A multiple quantum well laser and method of manufacture

Abstract

The application discloses a multiple quantum well laser, characterized in that the multiple quantum well laser comprises, from bottom to top, a substrate, a p-type confinement layer, a p-type waveguide layer, an active region, an n-type waveguide layer and an n-type confinement layer which are sequentially stacked, the active region comprises at least two quantum wells, a polarization gradient layer is arranged between two adjacent quantum wells, the Al component and the In component in the polarization gradient layer change along the epitaxial growth direction to generate a directional built-in electric field, and holes are induced in the polarization gradient layer, and the n-type confinement layer is etched to form a ridge waveguide structure. On the basis of sufficient carrier injection obtained by inversion epitaxy, the polarization gradient layer is arranged between adjacent quantum wells, and the component gradient is utilized to induce additional holes in the polarization gradient layer, so that the hole injection of the quantum well far away from the p-type layer is effectively supplemented, the problem of uneven hole distribution in the multiple quantum well structure is solved, and the electro-optical conversion efficiency and the working stability of the device are further improved.

Description

Technical Field

[0001] This invention relates to the field of semiconductor optoelectronic device technology, specifically to a multi-quantum-well laser and its fabrication method. Background Technology

[0002] GaN-based lasers have extremely wide applications in laser displays, solid-state lighting, optical storage, biomedicine, and optical communications. Among them, lasers with multiple quantum wells integrated on a single chip are the core devices for realizing wide color gamut laser displays.

[0003] However, single-chip multi-quantum-well lasers still have problems. First, the room-temperature activation rate of Mg doping in the p-type layer is only about 1%, and the hole injection capability is limited. In a multi-quantum-well chip, the gap in the total hole supply will be further amplified by the carrier distribution of multiple quantum wells, eventually resulting in all quantum wells being unable to obtain sufficient carrier injection. Second, there is an uneven carrier distribution among multiple quantum wells in GaN lasers, which can easily lead to problems such as lasing wavelength shift and imbalance of multi-quantum-well output power. Summary of the Invention

[0004] To address the aforementioned problems, this application provides a multi-quantum-well laser, comprising: The substrate, p-type confinement layer, p-type waveguide layer, active region, n-type waveguide layer, and n-type confinement layer are stacked sequentially from bottom to top. The active region includes at least two quantum wells, and a polarization gradient layer is disposed between two adjacent quantum wells. The Al composition and / or In composition in the polarization gradient layer varies along the epitaxial growth direction to generate a directional built-in electric field, thereby inducing the generation of holes in the polarization gradient layer. The n-type confinement layer and a portion of the n-type waveguide layer are etched to form a ridge waveguide structure.

[0005] In some possible implementations, the polarization gradient layer includes a first gradient layer and a second gradient layer; The first gradient layer is AlGaN, and the Al composition of the first gradient layer varies along the epitaxial growth direction; The second gradient layer is InGaN, and the In composition of the second gradient layer varies along the epitaxial growth direction; The second gradient layer is located on top of the first gradient layer.

[0006] In some possible implementations, the substrate is a GaN substrate with a Ga facet

[0001] crystal orientation; The Al composition of the first gradient layer decreases along the epitaxial growth direction, while the In composition of the second gradient layer increases along the epitaxial growth direction.

[0007] In some possible implementations, the substrate is a GaN substrate with an N-plane [000-1] crystal orientation; The Al composition of the first gradient layer increases along the epitaxial growth direction, and the In composition of the second gradient layer decreases along the epitaxial growth direction.

[0008] In some possible implementations, the quantum well is InGaN, and the In composition at the end of the second gradient layer is the same as the In composition of the adjacent quantum well.

[0009] In some possible implementations, the absolute value of the rate of change of the Al component in the first gradient layer is greater than or equal to 0.03 / nm, and the absolute value of the rate of change of the In component in the second gradient layer is greater than or equal to 0.02 / nm.

[0010] In some possible implementations, the substrate is a p-type doped substrate; A p-type electrode is disposed on the side of the substrate opposite to the p-type confinement layer, and the p-type electrode covers the substrate.

