In this paper, we investigated current (I)- and voltage (V)-sweeping properties in a double-stack structure, Ge2Sb2Te5/Ti/W-doped Ge8Sb2Te11, a candidate medium for applications to multilevel phase-change memory. 200-nm-thick Ge2Sb2Te5 and W-doped Ge8Sb2Te11 films were deposited on p-type Si(100) substrate using magnetron sputtering system, and the sheet resistance was measured using 4 point-probe method. The sheet resistance of amorphous-phase W-doped Ge8Sb2Te11 film was about 1 order larger than that of Ge2Sb2Te5 film. The I- and V-sweeping properties were measured using sourcemeter, pulse generator, and digital multimeter. The speed of amorphous-to-multilevel crystallization was evaluated from a graph of resistance vs. pulse duration (t) at a fixed applied voltage (12 V). All the double-stack cells exhibited a two-step phase change process with the multilevel memory states of high-middle-low resistance (HR-MR-LR). In particular, the stable MR state is required to guarantee the reliability of the multilevel phase-change memory. For the Ge2Sb2Te5 (150 nm)/Ti (20 nm)/WGe8Sb2Te11 (50 nm), the phase transformations of HR→MR and MR→LR were observed at t<30ns and t<65ns, respectively. We believe that a high speed and stable multilevel phase-change memory can be optimized by the double-stack structure of proper Ge-Sb-Te films separated by a barrier metal (Ti).
