Optical Properties of Multi-layer Chalcogenide Thin Film for Optical Recording Media
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Optical Properties of Multi-layer Chalcogenide Thin Film for Optical Recording Media Myung-Jin Kang1 Chan-Gyung Park2 and Se-Young Choi1 1 Department of Ceramic Engineering, Yonsei University 134, Shinchon-dong, Sudaemun-ku, Seoul, 120-749, Korea 2 Department of Material Science and Engineering and Center for Advance Aerospace Materials Pohang Univeristy of Science and Technology San 31, Hyoja-dong, Nam-gu, Pohang, Kyungbuk, 790-784, Korea ABSTRACT We present the results of optical properties of multi layer thin films as in the media of phase change optical disk data storage. Reflectance and optical contrast of multi layer thin films increased rapidly between 100 ℃ and 150 ℃. Moreover, optical contrasts at different wavelength were also studied. The refractive index and the optical band gap decreased, while the extinction coefficient increases as the crystallization occurs. The Egopt of crystalline thin film was ~0.6 eV lower than that of amorphous thin film. Egopt decreased as the number of stacked layer increased. INTRODUCTION In the last decade, there has been a great deal of interest in the study of chalcogenide amorphous semiconductors for use as optical and electrical device materials. One notable application of such materials is rewritable phase change optical disks, which takes advantage of the different optical properties in the amorphous and crystalline states [1-4]. As well known, data transfer rate is one of the most important parameter of the phase change optical disks, which is mostly limited by the crystallization speed of recording media. In order to enhance the data transfer rate, many methods have been developed such as adapting stoichiometric compound Ge1Sb2Te4 recording media or additional interface layer to phase change optical disk [5,6]. Recently, Chong et al. reported a superlattice-like (SLL) structure for high speed phase change optical recording media [7-10]. The signal differentiation in writing, reading and erasing was observed at the pulse width of 7 ns, which indicates that the SLL structure can effectively shorten the crystallization time. However, dependence of the optical properties on the multi layer stacking sequence and number of stacked layer has not been reported yet. In this research, effects of the stacking sequence and the number of stacked layers of GeTe/Sb2Te3 multi layer recording media on optical properties and nano structure of multi layer phase change thin films were discussed.
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EXPERIMENTAL PROCEDURES Evaporation targets were prepared with raw materials such as Ge, Sb, Te (99.99%, High purity chemicals, Japan). Raw materials were put in quartz ampoule which were sealed under vacuum of 1 × 10-5 torr. The raw materials were melted at 1000 ℃ for 24 hours in a rocking furnace for homogenization of the melted liquid. The ampoule was immediately air quenched after being removed from the furnace. Multi layer of GeTe and Sb2Te3 thin films of different stacking sequence with various number of stacks were prepared by e-beam evaporation of the bulk target onto
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