quantum cascade device

Abstract

To provide a quantum cascade element with high output or high sensitivity.SOLUTION: The quantum cascade element includes a substrate, a quantum cascade layer, and a semiconductor layer. The quantum cascade layer is provided on the substrate and includes a plurality of quantum well layers and a plurality of barrier layers stacked alternately in a direction perpendicular to the surface of the substrate. The semiconductor layer is provided on the quantum cascade layer and includes a photonic crystal having a periodicity composed of recesses provided on a surface in contact with the uppermost layer of the plurality of barrier layers.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The embodiments relate to quantum cascade devices. [Background technology]

[0002] Quantum cascade devices include quantum cascade lasers and quantum cascade photodetectors. Quantum cascade devices use multiple quantum wells and exhibit functions such as light emission and light reception through transitions between electron energy bands. For example, a surface-emitting quantum cascade laser or quantum cascade photodetector includes a quantum cascade layer, a photonic crystal, and a cladding layer stacked in this order on a semiconductor substrate. A surface-emitting quantum cascade laser emits laser light in a direction perpendicular to the substrate surface using a photonic crystal, while a quantum cascade photodetector converts light incident in a direction perpendicular to the substrate surface into a photocurrent in the quantum cascade layer.

[0003] The photonic crystal changes the propagation direction of laser light guided in the quantum cascade layer in a direction parallel to the substrate surface to a direction perpendicular to the substrate surface. Furthermore, the photonic crystal changes the propagation direction of light incident in a direction perpendicular to the substrate surface to a direction parallel to the substrate surface, and propagates the light through the quantum cascade layer. Therefore, in order to increase the output or sensitivity of a quantum cascade device, it is preferable to improve the coupling efficiency between the photonic crystal and the light in the quantum cascade layer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6513626 Summary of the Invention [Problem to be solved by the invention]

[0005] The embodiments provide a quantum cascade device with high output or high sensitivity. [Means for solving the problem]

[0006] A quantum cascade device according to an embodiment includes a first substrate, a quantum cascade layer, and a second substrate. The quantum cascade layer is provided on a surface of the first substrate and includes a plurality of quantum well layers and a plurality of barrier layers alternately stacked in a direction perpendicular to the surface of the first substrate. The second substrate is bonded to the quantum cascade layer and includes a photonic crystal formed by a recess provided in a bonding surface that contacts an uppermost layer of the plurality of barrier layers. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a schematic cross-sectional view showing a quantum cascade device according to an embodiment. [Figure 2] 1 is a schematic perspective view showing a quantum cascade device according to an embodiment. [Figure 3] 1 is a schematic perspective view showing a wafer used in manufacturing a quantum cascade device according to an embodiment. [Figure 4] 5A to 5C are schematic cross-sectional views showing a manufacturing process of the quantum cascade device according to the embodiment. [Figure 5] 5A to 5C are schematic cross-sectional views showing the manufacturing process following FIG. 4. [Figure 6] 10A and 10B are schematic perspective views showing a method for manufacturing a quantum cascade device according to a modified example of the embodiment. [Figure 7] FIG. 10 is a schematic cross-sectional view showing a quantum cascade device according to a modified example of the embodiment. [Figure 8] FIG. 10 is a schematic cross-sectional view showing a quantum cascade device according to another modified example of the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments will be described with reference to the drawings. Identical parts in the drawings are assigned the same numbers, and detailed descriptions thereof will be omitted as appropriate, and different parts will be described. Note that the drawings are schematic or conceptual, and the relationship between the thickness and width of each part, the size ratio between parts, etc., are not necessarily the same as those in reality. Furthermore, even when the same part is shown, the dimensions and ratios may be different depending on the drawing.

[0009] Furthermore, the arrangement and configuration of each part will be explained using the X-axis, Y-axis, and Z-axis shown in each figure. The X-axis, Y-axis, and Z-axis are mutually perpendicular and represent the X-direction, Y-direction, and Z-direction, respectively. In addition, the Z-direction may be explained as upward and the opposite direction as downward.

[0010] 1(a) and 1(b) are schematic cross-sectional views showing a quantum cascade device 1 according to an embodiment. The quantum cascade device 1 is, for example, a surface-emitting quantum cascade laser. Note that the quantum cascade device according to the embodiment is not limited to the following examples. The quantum cascade device according to the embodiment may be, for example, a quantum cascade photodetector.

