A DFB laser epitaxial structure and a preparation method thereof

Abstract

The application discloses a DFB laser epitaxial structure and a preparation method thereof, and relates to the technical field of semiconductors. The epitaxial structure comprises a substrate, an N-type doped layer, a lower limiting layer, an active layer, an upper limiting layer, a grating, a secondary epitaxial covering layer and an ohmic contact layer which are sequentially stacked on the substrate. The lower limiting layer is AlGaInAs or AlGaAs, and the Al component in the lower limiting layer is gradually increased towards the active layer. Through the arrangement, the electrons will first enter the lower limiting layer, the Al component of the lower limiting layer is gradually increased, the conduction band energy level presents a gradual distribution of smooth rising, the electron injection barrier formed between the traditional fixed component limiting layer and the active layer due to the conduction band mutation is avoided, the electrons can be injected into the active layer with higher energy efficiency, meanwhile, the high Al component area close to the active layer forms a wide band gap barrier, effectively prevents the electrons injected into the active layer from reverse spillover to the direction of the lower limiting layer, restricts the electrons in the active region to participate in radiation recombination, and reduces the threshold current density.

Description

Technical Field

[0001] This invention relates to the field of semiconductor technology, specifically to an epitaxial structure for a DFB laser and its fabrication method. Background Technology

[0002] DFB lasers typically employ separately confined heterostructures (SCHs), where confinement layers are placed on both the top and bottom of the active region to simultaneously confine the optical field and charge carriers near the active region. The bottom confinement layer, as the key region for controlling the optical field distribution, has a decisive impact on device performance due to its material composition design. In traditional DFB lasers, the bottom confinement layer often uses AlGaInAs or AlGaAs materials with a fixed composition, achieving optical field confinement through a single high-Al composition.

[0003] However, in order to obtain sufficient optical field confinement capability, existing technologies require the use of higher Al content. On the one hand, high Al content materials can lead to the formation of abrupt heterogeneous interfaces between the confinement layer and the active region, resulting in band discontinuities and hindering the effective injection of charge carriers from the lower confinement layer to the active region. On the other hand, high Al content materials have lower thermal conductivity, which exacerbates the heating problem in the active region of the device and affects high-temperature characteristics. Furthermore, the accumulation of charge carriers at the abrupt interface can also increase the probability of nonradiative recombination, leading to an increase in threshold current and a decrease in slope efficiency. Summary of the Invention

[0004] Based on this, the purpose of the present invention is to provide an epitaxial structure of a DFB laser and a method for fabricating the same, so as to solve at least one technical problem in the background art.

[0005] The present invention provides an epitaxial structure for a DFB laser, comprising a substrate, and an N-type doped layer, a lower confinement layer, an active layer, an upper confinement layer, a grating, a secondary epitaxial capping layer, and an ohmic contact layer sequentially stacked on the substrate; The lower confinement layer is AlGaInAs or AlGaAs; In the lower confinement layer, the Al component increases in a gradient towards the active layer.

[0006] Furthermore, the Al component in the lower confinement layer increases linearly towards the active layer.

[0007] Furthermore, the Al component in the lower confinement layer increases in a stepwise gradient towards the active layer.

[0008] Furthermore, the lower limiting layer comprises a plurality of sub-layers stacked sequentially; The Al composition in each of the sub-layers is different, and the Al composition in each of the sub-layers increases layer by layer along the direction closer to the active layer.

[0009] Furthermore, the thickness of the lower limiting layer is 50nm-500nm.

[0010] Furthermore, the Al composition in the lower confining layer varies from 0.3 to 0.7.

[0011] Furthermore, the active layer includes a well layer and a barrier layer stacked sequentially on top of the lower confining layer.

[0012] Furthermore, the upper confinement layer is ALlnPAs, and the AL component in the upper confinement layer together with the lower confinement layer forms a bidirectional confinement region for the active region.

[0013] Another aspect of the present invention is to provide a method for fabricating an epitaxial structure of a DFB laser, comprising: Provide a substrate; An N-type doped layer, a lower confinement layer, an active layer, an upper confinement layer, a grating, a secondary epitaxial capping layer, and an ohmic contact layer are sequentially grown on the substrate. In the step of growing the lower confinement layer, the Al component in the lower confinement layer is gradient-increased toward the active layer by adjusting the TMAl flow rate in real time.

[0014] Furthermore, the step of adjusting the TMA1 flow rate in real time also includes: By continuously increasing the TMAl flow rate, the Al component in the lower confinement layer increases linearly towards the active layer. Alternatively, by adjusting the TMAl flow rate segment by segment to a preset value, the Al component in the lower confinement layer can be increased in a stepwise gradient towards the active layer.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: In the DFB laser epitaxial structure provided by this invention, the lower confinement layer is made of AlGaInAs or AlGaAs, and the Al composition in the lower confinement layer increases gradually towards the active layer. That is, the closer to the subsequently grown active layer, the higher the Al composition in the lower confinement layer, the lower the refractive index, and the higher the conduction band energy level. In practice, electrons first enter the lower confinement layer. Utilizing the gradually increasing Al composition and the smooth, gradual distribution of the conduction band energy level, this avoids the electron injection barrier caused by abrupt conduction band changes between the traditional fixed-composition confinement layer and the active layer. This allows electrons to be injected into the active layer with higher energy efficiency. Simultaneously, the high Al composition region near the active layer forms a wide bandgap barrier, effectively preventing electrons injected into the active layer from leaking back towards the lower confinement layer. This confines electrons within the active region to participate in radiative recombination, reducing the threshold current. The increasing Al composition leads to a gradual decrease in refractive index, creating a smooth refractive index gradient from the active layer down to the confinement layer. This gradient refractive index distribution more effectively confines the optical field near the active region, reducing leakage losses towards the substrate and improving the transverse mode characteristics of the laser. Furthermore, the gradient Al composition eliminates the conduction band abrupt change between the traditional fixed-composition confinement layer and the active layer, avoiding electron scattering and thermal losses caused by the abrupt interface. This allows electrons to be injected into the active layer with higher energy efficiency, improving carrier injection uniformity. Simultaneously, the gradient Al composition structure eliminates interface band abrupt changes, reducing interface state density and nonradiative recombination probability, further improving internal quantum efficiency. Additionally, by suppressing electron leakage at high temperatures and optimizing electron injection uniformity, the characteristic temperature T0 is significantly increased, enabling the DFB laser to maintain low threshold and high efficiency operation even at high temperatures. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the epitaxial structure of a DFB laser in one embodiment of the present invention; Figure 2 This is a flowchart illustrating the fabrication method of a DFB laser epitaxial structure in one embodiment of the present invention.

[0017] In the figure: 10, substrate; 20, N-type doped layer; 30, lower confinement layer; 40, active layer; 50, upper confinement layer; 60, grating; 70, secondary epitaxial capping layer; 80, ohmic contact layer. Detailed Implementation

[0018] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Several embodiments of the invention are illustrated in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.

[0019] It should be noted that when a component is said to be "fixed to" another component, it can be directly on the other component or there may be an intervening component. When a component is said to be "connected to" another component, it can be directly connected to the other component or there may be an intervening component. The terms "vertical," "horizontal," "left," "right," and similar expressions used in this document are for illustrative purposes only.

[0020] 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 invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0021] The present invention provides an epitaxial structure for a DFB laser, comprising a substrate, and an N-type doped layer, a lower confinement layer, an active layer, an upper confinement layer, a grating, a secondary epitaxial capping layer, and an ohmic contact layer sequentially stacked on the substrate. The lower confinement layer is AlGaInAs or AlGaAs; In the lower confinement layer, the Al composition increases in a gradient towards the active layer.

[0022] Furthermore, the Al composition in the lower confinement layer increases linearly towards the active layer.

[0023] Furthermore, the Al composition in the lower confinement layer increases in a stepwise gradient towards the active layer.

[0024] Furthermore, the lower limiting layer comprises multiple sub-layers stacked sequentially; The Al composition in each sublayer is different, and the Al composition in each sublayer increases layer by layer along the direction closer to the active layer.

[0025] Furthermore, the thickness of the lower confinement layer is 50nm-500nm.

[0026] Furthermore, the Al composition in the lower confining layer varies from 0.3 to 0.7.

[0027] Furthermore, the active layer includes a trap layer and a barrier layer stacked sequentially on top of the lower confinement layer.

[0028] Furthermore, the upper confinement layer is ALlnPAs, and the AL components in the upper confinement layer together with the lower confinement layer form a bidirectional confinement region for the active region.

[0029] In addition, a method for fabricating an epitaxial structure of a DFB laser is also provided, including: Provide a substrate; An N-type doped layer, a lower confinement layer, an active layer, an upper confinement layer, a grating, a secondary epitaxial capping layer, and an ohmic contact layer are sequentially grown on the substrate. In the step of growing the lower confinement layer, the Al component in the lower confinement layer is increased in a gradient towards the active layer by adjusting the TMAl flow rate in real time.

