WO2023234760A1 - Optical sensor and memory device including ferroelectric thin film having pyroelectric properties - Google Patents

Abstract

An optical sensor for sensing a target object is disclosed. The optical sensor comprises: a light source that emits pulsed light toward a target object; a sensing element including a ferroelectric thin film that generates a current in response to a temperature change caused by the pulsed light reflected by the target object, and a first electrode disposed on the surface of the ferroelectric thin film; and a current measurer that is electrically connected to the first electrode to measure the current generated by the ferroelectric thin film.

Description

Optical sensor and memory device comprising a ferroelectric thin film with pyroelectric properties

The present invention relates to an optical sensor that detects light using a ferroelectric thin film with pyroelectric properties and a non-volatile memory device capable of optical reading and electrical writing.

Since the successful demonstration of ferroelectricity in hafnium oxide (HfO2), the scientific community has been very interested in technologies that exploit it for potential applications. One of the attractive properties of hafnium oxide (HfO ₂ ) is the pyroelectricity effect, which is related to changes in ferroelectric polarization depending on temperature. A slight change in temperature around a polar material changes the strength of its spontaneous polarization, which causes the creation of a large temporary surface charge, which in turn generates electricity, called the pyroelectric effect. The pyroelectric effect is a major method for detecting transient photon irradiation and is consequently widely applied in practical sensing applications such as infrared sensing and thermal imaging. Infrared photons interact thermally with the ferroelectric material by tuning the effective surface polarization, resulting in an electrical response.

Additionally, the dipole orientation of hafnium oxide (HfO ₂ ) can be switched by an electric field, which may make it possible to control pyroelectric behavior, but controlling pyroelectric behavior using spontaneous polarization has been little studied. It didn't work.

One object of the present invention is to provide an optical sensor capable of detecting a target using the pyroelectric properties of a ferroelectric thin film.

Another object of the present invention is to provide a non-volatile memory device capable of electromagnetic writing and optical reading using a ferroelectric thin film having pyroelectric properties.

An optical sensor according to an embodiment of the present invention is a device for detecting a target, comprising: a light source that irradiates pulsed light in the direction of the target; a sensing element including a ferroelectric thin film that generates a current by a temperature change caused by pulsed light reflected by the target and a first electrode disposed on the surface of the ferroelectric thin film; and a current meter electrically connected to the first electrode and measuring a current generated by the ferroelectric thin film.

In one embodiment, the ferroelectric thin film may include a polycrystalline hafnium oxide thin film including an orthorombic (Pca21) phase. In this case, the ferroelectric thin film may have an oxygen defect density of 9 to 15%.

In one embodiment, the light source has a pulse width of 0.01 to 100 ms and may irradiate pulsed light with a wavelength in the infrared region.

In one embodiment, the ferroelectric thin film is disposed on a substrate, and the first electrode is disposed to contact the upper surface of the ferroelectric thin film and may include a transparent electrode through which pulsed light can pass. For example, the first electrode may include a conductive metal nanowire electrode, a mesh-structured conductive metal electrode, a conductive carbon material electrode, a transparent conductive oxide electrode, or a transparent conductive polymer electrode.

The optical sensor includes a second electrode disposed between the substrate and the ferroelectric thin film; and an electric field applicator electrically connected to the second electrode and applying an electric field to the ferroelectric thin film to adjust the polarization state of the ferroelectric thin film.

A memory device according to an embodiment of the present invention includes a ferroelectric thin film having pyroelectric properties that generates a current by a temperature change caused by pulsed light, and a first electrode disposed to contact the first surface of the ferroelectric thin film. and a memory element including a second electrode disposed below a second surface of the ferroelectric thin film opposite to the first surface; an electric field applicator electrically connected to the second electrode and adjusting the polarization state of the ferroelectric thin film by applying an electric field to the ferroelectric thin film; a light source that irradiates pulsed light to the ferroelectric thin film; And a current detector electrically connected to the first electrode, generated by a temperature change caused by the pulsed light, and measuring a photocurrent by the ferroelectric thin film whose size changes depending on the polarization state of the ferroelectric thin film. You can.

In one embodiment, the electric field applicator may control the polarization state of the ferroelectric thin film by applying one or more pulse voltages with a width of 1 to 100 ms to the second electrode. For example, the one or more pulse voltages may include one or more positive pulse voltages and one or more negative pulse voltages.

In one embodiment, the ferroelectric thin film may include a polycrystalline hafnium oxide thin film including an orthorombic (Pca21) phase. In this case, the ferroelectric thin film may have an oxygen defect density of 9 to 15%.

In one embodiment, the light source has a pulse width of 0.01 to 100 ms and may irradiate pulsed light with a wavelength in the infrared region.

