Semiconductor light-emitting element

Abstract

To provide a semiconductor light-emitting element having high power-to-optical conversion efficiency and high optical output.SOLUTION: A semiconductor light-emitting element includes, on a substrate, an n-type cladding layer, a n-side optical guide layer, a light-emitting layer, a p-side optical guide layer, an electron barrier layer, and a p-type cladding layer. The electronic barrier layer contains a group III nitride semiconductor containing Al as a III-group element. A composition ratio of Al in the group III element is 20% or more, and a thickness is 5 nm or less.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present disclosure relates to semiconductor light emitting devices. [Background technology]

[0002] Due to their excellent features, such as compact size, low cost, and high output, semiconductor laser devices are used in a wide range of technological fields, including IT technologies such as communications and optical discs, as well as medical and some lighting applications. In recent years, high-power semiconductor laser devices have been used as light sources for processing a variety of materials, particularly metals, resins, and carbon fiber reinforced plastics (CFRP). In particular, laser processing systems with high optical output powers ranging from several hundred watts to several kilowatts have been developed and put into practical use using light synthesis technology, which combines multiple laser emitting points to generate high output. Laser processing offers many advantages, including remote processing, high throughput, low power consumption, and programmability, and is widely used in industrial production sites. GaAs-based near-infrared laser devices with wavelengths of approximately 1 μm are increasingly being used as semiconductor light sources for such laser processing. This is due to the maturity of semiconductor fabrication and processing technologies on GaAs substrates.

[0003] In recent years, copper, which has excellent properties such as low electrical resistance, easy workability, and high heat dissipation, has been widely used in various fields such as motors and batteries for electric vehicles (EVs). However, because copper reflects near-infrared laser light, laser processing of copper presents a problem in that the energy does not enter the material and heat is not transferred to the material, making processing difficult.

[0004] For this reason, attention is being paid to laser processing using a processing light source (semiconductor light-emitting element) with a wavelength of 405nm to 540nm that uses a GaN-based semiconductor laser element that emits light in the blue wavelength range where the reflectivity of copper material is low (absorption is high). Laser processing requires high optical output (processing energy), so high-power operation of GaN-based semiconductor laser elements is required.

[0005] To achieve such high-power operation, the semiconductor laser element must be sufficiently cooled and a large amount of current must be passed through it. This consumes a large amount of power, which places a heavy burden on the environment. Therefore, reducing the power consumption of semiconductor laser elements is particularly important in order to reduce the environmental burden.

[0006] To reduce the power consumption of semiconductor laser devices, it is necessary to efficiently convert the input power into light. In other words, it is important to increase the power-to-light conversion efficiency, which indicates the proportion of input energy converted into laser light. Furthermore, highly efficient conversion of input power into light not only increases the optical output, but also prevents excess energy from converting to heat, thereby reducing the reduction in optical output caused by heat generation and the adverse effects on long-term reliability.

[0007] The mechanism by which laser light is generated in a typical pn junction diode-type semiconductor laser device is as follows. A semiconductor laser device is formed on a substrate by stacking at least the following components in order: an n-type optical confinement layer (cladding layer), an n-side optical guide layer, an emission layer (active layer), a p-side optical guide layer, an electron barrier layer, and a p-type optical confinement layer (cladding layer). Electrons are injected from the n-type electrode layer of the semiconductor laser device, and holes are injected from the p-type electrode layer. The electrons and holes pass through the cladding layer and guide layer, respectively, to reach the emission layer. A certain percentage of them are consumed by recombination in the emission layer, while the remainder diffuse to the other guide layer and further cladding layer. Electrons and holes that do not recombine in the emission layer are converted to heat during diffusion through Schottky-Read-Hall (SRH) coupling and other mechanisms. This electron and hole diffusion does not contribute to laser oscillation, resulting in a decrease in power-to-light conversion efficiency. In particular, electrons with small effective mass easily pass through the emission layer and diffuse into the p-side optical guide layer and p-type cladding layer. Therefore, in order to reflect the electrons that have passed through the light-emitting layer and diffused into the p-side optical guide layer and return them to the light-emitting layer, an electron barrier layer is widely used, which provides a potential barrier in the valence band of the semiconductor through which the electrons move.

