Semiconductor Optical Devices

Abstract

This semiconductor optical device comprises: a lower cladding layer (102) formed on a substrate (101); a first semiconductor layer (103) composed of a first conductivity-type group III-V compound semiconductor and formed on the lower cladding layer (102); an active layer (104) composed of a group III-V compound semiconductor and formed on the first semiconductor layer (103); a second semiconductor layer (105) composed of a group III-V compound semiconductor and formed on the active layer (104); a resonator (106) composed of a diffraction grating and formed on the upper surface of the second semiconductor layer (105); and a third semiconductor layer (107) composed of a second conductivity-type group III-V compound semiconductor, formed above the formation region of the active layer (104), and having a mesa shape extending in the extending direction of the active layer (104). The mesa width of the third semiconductor layer (107) is made smaller than the width of the active layer (104).

Description

[Technical field]

[0001] The present invention relates to semiconductor optical devices. [Background technology]

[0002] A vertical thin-film structure has been proposed that has a thin mesa (typically 400 nm or less) for current injection above the active region of the thin-film structure, as a structure that enables both strong light confinement specific to the thin-film structure and efficient current injection from the vertical direction. The thin-film structure is, for example, a structure in which a III-V semiconductor thin film with a thickness of about 250 to 500 nm is surrounded by an insulating material with a low refractive index, such as SiO2 or air.

[0003] In a typical thin-film structure, a lateral type is widely adopted in which an n-type semiconductor layer and a p-type semiconductor layer made of, for example, InP are provided on the left and right of a buried active region, and carriers are injected into the active region from both of these layers (see Non-Patent Documents 1 to 3). For example, a multiple quantum well structure made of InGaAsP, InGaAlAs, etc. is used as the active region.

[0004] On the other hand, in the vertical thin-film structure described above, the first mesa for current injection above the active region is narrowed to, for example, about 400 nm or less, thereby preventing the optical mode in the cross section of the active region from being absorbed by the first mesa, and while enabling current injection from the vertical direction, it is possible to obtain high optical confinement that is comparable to that of the horizontal type. For convenience, in the following, the cross section of the active region is taken as the xy plane, and the horizontal direction is defined as the x direction and the vertical direction as the y direction. In this definition, the optical axis direction of the light propagating in the thin-film structure is the z direction.

[0005] A typical application example of such a thin film structure that allows current injection into the active region is a semiconductor laser diode (LD) having a surface diffraction grating in the cross section of the active region, as also realized in Non-Patent Documents 1 to 3. In these LDs, a diffraction grating is formed for the purpose of confining the mode of light propagating in the active region in the z direction by Bragg reflection to form a resonator. Particularly typical structures are the distributed feedback (DFB) laser in Non-Patent Document 1 and the distributed reflection (DR) laser in Non-Patent Document 2, which are widely used as structures that provide stable single-mode oscillation.

[0006] Here, in the lateral thin-film structure, since there is no first mesa on the semiconductor thin-film where the active region is formed, by etching the upper surface of a thin-film made of, for example, InP, it is possible to form a surface diffraction grating while maintaining strong light confinement in the active region. In this structure, light confinement is strong near the thin-film structure, and the refractive index difference between the convex parts (e.g., InP) and concave parts (e.g., SiO2 or air) of the surface diffraction grating is large. For this reason, in the lateral thin-film structure, a relatively shallow excavation of about 20 to 30 nm typically results in a surface diffraction grating with a depth of several hundred cm. -1 ~1000cm -1 It is possible to obtain a diffraction grating with a high coupling constant κ reaching κ (see Non-Patent Document 3). As shown in Non-Patent Document 3, achieving both strong optical confinement and a high coupling constant is particularly important in realizing a short cavity laser with low power consumption. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] S. Matsuo, T. Fujii, K. Hasebe, K. Takeda, T. Sato, and T. Kakitsuka, "Directly modulated buried heterostructure DFB laser on SiO2 / Si substrate fabricated by regrowth of InP using bonded active layer", OPTICS EXPRESS, vol. 22, no.10, pp. 12139-12147, 2014. [Non-Patent Document 2] S. Yamaoka et al., "Directly modulated membrane lasers with 108 GHz bandwidth on a high-thermal-conductivity silicon carbide substrate", Nature Photonics, vol. 15 pp. 28-35, 2021. [Non-Patent Document 3] E. Kanno et al., "Twin-mirror membrane distributed-reflector lasers using 20-μm-long active region on Si substrates", OPTICS EXPRESS, vol. 26, no. 2, pp. 1268-1277, 2018. Summary of the Invention [Problem to be solved by the invention]

[0008] On the other hand, in the vertical thin-film structure, because of the presence of the first mesa, it is not possible to form a surface diffraction grating identical to that in the horizontal thin-film structure. For this reason, in the vertical thin-film structure, a technique has been proposed in which the mode of a waveguide (e.g., a Si waveguide) buried in the undercladding (e.g., SiO2) below the semiconductor thin film is coupled with the mode on the semiconductor thin film side to form a diffraction grating on the buried waveguide side.

[0009] However, in this structure, since the location of the diffraction grating is far away from the active region, in order to obtain a high coupling constant, it is necessary to use a cross-sectional mode in which light is strongly localized on the buried waveguide side (a new supermode formed by the coupling of the mode on the thin film side and the mode on the buried waveguide side), and strong light confinement to the active region cannot be obtained. Thus, with the conventional vertical thin film structure, it was difficult to achieve both strong light confinement and a high coupling constant, which was possible with the horizontal structure.

