In a ferroelectric tunnel junction based on hafnium oxide, information is written by applying an external electric field, which changes the direction of the ferroelectric's polarization vector, and this in turn changes the shape of the potential barrier. Information is read by measuring the tunnel current, which depends on the shape of the barrier.Scientists from the Moscow Institute of Physics and Technology (MIPT) in Russia have succeeded in growing ultra-thin ferroelectric films, just 2.5nm thick, based on hafnium oxide that could potentially be used to develop non-volatile memory elements called ferroelectric tunnel junctions. This work is reported in a paper in ACS Applied Materials & Interfaces.
Scientists all over the world are trying to develop faster and more compact information storage devices. The ideal would be a ‘universal’ memory device with the speed of RAM, the capacity of a hard drive and the non-volatility of a flash drive, allowing it to store information even in the event of a power outage.
There are many known principles that can be used to build computer memory, but each one has its drawbacks. This is why modern computers and mobile devices use multiple types of memory.
Non-volatile memory based on ferroelectric tunnel junctions is a promising development that has not yet been fully implemented. A ferroelectric is a material that is able to ‘remember’ the direction of an externally-applied electric field by the residual polarization charge.
Thin-film ferroelectrics have for a long time been used to make non-volatile memory devices, but it is extremely difficult to miniaturize them in order to achieve a high density and thus a high storage capacity. In addition, they are made of materials that are ‘incompatible’ with the production processes used in modern microelectronics.
About 10 years ago, after ferroelectric properties were demonstrated in ultra-thin single-crystal films of perovskites, an alternative concept for memory devices was proposed – based on the use of the tunnel effect.
Ferroelectrics are mainly insulators and do not conduct electricity. However, if the ferroelectric layer is very thin, electrons can ‘slip’ through with a certain probability, thanks to the quantum tunneling effect. The probability of tunneling depends on the size and shape of the potential barrier (the energy characteristics of the structure), with the electrons that manage to ‘pass through’ creating a tunnel current.
The movement of the electrons is similar to an obstacle course race, and the size of the obstacle is determined by the direction of the polarization vector, which alters the shape of the potential barrier. In ferroelectric tunnel junctions, information is written by applying a voltage to electrodes bordering the ultra-thin ferroelectric, and it is read by measuring the tunneling current.
In theory, this type of memory should have an extremely high density, fast reading and writing speeds, and a low level of power consumption. It could become a non-volatile alternative to DRAM (dynamic random-access memory). Modern DRAM has a short retention time (approximately 0.1 seconds), after which the data is either lost or overwritten, and using this type of memory requires a large amount of power. Memory based on ferroelectric tunnel junctions will be able to save power, which is especially important for portable devices that run on batteries. However, until now, all prototypes of devices based on conventional ferroelectrics have not been compatible with the silicon-based technology used in the production of most modern computer chips.
The team of researchers from MIPT's Laboratory of Functional Materials and Devices for Nanoelectronics, together with colleagues from the University of Nebraska in the US and the University of Lausanne in Switzerland, have now for the first time developed polycrystalline alloyed films of hafnium oxide that can retain their ferroelectric properties on a silicon substrate. The films were also grown by atomic layer deposition (ALD), which is widely used today in the production of modern microprocessors; one of its particular benefits is that it allows functional layers to be grown in three-dimensional structures.
"Since the structures of this material are compatible with silicon technology, we can expect that new non-volatile memory devices with ferroelectric polycrystalline layers of hafnium oxide will be able to be built directly onto silicon in the near future," says Andrei Zenkevich, corresponding author of the paper and head of the Laboratory of Functional Materials and Devices for Nanoelectronics.
Furthermore, once ferroelectric tunnel junctions based on hafnium oxide are developed, they might also be able to demonstrate memristor properties. This is a prerequisite for creating electronic synapses, which could be used in neuromorphic systems that are currently being developed. These systems have a fundamentally different computing architecture that simulates the functioning principles of the human brain.
This story is adapted from material from the Moscow Institute of Physics and Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.