Photodetector and beat spectrometer

Abstract

To provide a photodetector capable of increasing the response speed and securing the intensity of an output signal and the reliability, and to provide a beat spectroscopic device capable of widening the wavelength sweeping range.SOLUTION: A photodetector 10 comprises: a semiconductor substrate 11; a mesa part 12 formed on a principal surface 11a of the semiconductor substrate 11 so as to extend along an optical waveguide direction A; a first contact layer 13; a second contact layer 14; a first electrode 15; and air bridge wiring 16 electrically connected with the first contact layer 13 and the first electrode 15. When seen from a direction perpendicular to the principal surface 11a of the semiconductor substrate 11, a length of the mesa part 12 in the optical waveguide direction A is longer than that of the mesa part 12 in a direction perpendicular to the optical waveguide direction A. The air bridge wiring 16 is drawn from the first contact layer 13 to one side in the direction perpendicular to the optical waveguide direction A, and stretched over between the first contact layer 13 and the first electrode 15.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a photodetector and a beat spectroscopic device. [Background technology]

[0002] Patent Document 1 discloses a quantum cascade detector, which detects light by utilizing intersubband transitions (intersubband absorption) in a quantum well structure. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2017-147428 A Summary of the Invention [Problem to be solved by the invention]

[0004] The above-mentioned photodetector is required to have a high response speed. It is also required to ensure the strength and reliability of the output signal. Therefore, the present invention aims to provide a photodetector that can realize a high response speed, a high output signal strength, and a high reliability, and a beat spectrometer that can widen the wavelength sweep range. [Means for solving the problem]

[0005] The photodetector of the present invention includes a semiconductor substrate having a main surface, and an active layer in which an absorption region that absorbs detection light by intersubband absorption and a transport region that transports electrons excited by intersubband absorption are alternately stacked, and the photodetector is provided with a mesa portion formed on the main surface of the semiconductor substrate so as to extend along an optical waveguide direction, a first contact layer formed on a surface of the mesa portion opposite the semiconductor substrate, a second contact layer formed between the main surface of the semiconductor substrate and the mesa portion, a first electrode formed on the main surface of the semiconductor substrate, and an air bridge wiring electrically connected to the first contact layer and the first electrode, wherein when viewed from a direction perpendicular to the main surface of the semiconductor substrate, the length of the mesa portion in the optical waveguide direction is longer than the length of the mesa portion in a direction perpendicular to the optical waveguide direction, and the air bridge wiring is drawn out from the first contact layer to one side in the direction perpendicular to the optical waveguide direction and bridges between the first contact layer and the first electrode.

[0006] In this photodetector, the first contact layer and the first electrode are connected by an air bridge wiring. As a result, the wiring length can be shortened and the area of ​​the mesa portion in a plan view can be reduced, as compared with the case where the first contact layer and the first electrode are connected by wire bonding, for example. By shortening the wiring length, the inductance can be reduced, and the response speed can be increased. In addition, by reducing the area of ​​the mesa portion, the parasitic capacitance can be reduced, and this can also increase the response speed. On the other hand, if the area of ​​the mesa portion is simply reduced, there is a concern that the intensity of the output signal will decrease. In this regard, in this photodetector, when viewed from a direction perpendicular to the main surface of the semiconductor substrate, the length of the mesa portion in the optical waveguide direction is longer than the length of the mesa portion in the direction perpendicular to the optical waveguide direction. As a result, light can be effectively absorbed in the mesa portion, and the intensity of the output signal can be ensured even if the area of ​​the mesa portion is reduced. Furthermore, in this photodetector, the air bridge wiring is drawn out from the first contact layer to one side in the direction perpendicular to the optical waveguide direction, and is bridged between the first contact layer and the first electrode. This makes it possible to ensure the width of the air bridge wiring (the length in the optical waveguide direction). When the wiring width is wide, it is possible to further reduce the inductance and ensure the strength of the air bridge wiring to ensure reliability. Therefore, with this photodetector, it is possible to achieve faster response speed, ensure the strength of the output signal, and ensure reliability.

[0007] The first electrode may have a connection portion located on one side of the mesa portion in a direction perpendicular to the optical waveguide direction, and the air bridge wiring may be drawn from the first contact layer to one side in the direction perpendicular to the optical waveguide direction and connected to the connection portion of the first electrode. In this case, the air bridge wiring can be drawn from the first contact layer to one side in the direction perpendicular to the optical waveguide direction, and the width of the air bridge wiring can be reliably secured.

[0008] The air bridge wiring has a bridge portion extending in the air, and the length of the bridge portion in the optical waveguide direction may be longer than the length of the bridge portion in the direction perpendicular to the optical waveguide direction. In this case, the width of the bridge portion (air bridge wiring) can be reliably secured.

[0009] The air bridge wiring has a bridge portion extending in the air, and the length of the mesa portion in the optical waveguide direction and the length of the bridge portion in the optical waveguide direction may each be 50 μm or more. In this case, the width of the bridge portion can be reliably secured. Also, the length of the mesa portion in the optical waveguide direction can be secured, and light can be absorbed more effectively in the mesa portion.

[0010] The ratio of the length of the mesa portion in the optical waveguide direction to the length of the mesa portion in the direction perpendicular to the optical waveguide direction may be greater than 1 and smaller than 100. In this case, the length of the mesa portion in the optical waveguide direction can be secured, and the mesa portion can absorb light more effectively.

[0011] The air bridge wiring has a bridge portion extending in the air, and the ratio of the length of the bridge portion in the optical waveguide direction to the length of the bridge portion in a direction perpendicular to the optical waveguide direction may be greater than 1 and less than 50. In this case, the width of the bridge portion (air bridge wiring) can be reliably secured.

[0012] The air bridge wiring may have a bridge portion extending in the air, and the thickness of the bridge portion may be 1 μm or more and 10 μm or less. In this case, reliability can be reliably ensured.

[0013] One end face of the mesa portion in the optical waveguide direction may be an incident face for the detection light, in which case the light can be absorbed more effectively in the mesa portion, and the intensity of the output signal can be reliably ensured.

[0014] One end face of the mesa portion in the optical waveguide direction may be flush with one end face of the semiconductor substrate in the optical waveguide direction, in which case the one end face of the mesa portion in the optical waveguide direction can be easily used as the incident surface of the detection light.

[0015] The photodetector of the present invention may further include a lens arranged to face the end face of the mesa portion and converging the detection light toward the end face of the mesa portion. In this case, the width of the end face of the mesa portion (the length in the direction perpendicular to the optical waveguide direction) can be narrowed, and the area of ​​the mesa portion in a plan view can be reduced. As a result, the parasitic capacitance can be reduced, and the response speed can be further increased. In addition, assuming that the area of ​​the mesa portion is constant, the length of the mesa portion in the optical waveguide direction can be increased by narrowing the width of the mesa portion. As a result, the light can be absorbed more effectively in the mesa portion, and the strength of the output signal can be reliably ensured.

[0016] Both side surfaces of the mesa in the direction perpendicular to the optical waveguide direction may be exposed, in which case the generation of parasitic capacitance can be suppressed and the response speed can be further increased.

[0017] The second contact layer may have a first portion located between the main surface of the semiconductor substrate and the mesa portion and a second portion located outside the mesa portion when viewed from a direction perpendicular to the main surface of the semiconductor substrate, and a second electrode may be formed on the second portion of the second contact layer. In this case, a large area of ​​the second electrode can be ensured, and the connecting member can be well connected to the second electrode.

