Optical sensor and method for manufacturing same

An optical sensor includes a graphene layer, a first electrode and a second electrode that are connected to the graphene layer, and an enhancement layer. The enhancement layer is disposed below the graphene layer to enhance the intensity of an optical electric field by surface plasmon resonance. The first electrode and the second electrode are arranged parallel to a first direction. The intensity of the optical electric field enhanced by the enhancement layer is greater on a first electrode side than on a second electrode side with respect to a centerline in the first direction of the graphene layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority to Japanese Patent Application No. 2020-184853, filed on Nov. 5, 2020, the entire contents of which are incorporated herein by reference.

FIELD

The disclosures herein relate to an optical sensor and a method for manufacturing the same.

BACKGROUND

Layered crystals such as graphite have a structure in which atomic layers, serving as structural units, are regularly stacked in a direction normal to the atomic layers. Such graphite exhibits high light absorption, high electrical conductivity, and high flexibility, and thus, the application of graphite to transparent conductive films and flexible devices has been studied. Recently, it has been found that a single atomic layer (graphene), obtained by exfoliation from a bulk graphite crystal, exhibits novel properties, such as high carrier mobility, that are different from those of the bulk crystal. As a result, the development of devices using graphene has been promoted. In particular, by utilizing graphene's high carrier mobility and ability to absorb various wavelengths of light from visible to infrared (2.3% of light per monolayer), high-speed, high-sensitivity infrared sensors that operate at room temperature have been developed.

Although it is preferable to use monolayer graphene in order to utilize its excellent electrical properties (high carrier mobility) when manufacturing a graphene-based optical sensor, it may be difficult to obtain sufficient sensitivity because the amount of light that can be absorbed by the monolayer graphene is small. For this reason, a technique that increases the amount of light absorption of a sensor by subjecting graphene, serving as a light receiver, to antidot patterning (forming a large number of minute holes in graphene) has been suggested.

However, such antidot patterning of graphene decreases carrier mobility in proportion to the amount of the holes formed.

Patent Documents

SUMMARY

According to an embodiment of the present disclosure, an optical sensor includes a graphene layer, a first electrode and a second electrode that are connected to the graphene layer, and an enhancement layer. The enhancement layer is disposed below the graphene layer to enhance the intensity of an optical electric field by surface plasmon resonance. The first electrode and the second electrode are arranged parallel to a first direction. The intensity of the optical electric field enhanced by the enhancement layer is greater on a first electrode side than on a second electrode side with respect to a centerline in the first direction of the graphene layer.

DESCRIPTION OF EMBODIMENTS

According to at least one embodiment, sensitivity can be improved while suppressing a decrease in carrier mobility.

In the following, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the specification and drawings, elements having substantially the same functions or configurations are denoted by the same reference numerals, and a duplicate description thereof may be omitted.

First Embodiment

First, a first embodiment will be described. The first embodiment relates to an optical sensor.FIG.1is a plan view of an optical sensor according to the first embodiment.FIG.2is a cross-sectional view of the optical sensor according to the first embodiment.FIG.2corresponds to a cross-sectional view taken through II-II ofFIG.1.

As illustrated inFIG.1andFIG.2, an optical sensor100according to the first embodiment includes a base103. The base103includes a silicon (Si) substrate101and a silicon oxide (SiO2) film102that is formed on the surface of the substrate101. The base103may be, for example, a Si substrate with a thermal oxide film. A first electrode131and a second electrode132are provided on the silicon oxide film102. The first electrode131and the second electrode132are arranged parallel to a first direction. The material of each of the first electrode131and the second electrode132is not particularly limited, and may be Au, Pd, Ni, Cr, or Ti. Each of the first electrode131and the second electrode132may include a stack of these metals. For example, each of the first electrode131and the second electrode132may include a stack of a Ti film and an Au film that is formed on the Ti film, or may include a stack of a Cr film and an Au film that is formed on the Cr film. The material of the first electrode131and the material of the second electrode132may be the same or may be different.

An enhancement layer120that enhances the intensity of an optical electric field by surface plasmon resonance is provided between the first electrode131and the second electrode132. The enhancement layer120will be described later.

