Light-emitting element

Abstract

To provide a light-emitting element having high carrier injection efficiency.SOLUTION: A light-emitting element includes a first semiconductor layer 11, a second semiconductor layer 12, a third semiconductor layer 13, and a fourth semiconductor layer 14 in this order. The second semiconductor layer 12 and the third semiconductor layer contain a p-type impurity. A thickness of the second semiconductor layer 12 is smaller than a thickness of the third semiconductor layer 13. An Al composition ratio x2 of the second semiconductor layer 12 is larger than any of an Al composition ratio x1 of the first semiconductor layer 11, an Al composition ratio x3 of the third semiconductor layer 13, and an Al composition ratio x4 of the fourth semiconductor layer 14. The Al composition ratio x3 of the third semiconductor layer 13 is larger than any of the Al composition ratio x1 of the first semiconductor layer 11, and the Al composition ratio x4 of the fourth semiconductor layer 14. The third semiconductor layer 13 may be used as an electron block layer for the fourth semiconductor layer 14.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a light-emitting element.

Background Art

[0002] Laser elements using nitride semiconductors are used as light sources for high-power in-vehicle headlamps and laser processing machines. To operate a high-power laser element efficiently, it is important to increase the carrier injection efficiency of the laser element. One of the factors that reduces the carrier injection efficiency is the phenomenon in which electrons injected into the active layer are thermally excited and overflow into the p-side semiconductor layer. In order to suppress the overflow of electrons into the p-side semiconductor layer, an electron blocking layer having a larger bandgap than the p-side semiconductor layer is inserted between the active layer and the p-side semiconductor layer.

[0003] A configuration having a peak in the aluminum (Al) composition ratio in a region of the electron blocking layer close to the active layer is known (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The present disclosure provides a light-emitting element with improved carrier injection efficiency.

Means for Solving the Problems

[0006] In one embodiment, the light-emitting element includes a first semiconductor layer, a second semiconductor layer, a third semiconductor layer, and a fourth semiconductor layer in this order. The first semiconductor layer is Al x1 In y1 Ga 1-x1-y1N(0 ≦ x1 ≦ 1, 0 ≦ y1 ≦ 1, 0 ≦ x1 + y1 ≦ 1), The second semiconductor layer is Al x2 In y2 Ga 1-x2-y2 N(0 ≦ x2 ≦ 1, 0 ≦ y2 ≦ 1, 0 ≦ x2 + y2 ≦ 1), The third semiconductor layer is Al x3 In y3 Ga 1-x3-y3 N(0 ≦ x3 ≦ 1, 0 ≦ y3 ≦ 1, 0 ≦ x3 + y3 ≦ 1), The fourth semiconductor layer is Al x4 In y4 Ga 1-x4-y4 N(0 ≦ x4 ≦ 1, 0 ≦ y4 ≦ 1, 0 ≦ x4 + y4 ≦ 1), The second semiconductor layer and the third semiconductor contain p-type impurities, The thickness of the second semiconductor layer is smaller than the thickness of the third semiconductor layer, The Al composition ratio x2 of the second semiconductor layer is larger than any of the Al composition ratio x1 of the first semiconductor layer, the Al composition ratio x3 of the third semiconductor layer, and the Al composition ratio x4 of the fourth semiconductor layer, The Al composition ratio x3 of the third semiconductor layer is larger than the Al composition ratio x1 of the first semiconductor layer and the Al composition ratio x4 of the fourth semiconductor layer.

Advantages of the Invention

[0007] A light-emitting device with improved carrier injection efficiency is realized.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0009] Hereinafter, embodiments for carrying out the present disclosure will be described with reference to the drawings. The following description is for embodying the technical idea of the present disclosure, and unless there is a specific description, the present disclosure is not limited to the following description. In each drawing, members having the same function may be denoted by the same reference numerals. For the sake of ease of explanation or understanding of the key points, the embodiments may be shown separately for convenience, but partial substitution or combination of the configurations shown in different embodiments or examples is possible. In the embodiments shown later, mainly the matters different from the previously shown embodiments will be described, and the overlapping explanations for the matters common to the previously shown embodiments may be omitted. The size, positional relationship, etc. of the members shown in each drawing may be exaggerated for clarity of the explanation.

[0010] <Embodiment> FIG. 1 is a schematic cross-sectional view of a light-emitting element according to an embodiment. The light-emitting element is, for example, a semiconductor laser element. Also, the light-emitting element is, for example, a light-emitting diode. Hereinafter, the semiconductor laser element 10 will be described as an example of the light-emitting element. The cross-section in FIG. 1 is a cross-section orthogonal to the resonance direction of the semiconductor laser element 10. On the substrate 1, an n-side semiconductor layer 3 is provided, and on the n-side semiconductor layer 3, an active layer 15, a first semiconductor layer 11, a second semiconductor layer 12, a third semiconductor layer 13, and a fourth semiconductor layer 14 are provided in this order. The first semiconductor layer 11 is on the side opposite to the n-side semiconductor layer 3 with the active layer 15 interposed therebetween, and is made of, for example, GaN or AlGaN. The second semiconductor layer 12 is between the first semiconductor layer 11 and the third semiconductor layer 13, and is the high Al composition layer having the highest Al composition ratio among the first semiconductor layer 11 to the fourth semiconductor layer 14. The third semiconductor layer 13 has a higher Al composition ratio than the fourth semiconductor layer 14, and can be used, for example, as an electron blocking layer for the fourth semiconductor layer 14. In this case, at least a part of the second semiconductor layer 12 may be included in the electron blocking layer. The fourth semiconductor layer 14 is the layer on the most p-side contact layer side, and can be, for example, a p-side optical guide layer. The second semiconductor layer 12 and the third semiconductor layer 13 provided between the first semiconductor layer 11 and the fourth semiconductor layer 14 have a larger bandgap energy than the first semiconductor layer 11 and the fourth semiconductor layer 14, and form a barrier for electrons at the lower end of the conduction band.

