Optical semiconductor devices

Abstract

To provide an optical semiconductor element capable of improving the gain.SOLUTION: An optical semiconductor element comprises an active layer 24 with a plurality of quantum dot layers. The plurality of quantum dot layers include: a first quantum dot layer 242 added with p-type impurities; and a second quantum dot layer 243 added with n-type impurities and having an emission wavelength different from that of the first quantum dot layer 242. Accordingly, the second quantum dot layer 243 added with the n-type impurities facilitates diffusion of positive holes, and thereby can increase the number of the quantum dot layers contributing to light emission and improve the gain.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to an optical semiconductor device including a quantum dot layer. [Background technology]

[0002] In semiconductor laser devices using optical semiconductor elements such as SOAs (semiconductor optical amplifiers), a method has been proposed to suppress the decrease in output power at high temperatures and obtain high output power over a wide temperature range by three-dimensionally confining electrons using a quantum dot layer and doping p-type impurities near the active layer.

[0003] Optical semiconductor devices using quantum dot layers are less susceptible to output degradation at high temperatures compared to those using quantum wells, but they have a narrow wavelength band. Regarding this issue, a technology has been proposed to obtain a large output over a wide band by combining multiple quantum dot layers with different emission wavelengths (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2008-244235 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, the quantum dot layer doped with p-type impurities has a reduced gain at low temperatures, so if only a quantum dot layer doped with p-type impurities is used as in Patent Document 1, there is a risk of insufficient output at low temperatures.

[0006] On the other hand, a quantum dot layer without p-type impurities can be used. Such a quantum dot layer has a smaller gain drop at low temperatures than a quantum dot layer with p-type impurities added, but a larger gain drop at high temperatures than a quantum dot layer with p-type impurities added. Therefore, if only a quantum dot layer without p-type impurities is used, there is a risk of insufficient output at high temperatures.

[0007] Therefore, the present inventors have proposed an optical semiconductor device including a quantum dot layer doped with p-type impurities and an undoped quantum dot layer containing almost no p-type impurities in Japanese Patent Application No. 2021-71809. This makes it possible to expand the emission band and suppress gain fluctuations associated with temperature changes when the operating wavelength is fixed.

[0008] However, in such an optical semiconductor device, the diffusion of carriers is insufficient due to the undoped layer, and there is a quantum dot layer that does not contribute to light emission. Therefore, there is room to increase the quantum dot layer that contributes to light emission and improve the gain of the optical semiconductor device.

[0009] In view of the above, an object of the present invention is to provide an optical semiconductor element capable of improving gain. [Means for solving the problem]

[0010] In order to achieve the above object, the present invention provides an optical semiconductor device, comprising: an active layer (24) having a plurality of quantum dot layers; a p-type cladding layer (25) laminated on the active layer; and an n-type cladding layer (23) laminated on the active layer. The multiple quantum dot layers include a first quantum dot layer (242) doped with p-type impurities and a second quantum dot layer (243) doped with n-type impurities and having a different emission wavelength from the first quantum dot layer. The first quantum dot layer is closer to the p-type cladding layer than the second quantum dot layer, and the multiple quantum dot layers have a higher p-type impurity concentration closer to the p-type cladding layer and a higher n-type impurity concentration closer to the n-type cladding layer. .

[0011] According to this, the diffusion of holes is promoted by the second quantum dot layer doped with n-type impurities, so that the quantum dot layer contributing to light emission can be increased, and the gain can be improved.

