Imaging device

An imaging device includes pixels. Each of the pixels includes a counter electrode, a pixel electrode, and a photoelectric conversion layer that includes carbon nanotubes. The pixels include a first pixel and a second pixel adjacent to the first pixel. The pixel electrode of the first pixel and the pixel electrode of the second pixel are isolated from each other. Carbon nanotubes included in the photoelectric conversion layer in at least one selected from the group consisting of the first pixel and the second pixel include at least one first carbon nanotube that satisfies A<B, where A denotes length of a carbon nanotube in a direction in which the pixel electrode of the first pixel and the pixel electrode of the second pixel are arranged and B denotes length of a gap between the pixel electrode of the first pixel and the pixel electrode of the second pixel.

BACKGROUND

1. Technical Field

The present disclosure relates to an imaging device.

2. Description of the Related Art

During these years, multilayer imaging devices in which photoelectric conversion elements are provided on a semiconductor substrate are being achieved. In a multilayer imaging device, a photoelectric conversion layer of photoelectric conversion elements can be composed of a material different from one used for a semiconductor substrate. The photoelectric conversion layer, therefore, can be composed of an inorganic material or an organic material different from a conventional semiconductor material such as silicon. As a result, imaging devices having physical properties or functions different from those of conventional imaging devices, such as sensitivity in a wavelength band different from ones in the case of conventional imaging devices, have been achieved. For example, Japanese Unexamined Patent Application Publication No. 2003-234460 discloses an imaging device having a high level of use efficiency of light because of two or more photoelectric conversion layers that are included in the imaging device and that have sensitivity in different wavelength bands. Japanese Patent No. 6161018 discloses an imaging device including carbon nanotubes as a photoelectric conversion material in a photoelectric conversion layer.

SUMMARY

In one general aspect, the techniques disclosed here feature an imaging device including a plurality of pixels. Each of the plurality of pixels includes a counter electrode that passes incident light, a pixel electrode that faces the counter electrode, and a photoelectric conversion layer that is located between the counter electrode and the pixel electrode and that includes carbon nanotubes. The plurality of pixels include a first pixel and a second pixel adjacent to the first pixel. The pixel electrode of the first pixel and the pixel electrode of the second pixel are isolated from each other. The photoelectric conversion layer is continuously provided between the first pixel and the second pixel. The carbon nanotubes included in the photoelectric conversion layer in at least one selected from the group consisting of the first pixel and the second pixel include at least one first carbon nanotube that satisfies A<B, where A denotes length of one carbon nanotube of the carbon nanotubes in a direction in which the pixel electrode of the first pixel and the pixel electrode of the second pixel are arranged and B denotes length of a gap between the pixel electrode of the first pixel and the pixel electrode of the second pixel.

DETAILED DESCRIPTION

Underlying Knowledge Forming Basis of Aspect of Present Disclosure

As illustrated in a schematic cross-sectional view ofFIG.1, an imaging device that employs carbon nanotubes as a photoelectric conversion material for a photoelectric conversion layer includes a photoelectric conversion layer80including a long, slightly curved carbon nanotube81. When a large amount of charge X that can cause leak current, such as dark current, is generated in the photoelectric conversion layer80of a pixel70b, a pixel electrode11of the pixel70bcollects a large amount of charge, and a white color is output in an image as if bright light is incident. This phenomenon will be referred to as a “white” sport, and a pixel corresponding to a white spot will be referred to as a “white pixel” hereinafter. When the photoelectric conversion layer80does not include the carbon nanotube81, only the pixel70bis a white pixel. Since the carbon nanotube81is long as illustrated inFIG.1and the charge conductivity of the carbon nanotube81is high, however, the carbon nanotube81can undesirably introduce leak current based on charge generated in the pixel70binto an adjacent pixel70a, thereby causing a white spot in the pixel70a, too. Because, unlike in the case of normal signal charge, the amount of charge that causes a white pixel is large, improvements need to be made in order to prevent the introduction of leak current into the adjacent pixel70a, even if charge conductivity inside the pixel70bis increased and generated charge is collected in the pixel electrode11of the pixel70bor the like.

When a white pixel occurs, the white pixel can be corrected by detecting the white pixel and performing interpolation on the basis of outputs of adjacent pixels. When the photoelectric conversion layer80includes the carbon nanotube81as described above, however, white spots also occur in adjacent pixels also become white pixels, thereby spreading white pixels.

The present inventors have thus found that in the case of an imaging device including a photoelectric conversion layer that employs carbon nanotubes as a photoelectric conversion material, the quality of the imaging device decreases because the carbon nanotubes spread white pixels and make correction difficult. In the present disclosure, therefore, a high-quality imaging device that causes few white pixels even though the imaging device includes, above a silicon semiconductor substrate, a photoelectric conversion layer that employs carbon nanotubes as a photoelectric conversion material.

An outline of aspects of the present disclosure is as follows.

An imaging device according to an aspect of the present disclosure includes a plurality of pixels. Each of the plurality of pixels includes a counter electrode that passes incident light, a pixel electrode that faces the counter electrode, and a photoelectric conversion layer that is located between the counter electrode and the pixel electrode and that includes carbon nanotubes. The plurality of pixels include a first pixel and a second pixel adjacent to the first pixel. The pixel electrode of the first pixel and the pixel electrode of the second pixel are isolated from each other. The photoelectric conversion layer is continuously provided between the first pixel and the second pixel. The carbon nanotubes included in the photoelectric conversion layer in at least one selected from the group consisting of the first pixel and the second pixel include at least one first carbon nanotube that satisfies A<B, where A denotes length of one carbon nanotube of the carbon nanotubes in a direction in which the pixel electrode of the first pixel and the pixel electrode of the second pixel are arranged and B denotes length of a gap between the pixel electrode of the first pixel and the pixel electrode of the second pixel.

As a result, a photoelectric conversion layer in at least one of two adjacent pixels includes at least one carbon nanotube that satisfies A<B. That is, the photoelectric conversion layer includes a carbon nanotube that does not extend beyond a gap between pixel electrodes of the two adjacent pixels when viewed in plan. Even if a large amount of signal charge that can cause a white spot is generated due to a pixel defect or the like, the carbon nanotube does not introduce the large amount of signal charge into an adjacent pixel as leak current. Even if carbon nanotubes other than the foregoing carbon nanotube introduce leak current into the adjacent pixel, therefore, signal charge generated by the carbon nanotubes can be obtained on the basis of a difference in the amount of signal charge between an on state and an off state. As a result, spread of white pixels is suppressed by performing correction using signal charge obtained from the difference as signal charge for imaging. In addition, since the charge conductivity of carbon nanotubes is high, normal signal charge also moves through the carbon nanotubes, but since at least one carbon nanotube that satisfies A<B is included, signal charge is hardly introduced from an adjacent pixel. As a result, color mixing between two adjacent pixels is suppressed. A high-quality imaging device is thus achieved.

In addition, for example, number of pixels that include the photoelectric conversion layer including the at least one first carbon nanotube among the plurality of pixels may be 50% or more of total number of the plurality of pixels.

As a result, signal charge obtained by more than half the pixels on the basis of differences can be used as signal charge for imaging, and color mixing is also suppressed. A modulation transfer function (MTF), which indicates optical resolution of an imaging device, therefore, becomes 0.5 or higher. When the MTF is 0.5 or higher, straight lines can be recognized through image processing, for example, and a high-quality imaging device can be achieved.

In addition, for example, number of pixels that include the photoelectric conversion layer including the at least one first carbon nanotube among the plurality of pixels may be 80% or more of total number of the plurality of pixels.

As a result, signal charge obtained by 80% or more of the pixels on the basis of differences can be used as signal charge for imaging, and color mixing is also suppressed. The image quality of the imaging device, therefore, becomes high enough to be able to use the imaging device as a focal plane array.

In addition, for example, number of pixels that include the photoelectric conversion layer including the at least one first carbon nanotube among the plurality of pixels may be 90% or more of total number of the plurality of pixels.

As a result, signal charge obtained by 90% or more of the pixels on the basis of differences can be used as signal charge for imaging, and color mixing is also suppressed. The MTF, which indicates optical resolution of an imaging device, therefore, becomes 0.9 or higher. High resolution, therefore, is maintained without performing post-processing such as image processing, thereby achieving a higher-quality imaging device.

In addition, for example, the at least one first carbon nanotube may include a plurality of first carbon nanotubes, and percentage of the plurality of first carbon nanotubes in the carbon nanotubes included in the photoelectric conversion layer in all the plurality of pixels may be 50% or more.

As a result, when the photoelectric conversion layer includes the same number of carbon nanotubes in each of the pixels, the number of pixels that include the photoelectric conversion layer including at least one carbon nanotube that satisfies A<B becomes 50% or more of the pixels, thereby easily achieving the imaging device that produces the above-described effect.

