Photodetector

Abstract

A photodetector according to the present invention is a photodetector including a photodiode (300) and a protection diode (301), wherein the photodiode (300) and the protection diode (301) are integrated on the same semiconductor substrate, the photodiode (300) includes a semiconductor portion made of at least one type of semiconductor material, an anode electrode, and a cathode electrode, the protection diode (301) includes a core layer, a semiconductor portion including a first type semiconductor region doped with first type impurity ions and a second type semiconductor region doped with second type impurity ions provided in the core layer, an anode electrode, and a cathode electrode, the semiconductor portion of the protection diode (301) is made of only one type of material among semiconductor materials used for the semiconductor portion of the photodiode (300), the anode electrode of the protection diode (301) and the anode electrode of the photodiode (300) are connected, and the cathode electrode of the protection diode (301) and the cathode electrode of the photodiode (300) are connected.

Description

Photodetector

[0001] The present invention relates to photodetectors used in optical communication systems and optical information processing systems, and more particularly to a structure for providing a photodetector that is resistant to electrostatic discharge.

[0002] With the recent spread of optical communications, there is a demand for lower costs for optical communications devices. One solution is to fabricate the optical circuits that make up optical communications devices on large-diameter wafers such as silicon wafers using micro-optical circuit technology such as silicon photonics. This reduces the material cost per chip, thereby enabling lower costs for optical communications devices.

[0003] A typical photodetector formed on a silicon (Si) substrate using such technology is a monolithically integrated germanium photodetector (hereinafter also referred to as GePD) (Patent Document 1). However, this photodetector does not have a protection circuit, and therefore has the problem of being vulnerable to electrostatic discharge.

[0004] In response to this, Patent Document 2 discloses a photodetector equipped with a protection circuit (protective Zener diode). The protective Zener diode of this photodetector is formed using both a Ge layer and a Si layer.

[0005] Patent No. 5370857 Patent No. 6981365

[0006] However, the photodetector equipped with a protective Zener diode of Patent Document 2 uses a mask to perform ion implantation (doping process) to form the Ge layer and the Si layer. During this process, due to manufacturing errors, misalignment may occur between the p-type semiconductor portion, n-type semiconductor portion (pn junction) or p-type semiconductor portion, intrinsic semiconductor portion, and n-type semiconductor portion (pin junction) on the Si core layer and the light absorption layer (Ge layer). This misalignment can have a negative effect on the rectification characteristics of the photodetector.

[0007] The present invention has been made in view of the above problems, and its object is to provide a photodetector that can eliminate the influence on rectification characteristics caused by misalignment of a protective Zener diode.

[0008] In order to achieve the above object, a first aspect of the present invention is a photodetector including a photodiode and a protection diode, the photodiode and the protection diode being integrated on a same semiconductor substrate, the photodiode including a semiconductor portion made of at least one type of semiconductor material, an anode electrode, and a cathode electrode, the protection diode including a core layer, a semiconductor portion including a first-type semiconductor region doped with first-type impurity ions and a second-type semiconductor region doped with second-type impurity ions which are provided in the core layer, the anode electrode, and the cathode electrode, the semiconductor portion of the protection diode being made of only one type of semiconductor material used for the semiconductor portion of the photodiode, the anode electrode of the protection diode being connected to the anode electrode of the photodiode, and the cathode electrode of the protection diode being connected to the cathode electrode of the photodiode.

[0009] The photodetector of the present invention can eliminate the influence on the rectification characteristics caused by misalignment of the protective Zener diode.

[0010] FIG. 1 is a diagram showing a top view according to a first embodiment of the present invention; FIG. 2 is a diagram showing a cross-sectional structure of a Zener diode provided in the photodetector according to the first embodiment of the present invention; FIG. 3 is a diagram showing the top surface of a photodetector according to a second embodiment of the present invention; FIG. 4 is a diagram showing a top view of a photodetector of a comparative example; and FIG. 5 is a diagram showing a cross-sectional structure of a Zener diode provided in the photodetector of the comparative example.

[0011] Hereinafter, the form of the photodetector of the present invention will be described in detail using embodiments and drawings. In the drawings, parts having the same function are assigned the same numbers to clarify the description. However, the present invention is not limited to the description of the embodiments shown below, and it will be obvious to those skilled in the art that the form and details can be changed in various ways without departing from the spirit of the invention disclosed in this specification, etc.

