Optical sensor, optical sensor array, and method for manufacturing an optical sensor

Abstract

The present invention provides an optical sensor with a configuration that prevents a point from occurring where the change in drain current due to the pyroelectric effect becomes zero. [Solution] The optical sensor 2A comprises a ferroelectric layer 111A having a front surface and a back surface, a graphene layer 114 provided on the front surface side of the ferroelectric layer 111A, a source electrode 115 and a drain electrode 116 electrically connected to each other by the graphene layer 114, and a base layer 112A bonded to the graphene layer 114 and injecting holes, which are charge carriers, into the graphene layer 114. The ferroelectric layer 111A has the characteristic of spontaneously polarizing in a first direction from the back surface toward the front surface, with the portion approaching the graphene layer 114 being positively biased and the portion away from the graphene layer 114 being negatively biased.

Description

Technical Field

[0001] The present invention relates to an optical sensor, an optical sensor array, and a method for manufacturing the optical sensor.

Background Art

[0002] Conventionally, as optical sensors, for example, infrared sensors used in infrared cameras capable of capturing images with long-wavelength infrared light having a wavelength of 8 μm to 15 μm are known.

[0003] In recent years, visualization of the distribution and emission of gas species such as propane (C3H8) and methane (CH4) has been desired. In particular, methane is one of the greenhouse gases, and its absorption wavelength is in the range of 7 μm to 8 μm. Therefore, the distribution and emission of methane can be visualized by an infrared camera capable of capturing images in a wavelength band of, for example, 6 μm to 20 μm including the long-wavelength infrared wavelength region.

[0004] As conventional techniques related to optical sensors, for example, Patent Documents 1 and 2 disclose graphene FET devices using a ferroelectric layer. A graphene FET device using a ferroelectric layer is an optical sensor in which the gate voltage changes due to polarization change caused by the pyroelectric effect of the ferroelectric layer, and a change in drain current with respect to the change in gate voltage is detected. In addition, Patent Document 3 discloses a technique for obtaining p-type graphene by forming a metal oxide film such as MoO3 (molybdenum oxide) or WO3 (tungsten oxide) on a graphene layer in a graphene FET device. Non-Patent Document 1 discloses a technique for obtaining n-type graphene by forming a layer of HfO2 (hafnium oxide) having oxygen vacancies as a layer below graphene in a graphene FET device. Non-Patent Document 2 discloses a technique for obtaining n-type graphene by forming a thin film of Si3N4 (silicon nitride) provided with an amine group on the graphene surface in a graphene FET device.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] International Publication No. 2018 / 163496 [Patent Document 2] International Publication No. 2024 / 100784 [Patent Document 3] Patent No. 05814348 [Non-patent literature]

[0006] [Non-Patent Document 1] Cheng Zhang, Dan Xie, Jian-Long Xu, Xin-Ming Li, Yi-Lin Sun, Rui-Xuan Dai, Xian Li, Hong-Wei Zhu3 "HfO2 dielectric thickness dependence of electrical properties in graphene field effect transistors with double conductance minima" JOURNAL OF APPLIED PHYSICS 118, 144301 (2015) [Non-Patent Document 2] Zegao Wang, Pingjian Li, Yuanfu Chen, Jingbo Liu, Fei Qi, Hongjun Tian, ​​Binjie Zheng, Jinhao Zhou "Air-stable n-type doping of graphene from overlying Si3N4film" Applied Surface Science 307 (2014) 712-715 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, depending on the configuration of the graphene FET device, the manufacturing process, the measurement environment, etc., the combination of p-doping or n-doping of graphene and the polarization direction of the ferroelectric layer can result in zero change in the drain current due to the pyroelectric effect of the ferroelectric layer, leading to a lack of stability as a sensor. The present invention has been made in view of the above problems, and aims to provide an optical sensor, an optical sensor array, and a method for manufacturing an optical sensor, with a configuration that does not result in a point where the change in drain current due to the pyroelectric effect becomes zero. [Means for solving the problem]

[0008] A light sensor according to one aspect of the present invention comprises a pyroelectric layer having a front surface and a back surface; a two-dimensional material layer provided on the front surface side of the pyroelectric layer and being p-type or n-type; a first electrode and a second electrode electrically connected to each other in the two-dimensional material layer; and an insulating layer provided on the two-dimensional material layer, wherein, when the two-dimensional material layer is p-type, the pyroelectric layer has the characteristic that when the temperature rises, the portion on the front surface becomes positively biased and the portion on the back surface becomes negatively biased, spontaneously polarizing in a first direction from the back surface toward the front surface; and when the two-dimensional material layer is n-type, the pyroelectric layer has the characteristic that when the temperature rises, the portion on the front surface becomes negatively biased and the portion on the back surface becomes positively biased, spontaneously polarizing in a second direction from the front surface toward the back surface. In the above configuration, the two-dimensional material layer may be formed from single-layer graphene, multilayer graphene, randomly stacked graphene, graphene nanoribbons, transition metal dichalcogenide, black phosphorus, silicene, or germanene. In the above configuration, the insulating layer may be a layer that dops the two-dimensional material layer into a p-type or n-type. In the above configuration, the insulating layer may consist of at least one of the following: a base layer bonded to the pyroelectric layer side of the two-dimensional material layer, or a protective layer bonded to the side of the two-dimensional material layer opposite to the pyroelectric layer side.

[0009] In the above configuration, the insulating layer doped into the p-type two-dimensional material layer may be made of a material with a work function greater than that of the two-dimensional material layer, and the insulating layer doped into the n-type two-dimensional material layer may be made of a material with a work function smaller than that of the graphene layer. In the above configuration, the insulating layer that dops the two-dimensional material layer in an n-type manner may be composed of a layer having amine groups. In the above configuration, the insulating layer that dops the two-dimensional material layer in an n-type manner may be made of a material having oxygen vacancies.

[0010] In the above configuration, the material having a work function greater than the work function of the two-dimensional material layer is one of the following: TiO2 (titanium oxide), Si3N4 (silicon nitride), HfO2 (hafnium oxide), SiO2 (silicon dioxide), Al2O3 (aluminum oxide), MoO3 (molybdenum oxide), and WO3 (tungsten oxide), and the material having a work function smaller than the work function of the two-dimensional material layer may be La2O3 (lanthanum oxide).

[0011] In the above configuration, when the two-dimensional material layer is p-type and the pyroelectric layer spontaneously polarizes in the first direction, the curve showing the relationship between the voltage applied to the third electrode and the current flowing from one of the first and second electrodes to the other may have a greater slope on the hole conduction side than the slope on the electron conduction side. Alternatively, when the two-dimensional material layer is n-type and the pyroelectric layer spontaneously polarizes in the second direction, the curve may have a greater slope on the electron conduction side than the slope on the hole conduction side. In the above configuration, the first or second direction may have an angular distribution such that 85% or more of the angles indicating the polarization directions at multiple polarization points are within the range of 70° to 110°. In the above configuration, when the two-dimensional material layer is p-type, the first electrode and the second electrode may be made of a material that has the function of injecting holes into the two-dimensional material layer, and when the two-dimensional material layer is n-type, the first electrode and the second electrode may be made of a material that has the function of injecting electrons into the two-dimensional material layer.

[0012] Furthermore, an optical sensor array according to one aspect of the present invention comprises a plurality of the above-mentioned optical sensors, and the plurality of the above-mentioned optical sensors are arranged in an array. Furthermore, a method for manufacturing a light sensor according to one aspect of the present invention includes the steps of forming a pyroelectric layer having a front surface and a back surface, wherein when the temperature rises, the front surface side becomes positively biased and the back surface side becomes negatively biased, and spontaneously polarizes in a first direction from the back surface toward the front surface, and forming a p-type two-dimensional material layer on the front surface side of the pyroelectric layer.

[0013] In the above configuration, the process further includes a step of forming an insulating layer that is bonded to the two-dimensional material layer and has the property of doping the two-dimensional material layer to a p-type, wherein in the step of forming the insulating layer, at least one of the following may be formed as the insulating layer: a base layer bonded to the pyroelectric layer side of the two-dimensional material layer, or a protective layer bonded to the side of the two-dimensional material layer opposite to the pyroelectric layer side.

[0014] Furthermore, a method for manufacturing a light sensor according to one aspect of the present invention includes the steps of forming a pyroelectric layer having a front surface and a back surface, wherein when the temperature rises, the front surface becomes negatively biased and the back surface becomes positively biased, and spontaneously polarizes in a second direction from the back surface toward the front surface, and forming an n-type two-dimensional material layer on the front surface side of the pyroelectric layer.