[0011] This application also provides a method for fabricating a multi-quantum-well laser, comprising the following steps: A p-type confinement layer and a p-type waveguide layer are epitaxially grown sequentially on a substrate; An active region comprising at least two quantum wells is sequentially grown on the p-type waveguide layer. A polarization gradient layer is also grown between two adjacent quantum wells. The polarization gradient layer comprises a first gradient layer and a second gradient layer that are sequentially grown along the epitaxial direction. The first gradient layer is made of AlGaN material, and the second gradient layer is made of InGaN material. The Al composition of the first gradient layer and the In composition of the second gradient layer vary along the epitaxial growth direction. An n-type waveguide layer and an n-type confinement layer are epitaxially grown sequentially on the active region; The n-type confinement layer and a portion of the n-type waveguide layer are etched to form a ridge waveguide structure.

[0012] In some possible implementations, the substrate is a GaN substrate with a Ga face

[0001] crystal orientation, and the Al composition of the first graded layer and the In composition of the second graded layer vary along the epitaxial growth direction, specifically including: The Al composition of the first gradient layer decreases along the epitaxial growth direction, while the In composition of the second gradient layer increases along the epitaxial growth direction.

[0013] In some possible implementations, the substrate is a GaN substrate with an N-plane [000-1] crystal orientation, and the Al composition of the first graded layer and the In composition of the second graded layer vary along the epitaxial growth direction, specifically including: The Al composition of the first gradient layer increases along the epitaxial growth direction, and the In composition of the second gradient layer decreases along the epitaxial growth direction.

[0014] In this application, an inverted epitaxial growth method is used, requiring only the etching of the n-type confinement layer without etching the p-type layer. This avoids the reduction of the effective conductive area of ​​the p-type layer due to ridge etching, thereby increasing the total supply of holes. Furthermore, the room-temperature activation rate of n-type doping is much higher than that of p-type doping, so ridge etching has less impact on electron injection efficiency and can still stably guarantee the electron supply to the active region, ensuring sufficient carrier injection for all quantum wells. Based on this sufficient carrier injection, a polarization gradient layer is placed between adjacent quantum wells. The polarization doping effect brought about by the compositional gradient induces additional holes in the polarization gradient layer, effectively supplementing the hole injection of quantum wells far from the p-type layer. This solves the problem of uneven hole distribution in multi-quantum-well structures, further improving the electro-optical conversion efficiency and operational stability of the device. Attached Figure Description

[0015] Figure 1 A schematic diagram of a multi-quantum-well laser is disclosed. Figure 2 A schematic diagram of a multi-quantum-well laser is disclosed. Figure 3 A schematic diagram of the energy band structure of an active region is disclosed. Figure 4 A flowchart of the fabrication process for a multi-quantum-well laser is disclosed.

[0016] Figure label: p-type electrode: 10; Substrate: 20; p-type confinement layer: 30; p-type waveguide layer: 40; Active region: 50; Quantum well: 51; Polarization gradient layer: 52; First gradient layer: 521; Second gradient layer: 522; Layer: 60; n-type waveguide layer: 70; n-type confinement layer: 80; Ridge waveguide structure: 90. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0018] In the following description, specific details such as particular system architectures and techniques are set forth for illustrative purposes and not for limitation, in order to provide a thorough understanding of the embodiments of this application. However, those skilled in the art will understand that this application may also be implemented in other embodiments without these specific details. In other instances, detailed descriptions of well-known systems, apparatuses, circuits, and methods have been omitted so as not to obscure the description of this application with unnecessary detail.

[0019] It should be understood that, when used in this application specification and the appended claims, the term "comprising" indicates the presence of the described features, integrals, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or a collection thereof.

[0020] It should also be understood that the term “and / or” as used in this application specification and the appended claims means any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0021] Furthermore, in the description of this application and the appended claims, the terms "first," "second," "third," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0022] References to "one embodiment" or "some embodiments" as described in this specification mean that one or more embodiments of this application include a specific feature, structure, or characteristic described in connection with that embodiment. Therefore, the phrases "in one embodiment," "in some embodiments," "in other embodiments," "in still other embodiments," etc., appearing in different parts of this specification do not necessarily refer to the same embodiment, but rather mean "one or more, but not all, embodiments," unless otherwise specifically emphasized. The terms "comprising," "including," "having," and variations thereof mean "including but not limited to," unless otherwise specifically emphasized.

[0023] There are two problems with single-chip integrated multi-quantum-well semiconductor lasers. First, existing conventional GaN-based lasers mostly adopt forward epitaxial structures, that is, epitaxial n-type layer, active region 50, and p-type layer sequentially along the growth direction. Ridge waveguide structure needs to be etched in the top p-type layer region. However, the room temperature activation rate of p-type Mg doping in GaN-based materials is only about 1%, which has the inherent defect of weak hole injection capability. Ridge etching will also cause lattice damage to p-type materials and reduce the effective conductive area, further aggravating the problem of insufficient hole injection, directly limiting the output power of the device and pushing up the threshold current. Moreover, in multi-quantum-well devices with single-chip integrated multiple quantum wells, the gap in the total hole supply will be further amplified by the carrier distribution of multiple quantum wells, and may even lead to multiple quantum wells being in a state of lack of carrier recombination.