[0008] The material used for the electron barrier layer is AlGaN, which has a larger band gap due to the addition of Al to GaN. Furthermore, it is known that adding Mg as an acceptor is effective in increasing the potential on the valence band side where electrons conduct. For example, Patent Document 1 states that the thickness of the electron barrier layer is 5 nm or more. Furthermore, although Patent Document 1 does not specify the concentration of Mg added, it is usually 1×10 19 cm -3 Some degree is considered necessary. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2023-121445 Summary of the Invention [Problem to be solved by the invention]

[0010] On the other hand, the inventors' investigations revealed that such an electron barrier layer has a region where the conduction band potential is reduced at the interface between the electron barrier layer and the p-side optical guide layer on the light-emitting layer side. This is thought to be due to piezoelectric polarization corresponding to the lattice distortion of AlGaN / GaN. The electron reflectivity of the electron barrier layer increases when the high-potential region is thick and continuous. Therefore, the potential reduction due to piezoelectric polarization poses the problem of reducing the electron reflectivity.

[0011] One way to prevent this potential drop is to add a large amount of Mg to form a high concentration of acceptors within the band, but this has the side effect of reducing the optical output because it absorbs the laser light.

[0012] An object of the present disclosure is to provide a semiconductor light-emitting device that has high power-to-light conversion efficiency and high optical output. [Means for solving the problem]

[0013] [1] A semiconductor light-emitting device having an n-type cladding layer, an n-side optical guide layer, a light-emitting layer, a p-side optical guide layer, an electron barrier layer, and a p-type cladding layer, in this order, on a substrate, wherein the electron barrier layer contains a III-V compound semiconductor containing Al as a Group III element, and the Al composition in the Group III element is 20% or more, and the thickness of the electron barrier layer is 5 nm or less. [2] The semiconductor light-emitting device according to [1], wherein the thickness of the electron barrier layer is 4 nm or less. [3] The semiconductor light-emitting device according to [1] or [2], wherein the electron barrier layer further contains Mg as a p-type impurity material, and the Mg concentration in the electron barrier layer is 0.5×10 19 cm -3 This is the semiconductor light emitting device. [4] The semiconductor light-emitting device according to any one of [1] to [3], wherein the Mg concentration in the electron barrier layer is 0.5×10 19 cm -3 Over 1.0 x 10 19 cm -3 The semiconductor light-emitting element is as follows: [5] The semiconductor light-emitting device according to any one of [1] to [3], wherein the electron barrier layer has a thickness of 2 nm or more. [6] A semiconductor light-emitting device according to any one of [1] to [3], wherein the p-side light guiding layer contains a III-V group compound semiconductor, and the group III element of the III-V group compound semiconductor contains In. [7] The semiconductor light-emitting element according to any one of [1] to [6], which is a semiconductor laser element. [Effects of the Invention]

[0014] According to the present disclosure, it is possible to provide a semiconductor light-emitting element having high power-to-light conversion efficiency and high optical output. [Brief explanation of the drawings]

[0015] [Figure 1]FIG. 1 is a schematic plan view showing a semiconductor light emitting device according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is a schematic cross-sectional view of the semiconductor light emitting device of FIG. 1 taken along line AB. [Figure 3] FIG. 3A is a graph showing an example of the results of calculating the semiconductor conduction band structure of a semiconductor light-emitting device, and FIG. 3B is a graph that enlarges the vicinity of the electron barrier layer of the conduction band side band structure shown in FIG. 3A and adds annotations to clarify the origin of the issue. [Figure 4] FIG. 4A is a graph showing the calculation results of the band structure when the thickness of the electron barrier layer is 6.5 nm, and FIG. 4B is a graph showing the calculation results of the band structure when the thickness of the electron barrier layer is 2.5 nm. [Figure 5] FIG. 5 is a graph showing the calculation results of the barrier height when the thickness of the electron barrier layer and the Mg doping concentration are changed. [Figure 6] FIG. 6 is a graph showing the calculation results of the tunneling probability when the thickness of the electron barrier layer and the Mg doping concentration are changed. [Figure 7] FIG. 7A is a graph showing the calculation results of the tunnel current density when the thickness of the electron barrier layer and the Mg doping concentration are changed, and FIG. 7B is a graph showing the calculation results of the bulk leakage current when there is no tunnel leakage current. [Figure 8] FIG. 8 is a graph showing the calculation results of the total leakage current density, which is the sum of the tunnel leakage current in FIG. 7A and the bulk leakage current in FIG. 7B. [Figure 9] FIG. 9 is a graph showing the calculation results of the optical output when a current of 1 A is passed through the semiconductor laser device of the first embodiment while changing the electron barrier layer and its Mg doping concentration. [Figure 10] FIG. 10 is a graph showing the calculation results of the total leakage current density when the thickness of the electron barrier layer is optimized to 4 nm or less and the Mg doping concentration is changed. DETAILED DESCRIPTION OF THE INVENTION

[0016] The semiconductor light emitting device of the present disclosure has, on a substrate, an n-type cladding layer, an n-side optical guide layer, a light emitting layer, a p-side optical guide layer, an electron barrier layer, and a p-type cladding layer in this order.

