Micro-led chip with double-sided microstructure sapphire substrate and preparation method and application thereof

Abstract

The application discloses a Micro-LED chip with a double-sided microstructure sapphire substrate and a preparation method and application thereof, and relates to the technical field of optoelectronic devices. The Micro-LED chip comprises a sapphire substrate and a flip GaN-based light-emitting unit arranged on one side of the sapphire substrate. The sapphire substrate has oppositely arranged first and second surfaces, the first surface faces the flip GaN-based light-emitting unit, and the second surface is a light-emitting upper surface. The first surface is formed with a microlens structure between the sapphire substrate and an n-type GaN layer, and the second surface is formed with a hexagonal pyramid microstructure array, which comprises a plurality of periodically arranged hexagonal pyramid structure units. The application can improve the surface light-emitting efficiency and the proportion of effective forward light emission while maintaining a high light extraction efficiency, and reduce the crosstalk influence of light emission in the sidewall direction on adjacent light-emitting pixels.

Description

Technical Field

[0001] This invention relates to the field of optoelectronic device technology, specifically to Micro-LED chips with double-sided microstructured sapphire substrates, their fabrication methods, and applications. Background Technology

[0002] GaN-based Micro-LEDs, as a new generation of core display devices, possess advantages such as high brightness, high contrast, long lifespan, and fast response speed, showing promising application prospects in micro-displays, high-resolution displays, and near-eye displays. For GaN-based Micro-LEDs, the light generated in the emitting region needs to pass through the GaN layer, sapphire substrate, and light-emitting interface before being emitted into the external environment. The light emission process is affected by factors such as material refractive index differences, interface reflection, electrode obstruction, and the substrate propagation path.

[0003] Due to the significant refractive index difference between GaN and air, some of the light generated in the emitting region is prone to Fresnel reflection or total internal reflection during propagation within the device, and is trapped inside the device in the form of lateral propagation or guided mode, thus limiting the light extraction efficiency. Although the metal reflective layer in flip-chip GaN-based Micro-LEDs can reflect some of the light propagating towards the bottom towards the sapphire substrate, the reflected light still needs to pass through the interface between the sapphire substrate and the GaN layer, as well as the light-emitting surface of the sapphire substrate, resulting in insufficient interface coupling and dispersion of the light emission direction.

[0004] As the size of Micro-LED chips continues to shrink, especially when the lateral dimension of the light-emitting mesa decreases to below 20μm, the surface area to volume ratio of the device increases, and the proportion of light emitted from the sidewalls significantly increases. Light emitted from the sidewalls can easily enter adjacent light-emitting pixel areas, causing optical crosstalk and reducing display contrast and clarity. Therefore, for small-sized Micro-LED chips, it is not enough to only focus on the total light output in all directions; it is also necessary to increase the proportion of light emitted from the upper surface of the sapphire substrate to improve effective display brightness and reduce the impact of side-emitting light.

[0005] Existing patterned sapphire substrate structures are mostly designed for larger LED devices, with their microstructure geometry parameters matched to light-emitting regions on the order of hundreds of micrometers. When device sizes shrink to below 20 μm, the matching degree between the number of periods, spatial coverage, and scattering area of ​​the patterned structure and the small-sized light-emitting mesa decreases, making it difficult to fully utilize the original scattering and waveguide mode disruption effects. Single-layer patterned structures typically only function at a single interface or in a single light-emitting stage, making it difficult to simultaneously address the optical path coupling at the interface between the sapphire substrate and the GaN layer, as well as the surface redirection of the light-emitting surface of the sapphire substrate. Summary of the Invention

[0006] One objective of the first aspect of this invention is to provide a Micro-LED chip with a double-sided microstructured sapphire substrate, which solves the technical problem in the prior art that the patterned substrate structure of small-sized flip-chip GaN-based Micro-LED chips is difficult to match with small-sized light-emitting mesa, resulting in difficulty in synergistically improving light extraction efficiency, surface light emission efficiency and lateral crosstalk.

[0007] Another objective of the first aspect of the present invention is to further improve the surface light extraction efficiency while maintaining a high light extraction efficiency.

[0008] The second aspect of this invention aims to provide a method for preparing a Micro-LED chip, which is used to prepare the aforementioned Micro-LED chip.

[0009] The third aspect of this invention aims to provide a Micro-LED display device.

[0010] According to a first aspect of the present invention, the present invention provides a Micro-LED chip with a double-sided microstructure sapphire substrate, wherein the lateral dimension of the light-emitting mesa of the Micro-LED chip is 5μm-20μm, and the Micro-LED chip includes a sapphire substrate and a flip-chip GaN-based light-emitting unit disposed on one side of the sapphire substrate. The sapphire substrate has a first surface and a second surface disposed opposite to each other. The first surface faces the flip-chip GaN-based light-emitting unit, and the second surface is the light-emitting upper surface. The flip-chip GaN-based light-emitting unit includes an n-type GaN layer, a multiple quantum well layer, a p-type GaN layer, a transparent conductive layer, and a metal reflective layer sequentially disposed along a direction away from the sapphire substrate. The first surface has a microlens structure located between the sapphire substrate and the n-type GaN layer, wherein the bottom radius of the microlens structure is any value between 1.9 μm and 2.2 μm, and the height is any value between 3.1 μm and 3.4 μm; The second surface is formed with a hexagonal pyramid microstructure array, which includes a plurality of periodically arranged hexagonal pyramid structural units. The base radius of each hexagonal pyramid structural unit is any value between 0.45μm and 0.55μm, and the height is any value between 0.75μm and 0.85μm.

[0011] Optionally, the arrangement period of the hexagonal pyramidal microstructure array is less than the bottom radius of the microlens structure, and when projected along the thickness direction of the sapphire substrate, the projection area of ​​one microlens structure corresponds to multiple hexagonal pyramidal structure units.

[0012] Optionally, the microlens structure is an arc-shaped structure formed by patterning the first surface of the sapphire substrate, and the hexagonal pyramidal structure unit is a cone-shaped protrusion structure formed by patterning the second surface of the sapphire substrate.

[0013] Optionally, the thickness of the n-type GaN layer is any value between 3.8 μm and 4.8 μm, the thickness of the multiple quantum well layer is any value between 0.05 μm and 0.20 μm, the thickness of the p-type GaN layer is any value between 0.05 μm and 0.20 μm, and the thickness of the transparent conductive layer is any value between 0.05 μm and 0.20 μm.

[0014] Optionally, the mesa sidewall of the flip-chip GaN-based light-emitting unit is covered with a SiO2 passivation layer, the thickness of which is any value between 100nm and 300nm.

[0015] Optionally, the metal reflective layer is an Au reflective layer, which is disposed on the side of the transparent conductive layer away from the p-type GaN layer, so as to reflect light propagating toward the metal reflective layer to the second surface direction of the sapphire substrate.