According to the optical sensor of the present invention, a target can be detected in real time by detecting a photocurrent generated from a ferroelectric thin film with pyroelectric properties without using external power.

In addition, according to the memory device of the present invention, it is possible to control the polarization state of the ferroelectric thin film with pyroelectric properties in various ways using electric fields of various sizes, making it easy to implement multiple storage states, and optically storing the stored information. Since it can be read, the reading speed of information can be significantly improved.

1 is a diagram for explaining an optical sensor according to an embodiment of the present invention.

Figure 2 is a diagram for explaining a memory device according to an embodiment of the present invention.

Figure 3 shows a cross-sectional TEM image of HfO ₂ /SiO ₂ /Si ('Figure 3a'), TEM images showing the orthorhombic crystal phase and monoclinic crystal phase of HfO ₂ ('Figure 3b', 'Figure 3c'), respectively, and Hf and EELS mapping results corresponding to O ('Figure 3d'), cross-sectional STEM image of HfO ₂ /SiO ₂ /Si ('Figure 3e'), EELS spectrum of HfO ₂ thin film ('Figure 3f') and HfO _{2 thin} film. The XRD spectrum ('Figure 3g') is shown.

Figure 4 shows the PV hysteresis curve ('Figure 4a'), the double pulse measurement result ('Figure 4b'), the polarization stability measurement result ('Figure 4c'), and the granule structure with different scan voltage regions for the _HfO 2 thin film. Showing surface morphology ('Figure 4d'), PFM phase and size ('Figure 4e'), loops on PFM cisterns as a function of voltage ('Figure 4f'), PFM image ('Figure 4g'), and oxygen defect ratio. The polarization state change ('Figure 4h') calculated according to and the relative energy change ('Figure 4i') according to atomic displacement are shown.

Figure 5 shows the instantaneous photoresponse ('FIG. 5a', 'FIG. 5b') measured under self-buyer conditions for AgNWs/HfO ₂ /SiO ₂ /Si device, and a schematic diagram showing the operating mechanism ('FIG. 5c'). , the change in photoresponse corresponding to a single pulse of light ('Figure 5d'), the change in photoresponse according to pulsed light intensity ('Figure 5e', 'Figure 5f'), and the temperature-dependent change of the pyrophotocurrent peak ('Figure 5g). ', 'Figure 5h').

Figure 6 is a graph showing the results of measuring the pyrophotonic response before and after applying a pulse voltage to the Si substrate of the AgNWs/HfO ₂ /SiO ₂ /Si device under the condition of continuous irradiation of pulsed light ('Figure 6a', ' Figure 6b'), to explain the operating mechanism of the AgNWs/HfO ₂ /SiO ₂ /Si device ('Figure 6c'), and the change in optical response according to the type of applied pulse voltage ('Figure 6d'). The measured results ('Figure 6e') are shown.

Hereinafter, embodiments of the present invention will be described in detail. Since the present invention can be subject to various changes and can have various forms, specific embodiments will be illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to a specific disclosed form, and should be understood to include all changes, equivalents, and substitutes included in the spirit and technical scope of the present invention. While describing each drawing, similar reference numerals are used for similar components. In the attached drawings, the dimensions of the structures are enlarged from the actual size for clarity of the present invention.

Terms such as first, second, etc. may be used to describe various components, but the components should not be limited by the terms. The above terms are used only for the purpose of distinguishing one component from another. For example, a first component may be named a second component, and similarly, the second component may also be named a first component without departing from the scope of the present invention.

The terms used in this application are only used to describe specific embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise. In this application, terms such as “comprise” or “have” are intended to designate the presence of features, steps, operations, components, parts, or combinations thereof described in the specification, but are not intended to indicate the presence of one or more other features or steps. , it should be understood that it does not exclude in advance the possibility of the existence or addition of operations, components, parts, or combinations thereof.

Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by a person of ordinary skill in the technical field to which the present invention pertains. Terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with the meaning in the context of the related technology, and unless explicitly defined in the present application, should not be interpreted in an ideal or excessively formal sense. No.

1 is a diagram for explaining an optical sensor according to an embodiment of the present invention.

Referring to FIG. 1, an optical sensor 100 according to an embodiment of the present invention may include a light source 110, a sensing element 120, and a current meter 130.

The light source 110 may radiate pulsed light to the target 10 to be sensed. For example, the light source 110 may irradiate pulsed light that belongs to the infrared wavelength range and has a width of about 0.01 to 100 ms in the direction of the target 10.

The sensing element 120 may generate current by receiving pulsed light reflected by the target 10. In one embodiment, the sensing element 120 may include a ferroelectric thin film 121 and an electrode 122.

The ferroelectric thin film 121 may have pyroelectric properties capable of generating current by a temperature change caused by pulsed light reflected from the target 10. For example, the temperature change may cause a change in the strength of the spontaneous polarization of the ferroelectric thin film 121, and a current may be generated by the temporary surface charge generated as a result.

