JP3966412B2 - Semiconductor optical device - Google Patents

Description

[0001]
[Technical field to which the invention belongs]
The present invention relates to a semiconductor optical device, for example, a semiconductor optical device having a light control function such as light modulation and light absorption.
[0002]
[Prior art]
One of key elements in high-speed optical communication systems and optical information processing systems is a semiconductor optical device having optical functions such as light modulation and light absorption. As an example of this semiconductor optical device, there is a high mesa type optical modulator which is an optical modulator having a high mesa type optical waveguide structure (see Non-Patent Document 1 below). FIG. 4 is a schematic structural diagram of an example of a conventional high-mesa optical modulator.
[0003]
As shown in the figure, a high-mesa optical modulator 400 includes an n-InP lower cladding layer 402, a multiple quantum well layer 403 as a core layer, a p-InP upper cladding layer 404, a p- The InGaAsP contact layer 405 is sequentially laminated, and the structure of the optical waveguide composed of the n-InP lower cladding layer 402, the multiple quantum well layer 403, the p-InP upper cladding layer 404, and the p-InGaAsP contact layer 405 is a high mesa ridge type. The ridge side surface of the optical waveguide is embedded with polyimide 408, and a p-electrode 406 is formed on the upper surface of the p-InGaAsP contact layer 405, and an n-electrode 407 is formed on the lower surface of the n-InP substrate 401. It has a structure.
[0004]
The multiple quantum well layer 403 functions as an absorption layer that absorbs light, and also functions as a core layer having a higher refractive index than the p-InP upper cladding layer 404 and the n-InP lower cladding layer 402.
[0005]
Next, the operation of the high mesa optical modulator 400 will be described. The light incident from the light incident end surface 409 is ON / OFF modulated and propagates from the light emitting end surface opposite to the light incident end surface 409 while propagating through the multiple quantum well layer 403 of the optical waveguide.
[0006]
FIG. 5 is a graph showing the dependence of the light absorption characteristic (absorption coefficient) of the multiple quantum well layer 403 functioning as a light absorption layer on the wavelength λ of incident light. This figure shows the wavelength dependence of the light absorption characteristics when the reverse bias voltage is applied between the p-electrode 406 and the n-electrode 407 (ON) and not applied (OFF). The operating wavelength λs shown in the figure is the wavelength of light incident on the optical modulator 400.
[0007]
As can be seen from the figure, when no reverse bias voltage is applied (OFF), the light absorption coefficient at the operating wavelength λs is sufficiently small, so that incident light is emitted as it is from the light emitting end face (light: ON state).
[0008]
However, when a reverse bias voltage is applied (ON), the light absorption characteristic of the multiple quantum well layer 403 shifts to the longer wavelength side, so that light absorption at the operating wavelength λs increases, and the light emission end face Is not emitted (light: OFF state).
[0009]
That is, in the high mesa optical modulator 400, the light propagating through the multiple quantum well layer 403 of the optical waveguide is turned OFF / ON in accordance with ON / OFF of the reverse bias voltage, so that the electric signal is converted into an optical signal. It becomes possible.
[0010]
In the high mesa type optical modulator 400, both side surfaces of the ridge of the multiple quantum well layer 403 that absorbs light are surrounded by the polyimide 408 having a small relative dielectric constant, and thus the parasitic capacitance C is small. On the other hand, when both sides of the ridge of the multiple quantum well layer 403 are embedded with a semiconductor instead of the polyimide 408, the semiconductor has a high relative dielectric constant, and thus the parasitic capacitance C increases.
[0011]
As a result, the structure embedded with a material such as polyimide having a low relative dielectric constant has a higher modulation frequency limit due to the CR constant limitation defined by the parasitic capacitance C and the load resistance R, and is therefore suitable for high-speed modulation. Yes.
[0012]
[Non-Patent Document 1]
T. Ido, H. Sano, S. Tanaka, DJ Moss, and H. Inoue, Performance of Strained InGaAs / InGlAs Multiple-Quantum-Well Electroabsorption Modulators, `` J. Lightwave Technology '', 1996, vol. 14, No. 10, p2324〜2331
[0013]
[Problems to be solved by the invention]
However, when adopting a structure in which the optical waveguide is embedded with polyimide, there is a problem with respect to heat because the thermal conductivity of polyimide is smaller than that of a semiconductor.
