Optical modulator integrated laser element and method for manufacturing optical modulator integrated laser element
Patent Information
- Application Number
- PCT/JP2026/009248
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2025-12-03
- Filing Date
- 2026-03-10
- Publication Date
- 2026-10-01
Smart Images
Figure JP2026009248_01102026_PF_FP_ABST
Abstract
Description
Optical modulator integrated laser element, and method for manufacturing an optical modulator integrated laser element
[0001] This disclosure relates to an optical modulator integrated laser element and a method for manufacturing an optical modulator integrated laser element. This application claims priority under Japanese application No. 2025-054152 filed on 27 March 2025 and Japanese application No. 2025-227235 filed on 3 December 2025, and incorporates all the provisions of the said Japanese applications.
[0002] Patent Document 1 discloses an optical modulator integrated laser element comprising a laser unit and an electric field absorption type optical modulator. The optical modulator comprises an n electrode and a p electrode. The n electrode and the p electrode are each input to a positive-sequence signal and an inverse-sequence signal that constitute a differential signal.
[0003] Japanese Patent Publication No. 2002-277840
[0004] An optical modulator-integrated laser element according to one embodiment of the present disclosure comprises a semi-insulating or insulating substrate, a laser unit provided on the substrate that outputs laser light, and an electro-absorption type optical modulator provided on the substrate that receives laser light from the laser unit. The optical modulator has, in order, a contact layer of a first conductivity type having a first impurity concentration, a first cladding layer of a first conductivity type having a second impurity concentration smaller than the first impurity concentration, an electro-absorption layer, a second cladding layer, and a contact layer of a second conductivity type on the substrate. The optical modulator further comprises a first electrode pad electrically connected to the contact layer of the first conductivity type via an ohmic electrode, and a second electrode pad electrically connected to the contact layer of the second conductivity type via an ohmic electrode.
[0005] Figure 1 is a plan view showing an optical modulator integrated laser element according to an embodiment of the present disclosure. Figure 2 is a cross-sectional view along line II-II in Figure 1. Figure 3 is a schematic diagram showing a cross-section along line III-III in Figure 1. Figure 4 is a schematic diagram showing a cross-section along line IV-IV in Figure 1. Figure 5 is a schematic diagram showing a cross-section along line V-V in Figure 1. Figure 6 is an enlarged view of parts of Figures 3, 4, and 5. Figure 7 is a further enlarged view of part of Figure 6. Figure 8 is a schematic diagram showing a cross-section along line VIII-VIII in Figure 1. Figure 9 is a schematic diagram showing a cross-section along line IX-IX in Figure 1. Figure 10 is a diagram showing the manufacturing process of an optical modulator integrated laser element. Figure 11 is a diagram showing the manufacturing process of an optical modulator integrated laser element. Figure 12 is a diagram showing the manufacturing process of an optical modulator integrated laser element. Figure 13 is a diagram showing the manufacturing process of an optical modulator integrated laser element. Figure 14 is a diagram showing the manufacturing process of an optical modulator integrated laser element. Figure 15 is a diagram showing the manufacturing process of an optical modulator integrated laser element. Figure 16 is a diagram showing the manufacturing process of an optical modulator integrated laser element. Figure 17 is a diagram showing the manufacturing process of an optical modulator integrated laser element. Figure 18 is a plan view showing an optical modulator integrated laser element according to the first modified example. Figure 19 is a cross-sectional view along the line XIX-XIX in Figure 18. Figure 20 is a cross-sectional view showing a modified example in the same cross-section as Figure 19. Figure 21 is a plan view showing an optical modulator integrated laser element according to the second modified example.
[0006] When performing differential drive of an electro-absorption type optical modulator, electrical connection is facilitated by providing electrode pads connected to the n electrode and electrode pads connected to the p electrode on the upper surface of the optical modulator. For this purpose, for example, the lower cladding layer, the electro-absorption layer, and the upper cladding layer are formed on an insulating or semi-insulating substrate. Then, the lower cladding layer is brought into contact with an ohmic electrode, and the upper cladding layer is brought into contact with another ohmic electrode. However, the lower cladding layer plays a role in optical confinement, and its impurity concentration is relatively low. Consequently, there is a problem that the connection resistance between the ohmic electrode and the lower cladding layer becomes high.
[0007] This disclosure aims to provide an optical modulator integrated laser element and a method for manufacturing the same that can reduce the connection resistance between the ohmic electrode and the lower cladding layer.
[0008] According to the optical modulator integrated laser element and its manufacturing method disclosed herein, the connection resistance between the ohmic electrode and the lower cladding layer (first cladding layer) can be reduced.
[0009] [Description of Embodiments of the Present Disclosure] First, the contents of embodiments of the present disclosure will be listed and described. (1) An optical modulator integrated laser element according to one embodiment of the present disclosure comprises a semi-insulating or insulating substrate, a laser unit provided on the substrate that outputs laser light, and an electro-absorption type optical modulator provided on the substrate that receives laser light from the laser unit. The optical modulator has, in order, a contact layer of a first conductivity type having a first impurity concentration, a first cladding layer of a first conductivity type having a second impurity concentration smaller than the first impurity concentration, an electro-absorption layer, a second cladding layer, and a contact layer of a second conductivity type on the substrate. The optical modulator further comprises a first electrode pad electrically connected to the contact layer of a first conductivity type via an ohmic electrode, and a second electrode pad electrically connected to the contact layer of a second conductivity type via an ohmic electrode.
