Morphological Instabilities during Explosive Crystallization of Germanium Films
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Morphological Instabilities during Explosive Crystallization of Germanium Films Aleksandra Chojnacka and Michael O. Thompson Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853 ABSTRACT Explosive crystallization, a self-sustaining transformation of an amorphous phase to a crystalline phase mediated by a thin liquid layer, exhibits three distinct kinetic and morphological regimes in germanium. Dynamics of these growth processes and the resulting morphologies have been examined in detail. Steady-state crystallization velocities were measured as a function of heat loss into the substrate. Dark field optical microscopy, tapping mode atomic force microscopy, transmission electron microscopy, and x-ray diffraction were used to examine the crystallized films. Analyses of the experimental results provide evidence for two distinct processes governing explosive crystallization in limits of high substrate temperature (low heat loss) and low substrate temperature (high heat loss). The low temperature growth mode produces a “scalloped” structure with propagation velocities that monotonically increase with temperature. At high substrate temperatures, the velocity is independent of temperature and a “columnar” pattern with preferred texture is formed. INTRODUCTION Explosive crystallization was first observed in antimony in 1855 [1,2]. Since then, the process has been studied in amorphous metallic films (e.g., Yb, Bi, and Ga in [3]), alloys (e.g., Se80Te20 in [4]), insulators (e.g., Si3N4 in [5]), and semiconductors [6]. Interest has been recently renewed for applications in laser processing of Si and Ge films for flat panel displays, solar cells, and infrared detectors [7-10]. Germanium exhibits an amorphous phase (a-Ge) that is energetically metastable relative to the crystalline phase (c-Ge). The explosive crystallization process takes advantage of the enthalpy difference between the two phases and of the fact that the melting temperature of the amorphous phase, which ranges from 960 to 975 K, is lower than that of the crystalline phase (1210 K) [11]. In germanium, explosive crystallization can be initiated by a thermal or mechanical shock [12,13], either at room temperature for free-standing films or at elevated temperature for thin films on substrates. Once initiated, the net enthalpy of crystallization drives the interface motion, with crystallization continuing as long as the net enthalpy released by the crystallization front exceeds the heat losses to the surroundings. By controlling the substrate temperature of the sample, these losses can be specified. For a germanium film of given thickness, preparation conditions, and substrate, self-sustaining explosive crystallization is thus possible only above a certain critical substrate temperature. This critical temperature is inversely proportional to the film thickness [14], with thinner films requiring higher substrate temperatures (lower losses). To account for the high crystal growth velocities observed during explosive crystallization in Ge
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