JP2019153671A - Optical waveguide type light receiving element - Google Patents

Abstract

To provide an optical waveguide type light receiving element capable of enhancing frequency response characteristics of a waveguide type photodiode.SOLUTION: An optical waveguide type light receiving element comprises: a first semiconductor layer 11 that has an n-type conductivity type; an optical waveguide structure 80 provided on a first region E of the first semiconductor layer 11; and a waveguide type photodiode structure 19 provided on a second region D adjacent to the first region E of the first semiconductor layer 11. The optical waveguide structure 80 has: an optical waveguide core layer 81 provided on the first semiconductor layer 11; and a clad layer 82 provided on the optical waveguide core layer 81. The waveguide type photodiode structure 19 comprises: a second semiconductor layer 12 provided side by side with the optical waveguide core layer 81 and having an n-type or i-type conductivity type of lower impurity concentration than the first semiconductor layer 11; a light absorption layer 13 provided on the second semiconductor layer 12 and optically coupled with the optical waveguide core layer 81; and a third semiconductor layer 14 provided on the light absorption layer 13 and having a p-type conductivity type.SELECTED DRAWING: Figure 4

Description

  The present invention relates to an optical waveguide type light receiving element.

  Patent Document 1 describes a technique related to an optical waveguide type light receiving element in which an optical waveguide structure and a waveguide type photodiode structure are integrated on a common substrate.

JP 2013-110207 A

  In recent years, an optical waveguide type light receiving element in which an optical waveguide structure and a waveguide type photodiode structure are integrated on a common substrate has been researched and developed. Such an optical waveguide type light receiving element is used as a reception front end of an optical transmission system that combines a multilevel modulation method and a digital coherent reception method, for example, having a high transmission rate of 40 Gb / s or more. An optical waveguide type light receiving element is formed by forming a butt joint structure on a semiconductor substrate with a semiconductor laminated portion for a photodiode including a light absorbing layer and a semiconductor laminated portion for an optical waveguide including an optical waveguide core layer. Produced.

  In the future, optical communication systems will require higher-speed optical communication technology that realizes a transmission speed of, for example, 400 Gb / s, and further increase in modulation speed (for example, 64 GBaud) and multi-level modulation format (for example, 64 QAM). Is needed. Therefore, higher frequency response characteristics are required for the waveguide type photodiode structure of the optical waveguide type light receiving element used in the receiving apparatus.

  The frequency response characteristic of the waveguide type photodiode structure is mainly determined by the CR time constant (C: capacitance, R: resistance) of the photodiode and the transit time of carriers (holes, electrons). In order to enhance the frequency response characteristics, the shorter the traveling time of the carrier, 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 carrier travel time, there is a trade-off that the capacitance increases and the CR time constant increases.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an optical waveguide type light receiving element that can further improve the frequency response characteristics of a waveguide type photodiode.

  In order to solve the above-described problems, an optical waveguide light receiving device according to an embodiment includes a first semiconductor layer having an n-type conductivity, and an optical waveguide structure provided on a first region of the first semiconductor layer. And a waveguide type photodiode structure provided on a second region adjacent to the first region of the first semiconductor layer, wherein the optical waveguide structure is an optical waveguide core layer provided on the first semiconductor layer And a cladding layer provided on the optical waveguide core layer, and the waveguide type photodiode structure is provided side by side with the optical waveguide core layer on the first semiconductor layer, and is lower than the first semiconductor layer A second semiconductor layer having an n-type or i-type conductivity of impurity concentration, a light absorption layer provided on the second semiconductor layer and optically coupled to the optical waveguide core layer, and provided on the light absorption layer and a third semiconductor layer having a p-type conductivity.

  According to the optical waveguide type light receiving element of the present invention, the frequency response characteristic of the waveguide type photodiode can be further improved.

FIG. 1 is a plan view showing a configuration of a light receiving device including an optical waveguide type light receiving element according to an embodiment of the present invention. FIG. 2 shows a cross section taken along the line II-II shown in FIG. FIG. 3 shows an enlarged part of FIG. FIG. 4 partially shows a cross section taken 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 the light receiving element portion and the optical waveguide portion in the comparative example.

