Light-emitting diode element

Abstract

To provide a high-efficiency light-emitting diode element which is capable of efficiently extracting lateral propagation light in an LED and has narrow-angle light distribution characteristics.SOLUTION: A light-emitting diode element 10 comprises: an NPSS substrate 11 having, on a surface, a moth-eye nanostructure 11M in which conical projections are cyclically formed; a first semiconductor layer 21 formed on the moth-eye nanostructure and including a photonic crystal layer 21P; an active layer 23 formed on the first semiconductor layer and including a light-emitting layer 23A; and a second semiconductor layer 25 formed on the active layer.SELECTED DRAWING: Figure 1B

Description

[Technical Field]

[0001] The present invention relates to a light-emitting diode device, and more particularly to a light-emitting diode device having a photonic crystal. [Background technology]

[0002] In recent years, efforts toward autonomous driving of vehicles and moving objects such as automobiles have progressed rapidly, and the requirements for lighting fixtures such as headlamps are also changing. For example, headlamps are expected to have value not as a single lamp but as a lamp system that incorporates sensors and other elements. However, while automobile headlamps sometimes use LED (Light Emitting Diode) elements arranged in parallel to achieve high output, there is a problem in that it is difficult to find space to install sensors and other elements.

[0003] Generally, with LEDs that emit spontaneous light, the light emission distribution is Lambertian, which causes the light distribution to spread, resulting in loss of light that escapes the lens inside the headlamp. In order to reduce light loss and further increase the amount of light, it is necessary to make the lens larger (i.e., increase the numerical aperture (NA)), but this is not desirable from the perspective of headlamp space. In order to increase the amount of LED light taken in without changing the NA of the lens, it is necessary to narrow the light emission distribution angle of the LED light source itself.

[0004] For example, Patent Document 1 discloses that a structure called a moth-eye nanostructure (NPSS: Nano pattern with sapphire substrate) is formed on a sapphire substrate or the like to increase light extraction efficiency.

[0005] Furthermore, it is disclosed that it is possible to extract a portion of light by forming a narrow angle depending on the period of the NPSS. However, the proportion of light that can be formed into a narrow angle by the effect of the NPSS is not high. Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide a highly efficient light emitting diode element having a light distribution characteristic with a highly efficient narrow angle. [Means for solving the problem]

[0007] A light emitting diode device according to one embodiment of the present invention is 、 a substrate having a moth-eye nanostructure on its surface, on which cone-shaped protrusions are periodically formed; a first semiconductor layer formed on the moth-eye nanostructure and having a photonic crystal layer; an active layer formed on the first semiconductor layer and having a light emitting layer; a second semiconductor layer formed on the active layer; With death, The protrusions of the moth-eye nanostructure are arranged in a lattice, and when the period of the protrusions is P, the emission wavelength of the light-emitting layer is λ, and the refractive index of the medium on the light-emitting layer side of the moth-eye nanostructure is ns, the light-emitting diode element satisfies P<λ / ns. do. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-109477 [Brief explanation of the drawings]

[0009] [Figure 1A] 1 is a plan view schematically showing the upper surface of a light-emitting diode element 10 according to a first embodiment of the present invention. [Figure 1B] FIG. 1B is a cross-sectional view schematically showing a cross-sectional structure taken along line AA shown in FIG. 1A. [Figure 2A] FIG. 1 is a diagram showing the dispersion relationship with the wave number kx (= k sin θ) on the horizontal axis and the normalized frequency a / λ on the vertical axis. [Figure 2B] FIG. 10 is an enlarged view of the maximum value portion of TE0. [Figure 3A] FIG. 10 is a diagram showing the experimental results of the dependence of PL detection intensity (vertical axis) on d / a (horizontal axis). [Figure 3B] FIG. 1 shows experimental results of the dependence of PL detection intensity on the lattice constant a. [Figure 4] 1 is a graph showing a comparison of the light distribution of the light-emitting diode 10 of the first embodiment (EMB1, solid line) with the light distribution of an LED of a comparative example (CMP, dashed line). [Figure 5A] FIG. 10 is a diagram showing that in light-emitting diode 10 of the present embodiment, the angle of a laterally propagating low-order mode is narrowed by photonic crystal layer 21P, thereby obtaining a narrow-angle light distribution characteristic. [Figure 5B] FIG. 10 is a diagram showing that the extraction efficiency of laterally propagating low-order mode light emitted from the light-emitting layer is low in the LED (CMP) of the comparative example. [Figure 6A] FIG. 10 is a plan view schematically showing the upper surface of a light-emitting diode 50 according to a second embodiment. [Figure 6B] FIG. 6B is a cross-sectional view schematically showing a cross-sectional structure taken along line AA shown in FIG. 6A. [Figure 7] 10 is a flowchart showing a procedure for determining the structure of a light-emitting diode 50. [Figure 8] FIG. 10 is a diagram showing the results of a waveguide mode simulation. [Figure 9A] 10 is a diagram schematically showing a light intensity distribution FO when a central position AC in the layer thickness direction of the light emitting layer 23A is taken as the x direction and a direction perpendicular to the light emitting layer 23A is taken as the x direction. [Figure 9B] FIG. 10 is a diagram showing the electric field distribution Ey(x) with respect to the position in the x direction. [Figure 10] 10 is a graph showing a difference Δx with respect to the layer thickness Ts of the spacer layer 23B. [Figure 11] 10 is a graph showing a comparison of the light distribution of the light-emitting diode 50 of the second embodiment (EMB2, solid line) with the light distribution of an LED of the comparative example (CMP, dashed line). [Figure 12] FIG. 10 is a plan view schematically showing the upper surface (light emitting surface) of a light emitting diode device 60 according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] In the following, preferred embodiments of the present invention will be described, but these may be modified and combined as appropriate. In the following description and accompanying drawings, substantially the same or equivalent parts are designated by the same reference numerals. [First embodiment] FIG. 1A is a plan view schematically showing the upper surface of a light-emitting diode element 10 according to a first embodiment of the present invention, and FIG. 1B is a cross-sectional view schematically showing the cross-sectional structure taken along line AA shown in FIG. 1A.

