JP2023163932A - Infrared detector and image sensor using the same - Google Patents

Abstract

To suppress the decline in a signal current in an infrared detector and improve light receiving sensitivity.SOLUTION: An infrared detector includes an etching stopper layer electrically connected to an electrode to which a common potential is applied, a first light-receiving layer provided on the etching stopper layer and absorbing infrared rays in a first wavelength band, and a second light-receiving layer provided on the first light-receiving layer and absorbing infrared rays in a second wavelength band, and the longest infrared wavelength detectable by the first light-receiving layer is shorter than the longest infrared wavelength detectable by the second light-receiving layer, and the bandgap of the etching stopper layer is the same as the bandgap of the first light-receiving layer, or smaller than the bandgap of the first light-receiving layer.SELECTED DRAWING: Figure 4

Description

The present disclosure relates to an infrared detector and an image sensor using the same.

Type II superlattice (T2SL) is expected to be a material for next-generation infrared detectors to replace mercury cadmium telluride (MCT), and development is progressing. Most infrared detectors using a type II superlattice have a superlattice structure formed on a GaSb substrate using a material such as GaSb, InAs, or AlSb, which has a lattice constant close to that of GaSb. By using an appropriately designed superlattice structure as an infrared receiving layer, it is possible to detect infrared rays in a desired wavelength band such as a medium wavelength band (3 to 5 μm) or a long wavelength band (8 to 12 μm).

As a configuration of a two-wavelength infrared detector, a structure has been proposed in which a unipolar barrier layer is placed between two light-receiving layers sensitive to infrared rays of different wavelengths to block the flow of carriers of either electrons or holes. (For example, see Non-Patent Document 1).

When forming an infrared detector array using a laminate including a light-receiving layer, the laminate is processed into a mesa shape in order to separate adjacent pixels. In order to improve etching depth accuracy, an etching stopper layer is often provided under the laminate. As the etching stopper layer, a material is used that has a lattice constant close to that of the film-forming substrate and a concentration of a specific element that is significantly different from that of the upper electrode layer (or contact layer) or light-receiving layer. This is because the etching depth can be controlled by monitoring signals indicating specific elemental components.

JP 2020-155513 Publication

SPIE Proceedings, Vol. 8155, pp. 815507 (2011)

When detecting infrared rays in two wavelength bands longer than 3 μm, there is a problem that a signal cannot be extracted well when the bandgap of the etching stopper layer is larger than the bandgap of the light-receiving layer. This is because the etching stopper layer acts as a potential barrier against minority carriers. Depending on the type of signal to be extracted and the energy of the conduction band and valence band of the etching stopper layer, it may act as a potential barrier for electrons or as a potential barrier for holes. The potential barrier obstructs the flow of signals, particularly on the long wavelength side, and reduces the light receiving sensitivity.

One aspect of the present disclosure aims to suppress a decrease in signal current in an infrared detector and improve light reception sensitivity.

According to one form of the present disclosure, the infrared detector includes:
an etching stopper layer electrically connected to an electrode to which a common potential is applied;
a first light-receiving layer provided on the etching stopper layer and absorbing infrared rays in a first wavelength band;
a second light-receiving layer provided on the first light-receiving layer and absorbing infrared rays in a second wavelength band;
The longest infrared wavelength that can be detected by the first light-receiving layer is shorter than the longest infrared wavelength that can be detected by the second light-receiving layer, and the band gap of the etching stopper layer is the same as that of the first light-receiving layer. The bandgap is equal to or smaller than the bandgap of the first light-receiving layer.

In an infrared detector, a decrease in signal current can be suppressed and light receiving sensitivity can be improved.

FIG. 2 is a diagram illustrating technical problems that may occur in a two-wavelength infrared detector. FIG. 2 is a diagram illustrating technical problems that may occur in a two-wavelength infrared detector. FIG. 2 is a schematic cross-sectional view of a two-wavelength infrared detector according to the first embodiment. FIG. 2 is an explanatory diagram of the operation of the two-wavelength infrared detector of the first embodiment. FIG. 2 is an explanatory diagram of the operation of the two-wavelength infrared detector of the first embodiment. It is a manufacturing process diagram of the two wavelength infrared detector of 1st Embodiment. It is a manufacturing process diagram of the two wavelength infrared detector of 1st Embodiment. It is a manufacturing process diagram of the two wavelength infrared detector of 1st Embodiment. It is a manufacturing process diagram of the two wavelength infrared detector of 1st Embodiment. It is a manufacturing process diagram of the two wavelength infrared detector of 1st Embodiment. FIG. 3 is a schematic cross-sectional view of a two-wavelength infrared detector according to a second embodiment. FIG. 7 is an explanatory diagram of the operation of the two-wavelength infrared detector of the second embodiment. FIG. 7 is an explanatory diagram of the operation of the two-wavelength infrared detector of the second embodiment. FIG. 1 is a schematic diagram of an image sensor according to an embodiment. FIG. 2 is a schematic cross-sectional view showing a part of an infrared detector array used in an image sensor. FIG. 1 is a schematic block diagram of an imaging system incorporating an image sensor.

