JPH07170020A - Semiconductor optical waveguide and its manufacture - Google Patents

Abstract

PURPOSE:To further eliminate or reduce imperfection caused by recombination of electrons and holes, in an optical waveguide using double hetero junction. CONSTITUTION:A semiconductor optical waveguide is provided with the following; an InGaAsP optical waveguide layer 11, an N-type InP first clad layer 15 and a P-type InP second clad layer 17 which sandwich the optical waveguide layer 11, an electrode 21 formed on the opposite side of the clad layer 15 to the optical waveguide layer 11, and an electrode 23 formed on the opposite side of the clad layer 17 to the optical waveguide layer 11. An insulating layer 25 is formed between the optical waveguide layer 11 and the first clad layer or the second clad layer. A protective layer 27 which prevents the quality deterioration of the optical waveguide layer 11 due to the insulating layer 25 is formed between the insulating layer 25 and the optical waveguide layer 11.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide having a variable refractive index and a manufacturing method thereof.
For example, the present invention relates to an optical waveguide suitable for use as a part of an optical device for optical communication or optical information processing, and a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, as an optical waveguide whose refractive index can be changed, for example, reference I (IE Photonics Technology Letters (IE) filed by the applicant of this application is used.
EEP Photonics Technology Le
terts), Vol. 4, p. 1096 (199
2)) was disclosed. However, this document I shows an example of an optical device in which an optical waveguide whose refractive index can be changed is integrated with an MQW (multiple quantum well) active layer functioning as a semiconductor laser. FIG. 4A is a side view schematically showing the optical device cited from Document I.

This optical device is basically shown in FIG.
As shown in, the optical waveguide layer 11 having the diffraction grating 11a
A structure in which the first cladding layer 15 and the second cladding layer 17 are sandwiched between the first cladding layer 15 and the second cladding layer 17, and a current can be externally injected into the optical waveguide layer 11 and the MQW active layer 13. there were. 19 in FIG. 4 (A)
The area indicated by is the portion forming the optical waveguide.

In this optical device, when a current is injected into the optical waveguide layer 11, the refractive index of the optical waveguide 11 changes (details will be described later), which changes the effective pitch of the diffraction grating 11a. The oscillation wavelength can be changed.

Although not described in detail, such an optical waveguide 19 whose refractive index can be varied is not limited to being used in combination with a semiconductor laser, but is also applied to an optical switch, a phase modulator or the like.

The conventional basic structure of the variable refractive index type optical waveguide having such a high utility value and the principle of varying the refractive index in this optical waveguide will be described below. FIG. 4B is a sectional view used for the explanation.

The conventional variable-refractive-index type optical waveguide is formed between the first conductivity type first cladding layer 15 and the second conductivity type second cladding layer 17 with a material having a refractive index larger than those of them. The optical waveguide layer 11 is sandwiched between the so-called double hetero structure, and the first electrode 21 and the second electrode 23 for injecting current into the double hetero structure. When a forward voltage is applied between the first and second electrodes 21 and 23, electrons and holes, which are free carriers, are injected into the optical waveguide layer 11 and the plasma effect due to them (for example, Reference II: “Optical Communication Device”). The refractive index of the optical waveguide layer is changed by "Optics" (Kogakusho Co., Ltd.), Hiroo Yonezu, (Showa 61), p.216.

[0008]

However, in the above-mentioned conventional optical waveguide, both electrons and holes are injected into the optical waveguide layer when the refractive index of the optical waveguide is changed. There are problems as explained.

(A) Since the electrons and holes injected into the optical waveguide layer are recombined, the carrier density in the optical waveguide layer fluctuates, which causes fluctuations in the refractive index of the optical waveguide layer, resulting in noise. The problem that it will occur.

(B) The rate of carrier recombination increases as the density of carriers accumulated in the optical waveguide layer increases. Therefore, even if the injection current to the optical waveguide layer is increased, the carrier density in the optical waveguide layer is saturated, and the change in the refractive index is also saturated. (There is naturally a limit to expanding the variable range of the refractive index. is there.).

(C) In the conventional optical waveguide, as a result of carrier recombination, a current flows through the device itself, so that heat is generated in the device. Such heat causes deterioration of the characteristics of the optical waveguide, and particularly when the optical waveguide and other devices are integrated, the temperature of the other devices also rises and causes the deterioration of the characteristics of the devices. The problem that will be.