[0012] First Embodiment FIG. 1 is a top view showing the configuration of a photodetector according to one embodiment of the present invention.

[0013] The photodetector according to one embodiment of the present invention is configured by electrically connecting a GePD 300 and a protective Zener diode 301 in parallel.

[0014] The GePD 300 includes a core layer 110 doped with a first-type impurity (n-type impurity), an (optical) waveguide 109 connected to the core layer 110, a light-absorbing layer (Ge layer) 114 doped with a second-type impurity on the core layer 110, a lower cladding layer 102 (see FIG. 2) below the core layer 110, a semiconductor substrate 101 (see FIG. 2) below the lower cladding layer 102, and an upper cladding layer 103 (see FIG. 2) above the core layer 110 and the light-absorbing layer. A p-type silicon slab 111a and p++ silicon contact regions 112 and 113 are formed in the core layer 110. An n-type Ge region 115 is formed in the light-absorbing layer 114. Electrodes 116, 117, and 118 are connected to the p++ silicon contact regions 112 and 113 and the n-type Ge region 115, respectively. In the GePD 300, when light is incident on the core layer 110 and absorbed by the light absorption layer 114, a current flows between the electrode 117 and the electrodes 116 and 118, and the light can be detected by detecting this current.

[0015] In this embodiment, the GePD 300 and the Zener diode 301 are arranged in a straight line when viewed from the waveguide 109 , and are arranged in the direction in which light from the waveguide 109 is incident.

[0016] The Zener diode 301 includes a core layer 110 extending from the GePD 300, in which a p-type silicon slab (region) 111b doped with p-type impurity ions, a p++ silicon contact region 112 heavily doped with p-type impurities and serving to make ohmic contact with the electrode 116, an n-type silicon slab 119 doped with n-type impurity ions, and an n++ silicon contact region 120 heavily doped with n-type impurities and serving to make ohmic contact with the electrode 117 are formed. The n-type silicon slab 119 is also referred to as an n-type Si slab or n-type silicon region. The n++ silicon contact region is also referred to as an n-portion silicon contact region or n-type electrode portion. An upper cladding layer 103 (see FIG. 2 ), which also continues from the GePD 300, is formed on the p++ silicon contact region 112 and the n++ silicon contact region 120, and an electrode 116 and an electrode 117 are formed so as to contact the p++ silicon contact region 112 and the n++ silicon contact region 120, respectively, through a plurality of openings formed in the upper cladding layer 103. The electrodes 116 and 117 are continuous with the electrodes 116 and 117 of the GePD 300.

[0017] FIG. 2 is a cross-sectional view of the Zener diode 301 taken along line II-II'. In the core layer 110, the p++ silicon contact region 112 is located on the p-type silicon region 111b, and the n++ silicon contact region 120 is located on the n-type silicon slab 119. The p++ silicon contact region 112 is connected to the electrode 116, and the n++ silicon contact region 120 is connected to the electrode 117. An intrinsic silicon region 125 without ion implantation exists between the p-type silicon region 111b and the n-type silicon slab 119. In FIG. 2, the operating threshold of the Zener diode is determined by the size of the intrinsic silicon region 125 indicated by the arrow and the doping concentrations of the p-type silicon region 111b and the n-type silicon slab 119. To adjust the operating threshold, the intrinsic silicon region 125 may be eliminated and the p-type silicon region 111b and the n-type silicon slab 119 may be brought into contact with each other to form a PN junction.

[0018] The upper cladding layer 103 is formed directly on the p-type semiconductor portion (p-type silicon region 111b and p++ silicon contact region 112), the i-type semiconductor portion (intrinsic silicon region 125), and the p-type semiconductor portion (n-type silicon slab 119 and n++ silicon contact region 120). As will be described later in comparison with the comparative examples shown in Figures 4 and 5, no Ge layer is formed in contact with the core layer 110. That is, in the photodetector of this embodiment, the upper cladding layer 103 is formed directly on the p-type and n-type semiconductor portions, and only the upper cladding layer is present between the electrodes connected to the p-type and n-type semiconductor portions. The semiconductor portion of the protection diode is made of only one of the semiconductor materials used in the semiconductor portion of the photodiode. This eliminates any misalignment between the pn junction consisting of the p-type and n-type semiconductor portions and the light absorption layer (Ge layer), thereby eliminating any impact on the rectification characteristics of the photodiode.

