CN115101609B - A SiGe photodetector based on a directional coupler - Google Patents

Abstract

The invention discloses a germanium-silicon photoelectric detector based on a directional coupler, which comprises a germanium absorption region, the directional coupler, a first conical sub-wavelength reflection grating, a second conical sub-wavelength reflection grating, a silicon flat plate and a substrate, wherein the silicon flat plate is arranged on the silicon flat plate; the germanium absorption region, the directional coupler, the first conical sub-wavelength reflection grating, the second conical sub-wavelength reflection grating and the silicon flat plate are integrated on the substrate. The germanium absorption region is arranged in two waveguides of a first output end and a second output end of the directional coupler, and the first conical sub-wavelength grating and the second conical sub-wavelength reflection grating are both connected with the output end of the directional coupler; the gaps of the first conical sub-wavelength reflection grating and the second conical sub-wavelength reflection grating are filled with silicon dioxide media; the silicon flat plate is arranged between the waveguide area where the germanium absorption area is positioned and the substrate; the invention increases the light absorption performance of the photoelectric detector, reduces the parasitic parameters of devices and improves the electrical bandwidth of the detector.

Description

A SiGe photodetector based on a directional coupler

technical field

The invention relates to the technical field of semiconductors, and more specifically, to a silicon germanium photodetector based on a directional coupler.

Background technique

In long-distance optical communication systems and short-distance on-chip optical interconnection systems, in order to achieve high-speed optical communication, detectors must have both high responsivity and large bandwidth. At present, high-performance detectors can be realized based on a variety of material systems, such as indium phosphide system and gallium arsenide system based on group III-V materials, and silicon optical systems based on group IV materials. Among them, the silicon photonics system has the advantages of being compatible with the standard semiconductor (CMOS) process, low cost, and high integration, and has gradually been widely adopted by the industry. Since the band gap of silicon is 1.12eV, it cannot effectively absorb light signals with a wavelength greater than 1.1 μm, so pure silicon photodetectors are not suitable for near-infrared light detection. The germanium material, which is also a group IV element, has a high response in the near-infrared and optical communication bands, and is compatible with the existing silicon CMOS process, which can effectively reduce costs.

In recent years, with the breakthrough of silicon-based germanium material epitaxy technology, germanium-silicon photodetectors are considered to be one of the best choices for silicon-based photodetectors. After decades of development, the structure of silicon germanium photodetectors has been continuously optimized, and the performance has been further improved. According to whether it is integrated with the waveguide, the detector can be divided into waveguide integrated photodetector and end incident photodetector. From the perspective of electrical structure, photodetectors can be divided into PN junction photodetectors, PIN photodetectors, APD photodetectors and MSM photodetectors.

There is a design conflict between the high responsivity and the large bandwidth of the SiGe detector. The responsivity is mainly related to the light absorption rate, while the bandwidth is limited by the carrier transit time and RC cut-off effect. Increasing the length of the germanium absorption region can increase the light absorption rate, thereby improving the responsivity, but a longer absorption region will be accompanied by a larger junction capacitance, causing the RC cut-off effect, thereby reducing the bandwidth. Similarly, increasing the thickness of the germanium absorption region can improve the responsivity, but due to the increase in the thickness of the absorption region, the photogenerated carriers need to travel a longer distance to be collected, which leads to an increase in their transit time and a decrease in bandwidth. Therefore, when designing a high-speed detector structure, there is often a trade-off between responsivity and bandwidth.

Current research focuses on simultaneously improving responsiveness and bandwidth in three different ways.

The first way is to adjust the junction area design. In order to increase the bandwidth, the traditional vertical PIN structure can be changed to a horizontal PIN structure. For lateral PINs based on germanium-metal contacts, in 2013, Léopold Virot et al. selectively epitaxially grown high-quality germanium materials on silicon, and achieved a bandwidth of more than 50 GHz under zero bias. The lateral PIN based on the silicon-metal contact can further reduce the loss caused by the doping of the germanium region, and can achieve high responsivity and large bandwidth to a certain extent. In 2017, Léopold Virot et al. published a lateral PIN detector based on a silicon-metal contact, and obtained a responsivity of 1.1A/W and a bandwidth of more than 50GHz experimentally.