[0011] As shown in FIG. 1(a), the quantum cascade device 1 includes a first substrate 10, a second substrate 20, a quantum cascade layer 30, a first electrode 40, and a second electrode 50. The first substrate 10 is a semiconductor substrate such as indium phosphide (InP). The second substrate 20 is a semiconductor substrate such as InP or silicon. The quantum cascade layer 30 is provided between the first substrate 10 and the second substrate 20. The second substrate 20 is provided so as to be in contact with the quantum cascade layer 30.

[0012] The second substrate 20 includes a photonic crystal PC in contact with the quantum cascade layer 30. The photonic crystal PC is provided on the front surface side of the second substrate 20. The photonic crystal PC includes, for example, a plurality of recesses 21 provided on the surface of the second substrate 20.

[0013] The first electrode 40 is provided, for example, on the back surface of the first substrate 10. The first electrode 40 is electrically connected to the quantum cascade layer 30 via the first substrate 10. The second electrode 50 is provided, for example, on the back surface of the second substrate 20 opposite the photonic crystal PC. The second electrode 50 is electrically connected to the quantum cascade layer 30 via the second substrate 20.

[0014] 1(b) is a cross-sectional view showing the configuration of the quantum cascade layer 30. The quantum cascade layer 30 includes a plurality of quantum well layers 31 and a plurality of barrier layers 33. The quantum well layers 31 and the barrier layers 33 are alternately stacked on the first substrate 10. The quantum well layer 31 is located between two barrier layers 33.

[0015] The second substrate 20 is provided, for example, so that the photonic crystal PC is in contact with the uppermost layer of the plurality of barrier layers 33. This can improve the coupling efficiency between the light propagating through the quantum cascade layer 30 and the photonic crystal PC.

[0016] The quantum cascade device 1 is not limited to this example, and may further include, for example, a cladding layer or a buffer layer provided between the first substrate 10 and the quantum cascade layer 30.

[0017] 2 is a schematic perspective view showing the quantum cascade device 1 according to the embodiment. The quantum cascade device 1 is mounted on, for example, a heat sink 60. The heat sink 60 is, for example, a metal block containing copper.

[0018] The quantum cascade device 1 is mounted, for example, with the back surface of the first substrate 10 facing upward. The second electrode 50 on the back surface side of the second substrate 20 is connected to a mounting pad (not shown) on the heat sink 60 via, for example, a solder material.

[0019] 2, the first electrode 40 is provided in a frame or ring shape having an opening that exposes the back surface side of the first substrate 10. The laser light LL emitted from the quantum cascade device 1 is emitted to the outside through the opening of the first electrode 40.

[0020] The first electrode 40 is electrically connected to a metal terminal 63 on the heat sink 60, for example, via a metal wire 41. The metal terminal 63 is electrically insulated from the heat sink 60, for example, by an insulating member 65. The quantum cascade device 1 is connected to an external driving circuit via the metal terminal 63 and a mount pad (not shown) on the heat sink 60.

[0021] 3(a) and (b) are perspective views schematically showing wafers used in manufacturing the quantum cascade device 1. The quantum cascade device 1 is formed by bonding the wafers shown in FIGS. 3(a) and (b).

[0022] 3(a) is a schematic perspective view showing a first substrate 10 of the quantum cascade device according to the embodiment. As shown in FIG. 3(a), the first substrate 10 includes a quantum cascade layer 30 provided on a surface 10F thereof.

[0023] The quantum cascade layer 30 is formed on the first substrate 10 by using, for example, MBE (Molecular Beam Epitaxy). The first substrate 10 is, for example, a growth substrate. The first substrate 10 may be, for example, an InP substrate.

[0024] The quantum cascade layer 30 includes a plurality of quantum well layers 31 and a plurality of barrier layers 33 stacked in a direction perpendicular to the surface 10F, for example, in the Z direction. The quantum well layers 31 and the barrier layers 33 are stacked alternately to form a plurality of quantum wells.

[0025] The quantum well layer 31 is, for example, a compound represented by the formula In X Ga 1-X The barrier layer 33 includes a semiconductor alloy crystal represented by the composition formula InGaAs (hereinafter referred to as InGaAs). Y Al1-Y The semiconductor alloy includes a semiconductor alloy represented by InAlAs (hereinafter referred to as InAlAs).

