JP7803209B2 - Infrared detector and image sensor using the same - Google Patents

Description

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

Type II superlattice (T2SL) is expected to be a next-generation infrared detector material to replace mercury cadmium telluride (MCT), and development is underway. Most infrared detectors using Type II superlattice have a superlattice structure formed on a GaSb substrate using materials with lattice constants close to GaSb, such as GaSb, InAs, and AlSb. By using an appropriately designed superlattice structure as the infrared-receiving layer, it is possible to detect infrared radiation in desired wavelength bands, such as the medium wavelength band (3-5 μm) and long wavelength band (8-12 μm).

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

When an infrared detector array is formed using a stack including a light-receiving layer, the stack is processed into a mesa shape to separate adjacent pixels. To improve the etching depth accuracy, an etching stopper layer is often provided below the stack. The etching stopper layer is made of a material that has a lattice constant close to that of the deposition substrate and a concentration of specific elements that differs significantly 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 that indicate specific elemental components.

Japanese Patent Application Laid-Open No. 2020-155513

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

When detecting infrared rays in two wavelength bands longer than 3 μm, if the band gap of the etching stopper layer is larger than the band gap of the light-receiving layer, there is a problem of not being able to extract the signal properly. This is because the etching stopper layer acts as a potential barrier against minority carriers. Depending on the type of signal being extracted and the energies of the conduction band and valence band of the etching stopper layer, it may act as a potential barrier against electrons or holes. The potential barrier impedes the flow of signals, especially on the longer wavelength side, reducing light-receiving sensitivity.

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

According to one aspect of the present disclosure, an infrared detector includes:
an etching stopper layer electrically connected to an electrode to which a common potential is applied;
a first absorption layer provided on the etching stopper layer and configured to absorb infrared light in a first wavelength band;
a second absorption layer provided on the first absorption layer and absorbing infrared light in a second wavelength band;
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 band gap of the etching stopper layer is the same as or smaller than the band gap of the first light-receiving layer.

In infrared detectors, it is possible to suppress a decrease in signal current and improve light receiving sensitivity.

1A and 1B are diagrams illustrating technical problems that may arise in a two-wavelength infrared detector. 1A and 1B are diagrams illustrating technical problems that may arise in a two-wavelength infrared detector. FIG. 1 is a cross-sectional schematic view of a two-wavelength infrared detector according to a first embodiment. 4A to 4C are diagrams illustrating the operation of the two-wavelength infrared detector of the first embodiment. 4A to 4C are diagrams illustrating the operation of the two-wavelength infrared detector of the first embodiment. 3A to 3C are diagrams illustrating steps for fabricating the two-wavelength infrared detector according to the first embodiment. 3A to 3C are diagrams illustrating steps for fabricating the two-wavelength infrared detector according to the first embodiment. 3A to 3C are diagrams illustrating steps for fabricating the two-wavelength infrared detector according to the first embodiment. 3A to 3C are diagrams illustrating steps for fabricating the two-wavelength infrared detector according to the first embodiment. 3A to 3C are diagrams illustrating steps for fabricating the two-wavelength infrared detector according to the first embodiment. FIG. 10 is a cross-sectional schematic view of a two-wavelength infrared detector according to a second embodiment. 10A and 10B are diagrams illustrating the operation of the two-wavelength infrared detector of the second embodiment. 10A and 10B are diagrams illustrating 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. 1 is a cross-sectional schematic diagram showing a portion 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 specific configuration examples of the present disclosure, technical issues that may arise in dual-wavelength infrared detectors will be described in more detail with reference to Figures 1A and 1B. The energy band diagrams in Figures 1A and 1B are based on a configuration in which an etching stopper layer (ES), a first electrode layer (EL1), a first absorption layer (L1) that absorbs light of wavelength λ1, a barrier layer (BR), a second absorption layer (L2) that absorbs light of wavelength λ2, and a second electrode layer (EL2) are stacked in this order.

To increase the light sensitivity of dual-wavelength infrared detectors using type II superlattices, they are often configured to detect highly mobile electrons as a signal. The configuration that extracts electrons as a signal is called the pBp (hole-blocking) type.