[0011] The semiconductor laser element 10 is formed of a nitride semiconductor. The first semiconductor layer 11 is Al x1 In y1 Ga 1-x1-y1 N (0≤x1≤1, 0≤y1≤1, 0≤x1 + y1≤1). The second semiconductor layer 12 is Al x2 In y2 Ga 1-x2-y2 N (0≤x2≤1, 0≤y2≤1, 0≤x2 + y2≤1). The third semiconductor layer 13 is Al x3 In y3 Ga 1-x3-y3 N (0≤x3≤1, 0≤y3≤1, 0≤x3 + y3≤1). The fourth semiconductor layer 14 is Al x4 In y4 Ga 1-x4-y4N satisfies 0≦x4≦1, 0≦y4≦1, and 0≦x4 + y4≦1. The second semiconductor layer 12 and the third semiconductor layer 13 contain p-type impurities. The thickness of the second semiconductor layer 12 is smaller than that of the third semiconductor layer 13. The Al composition ratio x2 of the second semiconductor layer 12 is larger than any of the Al composition ratio x1 of the first semiconductor layer 11, the Al composition ratio x3 of the third semiconductor layer 13, and the Al composition ratio x4 of the fourth semiconductor layer 14. The Al composition ratio x3 of the third semiconductor layer 13 is larger than any of the Al composition ratio x1 of the first semiconductor layer 11 and the Al composition ratio x4 of the fourth semiconductor layer 14.

[0012] As the substrate 1, a GaN substrate is used in Embodiment 1. The +c plane (0001) plane of the GaN substrate is used as the main plane, and the first semiconductor layer 11, the second semiconductor layer 12, the third semiconductor layer 13, and the fourth semiconductor layer 14 are grown on the +c plane. Here, when it is said that the “+c plane is the main plane”, it includes an offset within ±1°. By using a substrate with the +c plane as the main plane, mass productivity can be improved. An n-side electrode 17 is provided on the back surface of the substrate 1. An n-type impurity may be added to the substrate 1 to function as a contact layer.

[0013] An n-side semiconductor layer 3 and an active layer 15 are disposed between the substrate 1 and the first semiconductor layer 11. The n-side semiconductor layer 3 is formed of AlGaN, and a binary, ternary, or quaternary nitride semiconductor can be used. The n-side semiconductor layer 3 includes one or more n-side semiconductor layers. For example, it may include an n-side cladding layer and an n-side optical guide layer. The n-side cladding layer has a larger bandgap energy than the n-side optical guide layer. The n-side optical guide layer is preferably undoped or has a smaller n-type impurity concentration than the n-side cladding layer in order to suppress light absorption. Here, “undoped” means that the film is formed without intentional addition of impurities, and the impurity concentration is 1×10 16 cm -3 or less, or substantially free of impurities.

[0014] An undoped AlInGaN optical confinement layer may be provided in the interface region between the n-side semiconductor layer 3 and the active layer 15, that is, in the uppermost layer of the n-side semiconductor layer 3. In order to effectively confine light in the active layer in the stacking direction, it is desirable that the In composition ratio y1 of the undoped optical confinement layer be less than 0.05.

[0015] (First semiconductor layer 11) The first semiconductor layer 11 is provided between the active layer 15 and the second semiconductor layer 12 which is a part of the p-side semiconductor layer, and is made of Al x1 In y1 Ga 1-x1-y1 N (0 ≦ x1 ≦ 1, 0 ≦ y1 ≦ 1, 0 ≦ x1 + y1 ≦ 1). The first semiconductor layer 11 is provided, for example, for the purpose of confining light in the active layer 15. The first semiconductor layer 11 is, for example, formed undoped. The composition ratios x1 and y1 are determined so as to effectively confine light in the active layer 15. The In composition ratio y1 of the first semiconductor layer 11 may be 0. In that case, the refractive index of the first semiconductor layer 11 decreases, and the performance of blocking light can be enhanced.

[0016] The uppermost layer of the first semiconductor layer 11 may be a barrier layer or a well layer. From the viewpoint of suppressing the overflow of electrons to the fourth semiconductor layer 14 close to the p-side contact layer and promoting the diffusion of holes to the active layer 15, the uppermost layer of the first semiconductor layer 11 may be a well layer.

