Light receiving element, light receiving device, and light receiving module

A light receiving element includes a waveguide that includes a waveguide core, a multi-mode interference waveguide that has a width larger than a width of the waveguide, the multi-mode interference waveguide receiving a first light from the waveguide core at a first end, and a photodetection portion that includes a first semiconductor layer and an absorption layer disposed on the first semiconductor layer, the first semiconductor layer including at least one layer and receiving a second light from the multi-mode interference waveguide at a second end, the absorption layer being disposed above the first semiconductor layer and absorbing the second light. A distance from the first end of the multi-mode interference waveguide to the second end of the photodetection portion is longer than 70% of a first length and shorter than 100% of the first length, the first length being a length where self-imaging occurs in the multi-mode interference waveguide.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Applications No. 2011-061843, filed on Mar. 20, 2011, the entire contents of which are incorporated herein by reference.

FIELD

A technology disclosed in embodiments discussed herein is related to a light receiving element, a light receiving device, and a light receiving module.

BACKGROUND

FIG. 1is a perspective view illustrating a main part of a light receiving element100, which is an example of a light receiving element.FIG. 2is a cross-sectional view of the light receiving element100taken along line II-II ofFIG. 1.

The light receiving element100illustrated inFIGS. 1 and 2includes a photodetection portion101disposed on a substrate114and a waveguide portion111disposed on the same substrate114.

The waveguide portion111has a structure in which a waveguide core layer112and an upper clad layer113are stacked from the substrate114side. The waveguide portion111has a mesa structure including the upper clad layer113and the waveguide core layer112. Signal light propagates in the waveguide core layer112and enters the photodetection portion101.

The photodetection portion101has a structure in which the waveguide core layer112, an n-type semiconductor layer102, an i-type absorption layer103, a p-type upper clad layer104, and a p-type contact layer105are stacked from the substrate114side. The photodetection portion101has a mesa structure including the p-type contact layer105, the upper clad layer104, the i-type absorption layer103, and part of the n-type semiconductor layer102. The width of the mesa structure in the photodetection portion101is larger than the width of the mesa structure in the waveguide portion111. In this specification, the width is a length in the direction orthogonal to the direction in which a corresponding waveguide core layer extends, that is, the direction orthogonal to a signal light travel direction, and is a length in the direction parallel to a substrate. The photodetection portion101has a stacked structure including the waveguide core layer112and the n-type semiconductor layer102outside the mesa structure. The waveguide core layer112is shared by the photodetection portion101and the waveguide portion111.

As illustrated inFIG. 2, in the light receiving element100, signal light propagates in the waveguide core layer112in the waveguide portion111, and enters the waveguide core layer112in the photodetection portion101. The signal light then diffuses into the i-type absorption layer103via the n-type semiconductor layer102, which is so-called a spacer layer, and is absorbed by the i-type absorption layer103.

The n-type semiconductor layer102, the i-type absorption layer103, and the upper clad layer104form a PIN-type photodiode (PD). A p-side electrode and an n-side electrode (not illustrated) are connected to the p-type contact layer105and the n-type semiconductor layer102, respectively. A certain voltage for causing the p-side electrode to be at a negative potential and the n-side electrode to be at a positive potential is applied between the p-side electrode and the n-side electrode. Accordingly, photocarriers (holes and electrons) generated through light absorption in the i-type absorption layer103are detected via the upper clad layer104and the n-type semiconductor layer102. Accordingly, the photodetection portion101detects signal light as an electric signal (photocarrier current), and outputs a detection signal (photocarrier current) corresponding to the intensity of the signal light.

FIG. 3is a perspective view illustrating a main part of a light receiving element300, which is another example of a light receiving element.FIG. 4is a cross-sectional view of the light receiving element300taken along line VI-VI ofFIG. 3. The light receiving element300illustrated inFIGS. 3 and 4is different from the light receiving element100illustrated inFIGS. 1 and 2in that a slab region exists between a waveguide portion and a photodetection portion. Other than that point, the light receiving element300is similar to the light receiving element100.

The light receiving element300includes a photodetection portion301disposed on a substrate314and a waveguide portion311disposed on the same substrate314. Furthermore, the light receiving element300includes a slab region321on the substrate314between the waveguide portion311and the photodetection portion301.

The waveguide portion311has a structure in which a waveguide core layer312and an upper clad layer313are stacked from the substrate314side. The waveguide portion311has a mesa structure including the upper clad layer313and the waveguide core layer312. Signal light propagates in the waveguide core layer312and enters the slab region321.

The photodetection portion301has a structure in which the waveguide core layer312, an n-type semiconductor layer302, an i-type absorption layer303, a p-type upper clad layer304, and a p-type contact layer305are stacked from the substrate314side. The photodetection portion301has a mesa structure including the p-type contact layer305, the upper clad layer304, the i-type absorption layer303, and part of the n-type semiconductor layer302. The width of the mesa structure in the photodetection portion301is larger than the width of the mesa structure in the waveguide portion311. The photodetection portion301has a stacked structure including the waveguide core layer312and the n-type semiconductor layer302outside the mesa structure.

The slab region321includes the waveguide core layer312and the upper clad layer313. Part of the upper clad layer313in the slab region321forms a mesa structure of a shape similar to that of the mesa structure in the photodetection portion301. The width of the mesa structure in the slab region321is substantially the same as the width of the mesa structure in the photodetection portion301. Signal light that has entered the slab region321propagates in the waveguide core layer312and enters the photodetection portion301. The slab region321is generated as a result of taking measures for addressing a positioning error of a mask during photoresist exposure in a process of fabricating the photodetection portion301.

As illustrated inFIG. 4, in the light receiving element300, signal light propagates in the waveguide core layer312in the waveguide portion311and the slab region321, and enters the waveguide core layer321in the photodetection portion301. The signal light then diffuses into the i-type absorption layer303via the n-type semiconductor layer302, and is absorbed by the i-type absorption layer303.

The n-type semiconductor layer302, the i-type absorption layer303, and the upper clad layer304form a PIN-type PD. A p-side electrode and an n-side electrode (not illustrated) are connected to the p-type contact layer305and the n-type semiconductor layer302, respectively. A certain voltage for causing the p-side electrode to be at a negative potential and the n-side electrode to be at a positive potential is applied between the p-side electrode and the n-side electrode. Accordingly, photocarriers (holes and electrons) generated through light absorption in the i-type absorption layer303are detected via the upper clad layer304and the n-type semiconductor layer302. Accordingly, the photodetection portion301detects signal light as an electric signal (photocarrier current), and outputs a detection signal (photocarrier current) corresponding to the intensity of the signal light.

An example of the two light receiving elements illustrated inFIGS. 1 to 4is disclosed in Japanese Laid-open Patent Publication No. 07-183484.

FIG. 5illustrates a light intensity distribution of signal light in the light receiving element100illustrated inFIGS. 1 and 2. Referring toFIG. 5, solid lines represent the shape of the waveguide core layer112viewed from the upper side of the substrate114. Dotted-chain lines represent the shape of the i-type absorption layer103viewed from the upper side of the substrate114. Broken lines represent an example of a light intensity distribution of signal light. Arrows indicate radiation directions of signal light.

As described above, in the light receiving element100, the n-side electrode (not illustrated) is connected to the n-type semiconductor layer102. Thus, the width of the n-type semiconductor layer102is larger than the width of the i-type absorption layer103by at least a connection region for the n-side electrode. Accordingly, the width of the waveguide core layer112is larger than the width of the i-type absorption layer103. In contrast, the width of the waveguide core layer112in the waveguide portion111is smaller than the width of the i-type absorption layer103. As a result, signal light enters from the waveguide portion111having a small width into the photodetection portion101having a sufficiently large width.

The waveguide portion111has a mesa structure of a small width, and thus has a strong light confinement effect of confining signal light in the direction orthogonal to the signal light travel directions. In contrast, in the photodetection portion101, an effect of confining incident signal light in the direction orthogonal to the signal light travel direction is obtained by only the i-type absorption layer103, which is a part of the mesa structure, and a portion including a small protrusion of the n-type semiconductor layer102under the i-type absorption layer103. Thus, the light confinement effect is weak.

In the above-described structure, signal light enters from the waveguide portion111having a small width and a strong light confinement effect into the photodetection portion101having a large width and a weak light confinement effect. In this case, after signal light has entered the photodetection portion101, the light intensity distribution of the signal light expands in the direction orthogonal to the signal light travel direction. However, the photodetection portion101has low ability of suppressing the expansion.

Therefore, in the light receiving element100, the light intensity distribution of signal light expands in the direction orthogonal to the signal light travel direction when the signal light propagates in the photodetection portion101, as illustrated inFIG. 5. The radiation direction of the signal light is a direction in which the signal light diffuses in the direction orthogonal to the signal light travel direction, as indicated by the arrows inFIG. 5. That is, the signal light propagates in a diffusion direction.

As a result, part of incident signal light in the waveguide core layer112radiates to a region outside the i-type absorption layer103, not to a region below the i-type absorption layer103. The signal light radiated to a region outside the i-type absorption layer103is not absorbed by the i-type absorption layer103. Thus, in the light receiving element100, light absorption efficiency for incident signal light is not sufficiently increased.

FIG. 6illustrates a light intensity distribution of signal light in the light receiving element300illustrated inFIGS. 3 and 4. Referring toFIG. 6, solid lines represent the shape of the waveguide core layer312viewed from the upper side of the substrate314. Dotted-chain lines represent the shape of the i-type absorption layer303viewed from the upper side of the substrate314. Broken lines represent an example of a light intensity distribution of signal light. Arrows indicate radiation directions of signal light.

As described above, the light receiving element300includes the slab region321between the waveguide portion311and the photodetection portion301, in addition to the structure of the light receiving element100. The width of the waveguide core layer312in the slab region321is large, like the width of the waveguide core layer312in the photodetection portion301. Thus, as in the light receiving element100, signal light enters from the waveguide portion311having a small width into the slab region321having a sufficiently large width.

Like the waveguide portion111, the waveguide portion311has a strong light confinement effect of confining signal light in the direction orthogonal to the signal light travel direction. In contrast, the photodetection portion301has a weak light confinement effect, like the photodetection portion101.

Furthermore, in the slab region321, only the upper clad layer313exists on the waveguide core layer312. Thus, an element for confining signal light that has entered from the waveguide portion311in the direction orthogonal to the signal light travel direction hardly exists in the slab region321. Therefore, the light confinement effect of the slab region321is weaker than that of the photodetection portion301.

In the light receiving element300, signal light enters from the waveguide portion311having a small width and a strong light confinement effect into the slab region321having a sufficiently large width and a light confinement effect weaker than that of the photodetection portion301. In this case, after signal light has entered the slab region321, the light intensity distribution of the signal light expands in the direction orthogonal to the signal light travel direction. However, the slab region321has lower ability of suppressing the expansion than the photodetection portion301.

Therefore, in the light receiving element300, the light intensity distribution of signal light expands in the direction orthogonal to the signal light travel direction when the signal light propagates in the slab region321and the photodetection portion301, as illustrated inFIG. 6. The radiation direction of the signal light is a direction in which the signal light diffuses more significantly in the direction orthogonal to the signal light travel direction than in the light receiving element100(the light intensity distribution inFIG. 5). That is, the signal light propagates more significantly in the diffusion direction. When the propagation distance is the same, the range of the light intensity distribution of the signal light is wider.

As a result, in the light receiving element300, a larger part of incident signal light in the waveguide core layer312radiates to a region outside the i-type absorption layer303, not to a region below the i-type absorption layer303, compared to the light receiving element100. The signal light radiated to the region outside the i-type absorption layer303is not absorbed by the i-type absorption layer303. Thus, in the light receiving element300, it is more difficult to increase light absorption efficiency for incident signal light than in the light receiving element100.

SUMMARY

According to an aspect of the embodiments, a light receiving element includes a waveguide that includes a waveguide core, a multi-mode interference waveguide that has a width larger than a width of the waveguide, the multi-mode interference waveguide being configured to receive a first light from the waveguide core at a first end, and a photodetection portion that includes a first semiconductor layer and an absorption layer disposed on the first semiconductor layer, the first semiconductor layer including at least one layer and being configured to receive a second light from the multi-mode interference waveguide at a second end, the absorption layer being disposed above the first semiconductor layer and being configured to absorb the second light. In addition, a distance from the first end of the multi-mode interference waveguide to the second end of the photodetection portion is longer than (N−0.3)×100% of a first length and shorter than N×100% of the first length, the first length being the shortest length where self-imaging occurs in the multi-mode interference waveguide, N being a natural number.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments will be described.

