WO2023193280A1

WO2023193280A1 - Image sensor and electronic device including same

Info

Publication number: WO2023193280A1
Application number: PCT/CN2022/085979
Authority: WO
Inventors: Hidekazu Takahashi
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2022-04-08
Filing date: 2022-04-08
Publication date: 2023-10-12
Anticipated expiration: 2024-10-08
Also published as: CN118556290A

Abstract

An embodiment of the present application provides an image sensor including a quantum dot (QD) region configured to perform photoelectric conversion in short wavelength infrared (SWIR) range. The QD region has a stack of two or more QD layers. The two or more QD layers may include a first QD layer that contains first quantum dots and a second QD layer that contains second quantum dots, and a band gap of the first quantum dots may differ from a band gap of the second quantum dots. Alternatively, or additionally, the two or more QD layers may be doped differently from each other so as to generate a built-in electric field in the QD region for drifting photogenerated carriers out of the QD region.

Description

IMAGE SENSOR AND ELECTRONIC DEVICE INCLUDING SAME

TECHNICAL FIELD

The present application relates generally to image sensors, and more particularly to image sensors applicable to imaging in the Short Wavelength Infrared (SWIR) range.

BACKGROUND

Image sensors for SWIR imaging require a semiconductor material with a narrower band gap than silicon (Si) . As such, III-V compound semiconductors with high quantum efficiency in the IR range, such as InGaAs, have been used in high-performance cameras that require a high sensitivity. However, III-V compound semiconductor wafers are expensive and such a wafer generally needs to be mechanically and electrically bonded to a silicon read-out integrated circuit (ROIC) wafer. Consequently, image sensors using a III-V semiconductor are costly and have limitations in reducing the pixel size and increasing the resolution due to, for example, the size of bumps required for wafer bonding or some restrictions of hybrid bonding. Moreover, because the dark current is high at the room temperature, a cooling device need to be attached to the image sensor in order to reduce noise, and thus it may be difficult to miniaturize a camera with such an image sensor incorporated.

A SWIR sensor using quantum dots (QDs) has been proposed as being capable of solving at least some of the aforementioned problems with the traditional SWIR image sensors. Quantum dots utilize a spatial quantum confinement effect, thereby enabling the band gap to be controlled according to dot size. In addition, quantum dots can be deposited on the Si-ROIC wafer by using, for example, a spin coating process. Furthermore, in recent years, some QD materials have been developed that are comparable in quantum efficiency to compound semiconductors. However, in quantum dot image sensors, dark current may increase as the band gap becomes narrower to absorb longer wavelengths. Quantum dot image sensors may also require a relatively high drive voltage for high speed operation. Such a high bias voltage can increase power consumption and hence heat generation, leading to a further increase of the dark current.

Therefore, a technical solution that can avoid or mitigate the aforementioned problems in conventional SWIR image sensors is desired.

SUMMARY

An object of embodiments of the present application is to provide a QD image sensor for imaging in the SWIR range that can reduce the dark current and/or reduce the drive voltage. The embodiments of the present application further provide an electronic device including such a QD image sensor.

According to a first aspect, an image sensor is provided. The image sensor includes a QD region configured to perform photoelectric conversion in the SWIR range and the QD region has a stack of two or more QD layers. The two or more QD layers include a first QD layer that contains first quantum dots and a second QD layer that contains second quantum dots, wherein a band gap of the first quantum dots differs from a band gap of the second quantum dots.

In a possible implementation of the first aspect, the two or more QD layers may include the first QD layer and the second QD layer in order from a light-incident side of the QD region, and wherein the band gap of the first quantum dots may be wider than the band gap of the second quantum dots.

In a possible implementation of the first aspect, the second quantum dots may have a higher impurity concentration than the first quantum dots.

In a possible implementation of the first aspect, the second quantum dots may be P-type quantum dots and the first quantum dots may be P ^--type, intrinsic, or N-type quantum dots.

In a possible implementation of the first aspect, the two or more QD layers may include the first QD layer, the second QD layer, and a third QD layer in order from a light-incident side of the QD region, and wherein the band gap of the first quantum dots may be wider than the band gap of the second quantum dots, and the band gap of the second quantum dots may be wider than a band gap of third quantum dots contained in the third QD layer.

In a possible implementation of the first aspect, the second quantum dots may have a higher impurity concentration than the first quantum dots and the third quantum dots may have a higher impurity concentration than the second quantum dots. In one example, the third quantum dots are P-type quantum dots, the second quantum dots are P ^--type quantum dots, and the first quantum dots are intrinsic quantum dots. In another example, the third quantum dots are N-type quantum dots, the second quantum dots are N ^--type quantum dots, and the first quantum dots are intrinsic quantum dots.

In a possible implementation of the first aspect, the third quantum dots may be P-type quantum dots, the second quantum dots may be intrinsic quantum dots, and the first quantum dots may be N-type quantum dots.

In a possible implementation of the first aspect, the two or more QD layers may have a total thickness of 100 nm to 1000 nm inclusive. Respective thicknesses of the two or more QD layers may be different from one another.

In a possible implementation of the first aspect, the QD region may be a thin-film region deposited on a read-out integrated circuit (ROIC) .

According to a second aspect, an image sensor is provided. The image sensor includes a QD region configured to perform photoelectric conversion in the SWIR range and the QD region has a stack of two or more QD layers. The two or more QD layers are doped differently from each other so as to generate a built-in electric field in the QD region for drifting photogenerated carriers out of the QD region.

In a possible implementation of the second aspect, the two or more QD layers may be doped such that a lower end level (E _C) of a conduction band and an upper end level (E _V) of a valence band under thermal equilibrium both increase stepwise or both decrease stepwise throughout thickness of the QD region in a depth direction.

In a possible implementation of the second aspect, the two or more QD layers may include a first QD layer and a second QD layer in order from a light-incident side of the QD region, and wherein the second QD layer may contain P-type quantum dots and the first QD layer may contain intrinsic or N-type quantum dots.

