JPH06265741A - Optical semiconductor device and its production - Google Patents

Abstract

PURPOSE:To strictly control the coupling length of the vertical type directional coupler. CONSTITUTION:This vertical type directional coupler has waveguides at the top and bottom thereof. A buffer layer having a transparent region and absorption region to using light exists between the upper and lower waveguides. This process for production has stages for successively growing a lower clad layer 2, the lower waveguide 3 and an upper clad layer 4 of the lower waveguide on a substrate 1, opening the region including the lower waveguide in a stripe form on this upper clad layer, forming a selective growth mask 9 having the region of the large stripe width and region of the small stripe width in the direction perpendicular to the waveguides on both sides of the aperture, removing this selective growth mask, successively growing the lower clad layer 6 of the upper waveguide, the upper waveguide 7 and the upper clad layer of the upper waveguide on the substrate to cover the buffer layer and working the layers from the upper clad layer down to at least the buffer layer in the stripe form along the waveguide direction so as to make the width smaller than the opening width.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical semiconductor device, and more particularly to a vertical directional coupler for performing an optical switch in an optical communication system.

An optical communication system using an optical fiber as a transmission line has been put into practical use, and in recent years, it has been considered to support a subscriber system. In this system, an optical switch element that can freely branch optical signals plays an important role, and it is desirable to realize a compact optical switch element.

[0003]

2. Description of the Related Art In a directional coupler, two or more waveguides are optically coupled to each other, whereby light is transferred between the waveguides. The conventional vertical directional coupler is as shown in FIG.
Two upper and lower waveguides are formed on a semiconductor substrate, a voltage is applied to one of the waveguides to change the refractive index, and the coupling coefficient of both waveguides is changed to shift light between the waveguides (optical switching). )It is carried out.

In a structure having a plurality of waveguides such as a directional coupler, there are two odd target modes in the field distribution of light. Since the two modes travel at different speeds in the waveguide, the phases of the two modes differ depending on the distance from the starting point of the directional coupler. In a place where the phases of the two modes coincide with each other, the two modes strengthen each other, and, for example, the field distribution of light appears only in the upper waveguide. When the phases are deviated by π / 2, the two modes cancel each other and have a light field distribution only in the lower waveguide.

Usually, the distance from the position where the light field distribution exists only in the upper waveguide to the position where the light field distribution exists only in the lower waveguide is called the complete transition length. All the light entering the upper waveguide is transferred to the lower waveguide.

The complete transition length is determined by the difference in the traveling speeds of the two odd symmetrical modes in the structure having a plurality of waveguides, and this amount greatly depends on the distance between the waveguides and the refractive index. Therefore, when the voltage is not applied, the complete transition length is changed by changing the refractive index with the voltage, and when the voltage is not applied, the light that was put in the upper waveguide is transferred to the lower waveguide, but the voltage is applied. As a result, no light can be extracted from the lower waveguide. In this way, light switching can be performed.

[0007]

In the directional coupler, 2
The coupling length of the individual waveguides (coupling length) determines the proportion of light that travels from one waveguide to the other. In the conventional example, the coupling length is the element length, and it is difficult to strictly control the element length, and the coupling length varies from element to element. Further, in the conventional example, it was difficult to monolithically integrate the elements because the coupling length = the element length.

The object of the present invention is to strictly control the coupling length of a vertical directional coupler.

[0009]

Means for Solving the Problems To solve the above problems, 1) a vertical directional coupler in which two waveguides constituting a coupler are stacked one above the other is used between the upper and lower waveguides. An optical semiconductor device having a buffer layer having a transparent region and an absorption region for light, or 2) a lower clad layer for a lower waveguide on a semiconductor substrate 1,
The step of growing the lower waveguide 3 and the upper clad layer 4 of the lower waveguide in order, and the region including the lower waveguide is opened in a stripe shape on the upper clad layer of the lower waveguide, and on both sides of the opening. Forming a selective growth mask 9 having a region with a large stripe width and a region with a small stripe width in the direction perpendicular to the waveguide,
Growing the buffer layer 5, removing the selective growth mask, covering the buffer layer, the lower clad layer 6 of the upper waveguide on the substrate 6, the upper waveguide 7, the upper clad layer of the upper waveguide This is achieved by a method for manufacturing an optical semiconductor device, which includes a step of sequentially growing 8 and a step of processing from the upper cladding layer to at least the buffer layer in a stripe shape along the waveguide direction so as to be smaller than the opening width. It

[0010]

According to the present invention, when the buffer layers inserted in the upper and lower waveguides are grown, the stripe width of the selective growth mask having a stripe-shaped opening is narrowed in the transparent region and grown in the absorption region. As a result, the band gap of the growth film in both regions is changed by changing the ratio of the raw material taken into the wide area and the narrow area of the opening. That is, the band gaps of both regions are changed. Since there is an absorption region in the part other than the light coupling part, the field distribution does not overlap between the waveguides, and the light is not coupled. However, since the buffer layer is transparent at the coupling portion, coupling occurs between the upper and lower waveguides, and the length of the transparent region becomes the coupling length. As a result, the coupling length is determined by the precision of the mask and can be strictly controlled, and the light transfer ratio between the waveguides can also be strictly controlled.