In addition, for example, the at least one first carbon nanotube may include a plurality of first carbon nanotubes, and percentage of the plurality of first carbon nanotubes in the carbon nanotubes included in the photoelectric conversion layer in all the plurality of pixels may be 80% or more.

As a result, when the photoelectric conversion layer includes the same number of carbon nanotubes in each of the pixels, the number of pixels that include the photoelectric conversion layer including at least one carbon nanotube that satisfies A<B becomes 80% or more of the pixels, thereby easily achieving the imaging device that produces the above-described effect.

In addition, for example, the at least one first carbon nanotube may include a plurality of first carbon nanotubes, and percentage of the plurality of first carbon nanotubes in the carbon nanotubes included in the photoelectric conversion layer in all the plurality of pixels may be 90% or more.

As a result, when the photoelectric conversion layer includes the same number of carbon nanotubes in each of the pixels, the number of pixels that include the photoelectric conversion layer including at least one carbon nanotube that satisfies A<B becomes 90% or more of the pixels, thereby easily achieving the imaging device that produces the above-described effect.

In addition, for example, the photoelectric conversion layer may have a first part located over the pixel electrode of the first pixel in plan view and a second part located between the pixel electrode of the first pixel and the pixel electrode of the second pixel in plan view, the at least one first carbon nanotube may include first carbon nanotubes, and the second part may include more first carbon nanotubes than the first part.

As a result, even when the percentage of carbon nanotubes that satisfy A<B in the photoelectric conversion layer remains the same, carbon nanotubes that are not long enough to extend beyond a gap between pixel electrodes of adjacent pixels when viewed in plan tend to exist in the photoelectric conversion layer between the pixel electrodes of the adjacent pixels when viewed in plan. Spread of white pixels to adjacent pixels and color mixing between adjacent pixels, therefore, are suppressed more effectively.

In addition, for example, the carbon nanotubes included in the photoelectric conversion layer in at least one selected from the group consisting of the first pixel and the second pixel may include at least one second carbon nanotube that satisfies A<(B/2).

As a result, the photoelectric conversion layer in at least one of the pixels includes at least one carbon nanotube that satisfies A<(B/2). Even when regions within which corresponding pixel electrodes of adjacent pixels collect signal charge extend to the center of a gap between the pixel electrodes of the adjacent pixels, therefore, the photoelectric conversion layer includes a carbon nanotube that does not introduce leak current or the like from the regions within which the corresponding pixel electrodes of the adjacent pixels collect signal charge. Consequently, spread of white pixels and color mixing between adjacent pixels are suppressed.

In addition, for example, the carbon nanotubes included in the photoelectric conversion layer in at least one selected from the group consisting of the first pixel and the second pixel may include at least one third carbon nanotube that satisfies C<B, where a direct distance between two farthest points on one carbon nanotube of the carbon nanotubes is denoted by C.

As a result, the photoelectric conversion layer includes at least one carbon nanotube that does not extend across two adjacent pixels regardless of a direction in which carbon nanotubes are oriented in the photoelectric conversion layer. Spread of white pixels to adjacent pixels and color mixing between adjacent pixels, therefore, are suppressed, thereby easily achieving a high-quality imaging device.

In addition, for example, the carbon nanotubes included in the photoelectric conversion layer in at least one selected from the group consisting of the first pixel and the second pixel may include at least one fourth carbon nanotube that satisfies C<(B/2).

As a result, the photoelectric conversion layer in at least one of the pixels includes at least one carbon nanotube that satisfies C<(B/2). Even when regions within which corresponding pixel electrodes of adjacent pixels collect signal charge extend to the center of a gap between the pixel electrodes of the adjacent pixels, therefore, the photoelectric conversion layer includes a carbon nanotube that does not introduce leak current or the like from the regions within which the corresponding pixel electrodes of the adjacent pixels collect signal charge, regardless of a direction in which carbon nanotubes are oriented in the photoelectric conversion. Consequently, spread of white pixels and color mixing between adjacent pixels are suppressed, thereby easily achieving a high-quality imaging device.

In addition, for example, an imaging device according to another aspect of the present disclosure includes pixels and a barrier. Each of the pixels includes a counter electrode that passes incident light, a pixel electrode that faces the counter electrode, and a photoelectric conversion layer that is located between the counter electrode and the pixel electrode and that includes carbon nanotubes. The pixels include a first pixel and a second pixel adjacent to the first pixel. The pixel electrode of the first pixel and the pixel electrode of the second pixel are isolated from each other. The barrier is arranged in the photoelectric conversion layer and located between the pixels in plan view. The barrier has a first portion and a second portion adjacent to the first portion with the pixel electrode disposed between the first portion and the second portion. The carbon nanotubes included in the photoelectric conversion layer in one of the pixels include at least one carbon nanotube that satisfies A<D, where A denotes length of one carbon nanotube of the carbon nanotubes in a direction in which the pixel electrode of the first pixel and the pixel electrode of the second pixel are arranged and D denotes length of a gap between the first portion of the barrier and the second portion of the barrier. The imaging device may include a plurality of barriers.

As a result, since the photoelectric conversion layer includes a barrier that sandwiches the pixel electrode, movement of signal charge between adjacent pixels is suppressed, thereby suppressing color mixing between the adjacent pixels. In addition, since carbon nanotubes are shorter than a gap between two portions of the barrier, the carbon nanotubes hardly stick on upper surface of the barrier, thereby promoting an effect of preventing color mixing. In addition, the carbon nanotubes hardly get over the barrier, which improves the flatness of the upper surface of the barrier. The flatness of a counter electrode and the like formed over the barrier also improves, thereby reducing irregularity in sensitivity. A high-quality imaging device, therefore, is achieved.

Embodiments of the present disclosure will be described hereinafter with reference to the drawings. The present disclosure is not limited to the following embodiments. The following embodiments may be modified as necessary without deviating from a scope within which advantageous effects of the present disclosure are produced. Furthermore, an embodiment may be combined with another embodiment. In the following description, the same or similar components will be given the same reference numerals, and redundant description thereof might be omitted.

Terms indicating relationships between elements, such as “equal to each other”, terms indicating shapes of elements, such as “square” and “circle”, and ranges of values herein are not exact expressions but approximate expressions that can include errors of, say, several percent.

Terms “above” and “below” herein do not refer to upward (vertically above) and downward (vertically below) in absolute spatial recognition but are defined by relative positional relationships based on order of stacking of layers in a multilayer structure. The terms “above” and “below” will be used not only when two components are arranged in proximity to each other with another component provided therebetween but also when two components are arranged in contact with each other.

First Embodiment

Circuit Configuration of Imaging Device

First, the circuit configuration of an imaging device according to a first embodiment will be described with reference toFIG.2.

FIG.2is a schematic diagram illustrating an exemplary circuit configuration of the imaging device according to the present embodiment. An imaging device100illustrated inFIG.2includes a pixel array PA including pixels10arranged in two dimensions.FIG.2schematically illustrates an example in which the pixels10are arranged in a 2×2 matrix. The number and arrangement of pixels10of the imaging device100are not limited to the example illustrated inFIG.2. For example, the imaging device100may be a line sensor in which pixels10are arranged in a line, instead.

Each of the pixels10includes a photoelectric conversion unit13and a signal detection circuit14. As described later with reference to the drawings, the photoelectric conversion unit13includes a photoelectric conversion layer sandwiched between two electrodes facing each other. The photoelectric conversion unit13receives incident light and generates a signal. The photoelectric conversion unit13need not be an independent element for each of the pixels10, and a part of the photoelectric conversion unit13, for example, may be shared by two or more pixels10. The signal detection circuit14detects a signal generated by the photoelectric conversion unit13. In this example, the signal detection circuit14includes a signal detection transistor24and an address transistor26. The signal detection transistor24and the address transistor26are typically field-effect transistors (FETs). Here, n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) are used as an example of the signal detection transistor24and the address transistor26. Transistors such as the signal detection transistor24, the address transistor26, and a reset transistor28, which will be described later, each include a control terminal, an input terminal, and an output terminal. The control terminal is, for example, a gate. The input terminal is either a drain or a source, for example, and may be, for example, the drain. The output terminal is another of the drain and the source and may be, for example, the source.

As schematically illustrated inFIG.2, the control terminal of the signal detection transistor24is electrically connected to the photoelectric conversion unit13. Signal charge generated by the photoelectric conversion unit13is accumulated in a charge accumulation node41between the gate of the signal detection transistor24and the photoelectric conversion unit13. Here, the signal charge is holes or electrons. The charge accumulation node41is an example of a charge accumulation unit and also called a “floating diffusion node”. The charge accumulation node41will also be referred to as a “charge accumulation region” herein. Details of the structure of the photoelectric conversion unit13will be described later.

