A deep ultraviolet micro-LED chip with enhanced light emission from photonic crystals and a method for preparing the same

Abstract

The present invention relates to a deep ultraviolet micro-LED chip with enhanced light emission from photonic crystals and a method for preparing the same, and belongs to the field of light-emitting electronic technology. The present invention uses nanosphere etching technology to prepare a periodic photonic crystal structure on the n-AlGaN epitaxial layer exposed around the MESA table, thereby improving the light extraction of the deep ultraviolet micro-LED. The present invention proposes to increase the light scattering path through the grating diffraction effect and improve the light extraction efficiency of the deep ultraviolet micro-LED by introducing a photonic crystal structure with a period equivalent to the emission wavelength of the active layer into the exposed n-AlGaN layer; at the same time, an Al-based metal film with a high reflectivity for ultraviolet light is selected as the N-electrode connecting metal, which can not only connect the N electrodes around different table tops to form a complete loop, but also reflect ultraviolet light to one side of the substrate, further improving the light extraction efficiency of the chip, and ultimately improving the external quantum efficiency of the entire deep ultraviolet chip.

Description

Technical Field

[0001] The present invention belongs to the field of light-emitting electronic technology, and more specifically, relates to a deep ultraviolet micro-LED chip with enhanced light emission by photonic crystals and a preparation method thereof. Background Art

[0002] AlGaN-based deep ultraviolet LEDs are widely used in optical communications, sterilization, water purification, and medical diagnosis due to their small size, low power consumption, long life, easy integration, no mercury pollution, safety, and environmental protection. In recent years, due to the vigorous development of micro-LED display technology, more and more research has been done on deep ultraviolet micro-LEDs. Compared with traditional large-area deep ultraviolet LEDs, deep ultraviolet micro-LEDs have advantages such as more uniform current distribution, lower self-heating effect, and higher efficiency. Therefore, deep ultraviolet micro-LEDs have become the development direction of advanced and efficient deep ultraviolet light sources.

[0003] However, deep ultraviolet micro-LEDs are still facing the problem of difficulty in extracting ultraviolet light, mainly because the total internal reflection of light inside the chip leads to excessive light loss. Therefore, if some artificial microstructures can be introduced to improve the extraction efficiency of deep ultraviolet light, the performance of deep ultraviolet micro-LEDs can be further improved. For example, the patent with application number 202311819068.8 introduces a SiO2 vertical hole guiding layer into the deep ultraviolet micro-LED chip, which effectively converts the polarization mode of the multi-quantum well emitted light, thereby improving the light extraction efficiency of the chip. Although this method improves the light extraction efficiency at the bottom of the chip to a certain extent, this structure is difficult to prepare and the wafer process is complicated. Summary of the invention

[0004] The present invention adopts nanosphere etching technology to prepare photonic crystals, which solves the technical problem of low extraction efficiency of deep ultraviolet micro-LEDs in the prior art, and the process is relatively simple. The period of the photonic crystal structure of the n-AlGaN layer of the deep ultraviolet micro-LED of the present invention is equivalent to the emission wavelength of the active layer (multiple quantum wells), thereby producing a grating diffraction effect and increasing the scattering path of light. At the same time, an Al-based metal film is evaporated on the n-AlGaN layer, which not only connects the N electrode, but also reflects the ultraviolet light emitted from the active layer, effectively improving the light extraction efficiency of the deep ultraviolet micro-LED chip.

[0005] According to a first aspect of the present invention, a method for preparing a deep ultraviolet micro-LED chip with enhanced light emission by photonic crystals is provided, comprising the following steps:

[0006] (1) Depositing an AlN buffer layer, an n-AlGaN layer, an active layer, a p-AlGaN electron blocking layer and a p-type hole injection layer on a substrate in sequence;

[0007] (2) Spin-coating a photoresist on the p-type hole injection layer, then exposing and developing it, dry-etching it to the n-AlGaN layer, and then depositing a SiO2 coating layer to obtain a MESA table;

[0008] (3) dripping the polystyrene nanosphere solution onto the water surface along the obliquely inserted glass slide. Due to the surface tension, the nanospheres on the air-water interface will self-assemble and form a thin film. Then, the MESA table obtained in step (2) is placed underwater and pulled upward to transfer the nanosphere film on the water surface to the epitaxial wafer, and the nanosphere film is attached to the epitaxial wafer. After drying, the epitaxial wafer is treated with oxygen plasma to reduce the diameter of the nanospheres. Then, dry etching is performed to remove the nanosphere mask and the SiO2 layer of the table to obtain a photonic crystal structure.

