Can you imagine controlling the properties of a material by just shining light on it? We are used to see that the temperature of materials increases when exposed to the sun. But light may also have subtler effects. Indeed, light photons can create pairs of free charge carriers in otherwise insulating materials. This is the basic principle of the photovoltaic panels we use to harvest electrical energy from sun.
In a new twist, a light-induced change of materials’ properties could be used in memory devices, allowing more efficient storage of information and faster access and computing.
This, in fact, is one of our society’s current challenge: being able to develop high-performance commercially available electronic devices which are, at the same time, energy efficient. Smaller electronic devices having lower energy consumption and high performance and versatility
Non-volatile memory storage
Now, researchers from the Multifunctional Thin Films and Complex Structures (MULFOX) group at ICMAB have studied photoresponsive ferroelectric materials integrated in devices exploiting nanotechnologies and quantum effects. Memory elements have been engineered to store non-volatile information in distinct resistance states (ON/OFF). It has been discovered that, when properly designed, their electrical resistance can be modulated by pulsed light. This means that they can switch from a low-resistance to a high-resistance state just by the application of light pulses.
“Materials that show changes of resistance under illumination are abundant, although the effect is typically volatile and the material recovers its initial state after some dwell time” says ICMAB researcher Ignasi Fina, co-author of the study. “For devices to be used in computing and data storage, non-volatile optical control of electrical resistance is of potential interest” and adds “for non-volatile, we mean that the information can be retained stored in the device, even when the power is off”.
Two-in-one: photo-ferroelectric materials
Currently two different devices are required to use optical signals for non-volatile data storage: an optoelectronic sensor and a memory device. The ICMAB study features these properties combined in one single material able to modulate its resistance by pulsed light: a photo-ferroelectric material.
Ferroelectric materials have electrically switchable spontaneous non-volatile electric polarization. In ferroelectric ultrathin films of such material sandwiched between appropriate metals, a quantum mechanical phenomenon effect appears called the tunneling current. This effect allows a charge current flow across the ferroelectric layer, which is genuinely insulating, in an amount that depends on the direction of its polarization.
In the explored devices, first an electric field is used once to write the ON/OFF states, and it is combined with the optical stimulus to promote the ON/OFF change of states, and reversibly modulate the resistance (from high to low, and vice versa).
Energy efficient devices and applications
These devices are energy efficient for two main reasons: firstly, the energy consumption is reduced when the memory state is written, as it does not need any charge current flow. Secondly, as the information is stored in a non-volatile manner, the state is preserved and there is no need to refresh the information (re-writing) as continuously done in current RAMs memories of all computers, for example.
The observed optical switch is not restricted to the studied materials and, thus, opens a path towards further investigations on this phenomenon.
As for future applications, Ignasi Fina envisions the following: "The studied devices combine light sensor and memory functions. In addition, as shown in the study, the device behaves like a memristor. A memristor is a device that can display multiple resistance states according to the stimulus it has received, and is one of the basic devices for the development of neuromorphic computing systems. Therefore, the developed device opens a path to be explored in relation to its integration into neuromorphic vision systems, where the system learns to recognize images."
The study has been published in Nature Communications. One of the authors, Ignasi Fina, has been granted with a BBVA Foundation Leonardo Grant to further explore this topic.
Non-volatile optical switch of resistance in photoferroelectric tunnel junctions
Xiao Long, Huan Tan, Florencio Sánchez, Ignasi Fina, Josep Fontcuberta
Nature Communications, 2021
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