Nitride semiconductor light-emitting element

Abstract

To provide a nitride semiconductor light-emitting element capable of improving light emission efficiency by improving re-coupling efficiency of an electron and a hole in a quantum well layer.SOLUTION: A nitride semiconductor light-emitting element comprises in a thickness direction a plurality of barrier layers, a quantum well layer held between two barrier layers, and an active layer including a gradient layer disposed between the barrier layer and the quantum well layer. An In composition in the quantum well layer is higher than an In composition in the barrier layer, and an In composition in the gradient layer becomes higher from the side of the barrier layer toward the side of the quantum well layer in the thickness direction. When an average value of In compositions in the barrier layers is defined as In_b, an average value of In compositions in the quantum well layers is defined as In_w, a maximum value of In compositions in the gradient layers is defined as In_smax and a minimum value of the In compositions in the gradient layers is defined as In_smin, a relation of In_w>In_smax>In_Smin≥In_b is satisfied.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a light emitting device using a nitride semiconductor. [Background technology]

[0002] Light-emitting devices using nitride semiconductors have been developed. In general nitride semiconductor light-emitting devices, the active layer (light-emitting layer) that emits light has a barrier layer, a quantum well layer, an optical guide layer, etc. By arranging the quantum well layer so that it is sandwiched between the barrier layers, efficient light emission is achieved.

[0003] In light-emitting devices using nitride semiconductors, for example, when the quantum well layer included in the light-emitting layer (active layer) made of InGaN has a uniform In composition, compressive strain due to the difference in lattice constants can occur between the barrier layers or optical guide layers arranged on both sides of the quantum well layer. If the strain applied to the quantum well layer causes the energy band in the quantum well layer to tilt, the distributions of electrons and holes will be biased in opposite directions in the thickness direction of the light-emitting device (the growth direction of each layer). In this case, the probability of recombination between electrons and holes in the quantum well layer decreases, resulting in the quantum Stark effect, which reduces the light-emitting efficiency due to recombination.

[0004] Patent Document 1 discloses a technique for increasing the overlap between the wave functions in the valence band and the conduction band by increasing the indium content in the active quantum well in a stepwise and / or ramped manner in the growth direction, which suppresses the quantum Stark effect and increases the efficiency of the semiconductor chip. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2012-522390 Summary of the Invention [Problem to be solved by the invention]

[0006] In addition to the quantum Stark effect described above, strain in the quantum well layer of a nitride semiconductor light-emitting device can cause dislocations or fluctuations in the In composition within the quantum well layer. Dislocations or fluctuations in the In composition can trap some of the recombination of electrons and holes in deep levels created in the forbidden band (band gap) within the quantum well layer, preventing them from contributing to LED light emission or laser oscillation.

[0007] An object of the present disclosure is to provide a nitride semiconductor light-emitting element that can improve the recombination efficiency of electrons and holes in the quantum well layer and thereby improve the light-emitting efficiency by suppressing dislocations or fluctuations in the In composition that occur in the quantum well layer. [Means for solving the problem]

[0008] A nitride semiconductor light-emitting device according to one aspect of the present disclosure is a semiconductor light-emitting device having a structure in which nitride-based semiconductor layers including an active layer are stacked in a thickness direction, the active layer including, along the thickness direction, a plurality of barrier layers, a quantum well layer sandwiched between two of the barrier layers, and a gradient layer disposed between the barrier layer and the quantum well layer, the barrier layers, the quantum well layer, and the gradient layer each containing In, the In composition in the quantum well layer is higher than the In composition in the barrier layers, and the In composition in the gradient layer increases from the barrier layer side to the quantum well layer side along the thickness direction, and when an average value of the In composition in the barrier layers is In_b, an average value of the In composition in the quantum well layers is In_w, a maximum value of the In composition in the gradient layer is In_smax, and a minimum value of the In composition in the gradient layer is In_smin, In_w>In_smax>In_Smin≧In_b (1) The relationship is satisfied. [Effects of the Invention]