[0009] (4) Deposit a layer of SiO2 as the isolation of the N electrode, then spin-coat a photoresist, develop after exposure, and remove the resist in oxygen plasma; then use an etching solution to expose the location of the evaporated N electrode, and then evaporate the N electrode; after the N electrode is prepared, use a photoresist to protect the evaporated part of the P electrode, and then evaporate an Al-based metal film as the connection of the N electrode, which also plays a role in reflecting ultraviolet light;

[0010] (5) After the Al-based metal film is prepared, a layer of SiO2 is deposited as the insulating layer of the electrode, and photoresist is spin-coated. After exposure, it is developed and stripped in oxygen plasma. Then, the P-electrode vapor-deposited part is exposed by etching solution, and the P-electrode is evaporated. After the P-electrode is prepared, the P-electrode connecting electrode is evaporated, and SiO2 is deposited as a cladding layer. After a through hole is etched on it, a metal pad is evaporated to complete the chip preparation.

[0011] Preferably, in step (1), the p-type hole injection layer includes a lower p-AlGaN layer and an upper p-GaN layer.

[0012] Preferably, in step (2), the MESA table is a truncated cone with a top diameter of 20-90 μm.

[0013] Preferably, in step (3), the photonic crystal structure is an array structure of cylindrical protrusions.

[0014] Preferably, in step (1), the active layer is a multi-quantum well.

[0015] Preferably, the material of the quantum well of the active layer is Al x Ga 1-x N, where 0.4≤x≤0.5, and the thickness is 1-3nm; the material of the quantum barrier of the active layer is Al y Ga 1-yN, where 0.5≤y≤0.6, and the thickness is 10-12 nm; a layer of quantum well and a layer of quantum barrier are collectively referred to as a period, and the active layer has a total of 5-10 periods.

[0016] Preferably, the Al-based metal film is an Al / Cr / Pt metal film with an ultraviolet light reflectivity greater than 90%; wherein Al, Cr, and Pt metals are respectively evaporated on the chip from bottom to top.

[0017] Preferably, the thickness of the Al layer of the Al-based metal film is 1100-1300 nm, the thickness of the Cr layer is 40-60 nm, and the thickness of the Pt layer is 40-60 nm; the Al-based metal film is used to connect the N electrode and to reflect ultraviolet light to improve the light extraction efficiency of the chip.

[0018] According to another aspect of the present invention, a deep ultraviolet micro-LED chip with photonic crystal enhanced light emission obtained by any one of the preparation methods is provided.

[0019] In general, the above technical solution conceived by the present invention has the following technical advantages compared with the prior art:

[0020] (1) The present invention proposes to introduce a photonic crystal structure with a period equivalent to the emission wavelength of the active layer into the exposed n-AlGaN layer, thereby increasing the light scattering path through the grating diffraction effect and improving the light extraction efficiency of the deep ultraviolet micro-LED; at the same time, an Al-based metal film with a high reflectivity for ultraviolet light is selected as the N-electrode connecting metal, which can not only connect the N-electrodes around different mesas to form a complete loop, but also reflect ultraviolet light to the substrate side, further improving the light extraction efficiency of the chip, and ultimately improving the external quantum efficiency of the entire deep ultraviolet chip.

[0021] (2) The photonic crystal structure of the present invention is an artificial microstructure formed by the periodic arrangement of media with different refractive indices. When the period of the photonic crystal is equivalent to the wavelength of the emitted light, a grating diffraction effect can be generated, which increases the scattering path of light and allows more light to be extracted. This is a direct and effective method for improving the extraction efficiency of deep ultraviolet light. (3) The nanosphere etching technology of the present invention can form an etching mask that can be transferred to the epitaxial wafer only through the self-assembly of nanospheres at the interface of air and water. It is low-cost, simpler to operate, and has a processing accuracy of nanometers.