In one embodiment, the ferroelectric thin film 121 may be formed of hafnium oxide including an orthorombic (Pca2 ₁ ) phase having non-centrosymmetric properties. For example, the ferroelectric thin film 121 may include a polycrystalline hafnium oxide thin film including an orthorombic (Pca21) phase and a monoclinic (centrosymmetric space group P21/c) phase. In the hafnium oxide thin film, the orthorombic crystal phase may include hexagonal array atoms whose grains are slightly distorted, and this distortion is mainly caused by factors such as grain boundaries and oxygen vacancies, and the ferroelectricity of the hafnium oxide thin film It can play an important role in promoting movement.

In one embodiment, when the ferroelectric thin film 121 is a hafnium oxide thin film including an orthorombic crystal phase, the ferroelectric thin film 121 may have an oxygen defect density of about 9 to 15%, and the oxygen defects are It may include oxygen vacancies, etc.

In one embodiment, the ferroelectric thin film 121 may be formed to a thickness of about 5 to 50 nm on the substrate 140 through an RF sputtering process. In one embodiment, an amorphous hafnium oxide thin film is formed on the substrate 140 through an RF sputtering process, then subjected to a rapid thermal annealing process, and then cooled to form a crystalline hafnium oxide thin film containing an orthorombic phase. can be formed. Meanwhile, the material or structure of the substrate 140 is not particularly limited as long as it can support the ferroelectric thin film 121.

The electrode 122 may be formed on one surface of the ferroelectric thin film 121 and may include a transparent electrode through which pulsed light reflected from the target 10 can transmit. In one embodiment, the electrode 122 may include a conductive metal nanowire electrode, a conductive metal electrode with a mesh structure, a conductive carbon material electrode, a transparent conductive oxide electrode, a transparent conductive polymer electrode, etc.

The current meter 130 is electrically connected to the electrode 122 and measures the current generated by the pyroelectric characteristics of the ferroelectric thin film 121 by the reflected pulse light, thereby detecting the target 10. It can be sensed. The structure of the current meter 130 is not particularly limited as long as it can measure the current generated from the ferroelectric thin film 121.

In one embodiment, the optical sensor 100 is disposed between the ferroelectric thin film 121 and the substrate 140, and applies an electric field to the ferroelectric thin film 121 to increase the polarization strength of the ferroelectric thin film 121. It may further include an additional electrode (see '底' in FIG. 2) that adjusts and an electric field applicator (see '府' in FIG. 2) that applies a voltage to the additional electrode. By adjusting the polarization strength of the ferroelectric thin film 121 through the additional electrode and the electric field applicator, the current generated by the ferroelectric thin film 121 can be adjusted.

According to the optical sensor of the present invention, a target can be detected in real time by detecting a photocurrent generated from a ferroelectric thin film with pyroelectric properties without using external power.

Figure 2 is a diagram for explaining a memory device according to an embodiment of the present invention.

Referring to FIG. 2 , the memory device 200 according to an embodiment of the present invention may include a memory element 210, an electric field applicator 220, a light source 230, and a current detector 240.

The memory device 210 may include a ferroelectric thin film 211, a first electrode 212, and a second electrode 213. Meanwhile, the memory device 210 may further include an insulating film 214 disposed between the ferroelectric thin film 211 and the second electrode 213.

Since the ferroelectric thin film 211 and the first electrode 212 are substantially the same as the ferroelectric thin film 121 and the electrode 122 of the optical sensor 100 described with reference to FIG. 1, redundant detailed information about them is provided. The explanation is omitted.

The second electrode 213 may be disposed to face the first electrode 212 with the ferroelectric thin film 211 interposed therebetween, and the electric field applicator 220 applies an electric field to the ferroelectric thin film 121. A pulse voltage may be applied to the second electrode 213.

In one embodiment, the electric field applicator 220 may apply one or more positive pulse voltages and one or more negative pulse voltages to the second electrode 213. The positive pulse voltage and negative pulse voltage may have a width of about 1 to 100 ms. And when the positive pulse voltage includes a plurality of pulse voltages having different magnitudes, the dielectric thin film 211 may have different polarization states depending on the magnitude of each pulse voltage. And when the negative pulse voltage includes a plurality of pulse voltages having different magnitudes, the dielectric thin film 211 may have different polarization states depending on the magnitude of each pulse voltage. Meanwhile, when no voltage is applied to the second electrode 213, the polarization strength of the ferroelectric thin film 211 may have non-volatile characteristics that maintain the polarization state induced by the final applied pulse voltage.

