JP7371911B2 - Photoelectric conversion device - Google Patents

Description

The present invention relates to a photoelectric conversion device.

A rectenna consisting of an antenna and a diode is known as a device that converts electromagnetic waves into electric power. A rectenna performs photoelectric conversion by absorbing electromagnetic waves with an antenna and rectifying the internal oscillations of the electric field generated by the electromagnetic waves with a diode. With microwaves of several GHz, a high power conversion efficiency of about 85% has been achieved. In recent years, research has been progressing on optical rectennas that apply rectenna technology to the optical frequency domain, and for example, results of power generation using sunlight have been reported (Non-Patent Document 1).

Asha Sharma et al., nature nanotechnology, Vol.10, P.1027-1032, 2015.

A typical pn junction diode has a large parasitic capacitance and a large RC time constant, so it is difficult to respond at optical frequencies. On the other hand, if it is a metal-insulator-metal tunnel diode (MIM diode), it is theoretically possible to respond at the optical frequency, but in that case, the thickness of the insulator must be 5 nm or less. are required to do so. Currently reported optical rectennas have a structure that resonates with electromagnetic waves in the in-plane direction like a bow-tie antenna, and MIM diodes have a structure that resonates with electromagnetic waves in the in-plane direction, such as a bow-tie antenna. It is formed so that it is lined up with. In that case, production is difficult because it is necessary to form a very thin insulating layer of 5 nm or less or a spatial gap between the two metals in the in-plane direction.

On the other hand, there have been reports of solar power generation using MIM diodes stacked perpendicular to the surface, but in this example, the efficiency was only about 10 ⁻⁵ %. This is because the surface electrode formed on the MIM diode blocks light from entering the MIM diode, resulting in a low light coupling efficiency.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a photoelectric conversion device that has high power generation efficiency and can be easily manufactured.

In order to solve the above problems, the present invention employs the following means.

(1) A photoelectric conversion device according to one embodiment of the present invention includes at least a first metal layer connected to a lower electrode , an insulating layer thick enough to allow only tunneling current to flow , and a second metal layer connected to an upper electrode. and a laminated structure in which the first metal layer has a work function smaller than that of the second metal layer, and the hole or slit is formed in the laminated structure. The recess is formed of a space having the shape of a shape, and the recess communicates with an aperture provided in the second metal layer on the outermost surface of the laminated structure, through which light enters, and the aperture, and the recess is in communication with the aperture, and an insulating layer , a side wall formed on the first metal layer, and a bottom surface formed on the first metal layer,
Within the recess, a resonant wavelength of the light within the recess forms a standing wave .

(2) In the photoelectric conversion device according to (1) above, the position where the amplitude of the standing wave of the resonant wavelength formed in the recess is the maximum and the position of the insulating layer are different from each other in the recess. Preferably, they are at equal distances from the bottom surface.

(3) In the photoelectric conversion device according to any one of (1) above, it is preferable that the resonant wavelength is a harmonic of a second mode.

(4) In the photoelectric conversion device according to any one of (1) to (3) above, the aperture is preferably open .

(5) A photoelectric conversion device according to one aspect of the present invention includes a first metal layer connected to a lower electrode, an insulating layer having a thickness through which only a tunnel current flows, and a second metal layer connected to an upper electrode. It has a multi-layer stacked structure in which a plurality of stacked layers are stacked in order, and an inter-layer structure insulating layer is provided between the stacked structures of the multi-layer stacked structure, and the first metal layer The work function of the second metal layer is smaller than the work function of the second metal layer, and the recess formed in the multi-layer stacked structure is formed of a hole or a slit-like space, and It has an opening provided in the second metal layer on the surface through which light enters, and a side wall communicating with the opening and formed in the second metal layer, the insulating layer, and the first metal layer. , the opening becomes narrower for each lower laminated structure between the laminated structures, and includes a bottom surface formed in the first metal layer at the bottom of the multi-layered laminated structure; The resonant wavelength of the light forms a standing wave of the magnetic field .

According to the present invention, it is possible to provide a photoelectric conversion device that has high power generation efficiency and can be easily manufactured.