[0015] In the above configuration, the method further includes a step of forming an insulating layer that is bonded to the two-dimensional material layer and has a property of n-type doping the two-dimensional material layer, and in the step of forming the insulating layer, as the insulating layer, at least one of a base layer bonded to the surface of the two-dimensional material layer on the pyroelectric layer side, or a protective layer bonded to the surface of the two-dimensional material layer opposite to the pyroelectric layer side may be formed. [Effects of the Invention]

[0016] According to the present invention, a point at which the change in drain current due to the pyroelectric effect of the pyroelectric layer becomes zero does not occur, so the change value of the drain current due to the pyroelectric effect can be measured stably. [Brief Description of the Drawings]

[0017] [Figure 1] This is a schematic diagram showing the configuration of the sensor module according to the first embodiment. [Figure 2] (a) is a schematic diagram showing a first configuration example of the photosensor according to the first embodiment, and (b) is a diagram showing the drain current Id with respect to the gate voltage Vg in the first configuration example. [Figure 3] (a) is a schematic diagram showing a second configuration example of the photosensor 2 according to the first embodiment, and (b) is a diagram showing the drain current Id with respect to the gate voltage Vg in the second configuration example. [Figure 4] (a) and (b) are diagrams showing changes in the Id-Vg curve for the combination of +Z orientation and p-doping and the combination of -Z orientation and p-doping according to the first embodiment. [Figure 5] This is a diagram showing the Id-Vg curve of the ferroelectric layer 111 at each experimental temperature. [Figure 6] (a) and (b) are diagrams showing changes in the Id-Vg curve for the combination of +Z orientation and n-doping and the combination of -Z orientation and n-doping according to the first embodiment. [Figure 7A](a) is a diagram showing the Id-Vg characteristics of the first graphene FET fabricated using aluminum oxide as the base material, (b) is a cross-sectional view showing the structure of the first graphene FET, and (c) is a partial plan view of the first graphene FET. [Figure 7B] (a) is a figure showing the Id-Vg characteristics of a second graphene FET fabricated using silicon dioxide as the base material, (b) is a cross-sectional view showing the structure of the second graphene FET, and (c) is a partial plan view of the second graphene FET. [Figure 8] (a) is a graph showing the measurement results of the polarization direction of the ferroelectric material, and (b) is a diagram showing the configuration of the measurement substrate 200 used for the measurement. [Figure 9] (a) to (d) are energy band diagrams of the separated and joined states of the graphene layer and the doping layer. [Figure 10] This figure shows a method for manufacturing the optical sensor 2A according to the first embodiment. [Figure 11] This figure shows the manufacturing method for the optical sensor 2B of the first embodiment. [Figure 12] This figure shows the configuration and manufacturing method of the optical sensor 2C according to the second embodiment. [Figure 13] This figure shows the configuration and manufacturing method of the optical sensor 2D according to the second embodiment. [Figure 14] (a) and (b) are figures showing the changes in the Id-Vg curve for combinations of +Z orientation and p-doping and -Z orientation and p-doping according to the third embodiment. [Figure 15] (a) and (b) are figures showing the changes in the Id-Vg curve for combinations of +Z orientation and n-doping and -Z orientation and n-doping according to the third embodiment. [Figure 16] This is a schematic diagram showing the configuration of the sensor module in the modified example. [Modes for carrying out the invention]

[0018] Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the attached drawings. The embodiments described below are merely examples of means for realizing the present invention, and should be modified or changed as appropriate depending on the configuration of the apparatus to which the present invention is applied and various conditions. The present invention is not limited to the embodiments described below.

[0019] Furthermore, in the following drawings, identical or similar parts are denoted by the same or similar reference numerals. However, it should be noted that the drawings are schematic, and the vertical and horizontal dimensions and scale of the members or parts may differ from the actual dimensions. Therefore, specific dimensions and scales should be determined by referring to the following explanation. Also, it goes without saying that there may be parts where the dimensional relationships and ratios differ between drawings. [First Embodiment] 〔composition〕 First, a first embodiment of the present invention will be described. Figures 1 to 11 show the first embodiment. Figure 1 is a schematic diagram showing the configuration of the sensor module 1 according to the first embodiment. In Figure 1, the portion of the optical sensor 2 is shown in cross-section. The sensor module 1 according to the first embodiment comprises a light sensor 2 and a detection circuit 3, as shown in Figure 1. In Figure 1, the height direction of the light sensor 2 is the Z direction, the length direction of the light sensor 2 is the X direction, and the direction perpendicular to the Z direction and the X direction is the Y direction.

[0020] The optical sensor 2 comprises a semiconductor substrate 110, a ferroelectric layer 111, a base layer 112, a graphene layer 114, a source electrode 115, and a drain electrode 116. The ferroelectric layer 111 is provided on the semiconductor substrate 110, and the graphene layer 114 is provided on the base layer 112. The semiconductor substrate 110 and the graphene layer 114 are not in contact with each other because they are provided via the ferroelectric layer 111 and the base layer 112. The semiconductor substrate 110 is a semiconductor substrate containing impurities such as silicon, and functions as a gate electrode. This gate electrode may be formed of a metal film such as Au.

[0021] The source electrode 115 and drain electrode 116 are positioned on the substrate layer 112, sandwiching the graphene layer 114. The source electrode 115 and drain electrode 116 are electrically connected to the X-direction end of the graphene layer 114, respectively. The graphene layer 114 corresponds to a channel through which current flows. The source electrode 115 and drain electrode 116 are also referred to as contact electrodes. The gate terminal Tg is electrically connected to the semiconductor substrate 10, and the source terminal Ts and drain terminal Td are electrically connected to the source electrode 115 and drain electrode 116, respectively.

[0022] The semiconductor substrate 110 is, for example, an n-type or p-type silicon substrate doped with impurities. The ferroelectric layer 111 is, for example, a layer of LiNbO3 (lithium niobate), which is both a type of ferroelectric and a type of pyroelectric. LiNbO3 absorbs infrared light in the wavelength range of 6 μm to 20 μm and generates heat, causing a change in polarization due to the pyroelectric effect. When the optical sensor 2 detects infrared light or light of other wavelengths, a ferroelectric material other than LiNbO3 is used as the material for the ferroelectric layer 111. Examples of ferroelectric materials that can be used for the ferroelectric layer 111 include BaTiO3 (barium titanate), LiTaO3 (lithium tantalate), SrTiO3 (strontium titanate), PZT (lead zirconate titanate), SBT (strontium bismuth tantalate), BFO (bismuth ferrite), ZnO (zinc oxide), and HfO2 (hafnium oxide).

[0023] The base layer 112 is an insulating layer, and is an inorganic insulating film such as a silicon oxide film or a silicon nitride film. The source electrode 115 and the drain electrode 116 each have, for example, an adhesion film that adheres closely to the base layer 112 and a low-resistance film provided on the adhesion film. The adhesion film is, for example, a titanium film or a chromium film, and the low-resistance film is, for example, a gold film, a copper film, or an aluminum film.

[0024] The graphene layer 114 is an example of a two-dimensional material layer, and is a so-called monolayer (with a thickness of approximately 0.34 nm) consisting of a single atomic layer of graphene. In addition to single-layer graphene, the two-dimensional material layer may also be multilayer graphene, randomly stacked graphene, graphene nanoribbons, transition metal dichalcogenides, black phosphorus, silicene, germanene, etc. The detection circuit 3 is electrically connected to the gate terminal Tg, source terminal Ts, and drain terminal Td. The detection circuit 3 detects the drain current Id flowing between the source electrode 115 and the drain electrode 116 due to the incidence of light onto the ferroelectric layer 111.

[0025] Here, the ferroelectric layer 111 receives infrared light, which generates a voltage due to the pyroelectric effect, causing a change in the drain current Id flowing between the source electrode 115 and the drain electrode 116. In other words, the drain current Id flowing through the graphene layer 114 changes due to the polarization change in the ferroelectric layer 111 due to light reception. Thus, the graphene layer 114, the pair of source electrodes 115 and drain electrodes 116, and the ferroelectric layer 111 constitute a FET, and the polarization change in the ferroelectric layer 111 acts as a change in the gate voltage Vg.