In Figure 1A, to extract 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). This creates a reverse bias state for the p-type first light-receiving layer (L1), 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. Holes generated in the first light-receiving layer (L1) flow toward 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 recombine and disappear, or are negligibly small. Therefore, the λ1 infrared light incident on the first light-receiving layer (L1) is correctly detected.

1B, when electrons are extracted as a signal from the second absorption 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). This creates a reverse bias state for the p-type second absorption layer (L2), and electrons as minority carriers generated in the second absorption layer flow toward the first electrode layer (EL1). However, the conduction band of the etching stopper layer (ES), which has a larger band gap E _BG than the first absorption layer (L1), acts as a potential barrier, preventing the extraction of the signal (electrons).

To efficiently extract electrons as a signal from the second absorption layer (L2), the band gap _EBG of the etching stopper layer (ES) should be the same as or smaller than the band gap of the first absorption layer (L1). This configuration maintains the signal flow and increases the light-receiving sensitivity. Based on this knowledge, the specific configuration of the two-wavelength infrared detector of this embodiment will be described. In the following description, "above" or "below" refers to the top and bottom of the stacking direction or growth direction, and is not an absolute direction. In the drawings, the same components are denoted by the same symbols, and redundant explanations may be omitted.

First Embodiment
2 is a schematic cross-sectional view of a dual-wavelength infrared detector 10 according to the first embodiment. In the first embodiment, electrons are extracted as a signal. For convenience of illustration, the dual-wavelength infrared detector 10 is depicted as a single pixel 101. In actual use, a plurality of dual-wavelength infrared detectors 10 are arranged in one or two dimensions to form an infrared detector array.

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

The etching stopper layer 13 is used to improve the depth accuracy of the mesas 35 that form the individual pixels 101. The etching stopper layer 13 may function as part of the first electrode layer 14 as a common electrode shared by multiple pixels 101 included in the infrared detector array. In this case, the etching stopper layer (ES) may be doped with the same p-type impurity as the first electrode layer (EL1). In Figure 2, an electrode 22 connected to the etching stopper layer 13 is shown for the convenience of explaining the operation of the dual-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.

For example, a GaSb (100) substrate is used as the substrate 11. If a buffer layer 12 is provided, a GaSb layer that is lattice-matched to the substrate 11 may be used. The etching stopper layer 13 is formed as a superlattice, and its composition, film thickness, period, etc. are designed so that its band gap is the same as or smaller than the band gaps of the first electrode layer 14 and the first absorption layer 15.

The first electrode layer 14 may be designed to have the same band gap as the first absorption layer 15. Alternatively, as described below, the first electrode layer 14 and the first absorption layer 15 may be designed so that the energy difference between the conduction band minimums of the first electrode layer 14 and the first absorption layer 15 is 10 meV or less, and both are equal to or greater than the conduction band minimum of the etching stopper layer 13.

The first light receiving layer 15 is sensitive to a first wavelength band (e.g., 3 to 5 μm). The second light receiving layer 17 is sensitive to a second wavelength band (e.g., 8 to 12 μm). The longest infrared wavelength detectable by the first light receiving layer 15 is shorter than the longest infrared wavelength detectable by the second light receiving layer 17. In the first embodiment, electrons are detected as minority carriers, i.e., signals, so 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 to suppress dark current and extract signals separately from the first light receiving layer 15 and the second light receiving layer 17.

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

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

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

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

In Figure 4, the polarity of the applied voltage is switched to extract a detection signal for infrared light in the second wavelength band (λ2) from the second light receiving layer 17. Figure 4 (A) is a schematic cross-sectional view of the two-wavelength infrared detector 10, and (B) is an energy band diagram. A negative voltage is applied to electrode 21, and a positive voltage is applied to 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 along the electric field toward the first electrode layer 14 and are detected as a signal.

As shown in Figure 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 Figure 1B, the flow of electrons is not impeded, and the amount of light received by the second light-receiving layer 17 is correctly detected.

