Photodetector

Abstract

This photodetector comprises a plurality of columnar vias (106) that are connected through an upper cladding layer (105) to a second conductivity-type layer (104) formed on an optical absorption layer (103). The plurality of columnar vias (106) are formed arrayed in a waveguide direction. The covering ratios of the plurality of columnar vias (106) in a plan view gradually decrease from one end to the other end in a waveguide direction of the optical absorption layer (103). The cross-sectional areas of the plurality of columnar vias (106) in a plane parallel to the surface of a lower cladding layer (101) gradually decrease from one end to the other end in the waveguide direction.

Description

[Technical Field]

[0001] The present invention relates to a photodetector. [Background technology]

[0002] With the recent spread of optical communications, there is a demand for lower costs for optical communications devices. To meet this demand, for example, there is a technology that uses micro-optical circuit technology, such as silicon photonics, to form optical circuits constituting optical communications devices on large-diameter wafers, such as silicon wafers. This technology allows multiple optical circuit chips to be fabricated simultaneously, dramatically reducing the material cost per chip and enabling lower costs for optical communications devices. A representative device using this technology is a photodetector that combines a vertical photodiode and a waveguide, with each layer stacked (see Patent Document 1). In addition, there is a technology for this type of photodetector in which the electrode placed on the light absorption layer is divided into multiple columnar vias and positioned to avoid the optical mode distribution (see Patent Document 1 and Non-Patent Document 1).

[0003] Hereinafter, a photodetector in which a vertical photodiode and a waveguide are combined, in which an electrode for detecting photocurrent arranged on a light absorption layer is divided into multiple columnar vias, will be described with reference to Figures 6A and 6B. Note that the drawings are only a schematic representation.

[0004] This photodetector first includes a lower cladding layer 301 formed on a substrate 321, and a p-type layer 302 formed on the lower cladding layer 301. The p-type layer 302 is formed by introducing impurities into a predetermined region of a semiconductor layer 311 formed on and in contact with the lower cladding layer 301. The semiconductor layer 311 is made of, for example, silicon.

[0005] This photodetector also includes a light absorbing layer 303 extending in the waveguide direction and formed on the p-type layer 302, and an n-type layer 304 formed on the light absorbing layer 303. The light absorbing layer 303 is formed in a so-called core shape, and in this example, the cross section perpendicular to the waveguide direction has a trapezoidal shape. The light absorbing layer 303 is made of, for example, germanium and is either i-type or n-type. The n-type layer 304 is formed by introducing a high concentration of impurities into a predetermined region on the upper surface of the light absorbing layer 303.

[0006] The photodetector also includes an upper cladding layer 305 formed on the p-type layer 302 to cover the light absorption layer 303 and the n-type layer 304, and a plurality of columnar vias 306 that penetrate the upper cladding layer 305 and connect (by ohmic contact) to the n-type layer 304. The plurality of columnar vias 306 are connected to a first electrode 307 formed on the upper cladding layer 305. Also, on the upper cladding layer 305, second electrodes 314 and 315 are provided that penetrate the upper cladding layer 305 and connect to the p-type layer 302 on sides of the light absorption layer 303 in a direction intersecting the waveguide direction. Contact layers 312 and 313 into which impurities are introduced at a high concentration are formed in regions where the second electrodes 314 and 315 contact the p-type layer 302.

[0007] In addition, an optical waveguide 331 is optically connected to one end of the optical absorption layer 303 in the waveguiding direction of this photodetector. Signal light that is guided through the optical waveguide 331 is incident on the optical absorption layer 303 of the photodetector. A photodiode is formed by a stacked structure of a p-type layer 302, an optical absorption layer 303, and an n-type layer 304. If the optical absorption layer 303 is i-type, it becomes a pin photodiode. If the optical absorption layer 303 is n-type, it becomes a pn photodiode. Note that the plan view of FIG. 6A does not show the substrate 321 and lower cladding layer 301 shown in the cross-sectional view of FIG. 6B.

