Carrier Transport in Ultra-Thin Nano/Polycrystalline Silicon Films and Nanowires
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Carrier Transport in Ultra-Thin Nano/Polycrystalline Silicon Films and Nanowires Toshio Kamiya1,2,4, Yong T. Tan2,4, Yoshikazu Furuta3,4, Hiroshi Mizuta3,4, Zahid A.K. Durrani2,4 and Haroon Ahmed2,4 1 Materials and Structure Laboratory, Tokyo Institute of Technology, Japan 2 Microelectronics Research Centre, Cavendish Laboratory, University of Cambridge, UK 3 Hitachi-Cambridge Laboratory, UK 4 CREST, JST, Japan ABSTRACT Carrier transport was investigated in two different types of ultra-thin silicon films, polycrystalline silicon (poly-Si) films with large grains > 20 nm in size and hydrogenated nanocrystalline silicon (nc-Si:H) films with grains 4 nm – 8 nm in size. It was found that there were local non-uniformities in grain boundary potential barriers in both types of films. Single-electron charging effects were observed in 30 nm × 30 nm nanowires fabricated in 30 nm-thick nc-Si:H films, where the electrons were confined in crystalline silicon grains encapsulated by amorphous silicon. In contrast, the poly-Si nanowires of similar dimensions showed thermionic emission over the grain boundary potential barriers formed by carrier trapping in grain boundary defects. INTRODUCTION Recently, there has been considerable interest in the low temperature growth of hydrogenated microcrystalline silicon (µc-Si:H) thin films for their potential applications in thin film solar cells and thin film transistors for flat panel displays [1,2]. Plasma-enhanced chemical vapor deposition (PECVD) is a novel technique that allows the film microstructure to be controlled by selecting appropriate deposition conditions [3]. For example, it is possible to prepare small grain nanocrystalline silicon (nc-Si:H) using a large H2/SiF4 gas mixing ratio. This controllability of grain size is promising for novel devices utilizing quantum confinement and single-electron charging effects. It is difficult to fabricate very thin (20 nm in size. In contrast, the nc-Si:H film has much smaller single-domain grains 4-8 nm in size and contains a larger amorphous content. Carrier transport in films and very short nanowires Figure 4 shows the temperature dependence of conductivity for the films and the NWs. The carrier mobility measured by Hall measurements is ~1.8 cm2/Vs for the 50-nm-thick nc-Si:H films. In contrast, larger mobility of ~70 cm2/Vs is obtained for the poly-Si films. The larger mobility can be attributed to the larger grain size and the smaller amorphous content in the poly-Si films in part. We evaluate the activation energy of the conductivity (Ea) from the slope of the Arrhenius plot at around room temperature because the conductivity shows percolation behavior [10]. The Ea of the nc-Si:H films depends largely on the film thickness. Thinner films will have a larger activation energy. This is thought to be due to the destruction of carrier percolation paths, as discussed later. For NWs, the absolute values of the conductivity are much less than in the bulk film. It is thought that the low apparent conductivity in the NWs is due to geometrical e
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