[0018] The beat spectrometer of the present invention includes a fixed-wavelength light source, a variable-wavelength light source, and the above-mentioned photodetector that detects the light from the fixed-wavelength light source and the light from the variable-wavelength light source as detection light, and the light from the fixed-wavelength light source and the light from the variable-wavelength light source are detected by the photodetector while changing the wavelength of the light from the variable-wavelength light source so that the frequency of a beat signal having a frequency corresponding to the wavelength difference between the light from the fixed-wavelength light source and the light from the variable-wavelength light source is swept. The response speed of the photodetector included in this beat spectrometer is increased for the above-mentioned reasons. Therefore, this beat spectrometer can widen the wavelength sweep range in beat spectroscopy. Effect of the Invention

[0019] According to the present invention, it is possible to provide a photodetector capable of realizing high response speed, ensuring output signal strength, and ensuring reliability, and a beat spectrometer capable of widening the wavelength sweep range. [Brief description of the drawings]

[0020] [Figure 1] FIG. 2 is a front view of the light detection module according to the embodiment. [Diagram 2] FIG. [Diagram 3] FIG. 2 is a plan view of a photodetector. [Figure 4] FIG. [Diagram 5] 1 is a table showing a configuration of a photodetector. [Figure 6] FIG. [Figure 7] FIG. 2 is a perspective view showing a mounted state of the light detection module. [Figure 8] FIG. 2 is a plan view showing a mounting state of the light detection module. [Figure 9] FIG. 2 is a front view showing a mounted state of the light detection module. [Figure 10] 1 is a graph showing response characteristics of a photodetector. [Figure 11] 11 is a graph showing an example of an output signal in a high frequency range. [Figure 12] FIG. 2A is a front view of a photodetector according to an embodiment, and FIG. 2B is a front view of a photodetector according to a first modified example. [Figure 13] FIG. 11 is a cross-sectional view of a light detection module according to a second modified example. [Figure 14] FIG. 11 is a plan view of a fixing member according to a second modified example. [Figure 15] FIG. 13 is a plan view of a photodetector according to a third modified example. [Figure 16] FIG. 1 is a diagram illustrating a configuration of a beat spectrometer. [Figure 17] 4 is a graph showing the sensitivity characteristics of a photodetector and the oscillation wavelength of a fixed-wavelength light source. [Figure 18] 4 is a graph showing the oscillation wavelengths of a fixed wavelength light source and a tunable wavelength light source. [Figure 19] 1 is a graph showing the relationship between the injection current and the oscillation wavelength of a tunable light source. [Figure 20] 1 is a graph showing the relationship between the injection current into a tunable light source and the beat frequency. [Figure 21] 1 is a graph showing an example of a beat signal. [Figure 22] 13 is a graph showing a comparison result of response characteristics. [Figure 23] 13 is a graph showing the measurement results of beat spectroscopy. [Figure 24] 13 is a graph showing the measurement results of beat spectroscopy. [Diagram 25] 13 is a graph showing the measurement results of beat spectroscopy. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the following description, the same or corresponding elements are designated by the same reference numerals, and duplicated description will be omitted. [Photodetection module]

[0022] 1, the photodetector module 1 includes a photodetector 10 and a fixing member (submount) 50 to which the photodetector 10 is fixed. The photodetector 10 is, for example, a quantum cascade detector (QCD), and detects detection light DL by utilizing intersubband transition (intersubband absorption) in a quantum well structure. [Photodetector]

[0023] As shown in Figures 2, 3 and 4, the photodetector 10 includes a semiconductor substrate 11, a mesa portion 12, a first contact layer 13, a second contact layer 14, a first electrode 15, an air bridge wiring 16, and a second electrode 17.

[0024] The semiconductor substrate 11 is formed, for example, in a rectangular plate shape and has a flat main surface 11a. The semiconductor substrate 11 is, for example, a semi-insulating InP substrate. Hereinafter, the width direction, depth direction, and thickness direction of the semiconductor substrate 11 will be described as the X direction, Y direction, and Z direction, respectively. The X direction, Y direction, and Z direction are perpendicular to each other. Note that in Figs. 2 to 4, each component of the photodetector 10 is arranged in a manner inverted with respect to the X direction with respect to Fig. 1. In reality, each component of the photodetector 10 is arranged in a manner inverted with respect to the X direction with respect to the arrangement shown in Figs. 2 to 4. In short, it is sufficient that each element is arranged so that the first electrode 15 is connected to the first wiring 52 and the second electrode 17 is connected to the second wiring 53, as will be described later.

[0025] The mesa portion 12 is formed on the principal surface 11a of the semiconductor substrate 11 so as to extend along the optical waveguide direction A. The optical waveguide direction A is parallel to the Y direction. The mesa portion 12 is formed on the principal surface 11a via the second contact layer 14, and protrudes from the semiconductor substrate 11 in the Z direction. Each of a pair of side surfaces 12a of the mesa portion 12 in the X direction is exposed. That is, each side surface 12a is not covered by other elements. The side surface 12a is a surface that extends to intersect with the X direction, and is a flat surface perpendicular to the X direction in this example.

[0026] The mesa portion 12 includes an active layer 21. As shown in Fig. 5, the active layer 21 includes a plurality of absorption regions 22 and a plurality of transport regions 23 alternately stacked along the Z direction (a direction perpendicular to the main surface 11a of the semiconductor substrate 11), and has a quantum cascade structure. In the active layer 21, unit laminate structures 24 each consisting of a pair of an absorption region 22 and a transport region 23 are repeatedly stacked. In this example, the active layer 21 is configured so that the peak of the sensitivity wavelength is 4.5 µm, and includes 90 periods of unit laminate structures 24. The number of periods of the unit laminate structures 24 may be, for example, 10 or more and 150 or less.

[0027] The absorption region 22 includes a barrier layer 261 and a well layer 271, and absorbs the detection light DL by intersubband absorption. The transport region 23 includes multiple barrier layers 262-267 and multiple well layers 272-277, and transports electrons excited by intersubband absorption in the absorption region 22 to the next period of the absorption region 22. An example of the composition, layer thickness, and doping state of the barrier layers 261-267 and the well layers 272-277 is as shown in FIG.

[0028] When the detection light DL is incident on the active layer 21, the detection light DL is absorbed in the active layer 21. More specifically, electron excitation due to intersubband absorption, relaxation of the excited electrons, transport, and extraction of the electrons to the unit stack body 24 of the next period are repeated in the multiple unit stack bodies 24, causing cascaded light absorption in the active layer 21. In the light detection module 1, a current generated by this light absorption is extracted as an electrical signal, and the amount of the current is measured to detect the detection light DL. That is, the active layer 21 functions as a semiconductor region that generates an electrical signal according to the amount of incident detection light DL.

[0029] The mesa portion 12 is formed long in the Y direction, and is formed, for example, in a rectangular shape with its long sides parallel to the Y direction in plan view (when viewed from the Z direction). That is, in plan view, the length L1 of the mesa portion 12 in the Y direction (optical waveguide direction A) is longer than the length (width) L2 of the mesa portion 12 in the X direction (direction perpendicular to the optical waveguide direction A). The length L1 is, for example, 50 μm or more. The length L1 is, for example, about 50 μm to 3000 μm, and is 100 μm in this example. The length L2 is, for example, about 10 μm to 1000 μm, and is 25 μm in this example. The aspect ratio of the mesa portion 12 in plan view (the ratio of the length L1 to the length L2) is 1 to 100. The aspect ratio of the mesa portion 12 may be 1.5 to 50, and may be preferably 2 to 10. The lengths L1 and L2 may be 10 μm or more. If the length L1 is less than 50 μm, it may be difficult to ensure the strength of the output signal. If the length L2 is less than 10 μm, it may be difficult to form the air bridge wiring 16. If the length L1 is greater than 3000 μm or the length L2 is greater than 1000 μm, the element size becomes large, and it may be difficult to achieve a high cutoff.