[0010] The present invention has been made to solve the above problems, and has as its object to provide a semiconductor optical device having a vertical thin-film structure that can achieve both strong optical confinement and a high coupling constant. [Means for solving the problem]

[0011] The semiconductor optical device according to the present invention comprises a lower cladding layer formed on a substrate, a first semiconductor layer made of a first conductivity type III-V compound semiconductor formed on the lower cladding layer, an active layer made of a III-V compound semiconductor formed on the first semiconductor layer, a second semiconductor layer made of a III-V compound semiconductor formed on the active layer, a resonator made of a diffraction grating formed on an upper surface of the second semiconductor layer, a third semiconductor layer made of a second conductivity type III-V compound semiconductor formed above a region in which the active layer is formed and having a mesa shape extending in an extension direction of the active layer, an upper cladding layer formed on the second semiconductor layer to cover both side surfaces of the third semiconductor layer, a first electrode electrically connected to the first semiconductor layer, and a second electrode electrically connected to the third semiconductor layer, wherein the mesa width of the third semiconductor layer is smaller than the width of the active layer. Effect of the Invention

[0012] As described above, according to the present invention, in a semiconductor optical device having a vertical thin-film structure in which current is applied in the vertical direction of an active layer, a second semiconductor layer having a refractive index different from that of the active layer and a third semiconductor layer is provided between the active layer and the third semiconductor layer, and a diffraction grating is formed in the second semiconductor layer, so that the semiconductor optical device having a vertical thin-film structure can achieve both strong optical confinement and a high coupling constant. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a cross-sectional view showing a configuration of a semiconductor optical device according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a configuration diagram showing a structure assumed for calculating various characteristics of the structure of the semiconductor optical device shown in FIG. [Diagram 3] FIG. 3 is a distribution diagram showing the intensity distribution of the TEn0 mode (n=0, 1, 2) calculated using the structure shown in FIG. 2(a). [Figure 4] FIG. 4 is a characteristic diagram showing the equivalent refractive index of each mode (a), the Bragg wavelength of the diffraction grating (b), and the optical confinement coefficient in the active layer 104 (the entire MQW region) (c), calculated using the structure in FIG. 2(a). [Diagram 5] FIG. 5 is a characteristic diagram showing the coupling constant κ of the diffraction grating obtained using the equivalent refractive indices neq,1 and neq,2 in FIGS. 2(a) and 2(b), respectively. [Figure 6] FIG. 6 is a cross-sectional view showing a configuration of another semiconductor optical device according to an embodiment of the present invention. [Figure 7] FIG. 7 is a configuration diagram showing a structure assumed for calculating various characteristics of the structure of the semiconductor optical device shown in FIG. [Figure 8] FIG. 8 is a distribution diagram showing the intensity distribution of each mode calculated using the structure of FIG. [Figure 9] FIG. 9 is a characteristic diagram showing the equivalent refractive index neq (a) of each mode, the Bragg wavelength λB of the diffraction grating (b), and the optical confinement factor ΓMQW (c) in the active layer 104 (the entire MQW region), calculated using the structure in FIG. 7(a). [Figure 10] FIG. 10 is a characteristic diagram showing the coupling constant κ of the diffraction grating obtained using the equivalent refractive indices neq,1 and neq,2 in each of FIGS. 7(a) and 7(b). [Figure 11] FIG. 11 is a cross-sectional view showing the structure of another semiconductor optical device according to an embodiment of the present invention. [Figure 12]FIG. 12 is a configuration diagram showing a structure assumed for calculating various characteristics of the structure of the semiconductor optical device shown in FIG. [Figure 13] FIG. 13 shows the intensity distribution of each mode calculated using the structure of FIG. 12 with the mesa width of the third semiconductor layer 107 at one level of 200 nm and the mesa width of the active layer 104a at three levels of 800 nm, 2.0 μm, and 4.0 μm. [Figure 14] FIG. 14 is a characteristic diagram showing the equivalent refractive index of each mode calculated using the structure of FIG. [Figure 15] FIG. 15 is a characteristic diagram showing the Bragg wavelength of a diffraction grating for each mode calculated using the structure of FIG. [Figure 16] FIG. 16 is a characteristic diagram showing the optical confinement factor in the active layer 104 (the entire MQW region) for each mode calculated using the structure of FIG. [Figure 17] FIG. 17 is a characteristic diagram showing the coupling constant κ of the diffraction grating obtained using the structure of FIG. [Figure 18] FIG. 18 is a cross-sectional view showing the configuration of another semiconductor optical device according to an embodiment of the present invention. [Figure 19] FIG. 19 is a configuration diagram showing a structure assumed for calculating various characteristics of the structure of the semiconductor optical device shown in FIG. [Figure 20] FIG. 20 is a characteristic diagram showing the coupling constant κ of the diffraction grating when the width of the third semiconductor layer 107 is fixed at 200 nm and the mesa width of the active layer 104a (second semiconductor layer 105) is set to 0.8 μm. [Figure 21] FIG. 21 is a characteristic diagram showing the coupling constant κ of the diffraction grating when the width of the third semiconductor layer 107 is fixed at 200 nm and the mesa width of the active layer 104a (second semiconductor layer 105) is set to 2.0 μm. [Figure 22] FIG. 22 is a characteristic diagram showing the coupling constant κ of the diffraction grating when the width of the third semiconductor layer 107 is fixed at 200 nm and the mesa width of the active layer 104a (second semiconductor layer 105) is set to 4.0 μm. [Diagram 23] FIG. 23 is a cross-sectional view showing a configuration of another semiconductor optical device according to an embodiment of the present invention. [Figure 24] FIG. 24 is a cross-sectional view showing a configuration of a semiconductor optical device according to an embodiment of the present invention. [Diagram 25] FIG. 25 is a characteristic diagram showing the threshold material gain of the fundamental mode and the gain loss margin with respect to a higher order mode, calculated using the parameters of the buried structure of FIG. 2 and FIG. 7 and setting the grating depth to 20 nm. [Figure 26] FIG. 26 is a characteristic diagram showing the threshold material gain of the fundamental mode and the gain loss margin with respect to a higher mode, calculated using the parameters of the buried structure of FIG. 2 and FIG. 7 and setting the grating depth to 30 nm. [Figure 27] FIG. 27 is a characteristic diagram showing the threshold material gain of the fundamental mode and the gain loss margin with respect to the higher modes, calculated using the parameters of the ridge structure of the active layer 104 of the MQW structure shown in FIGS. 12 and 19, with the mesa width of the active layer 104 set to 0.8 μm and the grating depth set to 20 nm. [Figure 28] FIG. 28 is a characteristic diagram showing the threshold material gain and the gain loss margin with respect to the higher modes of the fundamental mode calculated using the parameters of the ridge structure of the active layer 104 of the MQW structure shown in FIGS. 12 and 19, with the mesa width of the active layer 104 set to 0.8 μm and the grating groove depth set to 30 nm. [Figure 29] FIG. 29 is a characteristic diagram showing the threshold material gain and the gain loss margin with respect to the higher mode of the fundamental mode calculated using the parameters of the ridge structure of the active layer 104 of the MQW structure shown in FIGS. 12 and 19, with the mesa width of the active layer 104 set to 2.0 μm and the grating groove depth set to 20 nm. [Diagram 30] FIG. 30 is a characteristic diagram showing the threshold material gain and the gain loss margin with respect to the higher modes of the fundamental mode calculated using the parameters of the ridge structure of the active layer 104 of the MQW structure shown in FIGS. 12 and 19, with the mesa width of the active layer 104 set to 2.0 μm and the grating groove depth set to 30 nm. [Diagram 31]FIG. 31 is a characteristic diagram showing the threshold material gain of the fundamental mode and the gain loss margin with respect to the higher mode, calculated using the parameters of the ridge structure of the active layer 104 of the MQW structure shown in FIGS. 12 and 19, with the mesa width of the active layer 104 set to 4.0 μm and the grating groove depth set to 20 nm. [Diagram 32] FIG. 32 is a characteristic diagram showing the threshold material gain of the fundamental mode and the gain loss margin with respect to the higher mode, calculated using the parameters of the ridge structure of the active layer 104 of the MQW structure shown in FIGS. 12 and 19, with the mesa width of the active layer 104 set to 4.0 μm and the grating groove depth set to 30 nm. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] A semiconductor optical device according to an embodiment of the present invention will be described below with reference to Fig. 1. This semiconductor optical device includes a lower cladding layer 102 formed on a substrate 101, a first semiconductor layer 103 made of a first conductivity type III-V compound semiconductor formed on the lower cladding layer 102, and an active layer 104 made of a III-V compound semiconductor formed on the first semiconductor layer 103.

