Graphene Optical Sensor

Abstract

To provide a graphene optical sensor that separately detects light rays of two wavelengths.SOLUTION: A graphene optical sensor includes: a first gate electrode and a second gate electrode, both formed on a substrate; an insulating film covering the first and second gate electrodes; a graphene layer stacked on top of the insulating film; a pair of electrodes connected to the graphene layer; a first periodic structure stacked in a first area of the graphene layer between the pair of electrodes; and a second periodic structure stacked in a second area of the graphene layer. The repetition periods of the first periodic structure and the second periodic structure are different.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to graphene optical sensors. [Background technology]

[0002] Graphene is a two-dimensional material in which carbon atoms are arranged in a two-dimensional honeycomb pattern, and has a unique energy band structure. Graphene has a certain degree of absorption for light in a wide range of wavelengths, and is also capable of absorbing terahertz light. Research and development of graphene as a material for visible light or infrared light sensors and terahertz light sensors is underway.

[0003] The physical properties of graphene, such as light absorption characteristics, resistivity, and Seebeck coefficient, depend on the carrier density of graphene. In order to obtain suitable physical properties of the optical sensor, a gate electrode may be provided to control the doping of graphene. A configuration is known in which gate electrodes for applying operating voltages to p-type and n-type graphene are provided to detect a differential current (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2018 / 163496 Summary of the Invention [Problem to be solved by the invention]

[0005] There is a demand for optical sensors to detect light of multiple wavelengths (or wavelength bands) separately. In solid-state infrared sensors, the transmission and radiation characteristics of infrared rays in the 3-5 μm band and infrared rays in the 8-12 μm band are different, so in some cases, infrared rays in the two wavelength bands are detected separately and the detection results are combined through calculation processing to obtain information. In general, materials radiate infrared rays through thermal radiation according to their temperature. When detecting infrared rays with a single wavelength, only the intensity of the infrared rays is obtained as information, and it is not possible to distinguish between a high-temperature material located far away and a low-temperature material located nearby. If infrared rays in two wavelength bands can be detected separately, it becomes possible to estimate the temperature and distance of the target material from the wavelength dependency of the radiation corresponding to the temperature.

[0006] An object of the present disclosure is to provide a graphene optical sensor that detects light of two wavelengths separately. [Means for solving the problem]

[0007] In one aspect of the present disclosure, the graphene optical sensor comprises: a first gate electrode and a second gate electrode formed on a substrate; an insulating film covering the first gate electrode and the second gate electrode; A graphene layer laminated on the insulating film; A pair of electrodes connected to the graphene layer; a first periodic structure laminated in a first region of the graphene layer between the pair of electrodes, and a second periodic structure laminated in a second region of the graphene layer; The first periodic structure and the second periodic structure have different repeating periods. Effect of the Invention

[0008] The graphene optical sensor can detect two wavelengths of light separately. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic plan view of a graphene optical sensor according to an embodiment. [Diagram 2]FIG. 2 is a schematic cross-sectional view taken along line II' in FIG. [Diagram 3] FIG. 2 is a diagram illustrating a plasmon antenna. [Figure 4] FIG. 1 is a diagram illustrating the optical absorption of graphene. [Diagram 5] FIG. 4 is a diagram illustrating the setting of a gate voltage. [Figure 6] 1 is a flowchart showing the operation of the graphene optical sensor. [Figure 7] FIG. 2 is a schematic cross-sectional view of another configuration example of a graphene optical sensor. [Figure 8] FIG. 13 is a schematic cross-sectional view of yet another configuration example of a graphene optical sensor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] In an embodiment, a graphene optical sensor is provided that detects light of two wavelengths or wavelength bands separately. Graphene absorbs light in a wide wavelength range due to its special energy band structure. In order to detect light of two wavelengths with a graphene optical sensor, a configuration for wavelength selection is required. In order to selectively detect light of a target wavelength, periodic structures with different periods are laminated on a graphene layer to give sensitivity to light of different wavelengths. Separate gate electrodes are provided according to detection regions of different wavelengths to control carrier doping.

[0011] Fig. 1 is a schematic plan view of a graphene optical sensor 100 according to an embodiment, and Fig. 2 is a schematic cross-sectional view taken along line I-I' in Fig. 1. The stacking direction of the graphene optical sensor 100 is the Z direction, and two directions perpendicular to each other in a plane perpendicular to the Z axis are the X direction and the Y direction. In Fig. 1 and Fig. 2, in addition to the device structure, electrical connection relationships are also depicted.

