A gallium nitride-based photonic crystal surface emitting blue laser and a preparation method thereof

Abstract

The application discloses a gallium nitride-based photonic crystal surface-emitting blue light laser and a preparation method thereof, and the structure comprises a substrate layer, a bottom GaN layer, a porous GaN layer, an AlGaN-n layer, an n electrode, a multilayer quantum well active layer formed by GaN / InGaN pairs, an AlGaN-p blocking layer, a GaN-p layer, a p electrode and a photonic crystal layer. The application proposes that the porous GaN with low and controllable refractive index is used as a cladding layer, the surface etching type photonic crystal structure is combined, and the layer structure design is optimized, so that the active layer and the surface photonic crystal layer can simultaneously have high resonant light field coupling strength, low threshold, high efficiency and high performance light emission are facilitated, and thus a new GaN-PCSEL is realized.

Description

Technical Field

[0001] This invention discloses a gallium nitride-based photonic crystal surface-emitting blue laser and its fabrication method, which belongs to the field of active photonic devices. Background Technology

[0002] Photonic crystal surface-emitting lasers (PCSELs) are a type of semiconductor laser with great potential. They utilize in-plane optical feedback within the band edges of a photonic crystal for resonance, optical gain, and lasing. The resonant cavity can encompass the entire photonic crystal structure, and the resulting standing waves oscillate within the photonic crystal, facilitating efficient coupling with the active layer for gain. PCSELs possess advantages such as large emission area, simple structure, good single-mode performance, low divergence angle, low absorption loss and series resistance, and high power output, making them highly promising and possessing significant market value in fields such as high-density optical storage, information transmission, micro-projector light sources, biomedical sensing, LiDAR, and solid-state laser lighting.

[0003] Gallium nitride (GaN)-based group III-V materials can be flexibly tunably covered in any wavelength range from ultraviolet to green light. With their large bandgap, high breakdown voltage, high saturated electron drift rate, and high thermal conductivity, they possess unique advantages in the fabrication of microwave and millimeter-wave devices, ultraviolet photodetectors, short-wavelength visible light-emitting diodes, and lasers. GaN-based short-wavelength lasers in ultraviolet / blue / green light have wide applications in high-speed communications, display systems, and high-density memory, demonstrating excellent performance and promising application prospects.

[0004] However, due to limitations in material growth, gallium nitride (GaN)-based PCSELs have consistently used AlGaN, which has a relatively high refractive index, as the cladding layer. This has led to reported electrically pumped GaN-based PCSELs all involving the fabrication of buried air holes within the GaN material to form photonic crystals (PCs) to achieve sufficient optical field distribution intensity. This method is complex, difficult, and costly, and also limits the coupling strength between the optical field and the PC and active regions. Furthermore, currently reported PCSELs that obtain photonic crystals by etching active quantum well layers through surface GaN are limited to optical pumping excitation and cause damage to the active layer, resulting in low quantum efficiency. All of these factors restrict breakthroughs in high-performance blue lasers. Summary of the Invention

[0005] To address the deficiencies in the aforementioned background technology, this invention provides a gallium nitride-based photonic crystal surface-emitting blue laser and its fabrication method, featuring a surface-etched PC structure and optimized layer structure design, which facilitates the realization of low-threshold, high-efficiency, and high-performance optical laser emission.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a gallium nitride-based photonic crystal surface-emitting blue laser, comprising:

[0007] Substrate layer, wherein the substrate layer is a sapphire substrate or a silicon substrate;

[0008] The bottom GaN layer is located above the substrate layer;

[0009] A porous GaN layer, located on top of the underlying GaN layer, has a refractive index of 1.6 to 2.2.

[0010] The AlGaN-n layer is located on top of the porous GaN layer;

[0011] The quantum well active layer is located above the AlGaN-n layer;

[0012] An AlGaN-p blocking layer is located above the quantum well active layer;

[0013] The GaN-p layer is located on top of the AlGaN-p barrier layer;

[0014] A photonic crystal layer with periodic holes of a certain depth is etched on the surface of the GaN-p layer;

[0015] The refractive index of the porous GaN layer is lower than that of other bottom GaN layers, AlGaN-n layers, quantum well active layers, AlGaN-p barrier layers, and GaN-p layers.

