Quantum Cascade Laser Structure and Its Preparation Method

Abstract

The present invention relates to the technical field of lasers, and discloses a quantum cascade laser structure and a preparation method thereof. The preparation method includes: forming an epitaxial structure including a lower confinement layer, a lower waveguide layer, an active layer, an upper waveguide layer, and an upper confinement layer on a substrate layer; forming a ridge structure including the upper confinement layer, the upper waveguide layer, the active layer, the lower waveguide layer, and a part of the lower confinement layer in the epitaxial structure, the ridge structure including a first surface and a second surface oppositely arranged in a first direction, and the first surface being the surface for light output; forming an isolation structure including a first isolation layer and a second isolation layer on the outer wall surface of the ridge structure, covering the first surface and the second surface respectively. The first isolation layer and the second isolation layer effectively reduce light absorption, thereby reducing the failure risk caused by the temperature rise of the cavity surface, and further reducing the process tolerance and thickness selection of the cavity surface dielectric film system in the subsequent light output direction, ensuring the light output effect and service life.

Description

Technical Field

[0001] The present invention relates to the technical field of lasers, and particularly relates to a quantum cascade laser structure and a preparation method thereof. Background Art

[0002] A quantum cascade laser (QCL for short) is a semiconductor laser capable of emitting lasers in the mid-infrared and far-infrared frequency bands. The output of the previous component in the active region is the input of the next component, and they are connected in series one by one. The spectral range covers the mid-infrared to far-infrared bands, and it can be used in many fields such as trace gas detection and free space optical communication. It has the advantages of high sensitivity and fast detection speed, and has broad market application prospects.

[0003] In the related art of the preparation process of quantum cascade lasers, first, the epitaxial structure is etched into a single-ridge structure; then, the secondary epitaxial growth technology is used to fill indium phosphide doped with iron on the left and right sides of the ridge structure. With its good electrical insulation characteristics and heat conduction characteristics, it ensures the heat dissipation ability and optical confinement effect of the device; then, the front and rear cavity surfaces of the device are cleaved and coated. An antireflection film (AR film) is evaporated on the front cavity surface of the device to ensure light output, and a high-reflection film (HR film) is evaporated on the rear cavity surface. Finally, a resonant cavity structure is formed. Among them, the antireflection film on the front cavity surface uses a yttrium oxide dielectric film to adjust the reflectivity of the front cavity surface, and the high-reflection film uses a composite film system of yttrium oxide + gold + yttrium oxide to provide a reflectivity of more than 90%.

[0004] However, the materials used in the cavity film systems (including AR film and HR film) of the above front and rear cavity surfaces, such as yttrium oxide, aluminum oxide, zirconium dioxide, etc., have strong light absorption in the mid-infrared band, which makes the cavity film systems prone to temperature rise during operation, and then leads to catastrophic optical mirror damage (COMD) on the front and rear cavity surfaces; in addition, in order to ensure the electrical isolation effect of the composite film system of the HR film, its insulating layer often increases in thickness, and the increase in thickness will further lead to enhanced light absorption, thereby increasing the probability of catastrophic optical damage on the rear cavity surface. Moreover, the increase in the thickness of the film layer will also increase the risk of peeling off, resulting in an increased risk of cavity surface failure of the quantum cascade laser structure, and the light output efficiency, service life and reliability are affected. Summary of the Invention

[0005] In view of this, the present invention provides a quantum cascade laser structure and a preparation method thereof to solve the problem that the existing quantum cascade laser structure is prone to cavity surface failure, affecting the light output efficiency, service life and reliability.

[0006] In the first aspect, the present invention provides a preparation method of a quantum cascade laser structure, including:

[0007] An epitaxial structure is formed on a substrate layer. The epitaxial structure includes a lower confinement layer, a lower waveguide layer, an active layer, an upper waveguide layer, and an upper confinement layer formed in sequence.

[0008] A ridge structure is formed on a side of the epitaxial structure facing away from the substrate layer. The ridge structure includes the upper confinement layer, the upper waveguide layer, the active layer, the lower waveguide layer, and a part of the lower confinement layer. The ridge structure includes a first surface and a second surface oppositely arranged in a first direction, and the first direction is the light-emitting direction.

[0009] An isolation structure is formed on an outer wall surface of the ridge structure. The isolation structure includes a first isolation layer and a second isolation layer. The first isolation layer covers the first surface, and the second isolation layer covers the second surface.

[0010] Beneficial effects: Compared with the conventional solution of only etching the two side surfaces (such as the left and right side surfaces) of the epitaxial structure parallel and opposite to the light-emitting direction to form a ridge structure, and then filling materials to limit the laser emission on these two sides; in the present invention, when etching the epitaxial structure, a part of the epitaxial structure is etched and removed in the light-emitting direction to form a ridge structure, and then a first isolation layer and a second isolation layer are respectively covered on the first surface and the second surface opposite to each other in the light-emitting direction (such as the front and back directions) of the ridge structure. The first isolation layer and the second isolation layer can effectively reduce the light absorption on the side of the light-emitting direction, that is, on one side of the front and back cavity surfaces, when the laser outputs, and the first isolation layer and the second isolation layer form a part of the outer wall surface in the light-emitting direction, and can have a certain isolation and insulation effect, effectively reducing the temperature rise of the cavity surface, thereby reducing the process tolerance and thickness selection of the cavity surface dielectric film system in the subsequent light-emitting direction, and finally reducing the cavity surface failure risk of the quantum cascade laser structure and ensuring the light-emitting effect and service life.

[0011] In an optional implementation manner, forming a ridge structure on a side of the epitaxial structure facing away from the substrate layer includes: etching a side of the epitaxial structure facing away from the substrate layer to form a plurality of spaced-apart ridge structures, and the plurality of ridge structures are arranged in a rectangular array. The ridge structure further includes a third surface and a fourth surface oppositely arranged in a second direction, and the second direction forms a preset angle with the first direction; the isolation structure further includes a third isolation layer and a fourth isolation layer, the third isolation layer covers the third surface of the ridge structure, and the fourth isolation layer covers the fourth surface of the ridge structure.

[0012] Beneficial effects: Forming a plurality of ridge structures on the epitaxial structure helps the batch production of the quantum cascade laser structure; and the plurality of ridge structures are arranged in a rectangular array, which is convenient for subsequent cleavage and cutting to form a plurality of quantum cascade laser structures. Each quantum cascade laser structure includes a substrate layer, a lower confinement layer with the same area as the substrate layer, and a ridge structure with a projection area smaller than that of the substrate layer and the lower confinement layer. The isolation structure realizes the all-round insulation isolation of the side wall surface of the ridge structure.

[0013] In an optional embodiment, the thickness of the first isolation layer and the second isolation layer in the first direction ranges from 0.2 μm to 5 μm.

[0014] Beneficial effect: The thickness of the first isolation layer and the second isolation layer is controlled between 0.2μm and 5μm, and preferably set close to 0.2μm, which can not only ensure effective insulation isolation and reduce light absorption effect, avoid leakage current causing short circuit of the laser structure, but also ensure efficient light transmission in the resonant cavity and the reflection performance of the cavity surface, ultimately ensuring the light output efficiency and performance of the quantum cascade laser.

[0015] In an optional embodiment, the isolation structure is annular and integrally formed on the outer wall surface of the ridge structure; the thickness of the third isolation layer and the fourth isolation layer in the second direction is equal to the thickness of the first isolation layer and the second isolation layer in the first direction.

[0016] Beneficial effects: The isolation structure is formed into an integrated annular column, which is simple to prepare and has good structural performance. Its inner wall is close to the outer wall of the ridge structure, and the thickness of the annular column is consistent, which helps to achieve uniform electrical isolation and insulation effect at all parts of the ridge structure and ensure the light output effect.

