Manufacturing Bits: Nov. 25

RF carbon nanotubes
For years, the industry has been working on logic and memory devices based on carbon nanotubes, although these technologies remain in R&D.

Now, there is a new device type using carbon nanotubes–RF. Startup Carbonics has developed an RF-based carbon nanotube technology that operates at frequencies over 100GHz.

The technology exceeds the cutoff frequency of today’ RF CMOS and is close to the performance of GaAs. Carbonics’ technology, which is funded by the Army Research Office, could be used by the U.S. Army for use in communications, radar, electronic warfare and related applications.

Carbon nanotubes are tube-shaped materials, which are 100,000 times smaller than the diameter of human hair. These structures have good electrical, chemical, thermal and mechanical properties.

In the fab, carbon nanotubes are formed using a deposition process. The challenge is that the nanotubes are prone to variations and misalignments during the process, making carbon nanotubes a difficult technology to put into mass production. That’s why carbon nanotubes are still in R&D for memory and logic devices.

For RF, though, Carbonics has apparently solved the problem. In 2014, Carbonics was spun-out from the joint center of UCLA-USC and King Abdulaziz City for Science and Technology.

Two years ago, Carbonics rolled out its carbon-on-silicon technology for RF applications. Used to develop single-walled carbon nanotubes (SWCNTs), the technology enables carbon nanotubes to be densely aligned and deposited onto a variety of substrates. This includes silicon, silicon-on-insulator (SOI), quartz and flexible materials.

“Aligned carbon nanotubes are proposed as an alternative to III–V technologies in such applications because of their highly linear signal amplification and compatibility with CMOS,” according to members from Carbonics and others in a recent edition of Nature Electronics, a technology journal. “Here we report the wafer-scalable fabrication of aligned carbon nanotube field-effect transistors operating at gigahertz frequencies. The devices have gate lengths of 110nm and are capable, in distinct devices, of an extrinsic cutoff frequency and maximum frequency of oscillation of over 100GHz, which surpasses the 90GHz cutoff frequency of radio-frequency CMOS devices with gate lengths of 100nm and is close to the performance of GaAs technology. Our devices also offer good linearity, with distinct devices capable of a peak output third-order intercept point of 26.5dB when normalized to the 1dB compression power, and 10.4dB when normalized to d.c. power.”

This Army-funded project is geared for 5G and mmWave technologies. “This milestone shows that carbon nanotubes, long thought to be a promising communications chip technology, can deliver,” said Joe Qiu, program manager of solid state and electromagnetics at the Army Research Office. “The next step is scaling this technology, proving that it can work in high-volume manufacturing. Ultimately, this technology could help the Army meet its needs in communications, radar, electronic warfare and other sensing applications.”

Solar nanotubes
The Skolkovo Institute of Science and Technology (Skoltech) and others have developed a p-type flexible transparent conductor using single-walled carbon nanotubes.

Flexible electronics are gaining steam. Most transparent conductive oxides are mainly based on n-type conductors, according to Skoltech.

Skoltech has devised a p-type transparent conductor for solar cells using SWCNTs. The research group also included Aalto University, DLR Institute of Networked Energy Systems and Tallinn University of Technology.

“We discovered the use of thin multicomponent layers and the introduction of carbon nanotube fibers in a dramatic improvement in the p-type transparent conductor development,” said Pramod Rajanna, a PhD student at Skoltech.

“Moreover, carbon nanotube fibers by themselves can be used as a replacement for traditional metal contacts. However, the most fascinating result was the solar cells fabricated at room temperature using the developed p-type transparent conductor and amorphous silicon, which are classified specially as hybrid devices and yield a record power conversion efficiency (conversion efficiency of sunlight to electricity) of 8.8%,” Rajanna said. “This is an effective 16% increase over the traditional amorphous silicon solar cells, thus highlighting the efficacy of the developed p-type transparent conductor. We have progressed from the initial 1.6% and 3.4% reported previously in 2016 and 2018 respectively to 8.8% in 2019 using our newly developed p-type transparent conductor for such hybrid thin film solar cells.”

Clean tubes
Aalto University has developed a fast way to fabricate hundreds of ultraclean carbon nanotube field‐effect transistors (FETs).

Researchers used aerosols of metal catalysts and gasses containing carbon in a chemical vapor deposition (CVD) system.

“The synthesis of the nanomaterial is performed by floating‐catalyst chemical vapor deposition, which is employed to fabricate high‐performance thin‐film transistors,” according to researchers from Aalto University and others in Advanced Functional Materials, a journal. “Combined with palladium metal bottom contacts, the transport properties of individual SWCNTs are directly unveiled.”

The carbon nanotube FETs exhibit a field‐effect mobility that is 3.3 times than solution‐processed CNTs. On–off current ratios reach over 30 million, according to researchers.