[0019] The applicant speculates that the reason the photodetector of this embodiment is operable is because the impurity doping concentration is set so that the Zener breakdown voltage of the protection diode is smaller than the avalanche breakdown voltage of the photodetector body.

[0020] The anode electrode of the Zener diode 301 and the anode electrode of the GePD 300 are connected, and the cathode electrode of the Zener diode 301 and the cathode electrode of the GePD 300 are connected.

[0021] When the electrode 116 functions as an anode electrode, the electrodes 117 and 118 function as cathode electrodes. When the electrode 116 functions as a cathode electrode, the electrodes 117 and 118 function as anode electrodes. Only the upper clad layer 103 is present between the electrodes 117 and 118.

[0022] The core layer 110 of the GePD 300 and the semiconductor portion (n-type silicon slab 119 and n++ silicon contact region 120) of the protection diode 301 are made of the same material.

[0023] As described above, the core layer 110 of the GePD 300 is the same layer as the semiconductor portion (n-type silicon slab 119 and n++ silicon contact region 120) of the protection diode 301.

[0024] In this embodiment, Ge is used as the main material of the light absorption layer 114, but other semiconductor materials such as GeSn, InGaAs, and InGaAsP may also be used.

[0025] 2, the GePD 300 and the protection diode 301 are on a continuous core layer 110, but this is not necessarily required, and they may be fabricated on separate Si slabs. The height and material of the layers used as the Si slabs are approximately the same as those of the core layer 110, but they may be separate.

[0026] In this embodiment, Si is used as the main material of the core layer 110, but other semiconductor materials such as InP and SiC may also be used. Furthermore, the positions of the p-type semiconductor portion and the n-type semiconductor portion may be reversed. As described above, the protection diode 301 may employ a pin junction or a pn junction. Furthermore, as described above, the protection diode 301 may be a single diode, or multiple diodes may be loaded in parallel.

[0027] Second Embodiment As shown in FIG. 3, a photodetector according to one embodiment of the present invention is configured by electrically connecting a GePD 300 and a Zener diode 301 serving as a protection circuit in series.

[0028] In the first embodiment of the GePD 300 shown in Fig. 1 , the GePD 300 and the Zener diode 301 are arranged in a straight line as viewed from the waveguide 109, whereas in the second embodiment of the GePD 300 shown in Fig. 3 , the GePD 300 and the Zener diode 301 are not arranged in a straight line as viewed from the waveguide 109, but are arranged in a direction perpendicular to the incident direction of light from the waveguide 109. The longitudinal direction of the Ge layer 114 of the GePD 300 and the longitudinal directions of the p++ silicon contact region 112 and the n++ silicon contact region 120 of the Zener diode 301 are approximately parallel to each other. The cross-sectional view taken along dashed line II-II' in Fig. 3 corresponds to Fig. 2 .

[0029] In the configuration of FIG. 3, the light traveling straight from the waveguide 109 does not enter the Zener diode 301, and therefore the Zener diode does not detect the photocurrent.

[0030] In this embodiment as well, the semiconductor portion of the protection diode 301 is made of only one type of semiconductor material that is used in the semiconductor portion of the photodiode 300. Therefore, it is possible to provide a photodetector having a protection circuit in which the rectification characteristics of the photodetector of this embodiment are not affected by a pn junction consisting of a p-type semiconductor portion and an n-type semiconductor portion or by misalignment between the p-type semiconductor portion, the I-type semiconductor portion, the n-type semiconductor portion (pin junction) and the light absorption layer (Ge layer).

[0031] 3, one Zener diode (protection diode) 301 is connected to the GePD 300, but multiple Zener diodes 301 may be connected. In this case, although it affects the high-speed operation characteristics of the GePD, it is possible to obtain high resistance to electrostatic discharge.

[0032] 4 and 5 show the configuration of a photodetector of a comparative example. The GePD 300 has the same configuration as the GePD 300 of Embodiments 1 and 2. In the comparative example, the GePD 300 and the Zener diode 401 are arranged in a straight line as viewed from the waveguide 109, and are arranged in the direction of incidence of light from the waveguide 109. The cross-sectional view taken along line VV' in FIG. 4 corresponds to FIG. 5.

[0033] The Zener diode 401 of this comparative example differs from the Zener diode 301 of the first and second embodiments in that the Zener diode for the protection circuit has a Ge layer 114. There is a problem that misalignment may occur between this Ge layer and the p-type semiconductor portion and n-type semiconductor portion (pn junction) or the p-type semiconductor portion, intrinsic semiconductor portion and n-type semiconductor portion (pin junction) on the Si core layer, and this misalignment affects the rectification characteristics of the photodetector.