[Description of Embodiment of the Present Invention]
First, the contents of the embodiment of the present invention will be listed and described. An optical waveguide light receiving device according to an embodiment includes a first semiconductor layer having an n-type conductivity, an optical waveguide structure provided on a first region of the first semiconductor layer, and a first semiconductor layer first. A waveguide type photodiode structure provided on a second region adjacent to the region, the optical waveguide structure provided on the optical waveguide core layer, and an optical waveguide core layer provided on the first semiconductor layer The waveguide type photodiode structure is provided side by side with the optical waveguide core layer on the first semiconductor layer, and has an n-type or i-type having an impurity concentration lower than that of the first semiconductor layer. A second semiconductor layer having a conductivity type; a light absorption layer provided on the second semiconductor layer and optically coupled to the optical waveguide core layer; and a third having a p-type conductivity type provided on the light absorption layer. And a semiconductor layer.

  In this optical waveguide type light receiving element, an n-type or i-type second semiconductor layer having a low impurity concentration is provided between the first semiconductor layer and the light absorption layer. In this case, the depletion region at the time of reverse bias application extends from the light absorption layer to the second semiconductor layer. Therefore, even when the light absorption layer is thinned and the transit time of minority carriers (holes) is shortened, The CR time constant can be kept small by suppressing the increase in capacity. As described above, according to the optical waveguide type light receiving element described above, the trade-off between the CR time constant of the waveguide type photodiode and the carrier traveling time can be solved, and the frequency response characteristic can be further improved.

  Further, in the optical waveguide type light receiving element, the second semiconductor layer is provided side by side 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 on the first semiconductor layer, if the light absorption layer is made thinner than the optical waveguide core layer in order to improve frequency response characteristics, the thickness is increased on the first semiconductor layer. Thus, the optical waveguide core layer and the light absorption layer having different values are aligned. Therefore, the center height of the light absorption layer with respect to the upper surface of the first semiconductor layer is different from the center height of the optical waveguide core layer, and the optical coupling efficiency between the light absorption layer and the optical waveguide core layer is reduced. . On the other hand, according to the above-described 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 brought close to the center height of the optical waveguide core layer. Therefore, the optical coupling efficiency between the light absorption layer and the optical waveguide core layer can be prevented from decreasing.

  In the optical waveguide type light receiving element, the center height of the optical waveguide core layer and the center height of the light absorption layer may be equal to each other with respect to the upper surface of the first semiconductor layer. Thereby, the fall of the optical coupling efficiency of a light absorption layer and an optical waveguide core layer can be suppressed more effectively.

In the above-described optical waveguide light-receiving element, the impurity concentration of the second semiconductor layer may be 1 × 10 ¹⁶ cm ⁻³ or less. For example, when the second semiconductor layer has such an impurity concentration, the depletion region when the reverse bias is applied can be more effectively extended to the second semiconductor layer.

  In the optical waveguide type light receiving element described above, the band gap of the second semiconductor layer may be larger than the band gap of the light absorption layer and may be equal to or smaller than the band gap of the first semiconductor layer. When the second semiconductor layer has such a band gap, for example, the depletion region when the reverse bias is applied can be more effectively expanded to the second semiconductor layer.

[Details of the embodiment of the present invention]
Specific examples 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. In addition, this invention is not limited to these illustrations, is shown by the claim, and intends that all the changes within the meaning and range equivalent to the claim are included. In the following description, the same reference numerals are given to the same elements in the description of the drawings, and redundant descriptions are omitted. In the following description, undoped means that the impurity concentration is extremely low, for example, 1 × 10 ¹⁵ cm ⁻³ or less.

  One embodiment of the present invention relates to an optical waveguide type light receiving element monolithically integrated with a 90 ° hybrid function mainly used in a coherent optical communication system, and in particular, to widen the high frequency response characteristics and light receiving sensitivity of the element. This relates to the enhancement of sensitivity of characteristics. FIG. 1 is a plan view showing a configuration of a light receiving device including an optical waveguide type light receiving element according to an embodiment of the present invention. 2 shows a cross section taken 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 taken along line IV-IV shown in FIG.

