JP2023010832A - Optical waveguide type light receiving element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical waveguide type light receiving element in which the frequency response characteristics of a waveguide type photodiode can be improved.

SOLUTION: An optical waveguide type light receiving element comprises: a first semiconductor layer having an n-type conductivity type; optical waveguide structure provided on a first region of the first semiconductor layer; and waveguide type photodiode structure provided on a second region adjacent to the first region of the first semiconductor layer. The optical waveguide structure has: an optical waveguide core layer provided on the first semiconductor layer; and a clad layer provided on the optical waveguide core layer. The waveguide type photodiode structure has: a second semiconductor layer provided side by side on the first semiconductor layer and having an n-type or i-type conductivity type having an impurity concentration lower than that of the first semiconductor layer; a light absorption layer provided on the second semiconductor layer and optically coupled with the optical waveguide core layer; and a third semiconductor layer having a p-type conductivity type and provided on the light absorption layer.

SELECTED DRAWING: Figure 4

COPYRIGHT: (C)2023,JPO&INPIT

Description

The present invention relates to an optical waveguide type photodetector.

Patent Literature 1 describes a technique related to an optical waveguide photodetector in which an optical waveguide structure and a waveguide photodiode structure are integrated on a common substrate.

JP 2013-110207 A

2. Description of the Related Art In recent years, research and development have been conducted on optical waveguide photodetectors in which an optical waveguide structure and a waveguide photodiode structure are integrated on a common substrate. Such an optical waveguide photodetector is used as a reception front end of an optical transmission system combining a multi-level modulation method and a digital coherent reception method, which has a high transmission speed of 40 Gb/s or more, for example. An optical waveguide type light receiving element is formed by forming a butt joint structure on a semiconductor substrate between a semiconductor lamination portion for a photodiode including a light absorption layer and a semiconductor lamination portion for an optical waveguide including an optical waveguide core layer. produced.

In the future, optical communication systems will require even faster optical communication technology that achieves transmission speeds of, for example, 400 Gb/s. is needed. Therefore, the waveguide photodiode structure of the optical waveguide photodetector used in the receiver is required to have a higher frequency response characteristic.

The frequency response characteristics of the waveguide photodiode structure are mainly determined by the CR time constant (C: capacitance, R: resistance) of the photodiode and transit time of carriers (holes, electrons). In order to improve frequency response characteristics, the shorter the carrier transit time, the better, and the smaller the CR time constant. However, if the light absorption layer of the photodiode is made thin in order to shorten the transit time of carriers, there is a trade-off that the capacitance increases and the CR time constant increases.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical waveguide photodetector capable of further enhancing the frequency response characteristics of a waveguide photodiode.

In order to solve the above-described problems, an optical waveguide photodetector according to one embodiment includes a first semiconductor layer having n-type conductivity, and an optical waveguide structure provided on a first region of the first semiconductor layer. and a waveguide photodiode structure provided on a second region adjacent to the first region of the first semiconductor layer, wherein the optical waveguide structure comprises an optical waveguide core layer provided on the first semiconductor layer and a clad layer provided on the optical waveguide core layer, wherein the waveguide photodiode structure is provided on the first semiconductor layer adjacent to a side surface of the optical waveguide core layer, the first semiconductor layer a second semiconductor layer having n-type or i-type conductivity with an impurity concentration lower than the second semiconductor layer, provided on the second semiconductor layer adjacent to a side surface of the optical waveguide core layer and optically coupled with the optical waveguide core layer; a light-absorbing layer thinner than the optical waveguide core layer; and a third semiconductor layer provided on the light-absorbing layer, in contact with the light-absorbing layer, and having p-type conductivity; The height of the center in the thickness direction of the optical waveguide core layer with respect to the upper surface and the height of the center in the thickness direction of the light absorption layer with the upper surface of the first semiconductor layer as a reference are equal to each other. The refractive index of the semiconductor layer is larger than that of the third semiconductor layer.

According to the optical waveguide photodetector of the present invention, the frequency response characteristics of the waveguide photodiode can be further enhanced.

FIG. 1 is a plan view showing the configuration of a light receiving device having an optical waveguide type light receiving element according to one embodiment of the present invention. FIG. 2 shows a cross section along line II--II shown in FIG. FIG. 3 shows an enlarged part of FIG. FIG. 4 partially shows a cross section along line IV--IV shown in FIG. FIG. 5 is a cross-sectional view of a joint portion between a light receiving element portion and an optical waveguide portion according to a modification. FIG. 6 is a cross-sectional view of a joint portion between a light receiving element portion and an optical waveguide portion in a comparative example.

[Description of the embodiment of the present invention]
First, the contents of the embodiments of the present invention will be listed and explained. An optical waveguide photodetector according to one embodiment includes a first semiconductor layer having n-type conductivity, an optical waveguide structure provided on a first region of the first semiconductor layer, and a first semiconductor layer of the first semiconductor layer. a waveguide photodiode structure provided on a second region adjacent to the region, wherein the optical waveguide structure comprises an optical waveguide core layer provided on the first semiconductor layer; and an optical waveguide core layer provided on the optical waveguide core layer. The waveguide photodiode structure is provided on the first semiconductor layer adjacent to the side surface of the optical waveguide core layer, and has an n-type or n-type impurity concentration lower than that of the first semiconductor layer. a second semiconductor layer having i-type conductivity, provided on the second semiconductor layer adjacent to the side surface of the optical waveguide core layer, optically coupled to the optical waveguide core layer, and thinner than the thickness of the optical waveguide core layer a light-absorbing layer; and a third semiconductor layer provided on the light-absorbing layer, in contact with the light-absorbing layer, and having p-type conductivity, wherein the optical waveguide core layer is based on the upper surface of the first semiconductor layer. and the center height in the thickness direction of the light absorption layer through the second semiconductor layer are equal to each other with respect to the upper surface of the first semiconductor layer, and the refractive index of the second semiconductor layer is the third Larger than the refractive index of the semiconductor layer.

