JP7173074B2 - photodetector - Google Patents

Description

TECHNICAL FIELD The present invention relates to a photodetector, and more particularly to a photodetector capable of detecting broadband light with high sensitivity.

Graphene is a sheet-like material consisting of one to several atomic layers of graphite crystals. It has extremely high carrier mobility at room temperature and can absorb light of all wavelengths from deep ultraviolet to terahertz. there is Therefore, attempts have been made to fabricate a broadband photodetector that is responsive to light from deep ultraviolet to infrared light by utilizing such unique properties of graphene. However, graphene is metallic, and the problem is that the sensitivity of photodetectors is low.

On the other hand, if graphene is refined to a size of several nanometers, it is possible to obtain graphene nanostructures such as graphene quantum dots that are made into semiconductors. Attempts have also been made to improve the sensitivity of photodetectors using such graphene nanostructures.

For example, Patent Document 1 discloses a photosensor comprising a field effect transistor (graphene FET) whose channel is made of graphene and a light absorbing layer formed on the surface of the channel, the light absorbing layer being made of nitrogen-functionalized nanographene. disclosed.
In the same document, the combination of graphene FET and nitrogen-functionalized nano-graphene provides a photosensor with high photoconductive gain, free of harmful elements, and responsive in the ultraviolet to near-infrared region. Have been described.

Patent Document 2 discloses nanographene, a π-conjugated functional group bonded to the nanographene via a pyrazine skeleton, and one or more Br groups and/or CN groups introduced into the π-conjugated functional group. Graphene nanostructures are disclosed.
In the same document,
(a) introduction of a π-conjugated functional group to nanographene via a pyrazine skeleton improves the sensitivity to light in the short wavelength region;
(b) When a Br group and/or a CN group is introduced into the π-conjugated functional group, the sensitivity to light in the short wavelength range is further improved, and at the same time, the light absorption edge is lengthened to 1000 nm or more, and the long wavelength range is improved. It is described that the sensitivity to light is remarkably improved.

Although Patent Document 3 does not aim to improve the sensitivity of the photodetector,
a graphene layer;
a source electrode coupled to one end of the graphene layer;
a drain electrode connected to the other end of the graphene layer;
a functional layer interposed between the graphene layer and the drain electrode;
a gate formed on the back surface of the graphene layer;
A graphene device comprising a graphene layer and a gate insulating layer interposed between the gate is disclosed.
The document describes that a multifunctional graphene element can be obtained by inserting a functional layer between the graphene layer and the drain electrode.

In Patent Documents 4 to 6, although it is not intended to improve the sensitivity of the photodetector, in an imaging device equipped with a photodetector (photoelectric conversion element), as a conductive layer (transport layer) of the photodetector The point of using graphene is described.
Furthermore, although it is not intended to improve the sensitivity of a photodetector, Patent Document 7 discloses that graphene is used as a layer formed under an electrode of an LED or a nitride semiconductor in a display element equipped with an LED. points are noted.

In existing imaging devices, the wavelength of light that can be imaged is divided depending on the application, such as the visible range and the near-infrared range. Therefore, when trying to image light in a plurality of bands, a plurality of elements are required. This is because the bandgap of the material used in the imaging device is fixed, which limits the wavelength of light that can respond.

On the other hand, the graphene nanostructures described in Patent Documents 1 and 2 can not only absorb light in a wide band, but also have high light sensitivity. Therefore, if this is applied to a photodetector for an imaging device, it is possible to obtain an imaging device having high sensitivity to broadband light.
However, in order to further improve the performance of the imaging device, it is desired to further improve the light sensitivity of the photodetector.

JP 2017-092210 A JP 2019-026528 A JP 2016-025356 A JP 2018-207273 A JP 2017-028682 A JP 2016-213823 A JP 2016-029794 A

A problem to be solved by the present invention is to provide a photodetector capable of detecting broadband light with high sensitivity.

In order to solve the above problems, a photodetector according to the present invention has the following configuration.
(1) the photodetector,
a channel made of graphene;
and a light absorbing layer including graphene nanostructures formed on the surface of the channel.
(2) The graphene nanostructure is
nanographene,
and an organic group bonded to the nanographene.
(3) the channel is
The path along which the carrier moves is curved or bent,
It has a planar shape capable of reabsorbing phonons emitted from the emitting portion of the channel in the reabsorbing portion of the channel that emitted the phonons .

