Formation of Polycrystalline Silicon Films Using Electrical-Current-Induced Joule Heating

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Formation of Polycrystalline Silicon Films Using Electrical-Current-Induced Joule Heating Nobuyuki Andoh, Hiroyuki Takahashi and Toshiyuki Sameshima Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588 JAPAN

ABSTRACT Electrical current induced joule heating method was applied to crystallization of 50 nm-thick amorphous silicon films formed on glass substrates. Voltages were applied to silicon films connected with a capacitance in parallel. Irradiation with 5 ns-pulsed-laser melted films partially and caused electrical current induced joule heating because of reduction of electrical resistance. Lateral grain growth with 12.6 µm was realized using the tapered-shaped-electrodes to apply voltages to silicon films with a capacitance of 0.33 µF. For 2.6×1017 cm-3 phosphorus-doped films, analysis of temperature change in the electrical conductivity gave that defect states at grain boundaries was low of ~1017 cm-3.

INTRODUCTION Polycrystalline silicon films are applied to many devices such as thin film transistors (TFTs) and solar cells [1-6]. Formation of polycrystalline silicon films at low cost has been demanded in resent years for fabrication of large-area devices. Many technologies have been reported for the formation of polycrystalline silicon films at low processing temperature. The pulsed laser crystallization method has an advantage for the formation of high-quality polycrystalline silicon films because of rapid melting followed by solidification. This method has therefore been applied to the fabrication of poly-Si TFTs and their electronic circuits. However, it is important to fabricate large crystalline grains with a low defect states, especially for solar cell application. Although several methods have been reported for the formation of large crystalline grains using the pulsed laser crystallization method, it is not easy to control the solidification parameters. In the crystallization by electrical current induced joule heating, because melting of silicon is controlled by the electrical current flowing in silicon, it would be easily possible to control the solidification parameters by a simple electrical circuits such as RC circuit [7-10]. In this paper, we report the crystallization by joule heating method using tapered-shaped-electrodes to apply voltages to silicon films in order to generate the temperature gradient at large area in the lateral direction of silicon films. Large grain growth >10 m in the lateral direction is demonstrated. We also report electrical properties of polycrystalline silicon formed by this method. The low density of defect states is demonstrated.

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EXPERIMENT Undoped 50 nm-thick-amorphous silicon films were deposited by low pressure chemical vapor deposition method (LPCVD) on the glass substrates. Some silicon films were doped with phosphorus atoms at 2.6×1017 cm-3 using the ion implantation method. Silicon films were defined to strip-shaped by lithography and etching method. Al/Cr double layered-electrodes were formed on the silico