Light-emitting device

Abstract

A light-emitting device according to an aspect of the present invention comprises an element body and a lens layer.　The element body includes a light-emitting layer, a light-emitting surface, and a reflective layer that reflects, toward the light-emitting surface, light emitted from the light-emitting layer. The lens layer is provided on the light-emitting surface, extracts prescribed polarized light from emitted light from the light-emitting surface, and has, with respect to the emitted light, a focal length corresponding to the optical path length from the light-emitting layer to the light-emitting surface via the reflective layer.

Description

Light-emitting devices

[0001] The present invention relates to a light-emitting device that emits light in a predetermined polarization state.

[0002] Circularly polarized light is expected to have applications in various fields, including optical communication, 3D displays, magnetic recording devices, and polarization imaging. However, conventional methods of generating circularly polarized light by combining multiple optical elements result in large optical systems, making it difficult to keep up with the latest technological trends such as thinner and higher resolution 3D displays, in-vivo imaging, and miniaturized magnetic recording devices. Therefore, there is a need for the development of a current-driven single-circularly polarized light-emitting element.

[0003] For example, Patent Document 1 discloses an illumination unit in which the light-emitting surface of an LED (Light Emitting Diode) is composed of nanostructures such as photonic crystals and pillar structures that convert emitted light into circularly polarized light. Non-Patent Document 1 discloses a bifocal metalens made of nanostructures having different focal lengths depending on the rotation direction of the incident circularly polarized light.

[0004] Special Publication No. 2022-532642

[0005] T. Shengnan et al., "Dielectric longitudinal bifocal metalens with adjustable intensity and high focusing efficiency" Opt. Express. 27, 680 (2019).

[0006] For example, a known GaN (gallium nitride) semiconductor device is used as an LED, having an n-type semiconductor layer, an active layer, and a p-type semiconductor layer sequentially stacked on a sapphire substrate, and emitting light from the active layer through a transparent electrode layer formed on the surface of the p-type semiconductor layer. In this type of semiconductor device, the distance between the active layer and the light-emitting surface (transparent electrode layer) is about several hundred nanometers. On the other hand, the focal length of the metalens described in Non-Patent Literature 1 is 2 μm to 4 μm, so even if such a metalens is simply provided on the light-emitting surface of the LED, it is not possible to efficiently extract collimated light with the desired polarization characteristics. Furthermore, it is not practical to manufacture LEDs to match the focal length of the metalens, and there is also the problem that it is extremely difficult to manufacture a metalens with a focal length of several hundred nanometers.

[0007] In view of the above circumstances, the object of the present invention is to provide a light-emitting device that can emit collimated light with desired polarization characteristics while easily fabricating a lens layer.

[0008] A light-emitting device according to one embodiment of the present invention comprises an element body and a lens layer. The element body has a light-emitting layer, a light-emitting surface, and a reflective layer that reflects light emitted from the light-emitting layer toward the light-emitting surface. The lens layer is provided on the light-emitting surface, extracts a predetermined polarization from the light emitted from the light-emitting surface, and has a focal length corresponding to the optical path length from the light-emitting layer through the reflective layer to the light-emitting surface with respect to the emitted light.

[0009] According to the above-described light-emitting device, the lens layer has a focal length corresponding to the optical path length from the light-emitting layer through the reflective layer to the light-emitting surface, thereby enabling the emission of collimated light with desired polarization characteristics. Furthermore, since the focal length of the lens layer can be designed to correspond to the optical path length (optical distance) from the light-emitting layer through the reflective layer to the light-emitting surface, the lens layer can be manufactured more easily compared to designing the focal length to correspond to the optical path length (optical distance) when light emitted from the light-emitting layer directly reaches the light-emitting surface.

[0010] The light-emitting layer may be placed between the light-emitting surface and the reflective layer.

[0011] The element body may further include a first conductivity type semiconductor layer formed between the light-emitting layer and the light-emitting surface, and a second conductivity type semiconductor layer formed between the light-emitting layer and the reflective layer, and the reflective layer may be a distributed Bragg reflector (DBR).

[0012] Alternatively, the element body may further include a first conductivity type semiconductor layer formed between the light-emitting layer and the reflective layer, and a second conductivity type semiconductor layer formed between the light-emitting layer and the light-emitting surface, wherein the reflective layer may be an electrode film laminated on the first conductivity type semiconductor layer.