The second way is to improve the electrode design. This aspect is mainly aimed at improving the contact between metal and semiconductor materials. In 2016, researchers from McGill University proposed to increase the size of the absorption region and use two eccentric small-sized metal contacts on the top of the germanium to reduce the absorption loss of the metal, increase the detector responsivity, and introduce peaking inductance to avoid Bandwidth reduction due to larger absorbing regions. The proposed detector achieves a responsivity of 1.09 A/W at 1550 nm and a bandwidth of 42.5 GHz at 2 V reverse bias voltage. In 2020, researchers at Seoul National University proposed to use interdigitated electrodes instead of traditional bulk metal contacts, which can further reduce metal absorption and improve responsiveness.

A third way is to add auxiliary structures. The light absorption ability of the photodetector is enhanced by adding an auxiliary structure, and the responsivity is additionally improved while maintaining a high bandwidth. In 2017, researchers from Peking University proposed that adding a Bragg reflection grating at the back end of the germanium region can effectively reduce the length of the detector, thereby increasing the bandwidth without reducing the responsivity. Similarly, in 2018, researchers from Huawei used a photonic crystal structure to enhance the light absorption of the detector, and experimentally proved that a responsivity of 0.75A/W can be achieved with only a 5μm-long germanium region.

In the detectors designed in the above-mentioned prior art, such as the lateral PIN structure, when the width of the germanium absorption region is small, it will bring high process difficulty, resulting in difficulty in guaranteeing the yield; and the responsivity gain brought by the addition of auxiliary structures is limited , is not enough to completely resolve the design contradiction. Therefore, it is difficult to further improve the responsiveness and bandwidth in the prior art.

Contents of the invention

In order to overcome the problem of difficulty in further improving the responsivity and bandwidth in the above-mentioned prior art, the present invention provides a silicon germanium photodetector based on a directional coupler, which can further improve the responsivity and bandwidth to meet the needs of long-distance optical communication systems and short-distance optical communication systems. Distance requirements for on-chip optical interconnect systems.

In order to solve the problems of the technologies described above, the technical solution adopted in the present invention is:

A silicon germanium photodetector based on a directional coupler, comprising a germanium absorption region, a directional coupler, a first tapered subwavelength reflection grating, a second tapered subwavelength reflection grating, a silicon plate and a substrate;

The germanium absorption region, the directional coupler, the first tapered subwavelength reflection grating, the second tapered subwavelength reflection grating, and the silicon plate are all integrated on the substrate;

The germanium absorption region is respectively arranged in the waveguide of the first output end and the second output end of the directional coupler, and there is no gap between the germanium absorption region and the waveguide of the directional coupler; the The surroundings of the germanium absorption region and the directional coupler are filled with silicon dioxide dielectric;

There is no gap connection between the first tapered sub-wavelength reflection grating and the waveguide at the third output end of the directional coupler; the gap of the first tapered sub-wavelength reflection grating is filled with a silicon dioxide medium;

The waveguides at the first output end and the second output end of the directional coupler are all connected to a second tapered sub-wavelength reflection grating without gaps, and the germanium absorption region in the waveguide is also connected to the second tapered The sub-wavelength reflection grating has no gap connection; the gap of the second tapered sub-wavelength reflection grating is filled with silicon dioxide medium;

The silicon slab is arranged between the waveguide region where the germanium absorption region is located and the substrate, and there is no gap connection between the silicon slab and the waveguide of the directional coupler; the silicon slab is in The length in the x direction is the same as that of the germanium absorption region.