[0022] (4) The MESA table of the deep ultraviolet micro-LED of the present invention is distributed in an array, and the N electrodes surrounding the table are not connected. It is necessary to select a suitable metal as the connecting electrode to form a complete circuit, and at the same time, it should not absorb too much ultraviolet light to cause light loss. Al metal has a high reflectivity to ultraviolet light, so Al-based metal film is the best choice as the connecting electrode.

[0023] (5) By combining the grating diffraction effect of the photonic crystal and the reflection effect of the Al-based metal film, the light extraction efficiency of the deep ultraviolet micro-LED chip can be improved, thereby improving the external quantum efficiency of the deep ultraviolet micro-LED chip. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic diagram of the deep ultraviolet micro-LED chip structure of an embodiment of the present invention; wherein: 1-c-plane sapphire substrate, 2-AlN buffer layer, 3-n-AlGaN layer, 4-multiple quantum well, 5-p-AlGaN electron blocking layer, 6-p-AlGaN layer, 7-p-GaN layer, 8-photonic crystal structure, 9-SiO2 layer, 10-N electrode, 11-Al-based metal film, 12-P electrode.

[0025] Figure 2 This is a flow chart for preparing a deep ultraviolet micro-LED chip according to an embodiment of the present invention.

[0026] Figure 3 A comparison chart of light extraction efficiency between an LED with a photonic crystal structure prepared using the present invention and a traditional LED. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0028] Figure 1 Schematic diagram of the structure of a deep ultraviolet micro-LED chip according to an embodiment of the present invention.

[0029] Figure 2 This is a flow chart for preparing a deep ultraviolet micro-LED chip according to an embodiment of the present invention.

[0030] The present invention discloses a deep ultraviolet micro-LED chip with enhanced light emission by photonic crystal and a preparation method thereof. The specific device structure and preparation method include: ① MOCVD epitaxial growth chip structure (sequentially depositing AlN buffer layer, n-AlGaN layer, active layer, p-AlGaN electron blocking layer and p-type hole injection layer on the substrate); ② surface cleaning of chip epitaxial wafer; ③ preparation and protection of MESA table; ④ preparation of photonic crystal structure by nanosphere etching; ⑤ preparation of N electrode and evaporation of Al-based metal film; ⑥ preparation of P electrode and insulating layer. The present invention proposes to introduce a photonic crystal structure with a period equivalent to the emission wavelength of the active layer into the exposed n-AlGaN layer, increase the scattering path of light through the grating diffraction effect, and improve the light extraction efficiency of the deep ultraviolet micro-LED; at the same time, select Al-based metal film with high reflectivity for ultraviolet light as the N electrode connection metal, which can not only connect the N electrodes around different table surfaces to form a complete loop, but also reflect ultraviolet light to the side of the substrate, further improve the light extraction efficiency of the chip, and finally improve the external quantum efficiency of the entire deep ultraviolet chip.

[0031] In some embodiments, the deep ultraviolet micro-LED chip MESA mesa is prepared by using photolithography technology and ICP dry etching, with an etching depth of 600-700 nm. The prepared MESA mesa is in the shape of a truncated cone with a top diameter of 20-90 μm and is arranged in a square array.

[0032] In some embodiments, the photonic crystal structure on the n-AlGaN layer is an array structure having a plurality of cylindrical protrusions, the cylindrical protrusions have a spacing of 200-300 nm, a height of 50-100 nm, a diameter of 150-250 nm, and are arranged in an equilateral triangle array.

[0033] In some embodiments, the preparation of the photonic crystal structure by nanosphere etching is specifically as follows: first, a polystyrene (PS) nanosphere solution is diluted with anhydrous ethanol; then, an appropriate amount of the diluted solution is dripped onto the water surface along an obliquely inserted glass slide, and due to the surface tension, the nanospheres on the air-water interface will self-assemble and form a thin film; then, the epitaxial wafer is placed underwater and pulled upward to transfer the nanosphere film on the water surface to the epitaxial wafer, and naturally air-dried so that the nanosphere film is firmly attached to the epitaxial wafer; after complete drying, the epitaxial wafer is treated with O plasma for 0.5-2 min to reduce the diameter of the nanospheres; then, ICP dry etching is performed for 20-30 s, and the nanosphere mask is removed with toluene to obtain a periodic photonic crystal structure.

