Light detector and light-beat spectrometer

Abstract

The light detector (10) of the present application is provided with a semiconductor substrate (11), a mesa portion (12) formed on a main surface (11a) of the semiconductor substrate (11) in a manner extending along a light waveguide direction (A), a first contact layer (13), a second contact layer (14), a first electrode (15), and an air-bridge wiring (16) electrically connected to the first contact layer (13) and the first electrode (15). When viewed from a direction perpendicular to the main surface (11a) of the semiconductor substrate (11), a length (L1) of the mesa portion (12) in the light waveguide direction (A) is longer than a length (L2) of the mesa portion in a direction perpendicular to the light waveguide direction (A). The air-bridge wiring (16) is drawn out from the first contact layer (13) to one side in the direction perpendicular to the light waveguide direction (A) and is erected between the first contact layer (13) and the first electrode (15).

Description

TECHNICAL FIELD

[0001] One aspect of the present disclosure relates to a light detector and a light beat spectrometer. BACKGROUND

[0002] A quantum cascade detector is disclosed in Patent Literature 1. In the quantum cascade detector, inter-subband transition (inter-subband absorption) of a quantum well structure is utilized to detect light.

[0003] PRIOR ART DOCUMENTS

[0004] PATENT LITERATURE

[0005] Patent Literature 1: Japanese Patent Application Publication No. 2017-147428 SUMMARY

[0006] PROBLEMS TO BE SOLVED BY THE INVENTION

[0007] There is a demand for increasing the response speed of the light detector described above. In addition, there is also a demand for ensuring the strength of the output signal and ensuring the reliability at the same time. Therefore, an object of one aspect of the present disclosure is to provide a light detector capable of achieving an increase in the response speed, ensuring the strength of the output signal, and ensuring the reliability, and a light beat spectrometer capable of expanding the wavelength scanning range.

[0008] MEANS FOR SOLVING THE PROBLEMS

[0009] The light detector of one aspect of the present disclosure includes a semiconductor substrate having a main surface; a mesa portion including an active layer in which an absorption region that absorbs a detection light by inter-subband absorption and a transport region that transports an electron excited by the inter-subband absorption are alternately laminated, and formed on the main surface of the semiconductor substrate in a manner extending in a light waveguide direction; a first contact layer formed on a surface of the mesa portion on the opposite side from 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, in which a length of the mesa portion in the light waveguide direction is longer than a length of the mesa portion in a direction perpendicular to the light waveguide direction when viewed in a direction perpendicular to the main surface of the semiconductor substrate, the air-bridge wiring is drawn from the first contact layer to one side in the direction perpendicular to the light waveguide direction, and is suspended between the first contact layer and the first electrode.

[0010] In this photodetector, the first contact layer and the first electrode are connected via an air bridge wiring. This shortens the wiring length and reduces the area of ​​the mesa when viewed from above, compared to connecting the first contact layer and the first electrode via wire bonding. Shortening the wiring length reduces inductance and increases response speed. Furthermore, reducing the mesa area reduces parasitic capacitance, further increasing response speed. However, reducing only the mesa area may decrease the output signal strength. In this photodetector, when viewed from a direction perpendicular to the main surface of the semiconductor substrate, the length of the mesa in the optical waveguide direction is longer than its length in the direction perpendicular to the optical waveguide direction. This allows for efficient light absorption at the mesa, ensuring output signal strength even with a reduced mesa area. Furthermore, in this photodetector, the air bridge wiring extends from the first contact layer to one side in a direction perpendicular to the optical waveguide direction and is positioned between the first contact layer and the first electrode. This ensures the width of the air bridge wiring (length in the optical waveguide direction). A wider wiring width further reduces inductance and ensures the strength of the air bridge wiring, thus guaranteeing reliability. Therefore, this photodetector enables high-speed response, ensures strong output signal, and guarantees reliability.

[0011] Alternatively, the first electrode may have a connection portion located on one side of the mesa perpendicular to the optical waveguide direction, and the air bridge wiring is led out from the first contact layer to the side 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 led out from the first contact layer to the side perpendicular to the optical waveguide direction, reliably ensuring the width of the air bridge wiring.

[0012] Alternatively, it can be an air-bridged wiring, having a bridge section extending in the air, the length of which in the direction of the optical waveguide is longer than its length in the direction perpendicular to the optical waveguide. In this case, the width of the bridge section (air-bridged wiring) can be reliably ensured.

[0013] Alternatively, the air bridge wiring may have a bridge section extending in the air, with both the length of the platform in the optical waveguide direction and the length of the bridge section in the optical waveguide direction being 50 μm or more. In this case, the width of the bridge section can be reliably ensured. Furthermore, ensuring the length of the platform in the optical waveguide direction allows for more efficient light absorption on the platform.

[0014] Alternatively, the ratio of the length of the mesa in the optical waveguide direction to the length of the mesa in the direction perpendicular to the optical waveguide direction is greater than 1 and less than 100. In this case, it can be ensured that the length of the mesa in the optical waveguide direction can absorb light more effectively on the mesa.

[0015] Alternatively, the air bridge wiring may have a bridge section extending in the air, wherein the ratio of the length of the bridge section in the optical waveguide direction to its length in the direction perpendicular to the optical waveguide direction is greater than 1 and less than 50. In this case, the width of the bridge section (air bridge wiring) can be reliably ensured.

[0016] Alternatively, the air bridge wiring may have a bridge section extending in the air, with a thickness of 1 μm or more and 10 μm or less. In this case, reliability can be reliably ensured.

[0017] Alternatively, one end face of the mesa along the optical waveguide direction can become the incident surface of the detection light. In this case, light can be absorbed more effectively on the mesa, reliably ensuring the strength of the output signal.

[0018] Alternatively, one end face of the mesa in the optical waveguide direction can be coplanar with one end face of the semiconductor substrate in the optical waveguide direction. In this case, one end face of the mesa in the optical waveguide direction can be easily used as the incident surface of the detection light.

[0019] Alternatively, the photodetector in one aspect of this disclosure may further include a lens configured to face the end face of the mesa, and to converge the detection light toward the end face of the mesa. In this case, the width of the end face of the mesa (its length in the direction perpendicular to the optical waveguide direction) can be narrowed, reducing the area of ​​the mesa when viewed from above. As a result, parasitic capacitance can be reduced, and the response speed can be further increased. Furthermore, assuming the area of ​​the mesa is constant, narrowing the width of the mesa increases the length of the mesa in the optical waveguide direction. As a result, light can be absorbed more effectively on the mesa, reliably ensuring the strength of the output signal.

[0020] Alternatively, the two sides of the mesa, perpendicular to the direction of the optical waveguide, can be exposed. In this case, the generation of parasitic capacitance can be suppressed, and the response speed can be further increased.

[0021] Alternatively, when viewed from a direction perpendicular to the main surface of the semiconductor substrate, the second contact layer has a first portion located between the main surface and the mesa of the semiconductor substrate, and a second portion located outside the mesa. A second electrode is formed on the second portion of the second contact layer. In this case, it is possible to ensure that the area of ​​the second electrode is large, and the connecting member can be well connected to the second electrode.

[0022] One aspect of this optical beat-splitting apparatus includes: a fixed-wavelength light source; a variable-wavelength light source; and a photodetector that detects light from the fixed-wavelength light source and light from the variable-wavelength light source as detection light. The apparatus scans the frequency of an optical 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, thereby changing the wavelength of the light from the variable-wavelength light source, and detecting both light from the fixed-wavelength light source and light from the variable-wavelength light source using the photodetector. Due to the aforementioned reasons, the response speed of the photodetector in this optical beat-splitting apparatus is increased. Therefore, the wavelength scanning range of optical beat-splitting can be expanded in this apparatus.