[0009] According to the present disclosure, by suppressing dislocations or fluctuations in the In composition occurring in the quantum well layer, the recombination efficiency of electrons and holes in the quantum well layer can be improved, thereby improving the light-emitting efficiency. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view illustrating the structure of a nitride semiconductor light-emitting evaluation sample as an example of a nitride semiconductor light-emitting device according to the present disclosure. [Figure 2] Energy band diagram of nitride semiconductor light-emitting device [Figure 3] FIG. 1 is a diagram showing an example of the structure of a general nitride semiconductor light-emitting device that does not have a gradient layer. [Figure 4] Graph showing the light emission characteristics of a typical nitride semiconductor light emitting device without a gradient layer. [Figure 5] 1 is a graph showing light emission characteristics of a nitride semiconductor light emitting device according to an embodiment of the present disclosure. [Figure 6] This diagram shows the energy band and In composition when two graded layers arranged on both sides of the quantum well layer in the thickness direction have In compositions that change symmetrically with respect to the quantum well layer. [Figure 7] This diagram shows the energy band and In composition when two graded layers arranged on both sides of the quantum well layer in the thickness direction have In compositions that change asymmetrically with respect to the quantum well layer. [Figure 8] FIG. 10 is a diagram for explaining the effect of the present disclosure when the In composition distribution and film thickness of the first gradient layer and the In composition distribution and film thickness of the second gradient layer are asymmetric with respect to the quantum well layer. [Figure 9] FIG. 1 is a diagram showing an example of application of the present disclosure to a nitride semiconductor light-emitting device having a DQW structure. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. However, more detailed explanation than necessary, such as detailed explanation of well-known matters or redundant explanation of substantially the same configuration, may be omitted.

[0012] <Configuration> 1 is a cross-sectional view illustrating the structure of a nitride semiconductor light-emitting evaluation sample 100 as an example of a nitride semiconductor light-emitting device according to the present disclosure. The nitride semiconductor light-emitting evaluation sample 100 has a structure in which a buffer layer 20, an active layer 30, and a cap layer 40 are stacked on a substrate 10. The stacking direction of each layer in FIG. 1 is referred to as the thickness direction in this specification.

[0013] The substrate 10 and the buffer layer 20 are made of, for example, n-GaN, and the cap layer 40 is made of, for example, u-GaN.

[0014] The active layer 30 includes a plurality of barrier layers 31, a quantum well layer 32 sandwiched between two barrier layers 31, and a gradient layer 33 disposed between the barrier layers 31 and the quantum well layer 32. The barrier layers 31, the quantum well layers 32, and the gradient layer 33 are made of, for example, InGaN. As will be described in detail later, the In compositions of the barrier layers 31, the quantum well layers 32, and the gradient layer 33 are all different from one another.

[0015] Next, the In composition in the active layer 30 will be described. The In composition in each of the barrier layer 31 and the quantum well layer 32 is approximately uniform in the thickness direction. The In composition in the quantum well layer 32 is higher than the In composition in the barrier layer 31. As an example, the In composition in the barrier layer 31 is 3%, and the In composition in the quantum well layer 32 is 17%.

[0016] The In composition in the gradient layer 33 varies along the thickness direction. Specifically, the In composition in the gradient layer 33 increases from the barrier layer 31 side toward the quantum well layer 32 side along the thickness direction. That is, the In composition on the barrier layer 31 side of the gradient layer 33 is the minimum value of the In composition in the gradient layer 33, and the In composition on the quantum well layer 32 side of the gradient layer 33 is the maximum value of the In composition in the gradient layer 33. As an example, the In composition (minimum value) on the barrier layer 31 side of the gradient layer 33 is 3.4%, and the In composition (maximum value) on the quantum well layer 32 side of the gradient layer 33 is 7.5%. As an example, the quantum well layer 32 has a thickness of 2.8 nm, and the gradient layer 33 has a thickness of 2.0 nm. The thicknesses of the multiple quantum well layers 32 included in the active layer 30 may be different from one another or the same. The same applies to the thicknesses of the barrier layers 31 and the gradient layers 33. FIG. 1 shows an example in which the thicknesses of the barrier layers 31, the quantum well layers 32, and the gradient layers 33 included in the active layer 30 are all the same.

[0017] In this way, it is desirable to set the In composition (maximum value) on the quantum well layer 32 side of the gradient layer 33 to be half or less of the In composition in the quantum well layer 32. This makes it possible to suppress the influence of strain in the quantum well layer 32 without causing a large change in the quantum level in the quantum well layer 32 due to the influence of the gradient layer 33.

[0018] The above can be summarized as follows: When the average value of the In composition in the barrier layer 31 is In_b, the average value of the In composition in the quantum well layer 32 is In_w, the maximum value of the In composition in the gradient layer 33 is In_smax, and the minimum value of the In composition in the gradient layer is In_smin, the relationship of the following formula (1) is satisfied. In_w>In_smax>In_Smin≧In_b (1)

[0019] <Effects> 2 is an energy band diagram of the nitride semiconductor light emission evaluation sample 100 shown in FIG. 1. FIG. 2A shows an energy band diagram of the entire active layer 30, and FIG. 2B shows an extracted portion of the conduction band (Ec) corresponding to a structure centered on one quantum well layer 32. In FIG. 2A, Egb and Egw indicate the band gaps between the barrier layer 31 and the quantum well layer 32. In FIG. 2A, as in FIG. 1, the vertical direction of the diagram corresponds to the thickness direction, but in FIG. 2B, for simplicity, the thickness direction is converted to the horizontal direction of the diagram.