In a possible implementation of the second aspect, the two or more QD layers may include a first QD layer, a second QD layer, and a third QD layer in order from a light-incident side of the QD region, and wherein the first, second, and third QD layers each may contain P-type or intrinsic quantum dots.

In a possible implementation of the second aspect, the two or more QD layers may include a first QD layer, a second QD layer, and a third QD layer in order from a light-incident side of the QD region, and wherein the first, second, and third QD layers each may contain N-type or intrinsic quantum dots.

In a possible implementation of the second aspect, the two or more QD layers may include a first QD layer, a second QD layer, and a third QD layer in order from a light-incident side of the QD region, and wherein one of the first and third QD layers may contain P-type quantum dots and the other of the first and third QD layers may contain N-type quantum dots. In one example, the second QD layer may contain intrinsic quantum dots.

According to a third aspect, an electronic device is provided. The electronic device includes the image sensor according to any one of the first aspect and the possible implementations thereof and the second aspect and the possible implementations thereof.

In a possible implementation of the third aspect, the electronic device includes no cooling devices configured to cool the image sensor.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating an image sensor according to embodiments of the present application.

FIG. 2 is a circuit diagram schematically illustrating a single pixel of the image sensor shown in FIG. 1.

FIG. 3A is a cross-sectional view schematically illustrating a QD photodiode of an image sensor according to a first embodiment of the present application.

FIG. 3B is a schematic diagram illustrating a potential profile at non-biasing for the QD photodiode shown in FIG. 3A.

FIG. 3C is a schematic diagram illustrating a potential profile at biasing for the QD photodiode shown in FIG. 3A.

FIG. 4A is a cross-sectional view schematically illustrating a QD photodiode of an image sensor according to a related art.

FIG. 4B is a schematic diagram illustrating a potential profile at non-biasing for the QD photodiode shown in FIG. 4A.

FIG. 4C is a schematic diagram illustrating a potential profile at biasing for the QD photodiode shown in FIG. 4A.

FIG. 5A is a cross-sectional view schematically illustrating a QD photodiode of an image sensor according to a second embodiment of the present application.

FIG. 5B is a schematic diagram illustrating a potential profile at non-biasing for the QD photodiode shown in FIG. 5A.

FIG. 6A is a cross-sectional view schematically illustrating a QD photodiode of an image sensor according to a third embodiment of the present application.

FIG. 6B is a schematic diagram illustrating a potential profile at non-biasing for the QD photodiode shown in FIG. 6A.

FIG. 7A is a cross-sectional view schematically illustrating a QD photodiode of an image sensor according to a fourth embodiment of the present application.

FIG. 7B is a schematic diagram illustrating a potential profile at non-biasing for the QD photodiode shown in FIG. 7A.

FIG. 8A is a cross-sectional view schematically illustrating a QD photodiode of an image sensor according to a fifth embodiment of the present application.

FIG. 8B is a schematic diagram illustrating a potential profile at non-biasing for the QD photodiode shown in FIG. 8A.

FIG. 9 is a schematic diagram of an electronic device according to an embodiment of the present application.

Throughout the drawings, same or similar elements are indicated by same or similar reference numerals.

DESCRIPTION OF EMBODIMENTS

To enable any person skilled in the art to better understand objectives, features, and advantages of embodiments of the present application, the following further describes the technical solutions in preferable embodiments of the present application in detail with reference to the accompanying drawings.

In the present application, the terms “include” , “comprise” , “have” and any other variants mean to cover the non-exclusive inclusion, for example, a process, method, device, or system that includes a list of steps or elements is not necessarily limited to those steps or elements, but may include other steps or elements not expressly listed or inherent to such a process, method, device, or system. Moreover, the articles “a” and “an” as used in the present application are intended to include one or more items, and may be used interchangeably with “one or more” .

FIG. 1 is a cross-sectional view schematically illustrating an image sensor 100 according to embodiments of the present application. The image sensor 100 includes a read-out integrated circuit (ROIC) 110 and a QD photodiode 150 formed on the ROIC 110. The ROIC 110 generally includes a silicon substrate 120 on or in which circuit elements (not shown in FIG. 1) , such as transistors and capacitors, are formed, and thus it may be referred to herein as Si-ROIC 110. The Si-ROIC 110 also includes a metallization layer 130 formed on the silicon substrate 120. The metallization layer 130 has a wiring structure 135 embedded in a dielectric material 132. The wiring structure 135 may include a number of traces, vias, contact plugs, and the like. The wiring structure 135 can connect the underlying circuit elements to each other and connect the QD photodiode 150 to the circuit elements.

The image sensor 100 also includes a plurality of pixels arranged in an array. A pixel electrode 140 corresponding to each pixel may be formed as a portion of the wiring structure 135 and exposed at a top surface of the ROIC 110. Alternatively, the pixel electrode 140 may be additionally formed on the top of the ROIC 110 to contact to the wiring structure 135. FIG. 1 shows a cross-section of a portion including three pixel electrodes 140, in other words, a portion including three pixels. It should be noted that each pixel of the image sensor 100 may include a plurality of sub-pixels, for example, in order to separately detect a plurality of different wavelengths, and in that case the pixel electrode 140 may be referred to as a sub-pixel electrode. In addition, herein, the pixel electrode 140 may also be referred to as a bottom pixel electrode or a bottom electrode in terms of the QD photodiode 150. The pixel electrode 140 may be made of one or more metals and/or alloys selected from the group including copper (Cu) , aluminum (Al) , tungsten (W) , gold (Au) , titanium nitride (TiN) , tantalum nitride (TaN) , tungsten silicide (WSi ₂) , titanium sulfide (TiS) , titanium oxide (TiO _x) , and the like. The pixel electrode 140 may be formed, for example, to a thickness of tens of nanometers, using any suitable deposition technique.

Referring now to FIG. 2, a circuit diagram is shown that schematically illustrates a single pixel 200 of the image sensor 100 shown in FIG. 1. A photodiode, which may be the QD photodiode 150 shown in FIG. 1, is schematically drawn in a cross-sectional view.

