Staircase germanium concentration gradient absorption enhanced silicon-on-germanium photodetector and preparation method thereof

Abstract

The application discloses a stepped germanium concentration gradient absorption enhanced photodetector and a preparation method thereof, and relates to the field of semiconductor photoelectric devices. The detector comprises an N-type Si substrate, an intrinsic Ge absorption zone located on the upper surface of the N-type Si substrate, and a P-type heavily doped Ge layer located on the surface layer of the intrinsic Ge absorption zone. The intrinsic Ge absorption zone is in a cylindrical stepped structure and has a stepped thickness distribution with a center thin area and an outer thick area in the radial direction, so that a stepped germanium concentration gradient is formed to drive the directional transport of photo-generated carriers in the outer thick area to the center thin area. The P-type heavily doped Ge layer is only arranged on the surface layer of the center thin area of the intrinsic Ge absorption zone. The application ensures the sufficient absorption of long-wave band photons by the outer thick area, improves the quantum efficiency and responsivity, shortens the carrier transit path by the center thin area, and effectively breaks the mutual restriction relationship between the bandwidth and the responsivity by cooperating with the concentration gradient driving mechanism. The application has the advantages of high process compatibility, simple structure, high reliability and the like.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor optoelectronic device technology, specifically relating to a stepped germanium concentration gradient absorption enhanced photodetector and its preparation method. Background Technology

[0002] Silicon photonics technology has emerged as a mainstream solution for building next-generation high-speed optical interconnect systems due to its significant advantages in device integration, cost, and compatibility with CMOS (Complementary Metal-Oxide-Semiconductor) processes. However, in the near-infrared communication band (1260-1625nm), silicon (Si) materials suffer from intrinsic absorption difficulties due to their inherent bandgap limitations. In contrast, germanium (Ge) materials possess absorption capabilities covering the entire optical communication band and are compatible with silicon processes, making silicon-based germanium detectors an ideal choice for achieving high-performance, low-cost on-chip optical reception. Among these, vertical-incident Ge-on-Si photodetectors are widely used in optical receiving modules due to their simple structure, ease of coupling and testing, and the elimination of the need for complex optical coupling systems.

[0003] However, traditional vertically incident Ge-on-Si photodetectors face an inherent contradiction: to improve responsivity, the thickness of the Ge absorption layer needs to be increased to fully absorb vertically incident light, but a thick absorption layer will prolong the transit time of photogenerated carriers, limiting the device bandwidth; conversely, thinning the Ge layer can increase the bandwidth, but it leads to insufficient light absorption and reduced responsivity.

[0004] To address this contradiction, researchers have proposed various structural designs. For example, the surface microstructure method reduces surface reflection by etching nanoscale conical or pyramidal structures (e.g., black germanium detectors) onto the detector surface, thereby lengthening the light path within the absorption layer and enhancing absorption. However, such methods easily introduce numerous surface defects and recombination centers during etching, leading to a significant increase in the device's dark current level. Furthermore, process damage can reduce the crystal quality of the material, negatively impacting the device's long-term stability and reliability. Another approach is the resonant cavity enhancement junction based on a distributed Bragg reflector (DBR). This involves integrating multiple layers of dielectric films such as SiO2 / Si3N4 on a germanium platform on an insulator as high-reflectivity mirrors, utilizing optical interference effects to enhance light-matter interactions within the Ge active region, thus improving responsivity. However, such devices are extremely sensitive to the wavelength and angle of incident light; even a small wavelength shift can cause a sharp drop in efficiency and result in a very narrow operating band. Simultaneously, the process for growing high-reflectivity DBR structures is extremely complex, significantly increasing fabrication costs and making it difficult to meet the demands of broadband detection.

[0005] In summary, existing methods have certain limitations in improving device performance, and suffer from problems such as complex structures, difficult manufacturing processes, and high costs. Therefore, how to significantly improve the absorption efficiency of devices without sacrificing bandwidth, while ensuring process simplicity and low cost, remains a pressing technical problem to be solved in the field of silicon-based germanium photodetectors. Summary of the Invention

[0006] To address the aforementioned problems in the prior art, this invention provides a stepped germanium concentration gradient absorption-enhanced photodetector and its fabrication method. The technical problem to be solved by this invention is achieved through the following technical solution: In a first aspect, the present invention proposes a silicon-based germanium photodetector with stepped germanium concentration gradient absorption enhancement, comprising: N-type Si substrate; The intrinsic Ge absorption region is located on the upper surface of the N-type Si substrate; A p-type heavily doped Ge layer is located on the surface of the intrinsic Ge absorption region; The intrinsic Ge absorption region has a cylindrical stepped structure with a stepped thickness distribution along the radial direction, which is thinner in the center and thicker in the outer ring, thus forming a stepped germanium concentration gradient to drive the photogenerated carriers in the thick outer ring to be transported directionally to the thin central region; the p-type heavily doped Ge layer is only set on the surface of the thin central region of the intrinsic Ge absorption region.

