We studied GeTe structures in superlattice phase change memories (superlattice PCMs) with a [GeTe/Sb2Te3] stacked structure by X-ray diffraction (XRD) analysis. We examined the electrical characteristics of superlattice PCMs with films deposited at different temperatures. It was found that XRD spectra differed between the films deposited at 200 °C and 240 °C; the differences corresponded to the differences in the GeTe sequences in the films. We applied first-principles calculations to calculate the total energy of three different GeTe sequences. The results showed the Ge-Te-Ge-Te sequence had the lowest total energy of the three and it was found that with this sequence the superlattice PCMs did not run.
A novel memory cell for phase-change memories (PCMs) that enables low-power operation has been developed. Power (i.e., current and voltage) for the cell is significantly reduced by inserting a very thin Ta 2 O 5 film between GeSbTe (GST) and a W plug. The Ta 2 O 5 interfacial layer works not only as a heat insulator enabling effective heat generation in GST but also as an adhesion layer between GST and SiO 2 underneath. Nonetheless, sufficient current flows through the interfacial layer due to direct tunneling. A low programming power of 1.5 V/100 µA can therefore be obtained even on a W plug with a diameter of 180 nm fabricated using standard 0.13-µm CMOS technology. In addition, the uniformity and repeatability of cell resistance are excellent because of the inherently stable Ta 2 O 5 film properties.
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