[0026] The detection circuit 3 detects the drain current Id flowing through the graphene layer 114 and performs infrared detection based on the detected drain current Id. Because the graphene layer 114 has high charge mobility, it has high sensitivity in infrared detection, i.e., high temperature resolution. Based on the detected drain current Id, the detection circuit 3 measures the change ΔId of this drain current Id with respect to the reference current Idr. Then, it detects infrared radiation based on the measurement result. Here, the reference current Idr is, for example, the drain current Id when the gate voltage Vg is 0V in a state where infrared radiation is not being received. The value of the reference current Idr is a value known at the time of design. [Configuration of Light Sensor 2A]

[0027] Next, the configuration of the optical sensor 2A, which is a first configuration example of the optical sensor 2, will be described based on Figures 2(a) and (b). Figure 2(a) is a schematic diagram showing the first configuration example of the optical sensor 2 according to the first embodiment, and Figure 2(b) is a diagram showing the drain current Id with respect to the gate voltage Vg in the first configuration example. In Figure 2(b), the horizontal axis is the gate voltage Vg, and the vertical axis is the drain current Id.

[0028] As shown in Figure 2(a), in the photosensor 2A, the ferroelectric layer 111 is configured such that the surface side, which is closer to the graphene layer 114, is positively biased, and the back side, which is further away from the graphene layer 114, is negatively biased, causing spontaneous polarization in a first direction from the back to the front. Positive bias indicates that positive charges are unevenly distributed, and negative bias indicates that negative charges are unevenly distributed. Hereinafter, the ferroelectric layer 111 that spontaneously polarizes in this first direction will be referred to as the "ferroelectric layer 111A," and the direction of this polarization in the first direction will be referred to as the +Z direction. Furthermore, the photosensor 2A is equipped with a base layer 112A instead of the base layer 112.

[0029] In its initial state, when not receiving infrared light, the light sensor 2A has an Id-Vg characteristic where the drain current Id is minimal when the gate voltage Vg is 0V, as shown by the solid curve in Figure 2(b). This point where the drain current Id is minimal is called the charge neutral point (CNP). In this state, when the ferroelectric layer 111A receives infrared light, a voltage of +ΔVpol is generated due to the polarization change caused by the pyroelectric effect, so the gate voltage Vg changes to Vg+ΔVpol. That is, as shown by the dashed curve in Figure 2(b), the gate voltage Vg at which the drain current Id is the charge neutral point shifts in the positive direction. This change in gate voltage Vg causes a current change of ΔId, as shown in Figure 2(b). [Configuration of Optical Sensor 2B]

[0030] Next, the configuration of the second configuration example of the optical sensor 2, optical sensor 2B, will be described based on Figures 3(a) and (b). Figure 3(a) is a schematic diagram showing the second configuration example of the optical sensor 2 according to the first embodiment, and Figure 3(b) is a diagram showing the drain current Id with respect to the gate voltage Vg in the second configuration example. In Figure 3(b), the horizontal axis is the gate voltage Vg, and the vertical axis is the drain current Id.

[0031] As shown in Figure 3(a), in the photosensor 2B, the ferroelectric layer 111 is configured such that the surface side, which is closer to the graphene layer 114, is negatively biased, and the back side, which is further away from the graphene layer 114, is positively biased, causing the ferroelectric layer 111 to spontaneously polarize in a second direction from the surface to the back. Hereinafter, the ferroelectric layer 111 that spontaneously polarizes in this second direction will be referred to as the "ferroelectric layer 111B," and the direction of this polarization in the second direction will be referred to as the -Z direction. Furthermore, the photosensor 2B is equipped with a base layer 112B instead of the base layer 112.

[0032] In its initial state, when not receiving infrared light, the optical sensor 2B has an Id-Vg characteristic where the drain current Id is minimized when the gate voltage Vg is 0V, as shown by the solid curve in Figure 3(b). In this state, when the ferroelectric layer 111A receives infrared light, a voltage of -ΔVpol is generated due to the polarization change caused by the pyroelectric effect, so the gate voltage Vg changes to Vg-ΔVpol. That is, as shown by the dashed curve in Figure 3(b), the gate voltage Vg at which the drain current Id is the charge neutral point shifts in the negative direction. This change in gate voltage Vg causes a current change of ΔId, as shown in Figure 3(b). [Relationship between polarization orientation of ferroelectric layer and p-doped graphene]

[0033] Next, based on Figures 4(a) and (b) and Figure 5, the changes in the Id-Vg curve when the graphene layer 114 is p-doped in the +Z and -Z polarization configurations of the optical sensors 2A and 2B will be explained. Figure 4(a) shows the changes in the Id-Vg curve for the combination of +Z orientation and p-doping, and Figure 4(b) shows the changes in the Id-Vg curve for the combination of -Z orientation and p-doping. Here, p-doping means that holes are injected into the graphene layer 114 as charge carriers. Also, in Figures 4(a) and (b), the horizontal axis is the gate voltage Vg and the vertical axis is the drain current Id. Figure 5 shows the Id-Vg curves of the ferroelectric layer 111 for each experimental temperature.

[0034] In the case of a photosensor 2A having a ferroelectric layer 111A that spontaneously polarizes in the +Z direction, when the graphene layer 114 is p-doped, as shown by the solid curve in Figure 4(a), in the initial state when infrared light is not being received, the temperature of the ferroelectric layer 111A becomes T°C and the gate voltage Vg at which the drain current Id is the charge neutral point shifts to the positive side from 0V. In this state, when infrared light is received and the temperature of the ferroelectric layer 111A rises to T+ΔT1°C, as shown by the dashed curve in Figure 4(a), the gate voltage Vg at which the drain current Id is the charge neutral point shifts further to the positive side. Subsequently, when infrared light is received and the temperature of the ferroelectric layer 111A rises to T+ΔT2°C, as shown by the dashed curve in Figure 4(a), the gate voltage Vg at which the drain current Id is the charge neutral point shifts even further to the positive side.

[0035] In other words, from the initial state where the gate voltage Vg, which is the charge neutral point at the drain current Id, is shifted to the positive side from 0V, a temperature rise in the ferroelectric layer 111A occurs due to infrared radiation. The voltage generated by the pyroelectric effect of the ferroelectric layer 111A due to infrared radiation reception increases as the temperature of the ferroelectric layer 111A increases. In particular, when the polarization direction is +Z, as shown in the experimental results in Figure 5, the Id-Vg curve shifts to the positive side as the temperature rises. Therefore, the change ΔId from the initial state of drain current Id at gate voltage Vg 0V is never 0. In Figure 5, the solid line shows the initial state, and the temperature of the ferroelectric layer is 24.2℃. In this initial state, as the temperature of the ferroelectric layer rises due to ultraviolet radiation reception, it can be seen that the Id-Vg curve shifts to the positive side, as shown by the dashed, dotted, and dotted lines in Figure 5.

[0036] On the other hand, in the case of the photosensor 2B having a ferroelectric layer 111B that spontaneously polarizes in the -Z direction, when the graphene layer 114 is p-doped, as shown by the solid curve in Figure 4(b), in the initial state when infrared light is not being received, the temperature of the ferroelectric layer 111A becomes T℃ and the gate voltage Vg at which the drain current Id is the charge neutral point shifts to the positive side from 0V. In this state, when infrared light is received and the temperature of the ferroelectric layer 111B rises to T+ΔT1℃, as shown by the dashed curve in Figure 4(b), the gate voltage Vg at which the drain current Id is the charge neutral point shifts to the negative side. Subsequently, when infrared light is received and the temperature of the ferroelectric layer 111B rises to T+ΔT2℃, ​​as shown by the dashed curve in Figure 4(b), the gate voltage Vg at which the drain current Id is the charge neutral point shifts further to the negative side.

[0037] In other words, as the temperature of the ferroelectric layer 111B rises due to infrared radiation reception, the gate voltage Vg at which the drain current Id is the charge neutral point shifts to the negative side. This means that when the polarization direction is -Z, the Id-Vg curve shifts to the negative side as the temperature of the ferroelectric layer 111B rises. As a result, a situation occurs where the drain current Id in the initial state at a gate voltage Vg of 0V and the drain current Id after the temperature rise coincide. In the example in Figure 4(b), the drain current Id in the initial state and the drain current Id when the temperature of the ferroelectric layer 111B rises to T+ΔT2℃ coincide. That is, a point occurs where the current change ΔId from the initial drain current Id is 0. [Relationship between polarization orientation of ferroelectric layer and n-doped graphene]

[0038] Next, based on Figures 6(a) and (b), the changes in the Id-Vg curve when the graphene layer 114 is n-doped in the +Z and -Z polarization configurations of the optical sensors 2A and 2B will be explained. Figure 6(a) shows the changes in the Id-Vg curve for the combination of +Z orientation and n-doping, and Figure 6(b) shows the changes in the Id-Vg curve for the combination of -Z orientation and n-doping. In Figures 6(a) and (b), the horizontal axis is the gate voltage Vg and the vertical axis is the drain current Id. Here, n-doping indicates that electrons are injected into the graphene layer 114 as charge carriers.

