Quantum cascade laser active region unit, active region, epitaxial structure and chip

Abstract

The present disclosure provides an active region unit of a quantum cascade laser having an inclined transition property. The active region unit sequentially includes, from top to bottom: an injection region for providing an injection energy state; a gain region for providing an upper emission energy state and a lower emission energy state, wherein the upper emission energy state is greater than the lower emission energy state, and the radiative transition from the upper emission energy state to the lower emission energy state is an inclined transition, and the injection energy state is greater than the upper emission energy state; and a relaxation region for providing relaxation energy states, the relaxation energy states including a top energy state and a bottom energy state, wherein the top energy state is less than the lower emission energy state; wherein the injection region, the gain region, and the relaxation region are all made of semiconductor materials. The present disclosure also provides an active region, an epitaxial structure, and a chip of a quantum cascade laser.

Description

Technical Field

[0001] The present disclosure relates to the field of semiconductor technologies, and more particularly, to an active region unit, an active region, an epitaxial structure, and a chip of a quantum cascade laser. Background Art

[0002] The quantum cascade laser (QCL) is currently a miniaturized and high-performance infrared laser source, which is widely used in fields such as environmental monitoring and industrial process control. Currently, QCLs based on the InP-based InGaAs / InAlAs material system can achieve continuous wave operation at room temperature in the 3-12 micron wavelength band.

[0003] However, when the laser wavelength needs to be extended to above 12 microns or below 3 microns, due to the significantly higher internal losses than those in the 3-12 micron wavelength band, the gain of the active region of the mature InP-based InGaAs / InAlAs material system is insufficient to compensate for the losses, and continuous wave operation at room temperature cannot be achieved. It is necessary to design a high-gain active region to overcome the above disadvantages. Summary of the Invention

[0004] In view of this, embodiments of the present disclosure provide an active region unit, an active region, an epitaxial structure, and a chip of a quantum cascade laser.

[0005] According to a first aspect of the present disclosure, there is provided an active region unit of a quantum cascade laser having a skew transition property. The active region unit sequentially includes, from top to bottom: an injection region for providing an injection energy state; a gain region for providing an upper emission energy state and a lower emission energy state, wherein the upper emission energy state is greater than the lower emission energy state, and the radiative transition from the upper emission energy state to the lower emission energy state is a skew transition, and the injection energy state is greater than the upper emission energy state; and a relaxation region for providing a relaxation energy state, the relaxation energy state including a top energy state and a bottom energy state, wherein the top energy state is less than the lower emission energy state; wherein the injection region, the gain region, and the relaxation region are all made of semiconductor materials.

[0006] According to an embodiment of the present disclosure, the semiconductor material is selected from at least one of InAs, InSb, GaSb, AlSb, InAsSb, or at least one of alloys formed by at least two of InAs, InSb, GaSb, AlSb, InAsSb.

[0007] According to an embodiment of the present disclosure, the energy difference between the injection energy state and the upper emission energy state is 5-7 meV.

[0008] According to an embodiment of the present disclosure, the energy difference between the injection energy state and the upper emission energy state is equal to an optical phonon energy.

[0009] According to an embodiment of the present disclosure, the relaxation energy state includes a miniband energy state or a localized energy state.

[0010] According to an embodiment of the present disclosure, the energy difference between the lower emission energy state and the top energy state is 0 - 5 meV.

[0011] According to an embodiment of the present disclosure, the energy difference between the lower emission energy state and the top energy state is equal to an optical phonon energy.

[0012] According to a second aspect of the present disclosure, there is provided an active region of a quantum cascade laser, the active region including a plurality of serially connected active region units as described in any one of the above, wherein, among the serially connected active region units, the bottom energy state of the relaxation region of the previous active region unit is greater than the upper emission energy state of the gain region of the next active region unit.

[0013] According to an embodiment of the present disclosure, the number of active region units in the active region is greater than or equal to 10.

[0014] According to a third aspect of the present disclosure, there is provided an epitaxial structure of a quantum cascade laser, the epitaxial structure including: a lower cladding layer, a lower superlattice waveguide layer, a lower isolation layer, the active region as described in any one of the above, an upper isolation layer, an upper superlattice waveguide layer, and an upper cladding layer that are sequentially grown on a substrate, wherein the active region uses semiconductor material.

[0015] According to an embodiment of the present disclosure, the substrate uses a material lattice - matched with the active region material, selected from one of InAs, GaSb, and InSb.

[0016] According to an embodiment of the present disclosure, the lower cladding layer and the upper cladding layer use materials lattice - matched with the substrate material, including semiconductor materials of binary, ternary, and quaternary compounds, and superlattice materials formed by narrow - bandgap quantum wells and wide - bandgap barriers.

[0017] According to an embodiment of the present disclosure, the doping concentration of the lower cladding layer and the upper cladding layer is 1×10 17 cm -3 ~5×10 18 cm -3 .

