Quantum cascade lasers and their fabrication methods

Abstract

This invention discloses a quantum cascade laser and its fabrication method, relating to the field of laser technology. The quantum cascade laser includes a substrate layer; a lower waveguide layer located on the substrate layer; an active layer located on the lower waveguide layer and including multiple injection layers located on the lower waveguide layer; and multiple light-emitting layers, wherein electrons in the light-emitting layers transition from high energy levels to low energy levels, suitable for exciting electrons to generate excitation light; wherein the light-emitting layers and injection layers are periodically stacked in an alternating cascade structure, each injection layer extracts low-energy electrons from the adjacent light-emitting layer above the injection layer and transports the low-energy electrons to the light-emitting layer of the previous period adjacent to the injection layer to form high-energy electrons, thereby transporting electrons between light-emitting layers belonging to two adjacent periods; and an upper waveguide layer located on the light-emitting layers, which can achieve a highly concentrated number of high-energy electrons, thereby realizing a large-scale, high-power quantum cascade laser.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and in particular to a quantum cascade laser and its fabrication method. Background Technology

[0002] Quantum cascade lasers are semiconductor lasers capable of emitting light in the mid-infrared and far-infrared frequency bands. With the development of high-power quantum cascade lasers, the long-wave infrared (LWIR, λ = 8-12 μm) region of the spectrum has recently attracted considerable interest and research. This is due to the abundance of molecules exhibiting absorption properties and low water absorption in this range. These properties allow for long-distance atmospheric transmission of infrared light, enabling applications in infrared spectroscopy, infrared countermeasures, free-space communication, chemical and biological sensing, and more. Quantum cascade lasers are the ideal single-pole quantum devices for realizing these applications.

[0003] Despite the rapid development of long-wavelength infrared quantum cascade lasers, several inherent technical challenges remain to meet the demands of large-scale production of high-power long-wavelength lasers. First, the higher longitudinal optical (LO) photon scattering rate leads to lower optical transition energies, resulting in a decreased lifetime for upper laser levels, making large population inversions more difficult to achieve. Second, the probability of leakage from the injector directly to lower laser levels is equally high. Third, low photon energy leads to low voltage efficiency, a significant factor affecting the ratio of photon energy drop to the total energy drop of the entire structure. Finally, waveguide losses due to free carrier absorption are roughly proportional to the square of the wavelength; longer wavelengths result in greater waveguide losses, significantly impacting device performance. Furthermore, thicker semiconductor active layers will lead to more challenging material growth, and long-term precise control over barrier and well composition, layer thickness, and heterojunction quality is crucial for achieving high-performance quantum cascade lasers. Summary of the Invention

[0004] To address the existing technical problems, this invention provides a quantum cascade laser and its fabrication method, which at least partially solves the above-mentioned technical problems.

[0005] This invention provides a quantum cascade laser, comprising: a substrate layer; a lower waveguide layer located on the substrate layer; an active layer located on the lower waveguide layer, and further comprising: multiple injection layers located on the lower waveguide layer; and multiple light-emitting layers, wherein electrons in the light-emitting layers transition from high energy levels to low energy levels, suitable for exciting electrons to generate excitation light; wherein the light-emitting layers and the injection layers are periodically stacked alternately in a cascade structure, each injection layer extracts low-energy electrons from the adjacent light-emitting layer above the injection layer and transports the low-energy electrons to the light-emitting layer of the previous period adjacent to the injection layer to form high-energy electrons, thereby transporting electrons between light-emitting layers belonging to two adjacent periods; and an upper waveguide layer located on the light-emitting layer.

[0006] According to embodiments of this disclosure, a single injection layer and a single light-emitting layer are stacked to form a cycle, and the active layer comprises 55 cycles stacked together. Both the injection layer and the light-emitting layer have a quantum well structure formed by alternating stacks of quantum barrier layers and quantum well layers.

[0007] According to embodiments of this disclosure, both the injection layer and the light-emitting layer are formed by alternating layers of strain-compensated materials including In.Ga.As and In.Al.As.

