Micro LED device

Abstract

A micro-LED with improved emission efficiency and collimation is provided. The micro-LED (100) includes a mesa substrate (110) including a recess; a semiconductor material (120) disposed in the recess including a first surface (121) adjacent to the mesa substrate, a second surface (122) opposite the first surface, and a light emitting region (126) configured to emit light in response to application of a current. The second surface includes a convex surface (127). If an angle of incidence with respect to a normal of the convex surface is less than a critical angle, light emitted by the light emitting region that is incident on the convex surface is transmitted through the convex surface and refracted upon transmission through the convex surface. The light emitting region (126) is located near a focal plane of the convex surface (127). An area of the light emitting region is less than a cross-sectional area of the convex surface in a plane parallel to the light emitting region.

Description

Technical Field

[0001] The present disclosure relates to the field of light emitting diodes (LEDs), and more particularly to a method for improving the luminous efficiency of LEDs. Background Art

[0002] LEDs convert electrical energy into light. In semiconductor LEDs, this typically occurs through electron-hole conversion, when electrons from an n-doped semiconductor layer recombine with holes from a p-doped semiconductor layer. The active region is the primary light-emitting region. Light generated in the quantum wells of an LED is emitted in all directions, but variations in the refractive index at the boundaries of the LED material mean that only light with an incident angle within a critical angle range can be emitted (although some light is still lost due to small Fresnel losses caused by angle variations). If the incident angle is outside this escape range, total internal reflection occurs.

[0003] Conventional cube-shaped LEDs have a long path length for emitted light, as most of the emitted light is outside the critical angle for escape and therefore has high absorption. By reducing the path length within the device, for example by using arrays of micro-LEDs, light absorption as heat can be reduced. This also has the effect of moving the emission region closer to a point source than the large electrodes typically used to maximize light generation. These provide a broad light source, as every point on the surface generates light emitted in all directions; therefore, external optics cannot be effectively used to focus the emitted light. This can be improved by reducing the size of the micro-LED by bringing the emission region closer to the point source; however, this also reduces the amount of light emitted and the luminous flux. Many micro-LEDs operate at current densities far below those at which the external quantum efficiency is at its maximum. Therefore, there is a need to improve optical efficiency while maintaining a good operating current density.

[0004] Some LEDs emit into air, and the emission efficiency depends on the fraction of photons that escape from the LED. The refractive index of the substrate material is typically much higher than that of air, so only light near the normal to the exit surface can escape. Typically, the LED is coupled to a light-collecting device, such as a projection lens. In this case, there are further losses in capturing the light escaping from the LED due to the divergence of the escaping light. The emission efficiency then depends on both the fraction of photons escaping from the LED and the fraction of those photons captured by the light-collecting device. The efficiency of capturing the escaping photons depends on the size of the diverging light angle (the solid angle formed by the half-power beam width of the emitted light) compared to the light-collection angle (the solid angle over which the light-collecting device captures at least half of the available photons). LEDs emit light with an angular distribution that approaches Lambertian emission, with a full-width half-maximum (FWHM) of 120 degrees. The acceptance angle of a lens is determined by its F-number, which for a typical projection lens can be F / 2.5 or F / 3, giving acceptance angles of 11.3° and 9.5°, respectively. Only 2.7% of the light emitted by a Lambertian LED is within the ±9.5° range, so 97.3% of the light is lost. It is clear that there is a need to improve the efficiency of the emission from the LED and to collimate the emitted light.

[0005] One approach to improving emission efficiency is to introduce random nanotextures onto the surface of LEDs, characterized by patterns on the scale of the wavelength of light, leading to chaotic behavior and improved emission efficiency (Applied Physics Letters, Vol. 63, 1993, pp. 2174-2176). Similarly, periodic or non-periodic patterns on the order of the wavelength of light can be introduced onto the emitting surface or internal interfaces of LEDs, where interference effects increase light extraction (U.S. Patent No. 5,779,924 A and U.S. Patent No. 6,831,302 B1). However, roughening can cause multiple internal reflections before light can escape, leading to losses.

[0006] Collimation is typically achieved by relying on secondary optics, which typically consist of an array of microlenses, each aligned with a single micro-LED to collimate the emitted light (e.g., US2009115970, US2007146655, and US2009050905 A1). These must be precisely aligned with the LED array.

[0007] Shaping the sidewalls of an LED can improve manufacturing and increase light extraction (e.g., US Patent No. 7,598,149 B2). Etching a mesa to form a parabolic mesa structure where the active layer resides can also collimate the emitted light (US2015236201 A1 and US2017271557 A1). Light reflects from the inner surface of the mesa and exits the LED from the emission surface opposite the mesa. This approach risks damaging the active layer and it is difficult to achieve a smooth finish when etching the mesa, so roughness on the mesa side of the active layer reduces the possible degree of collimation.