On the other hand, the p-type first absorption layer 15 is in a forward bias state, and the carriers (electron-hole pairs) generated by light absorption recombine and disappear, or are negligibly small. Holes generated in the first absorption 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 electrodes 21 and 22, it is possible to correctly detect the infrared light reception results of λ1 and the infrared light reception results of λ2.

Figures 5A to 5E are diagrams illustrating the manufacturing process for a two-wavelength infrared detector 10. The manufacturing process described with reference to Figures 5A to 5E is an example, and the materials, composition, film thickness, layer configuration, etc. can be changed as appropriate. In Figure 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 epitaxially grown in this order on a substrate 11.

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

Next, an etching stopper layer 13 is grown to a thickness of 300 nm. The etching stopper layer 13 is formed, for example, of a superlattice of InAs and InAsSb. The superlattice is formed by stacking 8.4 nm thick InAs and _2.2 nm thick _InAs0.5Sb0.5 in this order, with 30 periods (unit structures) being one period. The difference between the average lattice constant of this superlattice and the lattice constant of the GaSb substrate 11 is approximately 1600 ppm. If the lattice constant difference is within ±2000 ppm, defects associated with the lattice constant difference can be suppressed.

The band gap of the InAs/InAsSb superlattice of the etching stopper layer 13 is about 0.116 eV. The etching stopper layer 13 is doped with impurities such as Be, 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 using a superlattice structure in which 3.0 nm thick InAs and 1.2 nm thick GaSb are stacked in this order, with one period being a superlattice structure. The InAs/GaSb superlattice is repeated 80 times to grow to a thickness of 360 nm. 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, the first absorption layer 15 is formed using a superlattice of InAs and GaSb. A superlattice structure in which 3.0 nm thick InAs and 1.2 nm thick GaSb are stacked in this order is repeated 310 times to form a layer with a thickness of 1300 nm. The band gap 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 absorption layer 15 has, for example, a p-type conductivity with a hole concentration of 1×10 ¹⁶ cm ⁻³ .

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

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

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

In Figure 5B, the layered structure of Figure 5A is etched to form mesas 35. 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 portion of the surface of the etching stopper layer 13 is exposed. Although only one mesa 35 is shown in Figure 5B, multiple mesas 35 arranged in an array are formed in this etching process. The spaces between adjacent mesas 35 become pixel isolation grooves 36.

The mesa 35 is formed, for example, by reactive ion etching. During etching, the signal of Ga elements contained in the etching by-products is monitored. Ga elements are detected during etching from the second electrode layer 18 to the first electrode layer 14, but Ga elements are not contained in the etching stopper layer 13. By monitoring the signal of Ga elements, 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 silicon nitride film or silicon oxynitride film may also be formed. When forming a silicon oxide film, it can be formed by chemical vapor deposition using silane and nitrous oxide as reactive gases.

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

In Figure 5E, electrodes 21 and 22 are formed in openings 37-1 and 37-2. Electrode 21 is connected to the second electrode layer 18. Electrode 22 is connected to the etching stopper layer 13. Electrodes 21 and 22 are formed, for example, from Ti/Pt/Au. Ti can function as an adhesive film for the underlying film. Electrodes 21 and 22 may be formed by sputtering and milling a metal film, or by vapor deposition and lift-off. Thereafter, as described below, wiring connected to electrode 22 at the outermost pixels of the infrared detector array and electrode pads connected to electrode 21 at each effective pixel are formed, providing protruding electrodes at each pixel. If necessary, the substrate 11 and buffer layer 12 may be removed.

In the configuration of the first embodiment, the band gap of the first light receiving layer 15 and the first electrode layer 14 is approximately 0.256 eV, while the band gap of the etching stopper layer 13 is approximately 0.116 eV. The signal (electrons) generated by the absorption of infrared light in the second light receiving layer 17 passes through the barrier layer 16, the first light receiving layer 15, the first electrode layer 14, and the etching stopper layer 13, and is extracted to the outside from the electrode 22. Because the band gap of the etching stopper layer 13 is smaller than the band gaps of the first light receiving layer 15 and the first electrode layer 14, there is no potential barrier for the λ2 detection signal (electrons), and the signal can be easily extracted. When the polarity of the applied voltage is changed to extract the signal from the first light receiving layer 15, the signal can be detected with high sensitivity, similar to conventional methods, as shown in Figure 3.