[0007] Secondly, the present invention proposes a method for fabricating a silicon-based germanium photodetector with stepped germanium concentration gradient absorption enhancement, applicable to the stepped germanium concentration gradient absorption enhancement photodetector provided in the first aspect of the present invention. The fabrication method includes: Step 1: Select an N-type Si substrate and perform ion implantation to form an n-type heavily doped annular contact region; Step 2: Epitaxially grow a Ge buffer layer on an N-type Si substrate; Step 3: Define a stepped window on the Ge buffer layer using photolithography and etching processes, and selectively epitaxially grow a cylindrical stepped Ge structure to form an intrinsic Ge absorption region with a stepped thickness distribution along the radial direction, which is thinner at the center and thicker at the outer edge. Step 4: A p-type heavily doped Ge layer is formed on top of the thin central region of the intrinsic Ge absorption region by ion implantation; Step 5: Deposit a passivation layer across the entire device surface and define the contact window that exposes a portion of the P-type heavily doped Ge layer and a portion of the n-type heavily doped annular contact region; Step 6: Form the upper and lower electrodes within the contact window to form ohmic contacts with the p-type heavily doped Ge layer and the n-type heavily doped annular contact region, respectively.

[0008] The beneficial effects of this invention are: 1. The stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector provided by this invention includes an N-type Si substrate; an intrinsic Ge absorption region located on the upper surface of the N-type Si substrate; and a P-type heavily doped Ge layer located on the surface of the intrinsic Ge absorption region. The intrinsic Ge absorption region has a cylindrical stepped structure with a radially distributed thickness distribution that is thinner at the center and thicker at the outer ring, thus forming a stepped germanium concentration gradient to drive the directional transport of photogenerated carriers from the thicker outer ring to the thinner central ring. The heavily doped P-type Ge layer 3 only covers the surface of the thinner central ring of the intrinsic Ge absorption region. This invention, by employing a stepped Ge absorption region structure with a "thin center and thick outer ring," utilizes the thicker outer ring to ensure sufficient absorption of long-wavelength photons, significantly improving quantum efficiency and responsivity. Simultaneously, the thinner central ring shortens the vertical transit path of carriers, and combined with the concentration gradient-driven diffusion mechanism, maintains high responsivity while ensuring high bandwidth characteristics of the device, effectively overcoming the mutual constraint between bandwidth and responsivity in traditional detectors.

[0009] 2. The stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector provided by this invention has a fabrication process that effectively reduces the dislocation density at the Ge / Si interface and reduces the dark current caused by surface defects through stepwise selective epitaxy and high-temperature annealing. The entire process is highly compatible with standard silicon-based CMOS processes and has the advantages of simple structure, easy coupling, and high reliability. It can achieve large-scale monolithic integration at a low cost, meeting the needs of high-speed optical communication and broadband detection.

[0010] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0011] Figure 1 A cross-sectional schematic diagram of a stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector provided in an embodiment of the present invention; Figure 2 A top view of a stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector provided in an embodiment of the present invention; Figure 3 This is a schematic flowchart illustrating the fabrication method of the stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector provided in an embodiment of the present invention. Explanation of reference numerals in the attached figures: 1-N-type Si substrate, 11-n-type heavily doped annular contact region, 2-intrinsic Ge absorption region, 21-central thin region, 22-outer thick region, 3-P-type heavily doped Ge layer, 4-upper electrode, 41-first contact hole, 5-lower electrode, 51-second contact hole. Detailed Implementation

[0012] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0013] The first aspect of this invention provides a stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector. By designing a stepped Ge absorption region structure with a "thin center and thick outer ring" and combining it with a carrier concentration gradient distribution, the contradiction between responsivity and bandwidth in traditional Ge detectors is effectively solved. While achieving high-efficiency absorption in long wavelength bands (such as 1550 nm), the carrier transit time is significantly shortened. Moreover, the process is highly compatible with silicon-based CMOS technology, making it suitable for monolithic integration in the field of high-speed optical communication.