[0016] According to a second aspect of the present invention, the present invention also provides a method for fabricating a Micro-LED chip, for fabricating the Micro-LED chip described in any of the above claims, the method comprising the following steps: A sapphire substrate is provided, the sapphire substrate having a first surface and a second surface disposed opposite to each other; The first surface of the sapphire substrate is patterned to form a microlens structure with a first preset bottom radius and a first preset height; A flip-chip GaN-based light-emitting unit is fabricated on one side of the first surface where the microlens structure is formed. The flip-chip GaN-based light-emitting unit includes an n-type GaN layer, a multiple quantum well layer, a p-type GaN layer, a transparent conductive layer, and a metal reflective layer sequentially disposed along a direction away from the sapphire substrate. The second surface of the sapphire substrate is patterned to form a hexagonal pyramidal microstructure array. Each hexagonal pyramidal structural unit in the hexagonal pyramidal microstructure array has a second preset base radius and a second preset height, thereby preparing the Micro-LED chip.

[0017] Optionally, after fabricating the flip-chip GaN-based light-emitting unit, the flip-chip GaN-based light-emitting unit is subjected to mesa etching to form a light-emitting mesa with a lateral dimension of 5μm-20μm, and a SiO2 passivation layer of a predetermined thickness is formed on the sidewall of the light-emitting mesa.

[0018] According to a third aspect of the present invention, the present invention also provides a Micro-LED display device, including a driving substrate and a plurality of Micro-LED light-emitting pixels disposed on the driving substrate, wherein at least one of the Micro-LED light-emitting pixels includes the Micro-LED chip described in any of the preceding claims.

[0019] The Micro-LED chip of this invention forms a microlens structure and a hexagonal pyramidal microstructure array on opposite sides of a sapphire substrate, and combines them with a metal reflective layer in a flip-chip GaN-based light-emitting unit. This allows the light generated by the multi-quantum-well layer to form a continuous optical path within the device, consisting of down-reflection, interface coupling, substrate propagation, and surface redirection. Specifically, the metal reflective layer reflects light propagating away from the sapphire substrate back towards the sapphire substrate. The microlens structure on the first surface of the sapphire substrate adjusts the light propagation direction at the interface between the sapphire substrate and the n-type GaN layer, allowing more light to enter the sapphire substrate and participate in subsequent light emission. The hexagonal pyramidal microstructure array on the second surface of the sapphire substrate redirects the light propagating to the light-emitting upper surface again, allowing more light to be emitted from the light-emitting upper surface. In other words, by combining the bottom radius and height of the microlens structure, the bottom radius and height of the hexagonal pyramid structure unit, and their relative positions on opposite sides of the sapphire substrate, the Micro-LED chip can guide more emitting light to the light-emitting surface while maintaining high light extraction efficiency. This improves surface light extraction efficiency and the proportion of effective forward light extraction, and mitigates the display crosstalk problem caused by the high proportion of lateral light extraction in small-sized Micro-LED chips.

[0020] Furthermore, the arrangement period of the hexagonal pyramidal microstructure array of the present invention is smaller than the bottom radius of the microlens structure. When projected along the thickness direction of the sapphire substrate, the projection area of ​​one microlens structure corresponds to multiple hexagonal pyramidal structural units. This allows the microlens structure located on the first surface to form a larger-scale interface optical path control region, while the multiple hexagonal pyramidal structural units located on the second surface form a smaller-scale surface redirection region. The microlens structure first increases the number of photons entering the sapphire substrate and that can be extracted. Then, the multiple hexagonal pyramidal structural units locally control the direction of light propagating to the vicinity of the light-emitting upper surface, causing more light to be emitted from the light-emitting upper surface direction. The aforementioned dimensional relationship between the period and the bottom radius allows the bottom microlens structure and the top hexagonal pyramidal microstructure array to form a dimensional match within the same emitting platform, which is beneficial for further improving the surface light extraction efficiency while maintaining a high light extraction efficiency.

[0021] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. Attached Figure Description

[0022] The following sections will describe some specific embodiments of the invention in detail by way of example and not limitation, with reference to the accompanying drawings. The same reference numerals in the drawings denote the same or similar parts or portions. Those skilled in the art should understand that these drawings are not necessarily drawn to scale. In the drawings: Figure 1 This is a schematic diagram of a Micro-LED chip simulation model according to an embodiment of the present invention; Figure 2 This is a schematic flowchart of a method for fabricating a Micro-LED chip according to an embodiment of the present invention; Figure 3 This is a simulation test result diagram showing the effect of the microlens structure radius on the light extraction efficiency in a Micro-LED chip according to an embodiment of the present invention; Figure 4 This is a simulation test result diagram showing the effect of the height of the microlens structure on the light extraction efficiency in a Micro-LED chip according to an embodiment of the present invention; Figure 5 The results are simulation test results of the light extraction efficiency of the Micro-LED chips according to Embodiment 1 and Comparative Examples 1-3 of the present invention. Figure 6 This is a simulation test result diagram showing the effect of the base radius of the hexagonal pyramidal structure unit on the surface light extraction efficiency in a Micro-LED chip according to an embodiment of the present invention; Figure 7 This is a simulation test result diagram showing the effect of the height of the hexagonal pyramidal structure unit on the surface light extraction efficiency in a Micro-LED chip according to an embodiment of the present invention; Figure 8 The results are simulation test results of the surface light extraction efficiency of the Micro-LED chips according to Embodiment 1 and Comparative Examples 1-3 of the present invention.

[0023] Figure label: 100-Micro-LED chip, 10-Sapphire substrate, 20-Flip-chip GaN-based light-emitting unit, 21-n-type GaN layer, 22-Multiple quantum well layer, 23-p-type GaN layer, 24-Transparent conductive layer, 25-Metal reflective layer, 30-Microlens structure, 40-Hexagonal pyramidal microstructure array, 41-Hexagonal pyramidal structure unit, 50-SiO2 passivation layer, 200-Perfectly matched layer, 300-Top power monitor. Detailed Implementation

[0024] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0025] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, it should be noted that, for ease of description, only the parts relevant to this application are shown in the accompanying drawings, not the entire structure. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application.

[0026] The terms “comprising” and “having”, and any variations thereof, used in this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such process, method, product, or apparatus.

[0027] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0028] Figure 1 This is a schematic diagram of a Micro-LED chip simulation model according to an embodiment of the present invention. Figure 2 This is a schematic flowchart illustrating a method for fabricating a Micro-LED chip according to an embodiment of the present invention.

[0029] like Figure 1 As shown, this invention provides a Micro-LED chip 100 with a double-sided microstructured sapphire substrate 10. The lateral dimension of the light-emitting mesa of the Micro-LED chip 100 is 5μm-20μm, that is, the lateral dimension of the light-emitting mesa of the Micro-LED chip 100 can be 5μm, 6μm, 7μm, 8μm, 9μm, or 20μm, or any other value among 5μm-20μm. The lateral dimension of the light-emitting mesa refers to the characteristic dimension of the light-emitting mesa in the direction parallel to the surface of the sapphire substrate 10. When the light-emitting mesa is square or approximately square, the lateral dimension can be the side length of the light-emitting mesa; when the light-emitting mesa is circular or approximately circular, the lateral dimension can be the diameter of the light-emitting mesa; when the light-emitting mesa is rectangular or other irregular shapes, the lateral dimension can be the maximum circumscribed dimension of the light-emitting mesa in the direction parallel to the surface of the sapphire substrate 10.

