The hydrogen effects on the behavior of crystallization have been investigated in hydrogenated amorphous silicon (a-Si:H) films prepared by PECVD (Plasma Enhanced Chemical Vapor Deposition) using $H_2$ diluted $SiH_4$ gas.
First, a-Si:H films were deposited at various hydrogen dilution ratios ($H_2/SiH_4$) by varying $H_2$ gas flow rate and then those were annealed in the conventional furnace under the high vacuum condition ($5Ⅹ10^{-5}$ Torr) at 600℃. The structural changes in a-Si:H films were analyzed by Raman spectroscopy, hydrogens in as-deposited a-Si:H films by FT-IR (Fourier Transform InfraRed) spectroscopy, and hydrogen evolution behavior by modified G.C(Gas Chromatography), and the final grain sizes were measured from the TEM (Trasmission Electron Microscopy) images.
To study the effects of hydrogen dilution on the final grain size of crystallized films, a-Si:H films were deposited under $H_2/SiH_4$ dilution ratios ranging from 10:5 to 60:5 at constant substrate temperature and pressure of 200℃, 0.7 torr, respectively, and then annealed. As hydrogen dilution ratio was increased, the structural disorderness and the clustered $SiH_2$ in a-Si:H films decreased, the isolated SiH and hydrogen content in a-Si:H films increased, and the final grain size was increased more than 1.4㎛. TEM and Raman analysis revealed that the increased grain size was due to the lowered nucleation rate.
In order to illucidate the effects of hydrogen on the crystallization behavior, a-Si:H films were annealed for constant time, quenched and then analyzed by G.C, FT-IR spectroscopy, and Raman spectroscopy.
The analysis showed the hydrogens from SiH bonds had an dynamic disordering effect on a-Si:H network during hydrogen evolution and those from $SiH_2$ static disordering effect on as-deposited a-Si:H network, respectively. Because SiH bonds were domint in as-deposited a-Si:films, in the case of 60:5 dilution condition, amorphous silicon network was more disordered dynamically than in those of 10:5 dilution condition which caused the lower nucleation rate in 60:5 dilution condition during crystallization. On the contarary, in the condition of $SiH_2$ bonds being dominant, it is expected that the static disordering effect would be important, that is, the more disordered a-Si:H network of as-deposited a-Si:H causes the lower nucleation rate.
From above results and discussions, both of the hydrogens from SiH and $SiH_2$ bonds cause a-Si:H network disordering and lowered the nucleation rate during crystallization. These suggest that the more hydrogens in a-Si:H films cause the lower nucleation rate by disordering a-Si:H network.