[0017] (Second semiconductor layer 12 and third semiconductor layer 13) The second semiconductor layer 12 and the third semiconductor layer 13 are provided between the first semiconductor layer 11 and the fourth semiconductor layer. The second semiconductor layer 12 is provided closer to the n-side semiconductor layer 3 than the third semiconductor layer 13, and is made of Al x2 In y2 Ga 1-x2-y2 N (0 ≦ x2 ≦ 1, 0 ≦ y2 ≦ 1, 0 ≦ x2 + y2 ≦ 1). The third semiconductor layer 13 is provided closer to the fourth semiconductor layer 14 than the second semiconductor layer 12, and is made of Al x3 In y3 Ga 1-x3-y3It is formed by N(0≦x3≦1, 0≦y3≦1, 0≦x3 + y3≦1). The second semiconductor layer 12 and the third semiconductor layer 13 may be used as an electron blocking layer. The Al composition ratio x2 of the second semiconductor layer 12 is larger than any of the Al composition ratio x1 of the first semiconductor layer 11, the Al composition ratio x3 of the third semiconductor layer 13, and the Al composition ratio x4 of the fourth semiconductor layer 14. Also, the thickness of the second semiconductor layer 12 is smaller than the thickness of the third semiconductor layer 13. By reducing the thickness of the second semiconductor layer 12 having a relatively high Al composition ratio, the activation energy of the p-type impurities contained in the second semiconductor layer 12 becomes smaller, and the hole concentration can be improved. Also, the second semiconductor layer 12 and the third semiconductor layer 13 can reduce electron overflow and enhance the carrier injection efficiency. Also, the In composition ratio y2 of the second semiconductor layer 12 may be 0. In that case, the refractive index of the second semiconductor layer 12 decreases, and the performance of confining light can be enhanced. Also, the In composition ratio y3 of the third semiconductor layer 13 may be 0. In that case, the refractive index of the third semiconductor layer 13 decreases, and the performance of confining light can be enhanced.

[0018] The thickness of the second semiconductor layer 12 is, for example, greater than 0.5 nm and less than or equal to 2.5 nm. By making the second semiconductor layer 12 thicker than 0.5 nm, the electron overflow can be effectively reduced. Also, by making the thickness of the second semiconductor layer 12 less than or equal to 2.5 nm, an increase in the operating voltage can be reduced. The total thickness of the second semiconductor layer 12 and the third semiconductor layer 13 is, for example, greater than 5 nm and less than or equal to 30 nm. By making the total thickness of the second semiconductor layer 12 and the third semiconductor layer 13 greater than 5 nm, the possibility that the p-type impurities added to the fourth semiconductor layer 14 diffuse to the active layer 15 can be reduced. Also, by making the total thickness of the second semiconductor layer 12 and the third semiconductor layer less than or equal to 30 nm, the movement of holes in the third semiconductor layer 13 or the second semiconductor layer 12 is less likely to be inhibited, and an increase in the forward voltage can be reduced. The third semiconductor layer 13 may be doped with p-type impurities such as Mg. In that case, the p-type impurity concentration of the third semiconductor layer 13 is less than that of the fourth semiconductor layer 14 closer to the p-side contact layer. By adding p-type impurities to the third semiconductor layer 13, the hole injection efficiency from the fourth semiconductor layer 14 to the first semiconductor layer 11 can be increased. Also, since the p-type impurity concentration of the third semiconductor layer 13 is less than that of the fourth semiconductor layer 14 closer to the p-side contact layer, the optical loss in the third semiconductor layer 13 can be reduced.

[0019] Even when the second semiconductor layer 12 is formed undoped, p-type impurities (e.g., Mg) can diffuse from the third semiconductor layer 13 to the second semiconductor layer 12. To reduce the electron overflow from the first semiconductor layer 11 to the p-side fourth semiconductor layer 14, when the Al composition ratio x2 of the second semiconductor layer 12 is increased, the activation of the p-type impurities is reduced and the series resistance tends to increase. Therefore, by making the thickness of the second semiconductor layer 12 with a large Al composition ratio x2 such that the activation of the p-type impurities is less likely to be reduced, it is difficult to inhibit the injection of holes from the fourth semiconductor layer 14 to the first semiconductor layer 11 and further to the active layer 15, and an increase in the driving voltage can be reduced. The thickness of the second semiconductor layer 12 that makes it difficult to reduce the activation of the p-type impurities is, for example, less than or equal to 2.5 nm.

[0020] (Fourth semiconductor layer 14) The fourth semiconductor layer 14 is a semiconductor layer doped with p-type impurities. The fourth semiconductor layer 14 is made of Al x4 In y4 Ga 1-x4-y4 N (0 ≤ x4 ≤ 1, 0 ≤ y4 ≤ 1, 0 ≤ x4 + y4 ≤ 1), and depending on the composition of x4 and y4, a binary, ternary, or quaternary nitride semiconductor can be used. For example, Mg is added to the fourth semiconductor layer 14 as a p-type impurity. The In composition ratio y4 of the fourth semiconductor layer 14 may be 0. In that case, the refractive index of the fourth semiconductor layer 14 decreases, and the performance of blocking light can be enhanced.