[0030] In this embodiment, the Micro-LED chip 100 includes a sapphire substrate 10 and a flip-chip GaN-based light-emitting unit 20 disposed on one side of the sapphire substrate 10. The sapphire substrate 10 has a first surface and a second surface disposed opposite to each other. The first surface faces the flip-chip GaN-based light-emitting unit 20, and the second surface is the light-emitting upper surface. The flip-chip GaN-based light-emitting unit 20 includes an n-type GaN layer 21, a multiple quantum well layer 22, a p-type GaN layer 23, a transparent conductive layer 24, and a metal reflective layer 25 disposed sequentially along a direction away from the sapphire substrate 10. The first surface has a microlens structure 30 located between the sapphire substrate 10 and the n-type GaN layer 21. The microlens structure 30 has a base radius of any value between 1.9 μm and 2.2 μm and a height of any value between 3.1 μm and 3.4 μm. The second surface has a hexagonal pyramid microstructure array 40, which includes a plurality of periodically arranged hexagonal pyramid structural units 41. Each hexagonal pyramid structural unit 41 has a base radius of any value between 0.45 μm and 0.55 μm and a height of any value between 0.75 μm and 0.85 μm.

[0031] In this embodiment, by forming microlens structures 30 and hexagonal pyramidal microstructure arrays 40 on opposite sides of the sapphire substrate 10, and combining them with the metal reflective layer 25 in the flip-chip GaN-based light-emitting unit 20, the light generated by the multi-quantum-well layer 22 forms a continuous optical path control path within the device, consisting of down-reflection, interface coupling, substrate propagation, and surface redirection. Specifically, the metal reflective layer 25 can reflect light propagating away from the sapphire substrate 10 back towards the sapphire substrate 10; the microlens structure 30 located on the first surface of the sapphire substrate 10 can adjust the light propagation direction at the interface between the sapphire substrate 10 and the n-type GaN layer 21, allowing more light to enter the sapphire substrate 10 and participate in subsequent light emission; the hexagonal pyramidal microstructure array 40 located on the second surface of the sapphire substrate 10 can redirect the light propagating to the light-emitting upper surface again, allowing more light to be emitted from the light-emitting upper surface direction. Thus, the Micro-LED chip 100 can improve surface light emission efficiency and the proportion of effective forward light emission while maintaining high light extraction efficiency, and reduce the impact of sidewall-direction light emission on adjacent light-emitting pixels.

[0032] Furthermore, the bottom radius of the microlens structure 30 is any value between 1.9 μm and 2.2 μm, meaning the bottom radius of the microlens structure 30 can be 1.9 μm, 2.0 μm, 2.1 μm, or 2.2 μm, or any other value between 1.9 μm and 2.2 μm; the height of the microlens structure 30 is any value between 3.1 μm and 3.4 μm, meaning the height of the microlens structure 30 can be 3.1 μm, 3.2 μm, 3.3 μm, or 3.4 μm, or any other value between 3.1 μm and 3.4 μm. The aforementioned bottom radius and height allow the microlens structure 30 to match the main light-emitting area of ​​the 5 μm-20 μm light-emitting mesa, forming an arc-shaped modulation region suitable for small-sized light-emitting mesa between the sapphire substrate 10 and the n-type GaN layer 21. The bottom radius of the microlens structure 30 determines the main light propagation area it covers in the lateral direction, and the height determines its modulation depth on the interface optical path. By combining the bottom radius and height as described above, the microlens structure 30 can change the incident state of some of the oblique or lateral light emitted from the multi-quantum well layer 22 at the interface, making it easier for this part of the light to enter the sapphire substrate 10 and be extracted from the device, thereby improving the total light extraction capability of the small-sized Micro-LED chip 100.

[0033] Furthermore, the hexagonal pyramidal structure unit 41 has a base radius of 0.45μm-0.55μm and a height of 0.75μm-0.85μm, and multiple hexagonal pyramidal structure units 41 are periodically arranged on the light-emitting upper surface of the sapphire substrate 10. The base radius of each hexagonal pyramidal structure unit 41 can be 0.45μm, 0.50μm, or 0.55μm, or any other value within the range of 0.45μm-0.55μm; the height of each hexagonal pyramidal structure unit 41 can be 0.75μm, 0.80μm, or 0.85μm, or any other value within the range of 0.75μm-0.85μm. The above parameter range enables the hexagonal pyramidal structure unit 41 to form appropriate tilted sides and surface control dimensions on the light-emitting upper surface of the small-sized Micro-LED chip 100. When light propagating to the vicinity of the second surface of the sapphire substrate 10 encounters the hexagonal pyramidal structure unit 41, its emission direction can be adjusted by refraction or scattering of the pyramidal side surface, causing some of the light that originally tended to propagate laterally or be emitted laterally to be emitted directly above the chip, thereby improving the surface light emission efficiency and reducing the impact of light emitted from the sidewall direction on adjacent light-emitting pixels.

[0034] Furthermore, the microlens structure 30 and the hexagonal pyramidal microstructure array 40 correspond to different optical path stages in the small-sized Micro-LED chip 100. The microlens structure 30 has a micrometer-level base radius and a large modulation height, mainly acting on the interface optical path between the sapphire substrate 10 and the n-type GaN layer 21, to increase the number of photons that can enter the sapphire substrate 10 and be extracted; the hexagonal pyramidal structure unit 41 has a smaller base radius and height, mainly acting on the local emission direction of the light-emitting upper surface of the sapphire substrate 10, to control the direction of light that has propagated to the vicinity of the light-emitting upper surface. When the hexagonal pyramidal microstructure array 40 is set alone, its surface light emission enhancement effect is limited because the number of effective photons entering the sapphire substrate 10 and reaching the light-emitting upper surface is limited; after setting the microlens structure 30 on the first surface, more light can enter the sapphire substrate 10 and participate in light emission, and the hexagonal pyramidal microstructure array 40 on the second surface can further play a surface redirection role, enabling the double-layer structure to obtain higher surface light emission efficiency.

[0035] In a further embodiment, the arrangement period of the hexagonal pyramidal microstructure array 40 is smaller than the bottom radius of the microlens structure 30. When projected along the thickness direction of the sapphire substrate 10, the projection area of ​​one microlens structure 30 corresponds to multiple hexagonal pyramidal structural units 41. This allows the microlens structure 30 on the first surface to form a larger-scale interface optical path control region, while the multiple hexagonal pyramidal structural units 41 on the second surface form a smaller-scale surface redirection region. The microlens structure 30 first increases the number of photons entering the sapphire substrate 10 and that can be extracted. Then, the multiple hexagonal pyramidal structural units 41 perform local directional control on the light propagating to the vicinity of the light-emitting upper surface, causing more light to be emitted from the light-emitting upper surface direction. The aforementioned dimensional relationship between the period and the bottom radius allows the bottom microlens structure 30 and the top hexagonal pyramidal microstructure array 40 to form a dimensional match within the same light-emitting platform, which is beneficial for further improving the surface light extraction efficiency while maintaining a high light extraction efficiency.