[0030] The first contact layer 13 is an upper contact layer formed on a surface 12b of the mesa portion 12 opposite to the semiconductor substrate 11. The second contact layer 14 is a lower contact layer formed between the main surface 11a of the semiconductor substrate 11 and the mesa portion 12. An example of the composition, layer thickness, and doping state of the first contact layer 13 and the second contact layer 14 is as shown in FIG.

[0031] The second contact layer 14 is formed in a rectangular shape with each side longer than the mesa portion 12, and has a portion protruding from the mesa portion 12. That is, the second contact layer 14 has a first portion 14a located between the main surface 11a of the semiconductor substrate 11 and the mesa portion 12, and a second portion 14b located outside the mesa portion 12 in a plan view.

[0032] The mesa portion 12, the first contact layer 13, and the second contact layer 14 are formed on the main surface 11a of the semiconductor substrate 11 by, for example, crystal growth. The mesa portion 12, the first contact layer 13, and the second contact layer 14 are formed by performing etching using photolithography technology and dry etching technology after the crystal growth so as to reach the surface of the second contact layer 14 or the inside of the second contact layer 14. During manufacturing, for example, a plurality of portions corresponding to the mesa portion 12, the first contact layer 13, and the second contact layer 14 are formed on a wafer, and then the wafer is cut, thereby forming a plurality of elements collectively.

[0033] In this example, one end face 12c of the mesa portion 12 in the Y direction (optical waveguide direction A) (direction parallel to the main surface 11a of the semiconductor substrate 11) serves as an incident face (light receiving face) of the detection light DL. The detection light DL incident from the end face 12c travels through the mesa portion 12 along the optical waveguide direction A. The end face 12c is flush with the side face 11b of the semiconductor substrate 11. That is, the end face 12c and the side face 11b are located on the same plane. The side face 11b is a surface that extends so as to intersect with the Y direction, and in this example, is a flat surface perpendicular to the Y direction. In this example, the side face 11b and the end face 12c are cleavage planes formed by cutting the wafer.

[0034] The first electrode 15 is formed in a planar shape on the main surface 11a of the semiconductor substrate 11. The first electrode 15 is made of, for example, gold, and is formed in a square shape by patterning. The first electrode 15 has a connection portion 15a located on one side in the X direction with respect to the mesa portion 12. In this example, the connection portion 15a is the entire first electrode 15. The first electrode 15 is electrically connected to the first contact layer 13 via an air bridge wiring 16. The first electrode 15 is provided to extract the current output from the active layer 21 to the outside. The area of ​​the first electrode 15 is, for example, 10,000 μm 2 This is the end of the process. In this case, the first electrode 15 can be in excellent surface contact with the first wiring 52 described below. In this example, the dimensions of the first electrode 15 are 400 μm×400 μm.

[0035] The air bridge wiring 16 is a connection wiring electrically connected to the first contact layer 13 and the first electrode 15. The air bridge wiring 16 is drawn out from the first contact layer 13 to one side in the X direction and bridges between the first contact layer 13 and the first electrode 15. In this example, the air bridge wiring 16 is drawn out from the first contact layer 13 along a direction parallel to the X direction. The air bridge wiring 16 is drawn out from an end of the first contact layer 13 (mesa portion 12) in the X direction in a plan view. The air bridge wiring 16 is an aerial wiring (three-dimensional wiring) having a bridge portion 16a extending in the air. The bridge portion 16a is electrically connected to the first electrode 15 (connection portion 15a) via a plating layer 151 described later. The air bridge wiring 16 further has a planar portion 16b formed on the first contact layer 13. The bridge portion 16a is formed integrally with the portion 16b.

[0036] The air bridge wiring 16 is formed, for example, by the following process. First, a resist is formed by patterning in the region between the mesa portion 12 and the first electrode 15 on the main surface 11a of the semiconductor substrate 11. Next, a thin gold film with a thickness of about 5 μm is formed on the resist by plating, and then the resist is removed. In this way, a sheet-like air bridge wiring 16 is formed.

[0037] The bridge portion 16a is formed in a wide sheet shape (layer shape). The length (width) L3 of the bridge portion 16a in the Y direction (light guide direction A) is longer than the length L4 of the bridge portion 16a in the X direction. In this example, the bridge portion 16a extends in a curved manner when viewed from the Y direction. In this case, as shown in FIG. 4, the length L4 of the bridge portion 16a in the X direction is the length (actual length) of the bridge portion 16a along the extending direction. The length L3 is, for example, 50 μm or more. The length L3 is, for example, about 50 μm to 3000 μm, and is 80 μm in this example. The length L4 is, for example, about 5 μm to 200 μm, and is 30 μm in this example. The aspect ratio of the bridge portion 16a (the ratio of the length L3 to the length L4) may be 0.25 to 100, and may be preferably 1 to 50. The aspect ratio of the bridge portion 16a may be more preferably 1 to 20, and further preferably 2 to 10. The thickness of the bridge portion 16a (air bridge wiring 16) in the Z direction is 1 μm or more and 10 μm or less.

[0038] The second electrode 17 is formed in a planar shape on the second portion 14b of the second contact layer 14. In other words, the second electrode 17 is formed in a planar shape on the main surface 11a of the semiconductor substrate 11 via the second contact layer 14. The second electrode 17 is made of, for example, gold, and is formed in a square shape by patterning. The second electrode 17 is disposed on the other side in the X direction (the opposite side to the first electrode 15) of the mesa portion 12. The second electrode 17 is electrically connected to the second contact layer 14. The second electrode 17 is provided to extract the current output from the active layer 21 to the outside. The second electrode 17 is electrically isolated from the first electrode 15. The area of ​​the second electrode 17 is, for example, 10,000 μm 2 In this case, the second electrode 17 can be in good surface contact with the second wiring 53 described later. In this example, the dimensions of the second electrode 17 are 400 μm×400 μm, which are the same as those of the first electrode 15.

[0039] During plating in the above-mentioned process of forming the air bridge wiring 16, plating layers may also be formed on the first electrode 15 and the second electrode 17. In Figs. 2 to 4, these plating layers are indicated by reference numerals 151 and 171. The plating layers 151 and 171 can be regarded as constituting the first electrode 15 and the second electrode 17, respectively. In this example, the plating layer 151 is formed slightly smaller than the first electrode 15 (formed only on a part of the first electrode 15), but may be formed on the entire surface of the first electrode 15. Similarly, the plating layer 171 is formed slightly smaller than the second electrode 17, but may be formed on the entire surface of the second electrode 17. The plating layer 151 can also be regarded as constituting the air bridge wiring 16. The plating layers 151 and 171 do not have to be formed. [Fixed part]

[0040] As shown in FIG. 1 and FIG. 6, the fixing member 50 includes an insulating substrate 51, a first wiring 52, and a second wiring 53. In FIG. 6, the first wiring 52 and the second wiring 53 are hatched for ease of understanding. The insulating substrate 51 is formed, for example, in a rectangular flat plate shape, and has a main surface 51a and a main surface 51b opposite to the main surface 51a. In this example, the main surfaces 51a and 51b are flat surfaces parallel to each other. The base material of the insulating substrate 51 is, for example, alumina (aluminum oxide). In this case, it is possible to reduce dielectric loss in the high frequency band. The base material of the insulating substrate 51 may be SiC or ceramics, which are highly insulating materials.