In this optical waveguide type light-receiving device, an n-type or i-type second semiconductor layer with a low impurity concentration is provided between the first semiconductor layer and the light absorption layer. In this case, the depletion region when a reverse bias is applied spreads from the light absorption layer to the second semiconductor layer. The CR time constant can be kept small by suppressing an increase in capacitance. As described above, according to the optical waveguide photodetector, the trade-off between the CR time constant and the carrier transit time of the waveguide photodiode can be resolved, and the frequency response characteristics can be further improved.

Furthermore, in the above optical waveguide type light receiving element, the second semiconductor layer is provided in parallel with the optical waveguide core layer on the first semiconductor layer. When the light absorption layer and the optical waveguide core layer are simply arranged side by side on the first semiconductor layer, if the light absorption layer is made thinner than the optical waveguide core layer in order to improve the frequency response characteristic, the thickness on the first semiconductor layer will increase. The optical waveguide core layer and the light absorption layer are arranged with different . Therefore, the height of the center of the light absorption layer and the height of the center of the optical waveguide core layer differ from each other with reference to the upper surface of the first semiconductor layer, and the optical coupling efficiency between the light absorption layer and the optical waveguide core layer is lowered. . On the other hand, according to the optical waveguide type light-receiving element, the second semiconductor layer is provided between the first semiconductor layer and the light absorption layer. The center height of the absorption layer can be made close to the center height of the optical waveguide core layer. Therefore, a decrease in the optical coupling efficiency between the light absorption layer and the optical waveguide core layer can be suppressed.

In the above optical waveguide type light receiving element, the center height of the optical waveguide core layer and the center height of the light absorption layer are equal to each other with respect to the upper surface of the first semiconductor layer. This makes it possible to more effectively suppress a decrease in optical coupling efficiency between the light absorption layer and the optical waveguide core layer.

In the above optical waveguide photodetector, the second semiconductor layer may have an impurity concentration of 1×10 ¹⁶ cm ⁻³ or less. When the second semiconductor layer has such an impurity concentration, for example, the depleted region can be effectively extended to the second semiconductor layer when a reverse bias is applied.

In the above optical waveguide photodetector, the bandgap of the second semiconductor layer may be larger than the bandgap of the light absorption layer and equal to or smaller than the bandgap of the first semiconductor layer. When the second semiconductor layer has such a bandgap, for example, the depletion region can be effectively extended to the second semiconductor layer when a reverse bias is applied.

[Details of the embodiment of the present invention]
A specific example of the optical waveguide type light receiving element according to the embodiment of the present invention will be described below with reference to the drawings. The present invention is not limited to these examples, but is indicated by the scope of the claims, and is intended to include all modifications within the scope and meaning equivalent to the scope of the claims. In the following description, the same reference numerals are given to the same elements in the description of the drawings, and overlapping descriptions are omitted. In the following description, undoped means that the impurity concentration is extremely low, such as 1×10 ¹⁵ cm ⁻³ or less.

One embodiment of the present invention relates to an optical waveguide type photodetector monolithically integrated with a 90° hybrid function, which is mainly used in a coherent optical communication system. This relates to the enhancement of the sensitivity of characteristics. FIG. 1 is a plan view showing the configuration of a light receiving device having an optical waveguide type light receiving element according to one embodiment of the present invention. FIG. 2 shows a cross section along line II--II shown in FIG. 1, and FIG. 3 shows a part of FIG. 2 in an enlarged manner. FIG. 4 partially shows a cross section along line IV--IV shown in FIG.

As shown in FIG. 1, a light receiving device 1A of this embodiment includes an optical waveguide type light receiving element 2 and signal amplifiers 3A and 3B. The optical waveguide type light receiving element 2 has a planar shape such as a substantially rectangular shape, and is formed by forming an optical waveguide on a substrate made of a compound semiconductor such as InP, for example. The optical waveguide type light receiving element 2 has two input ports 4 a and 4 b and an optical branching section (optical coupler) 5 . The optical waveguide type light receiving element 2 further includes light receiving element portions 6a to 6d and capacitive element portions 7a to 7d formed on the substrate. That is, the optical waveguide type light receiving element 2 has a structure in which the optical waveguide and the light receiving element portions 6a to 6d are monolithically integrated on a common substrate.