[0021] The first semiconductor layer 11, the second semiconductor layer 12, the third semiconductor layer 13, and the fourth semiconductor layer 14 are processed into the shape of the ridge 16. In the cross-section of FIG. 1, the lateral change in refractive index due to the ridge 16, that is, the change in the medium of the insulating film 19, Al x4 In y4 Ga 1-x4-y4 N (0 ≤ x4 ≤ 1, 0 ≤ y4 ≤ 1, 0 ≤ x4 + y4 ≤ 1), and the change in the medium of the insulating film 19 causes light to be confined laterally in the active layer 15. A p-side electrode 18 is provided on the upper surface of the ridge 16. A p-side contact layer may be provided at the interface between the fourth semiconductor layer 14 and the p-side electrode 18. The p-side contact layer contains p-type impurities at a higher concentration than any layer of the fourth semiconductor layer 14. The first semiconductor layer 11 and the second semiconductor layer 12 are adjacent to each other. The second semiconductor layer 12 and the third semiconductor layer 13 are adjacent to each other. The third semiconductor layer 13 and the fourth semiconductor layer 14 are adjacent to each other. By making each layer continuous, a structure for reducing the increase in the operating voltage of the semiconductor laser element 10 can be accommodated in a narrow space.

[0022] (Distribution of Al composition ratio from the first semiconductor layer to the fourth semiconductor layer) FIG. 2 shows the distribution of the Al composition ratio from the first semiconductor layer 11 to the fourth semiconductor layer 14 of the semiconductor laser element 10 according to the embodiment. The horizontal axis represents the position in the stacking direction, and it gets closer to the n side as it goes to the right. The vertical axis is the Al composition ratio. Among the first semiconductor layer 11 to the fourth semiconductor layer 14, the Al composition ratio x2 of the second semiconductor layer 12 is the highest. The Al composition ratio x2 is, for example, 0.4 or more and 0.8 or less (denoted as "40% ≦ x2 ≦ 80%" in the figure), preferably 0.45 or more and 0.75 or less. When the Al composition ratio x2 of the second semiconductor layer 12 is 0.4 or more, electron overflow can be efficiently blocked at the lower end of the conduction band. Generally, the higher the Al composition ratio, the larger the bandgap and the higher the electron blocking effect. However, Al inactivates p-type impurities. When the Al composition ratio x2 exceeds 0.8, the contact resistivity between the second semiconductor layer 12 and the active layer increases due to the inactivation of p-type impurities, and there is a risk of an increase in the driving voltage. Therefore, the Al composition ratio x2 of the second semiconductor layer 12 is preferably 0.4 or more and 0.8 or less.

[0023] The Al composition ratio x3 of the third semiconductor layer 13 is smaller than the Al composition ratio x2 of the second semiconductor layer 12. The Al composition ratio x3 of the third semiconductor layer 13 is larger than the Al composition ratio x1 of the first semiconductor layer 11 and the Al composition ratio x4 of the fourth semiconductor layer 14. The third semiconductor layer 13 is a composition gradient layer. The Al composition ratio x3 of the third semiconductor layer 13 gradually decreases as it approaches the side closer to the fourth semiconductor layer 14 from the side closer to the second semiconductor layer 12, and sharply decreases at the boundary with the fourth semiconductor layer 14. The composition gradient of the third semiconductor layer 13 starts from the boundary between the second semiconductor layer 12 and the third semiconductor layer 13. Thereby, an increase in the operating voltage caused by the third semiconductor layer 13 can be reduced. The absolute value of the change rate of the Al composition ratio from the boundary between the third semiconductor layer 13 and the fourth semiconductor layer 14 toward the position where the Al composition ratio x4 shows the minimum value in the fourth semiconductor layer 14 is larger than the absolute value of the change rate of the Al composition ratio of the third semiconductor layer 13. In other words, in the third semiconductor layer 13, the Al composition ratio x3, which gradually decreased from the boundary with the second semiconductor layer 12 to the vicinity of the boundary with the fourth semiconductor layer 14, sharply decreases at the boundary with the fourth semiconductor layer 14, resulting in a stepped change in the distribution of the Al composition ratio. With this configuration, the electron blocking effect in the third semiconductor layer 13 can be enhanced.

[0024] Among the third semiconductor layer 13, the Al composition ratio in the region closest to the second semiconductor layer 12 is 25% ± 5%. Among the third semiconductor layer 13, the Al composition ratio in the region closest to the fourth semiconductor layer 14 is 18% ± 5% under the condition that it is smaller than the Al composition ratio in the vicinity of the boundary with the second semiconductor layer 12. The Al composition ratio at the point closest to the fourth semiconductor layer 14 among the third semiconductor layer 13 is larger than the Al composition ratio x4 of the fourth semiconductor layer 14. With this configuration, the electron blocking effect in the third semiconductor layer 13 can be enhanced. If the thickness of the third semiconductor layer 13 is, for example, 10 nm, the Al composition ratio x3 gradually decreases over this thickness range.

[0025] In the second semiconductor layer 12, the absolute value of the change rate of the Al composition ratio x2 from the position where the Al composition ratio shows the maximum value to the boundary between the second semiconductor layer 12 and the third semiconductor layer 13 is larger than the absolute value of the change rate of the Al composition ratio x3 of the third semiconductor layer 13. With this configuration, the electron blocking effect in the second semiconductor layer 12 can be enhanced. Also, from the boundary between the first semiconductor layer 11 and the second semiconductor layer 12 to the position where the Al composition ratio shows the maximum value in the second semiconductor layer 12, the absolute value of the change rate of the Al composition ratio is larger than the absolute value of the change rate of the Al composition ratio x3 of the third semiconductor layer. In other words, in the second semiconductor layer 12, the Al composition ratio x2 increases rapidly near the boundary with the first semiconductor layer 11, and the Al composition ratio x2 decreases rapidly near the boundary with the third semiconductor layer 13. With this configuration, the electron blocking effect in the second semiconductor layer 12 can be enhanced. However, looking at the change amount (absolute value) of the Al composition ratio, the change amount on the third semiconductor layer 13 side is smaller.