[0014]
That is, when a reverse bias voltage is applied between the electrodes, incident light is absorbed in the multiple quantum well 403, but carriers are generated by this light absorption and flow as current. For this reason, Joule heat represented by the product of the reverse bias voltage V and the generated current I is generated in the optical waveguide portion centering on the multiple quantum well layer 403.
[0015]
FIG. 6 is a graph showing the distribution of Joule heat generated in the width direction of the optical waveguide. When both sides of the ridge of the optical waveguide are surrounded by polyimide 408, the thermal conductivity of polyimide is several orders of magnitude smaller than that of a semiconductor. Therefore, as shown in the figure, the generated Joule heat is generated in the optical waveguide portion. Heat transfer to the outside of the pool hardly occurs.
[0016]
When the temperature of the optical waveguide portion rises due to the Joule heat accumulated in the optical waveguide portion, the light absorption characteristics of the multiple quantum well layer 403 change, and the modulation characteristics deteriorate. Furthermore, when the generated current I is increased by incident light of high power or when a high reverse bias voltage V is applied, the generated Joule heat increases, resulting in deterioration of the element.
[0017]
The present invention has been made in view of the above situation, and efficiently suppresses the temperature rise of the optical waveguide by efficiently dissipating the heat generated in the optical waveguide, while also preventing large power light incidence and high driving voltage. Another object of the present invention is to provide a semiconductor optical device capable of maintaining the optical waveguide, particularly the operating level of its core layer, in a high state. That is, an object of the present invention is to provide a semiconductor optical device with improved stability and reliability.
[0019]
[Means for solving the problems]
A first invention that solves the above-described problem is that an optical waveguide having a core layer and an upper clad layer and a lower clad layer sandwiching the core layer and shaped into a high mesa ridge shape, and power transfer between the lower clad layer In a semiconductor optical device having a first metal electrode to be performed and a second metal electrode to transfer power to and from the upper cladding layer,
The core layer is provided inside the ridge side surface so as to be concave with respect to the ridge side surface of the optical waveguide,
One or both of the first metal electrode and the second metal electrode are provided under the following conditions.
First metal electrodes: Ru provided in contact with the ridge side surface of the lower cladding layer.
Second metal electrode: Ru provided in contact with the ridge side surface of at least the upper cladding layer.
[0020]
By providing a metal electrode having high thermal conductivity on the ridge side surface in the cladding layer provided above and below the core layer, heat generated in the optical waveguide is radiated by the metal electrode. In order to obtain the effect of the present invention, the second metal electrode may be Kere set in contact with the ridge sides of at least the upper cladding layer, but when considering the function as an electrode through the contact layer It goes without saying that a metal electrode is also provided on the upper surface of the high mesa ridge. Further, the width of the core layer in the optical waveguide is made smaller than the ridge width (the width of the optical waveguide), and the core layer is provided on the inner side of the ridge so as to be concave with respect to the ridge side surface. Light loss due to the interaction between the metal electrode provided on the side surface and the optical electric field of light propagating through the core layer is suppressed.
[0021]
When the semiconductor optical element is a light emitting element such as a semiconductor laser, an element that converts an electrical signal into an optical signal, a light modulation element, or the like, the metal electrode is an electrode that supplies power from the outside of the element. Further, when the semiconductor optical element is an element (light receiving element) that converts an optical signal into an electric signal, the metal electrode serves as an electrode for extracting electric power to the outside of the element.
[0022]
A second invention for solving the above-described problems is a semiconductor optical device according to the first invention.
In the semiconductor optical device, the second metal electrode is formed in a U-shaped cross-sectional shape and is brought into contact with the ridge side surface of the upper clad layer, and is also brought into contact with the upper surface of the upper clad layer .