[0041] A recess 54 is formed on the main surface 51a of the insulating substrate 51. In this example, the recess 54 is a groove extending along the Y direction (optical waveguide direction A) and opens to a side surface 51c of the insulating substrate 51. That is, the recess 54 extends to reach the side surface 51c. The side surface 51c is a surface that extends to intersect with the Y direction, and in this example, is a flat surface perpendicular to the Y direction. As an example, the recess 54 has a rectangular shape in a plan view and a substantially semicircular shape in a cross section perpendicular to the Y direction.

[0042] The first wiring 52 is formed in a planar shape on the main surface 51a of the insulating substrate 51. The first wiring 52 is made of, for example, gold, and is formed into a predetermined shape by patterning. The first wiring 52 is disposed on one side in the X direction with respect to the recess 54. The first wiring 52 is formed in a rectangular shape with its long side parallel to the Y direction, and extends between the side surfaces 51c and 51d of the insulating substrate 51. The side surface 51d is the surface of the insulating substrate 51 opposite to the side surface 51c.

[0043] The second wiring 53 is formed in a planar shape on the main surface 51a of the insulating substrate 51. The second wiring 53 is electrically separated from the first wiring 52. The second wiring 53 is made of, for example, gold, and is formed into a predetermined shape by patterning. The second wiring 53 is formed so as to extend from the main surface 51a of the insulating substrate 51 through the side surface 51e to the main surface 51b (wrap around). That is, the second wiring 53 has a first portion 53a arranged on the main surface 51a and a second portion 53b arranged across the main surface 51b and the side surface 51e. The first portion 53a is arranged on the other side in the X direction (opposite the first wiring 52) with respect to the recess 54. The first portion 53a has a portion 53a1 formed in a rectangular shape with long sides parallel to the X direction, and a portion 53a2 formed in a rectangular shape with long sides parallel to the Y direction. The portion 53a2 extends from the portion 53a1 to the side surface 51d of the insulating substrate 51. The second portion 53b is formed on the entire main surface 51b and the side surface 51e.

[0044] A solder layer 55 is formed on the first wiring 52, and a solder layer 56 is formed on the second wiring 53. The solder layer 56 is formed on a portion 53a1 of the second wiring 53. Each of the solder layers 55, 56 is formed in a square shape from, for example, a metal material, and has a thickness of about 1 μm or more and 10 μm or less.

[0045] As shown in FIG. 1, the photodetector 10 is fixed to the fixing member 50 with the main surface 11a of the semiconductor substrate 11 facing the main surface 51a of the insulating substrate 51. In the fixed state in which the photodetector 10 is fixed to the fixing member 50, at least a part of the mesa portion 12 is disposed in the recess 54. In this example, the tip portion of the mesa portion 12 (the end portion opposite the semiconductor substrate 11 in the Y direction) is disposed in the recess 54. The mesa portion 12 is spaced apart from the inner surface 54a of the recess 54. The longitudinal direction of the mesa portion 12 is parallel to the longitudinal direction of the recess 54. The end surface 12c of the mesa portion 12 is exposed from the recess 54 when viewed from the Y direction. In the fixed state, a part of the air bridge wiring 16 (bridge portion 16a) is also disposed in the recess 54 so as to be spaced apart from the inner surface 54a. As a result, the connection portion between the air bridge wiring 16 and the first contact layer 13 is disposed in the recess 54.

[0046] The photodetector 10 is fixed to the fixing member 50 by connecting the first electrode 15 to the first wiring 52 and connecting the second electrode 17 to the second wiring 53. The first electrode 15 is fused to the first wiring 52 using a solder layer 55, and is electrically connected to the first wiring 52 in a state of surface contact with the first wiring 52 via the solder layer 55. The second electrode 17 is fused to the second wiring 53 using a solder layer 56, and is electrically connected to the second wiring 53 in a state of surface contact with the second wiring 53 via the solder layer 56. The contact area between the first electrode 15 and the first wiring 52 and the contact area between the second electrode 17 and the second wiring 53 are each 10,000 μm 2 In the fixed state, a part of the first wiring 52 is exposed so that a terminal 81 of a connector 80, which will be described later, can be connected (FIGS. 7 and 8). [Photodetector module installation status]

[0047] 7, 8 and 9, the light detection module 1 is used by being connected to, for example, a connector 80. The connector 80 is, for example, an SMA connector, and has a terminal 81 electrically connected to a signal line of an SMA cable. The terminal 81 is mechanically connected to the first wiring 52 of the fixing member 50, and is electrically connected to the first wiring 52. Alternatively, the terminal 81 may be connected to the first wiring 52 by soldering. Thus, in this example, the first wiring 52 is a signal side wiring electrically connected to the signal line.

[0048] The connector 80 further includes a main body member 82 and a support member 83 formed integrally with the main body member 82. The main body member 82 and the support member 83 are electrically connected to the ground line of the SMA cable. The light detection module 1 is fixed to the connector 80 by connecting the fixing member 50 to the support member 83. For example, the second portion 53b of the second wiring 53 of the fixing member 50 is connected to a gold layer formed on the surface of the support member 83 by soldering. The gold layer is formed so as to cover the entire surface of the support member 83, for example. Alternatively, the main body member 82 may be formed separately from the support member 83. In this case, the support member 83 to which the fixing member 50 is connected may be fixed to the main body member 82 by screwing or soldering. By the above connection, the second wiring 53 is electrically connected to the ground line. That is, in this example, the second wiring 53 is a ground side wiring electrically connected to the ground line. In this example, the electrical connection between the photodetector 10 and the connector 80 is achieved without using a bonding wire (wireless connection). This makes it possible to avoid the generation of inductance caused by the bonding wire. Furthermore, when a bonding wire is used, the wire may be damaged due to impact or being caught, so care must be taken when handling the device. In contrast, in this example, since no bonding wire is used, it is possible to facilitate handling of the device.

[0049] The shape (wiring pattern) of the first wiring 52 is set in consideration of impedance matching. In this example, the first wiring 52 is configured as a microstrip line. As an example, when the thickness of the insulating substrate 51 is 0.5 mm, the relative dielectric constant of the insulating substrate 51 is 9.8, the thickness of the first wiring 52 is 0.8 μm, and the matching resistance value at 20 GHz is 50Ω, the width W of the first wiring 52 is set to 0.5 mm or less, and the length L5 of the first wiring 52 is set to 1.45 mm or less (FIG. 6). As a result, the length L5 of the first wiring 52 is ¼ or less of the wavelength (electrical length) of the electrical signal propagating through the first wiring 52. In this way, by setting the length L5 of the first wiring 52 to ¼ or less of the electrical length calculated based on the design parameters of the first wiring 52, impedance matching in a desired frequency band is possible. The first wiring 52 may be configured as a coplanar line instead of a microstrip line.

