JP2018036088A - Polarization domain visualization/observation method and device of ferroelectric substance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a visualization/observation method and device of a polarization domain of a ferroelectric substance allowing observation at a high speed and high resolution by a simple observation method.SOLUTION: Positive and negative electric fields having a magnitude that does not invert a polarization domain are generated alternately at a cycle T in a ferroelectric substance thin film. The transmitted light or reflected light of monochromatic light that is radiated to the ferroelectric substance thin film synchronously with the cycle is imaged by a two-dimensional optical detector. A difference image between one set of images imaged in a positive electric field and negative electric field is obtained. An integrated image generated from a plurality of difference images is obtained to provide a contrast, and the cycle T is shifted to a short time side and a noise removal of the difference images is performed.SELECTED DRAWING: Figure 8

Description

  The present invention relates to a method and apparatus for visualizing and observing a polarization domain of a ferroelectric, and more particularly to a method and apparatus for visualizing and observing a polarization domain from a minute optical change when an electric field is applied to a ferroelectric.

  When an electric field is applied to a ferroelectric material in the direction opposite to the polarization direction, a polarization domain appears in the same direction as the electric field inside the ferroelectric material at a certain threshold value or more. On the other hand, the domain opposite to the electric field is gradually reversed so as to minimize the electrostatic energy, and finally the polarization is reversed in the entire ferroelectric substance. In this polarization inversion phenomenon, a domain that does not invert may remain due to the influence of impurities or lattice defects in the substance. In such a case, a hysteresis loop in the electric field-polarization characteristics cannot be obtained, and the memory characteristics as a memory element are greatly affected. Therefore, there is a need for a method capable of directly observing the polarization domain in confirming the operation of the ferroelectric and analyzing the deterioration process.

  Up to now, as a method for visualizing and observing the polarization domain, observation using a microscope such as a scanning electron microscope (SEM), a piezoelectric response microscope (PFM), a second harmonic generation (SHG) microscope, or a terahertz emission microscope is used. A method has been proposed. Microscopic electric field modulation spectroscopy has also been proposed. Among these, the microscopic electric field modulation spectroscopy is intended to visualize the polarization domain from a slight change in light transmittance and reflectance caused by applying an electric field to a ferroelectric.

For example, in Patent Document 1, by applying the application of an electric field, the position of the optical absorption peak of each polarization domain in the same direction and in the opposite direction to the direction of the electric field is shifted in the opposite direction with respect to the light wavelength. Is a method for performing polarization mapping by performing visualization under a microscope. Here, it is stated that the change in reflectance is only about 10 ⁻⁶ to 10 ⁻³ due to the magnitude of the electric field that can be applied, and it is difficult to directly observe such a minute change. In view of this, the observation accuracy for a minute change in reflectivity is improved by a lock-in detection method in which an alternating electric field is applied and a signal synchronized with the applied electric field is detected.

JP 2004-85399 A

  As described above, there is a need for a simple and fast method for observing a polarization domain of a ferroelectric material that can be used in the field such as an inspection process, for example, a method that can be easily observed without preparing a vacuum environment. Therefore, microscopic electric field modulation spectroscopy using an optical microscope is considered, but even if a slight optical change in the polarization domain of the ferroelectric material can be visualized, the resolution is high enough to determine the state of polarization inversion. It is not easy to observe.

  The present invention has been made in view of the above situation, and its object is to visualize the polarization domain of a ferroelectric material that can be observed at high speed and with high resolution while being a simple observation method. It is an object to provide a method and its apparatus.

  According to the method of visualizing and observing the polarization domain of a ferroelectric according to the present invention, a positive and negative electric field having a magnitude that does not invert the polarization domain is alternately formed in a ferroelectric thin film with a period T, and the ferroelectric domain is synchronized with the ferroelectric domain. In the visualization observation method, the transmitted light or reflected light of monochromatic light irradiated on the dielectric thin film is imaged by a two-dimensional photodetector, and a difference image between a set of images captured in each of a positive electric field and a negative electric field is obtained. An accumulated image from a plurality of difference images is obtained to give contrast, and the period T is shifted to a short time side to remove noise from the difference image.

  According to this invention, it is possible to observe easily without preparing a vacuum environment, and to observe with high resolution and high speed to such an extent that the state of polarization inversion of the polarization domain of the ferroelectric can be determined.