[0023]
By forming the second metal electrode in a U-shaped cross-sectional shape and bringing it into contact with the ridge side surface of the upper cladding layer, the second metal electrode can also be brought into contact with the upper surface of the upper cladding layer, thereby increasing the heat dissipation capability of the second metal electrode. Since the heat dissipation capability is high, the heat generated in the optical waveguide can be efficiently conducted to the outside, and the heat dissipation can be further improved.
[0024]
A third invention for solving the above-described problems is a semiconductor optical device according to the first or second invention.
In the semiconductor optical device, the width of the core layer is smaller than a ridge width of the optical waveguide so that an optical electric field of light guided through the core layer is confined inside in the width direction of the optical waveguide. .
[0025]
The optical electric field of light propagating through the core layer spreads outside the core layer within a range that depends on the wavelength of the light, the scale of the core layer, etc. By making the width smaller than the ridge width of the optical waveguide so as to be confined in the optical waveguide, optical loss due to the interaction between the metal electrode and the optical electric field is suppressed. In the present invention, since the metal electrode is provided in contact with (or close to) the side surface of the ridge, it is desirable that the optical electric field is completely confined without leaking outside the high mesa ridge (optical waveguide). Even if leakage occurs, there is no problem as long as the optical loss is within the allowable range.
[0026]
A fourth invention for solving the above-described problem is a semiconductor optical device according to any one of the first to third inventions,
The core layer, the optical waveguide of the ridge side or al 0. The semiconductor optical device is characterized in that it is arranged inside by 5 μm or more , or the width of the core layer is made 1 μm or less smaller than the ridge width of the optical waveguide .
[0027]
Ridge sides or al 0 of the core layer optical waveguide. The inner layer may be arranged 5 μm or more , or the core layer may be made 1 μm or more smaller than the ridge width of the optical waveguide. Preferably, placed inside the ridge side or al 1 [mu] m or more.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
<First Embodiment>
First, a first embodiment according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic structural diagram of a high-mesa optical modulator according to the first embodiment.
[0029]
As shown in the figure, in the high mesa optical modulator 100 according to the present embodiment, an n-InP lower cladding layer 102, a multiple quantum well layer 103 as a core layer, and a p-InP upper cladding on an n-InP substrate 101. An optical waveguide composed of an n-InP lower cladding layer 102, a multiple quantum well layer 103, a p-InP upper cladding layer 104, and a p-InGaAsP contact layer 105. Etching is performed so that the structure is a high mesa ridge type with a ridge width W.
[0030]
In this etching, the multiple quantum well layer 103 is processed inward by 1 μm from both side surfaces of the ridge so as to be concave with respect to the side surface of the ridge by a selective etching method. That is, the width of the multiple quantum well layer 103 is 2 μm smaller than the ridge width W.
[0031]
Further, in a portion other than the ridge portion (optical waveguide) on the n-InP substrate 101, an n electrode 107 as a “first electrode” is formed so as to be in contact with the ridge side surface of the n-InP lower cladding layer 102. At the same time, the multiple quantum well layer 103, the p-InP upper cladding layer 104, and the p-InGaAsP contact layer 105 are embedded with polyimide 108. Further, a p-electrode 106 as a “second electrode” is formed on the upper surface of the p-InGaAsP contact layer 105.
[0032]
The multiple quantum well layer 103 functions as an absorption layer that absorbs light, and also functions as a core layer having a higher refractive index than the p-InP upper cladding layer 104 and the n-InP lower cladding layer 102.
[0033]
The light incident from the light incident end surface 109 is ON / OFF modulated and propagates from the light emitting end surface opposite to the light incident end surface 109 while propagating through the optical waveguide. Here, when a reverse bias voltage is applied between the electrodes, incident light is absorbed in the multiple quantum well 103, but carriers are generated by this light absorption and flow as current. For this reason, Joule heat represented by the product of the reverse bias voltage V and the generated current I is generated in the optical waveguide portion centering on the multiple quantum well layer 103.