Before describing a specific configuration example of the present disclosure, technical problems that may occur in a two-wavelength infrared detector will be described in more detail with reference to FIGS. 1A and 1B. The energy band diagrams in FIGS. 1A and 1B include an etching stopper layer (ES), a first electrode layer (EL1), a first light-receiving layer (L1) that absorbs light with a wavelength of λ1, a barrier layer (BR), and a layer with a wavelength of λ2. It is assumed that a second light-receiving layer (L2) that absorbs light and a second electrode layer (EL2) are laminated in this order.

To increase the light reception sensitivity of a two-wavelength infrared detector using a type II superlattice, there are many configurations that detect high-mobility electrons as signals. A configuration that extracts electrons as a signal is called a pBp (hole block) type.

In FIG. 1A, when extracting electrons as a signal from the first light-receiving layer (L1), a positive voltage is applied to the second electrode layer (EL2), and a negative voltage is applied to the first electrode layer (EL1). The p-type first light-receiving layer (L1) is in a reverse bias state, and electrons as minority carriers generated in the first light-receiving layer (L1) flow toward the second electrode layer (EL2) and are detected as a signal. be done. Holes generated in the first light-receiving layer (L1) flow to the first electrode layer (EL1). At this time, the second light-receiving layer (L2) is in a forward bias state, and carriers (electron-hole pairs) generated by light absorption are recombined and disappear, or are negligibly small. Therefore, the infrared light of λ1 incident on the first light-receiving layer (L1) is correctly detected.

In FIG. 1B, when extracting electrons as a signal from the second light-receiving layer (L2), a positive voltage is applied to the first electrode layer (EL1) and a negative voltage is applied to the second electrode layer (EL2). The p-type second light-receiving layer (L2) is in a reverse bias state, and electrons as minority carriers generated in the second light-receiving layer flow toward the first electrode layer (EL1). However, the conduction band of the etching stopper layer (ES), which has a larger band gap _EBG than the first light-receiving layer (L1), acts as a potential barrier, making it impossible to extract signals (electrons).

In order to efficiently extract electrons as signals from the second light-receiving layer (L2), the band gap _EBG of the etching stopper layer (ES) must be the same as or smaller than the band gap of the first light-receiving layer (L1). do it. With this configuration, the signal flow can be maintained and the light receiving sensitivity can be increased. Based on this knowledge, the specific configuration of the two-wavelength infrared detector of the embodiment will be described. In the following description, "above" or "below" refers to the upper and lower sides of the stacking direction or the growth direction, and is not an absolute direction. In the drawings, the same components may be given the same reference numerals and redundant explanations may be omitted.

<First embodiment>
FIG. 2 is a schematic cross-sectional view of the two-wavelength infrared detector 10 of the first embodiment. In the first embodiment, electrons are extracted as signals. The two-wavelength infrared detector 10 is depicted as one pixel 101 for convenience of illustration. In actual use, a plurality of two-wavelength infrared detectors 10 are used as an infrared detector array arranged in one or two dimensions.

The two-wavelength infrared detector 10 includes an etching stopper layer 13, a first electrode layer 14, a first light-receiving layer 15, a barrier layer 16, a second light-receiving layer 17, and a second electrode layer 18 on a substrate 11 in this order. It is laminated with. A buffer layer 12 may be provided between the substrate 11 and the etching stopper layer 13. The mesa 35 includes a first electrode layer 14, a first light-receiving layer 15, a barrier layer 16, a second light-receiving layer 17, and a second electrode layer 18, except for electrodes 21 and 22 provided at predetermined locations. , the whole is covered with an insulating film 19.

The etching stopper layer 13 is used to improve the depth accuracy of the mesa 35 forming each pixel 101. The etching stopper layer 13 may function as a common electrode commonly used by the plurality of pixels 101 included in the infrared detector array, as part of the first electrode layer 14. In this case, the etching stopper layer (ES) may be doped with the same p-type impurity as the first electrode layer (EL1). In FIG. 2, an electrode 22 connected to the etching stopper layer 13 is illustrated for convenience of explaining the operation of the two-wavelength infrared detector 10. When the etching stopper layer 13 is used as a common electrode, the electrode 22 may be provided only in the outermost pixels of the infrared detector array.

As the substrate 11, for example, a GaSb (100) substrate is used. When providing the buffer layer 12, a GaSb layer that is lattice matched to the substrate 11 may be used. The etching stopper layer 13 is formed of a superlattice, and the composition, film thickness, period, etc. are adjusted so that its bandgap is the same as or smaller than that of the first electrode layer 14 and the first light-receiving layer 15. etc. are designed.

The first electrode layer 14 may be designed to have the same bandgap as the first light-receiving layer 15. Alternatively, as will be described later, the design is such that the energy difference between the lower ends of the conduction bands between the first electrode layer 14 and the first light-receiving layer 15 is 10 meV or less, and the energy of both is greater than or equal to the lower ends of the conduction bands of the etching stopper layer 13. You can.