[0020] In Figure 2B, the vertical direction of the figure corresponds to the energy level and the In composition of each structure. The In composition is higher at the top of the figure and lower at the bottom. Accordingly, the energy level is higher at the top of the figure and lower at the bottom.

[0021] 2, the In composition in the region between the end on the barrier layer 31 side and the end on the quantum well layer 32 side in the thickness direction of the gradient layer 33 varies monotonically, but the present disclosure is not limited to this. Details of how the In composition in the gradient layer 33 is varied in the present disclosure will be described later.

[0022] As shown in FIG. 2B , in the active layer 30, gradient layers 33 are arranged on both sides of the quantum well layer 32 between the barrier layer 31 and the quantum well layer 32 so as to contact the barrier layer 31 and the quantum well layer 32. The gradient layers 33 have a band gap that narrows from the band gap (Egb) of the barrier layer 31 to less than half the band gap (Egw) of the quantum well layer 32 from the barrier layer 31 toward the quantum well layer 32.

[0023] In this way, by making the In composition in the quantum well layer 32 approximately uniform in the thickness direction and inserting the gradient layer 33, in which the In composition varies along the thickness direction, between the barrier layer 31 and the quantum well layer 32, it is possible to suppress the occurrence of distortion in the quantum well layer 32 due to the difference in lattice constants between the barrier layer 31 and the quantum well layer 32 without significantly changing the quantum levels of electrons and holes formed in the quantum well layer 32.

[0024] It is also possible to suppress the generation of strain in the quantum well layer 32 by varying the In composition in the barrier layer or quantum well layer along the thickness direction, rather than providing the gradient layer 33 between the barrier layer 31 and the quantum well layer 32 as in the present disclosure. However, in this case, the effective well width of the quantum well layer changes significantly, resulting in a significant change in the quantum level in the quantum well layer. This results in a significant change in the wavelength of light generated from the active layer due to recombination of electrons and holes present due to the quantum level.

[0025] According to the present disclosure, by setting the maximum value of the In composition in the gradient layer 33 to half or less of the In composition in the quantum well layer 32, it is possible to suppress the occurrence of strain in the quantum well layer 32 without significantly changing the quantum level occurring in the quantum well layer 32. This makes it possible to suppress dislocations and fluctuations in the In composition in the quantum well layer 32 that are caused by strain.

[0026] (Comparison with comparative examples) In order to specifically demonstrate the effects obtained by the nitride semiconductor light emission evaluation sample 100 according to the embodiment of the present disclosure, a comparison will be made with a nitride semiconductor light emission evaluation sample 200 having a general structure.

[0027] FIG. 3 is a diagram showing an example of the structure of a general nitride semiconductor light emission evaluation sample 200 that does not have a gradient layer. The materials of each layer of the nitride semiconductor light emission evaluation sample 200 are the same as those of the nitride semiconductor light emission evaluation sample 100 according to the embodiment of the present disclosure. The nitride semiconductor light emission evaluation sample 200 does not have a structure equivalent to the gradient layer of the present disclosure, and the active layer 210 is composed of a barrier layer 211 and a quantum well layer 212. The active layer 210 and the barrier layer 211 have a substantially uniform In composition in the thickness direction. The active layer 210 has an average In composition of 3%, and the quantum well layer 212 has an average In composition of 17%. The nitride semiconductor light emission evaluation sample 200 has a double quantum well (DQW) structure with two quantum well layers 212.

[0028] Fig. 4 is a graph showing the emission characteristics of the nitride semiconductor light emission evaluation sample 200 shown in Fig. 3. Fig. 4 shows the PL (Photo Luminescence) wavelength spectrum when the nitride semiconductor light emission evaluation sample 200 shown in Fig. 3 is photoexcited by irradiating it with He—Cd laser light having a wavelength of 325 nm, thereby exciting the DQW structure of the sample and generating electrons and holes, and then emitting light through recombination of the generated electrons and holes. The vertical axis of Fig. 4 represents the PL intensity (logarithmically expressed), and the horizontal axis represents the wavelength. In Fig. 4, the emission characteristics of the nitride semiconductor light emission evaluation sample 200 when the temperature is changed from 10 K to 300 K (room temperature) are shown by separate lines.

[0029] 4, it can be seen that sub-peaks occur in addition to the highest main peak (areas surrounded by dashed lines in FIG. 4). This light emission is thought to be due to dislocations occurring in the quantum well layer 212 or fluctuations in the In composition.

