Semiconductor optical element

Abstract

This semiconductor optical element comprises: a substrate; and a laminate in which a semiconductor layer of a first conductivity type, an active layer, and a semiconductor layer of a second conductivity type are laminated in the stated order from the substrate side. At least one of the semiconductor layer of the first conductivity type and the semiconductor layer of the second conductivity type has a first waveguide layer and a second waveguide layer. The second waveguide layer has a higher doping concentration than the first waveguide layer and is disposed between the first waveguide layer and the active layer.

Description

Semiconductor optical device

[0001] The present invention relates to a semiconductor optical device. This application claims priority based on Japanese Patent Application No. 2023-219113 filed in Japan on December 26, 2023, and incorporates its content herein by reference.

[0002] One type of semiconductor optical device is an edge-emitting semiconductor laser. A general edge-emitting semiconductor laser is an element in which a first conductivity type (e.g., n-type) cladding layer, a first conductivity type waveguide layer, an active layer, a second conductivity type (e.g., p-type) waveguide layer, and a second conductivity type cladding layer are sequentially stacked in the vertical direction on a substrate. In such an edge-emitting semiconductor laser, most of the light generated in the active layer by current injection is confined and guided by the active layer and the waveguide layer.

[0003] In recent years, as a structure for suppressing laser light spread (or beam spread) in the vertical direction while having high output characteristics, a LOC (Large Optical Cavity) structure with a relatively large waveguide layer thickness may be used. Although there is no clear definition regarding the LOC structure, in a semiconductor laser with an oscillation wavelength in the 900 nm band, a structure in which the total thickness of the first conductivity type waveguide layer and the second conductivity type waveguide layer is approximately 1.5 to 2.5 μm is often used. For example, Patent Document 1 below discloses a LOC structure with a total waveguide layer thickness of 2.15 μm.

[0004] U.S. Patent No. 8976831

[0005] In a LOC structure with a large waveguide layer thickness, most of the generated laser light is guided by the active layer and the waveguide layer. When intentional doping is performed on the semiconductor layer through which the laser light is guided, while the resistance value of the element can be reduced, absorption of the laser light by the dopant called free carrier absorption occurs, and the laser emission efficiency is greatly reduced. In extreme cases, laser oscillation itself may become impossible due to free carrier absorption.

[0006] Therefore, due to the balance between the resistance of the element and the laser emission efficiency, the doping concentration of the waveguide layer in the LOC structure is generally kept low. For example, in the aforementioned Patent Document 1, the active layer and the layers adjacent to the active layer on both sides of the active layer are intentionally undoped layers, and the n-type waveguide layer has a doping concentration of approximately 1 × 10⁻⁶. 16 cm -3 It is said that...

[0007] Furthermore, in LOC structures, the large thickness of the waveguide layer, where doping concentrations are generally kept low, increases the resistance of the element, which tends to increase the operating voltage. In addition, in the above-described structure, since both the doping concentrations of the n-type and p-type waveguide layers are low, the depletion layer is extensive, which may further increase the operating voltage.

[0008] This invention has been made in view of the above circumstances, and aims to provide a semiconductor optical element that can reduce the operating voltage without causing a significant decrease in luminous efficiency.

[0009] To solve the above problems, a semiconductor optical element (1, 2) according to a first aspect of the present invention comprises a substrate (10) and a laminate (20) in which a first conductivity type semiconductor layer (30), an active layer (40), and a second conductivity type semiconductor layer (50) are stacked in order from the substrate side, wherein at least one of the first conductivity type semiconductor layer and the second conductivity type semiconductor layer has a first waveguide layer (32) and a second waveguide layer (33), the second waveguide layer has a higher doping concentration than the first waveguide layer and is disposed between the first waveguide layer and the active layer.

[0010] In a semiconductor optical device according to a first aspect of the present invention, a first waveguide layer and a second waveguide layer are provided in at least one of a first conductivity type semiconductor layer and a second conductivity type semiconductor layer stacked so as to sandwich an active layer. Since the second waveguide layer has a higher doping concentration than the first waveguide layer and is positioned between the first waveguide layer and the active layer, the operating voltage can be reduced without causing a significant decrease in luminescence efficiency.

[0011] According to a second aspect of the present invention, in the semiconductor optical device according to the first aspect of the present invention, the doping concentration of the second waveguide layer is two times or more the doping concentration of the first waveguide layer.

