Power/Performance Bits: Aug. 18

Flexible, hole-filled films
Researchers from Daegu Gyeongbuk Institute of Science and Technology (DGIST) and Hongik University propose a simple way to make flexible electrodes and thin film transistors last longer: adding lots of tiny holes.

A major problem with flexible electronics is the formation of microscopic cracks after repeated bending which can cause the device to lose its conductive properties and fail. To address the problem, the team looked to techniques used in civil engineering, said Jae Eun Jang, a professor at DGIST. “We happened to be passing by a construction site, when we saw steel plates with holes, often used in construction. We knew that these steel plates with holes are used to reduce stress. We thought that this method could also be a solution in the micrometer world and, based on this idea, we began conducting experiments.”

The researchers created micrometer-sized holes in a zigzag pattern on standard aluminum thin film electrodes and a-IGZO (amorphous indium-gallium-zinc oxide) thin film transistors. With the array of holes, the stress distribution of the material changes so that cracks only form at specific points near the edges of the holes and propagate over a short distance, rather than cracks forming in random locations and propagating widely. Because the crack sites were predictable and controllable, a fatal electrical breakdown in a conductive layer such as a metal electrode or the semiconducting junction of a TFT could be prevented by specifically arranging the hole arrays.

Testing showed the potential of the hole-filled films to withstand frequent bending, said Jang. “Our devices were able to maintain conductivity up to 300,000 bending cycles, which means that they can be bent over 80 times a day for 10 years.”

Plus, the team said the proposed approach is inexpensive and easy to adopt using equipment already employed in display fabrication.

28 Ghz transceiver for 5G
Researchers at Tokyo Institute of Technology and NEC Corporation built a 28 GHz phased-array transceiver supporting dual-polarized MIMO for 5G applications.

A class of antenna systems that can transmit data simultaneously through horizontal and vertical-polarized waves, dual-polarized phased-array transceivers have shown promise for improving the data rate and spectrum efficiency in 5G radio units. However, they can suffer from cross-polarization leakage, which results in degradation of signal quality particularly in the millimeter wave band.

Phased-array radio for polarization MIMO. (Source: © IEEE)

The new transceiver is capable of canceling cross-polarization interference using a built-in so-called horizontal and vertical (H/V) canceller. Tests showed that the error vector magnitude using the 256 QAM digital modulation method can be improved from 7.6% to a more desirable, lower figure of 3.3% using this new leakage cancellation technique. “The cancellation signals are generated for horizontal and vertical polarization at the transmission side so that it can cancel the cross-polarization leakage caused by all through the transmitter/receiver chip, package, printed circuit board and antenna,” the researchers say.

Fabricated using low-cost, mass-producible silicon CMOS technology, the transceiver has an area of 16mm2. The researchers anticipate that the new circuitry could be installed in a wide range of 5G applications and could improve spectrum efficiency while keeping equipment size and set-up costs to a minimum.

Optical chips from a Petri dish
Researchers at ITMO University, St. Petersburg Academic University, and Université de Lorraine developed a way to fabricate optical chips operating in the visible light range using a chemical solution in a Petri dish, rather than nanolithography.

The infrared range is commonly used in optical chips. “But to make the devices even more compact, we need to work in the visible range,” said Sergey Makarov, chief researcher at ITMO’s Department of Physics and Engineering, “as the size of a chip depends on the wavelength of its emission.”

While creating a source to emit in the green or red part of the spectrum is not difficult, creating waveguides for these wavelengths can be an issue.

“A microlaser is a source of emission that you need to guide somewhere,” said Ivan Sinev, senior researcher at ITMO’s Department of Physics and Engineering, “and this is what waveguides are for. But the standard silicon waveguides that are used in IR optics do not work in the visible range. They transmit the signal no further than several micrometers. For an optical chip, we need to transmit along tens of micrometers with a high localization, so that the waveguide would have a very small diameter and the light would go sufficiently far through it.”

The team settled on gallium phosphide as a material for the waveguides, as it has very low losses in the visible band.

“The chip’s important property is its ability to tune the emission color from green to red by using a very simple procedure: an anionic exchange between perovskite and hydrogen halides vapor,” noted Anatoly Pushkarev, senior researcher at ITMO’s Department of Physics and Engineering. “Importantly, you can change the emission color after the chip’s production, and this process is reversible. This could be useful for the devices that have to transmit many optical signals at different wavelengths. For example, you can create several lasers for such a device, connect them to a single waveguide, and use it for transmitting several signals of different colors at once.”

An optical nanoantenna made of perovskite that receives the signal travelling along the waveguide was also added to the chip, which enables uniting two chips in a single system.

“We added a nanoantenna at the other end of our waveguide,” explains Pavel Trofimov, PhD student at ITMO’s Department of Physics and Engineering, “so now we have a light source, a waveguide, and a nanoantenna that emits light when pumped by the microlaser’s emission. We added another waveguide to it: as a result, the emission from a single laser went into two waveguides. At the same time, the nanoantenna did not just connect these elements into a single system, but also converted part of the green light into the red spectrum.”

The size of the new chip’s elements is about three times smaller than that of its counterparts that work in the IR spectral range. Both the light source can be grown directly on a waveguide in a Petri dish using solution chemistry methods, which is much cheaper than the commonly used nanolithography.