  In the above-described invention, the direction of the electric field may be variable in the plane of the ferroelectric thin film. According to this invention, since the polarization domain direction and the electric field direction can be offset so as not to be perpendicular, observation can always be given.

  In the above-described invention, the wavelength of the monochromatic light may be in the range of 200 to 2000 nm. According to this invention, a wide range of ferroelectric materials can be visualized and observed.

In the above-described invention, the period T may correspond to a frequency of 10 Hz to 1000 Hz. According to this invention, an optical change of the order of 10 ⁻⁴ can usually be visualized in a short time.

  Further, the apparatus for visualizing and observing the polarization domain of the ferroelectric according to the present invention includes a light source for irradiating the ferroelectric thin film with monochromatic light, and a function generator for alternately forming positive and negative electric fields on the ferroelectric thin film with a period T. An optical system that collects the transmitted light or reflected light of the monochromatic light and forms an image on a two-dimensional photodetector, and an image processing means that processes an image signal of the two-dimensional photodetector, The image processing means obtains a difference image of a set of images captured in each of a positive electric field and a negative electric field in synchronization with the period T, obtains an integrated image from a plurality of the difference images, and gives contrast. The function generator is characterized in that the period T is shifted to a short time side to remove noise from the difference image.

  According to this invention, it is possible to observe easily without preparing a vacuum environment, and to observe with high resolution and high speed to such an extent that the state of polarization inversion of the polarization domain of the ferroelectric can be determined.

  In the above-described invention, a plurality of pairs of electrodes may be provided on the ferroelectric thin film so that the direction of the electric field is variable in the plane of the ferroelectric thin film. According to this invention, since the polarization domain direction and the electric field direction can be offset so as not to be perpendicular, observation can always be given.

  In the above-described invention, the wavelength of the monochromatic light may be in the range of 200 to 2000 nm. According to this invention, a wide range of ferroelectric materials can be visualized and observed.

In the above-described invention, the period T may correspond to a frequency of 10 Hz to 1000 Hz. According to this invention, an optical change of the order of 10 ⁻⁴ can usually be visualized in a short time.

It is a figure which shows the observation apparatus by this invention. It is a figure explaining the principle of this invention. FIG. 3 is a partially enlarged view of FIG. 2. It is a figure explaining the polarization inversion with respect to an electrode position. 3 is a structural formula of a ferroelectric material used in Examples. It is a figure which shows an example of the observation method of this invention. It is a time chart which shows an imaging timing. It is an example of visualization of a polarization domain. It is a graph which shows the relationship between the change rate of the light transmittance by an electric field, and light wavelength. It is an example of visualization of the time change of a polarization domain.

  First, a ferroelectric polarization domain observation apparatus according to one embodiment of the present invention will be described.

  As shown in FIG. 1, the polarization domain observation apparatus 1 guides and irradiates monochromatic light from a light source 20 to a thin film sample 10 made of a ferroelectric material provided on a substrate 11, and uses a pair of thin film samples 10 by a function generator 30. In this device, positive and negative electric fields are alternately formed with a predetermined period T between the electrodes 12. A voltage fluctuation generated by the function generator 30, for example, a rectangular pulse voltage having a period T, is applied from the booster amplifier 32 to the electrode 12 through the prober 34.

  Here, the light from the light source 20 is guided to either the optical fiber 24a or 24b by the optical path switch 23. In the transmission mode, the thin film sample 10 is irradiated with light by the lens 26a, and in the reflection mode. The thin film sample 10 is irradiated with light by the lens 26b.

  In either the transmission mode or the reflection mode, the light from the thin film sample 10 is guided to the two-dimensional photodetector 28 through the objective lens 27 and imaged. The image signal converted into an electric signal by the two-dimensional photodetector 28 is subjected to image processing by the image processing device 40 and displayed on the display 42.

  Here, the image processing in the image processing apparatus 40 obtains a difference image of a set of images captured in each of the positive electric field and the negative electric field in synchronization with the period T, and an integrated image from a plurality of the difference images. To obtain high contrast. This will be described later.

  The light source 20 may be a halogen lamp, a deuterium lamp, a xenon lamp, or the like, and irradiates the thin film sample 10 with light in the ultraviolet-near infrared region obtained by monochromatizing such lamp light through a spectroscope or bandpass filter 22. Moreover, LED which can take out the monochromatic light of the wavelength range of an ultraviolet-near infrared region directly can also be used suitably.

