Semiconductor optical element

Abstract

An embodiment semiconductor optical device includes an optical waveguide including a core, and an active layer extending in the waveguide direction of the optical waveguide for a predetermined distance and arranged in a state in which the active layer can be optically coupled to the core. The core and the active layer are arranged in contact with each other. The core is formed of a material with a refractive index of about 1.5 to 2.2, such as SiN, for example. In addition, the core is formed to a thickness at which a higher-order mode appears. The higher-order mode is an E.sub.12 mode, for example.

Claims

1. A semiconductor optical device comprising: an optical waveguide on a substrate, the optical waveguide including a core having a thickness at which a higher-order mode appears; an active layer above the substrate, the active layer extending along the core and configured to be optically coupled to the core, wherein the core and the active layer are in physical contact with each other; a p-type semiconductor layer and an n-type semiconductor layer in contact with the active layer above the substrate and sandwiching the active layer in a plan view; and a resonator structure configured to confine light in the active layer.

2. The semiconductor optical device according to claim 1, wherein the higher-order mode is an E12 mode.

3. The semiconductor optical device according to claim 1, wherein the core is between the substrate and the active layer.

4. The semiconductor optical device according to claim 1, wherein the core is above the active layer as viewed from a side of the substrate.

5. The semiconductor optical device according to claim 1, wherein the core comprises a material with a refractive index of 1.5 to 2.2.

6. The semiconductor optical device according to claim 5, wherein the core comprises SiN or SiON.

7. The semiconductor optical device according to claim 6, wherein the core comprises deuterium.

8. The semiconductor optical device according to claim 1, further comprising: an n-type electrode connected to the n-type semiconductor layer; and a p-type electrode connected to the p-type semiconductor layer.

9. The semiconductor optical device according to claim 1, wherein the resonator structure includes a diffraction grating in the core.

10. A semiconductor optical device comprising: an optical waveguide on a substrate, the optical waveguide including a core having a thickness at which a higher-order mode appears; an active layer above the substrate, the active layer extending along the core and optically coupled to the core, wherein the core and the active layer are in physical contact with each other; a p-type semiconductor layer and an n-type semiconductor layer in contact with the active layer above the substrate and sandwiching the active layer in a plan view; an n-type electrode connected to the n-type semiconductor layer; a p-type electrode connected to the p-type semiconductor layer; and a resonator structure configured to confine light in the active layer, the resonator structure including a diffraction grating in the core.

11. The semiconductor optical device according to claim 10, wherein the higher-order mode is an E12 mode.

12. The semiconductor optical device according to claim 10, wherein the core is between the substrate and the active layer.

13. The semiconductor optical device according to claim 12, further comprising a second core above the active layer as viewed from a side of the substrate.

14. The semiconductor optical device according to claim 10, wherein the core is above the active layer as viewed from a side of the substrate.

15. The semiconductor optical device according to claim 10, wherein the core comprises a material with a refractive index of 1.5 to 2.2.

16. The semiconductor optical device according to claim 10, wherein the core comprises SiN or SiON.

17. The semiconductor optical device according to claim 16, wherein the core comprises deuterium.

18. A method of forming a semiconductor optical device, the method comprising: forming an optical waveguide on a substrate, the optical waveguide including a core having a thickness at which a higher-order mode appears, the higher-order mode being an E12 mode or higher; forming an active layer above the substrate, the active layer extending along the core and being optically coupled to the core, wherein the core and the active layer are in physical contact with each other; forming a p-type semiconductor layer and an n-type semiconductor layer in contact with the active layer above the substrate and sandwiching the active layer in a plan view; and forming a resonator structure to confine light in the active layer.

19. The method according to claim 18, wherein the core comprises SiN or SiON.

20. The method according to claim 18, further comprising: forming an n-type electrode connected to the n-type semiconductor layer; and forming a p-type electrode connected to the p-type semiconductor layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A is a cross-sectional view of the configuration of a semiconductor optical device according to an embodiment of the present invention.

(2) FIG. 1B is a plan view of the configuration of a part of the semiconductor optical device according to an embodiment of the present invention.

(3) FIG. 2 is an illustration view for illustrating an E.sub.12 mode.

(4) FIG. 3 is a characteristic graph illustrating the calculation results in which the abscissa axis indicates the thickness of a core formed of SiN, and the ordinate axis indicates the effective refractive indices of E.sub.11, E.sub.12, and E.sub.22.

(5) FIG. 4A is a characteristic graph illustrating the results of calculation of light confinement in the core.

