Integrated optical semiconductor device

Abstract: In an integrated optical semiconductor device having an InGaAsP optical waveguide (13) and an InGaAs light absorption layer (14) integrated together therein, the light absorption layer (14) is formed to become gradually thicker in a traveling direction of light in the optical waveguide so that the effective absorption coefficient of the light absorption layer (14) with respect to the optical waveguide (13) can be set to become gradually larger in the traveling direction in the optical waveguide.

Description: This invention relates to an integrated optical semiconductor device having a semiconductor optical waveguide and a light absorption semiconductor layer integrated therein, and more particularly to an integrated optical semiconductor device having an improved interface portion between the optical waveguide and the light absorption layer. In recent years, an optical communication system using a wavelength of 1.55 µm at which the fiber loss becomes minimum has been put into a practical stage, and higher operation speed and larger capacity have been required. However, the present optical communication system is a system which simply utilizes ON and OFF of light and the system has limited operation speed and capacity. For this reason, coherent optical communication systems utilizing the property of light acting as wave are studied from various points of view. In order to put the optical communication system of high speed and large capacity into practice, it is necessary to integrate electronic devices and optical devices, and an OEIC (opto-electronic-integrated circuit) having electronic elements and optical input/output ports such as light emitting devices/photo detectors integrated therein has been studied for a long period of time. However, in the coherent optical communication system, since the property of light acting as wave is strongly exhibited, it becomes necessary to integrate an optical path such as an optical isolator and an optical waveguide in addition to the above elements, but the study of integration of the optical path and the like is still at the initial stage. Conventionally, as an example of an integrated optical semiconductor device which has long been studied, an integrated optical semiconductor device formed by evanescent wave coupling the optical waveguide and the photoelectric element to each other and integrating them is provided. According to the conventional device, an InGaAsP layer having a band gap corresponding to a wavelength of 1.1 µm is buried in an InP substrate to constitute an optical waveguide. An InGaAs light absorption layer, InAℓAs Schottky barrier layer and a pair of Schottky electrodes are laminated on a portion of the optical waveguide to constitute a light absorption region. In the conventional device with the above construction, the refractive indices of InP and InGaAsP are respectively 3.2 and 3.4, and if an interface at which the refractive index is sharply changed is present, light is reflected at the interface. Further, since the refractive index of InGaAsP is larger than that of InP, light travels inside the InGaAsP waveguide. The field strength E of light at this time exhibits such a distribution that the field is high at the center of the optical waveguide but part of the field leaks to the exterior. If the light absorption layer exists at the foot portion of the electric field distribution, part of the light is absorbed into the light absorbing layer. For this reason, when light is traveling in the light absorption region, the electric field E is gradually decreased. If, in this case, a reverse bias voltage is applied to the optical semiconductor element through a pair of electrodes and the light absorption region is depleted, absorbed light causes electron-hole pairs which are in turn derived out from the electrode as current. In the above integrated optical semiconductor device, some subjects which must be improved are present. First, when layers of different refractive indices are present on the optical waveguide, the effective refractive index of the optical waveguide is changed. Therefore, light which has traveled in the optical waveguide is subjected to sharp variation in the refractive index at the interface between the optical waveguide and light absorption region and consequently part of the light is reflected at the interface. This causes reduction in the absorption efficiency of the photoelectric element and an increase in noise of the laser due to the returned light. Further, since a large portion of incident light is absorbed on the incident side of the light absorption region, electron-hole pairs are densely generated on the incident side of the light absorption region. In the case of a coherent optical receiver, local oscillation light of more than several mW is incident, and if the incident light is so intense, deterioration in the response speed due to the space-charge effect and the breakdown of the Schottky barrier due to the concentration of current may be easily caused. Reverse bias of a PN junction is frequently used as means for depleting the light absorption layer, but also in this case, the PN junction may be easily broken down. If the absorption coefficient of the entire light absorption region is lowered in order to prevent the reflection and space-charge effect, the length of the light absorption region required for absorbing a preset amount of light becomes large, thereby increasing the element area and causing reduction in the response speed due to an increase