[0034] 5 is a cross-sectional view of the Zener diode 401 of the present invention taken along the line V-V'. The Ge layer 114 is formed on the n-type silicon slab 119, but not on the p-type silicon (Si) region 111b. The Ge layer 114 is also present between the electrodes 117 and 118.

[0035] The p-type silicon region 111b overlying the core layer 110 is not located directly below the Ge layer 114, and an n-type silicon slab 119 is located below the Ge layer 114. The p-type silicon region 111b may also be located directly below the Ge layer 114.

[0036] Because the Ge layer 114 is formed by implanting Ge material using a mask during the manufacturing process, misalignment may occur between the PN junction corresponding to the p-type and n-type semiconductor portions or the p-type, i-type, and n-type semiconductor portions (pin junction), which can affect the rectification characteristics of the photodetector. In contrast, the detector according to the embodiment of the present invention described above with reference to Figures 1 to 3 does not have a Ge layer below the upper cladding layer 103, eliminating any room for misalignment, thereby eliminating any impact on the rectification characteristics of the photodetector. Furthermore, the detector according to the embodiment of the present invention has the advantage of eliminating the issues that arise when the Ge layer 114 is present. Specifically, the interface between Ge and Si acts as a leakage current path, causing variations in reverse current when the PN junction and the Ge layer 114 are not aligned accurately. However, the detector according to the embodiment of the present invention can suppress this variation.

[0037] The present invention relates to a photodetector used in an optical communication system or an optical information processing system, and is particularly applicable to a photodetector that is resistant to electrostatic discharge.

[0038] 101 semiconductor substrate, 102 lower cladding layer, 103 upper cladding layer, 109 waveguide, 110 core layer, 111a p-type silicon slab, 111b p-type silicon slab (region), 112 silicon contact region, 113 p++ silicon contact region, 114 light absorption layer (Ge layer), 115 n-type Ge region, 116 electrode, 117 electrode, 118 electrode, 119 n-type silicon slab, 120 silicon contact region, 125 intrinsic silicon region, 300 GePD, 301 protection (Zener) diode, 401 protection (Zener) diode

Claims

1. A photodetector comprising a photodiode and a protection diode, wherein the photodiode and the protection diode are integrated on the same semiconductor substrate, the photodiode comprising a semiconductor portion made of at least one type of semiconductor material, an anode electrode, and a cathode electrode, the protection diode comprising a core layer, a semiconductor portion including a first-type semiconductor region doped with first-type impurity ions and a second-type semiconductor region doped with second-type impurity ions provided in the core layer, an anode electrode, and a cathode electrode, the semiconductor portion of the protection diode is made of only one type of semiconductor material used for the semiconductor portion of the photodiode, the anode electrode of the protection diode and the anode electrode of the photodiode are connected, and the cathode electrode of the protection diode and the cathode electrode of the photodiode are connected.

2. The photodetector according to claim 1, wherein the semiconductor portion of the photodiode comprises: a core layer doped with a first-type impurity; an optical waveguide connected to the core layer; a light absorption layer on the core layer doped with a second-type impurity; a lower cladding layer below the core layer; an upper cladding layer on the core layer and the light absorption layer; and electrodes connected to the core layer and the light absorption layer, respectively.

3. The photodetector according to claim 2, wherein the core layer of said photodiode and the semiconductor portion of said protection diode are made of the same material.

4. The photodetector according to claim 2 or 3, wherein the core layer of the photodiode and the semiconductor portion of the protection diode share the same layer.

5. The photodetector according to claim 2, wherein the main material of the core layer of said photodiode is silicon, and the main material of said light absorption layer is germanium.

6. A photodetector according to claim 1 or 2, wherein the photodiode and the protection diode are aligned in the incident direction of light passing through a silicon waveguide layer provided in the photodiode.

7. The photodetector according to claim 1 or 2, wherein the photodiode and the protection diode are aligned in a direction perpendicular to the incident direction of light passing through a silicon waveguide layer provided in the photodiode.

8. The photodetector according to claim 1 or 2, wherein a plurality of the protection diodes are connected to the photodiode.

9. A photodetector according to claim 1 or 2, wherein only an upper clad layer is formed on the semiconductor portion of said protection diode and in contact with the semiconductor portion and between the electrodes of said protection diode.