JP7593254B2 - Dual-wavelength infrared sensor and imaging system - Google Patents

Description

This disclosure relates to a dual-wavelength infrared sensor and an imaging system.

Infrared sensors are known that have a single pixel sensitive to different wavelength bands (see, for example, Patent Document 1 and Patent Document 2). In these dual-wavelength infrared sensors, a single pixel is made up of a stacked structure of light-receiving layers sensitive to different infrared wavelengths, and infrared responses of different wavelengths are detected by switching the direction of the bias applied to each pixel.

There is known an infrared detector in which four adjacent pixels in the plane of the pixel array are defined as one unit block, with one pixel in the unit block used as a pixel for detecting long wavelengths and the other three pixels used as pixels for detecting short wavelengths (see, for example, Patent Document 3).

1 shows the cross-sectional structure of a known infrared sensor. Cross section A is a schematic diagram of a row or column including only pixels for short wavelengths (λ _S ), and cross section B is a schematic diagram of a row or column including pixels for short wavelengths (λ _S ) and pixels for long wavelengths (λ _L ) alternately. In each pixel, a lower contact layer (B-NCT), a λ _L absorption layer, an intermediate contact layer (M-CNT), a λ _S absorption layer, and an upper contact layer (T-CNT) are laminated in this order from the light incident side.

A bias voltage applied between the intermediate contact layer (M-CNT) and the upper contact layer (T-CNT) reads out the charge accumulated in the λ _S absorption layer from the λ _S pixel. A bias voltage applied between the intermediate contact layer (M-CNT) and the lower contact layer (T-CNT) reads out the charge accumulated in the λ _L absorption layer from the λ _L pixel. With this configuration, detection results can be obtained simultaneously from the λ _S pixel and the λ _L pixel without switching the direction of the voltage applied to each pixel in a time-division manner. However, since each pixel can only extract a response output of either λ _L or λ _S , information on the missing wavelength is supplemented by an interpolation process based on the output information of adjacent pixels.

In the configuration of Fig. 1, a _λL absorption layer is disposed on the light incident side. _{The λL} absorption layer and the _λS absorption layer are formed of multiple quantum well layers, and are designed to have a steep absorption peak at the target wavelength, respectively. Since the absorption peaks of _{the λL} absorption layer and the _λS absorption layer are independent of each other, the _λS light reaches the _λS absorption layer with almost no absorption in the _λL absorption layer.

JP 2013-21032 A JP 2018-32663 A JP 2020-21846 A

In recent years, in order to improve the sensitivity of dual-wavelength infrared sensors, a configuration has been adopted in which T2SL (Type-II Super Lattice), which has high infrared absorption efficiency, is used as the light absorption layer. Since the light absorption layer of T2SL has a very broad response characteristic, the response characteristics of the λ _L absorption layer and the λ _S absorption layer overlap, and the λ _S infrared light is absorbed by the λ _L absorption layer. It is possible to switch the stacking order of the λ _L absorption layer and the λ _S absorption layer, but simply switching the stacking order does not allow independent wavelength information to be read out from the absorption layers of each wavelength.

The present disclosure aims to provide a wavelength infrared sensor that can maintain detection accuracy for light of different wavelengths without switching the bias voltage for charge readout at each pixel.

In one embodiment, the dual-wavelength infrared sensor includes a light receiving element array in which a plurality of pixels are arranged, the pixels each having, in order from a light incident side, a first contact layer, a first blocking layer, a second contact layer, a first light absorbing layer sensitive to light of a first wavelength, a second blocking layer, a second light absorbing layer sensitive to light of a second wavelength longer than the first wavelength, and a third contact layer;
At least one first pixel that detects light of the first wavelength and at least one second pixel that detects light of the second wavelength form a unit block, and the unit block is repeatedly arranged in the light receiving element array,
The first pixel is separated by a first separation trench penetrating the third contact layer, the second light absorption layer, the second blocking layer, and the first light absorption layer, and the second pixel is separated by a second separation trench extending from the third contact layer to the first contact layer.

The detection accuracy for light of different wavelengths can be maintained without switching the bias voltage at each pixel of the two-wavelength infrared sensor.

FIG. 1 is a schematic cross-sectional view of a conventional two-wavelength infrared sensor. 1A and 1B are diagrams for explaining problems that arise when T2SL is applied to a two-wavelength infrared sensor having a conventional configuration. 2 is a schematic plan view showing a configuration in which the λ _L absorption layer and the λ _S absorption layer are interchanged in the configuration of FIG. 1 . 2 is a schematic cross-sectional view showing a configuration in which the λ _L absorption layer and the λ _S absorption layer are interchanged in the configuration of FIG. 1 . 1A to 1C are schematic cross-sectional views of a light-receiving element array that can be considered in the process leading to an embodiment. 1 is a schematic plan view of a light receiving element array of a dual-wavelength infrared sensor according to an embodiment of the present invention; 7A and 7B are schematic diagrams of cross-sections A and B of the light-receiving element array in FIG. 6. FIG. 1 is an energy potential diagram of one pixel configuration. 1A and 1B are diagrams illustrating the operation of a short-wavelength pixel of a two-wavelength infrared sensor. 1A and 1B are diagrams illustrating the operation of a long wavelength pixel of a two-wavelength infrared sensor. 3A and 3B are diagrams illustrating a crystal growth structure of a light-receiving element array according to an embodiment. FIG. 12 is a diagram showing a processed cross section of the crystal growth structure of FIG. 11 . FIG. 2 is a schematic cross-sectional view of a two-wavelength infrared sensor. FIG. 13 is a schematic cross-sectional view of a modified example of the light-receiving element array. FIG. 1 is a schematic block diagram of an imaging system using a dual-wavelength infrared sensor. 5A to 5C are diagrams illustrating an interpolation process for two-wavelength signals according to an embodiment. 5A to 5C are diagrams illustrating an interpolation process for two-wavelength signals according to an embodiment.