[0017] In an optional embodiment, after a ridge structure is formed on a side of the epitaxial structure facing away from the substrate layer, and before an isolation structure is formed on an outer wall surface of the ridge structure, the preparation method further includes: forming a first cleavage groove and a second cleavage groove on a surface of the lower restriction layer facing away from the substrate layer, wherein the projection areas of the first cleavage groove and the second cleavage groove on the substrate layer are relatively isolated from the projection area of the ridge structure on the substrate layer; the first cleavage groove is arranged on both sides of the ridge structure along the first direction and extends along the second direction; the second cleavage groove is arranged between two adjacent ridge structures along the second direction and extends along the first direction.

[0018] Beneficial effects: In the first direction, a first cleavage groove is etched on the lower restriction layer between each two adjacent ridge structures, and the two cavity surfaces of the ridge structure in the light-emitting direction are precisely cleaved by controlling the width of the first cleavage groove; in the second direction, a second cleavage groove is etched between two adjacent ridge structures, which simply and quickly realizes the separation between each ridge structure in the second direction, without the need for precise cleavage etching to the third isolation layer and the fourth isolation layer, and the process requirements are relatively low.

[0019] In an optional embodiment, in the first direction, two adjacent ridge structures include two first cleavage grooves, and a distance between the two first cleavage grooves is greater than or equal to 0.5 mm.

[0020] Beneficial effects: Compared with the cleavage of the cavity surfaces of two ridge structures on both sides simultaneously achieved by one first cleavage groove, each ridge structure of the present invention includes a separately configured first cleavage groove on both sides in the first direction, so as to accurately achieve the cleavage of the cavity surfaces on both sides in the light-emitting direction of the ridge structure, and the ridge structures respectively close to one of them can be applicable to epitaxial structures with a variety of ridge structure spacing distances, with a wide application range. In the first direction, the width of the sacrificial area between two first cleavage grooves between two adjacent ridge structures is greater than or equal to 0.5 mm to ensure the smooth completion of the cleavage steps of two adjacent first cleavage channels.

[0021] In an optional implementation manner, the depth range of the first cleavage groove is 0.5 μm to 2 μm, and the width range of the first cleavage groove in the first direction is 0.5 μm to 5 μm; the depth range of the second cleavage groove is 0.2 μm to 2 μm, and the width range of the second cleavage groove in the second direction is 2 μm to 10 μm.

[0022] Beneficial effects: Set the etching width and etching depth of the first cleavage groove and the second cleavage groove to ensure that the epitaxial structure is accurately cleaved and segmented into multiple separate quantum cascade laser structures, improving the cavity surface performance of the quantum cascade laser structures.

[0023] In an optional implementation manner, forming an isolation structure on the outer wall surface of the ridge structure includes: forming the isolation structure by using hydride vapor phase epitaxy technology, and the first isolation layer and the second isolation layer of the isolation structure respectively cover two first cleavage grooves located on both sides of the ridge structure along the first direction.

[0024] Beneficial effects: Selectively grow an isolation structure on the outside of the ridge structure formed after etching by using hydride vapor phase epitaxy (HVPE) technology for electrical isolation and optical confinement, and realize the filling of steep sidewalls. The first isolation layer and the second isolation layer of the isolation structure cover the first cleavage groove to ensure that when cleavage etching is performed at the position of the first cleavage groove, the formed cavity surface includes the isolation structure, and at least one side cavity surface where the laser exits has the isolation structure, ensuring an effective reduction in the absorption of the emitted laser by the isolation structure.

[0025] In an optional implementation manner, after forming the isolation structure on the outer wall surface of the ridge structure, the preparation method further includes: cutting the epitaxial structure and the substrate layer at the first cleavage groove and the second cleavage groove to cleave and form multiple quantum cascade laser structures, and any one quantum cascade laser structure includes a first cavity surface and a second cavity surface oppositely arranged in the first direction, the first cavity surface is coplanar with the outer wall surface of the first isolation layer, and the second cavity surface is coplanar with the outer wall surface of the second isolation layer.

[0026] Beneficial effects: The second cleavage groove extending in the first direction and the first cleavage groove extending in the second direction cut the lower confinement layer and the substrate layer, separating each ridge structure arranged in a rectangular array to form multiple individual quantum cascade laser structures. And at least the cutting along the second direction in the first cleavage groove contacts the first isolation layer and the second isolation layer. The opposite outer wall surfaces formed by the quantum cascade laser structure in the first direction form a first cavity surface and a second cavity surface. The first cavity surface includes the outer wall surface of the first isolation layer, and the second cavity surface includes the outer wall surface of the second isolation layer, so that when a dielectric film system is provided on the first cavity surface and the second cavity surface, it can directly contact the first isolation layer and the second isolation layer, reducing the risk of the dielectric film system peeling off, ensuring that the isolation structure fully reduces the light absorption on the first cavity surface and the second cavity surface in the first direction, thereby avoiding cavity surface failure.

[0027] In an alternative embodiment, after forming the isolation structure on the outer wall surface of the ridge structure and before cutting the epitaxial structure and the substrate layer at the first cleavage groove and the second cleavage groove, it further includes:

[0028] Thinning the surface of the substrate layer on the side facing away from the epitaxial structure;

[0029] Forming a first electrode on the surface of the ridge structure facing away from the substrate layer, and forming a second electrode on the surface of the substrate layer facing away from the first electrode; The cutting of the epitaxial structure and the substrate layer at the first cleavage groove and the second cleavage groove further includes: cutting the second electrode.

[0030] Beneficial effects: Before cutting to form multiple individual quantum cascade laser structures, first thin the lower surface side of the substrate layer to facilitate a better conductive path after the second electrode is formed; then a first electrode is provided on the upper surface of the ridge structure, and a second electrode is provided on the lower surface side of the substrate layer. Both the first electrode and the second electrode are made of metal materials with good conductivity, such as gold, silver, aluminum, etc., and have a small contact resistance with the substrate layer and the surface of the junction structure. When performing cleavage cutting later, the second electrode layer on the lower surface of the substrate layer is cut simultaneously.

[0031] In an alternative embodiment, after cutting the epitaxial structure and the substrate layer at the first cleavage groove and the second cleavage groove, it further includes:

[0032] Forming an antireflection film layer on the first cavity surface, the antireflection film layer including a single-layer structure formed by a first insulating layer;

[0033] Forming a high-reflection film layer on the second cavity surface, the high-reflection film layer including a composite structure formed by sequentially stacking a second insulating layer, a reflective layer, and a third insulating layer.

[0034] Beneficial effects: On the first cavity surface for emitting laser, a single-layer yttrium oxide insulating material is used to form an antireflection film layer (AR film). On one side of the first cavity surface, there is a structure combining a first isolation layer and an antireflection film layer, which can not only enable the laser to fully emit, but also further reduce the thickness of the antireflection film layer. While reducing the temperature rise of the cavity surface, it also reduces the risk of the antireflection film layer peeling off, greatly increasing the cavity surface reliability, light output efficiency, and service life. On the second cavity surface for reflecting laser, a multi-layer composite structure is provided. Among them, the reflective layer uses a gold layer to achieve high reflection of the laser, especially mid-infrared light. A second insulating layer and a third insulating layer are respectively provided on both sides of the reflective layer to achieve an electrical isolation effect. Both the second insulating layer and the third insulating layer use the same yttrium oxide material as the first insulating layer. On one side of the second cavity surface, a structure combining a second isolation layer and a high-reflection film layer is used. The second isolation layer formed by iron-doped indium phosphide (Fe:InP) can effectively reduce the thickness of the second insulating layer and the third insulating layer. As the thickness decreases, the absorption of the second insulating layer and the third insulating layer for the laser also significantly decreases. Therefore, the temperature rise of the second cavity surface will be effectively suppressed, ultimately improving the catastrophic optical damage threshold of the second cavity surface and ensuring the performance of the second cavity surface. The dielectric film system on the first cavity surface and the second cavity surface in the light-emitting direction of the quantum cascade laser structure is improved, ultimately improving the light output efficiency, service life, and reliability of the quantum cascade laser structure.

