Japanese researchers have succeeded in producing functioning large-area, single-color LEDs by sputtering nitrides of indium and gallium onto a glass substrate.
The research suggests lower-cost manufacturing of large-area LEDs, but also eventually the possibility of light sources with OLED-like properties composed of inorganic LEDs.
Here is the research paper. It appeared in the journal Scientific Reports from the Nature Publishing Group, and I commend Nature for making it publicly available in full.
Drawbacks of glass
Glass is not a workable substrate for Group III nitride epitaxy using conventional MOCVD (metal oxide chemical vapor deposition) processes, for a number of reasons, not the least of which is a temperature mismatch. Glass begins to soften at temperatures in the 500°-700°C range, while MOCVD usually requires 1000°C.
The researchers' approach to epitaxy used pulsed sputtering deposition (PSD) at around 600°C. Sputtering is widely used in the manufacture of liquid-crystal displays, where glass is often the substrate of choice.
Bring the graphene
Laying down a GaN film directly on SiO2 does not result in high-quality crystal structures. The researchers interposed a multilayer graphene region and found that "the crystalline quality of a GaN film grown by PSD on amorphous SiO2 is considerably improved by the use of a multilayer graphene buffer layer." The figure at right shows the improved spectral quality of LEDs laid down on graphene.
Onward to white?
If we can produce red, green, and blue inorganic LEDs on glass, we might find a way to get to white LEDs. The researchers produced separate LEDs with emission peaks in the red, green, and blue, as shown in the figure at left. But I'm at a loss to imagine the geometry of a white LED produced by processes such as these.
I don't see how different colors could be engendered in different surface regions during epitaxial growth. The different colors require different mixes of In and GaN in the material being sputtered, and the nature of sputtering would seem to imply that only a single mix at a time is being distributed over a wide area. If we vary the In/GaN ratio over time, the resulting material will differentiate along the vertical axis, but not horizontally.
Would that get us to white? Those with insight into epitaxy and the workings of LEDs at the quantum level, please weigh in in the comments.
The researchers conclude: "It should also be noted that state-of-the-art technology in the glass industry can offer roll-to-roll processing of flexible glass foils. We think the combination of these techniques can lead to the development of large-area flexible inorganic devices."
— Keith Dawson , Editor-in-Chief, All LED Lighting