JP7601219B2 - Semiconductor Optical Devices - Google Patents

Description

The present invention relates to a semiconductor optical device.

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.

In a typical thin-film structure, a lateral type is widely used 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.

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, the cross section of the active region is taken as the xy plane below, 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 through the thin-film structure is the z direction.

A typical application of such a thin film structure that allows current injection into the active region is a semiconductor laser diode (LD) with 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 in order to confine 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.

Here, in the lateral thin film structure, since there is no first mesa on the semiconductor thin film on which the active region is formed, by etching the upper surface of a thin film made of, for example, InP, a surface diffraction grating can be formed while maintaining strong light confinement in the active region. In this structure, the light confinement is strong near the thin film structure, and the refractive index difference between the convex part (e.g., InP) and the concave part (e.g., SiO ₂ or air) of the surface diffraction grating is large. For this reason, in the lateral thin film structure, a diffraction grating with a high coupling constant κ reaching several hundred cm ^-1 to 1000 cm ^-1 can be obtained by a relatively shallow excavation of about 20 to 30 nm (see Non-Patent Document 3). As shown in Non-Patent Document 3, achieving both strong light confinement and a high coupling constant is particularly important in realizing a short cavity laser with low power consumption.

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. 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. 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.

On the other hand, in the vertical thin-film structure, since the first mesa exists, it is not possible to form a surface 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 an undercladding (e.g., _SiO2 ) below the semiconductor thin film is coupled with the mode on the semiconductor thin film side to form a grating on the buried waveguide side.

However, in this structure, the location where the diffraction grating is formed is far away from the active region, so 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 combining the mode on the thin film side with 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.

The present invention has been made to solve the above problems, and aims to achieve both strong optical confinement and a high coupling constant in a semiconductor optical device with a vertical thin-film structure.

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 the upper surface of the second semiconductor layer, a third semiconductor layer made of a second conductivity type III-V compound semiconductor formed above the formation region of the active layer and having a mesa shape extending in the extension direction of the active layer, an upper cladding layer formed on the second semiconductor layer and covering 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, the mesa width of the third semiconductor layer being smaller than the width of the active layer.

As described above, according to the present invention, in a semiconductor optical device with a vertical thin-film structure in which current is applied in the vertical direction of the 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 with a vertical thin-film structure can achieve both strong optical confinement and a high coupling constant.