[0016] The p-electrode is located on the surface of the GaN-p layer;

[0017] n-electrode, located on the surface of AlGaN-n layer;

[0018] p-type islands are formed by etching from top to bottom from the GaN-p layer to the AlGaN-n layer.

[0019] Furthermore, the thickness of the bottom GaN layer is 1500~50000nm, and the refractive index is 2.4~2.49.

[0020] Furthermore, the porous GaN layer has a thickness of 500~10000nm and a refractive index of 1.6~2.2.

[0021] Furthermore, the AlGaN-n layer has a thickness of 100~200nm and a refractive index of 2.4~2.49.

[0022] Furthermore, the active quantum well layer is a 3-12 layer quantum well layer composed of InGaN / GaN pairs, each layer including: an InGaN well layer of 2.5 nm and a GaN barrier layer of 12 nm, wherein the refractive index of the InGaN layer is 2.55-2.65 and the refractive index of the GaN barrier layer is 2.4-2.49.

[0023] Furthermore, the AlGaN-p barrier layer has a thickness of 20 nm, a refractive index of 2.3~2.4, and an Al content of 20%.

[0024] Furthermore, the GaN-p layer has a thickness of 50~500nm and a refractive index of 2.4~2.49.

[0025] Furthermore, the photonic crystal layer has a triangular lattice, a square lattice, or a honeycomb lattice, with an etching depth of 30~400nm, a period of 150~250nm, and a hole radius of 10~100nm.

[0026] Furthermore, the p electrode includes an ohmic contact metal electrode or a composite electrode of a metal electrode and ITO; the n electrode includes an ohmic contact metal electrode.

[0027] The fabrication method of a photonic crystal surface-emitting laser based on a porous gallium nitride cladding layer includes the following steps:

[0028] Step 1: Epitaxially grow GaN-based III-V group materials on a sapphire substrate or a silicon substrate, with the following layers sequentially on the substrate: bottom GaN, heavily doped GaN-n layer, AlGaN-n layer, quantum well active layer, AlGaN-p barrier layer, and GaN-p layer.

[0029] Step 2: Prepare a photonic crystal mask layer by electron beam lithography (EBL), then perform dry etching to form holes, and remove the mask layer to form the photonic crystal layer;

[0030] Step 3: Perform photolithography-dielectric protection-electrochemical etching process to transform the heavily doped GaN-n layer into a porous GaN layer;

[0031] Step 4: Fabricate the p-electrode using photolithography and lift-off processes;

[0032] Step 5: Photolithography, define the photolithography window, and dry etch down to the AlGaN-n layer to form mesa islands;

[0033] Step 6: Fabricate the n-electrode using photolithography and lift-off processes.

[0034] The working principle is as follows: When the frequency of the light emitted by the active layer (MQWs layer) satisfies the band gap condition of the photonic crystal, the wavelength resonates in the plane of the photonic crystal to generate a standing wave. The resonance of the standing wave in the plane will interact with the active layer, thereby forming a gain and population inversion, thus generating laser. At the same time, due to the first-order Bragg diffraction of the photonic crystal, the laser is emitted vertically out of the plane. Beneficial effects

[0035] 1. This invention utilizes low-refractive-index porous GaN as a cladding layer, which confines the light field more to the region above the porous GaN. Thus, the refractive index of the porous GaN can be controlled to achieve a greater distribution of the light field in the active region and the photonic crystal region. This enables the active layer and the surface-type photonic crystal layer to have high resonant light field coupling strength at the same time, which is beneficial for achieving low-threshold, high-efficiency, and high-performance photolasing.

[0036] 2. This invention employs direct etching of the GaN-p layer on its surface to form a photonic crystal structure, reducing both the fabrication process difficulty and cost, and facilitating the realization of high-quality photonic crystals. Compared to structures with buried air holes within the GaN layer, the surface etching of the PC structure in this invention eliminates the need for the complex "growth-etching-hole protection-re-growth" process. High-temperature re-growth after etching leads to a low-quality interface, introducing unnecessary non-radiative recombination, and the interface resistance of the secondary growth is very high, affecting device performance.

