Semiconductor light-emitting element

Abstract

This semiconductor light-emitting element comprises: an active layer that has second semiconductor layers sandwiching a first semiconductor layer, and third semiconductor layers sandwiching the first semiconductor layer and the second semiconductor layers; and fourth semiconductor layers sandwiching the active layer. The first semiconductor layer and the second semiconductor layers form a first quantum well with respect to electrons. The second semiconductor layers, the first semiconductor layer, and the third semiconductor layers form a second quantum well with respect to holes. The third semiconductor layers, the second semiconductor layers, and the fourth semiconductor layers form a third quantum well with respect to electrons. The energy difference between the conduction band minimum of the first semiconductor layer and the conduction band minimum of the third semiconductor layers is less than the quantization energy of the ground-state level in the first semiconductor layer. The wave functions of electrons are coupled between the first quantum well and the third quantum well. The ground-state level energy of pre-coupled electrons formed by the first quantum well is lower than the ground-state level energy of pre-coupled electrons formed by the third quantum well.

Description

Semiconductor light-emitting element

[0001] This invention relates to a semiconductor light-emitting element.

[0002] Semiconductor light-emitting devices are used, for example, as light sources that output signal light in optical communications. In optical communications, high-temperature operation is sometimes required for semiconductor light-emitting devices. Auger recombination of electrons and holes in the active layer of a semiconductor light-emitting device is a limiting factor for high-temperature operation of semiconductor light-emitting devices. In contrast, Non-Patent Literature 1 discloses that by using a quantum well formed by a type II heterojunction structure (hereinafter sometimes referred to as a type II quantum well) and appropriately adjusting the band offset of the conduction band and valence band, the reduction of Auger recombination can be theoretically predicted. A type II heterojunction is also called a staggered heterojunction.

[0003] Patent documents 1 and 2 disclose a technique for increasing the overlap integral of the wave functions of holes and electrons by coupling the wave functions of electrons in multiple type II quantum wells, thereby increasing the transition probability and, consequently, the gain.

[0004] Japanese Patent Publication No. 2000-164990 Japanese Patent Publication No. 2004-200463

[0005] GG Zegrya and A. D. Andreev "Mechanism of suppression of Auger recombination processes in type-II heterostructures" Applied Physics Letters vol.67, Issue 18, pp2681-2683, 1995

[0006] However, if the electron wavefunctions are coupled between multiple electron well layers, energy levels that do not contribute to stimulated emission may be formed, making it difficult to obtain population inversion. In this case, the threshold current of the semiconductor light-emitting device may increase.

[0007] The present invention has been made in view of the above, and aims to provide a semiconductor light-emitting element that suppresses the increase in threshold current and has good high-temperature operating characteristics.

[0008] One aspect of the present invention comprises a substrate, an active layer disposed on the substrate, having a first semiconductor layer, two second semiconductor layers disposed in contact with the first semiconductor layer and sandwiching the first semiconductor layer, and two third semiconductor layers, each in contact with each of the second semiconductor layers and sandwiching the first and second semiconductor layers, and two fourth semiconductor layers disposed on the substrate, each in contact with each of the third semiconductor layers and sandwiching the active layer, wherein the conduction band of the first semiconductor layer is The energy is lower than the conduction band lower edge energy of the second semiconductor layer, and the valence band upper edge energy of the first semiconductor layer is lower than the valence band upper edge energy of the second semiconductor layer, so that the first semiconductor layer and the second semiconductor layer form a type II heterojunction. The energy is higher than the conduction band lower edge energy of the third semiconductor layer, and the valence band upper edge energy of the second semiconductor layer is higher than the valence band upper edge energy of the third semiconductor layer, so that the second semiconductor layer and the third semiconductor layer form a type II heterojunction. The third semiconductor layer is formed such that the lower edge energy of the conduction band of the third semiconductor layer is lower than the lower edge energy of the conduction band of the fourth semiconductor layer, and the first and second semiconductor layers form a first quantum well with respect to electrons, with the first semiconductor layer as the well layer and the second semiconductor layer as the barrier layer, and the second semiconductor layer, the first semiconductor layer and the third semiconductor layer form a second quantum well with respect to holes, with the second semiconductor layer as the well layer and the first and third semiconductor layers as the barrier layer, and the third semiconductor layer, the second semiconductor layer and the fourth semiconductor layer form a first quantum well with respect to electrons This semiconductor light-emitting element forms a third quantum well with a third semiconductor layer as the well layer and the second and fourth semiconductor layers as barrier layers, the energy difference between the lower end of the conduction band of the first semiconductor layer and the lower end of the conduction band of the third semiconductor layer is smaller than the quantization energy of the ground state in the first semiconductor layer, the electron wavefunction is coupled between the first quantum well and the third quantum well, and the energy of the ground state of electrons before coupling formed by the first quantum well is lower than the energy of the ground state of electrons before coupling formed by the third quantum well.

[0009] The energy difference between the conduction band lower end of the first semiconductor layer and the conduction band lower end of the third semiconductor layer may be 270 meV or less.

[0010] The substrate is made of InP, and the first semiconductor layer is Ga ｘ In （１－ｘ） As ｙ P （１－ｙ） (where 0<x<1, 0<y<1), the second semiconductor layer may be made of Al ｒ Ga ｓ In （１－ｒ－ｓ） As (where 0<r<1-s, 0≦s<1), and the third semiconductor layer is made of Ga ｐ In （１－ｐ） As ｑ P （１－ｑ） (where 0<p<1, 0<q<1).

[0011] The lattice constant of the second semiconductor layer may be larger than the lattice constant of the substrate.

[0012] The lattice constant of the first semiconductor layer is smaller than the lattice constant of the substrate, the lattice constant of the third semiconductor layer is smaller than the lattice constant of the substrate, and the layer thickness weighted average strain in the laminated structure of the first semiconductor layer, the second semiconductor layer, and the third semiconductor layer may be less than 2%.

[0013] The fourth semiconductor layer may be made of Al ｖ In （１－ｖ） As ｗ P （１－ｗ） (where 0<v<1, 0<w<1).

[0014] The layer thickness of the first semiconductor layer may be larger than the layer thickness of the third semiconductor layer.

[0015] The layer thickness of the first semiconductor layer may be 120% or more and 550% or less of the layer thickness of the third semiconductor layer.

[0016] The conduction band lower end energy of the third semiconductor layer may be higher than the conduction band lower end energy of the first semiconductor layer.

[0017] The energy difference between the conduction band lower end of the third semiconductor layer and the conduction band lower end of the first semiconductor layer may be 27 meV or more and 270 meV or less.

[0018] The lower end energy of the conduction band of the third semiconductor layer may decrease toward the second semiconductor layer.

[0019] The thickness of the first semiconductor layer may be a thickness at which the wave functions of holes in the two second quantum wells do not couple to each other.

[0020] The thickness of the first semiconductor layer is 5 nm or more and 550% or less of the thickness of the third semiconductor layer, and the wave functions of holes in the two second quantum wells do not need to couple to each other.

[0021] According to the present invention, an increase in threshold current is suppressed, and a semiconductor light-emitting device with excellent high-temperature operation characteristics can be realized.