[0011] A light-emitting diode element (hereinafter simply referred to as a light-emitting diode) 10 has a moth-eye nanostructure (NPSS: Nano pattern with sapphire substrate) substrate (hereinafter referred to as an NPSS substrate) 11, and a semiconductor light-emitting structure layer (hereinafter referred to as an LED structure layer) 20 provided on the NPSS substrate 11. The LED structure layer 20 has a first semiconductor layer 21 having a photonic crystal layer 21P, an active layer 23, and a second semiconductor layer 25.

[0012] In this embodiment, the LED structure layer 20 is a nitride-based semiconductor layer (GaN system half However, it may be a semiconductor light emitting structure layer of another crystal system that operates as an LED.

[0013] Furthermore, the composition, layer thickness, impurities, doping concentration, etc. of each layer of the semiconductor structure layer shown below are merely examples, and can be appropriately selected, modified, etc. according to the desired characteristics.

[0014] 1A and 1B, the photonic crystal layer 21P is a structural layer having minute air holes 22 periodically formed two-dimensionally in a plane parallel to the first semiconductor layer 21, and having a refractive index that changes periodically.

[0015] The NPSS substrate 11 is a sapphire substrate having a light emitting surface 11S through which light from the LED structure layer 20 is emitted as emitted light LE. The NPSS substrate 11 has protrusions with a moth-eye structure on the surface opposite to the light emitting surface 11S.

[0016] More specifically, the NPSS substrate 11 has a protrusion structure (moth-eye structure) 11M on its surface, in which conical protrusions, each having a period of 200 nm, a height of 150 nm, and a bottom diameter of 120 nm, are periodically formed in a lattice pattern. The protrusions of the moth-eye structure 11M protrude toward the first semiconductor layer 21. The NPSS substrate 11 has a thickness of, for example, 150 μm.

[0017] The protrusions of the moth-eye structure 11M are not limited to the above example. It is sufficient that the protrusions are periodically formed in a lattice pattern. For example, protrusions having a period of 440 nm, a height of 400 nm, and a diameter at the bottom of 320 nm may be periodically formed in a lattice pattern.

[0018] The size and period of the protrusions of the moth-eye structure 11M can be set appropriately depending on the emission wavelength of the LED structure layer 20 and the light distribution pattern of the light-emitting diode 10. The moth-eye structure 11M enables light from the LED structure layer 20 to be extracted to the outside as narrow-angle light.

[0019] An n-GaN layer having a thickness of 5700 nm is provided as a first semiconductor layer 21 on the moth-eye structure 11M of the NPSS substrate 11. Each semiconductor layer on the first semiconductor layer 21 will be described below in the order of lamination.

[0020] The distance between photonic crystal layer 21P provided in first semiconductor layer 21 and the top surface of the protrusion of moth-eye structure 11M is 5740 nm. Holes 22 in photonic crystal layer 21P are arranged with two-dimensional periodicity in a plane parallel to first semiconductor layer 21.

[0021] More specifically, the holes 22 are arranged at square lattice positions, have a cylindrical shape, a lattice constant (period) of 185 nm, a height of 240 nm, and a diameter of 95 nm.

[0022] On photonic crystal layer 21 P, buried layer 21 B is formed to bury photonic crystal layer 21 P. Buried layer 21 B is made of n-GaN, an n-type semiconductor layer, and has a thickness of 120 nm.

[0023] Here, assuming that the lattice constant of the photonic crystal is a, the emission wavelength is λ (in vacuum), and the refractive index of the matrix is ​​neff, the following relationship is satisfied: a≠mλ / neff (m is a natural number) for a square lattice two-dimensional photonic crystal, and a≠mλ×2 / (3 / 2×neff) (m is a natural number) for a triangular lattice two-dimensional photonic crystal. Note that λ here applies not only to the peak wavelength but also to any wavelength λw (in vacuum) within the full width at half maximum of the emission spectrum of the active layer. In other words, if the emission spectrum peaks at 445 nm and the full width at half maximum is 10 nm, λ is considered to be in the range of 440 nm to 450 nm (440 nm≦λw≦450 nm). The present invention is preferably a light-emitting device that emits non-resonant LED light. By forming only photonic crystals that satisfy these conditions, the non-resonant light is narrowed.