[0037] 3. Compared with the quantum well etched PhC of etched GaN, the present invention does not introduce any defects into the III-V group active material system, and will not affect the uniformity of quantum well light emission, current injection and distribution. The defects introduced by etching into the quantum well will eventually lead to a decrease in the IQE of the device. Attached Figure Description

[0038] Figure 1 This is a side view of the structure of the present invention;

[0039] Figure 2 This is a flowchart of the preparation process of the present invention. Implementation

[0040] The implementation of the technical solution will be further described in detail below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0041] The concept of this invention is to use porous GaN with low and controllable refractive index as a cladding layer, combined with a surface-etched PC structure and the above-mentioned unique layer structure design, to achieve high coupling strength of the optical field between the active layer and the photonic crystal layer.

[0042] like Figure 1 One embodiment shown: This embodiment provides a structure for a surface-emitting blue laser from a photonic crystal based on a porous gallium nitride cladding layer, comprising:

[0043] Substrate layer;

[0044] The bottom GaN layer is located above the substrate layer;

[0045] A porous GaN layer is located on top of the underlying GaN layer;

[0046] The AlGaN-n layer is located on top of the porous GaN layer;

[0047] The quantum well active layer is located above the AlGaN-n layer;

[0048] An AlGaN-p blocking layer is located above the quantum well active layer;

[0049] The GaN-p layer is located on top of the AlGaN-p barrier layer;

[0050] The photonic crystal layer is located on top of the GaN-p layer;

[0051] p-electrode, the p-electrode is located on the surface of the GaN-p layer;

[0052] n-electrode, the n-electrode is located on the surface of the AlGaN-n layer;

[0053] The etching depth of the GaN-p layer into the photonic crystal is 100 nm; the lattice type of the photonic crystal layer is a triangular lattice with a period of 216 nm and a hole radius of 60 nm.

[0054] The bottom GaN layer has a thickness of 1500 nm and a refractive index of 2.46.

[0055] The porous GaN layer has a thickness of 1000 nm and a refractive index of 1.7.

[0056] The AlGaN-n layer has a thickness of 200 nm and a refractive index of 2.46.

[0057] The active quantum well layer is a five-layer quantum well layer composed of InGaN 2.5nm / GaN 12nm pairs, wherein the refractive index of the InGaN layer is 2.62 and the In content is 10%, and the refractive index of the GaN barrier layer is 2.46.

[0058] The AlGaN-p barrier layer has a thickness of 20 nm, a refractive index of 2.36, and an Al content of 20%.

[0059] The GaN-p layer has a thickness of 200 nm and a refractive index of 2.46.

[0060] Under these parameters, a resonant wavelength of 451.043 nm, a quality factor Q of 18277, a photonic crystal layer confinement factor ΓPhC of 4%, and an active region confinement factor Γact of 20% were obtained. The confinement factors are improved by approximately 30% and 50% respectively compared with the prior art.

[0061] The preparation process steps in this embodiment are as follows:

[0062] Step 1: Epitaxially grow GaN-based group III-V materials on a sapphire substrate. The substrate is layered in sequence as follows: bottom GaN, heavily doped GaN-n layer, AlGaN-n layer, quantum well active layer, AlGaN-p blocking layer, and GaN-p layer.

[0063] Step 2: Spin-coat PMMA, prepare a photonic crystal mask layer by electron beam lithography (EBL), and then perform dry etching to form holes with a target etching depth of 100nm. After removing the mask layer, a photonic crystal layer is formed.

[0064] Step 3: Perform photolithography-dielectric protection-electrochemical etching process to transform the heavily doped GaN-n layer into a porous GaN layer.

[0065] Step 4: Spin-coat a 5µm thick photoresist on the front side, then pre-bake at 90℃ for 3 minutes, use a mask with a certain pattern as a mask for UV exposure, develop to expose the specific area of ​​p-type gallium nitride to be deposited metal electrode, and dry; sputter a certain thickness of Ni / Au two-layer metal, lift-off to remove the photoresist and the metal on the photoresist, finally leaving a 100nm thick metal electrode in the specific area; dry.