[0013] The element body may be a gallium nitride-based semiconductor element.

[0014] The lens layer may be a metalens composed of nanostructures of dielectric material.

[0015] The lens layer may be configured to extract clockwise or counterclockwise circularly polarized light from the emitted light.

[0016] According to the present invention, it is possible to easily fabricate a lens layer while emitting collimated light with desired polarization characteristics.

[0017] This is a schematic side cross-sectional view showing the configuration of a light-emitting device according to the first embodiment of the present invention. This is a ray path diagram inside the light-emitting device. This is a simulation result showing the normalized electric field strength of right-handed circularly polarized and left-handed circularly polarized light for cases with and without DBR introduction. This shows the light distribution and degree of circular polarization (P) for cases where the focal length of the metalens is 1 μm (without DBR) and 10 μm (with DBR). ＣＰ This is a simulation result showing the respective results. This is a schematic side cross-sectional view showing the configuration of a light-emitting device according to the second embodiment of the present invention. This is a ray path diagram inside the above light-emitting device.

[0018] Embodiments of the present invention will be described below with reference to the drawings.

[0019] <First Embodiment> Figure 1 is a schematic side cross-sectional view showing the configuration of a light-emitting device 100 according to the first embodiment of the present invention. Figure 2 is a ray path diagram inside the light-emitting device 100.

[0020] [Overall Configuration] As shown in Figure 1, the light-emitting device 100 of this embodiment comprises an element body 10 and a lens layer 31.

[0021] (Element Body) The element body 10 is composed of an InGaN-based blue light-emitting diode. The element body 10 has a structure in which a buffer layer 16, a reflective layer 12, an n-type semiconductor layer 13 (first conductivity type semiconductor layer), an active layer 14 (light-emitting layer), a p-type semiconductor layer 15 (second conductivity type semiconductor layer), a transparent conductive film 17, and a p-type electrode layer 19 are sequentially stacked on a substrate 11.

[0022] The substrate 11 is a sapphire substrate with a thickness of 300 μm and a surface orientation (0001).

[0023] The reflective layer 12 is, for example, an AlInN / GaN distributed Bragg reflector (DBR) and is formed on the substrate 11 via a buffer layer 16 by epitaxial growth. The buffer layer 16 is GaN with a thickness of 20 nm and is formed on the substrate 11 by epitaxial growth.

[0024] The n-type semiconductor layer 13 is made of GaN (n-GaN) with a thickness of 1.6 μm and is formed on the reflective layer 12 by epitaxial growth. An n-type electrode layer 18 made of, for example, gold (Au) is connected to the n-type semiconductor layer 13. The n-type electrode layer 18 is formed on the n-type semiconductor layer 13 by, for example, sputtering.

[0025] The active layer 14 has, for example, an InGaN / GaN-based multiple quantum well (MQW) structure. The active layer 14 has a thickness of 75 nm and is formed on the n-type semiconductor layer 13 by epitaxial growth.

[0026] The p-type semiconductor layer 15 is made of GaN (p-GaN) with a thickness of 100 nm and is formed on the active layer 14 by epitaxial growth.

[0027] The transparent conductive film 17 is, for example, an ITO film with a thickness of 30 nm, and is formed on the p-type semiconductor layer 15, for example, by sputtering. The p-type electrode layer 19 is, for example, an electrode film made of gold (Au), and is formed on the transparent conductive film 17, for example, by sputtering. The p-type electrode layer 19 has an opening 19a, and the region of the transparent conductive film 17 exposed through this opening 19a forms the light-emitting surface 10a of the device body 10.

[0028] (Lens layer) The lens layer 31 is provided on the light-emitting surface 10a of the element body 10. The lens layer 31 is a metalens composed of a nanostructure of dielectric material capable of extracting a predetermined polarization (right-handed circular polarization in this embodiment) from the light emitted from the light-emitting surface 10a.

[0029] As the dielectric material constituting the lens layer 31, a material is used that has high transmittance (low absorption) to emitted light and a refractive index of, for example, 1.4 or higher. In this embodiment, GaN, which has a relatively high refractive index and a relatively wide band gap, is used as the dielectric material, but other materials such as ZrO are also used. ２ Si ３ N ４ , HfO ２ SiO ２ These can be adopted.