[0050] As shown in FIG. 8, the light detection module 1 (light detector 10) further includes a lens 40. The lens 40 is disposed to face the end face 12c of the mesa portion 12, which is a light incident surface, and focuses the detection light DL toward the end face 12c, thereby focusing the detection light DL on the end face 12c. The lens 40 has a numerical aperture (NA) of, for example, 0.4 or more, and a focused light diameter of several μm to several tens of μm. A low-reflection layer including a dielectric multilayer film may be formed on both sides of the lens 40 and on the end face 12c. In this case, the transmittance of these surfaces to light in the sensitivity wavelength range of the light detector 10 can be 95% or more. [Action and Effects]

[0051] In the photodetector 10, the first contact layer 13 and the first electrode 15 are connected by an air bridge wiring 16. This allows the wiring length to be shortened and the area of ​​the mesa portion 12 in a plan view to be reduced, as compared with the case where the first contact layer 13 and the first electrode 15 are connected by wire bonding, for example. In the case of wire bonding, the area required for wiring is determined by the area required for crimping the wire tip, and an area of ​​about 100 μm×100 μm is required, for example. By shortening the wiring length, the inductance can be reduced, and the response speed can be increased. In addition, by reducing the area of ​​the mesa portion 12, the parasitic capacitance can be reduced, and this also allows the response speed to be increased. On the other hand, there is a concern that simply reducing the area of ​​the mesa portion 12 will reduce the strength of the output signal. In this regard, in the photodetector 10, when viewed from the Z direction (direction perpendicular to the main surface 11a of the semiconductor substrate 11), the length L1 of the mesa portion 12 in the optical waveguide direction A (Y direction) is longer than the length L2 of the mesa portion 12 in the X direction (direction perpendicular to the optical waveguide direction A). This makes it possible to lengthen the path along which the detection light DL travels in the mesa portion 12, and the mesa portion 12 can effectively absorb the light. As a result, even if the area of ​​the mesa portion 12 is reduced, the intensity of the output signal can be ensured. Furthermore, in the photodetector 10, the air bridge wiring 16 is drawn out from the first contact layer 13 to one side in the X direction and bridges between the first contact layer 13 and the first electrode 15. This makes it possible to ensure the width (length in the optical waveguide direction A) of the air bridge wiring 16. When the wiring width is wide, the cross-sectional area becomes large, making it possible to further reduce the inductance, and also to ensure the strength of the air bridge wiring 16 and ensure reliability. Therefore, the photodetector 10 can achieve high response speed, ensure the strength of the output signal, and ensure reliability.

[0052] FIG. 10 is a graph showing the response characteristics of the photodetector 10, and FIG. 11 is a graph showing an example of an output signal in a high frequency region. As shown in FIG. 10, in the photodetector 10, the cutoff frequency, which is the frequency at which the signal strength drops by -3 dB, is 20 GHz or higher. For example, in the MCT (HgCdTe) sensor that has been used as a mid-infrared photodetector, only a cutoff frequency of about several hundred MHz (maximum about 1 GHz) could be realized. In contrast, in the photodetector 10, the cutoff frequency can be increased, and the response speed can be increased. Also, as shown in FIG. 11, a high signal strength exceeding 30 dB is obtained in a frequency band exceeding 26 GHz.

[0053] The first electrode 15 has a connection portion 15a located on one side in the X direction with respect to the mesa portion 12, and the air bridge wiring 16 is drawn out from the first contact layer 13 to one side in the X direction and connected to the connection portion 15a of the first electrode 15. This makes it possible to draw the air bridge wiring 16 from the first contact layer 13 to one side in the X direction, and ensure the width of the air bridge wiring 16.

[0054] The air bridge wiring 16 has a bridge portion 16a extending in the air, and a length L3 of the bridge portion 16a in the optical waveguide direction A is longer than a length L4 of the bridge portion 16a in the X direction. This makes it possible to reliably ensure the width of the bridge portion 16a (air bridge wiring 16).

[0055] The length L1 of the mesa portion 12 in the optical waveguide direction A and the length L3 of the bridge portion 16a in the optical waveguide direction A are each 50 μm or more. This makes it possible to reliably ensure the width of the bridge portion 16a. In addition, the length of the mesa portion 12 in the optical waveguide direction A can be ensured, and the mesa portion 12 can absorb light more effectively.

[0056] The ratio of the length L1 of the mesa portion 12 to the length L2 is greater than 1 and smaller than 100. This ensures the length L1 of the mesa portion 12 in the optical waveguide direction A, allowing the mesa portion 12 to absorb light more effectively.

[0057] The ratio of the length L3 of the bridge portion 16a to the length L4 is greater than 1 and smaller than 50. This makes it possible to reliably ensure the width of the bridge portion 16a (air bridge wiring 16).

[0058] The thickness of the bridge portion 16a is 1 μm or more and 10 μm or less. This can reliably ensure reliability. That is, by making the thickness of the bridge portion 16a 1 μm or more, the mechanical strength required for forming the bridge portion 16a and maintaining its shape can be ensured. Also, by making the thickness of the bridge portion 16a 10 μm or less, the shape can be maintained against its own weight.

[0059] One end face 12c of the mesa portion 12 in the optical waveguide direction A serves as the incident face of the detection light DL. This allows the mesa portion 12 to absorb light more effectively, and ensures the intensity of the output signal.

[0060] One end face 12c of the mesa portion 12 in the optical waveguide direction A is flush with the side face 11b of the semiconductor substrate 11. This makes it possible to easily use the end face 12c of the mesa portion 12 as an incident face for the detection light DL.

[0061] The photodetector 10 is disposed to face the end face 12c of the mesa portion 12, and includes a lens 40 that converges the detection light DL toward the end face 12c of the mesa portion 12. This allows the width (length in the X direction) of the end face 12c of the mesa portion 12 to be narrowed, and the area of ​​the mesa portion 12 in a plan view to be reduced. As a result, the parasitic capacitance can be reduced, and the response speed can be further increased. In addition, assuming that the area of ​​the mesa portion 12 is constant, the length of the mesa portion 12 in the optical waveguide direction A can be increased by narrowing the width of the mesa portion 12. As a result, the light can be absorbed more effectively in the mesa portion 12, and the intensity of the output signal can be reliably ensured. The absorption efficiency can also be increased by increasing the number of periods of the unit laminate body 24 in the active layer 21 to thicken the active layer 21. Increasing the thickness of the active layer 21 also contributes to reducing the parasitic capacitance and reducing noise due to an increase in the element resistance. On the other hand, if the number of periods is too large, the quantum efficiency decreases, so the number of periods is preferably about 10 or more and 150 or less.

[0062] The side surface 12a of the mesa portion 12 in the X direction is exposed. This makes it possible to suppress the occurrence of parasitic capacitance and to further increase the response speed. This point will be described below with reference to FIG.

[0063] FIG. 12(a) is a front view of the photodetector 10, and FIG. 12(b) is a front view of the photodetector 10A of the first modification. As shown in FIG. 12(a), in the photodetector 10, both side surfaces 12a of the mesa portion 12 in the X direction are exposed. In contrast, as shown in FIG. 12(b), in the photodetector 10A, both side surfaces 12a of the mesa portion 12 are covered with an insulating layer 90. The insulating layer 90 is, for example, a silicon nitride (SiN) film. The insulating layer 90 is embedded between the first contact layer 13 and the metal layer (part 16b of the air bridge wiring 16) on the first contact layer 13. Since this part has a structure in which an insulator is sandwiched between metals, for example, if misalignment occurs when forming the insulating layer 90, there is a possibility that a small amount of parasitic capacitance will occur in this part. In this regard, in the photodetector 10, since both side surfaces 12a of the mesa portion 12 are exposed, the occurrence of such parasitic capacitance can be suppressed.

[0064] The second contact layer 14 has a first portion 14a located between the main surface 11a of the semiconductor substrate 11 and the mesa portion 12 in a plan view, and a second portion 14b located outside the mesa portion 12, and the second electrode 17 is formed on the second portion 14b. This allows the area of ​​the second electrode 17 to be secured large, and the connection member can be well connected to the second electrode 17. For example, in the above example, the second wiring 53 is electrically connected to the second electrode 17 in a surface-to-surface contact state. This allows the inductance to be reduced compared to the case where the second electrode 17 and the second wiring 53 are connected by, for example, wire bonding, and the response speed can be further increased. In addition, by connecting the second electrode 17 and the second wiring 53 in a surface-to-surface contact state, the photodetector 10 and the fixing member 50 can be firmly fixed, and reliability can be reliably ensured. In addition, the large area of ​​the second electrode 17 allows the contact area between the second electrode 17 and the second wiring 53 to be secured large. The connection member connected to the second electrode 17 may be a ribbon-shaped electrode or a bonding wire. Even in such a case, the large area of ​​the second electrode 17 allows the connection member to be well connected to the second electrode 17.

