Provided is a photodetector including: an organic semiconductor (20) having protrusions; a metal layer (30) added onto the organic semiconductor (20), for promoting at least one of localized plasmon resonance and surface plasmon resonance in which electrons are excited through irradiation of detection light; and a semiconductor (40) forming a junction with the metal layer (30), for allowing electrons excited through the plasmon resonance to pass through the junction (40a) with the metal layer (30).

TECHNICAL FIELD

This disclosure relates to a photodetector.

BACKGROUND

In recent years, there have been proposed various photodetectors including, for example: the one having a quantum well structure (see, for example, Non-patent Literature (NPL) 1); the one having, in place of the quantum well, an infrared absorber using a metal (Au)/semiconductor material (Ge) (metamaterial type) (see, for example, NPL 2); the one utilizing a metal-semiconductor junction using silicon (see, for example, NPL 3); and the one having antenna layers for generating surface plasmon resonance (see, for example, Patent Literature (PTL) 1).

CITATION LIST

Patent Literature

SUMMARY

However, the photodetector disclosed in NPL 1 uses a compound semiconductor, which lacks affinity to the silicon process. In other words, the materials for the compound semiconductor need to be controlled finely and at high temperature, which means that the presence of contamination (contaminant), if any, may affect the material composition due to the diffusion of the contamination, resulting in failure to provide a desired material. For this reason, difficulty may be anticipated in integrating the aforementioned photodetectors onto a silicon device using the process of film formation or the like. Therefore, in consideration of forming a quantum well using a silicon device, NPL1 finds it theoretically impossible to laminate crystalline silicon and amorphous silicon through conventional evaporation process for the following reason. That is, a process of annealing amorphous silicon at high temperature for recrystallization is required in order to obtain crystalline silicon.

The photodetector disclosed in NPL 2 utilizes plasmonic resonance of metals to absorb infrared light. However, difficulty is anticipated in the fabrication of such photodetector, due to the aforementioned problem of metal contamination.

The photodetector disclosed in NPL 3 uses a metal-semiconductor junction using silicon, and thus can be fabricated with more ease as compared with the photodetectors disclosed in NPL 1 and NPL 2. However, silicon shows low detection sensitivity to light in the infrared region, and thus the wavelength of the detection light is to be subjected to limitation.

The photodetector disclosed in PTL 1 is configured to generate surface plasmon resonance in antenna layers to thereby output near field light from through holes of the antenna layers, so as to receive the near field light by a light receiving layer through light receiving surfaces over the areas of the through holes. Thus, according to the photodetector configured as above, the antenna layers may suitably be configured to generate surface plasmon by light at a desired wavelength, so as to detect light even if it is in the infrared region. However, in the aforementioned photodetector, the antenna layers constitute a grating structure, which means that the detection sensitivity will have incident angle dependence, failing to obtain stable detection sensitivity across a wide range of incident angles.

It could therefore be helpful to provide a photodetector, including:

an organic semiconductor having protrusions;

a metal layer added onto the organic semiconductor, for promoting at least one of localized plasmon resonance and surface plasmon resonance in which electrons are excited through irradiation of detection light; and

a semiconductor forming a junction with the metal layer, for allowing electrons excited through the plasmon resonance to pass through the junction with the metal layer.

The protrusions may each have a height including the metal layer being equal to or smaller than the wavelength of the detection light and the protrusions may each have a maximum dimension in thickness being equal to or smaller than the wavelength of the detection light.

The semiconductor may be an organic semiconductor.

The photodetector may further include a substrate for supporting the semiconductor.

The substrate may be a semiconductor substrate.

The substrate is a conductive substrate.

The substrate is an insulating substrate.

The substrate is an inorganic semiconductor.

The protrusions may each have a height, including the metal layer, of 20 nm or more.

The protrusions may each have a height, including the metal layer, of 50 nm or more.

The metal layer may have a concavo-convex structure having protrusions and recesses each being adjacent to the respective protrusions, and the protrusions in the concavo-convex structure may each have a dimension in height equal to or smaller than the wavelength of the detection light and have a maximum dimension in thickness that is equal to or smaller than the wavelength of the detection light.