[0008] It is an object of the present invention to provide a cheaper, simpler method of achieving collimation that does not require alignment of secondary optics or complex and expensive manufacturing. Etching a convex lens in the LED material instead of etching a mesa can be performed using conventional photolithography techniques, can avoid damage to the active layer and improve the degree of collimation. SUMMARY

[0009] In this context, there is provided:

[0010] A micro-LED comprising:

[0011] a mesa substrate comprising a recess;

[0012] a semiconductor material disposed in the recess, comprising a first surface adjacent to the mesa substrate, a second surface opposite the first surface, and a light emitting region configured to emit light in response to application of a current;

[0013] wherein the second surface comprises a convex surface;

[0014] wherein if an angle of incidence with respect to a normal to the convex surface is less than a critical angle, light emitted by the light emitting region that is incident on the convex surface is transmitted through the convex surface and refracted upon transmission through the convex surface;

[0015] wherein the light emitting region is located near a focal plane of the convex surface;

[0016] wherein an area of the light emitting region is less than a cross-sectional area of the convex surface in a plane parallel to the light emitting region.

[0017] In this way, by achieving a narrow collimated beam due to refraction of the emitted light, the overall efficiency of the micro-LED coupled to the light collection device can be improved.

[0018] A distance from the light emitting region to the focal plane of the convex surface can be less than 35% of a focal length of the convex surface.

[0019] The distance from the light emitting area to the focal plane of the convex surface can be preferably less than 25%, or more preferably less than 10%.

[0020] Advantageously, the light emitting area is close to the focal plane of the convex surface, such that light transmitted through the convex surface is refracted to be close to parallel to the central axis of the lens.

[0021] The area of the light emitting area can be less than 20% of the cross sectional area of the convex surface.

[0022] The area of the light emitting area can be preferably less than 10% of the cross sectional area of the convex surface, or more preferably less than 5% of the cross sectional area of the convex surface.

[0023] Advantageously, the light emitting area is less than the cross sectional area of the convex surface, such that light transmitted through the convex surface that is emitted from the edges of the light emitting area is refracted to be close to parallel to the central axis of the lens.

[0024] The full width half maximum of light transmitted through the convex surface emitted by the light emitting area can be less than 60 degrees.

[0025] The full width half maximum of light transmitted through the convex surface emitted by the light emitting area can be preferably less than 45 degrees, or more preferably less than 30 degrees, or still more preferably less than 25 degrees.

[0026] A narrower beam of light emitted from the micro-LED increases the proportion of the beam of light captured by a light collection device having a given acceptance angle, thereby increasing the overall efficiency of the micro-LED.

[0027] Optionally, the cross section of the convex surface can be circular.

[0028] The radius of curvature of the convex surface can be greater than the cross sectional radius of the widest part of the convex surface.

[0029] Advantageously, for ease of manufacture, the convex surface should be hemispherical or less than hemispherical.

[0030] The central axis of the light emitting area can be aligned with respect to the central axis of the convex surface.

[0031] Advantageously, aligning the light emitting area with the central axis of the convex surface produces a more uniform beam of light.

[0032] Optionally, there can be a reflective metal layer between the mesa substrate and the semiconductor material.

[0033] In this way, efficiency is improved since light emitted from the light emitting area that does not impinge on the convex surface can be reflected from the reflective surface and can subsequently impinge on the convex surface.

[0034] The light emitting region may be closer to the mesa substrate than to the convex surface.

[0035] Advantageously, the light emitting area is then closer to the focal plane of the convex surface.

[0036] Multiple micro-LEDs can be arranged in an array.

[0037] The micro-LEDs may be arranged in rows and columns.

[0038] In this way, micro-LEDs can form high-resolution displays.

[0039] The current may be applied using a first electrode adjacent to the first surface of the semiconductor material and a second electrode adjacent to the second surface of the semiconductor material.

[0040] A central axis of the first electrode is aligned with a central axis of the convex surface.

[0041] In this way, the central axis of the light emitting area is aligned relative to the central axis of the convex surface.

[0042] The radius of the light emitting area may be defined by the radius of the first electrode.

[0043] Advantageously, the light emitting area can be small to increase collimation and avoid edge effects.

[0044] An area of ​​the first electrode is less than 20% of a cross-sectional area of ​​the convex surface.