  As shown in FIG. 1, the light receiving device 1A of the present embodiment includes an optical waveguide type light receiving element 2 and signal amplification units 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. The optical waveguide type light receiving element 2 has two input ports 4 a and 4 b and an optical branching unit (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 light-receiving element 2 has a pair of end edges 2a and 2b extending along a predetermined direction A. The two input ports 4 a and 4 b are provided at one end edge 2 a of the end edges 2 a and 2 b of the optical waveguide type light receiving element 2. One of the two input ports 4a and 4b receives an optical signal La including four signal components modulated by a QPSK (Quadrature Phase Shift Keying) method outside the light receiving device 1A. Is input. The local oscillation light Lb is input to the other input port 4b. Each of the input ports 4a and 4b is optically coupled to the optical branching section 5 through the optical waveguide sections 8a and 8b. The optical waveguide portions 8a and 8b include a core layer made of a material having a relatively high refractive index (for example, InGaAsP), and a cladding layer made of a material having a refractive index smaller than that of the core layer (for example, InP) and covering the core layer. And is preferably configured.

  The optical branching unit 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 into QPSK. Each of the four signal components Lc1 to Lc4 modulated by the method branches. 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. 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. The signal components Lc3 and Lc4 have a quadrature relationship.

  The light receiving element portions 6 a to 6 d have a configuration as a PIN photodiode, and are arranged in this order along the edge 2 b of the optical waveguide type light receiving element 2. Each of the light receiving element portions 6a to 6d is optically coupled to the four output ends of the optical branching portion 5 through the optical waveguide portions 8c to 8f. 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 optical branching portion 5, and generates an electric signal (photocurrent) corresponding to the light intensity of each of the signal components Lc1 to Lc4. On the optical waveguide type light receiving element 2, signal output electrode pads 21a to 21d electrically connected to anodes of the light receiving element portions 6a to 6d are provided. The signal output electrode pads 21 a to 21 d are provided side by side along the direction A along the edge 2 b of the optical waveguide type light receiving element 2. The signal output electrode pads 21a to 21d are electrically connected to the signal input electrode pads 61a to 61d of the signal amplifiers 3A and 3B via bonding wires 20a to 20d, respectively.

  The capacitive element portions 7a to 7d are so-called MIM (Metal-Insulator-Metal) capacitors configured by a lower metal layer, an upper metal layer, and an insulating film 45 sandwiched between the lower metal layer and the upper metal layer. is there. The lower metal layer and the upper metal layer have a laminated structure such as TiW / Au or Ti / Pt / Au. Each of the capacitive element portions 7a to 7d is arranged side by side (adjacent) along the edge 2b with respect to each of the light receiving element portions 6a to 6d on the optical waveguide type light receiving element 2, and the light receiving element portions 6a to 6d. The bias wiring 42 for supplying a bias voltage to each cathode is electrically connected to 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. Further, the upper metal layer 43 of the capacitive element portions 7a to 7d is drawn 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 the reference potential side electrode pad. 23a-23d. The reference potential side electrode pads 23 a to 23 d are electrically connected to the back surface metal film 50 provided on the back surface of the substrate 10 through vias (not shown) penetrating the substrate 10. The lower metal layer 42 of the capacitive element portions 7 a to 7 d extends toward the inside of the substrate 10. By these capacitive element portions 7a to 7d, inductance components between the cathodes of the light receiving element portions 6a to 6d and a bypass capacitor (not shown) can be designed in a uniform manner.

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

  One end of each of the bonding wires 20i to 20m is connected to each of the bias voltage side electrode pads 22a to 22d. The other end of each of the bonding wires 20i to 20m is electrically connected to a bias voltage source (not shown). The bonding wires 20i to 20m constitute a part of wiring for supplying a bias voltage to each of the light receiving element portions 6a to 6d.

  One end of each of the bonding wires 20e to 20h is connected to each of the reference potential side electrode pads 23a to 23d. 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 the reference potential electrode pads 62a, 62c, 62d and 62f of the signal amplifying units 3A and 3B. Connected to each.

  In the present 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. Yes. In addition, one electrode (upper metal layer 43) of the capacitive element portions 7 a to 7 d is grounded to the back surface metal film 50 through a via penetrating the substrate 10, and the signal amplification unit 3 </ b> A through the back surface metal film 50. And 3B to the reference potential. Therefore, the quality of the reference potential of the light receiving element portions 6a to 6d can be improved.

