Method for buried heterostructure fabrication for distributed feedback lasers

Abstract

The application discloses a preparation method of a buried heterojunction for a distributed feedback laser, which can reduce the threshold current of a laser, greatly improve the consistency of a device and has strong process compatibility. The method introduces a selective Zn diffusion treatment process step after growing a buried layer on a top cladding layer. A high-concentration p-type region formed through the diffusion step can form a more ideal ohmic contact with a metal electrode. In addition, an extra thin gold layer with a thickness of 5-10 nm is evaporated on the top cladding layer, which can significantly reduce the ohmic contact. The ohmic contact is reduced from 1.2e ‑5 to 4.5e ‑7 , about 26.6 times. In addition, while reducing the ohmic contact, the current limiting effect is significantly enhanced. The threshold current of the laser prepared by the method can be reduced to 15-18 mA. In addition, the method has low process difficulty, high operability, high freedom and greatly improved device consistency. Moreover, the method has strong process compatibility. The method is suitable for popularization and application in the field of semiconductor lasers.

Description

Technical Field

[0001] This invention relates to the field of semiconductor laser technology, and more specifically, to a method for fabricating buried heterojunctions for distributed feedback lasers. Background Technology

[0002] Distributed feedback (DFB) lasers are core light sources in modern optical communication systems, widely used due to their advantages such as dynamic single longitudinal mode, narrow linewidth, and good wavelength stability. Buried heterojunction (BH) structures have been introduced into DFB lasers to better confine carriers and the optical field, thereby achieving low threshold current, high output power, and stable fundamental transverse mode characteristics. However, with the increasing demands on laser power consumption and efficiency from applications such as data centers and 5G bearer networks, further reducing the threshold current of BH structure DFB lasers to achieve lower power consumption and better performance has become a pressing technical problem in this field.

[0003] Prior art document (CN120109649B, authorization announcement date: 2025-09-09) discloses a method for fabricating a ridge waveguide analog laser with low threshold current. This prior art mainly achieves the reduction of threshold current through innovative design of the quantum well structure in the active region. Its core technical solution includes:

[0004] Active region design: Multiple tensile-strained wide quantum wells (e.g., 14 nm thick) are used to construct the active layer. This design utilizes tensile strain to alter the band structure, increasing the effective hole mass and reducing its mobility, thereby suppressing carrier lateral diffusion to some extent. Simultaneously, the wide quantum well design improves the optical field confinement factor and optical mode gain.

[0005] Preparation process: The preparation method focuses on the epitaxial growth process itself, including substrate cleaning, reactor preparation, metal-organic vapor phase epitaxy (MOVPE) growth at a specific temperature (720℃) and a specific V / III ratio (100:1), and verification of tensile strain and crystal quality by post-growth measurements (X-ray diffraction, photoluminescence).

[0006] Technical effect: This technology claims that by using the above tensile strain quantum well design, the threshold current of the laser can be reduced by more than 50% compared with the traditional compressive strain quantum well design.

[0007] The solution of this existing technology can be briefly understood as follows: the active region required for a low threshold is formed in one step during the epitaxial growth process through "material structure design" (tensile strain-broadened quantum well).

[0008] The existing technology suffers from the following problems in practical use: First, insufficient process flexibility and optimization freedom: The technical effect of the existing solution is highly dependent on specific parameters during the epitaxial growth stage, and its performance optimization is strongly coupled with the growth process. Once epitaxial growth is completed, the electrical characteristics of the active region and confinement layer are fixed, lacking the freedom and effective means to independently and finely control the current confinement structure in the back-end chip process. Second, high process difficulty and technical risk: The tensile strain quantum well structure itself has extremely stringent requirements for the precision and stability of epitaxial growth, with narrow process tolerance. Small fluctuations in actual production can easily lead to significant differences in device performance, posing challenges to yield and consistency. In addition, directly modifying the core light-emitting region of the quantum well is a fundamental structural change, which may trigger a chain of technical problems such as mode stability and reliability, resulting in high technical risks and uncertainties. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a method for fabricating buried heterojunctions for distributed feedback lasers that can reduce the threshold current of lasers, significantly improve device consistency, and have strong process compatibility.

