Evolution of Structural and Electronic Properties in Boron-doped NanocrystallineSilicon Thin Films

摘要

We report on the boron-doping dependence of the structural and electronic properties in nanocrystalline silicon (nc-Si:H) films directly deposited by plasmaenhanced chemical vapor deposition (PECVD). The crystallinity, micro-structure, and dark conductivity (σ_d) of the films were investigated by gradually varying a ratio of trimethylboron [B(CH_3)_3 or TMB] to silane (SiH_4) 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 σ_d of 1.11 S/cm 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 σd 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 (μp) of 0.24 cm~2/V•s.