Germanium photodetector embedded in a multimode interferometer

Abstract

Procedure comprising: Etching a silicon layer to form a silicon plate (26A) and an upper silicon region (26B) over the silicon plate (26A); Doping the silicon plate (26A) and the upper silicon region (26B) to form: - a p-region (30), - an n-region (32), and - an intrinsic region between the p-region (30) and the n-region (32); Etching the p-type region (30), the n-type region (32) and the intrinsic region to form a trench (44), with remaining portions of the upper silicon region (26B) forming a multimode interferometer, MMI, region; performing an epitaxial process to grow a germanium region (46) in the trench (44); and Forming electrical connections (56, 34) connecting to the p-region (30) and the n-region (32).

Description

PRIOR ARTA photodetector (FD) is a device which is crucial for optical-electrical signal conversion. Germanium FDs are used in sensing and high-speed communication, such as light detection and ranging (lidar), data center, telecommunications, or the like. Germanium has a high absorption coefficient for light having a wavelength smaller than about 1.55 μm, and therefore, it is a good material for forming a photodetector.A conventional lateral germanium photodetector may comprise a P-I-N diode having a germanium region overlying and contacting the PIN diode. The silicon layer in which the P-I-N diode is formed may be etched such that the sidewalls of the P-I-N diode are in contact with a dielectric. The germanium region has the capability to absorb light and convert the light into electron-hole pairs. In a conventional germanium photodetector, a germanium region is formed over the P-I-N diode. This type of germanium photodetector, in which the input light beam is easily redirected too much, has low sensitivity. To solve this problem, the P and N regions that are not overlapped by the germanium region are recessed to form side walls to narrow light in the vicinity of the germanium absorption region between the P-I-N diode and the dielectric region where the photodetector is located. This type of germanium photodetector, in which the light is absorbed by the germanium a short distance, has a low saturation power.With regard to the prior art, reference is made to the publications U.S. Pat. No. 2016 / 0 161 691 A1, U.S. Pat. No. 2007 / 0 116 398 A1 and U.S. Pat. No. 2001 / 0 021 299 A1.BRIEF DESCRIPTION OF THE DRAWINGSAspects of the present disclosure will be best understood from the following detailed description taken in conjunction with the accompanying drawings. It should be appreciated that, in accordance with practice in the industry, various features are not drawn to scale. Indeed, for clarity of explanation, the dimensions of the various features may be arbitrarily increased or decreased. FIGS. 1, 2A, 2B, 3A, 3B, 4-8, 9A, 9B, and 9C show the cross-sectional and top views of intermediate stages in forming a multimode germanium photodetector, in accordance with some embodiments. FIGS. 10, 11 and 12 show top views of multimode germanium photodetectors, in accordance with some embodiments. FIGS. 13 and 14 show the top views of multi-terminal germanium multimode photodetectors, in accordance with some embodiments. FIG. 15 illustrates the exit of two photodetectors (with and without taper structures) as functions of wavelengths, in accordance with some embodiments. FIG. 16 illustrates a process flow for forming a multimode germanium photodetector, in accordance with some embodiments.DETAILED DESCRIPTIONThe following disclosure provides many different embodiments, or examples, for implementing various features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are of course merely examples. For example, forming a first feature over or on a second feature in the description below may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and / or letters in the various examples. This repetition is for purposes of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and / or embodiments discussed.Moreover, terms relating to spatial relativeity, such as "underlying", "under", "lower", "overlying", "upper", and the like, may be used herein for ease of discussion to describe the relationship of one element or feature to another element or feature (to other elements or features) as depicted in the FIGS. The terms relating to spatial relativeness are intended to encompass different orientations of the device being used or operated in addition to the orientation depicted in the FIGS.. The device may be otherwise oriented (rotated 90 degrees or otherwise oriented) and the spatially relative terms used herein may likewise be arranged accordingly.A multimode germanium photodetector and the method of forming the same are provided in accordance with some embodiments. The intermediates in forming the multimode germanium photodetector are illustrated in accordance