JP7477766B2 - Photodetector - Google Patents

Description

The present invention relates to a light detection device.

Photodetectors that detect voltages generated in graphene when exposed to light (hereafter referred to as graphene photodetectors) have attracted attention. Graphene is a material in which carbon atoms are arranged in a two-dimensional honeycomb pattern, and has unique physical properties.

For example, graphene has a small heat capacity and a large thermoelectric effect. Therefore, when infrared or terahertz light is irradiated onto graphene, the temperature of the graphene rises significantly, resulting in the generation of a large thermoelectric power. A graphene photodetector is a photodetector that takes advantage of these characteristics (see, for example, Patent Document 1 and Non-Patent Document 1).

Light detection using graphene has been demonstrated by a graphene photodetector (hereafter referred to as a two-backgate photodetector) that has graphene and two backgates facing the graphene (see, for example, Non-Patent Document 1). When different voltages are applied to the two backgates, an asymmetric carrier distribution is formed in the graphene. Since the Seebeck coefficient depends on the carrier density, when graphene with an asymmetric carrier distribution is irradiated with infrared light or the like and the temperature rises, a thermoelectromotive force is generated. The two-backgate photodetector detects this thermoelectromotive force. Note that an asymmetric carrier distribution means that the type and/or density of the majority carriers are not uniform.

Asymmetric carrier distribution can also be achieved by attaching different substances to adjacent regions of graphene (see, for example, Patent Document 1). However, it is not easy to manufacture a graphene photodetector in which different substances are attached to graphene. The two-backgate photodetector does not have this problem.

Regarding the physical properties of graphene, a technology has been reported in which holes are provided in graphene to provide a band gap in graphene, which is a zero-gap semiconductor (see, for example, Patent Document 1). Regarding detectors that detect terahertz light, devices have been reported that use three-dimensional porous graphene as a light-receiving layer to improve sensitivity (see, for example, Patent Document 2).

Republished Publication No. 2013/018153 JP 2018-37617 A

Kei Kinoshita et al., Applied Physics Letters 113, 103102 (2018)

The two-back-gate photodetector is a device that can be manufactured without going through the complicated process of attaching different materials to graphene. However, the two-back-gate photodetector has the problem of being complex in structure.

Two-back-gate photodetectors also have the problem that the peripheral circuitry is complicated because different voltages are applied to the two back gates. These problems are related to the basic operation of the two-back-gate photodetector, which is to apply separate voltages to the two back gates.

Therefore, the objective of this invention is to solve these problems.

In one embodiment, the photodetector includes graphene including a light receiving region in which a first region having a hole is arranged at one end and a second region different from the first region is arranged at the other end, a gate electrode facing the light receiving region, an optical sensor that outputs an electrical signal in response to light irradiated to the light receiving region, and a gate voltage application circuit that applies a gate voltage between the light receiving region and the gate electrode, the ratio of the area of the first region other than the holes to the area of the first region being smaller than the ratio of the area of the second region other than the holes to the area of the second region, and the gate voltage is set so that the ratio of the intensity of the electrical signal to the intensity of the light is a specific value.

In one aspect, the present invention provides a graphene photodetector (i.e., a photodetector that converts light into electricity using graphene) that can detect light simply by applying one voltage to one backgate.

FIG. 1 is a diagram showing an example of a photodetector 2 according to the first embodiment. FIG. 2 is a perspective view showing an example of the optical sensor 4. As shown in FIG. FIG. 3 is a plan view showing an example of the light receiving region 10. As shown in FIG. FIG. 4 is a diagram showing the energy band structure of graphene that is not doped with impurities or has a foreign substance attached thereto. FIG. 5 is a diagram illustrating the conductivity type of the light receiving region 10 in the first embodiment. FIG. 6 is a diagram showing an example of the relationship between the sensitivity R1 and the gate voltage Vg of the photodetector 2 when light of a specific wavelength (for example, 8 μm) is irradiated onto the light-receiving region 10. In FIG. FIG. 7 is a diagram illustrating the density of electrons 34 induced in the first region 16a and the second region 16b. FIG. 8 is a diagram showing an example of a method for manufacturing the optical sensor 4 according to the first embodiment. FIG. 9 is a diagram showing an example of a method for manufacturing the optical sensor 4 according to the first embodiment. FIG. 10 is a diagram showing an example of the structure of a photodetection device 102 having a photosensor 104 with two gate electrodes. FIG. 11 is a diagram illustrating a modified example 216a of the first region 16a. FIG. 12 is a diagram illustrating an example of a photodetector 202 according to the second embodiment. FIG. 13 is a diagram illustrating the conductivity type of the light receiving region 10 in the second embodiment. FIG. 14 is a diagram showing an example of the relationship between the sensitivity R2 and the gate voltage Vg of the photodetector 202 when light of a specific wavelength is irradiated onto the light-receiving region 10. In FIG.

Below, embodiments of the present invention will be described with reference to the drawings. However, the technical scope of the present invention is not limited to these embodiments, but extends to the matters described in the claims and their equivalents. Parts having the same structure in different drawings will be given the same reference numerals, and their description will be omitted.

(Embodiment 1)
(1) Structure Fig. 1 is a diagram showing an example of a photodetector 2 according to embodiment 1. As shown in Fig. 1, the photodetector 2 according to embodiment 1 has a photosensor 4, a gate voltage application circuit 6, and an electric signal detection circuit 8.

-Optical sensor 4-
Fig. 2 is a perspective view showing an example of the optical sensor 4. Fig. 2 shows the vicinity of the graphene 12 (see Fig. 1) of the optical sensor 4.

The optical sensor 4 has a graphene 12 including an area 10 where light is irradiated (hereinafter referred to as a light receiving area), a first connection area 15a that contacts one end E1 of the light receiving area 10 (see FIG. 1), and a second connection area 15b that contacts the other end E2 of the light receiving area 10. The optical sensor 4 is a device that outputs an electrical signal in response to light irradiated to the light receiving area 10.

