Semiconductor light emitting device and optical coupling device

Abstract

Embodiments relate to semiconductor light-emitting devices and optical coupling devices. The semiconductor light-emitting device of the embodiment comprises a substrate, a first semiconductor layer of a first conductivity type, an active layer, and a second semiconductor layer of a second conductivity type. The substrate has a first energy band gap. The first semiconductor layer is provided on the substrate and has a second energy band gap wider than the first energy band gap. The active layer is provided on the first semiconductor layer and includes a quantum well layer having a third energy band gap narrower than the first and second energy band gaps and a first barrier layer provided between the first semiconductor layer and the quantum well layer. The second semiconductor layer is provided on the active layer. The refractive index of the substrate is greater than the refractive index of the first semiconductor layer, and the refractive index of the first semiconductor layer is the same as or greater than the refractive index of the first barrier layer.

Description

[0001] Related Application

[0002] This application claims priority based on Japanese Patent Application No. 2021-41036 (filing date: March 15, 2021), and the present application incorporates the entire contents of the basic application by reference. Technical Field

[0003] Embodiments relate to semiconductor light emitting devices and optical coupling devices. Background Art

[0004] LEDs (Light Emitting Diodes) using an indium gallium arsenide mixed crystal for a light-emitting layer and silicon photodiodes are widely used as light sources in a wavelength band (900 to 1000 nm) having sensitivity. Summary of the invention

[0005] An object of the present invention is to provide a semiconductor light emitting device and a light coupling device which can improve the light emitting output.

[0006] The semiconductor light-emitting device of the technical solution comprises a substrate, a first semiconductor layer of a first conductivity type, an active layer, and a second semiconductor layer of a second conductivity type. The substrate has a first energy band gap. The first semiconductor layer is provided on the substrate and has a second energy band gap wider than the first energy band gap. The active layer is provided on the first semiconductor layer and includes at least one quantum well layer having a third energy band gap narrower than the first and second energy band gaps, and a first barrier layer provided between the first semiconductor layer and the quantum well layer. The second semiconductor layer is provided on the active layer and has a fourth energy band gap wider than the third energy band gap. The refractive index of the substrate is greater than the refractive index of the first semiconductor layer at the wavelength of light radiated from the active layer; the refractive index of the first semiconductor layer is the same as or greater than the refractive index of the first barrier layer at the wavelength of the light. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 It is a schematic cross-sectional view showing the semiconductor light emitting device according to the first embodiment.

[0008] Figure 2 This is a schematic cross-sectional view showing an active layer of the semiconductor light-emitting device according to the first embodiment.

[0009] Figure 3 This is a graph showing the characteristics of the semiconductor light emitting device according to the first embodiment.

[0010] Figure 4 (a) and Figure 4The (b) is a schematic diagram showing a semiconductor light-emitting device according to the second embodiment.

[0011] Figure 5 It is a graph showing the characteristics of the semiconductor light-emitting device according to the second embodiment. Detailed Embodiments

[0012] Hereinafter, the embodiments will be described while referring to the appended Figure 1 drawings. The same parts in the drawings are given the same reference numerals, and the detailed description thereof is appropriately omitted, and different parts will be described. In addition, the drawings are schematic or conceptual, and the relationship between the thickness and width of each part, the ratio of the sizes between parts, etc. are not necessarily the same as in reality. In addition, even when showing the same part, there are cases where the mutual dimensions and ratios are shown differently in the drawings.

[0013] Furthermore, the X-axis, Y-axis, and Z-axis shown in each drawing are used to explain the arrangement and structure of each part. The X-axis, Y-axis, and Z-axis are orthogonal to each other and represent the X-direction, Y-direction, and Z-direction, respectively. In addition, there are cases where the Z-direction is set as the upper side and the opposite direction is set as the lower side for explanation.

[0014] (First Embodiment)

[0015] Figure 1 It is a schematic cross-sectional view showing a semiconductor light-emitting device 1 according to the first embodiment. The semiconductor light-emitting device 1 is, for example, an LED that emits infrared light with a wavelength of 900 to 1000 nm.