[0015] This semiconductor optical device also includes a second semiconductor layer 105 made of a III-V compound semiconductor formed on the active layer 104, a resonator 106 made of a diffraction grating formed on the upper surface of the second semiconductor layer 105, and a third semiconductor layer 107 made of a second conductivity type III-V compound semiconductor formed above the region where the active layer 104 is formed and in a mesa shape extending in the extension direction of the active layer 104. The mesa width of the third semiconductor layer 107 is smaller than the width of the active layer 104.

[0016] The resonator 106 is composed of a diffraction grating consisting of a concave-convex pattern formed by microfabricating the upper surface of the second semiconductor layer 105. By making the second semiconductor layer 105 and the third semiconductor layer 107 have different refractive indices, the concave-convex pattern on the upper surface of the second semiconductor layer 105 becomes a diffraction grating in which the convex parts are made of the second semiconductor layer 105 and the concave parts are made of the third semiconductor layer 107. In this example, an upper cladding layer 108, which will be described later, is also formed on the second semiconductor layer 105, and in this region, the concave-convex pattern on the upper surface of the second semiconductor layer 105 becomes a diffraction grating in which the convex parts are made of the second semiconductor layer 105 and the concave parts are made of the upper cladding layer 108.

[0017] This semiconductor optical device also includes an upper cladding layer 108 formed on the second semiconductor layer 105 to cover both side surfaces of the third semiconductor layer 107, a first electrode 131 electrically connected to the first semiconductor layer 103, and a second electrode 132 electrically connected to the third semiconductor layer 107. For example, the first electrode 131 is formed in contact with the top of the first semiconductor layer 103 in a region where no mesa such as the active layer 104 is formed. For example, the second electrode 132 is formed on the upper cladding layer 108 and electrically connected to the third semiconductor layer 107.

[0018] The semiconductor optical device according to the embodiment further includes a fourth semiconductor layer 121 and a fifth semiconductor layer 122 made of a non-conductive or low-conductive III-V compound semiconductor formed on the first semiconductor layer 103 in contact with both side surfaces of the mesa formed by the active layer 104. A sixth semiconductor layer 109 made of the same III-V compound semiconductor as the fourth semiconductor layer 121 and the fifth semiconductor layer 122 is formed on the active layer 104, the fourth semiconductor layer 121, and the fifth semiconductor layer 122. In this semiconductor optical device, a second electrode 132 is connected (formed) on the third semiconductor layer 107 via a contact layer 110.

[0019] The semiconductor optical device according to the embodiment is a so-called distributed feedback (DFB) semiconductor laser that uses a diffraction grating-based resonator 106. The diffraction grating is configured to be distributed along the resonance direction (waveguiding direction) of the active layer 104.

[0020] The substrate 101 may be made of, for example, silicon. The lower cladding layer 102 may be made of, for example, a material having a lower refractive index than the semiconductor that constitutes the active layer 104, such as silicon oxide, silicon nitride, silicon carbide, or diamond. The first semiconductor layer 103 may be made of, for example, n-type InP. In this case, the first conductivity type is n-type, and the second conductivity type is p-type.

[0021] The active layer 104 may have a multiple quantum well structure (MQW) made of well layers and barrier layers each made of InGaAlAs, InGaAs, InGaAsP, etc., each having a different composition. The active layer 104 may also be made of a bulk compound semiconductor such as InGaAlAs, InGaAs, InGaAsP, etc. The sixth semiconductor layer 109 may be made of p-type InP, for example. In this case, the second conductivity type is p-type. The second semiconductor layer 105 may be made of p-type InGaAsP.

[0022] The third semiconductor layer 107 may be made of, for example, p-type InP. The third semiconductor layer 107 may be made of, for example, n-type InP. The fourth semiconductor layer 121 and the fifth semiconductor layer 122 may be made of, for example, i-type InP (i-InP). The fourth semiconductor layer 121 and the fifth semiconductor layer 122 may also be made of semi-insulating InP (SI-InP) that is made highly resistive by doping with Fe, for example.

[0023] The upper cladding layer 108 can be made of a material having a lower refractive index than the semiconductor constituting the active layer 104, such as silicon oxide or silicon nitride. The upper cladding layer 108 can also be made of a resin such as benzocyclobutene (BCB). The upper cladding layer 108 can also be air. The upper cladding layer 108 functions as an upper cladding layer when viewed from the substrate 101. The semiconductor optical device according to the embodiment has an optical waveguide structure with the active layer 104 as a core.