[0024] Secondly, GaN-based materials suffer from carrier transport asymmetry: the room-temperature mobility of holes is only 1 / 50 to 1 / 100 that of electrons, resulting in extremely weak hole transport. In multi-quantum-well structures, the vast majority of holes preferentially recombine with electrons in quantum wells closer to the p-type layer, while quantum wells farther from the p-type layer struggle to obtain sufficient hole injection, leading to severe uneven carrier distribution among the quantum wells. This problem directly causes the quantum wells to fail to reach the lasing threshold synchronously, resulting in defects such as lasing wavelength shift, multi-quantum-well output power imbalance, and poor operational stability, severely limiting the performance of multi-quantum-well lasers for wide color gamut laser displays.

[0025] To address the aforementioned issues, this invention proposes a multi-quantum-well laser chip and its fabrication method. The core of this method lies in avoiding ridge etching of the p-type layer through an inverted epitaxial structure, thereby improving hole injection in the multi-quantum-well and increasing the total amount of carrier injection. Furthermore, a polarization gradient layer is set between the multi-quantum-wells. By generating a directional built-in electric field in the polarization gradient layer, holes are induced, supplementing the hole injection of quantum wells farther from the p-type layer, thus making the output power of the multi-quantum-well more balanced.

[0026] like Figure 1 As shown in the embodiment of this application, a multi-quantum-well laser is disclosed, including a substrate 20, a p-type confinement layer 30, a p-type waveguide layer 40, an active region 50, an n-type waveguide layer 70, and an n-type confinement layer 80 stacked sequentially from bottom to top; The active region 50 includes at least two quantum wells 51, and a polarization gradient layer 52 is disposed between two adjacent quantum wells 51. The Al composition and / or In composition in the polarization gradient layer 52 varies along the epitaxial growth direction to generate a directional built-in electric field, thereby inducing the generation of holes in the polarization gradient layer 52. The n-type confinement layer 80 and the n-type waveguide layer 70 of a certain thickness are etched to form a ridge waveguide structure 90. In some embodiments, only the n-type confinement layer 80 may be etched without etching the n-type waveguide layer 70 to form the ridge waveguide structure 90.

[0027] The substrate 20 is GaN, the p-type confinement layer 30 is mg-doped AlGaN, the p-type waveguide layer 40 is mg-doped InGaN, the active region 50 is InGaN, the n-type waveguide layer 70 is Si-doped InGaN, and the n-type confinement layer 80 is Si-doped AlGaN.

[0028] In this embodiment, the Al and In components of the polarization gradient layer continuously change along the epitaxial growth direction. The total polarization intensity changes continuously and synchronously along the growth direction with the continuous change in composition, forming a non-zero polarization intensity gradient. This continuous change in polarization intensity generates continuously distributed bulk polarization charges within the volumetric material of the gradient layer. These charges create a directional built-in electric field along the growth direction within the gradient layer. This built-in electric field induces holes, and the strength of the built-in electric field is positively correlated with the polarization intensity gradient: the steeper the composition change, the greater the polarization intensity gradient, the higher the strength of the built-in electric field, and the more holes are generated.

[0029] Understandably, when the laser is working, the holes generated in the polarization gradient layer 52 are transported towards the n-type layer (i.e., the n-type confinement layer and the n-type waveguide layer) under the action of the external electric field. They only enter the quantum well 51, which is far from the p-type layer, to supplement the hole injection. They will not be transported to the quantum well 51 in the opposite direction. Therefore, there will be no problem of holes being ineffectively injected into the quantum well 51, which originally had sufficient holes.

