JP2018082089A - Photodetector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photodetector capable of reducing dark current without degrading high speed characteristics.SOLUTION: Disclosed photodetector includes: an Si base plate; a lower clad layer formed over the base plate; an Si core layer which is formed on the lower clad layer and connected to a waveguide layer, including a p-type Si area which is doped with p-type impurities ion and an n-type Si area which is doped with n-type impurities ion; a germanium layer which is formed on at least one area of a first n-type Si area and a second n-type Si area; an upper clad layer formed over the Si core layer and the germanium layer; an electrode connected to the p-type Si area; and an electrode connected to the n-type Si area.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a photodetector, and more specifically, to a germanium photodetector formed using a micro optical circuit technique.

  With the spread of optical communication in recent years, there is a demand for cost reduction of optical communication devices. As one of the solutions, there is a method of forming an optical circuit constituting an optical communication device on a large-diameter wafer such as a silicon (Si) wafer by using a micro optical circuit technology such as silicon photonics. By such a method, the material cost per chip can be dramatically reduced, and the cost of the optical communication apparatus can be reduced.

  As a typical photodetector formed on a large-diameter silicon wafer using a micro optical circuit technique, there is a germanium photodetector (GePD) capable of monolithic integration. FIG. 1 is a top view showing the structure of a conventional photodetector. 2 is a cross-sectional view taken along the line II ′ of the photodetector in FIG. Here, in the following drawings, the substrate longitudinal direction is the x-axis direction, the direction perpendicular to the substrate longitudinal direction in the substrate plane direction is the y-axis direction, and the direction perpendicular to the substrate plane is the z-axis direction.

  1 and 2 includes a Si substrate 101, a lower clad layer 102 formed of an Si oxide film on the Si substrate 101, and an Si core layer 110 on the lower clad layer 102. The photodetector 100 is a waveguide-coupled vertical GePD, and is formed on an SOI (Silicon On Insulator) substrate composed of a Si substrate, a Si oxide film, and a surface Si layer using a lithography technique or the like. The Si core layer 110 is connected to an optical waveguide 109 that guides an optical signal.

  A p-type Si region 111 doped with a first semiconductor type (p-type in FIG. 1) impurity ion is formed on the Si core layer 110. On both sides of the upper part of the p-type Si region 111 in the x-axis direction, p-type impurities are doped at a high concentration to form p ++ Si electrode portions 112 and 113 that function as electrodes. A Ge layer 114 is formed at the center in the x-axis direction on the Si core layer 110. An n-type Ge region 115 doped with a second semiconductor type (n-type in FIG. 1) impurity is formed on the Ge layer 114. An upper cladding layer 103 is formed on the Si core layer 110 and the Ge layer 114, an electrode 116 is formed on the p ++ Si electrode portion 112, an electrode 118 is formed on the p ++ Si electrode portion 113, and an electrode is formed on the n-type Ge region 115. 117 is formed.

  In the photodetector 100, when light enters the Si core layer 110 from the optical waveguide 109 and light is absorbed in the Ge layer 114, a photocurrent flows between the electrodes 117 and 116 and the electrodes 117 and 118. The light is detected by detecting the current.

Japanese Patent No. 5370857

  In the photodetector 100, the depletion layer extends in a direction perpendicular to the growth direction (z-axis direction) of the Ge layer 114 (in the xy plane direction). Accordingly, carriers generated in the Ge layer 114 due to light absorption flow in the growth direction of the Ge layer 114.