[0026] Each of the multiple quantum wells includes subbands formed by quantizing the conduction band of the quantum well layer 31. The quantum cascade device 1 emits laser light generated by intersubband transition of electrons injected into the quantum cascade layer 30.

[0027] The quantum wells include, for example, a first group of quantum wells that function as light-emitting quantum wells and a second group of quantum wells that function as injection quantum wells. The first and second groups of quantum wells are alternately arranged in the Z direction. The quantum cascade layer 30 has a thickness in the Z direction of, for example, 0.6 to 4 micrometers (hereinafter, μm).

[0028] 3(b) is a schematic perspective view showing the second substrate 20 of the quantum cascade device 1 according to the embodiment. As shown in FIG. 3(b), the second substrate 20 has, for example, a plurality of recesses 21 provided on a surface 20F thereof.

[0029] The recesses 21 are arranged, for example, at equal intervals in the X and Y directions to form a two-dimensional diffraction grating. The multiple recesses 21 are located at the lattice points of the two-dimensional diffraction grating. The recesses 21 have, for example, circular or right-angled triangular openings. The recesses 21 preferably have an asymmetric shape with respect to the arrangement direction of the two-dimensional diffraction grating, for example, the X or Y direction. The recesses 21 are, for example, hollow and have a refractive index lower than that of the material of the second substrate 20. Alternatively, the recesses 21 may be filled with a material having a refractive index lower than that of the material of the second substrate 20.

[0030] Next, a method for manufacturing the quantum cascade device 1 will be described with reference to Figures 4(a) to 5(b). Figures 4(a) to 5(b) are schematic cross-sectional views showing the manufacturing process of the quantum cascade device 1 according to the embodiment.

[0031] 4(a), a dielectric film 22 and a metal film 23 are formed on a surface 20F of a second substrate 20. The second substrate 20 is, for example, an InP substrate. The dielectric film 22 is formed on the surface 20F, and the metal film 23 is formed on the dielectric film 22.

[0032] The dielectric film 22 is, for example, a silicon nitride film or a silicon oxide film. The dielectric film 22 is formed by using, for example, CVD (Chemical Vapor Deposition), sputtering, ECR (Electron Cyclotron Resonance), etc. The thickness of the dielectric film 22 is, for example, 0.3 μm to 1 μm.

[0033] The metal film 23 includes, for example, nickel (Ni). The metal film 23 is formed by, for example, sputtering. The thickness of the metal film 23 is, for example, 100 nanometers (hereinafter, nm).

[0034] As shown in FIG. 4(b), an etching mask 25 is formed on the metal film 23. The etching mask 25 is, for example, a photoresist. The etching mask 25 is applied to the metal film 23 and exposed using a photomask having a mask pattern corresponding to the photonic crystal PC. Subsequently, the etching mask 25 is developed to form openings 25a corresponding to the photonic crystal PC.

[0035] As shown in FIG. 4(c), the metal film 23 and the dielectric film 22 are selectively removed using an etching mask 25, and an opening 25a corresponding to the photonic crystal PC is transferred. The metal film 23 and the dielectric film 22 are selectively removed by, for example, dry etching. Subsequently, the etching mask 25 is removed. The etching mask 25 is removed by, for example, oxygen ashing.

[0036] 4(d), the second substrate 20 is etched through the openings 25a provided in the dielectric film 22 and the metal film 23 to form recesses 21. The second substrate 20 is selectively removed by, for example, dry etching.

[0037] Subsequently, the dielectric film 22 is removed from the surface 20F of the second substrate 20 by wet etching. At this time, the metal film 23 is also removed together with the dielectric film 22. As a result, a photonic crystal PC including a plurality of recesses 21 is formed on the surface 20F side of the second substrate 20.

[0038] 5(a), the first substrate 10 and the second substrate 20 are bonded via the quantum cascade layer 30. The second substrate 20 is bonded so that the photonic crystal PC is in contact with the quantum cascade layer 30.

[0039] After cleaning the surface 20F of the second substrate 20 and the surface 30F of the quantum cascade layer 30 by, for example, argon (Ar) sputtering, the surface 20F is brought into surface contact with the surface 30F, and pressure and heat are applied. The first substrate 10 and the second substrate 20 are heated to, for example, 250°C. As a result, the first substrate 10 and the second substrate 20 are bonded via the quantum cascade layer 30.