The present application has been made in view of the above points, and therefore, the object of the first invention of the present application is to provide a variable refractive index type optical waveguide capable of solving the above-mentioned problems or reducing the problems. Especially. Another object of the second invention of this application is to provide a method for easily manufacturing the optical waveguide of the first invention.

[0013]

In order to achieve the object of the first invention, according to the first invention, an optical waveguide layer made of a semiconductor material and a first conductivity type sandwiching the optical waveguide layer are provided. A semiconductor optical waveguide comprising a first clad layer and a second clad layer of a second conductivity type, and an electrode provided on each side of the respective clad layers opposite to the optical waveguide layer. An insulating layer is provided between the first clad layer and the second clad layer.

In carrying out the first aspect of the present invention, a protective layer is provided between the insulating layer and the optical waveguide layer to prevent deterioration of the quality of the optical waveguide layer due to the insulating layer. Is preferred.

In order to achieve the object of the second invention of this application, according to the second invention, when manufacturing the optical waveguide of the first invention, either one of the first substrate and the second substrate is used. Forming at least one of the first cladding layer and the second cladding layer and an optical waveguide layer thereon, and forming at least the first cladding layer and the second waveguide layer on the other of the first substrate and the second substrate. A step of forming the other of the two clad layers, a step of forming an insulating layer on one of the first substrate and the second substrate on which the clad layer or the like has been formed, and the insulating layer The step of arranging the substrate having already been formed and the other substrate so that the insulating layer and the optical waveguide layer or the clad layer are in contact with each other, and directly adhering the insulating layer and the optical waveguide layer or the clad layer. Characterize.

In implementing the second invention, the method further comprises the step of forming a protective layer on the optical waveguide layer for preventing the quality of the optical waveguide layer from being deteriorated due to the insulating layer. It is suitable. However, in this case, the insulating layer and the clad layer or the protective layer are directly bonded.

Further, in carrying out the second invention, the substrate of the first substrate and the second substrate on which the optical waveguide layer is not formed is an etching means capable of etching the substrate. First, an etching stop layer that is resistant to the clad layer and is made of a material that is selectively etched with respect to the clad layer is first formed, and the clad layer is formed on the etching stop layer, and the direct adhesion is finished. After that, it is preferable to remove the substrate and the etching stop layer, respectively.

Further, in carrying out the second aspect of the present invention, the direct adhesion is carried out by treating the surfaces of the respective layers to be directly adhered to make them hydrophilic, and the hydrophilically treated surfaces are brought into contact with each other. It is preferable to carry out by heat treatment.

[0019]

According to the structure of the first aspect of the present invention, when a voltage is applied between the electrodes, carriers (electrons or positive electrons) due to the first conductivity type impurities are formed on the surface of the insulating layer on the first cladding layer side due to the electric field effect. Holes) are accumulated, and carriers due to the second conductivity type impurities are accumulated on the surface of the insulating layer on the second cladding layer side. Further, here, the optical waveguide layer is in contact with the insulating layer directly or through a protective layer in a preferred example. Therefore, in the optical waveguide layer, either electrons or holes depending on the positional relationship between the optical waveguide layer and the insulating layer are confined according to the magnitude of the inter-electrode voltage. A carrier effect causes a plasma effect, which changes the refractive index of the optical waveguide layer. However, in the case of a structure with a protective layer, if the protective layer is too thick, the carriers will not reach the optical waveguide layer, so the thickness of the protective layer should be thin enough to allow the carrier to reach the optical waveguide layer. It is necessary to Thus, in the structure of the first invention, substantially only one carrier is accumulated in the optical waveguide layer, so that recombination of electrons and holes in the optical waveguide layer does not substantially occur, and No current flows in the wave layer. Therefore, the conventional problems can be avoided or reduced.

Further, in the constitution of providing the protective layer in the first aspect of the present invention, it is possible to prevent the occurrence of defects in the optical waveguide layer which would easily occur when the insulating layer is brought into direct contact with the optical waveguide layer.

