JP7505320B2 - Dual wavelength photodetector and image sensor using same - Google Patents

Description

This disclosure relates to a dual-wavelength photodetector 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). Most infrared detectors using type II superlattice have an InAs/GaSb superlattice on a GaSb substrate as a light-receiving layer, which is easy to lattice match with the substrate and has a large absorption coefficient.

A dual-wavelength infrared detection element is known in which two photosensitive layers with different photosensitive wavelengths formed by multiple quantum wells are separated by an intermediate layer, and the sensitivity wavelength is controlled by switching the polarity of the applied voltage (see, for example, Patent Document 1).An infrared detector is known in which an Al _x Ga _1-x Sb/InAs (x>0.3) superlattice made of III-V group compound semiconductors is used, and electrodes are attached to the end faces of the superlattice (see, for example, Patent Document 2).

JP 2000-295146 A Japanese Patent Application Laid-Open No. 6-196745

To increase the light receiving sensitivity of a two-wavelength photodetector using a type II superlattice, it is desirable to detect electrons with high mobility as a signal. On the other hand, to make the light receiving layer of a type II superlattice sensitive to light of different wavelengths, the thickness of the thin film forming the superlattice is controlled to control the size of the band gap of the superlattice. In an InAs/GaSb superlattice, the infrared detection wavelength is controlled by controlling the film thickness of the InAs. However, changing the film thickness of the InAs changes the energy position of the lower end of the conduction band of the superlattice, and the energy of the lower end of the conduction band of the superlattice of the light receiving layer on the long wavelength side becomes lower than the energy of the lower end of the conduction band of the superlattice of the light receiving layer on the short wavelength side. When detecting electrons as a signal, the short wavelength type light receiving layer becomes a potential barrier for the electrons in the light receiving layer on the long wavelength side, and the signal on the long wavelength side decreases.

The purpose of this disclosure is to suppress the decrease in signal current on the long wavelength side and improve the light receiving sensitivity in a dual-wavelength photodetector.

According to one aspect of the present disclosure, a dual wavelength photodetector includes:
a first light receiving layer having sensitivity to a first wavelength;
a second light receiving layer having sensitivity to a second wavelength longer than the first wavelength;
a barrier layer disposed between the first absorption layer and the second absorption layer, the barrier layer serving as a potential barrier against holes;
the first absorption layer is formed of a first superlattice including InAs, GaSb, and AlSb, and the second absorption layer is formed of a second superlattice of InAs and GaSb.

In a two-wavelength photodetector, the decrease in signal current on the longer wavelength side is suppressed, improving the light receiving sensitivity.

1 is a schematic cross-sectional view of a dual-wavelength photodetector according to a first embodiment. FIG. 3A to 3C are diagrams illustrating the operation principle of the two-wavelength photodetector of the first embodiment. 3A to 3C are diagrams illustrating the operation principle of the two-wavelength photodetector of the first embodiment. 5A to 5C are diagrams illustrating the effects of the dual-wavelength photodetector according to the first embodiment. 3A to 3C are diagrams illustrating steps for fabricating the dual-wavelength photodetector according to the first embodiment. 3A to 3C are diagrams illustrating steps for fabricating the dual-wavelength photodetector according to the first embodiment. 3A to 3C are diagrams illustrating steps for fabricating the dual-wavelength photodetector according to the first embodiment. 3A to 3C are diagrams illustrating steps for fabricating the dual-wavelength photodetector according to the first embodiment. 3A to 3C are diagrams illustrating steps for fabricating the dual-wavelength photodetector according to the first embodiment. FIG. 11 is a schematic cross-sectional view of a dual-wavelength photodetector according to a second embodiment. 13A to 13C are diagrams illustrating the effects of the two-wavelength photodetector according to the second embodiment. FIG. 1 is a schematic diagram of an image sensor according to an embodiment. 1 is a schematic cross-sectional view showing a portion of a photodetector array used in an image sensor. FIG. 1 is a schematic block diagram of an imaging system incorporating an image sensor.