FIG. 1 is a perspective view of a photoelectric conversion device according to a first embodiment of the present invention. 2 is an enlarged view of a partial cross section of the photoelectric conversion device of FIG. 1. FIG. 2 is an energy level diagram of each layer constituting the photoelectric conversion device of FIG. 1. FIG. (a) to (e) are process flows for manufacturing the main parts of a photoelectric conversion device. FIG. 3 is a perspective view of a photoelectric conversion device according to a second embodiment of the present invention. FIG. 3 is a plan view of a photoelectric conversion device according to a third embodiment of the present invention. FIG. 3 is a distribution diagram showing the dependence of light absorption rate on wavelength and depth of a recess in the photoelectric conversion device of Example 1. FIG. 3 is a graph showing the wavelength dependence of light absorption rate in the photoelectric conversion device of Example 2. 3 is a diagram showing a magnetic field strength distribution for each position in a recess in the photoelectric conversion device of Example 2. FIG. 3 is a graph of output current obtained by light irradiation in the photoelectric conversion device of Example 3. 3 is a graph of output voltage obtained by light irradiation in the photoelectric conversion device of Example 3.

Hereinafter, a photoelectric conversion device according to an embodiment to which the present invention is applied will be described in detail using the drawings. Note that the drawings used in the following explanations may show characteristic parts enlarged for convenience in order to make the characteristics easier to understand, and the dimensional ratio of each component may not be the same as the actual one. do not have. Furthermore, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto, and can be practiced with appropriate changes within the scope of the gist thereof.

<First embodiment>
FIG. 1 is a perspective view of a photoelectric conversion device 100 according to a first embodiment of the present invention. The photoelectric conversion device 100 has a plurality of recesses 105 recessed in the depth direction from one surface. FIG. 2 is an enlarged view of a cross section of a portion 100A of the photoelectric conversion device 100 of FIG. 1, showing a state where electromagnetic waves are incident. Note that the number of recesses 105 may be one, and in that case, the structure of the portion 100A shown in FIG. 2 will be referred to as a photoelectric conversion device. The photoelectric conversion device 100 mainly has a laminated structure 104 formed by laminating a first metal layer 101, an insulating layer 102, and a second metal layer 103 in this order. The configurations of the lower electrode 115 connected to the first metal layer 101 and the upper electrode 116 connected to the second metal layer 103 are not particularly limited. The lower electrode 115 here is integrated with the first metal layer 101. Hereinafter, a coordinate axis parallel to the stacking direction L of the laminated structure 104 will be referred to as a z-axis, and two coordinate axes orthogonal to this in a plane perpendicular to this will be referred to as an x-axis and a y-axis, respectively.

The laminated structure 104 has a recess (cavity) 105 that penetrates the second metal layer 103 and the insulating layer 102 and reaches the inside of the first metal layer 101 in the lamination direction L. Although the shape of the recessed portion 105 is not limited, here, an example in which the hollow portion is a square prism is illustrated.

The first metal layer 101 and the second metal layer 103 may be made of materials containing carbon, metals such as Ti, Au, Co, Ni, Se, Cu, or metal compounds thereof as main components. is selected such that the work function of the second metal layer is smaller than that of the second metal layer. For example, when two materials Ti and Pt are selected, Ti is used as the material for the first metal layer 101 and Pt is used as the material for the second metal layer 103. It is assumed that the material composition of the entire first metal layer 101 does not match the material composition of the entire second metal layer 103.

The insulating layer 102 is made of metal oxide, metal nitride, metal carbide, etc., more specifically, TiO ₂ , SiO ₂ , etc., which has an electron affinity smaller than the work function of the first metal layer 101 and the second metal layer 103 . made of an insulating material selected to have a The insulating layer 102 has a thickness that allows only a tunnel current to flow between the first metal layer 101 and the second metal layer 102.

In the photoelectric conversion device 100, the recess 105 formed in the laminated structure 104 operates as an antenna. Of the light (electromagnetic waves) that has entered the recess 105, only a component of a specific wavelength (resonant wavelength) forms a standing wave. The magnetic field Hx constituting this standing wave induces a current in the ±z direction parallel to the stacking direction L in the adjacent stacked structure 104 (side wall of the recess 105). The resonant wavelength λc forming the standing wave is expressed by the following equation (1) using the depth z of the recess 105. n represents the number of modes of the standing wave formed.