[0022] Figure 1 is a schematic diagram showing an example of the band structure and wave function of a W-type type II quantum well. Figure 2 is a schematic diagram showing an example of the relationship between the number of electron well layers and the overlap integral of the wave function between ground levels in a coupled type II quantum well. Figure 3 is a schematic diagram showing an example of the relationship between the number of electron well layers and the energy of each level in the electron well layer where the wave function is coupled. Figure 4 is a schematic diagram showing an example of the band structure and wave function of a VVV-type type II quantum well. Figure 5A is a schematic diagram showing the coupled wave function under the conditions of Figure 4. Figure 5B is a schematic diagram showing the coupled wave function under the conditions of Figure 4. Figure 5C is a schematic diagram showing the coupled wave function under the conditions of Figure 4. Figure 6 is a schematic cross-sectional view of a semiconductor light-emitting element according to Embodiment 1. Figure 7A is a schematic diagram showing an example of the laminated structure and band structure of the portion including the active layer of the semiconductor light-emitting element according to Embodiment 1. Figure 7B is a schematic diagram specifically showing the band structure. Figure 8 is a schematic diagram showing an example of the relationship between the layer thickness (well thickness) of the electron well layer and the energy of the ground state from the vacuum level. Figure 9A is a schematic diagram showing the probability density of electrons at the ground state when the well thickness ratio is different. Figure 9B is a schematic diagram showing the probability density of electrons at the ground state when the well thickness ratio is different. Figure 9C is a schematic diagram showing the probability density of electrons at the ground state when the well thickness ratio is different. Figure 10A is a schematic diagram showing the probability density of electrons at the second quantum level when the well thickness ratio is different. Figure 10B is a schematic diagram showing the probability density of electrons at the second quantum level when the well thickness ratio is different. Figure 10C is a schematic diagram showing the probability density of electrons at the second quantum level when the well thickness ratio is different. Figure 11A is a schematic diagram showing the probability density of electrons at the third quantum level when the well thickness ratio is different. Figure 11B is a schematic diagram showing the probability density of electrons at the third quantum level when the well thickness ratio is different. Figure 11C is a schematic diagram showing the probability density of electrons in the third quantum level for different well thickness ratios. Figure 12 is a schematic diagram showing an example of the energy of the ground level, second quantum level, and third quantum level from the vacuum level when the well thickness ratio is continuously varied. Figure 13 is a schematic diagram showing an example of the energy difference and overlap integral between the ground level and the second and third quantum levels when the well thickness ratio is continuously varied.Figure 14 is a schematic diagram showing a specific example of the band structure and wave function of the active layer and barrier layer in a semiconductor light-emitting device according to Embodiment 1. Figure 15A is a schematic diagram showing an example of the laminated structure and band structure of the portion including the active layer of a semiconductor light-emitting device according to Embodiment 2. Figure 15B is a schematic diagram specifically showing the band structure. Figure 16 is a schematic diagram showing an example of the relationship between the conduction band lower edge energy of the electron well layer from the vacuum level and the energy of the ground level from the vacuum level. Figure 17A is a schematic diagram showing the probability density of electrons at the ground level when the conduction band lower edge energies differ between electron well layers. Figure 17B is a schematic diagram showing the probability density of electrons at the ground level when the conduction band lower edge energies differ between electron well layers. Figure 17C is a schematic diagram showing the probability density of electrons at the ground level when the conduction band lower edge energies differ between electron well layers. Figure 18A is a schematic diagram showing the probability density of electrons at the second quantum level when the conduction band lower edge energies differ between electron well layers. Figure 18B is a schematic diagram showing the probability density of electrons in the second quantum level when the conduction band lower energy differs between electron well layers. Figure 18C is a schematic diagram showing the probability density of electrons in the second quantum level when the conduction band lower energy differs between electron well layers. Figure 19A is a schematic diagram showing the probability density of electrons in the third quantum level when the conduction band lower energy differs between electron well layers. Figure 19B is a schematic diagram showing the probability density of electrons in the third quantum level when the conduction band lower energy differs between electron well layers. Figure 19C is a schematic diagram showing the probability density of electrons in the third quantum level when the conduction band lower energy differs between electron well layers. Figure 20 is a schematic diagram showing an example of the energy difference and overlap integral between the ground level, the second quantum level, and the third quantum level when the energy difference of the conduction band lower energy is continuously varied. Figure 21 is a schematic diagram showing an example of the energy difference and overlap integral between the ground state, the second quantum level, and the third quantum level when the energy difference of the lower edge energy of the conduction band is continuously varied. Figure 22 is a schematic diagram showing a specific example of the band structure and wave function of the active layer and barrier layer in a semiconductor light-emitting device according to Embodiment 2. Figure 23 is a diagram illustrating the conditions under which the wave functions are coupled. Figure 24 shows that equation (3) holds, and k. ｓｉｄｅThis figure shows an example where is an imaginary number. Figure 25 shows that equation (3) does not hold, and k ｓｉｄｅ This figure shows an example where is a negative real number. Figure 26 is a schematic diagram showing an example of the relationship between the well thickness of the electron well layer and the quantization energy. Figure 27A compares the case where the well layer has a constant composition within the layer and the case where it has a graded composition layer. Figure 27B compares the case where the well layer has a constant composition within the layer and the case where it has a graded composition layer. Figure 28 is a schematic diagram showing a specific example of the band structure and wave function of the active layer and barrier layer in a semiconductor light-emitting device to which a graded composition layer is applied. Figure 29 is a schematic diagram showing a specific example of the band structure and wave function of the active layer and barrier layer in a semiconductor light-emitting device to which a graded composition layer is applied, similar to Figure 28 except that a graded composition layer is not applied. Figure 30 is a schematic diagram showing a specific example of the wave function of a hole. Figure 31 is a schematic diagram showing the relationship between the distance from the hole well end in the vertical direction and the relative probability density of holes in the case of Table 9.

[0023] Embodiments will be described below with reference to the drawings. However, this embodiment does not limit the present invention. Furthermore, in the drawings, the same or corresponding elements are denoted by the same reference numerals as appropriate, and redundant explanations are omitted as appropriate. It should also be noted that the drawings are schematic, and the dimensional relationships and ratios of each element may differ from reality. Even between drawings, there may be parts where the dimensional relationships and ratios differ.

[0024] (Coupled Type II Quantum Well) To describe the embodiment, we will first explain the characteristics of the coupled Type II quantum well. Figure 1 is a schematic diagram showing an example of the band structure and wave function of a W-type Type II quantum well. The horizontal axis shows the relative position of the quantum well in the vertical direction (stacking direction), and the vertical axis shows the energy from the vacuum level. In a W-type Type II quantum well, a hole well layer is located in the center, two electron well layers are located adjacent to the hole well layer and sandwiching the hole well layer, and two barrier layers are located adjacent to the electron well layer and sandwiching the hole well layer and the electron well layer.

[0025] Assuming the composition and thickness of the hole well layer, electron well layer, and barrier layer are set as shown in Table 1, the energy at the bottom of the conduction band Ec, the energy at the top of the valence band Ehh, the electron wave function φc, and the hole wave function φh are shown as illustrated. For the wave functions, the relative values ​​of the real part of the wave function are shown relative to the energy of the quantum level to facilitate visual understanding. Hereafter, when the wave functions of electrons and holes are shown simultaneously, the relative values ​​of the real part of the wave function are shown similarly relative to the energy of the quantum level to facilitate visual understanding.

[0026]

[0027] As shown in the diagram, the electron wavefunctions are coupled between the two electron wells. Furthermore, the energy difference between the ground level H1 in the valence band and the ground level E1 in the conduction band is 946.5 meV. This energy difference corresponds to a wavelength of 1310 nm, so this W-type type II quantum well emits light at a wavelength of 1310 nm. Additionally, the overlap integral of the hole-electron wavefunctions is 47.9%, which is less than 50%.

