JPH06268196A - Optical integrated device - Google Patents

Abstract

PURPOSE:To provide an optical integrated device with an integration board, on which a high-efficiency dual-valance-type receiver, a high-efficiency semiconductor laser unit, and an optical waveguide are assembled in a well-matched way. CONSTITUTION:An optical waveguide 421 is formed on a semi-insulating substrate 111 made of InP. On the InP substrate 111, an optical coupling element 424, a semiconductor laser 423, and a light receiving element 425 are integrated while they are connected with the optical waveguide 421. The optical waveguide 421, which is of semiconductor rib-type, is formed on an optical waveguide film made of laminated low-density semiconductor layers. Then, the semiconductor laser 423 and an active region 144d of the light-receiving element 425 are put selectively with a given width to the optical waveguide 421. A surrounding part of the active layer 114d is covered with a crystal layer that constitutes a rib with a smaller refractive index than that of the active region 114d. In this case, the optical waveguide 421, the optical coupling element 424, the semiconductor laser 423, and the light-receiving element 425 constitutes a mesa structure with a given width.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an optical waveguide and a light receiving element,
The present invention relates to an optical integrated device for coherent optical communication, which is formed by combining a directional coupling element, a semiconductor laser, and the like.

[0002]

2. Description of the Related Art In recent years, research and development of a coherent optical communication system capable of dramatically increasing the information transmission capacity by using optical frequency multiplexing technology have been actively conducted. In this frequency-multiplexed optical communication system, a method is proposed in which a hybrid configuration of an optical heterodyne receiving device including a semiconductor laser for local oscillation whose wavelength can be changed when selecting a channel, a dual-balance type light receiving element, etc., on the receiving side is proposed. ing. By integrating these on a semiconductor substrate, the connection between optical elements can be realized by a semiconductor optical waveguide, so connection of many optical components becomes unnecessary, and stability and reliability as a receiving system can be improved. It will be possible.

Heretofore, as a typical example of integrating optical components, an integrated optical semiconductor in which various functional elements such as an optical waveguide, a semiconductor laser, an optical coupling element, and a photodiode are integrated on an n-type semiconductor substrate. A device (optical integrated device) has been proposed (JP-A-3-44989). The optical waveguide has a buried rib type structure in which a clad layer is formed of a high resistance semiconductor. The performance of this device is at the stage where the basic operation as a heterodyne receiver has been confirmed, and the reception characteristic is extremely low.

In the above-described conventional optical integrated device, since each functional element is provided on the n-type semiconductor substrate, the semiconductor substrate has a common potential, and a dual balance type receiver in which photodiodes are connected in series is constructed. Was impossible. The function of the dual-balanced receiver is to superimpose a weak signal light containing information with local light with a high output level and receive it with high sensitivity by an optical detector. It is a very important issue to construct with good performance.

In the case of a semiconductor laser, when it is integrated with another functional element to form it, a special technique for forming a diffraction grating at a stepped portion or a very difficult work for controlling the active layer to a predetermined layer Was needed. In the optical waveguide and the optical coupling element, the buried structure of the high resistance semiconductor has a problem that the bending loss and the propagation loss in the waveguide are high, and the diameter is about 10 times the optical waveguide core width of about 1 μm. Since the light emitted from the optical fiber has to be aligned and introduced into the optical waveguide, there is a problem that advanced technology must be used.

In the manufacturing method, a fine patterning step such as the production of a diffraction grating on a step is indispensable. An extra additional layer is provided to prevent excessive etching in this step, a double exposure process, etc. Had to make full use of.
Crystal growth requires active layer formation, high-resistance semiconductor layer formation, and p-type layer formation three times. The first is 7 layers, the second is a high resistance layer, and the third is 4 layers, for a total of 12 crystal layers. It consisted of Overall, it was extremely complicated with more than 50 steps. Further, it is necessary to dope impurities such as iron and chromium to form the high resistance semiconductor layer, which becomes a contamination factor for other crystal layers. In other words, the high purification required to reduce the light propagation loss has been a major problem in crystal growth of the optical waveguide film.

On the other hand, in frequency-multiplexed coherent optical communication, in order to remove the in-phase component of the signal light and the local oscillation light,
A balanced optical receiver as shown in FIG. 14 is used. It is desirable that the two photodiodes used in the balanced optical receiver are monolithically formed on the same substrate in order to realize the removal of the high frequency in-phase component of the intermediate frequency signal. The monolithic integration makes it possible to minimize in-phase component mixing due to wiring imbalance. However,
Such a monolithically integrated photodiode has a problem as described below with reference to the drawings.

FIG. 15 shows a semi-insulating InP substrate 1 with two
125 μm of two mesa type pin photodiodes 2 and 3
It is a principal part cross-section structural diagram of the balanced optical receiving device integrated at intervals. Each photodiode 2, 3 has a thickness of 1.
5 μm n ⁺ Type InP layer 5, 1.4 μm thick n ⁻ Type I
nGaAs light absorption layer 6, 1 μm thick n ⁻ Type InP layer 7
Has a structure in which n are sequentially stacked, and n ⁻ In the InP layer 7, a Zn diffusion region (p ⁺ A mold region 8 is formed. The side surfaces of the n-type semiconductor layers 5, 6 and 7 of the photodiodes 2 and 3 and p ⁺ Electrodes 10 and 11 are formed in the mold region 8, respectively.
The surface of the portion where no is formed is covered with the insulating film 12.

The p-electrode 11 ₂ of the photodiode ₂ and the n-electrode 10 ₃ of the photodiode 3 are connected by the metal wiring 13 ₁ on the insulating film 12 and are not shown in the drawing.
Of the metal bump 14 ₁ . The n-electrode 10 ₂ of the photodiode 2 is connected to the _second metal bump 14 ₂ by the wiring 13 ₂ on the insulating film 12. The p-electrode 11 ₃ of the photodiode 3 is connected to the _third metal bump 14 ₃ by the wiring 13 ₃ on the insulating film 12. A microlens 15 is formed on the back surface of the substrate by dry etching and is covered with a non-reflection coating film. This device is flip-chip mounted on the circuit board with the surface on which the photodiodes 2 and 3 are formed facing down via the metal bumps 14.

