Structural phase transitions of Ge2Sb2Te5 cells with TiN electrodes using a homemade W heater tip

Abstract

The phase transitions of a Ge2Sb2Te5 cell with a volume of 20×20×0.1μm3 were carried out by applying a reset pulse (10V and 50ns) and a subsequent set pulse (5V and 300ns) using a homemade W heater tip fabricated by focused ion beam lithography. The phase transformation from a crystalline state to an amorphous state was confirmed by measuring the I-V curves and observation with a cross-sectional transmission electron microscope both before and after applying the reset pulse. The electron diffraction pattern obtained from the transformed area clearly showed the amorphous state. The resistance value of the transformed amorphous area was two orders higher than that of the original crystalline phase. This difference in the resistance value between the reset and set states was maintained for 20 reset/set pulse cycles. It is expected that this experimental setup can be used to evaluate the fatigue behavior of Ge2Sb2Te5 cells with reset/set pulse cycles.

Nanoscale mechanically induced structural and electrical changes in Ge2Sb2Te5 films

“…Among these attempts is the investigation on scanning probe based storage systems where, similarly to the PCRAM cells, writing of bits by conductive probes involves an electrothermal process in which Joule heating provides the energy required for film crystallization or amorphization. [9][10][11][12][13][14] In this work we report that microstructural and conductivity change of crystalline Ge 2 Sb 2 Te 5 films in the nanoscale range can also be obtained by a mechanical process via nanoscratching. In analogy with laser or electrical writing, the mechanically modified film regions have much lower conductivity than unmodified regions and show microstructural features that are consistent with the amorphous state of the Ge 2 Sb 2 Te 5 alloy.…”

Ultrahigh Storage Densities via the Scaling of Patterned Probe Phase-Change Memories

“…A Pt C-AFM probe with a 5 nm radius is used to make electrical contact with the TiN top electrode, and trapezoidal RESET and SET voltage pulses of various amplitudes (1 -2 V) and durations (40-100 ns) are applied to the top of the C-AFM probe, whilst the boundary of the Si substrate is maintained at ground potential (as shown previously in [5,7]). To avoid interference between neighboring cells, the top TiN electrode is patterned to cover the GST layer only (as proposed in [18]). …”

“…The choice of capping layer is crucial as it not only protects the active phase-change region from oxidation and wear, but also provides a conductive path for the current required for electrical switching, and consequently has a strong influence on the resulting temperature distributions inside the active region [5,7]. TiN is a well-known electrode material and has previously been used in GST-based patterned PCM cells to demonstrate electrical switching [18,19]. Amorphous to crystalline phase transitions in GST films as thin as 2 nm have also been reported in [2] using a trilayer TiN/GST/TiN structure.…”

“…Although electrical switching in PP-PCM cells has been demonstrated on the micrometer scale [18], and more recently storage densities of up to 207 Gb/in 2 have been reported using conical PP-PCM cells [19], it is yet to be determined whether PP-PCM cells can be scaled down to single-nanometer dimensions, and whether we can achieve ultrahigh (multi-Tb/in 2 ) storage densities, higher than those previously reported for phase-change based probe-type storage (i.e. 1.5 Tb/in 2 using electrical probes [7] and 3.3 Tb/in 2 using thermal probe storage [6]).…”

The Design of Rewritable Ultrahigh Density Scanning-Probe Phase-Change Memories

“…10 where a 20 nm diameter cylinder of GST is embedded in an SiO 2 matrix. Extended to a checkerboard type pattern, such a geometry would yield a storage density approaching 1 Tbit/sq.in.. Kim et al [39] have already demonstrated experimentally the ability to induce switching in patterned GST media, both from the crystalline state to the amorphous state and back again. However, they used patterning on the micrometer scale and a very large (18 m) tungsten tip.…”