Effect of Ni Thickness on Off-State Currents of Poly-Si TFTs Using Ni-Induced Lateral Crystallization of Amorphous Silicon

Abstract

We have studied the effect of metal thickness for the metal-induced lateral crystallization ͑MILC͒ of a-Si on the off-state currents of poly-Si thin-film transistors ͑TFTs͒. It is found that they decrease with decreasing the Ni thickness for MILC. The Ni-MILC poly-Si TFT with the Ni area density of 1.4 ϫ 10 14 cm −2 exhibited a field-effect mobility of 53.5 cm 2 /Vs and minimum leakage current of 0.44 pA/m at V ds = −10 V. However, the off-state current is 6.6 pA/m when the Ni density is 9.2 ϫ 10 15 cm −2 , corresponding to the average Ni thickness of 10 Å. It is concluded that the average Ni thickness for MILC should be less than 0.79 Å to have low off-state currents from our experimental data.Polycrystalline silicon ͑poly-Si͒ thin-film transistors ͑TFTs͒ are of increasing interest for an integrated active matrix liquid-crystal displays ͑AMLCDs͒ and integrated active-matrix organic lightemitting diodes ͑AMOLEDs͒. 1 Low-cost manufacturing technology of high-quality poly-Si TFTs over a large area is a key issue of applying low-temperature poly-Si ͑LTPS͒ to AMOLEDs and AMLCDs. 2,3 A common method for the poly-Si on glass is excimer laser annealing of a-Si, 4 but it still has some issues such as nonuniformity over large areas and high manufacturing cost. 5 Ni-mediated crystallization of a-Si is of increasing interest for low-cost production and uniform poly-Si layers on glass. However, the applications of Ni-mediated lateral crystallization of a-Si to TFTs have a critical issue because the off-state leakage currents are mostly high.The leakage current of poly-Si LTPS TFTs can be reduced by reducing both defect density and Ni density in the channel region. Poly-Si made by metal-induced crystallization with a cap layer ͑MICC͒ on a-Si layer can reduce metal contamination. 6,7 The Niinduced lateral crystallization ͑Ni-MILC͒ poly-Si TFT suffers from high leakage currents because Ni atoms stay in the channel region after the crystallization. 8,9 The Ni concentration in the MILC region is typically ϳ0.08 atom % by secondary ion mass spectroscopy ͑SIMS͒ analysis, 10 which induces the leakage currents related with Ni atoms. 11 Note that the leakage current does not depend on the Ni concentration in the poly-Si when Ni density is less than 1 ϫ 10 14 cm −2 , corresponding to bulk concentration of 2.5 ϫ 10 19 cm −3 ͑0.05%͒. 12 In our previous report, 13 disk-shaped grains of 42 m in poly-Si were achieved by silicide-mediated crystallization of a-Si using a thin Ni layer of 2.43 ϫ 10 14 cm −2 . The seeds can be formed by the aggregation of NiSi 2 and the grain shape is cylindrical due to diskshaped grain growth. 14 It is found that the grain size decreases with increasing the Ni density on the a-Si for metal-induced crystallization ͑MIC͒.In this work, we have studied the effect of Ni thickness for MILC on the performance of poly-Si TFTs. The Ni density was controlled from 3.4 ϫ 10 13 to 9.2 ϫ 10 15 cm −2 , corresponding to the average thickness of 0.037-10 Å, for MILC of a-Si. The a-Si was crystallized at 580°C. Experimental Fig...