Thin Film Transistors Using Nanocrystalline Silicon from Metal Induced Growth
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Thin film transistors (TFTs) have been well developed in over six decades. Nowadays, hydrogenated amorphous silicon (a-Si:H) has been largely used for active matrix liquid crystal display (AMLCD) in the industrial field. At the same time, nanocrystalline silicon (nc-Si) TFT has attracted more attention due to its better quality than a-Si and low costs. This thesis focused on the fabrication of nc-Si TFTs using metal-induced growth (MIG), which has been utilized in solar cell fabrication and nanowire growth. The 500 Å thickness palladium (Pd) was evaporated as the metal catalyst on the silicon substrate covered with silicon dioxide. The nc-Si films were deposited using two-step DC sputtering at 625 °C. The Pd was consumed to form Pd 2 Si at 50 W low power step, which provides the nucleation sites for nc-Si growth. Then, nc-Si films can be grown at a high deposition rate which was obtained in the 150 W high power step. As a result, the nc-Si with around 1000 nm thickness was deposited as the active channel of the TFTs by 45 min low power and 30 min high power sputtering. The output and transfer characteristics of the TFTs were illustrated in the thesis. The field effect mobility and the threshold voltage were calculated. The performance was difficult to get saturation due to the high leakage current. The field effect mobility values were 0.3 to 0.6 cm 2 /V.s, which were relatively low. The main reasons are the existence of the grain boundaries and the intragrain defects in the channel, the metal contamination introduced by MIG, and the thick channel layer. The threshold voltage values varied from 1.3 V to 1.9 V, which were partially affected by the short length effect (L= 5, 10 μm). In addition, the total resistances of semiconductor and metal-semiconductor contact were measured using the transmission line method (TLM). The linear TLM and the circular TLM were both used to calculate the contact resistance, transfer length, and the specific contact resistance. The results of three kinds of samples, MIG with anneal, MIG without anneal, and non MIG, are presented and compared. The MIG with anneal samples show much lower contact resistance than that of MIG without anneal samples. The quality of the nc-Si and the ohmic contact of the junction are improved after annealing at 700 °C for 2 hours in forming gas (15% H 2 and 85% N 2 ).