Nano-Crystalline Silicon-Based Bottom Gate Thin-Film Transistor Grown by LTPECVD With Hydrogen-Free He Diluted ${\hbox {SiH}} _{4}$

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 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 cm² V-s. The defect density in the nc-Si TFTs further decrease by one order of magnitude to 7.5 × 10¹⁶ cm^-3 eV, 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 growth has shown its compatibility with the conventional recipe for the high-mobility nc-Si TFT fabrication.