The optical sensor 4 further includes a first electrode 11a connected to the first connection region 15a and a second electrode 11b connected to the second connection region 15b. The optical sensor 4 further includes a gate electrode 5 facing the light receiving region 10. The gate electrode 5 is, for example, a conductive silicon substrate 7 (i.e., an n ⁺ silicon substrate or a p ⁺ silicon substrate).

The optical sensor 4 further comprises a _SiO2 film 9 disposed on the silicon substrate 7 and supporting the graphene 12. The optical sensor 4 further comprises a substrate electrode 13 (see FIG. 1) connected to the silicon substrate 7.

Figure 3 is a plan view showing an example of the light receiving region 10. A first region 16a having a hole 14 is arranged on one end E1 side of the light receiving region 10. A second region 16b different from the first region 16a is arranged on the other end E2 side of the light receiving region 10. The fill factor of the first region 16a is smaller than the fill factor of the second region 16b.

The fill factor of a certain region A (for example, the first region 16a) is the ratio (=S1/S2) of the area S1 of the region A other than the holes to the area S2 of the region A. Therefore, the fill factor of the first region 16a is the ratio (=S1/S2) of the area S1 of the region 3 filled with carbon atoms (hereinafter referred to as the filled region) of the first region 16a to the area S2 of the first region 16a. The area S2 of the first region 16a is the entire area inside the periphery of the first region 16a. In the example shown in FIG. 3, the fill factor of the first region 16a is approximately 0.5.

The same applies to the fill factor of the second region 16b. In the example shown in FIG. 3, the second region 16b has no holes, so the fill factor of the second region 16b is 1.

-Gate voltage application circuit 6-
The gate voltage application circuit 6 (see FIG. 1) is a circuit that applies a gate voltage Vg between the light receiving region 10 and the gate electrode 5. The gate voltage Vg of the photodetector 2 is a constant voltage V1 (e.g., 3 V). Details of the voltage V1 will be described in "(2) Operation".

In the example shown in FIG. 1, the gate voltage application circuit 6 has a constant voltage source 17 that is connected to the substrate electrode 13 and the second electrode 11b and outputs a voltage V1. The gate voltage Vg (i.e., voltage V1) is applied between the light receiving region 10 and the gate electrode 5 via the substrate electrode 13 and the second electrode 11b.

The conductivity type of the light-receiving region 10 is p-type (or n-type) when no voltage is applied between the light-receiving region 10 and the gate electrode 5. Application of voltage V1 between the light-receiving region 10 and the gate electrode 5 inverts the conductivity type of the first region 16a and maintains the conductivity type of the second region 16b.

- Electrical signal detection circuit 8 -
The electrical signal detection circuit 8 (see FIG. 1) is a circuit that detects an electrical signal output by the optical sensor 4 in response to light (for example, visible light, infrared light, or terahertz light) irradiated onto the light receiving region 10 .

The electrical signal detection circuit 8 in the first embodiment is a current detection circuit 19 that detects a current I flowing through the light receiving region 10 while applying a voltage Vbias to both ends E1, E2 of the light receiving region 10. The current detection circuit 19 has, for example, a constant voltage source 18 that applies a voltage Vbias to the light receiving region 10, and an ammeter 20 connected to the constant voltage source 18. One end of the current detection circuit 19 is connected to the first electrode 11a, and the other end is connected to the second electrode 11b.

The voltage Vbias is a voltage that makes the potential of one of the first and second regions 16a and 16b, which has n-type conductivity while voltage V1 is applied between the light-receiving region 10 and the gate electrode 5, higher than the potential of the other of the first and second regions 16a and 16b. The conductivity types of the first and second regions 16a and 16b are explained in "(2) Operation." The voltage Vbias is a reverse bias voltage that is applied to the pn junction (see "(2) Operation") formed in the light-receiving region 10.

The electrical signal output by the optical sensor 4 is the current I detected by the current detection circuit 19 while the voltage V1 is applied between the light receiving region 10 and the gate electrode 5 (i.e., the current flowing through the light receiving region 10 due to the irradiation of the reverse-biased light receiving region 10 with light).

(2) Operation Figure 4 shows the energy band structure of graphene that is not doped with impurities or has foreign substances attached. The _{k x-} axis 22 shows the wave number k _x in the x-axis direction. The _{k y-} axis 24 shows the wave number k _y in the y-axis direction. The energy axis 26 shows the energy E of the electron.

As shown in FIG. 4, the valence band 28 and conduction band 30 of graphene meet at the K point 32. For this reason, graphene is sometimes called a zero-gap semiconductor. The K point 32 in graphene is the Dirac point (i.e., the point where the bands are degenerate).

Figure 5 is a diagram explaining the conductivity type of the light-receiving region 10 in embodiment 1. Figure 5 shows an example of the energy band of the light-receiving region 10. In the example shown in Figure 5, the conductivity type of the light-receiving region 10 is p-type when no voltage is applied between the light-receiving region 10 and the gate electrode 5. To avoid complicating the explanation, the thermal energy of electrons is ignored in the explanation of Figure 5 (the same applies to Figure 13 described later).

In the following explanation, "between the light-receiving region 10 and the gate electrode 5" is abbreviated to "to the gate electrode 5." Therefore, for example, "when no voltage is applied between the light-receiving region 10 and the gate electrode 5" is abbreviated to "when no voltage is applied to the gate electrode 5." Similarly, "between the light-receiving region 10 and the gate electrode 5" is abbreviated to "to the gate electrode 5."