[0039] In the case of a photosensor 2A having a ferroelectric layer 111A that spontaneously polarizes in the +Z direction, when the graphene layer 114 is n-doped, as shown by the solid curve in Figure 6(a), in the initial state when infrared light is not received, the temperature of the ferroelectric layer 111A becomes T℃ and the gate voltage Vg at which the charge neutral point occurs shifts to the negative side from 0V. In this state, when infrared light is received and the temperature of the ferroelectric layer 111A rises to T+ΔT1℃, as shown by the dashed curve in Figure 6(a), the gate voltage Vg at which the charge neutral point occurs shifts to the positive side. Subsequently, when infrared light is received and the temperature of the ferroelectric layer 111A rises to T+ΔT2℃, ​​as shown by the dashed curve in Figure 6(a), the gate voltage Vg at which the charge neutral point occurs shifts further to the positive side.

[0040] In other words, as the temperature of the ferroelectric layer 111A rises due to infrared radiation reception, the gate voltage Vg at which the drain current Id is the charge neutral point shifts to the positive side. This means that when the polarization direction is +Z, the Id-Vg curve shifts to the positive side as the temperature of the ferroelectric layer 111A rises. As a result, a situation occurs where the drain current Id in the initial state at a gate voltage Vg of 0V and the drain current Id after the temperature rise coincide. In the example in Figure 6(a), the drain current Id in the initial state and the drain current Id when the temperature of the ferroelectric layer 111A rises to T+ΔT2℃ coincide. That is, a point occurs where the current change ΔId from the initial drain current Id is 0.

[0041] On the other hand, in the case of the photosensor 2B having a ferroelectric layer 111B that spontaneously polarizes in the -Z direction, when the graphene layer 114 is n-doped, as shown by the solid curve in Figure 6(b), in the initial state when infrared light is not being received, the temperature of the ferroelectric layer 111A becomes T℃ and the gate voltage Vg at which the drain current Id is the charge neutral point shifts to the negative side from 0V. In this state, when infrared light is received and the temperature of the ferroelectric layer 111B rises to T+ΔT1℃, as shown by the dashed curve in Figure 6(b), the gate voltage Vg at which the drain current Id is the charge neutral point shifts further to the negative side. Subsequently, when infrared light is received and the temperature of the ferroelectric layer 111B rises to T+ΔT2℃, ​​as shown by the dashed curve in Figure 6(b), the gate voltage Vg at which the drain current Id is the charge neutral point shifts even further to the negative side.

[0042] In other words, from the initial state where the gate voltage Vg, which is the charge neutral point at the drain current Id, is shifted to the negative side from 0V, a temperature rise in the ferroelectric layer 111B occurs due to infrared radiation. The voltage generated by the pyroelectric effect of the ferroelectric layer 111A due to the reception of infrared radiation increases as the temperature of the ferroelectric layer 111B rises. In particular, when the polarization direction is -Z, the Id-Vg curve shifts to the negative side as the temperature of the ferroelectric layer 111B rises. Therefore, the change ΔId of the drain current Id from the initial state at a gate voltage Vg of 0V is never 0.

[0043] Based on the above, for the photosensor 2A, the point at which the current change ΔId becomes zero can be avoided by p-doping the graphene layer 114, and for the photosensor 2B, the point at which the current change ΔId becomes zero can be avoided by n-doping the graphene layer 114. In other words, in the first embodiment, the underlayer 112A of the photosensor 2A is made of an insulating material that can inject holes into the graphene layer 114. The underlayer 112B of the photosensor 2B is made of an insulating material that can inject electrons into the graphene layer 114.

[0044] Whether graphene is p-doped or n-doped depends on various factors, such as the material properties of the components in contact with the graphene and the film deposition conditions. Furthermore, when an FET device is packaged in a certain atmosphere and used in a real environment, that atmosphere is thought to affect the graphene doping. In addition, the dielectric constant of the layer in contact with the graphene is also thought to affect the graphene doping. Generally, when graphene is p-doped and becomes a hole conductor, it can be said that the material in contact with the graphene is taking electrons from the graphene. Conversely, when graphene is n-doped and becomes an electron conductor, it can be said that the material in contact with the graphene is donating electrons to the graphene. Considering the controllability of the device, the inventors believe it is desirable to control the doping of graphene by using the material in contact with the graphene, particularly the underlayer and protective layer, or either or both, which have a large contact area with the graphene. Traditionally, it has been reported that doping is determined by the relationship between the work function of graphene and the work function of the layer in contact with the graphene. When the work function of the material is greater than that of graphene, the material in contact with the graphene removes electrons from the graphene, resulting in p-doping of the graphene. Conversely, when the work function of the material is smaller than that of graphene, the material in contact with the graphene donates electrons to the graphene, resulting in n-doping of the graphene. Furthermore, when there is oxygen depletion in the layer in contact with graphene, it is thought that graphene becomes n-doped in order to donate electrons to the graphene. This was reported in Non-Patent Document 1 mentioned above and was considered as the reason why GFET devices with HfO2 as the substrate are n-doped. Also, when Si3N4 with amine groups attached to the surface in contact with graphene is used as a protective layer to cover graphene, the graphene becomes n-doped. This was reported in Non-Patent Document 2 mentioned above. Here, it is considered that the presence of amine groups on the surface of Si3N4 in contact with graphene causes electrons to be taken away from the graphene. The work functions of HfO2 and Si3N4 are generally larger than those of graphene, but in the two reports mentioned above, it is thought that the properties of the material in contact with graphene, such as oxygen depletion and amine groups, which take away electrons, were dominant. Here, the material used to inject holes into the graphene layer 114 is, for example, a material having a work function greater than that of graphene. Since the work function of single-layer graphene is approximately 4.5 eV, the material must have a work function greater than this value. Examples of such materials that can be applied to the base layer 112A include TiO2, HfO2, SiO2, and Al2O3. On the other hand, examples of materials used to inject electrons into the graphene layer 114 include materials having a work function smaller than that of graphene, or materials having oxygen vacancies or amine groups. In the case of these materials, graphene can be controlled to be n-doped even if the material has a work function larger than that of graphene, or a work function very close to that of graphene.

[0045] Examples of materials that have a work function smaller than that of graphene and are applicable to the base layer 112B include La2O3. Examples of materials that have a work function larger than that of graphene but have oxygen vacancies and are applicable to the base layer 112B include HfO2. [Experimental results of the Id-Vg characteristics of p-doped graphene FETs]

[0046] Next, based on Figures 7A(a), (b), and (c), and Figures 7B(a), (b), and (c), we will describe the experimental results of the Id-Vg characteristics of graphene FETs in which the graphene layer is p-doped with a base layer made of different materials. Figure 7A(a) shows the Id-Vg characteristics of the first graphene FET fabricated using Al2O3 as the base layer material, Figure 7A(b) is a cross-sectional view showing the structure of the first graphene FET, and Figure 7A(c) is a partial plan view of the first graphene FET. Figure 7B(a) shows the Id-Vg characteristics of the second graphene FET fabricated using SiO2 as the base layer material, Figure 7B(b) is a cross-sectional view showing the structure of the second graphene FET, and Figure 7B(c) is a partial plan view of the second graphene FET.

[0047] The first prototype graphene FET, as shown in Figure 7A(b), comprises a semiconductor substrate 110, a base layer 112Aa, a graphene layer 114, a source electrode 115, and a drain electrode 116. The base layer 112Aa is provided on the semiconductor substrate 110, and the graphene layer 114 is provided on the base layer 112Aa. The semiconductor substrate 110 is composed of impurity-doped Si, and its thickness d1 is 0.525 mm. The base layer 112Aa is composed of Al2O3, and its thickness d2 is 100 nm. The source electrode 115 and the drain electrode 116 are composed of Au (gold), and their respective thicknesses d3 are 100 nm.