[0014] Please see Figure 1 and Figure 2 , Figure 1 This is a cross-sectional schematic diagram of a stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector provided in an embodiment of the present invention. Figure 2 This is a top view of a stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector provided in an embodiment of the present invention. The photodetector includes: N-type Si substrate 1; Intrinsic Ge absorption region 2 is located on the upper surface of N-type Si substrate 1; P-type heavily doped Ge layer 3 is located on the surface of intrinsic Ge absorption region 2; The intrinsic Ge absorption region 2 is a cylindrical stepped structure with a stepped thickness distribution along the radial direction, which is thinner in the center and thicker in the outer ring, thus forming a stepped germanium concentration gradient to drive the photogenerated carriers in the thick outer ring to be transported directionally to the thin central region; the p-type heavily doped Ge layer 3 only covers the surface of the thin central region of the intrinsic Ge absorption region 2.

[0015] Optionally, in this embodiment, the bottom of the device is an n-type doped Si substrate, which serves as the n region of the PIN structure. The outer ring of the surface layer of the N-type Si substrate 1 also includes an n-type heavily doped annular contact region 11 for forming an ohmic contact with the electrode.

[0016] It is understood that the N-type Si substrate 1 in this embodiment uses a substrate with extremely low resistivity, which can reduce the series resistance of the device and thus improve the 3dB bandwidth of the device.

[0017] As one implementation method, such as Figure 1 As shown, the N-type Si substrate 1 in this embodiment can be an SOI wafer, including a top silicon layer, a buried oxide layer, and a bulk silicon substrate; and the n-type heavily doped annular contact region 11 is located in the top silicon layer. The top silicon layer has a thickness of 220 nm, the buried oxide layer is made of SiO2 material with a thickness of 2 μm, and the bulk silicon substrate has a thickness of 400 nm.

[0018] Furthermore, the intrinsic Ge absorption region 2 has a stepped structure formed by selective epitaxial growth.

[0019] For example, please continue to see Figure 1 The intrinsic Ge absorption region 2 includes a two-stage stepped structure, namely a central thin region 21 located in the central region and an outer thick region 22 surrounding the central thin region 21, and the thickness of the outer thick region 22 is greater than the thickness of the central thin region 21.

[0020] Because the absorption coefficient of Ge material drops sharply after 1500 nm, a relatively thick Ge layer is required in vertically incident detectors near 1550 nm to achieve sufficient light absorption and obtain a high responsivity. However, an excessively thick absorption layer will significantly increase the transport distance of photogenerated carriers, thereby prolonging the transit time and reducing the bandwidth of the device.

[0021] Therefore, in this embodiment, the intrinsic Ge absorption region 2 is designed as a stepped structure, and the thickness of the central thin region 21 is further designed to be 200nm-400nm; the thickness of the outer thick region 22 is designed to be 1μm-1.2μm.

[0022] Preferably, the thickness of the central thin region 21 is 300 nm; the thickness of the outer thick region 22 is 1 μm.

[0023] It is understood that in this embodiment, the intrinsic Ge absorption region 2 is distributed in a stepped manner from the center to the outer ring in the radial direction, and the overall width (i.e., diameter) of the stepped structure does not exceed the annular contact region 11 of the n-type heavily doped region.

[0024] For example, the overall width of the stepped structure can be designed to be 50μm-200μm, which helps to reduce the junction capacitance of the device, thereby significantly improving the high-frequency characteristics of the device.

[0025] In this embodiment, a p-type heavily doped Ge layer 3 with a cylindrical stepped structure is designed. In the central region of the device, the Ge layer is relatively thin. The p-type heavily doped Ge layer 3 above it, together with the intrinsic Ge absorption region 2 and the N-type Si substrate 1 below, forms a PIN junction. Photogenerated carriers can be rapidly collected under the reverse bias electric field in this region. Due to the short carrier transport path in the thin Ge region, the device exhibits high bandwidth characteristics. Simultaneously, the thicker Ge layer at the periphery of the device can more fully absorb incident light in the 1550nm band, significantly increasing the total photon absorption and laying the foundation for high responsivity. Driven by the concentration gradient, the photogenerated carriers generated in this region can diffuse towards the central region and be effectively collected under the electric field of the PIN junction. The thin Ge layer in the central region provides the shortest transport path for the photogenerated carriers, ensuring the high bandwidth characteristics of the device, thereby significantly improving the overall responsivity of the device without sacrificing bandwidth.

