Red light Micro-LED with vertical structure and preparation method thereof

Abstract

The invention belongs to the technical field of semiconductor optoelectronic devices, and particularly relates to a vertical structure red light Micro-LED and a preparation method thereof. The red light Micro-LED comprises a p-type electrode, a Si substrate, a bonding metal layer, a Ni / Ag reflecting layer, a p-type GaN layer, a p-AlGaN electron blocking layer, an InGaN / GaN multi-quantum well layer, an InGaN / GaN superlattice layer, an n-type GaN layer and an n-type electrode which are sequentially arranged from bottom to top. Wherein the p-type GaN layer, the p-AlGaN electron barrier layer, the InGaN / GaN multi-quantum well layer, the InGaN / GaN superlattice and the n-type GaN layer form a p-n junction of the red light Micro-LED. According to the invention, the high-brightness vertical-structure red light Micro-LED is realized, and the vertical-structure red light Micro-LED has a wide application prospect in various fields such as high-resolution display and naked-eye 3D display.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor optoelectronic devices, and in particular relates to a vertical structure red light Micro-LED and a preparation method thereof. Background Art

[0002] Micro-LEDs have the advantages of high brightness, long life, and low power consumption. They can achieve high-density arrangement of micron-level pixel sizes, and can simultaneously meet new display requirements such as high resolution, fast response speed, and long life. They have broad application prospects in new fields such as large-size ultra-high-definition display, wearable display, and augmented reality / virtual reality (AR / VR) display. Although Micro-LED display has huge market prospects, there are still many technical problems to be solved in the commercialization of Micro-LED. Among them, the RGB pixel integration and full-color display of Micro-LED are a major difficulty. The implementation methods of Micro-LED full-color technology mainly include RGB three-primary color integration technology and color conversion technology. Among them, RGB three-primary color integrated white light Micro-LED is a method of placing red, green and blue primary color Micro-LED chips adjacent to each other to achieve wide-spectrum white light through color mixing. Since all three light-emitting devices are electroluminescent, this method can fundamentally solve the problems of Stokes loss and color conversion efficiency caused by color conversion technology.

[0003] Generally, green and blue Micro-LED chips use InGaN-based materials as the light-emitting active layer, while red Micro-LED chips use AlInGaP-based materials as the light-emitting active layer. Due to the different material systems, there are significant differences in their light absorption coefficients and refractive indices. Therefore, when the three chips are integrated, the spatial angle distribution of the light intensity of red Micro-LED and blue-green Micro-LED is not uniform, resulting in a certain color deviation when observed from different viewing angles, which reduces the color rendering quality of Micro-LED. In addition, the external quantum efficiency of AlInGaP red Micro-LED has a serious size effect. When the size of Micro-LED gradually decreases, the surface recombination of AlInGaP red Micro-LED will be significantly enhanced, resulting in a sharp drop in external quantum efficiency.

[0004] Compared with traditional AlInGaP-based red LEDs, InGaN-based red LEDs are easier to integrate with green and blue InGaN-based LEDs, and because carriers migrate less to sidewall defects, the device size can be better reduced. Therefore, InGaN-based red Micro-LEDs are an ideal choice for full-color Micro-LEDs. However, due to the large lattice mismatch between high-In component InGaN and GaN, a large number of defects are formed in the active region, resulting in a significant increase in non-radiative recombination in red multi-quantum wells, which causes a serious reduction in external quantum efficiency. After years of development, the external quantum efficiency of 630nm InGaN-based red Micro-LEDs is still less than 3%, which is much lower than the external quantum efficiency of blue-green Micro-LEDs, seriously hindering the development of red, green and blue primary color integrated full-color Micro-LED technology. Therefore, how to obtain efficient InGaN-based red Micro-LEDs is the key to the industrialization and large-scale application of Micro-LEDs. Due to the advantages of vertical structure LED chips in current expansion and heat dissipation, vertical structure Micro-LED chips also have broad application prospects in the field of new Micro-LED displays.

[0005] Therefore, it is of great significance to develop a vertical structured red light Micro-LED. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a high-brightness vertical structure red light Micro-LED in view of the shortcomings of the prior art, and to realize a high-brightness vertical structure red light Micro-LED by integrating a metal / semiconductor nanograting composite structure in the vertical structure red light Micro-LED; another object of the present invention is to provide a method for preparing a high-brightness vertical structure red light Micro-LED.