[0035] In an alternative embodiment, the thicknesses of the second insulating layer and the third insulating layer are equal, and the thicknesses of the second insulating layer and the third insulating layer are 30 nm.

[0036] Beneficial effects: Setting the same thicknesses of the second insulating layer and the third insulating layer in the high-reflection film layer facilitates achieving consistent electrical isolation on both sides of the reflective layer. And due to the setting of the second isolation layer, the thicknesses of the second insulating layer and the third insulating layer can be reduced from 200 nm in the conventional scheme to around 30 nm, greatly reducing the thicknesses of the second insulating layer and the third insulating layer. As the thickness decreases, the absorption of the second insulating layer and the third insulating layer for the laser significantly decreases, and the temperature rise on the second cavity surface will be effectively suppressed, ultimately improving the catastrophic optical damage threshold of the second cavity surface and ensuring the performance of the second cavity surface. Through the setting of the isolation structure, the cavity surface dielectric film system is overall improved, thereby effectively improving the light output power, service life, and reliability of the quantum cascade laser structure.

[0037] Second aspect, the present invention further provides a quantum cascade laser structure, which is prepared by using the preparation method of the above-mentioned quantum cascade laser structure, and includes: a substrate layer, an epitaxial structure, and an isolation structure; the epitaxial structure is disposed on one side surface of the substrate layer, and a ridge structure is provided on the side of the epitaxial structure facing away from the substrate layer. The ridge structure includes an upper confinement layer, an upper waveguide layer, an active layer, a lower waveguide layer, and a part of the lower confinement layer, and the ridge structure includes a first surface and a second surface oppositely disposed in a first direction. The first surface is the surface for light output; the isolation structure is disposed on the outer wall surface of the ridge structure, and the isolation structure includes a first isolation layer and a second isolation layer. The first isolation layer covers the first surface, and the second isolation layer covers the second surface.

[0038] Beneficial effects: In the quantum cascade laser structure of the present invention, the first isolation layer and the second isolation layer are respectively covered on the first surface and the second surface of the ridge structure opposite to each other along the light output direction. In the light output direction, the laser output first passes through the isolation structure and then reaches the dielectric film system at the cavity surface. The first isolation layer and the second isolation layer provided on the outer surface of the ridge structure can effectively reduce the light absorption of the laser directly output to the dielectric film system at the cavity surface, effectively reduce the temperature rise of the cavity surface, and can form a certain insulation isolation within the laser cavity surface, which helps to reduce the thickness of the dielectric film system on the cavity surface, thereby reducing the risk of cavity surface failure caused by the shedding of the dielectric film system. The process tolerance and thickness selection of the dielectric film system on the cavity surface in the subsequent light output direction are reduced, and finally the risk of cavity surface failure of the quantum cascade laser structure is reduced, ensuring the light output effect and service life. In addition, the isolation structure also reduces the probability of contact between the active layer material and the dielectric film system and air on the cavity surface, reducing the possibility of oxidation failure of the active layer material, and further improving the device life and reliability.

[0039] In an optional embodiment, it further includes:

[0040] A first electrode disposed on the side surface of the ridge structure facing away from the substrate layer;

[0041] A second electrode disposed on the side surface of the substrate layer facing away from the first electrode;

[0042] An antireflection film layer disposed on the first cavity surface, and the antireflection film layer includes a single-layer structure formed by a first insulating layer;

[0043] A high-reflection film layer disposed on the second cavity surface, and the high-reflection film layer includes a composite structure in which a second insulating layer, a reflective layer, and a third insulating layer are sequentially stacked; the first cavity surface and the second cavity surface are a set of outer wall surfaces of the quantum cascade laser structure oppositely disposed in the first direction. The first cavity surface includes the outer surface of the first isolation layer in a direction perpendicular to the first direction and a part of the surface on one side of the epitaxial structure, and the second cavity surface includes the outer surface of the second isolation layer in a direction perpendicular to the first direction and a part of the surface on the other side of the epitaxial structure.

[0044] Beneficial effects: A first electrode is disposed on the upper surface of the ridge structure, and a second electrode is disposed on one side of the lower surface of the substrate layer. Both the first electrode and the second electrode are made of metal materials with good electrical conductivity, such as gold, silver, aluminum, etc., and have a small contact resistance with the substrate layer and the surface of the junction structure. A transmissive film layer (AR film) is formed on the first cavity surface for emitting laser by using a yttrium oxide insulating monolayer material. The present invention has a structure in which a first isolation layer and a transmissive film layer are combined on one side of the first cavity surface, which can not only enable the laser to fully emit, but also further reduce the thickness of the transmissive film layer, reduce the risk of peeling of the transmissive film layer while reducing the temperature rise of the cavity surface, and greatly increase the cavity surface reliability, light output efficiency and service life. A multi-layer composite structure is disposed on the second cavity surface for reflecting laser. Among them, the reflective layer uses a gold layer to achieve a high reflection effect on laser, especially mid-infrared light. A second insulating layer and a third insulating layer are respectively disposed on both sides of the reflective layer to achieve an electrical isolation effect. Both the second insulating layer and the third insulating layer use the same yttrium oxide material as the first insulating layer. On one side of the second cavity surface, a structure combining a second isolation layer and a high-reflection film layer is adopted. The second isolation layer formed by iron-doped indium phosphide (Fe:InP) can effectively reduce the thickness of the second insulating layer and the third insulating layer. As the thickness decreases, the absorption of laser by the second insulating layer and the third insulating layer also significantly decreases. Therefore, the temperature rise of the second cavity surface will be effectively suppressed, and finally the catastrophic optical damage threshold of the second cavity surface will be improved to ensure the performance of the second cavity surface. The dielectric film system on the first cavity surface and the second cavity surface in the light-emitting direction of the quantum cascade laser structure is improved, and finally the light output efficiency, service life and reliability of the quantum cascade laser structure are improved. Brief Description of the Drawings

[0045] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0046] Figure 1 It is a schematic flow chart of the preparation method of the quantum cascade laser structure according to the embodiment of the present invention;

[0047] Figure 2 It is a schematic structural diagram after forming an epitaxial structure on a substrate layer according to the embodiment of the present invention;

[0048] Figure 3 It is a schematic structural diagram when forming a ridge structure on the epitaxial structure according to the embodiment of the present invention;

[0049] Figure 4 It is a schematic structural diagram after forming first cleavage grooves on both sides of the ridge structure in the first direction according to the embodiment of the present invention;

[0050] Figure 5 It is a schematic structural view after forming an isolation structure on the outer wall surface of the ridge structure in an embodiment of the present invention;

[0051] Figure 6 It is a schematic structural view after thinning the substrate layer and forming a first electrode and a second electrode in an embodiment of the present invention;

[0052] Figure 7 It is a top - view schematic view after forming a first electrode and a second electrode in an embodiment of the present invention;

[0053] Figure 8 It is a schematic structural view after cleavage cutting at a first cleavage groove and a second cleavage groove in an embodiment of the present invention;

[0054] Figure 9 It is a schematic structural view of a quantum cascade laser structure after setting an antireflection film layer on a first cavity surface and setting a high - reflection film layer on a second cavity surface in an embodiment of the present invention.