  Here, the Ge layer 114 is epitaxially grown from the Si layer core 110. However, since Ge and Si have different crystal lattice constants, when the Ge layer 114 is grown on the Si layer core 110, the Ge layer 114 generates dislocations due to lattice defects. The dislocation proceeds in parallel (z-axis direction) to the growth direction (z-axis direction) of the Ge layer 114. The dislocation may be a threading dislocation connected from the bottom surface to the top of the Ge layer 114. When an electric field is applied in the growth direction of the Ge layer 114, a current flows through the threading dislocation. That is, in the vertical GePD, the direction of current flowing through threading dislocations coincides with the direction of carriers generated by optical input. However, the current flowing through threading dislocations contributes to the dark current that flows when no light enters GePD. Accordingly, GePD using a Ge layer with threading dislocations has a dark current that is larger by the amount of current passing through the dislocations. GePD with a large dark current has a low minimum light receiving sensitivity of light intensity, and degrades performance in a long-term reliability test. As a result, vertical GePD has a problem that an increase in dark current due to threading dislocations cannot be avoided.

  In general, as one means for solving this problem, there is a thickening of Ge. As the thickness of Ge increases, the number of threading dislocations decreases, so the dark current can be reduced. However, increasing the thickness of Ge increases the distance traveled by carriers generated by light input, and thus there is a problem of degrading the high-speed characteristics of GePD.

  There is a lateral GePD as an existing photodetector for reducing the dark current. 3 and 4 are both cross-sectional views in the y-axis direction showing the structure of the lateral GePD photodetector. In the photodetector 300 of the first embodiment shown in FIG. 3, a Ge layer 314 is epitaxially grown on the Si core layer 310, and the p-type Ge region 321 is implanted on either side of the top surface of the Ge layer 314 in the x-axis direction. (Ion implantation) is performed, and an n-type Ge region 322 is implanted on the other side to form a pn structure. In the photodetector 400 of the second embodiment shown in FIG. 4, the Ge layer 414 is epitaxially grown at the center in the x-axis direction on the Si core layer 410, and the Si layer 410 is grown to the same height as the Ge layer 414. The Ge layer 414 is sandwiched. Next, a p-type Si region 411 is formed on one side of the Si layer 410 beside the Ge layer 414, and an n-type Si region 419 is formed on the other side. Further, a p ++ Si electrode portion 412 is formed on the p-type Si region 411, and an n ++ Si electrode portion 420 is formed on the n-type Si region 419 to obtain a pn structure.

  The photodetector 300 (and 400) has a pn structure formed in a direction (y-axis direction) orthogonal to the crystal growth direction (z-axis direction) of the Ge layer 314 (and 414), and the electric field is y-axis. Since the Ge layer 314 (and 414) enters in the direction, a decrease in dark current can be expected. However, additional steps specific to GePD fabrication, such as implantation on the top of the Ge layer 314 (photodetector 300 in FIG. 3) or regrowth of the Si layer 410 (photodetector 400 in FIG. 4), are required. There is a problem that the crystallinity of the film is reduced and the number of masks is increased.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a photodetector having a small dark current without degrading high-speed characteristics.

  In order to achieve such an object, a first aspect of the present invention is a photodetector, which is formed on a Si substrate, a lower cladding layer formed on the substrate, and the lower cladding layer. The Si core layer to which the waveguide layer is connected, the first semiconductor type Si region doped with the first semiconductor type impurity ions, and the second semiconductor type doped with the second semiconductor type impurity ions. A Si core layer including a semiconductor type Si region; a germanium layer formed on at least one of the first semiconductor type Si region and the second semiconductor type Si region; the Si core layer; An upper clad layer formed on the germanium layer, an electrode connected to the first semiconductor-type Si region, and an electrode connected to the second semiconductor-type Si region.

  The second aspect of the present invention is the photodetector according to the first aspect, wherein the first semiconductor-type Si region, the second semiconductor-type Si region, An authentic Si region that is not doped with impurities is directly below the germanium layer, and the authentic Si region is disposed between the first semiconductor type Si region and the second semiconductor type Si region. Features.

  A third aspect of the present invention is the photodetector according to the first aspect, wherein the second semiconductor-type Si region of the Si core layer is directly below the germanium layer, An authentic Si region that is not doped with impurities is disposed between the semiconductor-type Si region and the second semiconductor-type Si region.