  As the two-dimensional photodetector 28, a CMOS camera or a CCD camera having sensitivity in the range of 300 to 1000 nm, an InGaAs camera or a HgCdTe camera having sensitivity in the near infrared region of 1000 nm or more can be used. In the ultraviolet region of 300 nm or less, sensitivity may be given by attaching an image intensifier to a CCD or CMOS camera.

  As will be described later, such an imaging device can collectively acquire information on the area range determined by the field of view of the objective lens 27 with the two-dimensional photodetector 28, so that the measurement is performed by scanning the light collected in a spot shape. Compared with the method, the polarization domain can be observed at a high speed and in a wide range. Further, the visual field of the objective lens 27 can be applied to the thin film sample 10 having a large area of 10 cm square, for example. Furthermore, the spatial resolution of the image formed on the two-dimensional photodetector 28 depends on the pixel density of the two-dimensional photodetector 28, the numerical aperture of the objective lens 27, and the wavelength of light from the light source 20, and has a high numerical aperture. By using the objective lens 27 and the two-dimensional photodetector 28 having a high pixel density, a high spatial resolution corresponding to the diffraction limit of light, for example, the spatial resolution of about 10 μm is limited in the prior art (Patent Document 1). However, this can be increased to about several hundred nm.

  Next, the principle of the present invention will be described with reference to FIGS.

As shown in FIG. 2A, the signal intensity of light transmittance and reflectance of the ferroelectric thin film sample 10 slightly changes depending on the electric field. For example, let A be the signal strength when a positive electric field is formed, and B be the signal strength when switched to a negative electric field. Although this change d depends on the magnitude of the electric field, switching of the electric field that does not invert the polarization domain is usually very small on the order of 10 ^−4. A method is required. That is, the difference image d between when the positive electric field is formed on the ferroelectric thin film sample 10 and when the negative electric field is formed shows temporal fluctuations in the intensity of the light source 20 and the sensitivity of the two-dimensional photodetector 28. These are larger than changes in transmittance and reflectance on the order of 10 ⁻⁴ (for example, see D in FIG. 2A, D >> d). For this reason, even if the light intensities are integrated in a state in which a positive electric field is formed and in a state in which a negative electric field is formed (A and B in FIG. 2 (a)), the temporal fluctuation D in the difference d between these images It was canceled and could not be detected.

On the other hand, as shown in FIG. 2 (b), the positive electric field and negative electric field switches (modulation) are repeated on a time scale earlier than the temporal fluctuation D as described above, and the positive electric field and the negative electric field in each cycle are repeated. An image in the applied state (F1 and F2 in FIG. 2B) is taken and the respective differences d _1, d ₂ , d ₃ . . . . . . d _n is obtained and the difference image is integrated d ₁ + d ₂ + d ₃ +. . . . . + By d _n, it is to remove the effect of fluctuation.

  In other words, the method of visualizing and observing the polarization domain of the ferroelectric thin film 10 in the apparatus 1 described above causes the ferroelectric thin film 10 to alternately form positive and negative electric fields with a period T so as not to invert the polarization domain. The two-dimensional photodetector 28 images monochromatic transmitted or reflected light irradiated on the ferroelectric thin film 10 in synchronism with the image, and a difference image between a set of images captured in each of a positive electric field and a negative electric field is obtained. It is a visualization observation method to obtain. Then, a plurality of difference images of a pair of positive and negative electric fields are integrated to obtain an integrated image to give contrast.

  At this time, the noise of the difference image can be removed by shifting the period T to the short time side. The electric field is modulated by the function generator 30, and the modulation frequency is preferably T = 15 Hz-1 kHz. The high frequency is less susceptible to low frequency fluctuations, and the number of integration can be increased in a short time, which is advantageous for highly sensitive detection. For this purpose, it is preferable to use a two-dimensional photodetector 28 having a high frame rate (30 to 2000 fps) capable of imaging at a high frequency. The two-dimensional photodetector 28 preferably has a noise level as low as possible, a wide dynamic range, and a wider wavelength range having sensitivity. In addition to this, by using the high-speed image processing device 40, the difference between the positive electric field image and the negative electric field image is obtained in parallel with the imaging by the two-dimensional photodetector 28, and the integration process is performed. become able to. That is, the measurement can be completed in a significantly shorter time than when the difference image integration process is performed after the imaging is completed.