[0040] The surface 20F of the second substrate 20 and the surface 30F of the quantum cascade layer 30 each include a native oxide film Nox. The second substrate 20 and the quantum cascade layer 30 are preferably in surface contact via at least a portion of the native oxide film Nox on the surface 20F and the surface 30F. That is, the surface 20F of the second substrate 20 and the surface 30F of the quantum cascade layer 30 are preferably cleaned so as to leave at least a portion of the native oxide film Nox. This increases the bonding strength between the second substrate 20 and the quantum cascade layer 30 compared to when the native oxide film Nox is not present. In other words, the bonding surface between the second substrate 20 and the quantum cascade layer 30 preferably contains oxygen.

[0041] 5(b), a first electrode 40 and a second electrode 50 are formed on the rear surface 10B of the first substrate 10 and the rear surface 20B of the second substrate 20, respectively. Either the first electrode 40 or the second electrode 50 has an opening that exposes the radiation surface of the laser light LL.

[0042] Next, the first substrate 10 and the second substrate 20 are cut using, for example, a dicing blade to separate them into a plurality of quantum cascade devices 1. The quantum cascade devices 1 are separated into chips having a size of, for example, 500 μm square in plan view.

[0043] As described above, in the quantum cascade device 1 according to the embodiment, a photonic crystal PC can be provided in contact with the quantum cascade layer 30. This makes it possible to improve the coupling efficiency between the photonic crystal PC and light propagating in the quantum cascade layer 30 in a direction parallel to the surface 30F. As a result, the quantum cascade device 1 can have a high output. Furthermore, when the quantum cascade device 1 is operated as a photodetector, the photonic crystal PC allows incident light to propagate in a direction along the surface of the quantum cascade layer 30. This makes it possible to increase the light absorption in the quantum cascade layer 30, thereby increasing the light receiving sensitivity.

[0044] For example, it is difficult to form such a structure by using a method in which the quantum cascade layer 30 is formed on the first substrate 10 and then another semiconductor layer is grown on the quantum cascade layer 30. The photonic crystal PC is formed, for example, by etching a semiconductor layer formed on the quantum cascade layer 30, but considering the non-uniformity of the epitaxial film thickness and the non-uniformity of the etching rate within the wafer surface, it is difficult to form the photonic crystal PC directly on the quantum cascade layer 30 without etching the quantum cascade layer 30. Therefore, conventional quantum cascade devices have a structure in which the photonic crystal PC is spaced apart from the quantum cascade layer 30 so as not to reduce the manufacturing yield.

[0045] In the manufacturing method of the embodiment, a second substrate 20 including a photonic crystal PC is bonded to a first substrate 10 via a quantum cascade layer 30, thereby realizing a structure including a photonic crystal PC provided directly on the quantum cascade layer 30.

[0046] However, with this manufacturing method, it is inevitable that the wafer bonding interface will contain crystal defects due to discontinuities in the crystal structure. Such crystal defects, for example, form carrier trap levels located in the energy band gap. Non-radiative recombination of electrons and holes via such levels significantly reduces the luminous efficiency within the crystal. Therefore, in devices that use luminescence associated with electron-hole recombination, it is difficult to bond the photonic crystal PC to the light-emitting layer. In contrast, in quantum cascade devices 1 that utilize luminescence due to intersubband transitions of electrons, the carrier trap levels in the energy band gap have little effect on the luminous characteristics. Therefore, a manufacturing method can be used to bond the photonic crystal PC to the quantum cascade layer 30. In other words, the structure of the quantum cascade device according to the embodiment can achieve high output power for laser devices or high sensitivity for photodetectors.

[0047] Furthermore, by interposing oxygen at the interface between the quantum cascade layer 30 and the photonic crystal PC, it is possible to improve the uniformity of electron injection into the quantum cascade layer 30. That is, the presence of oxygen increases the electrical resistance at the junction interface, and it is possible to spread the electron injection to the region of the quantum cascade layer 30 facing the recess 21 of the photonic crystal PC. This makes it possible to improve the light emission efficiency in the quantum cascade layer 30.