<Possible configurations and problems in the process of arriving at the embodiment>
FIG. 2 is a diagram for explaining the problems that arise when T2SL is applied to the two-wavelength infrared sensor of FIG. 1. The dashed line in the figure shows the infrared response characteristic of the short wavelength (λ _S ) absorption layer formed by T2SL, and the solid line shows the infrared response characteristic of the long wavelength (λ _L ) absorption layer formed by T2SL. Unlike the light absorption layer using a multiple quantum well layer, the infrared response characteristic of the light absorption layer of T2SL is broad. Most of the response characteristic of the λ _S absorption layer overlaps with the response characteristic of the λ _L absorption layer. Therefore, when the λ _L absorption layer is located on the light incident side as shown in FIG. 1, most of the infrared light of λ _S is absorbed by the λ _L absorption layer. The infrared light of λ _S is attenuated by the λ _L absorption layer before reaching the λ _S absorption layer, and the detection sensitivity on the λ _S side decreases.

The first thing that can be thought of is to swap the positions of the λ _L absorption layer and the λ _S absorption layer in the configuration of Figure 1. That is, the λ _S absorption layer is provided on the light incident side, and the λ _L absorption layer is disposed on the side farther from the light incident surface. However, simply swapping the positions of the λ _L absorption layer and the λ _S absorption layer makes it impossible to properly extract the photodetection signals of each wavelength.

3 and 4 show a configuration in which the T2SL configuration is adopted and the λ _L absorption layer and the λ _S absorption layer are interchanged. FIG. 3 is a schematic plan view of the light receiving element array, and FIG. 4 is a schematic cross-sectional view of the light receiving element array. In FIG. 3, a plurality of pixels including a λ _L pixel and a λ _S pixel are arranged in the XY plane. Four pixels adjacent to each other in the X and Y directions of the light receiving element array form a unit block. Of the four pixels included in the unit block, one pixel is a λ _L pixel and the remaining three pixels are λ _S pixels.

Fig. 4A is a schematic diagram of cross section A in Fig. 3, and Fig. 4B is a schematic diagram of cross section B in Fig. 3. Unlike Fig. 1, a _λS absorption layer is disposed on the light incident side. Since the _λS pixel does not have a _λL absorption layer, the height of the short wavelength electrode EL _λS is made higher than the long wavelength electrode EL _λL to align the electrode surfaces.

The λ _L pixel is surrounded by the λ _S pixel. The middle contact layer (M-CNT) is divided for each pixel and cannot be used as a common contact layer. Therefore, the lower contact layer (B-CNT) is used as a common contact layer. In the λ _S pixel, the charge accumulated in the λ _S absorption layer can be read out by selecting each electrode EL _λS and applying a bias voltage. However, in the λ _L pixel, the output of the λ _S absorption layer is mixed with the output of the λ _L absorption layer, causing interference within one pixel. With this configuration, the amount of incident light of λ _L cannot be detected correctly.

5 shows yet another example of a configuration that can be considered in the process of arriving at the embodiment. In a configuration using a T2SL light absorption layer, a block layer (BLC) is inserted between the λ _S absorption layer and the λ _L absorption layer so that the output of the λ _S absorption layer is not mixed into the output current of the λ _L absorption layer. A lower contact layer (B-CNT) is used as a common contact layer, and a common bias voltage (for example, 0 V) is applied. By applying bias voltages of opposite polarity to the _{λ S} pixel and the λ _L pixel, the λ _S pixel and the λ _L pixel can be driven simultaneously without bias switching at each pixel.

When a positive bias is applied to the λ _S pixel and a negative bias is applied to the λ _L pixel, the electrons of the photoexcited carriers generated in the λ _S absorption layer of the λ _S pixel are drawn out from the electrode EL _{λ S} , and the holes are drawn out to the lower electrode (B-CNT). At this time, the holes, which are photoexcited carriers in the λ _L absorption layer of the λ _S pixel, are blocked by the block layer BLC. However, the holes drawn out from the λ _S absorption layer to the lower electrode (B-CNT) may enter the adjacent λ _L pixel, generating a current path in the direction of the arrow. Signals of unintended wavelengths may interfere with each other between adjacent pixels.

As such, in order to operate a two-wavelength infrared sensor using T2SL correctly without bias switching for each pixel, some ingenuity is required in the pixel structure. The specific configuration of the embodiment is described below. The same components are given the same reference numerals, and duplicate explanations may be omitted.

<Configuration of the embodiment>
Fig. 6 is a schematic plan view of the light receiving element array 20 of the dual-wavelength infrared sensor of the embodiment, and Fig. 7 is a schematic view of cross section A and cross section B of Fig. 6. In the embodiment, in the dual-wavelength infrared sensor using T2SL, mixing of signals of unintended wavelengths is prevented within a pixel and between adjacent pixels, and detection sensitivity to light of each wavelength is maintained without bias switching.