[0028] Quantum wells in which wave functions are coupled, such as W-type type II quantum wells, are called coupled type II quantum wells. Figure 2 is a schematic diagram showing an example of the relationship between the number of electron well layers and the overlap integral of wave functions between ground levels in a coupled type II quantum well. In the example shown, as in Table 1, the electron well layers are made of GaInAsP with a thickness of 5 nm, and the hole well layers are made of AlGaInAs with a thickness of 3 nm. As shown in Figure 2, the overlap integral increases as the number of electron well layers increases.

[0029] However, as mentioned above, when electron wave functions are coupled between multiple electron well layers, energy levels that do not contribute to stimulated emission are formed. Figure 3 is a schematic diagram showing an example of the relationship between the number of electron well layers and the energy of each level in the electron well layers where wave functions are coupled. The example shown is under the same conditions as in Figure 2. The vertical axis represents the energy relative to the lower end of the conduction band of the electron well layer. As shown in Figure 3, when there is one electron well layer, there is only one quantum level, the ground level E1. However, as the number of electron well layers increases, the number of quantum levels increases, and when there are seven electron well layers, there are seven quantum levels from E1 to E7.

[0030] The electron well layers with coupled wave functions will be explained using a configuration called a VVV type II quantum well as an example, in which two hole well layers are arranged between three electron well layers. Figure 4 is a schematic diagram showing an example of the band structure and wave function of a VVV type II quantum well. In a VVV type II quantum well, a central electron well layer is first arranged, two hole well layers (N side and P side) are arranged adjacent to the central electron well layer and flanking it, two electron well layers (N side and P side) are arranged adjacent to the two hole well layers and flanking the central electron well layer and the hole well layers, and two barrier layers (N side and P side) are arranged adjacent to the two electron well layers and flanking the three electron well layers and the two hole well layers.

[0031] Assuming the composition and thickness of the hole well layer, electron well layer, and barrier layer are set as shown in Table 2, the energy Ec at the bottom of the conduction band, the energy Ehh at the top of the valence band, the electron wave function φc, and the hole wave function φh are shown in the figure.

[0032]

[0033] As shown in the figure, the electron wave functions are coupled between the three electron wells. Furthermore, the energy difference between the ground level H1 in the valence band and the ground level E1 in the conduction band is 946.5 meV. This energy difference corresponds to a wavelength of 1310 nm, so this VVV type II quantum well emits light at a wavelength of 1310 nm. Additionally, the overlap integral of the hole-electron wave functions is 55.8%, which is larger than the overlap integral in the W type II quantum well shown in Figure 1.

[0034] In this case, three quantum levels are formed as explained with reference to Figure 3. The wave functions in this case are shown in Figures 5A, 5B, and 5C. That is, Figures 5A, 5B, and 5C are schematic diagrams showing the coupled wave functions under the conditions of Figure 4. Figure 5A shows the wave function corresponding to E1 in Figure 3, Figure 5B shows the wave function corresponding to E2 in Figure 3, and Figure 5C shows the wave function corresponding to E3 in Figure 3. Note that "E" on the vertical axis represents a power of 10, for example, "1E-06" is "1 × 10⁻⁶". －６ This means "[...]." The same applies to the following diagrams.

[0035] However, considering that stimulated emission occurs in accordance with the transition wavelength from the ground state, resulting in laser oscillation, only the E1 quantum level contributes to such laser oscillation. Therefore, the remaining two quantum levels are considered unnecessary. However, if quantum levels exist, carriers may occupy these levels. As a result, there will be carriers that do not contribute to laser oscillation, raising concerns about an increase in the threshold current.

[0036] Therefore, the inventors conceived of reducing the number of carriers occupying quantum levels that do not contribute to laser oscillation by increasing the energy difference between E1 and E2 and E3 in a VVV type II quantum well.

[0037] (Embodiment 1) [Overall Configuration of Semiconductor Light-Emitting Device] Figure 6 is a schematic cross-sectional view of a semiconductor light-emitting device according to Embodiment 1. The semiconductor light-emitting device 100 is configured as a semiconductor laser element. The semiconductor light-emitting device 100 comprises an N-type semiconductor portion 2 with an N-side electrode 1 formed on its back surface, an N-side barrier layer 3, an active layer 4, a P-side barrier layer 5, a P-type semiconductor portion 6, a guide layer 7, a current blocking layer 8, a contact layer 9, and a P-side electrode 10. The semiconductor light-emitting device 100 outputs laser light from the active layer 4 in a direction perpendicular to the drawing.

[0038] The N-type semiconductor portion 2 comprises a substrate 2a and an N-type cladding layer 2b located on the substrate 2a with the stacking direction facing upwards.

[0039] The substrate 2a is made of N-type InP (hereinafter referred to as N-InP as appropriate). The N-type cladding layer 2b is a layer made of n-InP that is laminated on the substrate 2a by epitaxial growth or the like.

[0040] In this specification, the N-type semiconductor layer contains, for example, silicon (Si), sulfur (S), and selenium (Se) as N-type impurities, but the N-type impurities are not particularly limited.

[0041] The N-side barrier layer 3, the active layer 4, and the P-side barrier layer 5 are arranged on the substrate 2a. The structure and characteristics of the N-side barrier layer 3, the active layer 4, and the P-side barrier layer 5 will be described in detail later.

[0042] The P-type semiconductor portion 6 has a stacked structure of semiconductor layers 6a, 6b, and 6c made of P-type InP (hereinafter referred to as P-InP as appropriate).

[0043] In this specification, the p-type semiconductor layer contains, for example, zinc (Zn) as a p-type impurity, but the p-type impurity is not particularly limited.

[0044] The guide layer 7 is located between the semiconductor layers 6a and 6b. The guide layer 7 has a higher refractive index than InP and is made of, for example, p-type GaInAsP. The guide layer 7 may be lattice-matched with InP; that is, the lattice constant of the GaInAsP constituting the guide layer 7 may be equal to the lattice constant of InP.

[0045] The guide layer 7 and a portion of the P-type semiconductor portion 6 have a stripe mesa structure. The stripe mesa structure is made to a width (for example, 2 μm) suitable for guiding the light emitted by the active layer 4 in single mode by etching or the like. Both sides of the stripe mesa structure (left and right directions in the drawing) are embedded by a current blocking layer 8, which is constructed by stacking a current blocking layer 8a made of P-InP and a current blocking layer 8b made of N-InP. The semiconductor layer 6c is formed to cover the semiconductor layer 6b and the current blocking layer 8.

[0046] The contact layer 9 is made of, for example, P-type GaInAsP and makes ohmic contact with the P-side electrode 10. The P-side electrode 10 contains, for example, titanium, platinum, or gold.

[0047] The N-side electrode 1 is provided to make ohmic contact with the substrate 2a. The N-side electrode 1 may contain, for example, gold or nickel.

[0048] Furthermore, both end faces of the semiconductor light-emitting element 100, parallel to the drawing, are end faces formed by cleavage. A relatively high reflectivity HR (High Reflection) film is formed on one end face, and an anti-reflection AR (Anti-Reflection) film is formed on the other end face. The HR film and the AR film form a laser resonator. When current is injected from the P-side electrode 10, the semiconductor light-emitting element 100 outputs laser light mainly from the end face where the AR film is formed.

[0049] [Configuration of the stacked structure of the portion including the active layer] Figure 7A is a schematic diagram showing an example of the stacked structure and band structure of the portion including the active layer 4 of the semiconductor light-emitting element 100. As shown in Figure 7A, the active layer 4 has a central electron well layer 41 as a first semiconductor layer, an N-side hole well layer 42a and a P-side hole well layer 42b as second semiconductor layers, and an N-side electron well layer 43a and a P-side electron well layer 43b as third semiconductor layers.