The QD photodiode 150 may be connected, via the bottom pixel electrode 140, to a circuit including a plurality of transistors, namely, a reset transistor 210, an amplifying transistor 220, and a select transistor 230. The

transistors

210, 220, and 230 can be formed in the Si-ROIC 110 shown in FIG. 1. It should be understood that the pixel 200 may include one or more additional transistors and/or other circuit elements.

In operation, the QD photodiode 150 can be biased with a drive voltage VTOP through a top transparent electrode 180 (which may be referred to as a top electrode) . Photogenerated carriers (e.g., holes) from the QD photodiode 150 are stored at a node 240 connected to the gate of the amplifying transistor 220. By turning on the select transistor 230, which is connected to the amplifying transistor 220 in serial, to select the pixel 200 for read-out, a current having a magnitude that depends on amount of the carriers stored at the node 240 can be output to a data line 250. Then, by turning on the reset transistor 210, which is also connected to the node 240, to drain the carriers stored at the node 240, the pixel 200 can be reset.

Referring back to FIG. 1, the QD photodiode 150 may include a stack of a first carrier blocking layer 155, a QD region 160, a second carrier blocking layer 175, and a top transparent electrode 180. The first carrier blocking layer 155 is formed on the ROIC chip 110 in contact with the bottom pixel electrode 140. The bottom pixel electrode 140 can be considered as a component of the QD photodiode 150.

The first carrier blocking layer 155 and the second carrier blocking layer 175 have an ability to block carriers that may flow into the QD photodiode 150 from outside by thermal excitation. As such, the

carrier blocking layers

155 and 175 may function to reduce dark current through the QD photodiode 150. Depending on polarity of the QD photodiode 150, one

carrier blocking layer

155 or 175 is formed as a hole blocking layer (HBL) and the other

carrier blocking layer

175 or 155 is formed as an electron blocking layer (EBL) . More specifically, the carrier blocking layer on the anode side of the photodiode may be the EBL and the carrier blocking layer on the cathode side may be the HBL. It can be understood in the art that the HBL and EBL may be referred to as ETL (electron transport layer) and HTL (hole transport layer) , respectively. The HBL (i.e., ETL) can form a barrier to hole injection into the QD region 160 from outside and can help to expel electrons generated within the QD region 160. The EBL (i.e., HTL) can form a barrier to electron injection into the QD region 160 from outside and can help to expel holes generated within the QD region 160.

The HBL can be made of a variety of materials, such as titanium oxide (TiO ₂) , C ₆₀ fullerene, organic materials, inorganic nanoparticles, and can be deposited using any suitable technique that may be selected depending on the specific material. The HBL may have a thickness of, for example, tens of nanometers. The EBL can also be made of a variety of materials, such as organic materials or inorganic nanoparticles, and can be deposited using any suitable technique that may be selected depending on the specific material. The EBL may have a thickness of, for example, tens of nanometers.

The QD region 160 is a layer containing quantum dots and is configured to perform photoelectric conversion for light incident on the image sensor 100. Details of the QD region 160 will be described below.

The top transparent electrode 180 is generally formed as a common electrode connected to all or some of the pixels of the image sensor 100. The top transparent electrode 180 may be made of a conductive material that can sufficiently transmit light in a desired wavelength range, such as from the visible range to the SWIR range. Examples of such conductive materials include indium tin oxide (ITO) , indium zinc oxide (IZO) , zinc oxide (ZnO) , tin oxide (SnO ₂) , and the like. The top transparent electrode 180 may have a thickness of, for example, about 10 nm to hundreds of nanometers, depending on the light transmittance and electrical conductivity of the specific material. The top transparent electrode 180 may be deposited using any suitable technique, such as sputtering.

The QD photodiode 150 illustrated in FIG. 1 further includes an array of microlenses 190 on the top transparent electrode 180, each microlens 190 aligned to a corresponding pixel (or sub-pixel) . The structure of the microlenses 190 is not limited to the illustrated structure. For example, a single microlens may be associated with several pixels. Further, microlenses 190 may be optional. In addition, although not shown, the QD photodiode 150 may optionally include one or more optical filters (e.g., a visible light cut filter, RGB filters when the visible light is also captured, or the like) corresponding to the absorption wavelength of the underlying QD region 160. Such an optical filter (s) may be disposed between the top transparent electrode 180 and the array of microlenses 190.

The following describes the QD region 160 in detail. It will be understood that quantum dots (QD) are also known as colloidal quantum dots (CQD) and each quantum dot has a core and ligands for capping the core. The core is an aggregate of hundreds to tens of thousands of semiconductor atoms or molecules and has a nanoscale diameter of, for example, 2 nm to 10 nm. The ligands can protect the core and can prevent the cores of neighboring quantum dots from contacting to each other. Otherwise, such contact would interfere with achieving the targeted quantum size effect.

In one embodiment, the cores of the quantum dots in the QD region 160 may include lead sulfide (PbS) . Additionally, or alternatively, the quantum dots in the QD region 160 may include nanoparticles of one or more substances selected from the group consisting of lead selenide (PbSe) , lead telluride (PbTe) , cadmium sulfide (CdS) , cadmium selenide (CdSe) , cadmium telluride (CdTe) , silicon (Si) , germanium (Ge) , and carbon (C) . Perovskite-based nanoparticles may also be used for the quantum dots.

In one embodiment, the ligands may include one or more substances selected from the group consisting of lead sulfate (PbSO ₄) or other sulfates, lead oxide (PbO) , lead selenite (PbSeO ₄) , lead metatellurate (PbTeO ₄) , silicon oxynitride (SiO _xN _y) , indium oxide (In ₂O ₃) , sulfur (S) , a variety of sulfoxides, carbon, and a variety of carbonates. The ligands can be used to dope quantum dots, as discussed below. Those ligands that can supply extra holes to the core can be used as P-type dopants. Those ligands that can supply extra electrons to the core can be used as N-type dopants.