The optical waveguide type light receiving element 2 has a pair of edges 2a and 2b extending along a predetermined direction A. As shown in FIG. The two input ports 4 a and 4 b are provided at one edge 2 a of the edges 2 a and 2 b of the optical waveguide type photodetector 2 . An optical signal La including four signal components modulated by a QPSK (Quadrature Phase Shift Keying) method is supplied to one input port 4a of the two input ports 4a and 4b to the outside of the light receiving device 1A. input from Local oscillation light Lb is input to the other input port 4b. The input ports 4a and 4b are optically coupled to the optical splitter 5 via optical waveguides 8a and 8b, respectively. The optical waveguide portions 8a and 8b include a core layer made of a material with a relatively high refractive index (eg, InGaAsP) and a clad layer made of a material (eg, InP) with a smaller refractive index than the core layer and covering the core layer. It is preferably configured by

The optical splitter 5 constitutes a 90° optical hybrid. That is, the optical branching unit 5 is configured by an MMI (Multi-Mode Interference) coupler, and by causing the optical signal La and the local oscillation light Lb to interfere with each other, the optical signal La is converted to QPSK It branches into four signal components Lc1 to Lc4 modulated according to the system. Of these four signal components Lc1 to Lc4, the signal components Lc1 and Lc2 have the same polarization state and have an in-phase relationship. Also, the polarization states of the signal components Lc3 and Lc4 are equal to each other and different from the polarization states of the signal components Lc1 and Lc2. Signal components Lc3 and Lc4 have a quadrature relationship.

The light receiving element portions 6a to 6d are configured as PIN photodiodes, and are arranged in this order along the edge 2b of the optical waveguide type light receiving element 2. FIG. The light receiving element portions 6a to 6d are optically coupled to the four output ends of the optical branching portion 5 via optical waveguide portions 8c to 8f, respectively. A constant bias voltage is supplied to the cathodes of the light receiving element portions 6a to 6d. Each of the light receiving element portions 6a to 6d receives the four signal components Lc1 to Lc4 from the light branching portion 5, and generates an electric signal (photocurrent) corresponding to the light intensity of each of these signal components Lc1 to Lc4. Signal output electrode pads 21a to 21d electrically connected to the anodes of the light receiving element portions 6a to 6d are provided on the optical waveguide type light receiving element 2. As shown in FIG. The signal output electrode pads 21 a to 21 d are arranged side by side in the direction A along the edge 2 b of the optical waveguide type photodetector 2 . The signal output electrode pads 21a-21d are electrically connected to the signal input electrode pads 61a-61d of the signal amplifiers 3A and 3B via bonding wires 20a-20d, respectively.

The capacitive element portions 7a to 7d are so-called MIM (Metal-Insulator-Metal) capacitors composed of a lower metal layer, an upper metal layer, and an insulating film 45 sandwiched between the lower metal layer and the upper metal layer. be. The lower metal layer and the upper metal layer have a laminated structure such as TiW/Au or Ti/Pt/Au. The capacitive element portions 7a to 7d are arranged side by side (adjacent to) the light receiving element portions 6a to 6d on the optical waveguide type light receiving element 2 along the edge 2b. It is electrically connected between a bias wiring 42 for supplying a bias voltage to each cathode and a reference potential wiring (GND line). The bias wiring 42 is used as a lower metal layer of the capacitive element portions 7a to 7d. The upper metal layers 43 of the capacitive element portions 7a to 7d are led out to the reference potential side electrode pads 23a to 23d arranged along the edge 2b of the optical waveguide type light receiving element 2, or 23a to 23d. The reference potential side electrode pads 23 a to 23 d are electrically connected to a back surface metal film 50 provided on the back surface of the substrate 10 via vias (not shown) penetrating the substrate 10 . The lower metal layers 42 of the capacitive element portions 7a-7d extend toward the inside of the substrate 10. As shown in FIG. These capacitive element portions 7a to 7d make it possible to match the inductance components between the cathodes of the light receiving element portions 6a to 6d and the bypass capacitors (not shown) in terms of design.

Each of the capacitive element portions 7a-7d has bias voltage side electrode pads 22a-22d connected to the lower metal layer 42, respectively. The reference potential side electrode pads 23a to 23d are arranged between the bias voltage side electrode pads 22a to 22d and the edge 2b of the optical waveguide type light receiving element 2 in a direction B that intersects (for example, is perpendicular to) the direction A. there is

One ends of bonding wires 20i to 20m are connected to the bias voltage side electrode pads 22a to 22d, respectively. The other ends of the bonding wires 20i-20m are electrically connected to a bias voltage source (not shown). The bonding wires 20i to 20m form part of wiring for supplying bias voltages to the light receiving element portions 6a to 6d, respectively.

One ends of bonding wires 20e to 20h are connected to the reference potential side electrode pads 23a to 23d, respectively. The bonding wires 20e to 20h are provided along the bonding wires 20a to 20d, and the other ends of the bonding wires 20e to 20h are connected to reference potential electrode pads 62a, 62c, 62d and 62f of the signal amplifiers 3A and 3B. connected to each other.

In this embodiment, the capacitive element portions 7a to 7d and the light receiving element portions 6a to 6d are monolithically integrated on the substrate 10, and the capacitive element portions 7a to 7d are arranged near the light receiving element portions 6a to 6d. there is In addition, one electrode (upper metal layer 43) of each of the capacitive element portions 7a to 7d is grounded to the rear metal film 50 through a via penetrating the substrate 10, and is connected to the signal amplification portion 3A through the rear metal film 50. and the reference potential of 3B. Therefore, the quality of the reference potential of the light receiving element portions 6a to 6d can be improved.