[0036] It should be further noted that the arrangement period of the hexagonal pyramidal microstructure array 40 is preferably 1 μm, but can also be further listed as 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, or 1.2 μm. If 1 μm is taken as the preferred period, it is smaller than the bottom radius range of 1.9 μm-2.2 μm of the microlens structure 30, which can satisfy the structural relationship that one projection area of ​​the microlens structure 30 corresponds to multiple hexagonal pyramidal structural units 41.

[0037] In a further embodiment, the microlens structure 30 is an arc-shaped structure formed by patterning the first surface of the sapphire substrate 10, and the hexagonal pyramidal structure unit 41 is a conical protrusion structure formed by patterning the second surface of the sapphire substrate 10. The arc-shaped structure can change the local interface orientation of the boundary region between the sapphire substrate 10 and the n-type GaN layer 21, making it easier for some oblique or laterally propagating light to enter the sapphire substrate 10 and participate in subsequent light extraction, thereby improving the overall light extraction capability. The conical protrusion structure has multiple inclined conical surfaces, which can refract or scatter the light propagating to the vicinity of the light-emitting upper surface, causing some light to be directed to the direction directly above the chip for emission, thereby improving the surface light extraction efficiency. The arc-shaped structure of the first surface and the conical protrusion structure of the second surface correspond to the interface coupling stage and the surface emission stage, respectively, and the two work together to form continuous optical path control. Here, the microlens structure 30 is an arc-shaped structure protruding relative to the first surface.

[0038] In a further embodiment, the thickness of the n-type GaN layer 21 is any value between 3.8 μm and 4.8 μm, that is, the thickness of the n-type GaN layer 21 can be 3.8 μm, 4.0 μm, 4.3 μm, 4.5 μm, or 4.8 μm, or any other value between 3.8 μm and 4.8 μm. The thickness of the multiple quantum well layer 22 is any value between 0.05 μm and 0.20 μm, that is, the thickness of the multiple quantum well layer 22 can be 0.05 μm, 0.10 μm, 0.15 μm, or 0.20 μm, or any other value between 0.05 μm and 0.20 μm. The thickness of the p-type GaN layer 23 is any value between 0.05 μm and 0.20 μm, that is, the thickness of the p-type GaN layer 23 can be 0.05 μm, 0.10 μm, 0.15 μm, or 0.20 μm, or any other value between 0.05 μm and 0.20 μm. The thickness of the transparent conductive layer 24 is any value between 0.05 μm and 0.20 μm, that is, the thickness of the transparent conductive layer 24 can be 0.05 μm, 0.10 μm, 0.15 μm, or 0.20 μm, or any other value between 0.05 μm and 0.20 μm.

[0039] In this embodiment, by controlling the n-type GaN layer 21, the multiple quantum well layer 22, the p-type GaN layer 23, and the transparent conductive layer 24 within the aforementioned thickness range, the flip-chip GaN-based light-emitting unit 20 can maintain a light-emitting structure and light propagation path suitable for a small-sized Micro-LED chip 100. The multiple quantum well layer 22 serves as the light-emitting region, and the n-type GaN layer 21 is located between the multiple quantum well layer 22 and the sapphire substrate 10. Its thickness affects the propagation state of light emitted from the multiple quantum well layer 22 when it reaches the vicinity of the first surface of the sapphire substrate 10. The aforementioned layer thickness range can cooperate with the microlens structure 30 on the first surface and the hexagonal pyramidal microstructure array 40 on the second surface, enabling the microlens structure 30 to effectively control the interface light path and further redirect the light entering the sapphire substrate 10 through the light-emitting upper surface.

[0040] In a further embodiment, the mesa sidewall of the flip-chip GaN-based light-emitting unit 20 is covered with a SiO2 passivation layer 50. The thickness of the SiO2 passivation layer 50 is any value between 100nm and 300nm, that is, the thickness of the SiO2 passivation layer 50 can be 100nm, 150nm, 200nm, 250nm, or 300nm, or any other value between 100nm and 300nm. In this embodiment, by covering the mesa sidewall of the flip-chip GaN-based light-emitting unit 20 with a SiO2 passivation layer 50, the mesa sidewall of the small-sized Micro-LED chip 100 can be insulated and passivated, reducing the impact of sidewall defects after mesa etching on device performance. For small-sized light-emitting mesa of 5μm-20μm, the sidewall area ratio is relatively high, and the impact of light emission in the sidewall direction on adjacent light-emitting pixels is more obvious. The SiO2 passivation layer 50 can cooperate with the microlens structure 30 on the first surface and the hexagonal pyramidal microstructure array 40 on the second surface to reduce the undesirable effects of the sidewall region and allow more light to be effectively emitted through the second surface direction of the sapphire substrate 10.

[0041] In a further embodiment, the metal reflective layer 25 is an Au reflective layer, disposed on the side of the transparent conductive layer 24 away from the p-type GaN layer 23, to reflect light propagating toward the metal reflective layer 25 to the second surface direction of the sapphire substrate 10. In this embodiment, by setting the metal reflective layer 25 as an Au reflective layer, light propagating away from the sapphire substrate 10 can be reflected, causing this portion of the light to propagate back toward the sapphire substrate 10. Thus, the Au reflective layer, the microlens structure 30 on the first surface, and the hexagonal pyramidal microstructure array 40 on the second surface can be sequentially coordinated along the light propagation path, which is beneficial to improving the utilization rate of photons inside the device and allowing more light to be emitted from the light-emitting upper surface direction.

[0042] like Figure 2As shown, the present invention also provides a method for fabricating a Micro-LED chip 100, which is used to fabricate the Micro-LED chip 100. The fabrication method includes the following steps: Step S100: Provide a sapphire substrate 10, the sapphire substrate 10 having a first surface and a second surface disposed opposite to each other; Step S200: The first surface of the sapphire substrate 10 is patterned to form a microlens structure 30 with a first preset bottom radius and a first preset height; Step S300: A flip-chip GaN-based light-emitting unit 20 is fabricated on one side of the first surface where the microlens structure 30 is formed. The flip-chip GaN-based light-emitting unit 20 includes an n-type GaN layer 21, a multiple quantum well layer 22, a p-type GaN layer 23, a transparent conductive layer 24, and a metal reflective layer 25 sequentially disposed along a direction away from the sapphire substrate 10. Step S400: The second surface of the sapphire substrate 10 is patterned to form a hexagonal pyramid microstructure array 40. Each hexagonal pyramid structural unit 41 in the hexagonal pyramid microstructure array 40 has a second preset base radius and a second preset height, thereby preparing a Micro-LED chip 100.