When the semiconductor light receiving element of FIG. 15 is used for optical heterodyne reception as the balanced optical receiver of FIG. 14, it is necessary to make the characteristics of the photodiode 2 and the photodiode 3 uniform to increase the common mode component removal ratio. Figure 16 shows this
It is a figure which shows the dark current characteristic of two photodiodes. It can be seen that in the region where the voltage is 20 V or less, the photodiode 2 has a larger dark current and the characteristics are not uniform. This is due to the semi-insulating substrate 1, mainly the substrate 1 and the insulating film 1.
N ^{+ of} two photodiodes 2 and 3 via a channel formed at the interface of 2 This is due to the leak current between the type InP layers 5. That is, as shown in the equivalent circuit of FIG. 17, the leak path through the substrate or the substrate surface has a conductance G _L in parallel with the photodiode 2, so that the photodiode 2 and the photodiode 3 are symmetrical as a balanced optical receiver. The sex is broken.

[0011]

As described above, in the conventional optical integrated device, the high performance dual balance type receiver,
It was difficult to incorporate a high-performance semiconductor laser, a low propagation loss optical waveguide, and the like into an optical integrated device with good matching. Furthermore, it has been difficult to manufacture the above-mentioned high-performance functional device in an optical integrated device by a simple process in which the formation of a high-resistance semiconductor layer which causes contamination in crystal growth is omitted.

The present invention has been made in consideration of the above circumstances, and an object of the present invention is to match a high-performance dual balance type receiver, a high-performance semiconductor laser, a low propagation loss optical waveguide and the like on the same substrate. An object of the present invention is to provide an optical integrated device that can be incorporated with good performance.

[0013]

SUMMARY OF THE INVENTION The essence of the present invention is to provide a highly purified rib type optical waveguide with a small propagation loss of light, an optical coupling element, a semiconductor laser and a waveguide type light receiving device formed in the optical waveguide. An optical integrated device including elements and the like has a structure in which each functional element can be configured with good matching even on a semi-insulating substrate.

That is, the present invention provides an optical integrated device in which an optical waveguide is formed on the main surface of a semiconductor substrate, and an optical coupling element, a semiconductor laser and a light receiving element are integrated by being optically connected to the optical waveguide. The waveguide is a semiconductor rib type waveguide formed on an optical waveguide film in which low-concentration semiconductor layers having different compositions are laminated.The active region of the semiconductor laser and the light receiving element has a predetermined width in a partial region on the optical waveguide. The active region is selectively provided, and the periphery of the active region is covered with a crystal layer forming a rib having a refractive index smaller than that of the active region, and the optical waveguide, the optical coupling device, the semiconductor laser, and the light receiving device have a predetermined width. Is a mesa structure formed by.

More specifically, in order to construct a high performance dual balanced receiver with good symmetry, a semi-insulating semiconductor substrate is used to eliminate optical and electrical crosstalk,
The pn junction area is greatly reduced in order to obtain high-speed response characteristics. In a semiconductor laser, a diffraction grating provided on the active layer is formed with high precision, and the active layer width size and the active layer side width are strictly controlled. In the optical waveguide and the optical coupling element, the height and width of the rib are precisely controlled by the same method as that of the semiconductor laser formation in order to form a rib-type waveguide with high coupling efficiency and little optical propagation loss due to bending or the like. . Further, in order to uniformly control the branching of light in the optical coupling element, electrodes are arranged on the rib and on the optical waveguide film beside the rib. As a whole structure, the structure of the semiconductor laser, the light receiving element, the optical coupling element, the optical waveguide, and the like is made into a mesa structure to prevent light crosstalk between the optical waveguides and current leakage between the functional elements.

In the manufacturing method, in order to precisely form the diffraction grating of the semiconductor laser, a step capable of patterning on a flat growth layer without steps is incorporated. Further, in order to maintain the high-purity crystal growth required for the optical waveguide layer and the light receiving element portion, the crystal growth can be performed twice in total without adopting the high resistance semiconductor crystal formation. The optical integrated device of the present invention is realized by many means. Further, the present invention is characterized by adopting the following configuration.

In an optical integrated device in which an optical waveguide is provided on the main surface of a semi-insulating semiconductor substrate and a light receiving element portion optically connected to the optical waveguide is integrated, the optical waveguide has a plurality of low concentration different compositions. It consists of a semiconductor rib type waveguide formed on the semiconductor laminated waveguide film and a groove provided in the peripheral region along the optical waveguide. An absorption region, an insulator region formed of a semiconductor layer having a bandgap energy larger than that of the low-concentration light absorption layer, and a p-type semiconductor and an n-type semiconductor are arranged in the vicinity of the insulator region sidewall through the insulator region and face the sidewall. And a waveguide type pin light receiving element obtained by removing a part of the p-type semiconductor.

Further, in an optical integrated device in which an optical waveguide is provided on the main surface of a semi-insulating semiconductor substrate and a light receiving element portion optically connected to the optical waveguide is integrated, the optical waveguide has a plurality of different compositions. It consists of a semiconductor rib type waveguide formed on a low concentration semiconductor laminated waveguide film and a groove provided in a peripheral region along the optical waveguide. An insulator region composed of a high density light absorption layer, a semiconductor layer capable of crystal growth in lattice match with the low density light absorption layer and capable of Schottky junction with a metal, and a high resistance formed so as to cover a sidewall of the insulator region. It is characterized by comprising a semiconductor embedded layer and a waveguide type MSM light receiving element made of a metal layer provided on the insulator region.

A first semiconductor layer of the first conductivity type, a second semiconductor layer formed on the first semiconductor layer and including a semiconductor having a bandgap narrower than that of the first semiconductor layer, and a second semiconductor layer In a semiconductor light receiving device in which a plurality of so-called pin photodiodes, which are formed of a third semiconductor layer of the second conductivity type formed on the semiconductor layer of, are integrated on a semi-insulating semiconductor substrate,
A fourth semiconductor layer of the second conductivity type is formed between the first semiconductor layer and the semi-insulating semiconductor substrate.