By the way, it is generally difficult to form a semiconductor layer for an optical waveguide layer on an insulating layer with good crystal quality. Also in the case of obtaining the optical waveguide of the first invention, for example, when the respective layers are formed in the order of the first cladding layer, the optical waveguide layer, the insulating layer, and the second cladding, they are formed on the insulating layer. It is difficult to obtain the desired crystal quality of the 2-clad layer. However, in the second invention, the first substrate and the second substrate are used, and the film formation order is devised so that the insulating layer becomes the outermost surface layer of these substrates. Then, the structure of the first invention is obtained by directly adhering the insulating layer and the semiconductor layer.
Therefore, the structure of the first invention can be obtained without growing crystals on the insulating layer.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the first invention and the second invention of this application will be described below with reference to the drawings. It should be noted that each of the drawings used for the description only schematically shows the shape, size, and arrangement relationship of each component so that the invention can be understood. Moreover, in each figure, the same components are denoted by the same reference numerals, and the duplicate description thereof will be omitted.

1. Description of Embodiment of First Invention (Semiconductor Optical Waveguide) FIG. 1 is a sectional view for explaining the structure of a semiconductor optical waveguide of an embodiment. The semiconductor optical waveguide 10 of this embodiment includes an optical waveguide layer 11 made of a semiconductor material, a first conductivity type first cladding layer 15 and a second conductivity type second cladding layer 17 sandwiching the optical waveguide layer 11. , Each of the cladding layers 15 and 17 is provided with electrodes 21 and 23 respectively provided on the opposite side of the optical waveguide layer 11, and between the optical waveguide layer 11 and the second cladding layer 17 in this case. It is characterized by having an insulating layer 25. Furthermore, the optical waveguide 10 of this embodiment
The protective layer 27 is provided between the insulating layer 25 and the optical waveguide layer 11 to prevent the quality of the optical waveguide layer 11 from being deteriorated due to the insulating layer 25. The deterioration of the quality of the optical waveguide layer 11 caused by the insulating layer 25 means, for example, the insulating layer 25
It is conceivable that crystal defects may be generated due to strain when the and the optical waveguide 11 are in direct contact with each other. Of course, the protective layer 27 may not be provided as long as the insulating layer 25 can be formed so that the deterioration of the quality of the optical waveguide layer 11 caused by the insulating layer 25 does not occur even if the insulating layer 25 and the optical waveguide 11 are in direct contact with each other.

Here, the optical waveguide layer 1 is not limited to this.
1 is composed of, for example, an i-InGaAsP layer, the first conductivity type first cladding layer 15 is composed of, for example, an n type InP substrate, and the second conductivity type second cladding layer 17 is composed of, for example, p type In.
The insulating layer 25 is composed of, for example, a SiO ₂ film, and the protective layer 27 is composed of an InP layer. Electrodes 23, 25
Is composed of a known material. The thickness of the i-InGaAsP optical waveguide layer 11 is, eg, 0.1-0.4 μm.
The thickness of the InP protective layer 27 is preferably as thin as possible so that the strain at the interface between the InP protective layer 27 and the insulating layer 25 does not reach the optical waveguide layer 11. InP protective layer 27
If the thickness is too thick, the carriers accumulated in the vicinity of the side surface of the protective layer 27 of the insulating layer 25 are substantially accumulated in the InP protective layer and are not accumulated in the optical waveguide layer 11, so this must be prevented. is there. Here, the InP protective layer 2
The thickness of 7 is about 5 to 10 nm. In addition, the thinner the insulating layer 25, the higher the carrier concentration accumulated on both surfaces of the insulating layer 25, which is preferable. However, if the insulating layer 25 is too thin, it is difficult to obtain an insulating layer having a uniform breakdown voltage and a uniform film thickness.
It is good to determine the thickness of. Here, the thickness of the insulating layer 25 is about 10 nm.

