JPH047509A - Semiconductor optical waveguide - Google Patents

Abstract

PURPOSE:To decrease the radiation losses of semiconductor optical waveguides by forming straight and curved rib-shaped optical waveguides without interruption and depositing thin films of hydrogenated amorphous silicon (a-Si:H) only in the bottom parts of the ribs of the straight optical waveguides. CONSTITUTION:The rib-shaped semiconductor optical waveguides are formed by deeply etching the entire part and thereafter, the thin films 19 of the a-Si:H are deposited on the etched surfaces of only the straight optical waveguide parts. The confining of the light in a horizontal direction is intensified in the curved optical waveguide parts 2 of the semiconductor optical waveguides produced in such a manner and the confining of the light in the horizontal direction in the straight optical waveguide parts is weakened as compared with the curved optical waveguide parts 2. The radiation losses of the curved optical waveguides 2 are decreased in this way even if the radius of curvature is successively decreased. Even a single mode condition is easily attained and there is no adverse influence on the other optical devices using the straight optical waveguides on the same substrate 11.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a semiconductor optical waveguide including a curved optical waveguide section.

(Prior Art and its Issues) Along with advances in optoelectronics, research and development into the integration of semiconductor optical devices has been actively promoted in recent years. In particular, semiconductor optical waveguides can be realized on semiconductor substrates by applying microfabrication technology cultivated in semiconductor electronic devices, and can be used for connections between each switch of a semiconductor optical matrix switch, and between semiconductor optical functional elements on the same substrate. (for example, connecting a light source to a switch or amplifier, etc.)
It is considered one of the important components of semiconductor optical integrated circuits. Such a semiconductor optical waveguide requires not only a straight optical waveguide but also a curved optical waveguide in order to connect each semiconductor optical device. Conventionally, it has been common practice to form this curved optical waveguide together with a straight optical waveguide using an ordinary one-shot photolithography method.

Incidentally, in order to downsize the integrated device, it is preferable to reduce the radius of curvature of the curved optical waveguide. However, since light inherently has the property of traveling in a straight line, if the radius of curvature of a curved optical waveguide is unnecessarily reduced, the conventional rib forming method has the problem of increasing radiation loss in the curved optical waveguide. Ta. Pree (R, J, DER) shows that if the radius of curvature is to a certain extent, radiation loss can be reduced by increasing the width of the waveguide and strengthening the light confinement.
I) et al., Electronics Letters, Vol. 23, p. 845 (ELECTRONIC8LETTER3Vo
1, 23 p. 845). However, as the width of the waveguide is increased, the guided light approaches the multiplexed condition, and at the same time the characteristics of optical devices made of other integrated linear optical waveguides, such as directional coupler type optical switches, deteriorate. have a negative impact. However, when the radius of curvature is reduced to the order of millimeters or less, there is a problem that radiation loss is not reduced much even if the width of the waveguide is increased to strengthen the light confinement.

FIG. 2(A) is a diagram showing the relationship between the radius of curvature and the radiation loss when only the waveguide width of the S-curve optical waveguide is changed within single mode conditions. This figure shows that when the radius of curvature is on the order of a few degrees, the radiation loss is reduced by widening the waveguide width, but when the radius of curvature is on the order of a few degrees or less, even if the waveguide width is widened, there is almost no radiation loss. It can be seen that it is not reduced. As described above, conventional semiconductor optical waveguides have had problems to be solved regarding reduction of radiation loss.

(Means for Solving the Problems) In order to solve the above-mentioned problems, a semiconductor optical waveguide according to the present invention is provided in which straight and curved rib-shaped optical waveguides are successively formed, and the linear rib-shaped optical waveguide A thin film of hydrogenated amorphous silicon (a-8i:H) is deposited only on the bottom of the rib.

(Function) Since light generally has the property of traveling in a straight line, radiation loss occurs in curved portions of a semiconductor optical waveguide. This radiation loss increases as the curvature of the curved optical waveguide decreases, so conventionally, curved optical waveguides had to be fabricated with a radius of curvature where the radiation loss was negligible compared to the waveguide loss. This was a major hindrance to reducing the overall length of the device. It is also possible to increase the waveguide width to strengthen light confinement and reduce radiation loss in a curved optical waveguide, but if the curvature is on the order of mm or less, this method does not reduce radiation loss much. Moreover, since it approaches a multi-mode condition, it is also undesirable in that it adversely affects other optical devices on the same substrate.