[0020]

According to the present invention, a plurality of light receiving elements are integrated while realizing optical connection between the functional elements in the optical waveguide, the optical coupling element, the semiconductor laser, the light receiving element and the like without light reflection. The balanced type light receiving elements can be arranged and configured on the same substrate. The optical integrated device in which each functional element is integrated can realize a small-sized, highly reliable and unprecedented high-performance receiver.

Regarding the light receiving element portion, since a light receiving element having no electrical crosstalk can be formed on the semi-insulating semiconductor substrate, a dual balance type light receiving element in which a plurality of light receiving elements are connected in series is provided in the optical integrated device. Can be configured with good symmetry. This light receiving element is 30G
High-speed response characteristics of Hz or higher are excellent, and leakage current is extremely small (1/1000 of the conventional value), which enables high-sensitivity reception.

In the semiconductor laser section, a patterning process on a flat active layer surface is possible, and a diffraction grating can be formed with high precision. Therefore, for example, DFB (Distributed Feedba) which raises diffraction efficiency and oscillates in a stable single axis mode.
ck) The laser can be configured with a low threshold and a high output (twice compared with the conventional one). In addition, the oscillation spectrum line width can be narrowed to 300 kHz or less. Therefore, coherent reception without phase noise is possible. On the other hand, since the width of the active layer can be strictly controlled, the disturbance of the effective refractive index of the active region can be suppressed, the selectivity of the oscillation wavelength can be improved, and the oscillation frequency can be highly stabilized.

In the optical waveguide section and the optical coupling element section, weak signal light is incident on the waveguide with high coupling efficiency, and optical propagation loss due to bending and scattering is reduced even in the waveguide by adopting the active layer removing step. Can be suppressed. In addition, the optical waveguide layer can be highly purified, the optical propagation loss can be reduced to one-third that of the conventional one, and the optical coupling with each functional element is reflected by the structure utilizing the exudation of light. It is possible to improve the coupling efficiency twice as much as the conventional one by eliminating the above.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. (Example 1)

1 to 5 show a manufacturing process of an optical integrated device according to a first embodiment of the present invention. (A) is a plan view, (b) is a semiconductor laser portion of (a). View B-
FIG. 6B is a sectional view taken along the line B ', FIG. 9C is a sectional view taken along the line CC' of the optical coupling element portion of FIG. 7A, and FIG. In this device, an optical coupling element, a dual-balanced type light receiving element section composed of two light receiving elements, and a multi-electrode laser are integrated in two optical waveguides. First, as shown in FIG. 1, semi-insulating InP
A semiconductor laminated film for forming an optical waveguide and a functional element is crystal-grown on the substrate 111. The semiconductor laminated film includes: 112; non-doped InGaAs crystal layer, 1.3 μm composition, thickness 0.3 to 0.6 μm 113; non-doped InP crystal layer, thickness 0.03 to 0.
2 μm 114; non-doped active layer, thickness 0.05-0.3 μm

Are sequentially grown and formed. Non-doped active layer 1
14 has the following layer structure, for example. That is, thickness 1
A multiple quantum well in which 10 layers of a 0-nm non-doped InGaAs crystal layer (1.3 μm composition) and an 8-nm-thick non-doped InGaAs crystal layer (1.5 μm composition) are alternately formed, and a non-doped InP layer of 50 to 200 nm thickness.
It is a crystal layer. If necessary, a non-doped InP buffer layer may be formed with a thickness of 10 nm or more at the interface between the semiconductor laminated film and the substrate.

Next, a first-order diffraction grating having a period of 240 nm is formed on the active layer 114 by the two-beam interference exposure method. At this time, the surface of the active layer is flat. Therefore, a very fine diffraction grating pattern can be formed.

Next, an SiO mask having a window is provided in the active region 114d of the light receiving element portion, and proton irradiation is performed to destroy the above quantum well. The proton irradiation reduces the bandgap energy of the active region 114d and improves the conversion efficiency of the light receiving element. Alternatively, the active layer in the portion where the window is opened is removed, and the InGaAs layer is formed to a thickness of about 0.5 μm by selective crystal growth. In this case, it is necessary to grow the crystal three times. The active region 114d may not be processed as described above.

Next, trenches are formed around the active layers 114b and 114d that are finally left by patterning the active layer 114. The width of the groove is 1 to 3 μm on the side of the active layer. The groove width in the optical waveguide portion is 3 to 8 μm, and the groove width in the optical coupling element portion is 4 to 16 μm. Since the thickness of the active layer is small in the groove forming step, lateral etching can be extremely reduced, and the mask pattern can be set. In the above steps, no active layer is left in the optical coupling element section and the optical waveguide.

Each of the crystal layers has a laminated structure formed by crystal growth using the low pressure MOCVD method or the chloride vapor phase epitaxy method, and the carrier concentrations are all set to 5 × 10 ¹⁵ cm ^-3 or less.

Next, as shown in FIG. 2, the semiconductor laminated film 211 is crystal-grown by the second crystal growth. The semiconductor laminated film 211 includes a non-doped InP crystal layer having a thickness of 0.05 to 0.4 μm and a p-type I layer having a thickness of 0.8 to 1.2 μm.
It is an nP crystal layer. An SiO mask 212 is selectively formed on the crystal layer 211, and then the InP crystal layer 21 is formed.
1 is removed by etching with a hydrochloric acid mixture. In the etching, side etching occurs in the lateral direction to form an inclined surface. This is effective for the electrode wiring described later. The active layers 114 having different compositions play an etching stop effect in the depth direction,
Etching stops.

Then, the active layer 114 is removed by etching. By using a mixed solution of sulfuric acid for etching the active layer 114, the InP layer 2 left under the mask in the above-described process.
11 is not etched and active regions 114b and 114d
Of the InP layer 211 remains in the groove portion described with reference to FIG. At this time, side etching does not occur at all. As a result, the width size of the InP layer 211 can be extremely controlled. Further, the InP layer 113 having a different composition in the depth direction also functions as an etching stop layer, and an extremely flat optical waveguide layer surface can be formed.