[0030] It should be noted that this application improves the overall hole injection of the multiple quantum wells 51 through an inverted epitaxial structure, and solves the problem of uneven carrier distribution in the multiple quantum wells 51 by setting a polarization gradient layer 52. The gains of the two are complementary. To clearly illustrate the necessity of the synergy of the technologies in this application, the limitations of a single technology and the complementary gain logic of the synergy between the two are explained in detail here: First, while using only an inverted epitaxial structure (with the p-type confinement layer 30 and p-type waveguide layer 40 adjacent to the substrate 20) can improve the p-type doping activation efficiency and increase the total hole injection supply by avoiding ridge etching damage to the p-type layer, thus solving the problem of insufficient hole supply in traditional forward epitaxial structures, the p-type layer in this application is located on the substrate 20 side, and in some embodiments, the electrode is located below the substrate 20, with holes injected into the active region 50 from the substrate 20 side. Compared to traditional forward epitaxial structures, this results in holes being directed away from the quantum well 51, which is further away from the p-type layer. The transport path is longer; and the hole mobility in GaN-based materials is much lower than that of electrons, only 1 / 50 to 1 / 100 of that of electrons. The extension of the transport path will further aggravate the problem of uneven carrier distribution among multiple quantum wells. Even if the total hole supply is increased, most holes will still preferentially recombine in quantum wells close to the p-type layer. Quantum wells far from the p-type layer still cannot obtain sufficient hole injection. The increase in the total number of holes cannot be converted into an overall lasing efficiency gain of multiple quantum wells, and the design goal of synchronous lasing and power balance of multiple quantum wells cannot be achieved.

[0031] Secondly, although setting a polarization gradient layer 52 only between quantum wells 51 can induce additional holes through the polarization doping effect and optimize the carrier distribution among multiple quantum wells to a certain extent, this design cannot solve the root problem of insufficient total hole supply caused by the etching of the p-type layer in the traditional forward epitaxial structure. The total upper limit of hole injection is limited by the low activation rate of p-type doping. Even if the limited number of holes are distributed through the polarization gradient layer 52, the carrier shunting effect of multiple sets of quantum wells will still cause each quantum 1 to fail to obtain the carrier concentration required for lasing, or even fail to reach the lasing threshold. The carrier regulation effect of the polarization gradient layer 52 cannot fully exert its design effect due to the lack of sufficient total hole support.

[0032] This application combines the above technologies to achieve a complementary synergistic gain effect: the inverted epitaxial structure solves the root cause of insufficient total hole supply, providing a sufficient hole base for carrier control of the polarization gradient layer 52, thus enabling the carrier allocation and transport optimization function of the polarization gradient layer 52 to play a role; while the polarization gradient layer 52 specifically addresses the inherent defects of prolonged hole transport paths and aggravated uneven carrier distribution in the inverted epitaxial structure, so that the increase in the total number of holes is converted into a lasing efficiency gain for all quantum wells 51. The synergistic effect of the two solves the problems of insufficient total hole injection and uneven carrier distribution in multi-quantum-well GaN-based lasers, achieving a simultaneous and significant improvement in device output power, electro-optic conversion efficiency, and multi-quantum-well lasing stability.

[0033] In some embodiments, the polarization gradient layer 52 includes a first gradient layer 521 and a second gradient layer 522 stacked sequentially along the epitaxial growth direction; the first gradient layer 521 is made of AlGaN material, and the Al composition of the first gradient layer 521 changes continuously along the epitaxial growth direction; the second gradient layer 522 is made of InGaN material, and the In composition of the second gradient layer 522 changes continuously along the epitaxial growth direction; the second gradient layer 522 is located above the first gradient layer 521, the lower surface of the first gradient layer 521 is in contact with the upper surface of the quantum well 51 near the p-type waveguide layer 40, and the upper surface of the second gradient layer 522 is in contact with the lower surface of the quantum well 51 away from the p-type waveguide layer 40.

[0034] Furthermore, the material of the interface between the first gradient layer 521 and the second gradient layer 522 can be GaN, that is, the first gradient layer 521 changes from AlGaN to GaN along the epitaxial direction, that is, the Al content decreases to 0, and the second gradient layer 522 changes from GaN to InGaN along the epitaxial direction, during which the In content gradually increases.

[0035] As an example, the substrate 20 is a GaN substrate with a Ga face

[0001] crystal orientation. The Al composition of the first gradient layer 521 decreases along the epitaxial growth direction, and the In composition of the second gradient layer 522 increases along the epitaxial growth direction.

[0036] The quantum well 51 near the p-type confinement layer 30 is InGaN with a thickness of 2-4 nm and an In composition of 0.15-0.2; the quantum well 51 away from the p-type confinement layer 30 is InGaN with a thickness of 2-4 nm and an In composition of 0.2-0.25; the first graded layer 521 has a thickness of 5-20 nm and an Al composition that linearly decreases from 0.2-0.3 to 0-0.1 along the epitaxial growth direction; the second graded layer 522 has a thickness of 2-10 nm and an In composition that linearly increases from 0.15-0.2 to 0.2-0.25 along the epitaxial growth direction.