In the semiconductor optical waveguide 10 of this embodiment, when a voltage is applied between the electrodes 21 and 23, the electric field effect is generated per unit area according to the relational expression: eN _S = εV / d _i (1) Free carriers (electrons or holes) are accumulated at both ends of the insulating layer 25. In the equation (1), e is the elementary charge, N _S is the surface carrier density, ε is the dielectric constant of the insulating layer 25, d _i is the thickness of the insulating layer 25, and V is the voltage applied to the insulating layer 25. . Of the free carriers accumulated at both ends of the insulating layer 25, those on the protective layer 27 side are confined in the optical waveguide layer 11 because the protective layer 27 has a small thickness (here, about 5 to 10 nm). become. Because the energy band gap of the InGaAsP optical waveguide layer is the InP protective layer 27.
Since it is smaller than that of InGaAs, the free carriers are InGaAs
This is because the energy is smaller when the light is confined in the P optical waveguide layer 11. Therefore, when a voltage is applied between the electrodes 21 and 23, free carriers are confined in the InGaAsP optical waveguide layer 11 due to the electric field effect and the carrier confinement effect, and the refractive index of the optical waveguide layer 11 changes due to the plasma effect. is there. Further, in this case, since the insulating layer 25 exists between the accumulated electrons and holes, the electrons and holes do not occupy the same position spatially, so that recombination of both does not occur. Therefore, fluctuations in carrier density due to carrier recombination do not occur, and noise due to the fluctuations does not occur. Further, since there is no recombination of carriers, saturation of the change in the refractive index associated therewith does not occur, and in the present invention, the refractive index of the optical waveguide layer 11 is changed according to the above equation (1) until the voltage at which the insulating layer 25 is destroyed is reached. You can In addition, a current is applied to the optical waveguide 10.
Since it does not flow through, the generation of heat can be avoided.

It is known that the plasma effect is produced more efficiently by electrons than when holes are carriers.
Therefore, in the case of the structure of the first invention, it is better to arrange the insulating layer 25 so that the electrons are confined in the optical waveguide layer 11.
It is preferable because a remarkable plasma effect can be obtained. That is, the insulating layer 2 is provided between the optical waveguide layer 11 and the p-type cladding layer.
It is preferable to provide 5.

2. Description of Embodiment of Second Invention (Manufacturing Method) Next, an embodiment of the second invention will be described with reference to FIGS. 2 and 3 and FIG. Here, FIG. 2 and FIG. 3 are process diagrams showing the state of the sample in the main steps of the manufacturing method of the embodiment in a sectional view at the same position as in FIG. The materials used, the chemicals used, and the numerical conditions in the processing described below are merely examples within the scope of the second invention.

In this embodiment, a first conductivity type InP substrate 15 (which also serves as the first cladding layer 15 in this case) is prepared as the first substrate. Then, the InGaAsP optical waveguide layer 1 is formed on the InP substrate 15 by a normal crystal growth method.
1 and InP protective layer 27 are sequentially grown (FIG. 2).
(A)). Of course, on the substrate 15 separately from the substrate, InP
The clad may be grown.

Separately from this, In is used as the second substrate.
A P substrate (conductivity type is not particularly limited) 31 is prepared. Then, the second substrate 31 is formed on the InP substrate 31.
Shows resistance to etching means capable of etching
In addition, the etching stop layer 33 made of a material that is selectively etched with respect to the second cladding layer 17.
In this case, the InGaAs layer or the InGaAsP layer is formed by the normal crystal growth method, and the second conductivity type InP layer 17 as the second cladding layer is further formed on the etching stop layer 33 by the normal crystal growth method ( FIG. 2B). The second substrate is not limited to the InP substrate as long as the clad layer can be grown well.
In addition, without forming the optical waveguide layer 11 on the first substrate 15 side,
In some cases, it may be formed on the second cladding layer 17 of the second substrate 31.

Next, in this embodiment, for example, a SiO ₂ film is formed as the insulating layer 25 on the protective layer 27 on the first substrate 15 side by, for example, the CVD method or the electron beam evaporation method (FIG. 2).
(C)). Of course, the insulating layer 25 may not be formed on the first substrate 15 side, but may be formed on the second cladding layer 17 on the second substrate 31 side.

Next, the surface of the InP clad layer 17 in the sample shown in FIG. 2B and the surface of the insulating layer 25 in the sample shown in FIG. 2C each contain sulfuric acid. Each surface is rendered hydrophilic by treatment with a deionized water solution.
Of course, the treatment liquid for hydrophilic treatment is not limited to the aqueous solution containing sulfuric acid, and may be any other suitable one.

Each of the samples thus hydrophilically treated is
The surfaces of the insulating layer 25 and the InP clad layer 17 are brought into contact with each other at room temperature and in the atmosphere (FIG. 3 (A)),
Then, for example, 450 to
Heat treatment is performed at a temperature of about 700 ° C. In this process, the insulating layer 25 and the InP clad layer 17 are directly bonded (FIG. 3 (B)).