A graphene layer111is provided above the enhancement layer120, and is connected to the first electrode131and to the second electrode132. The graphene layer111includes one or more stacked graphene layers. If the graphene layer111includes a plurality of graphene layers, the plurality of graphene layers are preferably twisted (randomly stacked). By twisting (randomly stacking) the graphene layers, relatively high carrier mobility can be obtained. Further, an insulating layer112is provided between the graphene layer111and the enhancement layer120. The insulating layer112may be made of hexagonal boron nitride (hBN) that is a two-dimensional material. The insulating layer112includes one or more stacked hBN layers. The length (dimension in the first direction) and the width (dimension in a second direction perpendicular to the first direction) of each of the graphene layer111and the insulating layer112are not particularly limited, and may be approximately 1 μm to 1000 μm.

Next, the enhancement layer120will be described. In the present embodiment, the enhancement layer120includes a plurality of graphene dot portions121. The graphene dot portions121are provided on the first electrode131side with respect to a centerline C in the first direction of the graphene layer111, and the graphene dot portions121are not provided on the second electrode132side with respect to the centerline C of the graphene layer111. That is, the density of the graphene dot portions121on the first electrode131side is higher than that on the second electrode132side with respect to the centerline C. Accordingly, the intensity of the optical electric field enhanced by the enhancement layer120is greater on the first electrode131side than on the second electrode132side with respect to the centerline C.

Each of the graphene dot portions121includes one or more stacked graphene layers. For example, the graphene dot portions121are arranged in an equilateral triangular lattice in plan view. The graphene dot portions121may be arranged in a square lattice in plan view. The shape of each of the graphene dot portions121in plan view is not particularly limited. Each of the graphene dot portions121may have a substantially circular shape or a substantially rectangular shape in plan view. In addition, the size of each of the graphene dot portions121in plan view is not particularly limited, and may be appropriately selected in accordance with the wavelength of an optical electric field to be enhanced. For example, in a case where an optical electric field in the mid-infrared region is to be enhanced, each of the graphene dot portions121has a substantially circular shape in plan view with a diameter of 100 nm to 200 nm, and the graphene dot portions121are regularly arranged with a period of 200 nm to 400 nm. As used herein, the period of the graphene dot portions121refers to the length of a line connecting the centers of the two closest graphene dot portions121.

In the optical sensor100having the above-described configuration according to the first embodiment, when light is incident on the graphene layer111, part of the light passes through the graphene layer111and the insulating layer112, and reaches the enhancement layer120. As described above, the graphene dot portions121are provided on the first electrode131side with respect to the centerline C, and are not provided on the second electrode132side with respect to the centerline C. Accordingly, the intensity of the optical electric field is enhanced near the graphene dot portions121on the first electrode131side, and is not enhanced on the second electrode132side with respect to the centerline C. That is, the intensity of the optical electric field enhanced by the enhancement layer120is greater on the first electrode131side than on the second electrode132with respect to the centerline C. Therefore, the amount of light absorbed by the graphene layer111is greater on the first electrode131side than on the second electrode132with respect to the centerline C. As a result, the density of optically excited carriers becomes different between the first electrode131side and the second electrode132side with respect to the centerline C of the graphene layer111, and thus, a potential difference (voltage) is generated between the first electrode131and the second electrode132. By measuring the potential difference (voltage), incident light can be detected.

Further, the graphene layer111is not necessarily subjected to any treatment that may decrease carrier mobility, such as antidot patterning. Accordingly, the sensitivity can be improved while suppressing a decrease in carrier mobility.

The amount of enhancement of the optical electric field by graphene dots is greater than that of antidot-patterned graphene. For example, while the amount of light absorbed by graphene without antidot patterning is approximately 2.3%, the amount of light absorbed by antidot-patterned graphene is approximately 60%. In contrast, the amount of light absorbed by graphene dots is approximately 90%. As will be described later, antidot-patterned graphene may be used for the enhancement layer120.

