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 element

[0001] The present invention relates to a semiconductor optical device. This application claims priority to Japanese Patent Application No. 2023-219113, filed on December 26, 2023, the contents of which are incorporated herein by reference.

[0002] One type of semiconductor optical device is an edge-emitting semiconductor laser. A typical 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 stacked in this order vertically 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 in the active layer and waveguide layer.

[0003] In recent years, large optical cavity (LOC) structures with relatively thick waveguide layers have been used as structures for suppressing laser light (or beam) divergence in the vertical direction while maintaining high output characteristics. Although there is no clear definition of an LOC structure, semiconductor lasers with an oscillation wavelength in the 900 nm band often use structures in which the first conductivity type waveguide layer and the second conductivity type waveguide layer have a combined thickness of approximately 1.5 to 2.5 μm. For example, Patent Document 1 listed below discloses an LOC structure with a combined waveguide layer thickness of 2.15 μm.

[0004] U.S. Patent No. 8,976,831

[0005] In an LOC structure with a large waveguide layer thickness, most of the generated laser light is guided by the active layer and the waveguide layer. Intentional doping of the semiconductor layer through which the laser light is guided can reduce the resistance of the device, but it also causes absorption of the laser light by the dopant, known as free carrier absorption, which significantly reduces the laser emission efficiency. In extreme cases, free carrier absorption can even make laser oscillation impossible.

[0006] For this reason, in order to balance the resistance value of the element and the laser light emission efficiency, the doping concentration of the waveguide layer in the LOC structure is generally kept low. For example, in the above-mentioned Patent Document 1, the active layer and the layers adjacent to the active layer on both sides thereof are undoped layers that are not intentionally doped, and the doping concentration of the n-type waveguide layer is about 1×10 16 cm -3 It is said that...

[0007] Furthermore, in the LOC structure, the thickness of the waveguide layer, which generally has a low doping concentration, is large, which tends to increase the resistance of the device and the operating voltage. In addition, in the above structure, the doping concentrations of both the n-type and p-type waveguide layers are low, which can lead to a large depletion layer and a further increase in the operating voltage.

[0008] SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to provide a semiconductor optical device capable of lowering the operating voltage without causing a significant decrease in light emission efficiency.

[0009] In order to solve the above-mentioned problems, a semiconductor optical device (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 laminated in this order from the substrate side, and 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), and the second waveguide layer has a doping concentration higher than that of the first waveguide layer and is disposed between the first waveguide layer and the active layer.

[0010] In the semiconductor optical device according to the 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 on either side of an active layer, and the second waveguide layer has a doping concentration higher than that of the first waveguide layer and is disposed between the first waveguide layer and the active layer, so that the operating voltage can be reduced without incurring a significant decrease in light emission efficiency.

[0011] A semiconductor optical device according to a second aspect of the present invention is the semiconductor optical device according to the first aspect of the present invention, wherein the doping concentration of the second waveguide layer is at least twice as high as the doping concentration of the first waveguide layer.

[0012] A semiconductor optical device according to a third aspect of the present invention is the semiconductor optical device according to the second aspect of the present invention, wherein the doping concentration of the second waveguide layer is 1×10 17 cm -3 That's it, 1 x 10 18 cm -3 The following is the result.

[0013] A semiconductor optical device according to a fourth aspect of the present invention is the semiconductor optical device according to the second aspect of the present invention, wherein at least one of the first conductive type semiconductor layer and the second conductive type semiconductor layer has, in addition to the first waveguide layer and the second waveguide layer, an undoped layer (34) that is not doped and is 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 That's it, 1 x 10 18 cm -3 The following is the result.

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

[0015] A semiconductor optical device according to a sixth aspect of the present invention is the semiconductor optical device according to any one of the first to fifth aspects of the present invention, wherein the doping concentration of the first waveguide layer is 3×10 16 cm -3 That's it, 5 x 10 16 cm -3 The following is the result.

[0016] According to the aspects of the present invention, it is possible to reduce the operating voltage without causing a significant decrease in light emission efficiency.

