Germanium-silicon photoelectric detector

Abstract

The invention discloses a germanium-silicon photoelectric detector, which relates to the field of photoelectric detectors and comprises a strip waveguide, a doped first silicon waveguide region, a doped second silicon waveguide region, a germanium layer, a doped germanium layer, a germanium upper electrode and a silicon upper electrode, the electrodes on the silicon surround all the side faces of the germanium layer, gaps at the vertex angles are reserved for transmission of optical signals, the full-surrounding electrodes reduce parasitic parameters of the photoelectric detector, and the bandwidth of the detector is improved. And the silicon strip waveguide is connected with the vertex angle of the second waveguide region corresponding to the silicon-on-electrode gap, and guides an optical signal to be input from the vertex angle of the second waveguide region and propagate along the diagonal line of the germanium layer, so that the propagation path of the optical signal is increased, the responsivity is improved, and the problem that the bandwidth and the responsivity are mutually restricted in the prior art is solved.

Description

Technical Field

[0001] The present invention relates to the field of photoelectric detectors, and in particular to a germanium silicon photoelectric detector. Background Art

[0002] There is an increasing demand for larger data capacity and higher data transmission rates. Optoelectronic devices have unique advantages due to their strong resistance to electromagnetic interference and the lower cost of optical fiber compared to traditional electrical cables. Among them, compared with other III-V materials, two-dimensional materials, etc., silicon-based platforms are widely used because they can be integrated on a large scale. Germanium materials can be directly epitaxially grown on silicon, and the manufacturing process is compatible with the currently very mature CMOS process.

[0003] As one of the cores of silicon-based optoelectronic devices, the performance of GeSi photodetectors is crucial. GeSi photodetectors with high bandwidth and high responsivity need to be designed to cope with the current explosive growth in data throughput demand and the need for high photoelectric conversion efficiency.

[0004] The bandwidth of the detector is mainly limited by the transit time of the carriers and parasitic parameters, among which the parasitic parameters include parasitic capacitance and parasitic resistance. The parasitic capacitance is proportional to the area of ​​germanium. If the height of the germanium layer remains unchanged, the smaller the area of ​​germanium, the smaller the parasitic capacitance, and the larger the corresponding bandwidth. The responsivity of the detector is mainly limited by the propagation path of light along the germanium. The smaller the area of ​​germanium, the lower the responsivity. Therefore, the bandwidth and responsivity of the germanium silicon photodetector restrict each other.

[0005] Non-patent document 1 (Photonics Research, 2021, 9(4): 605-609) reports a technology that sets high-concentration doping at the bottom of the silicon region to reduce parasitic resistance and improve the bandwidth of the germanium silicon photodetector. This document reduces the influence of parasitic resistance without affecting the parasitic capacitance of the germanium silicon detector. However, the improvement level of bandwidth by optimizing the bottom silicon doping is limited. The simulation fitting results show that the bandwidth of the optimized germanium silicon photodetector is only increased by 7GHz, and the bandwidth has not been maximized. In addition, the structure in which the light-incident strip waveguide is directly and vertically connected to the short side of the silicon waveguide area under the rectangular germanium layer is an optical coupling structure commonly used in the prior art. Since light is gradually absorbed by germanium in the propagation path along the germanium layer, the shorter the length of the germanium, the smaller the responsivity. A sufficiently long germanium region must be provided to ensure that the detector has a high responsivity, but the parasitic capacitance of the germanium region will also increase accordingly. Therefore, if the bandwidth is to be further improved, it will still be restricted by the responsivity.

[0006] In summary, fast data transmission rates require high-bandwidth, high-responsivity GeSi photodetectors. However, the bandwidth of GeSi detectors is still facing the problem of mutual restriction between responsivity and bandwidth. A new solution is needed to maximize the detector bandwidth while alleviating the problem of mutual restriction between responsivity and bandwidth of photodetectors. Summary of the invention

[0007] The present invention aims at the problem that the bandwidth and responsivity of the current germanium silicon photoelectric detector are mutually restricted, and proposes a germanium silicon photoelectric detector.