[0055] Explanation of reference numerals:

[0056] 1. Substrate layer; 2. Lower confinement layer; 3. Lower waveguide layer; 4. Active layer; 5. Upper waveguide layer; 6. Upper confinement layer; 7. Ridge structure; 701. First surface; 702. Second surface; 703. Third surface; 704. Fourth surface; 8. Isolation structure; 801. First isolation layer; 802. Second isolation layer; 803. Third isolation layer; 804. Fourth isolation layer; 9. First cleavage groove; 10. Second cleavage groove; 11. First cavity surface; 12. Second cavity surface; 13. First electrode; 14. Second electrode; 15. Antireflection film layer; 16. High - reflection film layer; 1601. Second insulating layer; 1602. Reflective layer; 1603. Third insulating layer;

[0057] 100. Mask layer. Detailed implementation manners

[0058] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It can be understood that the specific embodiments described herein are only for explaining the present invention, rather than limiting the present invention. Additionally, it should be noted that for the sake of description, only the parts related to the present invention rather than all the structures are shown in the drawings. In the following description, the description of well-known structures and technologies is omitted to avoid unnecessarily confusing the concepts of the present invention. Various schematic structural diagrams according to embodiments of the present invention are shown in the drawings. These figures are not drawn to scale, and for the purpose of clear expression, some details are enlarged and some details may be omitted. The shapes of various regions and layers shown in the figures, as well as their relative sizes and positional relationships, are only exemplary, and in practice, there may be deviations due to manufacturing tolerances or technical limitations. Those skilled in the art can additionally design regions / layers with different shapes, sizes, and relative positions according to actual requirements. In the context of the present invention, when a layer / component is referred to as being "on" another layer / component, the layer / component can be directly on the other layer / component, or there may be an intermediate layer / component between them. Additionally, if a layer / component is "on" another layer / component in one orientation, then when the orientation is reversed, the layer / component can be "under" the other layer / component.

[0059] In the related art, materials such as yttrium oxide, alumina, and zirconia used in the cavity film systems (including AR films and HR films) on the front and rear cavity surfaces of the quantum cascade laser structure have strong light absorption in the mid-infrared band, which makes the cavity film systems prone to temperature rise during operation, and further leads to catastrophic optical damage (abbreviated as COMD) on the front and rear cavity surfaces. In addition, to ensure the electrical isolation effect of the composite film system of the HR film, the thickness of the insulating layer often increases, and the increase in thickness further leads to enhanced light absorption, thereby increasing the probability of catastrophic optical damage on the rear cavity surface, ultimately resulting in the failure of the cavity surface of the quantum cascade laser structure and affecting its performance and service life.

[0060] It should be noted that the structure of a conventional quantum cascade laser is relatively similar to that of a conventional edge-emitting laser, with differences in the dielectric film system settings on the front and rear cavity surfaces. However, there are two main reasons why conventional edge-emitting lasers do not need to consider cavity surface failure: on the one hand, the laser wavelength of a conventional edge-emitting laser is usually 2μm, and the conventional multilayer dielectric films used in its AR and HR films, such as silicon dioxide, aluminum oxide, and titanium dioxide, do not have significant light absorption in the 2μm band, so there is no problem of cavity surface temperature rise failure; on the other hand, when using multilayer dielectric films, the total thickness of the required dielectric film material is positively correlated with the laser wavelength, that is, the shorter the laser wavelength, the smaller the total thickness of the required dielectric film, the simpler the dielectric film manufacturing process, and the less likely it is to have film layer shedding and other problems, which means that there is no need to consider cavity surface failure. On the one hand, the quantum cascade laser structure has light absorption in common dielectric films within this wavelength range because the laser wavelength is usually above 3.5μm. The longer the laser wavelength, the more severe the absorption of the dielectric film. Especially when the laser wavelength is higher than 9μm, severe light absorption will increase the temperature rise of the cavity surface, thereby causing cavity surface failure. On the other hand, when a multi-layer dielectric film structure is used, the thickness of the dielectric film required for the quantum cascade laser structure is much higher than that of a conventional edge-emitting laser structure, which makes it more prone to dielectric film shedding, thereby causing cavity surface failure of the quantum cascade laser structure. Therefore, a quantum cascade laser structure and its preparation method are urgently needed to avoid cavity surface failure of the quantum cascade laser structure, which affects the light output effect and service life.

[0061] Based on this, see Figures 1 to 9 This embodiment provides a method for preparing a quantum cascade laser structure. Figure 1 Schematic diagram of the preparation method, the preparation method comprises the following steps:

[0062] In step S101 , an epitaxial structure is formed on a substrate layer 1 . The epitaxial structure includes a lower confinement layer 2 , a lower waveguide layer 3 , an active layer 4 , an upper waveguide layer 5 and an upper confinement layer 6 , which are formed in sequence.

[0063] refer to Figure 2 For example, the substrate layer 1 is an indium phosphide substrate; the layers of the epitaxial structure are sequentially grown on the indium phosphide substrate, and the lower confinement layer 2 is 4 μm thick and has a doping concentration of 2×10 16 cm -3 Indium phosphide; the lower waveguide layer 3 has a thickness of 0.1 μm and a doping concentration of 2×10 16 cm -3 The active layer 4 has a thickness of 2 μm and includes a multi-layer quantum well composed of a GaAs-InAs barrier layer and an InAs-AlAs spacer layer; the upper waveguide layer 5 has a thickness of 0.1 μm and a doping concentration of 2×10 16 cm -3The upper confinement layer 6 has a thickness of 5 μm and a doping concentration of 2×10 16 cm -3 Indium phosphide; in addition, an indium phosphide contact layer is formed on the side of the upper limiting layer 6 away from the substrate layer 1, which has a thickness of 1 μm and a doping concentration of 5×10 18 cm -3 The contact layer helps to enhance the ohmic contact with the upper confinement layer 6, thereby ensuring the current injection effect.

[0064] In step S102, a ridge structure 7 is formed on a side of the epitaxial structure facing away from the substrate layer 1. The ridge structure 7 includes an upper confinement layer 6, an upper waveguide layer 5, an active layer 4, a lower waveguide layer 3, and a portion of the lower confinement layer 2. The ridge structure 7 includes a first surface 701 and a second surface 702 that are oppositely arranged in a first direction. The first surface 701 is a surface for emitting light.

[0065] refer to Figure 3 , by providing a patterned mask layer 100 on the side of the epitaxial structure away from the substrate layer 1, the exposed portion of the epitaxial structure is dry-etched to a depth of up to a portion of the lower confinement layer 2, for example, etching the lower confinement layer 2 to about 1 μm, to form a ridge structure 7 protruding from the unetched lower confinement layer 2. The first direction of this embodiment is the light-emitting direction of the quantum cascade laser structure with sidewall light emission, that is, Figure 3 The direction perpendicular to the first surface 701 and the second surface 702 in the horizontal direction. The first surface 701 and the second surface 702 of the ridge structure 7, which are arranged opposite to each other in the first direction, can serve as the front cavity surface and the back cavity surface of the laser structure, respectively, for light extraction and setting a reflective film layer. In addition, the angular deviation of the sidewall of the ridge structure 7 formed by etching from the vertical direction should be less than 2°, and the roughness of the sidewall surface should be less than 100nm to ensure the light extraction efficiency of the ridge structure 7. The formation of the above-mentioned mask layer 100 includes: using plasma enhanced chemical vapor deposition (PECVD) technology to deposit a 300nm silicon dioxide layer on the upper surface of the epitaxial structure; then performing photolithography and selectively etching the silicon dioxide layer using inductively coupled plasma etching (ICPE) technology; and then stripping the resist to obtain a patterned mask layer 100.

[0066] In step S103 , an isolation structure 8 is formed on the outer wall surface of the ridge structure 7 . The isolation structure 8 includes a first isolation layer 801 and a second isolation layer 802 . The first isolation layer 801 covers the first surface 701 , and the second isolation layer 802 covers the second surface 702 .

[0067] For example, in this embodiment, the ridge structure 7 is formed as a rectangular parallelepiped, and the isolation structure 8 is made of iron-doped indium phosphide, a semi-insulating material with low light absorption efficiency. When the isolation structure 8 is formed on the outer wall surface of the etched ridge structure 7, the isolation structure 8 covers at least two opposing sidewalls of the ridge structure 7 in the light-emitting direction. Specifically, the first isolation layer 801 of the isolation structure 8 covers the first surface 701, and the second isolation layer 802 covers the second surface 702. In the light-emitting direction, the laser output first passes through the isolation structure 8 before reaching the dielectric film system at the cavity surface. The provision of the first isolation layer 801 and the second isolation layer 802 on the outer surface of the ridge structure 7 effectively reduces light absorption by the dielectric film system at the cavity surface during direct laser output. Furthermore, the provision of a certain degree of insulation isolation within the laser cavity surface helps reduce the thickness of the dielectric film system at the cavity surface, thereby reducing the risk of cavity surface failure caused by dielectric film shedding. Furthermore, the isolation structure 8 reduces the probability of contact between the active layer 4 material and the cavity surface dielectric film system and air, reducing the possibility of oxidation failure of the active layer 4 material, thereby improving device life and reliability.