Further, because the insulating layer112is provided between the graphene dot portions121and the graphene layer111, the influence of carrier scattering on the graphene layer111by the graphene dot portions121can be suppressed. The intensity of the optical electric field to be enhanced is attenuated exponentially with the distance between the graphene dot portions121and the graphene layer111. Therefore, the insulating layer112is preferably thin. For example, the insulating layer112is preferably composed of one hBN layer. The material of the insulating layer112is not limited to hBN. For example, aluminum oxide (Al2O3) or hafnium oxide (HfO2) may be used as the material of the insulating layer112.

The wavelength of the optical electric field to be enhanced depends on the size and the period of the graphene dot portions121of the enhancement layer120. Therefore, the wavelength of the optical electric field to be enhanced can be modulated by adjusting the size and the period of the graphene dot portions121. For example, the wavelength of the optical electric field to be enhanced can be modulated in a range of approximately 8 μm to 10 μm by setting the diameter to 100 nm to 200 nm and the period to 200 nm to 400 nm.

Next, a method for manufacturing the optical sensor100according to the first embodiment will be described.FIG.3throughFIG.8are cross-sectional views illustrating a method for manufacturing the optical sensor100according to the first embodiment.FIG.9is a plan view illustrating the method for manufacturing the optical sensor100according to the first embodiment.FIG.4corresponds to a cross-sectional view taken through IV-IV ofFIG.9.

As illustrated inFIG.3, a base103is prepared first. The base103is, for example, a Si substrate with a thermal oxide film. Next, a graphene layer121A is formed on a silicon oxide film102. The graphene layer121A is formed so as to cover at least a region where a plurality of graphene dot portions121are to be formed. The graphene layer121A may be synthesized directly on the silicon oxide film102. Alternatively, the graphene layer121A may be separately synthesized, and the synthesized graphene layer121A may be transferred onto the silicon oxide film102.

Subsequently, as illustrated inFIG.4andFIG.9, the graphene layer121A is processed such that the plurality of graphene dot portions121constituting an enhancement layer120are formed. For example, when the graphene layer121A is processed, lithography is used to form a mask that covers portions where the plurality of graphene dot portions121are to be formed, and portions of the graphene layer121A exposed from the mask are removed by ashing using oxygen plasma or the like.

Next, as illustrated inFIG.5, an insulating layer112A is formed on the enhancement layer120. The insulating layer112A is made of, for example, hexagonal boron nitride (hBN) that is a two-dimensional material. The insulating layer112A is formed so as to cover at least a region where an insulating layer112is to be formed. The insulating layer112A may be separately synthesized and transferred onto the enhancement layer120. Alternatively, the insulating layer112A may be directly synthesized on the insulating layer112A. As the material of the insulating layer112A, aluminum oxide or hafnium oxide may be used. If aluminum oxide or hafnium oxide is used as the material of the insulating layer112A, the insulating layer112A may be formed by atomic layer deposition (ALD), vapor deposition, or the like.

Next, as illustrated inFIG.6, a graphene layer111A is formed on the insulating layer112A. The graphene layer111A is formed so as to cover at least a region where a graphene layer111is to be formed. The graphene layer111A may be directly synthesized on the insulating layer112A. Alternatively, the graphene layer111A may be separately synthesized and transferred onto the insulating layer112A.

Subsequently, as illustrated inFIG.7, the graphene layer111A and the insulating layer112A are processed such that the graphene layer111and the insulating layer112are formed. For example, when the graphene layer111A and the insulating layer112A are processed, lithography is used to form a mask that covers portions where the graphene layer111and the insulating layer112are to be formed, and portions of the graphene layer111A and the insulating layer112A exposed from the mask are removed by ashing using oxygen plasma or the like. Instead of ashing using oxygen plasma or the like, reactive ion etching (RIE) may be performed.

Next, as illustrated inFIG.8, a first electrode131and a second electrode132are formed so as to be connected to the graphene layer111. The first electrode131and the second electrode132are arranged in parallel to the first direction.

In this manner, the optical sensor100according to the first embodiment can be manufactured.