The first light-receiving layer 15 has sensitivity in a first wavelength band (for example, 3 to 5 μm). The second light-receiving layer 17 has sensitivity in a second wavelength band (for example, 8 to 12 μm). The longest infrared wavelength that can be detected by the first light-receiving layer 15 is shorter than the longest infrared wavelength that can be detected by the second light-receiving layer 17. In the first embodiment, since electrons are detected as minority carriers, that is, signals, the first light-receiving layer 15 and the second light-receiving layer 17 have p-type conductivity. The barrier layer 16 separates the first light-receiving layer 15 and the second light-receiving layer 17 in the stacking direction. The barrier layer 16 functions as a potential barrier against holes in order to suppress dark current and extract signals from the first light-receiving layer 15 and the second light-receiving layer 17 separately.

The substrate 11 side becomes a light incident surface. The substrate 11 and buffer layer 12 may be finally removed by polishing or the like.

3 and 4 are explanatory diagrams of the operation of the two-wavelength infrared detector 10 of the first embodiment. The two-wavelength infrared detector 10 detects infrared rays of different wavelengths separately by changing the polarity of the voltage applied to the electrodes 21 and 22.

In FIG. 3, a detection signal of infrared light in the first wavelength band (λ1) is extracted from the first light-receiving layer 15. FIG. 3A is a schematic cross-sectional view of the two-wavelength infrared detector 10, and FIG. 3B is an energy band diagram. When a positive voltage is applied to the electrode 21 and a negative voltage is applied to the electrode 22, the p-type first light-receiving layer 15 becomes reverse biased and functions as a light-receiving section. Minority carrier electrons generated by light absorption in the first light-receiving layer 15 flow toward the second electrode layer 18 along the electric field and are detected as a signal.

On the other hand, the p-type second light-receiving layer 17 is in a forward bias state, and carriers (electron-hole pairs) generated by light absorption are recombined and disappear, or are negligibly small. The barrier layer 16 blocks holes generated in the second light-receiving layer 17.

In FIG. 4, a detection signal of infrared light in the second wavelength band (λ2) is extracted from the second light-receiving layer 17 by switching the polarity of the applied voltage. FIG. 4A is a schematic cross-sectional view of the two-wavelength infrared detector 10, and FIG. 4B is an energy band diagram. A negative voltage is applied to the electrode 21 and a positive voltage is applied to the electrode 22. The p-type second light-receiving layer 17 is in a reverse bias state and functions as a light-receiving section. Minority carrier electrons generated by light absorption in the second light-receiving layer 17 flow toward the first electrode layer 14 along the electric field and are detected as a signal.

As shown in FIG. 4B, the band gap of the etching stopper layer 13 is smaller than or equal to the band gap of the first light-receiving layer 15, and there is no potential barrier for electrons in the conduction band. Unlike FIG. 1B, the flow of electrons is not inhibited, and the amount of light received by the second light-receiving layer 17 is detected correctly.

On the other hand, the p-type first light-receiving layer 15 is in a forward bias state, and carriers (electron-hole pairs) generated by light absorption are recombined and disappear, or are negligibly small. Holes generated in the first light-receiving layer 15 are blocked by the barrier layer 16.

In the two-wavelength infrared detector 10 of the first embodiment, by switching the polarity of the voltage applied to the electrodes 21 and 22, the results of receiving infrared light at λ1 and the results of receiving infrared light at λ2 are correctly detected. be able to.

5A to 5E are manufacturing process diagrams of the two-wavelength infrared detector 10. The manufacturing process described with reference to FIGS. 5A to 5E is an example, and the material, composition, film thickness, layer structure, etc. may be changed as appropriate. In FIG. 5A, a buffer layer 12, an etching stopper layer 13, a first electrode layer 14, a first light-receiving layer 15, a barrier layer 16, a second light-receiving layer 17, and a second electrode layer 18 are formed on a substrate 11 in this order. epitaxial growth.

An n-type GaSb (100) substrate is introduced into a substrate introduction chamber of a molecular beam epitaxy (MBE) device. The GaSb substrate 11 is degassed in a preparation chamber, and then transported to a growth chamber maintained at an ultra-high vacuum. The substrate 11 transported to the growth chamber is heated in an Sb atmosphere to remove the oxide film on the surface. After removing the oxide film, in order to improve the flatness of the surface of the substrate 11, a buffer layer 12 of, for example, GaSb is grown to a thickness of 100 nm at a substrate temperature of 500°C.

Next, an etching stopper layer 13 is grown to a thickness of 300 nm. The etching stopper layer 13 is formed of, for example, a superlattice of InAs and InAsSb. The superlattice has a structure in which InAs with a thickness of 8.4 nm and InAs _0.5 Sb _0.5 with a thickness of 2.2 nm are stacked in this order as one period (unit structure), and is repeated 30 times. The difference between the average lattice constant of this superlattice and the lattice constant of GaSb of the substrate 11 is about 1600 ppm. If the lattice constant difference is within ±2000 ppm, defects caused by the lattice constant difference can be suppressed.