[0024] However, even if a = mλ / neff is satisfied in the case of a square lattice two-dimensional photonic crystal, or a = mλ × 2 / (3 / 2 × neff) is satisfied in the case of a triangular lattice two-dimensional photonic crystal, the non-resonant light can be converted into narrow-angle light as long as the supplied current value is equal to or less than a certain value. In this case, by supplying a current value that does not generate resonant light from a power supply (not shown) connected to the light-emitting diode 10, the light-emitting device can emit non-resonant LED light for both narrow-angle light components (emission angle within 20°) and wide-angle light components (emission angle greater than 20°).

[0025] A light emitting layer 23A is formed on the first semiconductor layer 21. The light emitting layer 23A is a five-layer multiple quantum well structure layer (hereinafter referred to as an MQW layer) in which GaN barrier layers and InGaN well layers are alternately stacked.

[0026] A spacer layer 23B is formed on the light emitting layer 23A. The spacer layer 23B is made of GaN and has a thickness of 6 nm.

[0027] A second semiconductor layer 25 is formed on the spacer layer 23B. More specifically, the second semiconductor layer 25 is made of an electron blocking layer 25A and a p-GaN layer 25B, which is a p-type semiconductor layer, formed on the electron blocking layer 25A.

[0028] The electron blocking layer 25A is made of AlGaN and has a thickness of, for example, 10 nm, and the p-GaN layer 25B has a thickness of, for example, 116 nm.

[0029] In this specification, a layer consisting of the light emitting layer 23A and the spacer layer 23B is referred to as an active layer (core layer) 23. The first semiconductor layer 21 and the second semiconductor layer 25 sandwiching the active layer 23 from both sides function as a first cladding layer and a second cladding layer, respectively.

[0030] The first semiconductor layer (first cladding layer) 21 includes a photonic crystal layer 21P, i.e., voids 22, and therefore has a smaller refractive index than the base semiconductor (GaN in this embodiment), and therefore has a smaller effective refractive index than the active layer (core layer) 23.

[0031] In addition, the electron blocking layer 25A is made of a semiconductor (in this embodiment, AlGaN) that has a larger band gap and a smaller refractive index than the active layer 23 (i.e., the spacer layer 23B of the active layer 23), and the effective refractive index of the second semiconductor layer (second cladding layer) 25 is smaller than the effective refractive index of the active layer 23.

[0032] The first semiconductor layer 21 and the second semiconductor layer 25 have a function similar to that of the cladding of an optical fiber, that is, a function of confining light guided through the active layer (core layer) 23. The first semiconductor layer 21 may be composed of a plurality of semiconductor layers having different compositions. The second semiconductor layer 25 may also be composed of a plurality of semiconductor layers having different compositions.

[0033] Furthermore, the first semiconductor layer 21 is an n-type semiconductor layer, and the second semiconductor layer 25 is a p-type semiconductor layer (opposite conductivity type to the first semiconductor layer 21), but the first semiconductor layer 21 and the second semiconductor layer 25 may have an i-layer or an undoped layer. For example, the electron blocking layer 25A in the second semiconductor layer 25 may be configured as an undoped layer.

[0034] A p-electrode 31 is provided on the p-GaN layer 25B. The p-electrode 31 is formed, for example, as a Ni / Au structure (Au is the surface layer) made of Ni (nickel) formed on the p-GaN layer 25B and Au (gold) formed on the Ni.

[0035] The p-electrode 31 can be a metal layer that is in ohmic contact with the p-GaN layer 25B. For example, it can be configured using an ITO electrode with high reflectivity / Ag reflective film. Alternatively, a semiconductor layer (e.g., a highly doped layer) that can easily make ohmic contact with the metal layer may be provided on the p-GaN layer 25B.

[0036] Furthermore, an n-electrode 35 is provided on the exposed surface of first semiconductor layer 21 where buried layer 21B is partially exposed by etching. N-electrode 35 is in ohmic contact with first semiconductor layer 21. N-electrode 35 has, for example, an Al (aluminum) / Pt (platinum) / Au structure (Au is the surface layer).

[0037] 1A, light-emitting diode 10 is placed on a substrate or block provided with a wiring circuit, with p-electrode 31 and n-electrode 35 facing downward. When current is applied between p-electrode 31 and n-electrode 35, light-emitting diode 10 emits light. [Fabrication process of light-emitting diode 10] An example of a process for fabricating the light-emitting diode 10, which is a photonic crystal LED, will be described below.

[0038] First, an NPSS substrate 11 was prepared, in which a moth-eye structure 11M with a period of 356 nm was formed on a sapphire substrate. Then, an n-GaN layer was grown on the moth-eye structure 11M of the NPSS substrate 11 by MOCVD (metal organic chemical vapor deposition).

[0039] Next, a layer of SiO2, SiN, or other material was deposited on the n-GaN layer as a hard mask film using methods such as sputtering or plasma CVD. A resist film was then deposited on the hard mask film, and a square lattice resist pattern with a lattice constant of 185 nm and a hole diameter of 92.5 nm was formed by electron beam lithography.

[0040] Using the resist pattern as a mask, the hard mask was dry-etched with CF (fluorocarbon) gas to form a hard mask. The n-GaN layer was then further etched with chlorine (Cl)-based gas to a depth of 240 nm, forming holes arranged in a square lattice.