[0007] In order to solve the above technical problems, the present invention discloses a vertical structure red light Micro-LED, which includes a p-type electrode, a Si substrate, a bonding metal layer, a Ni / Ag reflective layer, a p-type GaN layer, a p-AlGaN electron blocking layer, an InGaN / GaN multi-quantum well layer, an InGaN / GaN superlattice layer, an n-type GaN layer and an n-type electrode arranged in sequence from bottom to top;

[0008] The p-type GaN layer, the p-AlGaN electron blocking layer, the InGaN / GaN multi-quantum well layer, the InGaN / GaN superlattice and the n-type GaN layer constitute the pn junction of the red light Micro-LED.

[0009] A composite grating is arranged in the n-type GaN layer and the InGaN / GaN superlattice layer; the composite grating is a metal / semiconductor nano-grating composite structure, which is obtained by etching the n-type GaN layer and the InGaN / GaN superlattice layer, evaporating metal Ag, and then stripping off the electron beam photoresist.

[0010] Furthermore, the size of the vertical structure red light Micro-LED is 10μm×10μm to 100μm×100μm.

[0011] The present invention also provides a method for preparing a vertical structure red light Micro-LED, the specific steps are:

[0012] S1. Prepare LED epitaxial wafers, and sequentially grow a buffer layer, a non-doped GaN layer, an n-type GaN layer, an InGaN / GaN superlattice layer, an InGaN / GaN multi-quantum well layer, a p-AlGaN electron blocking layer, and a p-type GaN layer on a sapphire substrate layer by vapor deposition technology to obtain LED epitaxial wafers;

[0013] S2. Evaporating Ni / Ag on the p-type GaN layer in the LED epitaxial wafer, and then rapidly annealing in an air atmosphere to form a Ni / Ag reflective layer;

[0014] S3. A metal In layer is evaporated on the Ni / Ag reflective layer, and a metal Ti layer, a Pt layer and an Au layer are sequentially deposited on the Si substrate, and the Ni / Ag reflective layer and the Si substrate are bonded together by Au-In eutectic bonding to form a bonding metal layer;

[0015] S4. On the other side of the Si substrate, Cr / Pt / Au layers are sequentially evaporated to form a p-type electrode, and the sapphire substrate layer is peeled off;

[0016] S5. Remove the buffer layer and the undoped GaN layer by chemical mechanical polishing technology, and thin the n-type GaN layer;

[0017] S6. Spin-coating electron beam photoresist ZEP520A on the thinned n-type GaN layer, and using electron beam exposure technology to define the position of the grating structure; using the electron beam photoresist as a mask, using inductively coupled plasma etching technology to transfer the grating pattern to the n-type GaN layer, wherein the etching is performed from top to bottom through the InGaN / GaN superlattice layer; then evaporating metal Ag on the etched InGaN / GaN superlattice layer and then peeling off ZEP520A to form a nanocomposite grating;

[0018] S7. Use electron beam evaporation technology to sequentially evaporate Cr / Pt / Au layers on the n-type GaN layer to form an n-type electrode.

[0019] Specifically, in S1, the buffer layer is AlN and has a thickness of 10 to 30 nm;

[0020] The thickness of the non-doped GaN layer is 3-4 μm;

[0021] The n-type GaN layer is silicon-doped n-type GaN with a thickness of 2.5 to 3 μm;

[0022] The number of periods of the InGaN / GaN superlattice layer is 24 to 36, the composition of In in InGaN is 0.05 to 0.15, the thickness of InGaN is 4 to 6 nm, and the thickness of GaN is 1 to 3 nm;

[0023] The number of periods of the InGaN / GaN multi-quantum well layer is 5 to 10, the composition of In in InGaN is 0.3 to 0.5, the emission wavelength is 620 to 740 nm, the thickness of the InGaN well layer is 2 to 3 nm, and the thickness of the GaN barrier layer is 13 to 14 nm;

[0024] The Al content in the p-AlGaN electron blocking layer is 0.1-0.3, and the thickness is 10-20 nm;

[0025] The thickness of the p-type GaN layer is 120-140 nm.

[0026] Specifically, in S2, the Ni layer thickness in the Ni / Ag reflective layer is 0.3-0.6 nm, and the Ag layer thickness is 80-120 nm.