[0068] In the method for preparing the quantum cascade laser structure of this embodiment, when etching the epitaxial structure, it is ensured that a portion of the epitaxial structure is etched away in the light-emitting direction to form a ridge structure 7. Subsequently, a first isolation layer 801 and a second isolation layer 802 are respectively covered on a first surface 701 and a second surface 702 of the ridge structure 7 that are opposite to each other in the light-emitting direction. The first isolation layer 801 and the second isolation layer 802 can effectively reduce light absorption of the laser output. The first isolation layer 801 and the second isolation layer 802 constitute part of the outer wall surface in the light-emitting direction, and can have a certain isolation and insulation effect, effectively reducing the increase in the wall surface temperature, thereby reducing the process tolerance and thickness selection of the cavity surface dielectric film system in the subsequent light-emitting direction, ultimately reducing the risk of cavity surface failure of the quantum cascade laser structure, and ensuring the light-emitting effect and service life.

[0069] In one embodiment, the above-mentioned step S101 of forming a ridge structure 7 on the side of the epitaxial structure away from the substrate layer 1 includes: etching the side of the epitaxial structure away from the substrate layer 1 to form a plurality of ridge structures 7 spaced apart from each other, the plurality of ridge structures 7 being arranged in a rectangular array, the ridge structure 7 also including a third surface 703 and a fourth surface 704 arranged relative to each other along a second direction, the second direction forming a preset angle with the first direction; the isolation structure 8 also including a third isolation layer 803 and a fourth isolation layer 804, the third isolation layer 803 covering the third surface 703 of the ridge structure 7, and the fourth isolation layer 804 covering the fourth surface 704 of the ridge structure 7.

[0070] refer to Figure 7, forming a plurality of ridge structures 7 on the epitaxial structure helps in the mass production of the quantum cascade laser structure; and the plurality of ridge structures 7 are arranged in a rectangular array, facilitating subsequent cleavage cutting to form a plurality of quantum cascade laser structures. Each quantum cascade laser structure includes a substrate layer 1, a lower confinement layer 2 having the same area as the substrate layer 1, and a ridge structure 7 with a projected area smaller than that of the substrate layer 1 and the lower confinement layer 2; the cutting can be only for the lower confinement layer 2 and the substrate layer 1, or can be carried out in contact with the isolation structure 8, but the cutting needs to be perpendicular to the surface of the substrate layer 1 to ensure the perpendicularity of the cavity surface of the formed quantum cascade laser structure in the light output direction. The ridge structure 7 is formed into a cuboid, including a first surface 701 and a second surface 702 oppositely arranged in a first direction, and a third surface 703 and a fourth surface 704 oppositely arranged in a second direction. Correspondingly, the isolation structure 8 further includes a third isolation layer 803 covering the third surface 703 and a fourth isolation layer 804 covering the fourth surface 704. The isolation structure 8 realizes the all-round insulation isolation of the side wall surface of the ridge structure 7. In this embodiment, it is preferably that the first direction is perpendicular to the second direction, that is, the second direction is the direction parallel to the first surface 701 and the second surface 702 in the horizontal direction.

[0071] In one embodiment, the thickness range of the above-mentioned first isolation layer 801 and second isolation layer 802 in the first direction is 0.2 μm to 5 μm.

[0072] If the thickness of the first isolation layer 801 and the second isolation layer 802 is less than 0.2 μm, the insulation isolation effect is not obvious, and leakage current may occur. For example, the injected current may cross the second isolation layer 802 and the thinner second insulating layer 1601 and directly transmit downward from the metal reflection layer 1602 in the middle of the high-reflectivity film layer 16 on the second cavity surface 12, resulting in a short circuit of the laser structure; however, if the thickness of the first isolation layer 801 and the second isolation layer 802 exceeds 5 μm, it will affect the light transmission of the resonant cavity of the quantum cascade laser structure, resulting in a weakening of the cavity surface reflection performance and ultimately affecting the performance of the laser structure. Therefore, controlling the thickness of the first isolation layer 801 and the second isolation layer 802 between 0.2 μm and 5 μm, and preferably setting it close to 0.2 μm, can not only ensure effective insulation isolation and reduce the light absorption effect, but also ensure the high-efficiency light transmission in the resonant cavity and the reflection performance of the cavity surface, and ultimately ensure the light output efficiency and performance of the quantum cascade laser.

[0073] In this embodiment, the above-defined thickness is the thickness of the first isolation layer 801 and the second isolation layer 802 in the laser structure after final cleavage. The thickness of the initially grown isolation structure 8 can be greater than the above-defined thickness, and the final thickness control is achieved through the cleavage position and the cleavage etching width.

[0074] Further, referring to Figure 7The isolation structure 8 is annular and integrally formed on the outer wall of the ridge structure 7; the thickness of the third isolation layer 803 and the fourth isolation layer 804 in the second direction is equal to the thickness of the first isolation layer 801 and the second isolation layer 802 in the first direction.

[0075] In this embodiment, the isolation structure 8 is formed as an integrated annular column, which is simple to prepare and has good structural performance. Its inner wall surface is close to the outer wall surface of the ridge structure 7, and the thickness of the annular column is consistent at all locations, which helps to achieve uniform electrical isolation and insulation effects at all locations of the ridge structure 7, thereby ensuring the light output effect.

[0076] On the basis of the above scheme, after step S102 of forming a ridge structure 7 on the side of the epitaxial structure away from the substrate layer 1, and before step S103 of forming an isolation structure 8 on the outer wall surface of the ridge structure 7, the preparation method also includes: forming a first cleavage groove 9 and a second cleavage groove 10 on the surface of the lower restriction layer 2 on the side away from the substrate layer 1, and the projection areas of the first cleavage groove 9 and the second cleavage groove 10 on the substrate layer 1 are relatively isolated from the projection area of the ridge structure 7 on the substrate layer 1; the first cleavage groove 9 is arranged on both sides of the ridge structure 7 along the first direction; the second cleavage groove 10 is arranged between two adjacent ridge structures 7 along the second direction, and extends along the first direction.

[0077] refer to Figure 2 and Figure 7 In the first direction, a first cleavage groove 9 is etched on the lower restriction layer 2 between each two adjacent ridge structures 7. By controlling the width of the first cleavage groove 9, the two cavity surfaces of the ridge structure 7 in the light-emitting direction are precisely cleaved, and the first cleavage groove 9 extends along the second direction; a second cleavage groove 10 is etched between the two adjacent ridge structures 7 in the second direction, which simply and quickly realizes the separation between each ridge structure 7 in the second direction, without the need for precise cleavage etching to the third isolation layer 803 and the fourth isolation layer 804, and the process requirements are relatively low. The second cleavage groove 10 extends along the first direction, and the first cleavage groove 9 and the second cleavage groove 10 of this embodiment both extend to the edge of the lower restriction layer 2 to facilitate cleavage etching positioning.

[0078] Furthermore, in the first direction, two adjacent ridge structures 7 include two first cleavage grooves 9 , and the distance between the two first cleavage grooves 9 is greater than or equal to 0.5 mm.

[0079] In the epitaxial structure of this embodiment, a plurality of ridge structures 7 spaced apart at least in the first direction are formed. Each ridge structure 7 includes a separately configured first cleavage groove 9 on both sides in the first direction, so as to accurately achieve cleavage of the cavity surfaces on both sides in the light-emitting direction of the ridge structure 7. Compared with simultaneously achieving cleavage of the cavity surfaces of two ridge structures 7 on both sides by one first cleavage groove 9, two first cleavage grooves 9 are provided between adjacent ridge structures 7 in the first direction and are respectively close to one of the ridge structures 7. It can be applied to epitaxial structures with various spacing distances of ridge structures 7, and has a wide application range. In the first direction, the area between the two first cleavage grooves 9 between two adjacent ridge structures 7 is a sacrificial area, and the width of this area should be greater than or equal to 0.5 mm to ensure the smooth completion of the cleavage steps of two adjacent first cleavage channels.