The light source 230 is disposed on top of the first electrode 212 of the memory element 210 and can irradiate pulsed light to the ferroelectric thin film 211, and the current detector 240 is configured to emit pulsed light. Detect the current generated from the ferroelectric thin film 211 by irradiation, and through this detect the polarization state of the ferroelectric thin film 211, that is, the non-volatile state input to the memory element 210 by the electric field applicator 220. Information can be read.

In one embodiment, the light source 230 may irradiate the ferroelectric thin film 211 with pulsed light having a wavelength in the infrared region and a width of about 0.01 to 100 ms.

According to the memory device of the present invention, it is possible to control the polarization state of a ferroelectric thin film with pyroelectric properties in various ways using electric fields of various sizes, making it easy to implement multiple storage states and optically read the stored information. This can significantly improve the reading speed of information.

Hereinafter, specific embodiments of the present invention will be described in detail. However, the following examples are only some embodiments of the present invention, and the scope of the present invention is not limited to the following examples.

[Example]

HfO ₂ thin films were deposited on low-resistance (1×10-3 Ωcm) p-Si substrates using a large-area RF sputtering technique using an ultra-high purity (99.999) HfO ₂ target. Before deposition, the Si substrate was ultrasonically cleaned using acetone, alcohol and propanol, and then dried using nitrogen.

Specifically, RF sputtering was performed at a power of 100 W and an operating pressure of 10 mTorr. To maintain the working pressure, ultra-high purity argon gas with a flow rate of 30 sccm was used. After HfO ₂ deposition at room temperature, rapid thermal annealing was performed for 30 minutes under nitrogen atmosphere at a flow rate of 50 sccm and a temperature of 600° C., whereby the HfO ₂ thin film was crystallized.

[Experimental example]

Figure 3 shows a cross-sectional TEM image of HfO ₂ /SiO ₂ /Si ('Figure 3a'), TEM images showing the orthorhombic crystal phase and monoclinic crystal phase of HfO ₂ ('Figure 3b', 'Figure 3c'), respectively, and Hf and EELS mapping results corresponding to O ('Figure 3d'), cross-sectional STEM image of HfO ₂ /SiO ₂ /Si ('Figure 3e'), EELS spectrum of HfO ₂ thin film ('Figure 3f') and HfO _{2 thin} film. The XRD spectrum ('Figure 3g') is shown.

As shown in Figure 3a, the thickness (d) of the HfO ₂ thin film was about 10 nm, and amorphous oxide (SiO ₂ , d = 3 nm) existed at the interface. In addition, low-resolution cross-sectional TEM confirmed that the HfO ₂ thin film is composed of tightly packed and elongated nanocolumn grains with a lateral size of approximately 12 to 20 nm, and that grain boundaries are formed perpendicular to the SiO ₂ /Si plane. .

As shown in Figures 3b and 3c, the HfO ₂ thin film was composed of a mixture of non-centrosymmetric allorhombic (Pca21) and monoclinic (centrosymmetric space group P21/c) phases, and the monoclinic phase was at room temperature and atmospheric pressure. It is the most common crystalline phase formed. As shown in the rectangle in Figure 3b, the enlarged TEM image showed that the grains exhibited an orthocomb crystal structure with slightly distorted hexagonal array atoms. This distortion is mainly due to factors such as grain boundaries and oxygen vacancies, which may play an important role in promoting the ferrostatic behavior of HfO ₂ . The average interlayer distance (about 0.29 nm) closely matched the (111) plane of the monoclinic HfO ₂ phase. (See the line profile in the inset of Figure 3c) The monoclinic phase of HfO ₂ is a stable form, and a complete phase transition can occur when the amorphous film changes to the monoclinic phase. On the other hand, through rapid thermal annealing and subsequent cooling, the amorphous HfO ₂ thin film can be converted to the tetragonal phase and then to the orthorombic phase.

As shown in Figure 3d, the deposition of HfO ₂ was confirmed by elemental mapping collected by cross-sectional electron energy loss spectroscopy (EELS) mapping. The elemental distributions corresponding to hafnium and oxygen elements did not match the device geometry, showing a uniform and cavitated distribution of hafnium and oxygen elements.

A cross-sectional EELS spectrum was recorded from SiO ₂ (interface) to the top, as illustrated by the vertical arrow in Figure 3e. The EELS spectrum from SiO ₂ showed a prominent peak at approximately 531.3 eV, confirming the OK edge as shown in Figure 3f. The OK edge peak moved toward low energy, and the peak corresponding to HfO ₂ appeared at 528.4 eV. Additionally, the EELS spectrum collected from the native oxide (SiO ₂ ) to HfO ₂ showed a pre-peak hump at 528.4 eV before in the HfO ₂ layer, as indicated by the background. These pre-peak projections are associated with electron transfer from O 1s to unfilled Hf 5d mixed with O 2p levels and indicate the presence of holes or oxygen vacancies on oxygen sites. As shown in the cross-sectional TEM image, the EELS spectrum also confirmed the growth of defective HfO ₂ thin films using sputtering.

