Quantum cascade laser

Abstract

To provide a quantum cascade laser capable of simplifying a structure and improving a strength.SOLUTION: A quantum cascade laser 1 includes: a semiconductor substrate 2 having a first surface 2a and a second surface 2b; a semiconductor stacked body 3 that includes an active layer 31 having a cascade structure and is formed on the first surface 2a of the semiconductor substrate 2; a first electrode 4 formed on a surface 3b of the semiconductor stacked body 3; and a second electrode 5 formed on the second surface 2b of the semiconductor substrate 2. The semiconductor substrate 2 is an InP substrate with a carrier density of 1×1017 cm-3 or less. A concave portion 21 that passes through the semiconductor substrate 2 and reaches the semiconductor stacked body 3 is formed in the second surface 2b of the semiconductor substrate 2. The second electrode 5 is continuously formed on the second surface 2b of the semiconductor substrate 2, on side surfaces 22 of the concave portion 21, and on an exposed surface 3a of the semiconductor stacked body 3 which is exposed from the semiconductor substrate 2 at a bottom 21a of the concave portion 21.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to quantum cascade lasers. [Background technology]

[0002] For example, in the quantum cascade laser described in Patent Document 1, a semiconductor laminate including an active layer is formed on a semiconductor substrate. In this quantum cascade laser, a pair of end faces of the active layer form a resonator that oscillates light of a first frequency and light of a second frequency, and a terahertz wave of the difference frequency between the first frequency and the second frequency is generated by difference frequency generation. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2021 / 125240 Summary of the Invention [Problem to be solved by the invention]

[0004] The quantum cascade laser described above may require a simplified structure and improved strength. Therefore, an object of the present invention is to provide a quantum cascade laser that can have a simplified structure and improved strength. [Means for solving the problem]

[0005] The quantum cascade laser of the present invention comprises: [1] "a semiconductor laminate including a semiconductor substrate having a first surface and a second surface opposite to the first surface; and an active layer having a cascade structure, the semiconductor laminate being formed on the first surface of the semiconductor substrate, the active layer generating and oscillating light of a first frequency and light of a second frequency, and generating a terahertz wave of a difference frequency between the first frequency and the second frequency by difference frequency generation; a first electrode formed on the surface of the semiconductor laminate opposite to the semiconductor substrate; and a second electrode formed on the second surface of the semiconductor substrate, the semiconductor substrate having a carrier density of 1×10 17 cm -3 a quantum cascade laser comprising an InP substrate as described below, a recess formed in the second surface of the semiconductor substrate that passes through the semiconductor substrate and reaches the semiconductor laminate, and the second electrode formed on the second surface of the semiconductor substrate, on a side surface of the recess, and on an exposed surface of the semiconductor laminate that is exposed from the semiconductor substrate at the bottom of the recess.

[0006] In this quantum cascade laser, the carrier density is 1×10 17 cm -3 A semiconductor laminate including an active layer is formed on a semiconductor substrate made of an InP substrate as described below. Since such a semiconductor substrate does not easily absorb terahertz waves, this quantum cascade laser allows terahertz waves generated by difference frequency generation to be transmitted through the semiconductor substrate and emitted. On the other hand, 17 cm -3Because the InP substrate described below has insulating or semi-insulating properties, when such a semiconductor substrate is used, a pair of electrodes (anode and cathode) has been provided on the first surface side (semiconductor laminate side) of the semiconductor substrate. However, in this case, the configuration of the first surface side may become complicated. In contrast, in this quantum cascade laser, a second electrode is formed on the second surface of the semiconductor substrate. More specifically, a recess that penetrates the semiconductor substrate to reach the semiconductor laminate is formed on the second surface of the semiconductor substrate, and the second electrode is formed on the second surface of the semiconductor substrate, on the side surfaces of the recess, and on the exposed surface of the semiconductor laminate exposed from the semiconductor substrate at the bottom of the recess. By providing the second electrode on the second surface side of the semiconductor substrate in this manner, the configuration of the first surface side can be simplified compared to when both the first electrode and the second electrode are provided on the first surface side. Furthermore, since the second electrode is formed on the second surface of the semiconductor substrate, on the side surfaces of the recess, and on the exposed surface of the semiconductor laminate, the strength of the device can be improved. Therefore, this quantum cascade laser can simplify the structure and improve the strength.