1. First Embodiment

1-1. Structure of Light Receiving Element700

FIG. 7is a perspective view illustrating an example of the structure of a light receiving element700according to a first embodiment, and illustrates only a main part of the light receiving element700.FIG. 8is a cross-sectional view of the light receiving element700taken along line VIII-VIII ofFIG. 7.FIG. 9Ais a cross-sectional view of the light receiving element700taken along line IXA-IXA ofFIG. 7.FIG. 9Bis a cross-sectional view of the light receiving element700taken along line IXB-IXB ofFIG. 7.FIG. 10Ais a cross-sectional view of the light receiving element700taken along line XA-XA ofFIG. 7.FIG. 10Bis a cross-sectional view of the light receiving element700taken along line XB-XB ofFIG. 7.

In this specification, regarding a surface of a substrate on which the structure of a light receiving element is disposed, a direction away from the surface of the substrate is referred to as “on” or “above”, and a direction of getting closer to the surface of the substrate is referred to as “below” or “under”.

As illustrated inFIGS. 7 and 8, the light receiving element700includes a photodetection portion701disposed on a substrate714, a waveguide portion711disposed on the same substrate714, a slab region721disposed on the same substrate714, and a multi-mode interference (MMI) portion731disposed on the same substrate714.

As illustrated inFIG. 9A, the waveguide portion711has a structure in which a waveguide core layer712and an upper clad layer713are stacked from the substrate714side. The material of the individual layers in the stacked structure is a semiconductor material, for example. The waveguide portion711has a mesa structure including the upper clad layer713and the waveguide core layer712. Signal light propagates in the waveguide core layer712and enters the MMI portion731.

As illustrated inFIG. 10B, the photodetection portion701has a structure in which the waveguide core layer712, an n-type semiconductor layer701, an i-type absorption layer703, a p-type upper clad layer704, and a p-type contact layer705are stacked from the substrate714side. The material of the individual layers in the stacked structure is a semiconductor material, for example. The photodetection portion701has a mesa structure including the p-type contact layer705, the upper clad layer704, the i-type absorption layer703, and part of the n-type semiconductor layer702. The width of the mesa structure in the photodetection portion701is larger than the width of the mesa structure in the waveguide portion711. The photodetection portion701has a stacked structure including the waveguide core layer712and the n-type semiconductor layer702outside the mesa structure. The n-type semiconductor layer702, the i-type absorption layer703, and the upper clad layer704form a PIN-type photodiode (PD).

The n-type semiconductor layer702has a refractive index which is higher than a refractive index of the waveguide core layer712and which is lower than a refractive index of the i-type absorption layer703. That is, the n-type semiconductor layer702has a band gap wavelength which is longer than the band gap wavelength of the waveguide core layer712and which is shorter than the band gap wavelength of the i-type absorption layer703. The n-type semiconductor layer702has a composition in which the absorptance with respect to signal light is sufficiently low.

As illustrated inFIG. 10A, the slab region721includes the waveguide core layer712and the upper clad layer713. In the slab region721, part of the upper clad layer713forms a mesa structure having a shape similar to that of the mesa structure in the photodetection portion701. Unlike the photodetection portion701, the slab region721includes the waveguide core layer712which is slab-shaped (flat), but does not have a mesa structure including an i-type absorption layer and an n-type semiconductor layer. The width of the mesa structure in the slab region721is substantially the same as the width of the mesa structure in the photodetection portion701. Signal light that has entered the slab region721propagates in the waveguide core layer712and enters the photodetection portion701.

The slab region721is generated as a result of providing a margin region at the time of forming a hard mask for forming the mesa structure in the photodetection portion701in order to address a positioning error of a mask during photoresist exposure in the process of fabricating the photodetection portion701, for example. Details will be given below.

As illustrated inFIG. 9B, the MMI portion731has a structure in which the waveguide core layer712and the upper clad layer713are stacked from the substrate714side. The material of the individual layers in the stacked structure is a semiconductor material, for example. The MMI portion731has a mesa structure including the upper clad layer713and the waveguide core layer712. Signal light that has entered the MMI portion731propagates in the waveguide core layer712and enters the slab region721. The waveguide core layer712is shared by the photodetection portion701, the waveguide portion711, the slab region721, and the MMI portion731.

The MMI portion731includes a 1×1 MMI waveguide732including one input and one output. At least part of the waveguide core layer712in the MMI portion731has a width larger than the width of the waveguide core layer712in the waveguide portion711. The large-width portion of the waveguide core layer712functions as the MMI waveguide732. The width of the MMI waveguide732is larger than the width of the mesa structure in each of the waveguide portion711and the photodetection portion701.

It is known that, in an MMI waveguide, a phenomenon called self-imaging occurs as a result of appropriately setting parameters of the MMI waveguide, such as width, length, and refractive index. In self-imaging, a light intensity distribution in an input portion of the MMI waveguide is reproduced in an output portion thereof. The length Lmmi of the MMI waveguide732is set so that a point at which self-imaging in the MMI waveguide732occurs (self-imaging point) is positioned in the photodetection portion701, particularly in a region below the i-type absorption layer703in the waveguide core layer712. More specifically, the length Lmmi of the MMI waveguide732is set so that the distance from an end of the MMI waveguide732on the side where signal light enters (input portion) to an end of the photodetection portion701on the side where signal light enters (incident end) is longer than 70% of a length Lsi at which self-imaging occurs in the MMI waveguide732and is shorter than 100% of the length Lsi. Likewise, the length Lmmi of the MMI waveguide is set so that the distance from an end of the MMI waveguide on the side where signal light enters (input portion) to an end of the photodetection portion on the side where signal light enters (incident end) is longer than (N−0.3)×100% of the length Lsi_min at which self-imaging occurs in the MMI waveguide and shorter than N×100% of the length Lsi_min. The length Lsi_min is the shortest length where self-imaging occurs in the MMI waveguide, and N is a natural number. Self-imaging in the MMI waveguide732and setting of the length Lmmi of the MMI waveguide732will be described below.

As illustrated inFIG. 8, in the light receiving element700, signal light propagates in the waveguide core layer712in the waveguide portion711, and enters the MMI waveguide732in the MMI portion731. As will be described below, the MMI waveguide732converges, at its output portion, signal light in the direction orthogonal to the signal light travel direction. Accordingly, the converged signal light enters the waveguide core layer712in the photodetection portion701via the waveguide core layer712in the slab region721. The signal light then diffuses into the i-type absorption layer703from the waveguide core layer712via the n-type semiconductor layer702, and is absorbed by the i-type absorption layer703.

A p-side electrode and an n-side electrode (not illustrated) are connected to the p-type contact layer705and the n-type semiconductor layer702, respectively. A certain voltage for causing the p-side electrode to be at a negative potential and the n-side electrode to be at a positive potential is applied between the p-side electrode and the n-side electrode. Accordingly, photocarriers (holes and electrons) generated through light absorption in the i-type absorption layer703are detected via the upper clad layer704and the n-type semiconductor layer702. Accordingly, the photodetection portion701detects signal light as an electric signal (photocarrier current), and outputs a detection signal (photocarrier current) corresponding to the intensity of the signal light to an electric circuit in the subsequent stage.

1-2. Setting of Length Lmmi of MMI Waveguide732

FIG. 11illustrates an example of a light intensity distribution in a case where self-imaging occurs in an MMI waveguide1100. Broken lines represent the light intensity distribution of signal light. Arrows indicate radiation directions of signal light.

The MMI waveguide1100illustrated inFIG. 11has parameters, such as width, length, and refractive index, which are set so as to cause self-imaging. The length Lsi at which self-imaging occurs is uniquely determined based on the width of an MMI waveguide and the refractive indices of a waveguide core layer and the layers near the waveguide core layer. It is known that the length Lsi at which self-imaging occurs is expressed by the following equation 1.
Lsi=N·nr·Weff2/λ

In equation 1, nr represents the effective refractive index of the waveguide core layer, Weff represents an effective waveguide width determined based on the width of the MMI waveguide and the refractive index distribution of the waveguide core layer and the layers near the waveguide core layer, and λ represents the wavelength of signal light.

From the viewpoint of decreasing the size and loss of the MMI waveguide, N is typically 1, and thus equation 1 expressing the length Lsi at which self-imaging occurs is as follows (equation 2).
Lsi=nr·Weff2/λ.

As illustrated inFIG. 11, the light intensity distribution of signal light gradually widens as the signal light propagates along the MMI waveguide1100just after the signal light enters the MMI waveguide1100from an input portion1101. The light intensity distribution is the widest at a point when the signal light reaches a position halfway (50%) along the entire length of the MMI waveguide1100(the length along the signal light travel direction or the direction in which the waveguide extends).

After the signal light has passed the position halfway (50%) along the entire length of the MMI waveguide1100, the light intensity distribution gradually narrows as the signal light propagates along the MMI waveguide1100. When the signal light reaches an output portion1102of the MMI waveguide1100, the light intensity distribution is the same as that in the input portion1101. In other words, the light intensity distribution in the input portion1101is reproduced in the output portion1102. That is, in the MMI waveguide1100, a self-imaging point is positioned in the output portion1102.

As is understood fromFIG. 11, the propagation state of signal light in the first-half region (region on the input portion1101side) and the propagation state of signal light in the latter-half region (region on the output portion1102side) are different from each other in the entire region of the MMI waveguide1100.

In the first-half region of the MMI waveguide1100, the radiation directions of the signal light is such directions that the signal light diffuses in the direction orthogonal to the signal light travel direction, as indicated by arrows inFIG. 11. That is, the signal light propagates in a diffusion direction.

In contrast, in the latter-half region of the MMI waveguide1100, the radiation directions of the signal light is such directions that the signal light converges in the direction orthogonal to the signal light travel direction, as indicated by arrows inFIG. 11. That is, the signal light propagates in a convergence direction. Particularly, near the output portion1102, the effect of converging signal light in the direction orthogonal to the signal light travel direction is strong, and the characteristic of converging signal light is remarkable.

In the light receiving element700according to the first embodiment, a point that the characteristic of converging signal light in the direction orthogonal to the signal light travel direction is remarkable near the output portion1102of the MMI waveguide1100is utilized. In the light receiving element700, this remarkable characteristic is utilized to converge a larger part of incident signal light to a region below the i-type absorption layer703in the waveguide core layer712in the photodetection portion701. Details will be given below.

1-2-2. Light Intensity Distribution of Signal Light in Light Receiving Element700

FIG. 12illustrates a light intensity distribution of signal light in the light receiving element700illustrated inFIGS. 7 and 8. Solid lines represent the shape of the waveguide core layer712viewed from the upper side of the substrate714. Dotted-chain lines represent the shape of the i-type absorption layer703viewed from the upper side of the substrate714. Broken lines represent an example of a light intensity distribution of signal light. Arrows indicate radiation directions of signal light.

Referring toFIG. 12, in the MMI waveguide732in the MMI portion731, a self-imaging point is positioned in a region below the i-type absorption layer703in the waveguide core layer712in the photodetection portion701. The length Lmmi of the MMI waveguide732in the MMI portion731is set so that, for example, the distance from an end of the MMI waveguide732on the side where signal light enters (input portion) to an end of the photodetection portion701on the side where signal light enters (incident end) is 85% of the length Lsi at which self-imaging occurs in the MMI waveguide732.

As illustrated inFIG. 12, signal light propagates in the waveguide core layer712and enters from the waveguide portion711into the MMI portion731. Then, as described above with reference toFIG. 11, the signal light converges in the direction orthogonal to the signal light travel direction in the output portion of the MMI waveguide732. As indicated by arrows, in the output portion of the MMI waveguide732, the signal light converges in the direction orthogonal to the signal light travel direction. Accordingly, the signal light enters the slab region721while maintaining a propagation state in the convergence direction.

As described above, in the light receiving element700, the n-side electrode (not illustrated) is connected to the n-type semiconductor layer702. Thus, the n-type semiconductor layer702desirably has a region connected to the n-side electrode in the region outside the mesa structure in the photodetection portion701. Accordingly, the n-type semiconductor layer702does not have a complete mesa structure unlike the i-type absorption layer703, and the width of the n-type semiconductor layer702is larger than the width of the i-type absorption layer703by at least the width of the region connected to the n-side electrode. In the photodetection portion701, the waveguide core layer712is under the n-type semiconductor layer702, and thus the width of the waveguide core layer712is larger than the width of the i-type absorption layer703by at least the width of the region connected to the n-side electrode. In addition, the width of the waveguide core layer712in the slab region721is large, like the width of the waveguide core layer712in the photodetection portion701.

