Quantum cascade laser element

Abstract

A quantum cascade laser of the present application includes a semiconductor substrate, an active layer provided on the semiconductor substrate, and an upper cladding layer provided on the opposite side of the semiconductor substrate from the active layer, the upper cladding layer having an impurity doping concentration of less than 1 x 1016 cm-2. Each unit stack contained in the active layer has a first luminescent upper energy level, a second luminescent upper energy level, and at least one luminescent lower energy level in its subband energy level structure. The active layer is configured such that, in each unit stack, light having a center wavelength of 10 μm or more is generated by electron transition between at least two energy levels among the first luminescent upper energy level, the second luminescent upper energy level, and the at least one luminescent lower energy level in the luminescent layer. 17 cm ‑3 -2.

Description

Technical Field

[0001] This disclosure relates to a quantum cascade laser element. Background Technology

[0002] Mid-infrared light (e.g., wavelengths of 5–30 μm) has become an important band in spectral analysis techniques. Quantum cascade laser (QCL) elements, which are semiconductor lasers capable of outputting light in this band, are well known (e.g., see Non-Patent Documents 1–5).

[0003] (Non-Patent Document 1) Vitiello MS, Scalari G, Williams B, De Natale P. Quantum Cascade Lasers: Two Decades of Challenges. Optics Letters 2015 Feb 23; 23(4): 5167-5182.

[0004] (Non-Patent Document 2) Michel Rochat, Daniel Hofstetter, Mattias Beck, and Jerome Faist. Single-frequency quantum cascade laser based on bound state to continuous state transition at room temperature in the long wavelength range (λ≒16μm). Applied Physics Information 2001; 79:4271-73.

[0005] (Non-Patent Document 3) Fujita K, Yamanishi M, Edamura T, Sugiyama A, Furuta S. Extremely high T0 (~450K) of long-wavelength (~15μm), low-threshold-current-density quantum-cascade lasers based on the indirect pump scheme. Applied Physics Information 2010; 97:201109.

[0006] (Non-Patent Document 4) Huang X, Charles WO, Gmachl C. Low-threshold, temperature-insensitive long-wavelength (λ≒14μm) quantum cascade lasers. Optics Letters 2011; 19:8297-302.

[0007] (Non-Patent Document 5) Fuchs P, Semmel J, Friedl J, Hofling S, Koeth J, Worschech L, Forchel A. Distributed feedback quantum cascade laser on indium phosphide at 13.8 μm. Applied Physics Messages 2011; 98:211118. Summary of the Invention

[0008] As disclosed in Non-Patent Document 1, it is difficult to achieve high optical output efficiency (slope efficiency) in existing quantum cascade laser devices at center wavelengths above 10 μm. For example, Non-Patent Document 2 discloses a quantum cascade laser device with an active layer having a bound-state-to-continuum (BTC) structure, but its slope efficiency is around 20 mW / A. Alternatively, Non-Patent Document 3 discloses a quantum cascade laser device with an active layer having indirect injection excitation (iDP), but its slope efficiency is around 346 mW / A. Non-Patent Document 4 discloses a quantum cascade laser device with an active layer having a two-phonon-continuum structure, but its slope efficiency is around 375 mW / A. Non-Patent Document 5 discloses a quantum cascade laser device with an active layer having the same BTC structure as Non-Patent Document 1, but its slope efficiency is around 200 mW / A.

[0009] Therefore, one aspect of this disclosure is to provide a quantum cascade laser element that can effectively improve slope efficiency.

[0010] One aspect of this disclosure relates to a quantum cascade laser element comprising: a semiconductor substrate; an active layer disposed on the semiconductor substrate, having a cascade structure in which the light-emitting layer and the injection layer are alternately stacked in a multi-level stacked unit stack containing a light-emitting layer and an injection layer; and a first cladding layer disposed on the opposite side of the active layer from the semiconductor substrate side, having an impurity doping concentration of less than 1 × 10⁻⁶. 17 cm -3 Each unit stack in the active layer has a first luminescent upper energy level, a second luminescent upper energy level higher than the first luminescent upper energy level, and at least one luminescent lower energy level lower than the first luminescent upper energy level in its subband energy level structure; the active layer is configured to generate light with a center wavelength of 10 μm or more by means of inter-subband energy level transitions of electrons between at least two of the first luminescent upper energy level, the second luminescent upper energy level, and at least one luminescent lower energy level in the luminescent layer.

[0011] In the aforementioned quantum cascade laser device, the impurity doping concentration of the first cladding layer adjacent to the active layer is set to be less than 1 × 10⁻⁶. 17 cm -3By suppressing the impurity doping concentration in the first cladding layer, the amount of current absorbed into the first cladding layer due to the absorption of free carriers generated on the active layer can be effectively suppressed. On the other hand, when the impurity doping concentration in the first cladding layer is reduced, the conductivity decreases, resulting in a drawback where current has difficulty flowing through the first cladding layer to the active layer. Therefore, in the quantum cascade laser element 1 described above, to overcome this drawback, a sub-band level structure (so-called dual-upper-state design) is adopted as the active layer structure, having a first upper emission level, a second upper emission level, and at least one lower emission level. Compared to a structure that provides sufficient carriers to both of the two upper emission levels, this sub-band level structure can achieve a lower threshold current density. That is, by adopting a DAU structure, the decrease in current caused by reducing the impurity doping concentration in the upper cladding layer 33 can be tolerated. As described above, by employing a DAU structure and reducing the impurity doping concentration in the first cladding layer, the slope efficiency of light (light with a center wavelength of 10 μm or more) that is difficult to obtain in existing quantum cascade laser elements is effectively improved.

[0012] The thickness of the first coating layer can also be 5 μm or more. By sufficiently increasing the thickness of the first coating layer, the light generated in the active layer can be effectively confined within the active layer. Therefore, light loss in the active layer can be suppressed more effectively, further improving the slope efficiency.

[0013] The doping concentration of impurities in the semiconductor substrate can also be less than 1×10⁻⁶. 17 cm -3 Therefore, it is possible to effectively reduce the amount of light absorbed by free carriers in the semiconductor substrate. Consequently, it is possible to more effectively suppress light loss in the active layer, achieving further improvement in slope efficiency.