The signal amplifiers 3A and 3B are amplifiers (TIA: Trans Impedance Amplifiers) for amplifying electrical signals (photocurrents) output from the light receiving elements 6a to 6d. The signal amplifiers 3A and 3B are arranged behind the optical waveguide photodetector 2 . The signal amplifying section 3A has two signal input electrode pads 61a and 61b, and performs differential amplification on electrical signals input to the signal input electrode pads 61a and 61b to generate one voltage signal. . The signal amplifier 3B has two signal input electrode pads 61c and 61d, and performs differential amplification on the electrical signals input to the signal input electrode pads 61c and 61d to generate one voltage signal. Generate. The signal input electrode pads 61a to 61d are arranged along the edge 2b of the optical waveguide photodetector 2 in the direction A in this order. As described above, the signal input electrode pads 61a-61d are electrically connected to the signal output electrode pads 21a-21d via the bonding wires 20a-20d, respectively.

The signal amplifier 3A further has three reference potential electrode pads 62a, 62b, and 62c. The reference potential electrode pads 62a to 62c are arranged in this order along the direction A along the edge 2b of the optical waveguide type light receiving element 2. As shown in FIG. The signal input electrode pad 61a is arranged between the reference potential electrode pads 62a and 62b, and the signal input electrode pad 61b is arranged between the reference potential electrode pads 62b and 62c. Similarly, the signal amplifier 3B further has three reference potential electrode pads 62d, 62e, and 62f. The reference potential electrode pads 62d to 62f are arranged in this order along the direction A along the edge 2b of the optical waveguide photodetector 2. As shown in FIG. The signal input electrode pad 61c is arranged between the reference potential electrode pads 62d and 62e, and the signal input electrode pad 61d is arranged between the reference potential electrode pads 62e and 62f. As described above, the reference potential electrode pads 62a, 62c, 62d and 62f of the signal amplifiers 3A and 3B are electrically connected to the reference potential side electrode pads 23a to 23d through bonding wires 20e to 20h, respectively. It is

FIG. 2 shows the cross-sectional structure of two of the four light-receiving element portions 6a to 6d, and FIG. 3 shows the cross-sectional structure of the light-receiving element portion 6d. The cross-sectional structures of the light receiving element portions 6a and 6b are the same as these. FIG. 4 shows the cross-sectional structure of the junction between the light receiving element portion 6d and the optical waveguide portion 8f. The cross-sectional structures of the joint portion between the element portion 6b and the optical waveguide portion 8d, and the joint portion between the light receiving element portion 6c and the optical waveguide portion 8e) are also the same. As shown in FIG. 4, the light receiving element portions 6a to 6d and the optical waveguide portions 8c to 8f are integrated on a common substrate 10. As shown in FIG. The substrate 10 is, for example, a semi-insulating InP substrate.

The cross-sectional structures of the light receiving element portions 6a to 6d will be described by taking the light receiving element portion 6d as an example. As shown in FIG. 3, the light-receiving element portion 6d includes a buffer layer 11 having a high-concentration n-type conductivity provided on a substrate 10, and a region D (second region in FIG. 3) of the n-type buffer layer 11. 4) and a waveguide photodiode structure 19 provided thereon. The waveguide photodiode structure 19 includes a low-concentration semiconductor layer 12 provided on the n-type buffer layer 11 , a light absorption layer 13 provided on the low-concentration semiconductor layer 12 , and a p-type semiconductor layer 13 provided on the light absorption layer 13 . and a p-type contact layer 15 provided on the p-type cladding layer 14 . The n-type buffer layer 11 is the first semiconductor layer in this embodiment, the low-concentration semiconductor layer 12 is the second semiconductor layer in this embodiment, and the p-type clad layer 14 is the third semiconductor layer in this embodiment. .

The n-type buffer layer 11 is, for example, a Si-doped InP layer. The Si doping concentration of the n-type buffer layer 11 is, for example, 1×10 ¹⁷ cm ⁻³ or more. The thickness of the n-type buffer layer 11 is, for example, 1 μm to 2 μm.

The low-concentration semiconductor layer 12 is a low-concentration n-type or i-type semiconductor layer provided between the n-type buffer layer 11 and the light absorption layer 13 . The low-concentration semiconductor layer 12 has an impurity concentration lower than that of the n-type buffer layer 11 or is undoped. In one example, the Si doping concentration of the low concentration semiconductor layer 12 is 1×10 ¹⁶ cm ⁻³ or less. Also, the bandgap of the low-concentration semiconductor layer 12 is larger than the bandgap of the light absorption layer 13 and smaller than the bandgap of the n-type buffer layer 11 . The low-concentration semiconductor layer 12 is, for example, a Si-doped InGaAsP layer. In this case, the bandgap wavelength is 1.4 μm. The thickness of the low-concentration semiconductor layer 12 is, for example, 0.1 μm to 0.2 μm. . In one example, lower surface 12b of low-concentration semiconductor layer 12 is in contact with upper surface 11a of n-type buffer layer 11 .

The light absorption layer 13 is, for example, an undoped InGaAs layer or a low-concentration n-type InGaAs layer with a Si doping concentration of 3×10 ¹⁶ cm ⁻³ or less. The thickness of the light absorbing layer 13 is, for example, 0.1 μm to 0.4 μm, and in one example is 0.25 μm. FIG. 4 shows the center height H1 of the light absorption layer 13 with the upper surface 11a of the n-type buffer layer 11 as a reference. The center height H1 is the distance between the upper surface 11a and a fictitious plane equidistant from the upper surface 13a and the lower surface 13b in the thickness direction of the light absorbing layer 13 . In one example, the lower surface 13b of the light absorption layer 13 is in contact with the upper surface 12a of the low-concentration semiconductor layer 12 .

