Explosive Crystallization of Amorphous Germanium Films
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EXPLOSIVE CRYSTALLIZATION OF AMORPHOUS GERMANIUM FILMS
H. J. LEAMY, W. L. BROWN, G. K. CELLER, G. FOTI', and G. H. GILMER
Bell Laboratories, Murray Hill, New Jersey 07974 and J. C. C. FAN
Lincoln Laboratory, Massachusetts Institute of Technology Lexington, Massachusetts 02173 ABSTRACT
Laser processing of amorphous thin films of amorphous Ge often results in an explosive or self-sustaining crystallization reaction. The reaction is sustained by the heat liberated during crystallization. In a theoretical analysis of the process that was presented at this symposium last year, Gilmer and Leamy postulated the existence of a thin layer of liquid at the propagating interface. The liquid layer forms at temperatures above T,, the melting point of amorphous Ge, and is predicted to be - 0.02 0.1 of the film thickness in width. We have obtained experimental confirmation of the presence of this liquid layer. INTRODUCTION
Explosive crystallization of amorphous thin films has been frequently observed'- 6, most recently in connection with laser processing of a-Ge"' 6 . The phenomenon manifests itself most startlingly when, following initiation, crystallization proceeds across the film at 100-300 cm/sec,6.7" 6 and is accompanied by heat generation and emission of visible light. Crystallization is self-sustaining only at substrate temperatures greater than some critical value, T*. For T, 0.7 and hence a more limited redistribution, as observed. Additional information concerning the nature of explosive crystallization can be obtained by examination of the surface of the crystallized film. The visibility of the crystallized grains in Fig. 3 is a consequence of scattering from surface undulations as shown in Fig. 5. These, undulations, which are similar to those reported by previous workers, are roughly parallel within each grain. The detailed morphology of the undulations is not visible at optical magiifications but is clearly revealed by scanning electron microscopy, as shown in Fig. 6. This figure shows that the undulations are composed of froth-like bubbles that are aligned in irregular rows. In most instances the bubbles have burst prior to solidification, but occasional unbroken examples can be found in Fig. 6. We believe that the.se bubbles are formed by coalescense of gas atoms within the film upon melting, and that they have been frozen before the
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Figure 4: A before and after comparison of the depth distribution of Pb and Sb impurities in the sample shown in Figure 3.
smoothing action of surface tension upon the liquid layer has had time to be effective. The presence of gaseous impurities within amorphous, vapor-deposited thin films is well documented 21, and the identification of frozen bubbles in rapidly solidified Si layers has recently been reported271,21 . The existence of bubbles below the surface of the sample shown in Fig. 6 is illustrated in Fig. 7. This figure is a transmission electr
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