FIG. 1 is a cross-sectional view showing the structure of a semiconductor optical device according to an embodiment of the present invention. FIG. 2 is a diagram showing a configuration assumed for calculating various characteristics of the structure of the semiconductor optical device shown in FIG. 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. 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 (c) in the active layer 104 (the entire MQW region), calculated using the structure of FIG. 2(a). FIG. 5 is a characteristic diagram showing the coupling constant κ of the diffraction grating obtained using the equivalent refractive indices n _eq,1 and n _eq,2 in FIGS. 2(a) and 2(b). FIG. 6 is a cross-sectional view showing the structure of another semiconductor optical device according to an embodiment of the present invention. FIG. 7 is a diagram showing a configuration assumed for calculating various characteristics of the structure of the semiconductor optical device shown in FIG. FIG. 8 is a distribution diagram showing the intensity distribution of each mode calculated using the structure of FIG. FIG. 9 is a characteristic diagram showing the equivalent refractive index n _eq (a) of each mode, the Bragg wavelength λ _B of the diffraction grating (b), and the optical confinement factor Γ _MQW (c) in the active layer 104 (the entire MQW region) calculated using the structure in FIG. 7(a). FIG. 10 is a characteristic diagram showing the coupling constant κ of the diffraction grating obtained using the equivalent refractive indices n _eq,1 and n _eq,2 in each of FIGS. 7(a) and 7(b). FIG. 11 is a cross-sectional view showing the structure of another semiconductor optical device according to an embodiment of the present invention. FIG. 12 is a diagram showing a configuration assumed for calculating various characteristics of the structure of the semiconductor optical device shown in FIG. 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. FIG. 14 is a characteristic diagram showing the equivalent refractive index of each mode calculated using the structure of FIG. FIG. 15 is a characteristic diagram showing the Bragg wavelength of a diffraction grating for each mode calculated using the structure of FIG. 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. FIG. 17 is a characteristic diagram showing the coupling constant κ of the diffraction grating obtained using the structure of FIG. FIG. 18 is a cross-sectional view showing the structure of another semiconductor optical device according to an embodiment of the present invention. FIG. 19 is a diagram showing a configuration assumed for calculating various characteristics of the structure of the semiconductor optical device shown in FIG. 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. 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. 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. FIG. 23 is a cross-sectional view showing the structure of another semiconductor optical device according to an embodiment of the present invention. FIG. 24 is a cross-sectional view showing the structure of a semiconductor optical device according to an embodiment of the present invention. 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 mode, calculated using the parameters of the buried structure of FIG. 2 and FIG. 7 and assuming the groove depth of the diffraction grating to be 20 nm. 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. FIG. 27 is a characteristic diagram showing the threshold material gain and the gain loss margin with respect to the higher mode in the fundamental mode calculated using the parameters of the ridge structure of the active layer 104 having 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 groove depth of the diffraction grating set to 20 nm. FIG. 28 is a characteristic diagram showing the threshold material gain and the gain loss margin with respect to the higher mode in the fundamental mode calculated using the parameters of the ridge structure of the active layer 104 having 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 groove depth of the diffraction grating set to 30 nm. FIG. 29 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 having 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 groove depth of the diffraction grating set to 20 nm. FIG. 30 is a characteristic diagram showing the threshold material gain and the gain loss margin with respect to the higher mode in the fundamental mode calculated using the parameters of the ridge structure of the active layer 104 having 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 groove depth of the diffraction grating set to 30 nm. FIG. 31 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 having 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 groove depth of the diffraction grating set to 20 nm. FIG. 32 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 having 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 groove depth of the diffraction grating set to 30 nm.

A semiconductor optical device according to an embodiment of the present invention will now be described with reference to Figure 1. This semiconductor optical device comprises 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.

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 having 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.

The resonator 106 is composed of a diffraction grating consisting of a pattern of protrusions and recesses formed by microfabrication of 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 pattern of protrusions and recesses on the upper surface of the second semiconductor layer 105 becomes a diffraction grating consisting of the second semiconductor layer 105 in the protruding parts and the third semiconductor layer 107 in the concave parts. In this example, the upper cladding layer 108 described later is also formed on the second semiconductor layer 105, and in this region, the pattern of protrusions and recesses on the upper surface of the second semiconductor layer 105 becomes a diffraction grating consisting of the second semiconductor layer 105 in the protruding parts and the upper cladding layer 108 in the concave parts.

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 first semiconductor layer 103 in an area where a mesa such as the active layer 104 is not formed. For example, the second electrode 132 is formed on the upper cladding layer 108 and electrically connected to the third semiconductor layer 107.

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, the second electrode 132 is connected (formed) on the third semiconductor layer 107 via the contact layer 110.

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

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 constituting 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.

The active layer 104 may have a multiple quantum well structure (MQW) consisting of well layers and barrier layers made of, for example, InGaAlAs, InGaAs, InGaAsP, etc., each of which has 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, for example, p-type InP. In this case, the second conductivity type is p-type. The second semiconductor layer 105 may be made of p-type InGaAsP.

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 be made of, for example, semi-insulating InP (SI-InP) that has been made highly resistive by doping with Fe, for example.

The upper cladding layer 108 can be made of a material with a lower refractive index than the semiconductor that makes up 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. When viewed from the substrate 101, the upper cladding layer 108 functions as an upper cladding layer. The semiconductor optical device according to the embodiment has an optical waveguide structure with the active layer 104 as the core.

It is important that the second semiconductor layer 105 is made of a III-V compound semiconductor with a refractive index different from that of the layers above and below it. It is also 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 that constitutes the lower layer and can be epitaxially grown.

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.