[0010] In the optical modulator integrated laser element described in (1) above, a contact layer having the same conductivity as the first cladding layer and a higher impurity concentration is provided between the substrate and the first cladding layer. The contact layer is electrically connected to the first electrode pad via an ohmic electrode. With this structure, the connection resistance between the ohmic electrode and the first cladding layer (lower cladding layer) can be reduced.
[0011] (2) The optical modulator integrated laser element described in (1) above may further include an etching stop layer of the first conductivity type, which is provided between the contact layer of the first conductivity type and the cladding layer and has a lower etching rate than the first cladding layer. In this case, even if the etching rate of the contact layer of the first conductivity type is the same as that of the cladding layer, etching to bring the contact layer of the first conductivity type into contact with the ohmic electrode can be accurately stopped before reaching the contact layer of the first conductivity type.
[0012] (3) In the optical modulator integrated laser element of (1) or (2) above, the thickness of the first conductivity type contact layer may be 0.1 μm or more and 0.5 μm or less. By forming the first conductivity type contact layer relatively thin in this way, the isolation resistance between the laser unit and the optical modulator can be increased, thereby reducing interference of the modulation signal of the optical modulator to the bias current of the laser unit.
[0013] (4) In any one of the optical modulator integrated laser elements described in (1) to (3) above, the first impurity concentration is 1 × 10 １８ cm －３ The above 1 x 10 １９ cm －３ The following may also apply. By having such a high impurity concentration in the first conductivity type contact layer, the connection resistance between the ohmic electrode and the first conductivity type contact layer can be effectively reduced.
[0014] (5) In any one of the optical modulator integrated laser elements described in (1) to (4) above, the optical modulator may include a first optical modulator and a second optical modulator.
[0015] (6) In the optical modulator integrated laser element described in (5) above, the first optical modulator has a first field absorption layer on a first cladding layer, and the second optical modulator has a second field absorption layer on a first cladding layer, and the first field absorption layer and the second field absorption layer may be made of the same material composition and formed continuously with respect to each other. In this case, since the first field absorption layer and the second field absorption layer are continuous, reflection and radiation at the interface can be reduced, and light loss can be reduced, compared to a structure in which a semiconductor layer of a different composition is interposed between them.
[0016] (7) In the optical modulator integrated laser element described in (5) above, the first optical modulator has a first field absorption layer on a first cladding layer, and the second optical modulator has a second field absorption layer on a first cladding layer, and a semiconductor layer made of a material with a different composition from the first and second field absorption layers may be interposed between the first and second field absorption layers. In this case, the optical coupling between the first and second optical modulators can be enhanced by appropriately selecting the composition of the semiconductor layer.
[0017] (8) A method for manufacturing an optical modulator integrated laser element according to one embodiment of the present disclosure is a method for manufacturing any one of the optical modulator integrated laser elements described in (2) to (7) above, comprising the steps of: growing a first conductivity type contact layer, an etching stop layer, a first cladding layer, an electric field absorption layer, a second cladding layer, and a second conductivity type contact layer on a substrate; etching the second conductivity type contact layer, the second cladding layer, the electric field absorption layer, and the first cladding layer to expose an etching stop layer; and forming an electrode on the etching stop layer that makes ohmic contact with the first conductivity type contact layer. In the etching step, etching is stopped by utilizing the difference in etching rates between the etching stop layer and the first cladding layer. According to the manufacturing method of (8) above, even if the etching rate of the first conductivity type contact layer is the same as that of the first cladding layer, etching to bring the first conductivity type contact layer into contact with the ohmic electrode can be accurately stopped before reaching the first conductivity type contact layer.
[0018] (9) A method for manufacturing an optical modulator integrated laser element according to one embodiment of the present disclosure is a method for manufacturing any one of the optical modulator integrated laser elements described in (2) to (7) above, comprising the steps of: growing a first conductivity type contact layer, an etching stop layer, a first cladding layer, an electric field absorption layer, a second cladding layer, and a second conductivity type contact layer on a substrate; etching the second conductivity type contact layer, the second cladding layer, the electric field absorption layer, and the first cladding layer to expose an etching stop layer; etching the etching stop layer to expose a first conductivity type contact layer; and forming an electrode on the first conductivity type contact layer that makes ohmic contact with the first conductivity type contact layer. According to the manufacturing method of (9), even when the etching rate of the first conductivity type contact layer is the same as the etching rate of the first cladding layer, etching to bring the first conductivity type contact layer into contact with the ohmic electrode can be accurately stopped before reaching the first conductivity type contact layer.
[0019] [Details of Embodiments of the Disclosure] Specific examples of the Disclosure will be described below with reference to the drawings. The present invention is not limited to these examples, but is shown in the claims, and all modifications within the meaning and scope of equivalence to the claims are intended. In the following description, the same elements in the description of the drawings will be denoted by the same reference numerals, and redundant descriptions will be omitted.