[0026] The Al composition ratio x4 of the fourth semiconductor layer 14 is 1% or more and 10% or less. With this configuration, the optical loss due to Al can be reduced. The fourth semiconductor layer 14 may be used, for example, as a p-side optical guide layer. The p-side optical guide layer has a lower refractive index and a larger bandgap than the first semiconductor layer 11.

[0027] In the example of FIG. 2, the Al composition ratio x1 in the first semiconductor layer 11 is 1% or more and 10% or less. With this configuration, the optical loss due to Al can be reduced. When the first semiconductor layer 11 is formed of In x1 In y1 Ga 1-x1-y1 N (0≦x1≦1, 0≦y1≦1, 0≦x1 + y1≦1), as described above, the target oscillation wavelength can be designed by adjusting the Al composition ratio x1 and the In composition ratio y1. The Al composition ratio x1 of the first semiconductor layer may be appropriately adjusted within the range of 1 / 50 or more and 1 / 5 or less of the peak Al composition ratio x2 of the second semiconductor layer 12.

[0028] By having the Al composition ratio distribution shown in Fig. 2, the semiconductor laser element 10 suppresses the overflow of electrons from the first semiconductor layer 11 to the fourth semiconductor layer 14, and also makes it difficult to inhibit the injection of holes from the fourth semiconductor layer 14 to the first semiconductor layer 11, improving the carrier injection efficiency. When the carrier injection efficiency is improved, oscillation can be achieved with a smaller current, the output at the same injection current is improved, and the power conversion efficiency is improved.

[0029] Note that the bandgap energies of the first semiconductor layer 11, the second semiconductor layer 12, the third semiconductor layer 13, and the fourth semiconductor layer 14 may have the following relationship. That is, the bandgap energy of the second semiconductor layer 12 is higher than the bandgap energies of the first semiconductor layer 11, the third semiconductor layer 13, and the fourth semiconductor layer 14. Also, the bandgap energy of the third semiconductor layer 13 is higher than the bandgap energies of the first semiconductor layer 11 and the fourth semiconductor layer 14. The bandgap energy of the third semiconductor layer 13 gradually decreases as it approaches the side closer to the fourth semiconductor layer 14 from the side closer to the second semiconductor layer 12.

[0030] (Fabrication and Evaluation of Samples) Based on the Al composition ratio distribution in Fig. 2, a sample of the semiconductor laser element 10 was fabricated. An n-side semiconductor layer 3 with a thickness of 3 μm was formed on a GaN substrate. Silicon-doped AlGaN was grown using ammonia, trimethylaluminum (TMA), trimethylgallium (TMG), and monosilane as source gases.

[0031] After the formation of the n-side semiconductor layer 3, the active layer 15 and the first semiconductor layer 11 were grown. The average Al composition ratio x1 of the first semiconductor layer 11 is 4.5%.

[0032] After the formation of the first semiconductor layer 11, the second semiconductor layer 12 was formed. The second semiconductor layer 12 may be used as part of an electron blocking layer. An Al 0.5 Ga 0.5 N second semiconductor layer 12 with a thickness of about 1.0 nm was formed using ammonia, TMA, and TMG.

[0033] Subsequently, ammonia, TMA, TMG, and biscyclopentadienylmagnesium (Cp 2 Mg) are used to form the third semiconductor layer 13. The third semiconductor layer 13 is disposed closer to the p-side contact layer than the second semiconductor layer 12 and can be used as part of the electron blocking layer. For example, a third semiconductor layer 13 of Mg-doped AlGaN with a thickness of 9.0 nm is formed. Cp 2 From the start of introducing CpMg, the flow rate of TMA is gradually reduced to adjust the thickness-direction distribution of the Al composition ratio x3 of the third semiconductor layer 13. The Al composition ratio immediately after the start of film formation of the third semiconductor layer 13, that is, the Al composition ratio near the boundary with the second semiconductor layer 12, is set to 25%. Thereafter, the flow rate of TMA is gradually decreased to make the Al composition ratio at the outermost surface of the third semiconductor layer 13 18%. Thereby, the second semiconductor layer 12 and the third semiconductor layer 13 having the Al composition ratio distribution of FIG. 2 are formed.

[0034] Subsequently, ammonia, TMA, TMG, and Cp 2 Mg are used to form a Mg-doped fourth semiconductor layer 14. The fourth semiconductor layer 14 is a p-side semiconductor layer close to the p-side contact layer and includes a p-side optical guide layer, a p-side clad layer, a p-side contact layer, and the like. The average value of the Al composition ratio x4 of the fourth semiconductor layer 14 is, for example, 4.5%. Thereby, a stack including the first semiconductor layer 11 to the fourth semiconductor layer 14 is formed on the substrate 1.

[0035] A mask for etching is formed in a predetermined region on the fourth semiconductor layer 14, and by dry etching, the fourth semiconductor layer 14 is removed up to the middle of the first semiconductor layer 11. Then, the mask is removed to obtain the ridge shape shown in FIG. 1. An insulating film 19 is formed to cover the entire surface of the fourth semiconductor layer 14. The insulating film 19 on the upper surface of the ridge 16 is removed, and a p-side electrode 18 is formed on the upper surface of the fourth semiconductor layer 14. The p-side electrode 18 is formed of a good conductor such as gold, platinum, titanium, aluminum, or an alloy thereof. Depending on the application, the p-side electrode 18 may be a transparent electrode such as ITO. Also, an n-side electrode 17 is formed on the back surface of the substrate 1. The n-side electrode 17 is formed of a good conductor such as gold, platinum, titanium, aluminum, or an alloy thereof.

[0036] FIG. 3 is a HAADF-STEM image of the fabricated sample. The vertical direction of the image in FIG. 3 is the same as the stacking direction in FIG. 1. The lower side in FIG. 3 is the first semiconductor layer 11, and the upper side is the fourth semiconductor layer 14. A second semiconductor layer 12 with a high Al composition ratio is confirmed between the third semiconductor layer 13 and the first semiconductor layer 11. The second semiconductor layer 12 may be used together with the third semiconductor layer 13 as part of an electron blocking layer.

[0037] FIG. 4 is a diagram showing the measured values of the Al composition ratio from the first semiconductor layer 11 to the fourth semiconductor layer 14. The horizontal axis represents the position [nm] in the depth direction from the fourth semiconductor layer 14 forming the ridge 16 toward the first semiconductor layer 11, and the vertical axis represents the Al composition ratio [atomic %]. The Al composition ratio is measured by secondary ion mass spectrometry (SIMS). The left end (0 nm) of the horizontal axis is the position 5 nm into the fourth semiconductor layer from the boundary between the third semiconductor layer 13 and the fourth semiconductor layer 14, and the right end (22 nm) is the position 7 nm into the first semiconductor layer from the boundary between the first semiconductor layer 11 and the second semiconductor layer. The ion beam is sputtered, and the mass analysis of the secondary ions generated in the order of the fourth semiconductor layer 14, the third semiconductor layer 13, the second semiconductor layer 12, and the first semiconductor layer 11 is repeated to measure the composition ratio in the depth direction.

[0038] The Al composition ratio changes steeply from 50% to 4.5% at the interface between the second semiconductor layer 12 and the first semiconductor layer 11, which is located between 14 nm and 15 nm. In the sample fabrication process, since the Al composition value is switched between the first semiconductor layer 11 and the second semiconductor layer 12, the change in the Al composition ratio becomes discontinuous and changes steeply. At the interface between the second semiconductor layer 12 and the third semiconductor layer 13, which is located between 12 nm and 14 nm, the Al composition value changes from 50% to 25%. Also between the second semiconductor layer 12 and the third semiconductor layer 13, since the Al composition value is switched in the sample fabrication process, the Al composition ratio changes steeply. In the third semiconductor layer 13, which is located between 5 nm and 12 nm, the Al composition ratio changes continuously from 18% to 25%. At the interface between the third semiconductor layer 13 and the fourth semiconductor layer 14, which is located near 5 nm, the Al composition ratio changes abruptly from 18% to 4.5%. In the sample fabrication process, when the fourth semiconductor layer 14 is formed on the third semiconductor layer 13, the Al composition value is switched, so the Al composition ratio changes abruptly. The Al composition ratio in FIG. 4 is measured continuously by SIMS and may include measurement errors, so there are also parts where the distribution profile of the Al composition ratio has a slope, but the measured values are in good agreement with the design values in FIG. 2.

[0039] As shown in FIGS. 3 and 4, by making the Al composition ratio of the second semiconductor layer 12 the highest, the energy barrier at the lower end of the conduction band becomes higher on the boundary side with the first semiconductor layer 11. From the measurement profile in the stacking direction of the Al composition ratio in FIG. 4, it can be seen that the overflow of electrons to the fourth semiconductor layer 14 is effectively reduced. With this structure, it is considered that electrons can be effectively blocked and the generation of carrier loss due to non-radiative recombination in the electron blocking layer can be reduced.

[0040] FIG. 5 is a diagram showing a configuration example having a peak in the Al composition ratio in a region near the boundary between an electron blocking layer and a p-side semiconductor layer close to the p-side contact layer as a comparative example. In this figure, on the horizontal axis, the n-side in the stacking direction is the side close to the active layer, and the p-side is the side close to the p-side contact layer. The region having a peak in the Al composition ratio corresponds to the electron blocking layer. The electron blocking layer contains, for example, a p-type impurity such as Mg. The layer of the first semiconductor layer closest to the electron blocking layer has a discontinuous change in the Al composition ratio and rises to 2% at the boundary between the first semiconductor layer and the electron blocking layer.

[0041] In the electron blocking layer, as the distance from the outermost surface of the first semiconductor layer 11 increases, the Al composition ratio transitions from 2% to 35%. In the electron blocking layer of the comparative example, the peak (35%) in the Al composition ratio is in the region of the electron blocking layer farthest from the active layer, and the Al composition ratio rapidly decreases in a region farther from the active layer than this region. In the comparative example, the Al composition ratio in the layer on the side farther from the active layer with reference to the location showing the peak in the Al composition ratio is 4.5%, the same as that of the fourth semiconductor layer 14 of the embodiment.