(6) FIG. 4B is a characteristic graph illustrating the results of calculation of light confinement in a p-type semiconductor layer.

(7) FIG. 4C is a characteristic graph illustrating the results of calculation of light confinement in an active layer.

(8) FIG. 5 is an illustration graph for illustrating a state in which the E.sub.12 mode is selected as a laser oscillation mode.

(9) FIG. 6A is a cross-sectional view of the configuration of another semiconductor optical device according to an embodiment of the present invention.

(10) FIG. 6B is a plan view of the configuration of a part of another semiconductor optical device according to an embodiment of the present invention.

(11) FIG. 7 is a cross-sectional view of the configuration of another semiconductor optical device according to an embodiment of the present invention.

(12) FIG. 8 is a cross-sectional view of the configuration of another semiconductor optical device according to an embodiment of the present invention.

(13) FIG. 9 is a cross-sectional view of the configuration of another semiconductor optical device according to an embodiment of the present invention.

(14) FIG. 10 is a cross-sectional view of the configuration of another semiconductor optical device according to an embodiment of the present invention.

(15) FIG. 11 is a cross-sectional view of the configuration of another semiconductor optical device according to an embodiment of the present invention.

(16) FIG. 12 is a cross-sectional view of the configuration of a semiconductor optical device.

(17) FIG. 13A is a cross-sectional view of the configuration of a semiconductor optical device.

(18) FIG. 13B is a perspective view of the configuration of the semiconductor optical device.

(19) FIGS. 14A-14C are illustration views for illustrating an E.sub.11 mode.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(20) Hereinafter, a semiconductor optical device according to an embodiment of the present invention will be described with reference to FIGS. 1A and 1B. The semiconductor optical device first includes an optical waveguide 104 formed on a substrate 101 and including a core 102, and an active layer 105 formed above the substrate 101 along the core 102 (by extending in the waveguide direction of the optical waveguide 104 for a predetermined distance) in a state in which the active layer 105 can be optically coupled to the core 102. The optical waveguide 104 includes the core 102 and a clad 103 formed to have the core 102 embedded therein. In this example, the core 102 and the active layer 105 are arranged in contact with each other. In addition, in this example, the core 102 is arranged between the substrate 101 and the active layer 105.

(21) The substrate 101 is formed of InP that has been made to have a semi-insulating property by being doped with iron, for example. The core 102 is formed of a material with a refractive index of about 1.5 to 2.2, such as SiN, for example. In addition, the core 102 is formed to a thickness at which a higher-order mode appears. It should be noted that the thickness of the core 102 is the height of the core 102 as seen from the side of the substrate 101. The higher-order mode is an E.sub.12 mode, for example. To allow the E.sub.12 mode to appear in the core 102, the width and thickness of the core 102 as seen in the cross-sectional view are set to 1.0 μm and 0.15 μm or more, respectively, when the refractive index of the core 102 is 2.00, for example. It should be noted that the core 102 may also be formed using SiON. SiN and SiON are materials that are unlikely to exhibit a nonlinear optical effect.

(22) The clad 103 is formed using InP, for example. The clad 103 may also be formed using GaAs. The active layer 105 is, for example, a quantum well structure obtained by stacking eight layers each including a well layer of InGaAsP with a thickness of 6 nm and a barrier layer with a thickness of 9 nm, and has a thickness of about 250 nm. In such a case, the light-emission wavelength of the active layer 105 is 1.55 μm. It should be noted that the active layer 105 may also be formed using InGaAlAs, for example. Such a structure is an embedded hetero structure in which the active layer 105 is embedded in the clad 103.

(23) In addition, the semiconductor optical device includes a p-type semiconductor layer 106 and an n-type semiconductor layer 107 formed in contact with the active layer 105, above the substrate 101. For example, the p-type semiconductor layer 106 is formed using p-type InP doped with about 1×10.sup.18 cm.sup.−3 of Zn, and the n-type semiconductor layer 107 is formed using n-type InP doped with about 1×10.sup.18 cm.sup.−3 of Si. The p-type semiconductor layer 106 and the n-type semiconductor layer 107 are formed sandwiching the active layer 105 therebetween as seen in a plan view. In this configuration, current is injected into the active layer 105 from a direction (i.e., transverse direction) parallel with the plane of the substrate 101.

(24) Each layer of the aforementioned compound semiconductor is formed through crystal growth using well-known metal organic chemical vapor deposition, for example. In addition, to form the core 102, the active layer 105, and diffraction gratings 121 described below, patterning, such as a known lithography technique and wet etching or dry etching, is used.

