JP2002131713A - Photosemiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the extinction ratio of the light emission from an optical modulation element in the optical modulation element or in a photosemiconductor device having a semiconductor substrate in which the above light modulation element and a laser element are assembled. SOLUTION: The semiconductor substrate including the optical modulation element has a ridge waveguide structure. A light absorbing layer to absorb the incident light according to an electric field is formed under the ridge waveguide structure, and auxiliary absorption layers having a higher absorption coefficient than that of the light absorbing layer are formed on both sides of the ridge waveguide structure.

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 used for a light source of an optical communication system.
In particular, the present invention relates to a light modulation element or an optical semiconductor device including a laser element in addition to the light modulation element.

[0002]

2. Description of the Related Art FIG. 9 shows a conventional optical semiconductor device having a light modulation element and a laser element. In this conventional optical semiconductor device, light generated in the active layer 1 of the laser element LD is guided to the light absorption layer 2 of the light modulation element LM. Light modulation element L
In M, a reverse bias voltage is applied to the light absorbing layer 2, and when this reverse bias voltage is changed, the amount of light absorption changes due to the Franz-Keldysh effect and the quantum confined Stark effect. The light modulated by the above can be output.

However, in the conventional light modulation device LM of an optical semiconductor device, the light absorption layer 2 extends from the lower part of the ridge-type waveguide structure 3 to the lower part of the step surface 4 on both sides thereof, and the ridge in the light absorbing state. The absorption coefficient of the absorption layer 2 in the lower part of the waveguide structure 3 becomes larger than the absorption coefficient in the lower part of the step surface 4, so that the light guided to the lower part of the ridge type waveguide structure 3 is a step having a smaller absorption coefficient. There is a tendency for the light to leak to the lower part of the surface 4, and the light leaking to the lower part of the step surface 4 is emitted with little absorption, and a sufficient extinction ratio indicating the on / off ratio of the emitted light cannot be obtained.

[0004]

SUMMARY OF THE INVENTION The present invention proposes an optical semiconductor device having an optical modulator capable of improving the extinction ratio.

Another object of the present invention is to propose an optical semiconductor device in which an optical modulation element and a laser element capable of improving the extinction ratio are integrated.

[0006]

An optical semiconductor device according to the present invention includes a semiconductor substrate having an optical modulator, the semiconductor substrate having a ridge-type waveguide structure on one principal surface, and the optical modulator includes A light absorbing layer provided on the semiconductor substrate below the waveguide structure for absorbing incident light according to an applied electric field and emitting the light, and the absorption layer provided on the semiconductor substrate on both sides of the ridge waveguide structure. And an auxiliary absorption layer having a larger light absorption coefficient.

Further, an optical semiconductor device according to the present invention comprises:
A semiconductor substrate on which a laser element and a light modulation element are integrated;
This semiconductor substrate has a ridge type waveguide structure on one principal surface,
The laser element has an active layer for generating light in the semiconductor substrate, and the light modulation element is provided on the semiconductor substrate below the ridge-type waveguide structure and receives light from the active layer. It has a light absorption layer that absorbs and emits light according to an electric field, and an auxiliary absorption layer that is provided on the semiconductor substrate on both sides of the ridge waveguide structure and has a larger light absorption coefficient than the absorption layer. .

An optical semiconductor device according to the present invention comprises:
The semiconductor substrate has a plurality of layers laminated on a semiconductor substrate, and the light absorption layer and the auxiliary absorption layer form one and the same layer among the plurality of layers.

An optical semiconductor device according to the present invention comprises:
The semiconductor substrate has a plurality of layers laminated on a semiconductor substrate, and the light absorption layer and the light auxiliary absorption layer form different layers in the plurality of layers.

An optical semiconductor device according to the present invention comprises:
The auxiliary absorption layer is made of the same material as the active layer.

Further, in the optical semiconductor device according to the present invention, the semiconductor substrate has the active layer, the light absorbing layer, and the auxiliary absorbing layer on the semiconductor substrate, and has the contact layer on the active layer and the light absorbing layer. The semiconductor substrate is provided with a common electrode, and the contact layer is provided with an electrode of the laser element and an electrode of the light modulation element.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. 1 is a perspective view showing Embodiment 1 of the optical semiconductor device according to the present invention. This optical semiconductor device has a laser element LD and a light modulation element L
M is an optical modulation element integrated semiconductor laser in which M is integrated on one semiconductor substrate 10.