[0026] Traditional vertical-incident detectors often require increased Ge layer thickness to improve absorption efficiency, which leads to longer carrier transit paths and thus limits bandwidth. The stepped germanium concentration gradient absorption-enhanced silicon-based germanium photodetector of this invention utilizes the geometric gain effect of the stepped structure to absorb incident light in a large outer thick region, while the collection process concentrates in a short-path central thin region. Due to the significant potential gradient and concentration difference between the central thin region 21 and the outer thick region 22, photogenerated holes rapidly diffuse laterally and vertically through the thin region to reach the P-type electrode. This three-dimensional carrier transport mode breaks the limitation of "absorption depth equals transit distance" in traditional PIN structures.

[0027] It is worth noting that in this embodiment, the p-type heavily doped Ge layer 3 only covers the top surface of the central thin region 21. This local heavy doping design establishes a lateral hole concentration gradient between the central and edge regions, inducing a lateral built-in electric field, which further accelerates the collection efficiency of carriers generated in the outer thick region to the central electrode.

[0028] This invention creates a two-dimensional concentration gradient within the Ge absorption region by placing a heavily p-type doped layer on top of the central thin region and utilizing the Fermi level shift generated by the heavy doping. This gradient field enhances the diffusion current component, enabling the device to maintain a high carrier collection rate even at low bias voltages.

[0029] Furthermore, the photodetector provided in this embodiment also includes a passivation layer (not shown in the figure), which is disposed on the sidewall of the stepped structure of the intrinsic Ge absorption region 2, the upper surface of the uncovered P-type heavily doped Ge layer 13, and the upper surface of the N-type Si substrate 11.

[0030] Optionally, in this embodiment, the material of the passivation layer 6 is at least one of Al2O3, SiO2, and SiN.

[0031] For example, the passivation layer can be a stacked structure consisting of Al2O3 grown by atomic layer deposition (ALD) and SiO2 grown by plasma enhanced chemical vapor deposition (PECVD). Al2O3 has excellent interfacial state passivation capabilities and can effectively fill the dangling bonds on the Ge surface, thereby controlling the dark current density to an extremely low level.

[0032] For further details, please continue to see Figure 1 The photodetector provided in this embodiment also includes an upper electrode 4 and a lower electrode 5; wherein, The upper electrode 4 is located on the upper surface of the P-type heavily doped Ge layer 3 and is connected to the P-type heavily doped Ge layer 3 through the first contact hole 41 to form an ohmic contact. The lower electrode 5 is a ring electrode arranged in a stepped structure around the intrinsic Ge absorption region 2, and is connected to the n-type heavily doped ring contact region 11 through the second contact hole 51 to form an ohmic contact.

[0033] Optionally, the upper electrode 4 and the lower electrode 5 are made of Ti / Pt / Au or Cr / Au multilayer metal electrodes.

[0034] In summary, this invention employs a stepped Ge absorption region structure with a "thin center and thick outer ring," utilizing the thick outer ring to ensure sufficient absorption of long-wavelength photons, significantly improving quantum efficiency and responsivity. Simultaneously, the thin center shortens the vertical transit path of charge carriers, and combined with a concentration gradient-driven diffusion mechanism, it maintains high responsivity while ensuring high bandwidth characteristics of the device, effectively overcoming the mutual constraint between bandwidth and responsivity in traditional detectors.

[0035] Based on the same inventive concept, a second aspect of the present invention also provides a method for fabricating a step-germanium concentration gradient absorption enhanced silicon-based germanium photodetector, applicable to the detector of the first aspect.

[0036] Please see Figure 3 , Figure 3This is a flowchart illustrating the fabrication process of a Si-based Ge photodetector with enhanced carrier concentration gradient absorption, as provided in an embodiment of the present invention. Figure 3 As shown, the preparation method of this embodiment includes the following steps: Step 1: Select an N-type Si substrate 1 and perform ion implantation to form an n-type heavily doped annular contact region 11.

[0037] Specifically, in this embodiment, a low-resistivity N-type SOI wafer is selected for cleaning. Then, high-dose implantation with phosphorus (P) or arsenic (As) ions is performed, followed by rapid thermal annealing to activate impurities and form the heavily doped n-type annular contact region 11 required for ohmic contacts.

[0038] Step 2: Epitaxially grow a Ge buffer layer on an N-type Si substrate 1.

[0039] Specifically, the Ge buffer layer can be grown using ultra-high vacuum chemical vapor deposition or depressurized chemical vapor deposition.

