Silicon wafer substrates can “copy” exotic semiconductor films
A range of compound semiconducting materials is commonly grown on silicon wafers. Their proerties are required for, for example, microwave transistor action, or for LEDs or laser diodes. Producing the non-silicon material in bulk and sawing it into wafers in the established manner, is prohibitively expensive, when all of the electrically-active elements are in a few nanometres of the surface. Normal practice is to grow a layer of the required material on a silicon wafer; the monocrystalline silicon imposes order on the atomic lattice of the non-silicon layer as it grows.
The resulting layer is ‘bonded’ at an atomic level to the silicon; so the monocrystalline silicon wafer, itself an expensive item, is employed merely as a carrier for the exotic semiconductor.
The MIT researchers have found that a layer of graphene applied to the surface of the silicon is thin enough that the grown layer – as before – takes on the desired crystal characteristics: but it also provides a discontinuity so that once grown, the wanted layer can be detached, and the silicon substrate re-used as a “pattern”. The deposited layer is so thin that it is flexible, and it “peels” off; it might then be applied to an inert, low-cost substrate for subsequent processing steps. In the photo above, the layer is of nickel, for demonstration purposes.
The group’s graphene-based peel-off technique may also advance the field of flexible electronics. In general, wafers are very rigid, making the devices they are fused to similarly inflexible. Kim says now, semiconductor devices such as LEDs and solar cells can be made to bend and twist. In fact, the group demonstrated this possibility by fabricating a flexible LED display (pictured), patterned in the MIT logo, using their technique.
“Let’s say you want to install solar cells on your car, which is not completely flat — the body has curves,” says Jeehwan Kim, Class of 1947 Career Development Assistant Professor in the departments of Mechanical Engineering and Materials Science and Engineering, “Can you coat your semiconductor on top of it? It’s impossible now, because it sticks to the thick wafer. Now, we can peel off, bend, and you can do conformal coating on cars, and even clothing.”
Going forward, the researchers plan to design a reusable “mother wafer” with regions made from different exotic materials. Using graphene as an intermediary, they hope to create multifunctional, high-performance devices. They are also investigating mixing and matching various semiconductors and stacking them up as a multimaterial structure.
“Now, exotic materials can be popular to use,” Kim says. “You don’t have to worry about the cost of the wafer. Let us give you the copy machine. You can grow your semiconductor device, peel it off, and reuse the wafer.”
This research was supported, in part, by the One to One Joint Research Project between the MI/MIT Cooperative Program and LG electronics R&D center.
The work was reported in a letter in Nature. Full item on the MIT news feed; here.
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