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A nonvolatile memory technology utilizing reversible changes between fcc and hcp crystalline phases is proposed. In this new type of phase-change memory, data are stored in different forms of crystalline phases of (Ge/sub 1/Sb/sub 2/Te/sub 4/)/sub 0.8/(Sn/sub 1/Bi/sub 2/Te/sub 4/)/sub 0.2/ chalcogenide alloy. RESET operation produces the less conductive metastable fcc phase via melt-quenching from the more conductive stable hcp phase and SET operation involves a phase change from fcc directly to hcp. Both RESET and SET operations can be completed as fast as 70 ns with large changes in cell resistance.

Amorphous Ge 2 Sb 2 Te 5 clusters with a size of 20 nm, self-enclosed by a thin layer of TiO x , were obtained by cosputtering Ge 2 Sb 2 Te 5 and TiO 2 targets at room temperature with the aim of reducing the reset current for phase change random access memory applications. Eutectic decomposition during the deposition caused a phase separation of Ge 2 Sb 2 Te 5 and TiO x . The temperature-dependent resistance change results showed that the activation energy for crystallization increased from 2.44 ± 0.76 to 3.84 ± 1.43 eV in the Ge 2 Sb 2 Te 5 film. The set resistance can be tuned within an acceptable range, and the reliability of this microstructure during repetitive laser melt-quenching cycles was tested.

The phase change material Ge2Sb2Te5 is widely investigated for use in nonvolatile memories. It has been reported that the crystallization speed depends on the thermal history, indicating that structural differences exist between amorphous states. The authors apply fluctuation electron microscopy to quantify differences in the nanometer-scale structural order between several amorphous states of Ge2Sb2Te5. All as-deposited films are found to contain ordered regions. Thermal annealing below the crystallization threshold increases the nanoscale order, and such samples crystallize slightly more rapidly. The authors hypothesize that the nanoscale ordered regions act as the nuclei for crystallization, with the largest regions being the most significant.

The thermal conductivity of thin films of the phase-change material Ge2Sb2Te5 is measured in the temperature range of 27°C

The electrical switching behaviors of an offset-type phase change memory device with a highly resistive TiON layer were investigated, where the TiON layer (7 nm thick) was formed at a 70 nm wide contact between Ge1Sb2Te4 and TiN layers. Reversible transitions between crystalline (set) and amorphous (reset) phases were found to occur at relatively lower reset and set voltages, as compared with a device having no TiON layer. These results hold a high promise for a low-power operation of a phase change memory device.