[0018] According to an embodiment of the present disclosure, the thickness of the lower cladding layer is 100 nm to 4000 nm, and the thickness of the upper cladding layer is 100 nm to 4000 nm.

[0019] According to an embodiment of the present disclosure, the materials used for the lower superlattice waveguide layer and the upper superlattice waveguide layer are strained superlattice materials.

[0020] According to an embodiment of the present disclosure, the refractive indices of the lower superlattice waveguide layer and the upper superlattice waveguide layer are less than that of the active region.

[0021] According to an embodiment of the present disclosure, the lower isolation layer and the upper isolation layer are made of a material lattice-matched with the substrate material.

[0022] According to an embodiment of the present disclosure, the refractive indices of the lower isolation layer and the upper isolation layer are greater than that of the active region.

[0023] According to an embodiment of the present disclosure, the thickness of the lower isolation layer is 100 nm to 4000 nm, and the thickness of the upper isolation layer is 100 nm to 4000 nm.

[0024] According to a fourth aspect of the present disclosure, there is provided a quantum cascade laser chip, which includes: the epitaxial structure of the quantum cascade laser according to any one of the above, wherein the active region, the upper isolation layer, the upper superlattice waveguide layer, and the upper cladding layer of the epitaxial structure form a ridge waveguide structure; a back electrode formed on the back surface of the substrate of the epitaxial structure away from the lower cladding layer; a dielectric insulating layer formed on the side walls of the ridge waveguide structure; and a surface electrode formed on the surface of the upper cladding layer of the epitaxial structure away from the upper superlattice waveguide layer.

[0025] According to an embodiment of the present disclosure, the width of the ridge waveguide structure is 5 μm to 50 μm.

[0026] According to an embodiment of the present disclosure, the thickness of the dielectric insulating layer is 50 nm to 1000 nm.

[0027] According to an embodiment of the present disclosure, the material used for the dielectric insulating layer is selected from SiO 2 , Si 3 N 4 , Si 3 N 4 / SiO 2 and the like.

[0028] According to an embodiment of the present disclosure, the material used for the surface electrode is selected from one of Au, Ti / Au, and Ti / Pt / Au.

[0029] It can be seen from the above technical solutions that the beneficial effects of the active region unit, the active region, the epitaxial structure, and the chip of the quantum cascade laser provided by the present disclosure are as follows:

[0030] 1. The active region unit of the quantum cascade laser with the property of oblique transition provided by the present disclosure adopts a semiconductor material and an oblique transition structure, which significantly improves the lifetime of the upper energy state of luminescence, thereby improving the gain of the active region and realizing a high-gain active region.

[0031] 2. The active region of the quantum cascade laser with the inclined transition property provided by the present disclosure is obtained by connecting in series active region units that adopt semiconductor materials and the structure of inclined transitions, which improves the gain of the active region and reduces the loss of the active region.

[0032] 3. The epitaxial structure of the quantum cascade laser with the inclined transition property provided by the present disclosure can effectively reduce the waveguide loss and increase the lifetime of the upper emitting energy state through a highly efficient active region design.

[0033] 4. The chip of the quantum cascade laser with the inclined transition property provided by the present disclosure adopts semiconductor materials and the structure of inclined transitions, significantly increasing the lifetime of the upper emitting energy state, thereby increasing the dynamic range and output power of the laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Through the following description of the embodiments of the present disclosure with reference to the drawings, the above and other objects, features, and advantages of the present disclosure will become clearer. In the drawings:

[0035] FIG. 1(a) schematically shows a diagram of the vertical transition of the local conduction band energy levels of the active region unit of the quantum cascade laser according to an embodiment of the present disclosure;

[0036] FIG. 1(b) schematically shows a diagram of the inclined transition of the local conduction band energy levels of the active region unit of the quantum cascade laser according to an embodiment of the present disclosure;

[0037] Figure 2 Schematically shows the conduction band energy level diagram of the active region unit of the quantum cascade laser according to an embodiment of the present disclosure when an electric field is applied;

[0038] Figure 3 Schematically shows the conduction band energy level diagram of the active region unit of the quantum cascade laser according to another embodiment of the present disclosure when an electric field is applied;

[0039] Figure 4 Schematically shows a schematic diagram of the stacked structure of the active region unit of the quantum cascade laser according to an embodiment of the present disclosure;

[0040] Figure 5 Schematically shows a schematic diagram of the epitaxial structure of the quantum cascade laser according to an embodiment of the present disclosure;

[0041] Figure 6 Schematically shows the high-resolution X-ray diffraction spectrum of the epitaxial structure of the quantum cascade laser according to an embodiment of the present disclosure;

[0042] Figure 7 Schematically shows the atomic force microscope topography of the surface of the epitaxial structure of the quantum cascade laser according to an embodiment of the present disclosure;

[0043] Figure 8 Schematically shows the lasing spectra of the quantum cascade laser chip according to embodiments of the present disclosure under different injection currents. Detailed implementation manners

[0044] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present disclosure. In the following detailed description, for the sake of explanation, numerous specific details are set forth in order to provide a comprehensive understanding of the embodiments of the present disclosure. However, it is obvious that one or more embodiments can also be implemented without these specific details. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessarily obscuring the concepts of the present disclosure.