In an embodiment, the light receiving layer of a dual wavelength photodetector sensitive to infrared rays of different wavelengths is formed with a type II superlattice, and a potential barrier against holes is provided between the two light receiving layers. The band gap of each layer is controlled so that the energy positions of the conduction band lower end of the barrier layer and the conduction band lower end of the short wavelength light receiving layer do not hinder the movement of electrons relative to the conduction band lower end of the long wavelength light receiving layer. Specifically, in a configuration in which a barrier layer is disposed between two light receiving layers, the magnitude (absolute value) of the energy difference of the conductor lower ends between adjacent layers is set to 20 meV or less, more preferably 10 meV or less. This suppresses the decrease in signal current on the long wavelength side, and improves light receiving sensitivity.

The specific configuration will be described below with reference to the drawings. In this specification and the accompanying drawings, components having substantially the same functional configuration may be designated by the same reference numerals to avoid redundant description.

First Embodiment
1 is a schematic cross-sectional view of a dual-wavelength photodetector 10 according to a first embodiment. For convenience of illustration, the dual-wavelength photodetector 10 is depicted as one pixel 101, but in actual use, a plurality of dual-wavelength photodetectors 10 may be used as a photodetector array arranged in one or two dimensions.

The two-wavelength photodetector 10 has a layered structure in which 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. At least one of a buffer layer 12 and an etching stopper layer 13 may be provided between the substrate 11 and the first electrode layer 14.

The first electrode layer 14 is used in common for multiple pixels 101 included in the photodetector array. A mesa 35 connected to the first electrode layer 14, i.e., a mesa 35 including a first light receiving layer 15, a barrier layer 16, a second light receiving layer 17, and a second electrode layer 18, forms one pixel 101. The mesa 35 and the first electrode layer 14 are entirely covered with an insulating film 19, except for electrodes 21 and 22 provided at predetermined locations.

For example, a GaSb (100) substrate is used as the substrate 11. When the buffer layer 12 and the etching stopper layer 13 are provided, a GaSb layer that is lattice-matched with the substrate 11 may be used as the buffer layer 12, and an InAsSb layer may be used as the etching stopper layer 13. The composition of the InAsSb layer is desirably set so as to lattice-match with GaSb, and for example, an InAs _0.91 Sb _0.09 layer is used.

The first electrode layer 14 is formed, for example, of a superlattice of InAs, GaSb, and AlSb. The first light receiving layer 15 has sensitivity to a first wavelength band (for example, 3 to 5 μm) and is formed of a superlattice of InAs, GaSb, and AlSb. The barrier layer 16 is formed, for example, of a superlattice of InAs and AlSb. The second light receiving layer 17 has sensitivity to a second wavelength band (for example, 8 to 12 μm) and is formed of a superlattice of InAs and GaSb. The second electrode layer 18 is formed, for example, of a superlattice of InGa and GaSb.

In the embodiment, electrons are detected as minority carriers, i.e., signals, so the first absorption layer 15 and the second absorption layer 17 have p-type conductivity. The first absorption layer 15 and the second absorption layer 17 are separated by a barrier layer 16 in the stacking direction. The difference between the energy position of the conduction band minimum of the superlattice of the first absorption layer 15 and the energy position of the conduction band minimum of the superlattice of the second absorption layer 17 is 20 meV or less, more preferably 10 meV or less.

The barrier layer 16 functions as a potential barrier against holes in order to suppress dark current and extract signals separately from the first light receiving layer 15 and the second light receiving layer 17. The energy band structures of the first light receiving layer 15, the barrier layer 16, and the second light receiving layer 17 will be described later with reference to FIG. 3.

Figures 2A and 2B are diagrams explaining the operating principle of the dual-wavelength photodetector 10. The first light receiving layer 15 is sensitive to a first wavelength (λ1), and the second light receiving layer 17 is sensitive to a second wavelength (λ2). λ2 is a longer wavelength than λ1. The dual-wavelength photodetector 10 can switch the wavelength of infrared light to be detected by changing the polarity of the voltage applied to the electrodes 21 and 22.