In contrast, the width of the waveguide core layer712in the waveguide portion711is smaller than the width of the i-type absorption layer703. Also, the width of the waveguide core layer712in the MMI portion731is larger than the width of the waveguide core layer712in the waveguide portion711and is sufficiently smaller than the width of the waveguide core layer712in the slab region721and the photodetection portion701. As a result, signal light enters from the waveguide portion711and the MMI portion731having a small width into the slab region721and the photodetection portion701having a sufficiently large width.

The waveguide portion711has a mesa structure of a small width, and thus has a strong light confinement effect of confining signal light in the direction orthogonal to the signal light travel direction. Also in the MMI portion731, the MMI waveguide732has a mesa structure, and thus the light confinement effect is strong.

In contrast, in the photodetection portion701, the effect of confining incident signal light in the direction orthogonal to the signal light travel direction is obtained by the i-type absorption layer703, which is part of the mesa structure, and a portion including a small protrusion of the n-type semiconductor layer702under the i-type absorption layer703. Thus, the light confinement effect is weak.

Furthermore, in the slab region721, only the upper clad layer713exists on the waveguide core layer312. Thus, an element for confining signal light that has entered from the MMI portion731in the direction orthogonal to the signal light travel direction hardly exists in the slab region721. Therefore, the light confinement effect of the slab region721is weaker than that of the photodetection portion701.

Accordingly, in the light receiving element700, as in the light receiving element300illustrated inFIGS. 3 and 4, signal light enters from the waveguide portion711and the MMI portion731having a small width and a strong light confinement effect into the slab region721and the photodetection portion701having a sufficiently large width and a weak light confinement effect. In this case, as in the light receiving element300, the slab region721and the photodetection portion701have low ability of suppressing expansion of the light intensity distribution of signal light in the direction orthogonal to the signal light travel direction.

However, in the light receiving element700, signal light enters the slab region721while maintaining a propagation state in the convergence direction, as described above. Since the self-imaging point of the MMI waveguide732is positioned in a region below the i-type absorption layer703in the waveguide core layer712in the photodetection portion701, signal light may be maintained in the propagation state in the convergence direction also in the slab region721, and thus the signal light does not diffuse. Accordingly, the signal light enters the photodetection portion701while maintaining the propagation state in the convergence direction.

The signal light that has entered the photodetection portion701is absorbed by the i-type absorption layer703. At this time, the self-imaging point of the MMI waveguide732is positioned in a region below the i-type absorption layer703in the waveguide core layer712in the photodetection portion701, and thus the signal light propagates in the convergence direction. Accordingly, a larger part of the incident signal light may be converged to the region below the i-type absorption layer703in the waveguide core layer712, compared to the light receiving elements100and300. Accordingly, a part of signal light that is not absorbed and is radiated to the outside of the i-type absorption layer703may be reduced.

Accordingly, in the light receiving element700, the light absorption efficiency may be increased compared to the light receiving elements100and300. Particularly, in the light receiving element700, the light absorption efficiency may be increased compared to the light receiving element100even if a slab region having a light confinement effect weaker than that of a photodetection portion is disposed between a waveguide portion and a photodetection portion, as in the light receiving element300.

For the reasons described above, when the self-imaging point of the MMI waveguide732is positioned in the photodetection portion701, particularly in a region below the i-type absorption layer703in the waveguide core layer712, the light absorption efficiency may be increased in the light receiving element700, compared to the light receiving element300. This is because, when the self-imaging point of the MMI waveguide732is positioned in the photodetection portion701, a characteristic in which signal light converges in the direction orthogonal to the signal light travel direction near the output portion of the MMI waveguide732may be utilized in a region below the i-type absorption layer703in the waveguide core layer712.

Thus, the length Lmmi of the MMI waveguide732in the MMI portion731is set so that the self-imaging point is positioned in the photodetection portion701, particularly in a region below the i-type absorption layer703in the waveguide core layer712.

1-2-3. Enhancement of Light Absorption Efficiency in Photodetection Portion701

Next, a simulation result will be described to further discuss the setting of the length Lmmi of the MMI waveguide732in the MMI portion731.

FIG. 13illustrates a result obtained by calculating, in a simulation, the relationship between the length Lmmi of the MMI waveguide732and the light absorption efficiency in the photodetection portion701. The horizontal axis indicates the length Lmmi of the MMI waveguide732, and the vertical axis indicates the quantum efficiency in the i-type absorption layer703.

In the simulation illustrated inFIG. 13, it is assumed that the width of the MMI waveguide732is 5 μm, the wavelength of signal light is 1.55 μm, and the waveguide core layer712is formed of an i-type InGaAsP. The width of the i-type absorption layer703in the photodetection portion701is 3 μm. In the simulation illustrated inFIG. 13, it is assumed for easy understanding that the length of the slab region721is 0 μm, that is, the slab region721does not exist.

Under the above-described simulation conditions, the value of the length Lsi at which self-imaging occurs in the MMI waveguide732may be calculated by using the above equation 2, and is about 59 μm (Lsi≈59 μm). InFIG. 13, a case where the length Lmmi of the MMI waveguide732is 0 μm corresponds to a case where the MMI waveguide732does not exist, that is, the structure of the light receiving element100illustrated inFIG. 1.

As is understood fromFIG. 13, when the length Lmmi of the MMI waveguide732is 100% of the length Lsi at which self-imaging occurs (≈59 μm), that is, when the length Lmmi is about 59 μm, the value of the quantum efficiency is substantially equal to the value obtained when the MMI waveguide732does not exist (Lmmi=0 μm), that is, the value obtained when the structure of the light receiving element100illustrated inFIG. 1is used. This is because self-imaging causes the light intensity distribution in the input portion of the MMI waveguide732to be reproduced in the output portion of the MMI waveguide732, and thus the light intensity distribution in a case where the MMI waveguide732does not exist is reproduced at the end of the photodetection portion701on the side where signal light enters (incident end).

When the length Lmmi of the MMI waveguide732is shorter than the length Lsi at which self-imaging occurs (≈59 μm), the value of the quantum efficiency is larger than the value obtained when the MMI waveguide732does not exist (Lmmi=0 μm), that is, the value obtained when the structure of the light receiving element100illustrated inFIG. 1is used. In this case, the self-imaging point of the MMI waveguide732is positioned in the photodetection portion701, particularly in a region below the i-type absorption layer703in the waveguide core layer712. Thus, a larger part of signal light that has entered the photodetection portion701may be converged to the region below the i-type absorption layer703in the waveguide core layer712. Here, a characteristic is utilized in which the effect of converging signal light in the direction orthogonal to the signal light travel direction is strong and the characteristic of converging signal light is remarkable near the output portion1102of the MMI waveguide1100having the length Lsi at which self-imaging occurs, as illustrated inFIG. 11.

When the length Lmmi of the MMI waveguide732is about 70% of the length Lsi at which self-imaging occurs (≈59 μm), that is, when the length Lmmi is about 41 μm, the value of the quantum efficiency is substantially equal to the value obtained when the MMI waveguide732does not exist (Lmmi=0 μm), that is, the value obtained when the structure of the light receiving element100illustrated inFIG. 1is used. Also in this case, the self-imaging point of the MMI waveguide732is positioned in a region below the i-type absorption layer703in the photodetection portion701, but the effect of converging signal light in the direction orthogonal to the signal light travel direction becomes weaker with the distance from the output portion1102of the MMI waveguide portion1100in a state where a light confinement structure in the lateral direction does not exist. Thus, incident signal light is not sufficiently converged to the region below the i-type absorption layer703in the waveguide core layer712in the photodetection portion701.

When the length Lmmi of the MMI waveguide732is smaller than 70% of the length Lsi at which self-imaging occurs (≈59 μm), that is, when the length Lmmi is shorter than about 41 μm, the value of the quantum efficiency is smaller than the value obtained when the MMI waveguide732does not exist (Lmmi=0 μm), that is, the value obtained when the structure of the light receiving element100illustrated inFIG. 1is used. This is because the effect of converging signal light in the direction orthogonal to the signal light travel direction becomes weaker with the distance from the output portion1102of the MMI waveguide1100. Also, this is because the signal light propagates in the diffusion direction when the length Lmmi of the MMI waveguide732is shorter than 50% of the length Lsi at which self-imaging occurs.

When the length Lmmi of the MMI waveguide732is larger than the length Lsi at which self-imaging occurs (≈59 μm), the value of the quantum efficiency is smaller than the value obtained when the MMI waveguide732does not exist (Lmmi=0 μm), that is, the value obtained when the structure of the light receiving element100illustrated inFIG. 1is used. This is because, when the length Lmmi of the MMI waveguide732is larger than the length Lsi at which self-imaging occurs (≈59 μm), signal light propagates in the diffusion direction as in the first-half region of the MMI waveguide1100.

It is considered from the above-described result that the relationship between the length Lmmi of the MMI waveguide732and the quantum efficiency (light absorption efficiency) is as follows. That is, when the length Lmmi of the MMI waveguide732is 100% of the length Lsi at which self-imaging occurs, the value of the quantum efficiency is substantially equal to the value obtained when the MMI waveguide732does not exist. As the length Lmmi of the MMI waveguide732is decreased to a value smaller than 100% of the length Lsi at which self-imaging occurs, the value of the quantum efficiency increases to a value larger than the value obtained when the MMI waveguide732does not exist. However, the value of the quantum efficiency becomes substantially equal to the value obtained when the MMI waveguide732does not exist when the length Lmmi of the MMI waveguide732is decreased to some extent. Then, when the length Lmmi of the MMI waveguide732is further decreased, the value of the quantum efficiency becomes smaller than the value obtained when the MMI waveguide732does not exist.

Accordingly, it is understood that, in the light receiving element700, the light absorption efficiency may be increased compared to the light receiving element100when the length Lmmi of the MMI waveguide732is smaller than 100% of the length Lsi at which self-imaging occurs and is larger than the length at which the same light absorption efficiency as the light absorption efficiency in a case where the MMI waveguide732does not exist may be obtained.

According to the above-described simulation result, in the light receiving element700, the length for obtaining a light absorption efficiency substantially the same as the light absorption efficiency obtained when the MMI waveguide732does not exist is 70% of the length Lsi at which self-imaging occurs (≈59 μm), that is, about 41 μm. Thus, the light absorption efficiency may be increased compared to the light receiving element100when the length Lmmi of the MMI waveguide732is larger than 70% of the length Lsi at which self-imaging occurs and is smaller than 100% of the length Lsi.

In the above-described simulation, it is assumed that the length of the slab region721is 0 μm and that the slab region721does not exist. The above-described simulation result may also be applied to a case where the slab region721having a finite length exists, if the length Lmmi of the MMI waveguide732is replaced with the distance from the end of the MMI waveguide732on the side where signal light enters (input portion) to the end of the photodetection portion701on the side where signal light enters (incident end). This is because, when the slab region721has a finite length, the light intensity distribution obtained when the MMI waveguide732does not exist (the light intensity distribution in the light receiving element100) is reproduced at the incident end of the photodetection portion701if the distance from the input portion of the MMI waveguide732to the incident end of the photodetection portion701matches the length Lsi at which self-imaging occurs in the MMI waveguide732.

Therefore, it is understood that, in the light receiving element700, the light absorption efficiency may be increased compared to the light receiving element100when the distance from the input portion of the MMI waveguide732to the incident end of the photodetection portion701is longer than 70% of the length Lsi at which self-imaging occurs in the MMI waveguide732and is shorter than 100% of the length Lsi. Likewise, it is understood that, in the light receiving element, the light absorption efficiency may be increased compared to the light receiving element100when the distance from the input portion of the MMI waveguide to the incident end of the photodetection portion is longer than (N−0.3)×100% of the length Lsi_min at which self-imaging occurs in the MMI waveguide and shorter than N×100% of the length Lsi_min. The length Lsi_min is the shortest length where self-imaging occurs in the MMI waveguide, and N is a natural number.

Furthermore, it is understood that, in the light receiving element700, the light absorption efficiency may be increased compared to the light receiving element300because the light absorption efficiency in the light receiving element300is lower than the light absorption efficiency in the light receiving element100due to the influence of the slab region321.

The light intensity distribution in the MMI waveguide illustrated inFIG. 11is obtained even if the size of the MMI waveguide is changed. Thus, the characteristic in which the light absorption efficiency may be increased by setting the length Lmmi of the MMI waveguide732to be smaller than 100% of the length Lsi at which self-imaging occurs and to be larger than 70% of the length Lsi is not limited to the specific structure of the MMI waveguide732in the above-described simulation.

1-2-4. Relationship Between Light Absorption Efficiency and Length of Slab Region

FIG. 14illustrates a result obtained by calculating, in a simulation, the relationship between the length of a slab region and the light absorption efficiency in a photodetection portion. The vertical axis indicates the quantum efficiency in an i-type absorption layer, and the horizontal axis indicates the length of a slab region. In this specification, the length is a length in the direction in which a corresponding waveguide extends, that is, the length along the signal light travel direction.