[0008] The signal light incident from the optical waveguide 331 is mainly absorbed by the light absorption layer 303, generating carriers. The generated carriers cause a photocurrent to flow between the first electrode 307 and the second electrodes 314 and 315 for detecting the photocurrent, and light is detected by detecting this photocurrent. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 6836547 Public Relations [Patent Document 2] U.S. Patent No. 7,613,369 [Non-patent literature]

[0010] [Non-Patent Document 1] G. Li et al., "Improving CMOS-compatible Germanium photodetectors", Optics Express, vol. 20, no. 24, pp. 26345-26350, 2012. Summary of the Invention [Problem to be solved by the invention]

[0011] The above-mentioned technology avoids optical mode distribution by dividing the electrode placed on the optical absorption layer into multiple columnar vias. However, reducing the planar area of ​​the via electrode near the optical mode (reducing the via electrode coverage) improves light sensitivity, but increases electrical resistance and reduces high-speed response. Furthermore, the volume fraction of the area where an electric field is generated in the optical absorption region decreases, making the space charge effect (the effect of charges generated by optical absorption weakening the electric field in the optical absorption region) more likely to occur. This causes the output photocurrent to saturate in response to high-power optical input, and reduces high-speed response.

[0012] On the other hand, if the area of ​​the via electrode in plan view is increased (the coverage rate of the via electrode is increased), the high-speed response improves but the light sensitivity decreases. Thus, in this type of photodetector, there is a trade-off between high-speed response and light sensitivity.

[0013] The present invention has been made to solve the above problems, and aims to improve the high-speed response and light-receiving sensitivity of a photodetector that combines a vertical photodiode and a waveguide while suppressing deterioration of either one of these characteristics. [Means for solving the problem]

[0014] The photodetector according to the present invention comprises a first-conductivity-type layer of a first conductivity type formed on a lower cladding layer, a light-absorbing layer extending in a waveguide direction and formed on the first conductivity-type layer, a second conductivity-type layer of a second conductivity type formed on the light-absorbing layer, an upper cladding layer formed on the first conductivity-type layer and covering the light-absorbing layer and the second conductivity-type layer, and a plurality of columnar vias penetrating the upper cladding layer and connecting to the second conductivity-type layer, the plurality of columnar vias being arranged in the waveguide direction and having a planar coverage that decreases from one end side to the other end side of the light-absorbing layer in the waveguide direction. [Effects of the Invention]

[0015] As described above, according to the present invention, a plurality of columnar vias are arranged in the waveguide direction, and the coverage rate in a planar view decreases from one end side to the other end side of the light absorption layer in the waveguide direction, thereby making it possible to suppress deterioration of either the high-speed response or the light-receiving sensitivity while improving the other characteristic. [Brief explanation of the drawings]

[0016] [Figure 1A] FIG. 1A is a plan view showing the configuration of a photodetector according to a first embodiment of the present invention. [Figure 1B] FIG. 1B is a cross-sectional view showing the configuration of the photodetector according to the first embodiment of the present invention. [Figure 2] FIG. 2 is a characteristic diagram showing simulation results of electrical resistance in the case where a plurality of columnar vias have the same cross-sectional area, in the case of the first embodiment, and in the case where the columnar vias are integrally formed. [Figure 3]FIG. 3 is a characteristic diagram showing simulation results of sensitivity when multiple columnar vias have the same cross-sectional area (dash line), in the case of embodiment 1 (solid line), and when the columnar vias are formed integrally (dashed line). [Figure 4] FIG. 4 is a plan view showing the configuration of a photodetector according to the second embodiment of the present invention. [Figure 5] FIG. 5 is a plan view showing the configuration of a photodetector according to the third embodiment of the present invention. [Figure 6A] FIG. 6A is a plan view showing the configuration of a conventional photodetector. [Figure 6B] FIG. 6B is a cross-sectional view showing the configuration of a conventional photodetector. DETAILED DESCRIPTION OF THE INVENTION

[0017] A photodetector according to an embodiment of the present invention will be described below.

[0018] [Embodiment 1] First, a photodetector according to a first embodiment of the present invention will be described with reference to Figures 1A and 1B. Note that the drawings are only schematic diagrams.