[0017] The p-side optical guiding layer preferably includes a III-V compound semiconductor, which is preferably a III-nitride semiconductor, more preferably GaN, InGaN, or a combination thereof. The group III element of the III-V compound semiconductor of the p-side light guide layer preferably contains In. By including In, it is possible to further strengthen the light confinement in the light emitting layer. When the group III element of the III-V compound semiconductor contains In, the In composition in the group III element is preferably, for example, 0.5% or more, and more preferably 3% to 10%. The In composition refers to the proportion of In atoms in the group III element. On the other hand, the group III elements of the III-V compound semiconductor of the p-side light guide layer preferably do not substantially contain Al. "Substantially not containing Al" means that the Al composition in the group III elements is, for example, 1% or less, preferably 0%. The Al composition indicates the proportion of Al atoms in the group III elements. The thickness of the p-side optical guide layer is not particularly limited, but can be, for example, 0.1 μm or more and 0.4 μm or less.

[0018] The electron barrier layer includes a III-V compound semiconductor containing Al as a group III element, which is preferably a group III nitride semiconductor, more preferably AlGaN or AlInGaN. The Al composition of the Group III elements in the III-V compound semiconductor is 20% or more. When the Al composition is 20% or more, the band gap energy increases, making it possible to increase the potential barrier height with respect to the p-side optical guide layer. The Al composition in the electron barrier layer is preferably 30% or more, and more preferably 35% or more. The upper limit of the Al composition in the electron barrier layer is not particularly limited, but may be, for example, 80% or less.

[0019] When the difference in Al composition ratio between the p-side optical guiding layer and the electron barrier layer is large, crystal distortion and piezoelectric polarization are particularly likely to occur, which tends to pull the conduction band potential toward the lower energy side and reduce the electron reflectivity.

[0020] In the present disclosure, the thickness of the electron barrier layer is reduced to 5 nm or less, preferably 4 nm or less. By reducing the thickness of the electron barrier layer, the amount of piezoelectric polarization can be reduced, thereby suppressing a decrease in the potential barrier height. This increases the electron reflectivity and provides a semiconductor light-emitting device with high power-to-light conversion efficiency.

[0021] Specifically, when the thickness of the electron barrier layer is 5 nm or less, preferably 4 nm or less, the potential barrier of the electron barrier layer can be increased, thereby reducing the bulk leak current and suppressing the decrease in electron reflectivity. The lower limit of the thickness of the electron barrier layer depends on the Mg concentration, but from the viewpoint of further reducing the tunnel leak current and further increasing the electron reflectivity, as will be described later, it is, for example, 0.8 nm or more, preferably 2 nm or more. In particular, when the Mg concentration is 1.0 × 10 19 cm -3 When the thickness is less than 100 nm, the lower limit of the thickness of the electron blocking layer is more preferably 3 nm or more. The thickness of the electron blocking layer can be measured using a transmission electron microscope.

[0022] In addition, the electron barrier layer preferably further contains Mg as a p-type impurity material. In the present disclosure, by reducing the thickness of the electron barrier layer, a decrease in the potential barrier height can be suppressed. Therefore, the Mg concentration is set to, for example, 1×10 19 cm -3 This can further reduce the light absorption due to the addition of Mg, thereby suppressing the resulting decrease in light output.

[0023] Specifically, the Mg concentration in the electron barrier layer is 1.6×10 19 cm -3 Preferably, it is 1.0 x 10 or less.19 cm -3 On the other hand, from the viewpoint of further increasing the potential barrier while further reducing the tunnel leakage current and further increasing the electron reflectivity, the Mg concentration is preferably 0.5×10 19 cm -3 That is, the Mg concentration in the electron barrier layer is preferably 0.5×10 19 cm -3 Over 1.0 x 10 19 cm -3 It is particularly preferable that the Mg concentration in the electron barrier layer is as follows: The Mg concentration in the electron barrier layer can be measured by secondary ion mass spectrometry (SIMS).

[0024] Hereinafter, preferred embodiments of the semiconductor light emitting device of the present disclosure will be specifically described with reference to the drawings.

[0025] [Embodiment 1] Hereinafter, a semiconductor light emitting device 100 according to an embodiment of the present disclosure will be described. In this example, a blue (wavelength 445 nm) semiconductor laser device using a hexagonal group III nitride semiconductor will be used as an example of the semiconductor light emitting device 100.

[0026] 1 and 2 are diagrams of a semiconductor light emitting device 100 according to this embodiment. Fig. 1 is a schematic plan view of the semiconductor light emitting device 100 as seen from above. Fig. 2 is a schematic cross-sectional view taken along line AB in Fig. 1.