As shown in Figure 3g, the XRD spectrum showed multiple characteristic peaks at 2θ of 30.33°31.44° and 35.36°. The peak at 30.33° represents the (111) plane of the orthocomb phase, and the other two peaks (31.44° and 35.56°) correspond to the (111) and (002) lattice planes of the monoclinic phase, respectively. The XRD spectrum confirmed that the film had a polycrystalline crystal structure and that orthocomb (#-HfO2) and monoclinic (m-HfO2) phases coexisted. The inset in Figure 3g shows a unit cell of monoclinic and orthorombic HfO ₂ .

TEM, _{EELS and}

Figure 4 shows the PV hysteresis curve ('Figure 4a'), the double pulse measurement result ('Figure 4b'), the polarization stability measurement result ('Figure 4c'), and the granule structure with different scan voltage regions for the _HfO 2 thin film. Showing surface morphology ('Figure 4d'), PFM phase and size ('Figure 4e'), loops on PFM cisterns as a function of voltage ('Figure 4f'), PFM image ('Figure 4g'), and oxygen defect ratio. The polarization state change ('Figure 4h') calculated according to and the relative energy change ('Figure 4i') according to atomic displacement are shown.

As shown in Figure 4a, the loop opening in the hysteresis loop was improved with increasing scanning voltage area for the HfO ₂ thin film. Specifically, the loop opening (i.e., 2P) was 2.52 μCcm ^-1 for scan voltages of 0→+1.0→0→-1.0→0V, while it improved to 4.93 μCcm ^-1 for ±8.0V scanning. Additionally, the current-voltage collected during PV measurements showed two distinct current peaks related to polarization switching. The apparent loop opening in the PV curve and the appearance of the corresponding peaks in the IV measurement indicate the ferroelectric behavior of the device. Although a hysteresis loop appeared in the PV results, it was not saturated. The ferroelectric properties of the HfO ₂ thin film were also confirmed by measurements of dual positive up-negative down pulses (V = ±8.0 V, width (w) = 10 μs).

As shown in Figure 4b, for the currents at point-1 and point-2 measured during the double pulse measurement, the current ( _IP+C ) corresponding to the first pulse is similar to that of the second pulse with similar magnitude and polarity. (I _C ) was relatively high compared to the corresponding current. The current generated by the first pulse (point-1) produced both polarization switching and capacitor charging current ( _IP+C ), whereas the second pulse (point-2) produced no polarization. Therefore, only the capacitor charging current ( _IC ) was shown. Therefore, double pulse measurements provided further evidence of polarization in AgNWs/HfO ₂ /SiO ₂ /Si devices.

As shown in Figure 4c, stabile flipping of the ferroelectric polarization corresponding to three voltages with multiple repetitions, namely +3.0, +5.0 and +8.0 V, was confirmed, and the polarization was reproducible. showed behavior.

As shown in Figure 4d, the surface topography collected by atomic force microscopy (AFM) measurements shows that the HfO ₂ film is closely packed, randomly distributed, and contains a granular phase structure with an average lateral size of 20 ± 10 nm. showed that

As shown in Figure 4e, the piezoelectric-force microscopy (PFM) analysis results provided microstructural information about the ferroelectric domain and switching behavior of the HfO ₂ thin film.

The out-of-plane phase maps shown in Figures 4e, 4f, and 4g showed spatial contrast changes, indicating nanoscale ferroelectricity with both upward (red contrast) and downward (blue contrast) polarization. The domain was demonstrated, suggesting spontaneous polarization of HfO ₂ . Additionally, phase changes (about 180°) at a forcing voltage of about 1.0 V and a local hysteresis loop appeared in voltage sweeping in the ±2.0 V region, which leads to polarization switching and thus the device exhibits ferroelectric properties even at the nanoscale. It indicates possession. As further evidence, scanning a small area (1.6 × 1 μm ² ) with a tip voltage (V _tip ) of -2.0 V resulted in polarization flipping in the HfO2 film along the preferred direction. Moving forward, PFM measurements were performed over a large area (4 × 4 μm ² ). The image image consists of bright/dark contrast regions corresponding to a well-defined polarization across the region by applying an external bias of -2 V (see central region in Figure 4g), which strongly demonstrates the ferroelectric properties of the HfO ₂ thin film. hints

As previously reported, the ferroelectricity of HfO ₂ is associated with the non-centrosymmetric allorhombic phase Pca21, inside which polarization occurs in the [001] direction. Therefore, the polarization was calculated as a function of defect density through density functional theory (DFT) using the orthocomb phase.