[0024] It is important that the second semiconductor layer 105 is made of a III-V compound semiconductor having a refractive index different from that of the layers above and below it. In addition, it is preferable that the second semiconductor layer 105 is made of a III-V compound semiconductor that is nearly lattice-matched to the III-V compound semiconductor constituting the lower layer and can be epitaxially grown.

[0025] The present invention aims to provide a structure that achieves both strong optical confinement and a high coupling constant in a semiconductor optical device (laser diode) with a so-called vertical thin-film structure. In addition, the present invention aims to provide a structure that suppresses oscillation of higher-order modes and stably oscillates only in a single mode in a case where the cross section of the active layer 104 does not satisfy the single mode condition and multiple transverse modes can oscillate.

[0026] A specific example of a structure assumed for calculating various characteristics of the structure of the semiconductor optical device according to the above-mentioned embodiment is shown in Fig. 2. In the following, the shape of the diffraction grating when viewed from the yz cross section is assumed to be rectangular.

[0027] Fig. 2(a) is an xy cross section of a convex portion of a rectangular diffraction grating serving as resonator 106, and Fig. 2(b) is an xy cross section of a concave portion. As can be seen from Fig. 2, in the diffraction grating that constitutes resonator 106, in the first mesa portion of third semiconductor layer 107, second semiconductor layer 105 is convex and third semiconductor layer 107 is concave. On the other hand, outside the first mesa in the x direction, second semiconductor layer 105 is convex and upper cladding layer 108 is concave.

[0028] In general, III-V mixed crystals such as InGaAsP have a higher refractive index than InP and the like, and using these materials to construct a diffraction grating contributes to the high coupling constant that is a feature of the present invention. In the calculations shown below, in each structure, the operating wavelength (laser oscillation wavelength) is assumed to be 1310 nm (a typical value as the oscillation wavelength of a light source for communication), the second semiconductor layer 105 is made of InGaAsP with a band gap wavelength of 1200 nm and lattice-matched to InP, and the third semiconductor layer 107 is made of InP.

[0029] Since the bandgap wavelength is sufficiently shorter than the operating wavelength, no light is generated or absorbed due to interband transition in the second semiconductor layer 105. The material refractive index of InGaAsP is calculated to be about 3.407, and the material refractive index of InP is calculated to be about 3.185, respectively, and there is a significant refractive index difference.

[0030] Moreover, on the sides of the third semiconductor layer 107, the concave portions of the diffraction grating formed on the third semiconductor layer 107 become a low refractive index material due to the upper cladding layer 108, so that an even larger refractive index difference is obtained, which also contributes greatly to a high coupling constant in the diffraction grating. Note that the wavelength of 1310 nm in this example is assumed for the convenience of calculating the specific example, and the applicable wavelength range of the present invention is not limited to this. For example, the present invention is also applicable to the 1550 nm band, which is also a typical wavelength band for communication.

[0031] Fig. 3 shows the intensity distribution of the TEn0 mode (n = 0, 1, 2) calculated using the structure (convex portion of the diffraction grating) shown in Fig. 2(a). The TE00 mode is the fundamental mode in which oscillation should occur, and modes with n of 1 or more are higher modes in which oscillation should be suppressed. In addition, four levels of 100 nm, 200 nm, 300 nm, and 400 nm, which are typical values ​​for obtaining strong light confinement in the active layer 104 (thin film structure), are shown as the mesa width W1 of the third semiconductor layer 107.

[0032] It can be seen that the mesa width of the active layer 104 made of MQW is relatively narrow at 800 nm, so that the optical confinement in the active layer 104 in higher modes is suppressed.

[0033] Fig. 4 shows the equivalent refractive index (a) of each mode, the Bragg wavelength of the diffraction grating (b), and the optical confinement coefficient (c) in the active layer 104 (whole MQW region), calculated using the structure in Fig. 2(a). The horizontal axis is the mesa width of the third semiconductor layer 107. Here, the Bragg wavelength in Fig. 4(b) is calculated from the following formula.

[0034]

number

[0035] This means that it is assumed that the diffraction grating is formed with a pitch such that the Bragg wavelength of the fundamental mode is 1310 nm. In order to suppress oscillation of higher-order modes, it is desirable to have a large detuning of the Bragg wavelengths between the modes. Figure 4(c) shows that the optical confinement of the fundamental mode is clearly greater than that of the higher-order modes. This is also important in suppressing oscillation of higher-order modes, as will be described later.

[0036] FIG. 5 shows the equivalent refractive index n eq,1 ,n eq,2 The coupling constant of the diffraction grating was calculated using the following formula, which gives the coupling constant of a rectangular diffraction grating with a duty ratio (ratio of the length of the convex part to the length of the concave part in the z direction) of 1:1.

[0037]

number

[0038] The horizontal axis of FIG. 5 is the recess depth of the diffraction grating (the difference in height between the convex portion and the recess portion). From FIG. 5(a), for example, when the mesa width of the third semiconductor layer 107 is 200 nm, the recess depth of the diffraction grating (depth of the recess) is 20 nm and 30 nm, respectively, and the recess depth is about 600 cm.-1 ,900cm -1 It can be seen that a coupling constant of about this magnitude is obtained in the fundamental mode. Furthermore, as already shown in FIG. 4(c), the optical confinement factor in the active layer 104 is a large value exceeding 0.4, and it can be seen that both strong optical confinement and a high coupling constant are achieved.

[0039] On the other hand, it can be seen from Figures 5(b) and 5(c) that an even larger coupling constant is obtained in the higher order mode. In order to suppress oscillation in the higher order mode, it is desirable to make only the fundamental mode have a large coupling constant. In the present invention, a structure as shown in Figure 7 is proposed as a structure for achieving this. In this semiconductor optical device, the width of the second semiconductor layer 105 in the structure of the semiconductor optical device described using Figure 1 is made to be approximately the same as the width of the active layer 104, forming a second semiconductor layer 105a, and a diffraction grating is formed only in this portion to form a resonator 106a.