with some embodiments. Some modifications of some embodiments are discussed. Embodiments discussed herein are intended to provide examples to enable making or using the subject matter of this disclosure, and one of ordinary skill in the art will readily understand modifications that may be made while retaining the scope of various embodiments contemplated. Like reference numerals are used throughout the several views and embodiments to refer to like elements. Although embodiments of methods may be discussed as being performed in a particular order, other embodiments of methods may be performed in any logical order.According to some embodiments of the present disclosure, the multimode germanium photodetector includes a germanium region in a multimode interferometer (MMI) structure. The MMI structure may be a silicon region that extends laterally sufficiently far beyond the germanium region so that higher order modes may be excited by a fundamental wave of an input light. The different modes having the fundamental and higher order modes interfere within the MMI structure, so that the optical field strength of the light can be redistributed. Consequently, the absorption along the germanium region is gradual, instead of the intensity being strongest in the front middle portion of the germanium region and most of the light energy being absorbed by the front portion. This results in increasing the saturation power of the light and in increasing the speed of the photodetector. Although silicon and germanium are used as example materials, it should be understood that other suitable materials may also be used to form a multimode photodetector.FIGS. 1, 2A, 2B, 3A, 3B, 4-8, 9A, 9B, and 9C show the cross-sectional and top views of intermediate stages in forming a multimode germanium photodetector, in accordance with some embodiments of the present disclosure. The corresponding processes are also schematically depicted in the process flow 200 illustrated in FIG. 16.In FIG. 1, a wafer 10 is provided that includes a substrate 20. The corresponding process is shown as process 202 in the process flow 200 illustrated in FIG. 16. The substrate 20 may have an SOI (semiconductor on insulator) structure including a semiconductor substrate 20A, a BOX (buried oxide) layer 20B over and associated with the semiconductor substrate 20A, and a semiconductor layer 20C over and associated with the BOX layer 20B. According to some embodiments, the semiconductor material of the semiconductor substrate 20A may include silicon, germanium, a compound semiconductor including silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and / or indium antimonide, an alloy semiconductor including SiGe, SiC, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and / or GaInAsP, or combinations thereof. The substrate 20A may also be formed of other materials, such as sapphire, indium tin oxide (ITO), or the like.The BOX layer 20B may be formed of a dielectric material having a refractive index that is smaller than the refractive index of silicon. In some embodiments, the BOX layer 20B is formed of or comprises silicon oxide. The semiconductor layer 20C may be formed of crystalline silicon, and will be referred to as a silicon layer 20C below. According to some embodiments of the present disclosure, silicon layer 20C is not doped with any of a p- or n-type impurity. According to alternative embodiments of the present disclosure, silicon layer 20C is lightly doped with a p-type impurity such as boron, indium, or the like, or an n-type impurity such as phosphorus, arsenic, antimony, or the like. The doping concentration of the lightly doped silicon layer 20C is low, for example, lower than about 5×10 15 / cm 3. Throughout the specification, the semiconductor layers (regions) that are undoped or lightly doped with a doping concentration lower than 5×10 15 / cm 3, are referred to as intrinsic semiconductor layers (regions). The thickness T 1 of the silicon layer 20C may be in the range between about 0.1 μm and about 1 μm.The silicon layer 20C is then patterned in a plurality of etching processes, for example, so that a plurality of device regions are formed. The corresponding process is shown as process 204 in the process flow 200 illustrated in FIG. 16. The example device regions are illustrated in FIG. 2A, which includes a grating coupler 22, a waveguide 24, and a silicon region 26. The silicon region 26 is used to form a multimode germanium photodetector in subsequent processes. According to some embodiments, the etching processes for patterning the silicon layer 20C include a time mode etching process to etch some portions of the silicon layer 20C, wherein the time mode etching process is stopped before the silicon layer 20C is etched through. Another etching process is performed to etch through the silicon layer 20C. As a result of the etching processes, the silicon region 26 