[0037] As an example, the substrate 20 is a GaN substrate with an N-plane [000-1] crystal orientation. The Al composition of the first gradient layer 521 increases linearly along the epitaxial growth direction, and the In composition of the second gradient layer 522 decreases linearly along the epitaxial growth direction.

[0038] The quantum well 51 near the p-type confinement layer 30 is InGaN with a thickness of 2-4 nm and an In composition of 0.15-0.2; the quantum well 51 away from the p-type confinement layer 30 is InGaN with a thickness of 2-4 nm and an In composition of 0.2-0.25; the first graded layer 521 has a thickness of 5-20 nm and an Al composition that increases from 0-0.1 to 0.2-0.3 along the epitaxial growth direction; the second graded layer 522 has a thickness of 2-10 nm and an In composition that decreases from 0.2-0.25 to 0.15-0.2 along the epitaxial growth direction.

[0039] In this embodiment, both the first gradient layer 521 and the second gradient layer 522 can generate continuous changes in polarization intensity through composition gradient. The two layers are superimposed to form a steeper total polarization intensity gradient, thereby obtaining a stronger directional built-in electric field and a more significant polarization doping effect.

[0040] The following description uses GaN substrate 20 with Ga face

[0001] crystal orientation as an example. The polarization intensity of AlGaN material increases significantly with increasing Al composition, and the starting point of polarization intensity is higher with high Al composition. The polarization intensity of InGaN material decreases with increasing In composition, and the ending point of polarization intensity is lower with high In composition. By using AlGaN with high Al composition as the starting point of polarization gradient layer and InGaN with high In composition as the ending point, a larger total change in polarization intensity than a single material gradient layer can be achieved within the same total thickness, resulting in a steeper polarization change rate and ultimately forming a stronger directional built-in electric field. At the same time, the bandgap difference between adjacent quantum wells 51 can be smoothed by the gradient change of In composition, avoiding the barrier peak formed by the abrupt change in bandgap that hinders hole transport.

[0041] In some embodiments, such as Figure 2 As shown, it also includes a barrier layer 60, which is located between the upper quantum well 51 and the n-type waveguide 70, and between the lower quantum well 51 and the p-type waveguide 40.

[0042] A barrier layer 60 may not be set between adjacent quantum wells 51, because the polarization gradient layer 52 can assume part of the function of the barrier layer.

[0043] In some embodiments, the substrate 20 is a GaN substrate 20 with Ga plane

[0001] crystal orientation, the Al composition of the first gradient layer 521 decreases linearly along the epitaxial growth direction, and the In composition of the second gradient layer 522 increases linearly along the epitaxial growth direction.

[0044] As an example, the quantum well 51 near the p-type confinement layer 30 is an InGaN quantum well 51 with a thickness of 2-4 nm and an In composition of 0.15-0.2; the quantum well 51 further away from the p-confinement layer is also an InGaN quantum well 51 with a thickness of 2-4 nm and an In composition of 0.2-0.25; the first graded layer 521 has a thickness of 5-15 nm, and the Al composition decreases linearly from 0.25 to 0.3 along the epitaxial growth direction to 0, corresponding to a gradual decrease in polarization intensity from a high absolute value towards the GaN reference value; the second graded layer... The thickness of 522 is 3~10nm. The In composition increases linearly from 0 to 0.2 to 0.25 along the epitaxial growth direction. The corresponding polarization intensity decreases further from the GaN reference value to the endpoint of a lower absolute value. That is, the polarization intensity changes in the two are consistent. After stacking, the total polarization intensity gradient is larger than that of a single gradient layer, which can form a stronger directional built-in electric field and obtain a more significant polarization doping hole induction effect. At the same time, the In composition of the second gradient layer 522 can be completely matched with the In composition of the adjacent quantum well 51, realizing a smooth transition of the band gap.

[0045] In some embodiments, the substrate 20 is a GaN substrate with an N-plane [000-1] crystal orientation, the Al composition of the first gradient layer 521 increases along the epitaxial growth direction, and the In composition of the second gradient layer 522 decreases along the epitaxial growth direction.

