Reaction Control in Amorphous Silicon Film Deposition by Hydrogen Chloride

HCl was added to SiH₄ containing plasmas to grow a-Si:H(Cl) films with dangling bonds terminated with Cl instead of H. Bulk and surface infrared spectra, film thickness and optical band gap were examined by in situ multiple total internal reflection Fourier transform infrared spectroscopy and in situ spectroscopic ellipsometry. SiH₂Cl₂ was also used as a conventional Cl source for reference a-Si:H(Cl) film deposition experiments. The introduction of HCl does not affect the deposition rate significantly, and the deposited a-Si:H(Cl) films contain over 10 ²¹cm^-3 Cl atoms. HCl addition to the gas phase changes the surface compositions of the growing films drastically from higher silicon hydride to chlorinated lower hydride. The surface reaction control eliminates unfavorable hydride bonding structures such as SiH₂ and/or SiH in voids in the deposited films. The a-Si:H(Cl) films deposited from mixtures of SiH₄ and HCl do not show significant optical band gap widening in spite of containing over 10²¹cm^-3 Cl atoms, a concentration that is comparable to that of hydrogen. In contrast, a conventional chlorine source of SiH₂Cl₂ increases the deposition rate significantly compared to HCl. The increase in the deposition rate results in monotonie decrease of the refractive index and the optical band gap widening.