[0030] Fig. 5 is a graph showing the emission characteristics of the nitride semiconductor emission evaluation sample 100 according to the embodiment of the present disclosure shown in Fig. 1 and Fig. 2. Like Fig. 4, Fig. 5 shows the PL wavelength spectrum obtained when light is emitted by optical excitation using a He—Cd laser beam having a wavelength of 325 nm. Like Fig. 4, the vertical axis of Fig. 5 represents the PL intensity (logarithmically expressed), and the horizontal axis represents the wavelength.

[0031] The graph shown in Figure 5 shows that the occurrence of peaks is suppressed at wavelengths where sub-peaks occur in Figure 4 (see arrows in Figures 4 and 5). That is, it can be seen that the nitride semiconductor light-emission evaluation sample 100 according to the embodiment of the present disclosure is able to suppress the influence on light emission of dislocations occurring in the quantum well layer 32 or fluctuations in the In composition. Furthermore, in the nitride semiconductor light-emission evaluation sample 100 according to the embodiment of the present disclosure, the occurrence of sub-peaks is suppressed, thereby improving the recombination efficiency of electrons and holes at the main peak and improving the light-emission efficiency at the main peak.

[0032] <About each parameter> The nitride semiconductor light emission evaluation sample 100 shown in FIGS. 1 and 2 is an example of a nitride semiconductor light emitting device according to the present disclosure, and in the present disclosure, parameters such as the In composition of each layer are not limited to the above-mentioned example.

[0033] The following describes in detail the relationships between the parameters that can be assumed by each component of the nitride semiconductor light emission evaluation sample 100 according to the present disclosure.

[0034] In the above example, it was explained that when the average value of the In composition in the barrier layer 31 is In_b, the average value of the In composition in the quantum well layer 32 is In_w, the maximum value of the In composition in the gradient layer 33 is In_smax, and the minimum value of the In composition in the gradient layer is In_smin, the relationship of the following formula (1) is satisfied. In_w>In_smax>In_Smin≧In_b (1)

[0035] As shown in formula (1), the minimum value of the In composition in the gradient layer may be the same as In_smin, and the average value of the In composition in the barrier layer 31 may be the same as In_b. In this case, the relationship in formula (2) below is satisfied. In_w>In_smax>In_Smin=In_b (2)

[0036] Of course, the minimum value of the In composition in the gradient layer and the average value of the In composition in the barrier layer 31 do not have to be the same value as In_smin.

[0037] In the above example, the maximum In composition in the gradient layer 33 is 7.5%, which is less than half the 17% In composition in the quantum well layer 32. If the maximum In composition in the gradient layer 33, In_smax, were as large as the average In composition in the quantum well layer 32, In_w, the quantum level in the quantum well layer 32 would shift significantly toward lower energy. In this case, the wavelength resulting from the recombination of electrons and holes would shift significantly toward longer wavelengths from the desired main peak wavelength. In addition, the increase in the number of layers with a high In composition would deteriorate the crystallinity near the quantum well layer 32 made of InGaN, increasing dislocations in the quantum well layer 32 and resulting in reduced luminous efficiency.

[0038] To prevent such a situation, it is desirable that the In composition In_b in the barrier layer 31, the In composition In_w in the quantum well layer 32, and the maximum In composition In_smax in the gradient layer 33 further satisfy the relationship of the following formula (3) in addition to the relationship of the above formula (1). In_w>2In_s max >In_b (3)

[0039] When the relationship of formula (3) is satisfied, if the emission wavelength (desired emission wavelength) of a quantum well layer with a uniform In composition in the thickness direction in a typical nitride semiconductor light-emitting device (e.g., see FIG. 3 ) that does not use a gradient layer is 440 nm, then in nitride semiconductor light-emitting evaluation sample 100 of the present disclosure, the shift of the quantum level to the lower energy side that may occur due to the provision of gradient layer 33 is limited to 5 nm, and the emission wavelength is 445 nm. This wavelength shift of about 5 nm can be sufficiently corrected to 440 nm by adjusting the growth temperature of the InGaN layer, etc.

[0040] Furthermore, according to the nitride semiconductor light-emitting evaluation sample 100 of the present disclosure, which satisfies the relationship of formula (3), the decrease in light-emitting intensity caused by strain in the quantum well layer 32 is suppressed, and the adoption of the gradient layer 33 is expected to reduce the threshold current. Therefore, it is considered that the light-emitting efficiency in the quantum well layer 32 can be improved compared to a general nitride semiconductor light-emitting device that does not use a gradient layer.

[0041] In addition to the relationship of formula (3), the relationship of formula (4) below may be satisfied. In_w>2In_s max >In_S min =In_b (4)

[0042] Furthermore, more preferably, in addition to the relationship of the above formula (1), the relationship of the following formula (4) may be satisfied. In_w>2.5In_s max >In_b (5)

[0043] When the relationship in formula (5) is satisfied, the quantum level shifts to the lower energy side by 4 nm, and the emission wavelength is 444 nm. This makes it possible to reduce the wavelength shift compared to when the relationship in formula (3) is satisfied, making it easier to adjust factors such as the growth temperature.