[0017] 1 is a cross-sectional view showing a schematic configuration of a semiconductor optical device according to a first embodiment of the present invention. 2 is a cross-sectional view showing a schematic configuration of a semiconductor optical device according to a second embodiment of the present invention. 3 is a view showing an example of a specific configuration of a semiconductor optical device according to an embodiment of the present invention. 4 is a view showing changes in current-voltage characteristics when the configuration of the n-side layer adjacent to the active layer is changed in a semiconductor optical device having the configuration shown in FIG. 3. 5 is a diagram summarizing the configurations of the n-side layer adjacent to the active layer in Examples 1 to 4 and the Comparative Example shown in FIG. 4. 6 is a view showing changes in output and PCE when the doping concentration of the second waveguide layer adjacent to the active layer is changed in a semiconductor optical device having the configuration shown in FIG. 3. 7 is a view showing changes in the relationship between current and voltage per unit length when the thickness of the second waveguide layer adjacent to the active layer is changed in a semiconductor optical device having the configuration shown in FIG. 3. 8 is a view showing changes in output, PCE, and resistance when the doping concentration of the first waveguide layer is changed in a semiconductor optical device having the configuration shown in FIG. 3.

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

[0019] 1 is a cross-sectional view showing a schematic configuration of a semiconductor optical device according to a first embodiment of the present invention. As shown in FIG. 1, the semiconductor optical device 1 of this embodiment includes 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 stacked body 20 is formed by stacking, in order from the substrate 10 side, a semiconductor layer 30 (first conductivity type semiconductor layer) which is a semiconductor layer of a first conductivity type, 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 device 1 is, for example, a semiconductor laser device or a light-emitting diode. Note that electrodes (not shown) may be provided on each of the substrate 10 and the stacked body 20 of the semiconductor optical device 1. In this embodiment, for ease of understanding, an example will be described in which the first conductivity type is n-type and the second conductivity type is p-type.

[0021] The semiconductor layer 30 includes a cladding layer 31, a first waveguide layer 32, and a second waveguide layer 33 stacked in this order from the substrate 10 side. The active layer 40 generates light of a predetermined wavelength when a current is injected into it. The semiconductor layer 50 includes a waveguide layer 51 and a cladding layer 52 stacked in this order from the active layer 40 side. The semiconductor optical device 1 of this embodiment has an LOC structure in order to suppress laser light divergence (or beam divergence) in the vertical direction while maintaining high output characteristics. That is, the semiconductor optical device 1 of this embodiment has a structure in which the waveguide layers (first waveguide layer 32, second waveguide layer 33, and waveguide layer 51) are relatively thick.

[0022] In this embodiment, the second waveguide layer 33 in the semiconductor layer 30 (a layer disposed between the first waveguide layer 32 and the active layer 40) has a doping concentration higher than that of the active layer 40 and the first waveguide layer 32. In other words, in this embodiment, a highly doped layer is disposed 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 devices, thereby enabling the operating voltage of the semiconductor optical device 1 to be lowered.

[0023] The semiconductor layer 30, the active layer 40, and the semiconductor layer 50 can be formed by, for example, metal organic chemical vapor deposition (MOCVD). In this case, the raw materials used may be selected from trimethylgallium (TMG), trimethylaluminum (TMA), trimethylindium (TMI), arsine gas (AsH), halomethanes such as carbon bromide (CBr) or carbon chloride (CCl), diethylzinc (DEZ), monosilane (SiH), and the like, 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 the compound semiconductor include III-V group compound semiconductors such as GaAs and InP. Examples of the dopant include elements such as Si, Ge, Sn, S, Se, and Te. These elements can be used alone 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] <Stacked body 20> As described above, the stacked body 20 includes, for example, the semiconductor layer 30, the active layer 40, and the semiconductor layer 50. The stacked body 20 includes a compound semiconductor. Examples of the compound semiconductor include GaAs, AlGaAs, InGaAs, InGaAlAs, InP, GaInP, AlInP, AlGaInP, and InGaAsP.

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

[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, the Al composition is x, and the Al to Ga composition ratio is x:(1-x). In this case, the first waveguide layer 32 and the second waveguide layer 33 of the semiconductor layer 30 preferably have an Al composition x that satisfies, for example, 0.1≦x≦0.2.