6, λ _S pixels 21 sensitive to λ _S and λ _L pixels 22 sensitive to λ _L are arranged in a periodic arrangement pattern within the XY plane. The center-to-center distance between adjacent pixels, i.e., the pixel pitch, is equal to or less than the optical resolution of λ _L among the response wavelengths of λ _S and λ _L. This is because the information on the λ _L light that cannot be collected at the λ _L pixels 22 and leaks out and spreads to the surrounding pixels is used in the interpolation process described later.

In the light receiving element array 20, a unit block 25 is formed of a plurality of pixels including at least one λ _S pixel 21 and at least one λ _L pixel 22. In Fig. 6, the unit block 25 is formed of 2 x 2 = 4 pixels, but is not limited to this example. As described later, the unit block 25 may be formed of two adjacent pixels, or may be formed of 3 x 3 = 9 pixels.

The arrangement of the unit blocks 25 is repeated within the XY plane of the light receiving element array 20. All pixels have a λ _S absorption layer and a λ _L absorption layer, and the λ _S pixels 21 included in the unit blocks 25 detect λ _S light, and the λ _L pixels 22 detect λ _L light. The amount of infrared light received at each pixel with a wavelength that is not detected is calculated by interpolating from the output values of surrounding pixels, thereby obtaining output signals of two wavelengths for each pixel. A specific example of the interpolation process will be described later.

7A shows the cross section A of FIG. 6, and FIG. 7B shows the cross section B of FIG. 6. The light receiving element array 20 includes a first contact layer 201, a first block layer 202, a second contact layer 203, a λ _S absorption layer 205, a second block layer 206, a λ _L absorption layer 207, and a third contact layer 209, in that order from the light incident side. The conductivity type of the light receiving layer and the contact layer is designed depending on whether electrons or holes are extracted as a signal among the photoexcited carriers. In the following, a configuration in which electrons are extracted as a signal, that is, a configuration in which a p-type conductor is applied to the stack, will be described as an example. In this case, the first block layer 202 and the second block layer 206 may be formed of p-type or i-type T2SL.

Cross section A includes only the λ _S pixels 21. The λ _S pixels 21 are separated from each other by shallow separation grooves 27. The separation grooves 27 reach the second contact layer 203 from the surface of the third contact layer 209. For the λ _S pixels 21, the first contact layer 201 and the second contact layer 203 can be used as a common contact layer.

Cross section B alternates between λ _S pixels 21 and λ _L pixels 22. The λ _L pixels 22 are separated by deep isolation trenches 28. The isolation trenches 28 reach the first contact layer 201 from the surface of the third contact layer 209. For the λ _L pixels 22, the first contact layer 201 can be used as a common contact layer.

The third contact layer 209 is electrically connected to the individual protruding electrodes 41 and 42 via the ohmic electrode 212 and the base electrode 213. For simplicity, the detailed configuration is omitted, but the laminated structure of each pixel, except for the electrode portion, may be covered with a protective film.

For convenience, the individual electrode provided in the λ _S pixel 21 is referred to as the protruding electrode 41, and the individual electrode provided in the λ _L pixel 22 is referred to as the protruding electrode 42, but the protruding electrodes 41 and 42 are electrodes of the same configuration formed in the same process. The light-receiving element array 20 is flip-chip bonded to the readout circuit by the protruding electrodes 41 and 42.

8 is an energy potential diagram of one pixel configuration. The energy band gap of the _{λ L} absorption layer 207 is narrower than that of the λ _S absorption layer 205. When electrons are extracted as a signal, the first block layer 202 and the second block layer 206 have a large potential barrier on the valence band side and a small potential barrier on the conduction band side with respect to the layers adjacent thereto above and below the stack. With respect to the barrier on the conduction band side, the potential difference B1 of the first block layer 202 with respect to the layers adjacent thereto above and below the stack is larger than the potential difference B2 of the second block layer 206 with respect to the layers adjacent thereto above and below the stack (B1>B2).

When holes are extracted as signals, the energy potential diagram is configured upside down compared to FIG. 8. That is, the contact layers of each pixel are formed of n-type conductors. The first block layer 202 and the second block layer 206 have a large potential barrier on the conduction band side and a small potential barrier on the valence band side with respect to the layers that contact them above and below the stack. On the valence band side where the potential barrier is small, the potential difference of the first block layer 202 with respect to the first contact layer 201 and the second contact layer 203 is made larger than the potential difference of the second block layer 206 with respect to the λ _S absorption layer 205 and the λ _L absorption layer 207.

Figures 9 and 10 are diagrams for explaining the operation of a two-wavelength infrared sensor using the light receiving element array 20 of Figure 7. Figure 9 shows the operation of the _λS pixel 21, and Figure 10 shows the operation of the _λL pixel 22. Both are based on a configuration in which electrons are extracted as a signal.

9, a positive bias, for example, +0.5 V, is applied to the protruding electrode 41 of the λ _S pixel 21, and a negative bias, for example, −0 V, is applied to the first contact layer 201 and the second contact layer 203 to read out a photocurrent from the λ _S absorption layer 205. The first contact layer 201 and the second contact layer 203 are set to the same potential and are connected to a common electrode on the outer periphery of the light receiving element array 20, as will be described later with reference to FIG.