[0042] In the comparative example of FIG. 5, the energy barrier at the lower end of the conduction band of the electron blocking layer having a peak in the Al composition ratio is formed in the region of the electron blocking layer farthest from the active layer and decreases while inclining toward the active layer. In this structure, it is considered that electrons injected into the active layer undergo non-radiative recombination in the electron blocking layer before reaching the region having a peak in the Al composition ratio, resulting in carrier loss.

[0043] FIG. 6 is a diagram showing the slope efficiency of the semiconductor laser elements of the example and the comparative example. The horizontal axis represents the injection current [mA], and the vertical axis represents the rate of increase in the optical output with respect to the current (dL / dI). This rate of increase in the optical output with respect to the injection current is the slope efficiency. The semiconductor laser element of the example maintains a substantially constant optical output even as the injection current increases. In contrast, in the semiconductor laser element of the comparative example, the slope efficiency decreases in the high current region. This is because electrons injected into the active layer flow out to the p-side and do not contribute to the generation of light.

[0044] According to the semiconductor laser element 10 of the embodiment, (a) The thickness of the second semiconductor layer 12 is made smaller than the thickness of the third semiconductor layer 13, (b) The Al composition ratio x2 of the second semiconductor layer 12 is made larger than any of the Al composition ratio x1 of the first semiconductor layer 11, the Al composition ratio x3 of the third semiconductor layer 13, and the Al composition ratio x4 of the fourth semiconductor layer 14, (c) The Al composition ratio x3 of the third semiconductor layer 13 is made larger than the Al composition ratio x1 of the first semiconductor layer and the Al composition ratio x4 of the fourth semiconductor layer, By doing so, the carrier injection efficiency can be improved.

[0045] By making the absolute value of the change rate of the Al composition ratio from the first semiconductor layer 11 to the second semiconductor layer 12 larger than the absolute value of the change rate of the Al composition ratio in the third semiconductor layer 13, the overflow of electrons to the p side can be effectively blocked.

[0046] By making the absolute value of the change rate of the Al composition ratio from the second semiconductor layer 12 to the third semiconductor layer 13 larger than the absolute value of the change rate of the Al composition ratio in the third semiconductor layer 13, the overflow of electrons to the p side can be effectively blocked.

[0047] By making the thickness of the second semiconductor layer 12 0.5 nm or more and 2.5 nm or less, an increase in the driving voltage can be reduced. By making the Al composition ratios of the first semiconductor layer 11 and the fourth semiconductor layer 14 1% or more and 10% or less, a decrease in the hole injection efficiency can be suppressed and the electrical conductivity can be improved.

[0048] Embodiments of the present disclosure may include, for example, the following configurations. (Item 1) A light-emitting element having a first semiconductor layer, a second semiconductor layer, a third semiconductor layer, and a fourth semiconductor layer in this order, The first semiconductor layer is Al x1 In y1 Ga 1-x1-y1 N (0 ≦ x1 ≦ 1, 0 ≦ y1 ≦ 1, 0 ≦ x1 + y1 ≦ 1), The second semiconductor layer is Alx2 In y2 Ga 1-x2-y2 N (0 ≤ x2 ≤ 1, 0 ≤ y2 ≤ 1, 0 ≤ x2 + y2 ≤ 1), wherein the third semiconductor layer is Al x3 In y3 Ga 1-x3-y3 N (0 ≤ x3 ≤ 1, 0 ≤ y3 ≤ 1, 0 ≤ x3 + y3 ≤ 1), wherein the fourth semiconductor layer is Al x4 In y4 Ga 1-x4-y4 N (0 ≤ x4 ≤ 1, 0 ≤ y4 ≤ 1, 0 ≤ x4 + y4 ≤ 1), wherein the second semiconductor layer and the third semiconductor layer contain p-type impurities, wherein the thickness of the second semiconductor layer is smaller than the thickness of the third semiconductor layer, wherein the Al composition ratio x2 of the second semiconductor layer is larger than any of the Al composition ratio x1 of the first semiconductor layer, the Al composition ratio x3 of the third semiconductor layer, and the Al composition ratio x4 of the fourth semiconductor layer, wherein the Al composition ratio x3 of the third semiconductor layer is larger than either the Al composition ratio x1 of the first semiconductor layer or the Al composition ratio x4 of the fourth semiconductor layer, A light-emitting element. (Item 2) wherein the Al composition ratio x3 of the third semiconductor layer decreases as approaching from the second semiconductor layer to the fourth semiconductor layer, wherein the absolute value of the change rate of the Al composition ratio from the boundary between the third semiconductor layer and the fourth semiconductor layer to the position where the Al composition ratio shows the minimum value in the fourth semiconductor layer is larger than the absolute value of the change rate of the Al composition ratio x3 of the third semiconductor layer, The light-emitting element according to Item 1. (Item 3) wherein the absolute value of the change rate of the Al composition ratio from the boundary between the first semiconductor layer and the second semiconductor layer to the position where the Al composition ratio shows the maximum value in the second semiconductor layer is larger than the absolute value of the change rate of the Al composition ratio x3 of the third semiconductor layer, The light-emitting element according to Item 1 or 2. (Item 4) The absolute value of the change rate of the Al composition ratio from the position where the Al composition ratio shows the maximum value in the second semiconductor layer to the boundary between the second semiconductor layer and the third semiconductor layer is greater than the absolute value of the change rate of the Al composition ratio x3 of the third semiconductor layer. The light-emitting element according to any one of Items 1 to 3. (Item 5) The thickness of the second semiconductor layer is greater than 0.5 nm and 2.5 nm or less. The light-emitting element according to any one of Items 1 to 4. (Item 6) The total thickness of the second semiconductor layer and the third semiconductor layer is greater than 5 nm and 30 nm or less. The light-emitting element according to any one of Items 1 to 5. (Item 7) The Al composition ratio x2 of the second semiconductor layer is 40% or more and 80% or less. The light-emitting element according to any one of Items 1 to 6. (Item 8) The Al composition ratio x1 of the first semiconductor layer is 1% or more and 10% or less. The light-emitting element according to any one of Items 1 to 7. (Item 9) The Al composition ratio x4 of the fourth semiconductor layer is 1% or more and 10% or less. The light-emitting element according to any one of Items 1 to 8. (Item 10) The fourth semiconductor layer contains an impurity of the first conductivity type. The third semiconductor layer contains a smaller amount of the impurity of the first conductivity type than the fourth semiconductor layer. The light-emitting element according to any one of Items 1 to 9.