[0010] The technical solution adopted by this invention to solve its technical problem is as follows: the method for fabricating a buried heterojunction for a distributed feedback laser includes the following steps:

[0011] A. An epitaxial growth process is performed on the upper surface of the wafer to obtain a grating epitaxial layer;

[0012] B. Perform grating pattern fabrication on the upper surface of the grating epitaxial layer;

[0013] C. A secondary epitaxial growth process is performed on the surface of the grating epitaxial layer after the grating pattern has been fabricated to obtain the active region;

[0014] D. Process the active area into a tabletop structure;

[0015] E. Current blocking layers are grown on both sides of the active region of the mesa structure;

[0016] F. A top cladding layer is grown on the upper surface of the active region of the mesa structure, the top cladding layer covering both the upper surfaces of the active region and the current blocking layer.

[0017] G. Perform Zn expansion treatment on the top cladding of the growth layer;

[0018] H. Evaporate a thin gold layer with a thickness of 5-10 nm onto the top cladding layer after Zn expansion treatment; and hold at 480℃-520℃ for 3-4 minutes;

[0019] I. Fabrication of metal electrodes on the surface of a thin gold layer and dicing / dissociation.

[0020] Furthermore, in step G, the Zn diffusion process is as follows: the Zn diffusion source is heat-treated at 600-650°C for 15-45 seconds.

[0021] Furthermore, the Zn diffusion source is a Zn-containing organometallic source or a solid source.

[0022] Furthermore, the Zn diffusion source is one or more of Zn3P2, ZnAs2, or DMZn.

[0023] Furthermore, the flow rate of the Zn diffusion source is 80 sccm-120 sccm.

[0024] Furthermore, in step H, the temperature is maintained at 500°C for 3 minutes.

[0025] Furthermore, the metal electrode has a three-layer structure, consisting of a 50nm Au layer, a 50nm Ti layer, and a 50nm Pt layer from top to bottom.

[0026] The beneficial effects of this invention are as follows: The method for fabricating buried heterojunctions for distributed feedback lasers described in this invention introduces a selective Zn diffusion process after growing the buried layer in the top cladding. The high-concentration p-type region formed through this diffusion step can achieve a more ideal ohmic contact with the metal electrode. Experimental verification shows that the ohmic resistivity is reduced from 1.2 eΩ. -5 Ω·m decreased to 6.0e -7 The ohmic contact resistance is reduced by approximately 20 times (Ω·m). Furthermore, depositing an additional 5-10 nm thick gold layer on the top cladding surface significantly reduces the ohmic contact resistance from 1.2 eΩ·m. -5 Reduced to 6e -6 This reduces the ohmic contact by approximately 50% when both processes are used simultaneously, from 1.2e. -5 Reduced to 4.5e -7 This reduces the ohmic contact by approximately 26.6 times; moreover, while reducing the ohmic contact, the expansion of Zn increases the p-type doping concentration in the current-limited region to 8 × 10⁻⁶. 18 -3×10 19 cm -3The current-limiting effect is significantly enhanced, reducing carrier leakage by more than 40%. Simultaneously, the lower doping density above the active region avoids impurity disturbances to the quantum well, improving carrier recombination efficiency by 25%. Actual tests show that the threshold current of the laser prepared using this invention can be reduced to 15-18 mA, a 25%-35% reduction compared to existing technologies (22-25 mA). Furthermore, this invention has low process difficulty, high operability, and high degree of freedom: by controlling the Zn diffusion process parameters (temperature, time, source concentration), the doping concentration distribution, junction depth, and lateral diffusion width of the p-type region can be adjusted to achieve… The optimal lateral current limitation is achieved in the active region of the mesa; the Zn expansion process is performed "after full epitaxy is completed", and Zn ions diffuse through a uniform p-type contact layer, avoiding the doping differences caused by "uneven growth rate of the ridge side surface during epitaxy" in the prior art; the threshold current fluctuation between devices is controlled within ±5%, and the yield rate is increased to over 95% (previous technology <80%), significantly improving device consistency; moreover, the Zn expansion process uses conventional diffusion furnace equipment, requiring no additional special equipment, and can be directly connected to existing MOCVD epitaxial production lines, with strong process compatibility: low modification cost, suitable for mass production. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the process for preparing a buried heterojunction for a distributed feedback laser according to the present invention. Detailed Implementation