The semiconductor substrate 10 has an upper main surface 10a and a lower main surface 1
0b and a pair of end faces 10c and 10d facing each other.
The semiconductor substrate 10 has a rectangular parallelepiped shape having a ridge type waveguide structure 11 substantially at the center of the upper main surface 10a. The ridge type waveguide structure 11 extends from the front end face 10c to the rear end face 10d. A pair of stepped surfaces 10e, one step down, on both sides of the ridge type waveguide structure 11 on the upper main surface 10a,
10f is formed.

On the right side of the semiconductor substrate 10, a laser element LD
However, a light modulation element LM is formed on the left side.
On the upper main surface 10a of the semiconductor substrate 10, the cathode electrode 12 of the laser element LD and the cathode electrode 1 of the light modulation element LM are provided.
3 are formed. Cathode electrode 1 of laser element LD
2 is a ridge type waveguide structure 11 and step surfaces 10e on both sides thereof,
10f, and the cathode electrode 13 of the light modulation element LM is connected to the ridge-type waveguide structure 11 and one of the steps 1
It is formed over 0f. On the lower main surface 10b of the semiconductor substrate 10, an anode electrode 14 common to the laser element LD and the light modulation element LM is formed.

The front end face 10c of the semiconductor substrate 10 is an emission face for outputting light, and has a low reflection coating 15 on its face.
Is formed. The low-reflection coating 15 has a low reflectance of, for example, about 3% with respect to light emitted from the semiconductor substrate 10a to the outside, and transmits the emitted light with a high transmittance of about 97%. A high-reflection coating 16 is applied to the rear end face 10d of the semiconductor substrate 10, and almost no light is emitted.

FIG. 2 is a sectional view taken along the line II-II of FIG. 1, which is a sectional view of the laser device LD. In FIG. 2, a semiconductor substrate 10 is configured by laminating a plurality of layers on an upper main surface of a semiconductor substrate 20. The semiconductor substrate 20 is an n-type InP
It is. On this substrate 20, first, InGaAsP-MQ
The W active layer 21 is formed as a first layer. MQW means multiple quantum well. A p-type InP cladding layer 22 is formed thereon as a second layer. The ridge-type waveguide structure 11 is formed at the center of the cladding layer 22,
Step surfaces 10e and 10f are formed on both sides thereof. In the ridge type waveguide structure 11 of the cladding layer 22, the third
A p-type InGaAsP light guide layer 23 is formed as a layer, and a p-type InP contact layer 24 is formed as a fourth layer.

On the upper surface 10a of the semiconductor substrate 10, the upper surface and side surfaces of the ridge-type waveguide structure 11 and the step surfaces 10e, 10f
Is covered with an SiO2 insulating layer 25 so as to cover the entire surface. A Ti / Au electrode layer 26 and an Au plating layer 27 are formed on the SiO 2 insulating layer 25, and the electrode layer 26 is formed on the upper surface of the ridge type waveguide structure 11 through a hole in the insulating film 25 through a contact layer. 24, and constitutes the cathode electrode 12 together with the plating layer 27. The lower surface of the semiconductor substrate 20 is the lower main surface 10b of the semiconductor substrate 10 and is covered with a Ti / Au electrode layer 28 and an Au plating layer 29. The electrode layer 28 and the plating layer 29 constitute the common anode electrode 14.

FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 1, and is a cross-section of the light modulation element LM. In FIG. 3, an InGaAsP-MQW light absorption layer 30 is formed in the center on a semiconductor substrate 20, and an InGaAsP light auxiliary absorption layer 31 is formed on both sides thereof. The light absorption layer 30 and the auxiliary absorption layer 31 of the light modulation element LM are formed by joining the active layer 21 of the laser element LD with the end face on the semiconductor substrate 20, and the first layer on the semiconductor substrate 20 is formed together with the active layer 21. In the first embodiment, the light absorption layer 30 and the auxiliary absorption layer 31 are both in the same first layer.