In manufacturing the semiconductor optical waveguide of the present invention, first, the entire structure is deeply etched to form a rib-type semiconductor optical waveguide, and then an a-8t:H thin film is deposited on the etched surface of the straight optical waveguide only. In the semiconductor optical waveguide manufactured in this way, the confinement of light in the horizontal direction can be made stronger in the curved optical waveguide section, and the confinement of light in the horizontal direction can be made weaker in the straight optical waveguide section than in the curved optical waveguide section. Therefore, even if the radius of curvature is made smaller, the radiation loss of the curved optical waveguide can be reduced, and single mode conditions can also be easily achieved, allowing other optical devices using straight optical waveguides on the same substrate. has no adverse effect on. Furthermore, the a-8t:H thin film can be deposited highly uniformly over a large area, and can be obtained at a constant thickness with good reproducibility regardless of the surface condition of the wafer. It is possible to precisely control the confinement state of light in the horizontal direction. In addition, the refractive index of the a-8t:H thin film for long wavelength bands can be easily controlled from around 2 to around 4 by changing the deposition conditions, making it suitable for optical waveguides using various semiconductor materials. do. Therefore, according to the present invention, a curved optical waveguide can be used in a matrix optical switch, etc. in which a large number of directional coupler type optical switches are integrated, which requires high accuracy in controlling the optical confinement state in the horizontal and horizontal directions of the rib waveguide section. It is possible to reduce the radius of curvature and downsize the element.

(Example) The present invention will be explained in more detail below with reference to the drawings.

FIG. 1 shows G a A s / A J G according to the present invention.
1(A) is a plan view, and FIG. 1(B) is a plane taken along the dotted line BB' in FIG. 1(A). A cross-sectional view showing the directional coupler section in FIG.
FIG. 3 is a cross-sectional view showing the curved optical waveguide section along the dotted line CC' plane in A).

First, the cross-sectional layer structure of the 4×4 matrix optical switch shown in FIG. 1 will be explained. n-A, I! on the n-G a As substrate 11; 0.5 G ao,s A
The S first cladding layer 12 is grown, n-AJ. .

A 1-GaAs waveguide layer 13 of the Gao,..., As first cladding layer 12 is grown. Said 1-GaAs waveguide layer 1
3 has a rib part iA J o, s G
a o, s A S second cladding layer 14 and p
AJ o, s G ao, 5A S third cladding layer 1
5 is formed. Said p A fJo, s G
a o, s A s On top of the third cladding layer 15,
A p"-GaAs cap layer 16 is formed. A P electrode 18 is formed on the P+GaAs scan 1 layer 16, and an n electrode 17 is formed below the n"-GaAs substrate 11. Here, in the directional coupler section 1, an a-8t:H thin film 19 is further formed on the etched surface of the iA16,5Gao,sAS second cladding layer 14.

A method for manufacturing the semiconductor optical waveguide shown in FIG. 1 will be described below. On the n''-GaAs substrate 11, using molecular beam epitaxial growth (MBE) or metal organic chemical vapor deposition (MO-CVD), n Alo, s Ga
o, s As first cladding layer 12.1 - GaAs waveguide 13, i A, Q o, s Gao6As second cladding layer 14, p AJ o, s Gao, s As third cladding layer 15, p" - A GaAs cap layer 16 is grown.The thickness of each layer is n A, I
! o, s Gao, s As first cladding layer 12 is 1
~2 μIn, 1-GaAs waveguide layer 13 is 0.2 μm
degree, i A, Q o, 5 Gao, s As second cladding layer 14 is about 0.5 μm, p AJo, sG
ao,s A S third cladding layer 15 is 0.5. Tun
The P''-GaAs cap layer 16 has a thickness of 0.2μllll.
That's about it. Also, regarding the carrier concentration of each layer, n AJ o, s Gao, s A S first cladding layer 12 has a carrier concentration of about 5×1717 CI11-3,
A, llo, s Gao, sAs third cladding layer 15 is about 5×l 7 +7all]-3, P''-GaAs
The cap layer 16 has a thickness of about 2×10 18 an −′. After crystal growth as described above, the rib portion of the directional coupler is formed by ordinary photolithography and etching methods. The etching depth is about 1.1 μm.