FIG. 3 is an energy level diagram of each layer, explaining the operation of the diode of the stacked structure 104. Since the first metal layer 101 and the second metal layer 103 are made of different metal materials, the shape of the tunnel barrier energy becomes asymmetrical due to the work function difference between the two layers. As a result, a rectifying effect is obtained in which the induced current flows in only one direction in the ±z direction. For example, when Ti is used as the material of the first metal layer 101 and Pt is used as the material of the second metal layer 103, as shown in FIG. More current flows in the direction of the second metal layer 103 (+z direction).

Since the induced current is proportional to the magnetic field Hx, from the viewpoint of increasing the amount of current obtained by rectification, the position of the insulating layer 102 constituting the diode is set so that the amplitude of the standing wave of the formed magnetic field Hx is It is preferable that it be located close to the position where it becomes large, preferably the position of the antinode. In other words, the position where the amplitude of a particular standing wave among the standing waves of the magnetic field Hx formed in the recess 105 is maximum and the position of the insulating layer 102 are approximately the same distance from the bottom surface 105a of the recess. It is preferable that the The specific standing wave to be aligned with the insulating layer 102 here is a constant harmonic of the second mode (n=2) that can be stably obtained regardless of the shape (depth) of the recess 105. It is preferable that there is a wave.

FIGS. 4(a) to 4(e) are diagrams showing a process flow for manufacturing the main part (around the laminated structure 104) of the photoelectric conversion device 100.

First, as shown in FIG. 4A, a first metal layer 101A having a uniform thickness is formed on one surface of the base material 106 using a known method such as sputtering.

Next, as shown in FIG. 4(b), an insulating layer 102A and a second metal layer 103A are sequentially formed on the exposed surfaces of the base material 106 and the first metal layer 101A. The insulating layer 102A is formed using an atomic layer deposition (ALD) method. The second metal layer 103A is formed using a known method such as sputtering.

Next, as shown in FIG. 4C, a resist film 108 having a two-layer structure sandwiching a metal layer 107 made of a metal such as Ti is formed for electron beam lithography. First, a first resist layer 109 is formed by applying a resist material. Subsequently, a metal layer 107 is formed by sputtering, and then a resist material is applied to form a second resist layer 110. By forming the resist film into such a two-layer structure, it is possible to prevent the resist film from collapsing during etching in a later step.

Next, as shown in FIG. 4D, the resist film 107 is exposed and etched to form a plurality of through holes 111 and partially expose the second metal layer 103A. The position where the through hole 111 is formed here is a position that overlaps with a recess that will be formed in the laminated structure in a later step.

Next, the recesses 105 and 112 that reach the inside of the first metal layer 101A from the exposed surface of the second metal layer 102A, and the recess 113 that reaches the inside of the base material 106 from the exposed surface of the second metal layer 103A, Formed by etching. A plurality of recesses 105 formed in the center of the region overlapping with the first metal layer 101 correspond to a microcavity that operates as an antenna. The end side of the recess 112 formed at the end of the region overlapping with the first metal layer 101 is designed to prevent conduction between the first metal layer 101 and the second metal layer 103 when a probe is inserted into the first metal layer 101. In addition, it becomes a groove for isolating the second metal layer 103 in the probe installation part and the microcavity part. On the other hand, the recess 113 formed in a region that does not overlap with the first metal layer 101 becomes a groove for isolating the microcavity from the surroundings. Finally, the photoelectric conversion device 100 can be obtained by removing the resist film 108 and the base material 106. Note that the base material 106 may be left as is, if necessary.

As described above, the photoelectric conversion device 100 of this embodiment has an aperture type antenna (cavity antenna) structure, and since the light incident path is not blocked by the electrode, it is possible to maintain a high optical coupling rate. I can do it. As a result, high power generation efficiency can be obtained. In addition, in the photoelectric conversion element 100 of this embodiment, an MIM diode is used, and each layer constituting the MIM diode can be formed individually in the order of lamination, so it can be easily manufactured using only a film forming process. can. Furthermore, since the space within the recess 105 is surrounded by side walls in all directions perpendicular to the stacking direction L, all polarized light incident on the recess 105 can be confined to form a standing wave, and this standing wave The amount of current induced by waves can be increased.

<Second embodiment>
FIG. 5 is a perspective view of a portion 200A of the photoelectric conversion device according to the second embodiment of the present invention. In the first embodiment, the recess 105 into which light enters is hole-shaped and surrounded by side walls in all directions in the xy plane. In contrast, the recess 105 of the present embodiment has a slit shape, and the space within the recess 105 is exposed in some directions (here, the y direction) among all directions perpendicular to the stacking direction L. There is. The other configurations are the same as the configuration of the photoelectric conversion device 100 of the first embodiment, and parts corresponding to the photoelectric conversion device 100 are indicated by the same reference numerals regardless of the difference in shape.

Here, a case is illustrated in which the width of the slit is constant in the x direction. In this case, the resistance of the electrodes connected to the first metal layer 101 and the second metal layer 102 can be uniformly designed, and the output efficiency is increased. Note that when the width of the slit is not constant in the x direction, light of various wavelengths corresponding to each width can be absorbed.

<Third embodiment>
FIG. 6 is a perspective view of a portion 300A of a photoelectric conversion device according to a third embodiment of the present invention. In this embodiment, the laminated structure 104 is stacked in multiple stages (here, three stages) in the stacking direction L. The recesses 105B and 105C of the second and subsequent layered structures 104B and 104C penetrate not only the second metal layer 103 and the insulating layer 102 but also the first metal layer 101, and the recesses 105A of the first layered structure 104A. It communicates with The laminated structures 104 adjacent to each other in the lamination direction D are electrically insulated from each other via the insulating layer 112 and the like. The other configurations are the same as the configuration of the photoelectric conversion device 100 of the first embodiment, and parts corresponding to the photoelectric conversion device 100 are indicated by the same reference numerals regardless of the difference in shape.

When the width of the recess (slit) 104 differs, the wavelength of electromagnetic waves that are easily absorbed differs. In this embodiment, it is possible to absorb a plurality of types (here, three types) of electromagnetic waves having different wavelengths, and it is possible to improve photoelectric conversion efficiency.

The output when operating the photoelectric conversion device of this embodiment is obtained by connecting electrodes to the first metal layer 101 of each stage, outputting from each electrode separately, and then adding up the output. Since the electrode resistance can be individually designed in each stage, the output efficiency is increased.

Hereinafter, the effects of the present invention will be made clearer by way of Examples. It should be noted that the present invention is not limited to the following examples, and can be implemented with appropriate changes without changing the gist thereof.

(Example 1)
Using the photoelectric conversion device according to the above embodiment, each recess (cavity) was made into a square prism, and a simulation was performed on the relationship between the depth of the recess and the resonant wavelength of electromagnetic waves. FIG. 7 is a graph showing simulation results. The horizontal axis of the graph represents the wavelength [μm] of the light incident on the recess, and the vertical axis of the graph represents the depth [μm] of the recess. The intensity of light absorption by the laminated structure (diode) is expressed by the intensity of the color.

The two regions with high absorption rates in the graph correspond to resonant wavelength regions corresponding to the first mode and the second mode in order of wavelength size. It can be seen that the occurrence of first-order mode resonance is limited to cases where the recess is relatively shallow, but the occurrence of second-order mode resonance does not depend on the depth of the recess.

(Example 2)
As an example of a photoelectric conversion device, in the case where the thickness of the concave portion of the first metal layer is 230 nm, the thickness of the insulating layer is 3 nm, the thickness of the second metal layer is 170 nm, and the pitch of the concave portions is 530 nm, the light absorption rate is We simulated the wavelength dependence of the magnetic field and the magnetic field strength distribution inside the recess.

FIG. 8 is a graph showing simulation results of wavelength dependence of absorption rate. The horizontal axis of the graph indicates the wavelength [μm] of light incident on the recess, and the vertical axis of the graph indicates the light absorption rate by the laminated structure. The peaks of the tertiary mode, the second mode, and the first mode are observed in order from the smallest wavelength. The peak of the second mode is higher than the peaks of other modes, and is preferable as a resonant mode from the viewpoint of increasing output. Further, the peak of the second-order mode has a narrow wavelength range, which is preferable because the error range when setting the resonance wavelength is narrow.

FIG. 9 is a graph showing simulation results of magnetic field strength distribution within the recess. The horizontal axis of the graph represents the width of the recess, and the vertical axis of the graph represents the depth of the recess. It can be seen that the magnetic field is locally strengthened near the insulating layer, increasing the amount of current induced in the laminated structure, making it possible to obtain high output.