The photoelectric conversion unit13of each of the pixels10is also connected to a counter electrode12. The counter electrode12is connected to a voltage supply circuit32. The voltage supply circuit32is also called a “counter electrode supply circuit”. The voltage supply circuit32is configured to be able to supply a variable voltage. During the operation of the imaging device100, the voltage supply circuit32supplies a certain voltage to the photoelectric conversion unit13via the counter electrode12. The voltage supply circuit32is not limited to a certain power supply circuit. The voltage supply circuit32may be a circuit that generates the certain voltage or a circuit that converts a voltage supplied from another power supply into the certain voltage, instead.

The voltage supplied from the voltage supply circuit32to the photoelectric conversion unit13switches between different voltages to control a start and an end of accumulation of signal charge from the photoelectric conversion unit13to the charge accumulation node41. Alternatively, this function can be achieved by controlling a voltage of the charge accumulation node41or a voltage of a pixel electrode, which will be described later. In other words, in the present embodiment, an operation of an electronic shutter is performed by switching the voltage supplied from the voltage supply circuit32to the photoelectric conversion unit13or an initial voltage of the charge accumulation node41or the pixel electrode. An example of the operation of the imaging device100will be described later. In the configuration illustrated inFIG.2, the charge accumulation node41and the pixel electrode are connected to each other and have the same potential.

The pixels10are each connected to a power supply line40used to supply a power supply voltage VDD. As illustrated inFIG.2, the input terminal of the signal detection transistor24is connected to the power supply line40. When the power supply line40functions as a source follower power supply, the signal detection transistor24amplifies a signal generated by the photoelectric conversion unit13and outputs the amplified signal.

The input terminal of the address transistor26is connected to the output terminal of the signal detection transistor24. The output terminal of the address transistor26is connected to one of vertical signal lines47provided for columns of the pixel array PA in one-to-one correspondence. The control terminal of the address transistor26is connected to an address control line46. An output of the signal detection transistor24can be selectively read by a corresponding vertical signal line47by controlling potential of the address control line46.

In the example illustrated inFIG.2, the address control line46is connected to a vertical scanning circuit36. The vertical scanning circuit36is also called a “row scanning circuit”. The vertical scanning circuit36applies a certain voltage to the address control lines46to select, in units of rows, the pixels10arranged in each of rows. As a result, reading of signals from the selected pixels10and resetting of the charge accumulation nodes41are performed.

The vertical signal lines47are main signal lines for transferring pixel signals from the pixel array PA to peripheral circuits. Column signal processing circuits37are connected to the vertical signal lines47. The column signal processing circuits37are also called “row signal accumulation circuits”. The column signal processing circuits37perform, for example, noise suppression signal processing, which is typified by correlated double sampling, and analog-to-digital (A/D) conversion. As illustrated inFIG.2, the column signal processing circuits37are provided for the columns of the pixels10in one-to-one correspondence. A horizontal signal reading circuit38is connected to the column signal processing circuits37. The horizontal signal reading circuit38is also called a “column scanning circuit”. The horizontal signal reading circuit38sequentially reads signals from the column signal processing circuits37and outputs the signals to a horizontal common signal line49.

The pixels10each include a reset transistor28. As with the signal detection transistor24and the address transistor26, for example, the reset transistor28is an FET. In the following description, an example in which an n-channel MOSFET is used as the reset transistor28will be described unless otherwise specified. As illustrated inFIG.2, the reset transistor28is connected between a reset voltage line44for supplying a reset voltage Vr and the charge accumulation node41. The control terminal of the reset transistor28is connected to a reset control line48. Potential of the charge accumulation node41can be reset to the reset voltage Vr by controlling potential of the reset control line48. In this example, the reset control line48is connected to the vertical scanning circuit36. The pixels10arranged in each of the rows, therefore, can be reset in units of rows by applying a certain voltage to the reset control lines48using the vertical scanning circuit36.

In this example, the reset voltage line44for supplying the reset voltage Vr to the reset transistor28is connected to a reset voltage source34. The reset voltage source34is also called a “reset voltage supply circuit”. The reset voltage source34may be configured to be able to supply the certain reset voltage Vr to the reset voltage line44during the operation of the imaging device100and, as with the voltage supply circuit32, is not limited to a certain power supply circuit. The voltage supply circuit32and the reset voltage source34may be parts of the same voltage supply circuit or may be separate voltage supply circuits. One or both of the voltage supply circuit32and the reset voltage source34may be a part of the vertical scanning circuit36. Alternatively, a counter electrode voltage from the voltage supply circuit32and/or the reset voltage Vr from the reset voltage source34may be supplied to the pixels10through the vertical scanning circuit36.

The power supply voltage VDD of the signal detection circuit14may be used as the reset voltage Vr, instead. In this case, a voltage supply circuit (not illustrated inFIG.2) that supplies a power supply voltage to the pixels10and the reset voltage source34can be integrated with each other. Because the power supply line40and the reset voltage line44can also be integrated with each other, wiring in the pixel array PA can be simplified. When the reset voltage Vr is different from the power supply voltage VDD supplied by the signal detection circuit14, however, the imaging device100can be controlled more flexibly.

Cross-Sectional Structure of Pixels

Next, the cross-sectional structure of the pixels10of the imaging device100according to the present embodiment will be described with reference toFIG.3.

FIG.3is a schematic diagram illustrating a cross-sectional structure of two adjacent of the pixels10illustrated inFIG.2. The two adjacent pixels10illustrated inFIG.3have the same structure. One of the two adjacent pixels10will be described hereinafter. The two adjacent pixels10may have partly different structures. In the structure illustrated inFIG.3, the signal detection transistor24, the address transistor26, and the reset transistor28are formed on a semiconductor substrate20. The semiconductor substrate20is not limited to a substrate entirely composed of a semiconductor. The semiconductor substrate20may be an insulating substrate for which a semiconductor layer is provided on a surface on a side where photosensitive region is formed, instead. An example in which a p-type silicon (Si) substrate is used as the semiconductor substrate20will be described hereinafter.

The semiconductor substrate20includes impurity regions26s,24s,24d,28d, and28sand element isolation regions20tfor electrically isolating the pixels10with one another. The impurity regions26s,24s,24d,28d, and28sare n-type regions. An element isolation region20tis also provided between the impurity regions24dand28d. The element isolation regions20tare formed, for example, by implanting an acceptor through ion implantation under certain implantation conditions.

The impurity regions26s,24s,24d,28d, and28sare, for example, an impurity diffusion layer formed in the semiconductor substrate20. As schematically illustrated inFIG.3, the signal detection transistor24includes the impurity regions24sand24dand a gate electrode24g. The gate electrode24gis composed of a conductive material. The conductive material is, for example, polysilicon that has conductivity as a result of impurity doping, but may be a metal material, instead. The impurity regions24sand24dfunction, for example, as a source region and a drain region of the signal detection transistor24, respectively. A channel region of the signal detection transistor24is formed between the impurity regions24sand24d.

Similarly, the address transistor26includes impurity regions26sand24sand a gate electrode26gconnected to one of the address control lines46. The gate electrode26gis composed of a conductive material. The conductive material is, for example, polysilicon that has conductivity as a result of impurity doping, but may be a metal material, instead. In this example, the signal detection transistor24and the address transistor26are electrically connected to each other by sharing the impurity region24s. The impurity region24sfunctions, for example, as a drain region of the address transistor26. The impurity region26sfunctions, for example, as a source region of the address transistor26. The impurity region26sis connected to one of the vertical signal lines47, which are not illustrated inFIG.3. The impurity region24sneed not be shared by the signal detection transistor24and the address transistor26. More specifically, the source region of the signal detection transistor24and the drain region of the address transistor26may be isolated from each other on the semiconductor substrate20and electrically connected to each other via wiring layers56provided in an interlayer insulation layer50.

The reset transistor28includes impurity regions28dand28sand a gate electrode28gconnected to one of the reset control lines48. The gate electrode28gis composed, for example, of a conductive material. The conductive material is, for example, polysilicon that has conductivity as a result of impurity doping, but may be a metal material, instead. The impurity region28sfunctions, for example, as a source region of the reset transistor28. The impurity region28sis connected to one of the reset voltage lines44, which are not illustrated inFIG.3. The impurity region28dfunctions, for example, as a drain region of the reset transistor28.

The interlayer insulation layer50is provided on the semiconductor substrate20in such a way as to cover the signal detection transistor24, the address transistor26, and the reset transistor28. The interlayer insulation layer50is an example of a first insulation layer. The interlayer insulation layer50is composed, for example, of an insulating material such as silicon dioxide. As illustrated inFIG.3, the wiring layers56are provided in the interlayer insulation layer50. The wiring layers56are typically composed of a metal such as copper. The wiring layers56may include, for example, signal lines such as the vertical signal lines47or power supply lines as a part thereof. The number of insulation layers in the interlayer insulation layer50and the number of layers included in the wiring layers56provided in the interlayer insulation layer50may be set as desired and are not limited to the example illustrated inFIG.3.