The protrusions including the metal layer may each be formed as being curved or bent into an arbitrary shape.

The protrusions including the metal layer may be in irregularly formed columnar shapes.

The organic semiconductor having protrusions may be formed through crystal growth.

The organic semiconductor may be formed of any of: a phthalocyanine-based material; a thiophene-based material; and Alq3.

The metal layer is formed of any of: Au; Pt; Al; and Ag.

The semiconductor may be formed of any of: a phthalocyanine-based material; a thiophene-based material; Alq3; and silicon.

The metal layer added onto the organic semiconductor may promote localized plasmon resonance.

DETAILED DESCRIPTION

Explained first is the principle of a photodetector disclosed herein.

FIG. 1is a sectional view illustrating a principal configuration of the disclosed photodetector. The photodetector10includes: an organic semiconductor20; a metal layer30; and a semiconductor40. The organic semiconductor20is formed as a plurality of protrusions on the semiconductor40, so as to form, together with the semiconductor40, a concavo-convex structure having protrusions and recesses each being adjacent to the respective protrusions. The metal layer30is added onto the organic semiconductor20and the semiconductor40in a concavo-convex structure, so as to promote at least one of localized plasmon resonance and surface plasmon resonance through irradiation of detection light. Accordingly, the protrusions of the concavo-convex structure each have a height h, including the metal layer30, equal to or smaller than the wavelength of the detection light, and have a thickness equal to or smaller than the wavelength of the detection light, where: the height h corresponds a width in the vertical direction between the protrusion; and the recess of the metal layer and the thickness corresponds to a maximum dimension d in a plane orthogonal to the extending direction (height direction) of the protrusion. This allows the plasmon resonance to be more effectively promoted. Here, the height h of the protrusion may preferably be 1/10 to ⅕ of the detection wavelength. In this case, the localized plasmon can be effectively excited (see, for example. J. J. Mock, M. Barbic, D. R. Smith, D. A. Schultz, and S. Schultz, “Shape effects in plasmon resonance of individual colloidal silver nanoparticles”, J. Chem. Phys. 116, 6755 (2002)). Further, the protrusions may be formed in arbitrary shapes such as columnar or prismatic, which may be regularly formed or may be irregularly formed as standing close together.

The semiconductor40is made of an organic semiconductor or an inorganic semiconductor, and allows the electrons excited by plasmon resonance occurring in the metal layer30to pass through via a junction40awith the metal layer30. This configuration allows for implementation of an operation of promoting light absorption through plasmon resonance. Note that the semiconductor40may preferably be supported on a substrate, so as to allow for various implementations.

FIGS. 2A to 2Dare views for illustrating an operation of the photodetector10ofFIG. 1. First, as illustrated inFIG. 2A, when light is incident on the metal layer30with a concavo-convex structure, the wavelength of the incident light conforms to the dimensions of the concavo-convex structure, to thereby generate localized plasmon resonance and/or surface plasmon resonance. As a result, as illustrated inFIG. 2B, electrons in the metal layer30are excited. A larger number of electrons are excited by the plasmon resonance, as compared with the case where no plasmon resonance is excited, that is, the case where the incident light is subjected to total reflection.

The electrons excited in the metal layer30pass through the junction40abetween the metal layer30and the semiconductor40as illustrated in FIG.2C. Here, an electric field is formed in the junction40adue to the contact between metal and semiconductor, and thus the excited electrons need to overcome a Schottky barrier at the metal-semiconductor at the junction40aso as to flow as a diffusion current. Therefore, as illustrated inFIG. 2D, a current flowing though the semiconductor40may be detected by a current detector50such as an ammeter, to thereby detect the incident light. The junction40amay be applied with a potential in order to make steep the electric field at metal-semiconductor interfaces at the junction40aFIG. 3is a graph showing a relation between the wavelength of the incident light and the detected current in the photodetector10.

