Diode and Atomic Layer Deposition approaches to nanoantenna solar energy and terahertz transistors

Infrared nanoantenna could in theory achieve 70% efficiency with solar energy conversion. There are several approaches to try to achieve that goal.

Diode Approach has had fundamental problems but adding a second insulator layer could help

The power conversion efficiency of broadband antennas, log-periodic, square-spiral, and archimedian-spiral antennas, coupled to Metal-Insulator-Metal and Esaki rectifying diodes has been obtained from both theoretical and numerical simulation perspectives. The results show efficiencies in the order of 10^−6 to 10^−9 for these rectifying mechanisms, which is very low for practical solar energy harvesting applications. This is mainly caused by the poor performance of diodes at the given frequencies and also due to the antenna-diode impedance mismatch. If only losses due to antenna-diode impedance mismatch are considered an efficiency of about 10^−3 would be obtained. In order to make optical antennas useful for solar energy harvesting new rectification devices or a different harvesting mechanism should be used.

MIM diodes use quantum tunneling, which permits electrons to jump from one metal electrode to the other without interacting with the intervening insulator layer — hence the power and heat reductions. So far, their development has been slow going.

Now Oregon State University (OSU) researchers claim to have invigorated the technology by adding a second insulator layer to produce an MIIM device that aims to solve the problems with MIM devices and come closer to taking the technology mainstream.

The two insulator layers — which for Conley’s work was hafnium oxide and aluminum oxide — enables what he called “step tunneling.” Step tunneling allows more precise control of the diode asymmetry and thus its rectification capabilities at low voltages.

As a result, Conley sees his MIIM devices are poised to improve all sorts of electronic devices in wide use today, from liquid crystal displays to cell phones and televisions, as well as new types of devices such as infrared solar cells that convert radiant heat into electricity.

The researchers hope to optimize their process, then tackle applications that use even more metal-insulator layers, such as transistors.

ALD to make nanoantenna arrays is another approach

For years, scientists have studied the potential benefits of a new branch of solar energy technology that relies on incredibly small nanosized antenna arrays that are theoretically capable of harvesting more than 70 percent of the sun’s electromagnetic radiation and simultaneously converting it into usable electric power.

Nextbigfuture covered this work back in February, 2013

The technology would be a vast improvement over the silicon solar panels in widespread use today. Even the best silicon panels collect only about 20 percent of available solar radiation, and separate mechanisms are needed to convert the stored energy to usable electricity for the commercial power grid.

But while nanosized antennas have shown promise in theory, scientists have lacked the technology required to construct and test them. The fabrication process is immensely challenging. The nano-antennas – known as “rectennas” because of their ability to both absorb and rectify solar energy from alternating current to direct current – must be capable of operating at the speed of visible light and be built in such a way that their core pair of electrodes is a mere 1 or 2 nanometers apart, a distance of approximately one millionth of a millimeter, or 30,000 times smaller than the diameter of human hair.

A potential breakthrough lies in a novel fabrication process called selective area atomic layer deposition (ALD) that was developed by Willis, an associate professor of chemical and biomolecular engineering and the previous director of UConn’s Chemical Engineering Program

It is through atomic layer deposition that scientists can finally fabricate a working rectenna device. In a rectenna device, one of the two interior electrodes must have a sharp tip, similar to the point of a triangle. The secret is getting the tip of that electrode within one or two nanometers of the opposite electrode, something similar to holding the point of a needle to the plane of a wall. Before the advent of ALD, existing lithographic fabrication techniques had been unable to create such a small space within a working electrical diode. Using sophisticated electronic equipment such as electron guns, the closest scientists could get was about 10 times the required separation. Through atomic layer deposition, Willis has shown he is able to precisely coat the tip of the rectenna with layers of individual copper atoms until a gap of about 1.5 nanometers is achieved. The process is self-limiting and stops at 1.5 nanometer separation.

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