High Mobility Nanocrystalline Silicon TFTs for Display Application

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0989-A11-01

High Mobility Nanocrystalline Silicon TFTs for Display Application Min-Koo Han, and Sang-Myeon Han School of Electrical Engineering, Seoul National University, School of Electric Engineering and Computer Science (#50), Seoul National University, Seoul, 151-742, Korea, Republic of ABSTRACT Nanocrystalline silicon (nc-Si) thin film transistors (TFTs) of which active layer thickness was 100nm were fabricated using inductively coupled plasma chemical vapor deposition (ICP-CVD) at 150oC. The fabricated nc-Si TFT exhibits rather high field effect mobility exceeding 22 cm2/Vs and excellent sub-threshold slope of 0.45 V/dec. The nc-Si film deposited 150oC as an active layer of the TFT shows high crystallinity more than 50% and very thin incubation layer less than 20 nm. ICP-CVD provides high density plasma with reduced ion bombardment during the deposition on nc-Si and He dilution can enhance the decomposition of SiH4 into Si, SiHX radicals and atomic H, so that high quality nc-Si film can be fabricated. The SiO2 film deposited by ICP-CVD at 150oC shows good electrical characteristics such as flat band voltage of -1.8 V and breakdown voltage of 6.2 MV/cm, which was used as a gate insulator. INTRODUCTION Recently, flexible displays have attracted considerable attention due to the robustness, lightweight, flexibility and low cost [1]. Various devices such as a-Si thin film transistors (TFTs), poly-Si TFTs and organic TFTs for flexible displays have been reported [2]. The conventional processes for low-temperature poly-Si (LTPS) TFT (< 450oC) on glass substrate may not be suitable for flexible substrates such as plastic because of a relatively high process temperature exceeding 200oC [3]. Therefore, complicated techniques and careful processes are required to fabricate poly-Si TFT under 200oC. On the other hand, peripheral driving circuits and pixel circuits are required in active matrix liquid crystal displays (AMLCDs) and active matrix organic light emitting displays (AMOLEDs). However, conventional a-Si TFTs and organic TFTs have difficulties in order to fabricate peripheral driving circuits, due to their inherent inferior electrical characteristics and their electrical instability [4,5]. Directly deposited nanocrystalline silicon (nc-Si) TFTs may be promising devices to fabricate various flat displays, due to a rather simple process and good uniformity compared with poly-Si TFTs, and superior performance and stability compared with a-Si and organic TFTs [6]. Inductively coupled plasma chemical vapor deposition (ICP-CVD) employs remote plasma [7-9] , which reduces problematic ion bombardment problems issues [10]. It may also be noted that ICP-CVD generates high density plasma [7,8]. ICP-CVD may also provide certain advantages such as a high deposition rate and improved film quality over plasma enhanced chemical vapor deposition (PECVD), which has been widely used to deposit Si film and SiO2 film at low temperature. The purpose of our work is to report the characteristics of nc-Si film deposited by ICPCVD (13.56

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