[0019] This photodetector first includes a lower cladding layer 101 formed on a substrate 121, and a first conductivity type layer 102 of a first conductivity type formed on the lower cladding layer 101. The first conductivity type layer 102 is formed by introducing impurities to impart the first conductivity type to a predetermined region of a semiconductor layer 111 formed on and in contact with the lower cladding layer 101. The first conductivity type can be, for example, p-type. In this case, the second conductivity type described below is n-type. The first conductivity type can also be n-type. In this case, the second conductivity type described below is p-type. The semiconductor layer 111 can be made of, for example, silicon. The lower cladding layer 101 can be made of, for example, silicon oxide.

[0020] The photodetector also includes a light absorbing layer 103 extending in the waveguide direction and formed on the first conductivity type layer 102, and a second conductivity type layer 104 of a second conductivity type formed on the light absorbing layer 103. The light absorbing layer 103 is formed in a so-called core shape, and in this example, the shape of a cross section perpendicular to the waveguide direction is trapezoidal. The light absorbing layer 103 is made of, for example, germanium and is of i-type or second conductivity type. The second conductivity type layer 104 is formed by introducing a high concentration of impurities into a predetermined region on the upper surface of the light absorbing layer 103.

[0021] The photodetector also includes an upper cladding layer 105 formed on the first conductivity type layer 102 to cover the light absorption layer 103 and the second conductivity type layer 104, and a plurality of columnar vias 106 that penetrate the upper cladding layer 105 and connect (by ohmic contact) to the second conductivity type layer 104. The upper cladding layer 105 may be made of, for example, silicon oxide. The columnar vias 106 may be made of a predetermined metal. The columnar vias 106 are located at positions such that light absorption or scattering does not occur near the columnar vias 106 when light incident on the light absorption layer 103 propagates through the light absorption layer 103.

[0022] Here, the multiple columnar vias 106 are arranged in the waveguide direction, and the coverage in a plan view gradually decreases from one end side to the other end side in the waveguide direction of the light absorbing layer 103. In other words, the proportion of the area occupied by the multiple columnar vias 106 changes from one end side to the other end side in the waveguide direction of the light absorbing layer 103.

[0023] In the first embodiment, the cross-sectional area of ​​the plurality of columnar vias 106 in a plane parallel to the surface of the lower cladding layer 101 gradually decreases from one end to the other end in the waveguide direction. In this example, the plurality of columnar vias 106 are arranged in two rows in the waveguide direction. In this example, the cross-sectional shape of each of the plurality of columnar vias 106 in a plane perpendicular to the direction in which the columnar vias 106 extend (a plane parallel to the surface of the lower cladding layer 101) is rectangular, but this is not limited thereto and the cross-sectional shape can be a polygon such as a circle, an ellipse, a pentagon, or a hexagon. The cross-sectional area can be changed linearly, but this is not limited thereto and the cross-sectional area can be changed nonlinearly.

[0024] The plurality of columnar vias 106 are connected to a first electrode 107 formed on the upper cladding layer 105. Second electrodes 114, 115 are provided on the upper cladding layer 105 on the sides of the light absorption layer 103 in a direction intersecting the waveguide direction, penetrating the upper cladding layer 105 and connecting to the first conductivity type layer 102. Contact layers 112, 113 into which first conductivity type impurities are introduced at a high concentration are formed in regions where the second electrodes 114, 115 contact the first conductivity type layer 102. The second electrodes 114, 115 can be made of a predetermined metal.

[0025] In this example, an optical waveguide 131 is optically connected to one end of the light absorption layer 103 in the waveguiding direction. Signal light that is guided through the optical waveguide 131 is incident on the light absorption layer 103 of the photodetector. A photodiode is formed by a stacked structure of a first conductivity type layer 102, a light absorption layer 103, and a second conductivity type layer 104. If the light absorption layer 103 is i-type, it becomes a pin photodiode. If the light absorption layer 103 is second conductivity type, it becomes a pn photodiode. Note that the substrate 121 and lower cladding layer 101 shown in the cross-sectional view of FIG. 1B are not shown in the plan view of FIG. 1A.

[0026] The signal light incident from the optical waveguide 131 is mainly absorbed by the light absorption layer 103, generating carriers. The generated carriers cause a photocurrent to flow between the first electrode 107 and the second electrodes 114 and 115 for detecting the photocurrent, and light is detected by detecting this. In the first embodiment, the signal light is incident from one end side of the light absorption layer 103 in the waveguiding direction.