[0048] 6 is a schematic perspective view showing a method for manufacturing the quantum cascade device 1 according to a modified example of the embodiment. The second substrate 20 may be a semiconductor substrate other than an InP substrate or a dielectric substrate. The second substrate 20 is, for example, a silicon substrate.

[0049] 6, a first substrate 10 and a second substrate 20 are bonded together via a quantum cascade layer 30 (not shown). The second substrate 20 is, for example, a silicon substrate. The second substrate 20 is, for example, cut out from a silicon wafer with a diameter of 300 millimeters (mm) on which a photonic crystal PC is formed.

[0050] Next, the back surface side of the second substrate 20 is thinned to a predetermined thickness, for example, by wet etching, and then a first electrode 40 and a second electrode 50 are formed on the back surface sides of the first substrate 10 and the second substrate 20, respectively.

[0051] In this way, using silicon wafers simplifies the manufacturing process and improves the processing accuracy of the photonic crystal PC. Furthermore, silicon wafers are cheaper than other substrates, which reduces the manufacturing cost of quantum cascade devices.

[0052] Furthermore, in a method in which a semiconductor layer is grown on the quantum cascade layer 30 and then a photonic crystal PC is formed in the semiconductor layer, if a defect occurs in the process of forming the photonic crystal PC or in a subsequent process, the first substrate 10 on which the quantum cascade layer 30 is formed becomes a defective product. The manufacturing method according to the embodiment makes it possible to minimize such losses.

[0053] 7(a) and 7(b) are schematic cross-sectional views showing modified quantum cascade devices 2 and 3 according to the embodiment. The quantum cascade devices 2 and 3 include a Bragg reflection multilayer DBR.

[0054] As shown in FIG. 7(a), the quantum cascade device 2 further includes a Bragg reflection multilayer film DBR provided on the back surface of the second substrate 20. A photonic crystal PC is provided on the surface opposite to the back surface of the second substrate 20. The Bragg reflection multilayer film DBR reflects, for example, laser light LL2 emitted from the quantum cascade layer 30 toward the first substrate 10. This can improve the extraction efficiency of the laser light LL from the back surface of the first substrate 10.

[0055] As shown in FIG. 7(b), the quantum cascade device 3 further includes a Bragg reflection multilayer film DBR and a metal film MR on the back surface side of the second substrate 20. The metal film MR is provided on the Bragg reflection multilayer film DBR. The metal film MR includes a material with high reflectivity to laser light, such as gold (Au). The metal film MR has a structure in which, for example, nickel with a thickness of 5 nm and gold with a thickness of 1 μm are stacked.

[0056] The Bragg reflection multilayer film DBR is located between the second substrate 20 and the metal film MR. The metal film MR improves the reflectivity of the laser light LL2 emitted from the quantum cascade layer 30, and further improves the extraction efficiency of the laser light LL from the back surface of the first substrate 10. The metal film MR also functions as, for example, the second electrode 50.

[0057] The Bragg-reflecting multilayer DBR is, for example, a semiconductor multilayer. The Bragg-reflecting multilayer DBR is, for example, a semiconductor multilayer formed by alternately stacking InP layers and InGaAs layers. The thickness of the InP layers is, for example, 364 nm. The thickness of the InGaAs layers is, for example, 331 nm. The Bragg-reflecting multilayer DBR includes, for example, 20 pairs of InP and InGaAs layers and functions as a reflective mirror for laser light with a wavelength of 4.5 μm. The semiconductor multilayer is, for example, electrically conductive. The semiconductor multilayer may also be polycrystalline. In other words, crystal defects contained in the semiconductor multilayer do not affect the characteristics of the quantum cascade devices 2 and 3.

[0058] The Bragg-reflecting multilayer DBR can be made of a ZnSe / ZnS pair, a Si / SiGe pair, or a GaAs / AlAs pair. These materials are formed by, for example, crystal growth or sputtering. The thickness of each layer constituting the Bragg-reflecting multilayer DBR is, for example, λ / 4Neff, where λ is the wavelength of the laser light, and Neff is the effective refractive index of each layer of the Bragg-reflecting multilayer DBR at the wavelength of the laser light.

[0059] The distance between the Bragg reflection multilayer film DBR and the quantum cascade layer 30 is preferably set so that, for example, laser light LL1 emitted from the quantum cascade layer 30 and propagating directly toward the first substrate 10 and laser light LL2 reflected by the Bragg reflection multilayer film DBR and propagating toward the first substrate 10 do not cancel each other out due to interference. This can further improve the extraction efficiency of the laser light LL.