[0020] Figure 1 is a plan view showing an optical modulator integrated laser element 1 according to an embodiment of the present disclosure. Figure 2 is a cross-sectional view along line II-II in Figure 1. Figures 3, 4, and 5 schematically show cross-sections along lines III-III, IV-IV, and V-V in Figure 1, respectively. Figure 6 is an enlarged view of parts of Figures 3, 4, and 5. Figure 7 is a further enlarged view of part of Figure 6.
[0021] The optical modulator integrated laser element 1 of this embodiment comprises an optical modulator 3, a laser unit 4, and an optical waveguide unit 5. The laser unit 4 outputs laser light. The optical modulator 3 receives laser light from the laser unit 4 and modulates the laser light. The optical waveguide unit 5 is provided between the optical modulator 3 and the laser unit 4 and optically couples the optical modulator 3 to the laser unit 4. The optical modulator 3, the laser unit 4, and the optical waveguide unit 5 are monolithically mounted on a common substrate 2. The laser unit 4 and the optical waveguide unit 5 are adjacent to each other on the substrate 2 along the optical waveguide direction D1. The optical waveguide unit 5 and the optical modulator 3 are adjacent to each other on the substrate 2 along the optical waveguide direction D1.
[0022] The optical modulator 3 is an electro-absorption type optical modulator and comprises a substrate 2, a contact layer 51, a first cladding layer 52, an electro-absorption layer 53, a second cladding layer 54, and a second conductive contact layer (not shown). The substrate 2 is semi-insulating or insulating. The substrate 2 includes, for example, a III-V compound semiconductor, and in one example, an InP substrate.
[0023] The contact layer 51 is provided on the substrate 2 and has a first conductivity type. The first conductivity type is, for example, n-type. The contact layer 51 includes a semiconductor that is lattice-matched with the substrate 2, and in one example, n ＋It is an InP layer of the type. The composition of the contact layer 51 may be the same as that of the first cladding layer 52. In this embodiment, the contact layer 51 is provided on the substrate 2 and is in contact with the substrate 2. The thickness of the contact layer 51 is smaller than that of the first cladding layer 52. The thickness of the contact layer 51 is not less than 0.1 µm and not more than 1 µm, or not less than 0.1 µm and not more than 0.5 µm, and is 0.4 µm in one example. The impurity concentration of the contact layer 51 is higher than that of the first cladding layer 52. The impurity concentration of the contact layer 51 is 1×10 １８ cm －３ -3 or more and 2×10 １９ cm －３ -3 or less, or 1×10 １８ cm －３ -3 or more and 1×10 １９ cm －３ -3 or less, and is 8×10 １８ cm －３ -3 in one example.
[0024] The first cladding layer 52 is provided on the contact layer 51 and has a first conductivity type. The first cladding layer 52 comprises a semiconductor lattice-matched with the contact layer 51, and in one example, it is an n-type InP layer. In this embodiment, the first cladding layer 52 is provided on the contact layer 51. The thickness of the first cladding layer 52 is not less than 0.5 µm and not more than 2 µm, and is 1.2 µm in one example. The impurity concentration of the first cladding layer 52 is 1×10 １７ cm －３ -3 or more and 2×10 １８ cm －３ -3 or less, and is 5×10 １７ cm －３ -3 in one example.
[0025] The field absorption layer 53 is an optical waveguide layer in the optical modulator 3. The field absorption layer 53 is provided on the first cladding layer 52. The band gap of the field absorption layer 53 is smaller than the band gap of the first cladding layer 52. The refractive index of the field absorption layer 53 is greater than the refractive index of the first cladding layer 52. The field absorption layer 53 includes a semiconductor that is lattice-matched with the first cladding layer 52. The field absorption layer 53 may have a multiple quantum well structure. In one example, the field absorption layer 53 is made up of alternating stacks of InGaAsP layers and InGaAlAs layers. In this embodiment, the field absorption layer 53 is provided on the first cladding layer 52 and is in contact with the first cladding layer 52. The distance between the field absorption layer 53 and the etching stop layer 60 or contact layer 51 is, for example, 0.5 μm or more and 2 μm or less, and in one embodiment it is 1.45 μm. By setting this distance to 0.5 μm or more, it is possible to suppress the decrease in light output due to light absorption in the etching stop layer 60 or the contact layer 51.
[0026] The second cladding layer 54 is provided on the field absorption layer 53 and has a second conductivity type. The second conductivity type is, for example, p-type. The second cladding layer 54 includes a semiconductor that is lattice-matched with the field absorption layer 53, and in one example is a p-type InP layer. In this embodiment, the second cladding layer 54 is provided on the field absorption layer 53 and is in contact with the field absorption layer 53. The contact layer of the second conductivity type is provided on the second cladding layer 54. The impurity concentration of the contact layer of the second conductivity type is greater than the impurity concentration of the second cladding layer 54.