[0012] Note that the reference symbols in parentheses attached to each component indicate an example of the correspondence between the component and specific components described in the embodiments described below. [Brief description of the drawings]

[0013] [Figure 1] 1 is a diagram showing a configuration of a semiconductor laser device according to a first embodiment. [Diagram 2] FIG. 2 is a cross-sectional view of the SOA shown in FIG. [Diagram 3] FIG. 3 is a cross-sectional view of the active layer shown in FIG. [Figure 4] 1 shows the gain spectrum of the active layer. [Diagram 5] FIG. 4 is a diagram showing temperature characteristics of gain. [Figure 6] FIG. 11 is a band diagram of a comparative example. [Figure 7] FIG. 2 is a band diagram of the first embodiment. [Figure 8] FIG. 11 is a cross-sectional view of an active layer in a second embodiment. [Figure 9] 13 is a gain spectrum of an active layer in the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiments, parts that are the same or equivalent to each other will be denoted by the same reference numerals.

[0015] (First embodiment) A first embodiment will be described. As shown in Fig. 1, a semiconductor laser device 1 of this embodiment includes an SOA 2, which is an optical semiconductor element, and a wavelength selection unit 3. The semiconductor laser device 1 is applied to, for example, laser radar and LiDAR. LiDAR is an abbreviation for Light Detection And Ranging. The SOA 2 and the wavelength selection unit 3 are formed, for example, by performing a semiconductor process on a semiconductor substrate (not shown).

[0016] The SOA 2 is a light source that generates laser light. As shown in Fig. 2, the SOA 2 is configured with a laminated structure of a lower electrode 21, a substrate 22, an undercladding layer 23, an active layer 24, an overcladding layer 25, a contact layer 26, and an upper electrode 27. Note that Fig. 1 shows only the undercladding layer 23, the active layer 24, and the overcladding layer 25 of the SOA 2.

[0017] As shown in Fig. 2, the lower electrode 21 is in contact with the back surface side of the substrate 22, that is, the side opposite to the undercladding layer 23. The substrate 22 is made of, for example, a GaAs substrate. The undercladding layer 23 is an n-type cladding layer made of n-type AlGaAs or the like. The active layer 24 is formed on the upper surface of the undercladding layer 23. Details of the active layer 24 will be described later.

[0018] The overcladding layer 25 is formed on the upper surface of the active layer 24. The overcladding layer 25 is a p-type cladding layer made of p-type AlGaAs or the like. The contact layer 26 is for making contact with the upper electrode 27, and is formed on the upper surface of the overcladding layer 25. The contact layer 26 is made of, for example, GaAs.

[0019] The upper electrode 27 is formed on the upper surface of the contact layer 26. A recess 28 is formed extending from the upper electrode 27 to the contact layer 26 and the surface portion of the overcladding layer 25, and the SOA2 has a mesa structure in which the upper electrode 27 and the contact layer 26 protrude into positions other than the recess 28.

[0020] With the SOA2 configured in this manner, by applying a voltage that generates a predetermined potential difference between the upper electrode 27 and the lower electrode 21, laser oscillation can be caused and laser light can be emitted from the end face of the active layer 24.

[0021] The detailed configuration of the active layer 24 will be described. As shown in FIG. 3, the active layer 24 includes an intermediate layer 241. The intermediate layer 241 is made of GaAs. The active layer 24 also includes a plurality of quantum dot layers. The quantum dot layer is made of, for example, InAs or InGaAs. The quantum dot layer has a structure including granular quantum dots formed by crystal growth or microfabrication, and the front and back sides are covered with the intermediate layer 241.

[0022] The gain spectrum of the active layer 24 has a maximum value that is formed by the emission due to the ground levels of the multiple quantum dot layers, and the emission wavelength and gain intensity of the active layer 24 are determined by the configuration of the multiple quantum dot layers. The gain spectrum can be measured by, for example, the Hakki-Paoli method.

[0023] The multiple quantum dot layers include a quantum dot layer 242 and a quantum dot layer 243. The quantum dot layer 242 is highly doped with p-type impurities, and the quantum dot layer 243 is highly doped with n-type impurities. The quantum dot layer 242 and the quantum dot layer 243 correspond to a first quantum dot layer and a second quantum dot layer, respectively.