[0041] After etching, crystal growth of an n-GaN layer was again performed using the MOCVD apparatus. This crystal growth formed photonic crystal layer 21P having vacancies 22 arranged at square lattice positions, and burying layer 21B burying photonic crystal layer 21P. Note that n-GaN layer 21A was formed between photonic crystal layer 21P and NPSS substrate 11.

[0042] Subsequently, the light emitting layer 23A, the spacer layer 23B, the electron blocking layer (p-cladding layer) 25A, and the p-GaN layer 25B were grown in this order on the burying layer 21B.

[0043] Furthermore, a p-electrode 31 was formed on p-GaN layer 25B using photolithography. Furthermore, buried layer 21B was partially exposed by photolithography and etching, and an n-electrode 35 was formed on the exposed buried layer 21B. [Narrowing angle with NPSS substrate] In the above, the NPSS substrate 11 is described as a sapphire substrate, but the same effect can be achieved regardless of the material as long as it has a tapered protrusion structure with a periodic pyramidal or frustum-shaped structure (hereinafter, these will be collectively referred to as pyramidal shapes). For example, semiconductor substrates such as GaN and Si (silicon) may also be used. In the present invention, materials other than sapphire are also referred to as NPSS for convenience.

[0044] Photonic crystal layer 21P has the function of irradiating narrow-angle light, but a cone-shaped periodic structure that can extract narrow-angle light as is is required for NPSS substrate 11. To achieve this, either (i) a cone-shaped periodic structure that does not diffract light narrowed by the photonic crystal and suppresses reflection at the interface between photonic crystal layer 21P and NPSS substrate 11, or (ii) a cone-shaped periodic structure that diffracts light narrowed by the photonic crystal so that the angle is less than a predetermined angle is required.

[0045] In the following, a case where light narrowed in angle by photonic crystal layer 21P is incident on NPSS substrate 11 will be described. The conditions for (i) to be true are as follows:

[0046] Wavelength λ (wavelength in vacuum), period P of the cone-shaped protrusions on the NPSS substrate 11, incident angle φ (when perpendicular to the NPSS substrate 11: φ=0), medium refractive index n s The condition for no diffraction when the substrate refractive index is n and the order is m is:

[0047]

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[0048]

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[0049] Here, the wavelength λ = 445 nm is used to determine the period at which no diffraction occurs. For example, if a photonic crystal is designed to generate light at φ = 10°, the NPSS assumes a condition where no diffraction occurs at the slightly larger angle of φ = 20°. This is because, when used as a headlamp fixture, for example, the φ = 20° component also contributes significantly to improving output, and so also plays a role in improving the extraction efficiency of some of the light other than the narrow-angle light generated by the action of the photonic crystal.

[0050] GaN (refractive index n s When light is incident on an NPSS structure formed of a refractive index n=2.5) and sapphire (refractive index n=1.8), the period P at which no diffraction occurs is 236 nm or less.

[0051] Assuming that the wavelength λ is 445 nm, the following formula (3) is obtained from the general relationship between the period and the wavelength.

[0052]

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[0053] Next, the conditions under which (ii) holds will be explained.

[0054] The photonic crystal is designed to generate light with a φ=10° angle. This is because, for example, when used as a headlamp fixture, the φ=20° component also contributes greatly to improving output, so it is necessary to extract and improve some of the light other than the narrow-angle light of the photonic crystal. Since both light that has been sandwiched by the photonic crystal and light that has not been sandwiched by the angle are incident on the NPSS, it is assumed that the combined light will be used. If the arrangement of the protrusions in the NPSS structure is a hexagonal lattice arrangement, the angle θ out The condition for diffracted light occurring in the direction is given by the following equation (4).

[0055]

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[0056]

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[0057] Furthermore, the light incident on the NPSS structure contains a reflected component, but this is reflected by the electrodes formed on the opposing surface and then re-enters the NPSS. Because the NPSS reflects light at an angle different from the incident angle, part of the light that re-enters the NPSS is also extracted as narrow-angle light, significantly increasing the output of narrow-angle light. The periodic structure of the NPSS can be arranged with point-like protrusions in a hexagonal lattice arrangement or a square lattice arrangement.

[0058] In this embodiment, the wavelength λ and period P of the NPSS structure are λ=445 nm and period P=440 nm, respectively, and P / λ=0.988, which satisfies formula (4). Therefore, this corresponds to the configuration (ii) above.

[0059] Furthermore, a comparative example without an NPSS structure was created, and the optical output was compared. The comparative example had the same configuration as this embodiment, except that the sapphire substrate had a flat surface without an NPSS structure. As a result, the comparative example output was 150 mW, and this embodiment output was 220 mW, demonstrating an optical output that was 1.5 times higher than that of a flat substrate without a concave-convex structure.

[0060] Next, we will explain the structure of the internal photonic crystal for narrowing the angle and the method for selecting the pattern, using a blue LED with a wavelength λ=445 nm (full width at half maximum 10 nm) as an example. We will calculate the band gap in a photonic crystal, and for information on calculation methods, please refer to references such as "Introduction to Photonic Crystals" (author: Kazuaki Sakoda, Morikita Publishing).

[0061] Photonic crystals have the ability to exclude light of a certain frequency from within the crystal, and the frequency range can be freely set, which is called the photonic band gap.