In FIG. 2A, when a positive voltage is applied to electrode 21 connected to second electrode layer 18 and a negative voltage is applied to electrode 22 connected to first electrode layer 14, the p-type first light receiving layer 15 is in a reverse bias state, and the first light receiving layer 15 functions as a light receiving section. Electrons as minority carriers 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. Meanwhile, the p-type second light receiving layer 17 is in a forward bias state, and carriers (electron-hole pairs) generated by light absorption recombine and disappear, or become negligibly small.

In FIG. 2B, when a negative voltage is applied to electrode 21 and a positive voltage is applied to electrode 22, p-type second light receiving layer 17 is in a reverse bias state, and second light receiving layer 17 functions as a light receiving section. Electrons as minority carriers generated by light absorption in second light receiving layer 17 flow along the electric field toward first electrode layer 14 and are detected as a signal. Meanwhile, p-type first light receiving layer 15 is in a forward bias state, and carriers (electron-hole pairs) generated by light absorption recombine and disappear, or become negligibly small.

By changing the polarity of the voltage applied to electrodes 21 and 22, infrared rays of different wavelengths can be detected. In the operation shown in Figures 2A and 2B, electrons generated by light absorption in one light-receiving layer cross the barrier layer 16 and the other light-receiving layer and are extracted from the corresponding electrode. In Figure 2A, of the electron-hole pairs generated in the first light-receiving layer 15, the electrons pass through the barrier layer 16 and the second light-receiving layer 17 and are extracted. In Figure 2B, of the electron-hole pairs generated in the second light-receiving layer 17, the electrons pass through the barrier layer 16 and the first light-receiving layer 15 and are extracted.

If there is a potential barrier against the electrons during the process of extracting them, the higher the potential barrier, the more the electrons are unable to overcome the barrier, resulting in a decrease in signal. In a conventional general configuration, when the first light receiving layer 15 and the second light receiving layer 17 are formed from an InAs/GaSb superlattice and the thickness of the InAs is changed to change the infrared wavelength to be detected, the energy position of the lower end of the conduction band of the superlattice changes, and the barrier against the electrons becomes higher.

For example, when a dual-wavelength photodetector sensitive to mid- and long-wavelength infrared light is constructed using an InAs/GaSb superlattice, an energy difference (0.1 eV or more) approximately equal to the band gap difference between the first and second absorption layers 15 and 17 occurs at the lower end of the conduction band of the first and second absorption layers 15 and 17.

The energy position of the conduction band minimum of an InAs/GaSb superlattice sensitive to the long wavelength band is lower than the energy position of the conduction band minimum of an InAs/GaSb superlattice sensitive to the medium wavelength band. In the process of electrons generated in the InAs/GaSb superlattice on the long wavelength side being extracted to the electrode, they are unable to overcome the potential barrier equivalent to the energy difference with the conduction band minimum of the InAs/GaSb superlattice sensitive to the medium wavelength band, and the signal on the long wavelength side cannot be extracted sufficiently.

In the embodiment, in order to suppress the decrease in the signal on the long wavelength side, the difference in energy between the lower end of the conduction band of the first absorption layer 15 and the lower end of the conduction band of the second absorption layer 17 is made small enough so as not to hinder the movement of electrons. In addition, the barrier layer 16 functions as a hole blocking layer to suppress dark current, while the band gap is designed so that it does not become a potential barrier for electrons.

Figure 3 is a diagram explaining the effect of the two-wavelength photodetector 10 of the first embodiment. In the first embodiment, the first absorption layer 15 on the short wavelength side is formed of a superlattice of InAs, GaSb, and AlSb, and the second absorption layer 17 is formed of a superlattice of InAs and GaSb. The band gap energy Eg of the superlattice is determined by the energy difference between the lower end of the conductor of the superlattice and the upper end of the valence band.

Unlike a typical InAs/GaSb superlattice dual-wavelength detector, in the dual-wavelength photodetector 10 of the first embodiment, the energy position of the conduction band lower end of the second absorption layer 17 on the long wavelength side is set higher than the energy position of the conduction band lower end of the first absorption layer 15. In addition, the difference between the energy position of the conduction band lower end of the first absorption layer 15 and the energy position of the conduction band lower end of the barrier layer 16, and the difference between the energy position of the conduction band lower end of the second absorption layer 17 and the energy position of the conduction band lower end of the barrier layer 16 are set to 20 meV or less, more preferably 10 meV or less.