[0027] As shown in Figure 6, the optical modulator 3 further comprises an etching stop layer 60. The etching stop layer 60 is provided between the contact layer 51 and the first cladding layer 52. In one example, the etching stop layer 60 is in contact with both the contact layer 51 and the first cladding layer 52. Under certain etching conditions, the etching rate of the etching stop layer 60 is smaller than the etching rate of the first cladding layer 52. The etching stop layer 60 is used to stop etching due to the difference in etching rates with the first cladding layer 52 when forming an ohmic electrode that contacts the contact layer 51. The etching stop layer 60 has a first conductivity type. When the first cladding layer 52 is an n-type InP layer, the etching stop layer 60 is, for example, a ternary or quaternary III-V compound semiconductor layer. In one embodiment, the etching stop layer 60 is n ＋ This is an InGaAsP layer of type [type].
[0028] The first cladding layer 52, the field absorption layer 53, the second cladding layer 54, and the second conductive contact layer form a mesa structure 58 extending along the optical waveguide direction D1. The width of the mesa structure 58 in the transverse direction is, for example, 1.4 μm. The base end of the mesa structure 58 is located in the first cladding layer 52. Therefore, the lower parts of the contact layer 51, the etching stop layer 60, and the first cladding layer 52 extend outside the mesa structure 58 in a plan view.
[0029] The contact layer 51 includes a sub-mesa region 51a and an electrode contact region 51b. The sub-mesa region 51a includes a region located between the mesa structure 58 and the substrate 2, and its surrounding region. The electrode contact region 51b is a region exposed from the first cladding layer 52 and the etching stop layer 60 for electrical connection with the wiring 73, which will be described later. The electrode contact region 51b is adjacent to the sub-mesa region 51a.
[0030] The optical modulator 3 further comprises an insulating film 81 and a buried region 82. The buried region 82 is provided on both sides of the mesa structure 58 and buries both side surfaces of the mesa structure 58. The buried region 82 has insulating or semi-insulating properties. The buried region 82 is, for example, a semi-insulating InP region. The material of the buried region 82 may be the same as or different from the material of the substrate 2. The insulating film 81 is provided on the buried region 82 and covers the buried region 82. The insulating film 81 is, for example, SiO ２ or a silicon compound film such as SiN.
[0031] The optical modulator 3 comprises a first electrode pad 71, a second electrode pad 72, a wiring 73, and a wiring 74. The first electrode pad 71, the second electrode pad 72, the wiring 73, and the wiring 74 are metal films, and in one example, are gold (Au) films. As shown in FIGS. 4 and 5, the first electrode pad 71 and the second electrode pad 72 are provided above the buried region 82 and on the insulating film 81. As shown in FIG. 1, the first electrode pad 71 and the second electrode pad 72 are arranged laterally with respect to the mesa structure 58. Further, the first electrode pad 71 and the second electrode pad 72 are arranged side by side along the optical waveguide direction D1. The planar shape of the first electrode pad 71 and the second electrode pad 72 is, for example, circular.
[0032] The wiring 73 electrically connects the first electrode pad 71 and the contact layer 51. The wiring 73 includes a portion (ohmic electrode 731) provided on the electrode contact region 51b of the contact layer 51 and in ohmic contact with the electrode contact region 51b, and a portion connected to the first electrode pad 71. A portion of the wiring 73 other than the portion in contact with the electrode contact region 51b is provided on the insulating film 81. The wiring 74 electrically connects the second electrode pad 72 and the contact layer of the second conductivity type. The wiring 74 includes a portion (ohmic electrode 741) provided on the contact layer of the second conductivity type and in ohmic contact with the contact layer of the second conductivity type, and a portion connected to the second electrode pad 72. A portion of the wiring 74 other than the portion in contact with the contact layer of the second conductivity type is provided on the insulating film 81. The wiring 73 and the wiring 74 may be air bridges.
[0033] A recess 50 is formed in the buried region 82. The recess 50 penetrates the buried region 82, the first clad layer 52, the etching stop layer 60, and the contact layer 51. The recess 50 is formed to partially remove the contact layer 51 on the substrate 2. In FIG. 1, a region 61 from which the contact layer 51 has been removed is indicated by halftone dots. That is, the region 61 is a region where the recess 50 is formed. As shown in FIG. 1, an integrated region composed of the under-mesa region 51a and the electrode contact region 51b is sandwiched between the regions 61 in the lateral direction. In the present embodiment, the region 61 reaches the light emission end face 1a. Accordingly, the under-mesa region 51a is sandwiched between the regions 61 up to the light emission end face 1a. An insulating film 81 is provided on the surface of the recess 50.
[0034] A protective mesa structure 62 is provided on the substrate 2. The protective mesa structure 62 is a region where the contact layer 51 is not removed, other than the under-mesa region 51a and the electrode contact region 51b. The recess 50 is not formed in the protective mesa structure 62. Therefore, the contact layer 51, the etching stop layer 60, the first clad layer 52, and the buried region 82 remain, and these form the protective mesa structure 62. The protective mesa structure 62 has an electrically isolated island shape.