[0014] The aforementioned quantum cascade laser device may also have an impurity doping concentration of less than 1 × 10⁻⁶, which is disposed between the active layer and the semiconductor substrate. 17 cm -3 The second cladding layer. By providing a second cladding layer between the active layer and the semiconductor substrate, the light generated in the active layer can be effectively confined within the active layer. Furthermore, similar to the first cladding layer, the second cladding layer can more effectively suppress light loss in the active layer due to its lower impurity doping concentration, thus achieving further improvement in slope efficiency.

[0015] The thickness of the second coating layer can also be 5 μm or more. By sufficiently increasing the thickness of the second coating layer, the light generated in the active layer can be more effectively confined within the active layer. Therefore, light loss in the active layer 31 can be more effectively suppressed, resulting in further improvement in slope efficiency.

[0016] The aforementioned quantum cascade laser element may also include: a first electrode disposed on the side of the semiconductor substrate with an active layer and electrically connected to the first cladding layer, and a second electrode disposed on the opposite side of the first electrode, sandwiching the semiconductor substrate and electrically connected to it. The impurity doping concentration in the semiconductor substrate may also be 5 × 10⁻⁶. 15 cm -3 Above and less than 1×10 17 cm -3 In this case, by arranging electrodes (a first electrode and a second electrode) on both sides of the semiconductor substrate 2, current can flow through the semiconductor substrate to the active layer. Therefore, compared to a structure in which current flows only on the side of the component opposite to the active layer on the semiconductor substrate (a so-called side-contact structure), the fabrication process of the quantum cascade laser element can be simplified. Furthermore, by setting the doping concentration of impurities in the semiconductor substrate to 5 × 10⁻⁶... 15 cm -3 Above and less than 1×10 17 cm -3 It can suppress the amount of light absorption caused by the absorption of free carriers in the semiconductor substrate, while allowing the current necessary to drive the quantum cascade laser device to flow properly through the semiconductor substrate.

[0017] According to one aspect of this disclosure, a quantum cascade laser element that effectively improves slope efficiency can be provided. Attached Figure Description

[0018] Figure 1 This is a cross-sectional view of a quantum cascade laser element according to one embodiment.

[0019] Figure 2 For along Figure 1 A cross-sectional view of a quantum cascade laser element in line II-II.

[0020] Figure 3 This is a diagram illustrating an example of the subband level structure in the active layer of a quantum cascade laser element.

[0021] Figure 4 This diagram illustrates an example of the structure of a unit stack that constitutes the active layer.

[0022] Figure 5A diagram illustrating an example of the construction of a unit stack of one cycle in the active layer.

[0023] Figure 6 A graph illustrating the current-optical output characteristics of the quantum cascade laser element involved in the embodiment. Detailed Implementation

[0024] The embodiments of this disclosure will now be described in detail with reference to the accompanying drawings. Furthermore, in the description of the drawings, the same symbols are used to denote the same elements, and repeated descriptions are omitted. Also, the scale of the drawings may not necessarily be consistent with the description in the specification.

[0025] Structure of quantum cascade laser components

[0026] like Figure 1 and Figure 2 As shown, the quantum cascade laser element 1 includes a semiconductor substrate 2, a semiconductor laminate 3, a buried layer 4, a dielectric layer 5, a first electrode 6, and a second electrode 7.

[0027] Semiconductor substrate 2 is, for example, a rectangular plate-shaped S-doped InP single-crystal substrate. As an example, the length of semiconductor substrate 2 is approximately 3 mm, the width is approximately 500 μm, and the thickness is approximately several hundred μm. In the following description, the width direction of semiconductor substrate 2 is defined as the X-axis, the length direction as the Y-axis, and the thickness direction as the Z-axis. The side of semiconductor stack 3 relative to semiconductor substrate 2 in the Z-axis direction is designated as the first side S1, and the side of semiconductor substrate 2 relative to semiconductor stack 3 in the Z-axis direction is designated as the second side S2. Semiconductor substrate 2 is a low-doped InP substrate. For example, the doping concentration of the impurity (S in this example) in semiconductor substrate 2 is less than 1 × 10⁻⁶. 17 cm -3 As an example, the impurity doping concentration in semiconductor substrate 2 is 5 × 10⁻⁶. 16 cm -3 about.

[0028] A semiconductor stack 3 is formed on the surface 2a of the first side S1 of the semiconductor substrate 2. The semiconductor stack 3 contains an active layer 31 having a quantum cascade structure. The semiconductor stack 3 is configured to oscillate with a laser having a center wavelength of 10 μm or more. In this embodiment, the semiconductor stack 3 is constructed by sequentially stacking a lower cladding layer 32 (second cladding layer), a lower guide layer (not shown), an active layer 31, an upper guide layer (not shown), an upper cladding layer 33 (first cladding layer), and a contact layer (not shown) from one side of the semiconductor substrate 2. The upper guide layer has a diffraction grating structure that functions as a distributed feedback (DFB) structure.

[0029] An active layer 31 is disposed on a semiconductor substrate 2 via a lower cladding layer 32 and a lower guiding layer. The active layer 31 may have, for example, an InGaAs / InAlAs multiple quantum well structure. The thickness of the active layer 31 is, for example, 3–7 μm. As an example, the thickness of the active layer 31 is approximately 5.7 μm. The active layer 31 will be described in detail later.

[0030] The lower cladding layer 32 is, for example, a Si-doped InP layer. The lower cladding layer 32 is disposed on the semiconductor substrate 2 via a lower guide layer. That is, the lower cladding layer 32 is disposed between the active layer 31 and the semiconductor substrate 2. The lower cladding layer 32 is constructed as a thinner, less-doped InP layer than is typically. The thickness of the lower cladding layer 32 is, for example, 5 μm or more. As an example, the thickness of the lower cladding layer 32 is approximately 5 μm. Here, the thickness of the lower cladding layer 32 refers to the thickness of the portion of the lower cladding layer 32 between the semiconductor substrate 2 and the active layer 31 (i.e., the portion overlapping with the active layer 31 as observed from the Z-axis direction). The doping concentration of the impurity (Si in this example) in the lower cladding layer 32 is, for example, less than 1 × 10⁻⁶. 17 cm -3 As an example, the impurity doping concentration in the lower cladding layer 32 is 4 × 10⁻⁶. 16 cm -3 about.