[0034]
On the other hand, in order to cope with high-speed modulation, it is necessary to reduce the junction capacitance, and the contact area of the multiple quantum well layer 103 with the n-InP lower cladding layer 102 or the p-InP upper cladding layer 104 is, for example, the width It is necessary to reduce it to about 1 μm and a length of 75 μm or less (at a modulation frequency of 50 GHz).
[0035]
For this reason, the photocurrent per unit area at the contact surface with the upper and lower cladding layers 104 and 102 of the multiple quantum well layer 103 increases as the modulation frequency increases, and the amount of heat generated per unit area increases.
[0036]
However, in this embodiment, the n electrode 107 is formed so as to be in contact with the ridge side surface of the n-InP lower cladding layer 102. For this reason, heat generated in the optical waveguide portion can be efficiently conducted to the outside and the heat dissipation can be greatly improved as compared with a conventional fully embedded modulator using only polyimide (see FIG. 4). Can do. This is because the n electrode 107, which is a metal electrode, has an extremely high thermal conductivity compared to polyimide, semiconductor, and the like.
[0037]
FIG. 2 is a graph showing the distribution of Joule heat generated in the width direction of the optical waveguide. As shown in the figure, compared to the conventional optical modulator in which Joule heat stays in the ridge portion, the optical modulator according to this embodiment efficiently dissipates heat to the outside of the ridge portion. The residence of Joule heat in the portion can be prevented. As a result, it is possible to suppress the deterioration and damage of the optical waveguide due to heat generated during operation.
[0038]
In this embodiment, the n-electrode 107 is provided so as to be in contact with the ridge side surface of the n-InP lower cladding layer 102. However, even if the n-electrode 107 is provided in the immediate vicinity of the ridge side without contacting, the heat dissipation effect by the electrode can be obtained. Can do.
[0039]
Further, in this embodiment, so that the concave of the multiple quantum well layer 103 relative to the ridge sides of the optical waveguide, since the pressurized Engineering inwardly from the ridge sides by 1 [mu] m, multiplexing the optical field distribution of the incident light It can be confined strongly in the vicinity of the quantum well layer 103 and prevented from expanding beyond the ridge width W of the optical waveguide.
[0040]
As a result, even when the n-electrode 107 is formed so as to be in contact with the ridge side surface of the n-InP lower cladding layer 102, the influence of the n-electrode 107 on the optical electric field can be suppressed and the light passes through the optical modulator. It is possible to suppress optical loss of the light to be transmitted.
[0041]
That is, the optical electric field of light propagating through the multiple quantum well layer 103 spreads outside the multiple quantum well layer 103 within a range depending on the wavelength of the light, the scale of the multiple quantum well layer 103, and the like. The optical loss is reduced by the interaction between the metal electrode and the optical electric field by setting the width of the electrode to “a width smaller than the ridge width of the optical waveguide so that the optical electric field is confined inside the optical waveguide in the width direction”. be able to.
[0042]
When the width of the multi-quantum well layer 103 is set to be approximately the same as the ridge width W, light absorption and light scattering by the metal occur due to the interaction between the n-electrode 107 which is a metal electrode and the optical electric field, and an extremely large amount of 10 dB or more. A large optical loss will occur. If the width of the multiple quantum well layer 103 is processed inward by 0.5 μm or more from both side surfaces of the ridge so as to be concave with respect to the side surface of the ridge, the portion of the electric field distribution of the optical electric field becomes slightly n-electrode. It is suppressed to the extent that it interacts with 107, and the deterioration of characteristics and the increase in optical loss are greatly reduced, and it is almost eliminated by setting it to 1 μm or more.
[0043]
In addition, since the periphery of the optical waveguide portion is embedded with polyimide 108 having a small relative dielectric constant, it is possible to suppress the additional capacitance that is parasitic directly under the pad portion 110, which can be realized at an operation frequency of 50 GHz. Has contributed.
[0044]
<Second Embodiment>
Next, a second embodiment according to the present invention will be described in detail with reference to the drawings. FIG. 3 is a schematic structural diagram of a high-mesa optical modulator according to the second embodiment.