[0030] The lens layer 31 is composed of an array of nanostructure elements, such as elliptical cylinder-shaped nanostructure elements (GaN nanopillar array), arranged two-dimensionally on the light-emitting surface 10a. In this embodiment, the height, period, ellipticity, orientation (direction), etc., of each nanostructure element are set so that only right-handed circularly polarized light can be collimated and emitted. An example of height is 800 nm, and an example of period is 200 nm.

[0031] The lens layer 31 is formed on the surface of the transparent conductive film 17 that forms the light-emitting surface 10a by sputtering. The patterning method for the lens layer 31 is not particularly limited, and known methods such as photolithography and lift-off methods can be used.

[0032] In the light-emitting device 100 of this embodiment, in which the lens layer 31 is arranged on the transparent conductive film 17, the distance between the lens layer 31 and the active layer 14 is very close, about 200 nm, making it extremely difficult to design a lens layer 31 with such a short focal length.

[0033] Therefore, in this embodiment, the focal length of the lens layer 31 is set to a distance corresponding to the optical path length (see ray L1 in Figure 2) from the active layer 14 through the reflective layer 12 to the light-emitting surface 10a with respect to the emitted light. Specifically, for example, the lens layer 31 is designed so that the focal length for right-handed circularly polarized light is 3.5 μm and the focal length for left-handed circularly polarized light is 0 μm. Examples of design methods for the lens layer 31 include simulations based on the finite-difference time-domain method (FDTD).

[0034] This allows the focal length of the lens layer 31 to be extended to a few micrometers, making it easier to design the lens layer 31 with the active layer 14 as the focal point, and, as will be described later, enabling efficient emission of collimated light with the desired right-handed circular polarization characteristics.

[0035] [Operation] In the light-emitting device 100 of this embodiment, configured as described above, when a forward voltage is applied between the n-type electrode layer 18 and the p-type electrode layer 19, electrons moving from the n-type semiconductor layer 13 toward the active layer 14 and holes moving from the p-type semiconductor layer 15 toward the active layer 14 combine and emit light (light-emitting recombination).

[0036] As shown in Figure 2, the light emitted from the active layer 14 includes synchrotron radiation L1 that is reflected by the reflective layer 12 and incident on the light emission surface 10a, and synchrotron radiation L2 that is directed directly from the active layer 14 towards the light emission surface 10a. A portion of the synchrotron radiation L2 passes through the lens layer 31 and is emitted to the outside as counterclockwise polarized light Lcp. On the other hand, most of the synchrotron radiation L2 is reflected towards the active layer 14 at the interface between the lens layer 31 and the p-type electrode layer 19, then reflected back by the reflector 12 and re-incident to the lens layer 31.

[0037] As described above, the focal length of the lens layer 31 is set to a distance corresponding to the optical path length from the active layer 14 to the light exit surface 10a via the reflective layer 12. Accordingly, clockwise circularly polarized light is extracted by the lens layer 31 from the re-incident light of the radiated light L1 and the radiated light L2, and is emitted to the outside as collimated light Rcp.

[0038] As described above, in the lens layer 31 of the present embodiment, a virtual focal point is set to have a focal length corresponding to the optical path length from the active layer 14 to the light exit surface 10a via the reflective layer 12, so that collimated light having desired polarization characteristics (clockwise circularly polarized light) can be emitted.

[0039] In addition, since the focal length of the lens layer 31 can be designed at a distance (optical distance) corresponding to the optical path length from the active layer 14 to the light exit surface via the reflective layer 12, compared with a case where the focal length is designed at a distance (optical distance) corresponding to the optical path length when light emitted from the active layer 14 directly reaches the light exit surface 10a, the lens layer 31 can be easily manufactured.

[0040] Furthermore, since the lens layer 31 is formed of an inorganic material, a circularly polarized light-emitting element that has a long element lifespan and stably emits light with a high circular polarization degree can be configured.

[0041] The inventors of the present invention designed a GaN metalens having a focal length of 3.5 μm for clockwise circularly polarized light and a focal length of 0 μm for counterclockwise circularly polarized light, placed a 450 nm wavelength dipole light source at the center of an InGaN / GaN MQR (active layer 14), and calculated the light distribution and circular polarization degree of the light.