[0048] Furthermore, as shown in Figure 7A(c), the channel size d5×d7 of the graphene layer 114 of the first graphene FET is 5μm×5μm, and the overlaps d4×d7 and d6×d7 between the source electrode 115 and the drain electrode 116 and the graphene layer 114 are both 5μm×5μm.

[0049] Two prototype graphene FETs with the configuration described above were fabricated. A semiconductor substrate 110 was placed on a voltage-applied stage, and terminals were connected to the source electrode 115 and drain electrode 116. The Id-Vg characteristics of each prototype were then measured. As shown in Figure 7A(a), in both cases, the position of the charge neutral point where the drain current Id is minimal was shifted to the positive side from Vg=0V. In other words, this experiment confirmed that using Al2O3 as the underlying material results in a p-doped state of the graphene layer 114.

[0050] On the other hand, the fabricated second graphene FET, as shown in Figure 7B(b), comprises a semiconductor substrate 110, a base layer 112Ab, a graphene layer 114, a source electrode 115, and a drain electrode 116. The base layer 112Ab is provided on the semiconductor substrate 110, and the graphene layer 114 is provided on the base layer 112Ab. The semiconductor substrate 110 is composed of impurity-doped Si, and its thickness d1 is 0.525 mm. The base layer 112Ab is composed of SiO2, and its thickness d2 is 100 nm. The source electrode 115 and the drain electrode 116 are composed of Au, and their respective thicknesses d3 are 100 nm.

[0051] Furthermore, as shown in Figure 7B(c), the channel size d5×d7 of the graphene layer 114 of the second graphene FET is 20μm×20μm, and the overlaps d4×d7 and d6×d7 between the source electrode 115 and the drain electrode 116 and the graphene layer 114 are both 5μm×20μm.

[0052] Two prototype second graphene FETs with the configuration described above were fabricated, and the Id-Vg characteristics of each prototype were measured using the same method as for the first graphene FET. As shown in Figure 7B(a), in both cases, the position of the charge neutral point where the drain current Id is minimal was shifted to the positive side from Vg=0V. In other words, this experiment confirmed that using SiO2 as the underlying material results in a p-doped state of the graphene layer 114. [Regarding the polarization direction of ferroelectric materials]

[0053] Next, the polarization direction of the ferroelectric material will be explained based on Figures 8(a) and (b). Figure 8(a) is a graph showing the measurement results of the polarization direction of the ferroelectric material, and Figure 8(b) is a diagram showing the configuration of the measurement substrate 200 used for the measurement. In Figure 8(a), the horizontal axis is angle [deg] and the vertical axis is percentage [%]. As shown in Figure 8(b), the measurement substrate 200 comprises a semiconductor substrate 201 made of Si, a base layer 202 made of SiO2, and a ferroelectric layer 203 made of LiNbO3. The base layer 202 is provided on the semiconductor substrate 201, and the ferroelectric layer 203 is provided on the base layer 202. The thickness of the semiconductor substrate 201 is 0.525 mm, the thickness of the base layer 202 is 500 nm, and the thickness of the ferroelectric layer 203 is 300 nm.

[0054] To measure the polarization direction, an atomic force microscope was used to visualize the polarization due to electric dipole action. Specifically, an AC voltage was applied to the surface of the ferroelectric layer 203 on the measurement substrate 200 using a conductive coated probe, and the vibration of the surface at this time was measured as the change in the probe tip. The phase of the measured vibration was examined with a lock-in amplifier to detect whether the phase was in phase with or out of phase with the AC frequency, and the polarization of the ferroelectric layer 203 was plotted for each measurement point. In this way, the polarization direction at each exploration position was measured.

[0055] As shown in Figure 8(a), the measurement results of the polarization direction are shown by the angle and percentage of the polarization direction at multiple polarization points, and it can be seen that it has a range of approximately 50° to 120°. In other words, the polarization direction is not regularly aligned in one direction at multiple polarization points, but is polarized in various directions at the ratio shown in Figure 8(a). Specifically, within the range of approximately 50° to 120°, the range of approximately 72° to 100° accounts for more than 5%, and the other ranges account for less than 5%. The polarization center is 84°, and the standard deviation σ is 7.6°. In other words, the example shown in Figure 8(a) has an angular distribution in which angles within the range of approximately 70° to 110° account for more than 85%, and the ferroelectric layer 203 spontaneously polarizes in a first direction in which the +Z direction side is positively biased and the -Z direction side is negatively biased. Thus, the direction of spontaneous polarization is not uniformly oriented in one direction; as shown in the examples in Figures 2(a) and 3(a), the ideal state is one in which polarization occurs in either the +Z or -Z direction. It is presumed that this angular distribution was observed because the ferroelectric layer 203 used in this measurement was a single crystal. If the ferroelectric layer were in a different crystalline state, such as polycrystalline, more than 50% of the angles would fall within the range of 70° to 110°, making the present invention applicable. [Relationship between the magnitude of the work function of the bonding material and the state of charge carrier injection]

[0056] Next, the relationship between the magnitude of the work functions of the graphene and doping layers and the charge carrier injection state will be explained based on Figures 9(a), (b), (c), and (d). Figures 9(a) to (d) are energy band diagrams of the separated and joined states of the graphene and doping layers. Note that in Figures 9(a) to (d), E vac This indicates the vacuum level, E F This indicates the Fermi level.

[0057] First, as shown in Figure 9(a), the work function qφ of the graphene layer G The work function qφ of the metal layer, which is the doping layer, is more important than the work function qφ of the metal layer, which is the doping layer. MLet's explain the case where the latter is larger. In this case, when a graphene layer and a metal layer are joined, electrons spontaneously move from the graphene layer to the metal layer due to the difference in their work functions, as shown in Figure 9(b). This electron movement creates holes in the graphene layer, which are vacancies left by electrons. In other words, the graphene layer becomes a state in which holes are injected as charge carriers, and is known as a p-doped state. A graphene layer in this state is sometimes called a p-type graphene layer.

[0058] Next, as shown in Figure 9(c), the work function qφ of the graphene layer G The work function qφ of the metal layer, which is the doping layer, is more important than the work function qφ of the metal layer, which is the doping layer. M Let's explain the case where the work function is smaller. In this case, when a graphene layer and a metal layer are joined, electrons spontaneously move from the metal layer to the graphene layer due to the difference in their work functions, as shown in Figure 9(d). This electron movement results in electrons being injected into the graphene layer as charge carriers, creating a so-called n-doped state. A graphene layer in this state is sometimes called an n-type graphene layer. [Manufacturing method for optical sensor 2A] Next, we will explain the manufacturing method of the optical sensor 2A. Figure 10 shows a method for manufacturing the optical sensor 2A according to the first embodiment. In step S101, a Si substrate doped with impurities is prepared as the semiconductor substrate 110. In step S102, a ferroelectric layer 111A that spontaneously polarizes in the +Z direction is formed on the semiconductor substrate 110, for example, by a ferroelectric material such as LiNbO3.

[0059] In step S103, a base layer 112A is formed on the ferroelectric layer 111A. Here, the base layer 112A is an acceptor layer that injects holes into the graphene layer 114. The acceptor layer is a layer formed from a material that has a work function greater than that of the graphene layer 114, such as SiO2 or Al2O3. In step S104, graphene is transferred to the surface of the underlying layer 112A to form a graphene layer 114. In step S105, the graphene layer 114 is patterned.

[0060] In step S106, a conductive layer, such as Au, which will serve as a contact electrode, is formed on the surface of the base layer 112A and the graphene layer 114. This conductive layer is then patterned to form a source electrode 115 and a drain electrode 116 with the graphene layer 114 in between. As a result, a photosensor 2A with the structure shown in Figure 2(a) is obtained. [Manufacturing method for optical sensor 2B] Next, we will describe the manufacturing method of the optical sensor 2B. Figure 11 shows a method for manufacturing the optical sensor 2B according to the first embodiment. In step S201, a Si substrate doped with impurities is prepared as the semiconductor substrate 110. In step S202, a ferroelectric layer 111B that spontaneously polarizes in the -Z direction is formed on the semiconductor substrate 110, for example, by a ferroelectric material such as LiNbO3.

[0061] In step S203, a base layer 112B is formed on the ferroelectric layer 111B. Here, the base layer 112B is a donor layer that injects electrons into the graphene layer 114. The donor layer is a layer formed from a material having oxygen vacancies, such as hafnium oxide. In step S204, graphene is transferred to the surface of the underlying layer 112B to form a graphene layer 114. In step S205, the graphene layer 114 is patterned.