[0040] In this embodiment, by epitaxially growing a low-temperature Ge buffer layer on the surface of an N-type Si substrate 1, the lattice mismatch between Si and Ge can be alleviated and the dislocation density reduced.

[0041] Step 3: Define a stepped window on the Ge buffer layer using photolithography and etching processes, and selectively grow a cylindrical stepped Ge structure to form an intrinsic Ge absorption region 2 with a stepped thickness distribution along the radial direction, which is thinner at the center and thicker at the outer ring.

[0042] First, define the staircase window.

[0043] Specifically, a SiO2 mask layer is deposited on the Ge buffer layer using plasma-enhanced chemical vapor deposition (PECVD); a stepped structure is defined on the SiO2 mask layer using electron beam lithography (EBRT) or deep ultraviolet lithography (DEUV); and the SiO2 mask layer is etched using reactive ion etching (RIE) to form a stepped window.

[0044] Then, a stepped intrinsic Ge absorption region 2 is selectively epitaxially grown.

[0045] Optionally, in this embodiment, a cylindrical stepped Ge structure can be selectively epitaxially grown in two steps to form an intrinsic Ge absorption region 2 with a stepped thickness distribution along the radial direction, which is thin at the center and thick at the outer ring.

[0046] Specifically, the first step is to use a SiO2 mask layer to block the outer region and grow a Ge layer in the central region of the device with a thickness of 300nm, forming a central thin region 21; the second step is to remove the SiO2 mask layer; redeposit SiO2 and expose the outer region of the device; grow a Ge layer in the outer region of the device with a thickness of 1μm, thereby forming an outer thick region 22 with a thickness greater than the central thin region 21.

[0047] Understandably, during the growth of the Ge layer, the thickness difference between the two can be precisely controlled by adjusting the growth time and the precursor flow rate. After growth, surface irregularities are removed by chemical mechanical polishing, followed by high-temperature cyclic annealing to further reduce dislocation density and improve crystal quality.

[0048] Step 4: A P-type heavily doped Ge layer 3 is formed on top of the thin central region of the intrinsic Ge absorption region 2 by ion implantation.

[0049] In this embodiment, boron (B) ion implantation is performed on the central thin region to form a p-type heavily doped Ge layer 3. The implantation energy can be determined according to the actual required junction depth.

[0050] Afterwards, high-temperature cyclic annealing is performed to use thermal stress to induce dislocation slip and annihilation, thereby improving the crystal quality of the Ge layer and reducing the bulk dark current.

[0051] Step 5: Deposit a passivation layer on the entire device surface and define a contact window that exposes part of the P-type heavily doped Ge layer 3 and part of the n-type heavily doped annular contact region 11.

[0052] Specifically, Al2O3 was deposited by ALD followed by SiN deposited by PECVD as a passivation layer. The contact window located at the top center of the p-type heavily doped Ge layer 3 and above the n-type heavily doped annular contact region 11 was precisely opened by photolithography and buffered hydrofluoric acid etching.

[0053] Step 6: Form an upper electrode 4 and a lower electrode 5 within the contact window, forming ohmic contacts with the p-type heavily doped Ge layer 3 and the n-type heavily doped annular contact region 11, respectively.

[0054] Specifically, an electron beam evaporation process is used to deposit Ti / Pt / Au or Cr / Au metal systems. Subsequently, alloying annealing is performed under nitrogen protection to create a good silicide / germanide contact between the metal and semiconductor interfaces, reducing contact resistance and forming electrodes.

[0055] The fabrication method of this invention directly constructs a non-planar stepped structure through selective epitaxy, avoiding large-scale dry etching of the Ge absorption region. This protects the sidewalls of the Ge material from plasma damage and significantly suppresses surface dark current caused by etching defects. Simultaneously, by combining cyclic annealing technology, the high dislocation density problem of Ge growth on Si is solved, ensuring the long-term stability of the device. Furthermore, the entire process is highly compatible with standard silicon-based CMOS processes, offering advantages such as simple structure, easy coupling, and high reliability. It enables large-scale monolithic integration at a lower cost, meeting the needs of high-speed optical communication and broadband detection.