[0029] The dopant may be the same as the dopant in the substrate 10. The second waveguide layer 33 has a higher doping concentration than the first waveguide layer 32. Specifically, the doping concentration of the second waveguide layer 33 is at least twice the doping concentration of the first waveguide layer 32. For example, the doping concentration of the first waveguide layer 32 is 1×10 16 ~1 x 10 17 cm -3The concentration is preferably in the range of 3×10 16 ~5 x 10 16 cm -3 The doping concentration of the second waveguide layer 33 is more preferably in the range of 1×10 17 ~1 x 10 18 cm -3 The concentration is preferably in the range of 1×10 17 ~5 x 10 17 cm -3 It is more desirable that the concentration be in the range of

[0030] The thicknesses of the cladding layer 31 and the first waveguide layer 32 are not particularly limited, but are preferably, for example, about 1 μm. The thickness of the second waveguide layer 33 is desirably about 25 to 50 nm. The doping concentration and thickness of the first waveguide layer 32 are designed in consideration of the balance between the resistance value of the semiconductor optical device 1 and the decrease in light emission 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 simultaneously suppressing the effect of an increase in the resistance value of the semiconductor optical device 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 when a current is injected. The active layer 40 includes a compound semiconductor. The compound semiconductor is appropriately selected depending on the wavelength of the light to be emitted from the semiconductor optical device 1. Examples of the compound semiconductor include InGaAs, GaAs, InGaAlAs, AlGaInP, and InGaAsP.

[0032] The active layer 40 may be composed of, for example, a stack including a quantum well layer between two barrier layers. The two barrier layers on either side of the quantum well layer are layers including a compound semiconductor having a band gap larger than that of the quantum well layer. The barrier layers may further include a dopant. The barrier layers may be composed of a layer with a constant doping concentration, a graded layer in which the doping concentration varies with distance from the quantum well layer, or a stack of these. Alternatively, the barrier layers may be composed of a layer with a constant composition of elements in the compound semiconductor, a graded layer in which the composition of elements in the compound semiconductor varies in the direction away from the quantum well layer, or a stack 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 have a multiple quantum well structure in which quantum well layers and barrier layers are alternately stacked over a plurality of layers.

[0034] <<Semiconductor Layer 50>> The semiconductor layer 50 includes, for example, a waveguide layer 51 and a cladding layer 52, as described above. The waveguide layer 51 is a layer that, together with the active layer 40, propagates light generated in the active layer 40. The cladding layer 52 is a layer that confines the light generated in the active layer 40. In other words, the light generated in the active layer 40 is confined by the cladding layer 52 and the cladding layer 31 of the semiconductor layer 30, and 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.

[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, the Al composition is represented by x, and the composition ratio of Al to Ga is represented by x:(1-x). The waveguide layer 51 of the semiconductor layer 50 preferably has a doping concentration of 0.1≦x≦0.2, for example. The dopant may be, for example, an element 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 may be 1×10 16 ~1 x 1017 cm -3 The concentration is preferably in the range of 3×10 16 ~5 x 10 16 cm -3 It is more desirable that the concentration be in the range of

[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. There are no particular restrictions on the thicknesses of the waveguide layer 51 and the cladding layer 52, but it is preferable that the thickness be approximately 1 μm, for example. As described above, the semiconductor optical device 1 of this embodiment has an LOC structure, and therefore the total thickness of the waveguide layer 51 and the first and second waveguide layers 32 and 33 in the semiconductor layer 30 is approximately 1.5 to 2.5 μm.

[0037] As described above, in this embodiment, the second waveguide layer 33 (a layer disposed between the first waveguide layer 32 and the active layer 40) in the semiconductor layer 30 has a doping concentration higher than that of 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 devices, thereby lowering the operating voltage of the semiconductor optical device 1. Furthermore, the thickness of the second waveguide layer 33 is sufficiently smaller than the thickness of the entire waveguide layer including the first waveguide layer 32, the active layer 40, and the waveguide layer 51, so that there is no reduction in light emission efficiency due to free carrier absorption (or the reduction is kept to a slight extent).