  According to a fourth aspect of the present invention, there is provided the photodetector according to any one of the first to third aspects, wherein the first semiconductor type Si region and the second semiconductor type Si region are: It is characterized by facing the comb shape.

  In the photodetector according to the present invention, it is possible to achieve a reduction in dark current without impairing the high-speed characteristics while requiring no change in film thickness as compared with the conventional vertical GePD. By reducing the dark current of GePD, the S / N ratio can be increased and the reception sensitivity is increased, so that the power consumption of the transmitter can be reduced and the transmission distance can be extended.

It is a top view which shows the structure of the conventional photodetector. It is sectional drawing in II 'of the photodetector of FIG. It is sectional drawing which shows the structure of the photodetector of horizontal type GePD. It is sectional drawing which shows the structure of the photodetector of horizontal type GePD. It is a top view which shows the structure of the photodetector which concerns on the 1st Embodiment of this invention. It is sectional drawing in VV 'of the photodetector of FIG. It is a top view which shows the structure of the photodetector which concerns on the 2nd Embodiment of this invention. It is sectional drawing in VII-VII 'of the photodetector of FIG. It is a top view which shows the structure of the photodetector which concerns on the modification of the 2nd Embodiment of this invention. It is a top view which shows the structure of the photodetector which concerns on the 3rd Embodiment of this invention. It is a top view which shows the structure of the photodetector which concerns on the modification of the 3rd Embodiment of this invention. It is a top view which shows the structure of the photodetector which concerns on the modification of the 3rd Embodiment of this invention. It is a top view which shows the structure of the photodetector which concerns on the 4th Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[First Embodiment]
FIG. 5 is a top view showing the structure of the photodetector according to the first embodiment of the present invention. 6 is a cross-sectional view taken along line VV ′ of the photodetector in FIG. 5 and 6 includes a Si substrate 501, a lower clad layer 502 formed of an Si oxide film on the Si substrate 501, and an Si core layer 510 on the lower clad layer 502. The photodetector 500 is a waveguide-coupled GePD, and is formed on a SOI substrate including a Si substrate, a Si oxide film, and a surface Si layer by using a lithography technique or the like. The Si core layer 510 is connected to an optical waveguide 509 that guides an optical signal.

  One side of the upper side of the upper part of the Si core layer 510 in the x-axis direction is doped with a first semiconductor type (p-type in this embodiment) impurity ion to form a p-type Si region 511. In addition, the other of the upper sides of the Si core layer 510 on both sides in the x-axis direction is doped with second semiconductor type (in this embodiment, n-type) impurity ions to form an n-type Si region 519. A p ++ Si electrode portion 512 that functions as an electrode is formed on the substrate edge side of the upper portion of the p-type Si region 511 by doping with a high concentration of p-type impurities. The p-type Si region 511 and the n-type Si region 519 are separated from each other by a certain distance, and this portion is a genuine Si region (i-Si region) 523 which is not doped with impurities. An n ++ Si electrode portion 520 that functions as an electrode is formed on the upper edge of the n-type Si region 519 on the substrate edge side in the x-axis direction by doping with n-type impurities at a high concentration. At the center in the x-axis direction on the Si core layer 510, a Ge layer 514 is formed by epitaxial growth.

  Impurities in the p-type Si region 511 and the n-type Si region 519 diffuse into the Ge layer 514 during the growth of the Ge layer 514 to create a p-type Ge region 521 and an n-type Ge region 522. The photodetector 500 absorbs light in a depletion layer formed by the p-type Ge region 521 and the n-type Ge region 522. An electrode 516 is formed on the p ++ Si electrode portion 512, and an electrode 518 is formed on the n ++ Si electrode portion 520. An upper cladding layer 503 is formed on the Si core layer 510 and the Ge layer 514.

  In the photodetector 500, light is incident on the Si core layer 510 from the optical waveguide 509, and the light is absorbed in the Ge layer 514 between the p-type Si region 511 and the n-type Si region 519. When light is absorbed in the Ge layer 514, carriers flow through the depletion layer to the electrodes 516 and 518. Light is detected by detecting the current.