The band gap energy Eg1 of the first absorption layer 15 is approximately 256 meV, the band gap energy Eg(bar) of the barrier layer 16 is approximately 484 meV, and the band gap energy Eg2 of the second absorption layer 17 is approximately 127 meV. The band gap of the barrier layer 16 is larger than the band gaps of both the first absorption layer 15 and the second absorption layer 17.

Looking at the energy difference at the conduction band minimum, the energy difference ΔEc1 between the conduction band minimum of the first absorption layer 15 and the conduction band minimum of the barrier layer 16 is approximately 9 meV, and the energy difference ΔEc2 between the conduction band minimum of the second absorption layer 17 and the conduction band minimum of the barrier layer 16 is small, approximately 3 meV.

When the second light receiving layer 17 is operated as shown in FIG. 2B, there is almost no energy difference between the second light receiving layer 17 and the barrier layer 16. In the process in which electrons generated in the second light receiving layer 17 are drawn to the electrode 22, the electrons easily overcome the slight energy difference in the conduction band lower end between the second light receiving layer 17 and the barrier layer 16 and are drawn to the electrode along the conduction band lower end of the first light receiving layer 15. The decrease in the signal on the long wavelength side is suppressed, and the light receiving sensitivity is improved. As shown in FIG. 2A, when the first light receiving layer 15 is operated, the energy difference ΔEc1 at the conduction band lower end between the first light receiving layer 15 and the barrier layer 16 is also only 9 meV, which is small enough for the electrons to overcome.

Looking at the energies of the upper ends of the valence bands, the energy difference ΔEv1 between the upper end of the valence band of the first absorption layer 15 and the upper end of the barrier layer 16 is approximately 219 meV, while the energy difference ΔEv2 between the upper end of the valence band of the second absorption layer 17 and the upper end of the barrier layer 16 is large, approximately 354 meV. The barrier layer 16 acts as a potential barrier against holes, and can suppress dark current.

In the quantum structure of the first absorption layer 15, in which GaSb and AlSb are barrier layers and InAs is a well layer, the energy position of the conductor bottom of the superlattice is determined by the quantum ground level of the electrons in the InAs layer. In the quantum structure of the barrier layers of InAs and AlSb and GaSb is a well layer, the energy position of the valence band top of the superlattice is determined by the quantum ground level of the holes formed in the GaSb layer. With this configuration, the energy of the valence band top can be lowered while maintaining a sufficiently small energy difference between the conduction band bottom of the first absorption layer 15 and the conduction band bottom of the second absorption layer 17.

In a conventional superlattice quantum structure with GaSb barrier layers and InAs well layers, the energy position of the lower end of the conduction band of the superlattice is determined by the ground level of the quantum level of electrons formed in the InAs layer. The energy position of the upper end of the valence band is determined by the ground level of the quantum level of holes formed in the GaSb layer. When the InAs film is made thinner, the energy position of the ground level of electrons becomes higher, so the energy position of the lower end of the conduction band of the superlattice becomes higher and the energy difference with the upper end of the valence band becomes larger. As a result, the band gap of the superlattice becomes larger and the detectable infrared wavelength becomes shorter.

On the other hand, when the thickness of the GaSb film is changed, the effective mass of the hole is heavy, so the energy position of the ground level of the hole, i.e., the energy position of the upper end of the valence band, does not change significantly. Since the band gap of the superlattice cannot be changed by controlling the thickness of the GaSb film, the band gap, i.e., the detection wavelength, is adjusted by controlling the thickness of the InAs. As mentioned above, the change in the thickness of the InAs film changes the energy position of the lower end of the conductor, decreasing the signal on the long wavelength side.

The dual-wavelength photodetector 10 of the first embodiment has an energy band gap designed as shown in FIG. 3, which suppresses signal degradation on the long wavelength side and improves light receiving sensitivity on the long wavelength side.

Figures 4A to 4D are diagrams showing the manufacturing process of the dual-wavelength photodetector 10. In Figure 4A, 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.