[0045] The terms used herein are merely for the purpose of describing specific embodiments and are not intended to limit the present disclosure. The terms "including", "comprising", etc. used herein indicate the presence of the described features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0046] In the case of using expressions such as "at least one of A, B, or C, etc.", generally, it should be interpreted according to the meaning that those skilled in the art usually understand this expression (for example, "a system having at least one of A, B, or C" should include but not be limited to a system having only A, only having B, only having C, having A and B, having A and C, having B and C, and / or having A, B, and C, etc.). The terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of the described features.

[0047] A quantum cascade laser (QCL) is based on intersubband transitions, and its gain g and the effective mass m* of the active region material satisfy the following relationship: g ∝ (m*) -3 / 2 , that is, the smaller the effective mass, the higher the material gain. Therefore, a new semiconductor material system with a small effective mass can be considered to replace the InGaAs / InAlAs material system, which can increase the gain by more than 2 times. However, the epitaxial growth difficulty of this new material system is significantly higher than that of the mature InP-based material system, and at the same time, there is a lack of high-efficiency active region design. Therefore, a quantum cascade laser based on a semiconductor material system has not achieved high-power continuous-wave lasing at room temperature so far.

[0048] An active region unit of a quantum cascade laser with skew transition properties is provided in an embodiment of the present disclosure. The active region unit sequentially includes, from top to bottom: an injection region, a gain region, and a relaxation region.

[0049] The injection region is configured to provide an injection energy state, and the wave function of the injection energy state optionally has a characteristic of strong coupling with the upper emission energy state of the gain region.

[0050] The gain region is configured to provide an upper emission energy state and a lower emission energy state. Among them, the upper emission energy state is greater than the lower emission energy state, the radiative transition from the upper emission energy state to the lower emission energy state is a skew transition, the injection energy state is greater than the upper emission energy state, the energy difference between the upper emission energy state and the lower emission energy state determines the wavelength of the laser, and the energy difference between the upper emission energy state and the lower emission energy state is determined by the thicknesses of the quantum wells and barriers providing the upper emission energy state and the lower emission energy state and the composition of the barriers. At least one barrier is provided between the quantum well providing the upper emission energy state and the quantum well providing the lower emission energy state. Optionally, 2 barriers are provided, and optionally, 3 barriers are provided, so that the radiative transition of electrons from the upper emission energy state to the lower emission energy state has the property of a skew transition. The property of the skew transition enables the lifetime of the upper emission energy state to be at least greater than 0.5 ps, and optionally, the lifetime of the upper emission energy state is greater than 1 ps.

[0051] The relaxation region is configured to provide relaxation energy states, and the relaxation energy states include a top energy state and a bottom energy state. Among them, the top energy state is less than the lower emission energy state; among them, the injection region, the gain region, and the relaxation region all adopt semiconductor materials. The semiconductor material system refers to semiconductor materials with a lattice constant near ...

[0052] According to an embodiment of the present disclosure, the semiconductor material is selected from at least one of InAs, InSb, GaSb, AlSb, InAsSb, or at least one of alloys formed by at least two of InAs, InSb, GaSb, AlSb, InAsSb.

[0053] According to an embodiment of the present disclosure, the energy difference between the injection energy state and the upper emission energy state is 5 - 7 meV. Optionally, the energy difference is 6 meV, which is beneficial to realizing resonant tunneling injection.

[0054] According to an embodiment of the present disclosure, the energy difference between the injection energy state and the upper emission energy state is equal to an optical phonon energy, which is beneficial to realizing phonon-assisted injection.

[0055] According to an embodiment of the present disclosure, the relaxation energy states include miniband energy states or localized energy states.

[0056] According to an embodiment of the present disclosure, the energy difference between the lower emitting energy state and the upper energy state is 0 - 5 meV, which is beneficial for the electrons in the lower emitting energy state to relax through resonant tunneling. Optionally, the energy difference may be 2 meV or 3 meV.

[0057] According to an embodiment of the present disclosure, the energy difference between the lower emitting energy state and the upper energy state is equal to an optical phonon energy, which is beneficial for the electrons in the lower emitting energy state to relax through phonon-assisted scattering.

[0058] Exemplarily, when the relaxation energy state is composed of minibands formed under an electric field, the energy state at the top of the miniband is below the lower emitting energy state; the energy difference between the energy state at the top of the miniband and the lower emitting energy state is not less than zero; the energy difference between the energy state at the bottom of the miniband and the upper emitting energy state of the next active region unit is not less than zero and can be small, so as to relax quickly through resonant tunneling or electron scattering.