in the capacity. Thus, in the conventional integrated optical semiconductor device having the semiconductor optical waveguide and the light absorption semiconductor layer laminated together, the degree of reflection of incident light at the interface between the optical waveguide and the light absorption layer is large, thereby causing reduction in the absorption efficiency and an increase in noise of the laser. Further, electron-hole pairs are densely generated on the light incident side (rear side of the optical waveguide with respect to the light traveling direction) of the light absorption region, thus causing deterioration in the response speed and breakdown of the device. JP-63014470 discloses an optical semiconductor device having an optical waveguide layer and an optical absorption layer which is formed in an inclined plane at the end of the optical waveguide layer. The absorption layer is inclined in the depth direction of the optical waveguide, so as to produce a device comprising a single substrate which carries integrally thereon a light-emitting element, the optical waveguide layer and a photodetector. Electronics Letters, Vol. 23, No. 10, pages 501, 502 discloses a monolithic integrated waveguide photodetector comprising a waveguide having a lateral ridge formed thereon. A circular InGaAs absorption layer is formed on the optical waveguide layer so as to partially intersect the ridge. Thus, an integrated waveguide-photoconductor can be produced. An object of this invention is to provide an integrated optical semiconductor device which can suppress the reflection of incident light at the interface between the optical waveguide and the light absorption region and prevent electron-hole pairs from being densely generated on the incident side of the light absorbing region and which is suitable for use in a coherent optical communication system. According to this invention as claimed in claim 1, the magnitude of an effective absorption coefficient of a light absorption region for absorbing light leaked out from an optical waveguide is changed in a traveling direction of light in the optical waveguide. That is, according to this invention as claimed in claim 1, in an integrated optical semiconductor device having a semiconductor optical waveguide and a light absorption semiconductor layer for absorbing light traveling in the optical waveguide integrated therein, the effective absorption coefficient of the light absorption layer with respect to the optical waveguide is set to gradually vary in the traveling direction of light in the optical waveguide. The integrated optical semiconductor device for permitting the effective absorption coefficient to increase in the traveling direction of light may be considered to have the following construction: (1) The light absorption layer is formed to be thin on the light incident side with respect to the traveling direction of the optical waveguide and thick on the light transmitting side. In addition, the light absorption layer is formed by laminating a plurality of layers having different etching properties. Further, the light absorption layer is formed by laminating a plurality of layers having different absorption wavelength ends. This is not claimed. (2) The end face on the light incident side of the light absorption layer with respect to the traveling direction of light in the optical waveguide is set to intersect the optical waveguide at an angle of 1 to 10°. In this case, the end face of the light absorption layer on the light incident side is formed in a stepped form so as to be set perpendicularly to and in parallel with the optical axis of the optical waveguide and the difference in level in the optical axis direction is set to be 1/4 of the wavelength of light traveling in the optical waveguide or an odd multiple thereof. This comes close to the invention as defined in the claims. (3) The optical waveguide and the light absorption layer are integrated in a lateral direction and a distance between the end face of the light absorption layer and the optical waveguide is set to gradually decrease. This is an additional feature of a dependent claim. (4) An electric field applied to the light is set to become gradually intense in the traveling direction of light in the optical waveguide from the light incident side. In this case, a quantum well structure is formed so that the absorption wavelength end of the light absorption layer may be set on the short wavelength side of an intended wavelength of light in the no electric field applying state and on the long wavelength side in the electric field applying state. This is not claimed. (5) The width or thickness of the optical waveguide is gradually reduced in the traveling direction of light. This is not claimed. As described below the effective absorption coefficient (optical coupling efficiency) of the light absorption layer with respect to the optical waveguide is set to gradually vary in the traveling direction of light in the optical waveguide. That is, the effective absorption coefficient of the light absorption region is small on the light incident side of the light absorption region and becomes gradually larger in the light traveling direction. As a result, electron-hole pairs are prevented from being densely generated on the light incident side of the light absorption region so that reduction in the response speed due to the space-charge effect and the breakdown of the device due to concentrated current can be prevented. Further, since the effective refractive index set at this time gradually varies, reflection of the incident light at the interface between the optical waveguide and light absorption region can be suppressed and reduction in the absorption efficiency and an increase in noise of a laser due to returned light can be prevented. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figs. 1A to 1C are perspective views of semiconductor structures in the manufacturing process for an integrated optical semiconductor device according to one embodiment of this invention; Fig. 2 is a plan view of an integrated optical semiconductor device according to another embodiment of this invention in which two sides of a light absorption layer are tapered; Fig. 3 is a perspective view of an integrated optical semiconductor device according to still another embodiment of this invention in which two sides of a light absorption layer are tapered; Fig. 4 is a plan view of an integrated optical semiconductor device according to another embodiment of this invention in which a light absorption layer is formed in a stepped configuration; Fig. 5 is a perspective view of an integrated optical semiconductor device according to still another embodiment of this invention having an optical waveguide of ridge structure; Fig. 6 is a perspective view of an integrated optical semiconductor device according to another embodiment of this invention having an optical waveguide of ridge structure; Fig. 7 is a perspective view of an integrated optical semiconductor device according to still another embodiment of this invention having an optical waveguide of multiple well structure. There will now be described a method for manufacturing an integrated optical semiconductor device according to an embodiment of this invention with reference to Figs. 1A to 1C. As shown in Fig. 1A, an undoped InGaAsP layer 12 having a band gap corresponding to a wavelength of 1.1 µm composition is formed to a thickness of 0.5 µm on an InP substrate 11 by epitaxial growth and then etched by ion etching, wet etching or the like with an SiO 2 film or the like used as a mask 21 so as to form an optical waveguide 13 having a width of 3 µm on the substrate 11. Next, as shown in Fig. 1B, an undoped InP layer 22 is formed by deposition to a thickness of 0.5 µm on the substrate 11 and thus the optical waveguide 13 is buried in the InP layer 22. After this, the mask 21 is removed, an undoped InP etching stop layer 23 is formed to a thickness of 0.4 µm, an undoped InGaAs light absorption layer 14 is formed to a thickness of 0.5 µm and an undoped InAℓAs Schottky barrier layer 15 is formed to a thickness of 0.2 µm in this order by epitaxial growth. Next, as shown in Fig. 1C, the InGaAs light absorption layer 14 and InAℓAs Schottky barrier layer 15 are selectively etched out so that the end faces thereof may intersect the underlying optical waveguide 13 to make a small angle (for example,  = 1°) with respect to the light propagating direction, that is, the lengthwise direction of the optical waveguide. After this, Schottky electrodes 16 are formed on the InAℓAs Schottky barrier layer 15 through the depositing process of gold or the like and the lift-off process, thus completing an integrated optical semiconductor device of this invention. If portions other than the InGaAs/InAℓAs photodiode portion are buried with InP, the integrated optical semiconductor device is formed with a planar structure and becomes more preferable in respect of wiring and reliability. Further, as shown in the drawing, light enters the optical waveguide 13 from the left side thereof, passes through the optical waveguide 13 and is incident on a light absorption region 17. As described before, if the light absorption layer 14 is formed on the optical waveguide 13, light having traveled along the optical waveguide 13 is absorbed into the light absorption layer 14. At this time, if the end face of the light absorption layer 14 intersects the optical waveguide 13 at a small angle as in this embodiment, the effective absorption coefficient of the light absorption layer 14 with respect to the optical waveguide 13, that is, the efficiency of optical coupling between the optical waveguide 13 and light absorption layer 14 is made gradually larger in the traveling direction of light in the optical waveguide 13. In other words, the effective light absorption coefficient of the light absorption region 17 is set to be small on the light incident side and becomes gradually larger as the light travels. Therefore, according to this invention, reduction in the absorption efficiency of the light absorption region 17 on the incident side and the breakdown of the Schottky barrier due to concentration of current which have been problems in the prior art can be prevented. Further, since the effective refractive index in the optical waveguide 13 gradually varies in a range in which the end face of the light absorption layer 14 intersects the optical waveguide 13, reflection of incident light at the interface between the optical waveguide and the light absorption region can be suppressed. In this case, if the angle  between the optical waveguide and the end face of the light absorption layer is set to be smaller, the effect of suppressing the light reflection can be made more effective, but if the angle is set to be too small, the length of the light absorption region becomes excessively large, thereby increasing the capacity and consequently lowering the response speed. In contrast, if the angle is set to be larger than 45°, light reflected at the interface in which the effective refractive