[0030] Furthermore, the step of adjusting the TMA1 flow rate in real time also includes: By continuously increasing the TMAl flow rate, the Al component in the lower confinement layer increases linearly towards the active layer. Alternatively, by adjusting the TMAl flow rate segment by segment to a preset value, the Al component in the lower confinement layer can be made to increase in a stepwise gradient towards the active layer.

[0031] First Embodiment Please see Figure 1 The first embodiment of the present invention provides a DFB laser epitaxial structure, including a substrate 10, and an N-type doped layer 20, a lower confinement layer 30, an active layer 40, an upper confinement layer 50, a grating 60, a secondary epitaxial capping layer 70 and an ohmic contact layer 80 sequentially stacked on the substrate 10. It should be noted that in this embodiment, the substrate 10 can be an InP substrate 10 or a GaAs substrate 10.

[0032] The lower confinement layer 30 is AlGaInAs or AlGaAs; The Al composition in the lower confinement layer 30 increases in a gradient toward the active layer 40.

[0033] Specifically, in this application, the lower confinement layer 30 is made of AlGaInAs or AlGaAs, and the Al composition in the lower confinement layer 30 is arranged in a gradient increasing direction toward the active layer 40. That is, the closer to the subsequently grown active layer 40, the higher the Al composition in the lower confinement layer 30 becomes, the lower the refractive index gradually decreases, and the higher the conduction band energy level. In practice, electrons will first enter the lower confinement layer 30. Utilizing the gradually increasing Al composition of the lower confinement layer 30 and the smooth upward distribution of the conduction band energy level, the electron injection barrier formed by the conduction band abrupt change between the traditional fixed-composition confinement layer and the active layer 40 is avoided. This allows electrons to be injected into the active layer 40 with higher energy efficiency. Simultaneously, the high Al composition region near the active layer 40 forms a wide bandgap barrier, effectively preventing electrons already injected into the active layer 40 from leaking back toward the lower confinement layer 30. This confines electrons within the active region to participate in radiative recombination, reducing the threshold voltage. Simultaneously, the gradual increase in Al composition leads to a gradual decrease in refractive index, forming a smooth refractive index gradient from the active layer 40 down to the confinement layer 30. This gradient refractive index distribution more effectively confines the optical field near the active region, reducing leakage losses of the optical field towards the substrate 10 and improving the transverse mode characteristics of the laser. Furthermore, the gradient Al composition eliminates the conduction band abrupt change between the traditional fixed-composition confinement layer and the active layer 40, avoiding electron scattering and thermal losses caused by the abrupt interface, allowing electrons to be injected into the active layer 40 with higher energy efficiency and improving carrier injection uniformity. Additionally, the gradient Al composition structure eliminates interface band abrupt changes, reducing interface state density and nonradiative recombination probability, further improving internal quantum efficiency. Moreover, by suppressing electron leakage at high temperatures and optimizing electron injection uniformity, the characteristic temperature T0 is significantly increased, enabling the DFB laser to maintain low threshold and high efficiency operation even at high temperatures.

[0034] In some preferred embodiments, the Al composition in the lower confinement layer 30 increases linearly toward the active layer 40, that is, the Al composition increases linearly along the growth direction. The linear gradient can minimize conduction band abrupt changes and achieve the smoothest electron transport channel.

[0035] In some other preferred embodiments, the Al composition in the lower confinement layer 30 increases in a stepwise gradient toward the active layer 40. That is, the lower confinement layer 30 includes multiple sub-layers stacked sequentially, wherein the Al composition in each sub-layer is different, and the Al composition in each sub-layer increases layer by layer along the direction closer to the active layer 40. The stepwise gradient structure can achieve a smooth transition of the conduction band while providing a more flexible space for adjusting growth parameters.

[0036] In addition, in the embodiments, the thickness of the lower confinement layer 30 is 50nm-500nm, and the Al composition in the lower confinement layer 30 varies from 0.3 to 0.7. Specifically, in this embodiment, the thickness of the lower confinement layer 30 is 200nm, and the Al composition in the lower confinement layer 30 varies from 0.3 to 0.5 toward the active layer 40.