[0080] In one embodiment, the depth range of the above-mentioned first cleavage groove 9 is 0.5 μm to 2 μm, and the width range of the first cleavage groove 9 in the first direction is 0.5 μm to 5 μm; the depth range of the second cleavage groove 10 is 0.2 μm to 2 μm, and the width range of the second cleavage groove 10 in the second direction is 2 μm to 10 μm.

[0081] Reference Figure 7 , set the etching width and etching depth of the first cleavage groove 9 and the second cleavage groove 10 to ensure that the epitaxial structure is accurately cleaved and divided into multiple separate quantum cascade laser structures, and improve the cavity surface performance of the quantum cascade laser structures.

[0082] Specifically, referring to Figure 5 and Figure 7 as shown, forming the isolation structure 8 on the outer wall surface of the ridge structure 7 includes: forming the isolation structure 8 by using hydride vapor phase epitaxy technology, and the first isolation layer 801 and the second isolation layer 802 of the isolation structure 8 respectively cover the first cleavage grooves 9 located on both sides of the ridge structure 7 along the first direction.

[0083] Using the hydride vapor phase epitaxy (HVPE) technique, a isolation structure 8 is selectively grown in the annular region outside the ridge structure 7 formed after etching, which can also be called the regrowth region, for electrical isolation and optical confinement. The width of the annular region is greater than the width of the isolation structure 8 layer. This step can only use HVPE for regrowth to achieve the filling of steep sidewalls. The first isolation layer 801 and the second isolation layer 802 of the isolation structure 8 cover the first cleavage groove 9 to ensure that when cleavage etching is performed at the position of the first cleavage groove 9, the cavity surface formed after etching includes the isolation structure 8, and at least on the cavity surface on the side where the laser exits, there is the isolation structure 8, ensuring an effective reduction in the absorption of the emitted laser by the isolation structure 8. In this embodiment, the width of the annular region outside the ridge structure 7 is defined as 20 μm to 40 μm, and the thicknesses in the first direction and the second direction are as close as possible, preferably the same.

[0084] In one embodiment, referring to Figure 7 and Figure 8 , after step S103 of forming the isolation structure 8 on the outer wall surface of the ridge structure 7, the following steps are further included:

[0085] Step S104, cutting the epitaxial structure and the substrate layer 1 at the first cleavage groove 9 and the second cleavage groove 10 to cleave and form a plurality of quantum cascade laser structures. Any quantum cascade laser structure includes a first cavity surface 11 and a second cavity surface 12 that are oppositely arranged along the first direction. The first cavity surface 11 is coplanar with the outer wall surface of the first isolation layer 801, and the second cavity surface 12 is coplanar with the outer wall surface of the second isolation layer 802.

[0086] The second cleavage groove 10 extending along the first direction and the first cleavage groove 9 extending along the second direction cut the lower confinement layer 2 and the substrate layer 1 to separate the ridge structures 7 arranged in a rectangular array, forming a plurality of separate quantum cascade laser structures. And at least the cutting along the second direction in the first cleavage groove 9 contacts the first isolation layer 801 and the second isolation layer 802. The relatively arranged outer wall surfaces of the quantum cascade laser structure formed in the first direction form the first cavity surface 11 and the second cavity surface 12. The first cavity surface 11 includes the outer wall surface of the first isolation layer 801, and the second cavity surface 12 includes the outer wall surface of the second isolation layer 802, so as to directly contact the first isolation layer 801 and the second isolation layer 802 when setting the dielectric film system on the first cavity surface 11 and the second cavity surface 12, reducing the risk of the dielectric film system falling off, and ensuring that the isolation structure 8 sufficiently reduces the light absorption on the first cavity surface 11 and the second cavity surface 12 in the first direction, thereby avoiding cavity surface failure.

[0087] Referring to Figure 6, after step S103 of forming the isolation structure 8 on the outer wall surface of the ridge structure 7 and before step S104 of cutting the epitaxial structure and the substrate layer 1 at the first cleavage groove 9 and the second cleavage groove 10, it further includes: thinning the surface of the substrate layer 1 on the side away from the epitaxial structure; forming a first electrode 13 on the surface of the ridge structure 7 on the side away from the substrate layer 1, and forming a second electrode 14 on the surface of the substrate layer 1 on the side away from the substrate layer 1; cutting the epitaxial structure and the substrate layer 1 at the first cleavage groove 9 and the second cleavage groove 10 further includes: cutting the second electrode 14.

[0088] Before cutting to form multiple separate quantum cascade laser structures, first thin the lower surface side of the substrate layer 1 to facilitate the formation of a better conductive path after the second electrode 14 is formed; then set the first electrode 13 on the upper surface of the ridge structure 7 and set the second electrode 14 on the lower surface side of the substrate layer 1. Both the first electrode 13 and the second electrode 14 are made of metal materials with good electrical conductivity, such as gold, silver, aluminum, etc., and have a small contact resistance with the substrate layer 1 and the surface of the connection structure. When performing cleavage cutting later, cut the second electrode 14 layer on the lower surface of the substrate layer 1 at the same time.

[0089] In one embodiment, refer to Figure 9 , after cutting the epitaxial structure and the substrate layer 1 at the first cleavage groove 9 and the second cleavage groove 10, the preparation method further includes the following steps:

[0090] Step S105, forming an antireflection film layer 15 on the first cavity surface 11, and the antireflection film layer 15 includes a single-layer structure formed by a first insulating layer.

[0091] The first cavity surface 11 of this embodiment is used for emitting laser. Therefore, a yttrium oxide insulating single-layer material is used to form an antireflection film layer 15 (AR film) on the first cavity surface 11. The single-layer yttrium oxide cooperates with the first isolation layer 801, greatly reducing the absorption of laser on the first cavity surface 11, ensuring the full emission of laser while reducing the temperature rise of the cavity surface, thereby reducing the risk of failure of the first cavity surface 11.

[0092] In the conventional solution for the first cavity surface 11, in order to avoid catastrophic optical damage (COMD), a dielectric-free uncoating structure is usually adopted. Briefly speaking, the first cavity surface 11 uses a natural cleavage surface as the reflection surface without additionally depositing an antireflection film layer 15, that is, no antireflection film layer 15 is deposited after cleavage; or an antireflection film layer 15 with a thickness less than 30 nm is deposited to reduce the light absorption of the antireflection film layer 15 on the first cavity surface 11. The structure of this embodiment having the first isolation layer 801 and the antireflection film layer 15 on one side of the first cavity surface 11 can not only achieve sufficient laser emission but also further reduce the thickness of the antireflection film layer 15, thereby greatly reducing the risk of peeling off of the antireflection film layer 15 and significantly increasing the light output efficiency and service life.

[0093] Step S106, form a high-reflection film layer 16 on the second cavity surface 12, and the high-reflection film layer 16 includes a composite structure formed by sequentially stacking a second insulating layer 1601, a reflection layer 1602, and a third insulating layer 1603.