Second Embodiment

Next, a second embodiment will be described. The second embodiment differs from the first embodiment in that a passivation layer is provided on the graphene layer111.FIG.10is a plan view of an optical sensor according to the second embodiment.FIG.11is a cross-sectional view of the optical sensor according to the second embodiment.FIG.11corresponds to a cross-sectional view taken through XI-XI ofFIG.10.

As illustrated inFIG.10andFIG.11, an optical sensor200according to the second embodiment includes a passivation layer212on the graphene layer111. The passivation layer212is made of, for example, hexagonal boron nitride (hBN) that is a two-dimensional material. The passivation layer212includes one or more stacked hBN layers.

Other configurations are the same as those of the first embodiment.

Accordingly, in the second embodiment, the sensitivity can be improved while suppressing a decrease in carrier mobility. Further, in the second embodiment, because the passivation layer212is formed, adsorption of moisture in the air by the graphene layer111can be suppressed. If the graphene layer111adsorbs moisture, the properties of the graphene layer111would change due to moisture doping. In the second embodiment, the graphene layer111is covered by the passivation layer212, the properties of the graphene layer111can be stabilized.

The material of the passivation layer212is not limited to hBN. As the material of the passivation layer212, aluminum oxide or hafnium oxide may be used, for example.

Next, a method for manufacturing the optical sensor200according to the second embodiment will be described.FIG.12throughFIG.14are cross-sectional views illustrating a method for manufacturing the optical sensor200according to the second embodiment.

First, in the same manner as in the first embodiment, the process as of the formation of the graphene layer111A is performed (seeFIG.6). Next, as illustrated inFIG.12, a passivation layer212A is formed on the graphene layer111A. The passivation layer212A is made of, for example, hexagonal boron nitride (hBN) that is a two-dimensional material. The passivation layer212A is formed so as to at least cover a region where a passivation layer212is to be formed. The passivation layer212A may be separately synthesized and transferred onto the graphene layer111A. Alternatively, the passivation layer212A may be directly synthesized on the graphene layer111A. As the material of the passivation layer212A, aluminum oxide or hafnium oxide may be used. If aluminum oxide or hafnium oxide is used as the material of the passivation layer212A, the passivation layer212A may be formed by ALD, vapor deposition, or the like.

Subsequently, as illustrated inFIG.13, the passivation layer212A, the graphene layer111A, and the insulating layer112A are processed such that the passivation layer212, the graphene layer111, and the insulating layer112are formed. For example, when the passivation layer212A, the graphene layer111A, and the insulating layer112A are processed, lithography is used to form a mask that covers portions where the passivation layer212, the graphene layer111, and the insulating layer112are to be formed, and portions of the passivation layer212A, the graphene layer111A, and the insulating layer112A exposed from the mask are removed by ashing using oxygen plasma or the like. Instead of ashing using oxygen plasma or the like, RIE may be performed.

Next, as illustrated inFIG.14, a first electrode131and a second electrode132are formed so as to be connected to the graphene layer111. The first electrode131and the second electrode132are arranged in parallel to the first direction.

In this manner, the optical sensor200according to the second embodiment can be manufactured.

Third Embodiment

Next, a third embodiment will be described. In the third embodiment, the configuration of an enhancement layer differs from that of the first embodiment.FIG.15is a plan view of an optical sensor according to the third embodiment.FIG.16is a perspective view of the optical sensor according to the third embodiment. InFIG.16, the graphene layer111and the insulating layer112are illustrated as being transparent.

As illustrated inFIG.15andFIG.16, an optical sensor300according to the third embodiment includes an enhancement layer320instead of the enhancement layer120. The enhancement layer320includes a plurality of linear graphene portions321. The linear graphene portions321are provided on the first electrode131side with respect to the centerline C of the graphene layer111, and the linear graphene portions321are not provided on the second electrode132side with respect to the centerline C of the graphene layer111. Accordingly, the intensity of an optical electric field enhanced by the enhancement layer320is greater on the first electrode131side than on the second electrode132side with respect to the centerline C.

Each of the linear graphene portions321includes one or more stacked graphene layers. The linear graphene portions321extend in the second direction perpendicular to the first direction, and are arranged parallel to the first direction. The size of each of the linear graphene portions321is not particularly limited, and can be selected in accordance with the wavelength of an optical electric field to be enhanced.