[0012] According to a third aspect of the present invention, in the semiconductor optical device according to the second aspect of the present invention, the doping concentration of the second waveguide layer is 1 × 10 17 cm -3 or more and 1 × 10 18 cm -3 or less.

[0013] According to a fourth aspect of the present invention, in the semiconductor optical device according to the second aspect of the present invention, at least one of the first conductivity type semiconductor layer and the second conductivity type semiconductor layer is not doped in addition to the first waveguide layer and the second waveguide layer, and has an undoped layer (34) disposed between the second waveguide layer and the active layer, and the doping concentration of the second waveguide layer is 5 × 10 17 cm -3 or more and 1 × 10 18 cm -3 or less.

[0014] According to a fifth aspect of the present invention, in the semiconductor optical device according to any one of the first to fourth aspects of the present invention, the thickness of the second waveguide layer is 25 nm or more and 50 nm or less.

[0015] According to a sixth aspect of the present invention, in the semiconductor optical device according to any one of the first to fifth aspects of the present invention, the doping concentration of the first waveguide layer is 3 × 10 16 cm -3 or more and 5 × 10 16 cm -3 or less.

[0016] According to an aspect of the present invention, there is an effect that the operating voltage can be reduced without causing a significant decrease in the light emission efficiency.

[0017] This is a cross-sectional view showing the schematic configuration of a semiconductor optical element according to the first embodiment of the present invention. This is a cross-sectional view showing the schematic configuration of a semiconductor optical element according to the second embodiment of the present invention. This is a diagram showing an example of a specific configuration of a semiconductor optical element according to an embodiment of the present invention. This is a diagram showing the change in current-voltage characteristics when the configuration of the n-side layer adjacent to the active layer is changed in the semiconductor optical element with the configuration shown in Figure 3. This is a diagram summarizing the configurations of the n-side layers adjacent to the active layer in Examples 1 to 4 and Comparative Examples shown in Figure 4. This is a diagram showing the change in output and PCE when the doping concentration of the second waveguide layer adjacent to the active layer is changed in the semiconductor optical element with the configuration shown in Figure 3. This is a diagram showing the change in the relationship between current per unit length and voltage when the thickness of the second waveguide layer adjacent to the active layer is changed in the semiconductor optical element with the configuration shown in Figure 3. This is a diagram showing the change in output, PCE, and resistance when the doping concentration of the first waveguide layer is changed in the semiconductor optical element with the configuration shown in Figure 3.

[0018] Hereinafter, semiconductor optical devices according to embodiments of the present invention will be described in detail with reference to the drawings.

[0019] [First Embodiment] Figure 1 is a cross-sectional view showing a schematic configuration of a semiconductor optical element according to the first embodiment of the present invention. As shown in Figure 1, the semiconductor optical element 1 of this embodiment comprises a substrate 10 and a laminate 20 provided on the substrate 10.

[0020] The substrate 10 is a semiconductor substrate of a first conductivity type. The laminate 20 is constructed by stacking, in order from the substrate 10 side, a semiconductor layer 30 (first conductivity type semiconductor layer), an active layer 40, and a semiconductor layer 50 (second conductivity type semiconductor layer), which is a semiconductor layer of a second conductivity type. Such a semiconductor optical element 1 is, for example, a semiconductor laser element or a light-emitting diode. Electrodes (not shown) may be provided on both the substrate 10 and the laminate 20 of the semiconductor optical element 1. In this embodiment, for ease of understanding, the case where the first conductivity type is n-type and the second conductivity type is p-type will be described as an example.

[0021] The semiconductor layer 30 consists of a cladding layer 31, a first waveguide layer 32, and a second waveguide layer 33, stacked in that order from the substrate 10 side. The active layer 40 generates light of a predetermined wavelength when current is injected. The semiconductor layer 50 consists of a waveguide layer 51 and a cladding layer 52, stacked in that order from the active layer 40 side. The semiconductor optical element 1 of this embodiment has a LOC structure in order to suppress vertical laser beam spreading (or beam spreading) while having high output characteristics. In other words, the semiconductor optical element 1 of this embodiment has a structure in which the waveguide layers (first waveguide layer 32, second waveguide layer 33, waveguide layer 51) have relatively large thicknesses.

