Explosive Crystallization in a-Ge and a-Si: A Review
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EXPLOSIVE CRYSTALLIZATION IN a-Ge AND a-Si : A REVIEW D. BENSAHEL, CNET-CNS,
G. AUVERT
BP.42, 38240, MEYLAN,
France
ABSTRACT Self-sustained or explosive crystallization results on a-Si and a-Ge are reviewed and compared. cw laser experiments on a-Si allow to follow all the phenomena the measure and to experimentally encountered, crystallization front velocity. The published theories agree well with the experimental results on a-Si. Moreover, a-Ge presents the same behaviour as a-Si, but for a-Si, the high nucleation rate can induce a specific explosive growth. I. INTRODUCTION : Explosive crystallization (XCR) is a generic term which has been used for over a century to describe the rapid crystallization of amorphous materials due to the free energy release during the amorphous to crystalline transition (1). This energy release heats the adjacent material which in turn crystallizes and drives the reaction outwards. The material properties of the film either (thickness, preparation of the samples ... ) or microscopic macroscopic (crystallinity, thermal history, nucleation and growth processes ... ) as well as its underlying substrate modify the features observed after XCR. In all the cases however, high crystallization front speeds which can vary from 2 to 16 m/sec. have been obtained. These high speeds seemed to be difficult to imagine taking place in the solid phase regime, and theories have concluded that a thin supercooled liquid layer must be present during the crystallization process. The aim of this paper is to review and compare the different results obtained in the XCR field in the case of a-Ge and a-Si since it appears that this kind of crystallization can have some technological interest for low temperature processes. All along this paper, we discuss more in detail the XCR of a-Si, that we will compare to the a-Ge behaviour when necessary. The differences between a-Si and a-Ge are explained as due to differences in their nucleation rates. We will show that XCR is a pure thermal process with three defined steps : initiation, propagation and quenching that we will describe using the two main published theoretical approaches to cw laser induced rapid crystallization. All the features observed in the following have been obtained after XCR of a-Si by cw Argon laser irradiation. The laser scan speed is then the parameter in the all the phenomena encountered that we will use to describe crystallization of a-Si from the furnace annealing to the Q-switched laser experiments. In particular, for slow scans, we observe isolated explosive phenomena that turn into cooperative movement leading to the formation of a crystallization front when the laser scan speed is increased. This results in periodic crescent shaped macroscopic features. The discussion is then based
upon the propagation and the quenching of these events. On the other hand , in the high scan speed regime, we obtain continuous crystallized sheets useful for Mat.
Res.
Soc.
Symp.
Proc. Vol.
13 (1983) QElsevier Science Publishing Co.,
Inc.
166
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