[0040] As a secondary effect of adopting this structure, current confinement occurs during carrier injection from the third semiconductor layer 107 to the active layer 104, which provides the advantage of more efficient current injection. Figures 7(a) and 7(b) show specific structural examples assumed for calculating various characteristics of the semiconductor optical device described with reference to Figure 6, and are the same structure as Figures 2(a) and 2(b) except that the width of the second semiconductor layer 105a is equal to the width of the active layer 104.

[0041] FIG. 8 shows the intensity distribution of each mode calculated using the structure of FIG. 7(a). FIG. 9 shows the equivalent refractive index n eq (a) Bragg wavelength λ of the diffraction grating B (b) Optical confinement factor Γ in the active layer 104 (entire MQW region) MQW 9(c). From FIG. 9(c), it can be seen that the fundamental mode still has a significantly larger optical confinement factor than the higher order modes.

[0042] FIG. 10 shows the equivalent refractive index neq,1 ,n eq,2 10A, for example, when the mesa width of the third semiconductor layer 107 is 200 nm, the grating has an indentation depth of about 500 cm for each of the indentation depths of 20 nm and 30 nm. -1 ,800cm -1 It can be seen that a coupling constant of about is obtained in the fundamental mode. Although this is somewhat lower than that in Fig. 5(a), it is still a high value for the coupling constant of a diffraction grating with a recess depth of 20 nm to 30 nm.

[0043] On the other hand, it can be seen from Figures 10(b) and 10(c) that the coupling constant of the higher-order mode is significantly lower than that of Figures 5(b) and 5(c), and as a result, it is a value significantly lower than that of the fundamental mode. Therefore, by using the semiconductor optical device described using Figure 6, it is possible to effectively reduce only the coupling constant of the higher-order mode while maintaining the coupling constant of the fundamental mode at a high level similar to that of the semiconductor optical device described using Figure 1.

[0044] Up to this point, we have assumed that the base vertical thin-film structure is one in which the active layer 104 is embedded in passive fourth semiconductor layer 121, fifth semiconductor layer 122, and sixth semiconductor layer 109, such as InP, but another vertical thin-film structure may also be considered, such as the structure shown in Fig. 11. This semiconductor optical device has a structure in which the active layer 104 is not embedded in a passive semiconductor, and a mesa shape is formed by the second semiconductor layer 105 and the active layer 104a.

[0045] Compared with the buried structure of the semiconductor optical device described with reference to FIGS. 1 and 6, the semiconductor optical device structure shown in FIG. 11 has the following advantages.

[0046] [Advantage 1] The crystal regrowth process for burying the active layer 104a is not required, and the fabrication is simpler. [Advantage 2] There are no passive semiconductor layers that can act as parallel current leakage paths, allowing efficient current injection.

[0047] Fig. 12 shows a specific example structure assumed for calculating various characteristics of the semiconductor optical device illustrated in Fig. 11. Note that the structure shown in Fig. 12 is an xy cross-sectional structure of the convex portion of the diffraction grating, and in the concave portion of the diffraction grating, the upper surface of the second semiconductor layer 105 is dug out similarly to Fig. 2(b) etc. (i.e., the width of the diffraction grating is equal to the mesa width of the active layer 104a and the second semiconductor layer 105).

[0048] Fig. 13 shows the intensity distribution of each mode calculated by setting the mesa width of the third semiconductor layer 107 to one level of 200 nm and the mesa width of the active layer 104a to three levels of 800 nm, 2.0 μm, and 4.0 μm in the structure of Fig. 12. As can be seen from these intensity distributions, in a semiconductor optical device in which the active layer 104a has a ridge structure, the side surfaces of the active layer 104a and the second semiconductor layer 105 are in contact with the upper cladding layer 108, which has a low refractive index, so that strong light confinement occurs also in the x direction, and not only the fundamental mode but also higher modes are strongly confined in the active layer 104a.

[0049] 14, 15, and 16 respectively show the equivalent refractive index of each mode, the Bragg wavelength of the diffraction grating, and the optical confinement coefficient in the active layer 104 (the entire MQW region) calculated using the structure of FIG.

[0050] In each figure, four levels of the mesa width of the third semiconductor layer 107, 100 nm, 200 nm, 300 nm, and 400 nm, are assumed, and the mesa width of the active layer 104a is plotted on the horizontal axis. From Figs. 14 and 15, it can be seen that as the mesa width of the active layer 104 becomes wider, the equivalent refractive index of the higher mode becomes closer to that of the fundamental mode, and the detuning of the Bragg wavelength becomes smaller. For example, when the mesa width of the third semiconductor layer 107 is 200 nm and the mesa width of the active layer 104 is 2.0 μm, the detuning of the Bragg wavelength between the fundamental mode and the TE10 mode is about 30 nm. This is not a sufficiently large value for the detuning when MQW is used for the active layer 104 (i.e., the same level of gain may be obtained at any Bragg wavelength), and if a significant difference cannot be obtained in the optical confinement or the coupling constant, there is a possibility of multi-mode oscillation.

[0051] 16, for example, when the mesa width of the third semiconductor layer 107 is 200 nm, an extremely high optical confinement factor exceeding 0.5 is obtained in the fundamental mode. However, under this condition, when the mesa width of the active layer 104a is wider than a certain degree, it is also found that the higher order modes have even stronger optical confinement.

[0052] Fig. 17 shows values ​​of coupling constants calculated in the same manner as in Fig. 5 using the structure of Fig. 12. The mesa width of the third semiconductor layer 107 is fixed at 200 nm, and three levels of mesa widths of 800 nm, 2.0 µm, and 4.0 µm are assumed for the active layer 104a, with the trench depth of the diffraction grating plotted on the horizontal axis. For any mesa width of the active layer 104a, the trench depths of about 700 cm were obtained at trench depths of 20 nm and 30 nm, respectively. -1 Over, about 1100cm -1 The above extremely high coupling constants are obtained for the fundamental mode. However, at the same time, the coupling constants for the higher modes are also very high.

[0053] Therefore, in the structure of Fig. 11, strong optical confinement and a high coupling constant are obtained in the fundamental mode, but at the same time, the higher modes also have similar optical confinement and coupling constants. Furthermore, as described above, when the mesa width of the active layer 104a is particularly wide, the detuning of the Bragg wavelength between the modes becomes small, so it may be difficult to obtain stable single-mode oscillation. As a structure to solve this multimode problem, the present invention proposes the structure shown in Fig. 18.