The reason why the band gap of the buffer layer changes depending on the stripe width of the mask is considered as follows. When the mask is formed on both sides of the opening, it does not grow on the mask, so that there is a concentration gradient of the raw material species from the vapor phase on the mask to the opening, and the concentration of the raw material species diffused by this concentration gradient. different. The wider the mask width, the more raw material species will gather in the openings. In such a growth method, the composition and film thickness of the material grown in the opening changes depending on the widths of the masks on both sides.

When a multiple quantum well (MQW) is grown by this method, the film thickness increases mainly in the wide mask width,
The energy gap shifts to the short wavelength side due to the quantum effect.

[0013]

Embodiments FIGS. 1A and 1B are explanatory views of an embodiment of the present invention. FIG. 1 (A) is a sectional view, and FIG. 1 (B) is a plan view of a selective growth mask for a buffer layer.

In FIG. 1A, a lower clad layer 2, a lower waveguide 3, and an upper clad layer 4 of the lower waveguide 4 are sequentially grown on a semiconductor substrate 1. Next, on the upper clad layer 4 of the lower waveguide, the silicon dioxide (S
The selective growth mask 9 made of iO ₂ ) film is formed, and the buffer layer is formed.
Grow up 5.

At this time, the composition EL (electroluminescence) wavelength of the buffer layer 5 changes depending on the stripe width W of the mask, and the narrow area of the mask stripe becomes the transparent area and the wide area becomes the absorption area.

Next, the mask 9 is removed, the buffer layer 5 processed into a stripe shape is covered, and the lower clad layer 6 of the upper waveguide 6, the upper waveguide 7, and the upper clad layer 8 of the upper waveguide are sequentially formed. grow up.

Next, the upper cladding layer 8 and the buffer layer 5 are etched so as to be smaller than the stripe width of the selectively grown buffer layer 5. Next, an example of specifications of each part is shown.

[0018]

[Table 1] Code composition Conductivity type dopant Impurity concentration (cm ^-3 ) Thickness (μm) 1 InP n Se 1E18 − 2 InP n Se 1E18 1 3 InGaAsP n Se 1E18 0.1 (λ _g = 1.45 μm) 4 InP n Se 1E18 0.2 5 MQW n Se 1E18 − 6 InP n Se 1E18 0.2 7 InGaAsP p Zn 1E18 0.1 (λ _g = 1.45 μm) 8 InP p Zn 1E18 0.1 9 SiO ₂ − − − 1 where the buffer MQW layer 5 is the well 10 nm thickness as a layer
20 layers of InGaAsP (λ _g = 1.55 μm) and 21 layers of InGaAsP (λ _g = 1.3 μm) with a thickness of 10 nm are alternately laminated as a barrier layer.

An example of growth conditions for the buffer MQW layer is shown below. Source gas: arsine (AH ₃ ), phosphine (PH ₃ ), trimethylidium (TMI), triethylgallium (TEG). Gas pressure: 75 Torr Substrate temperature: 700 ° C Mask stripe width: Transparent area (small) 3 μm / Absorption area (large) 10 μm The composition of the buffer layer at this time is energy display (wavelength display), 0.84 eV (1.48 μm) in the transparent area, and 0 in the absorption area.
It becomes 785 eV (1.58 μm).

The wavelength of light used in the directional coupler of the embodiment is 1.5 μm.

[0021]

According to the present invention, the coupling length of the vertical directional coupler can be strictly controlled by the width of the growth mask of the buffer layer. As a result, the rate of light transfer between the waveguides was kept constant and variations among the devices could be suppressed.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a conventional example.

[Explanation of symbols]

 1 semiconductor substrate 2 lower clad layer of lower waveguide 3 lower waveguide 4 upper clad layer of lower waveguide 5 buffer layer 6 lower clad layer of upper waveguide 7 upper waveguide 8 upper clad layer of upper waveguide 9 buffer Layer growth mask

Claims (2)

[Claims] 1. A vertical directional coupler in which two waveguides constituting a coupler are stacked one above the other and having a transparent region and an absorption region for the used light between the upper and lower waveguides. An optical semiconductor device having a buffer layer. 2. A step of growing a lower clad layer (2) of the lower waveguide, a lower waveguide (3), and an upper clad layer (4) of the lower waveguide in this order on a semiconductor substrate (1), A region including the lower waveguide is opened in a stripe shape on the upper clad layer of the waveguide, and a selective growth mask (9) having a region with a large stripe width and a region with a small stripe width in the direction perpendicular to the waveguide on both sides of the opening (9 ) Is formed and the buffer layer (5) is grown, the selective growth mask is removed, the buffer layer is covered, and the lower cladding layer (6) of the upper waveguide and the upper waveguide are formed on the substrate. (7) Step of sequentially growing the upper clad layer (8) of the upper waveguide, and processing in a stripe shape from the upper clad layer to at least the buffer layer along the waveguide direction so as to be smaller than the opening width. A method of manufacturing an optical semiconductor device, comprising: