Epitaxial structure and preparation method thereof, and VCSEL chip

Abstract

The invention relates to an epitaxial structure and a preparation method thereof, and a VCSEL chip, the epitaxial structure comprises a first Bragg reflection layer, a second Bragg reflection layer and an active layer, the first Bragg reflection layer and the second Bragg reflection layer are stacked, and the first Bragg reflection layer and the second Bragg reflection layer have different conduction types; the active layer is located between the first Bragg reflection layer and the second Bragg reflection layer and comprises a plurality of barrier layers and well layers which are stacked alternately, and each well layer and two adjacent barrier layers form a quantum well. The composition and / or thickness of the well layer and the barrier layer are designed such that the state densities of each quantum well formed at the predetermined energy level are substantially the same. According to the epitaxial structure, the preparation method thereof and the VCSEL chip provided by the invention, carriers of each quantum well are uniformly distributed, so that the radiation recombination efficiency and the light field uniformity of a laser device are remarkably improved.

Description

Technical Field

[0001] The present application relates to the technical field of semiconductor lasers, and particularly to an epitaxial structure, a preparation method thereof, and a VCSEL chip. Background Art

[0002] Vertical cavity surface emitting lasers (VCSELs) have many excellent characteristics, such as high resolution, low power consumption, easy integration, high reliability, etc., making them highly favored in scenarios such as short-distance optical communication, laser printing, 3D sensing, etc.

[0003] As the main light-emitting region of the VCSEL, the active layer of multiple quantum wells often has the problem of uneven carrier distribution in the quantum wells, causing carriers to escape from the quantum wells, which seriously affects the luminescence recombination efficiency of the carriers. Moreover, further from the perspective of energy conversion, unevenly distributed carriers cannot efficiently interact with photons in the effective region of the quantum wells. As a result, the energy of most carriers cannot be fully converted into light energy but is dissipated in the form of heat. This not only reduces the light-emitting efficiency of the laser but may also cause local overheating of the device, thereby affecting the stability and lifespan of the device.

[0004] In the prior art, the main means to suppress the electron overflow in the quantum wells is to increase the electron blocking layer. However, increasing the electron blocking layer will lead to an increase in the overall film thickness of the device, which not only increases the manufacturing process difficulty but also directly causes an increase in the overall resistance of the device, reduces the optoelectronic conversion efficiency of the device, and increases the heat dissipation difficulty of the device.

[0005] Therefore, how to achieve a uniform distribution of carriers in the active layer of the laser has become a technical problem that needs to be urgently solved by those skilled in the art. Summary of the Invention

[0006] Based on this, it is necessary to provide an epitaxial structure, a preparation method thereof, and a VCSEL chip for the problem of uneven carrier distribution in the quantum wells in the prior art.

[0007] To achieve the above object, on the one hand, the present application provides an epitaxial structure, including:

[0008] A first Bragg reflector and a second Bragg reflector stacked, wherein the first Bragg reflector and the second Bragg reflector have different conductivity types;

[0009] An active layer located between the first Bragg reflector and the second Bragg reflector. The active layer includes a plurality of alternately stacked well layers and barrier layers. Each well layer and the two adjacent barrier layers form a quantum well. The composition and / or thickness of the well layer and the barrier layer are designed so that the density of states formed at a preset energy level in each quantum well is substantially the same.

[0010] In one embodiment, the preset energy level is the ground state energy level.

[0011] In one embodiment, along the stacking direction of the epitaxial structure, the well widths of the quantum wells gradually change according to a first variation trend; and / or,

[0012] the well depths of the quantum wells gradually change according to a second variation trend, and one of the first variation trend and the second variation trend is a decreasing variation trend, and the other is an increasing variation trend.

[0013] In one embodiment, the physical thicknesses of the well layers of the quantum wells gradually change according to the first variation trend.

[0014] In one embodiment, the band gaps of the well layers of the active layer gradually change according to the first variation trend, and / or, the band gaps of the barrier layers of the active layer gradually change according to the second variation trend.

[0015] In one embodiment, the well layer includes a first element, the barrier layer includes a second element, and the composition of the first element in each well layer of the active layer gradually changes according to the second variation trend; and / or

[0016] the composition of the second element in each barrier layer of the active layer gradually changes according to the second variation trend.

[0017] In one embodiment, the composition of the second element in each barrier layer of the active layer changes linearly.

[0018] In one embodiment, the conductivity type of the first Bragg reflection layer is N-type, the conductivity type of the second Bragg reflection layer is P-type, the first variation trend is a decreasing variation trend, and the second variation trend is an increasing variation trend.

[0019] In one embodiment, in the quantum well closest to the first Bragg reflection layer, the band gap of the barrier layer close to the first Bragg reflection layer is greater than the band gap of the adjacent barrier layer far from the first Bragg reflection layer, and the band gap of the barrier layer closest to the first Bragg reflection layer is not greater than the band gap of the barrier layer closest to the second Bragg reflection layer.

[0020] This application also provides a method for preparing an epitaxial structure, including:

[0021] Providing a substrate, and epitaxially growing a first Bragg reflection layer on the substrate;

[0022] An active layer is epitaxially grown on the first Bragg reflection layer. The active layer includes a plurality of alternately stacked well layers and barrier layers. Each well layer and two adjacent barrier layers form a quantum well. The compositions and / or thicknesses of the well layer and the barrier layer are designed such that the density of states formed in each quantum well at a preset energy level is substantially the same;

[0023] A second Bragg reflection layer is epitaxially grown on the active layer. Among them, the first Bragg reflection layer and the second Bragg reflection layer have different conduction types.

