JP2000147284A - Semiconductor optical waveguide and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor optical waveguide having light enclosing structure based on an Al oxide layer on an InP substrate. SOLUTION: The main part of the waveguide 20 is laminated structure consisting of an InP layer 22, an AlInAs layer 23 having 100 nm thickness, a GaInAsP layer 24, an AlInAs layer having 50 nm thickness, a GaInAsP layer 26, an AlInAs layer 27 having 100 nm thickness, and an InP layer 28 and formed on the InP substrate as a ridge 30. The AlInAs layers 23, 25, 27 are held as non-oxidized AlInAs layers on the center part of the ridge, but on the side face part of the ridge, Al oxidized layers 31 are formed by selectively oxidizing Al in reslective AlInAs layers 23, 25, 27. The widths of non-oxidized areas of the layers 23, 27 are almost the same W1 and narrow and the width W2 of the non-oxidized area of the layer 25 is wider than the width W1. The center area 32 of the ridge constitutes an almost circular and non-oxidized core layer having a high refractive index and is functioned as an optical waveguide and the peripheral area of the ridge constitutes a light enclosing structure consisting of an Al oxide layer having a low refractive index.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor optical waveguide formed on an InP substrate and a method for manufacturing the same, and more particularly, to a semiconductor optical waveguide having an easy manufacturing structure formed on an InP substrate by a simple process. The present invention relates to a semiconductor optical waveguide and a method for manufacturing the same.

[0002]

2. Description of the Related Art In a semiconductor laser device, a semiconductor layer containing Al is formed as a part of a laminated structure of a light emitting region, and Al in the semiconductor layer containing Al is selectively oxidized to form an Al oxide layer. The Al oxide layer is used as a current blocking layer, that is, a current confinement structure. The current confinement structure using an Al oxide layer is often applied particularly to a surface-emitting type semiconductor laser device, and it is evaluated that the characteristics of the surface-emitting type semiconductor laser device have been dramatically improved.

[0003] In the selective oxidation technique of Al, Al in a semiconductor layer containing Al is selectively oxidized to form an Al layer containing Al ₂ O _3.
This is a technique for forming an oxide layer, and has a feature that a refractive index of a semiconductor layer is reduced by oxidizing a semiconductor layer containing Al. Therefore, it is possible to form a light confinement structure utilizing this feature on a GaAs substrate and make it function as a semiconductor optical waveguide structure.
The present applicant has already proposed a semiconductor optical waveguide structure having an Al oxide layer as disclosed in Japanese Patent Application Laid-Open No. Hei 10-135564.

Here, a semiconductor optical waveguide structure having an Al oxide layer will be briefly described based on the disclosure of Japanese Patent Application Laid-Open No. 10-135564. As shown in FIG. 4 (b), the optical waveguide structure 10 formed on the GaAs substrate has a film thickness of 1.0 μm, which is sequentially formed on the GaAs substrate 11.
_0.3 Ga _0.7 As clad layer 12, 2.0 μm thick A
l (Ga) As core layer 13, 1.0 μm thick Al _0.3
It has a laminated structure of a Ga _0.7 As cladding layer 14 and a GaAs cap layer 15 having a thickness of 0.2 μm. Al (G
a) The As (As) core layer 13 is an Al (Ga) As oxidized side region 1 obtained by selectively oxidizing Al of the Al (Ga) As layer.
6 and Al (Ga) As non-oxidized central region 17 having a substantially circular cross section
It is composed of

In the laminated structure, the GaAs cap layer 1
5, Al _0.3 Ga _0.7 As clad layer 14, Al (G
a) The As core layer 13 and the Al _0.3 Ga _0.7 As clad layer 12 are formed as ridges 19 having a width of 5 μm.