As illustrated inFIG.3, a plug52, a wire53, and contact plugs54and55are also provided in the interlayer insulation layer50. The wire53may be a part of the wiring layers56. The plug52, the wire53, and the contact plugs54and55are each composed of a conductive material. The plug52and the wire53, for example, are composed of a metal such as copper. The contact plugs54and55, for example, are composed of polysilicon that has conductivity as a result of impurity doping. The plug52, the wire53, and the contact plugs54and55may be composed of the same material or different materials.

The plug52, the wire53, and the contact plug54constitute at least a part of the charge accumulation node41between the signal detection transistor24and the photoelectric conversion unit13. In the structure illustrated inFIG.3, the gate electrode24gof the signal detection transistor24, the plug52, the wire53, the contact plugs54and55, and the impurity region28d, which is either the source region or the drain region of the reset transistor28function as a charge accumulation region for accumulating signal charge collected by a pixel electrode11of the photoelectric conversion unit13on the interlayer insulation layer50.

More specifically, the pixel electrode11of the photoelectric conversion unit13is connected to the gate electrode24gof the signal detection transistor24via the plug52, the wire53, and the contact plug54. In other words, the gate of the signal detection transistor24is electrically connected to the pixel electrode11. The pixel electrode11is also connected to the impurity region28dvia the plug52, the wire53, and the contact plug55.

As the pixel electrode11collects signal charge, a voltage according to the amount of signal charge accumulated in the charge accumulation region is applied to the gate of the signal detection transistor24. The signal detection transistor24amplifies the voltage. The voltage amplified by the signal detection transistor24is selectively read by the address transistor26as signal voltage.

The above-described photoelectric conversion unit13is arranged on the interlayer insulation layer50. In other words, in the present embodiment, the pixels10constituting the pixel array PA illustrated inFIG.2are formed in and on the semiconductor substrate20. The pixels10arranged in two dimensions when the semiconductor substrate20is viewed as a plan form a photosensitive region. The photosensitive region is also called a “pixel region”. A distance between two adjacent pixels10illustrated inFIG.3, that is, pixel pitch, may be, say, about 2 μm.

In a color filter layer19provided on the counter electrode12illustrated inFIG.3, not only color filters that each achieve a transmittance corresponding to the wavelength of red, green, or blue visible light as in conventional imaging devices but also a bandpass filter or a longpass filter for ultraviolet light or near-infrared light may be used.

Configuration of Photoelectric Conversion Unit

A specific configuration of the photoelectric conversion unit13on the interlayer insulation layer50will be described hereinafter.

The photoelectric conversion unit13includes the pixel electrode11and a photoelectric conversion layer15provided between the pixel electrode11and the counter electrode12. Furthermore, as illustrated inFIG.3, an electron blocking layer16, the photoelectric conversion layer15, and an acceptor layer17are stacked in this order between the counter electrode12and the pixel electrode11of the photoelectric conversion unit13from a side of the pixel electrode11. In this example, the counter electrode12, the photoelectric conversion layer15, the electron blocking layer16, and the acceptor layer17are formed across two adjacent pixels10. The counter electrode12, the photoelectric conversion layer15, the electron blocking layer16, and the acceptor layer17may further extend to other pixels10. The pixel electrode11is provided for each of two adjacent pixels10. The pixel electrodes11of two adjacent pixels10are spatially isolated from each other so that the pixel electrodes11are electrically isolated from each other. The same holds for pixel electrodes11of other pixels11that are not illustrated inFIG.3, that is, the pixel electrode11is provided for each of the pixels10. At least the counter electrode12, the photoelectric conversion layer15, the electron blocking layer16, or the acceptor layer17may be separately provided for each of the pixels10.

The pixel electrode11is an electrode for reading signal charge generated by the photoelectric conversion unit13. There is at least one pixel electrode11for each of the pixels10. The pixel electrode11is electrically connected to the gate electrode24gof the signal detection transistor24and the impurity region28d.

The pixel electrode11is composed of a conductive material. The conductive material is, for example, a metal such as aluminum or copper, a metal nitride, or polysilicon that has conductivity as a result of impurity doping.

The counter electrode12is, for example, a transparent electrode composed of a transparent conductive material. The counter electrode12is arranged on a side of the photoelectric conversion layer15where light is incident. Light that has passed through the counter electrode12, therefore, is incident on the photoelectric conversion layer15. Light detected by the imaging device100is not limited to light within a wavelength range of visible light. For example, the imaging device100may detect infrared light or ultraviolet light. The wavelength range of visible light is, for example, 380 nm to 780 nm.

A term “transparent” herein means that an object passes at least a part of light within a wavelength range to be detected and that an object need not pass light over the entirety of the wavelength range of visible light. A term “light” herein refers to electromagnetic waves in general including infrared light and ultraviolet light.

As described with reference toFIG.2, the counter electrode12is connected to the voltage supply circuit32. As illustrated inFIG.3, the counter electrode12is formed across two adjacent pixels10. The voltage supply circuit32, therefore, can collectively apply a desired counter electrode voltage to the two adjacent pixels10via the counter electrode12. The counter electrode12may also extend to pixels10that are not illustrated inFIG.3. The counter electrode12may be separately provided for each of the two adjacent pixels10and the other pixels10that are not illustrated inFIG.3, instead, insofar as the voltage supply circuit32can apply a desired counter electrode voltage.

When the voltage supply circuit32controls potential of the counter electrode12against potential of the pixel electrode11, the pixel electrode11can collect, as signal charge, either holes or electrons of hole-electron pairs generated in the photoelectric conversion layer15as a result of photoelectric conversion. When holes are used as signal charge, for example, the pixel electrode11can selectively collect the holes by making the potential of the counter electrode12higher than that of the pixel electrode11. A case where holes are used as signal charge will be described hereinafter. Electrons may be used as signal charge, instead. In this case, the potential of the counter electrode12may be made lower than that of the pixel electrode11. When an appropriate bias voltage is applied between the counter electrode12and the pixel electrode11, the pixel electrode11, which faces the counter electrode12, collects either positive or negative charge generated in the photoelectric conversion layer15as a result of photoelectric conversion.

The photoelectric conversion layer15receives incident light and generates hole-electron pairs. The photoelectric conversion layer15includes carbon nanotubes. The carbon nanotubes absorb light incident on the photoelectric conversion layer15within a certain wavelength range and generates hole-electron pairs. Chirality of the carbon nanotubes for achieving wavelength selectivity may be different between the pixels10or the same in all the pixels10.

With a structure in which the photoelectric conversion layer15is stacked above a circuit substrate, as in the case of the imaging device100according to the present embodiment, the photoelectric conversion unit13can be composed of a material different from Si or the like, which is used for the circuit substrate, unlike in the case of a complementary metal-oxide-semiconductor (CMOS) image sensor. An effect of achieving imaging that does not depend on wavelength characteristics of the circuit substrate can be produced.

The electron blocking layer16has a function of suppressing movement of electrons, which are opposite the signal charge, from an adjacent pixel electrode11to the photoelectric conversion layer15while transporting holes, which are the signal charge generated in the photoelectric conversion layer15, to the electrode. As a result, dark current is suppressed in the imaging device100. A material of the electron blocking layer16is a p-type semiconductor, for example, and, more specifically, may be a semiconductor composed of an inorganic material such as a nickel oxide, a copper oxide, a chromium oxide, a cobalt oxide, a titanium oxide, or a zinc oxide, but is not limited to these. The p-type semiconductor may be composed of an inorganic material obtained by doping a metal oxide or a metal nitride with an impurity, instead. More specifically, the p-type semiconductor may be, for example, a film obtained by doping a silicon oxide with phosphorus, arsenic, antimony, or the like, instead. The material of the electron blocking layer16may be a semiconductor composed of an organic material such as a hole-transporting organic compound.

The acceptor layer17has a function of receiving electrons, which are opposite the signal charge, from the photoelectric conversion layer15and transporting the electrons to the counter electrode12. A material of the acceptor layer17is, for example, a fullerene or a fullerene derivative.

The photoelectric conversion layer15and the acceptor layer17are in planar heterojunction. Electrons are extracted at an interface between the photoelectric conversion layer15and the acceptor layer17, and holes remaining in the photoelectric conversion layer15are collected on a side of the pixel electrode11because of the applied voltage.

