Phase change memory (PCM) progress report, part 2

EETimes recently carried a report [1] of new work reporting new low power phase change memory (PCM) devices. Workers at University of Illinois Urbana-Champaign have reported their results for phase change memory (PCM) devices formed by creating narrow 20–30nm gaps in 3nm diameter carbon nano-tubes (CNTs) and opened some new directions for further investigation [2].

Current Density
While the authors [2] comment optimistically on fabricating low power PCMs, there is no discussion of current density. They describe their “best” results, obtained from PCM devices with set/rest currents of 1uA/5uA respectively with 3V across a 20-30nm gap and their stated carbon nano-tube diameter as 3nm. Assuming that the contact is solid planar, the contact current density calculates as

 J = 7 x 10+7 Amps/sq-cm.

The authors [2] report higher values of reset currents 8uA, for what as described as not their “best” devices, for unspecified reasons, increasing the calculated current density value (J) to ~1 x 10+8 A/sq-cm. If the electrodes are truly tubular, as illustrated in Figure 1 (a) and the notional contact is as shown, the contact current density is likely to exceed this value.  Even if the region of molten chalcogenide penetrates into and around the end of the CNT tube during reset, enlarging the contact area as illustrated in Figure1, (b), the total contact area will still involve only a few hundred atoms.



Figure 1: Diagram showing two cutaway views of one electrode of a carbon nanotube (CNT- PCM, (a) the notional contact area of an annulus of atoms,(b) cutaway section CNT-PCM during reset.

Atomic force microscope (AFM) evidence from [2] appears to show a flat spherical shaped region of active material equal to about 2 to 3 diameters, approximately the gap width. This suggests the molten region may not extend very far into the CNT. As well as the active region, the calculated levels of current density will have a deleterious effect on other components in an array and that is already one of the serious and ongoing PCM problems.

With respect to current density, there is now good evidence [3], summarized by this author in [4] that during reset the molten GST material acts like an electrochemical cell (plating bath) and it is that process that accounts for element separation, composition change and eventual failure. Therefore an argument might be made that an emphasis on current density as the “villain in the piece” for the memory device, especially for the very small contact area of the CNT-PCM devices might be misplaced. Current density is in effect the signature of the presence of the “electrochemical cell” plating voltage, with the former necessary to maintain a very small volume of chalcogenide in a molten state.