  The signal amplifying units 3A and 3B are amplifiers (TIA: Trans Impedance Amplifiers) that amplify electric signals (photocurrents) output from the light receiving element units 6a to 6d. The signal amplification units 3A and 3B are arranged behind the optical waveguide type light receiving element 2. The signal amplifying unit 3A includes two signal input electrode pads 61a and 61b, and generates a single voltage signal by performing differential amplification of the electric signal input to the signal input electrode pads 61a and 61b. . The signal amplifying unit 3B includes two signal input electrode pads 61c and 61d, and performs differential amplification of the electric signal input to the signal input electrode pads 61c and 61d to generate one voltage signal. Generate. The signal input electrode pads 61 a to 61 d are arranged in this order in the direction A along the edge 2 b of the optical waveguide type light receiving element 2. As described above, each of the signal input electrode pads 61a to 61d is electrically connected to each of the signal output electrode pads 21a to 21d via the bonding wires 20a to 20d.

  The signal amplifying unit 3A further includes three reference potential electrode pads 62a, 62b, and 62c. The reference potential electrode pads 62 a to 62 c are arranged in this order along the direction A along the edge 2 b of the optical waveguide type light receiving element 2. The signal input electrode pad 61a is disposed between the reference potential electrode pads 62a and 62b, and the signal input electrode pad 61b is disposed between the reference potential electrode pads 62b and 62c. Similarly, the signal amplifying unit 3B further includes three reference potential electrode pads 62d, 62e, and 62f. The reference potential electrode pads 62 d to 62 f are arranged in this order along the direction A along the edge 2 b of the optical waveguide type light receiving element 2. The signal input electrode pad 61c is disposed between the reference potential electrode pads 62d and 62e, and the signal input electrode pad 61d is disposed 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 the bonding wires 20e to 20h, respectively. Has been.

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

  The cross-sectional structure 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 the substrate 10, and a region D (second region, FIG. 5) of the n-type buffer layer 11. 4) and a waveguide type photodiode structure 19 provided thereon. The waveguide type 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 provided on the light absorption layer 13. And a p-type contact layer 15 provided on the p-type clad layer 14. The n-type buffer layer 11 is a first semiconductor layer in this embodiment, the low-concentration semiconductor layer 12 is a second semiconductor layer in this embodiment, and the p-type cladding layer 14 is a 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 impurity concentration of the low-concentration semiconductor layer 12 is lower than that of the n-type buffer layer 11 or undoped. In one example, the Si doping concentration of the low-concentration semiconductor layer 12 is 1 × 10 ¹⁶ cm ⁻³ or less. The band gap of the low concentration semiconductor layer 12 is larger than the band gap of the light absorption layer 13 and smaller than the band gap 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 band gap 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, the lower surface 12 b of the low concentration semiconductor layer 12 is in contact with the upper surface 11 a of the 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 having a Si doping concentration of 3 × 10 ¹⁶ cm ⁻³ or less. The thickness of the light absorption layer 13 is, for example, 0.1 μm to 0.4 μm, and in one embodiment 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 the aerial plane equidistant from the upper surface 13a and the lower surface 13b in the thickness direction of the light absorption layer 13. In one example, the lower surface 13 b of the light absorption layer 13 is in contact with the upper surface 12 a of the low concentration semiconductor layer 12.

The p-type cladding layer 14 is, for example, a Zn-doped InP layer. The Zn doping concentration of the p-type cladding layer 14 is 2 × 10 ¹⁷ cm ⁻³ or more, for example. 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 hetero energy barrier (ΔEc: Conduction band) relaxation layer. Alternatively, the low-concentration semiconductor layer 12 may be a composition graded (gradient) layer that relaxes the heteroenergy barrier (ΔEc) between the n-type buffer layer 11 and the light absorption layer 13. In this case, the low-concentration semiconductor layer 12 is made of, for example, two layers of undoped or Si-doped InGaAsP, and the band gap wavelengths of the two layers are 1.3 μm and 1.1 μm, for example. The Si concentration is 1 × 10 ¹⁶ cm ⁻³ or less.