For example, an n-type GaSb (100) substrate is introduced into a substrate introduction chamber of a molecular beam epitaxy (MBE) apparatus. The GaSb substrate 11 is degassed in a preparation chamber and then transported to a growth chamber maintained at 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, for example, a GaSb buffer layer 12 is grown to a thickness of 100 nm at a substrate temperature of 500° C. Then, for example, an InAsSb etching stopper layer 13 is grown to a thickness of 300 nm. At this time, it is preferable to set the mixed crystal composition of InAsSb so as to be lattice-matched to GaSb, and as an example, an InAs _0.91 Sb _0.09 etching stopper layer 13 is formed.

Next, the first electrode layer 14 is formed of a superlattice of InAs, GaSb, and AlSb. For example, the first electrode layer 14 is formed of a superlattice having a structure in which 4.6 nm of InAs, 0.9 nm of GaSb, 1.5 nm of AlSb, and 0.9 nm of GaSb are stacked in this order as one period (unit structure). The superlattice of InAs/GaSb/AlSb/GaSb is repeated for example 40 periods and grown to a thickness of 360 nm. The band gap of this superlattice is about 0.256 eV. The first electrode layer 14 is doped with, for example, Be as an impurity, and has a p-type conductivity with a hole concentration of 1×10 ¹⁸ cm ⁻³ .

Next, the first absorption layer 15 is formed of a superlattice of InAs, GaSb, and AlSb. The first absorption layer 15 is formed of a superlattice having a structure in which, for example, 4.6 nm of InAs, 0.9 nm of GaSb, 1.5 nm of AlSb, and 0.9 nm of GaSb are stacked in this order as one period (unit structure). The InAs/GaSb/AlSb/GaSb superlattice is repeated for example 160 periods and grown to a thickness of about 1300 nm. The band gap of this superlattice is about 0.256 eV, and the first absorption layer 15 is sensitive to infrared light in the medium wavelength band (3 to 5 μm). The first absorption layer 15 has a p-type conductivity with a hole concentration of, for example, 1×10 ¹⁶ cm ⁻³ .

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

Then, the second light receiving layer 17 is formed from a superlattice of InAs and GaSb. The second light receiving layer 17 is formed from a superlattice in which, for example, 4.2 nm of InAs and 2.1 nm of GaSb are stacked in this order, forming one period (unit structure). The InAs/GaSb superlattice is repeated for example 200 periods and grown to a thickness of about 1300 nm. 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 thickness of the InAs in the InAs/GaSb superlattice of the second absorption layer 17 is thinner than the thickness of the InAs in the InAs/GaSb/AlSb/GaSb superlattice of the first absorption layer 15. By making the thickness of the InAs thinner than the thickness of the InAs in the superlattice of the first absorption layer 15, the energy position of the ground level of electrons becomes higher, and the energy position of the lower end of the conductor of the superlattice becomes higher. The second absorption layer has a conductivity type of p-type with a hole concentration of 1×10 ¹⁶ cm ^-3 , for example.

Next, the second electrode layer 18 is formed, for example, by a superlattice of InAs and GaSb. The second electrode layer 18 is formed by a superlattice having a structure in which, for example, 4.2 nm of InAs and 2.1 nm of GaSb are laminated in this order as one period (unit structure). The InAs/GaSb superlattice is repeated for example 60 periods and grown to a thickness of about 360 nm. The band gap of this superlattice is about 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 laminate structure of FIG. 4A.

In FIG. 4B, the layered structure of FIG. 4A is etched to form mesas 35. The second electrode layer 18, the second light receiving layer 17, the barrier layer 16, and the first light receiving layer 15 are selectively etched so that a portion of the surface of the first electrode layer 14 is exposed. Although only one mesa 35 is depicted in FIG. 4B, multiple mesas 35 arranged in an array are formed in this etching process. The spaces between adjacent mesas 35 become pixel separation grooves 36.

In FIG. 4C, an insulating film 19 is formed to cover the entire mesa 35 and the surface of the first electrode layer 14. The insulating film is, for example, a silicon oxide film having a thickness of 500 nm, but other insulating films such as a silicon nitride film or a 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. 4D, openings 26 are formed at predetermined locations in the insulating film 19 by selective etching using an etching mask, exposing a portion of the first electrode layer 14 and a portion of the second electrode layer 18.