[0045]
As shown in the figure, in the high mesa type optical modulator 300 according to the present embodiment, an n-InP lower clad layer 302, a multiple quantum well layer 303 as a core layer, and a p-InP upper clad on an n-InP substrate 301. Layer 304 and p-InGaAsP contact layer 305 are sequentially stacked, and an optical waveguide composed of n-InP lower cladding layer 302, multiple quantum well layer 303, p-InP upper cladding layer 304, and p-InGaAsP contact layer 305 is formed. Etching is performed so that the structure is a high mesa ridge type with a ridge width W.
[0046]
During this etching, the multiple quantum well layer 303 is processed inward by 1 μm from both side surfaces of the ridge so as to be concave with respect to the side surface of the ridge by a selective etching method. That is, the width of the multiple quantum well layer 303 is 2 μm smaller than the ridge width W.
[0047]
Further, in a portion other than the optical waveguide portion on the n-InP substrate 301, an n electrode 307 as a “first electrode” is formed so as to be in contact with the ridge side surface of the n-InP lower cladding layer 302, and A p-electrode 306, which is a “second electrode”, is formed so as to cover not only the upper surface of the p-InGaAsP contact layer 305 but also both ridge side surfaces of the upper portion of the optical waveguide, and a multiple quantum well layer 303 and a p-InP upper cladding layer The portions of the 304 and p-electrodes 306 that cover both side surfaces of the ridge above the optical waveguide are embedded with polyimide 308.
[0048]
In the present embodiment, the shape of the p-electrode 306 is different from that of the first embodiment, and the shape is such that it covers not only the upper surface of the p-InGaAsP contact layer 305 but also the ridges on both sides of the optical waveguide. Therefore, the p-electrode 306 is in contact with the upper surface and both ridge side surfaces of the p-InGaAsP contact layer 305 constituting the upper portion of the optical waveguide and the ridge side surfaces above the p-InP upper cladding layer 304.
[0049]
As a result, since the heat dissipation capability of the p-electrode 306 is higher than that of the first embodiment, the heat generated in the optical waveguide centering on the multiple quantum well layer 303 can be efficiently conducted to the outside, and the heat dissipation is further improved. Can be improved.
[0050]
In this embodiment, the p-electrode 306 is provided so as to be in contact with both side surfaces of the ridge above the optical waveguide. However, even if it is provided in the immediate vicinity of the side surface of the ridge without contact, the heat radiation effect by the electrode can be obtained.
[0051]
Similarly to the first embodiment, since the multiple quantum well layer 303 is processed inward by 1 μm from both side surfaces of the ridge so as to be concave with respect to the side surface of the ridge, the p electrode 306 is formed as a p-InGaAsP contact layer. Even when it is formed so as to cover not only the upper surface of 305 but also both sides of the ridge above the optical waveguide, the influence of the p-electrode 306 on the optical electric field can be suppressed, and the optical loss of light passing through the optical modulator can be reduced. It becomes possible to suppress.
[0052]
In the first and second embodiments, the periphery of the optical waveguide is directly embedded with polyimide. However, the present invention is not limited to this. For example, the periphery of the optical waveguide is thin silicon oxide film, silicon nitride film, or nitride. A structure may be adopted in which an insulating film such as an aluminum film is coated and then embedded with polyimide. Further, as a material other than polyimide, for example, a low dielectric constant material such as BCB (benzocyclobutene) resin may be embedded.
[0053]
Further, since the n-InP substrate is used as the substrate, an n electrode may be formed on the back surface of the substrate and used as the voltage application electrode. Since the resistance of the n-InP substrates 101 and 301 is about 0.1Ω, the n-electrodes 107 and 307 and the n-electrode formed on the back surface of the substrate are almost equipotential. In actual use, when the optical modulators 100 and 300 are mounted on the carrier, for example, the n electrodes 107 and 307 and the n electrode formed on the back surface of the substrate are connected by a conductive paste or the like, and used as a current path. The n-electrodes 107 and 307 are also used.
[0054]
Further, even if the structure is such that a semi-insulating InP substrate is used as a substrate, an n-InP contact layer or an n-InGaAsP contact layer is provided on the substrate, and an n-electrode is formed on the contact layer. Characteristics can be obtained.