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

The configuration of the first embodiment may be modified as needed, provided that the above-described effects are achieved. The superlattice forming the first absorption layer 15 and the second absorption layer 17 may have different thicknesses for each layer depending on the target absorption wavelength. However, it is desirable that the energy difference between the conduction band minimums of the first absorption layer 15, the barrier layer 16, and the second absorption layer 17 is small. Typically, the energy difference between the conduction band minimums of adjacent layers is desirably 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 8.4 nm thick InAs and 2.2 nm thick InAs _0.5 Sb _0.5 . The thickness of the InAs and InAsSb may be changed as appropriate as long as the band gap of the etching stopper layer 13 is the same as or smaller than the band gap of the first absorption layer 15. Instead of an InAs/InAsSb superlattice, the etching stopper layer 13 may be formed of InAs _x Sb _1-x (0≦x<1). The etching stopper layer 13 preferably has a small lattice constant difference from the GaSb substrate, and the lattice constant difference is preferably within ±2000 ppm, more preferably within ±1000 ppm.

As a p-type impurity, impurities other than Be, such as Zn, may be used. The deposition method for the dual-wavelength infrared detector 10 is not limited to MBE, and MOCVD or other growth methods capable of producing a layered structure may also be used. The insulating film 19 may be formed by atomic layer deposition, chemical vapor deposition, sputtering, or other methods.

Second Embodiment
6 is a cross-sectional schematic diagram 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 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 epitaxially grown in this order on a substrate 11. The stack is entirely covered with an insulating film 19, except for electrodes 21 and 22 provided at predetermined locations. The electrode 21 is connected to the second electrode layer 28. The electrode 22 is connected to the etching stopper layer 23.

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

The band gap of the InAs/InAsSb superlattice of the etching stopper layer 23 is approximately 0.116 eV. The etching stopper layer 23 is doped with, for example, Si as an impurity, 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 an InAs/GaSb superlattice. One period is a structure in which 2.4 nm thick InAs and 3.6 nm thick GaSb are stacked 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 absorption layer 25 is a 1,300 nm thick layer formed of an InAs/GaSb superlattice. One period is a structure in which 2.4 nm thick InAs and 3.6 nm thick GaSb are stacked in this order. The band gap of this InAs/GaSb superlattice is approximately 0.249 eV, and the first absorption layer 25 is sensitive to infrared light in the medium wavelength band (3 to 5 μm). The first absorption layer 25 has n-type conductivity, for example, doped with Si as an impurity, with an electron concentration of 1×10 ¹⁶ cm ⁻³ .

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

The second absorption layer 27 is a 1,300 nm thick layer formed of an InAs/GaSb superlattice. For example, one period of the superlattice is a structure in which 4.2 nm thick InAs and 2.1 nm thick GaSb are stacked in this order. The band gap of the second absorption layer 27 is approximately 0.127 eV, and the second absorption layer is sensitive to infrared light in the long wavelength band (8 to 12 μm). The second absorption layer 27 is, for example, an n-type layer with an electron concentration of 1×10 ¹⁶ cm ⁻³ .

The second electrode layer 28 is a 360 nm thick layer formed of an InAs/GaSb superlattice. One period of the superlattice is a structure in which 4.2 nm thick InAs and 2.1 nm thick GaSb are stacked 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 absorption layer 25, the barrier layer 26, and the second absorption layer 27 form an nBn (electron blocking) type infrared detection structure.

Figures 7A and 7B are diagrams illustrating 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 electrodes 21 and 22.