Referring toFIG. 14, a curve (a) indicates a result obtained in the light receiving element700illustrated inFIGS. 7 to 10B, that is, a result obtained when the MMI waveguide732is used. A curve (b) indicates a result obtained in the light receiving elements100and300illustrated inFIGS. 1 to 4, that is, a result obtained when no MMI waveguide exists.

The conditions used in the simulation illustrated inFIG. 14are the same as those used in the simulation illustrated inFIG. 13except that the value of the length of the slab region721is finite. In the curve (a), the length of the MMI waveguide732is 93% of the length Lsi at which self-imaging occurs (≈59 μm), that is, about 55 μm.

As indicated by the curve (a) inFIG. 14, in the light receiving element700, the value of the quantum efficiency is almost uniform regardless of the length of the slab region721. More specifically, when the length of the slab region721is 0 to about 18 μm, the value of the quantum efficiency is equal to or larger than the value of the quantum efficiency obtained when the slab region721does not exist (when the length of the slab region721is 0 μm). Thus, it is understood that, in the light receiving element700, the value of the quantum efficiency equal to or larger than the value obtained when the slab region721does not exist (the length of the slab region721is 0 μm) may be maintained by providing the MMI waveguide732, even if the length of the slab region721increases.

In contrast, when no MMI waveguide exists, the value of the quantum efficiency is maximum when no slab region exists (when the length of the slab region is 0 μm), that is, in the light receiving element100, as indicated by the curve (b) inFIG. 14. On the other hand, when the slab region321having a finite length exists, that is, in the light receiving element300, the value of the quantum efficiency monotonously decreases as the length of the slab region321increases.

When no slab region exists (when the length of the slab region is 0 μm), the value of the quantum efficiency in the light receiving element700(curve (a)) is larger than the value of the quantum efficiency in the light receiving element100(curve (b)). In addition, the difference in quantum efficiency between the light receiving element700(curve (a)) and the light receiving element300(curve (b)) becomes larger as the length of the slab region increases.

It is understood from the above-described result that, in the light receiving element700, the light absorption efficiency may be increased, as in a case where no slab region exists, by forming the MMI waveguide732even if the slab region721having a light confinement effect weaker than that of the photodetection portion701is formed between the waveguide portion711and the photodetection portion701. Furthermore, it is understood that, even if the length of the slab region721is increased in the light receiving element700, the influence of the slab region721may be decreased, and a quantum efficiency substantially the same as that in a case where no slab region exists may be obtained.

1-3. Specific Example of Structure of Light Receiving Element700

FIG. 15is a perspective view illustrating an example of the structure of a light receiving element1500, and illustrates only a main part of the light receiving element1500. The structure of the light receiving element1500illustrated inFIG. 15is a specific example of the structure of the light receiving element700illustrated inFIG. 7.FIG. 15specifically illustrates an exemplary structure of the individual layers in the light receiving element700.FIG. 16is a cross-sectional view of the light receiving element1500taken along line XVI-XVI ofFIG. 15.FIG. 17Ais a cross-sectional view of the light receiving element1500taken along line XVIIA-XVIIA ofFIG. 15, and illustrates a mesa structure and the vicinity thereof.FIG. 17Bis a cross-sectional view of the light receiving element1500taken along line XVIIB-XVIIB ofFIG. 15, and illustrates a mesa structure and the vicinity thereof.FIG. 18Ais a cross-sectional view of the light receiving element1500taken along line XVIIIA-XVIIIA ofFIG. 15, and illustrates a mesa structure and the vicinity thereof.FIG. 18Bis a cross-sectional view of the light receiving element1500taken along line XVIIIB-XVIIIB ofFIG. 15, and illustrates a mesa structure and the vicinity thereof.

As illustrated inFIGS. 15 to 18B, the light receiving element1500includes, for example, a photodetection portion1501disposed on a semi-insulating (SI) substrate1514composed of InP, a waveguide portion1511disposed on the same substrate1514, a slab region1521disposed on the same substrate1514, and an MMI portion1531disposed on the same substrate1514. The SI-InP substrate1514is a substrate doped with an element that forms a deep impurity level, such as Fe.

The waveguide portion1511has a structure in which a waveguide core layer1512and an upper clad layer1513are stacked from the SI-InP substrate1514side. The waveguide core layer1512is composed of i-type InGaAsP having a band gap wavelength of 1.05 μm. The upper clad layer1513is composed of i-type InP. The waveguide portion1511has a mesa structure including the i-InP upper clad layer1513, the i-InGaAsP waveguide core layer1512, and part of the SI-InP substrate1514, and has a high mesa waveguide structure the side surfaces of which are not embedded with a semiconductor material. The i-InGaAsP waveguide core layer1512has a band gap wavelength of 1.05 μm with respect to signal light of 1.55 μm, and thus has a low absorptance for signal light.

The MMI portion1531has a structure in which the i-InGaAsP waveguide core layer1512and the i-InP upper clad layer1513are stacked from the SI-InP substrate1514side. The MMI portion1531has a mesa structure including the i-InP upper clad layer1513, the i-InGaAsP waveguide core layer1512, and part of the SI-InP substrate1514. The MMI portion1531has the same stacked structure as that of the waveguide portion1511. However, the width of the i-InGaAsP waveguide core layer1512in the MMI portion1531is larger than the width of the i-InGaAsP waveguide core layer1512in the waveguide portion1511. Accordingly, the i-InGaAsP waveguide core layer1512in the MMI portion1531forms a 1×1 MMI waveguide1532including one input and one output.

The photodetection portion1501has a structure in which the i-InGaAsP waveguide core layer1512shared by the waveguide portion1511and the MMI portion1531, a semiconductor layer1502, an absorption layer1503, an upper clad layer1504, and a p-type contact layer1505are stacked from the SI-InP substrate1514side. The semiconductor layer1502is composed of n-type InGaAsP having a band gap wavelength of 1.3 μm. The absorption layer1503is composed of i-type InGaAs which is lattice-matched to InP. The upper clad layer1504is composed of p-type InP. The p-type contact layer1505has a two-layer structure of p-type InGaAs and InGaAsP. The n-InGaAsP semiconductor layer1502has a band gap wavelength of 1.3 μm with respect to signal light of 1.55 μm, and thus has a low absorptance for signal light.

The photodetection portion1501has a mesa structure including the p-type contact layer1505, the p-InP upper clad layer1504, the i-InGaAs absorption layer1503, and part of the n-InGaAsP semiconductor layer1502, and has a high mesa waveguide structure the side surfaces of which are not embedded with a semiconductor material. The n-InGaAsP semiconductor layer1502, the i-InGaAs absorption layer1503, and the p-InP upper clad layer1504form a PIN-type photodiode.

A p-side metal electrode1515is disposed on the p-type contact layer1505. An n-side metal electrode1516is disposed on the n-InGaAsP semiconductor layer1502. In the light receiving element1500, the portion that is not provided with the p-side metal electrode1515and the n-side metal electrode1516is covered with a passivation film1517composed of a dielectric material, such as a silicon-nitride film. InFIG. 15, the passivation film1517is not illustrated for easy understanding of the structure.

A certain voltage for causing the p-side metal electrode1515to be at a negative potential and the n-side metal electrode1516to be at a positive potential is applied between the p-side metal electrode1515and the n-side metal electrode1516. Accordingly, photocarriers (holes and electrons) generated through light absorption in the i-InGaAs absorption layer1503are detected via the p-InP upper clad layer1504and the n-InGaAsP semiconductor layer1502.

The slab region1521has a structure in which the i-InGaAsP waveguide core layer1512and the i-InP upper clad layer1513are stacked from the SI-InP substrate1514side. In the slab region1521, part of the i-InP upper clad layer1513forms a mesa structure having a shape similar to that of the mesa structure in the photodetection portion1501.

In the waveguide portion1511, the i-InGaAsP waveguide core layer1512has a thickness of 0.5 and the i-InP upper clad layer1513has a thickness of 1.0 μm, for example. In the MMI portion1531, as in the waveguide portion1511, the i-InGaAsP waveguide core layer1512has a thickness of 0.5 μm, and the i-InP upper clad layer1513has a thickness of 1.0 μm, for example.

In the photodetection portion1501, the i-InGaAsP waveguide core layer1512has a thickness of 0.5 μm, the n-InGaAsP semiconductor layer1502has a thickness of 0.5 μm, the i-InGaAs absorption layer1503has a thickness of 0.5 μm, and the p-InP upper clad layer1504and the p-type contact layer1505have a thickness of 1.0 μm in total, for example. In the slab region1521, as in the photodetection portion1501, the i-InGaAsP waveguide core layer1512has a thickness of 0.5 μm, for example. The thickness of the i-InP upper clad layer1513is 2.0 μm in the portion corresponding to the mesa structure in the photodetection portion1501, and is substantially the same as the thickness of the n-InGaAsP semiconductor layer1502in the photodetection portion1501in the other portion.

In the waveguide portion1511, the width of the mesa structure including the i-InGaAsP waveguide core layer1512and the i-InP upper clad layer1513is 2.5 μm, for example. In the MMI portion1531, the width of the mesa structure including the i-InGaAsP waveguide core layer1512and the i-InP upper clad layer1513is 5 μm, and the length of the mesa structure is 50 μm, for example.

In the photodetection portion1501, the width of the mesa structure including the i-InGaAs absorption layer1503, the p-InP upper clad layer1504, and the p-type contact layer1505is 3 μm, and the length of the mesa structure is 10 μm. In the slab region1521, the width of the mesa structure including the i-InP upper clad layer1513is 3 μm. The length of the slab region1521is 5 μm, for example.

In the light receiving element1500, when the MMI portion1531has the above-described structure, the length at which self-imaging occurs in the MMI waveguide1532is about 59 μm (Lsi≈59 μm). The distance from the input portion of the MMI waveguide732to the incident end of the photodetection portion731, that is, the total length of the MMI wavelength732and the slab region721(55 μm), is about 93% of the length at which self-imaging occurs (Lsi≈59 μm). In this case, as is understood from the simulation result illustrated inFIG. 13, the light absorption efficiency (quantum efficiency) increases in the i-InGaAs absorption layer1503in the photodetection portion1501, compared to a case where no MMI waveguide exists.

In the light receiving element1500having the above-described structure, the light absorption efficiency may be increased compared to the light receiving elements100and300illustrated inFIGS. 1 to 4. Particularly, in the light receiving element1500, the light absorption efficiency may be increased even if a slab region having a weak light confinement effect is disposed between a waveguide portion and a photodetection portion.

In the above-described embodiment, a description has been given of an example of a photodiode, which includes an absorption layer composed of InGaAs and a waveguide core layer and so forth composed of InGaAsP, serving as an example of a light receiving element for receiving signal light having a wavelength of 1.55 μm. Alternatively, in the light receiving element according to the above-described embodiment, the absorption layer may be composed of another material that absorbs light in the wavelength band of incident signal light, and the other layers may be composed of another material that does not absorb the light.

In the above-described embodiment, a layer such as an absorption layer is composed of i-type semiconductor. Alternatively, part of the absorption layer or the entire absorption layer may be composed of p-type or n-type semiconductor.

In the above-described embodiment, a waveguide core layer shared by a waveguide portion and an MMI portion exists below an absorption layer in a photodetection portion, but the structure is not limited thereto. Any layered structure may be used as long as signal light from a waveguide core layer in an MMI portion enters a layer below an absorption layer in a photodetection portion and does not directly enter the absorption layer. For example, a structure may be used in which signal light enters from the i-InGaAsP waveguide core layer1512in the MMI portion1531into the n-InGaAsP semiconductor layer1502in the photodetection portion1501via the slab region1521.

1-4. Method for Manufacturing Light Receiving Element1500

FIGS. 19A to 24Billustrate an example of a process of manufacturing the light receiving element1500illustrated inFIGS. 15 to 18B.FIGS. 19A,20A,21A,22A,23A, and24A are top plan views illustrating a main part of the light receiving element1500.FIGS. 19B,20B,21B,22B,23B, and24B are cross-sectional views taken along lines XIXB-XIXB, XXB-XXB, XXIB-XXIB, XXIIIB-XXIIIB, and XXIVB-XXIVB ofFIGS. 19A,20A,21A,22A,23A, and24A, respectively. Hereinafter, an example of the method for manufacturing the light receiving element1500will be described with reference toFIGS. 19A to 24B.