Explanation of Signs

[0049] 1 Substrate 3 n-side semiconductor layer 10 Semiconductor laser element 11 First semiconductor layer 12 Second semiconductor layer 13 Third semiconductor layer 14 Fourth semiconductor layer 15 Active layer 17 n-side electrode 18 p-side electrode x1 Al composition ratio of the first semiconductor layer x2 Al composition ratio of the second semiconductor layer x3 Al composition ratio of the third semiconductor layer x4 Al composition ratio of the fourth semiconductor layer

Claims

1. A light emitting device having a first semiconductor layer, a second semiconductor layer, a third semiconductor layer, and a fourth semiconductor layer in this order, The first semiconductor layer is Al ｘ１ In ｙ１ G １－ｘ１－ｙ１ N (0≦x1≦1, 0≦y1≦1, 0≦x1+y1≦1), The second semiconductor layer is Al ｘ２ In ｙ２ G １－ｘ２－ｙ２ N (0≦x2≦1, 0≦y2≦1, 0≦x2+y2≦1), The third semiconductor layer is Al ｘ３ In ｙ３ G １－ｘ３－ｙ３ N (0≦x3≦1, 0≦y3≦1, 0≦x3+y3≦1), The fourth semiconductor layer is Al ｘ４ In ｙ４ G １－ｘ４－ｙ４ N (0≦x4≦1, 0≦y4≦1, 0≦x4+y4≦1), the second semiconductor layer and the third semiconductor layer contain p-type impurities, the thickness of the second semiconductor layer is smaller than the thickness of the third semiconductor layer; an Al composition ratio x2 of the second semiconductor layer is greater than an Al composition ratio x1 of the first semiconductor layer, an Al composition ratio x3 of the third semiconductor layer, and an Al composition ratio x4 of the fourth semiconductor layer; an Al composition ratio x3 of the third semiconductor layer is greater than both an Al composition ratio x1 of the first semiconductor layer and an Al composition ratio x4 of the fourth semiconductor layer; Light emitting element.

2. the Al composition ratio x3 of the third semiconductor layer decreases from the second semiconductor layer toward the fourth semiconductor layer, an absolute value of a rate of change of the Al composition ratio from a boundary between the third semiconductor layer and the fourth semiconductor layer to a position in the fourth semiconductor layer where the Al composition ratio has a minimum value is greater than an absolute value of a rate of change of the Al composition ratio x3 of the third semiconductor layer; The light-emitting device according to claim 1 .

3. an absolute value of a rate of change of the Al composition ratio from a boundary between the first semiconductor layer and the second semiconductor layer to a position in the second semiconductor layer where the Al composition ratio has a maximum value is greater than an absolute value of a rate of change of the Al composition ratio x3 of the third semiconductor layer; The light-emitting device according to claim 1 .

4. an absolute value of a rate of change of the Al composition ratio from a position where the Al composition ratio in the second semiconductor layer has a maximum value to a boundary between the second semiconductor layer and the third semiconductor layer is greater than an absolute value of a rate of change of the Al composition ratio x3 of the third semiconductor layer; The light-emitting device according to claim 1 .

5. The thickness of the second semiconductor layer is greater than 0.5 nm and less than or equal to 2.5 nm. The light-emitting device according to claim 1 .

6. The sum of the thicknesses of the second semiconductor layer and the third semiconductor layer is greater than 5 nm and less than or equal to 30 nm. The light-emitting device according to claim 1 .

7. The Al composition ratio x2 of the second semiconductor layer is 40% or more and 80% or less. The light-emitting device according to claim 1 .

8. The Al composition ratio x1 of the first semiconductor layer is 1% or more and 10% or less. The light-emitting device according to claim 1 .

9. The Al composition ratio x4 of the fourth semiconductor layer is 1% or more and 10% or less. The light-emitting device according to claim 1 .

10. the fourth semiconductor layer contains impurities of a first conductivity type; the third semiconductor layer contains a smaller amount of impurities of the first conductivity type than the fourth semiconductor layer; The light-emitting device according to claim 1 .

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