[0022] In this embodiment, the second waveguide layer 33 in the semiconductor layer 30 (a layer positioned between the first waveguide layer 32 and the active layer 40) has a higher doping concentration than the active layer 40 and the first waveguide layer 32. In other words, in this embodiment, a high-concentration doping layer is positioned near the interface between the semiconductor layer 30 and the active layer 40. This makes it possible to reduce the extent of the depletion layer compared to conventional designs, and thus lower the operating voltage of the semiconductor optical element 1.

[0023] Furthermore, the semiconductor layer 30, the active layer 40, and the semiconductor layer 50 can be formed, for example, by metal-organic chemical vapor deposition (MOCVD). In this case, the raw materials can be appropriately selected from halomethanes such as trimethylgallium (TMG), trimethylaluminum (TMA), trimethylindium (TMI), arsine gas (AsH3), carbon bromide (CBr4), or carbon chloride (CCl4), diethylzinc (DEZ), monosilane (SiH4), etc., depending on the material constituting each layer.

[0024] The substrate 10 and the laminate 20 will be described in detail below.

[0025] <Substrate 10> The substrate 10 includes a compound semiconductor and a dopant. Examples of compound semiconductors include III-V compound semiconductors such as GaAs and InP. Examples of dopants include elements such as Si, Ge, Sn, S, Se, and Te. These can be used individually or in combination of two or more. The thickness of the substrate 10 is not particularly limited, but is, for example, about 250 to 450 μm.

[0026] <Laminate 20> The laminate 20 comprises, for example, a semiconductor layer 30, an active layer 40, and a semiconductor layer 50, as described above. The laminate 20 includes a compound semiconductor. Examples of compound semiconductors include GaAs, AlGaAs, InGaAs, InGaAlAs, InP, GaInP, AlInP, AlGaInP, and InGaAsP.

[0027] Semiconductor layer 30 The semiconductor layer 30 comprises, for example, a cladding layer 31, a first waveguide layer 32, and a second waveguide layer 33, as described above. The cladding layer 31 is a layer for confining the light generated in the active layer 40. The first waveguide layer 32 and the second waveguide layer 33 are layers through which the light generated in the active layer 40 propagates, together with the active layer 40.

[0028] The semiconductor layer 30 includes a compound semiconductor and a dopant. The compound semiconductor may be the same as or different from the compound semiconductor included in the substrate 10. For example, when the semiconductor layer 30 includes AlGaAs as the compound semiconductor, let the composition of Al be x, and the composition ratio of Al to Ga be x:(1-x). In this case, it is desirable that the composition of Al x in the first waveguide layer 32 and the second waveguide layer 33 of the semiconductor layer 30 is, for example, 0.1 ≤ x ≤ 0.2.

[0029] As the dopant, the same dopant as the dopant in the substrate 10 can be used. The doping concentration of the second waveguide layer 33 is higher than that of the first waveguide layer 32. Specifically, the doping concentration of the second waveguide layer 33 is more than twice that of the first waveguide layer 32. For example, the doping concentration of the first waveguide layer 32 is 1 × 10⁻¹⁶. 16 ~1 x 10 17 cm -3The concentration is preferably within the range of 3 × 10 16 ~5 x 10 16 cm -3 It is more desirable that the concentration be within the range of . The doping concentration of the second waveguide 33 is 1 × 10 17 ~1 x 10 18 cm -3 The concentration is preferably within the range of 1 × 10 17 ~5 x 10 17 cm -3 It is more desirable for the concentration to be within this range.

[0030] The thickness of the cladding layer 31 and the first waveguide layer 32 is not particularly limited, but is preferably about 1 μm. The thickness of the second waveguide layer 33 is preferably about 25 to 50 nm. The doping concentration and thickness of the first waveguide layer 32 are designed considering the balance between the resistance of the semiconductor optical element 1 and the decrease in luminescence efficiency due to free carrier absorption. The doping concentration and thickness of the second waveguide layer 33 are designed to sufficiently suppress the expansion of the depletion layer while suppressing the effect of the increase in the resistance of the semiconductor optical element 1.