[0059] Exemplarily, when the relaxation energy state is composed of a series of localized energy states formed under an electric field, the energy difference between the series of localized energy states is optionally an optical phonon energy; the highest energy state of the series of localized energy states is below the lower emitting energy state; the energy difference between the highest energy state of the series of localized energy states and the lower emitting energy state is not less than zero; the energy difference between the lowest energy state of the series of localized energy states and the upper emitting energy state of the next active region unit is not less than zero and can be small, so as to relax quickly through resonant tunneling or electron scattering.

[0060] The active region unit of the quantum cascade laser with the property of oblique transition provided by the embodiment of the present disclosure adopts semiconductor materials and an oblique transition structure, which significantly improves the lifetime of the upper emitting energy state, thereby improving the gain of the active region.

[0061] FIG. 1(a) schematically shows a schematic diagram of the vertical transition of the local conduction band energy band of the active region unit of the quantum cascade laser according to the embodiment of the present disclosure. FIG. 1(b) schematically shows a schematic diagram of the oblique transition of the local conduction band energy band of the active region unit of the quantum cascade laser according to the embodiment of the present disclosure.

[0062] As shown in FIG. 1(a), for the vertical transition of the prior art, the upper emitting energy state and the lower emitting energy state come from the same set of quantum wells, and the electron radiative transition is vertical in space, as shown by the vertical arrow in the figure.

[0063] As shown in FIG. 1(b), for the oblique transition of the embodiment of the present disclosure, the quantum well providing the upper emitting energy state and the quantum well providing the lower emitting energy state are spatially separated, and the electron radiative transition is oblique in space, as shown by the oblique arrow in the figure.

[0064] In the inclined transition energy band of the embodiment of the present disclosure and the vertical transition energy band of the prior art, the upper energy state lifetime of the inclined transition energy band is 1.6 ps, which is 3.3 times that of the upper energy state lifetime of the vertical transition. According to the known semiconductor laser theory, a long upper energy state lifetime means that population inversion and thus lasing can be achieved without a very high electron concentration in the injection region. The advantage of a lower electron concentration in the injection region is that the loss caused by free carrier absorption in the active region is smaller, which is beneficial to reducing the laser threshold and increasing the output power of the laser.

[0065] Figure 2 Schematically shows the conduction band energy band diagram of the active region unit of the quantum cascade laser of the embodiment of the present disclosure when an electric field is applied.

[0066] As Figure 2 shown, the conduction band energy band diagram includes a gain region 100, an injection region 200, and a relaxation region 300. The gain region 100 includes an upper emitting energy state 101 and a lower emitting energy state 102; the energy of the upper emitting energy state 101 is greater than the energy of the lower emitting energy state 102; the energy difference between the upper emitting energy state 101 and the lower emitting energy state 102 is equal to the energy of the emitted photons of the quantum cascade laser, that is, it determines the lasing wavelength of the laser; the energy difference between the upper emitting energy state 101 and the lower emitting energy state 102 varies due to factors such as the thickness of the quantum well layer, the thickness of the barrier layer, and the composition of the barrier material. Therefore, the emission wavelength of the QCL varies due to factors such as the thickness of the well layer, the thickness of the barrier layer, and the composition of the barrier layer material. The positions of the electron wave functions and energy states in the energy band diagram are obtained through simulation. The electron lifetime of the upper emitting energy state 101 is t1, and the electron lifetime of the lower emitting energy state 102 is t2. According to the embodiment of the present disclosure, t1 > t2; the maximum value of the wave function of the upper emitting energy state 101 is located in the quantum well U, and the maximum value of the wave function of the lower emitting energy state 102 is located in the quantum well L. Two barrier layers B1 and B2 are provided between the quantum well U and the quantum well L, such that the radiative transition from the upper emitting energy state 101 to the lower emitting energy state 102 is an inclined transition.

[0067] The injection region 200 includes a degenerate injection energy state 201; the energy of the injection energy state 201 is greater than the upper emitting energy state 101; the energy difference between the injection energy state 201 and the upper emitting energy state 101 is approximately the energy of an optical phonon, which is 30 - 35 meV. Optionally, the energy difference is 32 meV or 33 meV.

[0068] The relaxation region 300 includes a series of energy states that form a miniband under an applied electric field, where the top 301 of the miniband is below the lower lasing energy state 102. The energy difference between the top 301 of the miniband and the lower lasing energy state 102 is approximately the energy of an optical phonon, which is 30 - 35 meV. Optionally, the energy difference is 32 meV or 33 meV. The bottom 302 of the miniband is above the injection energy state 201 of the next active region unit, and the energy difference between the bottom 302 of the miniband and the injection energy state 201 of the next active region unit is 5 - 10 meV. Optionally, the energy difference is 7 meV or 8 meV. The energy state 401 that is above the injection energy state 201 and closest to it may become a leakage energy state for the injected electrons. According to the embodiments disclosed in the present invention, the energy difference between the energy state 401 and the injection energy state 201 is greater than the photon phonon energy, and the energy difference between the energy state 401 and the injection energy state 201 is 40 - 80 meV. Optionally, the energy difference is 55 meV or 60 meV.