[0007] The quantum cascade laser of the present invention may be [2] "the quantum cascade laser according to [1], wherein the side surface of the recess includes a first inclined surface that is inclined with respect to the stacking direction of the semiconductor stack so as to approach the outer edge of the semiconductor substrate as it moves away from the semiconductor stack." In this case, it is possible to prevent a step (fracture at a step portion) from occurring in the second electrode formed over the second surface of the semiconductor substrate, the side surface of the recess, and the exposed surface of the semiconductor stack.

[0008] The quantum cascade laser of the present invention may be [3] "the quantum cascade laser according to [2], wherein the semiconductor substrate further has a substrate end face connecting the first surface and the second surface, the substrate end face including an inclined end face inclined with respect to the stacking direction so as to face away from the semiconductor stack, and the inclination angle of the first inclined face with respect to the stacking direction is larger than the inclination angle of the inclined end face with respect to the stacking direction." In this case, the substrate end face can be used as an emission surface from which terahertz waves are emitted. Also, the occurrence of step discontinuities in the second electrode can be further suppressed.

[0009] The quantum cascade laser of the present invention may be [4] "the quantum cascade laser according to [3], wherein the length of the exposed surface of the semiconductor laminate in the oscillation direction of the light of the first frequency and the light of the second frequency in the active layer is longer than the length of the inclined end face in the oscillation direction." In this case, the second electrode can be in contact with the semiconductor laminate over a long distance in the oscillation direction, and the second electrode and the semiconductor laminate can be well electrically connected.

[0010] The quantum cascade laser of the present invention may be [5] "the quantum cascade laser according to any one of [2] to [4], wherein the first inclined surface is formed on one of the side surfaces of the recess from which the terahertz waves are emitted." In this case, it is possible to ensure a large size (length) along the oscillation direction of the portion of the semiconductor laminate from which the terahertz waves are emitted. By using this portion as a transmission path for the terahertz waves generated in the active layer, the terahertz waves can be emitted satisfactorily.

[0011] The quantum cascade laser of the present invention may be [6] "the quantum cascade laser according to any one of [1] to [5], wherein the side surface of the recess includes a second inclined surface that is inclined with respect to the stacking direction of the semiconductor stack so as to move away from the outer edge of the semiconductor substrate as it moves away from the semiconductor stack." In this case, the formation of the second inclined surface makes it possible to widen the exposed surface of the semiconductor stack. As a result, the second electrode can be brought into contact with the semiconductor stack over a wide area, and good electrical connection can be achieved between the second electrode and the semiconductor stack.

[0012] The quantum cascade laser of the present invention may be [7] "the quantum cascade laser according to any one of [1] to [6], wherein the side surface of the recess includes a first inclined surface inclined with respect to the stacking direction of the semiconductor stack so as to approach the outer edge of the semiconductor substrate as it moves away from the semiconductor stack, and a second inclined surface inclined with respect to the stacking direction of the semiconductor stack so as to move away from the outer edge of the semiconductor substrate as it moves away from the semiconductor stack, and the inclination angle of the first inclined surface with respect to the stacking direction is larger than the inclination angle of the second inclined surface with respect to the stacking direction." In this case, it is possible to further suppress the occurrence of a discontinuity in the second electrode.

[0013] The quantum cascade laser of the present invention may be [8] "the quantum cascade laser according to any one of [1] to [7], wherein the semiconductor laminate is formed on the semiconductor substrate so that a plurality of the active layers are arranged in a direction perpendicular to the lamination direction of the semiconductor laminate, and the recess is formed so as to overlap with the plurality of active layers when viewed from the lamination direction." In this case, a quantum cascade laser having a plurality of active layers can be successfully configured.

[0014] The quantum cascade laser of the present invention may be [9] "the quantum cascade laser according to any one of [1] to [8], wherein the surface of the first electrode opposite to the semiconductor laminate is flat." In this case, epi-side-down mounting can be successfully performed, in which the quantum cascade laser is mounted on a mounting target so that the first electrode faces the mounting surface of the mounting target. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a quantum cascade laser that can have a simplified structure and improved intensity. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a front view of a quantum cascade laser according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view of the quantum cascade laser taken along line II-II in FIG. [Figure 3] FIG. 2 is a diagram showing an example of the configuration of an active layer of a quantum cascade laser. [Figure 4] FIG. 10 is a perspective view of a quantum cascade laser according to a modified example, viewed from one side in the Z direction. [Figure 5] FIG. 10 is a perspective view of the quantum cascade laser according to the modified example, viewed from the other side in the Z direction. [Figure 6] FIG. 10 is a front view of a quantum cascade laser according to a modified example. [Figure 7] 6 is a side view of a quantum cascade laser according to a modified example taken along line VI-VI in FIG. 5. FIG. [Figure 8] FIG. 10 is a plan view of a quantum cascade laser according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0017] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description, the same or equivalent elements are designated by the same reference numerals, and redundant description will be omitted.