In FIG. 4E, an electrode 22 connected to the first electrode layer 14 and an electrode 21 connected to the second electrode layer 18 are formed in the opening 26. The electrodes 21 and 22 are formed of, for example, Ti/Pt/Au. Ti can function as an adhesive film with the electrode layer below. The electrodes 21 and 22 may be formed by sputtering and milling a metal film, or by deposition and lift-off. Thereafter, as described below, wiring connected to the electrode 22 at the outermost pixels of the photodetector array and electrode pads connected to the electrode 21 at each effective pixel are formed, and a protruding electrode is provided at each pixel, thereby fabricating the two-wavelength photodetector 10. If necessary, the substrate 11, the buffer layer 12, and the etching stopper layer 13 may be removed.

In the first embodiment, the energy difference between the conduction band bottoms of adjacent layers in the stack of the first absorption layer 15, the barrier layer 16, and the second absorption layer 17 is set to 20 meV or less, more preferably 10 meV or less, to efficiently extract electrons, which are signals, from the long wavelength side and improve the light receiving sensitivity. Specifically, the film thickness of InAs contained in the superlattice of InAs, GaSb, and AlSb in the first absorption layer 15 and the film thickness of InAs contained in the superlattice of InAs and GaSb in the second absorption layer 17 are close to each other with a thickness of 4 nm or more, so that the energy positions of the conductor bottoms are close to each other. In addition, by making the barrier layer 16 function as a hole blocking layer, dark current is suppressed and the light receiving sensitivity is improved.

Second Embodiment
5 is a schematic cross-sectional view of a dual-wavelength photodetector 20 of the second embodiment. The dual-wavelength photodetector 20 has a layered structure in which a first electrode layer 24, a first light receiving layer 25, 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. At least one of a buffer layer 12 and an etching stopper layer 13 may be disposed between the substrate 11 and the first electrode layer 24.

The dual-wavelength photodetector 20 has a similar configuration to the dual-wavelength photodetector 10 of the first embodiment, except for the configuration of the first electrode layer 24 and the first light receiving layer 25, so only the different parts will be described. The first electrode layer 24 and the first light receiving layer 25 are epitaxially grown in succession on the substrate 11. A buffer layer 12 and an etching stopper layer 13 may be inserted between the substrate 11 and the first electrode layer 24.

The first electrode layer 24 is formed of a superlattice of InAs, GaSb, and AlSb. For example, the first electrode layer 24 is formed of a superlattice having a structure in which 4.6 nm of InAs, 0.3 nm of GaSb, 0.9 nm of AlSb, and 0.3 nm of GaSb are laminated in this order as one period. The band gap of this superlattice is about 0.392 eV. The first electrode layer 24 is doped with, for example, Be as an impurity, and has a p-type conductivity with a hole concentration of 1×10 ¹⁸ cm ⁻³ . This InAs/GaSb/AlSb/GaSb superlattice is grown to a thickness of 360 nm by repeating, for example, 60 periods.

The first absorption layer 25 is formed of a superlattice of InAs, GaSb, and AlSb. For example, the first absorption layer 25 is formed of a superlattice having a structure in which 4.6 nm of InAs, 0.3 nm of GaSb, 0.9 nm of AlSb, and 0.3 nm of GaSb are laminated in this order as one period. The band gap of this superlattice is about 0.392 eV. The first absorption layer 25 is doped with, for example, Be as an impurity, and has a p-type conductivity with a hole concentration of 1×10 ¹⁸ cm ⁻³ . This InAs/GaSb/AlSb/GaSb superlattice is grown to a thickness of 1300 nm, repeating, for example, 210 periods.

Figure 6 is a diagram explaining the effect of the two-wavelength photodetector 20 of the second embodiment. In the second embodiment, the film thickness of one period of the superlattice of InAs, GaSb, and AlSb that forms the first absorption layer 25 is made thinner than in the first embodiment to raise the energy position of the lower end of the conduction band and further lower the energy position of the upper end of the valence band, and the band gap energy Eg1 is designed to be about 392 meV. In other words, the wavelength of infrared light that can be detected is shorter than that of the first absorption layer 15 of the first embodiment. The band gap energies of the barrier layer 16 and the second absorption layer 17 are about 484 meV and about 127 meV, respectively, as in the first embodiment.