[0027] Specifically, in S3, the thickness of the In layer is 2-3 μm; the thickness of the Ti layer is 40-60 nm, the thickness of the Pt layer is 40-60 nm, and the thickness of the Au layer is 1-1.5 μm.

[0028] Specifically, in S4, the thickness of the Cr layer in the p-type electrode is 8-12 nm, the thickness of the Pt layer is 180-220 nm, and the thickness of the Au layer is 1700-1900 nm.

[0029] Specifically, in S6, the thickness of the electron beam photoresist is 300-500 nm; the period of the composite grating is 600-700 nm, the depth is 100-400 nm, and the duty cycle is 0.5; the thickness of the metal Ag is 10-30 nm;

[0030] Furthermore, in S6, after the metal Ag evaporation is completed, a stripping process (ZEP stripping solution) is required to remove the remaining electron beam photoresist on the surface of the sample.

[0031] Specifically, in S6, the etching has a depth such that the distance between the bottom of the grating and the InGaN / GaN multi-quantum well layer is 8 to 12 nm.

[0032] Specifically, in S7, the thickness of the Cr layer in the n-type electrode is 8-12 nm, the thickness of the Pt layer is 180-220 nm, and the thickness of the Au layer is 1700-1900 nm.

[0033] Beneficial effects: The high-brightness vertical structure red light Micro-LED of the present invention uses a conductive substrate with high thermal conductivity to replace the sapphire substrate, so it has good heat dissipation performance. The p-electrode in the vertical structure adopts a Ni / Ag reflector with both high reflectivity and low contact resistance. Compared with the horizontal structure and flip-chip structure LED chips, the p and n electrodes are vertically distributed on both sides of the active layer. The vertical structure LED chip avoids the process of etching the n-electrode to contact the table, has a larger active layer area, and the current does not need to expand laterally, which improves the current concentration effect. The superiority of the vertical structure is also reflected in that the LED chip can have better photoelectric performance when the working current is a large current, which also enables the vertical structure LED chip to obtain higher light output power and larger light saturation current. In addition, the present invention also embeds the metal / semiconductor nano-grating composite structure into the red light Micro-LED, and uses the metal / semiconductor nano-grating composite structure to generate surface plasmons, which produce resonant coupling with the multiple quantum wells, thereby enhancing the spontaneous radiation recombination rate of the quantum wells, thereby improving the internal quantum efficiency of the LED. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments, and the above and / or other advantages of the present invention will become more clear.

[0035] Figure 1 This is a front view of the vertical structure red light Micro-LED of the present invention;

[0036] Figure 2 Schematic diagram of the red light Micro-LED structure obtained in step (1) of Example 2;

[0037] Figure 3 Schematic diagram of the red light Micro-LED structure obtained in step (2) of Example 2;

[0038] Figure 4 Schematic diagram of the red light Micro-LED structure obtained in step (3) of Example 2;

[0039] Figure 5 Schematic diagram of the red light Micro-LED structure obtained in step (4) of Example 2;

[0040] Figure 6 Schematic diagram of the red light Micro-LED structure obtained in step (5) of Example 2;

[0041] Figure 7 Schematic diagram of the red light Micro-LED structure obtained in step (6) of Example 2;

[0042] Figure 8 Schematic diagram of the red light Micro-LED structure obtained in step (7) of Example 2;

[0043] Fig. 9 Schematic diagram of the red light Micro-LED structure obtained in step (8) of Example 2;

[0044] Fig.10 Schematic diagram of the red light Micro-LED structure obtained in step (9) of Example 2;

[0045] Fig.11 Schematic diagram of the red light Micro-LED structure obtained in step (10) of Example 2;

[0046] Among them, 1 is a p-type GaN layer, 2 is a p-AlGaN electron blocking layer, 3 is an InGaN / GaN multiple quantum well layer, 4 is an InGaN / GaN superlattice layer, 5 is an n-type GaN layer, 6 is an undoped GaN layer, 7 is a buffer layer, 8 is a sapphire substrate layer, 9 is a Ni / Ag reflective layer, 10 is a bonding metal layer, 11 is a Si substrate layer, 12 is a p-type electrode, 13 is an electron beam photoresist, 14 is metal Ag, and 15 is an n-type electrode. DETAILED DESCRIPTION

[0047] The experimental methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials described are commercially available unless otherwise specified.