The working principle of the present invention is as follows: the incident light enters the inside of the directional coupler from the input end of the directional coupler; The second tapered sub-wavelength reflective grating is connected, the third output end of the directional coupler is connected with the first tapered sub-wavelength reflective grating, so light can propagate back and forth inside the directional coupler, and the The germanium absorption region absorbs the incident light multiple times, and converts the incident light into photogenerated electron-hole pairs, that is, photogenerated carriers.

Preferably, doped regions are formed on both sides of the germanium absorption region, and the doping concentration is greater than 1×10 ²⁰ cm ^-3 .

Preferably, the first tapered subwavelength reflection grating is composed of one section of tapered subwavelength reflection grating, and the second tapered subwavelength reflection grating is composed of two sections of tapered subwavelength reflection grating.

Preferably, it also includes a signal pole and two ground poles; the signal pole (7) is arranged on the silicon plate (5), between the two waveguides where the germanium absorption region (1) is located; the ground pole The poles (8) are arranged on the silicon plate (5), and are respectively located outside the two waveguides where the germanium absorption region (1) is located.

Preferably, the germanium absorption region has a length of 1-30 μm, a width of 200 nm, and a thickness of 160 nm.

Preferably, the straight waveguide in the coupling region of the directional coupler has a length of 5-30 μm, a width of 500 nm, a height of 220 nm, and a distance between two waveguides of 100-300 nm.

Preferably, the first tapered sub-wavelength reflective grating has a tapered area period number of 5-30, a tapered area period length of 0.1λ-λ, a duty ratio of 0.1-0.9, and an etching depth of 220nm. The narrowest width of the waveguide in the tapered region is 50-300nm, the period number of the reflective grating is 5-30, the period length is 0.1λ-λ, the duty ratio is 0.1-0.9, and the etching depth is 220nm, where λ represents the The working center wavelength of the silicon germanium photodetector described above.

Preferably, the second tapered sub-wavelength reflection grating has a period number of the tapered region of 5 to 30, a period length of the tapered region of 0.1λ to λ, a duty ratio of 0.1 to 0.9, and an etching depth of 220nm. The narrowest width of the waveguide in the tapered region is 50-300nm, the period number of the reflective grating is 5-30, the period length is 0.1λ-λ, the duty ratio is 0.1-0.9, and the etching depth is 220nm, where λ represents the The working center wavelength of the silicon germanium photodetector described above.

Preferably, the silicon slab has a length of 1-30 μm and a thickness of 60 nm.

Further, the waveguide of the directional coupler is formed with doped regions on both sides of the position where the germanium absorption region is located, and the silicon plate is also formed with doped regions, the doping concentration of which is greater than 1×10 ²⁰ cm ^{- 3} .

Preferably, the incident light enters the inside of the directional coupler from the input end of the directional coupler; since the first output end and the second output end of the directional coupler are in contact with the second cone Shaped sub-wavelength reflective grating connection, the third output end of the directional coupler is connected with the first tapered sub-wavelength reflective grating, so the light propagates back and forth inside the directional coupler, and the germanium absorption region is multiple times Absorb the incident light and convert the incident light into photogenerated electron-hole pairs, that is, photogenerated carriers.

Compared with prior art, the beneficial effect of the present invention is:

The present invention significantly improves the light absorption rate through the structure of the directional coupler combined with the tapered sub-wavelength grating, compared with the traditional detector with the same germanium absorption area, its effective absorption length is increased by four times, thereby achieving high responsivity .

The present invention utilizes the structure of the directional coupler combined with the tapered sub-wavelength grating to repeatedly absorb the incident light, so the absorption area can be further reduced, thus having shorter carrier transit time and smaller junction capacitance, The bandwidth of the detector can be greatly improved.

Description of drawings

Fig. 1 is a three-dimensional schematic diagram of a silicon germanium photodetector according to the present invention.

Fig. 2 is a schematic xz cross-sectional view of the SiGe photodetector according to the present invention.

Fig. 3 is a schematic xy cross-sectional view of the germanium absorption region of the present invention.

FIG. 4 is a diagram showing the difference between the technical solution of this embodiment and the traditional waveguide-integrated silicon-germanium detector in different lengths of germanium absorption regions.