[0024] The active layer 24 includes a plurality of quantum dot layers 242 and a plurality of quantum dot layers 243, and the plurality of quantum dot layers 242 are stacked on top of the plurality of quantum dot layers 243. That is, the quantum dot layers 242 are closer to the overcladding layer 25 on the active layer 24 than the quantum dot layers 243. For example, the number of layers of the quantum dot layers 242 and 243 is eight each.

[0025] As described above, the intermediate layer 241 is laminated on both sides of each quantum dot layer 242, 243. The intermediate layer 241 formed between the two quantum dot layers 242 and the intermediate layer 241 formed on the uppermost quantum dot layer 242 includes a p-type impurity layer 241a. The intermediate layer 241 formed between the two quantum dot layers 243 and the intermediate layer 241 formed below the lowermost quantum dot layer 243 includes an n-type impurity layer 241b.

[0026] The p-type impurity layer 241a is a layer formed by adding a p-type impurity during the formation of the intermediate layer 241, and is configured to contain a trace amount of p-type impurity in GaAs. The n-type impurity layer 241b is a layer formed by adding an n-type impurity during the formation of the intermediate layer 241, and is configured to contain a trace amount of n-type impurity in GaAs.

[0027] By disposing the p-type impurity layer 241a and the n-type impurity layer 241b in the vicinity of the quantum dot layers 242 and 243 in this manner, the p-type impurity diffused from the p-type impurity layer 241a and the n-type impurity diffused from the n-type impurity layer 241b are added to the quantum dot layers 242 and 243, respectively. Note that the intermediate layer 241 between the quantum dot layer 242 and the quantum dot layer 243 may include either the p-type impurity layer 241a or the n-type impurity layer 241b, or may not include either one.

[0028] The p-type impurity layer 241a has a higher p-type impurity concentration as it is closer to the overcladding layer 25. The n-type impurity layer 241b has a higher n-type impurity concentration as it is closer to the undercladding layer 23. With this configuration, the multiple quantum dot layers 242 and the multiple quantum dot layers 243 have a higher p-type impurity concentration as they are closer to the overcladding layer 25, and a higher n-type impurity concentration as they are closer to the undercladding layer 23.

[0029] The impurity concentration is set based on, for example, the number of holes confined in one quantum dot. Specifically, two or more holes are confined in one quantum dot. The p-type impurity concentration of the quantum dot layer 242 is set to be twice or more the surface density of the quantum dots 242a formed in the quantum dot layer 242. The n-type impurity concentration of the quantum dot layer 243 is set to be twice or more the surface density of the quantum dots 243a formed in the quantum dot layer 243.

[0030] In this embodiment, the p-type impurity concentration of the quantum dot layer 242 is higher the closer to the overcladding layer 25, for example, the p-type impurity concentration of the quantum dot layer 242 closest to the overcladding layer 25 is set to be at least twice the surface density of the quantum dots 242a. Also, in this embodiment, the n-type impurity concentration of the quantum dot layer 243 is higher the closer to the undercladding layer 23, for example, the n-type impurity concentration of the quantum dot layer 243 closest to the undercladding layer 23 is set to be at least twice the surface density of the quantum dots 243a.

[0031] The quantum dot layer 242 and the quantum dot layer 243 have different emission wavelengths. Specifically, the emission wavelength of the quantum dot layer 242 is shorter than the emission wavelength of the quantum dot layer 243. If the emission wavelength of the quantum dot layer 242, i.e., the wavelength at which the gain of the quantum dot layer 242 is at its peak, is λ1 and the emission wavelength of the quantum dot layer 243 is λ2, then, for example, the wavelengths λ1 and λ2 are 1210 nm and 1280 nm, respectively.

[0032] With this configuration, the gain spectrum of active layer 24 is as shown in Fig. 4. In Fig. 4, the solid line indicates the gain spectrum of active layer 24, the dashed line on the left indicates the gain spectrum of quantum dot layer 242, and the dashed line on the right indicates the gain spectrum of quantum dot layer 243. In this way, by combining the two gain spectra, a broadband spectrum with large gain at wavelengths between λ1 and λ2 is configured.

