Is there a bright optoelectronic future for PCM?

A team at the University of Oxford and the University of Exeter describes their work as an "optoelectronic framework".¹ This modest description hides what may well become recognised as the first steps on the road to a new and bright optoelectronic future for phase-change memory (PCM) materials. This is a future, not just for displays, but also for optoelectronic memory devices, optical switches, and modulators for intra- and inter-chip communications.

Almost everybody is familiar with the coloured rainbow of interference patterns that are observed in sunlight when a very thin film of oil is floating on water. What if you could place another thin film on top of the oil, which could by electrical means be used to modulate the colours, enhancing some, reducing others, and then make it all in solid state. That simplistic description is what the Oxford University team has done. And then they went the proverbial "extra mile."

As well as the large change in resistance that accompanies the transition between the amorphous and crystalline states for chalcogenide glasses, there is also a change in the optical refractive index. It is this refractive index change that has been exploited by the Oxford team.

The embryo structure for the reflection version (there is also a transmission option) of the proposed chalcogenide-based optical modulator or display, in this case employing a Ge₂Sb₂Te₅ (GST) composition, is illustrated in figure 1. From the bottom up, the active part of the device consists of a suitable substrate and a dielectric film overlaid with platinum film to serve as a reflector, a transparent film of conducting indium tin oxide (ITO), a film of GST, and a top film of transparent conducting ITO.

The two most important films in the stack are the bottom ITO electrode, with its thickness (t) a key variable, and the active memory material GST. In a typical stack, the thickness of the films would be as follows (10nmITO/7nmGST/t variable of ITO/100nm Pt) all deposited on a suitable substrate. By varying the thickness of the bottom ITO electrode, a particular colour can be reinforced. In addition, others can be reduced when the surface of the stack is illuminated with white light and the GST is in its amorphous state (figure 1).

Switching the GST to its crystalline state changes its refractive index and, by interference, changes the colour that is reinforced in the reflected beam. The films involved are all very thin compared with the wavelength range of visible light. Figure 2 shows the impressive range of colour changes that can be achieved over large areas with ITO thicknesses of 50 nm, 70 nm, 120 nm, and 150 nm. The wavelengths of visible incident light ranges from 350 to 750 nm.