Structural and optical transformations by laser irradiation of InSb-based thin films
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INTRODUCTION
Phase transformations are important for both technological applications and scientific study. One which is currently receiving attention is the amorphous-tocrystalline transformation. This structural transformation results in changes of physical properties such as optical constants. Materials which undergo changes of optical properties in a reproducible manner are of interest as potential recording media for phase change reversible optical storage disks.1'2 For historical reasons, most materials which have been studied for optical storage applications have been alloys and compounds containing Te and Se.3"5 There are other materials which undergo changes in optical properties as a result of crystallization. An example is InSb. There has been limited research performed on the amorphous state of this material. Goto et al.6 have discussed In0.4Sb0.6 alloy for phase change optical recording applications. They studied the morphology resulting from laser irradiation of previously crystallized thin films. A general comparison of amorphous as-deposited versus crystallized InSb thin films was discussed by Stuke.7 This paper presents a study of laser induced transformation of amorphous, as-deposited thin film alloys of (Ino.43Sbo.57V87Geo.13 and Ino.43Sbo.57- Both optical and microstructural changes resulting from a broad range of different irradiation conditions were studied. The relationship between the optical measurements and the observed microstructures is discussed. Characterization by in situ optical reflectivity and transmission "'Permanent address: I Optica, CSK, Serrano 121, Madrid 28006. 190
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J. Mater. Res., Vol. 5, No. 1, Jan 1990
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showed that several distinct optical states were formed. The results were conveniently summarized with Phase Transformation Kinetic (PTK) diagrams. The PTK diagram shows the changes in reflectivity versus laser power and pulse width.8 Depending on the laser conditions, the optical reflectivity increased or decreased. The observed reflectivity changes were correlated with microstructures resulting from phase transformations and/or segregation and surface morphology variations. The former were studied by transmission electron microscopy (TEM) aand the latter by scanning electron microscopy (SEM). II. EXPERIMENTAL
Films of 520 A thick, amorphous (Ino.43Sbo.57)o.87Geo.i3 and Ino.43Sbo.57 were prepared by simultaneous thermal evaporation from separate In, Sb, and Ge Knudsen cells. The base and evaporation pressures were 10"7 Torr and 10~6 Torr, respectively. The deposition rate was 1.9 A/s. The films were deposited on glass, carbon-coated mica, and silicon substrates and were not overcoated. The compositions of the films on silicon were measured by x-ray fluorescence and of films floated off carboncoated mica by EDAX in a JEM 200 analytical TEM. The evolution of the optical changes for times up to 20 /us was measured using a custom-built static tester.8 A krypton-ion (A = 647 nm) laser was focused to a diffraction-limited,
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