Unconventional piezoelectricity in ferroelectric hafnia

The effect: polarisation and electric field are pointing in the same direction. With positive d33, the sample expands,  whereas the material is contracting when d33 is negative.

The effect: polarisation and electric field are pointing in the same direction. With positive d33, the sample expands,  whereas the material is contracting when d33 is negative. © Laura Canil

PFM phase images of a W/HZO/W-capacitor. The pristine sample shows a positive piezoelectric coefficient (left). After more than 8000 cycles of an ac-electric field, the piezoelectric coefficient has changed its sign and is negative (right). The polarization is pointing downwards in both images.

PFM phase images of a W/HZO/W-capacitor. The pristine sample shows a positive piezoelectric coefficient (left). After more than 8000 cycles of an ac-electric field, the piezoelectric coefficient has changed its sign and is negative (right). The polarization is pointing downwards in both images. © HZB

Hafnium oxide thin films are a fascinating class of materials with robust ferroelectric properties in the nanometre range. While their ferroelectric behaviour is extensively studied, results on piezoelectric effects have so far remained mysterious. A new study now shows that the piezoelectricity in ferroelectric Hf0.5Zr0.5O2 thin films can be dynamically changed by electric field cycling. Another ground-breaking result is a possible occurrence of an intrinsic non-piezoelectric ferroelectric compound. These unconventional features in hafnia offer new options for use in microelectronics and information technology.

Since 2011, it has been known that certain hafnium oxides, are ferroelectric, that is, they possess a spontaneous electric polarization whose direction can be switched to the opposite one by applying an external electric field.  All ferroelectrics exhibit piezoelectricity with, most often, a positive longitudinal piezoelectric coefficient (d33). This means that the crystal expands if the applied electric field is in the same direction than the electrical polarization. However, for hafnia, studies have shown contradictory results, with different hafnia films expanding or contracting in the same experimental conditions. Moreover, it seems that the ferroelectric polarization can apparently switch against the electrical field, which was named “anomalous” switching.

An international collaboration led by Prof. Dr. Catherine Dubourdieu, HZB, has now elucidated for the first time some aspects of these mysterious results and discovered an unconventional behaviour in hafnia. They investigated Hf0.5Zr0.5O2 (HZO) capacitors using piezoresponse force microscopy (PFM): a conductive needle scans the sample surface under a small electrical voltage and measures the local piezoelectric response.

Their study revealed that piezoelectricity in HZO is not an invariable parameter but is a dynamic entity that can be changed, in the very same material, by an external stimulus such as electrical cycling. The ferroelectric HZO capacitors undergo a complete uniform inversion of the piezoelectric d33 coefficient sign, from positive to negative, upon electric field cycling. Every single location of the ferroelectric capacitor undergoes such a change passing through zero local piezoelectricity upon suitable number of ac cycles.

Density functional theory (DFT) calculations suggest that the positive d33 in the initial state is due to a metastable polar orthorhombic phase that gradually evolves, under ac cycling, towards the fully developed stable polar phase with negative d33. The DFT calculations not only suggest a mechanism for the d33 sign inversion but also predict a groundbreaking result: a possible occurrence of an intrinsic non-piezoelectric ferroelectric compound, which is observed experimentally.

"For the first time, we have been able to experimentally observe a sign inversion of the piezoelectric effect in the whole area of a capacitor with these Hafnia Zirconia ferroelectrics under applied ac electric field,” Catherine Dubourdieu states. This discovery has enormous potential for technological applications. “As the piezoelectricity in these materials can be dynamically changed and even nullified while the polarisation remains robust, we see fantastic prospects for developing ferroelectric HfO2-based devices with electromechanical functionalities. Moreover, on a fundamental standpoint, the possibility of a non-piezoelectric ferroelectric compound would revolutionize our vision of ferroelectricity." says Catherine Dubourdieu.

arö


You might also be interested in

  • BESSY II: How pulsed charging enhances the service time of batteries
    Science Highlight
    08.04.2024
    BESSY II: How pulsed charging enhances the service time of batteries
    An improved charging protocol might help lithium-ion batteries to last much longer. Charging with a high-frequency pulsed current reduces ageing effects, an international team demonstrated. The study was led by Philipp Adelhelm (HZB and Humboldt University) in collaboration with teams from the Technical University of Berlin and Aalborg University in Denmark. Experiments at the X-ray source BESSY II were particularly revealing.
  • Fuel Cells: Oxidation processes of phosphoric acid revealed by tender X-rays
    Science Highlight
    03.04.2024
    Fuel Cells: Oxidation processes of phosphoric acid revealed by tender X-rays
    The interactions between phosphoric acid and the platinum catalyst in high-temperature PEM fuel cells are more complex than previously assumed. Experiments at BESSY II with tender X-rays have decoded the multiple oxidation processes at the platinum-electrolyte interface. The results indicate that variations in humidity can influence some of these processes in order to increase the lifetime and efficiency of fuel cells. 
  • Neutron experiment at BER II reveals new spin phase in quantum materials
    Science Highlight
    18.03.2024
    Neutron experiment at BER II reveals new spin phase in quantum materials
    New states of order can arise in quantum magnetic materials under magnetic fields. An international team has now gained new insights into these special states of matter through experiments at the Berlin neutron source BER II and its High-Field Magnet. BER II served science until the end of 2019 and has since been shut down. Results from data at BER II are still being published.