The first contact layer 201 and the second contact layer 203 are at −0 V, and the third contact layer is at +0.5 V, so that a potential difference occurs between the conduction band of the λ _S absorption layer 205 and the conduction band of the λ _L absorption layer 207. On the conduction band side, the potential difference between the second block layer 206 and the λ _S absorption layer 205 and the λ _L absorption layer 207 is set small, as shown in FIG.

Among the photoexcited carriers generated in the λ _S absorption layer 205 by the incidence of infrared rays, electrons overcome the barrier of the second block layer 206 and move along the lower edge of the conduction band to the third contact layer 209. Holes generated in the _{λ S} absorption layer 205 move along the upper edge of the valence band to the second contact layer 203. At this time, holes generated in the λ _L absorption layer 207 are blocked by the second block layer 206 and do not flow to the second contact layer 203.

Since the first contact layer 201 and the second contact layer 203 are set to the same potential, excess carriers (electrons) from the first contact layer 201 are blocked by the first blocking layer 202. Even if a slight potential difference occurs between the first contact layer 201 and the second contact layer 203, the first blocking layer 202 is designed so that a potential barrier exists in both the conduction band and the valence band, and the flow of excess carriers between the first contact layer 201 and the second contact layer 203 is blocked. As a result, a photocurrent responsive to λ _S is drawn from the second contact layer 203.

10, a negative bias, for example, −0.5 V, is applied to the protruding electrode 42 of the λ _L pixel 22, and −0 V is applied to the first contact layer 201 to read out a photocurrent from the λ _L absorption layer 207. In the _{λ L} pixel 22, the second contact layer 203 is separated by a deep separation groove 28, and no external voltage is applied to the second contact layer 203 (floating state). On the other hand, in the λ _S pixel 21, the second contact layer 203 is continuous with the adjacent λ _S pixel 21 in the direction perpendicular to the paper surface, and is used as a common contact layer for the λ _S pixel 21.

In the λ _L pixel 22, the first contact layer 201 is at −0 V, the second contact layer 203 is in a floating state, and the third contact layer is at −0.5 V, and a potential difference is generated between the λ _S absorption layer 205 and the λ _L absorption layer 207 and between the first contact layer 201 and the second contact layer 203. When infrared light is incident, electrons of minority carriers generated in the λ _{L absorption layer 207 of the λ L pixel} 22 overcome the barriers of the second block layer 206 and the first block layer 202 and move to the first contact layer 201. Holes generated in the _{λ L} absorption layer 207 move to the third contact layer 209.

At this time, holes generated in the λ _S absorption layer 205 are blocked by the valence band of the second blocking layer 206 and cannot move toward the third contact layer 209. Therefore, a photocurrent responsive to λ _L is drawn from the protruding electrode 42. In this manner, it is possible to suppress both interference between different wavelength information within a pixel and interference between the λ _S pixel 21 and the λ _L pixel.

9 and 10, a positive bias is applied to the _λS protruding electrode 41, a negative bias is applied to the _λL protruding electrode 42, and a common potential is applied to the first contact layer 201 and the second contact layer 203 simultaneously. Without switching the bias application direction in each pixel, light of the target wavelength is correctly detected in each of the _λS pixel 21 and the _λL pixel 22.

Since the _λS absorption layer 205 is disposed on the side closer to the first contact layer 201, which is the light incident surface, and the _λL absorption layer 207 is disposed on the farther side, the _λS pixel 21 is not affected by light attenuation due to the _λL absorption layer 207. Therefore, the problem of reduced _λS sensitivity is also solved.

Figure 11 is a diagram showing the crystal growth structure of the light receiving element array 20 of the embodiment. The layer structure of the light receiving element array 20 of the dual wavelength infrared sensor is grown on the substrate 221 by a crystal growth method such as MBE (Molecular Beam Epitaxy). Specifically, each layer constituting the stack in Figure 11 is epitaxially grown in sequence on a GaSb (100) wafer. When detecting electrons as a signal, the stack in Figure 11 may be formed of a p-type conductor.

A GaSb buffer layer 222 with a thickness of 100 nm is grown on a GaSb substrate 221. The buffer layer 222 contains a p-type impurity such as boron at a concentration of 1E18 cm^-3.

On the buffer layer 222, a first contact layer 201, a first blocking layer 202, a second contact layer 203, a λ _S absorption layer 205, a second blocking layer 206, a λ _L absorption layer 207, a third contact layer 209, and a cap layer 210 are grown in this order.

The first contact layer 201 is formed of a superlattice of 100 repeated periods of InAs (3 nm)/GaSb (1 nm) and contains p-type impurities at a concentration of 1E18 cm^-3.

The first blocking layer 202 is formed of a superlattice consisting of 20 repeated periods of InAs (3 nm)/AlSb (1 nm) and contains p-type impurities at a concentration of 1E16 cm^-3. This superlattice has a deep, wide energy band gap on the valence band side.

The second contact layer 203 is formed of a superlattice of 100 repeated periods of InAs (3 nm)/GaSb (1 nm) and contains p-type impurities at a concentration of 1E18 cm^-3.

The _λS absorption layer 205 is formed of a superlattice consisting of 200 repeated periods of InAs (3 nm)/GaSb (1 nm) and contains p-type impurities at a concentration of 1E16 cm^-3. This superlattice has a response sensitivity over a band of 1 to 7 μm and a cutoff wavelength of response at 5 μm.