[0016] As Figure 1 shown, the semiconductor light-emitting device 1 includes, for example, a substrate 10, a light-emitting body LB, a first electrode 20, and a second electrode 30. The substrate 10 is, for example, a conductive semiconductor substrate. In addition, the substrate 10 may also be a structure including an insulating substrate and a semiconductor layer provided thereon. The substrate 10 includes, for example, gallium arsenide of the first conductivity type (hereinafter referred to as GaAs). Here, the first conductivity type is set as the n-type and the second conductivity type is set as the p-type for explanation, but the embodiments are not limited thereto.

[0017] The light-emitting body LB is provided on the substrate 10. The light-emitting body LB includes an intermediate layer 11, a first semiconductor layer 13, an active layer 15, a second semiconductor layer 17, and a third semiconductor layer 19.

[0018] The intermediate layer 11 is, for example, a buffer layer of the first conductivity type. The intermediate layer 11 is provided on the substrate 10. The intermediate layer 11 includes, for example, aluminum gallium arsenide mixed crystal (hereinafter referred to as AlGaAs) represented by GaAs or compositional Al x Ga 1－x As (0 < x < 1).

[0019] The first semiconductor layer 13 is, for example, a cladding layer of the first conductivity type. The first semiconductor layer 13 is provided on the intermediate layer 11. The first semiconductor layer 13 contains, for example, AlGaAs. The AlGaAs of the first semiconductor layer 13 has an Al composition ratio x1.

[0020] The active layer 15 includes, for example, at least one quantum well and is provided on the first semiconductor layer 13 .

[0021] The second semiconductor layer 17 is, for example, a cladding layer of the second conductivity type. The second semiconductor layer 17 is provided on the active layer 15. The second semiconductor layer 17 contains, for example, AlGaAs. The AlGaAs of the second semiconductor layer 17 has an Al composition ratio x2. The Al composition ratio x2 is, for example, the same as the Al composition ratio x1 of the first semiconductor layer 13, or greater than the Al composition ratio x1.

[0022] The third semiconductor layer 19 is, for example, a contact layer of the second conductivity type. The third semiconductor layer 19 is provided on the second semiconductor layer 17. The third semiconductor layer 19 includes, for example, GaAs.

[0023] The intermediate layer 11, the first semiconductor layer 13, the active layer 15, the second semiconductor layer 17 and the third semiconductor layer 19 are sequentially epitaxially grown on the substrate 10 using, for example, MOCVD. The light emitting body LB is formed by mesa etching each semiconductor layer epitaxially grown on the substrate 10.

[0024] The first electrode 20 is provided on the third semiconductor layer 19. The first electrode 20 is electrically connected to the third semiconductor layer 19. The first electrode 20 is made of a material having a high reflectivity with respect to light emitted from the active layer 15. The first electrode 20 contains, for example, aluminum (Al) or gold (Au).

[0025] The second electrode 30 is provided on the substrate 10 exposed by mesa etching, for example. The second electrode 30 is electrically connected to the first semiconductor layer 13 via the substrate 10 and the intermediate layer 11. In addition, the second electrode 30 is provided on the intermediate layer 11. For example, the light emitting body LB can also be formed by mesa etching the first semiconductor layer 13, the active layer 15, the second semiconductor layer 17, and the third semiconductor layer 19. In this case, the intermediate layer 11 is exposed after the mesa etching, and the second electrode 30 is formed on the intermediate layer 11. The second electrode 30 contains, for example, aluminum (Al) or gold (Au).

[0026] Figure 2 Schematic cross-sectional view showing the active layer 15 of the semiconductor light emitting device 1 according to the first embodiment. The active layer 15 includes a first quantum well layer WL1 and a second quantum well layer WL2. The embodiment is not limited to this example, and the active layer 15 may include one quantum well layer or three or more quantum well layers.

[0027] The first quantum well layer WL1 and the second quantum well layer WL2 include, for example, indium gallium arsenide mixed crystals (hereinafter referred to as InGaAs) represented by the composition formula In y Ga 1－y As (0 < y < 1).

[0028] As Figure 2 shown, the first quantum well layer WL1 and the second quantum well layer WL2 are arranged in the direction from the first semiconductor layer 13 toward the second semiconductor layer 17, for example, the Z direction.

[0029] The first quantum well layer WL1 is provided between the first semiconductor layer 13 and the second quantum well layer WL2. A first barrier layer BL1 is provided between the first semiconductor layer 13 and the first quantum well layer WL1. A second barrier layer BL2 is provided between the first quantum well layer WL1 and the second quantum well layer WL2.