[0066] Step 5: Photolithography. Define the photolithography window and dry etch down to the AlGaN-n layer to form a mesa island with a length and width of 300μm.

[0067] Step 6: Spin-coat an 8µm thick photoresist, bake the photoresist, and then overlay it with the photolithography from the previous step. Develop the photoresist to expose the specific area where the n-electrode will be deposited, and then dry it. Sputter a certain thickness of Ni / Au metal layers, lift-off the photoresist and the metal on the photoresist, and finally leave a 100nm thick metal electrode in the specific area; then dry it. Example

[0068] This embodiment provides a method for fabricating a surface-emitting blue laser from a photonic crystal based on a porous gallium nitride cladding layer, including:

[0069] Substrate layer;

[0070] The bottom GaN layer is located above the substrate layer;

[0071] A porous GaN layer is located on top of the underlying GaN layer;

[0072] The AlGaN-n layer is located on top of the porous GaN layer;

[0073] The quantum well active layer is located above the AlGaN-n layer;

[0074] An AlGaN-p blocking layer is located above the quantum well active layer;

[0075] The GaN-p layer is located on top of the AlGaN-p barrier layer;

[0076] The photonic crystal layer is located on top of the GaN-p layer;

[0077] p-electrode, the p-electrode is located on the surface of the GaN-p layer;

[0078] n-electrode, the n-electrode is located on the surface of the AlGaN-n layer;

[0079] The etching depth of the GaN-p layer into the photonic crystal is 200 nm; the lattice type of the photonic crystal layer is a square lattice with a period of 216 nm and a hole radius of 50 nm.

[0080] The bottom GaN layer has a thickness of 1500 nm and a refractive index of 2.46.

[0081] The porous GaN layer has a thickness of 1000 nm and a refractive index of 1.9.

[0082] The AlGaN-n layer has a thickness of 100 nm and a refractive index of 2.46.

[0083] The active quantum well layer is a five-layer quantum well layer composed of InGaN 2.5nm / GaN 12nm pairs, wherein the refractive index of the InGaN layer is 2.62 and the In content is 10%, and the refractive index of the GaN barrier layer is 2.46.

[0084] The AlGaN-p barrier layer has a thickness of 20 nm, a refractive index of 2.36, and an Al content of 20%.

[0085] The GaN-p layer has a thickness of 300 nm and a refractive index of 2.46.

[0086] Under these parameters, the resonant wavelength was 450.766 nm, the quality factor Q was 16714, the photonic crystal layer confinement factor ΓPhC was 3.9%, and the active region confinement factor Γact was 24%.

[0087] The preparation process steps of this embodiment are as follows:

[0088] Step 1: Epitaxially grow GaN-based III-V group materials on a silicon substrate. The substrate is layered in sequence as follows: lattice matching transition layer, bottom GaN, heavily doped GaN-n layer, AlGaN-n layer, quantum well active layer, AlGaN-p blocking layer, and GaN-p layer.

[0089] Step 2: Spin-coat 1μm of photoresist, prepare a photonic crystal mask layer by electron beam lithography (EBL), then perform dry etching to form holes with a target depth of 200nm, and remove the mask layer to form the photonic crystal layer.

[0090] Step 3: Perform photolithography-dielectric protection-electrochemical etching process to transform the heavily doped GaN-n layer into a porous GaN layer.

[0091] Step 4: Spin-coat an 8µm thick photoresist on the front side, then pre-bake at 90℃ for 3 minutes, use a mask with a certain pattern as a mask for UV exposure, develop to expose the specific area of ​​p-type gallium nitride to be deposited metal electrode, and dry; sputter a certain thickness of Ni / Au two-layer metal, lift-off to remove the photoresist and the metal on the photoresist, finally leaving an 80nm thick metal electrode in the specific area; dry.

[0092] Step 5: Photolithography. Define the photolithography window and dry etch down to the AlGaN-n layer to form a mesa island with a length and width of 400μm.