[0054] The semiconductor optical device shown in Fig. 18 has a structure in which the region where the diffraction grating is carved (formed) is limited to only a region of an appropriate width near the center in the direction of the mesa width of the second semiconductor layer 105, rather than the entire region of the mesa width of the second semiconductor layer 105 in the structure of Fig. 11. The width of this diffraction grating can be expressed as a ratio to the mesa width of the active layer 104a (second semiconductor layer 105). When the mesa width of the active layer 104a (second semiconductor layer 105) is 2.0 µm, a diffraction grating formation width of 1.5 µm corresponds to a diffraction grating occupancy rate of 75%.

[0055] A specific structural example assumed to calculate the coupling constant of this structure is shown in FIG. 19. The diffraction grating occupancy rates were assumed to be 25%, 50%, 75%, and 100%. As is clear from FIG. 19, a diffraction grating occupancy rate of 100% is equivalent to the structure of FIG. 11. The coupling constants when the width of the third semiconductor layer 107 is fixed to 200 nm and the mesa width of the active layer 104a (second semiconductor layer 105) is set to 0.8 μm, 2.0 μm, and 4.0 μm are shown in FIG. 20, FIG. 21, and FIG. 22, respectively. In each case, the coupling constant decreases overall as the diffraction grating occupancy rate decreases, but it can be seen that, particularly in the case of a diffraction grating occupancy rate of 50%, while the decrease in the coupling constant of the fundamental mode is limited, the coupling constant of the higher-order mode decreases significantly.

[0056] The coupling constant of the fundamental mode at a diffraction grating occupancy rate of 50% is about 500 cm when the mesa width of the active layer 104a (second semiconductor layer 105) is 0.8 μm and the recess depth is 20 nm and 30 nm. -1 ,800cm -1In addition, the coupling constant of the fundamental mode at a diffraction grating occupancy rate of 50% is about 650 cm when the mesa width of the active layer 104a (second semiconductor layer 105) is 2.0 μm and the recess depth is 20 nm and 30 nm. -1 ,1000cm -1 In addition, the coupling constant of the fundamental mode at a diffraction grating occupancy rate of 50% is about 650 cm when the mesa width of the active layer 104a (second semiconductor layer 105) is 4.0 μm and the recess depth is 20 nm and 30 nm. -1 ,800cm -1 It is about that level.

[0057] These values ​​are lower than those for the case of 100% grating occupancy, but are still high. On the other hand, the coupling constant of the higher-order modes in this condition is about 100 cm higher than that of the fundamental mode. -1 ~several hundred cm -1 It can be seen that this structure can effectively reduce only the coupling constant of the higher mode. Note that here, only four levels of diffraction grating occupancy, 25%, 50%, 75%, and 100%, are assumed, but it goes without saying that the scope of application of the present invention is not limited to these values, and any value may be used within a range in which the desired fundamental mode coupling constant and inter-mode coupling constant difference can be obtained.

[0058] As another configuration for suppressing oscillation of the higher-order mode, as shown in FIG. 23, it is possible to further provide structures 111, 112 for scattering or emitting light on the side surface of the mesa-shaped active layer 104a (second semiconductor layer 105). The structures 111, 112 can be, for example, a randomly rough uneven shape, a high-order sidewall diffraction grating, or the like. As can be seen from FIG. 13, the fundamental mode in the ridge-structured active layer 104a is localized near the center of the mesa of the active layer 104a, while the higher-order mode has a large overlap with the side surface of the mesa of the active layer 104a. Therefore, by providing the structures 111, 112 on the side surface of the mesa-shaped active layer 104a, it is possible to selectively increase only the propagation loss of the higher-order mode and suppress its oscillation.

[0059] In the following, in order to confirm the effects of the features of the structure of the present invention, namely, 1. strong light confinement in the active layer 104, 2. high coupling constant of the diffraction grating, and 3. single mode oscillation, the oscillation threshold is calculated when the present invention is applied to a λ / 4 shift type DFB laser as shown in Fig. 24 as one specific example. Fig. 24 shows a cross section parallel to the waveguiding direction. This DFB laser has a λ / 4 shift portion 113 in the middle (center) of the diffraction grating that constitutes the resonator 106.

[0060] When this semiconductor optical device is viewed along the z-axis, it is assumed that the end face of the active layer 104 is optically coupled to appropriate passive waveguides 141, 142, and that there is no reflection of light at this coupling point. Since the passive waveguides 141, 142 do not require the third semiconductor layer 107 for current injection, a structure in which the third semiconductor layer 107 is tapered to an appropriate length and terminated as shown in the figure is conceivable.

[0061] As described above, under ideal circumstances where there is no reflection from outside the DFB resonator, the laser oscillation conditions are given by:

[0062]

number

[0063] Here, L , r R are the amplitude reflectances of light due to Bragg reflection when viewed from the left and right of the λ / 4 shift portion 113 at the center of the resonator 106, Γ is the optical confinement coefficient of the portion (in the case of MQW, the well layer of each quantum well) that gives gain to light in the active layer 104, g is the material gain of that portion (in the case of MQW, the well layer of each quantum well), α i is the internal loss.

[0064] In this calculation, Γ was calculated assuming that there are six quantum wells (6-QW) in the active layer 104 with a thickness of 100 nm. i is a typical value of 10 cm -1As can be seen from equations (4) to (6), in a λ / 4 shift type DFB laser, the Bragg wavelength of the diffraction grating is λ = λ B The resonance condition is satisfied at λ = λ, and the resonance mode usually oscillates stably in a single mode in the axial direction. B and oscillation in this resonant mode is assumed.

[0065] Equation (4) expresses the balance between gain and loss as well as the resonance condition. The material gain that satisfies this is g = g th This is called the threshold material gain. Although it depends on the material system used, for example, in the case of MQW made of InGaAlAs or InGaAsP, it is typically g th For example, 500 cm -1 It is said that oscillation occurs at a sufficiently low threshold value if the th =500cm -1 is used as a guideline for the low oscillation threshold.