Fig. 5 is the bandwidth calculation result of this embodiment.

Among them: 1. Germanium absorption region; 2. Directional coupler; 3. The first tapered subwavelength reflection grating; 4. The second tapered subwavelength reflection grating; 5. Silicon plate; 6. Substrate; 7. Signal electrode 8. The poles of the earth.

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

The technical terms involved in this embodiment are explained as follows:

Responsivity: defined as the ratio of the unit incident light power to the generated photocurrent. The light incident on the absorption region of the detector generates photogenerated carriers, which drift to the electrodes on both sides under the action of the electric field, and are collected by the electrodes to form photocurrent. For PIN photodetectors, under ideal conditions, one photon can generate one photogenerated electron-hole pair. The responsivity characterizes the light absorption ability of the detector.

Bandwidth: defined as the corresponding frequency point after the response of the detector drops 3dB from the low frequency. It characterizes the high-speed response capability of the detector and is an index to measure the detector's ability to respond to changes in the power of the incident light signal. According to the basic principle of the PIN detector, the factors affecting the bandwidth can be classified into two categories: (1) carrier transit time; (2) RC cut-off effect. The slowest of these components determines the final bandwidth of the device.

Example 1

As shown in Figure 1 and Figure 2, a silicon germanium photodetector based on a directional coupler includes a germanium absorption region 1, a directional coupler 2, a first tapered subwavelength reflection grating 3, a second tapered subwavelength reflection grating grating 4, silicon plate 5 and substrate 6;

The germanium absorption region 1, the directional coupler 2, the first tapered subwavelength reflection grating 3, the second tapered subwavelength reflection grating 4, and the silicon plate 5 are all integrated on the substrate 6;

The germanium absorption region 1 is respectively arranged in the waveguides of the first output end and the second output end of the directional coupler 2, and between the germanium absorption region 1 and the waveguide of the directional coupler 2 No gap; the surroundings of the germanium absorption region 1 and the directional coupler 2 are filled with silicon dioxide medium;

There is no gap connection between the first tapered sub-wavelength reflection grating 3 and the waveguide at the third output end of the directional coupler 2; the gap of the first tapered sub-wavelength reflection grating 3 is filled with carbon dioxide silicon medium;

The waveguides of the first output end and the second output end of the directional coupler 2 are all connected with a second tapered sub-wavelength reflection grating 4 without a gap, and the germanium absorption region 1 in the waveguide is also connected with the first The second tapered subwavelength reflection grating 4 has no gap connection; the gap of the second tapered subwavelength reflection grating 4 is filled with a silicon dioxide medium;

The silicon plate 5 is arranged between the waveguide region where the germanium absorption region 1 is located and the substrate 6, and there is no gap connection between the silicon plate 5 and the waveguide of the directional coupler 2; The length of the silicon plate 5 in the x direction is the same as that of the germanium absorption region 1 .

The working principle of this embodiment is as follows: the incident light enters the inside of the directional coupler 2 from the input end of the directional coupler 2; end is connected with the second tapered sub-wavelength reflective grating 4, the third output end of the directional coupler 2 is connected with the first tapered sub-wavelength reflective grating 3, so the light can pass through the directional coupler 2. Internal back-and-forth propagation, the germanium absorption region 1 absorbs the incident light multiple times, and converts the incident light into photogenerated electron-hole pairs, that is, photogenerated carriers.

This embodiment proposes a silicon germanium photodetector based on a directional coupler, through which the directional coupler 2 combines the structure of the first tapered sub-wavelength grating 3 and the second tapered sub-wavelength reflection grating 4 to significantly improve the light absorption Compared with the traditional detector with germanium absorption region 1, its effective absorption length is increased by four times, thereby achieving high responsivity; since the directional coupler 2 combines the first tapered subwavelength grating 3 with the second tapered subwavelength The structure of the wavelength reflection grating 4 can repeatedly absorb the incident light, so the absorption area can be further reduced, thereby having a shorter carrier transit time and smaller junction capacitance, which can greatly improve the bandwidth of the detector. In this embodiment, the effects of improving responsivity and bandwidth can be achieved by adjusting the light field transmission inside the detector.

Example 2

As shown in Fig. 1, Fig. 2 and Fig. 3, on the basis of embodiment 1, doped regions are formed on both sides of the germanium absorption region 1, and the doping concentration is greater than 1×10 ²⁰ cm ^-3 .

More specifically, this embodiment also includes a signal pole (7) and two ground poles (8); said signal pole (7) is set on the silicon plate (5), located at the location of the germanium absorption region (1) between the two waveguides; the ground electrode (8) is arranged on the silicon plate (5), and is respectively located outside the two waveguides where the germanium absorption region (1) is located.

The photogenerated electron-hole pairs converted from the incident light drift to the signal electrode 7 and the ground electrode 8 under the action of the electric field jointly formed by the signal electrode 7, the ground electrode 8 and the doping, and finally enter the external circuit to form a current, thereby completing photoelectric conversion.

As shown in Figure 3, the directions of the signal pole 7 and the ground pole 8 are arranged perpendicular to the direction of the silicon plate 5, and the two ground poles 8 are outside the two waveguides of the directional coupler 2, and the outlet ends of the signal pole 7, The outlet ends of the two ground electrodes 8 are located on the same side of the SiGe photodetector.

More specifically, the germanium absorption region 1 has a length of 1-30 μm, a width of 200 nm, and a thickness of 160 nm.

More specifically, the straight waveguide in the coupling region of the directional coupler 2 has a length of 5-30 μm, a width of 500 nm, a height of 220 nm, and a distance between two waveguides of 100-300 nm.

In this embodiment, the first tapered sub-wavelength reflection grating 3 has a tapered area period number of 5-30, a tapered area period length of 0.1λ-λ, a duty ratio of 0.1-0.9, and the etching The depth is 220nm, the narrowest width of the waveguide in the tapered region is 50-300nm, the number of periods of the reflective grating is 5-30, the period length is 0.1λ-λ, the duty ratio is 0.1-0.9, and the etching depth is 220nm, where , λ represents the working center wavelength of the SiGe photodetector.

More specifically, the number of periods of the second tapered sub-wavelength reflection grating 4 in the tapered region is 5-30, the period length of the tapered region is 0.1λ-λ, the duty ratio is 0.1-0.9, and the etching depth is 220nm, the narrowest width of the waveguide in the tapered region is 50-300nm, the number of periods of the reflective grating is 5-30, the period length is 0.1λ-λ, the duty ratio is 0.1-0.9, and the etching depth is 220nm, where λ Indicates the working center wavelength of the silicon germanium photodetector.

In this embodiment, the silicon plate 5 has a length of 1-30 μm and a thickness of 60 nm.

In the above solution, by adjusting the length of the germanium absorption region 1 , different light absorption rates can be obtained for the silicon germanium photodetector, thereby affecting the responsivity of the silicon germanium detector. By adjusting the width of the germanium absorption region 1, the silicon germanium photodetector can obtain different carrier drift time and RC effect, thereby affecting the responsivity of the silicon germanium detector. By adjusting the length of the direct waveguide in the coupling region of the directional coupler 2 and the distance between the two waveguides, the silicon germanium photodetector can obtain different light absorption rates, thereby affecting the responsivity of the silicon germanium detector. By adjusting the number of periods of the tapered region, the period length of the tapered region, the duty ratio, the narrowest width of the waveguide in the tapered region, the number of periods of the reflective grating, the period length, and the duty cycle of the first tapered sub-wavelength reflection grating 3 The space ratio can obtain different light reflectivity and light absorptivity, thereby affecting the responsivity of the SiGe detector. By adjusting the number of periods of the tapered region of the second tapered sub-wavelength reflection grating 4, the period length of the tapered region, the duty ratio, the narrowest width of the waveguide in the tapered region, the number of periods of the reflective grating, the period length, the duty cycle The space ratio can obtain different light reflectivity and light absorptivity, thereby affecting the responsivity of the SiGe detector.

Example 3

Based on the silicon germanium photodetector based on the directional coupler described in embodiment 2, the specific technical scheme of the present embodiment is as follows:

As a further improvement of the embodiment, the germanium absorption region 1 has a length of 30 μm, a width of 200 nm, and a thickness of 160 nm.

As a further improvement of the embodiment, the straight waveguide in the coupling region of the directional coupler 2 has a length of 16.2 μm, a width of 500 nm, a height of 220 nm, and a distance between two waveguides of 200 nm.

As a further improvement of the embodiment, the first tapered sub-wavelength reflection grating 3 has a tapered region period number of 24, a tapered region period length of 429nm, a duty cycle of 0.5, an etching depth of 220nm, and a tapered The narrowest width of the waveguide in the area is 163nm, the period number of the reflection grating is 12, the period length is 345nm, the duty ratio is 0.5, and the etching depth is 220nm.

As a further improvement of the embodiment, the second tapered sub-wavelength reflection grating 4 has a tapered region period number of 22, a tapered region period length of 402nm, a duty cycle of 0.5, an etching depth of 220nm, and a tapered The narrowest width of the waveguide in the area is 103nm, the period number of the reflection grating is 12, the period length is 434nm, the duty ratio is 0.5, and the etching depth is 220nm.

As a further improvement of the embodiment, the silicon plate 5 has a length of 30 μm and a thickness of 60 nm.

As a further improvement of the embodiment, the waveguide of the directional coupler is formed with doped regions on both sides of the position where the germanium absorption region is located, and the silicon plate is also formed with doped regions with a doping concentration greater than 1× 10 ²⁰ cm ^-3 .

In order to further verify the technical effect of the silicon germanium photodetector based on the directional coupler described in this embodiment, the structural model of this embodiment is established by simulation software, and the technical solution of this embodiment is compared with the traditional waveguide integrated silicon germanium detection method by simulation calculation. The difference in the light absorption rate of the germanium absorption region with different lengths of the device is shown in Figure 4. The effect shows that the technical solution has a higher light absorption rate in comparison, thereby helping to improve the responsivity of the detector.

Furthermore, the bandwidth of the silicon germanium photodetector based on the directional coupler is simulated and calculated, and the effect is shown in Figure 5, and the bandwidth can reach at least 100 GHz. This effect shows that using the silicon germanium photodetector based on the directional coupler The detector structure can realize a germanium absorption region with a smaller width, thereby effectively reducing the carrier transit time and RC effect, which is conducive to increasing the bandwidth of the detector; and this effect shows that it can be further increased by adjusting the external inductance. Large detector bandwidth.

Apparently, the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, rather than limiting the implementation of the present invention. All modifications, equivalent replacements and improvements made within the spirit and principles of the present invention shall be included within the protection scope of the claims of the present invention.

Claims (10)

1. The germanium-silicon photoelectric detector based on the directional coupler is characterized by comprising a germanium absorption region (1), the directional coupler (2), a first conical sub-wavelength reflection grating (3), a second conical sub-wavelength reflection grating (4), a silicon flat plate (5) and a substrate (6);

the germanium absorption region (1), the directional coupler (2), the first conical sub-wavelength reflection grating (3), the second conical sub-wavelength reflection grating (4) and the silicon flat plate (5) are integrated on the substrate (6);

the germanium absorption region (1) is respectively arranged in the waveguides of the first output end and the second output end of the directional coupler (2), and the germanium absorption region (1) is connected with the waveguides of the directional coupler (2) in a gapless manner; the periphery of the germanium absorption region (1) and the periphery of the directional coupler (2) are filled with silicon dioxide media;

the first conical sub-wavelength reflection grating (3) is connected with the waveguide of the third output end of the directional coupler (2) in a gapless way; the gap of the first conical sub-wavelength reflection grating (3) is filled with silicon dioxide medium;