[0069] Figure 3 Schematically shows the conduction band energy band diagram of the active region unit of the quantum cascade laser according to another embodiment of the present disclosure when an electric field is applied.

[0070] As Figure 3 shown, the conduction band energy band diagram includes a gain region 100, an injection region 200, and a relaxation region 300. The gain region 100 includes an upper lasing energy state 101 and a lower lasing energy state 102. The energy of the upper lasing energy state 101 is greater than the energy of the lower lasing energy state 102. The energy gap between the upper lasing energy state 101 and the lower lasing energy state 102 is equal to the energy of the emitted photons of the quantum cascade laser, that is, it determines the lasing wavelength of the laser. The energy difference between the upper lasing energy state 101 and the lower lasing energy state 102 varies due to the thickness of the well layer, the thickness of the barrier layer, etc. Therefore, the emission wavelength of the quantum cascade laser varies due to the thickness of the well layer, the thickness of the barrier layer, etc. The positions of the electron wave functions and energy states in the energy band diagram are obtained through simulation. The electron lifetime of the upper lasing energy state 101 is t1, and the electron lifetime of the lower lasing energy state 102 is t2. According to the embodiments of the present disclosure, t1 > t2. The maximum value of the wave function of the upper lasing energy state 101 is located in the quantum well U, and the maximum value of the wave function of the lower lasing energy state 102 is located in the quantum wells Ll and L2. Two barrier layers B1 and B2 are provided between the quantum well U and the quantum well L1, such that the radiative transition from the upper lasing energy state 101 to the lower lasing energy state 102 is an inclined transition.

[0071] The injection region 200 includes an injection energy state 201. The energy of the injection energy state 201 is greater than the upper lasing energy state 101. The energy difference between the injection energy state 201 and the upper lasing energy state 101 is 5 - 10 meV.

[0072] The relaxation region 300 includes a series of energy states that form minibands under an applied electric field, where the top 301 of the miniband is below the lower emitting energy state 102, and the energy difference between the top 301 of the miniband and the lower emitting energy state 102 is approximately the energy of an optical phonon, which is 30 - 35 meV; the bottom 302 of the miniband is above the injection energy state 201 of the next active region unit. The energy state 401 that is above the injection energy state 201 and closest to it may become a leakage energy state for the injected electrons. According to an embodiment of the present disclosure, the energy difference between the energy state 401 and the injection energy state 201 is greater than the photon phonon energy, which is 40 - 80 meV.

[0073] Figure 4 Schematically shows a schematic diagram of the stack structure of the active region unit of the quantum cascade laser according to an embodiment of the present disclosure.

[0074] As Figure 4 shown, it is the stack structure of an active region unit of a quantum cascade laser, where the InAs layer forms the quantum well layer and the AlSb layer forms the barrier layer; the active region of the quantum cascade laser is composed of N stacked active region units, and generally N is greater than or equal to 10. Optionally, N is greater than or equal to 10 and less than or equal to 100. Optionally, N is equal to 80 or 90.

[0075] In this embodiment, the first 2.1 nm AlSb layer from top to bottom is the injection barrier; the adjacent 4 undoped InAs quantum wells form a light-emitting region that provides intersubband skew transitions; the 3 doped InAs quantum wells and the 2 undoped InAs quantum wells below the light-emitting region form a relaxation region, which is also the electron injection region of the next active region unit, and its electron energy states form a strong coupling with the upper emitting energy states of the next active region unit.

[0076] In another embodiment, the light-emitting region is formed by 3 or 5 undoped quantum wells and forms a strong coupling with the electron injection energy state of the previous active region unit; in another embodiment, the barrier can optionally be composed of AlAs x Sb 1-x such that the total strain of the quantum well layer and the barrier layer in the entire epitaxial structure is less than the critical strain value.

[0077] In another embodiment, the quantum well can optionally be composed of InAs x Sb 1-x such that the total strain of the quantum well layer and the barrier layer in the entire epitaxial structure is less than the critical strain value.

[0078] In another embodiment, the quantum well can optionally be composed of In 1-y Ga y Sb such that the proportion y of Ga makes the total strain of the quantum well layer and the barrier layer in the entire epitaxial structure less than the critical strain value.

[0079] In another embodiment, the quantum well may optionally be composed of In 1-y Ga y As x Sb 1-x , and the ratio y of Ga and the ratio x of As are such that the total strain of the quantum well layer and the barrier layer in the entire epitaxial structure is less than the critical strain value.

[0080] Embodiments of the present disclosure provide an active region of a quantum cascade laser. The active region includes a plurality of serially connected active region units as described in any one of the above. Among them, in the serially connected active region units, the bottom energy state of the relaxation region of the previous active region unit is greater than the upper emission energy state of the gain region of the next active region unit. Exemplarily, the relaxation region of the previous active region unit is also the electron injection region of the next active region unit.

[0081] According to an embodiment of the present disclosure, the number of active region units in the active region is greater than or equal to 10.