[0031] 《Active Layer 40》 The active layer 40 has a band gap smaller than the band gaps of the semiconductor layer 30 and the semiconductor layer 50, and is a layer that generates light by current injection. The active layer 40 contains a compound semiconductor. The compound semiconductor is appropriately selected according to the wavelength of the light emitted from the semiconductor photoelement 1. Examples of compound semiconductors include InGaAs, GaAs, InGaAlAs, AlGaInP, and InGaAsP.

[0032] The active layer 40 may be composed of a laminate including, for example, a quantum well layer between two barrier layers. The two barrier layers on either side of the quantum well layer are layers containing a compound semiconductor having a band gap larger than the band gap of the quantum well layer. The barrier layers may further contain dopants. The barrier layers can consist of a layer with a constant doping concentration, a graded layer in which the doping concentration changes as it moves away from the quantum well layer, or a laminate of these. Alternatively, the barrier layers can consist of a layer in which the elemental composition in the compound semiconductor is constant, a graded layer in which the elemental composition in the compound semiconductor changes along the direction away from the quantum well layer, or a laminate of these.

[0033] The thickness of the active layer 40 is not particularly limited, but is, for example, about 30 to 70 nm. The active layer 40 may also be a multiple quantum well structure in which quantum well layers and barrier layers are alternately stacked over multiple layers.

[0034] 《Semiconductor Layer 50》 The semiconductor layer 50 comprises, for example, a waveguide layer 51 and a cladding layer 52, as described above. The waveguide layer 51, together with the active layer 40, is a layer through which light generated in the active layer 40 propagates. The cladding layer 52 is a layer for confining the light generated in the active layer 40. In other words, the light generated in the active layer 40 propagates through the waveguide layer 51, the first waveguide layer 32 and the second waveguide layer 33 of the semiconductor layer 30, and the active layer 40, while being confined by the cladding layer 52 and the cladding layer 31 of the semiconductor layer 30.

[0035] The semiconductor layer 50 includes a compound semiconductor and a dopant. The compound semiconductor may be the same as or different from the compound semiconductor included in the substrate 10 or the semiconductor layer 30. For example, when the semiconductor layer 50 includes AlGaAs as the compound semiconductor, let the composition of Al be x, and the composition ratio of Al to Ga be x:(1-x). The waveguide layer 51 of the semiconductor layer 50 preferably has a dopant of, for example, 0.1 ≤ x ≤ 0.2. Examples of dopants include elements such as C. The doping concentration of the waveguide layer 51 may be the same as or different from the doping concentration of the first waveguide layer 32 in the semiconductor layer 30. For example, the doping concentration of the waveguide layer 51 is 1 × 10⁻¹⁶ 16 ~1 x 1017 cm -3 The concentration is preferably within the range of 3 × 10 16 ~5 x 10 16 cm -3 It is more desirable for the concentration to be within this range.

[0036] The thickness of the waveguide layer 51 may be the same as or different from the thickness of the first waveguide layer 32 in the semiconductor layer 30. The thickness of the cladding layer 52 may be the same as or different from the thickness of the cladding layer 31 in the semiconductor layer 30. The thicknesses of the waveguide layer 51 and the cladding layer 52 are not particularly limited, but are preferably about 1 μm. As mentioned above, the semiconductor optical element 1 of this embodiment has an LOC structure, so the sum of the thicknesses of the waveguide layer 51 and the first waveguide layer 32 and second waveguide layer 33 of the semiconductor layer 30 is approximately 1.5 to 2.5 μm.

[0037] As described above, in this embodiment, the second waveguide layer 33 in the semiconductor layer 30 (a layer positioned between the first waveguide layer 32 and the active layer 40) has a higher doping concentration than the active layer 40 and the first waveguide layer 32. This makes it possible to reduce the extent of the depletion layer compared to conventional designs, and thus lower the operating voltage of the semiconductor optical element 1. Furthermore, since the thickness of the second waveguide layer 33 is sufficiently small compared to the total thickness of the waveguide layer including the first waveguide layer 32, the active layer 40, and the waveguide layer 51, it does not cause a decrease in luminescence efficiency due to free carrier absorption (or the decrease is kept to a minimum).