includes a silicon plate 26A and upper silicon regions 26B, 26C, and 26D (shown in FIG. 2B ) over the silicon plate 26A. According to some embodiments of the present disclosure, a thickness T 2 of the silicon plate 26A may be in the range between about 0.05 μm and about 0.5 μm, and a thickness T 3 of the upper silicon regions 26B, 26C, and 26D may be in the range between about 0.05 μm and about 0.5 μm. The ratio T3 / T2may be in the range between about 0.1 and about 10.FIG. 2B shows a top view of the silicon region 26 including the silicon plate 26A, the silicon region 26B, the waveguide 26C, and the tapered region 26D. The cross-sectional view illustrated in FIG. 2A is obtained from the reference cross-section 2A- 2A as shown in FIG. 2B. The silicon plate 26A may be larger than the silicon region 26B and may extend beyond the edges of the silicon region 26B at least in the -X direction, the +Y direction, and the -Y direction. The silicon plate 26A may or may not extend beyond the edge of the silicon region 26B in the +X direction. Throughout the specification, the silicon region 26B is referred to as an MMI region, as discussed in detail in the sections below.FIG. 3A illustrates the formation of multiple doped regions including a p-type region 30, an n-type region 32, a heavily doped p-type region (p+ region) 34, and a heavily doped n-type region (n+ region) 36. The corresponding processes are shown as process 206 in the process flow 200 illustrated in FIG. 16. In some embodiments, p-type region 30 and n-type region 32 may each have p- and n-type impurity concentrations in the range between about 10 17 / cm 3 and about 10 19 / cm 3. The p+ region 34 and the n+ region 36 may each have p and n impurity concentrations ranging between about 10 19 / cm 3 and about 10 21 / cm 3. The p+ region 34 and the n+ region 36 are used as the electrical contact regions of the resulting photodetector. Each of the regions 30, 32, 34, and 36 may be doped using a photolithographic mask. For example, FIG. 3A illustrates the example formation process of the p+ region 34, which includes forming a patterned photoresist 38 having an opening 40 formed to expose a portion of the silicon region 26 and implanting a p-type impurity, such as boron and / or indium, to form the p+ region 34. The photoresist 38 is removed after the implantation.FIG. 3B shows a plan view of the structure shown in FIG. 3A, wherein the cross-sectional view shown in FIG. 3A has been obtained from the reference cross-section 3A- 3A as shown in FIG. 3B. As shown in FIG. 3B, some portions of the silicon region 26 are doped, while the remaining portions of the silicon region 26 are not doped. The undoped portions of the silicon region 26, which may include the portions of the silicon plate 26A and the upper silicon region 26B, may remain intrinsic.Referring to FIG. 4, a dielectric layer 42 is formed on silicon features. The corresponding process is shown as process 208 in process flow 200 illustrated in FIG. 16. According to some embodiments of the present disclosure, a dielectric layer 42 is formed of a dielectric material having a refractive index lower than the refractive index of silicon. In some embodiments, the dielectric layer 42 is formed of or includes silicon oxide (SiO 2), silicon oxynitride (SiON), aluminum oxide (Al 2 O 3) or the like. Other materials are also within the scope of the present disclosure. Forming the dielectric layer 42 may include atomic layer deposition (ALD), chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), or the like. The dielectric layer 42 may include portions covering the grating coupler 22, the waveguide 24, and the silicon region 26. A planarization process, such as a chemical mechanical polishing (CMP) process or a mechanical grinding process, may be performed to planarize the top surface of the dielectric layer 42.Referring to FIG. 5, the silicon region 26 is etched to form a trench 44 that extends into the undoped portion of the silicon region 26, the p-type region 30, and the n-type region 32. The corresponding process is shown as process 210 in the process flow 200 illustrated in FIG. 16. The etching may be performed using an anisotropic etching process. According to some embodiments, the etching is performed using C 2 F 6, CF 4, SO 2, the mixture of HBr, Cl 2 and O 2, the mixture of HBr, Cl2and O 2 or the mixture of HBr, Cl 2, O2and CF 2 etc. Other materials are also within the scope of the present disclosure. According to some embodiments, the bottom of the trench 44 is lower than the top surface of the silicon plate 26A. According to other embodiments, the bottom surface of the trench 44 is level with or higher than the top surface of the silicon plate 26A.FIG. 6 illustrates the epitaxial process for forming a germanium region 46 that is selectively grown from the exposed surfaces of the silicon region 26 but not from the dielectric layer 