Further, as described above, the Al (Ga) As core layer 13 is partially oxidized, and as shown in FIG. 4B, an Al (Ga) As oxidized region around the core layer is formed. 1
6 and an Al (Ga) As non-oxidized region 17 having a substantially circular cross section at the center. The non-oxidized region 17 has a substantially circular shape with a diameter of about 1.0 μm, and is used as an optical waveguide. That is, in the Al (Ga) As core layer 13, in the oxidized region 16, Al (Ga) As is Al _x O _y (AlO xid
The conversion to e) lowers the refractive index to about 1.6, which is sufficiently lower than the refractive index of the non-oxidized region 17 to be 2.9 to 3.4. FIG. 5B shows the distribution of the refractive index in the direction of the film surface and in the direction of the film thickness corresponding to the structure of the optical waveguide. Because of such a refractive index profile, the incident light is
That is, the light can be guided only in the central portion having a high refractive index. Further, the side surfaces of the ridge 19 are covered with the polyimide layer 18.

Hereinafter, a method for manufacturing the above-described optical waveguide structure will be described with reference to FIG. FIGS. 4 (a) and (b)
It is sectional drawing which shows the cross section of the laminated structure for every process at the time of manufacturing the above-mentioned semiconductor optical waveguide structure. First, as shown in FIG. 4 (a), A
l _0.3 Ga _0.7 As clad layer 12 is 1.0 μm
The composition of the Al (Ga) As core layer 13 is 2.0 μm, and the Al _0.3 Ga _0.7 As clad layer 14 is 1.0 μm, with the composition having the smallest composition at the center in the film thickness direction and gradually increasing toward the upper surface and the lower surface. , The GaAs cap layer 15 is 0.2 μm.
m, sequentially laminated. Here, in the Al (Ga) As core layer 13 in which the Al composition changes in the film thickness direction, as shown in FIG. 5A, the center part in the film thickness direction is Al _x Ga ₁ -x As.
(0.5 ≦ X ≦ 0.97), and A
It is preferable that l _x Ga _{1 -x} As (X ≒ 1.0).

In general, the oxidation rate of Al _x Ga ₁ -x As is determined by the ratio of Al, and the lower the ratio of Al, the lower the rate. For example, when Al is 97% (x = 0.97), The oxidation rate is about 1/10 of AlAs. Therefore, by setting the Al composition of the Al (Ga) As core layer at the central portion to 97% or less and the outermost portion to AlAs,
When the width of the optical waveguide is controlled by oxidizing the (Ga) As core layer 13 in a later step, the controllability is improved. The profile of the Al composition when X = 0.97 at the center and X = 1.0 at the outermost portions (the film front and back surfaces) is as shown in FIG. 5 (a).

The composition of the Al (Ga) As layer 13 is as _follows: Al _0.97 Ga _0.03 As in the center in the film thickness direction;
It is easy to set to 1As, for example, by employing the MBE method. In the MBE method, the Al composition of the Al (Ga) As core layer 13 can be continuously changed by continuously changing the temperature of the substrate. This is a commonly used method. is there. Ideally, the composition control of the Al (Ga) As core layer 13 is preferably continuously changed in the film thickness direction, but is not necessarily limited to this. In particular, when crystal growth is difficult where continuous composition control is difficult, such as when MOCVD is employed, or when a material other than AlGaAs is used for the core layer, the Al composition is changed stepwise. You may. In this case, a similar effect can be obtained.

Next, the GaAs cap layer 15, Al _0.3
The Ga _0.7 As clad layer 14, the Al (Ga) As core layer 13, and the Al _0.3 Ga _0.7 As clad layer 12 are
The ridge 19 is selectively removed by photolithography and dry etching to form a ridge 19 having a width of 5 μm.

Next, the entire wafer is placed in water vapor and heat-treated at a temperature of about 400 ° C. for 10 minutes. As a result, the Al (Ga) As core layer 13 is partially oxidized inward from both side surfaces of the ridge 19, and the Al (Ga) As oxidized region 16 around the core layer and the cross-section of the central portion are approximately formed. It is divided into a circular Al (Ga) As non-oxidized region 17. Here, assuming that the Al composition of AlGaAs at the center is 0.97, the oxidation rate is about 0.2 μm / min., And the oxidation rate of AlAs at the periphery is about 2 μm / mi on one side.
n. The non-oxidized region 17 is used as an optical waveguide and has a substantially circular shape with a diameter of about 1.0 μm.