[0033] The wavelength selection unit 3 selects the operating wavelength of the semiconductor laser device 1, specifically, the operating wavelength of the active layer 24, and, as shown in Fig. 1, includes an etalon filter 31 and a mirror 32. The operating wavelength of the active layer 24 is denoted as λop.

[0034] The etalon filter 31 transmits only light of a predetermined wavelength. The etalon filter 31 is disposed so that light emitted from the active layer 24 is incident on the etalon filter 31. As shown by an arrow A1 in FIG. 1, the light transmitted through the etalon filter 31 is incident on a mirror 32.

[0035] The mirror 32 is disposed so as to reflect the light incident from the etalon filter 31 towards the etalon filter 31. As indicated by arrow A2, the light reflected by the mirror 32 passes through the etalon filter 31, enters the active layer 24, and is emitted from the end face of the active layer 24 opposite to the etalon filter 31 and mirror 32. By adjusting the wavelength of the light to be transmitted through the etalon filter 31 by designing the etalon filter 31, the operating wavelength of the semiconductor laser device 1 can be selected.

[0036] In this embodiment, the wavelength selection unit 3 selects the operating wavelength λop so that the active layer 24 oscillates in a single mode, that is, at a single wavelength. Specifically, the wavelength selection unit 3 includes two etalon filters 31. The two etalon filters 31 are referred to as etalon filters 31a and 31b, respectively.

[0037] The etalon filters 31a and 31b have different free spectral intervals, and the plurality of wavelengths transmitted by the etalon filter 31a overlap with the plurality of wavelengths transmitted by the etalon filter 31b at only one wavelength. Therefore, as shown in Fig. 1, by placing the etalon filters 31a and 31b on the path of the light emitted from the active layer 24, the light of this one wavelength is incident on the mirror 32 and returns to the active layer 24. This causes the active layer 24 to oscillate in a single mode.

[0038] Although the operating wavelength λop may be selected so that the active layer 24 oscillates in multiple modes, gain fluctuations can be reduced by making the active layer 24 oscillate in a single mode.

[0039] Although the wavelength selection unit 3 is described here as being composed of the etalon filter 31 and the mirror 32, the wavelength selection unit 3 may be composed of a diffraction grating or the like that reflects only light of a predetermined wavelength. When the wavelength selection unit 3 is composed of a diffraction grating, the active layer 24 oscillates in a single mode. The operating wavelength of the semiconductor laser device 1 may be selected by applying a voltage or the like from the outside to the wavelength selection unit 3 composed of the etalon filter, the diffraction grating, or the like. The reflectance of the mirror 32 may be adjusted to emit light toward the mirror 32.

[0040] The operating wavelength λop is a wavelength between the wavelength λ1 at a predetermined temperature and the wavelength λ2 at a predetermined temperature. This predetermined temperature is room temperature or the middle temperature of the assumed environmental temperature. Room temperature is a temperature between 20° C. and 28° C., for example, 25° C. The assumed environmental temperature is, for example, a temperature between −40° C. and 85° C.

[0041] The temperature characteristics of the gain of the quantum dot layer will be explained. The gain spectrum of the quantum dot layer shifts in wavelength with temperature change. Specifically, when the temperature rises, the gain spectrum shifts to the longer wavelength side, and when the temperature drops, the gain spectrum shifts to the shorter wavelength side. Therefore, when the operating wavelength is fixed, the output of the quantum dot layer decreases with temperature change.

[0042] In contrast, by combining multiple quantum dot layers with different emission wavelengths, the gain spectrum can be broadened and the decrease in gain due to wavelength shift can be reduced. That is, as shown in Fig. 5, the gain of the entire active layer 24 is maintained by the gain of the quantum dot layer 243 at low temperatures, and the gain of the entire active layer 24 is maintained by the gain of the quantum dot layer 242 at high temperatures. In Fig. 5, the solid line, the dashed line, and the dashed double dot line represent the gain of the active layer 24 at room temperature, low temperature, and high temperature, respectively.

