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Is there a bright optoelectronic future for PCM?

Posted: 05 Aug 2014 ?? ?Print Version ?Bookmark and Share

Keywords:optoelectronic? phase-change memory? PCM? memory devices? optical switches?

To PCM memory watchers, the next step will be familiar. Make an orthogonal X-Y display formed of discrete memory devices. The initial form of a possible display is illustrated in figure 3. Here the two colours (enhanced blue and red) would be obtained by partially switching the memory material so that the GST becomes, in effect, two films of different refractive index. Alternatively, the array could be fabricated with each row having a different GST thickness.

ITO is not usually used as an electrode for PCM memory devices. It is normally used for data storage memory applications where the electrodes are a refractory metal and crystallized GST.

The Oxford/Exeter team initially answered the question as to the suitability and compatibility of ITO electrodes for phase-change switching by creating a large sheet of the sandwich structure. Then, they used the conductive tip of an atomic force microscope (CAFM) to step across the surface, to selectively switch regions under the point to create the pixels for a series of pictures in various colours, as shown in figure 4.

Figure 3: The initial form of a possible display.

If displays are the target application, then a transmission version will also be required in addition to a reflective one. In addition to displays, some of the other application options suggested by the Oxford team included contact-lens-type displays, smart eyeglasses, windshield displays, and possibly even synthetic retina devices.

Achieving an optical transmission structure requires constructing a PCM device without the platinum mirror and fabricating the device on a transparent substrate. Here again the CAFM technique was used to create large-area images on transparent quartz and (because the films are very thin) also on flexible plastic substrates.

Two of the many images demonstrated using the reflective technique are shown in figure 4.

Clearly, for an optical matrix operating in transmission mode, a gap-type PCM device would appear to be an option. To this end, an ITO/GST/ITO gap structure was evaluated that also served as a second vehicle for the evaluation and suitability of ITO electrodes. The demonstrated device had an active area of 300 x 300 nm, a threshold voltage of 2V, a maximum current of 20 uA, an on-to-off resistance ratio of 350, and a write/erase lifetime of 140 set/reset cycles. It was recognised that it will be necessary to switch all of the material in the gap and not just a filamentary region.

Figure 4: Two of the many images demonstrated using the reflective technique.

Next steps
This new work from the team at Oxford/Exeter was reported as a starting point or "framework" and acknowledged that, to date, outside of the CAFM work, no discrete planar device with ITO electrodes has been constructed or tested.

Therefore, to move this optoelectronic concept for PCM forward it will now be necessary to provide the support for the full evaluation and development of PCM memory devices with ITO electrodes. This will require a full characterisation, with special attention to write/erase lifetime, switching times, power dissipation, and data retention, added to which must be the optical characterisation for each step in the developmentall of which is a non trivial undertaking.

The history and literature of the development of PCM devices is replete with examples of claims of levels of performance and characteristics that, while achieved in the laboratory, are never reproduced in production. Care must be taken not to rely too heavily on that data except to learn from past mistakes.

For data memory, PCM past experience suggests to avoid initial threshold switching problems and drift from films deposited in the amorphous state. Experience shows that it is better to start with the active material in its crystallized state.

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