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

[0039] The semiconductor layer 30A has a cladding layer 31, a first waveguide layer 32, a second waveguide layer 33, and an undoped layer 34 laminated in this order from the substrate 10 side. That is, 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. The semiconductor optical device 1 shown in FIG. 1 has a configuration in which the second waveguide layer 33 of the laminate 20 is in direct contact with the active layer 40, but the semiconductor optical device 2 of this embodiment has a configuration in which the undoped layer 34 is disposed between the second waveguide layer 33 of the laminate 20 and the active layer 40.

[0040] In this embodiment, the thicknesses of the second waveguide layer 33 and the undoped layer 34 are not particularly limited, but are designed, for example, so that the sum of the thicknesses of the second waveguide layer 33 and the undoped layer 34 is the same as the thickness of the second waveguide layer 33 in the first embodiment. From the viewpoint of suppressing the expansion of the depletion layer, it is considered preferable that the thickness of the undoped layer 34 be 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, the thickness of the undoped layer 34 is, for example, 5 nm. In this embodiment, the doping concentration of the second waveguide layer 33 is 5×10 17 ~1 x 10 18 cm -3 The concentration is preferably in the range of

[0042] As described above, in this embodiment, although the undoped layer 34 is disposed between the second waveguide layer 33 and the active layer 40, the second waveguide layer 33 has a higher doping concentration than the active layer 40 and the first waveguide layer 32, so that the extent of the depletion layer can be made smaller than in the past. This makes it possible to lower the operating voltage of the semiconductor optical device 2. Furthermore, since the thickness of the second waveguide layer 33 is sufficiently smaller than the thickness of the entire waveguide layer including the first waveguide layer 32, the undoped layer 34, the active layer 40, and the waveguide layer 51, there is no decrease in light emission efficiency due to free carrier absorption (or the decrease is kept to a slight extent).

[0043] Although the semiconductor optical device according to the embodiments 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 above first and second embodiments, an example was described in which a heavily doped layer (second waveguide layer 33) is provided near the interface between the semiconductor layer 30, 30A and the active layer 40. However, such a heavily doped layer may be provided near the interface between the active layer 40 and the semiconductor layer 50, or may be provided both near the interface between the semiconductor layer 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 heavily doped layer be provided closer to the interface between the semiconductor layer 30, 30A and the active layer 40 than to the interface between the active layer 40 and the semiconductor layer 50. This is because the carrier absorption of a p-type AlGaAs semiconductor layer is about one order of magnitude larger than the carrier absorption of an n-type AlGaAs semiconductor layer, and therefore even if the heavily doped layer is only slightly thick, the light emission efficiency is significantly reduced.

[0045] In addition, in the above-described embodiment, for ease of understanding, the first conductivity type is n-type and the second conductivity type is p-type. 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 the semiconductor layers 30 and 30A may be p-type, and the semiconductor layer 50 may be n-type.

[0046] [Specific Configuration Example] Fig. 3 is a diagram showing an example of a specific configuration of a semiconductor optical device according to an embodiment of the present invention. Note that the configuration example shown in Fig. 3 is a configuration example for the semiconductor optical device 1 according to the first embodiment of the present invention, but the semiconductor optical device 2 according to the second embodiment of the present invention can also have a configuration similar to that shown in Fig. 3.

[0047] The semiconductor optical device shown in Fig. 3 uses a GaAs-based semiconductor compound. Specifically, the substrate 10 is formed using GaAs, and the active layer 40 is formed using InGaAs. 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 Fig. 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 compositions x in the cladding layer 31, the first waveguide layer 32, and the second waveguide layer 33 are 0.27, 0.16, and 0.16, respectively. The doping concentrations in the cladding layer 31, the first waveguide layer 32, and the second waveguide layer 33 are 1.0×10 18 cm -3 , 1.0×10 16 ~1.0 x 10 17 cm -3 , 5.0 × 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 ratio x in the waveguide layer 51 is 0.16, and the Al composition ratio x in all of the cladding layers 52a to 52c is 0.456. The doping concentrations in the waveguide layer 51 and the cladding layers 52a to 52c are each 5.0×10 16 cm -3 , 5.0 × 10 17 cm -3 , 2.0 × 10 18 cm -3 , 6.3 × 10 18 cm -3 The thicknesses of the waveguide layer 51 and the cladding layers 52a to 52c are 0.71 μm, 0.07 μm, 0.50 μm, and 0.35 μm, respectively.