In Figure 7A, a detection signal of λ1 infrared light is extracted from the first light receiving layer 25. A negative voltage is applied from electrode 21 to the second electrode layer 28, and a positive voltage is applied from 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 absorption layer 27 is in a forward bias state, and the carriers (electron-hole pairs) generated by light absorption recombine and disappear, or are negligibly small. The barrier layer 26 blocks the electrons generated in the second absorption layer 17.

In Figure 7B, the polarity of the applied voltage is switched to extract a detection signal of λ2 infrared light from the second light receiving layer 27. A positive voltage is applied from electrode 21 to the second electrode layer 28, and a negative voltage is applied from 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 Figure 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 impeded, and the amount of light received by the second light receiving layer 27 is correctly detected.

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

The two-wavelength infrared detector 20 of the second embodiment detects holes as a signal. By switching the polarity of the voltage applied to electrodes 21 and 22, the results of receiving λ1 infrared light and the results of receiving λ2 infrared light can be correctly detected.

In the configuration of the second embodiment, the band gap of the first light receiving layer 25 and the first electrode layer 24 is approximately 0.249 eV, while the band gap of the etching stopper layer 23 is approximately 0.116 eV. Signals (holes) generated by absorbing infrared light 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. Because the band gap of the etching stopper layer 23 is smaller than the band gaps of the first light receiving layer 25 and the first electrode layer 24, the etching stopper layer 23 does not act as a potential barrier to the signals (holes), allowing the signals to be extracted easily. The flow of signals generated in the second light receiving layer 27 is not reduced, maintaining high sensitivity. When the polarity of the applied voltage is changed to extract signals (holes) from the first light receiving layer 25, the signal flow is not impeded as in the conventional case, and high sensitivity is obtained.

As in the first embodiment, the lattice constant difference between the etching stopper layer 23 and the GaSb substrate 11 is within ±2000 ppm, and the etching depth can be precisely controlled by monitoring the Ga element signal.

The configuration of the second embodiment may be modified as appropriate as long as the desired effect is achieved. The thickness of each layer of the superlattice constituting the first absorption layer 25 and the second absorption layer 27 may be modified as appropriate depending on the absorption wavelength, but it is desirable that the energy difference between the top of the valence band of each of the first absorption layer 25, barrier layer 26, and second absorption layer 27 is small. Typically, it is preferable that the energy difference between the top of the valence band of adjacent layers is 10 meV or less, so that the energy difference can be easily overcome by thermal energy.

The thickness of InAs or InAsSb may be changed as appropriate as long as the band gap of the etching stopper layer 23 is the same as or smaller than the band gap of the first absorption layer 25. Alternatively, 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 it may also be Al _y Ga _1-y Sb (0<y≦1), AlAs _z Sb _1-z (0≦z≦1), etc. The dopant in the first electrode layer 24 and the second electrode layer 28 may be replaced with other n-type impurities, such as Te, instead of Si. As described in the first embodiment, the method for growing the stack and the method for forming the insulating film 19 can be selected appropriately.

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

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

The readout circuit 60 has a drive cell corresponding to each pixel 101 of the infrared detector array 50. Each drive cell alternately applies a bias that provides a positive potential and a bias that provides a negative potential to the corresponding pixel 101, reading out electrons from the first light receiving layer 15 and the second light receiving layer 17 as a signal. Each drive cell integrates the charges corresponding to the first wavelength and the second wavelength, respectively, 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, it may alternately output a time-series signal of the first wavelength (e.g., mid-infrared wavelength) and a time-series signal of the second wavelength (far-infrared wavelength).

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

The individual protruding electrodes 31p are disposed on a metal film 32 for surface wiring that is connected to the electrode 21 provided on the top of the mesa of each pixel 101. A bias voltage corresponding to the gate voltage V _IG applied to the gate of the transistor 61 of the corresponding driving cell is selectively applied to the pixel 101 from the protruding electrodes 31p.

The pixels on the outermost periphery of the infrared detector array 50 are dummy pixels 101D, which are connected to a common potential V _A by a common electrode 31 d. The common electrode 31 d is connected via a wiring 33 to an electrode 22 that is in ohmic contact with the etching stopper layer 13, and the common potential V _A is supplied to each pixel 101 via the etching stopper layer 13.