[0094] The second cavity surface 12 of this embodiment is used to reflect the laser so that the laser exits from the first cavity surface 11 opposite to the second cavity surface 12. A multi-layer stacked composite structure is provided on the second cavity surface 12, wherein the reflection layer 1602 uses a gold layer to achieve a high-reflection effect on the laser, especially mid-infrared light. However, the high electrical conductivity of gold easily causes the device to short-circuit. Therefore, the second insulating layer 1601 and the third insulating layer 1603 are respectively provided on both sides of the reflection layer 1602 to achieve an electrical isolation effect. Both the second insulating layer 1601 and the third insulating layer 1603 use the same yttrium oxide material as the first insulating layer. In the conventional solution, in order to achieve effective electrical isolation of the reflection layer 1602, the thicknesses of the second insulating layer 1601 and the third insulating layer 1603 on both sides of the reflection layer 1602 are usually not less than 200 nm, with large light absorption, and the overall thickness of the high-reflection film layer 16 is very thick, which is extremely easy to peel off and cause the failure of the second cavity surface 12. In this solution, on one side of the second cavity surface 12, the second isolation layer 802 and the high-reflection film layer 16 are combined. The second isolation layer 802 formed by iron-doped indium phosphide (Fe:InP) can effectively reduce the thicknesses of the second insulating layer 1601 and the third insulating layer 1603. As the thickness decreases, the absorption of the laser by the second insulating layer 1601 and the third insulating layer 1603 also significantly decreases. Therefore, the temperature rise of the high-reflection cavity, that is, the second cavity surface 12, can be effectively suppressed, and finally the catastrophic optical damage threshold of the second cavity surface 12 is improved to ensure the performance of the second cavity surface 12.

[0095] Specifically, the thicknesses of the second insulating layer 1601 and the third insulating layer 1603 are equal, and the thicknesses of the second insulating layer 1601 and the third insulating layer 1603 are 30 nm.

[0096] In this embodiment, the thicknesses of the second insulating layer 1601 and the third insulating layer 1603 in the high-reflection film layer 16 are set to be the same, which is convenient for achieving consistent electrical isolation on both sides of the reflective layer 1602. Moreover, due to the provision of the second isolation layer 802, the thicknesses of the second insulating layer 1601 and the third insulating layer 1603 can be reduced from 200 nm in the conventional solution to around 30 nm, greatly reducing the thicknesses of the second insulating layer 1601 and the third insulating layer 1603. As the thickness decreases, the absorption of the second insulating layer 1601 and the third insulating layer 1603 for the laser is significantly reduced, and the temperature rise on the second cavity surface 12 can be effectively suppressed, ultimately improving the catastrophic optical damage threshold of the second cavity surface 12 and ensuring the performance of the second cavity surface 12. Through the provision of the isolation structure 8, the cavity surface dielectric film system is improved as a whole, thereby effectively increasing the optical output power, service life and reliability of the quantum cascade laser structure.

[0097] As Figure 8 and Figure 9 shown, this embodiment further provides a quantum cascade laser structure, which is prepared by using the preparation method of the above-mentioned quantum cascade laser structure, and includes: a substrate layer 1, an epitaxial structure and an isolation structure 8; the epitaxial structure is disposed on one side surface of the substrate layer 1, and the side of the epitaxial structure facing away from the substrate layer 1 has a ridge structure 7, and the ridge structure 7 includes an upper confinement layer 6, an upper waveguide layer 5, an active layer 4, a lower waveguide layer 3 and a part of the lower confinement layer 2, and the ridge structure 7 includes a first surface 701 and a second surface 702 that are oppositely disposed in a first direction, and the first surface 701 is the surface for light output; the isolation structure 8 is disposed on the outer wall surface of the ridge structure 7, and the isolation structure 8 includes a first isolation layer 801 and a second isolation layer 802, the first isolation layer 801 covers the first surface 701, and the second isolation layer 802 covers the second surface 702.

[0098] In the quantum cascade laser structure of this embodiment, the first isolation layer 801 and the second isolation layer 802 are respectively covered on the first surface 701 and the second surface 702 of the ridge structure 7 that are opposite in the light output direction. In the light output direction, the laser output first passes through the isolation structure 8 and then reaches the dielectric film system at the cavity surface. The provision of the first isolation layer 801 and the second isolation layer 802 on the outer surface of the ridge structure 7 can effectively reduce the light absorption of the laser directly output to the dielectric film system at the cavity surface, effectively reduce the temperature rise of the cavity surface, and can form a certain insulating isolation in the laser cavity surface, which helps to reduce the thickness of the cavity surface dielectric film system, thereby reducing the risk of cavity surface failure caused by the shedding of the dielectric film system. The process tolerance and thickness selection of the cavity surface dielectric film system in the subsequent light output direction are reduced, ultimately reducing the risk of cavity surface failure of the quantum cascade laser structure and ensuring the light output effect and service life. In addition, the isolation structure 8 also reduces the probability of contact between the material of the active layer 4 and the dielectric film system and air at the cavity surface, reducing the possibility of oxidation failure of the material of the active layer 4, and further improving the device life and reliability.

[0099] In one embodiment, with reference to Figure 9 , the quantum cascade laser structure of this embodiment further includes: a first electrode 13, a second electrode 14, an antireflection film layer 15, and a high-reflection film layer 16. The first electrode 13 is disposed on a surface of the ridge structure 7 facing away from the substrate layer 1; the second electrode 14 is disposed on a surface of the substrate layer 1 facing away from the first electrode 13; the antireflection film layer 15 is disposed on the first cavity surface 11, and the antireflection film layer 15 includes a single-layer structure formed by a first insulating layer; the high-reflection film layer 16 is disposed on the second cavity surface 12, and the high-reflection film layer 16 includes a composite structure in which a second insulating layer 1601, a reflective layer 1602, and a third insulating layer 1603 are stacked in sequence; the first cavity surface 11 and the second cavity surface 12 are a set of outer wall surfaces of the quantum cascade laser structure oppositely disposed in a first direction. The first cavity surface 11 includes an outer surface of the first isolation layer 801 in a direction perpendicular to the first direction and a partial surface on one side of the epitaxial structure. The second cavity surface 12 includes an outer surface of the second isolation layer 802 in a direction perpendicular to the first direction and a partial surface on the other side of the epitaxial structure.

[0100] The first electrode 13 is disposed on the upper surface of the ridge structure 7, and the second electrode 14 is disposed on one side of the lower surface of the substrate layer 1. Both the first electrode 13 and the second electrode 14 are made of a metal material with good electrical conductivity, such as gold, silver, aluminum, etc., and have a small contact resistance with the substrate layer 1 and the surface of the contact structure. On the first cavity surface 11 for emitting laser, a yttrium oxide insulating single-layer material is used to form the antireflection film layer 15 (AR film). In this embodiment, a structure combining the first isolation layer 801 and the antireflection film layer 15 is provided on one side of the first cavity surface 11, which can not only enable the laser to fully emit, but also further reduce the thickness of the antireflection film layer 15, reduce the risk of peeling off of the antireflection film layer 15 while reducing the temperature rise of the cavity surface, and greatly increase the cavity surface reliability, light output efficiency, and service life. Figure 9The right arrow on the right indicates the emitted laser. A multi-layer composite structure is provided on the second cavity surface 12 for reflecting the laser. The reflective layer 1602 is made of a gold layer to achieve high reflection of the laser, especially mid-infrared light. The second insulating layer 1601 and the third insulating layer 1603 are respectively provided on both sides of the reflective layer 1602 to achieve an electrical isolation effect. Both the second insulating layer 1601 and the third insulating layer 1603 are made of the same yttrium oxide material as the first insulating layer. On one side of the second cavity surface 12, a structure combining the second isolation layer 802 and the high-reflection film layer 16 is adopted. The second isolation layer 802 formed by iron-doped indium phosphide (Fe:InP) can effectively reduce the thickness of the second insulating layer 1601 and the third insulating layer 1603. As the thickness decreases, the absorption of the laser by the second insulating layer 1601 and the third insulating layer 1603 also significantly decreases. Therefore, the temperature rise of the second cavity surface 12 can be effectively suppressed, ultimately improving the catastrophic optical damage threshold of the second cavity surface 12 and ensuring the performance of the second cavity surface 12. The dielectric film systems on the first cavity surface 11 and the second cavity surface 12 in the light-emitting direction of the quantum cascade laser structure are improved, ultimately improving the light output efficiency, service life, and reliability of the quantum cascade laser structure.