[0043] Although the doping of n-type impurities reduces the gain at high temperatures, if the quantum dot layer 243 doped with n-type impurities is located on the long wavelength side, the gain of the quantum dot layer 243 only affects low temperatures, so that the gain reduction at high temperatures at the operating wavelength λop can be suppressed. Also, since the quantum dot layer doped with p-type impurities has a smaller gain reduction at high temperatures than the quantum dot layer doped with n-type impurities, by placing the quantum dot layer 242 on the short wavelength side, the gain reduction at high temperatures at the operating wavelength λop can be suppressed.

[0044] The effect of this embodiment will be described. Assume that the quantum dot layer 243 is an undoped layer containing almost no p-type impurities and no n-type impurities. In this case, as shown in FIG. 6, electrons with high mobility diffuse from the undercladding layer 23 to the entire active layer 24. On the other hand, although holes with low mobility diffuse to a portion of the active layer 24 close to the overcladding layer 25, most of them recombine with electrons that have diffused uniformly to the quantum dot layer 242. Then, the newly supplied holes move to compensate for the holes that have disappeared in the quantum dot layer 242, and do not reach the portion close to the undercladding layer 23. Therefore, no combination of electrons and holes occurs in the quantum dot layer 243 near the undercladding layer 23, and this quantum dot layer 243 does not contribute to light emission.

[0045] In contrast, in this embodiment, n-type impurities are doped at a high concentration in the quantum dot layer 243. This increases the electron concentration in the quantum dot layer 243, and the probability of recombination with holes increases, attracting holes, and promoting the diffusion of holes in the quantum dot layer 243 as well, as shown in Fig. 7. Therefore, the quantum dot layer 243 near the undercladding layer 23 also contributes to light emission, and the gain of the entire active layer 24 is improved.

[0046] As described above, in this embodiment, the active layer 24 has a plurality of quantum dot layers, and the plurality of quantum dot layers include the quantum dot layer 242 doped with a p-type impurity and the quantum dot layer 243 doped with an n-type impurity and having an emission wavelength different from that of the quantum dot layer 242. This improves the gain of the quantum dot layer 242.

[0047] Furthermore, according to the above embodiment, the following effects can be obtained.

[0048] (1) The quantum dot layer 242 is closer to the overcladding layer 25 than the quantum dot layer 243. This promotes the diffusion of carriers in the quantum dot layer 242.

[0049] (2) The multiple quantum dot layers 242, 243 have a higher p-type impurity concentration toward the overcladding layer 25. By increasing the concentration of p-type impurities at the end of the active layer 24 in this manner, carrier diffusion is further promoted, and the gain of the active layer 24 is further improved.

[0050] (3) The multiple quantum dot layers 242, 243 have a higher n-type impurity concentration toward the undercladding layer 23. By increasing the concentration of n-type impurities at the end of the active layer 24 in this manner, carrier diffusion is further promoted, and the gain of the active layer 24 is further improved.

[0051] (4) The emission wavelength of the quantum dot layer 242 is shorter than the emission wavelength of the quantum dot layer 243. This makes it possible to suppress a decrease in gain at high temperatures at the operating wavelength λop.

[0052] Second embodiment The second embodiment will be described. This embodiment is different from the first embodiment in that the configuration of the active layer 24 is changed, and other aspects are the same as those of the first embodiment, so only the differences from the first embodiment will be described.

[0053] 8, in this embodiment, the multiple quantum dot layers included in the active layer 24 include a quantum dot layer 244 in addition to the quantum dot layer 242 and the quantum dot layer 243. The quantum dot layer 244 corresponds to a third quantum dot layer.