[0023] Invention Effects

[0024] According to one aspect of this disclosure, a photodetector capable of achieving high-speed response, ensuring output signal strength, and ensuring reliability, as well as a photodetector capable of expanding the wavelength scanning range, can be provided. Attached Figure Description

[0025] Figure 1 This is the front view of the light detection module in the implementation method.

[0026] Figure 2 This is a 3D view of the photodetector.

[0027] Figure 3 This is a top view of the photodetector.

[0028] Figure 4 This is the front view of the photodetector.

[0029] Figure 5 This is a table showing the structure of a photodetector.

[0030] Figure 6 This is a top view of the fixed component.

[0031] Figure 7 This is a 3D view showing the installation status of the light detection module.

[0032] Figure 8 This is a top view showing the installation status of the light detection module.

[0033] Figure 9 This is a front view showing the installation status of the light detection module.

[0034] Figure 10 This is a graph showing the response characteristics of the photodetector.

[0035] Figure 11 This is a graph showing an example of the output signal in the high-frequency region.

[0036] Figure 12 In the diagram, (a) is a front view of the photodetector of the embodiment, and (b) is a front view of the photodetector of the first modified example.

[0037] Figure 13 This is a cross-sectional view of the optical detection module in the second variation.

[0038] Figure 14 This is a top view of the fixed component in the second variation.

[0039] Figure 15 This is a top view of the photodetector in the third variant.

[0040] Figure 16 This is a diagram showing the structure of the optical beam splitter.

[0041] Figure 17 It is a graph showing the sensitivity characteristics of the photodetector and the oscillation wavelength of a fixed wavelength light source.

[0042] Figure 18 It is a graph showing the oscillation wavelength of a fixed-wavelength light source and a variable-wavelength light source.

[0043] Figure 19 It is a graph showing the relationship between the injection current and the oscillation wavelength of a wavelength-variable light source.

[0044] Figure 20 It is a graph showing the relationship between the injection current into a wavelength-variable light source and the beat frequency.

[0045] Figure 21 This is a graph showing an example of a photodetector signal.

[0046] Figure 22 This is a graph showing the comparison results of the response characteristics.

[0047] Figure 23 It is a graph showing the measurement results of light-induced beam splitting.

[0048] Figure 24 It is a graph showing the measurement results of light-induced beam splitting.

[0049] Figure 25 It is a graph showing the measurement results of light-induced beam splitting. Detailed Implementation

[0050] Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the accompanying drawings. In the following description, the same or equivalent elements will be referred to by the same reference numerals, and repeated descriptions will be omitted.

[0051] [Optical Detection Module]

[0052] likeFigure 1 As shown, the optical detection module 1 includes: a photodetector 10 and a mounting member (submount) 50 on which the photodetector 10 is fixed. The photodetector 10 is, for example, a quantum cascade detector (QCD), which uses the inter-subband transition (inter-subband absorption) of a quantum well structure to detect the detection light DL.

[0053] [Optical Detector]

[0054] like Figure 2 , Figure 3 and Figure 4 As shown, 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.

[0055] The semiconductor substrate 11 is formed, for example, into a rectangular plate shape, having 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, Y, and Z directions are orthogonal to each other. Furthermore, in Figures 2-4 In, relative to Figure 1 The structures of the photodetector 10 are configured in reverse order in the X direction. In fact, the structures of the photodetector 10 are relative to... Figures 2-4 The configuration shown is reversed in the X direction. In summary, as described below, the elements are configured such that the first electrode 15 is connected to the first wiring 52 and the second electrode 17 is connected to the second wiring 53.

[0056] The mesa 12 is formed on the main surface 11a of the semiconductor substrate 11, extending along the optical waveguide direction A. The optical waveguide direction A is parallel to the Y direction. The mesa 12 is formed on the main surface 11a through the second contact layer 14 and protrudes from the semiconductor substrate 11 in the Z direction. A pair of side surfaces 12a of the mesa 12 in the X direction are each exposed. That is, each side surface 12a is not covered by other elements. The side surface 12a is a surface that extends intersecting the X direction, and in this example, it is a flat surface perpendicular to the X direction.

[0057] The surface layer 12 includes an active layer 21. For example... Figure 5As shown, the active layer 21 comprises 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 stacks 24, composed of pairs of absorption regions 22 and transport regions 23, are repeatedly stacked. In this example, the active layer 21 is configured to have a peak sensitivity wavelength of 4.5 μm and comprises 90 cycles of unit stacks 24. The number of cycles of the unit stacks 24 may, for example, be 10 or more and 150 or less.

[0058] Absorption region 22 includes a barrier layer 261 and a well layer 271, absorbing the detection light DL through interband absorption. Transport region 23 includes multiple barrier layers 262-267 and multiple well layers 272-277, transporting electrons excited by interband absorption in absorption region 22 to the next cycle of absorption region 22. The composition, layer thickness, and doping state of barrier layers 261-267 and well layers 272-277 are, for example... Figure 5 As shown.

[0059] When the detection light DL is incident on the active layer 21, it is absorbed. More specifically, the electron excitation caused by inter-subband absorption, the relaxation of the excited electrons, the transport of electrons, and the extraction of electrons into the next unit stack 24 are repeated in multiple unit stacks 24, thereby causing cascaded light absorption in the active layer 21. In the photodetector module 1, the current generated by this light absorption is extracted as an electrical signal, and the detection light DL is detected by measuring its current magnitude. That is, the active layer 21 functions as a semiconductor region that generates an electrical signal based on the incident amount of the detection light DL.

[0060] The facet 12 is formed as a strip in the Y direction, for example, in a top view (when viewed from the Z direction), it is formed as a rectangle with its long side parallel to the Y direction. That is, in a top view, the length L1 of the facet 12 in the Y direction (optical waveguide direction A) is longer than the length (width) L2 of the facet 12 in the X direction (the direction perpendicular to the optical waveguide direction A). The length L1 is, for example, 50 μm or more. The length L1 is, for example, between 50 μm and 3000 μm, and in this example, it is 100 μm. The length L2 is, for example, between 10 μm and 1000 μm, and in this example, it is 25 μm. The aspect ratio (the ratio of length L1 to length L2) of the facet 12 in a top view is 1 to 100. The aspect ratio of the facet 12 can also be 1.5 to 50, and is preferably 2 to 10. The lengths L1 and L2 can also 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 component size becomes too large, and it may be difficult to achieve a high cut-off.

[0061] The first contact layer 13 is an upper contact layer formed on the surface 12b opposite to the mesa 12 of 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 12. For example, the composition, layer thickness, and doping state of the first contact layer 13 and the second contact layer 14... Figure 5 As shown.

[0062] The second contact layer 14 is formed as a rectangle with each side longer than the mesa 12, and has a portion exposed from the mesa 12. That is, when viewed from above, the second contact layer 14 has a first portion 14a located between the main surface 11a of the semiconductor substrate 11 and the mesa 12, and a second portion 14b located outside the mesa 12.

[0063] The mesa 12, the first contact layer 13, and the second contact layer 14 are formed on the main surface 11a of the semiconductor substrate 11, for example, by crystal growth. The mesa 12, the first contact layer 13, and the second contact layer 14 are formed by etching using photolithography and dry etching techniques after crystal growth to reach the surface or interior of the second contact layer 14. During manufacturing, for example, multiple components are formed simultaneously by dicing the wafer after forming multiple portions corresponding to the mesa 12, the first contact layer 13, and the second contact layer 14.

[0064] In this example, one end face 12c of the mesa 12 on the Y-direction (optical waveguide direction A) (parallel to the main surface 11a of the semiconductor substrate 11) becomes the incident surface (light-receiving surface) of the detection light DL. The detection light DL incident from the end face 12c travels along the optical waveguide direction A within the mesa 12. The end face 12c is coplanar with the side surface 11b of the semiconductor substrate 11. That is, the end face 12c and the side surface 11b are located on the same plane. The side surface 11b is a surface that extends in a manner intersecting the Y-direction, and in this example, it is a flat surface perpendicular to the Y-direction. In this example, the side surface 11b and the end face 12c are cleavage planes formed by cutting the wafer.

[0065] 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 patterned into a square shape. The first electrode 15 has a connection portion 15a on one side of the mesa 12 in the X direction. In this example, the connection portion 15a is an integral part of the 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, 10000 μm. 2 That's all. In this case, the first electrode 15 can make good ground contact with the first wiring 52 described later. In this example, the size of the first electrode 15 is 400μm × 400μm.

[0066] 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 extends from the first contact layer 13 to one side in the X direction and is positioned between the first contact layer 13 and the first electrode 15. In this example, the air bridge wiring 16 extends from the first contact layer 13 in a direction parallel to the X direction. When viewed from above, the air bridge wiring 16 extends from the end of the first contact layer 13 (platform 12) in the X direction. 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 also has a planar portion 16b formed on the first contact layer 13. The bridge portion 16a and the portion 16b are integrally formed.

[0067] The air bridge wiring 16 is formed, for example, by the following process. First, a resist is patterned and formed in the region between the mesa 12 and the first electrode 15 on the main surface 11a of the semiconductor substrate 11. Next, a thin film of gold with a thickness of about 5 μm is formed on the resist by plating, and then the resist is removed. Thus, a sheet-like air bridge wiring 16 is formed.

[0068] The bridge portion 16a is formed as a wide sheet (layer). The length (width) L3 of the bridge portion 16a in the Y direction (optical waveguide 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... Figure 4 As shown, the length L4 of the bridge portion 16a in the X direction is the length (actual length) of the bridge portion 16a along the extension direction. The length L3 is, for example, 50 μm or more. The length L3 is, for example, between 50 μm and 3000 μm, and in this example, it is 80 μm. The length L4 is, for example, between 5 μm and 200 μm, and in this example, it is 30 μm. The aspect ratio of the bridge portion 16a (the ratio of length L3 to length L4) can also be 0.25 to 100, and is preferably 1 to 50. The aspect ratio of the bridge portion 16a can also be more preferably 1 to 20, and is even more 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.

[0069] 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, with the second contact layer 14 in between. The second electrode 17 is made of, for example, gold and is formed into a square shape by patterning. The second electrode 17 is disposed on the opposite side in the X direction (opposite to the first electrode 15) relative to the mesa 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 separated from the first electrode 15. The area of ​​the second electrode 17 is, for example, 10000 μm. 2 That's all. In this case, the second electrode 17 can make good ground contact with the second wiring 53 described later. In this example, the dimensions of the second electrode 17 are 400μm × 400μm, the same as the first electrode 15.

[0070] During the plating process in the formation of the air bridge wiring 16 described above, electroplating layers may also be formed on the first electrode 15 and the second electrode 17. Figures 2-4 In the figures, these plating layers are shown by reference numerals 151 and 171. Plating layers 151 and 171 can be considered to constitute the first electrode 15 and the second electrode 17, respectively. In this example, plating layer 151 is formed to be slightly smaller than the first electrode 15 (formed only on a portion of the first electrode 15), but it can also be formed on the entire surface of the first electrode 15. Similarly, plating layer 171 is formed to be slightly smaller than the second electrode 17, but it can also be formed on the entire surface of the second electrode 17. Plating layer 151 can also be considered to constitute the air bridge wiring 16. Alternatively, plating layers 151 and 171 may not be formed.

[0071] [Fixed Components]

[0072] like Figure 1 and Figure 6 As shown, the fixing member 50 includes: an insulating substrate 51, a first wiring 52, and a second wiring 53. Figure 6 For ease of understanding, the first wiring 52 and the second wiring 53 are shaded. The insulating substrate 51 is, for example, formed as a rectangular plate, having a main surface 51a and a side 51b opposite to the main surface 51a. In this example, the main surfaces 51a and 51b are parallel flat surfaces. The base material of the insulating substrate 51 is, for example, alumina. In this case, dielectric loss in the high-frequency band can be reduced. The base material of the insulating substrate 51 can also be SiC or ceramic, which are highly insulating materials.

[0073] 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 opening into the 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 intersecting the Y direction, and in this example, it is a flat surface perpendicular to the Y direction. As an example, the recess 54 is rectangular in shape when viewed from above, and has a generally semi-circular shape in cross-section perpendicular to the Y direction.

[0074] The first wiring 52 is formed as a planar surface on the main surface 51a of the insulating substrate 51. The first wiring 52 is made of, for example, gold and is patterned into a predetermined shape. The first wiring 52 is positioned on one side in the X direction relative to the recess 54. The first wiring 52 is formed as a rectangle with its long side parallel to the Y direction and extends across the sides 51c and 51d of the insulating substrate 51. Side 51d is the surface of the insulating substrate 51 opposite to side 51c.

[0075] 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 to extend from the main surface 51a of the insulating substrate 51 through the side surface 51e to the main surface 51b (wound in). That is, the second wiring 53 has: a first portion 53a disposed on the main surface 51a, and a second portion 53b disposed throughout the main surface 51b and the side surface 51e. The first portion 53a is disposed on the opposite side in the X direction (opposite side of the first wiring 52) relative to the recess 54. The first portion 53a has: a portion 53a1 formed in a rectangular shape with its long side parallel to the X direction, and a portion 53a2 formed in a rectangular shape with its long side parallel to the Y direction. The portion 53a2 extends from the portion 53a1 to the side surface 51d of the insulating substrate 51. Part 2, 53b, is formed on the entire surface of the main face 51b and the side face 51e.

[0076] 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 solder layer 55 and 56 is formed into a square shape, for example, from a metal material, and has a thickness of more than 1 μm and less than 10 μm.

[0077] like Figure 1 As shown, 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 where the photodetector 10 is fixed to the fixing member 50, at least a portion of the mesa 12 is disposed within the recess 54. In this example, the front end of the mesa 12 (the end opposite to the semiconductor substrate 11 in the Y direction) is disposed within the recess 54. The mesa 12 is separated from the inner surface 54a of the recess 54. The long side direction of the mesa 12 is parallel to the long side direction of the recess 54. The end face 12c of the mesa 12 is exposed from the recess 54 when viewed from the Y direction. In the fixed state, a portion of the air bridge wiring 16 (bridge portion 16a) is also disposed within the recess 54 in a manner separated from the inner surface 54a. Thus, the connection between the air bridge wiring 16 and the first contact layer 13 is disposed within the recess 54.

[0078] The photodetector 10 is fixed to the fixing member 50 via a first electrode 15 connected to a first wiring 52 and a second electrode 17 connected to a 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 when in contact with its surface 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 when in contact with its surface 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 10000 μm. 2 That's all. In the fixed state, a portion of the first wiring 52 is exposed so that it can be connected to the terminal 81 of the connector 80 described later. Figure 7 , Figure 8 ).

[0079] [Installation status of the optical detection module]

[0080] like Figure 7 , Figure 8 and Figure 9As shown, the optical detection module 1 is used, for example, to connect to connector 80. Connector 80 is, for example, an SMA connector, having terminals 81 that are electrically connected to the signal lines of an SMA cable. Terminals 81 are mechanically connected to and electrically connected to the first wiring 52 of the fixing member 50. Alternatively, terminals 81 can also 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 lines.

[0081] The connector 80 also includes a main body member 82 and a support member 83 integrally formed with the main body member 82. The main body member 82 and the support member 83 are electrically connected to the grounding wire of the SMA cable. The photodetector module 1 is fixed to the connector 80 by connecting it to the support member 83 via a fixing member 50. 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, for example, in a manner that covers the entire surface of the support member 83. Alternatively, the main body member 82 may also be separately constructed from the support member 83. In this case, the support member 83, to which the fixing member 50 is connected, may also be fixed to the main body member 82 by screws or soldering. Through the above connections, the second wiring 53 is electrically connected to the grounding wire. That is, in this example, the second wiring 53 is a grounding side wiring electrically connected to the grounding wire. In this example, the electrical connection between the photodetector 10 and the connector 80 is achieved without using a bonding wire (wireless connection). Therefore, inductance caused by the bonding leads can be avoided. Furthermore, when using bonding leads, the wires may be damaged due to impacts, snagging, etc., so care must be taken when operating the device. In contrast, in this example, since bonding leads are not used, the operation of the device is simplified.

[0082] The shape (wiring pattern) of the first wiring 52 is set with impedance matching in mind. In this example, the first wiring 52 is configured as a microstrip line. As an example, with the thickness of the insulating substrate 51 being 0.5 mm, the relative permittivity of the insulating substrate 51 being 9.8, the thickness of the first wiring 52 being 0.8 μm, and the matching resistance value at 20 GHz being 50 Ω, the width W of the first wiring 52 is set to be 0.5 mm or less, and the length L5 of the first wiring 52 is set to be 1.45 mm or less. Figure 6Therefore, the length L5 of the first wiring 52 is less than 1 / 4 of the wavelength (electrical length) of the electrical signal transmitted in the first wiring 52. Thus, by setting the length L5 of the first wiring 52 to less than 1 / 4 of the electrical length calculated based on the design parameters of the first wiring 52, impedance matching of the desired frequency band can be achieved. Furthermore, the first wiring 52 can also be configured as a coplanar line instead of a microstrip line.

[0083] like Figure 8 As shown, the light detection module 1 (photodetector 10) also includes a lens 40. The lens 40 is configured to face the end face 12c of the mesa 12, which serves as the light incident surface, so that the detection light DL is focused towards the end face 12c, concentrating the detection light DL onto the end face 12c. The numerical aperture (NA) of the lens 40 is, for example, 0.4 or more, and the focusing diameter is several μm to tens of μm. A low-reflection layer comprising a dielectric multilayer film can also 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 within the sensitivity wavelength range of the photodetector 10 can be 95% or more.

[0084] [Functions and Effects]

[0085] In the photodetector 10, the first contact layer 13 and the first electrode 15 are connected via an air bridge wiring 16. This allows for a shorter wiring length and a smaller area of ​​the mesa 12 when viewed from above, compared to the case where the first contact layer 13 and the first electrode 15 are connected via wire bonding. With wire bonding, the area required for the wiring is determined by the area needed to crimp the wire tip, for example, requiring an area of ​​approximately 100 μm × 100 μm. Shortening the wiring length reduces inductance and allows for faster response. Furthermore, reducing the area of ​​the mesa 12 reduces parasitic capacitance, which also allows for faster response. However, if only the area of ​​the mesa 12 is reduced, the strength of the output signal may decrease. Regarding this, in the photodetector 10, when viewed from the Z direction (the direction perpendicular to the main surface 11a of the semiconductor substrate 11), the length L1 of the mesa 12 in the optical waveguide direction A (Y direction) is longer than the length L2 of the mesa 12 in the X direction (the direction perpendicular to the optical waveguide direction A). This extends the path of the detection light DL within the mesa 12, allowing for efficient light absorption within the mesa 12. As a result, even with a reduced area of ​​the mesa 12, the strength of the output signal can be ensured. Furthermore, in the photodetector 10, the air bridge wiring 16 extends from the first contact layer 13 to one side in the X direction and is positioned between the first contact layer 13 and the first electrode 15. This ensures the width (length in the optical waveguide direction A) of the air bridge wiring 16. With a wider wiring width, the inductance can be further reduced by increasing the cross-sectional area, and the strength of the air bridge wiring 16 can be ensured, thereby guaranteeing reliability. Therefore, according to the photodetector 10, it is possible to achieve high-speed response, ensure the strength of the output signal, and ensure reliability.

[0086] Figure 10 This is a graph showing the response characteristics of the photodetector 10. Figure 11 This is a graph showing an example of the output signal in the high-frequency region. For example... Figure 10 As shown, in the photodetector 10, the cutoff frequency, which is the frequency at which the signal strength decreases by -3dB, is above 20GHz. For example, in an MCT (HgCdTe) sensor used as a mid-infrared photodetector, only a cutoff frequency in the hundreds of MHz range (maximum 1GHz range) can be achieved. In contrast, in the photodetector 10, the cutoff frequency can be increased, enabling a faster response speed. Furthermore, as... Figure 11 As shown, a high signal strength of over 30dB was obtained in the frequency band exceeding 26GHz.

[0087] The first electrode 15 has a connection portion 15a on one side of the platform surface 12 in the X direction. The air bridge wiring 16 is led 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. Thus, the air bridge wiring 16 can be led out from the first contact layer 13 to one side in the X direction, and the width of the air bridge wiring 16 can be reliably ensured.

[0088] The air bridge wiring 16 has a bridge portion 16a extending in the air, and the length L3 of the bridge portion 16a in the optical waveguide direction A is longer than the length L4 of the bridge portion 16a in the X direction. As a result, the width of the bridge portion 16a (air bridge wiring 16) can be reliably ensured.

[0089] The length L1 of the mesa 12 in the optical waveguide direction A and the length L3 of the bridge portion 16a in the optical waveguide direction A are both 50 μm or more. This reliably ensures the width of the bridge portion 16a. Furthermore, ensuring the length of the mesa 12 in the optical waveguide direction A allows for more efficient light absorption on the mesa 12.

[0090] The ratio of the length L1 to the length L2 of the mesa 12 is greater than 1 and less than 100. This ensures that the length L1 of the mesa 12 in the optical waveguide direction A can absorb light more effectively on the mesa 12.

[0091] The ratio of the length L3 of the bridge section 16a to the length L4 is greater than 1 and less than 50. This ensures the width of the bridge section 16a (air bridge wiring 16) is reliably guaranteed.

[0092] The thickness of the bridge portion 16a is 1 μm or more and 10 μm or less. This ensures reliable performance. Specifically, the thickness of the bridge portion 16a being 1 μm or more ensures the mechanical strength required for its formation and shape maintenance. Furthermore, the thickness of the bridge portion 16a being 10 μm or less allows it to resist its own weight and maintain its shape.

[0093] One end face 12c of the mesa 12 on the optical waveguide direction A becomes the incident surface of the detection light DL. As a result, light can be absorbed more effectively on the mesa 12, and the strength of the output signal can be reliably ensured.

[0094] One end face 12c of the mesa portion 12 in the optical waveguide direction A is coplanar with the side surface 11b of the semiconductor substrate 11. Therefore, the end face 12c of the mesa portion 12 can be easily used as the incident surface of the detection light DL.

[0095] The photodetector 10 includes a lens 40 arranged facing the end face 12c of the stage 12, which converges the detection light DL toward the end face 12c of the stage 12. This narrows the width (length in the X direction) of the end face 12c of the stage 12, reducing the area of ​​the stage 12 when viewed from above. As a result, parasitic capacitance is reduced, and the response speed is further increased. Furthermore, assuming the area of ​​the stage 12 is constant, narrowing the width of the stage 12 increases the length of the stage 12 in the optical waveguide direction A. As a result, light can be absorbed more effectively on the stage 12, reliably ensuring the strength of the output signal. Increasing the number of periods of the unit stack 24 of the active layer 21, thus thickening the active layer 21, also improves absorption efficiency. The increased thickness of the active layer 21 also contributes to reducing parasitic capacitance and noise caused by increased element resistance. On the other hand, if the number of cycles is too high, it will lead to a decrease in quantum efficiency. Therefore, the number of cycles is preferably between 10 and 150.

[0096] The side surface 12a of the platform 12 in the X direction is exposed. This suppresses the generation of parasitic capacitance and allows for further acceleration of the response speed. Referring hereafter... Figure 12 This point needs clarification.

[0097] Figure 12 (a) is a front view of the photodetector 10. Figure 12 (b) is a front view of the photodetector 10A of the first modified example. Figure 12 As shown in (a), in the photodetector 10, the two side surfaces 12a of the mesa 12 in the X direction are exposed. In contrast, as... Figure 12 As shown in (b), in the photodetector 10A, the two side surfaces 12a of the mesa 12 are covered by an insulating layer 90. The insulating layer 90 is, for example, a silicon nitride (SiN) film. The insulating layer 90 extends between the first contact layer 13 and the metal layer (part 16b of the air bridge wiring 16) on the first contact layer 13. Because this part has a structure insulator held by metal, a small parasitic capacitance may be generated in this part, for example, if an alignment misalignment occurs during the formation of the insulating layer 90. In the photodetector 10, the two side surfaces 12a of the mesa 12 are exposed, thus suppressing the generation of such parasitic capacitance.

[0098] The second contact layer 14, when viewed from above, has a first portion 14a located between the main surface 11a and the mesa 12 of the semiconductor substrate 11, and a second portion 14b located outside the mesa 12, on which a second electrode 17 is formed. This allows for a larger area of ​​the second electrode 17, enabling a reliable connection of the connecting member 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 contact state. This reduces inductance compared to connecting the second electrode 17 and the second wiring 53 via wire bonding, allowing for a further increase in response speed. Furthermore, by connecting the second electrode 17 and the second wiring 53 in a surface contact state, the photodetector 10 and the fixing member 50 can be securely fixed, ensuring reliable performance. Additionally, because the second electrode 17 has a larger area, the contact area between the second electrode 17 and the second wiring 53 can be maximized. Furthermore, the connecting member connected to the second electrode 17 can be a strip electrode or a bonding lead. Even in such cases, due to the large area of ​​the second electrode 17, the connecting member can be well connected to the second electrode 17.

[0099] 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 on the insulating substrate 51 of the fixing member 50 in a state of surface contact. This avoids inductance caused by wire bonding, for example, compared to connecting the first electrode 15 and the first wiring 52 via lead bonding, thus reducing inductance. As a result, the response speed can be increased. Furthermore, by connecting the first electrode 15 and the first wiring 52 in a surface-contact state, the light detector 10 and the fixing member 50 can be securely fixed, ensuring reliability. Moreover, in the light detection module 1, at least a portion of the mesa portion 12 of the light detector 10 is disposed within a recess 54 formed in the insulating substrate 51. This protects the delicate mesa portion 12, ensuring reliability. For example, it protects the mesa portion 12 from damage caused by external contact, attachment of floating objects, wind pressure, etc. (mechanical damage caused by external factors).

[0100] The recess 54 opens into the side surface 51c of the insulating substrate 51. This allows the detection light to easily enter the end face 12c of the mesa 12 in the Y direction. At least a portion of the air bridge wiring 16 is disposed within the recess 54. This protects the delicate air bridge wiring 16 and reliably ensures reliability. The first wiring 52 has a length less than 1 / 4 of the wavelength of the electrical signal transmitted in the first wiring 52. This enables impedance matching. The mesa 12 is separated from the inner surface of the recess 54. This reliably protects the mesa 12. In the case where the photodetector 10 is a detector that detects the detection light DL using inter-subband absorption of a quantum well structure, since detection can be performed without applying an external voltage, heat is less likely to be generated on the mesa 12. Therefore, the mesa 12 can be separated from the insulating substrate 51.

[0101] The first electrode 15 is in surface contact with the first wiring 52 via solder layer 55, and the second electrode 17 is in surface contact with the second wiring 53 via solder layer 56. This allows for a secure electrical and mechanical connection between the first electrode 15 and the first wiring 52, and also a secure electrical and mechanical connection between the second electrode 17 and the second wiring 53. Furthermore, when connecting the first electrode 15 to the first wiring 52 and the second electrode 17 to the second wiring 53, even if there is a height difference between the first electrode 15 and the second electrode 17, the solder layers 55 and 56 can absorb this height difference, achieving good surface contact at each connection point.

[0102] [Variation Example]

[0103] exist Figure 13 and Figure 14 In the second modified example of the photodetector module 1 shown, the detection light DL travels within the semiconductor substrate 11 and then is incident on the mesa 12. In this second modified example, the side surface 11b of the semiconductor substrate 11 is an inclined surface tilted relative to the Z direction. As an example, when viewed from the X direction, the tilt angle θ of the side surface 11b relative to the Z direction is 45 degrees. The side surface 11b is, for example, a polished surface formed by grinding. The detection light DL is incident on the side surface 11b from a direction perpendicular to the side surface 11b, travels within the semiconductor substrate 11, and then is incident on the surface 12d of the mesa 12 via the second contact layer 14. The surface 12d is the surface of the mesa 12 on the semiconductor substrate 11 side. That is, in this example, the side surface 11b becomes the incident surface of the detection light DL. The end face 12c of the mesa 12 is not on the same plane as the side surface 11b of the semiconductor substrate 11. The detection light DL incident on the mesa 12 undergoes multiple reflections within the mesa 12, and the electric field vibration component parallel to the Z-direction in the detection light DL is absorbed in the active layer 21. The detection light DL can also undergo multiple reflections within the semiconductor substrate 11 and be incident multiple times on the surface 12d of the mesa 12. Furthermore, in Figure 13In the text, the first electrode 15, etc., are omitted, and the shading lines representing the cross-section are also omitted.

[0104] like Figure 14 As shown, in the second modification, the recess 54 is formed by a hole formed on the main surface 51a of the insulating substrate 51. The recess 54 does not open on the side surface 51c of the insulating substrate 51 and is separated from the outer edge of the main surface 51a. The recess 54 is approximately semi-circular in a cross-section perpendicular to the X direction. In the second modification, at least a portion of the mesa 12 is also disposed within the recess 54. The mesa 12 is formed in a shape that allows it to be disposed within the recess 54. According to this second modification, similarly to the embodiment described above, it is possible to achieve a high-speed response, ensure the strength of the output signal, and ensure reliability. Furthermore, since the recess 54 is formed as a hole without opening on the side surface 51c of the insulating substrate 51, the mesa 12 can be further reliably protected.

[0105] exist Figure 15 In the third variation shown, the photodetector 10 includes a height adjustment layer 19. The height adjustment layer 19 is formed planarly on the main surface 11a of the semiconductor substrate 11. The height adjustment layer 19 is formed on the main surface 11a separately 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 second contact layer 14 and has the same configuration as 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. Through this third variation, similar to the embodiments described above, it is possible to achieve high-speed response, ensure the strength of the output signal, and ensure reliability. Furthermore, when the first electrode 15 is connected to the first wiring 52 and the second electrode 17 is connected to the second wiring 53, even if there is a height difference between the first electrode 15 and the second electrode 17, the height difference between the first electrode 15 and the second electrode 17 can be reduced by the height adjustment layer 19, and good surface contact can be achieved at each connection point.

[0106] This disclosure is not limited to the embodiments described above. For example, the materials and shapes of each structure are not limited to those described above, and various materials and shapes can be used. 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 also be omitted. The second contact layer 14 may not have the second portion 14b and may only have the first portion 14a. The platform surface 12 may also further include: an upper cladding layer disposed between the active layer 21 and the first contact layer 13, and a lower cladding layer disposed between the active layer 21 and the second contact layer 14.

[0107] In the above embodiment, the air bridge wiring 16 is led out from the first contact layer 13 in a direction parallel to the X direction. However, the air bridge wiring 16 can be led out from the first contact layer 13 to one side in the X direction, or it can be led out from the first contact layer 13 in a direction inclined relative to the X and Y directions when viewed from above. That is, the air bridge wiring 16 can also be led out from the first contact layer 13 in a direction intersecting the Y direction when viewed from above.

[0108] In the above embodiment, the photodetector 10 is configured as a quantum cascade detector, but the photodetector 10 can also be configured as other photodetectors such as a quantum well infrared photodetector (QWIP). A quantum well infrared photodetector is an infrared detector that uses inter-subband absorption of a quantum well structure to detect light.

[0109] [Light-emitting beam splitter]

[0110] like Figure 16 As shown, the optical beat splitter 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 light detection module 1, a spectrum analyzer 104, and a gas cell 105. In the optical beat splitter 100, the wavelength of light P2 is varied by scanning the frequency of an optical beat signal having a frequency corresponding to the wavelength difference between light P1 from the first light source 101 and light P2 from the second light source 102, while the light detection module 1 detects light P1 and P2. This enables heterodyne detection and splitting.

[0111] The first light source 101 and the second light source 102 output light P1 and P2 with oscillation wavelengths that are within the sensitivity wavelength range of the light detection 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-QCL). 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 wavelength-fixed light source with a fixed wavelength for output light P1, and the second light source 102 is a wavelength-variable light source with a changing wavelength for output light P2. The operation of the second light source 102 is controlled, for example, by a control unit composed of a computer.

[0112] exist Figure 17 In the figure, the sensitivity characteristics of the photodetector 10 are represented by reference numeral R1, and the oscillation wavelength of the second light source 102 is represented by reference numeral R2.Figure 17 As shown, the oscillation wavelength of the second light source 102 is within the sensitivity wavelength range of the photodetector 10. Figure 18 This is a graph showing the oscillation wavelengths of the first light source 101 and the second light source 102. Reference numeral F indicates the oscillation wavelength of the first light source 101 when the injected current is 780 mA. Reference numerals T1 to T4 indicate the oscillation wavelengths of the second light source 102 when the injected current is 780 mA, 810 mA, 830 mA, and 850 mA, respectively. Thus, by changing the injected current, the oscillation wavelength of the second light source 102 can be modulated.

[0113] Light P1 from the first light source 101 is reflected by mirrors 111 and 112 after passing through lens 108 and then enters beam splitter 103. Light P2 from the second light source 102 is reflected by mirror 113 after passing through lens 109 and then enters beam splitter 103. Lights P1 and P2 are combined by beam splitter 103. The combined light P1 and P2 are reflected by mirror 116 after passing through aperture members 114 and 115 and then enter lens 40 and photodetector 10. Photodetector 10 is connected to spectrometer 104. A gas chamber 105 containing the gas to be measured is disposed between the second light source 102 and mirror 113. Light P2 from the second light source 102 enters mirror 113 after passing through gas chamber 105. In the case where the wavelength modulation range of light P2 from the second light source 102 spans the absorption line of the gas, light of a specific wavelength in light P2 is absorbed.

[0114] In the photodetector 10, an optical beat signal (beat frequency) with a frequency corresponding to the wavelength difference between light P1 from the first light source 101 and light P2 from the second light source 102 is detected. Directly measuring the frequency of light is difficult. For example, in the case of a wavelength of 4 μm, the response speed of the photodetector needs to be at least 75 THz. In contrast, in the optical beat splitting method using the optical beat signal, for example, in the case of a wavelength of 4.000 μm for light P1 and a wavelength of 4.001 μm for light P2, the response speed of the photodetector 10 only needs to be at least 18 GHz. As described above, a cutoff frequency of 20 GHz or higher can be obtained in the optical detection module 1. Therefore, by using the optical detection module 1 for optical beat splitting, a wide wavelength range can be split.

[0115] In the optical beat splitter 100, as described above, the wavelength of light P2 is changed by scanning the frequency of the optical beat signal, and light P1 and P2 are detected by the optical detection module 1. Figure 19 This is a graph showing the relationship between the injection current of the second light source 102 and the oscillation wavelength. (Example) Figure 19As shown, if the current of the second light source 102 is set as x, and the wavenumber of the light P2 from the second light source 102 is set as y, then the relationship y = 0.0147x + 2202.1 holds true. The dashed line B represents the oscillation wavelength of the first light source 101 when the injected current is 750mA. In this example, the operating temperature of both the first and second light sources 101 is 20℃, and both are driven by CW (Continuous Wave). The reference numeral C in the attached figure indicates the difference between the oscillation wavelengths of the first and second light sources 101 under a certain injected current. The faster the response speed of the photodetector 10, the greater this difference can be, and the wider the wavelength scanning range can be expanded. Figure 20 This is a graph showing the relationship between the injection current to the second light source 102 and the optical beat frequency. (Example) Figure 20 As shown, the optical beat frequency increases linearly with the increase of the injected current.

[0116] Figure 21 This is a graph showing an example of a photodetector signal. Figure 22 This is a graph showing the comparison results of the response characteristics. Figure 22 In the accompanying drawings, reference numeral S1 indicates the frequency response characteristics of the optical detection module 1, and reference numeral S2 indicates the frequency response characteristics of the optical detector of the comparison object. According to... Figure 22 It can be seen that in the optical detection module 1, the cutoff frequency is above 20GHz, and the response speed is faster compared with the optical detector of the comparison object.

[0117] Figure 23 , Figure 24 and Figure 25 It is a graph showing the measurement results of light-induced beam splitting. Figure 23 This is a graph showing the output signal of the spectrum analyzer 104. Figure 24 It is Figure 23 The horizontal axis of the graph shown is transformed into a wavelength graph. Figure 25 It is a curve obtained by subtracting the background light signal from the output signal.