The photodetector10ofFIG. 1uses the organic semiconductor30, which allows for easy fabrication and integration. Further, the concavo-convex structure may be designed with proper dimensions, to thereby change the light absorption wavelength λm ofFIG. 3, which allows for tuning of the photodetection wavelength. InFIG. 2C, electrons excited in the metal layer30overcome the barrier at the junction40a, which means that the wavelength bandwidth (λe ofFIG. 3) of light to be detected can be tuned by tuning the barrier. In particular, in the case of using an organic semiconductor to form the semiconductor40, the material can be selected from a wider variety of choices as compared with the case of using an inorganic semiconductor, which provides an effect that the barrier can be controlled with ease. In other words, the Schottky barrier at the junction with the metal layer can be tuned, which can allow a greater quantity of electrons to overcome the barrier to thereby improve the sensitivity while allowing for tuning of the sensitivity bandwidth. The organic semiconductor30is formed in a protruding shape (columnar shape), and thus light in the infrared region may even be detected with high sensitivity irrespective of the incident angle.

Here, the organic semiconductor20, the metal layer30, and the semiconductor40constituting the aforementioned photodetector10, and a substrate supporting the semiconductor40may be formed of, for example, an arbitrary combination of the materials shown in Table 1, but not limited thereto. For example, the organic semiconductor20is not limited to CuPc, and may be formed of any other phthalocyanine-based material or may be formed of a thiophene-based material and Alq3. The substrate may be a semiconductor substrate. This allows the use of a versatile substrate such as a silicon substrate, which leads to cost reduction and easy integration, making it possible to expand the range of application. The substrate may also be a conductive substrate. This allows the use of a flexible conductive substrate such as ITO/PET, ITO/polyimide, and aluminum foil, making it possible to implement a flexible photodetector. Alternatively, the substrate may be an insulating substrate. This allows for surface mounting on an insulating substrate such as a glass substrate, which expands the range of application.

In Table 1, assuming an exemplary case where: Al is used for the metal layer30; CuPc is used for the organic semiconductor20; and silicon is used for the semiconductor40and the substrate, the difference between the Al work function (4.1 eV) and the silicon electron affinity (4.15 eV) becomes equivalent to the barrier height (0.05 eV), and thus the detection light has a wavelength λ of 20 μm or less (λ≤20 μm). The selection of the materials from Table 1 may further be optimized so as reduce the barrier height, to thereby increase the wavelength of the detection light.

In the following, description is given of Examples the present disclosure. In Example below, the photodetectors are composed of the materials shown in Table 2.

EXAMPLES

FIG. 4is a sectional view illustrating a schematic configuration of a photodetector according to Example 1. The photodetector60ofFIG. 4includes a silicon substrate70as an inorganic semiconductor, formed on which is an organic semiconductor portion80made of PTCDA/CuPc, the organic semiconductor portion80constituting the organic semiconductor20and the semiconductor40ofFIG. 1, with a Au layer90, which constitutes the metal layer30ofFIG. 1, being formed on the concavo-convex structured surface of the organic semiconductor portion80. Formed on the rear surface of the silicon substrate70is a Al layer100for taking out an output current.

In the photodetector60ofFIG. 4, incidence of light onto the concavo-convex structured Au layer90generates localized plasmon resonance and/or surface plasmon resonance, which excites electrons in the Au layer90. The electrons excited in the Au layer90overcome a Schottky barrier at the junction80abetween the Au layer90and the organic semiconductor portion80to be injected into the organic semiconductor portion80, so as to be taken out from the Al layer100via the silicon substrate70.

FIGS. 5A and 5BandFIGS. 6A and 6Bare flowcharts for illustrating an exemplary method of manufacturing the photodetector60ofFIG. 4. First, as illustrated inFIG. 5A, two different organic semiconductors (PTCDA/CuPc) are each vapor-deposited to a thickness corresponding to, for example, 3 nm onto a surface of the n-type single crystal silicon substrate70having a plane direction of (100) with resistivity ρ of 40 Ωcm (ρ=40 Ωcm). PTCDA serves as identifying sites to grow CuPc forming protrusions. Thereafter, the silicon substrate70is heated at 80° C. to 230° C. for 1 hour so as to subject CuPc to crystal growth as illustrated inFIG. 5B, to thereby form the organic semiconductor portion80having a concavo-convex structure. Here,FIG. 5Bomits the illustration of CuPc formed on the recesses of the concavo-convex structured silicon substrate70.