The band gap of the InAs/InAsSb superlattice of the etching stopper layer 13 is approximately 0.116 eV. The etching stopper layer 13 is doped with, for example, Be as an impurity, and exhibits p-type conductivity with a hole concentration of 1×10 ¹⁸ cm ⁻³ .

Next, the first electrode layer 14 is formed using a superlattice of InAs and GaSb. For example, the first electrode layer 14 is formed of a superlattice in which one period is a structure in which InAs with a thickness of 3.0 nm and GaSb with a thickness of 1.2 nm are laminated in this order. The InAs/GaSb superlattice is grown to a thickness of 360 nm by repeating 80 cycles. The band gap of this superlattice is approximately 0.256 eV. The first electrode layer 14 is doped with, for example, Be as an impurity, and exhibits p-type conductivity with a hole concentration of 1×10 ¹⁸ cm ⁻³ .

Next, a first light-receiving layer 15 is formed using a superlattice of InAs and GaSb. A superlattice in which one period is a structure in which InAs with a thickness of 3.0 nm and GaSb with a thickness of 1.2 nm are laminated in this order is grown to a thickness of 1300 nm by repeating 310 periods. The bandgap of this superlattice is approximately 0.256 eV, and it is sensitive to infrared light in the medium wavelength band (3 to 5 μm). The first light-receiving layer 15 has, for example, a p-type conductivity type with a hole concentration of 1×10 ¹⁶ cm ⁻³ .

Next, a barrier layer 16 is formed of a superlattice of InAs and AlSb, for example. The barrier layer 16 is formed of, for example, a superlattice in which one period is a structure in which 4.6 nm of InAs and 1.2 nm of AlSb are laminated in this order. The InAs/AlSb superlattice is grown to a thickness of about 100 nm by repeating, for example, 20 cycles. The band gap of this superlattice is approximately 0.484 eV. The barrier layer 16 mainly functions as a barrier that blocks only holes, and has, for example, a p-type conductivity type with a hole concentration of 1×10 ¹⁶ cm ⁻³ .

Next, a second light-receiving layer 17 is formed using a superlattice of InAs and GaSb. The second light-receiving layer 17 is formed of, for example, a superlattice in which one period is a structure in which 4.2 nm of InAs and 2.1 nm of GaSb are laminated in this order. The InAs/GaSb superlattice is grown to a thickness of about 1300 nm by repeating, for example, 200 cycles. The band gap of the InAs/GaSb superlattice of the second light-receiving layer 17 is about 0.127 eV, and the second light-receiving layer 17 is sensitive to infrared rays in the long wavelength band (8 to 12 μm). The second light-receiving layer has, for example, a p-type conductivity type with a hole concentration of 1×10 ¹⁶ cm ⁻³ .

Next, the second electrode layer 18 is formed of, for example, a superlattice of InAs and GaSb. The second electrode layer 18 is formed of a superlattice in which one period is a structure in which, for example, 4.2 nm of InAs and 2.1 nm of GaSb are laminated in this order. The InAs/GaSb superlattice is grown to a thickness of about 360 nm by repeating, for example, 60 cycles. The band gap of this superlattice is approximately 0.127 eV. The second electrode layer 18 has, for example, a p-type conductivity type with a hole concentration of 1×10 ¹⁸ cm ⁻³ . Through the above steps, the laminated structure shown in FIG. 5A is obtained.

In FIG. 5B, a mesa 35 is formed by etching the stacked structure of FIG. 5A. The second electrode layer 18, second light-receiving layer 17, barrier layer 16, first light-receiving layer 15, and first electrode layer 14 are selectively etched so that a part of the surface of the etching stopper layer 13 is exposed. Although only one mesa 35 is depicted in FIG. 5B, a plurality of mesas 35 arranged in an array are formed in this etching process. The space between adjacent mesas 35 becomes a pixel isolation trench 36.

Mesa 35 is formed, for example, by reactive ion etching. During etching, a signal of Ga element contained in etching by-products is monitored. In the etching from the second electrode layer 18 to the first electrode layer 14, Ga element is detected, whereas the etching stopper layer 13 does not contain Ga element. By monitoring the signal of the Ga element, the etching depth can be controlled with high precision.

In FIG. 5C, an insulating film 19 is formed to cover the entire mesa 35 and the exposed surface of the etching stopper layer 13. The insulating film 19 is, for example, a 500 nm thick silicon oxide film, but other insulating films such as a silicon nitride film or a silicon oxynitride film may be formed. When forming a silicon oxide film, it can be formed by a chemical vapor deposition method using silane and dinitrogen monoxide as reaction gases.