[0062] In step S206, a conductive layer, such as Au, which will serve as a contact electrode, is formed on the surface of the base layer 112B and the graphene layer 114. This conductive layer is then patterned to form a source electrode 115 and a drain electrode 116 with the graphene layer 114 in between. As a result, a photosensor 2B with the structure shown in Figure 3(a) is obtained. [Effects of the First Embodiment]

[0063] As described above, the optical sensor 2A according to the first embodiment comprises a semiconductor substrate 110 that functions as a gate electrode, a ferroelectric layer 111A provided on the semiconductor substrate 110, a base layer 112A provided on the surface of the ferroelectric layer 111A, a graphene layer 114 provided on the surface of the base layer 112A, and a source electrode 115 and a drain electrode 116 that are electrically connected to each other by the graphene layer 114. In addition, the ferroelectric layer 111A has the characteristic that when the temperature rises, the surface side that is closer to the graphene layer 114 becomes positively biased and the back side that is further away from the graphene layer 114 becomes negatively biased, and spontaneous polarization occurs in a first direction (+Z direction) from the back side toward the surface. Furthermore, the base layer 112A is made of a material that has the function of injecting holes, which are charge carriers, into the graphene layer 114. Examples of materials for the base layer 112A include materials with a work function greater than that of the graphene layer 114, such as TiO2, HfO2, SiO2, and Al2O3.

[0064] In this configuration, the graphene layer 114 becomes p-doped by hole injection from the underlayer 112A, and the initial CNP is located at a position positive to 0V when the gate voltage is greater than 0V. In addition, the polarization change due to the pyroelectric effect of the ferroelectric layer 111A caused by infrared light reception results in a voltage change that increases the gate voltage Vg, so the CNP changes in the positive direction of the gate voltage Vg. Therefore, there is no point where the polarization change due to the pyroelectric effect of the ferroelectric layer 111A coincides with the initial drain current Id at 0V, thus eliminating the point where the change in drain current Id ΔId is zero. As a result, the change in drain current Id ΔId due to infrared light reception can be measured stably.

[0065] Furthermore, the optical sensor 2B according to the first embodiment includes a semiconductor substrate 110 that functions as a gate electrode, a ferroelectric layer 111B having a front and back surface provided on the semiconductor substrate 110, a base layer 112B provided on the front surface of the ferroelectric layer 111B, a graphene layer 114 provided on the front surface of the base layer 112B, and a source electrode 115 and a drain electrode 116 that are electrically connected to each other by the graphene layer 114. In addition, the ferroelectric layer 111B has the characteristic that when the temperature rises, the front side, which is the side approaching the graphene layer 114, becomes negatively biased and the back side, which is the side away from the graphene layer 114, becomes positively biased, and spontaneous polarization occurs in a second direction (-Z direction) from the front to the back surface. Furthermore, the base layer 112B is made of a material that has the function of injecting electrons, which are charge carriers, into the graphene layer 114. Examples of materials for the base layer 112B include materials having a work function smaller than that of the graphene layer 114, such as La2O3, and other materials having a work function larger than that of the graphene layer 114 but containing oxygen vacancies, such as HfO2.

[0066] In this configuration, the graphene layer becomes n-doped due to electron injection from the underlying layer 112B, and the initial CNP is located at a gate voltage negative than 0V. In addition, the polarization change due to the pyroelectric effect of the ferroelectric layer 111B caused by infrared light reception results in a voltage change that negatively affects the gate voltage, causing the CNP to change in the negative direction of the gate voltage. Therefore, with respect to the polarization change due to the pyroelectric effect of the ferroelectric layer 111B, there is no point where it coincides with the initial drain current Id at a gate voltage Vg of 0V, thus eliminating the point where the change in drain current Id ΔId is zero. As a result, the change in drain current Id ΔId due to infrared light reception can be measured stably. [Correspondence in the first embodiment] In the first embodiment, the ferroelectric layers 111, 111A, and 111B correspond to pyroelectric layers, the underlayers 112, 112A, and 112B correspond to insulating layers, the source electrode 115 corresponds to the first electrode, and the drain electrode 116 corresponds to the second electrode. [Second Embodiment] Next, a second embodiment of the present invention will be described. Figures 12 and 13 show the second embodiment. 〔composition〕

[0067] The second embodiment differs from the first embodiment in that a protective layer is provided on the graphene layer 114 as a doping layer for injecting charge carriers into the graphene layer 114. Hereinafter, the same reference numerals are used for components similar to those in the first embodiment, and explanations of parts similar to those in the first embodiment will be omitted as appropriate, while the different parts will be described in detail. The sensor module 1 according to the second embodiment includes an optical sensor 2C or 2D in place of the optical sensor 2A or 2B of the first embodiment.

[0068] Figure 12 shows the configuration and manufacturing method of the optical sensor 2C according to the second embodiment. As shown in Figure 12, the optical sensor 2C has a configuration in which a protective layer 117 is provided on the surface of the graphene layer 114, in which holes are injected as charge carriers, in the optical sensor 2A of the first embodiment. The protective layer 117 is an insulating layer, and examples of materials for it include insulating materials having a work function larger than that of graphene, such as TiO2, MoO3, WO3, and Si3N4. In addition, since the protective layer 117 in the optical sensor 2C has the function of injecting holes as charge carriers into the graphene layer 114, the base layer 112A may be made of a material that does not have the function of injecting holes, to the extent that the graphene layer 114 is p-doped by the protective layer 117.

[0069] Figure 13 shows the configuration and manufacturing method of the optical sensor 2D according to the second embodiment. As shown in Figure 13, the optical sensor 2D has a configuration in which a protective layer 118 is provided on the surface of the graphene layer 114 to inject electrons as charge carriers, in addition to the optical sensor 2B of the first embodiment described above. Examples of materials for the protective layer 118 include materials having a work function smaller than that of graphene, such as La2O3, or materials having amine groups, such as Si3N4 with amine groups attached, which have a work function larger than that of graphene. In addition, since the protective layer 118 in the optical sensor 2D has the function of injecting electrons into the graphene layer 114 as charge carriers, the base layer 112B may be made of a material that does not have the function of injecting electrons, to the extent that the graphene layer 114 is n-doped by the protective layer 118. [Manufacturing method for optical sensor 2C] Next, a method for manufacturing the optical sensor 2C will be described based on Figure 12. In step S301, a Si substrate doped with impurities is prepared as the semiconductor substrate 110. In step S302, a ferroelectric layer 111A that spontaneously polarizes in the +Z direction is formed on the semiconductor substrate 110, for example, by a ferroelectric material such as LiNbO3. In step S303, a base layer 112A is formed on the ferroelectric layer 111A. Here, the base layer 112A may be formed from a material that has the function of injecting holes into the graphene layer 114, or it may be formed from a material that does not have the function of injecting holes. In step S304, graphene is transferred to the surface of the underlying layer 112A to form a graphene layer 114. In step S305, the graphene layer 114 is patterned. In step S306, a conductive layer, such as Au, which will serve as a contact electrode, is formed on the surface of the base layer 112A and the graphene layer 114. This conductive layer is then patterned to form a source electrode 115 and a drain electrode 116 with the graphene layer 114 in between. In step S307, a protective layer 117 is formed on the surface of the graphene layer 114, as shown in Figure 12. As a result, an optical sensor 2C with the structure shown in Figure 12 is obtained.

[0070] Here, as the protective layer 117, an acceptor layer is formed that injects holes into the graphene layer 114. The acceptor layer is a layer formed from a material that has a work function greater than that of the graphene layer 114, such as SiO2 or Al2O3. [Manufacturing method for 2D optical sensors] Next, a method for manufacturing the optical sensor 2D will be described based on Figure 13. In step S401, a Si substrate doped with impurities is prepared as the semiconductor substrate 110. In step S402, a ferroelectric layer 111B that spontaneously polarizes in the -Z direction is formed on the semiconductor substrate 110, for example, by a ferroelectric material such as LiNbO3. In step S403, a base layer 112B is formed on the ferroelectric layer 111B. Here, the base layer 112B may be formed from a material that has the function of injecting electrons into the graphene layer 114, or it may be formed from a material that does not have the function of injecting electrons. In step S404, graphene is transferred to the surface of the underlying layer 112B to form a graphene layer 114. In step S405, the graphene layer 114 is patterned. In step S406, a conductive layer, such as Au, which will serve as a contact electrode, is formed on the surface of the base layer 112 and the graphene layer 114. This conductive layer is then patterned to form a source electrode 115 and a drain electrode 116 with the graphene layer 114 in between. In step S407, a protective layer 118 is formed on the surface of the graphene layer 114. As a result, an optical sensor 2D with the structure shown in Figure 13 is obtained.