[0030] The second quantum well layer WL2 is provided between the first quantum well layer WL1 and the second semiconductor layer 17. A third barrier layer BL3 is provided between the second quantum well layer WL2 and the second semiconductor layer 17.

[0031] The first semiconductor layer 13 contains a first conductive type impurity at a higher concentration than the impurity in the first barrier layer BL1. In addition, the second semiconductor layer 17 contains a second conductive type impurity at a higher concentration than the impurity in the third barrier layer BL3.

[0032] The first barrier layer BL1, the second barrier layer BL2, and the third barrier layer BL3 include, for example, aluminum gallium arsenide phosphide mixed crystals (hereinafter referred to as AlGaAsP) represented by the composition formula Al x Ga 1－x As 1－y P y (0 < x < 1, 0 < y < 1). In addition, at least one of the first barrier layer BL1 and the third barrier layer BL3 may contain AlGaAs.

[0033] The InGaAs used in the first and second quantum well layers WL1 and WL2 has a lattice constant larger than that of the GaAs used in the substrate 10 and the intermediate layer 11. In addition, the lattice constant of InGaAs is larger than that of the AlGaAs used in the first semiconductor layer 13 and the second semiconductor layer 17. In addition, AlGaAs has a lattice constant close to that of GaAs.

[0034] The AlGaAsP used in the first to third barrier layers BL1, BL2, and BL3 has a lattice constant smaller than that of GaAs. By using AlGaAsP in the first to third barrier layers BL1, BL2, and BL3, the stress caused by the lattice constant difference between InGaAs and GaAs and AlGaAs is compensated. As a result, in the second quantum well layer WL2 and each layer crystallized and grown thereafter, the occurrence of crystal defects such as transfer can be suppressed. In addition, by controlling the Al composition ratio x and P composition ratio y of AlGaAsP, the lattice distortion of the first and second quantum well layers WL1 and WL2 can be optimized, and the light emission efficiency of the active layer 15 can be improved.

[0035] In the semiconductor light-emitting device 1, by flowing a drive current between the first electrode 20 and the second electrode 30, electrons and holes are injected into the active layer 15. The active layer 15 emits light generated by the radiative recombination of electrons and holes in the first quantum well layer WL1 and the second quantum well layer WL2. The light propagating from the active layer 15 toward the first electrode 20 is reflected by the first electrode 20 and changes its propagation direction to advance toward the substrate 10. As a result, the light emitted from the active layer 15 propagates toward the back surface of the substrate 10 (see Figure 1 ).

[0036] For example, GaAs, AlGaAs, and AlGaAsP have an energy band gap (hereinafter referred to as Eg) wider than that of InGaAs. Therefore, the substrate 10, the intermediate layer 11, the first semiconductor layer 13, the second semiconductor layer 17, and the third semiconductor layer 19 are substantially transparent to the light emitted from the active layer 15. Thus, the light generated in the active layer 15 is emitted from the back surface of the substrate 10 on the side opposite to the light-emitting body LB. In addition, the Eg of GaAs is smaller than the Eg of AlGaAs and AlGaAsP.

[0037] In order to increase the light output of the semiconductor light-emitting device 1, it is desirable that the light is not reflected or scattered during the propagation from the active layer 15 to the back surface of the substrate 10. For example, the refractive index n0 of the substrate 10, the refractive index n1 of the intermediate layer 11, the refractive index n2 of the first semiconductor layer 13, and the refractive index nb of the first barrier layer BL1 preferably have the relationship of n0≥n1>n2≥nb. Furthermore, if the light propagating from the active layer 15 toward the first electrode 20 and reflected by the first electrode 20 is considered, it is more preferable that n3 < nb ≤ n2 < n1 ≤ n0 in the direction from the second semiconductor layer 17 (refractive index n3) toward the back surface of the substrate 10 (-Z direction). In other words, in the propagation path from the active layer 15 to the back surface of the substrate 10, it is desirable to suppress the reflection and scattering of light by reducing the refractive index difference between adjacent semiconductor layers.

[0038] (Example)

[0039] Aluminum gallium arsenic mixed crystal (Al-GaAs) may be used in the first semiconductor layer 13, the second semiconductor layer 17, the first barrier layer BL1, and the third barrier layer BL3. x Ga 1－x As). In the following example, the Al composition ratio of the first semiconductor layer 13 is x1, the Al composition ratio of the second semiconductor layer 17 is x2, and the Al composition ratio of the first barrier layer BL1 and the third barrier layer BL3 is xb. As the Al composition x of AlGaAs increases, the refractive index decreases and Eg becomes wider.

