Tellurium Alloys for Reversible Optical Data Storage
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TELLURIUM ALLOYS FOR REVERSIBLE OPTICAL DATA STORAGE R.T. YOUNG, D. STRAND, 3. GONZALEZ-HERNANDEZ AND S.R. OVSHINSKY Energy Conversion Devices, Inc., 1675 West Maple Road, Troy, Michigan 48084 ABSTRACT Tellurium-based chalcogenide alloys, which are used for electronic switch materials, also have the appropriate optical and thermal properties for reversible optical data recording. One of the major concerns encountered with this type of phase change material is that there can be two contradictory characteristics of the material, i.e., the amorphous phase thermal stability and the crystallization rate are involved. We have demonstrated, in this paper, that this problem can be solved. We report that certain transition metal elements added to Te alloy films can substantially improve the rate of crystallization without any reduction of the amorphous phase stability. INTRODUCTION It is widely recognized that optical recording of data by submicron or micron size laser spots provides the advantage of higher packing density and consequently a lower bit storage cost, compared to magnetic recording. Another advantage arises from the fact that the optical heads and the disc are not in direct contact, allowing disk removability. Considerable effort has been devoted to the development of suitable recording materials for commercialization; write-once optical discs are now on the market and erasable discs are predicted to appear shortly. The use of Te-based chalcogenide alloys, for electrical and optical data storage, was first reported by Ovshinsky in 1968 [1]. He and co-workers discovered that reversible phase change phenomena can be accomplished in several chalcogenide films, with either an electrical pulse or a focused laser beam [2,3]. In phase change reversible optical data recording, the material requirements are intimately related to various parameters associated with system design such as available power from the semiconductor laser, spot sizes used for recording and erasing, disk rotation speed, etc. Clearly, the material should be sensitive enough that laser power density and beam dwell times are sufficient to complete the amorphous-crystalline phase transformation. Furthermore, the optical properties of the recorded (amorphous) and erased (crystalline) areas should remain invariant upon cycling and be stable upon long term storage. Because of the appropriate optical and thermal properties, and the readily crystallizing behavior, tellurium-based alloys have been considered as the most attractive material for phase change reversible optical data storage. It is known that pure Te crystallizes rapidly at lO° [4]. In order to stabilize the amorphous Te phase, tellurium alloys containing certain amounts of Ge, Sn, Sb, As, Se, S, 0 etc., have been investigated [5-14]. The major difficulty encountered with this approach is that two contradictory characteristics of the material, i.e., the amorphous phase thermal stability vs. crystallization speed are involved. Finding an approach to enhance the crystallization speed while maint
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