[0038] [Second Embodiment] Figure 2 is a cross-sectional view showing the schematic configuration of a semiconductor optical element according to the second embodiment of the present invention. In Figure 2, components corresponding to the configuration shown in Figure 1 are denoted by the same reference numerals. As shown in Figure 2, the semiconductor optical element 2 of this embodiment differs from the semiconductor optical element 1 shown in Figure 1 in the configuration of the laminate 20. Specifically, the laminate 20 in this embodiment has a configuration in which the semiconductor layer 30 of the laminate 20 shown in Figure 1 is replaced with a semiconductor layer 30A.

[0039] The semiconductor layer 30A is constructed by stacking a cladding layer 31, a first waveguide layer 32, a second waveguide layer 33, and an undoped layer 34 in that order from the substrate 10 side. In other words, in the semiconductor layer 30A, the undoped layer 34 is provided on the side of the semiconductor layer 30 opposite to the substrate 10 side. In the semiconductor optical element 1 shown in Figure 1, the second waveguide layer 33 of the laminate 20 is in direct contact with the active layer 40, but in the semiconductor optical element 2 of this embodiment, the undoped layer 34 is arranged between the second waveguide layer 33 and the active layer 40 of the laminate 20.

[0040] In this embodiment, the thickness of the second waveguide layer 33 and the undoped layer 34 is not particularly limited, but for example, the sum of the thicknesses of the second waveguide layer 33 and the undoped layer 34 is designed to be the same as the thickness of the second waveguide layer 33 in the first embodiment. From the standpoint of suppressing the expansion of the depletion layer, it is considered desirable to make the thickness of the undoped layer 34 as small as possible.

[0041] For example, the thickness of the second waveguide layer 33 may be 25 nm or more, more preferably in the range of 25 to 50 nm, and even more preferably in the range of 45 to 50 nm. If the thickness of the second waveguide layer 33 is 45 nm, then the thickness of the undoping layer 34 is, for example, 5 nm. In this embodiment, the doping concentration of the second waveguide layer 33 is 5 × 10⁻¹⁶. 17 ~1 x 10 18 cm -3 It is desirable that the concentration be within this range.

[0042] As described above, in this embodiment, although an undoped layer 34 is arranged between the second waveguide layer 33 and the active layer 40, the doping concentration of the second waveguide layer 33 is higher than that of the active layer 40 and the first waveguide layer 32, which makes it possible to reduce the depletion layer's extent compared to conventional designs. This allows the operating voltage of the semiconductor optical element 2 to be lowered. Furthermore, since the thickness of the second waveguide layer 33 is sufficiently small compared to the total thickness of the waveguide layer including the first waveguide layer 32, the undoped layer 34, the active layer 40, and the waveguide layer 51, it does not lead to a decrease in luminescence efficiency due to free carrier absorption (or at least the decrease is kept to a minimum).

[0043] Although a semiconductor optical element according to an embodiment of the present invention has been described above, the present invention is not limited to the above embodiments and can be freely modified within the scope of the present invention. For example, in the first and second embodiments described above, an example was described in which a high-concentration doping layer (second waveguide layer 33) is provided near the interface between the semiconductor layers 30, 30A and the active layer 40. However, such a high-concentration doping layer may be provided near the interface between the active layer 40 and the semiconductor layer 50, or it may be provided near the interface between the semiconductor layers 30, 30A and the active layer 40, and near the interface between the active layer 40 and the semiconductor layer 50.

[0044] However, it is preferable that the high-concentration doping layer be located closer to the interface between the semiconductor layers 30, 30A and the active layer 40 than closer to the interface between the active layer 40 and the semiconductor layer 50. This is because the carrier absorption of the p-type AlGaAs semiconductor layer is about an order of magnitude larger than that of the n-type AlGaAs semiconductor layer, so even a small thickness of the high-concentration doping layer can significantly reduce the luminescence efficiency.

[0045] Furthermore, in the embodiments described above, the case where the first conductivity type is n-type and the second conductivity type is p-type was explained as an example for ease of understanding. However, the first conductivity type may be p-type and the second conductivity type may be n-type. In other words, the substrate 10 and semiconductor layers 30, 30A may be p-type, and the semiconductor layer 50 may be n-type.

[0046] [Specific Configuration Example] Figure 3 shows an example of a specific configuration of a semiconductor optical element according to an embodiment of the present invention. Although the configuration example shown in Figure 3 is an example of a semiconductor optical element 1 according to the first embodiment of the present invention, the semiconductor optical element 2 according to the second embodiment of the present invention can also be configured in the same way as shown in Figure 3.