5A shows an example of the energy band of the light-receiving region 10 when no voltage is applied to the gate electrode 5. When no voltage is applied to the gate electrode 5, the valence band 28 (see FIG. 5A) of the light-receiving region 10 is filled with electrons up to energy _E0 , and is filled with holes from energy _E0 to the top of the valence band (i.e., point K). That is, when no voltage is applied to the gate electrode 5, the conductivity type of the light-receiving region 10 is p-type.

Figure 5(b) shows the energy band of the first region 16a when a voltage V1 (i.e., the gate voltage Vg in embodiment 1) is applied to the gate electrode 5. Figure 5(c) shows the energy band of the second region 16b when a voltage V1 is applied to the gate electrode 5.

In the example shown in Figures 5(b) to (c), voltage V1 is a voltage that makes the potential of the light-receiving region 10 lower than the potential of the gate electrode 5. Therefore, when voltage V1 is applied to the gate electrode 5, electrons are induced in the first region 16a and the second region 16b.

At this time, electrons with a higher density than that of the second region 16b are induced in the filling region 3 (see FIG. 3) of the first region 16a, which has a low fill factor (see "Asymmetric carrier induction" below). As a result, the valence band 28 of the first region 16a is filled with the induced electrons, and the electrons that do not enter the valence band 28 fill the bottom of the conduction band 30 of the first region 16a (see FIG. 5(b)). Therefore, the conductivity type of the first region 16a is inverted to n-type by application of voltage V1 to the gate electrode 5.

On the other hand, even if a voltage V1 is applied to the gate electrode 5, sufficient electrons are not induced in the second region 16b to fill the conduction band 30, and the conductivity type of the second region 16b remains p-type (see FIG. 5(c)). Therefore, a pn junction is generated in the light-receiving region 10 by applying a voltage V1 to the gate electrode 5. The pn junction generated in the light-receiving region 10 can be detected, for example, by a scanning probe microscope.

In addition, in the second region 16b, the maximum value of the energy of electrons in the valence band 28 increases from _E0 to _E2 (see FIG. 5C) due to application of the voltage V1 to the gate electrode 5. Therefore, the hole density in the second region 16b decreases due to application of the voltage V1 to the gate electrode 5.

--Generation of photocurrent--
When a voltage Vbias (i.e., a reverse bias voltage) is applied to the light-receiving region 10, the depletion layer of the pn junction generated in the light-receiving region 10 expands. When the expanded depletion layer is irradiated with light, electron-hole pairs are generated. The generated electron-hole pairs are separated into electrons and holes by the electric field in the depletion layer. The separated electrons and holes become a flow of electrons and a flow of holes due to the electric field in the depletion layer. This flux of electron flow and hole flow is the current (hereinafter referred to as photocurrent) that flows in the light-receiving region 10 due to light irradiation.

The photocurrent flowing through the light-receiving region 10 is output from the light-receiving region 10 via the first electrode 11a and the second electrode 11b, and is detected by the ammeter 20 (see FIG. 1) of the current detection circuit 19. The electrical signal output by the optical sensor 4 is the photocurrent flowing through the light-receiving region 10 due to light irradiation while the voltage V1 is applied between the light-receiving region 10 and the gate electrode 5 and the voltage Vbias is applied to the light-receiving region 10.

-sensitivity-
6 is a diagram showing an example of the relationship between the sensitivity R1 and the gate voltage Vg of the photodetector 2 when light of a specific wavelength (e.g., 8 μm) is irradiated to the light receiving region 10. The horizontal axis is the gate voltage Vg. The vertical axis is the sensitivity R1 of the photodetector 2. The sensitivity of the photodetector 2 is the ratio (=A/B) of the intensity A of the electrical signal (photocurrent in the first embodiment) output by the optical sensor 4 in response to the light irradiated to the light receiving region 10 to the intensity B of the light irradiated to the light receiving region 10 (so-called optical power). As shown in FIG. 6, the sensitivity R1 can be either a positive or negative value.

The positive or negative sensitivity R1 of the photodetector 2 is determined by the positive or negative current flowing through the electrical signal detection circuit 8. For example, if the current is negative, the sensitivity is also negative. Since the positive or negative current is determined by the definition of the positive direction (of the current), the positive or negative sensitivity of the photodetector 2 is a matter of convenience. The same is true for the sensitivity of the photodetector 202 of embodiment 2.

In region I, where the gate voltage Vg (here, the potential of the gate electrode 5 relative to the light-receiving region 10) is small, the conductivity type of the first region 16a and the second region 16b of the light-receiving region 10 is not inverted, so no pn junction is formed in the light-receiving region 10. Therefore, the sensitivity of the photodetector 2 is approximately zero. On the other hand, in region III, where the gate voltage Vg is too large, the conductivity type of both the first region 16a and the second region 16b is inverted, so no pn junction is formed in the light-receiving region 10. Therefore, even in region III, the sensitivity of the photodetector 2 is approximately zero.

In region II, where the gate voltage Vg is neither too small nor too large, the conductivity type of only the first region 16a is inverted, forming a pn junction in the light receiving region 10. This increases the sensitivity of the photodetector 2, and a large peak 36 appears on the curve 35 showing the relationship between the gate voltage Vg and the sensitivity R1.

The peak value P1 of the sensitivity R1 (i.e., the sensitivity R1 with the maximum absolute value) depends not only on the gate voltage Vg but also on the voltage Vbias applied to the light receiving region 10. When the voltage Vbias is 1 V, the peak value P1 of the sensitivity R1 is, for example, 1 mA/W.

The gate voltage Vg in the first embodiment is a voltage V1 that is set so that the sensitivity R1 (see FIG. 6) of the photodetector 2 becomes a specific value (hereinafter referred to as the target sensitivity). The voltage V1 may be a changeable voltage. In other words, the constant voltage source 17 of the gate voltage application circuit 6 may be a variable voltage source.