In a photodetector in which a graphene nanostructure exhibiting broadband and high sensitivity is formed on a graphene channel, when the graphene nanostructure is irradiated with light, carriers are excited, and one of the excited carriers (for example, electrons) move to the graphene. Therefore, the presence or absence of light and the amount of light can be determined by detecting the amount of carriers that have moved to the graphene channel (that is, the amount of change in current).
However, when the planar shape of the channel is a linear shape, when the graphene nanostructure is irradiated with light, the light in the ultraviolet to visible region, which is a region of high photoresponse sensitivity, excites carriers, causing current flow. Although it is detected as a change amount, it is emitted from the channel as phonons in light with a wavelength longer than near-infrared, which is a region of low photoresponse sensitivity, and hardly contributes to the current change.

On the other hand, if the planar shape of the graphene channel is moderately curved or bent, the phonons emitted from a part of the channel (emission part) are reabsorbed in the other part of the channel (reabsorption part). can be done. Since graphene is a zero-gap semiconductor or a material having properties close thereto, carriers are excited in graphene when phonons are reabsorbed by graphene. As a result, the sensitivity to light is improved compared to when the graphene channel is linear.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of an organic group with a π-conjugated functional group and a Br group or a CN group, and a graphene nanostructure with the same. FIG. 2A is a schematic diagram of phonon behavior when a linear graphene channel is irradiated with light. FIG. 2B is a schematic diagram of the behavior of phonons when a wave-shaped graphene channel is irradiated with light. 1 is a schematic cross-sectional view of one pixel of an imaging device provided with a photodetector according to the present invention; FIG.

FIG. 4A is an SEM image of the pixel portion of the imaging device obtained in Example 1. FIG. FIG. 4(B) is an enlarged SEM image of a photodetector provided in the imaging device of FIG. 4(A). FIG. 5A is an optical microscope image of a photodetector having a straight graphene channel (Comparative Example 1). FIG. 5B is an optical microscope image of a photodetector (Example 1) having corrugated graphene channels. FIG. 4 is a diagram showing the relationship between the wavelength of light and the amount of change in current value ΔI when a photodetector having a linear (Comparative Example 1) or corrugated (Example 1) graphene channel is irradiated with light.

An embodiment of the present invention will be described in detail below.
[1. Graphene nanostructure]
In the present invention, the graphene nanostructure is
nanographene,
and an organic group bonded to the nanographene.

[1.1. Nanographene]
“Nanographene” is a substance having a two-dimensional sheet-like structure composed of a carbon ring structure and an sp ² -bonded aromatic ring. ). Nanographene may consist of a single-layer sheet or may consist of a multi-layer sheet. Generally, when nanographene exhibits semiconducting properties, graphene nanostructures into which organic groups are introduced also exhibit semiconducting properties.

[1.2. organic group]
The term “organic group” refers to a functional group that has the effect of lengthening the light absorption edge of nanographene and/or increasing the absorbance of light. In other words, the “organic group” has the effect of reducing the bandgap, lengthening the wavelength of the light absorption edge of nanographene, and/or increasing the absorbance of light by donating electrons to nanographene. A functional group.
Examples of such organic groups include
(a) a π-conjugated nitrogen functional group;
(b) a functional group comprising a π-conjugated functional group bonded to the nanographene via a pyrazine skeleton and one or more Br or CN groups introduced into the π-conjugated functional group;
and so on.
Any one of these organic groups may be bonded to the nanographene, or two or more thereof may be bonded.

[1.2.1. π-conjugated nitrogen functional group]
A first specific example of an organic group consists of a π-conjugated nitrogen functional group. Here, the term "π-conjugated nitrogen functional group" refers to a functional group containing a nitrogen atom having a lone pair of electrons as a constituent element.
Examples of the π-conjugated nitrogen functional group include an amine group, a dimethylamine group, an azo group, a naphthalenediamine group, a phenylenediamine group, and a methyl red group. The graphene nanostructure may contain any one of these π-conjugated nitrogen functional groups, or may contain two or more of them.
The π-conjugated nitrogen functional groups are preferably attached to the edge and/or basal plane of the nanographene.

[1.2.2. Organic group containing π-conjugated functional group and Br group or CN group]
A second specific example of the organic group is
a π-conjugated functional group bonded to the nanographene via a pyrazine skeleton;
and one or more Br groups or CN groups introduced into the π-conjugated functional group.
FIG. 1 shows a schematic diagram of an organic group having a π-conjugated functional group and a Br group or a CN group, and a graphene nanostructure having the same.

