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HH3.05 - Probing the Crystal Growth Velocity of Melt-Quenched Phase-Change Materials within a Memory Cell 
April 23, 2014   10:45am - 11:00am

Chalcogenide materials undergoing reversible crystal-to-amorphous phase transitions continue to play a key role in information technology. In particular, phase-change memory (PCM) has emerged as the most promising new nonvolatile solid-state memory technology. A key property that makes these materials attractive for such applications is the crystallization process that occurs at the nanometer length scale and the nanosecond time scale. Moreover, in spite of this fast crystallization rate at elevated temperatures, the melt-quenched amorphous state is stable at lower temperatures for several years. Given the length and time scales associated with this unique crystallization process, experimental measurements are extremely challenging. Most experimental investigations have been limited to low temperatures, at which the crystallization rate is small. A recent effort to measure the crystal growth rate using ultrafast differential scanning calorimetry was reported by Orava et al. [1]. These measurements were conducted on as-deposited phase-change materials. A more recent effort involved the use of laser-based time-resolved reflectivity measurements [2]. But both techniques were limited in terms of the temperature range over which crystallization rate was measured. In this talk, we present the measurement of crystallization rate with a PCM cell using electrical means. This approach is capable of measuring the crystallization rate down to nanometer dimensions as well as on the nanosecond time scale thanks to, respectively, the dimensions of the PCM cell and the fast thermal dynamics associated with such cells. We present a consistent description of the temperature dependence of the crystal growth velocity in the glass as well as the super-cooled liquid from room temperature up to the melting temperature. Isothermal measurements are conducted to measure the growth velocity in the glass state, which is shown to follow an Arrhenius relationship. The melting temperature, where the crystallization rate is zero, and the temperature at which this rate is maximum are estimated by exploiting the spatial and temporal evolution of temperature in the PCM cell created by the application of suitable voltage pulses. A cryogenic probe station to vary the ambient temperature and finite-element models to support the experimental findings are used. The overall temperature dependence of the growth velocity is quantified using these experiments in combination with an estimate of the evolution of the effective amorphous thickness as a function of the application of voltage pulses.[1] J. Orava et al., Nature Materials 11, 279 (2012)[2] M. Salinga et al., Nature Communications 4, 2371 (2013)

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