[0008] A germanium silicon photoelectric detector comprises: a silicon strip waveguide, a doped silicon waveguide region, a germanium layer, a doped germanium layer, a germanium upper electrode and a silicon upper electrode.

[0009] The doped silicon waveguide region includes a first silicon waveguide region and a second silicon waveguide region, the first silicon waveguide region is placed in the middle of the second silicon waveguide region, and the germanium layer is epitaxially grown on the first silicon waveguide region; the first silicon waveguide region and the second silicon waveguide region will form a ridge waveguide structure, which has a better confinement effect on the incident optical signal, and most of the optical signals will be coupled to the germanium layer through this ridge waveguide.

[0010] The silicon upper electrode is placed on the second silicon waveguide region, all sides of the germanium layer are surrounded by the silicon upper electrode, the silicon upper electrode is provided with a notch at the top corner of the germanium layer, and the silicon strip waveguide guides the optical signal to be transmitted to the germanium layer through the notch of the silicon upper electrode;

[0011] The silicon strip waveguide is connected to the vertex of the first silicon waveguide region, and guides the light to be incident from the vertex of the first silicon waveguide region corresponding to the notch of the silicon top electrode;

[0012] The doped silicon waveguide region is a first-type doped silicon waveguide region, and the doped germanium layer is a second-type doped germanium layer, wherein the second-type doping has a polarity opposite to that of the first-type doping.

[0013] The silicon strip waveguide guides the optical signal to the doped silicon waveguide area at the bottom of the germanium layer, and then couples to the germanium layer to be absorbed. After absorbing the optical signal, the germanium layer excites a large number of electron-hole pairs, which are called photogenerated carriers. The doped silicon waveguide area at the bottom of the germanium area has a higher conductivity due to doping. The doped germanium area and the doped silicon area will form a built-in electric field. The germanium upper electrode is the RF signal electrode, and the silicon upper electrode is connected to the ground electrode. The two can apply a bias voltage and output the current signal generated by the photodetector. When a reverse bias is applied to the germanium upper electrode and the silicon upper electrode, an external electric field will be formed inside the germanium area. Under the combined action of the external electric field and the built-in electric field, the electrons and holes generated in the germanium area move in a directional manner and are collected by the germanium upper electrode and the silicon upper electrode to form a current.

[0014] Among them, when the current flows through the doped silicon waveguide region to the silicon electrode, the doped silicon waveguide region can be equivalent to a parasitic resistor, which is related to the cross-sectional area through which the photogenerated carriers of the doped silicon waveguide region move to the silicon electrode and the length from the germanium region to the silicon electrode. The present invention sets the silicon electrode to surround all sides of the germanium region, leaving only a diagonal gap for optical signal input. The current flow area collected by the germanium layer to the silicon electrode is significantly increased, which is equivalent to connecting two additional resistors in parallel at the tail and front end of the germanium. When the doping concentration of the doped silicon waveguide region remains unchanged, the total parasitic resistance can be greatly reduced, reducing the influence of RC parasitic parameters.

[0015] The light transmission direction guided by the silicon strip waveguide is consistent with the diagonal direction of the germanium layer.

[0016] Compared with the traditional silicon strip waveguide that is directly coupled with the short side of the germanium region, this diagonal propagation method can, on the one hand, prevent the incident strip waveguide from crossing the silicon electrode, making the above-mentioned fully surrounded silicon electrode possible; on the other hand, the propagation path of light on the diagonal will be greater than the propagation path of the side length, and the proportion of germanium absorbing light will also be significantly improved, thereby improving the responsiveness of the photodetector.

[0017] Furthermore, the silicon top electrode is a square-shaped electrode with a notch at the top corner. The silicon top electrode is a square-shaped electrode, and the notch is arranged at the top corner of the square-shaped electrode.