[0101] In the above description, no detailed explanations are made for technical details such as the patterning and etching of each layer. However, those skilled in the art should understand that various technical means can be used to form layers, regions, etc. of the desired shape. In addition, in order to form the same structure, those skilled in the art can also design methods that are not exactly the same as the methods described above. In addition, although the above embodiments are described separately, this does not mean that the measures in each embodiment cannot be used advantageously in combination.

[0102] Although the embodiments of the present invention are described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the present invention, and such modifications and variations fall within the scope defined by the appended claims.

Claims

1. A method for preparing a quantum cascade laser structure, the laser wavelength of the quantum cascade laser structure being above 3.5 μm, characterized in that, Comprising: Forming an epitaxial structure on a substrate layer, the epitaxial structure including a lower confinement layer, a lower waveguide layer, an active layer, an upper waveguide layer, and an upper confinement layer formed in sequence; Forming a ridge structure on a side of the epitaxial structure facing away from the substrate layer, the ridge structure including the upper confinement layer, the upper waveguide layer, the active layer, the lower waveguide layer, and a part of the lower confinement layer, and the ridge structure including a first surface and a second surface oppositely arranged in a first direction, the first surface being a surface for light output; Forming an isolation structure on an outer wall surface of the ridge structure, the isolation structure including a first isolation layer and a second isolation layer, the first isolation layer covering the first surface, and the second isolation layer covering the second surface; an antireflection film layer is formed on an outer wall surface of the first isolation layer, and a high-reflection film layer is formed on an outer wall surface of the second isolation layer.

2. The preparation method of the quantum cascade laser structure according to claim 1, wherein, The forming the ridge structure on a side of the epitaxial structure facing away from the substrate layer includes: Etching a side of the epitaxial structure facing away from the substrate layer to form a plurality of spaced-apart ridge structures, the plurality of ridge structures being arranged in a rectangular array, the ridge structure further including a third surface and a fourth surface oppositely arranged in a second direction, the second direction forming a preset angle with the first direction; the isolation structure further includes a third isolation layer and a fourth isolation layer, the third isolation layer covering the third surface of the ridge structure, and the fourth isolation layer covering the fourth surface of the ridge structure.

3. The preparation method of the quantum cascade laser structure according to claim 2, characterized in that, The thickness range of the first isolation layer and the second isolation layer in the first direction is 0.2 μm to 5 μm.

4. The method for preparing the quantum cascade laser structure according to claim 3, characterized in that, The isolation structure is annular and integrally formed on the outer wall surface of the ridge structure; the thickness of the third isolation layer and the fourth isolation layer in the second direction is equal to the thickness of the first isolation layer and the second isolation layer in the first direction.

5. The preparation method of the quantum cascade laser structure according to any one of claims 2-4, characterized in that, After forming the ridge structure on a side of the epitaxial structure facing away from the substrate layer and before forming the isolation structure on the outer wall surface of the ridge structure, the preparation method further includes: Forming a first cleavage groove and a second cleavage groove on a surface of the lower confinement layer facing away from the substrate layer, a projection area of the first cleavage groove and the second cleavage groove on the substrate layer being relatively isolated from a projection area of the ridge structure on the substrate layer; the first cleavage groove is disposed on two sides of the ridge structure along the first direction and extends along the second direction; the second cleavage groove is disposed between two adjacent ridge structures along the second direction and extends along the first direction.

6. The preparation method of the quantum cascade laser structure according to claim 5, characterized in that, In the first direction, there are two first cleavage grooves between two adjacent ridge structures, and the distance between the two first cleavage grooves is greater than or equal to 0.5 mm.

7. The preparation method of the quantum cascade laser structure according to claim 6, characterized in that, The depth range of the first cleavage groove is 0.5 μm to 2 μm, and the width range of the first cleavage groove in the first direction is 0.5 μm to 5 μm; the depth range of the second cleavage groove is 0.2 μm to 2 μm, and the width range of the second cleavage groove in the second direction is 2 μm to 10 μm.

8. The manufacturing method of the quantum cascade laser structure according to claim 7, characterized in that, Forming an isolation structure on the outer wall surface of the ridge structure includes: forming the isolation structure by using metalorganic chemical vapor deposition technology, and the first isolation layer and the second isolation layer of the isolation structure respectively cover two first cleavage grooves located on both sides of the ridge structure along the first direction.

9. The preparation method of the quantum cascade laser structure according to claim 8, characterized in that, After forming the isolation structure on the outer wall surface of the ridge structure, the preparation method further includes: Cutting the epitaxial structure and the substrate layer at the first cleavage groove and the second cleavage groove to cleave and form a plurality of quantum cascade laser structures, and any one of the quantum cascade laser structures includes a first cavity surface and a second cavity surface oppositely arranged along the first direction, the first cavity surface is coplanar with the outer wall surface of the first isolation layer, and the second cavity surface is coplanar with the outer wall surface of the second isolation layer.

10. The preparation method of the quantum cascade laser structure according to claim 9, characterized in that, After forming the isolation structure on the outer wall surface of the ridge structure and before cutting the epitaxial structure and the substrate layer at the first cleavage groove and the second cleavage groove, it further includes: Thinning the surface of the substrate layer facing away from the epitaxial structure; Forming a first electrode on the surface of the ridge structure facing away from the substrate layer, and forming a second electrode on the surface of the substrate layer facing away from the first electrode; cutting the second electrode is also included in cutting the epitaxial structure and the substrate layer at the first cleavage groove and the second cleavage groove.

11. The method for preparing the quantum cascade laser structure according to claim 10, wherein After cutting the epitaxial structure and the substrate layer at the first cleavage groove and the second cleavage groove, it further includes: Forming an antireflection film layer on the first cavity surface, and the antireflection film layer includes a single-layer structure formed by a first insulating layer; Forming a high-reflection film layer on the second cavity surface, and the high-reflection film layer includes a composite structure formed by sequentially stacking a second insulating layer, a reflection layer, and a third insulating layer.

12. The preparation method of the quantum cascade laser structure according to claim 11, characterized in that, The thicknesses of the second insulating layer and the third insulating layer are equal, and the thicknesses of the second insulating layer and the third insulating layer are 30 nm.

13. A quantum cascade laser structure, characterized in that, Prepared by using the preparation method of the quantum cascade laser structure according to any one of claims 1-12, including: A substrate layer; An epitaxial structure disposed on one side surface of the substrate layer; a ridge structure is provided on the side surface of the epitaxial structure facing away from the substrate layer, the ridge structure includes an upper confinement layer, an upper waveguide layer, an active layer, a lower waveguide layer, and a part of the lower confinement layer, and the ridge structure includes a first surface and a second surface oppositely arranged in a first direction, and the first surface is the surface for light output; An isolation structure disposed on the outer wall surface of the ridge structure, the isolation structure includes a first isolation layer and a second isolation layer, the first isolation layer covers the first surface, and the second isolation layer covers the second surface; an antireflection film layer is formed on the outer wall surface of the first isolation layer, and a high-reflection film layer is formed on the outer wall surface of the second isolation layer.

14. The quantum cascade laser structure according to claim 13, wherein, It further includes: A first electrode disposed on the surface of the ridge structure facing away from the substrate layer; A second electrode disposed on the surface of the substrate layer facing away from the first electrode; An antireflection film layer disposed on the first cavity surface, and the antireflection film layer includes a single-layer structure formed by a first insulating layer; The high-reflection film layer is disposed on the second cavity surface. The high-reflection film layer includes a composite structure in which a second insulating layer, a reflective layer, and a third insulating layer are stacked in sequence; the first cavity surface and the second cavity surface are a set of outer wall surfaces of the quantum cascade laser structure that are oppositely disposed in the first direction. The first cavity surface includes the outer surface of the first isolation layer in a direction perpendicular to the first direction and a partial surface on one side of the epitaxial structure, and the second cavity surface includes the outer surface of the second isolation layer in a direction perpendicular to the first direction and a partial surface on the other side of the epitaxial structure.

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