[0066] On the other hand, the gain / loss margin given by the following formula is often used as a measure of single-mode oscillation.

[0067]

number

[0068] Here, the subscripts "main" and "side" refer to the main mode and the associated side mode, respectively. Equation (9) expresses the mirror loss of the resonator 106. It is known that the larger the gain loss margin, the better the single-mode oscillation in the main mode. Typically, when the gain loss margin is, for example, 100 cm -1 It is said that very good single mode characteristics, with a side mode suppression ratio exceeding 50 dB, can be obtained with some materials. The material gain g actually depends on the resonant wavelength (Bragg wavelength) of each mode, but in this case, in order to consider only the effect due to the difference in the optical confinement factor, g is commonly used as the threshold material gain value when the main mode oscillates. th TR00Therefore, when discussing the single mode characteristic between the fundamental mode (TE00) and the higher mode (TEn0), the following formula is given.

[0069]

number

[0070] In the following, the gain / loss margin between the TE00 mode and the TEn0 mode (n=1, 2) given by formula (10) is evaluated. In addition, in the following, the width of the third semiconductor layer 107 is fixed to 200 nm in all structures.

[0071] First, the threshold material gain of the fundamental mode calculated using the parameters of the embedded structure in FIG. 2 and FIG. 7, and the gain loss margin with respect to the higher modes are shown in FIG. 25 and FIG. 26. The horizontal axis is the cavity length L cav The solid line indicates the threshold material gain of the fundamental mode (vertical axis on the left), and the dashed and dotted lines indicate the gain-loss margin with the higher-order modes (vertical axis on the right). Figure 25 shows the case where the grating depth is 20 nm, and Figure 26 shows the case where it is 30 nm. In both figures, (a) corresponds to the structure in Figure 2, and (b) corresponds to the structure in Figure 7.

[0072] First, in terms of the threshold material gain of the fundamental mode, in all cases, it is 500 cm for short resonator lengths of approximately 40 μm to 60 μm. -1 It can be seen that a low threshold material gain of g is obtained. The reason why a short cavity laser that oscillates at such a low threshold is possible is none other than the fact that strong optical confinement and a high coupling constant are obtained. On the other hand, the gain loss margin is g th =500cm -1 In all cases, positive values ​​are obtained for the resonator length of 100 cm. -1It can be seen that the above margins can be easily obtained. Although the margin obtained in (a) is not as large as that in (b), as shown in the figure, in these buried structures, the Bragg wavelength detuning with the higher-order mode is a large value of 50 nm or more, so it is expected that sufficiently good single-mode characteristics can be obtained even in (a).

[0073] Next, the threshold material gain of the fundamental mode and the gain loss margin with respect to the higher mode, which are calculated in the same manner using the parameters of the ridge structure of the active layer 104 of the MQW structure in Figs. 12 and 19, are shown in Figs. 27 to 32. In Figs. 27 and 28, the mesa width of the active layer 104 and the second semiconductor layer 105 is 0.8 μm. In Figs. 29 and 30, the mesa width of the active layer 104 and the second semiconductor layer 105 is 2.0 μm. In Figs. 31 and 32, the mesa width of the active layer 104 and the second semiconductor layer 105 is 4.0 μm. In Figs. 27, 29, and 31, the grating is dug to a depth of 20 nm, and in Figs. 28, 30, and 32, the grating is dug to a depth of 30 nm.

[0074] First, looking at FIG. 27 and FIG. 28 (the mesa width of the active layer 104 and the second semiconductor layer 105 is 0.8 μm), it can be seen that a low oscillation threshold and a large gain-loss margin are well balanced, especially in the case of (b) where the diffraction grating occupancy is 50%. th =500cm -1 The cavity length required for this is a short value of approximately just under 50 μm to just over 60 μm, making it possible to realize a short cavity laser with low power consumption.

[0075] On the other hand, when considering the detuning of the Bragg wavelength between modes, as shown in the figure, the mesa width of the active layer 104 and the second semiconductor layer 105 is relatively narrow at 0.8 μm, which provides a very large detuning of 100 nm or more, and it is considered that the gain loss margin resulting from the difference between Γ and κ is not necessarily required. In this case, in order to further shorten the cavity, it is possible to adopt the diffraction grating occupancy rate of 100% in (d). When (d) is adopted, g th =500cm -1 The length of the cavity is shortened from approximately 40 μm to 50 μm.

[0076] 29 and 30 (active layer 104, second semiconductor layer 105 mesa width is 2.0 μm) and 31 and 32 (active layer 104, second semiconductor layer 105 mesa width is 4.0 μm), the Bragg wavelength detuning is relatively small, about 30 nm and about 10 nm, respectively, and it is considered that a sufficient gain-loss margin is necessary for stable single mode oscillation. As such, even when the mesa width of active layer 104 and second semiconductor layer 105 is relatively wide, it is found that a low oscillation threshold and a large gain-loss margin can be obtained in a well-balanced manner in the case of (b) where the diffraction grating occupancy is 50%.

[0077] On the other hand, when the grating occupancy is 100% in (d), the gain loss margin becomes a negative value. This means that it is easier to oscillate in higher order modes, and it is difficult to obtain stable single mode oscillation in the fundamental mode. This also shows the effect of the grating structure proposed by the present invention in FIG.

[0078] In the case of (b), g th =500cm -1 The cavity length at which this occurs is approximately just under 40 μm to just over 50 μm when the mesa width of the active layer 104 and the second semiconductor layer 105 is 2.0 μm, and approximately over 40 μm to just over 50 μm when the mesa width of the active layer 104 and the second semiconductor layer 105 is 4.0 μm, which shows that a short cavity laser with a low oscillation threshold can be realized. Furthermore, the gain-loss margin at this time is sufficiently large in either case, and good single mode characteristics can also be obtained.

[0079] In this description, a λ / 4 shifted DFB laser as shown in FIG. 24 is assumed as a typical embodiment. However, since the structure on which the present invention is based is a diffraction grating structure in the xy cross section of the active layer 104, the application of the device structure including the z-axis direction is not limited to the embodiment shown here, but can be generally applied to a device structure having a diffraction grating in the active layer 104.