[0043] In this embodiment, the present invention also provides a method for fabricating a Micro-LED chip 100, which is used to fabricate the aforementioned Micro-LED chip 100. The method includes: providing a sapphire substrate 10, the sapphire substrate 10 having a first surface and a second surface disposed opposite to each other; patterning the first surface of the sapphire substrate 10 to form a microlens structure 30 having a first preset bottom radius and a first preset height; fabricating a flip-chip GaN-based light-emitting unit 20 on one side of the first surface where the microlens structure 30 is formed, the flip-chip GaN-based light-emitting unit 20 including an n-type GaN layer 21, a multiple quantum well layer 22, a p-type GaN layer 23, a transparent conductive layer 24, and a metal reflective layer 25 sequentially disposed along a direction away from the sapphire substrate 10; patterning the second surface of the sapphire substrate 10 to form a hexagonal pyramidal microstructure array 40, each hexagonal pyramidal structure unit 41 in the hexagonal pyramidal microstructure array 40 having a second preset bottom radius and a second preset height, thereby fabricating a Micro-LED chip 100 with a double-layer structure of a hexagonal pyramidal structure array and a microlens structure 30.

[0044] In this embodiment, the above-described fabrication method enables the formation of a microlens structure 30 and a hexagonal pyramidal microstructure array 40 on opposite sides of the same sapphire substrate 10. The microlens structure 30 is positioned between the sapphire substrate 10 and the n-type GaN layer 21, while the hexagonal pyramidal microstructure array 40 is located on the light-emitting upper surface of the sapphire substrate 10. Consequently, the fabricated Micro-LED chip 100 can form a continuous optical path modulation structure involving the metal reflective layer 25, the first surface microlens structure 30, and the second surface hexagonal pyramidal microstructure array 40, thereby improving surface light extraction efficiency while maintaining high light extraction efficiency.

[0045] Specifically, the metal reflective layer 25 can reflect light propagating away from the sapphire substrate 10 back towards the sapphire substrate 10, improving the utilization rate of photons inside the device. The microlens structure 30 on the first surface is located between the sapphire substrate 10 and the n-type GaN layer 21, and can adjust the propagation path of light emitted from the multi-quantum well layer 22 when it enters the sapphire substrate 10, allowing more light to enter the sapphire substrate 10 and participate in subsequent light emission. The hexagonal pyramidal microstructure array 40 on the second surface is located on the light emission upper surface, and can control the direction of light propagating to the vicinity of the second surface of the sapphire substrate 10, allowing more light to be emitted from the light emission upper surface. The double-sided microstructure obtained through the above fabrication steps enables the interface optical path control of the first surface and the surface redirection of the second surface to be continuously coordinated in the light propagation path.

[0046] Furthermore, the first preset bottom radius and the first preset height are used to limit the lateral range of action and the optical path modulation depth of the microlens structure 30 at the first surface, enabling the microlens structure 30 to adapt to the light-emitting platform of the small-sized Micro-LED chip 100. The second preset bottom radius and the second preset height are used to limit the local redirection scale of the hexagonal pyramidal structure unit 41 on the light-emitting upper surface, enabling the hexagonal pyramidal microstructure array 40 to effectively adjust the direction of light propagating to the vicinity of the second surface. Thus, the size parameters of the microlens structure 30 and the size parameters of the hexagonal pyramidal structure unit 41 correspond to the interface coupling stage and the surface emission stage, respectively, and together they support the dual-sided microstructure coordinated light emission control of the Micro-LED chip 100.

[0047] In step S400, the patterning process can be achieved using photolithography and etching processes, or other patterning processes capable of forming periodic hexagonal pyramidal structure units 41 on the second surface of the sapphire substrate 10. The arrangement period of the hexagonal pyramidal microstructure array 40 can be 1 μm, or it can be adjusted according to the size of the light-emitting mesa of the Micro-LED chip 100 and the bottom radius of the microlens structure 30. In a preferred embodiment, the arrangement period of the hexagonal pyramidal microstructure array 40 is smaller than the bottom radius of the microlens structure 30, so that the projection area of ​​one microlens structure 30 can correspond to multiple hexagonal pyramidal structure units 41.

[0048] In a further embodiment, after fabricating the flip-chip GaN-based light-emitting unit 20, mesa etching is performed on the flip-chip GaN-based light-emitting unit 20 to form a light-emitting mesa with a lateral dimension of 5μm-20μm, and a SiO2 passivation layer 50 of a predetermined thickness is formed on the sidewall of the light-emitting mesa. Here, the predetermined thickness can be any value in the range of 100nm-300nm, for example, it can be 100nm, 150nm, 200nm, 250nm or 300nm, or any other value in the range of 100nm-300nm.

[0049] In this embodiment, a light-emitting mesa with a lateral dimension of 5μm is formed by mesa etching, making the Micro-LED chip 100 suitable for applications with small pixel pitch, such as high-resolution displays, micro-displays, or near-eye displays. Due to the small size of the light-emitting mesa, the area ratio of the mesa sidewalls is correspondingly increased, making the impact of sidewall defects, sidewall leakage, and lateral light emission on device performance and crosstalk between adjacent pixels more significant. Therefore, forming a SiO2 passivation layer 50 on the sidewalls of the light-emitting mesa can insulate and passivate the mesa sidewalls, reducing the impact of sidewall defects after mesa etching on the electrical and optical performance of the device.

[0050] Furthermore, the SiO2 passivation layer 50 works in conjunction with the microlens structure 30 on the first surface of the sapphire substrate 10 and the hexagonal pyramidal microstructure array 40 on the second surface of the sapphire substrate 10. The microlens structure 30 is used to adjust the interface optical path between the sapphire substrate 10 and the n-type GaN layer 21, allowing more light to enter the sapphire substrate 10 and participate in subsequent light emission. The hexagonal pyramidal microstructure array 40 is used to adjust the emission direction of light propagating to the vicinity of the light emission surface, allowing more light to be emitted from the light emission surface direction. The SiO2 passivation layer 50 is used to improve the sidewall state of the small-sized light-emitting mesa and reduce the undesired influence of the sidewall direction. Together, they improve the surface light emission efficiency and the proportion of effective forward light emission, and improve the influence of lateral light emission on adjacent light-emitting pixels in the small-sized Micro-LED chip 100.

[0051] This invention also provides a Micro-LED display device, including a driving substrate and a plurality of Micro-LED light-emitting pixels disposed on the driving substrate, wherein at least one Micro-LED light-emitting pixel includes the aforementioned Micro-LED chip 100. In this embodiment, in the Micro-LED display device, the spacing between the plurality of Micro-LED light-emitting pixels is small, and light emitted from the sidewall direction can easily enter the area of ​​adjacent light-emitting pixels and affect the clarity of the display boundary. This embodiment, by employing the aforementioned Micro-LED chip 100 in at least one Micro-LED light-emitting pixel, allows more light from the light-emitting upper surface of the sapphire substrate 10 to be emitted, reducing the impact of side-emitting light on adjacent light-emitting pixels, thereby improving the effective display brightness, contrast, and display clarity of the display device.