[0034] In some embodiments, the active layer is a multi-quantum well, and the material of the quantum well of the active layer is Al x Ga 1-xN, where 0.4≤x≤0.5, and the thickness is 1-3 nm; the material of the quantum barrier of the active layer is Al y Ga 1-y N, where 0.5≤y≤0.6, and the thickness is 10-12 nm; a layer of quantum well and a layer of quantum barrier are collectively referred to as a period, and the active layer has a total of 5-10 periods.

[0035] In some embodiments, the Al-based metal film is an Al / Cr / Pt metal film with a UV reflectivity greater than 90%, wherein Al, Cr, and Pt metals are evaporated on the chip from bottom to top, respectively, and the thickness of the Al layer is 1100-1300 nm, the thickness of the Cr layer is 40-60 nm, and the thickness of the Pt layer is 40-60 nm; this layer of metal film can not only connect to the N electrode, but also reflect ultraviolet light to improve the light extraction efficiency of the chip.

[0036] The following are specific examples and comparative examples.

[0037] Example 1

[0038] The present invention combines nano-ball etching technology with chip technology. After the MESA table of the chip is prepared, a photonic crystal structure is further prepared on the exposed n-AlGaN layer, and its period is equivalent to the wavelength of the light emitted by the active layer. Then, the electrode and other structures are prepared. The specific device structure includes: c-plane sapphire substrate 1; AlN buffer layer 2; n-AlGaN layer 3; multiple quantum well 4; p-AlGaN electron blocking layer 5; p-AlGaN layer 6; p-GaN layer 7; photonic crystal structure 8; SiO2 layer 9; N electrode 10; Al-based metal film 11; P electrode 12. The preparation process includes: MOCVD epitaxial growth chip structure; surface cleaning of chip epitaxial wafer; preparation and protection of MESA table; preparation of photonic crystal structure by nano-ball etching; preparation of N electrode and evaporation of Al-based metal film; preparation of insulating layer and P electrode.

[0039] The steps of the device preparation method are as follows:

[0040] (1) MOCVD epitaxial growth chip structure: First, a 3 μm thick AlN buffer layer 2 is grown on the c-plane sapphire substrate 1, and then a 2 μm thick n-AlGaN layer 3 is grown; then a 5-period multi-quantum well 4 is grown, whose quantum barriers and quantum wells have Al compositions of 0.55 and 0.45, and thicknesses of 11 nm and 2 nm, respectively; then a p-AlGaN electron blocking layer 5 with an Al composition of 0.68 and a thickness of 25 nm is grown; finally, a p-type hole injection layer is grown, which includes a 20 nm p-AlGaN layer 6 with an Al composition of 0.6 and a 100 nm p-GaN layer 7.

[0041] (2) Surface cleaning of chip epitaxial wafers: After the epitaxial wafers are grown, they are ultrasonically cleaned with acetone and isopropanol for 5 min, then rinsed with deionized water for 5 min, and finally dried using a spin dryer.

[0042] (3) Preparation and protection of MESA mesa: Spin-coat positive photoresist with a uniform thickness of about 3.5 μm, soft-bake at 100 °C for 20 min, expose for 6 s, and finally develop for 120 s; use ICP dry etching to etch deep ultraviolet epitaxial wafer to n-AlGaN layer 3, the etching gas is BCl3 and Cl2, the flow ratio is 1:3, and the etching power is 550 W; the etched epitaxial wafer is placed in 85 °C photoresist stripping solution, acetone, and isopropanol for 10 min, and finally rinsed with deionized water for 5 min and dried in a spin dryer. Finally, a layer of SiO2 is deposited to cover the prepared mesa to protect the mesa from the subsequent photonic crystal preparation process.