At the end of the process, as shown in FIG.
The polyimide layer 18 covers both sides of the ridge 19.

The above-mentioned optical waveguide structure has only one crystal growth of the AlGaAs layer and an oxidation step of the AlGaAs layer.
Since it can be manufactured, the manufacture is easy and the cost of the optical waveguide structure can be reduced. In addition, since the semiconductor optical waveguide manufactured as described above has a substantially circular mode field, the mode field difference can be reduced when coupling with an optical fiber, and the optical coupling efficiency can be reduced. Is good. Further, in this optical waveguide, the width in the direction perpendicular to the film surface, that is, the height, can be controlled in the crystal growth step, and the width in the film surface direction can be controlled by the relationship between the Al composition and the oxidation rate. Thus, a substantially circular light field can be obtained accurately and easily without requiring a regrowth step. Further, depending on the case, there is an effect on the polarization dependence. Note that AlGaAs is AlA
Since the lattice constant of s itself is close to the lattice constant of GaAs, A
Even if the l composition is changed from 0 to 1.0, it can be laminated on a GaAs substrate. However, AlInAs has A
l _0.48 In _0.52 As only for lattice matching with InP in In _0.52 As,
If the Al composition is changed, lattice matching will be lost.

[0014]

By the way, as described above, the optical confinement structure using the Al oxide layer has a low manufacturing cost. Therefore, in addition to the optical waveguide structure on the GaAs substrate described above, the light confinement structure is formed on an InP substrate. Also, an improved optical waveguide structure is desired. When an optical waveguide structure having the above-described structure is to be formed on an InP substrate, a semiconductor material that can be laminated on the InP substrate in lattice matching with the InP substrate and that contains Al is required.

However, at present, such semiconductor materials are limited to AlInAs and AlGaInAs, and when the Al composition is to be changed, InP
Since the lattice constant is shifted from the substrate, it is extremely difficult to stack a film having a thickness sufficient to function as an optical waveguide layer.

An object of the present invention is to provide a semiconductor optical waveguide having a light confinement structure using an Al oxide layer on an InP substrate.

[0017]

In order to achieve the above object, a semiconductor optical waveguide according to the present invention comprises a plurality of layers of AlIn.
As a ridge-like stacked structure of a semiconductor layer including an As layer, In
The AlInAs layer formed on the P substrate has a structure in which the Al in the AlInAs layer is selectively oxidized inward from both sides of the ridge over the entire width of the ridge or by a predetermined width.
It is characterized by being converted to an oxide layer.

Since the refractive index of the Al oxide layer formed by selectively oxidizing Al in the AlInAs layer is lower than that of the AlInAs layer, light is transmitted to the AlInAs layer region at the center of the ridge which is a non-oxidized region. Since only the layer is confined, the laminated structure functions as an optical waveguide layer.

The oxidation rate of the AlInAs layer is shown in FIG.
As shown in the figure, the oxidation reaction progresses rapidly in the thick AlInAs layer because the
In the lInAs layer, the progress of the oxidation reaction is slow. FIG.
AlIn when the AlInAs layer is oxidized at 500 ° C.
The thickness (nm) of the As layer and the oxidation rate (μ
m / √ (route) minutes). Therefore, the present inventor takes advantage of this fact and provides a preferred embodiment of the present invention with a plurality of AlInAs layers,
The thickness of the nAs layer is such that the thickness of the AlInAs layer is smaller at the core portion, and the thickness of the AlInAs layer further away from the core portion is larger. As a result, in the core portion, the non-oxidized layer extends widely, and the width of the non-oxidized layer becomes narrower as the core portion is separated. Can be formed. That is,
A waveguide having a substantially circular section for guiding light can be formed at the center of the ridge, and a light confinement layer can be formed around the waveguide.