The second blocking layer 206 is formed of a superlattice consisting of 20 repeated periods of InAs (5 nm)/AlSb (1 nm), and contains p-type impurities at a concentration of 1E16 cm^-3. This superlattice has a deep, wide energy band gap on the valence band side, and on the conduction band side, the potential difference between the second blocking layer 206 and the light absorption layer is smaller than the potential difference between the first blocking layer 202 and the contact layer.

The λ _L absorption layer 207 is formed of a superlattice consisting of 200 repeated periods of InAs (4 nm)/GaSb (2 nm), and contains p-type impurities at a concentration of 1E16 cm^-3. This superlattice has a response sensitivity over a band of 1 to 11 μm, and a cutoff wavelength of response at 9 μm. By adjusting the film thicknesses of InAs and GaSb in each of the superlattices of the λ _S absorption layer 205 and the λ _L absorption layer 207, a desired infrared spectral response can be obtained.

The third contact layer 209 is formed of a superlattice of 80 repeated periods of InAs (4 nm)/GaSb (2 nm) and contains p-type impurities at a concentration of 1E18 cm^-3. The cap layer 210 is an undoped InAs layer with a thickness of 30 nm.

The stack shown in Figure 11 is subjected to the element formation process described below to produce a light receiving element array 20 having the pixel structure shown in Figure 7.

(1) The cap layer 210 on the surface is etched to form a marker for pattern alignment, and then dry etching is performed from the surface to form a separation groove 27 that reaches the surface of the second contact layer 203. At this time, most of the λ _S absorption layer 205 is also removed by etching from the surface at the position where the common electrode is to be provided on the periphery of the pixel array. As a result, a shallow separation groove is formed over the entire stack. This separation groove becomes the separation groove 27 that separates the λ _S pixels 21.

(2) Patterning is performed to form deep separation grooves 28 around the λ _L pixels 22, and the second contact layer 203 and the first block layer 202 are dry etched to form the separation grooves 28 that reach the surface of the first contact layer 201. At this time, the second contact layer 203 and the first block layer 202 are simultaneously dry etched even at positions where common electrodes are provided on the periphery of the pixel array. As a result, a pixel separation structure is obtained in which cross sections A and B are divided by separation grooves of different depths, as shown in FIG.

(3) After covering the entire pixel separation structure obtained in FIG. 12 with a SiO2 protective film by CVD (Chemical Vapor Deposition), ohmic electrodes 212 that connect to the third contact layer 209 are formed in the required locations. For example, a resist pattern is formed on the SiO2 protective film, and openings are formed in the SiO2 protective film in the required locations to expose the third contact layer 209. An AuGe film is formed by vacuum deposition, and the AuGe film on the resist mask is removed together with the resist pattern by lift-off.

(4) A SiO2 film is formed again on the entire surface, and the SiO2 film in the region where the base electrode 213 of the protruding electrodes 41, 42 is provided and in the region where the wiring on the periphery of the array is formed is removed by dry etching. A Ti/Pt metal laminate film is formed on the entire surface by sputtering. Then, unnecessary portions of the Ti/Pt metal laminate film are removed by ion milling. In the pixel region including the _λS pixel 21 and the _λL pixel 22, a base electrode 213 (see FIG. 7) connected to the ohmic electrode 212 is formed. In the periphery of the pixel array, wiring connected to the first contact layer 201 and the second contact layer 203 is formed.

(5) Cover the entire pixel array with a SiO2 protective film, and form openings in the areas where the protruding electrodes 41, 42 and the common electrode will be formed. Using a lift-off method, form the protruding electrodes 41, 42 and the common electrodes 431, 432 (see Figure 13) made of indium or the like in the openings. This results in the configuration shown in Figure 7.

13 is a schematic cross-sectional view of a two-wavelength infrared sensor 50 using a light receiving element array 20. The light receiving element array 20 includes a pixel region 230 including λ _S pixels 21 and λ _L pixels 22, and a peripheral region 240 outside the pixel region 230. The light receiving element array 20 is flip-chip bonded to a readout circuit 30. The readout circuit 30 has a detection circuit for each pixel formed, for example, of a CMOS circuit.

In the pixel region 230 of the light receiving element array 20, the _λS pixel 21 and the _λL pixel 22 are connected to the corresponding detection circuits of the readout circuit 30 by protruding electrodes 41 and 42, respectively. In the peripheral region 240, the first contact layer 201 and the common electrode 431 are connected by an ohmic electrode 214 and a wiring 215. The second contact layer 203 and the common electrode 432 are connected by an ohmic electrode 216 and a wiring 217.

A positive bias V+ is applied to the λ _S pixels 21 from the readout circuit 30. A negative bias V- is applied to the λ _L pixels 22 from the readout circuit 30. A common bias potential V0 is applied to the common electrodes 431 and 432 from the readout circuit 30, and these electrodes are set to the same potential. These bias potentials are simultaneously applied to the light receiving element array 20, and a λ _S signal is extracted from the λ _S pixels 21, and a λ _L signal is extracted from the λ _L pixels 22. When holes are extracted as signals, a negative bias V- is applied to the λ _S pixels 21, and a positive bias V+ is applied to the λ _L pixels 22.

The obtained optical response output is sequentially scanned and read out by the readout circuit 30 and output from the two-wavelength infrared sensor 50 as a time-series signal.