When the peak value P1 of sensitivity R1 is a positive value, the target sensitivity is preferably 0.5 times or more the peak value P1 of sensitivity R1. More preferably, the target sensitivity is 0.7 times or more the peak value P1 of sensitivity R1. Most preferably, the target sensitivity is 0.9 times or more the peak value P1 of sensitivity R1 (e.g., peak value P1). The upper limit of the target sensitivity is the peak value P1 of sensitivity R1.

On the other hand, when the peak value P1 of sensitivity R1 is a negative value, the target sensitivity is preferably 0.5 times or less the peak value P1 of sensitivity R1. More preferably, the target sensitivity is 0.7 times or less the peak value P1 of sensitivity R1. Most preferably, the target sensitivity is 0.9 times or less the peak value P1 of sensitivity R1 (e.g., peak value P1). The lower limit of the target sensitivity is the peak value P1 of sensitivity R1.

Specifically, the absolute value of the target sensitivity is preferably 0.5 mA/W or more. More preferably, the absolute value of the target sensitivity is 0.7 mA/W or more. Most preferably, the absolute value of the target sensitivity is 0.9 mA/W or more.

As described in the second embodiment, when a gate voltage Vg is applied to the gate electrode 5, the Seebeck coefficient of the first region 16a of the light receiving region 10 and the Seebeck coefficient of the second region 16b diverge from each other. Therefore, the electrical signal output from the optical sensor 4 contains a current generated by the photothermoelectric effect (i.e., the thermoelectric effect caused by heat generated by light absorption).

However, in the photodetector 2 of embodiment 1, a reverse bias voltage Vbias is applied to the light receiving region 10, causing the depletion layer to expand, so that the photocurrent due to the photovoltaic effect (i.e., the photocurrent generated by the internal electric field of the pn junction) is often dominant over the current due to the photothermoelectric effect.

The photothermoelectric effect is explained in embodiment 2.

- Asymmetric carrier induction -
7 is a diagram for explaining the density of electrons 34 induced in the first region 16 a and the second region 16 b. Here, too, the conductivity type of the light-receiving region 10 is assumed to be p-type when no voltage is applied between the light-receiving region 10 and the gate electrode 5.

As is clear from Gauss's law regarding electrostatic fields, the number _N1 of electrons induced in the first region 16a by a certain gate voltage Vg is expressed by formula (1).

Here, ε is the dielectric constant of the SiO ₂ film 9, e is the elementary charge, and _{E n} is the component perpendicular to the first region 16 a among the components of the electric field vector from the first region 16 a toward the gate electrode 5. When the width of the hole 14 is sufficiently small compared with the distance d between the first region 16 a and the gate electrode 5 (i.e., the thickness of the SiO ₂ film 9), E _n is approximately Vg/d immediately below both the filling region 3 (i.e., the region filled with carbon atoms: see FIG. 2 ) and the hole 14.

Therefore, the macroscopic electron density _EDmac induced in the first region 16a by application of the gate voltage Vg is expressed by equation (2).

The macroscopic electron density is the total number of electrons induced in a macroscopic region divided by the area of the macroscopic region (i.e., the entire area inside the periphery of a region). Therefore, the macroscopic electron density ED _mac induced in the first region 16a by application of the gate voltage Vg is the total number N ₁ of electrons induced in the first region 16a by application of the gate voltage Vg divided by the area S _mac of the entire area inside the periphery of the first region 16a (= N ₁ /S _mac ).

On the other hand, the density of electrons ED _mic (hereinafter referred to as microscopic electron density) induced in the filled region 3 (see FIG. 2) filled with carbon atoms is N ₁ /S _mic , where S _mic is the area of the filled region and S _mic is the area of the portion of a certain region (here, first region 16a) other than holes 14.

Therefore, the microscopic electron density ED _mic in the first region 16a is expressed by equation (3).

Here, F is the fill factor (=S _mic /S _mac ) of the macroscopic region (here, the first region 16 a). The density of electrons induced in the graphene 12 in the first region 16 a is the microscopic electron density ED _mic rather than the macroscopic electron density ED _mac .

Similarly, the density of electrons induced in the graphene 12 in the second region 16b is not a macroscopic electron density but a microscopic electron density. Equations (1) to (3) also hold true for the electron density in the second region 16b.

Since the same gate voltage Vg is applied to the first region 16a and the second region 16b, as is clear from equation (2), the macroscopic electron density _EDmac of the first region 16a is equal to the macroscopic electron density _EDmac of the second region 16b. On the other hand, as shown in FIG. 3, the fill factor F of the first region 16a is smaller than the fill factor F of the second region 16b.

Therefore, the microscopic electron density ED _mic in the first region 16 a is higher than the microscopic electron density ED _mic in the second region 16 b. Therefore, the density of electrons induced in the graphene 12 in the first region 16 a is higher than the density of electrons induced in the graphene 12 in the second region 16 b. That is, application of one gate voltage Vg to one gate electrode 5 induces an asymmetric carrier distribution in the light-receiving region 10 (see FIG. 5 ).

In the above description, the width of the hole 14 is sufficiently smaller than the thickness of the _SiO2 film 9. However, even if such an ideal condition is not satisfied, the density of electrons (or holes) induced in the first region 16a will be higher than the density of electrons (or holes) induced in the second region 16b.

(3) Manufacturing Method FIGS. 8 and 9 are diagrams showing an example of a manufacturing method for the optical sensor 4 according to the first embodiment.

First, a SiO ₂ film 9 is formed on a conductive silicon substrate 7 (see FIG. 8A). The thickness of the SiO ₂ film 9 is, for example, 50 nm to 200 nm (preferably, 90 nm).