Next, a vacuum vapor deposition device (degree of vacuum=1.0×10−4Pa) is used to vapor-deposit Au onto the surface of the concavo-convex structure of the organic semiconductor portion80so as to form the Au layer90as illustrated inFIG. 6A. Lastly, a vacuum vapor deposition device (degree of vacuum=4.0×10−4Pa) is used to vapor-deposit Al onto the surface of the silicon substrate70so as to form the Al layer100as illustrated inFIG. 6B, to thereby complete the photodetector60.

FIGS. 7A and 7Bare electron microscope images of the photodetector60fabricated as described above, in which:FIG. 7Ashows a surface image of the concavo-convex structure of the photodetector60taken by a scanning electron microscope (SEM); andFIG. 7Bshows a sectional image of the photodetector60taken by a transmitting electron microscope (TEM). The electron microscope images ofFIGS. 7A and 7Bboth show the photodetector60having the organic semiconductor portion80formed by heating the silicon substrate70at 200° C. for 1 hour so as to subject CuPc to crystal growth.

As is apparent fromFIG. 7A, it can be seen that nano-sized protrusions are standing close together on the surface of the photodetector60. Further, as is apparent fromFIG. 7B, it can be seen that the Au layer90is vapor-deposited so as to surround the protrusion of the organic semiconductor portion80. As is also apparent fromFIG. 7B, the Au layer90and the silicon substrate70are not in direct contact with each other, with an organic semiconductor layer which constitutes the organic semiconductor portion80being interposed therebetween having a thickness of 10 nm or less. Meanwhile, it can be appreciated that the protrusion of the concavo-convex structure of the photodetector60has a height, which corresponds to the depth of the recess, of 50 nm or more. The silicon substrate70was heated at 110° C. for 1 hour to subject CuPc to crystal growth to thereby form the organic semiconductor portion80, so as to similarly form the photodetector60. The photodetector60thus formed was observed to find that the photodetector60in this case has a protrusion with a height of 20 nm or more.

Next, description is given of how we investigated the plasmon absorption effect of the photodetector60according to the disclosed Examples. The effect was investigated through an electromagnetic field simulation in the concavo-convex structure, with the use of “COMSOL Multiphysics” (trade name), a general-purpose physical software. Specifically, light was irradiated onto a substrate having Au protrusions standing close together, and it was investigated whether the absorption peak is present in the near-infrared region.

FIG. 8is an analytical photogrammetry showing a model of computation with the computation results of the electric field distribution. The model of computation includes; a silicon substrate; and a Au thin film vapor-deposited to 50 nm in thickness onto the silicon substrate, with an Au protrusion of 50 nm in width and h nm in height standing in the center. For the sake of simplicity, the organic semiconductor was omitted from the computation. As the boundary condition, the periodic boundary condition was applied in the lateral direction of the protrusion, and thus, in the case ofFIG. 8, the model will be configured to have protrusions standing close together at a pitch of 500 nm on the Au thin film so as to fill the plane. Here, TM (Transverse Magnetic) wave is vertically incident from the upper surface side onto the height h of the protrusion and the pitch of the protrusion, so as to calculate the absorptance of the substrate surface through 1-T-R computation based on the reflectance R and the transmittance T.

FIG. 9is a graph showing the absorptance spectra relative to different heights h in the aforementioned case. Referring toFIG. 9, it has turned out that the protrusions having h=150 nm to 250 nm are each increased in absorption at the resonance of a single peak in the red visible light region. Note that the protrusions ofFIG. 9have a pitch of 400 nm.