[0008] According to embodiments of this disclosure, the injection layer includes a plurality of injected quantum well layers and a plurality of injected quantum barrier layers, wherein the number of injected quantum well layers and the number of injected quantum barrier layers are equal and are stacked alternately layer by layer; the thickness of the plurality of injected quantum well layers of the injection layer gradually increases in the direction away from the substrate layer; the thickness of the plurality of injected quantum barrier layers of the injection layer gradually decreases in the direction away from the substrate layer, and the thickness is 0.5 nm to 1.9 nm.

[0009] According to embodiments of the present disclosure, the light-emitting layer includes a second quantum well layer, a second quantum barrier layer, a first quantum well layer, and a first quantum barrier layer stacked layer by layer in a direction away from the substrate layer.

[0010] According to embodiments of this disclosure, the thickness of the first quantum barrier layer is greater than the thickness of the plurality of injected quantum barrier layers of the injection layer; the thickness of the first quantum well layer is less than the thickness of the first quantum barrier layer and less than the thickness of the injected quantum well layer near the substrate layer; the thickness of the second quantum barrier layer is less than half the thickness of the first quantum barrier layer; and the thickness of the second quantum well layer is greater than the average thickness of the quantum well layers contained in the light-emitting layer and greater than the thickness of each injected quantum well layer of the injection layer, with a thickness of 2.8 nm to 6.2 nm.

[0011] According to embodiments of this disclosure, the thickness of the injected quantum well layer near the substrate layer in each period of the active layer is greater than the thickness of the injected quantum well layer adjacent to the injected quantum well layer near the substrate layer in the same period, and is greater than the thickness of the first quantum well layer.

[0012] This invention also provides a method for fabricating a quantum cascade laser, comprising:

[0013] S1. Place the substrate in the vapor phase epitaxial growth chamber, and introduce phosphine and trimethylindium into the vapor phase epitaxial growth chamber to grow the lower waveguide layer on the substrate using vapor phase epitaxial growth technology.

[0014] S2. Raise the temperature of the vapor phase epitaxial growth chamber to 650-750 degrees, introduce trimethylgallium and trimethylaluminum into the vapor phase epitaxial growth chamber respectively, maintain a constant flow rate of trimethylgallium and trimethylaluminum, and orderly control the time of introducing trimethylgallium and trimethylaluminum to grow an injection layer and a light-emitting layer on the lower waveguide layer. After 55 cycles of growth, stop introducing trimethylgallium and trimethylaluminum to grow and form an active layer.

[0015] S3. Cool the temperature of the vapor phase epitaxial growth chamber to 600 degrees, introduce trimethylindium and phosphine and maintain a constant flow rate, and grow the upper waveguide layer on the active layer for 5.5 hours.

[0016] S4. Stop the flow of trimethylindium and phosphine, and cool the vapor phase epitaxial growth chamber to room temperature.

[0017] According to an embodiment of this disclosure, in S2, the time required to introduce trimethylgallium for each layer of the active layer to grow In0.581Ga0.429As and the time required to introduce trimethylaluminum for each layer of the active layer to grow In0.369Al0.631As are calculated, and the time for introducing trimethylgallium and trimethylaluminum is switched in an orderly manner according to the order of each layer of the active layer.

[0018] According to embodiments of this disclosure, the growth environment of the vapor phase epitaxial growth chamber is a low-pressure environment of 50-100 mbar.

[0019] According to the quantum cascade laser provided by the present invention, the transition design of electrons in non-single quantum wells and adjacent quantum wells involves electrons in the light-emitting layer transitioning from a high energy level to a low energy level. The injection layer extracts low-energy electrons from the adjacent light-emitting layer located above the injection layer and transports the low-energy electrons to the light-emitting layer of the next cycle above the adjacent light-emitting layer of the injection layer to form high-energy electrons. This highly concentrates the high-energy electrons, thereby achieving efficient electron number inversion and enabling a highly concentrated number of high-energy electrons, thus realizing a large-scale, high-power quantum cascade laser. Attached Figure Description