Through the steps described above, the InP layer width size of the active region side of the light receiving element and laser section, the optical waveguide, the optical coupling element, etc. can be controlled, and the flatness of the surface of the InP crystal layer 113 can be controlled at the same time. Become. This optical waveguide forming step is very excellent as a method for forming an optical integrated device mainly including an optical waveguide.

Here, a modified example of the steps described with reference to FIGS. 1 and 2 will be described with reference to FIG. 3 (a)-(d)
2 corresponds to FIGS. 2A to 2D, in which the configuration of the optical waveguide is changed to further reduce the optical waveguide loss. As shown in FIG. 3, the optical waveguide layer which is the first semiconductor laminated film is provided on the semi-insulating InP substrate 111 with the following configuration. That is, 222; non-doped InGaAsP crystal layer, 1.3 μm composition, thickness 0.3 to 0.6 μm 223; non-doped InP crystal layer, thickness 0.03 to 0.
2 μm 224; non-doped InGaAsP crystal layer, 1.3 μm composition, thickness 0.03 to 0.3 μm 225; non-doped InP crystal layer, thickness 0.03 to 0.
3 μm is sequentially grown and formed. Then, the non-doped active layer 114
Grow to form. The non-doped active layer 114 has the structure described in FIG.

The forming method up to FIG. 3 may be performed by etching the non-doped InGaAs crystal layer 224 and the non-doped InP crystal layer 225 in addition to the method described in FIG. By etching, after shaping the InP crystal layer 211 formed in the second time, using this as a mask, the illustrated waveguide structure can be created in a self-aligned manner. In this case, the InP crystal layer 211
Need not be provided with a non-doped InP crystal layer, but may be a p-type InP crystal layer only.

Next, as shown in FIG. 4, a protective mask 311 is selectively formed while leaving the SiO mask 212. After that, the optical waveguide layers 112 and 113 are partially removed by etching to partially expose the semi-insulating substrate 111. By forming the electrode wiring described later on this semi-insulating substrate, the wiring capacitance of each functional element can be reduced. In addition, electrical and optical crosstalk between the functional elements can be significantly reduced.

Next, as shown in FIG. 5, a SiN insulator film 411 is formed by the plasma CVD method, and patterned and left selectively. Next, the electrodes 412 to 419 are formed by a metal vapor deposition method. Reference numerals 412 and 413 are electrodes of the semiconductor laser section, 414, 415 and 416 are electrodes of the optical coupling element section, and 417, 418 and 419 are electrodes of the dual balance type light receiving element section, and the elements are insulated from each other. The electrode 412 of the semiconductor laser section is made of Au for p-type semiconductor.
A Zn alloy is used as the electrode 413, and an AuGe alloy is used as an ohmic electrode for an n-type semiconductor, and a wiring electrode in which TiPtAu is laminated is laminated on the ohmic electrode. Similarly, the ohmic electrodes and the wiring electrodes of the optical coupling element portion and the light receiving element portion are also formed at the same time.

Here, in the light receiving element portion, wiring is performed so that the diodes are connected in series to form a dual balanced type light receiving element. As the electrodes of the optical coupling element portion, a plurality of n-type electrodes are provided for one p-type electrode. The semiconductor laser has a variable wavelength, long cavity (about 1 mm) semiconductor laser structure, and three p-type electrodes are provided here. The oscillation wavelength of the semiconductor laser can be changed by controlling the current applied to each electrode.

The optical waveguide portion and the optical coupling element portion are provided in a rib type on the optical waveguide layer 111 having a refractive index of 3.3 to 3.4.
Light is guided by the nP crystal layer 211 (refractive index 3.2). The effective waveguide region width under the rib is close to the core diameter size of the optical fiber, and the optical coupling efficiency is high. The guided light is gradually absorbed in the active layer 114d (refractive index 3.6) of the light receiving portion due to the light seepage effect. Therefore, it becomes possible to receive the light of high output oscillated by the semiconductor laser unit without reflection when the photo-electric conversion is performed by the light receiving element unit and with little unevenness of the place.

A brief description of the heterodyne reception with the above-mentioned configuration will be briefly described. A weak signal light is made incident on the optical waveguide 421 from the light introducing section 420 shown in FIG. 5A, and is oscillated by the semiconductor laser section 423 at the same time. Local light and optical coupling element unit 424
Overlap with. Then, the dual-balance type light receiving element 425 detects the modulated light component placed on the signal light as an intermediate frequency component. At this time, the combined light must be split evenly in the optical coupling element section. In order to perform uniform branching, it is necessary to partially change the refractive index in the optical coupling element by injecting current, and the optical coupling element is provided with five electrodes.

As described above, according to this embodiment, it is possible to construct an optical integrated device in which various functional elements are integrated by an extremely simple manufacturing process using two crystal growths. The functional element consists of a high performance semiconductor laser, a light receiving element, an optical coupling element, an optical waveguide, etc., and can be constructed in a series of steps with good matching.

As a result, a heterodyne beat spectrum generated between the signal light incident from the light introducing portion and the semiconductor laser incorporated in the optical integrated device is obtained, and the received light is electrically converted by the square-law detection of the dual balance type light receiving element. It enables the heterodyne reception that is converted to the beat signal with high accuracy. In particular, the monolithic telodyne receiver (optical integrated device) of the present embodiment in which a dual-balanced type light receiving element is mounted on a semi-insulating substrate can easily perform optical frequency multiplexing in coherent optical communication, and can be used for a multi-channel large-capacity transmission system. Applicable to Further, in the optical integrated device of the present embodiment, the connection between the optical elements can be realized by the short distance semiconductor optical waveguide, so that the light propagation time can be shortened and the strict control can be performed. (Example 2)

In this embodiment, in the optical integrated device, the light receiving element portion is arranged with the pin semiconductor layer in a direction orthogonal to the stacking direction of the semiconductor stacked films, so that electrodes having different potentials can be easily formed on the flattened substrate surface. It can be provided in the. This means that elements having different functions can be integrated on the semi-insulating substrate, and the dual-balance type light receiving element can be configured with good symmetry in the optical integrated device.
By forming the light receiving element portion on the semi-insulating substrate as a waveguide structure and reducing the PN junction area, the parasitic capacitance can be reduced to about 1/5 of that of the conventional one. Therefore, high-speed response is possible.