Next, the InP substrate which is the second substrate 31 is removed with hydrochloric acid. At this time, the etching stop layer 3
The second clad layer 17 is protected because of the presence of 3. next,
The InGaAs or InGaAsP etching stop layer 33 is removed by a mixed solution of hydrofluoric acid, water and nitric acid. Since the InP second cladding layer 17 is not etched by this etching solution, the second cladding layer 17 is exposed (not shown). After that, the first conductivity type electrode 21 is formed under the first substrate 15 and the second conductivity type electrode 23 is formed on the second cladding layer 17 by a known method.
The optical waveguide 10 of the embodiment of the first invention shown in FIG. 1 can be formed.

Although the embodiments of the inventions of this application have been described above, the inventions are not limited to the above-mentioned embodiments.

For example, InGaAs is used in the above embodiment.
Although the present invention is applied to the P / InP optical waveguide, the present invention can be applied to other material systems such as GaAs.

[0036]

As is apparent from the above description, according to the first invention of this application, an optical waveguide capable of producing a plasma effect by confining either one of electrons and holes in the optical waveguide layer. The structure is obtained. Therefore, recombination of electrons and holes does not substantially occur in the optical waveguide layer. Therefore, noise due to recombination of electrons and holes does not occur or is less noise than before, the variable width of the refractive index is wider than before, and characteristic deterioration due to heat is avoided or reduced. An optical waveguide can be provided.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of a first invention.

FIG. 2A to FIG. 2C are process drawings for explaining an embodiment of the second invention.

3 (A) and 3 (B) are process drawings following FIG. 2 for explaining an embodiment of the second invention.

4A and 4B are diagrams for explaining a conventional technique.

[Explanation of symbols]

11: Optical waveguide layer (for example, i-InGaAsP layer) 15: First conductivity type first cladding layer (for example, n-type InP)
substrate. First substrate 17: Second clad layer of second conductivity type (for example, p-type InP)
Layer: 21: Electrode (first conductivity type electrode) 23: Electrode (second conductivity type electrode) 25: Insulating layer (eg SiO ₂ etc.) 27: Protective layer (eg InP layer) 31: Second substrate (eg InP) Substrate) 33: Etching stop layer (for example, InGaAs layer or InGaAsP layer)

Front Page Continuation (72) Inventor Hiroshi Ogawa 1-7-12 Toranomon, Minato-ku, Tokyo Oki Electric Industry Co., Ltd.

Claims (5)

[Claims] 1. An optical waveguide layer made of a semiconductor material, and a first conductivity type first cladding layer and a second cladding layer sandwiching the optical waveguide layer.
Of the conductivity type second clad layer and each clad layer,
A semiconductor optical waveguide comprising electrodes provided on the opposite side of the optical waveguide layer, wherein an insulating layer is provided between the optical waveguide layer and the first cladding layer or the second cladding layer. Semiconductor optical waveguide. 2. The semiconductor optical waveguide according to claim 1, further comprising a protective layer between the insulating layer and the optical waveguide layer for preventing the quality of the optical waveguide layer from being deteriorated due to the insulating layer. A semiconductor optical waveguide characterized in that 3. The method for manufacturing the semiconductor optical waveguide according to claim 1, wherein at least one of the first cladding layer and the second cladding layer is provided on one of the first substrate and the second substrate. A step of forming an optical waveguide layer, a step of forming at least the other of the first cladding layer and the second cladding layer on the other of the first substrate and the second substrate, and formation of a cladding layer and the like. The step of forming an insulating layer on either one of the first and second substrates after completion of the above step, and the step of forming the insulating layer and the other substrate into the insulating layer and the optical waveguide layer. Or a step of directly arranging the insulating layer and the optical waveguide layer or the clad layer so as to be in contact with the clad layer, and manufacturing the semiconductor optical waveguide. 4. The method for manufacturing a semiconductor optical waveguide according to claim 3, further comprising a protective layer on the optical waveguide layer for preventing the quality of the optical waveguide layer from being deteriorated due to the insulating layer. A method of manufacturing a semiconductor optical waveguide, which further comprises a step of forming (in this case, an insulating layer and a cladding layer or a protective layer are directly bonded). 5. The method for manufacturing a semiconductor optical waveguide according to claim 3 or 4, wherein the direct bonding is a treatment for making the surface of each layer to be directly bonded hydrophilic. A method for manufacturing a semiconductor optical waveguide, characterized in that the heat treatment is performed while the surfaces are in contact with each other.