Al-ScN Foils: High Temperature Stability for Next-Generation Memory Devices

Imagine a thin layer, just nanometers thick, that could store gigabytes of data—enough for movies, video games, and movies. That’s the exciting potential of ferroelectric materials for memory storage. These materials have a unique arrangement of ions that results in two distinct polarization states, analogous to 0 and 1 in binary code, that can be used to store digital memory. These states are stable, meaning they can “remember” data without power, and can be efficiently switched by applying a small electric field. This property makes them extremely energy-efficient and capable of fast read and write speeds. However, some well-known ferroelectric materials, such as Pb(Zr,Ti)O₃ (PZT) and SrBi₂This₂ABOUT₉degrade and lose polarity when heat treated with hydrogen during production.

In a study published in the journal Applied Physics Letters, a research team led by Assistant Professor Kazuki Okamoto and Hiroshi Funakubo of Tokyo Institute of Technology (Tokyo Tech), in collaboration with Canon ANELVA Corporation and the Japan Synchrotron Radiation Research Institute (JASRI), has shown that ferroelectric aluminum scandium nitride (AlScN) films remain stable and retain their ferroelectric properties at temperatures up to 600°C.

“Our results confirm the high stability of ferroelectricity of films heat-treated in a hydrogen atmosphere, regardless of the electrode material. This is a very promising result for next-generation ferroelectric memory devices and offers more processing options,” Funakubo says.

For ferroelectric materials to be compatible with high-temperature manufacturing processes under H₂-switched atmosphere, ideally they should experience little or no degradation in their crystal structure and ferroelectric properties. Two key parameters in this regard are the residual polarization (P_R) and the coercive field (E_C).P_R refers to the polarization retained after the electric field is removed, while Ec is the electric field required to change the polarization state of the material. AlScN has a higher P_R (>100µC/cm²) than PZT (30-50 µC/cm²). However, the effect of heat treatment in an H atmosphere₂-the properties of the atmosphere contained in it have not been clear until now.

To investigate this, scientists deposited (Al_0.8Sc_0.2)N film on a silicon substrate using sputtering at 400°C. The films are sandwiched between two electrodes of platinum (Pt) and titanium nitride (TiN). The electrodes play a key role in the stability of the material. Pt helps incorporate hydrogen gas into the film, while TiN acts as a barrier to H₂ diffusion. ​​Therefore, evaluating its performance using different electrode materials is crucial.

The films were heat treated in a hydrogen and argon atmosphere for 30 min at temperatures ranging from 400 to 600 °C at 800 Torr. The researchers used X-ray diffraction (XRD) to study changes in the crystal structure in the bulk and at the film-electrode interface. Positive-up-negative-down (PUND) measurements were used to assess P_R and E_CThis technique involves applying positive and negative electric fields to the film and observing the resulting polarization reaction.

The films retained a stable wurtzite-type crystal structure._R remained stable above 120 µC/cm²regardless of the electrode or treatment atmosphere, a value five times greater than HfO₂-based on films and three times larger than PZT. In addition, E_C increased only slightly by about 9%. This increase was attributed to changes in the lattice constant of the film, rather than to the presence of hydrogen or the choice of electrode used. Remarkably, in contrast to other ferroelectric materials susceptible to hydrogen diffusion, the high binding energy between Al and N prevents hydrogen from penetrating the film.

“The results show that (Al_0.8Sc_0.2)N is much more resistant to degradation by heat treatment than conventional ferroelectrics and HfO₂-based on ferroelectric films,” says Funakubo. With a relatively stable crystal structure, high P_R value and a slight change in E_C(Al,Sc)N layers are a promising candidate for next-generation ferroelectric memory devices.