FIG. 6 is a bird's-eye view showing the structure of an optical integrated device according to the second embodiment of the present invention.
The light receiving element portion of the book is configured to be substantially symmetrical, and one of the optical waveguide and the light receiving portion is shown with a part cut away along the optical waveguide axis. In this embodiment, a dual-balance type light receiving element by a pin type light receiving element is integrated.

In the figure, 511 is a semi-insulating InP substrate,
On this InP substrate 511, a first semiconductor laminated film for forming an optical waveguide is crystal-grown. As the first semiconductor laminated film, 512; non-doped InGaAs crystal layer, thickness 0.3 to
0.6 μm, 513; non-doped InP crystal layer, 0.03 to 0.2 μm
m 514; non-doped InGaAsP crystal layer, thickness 0.0
3 to 0.2 μm 515; non-doped InP crystal layer, thickness 0.05 to 0.
2 μm is sequentially grown and formed. The non-doped InP buffer layer 516 may be formed at the interface with the substrate 511, if necessary. Further, the second semiconductor laminated film for forming the light receiving element portion is continuously crystal-grown on the first semiconductor laminated film. As the second semiconductor laminated film, 517; non-doped InGaAs crystal layer, thickness 0.3 to
0.8 μm 518; non-doped InP crystal layer, thickness 0.3-0.8
.mu.m is sequentially grown and formed.

The above crystal layers have a laminated structure formed by the crystal growth using the low pressure MOCVD method or the chloride vapor phase epitaxy method, and the carrier concentrations are all set to 5 × 10 ¹⁵ cm ^-3 or less.

The first semiconductor laminated film and the second semiconductor laminated film are shaped so that the optical waveguide 519 and the light receiving element portion 5 are formed.
A first groove for forming 20 is formed. A region that has been left by the groove formation at this time is named an optical waveguide mesa. The optical waveguide mesa is divided into an insulator region finally formed of the InGaAs layer 517 and the InP layer 518, and an optical waveguide portion 519 formed by removing the InGaAs layer 517 and the InP layer 518. The finally remaining insulator region absorbs the light propagating through the optical waveguide and converts it into an electric signal.

Next, the n-type semiconductor burying layer 521 is selectively formed by the above-described crystal growth method so as to flatten the first groove. The embedding layer 521 is formed and the unevenness disappears in the uppermost layer of the substrate. A p-type semiconductor 522 is formed by a selective diffusion method or selective crystal growth so as to be in contact with the sidewall surface of the insulator region. 523 is a SiN dielectric film, and 524 to 526 are electrodes. The electrodes are wired so that the light receiving elements are connected in series.

The rib type optical waveguide 519 is formed in a self-aligning manner by selectively etching away the second semiconductor laminated film and the semiconductor burying layer 521 in the region to be the optical waveguide. The groove 527 along the waveguide is formed by removing the portion including the side surface facing the insulator region of the p-type semiconductor, and optically and electrically prevents crosstalk between the two optical waveguides and between the light receiving elements. Therefore, the capacity of the light receiving element is further halved.

With such a structure, even if various elements are integrated on the semi-insulating substrate, they can be insulated from each other.
That is, it is possible to integrate a dual-balance type light receiving element portion for highly sensitive photodetection. The electrode wiring can be formed on a flattened substrate and has a structure suitable for integration. Crosstalk between elements can be eliminated electrically and optically. Next, a method of manufacturing the optical integrated device having the above configuration will be described. First, a method of shaping the laminated second semiconductor laminated film will be described below.

A SiN dielectric film 523 is selectively formed on the second semiconductor laminated film continuously formed on the substrate 511, and the laminated film is wet-etched or reactive-ion-etched by using the SiN dielectric film 523 as a mask. Form a groove. For example, when forming a groove by wet etching,
First, the InP layer 518 is removed with a hydrochloric acid mixed solution. The etching stops at the surface of the InGaAs layer 517. Then, In
The GaAs layer 518 is etched with a mixed solution of sulfuric acid, and the etching is stopped at the InP layer 515.

Thus, the depth can be easily controlled by performing the etching with the etching solution according to the crystal composition. The remaining laminated film is the optical waveguide mesa, and the portion where the InGaAs layer 517 is removed by etching in the process described later becomes the optical waveguide portion, and the remaining portion becomes the light receiving element portion. The optical waveguide mesa is finally a rib type optical waveguide portion with a width of about 2 to 6 μ.
m, and about 2 μm in the light receiving element section.

Next, the n-type semiconductor burying layer 521 is selectively formed so as to flatten the substrate surface. That is, the side surface of the optical waveguide mesa is embedded with an n-type semiconductor. At this time, the point is that the SiN dielectric film 523 described above is left and used as a self-aligned mask. The carrier concentration of the n-type semiconductor is InP set to about 1 to 8 × 10 ¹⁷ cm ^{−3, and the} n-type semiconductor is formed according to the total thickness of the second semiconductor stacked films 517 and 518.

Then, a p-type semiconductor 522 is formed by a selective diffusion method to form a light receiving element portion. p-type semiconductor 52
No. 2 is formed by thermal diffusion of Zn impurities through a mask that is selectively formed. At this time, the SiN dielectric film 523 that has already been formed by overlapping the boundary of the mask on the SiN dielectric film 523 described above. Acts as a diffusion mask, and the p-type semiconductor can be formed so as to contact the side wall surface of the optical waveguide mesa. That is, the p-type semiconductor 522 can be formed in a self-aligned manner in the vicinity of the sidewall of the insulator region, and the pin-type light receiving element portions arranged in the direction orthogonal to the stacking direction can be formed.

As described above, in the manufacturing method of this embodiment, there are four self-alignment steps related to the SiN dielectric film 523 including the connection between the optical waveguide portion and the light receiving element portion.
This enhances the ease of mask alignment work. Next, the electrodes 524, 525 and 526 are formed on the surface of the substrate by the vacuum deposition method and the shaping process.