In the present embodiment, a structure has been described in which holes are accumulated from the photoelectric conversion layer15and read as signal charge and the electron blocking layer16for reducing dark current is provided. When electrons are accumulated and read as signal charge, on the other hand, a hole blocking layer may be used instead of the electron blocking layer16, and a donor layer may be used instead of the acceptor layer17. The hole blocking layer has a function of suppressing movement of holes from an adjacent pixel electrode11to the photoelectric conversion layer15while transporting electrons generated in the photoelectric conversion layer15to the electrode. The donor layer has a function of receiving holes from the photoelectric conversion layer15and transporting the holes to the counter electrode12.

The photoelectric conversion unit13according to the present embodiment need not include the electron blocking layer16and the acceptor layer17. If light incident on the photoelectric conversion layer15generates hole-electron pairs inside the carbon nanotubes and generated signal charge moves in a direction perpendicular to a circumferential direction of the carbon nanotubes, that is, a longitudinal direction of the carbon nanotubes, before the pixel electrode11stacked under the photoelectric conversion layer15collects the signal charge, color mixing with an adjacent pixel or a decrease in resolution can occur. This effect is significant especially when the acceptor layer17is not provided between the photoelectric conversion layer15and the pixel electrode11. Because the length of the carbon nanotubes is limited in the present embodiment, details of which will be described later, color mixing with an adjacent pixel and a decrease in resolution can be suppressed, even if generated signal charge moves through the carbon nanotubes in the longitudinal direction of the carbon nanotubes.

Although a planar heterojunction structure, where the photoelectric conversion layer15is isolated from the acceptor layer17, is employed in the example illustrated inFIG.3, a photoelectric conversion layer having a bulk heterojunction structure, where the material of the photoelectric conversion layer15and the material of the acceptor layer17are mixed, may be employed, instead. When the bulk heterojunction structure is employed, charge extraction efficiency increases, which improves sensitivity.

Furthermore, inFIG.3, a hole blocking layer, which restricts movement of holes, which are signal charge, may be provided under the counter electrode12in order to reduce dark current caused by hole injection from the counter electrode12.

Next, details of the carbon nanotubes included in the photoelectric conversion layer15according to the present embodiment will be described.

FIG.4is a schematic diagram illustrating a cross-sectional structure of a photoelectric conversion unit13aof the imaging device100according to the present embodiment. BecauseFIG.4is intended for description of a carbon nanotube, only the photoelectric conversion layer15is provided between the counter electrode12and pixel electrodes11aand11b. A donor layer or an acceptor layer for extracting charge from the photoelectric conversion layer15and an electron blocking layer or a hole blocking layer for reducing dark current caused by injection of charge opposite signal charge from the pixel electrode11are not illustrated.FIG.4also illustrates a part of the interlayer insulation layer50. Although following schematic diagrams illustrating cross-sectional structures of a photoelectric conversion unit do not illustrate components other than part of the photoelectric conversion unit and an interlayer insulation layer, the components other than the photoelectric conversion unit are the same as those of each of the pixel10illustrated inFIG.3.

FIG.4illustrates two adjacent pixels10aand10b. The pixel10ais an example of a first pixel, and the pixel10bis an example of a second pixel.

As illustrated inFIG.4, the two adjacent pixels10aand10binclude a counter electrode12that passes incident light, pixel electrodes11aand11bfacing the counter electrode12, and a photoelectric conversion layer15sandwiched between the counter electrode12and the pixel electrodes11aand11b. More specifically, the counter electrode12and the photoelectric conversion layer15are formed across the two adjacent pixels10aand10b.

Each of the two adjacent pixels10aand10balso includes the pixel electrodes11aand11b, respectively. The interlayer insulation layer50is embedded between the pixel electrodes11aand11b. That is, the pixel electrodes11aand11bof the two adjacent pixels10aand10bare isolated from each other by an insulating material of the interlayer insulation layer50.FIG.4illustrates a case where the pixel electrode11aof the pixel10acollects signal charge within a collection region10a1and the pixel electrode11bof the pixel10bcollects signal charge within a collection region10b1.

The photoelectric conversion layer15in the pixels10aand10binclude carbon nanotubes.FIG.4illustrates only one60aof the carbon nanotubes and does not illustrate other carbon nanotubes. For example, carbon nanotubes may exist the photoelectric conversion layer15right above the pixel electrodes11aand11b.

When there is a pixel defect Y in the photoelectric conversion layer15in the pixel10b, the pixel defect Y can be a source of charge such as dark current of room temperature thermal excitation that can cause a white spot. As illustrated inFIG.4, length A of at least one carbon nanotube60ain the photoelectric conversion layer15is, when viewed in plan, smaller than a length B of a gap between the pixel electrode11aof the pixel10aand the pixel electrode11bof the pixel10bin the imaging device100according to the present embodiment in order not to introduce leak current, such as dark current, caused by charge generated in the pixel defect Y into the pixel10a. That is, the carbon nanotubes included in the photoelectric conversion layer15in the pixel10ainclude at least one carbon nanotube that satisfies the length A<the length B. Since the carbon nanotube that satisfies the length A<the length B is short, the carbon nanotube does not extend beyond the length B of the gap between the pixel electrodes11aand11bwhen viewed in plan. The carbon nanotubes included in the photoelectric conversion layer15in the pixel10aare carbon nanotubes that are at least partly included in the photoelectric conversion layer15in the pixel10a.

The carbon nanotubes included in the photoelectric conversion layer15in the pixel10b, too, may include at least one carbon nanotube that satisfies the length A<the length B.

Because a white spot occurs as a result of generation of a large amount of charge, it is difficult to prevent introduction of leak current into the adjacent pixel10aby collecting generated charge in the pixel electrode11bor the like. The pixel10aaccording to the present embodiment, however, includes, in the photoelectric conversion layer15, at least one carbon nanotube that satisfies the length A<the length B. Even if leak current, which can cause a white spot, is introduced from the adjacent pixel10bbecause carbon nanotubes other than the foregoing carbon nanotube do not satisfy the length A<the length B, therefore, signal charge generated by the carbon nanotubes can be obtained on the basis of differences in the amount of signal charge between an on state and an off state. Even if the adjacent pixel10bis a white pixel, an increase in the number of white pixels can be suppressed by performing correction using the signal charge obtained from the difference as signal charge for imaging. That is, since the carbon nanotubes included in the photoelectric conversion layer15in at least either the two adjacent pixels10aand10binclude at least one carbon nanotube that satisfies the length A<the length B, spread of white pixels can be suppressed, thereby achieving a high-quality imaging device with few white pixels.

Because charge conductivity of carbon nanotubes is high, normal signal charge move through the carbon nanotubes. Since the carbon nanotubes in the photoelectric conversion layer15in the pixel10ainclude at least one carbon nanotube that satisfies the length A<the length B, however, signal charge is hardly introduced from the adjacent pixel10b. Color mixing between the adjacent pixels10aand10bcan therefore be suppressed.

Since, as described above, leak current generated in the pixel10bis not introduced into the adjacent pixel10ain the imaging device100according to the present embodiment, white spots do not spread from the pixel10bto adjacent pixels such as the pixel10a. In addition, introduction of signal charge generated in the pixel10binto the adjacent pixel10aand the like can also be suppressed, that is, a decrease in resolution and color mixing can be suppressed.

The number of pixels that include the photoelectric conversion layer15including at least one carbon nanotube that satisfies the length A<the length B may be 50% or more of all the pixels10. In this case, signal charge obtained by more than half the pixels10on the basis of differences can be used as signal charge for imaging, and color mixing is suppressed. An MTF, which indicates optical resolution of an imaging device, therefore, becomes 0.5 or higher. The MTF is an index that falls within a range of 0 to 1. The higher the MTF, the higher the optical resolution. When the MTF is 0.5 or higher, straight lines can be recognized through image processing, for example, thereby achieving a high-quality imaging device can be achieved.

Alternatively, the number of pixels that include the photoelectric conversion layer15including at least one carbon nanotube that satisfies the length A<the length B may be 80% or more of all the pixels10. In this case, signal charge obtained by more than 80% of the pixels10on the basis of differences can be used as signal charge for imaging, and color mixing is suppressed. The image quality of the imaging device100, therefore, becomes high enough to be able to use the imaging device100as a focal plane array.

Alternatively, the number of pixels that include the photoelectric conversion layer15including at least one carbon nanotube that satisfies the length A<the length B may be 90% or more of all the pixels10. In this case, signal charge obtained by more than 90% of the pixels10on the basis of differences can be used as signal charge for imaging, and color mixing is suppressed. The MTF, which indicates the resolution of an imaging device, becomes 0.9 or higher. As a result, high resolution is maintained without performing post-processing such as image processing, thereby achieving a higher-quality imaging.