[0065] In the light detection module 1, the first electrode 15 on the semiconductor substrate 11 of the light detector 10 is electrically connected (surface-connected) to the first wiring 52 in a state of surface contact with the first wiring 52 on the insulating substrate 51 of the fixing member 50. As a result, compared to the case where the first electrode 15 and the first wiring 52 are connected by, for example, wire bonding, it is possible to avoid the occurrence of inductance due to the wire, and the inductance can be reduced. As a result, it is possible to increase the response speed. In addition, by connecting the first electrode 15 and the first wiring 52 in a surface-contact state, it is possible to firmly fix the light detector 10 and the fixing member 50, and reliability can be ensured. Furthermore, in the light detection module 1, at least a part of the mesa portion 12 of the light detector 10 is disposed in a recess 54 formed in the insulating substrate 51. As a result, it is possible to protect the delicate mesa portion 12, and it is possible to ensure reliability. For example, it is possible to protect the mesa portion 12 from damage (mechanical damage due to external factors) caused by contact from the outside, adhesion of floating objects, wind pressure, etc.

[0066] The recess 54 opens to the side surface 51c of the insulating substrate 51. This allows the detection light to be easily incident on the end surface 12c of the mesa portion 12 in the Y direction. At least a part of the air bridge wiring 16 is disposed in the recess 54. This allows the delicate air bridge wiring 16 to be protected, and the reliability can be reliably ensured. The first wiring 52 has a length of ¼ or less of the wavelength of the electrical signal propagating through the first wiring 52. This allows impedance matching to be realized. The mesa portion 12 is spaced from the inner surface of the recess 54. This allows the mesa portion 12 to be reliably protected. When the photodetector 10 is a detector that detects the detection light DL by utilizing intersubband absorption in a quantum well structure, the detection can be performed without applying an external voltage, so heat is unlikely to be generated in the mesa portion 12. Therefore, the mesa portion 12 can be spaced from the insulating substrate 51.

[0067] The first electrode 15 is in surface contact with the first wiring 52 via the solder layer 55, and the second electrode 17 is in surface contact with the second wiring 53 via the solder layer 56. This allows the first electrode 15 and the first wiring 52 to be electrically and mechanically firmly connected, and the second electrode 17 and the second wiring 53 to be electrically and mechanically firmly connected. Furthermore, even if there is a height difference between the first electrode 15 and the second electrode 17 when connecting the first electrode 15 and the first wiring 52 and connecting the second electrode 17 and the second wiring 53, the height difference can be absorbed by the solder layers 55 and 56, and good surface contact can be achieved at each connection point. [Variations]

[0068] In the light detection module 1 of the second modification shown in FIG. 13 and FIG. 14, the detection light DL travels through the semiconductor substrate 11 and then enters the mesa portion 12. In the second modification, the side surface 11b of the semiconductor substrate 11 is an inclined surface inclined with respect to the Z direction. As an example, the inclination angle θ of the side surface 11b with respect to the Z direction when viewed from the X direction is 45 degrees. The side surface 11b is a polished surface formed by polishing, for example. The detection light DL is incident on the side surface 11b from a direction perpendicular to the side surface 11b, travels through the semiconductor substrate 11, and then enters the surface 12d of the mesa portion 12 through the second contact layer 14. The surface 12d is the surface of the mesa portion 12 on the semiconductor substrate 11 side. That is, in this example, the side surface 11b is the incident surface of the detection light DL. The end surface 12c of the mesa portion 12 is not flush with the side surface 11b of the semiconductor substrate 11. The detection light DL incident on the mesa portion 12 is multiple-reflected within the mesa portion 12, and the electric field oscillation component of the detection light DL parallel to the Z direction is absorbed in the active layer 21. The detection light DL may be multiple-reflected within the semiconductor substrate 11 and may be incident on the surface 12d of the mesa portion 12 a number of times. Note that the first electrode 15 and the like are omitted in Fig. 13, and hatching indicating a cross section is also omitted.

[0069] As shown in FIG. 14, in the second modified example, the recess 54 is configured as a hole formed in the main surface 51a of the insulating substrate 51. The recess 54 does not open to the side surface 51c of the insulating substrate 51 and is spaced from the outer edge of the main surface 51a. The recess 54 has a substantially semicircular shape in a cross section perpendicular to the X direction. In the second modified example, at least a part of the mesa portion 12 is disposed in the recess 54. The mesa portion 12 is formed in a shape that allows it to be disposed in the recess 54. With such a second modified example, it is possible to realize high-speed response, ensure the strength of the output signal, and ensure reliability, as in the above embodiment. In addition, since the recess 54 does not open to the side surface 51c of the insulating substrate 51 and is configured as a hole, the mesa portion 12 can be protected more reliably.

[0070] In the third modified example shown in FIG. 15, the photodetector 10 includes a height adjustment layer 19. The height adjustment layer 19 is formed in a planar shape on the main surface 11a of the semiconductor substrate 11. The height adjustment layer 19 is formed on the main surface 11a so as to be spaced apart from the second contact layer 14, and is electrically isolated from the second contact layer 14. The height adjustment layer 19 is formed simultaneously with the formation of the second contact layer 14, and has a similar configuration to the second contact layer 14. That is, the height adjustment layer 19 has the same thickness as the second contact layer 14. The first electrode 15 is formed on the height adjustment layer 19. With this third modified example, it is possible to achieve high response speed, high strength of the output signal, and high reliability, as in the above embodiment. Furthermore, when connecting the first electrode 15 and the first wiring 52 and connecting the second electrode 17 and the second wiring 53, even if there is a height difference between the first electrode 15 and the second electrode 17, the height adjustment layer 19 can reduce the height difference between the first electrode 15 and the second electrode 17, and good surface contact can be achieved at each connection point.

[0071] The present invention is not limited to the above embodiment. For example, the material and shape of each component are not limited to the above-mentioned material and shape, and various materials and shapes can be adopted. The length L3 of the bridge portion 16a in the optical waveguide direction A does not necessarily have to be longer than the length L4 of the bridge portion 16a in the X direction. The lens 40 may be omitted. The second contact layer 14 may have only the first portion 14a without having the second portion 14b. The mesa portion 12 may further include an upper cladding layer arranged between the active layer 21 and the first contact layer 13, and a lower cladding layer arranged between the active layer 21 and the second contact layer 14.

[0072] In the above embodiment, the air bridge wiring 16 is drawn from the first contact layer 13 in a direction parallel to the X direction, but the air bridge wiring 16 may be drawn from the first contact layer 13 to one side in the X direction, and may be drawn from the first contact layer 13 in a direction inclined with respect to the X direction and the Y direction in a plan view. That is, the air bridge wiring 16 may be drawn from the first contact layer 13 in a direction intersecting with the Y direction in a plan view.

[0073] Although the photodetector 10 is configured as a quantum cascade detector in the above embodiment, the photodetector 10 may be configured as another photodetector such as a quantum well infrared photodetector (QWIP). A quantum well infrared photodetector is an infrared detector that detects light by utilizing intersubband absorption in a quantum well structure. [Beat Spectrometer]

[0074] 16, the beat spectrometer 100 includes a first light source (fixed wavelength light source) 101, a second light source (variable wavelength light source) 102, a beam splitter 103, a photodetection module 1, a spectrum analyzer 104, and a gas cell 105. In the beat spectrometer 100, the light P1 and P2 are detected by the photodetection module 1 while changing the wavelength of the light P2 so that the frequency of a beat signal having a frequency according to the wavelength difference between the light P1 from the first light source 101 and the light P2 from the second light source 102 is swept (scanned). This makes it possible to perform heterodyne detection spectroscopy.