Then, with the light receiving element portion selectively masked, a rib type optical waveguide is formed by, for example, wet etching. First, the SiN dielectric film 523 is removed. Then, the exposed InP layer is selectively etched. The InP layer is the InP layer 518 of the second semiconductor laminated film.
And the n-type semiconductor burying layer 521. By further etching, the n-type semiconductor burying layer 52 is formed.
The underlying InP layer 515 is etched. At this time, the InP layer 515 under the InGaAs layer is formed of InGa.
The As layer remains as a mask. Since this etching stops at the crystal layers having different compositions, it is the etching that exposes the InGaAs layer 517 and the InGaAs layer 5.
It is 14.

Next, the exposed InGaAs layer 517 and InGaAsP layer 514 are selectively etched.
In this case also, in the optical waveguide, the InP layer 515 under the InGaAsa layer 517 acts as a mask,
The P layer 514 is selectively left. Then, the InP layer 51
3 and 515 act as an etch stop layer to complete a rib type optical waveguide having a flat exposed surface as shown in the figure.

In the above etching process, the etching progresses in a self-aligned manner along the pattern of the optical waveguide mesa initially formed, and finally the rib waveguide 519 can be easily formed.

Finally, the second groove 527 along the waveguide is formed by etching with a hydrogen bromide mixed solution. This groove 52
7, the optical and electrical crosstalk between the waveguides and between the light receiving elements can be greatly reduced.

According to the above steps, the optical waveguide portion 519,
It is not necessary to perform alignment with the light receiving element section 520, and an optical integrated device incorporating the dual balance type light receiving element section can be configured very easily. (Example 3)

FIG. 7 is a sectional view for explaining the third embodiment of the present invention. This embodiment is an optical integrated device in which an MSM type light receiving element is incorporated, and FIG. 7 is a cross-sectional view of the light receiving element portion optically connected to the optical waveguide as seen from the optical waveguide direction, similarly to the second embodiment. .

In the figure, 611 is a semi-insulating InP substrate,
On this InP substrate 611, a first semiconductor laminated film for forming an optical waveguide is formed. The first semiconductor laminated film has the same laminated structure as that described in the second embodiment. Further, the second semiconductor laminated film for forming the light receiving element portion is continuously crystal-grown on the first semiconductor laminated film. As the second semiconductor laminated film, 617; non-doped InGaAs layer, thickness 0.3 to 0.
8 μm 618; undoped InAlAs layer, thickness 0.1-0.
3 μm is sequentially grown and formed. The above crystal layers have a laminated structure formed by crystal growth using the low pressure MOCVD method, and the carrier concentrations are all set to 5 × 10 ¹⁵ cm ⁻³ or less. InA
The lAs crystal layer can be Schottky-connected to a metal, and can form a so-called MSM type light-receiving element by metal-semiconductor-metal connection.

Next, the high resistance semiconductor burying layer 621 is selectively formed by the above-described crystal growth method so as to flatten the first groove. The buried layer 621 is formed and the uppermost layer of the substrate is planarized. 623 is a SiN dielectric film,
24 and 625 are Schottky electrodes.

The above manufacturing method is the same as that of the second embodiment, but there is an advantage that the step of forming the p-type semiconductor can be omitted. With respect to the characteristics of the light receiving element, a high speed response several times higher than that of the second embodiment can be obtained. (Example 4)

FIG. 8 is a sectional view for explaining an optical coupling element optically connected to the optical waveguide section described in the second and third embodiments of the present invention. This optical coupling element is configured by placing two optical waveguides 519 shown in FIG. 6 close to each other.
It has a function of evenly splitting the light propagating through one optical waveguide into two optical waveguides. In this case, it is necessary to apply an electric field or a current to the optical coupling element section to control the refractive index of the waveguide in order to achieve complete even branching.

In the figure, 711 is a semi-insulating InP substrate,
An optical waveguide is formed on the InP substrate 711 by the laminated structure and the manufacturing method described in the first embodiment. One electrode 724 and two electrodes 725 and 726 are provided on both sides of the rib 720 having a protruding shape, and are electrically separated from each other by a SiN dielectric film 723. Here, for example, when the light branching ratio is not 1: 1 when the current is not applied to the optical coupling element, when the current is applied between the electrodes 724 and 725, the refractive index becomes non-uniform in the rib, and the current is controlled by controlling the current. The propagated light can be split evenly.

The optical coupling element in this embodiment is composed of the electrode 72.
The non-uniformity of the current path given from No. 4 is excellent, and the optical branching ratio can be controlled with a current of 1/2 to 1/3 as compared with the conventional device.

As described above, according to the second to fourth embodiments, by arranging the pin semiconductor layers in the light receiving element portion in the direction orthogonal to the stacking direction of the semiconductor stacked films, the electrodes are formed on the same plane. Wiring could be formed, and the dual-balanced type light receiving element could be easily constructed in the optical integrated device.

The dual-balance type light receiving element formed on the semi-insulating substrate achieves a reduction of the pn connection area (1/5 of the conventional structure) as the structure of the present invention, and further, by partially removing the p-type semiconductor layer, the conventional structure. The area has been reduced by half. As a result, the diode capacity is reduced to about 1/10 of the conventional one, and high-speed modulation of 40 GHz or higher is possible. Further, the element isolation groove 527 can reduce the leakage current flowing through the substrate to 1/1000 to 1/10000 of the conventional one and the optical crosstalk to 1/50 to 1/100 of the conventional one. As described above, it is possible to provide an optical integrated device in which the high performance dual balance type receiver is incorporated. (Example 5)

FIG. 9 is a view for explaining the sectional structure of a semiconductor light receiving device according to the fifth embodiment of the present invention. This semiconductor light-receiving element has two pins on a semi-insulating InP substrate 1.
The photodiodes 2 and 3 are integrated. The photodiodes 2 and 3 are each 0.2 μm thick p-type In
P layer 4, undoped InP layer 9 having a thickness of 0.6 μm, n ^{+ having a} thickness of 1.2 μm Type InP layer 5 and n having a thickness of 1.0 μm
^- Type InGaAs light absorption layer 6 and an undoped InP layer 7 having a thickness of 0.8 μm are sequentially laminated,
In the undoped InP layer 7, a Zn diffusion region (p ⁺ A mold region 8 is formed.