[0035] Figure 8 is a schematic diagram showing a cross-section along the line VIII-VIII in Figure 1. The laser unit 4 has the same configuration as the optical modulator 3, except for the following: The laser unit 4 has an active layer 55 instead of the field absorption layer 53 of the optical modulator 3. The active layer 55 is the optical waveguide layer in the laser unit 4. The first cladding layer 52, the active layer 55, the second cladding layer 54, and the second conductive contact layer form a mesa structure 57 extending along the optical waveguide direction D1. The laser unit 4 has an electrode pad 75 instead of the first electrode pad 71 and wiring 73 of the optical modulator 3. The laser unit 4 has an electrode pad 76 instead of the second electrode pad 72 and wiring 74 of the optical modulator 3. The electrode pad 75 is provided above the embedded region 82 and on the insulating film 81, and extends to one side of the mesa structure 57. The electrode pad 75 includes an ohmic electrode 751 that makes ohmic contact with the exposed portion of the contact layer 51 via the etching stop layer 60. The electrode pad 76 is provided above the embedded region 82 and on the insulating film 81, and extends to the other side of the mesa structure 57. The electrode pad 76 includes an ohmic electrode 761 that makes ohmic contact with the exposed portion of the second conductivity type contact layer. As shown in Figure 1, in this embodiment, no region 61 (recess 50) is formed in the laser section 4. Therefore, the mesa sub-region 51a extends to both sides of the mesa structure 57.
[0036] Figure 9 is a schematic diagram showing a cross-section along the IX-IX line in Figure 1. The optical waveguide section 5 has the same configuration as the optical modulator 3, except for the following: The optical waveguide section 5 has an optical waveguide core layer 63 instead of the field absorption layer 53 of the optical modulator 3. The optical waveguide core layer 63 is the optical waveguide layer in the optical waveguide section 5. The first cladding layer 52, the optical waveguide core layer 63, the second cladding layer 54, and the second conductivity type contact layer form a mesa structure 64 extending along the optical waveguide direction D1. The optical waveguide section 5 does not have electrode pads or wiring. Therefore, the upper surface of the second conductivity type contact layer is covered by an insulating film 81, and the contact layer 51 is not exposed from the first cladding layer 52.
[0037] In the optical waveguide section 5, the contact layer 51 includes a sub-mesa region 51a located between the mesa structure 64 and the substrate 2. However, the width W1 of the sub-mesa region 51a of the optical waveguide section 5 in the lateral direction is smaller than both the width W2 of the sub-mesa region 51a of the laser section 4 in the lateral direction (see Figure 8) and the width W3 of the sub-mesa region 51a of the optical modulator 3 in the lateral direction (see Figure 4).
[0038] In the optical waveguide section 5, semi-insulating or insulating embedded regions 82 embed both sides of the mesa structure 64. The mesa sub-region 51a is sandwiched laterally between regions 61 from which the contact layer 51 has been removed. That is, both ends of the mesa sub-region 51a in the later direction are formed by recesses 50 that penetrate the embedded regions 82 and the contact layer 51.
[0039] The optical waveguide section 5 also has a protective mesa structure 62, similar to the optical modulator 3. The protective mesa structure 62 is provided on both sides of the mesa sub-region 51a in the lateral direction. The height of these protective mesa structures 62, with respect to the upper surface of the substrate 2, is greater than the height of the optical waveguide core layer 63, with respect to the upper surface of the substrate 2. In one example, the height of the protective mesa structure 62 is approximately equal to the height of the insulating film 81 on the mesa structure 64, with respect to the substrate 2. The protective mesa structure 62 is not limited to a structure including a contact layer 51 as shown in the illustrated example, but may be configured, for example, by directly forming an embedded region 82 on the substrate 2.
[0040] A method for manufacturing the optical modulator integrated laser element 1 will be described below. The following description will explain the manufacturing method of the optical modulator 3, but the manufacturing methods for the laser section 4 and the optical waveguide section 5 are the same. First, as shown in Figure 10, a contact layer 51, an etching stop layer 60, a first cladding layer 52, an electric field absorption layer 53, a second cladding layer 54, and a second conductive contact layer are epitaxially grown on the substrate 2. Next, as shown in Figure 11, an etching mask 80 is formed, and a mesa structure 58 is formed by etching (for example, dry etching).
[0041] Next, as shown in Figure 12, an embedded region 82 is deposited on the substrate 2 excluding the mesa structure 58 to embed the mesa structure 58. After embedding, the etching mask 80 is removed. Next, as shown in Figure 13, an etching mask 83 having an opening 831 is formed in the vicinity of the mesa structure 58. Then, etching (e.g., wet etching) is performed on the embedded region 82 and the first cladding layer 52 through the opening 831 to expose the etching stop layer 60. After etching, the etching mask 83 is removed. When the first cladding layer 52 is an InP layer and the etching stop layer 60 is an InGaAsP layer, for example, a hydrochloric acid-based etching solution can be used as the etchant. In this step, etching is stopped by utilizing the difference in etching rates between the etching stop layer 60 and the first cladding layer 52. The steps shown in Figures 11 and 13 correspond to the "step of etching the second conductive contact layer, the second cladding layer, the field absorption layer, and the first cladding layer to expose the etching stop layer" of this disclosure.
