High Mobility ZnO thin film transistors using the novel deposition of high-k dielectrics
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High Mobility ZnO thin film transistors using the novel deposition of high-k dielectrics D. K. Ngwashi1, R. B. M. Cross1, S. Paul1, Andrian P. Milanov2, Anjana Devi2 1
Emerging Technologies Research Centre, De Montfort University Leicester LE1 9BH, UK Inorganic Materials Chemistry Group, Inorganic Chemistry II, Ruhr-University Bochum, 44801 Bochum, Germany 2
ABSTRACT In order to investigate the performance of ZnO-based thin film transistors (ZnO-TFTs), we fabricate devices using amorphous hafnium dioxide (HfO2) high-k dielectrics. Sputtered ZnO was used as the active channel layer, and aluminium source/drain electrodes were deposited by thermal evaporation, and the HfO2 high-k dielectrics are deposited by metal-organic chemical vapour deposition (MOCVD). The ZnO-TFTs with high-k HfO2 gate insulators exhibit good performance metrics and effective channel mobility which is appreciably higher in comparison to SiO2-based ZnO TFTs fabricated under similar conditions. The average channel mobility, turnon voltage, on-off current ratio and subthreshold swing of the high-k TFTs are 31.2 cm2V-1s-1, 4.7 V, ~103, and 2.4 V/dec respectively. We compared the characteristics of a typical device consisting of HfO2 to those of a device consisting of thermally grown SiO2 to examine their potential for use as high-k dielectrics in future TFT devices. INTRODUCTION Polycrystalline silicon (poly-Si) thin film transistors (TFTs) have so far been the main alternative to amorphous silicon (a-Si) TFTs used in electronic flat panel displays by industrialists. This has been due to the mobility limitation imposed on amorphous silicon base TFTs [1, 2]. However, both the poly-Si and a-Si TFTs are opaque to visible light, thus would limit the aperture ratio of the active matrix arrays in flat panel displays [3, 4]. Semiconducting oxides with large bandgap (transparent to the visible regime, such as ZnO with a bandgap of ~3.37 eV), can be used to fabricate high channel mobility TFTs (at/near room temperatures) and offer a way to overcome both the mobility and the transparency issues with improved device stability to light. ZnO-based transparent (light stable) TFTs with improved performance have been demonstrated [5-10]. These transistors have been found to exhibit high effective channel electron mobility (μeff) that could lead to higher drive currents and faster device operating speeds (required for radio frequency applications and as driver TFTs required for organic light-emitting diode (LED) displays). EXPERIMENT Inverted-staggered TFTs incorporating high-k dielectric layers are demonstrated using hafnium oxide (HfO2), deposited by metal-organic chemical vapour deposition (MOCVD) as a gate insulating layer. The insulators were deposited from novel precursors at a temperature of 400 0C using the technique described in [11]. The source/drain (S/D) electrodes are formed by depositing 100 nm of aluminium patterned by shadow masks (giving rise to an active ZnO channel of 100 μm, and width/length ratio W/L=10:1). The ZnO channel layer (40nm) was
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