[0054] The active layer 24 includes a plurality of quantum dot layers 244, which are stacked between a plurality of quantum dot layers 242 and a plurality of quantum dot layers 243. For example, the number of layers of the quantum dot layers 242, 243, and 244 is 8, 5, and 3, respectively.

[0055] The quantum dot layer 244 has a lower p-type impurity concentration and an n-type impurity concentration than the quantum dot layer 242 , and has a lower p-type impurity concentration and an n-type impurity concentration than the quantum dot layer 243 .

[0056] Specifically, the intermediate layer 241 formed between the two quantum dot layers 244 does not include either the p-type impurity layer 241a or the n-type impurity layer 241b. Moreover, the intermediate layer 241 formed on the uppermost quantum dot layer 244 and the intermediate layer 241 formed below the lowermost quantum dot layer 244 do not include either the p-type impurity layer 241a or the n-type impurity layer 241b. With this configuration, the quantum dot layer 244 is an undoped layer that contains almost no p-type impurities or n-type impurities.

[0057] If the emission wavelength of the quantum dot layer 244 is λ3, then λ2<λ3. As a result, the gain spectrum of the active layer 24 is as shown in FIG. 9. In FIG. 9, the solid line indicates the gain spectrum of the active layer 24, and the three dashed dot lines indicate the gain spectra of the quantum dot layers 242, 243, and 244, from the left. The undoped layer has a smaller gain decrease at low temperatures than the quantum dot layer doped with p-type or n-type impurities. Therefore, by making the emission wavelength of the quantum dot layer 244 longer than the quantum dot layer 242 and the quantum dot layer 243, it is possible to suppress the gain decrease at low temperatures at the operating wavelength λop.

[0058] In this way, by configuring the multiple quantum dot layers in active layer 24 so that the shorter the emission wavelength is, the higher the p-type impurity concentration is, and the longer the emission wavelength is, the lower the p-type impurity concentration and the n-type impurity concentration are, it is possible to obtain a high gain over a wide wavelength range.

[0059] Alternatively, λ2≈λ3 or λ3<λ2, and the gain spectrum formed by the wavelengths λ2 and λ3 is single-peaked. That is, the gain spectrum of the active layer 24 has a shape similar to that of Fig. 4, and the gain spectrum indicated by the dashed dot line on the right side is formed by the gains of the quantum dot layers 243 and 244. In this case as well, it is possible to suppress the decrease in gain at low temperatures at the operating wavelength λop.

[0060] This embodiment has the same configuration and operation as the first embodiment, and therefore can obtain the same effects as the first embodiment.

[0061] Furthermore, according to the above embodiment, the following effects can be obtained.

[0062] (1) The multiple quantum dot layers included in the active layer 24 include the quantum dot layer 244 in addition to the quantum dot layer 242 and the quantum dot layer 243. The quantum dot layer 244 has a lower p-type impurity concentration and an n-type impurity concentration than the quantum dot layer 242, and a lower p-type impurity concentration and an n-type impurity concentration than the quantum dot layer 243. This can reduce a decrease in gain at low temperatures.

[0063] (2) λ2<λ3. Alternatively, the gain spectrum of the active layer 24, which is formed by the wavelengths λ2 and λ3, is single-peaked. This can further reduce the decrease in gain at low temperatures.

[0064] (3) The shorter the emission wavelength, the higher the p-type impurity concentration of the multiple quantum dot layers included in the active layer 24. With this configuration, it is possible to obtain a high gain over a wide wavelength range.

[0065] (4) The longer the emission wavelength, the lower the p-type impurity concentration and n-type impurity concentration of the multiple quantum dot layers included in the active layer 24. With this configuration, it is possible to obtain a high gain over a wide wavelength range.