index varies is returned to the laser directly or after being repeatedly reflected on the side surfaces of the waveguide. Therefore,  must be set to be less than 45°, and according to the invention,  is set in a range from 1°to 10°. As a result, reduction in the absorption efficiency and an increase in noise of the laser due to returned light can be previously prevented. Further, since the light absorption coefficient in the light absorption region 17 becomes larger as the light travels, it is not necessary to increase the length of the absorption region. Therefore, the area of the photoelectric element an be made small and the capacity of the element can be reduced. The end face of the light absorption layer 14 may be so formed that two sides thereof may be made to diverge in a tapered form and intersect the optical waveguide 13 instead of intersecting only one side thereof with the optical waveguide 13 at a small angle. At this time, as shown in Fig. 2, the effect of this invention can be further enhanced by forming the optical waveguide 13 narrower in a tapered form. This is because light propagating along the optical waveguide 13 is cut off by the tapered configuration of the optical waveguide 13 so that light can be leaked out to the exterior of the optical waveguide 13 without fail. Further, since the light absorption layer 14 is formed in a reversely tapered form, the mode of light can be easily matched between the optical waveguide 13 and the light absorption layer 14. As a result, a light absorption layer having reduced light reflection and loss and high absorption efficiency. As shown in Fig. 3, the absorption efficiency can be further enhanced by disposing a second light absorption layer 14' on both side portions of the optical waveguide 13 in addition to the first light absorption layer 14. At this time, a distance between the second light absorption layer 14' and the optical waveguide 13 is preferably set to be large on the incident side and become gradually smaller in the light traveling direction. That is, the second light absorption layer 14' is formed to have a tapered recess portion which is tapered in a tapering direction opposite to that of the front portion of the first light absorption layer 14. In the embodiment shown in Figs. 1A to 1C, the end face of the light absorption layer 14 which intersects the optical waveguide 13 is formed in a linear form, but the end face is not necessarily formed in a linear form and can be formed in a stepped form as shown in Fig. 4. At this time, if the difference d in level in the direction of the waveguide satisfies the condition that d = λ/4n where λ denotes the wavelength of light passing through the optical waveguide 13 and n denotes the effective refractive index given by the optical waveguide 13 and the light absorption layer 14, the effect of reducing the rate of reflection at the interface between the optical waveguide and the light absorption layer can be further enhanced. For example, if λ is set at 1.55 µm and n is set at 3.3, d is set to 0.12 µm. Since, in practice, it is difficult to attain such a fine patterning process, a value equal to an odd multiple of the above value d may be used. In this case, the same effect of reducing the rate of reflection at the interface as that obtained when d = 0.12 µm can be attained. Further, the optical waveguide 13 is not necessarily formed with the buried structure as shown in Fig. 1C, but it may be formed with a ridge structure as shown in Figs. 5 and 6. That is, with the structure shown in Fig. 5, an optical waveguide layer 11' having an optical waveguide 13 formed to protrude at the central portion thereof is formed on the InGaAsP layer 12 which is formed on the InP substrate 11. An InGaAs light absorption layer and an InAℓAs Schottky barrier layer having inclined end faces which intersect the underlaid optical waveguide 13 at a small angle (for example,  = 1°) are formed on the waveguide layer 13'. According to the structure shown in Fig. 6, the optical waveguide 13 is formed on the substrate 11 and an InP layer 11' is laminated on the optical waveguide 13. The light absorption layer and Schottky barrier layer having inclined end faces which intersect the optical waveguide 13 at a small angle (for example,  = 1°) are formed on the semiconductor structure shown in Fig. 6. Further, the optical waveguide 13 may be formed to have an MQW (multiple quantum well) structure 19 surrounded by a disordered region 18 formed by diffusing impurity such as Zn as shown in Fig. 7. This invention is not limited to the above embodiments. For example, the wavelength of light propagating in the optical waveguide is not limited to 1.55 pm, the composition of the semiconductor is not limited to InP, InGaAsP, and this invention can be applied to another wavelength and the semiconductor of another composition. In addition, another electronic element and optical semiconductor device can be easily integrated by use of this invention. As described above, according to this invention, since concentrated generation of electron-hole pairs on the incident side of the light absorption region can be suppressed, deterioration in the response speed due to the space charge effect and the breakage of the device due to concentration of current can be prevented. Further, since the reflection of incident light at the interface between the optical waveguide and the light absorption region can be suppressed, an increase in noise of the laser by the returned light and deterioration in the absorption efficiency can be prevented.