<Modification of the Light Receiving Element Array>
14 shows a cross-sectional configuration of a light-receiving element array 20A as a modified example of the light-receiving element array 20. Fig. 14(A) is a schematic diagram of cross section A in Fig. 6, and Fig. 14(B) is a schematic diagram of cross section B in Fig. 6.

In the modified example of FIG. 14, an n-type first block layer 232 is used instead of the wide-gap first block layer 202 used in the configuration of FIG. 7. The n-type first block layer 232 includes an n-type region 232a used in the λ _S pixel 21 and a p-type region 232b used in the λ _L pixel 22. The p-type region 232b may be formed by forming an n-type first block layer 232 as a whole, and then partially inverting it to p-type by ion-implanting p-type impurities. When the configuration of FIG. 14 is adopted, it is preferable to provide an electrode terminal for applying a bias voltage to the n-type first block layer 232 in the peripheral region 240 of the light receiving element array.

In the λ _S pixel 21, the first contact layer 201, the second contact layer 203, and the n-type region 232a of the first block layer 232 sandwiched between these contact layers form an electrical pnp-type connection 220. On the other hand, in the λ _L pixel 22, the first contact layer 201, the first block layer 232, and the second contact layer 203 are formed as p-type conductors, similarly to FIG.

A positive bias potential is applied to the _λS pixel 21, and a negative bias potential is applied to the _λL pixel 22. The same common bias voltage is applied to the first contact layer 201 and the second contact layer 203, and they are set to the same potential.

For example, +0.5V is applied to the protruding electrode 41 of the λ _S pixel 21, and −0V is applied to the first contact layer 201 and the second contact layer 203, to read out a photocurrent from the λ _S absorption layer 205. At this time, a positive potential is applied from the electrode terminal of the outer periphery 240 to the n-type first block layer 232. Of the photoexcited carriers generated in the λ _S absorption layer 205, electrons overcome a slight barrier in the conduction band of the second block layer 206 and move to the third contact layer 209, and holes move to the second contact layer 203. Holes generated in the λ _L absorption layer 207 are blocked by the wide-gap second block layer 206 and do not flow to the second contact layer 203.

When a positive potential is applied to the n-type first block layer between the first contact layer 201 and the second contact layer 203, a reverse bias state of a pn junction diode is generated, and the current between the first contact layer 201 and the second contact layer 203 is blocked. This makes it possible to prevent excess carriers (electrons) from flowing from the first contact layer 201 into the second contact layer 203. As a result, only the photocurrent responsive to λ _S is drawn out from the second contact layer 203.

For example, −0.5 V is applied to the protruding electrode 42 of the λ _L pixel 22, and −0 V is applied to the first contact layer 201, to read out a photocurrent from the λ _L absorption layer 207. In the λ _L pixel 22, the second contact layer 203 is in a floating state. Electrons of minority carriers generated in the λ _L absorption layer 207 of the λ _L pixel 22 overcome the barrier of the conduction band of the second block layer 206, pass through the first block layer 232 formed as a p-type region 232b, and move to the first contact layer 201. Holes generated in the λ _L absorption layer 207 move to the third contact layer 209.

Holes generated in the λ _S absorption layer 205 are blocked in the valence band of the second blocking layer 206. Therefore, only photocurrent responsive to λ _L is drawn out from the protruding electrode 42. In this way, with the configuration of Fig. 14 as with Fig. 7, it is possible to suppress both interference between two wavelengths within a pixel and interference between the λ _S pixel 21 and the λ _L pixel 22. As with the light receiving element array 20, the light receiving element array 20A can also realize a two-wavelength infrared sensor 50 with fine pixels using a highly sensitive T2SL.

<Imaging system>
15 is a schematic diagram of an imaging system 100 using a dual-wavelength infrared sensor 50. The imaging system 100 includes the dual-wavelength infrared sensor 50 and a signal processing circuit 60. The signal processing circuit 60 is realized by a DSP (Digital Signal Processor) or the like.

The charge amounts sequentially read out from the λ _S pixels 21 and the λ _L pixels 22 of the dual-wavelength infrared sensor 50 are input as analog electrical signals to the signal processing circuit 60. The analog electrical signals may be signals that have been subjected to processing such as noise cancellation and amplification by the readout circuit 30.

The signal processing circuit 60 has an AD converter 61, an interpolation processing circuit 62, a correction coefficient memory 63, a frame memory 64, and a sensitivity correction processing circuit 65. The AD converter 61 samples the input analog electrical signal at a predetermined rate and converts it into a digital signal. The frame memory 64 sequentially stores the digitally converted pixel output and holds one frame's worth of digital data.

The interpolation circuit 62 reads out the output value of each pixel and the output values of the surrounding pixels of different wavelengths from the frame memory 64, and performs a calculation process described later to interpolate the amount of received light of wavelengths not detected by the pixel of interest. Through this interpolation process, the interpolation circuit 62 outputs a λ _S digital signal and a λ _L digital signal for each pixel.

The sensitivity correction processing circuit 65 corrects the sensitivity variation in each pixel. The sensitivity of the dual-wavelength infrared sensor 50 is affected by the variation in response characteristics of the λ _S pixels 21 and the λ _L pixels 22, the variation in characteristics of the transistors in the readout circuit 30, and the like. The sensitivity correction processing circuit 65 uses the correction coefficients stored in the correction coefficient memory 63 to correct the sensitivity of the signals output in time series from the dual-wavelength infrared sensor 50 and converted into digital form.