As is known in the art, quantum dots have different properties from bulk crystalline semiconductors and enable the band gap to be controlled according to the dot size. Specifically, the larger the quantum dot size, the narrower the band gap. Therefore, increasing the quantum dot size allows for light having a longer wavelength be absorbed to generate electron-hole pairs. For example, adjusting the size of PbS quantum dots in the range of 2 nm to 10 nm may enable to realize band gaps that correspond to one or more desired wavelength ranges in the entire SWIR range (which may refer to the range from about 1 μm to about 2 μm) . Thus, a SWIR image sensor can be implemented by designing the QD size to match one or more desired wavelength ranges.

As in the known Si photoelectric conversion layer, the thicker the QD region 160, the higher the sensitivity for longer wavelengths. It is preferable to thicken the QD region 160 sufficiently, in particular, to perform SWIR imaging. Therefore, according to embodiments of the present application, the QD region 160 may be preferably formed to have a thickness of 100 nm to 1000 nm. The layer of quantum dots (i.e., the QD region 160) can be formed by, for example, spin coating. However, such a thick layer (e.g., 250 nm) is difficult to be formed at one time by spin coating. Accordingly, the QD region 160 may be formed in a layer-by-layer scheme by stacking a plurality of sublayers, each having a thickness of tens of nanometers.

In this manner, the image sensor 100 can be implemented as a QD image sensor for SWIR imaging. The QD photodiode 150 of the QD image sensor 100 can be fabricated as a thin-film deposited on the Si-ROIC 110, in other words, it can be fabricated without the need for any bonding process using bumps or the like. As such, the QD image sensor 100 not only saves the wafer cost but also reduces the pixel size and thereby increases the resolution, compared to traditional SWIR image sensors using a III-V compound semiconductor.

It should be noted that the fabrication process of the QD region 160 or the sublayers thereof is not limited to spin coating. For example, the QD region 160 or the sublayers thereof may be fabricated by spray coating or a printing technique such as ink jet printing.

As outlined in the Background section, however, conventional QD SWIR image sensors have the following problems:

(1) As the band gap becomes narrower to absorb longer wavelengths, the dark current will increase. A higher dark current may significantly reduce the signal-to-noise ratio (SNR) at low brightness, thereby making imaging in low brightness scenes virtually impossible.

(2) In order to expel photogenerated carriers from the QD layer prior to recombination of the photogenerated carriers occurring, a relatively high bias voltage is required to be applied to the QD photodiode. In other words, a relatively high drive voltage is required for fast imaging. However, such a higher drive voltage can increase power consumption and hence heat generation, leading to a further increase in of the dark current.

(3) As a result of the two problems described above, conventional quantum dot SWIR image sensors need to be cooled to a temperature below the room temperature in order to suppress the dark current, similar to the traditional SWIR image sensors using a III-V compound semiconductor. This cooling involves attaching an active cooling device, such as a Peltier device or another type of heat pump, to the QD image sensor. When such a cooling device is associated with the QD image sensor, it may be difficult or impossible to miniaturize an electronic device into which the QD image sensor is incorporated and/or to reduce the cost of the electronic device.

Embodiments of the present application provide a technical solution that can avoid or mitigate these problem.

Still referring to FIG. 1, according to the embodiments of the present application, the QD region 160 has a stack of a plurality of QD layers (QD1 (162) , QD2 (164) , ..., QDn (166) ) , where n is an integer equal to or greater than 2. In other words, the QD region 160 may include two or more stacked QD layers. The QD1 (162) is the QD layer closest to the top transparent electrode 180 and the QDn (166) is the QD layer closest to the bottom pixel electrode 140.

The two or more QD layers contain quantum dots that are different from one another in the band gap and/or impurity doping. Specifically, the quantum dots contained in the QD1 (162) may have a band gap BG1 and be applied with impurity doping DOPING1. The quantum dots contained in the QD2 (164) may have a band gap BG2 and be applied with impurity doping DOPING2. The quantum dots contained in the QDn (166) may have a band gap BGn and be applied with impurity doping DOPINGn. The combinations (BG1, DOPING1) , (BG2, DOPING2) , and (BGn, DOPINGn) differ from each other in at least one of the band gap and impurity doping. In this manner, the plurality of QD layers have different energy band structures from one another and at least one heterojunction is established in the QD region 160 including these QD layers.

It should be noted that one or more of the DOPING1 through DOPINGn may refer to non-doping that may result in intrinsic quantum dots. It should also be noted that each of the QD1, QD2, ..., QDn refers to a layer in the QD region 160 that contains substantially same quantum dots. In other words, each of the QD1, QD2, ..., QDn does not refer to a layer that can be deposited in a single iteration of a deposition process, such as a spin coating process. Rather, the QD1, QD2, ..., QDn may have different thicknesses from one another, and the QD1, QD2, ..., QDn each may be a single layer deposited in a single iteration or a stack of sublayers deposited in multiple iterations.

The following describes such band gap control and doping control separately for ease of explanation.

Regarding the band gap control, the respective band gaps for the two or more QD layers are set such that they become wider, as the QD layers are getting closer to the top transparent electrode 180, that is, to the light incident side of the QD region 160. Assuming herein that BG1, BG2, ..., and BGn are all different, the band gaps for the two or more QD layers can be controlled such that BG1 > BG2 > …> BGn. It can be understood that this can be accomplished by reducing the quantum dot size, as the QD layers are getting closer to the light incident side of the QD region 160.

Controlling the band gap for each QD layer in this manner allows the dark current generated in the QD region 160 to be reduced while keeping a high quantum efficiency of the QD region 160. Light in the SWIR range is less absorbed at shallower locations within the QD region 160 and can reach deeper locations within the QD region 160. Therefore, narrowing the band gap for a deeper QD layer (s) (e.g., QDn) can retain the high quantum efficiency in the QD region 160 as a whole. In addition, by widening the band gap for a shallower QD layer (s) (e.g., QD1) , the thermal excitation of carriers can be suppressed to reduce the dark current. This can improve the SNR at low brightness and allows for capturing a clear image even in low brightness scenes.