[0047] The semiconductor optical device shown in Figure 3 uses a GaAs-based semiconductor compound. Specifically, the substrate 10 is formed using GaAs, and the active layer 40 is formed using InGaAs. Furthermore, the semiconductor layer 30 (cladding layer 31, first waveguide layer 32, and second waveguide layer 33) and the semiconductor layer 50 (waveguide layer 51 and cladding layer 52) are formed using AlGaAs. In the semiconductor optical device shown in Figure 3, the cladding layer 52 of the semiconductor layer 50 has a three-layer structure consisting of cladding layers 52a, 52b, and 52c.

[0048] The Al composition x in the cladding layer 31, the first waveguide layer 32, and the second waveguide layer 33 is 0.27, 0.16, and 0.16, respectively. The doping concentration in the cladding layer 31, the first waveguide layer 32, and the second waveguide layer 33 is 1.0 × 10⁻⁶, respectively. 18 cm -3 , 1.0 × 10 16 ~1.0 x 10 17 cm -3 , 5.0 x 10 17 cm -3 The thicknesses of the cladding layer 31, the first waveguide layer 32, and the second waveguide layer 33 are 0.95 μm, 1.4 μm, and 0.045 μm, respectively.

[0049] The Al composition x in waveguide layer 51 is 0.16, and the Al composition x in cladding layers 52a to 52c is 0.456 in all of them. The doping concentrations in waveguide layer 51 and cladding layers 52a to 52c are 5.0 × 10⁻¹⁰, respectively. 16 cm -3 , 5.0 x 10 17 cm -3 , 2.0 x 10 18 cm -3 , 6.3 x 10 18 cm -3 The waveguide layer 51 and cladding layers 52a to 52c have thicknesses of 0.71 μm, 0.07 μm, 0.50 μm, and 0.35 μm, respectively.

[0050] Figure 4 shows the change in current-voltage characteristics when the configuration of the n-side layer adjacent to the active layer is changed in a semiconductor optical element with the configuration shown in Figure 3. Examples 1 and 2 in Figure 4 have a configuration in which the second waveguide layer 33 is adjacent to the active layer 40, as in the semiconductor optical element 1 shown in Figure 1. Examples 3 and 4 in Figure 4 have a configuration in which an undoped layer 34 is provided between the active layer 40 and the second waveguide layer 33, as in the semiconductor optical element 2 shown in Figure 2, and the undoped layer 34 is adjacent to the active layer 40. The comparative example in Figure 4 has a configuration in which there is no configuration corresponding to the second waveguide layer 33, as in the semiconductor optical element disclosed in Patent Document 1, and the undoped layer is adjacent to the active layer 40.

[0051] Figure 5 is a diagram summarizing the configuration of the n-side layer adjacent to the active layer in Examples 1 to 4 and the comparative example shown in Figure 4. In Examples 1 and 2, the thickness of the second waveguide layer 33 is 50 nm in both cases, but the doping concentration in the second waveguide layer 33 is different. Specifically, the doping concentration in the second waveguide layer 33 of Example 1 is 1 × 10⁻¹⁶. 17 cm -3 Therefore, the doping concentration in the second waveguide layer 33 of Example 2 is 5 × 10 17 cm -3 In addition, in Examples 1 and 2, since no undoping layer is provided, the thickness of the undoping layer is 0 nm, as shown in the figure.

[0052] In both Examples 3 and 4, the thickness of the undoped layer 34 is 5 nm and the thickness of the second waveguide layer 33 is 45 nm, but the doping concentration in the second waveguide layer 33 is different. Specifically, the doping concentration in the second waveguide layer 33 of Example 3 is 5 × 10⁻⁶. 17 cm -3 Therefore, the doping concentration in the second waveguide layer 33 of Example 4 is 1 × 10⁻⁶. 18 cm -3 That is the case.

[0053] In the comparative example, the thickness of the undoped layer is 50 nm. Furthermore, since the comparative example does not have a configuration corresponding to the second waveguide layer 33, the thickness of the second waveguide layer 33 is 0 nm, as shown in the figure. Thus, the thickness of the second waveguide layer 33 in Examples 1 and 2, the sum of the thicknesses of the undoped layer 34 and the second waveguide layer 33 in Examples 3 and 4, and the thickness of the undoped layer in the comparative example are all set to 50 nm.

