Evolution of structural and electronic properties in boron-doped nanocrystalline silicon thin films

We report on the boron-doping dependence of the structural and electronic properties in nanocrystalline silicon (nc-Si:H) films directly deposited by plasma-enhanced chemical vapor deposition (PECVD). The crystallinity, micro-structure, and dark conductivity (sigma(d)) of the films were investigated by gradually varying a ratio of trimethylboron [B(CH3)(3) or TMB] to silane (SiH4) from 0.1 to 2 %. It was found that the low level of boron doping (<= 0.2 %) first compensated the nc-Si:H material which demonstrates slightly n-type properties. As the doping increased up to 0.5 %, the maximum ad of 1.11 Skin was obtained while high crystalline fraction (X-c) of the films (over 70 %) was maintained. However, further increase in the TMB-to-SiH4 ratio reduced sigma(d) to the order of 10(-7) S/cm due to a phase transition of the films from nanocrystalline to amorphous, which was indicated by Raman spectra measurements. P-channel nc-Si:H thin film transistors (TFTs) with top gate and staggered source/drain contacts were fabricated using the developed p(+) nc-Si:H layer. The fabricated TFT exhibits a threshold voltage (V-Tp) of -26.2 V and field effective mobility of holes (mu(p)) of 0.24 cm(2)/V.s.