[0027] (Configuration of the semiconductor light emitting element 100) First, a simple configuration of the semiconductor light emitting device 100 will be described. 1 and 2, a semiconductor light emitting device 100 includes an n-type cladding layer 2 made of, for example, n-AlGaN, such as n-GaN / i-In, on a semiconductor substrate 1 which is, for example, an n-type hexagonal GaN substrate having a (0001) plane. x1 An n-side optical guide layer 3 is formed by stacking GaN (x1>0) in this order from the substrate side, an InGaN barrier layer, an InGaN quantum well, and an InGaN barrier layer in this order as the light emitting layer 4, a p-side optical guide layer 5 made of, for example, i-InGaN, and a p-Al 0.36An electron barrier layer 6 containing GaN and having a thickness of 3 nm, a p-type cladding layer 7 which is, for example, a p-AlGaN / GaN superlattice, and a p-type contact layer 9 which is, for example, p-GaN are laminated.

[0028] The optical waveguide 20 of the semiconductor light-emitting element 100 is insulated on both sides by an insulating film 8 such as SiO2, and on the top surface of the optical waveguide 20, a p-electrode 10 such as Pd / Pt, a wiring electrode 11 such as Ti / Pt / Au, and a pad electrode 12 such as Ti / Au are formed in a predetermined pattern.

[0029] A rear coat film 14, for example, made of a dielectric multilayer film, is formed on the front and rear sides of the optical waveguide 20 of the semiconductor light-emitting element 100 to reflect light within the optical waveguide 20, and a front coat film 13, for example, made of a dielectric multilayer film, is formed on the front and rear sides of the optical waveguide 20 to emit light.

[0030] An n-electrode 15 made of, for example, Ti / Au is formed on the opposite surface of the semiconductor substrate 1. The light emitting layer 4 may have a multiple quantum well structure in which barrier layers and quantum well layers are repeatedly stacked.

[0031] Next, the detailed configuration of the semiconductor light emitting device 100 will be described together with the manufacturing method.

[0032] First, on an n-type hexagonal GaN substrate 1 whose main surface is a (0001) plane, layers from an n-type cladding layer 2 to a p-type contact layer 9 are successively formed using, for example, metalorganic chemical vapor deposition (MOCVD).

[0033] First, n-type Al 0.03 An n-type cladding layer 2 made of a GaN cladding layer is deposited to a thickness of about 3 μm. The gaseous materials used for film formation include, for example, trimethylgallium (TMG), trimethylindium (TMI), and trimethylaluminum (TMA) as group III materials, silane as n-type impurities, and ammonia as group V materials. The Si concentration in the n-AlGaN n-type cladding layer is 1×10 18 cm -3It's best to keep it to that level.

[0034] Next, n-GaN, which constitutes the first layer of the n-side optical guide layer 3, is grown to 250 nm. Here, the Si concentration of the n-GaN layer is 1×10 18 cm -3 Furthermore, i-InGaN, which constitutes the remaining layers of the n-side optical guide layer 3, is laminated to a thickness of 150 nm.

[0035] Next, as part of the light-emitting layer 4 (active layer), In 0.03 A 7 nm thick GaN barrier layer is grown. Next, a first InGaN quantum well layer is grown to about 3 nm. Next, a 10 nm thick second InGaN barrier layer is grown, followed by a 3.0 nm thick second InGaN quantum well layer. Finally, a third In 0.03 The GaN barrier layer is grown to a thickness of 10 nm. If the InGaN barrier layer is too thick, the efficiency will decrease due to radiative recombination in the barrier layer. Therefore, it is preferable to limit the thickness of each barrier layer to a maximum of approximately 40 nm. Although the thickness is constant at 10 nm in the above example, the thicknesses of the first, second, and third barrier layers may differ from the viewpoint of uniformity of carrier injection.

[0036] Next, an i-InGaN layer, i-GaN, or a combination of these layers is deposited to a thickness of approximately 0.2 μm to form the p-side light guiding layer 5. In this embodiment, the p-side light guiding layer 5 is preferably an i-InGaN layer. From the perspective of light confinement within the device, the refractive index may be decreased from the light emitting layer 4 side to the p-type cladding layer 7 side. That is, the p-side light guiding layer 5 may be formed in the order of InGaN and GaN from the side closer to the light emitting layer 4 to increase light confinement. The In composition may be within the above-mentioned range.

[0037] Next, p-Al constituting the electron barrier layer 6 0.35 GaN (Mg concentration 0.5×10 19 cm -3 The p-AlGaN layer is formed by depositing a 3-nm thick layer of cyclopentadienyl magnesium (CpMg) with a Mg concentration of 0.5×1019 cm -3 The Al composition is as described above, but in this example it can be set to 36%.