[0018] The height of the first silicon waveguide region is 0.01-2 um, the height of the second silicon waveguide region is 0.01-2 um, and the height of the silicon strip waveguide is the sum of the heights of the first silicon waveguide region and the second silicon waveguide region.

[0019] The germanium upper electrode is disposed on the doped germanium layer.

[0020] The first type doping is N-type or P-type.

[0021] The second type doping is N-type or P-type.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] On the one hand, the full-surround electrode reduces the parasitic parameter influence of the germanium-silicon photodetector, which can improve the bandwidth of the germanium-silicon photodetector; on the other hand, the light is input from the silicon waveguide and propagates along the diagonal of the germanium, which can expand the propagation path of the light signal in the germanium and increase the proportion of the germanium layer absorbing the light signal, thereby expanding the responsivity. Therefore, we can go a step further and use a small area of ​​germanium to further reduce the parasitic capacitance and increase the bandwidth, while the responsivity can still be high, alleviating the constraint between responsivity and bandwidth. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A top view of an existing germanium silicon photodetector;

[0025] Figure 2 It is a side view of the existing germanium silicon photodetector except for the silicon upper electrode;

[0026] Figure 3 A top view of a germanium silicon photodetector provided in Example 1 of the present invention;

[0027] Figure 4 A side view of the germanium silicon photodetector provided in Example 1 of the present invention, excluding the silicon upper electrode;

[0028] Figure 5 This is a side view of the germanium silicon photodetector provided for Example 1 of the present invention.

[0029] Figure 6 It is the equivalent circuit diagram of the germanium silicon photodetector.

[0030] Figure 7 The frequency response of the existing germanium silicon photodetector changes with frequency under different inductances.

[0031] Figure 8 The frequency response of the germanium silicon photodetector provided in Example 1 of the present invention varies with frequency under different inductances.

[0032] Fig. 9 This is a comparison chart of the responsivity of the germanium silicon photodetector provided in Example 1 of the present invention and the existing germanium silicon photodetector.

[0033] Fig.10 It is a top view of another germanium silicon photodetector provided in Example 2 of the present invention.

[0034] Fig.11 It is a side view of another germanium silicon photodetector provided in Example 2 of the present invention, excluding the silicon upper electrode. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the working principle, implementation conditions and steps of the present invention are further described in detail below with reference to the accompanying drawings.

[0036] The embodiments described in the application examples are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] For the convenience of description, if the terms "up", "down", "left", "right", "vertical", "horizontal", "inside" and "outside" appear in this application, they only indicate that they are consistent with the up, down, left and right directions of the drawings themselves, and do not limit the structure. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operate in a specific orientation. Therefore, they cannot be understood as limiting the present application.

[0038] "Connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, or it can be the internal communication of two elements. It should be pointed out that all the drawings are exemplary representations. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0039] like Figure 1 and Figure 2 As shown, a germanium silicon photodetector of the prior art solution includes: a silicon strip waveguide 100, a germanium layer 130, a doped waveguide region 122, a doped germanium layer 131, a germanium upper electrode 151 and a silicon upper electrode 141. The silicon upper electrode 141 is a pair of metal electrodes, which are placed on the doped waveguide region 122, parallel to the long sides of the germanium layer, and collect the electron holes flowing out of the two long sides of the germanium layer to form a current. The silicon strip waveguide 100 is directly vertically connected to the short side of the silicon waveguide region under the rectangular germanium layer, guiding the light to couple to the germanium layer. The propagation path of the light in the germanium layer is the length of the germanium layer. If the germanium layer is not long enough, the proportion of light absorbed by the detector will be greatly reduced, and the responsivity will decrease accordingly, and the performance of the detector will be limited. If the detector is to have a higher responsivity, the parasitic capacitance of the germanium region will increase with the increase of the area of ​​the germanium layer.