A strip-shaped graphene 12 in which some of the carbon atoms are replaced with heteroatoms (e.g., nitrogen or boron) is placed on the _SiO2 film 9 (see FIG. 8(b)). When some of the carbon atoms are replaced with heteroatoms, the conductivity type of the graphene 12 becomes n-type or p-type. For example, the conductivity type of the graphene 12 in which some of the carbon atoms are replaced with nitrogen atoms is n-type. The conductivity type of the graphene 12 in which some of the carbon atoms are replaced with boron atoms is p-type.

The length of the graphene 12 is, for example, 5 μm to 20 μm (for example, 9 μm). The width of the graphene 12 is, for example, 2 μm to 10 μm (for example, 5 μm).

A photoresist film 38 having a plurality of openings 37 provided on one end side of the graphene 12 is formed on the _SiO2 film 9 and the graphene 12 (see FIG. 8C). The graphene 12 is etched through the photoresist film 38. The etching of the graphene 12 is performed by, for example, dry etching using oxygen as a reactive gas.

This etching forms multiple holes 14 on one end side of the graphene 12 (see FIG. 9(a)). The region in which the multiple holes 14 are formed becomes the first region 16a of the light receiving region 10. The region adjacent to the first region 16a becomes the second region 16b of the light receiving region 10. In FIG. 9(a), the first region 16a is longer than the second region 16b, but the lengths of the first region 16a and the second region 16b may be approximately the same. Alternatively, the first region 16a may be shorter than the second region 16b.

The width of the holes 14 is, for example, 10 nm to 1000 nm. The fill factor of the first region 16a is, for example, 0.7 to 0.95 (preferably, 0.9). The fill factor of the second region 16b is, for example, 1.0.

After the formation of the plurality of holes 14, a contact hole 40 penetrating the _SiO2 film 9 is formed (see FIG. 9B).

Then, a first electrode 11a is formed on one end of the graphene 12, and a second electrode 11b is formed on the other end. Furthermore, a substrate electrode 13 is formed in contact with the silicon substrate 7 exposed in the contact hole 40 and extending onto the _SiO2 film 9 (see FIG. 9C).

(4) Two-Back Gate Photodetector Fig. 10 is a diagram showing an example of the structure of a photodetector 102 having a photosensor 104 with two gate electrodes (i.e., a two-back gate photodetector). The photodetector 102 is similar to the photodetector 2 of the first embodiment described with reference to Fig. 1 etc. Therefore, a description of the parts common to the photodetector 2 will be omitted.

The light detection device 102 has a light sensor 104, a first bias circuit 106a, a second bias circuit 106b, and an electrical signal detection circuit 8. The electrical signal detection circuit 8 is the circuit described with reference to FIG. 1 etc.

-Optical sensor 104-
The graphene 112 of the optical sensor 104 does not have any holes. Therefore, the fill factor of the first region 116a of the light-receiving region 110 is 1. The fill factor of the second region 116b of the light-receiving region 110 is also 1.

The optical sensor 104 has a first gate electrode 105a facing the first region 116a and a second gate electrode 105b facing the second region 116b. The optical sensor 104 further has an insulating film 109 (e.g., a SiO ₂ film) disposed between the first gate electrode 105a and the second gate electrode 105b and the graphene 112. The first gate electrode 105a and the second gate electrode 105b are disposed on a SiO ₂ film 9 covering the silicon substrate 7.

--First bias circuit 106a and second bias circuit 106b--
The first bias circuit 106a applies a first gate voltage Vg1 between the first region 116a of the light-receiving region 110 and the first gate electrode 105a. The second bias circuit 106b applies a second gate voltage Vg2, which is different from the first gate voltage Vg1, between the second region 116b of the light-receiving region 110 and the second gate electrode 105b.

When no voltage is applied between the first region 116a and the first gate electrode 105a and between the second region 116b and the second gate electrode 105b, the conductivity type of the light receiving region 110 is p-type (or n-type).

The first gate voltage Vg1 and the second gate voltage Vg2 are different voltages. Therefore, the microscopic carrier density (e.g., electron density) induced in the first region 116a by the first gate voltage Vg1 and the microscopic carrier density induced in the second region 116b by the second gate voltage Vg2 are different from each other. Therefore, it is easy to form a pn junction in the light receiving region 110.

Therefore, the light irradiated to the graphene 112 can be detected by the photodetector 102 of FIG. 10 as well. However, since the photodetector 104 includes two gate electrodes (i.e., the first gate electrode 105a and the second gate electrode 105b), the structure of the photodetector 104 is complex. Furthermore, since separate voltages Vg1 and Vg2 are applied to the two gate electrodes 105a and 105b, the peripheral circuit 42 is also complex.

The photodetector 2 described with reference to Figure 1 does not have these problems.

(5) Modifications Fig. 11 is a diagram illustrating a modification 216a of the first region 16a. The hole 14 in the first region 16a shown in Fig. 3 is a band-shaped hole with rounded ends. On the other hand, the hole 114 in the first region 216a shown in Fig. 11 is a circular hole. According to this modification, the variety of the first region 16a is increased.

In the above example, holes 14 are not provided in the second region 16b. However, holes 14 may be provided not only in the first region 16a but also in the second region 16b.

In the above example, the conductivity type of the light-receiving region 10 is p-type when no voltage is applied between the light-receiving region 10 and the gate electrode 5. However, the conductivity type of the light-receiving region 10 may be n-type when no voltage is applied between the light-receiving region 10 and the gate electrode 5. In this case, the gate voltage V1 is a voltage that makes the potential of the light-receiving region 10 higher than the potential of the gate electrode 5.

In the above example, the light receiving region 10 has two regions (i.e., the first region 16a and the second region 16b) with different fill factors. However, the light receiving region 10 may have three or more regions with different fill factors. For example, the light receiving region 10 may have a region between the first region 16a and the second region 16b, whose fill factor is larger than that of the first region 16a and smaller than that of the second region 16b. With this structure, a pin junction can be formed in the light receiving region 10.

