Laser Induced Phase Transformations in Thin InSb Films
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LASER INDUCED PHASE TRANSFORMATIONS IN THIN InSb FILMS L.H. Chou and M.C. Kuo* Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, RIO.C. *Present address : Picvue Electronic Company, Hsin-Fung, Taiwan 304, R.O.C. ABSTRACT Thin In46SbM films have been prepared on glass substrates by flash evaporation. Films of 1000 A were deposited at room temperature. After 7 mins' thermal annealing at 450'C, an average grain size increased from 20 nm to 200 nm. Under exposure to a Fraunhofer diffracted pulsed Nd-YAG laser beam, the films showed different microstructures for different applied laser power. In general, five regions are identified: ablation region, regions with grain sizes less than 20 nm, grain sizes ranging from 400 to 1000 nm, grain sizes ranging from 200 to 400 nm and unchanged region. The previous five regions are counted from the center of the exposed area outward. The pulse duration is 16 ns. In this paper, an atomic migration mechanism is proposed to explain the power dependent microstructures changes. Transmission electron microscopy was mainly used to characterize the films. The application of this type of phase transformations in the phase change optical storage is also discussed. INTRODUCTION Thin In 40 Sb 60 films of 1000A thickness have been reported by Goto et al. [1] to be a candidate for application in phase change optical data storage media. They studied the microstructures changes of both written and erased bits, which are lIgm in diameter. By the application of electron probe microanalyzer (EPMA) and transmission electron microscopy (TEM), they found that large (or small) Sb grains are embedded in the matrix made of small In 50Sb 50 grains in the center of the written (or erased) bits. The grain sizes of the large Sb grains are about 400nm, while those of both InSb grains and the small Sb grains are about 20nm. They attributed the large Sb grains to the diffusion of Sb atoms, which had high diffusivity in the molten state during the cooling process, from outer region to the center of the written bit. Therefore, a thinner outer region made of In 50Sb50 was observed. However, the Sb atoms gathered in the center of the written bit during the cooling process didn't guarantee forming large grains from phase transformations point of view. This was not explained in their paper. Further, the formation of small Sb grains in the erased bit was not reasonably explained either. In this study, Fraunhofer diffracted laser beam with larger beam diameter, i.e., much larger than 1 gtm, was used. Therefore, different phenomenon from that of Goto et.al. (I], was obtained. By the application of transmission electron microscopy, an atomic migration mechanism was proposed to explain the microstructures changes observed. The application of this type of phase transformations in the phase change optical storage is also discussed. EXPERIMENT Thin ln46 Sb 54 films of 1O00A thickness were vacuum deposited on room temperature substrate, at a base pressure of 2x10"6 Tor'. Mica s
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