Research Bits: July 24

Protons improve ferroelectric memory

Researchers from King Abdullah University of Science and Technology (KAUST), Qingdao University, and Zhejiang University developed a method to produce multiple phase transitions in ferroelectric materials, which could increase storage capacity for neuromorphic memory.

The approach uses proton-mediation of the ferroelectric material indium selenide. The researchers incorporated the material in a transistor consisting of a silicon-supported stacked heterostructure. In the heterostructure, a platinum layer served as electrodes for the applied voltage and porous silica acted as an electrolyte and supplied protons to the ferroelectric film.

The researchers gradually injected or removed protons from the ferroelectric film by changing the applied voltage. This reversibly produced several ferroelectric phases with various degrees of protonation, which would enable multilevel memory devices with greater storage capacity. Higher positive applied voltages boosted protonation, whereas negative voltages of higher amplitudes depleted protonation levels to a greater extent.

Protonation levels also varied depending on the proximity of the film layer to silica. They reached maximum values in the bottom layer, which was in contact with silica, and decreased in stages to achieve minimum amounts in the top layer. Additionally, the proton-induced ferroelectric phases returned to their initial state when the applied voltage was turned off.

By manufacturing a film that displayed a smooth and continuous interface with silica, the team created a high proton-injection efficiency device that operated below 0.4 volts. “Our biggest challenge was to reduce the operating voltage, but we realized that the proton-injection efficiency over the interface governed operating voltages and could be tuned accordingly,” said Fei Xue of KAUST. “We are committed to developing ferroelectric neuromorphic computing chips that consume less energy and operate faster.”

Xin He et al., Proton-mediated reversible switching of metastable ferroelectric phases with low operation voltages. Sci. Adv. 9, eadg4561 (2023). DOI: https://doi.org/10.1126/sciadv.adg4561

Low-jitter all-digital PLL

Researchers from the University of Science and Technology of China designed a low-jitter millimeter-wave all-digital phase-locked loop (CSS-ADPLL) chip using a charge-steering sampling technique.

The CSS-ADPLL chip comprises a charge rudder discriminator (CSS-PD), a SAR-ADC, and digital filter in a compact structure. The frequency synthesizer could be used to deliver precise carrier signals for 5G/6G millimeter-wave communication systems.

The team said that combining charge-steering sampling with a successive approximation register-type ADC (SAR-ADC) using the charge rudder sampling technique enabled the construction of a digital phase discriminator with high phase-identification gain, linearity, and multi-bit digital outputs.

Test results demonstrated that the chip has achieved a clock jitter of 75.9 fs, a reference spurious level of -50.13 dBc, and a Figure of Merit (FoM) value of -252.4 dB. The chip’s core area measures 0.044 mm².

An 18.8-to-23.3 GHz ADPLL Based on Charge-Steering-Sampling Technique Achieving 75.9 fs RMS Jitter and -252 dB FoM, 2023 Symposium on VLSI Technology and Circuits

Composite semiconductors for flexible displays

Scientists at the Indian Institute of Science developed a flexible, composite semiconductor material with potential applications in flexible or curved displays, foldable phones, and wearable electronics.

The researchers used an inkjet printing technique to enable up to 40% of the composite material’s weight to be made up of a water-insoluble polymer such as ethyl cellulose without altering the semiconducting properties of the indium oxide. The resulting material was highly flexible and foldable without deteriorating its performance.

To design the material, the researchers mixed the polymer with the oxide precursor in such a way that interconnected oxide nanoparticle channels are formed around phase-separated polymer islands through which electrons can move from source to drain. Using a water-insoluble polymer that did not mix with the oxide lattice during fabrication was key.

“This ‘phase separation’ and the formation of polymer-rich islands helps in crack arrest, making it super flexible,” said Subho Dasgupta, associate professor in the Department of Materials Engineering at IISc.

Using inkjet printing, the researchers applied the material to a range of flexible substrates.

Next, the team plans to test its shelf-life and quality control from device to device before it can be scaled up for mass production. They also plan to look for other polymers that can help design such flexible semiconductors.

Divya, M., Cherukupally, N., Gogoi, S.K., Pradhan, J.R., Mondal, S.K., Jain, M., Senyshyn, A. and Dasgupta, S. (2023), Super Flexible and High Mobility Inorganic/Organic Composite Semiconductors for Printed Electronics on Polymer Substrates. Adv. Mater. Technol. 2300256. https://doi.org/10.1002/admt.202300256