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RESEARCHERS have discovered that epitaxial hafnium-oxide-based thin films maintain a stable ferroelectric phase up to 450 degrees C, allowing their use in next-generation electronics devices.
Ferroelectric materials have applications in next-generation electronics devices from optoelectronic modulators and random access memory to piezoelectric transducers and tunnel junctions.
With a stable ferroelectric phase up to 450 degrees, the researchers at Tokyo Institute of Technology say HfO2-based ferroelectric materials would be able to deliver stable device operation and processing.
Reports of ferroelectric properties in thin films of substituted hafnium-oxide – where some ions were replaced with other metals – have attracted particular interest because these films are compatible with the silicon fabrication techniques that dominate the industry.
HfO2 is considered a prime candidate for use in components such as dynamic random-access memory (DRAM) capacitors, thanks to its dielectric constant. In fact, they are already used for high-κ gates in devices.
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, the Tokyo Institute of Technology researchers created the HfO2 via 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. They then found that the orthorhombic phase is ferroelectric and stable for temperatures up to 450°C.