Substrate Effects on the Kinetics of Solid Phase Crystallization In a -Si
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SUBSTRATE EFFECTS ON THE KINETICS OF SOLID PHASE CRYSTALLIZATION IN a-Si L.HAJI*, P.JOUBERT*, M.GUENDOUZ**, N.DUHAMEL** and B.LOISEL** *Laboratoire Composants et Syst~mes de Visualisation, Universitd de Rennes I, I.U.T. de Lannion, 22302 Lannion Cedex, France "**Centre National d'Etudes des T616communications, LAB/OCM, 22301 Lannion Cedex, France ABSTRACT The effect of substrate nature on the solid phase crystallization at 600 *C of a-Si deposited by low pressure chemical vapor deposition is investigated by x-ray diffraction and transmission electron microscopy. The nucleation rate varies slightly resulting to a weak variation in the final grain sizes as a function of the substrate type. In all cases the grain growth mode is found to be three dimensional. In contrary, a drastic effect of the substrate is observed for films deposited by plasma enhanced CVD. Fast crystallization is obtained on indium tin oxide (ITO) resulting to small grain poly-Si, whereas the crystallization is retarded on glass leading to an increase in the grain size. INTRODUCTION Polycrystalline silicon thin film transistors (TFT's) on glass substrates have attracted a great importance in active matrix liquid crystal displays (AMLCD). PolySi offers much higher carrier mobilities, up to 100 cm 2 .V'l.s-1 , than hydrogenated amorphous silicon[l]. Nowadays large grain poly-Si is achieved by thermal crystallization of amorphous silicon (a-Si) at 600 'C. The a-Si starting material is obtained directly by low pressure chemical vapor deposition (LPCVD), plasma enhanced chemical vapor deposition (PECVD) or via implantation of as-deposited poly-Si films. Crystallization of these a-Si films occurs through nucleation and growth of crystallites during the thermal annealing. In the case where the a-Si film is deposited on patterned substrates, the crystallization process may be different according to the substrate nature. To our knowledge, very little is known so far on the substrate effects on the kinetics of crystallization in a-Si. In this report we will present the kinetics of crystallization of LPCVD and PECVD a-Si films deposited on glass and on glass covered with indium tin oxide or silicon nitride. EXPERIMENTAL PROCEDURE Undoped a-Si films 500-nm-thick were deposited by thermal decomposition of pure silane (LPCVD) at 535°C on glass substrate (Hoya NA40) uncovered or covered by a 230-nm-thick ITO film or a 250-nm-thick silicon nitride film deposited by PECVD. 100-nm-thick a-Si:H films were also deposited by PECVD at 180'C in a RF diode-type apparatus on patterned ITO onto glass substrate. These films were heated for dehydrogenation from RT to 500 0 C with a 1°C min-l heating rate. Crystallization annealing were performed at 600'C in nitrogen ambient. Kinetic of crystallization and texture of the films were deduced from x-ray diffraction measurements performed in a symmetrical 0-20 mode. To quantify the texture, the orientational factors OhkI were deduced from the measurements of the intensity of the three peaks {1 11), {220} and {311}, as previously r
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