[0028] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] like Figure 1 As shown, the method for fabricating buried heterojunctions for distributed feedback lasers according to the present invention includes the following steps:

[0030] A. An epitaxial growth process is performed on the upper surface of the wafer to obtain a grating epitaxial layer. The grating acts as an "optical frequency selector" and plays a role in the wavelength selection and feedback of light in subsequent devices. For example, it enables the laser to output a specific wavelength of 1310nm laser, laying the foundation for the optical performance of the structure.

[0031] B. Create a grating pattern on the upper surface of the grating epitaxial layer; by precisely defining the morphology and parameters of the grating pattern structure, ensure that it can be accurately matched with the active region to stably achieve the light control effect.

[0032] C. A secondary epitaxial growth process is performed on the surface of the grating epitaxial layer after the grating pattern has been fabricated to obtain the active region. The active region is the "optical energy core region" of the device, which can convert electrical energy into optical energy (such as the stimulated emission process of semiconductor lasers). This growth process tightly integrates the active region with the grating layer, creating conditions for light amplification and frequency selection.

[0033] D. Process the active region into a mesa structure; by means of etching, the active region is processed into a "mesa" structure. This design can facilitate the stacking and fabrication of subsequent layers, and can also spatially restrict the current and optical field to act only in the active region, reduce energy loss and improve device efficiency.

[0034] E. Current blocking layers are grown on both sides of the active region of the mesa structure. The current blocking layer acts like a "current gate", forcing the current to flow only through the active region and preventing it from spreading to the non-active region, thereby improving the power utilization rate and making the device performance more stable. The width of the active region can be precisely controlled within 2.0um ± 0.2um by dry etching or wet etching.

[0035] F. A top cladding layer is grown on the upper surface of the active region of the mesa structure. The top cladding layer covers both the upper surface of the active region and the upper surface of the current blocking layer. The top cladding layer is not only a "protective layer" for the active region, but also works in conjunction with other layers to constrain the light field in the vertical direction (locking the light in the active region). At the same time, it provides structural support for subsequent electrode fabrication and ensures the electrical connection function of the device.

[0036] G. Perform Zn diffusion treatment on the top cladding layer. The Zn diffusion treatment process is as follows: Since Zn diffusion remains in the top contact layer and cannot diffuse into the quantum well, it is necessary to control the degree of freedom of the process parameters. Preferably, the Zn diffusion source is heat-treated at 600-650℃ for 15-45 seconds. The Zn diffusion source is a Zn-containing organometallic source or a solid source, typically one or more of Zn3P2, ZnAs2, or DMZn. Typically, the Zn diffusion source is DMZn, and the DMZn flow rate is 80 sccm-120 sccm.

[0037] H. A thin gold layer with a thickness of 5-10 nm is deposited on the top cladding layer after Zn expansion treatment; and held at 480℃-520℃ for 3-4 minutes; preferably, held at 500℃ for 3 minutes.

[0038] I. Fabrication of metal electrodes on the surface of a thin gold layer and dicing / dissociation; the metal electrodes have a three-layer structure, consisting of a 50nm Au layer, a 50nm Ti layer, and a 50nm Pt layer from top to bottom.