[0037] It should also be noted that the active layer 40 includes a well layer and a barrier layer stacked sequentially on top of the lower confinement layer 30. In some practical cases, the emission wavelength of the active layer 40 can be tuned by adjusting the composition and thickness of the well layer. Specifically, an example is given below, where the emission wavelength of the active layer 40 is mainly determined by the bandgap of the well layer material and the quantum confinement effect. By changing the composition of the well layer, the intrinsic bandgap of the material can be changed, and by changing the thickness of the well layer, the energy blue shift caused by the quantum confinement effect can be finely adjusted. For InP-based DFB lasers in the communication band (1310nm / 1550nm), the commonly used active materials are AlGaInAs or InGaAsP multiple quantum wells (MQWs). The wavelength can be tuned by adjusting the composition of the well layer: the first is to increase the indium (In) content: compared to Ga or Al, the addition of In will narrow the bandgap, causing the emission wavelength to redshift (shift towards longer wavelengths); the second is to increase the arsenic (As) content: compared to P, As has a narrower bandgap, which also causes a redshift in wavelength.

[0038] In some preferred embodiments, the wavelength can also be controlled by adjusting the thickness of the well layer. Specifically, the thinner the well layer, the stronger the quantum confinement effect, the wider the effective bandgap, and the blue shift (moving towards shorter wavelengths) of the emission wavelength. For example, for InP-based materials, when the thickness of the well layer is reduced from 10 nm to 4 nm, the quantum confinement effect can cause a blue shift of 40 nm to 100 nm.

[0039] Furthermore, the upper confinement layer 50 is ALlnPAs, and the AL component in the upper confinement layer 50 together with the lower confinement layer 30 forms a bidirectional confinement region for the active region.

[0040] In some alternative embodiments, the grating layer 60 may be a refractive index-coupled grating 60 or a gain-coupled grating 60, disposed above the active layer 40, for the purpose of enabling single longitudinal mode selection.

[0041] In summary, the DFB laser epitaxial structure provided by this invention has at least the following advantages compared to traditional laser epitaxial structures: By setting the lower confinement layer 30 to be AlGaInAs or AlGaAs, and ensuring that the Al composition in the lower confinement layer 30 increases gradually towards the active layer 40—that is, the closer to the subsequently grown active layer 40, the higher the Al composition in the lower confinement layer 30, the lower the refractive index, and the higher the conduction band energy level—electrons will first enter the lower confinement layer 30. Utilizing the gradually increasing Al composition and the smooth, gradual distribution of the conduction band energy level in the lower confinement layer 30, the electron injection barrier caused by the conduction band abrupt change between the traditional fixed-composition confinement layer and the active layer 40 is avoided. This allows electrons to be injected into the active layer 40 with higher energy efficiency. Simultaneously, the high Al composition region near the active layer 40 forms a wide bandgap barrier, effectively preventing electrons already injected into the active layer 40 from leaking back towards the lower confinement layer 30, confining electrons within the active region to participate in radiative recombination, and reducing the threshold current density. As the Al composition gradually increases, the refractive index gradually decreases, forming a smooth refractive index gradient from the active layer 40 down to the confinement layer 30. This gradient refractive index distribution more effectively confines the optical field near the active region, reducing leakage losses of the optical field towards the substrate 10 and improving the transverse mode characteristics of the laser. At the same time, the gradient Al composition eliminates the conduction band abrupt change between the traditional fixed composition confinement layer and the active layer 40, avoiding electron scattering and thermal losses caused by the abrupt interface, allowing electrons to be injected into the active layer 40 with higher energy efficiency and improving carrier injection uniformity. In addition, the gradient Al composition structure eliminates the interface band abrupt change, reduces the interface state density and nonradiative recombination probability, and further improves the internal quantum efficiency. Furthermore, it can significantly increase the characteristic temperature T0 by suppressing electron leakage at high temperatures and optimizing electron injection uniformity, enabling the DFB laser to maintain low threshold and high efficiency operation even in high-temperature environments.

[0042] Second Embodiment The second embodiment of the present invention provides a DFB laser epitaxial structure. The difference between the DFB laser epitaxial structure in this embodiment and the DFB laser epitaxial structure in the first embodiment is that: In this embodiment, the thickness of the lower confinement layer is 200 nm, and the Al composition in the lower confinement layer changes towards the active layer by 0.3-0.6.

[0043] Third Embodiment The third embodiment of the present invention provides a DFB laser epitaxial structure. The difference between the DFB laser epitaxial structure in this embodiment and the DFB laser epitaxial structure in the first embodiment is that: In this embodiment, the thickness of the lower confinement layer is 200 nm, and the Al composition in the lower confinement layer changes towards the active layer by 0.3-0.7.