[0082] The active region of the quantum cascade laser with the property of skew transition provided by the embodiments of the present disclosure is obtained by serially connecting active region units with a semiconductor material and a skew transition structure. The intersubband transition of the emitting quantum well in the gain region adopts a skew transition mode, providing a long upper emission energy state lifetime, which is beneficial to achieving population inversion, reducing the electron concentration in the injection region, increasing the gain of the active region, and reducing the loss of the active region.

[0083] Embodiments of the present disclosure provide an epitaxial structure of a quantum cascade laser. The epitaxial structure includes: a lower cladding layer, a lower superlattice waveguide layer, a lower isolation layer, the active region as described in any one of the above, an upper isolation layer, an upper superlattice waveguide layer, and an upper cladding layer grown sequentially on a substrate. Among them, the active region uses a semiconductor material. Among them, the active region is composed of N serially connected active region units of the quantum cascade laser with the property of skew transition, where N = 10 - 100. The total strain of the lower superlattice waveguide layer and the upper superlattice waveguide layer is less than the critical strain value.

[0084] According to an embodiment of the present disclosure, the substrate uses a material lattice-matched with the active region material. For example, the effective mass at the room temperature Γ point is less than 0.03m 0 , where m 0 is the mass of a free electron. Exemplarily, it is selected from one of InAs, GaSb, and InSb.

[0085] ​According to an embodiment of the present disclosure, the lower cladding layer and the upper cladding layer are made of materials lattice-matched to the substrate material, including semiconductor materials such as binary, ternary, and quaternary compounds, and superlattice materials formed by narrow-bandgap quantum wells and wide-bandgap barriers. The upper isolation layer and the lower isolation layer are made of binary, ternary, or quaternary semiconductor materials lattice-matched to the substrate material.

[0086] According to an embodiment of the present disclosure, the doping concentration of the lower cladding layer and the upper cladding layer is 1×10 17 cm -3 ~5×10 18 cm -3 . Optionally, the doping concentration of the lower cladding layer and the upper cladding layer is 1×10 18 cm -3 .

[0087] According to an embodiment of the present disclosure, the thickness of the lower cladding layer is 100 nm to 4000 nm, and the thickness of the upper cladding layer is 100 nm to 4000 nm. Optionally, the thicknesses of both the lower cladding layer and the upper cladding layer are 2000 nm or both are 3000 nm.

[0088] According to an embodiment of the present disclosure, the materials used for the lower superlattice waveguide layer and the upper superlattice waveguide layer are strained superlattice materials.

[0089] According to an embodiment of the present disclosure, the refractive indices of the lower superlattice waveguide layer and the upper superlattice waveguide layer are less than that of the active region.

[0090] According to an embodiment of the present disclosure, the lower isolation layer and the upper isolation layer are made of materials lattice-matched to the substrate material.

[0091] According to an embodiment of the present disclosure, the refractive indices of the lower isolation layer and the upper isolation layer are greater than that of the active region.

[0092] According to an embodiment of the present disclosure, the thickness of the lower isolation layer is 100 nm to 4000 nm, and the thickness of the upper isolation layer is 100 nm to 4000 nm. Optionally, the thicknesses of both the lower isolation layer and the upper isolation layer are 2000 nm or both are 3000 nm.

[0093] According to an embodiment of the present disclosure, the refractive indices of the upper cladding layer and the lower cladding layer are less than or equal to those of the upper superlattice waveguide layer and the lower superlattice waveguide layer.

[0094] According to an embodiment of the present disclosure, the upper cladding layer and the lower cladding layer are doped more heavily, and the upper and lower spacer layers and the superlattice layers are doped less heavily.

[0095] According to an embodiment of the present disclosure, the upper superlattice waveguide layer and the lower superlattice waveguide layer are strained superlattice materials, and the total strain of the strained superlattice is not greater than a critical strain value; the critical strain value refers to that after the strain accumulation formed by the lattice mismatch between the epitaxial layer and the substrate reaches a critical value, defects such as dislocations will be generated in the epitaxial layer.

[0096] The epitaxial structure of the quantum cascade laser with the property of oblique transition provided by the embodiment of the present disclosure can effectively reduce the waveguide loss and improve the lifetime of the upper energy state of luminescence through the high-efficiency active region design, so as to improve the dynamic range and output power of the laser.

[0097] Figure 5 Schematically shows a schematic diagram of the epitaxial structure of the quantum cascade laser according to an embodiment of the present disclosure.

