Influence of the Structural Properties of Microcrystalline Silicon on the Performance of High Mobility Thin-Film Transis
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Influence of the Structural Properties of Microcrystalline Silicon on the Performance of High Mobility Thin-Film Transistors Kah-Yoong Chan1,2, Dietmar Knipp1, Reinhard Carius2, and Helmut Stiebig2 1 School of Engineering and Science, Jacobs University Bremen, Bremen, 28759, Germany 2 IEF5-Photovoltaics, Research Center Juelich, Juelich, 52425, Germany ABSTRACT The influence of the crystalline volume fraction of hydrogenated microcrystalline silicon (μc-Si:H) on the performance of thin-film transistors (TFTs) processed at temperatures below 180 °C was investigated. TFTs employing μc-Si:H channel material prepared near the transition to amorphous growth exhibit the highest electron charge carrier mobilities exceeding 50 cm2/Vs. The influence of the crystalline volume fraction of the intrinsic μc-Si:H material on the transistor parameters like the charge carrier mobility and the contact resistance will be discussed. INTRODUCTION Thin-film transistors (TFTs) are key elements in large area electronics. To date, TFTs based on amorphous silicon (a-Si:H) are widely used as pixel switches for display backpanels [1]. However, the realization of more complex peripheral circuitry is not possible due to the low charge carrier mobility and device instability of a-Si:H [1-2]. So far external drivers are needed or the circuitry has to be realized by polycrystalline silicon (poly-Si) TFTs with high charge carrier mobilities and stable threshold voltages [1]. However, the fabrication cost of poly-Si TFTs is higher due to high fabrication temperatures or additional laser crystallization steps. Hydrogenated microcrystalline silicon (μc-Si:H) is a promising alternative to existing technologies due to its high device charge carrier mobility [3]. The material consists of amorphous phases, crystallites and voids [4], and is usually deposited at low temperature by plasma-enhanced chemical vapor deposition (PECVD) using a high hydrogen dilution. The material properties are influenced by the silane concentration (SC) during the deposition (SC = SiH4/(SiH4+H2), where SiH4 is the silane flow rate and H2 is the hydrogen flow rate), the applied plasma power [4] and the plasma excitation frequency [5]. The microstructure of the μc-Si:H can be varied from highly crystalline to material where amorphous growth prevails. In recent years μc-Si:H has been deployed as active material in TFTs and high device charge carrier mobilities have been demonstrated [6-9]. This paper reports on the characterization of top-gate staggered TFTs employing intrinsic (i) μc-Si:H as channel layer. The properties of the channel layer were varied from highly crystalline to material where amorphous growth prevails. We investigated the dependence of the transistor parameters like the electron charge carrier mobility and the contact resistance on the crystalline volume fraction (XC) of the μc-Si:H channel material.
EXPERIMENT Fig. 1 shows the schematic cross-section of the investigated μc-Si:H TFT. The drain and source contacts were realized by chromium. Afterw
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