[0044] The nitride semiconductor light-emitting evaluation sample 100 of the present disclosure, which satisfies the relationship of formula (5), also suppresses the decrease in light-emitting intensity caused by strain in the quantum well layer 32, and the adoption of the gradient layer 33 is expected to reduce the threshold current. Therefore, it is believed that the light-emitting efficiency in the quantum well layer 32 can be improved compared to general nitride semiconductor light-emitting devices that do not use a gradient layer.

[0045] In addition to the relationship of formula (5), the relationship of the following formula (6) may be satisfied. In_w>2.5In_s max >In_S min =In_b (6)

[0046] In the above explanation, the average values ​​of the In composition (In_n and In_w) of the barrier layer 31 and the quantum well layer 32 are used. However, when the variation in the In composition within the barrier layer 31 and the quantum well layer 32 is taken into consideration, the In composition in the barrier layer 31 and the quantum well layer 32 can be expressed as follows: The In composition in the barrier layer 31 is expressed as In_b(t), and the In composition in the quantum well layer 32 is expressed as In_w(t). Here, t is a variable indicating the position in the thickness direction.

[0047] In the present disclosure, the variation in the In composition in the barrier layers 31 and the quantum well layers 32 may be, for example, within ±5% of the average value. In the example described above, the average In composition in the barrier layers 31 was set to 3.4%, but in this case, the actual In composition may be within a range of approximately 3.2% to approximately 3.6%. In the example described above, the average In composition in the quantum well layers was set to 17%, but in this case, the actual In composition may be within a range of approximately 16.2% to 17.9%. More preferably, the variation in the In composition in the barrier layers 31 and the quantum well layers 32 may be within a range of ±2% of the average value, and even more preferably, ±1%.

[0048] Next, the film thickness of each layer in the active layer 30 will be described.

[0049] In the above example, it has been described that the quantum well layer 32 has a thickness of 2.8 nm and the gradient layer 33 has a thickness of 2.0 nm. When the thickness of the quantum well layer 32 is L_w, the thickness of the gradient layer 33 in contact with the quantum well layer 32 and on the substrate 10 side as viewed from the quantum well layer 32 (an example of the first gradient layer of the present disclosure) is L_s1, and the thickness of the gradient layer 33 in contact with the quantum well layer 32 and on the cap layer 40 side as viewed from the quantum well layer 32 (an example of the second gradient layer of the present disclosure) is L_s2, it is desirable that the relationship of the following formula (7) or (8) be satisfied. L_w>L_s1 (7) L_w>L_s2 (8)

[0050] However, the thicknesses L_s1 and L_s2 of the gradient layer 33 must be equal to or greater than one atomic layer of InGaN, i.e., 0.25 nm = 2.5 Å or greater. This is because if the gradient layer 33 does not have a thickness equal to or greater than one atomic layer, it is difficult to obtain the effect of suppressing the influence of strain in the quantum well layer 32.

[0051] As long as the relationships of the above formulas (7) and (8) are satisfied, L_s1 and L_s2 do not need to have the same value.

[0052] In the above example, the case where the thickness L_w of the quantum well layer 32 is thicker than the thicknesses L_s1 and L_s2 of the grading layer 33 is described. However, in the present disclosure, the thickness L_w of the quantum well layer 32 may be thinner than the thicknesses L_s1 and L_s2 of the grading layer 33. Even in this case, the grading layer 33 can be expected to be effective in suppressing the influence of strain on the quantum well layer 32. However, if the grading layer 33 is made too thick, approaching or exceeding the critical thickness of InGaN, dislocations or fluctuations in the In composition of the grading layer 33 and the quantum well layer 32 will increase. To prevent this, the relationship of the following formula (9) or formula (10) must be satisfied. 2L_w>L_s1 (9) 2L_w>L_s2 (10)

[0053] <How to change the In composition in the gradient layer 33> As described above, in nitride semiconductor light emission evaluation sample 100 according to the embodiment of the present disclosure, the In composition of gradient layer 33 varies along the thickness direction. Hereinafter, a method for varying the In composition of gradient layer 33 in nitride semiconductor light emission evaluation sample 100 according to the embodiment of the present disclosure will be described.

[0054] In the example shown in Fig. 2B, in each of the two gradient layers 33 arranged on both sides of one quantum well layer 32 in the thickness direction, the In composition varies symmetrically with respect to the quantum well layer 32. The present disclosure is not limited to the example shown in Fig. 2B, and the In composition in each of the two gradient layers 33 arranged on both sides of one quantum well layer 32 in the thickness direction may vary in various ways as shown in Figs.