As illustrated inFIGS. 19A and 19B, an i-InGaAsP film1902, an n-InGaAsP film1903, an i-InGaAs film1904, a p-InP film1905, and a stacked film1906including two layers of p-InGaAs and p-InGaAsP are deposited on an SI-InP substrate1901by using, for example, a metal organic chemical vapor deposition (MOCVD) method. Here, deposition is performed so that the i-InGaAsP film1902has a thickness of 0.5 μm, the n-InGaAsP film1903has a thickness of 0.5 μm, and the i-InGaAs film1904has a thickness of 0.5 μm. Also, deposition is performed so that the total thickness of the p-InP film1905and the p-InGaAs/InGaAsP stacked film1906becomes 1.0 μm.

Subsequently, a mask2001for covering a region that is to become the photodetection portion1501illustrated inFIGS. 15 to 18Bis formed on the p-InGaAs/InGaAsP stacked film1906, and regions that are to become the waveguide portion1511and the MMI portion1531are selectively exposed. A silicon oxide film is used as the mask2001, for example. With known etching using the mask2001, the n-InGaAsP film1903, the i-InGaAs film1904, the p-InP film1905, and the p-InGaAs/InGaAsP stacked film1906are removed from the waveguide portion1511and the MMI portion1531, but remains in the photodetection portion1501, as illustrated inFIGS. 20A and 20B. As a result of this process, the i-InGaAsP film1902is exposed in the waveguide portion1511and the MMI portion1531.

Subsequently, as illustrated inFIGS. 21A and 21B, an i-InP film2101is deposited on the exposed i-InGaAsP film1902in the waveguide portion1511and the MMI portion1531by using the MOCVD method. Here, deposition is performed so that the i-InP film2101has a thickness of 1.0 μm. The photodetection portion1501is covered with the mask2001used for the above-described etching, and thus the growth of the i-InP film2101in the photodetection portion1501may be suppressed. At this time, the i-InP film2101rises and is deposited along the side surfaces of the n-InGaAsP film1903, the i-InGaAs film1904, the p-InP film1905, and the p-InGaAs/InGaAsP stacked film1906, so as to be formed in the shape illustrated inFIG. 21B. After the i-InP film2101has been deposited, the mask2001is removed.

Subsequently, a mask2201for covering a region that is to become a mesa structure in the photodetection portion1501is formed on the p-InGaAs/InGaAsP stacked film1906in the photodetection portion1501and the i-InP film2101in the waveguide portion1511and the MMI portion1531, as illustrated inFIGS. 22A and 22B. The mask2201is formed to cover the most part of the waveguide portion1511and the MMI portion1531, including the region other than the region that is to become a mesa structure. A silicon oxide film is used as the mask2201, for example.

At this time, as illustrated inFIGS. 22A and 22B, the mask2201is formed so that a non-covered region includes not only the p-InGaAs/InGaAsP stacked film1906but also part of the i-InP film2101, in view of a positioning error of the mask at photoresist exposure. That is, in the mask2201, a region to be etched is larger than a region to be removed to form a mesa structure in the photodetection portion1501, so that a margin region for canceling a mask positioning error is provided. The margin region is provided to suppress the occurrence of a phenomenon in which, as a result of displacement of the formation position of the mask2201due to a positioning error of the mask at photoresist exposure, the stacked film including the p-InGaAs/InGaAsP stacked film1906, the p-InP film1905, the i-InGaAs film1904, and the n-InGaAsP film1903remains at an end of the photodetection portion1501, and an InGaAs absorption layer remains over a large width.

With known etching using the mask2201, a mesa structure is formed in the photodetection portion1501, as illustrated inFIGS. 22A and 22B. In the photodetection portion1501, the p-InGaAs/InGaAsP stacked film1906, the p-InP film1905, and the i-InGaAs film1904are removed using the mask2201. Also, part of the n-InGaAsP film1903is removed to a certain width so that the other part remains. With this process, part of the n-InGaAsP film1903is exposed. Accordingly, the n-InGaAsP semiconductor layer1502, the i-InGaAs absorption layer1503, the p-InP upper clad layer1504, and the p-type contact layer1505illustrated inFIGS. 15 to 18Bare formed. As a result, the mesa structure illustrated inFIG. 18Bis formed.

As illustrated inFIGS. 22A and 22B, the part exposed by the mask2201in the i-InP film2101is removed at the same time. With this process, the mesa structure including the i-InP film2101adjacent to the mesa structure in the photodetection portion1501is formed at the same time. As a result, the mesa structure illustrated inFIG. 18Ais formed. After the etching, the mask2201is removed.

Subsequently, a mask2301for covering a region that is to become a mesa structure in the waveguide portion1511and the MMI portion1531is formed on the remaining part of the i-InP film2101, the mesa structure formed through the etching, and the n-InGaAsP semiconductor layer1502exposed through the etching, as illustrated inFIGS. 23A and 23B. The mask2301is formed so as to cover the most part of the mesa structure formed through the etching and the n-InGaAsP semiconductor layer1502exposed through the etching. A silicon oxide film is used as the mask2301, for example.

With known etching using the mask2301, mesa structures are formed in the waveguide portion1511and the MMI portion1531, as illustrated inFIGS. 23A and 23B. In the waveguide portion1511and the MMI portion1531, the i-InP film2101and the i-InGaAsP film1902are removed using the mask2301, and also part of the SI-InP substrate1901under the i-InGaAsP film1902is removed. With this process, part of the SI-InP substrate1901is exposed. Accordingly, the i-InGaAsP waveguide core layer1512and the i-InP upper clad layer1513illustrated inFIGS. 15 to 18Bare formed. As a result, the mesa structures illustrated inFIGS. 17A and 17Bare formed. After the etching, the mask2301is removed.

At this time, the portion covered by the mask2301in the mesa structure including the i-InP film2101is not etched and remains, as illustrated inFIGS. 23A and 23B. The remaining region corresponding to the mesa structure including the i-InP film2101serves as the slab region1521.

Subsequently, in the photodetection portion1501, the slab region1521, the MMI portion1531, and the waveguide portion1511, the passivation film1517composed of a dielectric material, such as a silicon-nitride film, is formed in the region except the region where a metal electrode is to be formed. After that, as illustrated inFIGS. 24A and 24B, the p-side metal electrode1515is formed in an exposed region of the p-type contact layer1505at the top of the mesa structure in the photodetection portion1501by using a known formation method, such as metal deposition or metal plating. Also, the n-side metal electrode1516is formed in an exposed region of the n-InGaAsP semiconductor layer1502using a known formation method, such as metal deposition or metal plating. In the plan view illustrated inFIG. 24A, the passivation film1517is not illustrated for easy understanding of the structure.

Referring toFIGS. 24A and 24B, a metal electrode having an air bridge structure is used as the p-side metal electrode1515. As is clear from the cross-sectional view inFIG. 24B, this structure causes the p-side metal electrode1515to be electrically insulated by air from the n-InGaAsP semiconductor layer1502connected to the n-side metal electrode1516.

Accordingly, the parasitic capacitance generated between the p-side metal electrode1515and the n-side metal electrode1516may be reduced. Thus, the capacitance generated in the photodetection portion1501may be reduced, and a cutoff frequency obtained based on a CR time constant increases in the transmission path between the light receiving element1500and an electric circuit in the subsequent stage. Accordingly, the light receiving element1500may supply a detection signal having a sufficient signal level to the electric circuit in the subsequent stage even in a high-frequency band, and the electric circuit in the subsequent stage may process the signal input thereto even in the high-frequency band.

Note that the structure of the p-side metal electrode1515is not limited to the air bridge structure. For example, an insulator may be formed in advance at the position where the p-side metal electrode1515is to be formed, and the p-side metal electrode1515may be formed on the insulator. InFIGS. 24A and 24B, part of the n-InGaAsP semiconductor layer1502remains in a portion opposite to the waveguide portion1511and the MMI portion1531regarding a PD, and the p-side metal electrode1515is formed on the part of the n-InGaAsP semiconductor layer1502via the passivation film1517. Alternatively, the part of the n-InGaAsP semiconductor layer1502may be removed. Accordingly, the capacitance of the p-side metal electrode1515may be decreased, and the characteristic of a high-frequency band may be further enhanced.

2. Second Embodiment

2-1. Structure of Light Receiving Element2500

FIG. 25is a perspective view illustrating an example of the structure of a light receiving element2500according to a second embodiment, and illustrates only a main part of the light receiving element2500.FIG. 26is a cross-sectional view of the light receiving element2500taken along line XXVI-XXVI ofFIG. 25.FIG. 27Ais a cross-sectional view of the light receiving element2500taken along line XXVIIA-XXVIIA ofFIG. 25.FIG. 27Bis a cross-sectional view of the light receiving element2500taken along line XXVIIB-XXVIIB ofFIG. 25.FIG. 28is a cross-sectional view of the light receiving element2500taken along line XXVIII-XXVIII ofFIG. 25.

The light receiving element2500illustrated inFIG. 25is different from the light receiving element700illustrated inFIG. 7in that no slab region exists between a photodetection portion and an MMI portion. Other than that, the light receiving element2500is similar to the light receiving element700.

As described above, the slab region721in the light receiving element700is generated as a result of providing a margin region when a hard mask for forming the mesa structure in the photodetection portion701is formed to address a positioning error of a mask at photoresist exposure in the process of fabricating the photodetection portion701, for example. However, it is not necessary to provide the margin region to the above-described hard mask if sufficient positioning accuracy of a mask at photoresist exposure is ensured in the process of manufacturing the light receiving element2500. Therefore, in this case, no slab region is formed. The light receiving element2500illustrated inFIG. 25has a structure without a slab region.

As illustrated inFIGS. 25 and 26, the light receiving element2500includes a photodetection portion2501disposed on a substrate2514, a waveguide portion2511disposed on the same substrate2514, and an MMI portion2531disposed on the same substrate2514.

As illustrated inFIG. 27A, the waveguide portion2511has a structure in which a waveguide core layer2512and an upper clad layer2513are staked from the substrate2514side. The material of the individual layers in this stacked structure is a semiconductor material, for example. The waveguide portion2511has a mesa structure including the upper clad layer2513and the waveguide core layer2512. Signal light propagates in the waveguide core layer2512and enters the MMI portion2531.

As illustrated inFIG. 28, the photodetection portion2501has a structure in which the waveguide core layer2512, an n-type semiconductor layer2502, an i-type absorption layer2503, a p-type upper clad layer2504, and a p-type contact layer2505are stacked from the substrate2514side. The material of the individual layers in this stacked structure is a semiconductor material, for example. The photodetection portion2501has a mesa structure including the p-type contact layer2505, the upper clad layer2504, the i-type absorption layer2503, and part of the n-type semiconductor layer2502. The width of the mesa structure in the photodetection portion2501is larger than the width of the mesa structure in the waveguide portion2511. In the photodetection portion2501, a stacked structure including the waveguide core layer2512and the n-type semiconductor layer2502exists outside the mesa structure. The n-type semiconductor layer2502, the i-type absorption layer2503, and the upper clad layer2504form a PIN-type photodiode.

The n-type semiconductor layer2502has a refractive index which is higher than the refractive index of the waveguide core layer2512and is lower than the refractive index of the i-type absorption layer2503. That is, the n-type semiconductor layer2502has a band gap wavelength which is longer than the band gap wavelength of the waveguide core layer2512and is shorter than the band gap wavelength of the i-type absorption layer2503. The n-type semiconductor layer2502has a composition in which the absorptance with respect to signal light is sufficiently low.

As illustrated inFIG. 27B, the MMI portion2531has a structure in which the waveguide core layer2512and the upper clad layer2513are staked from the substrate2514side. The material of the individual layers in this stacked structure is a semiconductor material, for example. The MMI portion2531has a mesa structure including the upper clad layer2513and the waveguide core layer2512. Signal light that has entered the MMI portion2531propagates in the waveguide core layer2512and directly enters the photodetection portion2501without via a slab region. The waveguide core layer2512is shared by the photodetection portion2501, the waveguide portion2511, and the MMI portion2531.

The MMI portion2531includes a 1×1 MMI waveguide2532including one input and one output. At least part of the waveguide core layer2512in the MMI portion2531has a width that is larger than the width of the waveguide core layer2512in the waveguide portion2511. The large-width portion of the waveguide core layer2512functions as the MMI waveguide2532. The width of the MMI waveguide2532is larger than the width of the mesa structure in each of the waveguide portion2511and the photodetection portion2501.