[0093] Step 6: Spin-coat an 8µm thick photoresist, bake the photoresist, and then overlay it with the photolithography from the previous step. Develop the photoresist to expose the specific area where the n-electrode will be deposited, and then dry it. Sputter two layers of Ti / Al / Ni / Au metal of a certain thickness, lift-off the photoresist and the metal on the photoresist, and finally leave an 80nm thick metal electrode in the specific area; then dry it.

[0094] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A gallium nitride-based photonic crystal surface-emitting blue laser, characterized in that, include: Substrate layer; The bottom GaN layer is located above the substrate layer; A porous GaN layer is located on top of the underlying GaN layer; The AlGaN-n layer is located on top of the porous GaN layer; The quantum well active layer is located above the AlGaN-n layer; An AlGaN-p blocking layer is located above the quantum well active layer; The GaN-p layer is located on top of the AlGaN-p barrier layer; A photonic crystal layer with periodic holes of a certain depth is etched on the surface of the GaN-p layer; The refractive index of the porous GaN layer is lower than that of other bottom GaN layers, AlGaN-n layers, quantum well active layers, AlGaN-p barrier layers, and GaN-p layers. The p-electrode is located on the surface of the GaN-p layer; n-electrode, located on the surface of AlGaN-n layer; Square p-type islands are formed by etching from top to bottom from the GaN-p layer to the AlGaN-n layer. The porous GaN layer has a thickness of 500~10000nm and a refractive index of 1.6~2.

2. The GaN-p layer has a thickness of 50~500nm and a refractive index of 2.4~2.

49.

2. A gallium nitride-based photonic crystal surface-emitting blue laser according to claim 1, characterized in that, The thickness of the bottom GaN layer is 1500~50000nm, and the refractive index is 2.4~2.

49.

3. A gallium nitride-based photonic crystal surface-emitting blue laser according to claim 1, characterized in that, The AlGaN-n layer has a thickness of 100~200nm and a refractive index of 2.4~2.

49.

4. A gallium nitride-based photonic crystal surface-emitting blue laser according to claim 1, characterized in that, The active quantum well layer is composed of 3 to 12 quantum well layers consisting of InGaN / GaN pairs. Each layer includes an InGaN well layer of 2.5 nm and a GaN barrier layer of 12 nm, wherein the refractive index of the InGaN layer is 2.55 to 2.65 and the refractive index of the GaN barrier layer is 2.4 to 2.

49.

5. A gallium nitride-based photonic crystal surface-emitting blue laser according to claim 1, characterized in that, The AlGaN-p barrier layer has a thickness of 20 nm, a refractive index of 2.3~2.4, and an Al content of 20%.

6. A gallium nitride-based photonic crystal surface-emitting blue laser according to claim 1, characterized in that, The photonic crystal layer has a triangular lattice, a square lattice, or a honeycomb lattice, with an etching depth of 30~400nm, a period of 150~250nm, and a hole radius of 10~100nm.

7. A gallium nitride-based photonic crystal surface-emitting blue laser according to claim 1, characterized in that, The p electrode includes an ohmic contact metal electrode or a composite electrode of a metal electrode and ITO; the n electrode includes an ohmic contact metal electrode.

8. A method for fabricating a photonic crystal surface-emitting laser based on a porous gallium nitride cladding layer according to any one of claims 1 to 7, characterized in that, Includes the following steps: Step 1: Epitaxially grow GaN-based III-V group materials on a sapphire substrate or a silicon substrate, with the following layers sequentially on the substrate: bottom GaN, heavily doped GaN-n layer, AlGaN-n layer, quantum well active layer, AlGaN-p barrier layer, and GaN-p layer. Step 2: Prepare a photonic crystal mask layer by electron beam lithography (EBL), then perform dry etching to form holes, and remove the mask layer to form the photonic crystal layer; Step 3: Perform photolithography-dielectric protection-electrochemical etching process to transform the heavily doped GaN-n layer into a porous GaN layer; Step 4: Fabricate the p-electrode using photolithography and lift-off processes; Step 5: Photolithography, define the photolithography window, and dry etch down to the AlGaN-n layer to form mesa islands; Step 6: Fabricate the n-electrode using photolithography and lift-off processes.

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