Using the photoelectric conversion device according to the above embodiment, photoelectric conversion was performed on light irradiated from various light sources. Each component of the photoelectric conversion device was manufactured as follows.
・Depth of recess: 400 [nm]
・Width of recess: 320 [nm]
・Concave pitch: 530 [nm]
・Thickness of the recessed portion of the first metal layer: 230 [nm]
・Thickness of insulating layer: 3 [nm]
・Thickness of second metal layer: 170 [nm]

10 and 11 are graphs showing measurement results of output current and output voltage obtained by light irradiation in the photoelectric conversion device of Example 3. The horizontal axis of the graph indicates elapsed time [s], and the vertical axis of the graph indicates output current [A/m ² ] (FIG. 10) and output voltage [V] (FIG. 11). It can be seen that sufficient output is obtained in the time period (Irum) during which light is irradiated, regardless of which light source is used. Furthermore, both the rise and fall of the output are steep, indicating that the sensor has high sensitivity. It can be seen that in the time zone (Dark) when no light is irradiated, the output is almost suppressed, and leakage is sufficiently suppressed.

The conditions of the irradiation light used in Example 3 and the measurement results of the obtained output characteristics are summarized in Table 1. In a conventional structure that resonates with electromagnetic waves in the xy-plane direction, the efficiency obtained using sunlight is about 1.7×10 ⁻⁹ . Since the obtained efficiency is on the same level as this, it can be confirmed that the photoelectric conversion device of the present invention performs normal photoelectric conversion for any light. .

100...Photoelectric conversion device 100A, 200A, 300A...Part of structure of photoelectric conversion device 101, 101A...First metal layer 102, 102A...Insulating layer 103, 103A...Second Metal layers 104, 104A, 104B, 104C... Laminated structure 105, 105A, 105B, 105C... Recess 105a... Bottom surface of recess 106... Base material 107... Metal layer 108... Resist film 109...First resist layer 110...Second resist layer 111...Through holes 112, 113...Concave portion 114...Insulating layer 115...Lower electrode 116...Upper electrode L...・Stacking direction

Claims (5)

It has a laminated structure in which a first metal layer connected to the lower electrode , an insulating layer thick enough to allow only tunneling current to flow , and a second metal layer connected to the upper electrode are laminated in order,
The work function of the first metal layer is smaller than the work function of the second metal layer,
A recess formed in the laminated structure and consisting of a hole or slit-like space,
The recess communicates with an opening provided in the second metal layer on the outermost surface of the laminated structure through which light enters, and communicates with the opening, and the recess communicates with the opening provided in the second metal layer on the outermost surface of the laminated structure, and connects the second metal layer, the insulating layer, and the first metal layer . a side wall formed on the first metal layer, and a bottom surface formed on the first metal layer,
A photoelectric conversion device characterized in that, within the recess, a resonance wavelength of the light within the recess forms a standing wave of a magnetic field . 2. The method according to claim 1, wherein a position where the amplitude of a standing wave having the resonant wavelength formed in the recess is maximized and a position of the insulating layer are at the same distance from the bottom surface of the recess . The photoelectric conversion device described. 3. The photoelectric conversion device according to claim 2, wherein the resonant wavelength is a harmonic of a second mode. 4. The photoelectric conversion device according to claim 1, wherein the aperture is open. A multi-layer structure in which a first metal layer connected to a lower electrode, an insulating layer thick enough to allow only tunneling current to flow, and a second metal layer connected to an upper electrode are laminated in order. It has a laminated structure,
An inter-laminated structure insulating layer is provided between the laminated structures of the multi-layered laminated structure,
The work function of the first metal layer is smaller than the work function of the second metal layer,
A recess formed in the multi-layer stacked structure and consisting of a hole or slit-like space,
an aperture provided in the second metal layer on the outermost surface of the multi-layer stacked structure through which light enters; and an aperture that communicates with the aperture and is formed in the second metal layer, the insulating layer, and the first metal layer. the opening is narrower for each lower layer of the layered structure between the layered structures, and a bottom surface is formed in the first metal layer at the lowest layer of the multi-layer layered structure; ,
A photoelectric conversion device characterized in that the resonant wavelength of the light forms a standing wave of a magnetic field within the recess.

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