Between the first absorption layer 25 and the second absorption layer 17, the energy difference in the conduction band minimum is smaller than that in the first embodiment, and is at almost the same energy position. Looking at the energy positions of the conduction band minimum between the first absorption layer 25, the barrier layer 16, and the second absorption layer 17, there is almost no potential barrier for electrons. Electrons generated in the second absorption layer 17 on the long wavelength side can easily overcome the energy difference in the conduction band minimum between the barrier layer 16 and the first absorption layer 25 in the process of being drawn out to the electrode 22. The decrease in the signal on the long wavelength side is suppressed, and the light receiving sensitivity can be improved. There is almost no potential barrier for electrons generated in the first absorption layer 25, and the light receiving sensitivity on the short wavelength side is also improved.

<Image sensor using a two-wavelength photodetector>
7 is a schematic diagram of an image sensor 100 using the dual-wavelength photodetector 10. The image sensor 100 has a photodetector array 50 including an array of a plurality of pixels 101, and a readout circuit 60. The photodetector array 50 is flip-chip bonded to the readout circuit 60 by protruding electrodes 31. An underfill may be filled and cured between the photodetector array 50 and the readout circuit 60. In FIG. 7, the photodetector array 50 uses the dual-wavelength photodetector 10 of the first embodiment, but the dual-wavelength photodetector 20 of the second embodiment may also be used.

The surface of the photodetector array 50 opposite the protruding electrodes 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 positioned closer to the light incident surface.

The readout circuit 60 has a driving cell corresponding to each pixel 101 of the photodetector array 50, and alternately applies a bias that provides a positive potential and a bias that provides a negative potential to each pixel 101 to read out electrons as signals from the first light receiving layer 15 and the second light receiving layer 17. Each driving 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, a time-series signal of the first wavelength (for example, a mid-infrared wavelength) and a time-series signal of the second wavelength (far-infrared wavelength) may be output alternately.

Figure 8 is a schematic cross-sectional view showing a portion of a photodetector array 50 used in an image sensor 100. For example, a plurality of pixels 101 are arranged two-dimensionally, with each pixel 101 being the dual-wavelength photodetector 10 of the first embodiment. Each pixel 101 is separated from the others by a pixel separation groove 36, and is electrically connected to a transistor 61 of a corresponding driving cell of a readout circuit 60 via an individual protruding electrode 31p formed of In or the like.

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

The pixels on the outermost periphery of the photodetector array 50 are dummy pixels and 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 first electrode layer 14, and the common potential V _A is supplied to each pixel 101 via the first electrode layer 14.

The metal film 32 and the wiring 33 are formed in the same process after the process of FIG. 4E. On the metal film 32 and the wiring 33, the individual protruding electrodes 31p and the common electrode 31d are formed in the same process. After the photodetector array 50 is flip-chip bonded to the readout circuit 60 via the protruding electrodes 31p and the common electrode 31d, the substrate 11, the buffer layer 12, and the etching stopper layer 13 may be removed as necessary. This results in the image sensor 100 of FIG. 7.

Figure 9 is a schematic block diagram of an imaging system 1 incorporating an image sensor 100. The imaging system 1 has 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 image of the incident infrared light on the light incident surface of the photodetector 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 photodetector 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 to 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 correction of variations in sensitivity and nonlinearity for each pixel 101.

The photodetector array 50 using the dual-wavelength photodetector 10 or 20 of the embodiment suppresses signal drop on the long wavelength side in each pixel 101 or 201, and also suppresses dark current, so that a high-quality dual infrared image can be obtained. The image generated by the control and calculation unit 2 is displayed on the display unit 3, making it possible to visually recognize the temperature distribution and gas distribution.

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 photodetector 10 or 20 used in the image sensor 100 detects electrons with high mobility as a signal, suppressing signal drop and dark current on the long wavelength side, and providing high sensitivity, i.e., high accuracy in identifying the object being observed.