After etching, a p-electrode 18 is formed on top of the P+-GaAS cap layer 7, and an n-electrode 1 is formed below the n''-GaAs substrate 11.
7 are respectively provided by metal vapor deposition. After that, only the curved optical waveguide section 2 is masked using the usual photolithography method, and then the parallel plate glow discharge decomposition method (GD-
A-3i:H thin film 19 is deposited on the entire surface using CVD).
By depositing about 1 μm, an aSt:H thin film 19 is formed on the etched surface only in the directional coupler portion 1. The refractive index of the a-3i:H thin film 19 is controlled by the substrate temperature during deposition, which is approximately 250 degrees Celsius here, and the raw material gas used is SiH<100%. In this case, the refractive index of a-3i:H is A, Q o, s Gao, s
It is almost the same as As.

The above is an explanation of the structure of a semiconductor optical waveguide and its manufacturing method, which is an embodiment of the present invention. In the semiconductor optical waveguide described above, the radiation loss of the curved optical waveguide is improved compared to the conventional one,
The principle by which the single mode condition can also be easily realized will be explained below.

In this embodiment, as shown in FIG. 1(B), the lip height of the directional coupler section 1 is i-A, 1lO0, Ga, . 8i:H thin film 19 effectively adjusts the height, whereas the rib height of the curved optical waveguide section 2 is set slightly higher. Therefore, light is strongly confined in the curved optical waveguide section 2, and radiation loss is reduced. FIG. 2(A) shows an example of the relationship between the radius of curvature and radiation loss when the waveguide width is changed within single mode conditions. The radiation loss is calculated as the total radiation loss per S-curve as shown in Figure 3 (A), and the waveguide structure is calculated according to the layered structure model as shown in Figure 3 (B). Calculate, here AJ), .

G ao, s AS The layer thickness of the first cladding layer 22 is 1.5 μm, and the layer thickness of the GaAs waveguide layer 23 is 0.2 μl.
ll, AJ o, s G ao, s A The layer thickness of the second cladding layer 24 is 1.2 μm, and the etching depth t is 0.
．． Assuming 9 μm, the waveguide width W is 2 μrn and 2.5 μm.
rn was varied to 3 μn. From Figure 2 (A>), when the radius of curvature is several ff1m, the radiation loss can be reduced by widening the waveguide width, but when the radius of curvature is on the order of 1m or less, the radiation loss does not change much even if the waveguide width is widened. It turns out that there isn't.

FIG. 2(B) shows an example of the relationship between the radius of curvature and the radiation loss when the rib height of the waveguide is changed within the single mode condition with the same structure as in FIG. 2(A).

As in Figure 2 (A), calculation of radiation loss is shown in Figure 3 (A).
), and the waveguide structure is calculated according to the mm model as shown in Figure 3 (B).
16. s G a o, s A The layer thickness of the first cladding layer 22 is 1.5 μm, and the layer thickness of the waveguide layer 23 is 0.2 μm.
Ill, A, ll Q, 5 Gao, s As 2nd
The layer thickness of the cladding layer 24 is 1.2 μm, and the waveguide width W is 2 μm.
m, the etching depth t is 0.9μrn, 0.
The thickness was changed to 95 μm and 1 μm. From FIG. 2(B), it can be seen that when the rib height of the waveguide is increased, the radiation loss can be effectively reduced even if the radius of curvature is on the order of nm or less. In this example, the height of the ribs is increased to strengthen the confinement of light, so even if the radius of curvature of the curved optical waveguide section 2 is shortened to less than 100 degrees, the width of the waveguide can be widened to improve the confinement of light. It is possible to reduce radiation loss more effectively than in the case of increasing the strength. In addition, the directional coupler section 1
is adjusted by the a-3t:H thin film 19, the single mode condition can be easily maintained in the directional coupler section 1, and the thickness of the a-8i:H thin film 19, the waveguide width, and the The degree of freedom in design is increased because the complete coupling length can be controlled by the waveguide spacing, and the single mode condition of the curved optical waveguide section 2 can be easily achieved even if the rib height is higher than that of the directional coupler section 1. single-mode conditions can be achieved. Further, since the waveguide width of the directional coupler section 1 and the waveguide width of the curved optical waveguide section 2 are the same, no particular problem arises regarding connection loss between the directional coupler section 1 and the curved optical waveguide section 2. Furthermore, the a-8t:H4 film can be deposited over a large area and highly uniformly, and can be obtained at a constant thickness with good reproducibility regardless of the surface condition of the wafer. In the optical waveguide, it is possible to precisely control the confinement state of light in the horizontal direction of the straight optical waveguide section. Also,
Since the a-8i:H thin film can easily control the refractive index for long wavelength bands from around 2 to around 4 by changing the deposition conditions, it is suitable for optical waveguides using various semiconductor materials. Therefore, according to the present invention, the radius of curvature of a curved optical waveguide can be improved in a matrix optical switch, etc. in which directional coupler type optical switches are simultaneously integrated, which requires high accuracy in confining light in the lateral direction of the rib waveguide. It is possible to reduce the size of the device.