[0056] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0057] Furthermore, unless otherwise expressly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Moreover, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0058] In the description of this invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0059] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A silicon-based germanium photodetector with stepped germanium concentration gradient absorption enhancement, characterized in that, include: N-type Si substrate (1); The intrinsic Ge absorption region (2) is located on the upper surface of the N-type Si substrate (1); A p-type heavily doped Ge layer (3) is located on the surface of the intrinsic Ge absorption region (2); The intrinsic Ge absorption region (2) is a cylindrical stepped structure with a stepped thickness distribution along the radial direction, which is thinner in the center and thicker in the outer ring, thereby forming a stepped germanium concentration gradient to drive the photogenerated carriers in the thick outer ring to be transported directionally to the thin center region; the p-type heavily doped Ge layer (3) is only disposed on the surface of the thin center region of the intrinsic Ge absorption region (2).

2. The stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector according to claim 1, characterized in that, The N-type Si substrate (1) includes an n-type heavily doped annular contact region (11), which is located on the surface of the N-type Si substrate (1).

3. The stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector according to claim 2, characterized in that, The N-type Si substrate (1) is an SOI wafer, including a top silicon layer, a buried oxide layer and a bulk silicon substrate, and the n-type heavily doped annular contact region (11) is located in the top silicon layer.

4. The stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector according to claim 1, characterized in that, The intrinsic Ge absorption region (2) includes a two-level stepped structure, namely a central thin region (21) located in the central region and an outer thick region (22) surrounding the central thin region (21), and the thickness of the outer thick region (22) is greater than the thickness of the central thin region (21).

5. The stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector according to claim 1, characterized in that, The thickness of the central thin region (21) is 200nm-400nm; the thickness of the outer thick region (22) is 800nm-1200nm.

6. The stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector according to claim 2, characterized in that, The overall width of the stepped structure of the intrinsic Ge absorption region (2) does not exceed that of the n-type heavily doped annular contact region (11).

7. The stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector according to claim 2, characterized in that, It also includes an upper electrode (4) and a lower electrode (5); The upper electrode (4) is located on the upper surface of the P-type heavily doped Ge layer (3) and is connected to the P-type heavily doped Ge layer (3) through the first contact hole (41) to form an ohmic contact; The lower electrode (5) is an annular electrode arranged in a stepped structure around the intrinsic Ge absorption region (2), and is connected to the n-type heavily doped annular contact region (11) through the second contact hole (51) to form an ohmic contact.

8. A method for fabricating a silicon-based germanium photodetector with stepped germanium concentration gradient absorption enhancement, characterized in that, The method for preparing the step-germanium concentration gradient absorption-enhanced photodetector according to any one of claims 1-7 comprises: Step 1: Select an N-type Si substrate (1) and perform ion implantation to form an n-type heavily doped annular contact region (11); Step 2: Epitaxially grow a Ge buffer layer on the N-type Si substrate (1); Step 3: Define a stepped window on the Ge buffer layer by photolithography and etching process, and selectively grow a cylindrical stepped Ge structure to form an intrinsic Ge absorption region with a stepped thickness distribution along the radial direction, which is thin in the center and thick in the outer ring (2). Step 4: A P-type heavily doped Ge layer (3) is formed on top of the central thin region of the intrinsic Ge absorption region (2) by ion implantation; Step 5: Deposit a passivation layer across the entire device surface and define a contact window that exposes a portion of the p-type heavily doped Ge layer (3) and a portion of the n-type heavily doped annular contact region (11); Step 6: Form an upper electrode (4) and a lower electrode (5) within the contact window, and form ohmic contacts with the p-type heavily doped Ge layer (3) and the n-type heavily doped annular contact region (11), respectively.

9. The stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector according to claim 8, characterized in that, In step 3, a stepped window is defined on the Ge buffer layer using photolithography and etching processes, including: A SiO2 mask layer was deposited on the Ge buffer layer using plasma-enhanced chemical vapor deposition. A stepped structure is defined on the SiO2 mask layer using electron beam lithography or deep ultraviolet lithography. The SiO2 mask layer is etched using reactive ion etching to form a stepped window.

10. The stepped germanium concentration gradient absorption enhanced silicon-based germanium photodetector according to claim 9, characterized in that, In step 3, a cylindrical stepped Ge structure is selectively epitaxially grown in two steps to form an intrinsic Ge absorption region (2) with a stepped thickness distribution along the radial direction, which is thinner at the center and thicker at the outer ring. Specifically, this includes: The first step is to use the SiO2 mask layer to block the peripheral area and grow a Ge layer in the central region of the device to form a central thin region (21); The second step is to remove the SiO2 mask layer; redeposit SiO2 and expose the outer ring region of the device; grow a Ge layer in the outer ring region of the device to form a thicker outer ring region (22) than the central thin region (21).

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