[A. π-Conjugating Functional Group, Pyrazine Skeleton]
A "π-conjugated functional group" refers to a functional group derived from an unsaturated cyclic compound having a structure in which atoms having π electrons are arranged in a ring. Atoms having π electrons include, for example, C and N. The π-conjugated functional groups are attached to the nanographene through the pyrazine skeleton.

“Pyrazine skeleton” refers to a pyrazine ring (=C ₄ N ₂ =) covalently bonded to the edge and/or basal plane of nanographene on only one side. Either one of the “carbons at the 2nd and 3rd positions” or the “carbons at the 5th and 6th positions” of the pyrazine ring is shared between the nanographene and the pyrazine skeleton, and the other is shared between the pyrazine skeleton and the π-conjugated functional group. shared between.
The upper part of FIG. 1 is a schematic diagram showing how the pyrazine skeleton is bound to the edge or basal plane of nanographene. A pyrazine skeleton and a π-conjugated functional group substituted with a Br group or a CN group are bonded in such a manner that two adjacent Rs at the tip of the pyrazine skeleton are shared.

Examples of unsaturated cyclic compounds that form the basis of the π-conjugated functional group include:
(a) an unsaturated cyclic compound whose cyclic structure consists only of C (e.g., benzene (C ₆ H ₆ ), azulene (C ₁₀ H ₈ ), cyclobutadiene (C ₄ H ₄ ), etc.);
(b) Unsaturated cyclic compounds whose cyclic structures are composed of C and _{N (} e.g., pyridine ( _C5H5N ), pyrazine _{( C4H4N2} ), _{phenazine ( C12H8N2} ), pyridazine _{( C4H4N2} ), _{pyrimidine ( C4H4N2} ), _etc. ),
(c) unsaturated cyclic compounds whose cyclic structures are composed of C and O (for example, furan (C ₄ H ₄ O), etc.);
(d) Unsaturated cyclic compounds whose cyclic structures are composed of C and N and contain N—H bonds (e.g., pyrrole (C ₄ H ₅ N), pyrazole (C ₃ H ₄ N ₂ ), imidazole (C _{3H4N2 )} , _etc. ),
(e) annulene having more than 6 carbon atoms (e.g., cyclooctatetraene _{( C8H8} ), cyclotetradecaheptaene _{( C14H14} ), cyclooctadecanonaene _{( C18H18} ), etc.);
and so on.
The graphene nanostructure may contain any one of these π-conjugated functional groups, or may contain two or more of them.

[B. Br group, CN group]
A Br group and/or a CN group is introduced into the π-conjugated functional group. “Br group and / or CN group is introduced” means that the hydrogen atoms bonded to the atoms constituting the ring structure of the π-conjugated functional group are substituted with Br group and / or CN group Say. Either a Br group or a CN group may be introduced into the π-conjugated functional group, or both may be introduced.

Furthermore, one Br group or CN group may be introduced into the π-conjugated functional group, or two or more Br groups and/or CN groups may be introduced. Since both Br and CN groups are strongly electron-attracting, the greater the number of Br and CN groups introduced into the π-conjugated functional group, the greater the push-pull effect with the π electrons of nanographene. , the light absorption wavelength becomes longer. However, even if Br groups or CN groups are introduced more than necessary, there is no difference in the effect and there is no practical benefit. Therefore, it is preferable to select the optimum number of Br groups and CN groups depending on the purpose.

Examples of the π-conjugated functional group introduced with a Br group and/or a CN group include a 4-bromobenzene group (C ₆ H ₃ Br=), a 4,5-dibromobenzene group (C ₆ H ₂ Br ₂ = ), 5-bromopyridine group (C ₅ H ₂ BrN=), 5-bromopyrazine group (C ₄ HBrN ₂ =), benzonitrile group (C ₆ H ₃ CN=), phthalonitrile group (C ₆ H ₂ ( CN) ₂ =), 2,3-dicyanopyrazine group (C ₄ N ₂ (CN) ₂ =), etc. (see FIG. 1).

[1.3. average size]
The “size of the graphene nanostructure” refers to the maximum length of the sheet when the sheet is viewed from the normal direction.
“Average size” refers to the average size of n randomly selected graphene nanostructures (n≧5).

The average size of graphene nanostructures affects the bandgap Eg. If the average size of the graphene nanostructures becomes too small, the Eg becomes excessively large and the semiconductor property may be lost. Therefore, the average size is preferably 1 nm or more. The average size is preferably 3 nm or more.
On the other hand, when the average size of the graphene nanostructures becomes too large, the semiconducting properties are lost. Therefore, the average size is preferably 100 nm or less. The average size is preferably 50 nm or less, more preferably 30 nm or less.