[0012] The graphene optical sensor 100 includes a graphene layer 15, a pair of electrodes 13 and 14 connected to the graphene layer 15, a first plasmon antenna 17 stacked in a first region 151 of a light incident surface 155 of the graphene layer 15 between the electrodes 13 and 14, and a second plasmon antenna 18 stacked in a second region 152 of the light incident surface 155 of the graphene layer 15. The first plasmon antenna 17 has a predetermined periodic structure and is an example of a "first periodic structure." The second plasmon antenna 18 has a periodic structure different from that of the first plasmon antenna 17 and is an example of a "second periodic structure."

[0013] The graphene optical sensor 100 has a first gate electrode 21 and a second gate electrode 22 provided on the surface of the graphene layer 15 opposite to the light incident surface 155 via an insulating film 16. The first gate electrode 21 is provided at a position corresponding to the first region 151, and the second gate electrode 22 is provided at a position corresponding to the second region 152.

[0014] The first gate voltage V applied to the first gate electrode 21 Gate1 and a second gate voltage V applied to the second gate electrode 22. Gate2 is controlled by the control circuit 101. The bias voltage V between the electrodes 13 and 14 BIAS The application timing of the voltage Vcc is also controlled by the control circuit 101.

[0015] One of the electrodes 13 and 14 is used as a source electrode, and the other is used as a drain electrode. A bias voltage V BIAS When a bias voltage V is applied, a gradient is generated in the energy band at the interface between the electrode 13 and the graphene layer 15 and at the interface between the electrode 14 and the graphene layer 15. BIAS is, for example, 0.1 V to 0.5 V. When light is incident on the graphene layer 15 in this state, the resistance value changes due to light absorption, and the photocurrent flowing between the electrodes 13 and 14 changes. The change in photocurrent correlates with the amount of incident light. The amount of incident light can be obtained by reading the change in photocurrent with an ammeter 102 (indicated by the symbol "A" in the figure).

[0016] The first plasmon antenna 17 and the second plasmon antenna 18 are periodic patterns formed of a conductor such as a metal on the light incident surface 155 of the graphene layer 15. The repetition period of the patterns of the first plasmon antenna 17 and the second plasmon antenna 18 is set according to the target wavelength band. The first plasmon antenna 17, which is a first periodic structure, and the second plasmon antenna 18, which is a second periodic structure, are plasmon antennas that resonate with light of different wavelengths.

[0017] FIG. 3 is a diagram illustrating a plasmon antenna. For example, when light of a specific wavelength is incident on a periodic structure formed by a metal pattern 171, plasmon resonance is induced. Surface plasmons (sometimes simply called "plasmons") are collective oscillations of electrons caused by incident light. The gap G between two adjacent metal patterns 171 is set to be equal to or less than the target wavelength. Evanescent waves that pass through a minute gap equal to or less than the wavelength are enhanced by the surface plasmons. In other words, the optical electric field increases near the edge 172 of the periodic metal pattern 171.

[0018] 1 and 2, the first region 151 and the second region 152 have different plasmon antennas 17 and 18, respectively, provided on the graphene layer 15, and therefore have different plasmon resonance wavelengths. This allows the first region 151 and the second region 152 of the graphene layer 15 to absorb light of different wavelengths.

[0019] A first gate electrode 21 and a second gate electrode 22 are provided at positions corresponding to the first region 151 and the second region 152. Appropriate gate voltages are applied individually from the first gate electrode 21 and the second gate electrode 22 to the first region 151 and the second region 152 of the graphene layer 15. The appropriate gate voltage is a gate voltage that satisfies a condition for light absorption at the plasmon resonance wavelength to occur in the region to be sensed, out of the first region 151 and the second region 152, and satisfies a condition for light absorption at the plasmon resonance wavelength not to occur in the other region.

[0020] Figure 4 is a diagram explaining the optical absorption of graphene. Single-layer graphene has an energy band structure in which the upper end of the valence band and the lower end of the conduction band coincide or overlap. Single-layer graphene is represented by inverted cones whose tips coincide as shown in Figure 4. This energy band structure is called a Dirac cone, and the point where the tips of the cones meet is called the Dirac point.

[0021] The optical absorption of graphene is mainly caused by the interband transition of electrons in graphene. The wavelength at which optical absorption occurs due to the interband transition of graphene is the Fermi level E F The energy of the photon incident on graphene, hω, depends on the Fermi level E F When the magnitude exceeds twice that of (hω>2|E F |), the light is absorbed by the graphene. The electrons in the graphene become dominant over the holes, and the Fermi level E F shifts to the conduction band, and the energy of the incident light hω rises to the Fermi level E F If hω<2|E F |), no light absorption occurs according to the Pauli exclusion principle.