[0062] Here, the dispersion relation is obtained from photonic band gap calculations. The refractive index of the photonic crystal medium is n = 2.5 (e.g., GaN), the refractive index of the medium (air) of the photonic crystal air holes is n = 1, and the lattice constant a (or period) and the diameter d of the air holes are d / a = 0.5. The dispersion relation of the TE mode (Transverse Electric mode) is calculated when the air holes are arranged in a square lattice.

[0063] Figure 2A shows the dispersion relationship, with the horizontal axis representing the wave number kx (= k sin θ) and the vertical axis representing the normalized frequency a / λ. Here, the diffraction angle is θ, and the wave number k = 2π / λ. Here, we focus on the lowest-order TE0 mode among multiple TE modes.

[0064] 2B is an enlarged view of the maximum value of TE0. As shown in Fig. 2B, matching the normalized frequency to the optical mode edge (white circle in the figure) corresponds to extracting narrow-angle light in the 0° direction.

[0065] Here, let us consider the design of photonic crystal layer 21P, assuming that narrow-angle light is permitted from θ=−10° to 10°. Diffraction lines obtained at θ=−10° and θ=10° are plotted in Fig. 2B.

[0066] A lattice constant design value of 185 nm is obtained from the normalized frequency of 0.415 at the point where the optical mode (TE0) and the diffraction line intersect (black circle in the figure). Therefore, photonic crystal layer 21P can be configured by arranging air holes 22 in a square lattice with air hole diameter d = 92.5 nm and lattice constant a = 185 nm, and arranged in an in-plane area of ​​1 mm square. Note that the air hole arrangement in photonic crystal layer 21P is not limited to a square lattice, and can be appropriately selected from triangular lattices, hexagonal lattices, etc., and obtained using a similar procedure.

[0067] The design value may be determined from the intersection of the diffraction lines and the theoretical value, but may also be determined after actually fabricating a number of photonic crystal structures with slightly different lattice constants and determining the conditions.

[0068] For example, by forming an LED having a photonic crystal and measuring the on-axis PL (Photoluminescence) intensity, it is possible to obtain the optimal values ​​for the lattice constant a and the diameter d of the holes 22 so that the peak intensity value is within the desired range.

[0069] By determining the lattice constant a of the photonic crystal and the diameter d of the air holes 22, the angle of the included angle light by the photonic crystal (the angle at which the intensity peaks) is determined, and in the case of the above-mentioned condition (ii), the period of the NPSS protrusions can also be determined so that θin is slightly wider than the peak angle (peak value + 10°).

[0070] Figure 3A shows the experimental results of the dependence of PL intensity (vertical axis) on d / a (horizontal axis). Figure 3B shows the experimental results of the dependence of PL intensity (vertical axis) on lattice constant a (horizontal axis). The PL intensity reference level (REF) is also shown in Figures 3A and 3B.

[0071] As a result of this research, the experimental results shown in Figure 3A indicate that the PL intensity increases as the d / a ratio increases, and that it is desirable for d / a to be 0.3 or greater (0.3≦d / a). Furthermore, the experimental results shown in Figure 3B indicate that the PL intensity has a peak value depending on the lattice constant a. Based on these experimental results, the lattice constant can be determined to be 190 nm (dashed line in Figure 3B) to maximize the PL intensity.

[0072] As explained above, in the photonic crystal layer 21P of this embodiment, d / a (=hole diameter / lattice constant) is 0.3 or more, and the holes 22 are formed at a period a that extracts narrow-angle light within ±10° (-10°≦θ≦10°) in the TE0 mode.

[0073] In addition, the required height of holes 22 in photonic crystal layer 21P will be explained. Generally, a region with a high optical refractive index (e.g., GaN, n=2.5) such as a light-emitting layer is called a core, and a surrounding region with a low refractive index (n≦2.4) including the photonic crystal layer is called a cladding.

[0074] Considering the waveguide around the light emitting layer, the light intensity in the cladding has an evanescent light distribution that decreases exponentially with distance. For the TE (transverse electric) mode, where the electric field oscillates horizontally in the waveguide, consider the penetration length q of the evanescent light of the mode corresponding to the lowest order TE0 mode. Light intensity I o The intensity distribution I(x) of the evanescent light at a distance x is given by equation (6).

[0075]

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[0076]

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[0077] In other words, the height of the holes 22 is preferably equal to or greater than the seepage length q of the evanescent light in the TE0 mode light from the light emitting layer 23A.

[0078] FIG. 4 is a graph showing a comparison of the light distribution of the light-emitting diode 10 of the first embodiment (EMB1, solid line) with the light distribution of an LED of the comparative example (CMP, dashed line).

[0079] The LED (CMP) of the comparative example differs from the light-emitting diode 10 (EMB1) in that it does not have the photonic crystal layer 21P, but the other configurations are the same as those of the light-emitting diode 10.

[0080] As shown in FIG. 4, in the light-emitting diode 10 of this embodiment, the narrow-angle light component within ±15° is significantly increased compared to conventional LEDs, and it can be seen that a light-emitting diode with narrow-angle light distribution characteristics has been realized.