[0018] 1 and 2, the quantum cascade laser 1 includes a semiconductor substrate 2 (hereinafter also referred to as substrate 2), a semiconductor stack 3 (hereinafter also referred to as stack 3), a first electrode 4, and a second electrode 5. Hereinafter, the width direction of the substrate 2 will be referred to as the X direction, the length direction of the substrate 2 will be referred to as the Y direction, and the thickness direction of the substrate 2 will be referred to as the Z direction. As will be described later, the Y direction is the oscillation direction of light in the active layer 31 included in the stack 3, and the Z direction is the stacking direction in which multiple layers constituting the stack 3 are stacked. The quantum cascade laser 1 outputs output light LT which is a terahertz wave.

[0019] The substrate 2 is a rectangular plate-shaped InP substrate (single crystal substrate of indium phosphide). The carrier density in the substrate 2 is 1×10 17 cm -3 or less, and the substrate 2 has insulating or semi-insulating properties. The substrate 2 may be an undoped InP substrate that is not doped with impurities, or may be a lightly doped InP substrate that is doped with impurities (e.g., Fe) at a low concentration. The substrate 2 is transparent to the output light LT. The carrier density in the substrate 2 is 1×10 16 cm -3 In this case, the substrate 2 hardly absorbs the output light LT (terahertz waves). The carrier density in the substrate 2 may be 1×10 16 cm -3 From 1×10 17 cm -3 As the temperature approaches , the absorption of the output light LT in the substrate 2 increases.

[0020] The substrate 2 has a first surface 2a (front surface), a second surface 2b (back surface) opposite to the first surface 2a, and a substrate end surface 2c connecting the first surface 2a and the second surface 2b. The first surface 2a and the second surface 2b are, for example, flat surfaces perpendicular to the Z direction.

[0021] The substrate end surface 2c is the end surface of the substrate 2 in the Y direction, and more specifically, the end surface on the side from which the output light LT is emitted (the right side in FIG. 2). The substrate end surface 2c includes a vertical surface 2d perpendicular to the Y direction (parallel to the Z direction) and an inclined end surface 2e inclined with respect to the Z direction. The vertical surface 2d is connected to the first surface 2a, and the inclined end surface 2e is connected to the second surface 2b. The inclined end surface 2e is inclined with respect to the Z direction so as to face away from the laminate 3. In other words, the inclined end surface 2e is inclined so as to move inward of the substrate 2 as it approaches the second surface 2b. Here, the "inner side" refers to the center side of the substrate 2 in a plan view (when viewed from the Z direction). In the quantum cascade laser 1, the output light LT is emitted from the inclined end surface 2e. In other words, the inclined end surface 2e is the emission surface of the quantum cascade laser 1.

[0022] A recess 21 (through hole) is formed on the second surface 2b of the substrate 2. The recess 21 penetrates the substrate 2 and reaches the laminate 3. As a result, at the bottom 21a of the recess 21, an exposed surface 3a of the laminate 3 is exposed from the substrate 2.

[0023] The side surface 22 of the recess 21 includes a pair of first inclined surfaces 22a and a pair of second inclined surfaces 22b. The pair of first inclined surfaces 22a are side surfaces in the Y direction and face each other in the Y direction. One of the first inclined surfaces 22a is formed on the side surface of the side surface 22 of the recess 21 on the side from which the output light LT is emitted (the right side in FIG. 2). The pair of second inclined surfaces 22b are side surfaces in the X direction and face each other in the X direction. Each first inclined surface 22a is inclined with respect to the Z direction so as to approach the outer edge of the substrate 2 as it moves away from the laminate 3. That is, the pair of first inclined surfaces 22a are formed in a forward tapered shape so as to approach each other as it moves closer to the laminate 3. Each second inclined surface 22b is inclined with respect to the Z direction so as to move away from the outer edge of the substrate 2 as it moves away from the laminate 3. That is, the pair of second inclined surfaces 22b are formed in an inverse tapered shape so that the closer they are to the stacked body 3, the further apart they become from each other.