In FIG. 3, the same p-type InP cladding layer 22 as the laser element LD is formed as a second layer on the light absorption layer 30 and the auxiliary absorption layer 31.
2, a ridge-type waveguide structure 11 is formed at the center of the laser element LD in the same manner as the laser element LD.
f is formed. In the ridge-type waveguide structure 11, the same p as the fourth layer of the laser device LD is formed on the cladding layer 22.
Type InP contact layer 24 is formed, and the upper surface, side surfaces and step surfaces 10e, 10f of the ridge type waveguide structure 11 are formed.
Is coated with an SiO2 insulating layer 25 as in the case of the laser element LD. Further, a Ti / Au electrode layer 36 and an Au plating layer 37 are formed over the ridge-type waveguide structure 11 and one of the step surfaces 10f. A contact is made with the contact layer 24 to form the cathode electrode 13 of the light modulation element LM.

The auxiliary absorption layer 31 is made of a semiconductor material having a longer band gap wavelength than the light absorption layer 30 and has a larger light absorption coefficient than the light absorption layer 30. The light absorbing layer 30 is made of In
In the case of a GaAsP-MQW layer, the auxiliary absorption layer 31 is composed of, for example, an InGaAsP or InGaAs layer. As a result, the light absorption layer 30 of the light modulation element LM
Is absorbed, the light leaking from the lower portion of the ridge-type waveguide structure 11 to both sides thereof is absorbed by the auxiliary absorption layer 31 with a high absorption coefficient, and the amount of light leaking from the emission surface is small. As a result, the extinction ratio of the optical semiconductor device is improved.

In the first embodiment, when the laser element LD is forward-biased and a current is injected into the active layer 21,
Laser oscillation occurs below the ridge waveguide structure 11 at a wavelength determined by the diffraction grating pitch of the light guide layer 23. As a result, light is generated in the active layer 21 below the ridge-type waveguide structure 11. Since the rear end face 10d of the semiconductor substrate 10 has the high reflection coating 16, the generated light travels from the active layer 21 toward the front end face 10c and enters the light absorption layer 30 of the light modulation element LM. In the light modulation element LM, a reverse bias voltage is applied between the substrate 20 and the light absorbing layer 30, and by changing the reverse bias voltage, the electric field applied to the absorbing layer 30 changes. The amount of light absorbed in the light absorption layer 30 changes due to the Franz-Keldysh effect and the quantum confined Stark effect, and the modulated light is emitted from the front end face 10 a of the semiconductor substrate 10. In this light absorption state, the auxiliary absorption layer 31 has a larger ridge-type waveguide structure 1 due to its larger light absorption coefficient.
1 absorbs light that leaks to both sides and increases the extinction ratio.

Embodiment 2 FIG. FIG. 4 shows a second embodiment of the method for manufacturing an optical semiconductor device according to the present invention. Specifically, the method for manufacturing the first embodiment shown in FIGS. 1 to 3 is shown along the process. 4 (a) and (d) to (h) are cross-sectional views taken along the line II-II in FIG. 1, and FIGS. 4 (b) and 4 (c) are perspective views.

In the step of FIG. 4A, a semiconductor substrate 20 made of n-type InP is prepared, and an MOC
The InGaAsP-MQW active layer 21 is formed by a VD method or the like.
Is epitaxially grown.

In the step of FIG. 4B, the active layer 2 is formed on the left side of the semiconductor substrate 20 corresponding to the light modulation element LM.
1 is selectively removed by etching, and the light absorbing layer 30 is selectively epitaxially grown on the removed portion. In this step, a selective MOCVD method or the like is used, and selective etching and selective burying growth are performed.

In FIG. 4C, the light absorbing layer 30 is etched away on both sides thereof, leaving a central portion corresponding to the ridge type waveguide structure 11 in the light absorbing layer 30, and the removed portion is selected. The auxiliary absorption layer 31 is epitaxially grown. Also in this step, a selective MOCVD method or the like is used.

