“This is a fundamental change in the way you could produce electronic products, at high speed on a huge scale at very low cost, even less than with conventional methods,” said Douglas Keszler, a distinguished professor of chemistry at Oregon State University. “It’s a basic way to eliminate the current speed limitations of electrons that have to move through materials.”
The discovery, just reported online this week in the professional journal Advanced Materials, demonstrated the creation for the first time of a high-performance MIM or “metal-insulator-metal” diode.
What is a diode, and why does this matter? The kind you know best is LEDs, light-emitting diodes. These include the recently available white LEDs that work through a process where an actual light-emitting semiconductor produces light in the blue part of the spectrum, which strikes a phosphor compound deposited on a reflector. The phosphor then fluoresces in the orange part of the spectrum. The combination of the two colors produces a net effect that appears as white light. Usually “diode” means some combination of three things: (1) An electronic device that restricts current flow chiefly to one direction; (2) in the old pre-transistor days of vacuum tube, an electron tube having a cathode and an anode; (3) two-terminal semiconductor device used chiefly as a rectifier.
That’s why display technology for commercial electronics has driven the research. “When they first started to develop more sophisticated materials for the display industry, they knew this type of MIM diode was what they needed, but they couldn’t make it work,” Keszler explained. “Now we can, and it could probably be used with a range of metals that are inexpensive and easily available, like copper, nickel or aluminum. It’s also much simpler, less costly and easier to fabricate.”
But MIM diodes matter even if no light shines from them. “Rectifying diodes are the fundamental building blocks in electronics,” says Tuo-Hung Hou, researcher at at Taiwan’s National Chiao Tung University. “Diodes made of oxide materials instead of traditional silicon are especially interesting because they can be fabricated at room temperature, as opposed to the 1,000°C typically required for silicon diodes. Besides complex materials engineering, our work shows a new route to greatly improve the rectification efficiency of oxide diodes by forming nanoscale current paths in oxides.” Further, by carefully controlling the nanoscale paths, the Taiwan group creates either a resistive nonvolatile memory, so-called RRAM, or a rectifying diode in the same structure. RRAM simply consists of a layer of transition metal oxides sandwiched between two metal electrodes. It’s being vigorously pursued by many companies as the “next big thing” in memory.
Until nanotechnology devices, or spintronic devices, are commercialized, much of the trillion dollar electronics industry is all about controlling the flow of electrons in solids. That’s what the silicon does in your microcomputer chip. MIM diode can be used to perform some of the same functions as a transistor in a integrated circuit, but in a essentially different way. In the MIM system, the device is like a sandwich, with the insulator in the middle and two layers of metal above and below it. In order to function, the electron doesn’t so much move through the materials as it “quantum tunnels” through the insulator — appearing almost instantly on the other side.
Oregon State University researchers have been leaders in a number of important material science advances in recent years, including the field of transparent electronics. University scientists will do some initial work with the new technology in electronic displays, but many applications are possible.
One possibility is high speed computers and electronics that don’t depend on transistors. Also on the horizon are energy harvesting technologies such as the nighttime capture of re-radiated solar energy, a way to produce energy from the Earth as it cools during the night.
OSU officials say they have applied for a patent. New companies, industries and high-tech jobs may ultimately emerge from this breakthrough. The research took place at OSU’s Center for Green Materials Chemistry and was supported by the National Science Foundation, the Army Research Laboratory and the Oregon Nanoscience and Microtechnologies Institute.
Keszler: “For a long time, everyone has wanted something that takes us beyond silicon. This could be a way to simply print electronics on a huge size scale even less expensively than we can now. And when the products begin to emerge the increase in speed of operation could be enormous.”
E. William Cowell, Nasir Alimardani, Christopher C. Knutson, John F. Conley, Douglas A. Keszler, Brady J. Gibbons, John F. Wager. Advancing MIM Electronics: Amorphous Metal Electrodes. Advanced Materials, 2010; DOI: 10.1002/adma.201002678