An electrode 10 is formed on the exposed surface of the n-type semiconductor layer 5 of each of the photodiodes 2 and 3 by p ^+. An electrode 11 is formed on the mold region 8. A part of the portion where the electrodes 10 and 11 are not formed is covered with the insulating film 12. The p electrode 11 ₂ of the photodiode ₂ and the n electrode 10 _{3 of the} photodiode 3 are connected to the metal wiring 1 on the insulating film 12.
It is connected by 3 ₁ and is connected to the first metal bump 14 ₁ not shown in the figure. Photodiode 2
N electrode 10 ₂ of the by wiring 13 ₂ on the insulating film 12,
It is connected to the second metal bump 14 at ₂ . The p-electrode 11 ₃ of the photodiode 3 is the wiring 1 on the insulating film 12.
It is connected to the _third metal bump 14 ₃ by 3 ₃ .

The wiring between the photodiodes and between the photodiodes and the bumps is a bridge wiring as shown in the figure. A microlens 15 is formed on the back surface of the substrate by dry etching, and is covered with an antireflection coating film 16. This device is flip-chip mounted on the circuit board with the surface on which the photodiodes 2 and 3 are formed facing down via the metal bumps 14.

A manufacturing process of the semiconductor light receiving device according to the fifth embodiment will be described with reference to FIG. First, FIG.
As shown in (a), a 0.2 μm thick p-type InP layer 4, a 0.6 μm thick undoped InP layer 9, and a 1.2 μm thick n ⁺ layer are formed on a semi-insulating InP substrate 1. Type InP layer 5 and 1.0 μm thick n ⁻ Type InGaAs light absorption layer 6
And an undoped InP layer 7 having a thickness of 0.8 μm are stacked by a metal organic chemical vapor deposition (MOCVD) method.

Next, as shown in FIG. 11B, a SiNx film 21 is deposited on the entire surface, and a hole is opened at a predetermined position to form a Zn selective diffusion region 8. Then SiN
The x film 21 is removed, and as shown in FIG. 11C, the photodiode part and the bump pad part are mesa processed to form the SiNx film 12 on the entire surface. After that, the p-electrode 11 and then the n-electrode 10 are formed by lift-off, and the metal wiring 13 is formed at a predetermined position.

Then, as shown in FIG. 11D, the photoresist 22 is used as a mask and the SiNx film 12 and the n ⁺ film are left, leaving a predetermined mesa portion. The type InP layer 5, the undoped InP layer 9, and the p-type InP layer 4 are removed by etching. Of course, a part of the semi-insulating substrate 1 may be removed by etching. At this time, the semiconductor layer under the metal wiring 13 is also removed by side etching, so that a bridge wiring is formed. Finally, the photoresist film 22 is removed,
After polishing the back surface and forming the microlens 15, the semiconductor light receiving device of the first embodiment of the present invention is completed by cutting out into chips and mounting the back surface on the circuit board by flip chip mounting.

FIG. 10 shows an equivalent circuit of the semiconductor light receiving device of the fifth embodiment. Compared with the equivalent circuit of the conventional semiconductor photodetector shown in FIG. 17, the p-type InP layer 4, the undoped InP layer 9 and the n ⁺ The diode D _S formed of the type InP layer 5 is featured in series with the substrate leak path G _L. As shown, a positive voltage to the terminal A, when a negative voltage is applied to the terminal B, it means that a reverse bias is applied to the diode D _S made under the photodiode 2, via the G _L It is possible to reduce current leakage.

The semiconductor layers 4, 5, 9 forming the diode D _S are n ⁻ Since it is transparent to the light of 1.2 to 1.6 μm that is to be received by the type InGaAs light absorption layer 6, no photocurrent flows due to the incident light from the optical fiber. In addition, by packaging so that only light from the optical fiber enters, no photocurrent flows due to external light.

The surface of the semiconductor layers 1, 4, 9 and Si
Since the Nx film 12 and the metal wiring 13 are separated from each other by air, a channel is not formed by the charges induced on the surface of the semiconductor layer. Of course, all or part of this space may be filled with an organic insulating film, such as polyimide, which is unlikely to induce charges.

As a result, the dark current seen from the outside of the photodiode 2 and the photodiode 3, that is, the dark current between the terminals AC and between the terminals BC is small, and the characteristics can be made almost equal. Therefore, a balanced optical receiver having excellent characteristics can be realized as compared with the case of using the conventional semiconductor light receiving device. (Example 6)

FIG. 12 is a sectional view of an element structure showing a planar type semiconductor light receiving device according to the sixth embodiment of the present invention. The structure is almost the same as that of the fifth embodiment, except that the photodiodes 2 and 3 are formed in the recesses previously formed in the semi-insulating substrate 1 so that the whole structure becomes substantially flat. ing. That is, in the groove of the semi-insulating InP substrate 1, n ⁻ Type InP buffer layer 18, p type InP layer 4, undoped InP layer 9, n ⁺ Type InP layer 5, undoped I
The nGaAs light absorption layer 6 and the p-type InP layer 8 are embedded and laminated by a metal organic chemical vapor deposition (MOCVD) method. n ⁺ The electrode 10 is provided on the p-type InP layer 5 and the p-type InP layer 8 is provided.
An electrode 11 is formed on the. In this example, after burying the recesses that are formed by removing a part of the semiconductor surface with polyimide 17,
The wiring 13 is formed. In this embodiment, since the light introduced from the optical fiber is incident from the front surface rather than the back surface, the insulating film 12 also serves as an antireflection coating.

[0081] In this sixth embodiment, as shown in FIG. 12, n ^- The diode D _S2 is also formed between the p-type InP buffer layer 18 and the p-type InP layer 4. Also in this case, the fifth
Similarly to the above embodiment, the leak current through the surface of the substrate 1 can be effectively prevented. (Example 7)

FIG. 13 shows a semiconductor light receiving device using a Schottky photodiode according to the seventh embodiment of the present invention. Thin undoped InAlAs is used as the third semiconductor layer 7, there is no p-type region 8, and the Schottky electrode 3 is used instead of the ohmic electrode 11.
The structure is almost the same as that of the sixth embodiment except that 1 is used.