[0031] The upper cladding layer 33 is, for example, a Si-doped InP layer. The upper cladding layer 33 is disposed on the surface 31a of the first side S1 of the active layer 31 via an upper guide layer. That is, the upper cladding layer 33 is disposed on the opposite side of the active layer 31 from the semiconductor substrate 2 side. Like the lower cladding layer 32, the upper cladding layer 33 is constructed as a thinner, less-doped InP layer than is typically. The thickness of the upper cladding layer 33 is, for example, 5 μm or more. As an example, the thickness of the upper cladding layer 33 is approximately 5 μm. The doping concentration of the impurity (Si in this example) in the upper cladding layer 33 is, for example, less than 1 × 10⁻⁶. 17 cm -3As an example, the impurity doping concentration in the upper cladding layer 33 is 4 × 10⁻⁶. 16 cm -3 about.

[0032] The lower guiding layer, disposed between the active layer 31 and the lower covering layer 32, is, for example, a Si-doped InGaAs layer. The thickness of the lower guiding layer is, for example, approximately 0.20 μm. The doping concentration of the impurity (Si in this example) in the lower guiding layer is, for example, 5 × 10⁻⁶. 16 cm -3 about.

[0033] The upper guiding layer disposed between the active layer 31 and the upper covering layer 33 is, for example, a Si-doped InGaAs layer. The thickness of the upper guiding layer is, for example, about 0.45 μm. The doping concentration of the impurity (Si in this example) in the upper guiding layer is, for example, 5 × 10⁻⁶. 16 cm -3 about.

[0034] The contact layer disposed on the surface of the first side S1 of the upper cladding layer 33 is, for example, a Si-doped InGaAs layer. The thickness of the contact layer is, for example, about 10 nm. The doping concentration of the impurity (Si in this example) in the contact layer is, for example, 3 × 10⁻⁶. 18 cm -3 about.

[0035] The semiconductor laminate 3 has a ridge 30 extending along the Y-axis. The ridge 30 is composed of a portion of the first side S1 of the lower cladding layer 32, a lower guiding layer, an active layer 31, an upper guiding layer, an upper cladding layer 33, and a contact layer. The width of the ridge 30 in the X-axis direction is narrower than the width of the semiconductor substrate 2 in the X-axis direction. The length of the ridge 30 in the Y-axis direction is equal to the length of the semiconductor substrate 2 in the Y-axis direction. As an example, the length of the ridge 30 is approximately 3 mm, the width of the ridge 30 is approximately 12 μm, and the thickness of the ridge 30 is approximately 11 μm. The ridge 30 is located at the center of the semiconductor substrate 2 in the X-axis direction. The layers constituting the semiconductor laminate 3 are not present on either side of the ridge 30 in the X-axis direction.

[0036] The ridge 30 has a top surface 30a and a pair of side surfaces 30b. The top surface 30a is the surface of the first side S1 of the ridge 30. The pair of side surfaces 30b are the surfaces of both sides of the ridge 30 in the X-axis direction. In this example, both the top surface 30a and the side surfaces 30b are flat surfaces. When viewed from the Y-axis direction... Figure 1 The center line CL is a straight line passing through the center (geometric center) of the ridge 30 and parallel to the Z-axis. When viewed from the Y-axis direction, the quantum cascade laser element 1 is line-symmetric about the center line CL.

[0037] The semiconductor laminate 3 has two end faces, namely a first end face 3a and a second end face 3b, of a ridge 30 in the optical waveguide direction A. The optical waveguide direction A is parallel to the extension direction of the ridge 30, i.e., the Y-axis direction. The first end face 3a and the second end face 3b function as light emission end faces. The first end face 3a and the second end face 3b are located on the same plane as the two end faces of the semiconductor substrate 2 in the Y-axis direction.

[0038] The buried layer 4 is, for example, a semiconductor layer formed from an Fe-doped InP layer. The buried layer 4 has a pair of first portions 41 and a pair of second portions 42. The pair of first portions 41 are formed on a pair of side surfaces 30b of the ridge 30. The pair of second portions 42 extend along the X-axis from the edge portions of the second side surfaces S2 of the pair of first portions 41. Each second portion 42 is formed on the surface 32a of the lower cladding layer 32. Surface 32a is the surface of the first side surface S1 of the portion of the lower cladding layer 32 that does not form the ridge 30. The thickness of the first portion 41 is, for example, about 1 to 2 μm. The thickness of the second portion 42 is, for example, about 3 μm.

[0039] In the Z-axis direction, the surface 42a of the first side S1 in each of the second portions 42 is located between the surface 31a of the first side S1 and the surface 31b of the second side S2 in the active layer 31. In other words, when viewed from the X-axis direction, a portion of the first side S1 in the second portion 42 coincides with a portion of the second side S2 in the active layer 31.

[0040] The dielectric layer 5 is, for example, an insulating layer made of a SiN film or a SiO2 film. The dielectric layer 5 is formed on the surface 42a of the outer portion 42c of the second portion 42, such that the top surface 30a of the ridge 30, the surface 41a of the first portion 41, and the surface 42a of the inner portion 42b of the second portion 42 are exposed from the dielectric layer 5. The inner portion 42b is the part of the second portion 42 that is connected to the first portion 41, and the outer portion 42c is the part of the second portion 42 that is located further outward in the X-axis direction than the inner portion 42b.

[0041] The dielectric layer 5 is formed on the surface 42a of the outer portion 42c, but not on the surface 42a of the inner portion 42b. Therefore, the surface 42a of the inner portion 42b is exposed. In other words, an opening 5a is formed in the dielectric layer 5 to expose the inner portion 42b. The opening 5a exposes the top surface 30a of the ridge 30, the surface 41a of the first portion 41, and the surface 42a of the inner portion 42b of the second portion 42 from the dielectric layer 5. The outer edge of the dielectric layer 5 also reaches the outer edge of the buried layer 4 in either the X-axis or Y-axis direction. The dielectric layer 5 also functions as a bonding layer to improve the adhesion between the buried layer 4 and the metal layer 61 described below.