[0024] In this example, the inclination angle θ1 of the first inclined surface 22a relative to the Z direction is greater than the inclination angle θ3 of the inclined end surface 2e relative to the Z direction. The inclination angle θ1 of the first inclined surface 22a relative to the Z direction is greater than the inclination angle θ2 of the second inclined surface 22b relative to the Z direction. The length L3a of the exposed surface 3a of the laminate 3 in the Y direction is greater than the length L2e of the inclined end surface 2e in the Y direction. As an example, the inclination angle θ1 of the first inclined surface 22a is approximately 60°, and the inclination angle θ2 of the second inclined surface 22b is approximately 20°. Such an inclined surface can be formed, for example, by anisotropic etching that utilizes differences in etching rate depending on the crystal plane of the substrate 2. That is, the laminate 3 is formed by determining the formation direction (the light oscillation direction in the active layer 31) so that the orientation of the crystal plane of the substrate 2 is along the desired formation direction of the first inclined surface and the second inclined surface, and then etching is performed from the second surface 2b side, thereby obtaining a surface with the above inclination angle. For example, the inclination angle θ3 of the inclined end face 2e is approximately 10°. The inclined end face 2e is formed by, for example, polishing.

[0025] The laminate 3 is formed on the first surface 2a of the substrate 2. The laminate 3 has an active layer 31, a first cladding layer 32, a second cladding layer 33, a first guide layer 34, a second guide layer 35, a contact layer 36, and a support layer 37. The contact layer 36, the second cladding layer 33, the second guide layer 35, the active layer 31, the first guide layer 34, and the first cladding layer 32 are stacked in this order on the first surface 2a of the substrate 2. The support layer 37 is formed to sandwich the active layer 31, the first guide layer 34, and the second guide layer 35 in the X direction. The first cladding layer 32 and the second cladding layer 33 are a pair of cladding layers that sandwich the active layer 31 in the Z direction. Each layer constituting the laminate 3 is formed on the substrate 2 by crystal growth using, for example, a metalorganic vapor phase epitaxy (MOVPE) method, a molecular beam epitaxy (MBE) method, or the like. A first electrode 4 is formed on a surface 3b of the laminate 3 opposite to the substrate 2.

[0026] An example of the configuration of the laminated body 3 will be described. The contact layer 36 is made of InGaAs (Si doping concentration: 1.5×10 18 cm -3) and has a thickness of about 400 nm. The contact layer 36 is formed on the first surface 2a of the substrate 2. The first cladding layer 32 and the second cladding layer 33 are each made of InP (Si doping concentration: 1.5×10 16 cm -3 The first guide layer 34 is made of InGaAs (Si doping concentration: 1.5×10 16 cm -3 The second guide layer 35 is made of InGaAs (Si doping concentration: 1.5×10 16 cm -3 ) and has a thickness of about 250 nm. The support layer 37 is an InP layer doped with Fe, and is disposed between the first cladding layer 32 and the second cladding layer 33 on both sides of the active layer 31, the first guide layer 34, and the second guide layer 35 in the X direction. A contact layer may be formed between the first cladding layer 32 and the first electrode 4. The contact layer is made of InGaAs (Si doping concentration: 1.5×10 18 cm -3 ) and may have a thickness of about 15 nm.

[0027] The first guide layer 34 has a diffraction grating structure that functions as a distributed feedback (DFB) structure formed along the Y direction, which is the oscillation direction of the first pump light and the second pump light (details will be described later). The first guide layer 34 includes, as the diffraction grating structure, a first diffraction grating structure 34a and a second diffraction grating structure 34b that are aligned in the Y direction. The first diffraction grating structure 34a causes the first pump light to oscillate in a single mode, and the second diffraction grating structure 34b causes the second pump light to oscillate in a single mode. The first diffraction grating structure 34a and the second diffraction grating structure 34b are configured, for example, by forming a plurality of grooves extending in the X direction and aligned at a constant pitch in the Y direction. The pitch of these grooves differs between the first diffraction grating structure 34a and the second diffraction grating structure 34b.