[0040] For example, GaAs of the substrate 10 and the intermediate layer 11 has an Eg of about 1.4 eV. The first and second quantum well layers WL1 and WL2 have an Eg of about 1.1 eV and emit light with a wavelength of 900 to 1000 nm. The difference in Eg between the two is about 0.3 eV. For example, by using AlGaAs having an Eg wider than that of GaAs in the cladding layer (the first semiconductor layer 13 and the second semiconductor layer 17), the difference in Eg between the first and second quantum well layers WL1 and WL2 and the cladding layer can be increased. As a result, the effect of encapsulating carriers (holes) into the active layer 15 can be improved, thereby improving the luminous efficiency of the active layer 15.

[0041] In this example, AlGaAs is used in the first barrier layer BL1 and the third barrier layer BL3 in addition to the first semiconductor layer 13 and the second semiconductor layer 17 as cladding layers, thereby enabling more intentional confinement of carriers in the first and second quantum well layers WL1 and WL2 .

[0042] The substrate 10 is a GaAs substrate, and the intermediate layer 11 is a GaAs buffer layer. The first quantum well layer WL1 and the second quantum well layer WL2 contain In 0.2 Ga 0.8 As. The second barrier layer BL2 includes Al 0.15 Ga 0.85 As 0.9 P 0.1 .

[0043] Figure 3 Graph showing the characteristics of the semiconductor light emitting device 1 according to the first embodiment. The horizontal axis is the Al composition ratio x1 of the first semiconductor layer 13 (n-type cladding layer). The vertical axis is the light output. Figure 3 In FIG. 1 , the light output for the Al composition ratio x1 is represented by using the Al composition ratio xb of the first barrier layer BL1 and the third barrier layer BL3 as a parameter. The Al composition ratio x2 of the second semiconductor layer 17 is 0.5.

[0044] The wavelength of light emitted from the active layer 15 is, for example, 950 nm. The refractive index of GaAs is 3.54. The refractive index of AlGaAs varies from 3.48 (Al composition ratio x = 0.1), 3.47 (Al composition ratio x = 0.15), 3.42 (Al composition ratio x = 0.2), 3.36 (Al composition ratio x = 0.3), 3.31 (Al composition ratio x = 0.4), and 3.25 (Al composition ratio x = 0.5).

[0045] When the Al composition ratio xb=0, that is, when the first barrier layer BL1 and the third barrier layer BL3 are GaAs, n0=n1=nb>n2. That is, the magnitude relationship of the refractive index of the first barrier layer BL1 and the first semiconductor layer 13 is reversed. In this example, the light reflection at the interface between the first semiconductor layer 13 and the active layer 15 becomes larger. In addition, the leakage of holes from the active layer 15 to the first semiconductor layer 13 becomes significant, and the luminous efficiency also decreases. Therefore, the light output is low.

[0046] In the case of the Al composition ratio xb=0.1, the light output is maximum when the Al composition ratio x1 of the first semiconductor layer 13 is 0.1, and the light output decreases as the Al composition ratio x1 of the first semiconductor layer 13 increases from 0.1. This is because the refractive index difference between the first semiconductor layer 13 and the intermediate layer 11 containing GaAs increases, and the light reflection at the interface between the intermediate layer 11 and the first semiconductor layer 13 increases. In addition, if the Al composition ratio x1 is less than 0.1, the energy barrier between the active layer 15 and the first semiconductor layer 13 becomes smaller. Therefore, the leakage of holes from the active layer 15 to the first semiconductor layer 13 becomes significant, the light emission recoupling in the first quantum well layer WL1 and the second quantum well layer WL2 is reduced, and the light output decreases.

[0047] In the case of Al composition ratio xb=0.15, the light output is maximum when the Al composition ratio x1 of the first semiconductor layer 13 is 0.15, and if the Al composition ratio x1 is greater than 0.2, the light output decreases. This is because the refractive index difference between the first semiconductor layer 13 and the intermediate layer 11 containing GaAs becomes larger, and the light reflection at the interface between the first semiconductor layer 13 and the intermediate layer 11 increases. On the other hand, if the Al composition ratio x1 is less than 0.1, the light output decreases due to the leakage of holes from the active layer 15 to the first semiconductor layer 13.

