The electrical, optical, and structural properties of microcrystalline silicon (μc-Si) films prepared by rf plasma-enhanced chemical vapor deposition using $SiF_4$ and $H_2$ gases have been investigated. We have deposited μc-Si films with varying deposition conditions such as substrate temperature and hydrogen dilution ratios.
In the ranges of $300~400^oC $ substrate temperature, the structural properties of μc-Si films don't change and the deposition rate of the films decreases as the substrate temperature increased by effective etching of week bonded adsorbates. As the hydrogen dilution ratio of source gases increases, the film structure changes from amorphous to microcrystalline. The crystalline volume fraction of μc-Si films also increases with increasing the hydrogen dilution ratio and reaches about 80 % at the hydrogen dilution ratio above 0.2. With more increasing the hydrogen dilution ratio we don't observe the structural changes of μc-Si films, but increase of the hydrogen content in the film was observed.
To investigate the growth mechanism of the μc-Si films, we have deposited the μc-Si films on two different types of substrates : Corning 7059 glass and $SiO_2$ film. We observed that on the glass substrates, small grain grew and grain boundaries were not clear at the early stage of deposition (≤ 100 nm) before the growth of a columnar crystalline layer. However, on the $SiO_2$ substrates, after the growth of a thin amorphous layer (~10 nm), direct growth of a columnar crystalline layer was observed. Surface roughness of the films increased with the film thickness on the both substrates and was larger on glass substrates than on $SiO_2$ substrates.
We have also studied the electrical properties of μc-Si films prepared with various deposition conditions. Dark conductivities of the μc-Si films are as large as $~10^{-3}$ S/cm and activation energies of the films are as small as ~0.2 eV. Electrical conduction of undoped μc-Si films is explained by the combination of the extended conduction at high temperatures and the variable range hopping at low temperatures. Hydrogen plasma treatment to the films affect the electrical conduction of μc-Si. After hydrogen treatment, activation energies of μc-Si become larger than those of as-deposited films and one electrical conduction path in μc-Si are shown. To explain the electrical conduction of μc-Si before and after hydrogen plasma treatment, we propose a band diagram of μc-Si such that the Fermi level located at 0.15 eV below the conduction band due to the donor-like defects.