Other configurations are the same as those of the first embodiment.

Accordingly, in the third embodiment, the sensitivity can be improved while suppressing a decrease in carrier mobility. Further, in the third embodiment, a mask for forming the enhancement layer320can be easily designed and created.

Fourth Embodiment

Next, a fourth embodiment will be described. In the fourth embodiment, the configuration of an enhancement layer differs from that of the first embodiment.FIG.17is a plan view of an optical sensor according to the fourth embodiment.FIG.18is a perspective view of the optical sensor according to the fourth embodiment. InFIG.18, the graphene layer111and the insulating layer112are illustrated as being transparent.

As illustrated inFIG.17andFIG.18, an optical sensor400according to the fourth embodiment includes an enhancement layer420instead of the enhancement layer120. The enhancement layer420includes an antidot-patterned graphene layer421. The graphene layer421is provided on the first electrode131side with respect to the centerline C of the graphene layer111, and the graphene layer421is not provided on the second electrode132side with respect to the centerline C. Accordingly, the intensity of an optical electric field enhanced by the enhancement layer420is greater on the first electrode131side than on the second electrode132side of the centerline C.

The graphene layer421includes one or more stacked graphene layers. A plurality of openings422are formed in the graphene layer421by antidot patterning. The shape of each of the openings422is not particularly limited. Each of the openings422may have a substantially circular shape or a substantially rectangular shape. The size of each of the openings422is not particularly limited and may be appropriately selected in accordance with the wavelength of an optical electric field to be enhanced.

Accordingly, in the fourth embodiment, the sensitivity can be improved while suppressing a decrease in carrier mobility.

Fifth Embodiment

Next, a fifth embodiment will be described. The fifth embodiment differs from the first embodiment in that a control electrode is provided.FIG.19is a cross-sectional view of an optical sensor according to the fifth embodiment.

As illustrated inFIG.19, an optical sensor500according to the fifth embodiment includes a gate electrode531that modulates the carrier concentration of the graphene layer111. The gate electrode531is an example of the control electrode. For example, the gate electrode531is provided below the silicon substrate101. That is, the optical sensor500has a back-gate structure. The gate electrode531can modulate not only the carrier concentration of the graphene layer111, but also the carrier concentration of the graphene dot portions121.

Other configurations are the same as those of the first embodiment.

Accordingly, in the fifth embodiment, the sensitivity can be improved while suppressing a decrease in carrier mobility. Further, in the fifth embodiment, the gate electrode531can modulate the wavelength of an optical electric field to be enhanced by modulating the carrier concentration of the graphene layer111and the carrier concentration of the graphene dot portions121.

The gate electrode531may be provided in the optical sensors according to the second embodiment, the third embodiment and the fourth embodiment.

The configurations of the enhancement layer are not limited to those of the above-described embodiments. For example, the boundary between a region where graphene portions constituting the enhancement layer are provided and a region where graphene portions are not provided need not coincide with the centerline C in plan view. Further, the number of graphene portions provided on the second electrode132side is not necessarily zero. For example, if the intensity of an optical electric field to be enhanced is greater on the first electrode131side than on the second electrode132side, graphene portions may also be provided on the second electrode132side with respect to the centerline C. That is, the boundary between a region where graphene portions constituting the enhancement layer are provided and a region where graphene portions are not provided may be located on the second electrode132side with respect to the centerline C. Conversely, the boundary between a region where graphene portions are provided and a region where graphene portions are not provided may be located on the first electrode131side with respect to the centerline C. Further, a plurality of graphene dot portions may be provided on the second electrode132side at a density lower than that on the first electrode131side.

Further, hBN may be provided between the graphene constituting enhancement layer120and the silicon oxide film102. By providing hBN, doping from the silicon oxide film to the graphene constituting the enhancement layer can be suppressed.

Further, the base103is not limited to a stack of the silicon substrate101and the silicon oxide film102.

Although the embodiments of the present invention have been described in detail, the present invention is not limited to the above-described embodiments. Variations and modifications may be made without departing from the scope of the present invention described in the claims.