[0048] In the case of Al composition ratio xb=0.2, the light output is maximum when the Al composition ratio x1 of the first semiconductor layer 13 is 0.15 to 0.2, and if the Al composition ratio x1 is greater than 0.2, the light output gradually decreases. This is because the refractive index difference between the first semiconductor layer 13 and the intermediate layer 11 containing GaAs becomes larger, and the light reflection at the interface between the intermediate layer 11 and the first semiconductor layer 13 increases. On the other hand, if the Al composition ratio x1 is less than 0.1, the light output decreases due to the leakage of holes from the active layer 15 to the first semiconductor layer 13.

[0049] Thus, by appropriately controlling the Al composition ratio in the first semiconductor layer 13, the first barrier layer BL1, and the second barrier layer BL2, it is possible to suppress, for example, light reflection at the interface between the first semiconductor layer 13 and the intermediate layer 11, thereby increasing light output. In addition, by setting the Al composition ratio x1 of the first semiconductor layer 13 to be greater than 0.1, it is possible to improve the effect of sealing holes in the active layer 15, thereby increasing light output. For example, by using Figure 3 The Al composition ratio in the region indicated by the dotted line can improve the light output of the semiconductor light emitting device 1. In this region, the Al composition ratio becomes x2>xb≥x1, which can satisfy the relationship of n0≥n1>n2≥nb>n3. In addition, the Eg of the first barrier layer BL1 and the third barrier layer BL3 becomes larger than the Eg of the first semiconductor layer 13, which can improve the carrier confinement effect in the first quantum well layer WL1 and the second quantum well layer WL2.

[0050] (Second embodiment)

[0051] Figure 4 (a) and (b) are schematic diagrams showing an optical coupling device 2 according to the second embodiment. Figure 4 (a) is a perspective view showing the optical coupling device 2 . Figure 4 (b) is along Figure 4 2 is a cross-sectional view of the cutting plane CP shown in (a). The optical coupling device 2 is a so-called photocoupler.

[0052] The optical coupling device 2 includes a substrate 10, a light emitting body LB, a first electrode 20, and a second electrode 30. The light emitting body LB is provided on the substrate 10. The first electrode 20 is provided on the light emitting body LB. The second electrode 30 is provided, for example, on the surface of the substrate 10. The second electrode 30 is provided, for example, at one corner of the surface of the quadrilateral of the substrate 10. The light emitting body LB and the first electrode 20 are provided so as to cover the rest of the substrate 10.

[0053] like Figure 4As shown in (b), the optical coupling device 2 further includes a light receiving element 40. The light receiving element 40 is bonded to the back surface of the substrate 10 opposite to the light emitting body LB via a resin layer 50. The light receiving element 40 is, for example, a silicon photodiode. The resin layer 50 is, for example, a resin layer containing polyimide.

[0054] The resin layer 50 is connected to the back side of the substrate 10 via an insulating film 53. The insulating film 53 is, for example, a silicon oxide film or a silicon nitride film. The insulating film 53 improves the adhesion between the substrate 10 and the resin layer 50. In addition, the insulating film 53 functions as an anti-reflection film between the substrate 10 and the resin layer 50. In addition, the back side of the substrate 10 may also have a concave-convex shape for improving the light extraction efficiency.

[0055] The light receiving element 40 includes a light receiving layer 43 . The light receiving layer 43 is, for example, an epitaxial layer provided on a silicon substrate and is connected to the resin layer 50 .

[0056] The resin layer 50 and the insulating film 53 are provided to the active layer 15 (see Figure 1 ) is substantially transparent to the light emitted from the active layer 15. The light emitted from the active layer 15 passes through the resin layer 50 and the insulating film 53 and enters the light receiving layer 43.

[0057] Since the optical coupling device 2 has a structure in which the light emitting element and the light receiving element are stacked, for example, the mounting area on the circuit board can be reduced. In addition, the distance between the light emitting element and the light receiving element can be further controlled by the thickness of the resin layer 50, thereby reducing the height of the package.