[0043] (4) Preparation of photonic crystal structure by etching of nanospheres: First, a 5 wt.% polystyrene (PS) nanosphere solution was diluted with anhydrous ethanol at a dilution ratio of 1:4, where the diameter of the nanosphere was 300 nm. Then, an appropriate amount of deionized water was placed in a culture dish, and a glass slide was inserted obliquely into the culture dish so that it was partially immersed in the water. Then, an appropriate amount of diluted PS ball solution was dripped onto the water surface along the obliquely inserted glass slide. Due to the surface tension, the nanospheres on the air-water interface will self-assemble and form a thin film. Then, the epitaxial wafer with MESA table preparation and protection was placed underwater and pulled upward to transfer the nanosphere film on the water surface to the epitaxial wafer, and naturally air-dried so that the nanosphere film was firmly attached to the epitaxial wafer. After complete drying, the epitaxial wafer was treated with O plasma for 1 min to reduce the diameter of the nanospheres. Then, ICP dry etching was performed for 25 s, the etching gases are BCl3 and Cl2, and toluene and buffered oxide etching solution are used to remove the nanosphere mask and the SiO2 layer protecting the mesa, and finally the photonic crystal structure 8 is obtained.

[0044] (5) Preparation of N electrode and evaporation of Al-based metal film: First, a SiO2 layer 9 with a thickness of about 500 nm was deposited as the isolation of the N electrode, and then a positive photoresist was spin-coated with a uniform thickness of about 3 μm. After soft baking at 95 °C for 1 min, the layer was exposed for 3 s, and finally developed for 135 s, and then stripped in 100 W O plasma for 10 min. Then, the position where the N electrode was to be evaporated was exposed using a buffered oxide etchant, and then an electron beam evaporation device was used to evaporate the N electrode 10, which had a composition of Cr / Al / Ti / Au with a thickness of 2.5 / 120 / 40 / 60 nm, where each metal was evaporated from bottom to top in sequence; finally, the blue film was peeled off, and the layer was cleaned in an 85 °C stripping solution for 10 min, and then rapidly thermally annealed in a nitrogen environment to obtain the N electrode 10. After the N electrode 10 is prepared, a positive photoresist and similar photolithography steps are used to protect the P electrode vapor deposition part with the photoresist, and then an Al / Cr / Pt metal film 11 is vapor deposited as a connection for the N electrode, which can also reflect ultraviolet light. The thickness is 1200 / 50 / 50 nm, and each metal is vapor deposited from bottom to top in sequence. Finally, the blue film is stripped and the glue is removed to obtain the Al-based metal film 11 connected to the N electrode 10.

[0045] (6) Preparation of P electrode and pad: After the Al-based metal film is prepared, a layer of SiO2 of about 400 nm is deposited as the insulating layer of the N / P electrode, and a positive photoresist is spin-coated with a uniform thickness of about 3.5 μm. After soft baking at 100 °C for 20 min, it is exposed for 6 s, and finally developed for 120 s. The photoresist is stripped in 100 W O plasma for 10 min. Then, the P electrode 12 is exposed by using a buffered oxide etchant, and then the P electrode 12 is evaporated by an electron beam evaporation device. Its composition is Ni / Au with a thickness of 20 / 50 nm, where each metal is evaporated from bottom to top. Finally, the blue film is peeled off, and it is cleaned in an 85 °C degumming solution for 10 min, and then rapidly thermally annealed in an oxygen environment to obtain the P electrode 12. Finally, the P electrode connection electrode is evaporated to connect the P electrodes on all discrete MESA tables, and a 1000 nm SiO2 cladding is deposited. After etching through holes on it, a metal pad is evaporated to complete the chip preparation.

[0046] Comparative Example

[0047] In order to verify the effect of photonic crystals in enhancing light emission, a deep ultraviolet micro-LED chip with the same structure as the present invention except that it does not have a photonic crystal structure was prepared. Figure 3 As shown in the figure, by calculating the light extraction efficiency of the chip, it is found that the photonic crystal structure can effectively improve the light extraction efficiency of the chip. Among them, the extraction efficiency of transverse magnetic (TM) mode light can be increased by about 21%, and the extraction efficiency of transverse electric (TE) mode light can be increased by about 10%.