[0055]
In the first and second embodiments, the light modulation element having a light modulation function in which the core layer is a multiple quantum well layer has been described. However, the present invention is not limited to this, and the core layer has a light receiving function. The same electrode structure and core layer structure can be applied to a light-receiving element having a light emitting element and a light-emitting element having a light emitting function.
[0056]
In the first and second embodiments, a single semiconductor electroabsorption modulator has been described. However, even in the case of an element in which a semiconductor electroabsorption modulator and other semiconductor devices are integrated, electroabsorption modulation is used. The structure according to the present invention may be adopted as the structure of the vessel portion.
[0057]
【The invention's effect】
As described above, in the semiconductor optical device of the present invention, both the Ru provided by contacting a metal electrode on the ridge side of the optical waveguide, an inner optical field of light guided through the core layer in the width direction of the optical waveguide The core layer was made smaller than the ridge width of the optical waveguide so as to be confined in the optical waveguide.
[0058]
As a result, it is possible to efficiently dissipate the heat generated in the optical waveguide centering on the core layer to the outside to suppress the temperature rise of the core layer, and to prevent characteristic deterioration and element damage due to the heat of the semiconductor optical device. Can be prevented.
[0059]
Furthermore, the operating level of the optical waveguide, especially the core layer, is guaranteed at a high level against the increase in Joule heat that occurs when the generated current increases due to incident light of high power or when a high reverse bias voltage is applied. In addition, the operational stability and reliability of the element can be improved.
[Brief description of the drawings]
FIG. 1 is a schematic structural diagram of a high-mesa optical modulator according to a first embodiment.
FIG. 2 is a graph showing Joule heat distribution in the width direction of the optical waveguide.
FIG. 3 is a schematic structural diagram of a high-mesa optical modulator according to a second embodiment.
FIG. 4 is a schematic structural diagram of an example of a conventional high-mesa optical modulator.
FIG. 5 is a graph showing the wavelength dependence of the light absorption characteristics of a multiple quantum well layer.
FIG. 6 is a graph showing Joule heat distribution in the width direction of the optical waveguide.
[Explanation of symbols]
100,300,400 Hi-Mesa optical modulator
101,301,401 n-InP substrate
102,302,402 n-InP lower cladding layer
103,303,403 Multiple quantum well layers
104,304,404 p-InP upper cladding layer
105,305,405 p-InGaAsP contact layer
106,306,406 p-electrode
107,307,407 n-electrode
108,308,408 Polyimide
109,309,409 Light incident end face
110,310,410 Pad part

Claims (4)

An optical waveguide having a core layer and an upper clad layer and a lower clad layer sandwiching the core layer and molded into a high mesa ridge shape;
A first metal electrode that transfers power to and from the lower cladding layer;
In a semiconductor optical device having a second metal electrode that transfers power to and from the upper cladding layer,
The core layer is provided inside the ridge side surface so as to be concave with respect to the ridge side surface of the optical waveguide,
One or both of said 1st or 2nd metal electrodes are provided on condition of the following, The semiconductor optical element characterized by the above-mentioned.
First metal electrodes: Ru provided in contact with the ridge side surface of the lower cladding layer.
Second metal electrode: Ru provided in contact with the ridge side surface of at least the upper cladding layer. The semiconductor optical device according to claim 1,
A semiconductor optical device, wherein the second metal electrode is formed in a U-shaped cross-sectional shape and is brought into contact with a ridge side surface of the upper clad layer and is also brought into contact with an upper surface of the upper clad layer. In the semiconductor optical device according to claim 1 or 2 ,
The semiconductor optical device, wherein the width of the core layer is smaller than the ridge width of the optical waveguide so that an optical electric field of light guided through the core layer is confined inside in the width direction of the optical waveguide. In the semiconductor optical device according to any one of claims 1 to 3 ,
The core layer, the optical waveguide of the ridge side or al 0. A semiconductor optical device, wherein the semiconductor optical device is arranged 5 μm or more inside, or the width of the core layer is made 1 μm or more smaller than the ridge width of the optical waveguide .

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