In the above examples, some of the carbon atoms contained in the graphene 12 are replaced by heteroatoms (see "(3) Manufacturing method"). However, it is possible to make the conductivity type of the light-receiving region 10 n-type or p-type without the substitution of carbon atoms. For example, the conductivity type of the graphene 12 can be controlled by attaching any one of an organic substance such as F4-TCNQ (tetrafluoro-tetracyanoquin-odimethane), metal particles such as Au, and an Al ₂ O ₃ film to the light-receiving region 10. Alternatively, the conductivity type of the graphene 12 can be controlled by introducing defects into the graphene 12. The substitution of carbon atoms and the attachment of an organic substance or the like do not have to be intentional.

In the above example, the gate electrode 5 is a conductive silicon substrate 7. However, the gate electrode 5 may also be a metal film.

Furthermore, the insulating film disposed between the graphene 12 and the gate electrode 5 may be an insulating film other than a SiO ₂ film (for example, a SiN film).

In the first embodiment, a single voltage V1 is applied between the light receiving region 10, which includes regions with different fill factors, and the gate electrode 5, to induce an asymmetric carrier distribution in the light receiving region 10 and form a pn junction. Therefore, according to the first embodiment, a graphene photodetector capable of detecting light can be realized by simply applying a single voltage V1 to a single gate electrode 5.

(Embodiment 2)
The second embodiment is similar to the first embodiment. Therefore, the description of the same configuration as the first embodiment will be omitted or simplified.

(1) Structure Fig. 12 is a diagram showing an example of a photodetector 202 according to embodiment 2. As shown in Fig. 12, the photodetector 202 has a photosensor 4, a gate voltage application circuit 6, and an electric signal detection circuit 208.

The optical sensor 4 is the sensor described in the first embodiment (see FIG. 1). Similarly, the gate voltage application circuit 6 is the circuit described in the first embodiment. On the other hand, the electrical signal detection circuit 208 is a circuit different from the electrical signal detection circuit 8 in the first embodiment.

The electrical signal detection circuit 208 is a voltage detection circuit 219 that detects a potential difference V _PTE generated between the first connection region 15 a in contact with the first region 16 a of the light receiving region 10 and the second connection region 15 b in contact with the second region 16 b of the light receiving region 10.

12, the voltage detection circuit 219 is a circuit having a voltmeter 220 connected to the first connection region 15a and the second connection region 15b. The electrical signal output by the optical sensor 4 of the second embodiment is a potential difference V _PTE (i.e., a potential difference generated across the light-receiving region 10 illuminated with light) detected by the voltage detection circuit 219 while a voltage V2 is applied to the gate electrode 5 of the optical sensor 4. The voltage V2 is a gate voltage Vg output by the gate voltage application circuit 6. Details of the voltage V2 will be described in "(2) Operation".

(2) Operation Fig. 13 is a diagram for explaining the conductivity type of the light-receiving region 10 in the second embodiment. Fig. 13 shows an example of the energy band of the light-receiving region 10. In the example shown in Fig. 13, the conductivity type of the light-receiving region 10 is p-type when no voltage is applied to the gate electrode 5.

Fig. 13(a) shows the energy band of the light-receiving region 10 when no voltage is applied to the gate electrode 5. As shown in Fig. 13(a), when no voltage is applied to the gate electrode 5, the valence band of the light-receiving region 10 is filled with electrons up to energy _E0 , and is filled with holes from energy _E0 to the top of the valence band (i.e., point K).

Figure 13(b) shows the energy band of graphene 12 in the first region 16a when a voltage V2 (i.e., the gate voltage Vg in embodiment 2) is applied to the gate electrode 5. Figure 13(c) shows the energy band of graphene 12 in the second region 16b when a voltage V2 is applied to the gate electrode 5.

When a voltage V2 is applied to the gate electrode 5, electrons are induced in the first region 16a and the second region 16b. At this time, the density of electrons induced in the graphene 12 in the first region 16a, which has a small fill factor, is higher than the density of electrons induced in the graphene in the second region 16b (see embodiment 1). Therefore, the maximum value E1' of the electron energy in the valence band in the first region 16a is higher than the maximum value E2' of the electron energy in the valence band in the second region 16b. In other words, the hole density in the first region 16a is lower than the hole density in the second region 16b.

Therefore, while voltage V2 is applied to gate electrode 5, the density of majority carriers (here, holes) in first region 16a is lower than the density of majority carriers in second region 16b.

The Seebeck coefficient, which indicates the magnitude of the thermoelectric effect, depends on the density of majority carriers. Therefore, when a gate voltage V2 is applied to the gate electrode 5, the Seebeck coefficient of the first region 16a and the Seebeck coefficient of the second region 16b deviate from each other. Therefore, when the temperature of the light-receiving region 10 increases due to light irradiation, a potential difference V _PTE occurs between the first connection region 15a and the second connection region 15b (see "-- Generation of potential difference V _PTE due to thermoelectric effect--"). The potential difference V _PTE is detected by the voltmeter 220 of the electric signal detection circuit 208.

-Generation of potential difference V _PTE due to thermoelectric effect-
The light irradiated to the light-receiving region 10 is absorbed by the graphene 12 in the light-receiving region 10 , causing the temperature of the light-receiving region 10 to increase.

On the other hand, light irradiated from above the optical sensor 4 toward the first connection region 15a is blocked by the first electrode 11a and does not reach the first connection region 15a. Similarly, light irradiated from above the optical sensor 4 toward the second connection region 15b does not reach the second connection region 15b. Therefore, light irradiation of the first and second connection regions 15a and 15b does not increase the temperature of the first and second connection regions 15a and 15b.