  In the photodetector 500 of this embodiment, carriers generated by light absorption travel in the y-axis direction in FIG. 6, and therefore do not coincide with the threading dislocation direction (x-axis direction) in the Ge layer 514. Therefore, it is possible to suppress an increase in dark current due to threading dislocation like the vertical GePD 100 of FIGS. By suppressing the increase in dark current, the S / N ratio can be increased and the reception sensitivity of GePD light can be increased, so that the power consumption of the transmitter can be reduced and the transmission distance can be extended.

  In this embodiment, the doping of impurity ions into the p-type Si region 511 and the n-type Si region 519 can be performed simultaneously with the implantation of other active devices, so that the number of masks and the number of processes can be reduced. Further, since the implantation to the Ge layer 514 is not necessary, the Ge crystallinity is not deteriorated.

  In addition, the photodetector 500 according to the present embodiment has a structure that can adjust the travel distance of carriers generated by light input by adjusting the distance 523 between the p-type Si region 511 and the n-type Si region 519. It has become. That is, it is not necessary to increase the thickness of the Ge layer in order to reduce the dark current. Therefore, the photodetector 500 according to the present embodiment can achieve a reduction in dark current without impairing the high-speed characteristics. As in the vertical GePD (photodetector 100) shown in FIGS. Therefore, it is not necessary to adjust the film thickness of the Ge layer in order to adjust the high speed and the reduction in dark current.

  In all the following embodiments, the first semiconductor type and the second semiconductor type (in this embodiment, the p-type Si region (511), the p ++ Si electrode portion (512), the n-type Si region (519), and The n ++ Si electrode portion (520) may be at an inverted position.

[Second Embodiment]
FIG. 7 is a top view showing the structure of the photodetector according to the second embodiment of the present invention. FIG. 8 is a cross-sectional view taken along the line VII-VII ′ of the photodetector in FIG. The photodetector 700 according to the second embodiment shown in FIGS. 7 and 8 has the first semiconductor type (p-type in this embodiment) Si region of the photodetector 500 according to the first embodiment from the bottom of the Ge layer. This is an example in which the second semiconductor type (n-type in the present embodiment) Si region is arranged on the bottom surface of the Ge layer with a distance therebetween.

  7 and 8 includes a Si substrate 701, a lower clad layer 702 formed of an Si oxide film on the Si substrate 701, and an Si core layer 710 on the lower clad layer 702. The photodetector 700 is a waveguide coupled GePD, and is connected to the Si core layer 710 to an optical waveguide 709 that guides an optical signal.

  One of the upper sides of the Si core layer 710 on both sides in the x-axis direction is doped with p-type impurity ions to form a p-type Si region 711. Further, n-type impurity ions are doped from the other side of the upper part of the Si core layer 710 on both sides in the x-axis direction to the center to form an n-type Si region 719. The p-type Si region 711 and the n-type Si region 719 are separated from each other, and an i-Si region 725 is formed therebetween. A p ++ Si electrode portion 712 that functions as an electrode is formed on the upper edge of the p-type Si region 711 on the substrate edge side in the x-axis direction by doping with a high concentration of p-type impurities. An n ++ Si electrode portion 720 that functions as an electrode is formed on the substrate edge side of the upper portion of the n-type Si region 719 with an n-type impurity at a high concentration. At the center in the x-axis direction on the Si core layer 710, a Ge layer 714 is formed by epitaxial growth.

  Impurities in the n-type Si region 719 diffuse into the Ge layer 714 during the Ge layer 714 growth, creating an n-type Ge region 722. The photodetector 700 absorbs light in the depletion layer formed by the n-type Ge region 722. An electrode 716 is formed on the p ++ Si electrode portion 712, and an electrode 718 is formed on the n ++ Si electrode portion 720. An upper cladding layer 703 is formed on the Si core layer 710 and the Ge layer 714.