[0058] In the optical coupling device 2, it is desirable to irradiate light uniformly to the light receiving layer 43 of the light receiving element 40. For example, if the light intensity on the surface of the light receiving layer 43 is locally high, the current density in that part may increase, and the output voltage of the light receiving element 40 may become unstable.

[0059] Figure 5 It is a graph showing the characteristics of the optical coupling device 2 according to the second embodiment. Figure 5 express Figure 4 The light intensity distribution on the light receiving surface of the light receiving element 40 in the cross section shown in (b). Figure 5 , a light intensity distribution EM of the optical coupling device 2 and a light intensity distribution CE of a semiconductor light emitting device according to a comparative example are shown.

[0060] Figure 5The light intensity distribution EM and CE shown can be obtained by, for example, a ray tracing method. In this example, the calculation is performed by assuming that the refractive index of the resin layer 50 is 1.7 and the thickness of the resin layer 50 is 10 μm. In addition, in order to obtain the light intensity distribution at a depth of 1 μm from the light receiving surface, the thickness of the light receiving layer 43 is set to 1 μm. In addition, in order to consider the absorption of silicon, the complex refractive index at a wavelength of 950 nm (3.59, 1.19×10 －3 ).

[0061] The light emitting body LB of the optical coupling device 2 has the same structure as the light emitting body LB of the semiconductor light emitting device 1. The refractive indexes of the substrate 10, the intermediate layer 11, the first semiconductor layer 13 and the active layer 15 have the relationship of n0=n1>n2≥nb.

[0062] In contrast, in the semiconductor light-emitting device of the comparative example, the first semiconductor layer 13 has an Al composition x1=0.5. The refractive index of each layer of the substrate 10, the intermediate layer 11, the first semiconductor layer 13, and the active layer 15 has a relationship of n0=n1>nb>n2. That is, in the propagation path of light from the active layer 15 toward the back side of the substrate 10, the refractive index of the first conductive type cladding layer (first semiconductor layer 13) decreases, and the refractive index of the first conductive type buffer layer (intermediate layer 11) increases.

[0063] like Figure 5 As shown, the light intensity distribution CE of the comparative example has two intensity peaks P1 and P2. The intensity peak P1 can be seen on the light receiving surface at a position close to the end of the substrate 10 (GaAs substrate) and below the light emitting body LB. On the other hand, the intensity peak P2 can be seen on the light receiving surface at a position below the second electrode 30.

[0064] These intensity peaks P1 and P2 are considered to be generated by multiple reflections of light at the interface between the intermediate layer 11 and the first semiconductor layer 13 and at the interface between the first semiconductor layer 13 and the active layer 15 , and by reflections on the side surfaces of the substrate 10 .

[0065] In an optical coupling device in which a light emitting element and a light receiving element are stacked, for example, the distance from the substrate 10 to the light receiving element 40 in the Z direction is shorter than the thickness of the substrate 10, and the refractive index of the resin layer 50 is larger than the refractive index (about 1.4) of a general transparent resin and an insulating film. Therefore, it is difficult to reduce the influence of reflection caused by the refractive index difference in the light emitting body LB by optimizing the stacked structure of the light emitting element and the light receiving element.

[0066] In contrast, the intensity peaks P1 and P2 are not seen in the light intensity distribution EM of the optical coupling device 2, and it can be seen that the light reflection in the light emitting body LB is suppressed. As a result, the output voltage of the light receiving element 40 can be stabilized. In addition, by suppressing the reflection and scattering in the propagation path of the light from the active layer 15 to the back side of the substrate 10, the amount of light incident from the substrate 10 to the light receiving element 40 is also increased.

[0067] Several embodiments of the present invention have been described, but these embodiments are provided as examples and are not intended to limit the scope of the invention. These new embodiments can be implemented in various other forms, and various omissions, substitutions, and changes can be made without departing from the scope of the invention. These embodiments and their variations are included in the scope or spirit of the invention, and are included in the invention described in the claims and the scope of their equivalents.

Claims

1. A semiconductor light emitting device, wherein: have: A substrate having a first energy band gap; a first semiconductor layer of the first conductivity type, provided on the substrate, having a second energy band gap wider than the first energy band gap; an active layer provided on the first semiconductor layer, comprising at least one quantum well layer having a third energy band gap narrower than the first energy band gap and the second energy band gap, and a first barrier layer provided between the first semiconductor layer and the quantum well layer; and a second semiconductor layer of the second conductivity type, provided on the active layer, having a fourth energy band gap wider than the third energy band gap; At the wavelength of light emitted from the active layer, the refractive index of the substrate is greater than the refractive index of the first semiconductor layer. At the wavelength of the light, the refractive index of the first semiconductor layer is the same as or greater than the refractive index of the first barrier layer.