Although the embodiment of the invention has been described above based on a specific configuration example, the present invention is not limited to the above configuration example. The thickness of each layer of the superlattice used in the first and second absorption layers can be changed as appropriate as long as the energy difference between the conduction band minimums of the first and second absorption layers is 20 meV or less, more preferably 10 meV or less. In addition, if the thickness of the InAs used in the superlattice of the first absorption layer on the short wavelength side is larger than the thickness of the InAs used in the superlattice of the second absorption layer on the long wavelength side, each may be set to an appropriate thickness of 4 nm or more. The superlattice forming the barrier layer 16 may be formed of an appropriate material and thickness as long as it does not become a potential barrier for electrons generated in the second absorption layer and functions as a blocking layer for holes.

Zn, Mg, etc. may be used instead of or together with Be as an impurity (acceptor) that changes the conductivity type of the first and second light receiving layers to p-type. The formation of the stacked layers of the two-wavelength photodetector is not limited to the MBE method, and metal organic chemical vapor deposition (MOCVD) method or other methods capable of forming stacked layers may be adopted. The insulating film covering the mesa 35 may be formed by atomic layer deposition method, sputtering method, etc. In either case, by appropriately designing the energy band structures of the first light receiving layer, barrier layer, and second light receiving layer, the potential barrier against electrons on the long wavelength side can be reduced, improving the light receiving sensitivity.

REFERENCE SIGNS LIST 1 Imaging system 10, 20 Dual-wavelength photodetector 11 Substrate 14, 24 First electrode layer 15, 25 First light-receiving layer 16 Barrier layer 17 Second light-receiving layer 18 Second electrode layer 21, 22 Electrode 31, 31p Protruding electrode 31d Common electrode 32 Metal film 33 Wiring 50 Photodetector array 60 Readout circuit 100 Image sensor 101, 201 Pixel

Claims (8)

a first light receiving layer having sensitivity to a first wavelength;
a second light receiving layer having sensitivity to a second wavelength longer than the first wavelength;
a barrier layer disposed between the first absorption layer and the second absorption layer, the barrier layer serving as a potential barrier against holes;
having
the first absorption layer is formed of a first superlattice including InAs, GaSb, and AlSb;
the second absorption layer is formed of a second superlattice of InAs and GaSb ;
a thickness of the InAs included in the first superlattice is greater than a thickness of the InAs included in the second superlattice ;
Dual wavelength photodetector. a first light receiving layer having sensitivity to a first wavelength;
a second light receiving layer having sensitivity to a second wavelength longer than the first wavelength;
a barrier layer disposed between the first absorption layer and the second absorption layer, the barrier layer serving as a potential barrier against holes;
having
the first absorption layer is formed of a first superlattice including InAs, GaSb, and AlSb;
the second absorption layer is formed of a second superlattice of InAs and GaSb;
the InAs contained in the first superlattice and the InAs contained in the second superlattice have a film thickness of 4 nm or more;
Dual wavelength photodetector . an energy difference between the conduction band minimums of the first superlattice and the second superlattice is 20 meV or less;
3. The dual-wavelength photodetector according to claim 1 or 2 . The first superlattice is a superlattice having a unit structure in which InAs, GaSb, AlSb, and GaSb are laminated in this order.
The dual-wavelength photodetector according to any one of claims 1 to 3 . The barrier layer is formed of a superlattice of InAs and AlSb,
the band gap of the barrier layer is larger than the band gaps of the first absorption layer and the second absorption layer;
The dual-wavelength photodetector according to any one of claims 1 to 4 . The first absorption layer and the second absorption layer are doped with Be, Zn, or Mg as an impurity element.
The dual-wavelength photodetector according to any one of claims 1 to 5 . the barrier layer has the same conductivity type as the first absorption layer and the second absorption layer;
The dual-wavelength photodetector according to any one of claims 1 to 6 . a photodetector array in which a plurality of dual-wavelength photodetectors according to any one of claims 1 to 7 are arranged;
a readout circuit electrically connected to the photodetector array;
An image sensor having

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