[0039] The buried heterojunction fabrication method for distributed feedback lasers described in this invention introduces a selective Zn diffusion process after growing a buried layer in the top cladding. This diffusion process creates a high-concentration p-type region that can form a more ideal ohmic contact with the metal electrode. Experimental verification shows that the ohmic resistivity is reduced from 1.2 eΩ. -5 Ω·m decreased to 6.0e -7 The ohmic contact resistance is reduced by approximately 20 times (Ω·m). Furthermore, depositing an additional 5-10 nm thick gold layer on the top cladding surface significantly reduces the ohmic contact resistance from 1.2 eΩ·m. -5 Reduced to 6e -6 This reduces the ohmic contact by approximately 50% when both processes are used simultaneously, from 1.2e. -5 Reduced to 4.5e -7 This reduces the ohmic contact by approximately 26.6 times; moreover, while reducing the ohmic contact, the expansion of Zn increases the p-type doping concentration in the current-limited region to 8 × 10⁻⁶. 18 -3×10 19 cm -3 The current-limiting effect is significantly enhanced, reducing carrier leakage by more than 40%. Simultaneously, the lower doping density above the active region avoids impurity disturbances to the quantum well, improving carrier recombination efficiency by 25%. Actual tests show that the threshold current of the laser prepared using this invention can be reduced to 15-18 mA, a 25%-35% reduction compared to existing technologies (22-25 mA). Furthermore, this invention has low process difficulty, high operability, and high degree of freedom: by controlling the Zn diffusion process parameters (temperature, time, source concentration), the doping concentration distribution, junction depth, and lateral diffusion width of the p-type region can be adjusted to achieve… The optimal lateral current limitation is achieved in the active region of the mesa; the Zn expansion process is performed "after full epitaxy is completed", and Zn ions diffuse through a uniform p-type contact layer, avoiding the doping differences caused by "uneven growth rate of the ridge side surface during epitaxy" in the prior art; the threshold current fluctuation between devices is controlled within ±5%, and the yield rate is increased to over 95% (previous technology <80%), significantly improving device consistency; moreover, the Zn expansion process uses conventional diffusion furnace equipment, requiring no additional special equipment, and can be directly connected to existing MOCVD epitaxial production lines, with strong process compatibility: low modification cost, suitable for mass production.

[0040] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for fabricating buried heterojunctions for distributed feedback lasers, comprising the following steps: A. An epitaxial growth process is performed on the upper surface of the wafer to obtain a grating epitaxial layer; B. Perform grating pattern fabrication on the upper surface of the grating epitaxial layer; C. A secondary epitaxial growth process is performed on the surface of the grating epitaxial layer after the grating pattern has been fabricated to obtain the active region; D. Process the active area into a tabletop structure; E. Current blocking layers are grown on both sides of the active region of the mesa structure; F. A top cladding layer is grown on the upper surface of the active region of the mesa structure, the top cladding layer covering both the upper surfaces of the active region and the current blocking layer. Its features are: G. Perform Zn expansion treatment on the top cladding of the growth layer; H. Evaporate a thin gold layer with a thickness of 5-10 nm onto the top cladding layer after Zn expansion treatment; and hold at 480℃-520℃ for 3-4 minutes; I. Fabrication of metal electrodes on the surface of a thin gold layer and dicing / dissociation.

2. The method for fabricating a buried heterojunction for a distributed feedback laser according to claim 1, characterized in that: In step G, the Zn diffusion process is as follows: the Zn diffusion source is heat-treated at 600-650°C for 15-45 seconds.

3. The method for fabricating a buried heterojunction for a distributed feedback laser according to claim 2, characterized in that: The Zn diffusion source is a Zn-containing organometallic source or a solid source.

4. The method for fabricating a buried heterojunction for a distributed feedback laser according to claim 3, characterized in that: The Zn diffusion source is one or more of Zn3P2, ZnAs2, or DMZn.

5. The method for fabricating a buried heterojunction for a distributed feedback laser according to claim 4, characterized in that: The flow rate of the Zn diffusion source is 80 sccm-120 sccm.

6. The method for fabricating a buried heterojunction for a distributed feedback laser according to claim 1, characterized in that: In step H, the temperature is maintained at 500°C for 3 minutes.

7. The method for fabricating a buried heterojunction for a distributed feedback laser according to claim 1, characterized in that: The metal electrode has a three-layer structure, consisting of a 50nm Au layer, a 50nm Ti layer, and a 50nm Pt layer from top to bottom.

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