[0055] Fig. 6 is a diagram showing the energy band and In composition when two gradient layers 33 arranged on both sides of the quantum well layer 32 in the thickness direction have In compositions that change symmetrically with respect to the quantum well layer 32. Fig. 7 is a diagram showing the energy band and In composition when two gradient layers 33 arranged on both sides of the quantum well layer 32 in the thickness direction have In compositions that change asymmetrically with respect to the quantum well layer 32.

[0056] 6A and 7A show examples in which the In composition in the two gradient layers 33 monotonically decreases from the barrier layer 31 side toward the quantum well layer 32 side, and the minimum value of the In composition in the gradient layer 33 is the same as the In composition in the barrier layer 31.

[0057] 6B and 7B show examples in which the In composition in the two gradient layers 33 monotonically decreases from the barrier layer 31 side toward the quantum well layer 32 side, and the minimum value of the In composition in the gradient layer 33 is slightly higher than the In composition in the barrier layer 31.

[0058] 6C and 7C show examples in which the In composition in the two graded layers 33 decreases stepwise from the barrier layer 31 side toward the quantum well layer 32 side.

[0059] 6D and 7D, and 6E and 7E show examples in which the In composition in the two gradient layers 33 decreases in two or more stages from the barrier layer 31 side toward the quantum well layer 32 side.

[0060] 6F and 7F, and 6G and 7G show examples in which the In composition in the two gradient layers 33 decreases with a gradual change in the rate of change from the barrier layer 31 side to the quantum well layer 32 side.

[0061] However, each example shown in FIG. 6 satisfies the relationship of the following formula (11) or formula (12). In_w×L_w>∫ L_s In_s(t)dt (11) ∫ L_w In_w(t)dt>∫ L_s In_s(t)dt (12)

[0062] Note that In_w is the average In composition in the quantum well layer 32, L_w is the film thickness of the quantum well layer 32, L_s is the film thickness of the gradient layer 33, In_s(t) is the In composition distribution in the gradient layer 33, and In_w(t) is the In composition distribution in the quantum well layer 32 taking into account the variation in the In composition.

[0063] Moreover, each example shown in FIG. 7 satisfies the relationship of the following formula (13) or formula (14). In_w×L_w>∫ L_s1 In_s1(t)dt=∫ L_s2 In_s2(t)dt (13) ∫ L_w In_w(t)dt>∫ L_s1 In_s1(t)dt=∫ L_s2 In_s2(t)dt (14)

[0064] Note that In_w is the average In composition in the quantum well layer 32, L_w is the thickness of the quantum well layer 32, L_s1 is the thickness of the gradient layer 33 (first gradient layer) on the substrate side of the quantum well layer 32, L_s2 is the thickness of the gradient layer 33 (second gradient layer) on the cap layer 40 side of the quantum well layer 32, In_s1(t) is the In composition distribution in the first gradient layer, In_s2(t) is the In composition distribution in the second gradient layer, and In_w(t) is the In composition distribution in the quantum well layer 32 taking into account variations in the In composition.

[0065] In nitride semiconductor light emission evaluation sample 100 according to an embodiment of the present disclosure, when the relationship of formula (11) or (12), or formula (13) or (14) is satisfied, the product or integral of the In composition and film thickness in quantum well layer 32 becomes larger than the integral of the In composition distribution and film thickness in gradient layers 33 disposed on both sides of quantum well layer 32. In this way, the product or integral of the In composition and film thickness in quantum well layer 32 is sufficiently larger than the integral of the In composition distribution and film thickness in gradient layers 33 disposed on both sides of quantum well layer 32, so that gradient layers 33 can suppress the influence of strain in quantum well layer 32 without significantly changing the emission wavelength of the quantum well layer.

[0066] Furthermore, in nitride semiconductor light emission evaluation sample 100 according to the embodiment of the present disclosure, when the relationship of formula (13) or formula (14) is satisfied, the integrals of the In composition distribution and film thickness in gradient layers 33 disposed on both sides of quantum well layer 32 are the same. As a result, even if the change rates of the In composition distribution in gradient layers 33 disposed on both sides of quantum well layer 32 are different, the effect of suppressing the influence of strain in quantum well layer 32 (the occurrence of dislocations and fluctuations in the In composition in quantum well layer 32) by the two gradient layers 33 can be made approximately equal on both sides of the quantum well layer. This enhances the effect of suppressing the influence of strain in quantum well layer 32, which is achieved by providing gradient layers 33 on both sides of quantum well layer 32.

[0067] 8A to 8G correspond to FIGS. 7A to 7G, respectively. In each of the shaded regions in FIGS. 8A to 8G, the left side corresponds to the integral of the In composition distribution and film thickness of the first gradient layer, and the right side corresponds to the integral of the In composition distribution and film thickness of the second gradient layer. When these integrals are equal to each other, the above-described effect can be obtained.