[0060] The distance from the junction plane between the quantum cascade layer 30 and the photonic crystal PC to the Bragg reflection multilayer film DBR, in other words, the thickness d of the second substrate 20 in the stacking direction (Z direction) is preferably expressed by the following equation. d=dact / 2+(λ / Nseff)×M Here, dact is the thickness of the quantum cascade layer 30 in the stacking direction, λ is the wavelength of the laser light LL, Nseff is the effective refractive index of the material of the second substrate 20 at the wavelength of the laser light LL, and M is any integer.

[0061] 8 is a schematic cross-sectional view showing a quantum cascade device 4 according to another modified example of the embodiment. The quantum cascade device 4 is, for example, a photodetector. The quantum cascade device 4 applies a predetermined voltage between the first electrode 40 and the second electrode 50, and detects, for example, a change in current value due to light IL incident from the back surface of the second substrate 20.

[0062] For example, the optical absorption coefficient of infrared light detected by intersubband transitions in the quantum cascade layer 30 is small. Therefore, in order to increase the photosensitivity of the quantum cascade device 4, it is preferable to increase the propagation distance of light in the quantum cascade layer 30. That is, in order to increase the photosensitivity of the quantum cascade device 4, it is preferable to propagate light in the quantum cascade layer 30 in a direction parallel to the surface of the first substrate 10.

[0063] In order to propagate light in a direction parallel to the surface of the first substrate 10, for example, light may be incident on the quantum cascade layer 30 from the side of the quantum cascade device 4. However, the thickness of the quantum cascade layer 30 in the stacking direction (Z direction) is, for example, 2 to 5 μm, making it difficult to align light incident on the quantum cascade layer 30 from the side. In contrast, in the quantum cascade device 4 according to the embodiment, light IL incident from the back surface of the second substrate 20 is converted by the photonic crystal PC into light propagating in a direction parallel to the surface of the first substrate 10. This eliminates the need for precise alignment of the incident light IL, and improves light detection sensitivity. Furthermore, by arranging the photonic crystal PC so that it is in contact with the quantum cascade layer 30, light receiving sensitivity can be further improved.

[0064] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, and are also included in the scope of the invention and its equivalents as defined in the claims. [Explanation of symbols]

[0065] 1, 2, 3, 4...quantum cascade element, 10...first substrate, 10B, 20B...rear surface, 10F, 20F, 30F...front surface, 20...second substrate, 21...recess, 22...dielectric film, 23...metal film, 25...etching mask, 25a...opening, 30...quantum cascade layer, 31...quantum well layer, 33...barrier layer, 40...first electrode, 41...metal wire, 50...second electrode, 60...heat sink, 63...metal terminal, 65...insulating member, DBR...Bragg reflector multilayer film, IL...light, LL...laser light, MR...metal film, Nox...native oxide film, PC...photonic crystal

Claims

1. a first substrate; a quantum cascade layer provided on the surface of the first substrate, the quantum cascade layer including a plurality of quantum well layers and a plurality of barrier layers alternately stacked in a direction perpendicular to the surface of the first substrate; a second substrate bonded to the quantum cascade layer and including a photonic crystal having a recess provided on a bonding surface in contact with an uppermost layer of the plurality of barrier layers; Equipped with the quantum cascade layer and the photonic crystal are in contact with each other via an oxide film; The quantum cascade device, wherein the oxide film is not provided inside the recess.

2. an electrode for passing a current through the quantum cascade layer; 2. The quantum cascade device according to claim 1, which emits laser light due to intersubband transition in the quantum cascade layer.

3. The quantum cascade device according to claim 2 , further comprising a reflective layer made of a multilayer film laminated on the surface of the second substrate opposite to the bonding surface.

4. The quantum cascade device according to claim 3 , further comprising a metal film provided on the reflective layer to reflect light emitted from the quantum cascade layer.

5. 10. The quantum cascade device according to claim 1, further comprising an electrode for detecting a current generated by light absorption in the quantum cascade layer.

6. the first substrate comprises indium phosphide; 6. The quantum cascade device according to claim 1, wherein the second substrate comprises silicon.

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