  In the photodetector 700 of this embodiment, among the carriers generated in the Ge layer 714, electrons pass through the route of the Ge layer 714-i-Si region 725-p-type Si region 711, and holes are in the Ge layer 714-n. It passes through the route of the type Ge layer 722-n type Si region 719. The structure between pn is the structure of n-type Si region 719 / n-type Ge region 722 / Ge layer (i-Ge region) 714 / i-Si region 725 / p-type Si region 711, and p-type Si region Since the i-Si region 725 exists at a position connecting the 711 and the n-type Si region 719 with the shortest distance, when the electric field is applied between the electrodes 716-718, the i-Si region 725 is most strongly subjected to the electric field. As described above, since electrons pass through the i-Si region 725, an avalanche amplification occurs when a strong electric field is applied to the i-Si region 725. Therefore, the photodetector having the structure of this embodiment functions as an avalanche photodiode (APD).

  The photodetector 700 according to the present embodiment has a structure in which, as with the photodetector 500 according to the first embodiment, the dark current can be reduced and the sensitivity can be improved by avalanche amplification. Here, the photodetector 700 has the most electric field in the i-Si region 725 among the n-type Si region 719 / n-type Ge region 722 / i-Ge region 714 / i-Si region 725 / p-type Si region 711. Therefore, in order to obtain good APD characteristics in the photodetector 700, it is desirable that the i-Si region 725 be as free of impurities as possible.

  The Ge layer 714 may have a structure arranged only on the n-type Si region 719. FIG. 9 is a top view showing the structure of the photodetector according to the modification of the second embodiment of the present invention. The photodetector 700 in FIG. 9 has a structure in which the Ge layer 714 is disposed only on the n-type Si region 719. When the Ge layer 714 is grown on the n-type Si region 719 as shown in FIG. 9, impurities in the n-type Si region 719 diffuse into the Ge layer 714 while the Ge layer 714 grows, and the Ge layer An n-type Ge region 722 is formed on the entire lower surface of 714.

  However, when the p-type Si region 711 comes into contact with the bottom surface of the Ge layer 714, the carriers conduct through the path of the n-type Si region 719-n-type Ge region 722-i-Ge region 714-p-type Si region 711. , The efficiency of avalanche amplification decreases because it does not pass through the i-Si region 725. Therefore, it is desirable that the p-type Si region 711 is not in contact with the bottom surface of the Ge layer 714.

  In this structure, the n-type Si region 719 may be spaced from the bottom surface of the Ge layer 714, and the p-type Si region 711 may be disposed on the Ge bottom surface. In this case, holes are amplified.

[Third Embodiment]
FIG. 10 is a top view showing the structure of the photodetector according to the third embodiment of the present invention. The photodetector 1000 in FIG. 10 includes a first semiconductor type (p-type in the present embodiment) Si region 511 and a second semiconductor type (n in the present embodiment) of the photodetector 500 of the first embodiment. This is a photodetector having a shape in which the Si region 519 is opposed to a comb shape. In order to make the figure easy to see, only the bottom surface of the Ge layer 1014 is drawn with a broken line. By making the p-type Si region 1011 and the n-type Si region 1019 face each other in a comb shape, both the p-type Ge region and the n-type Ge region diffused in the Ge layer 1014 are opposed to the comb shape. The depletion layer region created by the region widens, improving the sensitivity of the photodetector.

  11 and 12 are top views showing the structure of a photodetector according to a modification of the third embodiment of the present invention. In the present embodiment, as shown in FIG. 11, the p-type Si region 1111 and the n-type Si region 1119 may be arranged by providing a portion that is perpendicular to the light input direction. In addition, as shown in FIG. 12, a portion that is perpendicular to the light input direction may be opposed to a comb shape, and may be arranged as a p-type Si region 1211 and an n-type Si region 1219.