[0098] As Figure 5 shown, a substrate 600, an InAs single crystal substrate is adopted;

[0099] An active region 604, composed of an InAs / AlSb quantum well superlattice;

[0100] A lower cladding layer 601a and an upper cladding layer 601b, an epitaxial InAs thin film is adopted, with a thickness of 100 nm to 4000 nm and an n-type doping concentration of 1×10 17 cm -3 ~5×10 18 cm -3 , and its refractive index is less than that of the active region;

[0101] A lower superlattice waveguide layer 602a and an upper superlattice waveguide layer 602b, an epitaxial InAs / AlSb superlattice is adopted, with a thickness of 500 nm to 5000 nm, and its refractive index is less than that of the active region;

[0102] According to an embodiment of the disclosure, by adopting the lower superlattice waveguide layer 602a and the upper superlattice waveguide layer 602b, compared with the heavily doped InAs waveguide layer in the prior art, the waveguide loss can be effectively reduced, and thus the performance such as the dynamic range and output power of the laser can be improved;

[0103] A lower isolation layer 603a and an upper isolation layer 603b, an epitaxial InAs thin film is adopted, with a thickness of 500 nm to 4000 nm and an n-type doping concentration of 1×10 16 cm -3 ~1×10 17 cm -3 , and the refractive index is greater than that of the active region.

[0104] Figure 6 Schematically shows the high-resolution X-ray diffraction spectrum of the epitaxial structure of the quantum cascade laser according to an embodiment of the present disclosure.

[0105] As Figure 6 shown, the spectrum contains distinct satellite peaks with an average full width at half maximum (FWHM) of 26 arcsec. The thickness of one cascade period calculated from the spacing between adjacent satellite peaks is 98.5 nm, which is basically consistent with the designed 100 nm, indicating a high crystal quality of the epitaxial layer.

[0106] Figure 7 Schematically shows the atomic force microscope topography of the surface of the epitaxial structure of the quantum cascade laser according to an embodiment of the present disclosure.

[0107] As Figure 7 shown, distinct atomic steps can be seen on the surface of the epitaxial structure of the quantum cascade laser, with an RMS of 0.17 nm, indicating a high surface flatness of the epitaxial layer.

[0108] Embodiments of the present disclosure provide a quantum cascade laser chip, which includes: the epitaxial structure of the quantum cascade laser according to any one of the above, wherein the active region, the upper isolation layer, the upper superlattice waveguide layer, and the upper cladding layer of the epitaxial structure form a ridge waveguide structure; a back electrode formed on the back surface of the substrate of the epitaxial structure away from the lower cladding layer; a dielectric insulating layer formed on the sidewalls of the ridge waveguide structure; and a surface electrode formed on the surface of the upper cladding layer of the epitaxial structure away from the upper superlattice waveguide layer.

[0109] According to an embodiment of the present disclosure, the width of the ridge waveguide structure is 5 μm to 50 μm. Optionally, the width is 30 μm or 35 μm.

[0110] According to an embodiment of the present disclosure, the thickness of the dielectric insulating layer is 50 nm to 1000 nm. Optionally, the thickness is 500 nm or 600 nm.

[0111] According to an embodiment of the present disclosure, the material used for the dielectric insulating layer is selected from SiO 2 、Si 3 N 4 、Si 3 N 4 / SiO 2 one of them.

[0112] According to an embodiment of the present disclosure, the material used for the surface electrode is selected from one of Au, Ti / Au, and Ti / Pt / Au.

[0113] The chip of the quantum cascade laser with the property of oblique transition provided by the embodiments of the present disclosure uses The structure of the semiconductor material and the inclined transition significantly improves the lifetime of the upper luminescent energy state, which is beneficial to reducing the loss in the active region, thereby improving the dynamic range and output power of the laser. The 15-μm lasing generated by the designed QCL with an inclined transition energy band structure has been achieved.

[0114] Figure 8 Schematically shows the lasing spectra of the quantum cascade laser chip according to the embodiments of the present disclosure under different injection currents.

[0115] As Figure 8 shown, the lasing wavelength of the laser is located near 668 cm -1 , that is, 14.9 μm, which is consistent with the theoretically designed 15 μm. The depression in the following single lasing spectrum is caused by the absorption of CO in the air. 2 in the air.

[0116] It should be noted that the embodiments of the present disclosure give the designs of the energy band structures of two active regions as non-limiting examples. It can be envisioned that in some embodiments of the present invention, the active region has an inclined transition energy band structure, and the relative positions and energy differences between the upper luminescent energy state and the injection energy state can adopt similar designs. The relaxation region below the lower luminescent energy state can adopt the form of local relaxation energy states instead of the microband form.

[0117] Those skilled in the art can understand that the features recited in the various embodiments and / or claims of the present disclosure can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly recited in the present disclosure. In particular, without departing from the spirit and teachings of the present disclosure, the features recited in the various embodiments and / or claims of the present disclosure can be combined and / or combined in various ways. All such combinations and / or combinations fall within the scope of the present disclosure.

[0118] The above describes the embodiments of the present disclosure. However, these embodiments are only for illustrative purposes and do not limit the scope of the present disclosure. Although the embodiments are described separately above, this does not mean that the measures in the respective embodiments cannot be used advantageously in combination. The scope of the present disclosure is defined by the appended claims and their equivalents. Without departing from the scope of the present disclosure, those skilled in the art can make various substitutions and modifications, and all such substitutions and modifications should fall within the scope of the present disclosure.