(25) The semiconductor optical device also includes an n-type electrode 109 connected to the n-type semiconductor layer 107, and a p-type electrode 108 connected to the p-type semiconductor layer 106. In addition, the semiconductor optical device has the diffraction gratings 121 formed in the core 102, as a resonator structure for confining light in the active layer 105, and thus is formed as a distributed feedback laser. The diffraction gratings 121 are formed on the lateral portions of the core 102.

(26) The E.sub.12 mode is used to strongly confine light in the core 102, which is formed of a material with a refractive index lower than those of semiconductors, and to suppress overlap of optical modes with the p-type semiconductor layer 106. In this mode, as illustrated in FIG. 2, the number of waves of the transverse electromagnetic field components in the y-axis direction is two, and the optical mode is strongly distributed in the core of SiN (the first wave), and thus, the spread of light to p-type InP (p-InP) is suppressed. In addition, since the light overlaps with the active layer (the second wave), a gain is secured.

(27) To select the aforementioned E.sub.12 mode as a laser oscillation mode, the periods of the diffraction gratings 121 are determined so that the reflectivity of the E.sub.12 mode becomes high and the Bragg wavelength of the E.sub.12 mode overlaps with the gain distribution of the active layer 105.

(28) FIG. 3 illustrates the calculation results in which the abscissa axis indicates the thickness of the core 102 formed of SiN, and the ordinate axis indicates the effective refractive indices of E.sub.11, E.sub.12, and E.sub.22. From the results, it is found that the E.sub.12 mode is generated when the thickness of the core 102 formed of SiN is greater than or equal to 0.15 μm.

(29) FIG. 4A illustrates the results of calculation of confinement of light in the core 102. FIG. 4B illustrates the results of calculation of confinement of light in the p-type semiconductor layer 106. FIG. 4C illustrates the results of calculation of confinement of light in the active layer 105. As shown in the calculation results, in the E.sub.11 mode, almost no portion of light is confined in the core 102, and thus, the mode overlap with the p-type semiconductor layer 106 is large. Meanwhile, in the E.sub.12 mode, almost all portions of light are confined in the core 102 in contrast to the E.sub.11 mode, and thus, the mode overlap with the p-type semiconductor layer 106 is found to be suppressed. In addition, in the E.sub.12 mode, light overlaps with the active layer 105. Thus, a gain for laser oscillation is secured.

(30) Next, the diffraction gratings 121 will be described. First, the period of each diffraction grating 121 is determined so that the Bragg wavelength of the diffraction grating 121 for the E.sub.12 mode overlaps with the gain wavelength of the active layer 105. For example, suppose a case where the peak gain wavelength of the active layer 105 is 1.55 μm. The effective refractive index of the E.sub.12 mode is 1.70 when the thickness of the core 102 formed of SiN is 0.6 μm. The Bragg wavelength is given by λ.sub.B=2n.sub.effΛ/m, where n.sub.eff is the effective refractive index, Λ is the period of the diffraction grating, and m is the order (a positive integer) of the diffraction grating. Thus, the period of the diffraction grating 121 for setting the Bragg wavelength to 1.55 μm is determined as Λ=0.456 μm. Herein, the order of the diffraction grating 121 was set to 1.

(31) It should be noted that since the effective refractive index of E.sub.11 is 2.86, when a diffraction grating with Λ=0.456 μm is used, the Bragg wavelength for the E.sub.11 mode is determined as λ.sub.B=2.61 μm (when m=1), 1.30 μm (when m=2), or 0.869 μm (when m=3). The Bragg wavelength that overlaps with the gain wavelength of the active layer 105 is only the Bragg wavelength for the E.sub.12 mode when m=1. Thus, the E.sub.12 mode is selected as a laser oscillation mode.

(32) FIG. 5 illustrates the view described above. As illustrated in FIG. 5, it is necessary to prevent the higher-order mode (m>1) of the diffraction grating for E.sub.11 from overlapping with the gain distribution. To this end, a combination of the active layer 105 formed of a group III-V compound semiconductor with a thickness of 250 nm and the core 102 formed of SiN with a thickness of 0.6 μm is used, for example.

(33) Next, each diffraction grating 121 is arranged at a position where it has high reflectivity (that is, a high coupling coefficient) for the E.sub.12 mode, and has low reflectivity (that is, a low coupling coefficient) for the E.sub.11 mode. For example, the diffraction grating 121 is arranged at a position close to the core 102 where in the E.sub.12 mode, the greater part of the mode is confined. Specifically, as illustrated in FIG. 1B, the width of the core 102 is periodically changed to obtain the diffraction grating 121. Accordingly, the threshold gain of the E.sub.12 mode becomes lower than those of the other modes, and the E.sub.12 mode is selected as a laser oscillation mode.

