Research Bits: August 29

Memory focus: Resistive switching with hafnium oxide; bottom contact for ferroelectric memory; gallium oxide in space.


Resistive switching with hafnium oxide

Researchers from the University of Cambridge, Purdue University, University College London, Los Alamos National Laboratory, and University at Buffalo used hafnium oxide to build a resistive switching memory device that processes data in a similar way as the synapses in the human brain.

At the atomic level, hafnium oxide has no structure, with the hafnium and oxygen atoms randomly mixed. The team found that by adding barium to thin films of hafnium oxide, some unusual structures started to form, perpendicular to the hafnium oxide plane, in the composite material.

These vertical barium-rich ‘bridges’ are highly structured, and allow electrons to pass through, while the surrounding hafnium oxide remains unstructured. At the point where these bridges meet the device contacts, an energy barrier was created, which electrons can cross. The researchers were able to control the height of this barrier, which in turn changes the electrical resistance of the composite material.

“This allows multiple states to exist in the material, unlike conventional memory which has only two states,” said Markus Hellenbrand, from Cambridge’s Department of Materials Science and Metallurgy. “What’s really exciting about these materials is they can work like a synapse in the brain: they can store and process information in the same place, like our brains can, making them highly promising for the rapidly growing AI and machine learning fields.”

The hafnium oxide composites self-assemble at low temperatures and showed high levels of performance and uniformity. The researchers are now working with industry to carry out larger feasibility studies on the materials, in order to understand more clearly how the high-performance structures form.

Markus Hellenbrand et al., Thin film design of amorphous hafnium oxide nanocomposites enabling strong interfacial resistive switching uniformity. Sci. Adv. 9, eadg1946 (2023).

Bottom contact for ferroelectric memory

Researchers from Tokyo Institute of Technology propose a ferroelectric memory built with the 2D van der Waals material indium selenide (α-In2Se3) by using a two-terminal nanogap-structured bottom contact.

In the device, α-In2Se3 is exfoliated on electrodes as the bottom contact. The in-plane polarization can be reversed by applying a drain voltage via a channel with a relatively narrow length of 100 nm. This lateral channel design allows for higher memory density, enabling the integration of many memory cells on a single chip.

Two-dimensional ferroelectric semiconductor memory with a bottom contact 100 nm channel using in-plane polarization. (Credit: Tokyo Tech)

The researchers found that the α-In2Se3 ferroelectric memory exhibits typical resistive switching, a high on/off ratio of over 103, a large memory window of 13 V, good retention for 17 hours, and endurance for 1,200 cycles.

Additionally, the team says the lateral memory configuration enables seamless integration with existing semiconductor device fabrication techniques.

Miao, S., Nitta, R., Izawa, S., Majima, Y., Bottom Contact 100 nm Channel-Length α-In2Se3 In-Plane Ferroelectric Memory. Adv. Sci. 2023, 2303032.

Gallium oxide space memory

Researchers from King Abdullah University of Science and Technology (KAUST) developed a flash memory device made from gallium oxide that can withstand the harsh conditions of space probes, such as radiation and high temperatures.

The device contains a layer of gallium oxide 50nm thick. Above the gallium oxide is a minuscule fragment of titanium nitride, encased in a very thin layer of insulating material, which serves as the floating gate.

To program data into the floating gate, the researchers apply a positive voltage pulse that sends electrons from the gallium oxide through the insulator and into the floating gate, where they are trapped. A negative voltage can erase the data by sending the electrons back into the gallium oxide. The location of these electrons affects how well the gallium oxide conducts electricity, which can be used to read the state of the memory device.

Thanks to gallium oxide’s wide bandgap, the prototype device could retain its data for more than 80 minutes.

Currently, programming and erasing the device requires relatively long voltage pulses of about 100 milliseconds, which the team hopes to shorten. “Further development in gallium oxide material quality and device design will give better memory properties for practical extreme-environment applications,” said Xiaohang Li, an associate professor of electrical and computer engineering at KAUST.

Khandelwal, V., Rajbhar, M.K., García, G.I.M., Tang, X., Sarkar, B. & Li, X. Demonstration of β-Ga2O3 nonvolatile flash memory for oxide electronics. Japanese Journal of Applied Physics 62, 060902 (2023).

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