[0042] Next, as shown in Figure 14, after removing the etching mask 83, an etching mask 84 is formed having openings 841 in the exposed portion of the etching stop layer 60 and the peripheral region of the mesa structure 58. Then, a recess 50 is formed. That is, etching (for example, dry etching or wet etching) is performed on the embedded region 82, the first cladding layer 52, the etching stop layer 60, and the contact layer 51 through the openings 841 to expose the substrate 2 (etch the substrate 2). Next, as shown in Figure 15, after removing the etching mask 84, an insulating film 81 is formed by plasma CVD. A resist mask (not shown) with an opening on the exposed portion of the etching stop layer 60 is formed, and a part of the insulating film 81 is opened by fluorine-based dry etching or wet etching with a hydrofluoric acid-based etching solution. An ohmic electrode 731 that makes ohmic contact with the contact layer 51 via the etching stop layer 60 is formed by deposition and lift-off method. Similarly, a portion of the insulating film 81 on the second conductivity type contact layer is opened to form an ohmic electrode 741 that makes ohmic contact with the second conductivity type contact layer. Next, as shown in Figure 16, a resist opening (not shown) is formed on the portion of the insulating film 81 above the ohmic electrode 731 and the portion above the ohmic electrode 741, and the first electrode pad 71, the second electrode pad 72, the wiring 73, and the wiring 74 are formed on the insulating film 81 by a plating method. Through these steps, the optical modulator 3, the laser section 4, and the optical waveguide section 5 are manufactured. As shown in Figure 17, the ohmic electrode 731 can also be formed by the following procedure. After opening a portion of the insulating film 81, the etching stop layer 60 is etched with a hydrogen peroxide-based etching solution to expose the contact layer 51. Next, the ohmic electrode 731 is formed on the contact layer 51 by deposition and lift-off method.
[0043] Butt joint structures may be formed between the laser section 4 and the optical waveguide section 5, and between the optical waveguide section 5 and the optical modulator 3. That is, in the initial epitaxial growth process, after growing up to the second conductivity type contact layer of the laser section 4, the areas of the second conductivity type contact layer, the second cladding layer 54, and the active layer 55 other than the area that will become the laser section 4 are etched and removed. Then, the optical waveguide core layer 63, the second cladding layer 54, and the second conductivity type contact layer are selectively grown in these other areas. Then, the areas of the second conductivity type contact layer, the second cladding layer 54, and the optical waveguide core layer 63 other than the area that will become the optical waveguide section 5 are etched and removed. Then, the field absorption layer 53, the second cladding layer 54, and the second conductivity type contact layer are selectively grown in these other areas. After that, mesa structures 58, 57, and 64 are formed by etching (e.g., dry etching). With this structure, the composition of the electric field absorption layer 53, the composition of the active layer 55, and the composition of the optical waveguide core layer 63 can be made different from each other.
[0044] The effects obtained by the optical modulator integrated laser element 1 according to this embodiment, as described above, will now be explained. In the optical modulator integrated laser element 1 of this embodiment, a contact layer 51 having the same conductivity type as the first cladding layer 52 and a higher impurity concentration than the first cladding layer 52 is provided between the substrate 2 and the first cladding layer 52. The contact layer 51 is electrically connected to the first electrode pad 71 via an ohmic electrode. With this structure, the connection resistance between the ohmic electrode and the first cladding layer 52 can be reduced.
[0045] As in this embodiment, the optical modulator integrated laser element 1 may include a first conductivity type etching stop layer 60 provided between the contact layer 51 and the first cladding layer 52, having a lower etching rate than the first cladding layer 52. In this case, even if the etching rate of the contact layer 51 is the same as that of the first cladding layer 52, etching to bring the contact layer 51 into contact with the ohmic electrode 731 can be accurately stopped before reaching the contact layer 51.
[0046] As in this embodiment, the thickness of the contact layer 51 may be 0.1 μm or more and 0.5 μm or less. By forming the contact layer 51 relatively thinly in this way, the isolation resistance between the laser unit 4 and the optical modulator 3 can be increased (for example, 1 kΩ or more), thereby reducing interference between the modulation signal of the optical modulator 3 and the bias current of the laser unit 4.
[0047] As in this embodiment, the first impurity concentration is 1 × 10⁻⁶ １８ cm －３ The above 1 x 10 １９ cm －３ The following may also be true. By having such a high impurity concentration in the contact layer 51, the connection resistance between the ohmic electrode 731 and the contact layer 51 can be effectively reduced.
[0048] According to the manufacturing method of this embodiment, even when the etching rate of the contact layer 51 is the same as that of the first cladding layer 52, etching to bring the contact layer 51 into contact with the ohmic electrode can be accurately stopped before reaching the contact layer 51.
[0049] (First Modification) Figure 18 is a plan view showing an optical modulator integrated laser element 1A according to the first modification. Figure 19 is a cross-sectional view along the line XIX-XIX in Figure 18. The optical modulator integrated laser element 1A of this modification differs from the above embodiment in that it includes an optical modulator 3A instead of the optical modulator 3, and optical waveguide sections 5A and 5B instead of the optical waveguide section 5, and is otherwise consistent with the above embodiment. The optical modulator 3A includes a first optical modulator 31 and a second optical modulator 32 that are optically coupled in series. As shown in Figure 19, the laser section 4, optical waveguide section 5A, first optical modulator 31, optical waveguide section 5B, and second optical modulator 32 are arranged in this order along the optical waveguide direction D1.