[0118] In this example, the gas being measured is carbon monoxide, and the observed wavenumber is 2190 cm⁻¹. -1 Nearby absorption lines. In cases where the wavelength scan range spans the absorption lines of the gas being measured, such as... Figure 23 As indicated by arrow D, wavelength-dependent gas absorption is observed. This is because, when the wavelength of light P2 from the second light source 102 matches the absorption wavelength of the gas, the light intensity decreases due to absorption. The intensity of the optical beat signal is proportional to the product of the electric field amplitude of light P1 from the first light source 101 and the electric field amplitude of light P2 from the second light source 102. Therefore, the intensity of light P2 decreases, resulting in the observed dip as indicated by arrow D.Figure 24 In the figure, SG indicates the output signal when carbon monoxide is sealed in the gas chamber 105, and BG indicates the output signal when carbon monoxide is absent from the gas chamber 105. Furthermore, the absorption spectrum of carbon monoxide is represented by dashed lines. FT indicates an example of the resolution range of the Fourier Transform Infrared Spectrometer (FTIR). The resolution relative to FTIR is 3 GHz (0.1 cm⁻¹). -1 With this level of resolution, the optical beat-and-splitter 100 can achieve a resolution of around 20 MHz. This is more than 150 times the resolution of FTIR. Thus, the optical beat-and-splitter 100 can expand the wavelength scanning range of optical beat-and-splitter.

[0119] Furthermore, in the optical beam splitter 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. The output light of the quantum cascade laser is linearly polarized light parallel to the growth direction of the active layer. Therefore, by arranging the first light source 101, the second light source 102, and the photodetector 10 in such a way that the polarization directions of the first light source 101 and the second light source 102 are consistent with the polarization direction of the sensitivity of the photodetector 10, 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. 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 together like polarization filters, thereby suppressing the influence of background light with random polarization, resulting in an improved signal-to-noise ratio (SN ratio).

[0120] Explanation of reference numerals in the attached figures

[0121] 10, 10A…Photodetector, 11…Semiconductor substrate, 11a…Main surface, 11b…Side surface, 12…Medium surface, 12a…Side surface, 12c…End surface, 13…First contact layer, 14…Second contact layer, 14a…First part, 14b…Second part, 15…First electrode, 15a…Connection part, 16…Air bridge wiring, 16a…Bridge part, 17…Second electrode, 21…Active layer, 22…Absorption region, 23…Transport region, 40…Lens, 100…Optical beam splitter, 101…First light source (fixed wavelength light source), 102…Second light source (variable wavelength light source), A…Optical waveguide direction, DL…Detection light, P1…Light from the first light source, P2…Light from the second light source.

Claims

1. A photodetector, wherein, provided with: a semiconductor substrate having a main surface; a mesa portion including an active layer in which an absorption region that absorbs a detection light by inter-subband absorption and a transport region that transports an electron excited by the inter-subband absorption are alternately laminated, and formed on the main surface of the semiconductor substrate in a manner extending along a light waveguide direction; a first contact layer formed on a surface of the mesa portion on the opposite side from 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, a length of the mesa portion in the light waveguide direction is longer than a length of the mesa portion in a direction perpendicular to the light waveguide direction when viewed from a direction perpendicular to the main surface of the semiconductor substrate, the air-bridge wiring is drawn out from the first contact layer to one side in the direction perpendicular to the light waveguide direction and is erected between the first contact layer and the first electrode, the air-bridge wiring has a bridge portion extending in the air, a length of the bridge portion in the light waveguide direction is longer than a length of the bridge portion in the direction perpendicular to the light waveguide direction.

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

3. The photodetector according to claim 1, wherein, the air-bridge wiring has a bridge portion extending in the air, the length of the mesa portion in the light waveguide direction and the length of the bridge portion in the light waveguide direction are each 50 μm or more.

4. The photodetector according to claim 2, wherein, the air-bridge wiring has a bridge portion extending in the air, the length of the mesa portion in the light waveguide direction and the length of the bridge portion in the light waveguide direction are each 50 μm or more.

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

6. The photodetector according to claim 2, wherein, a ratio of the length of the mesa portion in the light waveguide direction with respect to the length of the mesa portion in the direction perpendicular to the light waveguide direction is greater than 1 and less than 100.

7. The photodetector according to claim 3, wherein, a ratio of the length of the mesa portion in the light waveguide direction with respect to the length of the mesa portion in the direction perpendicular to the light waveguide direction is greater than 1 and less than 100.

8. The photodetector according to claim 4, wherein, ​ 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 is greater than 1 and less than 100.

9. The photodetector according to claim 1, wherein The air-bridge wiring has a bridge portion extending in air, The ratio of the length of the bridge portion in the optical waveguide direction to the length of the bridge portion in the direction perpendicular to the optical waveguide direction is greater than 1 and less than 50.

10. The photodetector according to claim 2, wherein The air-bridge wiring has a bridge portion extending in air, The ratio of the length of the bridge portion in the optical waveguide direction to the length of the bridge portion in the direction perpendicular to the optical waveguide direction is greater than 1 and less than 50.

11. The photodetector according to claim 3, wherein The air-bridge wiring has a bridge portion extending in air, The ratio of the length of the bridge portion in the optical waveguide direction to the length of the bridge portion in the direction perpendicular to the optical waveguide direction is greater than 1 and less than 50.

12. The photodetector according to claim 4, wherein The air-bridge wiring has a bridge portion extending in air, The ratio of the length of the bridge portion in the optical waveguide direction to the length of the bridge portion in the direction perpendicular to the optical waveguide direction is greater than 1 and less than 50.

13. The photodetector according to claim 5, wherein The air-bridge wiring has a bridge portion extending in air, The ratio of the length of the bridge portion in the optical waveguide direction to the length of the bridge portion in the direction perpendicular to the optical waveguide direction is greater than 1 and less than 50.

14. The photodetector according to claim 6, wherein The air-bridge wiring has a bridge portion extending in air, The ratio of the length of the bridge portion in the optical waveguide direction to the length of the bridge portion in the direction perpendicular to the optical waveguide direction is greater than 1 and less than 50.

15. The photodetector according to claim 7, wherein The air-bridge wiring has a bridge portion extending in air, The ratio of the length of the bridge portion in the optical waveguide direction to the length of the bridge portion in the direction perpendicular to the optical waveguide direction is greater than 1 and less than 50.

16. The photodetector according to claim 8, wherein The air-bridge wiring has a bridge portion extending in air, The ratio of the length of the bridge portion in the optical waveguide direction to the length of the bridge portion in the direction perpendicular to the optical waveguide direction is greater than 1 and less than 50.

17. The photodetector according to any one of claims 1 to 16, wherein The air-bridge wiring has a bridge portion extending in air, The thickness of the bridge portion is 1 μm or more and 10 μm or less.

18. The photodetector according to any one of claims 1 to 16, wherein One end surface of the mesa portion in the optical waveguide direction becomes an incident surface of the detection light.

19. The photodetector according to claim 17, wherein One end surface of the mesa portion in the direction of the optical waveguide becomes an incident surface of the detection light.

20. The photodetector according to any one of claims 1 to 16, wherein One end surface of the mesa portion in the direction of the optical waveguide and a side surface of the semiconductor substrate become the same plane.

21. The photodetector according to claim 17, wherein One end surface of the mesa portion in the direction of the optical waveguide and a side surface of the semiconductor substrate become the same plane.

22. The photodetector according to claim 18, wherein One end surface of the mesa portion in the direction of the optical waveguide and a side surface of the semiconductor substrate become the same plane.

23. The photodetector according to claim 19, wherein One end surface of the mesa portion in the direction of the optical waveguide and a side surface of the semiconductor substrate become the same plane.