Further, an InGaAsP layer may be provided between the light absorption layer 13 and the p-type cladding layer 14 for the purpose of reducing traveling delay of minority carriers (holes) in order to realize a high-speed response. Further, a composition graded (gradient) layer that relaxes a hetero energy barrier (ΔEv: Valence band (valence band)) between both layers may be provided between the light absorption layer 13 and the p-type cladding layer 14. . This composition graded layer is made of, for example, two layers of undoped or Zn-doped InGaAsP, and the band gap wavelengths of the two layers are, for example, 1.3 μm and 1.1 μm, respectively. The Zn concentration is 1 × 10 ¹⁷ cm ⁻³ or less.

In addition, 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 the hetero barrier relaxation layer is, for example, 1 × 10 ¹⁸ cm ⁻³ or more. The hetero barrier relaxation layer is made of, for example, two layers of Zn-doped InGaAsP, and the band gap 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 that extends in a predetermined optical waveguide direction (in this embodiment, the direction B in FIG. 1). The mesa structure has a pair of side surfaces. A pair of side surfaces of this mesa structure is buried with a buried region 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 to 3.5 μm.

The light receiving element portion 6d further includes 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 an opening on the top surface of the mesa structure, 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 opening. ing. The p-type ohmic electrode 31 is made of, for example, AuZn or an alloy of Pt and the p-type contact layer 15. 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 pad 21d is formed by Au plating, for example.

  The insulating films 16 and 17 have another opening also on the n-type buffer layer 11 separated from the mesa structure of the light receiving element portion 6d. An n-type ohmic electrode 41 as a cathode is provided 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, for example, an alloy of AuGe or AuGeNi and the n-type buffer layer 11. 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 7 d, and electrically connects the lower metal layer and the n-type ohmic electrode 41.

  Subsequently, a cross-sectional structure of the optical waveguide portions 8c to 8f will be described. FIG. 4 includes a cross-sectional structure perpendicular to the optical waveguide direction of the optical waveguide portion 8f. The other optical waveguide portions 8c to 8e have the same cross-sectional structure as the optical waveguide portion 8f. The optical waveguide portion 8 f 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. It 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 common to the light receiving element portion 6d, and functions as a first lower cladding layer in the optical waveguide portion 8f. The n-type buffer layer 11 is provided from the substrate 10 in the light receiving element portion 6d to 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 aligned with each other in the predetermined optical waveguide direction (direction B in FIG. 1) on the n-type buffer layer 11. In one example, the end surface of the low concentration semiconductor layer 12 and the end surface 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 (in this embodiment, the n-type buffer layer 11).

  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 a distance between the upper surface 11a and an imaginary plane equidistant from the upper surface 81a and the lower surface 81b in the thickness direction of the optical waveguide core layer 81. In one example, the center height H1 and the center height H2 are equal to each other. That is, the center in the thickness direction of the optical waveguide core layer 81 and the center in the thickness direction of the light absorption layer 13 are aligned with each other.

  The optical waveguide core layer 81 is made of a material (for example, InGaAsP) having a refractive index larger than that of the n-type buffer layer 11 and capable of lattice matching with the buffer layer 11. In one example, the band gap wavelength of InGaAsP in 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) having a refractive index smaller than that of the optical waveguide core layer 81 and capable of lattice matching with the optical waveguide core layer 81. The thickness of the cladding layer 82 is, for example, 1 μm to 3 μm, and the height of the upper surface of the cladding layer 82 with respect to the upper surface 11a of the n-type buffer layer 11 and the height of the upper surface of the p-type contact layer 15 are aligned with each other. A part of the n-type buffer layer 11, the optical waveguide core layer 81, and the cladding layer 82 form a mesa structure extending in a predetermined optical waveguide direction. The optical signal is confined in the optical waveguide core layer 81 by the difference in refractive index between the buffer layer 11 and the cladding layer 82 and the optical waveguide core layer 81 and this mesa structure, and the optical signal can be propagated to the light receiving element portion 6d. it can. The side surface and the upper surface of the mesa structure are protected by being covered with two layers of insulating films 16 and 17 (see FIG. 3).

