Kinetic Study of Crystallisation in Amorphous Thin Lpcvd si Films
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ABSTRACT The crystallisation behaviour of LPCVD silicon films has been investigated by TEM from in situ isothermal annealing of undoped a-Si films deposited from disilane (Si2H6) at temperatures 450, 465 and 4 0 0C and at gas pressure of 200 mTorr. Nucleation kinetics, grain growth rates and crystallisation kinetics were determined for temperatures ranging from 600 to 675 *C. Nucleation kinetics have been experimentally determined in the early first stages of annealing : they do not show any steady-state rate and are fitted according to a power law. Experimental data for crystallisation kinetics are fitted by an Avrami law without introducing any incubation time.
INTRODUCTION The crystallisation of amorphous silicon (a-Si) films is an important and popular research topic because of its potential applications in microelectronics such as thin film transistors (TFTs). Most of previous studies have been performed on silicon films deposited by low pressure chemical vapor deposition (LPCVD) from silane (SiH4) or by physical deposition such as electron-beam evaporation [1,2,3]. Nucleation kinetics are usually derived by counting the number of Si crysthllites after isothermal annealings for different times assuming a constant nucleation rate after some transient period [4]. Crystallite growth kinetics are extracted, through a kinetic model, from the nucleation rate and global transformation kinetics [5]. The activation energies for nucleation and growth are then deduced from the activation energies of both the grain size and a characteristic crystallisation time [ 1,2,6]. This paper concerns LPCVD Si films deposited at low temperature from disilane (Si2H6). Far fewer kinetic and thermodynamic data have been previously published on the crystallisation of such films [6,7,8,9,10] than on films deposited from Sil4. They showed that a large grain size could be obtained from crystallisation at a temperature as low as 600'C. In this study, the crystallite growth kinetics, the nucleation and the global crystallisation kinetics were separately determined without referring to a kinetic model. This was possible through an initial determination of the crystallite growth rate by in situ transmission electron microscopy (TEM) annealing experiments [11]. The knowledge of the crystallite growth kinetics permitted us to accurately determine the nucleation kinetics [12]. The crystallisation kinetics were then experimentally determined and compared to the crystallisation kinetics calculated through a classical nucleation and growth model.
EXPERIMENTAL PROCEDURES The investigated materials were undoped a-Si thin films, 150 nm thick, grown from disilane by LPCVD, at deposition temperatures Td of 450, 465 and 480 *C and total gas pressure Pd of 200 mTorr, onto thermally oxidized (111) silicon wafers. Plan-view samples were prepared by chemical dissolution of the silicon oxide (SiO2) interlayer and by subsequent cleaning. The in situ annealing experiments were performed in a heating JEOL specimen stage permitting the control of specimen temper
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