In FIG. 5D, openings 37-1 and 37-2 are formed in predetermined locations of the insulating film 19 by selective etching using an etching mask, and a part of the second electrode layer 18 and the etching stopper layer 13 are formed. expose a part of

In FIG. 5E, electrodes 21 and 22 are formed in openings 37-1 and 37-2. Electrode 21 is connected to second electrode layer 18 . Electrode 22 is connected to etching stopper layer 13 . The electrodes 21 and 22 are made of Ti/Pt/Au, for example. Ti can function as an adhesive film with the underlying film. The electrodes 21 and 22 may be formed by sputtering and milling a metal film, or may be formed by vapor deposition and a lift-off method. After this, as will be described later, wiring connected to the electrode 22 at the outermost pixel of the infrared detector array and electrode pads connected to the electrode 21 at each effective pixel are formed, and a protruding electrode is provided at each pixel. . The substrate 11 and buffer layer 12 may be removed if necessary.

In the configuration of the first embodiment, the band gap between the first light receiving layer 15 and the first electrode layer 14 is approximately 0.256 eV, whereas the band gap of the etching stopper layer 13 is approximately 0.116 eV. Signals (electrons) generated by absorption of infrared light in the second light receiving layer 17 are transmitted from the electrode 22 via the barrier layer 16, the first light receiving layer 15, the first electrode layer 14, and the etching stopper layer 13. pulled out to the outside. At this time, since the bandgap of the etching stopper layer 13 is smaller than the bandgap of the first light-receiving layer 15 and the first electrode layer 14, there is no potential barrier to the detection signal (electron) of λ2, and the signal can be easily extracted. When a signal is extracted from the first light-receiving layer 15 by changing the polarity of the applied voltage, the signal can be detected with high sensitivity as in the conventional case, as shown in FIG.

Since the difference in lattice constant between the etching stopper layer 13 and the GaSb substrate 11 is within ±2000 ppm, defects due to the difference in lattice constant can be suppressed, and the etching stopper layer 13 satisfies the requirements from the viewpoint of film formation. While the upper first electrode layer 14 and first light-receiving layer 15 contain Ga, the etching stopper layer 13 does not contain Ga. By monitoring the Ga element signal during etching, the etching depth can be controlled with high precision. The etching stopper layer 13 also satisfies requirements from a process perspective.

The configuration of the first embodiment may be modified as appropriate as long as the above-mentioned effects can be obtained. The thickness of each layer of the superlattice forming the first light-receiving layer 15 and the second light-receiving layer 17 may be changed as appropriate depending on the target absorption wavelength. However, it is desirable that the energy difference between the lower ends of the conduction bands of the first light-receiving layer 15, the barrier layer 16, and the second light-receiving layer 17 is small. Typically, it is desirable that the energy difference at the bottom of the conduction band between adjacent layers is 10 meV or less so that the energy difference can be easily overcome by thermal energy.

The etching stopper layer 13 is not limited to a superlattice of InAs with a thickness of 8.4 nm and InAs _0.5 Sb _0.5 with a thickness of 2.2 nm. As long as the bandgap of the etching stopper layer 13 is the same as or smaller than the bandgap of the first light-receiving layer 15, the film thickness of InAs and the film thickness of InAsSb may be changed as appropriate. The etching stopper layer 13 may be formed of InAs _x Sb _1-x (0≦x<1) instead of the InAs/InAsSb superlattice. The etching stopper layer 13 preferably has a small difference in lattice constant from the GaSb substrate, and the difference in lattice constant is within ±2000 ppm, more preferably within ±1000 ppm.

As the p-type impurity, impurities other than Be, such as Zn, may be used. The method for stacking the two-wavelength infrared detector 10 is not limited to the MBE method, but may be the MOCVD method or other growth methods that can produce a stacked structure. The insulating film 19 may be formed by atomic layer deposition, chemical vapor deposition, sputtering, or the like.

<Second embodiment>
FIG. 6 is a schematic cross-sectional view of a two-wavelength infrared detector 20 according to the second embodiment. In the second embodiment, holes are extracted as signals. The two-wavelength infrared detector 20 includes, on the substrate 11, a buffer layer 12, an etching stopper layer 23, a first electrode layer 24, a first light-receiving layer 25, a barrier layer 26, a second light-receiving layer 27, and a second electrode layer. 28 are epitaxially grown in this order. The entire stacked body is covered with an insulating film 19 except for electrodes 21 and 22 provided at predetermined locations. Electrode 21 is connected to second electrode layer 28 . The electrode 22 is connected to an etching stopper layer 23.

The substrate 11 is an n-type GaSb (100) substrate similarly to the first embodiment. The buffer layer 12 is also a GaSb layer with a thickness of about 100 nm, similar to the first embodiment. The etching stopper layer 23 is a 300 nm thick layer formed of InAs/InAsSb superlattice. The superlattice is, for example, a superlattice in which one period is a structure in which InAs is laminated in the order of 8.4 nm and InAs _0.5 Sb _0.5 is laminated in the order of 2.2 nm. The difference between the average lattice constant of this superlattice and the lattice constant of GaSb, which is the substrate, is about 1600 ppm. If the lattice constant difference is within ±2000 ppm, defects associated with the lattice constant difference can be suppressed, so the requirements from the viewpoint of film formation are met.