  The effect obtained by the optical waveguide type light receiving element 2 of the present embodiment having the above configuration will be described. FIG. 6 is a diagram illustrating a cross section of a joint portion between the light receiving element portion 106 and the optical waveguide portion 108 of the optical waveguide type light receiving device 100 according to the comparative example. In this optical waveguide type light receiving element 100, an n-type buffer layer 11 is provided on a common substrate 10. On the region D of the n-type buffer layer 11, a light absorption layer 13, a p-type cladding layer 14, and a p-type contact layer 15 for the light receiving element portion 106 are laminated in this order. On the region E of the n-type buffer layer 11, an optical waveguide core layer 81 and a cladding layer 82 for the optical waveguide unit 108 are laminated in this order. When the light absorption layer 13 and the optical waveguide core layer 81 are grown on different layers, the optical coupling efficiency of the junction portion is lowered due to abnormal growth. For this reason, 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. Thereby, the fall of the optical coupling efficiency of a junction part can be suppressed.

  Here, when enhancing the frequency response characteristics of the light receiving element unit 106, a trade-off between the CR time constant of the light receiving element unit 106 and the carrier traveling 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 CR time constant, but the traveling time of minority carriers (holes) generated in the light absorption layer 13 becomes longer. Further, the thinner the light absorption layer 13 of the light receiving element portion 106, the shorter the traveling time of minority carriers (holes), but the capacitance increases and the CR time constant increases. Therefore, it is desirable to improve the frequency response characteristics by solving such a trade-off.

  In order to solve such a problem, in the optical waveguide type light receiving element 2 of the present embodiment, a low impurity concentration n-type or i-type low-concentration semiconductor layer 12 is provided between the n-type buffer layer 11 and the light absorption layer 13. Is provided. In this case, since the depletion region at the time of reverse bias application extends from the light absorption layer 13 to the low concentration semiconductor layer 12, even when the light absorption layer 13 is thinned and the traveling time of minority carriers (holes) is shortened. , Thinning of the depleted region can be suppressed. Therefore, an increase in capacitance can be suppressed (or reduced or maintained), and the CR time constant can be reduced. Thus, according to the optical waveguide type light receiving element 2 of the present embodiment, the trade-off between the CR time constant of the light receiving element portions 6a to 6d and the carrier travel time can be solved, and the frequency response characteristics can be further improved.

  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 changed to the optical waveguide core layer 81 in order to improve frequency response characteristics. When the thickness is made thinner, 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 with respect to the upper surface 11a of the n-type buffer layer 11 and the center height H2 of the optical waveguide core layer 81 are different from each other, and the light absorption layer 13 and the optical waveguide core layer are different from each other. The optical coupling efficiency with 81 is lowered, and the light receiving sensitivity is deteriorated. On the other hand, 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 thin, no light is emitted. The center height H1 of the absorption layer 13 can be brought close to the center height H2 of the optical waveguide core layer 81. Therefore, it is possible to suppress a decrease in optical coupling efficiency between the light absorption layer 13 and the optical waveguide core layer 81 and to suppress a deterioration in the light receiving sensitivity (or improve the light receiving sensitivity).

  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. Thereby, the fall of the optical coupling efficiency of the light absorption layer 13 and the optical waveguide core layer 81 can be suppressed more effectively.

  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 constituting 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 respectively provided due to material properties. Compared with the n-type InP and p-type InGaAsP that are formed, the loss due to free carrier absorption is large. As in this embodiment, the low-concentration semiconductor layer 12 relaxes the refractive index difference at the heterointerface between the n-type buffer layer 11 and the light absorption layer 13, so that the heterogeneity between the p-type cladding layer 14 and the light absorption layer 13 is achieved. By making the refractive index difference at the interface relatively large, it is possible to reduce the oozing of light to the p-type cladding layer 14 during the light absorption waveguide in the light absorption layer 13. Thereby, light loss can be reduced and it can contribute to the improvement of a light reception sensitivity.

Further, 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 when the reverse bias is applied can be more effectively extended to the low concentration semiconductor layer 12.

  Further, as in this embodiment, the band gap of the low concentration semiconductor layer 12 may be larger than the band gap of the light absorption layer 13 and smaller than the band gap of the n-type buffer layer 11. When the low-concentration semiconductor layer 12 has such a band gap, for example, the depletion region at the time of reverse bias application can be more effectively extended to the low-concentration semiconductor layer 12.

