Thin films offer promise for ferroelectric devices

Ferroelectric materials have applications in next-gen electronics devices from optoelectronic modulators and RAM to piezoelectric transducers and tunnel junctions. Now researchers at the Tokyo Institute of Technology report insights into the properties of epitaxial hafnium-oxide-based (HfO2-based) thin films, confirming a stable ferroelectric phase up to 450°C.

Reports of ferroelectric properties in thin films of substituted hafnium-oxide have attracted particular interest because these films are already used in electronics and are compatible with the silicon fabrication techniques that dominate the industry. However, attempts to study the crystal structure of HfO2-based thin films in detail to understand these ferroelectric properties have met with challenges due to the random orientation of the polycrystalline films.

In order to obtain thin films with a well-defined crystal orientation, Takao Shimizu, Hiroshi Funakubo and colleagues at the Tokyo Institute of Technology turned to a growth approach that had not been tried with HfO2-based materials before: epitaxial film growth. They then used a range of characterisation techniques, including x-ray diffraction analysis and wide-area reciprocal space mapping, to identify changes in the crystal structure as the yttrium content increased. They found a change from a low- to a high-symmetry phase via an interim orthorhombic phase with increasing yttrium from -15% substituted yttrium oxide. Further studies confirmed that this orthorhombic phase is ferroelectric and stable for temperatures up to 450°C.