[0024] In one embodiment, the epitaxial growth of the active layer on the first Bragg reflection layer includes:

[0025] The well layer and the barrier layer are alternately grown on the first Bragg reflection layer. In the epitaxial growth direction, the well widths of the quantum wells are controlled to gradually change according to a first variation trend, and / or,

[0026] The well depths of the quantum wells are controlled to gradually change according to a second variation trend. One of the first variation trend and the second variation trend is a decreasing variation trend, and the other is an increasing variation trend.

[0027] In one embodiment, the control of the well widths of the quantum wells to gradually change according to a first variation trend includes:

[0028] The growth time of the well layer of each quantum well is regulated to gradually change according to the first variation trend, so that the physical thickness of the well layer of each quantum well gradually changes according to the first variation trend.

[0029] In one embodiment, the control of the well depths of the quantum wells to gradually change according to a second variation trend includes:

[0030] The band gaps of the well layers of the active layer are controlled to gradually change according to a first variation trend, and / or, the band gaps of the barrier layers of the active layer are controlled to gradually change according to a second variation trend.

[0031] In one embodiment, the well layer includes a first element, the barrier layer includes a second element, and the control of the band gaps of the well layers of the active layer to gradually change according to a first variation trend includes:

[0032] The flow rate of the first gas source for generating the first element in each well layer is regulated to gradually change according to the second variation trend, so that the composition of the first element in each well layer of the active layer gradually changes according to the second variation trend;

[0033] The bandgap widths of the barrier layers that control the active layer gradually change according to a second changing trend, including:

[0034] Adjusting the flow rate of the second gas source that regulates the generation of the second element in each of the barrier layers to gradually change according to the second changing trend, so that the composition of the second element in each of the barrier layers of the active layer gradually changes according to the second changing trend.

[0035] This application also provides a VCSEL chip, including the epitaxial structure described in any of the above embodiments.

[0036] In one embodiment, the VCSEL chip further includes a photoelectric confinement layer, the photoelectric confinement layer is located on the side of the second Bragg reflection layer close to the active layer, and the photoelectric confinement layer is configured to define the light-emitting area of the VCSEL chip.

[0037] In one embodiment, the VCSEL chip further includes a first electrode and a second electrode. The first electrode is an N-type electrode for connecting the negative pole of an external circuit, and the second electrode is a P-type electrode for connecting the positive pole of an external circuit.

[0038] For the above epitaxial structure and its preparation method, and the VCSEL chip, by arranging an active layer between a first Bragg reflection layer and a second Bragg reflection layer with opposite conduction types, and the active layer is composed of multiple alternating well layers and barrier layers, each well layer and the adjacent barrier layer form a quantum well, the active layer includes multiple quantum wells, and the composition and / or thickness of the well layer and the barrier layer are designed so that the density of states formed at a preset energy level in each quantum well is basically the same, ensuring that the injected carriers are evenly distributed among the quantum wells, effectively avoiding the non-radiative recombination loss caused by the aggregation of carriers in local quantum wells, thereby significantly improving the radiative recombination efficiency and light field uniformity of the device. At the same time, the above epitaxial structure is conducive to enhancing the interaction between carriers and photons in the active layer, improving the photoelectric conversion efficiency of the laser, and is conducive to the high-power and high-efficiency application of vertical-cavity surface-emitting laser devices. And in some embodiments, by setting in the quantum well closest to the first Bragg reflection layer, the energy band height of the barrier layer close to the first Bragg reflection layer is higher than that of the adjacent barrier layer far from the first Bragg reflection layer, and the energy band height of the barrier layer close to the first Bragg reflection layer is not higher than that of the barrier layer closest to the second Bragg reflection layer, ensuring the smooth injection of carriers, improving the injection efficiency of carriers, enhancing the confinement ability of the quantum well to carriers, effectively suppressing the leakage or escape of carriers, and at the same time, in the quantum well closest to the first Bragg reflection layer, the barrier layer close to the first Bragg reflection layer can also serve as an electron injection barrier to effectively suppress the carrier backflow. Description of the Drawings

[0039] To more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0040] Figure 1 It is a schematic cross-sectional structure diagram of an epitaxial structure provided in an embodiment;

[0041] Figure 2 It is a schematic energy band structure diagram of an epitaxial structure provided in an embodiment;

[0042] Figure 3 It is a flowchart of a preparation method of an epitaxial structure provided in an embodiment;

[0043] Figure 4 It is a schematic cross-sectional structure diagram of a substrate in a preparation method of an epitaxial structure provided in an embodiment;

[0044] Figure 5 It is a schematic cross-sectional structure diagram after forming a first Bragg reflector layer in a preparation method of an epitaxial structure provided in an embodiment;

[0045] Figure 6 It is a schematic cross-sectional structure diagram after forming an active layer in a preparation method of an epitaxial structure provided in an embodiment;

[0046] Figure 7 It is a schematic cross-sectional structure diagram of a VCSEL chip provided in an embodiment.

[0047] Explanation of reference numerals:

[0048] 1 - First Bragg reflector layer, 2 - Second Bragg reflector layer, 3 - Active layer, 31 - Well layer, 32 - Barrier layer, 33 - Quantum well, 4 - Substrate, 5 - Optical confinement layer, 6 - First electrode, 7 - Second electrode. Detailed implementation manners

[0049] To facilitate the understanding of the present application, the following will describe the present application more comprehensively with reference to the relevant drawings. Embodiments of the present application are given in the drawings. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thorough and comprehensive.

