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A phase-change alloy that crystallizes without shrinking

Eliminating density changes that accompany changes in reflectivity and resistance would make a promising class of data-storage materials more effective.
In the race to become the next technology for computer memory, phase-change memory (PCM) is among the leading contenders. The basis of PCM is a reversible, temperature-induced change between a shiny, low-resistance crystalline phase and a dull, high-resistance amorphous phase. In prototype devices, either the change in reflectivity or the change in resistance is exploited to write and store data. Unfortunately, in most PCM materials, the phase change is accompanied by a change in density that's large enough to create performance-sapping voids beneath electrodes. And the materials that exhibit the biggest change in reflectivity or resistance also exhibit the biggest change in density. Magali Putero of Aix-Marseille University in France and her colleagues from CNRS and IBM have been systematically investigating the phase-change behavior of a wide range of PCM materials. Most of them shrink when they crystallize, but others swell. Using a beamline at the European Synchrotron Radiation Facility, Putero and her colleagues found that 50-nanometer-thick films of one PCM material, a gallium–antimony alloy, can do both, depending on the composition ratio. Films of the lowest Sb composition that they studied, 55%, expanded in thickness by 3%, and films of the highest Sb composition, 95%, shrank by 4%. At 70% Sb, the thickness remained the same. That desirable property came with another: The change in resistance at that composition was large enough to qualify the alloy as a good PCM material. The researchers are now looking for other PCM materials that behave in the same way. (M. Putero et al., APL Mat. 1, 062101, 2013.)—Charles Day


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