[0081] As shown in FIG. 5A, in the light-emitting diode 10 of this embodiment, the laterally propagating low-order mode (indicated by the arrow in the figure) is narrowed in angle by the photonic crystal layer 21P, and the narrow-angle light is maintained by the NPSS substrate 11, thereby obtaining a narrow-angle light distribution characteristic.

[0082] On the other hand, as shown in FIG. 5B, the LED (CMP) of the comparative example has an NPSS substrate 11, but does not have a photonic crystal structure in the n-GaN layer 91 between the light-emitting layer 23A and the NPSS substrate 11, and therefore has low extraction efficiency of the lower-order mode light (indicated by the arrow in the figure) emitted from the light-emitting layer and propagating laterally.

[0083] Therefore, according to the light-emitting diode 10 of this embodiment, the photonic crystal layer 21P and the NPSS substrate 11 can efficiently extract the laterally propagating light within the LED, thereby realizing a highly efficient light-emitting diode element with narrow-angle light distribution characteristics. [Second embodiment] Fig. 6A is a plan view schematically showing the upper surface of light-emitting diode 50 according to the second embodiment of the present invention, and Fig. 6B is a cross-sectional view schematically showing the cross-sectional structure taken along line AA shown in Fig. 6A. Also, Fig. 7 is a flowchart showing the procedure for determining the structure of light-emitting diode 50.

[0084] As shown in FIGS. 6A and 6B, in the light-emitting diode 50 of the second embodiment, the LED structure layer 20 has a rectangular parallelepiped shape, and a reflective film 51 is formed to cover all side surfaces of the LED structure layer 20 of the light-emitting diode 50.

[0085] It is sufficient that the reflective film 51 is formed so as to cover at least four side surfaces of the light emitting layer 23A of the LED structure layer 20.

[0086] In an LED element, the region with a high optical refractive index (for example, n=2.5 to 2.7), such as the light-emitting layer, is called the core, and the surrounding region with a low refractive index (for example, n≦2.4), including the photonic crystal portion, is called the cladding.

[0087] In LED elements with a sufficiently thick core film (for example, several tens of microns or more), there are numerous modes (several tens or more) that can be handled by geometric optics. As a result, light with all angular components is emitted from the light-emitting layer and guided into the waveguide, resulting in a wide angular dispersion range of the diffraction angle when extracted. On the other hand, if the film thickness of the core region is limited, only solutions that are acceptable from a wave optics perspective can exist, limiting the number of light modes.

[0088] Here, we calculate the number of modes of light. When approximating it with a three-layer waveguide consisting of a core and cladding, the electric field distribution E of light traveling in the z direction (horizontal direction) among the xyz directions is y (x) is given by equation (8).

[0089]

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[0090]

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[0091]

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[0092] For example, when the core layer thickness is 1 μm, the number of modes can be 6 (TE0 to TE5), when the core layer thickness is 500 nm, the number of modes can be 3 (TE0 to TE2), and when the core layer thickness is 360 nm, the number of modes can be 2 (TE0, TE1).

[0093] By limiting the mode to low-order modes (the lowest mode is TE0), it is possible to reduce the angular variations within the waveguide.

[0094] 8, when the layer thickness is selected in a region where the optical confinement factor is high (especially, TE0 is 90% or more), only a small number of low-order modes (6 or less) propagate within the waveguide. That is, in this embodiment, the core layer is formed under the condition that the number of modes is 6 or less (TE0 to TE5) and the optical confinement factor is 90% or more at TE0.

[0095] For example, when the core layer thickness is 360 nm, the optical confinement factor f reaches 97% at TE0 and 90% at TE1. However, under these conditions alone, loss increases with each propagation through the waveguide, and the light is attenuated.

[0096] In order to increase the recombination luminous efficiency and suppress light attenuation, it is important to align the center of the light-emitting layer 23A with the center of the light intensity distribution. A light intensity distribution simulation was performed to estimate the relationship between the center position of the light intensity distribution and the central coordinates of the light-emitting layer (FIG. 7, step S12).

[0097] FIG. 9A is a diagram schematically showing the light intensity distribution FO when the center position (center line: AC) in the layer thickness direction of the light emitting layer 23A and the direction perpendicular to the light emitting layer 23A (the direction of the NPSS substrate 11) are defined as the x direction.

[0098] Also, FIG. 9B shows the electric field distribution E y 1(x) is a diagram showing the electric field distribution E y The difference Δx in the peak position of (x) is shown schematically.

[0099] The layer thickness is finally determined so as to minimize this difference Δx (FIG. 7, step S13). In this embodiment, the layer thickness of the active layer (core layer) 23 (light emitting layer 23A+spacer layer 23B) and the photonic crystal layer The total thickness of the buried layer 21B on 21P was assumed to be 500 nm, and optimization was performed using this condition as an example.

[0100] FIG. 9B shows the light intensity distribution of TE0 mode light (electric field distribution E y The electric field distribution E y It can be seen that the position FC of the peak of (x) is shifted Δx from the center AC of the light-emitting layer 23A by about 40 nm.

[0101] 10 is a graph showing the difference Δx versus the thickness Ts of the spacer layer 23B. It can be seen that Δx increases as the thickness Ts of the spacer layer 23B increases. Since Δx=0 when the thickness Ts of the spacer layer 23B is approximately 5 to 6 nm, optimization can be achieved at this thickness Ts.