[0050] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terms used in the description of this application herein are for the purpose of describing specific embodiments only and are not intended to limit this application.

[0051] It should be understood that when an element or layer is referred to as "on", "adjacent to", "connected to" or "coupled to" another element or layer, it can be directly on, adjacent to, connected or coupled to the other element or layer, or there may be intervening elements or layers. In contrast, when an element is referred to as "directly on", "directly adjacent to", "directly connected to" or "directly coupled to" another element or layer, there are no intervening elements or layers. It should be understood that although the terms first, second, third, etc. may be used to describe various elements, components, regions, layers, doping types and / or parts, these elements, components, regions, layers, doping types and / or parts should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, doping type or part from another element, component, region, layer, doping type or part. Thus, without departing from the teachings of the present invention, the first element, component, region, layer, doping type or part discussed below may be denoted as the second element, component, region, layer or part; for example, the first doping type may be referred to as the second doping type, and similarly, the second doping type may be referred to as the first doping type; the first doping type and the second doping type are different doping types. For example, the first doping type may be P-type and the second doping type may be N-type, or the first doping type may be N-type and the second doping type may be P-type.

[0052] Spatial relationship terms such as "under", "below", "lower", "beneath", "above", "upper", etc. may be used herein to describe the relationship of one element or feature shown in the figures to other elements or features. It should be understood that, in addition to the orientation shown in the figures, spatial relationship terms also include different orientations of the device in use and operation. For example, if the device in the figures is flipped, an element or feature described as "under" or "beneath" or "underneath" another element or feature will be oriented "on" the other element or feature. Thus, the exemplary terms "under" and "beneath" can include both an upper and a lower orientation. In addition, the device may also include additional orientations (such as rotating 90 degrees or other orientations), and the spatial descriptors used herein are to be interpreted accordingly.

[0053] As used herein, the singular forms "a", "an" and "the" may also include the plural forms unless the context clearly dictates otherwise. It should also be understood that the terms "comprising", "including" or "having", etc., specify the presence of the stated features, integers, steps, operations, components, parts or combinations thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, components, parts or combinations thereof. At the same time, in this specification, the term "and / or" includes any and all combinations of the related listed items.

[0054] Please refer to Figures 1 to 2 (wherein Figure 2 the energy band diagram shown above represents the bottom of the conduction band, and the energy band diagram shown below represents the top of the valence band), the present application provides an epitaxial structure, including: a first Bragg reflection layer 1, an active layer 3 and a second Bragg reflection layer 2 which are stacked, wherein the first Bragg reflection layer 1 and the second Bragg reflection layer 2 have different conduction types; the active layer 3 is located between the first Bragg reflection layer 1 and the second Bragg reflection layer 2, and the active layer 3 includes a plurality of alternately stacked well layers 31 and barrier layers 32, and each well layer 31 and two adjacent barrier layers form a quantum well 33, and the composition and / or thickness of the well layer 31 and the barrier layer 32 are designed so that the density of states formed by each quantum well 33 at a preset energy level is substantially the same.

[0055] It should be noted that the "substantially the same" here is because in the actual epitaxial growth and device operation process, the factors affecting the density of states have a certain degree of complexity and uncontrollability. For example, in the actual epitaxial process, due to composition, temperature fluctuations or slight differences in growth rate, the strain state or energy band fine-tuning between different quantum wells 33 is slightly different, affecting the density distribution of carriers at the energy level; the setting of multiple well layers 31 and barrier layers 32 in the active layer 3 will inevitably have sub-nanometer thickness fluctuations and interfacial mixing layers, which will further cause slight differences between the actual well depth and well width of each quantum well 33 and the ideal state, resulting in slight deviations in the electron and hole wave function distributions and density of states of different quantum wells 33; during the operation of the device, especially under high power density conditions, there may be an electric field or a thermal field gradient at the positions where each quantum well 33 is located, which will affect the energy band structure and carrier distribution, and further cause slight differences in the dynamic density of states; and in the present application, by optimizing the design of the thickness and composition, it is ensured that near the preset energy level, the effective density of states distributions of each quantum well 33 are at the same level within the process error range, which is sufficient to meet the key performance requirements such as luminescence consistency and gain consistency of the laser at the working wavelength. Furthermore, "substantially the same" reflects the engineering optimization strategy of this epitaxial structure under the actual realizable conditions, rather than the theoretical ideal state, which not only ensures the performance but also has practical manufacturability, reflecting the technical feasibility and industrial adaptability of the present application.

[0056] In the above example, by disposing an active layer 3 between a first Bragg reflection layer 1 and a second Bragg reflection layer 2 with opposite conduction types, and the active layer 3 is composed of a plurality of alternating well layers 31 and barrier layers 32, each well layer 31 and two adjacent barrier layers 32 form a quantum well 33, and the energy levels of the plurality of quantum wells 33 are kept consistent, ensuring the uniform distribution of the injected carriers among the quantum wells 33, effectively avoiding the non-radiative recombination loss caused by the aggregation of carriers in local quantum wells 33, thereby significantly improving the radiative recombination efficiency and the light field uniformity of the device. At the same time, the above epitaxial structure is conducive to enhancing the interaction between carriers and photons in the active layer 3, improving the photoelectric conversion efficiency of the laser, and is conducive to the high-power and high-efficiency application of the vertical cavity surface emitting laser device.