Figure 2 shows a specific example structure assumed for calculating various characteristics of the structure of the semiconductor optical device according to the above-mentioned embodiment. In the following, a rectangular shape is assumed as the diffraction grating when viewed in the yz cross section.

2A is an xy cross section of the convex portion of the rectangular diffraction grating that serves as the resonator 106, and FIG. 2B is an xy cross section of the concave portion. As can be seen from FIG. 2, in the diffraction grating that serves as the resonator 106, in the first mesa portion of the third semiconductor layer 107, the second semiconductor layer 105 is convex and the third semiconductor layer 107 is concave. On the other hand, outside the first mesa in the x direction, the second semiconductor layer 105 is convex and the upper cladding layer 108 is concave.

In general, III-V mixed crystals such as InGaAsP have a higher refractive index than InP and the like, and constructing a diffraction grating from these materials contributes to the high coupling constant that is a feature of the present invention. In the calculations shown below, in both structures, the operating wavelength (laser oscillation wavelength) is assumed to be 1310 nm (a typical value for the oscillation wavelength of a communication light source), 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.

Since the band gap wavelength is sufficiently shorter than the operating wavelength, no light is generated or absorbed due to band-to-band 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.

In addition, 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 an even larger refractive index difference is obtained, which also contributes greatly to the 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 can also be applied to the 1550 nm band, which is also a typical wavelength band for communications.

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 to be oscillated, and the modes with n of 1 or more are higher modes to suppress oscillation. 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.

Since the mesa width of the active layer 104 made of MQW is relatively narrow at 800 nm, it can be seen that the optical confinement in the higher mode active layer 104 is suppressed.

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

This means that the diffraction grating is assumed to be 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.

Fig. 5 shows the coupling constant of the diffraction grating calculated using the equivalent refractive indices n _eq,1 and n _eq,2 in Fig. 2(a) and Fig. 2(b). The following equation was used for the calculation, which gives the coupling constant of a rectangular diffraction grating with a duty ratio (ratio of the length of the convex portion to the length of the concave portion in the z direction) of 1:1.

The horizontal axis of Fig. 5 is the recess depth of the diffraction grating (the difference between the heights of the convex and concave portions). Fig. 5(a) shows that, 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 concave portion) is 20 nm and 30 nm, respectively, and coupling constants of about 600 cm ^-1 and 900 cm ^-1 are obtained in the fundamental mode. In addition, 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 is seen that strong optical confinement and a high coupling constant are both achieved.

On the other hand, from (b) and (c) of FIG. 5, it can be seen that an even larger coupling constant is obtained in the higher-order mode. In order to suppress the oscillation of 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 FIG. 7 is proposed as a structure to achieve this. In this semiconductor optical device, the width of the second semiconductor layer 105 in the structure of the semiconductor optical device described using FIG. 1 is set to the same as the width of the active layer 104, and a diffraction grating is formed only in this portion to form a resonator 106a.

As a secondary effect of adopting this structure, current constriction occurs during carrier injection from the third semiconductor layer 107 to the active layer 104, and current injection can be performed more efficiently. Figures 7(a) and 7(b) are 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.

Fig. 8 shows the intensity distribution of each mode calculated using the structure of Fig. 7(a). Also, Fig. 9 shows the equivalent refractive index n _eq (a), the Bragg wavelength λ _B of the diffraction grating (b), and the optical confinement factor Γ _MQW (c) in the active layer 104 (whole MQW region) calculated using the structure of Fig. 7(a). It can be seen from Fig. 9(c) that the fundamental mode still has a significantly larger optical confinement factor than the higher-order modes.

Fig. 10 shows the coupling constant κ of the diffraction grating calculated using the equivalent refractive indices n _eq,1 and n _eq,2 in Fig. 7(a) and Fig. 7(b) in the same manner as Fig. 5. Fig. 10(a) shows that, for example, when the mesa width of the third semiconductor layer 107 is 200 nm, coupling constants of about 500 cm ^-1 and 800 cm ^-1 are obtained in the fundamental mode when the grating is dug to a depth of 20 nm and 30 nm, respectively. Although this is somewhat lower than Fig. 5(a), it is still a high value for the coupling constant of a diffraction grating with a dug to a depth of 20 nm to 30 nm.