[0068] More preferably, the relationship of the following formula (15), formula (16), formula (17), or formula (18) should be satisfied. In_w×L_w>2∫ L_s In_s(t)dt (15) ∫ L_w In_w(t)dt>2∫ L_s In_s(t)dt (16) In_w×L_w>∫ L_s1 In_s1(t)dt=2∫ L_s2 In_s2(t)dt (17) ∫ L_w In_w(t)dt>∫ L_s1 In_s1(t)dt=2∫ L_s2 In_s2(t)dt (18)

[0069] By satisfying these conditions, it is possible to further enhance the effect of suppressing the influence of strain in the quantum well layer 32 while preventing the quantum level in the quantum well layer 32 from being changed.

[0070] <Modification> The nitride semiconductor light emission evaluation sample 100 according to the embodiment of the present disclosure has been described above, but the present disclosure can take various forms other than the above-described embodiment.

[0071] In the above-described embodiment, FIG. 1 shows the nitride semiconductor light emission evaluation sample 100 having a DQW (Double Quantum Well) structure including two quantum well layers 32. However, the present disclosure can also be applied to nitride semiconductor light-emitting devices having an SQW (Single Quantum Well) structure including a single quantum well layer, or an MQW (Multiple Quantum Well) structure including three or more quantum well layers, and similar effects can be obtained.

[0072] 9A and 9B are diagrams showing examples of application of the present disclosure to a nitride semiconductor light-emitting device having a DQW structure. Fig. 9A shows an example in which the In composition distribution in the gradient layers arranged on both sides of the quantum well layer is symmetric with respect to the quantum well layer. Fig. 9B shows an example in which the In composition distribution in the gradient layers arranged on both sides of the quantum well layer is asymmetric with respect to the quantum well layer. Fig. 9C shows an example in which the barrier layer between the two quantum well layers is very thin.

[0073] When the present disclosure is applied to a nitride semiconductor light-emitting device having an MQW structure, if the quantum well layers are too far apart, resonance of the wave functions cannot be obtained, and the probability of the existence of electrons and holes is determined solely by the effect of the wave functions in each quantum well layer. In this case, it is difficult to efficiently obtain the effects of the MQW structure. From this perspective, in order to prevent the quantum well layers from being too far apart, the distance between two adjacent quantum well layers should be no more than three times the film thickness of the quantum well layer.

[0074] In the above-described embodiment, an example was shown in which InGaN containing In was used for the barrier layer 31, but in the present disclosure, for example, AlGaN or GaN not containing In may be used for the barrier layer or the optical guide layer. In this case, however, it is necessary to ensure a refractive index difference with the n-side or p-side cladding layer in which light of the wavelength that causes laser oscillation is confined in the thickness direction.

[0075] Furthermore, when the present disclosure is applied to a nitride semiconductor light-emitting device having a DQW structure or an MQW structure, the film thickness of each layer included in the active layer may satisfy, for example, the following relationship: That is, when the active layer has a structure in which a plurality of gradient layers are stacked between a plurality of barrier layers and a plurality of quantum well layers, and the film thickness of the barrier layers is L_b, the film thickness of the quantum well layers is L_w, and the film thickness of the gradient layers is L_s, the relationship of the following formula (19) or (20) may be satisfied. L_w≧L_b≧L_s (19) L_w≧L_s≧L_b (20)

[0076] By making the quantum well layers thicker than the barrier layers and the gradient layers in this way, it is possible to prevent the quantum well layers from being too far apart. [Industrial Applicability]

[0077] The present disclosure is useful for nitride semiconductor light-emitting devices having a quantum well structure. [Explanation of symbols]

[0078] 100 nitride semiconductor light emitting evaluation samples (nitride semiconductor light emitting devices) 10 Substrate 20 Buffer Layer 30 Active layer 31 Barrier Layer 32 quantum well layer 33 Gradient Layer 40 Cap Layer

Claims

1. A nitride semiconductor light emitting device having a structure in which nitride semiconductor layers including an active layer are stacked in a thickness direction, the active layer includes, along the thickness direction, a plurality of barrier layers, a quantum well layer sandwiched between two of the barrier layers, and a graded layer disposed between the barrier layers and the quantum well layers; the barrier layer, the quantum well layer, and the gradient layer each contain In; the quantum well layer has an In composition higher than the barrier layer; the In composition in the gradient layer increases from the barrier layer side toward the quantum well layer side along the thickness direction, The average value of the In composition in the barrier layer is In_b, the average value of the In composition in the quantum well layer is In_w, and the maximum value of the In composition in the gradient layer is In_s. ｍａｘ The minimum value of the In composition in the gradient layer is In_s ｍｉｎ When In_w>In_s ｍａｘ >In_S ｍｉｎ ≧In_b (1) The relationship is satisfied, Nitride semiconductor light emitting device.