[0054] In Figure 4, focusing on Examples 1 and 2 and the Comparative Example, the voltage decreases in the order of Comparative Example, Example 1, and Example 2. As a result, in the configuration of the semiconductor optical element 1 shown in Figure 1 (a configuration in which the second waveguide layer 33 is adjacent to the active layer 40), the doping concentration of the second waveguide layer 33 is 1 × 10⁻¹⁶. 17 cm -3 The above demonstrates that the voltage reduction effect is achieved. Furthermore, since the voltage is significantly reduced in Example 2 compared to Example 1, the doping concentration of the second waveguide layer 33 should be set to 5 × 10⁻⁶. 17 cm -3 It can be seen that a greater voltage reduction effect can be obtained by adjusting the setting to a certain extent.

[0055] Next, looking at Examples 3 and 4 in Figure 4, the voltage is lower than that of Example 1 and higher than that of Example 2. In other words, Examples 3 and 4 achieve the voltage reduction effect between Examples 1 and 2. Specifically, in the configuration of the semiconductor optical element 2 shown in Figure 3 (a configuration in which an undoping layer 34 is provided between the active layer 40 and the second waveguide layer 33), the doping concentration of the second waveguide layer 33 is 5 × 10⁻⁶ 17 ~1 x 10 18 It can be seen that a voltage reduction effect can be obtained within this concentration range.

[0056] Figure 6 shows the changes in output and power-to-optical conversion efficiency (PCE) when the doping concentration of the second waveguide layer adjacent to the active layer is changed in a semiconductor optical device with the configuration shown in Figure 3. In Figure 6, the doping concentration of the second waveguide layer 33 adjacent to the active layer 40 is set to 5 × 10⁻¹⁰. 17 ~5 x 10 18 cm -3It is being changed within that range. Also, the drive current for the semiconductor element is set to 25A.

[0057] Here, PCE (Personal Computer Equivalent) is the ratio of the optical output that can be extracted to the amount of electrical energy input to a semiconductor optical element. Specifically, PCE is calculated using the formula: (Optical output) ÷ {(Drive current) × (Drive voltage)}. A larger PCE indicates a higher optical output relative to the input power, and therefore higher efficiency.

[0058] As shown in Figure 6, the output and PCE of the optical semiconductor device decrease as the doping concentration of the second waveguide layer 33 increases. 18 At this point, it can be seen that the output and PCE of the optical semiconductor element decrease sharply. Therefore, in the configuration of the semiconductor optical element 1 shown in Figure 1, the upper limit of the doping concentration of the second waveguide layer 33 is 1 × 10⁻⁶. 18 cm -3 It is thought to be to that extent.

[0059] Referring to Figure 4, in the configuration of the semiconductor optical element 1 shown in Figure 1 (a configuration in which the second waveguide layer 33 is adjacent to the active layer 40), as described above, the doping concentration of the second waveguide layer 33 is 1 × 10⁻¹⁶. 17 cm -3 If the above is true, it can be seen that the effect of voltage reduction can be obtained. Also, referring to Figure 6 in addition to Figure 4, the doping concentration of the second waveguide layer 33 is 5 × 10 17 ~1 x 10 8 cm -3 It can be seen that within this concentration range, a significant voltage reduction effect can be obtained without causing a substantial decrease in output or PCE.

[0060] Figure 7 shows the change in the relationship between current per unit length and voltage when the thickness of the second waveguide layer adjacent to the active layer is changed in a semiconductor optical device with the configuration shown in Figure 3. In Figure 7, the thickness of the second waveguide layer 33 adjacent to the active layer 40 is varied in the range of 0 to 50 nm. As shown in Figure 7, it can be seen that the voltage decreases as the thickness of the second waveguide layer 33 adjacent to the active layer 40 increases. It can also be seen that the voltage remains almost the same when the thickness of the second waveguide layer 33 is in the range of 45 to 50 nm. From the above, it can be seen that a voltage reduction effect can be obtained when the thickness of the second waveguide layer 33 adjacent to the active layer 40 is in the range of 25 to 50 nm, and an even greater voltage reduction effect can be obtained when the thickness of the second waveguide layer 33 adjacent to the active layer 40 is in the range of 45 to 50 nm.

