The configuration achieves programming currents as low as 0.5 microamps for set and 5 microamps for reset operations, two orders of magnitude lower than state-of-the-art devices, the authors claim in their ScienceExpress paper, published March 10.
The researchers, led by Professor Eric Pop of University of Illinois, grew single-wall and multi-wall carbon nanotubes with diameters of between 1 and 6 nanometers and then used electrical breakdown, e-beam writing or an atomic force microscope to form a gap of less than 100 nanometers in each wire. The devices than received a sputtered layer of germanium-antimony-tellurium (GST) so that the gaps in the CNTs are filled with the phase-change material. Although amorphous GST is laid down over the entire device, the switching occurs only in the nanogap of the CNT, which is the location of highest electric field and heating effect.
The authors claim their results address the scaling of both the size and power reduction that is possible with programmable PCM bits. The paper reports on reversible switching with programming currents between 1 and 8 microamps, two orders of magnitude lower than state-of-the-art PCM devices. The PCM material switches between an off resistance of 50-megaohm to an on resistance of 2-Mohm but which can be as low 500-kohm, depending on sample preparation.
The authors report hundreds of switching cycles. Conventional PCM devices have been reported at more than 1 million cycles endurance.
The creation of nano-debris in the formation of the CNT gaps does not appear to be a problem, according to Professor Pop. "The nanogaps are formed by an oxidation process, so it is possible that the C atoms combine with O atoms forming gas (CO or CO2), not solid debris. Regardless, debris does not seem to be a problem since the nanotubes show a “clean cut” after imaging with atomic force microscopy (AFM)," Professor said in email correspondence with EE Times .
Professor Pop and his team only worked