Power/Performance Bits: Nov. 16

Light-emitting memory; random number generation; finding ferroelectric materials.


Light-emitting memory
Researchers from Kyushu University and National Taiwan Normal University propose a ‘light-emitting memory’ based on a perovskite that can simultaneously store and visually transmit data. The team used the idea in conjunction with resistive RAM (RRAM), in which states of high and low resistance represent ones and zeros.

“The electrical measurements needed to check the resistance and read zeros and ones from RRAM can limit the overall speed,” said Chun-Chieh Chang, professor at National Taiwan Normal University. “Recently, to overcome this issue, RRAMs have been combined with LEDs to develop something called light-emitting memories. In this case, the data can also be read by checking if the LED is on or off. This additional optical reading also opens new routes for carrying large amounts of information.”

However, manufacturing such a device has proven difficult. The team turned to a perovskite consisting of cesium lead bromide (CsPbBr3) and showed that data can be electrically written, erased, and read in one of the perovskite devices acting as an RRAM. Simultaneously, the second perovskite device can optically transmit whether data is being written or erased through light emission by working as a light-emitting electrochemical cell with a high transmission speed.

“Using just one perovskite layer between contacts, we could fabricate a device that works both as a RRAM and a light-emitting electrochemical cell,” explains National Taiwan Normal University’s Ya-Ju Lee. “By taking advantage of the fast, electrically switchable ionic motion that enables this dual functionality in a single layer of perovskite, we were able to connect two devices together and develop an all-inorganic perovskite light-emitting memory.”

A scanning electron microscope image of the light emitting memory, with each key layer labeled. (Credit: Kyushu University / Ya-Ju Lee)

Additionally, the researchers used perovskite quantum dots of two different sizes for the two devices in the light-emitting memory to achieve different emission colors depending on whether the memory was being written or erased, providing a real-time indicator of the ones and zeros.

“This demonstration significantly broadens the scope of applications of the developed all-perovskite light-emitting memory and can serve as a new paradigm of synergistic combination between electronic and photonic degrees of freedom in perovskite materials,” said Kaoru Tamada, a distinguished professor at Kyushu University’s Institute for Materials Chemistry and Engineering. “From multicast mesh network to data encryption systems, these findings have the potential for numerous applications in next-generation technologies.”

Random number generation
Researchers from King Abdullah University of Science and Technology (KAUST), Soochow University, Università di Modena e Reggio Emilia, Imec, Catalan Institute of Nanoscience and Nanotechnology, Universidad de Granada, ShanghaiTech University, Stanford University, Universitat de Barcelona, and Israel Institute of Technology propose using memristors as a random number generator.

“Memristors are meta/insulator/metal nanocells based on two-dimensional materials that have fast operation speed, low energy consumption and very long endurance and data retention time, as well as being very easy and cheap to fabricate,” said Mario Lanza of KAUST. “For this reason, memristors are being intensively explored for applications such as high-density electronic memories. They are also particularly useful for encryption systems because they can produce fluctuating electronic signals with an extraordinarily high degree of randomness.”

Memristors produce a type of electrical noise called random telegraphic noise (RTN) which can be used for random number generation. The team sought to design and fabricate a memristor device that has stable RTN over time.

“The main challenge was that the atomic structure of the resistive thin film degrades over time, which causes the RTN signal to disappear,” said Lanza. “In our devices, we used two-dimensional multilayer hexagonal boron nitride, which is a two-dimensional material that has a very stable atomic structure and is immune to this effect.”

The team fabricated hundreds of devices using industry-compatible methods and characterized them using a range of techniques including a randomness test involving the generation of one-time passwords.

“A key aspect of our work was the use of fabrication processes that are compatible with industry, which facilitates integration in commercial products,” said Lanza. “We also presented yield and variability information for hundreds of devices; it was a tremendous effort, but it gives more reliability to our study.”

Finding ferroelectric materials
Researchers from Pennsylvania State University demonstrated ferroelectricity in magnesium-substituted zinc oxide.

Ferroelectric materials possess a spontaneous electric polarization as the result of shifts of negative and positive charges within the material that can be reoriented via the application of an external electric field. They could be useful for data storage and memory because they remain in one polarized state without additional power.

“We’ve identified a new family of materials from which we can make tiny capacitors and we can set their polarization orientation so that their surface charge is either plus or minus,” said Jon-Paul Maria, a professor of materials science and engineering at Penn State. “That setting is nonvolatile, meaning we can set the capacitor to plus, and it stays plus, we can set it to minus, it stays minus. And then we can come back and identify how we set that capacitor, at say an hour ago.”

The new materials are made with magnesium-substituted zinc oxide thin films. The film was grown via sputter deposition, a process where argon ions are accelerated towards the target materials, impacting it with a high enough energy to break atoms free from the target that contains magnesium and zinc. The freed magnesium and zinc atoms travel in a vapor phase until they react with oxygen and collect on a platinum-coated aluminum oxide substrate and form the thin films.

“This type of storage requires no additional energy,” Maria said. “And that’s important because many of the computer memories that we use today require additional electricity to sustain the information, and we use a substantial amount of the American energy budget on information.”

“Generally speaking, ferroelectricity often occurs in minerals that are complicated from a structure and chemistry point of view,” Maria said. “And our team proposed the idea about two years ago, that there are other simpler crystals in which this useful phenomenon could be identified, as there were some clues that made us propose this possibility. To say ‘ferroelectrics everywhere’ is a bit of a play on words, but it captures the idea that there were materials around us that were giving us hints, and we were ignoring those hints for a long time.”

Additionally, the magnesium-substituted zinc oxide thin films can be deposited at much lower temperatures than other ferroelectric materials.

“The overwhelming majority of electronic materials are prepared with the assistance of high temperatures, and high temperatures means anywhere from 300 to 1000 degrees Celsius (572 to 1835 degrees Fahrenheit),” said Maria. “Whenever you make materials at elevated temperatures, it comes with a lot of difficulties. They tend to be engineering difficulties, but nonetheless they make everything more challenging. Consider that every capacitor needs two electrical contacts — if I prepare my ferroelectric layer at high temperatures on at least one of these contacts, at some point an unwanted chemical reaction will occur. So, when you can make things at low temperatures, you can integrate them much more easily.”

The researchers plan to continue work on the material, making it into capacitors and assessing reliability and manufacturability.

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