[0040] like Figure 3 FIG. 1 is a top view of a photoelectric detector provided in an embodiment of the present application. Figure 4 This is a side view of the remaining structures of the photodetector provided in the embodiment of the present application except for the silicon upper electrode 241. Figure 5 The photodetector provided in the embodiment of the present application includes a side view of the silicon upper electrode 241. The photodetector provided in the embodiment of the present application includes: a silicon strip waveguide 200, a doped silicon waveguide region 220, a germanium layer 230, a doped germanium layer 231, a germanium upper electrode 251 and a silicon upper electrode 241. Among them:

[0041] The doped silicon waveguide region 220 includes a first silicon waveguide region 221 and a second silicon waveguide region 222 , and the second silicon waveguide region 222 is placed above the first silicon waveguide region 221 .

[0042] Optionally, the height of the first silicon waveguide region is 0.09um, and the height of the second silicon waveguide region is 0.13um. The strip waveguide height is 0.22um. The first silicon waveguide region 221 and the second silicon waveguide region 222 together form a ridge waveguide, which is conducive to confining light in the central area where the germanium layer is located, and preventing light from diffusing to the area where the silicon electrode is located.

[0043] The germanium layer 230 and the doped germanium layer 231 are epitaxially grown on the second silicon waveguide region 221, with a length and width of 7 um and a thickness of 0.5 um;

[0044] The doped silicon waveguide region 220 is P-type doped, and the doped germanium layer 231 is N-type doped, and together with the germanium layer, forms a PIN type photodetector.

[0045] The silicon top electrode 241 surrounds the four sides of the germanium layer, maximizing the flow area of ​​the current flowing to the silicon top electrode, leaving only a gap at the top corner to facilitate the flow of optical signals.

[0046] The photodetector generates current after absorbing light signals, which is similar to a current source. When the germanium layer absorbs light, electron-hole pairs are generated. When a bias voltage is applied to the electrode, an external electric field is generated. Under the action of the external electric field, the electrons generated by the germanium layer move toward the anode, and the holes generated move toward the cathode. The two work together to form a current, and the light energy is converted into electrical energy.

[0047] like Figure 6 The figure shows an equivalent circuit model of a germanium-silicon photodetector in the prior art, in which the process of the germanium region absorbing light signals to excite electron-hole pairs to generate current is equivalent to an AC current source, C pd and R pd R is the equivalent parasitic capacitance of the germanium region and the equivalent parasitic resistance of the current flowing through the silicon region. load is the load end resistance, usually fixed at 50 ohms, C load is the parasitic capacitance at the load end. ind The parasitic inductance introduced by the germanium upper electrode and the silicon upper electrode, R ind and C ind These are the parasitic resistance and capacitance introduced by the germanium top electrode and the silicon top electrode respectively.

[0048] The parasitic capacitance C of the germanium region pd The corresponding calculation formula can be expressed as:

[0049]

[0050] In formula (1), W Ge is the width of the germanium layer, L Ge is the length of the germanium layer, k is the electrostatic force constant, d is the thickness of the germanium layer, ε ris the relative dielectric constant of germanium material

[0051] The electron-hole pairs generated by the germanium layer will flow from the germanium layer to the doped silicon waveguide region 220 at the bottom of the germanium layer 230, and then reach the corresponding silicon electrode through the doped silicon waveguide region. The corresponding doped silicon waveguide region will produce a parasitic resistance. The circuit principle shows that this parasitic resistance is proportional to the current flow path and inversely proportional to the current flow area.

[0052] The current flows through the parasitic resistance R of the silicon region pd The corresponding calculation formula can be expressed as:

[0053]

[0054] In formula (2), L is the length of the path that the current in the doped silicon waveguide region passes through from the germanium region to the silicon electrode. S is the cross-sectional area through which the current on silicon flows, that is, the sum of the cross-sectional areas of the doped silicon waveguide region at the bottom of the germanium region surrounded by the silicon electrode. ρ is the resistivity of the silicon material in the doped silicon waveguide region, which is related to the doping concentration and the properties of the silicon material. If the width and length of the germanium region are set to be equal, since the fully enclosed electrode of Example 1 of the present invention makes the two surfaces in front and behind the germanium region also have silicon electrodes of equal length to the side surfaces, the current flow area flowing to the silicon electrode can be doubled. When the distance between the germanium layer and the silicon electrode remains unchanged, it is equivalent to connecting a resistor with the same resistance value as the original parasitic resistor in parallel, and the parasitic resistance of the silicon waveguide region of the photodetector can be reduced by half at most; if only one electrode is added, the parasitic resistance of the silicon waveguide region of the photodetector can only be reduced by 1 / 3 at most.