[0045] The area of ​​the first electrode may preferably be less than 10% of the cross-sectional area of ​​the convex surface, or more preferably less than 5% of the cross-sectional area of ​​the convex surface.

[0046] Advantageously, the electrodes, and therefore the light emitting area, are smaller than the cross-sectional area of ​​the convex surface, so that light emitted from the edge of the light emitting area that is transmitted through the convex surface is refracted to be nearly parallel to the central axis of the lens. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Specific embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0048] Figure 1 A schematic cross-sectional view of a micro-LED according to the present disclosure is shown.

[0049] Figure 2 A schematic cross-sectional view of a micro-LED with ray traces of emitted light according to the present disclosure is shown.

[0050] Figure 3 A schematic cross-sectional view of a micro LED array according to an embodiment of the present disclosure is shown.

[0051] Figure 4 A shows a schematic perspective view of a micro LED array according to an embodiment of the present disclosure.

[0052] Figure 4 B shows a schematic plan view of a micro LED array according to an embodiment of the present disclosure.

[0053] Figure 5 A shows a schematic cross-sectional view of ray tracing of a micro-LED with the light emitting area at the center of a convex lens.

[0054] Figure 5 B shows a schematic cross-sectional view of ray tracing of a micro-LED at the focal plane of a convex lens in a light-emitting area according to an embodiment of the present disclosure.

[0055] Figure 6A A schematic cross-sectional view illustrating ray tracing of a micro-LED with a light emitting area at the focal plane of a convex lens according to an embodiment of the present disclosure is shown.

[0056] Figure 6B shows the characteristic size ratio of the light emitting area Figure 6A Schematic cross-sectional view of ray tracing of a micro-LED with a larger characteristic size at the focal plane of a convex lens.

[0057] Figure 7A The effect of varying the ratio of the radius of curvature of the lens to the lens height on the full-width half-maximum of the extracted light is shown for a micro-LED according to an embodiment of the present disclosure with an 8 μm pitch and a light emitting area diameter of 2 μm.

[0058] Figure 7B The effect of changing the ratio of the lens curvature radius to the lens height on light extraction efficiency is shown for a micro-LED according to an embodiment of the present disclosure with a pitch of 8 μm and a light emitting area diameter of 2 μm.

[0059] Figure 7C The effect of changing the ratio of the lens's radius of curvature to the lens's height on the coupling efficiency of an F / 1 lens is shown for a micro-LED according to an embodiment of the present disclosure with a pitch of 8 μm and a light-emitting area diameter of 2 μm.

[0060] Figure 7D The effect of changing the ratio of the lens's radius of curvature to the lens's height on the coupling efficiency of an F / 2 lens is shown for a micro-LED according to an embodiment of the present disclosure with a pitch of 8 μm and a light emitting area diameter of 2 μm.

[0061] Figure 7E The effect of changing the ratio of the lens's radius of curvature to the lens's height on the coupling efficiency of an F / 3 lens is shown for a micro-LED according to an embodiment of the present disclosure with a pitch of 8 μm and a light emitting area diameter of 2 μm.

[0062] Figure 8A The effect of varying the radius of curvature of the lens and the size of the light emitting area on the full width half maximum of the extracted light is shown for a micro-LED according to an embodiment of the present disclosure with a pitch of 8 μm.

[0063] Figure 8B The effect of changing the curvature radius of the lens and the size of the light emitting area on the light extraction efficiency is shown for a micro-LED according to an embodiment of the present disclosure with a pitch of 8 μm.

[0064] Figure 8C The effect of changing the curvature radius of the lens and the size of the light emitting area on the F / 1 lens coupling efficiency is shown for a micro-LED according to an embodiment of the present disclosure with a pitch of 8 μm.

[0065] Figure 8D The effect of changing the curvature radius of the lens and the size of the light emitting area on the coupling efficiency of the F / 2 lens is shown for a micro-LED according to an embodiment of the present disclosure with a pitch of 8 μm.

[0066] Figure 8E The effect of changing the curvature radius of the lens and the size of the light emitting area on the coupling efficiency of the F / 3 lens is shown for a micro-LED according to an embodiment of the present disclosure with a pitch of 8 μm.

[0067] Figure 9 A microscope image showing an external appearance view of a micro LED array according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0068] According to an embodiment of the present disclosure, a micro LED 100 is provided. Figure 1 The micro LED 100 includes a mesa substrate 110 having a recess 111 on a first surface thereof. A semiconductor material 120 is disposed on the first surface of the mesa substrate 110. The semiconductor material 120 includes a first surface 121 adjacent to the mesa substrate 110 and a second surface 122 opposite to the first surface 121.