[0046] As an example, in this embodiment, the quantum well 51 near the p-type confinement layer 30 is an InGaN quantum well 51 with a thickness of 2-4 nm and an In composition of 0.15-0.2; the quantum well 51 away from the p-type confinement layer 30 is an InGaN quantum well 51 with a thickness of 2-4 nm and an In composition of 0.2-0.25; the thickness of the first gradient layer 521 is 5-15 nm, and the Al composition increases linearly from 0 to 0.25-0.3 along the epitaxial growth direction, corresponding to a gradual increase in polarization intensity under the N-plane from the GaN reference value to the starting point of a high absolute value; the thickness of the second gradient layer 522 is 3-10 nm, and the In composition decreases linearly from 0.2-0.25 to 0.15-0.2 along the epitaxial growth direction, corresponding to a further increase in polarization intensity from the end point of a low absolute value to the GaN reference value.

[0047] As an example, such as Figure 3 As shown, Figure 3 The diagram shows the energy band diagram of the active region along the epitaxial growth direction, i.e. from the p-type confinement layer 30 to the n-type confinement layer 80, when the substrate 20 is a GaN substrate with the Ga face

[0001] crystal orientation. The energy band of the first gradient layer 521 decreases linearly with the linear decrease of Al composition, and the energy band of the second gradient layer 522 increases linearly with the linear increase of In composition.

[0048] In this embodiment, the polarization intensity of the first gradient layer 521 and the second gradient layer 522 have the same trend. When superimposed, they form a steeper total polarization intensity gradient, which at the same time achieves a stronger hole induction and directional transport effect. The In component of the second gradient layer 522 is smoothly connected to the adjacent quantum well 51 to avoid adding an extra hole transport barrier.

[0049] In some embodiments, the quantum well 51 is InGaN, and the In composition at the end of the second gradient layer 522 is the same as the In composition of the adjacent quantum well 51.

[0050] In some embodiments, the quantum well 51 is made of InGaN material, and the In composition at the end of the second graded layer 522 is the same as that of the adjacent quantum well 51. Specifically, the second graded layer 522 is located between the first graded layer 521 and the quantum well 51 away from the p-type waveguide layer 40. The end refers to the top of the second graded layer 522 along the epitaxial growth direction, that is, the end of the second graded layer 522 adjacent to the quantum well 51 away from the p-type waveguide layer 40, and the In composition at this end is completely consistent with the In composition of the adjacent quantum well 51.

[0051] Specifically, if the substrate 20 is a GaN substrate 20 with Ga face

[0001] crystal orientation, the first gradient layer 521 is an AlGaN material, and its Al composition decreases linearly along the epitaxial growth direction; the second gradient layer 522 is an InGaN material, and its In composition increases linearly along the epitaxial growth direction. The In composition at the upper end of the second gradient layer 522 is the same as or differs by no more than 15% from the In composition of the adjacent quantum well 51 that is far from the p-type confinement layer 30.

[0052] As an example, the In composition of the adjacent quantum well 51 is 0.22, and the In composition of the second graded layer 522 increases linearly from 0.18 to 0.22 along the epitaxial growth direction, with the In composition at its upper end being consistent with that of the adjacent quantum well 51.

[0053] If the substrate 20 is a p-type GaN substrate 20 with N-plane [000-1] crystal orientation, the first gradient layer 521 is made of AlGaN material, and its Al composition increases linearly along the epitaxial growth direction; the second gradient layer 522 is made of InGaN material, and its In composition decreases linearly along the epitaxial growth direction. The In composition at the end of the second gradient layer 522 is the same as or differs by no more than 15% from the In composition of the adjacent quantum well 51 that is far away from the p-type waveguide layer 40.

[0054] As an example, the In composition of the adjacent quantum well 51 is 0.22, and the In composition of the second graded layer 522 decreases linearly from 0.25 to 0.22 along the epitaxial growth direction. The In composition of its upper end is consistent with the In composition of the ringing quantum well 51, which realizes a smooth transition of the band gap and avoids the obstruction of hole transport by the interface barrier.

[0055] This embodiment, through the aforementioned component matching design, achieves a smooth bandgap transition between the second gradient layer 522 and the adjacent quantum well 51, eliminating the heterojunction interface bandgap spikes caused by In compositional abrupt changes, avoiding the barrier effect of potential peaks on hole transport, further reducing the potential barrier for hole transport to the quantum well 51 far from the p-type layer, and improving hole injection efficiency. Simultaneously, the perfectly matched interface avoids interface defects and non-radiative recombination centers caused by lattice mismatch abrupt changes, reducing carrier interface losses and improving the device's electro-optic conversion efficiency and long-term operational reliability. Furthermore, the component-free interface avoids the appearance of additional polarization spikes, preventing disruption of the polarization intensity gradient formed by the superposition of the first gradient layer 521 and the second gradient layer 522, ensuring the continuity and intensity of the directional built-in electric field formed by the polarization gradient layer 52, and fully leveraging the effects of polarization doping-induced holes and directional hole transport.