[0044] Fourth embodiment The fourth embodiment of the present invention provides a DFB laser epitaxial structure. The difference between the DFB laser epitaxial structure in this embodiment and the DFB laser epitaxial structure in the first embodiment is that: In this embodiment, the thickness of the lower confinement layer is 300 nm, and the Al composition in the lower confinement layer changes towards the active layer by 0.3-0.5.

[0045] Fifth Embodiment The fifth embodiment of the present invention provides a DFB laser epitaxial structure. The difference between the DFB laser epitaxial structure in this embodiment and the DFB laser epitaxial structure in the first embodiment is that: In this embodiment, the thickness of the lower confinement layer is 300 nm, and the Al composition in the lower confinement layer changes towards the active layer by 0.3-0.6.

[0046] Sixth Embodiment The sixth embodiment of the present invention provides a DFB laser epitaxial structure. The difference between the DFB laser epitaxial structure in this embodiment and the DFB laser epitaxial structure in the first embodiment is that: In this embodiment, the thickness of the lower confinement layer is 300 nm, and the Al composition in the lower confinement layer changes towards the active layer by 0.3-0.7.

[0047] Seventh Embodiment The seventh embodiment of the present invention provides a DFB laser epitaxial structure. The difference between the DFB laser epitaxial structure in this embodiment and the DFB laser epitaxial structure in the first embodiment is that: In this embodiment, the thickness of the lower confinement layer is 400 nm, and the Al composition in the lower confinement layer changes towards the active layer by 0.3-0.5.

[0048] Eighth embodiment The eighth embodiment of the present invention provides a DFB laser epitaxial structure. The difference between the DFB laser epitaxial structure in this embodiment and the DFB laser epitaxial structure in the first embodiment is that: In this embodiment, the thickness of the lower confinement layer is 400 nm, and the Al composition in the lower confinement layer changes towards the active layer by 0.3-0.6.

[0049] Ninth Embodiment The ninth embodiment of the present invention provides a DFB laser epitaxial structure. The difference between the DFB laser epitaxial structure in this embodiment and the DFB laser epitaxial structure in the first embodiment is that: In this embodiment, the thickness of the lower confinement layer is 400 nm, and the Al composition in the lower confinement layer changes towards the active layer by 0.3-0.7.

[0050] First comparison The present invention provides a first comparative example, in which a first epitaxial structure of a DFB laser in the prior art is provided. The first epitaxial structure in the prior art is basically the same as the DFB laser epitaxial structure shown in the first embodiment above. The difference is that the lower confining layer in the first comparative example is an AlGaInAs material, the Al composition in the lower confining layer is 0.8, and there is no Al composition gradient growth method.

[0051] Second pair of proportions The present invention provides a second comparative example, in which a second epitaxial structure of a DFB laser in the prior art is provided. The second epitaxial structure in the prior art is basically the same as the DFB laser epitaxial structure shown in the first embodiment above, except that the lower confinement layer in the second comparative example is made of lnGaAsP material and the Al composition in the lower confinement layer is 0.

[0052] To effectively verify the performance of the DFB laser epitaxial structure in the embodiments of the present invention, the DFB laser epitaxial structures prepared in the first to ninth embodiments and the first and second comparative examples were respectively subjected to the relevant performance tests shown in Table 1 below. The test results are shown in Table 1 below: Table 1 below shows a data comparison between the first embodiment, the ninth embodiment, the first comparative example, and the second comparative example;

[0053] Table 1 Overall, the experiments in Table 1 show that, compared with the traditional DFB laser epitaxial structure, the DFB laser epitaxial structure in this embodiment achieves a decrease in wavelength redshift rate and an increase in relaxation frequency by using AlGaInAs or AlGaAs as the lower confinement layer and having the Al composition in the lower confinement layer increase in a gradient towards the active layer. Compared with the first and second comparative examples, it has a significant improvement in wavelength stability and high-frequency response.

[0054] Tenth Embodiment Please see Figure 2 The twelfth embodiment of the present invention provides a method for fabricating a DFB laser epitaxial structure. This method is used to fabricate the DFB laser epitaxial structure described in the first embodiment above. The fabrication method includes: Provide a substrate; An N-type doped layer, a lower confinement layer, an active layer, an upper confinement layer, a grating, a secondary epitaxial capping layer, and an ohmic contact layer are sequentially grown on the substrate. In the step of growing the lower confinement layer, the Al component in the lower confinement layer is increased in a gradient towards the active layer by adjusting the TMAl flow rate in real time.