[0038] Next, p-Al 0.03 The p-type cladding layer 7 made of a 600 nm GaN layer is grown by, for example, 19 cm -3 The layers are laminated as such.

[0039] Furthermore, the p-GaN contact layer 9 having a thickness of 10 nm was grown by oxidizing the p-GaN contact layer 9 so that the Mg concentration was 1×10 20 cm -3 The layers are laminated as follows.

[0040] The grown wafer is then processed into a ridge stripe laser. First, a 0.3 μm-thick SiO2 insulating film (not shown) made of SiO2 is formed on the p-type contact layer 9, for example, by thermal CVD. Then, photolithography and an etching method using hydrofluoric acid are used to etch away the SiO2 insulating film, leaving it in stripes 16 μm wide. At this time, the stripes are oriented parallel to the m-axis direction of the hexagonal GaN, taking into consideration that the laser facets will be formed using the natural cleavage planes (m-planes) of the hexagonal nitride semiconductor.

[0041] Next, the upper part of the stacked structure is etched to a depth of 1.0 μm using an inductively coupled plasma (ICP) etching method using an SiO2 insulating film to form a ridge stripe portion that constitutes the optical waveguide 20 from the upper part of the p-type contact layer 9 and the p-type cladding layer 7. Thereafter, the second mask film is removed using hydrofluoric acid, and again, a 200 nm thick insulating film 8 made of SiO2 is formed by thermal CVD method over the entire surface of the exposed p-type cladding layer 7, including the ridge stripe portion.

[0042] Next, a resist pattern (not shown) having an opening with a width of 15.5 μm along the ridge stripe portion (optical waveguide 20) is formed by lithography on the upper surface of the ridge stripe portion in the insulating film 8. Subsequently, the SiO2 insulating film is etched using the resist pattern as a mask by reactive ion etching (RIE) using, for example, methane trifluoride (CHF3) gas, thereby exposing the p-type contact layer 9 from the upper surface of the ridge stripe portion.

[0043] Next, a metal laminate film constituting p-electrode 10 made of, for example, palladium (Pd) with a thickness of 40 nm and platinum (Pt) with a thickness of 35 nm is formed on at least the p-type contact layer 9 exposed from the upper surface of the ridge stripe portion by, for example, electron beam (EB) evaporation. Thereafter, the metal laminate film in the region other than the upper part of the ridge stripe is removed by a lift-off method that removes the resist pattern, thereby forming p-electrode 10.

[0044] Next, as shown in FIG. 2, a wiring electrode 11 made of, for example, Ti / Pt / Au is selectively formed on the insulating film 8 by lithography and lift-off, so as to cover the p-electrode 10 on the top of the ridge stripe. The wiring electrode 11 has a planar dimension of, for example, 750 μm parallel to the ridge stripe and a planar dimension of 150 μm perpendicular to the ridge stripe. Here, the wiring electrode 11 is formed of a metal laminate film of titanium (Ti), platinum (Pt), and gold (Au) with thicknesses of 50 nm, 200 nm, and 100 nm, respectively. Generally, multiple laser devices are formed in a matrix on the main surface of a wafer. Therefore, when dividing the wafer into individual laser chips, cutting the wiring electrode 11 may cause the p-electrode 10, which is in close contact with the wiring electrode 11, to peel off from the p-type contact layer 9. Therefore, as shown in FIG. 2, it is desirable that the wiring electrodes 11 are not connected to adjacent chips. Subsequently, an Au layer having a thickness of, for example, 10 μm is formed on the wiring electrode 11 by electrolytic plating to form the pad electrode 12. This makes it possible to mount a laser chip by wire bonding and also to effectively dissipate heat generated in the light-emitting layer 4, thereby improving the reliability of the semiconductor light-emitting element 100.

[0045] Next, the back surface of the semiconductor light emitting element 100 in wafer form, on which the Au pad electrode has been formed, is polished with diamond slurry to thin the semiconductor substrate 1 to a thickness of about 100 μm. Thereafter, a metal laminate film made of, for example, 5 nm of Ti, 10 nm of platinum, and 1000 nm of Au is formed on the back surface of the semiconductor substrate 1 (the surface opposite to the surface on which the optical waveguide 20 is formed) by, for example, EB evaporation, to form the n-electrode 15.

[0046] Next, the semiconductor light-emitting element 100 in wafer form is cleaved (primary cleavage) along the m-plane so that the length in the m-axis direction is, for example, 1200 μm. Subsequently, a front coat film 13 is formed on the cleavage plane from which the laser light is emitted, and a rear coat film 14 is formed on the opposite cleavage plane, for example, using electron cyclotron resonance (ECR) sputtering. The front coat film 13 is made of a dielectric film, such as a single-layer SiO2 film. The rear coat film 14 is made of a dielectric film, such as a ZrO2 / SiO2 stacked film. A highly efficient semiconductor light-emitting element 100 can be achieved by setting the reflectance of the front side (light-emitting side) of the semiconductor light-emitting element 100 to, for example, 6% and the rear side (opposite the light-emitting side) to, for example, 95%. When using an external resonant optical system that combines a semiconductor laser with a focusing lens or the like positioned on the laser optical axis outside the semiconductor to generate oscillation, the front-side reflectance is preferably 1% or less. Next, the semiconductor light emitting element 100 that has been subjected to the first cleavage is cleaved (secondary cleavage) along the a-plane between the optical waveguides 20 that are formed at a pitch of 200 μm in the a-axis direction, for example, to complete a laser chip.

[0047] In the first embodiment, in a pn junction semiconductor light emitting device, particularly a semiconductor laser device, the thickness of the electron barrier layer 6 is set to a thin film of, for example, 3 nm, and further, the Mg doping concentration is set to, for example, 0.5×10 19 cm -3 This makes it possible to increase the light output and improve the luminous efficiency by suppressing the light absorption of laser light while enhancing the electron reflection effect of the electron barrier layer 6.

[0048] (mechanism) Next, the mechanism by which the luminous efficiency increases in the first embodiment will be described.

[0049] First, the role of the electron barrier layer in a GaN laser will be described with reference to FIG. 3A. 3A is a graph showing an example of the results of calculating the semiconductor conduction band structure of a semiconductor light-emitting device. Specifically, in the GaN laser of the first embodiment, the thickness of the electron barrier layer is 5 nm, the Mg concentration is 1.6×10 19 cm -3 This is an example of calculating the semiconductor conduction band structure from the n-side InGaN optical guide layer, light-emitting layer, p-side InGaN optical guide layer (In=3%), electron barrier layer (p-AlGaN, Al=36%), to p-type AlGaN (Al=2.6%) cladding layer, assuming the above. The calculation was performed by solving the Schrodinger-Poisson equation, taking into account the polarization charge of the semiconductor layer. The values ​​used for the physical property parameters of the nitride-based semiconductor used in the calculation are given in Non-Patent Document 1 (Nitride Semiconductor Devices, J. Piprek, J Principles and Simulation, 2007, Wiley-VCH).

[0050] Electrons are injected into the conduction band side band shown in Figure 3A from the left side of the figure. The injected electrons pass through the n-side optical guide layer and reach the light-emitting layer, as represented by the "electron flow" in the figure. Because there is a depression in the conduction band side potential above the light-emitting layer, the electrons fall into this depression and become bound. A certain proportion of these bound electrons diffuse through the valence band from the p-type cladding layer side and combine with holes (not shown) that have also fallen into the potential depression in the light-emitting layer in the valence band, thereby emitting light. A portion of this light emission then becomes laser light. Therefore, supplying a large number of electrons and holes to the light-emitting layer and allowing them to combine is important for achieving highly efficient laser oscillation.

[0051] However, electrons in GaN-based semiconductors have an effective mass of approximately 0.21 × m0 (m0 is the electron mass in a vacuum), which is smaller than the 1.8 × m0 of holes, and they easily escape from the potential well in the light-emitting layer and diffuse into the p-side optical guiding layer. As shown in Figure 3A, these escaping electrons are reflected by the electron barrier layer, which forms a potential barrier in the conduction band, and then pass through the p-side optical guiding layer again to reach the active layer, where they can recombine with holes.

[0052] Next, we will explain the issues and solutions for these electron barrier layers with reference to Figure 3B. Figure 3B is an enlarged view of the electron barrier layer in the conduction band structure shown in Figure 3A, with annotations added to clarify the origin of the issues. Note that in this figure, the electron barrier layer is referred to as the OFS layer. In Figure 3B, "+" indicates a positive charge induced by piezoelectric polarization due to the difference in lattice constants between AlGaN and InGaN. "-" indicates a depleted acceptor formed by Mg doping. The function of the electron barrier layer is to reflect electrons diffusing from the left side of the figure, and the amount of electrons that can be reflected increases as the potential of the electron barrier layer increases and its width increases. When AlGaN is used for the electron barrier layer, as mentioned above, increasing the Al composition increases the band gap energy, making it possible to increase the potential barrier height that can be achieved with respect to the optical guiding layer. Therefore, the Al composition is sometimes set to 20% or more, more preferably 30% or more, and even more preferably 35% or more.

[0053] On the other hand, in AlGaN layers with a high Al composition, the lattice constant of AlGaN is smaller than that of GaN, which is known to cause distortion in the crystal's A- and M-axis directions. This distortion induces fixed charges (+ and -) at the top and bottom of the AlGaN crystal, respectively. As the AlGaN thickness increases, the crystal distortion increases, and so does the amount of fixed charge. These positive charges have the effect of pulling the conduction band potential of the semiconductor toward lower energy, distorting the band structure and reducing the potential for electrons, as shown in Figure 3B. This reduces the electron reflection ability of the electron barrier layer, resulting in an increase in bulk leakage current due to electrons penetrating the electron barrier layer from left to right in the figure. Because the amount of positive charge induced by piezoelectric polarization increases with AlGaN thickness, thinning the AlGaN layer is expected to reduce the amount of positive charge and reduce the effect of pulling the potential toward lower energy.

[0054] The reduction in the potential pull-in effect due to thinning will be explained with reference to Figures 4A and 4B. Figure 4A is a graph showing the calculation results of the band structure when the thickness of the electron barrier layer is 6.5 nm, and Figure 4B is a graph showing the calculation results of the band structure when the thickness of the electron barrier layer is 2.5 nm. Specifically, the electron barrier layer is made of p-AlGaN (Al = 36%) with a Mg concentration of 1.0 × 10 19 cm -3 This is the calculation result when ΔE OFS represents the potential height of the top of the electron barrier layer as viewed from the bottom of the conduction band. It can be seen that the potential height increases by thinning the electron barrier layer (compare Figure 4A and Figure 4B). This means that in order to enhance the effectiveness of the electron barrier layer, it is necessary to appropriately control the thickness. Furthermore, since Mg doping into the electron barrier layer places negative (-) charges in the AlGaN, it is expected that Mg doping can also adjust the potential height.

[0055] Figure 5 is a graph showing the calculation results of the barrier height when the film thickness and Mg doping concentration of the electron barrier layer are changed. Here, the horizontal axis of Figure 5 is the thickness (nm) of the AlGaN (Al=36%):Mg layer, which corresponds to the thickness of the electron barrier layer. The Mg doping concentration is 0.25 × 10 19 cm -3 (T1), 0.375 × 10 19 cm -3 (T2), 0.5 × 10 19 cm -3 (T3), 1.0 × 10 19 cm -3 (T4), 1.6×10 19 cm -3 (T5). The same applies to Figures 6 to 9.

[0056] As shown in Figure 5, the barrier height increases as the thickness decreases. Increasing the Mg doping concentration is also effective in increasing the barrier height. As mentioned earlier, increasing the Mg doping concentration increases the amount of negative (-) charge in Figure 3B, partially offsetting the positive (+) charge resulting from piezoelectric polarization. However, increasing the Mg concentration has the side effect of increasing the optical absorption of laser light. Therefore, from the perspective of optical design, it is preferable to keep the Mg concentration as low as possible. From this perspective, reducing the thickness of the electron barrier layer is promising from the perspective of increasing the potential barrier while keeping the Mg concentration as low as possible.

[0057] However, thinning the electron barrier layer also poses challenges. Returning to Figure 3, we will explain these challenges. Thinning the electron barrier layer means that the OFS layer in Figure 3B becomes physically thinner. However, because electrons have a quantum leakage of several nanometers, electron leakage due to quantum tunneling may occur when the layer is thinned, regardless of the potential height.

[0058] In order to calculate the leakage electron current due to such quantum tunneling, the tunneling probability was calculated using the WKB approximation described in Non-Patent Document 2 (Detailed Theory and Applied Quantum Mechanics Exercises (Kyoritsu Shuppan, 1985) pp. 56-67), and the results are shown in Figure 6. Figure 6 is a graph showing the calculation results of the tunneling probability when the thickness of the electron barrier layer and the Mg doping concentration are changed. Furthermore, from the calculated tunneling probability, the threshold current density (1 kA / cm 2) is shown in Figure 7A. It can be seen that the tunneling current density increases when the electron barrier layer is made thinner. It can also be seen that the tunneling current density increases when the Mg doping concentration is reduced. This is because the tunneling probability also depends on the potential height. Figure 7B shows the bulk leakage current calculation results for the absence of a tunnel corresponding to issue 1 in Figure 3B. The combination of the tunnel leakage current in Figure 7A and the bulk leakage current in Figure 7B shows the dependence of the total leakage current density on the electron barrier layer thickness, as shown in Figure 8. Since the bulk leakage current and tunnel leakage current have opposite trends with the electron barrier layer thickness, there exists a thickness of the electron barrier layer that minimizes the total leakage current density.