[0042] The width of the opening 5a in the X-axis direction is, for example, more than twice the width of the active layer 31 in the X-axis direction. The width of the opening 5a in the X-axis direction can also be more than five times the width of the active layer 31 in the X-axis direction. As an example, the width of the opening 5a in the X-axis direction is approximately 50 μm, and the width of the active layer 31 in the X-axis direction is approximately 9 μm. Furthermore, in Figure 1 In this case, although the width of the active layer 31 is a fixed value, the width of the active layer 31 can also gradually narrow from the second side S2 to the first side S1 to form a cone shape. In this case, the "width of the active layer 31 in the X-axis direction" mentioned above refers to the width of the end of the first side S1 of the active layer 31.

[0043] The width of the opening 5a in the X-axis direction can also be more than 10 times the thickness of the embedded layer 4 in the Z-axis direction. Here, "thickness of the embedded layer 4" refers to the thicker of the first part 41 and the second part 42, which in this example is the thickness of the second part (approximately 3 μm). That is to say, the width of the opening 5a can also be more than 10 times the thickness of the second part 42.

[0044] The first electrode 6 has a metal layer 61 and a plating layer 62. The metal layer 61 is, for example, a Ti / Au layer, which serves as a base layer for forming the plating layer 62. The plating layer 62 is formed on the metal layer 61. The plating layer 62 is, for example, an Au plating layer. The thickness of the first electrode 6 in the Z-axis direction is, for example, 6 μm or more.

[0045] A metal layer 61 is integrally formed on the top surface 30a of the ridge 30 and extends across the first portion 41 and the second portion 42 of the buried layer 4. The metal layer 61 is in contact with the top surface 30a of the ridge 30. Therefore, the first electrode 6 is electrically connected to the upper cladding layer 33 via the contact layer. In either the X-axis or Y-axis direction, the outer edge of the metal layer 61 is located inside the outer edges of the buried layer 4 and the dielectric layer 5. The distance between the outer edge of the metal layer 61 and the outer edge of the dielectric layer 5 (the outer edges of the semiconductor substrate 2, the semiconductor stack 3, and the buried layer 4) in the X-axis direction is, for example, about 50 μm.

[0046] The metal layer 61 is formed directly on the first portion 41. That is, no other layer (e.g., dielectric layer or insulating layer) is formed between the metal layer 61 and the first portion 41. The metal layer 61 is formed over the entire surface 41a of the first portion 41.

[0047] The metal layer 61 is on the inner portion 42b of the second portion 42, and contacts the surface 42a of the inner portion 42b via an opening 5a formed in the dielectric layer 5. The metal layer 61 is formed on the outer portion 42c of the second portion 42 via the dielectric layer 5. That is, the dielectric layer 5 is disposed between the outer portion 42c of the second portion 42 and the first electrode 6. When viewed from the Z-axis direction, the outer edge of the first electrode 6 is located further inward than the outer edges of the semiconductor substrate 2, the semiconductor stack 3, the buried layer 4, and the dielectric layer 5.

[0048] The surface 62a of the first side S1 in the plating layer 62 is electrically connected to multiple wires 8. Each wire 8 is formed, for example, by wire bonding, and is electrically connected to the metal layer 61 via the plating layer 62. When viewed from the Z-axis direction, the connection position between the first electrode 6 (plating layer 62) and each wire 8 overlaps with the dielectric layer 5. Furthermore, the number of wires 8 is not limited, and only one wire 8 may be provided.

[0049] The second electrode 7 is formed on the surface 2b of the second side S2 in the semiconductor substrate 2. The second electrode 7 is, for example, an AuGe / Au film, an AuGe / Ni / Au film, or an Au film. The second electrode 7 is electrically connected to the lower cladding layer 32 via the semiconductor substrate 2.

[0050] In the quantum cascade laser element 1, a bias voltage is applied to the active layer 31 via the first electrode 6 and the second electrode 7, causing light to be emitted from the active layer 31. Light with a predetermined center wavelength resonates in the distributed feedback structure. Therefore, laser light with a predetermined center wavelength is emitted from each of the first end face 3a and the second end face 3b. Alternatively, a high-reflectivity film can be formed on one end face of the first end face 3a and the second end face 3b. In this case, laser light with a predetermined center wavelength is emitted from the other end face of the first end face 3a and the second end face 3b. Alternatively, a low-reflectivity film can also be formed on one end face of the first end face 3a and the second end face 3b. Furthermore, a high-reflectivity film can be formed on another end face different from the one on which the low-reflectivity film is formed. In either of these cases, laser light with a predetermined center wavelength is emitted from one end face of the first end face 3a and the second end face 3b. In the former case, laser light is emitted from both ends of the first end face 3a and the second end face 3b.

[0051] The quantum cascade laser element 1 and the driving unit that drives the quantum cascade laser element 1 together constitute a quantum cascade laser device. The driving unit is electrically connected to the first electrode 6 and the second electrode 7. The driving unit is, for example, a pulse driving unit that drives the quantum cascade laser element 1 so that the quantum cascade laser element 1 oscillates laser pulses.

[0052] [Structure of the active layer]

[0053] likeFigure 3 As shown, the active layer 31 has a cascaded structure of multiple stacked unit stacks 16. Each unit stack 16 contains a light-emitting layer 17 that generates light and an injection layer 18 that transfers electrons from the light-emitting layer 17. The light-emitting layer 17 is the part that primarily performs the function of generating light. The injection layer 18 is the part that primarily performs the function of electron transfer, which refers to injecting electrons from the light-emitting layer 17 into the upper energy level of the light-emitting layer 17 in the subsequent unit stack 16. Both the light-emitting layer 17 and the injection layer 18 have quantum well structures formed by alternating quantum well layers and barrier layers. Therefore, a sub-band energy level structure based on the quantum well structure is formed in each unit stack 16.

[0054] The unit stack 16 has a subband structure with a combined dual-upper-state design (DAU) and two luminescent upper levels. Figure 3 In the example, the unit stack 16 has a subband level structure comprising a DAU / MS (dual-upper-state to multiple lower state) structure containing two emitting upper levels and multiple (here, three) emitting lower levels. The unit stack 16 has a first emitting upper level (level 4) L in this subband level structure. up1 (L4) and the second luminous superior level (level 5) which is a higher energy level than the first luminous superior level. up2 (L5). Furthermore, the unit stack 16 in this sub-band energy level structure also possesses a first luminescent lower energy level (level 1) that is lower than the first luminescent upper energy level. low1 (L1), the second luminescent lower level (level 2) is lower than the first luminescent upper level and higher than the first luminescent lower level. low2 (L2) A third luminescent lower level (level 3) with an energy level lower than the first luminescent upper level and higher than the second luminescent lower level. low3 (L3), and the mild level Lr, which is lower than these luminescent lower level energy levels.