The band gap of the InAs/InAsSb superlattice of the etching stopper layer 23 is about 0.116 eV. The etching stopper layer 23 is doped with Si as an impurity, for example, and exhibits n-type conductivity with an electron concentration of 1×10 ¹⁸ cm ⁻³ .

The first electrode layer 24 is a 360 nm thick layer formed of InAs/GaSb superlattice. One period has a structure in which InAs with a thickness of 2.4 nm and GaSb with a thickness of 3.6 nm are laminated in this order. The band gap of this InAs/GaSb superlattice is approximately 0.249 eV. The first electrode layer 24 is an n-type conductive layer doped with Si and having an electron concentration of 1×10 ¹⁸ cm ⁻³ .

The first light receiving layer 25 is a 1300 nm thick layer formed of InAs/GaSb superlattice. One period has a structure in which InAs with a thickness of 2.4 nm and GaSb with a thickness of 3.6 nm are laminated in this order. The bandgap of this InAs/GaSb superlattice is approximately 0.249 eV, and the first light receiving layer 25 is sensitive to infrared rays in the medium wavelength band (3 to 5 μm). The first light-receiving layer 25 has an n-type conductivity type with an electron concentration of 1×10 ¹⁶ cm ⁻³ and is doped with Si as an impurity, for example.

The barrier layer 26 is a 100 nm thick layer made of Al _0.2 Ga _0.8 Sb. The bandgap of Al _0.2 Ga _0.8 Sb is approximately 1.083 eV. The barrier layer 26 mainly blocks only electrons and has an n-type conductivity type with an electron concentration of 1×10 ¹⁶ cm ⁻³ .

The second light receiving layer 27 is a 1300 nm thick layer formed of InAs/GaSb superlattice. For example, one period of the superlattice has a structure in which InAs with a thickness of 4.2 nm and GaSb with a thickness of 2.1 nm are laminated in this order. The bandgap of the second light-receiving layer 27 is approximately 0.127 eV, and the second light-receiving layer is sensitive to infrared rays in the long wavelength band (8 to 12 μm). The second light-receiving layer 27 is, for example, an n-type with an electron concentration of 1×10 ¹⁶ cm ⁻³ .

The second electrode layer 28 is a 360 nm thick layer formed of InAs/GaSb superlattice. One period of the superlattice has a structure in which InAs with a thickness of 4.2 nm and GaSb with a thickness of 2.1 nm are laminated in this order. The band gap of the second electrode layer 28 is approximately 0.127 eV. The second electrode layer 28 is, for example, an n-type with an electron concentration of 1×10 ¹⁸ cm ⁻³ . The first light-receiving layer 25, the barrier layer 26, and the second light-receiving layer 27 form an nBn (electron blocking) type infrared detection structure.

7A and 7B are explanatory diagrams of the operation of the two-wavelength infrared detector 20 of the second embodiment. The two-wavelength infrared detector 20 detects infrared rays of different wavelengths by changing the polarity of the voltage applied to the electrodes 21 and 22.

In FIG. 7A, a detection signal of infrared light of λ1 is extracted from the first light-receiving layer 25. A negative voltage is applied from the electrode 21 to the second electrode layer 28, and a positive voltage is applied from the electrode 22 to the etching stopper layer 23. The n-type first light-receiving layer 25 is in a reverse bias state and functions as a light-receiving section. Minority carrier holes generated by light absorption in the first light-receiving layer 25 flow toward the second electrode layer 28 and are detected as a signal.

On the other hand, the n-type second light-receiving layer 27 is in a forward bias state, and carriers (electron-hole pairs) generated by light absorption are recombined and disappear, or are negligibly small. Barrier layer 26 blocks electrons generated in second light-receiving layer 17 .

In FIG. 7B, a detection signal of infrared light of λ2 is extracted from the second light-receiving layer 27 by switching the polarity of the applied voltage. A positive voltage is applied from the electrode 21 to the second electrode layer 28, and a negative voltage is applied from the electrode 22 to the etching stopper layer 23. The n-type second light-receiving layer 27 is in a reverse bias state and functions as a light-receiving section. Minority carrier holes generated by light absorption in the second light-receiving layer 27 flow toward the first electrode layer 24 and are detected as a signal.

As shown in FIG. 7B, the band gap of the etching stopper layer 23 is smaller than or equal to the band gap of the first light receiving layer 25, and there is no potential barrier for holes in the valence band. The flow of holes is not inhibited, and the amount of light received by the second light-receiving layer 27 is correctly detected.

On the other hand, the n-type first light-receiving layer 25 is in a forward bias state, and carriers (electron-hole pairs) generated by light absorption are recombined and disappear, or are negligibly small. Electrons generated in the first light-receiving layer 25 are blocked by the barrier layer 26.

The two-wavelength infrared detector 20 of the second embodiment detects holes as signals. By switching the polarity of the voltages applied to the electrodes 21 and 22, it is possible to correctly detect the results of receiving infrared light of λ1 and the results of receiving infrared light of λ2.