As shown in Figure 4h, the magnitude of polarization increased with increasing defect density and reached a maximum value of 62 μCcm ^-2 at a defect density of 16.6%. On the other hand, as the defect density further increased, the polarization decreased rapidly.

As shown in Figure 4i, the potential barrier in orthorombic HfO2 was calculated to be about 177.4 meV, which was reduced to 153 meV at a defect density of 8.3%. However, the barrier height increased rapidly with further increase in defect density (see inset). DFT calculations show that the presence of oxygen vacancies in the HfO ₂ thin film not only enhances the polarization of the HfO ₂ thin film but also reduces the effective barrier to oxygen migration. Sikkim is shown.

Figure 5 shows the instantaneous photoresponse ('FIG. 5a', 'FIG. 5b') measured under self-buyer conditions for AgNWs/HfO ₂ /SiO ₂ /Si device, and a schematic diagram showing the operating mechanism ('FIG. 5c'). , the change in photoresponse corresponding to a single pulse of light ('Figure 5d'), the change in photoresponse according to pulsed light intensity ('Figure 5e', 'Figure 5f'), and the temperature-dependent change of the pyrophotocurrent peak ('Figure 5g). ', 'Figure 5h').

As shown in Figures 5a and 5b, since HfO ₂ (d = 10 nm) is an insulator, AgNWs/HfO 2 /SiO 2 /Si devices are expected to show no photocurrent signal upon near-infrared photon irradiation unless voltage is applied. , in fact, the device did not show photocurrent due to the photoelectric effect upon continuous and/or pulsed light irradiation (λ=940 nm, intensity (L)=4 mWcm ^-2 ). However, interestingly, as indicated by the vertical arrows in Fig. 5a, two distinct and sharp current peaks appeared during photoirradiation on/off at a switching speed of 100 μs under zero bias conditions, which correspond to the instantaneous photon irradiation-induced surface temperature. It is believed that it appeared due to change. In fact, with instantaneous photon irradiation, a change in surface temperature occurs, resulting in a potential difference between the top and bottom of the non-centrosymmetric material, an effect known as the pyro-electro-photoelectric effect.

The current on/off ratio (I _py /I _dark , I _py and I _dark represent the pyroelectric photocurrent and dark current of the device, respectively) under zero bias conditions (λ = 940 nm, intensity = 4 mWcm ^-2 ) is 2 It was measured to be 10 ³ , and this high I _py /I _dark value confirms that the device is highly sensitive to photoresponse. Additionally, the reproducible behavior was tested with a large number of pulsed light irradiations, and surprisingly, the current peaks appeared robustly and there was no notable peak decay during the test. The appearance of these peaks clearly shows that the device can sense light irradiation under self-bias conditions.

The generation of these two current peaks during light irradiation on/off conditions is due to the instantaneous light irradiation-induced surface temperature change, which may cause a change in the effective polarization and thereby the surface potential. In fact, instantaneous light irradiation increases the surface temperature from T to T+ΔT, which can adjust the effective polarization from P to P−ΔP, as schematically shown in Figure 5c. This change in polarization creates a difference in potential (i.e., pyroelectric-photoelectric potential) between the top (AgNWs) and bottom (low resistive Si) electrodes, which can cause the appearance of sharp peaks.

A sharp reverse current peak (-I _py ) was found under light irradiation OFF conditions, which mainly originates from the surface temperature flow and the resulting instantaneous reverse pyropotential. Meanwhile, continuous irradiation of light did not generate any photocurrent, confirming that isothermal heating cannot generate pyroelectric potential, and these results are in good agreement with the pyroelectric behavior of ferroelectric-based devices.

As a key requirement for practical application of the device, the response speed of the pyroelectric-optical detector was measured by instantaneous response.

An enlarged view of I _py during light irradiation ON conditions is shown in Figure 5d. The current reached 10 to 90% of the peak value within 60 μs (i.e., rise time) and gradually decayed. This ultrafast response time confirms that AgNWs/HfO2/SiO2/Si devices can be used to design ultrafast IR photodetectors. The current gradually decreased after the light on/off condition and returned to the initial level (I _dark ) within a short period of time (20 ms). This gradual change _{in Ipy} is related to the rate of change in surface temperature to return to thermal equilibrium. Therefore, this rapid decrease suggests that the depth of the conduction region is shallow. As further evidence of dominant dynamics, the current decrease beyond the peak value (see fitting dashed line in Figure 5d)

(t=time), which confirms that the pyroelectric current originates close to the surface of the HfO ₂ thin film.

Additionally, to confirm that the photocurrent originates purely due to HfO2, AgNWs/Al ₂ O ₃ /HfO ₂ /SiO ₂ /Si and AgNWs/Al ₂ O ₃ /SiO ₂ /Si were prepared, and similar conditions (L = The photocurrent was measured under 4 mWcm ^-2 ). However, none of these devices showed an Ipy peak, confirming the important and direct role of ferroelectric HfO ₂ in the pyroelectric-photonic response.

