Hafnium oxide material could store non-volatile memory

Non-volatile memory based on ferroelectric tunnel junctions is a promising future development. A ferroelectric is a material can remember the direction of an externally applied electric field by the residual polarisation charge.

Thin-film ferroelectrics have for a long time been used to make non-volatile memory devices, but it is extremely difficult to miniaturise them in order to achieve high density storage capacity; in addition to this, 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 generally 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), and the electrons that pass through create a tunnel current.

The movement of the electrons in this case is similar to an obstacle course race, and the size of the obstacle is determined by the direction of the polarisation vector, which alters the shape of the potential barrier. Information is written by applying a voltage to the electrodes next to the ultra-thin ferroelectric, and it is read by measuring the tunnelling current.

In theory, this type of memory may 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. Using this type of memory requires a large amount of power. Memory based on ferroelectric tunnel junctions could 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 been incompatible with silicon technology, which is used in the production of most modern chips.

The team of researchers from MIPT's Laboratory of Functional Materials and Devices for Nanoelectronics and their collaborators have for the first time experimentally demonstrated that polycrystalline alloyed films of hafnium and zirconium oxides with a thickness of just 2.5 nm retain their ferroelectric properties.

While hafnium oxide is already used in the production of modern silicon logic chips, a few years ago, ferroelectric properties were discovered in one of its modifications. In their study, the scientists from MIPT succeeded in growing an ultra-thin, tunnel-transparent film of this material on a silicon substrate, while maintaining the ferroelectric properties.

It is important to note that the the film was grown by Atomic Layer Deposition (ALD) technique. which is widely used today in the production of modern microprocessors, and one of its particular benefits is that it enables functional layers to be grown in three-dimensional structures.

Furthermore, if ferroelectric tunnel junctions based on hafnium oxide are developed, it can be hoped that they will 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.