The p-type clad layer 14 is, for example, a Zn-doped InP layer. The Zn doping concentration of the p-type cladding layer 14 is, for example, 2×10 ¹⁷ cm ⁻³ or more. The thickness of the p-type cladding layer 14 is, for example, 1 μm to 2.5 μm. The p-type contact layer 15 is, for example, a Zn-doped InGaAs layer. The Zn doping concentration of the p-type contact layer 15 is, for example, 1×10 ¹⁸ cm ⁻³ or more. The thickness of the p-type contact layer 15 is, for example, 0.1 μm to 0.3 μm.

The low-concentration semiconductor layer 12 also functions as a relaxation layer for a hetero energy barrier (ΔEc: conduction band). Alternatively, the low-concentration semiconductor layer 12 may be a composition-graded (graded) layer that relaxes the hetero-energy barrier (ΔEc) between the n-type buffer layer 11 and the light absorption layer 13 . In that case, the low-concentration semiconductor layer 12 consists of, for example, two layers of undoped or Si-doped InGaAsP, and the bandgap wavelengths of the two layers are, for example, 1.3 μm and 1.1 μm, respectively. The Si concentration is 1×10 ¹⁶ cm ⁻³ or less.

In addition, an InGaAsP layer may be provided between the light absorption layer 13 and the p-type clad layer 14 for the purpose of reducing travel delay of minority carriers (holes) in order to achieve high-speed response. A composition-graded (graded) layer may be provided between the light absorption layer 13 and the p-type clad layer 14 to relax the hetero-energy barrier (ΔEv: Valence band) between the layers. . This composition-graded layer consists of, for example, two layers of undoped or Zn-doped InGaAsP, and the respective bandgap wavelengths of the two layers are, for example, 1.3 μm and 1.1 μm. The Zn concentration is 1×10 ¹⁷ cm ⁻³ or less.

A p-type hetero-barrier relaxation layer may be provided between the p-type cladding layer 14 and the p-type contact layer 15 . The Zn doping concentration of this hetero-barrier relaxation layer is, for example, 1×10 ¹⁸ cm ⁻³ or more. This hetero-barrier relaxation layer is made of, for example, two layers of Zn-doped InGaAsP, and the respective bandgap wavelengths of the two layers are, for example, 1.1 μm and 1.3 μm.

The low-concentration semiconductor layer 12, the light absorption layer 13, the p-type cladding layer 14, and the p-type contact layer 15 form a mesa structure extending in a predetermined optical waveguide direction (direction B in FIG. 1 in this embodiment). , the mesa structure has a pair of sides. A pair of side surfaces of this mesa structure are buried with buried regions 18 made of a semi-insulating material such as Fe-doped InP. The width of the mesa structure in the direction orthogonal to the optical waveguide direction is, for example, 1.5 to 3 μm. The height of the mesa structure is, for example, 2-3.5 μm.

The light receiving element section 6d further has two layers of insulating films 16 and 17 . The insulating films 16 and 17 are provided from the upper surface of the mesa structure to the buried region 18 to cover and protect them. The insulating films 16 and 17 are, for example, insulating silicon compound (SiN, SiON, or SiO ₂ ) films. The insulating films 16 and 17 have openings on the upper surfaces of the mesa structures, and a p-type ohmic electrode 31 is provided on the p-type contact layer 15 exposed from the insulating films 16 and 17 through the openings. ing. The p-type ohmic electrode 31 is made of an alloy of AuZn or Pt and the p-type contact layer 15, for example. A wiring 32 is provided on the p-type ohmic electrode 31 . The wiring 32 extends in the optical waveguide direction (second direction B) and electrically connects the p-type ohmic electrode 31 and the signal output electrode pad 21d. The wiring 32 has a laminated structure such as TiW/Au or Ti/Pt/Au, and the signal output electrode pads 21d are formed by Au plating, for example.

The insulating films 16 and 17 also have another opening on the n-type buffer layer 11 away from the mesa structure of the light receiving element portion 6d. An n-type ohmic electrode 41 is provided as a cathode on the n-type buffer layer 11 exposed from the insulating films 16 and 17 through the openings. The n-type ohmic electrode 41 is made of AuGe or an alloy of AuGeNi and the n-type buffer layer 11, for example. A bias wiring 42 is provided on the n-type ohmic electrode 41 . As shown in FIG. 2, the bias wiring 42 extends to the lower metal layer of the capacitive element portion 7d and electrically connects the lower metal layer and the n-type ohmic electrode 41. As shown in FIG.

Next, the cross-sectional structures of the optical waveguide portions 8c to 8f will be described. FIG. 4 includes the structure of the cross section perpendicular to the optical waveguide direction of the optical waveguide portion 8f. Other optical waveguide portions 8c to 8e have the same cross-sectional structure as the optical waveguide portion 8f. The optical waveguide portion 8f includes an n-type buffer layer 11 provided on the substrate 10, and an optical waveguide structure 80 provided on a region E (first region) adjacent to the region D of the n-type buffer layer 11. consists of The optical waveguide structure 80 includes an optical waveguide core layer 81 provided on the n-type buffer layer 11 and a clad layer 82 provided on the optical waveguide core layer 81 .