In FIG. 4D, the active layer 21, the light absorbing layer 3
0, a p-type InP cladding layer 22 is grown on the entire upper surface of the auxiliary absorption layer 31, and subsequently, a p-type
An nGaAsP light guide layer 23 is grown. An MOCVD method or the like is used for these growths.

In the next step shown in FIG. 4E, the light guide layer 23 is patterned in a diffraction grating pattern using an interference exposure method. The light guide layer 23 is left in a portion corresponding to the ridge waveguide 11 of the laser device LD, and is removed in other portions.

In FIG. 4F, a p-type InP contact layer 24 is epitaxially grown on the entire upper surface of the semiconductor substrate 20 by MOCVD or the like so as to cover the light guide layer 23. In the process, CH4 or C
layers 22 and 2 by dry etching using l2 or the like.
A ridge type waveguide structure 11 is formed at the center of 3, 24,
Step surfaces 10e and 10f are formed on both sides thereof.

The process shown in FIG. 4G is completed as shown in FIG. First, an SiO2 insulating film 25 is formed on the upper surface, the side surfaces, and the entire surface of the step surfaces 10e and 10f of the ridge-type waveguide structure 11, and holes are formed at positions corresponding to the contact portions of the cathode electrodes 12 and 13. Ti / Au
An electrode layer and an Au plating layer are deposited on the entire surface, and are selectively removed to form cathode electrodes 12 and 13.
A Ti / Au electrode layer and an Au plating layer are also applied to the lower surface of the substrate 20, and the common anode electrode 14 is formed.

Embodiment 3 FIG. 5 is a sectional view showing Embodiment 3 of the present invention. Embodiment 3 is an embodiment of an optical semiconductor device according to the present invention. In the third embodiment, the same laser element LD and light modulation element LM as those in the first embodiment are formed in one semiconductor substrate 10, but the third embodiment is different from the first embodiment. FIG. 5 is a diagram in which the light modulation element LM is changed.
FIG. 13 is a cross-sectional view of the light modulation element LM corresponding to line III. The laser element LD has the same configuration as in the first embodiment.
5, the same parts as those in FIGS. 1 to 3 are denoted by the same reference numerals.

The light modulation element LD shown in FIG.
0 is formed on the entire surface of the light modulation element LM on the semiconductor substrate 20, and the light absorption layer 30 is formed on the ridge-type waveguide structure 11
From the lower part to the lower part of the step surfaces 10e and 10f on both sides thereof. The entire upper surface of the light absorption layer 30 is p
A mold InP cladding layer 22A is applied, and an auxiliary absorption layer 31 is formed on the cladding layer 22A. This auxiliary absorption layer 31 is formed as a layer different from the absorption layer 30. The auxiliary absorption layer 31 is not provided below the ridge type waveguide structure 11, but is provided below the step surfaces 10e and 10f on both sides thereof. A p-type InP cladding layer 22B is coated on the entire upper surface of the auxiliary absorption layer 31, and this cladding layer 22B constitutes the ridge-type waveguide structure 11, and the lower portion is in direct contact with the cladding layer 22A. The ridge-type waveguide structure 11 has a contact layer 24 on the cladding layer 22B.

In the third embodiment, the light absorbing layer 30
Is bonded to the active layer 21 of the laser element LD at its end face, and light generated mainly in the active layer 21 below the ridge waveguide structure 11 is applied to the light absorbing layer 30 and the substrate 20. Although the light is absorbed according to the electric field based on the reverse voltage, the light leaking to both sides of the ridge-type waveguide structure 11 is absorbed by the auxiliary absorption layer 31 with a larger absorption coefficient, which contributes to the improvement of the extinction ratio.

Embodiment 4 FIG. 6 is a sectional view showing Embodiment 4 of the optical semiconductor device according to the present invention. The fourth embodiment is a modification of the light modulation element LM of the first embodiment, similarly to the third embodiment shown in FIG. In the figure, the same parts as those in the first and third embodiments are denoted by the same reference numerals.