[0020] Figure 1This is a schematic diagram of the structure of a quantum cascade laser according to an embodiment of the present invention;

[0021] Figure 2 This is an energy level diagram of a quantum cascade laser according to an embodiment of the present invention;

[0022] Figure 3(a) is a high-resolution diffraction peak diagram of the active layer of a quantum cascade laser according to an embodiment of the present invention;

[0023] Figure 3(b) is an enlarged view of the dashed box portion in 3(a);

[0024] Figure 4(a) is an atomic force microscopy scanning image of the active layer test area of ​​the quantum cascade laser according to an embodiment of the present invention, with a measurement area of ​​5 μm × 5 μm; and

[0025] Figure 4(b) is an atomic force microscopy scan image of the active layer test area of ​​the quantum cascade laser according to an embodiment of the present invention, which is 1 μm × 1 μm.

[0026] Figure Labels

[0027] 1. Substrate layer;

[0028] 2. Lower waveguide layer;

[0029] 3. Active layer;

[0030] 31. Injection layer;

[0031] 311. Injecting a quantum barrier layer; 312. Injecting a quantum well layer;

[0032] 32. Emissive layer;

[0033] 321. First quantum barrier layer; 322. First quantum well layer; 323. Second quantum barrier layer; 324. Second quantum well layer;

[0034] 4. Upper waveguide layer;

[0035] 5. Low energy level;

[0036] 6. High energy level;

[0037] 7. Parasitic energy level. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0039] The present invention is described herein with respect to structural embodiments and methods. It should be understood that this is not intended to limit the invention to the specific disclosed embodiments; the invention can be practiced using other features, elements, methods, and embodiments. Similar elements in different embodiments are typically designated with similar numbers.

[0040] This invention proposes a quantum cascade laser.

[0041] Figure 1 This is a schematic diagram of the structure of a quantum cascade laser according to an embodiment of the present invention; Figure 2 Figure 3(a) is an energy level diagram of a quantum cascade laser according to an embodiment of the present invention; Figure 3(b) is an enlarged view of the dashed box portion in Figure 3(a); Figure 4(a) is an atomic force microscopy scan image of the active layer of the quantum cascade laser according to an embodiment of the present invention with a test area of ​​5 μm × 5 μm; Figure 4(b) is an atomic force microscopy scan image of the active layer of the quantum cascade laser according to an embodiment of the present invention with a test area of ​​1 μm × 1 μm.

[0042] refer to Figure 1 and Figure 2 In one illustrative embodiment, a quantum cascade laser includes a substrate layer 1, a lower waveguide layer 2, an active layer 3, and an upper waveguide layer 4 stacked sequentially from bottom to top.

[0043] In one illustrative embodiment, a quantum cascade laser includes a substrate 1 made of indium phosphide; a lower waveguide layer 2 on the substrate 1; and an active layer 3 on the lower waveguide layer 2. The active layer 3 includes multiple injection layers 31 and multiple light-emitting layers 32. Electrons in the light-emitting layers 32 transition from a high energy level 6 to a low energy level 5, which is suitable for exciting electrons to generate excitation light. The light-emitting layers 32 and injection layers 31 are stacked alternately in a cascaded structure. A single injection layer 31 and a single light-emitting layer 32 are stacked to form one cycle. In this embodiment, the injection layers 31 and light-emitting layers 32 are stacked alternately for 55 cycles to form the active layer 3. In each cycle, the injection layer 31 extracts low-energy level 5 electrons from the adjacent light-emitting layer 32 located above the injection layer 31 and transports the low-energy level 5 electrons to the light-emitting layer 32 of the previous cycle adjacent to the injection layer 31 to form high-energy level 6 electrons, thereby transporting electrons between the light-emitting layers 32 belonging to two adjacent cycles. The active layer 3 is closer to the substrate layer 1 as the injection layer 31, which is stacked on the lower waveguide layer 2. The active layer 3 is farther from the substrate layer 1 as the light-emitting layer 32, and the upper waveguide layer 4 is located on the light-emitting layer 32.