2. When a variable indicating a position in the thickness direction is t, the thickness of the gradient layer is L_s, the In composition distribution in the gradient layer is Is_s(t), the thickness of the quantum well layer is L_w, and the In composition distribution in the quantum well layer is Is_w(t), In_w×L_w>∫ Ｌ＿ｓ In_s(t)dt (2) or ∫ Ｌ＿ｗ In_w(t)dt>∫ Ｌ＿ｓ In_s(t)dt (3) The relationship is satisfied, The nitride semiconductor light emitting device according to claim 1 .

3. In_w×L_w>2∫ Ｌ＿ｓ In_s(t)dt (4) or ∫ Ｌ＿ｗ In_w(t)dt>2∫ Ｌ＿ｓ In_s(t)dt (5) The relationship between The nitride semiconductor light emitting device according to claim 2 .

4. the active layer has a structure in which a first barrier layer, a first gradient layer, the quantum well layer, a second gradient layer, and a second barrier layer are stacked in this order along a thickness direction; When a variable indicating a position in the thickness direction is t, a thickness of the first gradient layer is L_s1, an In composition distribution in the first gradient layer is Is_s1(t), a thickness of the second gradient layer is L_s2, an In composition distribution in the second gradient layer is Is_s2(t), a thickness of the quantum well layer is L_w, and an In composition distribution in the quantum well layer is Is_w(t), In_w×L_w>∫ Ｌ＿ｓ１ In_s1(t)dt=∫ Ｌ＿ｓ２ In_s2(t)dt (6) or ∫ Ｌ＿ｗ In_w(t)dt>∫ Ｌ＿ｓ１ In_s1(t)dt=∫ Ｌ＿ｓ２ In_s2(t)dt (7) The relationship is satisfied, The nitride semiconductor light emitting device according to claim 1 .

5. In_w×L_w>2∫ Ｌ＿ｓ１ In_s1(t)dt=2∫ Ｌ＿ｓ２ In_s2(t)dt (8) or ∫ Ｌ＿ｗ In_w(t)dt>2∫ Ｌ＿ｓ１ In_s1(t)dt=2∫ Ｌ＿ｓ２ In_s2(t)dt (9) The relationship between The nitride semiconductor light emitting device according to claim 4 .

6. L_w>L_s1 (10) or L_w>L_s2 (11) The relationship between The nitride semiconductor light emitting device according to claim 4 .

7. 2L_w>L_s1 (12) or 2L_w>L_s2 (13) The relationship between The nitride semiconductor light emitting device according to claim 4 .

8. L_w>L_s1 (10) and, L_w>L_s2 (11) The relationship between The nitride semiconductor light emitting device according to claim 4 .

9. 2L_w>L_s1 (12) and, 2L_w>L_s2 (13) The relationship between The nitride semiconductor light emitting device according to claim 4 .

10. The thickness L_s1 of the first gradient layer or the thickness L_s2 of the second gradient layer has a thickness of 0.25 nm or more. The nitride semiconductor light-emitting device according to claim 6 .

11. the In composition in the barrier layer and the In composition in the quantum well layer are substantially uniform in the thickness direction; The nitride semiconductor light emitting device according to claim 1 .

12. In_w>2In_s ｍａｘ >In_b (14) The relationship between The nitride semiconductor light emitting device according to claim 1 .

13. In_w>In_s ｍａｘ >In_S ｍｉｎ =In_b (15) The relationship between The nitride semiconductor light emitting device according to claim 1 .

14. In_w>2In_s ｍａｘ >In_S ｍｉｎ =In_b (16) The relationship between The nitride semiconductor light emitting device according to claim 1 .

15. In_w>2.5In_smax>In_smin=In_b (17) The relationship between The nitride semiconductor light emitting device according to claim 1 .

16. the active layer has a structure in which a plurality of the gradient layers are stacked between a plurality of the barrier layers and a plurality of the quantum well layers, When the thickness of the barrier layer is L_b, the thickness of the quantum well layer is L_w, and the thickness of the gradient layer is L_s, L_w≧L_b≧L_s (18) or L_w≧L_s≧L_b (19) The relationship is satisfied, The nitride semiconductor light emitting device according to claim 1 .

17. the active layer has a structure in which a plurality of the gradient layers are stacked between a plurality of the barrier layers and a plurality of the quantum well layers, a distance between two adjacent quantum well layers among the plurality of quantum well layers is three times or less the film thickness of the quantum well layer; The nitride semiconductor light emitting device according to claim 1 .

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