis results. The Fourier transform infrared (FTIR) spectroscopic analysis showed the hydrogen was predominantly incorporated in the silicon films in the mono-hydrogen (Si-H) bonding configuration. With increasing the distance, the hydrogen content in the silicon films exhibited similar behavior with crystallinity of silicon films. Based on the results, a gas-phase transport process was suggested for the deposition of silicon films in ICP-CVD system. Furthermore, the Tauc's optical band gap achieved the maximum of 1.68 eV with the distance of 5 cm. Besides the effect of hydrogen content, the quantum size effect might also be responsible for higher band gap in crystalline silicon films.