[0057] In one embodiment, the epitaxial structure further includes a substrate 4 for serving as a growth basis for other structural layers in the epitaxial structure. Therefore, the substrate 4 has good lattice matching with other structural layers in the epitaxial structure grown thereon to suppress dislocation generation and improve the material quality. And in order to meet the light-emitting performance of the laser, the substrate 4 also has a high light transmittance. The material of the substrate 4 includes gallium arsenide, indium phosphide, sapphire, silicon carbide, and silicon to adapt to the growth of the epitaxial structure of lasers in different systems. In this embodiment, the material of the substrate 4 is gallium arsenide. It should be noted that the size of the epitaxial structure can be selected according to the actual situation without limitation as long as the performance of the epitaxial structure is satisfied.

[0058] In one embodiment, in the epitaxial structure, a first Bragg reflection layer 1 and a second Bragg reflection layer 2 with opposite conduction types and located on opposite sides of the active layer 3 are formed to construct an optical resonant cavity of the laser, and photons are confined in the active layer 3 through Bragg interference to achieve resonance-enhanced light emission.

[0059] In the laser, the first Bragg reflection layer 1 and the second Bragg reflection layer 2 can be respectively used as an electron injection layer and a hole injection layer through doping design and become part of the electrical injection path. It should be noted that the main functions of the first Bragg reflection layer 1 and the second Bragg reflection layer 2 are optical reflection mirrors. While achieving good electrical injection characteristics, doping, thickness, material selection, and structure optimization are required to avoid affecting the performance of the first Bragg reflection layer 1 and the second Bragg reflection layer 2. For example, the element concentration in the first Bragg reflection layer 1 and the second Bragg reflection layer 2 can be designed to be gradually changed to ensure a high reflectivity while reducing the series resistance of the device and optimizing the electrical injection efficiency.

[0060] Among them, the first Bragg reflection layer 1 is located above the substrate 4. The first Bragg reflection layer 1 includes multiple layers of alternately grown high- and low-refractive-index materials, such as GaAs, AlGaAs, InGaAsP, InP, GaN, AlGaN, etc. Moreover, the thickness of each layer of material satisfies the Bragg condition, and the thickness is the ratio of the wavelength to four times the refractive index. The first Bragg reflection layer 1 has a high lattice matching degree with the active layer 3 to ensure that the device has a high quantum efficiency. In addition, the first Bragg reflection layer 1 also has a high reflectivity and can confine photons in the active layer 3 through Bragg interference. Furthermore, the conductivity type of the first Bragg reflection layer 1 is N-type, and the first Bragg reflection layer 1 has a relatively low energy band barrier, which is beneficial to electron injection.

[0061] The second Bragg reflection layer 2 is located on the side of the active layer 3 away from the substrate 4. The second Bragg reflection layer 2 also includes multiple layers of alternately grown high- and low-refractive-index materials, such as GaAs, AlGaAs, InGaAsP, InP, GaN, AlGaN, etc. While the second Bragg reflection layer 2 has a high lattice matching degree with the active layer 3, the thickness of each layer of material satisfies the Bragg condition, and the thickness is the ratio of the wavelength to four times the refractive index, so as to ensure that the device has a high quantum efficiency. The second Bragg reflection layer 2 also has a high reflectivity and can confine photons in the active layer 3 through Bragg interference. In addition, the conductivity type of the second Bragg reflection layer 2 is P-type, and the second Bragg reflection layer 2 has a relatively high energy band barrier, which is beneficial to hole injection.

[0062] In one embodiment, the active layer 3 is located between the first Bragg reflection layer 1 and the second Bragg reflection layer 2, and the active layer 3 is composed of multiple alternately grown well layers 31 and barrier layers 32. Each well layer 31 and the two adjacent barrier layers 32 form a quantum well 33. The active layer 3 includes multiple quantum wells 33. Since the bandgap width of the well layer 31 is lower than that of the barrier layer 32, in a semiconductor heterojunction, when two materials with different bandgap widths are in contact, their energy bands will bend to match the Fermi level. Among them, for the material with a narrow bandgap width, such as the well layer 31, the bottom of the conduction band is lower and the top of the valence band is higher, and the overall energy band "sags"; for the material with a wide bandgap width, such as the barrier layer 32, the bottom of the conduction band is higher and the top of the valence band is lower, and the overall energy band "bulges". Then, the well layer 31 and the two adjacent barrier layers 32 form a potential energy trap (similar to a "well-shaped" structure) in the active layer 3, which is called a quantum well 33.

[0063] In one embodiment, in the stacking direction from the first Bragg reflection layer 1 to the second Bragg reflection layer 2, the well width of each quantum well 33 gradually changes according to a first variation trend, and / or the well depth of each quantum well 33 gradually changes according to a second variation trend. One of the first variation trend and the second variation trend is a decreasing variation trend, and the other is an increasing variation trend. That is, by adjusting the well width and / or the well depth, the energy levels of the cascaded quantum wells 33 can be aligned, and the density of states formed at the energy levels is substantially the same.