[0075] The first light source 101 and the second light source 102 output light P1, P2 having oscillation wavelengths that are included in the wavelength sensitivity range of the photodetection module 1 and are close to each other. The first light source 101 and the second light source 102 are, for example, distributed feedback quantum cascade lasers (DFB-QCLs). When in use, the oscillation wavelength of the first light source 101 is fixed and the oscillation wavelength of the second light source 102 is modulated. For example, the oscillation wavelength of the second light source 102 can be modulated by changing the injection current. That is, the first light source 101 is a fixed wavelength light source in which the wavelength of the output light P1 is fixed, and the second light source 102 is a variable wavelength light source in which the wavelength of the output light P2 is variable. The operation of the second light source 102 is controlled by a control unit configured by, for example, a computer.

[0076] In FIG. 17, the sensitivity characteristic of the photodetector 10 is indicated by the symbol R1, and the oscillation wavelength of the second light source 102 is indicated by the symbol R2. As shown in FIG. 17, the oscillation wavelength of the second light source 102 is included in the sensitivity wavelength range of the photodetector 10. FIG. 18 is a graph showing the oscillation wavelengths of the first light source 101 and the second light source 102. Symbol F indicates the oscillation wavelength of the first light source 101 when the injection current is 780 mA. Symbols T1 to T4 indicate the oscillation wavelengths of the second light source 102 when the injection current is 780 mA, 810 mA, 830 mA, and 850 mA, respectively. In this way, the oscillation wavelength of the second light source 102 can be modulated by changing the injection current.

[0077] Light P1 from the first light source 101 passes through a lens 108, is reflected by mirrors 111 and 112, and enters the beam splitter 103. Light P2 from the second light source 102 passes through a lens 109, is reflected by a mirror 113, and enters the beam splitter 103. The lights P1 and P2 are combined by the beam splitter 103. The combined lights P1 and P2 pass through iris members (irises) 114 and 115, are reflected by a mirror 116, and enter the lens 40, and then enter the photodetector 10. A spectrum analyzer 104 is connected to the photodetector 10. A gas cell 105 containing a gas to be measured is disposed between the second light source 102 and the mirror 113. The light P2 from the second light source 102 passes through the gas cell 105 and enters the mirror 113. When the wavelength modulation range of the light P2 from the second light source 102 crosses the absorption line of the gas, light of a specific wavelength of the light P2 is absorbed.

[0078] In the photodetector 10, a beat signal (beat) having a frequency according to the wavelength difference between the light P1 from the first light source 101 and the light P2 from the second light source 102 is detected. It is difficult to directly measure the frequency of light. For example, when the wavelength is 4 μm, the response speed of the photodetector needs to be about 75 THz or higher. In contrast, in a beat spectroscopy method using a beat signal, when the wavelength of the light P1 is 4.000 μm and the wavelength of the light P2 is 4.001 μm, the response speed of the photodetector 10 only needs to be about 18 GHz or higher. As described above, the photodetector module 1 has a cutoff frequency of 20 GHz or higher. Therefore, by performing beat spectroscopy using the photodetector module 1, it is possible to perform spectroscopy over a wide wavelength range.

[0079] In the beat spectroscopic device 100, as described above, the wavelength of the light P2 is changed so that the frequency of the beat signal is swept, while the light P1 and P2 are detected by the photodetection module 1. FIG. 19 is a graph showing the relationship between the injection current and the oscillation wavelength of the second light source 102. As shown in FIG. 19, if the current of the second light source 102 is x and the wave number of the light P2 from the second light source 102 is y, the relationship y=0.0147x+2202.1 is established. The dashed line B shows the oscillation wavelength of the first light source 101 when the injection current is 750 mA. In this example, the operating temperature of the first light source 101 and the second light source 102 is 20° C., and both are driven by CW (Continuous Wave). The symbol C shows the difference in the oscillation wavelength of the first light source 101 and the second light source 102 at a certain injection current. The faster the response speed of the photodetector 10, the larger this difference can be, and the wider the wavelength sweep range can be. Fig. 20 is a graph showing the relationship between the beat frequency and the injection current to the second light source 102. As shown in Fig. 20, the beat frequency increases linearly as the injection current increases.

[0080] Fig. 21 is a graph showing an example of a beat signal. Fig. 22 is a graph showing a comparison result of response characteristics. In Fig. 22, symbol S1 indicates the frequency response characteristic of the photodetector module 1, and symbol S2 indicates the frequency response characteristic of the comparative photodetector. It can be seen from Fig. 22 that the cutoff frequency of the photodetector module 1 is 20 GHz or higher, and the response speed is faster than that of the comparative photodetector.

[0081] Figures 23, 24, and 25 are graphs showing the measurement results of beat spectroscopy. Figure 23 is a graph showing the output signal of spectrum analyzer 104, and Figure 24 is a graph in which the horizontal axis of the graph shown in Figure 23 is converted into wavelength. Figure 25 is a graph in which the signal of background light has been subtracted from the output signal.

[0082] In this example, the gas to be measured is carbon monoxide, with a wave number of 2190 cm -1The absorption line near the gas was observed. When the wavelength sweep range crosses the absorption line of the gas to be measured, the gas absorption depending on the wavelength is observed as shown by the arrow D in FIG. 23. This is because when the wavelength of the light P2 from the second light source 102 matches the absorption wavelength of the gas, the light intensity decreases due to the absorption. The intensity of the beat signal is proportional to the product of the electric field amplitude of the light P1 from the first light source 101 and the electric field amplitude of the light P2 from the second light source 102. Therefore, a dip as shown by the arrow D is observed due to the decrease in the intensity of the light P2. In FIG. 24, the output signal when carbon monoxide is sealed in the gas cell 105 is indicated by the symbol SG, and the output signal when carbon monoxide is not present in the gas cell 105 is indicated by the symbol BG. The absorption spectrum of carbon monoxide is also indicated by a dashed line. The symbol FT indicates an example of the range of resolution of a Fourier Transform Infrared Spectrometer (FTIR). The resolution of the FTIR is 3GHz (0.1cm -1 ), whereas the beat spectrometer 100 can obtain a resolution of about 20 MHz. This is more than 150 times the resolution of FTIR. Thus, the beat spectrometer 100 can widen the wavelength sweep range in beat spectroscopy.

[0083] In the beat spectrometer 100, the first light source 101 and the second light source 102 are quantum cascade lasers, and the photodetector 10 is a quantum cascade detector. Since the output light of the quantum cascade laser is linearly polarized light parallel to the growth direction of the active layer, the first light source 101, the second light source 102, and the photodetector 10 are arranged so that the polarization directions of the first light source 101 and the second light source 102 and the polarization direction to which the photodetector 10 is sensitive coincide with each other, so that the light P1 and P2 from the first light source 101 and the second light source 102 can be effectively absorbed in the active layer 21 of the photodetector 10. Furthermore, in this case, the polarization characteristics of the first light source 101 and the second light source 102 and the polarization characteristics of the photodetector 10 function like a polarizing filter, so that the influence of background light having random polarization can be suppressed, and as a result, the signal-to-noise ratio can be improved. [Explanation of symbols]

[0084] 10, 10A...photodetector, 11...semiconductor substrate, 11a...main surface, 11b...side surface, 12...mesa portion, 12a...side surface, 12c...end surface, 13...first contact layer, 14...second contact layer, 14a...first portion, 14b...second portion, 15...first electrode, 15a...connection portion, 16...air bridge wiring, 16a...bridge portion, 17...second electrode, 21...active layer, 22...absorption region, 23...transport region, 40...lens, 100...beat spectrometer, 101...first light source (fixed wavelength light source), 102...second light source (tunable wavelength light source), A...optical waveguide direction, DL...detected light, P1...light from first light source, P2...light from second light source.

