Effect of Hydrogen Neutral Beam on the Growth of Nanocrystalline Silicon

We have investigated the effect of hydrogen neutral beam(H-NB) on the nano-crystalline silicon (nc-Si) thin films using the neutral beam assisted chemical vapor deposition (NBaCVD) system. In traditional hydrogenated silicon deposition, the gas mixture ratio of hydrogen and silane (SiH₄) controls the phase of the thin films, including that from hydrogenated amorphous silicon (a-Si:H) to microcrystalline silicon (μc-Si). However, poor migration of deposition precursor and the absence of nucleation sites cause silicon background to be amorphous in the early stage of growth. Conversely, The NBaCVD technology controls the energy of neutral particles (mainly hydrogen atoms) in the range of 0-300 eV to enhance the crystallinity during thin film deposition at low temperature (<80 °C). The growth mechanism of the nc-Si thin film by the NBaCVD process differs from that of the nc-Si by conventional PECVD. The nc-Si thin films crystal volume fraction increases rapidly and saturates near 80 % while the grain size becomes gradually larger in a range of 4~5.5 nm as the H-NB kinetic energy (reflector bias) increases. Because the nucleation and nc-grain growth should be initiated on the sub-layer surface due to assistance of the energetic H-NB particles, an incubation layer is eliminated effectively. The structural state was estimated by micro Raman spectroscopy. The microstructure was analyzed by field emission transmission electron microscopy (FE-TEM). Finally, the very low temperature processed nc-Si TFT show much improved stabilities against sever positive bias light stress (PBIS; Vg= 20 V, Vd= 1 V, 100 mW/cm² Halogen Lamp) condition, while the field effect mobility was up to 3.2 cm²/Vs: threshold voltage shift was smaller than 1 V after 10⁴sec under the PBIS conditions.