Next, the doping control is described. The two or more QD layers (QD1 (162) , QD2 (164) , …, and QDn (166) ) are doped differently from each other so as to generate a built-in electric field in the QD region 160 for drifting photogenerated carriers out of the QD region 160. More specifically, the two or more QD layers are doped such that the lower end level (E _C) of the conduction band and the upper end level (E _V) of the valence band under thermal equilibrium both increase stepwise or both decrease stepwise throughout thickness of the QD region 160 in the depth direction. Whether they increase or decrease in the depth direction will depend on the polarity of the QD photodiode 150.

Such doping can be accomplished by properly selecting a ligand (s) of the quantum dots. For example, for each QD layer, one or more ligands acting as P-type dopants or N-type dopants can be selected such that the ligands supply a desired quantity of holes or electrons to the cores of the quantum dots. This can create an energy band structure having one or more heterojunctions that produce a desired built-in electric field in the QD region 160.

Such a built-in electric field may help rapidly drive or move the photogenerated carriers out of the QD region 160. Accordingly, in order to achieve high speed operation of the QD photodiode 150, a low bias voltage needs to be applied to the QD photodiode 150, compared to cases in which such a built-in electric field is not produced. In other words, the drive voltage required for fast imaging can be lowered. The lower drive voltage may also reduce heat generation, leading to a further reduction of the dark current.

In this manner, both the band gap control and the doping control for each QD layer may result in reducing the dark current. This may eliminate the need for a cooling device, such as a Peltier device or another type of heat pump, required in the related art. As a result, it may be enabled to miniaturize an electronic device into which the QD image sensor 100 is incorporated and/or to reduce the cost of the electronic device.

Now, some embodiments of the present application are described in more detail.

FIRST EMBODIMENT

FIG. 3A is a cross-sectional view schematically illustrating a QD photodiode 350 of an image sensor according to a first embodiment of the present application. FIG. 3B is a schematic diagram illustrating a potential profile at non-biasing for the QD photodiode 350. FIG. 3C is a schematic diagram illustrating a potential profile at biasing for the QD photodiode 350.

The QD photodiode 350 includes a bottom electrode 140, an EBL 355, a QD region 360, an HBL 375, and a top electrode 180. The bottom electrode 140, the EBL 355, the HBL 375, and the top electrode 180 correspond to the bottom pixel electrode 140, the first carrier blocking layer 155, the second carrier blocking layer 175, and the top transparent electrode 180 shown in FIG. 1, respectively. The QD region 360 is one embodiment of the QD region 160 shown in FIG. 1.

The QD region 360 includes three stacked QD layers: QD1 (362) , QD2 (364) , and QD3 (366) . The layer QD1 (362) contains intrinsic quantum dots with a band gap BG1. The layer QD2 (364) contains P ^--type quantum dots with a band gap BG2. The layer QD3 (366) contains P-type quantum dots with a band gap BG3. These band gaps satisfy the relationship: BG1 > BG2 > BG3, as best seen in FIG. 3B. Thus, these three QD layers are designed such that the impurity concentration increases as the band gap narrows.

As an example, the band gaps BG1, BG2, and BG3 may be realized by PbS quantum dots having diameters of about 5 nm, about 6 nm, and about 7 nm, respectively. By making the band gaps for the layers QD1 and QD2 at shallower locations wider than the band gap for the layer QD3 at a deeper location, thermal excitation of carriers at the shallower locations can be suppressed, thereby reducing the dark current. In addition, since light in the SWIR range can reach deeper locations within the QD region 360, the narrower band gap for the layer QD3 may retain a high quantum efficiency in the QD region 360 as a whole.

In FIG. 3B, the Fermi level E _F, the lower end level E _C of the conduction band, and the upper end level E _V of the valence band are also shown schematically. In addition, generation of charge carriers (electron-hole pairs, as illustrated) by photons incident through the top electrode 180 is schematically shown in FIG. 3B. It can be understood that the intrinsic quantum dots in the layer QD1 have the Fermi level E _F in the middle of the band gap defined between E _C and E _V. The P ^--type quantum dots in the layer QD2 have E _F slightly shifted toward E _V and the P-type quantum dots in the layer QD3 have E _F further shifted toward E _V. As a result, E _C and E _V under thermal equilibrium both increase stepwise throughout thickness of the QD region 360 in the depth direction. The resulting built-in electric field in the QD region 360 creates a potential gradient that promotes the movement of holes toward the bottom electrode 140 and the movement of electrons toward the top electrode 180.

In operation, a bias voltage is applied across the bottom electrode 140 and the top electrode 180 to achieve high speed operation as desired. As shown in FIG. 3C, the bias voltage acts to increase the potential gradient across the QD region 360 and move the photogenerated carriers more rapidly. It should be understood that the bias voltage (which may be about 2.5 V as an example) required for the QD photodiode 350 is lower than the bias voltage (which may be about 3.3 V as an example) required for conventional QD photodiodes (an example of which is illustrated in FIG. s4A-4C) .

COMPARATIVE EXAMPLE

FIG. 4A is a cross-sectional view schematically illustrating a QD photodiode 50 of an image sensor according to a related art. FIG. 4B is a schematic diagram illustrating a potential profile at non-biasing for the QD photodiode 50. FIG. 4C is a schematic diagram illustrating a potential profile at biasing for the QD photodiode 50.

The QD photodiode 50 includes a bottom electrode 40, an EBL 55, a QD region 60, an HBL 75, and a top electrode 80. In one example, the bottom electrode 40, the EBL 55, the HBL 75, and the top electrode 80 may be the same as the bottom electrode 340, the EBL 355, the HBL 375, and the top electrode 380, respectively, of the QD photodiode 350 shown in FIG. 3A.