Claims

1. a semiconductor substrate having a main surface; a mesa portion formed on the main surface of the semiconductor substrate, the mesa portion including an active layer in which an absorption region that absorbs detection light by intersubband absorption and a transport region that transports electrons excited by the intersubband absorption are alternately stacked, the mesa portion extending along an optical waveguide direction; a first contact layer formed on a surface of the mesa portion opposite to the semiconductor substrate; a second contact layer formed between the main surface of the semiconductor substrate and the mesa portion; a first electrode formed on the main surface of the semiconductor substrate; an air bridge wiring electrically connected to the first contact layer and the first electrode; When viewed from a direction perpendicular to the principal surface of the semiconductor substrate, a length of the mesa portion in the optical waveguide direction is longer than a length of the mesa portion in a direction perpendicular to the optical waveguide direction; the air bridge wiring is drawn out from the first contact layer to one side in the direction perpendicular to the optical waveguide direction and bridges between the first contact layer and the first electrode, the air bridge wiring has a bridge portion formed in a sheet shape and extending in the air, A photodetector, wherein a length of the bridge portion in the optical waveguide direction is longer than a length of the bridge portion in the direction perpendicular to the optical waveguide direction.

2. the first electrode has a connection portion located on the one side of the mesa portion in the direction perpendicular to the optical waveguide direction, 2 . The photodetector according to claim 1 , wherein the air bridge wiring is drawn out from the first contact layer to the one side in the direction perpendicular to the optical waveguide direction and connected to the connection portion of the first electrode.

3. 3. The photodetector according to claim 1, wherein each of the length of the mesa portion in the optical waveguide direction and the length of the bridge portion in the optical waveguide direction is 50 [mu]m or more.

4. 4. The photodetector according to claim 1, wherein a ratio of a length of the mesa portion in the optical waveguide direction to a length of the mesa portion in the direction perpendicular to the optical waveguide direction is greater than 1 and less than 100.

5. 5. The optical detector of claim 1, wherein a ratio of a length of the bridge portion in the optical waveguide direction to a length of the bridge portion in the direction perpendicular to the optical waveguide direction is greater than 1 and less than 50.

6. 6. The photodetector according to claim 1, wherein the bridge portion has a thickness of 1 μm or more and 10 μm or less.

7. 7. The photodetector according to claim 1, wherein one end face of the mesa portion in the optical waveguide direction serves as an incident face of the detection light.

8. 8. The photodetector according to claim 1, wherein one end face of the mesa portion in the optical waveguide direction is flush with a side face of the semiconductor substrate.

9. The photodetector according to claim 7 , further comprising a lens arranged to face the end face of the mesa portion and configured to converge the detection light toward the end face of the mesa portion.

10. 10. The photodetector according to claim 1, wherein both side surfaces of the mesa portion in the direction perpendicular to the optical waveguide direction are exposed.

11. the second contact layer has a first portion located between the main surface of the semiconductor substrate and the mesa portion, and a second portion located outside the mesa portion, when viewed in a direction perpendicular to the main surface of the semiconductor substrate; The photodetector according to any one of claims 1 to 10, further comprising a second electrode formed on the second portion of the second contact layer.

12. A fixed wavelength light source; A tunable light source; and a photodetector according to any one of claims 1 to 11, which detects light from the fixed wavelength light source and light from the variable wavelength light source as the detection light, a wavelength of the light from the tunable wavelength light source is changed so that a frequency of a beat signal having a frequency corresponding to a wavelength difference between the light from the fixed wavelength light source and the light from the tunable wavelength light source is swept, while the light from the fixed wavelength light source and the light from the tunable wavelength light source are detected by the photodetector.

13. a semiconductor substrate having a main surface; a mesa portion formed on the main surface of the semiconductor substrate, the mesa portion including an active layer in which an absorption region that absorbs detection light by intersubband absorption and a transport region that transports electrons excited by the intersubband absorption are alternately stacked, the mesa portion extending along an optical waveguide direction; a first contact layer formed on a surface of the mesa portion opposite to the semiconductor substrate; a second contact layer formed between the main surface of the semiconductor substrate and the mesa portion; a first electrode formed on the main surface of the semiconductor substrate; an air bridge wiring electrically connected to the first contact layer and the first electrode; When viewed from a direction perpendicular to the principal surface of the semiconductor substrate, a length of the mesa portion in the optical waveguide direction is longer than a length of the mesa portion in a direction perpendicular to the optical waveguide direction; the air bridge wiring is drawn out from the first contact layer to one side in the direction perpendicular to the optical waveguide direction and bridges between the first contact layer and the first electrode, one end face of the mesa portion in the optical waveguide direction serves as an incident surface of the detection light, the photodetector further comprising a lens disposed opposite the end face of the mesa portion and configured to converge the detection light toward the end face of the mesa portion.

14. a semiconductor substrate having a main surface; a mesa portion formed on the main surface of the semiconductor substrate, the mesa portion including an active layer in which an absorption region that absorbs detection light by intersubband absorption and a transport region that transports electrons excited by the intersubband absorption are alternately stacked, the mesa portion extending along an optical waveguide direction; a first contact layer formed on a surface of the mesa portion opposite to the semiconductor substrate; a second contact layer formed between the main surface of the semiconductor substrate and the mesa portion; a first electrode formed on the main surface of the semiconductor substrate; an air bridge wiring electrically connected to the first contact layer and the first electrode; When viewed from a direction perpendicular to the principal surface of the semiconductor substrate, a length of the mesa portion in the optical waveguide direction is longer than a length of the mesa portion in a direction perpendicular to the optical waveguide direction; the air bridge wiring is drawn out from the first contact layer to one side in the direction perpendicular to the optical waveguide direction and bridges between the first contact layer and the first electrode, one end face of the mesa portion in the optical waveguide direction is flush with a side face of the semiconductor substrate, the photodetector further comprising a lens disposed opposite the end face of the mesa portion and configured to converge the detection light toward the end face of the mesa portion.

15. A fixed wavelength light source; A tunable light source; a photodetector configured to detect light from the fixed wavelength light source and light from the variable wavelength light source as detection light, the wavelength of the light from the tunable wavelength light source is changed so that a frequency of a beat signal having a frequency corresponding to a wavelength difference between the light from the fixed wavelength light source and the light from the tunable wavelength light source is swept, while the light from the fixed wavelength light source and the light from the tunable wavelength light source are detected by the photodetector; The photodetector includes: a semiconductor substrate having a main surface; a mesa portion formed on the main surface of the semiconductor substrate, the mesa portion including an active layer in which an absorption region that absorbs the detection light by intersubband absorption and a transport region that transports electrons excited by the intersubband absorption are alternately stacked, the mesa portion extending along an optical waveguide direction; a first contact layer formed on a surface of the mesa portion opposite to the semiconductor substrate; a second contact layer formed between the main surface of the semiconductor substrate and the mesa portion; a first electrode formed on the main surface of the semiconductor substrate; an air bridge wiring electrically connected to the first contact layer and the first electrode; When viewed from a direction perpendicular to the principal surface of the semiconductor substrate, a length of the mesa portion in the optical waveguide direction is longer than a length of the mesa portion in a direction perpendicular to the optical waveguide direction; a first electrode connected to the first contact layer and extending from the first contact layer to a first electrode in a direction perpendicular to the optical waveguide direction;

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