Researchers at the UCSD Applied Electromagnetics Group have designed the first semiconductor-free microelectronic device, showing an 1000-percent increase in conductivity after being activated by a low-voltage and low-power laser. The findings were published in the journal Nature Communications on Nov. 4.
The device has a layer of mushroom-like nanostructures made of gold called which the authors called the metasurface.
Dan Sievenpiper, director of the Applied Electromagnetics Group, co-author of the paper and a professor of electrical and computer engineering, explained how the the surface of the microelectronic device works.
“The metasurface is made up of an array of resonators, which are designed to work at optical frequencies,” Sievenpiper told the UCSD Guardian. “When light is shined on the surface, the optical electric field builds up within these resonators and creates a strong field at the narrow gap between the patches. This is combined with a static bias field, and the two together cause electrons to be emitted from the surface.”
Sievenpiper added that the increased conductivity was due to the features of the components as well as the design.
“The unique property of our structure is that it provides very high electric field enhancement through a combination of geometry and plasmonic effects in the metal,” Sievenpiper said. “This leads to orders of magnitude increase in photoemission so that we can use low-power lasers and low-voltage bias to control a large flow of electrons.”
Postdoctoral researcher and co-author of the paper Ebrahim Forati elaborated on Sievenpiper’s explanation, telling the Guardian that microelectronic devices are limited by their transistors, such as semiconductors, because of their constituent parts.
“Electrons moving inside of the semiconductors are repeatedly colliding with atoms in the lattice, generating heat, which can be quantified as resistance,” Forati said. “So if we remove the lattice, that causes the resistance to also be removed. That can improve conductivity versus semiconductors.”
All materials have a property called “work function,” which is the amount of energy needed to be given to electrons to liberate them from the material. Some materials have lower work functions and are used in electron-emitting devices such as electron microscopes.
Forati added that the metasurface combines two methods of lowering work function to free the electrons from metal.
“The typical method of decreasing work function is either by heating, or by applying very high voltage, or by using very high intensity lasers.” Forati said. “This surface that we designed is combining two of these phenomena — applying voltage and applying lasers — and decreasing the required value of these two.”
Although there is a significant increase in conductivity, Sievenpiper believes that the metasurface will probably remain a niche solution.
“It won’t replace most semiconductor devices, but it will be useful for specialty applications such as high-speed or high-power devices,” Sievenpiper said. “Since it can be built right on top of conventional semiconductor circuits, it could serve as a front end to sensitive high-speed circuits, for example, supported by highly integrated conventional circuits for signal processing.”
Forati also mentioned that the metasurface has hopeful implications as photovoltaic cells, which is something the group may be studying next.
“By adjusting the dimension of the inclusions of the surface, which are those mushrooms, we can adjust the wavelength or frequency at which the surface interacts with optical frequencies to interact with light as visible wavelengths,” Forati said. “Our next step is to investigate this application because we think it’s interesting and can be a potential high-efficiency solar cell.”