[0064] In one embodiment, the physical thickness of each well layer 31 of the active layer 3 gradually changes according to a first variation trend. The physical thickness of each well layer 31 represents the actual well width of the quantum well 33. The narrower the well, the higher the quantization energy level and the larger the energy level spacing. Therefore, by adjusting the physical thickness of each well layer 31, the well width of the corresponding quantum well 33 can be adjusted. Exemplarily, the physical thickness of the well layer 31 is 5-30 nm; the physical thickness of the barrier layer 32 is 10-40 nm, and the physical thickness of the barrier layer 32 in each quantum well 33 is greater than the physical thickness of the well layer 31. That is, the physical thicknesses of the well layer 31 and the barrier layer 32 are both in the nanometer range, which ensures the effect of the quantum well 33 while the thick enough barrier layer 32 also avoids carrier tunneling between the quantum wells 33.

[0065] In one embodiment, the band gap of each well layer 31 of the active layer 3 gradually changes according to a first variation trend, and / or the band gap of each barrier layer 32 of the active layer 3 gradually changes according to a second variation trend. The well depth of the quantum well 33 is mainly related to the band gaps of the barrier layer 32 and the well layer 31 that make up the corresponding quantum well 33. The narrower the band gap of the well layer 31, that is, the lower the "bottom of the well" of the quantum well 33, and the wider the band gap of the barrier layer 32, that is, the higher the "well wall" of the quantum well 33. Therefore, by adjusting the band gap of the well layer 31 to be narrower and / or the band gap of the barrier layer 32 to be wider, the well depth of the quantum well 33 can be made deeper.

[0066] Further, the well layer 31 includes a first element, the barrier layer 32 includes a second element, the composition of the first element in each well layer 31 of the active layer 3 gradually changes according to a second variation trend, and / or the composition of the second element in each barrier layer 32 of the active layer 3 gradually changes according to a second variation trend. The band gaps of the well layer 31 and the barrier layer 32 are mainly achieved by adjusting the composition of the elements in the well layer 31 and the barrier layer 32.

[0067] Exemplarily, the well layer 31 is In x Ga 1-x As, the first element is the In element, where x represents the composition of the In element, 0≤x≤0.4; specifically, when the content of the In element increases, the band gap of the well layer 31 becomes narrower, that is, the "bottom of the well" energy decreases and the bound state energy level drops; the barrier layer 32 includes Aly Ga 1-y As, the second element is Al, and y represents the component of Al, where 0 ≤ y ≤ 0.4; among them, when the component of Al increases, the bandgap width of the barrier layer 32 becomes higher, the height of the "well wall" increases, and the binding ability to carriers increases. Therefore, by controlling the component of Al in the barrier layer 32 and the change of In in the well layer 31, the quantum confinement effect in the quantum well 33 can be further regulated to make the energy levels of each quantum well 33 consistent.

[0068] In this embodiment, the conduction type of the first Bragg reflection layer 1 is N-type, the conduction type of the second Bragg reflection layer 2 is P-type, the first change trend is a decreasing change trend, and the second change trend is an increasing change trend. Therefore, according to the above change trends, by adjusting the physical thickness of the well layer 31 to regulate the well width of the quantum well 33, and by adjusting the components of the first element and the second element, the well depth of the quantum well 33 is regulated to form a quantum well 33 with precisely controlled depth and width, so as to accurately regulate the alignment of the energy levels of each quantum well 33, and the density of states formed at the energy levels is basically the same, realizing the uniform distribution of carriers, improving the carrier luminescence recombination efficiency; realizing the suppression of carrier overflow and enhancing the retention of carriers in the quantum well; enhancing the optoelectronic conversion efficiency of carriers and photons in the region with high light field intensity.

[0069] Exemplarily, the preset energy level is the ground state energy level (M = 1 energy level), that is, the energy level position corresponding to the first sub-band of electrons and the first sub-band of holes. By reasonably regulating the component ratio and thickness parameters of each well layer 31 and its adjacent barrier layer 32, so that all quantum wells 33 have basically the same electron and hole density of states at this ground state energy level, thus realizing the high alignment of the energy levels between the electron ground state and the hole ground state, which is conducive to exciton formation; the overlap degree of the wave functions of electrons and holes in space is the largest, enhancing the probability of radiative recombination; improving the optical gain and luminescence intensity of the device, while suppressing the non-radiative recombination path; ensuring the consistency and synchronization of each quantum well 33 in the multi-quantum well structure for laser output. It should be noted that although the above structure takes alignment as the preferred implementation mode, the present application is not limited to this configuration of the ground state energy level. For some special application scenarios, such as the need to achieve multi-wavelength emission, enhance the recombination efficiency of high-order excited states or regulate the gain spectrum width, the energy level alignment of high-order excited states (such as the first excited state, the second excited state) can also be further introduced in the design. At this time, by further refining and regulating the quantum well layer structure parameters, the quantum well 33 can form a density of states distribution that tends to be consistent at one or more non-ground state energy levels, so as to achieve specific functional goals, which will not be elaborated here.

[0070] In one embodiment, the composition of the second element in each barrier layer 32 of the active layer 3 varies linearly. That is, each barrier layer 32 exhibits a band tilt structure. The end with a higher energy band effectively suppresses electron overflow, improving the retention rate of carriers in the active layer 3. The end with a lower energy band helps reduce the electron injection barrier, improving the injection efficiency. Therefore, by guiding the carrier flow path through the band slope, the carriers can be more evenly distributed among the multiple quantum wells 33, reducing excessive recombination in a single quantum well, improving the overall luminescence efficiency, and effectively reducing the collision and non-radiative recombination of high-energy electrons among the multiple quantum wells 33. When the performance of the subsequent formed device is satisfied, the composition change within each barrier layer 32 can also remain unchanged or exhibit other change trends, not limited to the above linear change.