The n-type buffer layer 11 is a semiconductor layer shared with the light receiving element portion 6d, and functions as a first lower clad layer in the optical waveguide portion 8f. The n-type buffer layer 11 is provided over the substrate 10 in the light receiving element portion 6d and over the substrate 10 in the optical waveguide portion 8f.

The optical waveguide portion 8f and the light receiving element portion 6d have a butt joint structure, and the optical waveguide core layer 81 and the light absorption layer 13 are in contact with each other. Thereby, the optical waveguide core layer 81 and the light absorption layer 13 are optically coupled to each other. The optical waveguide core layer 81 and the low-concentration semiconductor layer 12 are arranged side by side in a predetermined optical waveguide direction (direction B in FIG. 1) on the n-type buffer layer 11 . In one example, the end face of the low-concentration semiconductor layer 12 and the end face of the optical waveguide core layer 81 are in contact with each other at the butt joint interface. The lower surface 81b of the optical waveguide core layer 81 and the lower surface 12b of the low-concentration semiconductor layer 12 are in contact with a common semiconductor layer (the n-type buffer layer 11 in this embodiment).

FIG. 4 shows the center height H2 of the optical waveguide core layer 81 with the upper surface 11a of the n-type buffer layer 11 as a reference. The center height H2 is the distance between the top surface 11a and an imaginary plane equidistant from the top surface 81a and the bottom surface 81b in the thickness direction of the optical waveguide core layer 81 . In one example, center height H1 and center height H2 are equal to each other. That is, the center of the optical waveguide core layer 81 in the thickness direction and the center of the light absorption layer 13 in the thickness direction are aligned with each other.

The optical waveguide core layer 81 is made of a material (for example, InGaAsP) that has a higher refractive index than the n-type buffer layer 11 and can be lattice-matched with the buffer layer 11 . In one example, the bandgap wavelength of InGaAsP of the optical waveguide core layer 81 is 1.05 μm. The thickness of the optical waveguide core layer 81 is, for example, 0.3 μm to 0.5 μm, and in one embodiment is 0.5 μm. The clad layer 82 is made of a material (for example, undoped InP) that has a lower refractive index than the optical waveguide core layer 81 and can be lattice-matched with the optical waveguide core layer 81 . The thickness of the clad layer 82 is, for example, 1 μm to 3 μm, and the height of the top surface of the clad layer 82 and the height of the top surface of the p-type contact layer 15 with respect to the top surface 11a of the n-type buffer layer 11 are aligned with each other. A portion of the n-type buffer layer 11, the optical waveguide core layer 81, and the clad layer 82 form a mesa structure extending in a predetermined optical waveguide direction. An optical signal is confined in the optical waveguide core layer 81 by the refractive index difference between the buffer layer 11 and the cladding layer 82 and the optical waveguide core layer 81 and the mesa structure, and the optical signal can be propagated to the light receiving element portion 6d. can. The side and top surfaces of this mesa structure are protected by being covered with two layers of insulating films 16 and 17 (see FIG. 3).

Effects obtained by the optical waveguide type light receiving element 2 of this embodiment having the above configuration will be described. FIG. 6 is a cross-sectional view of a joint portion between the light receiving element section 106 and the optical waveguide section 108 of the optical waveguide type light receiving element 100 according to the comparative example. In this optical waveguide photodetector 100, an n-type buffer layer 11 is provided on a common substrate 10. As shown in FIG. A light absorption layer 13, a p-type cladding layer 14, and a p-type contact layer 15 for the light receiving element section 106 are stacked in this order on the region D of the n-type buffer layer 11. As shown in FIG. An optical waveguide core layer 81 and a clad layer 82 for the optical waveguide section 108 are laminated in this order on the region E of the n-type buffer layer 11 . If the light absorption layer 13 and the optical waveguide core layer 81 are grown on different layers, the optical coupling efficiency at the junction will decrease due to abnormal growth. Therefore, by providing the light absorption layer 13 and the optical waveguide core layer 81 on the common n-type buffer layer 11, the butt joint structure can be regrown without abnormal growth. As a result, it is possible to suppress a decrease in the optical coupling efficiency of the joint portion.

Here, when improving the frequency response characteristic of the light receiving element section 106, a trade-off between the CR time constant of the light receiving element section 106 and the carrier transit time becomes a problem. That is, the thicker the light absorption layer 13 of the light receiving element portion 106, the smaller the capacitance and the smaller the CR time constant, but the transit time of minority carriers (holes) generated in the light absorption layer 13 becomes longer. Also, the thinner the light absorption layer 13 of the light receiving element section 106 is, the shorter the transit time of minority carriers (holes) can be, but the capacitance is increased and the CR time constant is increased. Therefore, it is desired to solve such a trade-off and improve the frequency response characteristics.