The percentage of carbon nanotubes that satisfy the length A<the length B in the photoelectric conversion layer15in all the pixels10may be 50% or more. The percentage refers to a ratio of the number of carbon nanotubes that satisfy the length A<the length B to the total number of carbon nanotubes. In this case, when the photoelectric conversion layer15includes substantially the same number of carbon nanotubes in each of the pixels10, the number of pixels in which the photoelectric conversion layer15includes at least one carbon nanotube that satisfies the length A<the length B becomes 50% or more of the total number of pixels10, thereby easily an imaging device that produces the above effect. Furthermore, when the carbon nanotubes are uniformly dispersed in the photoelectric conversion layer15, the percentage of pixels including at least one carbon nanotube that satisfies the length A<the length B is expected to increase further, thereby achieving a high-quality imaging device.

Alternatively, the percentage of carbon nanotubes that satisfy the length A<the length B in the photoelectric conversion layer15in all the pixels10may be 80% or more. In this case, when the photoelectric conversion layer15includes substantially the same number of carbon nanotubes in each of the pixels10, the number of pixels in which the photoelectric conversion layer15includes at least one carbon nanotube that satisfies the length A<the length B becomes 80% or more of the total number of pixels10, thereby easily achieving an imaging device that produces the above advantageous effect. Furthermore, when the carbon nanotubes are uniformly dispersed in the photoelectric conversion layer15, the percentage of pixels including at least one carbon nanotube that satisfies the length A<the length B is expected to increase further, thereby achieving a high-quality imaging device.

Alternatively, the percentage of carbon nanotubes that satisfy the length A<the length B in the photoelectric conversion layer15in all the pixels10may be 90% or more. In this case, when the photoelectric conversion layer15includes substantially the same number of carbon nanotubes in each of the pixels10, the number of pixels in which the photoelectric conversion layer15includes at least one carbon nanotube that satisfies the length A<the length B becomes 90% or more of the total number of pixels10, thereby easily achieving an imaging device that produces the above advantageous effect. Furthermore, when the carbon nanotubes are uniformly dispersed in the photoelectric conversion layer15, the percentage of pixels including at least one carbon nanotube that satisfies the length A<the length B is expected to increase further, thereby achieving a high-quality imaging device.

A second part of the photoelectric conversion layer15located between the pixel electrode11aof the pixel10aand the pixel electrode11bof the pixel10bwhen viewed in plan may include more carbon nanotubes that satisfy the length A<the length B than a first part of the photoelectric conversion layer15located over the pixel electrode11aof the pixel10awhen viewed in plan. In this case, even when the percentage of carbon nanotubes that satisfy the length A<the length B in the photoelectric conversion layer15remains the same, carbon nanotubes having the length A shorter than the length B tend to exist in the photoelectric conversion layer15between the pixel electrode11aof the pixel10aand the pixel electrode11bof the pixel10bwhen viewed in plan. Spread of white pixels to adjacent pixels and color mixing between adjacent pixels, therefore, can be further suppressed. By forming the photoelectric conversion layer15using inks including carbon nanotubes having different lengths, for example, the carbon nanotubes having different lengths can be provided for the photoelectric conversion layer15over the pixel electrode11awhen viewed in plan and the photoelectric conversion layer15between the pixel electrodes11aand11bwhen viewed in plan.

The length of carbon nanotubes will be described hereinafter.FIGS.5A to5Dare schematic diagrams illustrating the length of carbon nanotubes according to the present embodiment. The length A of a carbon nanotube60herein does not refer to an effective length between two ends of the carbon nanotube60but, as illustrated inFIG.5A, refers to the length of the carbon nanotube60in an arrangement direction, which is a direction in which pixel electrodes11cand11dare arranged when viewed in plan. In an example illustrated inFIG.5A, an arrangement direction of the pixel electrodes11cand11dand a longitudinal direction of the carbon nanotube60are the same, and a direct distance between the two ends of the carbon nanotube60and the length A are the same. When the longitudinal direction of the carbon nanotube60is inclined with respect to the arrangement direction of the pixel electrodes11cand11das illustrated inFIG.5B, the length A is shorter than the direct distance between the two ends of the carbon nanotube60.

The photoelectric conversion layer15according to the present embodiment is fabricated, for example, by applying ink including carbon nanotubes to an upper surface of the pixel electrode11and the like. The length A of the carbon nanotubes may be achieved by adjusting the effective length of the carbon nanotubes included in the ink used to fabricate the photoelectric conversion layer15or, as illustrated inFIGS.5A and5B, by controlling the longitudinal direction of the carbon nanotubes with respect to the arrangement direction of pixel electrodes in the photoelectric conversion layer15without adjusting the effective length of the carbon nanotubes. Although the length A changes depending on the longitudinal direction of the carbon nanotube60when viewed in plan inFIGS.5A and5B, the length A decreases, too, when viewed in cross section as the longitudinal direction of the carbon nanotube60is inclined more steeply with respect to the arrangement direction of the pixel electrodes.

The inclination of the carbon nanotubes in the photoelectric conversion layer15with respect to an arrangement direction of pixel electrodes can be controlled using various methods. For example, the inclination can be simply controlled on the basis of spin coating speed or viscosity at a time when the ink including the carbon nanotubes is applied. More specifically, when the spin coating speed is high when the ink including the carbon nanotubes is dropped onto the upper surface of the pixel electrode11and the like, the longitudinal direction of the carbon nanotubes tends to be perpendicular to a stacking direction. When the viscosity of the ink including the carbon nanotubes is high or the spin coating speed is low, on the other hand, the longitudinal direction of the carbon nanotubes tends to align with the stacking direction. Whether the spin coating speed is high or low is determined in relation to a reference state on the basis of the viscosity of the ink including the carbon nanotubes and a target film thickness.

The effective length of the carbon nanotubes may be adjusted by selecting available carbon nanotubes having a target length or by obtaining longer carbon nanotubes and then cutting the carbon nanotubes through stirring or the like. Because common carbon nanotubes are a mixture of carbon nanotubes of various lengths, carbon nanotubes having a target length may be extracted through filtration or the like. Alternatively, carbon nanotubes having a target length may be synthesized using a known method such as microplasma chemical vapor deposition (CVD), a carbon penetration method, or surface decomposition of SiC.

In the imaging device100according to the present embodiment, not the length A of the carbon nanotube60but, as illustrated inFIG.5C, a direct distance C between two points on the carbon nanotube60spatially farthest from each other may be shorter than the length B inFIG.4. That is, the carbon nanotubes included in the photoelectric conversion layer15in the pixel10aillustrated inFIG.4may include at least one carbon nanotube that satisfies the direct distance C<the length B. In this case, the photoelectric conversion layer15includes at least one carbon nanotube that does not extend across two adjacent pixels regardless of a direction in which the carbon nanotubes are oriented in the photoelectric conversion layer15. Spread of white pixels to adjacent pixels and color mixing between adjacent pixels, therefore, are suppressed, thereby easily achieving a high-quality imaging device.

In an example illustrated inFIG.5C, a direct distance between the two ends of the carbon nanotube60and the direct distance C between the two spatially farthest points are the same. When a carbon nanotube61has a curved shape where two ends are coming closer to each other as illustrated inFIG.5D, on the other hand, the direct distance C between two spatially farthest points is longer than a distance between the two ends of the carbon nanotube61.

Next, the length B of a gap between two adjacent pixels will be described.FIG.6Ais a diagram illustrating a planar layout of pixel electrodes of the imaging device100according to the present embodiment. As illustrated inFIG.6A, pixel electrodes11e1to11e9are arranged in a matrix. When pixel electrodes are arranged in a matrix, a length B1of a gap between pixel electrodes of two pixels adjacent to each other in a horizontal or vertical direction, such as pixel electrodes11e5and11e6, is the smallest. A length B2of a gap between pixel electrodes of two pixels adjacent to each other at an angle of 45 degrees, such as pixel electrodes11e4and11e8, is greater than the length B1. In the case of arrangement of pixel electrodes where the length of a gap varies depending on positions, such as in the case of the pixel electrodes11e4and11e8of two pixels adjacent to each other at an angle of 45 degrees, a “gap” refers to a gap between closest points on the two adjacent pixel electrodes.

The length B of the gap that contributes to reduction in color mixing and the like illustrated inFIG.4may be the length B1of the gap between the pixel electrodes11e5and11e6, which are arranged adjacent to each other in the horizontal direction illustrated inFIG.6A, or the length B2of the gap between the pixel electrodes11e4and11e8, which are arranged adjacent to each other at an angle of 45 degrees illustrated inFIG.6A, depending on a position at which the carbon nanotubes are provided.