The length Lmmi of the MMI waveguide2532is set so that a point at which self-imaging occurs (self-imaging point) is positioned in the photodetection portion2501, particularly in a region below the i-type absorption layer2503in the waveguide core layer2512. More specifically, the length Lmmi of the MMI waveguide2532is set so as to be smaller than 100% of the length Lsi at which self-imaging occurs and larger than 70% of the length Lsi. Likewise, the length Lmmi of the MMI waveguide is set so as to be longer than (N−0.3)×100% of the length Lsi_mins at which self-imaging occurs in the MMI waveguide and shorter than N×100% of the length Lsi_min. The length Lsi_min is the shortest length where self-imaging occurs in the MMI waveguide, and N is a natural number. The manner of setting the length Lmmi of the MMI waveguide2532is the same as that in the light receiving element700according to the first embodiment described above.

As illustrated inFIG. 26, in the light receiving element2500, signal light propagates in the waveguide core layer2512in the waveguide portion2511, and enters the MMI waveguide2532in the MMI portion2531. In the MMI waveguide2532, as in the MMI waveguide732, signal light converges in the direction orthogonal to the signal light travel direction in the output portion. Accordingly, the converged signal light enters the waveguide core layer2512in the photodetection portion2501. The signal light then diffuses from the waveguide core layer2512into the i-type absorption layer2503via the n-type semiconductor layer2502, and is absorbed by the i-type absorption layer2503.

A p-side electrode and an n-side electrode (not illustrated) are connected to the p-type contact layer2505and the n-type semiconductor layer2502, respectively. A certain voltage for causing the p-side electrode to be at a negative potential and the n-side electrode to be at a positive potential is applied between the p-side electrode and the n-side electrode. Accordingly, photocarriers (holes and electrons) generated through light absorption in the i-type absorption layer2503are detected via the upper clad layer2504and the n-type semiconductor layer2502. Accordingly, the photodetection portion2501detects signal light as an electric signal (photocarrier current), and outputs a detection signal (photocarrier current) corresponding to the intensity of the signal light to an electric circuit in the subsequent stage.

The structure described above as a specific example of the structure of the light receiving element700according to the first embodiment may be used as a specific example of the structure of the light receiving element2500according to the second embodiment. Note that no slab region is formed, as described above.

Also, the method described above as a method for manufacturing the light receiving element700may be used as a method for manufacturing the light receiving element2500. Note that no margin region is provided in a hard mask for forming the mesa structure in the photodetection portion2501.

2-2. Light Intensity Distribution of Signal Light in Light Receiving Element2500

FIG. 29illustrates a light intensity distribution of signal light in the light receiving element2500illustrated inFIGS. 25 and 26. Solid lines represent the shape of the waveguide core layer2512viewed from the upper side of the substrate2514. Dotted-chain lines represent the shape of the i-type absorption layer2503viewed from the upper side of the substrate2514. Broken lines represent an example of a light intensity distribution of signal light. Arrows indicate radiation directions of signal light.

Referring toFIG. 29, in the MMI waveguide2532in the MMI portion2531, a self-imaging point is positioned in a region below the i-type absorption layer2503in the waveguide core layer2512in the photodetection portion2501. The length Lmmi of the MMI waveguide2532is 85% of the length Lsi at which self-imaging occurs, for example.

As illustrated inFIG. 29, signal light propagates in the waveguide core layer2512and enters from the waveguide portion2511into the MMI portion2531. After that, as described above with reference toFIG. 11, the signal light converges in the direction orthogonal to the signal light travel direction in the output portion of the MMI waveguide2532. In the output portion of the MMI waveguide2532, the signal light converges in the direction orthogonal to the signal light travel direction, as indicated by the arrows. Accordingly, the signal light enters the photodetection portion2501while maintaining a propagation state in the convergence direction.

In the photodetection portion2501, as in the photodetection portion701in the light receiving element700, the width of the waveguide core layer2512is larger than the width of the i-type absorption layer2503by at least the region connected to the n-side electrode.

In contrast, in the waveguide portion2511, the width of the waveguide core layer2512is smaller than the width of the i-type absorption layer2503. In the MMI portion2531, the width of the waveguide core layer2512is larger than the width of the waveguide core layer2512in the waveguide portion2511, and is sufficiently smaller than the width of the waveguide core layer2512in the photodetection portion2501. As a result, signal light enters from the waveguide portion2511and the MMI portion2531having a small width into the photodetection portion2501having a sufficiently large width.

The waveguide portion2511has a mesa structure of a small width, and thus has a strong light confinement effect of confining signal light in the direction orthogonal to the signal light travel direction. Also, the MMI portion2531has a strong light confinement effect because the MMI waveguide2532has a mesa structure. In contrast, the photodetection portion2501has a weak effect of confining incident signal light in the direction orthogonal to the signal light travel direction, like the photodetection portion701.

Therefore, in the light receiving element2500, as in the light receiving element100illustrated inFIGS. 1 and 2(a case where no slab region is provided), signal light enters from the waveguide portion2511and the MMI portion2531having a small width and a strong light confinement effect into the photodetection portion2501having a sufficiently large width and a weak light confinement effect. In this case, as in the case of the light receiving element100, the photodetection portion2501has low ability of suppressing expansion of the light intensity distribution of signal light in the direction orthogonal to the signal light travel direction.

However, in the light receiving element2500, signal light enters the photodetection portion2501while maintaining a propagation state in the convergence direction, as described above. The signal light that has entered the photodetection portion2501is absorbed by the i-type absorption layer2503. At this time, the self-imaging point of the MMI waveguide2532is positioned in a region below the i-type absorption layer2503in the waveguide core layer2512in the photodetection portion2501, and the signal light propagates in the convergence direction. Accordingly, a larger part of incident signal light may be converged to the region below the i-type absorption layer2503in the waveguide core layer2512, compared to the light receiving element100. Accordingly, the part of signal light that is not absorbed and is radiated to the outside of the i-type absorption layer2503may be decreased.

Accordingly, in the light receiving element2500, the light absorption efficiency may be increased compared to the light receiving element100(without MMI waveguide).

More specifically, in the light receiving element2500, as is clear from the simulation result illustrated inFIG. 13, the length Lmmi of the MMI waveguide2532is set to be larger than 70% of the length Lsi at which self-imaging occurs and to be smaller than 100% of the length Lsi, so that the light absorption efficiency may be increased compared to the light receiving element100(without MMI waveguide).

FIG. 30is a perspective view illustrating an example of the structure of a light receiving element3000according to a third embodiment, and illustrates only a main part of the light receiving element3000.FIG. 31is a cross-sectional view of the light receiving element3000taken along line XXXI-XXXI ofFIG. 30.FIG. 32Ais a cross-sectional view of the light receiving element3000taken along line XXXIIA-XXXIIA ofFIG. 30.FIG. 32Bis a cross-sectional view of the light receiving element3000taken along line XXXIIB-XXXIIB ofFIG. 30.FIG. 33Ais a cross-sectional view of the light receiving element3000taken along line XXXIIIA-XXXIIIA ofFIG. 30.FIG. 33Bis a cross-sectional view of the light receiving element3000taken along line XXXIIIB-XXXIIIB ofFIG. 30.

The light receiving element3000illustrated inFIGS. 30 and 31is called an embedded-type light receiving element. The light receiving element3000is different from the light receiving element700illustrated inFIG. 7in that an embedded layer is formed in a side surface portion of the mesa structure in each of a waveguide portion, an MMI portion, a slab region, and a photodetection portion. Other than that, the light receiving element3000is similar to the light receiving element700.

As illustrated inFIGS. 32A and 32B, an embedded layer3041is formed in a side surface portion of the mesa structure in each of a waveguide portion3011and an MMI portion3031. As illustrated inFIGS. 33A and 33B, an embedded layer3042is formed in a side surface portion of the mesa structure in each of a slab region3021and a photodetection portion3001, except a region provided with an n-side electrode on an n-type semiconductor layer3002. Preferably, the material of the embedded layers3041and3042is a semiconductor material that is the same as or similar to the semiconductor material of at least one of upper clad layers3004and3013.

The structure of the light receiving element3000is the same as that of the light receiving element700illustrated inFIGS. 7 to 10Bexcept the point described above, and thus the corresponding description is omitted. In the light receiving elements3000and700, the elements denoted by reference numerals with the same two last digits correspond to each other.

In the light receiving element3000, as in the light receiving element700, an MMI waveguide3032is provided. Thus, signal light enters the slab region3021and the photodetection portion3001while maintaining a propagation state in the convergence direction. Accordingly, in the light receiving element3000, the light absorption efficiency may be increased compared to the light receiving elements100and300illustrated inFIGS. 1 to 4. Particularly, in the light receiving element3000, the light absorption efficiency may be increased even though the slab region3021having a weak light confinement effect is disposed between the waveguide portion3011and the photodetection portion3001.

Also, when the distance from an end of the MMI waveguide3032on the side where signal light enters (input portion) to an end of the photodetection portion3001on the side where signal light enters (incident end) is longer than 70% of the length Lsi at which self-imaging occurs in the MMI waveguide3032and is shorter than 100% of the length Lsi, the light absorption efficiency may be increased compared to the light receiving elements100and300. Likewise, when the distance from an end of the MMI waveguide on the side where signal light enters (input portion) to an end of the photodetection portion on the side where signal light enters (incident end) is longer than (N−0.3)×100% of the length Lsi_min at which self-imaging occurs in the MMI waveguide and shorter than N×100% of the length Lsi_min, the light absorption efficiency may be increased compared to the light receiving element100. The length Lsi_min is the shortest length where self-imaging occurs in the MMI waveguide, and N is a natural number.

In addition, in the light receiving element3000, when the material of the embedded layer3042is a semiconductor material that is the same as or similar to the material of the upper clad layer3013in the slab region3021, the structure of the slab region3021is a more complete slab structure from the optical viewpoint, as illustrated inFIG. 33A.

That is, in the slab region721illustrated inFIG. 10A, the side surface portion of the mesa structure is filled with air, and thus a difference in refractive index occurs between the air in the side surface portion of the mesa structure and the semiconductor material of the upper clad layer713that forms the mesa structure. In the slab region721, a certain light confinement effect is generated, though slightly, due to the difference in refractive index.

In contrast, in the slab region3021, when the semiconductor material of the upper clad layer3013that forms the mesa structure and the semiconductor material of the embedded layer3042are the same or are similar to each other, a difference in refractive index does not occur between both the semiconductor materials. For this reason, the slab region3021hardly has a light confinement effect. Thus, the light confinement effect in the slab region3021is weaker than the light confinement effect in the slab region721in the light receiving element700.

Accordingly, in the light receiving element3000, the ability of the slab region3021of suppressing expansion of the light intensity distribution of signal light in the direction orthogonal to the light signal travel direction is lower than that of the slab region721. Therefore, in the light receiving element3000, degradation of light absorption efficiency caused by the slab region is more likely to occur than in the light receiving element700.

However, in the light receiving element3000, signal light enters the slab region3021while maintaining a propagation state in the convergence direction, due to the effect of the MMI waveguide3032, as in the light receiving element700. Accordingly, the signal light may be maintained in a propagation state in the convergence direction also in the slab region3021, and the signal light enters from the slab region3021into the photodetection portion3001while maintaining the propagation state in the convergence direction. Accordingly, a large part of incident signal light may be converged to a region below an i-type absorption layer3003in a waveguide core layer3012.

Therefore, in the light receiving element3000, which is an embedded-type light receiving element in which degradation of light absorption efficiency is more likely to occur, degradation of light absorption efficiency caused by the slab region may be suppressed, and the light absorption efficiency may be increased.

The structure described above as a specific example of the structure of the light receiving element700according to the first embodiment may be used as a specific example of the structure of the light receiving element3000according to the third embodiment. Note that an SI-InP embedded layer is formed in a side surface portion of the mesa structure in each of the waveguide portion3011, the MMI portion3031, the slab region3021, and the photodetection portion3001.

Also, the method described above as a method for manufacturing the light receiving element700may be used as a method for manufacturing the light receiving element3000. Note that a step of forming an SI-InP embedded layer is performed before forming a passivation film in the photodetection portion3001, the slab region3021, the MMI portion3031, and the waveguide portion3011.

4-1. Structure of Light Receiving Element3400

FIG. 34is a perspective view illustrating an example of the structure of a light receiving element3400according to a fourth embodiment, and illustrates only a main part of the light receiving element3400.FIG. 35is a cross-sectional view of the light receiving element3400taken along line XXXV-XXXV ofFIG. 34.FIG. 36Ais a cross-sectional view of the light receiving element3400taken along line XXXVIA-XXXVIA ofFIG. 34.FIG. 36Bis a cross-sectional view of the light receiving element3400taken along line XXXVIB-XXXVIB ofFIG. 34.FIG. 37Ais a cross-sectional view of the light receiving element3400taken along line XXXVIIA-XXXVIIA ofFIG. 34.FIG. 37Bis a cross-sectional view of the light receiving element3400taken along line XXXVIIB-XXXVIIB ofFIG. 34.