[0056] In some embodiments, the absolute value of the rate of change of the Al component in the first gradient layer 521 is greater than or equal to 0.03 / nm, and the absolute value of the rate of change of the In component in the second gradient layer 522 is greater than or equal to 0.02 / nm. The absolute value of the rate of change of the components refers to the ratio of the absolute value of the difference in components at both ends of the gradient layer to the thickness of the gradient layer.

[0057] This embodiment can effectively control the polarization intensity gradient of the polarization gradient layer 52 by setting the composition change rate of the two-component layer, thus ensuring that a sufficiently strong directional built-in electric field and polarization doping effect are obtained.

[0058] In some embodiments, the substrate 20 is a p-type doped substrate 20, and a p-type electrode 10 is disposed on the side of the substrate 20 away from the p-type confinement layer 30, and the p-type electrode 10 covers the substrate 20.

[0059] Specifically, the p-type confinement layer 30 is directly epitaxially grown on the upper surface of the p-type doped substrate 20 along the epitaxial growth direction, and the p-type electrode 10 is fabricated on the lower surface of the substrate 20 (i.e. the side of the substrate 20 away from the p-type confinement layer 30), and the p-type electrode 10 completely covers the entire lower surface of the substrate 20, forming a surface-contact p-type ohmic electrode structure.

[0060] The core advantages of this embodiment are as follows: First, by using a p-type doped substrate 20 in conjunction with a p-type electrode 10 that fully covers the back side, uniform surface injection of p-type current can be achieved across the entire domain. Compared with the top strip p-type electrode 10 of the traditional forward epitaxial structure, the uniformity of hole injection is greatly improved, while the series resistance and Joule heat loss of the device are significantly reduced, the heat dissipation performance of the device is optimized, and the stability under high power operation is improved. Second, the p-type doped substrate 20 and the p-type confinement layer 30 are doped with the same type, and the heterojunction interface barrier between the two is extremely low, which can achieve efficient hole transport. With the fully covered surface electrode, holes can be uniformly injected into the entire p-type confinement layer 30 and the p-type waveguide layer 40, further amplifying the advantage of the inverted epitaxial structure of this invention in increasing the total amount of hole injection, and providing sufficient and uniform hole supply for the active region 50.

[0061] As an example, substrate 20 is a p-type Mg-doped GaN substrate with a

[0001] crystal orientation and a doping concentration of 1×10¹. 8 cm⁻³~5×10¹ 9 cm⁻³; the p-type electrode 10 is a Ni / Au metal stack structure, which is prepared on the back side of the substrate 20 by electron beam evaporation process, and the p-type electrode 10 completely covers the entire back side area of ​​the substrate 20. After rapid thermal annealing, it forms a low-resistance ohmic contact with the p-type substrate 20.

[0062] As an example, substrate 20 is a p-type Mg-doped N-plane [000-1] oriented GaN substrate 20 with a doping concentration of 5 × 10¹. 8 cm⁻³~1×10² 0 cm⁻³; The p-type electrode 10 is a Pd / Au metal stack structure that completely covers the entire back side of the substrate 20.

[0063] like Figure 4 As shown in the embodiments, this application also discloses a method for fabricating a multi-quantum-well laser, including the following steps: S101. A p-type confinement layer 30 and a p-type waveguide layer 40 are epitaxially grown sequentially on the substrate 20. S102. An active region 50, including a quantum well 51, a polarization gradient layer 52, and the quantum well 51, is sequentially grown on the p-type waveguide layer 40. The polarization gradient layer 52 includes a first gradient layer 521 and a second gradient layer 522, which are sequentially grown along the epitaxial direction. The first gradient layer 521 is made of AlGaN material, and the second gradient layer 522 is made of InGaN material. The Al composition of the first gradient layer 521 and the In composition of the second gradient layer 522 change along the epitaxial growth direction. S103. An n-type waveguide layer 70 and an n-type confinement layer 80 are epitaxially grown sequentially on the active region 50. S104. Etch the n-type confinement layer 80 and the n-type waveguide layer 70 of a certain thickness to form a ridge waveguide structure 90.