[0071] In one embodiment, to suppress the electron backflow at the electron injection end, i.e., the side close to the first Bragg reflection layer 1, in the quantum well 33 closest to the first Bragg reflection layer 1, the bandgap of the barrier layer 32 close to the first Bragg reflection layer 1 is greater than the bandgap of the adjacent barrier layer 32 far from the first Bragg reflection layer 1, and the bandgap of the barrier layer 32 closest to the first Bragg reflection layer 1 is not greater than the bandgap of the barrier layer 32 closest to the second Bragg reflection layer 2. While ensuring the electron injection efficiency, the barrier layer 32 closest to the first Bragg reflection layer 1 can also act as an electron barrier to effectively suppress electron backflow.

[0072] Furthermore, it should be noted that the bandgap of each barrier layer 32 in the active layer 3 is smaller than the bandgap of the first Bragg reflection layer 1 and the second Bragg reflection layer 2, so as to effectively confine the carriers within the active layer 3, thereby maximizing the quantum efficiency of the device and ensuring the luminescence intensity and photoelectric conversion efficiency.

[0073] Among them, the number of quantum wells 33 is not less than 3 to meet the collaborative optimization of carrier transport, optical gain, and thermal management, taking into account the device lifetime, heat control, and high-frequency response ability while ensuring the device working efficiency.

[0074] Please refer to Figure 3 , in one embodiment, the present application also provides a method for preparing an epitaxial structure, including the following steps:

[0075] Step S1: Provide a substrate 4, and epitaxially grow a first Bragg reflection layer 1 on the substrate 4;

[0076] Step S2: Epitaxially grow an active layer 3 on the first Bragg reflection layer 1. The active layer 3 includes a plurality of alternately stacked well layers 31 and barrier layers 32. Each well layer 31 and the two adjacent barrier layers 32 thereto form a quantum well 33. The composition and / or thickness of the well layer 31 and the barrier layer 32 are designed so that the density of states formed at a preset energy level in each quantum well 33 is substantially the same;

[0077] Step S3: Epitaxially grow a second Bragg reflector layer 2 on the active layer 3.

[0078] In the above example, the preparation method of the epitaxial structure realizes the preparation of the active layer 3 of the multi-quantum well 33 with energy level alignment, and the first Bragg reflector layer 1 and the second Bragg reflector layer 2, which is applicable to the application of high-performance optical devices such as vertical cavity surface emitting lasers.

[0079] Specifically, please refer to Figures 4 to 5 , perform step S1 to provide a substrate 4 and epitaxially grow a first Bragg reflector layer 1 on the substrate 4.

[0080] In one embodiment, before epitaxially growing the first Bragg reflector layer 1 on the substrate 4, it further includes:

[0081] Pre-treat the substrate 4. Exemplarily, pre-treating the substrate 4 includes cleaning with a solvent and drying with a gas to remove contaminants and ensure the quality of subsequent epitaxial growth.

[0082] In one embodiment, epitaxially growing the first Bragg reflector layer 1 on the substrate 4 includes:

[0083] Alternately grow materials with high and low refractive indices on the substrate 4 to obtain the first Bragg reflector layer 1. Among them, the method of alternately growing materials with high and low refractive indices on the substrate 4 includes metal organic chemical vapor deposition or other suitable methods.

[0084] Specifically, please refer to Figure 6 , perform step S2 to epitaxially grow an active layer 3 on the first Bragg reflector layer 1. The active layer 3 includes a plurality of alternately stacked well layers 31 and barrier layers 32. Each well layer 31 and two adjacent barrier layers 32 form a quantum well 33. The composition and / or thickness of the well layer 31 and the barrier layer 32 are designed so that the density of states formed by each quantum well 33 at a preset energy level is substantially the same.

[0085] In one embodiment, epitaxially growing the active layer 3 on the first Bragg reflector layer 1 includes:

[0086] Alternately grow the barrier layer 32 and the well layer 31 on the substrate 4, and in the epitaxial growth direction, control the well width of each quantum well to gradually change according to a first variation trend, and / or control the well depth of each quantum well 33 to gradually change according to a second variation trend. One of the first variation trend and the second variation trend is a decreasing variation trend, and the other is an increasing variation trend. The method of growing the barrier layer 32 and the well layer 31 on the substrate 4 includes a metal organic chemical vapor deposition process or other suitable methods.

[0087] Further, controlling the well widths of the quantum wells 33 to gradually change according to a first variation trend includes:

[0088] Adjusting the growth time of the well layers 31 of each quantum well 33 to gradually change according to a first variation trend, so that the physical thickness of the well layers 31 of each quantum well 33 gradually changes according to the first variation trend.

[0089] Further, controlling the well depths of the quantum wells 33 to gradually change according to a second variation trend includes:

[0090] Controlling the bandgap widths of the well layers of the active layer 3 to gradually change according to a first variation trend, and / or controlling the bandgap widths of the barrier layers 32 of the active layer 3 to gradually change according to a second variation trend.

[0091] Further, the well layer 31 includes a first element, the barrier layer 32 includes a second element, and controlling the bandgap widths of the well layers 31 of the active layer 3 to gradually change according to a first variation trend includes:

[0092] Adjusting the flow rate of the first gas source for generating the first element in each well layer 31 to gradually change according to a second variation trend, so that the composition of the first element in each well layer 31 of the active layer 3 gradually changes according to the second variation trend;

[0093] Controlling the bandgap widths of the barrier layers 32 of the active layer 3 to gradually change according to a second variation trend includes:

[0094] Adjusting the flow rate of the second gas source for generating the second element in each barrier layer 32 to gradually change according to a second variation trend, so that the composition of the second element in each barrier layer of the active layer 3 gradually changes according to the second variation trend.

