The 70th Japan Society of Applied Physics Spring Meeting 2023
Highlighted Presentations Press Release
April 18, 2023

Development of an operando laser-based photoemission electron microscope capable of characterizing
ferroelectric properties

Established a new method for observing and evaluating
ferroelectric properties of next-generation memorymaterial, such as hafnia-based ferroelectrics
ISSP, Univ. of Tokyo 1, IIS, Univ. of Tokyo 2, d. lab, Univ. of Tokyo3, Graduate school of Frontier Sciences, Univ. of Tokyo4,MIRC, Univ. of Tokyo5, Office of University Professor, Univ. of Tokyo 6Hirokazu Fujiwara1 , Yuki Itoya2 , Masaharu Kobayashi3 , Cédric Bareille4,5 , Shin Shik5,6 , Toshiyuki Taniuchi 4,5E-mail: hfujiwara@issp.u-tokyo.ac.jp

Summary• Established an evaluation method based on directly observing devices in operation toevaluate their ferroelectric properties, which is essential for applying hafnia-basedmaterial, one of the most promising candidates for next-generation memory material.
• This is the first report on ferroelectric material characterization using an operando laserbased photoemission electron microscope (laser-PEEM) system with a Sawyer-Tower (ST)circuit.
In 2011, a German research group reported that a thin film of hafnium dioxide (HfO2), one of the most promising
candidates for next-generation memory material, show ferroelectricity, and a research team led by Hirokazu Fujiwara at
the Institute for Solid State Physics in the University of Tokyo has been working on the research and development ofobservation techniques to characterize its properties. In particular, they have been developing an observation systemusing operando laser-based photoemission electron microscope (laser-PEEM), which enables the direct observation ofcharacteristic modulations. In this study, a Sawyer-Tower (ST) circuit was implemented in a laser-PEEM to evaluate thecharacteristics of ferroelectric devices during operation, and the world's first ferroelectric property evaluation wassuccessfully demonstrated. This technology contributes to establishing observation and evaluation techniques that areuseful for clarifying the mechanism of characteristic modulations, which is indispensable for the practical application ofhafnia-based ferroelectrics.

Details

Hafnia-based ferroelectrics, a new material for next-generation memory
A German research group reported the ferroelectricity of a thin film of hafnium dioxide (HfO2) in2011; the film thickness is about 10 nm and has attracted much attention. Thus, research anddevelopment of "hafnia-based ferroelectrics" (*1) have been active as a new material for nextgeneration memories.
Applying voltage to ferroelectric material produces macroscopic "polarization", which remains evenwhen the voltage is reduced to zero. Ferroelectric random-access memory (FeRAM) is anapplication of this physical storage media feature. Among ferroelectric memories, "perovskite-typeoxide ferroelectrics," which use inorganic oxides with perovskite structures (*2) are currently themost used for practical applications. They are widely used in transportation IC cards owing to their
low-voltage operation and high-speed response.
Due to features such as low-voltage drive and high-speed response performance, ferroelectric
memory is expected to be a technology that will contribute to lower power consumption andgreater computing efficiency through artificial intelligence if it can replace DRAM (*3), the main
memory of computers. However, perovskite-type oxide ferroelectrics in practical use have thedisadvantage of not being able to retain their ferroelectricity when miniaturized. Thus, it is not easy
to apply them to large-scale integrated circuits, such as smartphones, where miniaturization isprogressing. However, the new Hafnia-based ferroelectric material can exhibit stableferroelectricity even in thin films of 10 nm or less, and if applied, it can replace the main memory incomputers.
However, hafnia-based ferroelectrics have disadvantages, such as durability. By 105 –1010 times ofwrite/erase operations, after the polarization is modulated, eventually it causes dielectric
breakdown. When this breakdown occurs, the memory function is no longer retained. DRAM used
as a working memory must withstand information rewriting 1015 times or more; however, how theamount of ferroelectricity polarization changes owing to repeated write/erase operations(information rewriting), leading to dielectric breakdowns, remains unresolved from a physicalstandpoint.
Therefore, fundamental physical studies are required to elucidate the mechanism of characteristic
modulations to improve the performance of hafnia-based ferroelectrics. Hirokazu Fujiwara and his
research team at the University of Tokyo implemented a "Sawyer-Tower (ST) circuit" (*5) in alaser-based photoemission electron microscope (laser-PEEM*4) and performed characterization of
the ferroelectric devices in operation. This technology contributes to establishing observation andevaluation techniques helpful in elucidating the mechanisms of characteristic modulations in
hafnia-based ferroelectrics.
Enables the advanced observation of devices and catalysts in operation
In order to investigate the mechanism of characteristic modulations in hafnia-based ferroelectrics,
it is important to directly observe how the characteristics are modulated in an operating device, orperform 'operando observation'. Our research team has achieved this observation by utilizing'operando laser-PEEM,' says Fujiwara.
The word "operando" means "working" or "operating" in Latin, and operando observation is atechnique for directly observing a device or catalyst in operation.