[0044] According to the quantum cascade laser of the present invention, the active layer 3 has an alternating periodic stacked cascade structure of light-emitting layer 32 and injection layer 31. That is, in each cycle, the injection layer 31 extracts low-energy level 5 electrons from the adjacent light-emitting layer 32 located above the injection layer 31 and transports the low-energy level 5 electrons to the light-emitting layer 32 of the previous cycle adjacent to the injection layer 31 to form high-energy level 6 electrons. Thus, electrons are transported between the light-emitting layers 32 belonging to two adjacent cycles to achieve a highly concentrated number of high-energy level 6 electrons, thereby realizing a large-scale, high-power quantum cascade laser.

[0045] In one illustrative embodiment, both the injection layer 31 and the light-emitting layer 32 are formed by alternating layers of a strain-compensated material system comprising In0.581Ga0.429As and In0.369Al0.631As.

[0046] By employing a strain-compensated material system, the band shift can be made sufficient to fully utilize the relatively small transition energies of long-wavelength infrared quantum cascade lasers (LWLS) to suppress electron leakage from high-energy level 6, thereby improving the electro-optical conversion efficiency of LWLS. However, conventional LWLS are designed and manufactured using lattice-matched In0.43Ga0.57As / In0.42Al0.58As compositions, resulting in a relatively small band shift of 520 meV. The emission wavelength of the emitting layer 32 in this conventional LWLS is 9 μm, and this band shift leads to an energy gap of 250 meV between the electron energy of high-energy level 6 and the continuous state above the barrier. Therefore, the band shift of the lattice-matched component is insufficient to fully utilize the relatively small transition energies of the LWLS to suppress electron leakage from high-energy level 6.

[0047] In one illustrative embodiment, both the injection layer 31 and the light-emitting layer 32 have a quantum well structure formed by alternating layers of quantum barrier layers and quantum well layers.

[0048] In one illustrative embodiment, see [link to example]. Figure 1The injection layer 31 includes multiple injection quantum well layers 312 and multiple injection quantum barrier layers 311. The injection quantum well layers 312 are made of In0.581Ga0.429As material, and the injection quantum barrier layers 311 are made of In0.369Al0.631As material. The number of injection quantum well layers 312 and injection quantum barrier layers 311 is equal. The injection quantum well layers 312 of the injection layer 31 are located on the lower waveguide layer 2, and the injection quantum well layers 312 and injection quantum barrier layers 311 are stacked alternately. In this embodiment, both the injection quantum well layers 312 and injection quantum barrier layers 311 are set to 6 layers. The light-emitting layer 32 includes a second quantum well layer 324, a second quantum barrier layer 323, a first quantum well layer 322, and a first quantum barrier layer 321 stacked in a direction away from the substrate layer 1. The second quantum well layer 324 is located on the injection quantum barrier layer 311 of the injection layer 31 away from the substrate layer 1.

[0049] In one illustrative embodiment, the thickness of the plurality of injected quantum well layers 312 of the injected layer 31 gradually increases towards the direction away from the substrate layer 1, with a thickness of 3.3 nm to 5.24 nm. To improve the performance of the quantum cascade laser, the thickness of the injected quantum well layer 312 near the substrate layer 1 in each cycle of the active layer 3 is adjusted so that the thickness of the injected quantum well layer 312 near the substrate layer 1 in each cycle of the active layer 3 is greater than the thickness of the injected quantum well layer 312 adjacent to the injected quantum well layer 312 near the substrate layer 1 in the same cycle, and greater than the thickness of the first quantum well layer 322. For example, Figure 1 The thicknesses of the injected quantum well layer 312, from bottom to top, are 3.43 nm, 3.05 nm, 3.88 nm, 4.22 nm, 4.87 nm, and 5.23 nm. The thicknesses of the multiple injected quantum barrier layers 311 of the injection layer 31 gradually decrease towards the direction away from the substrate layer 1, ranging from 0.5 nm to 1.9 nm. Figure 1 The thickness of the injected quantum barrier layer 311 from bottom to top is 1.83nm, 1.57nm, 1.45nm, 1.23nm, 0.85nm and 0.59nm respectively.