[0071] Here, a donor layer is formed as the protective layer 118, which injects electrons into the graphene layer 114. The donor layer is a layer formed from a material having a work function smaller than that of the graphene layer 114, such as La2O3, or a material having amine groups, such as Si3N4, on the surface in contact with the graphene layer 114 by a special film deposition process. [Effects of the second embodiment] As described above, the optical sensor 2C according to the second embodiment is configured such that, in the optical sensor 2A of the first embodiment, a protective layer 117 is provided on the surface of the graphene layer 114, which injects holes as charge carriers into the graphene layer 114. Examples of materials for the protective layer 117 include insulating materials having a work function larger than that of graphene, such as TiO2, Si3N4, MoO3, and WO3.

[0072] In this configuration, the graphene layer becomes p-doped due to hole injection from the protective layer 117, and the initial CNP is located at a position positive to 0V when the gate voltage is greater than 0V. In addition, the polarization change due to the pyroelectric effect of the ferroelectric layer 111A caused by infrared light reception results in a voltage change that increases the gate voltage, so the CNP changes in the positive direction of the gate voltage. Therefore, there is no point where the polarization change due to the pyroelectric effect of the ferroelectric layer 111A coincides with the initial drain current Id at a gate voltage Vg of 0V, thus eliminating the point where the change in drain current Id ΔId is zero. As a result, the change in drain current Id ΔId due to infrared light reception can be measured stably. Furthermore, the optical sensor 2D according to the second embodiment is configured such that, in the optical sensor 2B of the first embodiment, a protective layer 118 is provided on the surface of the graphene layer 114 to inject electrons into the graphene layer 114 as charge carriers. Examples of materials for the protective layer 118 include a material having a work function smaller than the work function of the graphene layer 114, such as La2O3, or a material having amine groups, such as Si3N4, on the surface in contact with the graphene layer 114.

[0073] In this configuration, the graphene layer becomes n-doped due to electron injection from the protective layer 118, and the initial CNP is located at a gate voltage negative than 0V. In addition, the polarization change due to the pyroelectric effect of the ferroelectric layer 111B caused by infrared light reception results in a voltage change in the direction of negative gate voltage, so the CNP changes in the negative direction of gate voltage. Therefore, there is no point where the polarization change due to the pyroelectric effect of the ferroelectric layer 111B coincides with the initial drain current Id at a gate voltage Vg of 0V, thus eliminating the point where the change in drain current Id ΔId is zero. As a result, the change in drain current Id ΔId due to infrared light reception can be measured stably. [Correspondence in the second embodiment] In the second embodiment, the ferroelectric layers 111, 111A, and 111B correspond to pyroelectric layers, the underlayers 112A and 112B and the protective layers 117 and 118 correspond to insulating layers, the source electrode 115 corresponds to the first electrode, and the drain electrode 116 corresponds to the second electrode. [Third Embodiment] Next, a third embodiment of the present invention will be described. Figures 14 and 15 show the third embodiment. 〔composition〕 The third embodiment differs from the first and second embodiments in that it changes the slope of the Id-Vg curve of the optical sensor by a process such as annealing. Hereinafter, parts similar to the first and second embodiments will be omitted from the explanation as appropriate, and the differences will be described in detail. [Relationship between polarization orientation of ferroelectric layer and p-doped graphene]

[0074] First, based on Figures 14(a) and (b), the changes in the Id-Vg curves according to the third embodiment when the graphene layer 114 is p-doped in the +Z and -Z polarization configurations of the optical sensors 2A and 2B will be explained. Figure 14(a) is a diagram showing the changes in the Id-Vg curve for the combination of +Z orientation and p-doping according to the third embodiment, and Figure 14(b) is a diagram showing the changes in the Id-Vg curve for the combination of -Z orientation and p-doping according to the third embodiment. In Figures 14(a) and (b), the horizontal axis is the gate voltage Vg and the vertical axis is the drain current Id.

[0075] In the examples shown in Figures 14(a) and (b), the Id-Vg curve of the optical sensor 2A is configured such that the slope on the hole conduction side is greater than the slope on the electron conduction side. This difference in slope can be intentionally created, for example, by annealing.

[0076] Even when the Id-Vg curve has such characteristics, as shown in Figure 14(a), by p-doping the graphene layer 114 in the photosensor 2A, it is possible to eliminate the point where the current change ΔId of the drain current Id due to the polarization change of the ferroelectric layer 111A becomes zero. On the other hand, as shown in Figure 14(b), when the graphene layer 114 in the photosensor 2B is p-doped, a point occurs where the current change ΔId due to the polarization change of the ferroelectric layer 111B becomes zero.

[0077] Furthermore, as shown in Figures 14(a) and (b), the current change ΔId when the temperature of the ferroelectric layer 111A of the photosensor 2A changes from T℃ to T+ΔT1℃ is larger than the current change ΔId when the temperature of the ferroelectric layer 111B of the photosensor 2B changes from T℃ to T+ΔT1℃. [Relationship between polarization orientation of ferroelectric layer and n-doped graphene]

[0078] Next, based on Figures 15(a) and (b), the changes in the Id-Vg curves according to the third embodiment when the graphene layer 114 is n-doped in the +Z and -Z polarization configurations of the optical sensors 2A and 2B will be described. Figure 15(a) is a diagram showing the changes in the Id-Vg curve for the combination of +Z orientation and n-doping according to the third embodiment, and Figure 15(b) is a diagram showing the changes in the Id-Vg curve for the combination of -Z orientation and n-doping according to the third embodiment. In Figures 15(a) and (b), the horizontal axis is the gate voltage Vg and the vertical axis is the drain current Id.

[0079] In the examples shown in Figures 15(a) and (b), the Id-Vg curve of the optical sensor 2B is configured such that the slope on the electron conduction side is greater than the slope on the hole conduction side. This difference in slope can be intentionally created, for example, by annealing.

[0080] Even when the Id-Vg curve has such characteristics, doping the graphene layer 114 in the photosensor 2A with n doping results in a point where the current change ΔId is zero. On the other hand, when the graphene layer 114 in the photosensor 2B is doped with n doping, the point where the current change ΔId is zero can be eliminated.

[0081] Furthermore, as shown in Figures 15(a) and (b), the current change ΔId when the temperature of the ferroelectric layer 111B of the photosensor 2B changes from T℃ to T+ΔT1℃ is larger than the current change ΔId when the temperature of the ferroelectric layer 111A of the photosensor 2A changes from T℃ to T+ΔT1℃. Although an example of applying the invention according to the third embodiment to the optical sensors 2A and 2B according to the first embodiment has been described, the invention according to the third embodiment is also applicable to the optical sensors 2C and 2D according to the second embodiment. [Effects of the third embodiment] As described above, the optical sensor 2A according to the third embodiment is configured such that, in the optical sensor 2A of the first embodiment in which the graphene layer 114 is p-doped, the slope of the Id-Vg curve is greater on the hole conduction side than on the electron conduction side.

[0082] With this configuration, the same effects as the optical sensor 2A of the first embodiment can be obtained. In addition, the change in drain current Id ΔId due to the polarization change of the ferroelectric layer 111A can be made larger compared to the case where the graphene layer 114 is n-doped, so that current changes can be detected stably and with high sensitivity to polarization changes. Furthermore, in the third embodiment, the optical sensor 2B is configured such that, in the optical sensor 2B of the first embodiment in which the graphene layer 114 is n-doped, the slope of the Id-Vg curve is greater on the electron conduction side than on the hole conduction side.