To address such a problem, in the optical waveguide photodetector 2 of the present embodiment, an n-type or i-type low-concentration semiconductor layer 12 having a low impurity concentration is provided between the n-type buffer layer 11 and the light absorption layer 13. is provided. In this case, since the depletion region when the reverse bias is applied spreads from the light absorption layer 13 to the low-concentration semiconductor layer 12, even if the light absorption layer 13 is thinned to shorten the transit time of minority carriers (holes), , the thinning of the depleted region can be suppressed. Therefore, it is possible to suppress (or reduce or maintain) an increase in capacitance and keep the CR time constant small. As described above, according to the optical waveguide type photodetector 2 of the present embodiment, the trade-off between the CR time constant and the carrier transit time of the photodetector portions 6a to 6d can be resolved, and the frequency response characteristics can be further improved.

Further, in the present embodiment, the low-concentration semiconductor layer 12 is provided side by side with the optical waveguide core layer 81 on the n-type buffer layer 11 . As shown in FIG. 5, when the light absorption layer 13 and the optical waveguide core layer 81 are simply arranged on the n-type buffer layer 11, the light absorption layer 13 is replaced with the optical waveguide core layer 81 in order to improve the frequency response characteristics. , the optical waveguide core layer 81 and the light absorption layer 13 having different thicknesses are arranged on the n-type buffer layer 11 . Therefore, the center height H1 of the light absorption layer 13 and the center height H2 of the optical waveguide core layer 81 with respect to the upper surface 11a of the n-type buffer layer 11 are different from each other. The optical coupling efficiency with 81 is lowered, and the light receiving sensitivity is deteriorated. In contrast, according to the present embodiment, since the low-concentration semiconductor layer 12 is provided between the n-type buffer layer 11 and the light absorption layer 13, even if the light absorption layer 13 is made thin, the light can still be absorbed. The center height H1 of the absorption layer 13 can be brought closer to the center height H2 of the optical waveguide core layer 81 . Therefore, it is possible to suppress the deterioration of the light receiving sensitivity (or improve the light receiving sensitivity) by suppressing the deterioration of the optical coupling efficiency between the light absorption layer 13 and the optical waveguide core layer 81 .

As in this embodiment, the center height H1 of the light absorption layer 13 and the center height H2 of the optical waveguide core layer 81 with respect to the upper surface 11a of the n-type buffer layer 11 may be equal to each other. This makes it possible to more effectively suppress a decrease in optical coupling efficiency between the light absorption layer 13 and the optical waveguide core layer 81 .

Further, as in the present embodiment, the upper and lower layer structures sandwiching the light absorption layer 13 may be asymmetric. In the p-type InP forming the p-type cladding layer 14 located above the light absorption layer 13, the n-type buffer layer 11 and the low-concentration semiconductor layer 12 located below the light absorption layer 13 are separated from each other in terms of material properties. Compared to n-type InP and p-type InGaAsP, the loss due to free carrier absorption is large. As in the present embodiment, the low-concentration semiconductor layer 12 relaxes the refractive index difference at the hetero interface between the n-type buffer layer 11 and the light absorption layer 13 , and the hetero interface between the p-type cladding layer 14 and the light absorption layer 13 . By making the refractive index difference at the interface relatively large, it is possible to reduce the leakage of light into the p-type cladding layer 14 when the light is guided by absorption in the light absorption layer 13 . Thereby, it is possible to reduce the optical loss and contribute to the improvement of the light receiving sensitivity.

Also, as in the present embodiment, the impurity concentration of the low-concentration semiconductor layer 12 may be 1×10 ¹⁶ cm ⁻³ or less. When the low-concentration semiconductor layer 12 has such an impurity concentration, for example, the depletion region can be effectively extended to the low-concentration semiconductor layer 12 when a reverse bias is applied.

Further, as in the present embodiment, the bandgap of the low-concentration semiconductor layer 12 may be larger than the bandgap of the light absorption layer 13 and smaller than the bandgap of the n-type buffer layer 11 . When the low-concentration semiconductor layer 12 has such a bandgap, for example, the depletion region can be effectively extended to the low-concentration semiconductor layer 12 when a reverse bias is applied.

Further, as in the present embodiment, the n-type buffer layer 11 and the low-concentration semiconductor layer 12, which are semiconductor layers common to the light receiving element portions 6a to 6d and the optical waveguide portions 8a to 8f, have different compositions. may As a result, the etching selectivity between the n-type buffer layer 11 and the low-concentration semiconductor layer 12 can be increased when the light-receiving element portions 6a to 6d are regrown, and the n-type buffer layer 11 can effectively function as an etching stop layer. can. Therefore, the height of the upper surface 11a of the n-type buffer layer 11 in the light receiving element portions 6a to 6d can be formed with high precision, and the height of the upper surface 11a of the n-type buffer layer 11 in the optical waveguide portions 8a to 8f can be aligned. Therefore, the center height H1 of the light absorption layer 13 and the center height H2 of the optical waveguide core layer 81 can be more accurately matched.

(Modification)
FIG. 5 is a cross-sectional view of a joint portion between a light receiving element portion 6d and an optical waveguide portion 8f according to a modification. In the above embodiment, an example in which the low-concentration semiconductor layer 12 and the optical waveguide core layer 81 are in contact with the n-type buffer layer 11 has been described. Another buffer layer 11A may be provided in between. The buffer layer 11A is provided on the n-type buffer layer 11 and formed from the region D to the region E. As shown in FIG. The buffer layer 11A has the same conductivity type as the n-type buffer layer 11, and its Si doping concentration is lower than that of the n-type buffer layer 11 or undoped. The Si doping concentration of the buffer layer 11A is, for example, 1×10 ¹⁶ cm ⁻³ or less. The bandgap of buffer layer 11A is the same as or smaller than that of n-type buffer layer 11 . The buffer layer 11A is, for example, an n-type or undoped InP layer. The thickness of the buffer layer 11A is, for example, 0.1 μm to 0.3 μm.