[0069] The semiconductor material 120 also includes a first doped region 123 and a second doped region 124. An interface 125 between the first doped region 123 and the second doped region 124 is configured to emit light when a current is applied. The interface 125 is closer to the first surface 121 than the second surface 122 of the semiconductor material 120. Electrodes 130 and 140 for applying the current are disposed on both sides of the semiconductor material 120 outside the semiconductor material 120. The electrode 130 is disposed on the first surface 121 of the semiconductor material 120 between the mesa substrate 110 and the semiconductor material 120. A central axis of the electrode 130 is aligned with a central axis of the recess 111 of the mesa substrate 110. The electrode 140 is disposed on the second surface 122 of the semiconductor material 120.

[0070] When a current is applied to the electrodes 130 and 140, the portion of the interface 125 that emits light is the portion that is proximate to the electrode 130 and is given by the light emitting region 126. The size and shape of the electrode 130 define the size and shape of the light emitting region 126. Thus, the characteristic dimension of the light emitting region 126 is defined by the characteristic dimension of the electrode 130. In embodiments where the electrode 130 is circular, the characteristic dimension can be the diameter of the circular electrode.

[0071] The second surface 122 of the semiconductor material 120 includes a convex surface 127. A central axis of the convex surface 127 can be aligned with the central axis of the recess 111. A central axis of the light emitting region 126 can be aligned with the central axis of the convex surface 127, and a characteristic dimension of the light emitting region 126 can be less than a widest dimension of the convex surface 127.

[0072] Reference is made to Figure 2 The light emitting region 126 is configured to emit light rays 210 when a current is applied to the electrodes 130 and 140. The light rays are emitted by the entire light emitting region 126, but for clarity of illustration, only three exemplary light rays 210 are shown as being emitted from only a single point. Figure 2 The exemplary light rays 210 are incident on the convex surface 127 and are transmitted as light rays 220.

[0073] Embodiments of the present disclosure can include a plurality of micro-LEDs 100 arranged in an array. A cross-sectional schematic view of a portion of the array 300 is shown in Figure 3 In this embodiment, the mesa substrate 110 has a plurality of recesses 111 on the first surface. The semiconductor material 120 is disposed on the first surface of the mesa substrate 110. The semiconductor material 120 includes a first surface 121 adjacent to the mesa substrate 110 and a second surface 122 opposite the first surface 121. The semiconductor material and the structure associated with an individual recess 111 includes an individual micro-LED 100.

[0074] Within each recess 111, the semiconductor material 120 further includes a first doped region 123 and a second doped region 124. An interface 125 between the first doped region 123 and the second doped region 124 is configured to emit light when an electric current is applied. Electrodes 130 and 140 for applying electric current are disposed on both sides of the semiconductor material 120 outside the semiconductor material 120. The electrode 130 is disposed on the first surface 121 of the semiconductor material 120 and is located between the mesa substrate 110 and the semiconductor material 120. The central axis of each electrode 130 can be aligned with the central axis of the corresponding recess 111 in the mesa substrate 110. The electrode 140 is disposed on the second surface 122 of the semiconductor material 120. The electrodes 140 can be equidistant between adjacent recesses 111 of the mesa substrate 110.

[0075] The second surface 122 of the semiconductor material 120 includes a plurality of raised regions that define a plurality of convex surfaces 127. The central axis of each convex surface 127 can be aligned with the central axis of the concave portion 111 in the mesa substrate 110.

[0076] refer to Figure 4 A-4B shows an array 400 including a plurality of micro-LEDs 100 . Figure 4 A shows a perspective view, Figure 4 B shows a plan view. Electrode tracks 410 may include electrodes 140. Electrode tracks 410 may extend equally between adjacent micro-LEDs 100, with convex surfaces 127 centered between adjacent electrode tracks 410. Electrode 140 may be disposed on second surface 122 of semiconductor material 120.

[0077] Reference again Figure 2 In use, when current is applied using electrodes 130 and 140, light emitting region 126 emits light. If the angle of incidence relative to the normal of convex surface 127 is less than the critical angle, light incident on convex surface 127 is transmitted. Figure 2 127. A transmitted ray 220 is shown in FIG. If the angle of incidence of ray 210 relative to the normal to convex surface 127 is greater than the critical angle, ray 210 undergoes total internal reflection (not shown). The proportion of emitted light that is transmitted benefits extraction efficiency and is greatest when ray 210 is incident on convex surface 127 at normal, as this also reduces Fresnel reflections that occur when the light is refracted.