[0050] The N-side hole well layer 42a and the P-side hole well layer 42b are arranged in contact with the central electron well layer 41 and sandwich the central electron well layer 41.

[0051] The N-side electron well layer 43a and the P-side electron well layer 43b are arranged in contact with the N-side hole well layer 42a and the P-side hole well layer 42b, respectively, and sandwich the central electron well layer 41, the N-side hole well layer 42a, and the P-side hole well layer 42b.

[0052] Furthermore, the N-side barrier layer 3 and P-side barrier layer 5, which serve as the fourth semiconductor layers, are arranged so as to be in contact with the N-side electron well layer 43a and the P-side electron well layer 43b, respectively, and sandwich the active layer 4 between them.

[0053] The central electron well layer 41 is, for example, Ga ｘ In （１－ｘ） As ｙ P （１－ｙ） (where 0 < x < 1, 0 < y < 1). The N-side hole well layer 42a and the P-side hole well layer 42b are made of, for example, Al ｒ Ga ｓ In （１－ｒ－ｓ） It consists of As (where 0 < r < 1 - s, 0 ≤ s < 1). The N-side electron well layer 43a and the P-side electron well layer 43b are, for example, Ga ｐ In （１－ｐ） As ｑ P （１－ｑ） (However, 0 < p < 1 and 0 < q < 1) are the conditions. Also, the N-side barrier layer 3 and the P-side barrier layer 5 are made of, for example, Al ｖ In （１－ｖ） As ｗ P （１－ｗ） (However, 0 < v < 1 and 0 < w < 1) are the conditions.

[0054] The lattice constants of the N-side hole well layer 42a and the P-side hole well layer 42b may be greater than the lattice constant of the substrate 2a. This allows for a relatively high barrier height to the electron well layer. Therefore, in addition to the general effects of strain such as separation of heavy and light hole bands and reduction of effective mass, an effect of reducing carrier leakage can be obtained. In this case, furthermore, the lattice constant of the central electron well layer 41 may be smaller than the lattice constant of the substrate 2a, and the lattice constants of the N-side electron well layer 43a and the P-side electron well layer 43b may be smaller than the lattice constant of the substrate 2a, and the layer thickness-weighted average strain in the laminated structure of the central electron well layer 41 and the N-side hole well layer 42a and P-side hole well layer 42b and the N-side electron well layer 43a and P-side electron well layer 43b may be less than 2%. As a result, the strain is compensated, and a laminated structure with good crystal quality can be constructed.

[0055] The lower conduction band energy of the central electron well layer 41 is lower than the lower conduction band energy of the N-side hole well layer 42a and the P-side hole well layer 42b, and the upper valence band energy of the central electron well layer 41 is lower than the upper valence band energy of the N-side hole well layer 42a and the P-side hole well layer 42b. As a result, the central electron well layer 41 and the N-side hole well layer 42a and the P-side hole well layer 42b form a type II heterojunction. The central electron well layer 41 and the N-side hole well layer 42a and the P-side hole well layer 42b then form a first quantum well with respect to electrons, with the central electron well layer 41 as the well layer and the N-side hole well layer 42a and the P-side hole well layer 42b as barrier layers.

[0056] The lower conduction band energies of the N-side hole well layer 42a and the P-side hole well layer 42b are higher than the lower conduction band energies of the N-side electron well layer 43a and the P-side electron well layer 43b, and the upper valence band energies of the N-side hole well layer 42a and the P-side hole well layer 42b are higher than the upper valence band energies of the N-side electron well layer 43a and the P-side electron well layer 43b. As a result, the N-side hole well layer 42a and the P-side hole well layer 42b form a type II heterojunction with respect to holes. Furthermore, the P-side hole well layer 42b, the central electron well layer 41, and the P-side electron well layer 43b form another second quantum well with respect to holes, with the P-side hole well layer 42b acting as the well layer and the central electron well layer 41 and P-side electron well layer 43b acting as barrier layers.

[0057] Furthermore, the lower conduction band energies of the N-side electron well layer 43a and the P-side electron well layer 43b are lower than the lower conduction band energies of the N-side barrier layer 3 and the P-side barrier layer 5. As a result, the N-side electron well layer 43a, the N-side hole well layer 42a, and the N-side barrier layer 3 form one third quantum well with respect to electrons, with the N-side electron well layer 43a as the well layer and the N-side hole well layer 42a and the N-side barrier layer 3 as the barrier layers. In addition, the P-side electron well layer 43b, the P-side hole well layer 42b, and the P-side barrier layer 5 form another third quantum well with respect to electrons, with the P-side electron well layer 43b as the well layer and the P-side hole well layer 42b and the P-side barrier layer 5 as the barrier layers.

[0058] In this embodiment 1, the valence band upper energy of the N-side electron well layer 43a and the P-side electron well layer 43b is higher than, but may be lower than, the valence band upper energy of the N-side barrier layer 3 and the P-side barrier layer 5.

[0059] In this embodiment 1, the energy difference between the lower end of the conduction band of the central electron well layer 41 and the lower ends of the conduction bands of the N-side electron well layer 43a and the P-side electron well layer 43b is smaller than the quantization energy of the ground state in the central electron well layer 41, for example, 270 meV or less. As a result, the electron wave function is coupled between the first quantum well and the third quantum well.

[0060] Furthermore, the thickness of the central electron well layer 41 is greater than the thickness of the N-side electron well layer 43a and the P-side electron well layer 43b. This results in a state where the energy of the ground state of electrons before coupling, formed by the first quantum well, is lower than the energy of the ground state of electrons before coupling, formed by the third quantum well.

[0061] Specifically, in Figure 7B, band B1 represents the first quantum well in the conduction band, and band B2 represents the third quantum well in the conduction band. The dashed lines indicate the energy levels of the electron ground state before the wave functions are coupled. In the first quantum well, the quantum confinement effect is lower than in the third quantum well due to the relatively thicker layer, so the ground state energy is lower.

[0062] For example, Figure 8 is a schematic diagram showing an example of the relationship between the thickness of the electron well layer (well thickness) and the energy of the ground state relative to the vacuum level. As can be seen from Figure 8, the thicker the well, the lower the energy of the ground state.

[0063] In the semiconductor light-emitting element 100 configured as described above, the energy of the ground state of electrons before coupling formed by the first quantum well is lower than the energy of the ground state of electrons before coupling formed by the third quantum well, so the energy difference between E1 and E2 and E3 can be increased.

[0064] The details are explained below. Figures 9A, 9B, and 9C are schematic diagrams showing the probability density of electrons at the ground level (corresponding to E1) when the well thickness ratio is different. Here, the well thickness ratio is the ratio of the thickness of the central electron well layer 41 to the thickness of the N-side electron well layer 43a and the P-side electron well layer 43b. In the following, the N-side electron well layer 43a and the P-side electron well layer 43b may be referred to as the end electron well layers. Figures 9A, 9B, and 9C show the cases where the well thickness ratio is 0.5, 1.0, and 2.15, respectively. In Figures 9A, 9B, and 9C, the absolute value of the layer thickness is adjusted so that the transition wavelength is 1310 nm in all cases.

[0065] As shown in Figures 9A, 9B, and 9C, electrons in the ground state have a high probability density in well layers with thicker layers and lower quantization energies.

