New Type of Hyper-Efficient Synapse-like Computer Memory Design

A new design for computer memory that could both greatly improve performance and reduce the energy demands of internet and communications technologies, which are predicted to consume nearly a third of global electricity within the next ten years.

“A typical USB stick based on continuous range would be able to hold between ten and 100 times more information, for example,” said Hellenbrand.

Researchers, led by the University of Cambridge, developed a device that processes data in a similar way as the synapses in the human brain. The devices are based on hafnium oxide, a material already used in the semiconductor industry, and tiny self-assembled barriers, which can be raised or lowered to allow electrons to pass.

This method of changing the electrical resistance in computer memory devices, and allowing information processing and memory to exist in the same place, could lead to the development of computer memory devices with far greater density, higher performance and lower energy consumption.

Note: seems like planar memory could give serious competition to 3D memory.

One potential solution to the problem of inefficient computer memory is a new type of technology known as resistive switching memory. Conventional memory devices are capable of two states: one or zero. A functioning resistive switching memory device however, would be capable of a continuous range of states – computer memory devices based on this principle would be capable of far greater density and speed.

Hellenbrand and his colleagues developed a prototype device based on hafnium oxide, an insulating material that is already used in the semiconductor industry. The issue with using this material for resistive switching memory applications is known as the uniformity problem. At the atomic level, hafnium oxide has no structure, with the hafnium and oxygen atoms randomly mixed, making it challenging to use for memory applications.

However, the researchers 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 Hellenbrand.

Unlike other composite materials, which require expensive high-temperature manufacturing methods, these hafnium oxide composites self-assemble at low temperatures. The composite material showed high levels of performance and uniformity, making them highly promising for next-generation memory applications.

A patent on the technology has been filed by Cambridge Enterprise, the University’s commercialization arm.

“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,” said Hellenbrand.

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. Since hafnium oxide is a material already used in the semiconductor industry, the researchers say it would not be difficult to integrate into existing manufacturing processes.

Markus Hellenbrand et al. ‘Thin-film design of amorphous hafnium oxide nanocomposites enabling strong interfacial resistive switching uniformity.’ Science Advances (2023).

Abstract
A design concept of phase-separated amorphous nanocomposite thin films is presented that realizes interfacial resistive switching (RS) in hafnium oxide–based devices. The films are formed by incorporating an average of 7% Ba into hafnium oxide during pulsed laser deposition at temperatures ≤400°C. The added Ba prevents the films from crystallizing and leads to ∼20-nm-thin films consisting of an amorphous HfOx host matrix interspersed with ∼2-nm-wide, ∼5-to-10-nm-pitch Ba-rich amorphous nanocolumns penetrating approximately two-thirds through the films. This restricts the RS to an interfacial Schottky-like energy barrier whose magnitude is tuned by ionic migration under an applied electric field. Resulting devices achieve stable cycle-to-cycle, device-to-device, and sample-to-sample reproducibility with a measured switching endurance of ≥104 cycles for a memory window ≥10 at switching voltages of ±2 V. Each device can be set to multiple intermediate resistance states, which enables synaptic spike-timing–dependent plasticity. The presented concept unlocks additional design variables for RS devices.

Devices and electrical performance
Ba:HfOx thin films were deposited directly on electrically conductive single-crystal (001)-oriented Nb-doped strontium titanate (Nb:STO) substrates with a supplier-provided resistivity ρ ≈ 5.5 milliohm·cmat 300 K. Nb:STO was chosen as the initial substrate to study our thin-film design concept with a ‘clean’ (conductive, no native oxides, minimal surface roughness) reference bottom electrode (BE). While a commercial process for the integration of STO on 200-mm Si wafers has been demonstrated (24), Nb:STO is typically not regarded as an industry-compatible electrode.