In the configuration of the second embodiment, the band gap between the first light receiving layer 25 and the first electrode layer 24 is about 0.249 eV, while the band gap of the etching stopper layer 23 is about 0.116 eV. Signals (holes) generated by absorbing infrared rays in the second light receiving layer 27 are extracted to the outside via the barrier layer 26, the first light receiving layer 25, the first electrode layer 24, and the etching stopper layer 23. Since the bandgap of the etching stopper layer 23 is smaller than the bandgap of the first light-receiving layer 25 and the first electrode layer 24, the etching stopper layer 23 does not become a potential barrier to signals (holes) and can easily extract signals. The flow of signals generated in the second light-receiving layer 27 is not reduced, and high sensitivity is maintained. When a signal (hole) is extracted from the first light-receiving layer 25 by changing the polarity of the applied voltage, the flow of the signal is not hindered as in the conventional case, and high sensitivity can be obtained.

As in the first embodiment, the etching stopper layer 23 has a lattice constant difference of within ±2000 ppm with GaSb of the substrate 11, and the etching depth can be controlled with high accuracy by monitoring the signal of the Ga element. It is.

The configuration of the second embodiment may be modified as appropriate within a range where effects can be obtained. Regarding the superlattice constituting the first light-receiving layer 25 and the second light-receiving layer 27, the thickness of each layer may be changed as appropriate depending on the absorption wavelength. It is desirable that the energy difference between the upper ends of the valence bands of the layers 27 is small. Typically, it is preferable that the energy difference at the top of the valence band between adjacent layers is 10 meV or less so that the energy difference can be easily overcome by thermal energy.

As long as the bandgap of the etching stopper layer 23 is the same as or smaller than the bandgap of the first light-receiving layer 25, the thickness of InAs and InAsSb may be changed as appropriate. Further, the etching stopper layer 23 may be formed of InAs _x Sb _1-x (0≦x<1).

In the second embodiment, the barrier layer 26 is made of Al _0.2 Ga _0.8 Sb, but Al _y Ga _1-y Sb (0<y≦1), AlAs _z Sb _1-z (0≦z≦1) ) etc. The dopants of the first electrode layer 24 and the second electrode layer 28 may be replaced with Si and other n-type impurities, such as Te, may be used. As described in the first embodiment, the method for growing the stack and the method for forming the insulating film 19 can be selected as appropriate.

<Image sensor using a two-wavelength infrared detector>
FIG. 8 is a schematic diagram of an image sensor 100 using a two-wavelength infrared detector 10. The image sensor 100 includes an infrared detector array 50 including an array of pixels 101 and a readout circuit 60. The infrared detector array 50 is flip-chip bonded to the readout circuit 60 by the protruding electrodes 31. An underfill may be filled between the infrared detector array 50 and the readout circuit 60 and cured. In FIG. 8, the infrared detector array 50 uses the two-wavelength infrared detector 10 of the first embodiment, but the two-wavelength infrared detector 20 of the second embodiment may also be used.

The surface of the infrared detector array 50 opposite to the protruding electrode 31 is a light incident surface, and light enters from the first light receiving layer 15 (or 25) side. In the case of a type II superlattice, the first light-receiving layer 15 having a short response wavelength is provided on the side closer to the light incidence surface, due to the relationship between the band gap of the light-receiving layer and the wavelength response characteristics.

The readout circuit 60 has a drive cell corresponding to each pixel 101 of the infrared detector array 50. Each drive cell reads electrons as signals from the first light-receiving layer 15 and the second light-receiving layer 17 by alternately applying a bias that gives a positive potential and a bias that gives a negative potential to the corresponding pixel 101. Each drive cell integrates each of the charges corresponding to the first wavelength and the second wavelength, and outputs a voltage signal corresponding to the incident intensity of each wavelength. The readout circuit 60 may sequentially scan the response output from each pixel 101 and output a signal containing information on the temperature distribution and gas distribution from the observation target as a time-series signal. In this case, a time-series signal of a first wavelength (for example, a mid-infrared wavelength) and a time-series signal of a second wavelength (far-infrared wavelength) may be output alternately.

FIG. 9 is a schematic cross-sectional view showing a part of the infrared detector array 50 used in the image sensor 100. For example, the two-wavelength infrared detector 10 of the first embodiment is one pixel 101, and a plurality of pixels 101 are arranged two-dimensionally. Each pixel 101 is separated from each other by a pixel isolation trench 36, and is electrically connected to a transistor 61 of a corresponding drive cell of the readout circuit 60 via an individual protruding electrode 31p made of In or the like.

The individual projecting electrodes 31p are arranged on the metal film 32 for surface wiring connected to the electrode 21 provided on the mesa of each pixel 101. A bias voltage corresponding to the gate voltage _VIG applied to the gate of the transistor 61 of the corresponding drive cell is selectively applied to the pixel 101 from the protrusion electrode 31p.

The outermost pixel of the infrared detector array 50 is connected to a common potential _VA by a common electrode 31d as a dummy pixel 101D. The common electrode 31d is connected to the electrode 22 in ohmic contact with the etching stopper layer 13 via the wiring 33, and a common potential _VA is supplied to each pixel 101 via the etching stopper layer 13.