[0055] Furthermore, the step of adjusting the TMA1 flow rate in real time also includes: By continuously increasing the TMAl flow rate, the Al component in the lower confinement layer increases linearly towards the active layer. Alternatively, by adjusting the TMAl flow rate segment by segment to a preset value, the Al component in the lower confinement layer can be made to increase in a stepwise gradient towards the active layer.

[0056] Specifically, the preparation method of the present invention can be carried out using the following steps, S01-S07; S01, provides a substrate; Specifically, the substrate undergoes surface cleaning.

[0057] It should be noted that the substrate includes, but is not limited to, InP substrates or GaAs substrates. In some optional embodiments, the use of InP substrates or GaAs substrates can be used to fabricate DFB lasers in different wavelength bands.

[0058] SO2, grows an N-type doped layer on the substrate; It should be noted that an n-type doped layer was grown on the substrate using metal-organic chemical vapor deposition (MOCVD), with SIH4 doping to provide charge carriers.

[0059] S03, a lower confinement layer is grown on an N-type doped layer; It should be noted that a lower confinement layer is grown on the N-type doped layer. During growth, the Al composition is gradually increased along the growth direction by adjusting the TMAl flow rate in real time. If a linear gradient is used, the TMAl flow rate is continuously increased; if a step gradient is used, the TMAl flow rate is adjusted step by step to the preset value. Parameters such as growth temperature, pressure, and V / III ratio need to be optimized synergistically to ensure crystal quality.

[0060] S04, an active layer is grown on the lower confinement layer; It should be noted that the active layer specifically refers to a multi-quantum-well active layer, which includes alternately grown well layers and barrier layers.

[0061] S05, grow an upper confinement layer on the active layer; S06, A grating layer is formed on the upper confinement layer. After photolithography and etching to prepare the grating pattern, a second epitaxial cover layer is formed. S07, an ohmic contact layer is formed on the secondary epitaxial capping layer.

[0062] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0063] The above embodiments merely illustrate several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. An epitaxial structure for a DFB laser, characterized in that, It includes a substrate, and an N-type doped layer, a lower confinement layer, an active layer, an upper confinement layer, a grating, a secondary epitaxial capping layer, and an ohmic contact layer sequentially stacked on the substrate; The lower confinement layer is AlGaInAs or AlGaAs; In the lower confinement layer, the Al component increases in a gradient towards the active layer.

2. The DFB laser epitaxial structure according to claim 1, characterized in that, In the lower confinement layer, the Al component increases linearly towards the active layer.

3. The DFB laser epitaxial structure according to claim 1, characterized in that, In the lower confinement layer, the Al component increases in a stepwise gradient towards the active layer.

4. The DFB laser epitaxial structure according to claim 3, characterized in that, The lower limiting layer comprises multiple sub-layers stacked sequentially; The Al composition in each of the sub-layers is different, and the Al composition in each of the sub-layers increases layer by layer along the direction closer to the active layer.

5. The DFB laser epitaxial structure according to claim 1, characterized in that, The thickness of the lower confinement layer is 50nm-500nm.

6. The DFB laser epitaxial structure according to claim 1, characterized in that, The Al composition in the lower confinement layer varies from 0.3 to 0.

7.

7. The DFB laser epitaxial structure according to claim 1, characterized in that, The active layer includes a well layer and a barrier layer stacked sequentially on top of the lower confinement layer.

8. The DFB laser epitaxial structure according to claim 1, characterized in that, The upper confinement layer is ALlnPAs, and the AL component in the upper confinement layer together with the lower confinement layer forms a bidirectional confinement region for the active region.

9. A method for fabricating an epitaxial structure of a DFB laser, characterized in that, include: Provide a substrate; An N-type doped layer, a lower confinement layer, an active layer, an upper confinement layer, a grating, a secondary epitaxial capping layer, and an ohmic contact layer are sequentially grown on the substrate. In the step of growing the lower confinement layer, the Al component in the lower confinement layer is gradient-increased toward the active layer by adjusting the TMAl flow rate in real time.

10. The method for fabricating the epitaxial structure of a DFB laser according to claim 9, characterized in that, The steps for real-time adjustment of TMA1 flow also include: By continuously increasing the TMAl flow rate, the Al component in the lower confinement layer increases linearly towards the active layer. Alternatively, by adjusting the TMAl flow rate segment by segment to a preset value, the Al component in the lower confinement layer can be increased in a stepwise gradient towards the active layer.

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