[0050] As shown in Figure 19, the optical waveguide sections 5A and 5B have an optical waveguide core layer 63. The first optical modulator 31 has an electric field absorption layer 53A, and the second optical modulator 32 has an electric field absorption layer 53B. The optical waveguide core layer 63 of the optical waveguide section 5B is interposed between the electric field absorption layer 53A and the electric field absorption layer 53B. That is, the light output from the first optical modulator 31 is guided to the second optical modulator 32 via the optical waveguide core layer 63.
[0051] The optical modulator 3A has electrode pads 71A, 71B, and 72A, and wiring 73A, 73B, 74A, and 74B instead of the first electrode pad 71, second electrode pad 72, wiring 73, and wiring 74 of the above embodiment.
[0052] Electrode pad 72A is an electrode pad belonging to the first optical modulator 31. Electrode pad 71B is an electrode pad belonging to the second optical modulator 32. Electrode pad 71A is an electrode pad belonging to both the first optical modulator 31 and the second optical modulator 32. In the first optical modulator 31, electrode pad 71A corresponds to the first electrode pad, and electrode pad 72A corresponds to the second electrode pad. In the second optical modulator 32, electrode pad 71B corresponds to the first electrode pad, and electrode pad 71A corresponds to the second electrode pad.
[0053] The electrode pads 71A, 71B, and 72A, and the wirings 73A, 73B, 74A, and 74B are metal films, in one example being gold (Au) films. The electrode pads 71A, 71B, and 72A are provided above the embedded region 82 and on the insulating film 81. As shown in Figure 18, electrode pad 71A is positioned on one side of the mesa structure 58. Electrode pads 71B and 72A are positioned on the other side of the mesa structure 58. The planar shape of the electrode pads 71A, 71B, and 72A is, for example, circular.
[0054] Wirings 73A and 74A belong to the first optical modulator 31. Wiring 73A electrically connects the electrode pad 71A to the contact layer 51 of the first optical modulator 31. Wiring 73A includes a portion that is provided on the electrode contact area 51b of the contact layer 51 and makes ohmic contact with the electrode contact area 51b, and a portion that is connected to the electrode pad 71A. Wiring 74A electrically connects the electrode pad 72A to the second conductivity type contact layer of the first optical modulator 31.
[0055] Wirings 73B and 74B are wirings belonging to the second optical modulator 32. Wiring 73B electrically connects the electrode pad 71B to the contact layer 51 of the second optical modulator 32. Wiring 73B includes a portion that is provided on the electrode contact area 51b of the contact layer 51 and makes ohmic contact with the electrode contact area 51b, and a portion that is connected to the electrode pad 71B. Wiring 74B electrically connects the electrode pad 71A to the second conductivity type contact layer of the second optical modulator 32.
[0056] Even when the optical modulator 3A has a first optical modulator 31 and a second optical modulator 32, as in this modified example, the same effects as in the above embodiment can be obtained. In addition, as in this modified example, by interposing an optical waveguide core layer 63 made of a material with a different composition from the electric field absorption layer 53A and the electric field absorption layer 53B between the electric field absorption layer 53A and the electric field absorption layer 53B, the optical coupling between the first optical modulator 31 and the second optical modulator 32 can be enhanced by appropriately selecting the composition of the optical waveguide core layer 63.
[0057] Figure 20 is a cross-sectional view of a modified optical modulator integrated laser element 1AA, showing the same cross-section as in Figure 19. The optical modulator integrated laser element 1AA differs from the optical modulator integrated laser element 1A in that the field absorption layer of the first optical modulator 31 and the field absorption layer of the second optical modulator 32 are formed continuously as a field absorption layer 53 of the same composition. That is, in the optical modulator integrated laser element 1AA, there is no optical waveguide core layer 63 interposed between the first optical modulator 31 and the second optical modulator 32, and the field absorption layer 53 extends continuously from the first optical modulator 31 to the second optical modulator 32. Of the field absorption layer 53, the portion located below the wiring 74A functions as the optical waveguide layer of the first optical modulator 31, and the portion located below the wiring 74B functions as the optical waveguide layer of the second optical modulator 32. The light output from the first optical modulator 31 is guided to the second optical modulator 32 through the continuous field absorption layer 53. With this structure, compared to the structure in Figure 19 in which the field absorption layer 53A and the field absorption layer 53B are optically coupled via optical waveguide core layers 63 of different compositions, reflection and radiation at the interface between the field absorption layer and the optical waveguide core layer 63 can be reduced, thereby reducing optical loss. Even with this structure, the same effects as the optical modulator integrated laser element 1A can be obtained.