[0028] The active layer 31 extends along the Y direction. The active layer 31 includes, for example, unit laminate structures stacked in multiple stages, and has a multiple quantum well structure. The multiple quantum well structure includes multiple well layers made of InGaAs and multiple barrier layers made of InAlAs. The active layer 31 has a cascade structure in which quantum well light-emitting layers used to generate light and electron injection layers used to inject electrons into the light-emitting layers are alternately stacked in multiple stages. More specifically, one period of a semiconductor laminate structure made of a light-emitting layer and an injection layer is used as a unit laminate structure, and these unit laminate structures are stacked in multiple stages to form the active layer 31 having a cascade structure. The number of unit laminate structures stacked is appropriately determined depending on the specific configuration, characteristics, etc. of the laser element.

[0029] 3, the active layer 31 is formed by stacking unit laminate structures, each including a quantum well light emitting layer 17 and an electron injection layer 18. One period of unit laminate structures is formed as a quantum well structure in which eleven quantum well layers 161-164, 181-187 and eleven quantum barrier layers 171-174, 191-197 are alternately stacked. For example, the quantum well layers are formed of InGaAs layers that are lattice-matched with the substrate 2 made of InP, and the quantum barrier layers are formed of InAlAs layers that are lattice-matched with the substrate 2.

[0030] A stacked portion consisting of the well layers 161-164 and the barrier layers 171-174 mainly functions as the quantum well light emitting layer 17. A stacked portion consisting of the well layers 181-187 and the barrier layers 191-197 mainly functions as the electron injection layer 18. Of the semiconductor layers of the quantum well light emitting layer 17, the first quantum barrier layer 171 functions as an injection barrier layer for electrons injected from the electron injection layer 18 into the quantum well light emitting layer 17. Of the semiconductor layers of the electron injection layer 18, the first quantum well layer 161 functions as an exit barrier layer for electrons from the quantum well light emitting layer 17 to the electron injection layer 18. The quantum well layer 161 does not necessarily have to function as an exit barrier layer.

[0031] When a bias is applied between the first electrode 4 and the second electrode 5, electron injection, electron radiative transition, and electron relaxation are repeated in the multiple unit laminate structures of the active layer 31, resulting in cascaded light generation. As electrons move in a cascaded manner through the multiple unit laminate structures, first pump light with a first frequency ω1 and second pump light with a second frequency ω2 are generated by intersubband radiative transition of electrons in each unit laminate structure. The generated first pump light and second pump light oscillate between a pair of end faces 31a of the active layer 31 in the Y direction. That is, the pair of end faces 31a of the active layer 31 form a resonator that oscillates the first pump light and the second pump light. The first pump light is oscillated in a single mode by the first diffraction grating structure 34a, and the second pump light is oscillated in a single mode by the second diffraction grating structure 34b. Then, by difference frequency generation (DFG) using Cherenkov phase matching, a terahertz wave (output light LT) having a difference frequency (|ω1-ω2|) between the first frequency ω1 and the second frequency ω2 is generated. For example, the first pump light and the second pump light are mid-infrared light, and the frequency range of the generated terahertz wave is 1 THz to 6 THz.

[0032] In the quantum cascade laser 1, Cherenkov phase matching is used to generate and output light of the difference frequency. Cherenkov phase matching is a pseudo-phase matching method in which the output light LT is emitted in a direction inclined with respect to the traveling direction (Y direction) of the first pump light and the second pump light. Therefore, in the quantum cascade laser 1, the output light LT is emitted from the inclined end face 2e inclined with respect to the Y direction. The output light LT passes through the substrate 2 and is emitted to the outside.

[0033] The first electrode 4 is a surface electrode formed on the surface 3b of the laminate 3 opposite to the substrate 2. The surface 3b may be formed of the first cladding layer 32 or the contact layer described above. The first electrode 4 is formed of a conductive metal material. An insulating layer 6 is formed on the surface 3b of the laminate 3. The insulating layer 6 is formed so as to expose the surface 3b at the center in the X direction, and the first electrode 4 is in contact with the laminate 3 at this exposed portion and is electrically connected thereto. The surface 4a of the first electrode 4 opposite to the laminate 3 is flat. This allows for good epi-side-down mounting, in which the quantum cascade laser 1 is mounted on a mounting target (e.g., a submount) so that the first electrode 4 faces the mounting surface of the mounting target.