[0066] (Other embodiments) The present invention is not limited to the above-mentioned embodiments, and can be modified as appropriate within the scope of the claims. In addition, in each of the above-mentioned embodiments, it goes without saying that the elements constituting the embodiments are not necessarily essential, except when they are specifically stated as essential or when they are clearly considered essential in principle. In addition, in each of the above-mentioned embodiments, when the numbers, values, amounts, ranges, etc. of the components of the embodiments are mentioned, they are not limited to the specific numbers, except when they are specifically stated as essential or when they are clearly limited to a specific number in principle. [Explanation of symbols]

[0067] 24 Active layer 242 Quantum dot layer 243 Quantum dot layer

Claims

1. An optical semiconductor element, an active layer (24) having a plurality of quantum dot layers; A p-type cladding layer (25) laminated on the active layer; An n-type clad layer (23) laminated on the active layer, The plurality of quantum dot layers include a first quantum dot layer (242) doped with p-type impurities; and a second quantum dot layer (243) having an emission wavelength different from that of the first quantum dot layer and doped with n-type impurities; the first quantum dot layer is closer to the p-type cladding layer than the second quantum dot layer; In the plurality of quantum dot layers, the closer to the p-type cladding layer the higher the p-type impurity concentration, and the closer to the n-type cladding layer the higher the n-type impurity concentration.

2. The optical semiconductor device according to claim 1 , wherein the emission wavelength of the first quantum dot layer is shorter than the emission wavelength of the second quantum dot layer.

3. the plurality of quantum dot layers includes a third quantum dot layer (244) in addition to the first quantum dot layer and the second quantum dot layer; 3 . The optical semiconductor device according to claim 1 , wherein the third quantum dot layer has a p-type impurity concentration and an n-type impurity concentration lower than those of the first quantum dot layer and a p-type impurity concentration and an n-type impurity concentration lower than those of the second quantum dot layer.

4. The plurality of quantum dot layers includes a third quantum dot layer (244) in addition to the first quantum dot layer and the second quantum dot layer; The emission wavelength of the second quantum dot layer is λ2, The emission wavelength of the third quantum dot layer is λ3, λ2<λ3, or 4. The optical semiconductor device according to claim 1, wherein the gain spectrum of the active layer, which is formed by the emission wavelengths .lambda.2 and .lambda.3, has a single peak.

5. An optical semiconductor element, an active layer (24) having a plurality of quantum dot layers; A p-type cladding layer (25) laminated on the active layer, The plurality of quantum dot layers include a first quantum dot layer (242) doped with p-type impurities; and a second quantum dot layer (243) having an emission wavelength different from that of the first quantum dot layer and doped with n-type impurities; the first quantum dot layer is closer to the p-type cladding layer than the second quantum dot layer; an emission wavelength of the first quantum dot layer is shorter than an emission wavelength of the second quantum dot layer; the plurality of quantum dot layers includes a third quantum dot layer (244) in addition to the first quantum dot layer and the second quantum dot layer; the third quantum dot layer has a p-type impurity concentration and an n-type impurity concentration lower than those of the first quantum dot layer, and has a p-type impurity concentration and an n-type impurity concentration lower than those of the second quantum dot layer; The emission wavelength of the second quantum dot layer is λ2, The emission wavelength of the third quantum dot layer is λ3, λ2<λ3, or An optical semiconductor element in which the gain spectrum of the active layer, which is formed by the emission wavelength λ2 and the emission wavelength λ3, has a single peak.

6. The optical semiconductor device according to claim 5 , wherein the plurality of quantum dot layers have a higher p-type impurity concentration as they are closer to the p-type cladding layer.

7. 7. The optical semiconductor device according to claim 1, wherein the p-type impurity concentration of the plurality of quantum dot layers increases as the emission wavelength decreases.

8. 8. The optical semiconductor device according to claim 1, wherein the p-type impurity concentration and the n-type impurity concentration of the plurality of quantum dot layers decrease as the emission wavelength increases.

9. The p-type impurity concentration of the first quantum dot layer is set to be at least twice the surface density of the quantum dots (242a); 9. The optical semiconductor device according to claim 1, wherein the n-type impurity concentration of the second quantum dot layer is at least twice the surface density of the quantum dots.

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