The light modulation element LM according to the fourth embodiment has an auxiliary absorption layer 31 on the upper surface of the semiconductor substrate 20 as shown in FIG.
Is provided. The auxiliary absorption layer 31 is not provided below the ridge-type waveguide structure 11, and the step surfaces 10 on both sides thereof are not provided.
e, provided below 10f. This auxiliary absorption layer 31
A p-type InP cladding layer 22C is provided thereon, and the cladding layer 22C is directly bonded to the substrate 20 below the ridge-type waveguide structure 11. The light absorption layer 30 is provided on the entire upper surface of the clad layer 22C, and the clad layer 22 is provided thereon. Also in the fourth embodiment, the auxiliary absorption layer 31 is formed as a layer different from the absorption layer 30.

Also in the fourth embodiment, the light absorbing layer 3
Reference numeral 0 denotes an active layer 21 of the laser element LD, which is bonded at the end face thereof.
Is absorbed by the light absorbing layer 30 in response to an electric field based on the reverse voltage applied between the substrate 20 and the light leaking to both sides of the ridge type waveguide structure 11.
Is absorbed with a larger absorption coefficient, and contributes to the improvement of the extinction ratio.

Embodiment 5 FIG. 7 is a perspective view showing a state in one manufacturing step of an optical semiconductor device according to Embodiment 5 of the present invention. The fifth embodiment is also the first embodiment.
Similarly, the laser element LD and the light modulation element LM are formed in the same semiconductor substrate 10, and FIG.
The step of forming the active layer 21 of D is shown. Other parts are shown in FIG.
This is the same as Embodiments 2 and 3 and Embodiment 2 in FIG.

In the fifth embodiment, the active layer 21 of the laser element LD is directly used as the auxiliary absorption layer 31 of the light modulation element LM.
Used as In the step of FIG. 7, the active layer 21 (auxiliary absorption layer 31) is formed on the entire upper surface of the substrate 20, and thereafter, in the portion corresponding to the ridge-type waveguide structure 11 of the light modulation element LM, the active layer 21 is selected. The light absorbing layer 30 is selectively grown on the removed portion.

According to the fifth embodiment, since the active layer 21 is used as it is as the auxiliary absorption layer 31, the manufacturing process can be simplified and the product cost can be reduced.

Embodiment 6 FIG. FIG. 8 is a perspective view showing Embodiment 6 of the optical semiconductor device according to the present invention. In the sixth embodiment, the optical semiconductor device is configured as an optical modulator. This modulator is basically configured by separating the light modulation element LM shown in FIG. 1 from the laser element LD,
It has the same configuration as the light modulation element LM of FIG. In the figure, the same parts as those of the light modulation element LM in FIGS.

In the sixth embodiment, a low-reflection coating 15 is also formed on a rear end face 10d of the semiconductor substrate 10 opposite to the front end face 10c, so that light to be modulated is incident on the rear end face 10d. Is done. This incident light is incident on the light absorbing layer 30, and the light absorbing layer 30 is
On the way to c, the light emitted from the front end face 10c is modulated according to the strength of the electric field due to the reverse bias voltage applied between the light absorbing layer 30 and the semiconductor substrate 20.

[0041]

As described above, according to the present invention, in a semiconductor substrate having a light modulation element, a light absorbing layer is provided below a ridge-type waveguide structure, and auxiliary absorption layers are provided on both sides of the ridge-type waveguide structure. Since the auxiliary absorption layer has a larger light absorption coefficient than the light absorption layer, the extinction ratio of the light emitted from the light modulation element can be greatly improved.

Further, according to the present invention, in a semiconductor substrate on which a laser element and a light modulation element are integrated, a light absorption layer for absorbing light from an active layer of the laser element is provided below the ridge waveguide structure of the light modulation element. Since the auxiliary absorption layer having a larger light absorption coefficient than the light absorption layer is provided on both sides, the extinction ratio with respect to the light output obtained by modulating the light from the laser element by the light modulation element can be improved.

In the case where the light absorption layer and the auxiliary absorption layer are formed as the same or different layers on the semiconductor substrate as in the present invention, the auxiliary absorption layers are effectively disposed on both sides of the light absorption layer. And the extinction ratio can be effectively improved.