[0027] Here, the highest optical power exists at one end in the waveguiding direction of the optical absorption layer 303, which is the optical input portion where the signal light from the optical waveguide 131 is incident, but the optical power remaining in the photodiode including the optical absorption layer 103 decreases exponentially as the light is guided (propagated) in the waveguiding direction.

[0028] In the first embodiment, the columnar vias 106 near the optical input portion have a smaller cross-sectional area in a plane parallel to the surface of the lower cladding layer 101 than the columnar vias 106 at positions away from the optical input portion, so that optical loss when the remaining optical power is large is kept small. Also, the columnar vias 106 at positions away from the optical input portion have a larger cross-sectional area than the columnar vias 106 near the optical input portion, so that an increase in electrical resistance due to division is kept small. On the other hand, although optical loss is large at positions away from the optical input portion, the remaining optical power is already small, so that the proportion of power lost in this portion is small relative to the total input optical power.

[0029] As a result, according to the first embodiment, the increase in electrical resistance is minimized while achieving the same effect of improving photodetection sensitivity as the prior art. Figure 2 shows the simulation results for electrical resistance. A comparison is made between a case where multiple columnar vias have the same cross-sectional area, the first embodiment, and a case where the columnar vias are integrally formed. As shown in Figure 2, the first embodiment can reduce electrical resistance compared to a case where multiple columnar vias have the same cross-sectional area.

[0030] 3 shows the results of a simulation of sensitivity. The simulation compares the results when multiple columnar vias have the same cross-sectional area (dash-dotted line), the results of the first embodiment (solid line), and the results of integrally forming the columnar vias (dashed line). As shown in FIG. 3, the first embodiment can improve sensitivity compared to the results of integrally forming the columnar vias.

[0031] In the above example, the coverage of the columnar via 106 is reduced near the optical input section where the residual optical power is high, and the coverage is increased in the rear section where the residual optical power is low, thereby improving the light receiving sensitivity while minimizing the deterioration of high-speed responsiveness, but the present invention is not limited to this.

[0032] For example, by configuring the signal light to enter from the other end of the optical absorption layer 103 in the waveguide direction, the coverage of the columnar vias 106 can be increased near the optical input section where the residual optical power is high and a large amount of charge is likely to be generated, while the coverage of the columnar vias 106 can be reduced in the rear section where only a small amount of charge is generated.This makes it possible to improve the linearity and high-speed responsiveness of the output photocurrent in response to high-power optical input while minimizing the decrease in light receiving sensitivity.

[0033] As described above, according to the first embodiment, in a photodetector in which a vertical photodiode and a waveguide are coupled together, it is possible to suppress deterioration of one of the characteristics, high-speed response and light sensitivity, while improving the other characteristic.

[0034] [Embodiment 2] Next, a photodetector according to a second embodiment of the present invention will be described with reference to Fig. 4. Note that the drawing is a schematic diagram.

[0035] This photodetector has the same configuration as the above-described first embodiment, and in the second embodiment, it includes a plurality of columnar vias 106a that penetrate the upper cladding layer 105 and connect to the second conductivity type layer 104. In the second embodiment as well, the plurality of columnar vias 106a are formed and arranged in the waveguide direction, and the coverage in a plan view gradually decreases from one end side to the other end side of the light absorption layer 103 in the waveguide direction.

[0036] In the second embodiment, the spacing between the adjacent columnar vias 106a in the waveguide direction gradually increases from one end to the other end in the waveguide direction. Also in this example, the columnar vias 106a are arranged in two rows in the waveguide direction. In this example, each of the columnar vias 106a has a rectangular cross-sectional shape in a plane perpendicular to the direction in which the columnar vias 106a extend (a plane parallel to the surface of the lower cladding layer 101). However, this is not a limitation and the cross-sectional shape may be a circle, ellipse, pentagon, hexagon, or other polygon. The spacing between the adjacent columnar vias 106a in the waveguide direction may be changed linearly, but this is not a limitation and the spacing may be changed nonlinearly.