The correction coefficient memory 63 stores a correction coefficient (including an offset value and a gain value) for each pixel. The sensitivity correction processing circuit 65 may perform sensitivity correction by multiplying the output of the interpolation processing circuit 62 by a correction coefficient read from the correction coefficient memory 63. The sensitivity correction processing circuit 65 performs sensitivity correction on each of the λ _S signal and the λ _L signal for each pixel, and outputs the result as an image signal.

The imaging system 100 uses a small, high-resolution two-wavelength infrared sensor 50, allowing the entire device to be made compact. The absolute temperature of the object being observed can be measured with high precision based on the correlation between the two wavelength outputs for each pixel. In addition, the reflected light component of natural light (sunlight) can be distinguished from the infrared information received from the object being observed, from the temperature radiation component from the object itself.

<Interpolation processing>
16 and 17 are diagrams for explaining the interpolation process of the two-wavelength signal in the embodiment. This interpolation process is performed by the interpolation process circuit 62 of the signal processing circuit 60. As shown in FIG. 16, four adjacent pixels are defined as one unit block 25. The unit block 25 has one λ _L pixel 22 and three λ _S pixels 21. Within the unit block 25A, the position of the λ _L pixel 22 is defined as (3), the position of the λ S pixel 21 adjacent to the λ _L pixel 22 in the horizontal direction (X direction) or vertical direction (Y direction) is defined as (1), and the position of the λ _S pixel 21 diagonally opposite the _{λ L} pixel 22 is defined as (2). At each of the pixel positions (1) to (3), the current value output is a detection value of a corresponding single wavelength, but an interpolation process is performed in order to allow the light receiving element array 20 to function as a light receiving unit of the two-wavelength infrared sensor 50.

Fig. 17 shows the interpolation process performed on each pixel at pixel positions (1) to (3). At pixel position (1) in Fig. 17(A), the output value from this λ _S pixel 21 is used as is as the λ _S output. This λ _S pixel 21 does not output a detected value of λ _L. Therefore, the average value of pixels A and B of the adjacent λ _L pixels 22 is used as the λ _L output of the λ _S pixel 21. This makes it possible to obtain detected values corresponding to two wavelengths from one pixel.

The pixel size or pixel pitch is set to be larger than the infrared resolution on the short wavelength (λ _S ) side and smaller than the infrared resolution on the long wavelength (λ _L ) side. In this case, the light of λ _S is collected in the λ _S pixel 21 at pixel position (1) and reflected in the detection output of pixel position (1), and the leakage of light to adjacent pixels is small. On the other hand, since the pixel pitch (size) of the incident light of λ _L is smaller than the optical resolution of this wavelength, the light cannot be collected to the size of one pixel, and the incident light is imaged across multiple pixels. The λ _L incident light to pixel position (1) leaks and enters the adjacent λ _L pixel 22 (two pixels, pixel A and pixel B in the figure). Therefore, the detection outputs of the two pixels A and B adjacent to pixel position (1) partially reflect the component of the λ _L incident light to pixel position (1). By utilizing this, even if the detection output of λ _L cannot be directly obtained from the λ _S pixel 21 at pixel position (1), the amount of λ _L incident on the λ _S pixel 21 can be roughly estimated by averaging the outputs of the adjacent pixels A and B.

At pixel position (2) in FIG. 17B, the output value from this λ _S pixel 21 is used as the λ _S output as is. This λ _S pixel 21 does not output a detection value of λ _L. Therefore, the average value of the four diagonally adjacent λ _L pixels 22 (pixels A to D) is used as the λ _L output of this pixel. The λ _L infrared light incident on the λ _S pixel 21 at pixel position (2) is not condensed to this pixel size but is also incident on the surrounding pixels A to D, and the amount of λ _L incident on pixel position (2) can be estimated from the outputs of pixels A to D. This makes it possible to obtain detection values corresponding to two wavelengths from one pixel.

At pixel position (3) in Fig. 17C, the output value from this λ _L pixel 22 is used as the λ _L output as is. This λ _L pixel 22 does not output a detection value of the short wavelength λ _S. Therefore, the average value of the four λ _S pixels 21 (pixels A to D) adjacent in the horizontal and vertical directions is used as the short wavelength output. It is considered that the λ _S infrared light incident on the λ _L pixel 22 at pixel position (3) has an intensity distribution that is continuous with the λ _S incident light on the surrounding pixels A to D. Therefore, by using the detection values of the surrounding pixels, it is possible to obtain detection values corresponding to two wavelengths from one pixel.

At each pixel position, the missing wavelength output can be compensated for by the above-mentioned interpolation process, thereby generating output signals for two wavelengths. This allows the detection sensitivity of two wavelengths to be maintained without switching the bias application direction at each pixel using T2SL. The two-wavelength infrared sensor 50 and imaging system 100 of the embodiment can be applied to security systems, unmanned exploration systems, nighttime surveillance systems, etc.

Although the present invention has been described above with reference to specific examples, the present invention is not limited to the configurations exemplified in the examples. The configuration of the T2SL used in each pixel is not limited to the above-mentioned example. By adjusting the composition and thickness of the thin film constituting the T2SL, it is possible to give sensitivity to the target wavelength. It is not necessary to form both the λ _S pixel 21 and the λ _L pixel 2 with T2SL. If either one of the light absorption layers is T2SL, attenuation of one wavelength and interference problems occur due to broad response characteristics, but this problem is solved by the configuration of the embodiment. The conductivity types of the contact layer and the light absorption layer are appropriately determined according to the carrier to be detected. The bias voltage value applied to each pixel can be appropriately adjusted. When an n-type conductor is used as a whole in the configuration of FIG. 14, an npn junction may be formed with the first contact layer 201, the first block layer 232, and the second contact layer 203 of the λ _S pixel 21.