As shown in Figures 5e and 5f, as the irradiation intensity of light increased from 0.5 mWcm ^-2 to 4 mWcm ^-2 , the current size gradually increased from 8.3 nA to 68.67 nA. This light intensity-dependent change in _Ipy is due to the enhanced temperature change rate depending on light intensity. Additionally, the change in current can be expressed by the value [δ/I _mean ]×100%, where δ is the full width at half maxima of the distribution of _Ipy and the average current value, _Imean . , FWHM]. Interestingly, the variation remained below 3% in Ipy generation for all devices. In particular, these minimal changes enabled more precise control of reproducible device operation.

Since the instantaneous light irradiation-induced surface temperature change is the main cause of I _py , the effect of external temperature on I _py was additionally investigated. Figure 5g shows the change in _Ipy behavior with temperature change. The size of the I _py peak decreased as the overall device temperature increased, showing strong temperature-dependent performance. This change in I _py indicates that the effective polarization of the AgNWs/HfO ₂ /SiO ₂ /Si device can be controlled by external temperature. For clarity, the change in maximum I _py is plotted in Figure 5h, showing a gradual decrease with temperature, a result showing that the device can be applied to read non-contact temperature sensors.

Figure 6 is a graph showing the results of measuring the pyrophotonic response before and after applying a pulse voltage to the Si substrate of the AgNWs/HfO ₂ /SiO ₂ /Si device under the condition of continuous irradiation of pulsed light ('Figure 6a', ' Figure 6b'), to explain the operating mechanism of the AgNWs/HfO ₂ /SiO ₂ /Si device ('Figure 6c'), and the change in optical response according to the type of applied pulse voltage ('Figure 6d'). The measured results ('Figure 6e') are shown.

As shown in Figures 6a and 6b, the device showed reproducible Ipy generation upon pulsed light irradiation (3.2 mWcm ^-2 ). After a pulse voltage of +0.8 V was applied, the Ipy of the device decreased below the system measurement limit, and after a pulse voltage of -8.0 V of opposite polarity (w=10 ms) was applied, the Ipy of the device decreased to the initial level (42 nA, L=3.2 mWcm ^-2 ) was reset.

To obtain better control over Ipy tuning, when voltage pulses (w=10 ms) of different magnitudes of +6.0 V (see middle panel) and +4.0 V (see right panel) were applied to the device, the Ipy It was found that the size changes depending on the size of the pulse voltage. For example, Ipy was reduced to 3.7 nA and 7.96 nA by +6.0V and +4.0V, respectively. This dynamic and controlled change in Ipy indicates that the photosensing properties of the device can be tuned by short electrical pulses. In particular, as shown in Figure 6b, Ipy was reduced from 42 to 3.7 nA by a short electrical pulse (+6.0 V, w=10 ms), whereas an electrical pulse of opposite polarity (-6.0 V, m=10 ms) The current increased nominally (about 7.6 nA).

To understand the dynamic regulation of Ipy using pulse voltage, the IV curve of the device was measured by double sweeping within ±3.0 V, and the results showed that the IV curve was asymmetric with respect to the applied pulse voltage and showed a well-defined hysteresis loop opening. showed it Additionally, the current ratio from -3.0 to +3.0 V was measured to be 2×10 ⁴ , indicating the formation of a Schottky-like junction in the AgNWs/HfO ₂ /SiO ₂ /Si device. Therefore, the dominant charge transfer mechanism across the device can be described by Schottky emission, and mathematically, the Schottky emission that dominates charge transfer can be defined as Equation 1 below.

[Formula 1]

In Equation 1 above, A is a constant, T is the absolute temperature, Φ _b is the barrier height, k _B is the Boltzmann constant, e is the electrical charge, ε _r is the effective dielectric constant, and ε ₀ is the vacuum permittivity. .

It has been previously reported that migration of oxygen vacancies can cause hysteresis loop opening in the IV curve of HfO ₂ -based devices. In fact, the redox reaction that occurred within the device (

), the migration and accumulation of oxygen ions will regulate the effective Schottky barrier. The slope of the ln(I) versus V1/2 curve is

Effective barrier thickness (

), the slopes of the forward (0→+Ve V) and opposite (+Ve→OV) voltage sweepings were measured by linear fitting, and the slopes for forward and reverse scanning ranged from 5.84 to 4.90. changed to , which indicates adjustment of the effective barrier thickness. Based on experimental analysis, the proposed photosensing mechanism is schematically illustrated in Figure 6c, which suggests quantitative migration of oxygen ions under the influence of an applied electric field.