[0078] If the micro-LED 100 is intended to be coupled to a light collecting device, only the transmitted light rays within the light collecting device's acceptance angle will be captured. Any light rays outside the light collecting device's acceptance angle will be lost. Therefore, collimation of the transmitted light is necessary to improve the overall efficiency of the micro-LED 100, which can be achieved by refracting the transmitted light rays.

[0079] Convex surface 127 acts as a lens for light ray 210, refracting transmitted light ray 220 toward the central axis of the lens. Interface 125 is located near the focal plane of convex surface 127, refracting transmitted light ray 220 parallel to each other and the central axis of the lens. Collimation is maximized, and therefore capture efficiency is maximized.

[0080] The proportion of the emitted light transmitted by the convex surface 127 and the degree of refraction of the transmitted light both depend on the angle of incidence of the light ray 210 relative to the normal of the convex surface 127. Therefore, the transmission and refraction also depend on the distance between the light emitting region 126 and the convex surface 127. Figure 5 A-5B, the cross section of the convex surface 127 forms an arc of circle 510, as shown by the dashed line. The effect of the position of the interface 125 relative to the radius of curvature of the convex surface 127 is shown as follows Figure 5 As shown in A-5B. Figure 5 A shows a configuration that maximizes the transmission of emitted light 210, while Figure 5 B shows a configuration that maximizes the collimation of the transmitted light.

[0081] Figure 5 A shows interface 125 intersecting the center of circle 510. Therefore, the distance from the center of interface 125 to the center of convex surface 127 is equal to the radius of curvature of convex surface 127. A ray emitted from the center of interface 125 is incident on convex surface 127 at the convex surface normal and is therefore transmitted without refraction. Since all light rays 210 are transmitted, emission is maximized, but the light is not collimated.

[0082] Figure 5 B shows an embodiment of the present disclosure. Interface 125 is located near the focal plane of the lens defined by convex surface 127, so that transmitted light rays 220 are refracted to be parallel or nearly parallel to each other. Light rays 220 are refracted so that their angle relative to the central axis of the lens is zero or minimized, and the collimation of transmitted light rays 220 is maximized. Therefore, the light coupling efficiency is maximized.

[0083] As described above, the angle of incidence of light ray 210 on convex surface 127 affects whether the light is transmitted and the degree to which the transmitted light ray 220 is refracted. Therefore, light emitted from a point on the light emitting area 126 that is a finite distance from the central axis of the lens defined by convex surface 127 will experience different transmission and refraction than light emitted from a point on the light emitting area 126 that is on the central axis of the lens. In particular, if the light emitting area 126 is located at the focal plane of the lens, the point on the light emitting area 126 that is on the central axis of the lens is the focus of the lens. Light emitted from the focal point of the lens will undergo refraction so that the transmitted light rays 220 are parallel to each other and to the central axis of the lens. Light emitted from a point on the focal plane of the lens but at a finite distance from the central axis of the lens will also be refracted, but the transmitted light rays 220 will no longer be parallel to the central axis of the lens. For emission points on the focal plane of the lens, the angle between the transmitted light and the central axis increases as the distance between the emission point and the central axis increases. Therefore, it is preferable to reduce the size of the light emitting area 126 so that light emitted from the edge of the light emitting area 126 is refracted to form a smaller angle with the central axis of the lens. This is in Figure 6A-6B It is proved in Figure 6A-6B Two schematic cross-sectional views of a micro-LED 100 are shown. Figure 6A The characteristic size of the light emitting region 126 is less than Figure 6B The characteristic size of the light emitting region 126 is thus Figure 6A The light 610 emitted from the edge of the light emitting area 126 is Figure 6B Compared with the light 620 emitted from the edge of the larger light emitting area 126, the angle with the central axis of the lens after being refracted is smaller.

[0084] Improving the light collection efficiency of the micro-LED 100 is a trade-off between maximizing the extraction efficiency and the light coupling efficiency to the light collection device. Figure 7A-7E It was proved that Figure 7A-7E The effect of varying the radius of curvature of the convex surface 127 for certain embodiments is shown. Results are given for a micro-LED 100 having a total width of 8µm, a depth of 2.2µm for the recess 111, a distance of 2.45µm from the base of the recess 111 to the flat portion of the second surface 122 of the semiconductor material 120, and a characteristic dimension of 2µm for the light emitting region 126. At its widest portion, the diameter A of the convex surface 127 is 6.5µm. The radius of curvature R of the convex surface 127 is varied, and the vertical dimension B of the convex surface 127 is varied from The graph is plotted on the x-axis as R / H, where H is the distance between the center of the convex surface 127 and the light-emitting area 126, and is therefore given by (2.45 + B) µm. If all dimensions are scaled by the same factor, the result remains unchanged.