The metal film 32 and wiring 33 are formed in the same process after the process of Figure 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 electrodes 31p and common electrodes 31d, the substrate 11 and buffer layer 12 may be removed as necessary. This results in the image sensor 100 of Figure 8.

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

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

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

The infrared detector array 50 using the two-wavelength infrared detector 10 or 20 of the embodiment suppresses the decline in signal flow on the long wavelength side at each pixel, producing high-quality infrared images. The image generated by the control and calculation unit 2 is displayed on the display unit 3, allowing the temperature distribution and gas distribution to be visually recognized.

The image sensor 100 and 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 impede the flow of minority carriers, suppresses signal reduction on the long wavelength side, and provides high accuracy in identifying the object being observed.

REFERENCE SIGNS LIST 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 Protruding electrode 31d Common electrode 32 Metal film 33 Wiring 50 Two-wavelength infrared detector array 60 Readout circuit 100 Image sensor 101 Pixel

Claims (10)

an etching stopper layer electrically connected to an electrode to which a common potential is applied;
a first absorption layer provided on the etching stopper layer and configured to absorb infrared light in a first wavelength band;
a second absorption layer provided on the first absorption layer and absorbing infrared light 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;
the band gap of the etching stopper layer is the same as or smaller than the band gap of the first absorption layer;
the conductivity type of the first absorption layer and the second absorption layer is p-type,
energy of electrons at the bottom of the conduction band of the etching stopper layer is lower than energy of electrons of the first absorption layer;
Infrared detector. an etching stopper layer electrically connected to an electrode to which a common potential is applied;
a first absorption layer provided on the etching stopper layer and configured to absorb infrared light in a first wavelength band;
a second absorption layer provided on the first absorption layer and absorbing infrared light 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;
the band gap of the etching stopper layer is the same as or smaller than the band gap of the first absorption layer;
the conductivity type of the first absorption layer and the second absorption layer is n-type;
the energy of holes at the top of the valence band of the etching stopper layer is higher than the energy of holes in the first absorption layer;
Infrared detector. the first absorption layer is formed of a material containing Ga, and the etching stopper layer is formed of a material not containing Ga;
3. The infrared detector according to claim 1 or 2 . an etching stopper layer electrically connected to an electrode to which a common potential is applied;
a first absorption layer provided on the etching stopper layer and configured to absorb infrared light in a first wavelength band;
a second absorption layer provided on the first absorption layer and absorbing infrared light 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;
the band gap of the etching stopper layer is the same as or smaller than the band gap of the first absorption layer;
the first absorption layer is formed of an InAs/GaSb superlattice, and the etching stopper layer is formed of an InAs/InAsSb superlattice;
Infrared detector . a barrier layer provided between the first absorption layer and the second absorption layer, the barrier layer serving as a potential barrier against either electrons or holes;
5. The infrared detector of claim 4 , wherein a barrier layer provided between the first absorption layer and the second absorption layer, the barrier layer serving as a potential barrier against holes;
the energy of electrons at the bottom of the conduction band of the etching stopper layer is lower than the energy of electrons of the barrier layer and the second absorption layer;
10. The infrared detector of claim 1 . a barrier layer provided between the first absorption layer and the second absorption layer, the barrier layer serving as a potential barrier against electrons;
the energy of holes at the top of the valence band of the etching stopper layer is higher than the energy of holes in the barrier layer and the second absorption layer;
3. The infrared detector of claim 2 . an energy difference between the conduction band minimums or valence band maximums of the first absorption layer, the barrier layer, and the second absorption layer is 10 meV or less;
The infrared detector according to any one of claims 5 to 7 . a long wavelength band, and the second wavelength band is a wavelength band longer than 8 μm;
The infrared detector according to any one of claims 1 , 2 and 4 to 7 . The infrared detector according to any one of claims 1 , 2 and 4 to 7 ,
a readout circuit electrically connected to the infrared detector for reading out a signal from the infrared detector;
An image sensor having: