Semiconductor photodetector and semiconductor element

Abstract

This semiconductor light receiver comprises: cladding layers (2, 4); a group III-V compound semiconductor layer (3); an absorption layer (30) composed of a group III-V compound semiconductor; a p-type layer (31) composed of a group III-V compound semiconductor; an n-type layer (32); and electrodes (5, 6). The absorption layer (30) is constituted by: a non-doped absorption layer (30-1) composed of a non-doped group III-V compound semiconductor; a p-type absorption layer (30-2) formed in a region of contact between the absorption layer (30) and the p-type layer (31); and an n-type absorption layer (30-3) formed in a region of contact between the absorption layer (30) and the n-type layer (32). An absorption edge wavelength of the non-doped absorption layer (30-1) is shorter than an incident light wavelength.

Description

[Technical Field]

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

[0002] The technology of integrating III-V semiconductors onto Si optical waveguide circuits is a key technology for realizing miniaturization and cost reduction of optical communication transceivers, including lasers. Until now, lasers have been realized on Si optical waveguide circuits using techniques such as direct bonding of III-V semiconductor materials onto Si substrates.

[0003] In recent years, III-V semiconductors on Si have attracted attention not only as materials for lasers but also for fabricating high-speed, high-efficiency external modulators and photodetectors. In particular, photodetectors fabricated using thin-film InP-based materials bonded to a Si substrate can easily achieve high speeds due to their extremely small device capacitance and strong optical confinement factor. In the thin-film InP-based photodetector disclosed in Non-Patent Document 1, a core exhibiting a high absorption coefficient in the telecommunication wavelength band is embedded in an InP thin film, and a lateral diode is formed by doping the InP on both sides of the core with donors and acceptors. This results in a smaller device capacitance than conventional III-V semiconductor devices in which pin junctions are formed perpendicular to the substrate, making it easier to realize high-speed photodetectors.

[0004] To achieve both high sensitivity and high speed in the waveguide-coupled thin-film photodetector disclosed in Non-Patent Document 1, it is necessary to shorten the length of the device while increasing the optical confinement in the absorption layer. However, increasing the optical confinement in the absorption layer requires a wide core width, and as the core width increases, the carrier drift time in the depletion layer increases. Therefore, even in a thin-film device with a small RC bandwidth, the maximum value of the 3 dB bandwidth is limited by the drift time in the depletion layer. On the other hand, narrowing the core width to shorten the drift time weakens the optical confinement in the absorption layer, resulting in reduced photosensitivity. In other words, waveguide-coupled thin-film photodetectors with buried cores have the problem of being unable to achieve both high speed and high sensitivity. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] YANNICK BAUMGARTNER,et al.,“High-speed CMOS-compatible III-V on Si membrane photodetectors”,Optics Express,Vol.29,No.1,pp.509-516,2021 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made to solve the above problems, and an object of the present invention is to provide a semiconductor photodetector that can achieve both high speed and high sensitivity. [Means for solving the problem]

[0007] a p-type layer made of a III-V compound semiconductor formed in the semiconductor layer along the horizontal direction so as to be adjacent to the absorption layer and in contact with the absorption layer; an n-type layer made of a III-V compound semiconductor formed in the semiconductor layer on the opposite side to the p-type layer with the absorption layer sandwiched therebetween; a first electrode formed so as to be connected to the p-type layer; and a second electrode formed so as to be connected to the n-type layer, wherein the absorption layer is composed of an undoped absorption layer made of an undoped III-V compound semiconductor, a p-type absorption layer formed in a region where the absorption layer and the p-type layer are in contact with each other, and an n-type absorption layer formed in a region where the absorption layer and the n-type layer are in contact with each other, and the absorption edge wavelength of the undoped absorption layer is shorter than the wavelength of incident light. The optical confinement coefficient of the non-doped absorption layer has a characteristic that varies depending on the total width of the absorption layer, and the total width of the absorption layer is set so that the optical confinement coefficient is maximized under the condition that the width of the non-doped absorption layer is set to a specific value. It is characterized by the following. [Effects of the Invention]

[0008] According to the present invention, by configuring the absorption layer with an undoped absorption layer, a p-type absorption layer, and an n-type absorption layer and by making the absorption edge wavelength of the undoped absorption layer shorter than the wavelength of the incident light, it is possible to realize a semiconductor photodetector that is capable of achieving both high speed and high sensitivity. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view of a semiconductor optical receiver according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view of a semiconductor optical receiver according to a second embodiment of the present invention. [Figure 3] FIG. 3 is a diagram showing the core width dependency of the optical confinement factor in a conventional semiconductor photodetector. [Figure 4] FIG. 4 is a diagram showing the core width dependency of the optical confinement factor in a semiconductor optical receiver according to the second embodiment of the present invention. [Figure 5] FIG. 5 is a cross-sectional view of a semiconductor optical receiver according to a third embodiment of the present invention. [Figure 6] FIG. 6 is a cross-sectional view of a semiconductor device according to a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] [First Example] To solve the above-mentioned problems, it is necessary to achieve both a reduced depletion layer width and high optical confinement. In the present invention, the requirement for both a reduced depletion layer width and high optical confinement is met by injecting donors and acceptors into a part of the thin film core.

[0011] 1 is a cross-sectional view of a semiconductor photodetector according to a first embodiment of the present invention. The semiconductor photodetector includes a Si substrate 1, a cladding layer 2 made of SiO formed on the Si substrate 1, a III-V compound semiconductor layer 3 made of InP formed on the cladding layer 2, a cladding layer 4 made of SiO formed on the III-V compound semiconductor layer 3, an electrode 5 formed to connect with a p-type layer 31 in the III-V compound semiconductor layer 3, and an electrode 6 formed to connect with an n-type layer 32 in the III-V compound semiconductor layer 3.

[0012] An absorption layer 30 having a width Wc and serving as a core is embedded in the III-V compound semiconductor layer 3. Furthermore, in the III-V compound semiconductor layer 3, a p-type layer 31 made of p-type InP is formed adjacent to the absorption layer 30 along the horizontal direction (horizontal direction in FIG. 1) perpendicular to the stacking direction of the cladding layer 2, the III-V compound semiconductor layer 3, and the cladding layer 4 (vertical direction in FIG. 1), so as to be in contact with the absorption layer 30, and an n-type layer 32 made of n-type InP is formed on the opposite side of the p-type layer 31 so as to be in contact with the absorption layer 30, with the absorption layer 30 sandwiched therebetween.

[0013] The absorption layer 30 is composed of an undoped absorption layer 30-1 made of an undoped III-V compound semiconductor having a width Wi (Wc>Wi), a p-type absorption layer 30-2 made of an acceptor-doped III-V compound semiconductor formed between the undoped absorption layer 30-1 and the p-type layer 31 so as to be in contact with the undoped absorption layer 30-1 and the p-type layer 31, and an n-type absorption layer 30-3 made of a donor-doped III-V compound semiconductor formed between the undoped absorption layer 30-1 and the n-type layer 32 so as to be in contact with the undoped absorption layer 30-1 and the n-type layer 32.

[0014] As shown in Figure 1, by connecting a power supply 7 to electrodes 5 and 6 and applying a reverse bias to the semiconductor photodetector, the distance the carriers drift becomes Wi in Figure 1. Therefore, by controlling the reverse bias voltage and thereby the carrier profile, it is possible to reduce the drift time regardless of the core width Wc.

[0015] In the structure of this embodiment (Wc>Wi), the optical confinement factor for the same undoped absorption layer width is larger than that of the conventional structure (Wc=Wi). As a result, in this embodiment, it is possible to increase the optical confinement even in a structure having an undoped absorption layer width with a short carrier drift time, thereby achieving both high sensitivity and high speed.

[0016] If the absorption layer 30 is made of a material with a small band gap, such as an InGaAs core, which is widely used in the communication wavelength band, light is also absorbed in the doped regions (the p-type absorption layer 30-2 and the n-type absorption layer 30-3), generating slow carriers in the doped regions, which limits the OE (optical-to-electrical) bandwidth.

[0017] Therefore, in this embodiment, the absorption edge wavelength of the material constituting the undoped absorption layer 30-1 is set to be shorter than the wavelength of the incident light. As a result, photocarriers are not generated in the doped region to which no electric field is applied. On the other hand, the absorption coefficient increases when a horizontal electric field is applied within the undoped absorption layer 30-1, and incident light is selectively absorbed only within the undoped absorption layer 30-1. With this structure, this embodiment can achieve both high speed and high sensitivity of the semiconductor photodetector while suppressing the generation of slow carriers in the doped region.

[0018] In this embodiment, the material of the absorption layer 30-1 is a non-doped III-V compound semiconductor, but it may be a lightly doped III-V compound semiconductor so that it is depleted by a reverse bias. In this case, the carrier concentration should be such that the desired absorption layer width Wi is obtained by applying a reverse bias voltage.

[0019] [Second Example] Next, a second embodiment of the present invention will be described. This embodiment is a specific example of the first embodiment. FIG. 2 is a cross-sectional view of a semiconductor photodetector according to this embodiment, and the same components as those in FIG. 1 are designated by the same reference numerals. The semiconductor photodetector includes a Si substrate 1, a cladding layer 2 made of SiO formed on the Si substrate 1, a III-V compound semiconductor layer 3 made of InP formed on the cladding layer 2, a cladding layer 4 made of SiO formed on the III-V compound semiconductor layer 3, an electrode 5 formed to connect with a p-type layer 31 in the III-V compound semiconductor layer 3, and an electrode 6 formed to connect with an n-type layer 32 in the III-V compound semiconductor layer 3.

[0020] An absorption layer 30a having a width Wc and serving as a core is embedded in a 230-nm-thick III-V compound semiconductor layer 3 made of InP. In this embodiment, an InGaAlAs-based multiple quantum well (MQW) layer is used as the absorption layer 30a. Furthermore, a p-type layer 31 made of p-type InP is formed adjacent to the absorption layer 30a along the horizontal direction (the left-right direction in FIG. 2 ) so as to be in contact with the absorption layer 30a, and an n-type layer 32 made of n-type InP is formed on the opposite side of the p-type layer 31 with the absorption layer 30a sandwiched therebetween so as to be in contact with the absorption layer 30a.

[0021] The absorption layer 30a is composed of an undoped absorption layer 30a-1 made of an undoped MQW layer having a width Wi (Wc>Wi), a p-type absorption layer 30a-2 made of an MQW layer into which acceptors have been implanted and formed between the undoped absorption layer 30a-1 and the p-type layer 31 so as to be in contact with the undoped absorption layer 30a-1 and the p-type layer 31, and an n-type absorption layer 30a-3 made of an MQW layer into which donors have been implanted and formed between the undoped absorption layer 30a-1 and the n-type layer 32 so as to be in contact with the undoped absorption layer 30a-1 and the n-type layer 32. Cladding layers 2 and 4 made of SiO2, which has a refractive index smaller than that of InP, are formed above and below the III-V compound semiconductor layer 3 made of InP, thereby realizing strong optical confinement.

[0022] Figure 3 shows the core width dependence of the optical confinement factor in a semiconductor photodetector with a conventional structure where Wc = Wi, i.e., an undoped core. Here, the optical confinement factor is the optical confinement factor for the well layer contained in the absorption layer. Figure 3 shows that while increasing the core thickness increases the optical confinement, narrowing the core to shorten the drift time results in a significant decrease in the optical confinement factor.

[0023] The core width dependency of the optical confinement factor in the semiconductor optical receiver of this embodiment is shown in Fig. 4. In Fig. 4, 40 shows the core width dependency of the optical confinement factor in the undoped absorption layer 30a-1 when the width of the undoped absorption layer 30a-1 is set to Wi = 100 nm, 41 shows the core width dependency of the optical confinement factor when Wi = 200 nm, and 42 shows the core width dependency of the optical confinement factor when Wi = 300 nm.

[0024] It can be seen that, in any case where the width Wi of the undoped absorption layer 30a-1 is 100 nm, 200 nm, or 300 nm, the optical confinement factor can be larger when Wc>Wi as in this embodiment than when Wc=Wi. This effect of optical confinement enhancement becomes more pronounced as the width Wi of the undoped absorption layer 30a-1 becomes smaller, and therefore the benefit becomes more pronounced when an element with a shorter drift time is produced.

[0025] 4, it can be seen that there exists a core width Wc at which the optical confinement factor is maximized regardless of whether the width Wi of the undoped absorption layer 30a-1 is 100 nm, 200 nm, or 300 nm. When the width Wi of the undoped absorption layer 30a-1 is set to a specific value, for example, 100 nm, the core width Wc should be set to approximately 400 nm to maximize the optical confinement factor. By appropriately designing the core width Wc and the width Wi of the undoped absorption layer 30a-1, it is possible to achieve both an improved drift band and high optical confinement.

[0026] As in the first embodiment, the absorption edge wavelength of the undoped InGaAlAs-based MQW absorbing layer 30a-1 is set shorter than the wavelength of the incident light. This results in almost no light absorption in the acceptor-implanted p-type absorbing layer 30a-2 and the donor-implanted n-type absorbing layer 30a-3, suppressing the generation of slow carriers in these doped regions. Meanwhile, applying a reverse bias to the semiconductor photodetector using the power supply 7 applies a horizontal electric field to the undoped absorbing layer 30a-1, selectively increasing the absorption coefficient only in the undoped absorbing layer 30a-1 due to the Franz-Keldysh effect. It is known that when an MQW layer is used as an absorbing layer, the application of a horizontal electric field significantly changes the absorption coefficient at wavelengths near the absorption edge, making it possible to increase the absorption coefficient with a relatively low bias voltage.

[0027] The semiconductor photodetector of this embodiment can be fabricated using the following known process. First, an InP substrate, on which an InP layer, an absorption layer, and an InP layer are grown in this order, is bonded to a Si substrate 1 on which an SiO2 film, which will become the cladding layer 2, is formed, and the InP substrate is then removed. The absorption layer remaining on the SiO2 film is processed into the shape of a core, and the InP layer formed below the absorption layer is left on the entire wafer surface. A new undoped InP layer is grown from this InP layer, and the absorption layer 30a, processed into the shape of a core, is embedded in the InP layer.

[0028] Next, the p-type absorption layer 30a-2 and the n-type absorption layer 30a-3 are formed by ion implantation and thermal diffusion processes. At this time, the diffusion of donors and acceptors into the absorption layer 30a is controlled by controlling the patterning and heat treatment conditions of the p-type absorption layer 30a-2 and the n-type absorption layer 30a-3. Finally, metal electrodes 5 and 6 are formed so that a horizontal electric field can be applied to the non-doped absorption layer 30a-1 via the p-type absorption layer 30a-2 and the n-type absorption layer 30a-3.

[0029] In the present invention, as the temperature increases, the band gap of the entire absorption layer becomes smaller, making it easier for light absorption to occur in the doped region. To prevent light absorption in the doped region even at high temperatures, the difference between the absorption edge wavelength and the wavelength of the incident light at room temperature (detuning amount) can be increased. However, increasing the detuning amount increases the voltage required to increase the absorption coefficient of the non-doped absorption layer at room temperature. Therefore, it is promising to increase the detuning amount of only the doped region at room temperature without increasing the detuning amount of the non-doped absorption layer.

[0030] When an MQW layer is used as the absorption layer 30a as in this embodiment, it is possible to increase only the band gap of the doped regions (the p-type absorption layer 30a-2 and the n-type absorption layer 30a-3) by intermixing the MQW through impurity introduction. That is, the absorption edge wavelength of at least one of the p-type absorption layer 30a-2 and the n-type absorption layer 30a-3 becomes shorter than the absorption edge wavelength of the undoped absorption layer 30a-1. According to this embodiment, the band gap of only the doped regions can be selectively increased without increasing the amount of detuning of the undoped absorption layer 30a-1 at room temperature, thereby suppressing the generation of slow carriers at high temperatures.

[0031] The temperature of the absorption layer rises not only due to the external environmental temperature but also due to self-heating caused by photocurrent. In particular, since the thin-film photodetector is surrounded by SiO2, which has low thermal conductivity, the temperature rise due to self-heating when photocurrent is generated is likely to be significant. Furthermore, the quaternary material that constitutes the absorption layer has low thermal conductivity, so the temperature rise is likely to be significant. Therefore, it is desirable to embed the absorption layers 30, 30a in an InP material with high heat dissipation properties, as in the first and second embodiments.

[0032] In this embodiment, the absorption layer 30a-1 is an undoped MQW layer, but it may be a lightly doped MQW layer that is depleted by reverse bias. In this case, the carrier concentration should be such that the desired absorption layer width Wi is obtained by applying a reverse bias voltage.

[0033] Furthermore, in this example, a structure using an MQW layer as the absorption layer has been shown, but this is not necessarily limited to MQW, and a bulk layer may also be used as the absorption layer. In the case of a bulk layer, the cross-sectional area of ​​the absorption layer is larger than that of an MQW, which is promising for improving the electric field shielding resistance when high-intensity light is input. It is also possible to use an absorption layer with a quantum wire structure or quantum dot structure formed by processing an MQW into a fine-wire pattern.

[0034] In this embodiment, InGaAlAs was used as the material for the absorption layer 30a, but this is not necessarily limited to InGaAlAs. Any appropriate material may be selected depending on the wavelength of the incident light. For example, an MQW layer may be used in which InAs, InGaAs, or InGaAsP is used as the well layer or barrier layer, or InAlAs, which forms a high barrier against the well layer in the conduction band, may be used as the MQW barrier layer material. Furthermore, when a bulk layer is used as the absorption layer, the same material as the MQW well layer may be used.

[0035] In this example, the thickness of the III-V compound semiconductor layer 3 made of InP was set to 230 nm, but this is not necessarily limited to this. However, when a new non-doped InP layer is epitaxially grown from the InP layer formed below the absorption layer as described above, it is desirable to set the thickness of the III-V compound semiconductor layer 3 to be equal to or less than the critical film thickness (up to 430 nm) resulting from the difference in thermal expansion coefficient between InP and the Si substrate.

[0036] [Third Example] In the first and second embodiments, as shown in Fig. 5, a structure in which a Si core 8 optically coupled to the absorption layer 30a or 30 of the semiconductor photodetector is formed in the cladding layer 2 is also possible. By coupling the absorption layer 30a or 30 to the low-loss Si core 8, free carrier absorption in the doped regions (p-type absorption layer and n-type absorption layer) is suppressed and integration with a Si optical waveguide circuit is facilitated. Furthermore, the core material optically coupled to the photodetector does not necessarily have to be Si, and other low-loss waveguide materials such as SiN or lithium niobate may also be used.

[0037] [Fourth Example] 6, it is also possible to integrate a semiconductor laser having an active layer made of the same material as the absorption layer 30a or 30 in the first and second embodiments with a semiconductor photodetector. The structure of the semiconductor photodetector 10 is as described in the first and second embodiments.

[0038] The semiconductor laser 11 includes a Si substrate 1, a cladding layer 2 made of SiO formed on the Si substrate 1, a III-V compound semiconductor layer 3 made of InP formed on the cladding layer 2, a cladding layer 4 made of SiO formed on the III-V compound semiconductor layer 3, an electrode 5b formed to connect to a p-type layer 31b in the III-V compound semiconductor layer 3, and an electrode 6b formed to connect to an n-type layer 32b in the III-V compound semiconductor layer 3. A forward bias is applied to the semiconductor laser 11 by connecting a power supply 9 to the electrodes 5b and 6b as shown in FIG.

[0039] An active layer 30b made of a non-doped III-V compound semiconductor is buried in the III-V compound semiconductor layer 3. The semiconductor laser 11 has an active layer 30b of the same material and thickness as the absorption layer 30a or 30 of the semiconductor photodetector 10, and a III-V compound semiconductor layer 3 of the same thickness as the semiconductor photodetector 10. Since the semiconductor laser 11 and the semiconductor photodetector 10 share a lateral pin diode structure, they can be integrated together using the same manufacturing process.

[0040] However, the core width and carrier profile are designed differently. The semiconductor photodetector 10 is designed as described above, with the absorption layers 30a and 30a-1 partially doped, and the core width Wc is designed to maximize light confinement in the depletion layers (non-doped absorption layers 30a-1 and 30a-1) with width Wia.

[0041] On the other hand, in the semiconductor laser 11, because the carrier drift time does not contribute to the performance, the core has a wider depletion layer width Wib (Wib>Wia) than in the semiconductor photodetector 10. In this way, in this embodiment, it is possible to integrate, for example, a high-speed, low-power directly modulated laser with strong optical confinement and a semiconductor photodetector. The semiconductor laser 11 becomes a DFB (distributed feedback) laser by forming a diffraction grating on the surface of the InP layer (III-V compound semiconductor layer 3) in FIG.

[0042] Furthermore, similarly to the third embodiment, a low-loss laser structure can be realized in the semiconductor laser 11 by forming a low-loss core, such as Si, optically coupled to the active layer 30b in the cladding layer 2. When a Si waveguide is formed, it is also possible to form the laser resonator using the Si waveguide layer.

[0043] The oscillation wavelength of the semiconductor laser 11 may be designed to be longer than the band edge wavelength of the active layer 30b at room temperature. This is because thin-film lasers have high thermal resistance, and the temperature rise in the active layer due to current injection is significant. By using such a semiconductor laser 11 as the light source of the transmitter, the wavelength of light incident on the semiconductor photodetector 10, which has an absorption layer 30a or 30 made of the same material as the active layer 30b, becomes longer than the absorption edge wavelength of the absorption layer 30a or 30, thereby achieving the effect of suppressing light absorption in the doped region, as described above.

[0044] In this embodiment, a semiconductor device in which a semiconductor laser and a semiconductor photodetector are integrated has been described, but the present invention is not limited to a semiconductor laser; it is also possible to integrate a semiconductor optical amplifier and a semiconductor photodetector. [Industrial Applicability]

[0045] The present invention can be applied to semiconductor devices. [Explanation of symbols]

[0046] 1...Si substrate, 2, 4...cladding layer, 3...III-V compound semiconductor layer, 5, 5b, 6, 6b...electrodes, 8...Si core, 10...semiconductor photodetector, 11...semiconductor laser, 30, 30a...absorption layer, 30b...active layer, 31, 31b...p-type layer, 32, 32b...n-type layer, 30-1, 30a-1...non-doped absorption layer, 30-2, 30a-2...p-type absorption layer, 30-3, 30a-3...n-type absorption layer.

Claims

1. a first cladding layer formed on the substrate; a III-V compound semiconductor layer formed on the first cladding layer; a second cladding layer formed on the semiconductor layer; an absorption layer made of a III-V compound semiconductor embedded in the semiconductor layer; a p-type layer made of a III-V compound semiconductor formed adjacent to the absorption layer and in contact with the absorption layer along the horizontal direction in the semiconductor layer; an n-type layer made of a III-V compound semiconductor formed in the semiconductor layer on the opposite side of the p-type layer with the absorption layer sandwiched therebetween so as to be in contact with the absorption layer; a first electrode formed to be connected to the p-type layer; a second electrode formed to be connected to the n-type layer; The absorbent layer comprises: an undoped absorption layer made of an undoped III-V compound semiconductor; a p-type absorption layer formed in a region where the absorption layer and the p-type layer are in contact with each other; an n-type absorption layer formed in a region where the absorption layer and the n-type layer are in contact with each other; the absorption edge wavelength of the non-doped absorption layer is shorter than the wavelength of the incident light, the optical confinement coefficient of the non-doped absorption layer varies depending on the overall width of the absorption layer; A semiconductor photodetector, characterized in that the total width of the absorption layer is set so that the optical confinement factor is maximized under the condition that the width of the non-doped absorption layer is set to a specific value.

2. 2. The semiconductor photodetector according to claim 1, 2. A semiconductor photodetector, wherein the absorption layer is a multi-quantum well layer.

3. 3. The semiconductor photodetector according to claim 2, a semiconductor photodetector, wherein the absorption edge wavelength of at least one of the n-type absorption layer and the p-type absorption layer is shorter than the absorption edge wavelength of the non-doped absorption layer;

4. The semiconductor photodetector according to any one of claims 1 to 3, a semiconductor laser having an active layer made of a material having the same composition as the absorption layer of the semiconductor photodetector, a depletion layer width in the active layer of the semiconductor laser is wider than a depletion layer width in the absorption layer of the semiconductor photodetector; A semiconductor device, wherein the oscillation wavelength of the semiconductor laser is longer than the band edge wavelength of the active layer.

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