[0037] In the second embodiment, the number of columnar vias 106a per unit area near the optical input section is smaller than the columnar vias 106a at positions further from the optical input section, so optical loss when the remaining optical power is large is kept small. Furthermore, the number of columnar vias 106a per unit area at positions further from the optical input section is greater than the columnar vias 106a near the optical input section, so the increase in electrical resistance due to division is kept small. On the other hand, although optical loss is large at positions far from the optical input section, the remaining optical power is already small, so the proportion of power lost in this area is small relative to the total input optical power. As a result, the second embodiment also minimizes the increase in electrical resistance while achieving the same improvement in optical detection sensitivity as the prior art.

[0038] In the above example, the coverage is reduced by reducing the number of columnar vias 106a per unit area near the optical input section where the residual optical power is high, and the coverage is increased by increasing the number per unit area in the rear where the residual optical power is low, thereby improving the light receiving sensitivity while minimizing the deterioration of high-speed responsiveness, but the present invention is not limited to this.

[0039] For example, by configuring the signal light to enter from the other end of the optical absorption layer 103 in the waveguide direction, the coverage of the columnar vias 106a can be increased near the optical input section where the residual optical power is high and a large amount of charge is likely to be generated, while the coverage of the columnar vias 106a can be reduced in the rear section where only a small amount of charge is generated.This makes it possible to improve the linearity and high-speed responsiveness of the output photocurrent in response to high-power optical input while minimizing the decrease in light receiving sensitivity.

[0040] As described above, in the second embodiment as well, in a photodetector in which a vertical photodiode and a waveguide are coupled, it is possible to suppress deterioration of either the high-speed response or the light-receiving sensitivity while improving the other characteristic.

[0041] [Embodiment 3] Next, a photodetector according to a third embodiment of the present invention will be described with reference to Fig. 5. Note that the drawing is a schematic diagram.

[0042] This photodetector has the same configuration as the above-described first embodiment, and in the third embodiment, it includes a plurality of columnar vias 106b that penetrate the upper cladding layer 105 and connect to the second conductivity type layer 104. In the third embodiment as well, the plurality of columnar vias 106b are formed and arranged in the waveguide direction, and the coverage in a plan view gradually decreases from one end side to the other end side of the light absorption layer 103 in the waveguide direction.

[0043] In the third embodiment, the columnar vias 106b are arranged in two rows in the waveguide direction, and the center of gravity in a plan view is shifted more toward the center of the two rows from one end to the other end in the waveguide direction. In this example, the columnar vias 106b arranged in two rows have the same outer side surface position, and the length in a direction perpendicular to the waveguide direction in a plane parallel to the surface of the lower cladding layer 101 gradually increases from one end to the other end in the waveguide direction, thereby shifting the center of gravity. The cross-sectional area of ​​the columnar vias 106b in a plane parallel to the surface of the lower cladding layer 101 gradually increases from one end to the other end in the waveguide direction.

[0044] In this example, the plurality of columnar vias 106b are also arranged in two rows in the waveguide direction. In this example, the cross-sectional shape of each of the plurality of columnar vias 106b in a plane perpendicular to the direction in which the columnar vias 106b extend (a plane parallel to the surface of the lower cladding layer 101) is rectangular, but this is not limited thereto and the cross-sectional shape may be a polygon such as a circle, an ellipse, a pentagon, or a hexagon. In addition, the position of the center of gravity in a plan view may be changed so as to be more shifted toward the center of the two rows from the other end side to one end side in the waveguide direction.

[0045] In the third embodiment, the columnar vias 106b near the optical input section have a small cross-sectional area and the distance between the optical mode and the center of gravity of the cross section is large, so that optical loss is kept small even when the optical power is high. The columnar vias 106b farther from the optical input section have a large cross-sectional area and the distance between the optical mode and the center of gravity of the via cross section is small, so that the increase in electrical resistance due to division is kept small. On the other hand, although the optical loss is large, the remaining optical power is already small, so the proportion of the power lost in this portion is small relative to the total input optical power. As a result, the third embodiment also minimizes the increase in electrical resistance while maintaining the same effect of improving optical detection sensitivity as the prior art.

[0046] In the above example, the coverage is reduced by reducing the number of columnar vias 106b per unit area near the optical input section where the residual optical power is high, and the coverage is increased by increasing the number per unit area in the rear where the residual optical power is low, thereby improving the light receiving sensitivity while minimizing the deterioration of high-speed responsiveness, but the present invention is not limited to this.