[1.4. Average thickness]
“Average thickness of graphene nanostructures” refers to the average thickness of randomly selected n (n≧5) graphene nanostructures.
As a method of measuring the thickness, for example,
(a) a method of directly measuring the thickness of the sheet using an atomic force microscope (AFM);
(b) There is a method of determining the thickness by considering the ideal thickness of one layer (0.34 nm) from the number of layers of the graphene nanostructure observed with a transmission electron microscope (TEM). Almost the same results are obtained using either method.

The average thickness of graphene nanostructures affects semiconductivity and solar transmittance. If the graphene nanostructures become too thick, they may lose their semiconducting properties. Therefore, the average thickness of the graphene nanostructures is preferably 100 nm or less. The average thickness is preferably 50 nm or less, more preferably 10 nm or less.

[1.5. bandgap]
When applying the graphene nanostructure according to the present invention to a photodetector, the graphene nanostructure should be capable of forming electron-hole pairs by absorbing light. For that purpose, the graphene nanostructure needs to exhibit semiconducting properties (Eg>0 eV).
Whether the graphene nanostructure exhibits semiconductivity and the size of its bandgap (Eg) are mainly determined by the average size (or average mass) of the graphene nanostructure, the type of organic group (for example, Br group and / or number of CN groups).

In general, the lower the Eg of the graphene nanostructure, the longer the wavelength of light that can be absorbed. However, if Eg becomes too small, thermal excitation of carriers becomes possible, and it cannot be used as a broadband photodetector. Therefore, Eg is preferably 0.01 eV or more.
On the other hand, if Eg becomes too large, it becomes impossible to absorb long-wavelength light. Therefore, Eg is preferably 1.2 eV or less. Eg is preferably 1.0 eV or less, more preferably 0.8 eV or less.

[1.6. HOMO level]
The graphene nanostructure according to the present invention can be used for various purposes, and is particularly suitable as a material for a light absorption layer of a photodetector (for example, a phototransistor) using graphene for the channel.
In general, photoexcited carriers have a short lifetime and are likely to disappear due to recombination. On the other hand, when a light absorption layer with a long photoexcited carrier lifetime is combined with a graphene field effect transistor (graphene FET), one of the carriers (for example, electrons) excited in the light absorption layer quickly moves to the graphene layer. , the lifetime of the other carriers (eg, holes) remaining in the light absorption layer is lengthened. As a result, the photoconductive gain of the graphene FET is improved.

In general, the smaller the LUMO (lowest unoccupied molecular orbital) level difference or the HOMO (highest occupied molecular orbital) level difference between the light absorption layer and the graphene, the easier it is for one carrier to move to the graphene layer. In order to obtain a high photoconductive gain, the LUMO level difference or HOMO level difference between them should be 2 eV or less. The LUMO level difference or HOMO level difference is preferably 1 eV or less.

The LUMO level difference or HOMO level difference between the graphene nanostructure according to the present invention and the graphene used for the channel of the graphene FET mainly depends on the type of organic groups introduced into the graphene nanostructure (for example, type of π-conjugated functional group, number of Br groups and/or CN groups, etc.).
The HOMO level of graphene is about −4.6 eV. On the other hand, if the type of organic group is optimized, the HOMO level of the graphene nanostructure will be −6.0 eV or more and −4.0 eV or less. Therefore, when a graphene nanostructure having such a HOMO level is combined with a graphene FET, electrons can move quickly to the graphene layer, and the photoconductive gain is improved.

[2. Method for producing graphene nanostructure]
The method for producing a graphene nanostructure according to the present invention comprises
Dispersing nanographene in an aqueous solution in which (a) a compound having a π-conjugated nitrogen functional group or (b) a diamine compound containing a π-conjugated functional group substituted with a Br group and/or a CN group is dissolved; a dispersion step of obtaining a dispersion;
and a heating step of heating the dispersion at 60° C. or higher.

[2.1. Dispersion process]
first,
(a) a compound having a π-conjugated nitrogen functional group, or
(b) Nanographene is dispersed in an aqueous solution in which a diamine compound containing a π-conjugated functional group substituted with a Br group and/or a CN group is dissolved to obtain a dispersion (dispersion step).

[2.1.1. Compound having π-conjugated nitrogen functional group]
"Compound with π-conjugated nitrogen functional group (hereinafter also referred to as "nitrogen-containing compound")" means a compound having a functional group containing a nitrogen atom having a lone pair as a constituent element, which is dissolved in water. or dispersible. As a starting material, any one kind of nitrogen-containing compound may be used, or two or more kinds may be used.