[0042] FIG. 3 shows the normalized electric field intensities of clockwise circularly polarized light and counterclockwise circularly polarized light for the case where a DBR (reflector 12) is introduced and the case where no DBR is introduced, respectively. In this figure, (A) shows the case without DBR, and (B) shows the case with DBR. In addition, in the figure, I Ｒ represents the normalized electric field intensity of clockwise circularly polarized light, and I Ｌ represents the normalized electric field intensity of counterclockwise circularly polarized light.

[0043] As shown in the figure, when DBR is introduced, it can be clearly observed that only right-handed circularly polarized light is collimated due to the effect of the virtual focus. In addition, it was confirmed that the effective degree of circular polarization with respect to the radiation angle is 0.78, and the proportion of the right-handed circularly polarized light component is extremely high (see FIG. 3(B)).

[0044] Subsequently, FIGS. 4(A) and 4(B) show the light distribution and the degree of circular polarization (P ＣＰ ) when the focal length (f) of the metalens is 1 μm (˜400 nm in GaN, no DBR) and 10 μm (˜4 μm in GaN, with DBR), respectively. As shown in the figure, providing DBR can extend the virtual focal length of the metalens, so collimated light of the right-handed circularly polarized light component can be emitted efficiently (see FIG. 4(B)).

[0045] <Second Embodiment> FIG. 5 is a schematic side cross-sectional view showing the configuration of a light-emitting device 200 according to a second embodiment of the present invention. FIG. 6 is a ray path diagram inside the light-emitting device 200.

[0046] As shown in FIG. 5, the light-emitting device 100 of the present embodiment includes an element body 20 and a lens layer 32.

[0047] The element body 20 is configured of an InGaN-based blue light-emitting diode. The element body 20 has a structure in which a buffer layer 26, an n-type semiconductor layer 23 (first conductivity type semiconductor layer), an active layer 24 (light-emitting layer), a p-type semiconductor layer 25 (second conductivity type semiconductor layer), and a P-type electrode layer 29 (reflective layer) are sequentially stacked on a substrate 21.

[0048] The substrate 21 is a sapphire substrate having a thickness of 300 μm and a (0001) plane orientation. The back surface of the substrate 21 forms the light emission surface 20a of the element body 20.

[0049] The n-type semiconductor layer 23 is made of GaN (n-GaN) with a thickness of 1.6 μm and is formed on the substrate 21 via a buffer layer 26 by epitaxial growth. The buffer layer 26 is made of GaN with a thickness of 20 nm and is formed on the substrate 21 by epitaxial growth. An n-type electrode layer 28 made of, for example, gold (Au) is connected to the n-type semiconductor layer 23. The n-type electrode layer 28 is formed on the n-type semiconductor layer 13 by, for example, sputtering.

[0050] The active layer 24 has, for example, an InGaN / GaN-based multiple quantum well (MQW) structure. The active layer 24 has a thickness of 75 nm and is formed on the n-type semiconductor layer 23 by epitaxial growth.

[0051] The p-type semiconductor layer 25 is GaN (p-GaN) with a thickness of 100 nm and is formed on the active layer 24 by epitaxial growth. The p-type electrode layer 29 is an electrode film made of, for example, gold (Au) and is formed on the p-type semiconductor layer 25 by, for example, sputtering.

[0052] The lens layer 32 is provided on the light-emitting surface 20a of the element body 20. The lens layer 32 is a metalens composed of a nanostructure of dielectric material capable of extracting a predetermined polarization (right-handed circular polarization in this embodiment) from the light emitted from the light-emitting surface 20a. In this embodiment, the focal length of the lens layer 32 is set to a distance corresponding to the optical path length (see ray L3 in Figure 6) from the active layer 24 through the p-type electrode layer 29 to the light-emitting surface 20a with respect to the emitted light.

[0053] The dielectric material constituting the lens layer 32 is the same material as in the first embodiment described above. The lens layer 32 is composed of an array of nanostructure elements, for example, elliptical cylinder-shaped nanostructure elements (GaN nanopillar array) arranged two-dimensionally on the light emission surface 20a. In this embodiment, the height, period, ellipticity, orientation (direction), etc., of each nanostructure element are set so that only right-handed circularly polarized light can be collimated and emitted.