Claims: An integrated optical semiconductor device comprising: a semiconductor optical waveguide (13) having a light incident end on which light is incident, for guiding the incident light in a preset direction; and a light absorption semiconductor layer (14) integrated together with said semiconductor optical waveguide (13), for absorbing light travelling in said preset direction in said optical waveguide (13); wherein said light absorption layer (14) is formed on said optical waveguide (13) and has an end face inclined at a preset angle toward said travelling direction of light in said optical waveguide along the width direction of the optical waveguide (13) to have an effective absorption coefficient which is small on the incident end side and gradually larger in the light travelling direction, characterized in that said end face of said light absorption layer (14) is set to make an angle of 1° to 10° with respect to the lengthwise direction of said optical waveguide (13). An integrated optical semiconductor device according to claim 1, characterized in that said light absorption layer (14) is formed on said optical waveguide (13) to have a tapered front end portion which becomes wider in the travelling direction of light in said optical waveguide, said front end portion having two sides which intersect said optical waveguide. An integrated optical semiconductor device according to claim 2, characterized in that said optical waveguide (13) is formed to have a front end portion which is tapered in a direction opposite to the tapered direction of said front end portion of said light absorption layer (14). An integrated optical semiconductor device according to claim 3, characterized by further comprising a second light absorption layer (14') disposed near said front end portion of said optical waveguide (13), said second light absorption layer (14') being formed in such a shape that a distance between said second light absorption layer and said front end portion of said optical waveguide is large on the light incident side and becomes gradually smaller in the light travelling direction. An integrated optical semiconductor device according to claim 1, characterised in that said light absorption layer (14) has an end face formed in a stepped configuration whose crest line is inclined with respect to the light travelling direction. An integrated optical semiconductor device according to claim 5, characterized in that said stepped end face has a difference in level equal to λ/4n if the wavelength of light travelling in said optical waveguide (13) is λ and the effective refractive index determined by said optical waveguide (13) and light absorption layer (14) is n. An integrated optical semiconductor device according to claim 1, characterized in that said optical waveguide (13) is formed of an optical waveguide having a buried type structure. An integrated optical semiconductor device according to claim 1, characterized in that said optical waveguide (13) is formed of an optical waveguide having a ridge structure. An integrated optical semiconductor device according to claim 1, characterized in that said optical waveguide (13) is formed of an optical waveguide having a multiple quantum well structure surrounded by an impurity region. An integrated optical semiconductor device according to claim 1, characterized by comprising a second light absorption layer (14'), wherein said optical waveguide layer (13) and second light absorption layer (14') are arranged in a lateral direction and said second light absorption layer (14') has an end face which is slightly inclined with respect to said optical waveguide. An integrated optical semiconductor device according to claim 1, characterized by comprising a second light absorption layer (14'), wherein said optical waveguide layer (13) and second light absorption layer (14) are arranged in a lateral direction and said optical waveguide is formed to become gradually closer to said second light absorption layer (14') in the light traveling direction. An integrated optical semiconductor device according to claim 1, characterized by a semiconductor substrate (11) on which the semiconductor optical waveguide (13) is formed.

Inventors: KURODA FUMIHIKO, SUZUKI NOBUO, KURODA, FUMIHIKO, SUZUKI, NOBUO, KURODA, FUMIHIKO, C/O INTELLECTUAL PROPERTY DIV., SUZUKI, NOBUO, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H01L27
IPC codes: H01L27, G02B6, H01S5