Therefore, when light is irradiated from the front side of the silicon substrate 7, rather than from the back side, toward the entire graphene 12, the temperature rises in the center of the light-receiving region 10, and the temperature rise at both ends of the light-receiving region 10 is suppressed by the first and second connection regions 15a, 15b. As a result, a temperature gradient occurs in the first region 16a and the second region 16b. Naturally, a temperature gradient TG occurs in the first region 16a and the second region 16b when light is irradiated only to the center of the light-receiving region 10.

When light is irradiated onto the light-receiving region 10 in which the Seebeck coefficient of the first region 16a and the Seebeck coefficient of the second region 16b have diverged due to the application of voltage V2, and a temperature gradient TG is generated, a potential difference V _PTE is generated between the first connection region 15a and the second connection region 15b due to the thermoelectric effect. In other words, when light is irradiated onto the light-receiving region 10 in which regions with different Seebeck coefficients have been generated by the application of voltage V2 to the gate electrode 5, a potential difference V _PTE is generated due to the photothermoelectric effect.

-sensitivity-
As is apparent from the above description, the sensitivity of the photodetector 202 can be controlled by the gate voltage Vg. Like the gate voltage Vg of the first embodiment, the gate voltage Vg of the second embodiment is set so that the sensitivity of the photodetector 202 becomes a specific value (i.e., target sensitivity).

When the target sensitivity is a positive value, the target sensitivity in embodiment 2 is preferably 0.5 V/W or more. More preferably, the target sensitivity is 0.7 V/W or more. Most preferably, the target sensitivity is 0.9 V/W or more. The upper limit of the target sensitivity is the maximum sensitivity of the photodetector 202 (e.g., peak value P2 in FIG. 14 described later).

When the target sensitivity is a negative value, the target sensitivity in embodiment 2 is preferably -0.5 V/W or less. More preferably, the target sensitivity is -0.7 V/W or less. Most preferably, the target sensitivity is -0.9 V/W or less. The lower limit of the target sensitivity is the minimum sensitivity of the photodetector 202.

(3) Modifications (3-1) Modification 1
13, the conductivity type of the light-receiving region 10 is p-type when no voltage is applied to the gate electrode 5. However, the conductivity type of the light-receiving region 10 may be n-type when no voltage is applied to the gate electrode 5. In this case, the carriers induced in the light-receiving region 10 by application of voltage V2 to the gate electrode 5 are holes.

Alternatively, the conductivity type of the light receiving region 10 may be i-type when no voltage is applied to the gate electrode 5. In this case, the carriers induced in the light receiving region 10 by application of the voltage V2 to the gate electrode 5 may be either electrons or holes. According to the first variant, the variety of the light detection device 202 is increased.

(3-2) Modification 2
In the above example, the conductivity types of the first region 16a and the second region 16b are both n-type or p-type while the voltage V2 is applied to the gate electrode 5. However, one of the conductivity types of the first region 16a and the second region 16b may be n-type and the other may be p-type while the voltage V2 is applied to the gate electrode 5.

The Seebeck coefficient depends not only on the carrier density of the thermoelectric exchange material but also on its conductivity type, so even with variant 2, it is possible to detect light irradiated onto the light receiving region 10.

As is well known, the sensitivity of a temperature sensor in which an n-type thermoelectric exchange material and a p-type thermoelectric exchange material are connected is higher than the sensitivity of a temperature sensor in which thermoelectric exchange materials of the same conductivity type are connected. Therefore, according to the second modification, the sensitivity of the light detection device 202 can be improved.

Since a pn junction is formed in the light receiving region 10 of the second modification, the electrical signal output from the optical sensor 4 contains a voltage generated by the photovoltaic effect. However, since a reverse bias voltage is not applied to the light receiving region 10 in the second modification, in many cases the voltage generated by the photothermoelectric effect is dominant over the voltage generated by the photovoltaic effect.

14 is a diagram showing an example of the relationship between the sensitivity R2 and the gate voltage Vg of the photodetector 202 when light of a specific wavelength (hereinafter referred to as the irradiated light) is irradiated to the light receiving region 10. The wavelength of the irradiated light is, for example, 8 μm. The intensity of the irradiated light is, for example, 1 W/ ^cm2 .

The horizontal axis is the gate voltage Vg. In FIG. 14, the gate voltage Vg is the difference (=Φ1-Φ2) between the potential Φ1 of the gate electrode 5 and the potential Φ2 of the graphene 12. The vertical axis is the sensitivity R2 of the photodetector 202. The sensitivity of the photodetector 202 is the ratio (=a/b) of the intensity a of the electrical signal (voltage in the second embodiment) output by the optical sensor 4 in response to the light irradiated to the light receiving region 10 to the intensity b of the light irradiated to the light receiving region 10.

In the region 240 where the gate voltage Vg is small, the conductivity type of the first region 16a and the second region 16b is not inverted (see FIG. 13). Therefore, in the region 240 where the gate voltage Vg is small, a pn junction is not formed in the light receiving region 10. For this reason, in the region 240, the absolute value of the sensitivity R2 of the photodetector 202 is not very large.

In a region (not shown) where the gate voltage Vg is too large, the conductivity types of both the first region 16a and the second region 16b are inverted, so that no pn junction is formed in the light receiving region 10. Even in this region, the absolute value of the sensitivity R2 of the photodetector 202 is not very large.

In the region 242 where the gate voltage Vg is neither too small nor too large, only the conductivity type of the first region 16a is inverted and a pn junction is formed. When the pn junction is formed, the absolute value of the sensitivity R2 increases significantly. Therefore, a large peak 236 appears on the curve 235 showing the relationship between the gate voltage Vg and the sensitivity R2. The peak value P2 of the sensitivity R2 (i.e., the sensitivity with the maximum absolute value among the sensitivity R2) is, for example, 1 V/W.

