In response to an external electric field, 'ferroelectric' materials tend to switch between different states of electrical polarization. This flexibility indicates that they hold promise for several applications. including computer memory and electronic devices.
Existing ferroelectric materials are highly valued for their chemical and thermal stability and fast electro-mechanical responses, but developing a material that can be scaled down to the miniature sizes required for technologies such as silicon-based semiconductors (Si-based CMOS) has been quite challenging.
Recently, Hiroshi Funakubo and co-workers at the Tokyo Institute of Technology, in partnership with researchers from many parts of Japan, have performed experiments to establish the ferroelectric properties of an inorganic compound known as hafnium oxide (HfO2) for the first time.
HfO2 can be deposited in ultra-thin films because of its crystal structure. This may be invaluable for next-generation technologies.
Ferroelectric properties stem from the structure and shape of the crystal used. The team was aware that an 'orthorhombic' crystal of HfO2 would probably display ferroelectricity.
Funakubo's team wanted to identify the material's Curie temperature and spontaneous polarization. To achieve this, they had to grow a cautiously-ordered crystal on a substrate using a process called epitaxy, which would provide them with definite data on an atomic scale.
The researchers discovered that one particular epitaxial film, named YHO-7, displayed ferroelectricity a Curie temperature of 450°C and with a spontaneous polarization of 45 μC/cm.
The results of the experiments validate previous predictions using first principle calculations.
From an industrial and scientific standpoint, a Curie temperature of 450°C is very appealing as it would means that the material can accomplish functions for future technologies. In contrast to several current ferroelectric materials, the new thin-film displays compatibility with Si-based CMOS and is robust in tiny forms.
Ferroelectric materials are different from other materials as their polarization can be reversed using an external electric field applied in the opposite direction to the current polarization. This property is possible due to the specific crystal structure of the material.
Ferroelectric materials are extremely valuable for advanced electronics. The science world is aware of many ferroelectric materials and some of them are already applied in various applications. However their crystal structure does not allow them to be scaled down to a small enough, ultra-thin film for use in miniaturized gadgets.
HfO2 used by Funakubo and collaborators had previously been predicted to display ferroelectric properties via first principle calculations.
However, no research team conducted any experiments to verify and analyze these predictions. Funakubo's team worked at measuring the material’s properties when it was deposited in thin-film crystal form onto a substrate. The crystal structure’s precise nature enabled the team to identify the full properties of the material for the first time.
Their detection of a specific epitaxial thin-film crystal of HfO2 that is capable of ferroelectricity below 450°C will be of huge importance in the field.
Implications of the current study
Funakubo's team hope that their new thin film ferroelectric material will be applied in novel random-access memory and transistors as well as in quantum computing. Their material is the first ferroelectric material that is well-matched with Si-based CMOS.