[0048] Example 1: High-brightness vertical structure red Micro-LED

[0049] A high-brightness vertical structure red light Micro-LED, using LED epitaxial wafer as carrier, the epitaxial wafer structure is as follows Figure 2 As shown, it includes a p-type GaN layer 1, a p-AlGaN electron blocking layer 2, an InGaN / GaN multi-quantum well layer 3, an InGaN / GaN superlattice layer 4, an n-type GaN layer 5, an undoped GaN layer 6, a buffer layer 7 and a sapphire substrate layer 8, wherein the p-type GaN layer 1, the p-AlGaN electron blocking layer 2, the InGaN / GaN multi-quantum well layer 3, the InGaN / GaN superlattice layer 4, the n-type GaN layer 5, the undoped GaN layer 6, the buffer layer 7 and the sapphire substrate layer 8 are arranged in sequence from bottom to top.

[0050] Fig.11It is a schematic diagram of the structure of a Micro-LED device, wherein the pn junction of the Micro-LED device is connected in sequence from bottom to top and includes a p-type GaN layer 1, a p-AlGaN electron blocking layer 2, an InGaN / GaN multi-quantum well layer 3, an InGaN / GaN superlattice layer 4 and an n-type GaN layer 5.

[0051] Among them, the LED device is provided with a one-dimensional grating structure in the n-type region, a layer of metal Ag14 is evaporated in the middle of the grating, which is isolated from the InGaN / GaN multi-quantum well layer 3 by the InGaN / GaN superlattice layer 4, and a one-dimensional metal grating structure is provided at the bottom of the grating; the grating of the Micro-LED device is etched from the n-type GaN layer 5 to the InGaN / GaN superlattice layer 4 and leaves a distance from the InGaN / GaN multi-quantum well layer 3, the n-type electrode 15 is arranged on the n-type GaN layer 5, and the p-type electrode 12 is arranged under the Si substrate layer 11; wherein the p-type electrode and the n-type electrode are both metal materials chromium / platinum / gold (Cr / Pt / Au).

[0052] The In composition of the InGaN / GaN multi-quantum well layer 3 is 0.40, the multi-quantum well active layer emits light at a wavelength of 620nm to 740nm, the number of multi-quantum well periods is 5 to 10, the thickness of the p-AlGaN electron blocking layer 2 is 10 to 20nm, and the thickness of the p-type GaN layer is 130nm.

[0053] The period of the grating is 600-700nm and the depth is 100-400nm.

[0054] The size of Micro-LED devices is 10μm×10μm~100μm×100μm.

[0055] Example 2: Preparation method of high-brightness vertical structure red light Micro-LED

[0056] (1) Figure 2 As shown, an LED epitaxial wafer is prepared, and an epitaxial wafer structure of a GaN-based LED chip is epitaxially grown on a crystal plane of a sapphire substrate layer 8 using an MOCVD device. Specifically, a buffer layer 7, a non-doped GaN layer 6, an n-type GaN layer 5, an InGaN / GaN superlattice layer 4, an InGaN / GaN multi-quantum well layer 3, a p-AlGaN electron blocking layer 2, and a p-type GaN layer 1 are sequentially grown on the sapphire substrate layer 8 to obtain an LED epitaxial wafer.

[0057] The buffer layer 7 is about 20 nm thick and made of AlN. The function of this layer is to reduce the stress between the growth substrate and the GaN epitaxial layer. The thickness of the undoped GaN layer 6 is about 3 μm. The thickness of the n-type GaN layer 5 is about 3 μm and made of silicon-doped n-type GaN with a doping concentration of 8×10 18 / cm 3 ; The number of periods of InGaN / GaN superlattice layer 4 is 32, of which the In composition of InGaN is 0.1, the thickness of InGaN is 5nm, and the thickness of GaN is 2nm; the number of periods of InGaN / GaN multi-quantum well layer 3 is 5, of which the In composition of InGaN is 0.4, the emission wavelength is 630nm, the thickness of InGaN well layer is 2.5nm, and the thickness of GaN barrier layer is 13.5nm; the thickness of p-AlGaN electron blocking layer (EBL) 2 is about 10nm, of which the Al composition is 0.2; the thickness of p-type GaN layer 1 is about 130nm.