Furthermore, when optical filters such as color filters, bandpass filters, or longpass filters are provided above the pixel electrodes11e5and11e6, respectively, and optical characteristics of the optical filters provided above the pixel electrodes11e5and11e6are the same, color mixing is reduced compared to when optical filters having different optical characteristics are provided above adjacent pixel electrodes, even if the length A of carbon nanotubes is greater than the length B1of the gap between the pixel electrodes11e5and11e6. When optical filters having different optical characteristics are provided above pixel electrodes, on the other hand, the length A of carbon nanotubes is desirably set in accordance with the length of a gap between the pixel electrodes of pixels for which the optical filters having different optical characteristics are provided. When optical filters having different optical characteristics are provided above the pixel electrodes11e4and11e8, for example, the length A of carbon nanotubes may be set in accordance with the length B2of the gap between the pixel electrodes11e4and11e8, which are adjacent to each other at an angle of 45 degrees.

A planar shape of pixel electrodes is not particularly limited. For example, pixel electrodes may be circles or regular polygons such as regular hexagons or regular octagons. The arrangement of pixel electrodes is not particularly limited, either. For example, pixel electrodes may be arranged in a diagonal direction.

FIGS.6B,6C, and6Dare diagrams illustrating examples of the planar layout of pixel electrodes for describing the lengths of gaps between the pixel electrodes. When pixel electrodes111ahaving a planar shape of a regular octagon are arranged in a matrix as illustrated inFIG.6B, for example, a length B4of a gap between pixel electrodes111aadjacent to each other at an angle of 45 degrees is greater than a length B3of a gap between pixel electrodes111aadjacent to each other in the horizontal direction. When pixel electrodes111bhaving a planar shape of a regular octagon are arranged in such a way as to form lines at an angle of 45 degrees as illustrated inFIG.6C, for example, a length B6of a gap between pixel electrodes111badjacent to each other in the horizontal direction is greater than a length B5of a gap between pixel electrodes111badjacent to each other at an angle of 45 degrees. When larger pixel electrodes111L and smaller pixel electrodes111S having a planar shape of a regular octagon are alternately arranged in a matrix as illustrated inFIG.6D, for example, a length B8of a gap between pixel electrodes111L adjacent to each other at an angle of 45 degrees is greater than a length B7of a gap between pixel electrodes111L and111S adjacent to each other in the horizontal direction.

As illustrated inFIGS.6A to6D, at least one of the lengths81to B8in directions in which pixel electrodes are arranged is thus used as the length of gaps between pixel electrodes. A length in any arrangement direction may be used in accordance with a position at which carbon nanotubes are provided.

Next, an imaging device in another example of the present embodiment will be described.FIGS.7A and7Bare schematic diagrams illustrating cross-sectional structures of photoelectric conversion units of the imaging device in the other example of the present embodiment.FIG.7Aillustrates a photoelectric conversion unit13bformed for two adjacent pixels10cand10d.FIG.7Billustrates a photoelectric conversion unit13cformed for two adjacent pixels10eand10f.FIGS.7A and7Balso illustrate a part of the interlayer insulation layer50. The photoelectric conversion units13band13cillustrated inFIGS.7A and78, respectively, are different from the photoelectric conversion unit13aillustrated inFIG.4in that collection regions, in which pixel electrodes collect signal charge, are larger. The pixels10cand10eare other examples of the first pixel, and the pixels10dand10fare other examples of the second pixel.FIGS.7A and7Billustrate only carbon nanotubes60band60c, respectively, and do not illustrate other carbon nanotubes.

The size of regions in which pixel electrodes collect signal charge generated in a photoelectric conversion layer can be adjusted to a certain degree with electric fields generated by voltages applied between a counter electrode and the pixel electrodes. In the case of the pixels10cand10dillustrated inFIG.7A, a pixel electrode11fof the pixel10ccollects signal charge in a collection region10c1, and a pixel electrode11gof the pixel10dcollects signal charge in a collection region10d1. In the photoelectric conversion unit13b, a boundary between the collection regions10c1and10d1is a middle point of the length B of a gap between the pixel electrodes11fand11g. At this time, reset voltages, which are initial potentials of the pixel electrodes11fand11g, are assumed to be the same.

In order not to introduce, into the adjacent pixel10c, leak current caused by charge generated at a pixel defect Y above the pixel electrode11gof the pixel10d, therefore, carbon nanotubes included in the photoelectric conversion layer15in the pixel10cdesirably include, as illustrated inFIG.7A, at least one carbon nanotube60bwhose length A is smaller than half (B/2) the length B of the gap between the pixel electrodes11fand11g. In this case, since the photoelectric conversion layer15in the pixel10cincludes the carbon nanotube60bthat does not introduce leak current or the like into the photoelectric conversion layer15in the pixel10cfrom the photoelectric conversion layer15in the adjacent pixel10dover the pixel electrode11g, spread of white pixels and color mixing between adjacent pixels are suppressed. The carbon nanotubes included in the photoelectric conversion layer15in the pixel10cmay include at least one carbon nanotube whose direct distance C is smaller than half (B/2) the length B of the gap between the pixel electrodes11fand11g.

In order to increase an allowable length A of carbon nanotubes, the size of pixel electrodes11hand11imay, as illustrated inFIG.7B, be made smaller than that of the pixel electrodes11fand11gillustrated inFIG.7A. In general, when pixel electrodes in a multilayer imaging device are reduced in size, collection regions for signal charge are also reduced in size, which causes a decrease in sensitivity. In the case of the multilayer imaging device including carbon nanotubes in a photoelectric conversion layer, however, the collection regions for signal charge can be made larger than the pixel electrodes because the carbon nanotubes are long. As illustrated inFIG.78, therefore, the length A of a carbon nanotube60cmay be greater than the width of the pixel electrodes11hand11i. The length A of the carbon nanotube60cmay be smaller than half (B/2) the length B of a gap between the pixel electrodes11hand11i. The width of the pixel electrodes11hand11imay be smaller than the length B of the gap between the pixel electrodes11hand11ior smaller than half (B/2) the length B of the gap between the pixel electrodes11hand11i.

Next, another effect due to the length of carbon nanotubes will be described.FIGS.8A and8Bare schematic cross-sectional diagrams for describing the flatness of a photoelectric conversion layer.FIG.8Aillustrates a case where a carbon nanotube81aincluded in a photoelectric conversion layer80ais relatively long.FIG.8Billustrates a case where a carbon nanotube60dincluded in the photoelectric conversion layer15is relatively short. When an imaging device that includes a photoelectric conversion layer including carbon nanotubes is fabricated, the density of carbon nanotubes in the photoelectric conversion layer may be increased by applying ink including carbon nanotubes two or more times or two or more layers in order to increase sensitivity. Since the carbon nanotubes dispersed in a solvent are rigid and long, a surface of the photoelectric conversion layer80aincluding, as illustrated inFIG.8A, the carbon nanotube81athat is long with respect to the size of the pixel electrode11and the gap, that is, the carbon nanotube81athat satisfies the length A>the length B, is undulating after a drying process. The cycle of undulations on the surface of the photoelectric conversion layer80ais larger than pixel pitch, which can result in irregular sensitivity, that is, different levels of sensitivity are observed in different pixels. As illustrated inFIG.8B, on the other hand, when the photoelectric conversion layer15includes a carbon nanotube60dwhose length A is smaller than the length B of a gap between adjacent pixel electrodes11, the cycle of undulations on a surface of the photoelectric conversion layer15falls below the pixel pitch. As a result, irregularity in sensitivity between pixels can be reduced. Furthermore, when a direct distance C between two farthest points on the carbon nanotube60dis smaller than the length B of the gap between adjacent pixel electrodes11, the size of the undulations on the surface of the photoelectric conversion layer15is also reduced, thereby making it easier to stack a counter electrode and the like on the photoelectric conversion layer15flatly.

Second Embodiment

Next, a second embodiment will be described. The second embodiment is different from the first embodiment in that a photoelectric conversion layer includes a barrier. Differences from the first embodiment will be mainly described hereinafter, and description of common points is omitted or simplified.

First, an imaging device in a first comparative example for describing the second embodiment will be described.FIG.9is a schematic diagram illustrating a cross-sectional structure of a photoelectric conversion unit90of the imaging device in the first comparative example.FIG.9illustrates two adjacent pixels70cand70d. InFIG.9, a photoelectric conversion layer80band the counter electrode12are isolated from each other.

As illustrated inFIG.9, the pixels70cand70dinclude the counter electrode12that passes incident light, pixel electrodes11jand11kthat face the counter electrode12, and the photoelectric conversion layer80bsandwiched between the counter electrode12and the pixel electrodes11jand11k. The pixels70cand70dinclude separate pixel electrodes11jand11k, respectively.

The photoelectric conversion layer80bincludes carbon nanotubes.FIG.9only illustrates only one81bof the carbon nanotubes and does not illustrate other carbon nanotubes.