The result shows that the shape of the concavo-convex structure determines the absorption wavelength (λm ofFIG. 3). Further, referring toFIG. 8, it can be identified that the electric field strength distribution is generated around the protrusion when the resonance is being generated, which suggests that a dipolar plasmon mode is being excited. In order to investigate whether the vibration at the absorption peak is a dipolar response in reality, the vector representation of the electric field strength was analyzed.

FIG. 10is an analytical photogrammetry showing the electric field strength distribution. Referring toFIG. 10, it can be identified that an electric field starting from the tip of each of the protrusion is vertically incident on the substrate surface to terminate thereon. This means that the protrusion formed upright from the Au thin film surface excites negative charges at the mirror image position within the substrate, leading to a possibility that a dipolar electric field strength distribution be formed in the upper half.

FIG. 11is an analytical photogrammetry visualizing the current distribution of the aforementioned case.FIG. 11shows that currents are flowing toward the tip of the protrusion, which suggested increase in charge density at the tip of the protrusion.

Based on the aforementioned analysis, we have found that the charge distribution was generated at the tip of the protrusion during resonance, exciting a dipolar plasmon mode. Accordingly, in the photodetector60according to this Example, the plasmon resonance in the surface structure may possibly be promoting light absorption/detection.

Further, we obtained the electric characteristic of the photodetector60in order to confirm the effect thereof. First, the Au layer90side and the n-type silicon substrate70side of the photodetector are each set as a positive electrode and a negative electrode, respectively, so as to obtain current-voltage characteristics. The result thereof is shown inFIG. 12A. Referring toFIG. 12A, Example 1-1 shows the photodetector60obtained by heating the silicon substrate70at 200° C. to form the organic semiconductor portion80having a concavo-convex structure. Further, Example 1-2 shows a photodetector60obtained by heating the silicon substrate70at 110° C. to form the organic semiconductor portion80having a concavo-convex structure.

FIG. 12Aalso shows the results obtained by measuring a reference diode serving as a reference device, the reference diode having no concavo-convex structure and being formed of a Au/n-type silicon junction (the Au has a thickness of 50 nm).

According to the results shown inFIG. 12A, the carriers flowing in the forward direction can be considered as electrons (see Sze S M 1981 Physics of Semiconductor Devices 2nd edn (NewYork: Wiley)). Further, in investigating the properties of these diodes, it is important to obtain the parasitic resistance Rs, the Schottky barrier height Φb, and n value of each of these diodes. In particular, the Schottky barrier height Φb is an important parameter for determining the detection area of the infrared light sensor. In consideration thereof, these parameters were obtained. The measurement result is shown in Table 3. The parameters were calculated based on a method described in “Guirong Liang, Tianhong Cui, Kody Varahramyan, “Fabrication and electrical characteristics of polymer-based Schottky diode” Solid-State Electronics 47 (2003) 691-694”.

As is apparent from Table 3, when Examples 1-1, 1-2 are compared with the reference diode without having the concavo-convex structure, it can be found that (1) the photodetectors60of Examples 1-1, 1-2 are lower in parasitic resistance R2, and (2) Examples 1-1, 1-2 are almost the same as the reference diode in the Schottky barrier height Φb. As for the reason of (1), it may be assumed that the contact area between Au and the organic semiconductor forming the concavo-convex structure was increased to be larger than the contact area between Au and the n-type silicon, which increased the number of current paths. Meanwhile, as for (2), a conceivable reason is that the detection areas of the photodetectors60of Examples 1-1, 1-2 are almost the same as the detection area of the reference diode when the photodetectors60of Examples 1-1, 1-2 are used as infrared sensors. Further, all the n values are 1.5 or less. This means that the predominant drive current in the photodetectors60of Examples 1-1, 1-2 is a diffusion current, indicating that an excellent diode operation with little defect is implemented.

We further evaluated the spectral sensitivity characteristic of the photodetector60. In the evaluation, the sensitivity was evaluated in a wavelength range λ which is targeted to λ=1000 nm to 1500 nm in order to make the evaluation in a long-wavelength range. Meanwhile, a source measure unit (Model 2400 manufactured by Keithley Instruments Inc.) was used for current detection, and the current detection was performed with the application voltage of 0 v (i.e., during short circuit). The result thereof is shown inFIG. 12B.