2. The semiconductor light emitting device according to claim 1, wherein: At a wavelength of light emitted from the active layer, a refractive index of the second semiconductor layer is smaller than a refractive index of the first semiconductor layer.

3. The semiconductor light emitting device according to claim 1, wherein: It also has a first conductive type intermediate layer, which is arranged between the substrate and the first semiconductor layer and has a fifth energy band gap wider than the third energy band gap. At the wavelength of light emitted from the active layer, the intermediate layer has a refractive index greater than the refractive index of the first semiconductor layer.

4. The semiconductor light emitting device according to claim 3, wherein: The refractive index of the intermediate layer is smaller than the refractive index of the substrate.

5. The semiconductor light emitting device according to claim 1, wherein: The active layer further includes a second barrier layer provided between the quantum well layer and the second semiconductor layer. The second barrier layer includes a material having the same composition as that of the first barrier layer.

6. The semiconductor light emitting device according to claim 1, wherein: The active layer further includes a second barrier layer provided between the quantum well layer and the second semiconductor layer. The second barrier layer includes a material having a different composition from that of the first barrier layer.

7. The semiconductor light emitting device according to claim 1, wherein: The active layer includes a plurality of quantum well layers and a plurality of barrier layers. The plurality of quantum well layers include a first quantum well layer and a second quantum well layer. The plurality of barrier layers include the first barrier layer, the second barrier layer and the third barrier layer. The first quantum well layer is provided between the first semiconductor layer and the second quantum well layer. The second quantum well layer is provided between the second semiconductor layer and the first quantum well layer. The first barrier layer is provided between the first semiconductor layer and the first quantum well layer. The second barrier layer is provided between the second quantum well layer and the second semiconductor layer. The third barrier layer is provided between the first quantum well layer and the second quantum well layer.

8. The semiconductor light emitting device according to claim 7, wherein: The first barrier layer to the third barrier layer include materials having the same composition.

9. The semiconductor light emitting device according to claim 7, wherein: At least one of the first barrier layer and the third barrier layer includes a material having a different composition from that of the second barrier layer.

10. The semiconductor light emitting device according to claim 1, wherein: The substrate comprises gallium arsenide, The first semiconductor layer includes a first aluminum-gallium-arsenic mixed crystal. The first barrier layer includes a second aluminum gallium arsenic mixed crystal, The aluminum composition ratio of the first aluminum gallium arsenic mixed crystal is the same as or smaller than the aluminum composition ratio of the second aluminum gallium arsenic mixed crystal.

11. The semiconductor light emitting device according to claim 10, wherein: The second semiconductor layer comprises a third aluminum-gallium-arsenic mixed crystal; The aluminum composition ratio of the third aluminum gallium arsenic mixed crystal is greater than the aluminum composition ratio of the first aluminum gallium arsenic mixed crystal and the aluminum composition ratio of the second aluminum gallium arsenic mixed crystal.

12. The semiconductor light emitting device according to claim 1, wherein: The active layer further includes a second barrier layer provided between the quantum well layer and the second semiconductor layer. The quantum well layer comprises an indium gallium arsenide mixed crystal. The first barrier layer comprises an aluminum-gallium-arsenic mixed crystal. The second barrier layer includes an aluminum gallium arsenic phosphorus mixed crystal.

13. The semiconductor light emitting device according to claim 1, wherein: The active layer further includes a second barrier layer provided between the quantum well layer and the second semiconductor layer. The quantum well layer comprises an indium gallium arsenide mixed crystal. The first barrier layer and the second barrier layer include aluminum gallium arsenic phosphorus mixed crystal.

14. An optical coupling device, wherein: have: The semiconductor light emitting device according to any one of claims 1 to 13; and A light receiving element configured to receive light radiated from the active layer of the semiconductor light emitting device; The substrate of the semiconductor light emitting device is located between the active layer and the light receiving element.

15. The optical coupling device according to claim 14, wherein: The light receiving element is bonded to the substrate via a resin layer that transmits light emitted from the active layer.

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