Unlike the QD region 360 shown in FIG. 3A, the QD region 60 consists of a single thicker QD layer. As an example, the QD region 60 contains P-type PbS quantum dots having a diameter of about 7 nm. Consequently. The entire QD region 60 contains substantially the same quantum dots having a relatively narrow band gap BG. Thermally excited carriers generated throughout the QD region 60 can result in a large dark current even at the room temperature.

In the thermal equilibrium state (FIG. 4B) , there is no built-in electric field in the QD region 60. Accordingly, in operation (FIG. 4C) , a bias voltage higher than the bias voltage required in FIG. 3C is required to achieve high speed operation as desired.

SECOND EMBODIMENT

FIG. 5A is a cross-sectional view schematically illustrating a QD photodiode 550 of an image sensor according to a second embodiment of the present application. FIG. 5B is a schematic diagram illustrating a potential profile at non-biasing for the QD photodiode 550.

The QD photodiode 550 is the same as the QD photodiode 350 according to the first embodiment, except that the QD region 360 is replaced with a QD region 560. The same components are given the same reference numerals and are not described again in detail.

The QD region 560 is one embodiment of the QD region 160 shown in FIG. 1 and includes two stacked QD layers: QD1 (562) and QD2 (566) . The layer QD1 (562) contains intrinsic quantum dots with a band gap BG1. The layer QD2 (566) contains P-type quantum dots with a band gap BG2. These band gaps satisfy the relationship: BG1 > BG2 (FIG. 5B) . Thus, these two QD layers are designed such that the impurity concentration increases as the band gap narrows.

As an example, the band gaps BG1 and BG2 may be realized by PbS quantum dots having diameters of about 5 nm and about 7 nm, respectively. As in the first embodiment, the dark current generated in the QD region 560 can be suppressed while retaining a high quantum efficiency in the QD region 560.

As shown in FIG. 5B, E _C and E _V under thermal equilibrium both increase stepwise throughout thickness of the QD region 560 in the depth direction. The resulting built-in electric field in the QD region 560 creates a potential gradient that promotes the movement of holes toward the bottom electrode 140 and the movement of electrons toward the top electrode 180. As in the first embodiment, the bias voltage required for the QD photodiode 550 according to this embodiment is lower than the bias voltage required for conventional QD photodiodes.

The second embodiment has reduced the three QD layers in the first embodiment to the two QD layers. It can be understood, however, that other embodiments may include four or more QD layers. Details are not discussed herein.

THIRD EMBODIMENT

FIG. 6A is a cross-sectional view schematically illustrating a QD photodiode 650 of an image sensor according to a third embodiment of the present application. FIG. 6B is a schematic diagram illustrating a potential profile at non-biasing for the QD photodiode 650.

While the first and second embodiments are directed to QD photodiodes of the hole-accumulation type, which outputs holes to the ROIC, the QD photodiodes 650 according to this embodiment is of the electron-accumulation type.

The QD photodiode 650 includes a bottom electrode 140, an HBL 655, a QD region 660, an EBL 675, and a top electrode 180. The bottom electrode 140, the HBL 655, the EBL 675, and the top electrode 180 correspond to the bottom pixel electrode 140, the first carrier blocking layer 155, the second carrier blocking layer 175, and the top transparent electrode 180 shown in FIG. 1, respectively. The QD region 660 is one embodiment of the QD region 160 shown in FIG. 1. It should be noted that in this embodiment, which is of the electron-accumulation type, the carrier blocking layer 655 adjacent the bottom electrode 140 is the HBL (i.e., ETL) and the carrier blocking layer 675 adjacent the top electrode 180 is the EBL (i.e., HTL) .

The QD region 660 includes three stacked QD layers: QD1 (662) , QD2 (664) , and QD3 (666) . The layer QD1 (662) contains intrinsic quantum dots with a band gap BG1. The layer QD2 (664) contains N ^--type quantum dots with a band gap BG2. The layer QD3 (666) contains N-type quantum dots with a band gap BG3. These band gaps satisfy the relationship: BG1 > BG2 > BG3 (FIG. 6B) . Thus, these three QD layers are designed such that the impurity concentration increases as the band gap narrows.

As an example, the band gaps BG1, BG2, and BG3 may be realized by PbS quantum dots having diameters of about 5 nm, about 6 nm, and about 7 nm, respectively. As in the first embodiment, by making the band gaps for the layers QD1 and QD2 at shallower locations wider than the band gap for the layer QD3 at a deeper location, the dark current generated in the QD region 660 can be suppressed while retaining a high quantum efficiency in the QD region 660.

As shown in FIG. 6B, E _C and E _V under thermal equilibrium both decrease stepwise throughout thickness of the QD region 660 in the depth direction. The resulting built-in electric field in the QD region 660 creates a potential gradient that promotes the movement of electrons toward the bottom electrode 140 and the movement of holes toward the top electrode 180. As in the first embodiment, the bias voltage required for the QD photodiode 650 according to this embodiment is lower than the bias voltage required for conventional QD photodiodes.

FOURTH EMBODIMENT

FIG. 7A is a cross-sectional view schematically illustrating a QD photodiode 750 of an image sensor according to a fourth embodiment of the present application. FIG. 7B is a schematic diagram illustrating a potential profile at non-biasing for the QD photodiode 750.

The QD photodiode 750 is the same as the QD photodiode 350 according to the first embodiment, except that the QD region 360 is replaced with a QD region 760. The same components are given the same reference numerals and are not described again in detail.

The QD region 760 is one embodiment of the QD region 160 shown in FIG. 1 and includes three stacked QD layers: QD1 (762) , QD2 (764) , and QD3 (766) . The layer QD1 (762) contains N-type quantum dots with a band gap BG1. The layer QD2 (764) contains intrinsic quantum dots with a band gap BG2. The layer QD3 (766) contains P-type quantum dots with a band gap BG3. Thus, the QD photodiode 750 is a hole-accumulation type PIN photodiode. These band gaps satisfy the relationship: BG1 > BG2 > BG3 (FIG. 7B) .