[Fourth Embodiment]
FIG. 13 is a top view showing the structure of the photodetector according to the fourth embodiment of the present invention. FIG. 13 shows a first semiconductor type (p-type in this embodiment) Si region 711 and a second semiconductor type (n-type in this embodiment) Si region 719 of the photodetector 700 of the second embodiment. Is a GePD having a shape facing the comb. In order to make the figure easy to see, only the bottom surface of the Ge layer 1314 is drawn with a broken line. By facing the comb shape, the p-type Ge region 1321 and the n-type Ge region 1322 diffusing in the Ge layer 1314 are also opposed to the comb shape, and the depletion layer region formed by both regions is expanded, thereby improving the sensitivity of the photodetector.

  In the present embodiment, as shown in FIG. 11 of the third embodiment, the p-type Si region 1311 and the n-type Si region 1319 may be disposed by providing a portion that is perpendicular to the light input direction. . In addition, as shown in FIG. 12, a portion that is perpendicular to the light input direction may be opposed to a comb shape, and may be arranged as a p-type Si region 1211 and an n-type Si region 1219.

  However, in an arrangement in which the p-type Si region 1311 is in contact with the bottom surface of the Ge layer 1314, the n-type Si region 1319 / n-type Ge region 1322 / i-Ge region 1314 in which carriers conduct without passing through the i-Si region. Since the path of / p-type Si region 1311 is born, the efficiency of avalanche amplification decreases. Therefore, it is desirable that the p-type Si region 1311 is not in contact with the bottom surface of the Ge layer 1314.

100, 300, 400, 500, 700, 1000, 1100, 1200, 1300 Photodetector 101, 301, 401, 501, 701, 1001, 1101, 1201, 1301 Si substrate 102, 302, 402, 502, 702, 1002 1102, 1202, 1302 Lower cladding layer 103, 303, 403, 503, 703, 1003, 1103, 1203, 1303 Upper cladding layer 109, 309, 409, 509, 709, 1009, 1109, 1209, 1309 Optical waveguide 110, 310, 410, 510, 710, 1010, 1110, 1210, 1310 Si core layer 111, 311, 411, 511, 711, 1011, 1111, 1211, 1311 p-type Si regions 112, 113, 312, 412, 512, 7 12, 1012, 1112, 1212, 1312 p ++ Si electrode part 114, 314, 414, 514, 714, 1014, 1114, 1214, 1314 Ge layer 115, 322, 522, 722 n-type Ge region 116, 117, 118, 316, 318, 416, 418, 516, 518, 716, 718, 1016, 1018, 1116, 1118, 1216, 1218, 1316, 1318 Electrodes 419, 719, 1019, 1119, 1219, 1319 n-type Si regions 420, 720, 1020 1120, 1220, 1320 n ++ Si electrode portions 421, 521, 721 p-type Ge regions 523, 725, 1025, 1125, 1225, 1325 i-Si regions

Claims (4)

A Si substrate;
A lower cladding layer formed on the substrate;
A Si core layer formed on the lower cladding layer and connected to a waveguide layer, the first semiconductor type Si region doped with a first semiconductor type impurity ion, and a second semiconductor type impurity A Si core layer comprising a second semiconductor-type Si region doped with ions;
A germanium layer formed on at least one of the first semiconductor-type Si region and the second semiconductor-type Si region;
An upper cladding layer formed on the Si core layer and the germanium layer;
An electrode connected to the first semiconductor-type Si region;
And an electrode connected to the second semiconductor-type Si region. Of the Si core layer, the first semiconductor type Si region, the second semiconductor type Si region, and a genuine Si region that is not doped with impurities are directly below the germanium layer,
The photodetector according to claim 1, wherein the genuine Si region is disposed between the first semiconductor type Si region and the second semiconductor type Si region. The second semiconductor type Si region of the Si core layer is directly under the germanium layer;
2. The photodetector according to claim 1, wherein a genuine Si region not doped with an impurity is disposed between the first semiconductor type Si region and the second semiconductor type Si region.   4. The photodetector according to claim 1, wherein the first semiconductor-type Si region and the second semiconductor-type Si region face each other in a comb shape. 5.

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