Nitrogen-containing compounds include, for example, 2,3-diaminonaphthalene (DAN), o-phenylenediamine (o-PD), aniline, ammonia, dimethylformamide, diphenyl phosphate azide, and paramethyl red.

[2.1.2. diamine compound]
A “diamine compound containing a π-conjugated functional group substituted with a Br group and/or a CN group (hereinafter also simply referred to as a “diamine compound”)” means a π-conjugated functional group substituted with a Br group and/or a CN group. A compound in which primary amine groups are bonded to two adjacent positions of the group, respectively, and is soluble or dispersible in water.

Examples of such diamine compounds include
4-bromo-1,2-diaminobenzene (C ₆ H ₃ Br(NH ₂ ) ₂ ),
4,5-dibromo-1,2-phenylenediamine (C ₆ H ₂ Br ₂ (NH ₂ ) ₂ ),
2,3-diamino-5-bromopyridine (C ₅ HNBr(NH ₂ ) ₂ ),
2,3-diamino-5 _- bromopyrazine ( _C4HN2Br ( _NH2 ) ₂ ),
3,4-diaminobenzonitrile (C ₆ H ₃ CN(NH ₂ ) ₂ ),
4,5-diaminophthalonitrile (C ₆ H ₂ (CN) ₂ (NH ₂ ) ₂ ),
_5,6 -diamino-2,3-dicyanopyrazine ( _C6N2 (CN) ₂ ( _NH2 ) ₂ ),
and so on.
Any one of these diamine compounds may be used as the starting material, or two or more thereof may be used.

[2.1.3. Concentration of aqueous solution]
Both the nitrogen-containing compound and the diamine compound are used in the form of an aqueous solution dissolved or dispersed in water. The concentration of the nitrogen-containing compound or diamine compound contained in the aqueous solution is not particularly limited, and an optimum concentration may be selected according to the type of starting material, required properties, and the like. The concentration of the nitrogen-containing compound or diamine compound is usually 0.1-10 mol/L.

[2.1.4. Nanographene]
Nanographene is
(a) dispersing graphite oxide or graphene oxide in an alkaline aqueous solution and heating to 60° C. or higher in a closed container; or
(b) oxidizing the nanographite particles with an oxidizing agent (potassium permanganate, potassium nitrate, etc.), for example, in strong acid (concentrated sulfuric acid);
can be synthesized by

Here, “graphite oxide” means oxygen-containing functional groups (for example, —COOH groups, —OH groups, —C—O—C— groups, etc.) on the edges and/or base surfaces of the graphene layers that constitute graphite. means that is bound to Graphite oxide is obtained, for example, by oxidizing graphite using an oxidizing agent (potassium permanganate, potassium nitrate, etc.) in a strong acid (concentrated sulfuric acid).

“Graphene oxide” refers to a sheet material obtained by exfoliating graphite oxide layers. Graphene oxide can be obtained, for example, by dispersing graphite oxide in an aqueous solution and irradiating it with ultrasonic waves.
In the present invention, the starting material may be either graphite oxide before delamination or graphene oxide after delamination, or both of them may be used.

[2.1.5. Dispersion]
Nanographene is added to an aqueous solution containing nitrogen-containing compounds or diamine compounds. The amount of nanographene contained in the dispersion liquid is not particularly limited, and an optimum amount may be selected according to the type of starting material, required properties, and the like. The amount of nanographene is typically 0.1-50 mg/mL.

[2.2. Heating process]
Next, after dispersing nanographene in an aqueous solution in which a nitrogen-containing compound or a diamine compound is dissolved or dispersed, the dispersion is heated (heating step).
Heating is performed to speed up the reaction rate. If the heating temperature exceeds the boiling point of the dispersion, heating is done in a closed container.

The optimum heating temperature varies depending on the type of organic base. For example, when a nitrogen-containing compound is used as the organic radical source, if the heating temperature is too low, the reaction will not proceed sufficiently within a practical time. Therefore, the heating temperature is preferably 60° C. or higher. The heating temperature is more preferably 70° C. or higher, more preferably 80° C. or higher.
On the other hand, if the heating temperature is too high, the bonded nitrogen may be desorbed. Moreover, an expensive pressure-resistant container is required, which increases the manufacturing cost. Therefore, the heating temperature is preferably 200° C. or lower. The heating temperature is more preferably 180° C. or lower, more preferably 160° C. or lower.