[0095] Specifically, the first gas source and the second gas source are respectively used as precursors for generating the first element and the second element. The first gas source includes TMIn, and the second gas source includes TMAl.

[0096] In addition, the adjustment of the flow rate of the second gas source for each barrier layer 32 changes linearly, so that the composition of the second element in each barrier layer 32 of the active layer 3 changes linearly.

[0097] In addition, during the epitaxial growth of the active layer 3, it is also necessary to ensure temperature uniformity and prevent composition mismatch.

[0098] Specifically, please refer to Figure 1 , perform step S3, and epitaxially grow the second Bragg reflector layer 2 on the active layer 3.

[0099] In one embodiment, epitaxially growing the second Bragg reflector layer 2 on the active layer 3 includes:

[0100] Using metal-organic chemical vapor deposition process, materials with high and low refractive indices are alternately grown on the active layer 3 to obtain the second Bragg reflector 2. In this embodiment, the second Bragg reflector 2 is formed on the optical confinement layer 5.

[0101] For the specific structures and related descriptions in the examples of the preparation method of the above epitaxial structure, reference can be made to the relevant content in the embodiments of the epitaxial structure, which will not be elaborated here.

[0102] Please refer to Figure 7 , in another embodiment, the present application also provides a VCSEL chip, including the epitaxial structure described in any of the above embodiments.

[0103] In one embodiment, the VCSEL chip further includes an optical confinement layer 5. The optical confinement layer 5 is located on the side of the second Bragg reflector 2 close to the active layer 3, and the optical confinement layer 5 is configured to define the light-emitting area of the VCSEL chip. Among them, the optical confinement layer 5 is obtained by selectively oxidizing one or more material layers in the second Bragg reflector 2 close to the active layer 3 through wet oxidation. The unoxidized part serves as the light-emitting area of the VCSEL chip. For example, when the material of the second Bragg reflector 2 is AlGaAs, the material of the optical confinement layer 5 is Al2O3. The optical confinement layer 5 can achieve triple confinement of current, carriers, and photons, enabling the current to only pass through the unoxidized part of the second Bragg reflector 2, reducing the threshold current, achieving strong optical field confinement through the reflectivity difference between the unoxidized part of the second Bragg reflector 2 and the optical confinement layer 5, and forming a potential barrier through the optical confinement layer 5 to block the lateral diffusion of holes and improve the quantum efficiency.

[0104] In one embodiment, the VCSEL chip further includes a first electrode 6 and a second electrode 7. The first electrode 6 is an N-type electrode for connecting the negative pole of the external circuit, and the second electrode 7 is a P-type electrode for connecting the positive pole of the external circuit.

[0105] Exemplarily, the first electrode 6 and the second electrode 7 can be located on the same side of the active layer 3 or on both sides of the active layer 3, which can be selected according to the actual layout. For example, when the first electrode 6 and the second electrode 7 are located on both sides of the active layer 3, the first electrode 6 is located on the surface of the substrate 4 away from the active layer 3, and the second electrode 7 is located on the surface of the second Bragg reflector 2 away from the active layer 3. The material of the first electrode 6 includes gold, nickel, cadmium, or other suitable conductive materials; the material of the second electrode 7 includes gold, platinum, titanium, or other suitable conductive materials.

[0106] During the working process, the current flows in from the second electrode 7, passes through the second Bragg reflector 2, the active layer 3, and the first Bragg reflector 1 in sequence, and flows out from the first electrode 6 to form a complete current path.

[0107] Among them, the shapes of the first electrode 6 and the second electrode 7 can be selected according to the actual situation. Exemplarily, the second electrode 7 can be designed to be annular, which can define the optical path channel while improving the uniformity of current transmission and enhancing the chip performance.

[0108] It should be noted that the VCSEL chip includes the above-mentioned epitaxial structure, specifically, the epitaxial structure can be applied to the preparation of the VCSEL chip, and a VCSEL chip with high gain, high stability, low threshold and good temperature control characteristics can be prepared by cooperating with subsequent processing technologies (such as etching, forming an optical and electrical confinement layer, depositing electrodes, processing the cavity surface, etc.).

[0109] The above-mentioned VCSEL chips have broad application prospects in high-end optoelectronic application scenarios such as communication, sensing, lidar (LiDAR), and facial recognition.

[0110] It should be understood that although Figure 3 the steps in the flowchart of Figure 3 are shown in sequence according to the indication of the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise clearly stated in this article, the execution of these steps has no strict order restriction, and these steps can be executed in other orders. Moreover,

[0111] in the description of this specification, the descriptions referring to terms such as "some embodiments", "other embodiments", "ideal embodiments", etc. mean that the specific features, structures, materials or features described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic descriptions of the above terms do not necessarily refer to the same embodiment or example.

[0112] The technical features of the above-mentioned embodiments can be combined arbitrarily. For the sake of concise description, not all possible combinations of the technical features of the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered to be within the scope described in this specification.

[0113] The above-described embodiments merely represent several implementation manners of the present application. The description thereof is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several modifications and improvements can still be made, and these all fall within the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the appended claims.