[0102] As described above, by performing a waveguide mode simulation to extract the conditions under which only a small number of low-order modes propagate within the waveguide (FIG. 7, step S11), and then performing a light intensity distribution simulation (FIG. 7, step S12) to match the center of light-emitting layer 23A with the center of the light intensity distribution for the guided light of the low-order mode, particularly TE0 mode light, it is possible to provide a light-emitting diode having high light-emitting efficiency and narrow-angle light distribution characteristics.

[0103] As described above, by forming a core with a high refractive index including a light-emitting layer and a cladding with a low refractive index using the photonic crystal section and other layers such as the electron blocking layer 25A, it is possible to efficiently create propagating light that forms a narrow angle in the photonic crystal section. (reflective film) The reflective film 51 provided on the side surface of the LED structural layer 20 will be described below. Here, we will assume an LED with an emission wavelength λ of 445 nm and an active layer (core layer) 23 including a light-emitting layer 23A with a thickness of 1 μm. As mentioned above, the description will be given taking the case of six TE modes as an example.

[0104] Using the propagation constants calculated from the simulation, the reflection angles when reaching the waveguide end faces (side faces) can be estimated as follows: TE0: 84.9°, TE1: 79.9°, TE2: 74.8°, TE3: 69.6°, TE4: 64.2°, and TE5: 58.8°.

[0105] When these six waveguide modes reach the end face of the waveguide (i.e., the side face of the LED structural layer 20), the incident angle α of the light with respect to the end face is 5.1° for TE0, 10.1° for TE1, 15.2° for TE2, 20.4° for TE3, 25.8° for TE4, and 31.2° for TE5.

[0106] Here, the total reflection angle of light from the semiconductor layer to the air interface is 23.6°, and if the incident angle is equal to or greater than the total reflection angle, the light will be reflected even without a periodic reflective structure. For this reason, it is important that, of the six modes, low-order mode light up to TE3 is reflected by the reflective film 51.

[0107] The reflective film 51 can be realized by forming a dielectric multilayer film such as TiO2 / SiO2 (titanium oxide film / silicon oxide film). If a metal film is used, problems such as absorption loss during reflection occur, so it is important that the film be a dielectric multilayer film.

[0108] Here, the conditions for efficient mode reflection are explained. Consider that the optical mode of the waveguide reaches the edge of the substrate and enters at the above-mentioned incident angle α. If a semiconductor layer with a thickness of d h High refractive index material (refractive index: n h ) and the film thickness d l Low refractive index material (refractive index: n l The condition for the reflected light to be constructively reflected by the multilayer film formed by

[0109]

number

[0110]

number

[0111] Furthermore, n in Equation (11) and Equation (12) h n l Replace with d h d l By replacing it with, the film thickness condition of the low refractive index material can be determined in a similar manner.

[0112] The dielectric multilayer film is basically designed to have a thickness of α=5.1°, focusing on the lowest-order TE0 mode. However, it is also possible to change the incident angle α to match the reflection of multiple lower-order modes such as TE1 (α: 10.1°), TE2 (α: 15.2°), and TE3 (α: 20.4°), and form multiple types of multilayer films.

[0113] Furthermore, the size of the rectangular LED structure layer 20 (width L x , vertical width L y ) is an integer multiple of the wavelength λs in the medium, that is, L x =n x λs, L y =n y λs(n x , n y If you determine it from the above formula (where is an integer), you can get a better amplification effect.

[0114] FIG. 11 is a graph showing a comparison of the light distribution of the light-emitting diode 50 of the second embodiment (EMB2, solid line) with the light distribution of an LED of the comparative example (CMP, dashed line).

[0115] The LED (CMP) of the comparative example differs from the light-emitting diode 50 (EMB2) in that it does not have the photonic crystal layer 21P and the reflective film 51, but the other configurations are the same as those of the light-emitting diode 50.

[0116] 11, in the light-emitting diode 50 of the second embodiment, the narrow-angle light component within ±15° is significantly increased compared to the conventional LED, and the angle is narrowed. Furthermore, it can be seen that a light-emitting diode having a light distribution characteristic that is even narrower than the light-emitting diode 10 of the first embodiment (FIG. 4) has been realized. [Third embodiment] FIG. 12 is a plan view schematically showing the upper surface (light emitting surface) of a light emitting diode device 60 according to the third embodiment of the present invention.

[0117] The light emitting diode device 60 has the same structure as the light emitting diode 50 of the second embodiment, but is configured by arranging a plurality of light emitting diodes 61, each having a small element size, adjacent to each other in a matrix.

[0118] When a photonic crystal is fabricated with a large area, the resistance increases due to the voids in the photonic crystal, which may make it difficult to spread current. In this embodiment, each of the plurality of light-emitting diodes 61 has a small size that allows sufficient current spreading.

[0119] Therefore, each of the light-emitting diodes 61 has uniform and highly efficient light-emitting characteristics. Furthermore, by making the overall size of the light-emitting diode device 60 approximately the same as that of the single light-emitting diode 50 of the second embodiment, for example, it is possible to realize a light-emitting diode device having narrow-angle light distribution and uniform and highly efficient light-emitting characteristics.