On the other hand, from Figures 10(b) and 10(c), it can be seen 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 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.

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 the fourth semiconductor layer 121, the fifth semiconductor layer 122, and the sixth semiconductor layer 109, which are passive layers such as InP, but other vertical thin-film structures such as the structure shown in Figure 11 are also possible. 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.

Compared to an embedded structure such as the semiconductor optical device described using Figures 1 and 6, the advantages of using the semiconductor optical device structure shown in Figure 11 include the following:

[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.

Figure 12 shows a specific example structure assumed for calculating various characteristics of the semiconductor optical device illustrated in Figure 11. Note that the structure shown in Figure 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 top surface of the second semiconductor layer 105 is dug out (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) as in Figure 2(b) and the like.

13 shows the intensity distribution of each mode calculated for 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. As can be seen from these intensity distributions, in a semiconductor optical device with the active layer 104a having a ridge structure, the sides 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 in the x direction as well, and not only the fundamental mode but also higher modes are strongly confined in the active layer 104a.

Figures 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 Figure 12.

In each figure, the mesa width of the third semiconductor layer 107 is assumed to be 100 nm, 200 nm, 300 nm, and 400 nm, and the mesa width of the active layer 104a is taken on the horizontal axis. From Figures 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 detuning when using MQW in 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 coupling constant, the possibility of multi-mode oscillation occurs.

16, for example, when the mesa width of the third semiconductor layer 107 is 200 nm, a very high optical confinement factor exceeding 0.5 is obtained in the fundamental mode. However, under these conditions, when the mesa width of the active layer 104a is wider than a certain degree, the higher-order modes have even stronger optical confinement.

Figure 17 shows the coupling constant values calculated in the same manner as in Figure 5 using the structure of Figure 12. The mesa width of the third semiconductor layer 107 was fixed at 200 nm, and three levels of 800 nm, 2.0 μm, and 4.0 μm were assumed as the mesa width of 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, extremely high coupling constants of approximately 700 cm ^-1 or more and approximately 1100 cm ^-1 or more were obtained in the fundamental mode at trench depths of 20 nm and 30 nm, respectively. However, at the same time, the coupling constants of the higher modes also have extremely high values of the same order.

Therefore, in the structure of Figure 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, making it difficult to obtain stable single-mode oscillation. As a structure to solve this multi-mode problem, the present invention proposes the structure shown in Figure 18.

The semiconductor optical device shown in Figure 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 mesa width of the second semiconductor layer 105 in the structure of Figure 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%.

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 in FIG. 11. The coupling constants 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, 2.0 μm, and 4.0 μm are shown in FIG. 20, FIG. 21, and FIG. 22, respectively. In all cases, 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.

The coupling constant of the fundamental mode at a diffraction grating occupancy of 50% is approximately 500 cm ^-1 and slightly less than 800 cm ^-1 at the excavation depths of 20 nm and 30 nm when the mesa width of the active layer 104a (second semiconductor layer 105) is 0.8 μm. The coupling constant of the fundamental mode at a diffraction grating occupancy of 50% is approximately 650 cm ^-1 and 1000 cm -1 at the excavation depths of 20 nm and 30 nm when the mesa width of the active layer 104a (second semiconductor layer 105) is 2.0 μm. The coupling constant of the fundamental mode at a diffraction grating occupancy of 50% is approximately 650 cm ^-1 and 800 cm ^-1 at the excavation depths of 20 nm and 30 nm when the mesa width of the active layer ^104a (second semiconductor layer 105) is 4.0 μm.

These values are lower than those in the case of a diffraction grating occupancy of 100%, but are still high. On the other hand, the coupling constant of the higher-order mode under these conditions is smaller than that of the fundamental mode by about 100 cm ^-1 to several hundred cm ^-1 , and it can be seen that only the coupling constant of the higher-order mode can be effectively reduced by this structure. Note that here, only four levels of the 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.

