Copper vanadates as candidate materials for phase change optical memory
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Several copper vanadium oxide melts were tested for possible application as the active medium in phase-change optical data storage devices. These materials were melted in the bulk and then quenched. Their phase development was characterized to help determine their applicability to optical data storage. It was found that they satisfy many of the criteria necessary for successful phase-change data storage; further studies of their behavior in thin film geometry would be warranted.
I. INTRODUCTION Optical memory systems consist of a thin film material that is sensitive to laser radiation and can be written into two optically differentiable states. Some systems are based on magneto-optic thin films which cause slight polarization rotations of the reflected light.1 Most systems of this type use rare earth-transition metal (RE-TM) alloys as their optically active medium. Other systems are based on writing the active material into either crystalline or amorphous states and then sensing the reflectivity difference between the two phases.2"4 These phase change systems have been developed to a lesser degree, partly because of cyclability and stability problems.5 The present research focuses on finding new materials for use as the active medium in phase change data storage systems. For a system to be useful as a phase change memory many criteria must be met. The primary considerations center on the reversible cycling between the two phase states, amorphous and crystalline. First, these two states must have different reflectivities at the desired wavelength (usually in the range 600-900 nm where semiconductor diode lasers operate). Second, this same laser wavelength must be absorbed strongly enough to ensure that the active layer in the optical disk can be heated sufficiently to cause melting, thereby writing or erasing data. Third, the material must be able to form either the amorphous or crystalline phases after laser melting. Finally, these materials must be cycled millions of times without degradation of their melting, freezing, or reflectivity. It should be noted that this provides a narrow window of applicability where the material must be sensitive enough to melt using short laser pulses, yet must also be stable enough that it will have long shelf life and good cyclability. The presently popular phase change materials use semiconductor alloys or compound phases based primarily on Te, Sb, or Ge, and other alloying elements.6"11 Unfortunately, these materials undergo degradation during J. Mater. Res., Vol. 7, No. 3, Mar 1992 http://journals.cambridge.org
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cycling.9'12 Much research has been directed toward improving composition, laser pulsing conditions, and thin film design, in order to optimize the cycling behavior of these systems.12'13 However, less emphasis has been placed on searching for significantly different alternative materials systems for use as the active medium in phasechange memory. The present work reports some initial efforts toward finding a novel replacement material that can
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