Microscopes are used for the direct observation of devices. Optical microscopes have excellent
physical properties and depth of detection (several micrometers) but low resolution (~500 nm).
Electron microscopes have moderate observation of physical properties and high resolution (>1nm) but have a problem with the detection depth (<10 nm). Fujiwara et al. used PEEM, which
combines optical and electron microscopy.
PEEM uses an imaging technique to magnify and image electrons emitted by the photoelectriceffect (a phenomenon in which electrons are ejected from the material when light is exposed on
the material surface) caused by irradiating the material with an ultraviolet (UV) laser. Unlike otherelectron microscopes, this technique is sensitive to the electronic and chemical states of thematerial. Furthermore, it has the advantage of nondestructive observation of films embedded
under electrodes and other materials, making it ideal for observing ferroelectric material.
Conventional PEEMs have a resolution of only 20–50 nm, making them unsuitable for research on
semiconductor devices in which advanced microfabrication is applied. However, the cutting-edge
PEEM at the Institute for Solid State Physics in the University of Tokyo, which uses a deepultraviolet continuous-wave (CW) laser (*6) as its light source, can achieve the world's highest
resolution 2.6 nm, thereby enabling research on semiconductor devices.
The group of Fujiwara et al. utilized this operando laser-PEEM to report operando observations ofresistance change memory in 2018 and the dielectric breakdown process observations of hafniabased ferroelectric capacitors in 2022.
Operando observation under AC voltage, the key to ferroelectricity evaluation, is realized.
“Previous studies used operando laser-PEEM to characterize physical properties based on DCvoltage. In this study, we developed the AC-voltage-based system. The novelty is that we can now
quantitatively evaluate the amount of device polarization, which is important in ferroelectricity
evaluation based on capacitance and inductance.,” says Fujiwara.
Although the electrical resistance can be mainly evaluated under DC voltage, the actual amount offerroelectric polarization cannot be fully evaluated only from the electrical resistance. Fujiwara's
research group implemented the "Sawyer-Tower (ST) circuit" in an operando laser-PEEM system.
It succeeded for the first time in the world in evaluating electrical polarization while applying 1 MHzclass cycling stress (voltage application that reverses polarization many times) by AC –voltageapplication system (Figure).
“The strengths of the observation and evaluation methods we have established thus are that they
enable us to get closer to the mechanisms of characteristic modulations that lead to the dielectric
breakdown of ferroelectric material. In the future, we would like to elucidate the chemical changes
that cause characteristic modulations in hafnia-based ferroelectric capacitors, particularly
decrease of polarization. In the future, I would like also to expand my research into themechanisms of characteristic modulations in various ferroelectric memory devices and contributeto their practical applications.” says, Fujiwara.
The research group will continue its research to characterize "ferroelectric field effect transistors(FeFETs)" and "ferroelectric tunnel junctions (FTJs)," which are expected to be used as nonvolatile memories in the future.

Annotation*1 Hafnia-based ferroelectrics: Inorganic compounds with a basic composition of hafnium oxide (HfO2 ) are known for
their ferroelectricity in thin films with a thickness of approximately 10 nm.
*2 Perovskite structures: A type of crystal structure is found in inorganic compounds with an ABX3 composition. BaTiO3and PbTiO3, which exhibit ferroelectricity, also exhibit perovskite structures.
*3 Dynamic random-access memory (DRAM): A type of read/write semiconductor memory, used in many modern PCs
as main memory.
*4 Laser-PEEM / Laser-based photoemission electron microscope: An electron projection microscope that performs
imaging by magnifying and imaging the electrons (photoelectrons) emitted by the photoelectric effect from the object
under observation when irradiated with light having energy greater than or equal to the work function.
5 Sawyer-Tower (ST) circuit: A circuit that generates electric field hysteresis, which is necessary to evaluate
ferroelectricity in ferroelectric materials
6 Deep ultraviolet continuous wave (CW) laser: A laser light source using deep ultraviolet light. Because there is no
instantaneous photoelectron excitation as in conventional pulsed light sources, "Coulomb repulsion" between excited
photoelectrons can be suppressed and high resolution can be obtained.

Figures

Field cycling characteristics of a 100 µm x 100 µm capacitor with 10 nm thick Hf0.5Zr0.5ZO2 were observed without
applying high voltage between the sample and the objective lens. (a) P-V hysteresis loop obtained by the PUND method;(b) extracted changes in 2Pr due to electric field cycling.