[0083] With this configuration, the same effects as the optical sensor 2B of the first embodiment can be obtained. In addition, the change in drain current Id ΔId due to the polarization change of the ferroelectric layer 111B can be made larger compared to the case where the graphene layer 114 is p-doped, so that current changes can be detected stably and with high sensitivity to polarization changes. [Correspondence in the third embodiment] In the third embodiment, the ferroelectric layers 111, 111A, and 111B correspond to pyroelectric layers, the underlayers 112A and 112B correspond to insulating layers, the source electrode 115 corresponds to the first electrode, the drain electrode 116 corresponds to the second electrode, and the semiconductor substrate 110 corresponds to the third electrode. [Variation]

[0084] In the above embodiment, the optical sensor is configured with a conductive semiconductor substrate that functions as a gate electrode on the back surface of the ferroelectric layer. However, the configuration is not limited to this, and a configuration without the semiconductor substrate may also be used. For example, as shown in Figure 16, the modified sensor module 1A includes an optical sensor 2E in place of the optical sensor 2 in the sensor module 1 of the first embodiment. The optical sensor 2E does not include the semiconductor substrate 110 in the optical sensor 2 of the first embodiment. In this configuration, if a protective layer is provided as an insulating layer, the underlayer 112 does not need to be provided. In these configurations, since the reference current Idr in the initial state when infrared light is not being received is known, the current change ΔId due to the pyroelectric effect is detected relative to this reference current Idr. Furthermore, in the above embodiments and their modifications, a layer of single-layer graphene was used as an example of a two-dimensional material layer. However, the invention is not limited to this configuration, and any material other than single-layer graphene mentioned above may be used as the material for the two-dimensional material layer. That is, the work function of single-layer graphene is approximately 4.5 eV, but in the case of the materials other than single-layer graphene mentioned above, the work function is in the range of approximately 4.0 eV to 4.8 eV. Therefore, it can be understood that the present invention can be realized even without using single-layer graphene as the two-dimensional material, by appropriately selecting the component that comes into contact with the two-dimensional material. Furthermore, in the above embodiment and its variations, the source electrodes 115 and drain electrodes 116 of the optical sensors 2A and 2D may be made of a material having a work function greater than that of the graphene layer 114. With this configuration, the graphene layer 114 is p-doped in the base layer 112A or protective layer 117, and since the electrodes are made of a material that contributes to p-doping, they can be further p-doped. Furthermore, in the above embodiments and their variations, the source electrodes 115 and drain electrodes 116 of the optical sensors 2B and 2D may be made of a material having a work function smaller than that of the graphene layer 114, or a material having a work function larger than that of the graphene layer 114 but containing amine groups. With this configuration, the graphene layer 114 is n-doped in the base layer 112B or protective layer 118, and since the electrodes are made of a material that contributes to n-doping, they can be further n-doped. Furthermore, although the above embodiments and their modifications were described using a sensor module composed of a single optical sensor as an example, the system is not limited to this configuration. For example, the sensor module may be composed of an optical sensor array in which multiple optical sensors are arranged in an array. Since each optical sensor has the configuration of the present invention and is connected to a readout circuit to output its drain current, the change in drain current due to the pyroelectric effect can be measured stably. Furthermore, in the above embodiments and their modifications, an optical sensor for detecting infrared light is shown, and an optical sensor particularly suitable for detecting infrared light in the wavelength band of 6 μm to 20 μm is exemplified. However, the optical sensor of the present invention may be applied to optical sensors for detecting infrared light in other wavelength bands, or to optical sensors for detecting visible light. [Explanation of Symbols]

[0085] 1...Sensor module, 2,2A~2E...Optical sensor, 3...Detection circuit, 100,201...Semiconductor substrate, 111,111A,111B,203...Ferroelectric layer, 112,112A,112Aa,112Ab,112B,202...Underlayer, 114...Graphene layer, 115...Source electrode, 116...Drain electrode, 117,118...Protective layer

Claims

1. A pyroelectric layer having a front surface and a back surface, A two-dimensional material layer, which is p-type or n-type, is provided on the surface side of the pyroelectric layer, The two-dimensional material layer comprises a first electrode and a second electrode that are electrically connected to each other, The two-dimensional material layer comprises an insulating layer provided on the aforementioned two-dimensional material layer, The pyroelectric layer has the characteristic that, when the two-dimensional material layer is p-type, when the temperature rises, the portion on the surface side becomes positively biased and the portion on the back side becomes negatively biased, spontaneously polarizing in a first direction from the back side toward the surface, and when the two-dimensional material layer is n-type, when the temperature rises, the portion on the surface side becomes negatively biased and the portion on the back side becomes positively biased, spontaneously polarizing in a second direction from the surface toward the back side, an optical sensor.

2. The optical sensor according to claim 1, wherein the two-dimensional material layer is formed from single-layer graphene, multilayer graphene, randomly stacked graphene, graphene nanoribbons, transition metal dichalcogenide, black phosphorus, silicene, or germanene.

3. The optical sensor according to claim 1, wherein the insulating layer dops the two-dimensional material layer in a p-type or n-type manner.

4. The optical sensor according to claim 3, wherein the insulating layer is composed of at least one of the following: an underlayer bonded to the pyroelectric layer side of the two-dimensional material layer, or a protective layer bonded to the side of the two-dimensional material layer opposite to the pyroelectric layer side.

5. The optical sensor according to claim 3, wherein the insulating layer doping the two-dimensional material layer in a p-type manner is made of a material having a work function greater than that of the two-dimensional material layer, and the insulating layer doping the two-dimensional material layer in an n-type manner is made of a material having a work function smaller than that of the two-dimensional material layer.

6. The optical sensor according to claim 3, wherein the insulating layer doping the two-dimensional material layer in an n-type manner is composed of a layer having amine groups.

7. The optical sensor according to claim 3, wherein the insulating layer doping the two-dimensional material layer in an n-type manner is made of a material having oxygen vacancies.

8. The material with a work function greater than the work function of the two-dimensional material layer is TiO ２ (titanium oxide), Si ３ N ４ (silicon nitride), HfO ２ (hafnium oxide), SiO ２ (silicon dioxide), Al ２ O ３ (aluminum oxide), MoO ３ (molybdenum oxide), WO ３ (tungsten oxide), any one of them, and the material with a work function smaller than the work function of the two-dimensional material layer is La ２ O ３ (lanthanum oxide), the photosensor according to claim 5.

9. When the two-dimensional material layer is p-type and the pyroelectric layer spontaneously polarizes in the first direction, the curve showing the relationship between the voltage applied to the third electrode and the current flowing from one of the first and second electrodes to the other has a configuration in which the slope on the hole conduction side is greater than the slope on the electron conduction side. The optical sensor according to any one of claims 1 to 8, wherein when the two-dimensional material layer is n-type and the pyroelectric layer spontaneously polarizes in the second direction, the slope of the curve on the electron conduction side is greater than the slope on the hole conduction side.

10. The optical sensor according to claim 1, wherein the first direction or the second direction has an angular distribution such that 85% or more of the angles indicating the polarization directions at multiple polarization points are within the range of 70° to 110°.

11. When the two-dimensional material layer is p-type, the first electrode and the second electrode are made of a material that has the function of injecting holes into the two-dimensional material layer. The optical sensor according to claim 1, wherein, when the two-dimensional material layer is n-type, the first electrode and the second electrode are made of a material having the function of injecting electrons into the two-dimensional material layer.

12. An optical sensor array comprising a plurality of optical sensors as described in claim 1, wherein the plurality of optical sensors are arranged in an array.

13. A step of forming a pyroelectric layer having a front surface and a back surface, wherein when the temperature rises, the front surface side becomes positively biased and the back surface side becomes negatively biased, and spontaneously polarizes in a first direction from the back surface toward the front surface, A method for manufacturing an optical sensor, comprising the step of forming a p-type two-dimensional material layer on the surface side of the pyroelectric layer.

14. The process further includes a step of forming an insulating layer that is bonded to the two-dimensional material layer and has the property of doping the two-dimensional material layer in a p-type manner, The method for manufacturing an optical sensor according to claim 13, wherein in the step of forming the insulating layer, at least one of the following is formed as the insulating layer: an underlayer bonded to the pyroelectric layer side of the two-dimensional material layer, or a protective layer bonded to the side of the two-dimensional material layer opposite to the pyroelectric layer side.

15. A step of forming a pyroelectric layer having a front surface and a back surface, wherein when the temperature rises, the front surface becomes negatively biased and the back surface becomes positively biased, and spontaneously polarizes in a second direction from the front surface to the back surface, A method for manufacturing an optical sensor, comprising the step of forming an n-type two-dimensional material layer on the surface side of the pyroelectric layer.

16. The process further includes a step of forming an insulating layer that is bonded to the two-dimensional material layer and has the property of doping the two-dimensional material layer in an n-type manner, The method for manufacturing an optical sensor according to claim 15, wherein in the step of forming the insulating layer, at least one of the following is formed as the insulating layer: an underlayer bonded to the pyroelectric layer side of the two-dimensional material layer, or a protective layer bonded to the side of the two-dimensional material layer opposite to the pyroelectric layer side.

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