In the light receiving element3400illustrated inFIG. 34, the structures of the waveguide portion, the MMI portion, and the slab region are different from those in the light receiving element700illustrated inFIG. 7. Other than that, the light receiving element3400is similar to the light receiving element700.

As illustrated inFIGS. 34 and 35, the light receiving element3400includes a photodetection portion3401disposed on a substrate3414, a waveguide portion3411disposed on the same substrate3414, a slab region3421disposed on the same substrate3414, and an MMI portion3431disposed on the same substrate3414.

As illustrated inFIG. 36A, the waveguide portion3411includes a rib-type waveguide layer3412disposed on the substrate3414. The material of the waveguide layer3412is a semiconductor material, for example. In the waveguide portion3411, no upper clad layer is disposed on the waveguide layer3412. Signal light propagates in a protrusion (core layer)3413of the waveguide layer3412and enters the MMI portion3431.

As illustrated inFIG. 37B, the photodetection portion3401has a structure in which the waveguide layer (core layer)3412, an n-type semiconductor layer3402, an i-type absorption layer3403, a p-type upper clad layer3404, and a p-type contact layer3405are stacked from the substrate3414side. The material of the individual layers in this stacked structure is a semiconductor material, for example. The photodetection portion3401has a mesa structure including the p-type contact layer3405, the p-type upper clad layer3404, the i-type absorption layer3403, and part of the n-type semiconductor layer3402. The width of the mesa structure in the photodetection portion3401is larger than the width of the mesa structure in the waveguide portion3411. In the photodetection portion3401, a stacked structure including the waveguide layer (core layer)3412and the n-type semiconductor layer3402exists outside the mesa structure. The n-type semiconductor layer3402, the i-type absorption layer3403, and the p-type upper clad layer3404form a PIN-type photodiode.

The n-type semiconductor layer3402has a refractive index which is higher than the refractive index of the waveguide layer3412and is lower than the refractive index of the i-type absorption layer3403. That is, the n-type semiconductor layer3402has a band gap wavelength which is longer than the band gap wavelength of the waveguide layer3412and is shorter than the band gap wavelength of the i-type absorption layer3403. The n-type semiconductor layer3402has a composition in which the absorptance with respect to signal light is sufficiently low.

As illustrated inFIG. 37A, the slab region3421includes the slab-shaped (flat) waveguide layer (core layer)3412on the substrate3414. In the slab region3421, no upper clad layer is disposed on the waveguide layer (core layer)3412. Unlike the photodetection portion3401, the slab region3421includes the slab-shaped (flat) waveguide layer (core layer)3412but does not have a mesa structure including an i-type absorption layer and an n-type semiconductor layer. Signal light that has entered the slab region3421propagates in the waveguide layer3412and enters the photodetection portion3401.

The slab region3421is generated as a result of providing a margin region at the time of forming a hard mask for forming the mesa structure in the photodetection portion3401in order to address a positioning error of a mask during photoresist exposure in the process of fabricating the photodetection portion3401, for example. Details will be given below.

As illustrated inFIG. 36B, the MMI portion3431includes the rib-type waveguide layer3412on the substrate3414. The material of the waveguide layer3412is a semiconductor material, for example. In the MMI portion3431, no upper clad layer is disposed on the waveguide layer3412. Signal light that has entered the MMI portion3431propagates in the protrusion (core layer)3413of the waveguide layer3412and enters the photodetection portion3401. The waveguide layer3412is shared by the photodetection portion3401, the waveguide portion3411, the slab region3421, and the MMI portion3431.

The MMI portion3431includes a 1×1 MMI waveguide3432including one input and one output. At least part of the protrusion (core layer)3413of the waveguide layer3412in the MMI portion3431has a width larger than the width of the protrusion (core layer)3413of the waveguide layer3412in the waveguide portion3411. The large-width portion of the protrusion (core layer)3413of the waveguide layer3412functions as the MMI waveguide3432. The width of the MMI waveguide3432is larger than the width of the protrusion (core layer)3413of the waveguide layer3412in the waveguide portion3411and the mesa structure in the photodetection portion3401.

The length Lmmi of the MMI waveguide3432is set so that a point at which self-imaging in the MMI waveguide3432occurs (self-imaging point) is positioned in the photodetection portion3401, particularly in a region below the i-type absorption layer3403in the waveguide layer3412in the photodetection portion3401. More specifically, the length Lmmi of the MMI waveguide3432is set so that the distance from an end of the MMI waveguide3432on the side where signal light enters (input portion) to an end of the photodetection portion3401on the side where signal light enters (incident end) is longer than 70% of the length Lsi at which self-imaging occurs in the MMI waveguide3432and is shorter than 100% of the length Lsi. Likewise, the length Lmmi of the MMI waveguide is set so that the distance from an end of the MMI waveguide on the side where signal light enters (input portion) to an end of the photodetection portion on the side where signal light enters (incident end) is longer than (N−0.3)×100% of the length Lsi_min at which self-imaging occurs in the MMI waveguide and shorter than N×100% of the length Lsi_min. The length Lsi_min is the shortest length where self-imaging occurs in the MMI waveguide, and N is a natural number.

The manner of setting the length Lmmi of the MMI waveguide3432is the same as that described above regarding the light receiving element700according to the first embodiment.

As illustrated inFIG. 35, in the light receiving element3400, signal light propagates in the protrusion (core layer)3413of the waveguide layer3412in the waveguide portion3411, and enters the MMI waveguide3432in the MMI portion3431. Like the MMI waveguide732, the MMI waveguide3432converges, in its output portion, signal light in the direction orthogonal to the signal light travel direction. Accordingly, the converged signal light enters the waveguide layer (core layer)3412in the photodetection portion3401. The signal light then diffuses from the waveguide layer3412into the i-type absorption layer3403via the n-type semiconductor layer3402, and is absorbed by the i-type absorption layer3403.

A p-side electrode and an n-side electrode (not illustrated) are connected to the p-type contact layer3405and the n-type semiconductor layer3402, respectively. A certain voltage for causing the p-side electrode to be at a negative potential and the n-side electrode to be at a positive potential is applied between the p-side electrode and the n-side electrode. Accordingly, photocarriers (holes and electrons) generated through light absorption in the i-type absorption layer3403are detected via the p-type upper clad layer3404and the n-type semiconductor layer3402. Accordingly, the photodetection portion3401detects signal light as an electric signal (photocarrier current), and outputs a detection signal (photocarrier current) corresponding to the intensity of the signal light to an electric circuit in the subsequent stage.

In the light receiving element3400described above, as in the light receiving element700, the MMI waveguide3432is provided, so that signal light enters the slab region3421and the photodetection portion3401while maintaining a propagation state in the convergence direction. Accordingly, the light absorption efficiency may be increased in the light receiving element3400. Particularly, in the light receiving element3400, the light absorption efficiency may be increased even though the slab region3421having a weak light confinement effect is disposed between the waveguide portion3411and the photodetection portion3401.

When the distance from an end of the MMI waveguide3432on the side where signal light enters (input portion) to an end of the photodetection portion3401on the side where signal light enters (incident end) is longer than 70% of the length Lsi at which self-imaging occurs in the MMI waveguide3432and is shorter than 100% of the length Lsi, the light absorption efficiency may be increased compared to the light receiving elements100and300. Likewise, when the distance from an end of the MMI waveguide on the side where signal light enters (input portion) to an end of the photodetection portion on the side where signal light enters (incident end) is longer than (N−0.3)×100% of the length Lsi_min at which self-imaging occurs in the MMI waveguide and shorter than N×100% of the length Lsi_min, the light absorption efficiency may be increased compared to the light receiving elements100and300. The length Lsi_min is the shortest length where self-imaging occurs in the MMI waveguide, and N is a natural number.

In addition, in the light receiving element3400, nothing is disposed on the waveguide layer3412in the slab region3421, and the upper surface of the waveguide layer3412is filled with air, as illustrated inFIG. 37A. Thus, the slab region3421has an optically complete slab structure.

That is, the slab region3421has no light confinement effect. Thus, the light confinement effect in the slab region3421is weaker than the light confinement effect in the slab region721of the light receiving element700. Therefore, in the light receiving element3400, the ability of the slab region3421of suppressing expansion of the light intensity distribution of signal light in the direction orthogonal to the signal light travel direction is lower than that of the slab region721. As a result, in the light receiving element3400, degradation of light absorption efficiency caused by the slab region is more likely to occur than in the light receiving element700.

However, in the light receiving element3400, as in the light receiving element700, signal light enters the slab region3421while maintaining a propagation state in the convergence direction due to the effect of the MMI waveguide3432. Accordingly, in the slab region3421, signal light may be maintained in the propagation state in the convergence direction, and enters the photodetection portion3401from the slab region3421while maintaining the propagation state in the convergence direction. Accordingly, a large part of incident signal light may be converged in a region below the i-type absorption layer3403in the waveguide layer3412.

Therefore, in the light receiving element3400, which includes a rib-type waveguide that is likely to cause degradation of light absorption efficiency, degradation of light absorption efficiency caused by a slab region may be suppressed, and the light absorption efficiency may be increased.

A structure similar to the structure described above as a specific example of the structure of the light receiving element700according to the first embodiment may be used as a specific example of the structure of the light receiving element3400according to the fourth embodiment. Note that no upper clad layer is disposed on the waveguide layer3412in the waveguide portion3411, as described above. In the waveguide portion3411, the width of the protrusion (core layer)3413of the waveguide layer3412is 2.5 μm, for example. In the MMI portion3431, the width of the protrusion (core layer)3413of the waveguide layer3412is 5 μm, and the length thereof is 50 μm, for example.

4-2. Method for Manufacturing Light Receiving Element3400

FIGS. 38A to 42Billustrate an example of a process of manufacturing the light receiving element3400illustrated inFIGS. 34 to 37B.FIGS. 38A,39A,40A,41A, and42A are top plan views illustrating a main part of the light receiving element3400.FIGS. 38B,39B,40B,41B, and42B are cross-sectional views taken along lines XXXVIIIB-XXXVIIIB, XXXIXB-XXXIXB, XLB-XLB, XLIB-XLIB, and XLIIB-XLIIB ofFIGS. 38A,39A,40A,41A, and42A, respectively. Hereinafter, an example of the method for manufacturing the light receiving element3400will be described with reference toFIGS. 38A to 42B.

As illustrated inFIGS. 38A and 38B, an i-InGaAsP film3802, an n-InGaAsP film3803, an i-InGaAs film3804, a p-InP film3805, and a stacked film3806including two layers of p-InGaAs and p-InGaAsP are deposited on an SI-InP substrate3801by using, for example, the MOCVD method. Here, deposition is performed so that the i-InGaAsP film3802has a thickness of 0.5 μm, the n-InGaAsP film3803has a thickness of 0.5 μm, and the i-InGaAs film3804has a thickness of 0.5 μm. Also, deposition is performed so that the total thickness of the p-InP film3805and the p-InGaAs/InGaAsP stacked film3806becomes 1.0 μm.

Subsequently, a mask3901for covering a region that is to become the photodetection portion3401illustrated inFIGS. 34 to 37Bis formed on the p-InGaAs/InGaAsP stacked film3806, and regions that are to become the waveguide portion3411and the MMI portion3431are selectively exposed. A silicon oxide film is used as the mask3901, for example. With known etching using the mask3901, the n-InGaAsP film3803, the i-InGaAs film3804, the p-InP film3805, and the p-InGaAs/InGaAsP stacked film3806are removed from the waveguide portion3411and the MMI portion3431, but remains in the photodetection portion3401, as illustrated inFIGS. 39A and 39B. As a result of this process, the i-InGaAsP film3802is exposed in the waveguide portion3411and the MMI portion3431. After the etching, the mask3901is removed.

Subsequently, a mask4001for covering a region that is to become a mesa structure in the photodetection portion3401is formed on the p-InGaAs/InGaAsP stacked film3806and the exposed n-InGaAsP film3402, as illustrated inFIGS. 40A and 40B. The mask4001is formed to cover the most part of the waveguide portion3411and the MMI portion3431, including the region other than the region that is to become a protrusion. A silicon oxide film is used as the mask4001, for example.

At this time, as illustrated inFIGS. 40A and 40B, the mask4001is formed so that a non-covered region includes not only the p-InGaAs/InGaAsP stacked film3806but also part of the exposed i-InGaAsP film3802, in view of a positioning error of the mask at photoresist exposure. That is, in the mask4001, a region to be etched is larger than a region to be removed to form a mesa structure in the photodetection portion3401, so that a margin region for canceling a mask positioning error is provided. The margin region is provided to suppress the occurrence of a phenomenon in which, as a result of displacement of the formation position of the mask4001due to a positioning error of the mask at photoresist exposure, the stacked film including the p-InGaAs/InGaAsP stacked film3806, the p-InP film3805, the i-InGaAs film3804, and the n-InGaAsP film3803remains at an end of the photodetection portion3401, and an absorption layer remains over a large width.