[0048] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a deep ultraviolet micro-LED chip with enhanced light emission by photonic crystals, characterized in that: The following steps are involved: (1) Depositing an AlN buffer layer, an n-AlGaN layer, an active layer, a p-AlGaN electron blocking layer and a p-type hole injection layer on a substrate in sequence; (2) Spin-coating a photoresist on the p-type hole injection layer, then exposing and developing it, dry-etching it to the n-AlGaN layer, and then depositing a SiO2 coating layer to obtain a MESA table; (3) dripping the polystyrene nanosphere solution onto the water surface along the obliquely inserted glass slide. Due to the surface tension, the nanospheres on the air-water interface will self-assemble and form a thin film. Then, the MESA table obtained in step (2) is placed underwater and pulled upward to transfer the nanosphere film on the water surface to the epitaxial wafer, and the nanosphere film is attached to the epitaxial wafer. After drying, the epitaxial wafer is treated with oxygen plasma to reduce the diameter of the nanospheres. Then, dry etching is performed to remove the nanosphere mask and the SiO2 layer of the table to obtain a photonic crystal structure. (4) Deposit a layer of SiO2 as the isolation of the N electrode, then spin-coat photoresist, develop after exposure, and remove the resist in oxygen plasma; Then, an etching solution is used to expose the position of the evaporated N electrode, and then the N electrode is evaporated; after the N electrode is prepared, a photoresist is used to protect the evaporated part of the P electrode, and then an Al-based metal film is evaporated as a connection of the N electrode, which also plays a role in reflecting ultraviolet light; (5) After the Al-based metal film is prepared, a layer of SiO2 is deposited as an insulating layer for the electrode, and a photoresist is spin-coated, developed after exposure, and stripped in oxygen plasma; Then, the P-electrode vapor-deposited part is exposed through an etching solution, and then the P-electrode is vapor-deposited. After the P-electrode is prepared, the P-electrode connecting electrode is vapor-deposited, and SiO2 is deposited as a cladding layer. After a through hole is etched on it, a metal pad is vapor-deposited to complete the preparation of the chip.

2. The method for preparing a deep ultraviolet micro-LED chip with enhanced light emission from photonic crystals according to claim 1, characterized in that: In step (1), the p-type hole injection layer includes a lower p-AlGaN layer and an upper p-GaN layer.

3. The method for preparing a deep ultraviolet micro-LED chip with enhanced light emission from photonic crystals according to claim 1, characterized in that: In step (2), the MESA table is a truncated cone with a top diameter of 20-90 μm.

4. The method for preparing a deep ultraviolet micro-LED chip with enhanced light emission from photonic crystals according to claim 1, characterized in that: In step (3), the photonic crystal structure is an array structure of cylindrical protrusions.

5. The method for preparing a deep ultraviolet micro-LED chip with enhanced light emission from photonic crystals according to claim 1, characterized in that: In step (1), the active layer is a multi-quantum well.

6. The method for preparing a deep ultraviolet micro-LED chip with enhanced light emission from photonic crystals as claimed in claim 5, characterized in that: The material of the quantum well of the active layer is Al x Ga 1-x N, where 0.4≤x≤0.5, and the thickness is 1-3 nm; the material of the quantum barrier of the active layer is Al y Ga 1-y N, where 0.5≤y≤0.6, and the thickness is 10-12 nm; a layer of quantum well and a layer of quantum barrier are collectively referred to as a period, and the active layer has a total of 5-10 periods.

7. The method for preparing a deep ultraviolet micro-LED chip with enhanced light emission from photonic crystals according to claim 1, characterized in that: The Al-based metal film is an Al / Cr / Pt metal film with an ultraviolet light reflectivity greater than 90%; wherein Al, Cr, and Pt metals are evaporated on the chip from bottom to top.

8. The method for preparing a deep ultraviolet micro-LED chip with enhanced light emission from photonic crystals as claimed in claim 7, characterized in that: The thickness of the Al layer of the Al-based metal film is 1100-1300 nm, the thickness of the Cr layer is 40-60 nm, and the thickness of the Pt layer is 40-60 nm; the Al-based metal film is used to connect the N electrode and to reflect ultraviolet light to improve the light extraction efficiency of the chip.

9. A deep ultraviolet micro-LED chip with photonic crystal enhanced light emission obtained by any one of the preparation methods of claims 1-8.

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