[0054] The lens layer 32 is formed on the back surface of the substrate 21 that forms the light-emitting surface 20a by sputtering. The patterning method for the lens layer 32 is not particularly limited, and known methods such as photolithography and lift-off methods can be used.

[0055] In the light-emitting device 200 of this embodiment, configured as described above, as shown in Figure 6, the light emitted from the active layer 24 includes synchrotron radiation L3 that is reflected by the p-type electrode layer 29 and incident on the light-emitting surface 20a, and synchrotron radiation L4 that is directed directly from the active layer 24 toward the light-emitting surface 20a.

[0056] As described above, the focal length of the lens layer 32 is set to a distance corresponding to the optical path length from the active layer 24 through the p-type electrode layer 29 to the light emission surface 10a. As a result, right-handed circularly polarized light is extracted from the re-incident synchrotron radiation L1 and L2 by the lens layer 31 and emitted to the outside as collimated light Rcp.

[0057] As described above, in the lens layer 32 of this embodiment, a virtual focus is set to have a focal length corresponding to the optical path length from the active layer 24 through the p-type electrode layer 29 to the light emission surface 20a, so that collimated light with the desired polarization characteristics (right-handed circular polarization) can be emitted. Furthermore, since the lens layer 32 is provided on the back surface of the substrate 21, it can be designed relatively easily to obtain the desired lens function at a focal length, thereby facilitating the fabrication of the lens layer 32.

[0058] Although embodiments of the present invention have been described above, it goes without saying that the present invention is not limited to the embodiments described above and can be modified in various ways.

[0059] For example, in the embodiments described above, a light-emitting device that emits right-handed circularly polarized collimated light was used as an example, but the present invention is not limited to this, and can also be applied to light-emitting devices that emit left-handed circularly polarized light, linearly polarized light, and even elliptically polarized light.

[0060] Furthermore, although the above embodiments have described an InGaN-based blue semiconductor light-emitting element (LED) as an example of the element body, the present invention is not limited to this and can be applied to light-emitting elements capable of emitting light in other wavelength ranges. In addition, the element body is not limited to inorganic light-emitting elements, but may also be an organic light-emitting element (OLED).

[0061] 10, 20... Element body 10a, 20a... Light-emitting surface 11, 21... Substrate 12... Reflective layer 13, 23... n-type semiconductor layer 14, 24... Active layer (light-emitting layer) 15, 25... p-type semiconductor layer 29... p-type electrode layer (reflective layer) 31, 32... Lens layer 100, 200... Light-emitting device

Claims

1. A light-emitting device comprising: an element body having a light-emitting layer, a light-emitting surface, and a reflective layer that reflects light emitted from the light-emitting layer toward the light-emitting surface; and a lens layer provided on the light-emitting surface, which extracts a predetermined polarization from the light emitted from the light-emitting surface and has a focal length corresponding to the optical path length from the light-emitting layer through the reflective layer to the light-emitting surface with respect to the emitted light.

2. A light-emitting device according to claim 1, wherein the light-emitting layer is disposed between the light-emitting surface and the reflective layer.

3. A light-emitting device according to claim 2, wherein the element body further comprises a first conductivity type semiconductor layer formed between the light-emitting layer and the light-emitting surface, and a second conductivity type semiconductor layer formed between the light-emitting layer and the reflective layer, wherein the reflective layer is a distributed Bragg reflector (DBR).

4. A light-emitting device according to claim 2, wherein the element body further comprises a first conductivity type semiconductor layer formed between the light-emitting layer and the reflective layer, and a second conductivity type semiconductor layer formed between the light-emitting layer and the light-emitting surface, the reflective layer being an electrode film laminated on the first conductivity type semiconductor layer.

5. A light-emitting device according to claim 1, wherein the element body is a gallium nitride-based semiconductor element.

6. A light-emitting device according to claim 1, wherein the lens layer is a metalens composed of nanostructures of a dielectric material.

7. A light-emitting device according to claim 6, wherein the lens layer extracts clockwise or counterclockwise circularly polarized light from the emitted light.

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