With known etching using the mask4001, a mesa structure is formed in the photodetection portion3401, as illustrated inFIGS. 40A and 40B. In the photodetection portion3401, the p-InGaAs/InGaAsP stacked film3806, the p-InP film3805, and the i-InGaAs film3804are removed using the mask4001. Also, part of the n-InGaAsP film3803is removed to a certain width so that the other part remains. With this process, part of the n-InGaAsP film3803is exposed. Accordingly, the n-InGaAsP semiconductor layer3402, the i-InGaAs absorption layer3403, the p-InP upper clad layer3404, and the p-type contact layer3405illustrated inFIGS. 34 to 37Bare formed. As a result, the mesa structure illustrated inFIG. 37Bis formed. After the etching, the mask4001is removed.

Subsequently, a mask4101for covering a region that is to become a mesa structure in the waveguide portion3411and the MMI portion3431is formed on the exposed i-InGaAsP film3802, the mesa structure formed through the etching, and the n-InGaAsP semiconductor layer3402exposed through the etching, as illustrated inFIGS. 41A and 41B. The mask4101is formed so as to cover the entire mesa structure formed through the etching and the entire n-InGaAsP semiconductor layer3402exposed through the etching. A silicon oxide film is used as the mask4101, for example.

With known etching using the mask4101, a protrusion (core layer) of a waveguide layer is formed in the waveguide portion3411and the MMI portion3431, as illustrated inFIGS. 41A and 41B. In the waveguide portion3411and the MMI portion3431, part of the i-InGaAsP film3802is removed using the mask4101. Accordingly, the i-InGaAsP waveguide layer3412and the protrusion (core layer)3413thereof illustrated inFIGS. 34 to 37Bare formed. As a result, the protrusion-shaped structure illustrated inFIGS. 36A and 36Bis formed. After the etching, the mask4101is removed.

At this time, the portion covered by the mask4101in the exposed i-InGaAsP film3802is not etched and remains, as illustrated inFIGS. 41A and 41B. The remaining region corresponding to the i-InGaAsP waveguide layer3412composed of the i-InGaAsP film3802serves as the slab region3421.

Subsequently, in the photodetection portion3401, the slab region3421, the MMI portion3431, and the waveguide portion3411, a passivation film4201composed of a dielectric material, such as a silicon-nitride film, is formed in the region except the region where a metal electrode is to be formed. After that, as illustrated inFIGS. 42A and 42B, a p-side metal electrode4202is formed in an exposed region of the p-type contact layer3405at the top of the mesa structure in the photodetection portion3401by using a known formation method, such as metal deposition or metal plating. Also, an n-side metal electrode4203is formed in an exposed region of the n-InGaAsP semiconductor layer3402using a known formation method, such as metal deposition or metal plating. In the plan view illustrated inFIG. 42A, the passivation film4201is not illustrated for easy understanding of the structure.

Referring toFIGS. 42A and 42B, a metal electrode having an air bridge structure is used as the p-side metal electrode4202. As is clear from the cross-sectional view inFIG. 42B, this structure causes the p-side metal electrode4202to be electrically insulated by air from the n-InGaAsP semiconductor layer3402connected to the n-side metal electrode4203.

Accordingly, the parasitic capacitance generated between the p-side metal electrode4202and the n-side metal electrode4203may be reduced. Thus, the capacitance generated in the photodetection section3401may be reduced, and a cutoff frequency obtained based on a CR time constant increases in the transmission path between the light receiving element3400and an electric circuit in the subsequent stage. Accordingly, the light receiving element3400may supply a detection signal having a sufficient signal level to the electric circuit in the subsequent stage even in a high-frequency band, and the electric circuit in the subsequent stage may process the signal input thereto even in the high-frequency band.

Note that the structure of the p-side metal electrode4202is not limited to the air bridge structure. For example, an insulator may be formed in advance at the position where the p-side metal electrode4202is to be formed, and the p-side metal electrode4202may be formed on the insulator. InFIGS. 42A and 42B, part of the n-InGaAsP semiconductor layer3402remains in a portion opposite to the waveguide section3411and the MMI section3431regarding a PD, and the p-side metal electrode4202is formed on the part of the n-InGaAsP semiconductor layer3402via the passivation film4201. Alternatively, the part of the n-InGaAsP semiconductor layer3402may be removed. Accordingly, the capacitance of the p-side metal electrode4202may be decreased, and the characteristic of a high-frequency band may be further enhanced.

In the above-described method for manufacturing the light receiving element3400, it is not necessary to deposit an upper clad layer on the waveguide layer3412in the waveguide section3411and the MMI section3431. Accordingly, the number of steps in the process may be decreased, and the light receiving element3400may be easily manufactured.

FIG. 43illustrates an example the configuration of a light receiving device4300according to a fifth embodiment.

The light receiving device4300illustrated inFIG. 43is an example of an optical coherent receiver for demodulating a signal on which quadrature phase shift keying (QPSK) modulation has been performed. The light receiving device4300is a promising waveguide-integrated light receiving device including an optical hybrid waveguide that converts phase-modulated signal light into intensity-modulated signal light and a photodiode (PD) which are integrated on the same substrate, from the viewpoint of decreasing the size and assembly cost.

As illustrated inFIG. 43, the light receiving device4300includes a photodetection section4301, a connection waveguide portion4302, an optical hybrid waveguide portion4303, and an input waveguide portion4304. The photodetection portion4301includes four photodiode (PD) elements4305to4308. The connection waveguide portion4305includes four connection waveguides4309to4312. The optical hybrid waveguide portion4303includes a 90-degree optical hybrid waveguide4313including two inputs and four outputs. The input waveguide portion4303includes two input waveguides4314and4315.

The two input waveguides4314and4315in the input waveguide portion4303are connected to the two inputs of the 90-degree optical hybrid waveguide4313, respectively. The four connection waveguides4309to4312in the connection waveguide portion4305are connected to the four outputs of the 90-degree optical hybrid waveguide4313, respectively. The four connection waveguides4309to4312are connected to the PD elements4305to4308in the photodetection portion4301, respectively.

In the light receiving device4300, the photodetection portion, the MMI portion, the slab region, and the waveguide portion of any one of the light receiving elements700,2500,3000, and3400according to the first to fourth embodiments illustrated inFIGS. 7,25,30, and34may be used for the PD element4305and the connection waveguide4309. This is the same in the other PD elements4306to4308and the connection waveguides4310to4312. The optical hybrid waveguide portion4303and the input waveguide portion4304have the same layered structure as the connection waveguide portion4302, and are disposed on the same substrate on which the photodetection portion4301and the connection waveguide portion4302are disposed.

Now, operation of the light receiving device4300will be described. QPSK-modulated signal light enters the input waveguide4314, and local oscillator (LO) light enters the input waveguide4315as reference light. The 90-degree optical hybrid waveguide4313receives the signal light and the LO light via the input waveguides4314and4315. The 90-degree optical hybrid waveguide4313causes the LO light and the signal light to interfere with each other, thereby demodulating the QPSK-modulated signal light, so as to generate complementary I-channel signal light beams having a phase difference of 180 degrees and to generate complementary Q-channel signal light beams having a phase difference of 180 degrees. The 90-degree optical hybrid waveguide4313outputs the complementary I-channel signal light beams to the connection waveguides4309and4310, and outputs the complementary Q-channel signal light beams to the connection waveguides4311and4312.

The PD elements4305and4306receive the complementary I-channel signal light beams from the 90-degree optical hybrid waveguide4313via the connection waveguides4309and4310, respectively. Each of the PD elements4305and4306detects the received I-channel signal light beam as an electric signal and generates an I-channel signal (electric signal). The PD elements4307and4308receive the complementary Q-channel signal light beams from the 90-degree optical hybrid waveguide4313via the connection waveguides4311and4312, respectively. Each of the PD elements4307and4308detects the received Q-channel signal light beam as an electric signal and generates a Q-channel signal (electric signal).

A 4×4 MMI waveguide including four inputs and four outputs may be used as the 90-degree optical hybrid waveguide4313, for example. In this case, the input waveguides4314and4315are connected to two inputs of the 4×4 MMI waveguide. The connection waveguides4309to4312are connected to the four outputs of the 4×4 MMI waveguide.

In the light receiving device4300according to the fifth embodiment, as in the light receiving elements700,2500,3000, and3400according to the first to fourth embodiments, light absorption efficiency may be increased. Particularly, in the light receiving device4300, light absorption efficiency may be increased even when a slab region having a weak light confinement effect is disposed between a waveguide portion and a photodetection portion.

In the four PD elements4305to4308included in the photodetection portion4301, corresponding spacer layers are disposed independently. Accordingly, among the PD elements4305to4308, not only a p-side electrode but also an n-side electrode may be disposed in an electrically separated manner, and sufficient electrical separation among the PD elements4305to4308may be realized. Thus, the occurrence of undesired crosstalk among the PD elements4305to4308may be suppressed, so that signal light with a reduced error may be received by the light receiving device4300.

In the fifth embodiment, an example of an optical coherent receiver has been described as a waveguide-integrated light receiving device. Alternatively, any type of element may be used as long as a PD and a waveguide are integrated together. The light receiving elements according to the first to fourth embodiments may be used as such an element.

FIG. 44illustrates an example of the configuration of a light receiving module4400according to a sixth embodiment.

The light receiving module4400illustrated inFIG. 44is an example of an optical coherent receiver module for demodulating a signal on which dual polarization-quadrature phase shift keying (DP-QPSK) modulation has been performed.

As illustrated inFIG. 44, the light receiving module4400includes optical coherent receivers4401and4402, transimpedance amplifiers (TIAs)4403to4406, polarizing beam splitters (PBSs)4407and4408, lenses4409to4414, and mirrors4415and4416. Optical fiber cables4417and4418are connected to the light receiving module4400.

The light receiving module4400receives DP-QPSK-modulated signal light via the optical fiber cable4417, and receives LO light, which is reference light, via the optical fiber cable4418. The DP-QPSK-modulated signal light includes two signal light beams polarized in different directions orthogonal to each other. The two signal light beams transmit signals different from each other.

The DP-QPSK-modulated signal light enters the PBS4407via the lens4409and is separated into two signal light beams polarized in different directions by the PBS4407. One of the two signal light beams enters the optical coherent receiver4401via the lens4411, and the other enters the optical coherent receiver4402via the mirror4415and the lens4413. Likewise, LO light beams are supplied to the optical coherent receivers4401and4402.

The light receiving device4300according to the fifth embodiment illustrated inFIG. 43may be used as each of the optical coherent receivers4401and4402. Each of the optical coherent receivers4401and4402receives QPSK-modulated signal light and LO light and causes the signal light and the LO light to interfere with each other, thereby demodulating the QPSK-modulated signal light.

The optical coherent receiver4401detects complementary I-channel signal light obtained through demodulation as a complementary electric signal (I-channel signal). The optical coherent receiver4401detects complementary Q-channel signal light obtained through demodulation as a complementary electric signal (Q-channel signal). The optical coherent receiver4401supplies the complementary I-channel signal (electric signal) obtained through detection to the TIA4403, and supplies the complementary Q-channel signal (electric signal) obtained through detection to the TIA4404. Likewise, the optical coherent receiver4402supplies the complementary I-channel signal to the TIA4405, and supplies the complementary Q-channel signal to the TIA4406.

Each of the TIAs4403to4406receives a complementary I-channel signal or a complementary Q-channel signal, and performs differential amplification on the signal level of the signal.

In the light receiving module4400according to the sixth embodiment, as in the light receiving device4300according to the fifth embodiment, light absorption efficiency may be increased in an optical coherent receiver. Particularly, in the light receiving module4400, light absorption efficiency may be increased even when a slab region having a weak light confinement effect is disposed between a waveguide portion and a photodetection portion.

In a plurality of PD elements included in each optical coherent receiver, corresponding spacer layers are disposed independently. Accordingly, among the PD elements, not only a p-side electrode but also an n-side electrode may be disposed in an electrically separated manner, and sufficient electrical separation among the PD elements may be realized. Thus, the occurrence of undesired crosstalk among the PD elements may be suppressed, so that signal light with a reduced error may be received by the light receiving module4400.

In the light receiving element disclosed in the embodiments, light absorption efficiency may be increased. Particularly, in the light receiving element, light absorption efficiency may be increased even when a slab region having a weak light confinement effect is disposed between a waveguide portion and a photodetection portion.