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Anomalous crystallization of hydrogenated amorphous silicon during fast heating ramps J. Farjas, J. Serra-Miralles, P. Rouraa) GRMT, Department of Physics, University of Girona, Campus Montilivi, E17071-Girona, Catalonia, Spain

E. Bertran FEMAN, Departament de Física Aplicada i Optica, Universitat de Barcelona, E08028-Barcelona, Catalonia, Spain

P. Roca i Cabarrocas LPICM (UMR 7647 CNRS) Ecole Polytechnique, 91128 Palaiseau Cedex, France (Received 14 July 2004; accepted 28 October 2004)

Thermal crystallization experiments carried out using calorimetry on several a-Si:H materials with different microstructures are reported. The samples were crystallized during heating ramps at constant heating rates up to 100 K/min. Under these conditions, crystallization takes place above 700 °C and progressively deviates from the standard kinetics. In particular, two crystallization processes were detected in conventional a-Si:H, which reveal an enhancement of the crystallization rate. At 100 K/min, such enhancement is consistent with a diminution of the crystallization time by a factor of 7. In contrast, no systematic variation of the resulting grain size was observed. Similar behavior was also detected in polymorphous silicon and silicon nanoparticles, thus showing that it is characteristic of a variety of hydrogenated amorphous silicon materials.

Crystallization of amorphous silicon (a-Si) is a subject that has been studied extensively during the last three decades. It improves the electrical conductivity substantially,1 making the resulting polycrystalline silicon useful for a number of applications, such as thin film transistors (TFT), liquid crystal displays (LCD), and image sensors.2 The kinetics of this process follows the nucleation and growth steps with rate constants characterized by transmission electron microscopy.3–6 The relative values of nucleation and growth rates determine the final grain size that can be controlled through the crystallization temperature or by ion irradiation during crystallization.4 The temperature range in the published results seldom extends above 700 °C because of the difficulty in reaching the isothermal conditions without any incipient crystallization taking place during the heating ramp. This difficulty is partially overcome by calorimetric experiments where the material is heated at a constant rate.

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0037 J. Mater. Res., Vol. 20, No. 2, Feb 2005

http://journals.cambridge.org

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From the heat that evolves the crystallization rate can be obtained. Kinetic studies using calorimetry have only been performed during the epitaxial crystallization of thin a-Si films obtained from rapid solidification.7 With the exception of our preliminary work,8 no systematic studies on hydrogenated a-Si (a-Si:H) have been published. In this paper we extend the temperature range during the heating ramps above 700 °C up to 100 K/min. Under these conditions, an unexpected and substantial enha