Laser-Induced Phase Transformation of Sb-Te Alloy Films for Optical Storage
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LASER-INDUCED PHASE TRANSFORMATION OF Sb-Te ALLOY FILMS FOR OPTICAL STORAGE Susumu Fujimori,
Shogo Yagi, Hiroki Yamazaki and Nobuhiro Funakoshi NTT Electrical Communications Laboratories, Tokai, Ibaraki-ken, 319-11, Japan ABSTRACT Sb-Te alloy films are developed as rewritable optical recording materials based on amorphous-crystalline phase transformations. Sb 2 Te 3 is shown to be the best practical phase change medium. INTRODUCTION Optical Recording using amorphous - crystalline phase transformations has generated considerable interest, since it is expected to provide rewritable optical disk data storage. The performance of rewritable (erasable) phase change recording depends on : (1) the writing (amorphizing) sensitivity and erasing (crystallizing) sensitivity (2) the data stability (the lifetime of the amorphous state) (3) the repeatability of writing and erasing. Chalcogenide thin films, usually Te alloys, have been used as recording materials because they can easily be quenched into a glassy state by laser pulse irradiation. At present, the most significant problem with these materials is the trade off between data (amorphous phase) stability and erasing (crystallization) sensitivity [1-5]. In this report, Sb 2 Te 3 films are shown to have room temperature stability in amorphous states, short erasure times and potentially good rewritability. MATERIAL SELECTION A variety of materials, mostly based on Te or Se alloy films with additives such as Ge, Sn, Pb, As, Sb and Bi have been studied so far for erasable phase change recording medium. SbTe alloy films are considered to have the best reversibility between the amorphous and crystalline phases because they have a smaller volume change ig the phase transformation than other alloys.[6] More than 10 repitition cycles were achieved for Sb-Te alloy films in our preliminary material examination. Accordingly, we chose the Sb-Te films as testing samples in our experiments to study phase-transformation kinetics of rewritable recording media. The phase diagrams of the Sb-Te alloys were studied many years ago. The single phase Sb 2 Te 3 was reported to be obtained over the wide range between 4d and 89 at.% Sb. [7] SAMPLE PREPARATION We used RF-sputtering the following structure.
to prepare
Mat. Res. Soc. Symp. Proc. Vol. 100. 01968 Materials Research Society
thin film samples with
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(1) A SiO undercoat layer 100 nm thick deposited on a glass substrate by magnetron sputtering. (2) A Sb-Te alloy recording layer 80-100 nm thick deposited by conventional sputtering. (3) A SiO 2 overcoat layer 100 nm thick deposited by magnetron sputtering. The Sb-Te target in our sputtering system consisted of a Te plate with many pieces of Sb on it. The Sb content in the alloy films was controlled by changing the number of the Sb pieces, and confirmed by X-ray photo-electron spectroscopy (XPS). The structure of the films prepared was examined by X-ray diffraction. Most of the as-deposited films had an amorphous structure. These samples were crystallized by a heat treatment. They show
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