[0022] Since the Fermi level changes depending on the amount of carriers, i.e., electrons or holes, in graphene, by individually controlling the gate voltages applied to the first gate electrode 21 and the second gate electrode 22, it is possible to create conditions under which light absorption occurs at a wavelength of interest and no light absorption occurs at the other wavelength.

[0023] 5 is a diagram for explaining the setting of the gate voltage. Consider the case where a gate voltage is applied from the first gate electrode 21 to the graphene layer 15 via the insulating film 16. First, under ideal conditions, when no gate voltage is applied, the Fermi level E F Consider the condition where is zero, i.e., at the Dirac point.

[0024] Gate voltage V Gate As a result, a charge Q is accumulated at the interface of the graphene layer 15 and the interface of the first gate electrode 21. The charge Q is expressed as follows:

[0025]

number

[0026]

number

[0027]

number

[0028]

number

[0029]

number

[0030]

number

[0031] 1 and 2, for example, a condition is set in which the first region 151 absorbs light of wavelength λ1 and the second region 152 does not absorb light of wavelength λ2. In the first region 151, the Fermi level E F near the Dirac point, hω>2|E F The first gate voltage V applied to the first gate electrode 21 satisfies | Gate1 At the same time, the second region 152 is highly doped so that hω<2|E F The second gate voltage V applied to the second gate electrode 22 so as to satisfy | Gate2 Set.

[0032] The second region 152 is connected to a second gate voltage V Gate2 When doped by applying a voltage, the resistance of the first region 151 becomes low, and the region is short-circuited and does not respond. In the high-resistance first region, the light of wavelength λ1 that resonates with the first plasmon antenna 17 is absorbed, and carriers are generated. This changes the resistance of the first region 151, and the first region 151 operates as a photoconductor.

[0033] Conversely, when the second region 152 absorbs light of wavelength λ2 and the first region 151 does not absorb light of wavelength λ1, the Fermi level E F near the Dirac point, hω>2|E F | is satisfied, the first region 151 is made highly doped, and hω<2|E F In order to satisfy these conditions, the first gate voltage V Gate1 and the second gate voltage V applied to the second gate electrode 22. Gate2 In the second region 152, the resistance becomes low due to doping with carriers, causing a short circuit, and no change in the photocurrent is detected. In the high-resistance second region, the light of wavelength λ2 that resonates with the second plasmon antenna 18 is absorbed, and carriers are generated. This changes the resistance value of the second region 152, causing it to act as a photoconductor.

[0034] 1 and 2, different wavelengths can be detected while using graphene that absorbs light in a wide wavelength range. The graphene optical sensor 100 has a mobility much faster than that of a solid-state infrared sensor. In addition, the element size is small and easy to fabricate. A high-speed, ultra-compact, two-wavelength infrared sensor is realized.

[0035] The graphene optical sensor 100 can be fabricated by an already established semiconductor process. A first gate electrode 21 and a second gate electrode 22 are formed in a predetermined pattern on the surface of an insulating substrate 11. As described later, the substrate 11 may be an undoped semi-insulating semiconductor substrate on which an oxide film is formed. As an example, the first gate electrode 21 and the second gate electrode 22 are formed by transferring and etching graphene. The first gate electrode 21 and the second gate electrode 22 are connected to a gate voltage source 103 and a gate voltage source 104 by terminals that are individually drawn out, respectively.

[0036] The gate voltage source 103 has an output capacitance capable of applying a potential at which light absorption in the graphene layer 15 is inhibited in the first region 151. The gate voltage source 104 has an output capacitance capable of applying a potential at which light absorption in the graphene layer 15 is inhibited in the second region 152.

[0037] An insulating film 16 is formed on the entire surface of the substrate 11 including the first gate electrode 21 and the second gate electrode 22. The insulating film 16 in the portion covering the first gate electrode 21 and the second gate electrode 22 becomes a gate insulating film. The insulating film 16 is formed of, for example, an aluminum oxide (Al2O3) film having a thickness of 20 nm.