[0055] An injection barrier is provided between the light-emitting layer 17 and the injection layer 18a of the preceding unit stack 16 to prevent electrons from being injected from the injection layer 18a into the light-emitting layer 17. A thin barrier layer, sufficient to allow sufficient wavefunction overflow, is provided between the light-emitting layer 17 and the injection layer 18. Furthermore, an exit barrier layer may also be provided between the light-emitting layer 17 and the injection layer 18 to prevent electrons from being extracted from the light-emitting layer 17 into the injection layer 18.

[0056] The spacing structure of each energy level in the sub-band energy level structure of the unit stack 16 is as follows. That is, from the first luminescent upper energy level L...up1 To the first luminescent lower level L low1 The energy ΔE of the luminescent transition (4→1) 41 and from the second luminescent upper energy level L up2 To the second luminescent lower level L low2 The energy ΔE of the luminescent transition (5→2) 52 The energy E1 of the pump light with the first frequency ω1 is approximately the same (ΔE) 41 =ΔE 52 =E1). Additionally, from the first luminescent upper energy level L... up1 To the second luminescent lower level L low2 The energy ΔE of the luminescent transition (4→2) 42 and the second luminescent upper energy level L up2 To the third lower energy level L low3 The energy ΔE of the luminescent transition (5→3) 53 The energy E2 of the pump light with the second frequency ω2 is approximately the same (ΔE) 42 =ΔE 53 =E2). Additionally, the first luminescent lower energy level L low1 With the second luminescent lower level L low2 Energy difference ΔE 21 Second luminescent lower energy level L low2 With the third luminescent lower energy level L low3 Energy difference ΔE 32 and the first luminescent upper energy level L up1 Second luminescent upper energy level L up2 Energy difference ΔE 54 The energy E = E1 - E2 of the terahertz wave with a difference frequency ω between the first frequency ω1 and the second frequency ω2 is approximately the same (ΔE). 21 =ΔE 32 =ΔE 54 =E). In this embodiment, the first frequency ω1 is greater than the second frequency ω2, and the difference frequency ω = ω1 - ω2.

[0057] In the aforementioned sub-band energy level structure, electrons are injected from the easing energy level Lr of the injection layer 18a of the preceding stage into the luminescent layer 17 via the injection barrier. Therefore, the second luminescent upper energy level L, which is combined with the easing energy level Lr, is... up2 It is strongly excited. At this time, through high-speed scattering processes such as electron-electron scattering, the first luminescent upper energy level L... up1 Sufficient electrons were also provided to the first luminescent upper energy level L. up1 and the second luminescent upper energy level L up2 Both were provided with sufficient charge carriers.

[0058] Injected into the first luminescent upper energy level Lup1 and the second luminescent upper energy level L up2 The electrons transition to the first luminescent lower level L, respectively. low1 Second luminescent lower energy level L low2 and the third luminescent lower energy level L low3 At this point, light equivalent to the energy difference between the sub-band energy levels of the upper and lower luminous energy levels is generated and released. Specifically, a first pump light with a first frequency ω1 and an energy of E1, and a second pump light with a second frequency ω2 and an energy of E2 are generated and released.

[0059] Transition to the first luminescent lower level L low1 Second luminescent lower energy level L low2 and the third luminescent lower energy level L low3 The electrons are moderated in the moderated energy level Lr. This is achieved through the transition from the first luminescent lower energy level Lr. low1 Second luminescent lower energy level L low2 and the third luminescent lower energy level L low3 Extract electrons at the first luminescent upper energy level L up1 and the second luminescent upper energy level L up2 First luminescent lower energy level L low1 Second light emission L low2 and the third light source L low3 Between these, an inverted distribution is formed to achieve laser oscillation. Electrons moderated at the moderated energy level Lr pass through the injection layer 18 to the first luminescent upper energy level L of the subsequent luminescent layer 17b. up1 and the second luminescent upper energy level L up2 Intermediate cascade injection. Furthermore, the moderate energy level Lr is not limited to being composed of only one energy level; it can also be composed of multiple energy levels, or even mini bands.

[0060] The aforementioned electron injection, electron luminescence transitions, and electron tempering are repeated in multiple unit stacks 16 constituting the active layer 31, resulting in cascaded light generation in the active layer 31. As electrons cascade through the multiple unit stacks 16, inter-band luminescence transitions of electrons in each unit stack 16 generate a first pump light with a first frequency ω1 and a second pump light with a second frequency ω2. Furthermore, a terahertz wave with a difference frequency ω (=|ω1-ω2|) between the first frequency ω1 and the second frequency ω2 is generated based on the difference frequency generated by Cherenkov phase matching.

[0061] Reference Figure 4 and Figure 5 The structure of the active layer 31 will be further explained. Furthermore, in Figure 4In the diagram, a portion of the repeating structure formed by the emitting layer 17 and the injection layer 18 is shown, illustrating the quantum well structure and subband energy level structure in its operating electric field.

[0062] In this structural example, the active layer 31 is constructed by stacking 70 cycles of unit stack 16, and the center wavelength of the gain in the active layer 31 is specified to be a wavelength of 10 μm or more. As an example, a unit stack 16 of one cycle is constructed as a quantum well structure by alternately stacking 11 quantum well layers (well layers 161-164, 181-187) and 11 quantum barrier layers (barrier layers 171-174, 191-197). Each well layer 161-164 and 181-187 is, for example, an InGaAs layer, and each barrier layer 171-174 and 191-197 is, for example, an InAlAs layer.

[0063] In the unit stack 16, the alternating stacked portions of the four well layers 161-164 and the four barrier layers 171-174 primarily function as the light-emitting layer 17, while the alternating stacked portions of the seven well layers 181-187 and the seven barrier layers 191-197 primarily function as the injection layer 18. The first-stage barrier layer 171 contained in the light-emitting layer 17 functions as the injection barrier layer. Furthermore, in this structural example, an extraction barrier layer that effectively functions as an extraction barrier is not disposed between the light-emitting layer 17 and the injection layer 18. In this structural example, the barrier layer 191 is defined as a formal extraction barrier layer, and the light-emitting layer 17 and the injection layer 18 before and after it are distinguished by function.

[0064] In active layer 31, to generate terahertz waves caused by difference frequency generation, it is necessary to generate pump light components of two wavelengths and to have a high second-order nonlinear resusceptibility χ(2) relative to the two pump light components. For example, by setting two diffraction grating layers in the upper guide layer, the generation of a first pump light with a first frequency ω1 and a second pump light with a second frequency ω2, as well as the generation of terahertz waves with difference frequency ω, can be achieved through a single active layer design. Furthermore, when realizing a quantum cascade laser element that outputs only either the first or second pump light, a diffraction grating layer corresponding to the first or second pump light can be set in the upper guide layer. In addition, these diffraction grating layers can also be set inside the cladding layer (e.g., the upper cladding layer 33).

[0065] like Figure 4 As shown, the subband energy level structure of this example is designed to enable electrons to optically transition from the strongly bound first luminescent upper energy level L4 and second luminescent upper energy level L5 to the first luminescent lower energy level L1, second luminescent lower energy level L2, and third luminescent lower energy level L3. In this example, the energy interval between the first luminescent upper energy level L4 and the second luminescent upper energy level L5 is ΔE.54 =18 meV. The energy interval between other energy levels is ΔE. 53 =121meV, ΔE 52 =136meV, ΔE 51 =149meV, ΔE 43 =102meV, ΔE 42 =117meV, ΔE 41 =131meV.

[0066] In this structural example, electrons injected from the injection layer 18 to the light-emitting layer 17 undergo high-speed electron-electron scattering at the first light-emitting upper energy level L4 and the second light-emitting upper energy level L5. 5中 With equal distribution, the first luminescent upper level L4 and the second luminescent upper level L5 operate like an extended single upper level. Therefore, the gain achieved by the transition of electrons from the first luminescent upper level L4 to the first luminescent lower level L1, the second luminescent lower level L2, and the third luminescent lower level L3, and the gain achieved by the transition of electrons from the second luminescent upper level L5 to the first luminescent lower level L1, the second luminescent lower level L2, and the third luminescent lower level L3, overlap with equal contribution, resulting in a broadband emission spectrum on a single peak.

[0067] In this way, the structure using a single active layer differs from the structure using multiple stacked active layers, resulting in uniform nonlinear optical properties across the entire area of ​​the active layer 31, achieving high-efficiency wavelength conversion. Assuming the carrier concentrations of the first lower emission level L1, the second lower emission level L2, the third lower emission level L3, the first upper emission level L4, and the second upper emission level L5 are n1 to n5 respectively, and assuming n1 = n2 = n3, if the condition n5 - n... i =1.0×10 15 / cm 3 n4-n i =1.3×10 15 / cm 3 (i = 1, 2, 3), representing the second-order nonlinear responsibility χ(i = 1, 2, 3) generated by the DAU. 2 The absolute value of the sum of ) is |χ( 2 )|=23.3nm / V.

[0068] When the quantum cascade laser element 1 is used as an optical element for outputting terahertz waves, the design frequency ωTHz, the first frequency ω1, and the second frequency ω2 are determined by the DFB configuration. The final terahertz wave is determined by ωTHz (=ω1-ω2). For example, the design frequency ωTHz is set to approximately 3THz. In this case, using a two-cycle DFB configuration allows the first frequency ω1 and the second frequency ω2 to operate in single-mode simultaneously, enabling single-mode operation of the terahertz wave. In this structural example, the configuration is such that the wavelength corresponding to at least one of the light at the first frequency ω1, the second frequency ω2, and the difference frequency ωTHz (terahertz wave) is 10 μm or more.

[0069] [Functions and Effects]

[0070] In the quantum cascade laser element 1, the doping concentration of the impurity (Si in this example) in the upper cladding layer 33 adjacent to the active layer 31 is less than 1 × 10⁻⁶. 17 cm -3 By suppressing the impurity doping concentration in the upper cladding layer 33, the amount of light absorbed by the upper cladding layer 33 due to free carrier absorption generated by the active layer 31 can be effectively suppressed. On the other hand, when the impurity doping concentration in the upper cladding layer 33 is reduced, there is a disadvantage that current is difficult to flow through the upper cladding layer 33 to the active layer 31 due to the reduced conductivity. Therefore, in the quantum cascade laser element 1, in order to overcome this disadvantage, a DAU structure as described above (DAU / MS structure as an example in this embodiment) is used as the active layer structure. In this subband energy level structure, a relatively low threshold current density can be achieved compared to a structure that provides sufficient carriers to both sides of the two emitting upper energy levels. That is, by using a DAU structure, the disadvantage caused by reducing the impurity doping concentration in the upper cladding layer 33 (i.e., the low amount of current flowing through the active layer 31) can be tolerated. As described above, by adopting the DAU structure and reducing the impurity doping concentration in the upper cladding layer 33, the slope efficiency of light (light with a center wavelength of 10 μm or more) that is difficult to obtain in existing quantum cascade laser elements is effectively improved.

[0071] Furthermore, the thickness of the upper coating layer 33 can also be 5 μm or more. By sufficiently increasing the thickness of the upper coating layer 33, the light generated in the active layer 31 can be effectively confined within the active layer 31. Therefore, light loss in the active layer 31 can be suppressed more effectively, further improving the slope efficiency.

[0072] Furthermore, the doping concentration of impurities (S in this example) in semiconductor substrate 2 can also be less than 1 × 10⁻⁶. 17 cm -3As described above, in this embodiment, a doping concentration of 5 × 10⁻⁶ is used. 16 cm -3 The left and right low-doped InP substrates serve as semiconductor substrates 2. By using this low-doped semiconductor substrate 2, the amount of light absorbed by free carriers in the semiconductor substrate 2 can be effectively reduced. Therefore, light loss in the active layer 31 can be suppressed more effectively, and slope efficiency can be further improved.

[0073] Furthermore, the quantum cascade laser element 1, disposed between the active layer 31 and the semiconductor substrate 2, may also have an impurity (Si in this example) doping concentration of less than 1 × 10⁻⁶. 17 cm -3 The lower cladding layer 32. By providing the lower cladding layer 32 between the active layer 31 and the semiconductor substrate 2, the light generated in the active layer 31 can be effectively confined within the active layer 31. In addition, the lower cladding layer 32, like the upper cladding layer 33, can more effectively suppress light loss in the active layer 31 by suppressing the impurity doping concentration, thereby achieving further improvement in slope efficiency.