In the method of manufacturing a semiconductor optical waveguide according to the present invention, a semiconductor layer including a plurality of AlInAs layers is stacked on an InP substrate to form a stacked structure, and at least the AlInAs layer is exposed on the side surface of the ridge. Thus, a step of processing the laminated structure into a ridge shape, a step of performing a heat treatment in water vapor, and selectively oxidizing Al of the AlInAs layer from the side surface of the ridge to generate an Al oxide layer extending inward. It is characterized by having.

[0021]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. Embodiment Example This embodiment is an example of an embodiment of a semiconductor optical waveguide formed on an InP substrate according to the present invention, and FIG. 1 is a cross-sectional view showing a configuration of the semiconductor optical waveguide of this embodiment. .
As shown in FIG. 1, a main part of the semiconductor optical waveguide 20 of the present embodiment is constituted by a ridge-shaped semiconductor laminated structure 29 formed on an InP substrate 21. The semiconductor laminated structure 29 has an InP layer 22 having a thickness of 1 μm and a thickness of 100 nm.
AlInAs layer 23, GaInAsP layer 24 having a thickness of 0.2 μm and a band gap wavelength of 1.1 μm, and a thickness of 5
An AlInAs layer 25 of 0 nm, a GaInAsP layer 26 having a thickness of 0.2 μm and a band gap wavelength of 1.1 μm,
AlInAs layer 27 having a thickness of 100 nm and a thickness of 1 μm
This is a stacked structure of m InP layers 28 and is formed as a ridge 30.

The AlInAs layers 23, 25 and 27 have a non-oxidized A
The AlInAs layer remains, and on both sides of the ridge 30, A in which Al in each AlInAs layer is selectively oxidized.
1 oxide layer 31. AlInAs layer 23 and Al
The non-oxidized region of the InAs layer 27 is located at the center and its width is narrow at about the same W ₁ , and the non-oxidized region of the AlInAs layer 25 is also located at the center and its width W ₂ is AlInAs
Wider than the width W ₁ of the layers 23, 27. Al oxide layer 31
Has a low light refractive index, while the non-oxidized AlInAs layer has a higher light refractive index than the Al oxide layer 31. This allows
When viewed in a vertical cross section of the ridge 30, the ridge central region 32 functions as an optical waveguide by forming a substantially circular non-oxidized core layer having a high refractive index. To form a light confinement layer made of an Al oxide layer having a low thickness. In other words, the Al oxide layer 31 and the non-oxidized A
The lInAs layer forms an optical waveguide structure.

Hereinafter, a method for fabricating the semiconductor optical waveguide of this embodiment will be described with reference to FIG. FIG.
FIG. 3C is a cross-sectional view for each step in manufacturing the semiconductor optical waveguide of the present embodiment. First, as shown in FIG. 2A, an InP layer 22 having a thickness of 1 μm and an AlInA layer having a thickness of 100 nm are sequentially formed on an InP substrate 21 by MOCVD.
s layer 23, thickness 0.2 μm, band gap wavelength 1.1
GaInAsP layer 24 having a composition of μm, Al having a thickness of 50 nm
InAs layer 25, GaInAsP layer 26 having a thickness of 0.2 μm and a band gap wavelength of 1.1 μm, and a thickness of 100 n
An AlInAs layer 27 having a thickness of m and an InP layer having a thickness of 1 μm are formed to form a laminated structure.

Next, as shown in FIG. 2B, a SiO ₂ film is formed on the InP layer 28 and patterned to form a mask 33, and the InP layer 28, AlInAs layer 27, G
aInAsP layer 26, AlInAs layer 25, GaInA
The laminated structure of the s layer 24, the AlInAs layer 23, and the InP layer 22 is etched to form a stripe-shaped ridge 30 having a width of 10 μm. Next, the mask 33 is removed, and a heat treatment is performed in a steam atmosphere at a temperature of about 500 ° C., thereby simultaneously forming the AlInAs layers 23, 25, and 27 from both sides of the ridge 30, as shown in FIG. A in
1 is selectively oxidized to form an Al oxide layer 31. At the end of the process, although not shown, both sides of the ridge 30 are covered with a polyimide layer or the like to form a protective film. Thereby, the semiconductor optical waveguide 20 of the embodiment can be formed.