[0038] The graphene layer 15 is formed at a predetermined position on the insulating film 16. The graphene layer 15 may be disposed on the insulating film 16 by a mechanical peeling method, a transfer method, or the like. Alternatively, a catalytic metal may be deposited on the insulating film 16 by sputtering, and graphene may be grown directly on the insulating film 16 by a CVD method. The graphene layer 15 may be a single layer of graphene, or may contain two or more layers of graphene. After the graphene layer 15 is provided, it may be annealed at a temperature of 150 to 200° C. to enhance adhesion between the insulating film 16 and the graphene.

[0039] On the graphene layer 15, the electrodes 13 and 14, the first plasmon antenna 17, and the second plasmon antenna 18 are formed. The order in which the electrodes 13 and 14, the first plasmon antenna 17, and the second plasmon antenna 18 are formed does not matter. The electrodes 13 and 14 are formed of a good conductor such as Au, Ti, palladium (Pd), chromium (Cr), platinum (Pt), or a combination of these. The electrodes 13 and 14 are connected to a bias voltage source 105.

[0040] The first plasmon antenna and the second plasmon antenna 18 may be formed of the same material as the electrodes 13 and 14, or may be formed of a different material. The first plasmon antenna 17 is, for example, a rectangular metal thin film of 950 nm x 240 nm arranged in the first region 151 at intervals of 60 nm in the long axis direction and at intervals of 960 nm in the short axis direction. The second plasmon antenna 18 is a rectangular metal thin film of 2350 nm x 480 nm arranged in the second region 152 at intervals of 50 nm in the long axis direction and at intervals of 1920 nm in the short axis direction. The graphene optical sensor 100 thus fabricated may be entirely covered with a transparent protective layer.

[0041] 6 is a flowchart of the operation of the graphene optical sensor 10. When the operation of the graphene optical sensor 100 is started, the control circuit 101 controls the first gate voltage V Gate1 is set to a voltage at which light absorption in the first region 151 is permitted, and the second gate voltage V Gate2 is set to a voltage at which light absorption is inhibited in the second region 152 (S11).

[0042] For example, the first gate voltage V Gate1 hω>2|E F1 | and the second gate voltage V Gate2 hω<2|E F2 Set the voltage to satisfy |, where E F1 is the Fermi level of the graphene in the first region 151, and E F2 is the Fermi level of the graphene in the second region 152. Then, a bias voltage V BIAS is applied and the light of λ1 is detected (S12).

[0043] Next, the control circuit 101 controls the second gate voltage V Gate2 is set to a voltage at which light absorption is permitted in the second region 152, and the first gate voltage V Gate1 is set to a voltage at which light absorption is inhibited in the first region 151 (S13). For example, the second gate voltage V Gate2 hω<2|E F2| and the first gate voltage V Gate1 hω>2|E F1 Then, a bias voltage V BIAS is applied and the light of λ2 is detected (S14).

[0044] It is determined whether the operation of the graphene optical sensor 100 has ended (S15), and steps S11 to S15 are repeated until the operation ends (Yes in S15). According to the operation flow in Fig. 6, infrared rays of two wavelengths can be detected separately with simple control.

[0045] FIG. 7 is a schematic cross-sectional view of a graphene optical sensor 100A having another configuration. In the graphene optical sensor 100A, an insulating layer 19 is laminated on a substrate 11A, and a first gate electrode 21 and a second gate electrode 22 are laminated on the insulating layer 19. The substrate 11A is a semi-insulating substrate such as undoped silicon. The insulating layer 19 may be a thermal oxide film (SiO2) formed on the substrate 11A in advance, or may be a layer formed on the substrate 11A by sputtering or the like. An insulating resin film may be used. An insulating film 16 is provided to cover the entire surfaces of the insulating layer 19, the first gate electrode 21, and the second gate electrode 22 and to be used as a gate insulating film. A graphene layer 15 is laminated on the insulating film 16.

[0046] 1 and 2, the first plasmon antenna 17 is laminated in a first periodic structure in a first region 151 of a light incident surface 155 of a graphene layer 15 between a pair of electrodes 13 and 14, and the second plasmon antenna 18 is disposed in a second periodic structure in a second region 152. In the configuration of FIG. 6, the gate voltage V applied to the first gate electrode 21 is Gate1 and the gate voltage V applied to the second gate electrode 22. Gate2 By controlling these individually, two different wavelengths can be detected.