Further, when a diamine compound is used as the organic group source, if the heating temperature is too low, the reaction will not proceed sufficiently within a realistic time. Therefore, the heating temperature is preferably 100° C. or higher. The heating temperature is more preferably 120° C. or higher, more preferably 140° C. or higher.
On the other hand, if the heating temperature is too high, the substituted or bonded Br group, CN group, or primary amine group may be eliminated. Moreover, an expensive pressure-resistant container is required, which increases the manufacturing cost. Therefore, the heating temperature is preferably 260° C. or less. The heating temperature is more preferably 240° C. or lower, more preferably 220° C. or lower.

The optimum heating time is selected according to the heating temperature. In general, the higher the heating temperature, the shorter the reaction time. The heating time is usually 1 to 20 hours.
The obtained graphene nanostructure may be used for various applications as it is, or may be washed, filtered and/or dialyzed as necessary.

[3. Photodetector]
The photodetector according to the present invention is
a channel made of graphene;
and a light absorbing layer including graphene nanostructures formed on the surface of the channel.

[3.1. channel]
[3.1.1. material]
The channel consists of graphene. “Graphene” is a substance having a two-dimensional sheet-like structure composed of carbon ring structures and sp ² -bonded aromatic rings (in other words, a sheet-like structure consisting of one to several atomic layers of graphite crystals). substance) that exhibits semi-metallic properties. Depending on the size and the number of atomic layers, graphene exists in a range from semimetallic properties to semiconducting properties.
In the present invention, graphene consists of a semimetal (bandgap (Eg)≈0 eV). Graphene, which is a semimetal, has high carrier mobility and a short transition time (τ _transit ), and is suitable as a channel material.

In the present invention, the term “(broadly defined) semimetal” means
(a) materials whose conduction and valence bands are perfectly matched at the Fermi level (i.e., zero-gap semiconductors), or
(b) a substance with a band structure in which the lower end of the conductor and the upper end of the valence band slightly overlap across the Fermi level (that is, a semimetal in the narrow sense)
Say.
In the present invention, the term "semimetal" means "semimetal in a broad sense" unless otherwise specified.

In order for graphene to have semi-metallic properties, the size of graphene should be greater than 100 nm in both width and height. The size of the graphene is 10 μm or more, more preferably 50 μm or more in both width and height.

Single-layer graphene is a zero-gap semiconductor. On the other hand, graphene, in which two or more atomic layers are superimposed, exhibits semimetallic properties in the narrow sense. However, if the number of atomic layers becomes too large, the electron-like body approaches a bulk, resulting in low carrier mobility. Therefore, the number of atomic layers of graphene is preferably two or less.

In order to obtain a high photoconductive gain, the higher the carrier mobility of graphene, the better. The carrier mobility of graphene is preferably 100 cm ² V ⁻¹ s ⁻¹ or more, more preferably 500 cm ² V ⁻¹ s ⁻¹ or more, still more preferably 1000 cm ² V ⁻¹ s ⁻¹ or more.

[3.1.2. shape]
In the present invention, a channel is
The path along which the carrier moves is curved or bent,
It has a planar shape capable of reabsorbing phonons emitted from the emitting portion of the channel in the reabsorbing portion of the channel.
This point is different from the conventional one.

FIG. 2A shows a schematic diagram of phonon behavior when a linear graphene channel is irradiated with light. FIG. 2B shows a schematic diagram of the behavior of phonons when a wave-shaped (S-shaped) graphene channel is irradiated with light.
When the graphene nanostructure is irradiated with light, light with a wavelength longer than near-infrared, which is in a region of low photoresponse sensitivity, is emitted from the channel as phonons. In this case, as shown in FIG. 2(A), when the channel is straight, the phonons are directly emitted to the outside. Therefore, the emitted phonons (ie, excess energy) contribute little to improving the sensitivity of the photodetector.

On the other hand, as shown in FIG. 2(B), when the channel is curved or bent appropriately, phonons emitted from a part of the channel (hereinafter also referred to as an “emission part”) are emitted from other parts of the channel. (hereinafter also referred to as “reabsorbing portion”). Since graphene is a zero-gap semiconductor or a material having properties close thereto, carriers are newly excited in graphene when phonons are reabsorbed by graphene. As a result, the sensitivity to light is improved compared to when the graphene channel is linear.