A barrier18is provided in the photoelectric conversion layer80bbetween, when viewed in plan, the pixels70cand70dand other pixels that are not illustrated inFIG.9. The barrier18has portions that sandwich pixel electrodes such as the pixel electrodes11jand11k. The length A of a carbon nanotube81bis greater than a length D of a gap between a first portion of a barrier18on the left of the pixel electrode11jand a second portion of a barrier18on the right of the pixel electrode11j. A plurality of barriers18may be provided in the photoelectric conversion layer80b.

When a barrier18is provided for each pixel in order to avoid color mixing optically or electrically and the length A of the carbon nanotube81bis greater than the length D of the gap between, when viewed in plan, the first portion of the barrier18on the left of the pixel electrode11jand the second portion of the barrier18on the right of the pixel electrode11j, the carbon nanotube81bmight get over the barrier18as illustrated inFIG.9, thereby impairing the flatness of an upper surface of the barrier18. In addition, the carbon nanotube81bmight stick on the upper surface of the barrier18, thereby connecting the pixels11jand11kto each other and reducing an effect of preventing color mixing.

Next, the imaging apparatus according to the present embodiment will be described with reference toFIG.10.FIG.10is a schematic diagram illustrating a cross-sectional structure of a photoelectric conversion unit13dof the imaging device according to the present embodiment.FIG.10also illustrates a part of the interlayer insulation layer50.FIG.10illustrates two adjacent pixels10gand10h.

As illustrated inFIG.10, the pixels10gand10hinclude the counter electrode12that passes incident light, pixel electrodes11mand11nthat face the counter electrode12, respectively, and a photoelectric conversion layer15asandwiched between the counter electrode12and the pixel electrodes11mand11n. The counter electrode12and the photoelectric conversion layer15aare formed across the two adjacent pixels10gand10h.

The pixels10gand10hinclude the separate pixel electrodes11mand11n, respectively. The interlayer insulation layer50is embedded between the two separate pixel electrodes11mand11n. That is, the pixel electrodes11mand11nof the two adjacent pixels10gand10hare isolated from each other by the insulating material of the interlayer insulation layer50.

The photoelectric conversion layer15ain the pixels10gand10hincludes carbon nanotubes.FIG.10illustrates only one60eof the carbon nanotubes and does not illustrate other carbon nanotubes.

A barrier18is provided in the photoelectric conversion layer15abetween, when viewed in plan, the pixels10gand10hand other pixels that are not illustrated inFIG.10. The barrier18has portions that sandwich pixel electrodes such as the pixel electrodes11mand11n. A length D of a gap between a first portion of the barrier18on the left of the pixel electrode11mand a second portion of the barrier18on the right of the pixel electrode11mis smaller than the length A of a carbon nanotube60eincluded in the photoelectric conversion layer15ain the pixel10g. That is, the carbon nanotubes included in the photoelectric conversion layer15ain the pixel10ginclude the carbon nanotube60ethat satisfies the length A<the length D. The carbon nanotubes included in the photoelectric conversion layer15ain the pixel10h, too, may include the carbon nanotube60ethat satisfies the length A<the length D. A plurality of barriers18may be provided in the photoelectric conversion layer15a.

The barrier18is composed of a material whose resistance is higher than that of the photoelectric conversion layer15aand whose charge conductivity is lower than that of the photoelectric conversion layer15a. As a result, movement of signal charge between the adjacent pixels10gand10his suppressed, thereby suppressing color mixing between the adjacent pixels10gand10h. The material of the barrier18may be a material whose resistivity is higher than that of the photoelectric conversion layer15aincluding the carbon nanotubes. An insulating material such as SiO2, AlO, or SiN is used for the barrier18. The material of the barrier18is not limited to an insulating material insofar as the resistance of the material is higher than that of the photoelectric conversion layer15a. One of various materials, therefore, may be selected in accordance with desired physical properties. When a flattening process such as chemical mechanical polishing (CMP) is provided after formation of the barrier18and the photoelectric conversion layer15a, for example, the hardness of the barrier18and the photoelectric conversion layer15aaffects flatness. A material having an appropriate level of hardness, therefore, may be used for the barrier18.

When a barrier18is provided for each pixel in order to avoid color mixing optically or electrically and the length A of the carbon nanotube60eis shorter than the length D of the gap between, when viewed in plan, the first portion of the barrier18on the left of the pixel electrode11mand the second portion of the barrier18on the right of the pixel electrode11m, the carbon nanotube60ehardly sticks on an upper surface of the barrier18, thereby promoting the effect of preventing color mixing. In addition, the carbon nanotube60ehardly gets over the barrier18, thereby improving the flatness of upper surface of the barrier18. As a result, the flatness of a layer formed over the photoelectric conversion layer15a, that is, for example, the counter electrode12, and evenness in the thickness of the layer improve. As a result of the improved flatness of the counter electrode12, the thickness of the photoelectric conversion unit13dat points in the pixels at which voltages are applied from the counter electrode12to the pixel electrodes11mand11nbecomes even, thereby reducing irregularity in sensitivity. When the thickness of the counter electrode12varies between pixels, the resistance of the counter electrode12also varies. By making the thickness of the counter electrode12even, variation in a time constant is reduced, variation between imaging devices is reduced, and yields improve when the potential of the counter electrode12is changed in accordance with the operation of the imaging devices.

Although the photoelectric conversion layer15ais formed even over the barrier18inFIG.10, the photoelectric conversion layer15aneed not exist over the barrier18and the counter electrode12may be directly stacked on the barrier18, instead. In addition, although the length D of the gap between the first portion of the barrier18on the left of the pixel electrode11mand the second portion of the barrier18on the right of the pixel electrode11mand the width of the pixel electrode11mare the same inFIG.10, the length D and the width of the pixel electrode11mmay be different from each other, instead. By making the length D greater than the width of the pixel electrode11m, for example, functioning regions in the photoelectric conversion layer15aare increased in size, thereby improving sensitivity and increasing the allowable length A of the carbon nanotube60e. Although the length A of the carbon nanotube60eis greater than the length of a gap between the pixel electrodes11mand11ninFIG.10, the length A of the carbon nanotube60emay be smaller than the length of the gap between the pixel electrodes11mand11n, instead.

As in the first embodiment, the number of pixels that include the photoelectric conversion layer15aincluding at least one carbon nanotube that satisfies the length A<the length D may be 50% or more, 80% or more, or 90% or more of the total number of pixels.

In addition, the percentage of carbon nanotubes that satisfy the length A<the length D may be 50% or more, 80% or more, or 90% or more of all the carbon nanotubes included in the photoelectric conversion layer15ain all the pixels.

Third Embodiment

Next, a third embodiment will be described.

FIG.11is a block diagram illustrating the structure of a camera system600according to the present embodiment. The camera system600includes a lens optical system601, an imaging device602, a system controller603, and a camera signal processing unit604.

The lens optical system601includes, for example, an autofocus lens, a zoom lens, and a diaphragm. The lens optical system601focuses light onto an imaging surface of the imaging device602. Light that has passed the lens optical system601enters the photoelectric conversion unit13and is subjected to photoelectric conversion, thereby generating signal charge. A reading circuit30reads the signal charge and outputs an imaging signal. The imaging device602is the imaging device according to the first or second embodiment. The reading circuit30includes, for example, the circuits illustrated inFIG.2.

The system controller603controls the entirety of the camera system600. The system controller603can be achieved, for example, by a microcomputer.

The camera signal processing unit604functions as a signal processing circuit that processes signals output from the imaging device602. The camera signal processing unit604performs processing such as gamma correction, color interpolation, spatial interpolation, auto white balance, distance measurement calculation, and wavelength information separation. The camera signal processing unit604is achieved, for example, by a digital signal processor (DSP).

With the camera system600according to the present embodiment, a high-quality imaging device can be achieved using the imaging device according to the first or second embodiment.

Other Embodiments

Although an imaging device according to one or more aspects have been described on the basis of some embodiments, the present disclosure is not limited to these embodiments. The scope of the present disclosure also includes modes obtained by modifying the embodiments in various ways conceivable by those skilled in the art and modes constructed by combining together components from different embodiments, insofar as the spirit of the present disclosure is not deviated from.

For example, the imaging device may also include pixels including a photoelectric conversion layer that does not include carbon nanotubes. In the imaging device, pixels that include a photoelectric conversion layer including carbon nanotubes and the pixels that include the photoelectric conversion layer that does not include carbon nanotubes may be arranged.

In addition, for example, a photoelectric conversion layer may include, as well as carbon nanotubes, another photoelectric conversion material such as an organic semiconductor and a semiconductor polymer for improving the dispersibility of the carbon nanotubes.

The imaging device in the present disclosure can be used for various camera systems and sensor systems such as medical cameras, monitoring cameras, vehicle cameras, distance measuring cameras, microscope cameras, drone cameras, and robot cameras.