[0061] Figure 8 shows the changes in output, PCE (power-to-optical efficiency), and resistance when the doping concentration of the first waveguide layer is changed in a semiconductor optical device with the configuration shown in Figure 3. In Figure 8, the doping concentration of the first waveguide layer 32 is set to 1 × 10⁻¹⁶. 16 ~1 x 10 17 cm -3 It is being changed within that range. Also, the drive current for the semiconductor element is set to 25A.

[0062] As shown in Figure 8, the resistance and output of the semiconductor optical element gradually decrease as the doping concentration of the first waveguide layer 32 increases. In contrast, the PCE of the semiconductor optical element decreases as the doping concentration of the first waveguide layer 32 increases. 16 ~3 x 10 16 cm -3 When within the range, the doping concentration increases as the doping concentration increases, but when the doping concentration is 3 × 10 16 cm -3 As the doping concentration increases, the levels decrease.

[0063] As shown in Figure 8, the doping concentration of the first waveguide layer 32 is 2 × 10 16 ~5 x 10 16 cm -3 It can be seen that a high PCE can be obtained when it is within the range. On the other hand, the resistance value of the semiconductor optical element is when the doping concentration is 2 × 10 16is lower when it is within the range of 3×10 16 ~5×10 16 cm -3 than in the case described above. From the above, the doping concentration of the first waveguide layer 32 is preferably within the range of 3×10 16 ~5×10 16 cm -3 .

[0064] Incidentally, the semiconductor optical device of the present invention is of course not limited to the above-described embodiments, nor is it limited to the above-described specific configuration examples. That is, the present invention can be freely changed within the scope of the present invention. For example, in the above-described embodiments and specific configuration examples, the stacked structure of the semiconductor optical device has been mainly described, but the structure other than the stacked structure is arbitrary. For example, when the semiconductor optical device is an end-face-emitting semiconductor laser, it may be a semiconductor laser having a Fabry-Perot resonator structure or a distributed feedback (DFB) semiconductor laser.

[0065] Further, the semiconductor optical device may be provided with a structure for restricting the supply region of the current supplied from the electrode to the active layer 40, if necessary. For example, by forming the electrode provided on the semiconductor layer 50 side in a stripe shape, the supply region of the current supplied from the electrode to the active layer 40 may be restricted. Alternatively, in the clad layer 52 of the semiconductor layer 50, a current blocking layer having an opening for injecting current and blocking current may be provided to restrict the supply region of the current supplied from the electrode to the active layer 40. Or, instead of the current stripe structure, a ridge structure having a current injection region and a current non-injection region may be provided to restrict the supply region of the current supplied from the electrode to the active layer 40.

[0066] 1, 2... semiconductor optical device, 10... substrate, 20... laminate, 30... semiconductor layer, 32... first waveguide layer, 33... second waveguide layer, 34... undoped layer, 40... active layer, 50... semiconductor layer

Claims

1. A semiconductor optical element comprising a substrate and a laminate in which a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer are stacked in order from the substrate side, wherein at least one of the first conductivity type semiconductor layer and the second conductivity type semiconductor layer has a first waveguide layer and a second waveguide layer, the second waveguide layer has a higher doping concentration than the first waveguide layer and is disposed between the first waveguide layer and the active layer.

2. The semiconductor optical element according to claim 1, wherein the doping concentration of the second waveguide is twice or more the doping concentration of the first waveguide.

3. The doping concentration of the second waveguide is 1 × 10⁻⁶ 17 cm -3 The above is 1 x 10 18 cm -3 The semiconductor optical element according to claim 2, which is as follows:

4. At least one of the first conductivity type semiconductor layer and the second conductivity type semiconductor layer has, in addition to the first waveguide layer and the second waveguide layer, an undoped layer disposed between the second waveguide layer and the active layer, wherein the doping concentration of the second waveguide layer is 5 × 10⁻¹⁰. 17 cm -3 The above is 1 x 10 18 cm -3 The semiconductor optical element according to claim 2, which is as follows:

5. The semiconductor optical element according to any one of claims 1 to 4, wherein the thickness of the second waveguide layer is 25 nm or more and 50 nm or less.

6. The doping concentration of the first waveguide layer is 3×10 16 cm -3 or more and 5×10 16 cm -3 or less. The semiconductor optical device according to claim 5.

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