Referring toFIG. 12B, Examples are compared with one another for the sensitivity characteristic at λ=1200 nm, and found that the sensitivity for Example 1-1 was 1.79 mA/W, for Example 1-2 was 0.945 mA/W, and for the reference diode was 0.141 mA/W. The result has revealed that the presence of the concavo-convex structure increases the sensitivity at this wavelength. Specifically, Example 1-1 was confirmed to be improved in sensitivity by approximately 12.7-fold, which is an increase of one or more digits, with respect to the reference diode. Example 1-2 was confirmed to be improved in sensitivity by approximately 6.7-fold with respect to the reference diode. The light having this wavelength (λ=1200 nm) has an energy (hv=1.03 eV) that is equal to or smaller than the band gap of silicon (Eg=1.12 eV), and thus, it can be assumed that the light having this wavelength is absorbed in the Au layer90. In view of this, it can be identified that the light absorption in the Au layer90has been increased because the diode having a concavo-convex structure is increased in sensitivity at this wavelength as compared with the sensitivity of the reference diode.

Considering the above, Example 1 allows for providing a photodetector that is easy to manufacture and capable of performing highly-sensitive detection of light even in the infrared region irrespective of the incident angle. Further, the use of the silicon substrate70, which is highly versatile, allows for cost reduction and facilitates integration, making it possible to provide a wider range of application.

FIG. 13is a sectional view illustrating a schematic configuration of the photodetector of Example 2. The photodetector61of Example 2 includes a conductive substrate71as a replacement for the silicone substrate70and the Al layer100of the photodetector60ofFIG. 4. The conductive substrate71is formed by vapor-depositing a transparent electrode (indium tin oxide (ITO))73onto a polyimide substrate72, with an organic semiconductor portion80being formed on the transparent electrode73. The rest of the configuration is the same as that ofFIG. 4, and thus the same constituent elements as those ofFIG. 4are denoted by the same reference symbols to omit the description thereof.

In Example 2, electrons excited in the Au layer90by localized plasmon resonance and/or surface plasmon resonance overcome a Schottky barrier at the junction80abetween the Au layer90and the organic semiconductor portion80to be injected into the organic semiconductor portion80. Then, the electrons injected into the organic semiconductor portion80are taken out therefrom via the transparent electrode73of the conductive substrate71.

In manufacturing the photodetector61ofFIG. 13, a transparent oxide material (ITO) is first vapor-deposited onto a commercially-available polyimide substrate72to form a transparent electrode73, to thereby prepare a conductive substrate71. Thereafter, as in the case of Example 1, the organic semiconductor portion80having a concavo-convex structure formed of PTCDA/CuP is constructed on the transparent electrode73, and then Au is further vapor-deposited onto the concavo-convex side of the organic semiconductor portion80to form the Au layer90, to thereby fabricate the photodetector61.

The photodetector61of Example 2 was measured for spectral sensitivity characteristic with the use of a source measure unit, in the same manner as in Example 1. As a result, optical response was identified in the infrared region (λ=1200 nm). Further, in Example 2, the conductive substrate71is not limited to ITO/polyimide, and may be formed of a flexible conductive substrate such as ITO/PET (polyethylene terephthalate) or aluminum foil. Accordingly, the photodetector61of Example 2 is capable of not only providing the same effect as in Example 1 but also being implemented as a flexible sensor, and thus can be developed for ubiquitous applications.

FIG. 14is a sectional view illustrating a schematic configuration of the photodetector of Example 3. The photodetector62of Example 3 includes a glass substrate75and a metal junction110being as replacements for the silicon substrate70and the Al layer100of the photodetector60ofFIG. 4. The metal junction110may be made of, for example, platinum (Pt), which forms an Ohmic junction with part of the organic semiconductor portion80. The rest of the configuration is the same as that ofFIG. 4, and thus the same constituent elements as those ofFIG. 4are denoted by the same reference symbols to omit the description thereof.