[0059] Figure 9 shows the calculation results of the optical output when a current of 1 A is applied to the semiconductor laser of embodiment 1 while varying the thickness of the electron barrier layer and its Mg doping concentration. In the calculation, the laser has a stripe width of 16 μm, a cavity length of 1200 μm, a front coat reflectivity of 6%, and a rear coat reflectivity of 95%. Also, assuming that the internal loss αi increases in proportion to the Mg doping concentration, the threshold and slope efficiency were calculated using classical laser theory described in Non-Patent Document 3 (Semiconductor Lasers and Photonics Integrated Circuits (Ohmsha), Chapter 2). From FIG. 9, it is preferable that the thickness of the electron barrier layer is set to 5 nm or less, preferably 4 nm or less, and further, the Mg doping concentration is set to 0.5×10 19 cm -3 Over 1.0 x 10 19 cm -3 It can be seen that the light output can be increased more effectively by setting the thickness to about 1 / 2 mm or less.

[0060] Figure 10 shows the calculation results of the total leakage current density when the Mg doping concentration is changed while optimizing the thickness of the electron barrier layer to 4 nm or less. 19 cm -3 Over 1.0 x 10 19 cm -3 It can be seen that the light output can be further increased if the following is true:

[0061] The above study was carried out by setting the Mg doping concentration of the p-type cladding layer other than the electron barrier layer to 5 × 10 19 cm -3 However, the Mg concentration in the portions other than the electron barrier layer may be different. Also, the electron barrier layer is calculated based on the assumption that the entire layer is uniformly doped with a certain amount of Mg, but as is often seen in actual crystals, the composition may fluctuate due to diffusion back and forth.

[0062] In addition to MOCVD, crystal growth methods for forming the semiconductor stack structure described above, such as molecular beam epitaxy (MBE) or chemical beam epitaxy (CBE), which are capable of growing a GaN-based blue-violet semiconductor laser structure, may also be used. Furthermore, as can be seen from the description of the case where AlGaN and InGaN are in contact in this example, it is preferable that there is substantially no region with a gradient in In or Al composition between the AlGaN and InGaN. This is because piezoelectric polarization is more likely to occur in such cases, making the configuration of the present disclosure particularly effective. Furthermore, while this example describes GaN-based materials, other material systems that generate polarization by changing the composition, such as AlGaAs / InGaAs on GaAs, InGaAsP on InP, AlGaAsP, and GaInSb on GaSb, may also be used.

[0063] Furthermore, in this embodiment, the chip is divided into pieces with a width of 200 μm, but it may also be an array element in which a plurality of light emitting elements are formed continuously.

[0064] In this embodiment, a pn junction semiconductor laser element has been described, but the element may be a pn junction semiconductor LED element or a unipolar electron conduction quantum cascade laser element as long as it has a structure in which electrons are conducted in the stacking direction and there is potential in layers with different compositions. [Industrial Applicability]

[0065] According to the present disclosure, it is possible to provide a semiconductor light emitting device with high power-to-light conversion efficiency and high optical output, thereby realizing a light source that can operate with low power consumption and high optical output. [Explanation of symbols]

[0066] 1. Semiconductor substrate 2 n-type cladding layer 3 n-side optical guide layer 4. Light-emitting layer 5 p-side optical guide layer 6. Electron barrier layer 7 p-type cladding layer 8. Insulating film 9 p-type contact layer 10p electrode 11 Wiring electrode 12 Pad electrode 13 Front coat film 14 Rear coat film 15n electrode

Claims

1. an n-type cladding layer, an n-side optical guide layer, a light emitting layer, a p-side optical guide layer, an electron barrier layer, and a p-type cladding layer are provided on a substrate in this order; the electron barrier layer includes a III-V group compound semiconductor containing Al as a group III element, and the Al composition in the group III element is 20% or more; The thickness of the electron barrier layer is 5 nm or less. Semiconductor light emitting element.

2. The semiconductor light emitting device according to claim 1 , The thickness of the electron barrier layer is 4 nm or less. Semiconductor light emitting element.

3. The semiconductor light emitting device according to claim 1 , the electron barrier layer further contains Mg as a p-type impurity material; The Mg concentration in the electron barrier layer is 0.5×10 １９ cm －３ That's all. Semiconductor light emitting element.

4. The semiconductor light emitting device according to claim 2, The Mg concentration in the electron barrier layer is 0.5×10 １９ cm －３ Above 1.0 x 10 １９ cm －３ Below is the Semiconductor light emitting element.

5. The semiconductor light emitting device according to claim 1 , The thickness of the electron barrier layer is 2 nm or more. Semiconductor light emitting element.

6. The semiconductor light emitting device according to claim 1 , the p-side optical guiding layer includes a III-V group compound semiconductor; The group III element of the group III-V compound semiconductor includes In. Semiconductor light emitting element.

7. The semiconductor light emitting device according to any one of claims 1 to 6, A semiconductor laser element, Semiconductor light emitting element.

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