[0058] (2) Figure 3 As shown, after the epitaxial growth is completed, the epitaxial wafer is cleaned, and Ni / Ag (0.5 / 100 nm) is evaporated on the p-type GaN layer 1 in the LED epitaxial wafer using an electron beam evaporation system, and then rapidly annealed in an air atmosphere to form a Ni / Ag reflective layer 9;

[0059] (3) Figure 4 As shown, a metal In layer with a thickness of 2.5 μm is deposited on the Ni / Ag reflective layer 9 by electron beam evaporation, a metal Ti layer with a thickness of 50 nm, a metal Pt layer with a thickness of 50 nm and a metal Au layer with a thickness of 1.2 μm are sequentially deposited on the p-type Si substrate 11, and the Ni / Ag reflective layer 9 and the Si substrate 11 are bonded together by Au-In eutectic bonding. At this time, a bonding metal layer 10 is formed between the Si substrate 11 and the Ni / Ag reflective layer 9;

[0060] (4) Figure 5 As shown, on the other side of the Si substrate 11, metal Cr / Pt / Au is deposited by electron beam evaporation with thicknesses of 10 / 200 / 1800 nm respectively to form a p-type electrode 12;

[0061] (5) Figure 6 As shown, the sapphire substrate 8 is separated by laser lift-off technology;

[0062] (6) Figure 7 As shown, the buffer layer 7, the undoped GaN layer 6, and the n-type GaN layer 5 are directly thinned by chemical mechanical polishing technology until the remaining n-type layer is 100 nm;

[0063] (7) Figure 8 As shown, ZEP520A is spin-coated on the LED epitaxial wafer, i.e., the n-type GaN layer 5 to form an electron beam photoresist 13, the thickness of the electron beam photoresist 13 is 500nm, and the position of the grating structure is defined by electron beam exposure technology, the period of the grating is 630nm, the duty cycle is 0.5, and the exposure area size is 60μm×60μm;

[0064] (8) Fig. 9 As shown, the electron beam photoresist 13 is used as a mask and the ICP etching technology is used to transfer the grating pattern to the n-type GaN layer 5. The LED epitaxial wafer is etched downward to the InGaN / GaN superlattice layer 4 by ICP, and the etching depth is controlled so that the distance between the bottom of the grating and the InGaN / GaN multi-quantum well layer 3 is kept at about 10nm;

[0065] (9) Fig.10 As shown, an electron beam evaporation system (PVD) is used to evaporate a layer of metal silver 14 with a thickness of 20 nm on the surface of the LED epitaxial wafer, that is, on the etched InGaN / GaN superlattice layer 4, and then a stripping process (ZEP stripping liquid) is used to remove the remaining electron beam photoresist 13 on the surface of the sample, and an Ag grating structure is formed at the bottom of the n-type GaN grating. The period of the Ag grating is 630 nm, and the duty cycle is 0.5;

[0066] (10) Fig.11 As shown, the n-type electrode region of the Micro-LED device is defined by photolithography alignment technology, and Cr / Pt / Au (10 / 200 / 1800nm) metal layers are sequentially evaporated by electron beam evaporation technology as the n-type electrode 15, wherein the n-type electrode is evaporated on the n-type GaN layer.

[0067] The present invention provides a vertical structure red light Micro-LED and its preparation method. There are many methods and ways to implement the technical solution. The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the protection scope of the present invention. All components not specified in this embodiment can be implemented by existing technologies.

Claims

1. A vertical structured red light Micro-LED, characterized in that: The red light Micro-LED includes a p-type electrode, a Si substrate, a bonding metal layer, a Ni / Ag reflective layer, a p-type GaN layer, a p-AlGaN electron blocking layer, an InGaN / GaN multi-quantum well layer, an InGaN / GaN superlattice layer, an n-type GaN layer and an n-type electrode arranged in sequence from bottom to top; The p-type GaN layer, the p-AlGaN electron blocking layer, the InGaN / GaN multi-quantum well layer, the InGaN / GaN superlattice and the n-type GaN layer constitute the pn junction of the red light Micro-LED.