This light receiving device is made of In, which is not shown in the figure.
InGaAs / In formed at another position on the P substrate 1
It is monolithically integrated with HEMT of AlAs. Each element is also surrounded by a groove 32 in order to prevent leakage due to a channel formed at the interface between the semi-insulating substrate 1 and the insulating film 12. The groove 32 is also filled with the polyimide 17.

Even when such a Schottky photodiode is used, as in the case of the pin photodiode, the p-type InP layer 4 and the undoped In of the present invention are used.
P layer 9, n ⁺ The diode D _S formed of the InP layer 5 is
Effective in reducing substrate leakage current. Further, this diode D _S , together with the groove 32, includes a photodiode and a HEM.
It is also effective in reducing the leak current through the substrate between T.

As described above, according to the fifth to seventh embodiments,
In a semiconductor light receiving element in which a plurality of photodiodes are monolithically integrated, a leak current between the photodiodes via a substrate or a substrate surface can be significantly reduced. As a result, it is possible to realize a balanced optical receiver which is necessary for optical heterodyne reception and has good element symmetry and excellent characteristics.

The present invention is not limited to the above embodiments, but can be variously modified and applied without departing from the scope of the invention. For example, it is not necessary to use a single semiconductor layer as the light absorption layer, and it can be applied to, for example, a light receiving element using a quantum well light absorption layer. In the above embodiment, the light is supposed to enter the substrate vertically,
The present invention can be applied to, for example, a semiconductor light receiving device in which light is incident from the side surface, which is monolithically integrated with an optical waveguide. Also, the semiconductor material is not limited to InP and InGaAs, but InGaAsP, GaAs, and AlG.
aAs, InGaP, InGaAlP, GaSb, In
It can be applied to various semiconductor materials such as As, HgCdTe, and ZnSSe. In addition to the optical heterodyne receiving device, the present invention can be applied to a light receiving device in which a plurality of photodiodes are integrated, for example, isolation between elements of a pin photodiode array for parallel optical transmission.

[0087]

As described in detail above, according to the present invention, 2
It is possible to construct an optical integrated device in which various functional elements are integrated by an extremely simple manufacturing process using single crystal growth. The functional elements are high-performance semiconductor lasers, light-receiving elements,
It consists of an optical coupling element, an optical waveguide, etc., and can be constructed in a series of steps with good matching.

As a result, a heterodyne beat spectrum generated between the signal light incident from the light introducing section and the semiconductor laser incorporated in the optical integrated device is obtained, and the received light is electrically converted by the square-law detection of the dual balance type light receiving element. It enables the heterodyne reception that is converted to the beat signal with high accuracy. In particular, the monolithic telodyne receiver (optical integrated device) of the present invention in which a dual-balance type light receiving element is mounted on a semi-insulating substrate can easily perform optical frequency multiplexing in coherent optical communication, and can be used as a multi-channel large-capacity transmission system. Applicable. Since the optical integrated device of the present invention can realize the connection between the optical elements by the short-distance semiconductor optical waveguide, the optical propagation time can be shortened and strict control can be performed.

[Brief description of drawings]

FIG. 1 is a diagram showing a manufacturing process of an optical integrated device according to a first embodiment.

FIG. 2 is a view showing a manufacturing process of the optical integrated device according to the first embodiment.

FIG. 3 is a diagram showing a modification of the first embodiment.

FIG. 4 is a view showing a manufacturing process of the optical integrated device according to the first embodiment.

FIG. 5 is a diagram showing a manufacturing process of the optical integrated device according to the first embodiment.

FIG. 6 is a bird's-eye view showing an element structure of an optical integrated device according to a second embodiment.

FIG. 7 is a sectional view showing an element structure of an optical integrated device according to a third embodiment.

FIG. 8 is a sectional view showing an element structure of an optical integrated device according to a fourth embodiment.

FIG. 9 is a sectional view of an element structure showing a semiconductor light receiving device according to a fifth embodiment.

FIG. 10 is a diagram showing an equivalent circuit of a semiconductor light receiving device according to a fifth embodiment.

FIG. 11 is a cross-sectional view showing the manufacturing process of the fifth embodiment.

FIG. 12 is a sectional view of an element structure showing a planar type semiconductor light receiving device according to a sixth embodiment.

FIG. 13 is a sectional view of an element structure showing a semiconductor light receiving device according to a seventh embodiment.

FIG. 14 is a diagram showing a configuration of a balanced optical receiver in conventional optical heterodyne reception.

FIG. 15 is a sectional view showing a main part structure of a conventional balanced optical receiving device.

FIG. 16 is a diagram showing dark current characteristics of two photodiodes.

FIG. 17 is a diagram showing an equivalent circuit of a conventional semiconductor light receiving device.

[Explanation of symbols]

111 ... Semi-insulating InP substrate 112 ... Non-doped InGaAs crystal layer 113 ... Non-doped InP crystal layer 114 ... Non-doped active layer 211 ... Semiconductor laminated film 212 ... SiO mask 222 ... Non-doped InGaAsP crystal layer 223 ... Non-doped InP crystal layer 224 ... Non-doped InGaAsP crystal Layer 225 ... Non-doped InP crystal layer 411 ... SiN insulator film 412, 413 ... Electrode of semiconductor laser section 414, 415, 416 ... Electrode of optical coupling element section 417, 418, 419 ... Electrode of dual balance type light receiving element section 420 ... Light introducing section 421 ... Optical waveguide 423 ... Semiconductor laser section 424 ... Optical coupling element section 425 ... Dual balance type light receiving element

Claims (1)

[Claims] 1. An optical coupling element which is formed on a main surface of a semiconductor substrate and is optically connected to the optical waveguide.
In an optical integrated device in which a semiconductor laser and a light receiving element are integrated, the optical waveguide is a semiconductor rib type waveguide formed on an optical waveguide film in which low-concentration semiconductor layers having different compositions are laminated, The active region of the element is selectively provided in a partial region on the optical waveguide with a predetermined width, and the periphery of the active region is formed by a crystal layer forming the rib type waveguide having a smaller refractive index than the active region. An optical integrated device, wherein the optical waveguide, the optical coupling element, the semiconductor laser, and the light receiving element are covered with a mesa structure having a predetermined width.