Examples of planar shapes of channels that can improve light sensitivity include:
(a) A waveform in which the channel is periodically curved or bent (e.g., U-shaped, V-shaped, N-shaped, S-shaped, W-shaped, sinusoidal, rectangular, triangular, sawtooth wave type, etc.)
(b) spiral, in which the channel is spirally curved or bent;
and so on.
A planar shape of the channel is particularly preferably a corrugated shape. This is because parameters such as resistance and channel length can be easily adjusted depending on the number and shape of curved portions or bent portions.

[3.1.3. average distance D between the emitting part and the reabsorbing part]
"Average distance D between the emitting part and the reabsorbing part" means the average value of the shortest distance D _min and the longest distance D _max between the emitting part and the reabsorbing part where phonons are exchanged.
The average distance D between the emitter and reabsorber affects the performance of the photodetector when the channel is curved or bent. If the average distance D is too short, the tunnel current increases, the resistance of graphene in the absence of light irradiation decreases, and the photodetection function may significantly deteriorate. Therefore, the average distance D is preferably 10 nm or more.
On the other hand, if the average distance D is too long, the rate at which phonons are reabsorbed decreases. Therefore, the average distance D is preferably 10 μm or less.
Furthermore, in order to efficiently transmit and receive phonons, it is preferable to have a shape in which the distance between the emitting portion and the reabsorbing portion facing each other is constant, as shown in FIG. 2(B).

[3.2. Light absorption layer]
A light absorbing layer is formed on the surface of the channel. In the present invention, the light absorbing layer includes graphene nanostructures. The light-absorbing layer may consist only of graphene nanostructures, or may further contain other materials. The details of the graphene nanostructure are as described above, so the description is omitted.

[3.3. Type of photodetector]
In the present invention, the type of photodetector is not particularly limited.
Specifically, as a photodetector,
(a) a photodiode with an anode and a cathode joined across the channel;
(b) a field effect transistor in which a source electrode and a drain electrode are joined to both ends of a channel, and a gate electrode is arranged on the upper surface or the lower surface of the channel via an insulating film;
(c) a photoresistor with an anode and a cathode bonded across the channel;
and so on.

[4. Image sensor]
A photodetector according to the present invention can be used as a photodetector for an imaging device. An imaging device generally includes a semiconductor circuit board and a plurality of pixels arranged in a two-dimensional regular pattern formed on the surface of the semiconductor circuit board. Each pixel includes a photodetector for detecting light, a source follower circuit for extracting output from the photodetector, and a reset circuit for resetting the source follower circuit. The structures of the source follower circuit and the reset circuit are not particularly limited, and known circuits can be used.

FIG. 3 shows a schematic cross-sectional view of one pixel of an imaging device equipped with a photodetector according to the present invention. In FIG. 3, a pixel 10 comprises a photodetector 20 according to the invention and a metal-oxide-semiconductor field effect transistor (MOSFET) 40 that serves to amplify current or voltage. The photodetector 20 comprises a channel 22 and a light absorption layer 24 formed on the surface of the channel 22 . As noted above, channel 22 is composed of graphene and light absorbing layer 24 includes graphene nanostructures.

A source electrode 26 is connected to one end of the channel 22 and a drain electrode 28 is connected to the other end. Further, a gate electrode 30 is arranged below the channel 22 . That is, in FIG. 3, the photodetector 20 consists of a field effect transistor. Furthermore, the source electrode 44 of MOSFET 40 is connected to the drain electrode 28 of photodetector 20 via line 32 .

[5. action]
In a photodetector in which a graphene nanostructure exhibiting broadband and high sensitivity is formed on a graphene channel, when the graphene nanostructure is irradiated with light, carriers are excited, and one of the excited carriers (for example, electrons) move to the graphene. Therefore, the presence or absence of light and the amount of light can be determined by detecting the amount of carriers that have moved to the graphene channel (that is, the amount of change in current).
However, when the planar shape of the channel is a linear shape, when the graphene nanostructure is irradiated with light, the light in the ultraviolet to visible region, which is a region of high photoresponse sensitivity, excites carriers, causing current flow. Although it is detected as a change amount, it is emitted from the channel as phonons in light with a wavelength longer than near-infrared, which is a region of low photoresponse sensitivity, and hardly contributes to the current change.

On the other hand, if the planar shape of the graphene channel is moderately curved, phonons generated in the graphene nanostructure can be efficiently read out as current. This is considered to be due to the following reasons.
That is, since graphene nanostructures have a flat shape, they are easily deposited on graphene, and energy is easily transferred to graphene. Therefore, the graphene nanostructure exhibits not only a photon response but also a phonon response when irradiated with light. It is thought that phonons are conducted from the graphene nanostructure to the graphene channel surface, and the conduction on the graphene surface becomes dominant.

At this time, by making the planar shape of the graphene channel appropriately curved or crooked (for example, corrugated), phonons are less likely to escape when moving through the channel, and are easier to detect as current. In addition, by curving or bending the planar shape of the channel, the light receiving area can be increased without phonons escaping, and the resistance value can be adjusted.

Furthermore, in order to maximize the use of phonon energy, it is effective to eliminate the influence of the substrate on the graphene. Specifically, the structure of the photodetector is a field effect transistor structure, and the effect of the substrate can be eliminated by applying a gate voltage to the channel.
Graphene is a zero-bandgap semiconductor in its ideal state, but electrostatic interactions with substrates and graphene nanostructures can change the electron density of states and change the bandgap. The phonon sensitivity is improved by adjusting the electron state change by applying an external gate voltage to bring the bandgap closer to the ideal state.

(Example 1, Comparative Example 1)
[1. Production of image sensor]
An imaging device was fabricated in which a graphene photodetector (graphene PD) composed of graphene and graphene nanostructures was formed on an NMOS semiconductor structure. Nanographene modified with phenylenediamine groups was used as the graphene nanostructure. The channel shape is
(a) Corrugated shape (width 8 μm, length 90 μm, area 657 μm ² ) (Example 1), or
(b) Linear type (width 10 μm, length 100 μm, area 1000 μm ² ) (Comparative Example 1)
and

[2. Test method]
[2.1. Observation with an optical microscope]
The obtained imaging device was observed with an optical microscope.
[2.2. Amount of change in current value]
While applying a voltage of 0.5 V between the source electrode and the drain electrode of the graphene PD, the graphene channel was irradiated with light having a wavelength of 406 nm, 488 nm, 850 nm, or 1310 nm, and the amount of change in current value ΔI was measured.

[3. result]
[3.1. Observation with an optical microscope]
FIG. 4A shows an SEM image of the pixel portion of the imaging element obtained in Example 1. FIG. FIG. 4B shows an enlarged SEM image of a photodetector provided in the imaging device of FIG. 4A. FIG. 5A shows an optical microscope image of a photodetector having a straight graphene channel (Comparative Example 1). FIG. 5B shows an optical microscope image of the photodetector (Example 1) having corrugated graphene channels. From FIGS. 4 and 5, it can be seen that the graphene PD having the desired channel shape was obtained.

[3.2. Amount of change in current value]
FIG. 6 shows the relationship between the wavelength of light and the amount of change in current value ΔI when a photodetector having a linear (Comparative Example 1) or corrugated (Example 1) graphene channel is irradiated with light. 6 that ΔI of Example 1 is larger than ΔI of Comparative Example 1. FIG. This is probably because the planar shape of the graphene channel is wavy, making it difficult for phonons to escape.
In addition, in FIG. 6, ΔI at the wavelength of 850 nm is extremely low. Although the details of the reason are unknown, it is considered that the electronic state of the graphene nanostructure and the electronic state of graphene when combined with graphene have an effect.

Although the embodiments of the present invention have been described in detail above, the present invention is by no means limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present invention.

INDUSTRIAL APPLICABILITY The photodetector according to the present invention can be used for sensing for health care, such as an imaging device, product inspection for flaws, stains, and defects, human sensors, and detection of pulse and red blood cell count.

Claims (6)

A photodetector with the following configuration.
(1) the photodetector,
a channel made of graphene;
and a light absorbing layer including graphene nanostructures formed on the surface of the channel.
(2) The graphene nanostructure is
nanographene,
and an organic group bonded to the nanographene.
(3) the channel is
The path along which the carrier moves is curved or bent,
having a wavy or spiral planar shape capable of reabsorbing the phonons emitted from the emission part of the channel in the reabsorption part of the channel that emitted the phonons ;
An average distance D between the emitting portion and the reabsorbing portion is 10 μm or less. 2. The photodetector according to claim 1, wherein the planar shape of the channel is corrugated. 3. The photodetector according to claim 1, wherein the average distance D between the emitting part and the reabsorbing part is 10 nm or more and 10 [mu]m or less. 4. A photodetector according to any one of claims 1 to 3, wherein said organic group comprises a [pi]-conjugated nitrogen functional group. The organic group is
a π-conjugated functional group bonded to the nanographene via a pyrazine skeleton;
5. The photodetector according to any one of claims 1 to 4, comprising one or more Br groups or CN groups introduced into the π-conjugated functional group. 6. A photodetector according to any one of claims 1 to 5, comprising a photodiode or a field effect transistor.

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