Claims

1. An epitaxial structure, characterized in that, Comprising: A first Bragg reflection layer and a second Bragg reflection layer which are stacked, wherein the first Bragg reflection layer and the second Bragg reflection layer have different conduction types; An active layer located between the first Bragg reflection layer and the second Bragg reflection layer, the active layer comprising a plurality of alternately stacked well layers and barrier layers, each well layer and two adjacent barrier layers thereof constituting a quantum well, and the composition and / or thickness of the well layer and the barrier layer being designed such that the density of states formed by each quantum well at a preset energy level is substantially the same.

2. The epitaxial structure according to claim 1, wherein The preset energy level is the ground state energy level.

3. The epitaxial structure according to claim 1, characterized in that, Along the stacking direction of the epitaxial structure, the well widths of the quantum wells gradually change according to a first variation trend; and / or The well depths of the quantum wells gradually change according to a second variation trend, and one of the first variation trend and the second variation trend is a decreasing variation trend and the other is an increasing variation trend.

4. The epitaxial structure according to claim 3, characterized in that, The physical thicknesses of the well layers of the quantum wells gradually change according to the first variation trend.

5. The epitaxial structure according to claim 3, wherein The band gaps of the well layers of the active layer gradually change according to the first variation trend, and / or the band gaps of the barrier layers of the active layer gradually change according to the second variation trend.

6. The epitaxial structure according to claim 5, characterized in that, The well layer comprises a first element, the barrier layer comprises a second element, and the composition of the first element in each well layer of the active layer gradually changes according to the second variation trend; and / or The composition of the second element in each barrier layer of the active layer gradually changes according to the second variation trend.

7. The epitaxial structure according to claim 6, wherein, The composition of the second element in each barrier layer of the active layer changes linearly.

8. The epitaxial structure according to claim 3, wherein The conduction type of the first Bragg reflection layer is N-type, the conduction type of the second Bragg reflection layer is P-type, the first variation trend is a decreasing variation trend, and the second variation trend is an increasing variation trend.

9. The epitaxial structure according to claim 3, wherein In the quantum well closest to the first Bragg reflection layer, the band gap of the barrier layer close to the first Bragg reflection layer is greater than the band gap of the adjacent barrier layer far from the first Bragg reflection layer, and the band gap of the barrier layer closest to the first Bragg reflection layer is not greater than the band gap of the barrier layer closest to the second Bragg reflection layer.

10. A method for preparing an epitaxial structure, characterized in that, Comprising: Providing a substrate, and epitaxially growing a first Bragg reflection layer on the substrate; Epitaxially growing an active layer on the first Bragg reflection layer, the active layer comprising a plurality of alternately stacked barrier layers and well layers, each well layer and two adjacent barrier layers thereof constituting a quantum well, and the composition and / or thickness of the well layer and the barrier layer being designed such that the density of states formed by each quantum well at a preset energy level is substantially the same; Epitaxially growing a second Bragg reflection layer on the active layer, wherein the first Bragg reflection layer and the second Bragg reflection layer have different conduction types.

11. The method for preparing the epitaxial structure according to claim 10, characterized in that, The epitaxially growing the active layer on the first Bragg reflection layer comprises: Alternately growing the barrier layer and the well layer on the first Bragg reflection layer, and controlling the well widths of the quantum wells to gradually change according to a first variation trend in the epitaxial growth direction; and / or Controlling the well depth of each of the quantum wells to gradually change according to a second variation trend, where one of the first variation trend and the second variation trend is a decreasing variation trend and the other is an increasing variation trend.

12. The method for preparing the epitaxial structure according to claim 11, wherein The controlling the well width of each of the quantum wells to gradually change according to a first variation trend includes: Regulating the growth time of the well layers of each of the quantum wells to gradually change according to the first variation trend, so that the physical thickness of the well layers of each of the quantum wells gradually changes according to the first variation trend.

13. The manufacturing method of the epitaxial structure according to claim 11, characterized in that, The controlling the well depth of each of the quantum wells to gradually change according to a second variation trend includes: Controlling the bandgap width of each of the well layers of the active layer to gradually change according to a first variation trend, and / or controlling the bandgap width of each of the barrier layers of the active layer to gradually change according to a second variation trend.

14. The method for preparing the epitaxial structure according to claim 13, wherein, The well layer includes a first element, the barrier layer includes a second element, and the controlling the bandgap width of each of the well layers of the active layer to gradually change according to a first variation trend includes: Regulating the flow rate of the first gas source for generating the first element in each of the well layers to gradually change according to the second variation trend, so that the composition of the first element in each of the well layers of the active layer gradually changes according to the second variation trend; The controlling the bandgap width of each of the barrier layers of the active layer to gradually change according to a second variation trend includes: Regulating the flow rate of the second gas source for generating the second element in each of the barrier layers to gradually change according to the second variation trend, so that the composition of the second element in each of the barrier layers of the active layer gradually changes according to the second variation trend.

15. A VCSEL chip, comprising the epitaxial structure according to any one of claims 1 to 9.

16. The VCSEL chip according to claim 15, wherein, The VCSEL chip further includes a photoelectric confinement layer, the photoelectric confinement layer is located on the side of the second Bragg reflector close to the active layer, and the photoelectric confinement layer is configured to define the light-emitting region of the VCSEL chip.

17. According to the VCSEL chip of claim 15, the VCSEL chip further includes a first electrode and a second electrode, the first electrode is an N-type electrode for connecting the negative pole of an external circuit, and the second electrode is a P-type electrode for connecting the positive pole of an external circuit.

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