[0066] Next, Figures 10A, 10B, and 10C are schematic diagrams showing the probability density of electrons in the second quantum level (corresponding to E2) for different well thickness ratios. Figures 10A, 10B, and 10C show the cases where the well thickness ratios are 0.5, 1.0, and 2.15, respectively. As shown in Figures 10A, 10B, and 10C, the probability density of electrons in the second quantum level is high in the double-ended electron well layer.

[0067] Next, Figures 11A, 11B, and 11C are schematic diagrams showing the probability density of electrons in the third quantum level (corresponding to E3) for different well thickness ratios. Figures 11A, 11B, and 11C show the cases where the well thickness ratio is 0.5, 1.0, and 2.15, respectively. As shown in Figures 11A, 11B, and 11C, the probability density of electrons in the third quantum level is high in well layers with thin layer thickness and high quantization energy.

[0068] As described above, the well layer with a high electron probability density differs depending on the quantum level. In this embodiment, this is utilized to increase the energy difference between the ground level and the second quantum level, and between the ground level and the third quantum level. Specifically, the probability density of electrons at the ground level is higher in the central electron well layer than in the end electron well layer, while the probability density of electrons at the second quantum level is higher in the end electron well layer than in the central electron well layer. This is used to increase the energy difference between the ground level and the second quantum level. Furthermore, the probability density of electrons at the third quantum level is also higher in the end electron well layer than in the central electron well layer. This is used to increase the energy difference between the ground level and the third quantum level.

[0069] As shown in the following equation (1), the ground level (energy E １ The nth quantum level (energy E) for ) ｎ The ratio R(E) of carrier occupation probabilities in ) decreases exponentially as the energy difference from the ground level increases. Note that in equation (1), T is the absolute temperature, and k Ｂ is the Boltzmann constant. Therefore, by increasing the energy difference between the ground state and the second quantum state, and the energy difference between the ground state and the third quantum state, the increase in the threshold current of the semiconductor light-emitting device 100 can be suppressed.

[0070] Figure 12 is a schematic diagram showing an example of the energy differences between the ground level, second quantum level, and third quantum level from the vacuum level when the well thickness ratio is continuously varied. The absolute values ​​of the layer thickness are adjusted so that the transition wavelengths are all 1310 nm. As shown in the figure, if the well thickness ratio is greater than 1, the larger the well thickness ratio, the larger the energy difference between the ground level and the second quantum level, and the larger the energy difference between the ground level and the third quantum level can be.

[0071] Figure 13 is a schematic diagram showing an example of the energy difference between the ground state and the second quantum state (E2-E1), the energy difference between the ground state and the third quantum state (E3-E1), and the overlap integral when the well thickness ratio is continuously varied. As shown in the figure, if the well thickness ratio is 1.2 or more, that is, if the thickness of the central electron well layer 41 is 120% or more of the thickness of the N-side electron well layer 43a and the P-side electron well layer 43b, then (E2-E1) and (E3-E1) can be made larger than the thermal energy at room temperature of 26 meV, so the effect of increasing the energy difference is sufficiently obtained. Also, if the well thickness ratio is 5.5 or less, that is, if the thickness of the central electron well layer 41 is 550% or less of the thickness of the N-side electron well layer 43a and the P-side electron well layer 43b, then the overlap integral can be made 50% or more. Note that the overlap integral is maximum when the well thickness ratio is approximately 1.15.

[0072] Figure 14 is a schematic diagram showing a specific example of the band structure and wave function of the active layer and barrier layer in a semiconductor light-emitting element 100 according to Embodiment 1.

[0073] In Figure 14, the composition and thickness of the hole well layer, electron well layer, and barrier layer are set as shown in Table 3. Therefore, the well thickness ratio is 2.15. In this case, the energy Ec at the bottom of the conduction band, the energy Ehh at the top of the valence band, the electron wave function φc, and the hole wave function φh are shown as illustrated. Note that the energy at the bottom of the conduction band of the central electron well layer 41 is the same as the energy at the bottom of the conduction band of the N-side electron well layer 43a and the P-side electron well layer 43b.

[0074]

[0075] As shown in the figure, the electron wave functions are coupled between the three electron wells. Furthermore, the energy difference between the ground level H1 in the valence band and the ground level E1 in the conduction band is 946.5 meV. Therefore, the semiconductor light-emitting element 100 emits light at 1310 nm. In addition, the overlap integral of the wave functions between holes and electrons is 51.1%, which is larger than the overlap integral in the W-type II quantum well shown in Figure 1. Furthermore, according to the settings in Table 3, the energy difference between the ground level and the second quantum level (E2-E1) is 64.0 meV, and the energy difference between the ground level and the third quantum level (E3-E1) is 81.7 meV, ensuring a sufficient energy difference.

[0076] (Embodiment 2) The semiconductor light-emitting element according to Embodiment 2 has a configuration in which the active layer 4 is replaced with an active layer 4A in the semiconductor light-emitting element 100 according to Embodiment 1 shown in Figures 6 and 7. Therefore, the active layer 4A will be described mainly below, and the descriptions of other components will be omitted as appropriate.

[0077] Figure 15A is a schematic diagram showing an example of the stacked structure and band structure of the portion of the semiconductor light-emitting element according to Embodiment 2 that includes the active layer 4A. As shown in Figure 15A, the active layer 4A has a configuration in which the N-side electron well layer 43a and P-side electron well layer 43b in the active layer 4 are replaced with an N-side electron well layer 43Aa and a P-side electron well layer 43Ab as a third semiconductor layer.

[0078] The N-side electron well layer 43Aa and the P-side electron well layer 43Ab are arranged so as to be adjacent to the N-side hole well layer 42a and the P-side hole well layer 42b, respectively, and sandwich the central electron well layer 41, and the N-side hole well layer 42a and the P-side hole well layer 42b. The N-side electron well layer 43Aa and the P-side electron well layer 43Ab are, for example, Ga ｐ In （１－ｐ） As ｑ P （１－ｑ）(wherein 0 < p < 1, 0 < q < 1). Also, the lattice constant of the central electron well layer 41 is smaller than the lattice constant of the substrate 2a, the lattice constants of the N-side electron well layer 43Aa and the P-side electron well layer 43Ab are smaller than the lattice constant of the substrate 2a, and the layer thickness-weighted average strain in the laminated structure of the central electron well layer 41 and the N-side hole well layer 42a, the P-side hole well layer 42b and the N-side electron well layer 43Aa and the P-side electron well layer 43Ab may be less than 2%.

[0079] The lower conduction band energies of the N-side hole well layer 42a and the P-side hole well layer 42b are higher than the lower conduction band energies of the N-side electron well layer 43Aa and the P-side electron well layer 43Ab, and the upper valence band energies of the N-side hole well layer 42a and the P-side hole well layer 42b are higher than the upper valence band energies of the N-side electron well layer 43Aa and the P-side electron well layer 43Ab. As a result, the N-side hole well layer 42a and the P-side hole well layer 42b form a type II heterojunction with respect to holes. Furthermore, the P-side hole well layer 42b, the central electron well layer 41, and the P-side electron well layer 43Ab form another second quantum well with respect to holes, with the P-side hole well layer 42b acting as the well layer and the central electron well layer 41 and P-side electron well layer 43Ab acting as barrier layers.

[0080] Furthermore, the lower conduction band energies of the N-side electron well layer 43Aa and the P-side electron well layer 43Ab are lower than the lower conduction band energies of the N-side barrier layer 3 and the P-side barrier layer 5. As a result, the N-side electron well layer 43Aa, the N-side hole well layer 42a, and the N-side barrier layer 3 form one third quantum well with respect to electrons, with the N-side electron well layer 43Aa as the well layer and the N-side hole well layer 42a and the N-side barrier layer 3 as the barrier layers. In addition, the P-side electron well layer 43Ab, the P-side hole well layer 42b, and the P-side barrier layer 5 form another third quantum well with respect to electrons, with the P-side electron well layer 43Ab as the well layer and the P-side hole well layer 42b and the P-side barrier layer 5 as the barrier layers.