waveguides of the first output end and the second output end of the directional coupler (2) are connected with one second conical sub-wavelength reflection grating (4) in a gapless manner, and a germanium absorption area (1) in the waveguide is also connected with the second conical sub-wavelength reflection grating (4) in a gapless manner; the gap of the second conical sub-wavelength reflection grating (4) is filled with silicon dioxide medium;

the silicon flat plate (5) is arranged between the waveguide area where the germanium absorption area (1) is positioned and the substrate (6), and the silicon flat plate (5) is connected with the waveguide of the directional coupler (2) in a gapless way; the length of the silicon flat plate (5) in the x direction is the same as that of the germanium absorption region (1).

2. The directional coupler-based silicon germanium photodetector of claim 1, wherein the germanium absorption region (1) is formed with a doped region having a doping concentration of greater than 1 x 10 ²⁰ cm ^-3 。

3. The directional coupler-based silicon germanium photodetector according to claim 1, further comprising one signal pole (7) and two ground poles (8); the signal electrode (7) is arranged on the silicon flat plate (5) and is positioned between the two waveguides where the germanium absorption region (1) is positioned; the ground electrode (8) is arranged on the silicon flat plate (5) and is respectively positioned at the outer sides of the two waveguides where the germanium absorption region (1) is positioned.

4. The directional coupler-based silicon germanium photodetector according to claim 1, wherein the germanium absorption region (1) has a length of 1-30 μm, a width of 200nm and a thickness of 160nm.

5. The directional coupler-based germanium-silicon photodetector according to claim 1, wherein the coupling region of the directional coupler (2) has a straight waveguide length of 5-30 μm, a width of 500nm, a height of 220nm, and a spacing of 100-300 nm.

6. The directional coupler-based silicon germanium photodetector according to claim 1, wherein the period number of the tapered region of the first tapered sub-wavelength reflection grating (3) is 5-30, the period length of the tapered region is 0.1λ - λ, the duty cycle is 0.1-0.9, the etching depth is 220nm, the narrowest waveguide width of the tapered region is 50-300 nm, the period number of the reflection grating is 5-30, the period length is 0.1λ - λ, the duty cycle is 0.1-0.9, and the etching depth is 220nm, wherein λ represents the working center wavelength of the silicon germanium photodetector.

7. The directional coupler-based silicon-germanium photodetector of claim 1, wherein: the period number of the conical region of the second conical sub-wavelength reflection grating (4) is 5-30, the period length of the conical region is 0.1lambda-lambda, the duty cycle is 0.1-0.9, the etching depth is 220nm, the narrowest waveguide width of the conical region is 50-300 nm, the period number of the reflection grating is 5-30, the period length is 0.1lambda-lambda, the duty cycle is 0.1-0.9, and the etching depth is 220nm, wherein lambda represents the working center wavelength of the germanium-silicon photoelectric detector.

8. The silicon germanium photodetector according to claim 1, wherein the silicon plate (5) has a length of 1-30 μm and a thickness of 60nm.

9. The directional coupler-based silicon-germanium photodetector of claim 2, wherein the waveguide of the directional coupler (2) is formed with doped regions on both sides of the germanium absorption region (1), and the silicon plate (5) is also formed with doped regions having a doping concentration of greater than 1 x 10 ²⁰ cm ^-3 。

10. The directional coupler-based silicon-germanium photodetector according to any one of claims 1 to 9, wherein incident light is incident from an input end of said directional coupler (2) into an interior of said directional coupler (2); since the first output end and the second output end of the directional coupler (2) are connected with the second conical sub-wavelength reflection grating (4), and the third output end of the directional coupler (2) is connected with the first conical sub-wavelength reflection grating (3), light propagates back and forth inside the directional coupler (2), and the germanium absorption region (1) absorbs incident light for a plurality of times to convert the incident light into photo-generated electron-hole pairs, namely photo-generated carriers.