[0074] Furthermore, the thickness of the lower coating layer 32 can also be 5 μm or more. By sufficiently increasing the thickness of the lower coating layer 32, the light generated in the active layer 31 can be more effectively confined within the active layer 31. Therefore, light loss in the active layer 31 can be more effectively suppressed, resulting in further improvement in slope efficiency.

[0075] Alternatively, the quantum cascade laser element 1 can be constructed using a so-called top-and-bottom conductive element. Specifically, the quantum cascade laser element 1 may have: a first electrode 6 disposed on the side opposite to the active layer 31 of the semiconductor substrate 2 (i.e., the first side S1) and electrically connected to the upper cladding layer 33; and a second electrode 7 disposed on the opposite side of the first electrode 6 (i.e., the second side S2) and electrically connected to the semiconductor substrate 2, sandwiching the semiconductor substrate 2. Furthermore, the impurity doping concentration in the semiconductor substrate 2 may be 5 × 10⁻⁶. 15 cm -3 Above and less than 1×10 17 cm -3In this case, by arranging electrodes (first electrode 6 and second electrode 7) on both sides (i.e., the first side S1 and the second side S2) sandwiching the semiconductor substrate 2, current can flow through the semiconductor substrate 2 to the active layer 31. Therefore, compared to the structure of a component that only allows current to flow to the side (i.e., the first side S1) where the active layer 31 of the semiconductor substrate 2 is disposed (so-called side contact structure), the fabrication process of the quantum cascade laser device can be simplified. Furthermore, by setting the doping concentration of the impurity (S in this example) in the semiconductor substrate 2 to 5 × 10⁻⁶, the process can be simplified. 15 cm -3 Above and less than 1×10 17 cm -3 It can suppress the amount of light absorption caused by the absorption of free carriers in the semiconductor substrate 2, while allowing the current necessary to drive the quantum cascade laser element 1 to flow appropriately through the semiconductor substrate 2.

[0076] Specifically, in the side contact structure, an electrode electrically connected to the lower cladding layer 32 is provided as an alternative electrode to the second electrode 7. To provide this electrode, a contact layer (e.g., a Si-doped InGaAs layer) needs to be provided between the semiconductor substrate 2 and the lower cladding layer 32. Furthermore, to electrically connect the contact layer and the electrode, contact holes need to be formed in a portion of the buried layer 4 and the dielectric layer 5 in the portion where the first electrode 6 is not provided. Moreover, in this side contact structure, both the first electrode 6 and the electrode (the electrode provided instead of the second electrode 7) are disposed on the first side S1 of the semiconductor substrate 2. Therefore, to prevent these electrodes from contacting each other, it is necessary to assemble the electrode components with high precision. On the other hand, as shown in this embodiment, by constructing the quantum cascade laser element 1 using an up-and-down conduction type element, the fabrication process can be simplified compared to the side contact structure described above. Additionally, according to the up-and-down conduction type, the driving voltage can be reduced compared to the side contact structure.

[0077] [Example]

[0078] Figure 6A graph illustrating the current-light output characteristics of the embodiment (i.e., the quantum cascade laser element 1 described above) is provided. Solid lines in the graph represent light output (W), and dashed lines represent driving voltage (V). Furthermore, in this embodiment, a highly reflective film is formed on one end face of the first end face 3a and the second end face 3b, and light is emitted from the other end face (emission surface) of the first end face 3a and the second end face 3b. Additionally, in this embodiment, a diffraction grating layer corresponding to a wavelength of 12.9 μm is provided in the semiconductor stack 3 (upper guiding layer), so that light with a center wavelength of 12.9 μm is emitted from the aforementioned emission surface. Furthermore, the structures and slope efficiencies disclosed in the comparative examples (the aforementioned non-patent documents 2-5) are shown below. Figure 6 As shown, compared with Comparative Examples 1 to 4 below, high slope efficiency (around 1 W / A) was confirmed in the examples.

[0079] [Comparative Example 1: Non-Patent Document 2]

[0080] • Active layer construction: BTC construction

[0081] • Thickness of the upper coating layer (InP): 1.75 μm

[0082] • Impurity doping concentration in the upper coating layer: 6 × 10 16 cm -3

[0083] • Thickness of the lower coating layer (InP): 0.6 μm

[0084] • Impurity doping concentration in the lower cladding layer: 6 × 10 16 cm -3

[0085] • Impurity doping concentration in the semiconductor substrate (InP): 1×10 17 cm -3

[0086] • Output light wavelength: approximately 16μm

[0087] • Slope efficiency: approximately 20 mW / A

[0088] [Comparative Example 2: Non-Patent Document 3]

[0089] • Active layer structure: iDP structure

[0090] • Thickness of the upper coating layer (InP): 5 μm

[0091] • Impurity doping concentration in the upper coating layer: 5 × 10 16 cm -3

[0092] • Thickness of the lower coating layer (InP): 5 μm

[0093] • Impurity doping concentration in the lower cladding layer: 5 × 10 16 cm -3

[0094] • Semiconductor substrate (InP): Impurity doping concentration: 1×10 18 cm -3 above

[0095] • Output light wavelength: approximately 14–15.5 μm

[0096] • Slope efficiency: 346mW / A

[0097] [Comparative Example 3: Non-Patent Document 4]

[0098] • Active layer: 2-phonon continuous state structure

[0099] • Thickness of the top coating layer (InP): 2.4 μm

[0100] • Impurity doping concentration in the upper coating layer: 5 × 10 16 cm -3

[0101] • Bottom cladding layer: None (the function of the bottom cladding layer is performed by a semiconductor substrate).

