Nd:YVO 4 Laser Crystallization for Thin Film Transistors with a High Mobility
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Nd:YVO4 Laser Crystallization for Thin Film Transistors with a High Mobility Ralf Dassow1, Jürgen R. Köhler1, Melanie Nerding2, Horst P. Strunk2, Youri Helen3, Karine Mourgues3, Olivier Bonnaud3, Tayeb Mohammed-Brahim3, and Jürgen H. Werner1 1 Universität Stuttgart, Institut für Physikalische Elektronik, Pfaffenwaldring 47, D-70569 Stuttgart, Germany, [email protected] 2 Universität Erlangen-Nürnberg, Institut für Werkstoffwissenschaften, Lehrstuhl für Mikrocharakterisierung, Cauerstr. 6, D-91058 Erlangen, Germany 3 Groupe de Microélectronique et Visualisation, Université de Rennes 1, Campus de Beaulieu, F35042 Rennes Cedex, France ABSTRACT We crystallize amorphous silicon films with a frequency doubled Nd:YVO4 laser operating at a repetition frequency of up to 50 kHz. A sequential lateral solidification process yields polycrystalline silicon with grains longer than 100 µm and a width between 0.27 and 1.7 µm depending on film thickness and laser repetition frequency. The average grain size is constant over the whole crystallized area of 25 cm2. Thin film transistors with n- type and p-type channels fabricated from the polycrystalline films have average field effect mobilities of µn = 467 cm2/Vs and µp = 217 cm2/Vs respectively. As a result of the homogeneous grain size distribution, the standard deviation of the mobility is only 5 %. INTRODUCTION Polycrystalline silicon fabricated at low temperatures is of great interest for future electrical devices on cheap and /or transparent substrates (e. g. glass). Laser crystallization of amorphous silicon deposited at low temperatures is the most promising technique to produce the desired material with excellent electrical properties. The established process is based on the irradiation of each part of the silicon film with a number of n = 10 ... 50 pulses of an excimer laser [1]. To improve the electrical properties of the devices, crystallization of the active layer is often performed in vacuum [2]. Thin film transistors (TFTs) with the highest reported electron mobility µ = 640 cm2/Vs were deposited and crystallized completely in ultra high vacuum [3]. Unfortunately, with the established processes the grain sizes and therefore also (due to grain boundaries [4]) the electrical properties of the TFTs depend strongly on the laser’s local energy density D. For example, an increase of D from D = 406 mJ/cm2 to D = 420 mJ/cm2 results in a decrease of the grain size d from d = 500 nm to d = 30 nm [5]. Due to this sensitivity of d on D, temporal and spatial fluctuations of the laser energy lead to considerable scatter in TFT properties. Sequential lateral solidification (SLS) increases the grain size [6] and hence reduces the number of grain boundaries within the devices. This reduction should significantly improve the electrical properties. Simultaneously the SLS process reduces the sensitivity of the grain size on the energy density D. Unfortunately, up to now only excimer lasers were used in combination with an SLS process. The repetition rate of this type of laser ha
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