[0081] In this embodiment 2, the valence band upper energy of the N-side electron well layer 43Aa and the P-side electron well layer 43Ab is higher than, but may be lower than, the valence band upper energy of the N-side barrier layer 3 and the P-side barrier layer 5.

[0082] Here, in this second embodiment, as in the first embodiment, the energy difference between the lower end of the conduction band of the central electron well layer 41 and the lower ends of the conduction bands of the N-side electron well layer 43Aa and the P-side electron well layer 43Ab is 270 meV or less. As a result, the electron wave function is coupled between the first quantum well and the third quantum well.

[0083] However, unlike Embodiment 1, the thickness of the central electron well layer 41 is the same as the thickness of the N-side electron well layer 43Aa and the P-side electron well layer 43Ab. However, the lower conduction band energy of the N-side electron well layer 43Aa and the P-side electron well layer 43Ab is higher than the lower conduction band energy of the central electron well layer 41. As a result, a state is achieved in which the energy of the ground state of electrons before coupling formed by the first quantum well is lower than the energy of the ground state of electrons before coupling formed by the third quantum well.

[0084] Specifically, in Figure 15B, band B1A represents the first quantum well in the conduction band, and band B2A represents the third quantum well in the conduction band. The dashed lines indicate the energy levels of the electron ground state before the wave functions are coupled. In the third quantum well, the energy of the ground state is higher because the lower end energy of the conduction band is relatively high.

[0085] For example, Figure 16 is a schematic diagram showing an example of the relationship between the lower edge energy of the conduction band of the electron well layer from the vacuum level and the energy of the ground level from the vacuum level. As can be seen from Figure 16, the higher the lower edge energy of the conduction band, the higher the energy of the ground level.

[0086] In the semiconductor light-emitting element according to Embodiment 2 configured as described above, the energy of the ground state of electrons before coupling formed by the first quantum well is lower than the energy of the ground state of electrons before coupling formed by the third quantum well, so the energy difference between E1 and E2 and E3 can be increased.

[0087] The details are explained below. Figures 17A, 17B, and 17C are schematic diagrams showing the probability density of electrons at the ground level (corresponding to E1) when the conduction band lower edge energies differ between electron well layers. Here, the energy difference of the conduction band lower edge energies is the value obtained by subtracting the conduction band lower edge energy of the central electron well layer 41 from the conduction band lower edge energies of the N-side electron well layer 43Aa and the P-side electron well layer 43Ab. Figures 17A, 17B, and 17C show the cases where the energy difference is -110 meV, 0 meV, and 97 meV, respectively. Note that in Figures 17A, 17B, and 17C, the absolute value of the conduction band lower edge energies is adjusted so that the transition wavelength is 1310 nm in all cases.

[0088] As shown in Figures 17A, 17B, and 17C, electrons in the ground state have a high probability density in well layers where the lower energy of the conduction band is low and the energy of the quantum levels is low.

[0089] Next, Figures 18A, 18B, and 18C are schematic diagrams showing the probability density of electrons in the second quantum level (corresponding to E2) when the conduction band lower edge energy differs between electron well layers. Figures 18A, 18B, and 18C show the cases where the energy difference is -110 meV, 0 meV, and 97 meV, respectively. As shown in Figures 18A, 18B, and 18C, the probability density of electrons in the second quantum level is high in the electron well layers at both ends.

[0090] Next, Figures 19A, 19B, and 19C are schematic diagrams showing the probability density of electrons in the third quantum level (corresponding to E3) when the conduction band lower edge energy differs between electron well layers. Figures 19A, 19B, and 19C show the cases where the energy difference is -110 meV, 0 meV, and 97 meV, respectively. As shown in Figures 19A, 19B, and 19C, the probability density of electrons in the third quantum level is high in well layers where the conduction band lower edge energy is high and the quantum level energy is high.

[0091] In this embodiment as well, the energy difference between the ground state and the second quantum state, and between the ground state and the third quantum state, is increased by utilizing the fact that the well layer with a high electron probability density differs depending on the quantum level. By increasing the energy difference between the ground state and the second quantum level, and between the ground state and the third quantum level in this way, the increase in the threshold current of the semiconductor light-emitting device according to Embodiment 2 can be suppressed.

[0092] Figure 20 is a schematic diagram showing an example of the energies of the ground level, second quantum level, and third quantum level from the vacuum level when the energy difference of the lower edge energy of the conduction band is continuously varied. The absolute value of the layer thickness is adjusted so that the transition wavelength is 1310 nm in all cases. As shown in the figure, if the energy difference is greater than zero, the larger the energy difference, the larger the energy difference between the ground level and the second quantum level, and the larger the energy difference between the ground level and the third quantum level can be.

[0093] Figure 21 is a schematic diagram showing an example of the energy difference between the ground state and the second quantum state (E2-E1), the energy difference between the ground state and the third quantum state (E3-E1), and the overlap integral when the energy difference of the conduction band lower edge energy is continuously varied. As shown in the figure, if the energy difference between the conduction band lower edge energy of the central electron well layer 41 and the conduction band lower edge energies of the N-side electron well layer 43Aa and P-side electron well layer 43Ab is 27 meV or more, then (E2-E1) and (E3-E1) can be made larger than the thermal energy at room temperature, which is 26 meV. Furthermore, the overlap integral is maximum when the energy difference is 40 meV, but even if the energy difference is greater than that, the decrease in the overlap integral is very small, and 55% or more is ensured. However, as mentioned above, the energy difference is 270 meV or less.

[0094] Figure 22 is a schematic diagram showing a specific example of the band structure and wave function of the active layer and barrier layer in a semiconductor light-emitting element according to Embodiment 2.

[0095] In Figure 22, the composition and film thickness of the hole well layer, electron well layer, and barrier layer are set as shown in Table 4. In this case, the energy difference between the lower conduction band energy of the N-side electron well layer 43Aa and the P-side electron well layer 43Ab and the lower conduction band energy of the central electron well layer 41 is 97 meV, and the energy of the lower conduction band Ec, the energy of the upper valence band Ehh, the electron wave function φc, and the hole wave function φh are shown as illustrated.

[0096]

[0097] As shown in the figure, the electron wave functions are coupled between the three electron wells. Furthermore, the energy difference between the ground level H1 in the valence band and the ground level E1 in the conduction band is 946.5 meV. Therefore, the semiconductor light-emitting device 100 emits light with a wavelength of 1310 nm. In addition, the overlap integral of the wave functions between holes and electrons is 58.2%, which is larger than the overlap integral in the W-type II quantum well shown in Figure 1. Moreover, the energy difference between the ground level and the second quantum level (E2-E1) is 74.3 meV, and the energy difference between the ground level and the third quantum level (E3-E1) is 84.0 meV, ensuring a sufficient energy difference.

[0098] (Conditions for the coupling of two wave functions) However, as in embodiments 1 and 2 above, when the ground state energies of the electrons before coupling are different, certain conditions exist for the two wave functions to couple. Specifically, the wave number of the ground state wave function in the electron well layer to be coupled must be imaginary for the wave functions to couple.

[0099] For example, Figure 23 is a diagram illustrating the conditions under which wave functions are coupled. In Figure 23, the electron well layers are shown as the central electron well layer and one of the end electron well layers of a VVV type II quantum well. In Figure 23, the difference between the lower conduction band energy of the central electron well layer and the lower conduction band energy of the end electron well layers is E. ｃ＿ｃｅｎｔｅｒ－ｓｉｄｅ Let ΔE be the quantization energy of the ground state in the central electron well layer. １ Let's assume that the wave number k of the wave function of the ground state in the double-ended electron well layer, which is the electron well layer to be coupled, is... ｓｉｄｅThey are in a proportional relationship, as expressed by equation (2).

[0100] k ｓｉｄｅ For it to be an imaginary number, equation (3) must hold true.

[0101] For example, Figure 24 shows that equation (3) holds true, that is, the energy difference between the lower end of the conduction band of the first semiconductor layer and the lower end of the conduction band of the third semiconductor layer is smaller than the quantization energy of the ground state in the first semiconductor layer, and k ｓｉｄｅ This figure shows an example where is an imaginary number. In this case, the wave function of the ground level in the two-ended electron well layer is sinusoidal, and the wave function of the electrons in the central electron well layer and the wave function of the electrons in the two-ended electron well layer are coupled. In this case, the overlap integral can be made as high as 43.5%.

[0102] In contrast, in Figure 25, equation (3) does not hold, and k ｓｉｄｅ This figure shows an example where is a negative real number. In this case, the wave function of the ground level in the two-ended electron well layer has a shape that decreases monotonically with position, and the wave function of electrons in the central electron well layer does not couple with the wave function of electrons in the two-ended electron well layer. In this case, the overlap integral is 35.9%, and it cannot be increased.

[0103] Quantization energy ΔE of the ground level １ This affects the composition of the electron well layer and hole well layer, band discontinuities in the conduction band, and the thickness of the electron well layer. Therefore, the inventor made the electron well layer Ga ０．３４ In ０．６６ As ０．６４ P ０．３６ It is composed of, and the hole well layer is Al ０．３０ Ga ０．０３ In ０．６７ The relationship between the layer thickness (well thickness) of the electron well layer and the quantization energy was investigated by constructing the material with As and setting the conduction band discontinuity to 301 meV. The value of 301 meV is an example of a conduction band discontinuity realized when the electron well layer / hole well layer is constructed with GaInAsP / AlGaInAs.

[0104] Figure 26 shows the well thickness of the electron well layer and the quantization energy ΔE of the ground level. １ This is a schematic diagram illustrating an example of the relationship. As shown in Figure 26, the quantization energy at a well thickness of 1 nm is approximately 270 meV, but the quantization energy decreases as the well thickness increases. From this result, it is preferable that the energy difference between the lower end of the conduction band of the central electron well layer and the lower end of the conduction band of the electron well layers at both ends is 270 meV or less for the electron wavefunction to couple between the central electron well layer and the electron well layers at both ends.

[0105] (Other Embodiments) As a semiconductor light-emitting element according to the embodiment, for example, some of the well layers of the semiconductor light-emitting element according to Embodiments 1 and 2 may be replaced with gradient composition layers.

[0106] For example, Figures 27A and 27B compare the case where the well layer has a constant composition within the layer and the case where it has a gradient composition. Figure 27A shows the case where the barrier layer, electron well layer, and hole well layer are arranged from left to right, and the composition and film thickness are set as shown in Table 5. Figure 27B shows the case where the barrier layer, electron well layer, and hole well layer are arranged from left to right, and the composition and film thickness are set as shown in Table 6.

[0107]

[0108] The electron well layer shown in Figure 27A and Table 5 has a constant composition within the layer. On the other hand, the electron well layer shown in Figure 27B and Table 6 is a gradient composition layer, with each composition ratio changing in five stages from the barrier layer to the hole well layer. Specifically, the Ga composition ratio changes from 0.23 to 0.28, the In composition ratio changes from 0.77 to 0.72, the As composition ratio changes from 0.40 to 0.52, and the P composition ratio changes from 0.60 to 0.48.

[0109] In Figure 27A, the ground level is -4.8592 eV, but in Figure 27B, which is a graded composition layer, the ground level is -4.8463 eV, which is higher than in Figure 27A. Therefore, such graded composition layers can be applied, for example, to quantum well layers at both ends.

[0110] Figure 28 is a schematic diagram showing a specific example of the band structure and wave function of the active layer and barrier layer in a semiconductor light-emitting device using a gradient composition layer. The gradient composition layer is applied to the N-side electron well layer and the P-side electron well layer. In other words, this example shows a case where the conduction band lower edge energy of the third semiconductor layer decreases as it approaches the second semiconductor layer.

[0111] In Figure 28, the composition and thickness of the hole well layer, electron well layer, and barrier layer are set as shown in Table 7. In this case, the energy Ec at the bottom of the conduction band, the energy Ehh at the top of the valence band, the electron wave function φc, and the hole wave function φh are represented as shown in the figure.

[0112]

[0113] In this case, the energy difference between the ground level H1 in the valence band and the ground level E1 in the conduction band is 946.5 meV. Therefore, the semiconductor light-emitting element 100 emits light with a wavelength of 1310 nm. Also, the overlap integral of the wave functions of holes and electrons is large at 60.0%. Furthermore, according to the settings in Table 7, the energy difference between the ground level and the second quantum level (E2-E1) is 42.5 meV, and the energy difference between the ground level and the third quantum level (E3-E1) is 57.6 meV, ensuring a sufficient energy difference.

[0114] On the other hand, Figure 29 is a schematic diagram showing a specific example of the band structure and wave function of the active layer and barrier layer in a semiconductor light-emitting device, similar to Figure 28 except that a gradient composition layer is not applied.

[0115] In Figure 29, the composition and thickness of the hole well layer, electron well layer, and barrier layer are set as shown in Table 8. In this case, the energy Ec at the bottom of the conduction band, the energy Ehh at the top of the valence band, the electron wave function φc, and the hole wave function φh are represented as shown in the figure.

[0116]

[0117] In this case, the energy difference between the ground level H1 in the valence band and the ground level E1 in the conduction band is 946.5 meV. Therefore, the semiconductor light-emitting element 100 emits light with a wavelength of 1310 nm. Furthermore, the overlap integral of the wave functions of holes and electrons is large at 58.6%. Also, according to the settings in Table 8, the energy difference between the ground level and the second quantum level (E2-E1) is 41.0 meV, and the energy difference between the ground level and the third quantum level (E3-E1) is 51.5 meV, ensuring a sufficient energy difference.

[0118] Comparing Figure 28 and Figure 29, it can be seen that by applying gradient composition layers, a sufficient energy difference can be secured and the overlap integral can be increased.

[0119] (Coupling of Hole Wave Functions) The coupling of electron wave functions between electron wells has been explained above, but for hole wave functions, it is preferable that hole wave functions do not couple between hole wells. The reason for this is that, as with electrons, if hole wave functions couple, energy levels that do not contribute to stimulated emission are formed.

[0120] Therefore, in the above embodiment, it is preferable that the thickness of the central electron well layer 41 as the first semiconductor layer is such that the wave functions of the holes in the N-side hole well layer 42a and the P-side hole well layer 42b, which are the two second quantum wells, do not couple with each other.

[0121] Figure 30 is a schematic diagram showing the probability density of holes in the hole well layer. In Figure 30, the composition and film thickness of the hole well layer and electron well layer are set as shown in Table 9.