[0085] Figure 7AThe variation of the full width at half maximum (FWHM) of the emitted light beam is shown. For the narrowest emitted light beam, the light coupling efficiency is the greatest, and therefore the FWHM is the smallest. This corresponds to the light emitting region 126 being located at the focal plane of the convex surface 127. For a lens in air, the focal length is given by

[0086]

[0087] Where f is the focal length, R is the radius of curvature of the lens, and n is the refractive index of the semiconductor material 120 . Figure 7A The results shown in Figure 1 are for a material with a refractive index of n = 2.44. The graph plots FWHM versus R / H, where H is the distance between the center of convex surface 127 and light-emitting region 126. The lower limit for R is when convex surface 127 is hemispherical, corresponding to R = 3.25µm. The minimum FWHM should occur when H = f, resulting in R / H = 0.59. This is visible in the data.

[0088] Figure 7B The change of light extraction efficiency is shown. When the center of the lens is located at the light emitting area 126, the light extraction efficiency should be the maximum value, such as Figure 5 A. This should happen when R / H=1, however, there is a local minimum at R / H=1 because the mesa substrate 110 reflects some light.

[0089] The total efficiency of the micro-LED 100 coupled to the light collecting device is the extraction efficiency ( Figure 7B ) and the FWHM of the emitted beam ( Figure 7A ) is a compromise between. Figure 7C-7E Shown is the total efficiency of three different condenser lenses with different F-numbers. The F-number is the ratio of the focal length of a lens to its diameter. It is a measure of the lens's acceptance angle and is given by θ = arctan(1 / (2F)). A lower F-number corresponds to a larger acceptance angle and, therefore, higher coupling efficiency.

[0090] The highest overall efficiency for each lens occurs at the same R / H value as the minimum FWHM of the extracted light beam. This corresponds to the radius of curvature of the light-emitting region 126 at the focal plane of the convex surface 127. This R / H value does not provide the maximum extraction efficiency. Depending on the intended application, it may be worth sacrificing some extraction efficiency to obtain a narrower beam and maximize overall efficiency; beam collimation is a more important factor for the overall efficiency of light capture than the light extracted from the LED.

[0091] Figure 8A-8E The changes in FWHM, light extraction efficiency, and overall efficiency are also shown. Figure 8A-8E The graph in φ shows the variation of the curvature radius and the characteristic size P of the light emitting region 126 . The size of the light emitting region 126 is defined by the size of the electrode 130 .

[0092] Figure 8A The variation of the FWHM of the emitted light beam is shown. The minimum FWHM occurs at the radius R, which corresponds to the light emitting region 126 being located at the focal plane of the convex surface 127 and is given by R = 3.25µm. In addition, the FWHM decreases by reducing the characteristic size P of the light emitting region 126. This is Figure 6A-6B It is schematically shown in FIG. Figure 8A The graph in also shows that for any given value of the radius of curvature, the FWHM generally decreases as P decreases. Figure 8A As shown, the FWHM is low at the expected R = 3.25µm, but also for R values ​​other than those corresponding to the location of the light-emitting region 126 at the focal plane of the convex surface 127. This is due to Fresnel reflection occurring at the convex surface 127 when light is refracted after being transmitted through the surface. As a result of Fresnel reflection, a portion of the light beam incident on the convex surface 127 (i.e., incident at an angle relative to the surface normal that is greater than zero but less than the critical angle for total internal reflection) is transmitted and refracted, while a portion is reflected at the convex surface 127. The reflected portion is incident on the mesa substrate 110 and reflected from it back toward the convex surface 127, ultimately transmitting through the convex surface 127. Compared to a case where Fresnel reflection does not occur, the extracted light can be collimated with a lower FWHM, but the extraction efficiency is lower and the angular distribution of the light beam is more uneven. As a result, despite the lower FWHM, the overall efficiency is lower than when the radius R corresponding to the light-emitting area 126 is located at the focal plane of the convex surface 127. For example, for R = 9µm and P = 1µm, when Fresnel reflection is considered, the FWHM is 26 degrees (when Fresnel reflection is not considered, it is 76 degrees). However, when Fresnel reflection is considered, only 13% of the emitted light is within the FWHM, so the overall efficiency is very low. For R = 3.25µm and P = 1µm, the light-emitting area 126 is located at the focal plane of the convex surface 127. In this case, when Fresnel reflection is considered, the FWHM is 25 degrees, and 28% of the emitted light is within the FWHM, so the overall efficiency is higher than when R = 9µm. This will Figure 8C - 8E shows, Figure 8C - 8E shows the total efficiency and takes into account both FWHM and extraction efficiency.