  Further, as in the present embodiment, the n-type buffer layer 11 which is a semiconductor layer common to the light receiving element portions 6a to 6d and the optical waveguide portions 8a to 8f and the low concentration semiconductor layer 12 have different compositions. May be. Thus, the etching selectivity between the n-type buffer layer 11 and the low-concentration semiconductor layer 12 can be improved during the regrowth of the light receiving element portions 6a to 6d, and the n-type buffer layer 11 can effectively function as an etching stop layer. it can. Accordingly, 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 accuracy and can be made equal to the height of the upper surface 11a of the n-type buffer layer 11 in the optical waveguide portions 8a to 8f. 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 matched more accurately.

(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, the 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. However, the low concentration semiconductor layer 12 and the optical waveguide core layer 81 and the n-type buffer layer 11 Another buffer layer 11A may be provided therebetween. The buffer layer 11A is provided on the n-type buffer layer 11 and is formed from the region D to the region E. 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 band gap of the buffer layer 11 </ b> A is the same as or smaller than that of the 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 at the time of reverse bias application extends from the light absorption layer 13 to the buffer layer 11A beyond the low-concentration semiconductor layer 12, so that an increase in capacitance can be more effectively suppressed. The light absorption layer 13 can be made thinner to further reduce the traveling time of minority carriers (holes). Therefore, according to the present modification, the frequency response characteristic can be further improved.

  In the present modification, the buffer layer 11 </ b> A 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 (for example, 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, a low free carrier absorption coefficient) is provided between the n-type buffer layer 11 and the optical waveguide core layer 81, the buffer layer 11A The optical loss in the optical waveguide core layer 81 is small, and the n-type buffer layer 11 is moved away from the optical waveguide core layer 81. Therefore, 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 to 8f is improved. Therefore, the light receiving sensitivity of the optical waveguide type light receiving element 2 is further improved.

  The present invention has been specifically described above based on the embodiments. However, the present invention is not limited to the above-described embodiments, and can be changed without departing from the gist thereof. For example, the composition of the optical waveguide core layer 81 of the above embodiment is not limited to the InGaAsP system, and may be an AlGaInAs system, for example. 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 is illustrated. However, other InP-based electronic devices (for example, A photoelectric conversion circuit including a heterojunction bipolar transistor), a capacitor, and a resistor may be further integrated. 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 serves as 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 ... End edge, 3A, 3B ... Signal amplification part, 4a, 4b ... Input port, 5 ... Optical branching part, 6a-6d ... Light receiving element part, 7a ˜7d ... capacitance element portion, 8a˜8f ... optical waveguide portion, 10 ... substrate, 11 ... n-type buffer layer, 11A ... buffer layer, 12 ... low concentration semiconductor layer, 13 ... light absorption layer, 14 ... p-type cladding 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 pad, 23a to 23d ... Reference potential side electrode pad, 31 ... p-type ohmic electrode, 32 ... wiring, 41 ... n-type ohmic electrode, 42 ... bias wiring (lower metal layer), 43 ... upper Metal layer, 50 ... backside metal film, 61a-61d ... signal input electrode pad, 62a-62f ... reference potential electrode pad, 80 ... optical waveguide structure, 81 ... optical waveguide core layer, 82 ... clad layer, D ... region , E ... area, H1, H2 ... center height, La ... optical signal, Lb ... local oscillation light, Lc1-Lc4 ... signal components.

Claims (4)

a first semiconductor layer having an n-type conductivity type;
An optical waveguide structure provided on the 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 type photodiode structure is
A second semiconductor layer provided on the first semiconductor layer side by side with the optical waveguide core 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 to the optical waveguide core layer;
A third semiconductor layer having a p-type conductivity type provided on the light absorption layer;
An optical waveguide type light receiving element having:   2. The optical waveguide type light receiving element according to claim 1, wherein a center height of the optical waveguide core layer and a center height of the light absorption layer with respect to the upper surface of the first semiconductor layer are equal to each other. 3. The optical waveguide type light receiving element according to claim 1, wherein an impurity concentration of the second semiconductor layer is 1 × 10 ¹⁶ cm ⁻³ or less.   4. The optical waveguide according to claim 1, wherein a band gap of the second semiconductor layer is larger than a band gap of the light absorption layer and is equal to or smaller than a band gap of the first semiconductor layer. Type light receiving element.

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