In this modification, the depletion region when a reverse bias is applied spreads from the light absorption layer 13 beyond the low-concentration semiconductor layer 12 to the buffer layer 11A. The transit time of minority carriers (holes) can be further shortened by making the light absorption layer 13 thinner. Therefore, according to this modified example, the frequency response characteristic can be further improved.

Further, in this modification, the buffer layer 11A extends between the n-type buffer layer 11 and the optical waveguide core layer 81 in the optical waveguide structure. Since the n-type buffer layer 11 contains a large amount of n-type impurities (eg, Si), the free carrier absorption coefficient is large, and the loss of light guided through the optical waveguide core layer 81 is increased. On the other hand, if an n-type or undoped buffer layer 11A having a low impurity concentration (that is, having a small free carrier absorption coefficient) is provided between the n-type buffer layer 11 and the optical waveguide core layer 81, the buffer layer 11A , and the n-type buffer layer 11 is distant from the optical waveguide core layer 81, so the loss of light guided through the optical waveguide core layer 81 can be reduced. Therefore, the propagation loss of the optical waveguide portions 8c-8f is improved. Therefore, the light receiving sensitivity of the optical waveguide type light receiving element 2 is further improved.

Although the present invention has been specifically described above based on the embodiments, the present invention is not limited to the above embodiments, and can be modified without departing from the scope of the invention. For example, the composition of the optical waveguide core layer 81 in the above embodiment is not limited to the InGaAsP system, and may be, for example, an AlGaInAs system. Further, in the above embodiment, the configuration in which the optical waveguide portions 8a to 8f and the light receiving element portions 6a to 6d are integrated on the common substrate 10 was exemplified. Photoelectric conversion circuits including heterojunction bipolar transistors), capacitors and resistors may also be integrated. Further, in the above embodiment, the buffer layer 11 is provided on the substrate 10, but the buffer layer 11 may be omitted when the substrate is an n-type semiconductor substrate. In that case, the n-type semiconductor substrate becomes the first semiconductor layer, and the relationship between the buffer layer 11 and the other semiconductor layers in the above description can be read as the relationship between the n-type semiconductor substrate and the other semiconductor layers.

DESCRIPTION OF SYMBOLS 1A... Light receiving device, 2... Optical waveguide type light receiving element, 2a, 2b... Edge, 3A, 3B... Signal amplification part, 4a, 4b... Input port, 5... Optical branching part, 6a to 6d... Light receiving element part, 7a 7d... capacitive element part, 8a to 8f... optical waveguide part, 10... substrate, 11... n-type buffer layer, 11A... buffer layer, 12... low concentration semiconductor layer, 13... light absorption layer, 14... p-type clad layer , 15... p-type contact layer, 16, 17... insulating film, 18... buried region, 19... waveguide type photodiode structure, 20a to 20m... bonding wire, 21a to 21d... electrode pad for signal output, 22a to 22d Bias voltage side electrode pads 23a to 23d Reference potential side electrode pads 31 p-type ohmic electrode 32 wiring 41 n-type ohmic electrode 42 bias wiring (lower metal layer) 43 upper metal layer , 50... back surface metal film 61a to 61d... signal input electrode pads 62a to 62f... reference potential electrode pads 80... optical waveguide structure 81... optical waveguide core layer 82... clad layer D... region E . . . area, H1, H2 .. center height, La .

Claims (3)

a first semiconductor layer having n-type conductivity;
an optical waveguide structure provided on a first region of the first semiconductor layer;
a waveguide photodiode structure provided on a second region adjacent to the first region of the first semiconductor layer;
The optical waveguide structure is
an optical waveguide core layer provided on the first semiconductor layer;
a clad layer provided on the optical waveguide core layer;
The waveguide photodiode structure includes:
a second semiconductor layer provided on the first semiconductor layer adjacent to the side surface of the optical waveguide core layer and having n-type or i-type conductivity with a lower impurity concentration than the first semiconductor layer;
a light absorption layer provided on the second semiconductor layer adjacent to a side surface of the optical waveguide core layer, optically coupled to the optical waveguide core layer, and thinner than the thickness of the optical waveguide core layer;
a third semiconductor layer provided on the light absorption layer, in contact with the light absorption layer, and having p-type conductivity;
and the center height in the thickness direction of the optical waveguide core layer with reference to the upper surface of the first semiconductor layer, and the light absorption through the second semiconductor layer with reference to the upper surface of the first semiconductor layer the center heights in the thickness direction of the layers are equal to each other,
The optical waveguide type light receiving element, wherein the refractive index of the second semiconductor layer is larger than the refractive index of the third semiconductor layer. 2. The optical waveguide photodetector according to claim 1, wherein said second semiconductor layer has an impurity concentration of 1×10 ¹⁶ cm ⁻³ or less. 3. The optical waveguide photodetector according to claim 1, wherein a bandgap of said second semiconductor layer is larger than a bandgap of said light absorption layer and equal to or smaller than a bandgap of said first semiconductor layer. .

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