[0047] For example, by configuring the signal light to enter from the other end of the optical absorption layer 103 in the waveguide direction, the coverage of the columnar vias 106b can be increased near the optical input section where the residual optical power is high and a large amount of charge is likely to be generated, while the coverage of the columnar vias 106b can be reduced in the rear section where only a small amount of charge is generated.This makes it possible to improve the linearity and high-speed responsiveness of the output photocurrent in response to high-power optical input while minimizing the decrease in light receiving sensitivity.

[0048] As described above, in the third embodiment as well, in a photodetector in which a vertical photodiode and a waveguide are coupled together, it is possible to suppress deterioration of either the high-speed response or the light-receiving sensitivity while improving the other characteristic.

[0049] As described above, according to the present invention, a plurality of columnar vias connected to the second semiconductor layer formed on the light absorption layer are arranged in the waveguide direction, and the coverage rate in a planar view is reduced from one end side to the other end side of the light absorption layer in the waveguide direction. This makes it possible to suppress deterioration of either the high-speed response or the light-receiving sensitivity while improving the other characteristic.

[0050] Some or all of the above-described embodiments may also be described as, but are not limited to, the following supplementary notes.

[0051] [Appendix 1] a first semiconductor layer of a first conductivity type formed on the lower cladding layer; a light absorbing layer extending in a waveguiding direction and formed on the first semiconductor layer; a second semiconductor layer of a second conductivity type formed on the light absorption layer; an upper cladding layer formed on the first semiconductor layer to cover the light absorption layer and the second semiconductor layer; a plurality of columnar vias that penetrate the upper cladding layer and connect to the second semiconductor layer; Equipped with The photodetector is characterized in that the plurality of columnar vias are arranged in a waveguide direction, and the coverage in a plan view decreases from one end side to the other end side in the waveguide direction of the light absorption layer.

[0052] [Appendix 2] 2. The photodetector according to claim 1, The photodetector is characterized in that the cross-sectional area of ​​the plurality of columnar vias in a plane parallel to the surface of the lower cladding layer decreases from one end side to the other end side in the wave-guiding direction.

[0053] [Appendix 3] 3. The photodetector according to claim 1 or 2, The photodetector is characterized in that the spacing between the plurality of columnar vias adjacent to each other in the waveguide direction increases from one end to the other end in the waveguide direction.

[0054] [Appendix 4] In the photodetector according to any one of Supplementary Notes 1 to 3, The photodetector is characterized in that the plurality of columnar vias are arranged in two rows in the waveguide direction, and the center of gravity position in a planar view changes so that it is shifted more toward the center of the two rows from one end side to the other end side in the waveguide direction.

[0055] [Appendix 5] In the photodetector according to any one of Supplementary Notes 1 to 4, The photodetector is characterized in that the plurality of columnar vias are arranged at positions where no light absorption or scattering occurs in the vicinity of the plurality of columnar vias when light incident on the light absorption layer propagates through the light absorption layer.

[0056] [Appendix 6] 6. The photodetector according to any one of Supplementary Notes 1 to 5, A photodetector characterized in that signal light is incident on one end or the other end of the light absorption layer in the waveguiding direction.

[0057] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention. [Explanation of symbols]

[0058] 101...lower cladding layer, 102...first conductivity type layer, 103...light absorption layer, 104...second conductivity type layer, 105...upper cladding layer, 106...columnar via, 107...first electrode, 111...semiconductor layer, 112...contact layer, 113...contact layer, 114...second electrode, 115...second electrode, 131...optical waveguide.

Claims

[Claim 1] a first semiconductor layer of a first conductivity type formed on the lower cladding layer; a light absorbing layer extending in a waveguiding direction and formed on the first semiconductor layer; a second semiconductor layer of a second conductivity type formed on the light absorbing layer; an upper cladding layer formed on the first semiconductor layer to cover the light absorption layer and the second semiconductor layer; a plurality of columnar vias that penetrate the upper cladding layer and connect to the second semiconductor layer; Equipped with the plurality of columnar vias are arranged in a waveguide direction, and a coverage ratio in a plan view decreases from one end side to the other end side of the light absorption layer in the waveguide direction; The photodetector is characterized in that the spacing between the plurality of columnar vias adjacent to each other in the waveguide direction increases from one end to the other end in the waveguide direction.

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