[0052] It should be noted that each of the multiple Micro-LED light-emitting pixels may include the aforementioned Micro-LED chip 100, or only some Micro-LED light-emitting pixels may include the aforementioned Micro-LED chip 100. For each light-emitting pixel employing the aforementioned Micro-LED chip 100, the microlens structure 30 may be correspondingly arranged with the light-emitting platform of the corresponding light-emitting pixel, and the hexagonal pyramidal microstructure array 40 may be arranged in the light-emitting region of the corresponding light-emitting pixel, so that interface light path modulation and surface light emission redirection act on the light emission path of the same light-emitting pixel.

[0053] The technical solution of this application will be further described below with reference to specific embodiments.

[0054] In some embodiments, the Micro-LED chip 100 with a lateral size of 5μm-20μm on the light-emitting platform includes a sapphire substrate 10 and a flip-chip GaN-based light-emitting unit 20 disposed on one side of the sapphire substrate 10. The sapphire substrate 10 has a first surface and a second surface disposed opposite to each other. The first surface faces the flip-chip GaN-based light-emitting unit 20, and the second surface is the light-emitting upper surface. The flip-chip GaN-based light-emitting unit 20 includes an n-type GaN layer 21, a multiple quantum well layer 22, a p-type GaN layer 23, a transparent conductive layer 24, and a metal reflective layer 25 disposed sequentially along a direction away from the sapphire substrate 10.

[0055] The first surface has a microlens structure 30 located between the sapphire substrate 10 and the n-type GaN layer 21. The microlens structure 30 has a base radius of any value between 1.9 μm and 2.2 μm and a height of any value between 3.1 μm and 3.4 μm. The second surface has a hexagonal pyramid microstructure array 40, which includes a plurality of periodically arranged hexagonal pyramid structural units 41. Each hexagonal pyramid structural unit 41 has a base radius of any value between 0.45 μm and 0.55 μm and a height of any value between 0.75 μm and 0.85 μm.

[0056] Example 1 The Micro-LED chip 100 with a lateral dimension of 5μm includes a sapphire substrate 10 and a flip-chip GaN-based light-emitting unit 20 disposed on one side of the sapphire substrate 10. The sapphire substrate 10 has a first surface and a second surface disposed opposite to each other. The first surface faces the flip-chip GaN-based light-emitting unit 20 and the second surface is the light-emitting upper surface. The flip-chip GaN-based light-emitting unit 20 includes an n-type GaN layer 21, a multiple quantum well layer 22, a p-type GaN layer 23, a transparent conductive layer 24, and a metal reflective layer 25 disposed sequentially along a direction away from the sapphire substrate 10. The first surface has a microlens structure 30 located between the sapphire substrate 10 and the n-type GaN layer 21. The microlens structure 30 has a bottom radius of 2.0 μm and a height of 3.3 μm. The second surface has a hexagonal pyramid microstructure array 40, which includes a plurality of periodically arranged hexagonal pyramid structural units 41. Each hexagonal pyramid structural unit 41 has a bottom radius of 0.45 μm and a height of 0.8 μm.

[0057] Example 2 The only difference between Example 2 and Example 1 is that the bottom radius of the microlens structure 30 is 1.9 μm and the height is 3.1 μm.

[0058] Example 3 The only difference between Example 3 and Example 1 is that the bottom radius of the microlens structure 30 is 2.1 μm and the height is 3.2 μm.

[0059] Example 4 The only difference between Example 4 and Example 1 is that the bottom radius of the microlens structure 30 is 2.2 μm and the height is 3.4 μm.

[0060] Example 5 The only difference between Example 5 and Example 1 is that the base radius of the hexagonal pyramidal structural unit 41 is 0.5 μm.

[0061] Example 6 The only difference between Example 6 and Example 1 is that the base radius of the hexagonal pyramidal structural unit 41 is 0.55 μm.

[0062] Example 7 The only difference between Example 7 and Example 1 is that the height of the hexagonal pyramidal structural unit 41 is 0.75 μm.

[0063] Example 8 The only difference between Example 8 and Example 1 is that the height of the hexagonal pyramidal structural unit 41 is 0.85 μm.

[0064] Comparative Example 1 The only difference between Comparative Example 1 and Example 1 is that the first surface of the sapphire substrate 10 does not have a microlens structure 30 and the second surface does not have a hexagonal pyramidal microstructure array 40.

[0065] Comparative Example 2 The only difference between Comparative Example 2 and Example 1 is that the first surface of the sapphire substrate 10 does not have a microlens structure 30.

[0066] Comparative Example 3 The only difference between Comparative Example 3 and Example 1 is that the second surface of the sapphire substrate 10 does not have a hexagonal pyramidal microstructure array 40.

[0067] Comparative Example 4 The only difference between Comparative Example 4 and Example 1 is that the bottom radius of the microlens structure 30 is 1.0 μm.

[0068] Comparative Example 5 The only difference between Comparative Example 5 and Example 1 is that the bottom radius of the microlens structure 30 is 2.3 μm.

[0069] Comparative Example 6 The only difference between Comparative Example 6 and Example 1 is that the height of the microlens structure 30 is 1.0 μm.

[0070] Comparative Example 7 The only difference between Comparative Example 7 and Example 1 is that the height of the microlens structure 30 is 4.0 μm.

[0071] Comparative Example 8 The only difference between Comparative Example 8 and Example 1 is that the base radius of the hexagonal pyramidal structural unit 41 is 0.25 μm.

[0072] Comparative Example 9 The only difference between Comparative Example 9 and Example 1 is that the base radius of the hexagonal pyramidal structural unit 41 is 0.3 μm.

[0073] Comparative Example 10 The only difference between Comparative Example 10 and Example 1 is that the height of the hexagonal pyramidal structural unit 41 is 0.40 μm.

[0074] Comparative Example 11 The only difference between Comparative Example 11 and Example 1 is that the height of the hexagonal pyramidal structural unit 41 is 0.90 μm.

[0075] Figure 3 This is a simulation test result diagram showing the effect of the microlens structure radius on the light extraction efficiency in a Micro-LED chip according to an embodiment of the present invention. Figure 4 This is a simulation test result diagram showing the effect of the height of the microlens structure on the light extraction efficiency in a Micro-LED chip according to an embodiment of the present invention. Figure 5 These are simulation test results of the light extraction efficiency of the Micro-LED chips according to Embodiment 1 and Comparative Examples 1-3 of the present invention. Figure 6 This is a simulation test result diagram showing the effect of the base radius of the hexagonal pyramidal structure unit on the surface light extraction efficiency in a Micro-LED chip according to an embodiment of the present invention. Figure 7 This is a simulation test result diagram showing the effect of the height of the hexagonal pyramidal structure unit on the surface light extraction efficiency in a Micro-LED chip according to an embodiment of the present invention. Figure 8 These are simulation test results of the surface light extraction efficiency of the Micro-LED chips according to Embodiment 1 and Comparative Examples 1-3 of the present invention. Figures 3-5 The vertical axis in the figure is LEE (Light Extraction Efficiency). Figures 6-8 The vertical axis LSE in the figure is (Lateral Surface Emission).

[0076] Optical simulation tests were performed on the Micro-LED chips 100 of Examples 1-8 and Comparative Examples 1-11, and the results were as follows: Figure 3-8 The test results are shown. In the simulation embodiment, the Micro-LED chip 100 is disposed within the simulation area enclosed by the perfectly matched layer 200. The perfectly matched layer 200 serves as a simulation boundary condition to absorb light waves incident on the boundary of the simulation area, reducing the impact of boundary reflection on the simulation results. A top power monitor 300 is disposed on one side of the light-emitting upper surface of the Micro-LED chip 100, spaced 1 μm apart from the Micro-LED chip 100, for collecting the light power emitted from the direction of the light-emitting upper surface; a lateral power monitor is disposed on the lateral outer periphery of the Micro-LED chip 100, spaced 0.5 μm apart from the Micro-LED chip 100, for collecting the lateral emitted light power.

[0077] like Figure 3 As shown, with the increase of the bottom radius of the microlens structure 30, the light extraction efficiency of the Micro-LED chip 100 generally shows a trend of first rapidly increasing and then stabilizing. When the bottom radius of the microlens structure 30 is small, the lateral optical path control range formed by it in the interface region between the sapphire substrate 10 and the n-type GaN layer 21 is insufficient, making it difficult to fully act on the main light-emitting area below the light-emitting mesa. As the bottom radius increases, the microlens structure 30 can cover more oblique or laterally propagating light from the multi-quantum well layer 22, allowing more light to enter the sapphire substrate 10 and participate in subsequent light emission, thus significantly improving the light extraction efficiency. When the bottom radius of the microlens structure 30 is in the range of 1.9μm-2.2μm, the light extraction efficiency is at a relatively high and stable level, indicating that this bottom radius range can form a good scale match with the main light-emitting area of ​​the 5μm light-emitting mesa. In Comparative Example 4, the bottom radius of the microlens structure 30 is 1.0 μm. Although it can produce a certain interface control effect, it does not belong to the better stability range defined in this application, which is not conducive to further demonstrating the two-layer scale coordination relationship with the top hexagonal pyramid microstructure array 40.

[0078] like Figure 4 As shown, with the increase in height of the microlens structure 30, the light extraction efficiency of the Micro-LED chip 100 generally shows an upward trend and remains at a high level within a relatively large height range. When the height of the microlens structure 30 is small, the modulation depth of the curved structure at the interface between the sapphire substrate 10 and the n-type GaN layer 21 is insufficient, and some oblique or lateral propagating light still has difficulty effectively changing its propagation path. As the height of the microlens structure 30 increases, the adjustment effect of the curved structure on the local incident state of the interface is enhanced, which is beneficial to increasing the probability of light entering the sapphire substrate 10 and being extracted. When the height of the microlens structure 30 is in the range of 3.1μm-3.4μm, the light extraction efficiency is at a high level, indicating that this height range can provide sufficient modulation depth for interface optical path control. In Comparative Example 6, the height of the microlens structure 30 is 1.0μm, and its modulation depth is lower than the range defined in this application, making it difficult to obtain the full improvement effect of the microlens structure 30 on the total light extraction capability in the embodiment.

[0079] It should be noted that, Figure 4The local fluctuations in light extraction efficiency as the height of the microlens structure 30 changes may be related to the phase conditions of the simulated light source and the optical path difference in the multilayer structure of the device. When the height of the microlens structure, the thickness of the GaN layer, or the local propagation optical path is on the same order of magnitude as the wavelength of blue light, light from different propagation paths may produce constructive or destructive superposition, resulting in local fluctuations in the simulation curve. The actual light emission of Micro-LED chips comes from spontaneous emission from multiple spatial locations and multiple radiation directions within the multi-quantum well layer, exhibiting incoherent characteristics, and the emission band usually has a certain spectral width. Therefore, the interference peaks and valleys formed under single-phase conditions will be affected by the averaging of spatial location, polarization direction, and wavelength distribution in the actual device. Based on this, the height of the microlens structure 30 should not be determined solely based on a single peak point, but should be selected in conjunction with the stability of light extraction efficiency over a wider height range. In this embodiment, the height of the microlens structure 30 is limited to 3.1μm-3.4μm because within this range, the microlens structure 30 can provide sufficient interfacial optical path modulation depth and keep the light extraction efficiency in a high and relatively stable range, thus making it more suitable for characterizing the stable improvement effect of the microlens structure 30 on the total light extraction capability in the small-sized Micro-LED chip 100.

[0080] like Figure 5 As shown, the planar structure in Comparative Example 1 has a low light extraction efficiency, while the single-layer hexagonal pyramid structure in Comparative Example 2 fails to effectively improve the light extraction efficiency, and its light extraction efficiency is lower than that of the planar structure in some wavelength bands. The single-layer microlens structure 30 in Comparative Example 3 exhibits a high light extraction efficiency in the 435nm-465nm wavelength band, indicating that the microlens structure 30 on the first surface is the main structure for improving the overall light extraction efficiency. Although the light extraction efficiency of the double-layer structure in Example 1 is lower than that of the single-layer microlens structure 30 in Comparative Example 3, it is still significantly higher than that of the planar structure and the single-layer hexagonal pyramid structure, indicating that after the hexagonal pyramid microstructure array 40 is superimposed on the second surface, the double-layer structure can still maintain a high overall light extraction capability. The results show that the hexagonal pyramid microstructure array 40 is not primarily used to improve light extraction efficiency alone; it is more suitable for further improving the light emission direction distribution based on the increased number of extractable photons achieved by the microlens structure 30.

[0081] like Figure 6As shown, with the increase of the base radius of the hexagonal pyramidal structure unit 41, the surface light extraction efficiency of the Micro-LED chip 100 generally shows an increasing trend, reaching a relatively high level near 0.45 μm, and then tending to stabilize or slightly decrease. When the base radius of the hexagonal pyramidal structure unit 41 is small, the scale of the inclined conical surface of the light-emitting upper surface is insufficient, limiting its ability to adjust the direction of oblique or lateral light propagating to the vicinity of the second surface of the sapphire substrate 10. As the base radius increases, the hexagonal pyramidal structure unit 41 can form a more effective local surface redirection structure, causing more light to be directed towards the direction directly above the chip. When the base radius of the hexagonal pyramidal structure unit 41 is in the range of 0.45 μm-0.55 μm, the surface light extraction efficiency is at a relatively high level. In Comparative Example 8, the base radius of the hexagonal pyramidal structure unit 41 is 0.25 μm, and its surface redirection scale is insufficient, making it difficult to achieve the high surface light extraction efficiency of the embodiment.

[0082] like Figure 7 As shown, with the increase in height of the hexagonal pyramidal structure unit 41, the surface light extraction efficiency of the Micro-LED chip 100 generally shows an upward trend, entering a relatively stable range around 0.75μm-0.85μm. When the height of the hexagonal pyramidal structure unit 41 is small, the inclined side of the conical protrusion structure has insufficient ability to adjust the emission direction; as the height increases, the conical side has a more obvious refraction or scattering adjustment effect on the light propagating to the light-emitting upper surface, causing some of the light that originally tended to be emitted laterally to turn towards the light-emitting upper surface. When the height of the hexagonal pyramidal structure unit 41 is in the range of 0.75μm-0.85μm, a good balance can be achieved between surface redirection ability and structural scale. In Comparative Example 10, the height of the hexagonal pyramidal structure unit 41 is 0.40μm, and its conical side height is insufficient, limiting the improvement of surface light extraction efficiency.

[0083] like Figure 8As shown, within the 435nm-465nm wavelength range, the surface light extraction efficiency of the double-layer structure in Example 1 is significantly higher than that of the planar structure in Comparative Example 1, the single-layer hexagonal pyramid structure in Comparative Example 2, and the single-layer microlens structure 30 in Comparative Example 3. In Comparative Example 2, which only includes the hexagonal pyramid microstructure array 40, the lack of the first surface microlens structure 30 to regulate the interface light path limits the number of effective photons entering the sapphire substrate 10 and reaching the light-emitting upper surface, thus failing to significantly improve its surface light extraction efficiency. In Comparative Example 3, which only includes the microlens structure 30, although it can improve the overall light extraction capability, its directional control effect on the light-emitting upper surface is limited, and the surface light extraction efficiency is still lower than that of Example 1. Example 1 simultaneously includes the microlens structure 30 and the hexagonal pyramid microstructure array 40. The microlens structure 30 first increases the number of photons entering the sapphire substrate 10 and that can be extracted, and the hexagonal pyramid microstructure array 40 then redirects the light propagating to the vicinity of the second surface, causing more light to exit from the light-emitting upper surface, thus achieving a higher surface light extraction efficiency.

[0084] In summary, this application forms a microlens structure with a specific base radius and height on the first surface of a sapphire substrate facing the flip-chip GaN-based light-emitting unit, and forms a hexagonal pyramidal microstructure array with a specific base radius and height on the light-emitting upper surface of the sapphire substrate. The microlens structure acts in the interface optical path modulation stage between the sapphire substrate and the n-type GaN layer, while the hexagonal pyramidal microstructure array acts in the surface redirection stage of the light-emitting upper surface of the sapphire substrate. Therefore, the microlens structure can increase the number of photons entering the sapphire substrate and that can be extracted, and the hexagonal pyramidal microstructure array can further adjust the emission direction of this portion of light on the light-emitting upper surface, enabling the Micro-LED chip to improve surface light extraction efficiency and the proportion of effective forward light extraction while maintaining high light extraction efficiency.

[0085] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0086] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A Micro-LED chip with a double-sided microstructured sapphire substrate, characterized in that, The lateral dimension of the light-emitting mesa of the Micro-LED chip is 5μm-20μm. The Micro-LED chip includes a sapphire substrate and a flip-chip GaN-based light-emitting unit disposed on one side of the sapphire substrate. The sapphire substrate has a first surface and a second surface disposed opposite to each other. The first surface faces the flip-chip GaN-based light-emitting unit, and the second surface is the light-emitting upper surface. The flip-chip GaN-based light-emitting unit includes an n-type GaN layer, a multiple quantum well layer, a p-type GaN layer, a transparent conductive layer, and a metal reflective layer sequentially disposed along a direction away from the sapphire substrate. The first surface has a microlens structure located between the sapphire substrate and the n-type GaN layer, wherein the bottom radius of the microlens structure is any value between 1.9 μm and 2.2 μm, and the height is any value between 3.1 μm and 3.4 μm; The second surface is formed with a hexagonal pyramid microstructure array, which includes a plurality of periodically arranged hexagonal pyramid structural units. The base radius of each hexagonal pyramid structural unit is any value between 0.45μm and 0.55μm, and the height is any value between 0.75μm and 0.85μm.

2. The Micro-LED chip with a double-sided microstructured sapphire substrate according to claim 1, characterized in that, The arrangement period of the hexagonal pyramidal microstructure array is less than the bottom radius of the microlens structure, and when projected along the thickness direction of the sapphire substrate, the projection area of ​​one microlens structure corresponds to multiple hexagonal pyramidal structure units.

3. The Micro-LED chip with a double-sided microstructured sapphire substrate according to claim 2, characterized in that, The microlens structure is an arc-shaped structure formed by patterning the first surface of the sapphire substrate, and the hexagonal pyramidal structure unit is a cone-shaped protrusion formed by patterning the second surface of the sapphire substrate.

4. The Micro-LED chip with a double-sided microstructured sapphire substrate according to claim 3, characterized in that, The thickness of the n-type GaN layer is any value between 3.8 μm and 4.8 μm, the thickness of the multiple quantum well layer is any value between 0.05 μm and 0.20 μm, the thickness of the p-type GaN layer is any value between 0.05 μm and 0.20 μm, and the thickness of the transparent conductive layer is any value between 0.05 μm and 0.20 μm.

5. The Micro-LED chip with a double-sided microstructured sapphire substrate according to claim 4, characterized in that, The mesa sidewall of the flip-chip GaN-based light-emitting unit is covered with a SiO2 passivation layer, the thickness of which is any value between 100nm and 300nm.

6. The Micro-LED chip with a double-sided microstructured sapphire substrate according to any one of claims 1-5, characterized in that, The metal reflective layer is an Au reflective layer, which is disposed on the side of the transparent conductive layer away from the p-type GaN layer, so as to reflect light propagating toward the metal reflective layer to the second surface direction of the sapphire substrate.

7. A method for fabricating a Micro-LED chip, characterized in that, The method for preparing the Micro-LED chip according to any one of claims 1-6 comprises the following steps: A sapphire substrate is provided, the sapphire substrate having a first surface and a second surface disposed opposite to each other; The first surface of the sapphire substrate is patterned to form a microlens structure with a first preset bottom radius and a first preset height; A flip-chip GaN-based light-emitting unit is fabricated on one side of the first surface where the microlens structure is formed. The flip-chip GaN-based light-emitting unit includes an n-type GaN layer, a multiple quantum well layer, a p-type GaN layer, a transparent conductive layer, and a metal reflective layer sequentially disposed along a direction away from the sapphire substrate. The second surface of the sapphire substrate is patterned to form a hexagonal pyramidal microstructure array. Each hexagonal pyramidal structural unit in the hexagonal pyramidal microstructure array has a second preset base radius and a second preset height, thereby preparing the Micro-LED chip.

8. The method for fabricating a Micro-LED chip according to claim 7, characterized in that, After fabricating the flip-chip GaN-based light-emitting unit, the flip-chip GaN-based light-emitting unit is subjected to mesa etching to form a light-emitting mesa with a lateral dimension of 5μm-20μm, and a SiO2 passivation layer of a predetermined thickness is formed on the sidewall of the light-emitting mesa.

9. A Micro-LED display device, characterized in that, It includes a driving substrate and a plurality of Micro-LED light-emitting pixels disposed on the driving substrate, wherein at least one of the Micro-LED light-emitting pixels includes a Micro-LED chip according to any one of claims 1-6.

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