Abstract

The bottom-gate nc-Si based thin-film-transistors (TFTs) grown by using the low-temperature plasma-enhanced chemical vapor deposition (LT-PECVD) system with He diluted ${{SiH}} _{4}$ are demonstrated. With the RF plasma power increasing from 20 to 100 W, the crystalline volume ratio of the nc-Si inside the a-Si:H film significantly increases from 12.5% to 32%, and its deposition rate is also enhanced from 9.5 to 14.5 nm/min. The faster deposition at higher plasma greatly suppresses the residual oxygen content in nc-Si film to 4% or less, which reduces the flat-band shifted voltage of the MOS diode by 2 volts. The increased crystalline volume with suppressed oxide in nc-Si films contribute to the enhanced Hall mobility and conductivity. The nc:Si TFT decreases its threshold voltage from 3.3 V to 2.7 V, and enlarges its field mobility from 0.3 to 1.3 ${{cm}}^{2} /{{V}} \mathchar"702D {{s}}$. The defect density in the nc-Si TFTs further decrease by one order of magnitude to ${{7.5}}\times {{10}} ^{16}~{{cm}}^{-3}\mathchar"702D{{eV}} ^{-1}$, which causes a shrinkage on the sub-threshold operation range to make easier the operation of the nc-Si TFTs entering into the above-threshold regime at lower voltage. The hydrogen-free He diluted ${{SiH}} _{4}$ growth has shown its compatibility with the conventional recipe for the high-mobility nc-Si TFT fabrication.

© 2013 IEEE

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