The voltage V2 applied to the gate electrode 5 is selected from the gate voltage Vg in the region 242, so that according to the second modification, the sensitivity of the photodetector 202 can be improved.

In the second embodiment, a single voltage V2 is applied between the light receiving region 10, which includes regions with different fill factors, and the gate electrode 5, to induce an asymmetric carrier distribution in the light receiving region 10, thereby forming regions with different Seebeck coefficients in the light receiving region 10. Therefore, according to the second embodiment, as in the first embodiment, a graphene photodetector capable of detecting light can be realized by simply applying a single voltage V2 to a single gate electrode 5.

Although the embodiments of the present invention have been described above, embodiments 1 and 2 are illustrative and not limiting. For example, in the above examples, the fill factor of the first region 16a is smaller than the fill factor of the second region 16b. However, the fill factor of the first region 16a may be larger than the fill factor of the second region 16b.

In the above example, the electrodes provided on the graphene 12 (i.e., the first electrode 11a and the second electrode 11b) are strip-shaped electrodes. However, the shape of the electrodes provided on the graphene 12 is not limited to strip-shaped. The shape of the electrodes provided on the graphene 12 may be, for example, a comb-shaped electrode.

The following notes are further provided with respect to the above embodiments 1 and 2.

(Appendix 1)
an optical sensor including: graphene including a light receiving region having a first region with a hole disposed on one end side and a second region different from the first region disposed on the other end side; and a gate electrode facing the light receiving region, the optical sensor outputting an electrical signal in response to light irradiated to the light receiving region;
a gate voltage application circuit for applying a gate voltage between the light receiving region and the gate electrode;
a ratio of an area of the first region other than the holes to an area of the first region is smaller than a ratio of an area of the second region other than the holes to an area of the second region;
The gate voltage is set so that the ratio of the intensity of the electrical signal to the intensity of the light becomes a specific value.

(Appendix 2)
the conductivity type of the light receiving region is n-type or p-type when no voltage is applied between the light receiving region and the gate electrode;
2. The photodetection device described in claim 1, wherein application of the gate voltage between the light receiving region and the gate electrode inverts the conductivity type of the first region and maintains the conductivity type of the second region.

(Appendix 3)
a current detection circuit that detects a current flowing through the light-receiving region while applying a voltage to both ends of the light-receiving region that makes a potential of one of the first region and the second region, which is an n-type conductive region while the gate voltage is being applied between the light-receiving region and the gate electrode, higher than a potential of the other of the first region and the second region;
3. The photodetector according to claim 2, wherein the electrical signal is the current detected by the current detection circuit while the gate voltage is applied between the light receiving region and the gate electrode.

(Appendix 4)
4. The photodetector according to claim 3, wherein the absolute value of the specific value is 0.5 mA/W or more.

(Appendix 5)
Further, a voltage detection circuit is provided for detecting a potential difference generated between a first connection region included in the graphene and in contact with the first region and a second connection region included in the graphene and in contact with the second region,
3. The photodetector according to claim 1, wherein the electrical signal is the potential difference detected by the voltage detection circuit while the gate voltage is applied between the light receiving region and the gate electrode.

(Appendix 6)
The photodetector according to claim 5, wherein the absolute value of the specific value is 0.5 V/W or more.

2, 202: Photodetector 4: Photosensor 5: Gate electrode 6: Gate voltage application circuit 8, 208: Electric signal detection circuit 10: Light receiving region 12: Graphene 14: Hole 15a: First connection region 15b: Second connection region 16a: First region 16b: Second region

Claims (5)

an optical sensor including: graphene including a light receiving region having a first region with a hole disposed on one end side and a second region different from the first region disposed on the other end side; and a gate electrode facing the light receiving region, the optical sensor outputting an electrical signal in response to light irradiated to the light receiving region;
a gate voltage application circuit for applying a gate voltage between the light receiving region and the gate electrode;
a ratio of an area of the first region other than the holes to an area of the first region is smaller than a ratio of an area of the second region other than the holes to an area of the second region;
The gate voltage is set so that the ratio of the intensity of the electrical signal to the intensity of the light becomes a specific value. the conductivity type of the light receiving region is n-type or p-type when no voltage is applied between the light receiving region and the gate electrode;
2. The photodetector according to claim 1, wherein application of the gate voltage between the light receiving region and the gate electrode inverts the conductivity type of the first region and maintains the conductivity type of the second region. a current detection circuit that detects a current flowing through the light-receiving region while applying a voltage to both ends of the light-receiving region that makes a potential of one of the first region and the second region, which is an n-type conductive region while the gate voltage is being applied between the light-receiving region and the gate electrode, higher than a potential of the other of the first region and the second region;
3. The photodetection device according to claim 2, wherein the electrical signal is the current detected by the current detection circuit while the gate voltage is applied between the light receiving region and the gate electrode. Further, a voltage detection circuit is provided for detecting a potential difference generated between a first connection region included in the graphene and in contact with the first region and a second connection region included in the graphene and in contact with the second region,
3. The photodetector according to claim 1, wherein the electrical signal is the potential difference detected by the voltage detection circuit while the gate voltage is applied between the light receiving region and the gate electrode. an optical sensor including: graphene including a light receiving region, the light receiving region having a first region with a hole disposed on one end side and a second region in contact with the first region disposed on the other end side; and a gate electrode facing the light receiving region, the optical sensor outputting an electrical signal in response to light irradiated to the light receiving region;
a gate voltage application circuit for applying a gate voltage between the light receiving region and the gate electrode;
a ratio of an area of the first region other than the holes to an area of the first region is smaller than a ratio of an area of the second region other than the holes to an area of the second region;
The gate voltage is set so that the ratio of the intensity of the electrical signal to the intensity of the light is a specific value.
Light detection device.

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