[0055] The equivalent impedance Z corresponding to the silicon upper electrode and the germanium upper electrode ind for:

[0056]

[0057] The equivalent impedance Z corresponding to the load end load for:

[0058]

[0059] Normalized frequency response S corresponding to the power at the output port of the SiGe photodetector 21 for:

[0060]

[0061] In equation (5), w is the frequency of the current source equivalent to the SiGe photodetector, S 21 The frequency value corresponding to a 3 dB drop is the bandwidth of the photodetector. From equation (5), it can be seen that the parasitic capacitance and parasitic resistance of the germanium region both limit the transmission coefficient.

[0062] Non-patent document 2 (J. Semicond. 2023, 44, 012301) is a germanium-silicon photodetector using existing technology. The equivalent parasitic capacitance of the germanium region is about 15fF, and the parasitic resistance on silicon is about 110Ω. In the example of the present invention, the width and length of the germanium region are equal, and a fully enclosed electrode is used, so the parasitic resistance can be reduced by half to 55Ω.

[0063] When designing a germanium-silicon photodetector, it is usually necessary to consider designing an optimal inductance value at the germanium upper electrode and the silicon upper electrode to match the parasitic capacitance of the germanium region and the parasitic resistance of the silicon region, thereby further maximizing the bandwidth. Figure 7 When an inductance of 50-225pH is introduced for the germanium upper electrode, the normalized frequency response of the prior art structure changes with frequency. Its maximum 3dB bandwidth is 87GHz, and the corresponding optimal inductance value is 175pH. Figure 8 When an inductance of 50-225pH is introduced to the germanium upper electrode, the normalized frequency response of a germanium-silicon photodetector proposed in the example of the present invention changes with frequency, and its maximum 3dB bandwidth can reach 126GHz, and the corresponding optimal inductance value is 75pH, which obtains a bandwidth improvement of 39GHz. The specific parasitic parameters corresponding to the prior art and the example of the present invention are shown in Table 1. Except for the parasitic resistance corresponding to the silicon region, other parameters remain unchanged.

[0064] Table 1. Parasitic parameters of photodetectors in the prior art and in the present invention

[0065]

[0066] This example uses diagonal oblique incidence to couple the light input. This diagonal oblique incidence design does not occupy any surface of the germanium region, and allows the electrode on the silicon to completely surround the four surfaces of the germanium without any surface missing, making the fully surrounded electrode structure feasible.

[0067] The silicon strip waveguide is connected to the vertex of the second silicon waveguide region, and guides the light to be incident from the vertex of the second silicon waveguide region corresponding to the notch of the silicon upper electrode;

[0068] The silicon strip waveguide is a waveguide for guiding the incident light signal. It is connected along the vertex of the second silicon waveguide area corresponding to the gap of the silicon electrode, and the incident direction of the light signal is parallel to the diagonal direction of the germanium layer. The light signal propagates along the incident strip waveguide to the diagonal direction of the germanium, which can maximize the propagation path of the light in the germanium layer 231, and the germanium layer can fully absorb the light. Under the same germanium area, the diagonal oblique incidence method can improve the responsiveness of the germanium silicon detector. Fig. 9 As shown, the detector responsivity changes with the bias voltage. The circled line is the photodetector of the present invention, and the curve is the photodetector of the prior art. It can be seen that the responsivity of the germanium silicon photodetector with diagonal incidence is significantly improved.

[0069] Furthermore, due to the diagonal incidence method, the detector can also use a smaller germanium structure. The responsiveness of the germanium detector can still be maintained at a high level, but the parasitic capacitance of the germanium area will be smaller, thereby further improving the bandwidth of the detector.

