An important Research breakthrough, enabled by the Royce Equipment Access Scheme, and use of its unique Hard X-ray Photoelectron Spectroscopy (HAXPES) facility has been published in the prestigious journal Advanced Materials (Wiley).
The research is set out in a new paper which details the important work on ferroelectricity in nanoscale hafnia films, using HAXPES to elucidate the relationship between polarization (memory applications) and electrochemical state, which is a significant research advancement. HAXPES, which uses high energy X-rays to measure the chemical composition in materials non-destructively, was the key characterisation technique that enabled this, using both the Royce lab-based instrument as well as synchroton radiation.
Ferroelectrics, owing to their electrically switchable spontaneous polarization, are prospective building blocks in a wide array of electronic and optical applications. In the last decade, the discovery of ferroelectricity in nanoscale CMOS‐compatible, hafnia‐based films has led to considerable interest in this material for high‐performance ferroelectric‐based memory and computing devices
Hafnium (IV) oxide (HfO2) thin films have been utilised in the semiconductor industry as a high-k dielectric for decades. In 2011, the discovery of ferroelectricity in this industry-compatible film escalated HfO2 research for numerous ferroelectric devices such as ferroelectric memory and ferroelectric field effect transistors. However, researchers have yet to fully understand the ferroelectric nature of HfO2, and therefore controlling it for industrial applications is a challenge.
Numerous publications have connected ferroelectricity in HfO2 to the migration of oxygen vacancies within the material, which if true, by its nature, would limit device performance. However, this research provides evidence of intrinsic ferroelectricity in HfO2, with oxygen migration only occurring alongside, not directly being linked to, ferroelectric switching. This gives evidence that HfO2 is still a promising candidate for ferroelectric devices suitable for industrial scalability.
The Process
The research team used a unique comparison of a traditional hafnia-based ferroelectric (HZO) and a newly developed ferroelectric stoichiometry (HLTO). Films were integrated into identical device stacks, which was critical for directly comparing the observed electrochemical behaviours and the resultant ferroelectric performance. They discovered that hafnia-based films with higher polarisation exhibited less electrochemical changes in a single-cycle measurement, evidencing the intrinsic nature of ferroelectricity.
A unique approach was taken to study the effect of the ferroelectric switching – large regions (~300 µm x 300 µm) were poled using piezoresponse force microscopy (PFM) which allowed the team to compare up and down polarized regions of the film without making top electrodes that would block a significant portion of the deeper photoelectrons.
Films of Hf0.88La0.04Ta0.08O2 (HLTO) show significantly less electrochemical modification upon electric field switching as compared to traditional Hf0.5Zr0.5O2 (HZO). This occurs despite the fact that HLTO presents a higher remnant polarization (Pr) than HZO. Specifically, the researchers observed no redox process in either the ferroelectric layer or the bottom electrode for HLTO (Pr = 6 μC cm−2), while we do observe redox in the HZO film (Pr = 4 μC cm−2).
Results
The research contradicts the expectation that a larger electrochemical modification would occur for a film having larger polarisation, suggesting that ferroelectric reversal is not the sole cause of oxygen reorganisation.
Consequently, the researchers conclude that changes in oxygen electrochemistry within the device occur adjacent to ferroelectric switching as an effect of the electric field. They found that the electric field drives oxygen reorganization, with the material chemistry and charge state, as well as the device structure, determining the resulting changes.
Megan O. Hill, Beamline Scientist, NanoMAX beamline, MAX IV Laboratory said:
“After first collecting HAXPES data on the beamline at Diamond Light Source, we realized that interpretation of the data would not be possible without a reference measurement on HZO. Instead of waiting over six months for additional beamtime, we performed these measurements at the laboratory-based HAXPES at Royce at the University of Manchester.
“This provided us the needed comparison of HZO, which showed significantly different electrochemical behaviour from our HLTO film. While the data collected from the Synchrotron was faster, the lab-based HAXPES provides sufficiently high quality of data to directly compare our HZO and HLTO films. Further, this research could not be completed without the additional HZO data, and therefore faster and easier access to the lab-based HAXPES was critical to our work.”
Dr. Ben Spencer Senior Technical Specialist: Faculty lead in Surface Characterisation at the University of Manchester said:
“The use of the Royce HAXPES facility to provide new evidence that ferroelectricity in HfO2 is not directly dependent on oxygen reorganization and redox processes, as previously suggested, is another great example of its capability.
“We are proud to have worked closely with the researchers of this paper and to demonstrate that the HAXPES Lab can handle a wide range of experiments and measure a wide range of important materials.”
Megan Hill will present this work at an upcoming industrial showcase for the HAXPES Lab on October 23rd, for more information: Royce Industrial Showcase | Applications of Hard X-ray Photoelectron Spectroscopy – Henry Royce Institute Participants are welcome in person (Registration deadline 16/10) as well as online (Registration deadline 22/10).
More information: Megan O. Hill, Ji Soo Kim, Moritz L. Müller, Dibya Phuyal , Sunil Taper, Manisha Bansal, Maximilian T. Becker, Babak Bakhit, Tuhin Maity, Bartomeu Monserrat, Giuliana Di Martino, Nives Strkalj, Judith L. MacManus‐Driscoll
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