Note that the present invention is not limited to the above embodiments. Although a GaAs-based material is used in the embodiment, the invention is not limited to this, and the present invention is applicable to other materials such as InP-based materials as long as they are materials for optical waveguides. Further, in this example, etching is performed by dry etching using the RIBE method, but other etching methods may be used as long as the rib portions are formed. For example, reactive ion etching (RIE method) may be used. You can also use wet etching. Furthermore, although parallel plate glow discharge decomposition (GDCVD) was used to deposit the a-3t:H thin film, the method is not limited to this; for example, a photo-CVD method may be used as another deposition method. However, an ECR-plasma CVD method may also be used. In addition, the refractive index of the a-8t:H thin film was controlled by controlling the substrate temperature during deposition.
It is also possible to use a method of controlling the refractive index by using i H4 gas and H2 gas and changing the gas flow rate ratio.

(Effects of the Invention) As described above, according to the present invention, in a semiconductor optical waveguide, even in a curved optical waveguide with a curvature on the order of mm or less, the radiation loss is improved compared to the conventional one, and single mode conditions can be easily achieved. The present invention provides a structure of a semiconductor optical waveguide that can be realized. For this reason, it is possible to know the length occupied by the curved optical waveguide of a semiconductor optical waveguide in an optical functional device such as an optical switch, making it possible to shorten the device, and by minimizing the device length, the optical path length can be shortened. Therefore, it is possible to reduce waveguide loss, and it does not adversely affect other optical devices made of straight optical waveguides on the same substrate.

Furthermore, the a-3t:H thin film can be deposited over a large area and highly uniformly, and can be obtained at a constant thickness with good reproducibility regardless of the surface condition of the wafer. It is possible to precisely control the confinement state of light in the horizontal direction. In addition, the a-3i:H thin film can easily control the refractive index for long wavelength bands from around 2 to around 4 by changing the deposition conditions, making it suitable for optical waveguides using various semiconductor materials. do. Therefore, according to the present invention, the radius of curvature of a curved optical waveguide can be improved in a matrix optical switch in which directional coupler type optical switches that require high accuracy in confining light in the lateral direction of the rib waveguide are simultaneously integrated. It is possible to reduce the size of the device.

[Brief explanation of drawings]

FIG. 1 shows a 4×4 directional coupler 1 consisting of a curved optical waveguide portion 2 and two straight optical waveguides, in which GaAs/All GaAs semiconductor optical waveguides according to the present invention are integrated.
1(A) is a plan view, FIG. 1(B) is a sectional view of the directional coupler section along the dotted line BB' plane in FIG. 1(A), and FIG. (C) shows a cross-sectional view of the curved optical waveguide section along the dotted line CC' plane in FIG. 1(A). Figure 2 shows the radiation loss and radius of curvature R of a curved optical waveguide.
FIG. 2(A) is a characteristic diagram showing the relationship between radiation loss and R when changing the waveguide width within single mode conditions, and FIG. 2(B) is a characteristic diagram showing the relationship between R and FIG. It is a characteristic diagram which shows the relationship between radiation loss and R when the rib height is changed within single mode conditions with the same structure as (A). Figure 3 (A
) is a plan view showing an outline of the figure-8 curve used in the calculation in FIG. 2, and FIG. 3(B) is a cross-sectional view showing the layer structure used in the calculation in FIG. DESCRIPTION OF SYMBOLS 1... Directional coupler part, 2... Curved optical waveguide part, 1
1-n”-GaAs substrate, 12-・n-A, Ilo,
5Gao, sAs first cladding layer, 13-i-G
aAs waveguide layer, 14-1-AfJo, Gao, s A
s Second cladding layer, 15”・p −A, Ilo, s
Gao, s As third cladding layer, 16...p+-G
aAs-q tough 1 layer, 17...n electrode, 18...p
Electrode, 19... aSi:H thin film, 72... Radius of curvature R173... Waveguide width W, 74... Etching depth t.

Claims (1)

[Claims] A semiconductor optical waveguide characterized in that straight and curved rib-shaped optical waveguides are continuously formed, and a hydrogenated amorphous silicon thin film is deposited only on the bottom of the rib of the linear rib-shaped optical waveguide. .