[0050] This structure enables electron extraction using the concept of irregular lower energy level arrangement. This design allows the low energy level 5 to permeate the entire structure, providing a pathway for electrons to travel from the emitting layer 32 to the injection layer 31. Furthermore, the voltage defect is a crucial design element, representing the process of electrons moving from the low energy level 5 to the injection level, a process in which energy is converted into heat rather than optical power. The voltage defect designed in this invention is approximately 66 meV, effectively improving voltage efficiency, preventing the thermal backfill effect, and thus promoting the electro-optical conversion efficiency of the quantum cascade laser.

[0051] The first quantum barrier layer 321 and the second quantum barrier layer 323 are made of In0.369Al0.631As material, and the Al composition in the first quantum barrier layer 321 and the second quantum barrier layer 323 is higher than that in conventional infrared long-wavelength quantum cascade lasers. According to such quantum barrier layers, the high proportion of Al composition results in a high barrier potential in the system, which is beneficial for suppressing the leakage of charge carriers from the high-temperature lower energy level 5 across the barrier.

[0052] In one illustrative embodiment, the thickness of the first quantum barrier layer 321 is greater than the thickness of the plurality of injected quantum barrier layers 311 of the injection layer 31; the thickness of the first quantum well layer 322 is less than the thickness of the first quantum barrier layer 321 and less than the thickness of the injected quantum well layer 312 near the substrate layer 1; the thickness of the second quantum barrier layer 323 is less than or equal to the thickness of the first quantum barrier layer 321; and the thickness of the second quantum well layer 324 is greater than the average layer thickness of the quantum well layers contained in the light-emitting layer 32 and greater than the thickness of each injected quantum well layer 312 of the injection layer 31, with a thickness of 2.8 nm to 6.2 nm.

[0053] High-resolution X-ray diffraction measurements are commonly used to evaluate the quality of multi-quantum-well structures and heterostructure interfaces. In Figure 3(a), the high-resolution X-ray diffraction rocking curves, obtained by fitting the peaks with a Gaussian distribution, reveal high-resolution superlattice peaks up to order 15. Combined with the full width at half maximum (FWHM) marked in Figure 3(b), this indicates that active layer 3 possesses high structural quality and periodicity. The FWHM of satellite diffraction peaks and inter-satellite interference fringes is an indicator of overall material quality and further demonstrates the high quality of the heterostructure. In Figure 3(b), the FWHM of the satellite peaks is 13–17 arcseconds. The narrow FWHM of the measured scans indicates a low interfacial roughness (IFR) in active layer 3. This guarantees the high performance of the quantum cascade laser. Furthermore, the stage period of the quantum cascade laser, measured from the satellite peak spacing, is 44.8 nm, very close to the nominal value of 45.1 nm. The closeness between the simulation and experimental data also indicates that the layer thickness, material composition, and interface switching exhibit excellent uniformity and control in the 55-cycle layer structure.

[0054] In Figures 4(a) and 4(b), the test area sizes on the upper surface of active layer 3 are 5 μm × 5 μm and 1 μm × 1 μm, respectively. Figure 4(b) shows smooth step edges with very distinct atomic steps, approximately 210 nm in diameter. Overall, both samples exhibit a clear step flow pattern surface morphology, indicating good surface growth. Furthermore, changing the number of periods in the active region has no significant impact on the surface morphology of the samples.