[0102] • Semiconductor substrate (InP): Impurity doping concentration: 1×10 17 cm -3

[0103] • Output light wavelength: approximately 14μm

[0104] • Slope efficiency: 375mW / A

[0105] [Comparative Example 4: Non-Patent Document 5]

[0106] • Active layer construction: BTC construction

[0107] • Thickness of the upper coating layer (InP): 4μm

[0108] • Impurity doping concentration in the upper coating layer: 5.5 × 10 16 cm -3

[0109] • Thickness of the lower coating layer (InP): 4 μm

[0110] • Impurity doping concentration in the lower cladding layer: 5.5 × 10⁻⁶ 16 cm -3

[0111] • Structure of the semiconductor substrate: unknown

[0112] • Output light wavelength: approximately 13.8μm

[0113] • Slope efficiency: approximately 200 mW / A

[0114] [Variation Example]

[0115] The above describes one embodiment of this disclosure, but this disclosure is not limited to the above embodiment. The materials and shapes of each structure are not limited to the materials and shapes described above, and various materials and shapes can be used.

[0116] For example, a diffraction grating layer can be provided in the semiconductor stack 3 (upper guiding layer), or three or more diffraction grating layers can be provided in the semiconductor stack 3. The diffraction grating layer, which functions as a distributed feedback structure, can cause at least one of the first pump light and the second pump light to oscillate in a single mode.

[0117] In addition, the active layer 31 is not limited to, for example Figures 3 to 5 The structure shown contains only one unit stack 16, but it can also contain two or more active layer structures (unit stacks). Additionally, the lower cladding layer 32 can be omitted. In this case, a portion of the semiconductor substrate 2 (the portion adjacent to the active layer 31) can also function as a cladding layer.

[0118] Furthermore, although the active layer 31 in the above embodiment is shown as a structure for lattice matching of an InP single-crystal substrate, the active layer 31 can also use a structure incorporating distortion compensation. Additionally, the semiconductor material system of the active layer 31 is not limited to InGaAs / InAlAs described above, and various semiconductor material systems such as GaAs / AlGaAs, InAs / AlSb, GaN / AlGaN, and SiGe / Si can also be used. Furthermore, various methods can be applied to the semiconductor crystal growth process.

[0119] Furthermore, the quantum cascade laser element is not limited to the top-and-bottom conduction type described in the above embodiments, and may also have a side-contact structure. In this case, a DAU structure can be used as the active layer structure, and by employing at least one of the structures of the upper cladding layer 33, the lower cladding layer 32, and the semiconductor substrate 2 (mainly in terms of thickness and doping concentration), the same effect as the quantum cascade laser element 1 described above can be achieved. In addition, when a side-contact structure is used, since there is no need for current to flow into the semiconductor substrate, an undoped semi-insulator substrate can be used as the semiconductor substrate. Therefore, the absorption loss of output light on the semiconductor substrate can be effectively reduced.

[0120] Additionally, the outer edge of the metal layer 61 in the Y-axis direction can extend to the outer edges of the embedded layer 4 and the dielectric layer 5. In this case, heat dissipation on the first end face 3a and the second end face 3b can be improved. The side faces 30b of the ridge 30 can also extend parallel to the center line CL. The metal layer 61 can also be constructed comprising multiple mutually separate portions. For example, the metal layer 61 on the first portion 41 can be separately disposed from the metal layer 61 on the second portion 42.

[0121] Alternatively, the first electrode 6 can be formed solely by the metal layer 61 without the plating layer 62. In this case, the wire 8 can also be connected to the surface of the first side S1 in the metal layer 61. In the above embodiment, the inner portion 42b of the second portion 42 is exposed from the dielectric layer 5, and the metal layer 61 contacts the inner portion 42b, but a portion of the second portion 42 is exposed from the dielectric layer 5, and the metal layer 61 contacts the second portion 42 on this portion. In the above embodiment, a portion of the surface 62a of the plating layer 62 is located further to the second side S2 than the top surface 30a of the ridge 30, and the entire surface 62a of the plating layer 62 can also be located further to the first side S1 than the top surface 30a. For example, the plating layer 62 can be formed by electroplating so that the entire surface 62a is located further to the first side S1 than the top surface 30a, and then the surface 62a can be planarized by grinding.

[0122] [Explanation of Symbols]

[0123] 1…Quantum cascade laser, 2…Semiconductor substrate, 6…First electrode, 7…Second electrode, 16…Unit stack, 17, 17b…Emitting layers, 18, 18a…Implantation layers, 31…Active layer, 32…Lower cladding layer (second cladding layer), 33…Upper cladding layer (first cladding layer), L up1 ...the first luminescent upper energy level, L up2 ...the second luminescent upper energy level, L low1 ...the first luminescent lower level (luminescent lower level), L low2 ...the second luminescent lower level (luminescent lower level), L low3 ...the third luminescent lower level (luminescent lower level).

Claims

1. A quantum cascade laser element, wherein, have: A semiconductor substrate having a first surface and a second surface that is flat and opposite to the first surface; An active layer is disposed on the first surface of the semiconductor substrate and is formed in a cascaded structure in which the light-emitting layer and the injection layer are alternately stacked in a multi-level stacked unit stack containing a light-emitting layer and an injection layer. The first coating layer is disposed on the side of the active layer opposite to the semiconductor substrate side, and the impurity doping concentration is less than 1×10⁻⁶. 17 cm -3 ; The first electrode is disposed on the side of the semiconductor substrate on which the active layer is disposed, and is electrically connected to the first coating layer; The second electrode is disposed on the second surface of the semiconductor substrate and is electrically connected to the semiconductor substrate. Each of the unit stacks contained in the active layer has the following subband energy level structure: a first luminescent upper energy level, a second luminescent upper energy level with an energy level higher than the first luminescent upper energy level, and at least one luminescent lower energy level with an energy level lower than the first luminescent upper energy level. The active layer is configured such that, in each of the unit stacks, light with a center wavelength of 10 μm or greater is generated by electron transitions between at least two energy levels in the light-emitting layer, namely the first upper light-emitting energy level, the second upper light-emitting energy level, and the lower light-emitting energy level of the at least one unit stack. The impurity doping concentration in the semiconductor substrate is 5 × 10⁻⁶. 15 cm -3 Above and less than 1×10 17 cm -3 .

2. The quantum cascade laser element according to claim 1, wherein, The thickness of the first coating layer is 5 μm or more.

3. The quantum cascade laser element according to claim 1 or 2, wherein, It also has: The impurity doping concentration is less than 1×10⁻⁶ and is disposed between the active layer and the semiconductor substrate. 17 cm -3 The second coating layer.

4. The quantum cascade laser element according to claim 3, wherein, The thickness of the second coating layer is 5 μm or more.

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