[0034] The second electrode 5 is a back electrode formed on the second surface 2b of the substrate 2. The first electrode 4 is formed of a conductive metal material. The second electrode 5 is formed on the second surface 2b of the substrate 2, on the side surfaces 22 (first inclined surface 22a and second inclined surface 22b) of the recess 21, and on the exposed surface 3a of the laminate 3 exposed from the substrate 2 at the bottom 21a of the recess 21. The exposed surface 3a is formed, for example, by a contact layer 36. In this example, the second electrode 5 is formed on the entire second surface 2b, the entire side surfaces 22, and the entire exposed surface 3a. Since the output light LT is reflected by the second electrode 5 made of a metal material on the second surface 2b, the side surfaces 22, and the exposed surface 3a, the output light LT is not emitted from the second surface 2b, the side surfaces 22, and the exposed surface 3a. The second electrode 5 is in contact with and electrically connected to the laminate 3 (contact layer 36) on the exposed surface 3a. In order to ensure the strength of the quantum cascade laser 1, the thickness of the second electrode 5 is preferably, for example, 3 μm or more. [Action and effect]

[0035] In quantum cascade laser 1, the carrier density is 1×10 17 cm -3A laminate 3 including an active layer 31 is formed on a substrate 2 made of an InP substrate as described below. Since such a substrate 2 does not easily absorb terahertz waves, the quantum cascade laser 1 can transmit and emit terahertz waves (output light LT) generated by difference frequency generation through the substrate 2. On the other hand, when the carrier density is 1×10 17 cm -3 Because the InP substrate described below has insulating or semi-insulating properties, when such a semiconductor substrate is used, a pair of electrodes (anode and cathode) has been provided on the first surface side (semiconductor stack side) of the semiconductor substrate. However, in this case, the configuration on the first surface side may become complicated. For example, a separation groove must be formed in the semiconductor stack to electrically insulate the pair of electrodes, which may complicate the manufacturing process. Furthermore, the complicated configuration on the first surface side may make it unsuitable for first surface side mounting (epi-side down mounting) to a mounting target. In contrast, in the quantum cascade laser 1, a second electrode 5 is formed on the second surface 2b of the substrate 2. More specifically, a recess 21 that penetrates the substrate 2 and reaches the stack 3 is formed on the second surface 2b of the substrate 2, and the second electrode 5 is formed over the second surface 2b of the substrate 2, the side surface 22 of the recess 21, and the exposed surface 3a of the stack 3 that is exposed from the substrate 2 at the bottom 21a of the recess 21. By providing the second electrode 5 on the second surface 2b side of the substrate 2 in this manner, the configuration on the first surface 2a side can be simplified compared to when both the first electrode 4 and the second electrode 5 are provided on the first surface 2a side. Furthermore, since the second electrode 5 is formed over the second surface 2b of the substrate 2, the side surface 22 of the recess 21, and the exposed surface 3a of the laminate 3, the strength of the element can be improved. Therefore, according to the quantum cascade laser 1, the structure can be simplified and the strength can be improved.

[0036] In the quantum cascade laser 1, the stack 3 is formed thicker, mainly due to the thickness of the cladding layer, compared to other semiconductor lasers (for example, semiconductor lasers that emit near-infrared light). For example, the stack 3 has a thickness of about 10 μm. Therefore, the strength of the stack 3 is relatively high. Because the strength of the stack 3 is relatively high in this way, the quantum cascade laser 1 can maintain its strength even if a recess 21 is formed in the substrate 2 or the exposed surface 3 a is formed widely. Furthermore, the strength can be supplemented by the fact that the second electrode 5 is formed over the second surface 2 b of the substrate 2, the side surface 22 of the recess 21, and the exposed surface 3 a of the stack 3.

[0037] The side surface 22 of the recess 21 includes a first inclined surface 22a that is inclined with respect to the Z direction (the stacking direction of the laminate 3) so as to approach the outer edge of the substrate 2 with increasing distance from the laminate 3. This makes it possible to prevent the second electrode 5 formed on the second surface 2b of the substrate 2, the side surface 22 of the recess 21, and the exposed surface 3a of the laminate 3 from being discontinuous (fracture at the step portion).

[0038] The substrate end surface 2c of the substrate 2 includes an inclined end surface 2e inclined with respect to the Z direction so as to face away from the laminate 3, and the inclination angle θ1 of the first inclined surface 22a with respect to the Z direction is larger than the inclination angle θ3 of the inclined end surface 2e with respect to the Z direction. This allows the substrate end surface 2c to be used as an emission surface from which the output light LT is emitted. Also, the occurrence of step discontinuities in the second electrode 5 can be further suppressed.