[0055] According to the quantum cascade laser of the above embodiment of the present invention, this structural design results in strong coupling between low-energy level 5 electrons and high-energy level 6 electrons in the injection layer 31, with a range of hQ to 6.2 meV. The electron transition design involves electrons in adjacent quantum wells (not a single quantum well), where electrons in the emitting layer 32 transition from high energy level 6 to low energy level 5. The injection layer 31 extracts low-energy level 5 electrons from the adjacent emitting layer 32 located above the injection layer 31 and transports these low-energy level 5 electrons to the emitting layer 32 of the previous cycle above the adjacent emitting layer 32 of the injection layer 31, forming high-energy level 6 electrons. This highly concentrated high-energy level 6 electrons achieve efficient electron population inversion and prevent electrons from flowing back or diffusing forward to the injection layer 31 through resonant tunneling. This transition design in adjacent quantum wells increases the electron lifetime of high energy level 6 to 2.3 ps, making electron population inversion easier to achieve and thus reducing the threshold current density of the laser. Furthermore, the energy difference between high energy level 6 and the parasitic energy level 7 located above it is 72 meV. This helps to minimize electron thermal activation leakage. The energy difference between the high-energy level 6 and the top of the barrier is 473 meV. This energy difference reduces the likelihood of electrons escaping through the parasitic energy level 7 or over the barrier at high temperatures. Therefore, this design ensures high internal quantum efficiency when the quantum cascade laser operates at high temperatures. Consequently, a highly concentrated number of electrons in the high-energy level 6 can be achieved, enabling large-scale, high-power quantum cascade lasers.

[0056] This invention also provides a method for fabricating a quantum cascade laser, comprising:

[0057] S1. Place the substrate 1 in the vapor phase epitaxial growth chamber, and introduce phosphine and trimethylindium into the vapor phase epitaxial growth chamber to grow the lower waveguide layer 2 on the substrate 1 using vapor phase epitaxial growth technology.

[0058] S2. Raise the temperature of the vapor phase epitaxial growth chamber to 650-750 degrees, introduce trimethylgallium and trimethylaluminum into the vapor phase epitaxial growth chamber respectively, maintain a constant flow rate of trimethylgallium and trimethylaluminum, and orderly control the time of introducing trimethylgallium and trimethylaluminum. Grow the injection layer 31 and the light-emitting layer 32 on the lower waveguide layer 2. After 55 cycles of growth, stop introducing trimethylgallium and trimethylaluminum, and grow to form the active layer 3.

[0059] S3. Cool the temperature of the vapor phase epitaxial growth chamber to 600 degrees, introduce trimethylindium and phosphine and maintain a constant flow rate, and grow the upper waveguide layer 4 on the active layer 3 for 5.5 hours.

[0060] S4. Stop the flow of trimethylindium and phosphine, and cool the vapor phase epitaxial growth chamber to room temperature.

[0061] According to an embodiment of this disclosure, in S2, the time required to introduce trimethylgallium for each layer of the active layer 3 to grow In0.581Ga0.429As and the time required to introduce trimethylaluminum for each layer of the active layer 3 to grow In0.369Al0.631As are calculated, and the time for introducing trimethylgallium and trimethylaluminum is switched in an orderly manner according to the order of each layer of the active layer 3.

[0062] According to embodiments of this disclosure, the growth environment of the vapor phase epitaxial growth chamber is a low-pressure environment of 50-100 mbar.

[0063] The above specific embodiments further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A quantum cascade laser, characterized by, include: Substrate (1); The lower waveguide layer (2) is located on the substrate layer (1); An active layer (3) is located on the lower waveguide layer (2) and includes: Multiple injection layers (31) are located on the lower waveguide layer (2). Each injection layer (31) includes multiple injection quantum well layers (312) and multiple injection quantum barrier layers (311). The number of injection quantum well layers (312) and injection quantum barrier layers (311) is equal, and they are stacked alternately layer by layer. Multiple light-emitting layers (32) are provided, comprising a second quantum well layer (324), a second quantum barrier layer (323), a first quantum well layer (322), and a first quantum barrier layer (321) stacked in a direction away from the substrate layer (1). Electrons in the light-emitting layer (32) undergo oblique transitions from the high energy level (6) of the first quantum well layer (322) to the low energy level (5) of the second quantum well layer (324), which is suitable for exciting electrons to generate excitation light. The low energy level (5) penetrates the light-emitting layer (32) and the injection layer (31), providing a channel for the transmission of electrons from the light-emitting layer (32) to the injection layer (31). The injection layer (31) and the light-emitting layer (32) are both formed by alternating layers of a strain-compensated material system including In0.581Ga0.429As and In0.369Al0.631As. The light-emitting layer (32) and the injection layer (31) are periodically stacked in a cascade structure. Each injection layer (31) extracts low-energy electrons from the adjacent light-emitting layer (32) above it and transports these low-energy electrons to the light-emitting layer (32) of the previous cycle adjacent to the injection layer (31) to form high-energy electrons, thereby transporting electrons between two adjacent light-emitting layers (32). The upper waveguide layer (4) is located on the light-emitting layer (32).