  The direction of the electric field applied by the pair of electrodes 12 is variable in the plane of the ferroelectric thin film 10 so that the direction of the polarization domain and the direction of the electric field are offset so as not to be perpendicular so that observation is always possible. It is preferable to do. For example, it can be considered to perform observation by providing a plurality of pairs of electrodes 12 to be described later.

  By the way, as shown in FIG. 4, in the ferroelectric thin film sample 10, a pair of electrodes 12 are provided on the same surface with respect to the main surface of the thin film (horizontal type, see FIG. 4 (a)), or applied to both surfaces. (Vertical type, see FIG. 4B). In the former, a domain having polarization in a direction parallel to the main surface can be visualized, and in a vertical arrangement, a domain having polarization in a direction perpendicular to the substrate can be visualized.

  Here, in the vertical arrangement as shown in FIG. 4B, when imaging with transmitted light is performed, the pair of upper and lower electrodes 12 are either transparent electrodes with respect to the light from the light source 20 such as ITO or light. A thin metal film that is thin enough to transmit is used. On the other hand, when imaging by reflected light is performed, the electrodes as described above are provided only on the light incident side. In any case, when imaging with transmitted light is performed, the thickness of the ferroelectric thin film sample 10 must be thin enough to transmit light.

  According to the polarization domain observation apparatus 1 and the observation method described above, observation can be performed easily without preparing a vacuum environment, and at a high resolution and at a high speed so that the state of polarization inversion of the polarization domain of the ferroelectric can be determined. It can be done.

  Next, as shown in FIG. 5, a ferroelectric thin film made of a proton transfer salt made of [H-dppz] [Hca] (2,3-di (2-pyridinyl) pyrazine (dppz) and chloranilic acid (H2ca). (For details, see, for example, S. Horiuchi et al., J. Am. Chem. Soc. 135, 4492 (2013).) An observation example in which polarization domain observation is performed with the above-described apparatus is described. To do.

  First, as shown in FIG. 6, a glass substrate 100 on which a plurality of parallel electrodes 101 are deposited in a stripe shape is prepared, and a solution 104 in which an organic ferroelectric substance is dissolved in a solvent is dropped onto the pipette 103 from the tip of the pipette 103. (See FIG. 6 (a)). Here, acetone was used as the solvent, the concentration was 0.2 wt%, and the amount dropped onto the substrate was 30 μL.

  The glass substrate 105 that has been subjected to water repellent treatment is covered from above, so that the solution 104 is thinly spread over the glass substrate 100 (see FIG. 6B). In this state, it is allowed to stand at room temperature, and the solvent is allowed to evaporate for several hours to several days, and the glass plate 105 is removed (see FIG. 6C). As a result, the flaky ferroelectric single crystal thin film 104a in a state straddling between the electrodes 101 on the substrate 100 can be grown.

  The observation method is as follows. Here, description will be given with reference to FIG.

  A substrate 100 provided with the single crystal thin film 10 (corresponding to 104a in FIG. 6) is arranged to face the optical lens 26a of the apparatus 1, and a prober 34 is applied to the electrode 12 (corresponding to the electrode 101 in FIG. 6) to perform electricity. Give a positive contact. The single-crystal thin film 11 is focused by adjusting the optical lens 26a, and light having a wavelength of 530 nm is irradiated. A voltage of +15 V and −15 V is alternately applied between the electrodes 12 using a function generator 30 and a boost amplifier 32 at a repetition period of 45 Hz.

  On the other hand, a trigger with a double repetition period (90 Hz) is input to the two-dimensional photodetector (CMOS camera) 28, and imaging is performed for each of a positive state to which a voltage of + 15V is applied and a negative state to which a voltage of -15V is applied. I do.

  FIG. 7 shows a time chart of applied voltage and a time chart of imaging trigger. In a rectangular positive / negative pulse repeated every 22 ms, images are taken every 11 ms at the same timing.

  In parallel with the imaging, the image processing apparatus 40 performs a difference and integration process between positive and negative images captured in each cycle. The results are shown below.