(34) Although the present embodiment has illustrated an example in which the E.sub.12 mode is selected, the present invention is not limited thereto. For example, an even higher-order mode, such as an E.sub.13 mode, may be used. To allow the E.sub.13 mode to appear in the core, the thickness of the core formed of SiN is set to greater than or equal to 0.7 μm.

(35) In addition, as illustrated in FIGS. 6A and 6B, a diffraction grating 121a may be provided between a core 102a and the substrate 101. Alternatively, as illustrated in FIG. 7, diffraction gratings 121a may be arranged around the opposite sides of the core 102a. Adjusting the gap between the core 102a and each diffraction grating 121a can adjust the coupling coefficient of the diffraction grating 121a. Examples of the material of the diffraction grating 121a include SiN, Si, and SiO.sub.x.

(36) As illustrated in FIG. 8, the core 102 and the active layer 105 may be arranged apart from each other. It is acceptable as long as the core 102 and the active layer 105 are arranged in a state in which they can be optically coupled and the distance between the core 102 and the active layer 105 is 0 to 0.5 μm, for example.

(37) As illustrated in FIG. 9, a configuration may also be provided in which a core 102b arranged in contact with the active layer 105 includes a connection portion 102c that reaches the substrate 101. Such a configuration can provide a path for the flow of heat generated in the active layer 105 during operation, specifically, from the active layer 105.fwdarw.the core 102b.fwdarw.the connection portion 102c.fwdarw.the substrate 101, and thus, an improvement in the temperature characteristics can also be expected.

(38) As illustrated in FIG. 10, a configuration may also be provided in which a core 102d is provided above the active layer 105 as seen from the side of the substrate 101. Alternatively, as illustrated in FIG. 11, a configuration may also be provided in which the core 102 is arranged below the active layer 105, and the core 102d is arranged above the active layer 105. When the core 102d and the core 102 are arranged above and below the active layer 105, the E.sub.13 mode appears in the core 102d and the core 102, and the peak of the electric field intensity appears in the core 102d, the core 102, and the active layer 105. With such a configuration, advantageous effects similar to those of the embodiment described with reference to FIGS. 1A and 1B are expected to be obtained.

(39) By the way, when the core 102d of SiN is formed above the active layer 105, a SiN film for forming the core 102d is formed (deposited) by ECR plasma CVD as described below. When ECR plasma CVD is used, the film formation reaction is allowed to proceed using ions with high electron energy. Thus, the substrate need not be heated and low-temperature film formation is possible. When a SiN film is formed with such a film formation method, there is no possibility that an active element portion, such as the active layer 105, that has been already formed will be damaged.

(40) Herein, to form a SiN film using ECR plasma CVD, SiH.sub.4, Si.sub.2H.sub.6, or the like is used as a source gas for Si. In such a case, an N—H group is formed in the SiN film to be formed. Absorption of light by the N—H group appears at a wavelength of about 1510 nm. Thus, the optical waveguide including the core 102d formed of a SiN film containing an N—H group is not suitable as a constituent element of a semiconductor laser used for optical communication.

(41) To solve such a problem, the N—H group in the SiN film has only to be reduced. To this end, a deuterated silane gas not containing H is used as a source gas for Si. According to ECR plasma CVD using a deuterated silane gas, formation of an N—H group in the SiN film can be suppressed. Consequently, the optical waveguide including the core 102d formed of such a SiN film can suppress absorption of light with a wavelength of about 1510 nm. It should be noted that a SiN film formed by ECR plasma CVD using a deuterated silane gas contains deuterium. This is also true of a case where the core 102d is formed using SiON.

(42) As described above, according to embodiments of the present invention, a core, which is arranged in a state in which the core can be optically coupled to an active layer, is formed to a thickness at which a higher-order mode appears. Thus, even when a material that is unlikely to exhibit a nonlinear optical effect is used as a material of the core, it is possible to reduce waveguide loss of the semiconductor optical device with the embedded optical waveguide structure.

(43) It should be noted that the present invention is not limited to the embodiments described above, and it is apparent that one of ordinary skill in the art can apply various modifications and combinations within the technical idea of the present invention.

REFERENCE SIGNS LIST

(44) 101 Substrate 102 Core 103 Clad 104 Optical waveguide 105 Active layer 106 p-type semiconductor layer 107 n-type semiconductor layer 108 p-type electrode 109 n-type electrode 121 Diffraction grating