As an example, the band gaps BG1, BG2, and BG3 may be realized by PbS quantum dots having diameters of about 5 nm, about 6 nm, and about 7 nm, respectively. As in the first embodiment, by making the band gaps for the layers QD1 and QD2 at shallower locations wider than the band gap for the layer QD3 at a deeper location, the dark current generated in the QD region 760 can be suppressed while retaining a high quantum efficiency in the QD region 760.

As shown in FIG. 7B, E _C and E _V under thermal equilibrium both increase stepwise throughout thickness of the QD region 760 in the depth direction. The PIN structure enables a larger built-in electric field to be produced in the QD region 760. Accordingly, the bias voltage required for the QD photodiode 750 according to this embodiment may further be lower than the bias voltage required for the QD photodiode 350, which has a PI (or PP ^-I) structure, according to the first embodiment.

The PIN photodiode 750 of the hole-accumulation type has been described herein. It can be understood from the foregoing (in particular, the first and third embodiments) that another embodiment may implement a PIN photodiode of the electron-accumulation type.

FIFTH EMBODIMENT

FIG. 8A is a cross-sectional view schematically illustrating a QD photodiode 850 of an image sensor according to a fifth embodiment of the present application. FIG. 8B is a schematic diagram illustrating a potential profile at non-biasing for the QD photodiode 850.

The QD photodiode 850 is the same as the QD photodiode 750 according to the fourth embodiment, except that the QD region 760 is replaced with a QD region 860. The same components are given the same reference numerals and are not described again in detail.

The QD region 860 is one embodiment of the QD region 160 shown in FIG. 1 and includes two stacked QD layers: QD1 (862) and QD2 (866) . The layer QD1 (862) contains N-type quantum dots with a band gap BG1. The layer QD2 (866) contains P-type quantum dots with a band gap BG2. Thus, the QD photodiode 850 is a hole-accumulation type PN photodiode. These band gaps satisfy the relationship: BG1 > BG2 (FIG. 8B) .

As an example, the band gaps BG1 and BG2 may be realized by PbS quantum dots having diameters of about 5 nm and about 7 nm, respectively. As in the fourth embodiment, the dark current generated in the QD region 860 can be suppressed while retaining a high quantum efficiency in the QD region 860.

As shown in FIG. 8B, E _C and E _V under thermal equilibrium both increase stepwise throughout thickness of the QD region 860 in the depth direction. Similar to the PIN structure as described above, the PN structure enables a larger built-in electric field to be produced in the QD region 860. Accordingly, the bias voltage required for the QD photodiode 850 according to this embodiment may also be lower than the bias voltage required for the QD photodiode 350, which has a PI (or PP ^-I) structure, according to the first embodiment.

The PN photodiode 850 of the hole-accumulation type has been described herein. It can be understood from the foregoing (in particular, the first and third embodiments) that another embodiment may implement a PN photodiode of the electron-accumulation type.

In the foregoing, exemplary embodiments of the QD image sensor for imaging in the SWIR range according to the technical solution in the present application have been described. As detailed in detail above, the QD image sensor can reduce the dark current and improve the SNR, thereby may allow for capturing a clear image even in low brightness scenes. In addition, the QD image sensors enables fast imaging at a lower drive voltage. The reduced dark current and the lower drive voltage can also reduce heat generation and eliminate the need for a cooling device to cool the image sensor.

Furthermore, the QD image sensor can be fabricated integrally on the Si-ROIC in a layer-by-layer scheme. This can save the wafer cost, reduce the pixel size, and increase the resolution, compared to the traditional SWIR image sensors, which use a compound semiconductor, for example, InGaAs.

The QD image sensor can be incorporated into a wide variety of electronic devices that involve imaging in the SWIR range so that the above advantages are exploited.

For example, the QD image sensor may be applied to cameras, such as general-purpose cameras, security cameras, or in-vehicle cameras, thereby implementing compact, low-cost, and high-performance SWIR cameras. In such a camera, the QD image sensor for imaging in the SWIR range may be used alone, or used with an image sensor for imaging in the visible range, or integrated with an image sensor for imaging in the visible range. As an example, an in-vehicle camera that detects pedestrians, obstacles, and other objects outside the vehicle may implement an image sensor for the visible range and an image sensor for the infrared (IR) range (including the SWIR range) separately or implement a single image sensor capable of capturing both the visible and infrared ranges. As another example of in-vehicle cameras, an in-cabin camera that detects a driver's line of sight or face orientation can also use the QD image sensor for imaging in the SWIR range.

Furthermore, the QD image sensor, which may eliminate the need for a cooling device, such as a Peltier device or another type of heat pump, is applicable to small, thin, and/or battery-powered mobile devices, such as smartphones or tablets. One such example is shown in FIG. 9.

FIG. 9 is a schematic diagram of an electronic device 900 according to an embodiment of the present application. In one example, the electronic device 900 may be a smartphone or the like. The electronic device 900 includes a processor 910, a memory 920, a battery 930, a display panel 940, and a camera 950. The display panel 940 may be assembled with a housing of the electronic device 900 in such a manner that the front surface of the display panel 940 is visible from a user of the electronic device 900, as shown by solid lines in FIG. 9. The processor 910, the memory 920, the battery 930, and the camera 950 may be contained within the housing, as indicated by dashed lines in FIG. 9. The processor 910, the memory 920, the battery 930, the display panel 940, and the camera 950 may be electrically connected to each other.

Although not shown, the electronic device 900 may optionally include a radio frequency (RF) circuit, a speaker, a microphone, an input device, a sensor, an antenna, a near field communication module, and/or the like.

The processor 910 may be configured to invoke a software program and data stored in the memory 920 and execute the software program to perform various functions and/or data processing of the electronic device 900. The processor 910 may include any suitable special-purpose or general-purpose processing device or unit. Additionally, the processor 910 may include any suitable number of processors. For example, the processor 910 may include one or more of a microprocessor, a microcontroller, an application processor, a central processing unit (CPU) , a graphics processing unit (GPU) , a digital signal processor (DSP) , an application-specific integrated circuit (ASIC) , a Field Programmable Gate Array (FPGA) , and the like.