24. The photodetector according to claim 18, wherein Further comprising a lens disposed so as to face the end surface of the mesa portion and converging the detection light toward the end surface of the mesa portion.

25. The photodetector according to claim 19, wherein Further comprising a lens disposed so as to face the end surface of the mesa portion and converging the detection light toward the end surface of the mesa portion.

26. The photodetector according to claim 20, wherein Further comprising a lens disposed so as to face the end surface of the mesa portion and converging the detection light toward the end surface of the mesa portion.

27. The photodetector according to claim 21, wherein Further comprising a lens disposed so as to face the end surface of the mesa portion and converging the detection light toward the end surface of the mesa portion.

28. The photodetector according to claim 22, wherein Further comprising a lens disposed so as to face the end surface of the mesa portion and converging the detection light toward the end surface of the mesa portion.

29. The photodetector according to claim 23, wherein Further comprising a lens disposed so as to face the end surface of the mesa portion and converging the detection light toward the end surface of the mesa portion.

30. The photodetector according to any one of claims 1 to 16, wherein Two side surfaces of the mesa portion in the direction perpendicular to the direction of the optical waveguide are exposed.

31. The photodetector according to claim 17, wherein Two side surfaces of the mesa portion in the direction perpendicular to the direction of the optical waveguide are exposed.

32. The photodetector according to claim 18, wherein Two side surfaces of the mesa portion in the direction perpendicular to the direction of the optical waveguide are exposed.

33. The photodetector according to claim 19, wherein Two side surfaces of the mesa portion in the direction perpendicular to the direction of the optical waveguide are exposed.

34. The photodetector according to claim 20, wherein Two side surfaces of the mesa portion in the direction perpendicular to the direction of the optical waveguide are exposed.

35. The photodetector according to claim 21, wherein Both side surfaces of the mesa portion in the direction perpendicular to the direction of the optical waveguide are exposed.

36. The photodetector according to claim 22, wherein Both side surfaces of the mesa portion in the direction perpendicular to the direction of the optical waveguide are exposed.

37. The photodetector according to claim 23, wherein Both side surfaces of the mesa portion in the direction perpendicular to the direction of the optical waveguide are exposed.

38. The photodetector according to claim 24, wherein Both side surfaces of the mesa portion in the direction perpendicular to the direction of the optical waveguide are exposed.

39. The photodetector according to claim 25, wherein Both side surfaces of the mesa portion in the direction perpendicular to the direction of the optical waveguide are exposed.

40. The photodetector according to claim 26, wherein Both side surfaces of the mesa portion in the direction perpendicular to the direction of the optical waveguide are exposed.

41. The photodetector according to claim 27, wherein Both side surfaces of the mesa portion in the direction perpendicular to the direction of the optical waveguide are exposed.

42. The photodetector according to claim 28, wherein Both side surfaces of the mesa portion in the direction perpendicular to the direction of the optical waveguide are exposed.

43. The photodetector according to claim 29, wherein Both side surfaces of the mesa portion in the direction perpendicular to the direction of the optical waveguide are exposed.

44. The photodetector according to any one of claims 1 to 16, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed from a direction perpendicular to the main surface of the semiconductor substrate, A second electrode is formed on the second portion of the second contact layer.

45. The photodetector according to claim 17, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed from a direction perpendicular to the main surface of the semiconductor substrate, A second electrode is formed on the second portion of the second contact layer.

46. The photodetector according to claim 18, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed from a direction perpendicular to the main surface of the semiconductor substrate, A second electrode is formed on the second portion of the second contact layer.

47. The photodetector according to claim 19, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed from a direction perpendicular to the main surface of the semiconductor substrate, A second electrode is formed on the second portion of the second contact layer.

48. The photodetector according to claim 20, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

49. The photodetector of claim 21, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

50. The photodetector of claim 22, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

51. The photodetector of claim 23, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

52. The photodetector of claim 24, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

53. The photodetector of claim 25, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

54. The photodetector of claim 26, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

55. The photodetector of claim 27, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

56. The photodetector according to claim 28, wherein The second contact layer has, when viewed from a direction perpendicular to the main surface of the semiconductor substrate, a first portion between the main surface of the semiconductor substrate and the mesa portion, and a second portion outside the mesa portion, A second electrode is formed on the second portion of the second contact layer.

57. The photodetector according to claim 29, wherein The second contact layer has, when viewed from a direction perpendicular to the main surface of the semiconductor substrate, a first portion between the main surface of the semiconductor substrate and the mesa portion, and a second portion outside the mesa portion, A second electrode is formed on the second portion of the second contact layer.

58. The photodetector according to claim 30, wherein The second contact layer has, when viewed from a direction perpendicular to the main surface of the semiconductor substrate, a first portion between the main surface of the semiconductor substrate and the mesa portion, and a second portion outside the mesa portion, A second electrode is formed on the second portion of the second contact layer.

59. The photodetector according to claim 31, wherein The second contact layer has, when viewed from a direction perpendicular to the main surface of the semiconductor substrate, a first portion between the main surface of the semiconductor substrate and the mesa portion, and a second portion outside the mesa portion, A second electrode is formed on the second portion of the second contact layer.

60. The photodetector according to claim 32, wherein The second contact layer has, when viewed from a direction perpendicular to the main surface of the semiconductor substrate, a first portion between the main surface of the semiconductor substrate and the mesa portion, and a second portion outside the mesa portion, A second electrode is formed on the second portion of the second contact layer.

61. The photodetector according to claim 33, wherein The second contact layer has, when viewed from a direction perpendicular to the main surface of the semiconductor substrate, a first portion between the main surface of the semiconductor substrate and the mesa portion, and a second portion outside the mesa portion, A second electrode is formed on the second portion of the second contact layer.

62. The photodetector according to claim 34, wherein The second contact layer has, when viewed from a direction perpendicular to the main surface of the semiconductor substrate, a first portion between the main surface of the semiconductor substrate and the mesa portion, and a second portion outside the mesa portion, A second electrode is formed on the second portion of the second contact layer.

63. The photodetector according to claim 35, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

64. The photodetector of claim 36, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

65. The photodetector of claim 37, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

66. The photodetector of claim 38, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

67. The photodetector of claim 39, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

68. The photodetector of claim 40, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

69. The photodetector of claim 41, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

70. The photodetector of claim 42, wherein The second contact layer has a first portion between the main surface of the semiconductor substrate and the mesa portion and a second portion outside the mesa portion when viewed in a direction perpendicular to the main surface of the semiconductor substrate. A second electrode is formed on the second portion of the second contact layer.

71. The photodetector according to claim 43, wherein The second contact layer has, when viewed from a direction perpendicular to the main surface of the semiconductor substrate, a first portion between the main surface of the semiconductor substrate and the mesa portion, and a second portion outside the mesa portion, A second electrode is formed on the second portion of the second contact layer.

72. An optical beat spectrometer, wherein provided with: a wavelength-fixed light source; a wavelength-variable light source; and the photodetector according to any one of claims 1 to 71 that detects light from the wavelength-fixed light source and light from the wavelength-variable light source as the detection light, the wavelength of the light from the wavelength-variable light source is varied in a manner that scans a frequency of an optical beat signal having a frequency corresponding to a wavelength difference between the light from the wavelength-fixed light source and the light from the wavelength-variable light source, and the light from the wavelength-fixed light source and the light from the wavelength-variable light source are detected by the photodetector.

Images