[0120] In the above embodiment, the light emitting diode having a photonic crystal layer with a single lattice structure has been described, but the present invention is not limited to this and can also be applied to a light emitting diode having a photonic crystal layer with a multiple lattice structure.

[0121] Furthermore, in the above-described embodiment, a photonic crystal light-emitting diode made of a nitride semiconductor has been described, but the present invention is not limited to this and can also be applied to photonic crystal light-emitting diodes made of semiconductors of other crystal systems.

[0122] Furthermore, the numerical values ​​in the above examples are merely examples and can be appropriately modified and applied depending on the composition of the semiconductor used, the emission wavelength, etc. [Explanation of symbols]

[0123] 10, 50, 61: light-emitting diode element, 11: NPSS substrate, 11M: moth-eye structure, 11S: light-emitting surface, 20: LED structure layer, 21: first semiconductor layer, 21B: buried layer, 21P: photonic crystal layer, 22: holes, 23: active layer, 23A: light-emitting layer, 23B: spacer layer, 25: second semiconductor layer, 25A: electron blocking layer, 25B: p-GaN layer, 31: p-electrode, 35: n-electrode, 51: reflective film, 60: light-emitting diode device, CMP: comparative example

Claims

1. a substrate having a moth-eye nanostructure on its surface, on which cone-shaped protrusions are periodically formed; a first semiconductor layer formed on the moth-eye nanostructure and having a photonic crystal layer; an active layer formed on the first semiconductor layer and having a light emitting layer; a second semiconductor layer formed on the active layer; and the holes of the photonic crystal layer have a cylindrical shape, are arranged at square lattice points, and satisfy the relationship 0.3≦d / a, where a is the lattice constant of the holes and d is the diameter of the holes; Light-emitting diode element.

2. When the lattice constant of the photonic crystal layer is a, the refractive index of the material of the first semiconductor layer is n, and an arbitrary wavelength (in vacuum) within the full width at half maximum of the emission spectrum of the light emitting layer is λ, 2. The light-emitting diode element according to claim 1, wherein the photonic crystal layer has only photonic crystals in a square lattice arrangement that satisfies a≠mλw / neff (m is a natural number), or the photonic crystal layer has only photonic crystals in a triangular lattice arrangement that satisfies a≠mλw×2 / (3 / 2×neff) (m is a natural number).

3. a first low-refractive index layer and a second low-refractive index layer, each having an effective refractive index lower than that of the active layer, are formed to sandwich the active layer; 3. The light-emitting diode device according to claim 1, wherein the photonic crystal layer is the first low refractive index layer.

4. 2. The light-emitting diode element according to claim 1, wherein the first semiconductor layer is made of GaN, and the depth of the holes in the photonic crystal layer is equal to or greater than the seepage length of evanescent light of TE0 mode light from the light-emitting layer.

5. A substrate having a moth-eye nanostructure on its surface, on which cone-shaped protrusions are periodically formed; a first semiconductor layer formed on the moth-eye nanostructure and having a photonic crystal layer; an active layer formed on the first semiconductor layer and having a light emitting layer; a second semiconductor layer formed on the active layer; and the protrusions of the moth-eye nanostructure are arranged in a lattice, and satisfy P<λ / ns, where P is a period of the protrusions, λ is an emission wavelength of the light-emitting layer, and ns is a refractive index of a medium on the light-emitting layer side of the moth-eye nanostructure; The active layer has a spacer layer formed on the light emitting layer, and has a layer thickness that allows only TE0 to TE5 mode light to be guided.

6. A substrate having a moth-eye nanostructure on its surface, on which cone-shaped protrusions are periodically formed; a first semiconductor layer formed on the moth-eye nanostructure and having a photonic crystal layer; an active layer formed on the first semiconductor layer and having a light emitting layer; a second semiconductor layer formed on the active layer; and the protrusions of the moth-eye nanostructure are arranged in a lattice, and satisfy P<λ / ns, where P is a period of the protrusions, λ is an emission wavelength of the light-emitting layer, and ns is a refractive index of a medium on the light-emitting layer side of the moth-eye nanostructure; an LED structure layer including the first semiconductor layer, the active layer, and the second semiconductor layer has a rectangular parallelepiped shape, and a reflective film is formed to cover at least four side surfaces of the light emitting layer; The reflective film is a dielectric multilayer film in which high-refractive-index films and low-refractive-index films are alternately laminated, and when the emission wavelength of the light-emitting layer is λ, the incident angle of TE0 mode light is α, the refractive index of the high-refractive-index film is nh, and the film thickness is dh, A light-emitting diode element in which

7. A substrate having a moth-eye nanostructure on its surface, on which cone-shaped protrusions are periodically formed; a first semiconductor layer formed on the moth-eye nanostructure and having a photonic crystal layer; an active layer formed on the first semiconductor layer and having a light emitting layer; a second semiconductor layer formed on the active layer; and the protrusions of the moth-eye nanostructure are arranged in a lattice, and satisfy P<λ / ns, where P is a period of the protrusions, λ is an emission wavelength of the light-emitting layer, and ns is a refractive index of a medium on the light-emitting layer side of the moth-eye nanostructure; a center position of the light emitting layer coincides with a peak position of the light intensity distribution of TE0 mode light from the active layer;

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