[0039] The length L3a of the exposed surface 3a of the laminate 3 in the Y direction (the oscillation direction of the light of the first frequency (first pump light) and the light of the second frequency (second pump light) in the active layer 31) is longer than the length L2e of the inclined end face 2e in the Y direction. This allows the second electrode 5 to be in contact with the laminate 3 over a long distance in the Y direction, and allows for good electrical connection between the second electrode 5 and the laminate 3.

[0040] The first inclined surface 22a is formed on the side surface from which the output light LT is emitted, among the side surfaces 22 of the recess 21. This makes it possible to ensure a large size (length) along the oscillation direction of the portion of the laminate 3 from which the output light LT is emitted. By using this portion as a transmission path for the output light LT generated in the active layer 31, the output light LT can be emitted satisfactorily.

[0041] The side surface 22 of the recess 21 includes a second inclined surface 22b that is inclined with respect to the Z direction so as to move away from the outer edge of the substrate 2 as it moves away from the laminate 3. The formation of the second inclined surface 22b makes it possible to widen the exposed surface 3a of the laminate 3. As a result, the second electrode 5 can be brought into contact with the laminate 3 over a wide range, and the second electrode 5 and the laminate 3 can be electrically connected well.

[0042] The inclination angle θ1 of the first inclined surface 22a with respect to the Z direction is smaller than the inclination angle θ2 of the second inclined surface 22b with respect to the Z direction. This further reduces the occurrence of discontinuities in the second electrode 5. That is, first, the large inclination angle θ1 reduces the occurrence of discontinuities in the second electrode 5 during formation of the second electrode 5. Furthermore, for example, when the second electrode 5 is formed by vapor deposition from the back surface side (the lower side in FIG. 1 ), if the inclination angle θ2 of the second inclined surface 22b is large, the second inclined surface 22b may interfere with the vapor deposition, preventing the second electrode 5 from being properly formed up to the boundary between the second inclined surface 22b and the exposed surface 3a. However, in this embodiment, the small inclination angle θ2 reduces the occurrence of such a situation.

[0043] The surface 4a of the first electrode 4 opposite to the laminate 3 is flat. This allows for good epi-side-down mounting of the quantum cascade laser 1 onto the mounting target so that the first electrode 4 faces the mounting surface of the mounting target. [Variations]

[0044] As in the modified quantum cascade laser 1 shown in FIGS. 4 to 8, the laminate 3 may be formed on the substrate 2 so that a plurality of active layers 31 are aligned along the X direction (a direction perpendicular to the stacking direction of the laminate 3). In this modified example, the laminate 3 has a plurality (five in this example) of active layers 31 extending along the Y direction. The active layers 31 are aligned at regular intervals along the X direction. A plurality (five in this example) of first electrodes 4 corresponding to the active layers 31 are formed on the surface 3b of the laminate 3. The first electrodes 4 are arranged so as to overlap the active layers 31, respectively, in the Z direction. The recesses 21 are formed so as to overlap the active layers 31 in a planar view. That is, one recess 21 is formed in the substrate 2, and one recess 21 overlaps all of the active layers 31.

[0045] As with the above embodiment, this modification also simplifies the structure and improves the intensity. Furthermore, it is possible to favorably configure a quantum cascade laser 1 that includes a plurality of active layers 31. Such a quantum cascade laser 1 may be driven and used so that output light LT is generated in only one of the plurality of active layers 31, or may be driven and used so that output light LT is generated in two or more (for example, all) of the plurality of active layers 31.

[0046] The present invention is not limited to the above-described embodiment and modifications. For example, the materials and shapes of the components are not limited to those described above, and various materials and shapes can be adopted.

[0047] In the above embodiment, the first inclined surface 22a was formed on the side surface (side surface in the Y direction) of the side surface 22 of the recess 21 on the side from which the output light LT is emitted, but the first inclined surface 22a may be formed on the side surface of the recess 21 in the X direction. In the above embodiment, the second inclined surface 22b was formed on the side surface of the side surface 22 of the recess 21 in the X direction, but the second inclined surface 22b may be formed on the side surface of the recess 21 in the Y direction. The side surface 22 may not have the first inclined surface 22a. The side surface 22 may not have the second inclined surface 22b. For example, any or all of the side surfaces 22 may be surfaces parallel to the Z direction.