[0058] (Second Modification) Figure 21 is a plan view showing an optical modulator integrated laser element 1B according to the second modification. The optical modulator integrated laser element 1B has the same configuration as the optical modulator integrated laser element 1 of the above embodiment, in addition to further comprising a semiconductor optical amplifier (SOA) 7. The semiconductor optical amplifier 7 is monolithically provided on a substrate 2 common to the optical modulator 3 and the laser unit 4. The semiconductor optical amplifier 7 is provided on the opposite side of the laser unit 4 from the optical modulator 3 and is adjacent to the optical modulator 3 in the optical waveguide direction D1. The semiconductor optical amplifier 7 receives the modulated laser light from the optical modulator 3, amplifies the modulated laser light, and outputs it. The semiconductor optical amplifier 7 has an electrode pad 77 that makes ohmic contact with a contact layer 51 and an electrode pad 78 that makes ohmic contact with a second conductivity type contact layer. The same effects as the above embodiment can be obtained in this modification as well. The semiconductor optical amplifier 7 may also be provided in the first modification described above.
[0059] The optical modulator integrated laser element and its manufacturing method according to this disclosure are not limited to the embodiments and modifications described above, and various other modifications are possible. For example, in the embodiments and modifications described above, the first conductivity type is n-type and the second conductivity type is p-type, but the first conductivity type may be p-type and the second conductivity type may be n-type.
[0060] 1, 1A, 1AA, 1B... Optical modulator integrated laser element 1a... Light output end face 2... Substrate 3, 3A... Optical modulator 4... Laser section 5, 5A, 5B... Optical waveguide section 7... Semiconductor optical amplifier 31... First optical modulator 32... Second optical modulator 50... Recess 51... Contact layer 51a... Sub-mesa region 51b... Electrode contact region 52... First cladding layer 53, 53A, 53B... Field absorption layer 54... Second cladding layer 55... Active layer 57, 58, 64... Mesa structure 60... Etching stop layer 61... Region 62... Protective mesa structure 63... Optical waveguide core layer 71... First electrode pad 71A, 71B, 72A, 75, 76, 77, 78... Electrode pad 72... Second electrode pad 73, 73A, 73B, 74, 74A, 74B...Wiring 80...Etching mask 81...Insulating film 82...Buried area 83, 84...Etching mask 731, 741, 751, 761...Ohmic electrode 831, 841...Aperture D1...Optical guidance direction W1, W2, W3...Width
Claims
1. An optical modulator integrated laser element comprising: a semi-insulating or insulating substrate; a laser unit provided on the substrate and outputting laser light; and an electro-absorption type optical modulator provided on the substrate and receiving the laser light from the laser unit, wherein the optical modulator has, in order, a first conductivity type contact layer having a first impurity concentration; a first conductivity type cladding layer having a second impurity concentration smaller than the first impurity concentration; an electro-absorption layer; a second cladding layer; and a second conductivity type contact layer on the substrate, wherein the optical modulator further comprises: a first electrode pad electrically connected to the first conductivity type contact layer via an ohmic electrode; and a second electrode pad electrically connected to the second conductivity type contact layer via an ohmic electrode.
2. The optical modulator integrated laser element according to claim 1, further comprising an etching stop layer of the first conductivity type provided between the first conductivity type contact layer and the first cladding layer, the etching stop layer having a lower etching rate than the first cladding layer.
3. The optical modulator integrated laser element according to claim 1 or claim 2, wherein the thickness of the first conductive contact layer is 0.1 μm or more and 0.5 μm or less.
4. The concentration of the first impurity is 1 × 10⁻⁶ １８ cm －３ The above 1 x 10 １９ cm －３ The optical modulator integrated laser element according to any one of claims 1 to 3, which is as follows:
5. The optical modulator integrated laser element according to any one of claims 1 to 4, wherein the optical modulator includes a first optical modulator and a second optical modulator.
6. The optical modulator integrated laser element according to claim 5, wherein the first optical modulator has a first field absorption layer on the first cladding layer, and the second optical modulator has a second field absorption layer on the first cladding layer, and the first field absorption layer and the second field absorption layer are made of a material of the same composition and are formed continuously with respect to each other.
7. The optical modulator integrated laser element according to claim 5, wherein the first optical modulator has a first field absorption layer on the first cladding layer, the second optical modulator has a second field absorption layer on the first cladding layer, and a semiconductor layer made of a material with a different composition from the first field absorption layer and the second field absorption layer is interposed between the first field absorption layer and the second field absorption layer.
8. A method for manufacturing an optical modulator integrated laser element according to claim 2, comprising the steps of: growing the first conductivity type contact layer, the etching stop layer, the first cladding layer, the field absorption layer, the second cladding layer, and the second conductivity type contact layer on the substrate; etching the second conductivity type contact layer, the second cladding layer, the field absorption layer, and the first cladding layer to expose the etching stop layer; and forming an electrode on the etching stop layer that makes ohmic contact with the first conductivity type contact layer, wherein in the etching step, etching is stopped by utilizing the difference in etching rates between the etching stop layer and the first cladding layer.
9. A method for manufacturing an optical modulator integrated laser element according to claim 2, comprising the steps of: growing the first conductivity type contact layer, the etching stop layer, the first cladding layer, the field absorption layer, the second cladding layer, and the second conductivity type contact layer on the substrate; etching the second conductivity type contact layer, the second cladding layer, the field absorption layer, and the first cladding layer to expose the etching stop layer; etching the etching stop layer to expose the first conductivity type contact layer; and forming an electrode on the first conductivity type contact layer that makes ohmic contact with the first conductivity type contact layer.