[0050] Fig. 4 is a diagram showing changes in current-voltage characteristics when the configuration of the n-side layer adjacent to the active layer is changed in the semiconductor optical device configured as shown in Fig. 3. Examples 1 and 2 in Fig. 4 have a configuration in which the second waveguide layer 33 is adjacent to the active layer 40, as in the semiconductor optical device 1 shown in Fig. 1. Examples 3 and 4 in Fig. 4 have a configuration in which an undoped layer 34 is provided between the active layer 40 and the second waveguide layer 33, and the undoped layer 34 is adjacent to the active layer 40, as in the semiconductor optical device 2 shown in Fig. 2. The comparative example in Fig. 4 has a configuration in which a configuration equivalent to the second waveguide layer 33 is not provided, as in the semiconductor optical device disclosed in Patent Document 1, and an undoped layer is adjacent to the active layer 40.

[0051] 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 FIG. 4. In Examples 1 and 2, the thickness of the second waveguide layer 33 is both 50 nm, but the doping concentration in the second waveguide layer 33 is different. Specifically, the doping concentration in the second waveguide layer 33 in Example 1 is 1×10 17 cm -3 The doping concentration in the second waveguide layer 33 of the second embodiment is 5×10 17 cm -3 In Examples 1 and 2, since no undoped layer is provided, the thickness of the undoped 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 in Example 3 is 5×10 17 cm -3 The doping concentration in the second waveguide layer 33 of the fourth embodiment is 1×10 18 cm -3 is.

[0053] In the comparative example, the thickness of the undoped layer is 50 nm. Note that in the comparative example, a configuration corresponding to the second waveguide layer 33 is not provided, and therefore, as shown in the figure, the thickness of the second waveguide layer 33 is 0 nm. In this way, 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] 4, when attention is paid to 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 device 1 shown in FIG. 1 (the 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 17 cm -3 It can be seen from the above that the voltage reduction effect is obtained. Furthermore, since the voltage is significantly reduced in Example 2 compared to Example 1, it is considered that the doping concentration of the second waveguide layer 33 is 5×10 17 cm -3 It can be seen that a larger voltage reduction effect can be obtained by setting the voltage to about 1 / 200V.

[0055] 4, in Examples 3 and 4, the voltage is lower than that of Example 1 and higher than that of Example 2. In other words, it can be seen that Examples 3 and 4 achieve the effect of reducing the voltage between that of Example 1 and Example 2. That is, in the configuration of the semiconductor optical device 2 shown in FIG. 3 (the configuration in which the undoped layer 34 is provided between the active layer 40 and the second waveguide layer 33), when the doping concentration of the second waveguide layer 33 is 5×10 17 ~1 x 10 18 It can be seen that the voltage reduction effect can be obtained if the concentration is in the range.

[0056] 6 is a graph showing the change in output power and power conversion efficiency (PCE) when the doping concentration of the second waveguide layer 33 adjacent to the active layer is changed in the semiconductor optical device having the configuration shown in FIG. 6. In FIG. 6, the doping concentration of the second waveguide layer 33 adjacent to the active layer 40 is set to 5×10 17 ~5 x 10 18 cm -3The driving current of the semiconductor element is set to 25A.

[0057] Here, PCE is the ratio of optical output power that can be extracted to the amount of power input to a semiconductor optical device. Specifically, PCE is a value calculated by the formula: (optical output power) ÷ {(driving current) × (driving voltage)}. The larger the PCE, the larger the optical output power relative to the input power, and the higher the efficiency.