The number of pixels forming the unit block 25 is not limited to 4 pixels of 2×2, and the unit block 25 may be formed of 2×1, 3×3, 4×4, 5×5, etc. The 2×1 unit block is formed of one λ _S pixel 21 and one λ _L pixel 22. In this case, the planar pixel arrangement of the light receiving element array 20 is a checkerboard pattern. The interpolation process is the same as that of FIG. 17A. The 3×3 unit block includes three λ _L pixels 22 arranged on the diagonal of the matrix and six λ _S pixels 21. The interpolation process is similar to that of FIG. 17, but if the distance from the λ _S pixel 21 of interest to the surrounding λ _L pixels 22 for interpolation varies depending on the position of the λ _L pixel 22, weighting may be applied to the calculation of the average value. In the case of a 4×4 unit block, the interpolation process of the 2×2 unit block is repeated.

Even when these replacements and modifications are made, the λ _S pixel 21 and the λ _L pixel 22 operate simultaneously in the two-wavelength infrared sensor 50, and an interpolation process is performed for each pixel, enabling two-wavelength imaging. This function cannot be achieved in a configuration in which the sensor operates as a two-wavelength image sensor by switching bias for each pixel. When performing correlation processing of two-wavelength information as in the embodiment, it is important that the data are acquired simultaneously. When focusing on one wavelength, it appears that a pixel is missing spatially, but for the λ _L pixel 22, which is arranged in a small number, the λ _L infrared ray collected and imaged by the lens optical system cannot be collected within one pixel, and is imaged across multiple pixels. Therefore, the λ _L response at the missing pixel position can be sufficiently estimated by the interpolation process.

20 Light receiving element array 21 λ _S pixel (first pixel)
22 λ _L pixel (second pixel)
27 First isolation groove 28 Second isolation groove 201 First contact layer 202, 232 First block layer 232a n-type region 232b p-type region 203 Second contact layer 205 λ _S absorption layer (first light absorption layer)
206 Second blocking layer 207 λ _L absorption layer (second light absorption layer)
209 Third contact layer 30 Readout circuit 41, 42 Protruding electrode 50 Two-wavelength infrared sensor 60 Signal processing circuit 100 Imaging system

Claims (10)

a light receiving element array in which a plurality of pixels are arranged, each pixel having, in order from a light incident side, a first contact layer, a first blocking layer, a second contact layer , a first light absorbing layer sensitive to light of a first wavelength, a second blocking layer, a second light absorbing layer sensitive to light of a second wavelength longer than the first wavelength, and a third contact layer;
having
At least one first pixel that detects light of the first wavelength and at least one second pixel that detects light of the second wavelength form a unit block, and the unit block is repeatedly arranged in the light receiving element array,
the first pixel is divided by a first separation groove penetrating the third contact layer, the second light absorption layer, the second blocking layer, and the first light absorption layer;
the second pixel is separated by a second separation trench extending from the third contact layer to the first contact layer;
Dual wavelength infrared sensor. the first blocking layer and the second blocking layer have a first potential barrier in one of a conduction band and a valence band with respect to the first light absorbing layer and the second light absorbing layer, and have a second potential barrier lower than the first potential barrier in the other of the valence band and the conduction band,
a potential difference between the first blocking layer and the first contact layer and between the second blocking layer and the first light absorbing layer and between the second blocking layer and the first contact layer and between the second blocking layer and the first light absorbing layer and between the second blocking layer and the second light absorbing layer on the side of the second potential barrier;
2. The dual-wavelength infrared sensor according to claim 1. the first contact layer, the second contact layer, the first light absorbing layer, the second light absorbing layer, and the third contact layer are of the same conductivity type;
3. The two-wavelength infrared sensor according to claim 1. the first contact layer and the second contact layer have the same conductivity type, and the first block layer has a conductivity type opposite to that of the first contact layer and the second contact layer;
2. The dual-wavelength infrared sensor according to claim 1. the first contact layer and the second contact layer are each connected to a common electrode, and the first contact layer and the second contact layer are set to the same potential by the common electrode;
The two-wavelength infrared sensor according to claim 1 . A reverse bias is applied to the first blocking layer.
5. The dual-wavelength infrared sensor according to claim 4. Bias voltages having different polarities are simultaneously applied to the first pixel and the second pixel.
The dual-wavelength infrared sensor according to claim 1 . At least one of the first light absorbing layer and the second light absorbing layer is formed of a type II superlattice.
The two-wavelength infrared sensor according to claim 1 . The two-wavelength infrared sensor according to any one of claims 1 to 8, wherein the pixel pitch of the light receiving element array is equal to or smaller than the optical resolution of the second pixel. The two-wavelength infrared sensor according to claim 1 ,
a signal processing circuit connected to an output of the dual-wavelength infrared sensor;
having
the signal processing circuit interpolates an amount of light having a second wavelength received at the first pixel using outputs of the second pixels surrounding the first pixel, and generates first wavelength information and second wavelength information at the first pixel.
Imaging system.

Images