Phosphorus-doped hydrogenated microcrystalline silicon (μc:Si:H) films for thin film transistors were deposited by plasma enhanced chemical vapor deposition at various deposition conditions such as $PH_3$/$SiH_4$ ratio ($7.55\times10^{-3}$ to $1.21\times10^{-2}$), substrate temperature (150 to 400℃) and RF power (10 to 120 W). 10.2% silane ($SiH_4$) gas diluted in Ar and 308 ppm phosphine ($PH_3$) gas diluted in Ar were used as source gases. Formation microcrystalline phase and its volume fraction were investigated by using X-ray diffractometor and Raman spectroscope. The electrical and optical properties of the films were investigated by FT-IR transmittance, UV transmittance and electrical resistivity measurements. The effects of the deposition conditions and the volume fraction of microcrystalline phase on the electrical and optical properties were discussed.
Microcrystalline phase started to form when the $PH_3$/$SiH_4$ ratio exceeded $9.06\times10^{-3}$, below which only amorphous films were deposited. The volume fraction of microcrystalline phase increased with further increase in the $PH_3$/$SiH_4$ ratio, and reached up to 59.1% at the $PH_3$/$SiH_4$ ratio of $1.21\times10^{-2}$, at which substrate temperature and RF power were fixed at 250℃ and 120 W, respectively. Hydrogen contents of the films increased to 13.03 at % with increase in the $PH_3$/$SiH_4$ up to $1.51\times10^{-3}$, and then decreased with further increase in the $PH_3$/$SiH_4$ ratio. Above $9.06\times10^{-3}$ of the $PH_3$/$SiH_4$ ratio, hydrogen contents sharply decreased due to the sudden increase in volume fraction of microcrystalline phase. The electrical resistivity of the films decreased monotonically with increase in the $PH_3$/$SiH_4$ ratio and sharply decreased due to forming microcrystalline phase as $PH_3$/$SiH_4$ exceeded $9.06\times10^{-3}$.
The thin films deposited at the RF power of 80 W and the $PH_3$/$SiH_4$ ratio of $1.21\times10^{-2}$, had a columnar structure and a microcrystalline phase regardless of substrate temperature. The volume fraction of microcrystalline phase in the films increased with increase in substrate temperature up to 200℃, and then decreased with further increase in substrate temperature. Hydrogen contents decreased to 0.3 at % monotonically with increasing substrate temperature. Dangling bond density did not change significantly as substrate temperature increased to 300℃, but the density increased with further increase in substrate temperature.
Volume fraction of microcrystalline phase increased monotonically with increase in RF power at the substrate temperature of 250℃ and $PH_3$/$SiH_4$ ratio of $1.21\times10^{-2}$. Hydrogen contents and electrical resistivity decreased monotonically with increasing RF power due to the increasing volume fraction of microcrystalline phase.
The optical band gap of the films became smaller as the $PH_3$/$SiH_4$ ratio increased, while the value became larger as the hydrogen contents increased. The increase of the microcrystalline volume fraction widened the optical band gap.