[0047] FIG. 8 is a schematic cross-sectional view of a graphene optical sensor 100B having yet another configuration. In the graphene optical sensor 100B, a first gate electrode 21B and a second gate electrode 22B are formed in a predetermined region inside a substrate 11B. The substrate 11B is a semi-insulating substrate such as undoped silicon. The first gate electrode 21B and the second gate electrode 22B are formed by doping a high concentration of impurities in a predetermined region of the substrate 11B, and only the region into which the impurities are injected becomes a back gate. A first gate voltage V Gate1 and the second gate voltage V Gate2 are applied to the first gate electrode 21B and the second gate electrode 22B, respectively, individually.

[0048] An insulating film 16 is laminated on the surface of the substrate 11B. The insulating film 16 in the portion covering the first gate electrode 21B and the second gate electrode 22B becomes a gate insulating film. A graphene layer 15 is laminated on the insulating film 16. Between the pair of electrodes 13 and 14, a first plasmon antenna 17 is provided in a first region 151 of a light incident surface 155 of the graphene layer 15, and a second plasmon antenna 18 is provided in a second region 152. The first region 151 corresponds to the region where the first gate electrode 21B of the substrate 11B is formed. The second region 152 corresponds to the region where the second gate electrode 22B of the substrate 11B is formed.

[0049] In the configuration of FIG. 8, the first gate voltage V Gate1 and the second gate voltage V applied to the second gate electrode 22B. Gate2 By controlling these individually, two different wavelengths can be detected.

[0050] Although the present disclosure has been described above based on specific examples, the present disclosure is not limited to the above examples. The insulating film 16 is not limited to aluminum oxide, and may be a silicon oxide film, a silicon nitride film, or a boron nitride (BN) film. The first plasmon antenna 17 and the second plasmon antenna 18 are not limited to an arrangement of rectangular metal films, and may be a polygonal pattern such as a hexagon or octagon, or a circular pattern, as long as the interval between adjacent patterns is set to a desired wavelength or less. The graphene optical sensor 100 (or 100A or 100B) may be arranged in a one-dimensional or two-dimensional direction to form a graphene optical sensor array. [Explanation of symbols]

[0051] 11, 11A, 11B Board 13 electrodes 14 electrodes 15 graphene layers 151 First area 152 Second area 155 Light incidence surface 16 Insulating film 17 First plasmon antenna (first periodic structure) 18 Second plasmon antenna (second periodic structure) 19 Insulating layer 21, 21B First gate electrode 22, 22B Second gate electrode 100, 100A, 100B Graphene Light Sensor 101 Control circuit V Gate1 First Gate Voltage V Gate2 Second Gate Voltage

Claims

1. a first gate electrode and a second gate electrode provided on a substrate; an insulating film covering the first gate electrode and the second gate electrode; A graphene layer laminated on the insulating film; A pair of electrodes connected to the graphene layer; a first periodic structure laminated on a first region of the graphene layer between the pair of electrodes, and a second periodic structure laminated on a second region of the graphene layer; the first periodic structure and the second periodic structure have different repeating periods, the first gate electrode is provided at a position corresponding to the first region, and the second gate electrode is provided at a position corresponding to the second region; a gate voltage is applied to the first gate electrode and the second gate electrode individually; the first periodic structure is a first plasmon antenna that resonates with light of a first wavelength, and the second periodic structure is a second plasmon antenna that resonates with light of a second wavelength different from the first wavelength, when detecting light of the first wavelength, a first gate voltage that does not inhibit optical absorption of light of the first wavelength in the first region is applied to the first gate electrode, and a second gate voltage that inhibits optical absorption of light of the first wavelength in the second region is applied to the second gate electrode; when detecting light of the second wavelength, a third gate voltage that does not inhibit optical absorption of the light of the second wavelength in the second region is applied to the second gate electrode, and a fourth gate voltage that inhibits optical absorption of the light of the second wavelength in the first region is applied to the first gate electrode; The graphene photosensor, wherein the first gate voltage is different from the third gate voltage, and the second gate voltage is different from the fourth gate voltage.

2. the first gate electrode is connected to a first gate voltage source, and the first gate voltage source is capable of outputting the first gate voltage and the fourth gate voltage; The graphene optical sensor of claim 1 .

3. the second gate electrode is connected to a second gate voltage source, the second gate voltage source being capable of outputting the second gate voltage and the third gate voltage; The graphene optical sensor of claim 2.

4. the substrate is a semiconductor substrate; the first gate electrode and the second gate electrode are stacked on an insulating layer stacked on the substrate; The graphene optical sensor according to any one of claims 1 to 3.

5. the substrate is an undoped substrate, The second gate electrode and the second gate electrode are high concentration impurity doped regions formed in the substrate; The graphene optical sensor according to any one of claims 1 to 3.

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