Poly-Si can be doped by diffusion, implantation, or the addition of dopant gases during deposition (in-situ doping). Diffusion and implantation are high temperature processes, so that it is improper for gate poly-Si in TFT LCD. In-situ doping offers the advantage of lower processing temperature, but its mobility is often low, 10 to 30 ㎠/V$\sec$. To develop P-doped poly-Si which is low temperature process, low resistivity, and high mobility, in-situ P-doped hydrogenated amorphous Si (α-Si:H) films anneals at 600℃ for solid phase crystallization (SPC).
To investigate the crystallization behavior of P-doped α-Si:H films and to find out the optimum deposition and annealing conditions for low resistivity and high mobility poly-Si, P-doped α-Si films were deposited at various conditions such as $PH_3/SiH_4$ (7.5×$10^{-5}$ to 2.5×$10^{-2}$), substrate temperature (200 to 400℃), RF power (10 to 90 W) and annealed at 600℃.
P-doped α-Si:H films were deposited by plasma enhanced chemical vapor deposition (PECVD) below 400℃ with 20% $SiH_4$ gas, 1% and 308 ppm $PH_3$ gas and Ar. Then P-doped α-Si:H films annealed at 600℃ in Ar atmosphere to make poly-Si films by the solid phase crystallization (SPC).
As the $PH_3/SiH_4$ ratio increased, nucleation rate decreased and grain growth rate increased. The final grain size had a maximum value of 2.08 μm when the $PH_3/SiH_4$ ratio was 8×$10^{-3}$. The resistivity of the poly-Si decreased from 23.57 Ωcm to 2.36×$10^{-3}$Ωcm when the $PH_3/SiH_4$ ratio increased from 7.5×$10^{-5}$ to 2.5×$10^{-3}$. The resistivity was constant with further increase in the $PH_3/SiH_4$ ratio above 2.5×$10^{-3}$. And the carrier concentration was saturated to 7×$10^{19}cm^{-3}$ as the $PH_3/SiH_4$ ratio was above 2.5×$10^{-3}$ because of the solid solubility of phosphorus in Si. The Hall mobility of poly-Si increased with increase in $PH_3/SiH_4$ ratio and had a maximum value of 35.65 ㎠/V$\sec$ when the $PH_3/SiH_4$ ratio was 8×$10^{-3}$ because of the largest grain size.
As the substrate temperature decreased from 400℃ to 200℃, the hydrogen contents of films increased from 0.76% to 15.45%. The P-doped α-Si:H films deposited above 250℃ had only Si-H bonds, but the films deposited at 200℃ had a number of Si-$H_2$ bonds. With increase in substrate temperature, nucleation rate increased, and grain growth rate decreased. Therefore, final grain size increased as substrate temperature decreased. In spite of the change of substrate temperature, the carrier concentration was almost constant, with a value of 6×$10^{19}cm^{-3}$. The mobility increased with decrease in substrate temperature, and the mobility had a maximum value of 37.8 ㎠/V$\sec$ when deposited at 200℃.
With increase in RF power, the rate of crystallization increased. The resistivity and the carrier concentration of poly-Si maintained 2.5×$10^{-3}$Ωcm, 6×$10^{19}cm^{-3}$ respectively. The mobility decreased with the increase in the RF power.
The films were made a high mobility of 37.8 ㎠/Vsec and low processing temperature of 600℃ when the deposition condition was the $PH_3/SiH_4$ ratio 8×$10^{-3}$, substrate temperature 200℃, RF power 10 W.