The memory 920 may be configured to store a software program and data, and may include any suitable medium that may be accessed by the processor 910. Additionally, the memory 920 may include memory in any suitable number. The memory 920 can include volatile memory and/or non-volatile memory, and may include, for example, a random access memory (RAM) , a read-only memory (ROM) , and/or a flash memory. It should be noted that the term “memory” as used herein may refer to a mass storage that can store large amounts of data. Therefore, the memory 920 may also include, for example, a hard disk drive (HDD) , a solid state drive (SDD) , an optical disk drive, or the like.

The battery 930 may be configured to supply power to each of components of the display device 900, such as the processor 910, the memory 920, the display panel 940, and the camera 950. The processor 910 may run a power management program or module stored in the memory 920 to control power consumption of one or more components, as well as, charging and discharging of the battery 930. In addition to, or instead of, the battery 930, the electronic device 900 may have a power connector, adapter, or the like, which is connected to an external power supply, such as utility power.

The display panel 940 may be configured to display a variety of information and content, including information entered by a user and information provided for the user. The display panel 940 may include a user input device, such as a touch screen, on at least a part of the surface exposed from the housing.

The camera 950 may include, for example, the QD image sensor 100 shown in FIG. 1, which is capable of capturing the SWIR range. As an example, the QD image sensor 100 in the camera 950 may be utilized to monitor a line of sight and/or a facial expression of a user who is viewing a content displayed on the display panel 940. As another example, the QD image sensor 100 in the camera 950 may be used to image surrounding scenery and/or objects. The camera 950 that includes the QD image sensor 100 can capture a clear image even at low brightness.

Although some preferred embodiments of the present application have been described, persons skilled in the art may make changes and modifications to these embodiments without departing from the scope of present disclosure. Therefore, the following claims are intended to be construed as to cover all changes and modifications falling within the scope of the present disclosure.

Claims

An image sensor comprising:

a quantum dot (QD) region configured to perform photoelectric conversion in short wavelength infrared (SWIR) range, the QD region comprising a stack of two or more QD layers, the two or more QD layers including a first QD layer that contains first quantum dots and a second QD layer that contains second quantum dots, wherein a band gap of the first quantum dots differs from a band gap of the second quantum dots.
The image sensor of claim 1, wherein the two or more QD layers comprise the first QD layer and the second QD layer in order from a light-incident side of the QD region, and wherein the band gap of the first quantum dots is wider than the band gap of the second quantum dots.
The image sensor of claim 2, wherein the second quantum dots have a higher impurity concentration than the first quantum dots.
The image sensor of claim 3, wherein the second quantum dots are P-type quantum dots and the first quantum dots are P ^--type, intrinsic, or N-type quantum dots.
The image sensor of claim 1, wherein the two or more QD layers comprise the first QD layer, the second QD layer, and a third QD layer in order from a light-incident side of the QD region, and wherein the band gap of the first quantum dots is wider than the band gap of the second quantum dots, and the band gap of the second quantum dots is wider than a band gap of third quantum dots contained in the third QD layer.
The image sensor of claim 5, wherein the second quantum dots have a higher impurity concentration than the first quantum dots and the third quantum dots have a higher impurity concentration than the second quantum dots.
The image sensor of claim 6, wherein the third quantum dots are P-type quantum dots, the second quantum dots are P ^--type quantum dots, and the first quantum dots are intrinsic quantum dots.
The image sensor of claim 6, wherein the third quantum dots are N-type quantum dots, the second quantum dots are N ^--type quantum dots, and the first quantum dots are intrinsic quantum dots.
The image sensor of claim 6, wherein the third quantum dots are P-type quantum dots, the second quantum dots are intrinsic quantum dots, and the first quantum dots are N-type quantum dots.
The image sensor of claim 1, wherein the two or more QD layers have a total thickness of 100 nm to 1000 nm inclusive.
The image sensor of claim 1, wherein the QD region is a thin-film region deposited on a read-out integrated circuit (ROIC) .
An image sensor comprising:

a quantum dot (QD) region configured to perform photoelectric conversion in short wavelength infrared (SWIR) range, the QD region comprising a stack of two or more QD layers, the two or more QD layers are doped differently from each other so as to generate a built-in electric field in the QD region for drifting photogenerated carriers out of the QD region.
The image sensor of claim 12, wherein the two or more QD layers are doped such that a lower end level (E _C) of a conduction band and an upper end level (E _V) of a valence band under thermal equilibrium both increase stepwise or both decrease stepwise throughout thickness of the QD region in a depth direction.
The image sensor of claim 12, wherein the two or more QD layers comprise a first QD layer and a second QD layer in order from a light-incident side of the QD region, and wherein the second QD layer contains P-type quantum dots and the first QD layer contains intrinsic or N-type quantum dots.
The image sensor of claim 12, wherein the two or more QD layers comprise a first QD layer, a second QD layer, and a third QD layer in order from a light-incident side of the QD region, and wherein the first, second, and third QD layers each contain P-type or intrinsic quantum dots.
The image sensor of claim 12, wherein the two or more QD layers comprise a first QD layer, a second QD layer, and a third QD layer in order from a light-incident side of the QD region, and wherein the first, second, and third QD layers each contain N-type or intrinsic quantum dots.
The image sensor of claim 12, wherein the two or more QD layers comprise a first QD layer, a second QD layer, and a third QD layer in order from a light-incident side of the QD region, and wherein one of the first and third QD layers contains P-type quantum dots and the other of the first and third QD layers contains N-type quantum dots.
The image sensor of claim 17, wherein the second QD layer contains intrinsic quantum dots.
An electronic device including the image sensor of any one of claims 1 to 18.
The electronic device of claim 19, wherein the electronic device comprises no cooling devices configured to cool the image sensor.