[0025]

According to the present invention, a plurality of layers of AlInAs are provided.
A ridge-like laminated structure of a semiconductor layer including a layer is formed on an InP substrate, and AlInAs
By oxidizing the layer over the entire width of the ridge width or by a predetermined width, a semiconductor optical waveguide using an Al oxide layer as an optical confinement layer on an InP substrate can be realized.
Since the semiconductor optical waveguide of the present invention forms an optical confinement structure using an Al oxide layer, it is easy to manufacture.
This can contribute to cost reduction of an optical device that requires an optical waveguide on an InP substrate.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a configuration of a semiconductor optical waveguide according to an embodiment.

FIGS. 2A to 2C are cross-sectional views for respective steps when fabricating the semiconductor optical waveguide of the embodiment.

FIG. 3 is a graph showing the relationship between the thickness of an AlInAs layer and the oxidation rate.

FIGS. 4A and 4B are cross-sectional views each showing a cross section of a laminated structure in each step when a semiconductor optical waveguide structure is formed on a GaAs substrate.

5 (a) is a profile of the Al composition of the core layer in the optical waveguide structure of FIG. 4 (b), and FIG. 5 (b) is a refractive index profile of the core layer after oxidation.

[Explanation of symbols]

Reference Signs List 11 Ga As substrate 12 Al _0.3 Ga _0.7 As clad layer 13 Al (Ga) As core layer 14 Al _0.3 Ga _0.7 As clad layer 15 Ga As cap layer 16 Al (Ga) As oxidized region 17 Al (Ga) As non-oxidized region Reference Signs List 18 polyimide layer 20 main part of semiconductor optical waveguide of embodiment 21 InP substrate 22 InP layer 23 AlInAs layer 24 GaInA having band gap wavelength of 1.1 μm composition
sP layer 25 AlInAs layer 26 GaInA having band gap wavelength of 1.1 μm composition
sP layer 27 AlInAs layer 28 InP layer 29 ridge-shaped semiconductor laminated structure 30 ridge 31 Al oxide layer 32 ridge central part 33 mask

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Akihiko Kasukawa 2-6-1 Marunouchi, Chiyoda-ku, Tokyo Furukawa Electric Co., Ltd. F-term (reference) 2H047 KA04 KA05 MA05 MA07 PA03 PA06 PA24 QA02 TA42 TA43 TA44 5F073 AA12 CA07 CA12 CB05 DA05 DA21

Claims (4)

[Claims] 1. An AlInAs layer is formed on an InP substrate as a ridge-like laminated structure of a semiconductor layer including a plurality of AlInAs layers. Only the Al in the AlInAs layer is selectively oxidized and converted into an Al oxide layer. 2. A semiconductor device comprising a plurality of AlInAs layers, wherein the thickness of the AlInAs layer in the stacking direction is AlInAs in the core portion.
2. The semiconductor optical waveguide according to claim 1, wherein the thickness of the layer is smaller, and the thickness of the AlInAs layer further away from the core portion is larger. 3. A step of laminating a semiconductor layer including a plurality of AlInAs layers on an InP substrate to form a laminated structure, and processing the laminated structure into a ridge shape so that at least the AlInAs layer is exposed on a side surface of the ridge. Process and heat treatment in water vapor, AlInA
A extending inward by selectively oxidizing Al in the s layer
a step of forming an oxide layer. 4. The method according to claim 1, wherein in the step of forming the laminated structure, the AlInAs layer has a small thickness at the core portion and the AlIAs layer separated from the core portion has a small thickness.
A plurality of AlI layers are formed so that the nAs layer becomes thicker.
4. The method according to claim 3, wherein an nAs layer is laminated.