2. The quantum cascade laser according to claim 1, characterized in that, A single injection layer (31) and a single light-emitting layer (32) are stacked to form a cycle, and the active layer (3) consists of 55 cycles stacked together. Both the injection layer (31) and the light-emitting layer (32) have a quantum well structure formed by alternating stacking of quantum barrier layers and quantum well layers.

3. The quantum cascade laser according to claim 1, characterized in that, The thickness of the plurality of injected quantum well layers (312) of the injected layer (31) gradually increases in the direction away from the substrate layer (1); The thickness of the plurality of injected quantum barrier layers (311) of the injected layer (31) gradually decreases in the direction away from the substrate layer (1), and the thickness is 0.5 nm to 1.9 nm.

4. The quantum cascade laser according to claim 3, characterized in that, The thickness of the first quantum barrier layer (321) is greater than the thickness of the plurality of injected quantum barrier layers (311) of the injection layer (31); The thickness of the first quantum well layer (322) is less than the thickness of the first quantum barrier layer (321) and less than the thickness of the injected quantum well layer (312) near the substrate layer (1); The thickness of the second quantum barrier layer (323) is less than half the thickness of the first quantum barrier layer (321); as well as The thickness of the second quantum well layer (324) is greater than the average thickness of the quantum well layers contained in the light-emitting layer (32) and greater than the thickness of each injected quantum well layer (312) of the injection layer (31), with a thickness of 2.8 nm to 6.2 nm.

5. The quantum cascade laser according to claim 3, characterized in that, The thickness of the injected quantum well layer (312) near the substrate layer (1) in each period of the active layer (3) is greater than the thickness of the injected quantum well layer (312) adjacent to the injected quantum well layer (312) near the substrate layer (1) in the same period, and is greater than the thickness of the first quantum well layer (322).

6. A method for fabricating a quantum cascade laser according to any one of claims 1-5, characterized in that, include: S1. Place the substrate layer (1) in the vapor phase epitaxial growth chamber, and introduce phosphine and trimethylindium into the vapor phase epitaxial growth chamber to grow the lower waveguide layer (2) on the substrate layer (1) using vapor phase epitaxial growth technology. S2. Raise the temperature of the vapor phase epitaxial growth chamber to 650-750 degrees, introduce trimethylgallium and trimethylaluminum into the vapor phase epitaxial growth chamber respectively, maintain the flow rate of trimethylgallium and trimethylaluminum constant, and orderly control the time of introducing trimethylgallium and trimethylaluminum. Grow the injection layer (31) and the light-emitting layer (32) on the lower waveguide layer (2). After growing for 55 cycles, stop introducing trimethylgallium and trimethylaluminum to grow and form the active layer (3). S3. Cool the temperature of the vapor phase epitaxial growth chamber to 600 degrees, introduce trimethylindium and phosphine and maintain a constant flow rate, and grow the upper waveguide layer (4) on the active layer (3) for 5.5 hours. S4. Stop the flow of trimethylindium and phosphine, and cool the vapor phase epitaxial growth chamber to room temperature.

7. The method for fabricating a quantum cascade laser according to claim 6, characterized in that, In S2, the time required to introduce trimethylgallium for each layer of the active layer (3) to grow In0.581Ga0.429As and the time required to introduce trimethylaluminum for each layer of In0.369Al0.631As are calculated. The time for introducing trimethylgallium and trimethylaluminum is switched in an orderly manner according to the order of each layer of the active layer (3).

8. The method for fabricating a quantum cascade laser according to claim 6, characterized in that, The growth environment of the vapor phase epitaxial growth chamber is a low-pressure environment of 50-100 mbar.

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