  First, as shown in FIG. 8A, it was possible to obtain an image clearly showing the black and white boundary line in a measurement time of about 3 minutes. Further, as shown in FIG. 8 (b), a similar image is obtained by observation with a known PFM (for example, refer to SV Kalinin et al., Jpn. J. Appl. Phys. 46, 5674 (2007)). Was obtained. That is, the gray and white portions shown in FIG. 8A correspond to domains in which the direction of polarization is reversed by 180 degrees.

  Further, in FIG. 8A, the change rate of the light transmittance due to the electric field for each of the domain having an upward polarization (see gray part and upward arrow) and the domain having a downward polarization (see white part and downward arrow). (ΔT / T) was measured by changing the wavelength of light.

  As shown in FIG. 9, at an irradiation wavelength of 530 nm, the upward domain shows a negative rate of change (see slope p1), while the downward domain shows a positive rate of change (see slope p2). The contrast of the polarization domain in the difference image is due to this difference in rate of change.

  Next, the ferroelectric substance ([H-dppz] [Hca]) was applied with a voltage (100 V) that is equal to or higher than the threshold value for inverting the polarization domain, and imaged over time in the same manner as described above.

  As shown in FIG. 10, it was observed that the polarization domain gradually reversed and changed over time. Furthermore, it was possible to visualize the domain that remained without being inverted (see the r portion, where the upper and lower black portions are the portions indicating electrodes). Such visualization of the domain is considered to be very useful for the purpose of clarifying the cause of performance deterioration of the ferroelectric material.

  As mentioned above, although the Example by this invention and the modification based on this were demonstrated, this invention is not necessarily limited to this, A person skilled in the art will deviate from the main point of this invention, or the attached claim. Various alternative embodiments and modifications could be found without doing so.

DESCRIPTION OF SYMBOLS 1 Polarization domain observation apparatus 10 Ferroelectric thin film sample 12 Electrode 20 Light source 22 Filter 23 Optical path switcher 24 Optical fiber 26 Lens 28 Two-dimensional photodetector 30 Function generator 32 Booster amplifier 40 Image processing apparatus 42 Display

Claims (8)

A method for visualizing and observing the polarization domain of a ferroelectric,
In the ferroelectric thin film, positive and negative electric fields having a magnitude that does not invert the polarization domain are alternately formed with a period T, and in synchronism with this, transmitted or reflected monochromatic light irradiated on the ferroelectric thin film is In a visualization observation method for obtaining a difference image of a set of images imaged by each of a positive electric field and a negative electric field by imaging with a two-dimensional photodetector, obtaining an integrated image from a plurality of the difference images and providing contrast, A method for visualizing and observing a polarization domain of a ferroelectric substance, wherein the period T is shifted to a short time side to remove noise from the difference image.   2. The ferroelectric polarization domain visualization observation method according to claim 1, wherein the direction of the electric field is variable in the plane of the ferroelectric thin film.   The polarization domain visualization observation method according to claim 1 or 2, wherein the wavelength of the monochromatic light is in a range of 200 to 2000 nm.   4. The polarization domain visualization observation method according to claim 1, wherein the period T corresponds to a frequency of 10 Hz to 1000 Hz. An apparatus for visualizing and observing the polarization domain of a ferroelectric,
A light source for irradiating the ferroelectric thin film with monochromatic light;
A function generator that alternately forms positive and negative electric fields in a ferroelectric thin film with a period T;
An optical system that focuses the transmitted light or reflected light of the monochromatic light and forms an image on a two-dimensional photodetector;
Image processing means for processing an image signal of the two-dimensional photodetector,
The image processing means obtains a difference image of a set of images captured in each of a positive electric field and a negative electric field in synchronization with the period T, obtains an integrated image from a plurality of the difference images, and gives contrast. ,
The ferroelectric generator domain visualization observation apparatus, wherein the function generator shifts the period T to a short time side to remove noise from the difference image.   6. The ferroelectric polarization domain visualization observation apparatus according to claim 5, wherein a plurality of pairs of electrodes are provided on the ferroelectric thin film so that the direction of the electric field is variable in the plane of the ferroelectric thin film.   The polarization domain visualization observation apparatus according to claim 5 or 6, wherein the wavelength of the monochromatic light is in a range of 200 to 2000 nm. The polarization domain visualization observation apparatus according to claim 5, wherein the period T corresponds to a frequency of 10 Hz to 1000 Hz.

Images