[0048] The inclination angle θ1 of the first inclined surface 22a may be less than the inclination angle θ3 of the inclined end surface 2e. The substrate end surface 2c of the substrate 2 may not include the inclined end surface 2e, and the entire substrate end surface 2c may be a vertical surface 2d. The substrate end surface 2c of the substrate 2 may not include the vertical surface 2d, and the entire substrate end surface 2c may be an inclined end surface 2e. The length L3a of the exposed surface 3a of the laminate 3 may be less than the length L2e of the inclined end surface 2e in the Y direction. The surface 4a of the first electrode 4 does not necessarily have to be flat. The second electrode 5 does not have to be formed over the entire second surface 2b, the entire side surface 22, or the entire exposed surface 3a. For example, it may be formed on a part of the second surface 2b or on a part of the exposed surface 3a. However, forming the second electrode 5 over these entire surfaces is advantageous in terms of both ensuring strength and electrical continuity. [Explanation of symbols]

[0049] 1...Quantum cascade laser, 2...Semiconductor substrate, 2a...First surface, 2b...Second surface, 2c...Substrate end face, 2e...Slanted end face, 3...Semiconductor stack, 3a...Exposed surface, 3b...Surface, 4... First electrode, 4a...surface, 5...second electrode, 21...recess, 21a...bottom, 22...side surface, 22a...first inclined surface, 22b...second inclined surface, 31...active layer, θ1, θ2, θ3... inclined angle.

Claims

1. a semiconductor substrate having a first surface and a second surface opposite the first surface; a semiconductor stack formed on the first surface of the semiconductor substrate, the semiconductor stack including an active layer having a cascade structure, the active layer generating and oscillating light of a first frequency and light of a second frequency, and generating a terahertz wave of a difference frequency between the first frequency and the second frequency by difference frequency generation; a first electrode formed on a surface of the semiconductor laminate opposite to the semiconductor substrate; a second electrode formed on the second surface of the semiconductor substrate; The semiconductor substrate has a carrier density of 1×10 １７ cm －３ An InP substrate having the following structure: a recessed portion is formed on the second surface of the semiconductor substrate, the recessed portion penetrating the semiconductor substrate and reaching the semiconductor laminate; A quantum cascade laser, wherein the second electrode is formed over the second surface of the semiconductor substrate, over the side surface of the recess, and over the exposed surface of the semiconductor stack exposed from the semiconductor substrate at the bottom of the recess.

2. 2. The quantum cascade laser according to claim 1, wherein the side surface of the recess includes a first inclined surface inclined with respect to a stacking direction of the semiconductor stack so as to approach an outer edge of the semiconductor substrate as it moves away from the semiconductor stack.

3. the semiconductor substrate further has a substrate end surface connecting the first surface and the second surface, the substrate end surface includes an inclined end surface inclined with respect to the stacking direction so as to face a side opposite to the semiconductor stacked body, The quantum cascade laser according to claim 2 , wherein an inclination angle of the first inclined surface with respect to the stacking direction is larger than an inclination angle of the inclined end surface with respect to the stacking direction.

4. 4. The quantum cascade laser according to claim 3, wherein a length of the exposed surface of the semiconductor laminate in the oscillation direction of the light of the first frequency and the light of the second frequency in the active layer is longer than a length of the inclined end face in the oscillation direction.

5. The quantum cascade laser according to claim 2 , wherein the first inclined surface is formed on one of the side surfaces of the recess, the side surface being on a side from which the terahertz wave is emitted.

6. 3. The quantum cascade laser according to claim 1, wherein the side surface of the recess includes a second inclined surface inclined with respect to the stacking direction of the semiconductor stack so as to move away from the outer edge of the semiconductor substrate as it moves away from the semiconductor stack.

7. the side surface of the recess includes a first inclined surface inclined with respect to the stacking direction of the semiconductor stack so as to approach the outer edge of the semiconductor substrate as it becomes farther away from the semiconductor stack, and a second inclined surface inclined with respect to the stacking direction of the semiconductor stack so as to become farther away from the outer edge of the semiconductor substrate as it becomes farther away from the semiconductor stack, The quantum cascade laser according to claim 1 , wherein an inclination angle of the first inclined surface with respect to the stacking direction is larger than an inclination angle of the second inclined surface with respect to the stacking direction.

8. the semiconductor laminate is formed on the semiconductor substrate such that a plurality of the active layers are arranged along a direction perpendicular to a lamination direction of the semiconductor laminate, 3. The quantum cascade laser according to claim 1, wherein the recess is formed so as to overlap with the plurality of active layers when viewed from the stacking direction.

9. The quantum cascade laser according to claim 1 , wherein the surface of the first electrode opposite to the semiconductor stack is flat.

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