Claims

1. An active region unit of a quantum cascade laser with inclined transition properties, characterized in that, the active region unit successively includes from top to bottom: an injection region for providing injection energy states; a gain region for providing upper emission energy states and lower emission energy states, wherein the upper emission energy state is greater than the lower emission energy state, the radiative transition from the upper emission energy state to the lower emission energy state is an inclined transition, the injection energy state is greater than the upper emission energy state, and the energy difference between the injection energy state and the upper emission energy state is 5 - 7 meV or equal to an optical phonon energy; and a relaxation region for providing relaxation energy states, the relaxation energy states including a top energy state and a bottom energy state, wherein the top energy state is less than the lower emission energy state; wherein, the injection region, the gain region and the relaxation region all adopt a 6.1 Å semiconductor material.

2. The active region unit according to claim 1, characterized in that, the 6.1 Å semiconductor material is selected from at least one of InAs, InSb, GaSb, AlSb, InAsSb, or at least one of alloys formed by at least two of InAs, InSb, GaSb, AlSb, InAsSb.

3. The active region unit according to claim 1, characterized in that, the relaxation energy states include miniband energy states or localized energy states.

4. The active region unit according to claim 3, characterized in that, the energy difference between the lower emission energy state and the top energy state is 0 - 5 meV.

5. The active region unit according to claim 3, characterized in that, the energy difference between the lower emission energy state and the top energy state is equal to an optical phonon energy.

6. An active region of a quantum cascade laser, characterized in that, the active region includes a plurality of serially connected active region units as described in any one of claims 1 - 5, wherein, among the serially connected active region units, the bottom energy state of the relaxation region of the previous active region unit is greater than the upper emission energy state of the gain region of the next active region unit.

7. The active region according to claim 6, characterized in that, the number of active region units in the active region is greater than or equal to 10.

8. An epitaxial structure of a quantum cascade laser, characterized in that, the epitaxial structure includes: a lower cladding layer, a lower superlattice waveguide layer, a lower isolation layer, the active region as described in any one of claims 6 - 7, an upper isolation layer, an upper superlattice waveguide layer and an upper cladding layer successively grown on a substrate, wherein the active region adopts a 6.1 Å semiconductor material.

9. The epitaxial structure according to claim 8, characterized in that, the substrate adopts a material lattice - matched with the active region material, and is selected from one of InAs, GaSb, InSb.

10. The epitaxial structure according to claim 9, characterized in that, the lower cladding layer and the upper cladding layer adopt materials lattice - matched with the substrate material, including binary, ternary, quaternary compound semiconductor materials, and superlattice materials formed by narrow - bandgap quantum wells and wide - bandgap barriers.

11. The epitaxial structure according to claim 10, characterized in that, The doping concentrations of the lower cladding and the upper cladding are 1×10 17 cm -3 ~5×10 18 cm -3 .

12. The epitaxial structure according to claim 11, characterized in that, The thickness of the lower cladding layer is 100 nm to 4000 nm, and the thickness of the upper cladding layer is 100 nm to 4000 nm.

13. The epitaxial structure according to claim 8, wherein, the materials used for the lower superlattice waveguide layer and the upper superlattice waveguide layer are strained superlattice materials.

14. The epitaxial structure according to claim 13, wherein, the refractive indices of the lower superlattice waveguide layer and the upper superlattice waveguide layer are less than that of the active region.

15. The epitaxial structure according to claim 9, wherein, the materials used for the lower isolation layer and the upper isolation layer are lattice-matched to the substrate material.

16. The epitaxial structure according to claim 15, wherein, the refractive indices of the lower isolation layer and the upper isolation layer are greater than that of the active region.

17. The epitaxial structure according to claim 16, wherein, the thickness of the lower isolation layer is 100 nm to 4000 nm, and the thickness of the upper isolation layer is 100 nm to 4000 nm.

18. A quantum cascade laser chip, wherein, the chip includes: the epitaxial structure of the quantum cascade laser according to any one of claims 8 - 17, wherein the active region, the upper isolation layer, the upper superlattice waveguide layer and the upper cladding layer of the epitaxial structure form a ridge waveguide structure; a back electrode formed on the back of the substrate of the epitaxial structure away from the lower cladding layer; a dielectric insulating layer formed on the sidewalls of the ridge waveguide structure; and a surface electrode formed on the surface of the upper cladding layer of the epitaxial structure away from the upper superlattice waveguide layer.

19. The quantum cascade laser chip according to claim 18, wherein, the width of the ridge waveguide structure is 5 μm to 50 μm.

20. The quantum cascade laser chip according to claim 18, wherein, the thickness of the dielectric insulating layer is 50 nm to 1000 nm.

21. The quantum cascade laser chip according to claim 20, wherein, The material used for the dielectric insulating layer is selected from SiO 2 , Si 3 N 4 , Si 3 N 4 / SiO 2 and one of them.

22. The quantum cascade laser chip according to claim 18, wherein, the material used for the surface electrode is selected from one of Au, Ti / Au, and Ti / Pt / Au.

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