[0070] like Fig.10 , which is a top view of a germanium silicon photodetector provided in Example 2 of the present invention. Fig.11 A side cross-sectional view of the remaining structures of a germanium silicon photodetector provided in Example 2 of the present invention except for the silicon upper electrode 341. A germanium silicon photodetector provided in Example 2 of the present invention comprises: a silicon strip waveguide 300, a germanium layer 330, a doped waveguide region 322, a doped germanium layer 331, a germanium upper electrode 351 and a silicon upper electrode 341. The difference between Example 2 of the present invention and Example 1 above is that the silicon upper electrode 351 has two notches. The optical signal is divided into two equal parts, enters the silicon strip waveguide 322 respectively, and is transmitted to the germanium layer 330 through the notch of the silicon upper electrode 341. The double-end input of the optical signal can reduce the uniform distribution of the optical field of the germanium layer caused by the single waveguide input, thereby suppressing the negative impact of the space charge effect of the germanium layer on the bandwidth and improving the bandwidth.

[0071] The present invention is based on a fully enclosed electrode structure and adopts a diagonal incidence method. Compared with the germanium silicon detector in the prior art, the fully enclosed electrode structure reduces the influence of parasitic parameters and can obtain a higher bandwidth. At the same time, the diagonal incidence method further alleviates the constraint between bandwidth and responsivity and can obtain a higher responsivity.

[0072] It should be noted that the projection of the fully enclosed electrode and the germanium layer in the second silicon waveguide region can also adopt other arbitrary shapes, such as polygon, arc, square, etc., which can achieve the beneficial effects of the present invention and are not limited to rectangle. At the same time, there can be multiple gaps in the silicon electrode and silicon strip waveguides, not limited to one or two.

[0073] The above description is only a preferred example of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A germanium silicon photodetector, include: A silicon strip waveguide, a doped silicon waveguide region, a germanium layer, a doped germanium layer, a germanium upper electrode and a silicon upper electrode, wherein the doped silicon waveguide region comprises a first silicon waveguide region and a second silicon waveguide region, the first silicon waveguide region is placed in the middle of the second silicon waveguide region, and the germanium layer is epitaxially grown on the first silicon waveguide region; The silicon upper electrode is placed on the second silicon waveguide region, all sides of the germanium layer are surrounded by the silicon upper electrode, the silicon upper electrode is provided with a notch at the top corner of the germanium layer, and the silicon strip waveguide guides the optical signal to be transmitted to the germanium layer through the notch of the silicon upper electrode; The silicon strip waveguide is connected to the vertex of the first silicon waveguide region, and guides the light to be incident from the vertex of the first silicon waveguide region corresponding to the notch of the silicon top electrode; The doped silicon waveguide region is a first-type doped silicon waveguide region, and the doped germanium layer is a second-type doped germanium layer, wherein the second-type doping has a polarity opposite to that of the first-type doping.

2. The silicon-germanium photodetector according to claim 1, It is characterized in that The light transmission direction guided by the silicon strip waveguide is consistent with the diagonal direction of the germanium layer.

3. The germanium silicon photodetector according to claim 1, It is characterized in that The silicon top electrode is a square-shaped electrode, and the notch is arranged at the top corner of the square-shaped electrode.

4. The silicon-germanium photodetector according to claim 1, It is characterized in that The height of the first silicon waveguide region is 0.01-2 μm, the height of the second silicon waveguide region is 0.01-2 μm, and the height of the silicon strip waveguide is the sum of the heights of the first silicon waveguide region and the second silicon waveguide region.

5. The germanium silicon photodetector according to claim 1, It is characterized in that The germanium upper electrode is disposed on the doped germanium layer.

6. The silicon-germanium photodetector according to claim 1, It is characterized in that The first type doping is N-type or P-type.

7. The silicon-germanium photodetector according to claim 1, It is characterized in that The second type doping is N-type or P-type.

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