[0093] Figure 8B The variation of light extraction efficiency is shown. When the light emitting area 126 is located at the center of the lens, the extraction efficiency should be the maximum value, as shown in FIG. Figure 5 As shown in A. Figure 8B For the system simulated in , this should correspond to a radius of curvature of 4.7 µm. There is some variation from the actual maximum value of R due to reflections of light from the mesa substrate 110. Figure 8BIt is also shown that light extraction efficiency generally increases with decreasing P. This is to be expected because light emitted from a point on the light emitting area 126 located on the central axis of the lens will be incident on the convex surface 127 at a smaller angle than light emitted from a point on the light emitting area located at a finite distance from the central axis of the lens.

[0094] Figure 8C 、 8D Figures 8 and 8E show the total efficiency of an LED when coupled to lenses with three different F numbers. As mentioned above, the total efficiency is a trade-off between light extraction efficiency and the collimation of the extracted light (measured by FWHM). Figure 8C-8E It is shown that the maximum efficiency is obtained when the curvature radius R of the light emitting area 126 is located at the focal plane = 3.25 μm, as shown in FIG. Figure 7C-7E This corresponds to the minimum FWHM but not to the best light extraction efficiency. Figure 8C-8E It is shown that the maximum efficiency is obtained with the smallest possible light emitting area 126, which corresponds to the best FWHM and the best light extraction efficiency. The choice of the radius of curvature is a compromise between reducing the FWHM and improving the light extraction efficiency, and a small light emitting area 126 may be preferable for reducing the FWHM and improving the light extraction efficiency. Therefore, although the number of emitted photons decreases when the size of the light emitting area 126 is reduced, the improvement in extraction efficiency and collimation is sufficient to allow the size of the light emitting area 126 to be reduced, resulting in more photons being captured by the lens. Therefore, when a lens is coupled to the micro-LED 100 of the present disclosure, less electrical power will be required to capture the same number of photons than when coupled to the micro-LED 100 without the collimation from the lens and the smaller light emitting area 126.

[0095] In embodiments of the present disclosure, the distance from the light-emitting region 126 to the focal plane of the convex surface 127 may be less than 35% of the focal length of the convex surface 127. The distance from the light-emitting region 126 to the focal plane of the convex surface 127 may preferably be less than 25%, or more preferably less than 10%. In certain embodiments of the present disclosure, the distance from the light-emitting region 126 to the focal plane of the convex surface 127 is less than 2µm of the focal length of the convex surface 127. The distance from the light-emitting region 126 to the focal plane of the convex surface 127 may preferably be less than 1.5µm, or more preferably less than 0.5µm.

[0096] In an embodiment of the present disclosure, the area of ​​the light-emitting region 126 may be less than 20% of the cross-sectional area of ​​the convex surface 127. The area of ​​the light-emitting region 126 may preferably be less than 10% of the cross-sectional area of ​​the convex surface 127, or more preferably less than 5% of the cross-sectional area of ​​the convex surface 127. In certain embodiments of the present disclosure, the diameter of the light-emitting region 126 may be less than 3 μm. The diameter of the light-emitting region 126 may preferably be less than 2 μm, or more preferably less than 1.5 μm.

[0097] In an embodiment of the present disclosure, the area of ​​the electrode 130 may be less than 20% of the cross-sectional area of ​​the convex surface 127. The area of ​​the electrode 130 may preferably be less than 10% of the cross-sectional area of ​​the convex surface 127, or more preferably less than 5% of the cross-sectional area of ​​the convex surface 127.

[0098] In an embodiment of the present disclosure, the full width half maximum of the light emitted by the light emitting region 126 and transmitted through the convex surface 127 may be less than 60 degrees. The full width half maximum of the light emitted by the light emitting region 126 and transmitted through the convex surface 127 may preferably be less than 45 degrees, or more preferably less than 30 degrees, or even more preferably less than 25 degrees. In some embodiments, the full width half maximum of the light emitted by the light emitting region 126 and transmitted through the convex surface 127 may be 20 degrees.

[0099] The mesa substrate 110 may comprise an oxide material, such as SiO 2 . A thin reflective layer, such as aluminum or silver, may be provided on the surface of the mesa substrate 110 between the mesa substrate 110 and the semiconductor material 120 . The semiconductor material 120 may be GaN. The convex surface 127 may be etched.