The metal film 32 and the wiring 33 are formed in the same process after the process shown in FIG. 5E. Individual protruding electrodes 31p and common electrodes 31d are formed on the metal film 32 and wiring 33 in the same process. After the infrared detector array 50 is flip-chip bonded to the readout circuit 60 via the protruding electrode 31p and the common electrode 31d, the substrate 11 and buffer layer 12 may be removed, if necessary. As a result, the image sensor 100 shown in FIG. 8 is obtained.

FIG. 10 is a schematic block diagram of an imaging system 1 incorporating an image sensor 100. The imaging system 1 includes an image sensor 100, a control calculation section 2 connected to the image sensor 100, and a display section 3 connected to the control calculation section 2. The optical system 110 may be placed on the light incident side of the image sensor 100.

The optical system 110 forms an optical image of the incident infrared rays on the light incidence surface of the infrared detector array 50 of the image sensor 100 . Infrared intensity information detected at two wavelengths for each pixel 101 is alternately read out to the readout circuit 60. Infrared intensity information obtained from the infrared detector array 50 represents infrared radiation from the observation target, that is, temperature distribution and gas concentration distribution.

A voltage signal output from the readout circuit 60 of the image sensor 100 is connected to an input of the control calculation section 2. Before the voltage signal is input to the control calculation section 2, it may be converted into a digital signal. The control calculation unit 2 performs correction processing, image processing, etc. on the output signal from the readout circuit 60 to generate an image according to the temperature distribution and gas distribution of the observation target. The correction processing may include correction of variations in sensitivity and nonlinearity for each pixel 101.

In the infrared detector array 50 using the two-wavelength infrared detector 10 or 20 of the embodiment, a decline in signal flow on the longer wavelength side is suppressed in each pixel, and a high-quality infrared image can be obtained. The image generated by the control calculation unit 2 is displayed on the display unit 3, and the temperature distribution and gas distribution can be visually recognized.

The image sensor 100 and the imaging system 1 of the embodiment can be applied to fields such as security and infrastructure inspection. The two-wavelength infrared detector 10 or 20 used in the image sensor 100 has an energy band structure that does not inhibit the flow of minority carriers, suppresses signal degradation on the long wavelength side, and has high identification accuracy for the observed object.

1 imaging system 10, 20 two-wavelength infrared detector 11 substrate 12 buffer layer 13, 23 etching stopper layer 14, 24 first electrode layer 15, 25 first light-receiving layer 16, 26 barrier layer 17, 27 second light-receiving layer 18, 28 Second electrode layer 21, 22 Electrode 31, 31p Projection electrode 31d Common electrode 32 Metal film 33 Wiring 50 Two-wavelength infrared detector array 60 Readout circuit 100 Image sensor 101 Pixel

Claims (9)

an etching stopper layer electrically connected to an electrode to which a common potential is applied;
a first light-receiving layer provided on the etching stopper layer and absorbing infrared rays in a first wavelength band;
a second light-receiving layer provided on the first light-receiving layer and absorbing infrared rays in a second wavelength band;
has
The longest infrared wavelength detectable by the first light-receiving layer is shorter than the longest infrared wavelength detectable by the second light-receiving layer,
The bandgap of the etching stopper layer is the same as the bandgap of the first light-receiving layer, or smaller than the bandgap of the first light-receiving layer.
Infrared detector. The first light-receiving layer is made of a material containing Ga, and the etching stopper layer is made of a material not containing Ga.
The infrared detector according to claim 1. The first light-receiving layer is formed of an InAs/GaSb superlattice, and the etching stopper layer is formed of an InAs/InAsSb superlattice.
The infrared detector according to claim 1 or 2. a barrier layer provided between the first light-receiving layer and the second light-receiving layer and serving as a potential barrier for either electrons or holes;
The infrared detector according to any one of claims 1 to 3, comprising: The conductivity type of the first light-receiving layer and the second light-receiving layer is p-type,
The energy for electrons at the lower end of the conduction band of the etching stopper layer is lower than the energy for electrons in the first light-receiving layer, the barrier layer, and the second light-receiving layer.
The infrared detector according to claim 4. The conductivity type of the first light-receiving layer and the second light-receiving layer is n-type,
The energy for holes at the upper end of the valence band of the etching stopper layer is higher than the energy for holes in the first light-receiving layer, the barrier layer, and the second light-receiving layer.
The infrared detector according to claim 4. The energy difference at the lower end of the conduction band or the energy difference at the upper end of the valence band of the first light-receiving layer, the barrier layer, and the second light-receiving layer is 10 meV or less,
The infrared detector according to any one of claims 4 to 6. The first wavelength band is a wavelength band longer than 3 μm, and the second wavelength band is a wavelength band longer than 8 μm.
The infrared detector according to any one of claims 1 to 7. An infrared detector according to any one of claims 1 to 8,
a readout circuit that is electrically connected to the infrared detector and reads out a signal from the infrared detector;
An image sensor with

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