The effect of active-layer thickness on the characteristic of nanocrystalline silicon thin film transistor
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1153-A21-04
The effect of active-layer thickness on the characteristic of nanocrystalline silicon thin film transistor Sun-Jae Kim, Sang-Myeon Han, Seung-Hee Kuk, Jeong-Soo Lee and Min-Koo Han School of Electrical Engineering, Seoul National University, Gwanak-gu, Seoul 151-742, Korea ABSTRACT We fabricated nc-Si TFTs in order to investigate the effect of the active-layer thickness on the characteristic of the nc-Si TFT. Bottom gate nc-Si TFTs were fabricated at 350oC using ICP-CVD. The thicknesses of the nc-Si layer were remained to 700, 1200 and 1700 Å. As the active-layer thickness increases, the mobility and the on-current level were not altered. However, the off-current level increased considerably and on/off ratio decreased. It may be attributed to highly doped characteristic of thick nc-Si film. As the nc-Si film thicker, the conductivity increases considerably and the Fermi level approaches to the conduction band minimum, which indicates the increases of doping level. The oxygen concentration shows high level of unintentional doping. Also, columnar growth of nc-Si film makes that the crystallinity of top region is much higher than that of bottom region. So, the conductivity of thick nc-Si film becomes high compared to that of thin nc-Si film. The structure of the nc-Si TFT with thick ncSi film can be similar to the serial connection of N+, N- and N+ resistance, so that it suffers difficulty to suppress the off current and to secure high on/off ratio. Therefore, the off current can be suppressed by thinning of the high conducting active nc-Si layer and nc-Si TFT with channel thickness of 700 Å shows good on/off characteristic. It is deduced that bottom gate nc-Si TFT is necessary to have intrinsic channel layer as well as thin channel layer to reduce the leakage current. INTRODUCTION Nanocrystalline silicon (nc-Si) thin film transistor (TFT) has attracted considerable attention [1]. It can be fabricated with rather simple process as compared with polycrystalline silicon (poly-Si) TFT since it doesn’t need additional recrystallization process. And it also shows better uniformity while recrystallization process causes severe non-uniformity problem in polySi TFT [2,3]. nc-Si TFT also has advantage compared to hydrogenated amorphous silicon (aSi:H) TFT due to better electrical stability [4]. nc-Si TFT can be fabricated by simple process compatible to widely used a-Si:H TFT fabrication process. However, off-current level is rather high in nc-Si TFT as compared with aSi:H TFT. Several efforts were done for the suppression of leakage current level of nc-Si TFT [5-7]. However, since nc-Si structure has special feature of columnar crystalline growth, which may attribute the conduction mechanism, intrinsic properties of nc-Si film with various thicknesses need to be concretely. The purpose of our work is to report the effect of active-layer thickness on the off-current characteristic of nc-Si TFT. Using etch-back process, nc-Si TFTs with various active-layer thicknesses were fabricated employing inductively coupled pl
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