As measured by cross-section TEM, the HfO ₂ thin film contains abundant oxygen defects and loosely bound oxygen, so when a positive voltage is applied to the top electrode of the device, oxygen ions are generated and migrate, which increases the effective barrier thickness. (

) decreases. This migration of oxygen ions changes the effective polarization of the device, which can cause the photosensing behavior of the device.

As shown in FIGS. 6D and 6E, the change in _Ipy is measured by sequential application of positive pulse voltages (w=10 ms) with positive polarity from +1.0 to +8.0 and different magnitudes. , based on this

The responsivity (R) defined by (A is the effective area of the device (0.6 cm ² ), and P is the irradiation intensity of light) was calculated.

The responsivity (R) gradually decreased from 68.78 to 1.87 μWcm ^-2 as the magnitude of the pulse voltage increased from +1.0 to +8.0V (see the dotted arrow in Figure 6e), but as the black dotted arrow in Figure 6e, As can be seen, the responsivity (R) did not follow a similar behavior in the case of a decrease in the magnitude of the applied pulse voltage. Similarly, the responsivity (R) increased for negative polarity pulses, and a negative voltage was required to reset the R value set to the initial level (R=68.87 μWcm ^-2 ).

In conclusion, loop opening appeared in the responsivity (R) versus pulse voltage curve, indicating that the device of the present invention has the potential to store quantitative memory. That is, in the devices of the present invention, controllable manipulation of photoresponsive behavior using electrical pulses could not only provide optically readable ferroelectric memory, but also provide the opportunity to construct trainable artificial versions.

Although the present invention has been described above with reference to preferred embodiments, those skilled in the art can make various modifications and changes to the present invention without departing from the spirit and scope of the present invention as set forth in the following patent claims. You will understand that it is possible.

Claims (13)

1. In an optical sensor that detects a target,

   a light source that irradiates pulsed light in the direction of the target;

   a sensing element including a ferroelectric thin film that generates a current by a temperature change caused by pulsed light reflected by the target and a first electrode disposed on the surface of the ferroelectric thin film; and

   An optical sensor comprising: a current meter electrically connected to the first electrode and measuring a current generated by the ferroelectric thin film.
2. According to paragraph 1,

   An optical sensor, wherein the ferroelectric thin film includes a polycrystalline hafnium oxide thin film containing an orthorombic (Pca21) phase.
3. According to paragraph 2,

   An optical sensor, wherein the ferroelectric thin film has an oxygen defect density of 9 to 15%.
4. According to paragraph 1,

   An optical sensor, wherein the light source has a pulse width of 0.01 to 100 ms and irradiates pulsed light with a wavelength in the infrared region.
5. According to paragraph 1,

   The ferroelectric thin film is disposed on a substrate,

   The first electrode is disposed to contact the upper surface of the ferroelectric thin film and includes a transparent electrode through which pulsed light can pass.
6. According to clause 5,

   The first electrode is an optical sensor, characterized in that it includes a conductive metal nanowire electrode, a conductive metal electrode with a mesh structure, a conductive carbon material electrode, a transparent conductive oxide electrode, or a transparent conductive polymer electrode.
7. According to clause 5,

   a second electrode disposed between the substrate and the ferroelectric thin film; and

   The optical sensor is electrically connected to the second electrode and further comprises an electric field applicator that applies an electric field to the ferroelectric thin film to adjust the polarization state of the ferroelectric thin film.
8. A ferroelectric thin film having pyroelectric properties that generates an electric current by a temperature change caused by pulsed light, a first electrode disposed in contact with a first side of the ferroelectric thin film, and the ferroelectric thin film opposing the first side. a memory element including a second electrode disposed below the second surface of;

   an electric field applicator electrically connected to the second electrode and adjusting the polarization state of the ferroelectric thin film by applying an electric field to the ferroelectric thin film;

   a light source that irradiates pulsed light to the ferroelectric thin film; and

   A current detector electrically connected to the first electrode, generated by a temperature change caused by the pulsed light, and measuring a photocurrent generated by the ferroelectric thin film whose size changes depending on the polarization state of the ferroelectric thin film. Including, memory device.
9. According to clause 8,

   The electric field applicator controls the polarization state of the ferroelectric thin film by applying one or more pulse voltages with a width of 1 to 100 ms to the second electrode.
10. According to clause 8,

    wherein the one or more pulse voltages include one or more positive pulse voltages and one or more negative pulse voltages.
11. According to clause 8,

    An optical sensor, wherein the ferroelectric thin film includes a polycrystalline hafnium oxide thin film containing an orthorombic (Pca21) phase.
12. According to clause 11,

    An optical sensor, wherein the ferroelectric thin film has an oxygen defect density of 9 to 15%.
13. According to clause 8,

    An optical sensor, wherein the light source has a pulse width of 0.01 to 100 ms and irradiates pulsed light with a wavelength in the infrared region.

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