2. The red light Micro-LED according to claim 1, characterized in that: A composite grating is arranged in the n-type GaN layer and the InGaN / GaN superlattice layer; the composite grating is a metal / semiconductor nano-grating composite structure, which is obtained by etching the n-type GaN layer and the InGaN / GaN superlattice layer, evaporating metal Ag, and then stripping off the electron beam photoresist.

3. A method for preparing a vertical structured red light Micro-LED, characterized in that: The specific steps are: S1. Prepare LED epitaxial wafers, and sequentially grow a buffer layer, a non-doped GaN layer, an n-type GaN layer, an InGaN / GaN superlattice layer, an InGaN / GaN multi-quantum well layer, a p-AlGaN electron blocking layer, and a p-type GaN layer on a sapphire substrate layer by vapor deposition technology to obtain LED epitaxial wafers; S2. Evaporating Ni / Ag on the p-type GaN layer in the LED epitaxial wafer, and then rapidly annealing in an air atmosphere to form a Ni / Ag reflective layer; S3. A metal In layer is evaporated on the Ni / Ag reflective layer, and a metal Ti layer, a Pt layer and an Au layer are sequentially deposited on the Si substrate, and the Ni / Ag reflective layer and the Si substrate are bonded together by Au-In eutectic bonding to form a bonding metal layer; S4. On the other side of the Si substrate, Cr / Pt / Au layers are sequentially deposited to form a p-type electrode, and the sapphire substrate layer is peeled off; S5. Remove the buffer layer and the undoped GaN layer by chemical mechanical polishing technology, and thin the n-type GaN layer; S6. Spin-coating electron beam photoresist ZEP520A on the thinned n-type GaN layer, and defining the position of the grating structure using electron beam exposure technology; Using electron beam photoresist as a mask, the grating pattern is transferred to the n-type GaN layer by inductively coupled plasma etching technology, wherein the etching is carried out from top to bottom through the InGaN / GaN superlattice layer; then, metal Ag is evaporated on the etched InGaN / GaN superlattice layer, and then ZEP520A is peeled off to form a nanocomposite grating; S7. Use electron beam evaporation technology to sequentially evaporate Cr / Pt / Au layers on the n-type GaN layer to form an n-type electrode.

4. The preparation method according to claim 3, characterized in that: In S1, the buffer layer is AlN with a thickness of 10 to 30 nm; the thickness of the undoped GaN layer is 3 to 4 μm; the n-type GaN layer is silicon-doped n-type GaN with a thickness of 2.5 to 3 μm; the number of periods of the InGaN / GaN superlattice layer is 24 to 36, the In component of InGaN is 0.05 to 0.15, the thickness of InGaN is 4 to 6 nm, and the thickness of GaN is 1 to 3 nm; the number of periods of the InGaN / GaN multi-quantum well layer is 5 to 10, the In component of InGaN is 0.3 to 0.5, the emission wavelength is 620 to 740 nm, the thickness of the InGaN well layer is 2 to 3 nm, and the thickness of the GaN barrier layer is 9 to 14 nm; the Al component of the p-AlGaN electron blocking layer is 0.1 to 0.3, and the thickness is 10 to 20 nm; the thickness of the p-type GaN layer is 120 to 150 nm.

5. The preparation method according to claim 3, characterized in that: In S3, the thickness of the In layer is 2-3 μm; the thickness of the Ti layer is 40-60 nm, the thickness of the Pt layer is 40-60 nm, and the thickness of the Au layer is 1-1.5 μm.

6. The preparation method according to claim 3, characterized in that: In S4, the thickness of the Cr layer in the p-type electrode is 8-12 nm, the thickness of the Pt layer is 180-220 nm, and the thickness of the Au layer is 1700-1900 nm.

7. The preparation method according to claim 3, characterized in that: In S6, the thickness of the electron beam photoresist is 300-500 nm; the period of the composite grating is 600-700 nm, the depth is 100-400 nm, and the duty cycle is 0.5; and the thickness of the metal Ag is 10-30 nm.

8. The preparation method according to claim 3, characterized in that: In S6, the etching has a depth such that the distance between the bottom of the grating and the InGaN / GaN multi-quantum well layer is 8-12 nm.

9. The preparation method according to claim 3, characterized in that: In S7, the thickness of the Cr layer in the n-type electrode is 8-12 nm, the thickness of the Pt layer is 180-220 nm, and the thickness of the Au layer is 1700-1900 nm.

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