As another configuration for suppressing oscillation of the higher-order mode, as shown in FIG. 23, it is possible to further provide structures 111 and 112 that scatter or radiate light on the side of the mesa-shaped active layer 104a (second semiconductor layer 105). The structures 111 and 112 can be, for example, a randomly rough uneven shape, a high-order sidewall diffraction grating, etc. 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 of the mesa of the active layer 104a. Therefore, by providing structures 111 and 112 on the side 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.

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, as a specific example, the oscillation threshold is calculated when the present invention is applied to a λ/4 shift type DFB laser as shown in Figure 24. Figure 24 shows a cross section parallel to the waveguiding direction. This DFB laser has a λ/4 shift section 113 in the middle (center) of the diffraction grating that constitutes the resonator 106.

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 the appropriate passive waveguides 141, 142, and that there is no reflection of light at this coupling point. Since the third semiconductor layer 107 for current injection is not required in the passive waveguides 141, 142, a structure in which the third semiconductor layer 107 is tapered to an appropriate length and terminated as shown in the figure is conceivable.

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

Here, _rL and _rR are the amplitude reflectance of light due to Bragg reflection when looking to 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 active layer 104 that provides gain to light (in the case of MQW, the well layer of each quantum well), g is the material gain of that portion (in the case of MQW, the well layer of each quantum well), and _αi is the internal loss.

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. In addition, α _i was set to a typical value of 10 cm ^-1 . As can be seen from equations (4) to (6), in a λ/4 shifted DFB laser, the resonance condition is satisfied at the Bragg wavelength λ of the diffraction grating = λ _B , and the resonance mode usually oscillates stably in a single mode in the axial direction. In this calculation, λ = λ _B is also set, and oscillation in that resonance mode is assumed.

Equation (4) expresses the balance between gain and loss as well as the resonance condition, and the material gain that satisfies this is expressed as g = g _th and 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 said that oscillation occurs at a sufficiently low threshold if g _th is, for example, about 500 cm ^-1 . Therefore, in the following, g _th = 500 cm ^-1 is used as a guideline for the low oscillation threshold.

On the other hand, the gain-loss margin given by the following formula is often used as a guideline for single-mode oscillation.

Here, the subscripts "main" and "side" mean the main mode and the side mode associated therewith, respectively, that oscillate. Moreover, formula (9) represents 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, it is said that if the gain loss margin is, for example, 100 cm ^-1 , a very good single mode characteristic with a side mode suppression ratio exceeding 50 dB can be obtained. Although the material gain g actually depends on the resonant wavelength (Bragg wavelength) of each mode, in this case, in order to consider only the effect due to the difference in the optical confinement factor, it is assumed that g is commonly the threshold material gain value g _th ^TR00 when the main mode oscillates. Therefore, when discussing the single mode characteristic between the fundamental mode (TE00) and the higher-order mode (TEn0), it is shown in the following formula.

Below, we evaluate the gain loss margin between the TE00 mode and the TEn0 mode (n = 1, 2) given by equation (10). In addition, the width of the third semiconductor layer 107 is fixed to 200 nm in all structures below.

First, the threshold material gain of the fundamental mode and the gain loss margin with the higher order mode calculated using the parameters of the buried structure in Fig. 2 and Fig. 7 are shown in Fig. 25 and Fig. 26. The horizontal axis is the cavity length _Lcav , the solid line is the threshold material gain of the fundamental mode (vertical axis on the left), and the dashed and dotted lines are the gain loss margin with the higher order mode (vertical axis on the right). Fig. 25 shows the case where the grating is dug to a depth of 20 nm, and Fig. 26 shows the case where the grating is dug to a depth of 30 nm. In both figures, (a) corresponds to the structure in Fig. 2, and (b) corresponds to the structure in Fig. 7.