In the present invention, when the active layer of the laser element and the auxiliary absorption layer of the light modulation element are formed of the same material, they can be formed simultaneously of the same material.
The manufacturing process can be simplified and the product cost can be reduced.

Further, in the present invention, a contact layer is provided on the active layer of the laser element and the light absorbing layer of the light modulation element, an electrode of the laser element and an electrode of the light modulation element are provided on the contact layer, and a common electrode is provided on the semiconductor substrate. Is provided, an optical semiconductor device having an improved extinction ratio can be realized under an effective electrode arrangement.

[Brief description of the drawings]

FIG. 1 is a perspective view showing Embodiment 1 of an optical semiconductor device according to the present invention.

FIG. 2 is a cross-sectional view of the laser device taken along line II-II in FIG.

FIG. 3 is a cross-sectional view of the light modulation element along line III-III in FIG.

FIGS. 4A to 4D are views showing a manufacturing process of an optical semiconductor device according to the present invention as a second embodiment, wherein FIGS.
(E), (f), and (g) are cross-sectional views taken along the line II-II in FIG.
(C) is a perspective view.

FIG. 5 is a sectional view showing an optical modulation element according to a third embodiment of the optical semiconductor device according to the present invention.

FIG. 6 is a sectional view showing an optical modulation device according to a fourth embodiment of the optical semiconductor device according to the present invention.

FIG. 7 is a perspective view showing a state of one manufacturing step of an optical semiconductor device according to a fifth embodiment of the present invention;

FIG. 8 is a perspective view showing a sixth embodiment of the optical semiconductor device according to the present invention.

FIG. 9 is a perspective view of a conventional optical semiconductor device.

[Explanation of symbols]

LD laser device, LM light modulation device, 10
Semiconductor substrate, 11 ridge type waveguide structure, 12
Cathode electrode of laser element, 13 Cathode electrode of light modulation element, 14 Common anode electrode, 20
Semiconductor substrate, 21 active layer of laser element, 30 light absorption layer of light modulation element, 31 auxiliary absorption layer of light modulation element, 24 contact layer.

Claims (6)

[Claims] A semiconductor substrate having a light modulation element;
This semiconductor substrate has a ridge type waveguide structure on one principal surface,
The light modulation element is provided on the semiconductor substrate below the ridge-type waveguide structure, a light-absorbing layer that absorbs incident light according to an applied electric field and emits the light, and the light-absorbing layer on both sides of the ridge-type waveguide structure. An optical semiconductor device comprising: an auxiliary absorption layer provided on a semiconductor substrate and having a larger light absorption coefficient than the absorption layer. 2. A semiconductor substrate having a laser element and a light modulation element integrated thereon, the semiconductor substrate having a ridge waveguide structure on one main surface, and the laser element generating light in the semiconductor substrate. A light absorption layer having an active layer, wherein the light modulation element is provided on the semiconductor substrate below the ridge-type waveguide structure, and absorbs and emits light from the active layer according to an applied electric field; An optical semiconductor device comprising: an auxiliary absorption layer provided on the semiconductor substrate on both sides of the ridge waveguide structure and having a light absorption coefficient larger than that of the absorption layer. 3. The semiconductor substrate according to claim 1, wherein the semiconductor substrate has a plurality of layers laminated on the semiconductor substrate, and the light absorption layer and the auxiliary absorption layer form one and the same layer among the plurality of layers. 3. The optical semiconductor device according to 2. 4. The semiconductor substrate according to claim 1, wherein the semiconductor substrate has a plurality of layers laminated on a semiconductor substrate, and the light absorption layer and the light auxiliary absorption layer form different layers in the plurality of layers.
The optical semiconductor device according to the above. 5. The optical semiconductor device according to claim 2, wherein said auxiliary absorption layer is made of the same material as said active layer. 6. The semiconductor substrate has the active layer, the light absorbing layer, and the auxiliary absorbing layer on a semiconductor substrate, has a contact layer on the active layer and the light absorbing layer, and is common to the semiconductor substrate. 6. The optical semiconductor device according to claim 2, wherein an electrode is disposed on the contact layer, and an electrode of the laser element and an electrode of the light modulation element are disposed on the contact layer, respectively.