[0100] The fabrication process may include photolithography combined with plasma etching. In some embodiments, a 400 nm undoped silicon glass (USG) hard mask and a 1780 nm photoresist may be used. A two-step plasma etch may then be used. This may include using 20% ​​Ar and 30% Cl2 at 720 W for 90 seconds, followed by 20% Ar and 50% Cl2 at 720 W for 140 seconds. This process gives Figure 9 The structure shown.

[0101] The mesa substrate 110 can be shaped by etching, which can damage the edges of the mesa substrate 110. This can adversely affect the internal quantum efficiency (and therefore the external quantum efficiency) of the micro-LED 100. This effect can be limited to the edges of the semiconductor material 120 near the mesa substrate surface. Therefore, the external quantum efficiency is only reduced due to edge effects when the light-emitting region 126 is close to the mesa substrate 110. A parabolic mesa can be narrower near the light-emitting region 126, so the light-emitting region 126 may be susceptible to edge effects. In the present disclosure, the shape of the mesa substrate 110 is not parabolic, and the mesa substrate 110 is designed to protect the micro-LED 100 from degradation caused by mesa etching. The recess 111 of the mesa substrate 110 can be shaped so that the width of the interface 125 between the first doped region 123 and the second doped region 124 is greater than that of the light-emitting region 126, so that the damaged region of the semiconductor material 120 caused by etching does not overlap with the light-emitting region 126.

Claims

1. A micro LED, characterized in that: include: a mesa substrate including a recess; a semiconductor material disposed in the recess, comprising a first surface adjacent to the mesa substrate, a second surface opposite the first surface, and a light emitting region configured to emit light in response to application of an electric current; wherein the second surface comprises a convex surface; wherein if the incident angle relative to the normal of the convex surface is less than a critical angle, light emitted by the light emitting area and incident on the convex surface is transmitted through the convex surface and refracted when transmitted through the convex surface; wherein the light emitting region is located near a focal plane of the convex surface, such that a distance from the light emitting region to the focal plane of the convex surface is less than 35% of a focal length of the convex surface; The area of ​​the light emitting region is smaller than the cross-sectional area of ​​the convex surface in a plane parallel to the light emitting region.

2. The micro-LED according to claim 1, wherein: A distance from the light emitting area to the focal plane of the convex surface is less than 25% of the focal length of the convex surface.

3. The micro-LED according to claim 1, wherein: A distance from the light emitting area to a focal plane of the convex surface is less than 10% of a focal length of the convex surface. 4 . The micro-LED according to claim 1 , wherein an area of ​​the light-emitting region is less than 20% of a cross-sectional area of ​​the convex surface. The micro-LED according to claim 4 , wherein an area of ​​the light-emitting region is less than 10% of a cross-sectional area of ​​the convex surface. 6 . The micro-LED according to claim 1 , wherein a full width half maximum of light emitted by the light emitting region and transmitted through the convex surface is less than 60 degrees.

7. The micro-LED according to any one of claims 1 to 5, wherein the full width half maximum of light emitted by the light emitting area and transmitted through the convex surface is less than 45 degrees. The micro-LED according to claim 1 , wherein a cross-section of the convex surface is circular. 9 . The micro-LED according to claim 1 , wherein a radius of curvature of the convex surface is larger than a cross-sectional radius of a widest portion of the convex surface. 10 . The micro-LED according to claim 1 , wherein a central axis of the light emitting area is aligned with a central axis of the convex surface.

11. The micro-LED according to any one of claims 1 to 5, further comprising a reflective metal layer between the mesa substrate and the semiconductor material. 12 . The micro-LED according to claim 1 , wherein the light emitting region is closer to the mesa substrate than to the convex surface.

13. The micro-LED according to any one of claims 1 to 5, wherein a plurality of micro-LEDs are arranged in an array. The micro-LED array of claim 13 , wherein the micro-LEDs are arranged in rows and columns. 15 . The micro-LED of claim 1 , wherein the current is applied using a first electrode adjacent to the first surface of the semiconductor material and a second electrode adjacent to the second surface of the semiconductor material. The micro-LED according to claim 15 , wherein a central axis of the first electrode is aligned with a central axis of the convex surface. The micro-LED according to claim 16 , wherein a radius of the light emitting area is defined by a radius of the first electrode. The micro LED according to claim 17 , wherein an area of ​​the first electrode is less than 20% of a cross-sectional area of ​​the convex surface. The micro LED according to claim 18 , wherein an area of ​​the first electrode is less than 10% of a cross-sectional area of ​​the convex surface. 20 . The micro-LED according to claim 1 , wherein a cross-sectional area of ​​the recess is larger than an area of ​​the light emitting region.

21. The micro-LED according to claim 20, wherein the edge of the light emitting area is greater than 1 μm away from the edge of the recess.

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