First, looking at the threshold material gain of the fundamental mode, it can be seen that in all cases, a low threshold material gain of 500 cm ⁻¹ is obtained at a short cavity length of approximately 40 μm to 60 μm. 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, regarding the gain loss margin, a positive value is obtained in all cases at a cavity length where g _th =500 cm ⁻¹ . It can be seen that a margin of 100 cm ⁻¹ or more can be easily obtained in particular in (b). Although a margin as large as that in (a) is not obtained 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 a sufficiently good single mode characteristic can be obtained even in (a).

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

27 and 28 (where 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 obtained in a well-balanced manner, particularly in the case of (b) where the diffraction grating occupancy is 50%. The cavity length at which g _th = 500 cm ^-1 in (b) is a short value of approximately just under 50 μm to just over 60 μm, and a low-power-consumption short cavity laser can also be realized.

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, the diffraction grating occupancy of 100% of (d) can be adopted. When (d) is adopted, the cavity length at which g _th = 500 cm ^-1 is shortened from approximately 40 μm to 50 μm.

Next, in Figures 29 and 30 (active layer 104, second semiconductor layer 105 mesa width 2.0 μm) and Figures 31 and 32 (active layer 104, second semiconductor layer 105 mesa width 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 the active layer 104 and second semiconductor layer 105 is relatively wide, it is clear that a low oscillation threshold and a large gain loss margin can be obtained in a well-balanced manner in the case of the diffraction grating occupancy of 50% in (b).

On the other hand, at a diffraction grating occupancy of 100% in (d), the gain loss margin is a negative value. This means that higher-order modes are more likely to oscillate, and it is difficult to obtain stable single-mode oscillation in the fundamental mode. This also explains the effect of the diffraction grating structure proposed by the present invention in Figure 18.

In the case of (b), the cavity length at which g _th =500 cm ^-1 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.

In this explanation, a λ/4 shifted DFB laser as shown in Figure 24 has been assumed as a typical example, but since the basic structure of the present invention is a diffraction grating structure in the xy cross section of the active layer 104, the application of device structures including the z-axis direction is not limited to the example shown here, but can be applied to general device structures having a diffraction grating in the active layer 104.

Other typical examples include the following:

1. DR laser structure: A uniform diffraction grating without shift is uniformly formed in the active layer 104 to create 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.

2. Two-section DFB laser structure: A structure that enables single-mode oscillation and asymmetric light extraction at the front and rear sides (outputting more light from the front side) by connecting two DFB sections with different Bragg wavelengths and lengths.

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 creating 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.

The features of the above-mentioned embodiment are as follows:

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 the second semiconductor layer 105 and the semiconductor constituting the third semiconductor layer 107, such as InP. This makes it possible to form a diffraction grating near the active layer 104, even with a vertical thin-film structure, and achieves both strong optical confinement and a high coupling constant.

2. The structure proposed effectively suppresses the oscillation of m higher-order modes by appropriately designing the cross-sectional structure and diffraction grating shape, and a stable single-mode oscillation is obtained. 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 out in the x direction, and the suppression of oscillation of higher-order modes has been ensured by the cutoff of the guided mode. In the above-mentioned embodiment, a new local diffraction grating structure in which only the overlap with the fundamental mode is large is 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 an MQW ridge structure, which has strong optical confinement in the x direction and strong multi-mode properties.

As described above, according to the present invention, in a semiconductor optical device with a vertical thin-film structure in which current is applied in the vertical direction of the 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, thereby making it possible to achieve both strong optical confinement and a high coupling constant in a semiconductor optical device with a vertical thin-film structure.

It is to be understood that the present invention is not limited to the embodiments described above, and that many modifications and combinations can be implemented by a person having ordinary knowledge in the art within the technical concept of the present invention.

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 (5)

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;
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 ; 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;
A semiconductor optical device further comprising a light scattering or emitting structure formed on a side surface of the active layer. 3. The semiconductor optical device according to claim 1,
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. 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. The semiconductor optical device according to any one of claims 1 to 4 ,
The second electrode is formed on the third semiconductor layer via a contact layer.

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