In Example 3, electrons excited in the Au layer90through localized plasmon resonance and/or surface plasmon resonance overcome a Schottky barrier at the junction80abetween the Au layer90and the organic semiconductor portion80to be injected into the organic semiconductor portion80. Then, the electrons injected into the organic semiconductor portion80are taken out therefrom via the metal junction110.

In manufacturing the photodetector62ofFIG. 14, the organic semiconductor portion80having a concavo-convex structure formed of PTCDA/CuP is first constructed on the glass substrate75as in the case of Example 1, and further Au is vapor-deposited onto the concavo-convex side of the organic semiconductor portion80to form the Au layer90. Thereafter, the metal junction110is vapor-deposited onto part of the organic semiconductor portion80to form an Ohmic junction therewith, to thereby fabricate the photodetector62.

The photodetector62of Example 3 was measured for spectral sensitivity characteristic with the use of a source measure unit, as in the case of Example 1, with the Au layer90side being set as a positive electrode and the metal junction110side being set a negative electrode. As a result, optical response was identified in the infrared region (λ=1200 nm). Accordingly, the photodetector62of Example 3 is capable of providing the same effect as in Example 1. In addition, the photodetector62of Example 3 can be surface-mounted onto an insulating substrate such as the glass substrate75, and thus can be directly mounted onto, for example, the windshield of a vehicle, which provides an effect of increasing applicability of the system.

FIG. 15is a sectional view illustrating a schematic configuration of the photodetector of Example 4. The photodetector63of Example 4 has the organic semiconductor20of the photodetector60ofFIG. 4formed in a protruding shape (columnar shape) directly on the silicon substrate70. In other words, the semiconductor40ofFIG. 1is replaced with the silicon substrate70. Therefore, in Example 4, the Au layer90comes into contact with the organic semiconductor20and the silicon substrate70, and thus, electrons excited in the Au layer90by localized plasmon resonance and/or surface plasmon resonance overcome a Schottky barrier at the junction70abetween the Au layer90and the silicon substrate70to be taken out from the Al layer100via the silicon substrate70.

In manufacturing the photodetector62ofFIG. 15, the substrate temperature of the silicon substrate70and the conditions of vapor-depositing PTCDA/CuPc constituting the organic semiconductor20are optimized, to thereby form, on the silicon substrate70, portions with or without the organic semiconductor20in a protruding shape. Thereafter, the Au layer90is vapor-deposited on the organic semiconductor20side of the silicon substrate70while vapor-depositing the Al layer100on the other side thereof, to thereby fabricate the photodetector63.

The photodetector63of Example 4 was measured for spectral sensitivity characteristic with the use of a source measure unit, in the same manner as in Example 1. As a result, optical response was identified in the infrared region (λ=1200 nm). Therefore, an effect of promoting light absorption through plasmon resonance can similarly be attained in Example 4, providing the same effect as in Example 1.

It should be noted that the disclosed photodetector is not limited to Examples above, and may be subjected to various modifications and alterations without departing from the gist of the present disclosure. For example, according to the disclosed photodetector, as illustrated in the schematic perspective view ofFIG. 16, the plurality of columnar protrusions formed into a concavo-convex structure including the metal layer30each may be curved or bent at an arbitrary curvature or angle so as to be formed into an arbitrary shape. This configuration facilitates the fabrication. In this case, the protrusion of the concavo-convex structure may have a height h including the metal layer30, i.e., a width in a vertical direction between the protrusion and the recess of the metal layer30, the height h or the width being defined to be in a dimension equal to or smaller than the wavelength of the detection light, as in the cases of the aforementioned disclosed photodetectors, and may more preferably be defined in a dimension equal to or smaller than 1/10 to ⅕ of the detection wavelength, while the protrusion has a thickness or a maximum dimension d in section perpendicular to the height direction of the protrusion being defined to be equal to or smaller than the wavelength of the detection light.

REFERENCE SIGNS LIST