[0122]

[0123] Figure 31 is a schematic diagram showing the relationship between the distance from the hole well end in the vertical direction and the relative probability density of holes when normalized in one dimension, in the case of Table 9. Here, the hole well end is the interface between the hole well layer and the electron well layer. As shown in Figure 31, if the distance from the hole well end is 5 nm or more, the probability density of holes is zero. Therefore, if the thickness of the central electron well layer 41 is 5 nm or more, the wave functions of holes in the N-side hole well layer 42a and the P-side hole well layer 42b do not couple to each other and are separated. However, as mentioned above, it is preferable that the thickness of the central electron well layer 41 is 550% or less of the thickness of the N-side electron well layer 43a and the P-side electron well layer 43b.

[0124] In the above embodiment, the substrate is made of N-InP, but it may also be made of P-InP. In the case of a substrate made of P-InP, the stacking order of the P-type semiconductor layer and the N-type semiconductor layer relative to the active layer is the reverse of that in Figure 6.

[0125] Furthermore, in Embodiment 1, the central quantum well layer and the quantum well layers at both ends have different layer thicknesses, and in Embodiment 2, the ground state energy of the central quantum well layer and the quantum well layers at both ends have different layer thicknesses and ground state energy of the central quantum well layer and the quantum well layers at both ends may also differ.

[0126] Furthermore, in the above embodiment, the semiconductor light-emitting element is configured as a semiconductor laser element, but the semiconductor light-emitting element may also be configured as a semiconductor optical amplifier. When configured as a semiconductor optical amplifier, the semiconductor light-emitting element is configured without a laser resonator. Also, when the semiconductor light-emitting element is configured as a distributed feedback (DFB) laser element, a diffraction grating layer is provided near the active layer. Also, when the semiconductor light-emitting element is configured as a surface-emitting laser element, the active layer is placed inside a vertical laser resonator.

[0127] Furthermore, the present invention is not limited by the embodiments described above. Configurations that appropriately combine the above-described components are also included in the present invention. Moreover, further effects and modifications can be easily derived by those skilled in the art. Therefore, broader aspects of the present invention are not limited to the embodiments described above, and various modifications are possible.

[0128] This invention can be used in semiconductor light-emitting devices.

[0129] 1: N-side electrode 2: N-type semiconductor portion 2a: Substrate 2b: N-type cladding layer 3: N-side barrier layer 4, 4A: Active layer 5: P-side barrier layer 6: P-type semiconductor portion 6a, 6b, 6c: Semiconductor layer 7: Guide layer 8, 8a, 8b: Current blocking layer 9: Contact layer 10: P-side electrode 41: Central electron well layer 42a: N-side hole well layer 42b: P-side hole well layer 43a, 43Aa: N-side electron well layer 43b, 43Ab: P-side electron well layer 100: Semiconductor light-emitting element B1, B1A, B2, B2A: Band

Claims

1. A substrate; an active layer disposed on the substrate, comprising: a first semiconductor layer; two second semiconductor layers disposed in contact with the first semiconductor layer and sandwiching the first semiconductor layer; and two third semiconductor layers, each in contact with each of the second semiconductor layers and sandwiching the first and second semiconductor layers; and two fourth semiconductor layers disposed on the substrate, each in contact with each of the third semiconductor layers and sandwiching the active layer, wherein the conduction band lower edge energy of the first semiconductor layer is lower than the conduction band lower edge energy of the second semiconductor layer, and the valence band upper edge energy of the first semiconductor layer is lower than the valence band upper edge energy of the second semiconductor layer, the first semiconductor layer and the second semiconductor layer form a type II heterojunction, the conduction band lower edge energy of the second semiconductor layer is higher than the conduction band lower edge energy of the third semiconductor layer, and the valence band upper edge energy of the second semiconductor layer is higher than the valence band upper edge energy of the third semiconductor layer. The second semiconductor layer and the third semiconductor layer form a type II heterojunction, the lower conduction band energy of the third semiconductor layer is lower than the lower conduction band energy of the fourth semiconductor layer, the first semiconductor layer and the second semiconductor layer form a first quantum well for electrons with the first semiconductor layer as the well layer and the second semiconductor layer as the barrier layer, the second semiconductor layer, the first semiconductor layer and the third semiconductor layer form a second quantum well for holes with the second semiconductor layer as the well layer and the first and third semiconductor layers as the barrier layer, the third semiconductor layer, the second semiconductor layer and the fourth semiconductor layer form a third quantum well for electrons with the third semiconductor layer as the well layer and the second and fourth semiconductor layers as the barrier layer, the energy difference between the lower conduction band of the first semiconductor layer and the lower conduction band of the third semiconductor layer is smaller than the quantization energy of the ground state in the first semiconductor layer, and the wave function of the electron is coupled between the first and third quantum wells. A semiconductor light-emitting element in which the energy of the ground state of electrons before coupling, formed by the first quantum well, is lower than the energy of the ground state of electrons before coupling, formed by the third quantum well.

2. The semiconductor light-emitting element according to claim 1, wherein the energy difference between the lower end of the conduction band of the first semiconductor layer and the lower end of the conduction band of the third semiconductor layer is 270 meV or less.

3. The substrate is made of InP, and the first semiconductor layer is Ga ｘ In （１－ｘ） As ｙ P （１－ｙ） (where 0<x<1, 0<y<1), the second semiconductor layer is made of Al ｒ Ga ｓ In （１－ｒ－ｓ） As (where 0<r<1-s, 0≦s<1), and the third semiconductor layer is made of Ga ｐ In （１－ｐ） As ｑ P （１－ｑ） (where 0<p<1, 0<q<1), The semiconductor light-emitting device according to claim 1.

4. The semiconductor light-emitting element according to claim 3, wherein the lattice constant of the second semiconductor layer is greater than the lattice constant of the substrate.

5. The semiconductor light-emitting element according to claim 4, wherein the lattice constant of the first semiconductor layer is smaller than the lattice constant of the substrate, the lattice constant of the third semiconductor layer is smaller than the lattice constant of the substrate, and the layer thickness-weighted average strain in the stacked structure of the first semiconductor layer, the second semiconductor layer, and the third semiconductor layer is less than 2%.

6. The fourth semiconductor layer is Al ｖ In （１－ｖ） As ｗ P （１－ｗ） (However, the semiconductor light-emitting element according to claim 3, wherein 0 < v < 1 and 0 < w < 1.) 7. The semiconductor light-emitting element according to claim 1, wherein the thickness of the first semiconductor layer is greater than the thickness of the third semiconductor layer.

8. The semiconductor light-emitting element according to claim 7, wherein the thickness of the first semiconductor layer is 120% or more and 550% or less of the thickness of the third semiconductor layer.

9. The semiconductor light-emitting element according to claim 1, wherein the conduction band lower edge energy of the third semiconductor layer is higher than the conduction band lower edge energy of the first semiconductor layer.

10. The semiconductor light-emitting element according to claim 9, wherein the energy difference between the lower end of the conduction band of the third semiconductor layer and the lower end of the conduction band of the first semiconductor layer is 27 meV or more and 270 meV or less.

11. The semiconductor light-emitting element according to claim 1, wherein the conduction band lower edge energy of the third semiconductor layer decreases as it approaches the second semiconductor layer.

12. The semiconductor light-emitting element according to claim 1, wherein the thickness of the first semiconductor layer is such that the wave functions of holes in the two second quantum wells do not couple with each other.

13. The semiconductor light-emitting element according to claim 1, wherein the thickness of the first semiconductor layer is 5 nm or more and is 550% or less of the thickness of the third semiconductor layer, and the wave functions of holes in the two second quantum wells are not coupled to each other.

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