[0080] Other typical examples include the following:

[0081] 1. DR laser structure: A uniform diffraction grating without shift is formed uniformly in the active layer 104 to form a uniform DFB structure, and a distributed Bragg reflector (DBR) is formed in the passive waveguide on the rear side, enabling single-mode oscillation and optical output only from the front side.

[0082] 2.2-section DFB laser structure: A structure that enables single-mode oscillation and asymmetric light extraction from the front and rear sides (outputting more light from the front side) by linking two DFB sections with different Bragg wavelengths and lengths.

[0083] 3. Apodized DFB laser structure: A structure in which the pitch, coupling constant, equivalent refractive index, etc. of the diffraction grating in the DFB region are continuously changed along the z-axis to form a mode gap type resonator with a small bias in the z-axis direction of the oscillation mode intensity distribution, thereby reducing the effect of spatial hole burning, which is prominent in λ / 4 shifted DFB laser structures. The continuous change in pitch can be directly controlled by the drawing pattern of the diffraction grating. The continuous change in coupling constant can be controlled, for example, by changing the duty ratio of the unevenness of the diffraction grating. The continuous change in equivalent refractive index can be controlled, for example, by fabricating a structure in which the width of the third semiconductor layer 107, the width of the active layer 104, and the width of the mesa of the second semiconductor layer 105 change continuously along the z-axis direction.

[0084] The features of the above-described embodiment are as follows.

[0085] 1. By disposing the second semiconductor layer 105 made of a mixed crystal semiconductor such as InGaAsP above the active layer 104, a refractive index difference is achieved between it and the semiconductor constituting the third semiconductor layer 107, such as InP. This makes it possible to form a diffraction grating in the vicinity of the active layer 104, even with a vertical thin film structure, and makes it possible to achieve both strong light confinement and a high coupling constant.

[0086] 2. The structure proposed effectively suppresses the oscillation of m-order modes by appropriately designing the cross-sectional structure and the diffraction grating shape, and a single-mode oscillation can be obtained stably. Conventionally, the formation of a surface diffraction grating in a thin-film structure has been standardized to have a structure in which the entire upper surface of the thin film is dug down in the x direction, and the suppression of oscillation of higher-order modes has been guaranteed by the cutoff of the guided mode. In the above-mentioned embodiment, a local diffraction grating structure that only increases the overlap with the fundamental mode is newly proposed, so that excellent single-mode oscillation can be obtained even with a multi-mode cross-sectional structure in which the cutoff condition is not satisfied. This is a particularly important point for the active layer 104 of the MQW ridge structure, which has strong optical confinement in the x direction and strong multi-mode properties.

[0087] As described above, according to the present invention, in a semiconductor optical device having a vertical thin-film structure in which current is applied in the vertical direction of an active layer, a second semiconductor layer having a refractive index different from that of the active layer and a third semiconductor layer is provided between the active layer and the third semiconductor layer, and a diffraction grating is formed in the second semiconductor layer. Therefore, the semiconductor optical device having a vertical thin-film structure can achieve both strong optical confinement and a high coupling constant.

[0088] It should be noted that the present invention is not limited to the above-described embodiments, and it is apparent that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention. [Explanation of symbols]

[0089] 101...substrate, 102...lower cladding layer, 103...first semiconductor layer, 104...active layer, 105...second semiconductor layer, 106...resonator, 107...third semiconductor layer, 108...upper cladding layer, 109...sixth semiconductor layer, 110...contact layer, 121...fourth semiconductor layer, 122...fifth semiconductor layer, 131...first electrode, 132...second electrode.

Claims

1. a lower cladding layer formed on the substrate; a first semiconductor layer made of a first conductivity type III-V compound semiconductor formed on the lower cladding layer; an active layer made of a III-V compound semiconductor formed on the first semiconductor layer; a second semiconductor layer made of a III-V compound semiconductor formed on the active layer; a resonator formed of a diffraction grating formed on an upper surface of the second semiconductor layer; a third semiconductor layer made of a second conductivity type III-V compound semiconductor, the third semiconductor layer being formed above a region where the active layer is formed and having a mesa shape extending in the extending direction of the active layer; an upper cladding layer formed on the second semiconductor layer to cover both side surfaces of the third semiconductor layer; A first electrode electrically connected to the first semiconductor layer; a second electrode electrically connected to the third semiconductor layer; Equipped with a mesa width of the third semiconductor layer is made smaller than a width of the active layer; the first semiconductor layer and the third semiconductor layer are made of InP; 2. A semiconductor optical device, comprising: a first semiconductor layer and a second semiconductor layer;

2. A lower cladding layer formed on a substrate; a first semiconductor layer made of a first conductivity type III-V compound semiconductor formed on the lower cladding layer; an active layer made of a III-V compound semiconductor formed on the first semiconductor layer; a second semiconductor layer made of a III-V compound semiconductor formed on the active layer; a resonator formed of a diffraction grating formed on an upper surface of the second semiconductor layer; a third semiconductor layer made of a second conductivity type III-V compound semiconductor, the third semiconductor layer being formed above a region where the active layer is formed and having a mesa shape extending in the extending direction of the active layer; an upper cladding layer formed on the second semiconductor layer to cover both side surfaces of the third semiconductor layer; A first electrode electrically connected to the first semiconductor layer; a second electrode electrically connected to the third semiconductor layer; Equipped with a mesa width of the third semiconductor layer is made smaller than a width of the active layer; A semiconductor optical device further comprising a light scattering or emitting structure formed on a side surface of the active layer.

3. 3. The semiconductor optical device according to claim 1, The semiconductor device further includes a fourth semiconductor layer and a fifth semiconductor layer, each made of a non-conductive or low-conductive III-V compound semiconductor, formed on the first semiconductor layer in contact with both side surfaces of the mesa defined by the active layer. A semiconductor optical device comprising:

4. 4. The semiconductor optical device according to claim 3, 2. A semiconductor optical device, comprising: the fourth semiconductor layer and the fifth semiconductor layer made of InP.

5. The semiconductor optical device according to any one of claims 1 to 4, The semiconductor optical device according to claim 1, wherein the second electrode is formed on the third semiconductor layer via a contact layer.

Images