[0058] 6, it can be seen that the output power and PCE of the optical semiconductor device decrease as the doping concentration of the second waveguide layer 33 increases. 18 Above this value, it can be seen that the output power and PCE of the optical semiconductor device drop sharply. For this reason, in the configuration of the semiconductor optical device 1 shown in FIG. 1, the upper limit of the doping concentration of the second waveguide layer 33 is set to 1×10 18 cm -3 It is thought to be about that extent.

[0059] 4, in the configuration of the semiconductor optical device 1 shown in FIG. 1 (the 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 17 cm -3 6 in addition to FIG. 4, when the doping concentration of the second waveguide layer 33 is 5×10 17 ~1 x 10 8 cm -3 It can be seen that a concentration in this range can achieve a significant voltage reduction effect without causing a significant decrease in output and PCE.

[0060] Figure 7 shows 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 device shown in Figure 3. In Figure 7, the thickness of the second waveguide layer 33 adjacent to the active layer 40 is changed in the range of 0 to 50 nm. As shown in Figure 7, 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 approximately constant 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] 8 is a graph showing the changes in output, PCE (power-to-light conversion efficiency), and resistance when the doping concentration of the first waveguide layer 32 is changed in the semiconductor optical device having the configuration shown in FIG. 8. In FIG. 8, the doping concentration of the first waveguide layer 32 is set to 1×10 16 ~1 x 10 17 cm -3 The driving current of the semiconductor element is set to 25A.

[0062] 8, the resistance and output of the semiconductor optical device gradually decrease as the doping concentration of the first waveguide layer 32 increases. In contrast, the PCE of the semiconductor optical device decreases when the doping concentration of the first waveguide layer 32 increases from 1×10 16 ~3 x 10 16 cm -3 When the doping concentration is within the range of 3×10, the doping concentration increases. 16 cm -3 When the doping concentration is higher than 1000 kJ / s, it decreases as the doping concentration increases.

[0063] From the results shown in FIG. 8, it can be seen that the doping concentration of the first waveguide layer 32 is 2×10 16 ~5 x 10 16 cm -3 On the other hand, the resistance of the semiconductor optical device increases when the doping concentration is within the range of 2×10 16than 3 x 10 16 ~5 x 10 16 cm -3 From the above, the doping concentration of the first waveguide layer 32 is lower when it is within the range of 3×10 16 ~5 x 10 16 cm -3 It is desirable that the range is within the range.

[0064] The semiconductor optical device of the present invention is not limited to the above-described embodiment, nor is it limited to the above-described specific configuration examples. In other words, the present invention can be freely modified within the scope of the present invention. For example, in the above-described embodiment 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, if the semiconductor optical device is an edge-emitting semiconductor laser, it may be a semiconductor laser having a Fabry-Perot resonator structure or a distributed feedback (DFB) semiconductor laser.

[0065] Furthermore, the semiconductor optical device may be provided with a structure for limiting the supply area of ​​the current supplied from the electrode to the active layer 40, as necessary. For example, the electrode provided on the semiconductor layer 50 side may be formed in a stripe shape to limit the supply area of ​​the current supplied from the electrode to the active layer 40. Alternatively, the cladding layer 52 of the semiconductor layer 50 may be provided with a current blocking layer that has an opening for injecting current and blocks the current, to limit the supply area of ​​the current supplied from the electrode to the active layer 40. Alternatively, instead of the current stripe structure, a ridge structure having a current injection area and a current non-injection area may be provided to limit the supply area of ​​the current supplied from the electrode to the active layer 40.

[0066] Reference Signs List 1, 2... semiconductor optical element, 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 device 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 laminated in this 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 device according to claim 1, wherein the doping concentration of the second waveguide layer is two times or more the doping concentration of the first waveguide layer.

3. The doping concentration of the second waveguide layer is 1 × 10 17 cm -3 or more and 1 × 10 18 cm -3 or less. The semiconductor optical device according to claim 2.

4. At least one of the first conductivity type semiconductor layer and the second conductivity type semiconductor layer has an undoped layer disposed between the second waveguide layer and the active layer, without being doped in addition to the first waveguide layer and the second waveguide 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. The semiconductor optical element according to claim 2.

5. The semiconductor optical device 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 element according to claim 5.

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