[0064] In some embodiments, the substrate 20 is a GaN substrate 20 with a Ga plane

[0001] crystal orientation, and the Al composition of the first gradient layer 521 and the In composition of the second gradient layer 522 vary along the epitaxial growth direction, specifically including: The Al content of the first graded layer 521 decreases along the epitaxial growth direction, while the In content of the second graded layer 522 increases along the epitaxial growth direction.

[0065] In some embodiments, the substrate 20 is a GaN substrate 20 with an N-plane [000-1] crystal orientation, and the Al composition of the first gradient layer 521 and the In composition of the second gradient layer 522 vary along the epitaxial growth direction, specifically including: The Al composition of the first graded layer 521 increases along the epitaxial growth direction, while the In composition of the second graded layer 522 decreases along the epitaxial growth direction.

[0066] In the above embodiments, the descriptions of each embodiment have different focuses. For parts that are not described in detail or recorded in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0067] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A multi-quantum-well laser, characterized in that, It includes a substrate, a p-type confinement layer, a p-type waveguide layer, an active region, an n-type waveguide layer, and an n-type confinement layer stacked sequentially from bottom to top; The active region includes at least two quantum wells, and a polarization gradient layer is disposed between two adjacent quantum wells. The Al and In components in the polarization gradient layer vary along the epitaxial growth direction to generate a directional built-in electric field, thereby inducing the generation of holes in the polarization gradient layer. The n-type confinement layer is etched to form a ridge waveguide structure.

2. The multi-quantum-well laser according to claim 1, characterized in that, The polarization gradient layer includes a first gradient layer and a second gradient layer; The first gradient layer is AlGaN, and the Al composition of the first gradient layer varies along the epitaxial growth direction; The second gradient layer is InGaN, and the In composition of the second gradient layer varies along the epitaxial growth direction; The second gradient layer is located on top of the first gradient layer.

3. The multi-quantum-well laser according to claim 2, characterized in that, The substrate is a GaN substrate with a Ga face [0001] crystal orientation; The Al composition of the first gradient layer decreases along the epitaxial growth direction, while the In composition of the second gradient layer increases along the epitaxial growth direction.

4. The multi-quantum-well laser according to claim 2, characterized in that, The substrate is a GaN substrate with an N-plane [000-1] crystal orientation; The Al composition of the first gradient layer increases along the epitaxial growth direction, and the In composition of the second gradient layer decreases along the epitaxial growth direction.

5. The multi-quantum-well laser according to claim 2, characterized in that, The quantum well is InGaN, and the In composition at the end of the second graded layer is the same as the In composition of the adjacent quantum well.

6. The multi-quantum-well laser according to claim 2, characterized in that, The absolute value of the rate of change of the Al component in the first gradient layer is greater than or equal to 0.03 / nm, and the absolute value of the rate of change of the In component in the second gradient layer is greater than or equal to 0.02 / nm.

7. The multi-quantum-well laser according to any one of claims 1 to 6, characterized in that, The substrate is a p-type doped substrate; A p-type electrode is disposed on the side of the substrate opposite to the p-type confinement layer, and the p-type electrode covers the substrate.

8. A method for fabricating a multi-quantum-well laser, characterized in that, Includes the following steps: A p-type confinement layer and a p-type waveguide layer are epitaxially grown sequentially on a substrate; An active region comprising at least two quantum wells is sequentially grown on the p-type waveguide layer. A polarization gradient layer is also grown between two adjacent quantum wells. The polarization gradient layer comprises a first gradient layer and a second gradient layer that are sequentially grown along the epitaxial direction. The first gradient layer is made of AlGaN material, and the second gradient layer is made of InGaN material. The Al composition of the first gradient layer and the In composition of the second gradient layer vary along the epitaxial growth direction. An n-type waveguide layer and an n-type confinement layer are epitaxially grown sequentially on the active region; The n-type confinement layer is etched to form a ridge waveguide structure.

9. The preparation method according to claim 8, characterized in that, The substrate is a GaN substrate with a Ga face [0001] crystal orientation, and the Al composition of the first graded layer and the In composition of the second graded layer vary along the epitaxial growth direction, specifically including: The Al composition of the first gradient layer decreases along the epitaxial growth direction, while the In composition of the second gradient layer increases along the epitaxial growth direction.

10. The preparation method according to claim 8, characterized in that, The substrate is a GaN substrate with an N-plane [000-1] crystal orientation. The Al composition of the first graded layer and the In composition of the second graded layer vary along the epitaxial growth direction, specifically including: The Al composition of the first gradient layer increases along the epitaxial growth direction, and the In composition of the second gradient layer decreases along the epitaxial growth direction.

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