42. The corresponding process is shown as process 212 in the process flow 200 illustrated in FIG. 16. According to some embodiments, epitaxy is performed using monogermane (GeH 4) as a process gas, adding an etching gas such as HCl to achieve selective growth. The germanium region 46 may include pure germanium or substantially pure germanium (e.g., with a germanium atom percentage that is greater than 99 percent). According to some embodiments of the present disclosure, the germanium region 46 is intrinsic, wherein no p-type impurity and no n-type impurity are added in situ during epitaxy. According to alternative embodiments, the germanium region 46 is lightly in situ doped with p- and / or n-type impurities having a doping concentration lower than, for example, about 1×10 15 / cm 3. The epitaxial process is performed until the upper surface of the germanium region 46 is at least equal to or higher than the upper surface 26T of the silicon region 26. For example, FIG. 6 shows an example germanium region 46 where the top surface is higher than the top surface of the dielectric layer 42. the germanium region 46 may have, for example, surfaces as shown using dashed lines 47. According to some embodiments of the present disclosure, a planarization process is performed after the epitaxy process to planarize the top surfaces of the germanium region 46 and the dielectric layer 42. According to alternative embodiments, a planarization process is not performed and the top surface of the germanium region 46 may be lower, equal to or higher than the top surface of the dielectric layer 42.FIG. 7 illustrates the formation of a capping layer 50 in accordance with some embodiments of the present disclosure. The corresponding process is shown as process 214 in the process flow 200 illustrated in FIG. 16. In accordance with some embodiments, the capping layer 50 is formed of a dense material that can prevent the germanium in the germanium region 46 from undesirably diffusing upward into and contaminating overlying regions. Accordingly, in forming the capping layer, the forming process of the present disclosure is more compatible with forming other integrated circuits, such as transistors. The capping layer 50 may be formed of or include silicon nitride, silicon oxynitride, aluminum oxide, or the like. Other materials are also within the scope of the present disclosure. The thickness of the capping layer 50 may be in the range of between about 20 nm and about 600 nm.The material and formation process of the capping layer 50 may be configured to allow the capping layer 50 to apply stress to the underlying germanium region 46. In some embodiments, the stress is a tension stress. In some embodiments, the stress is compressive stress. The strain may be greater than about 1 Gpa and may range between about 0.2 Gpa and about 1.7 Gpa. Other values are also within the scope of the present disclosure. A tensile stress can effectively reduce the direct band gap of germanium, which means widening the strong absorption band to the longer wavelength. For example, with a 0.5 Gpa stress, the Ge absorption edge can be shifted 90 nm to the longer wavelength (from ~1550 nm to ~1640 nm), causing the Ge high absorption band to fully cover the C band used in telecommunications. When the capping layer 50 applies stress to the germanium region 46, it will be appreciated that the type (compressive or tensile stress) and magnitude of the stress are related to both the material and the formation process of the capping layer 50. Both the material and the forming process of the capping layer 50 are chosen to achieve the desired stress.According to some embodiments, forming the capping layer 50 includes depositing a blanket capping layer and then patterning the blanket capping layer using etching. A tempering process may be performed to adjust the strain. During the patterning, the portions of the blanket capping layer on the dielectric layer 52 are removed. According to alternative embodiments, forming the capping layer 50 includes selectively depositing an inhibitor film on the surface of the dielectric layer 42, for example, using a silylation process, and then selectively depositing the capping layer 50 on the germanium region 46. The inhibitor film can be removed by annealing.FIG. 8 illustrates the implantation process for forming a p-type region 46 pin the germanium region 46, and the corresponding process is shown as process 216 in the process flow 200 illustrated in FIG. 16. According to some embodiments, forming the p-type region 46 pmay include forming a patterned implantation mask 48 (such as a photoresist) to define the implantation region and implanting a p-type impurity such as boron and / or indium into an edge portion of the germanium region 46. Doping the edge portion of the germanium region 46 as the p-type region 46 pmay reduce the barrier height between the germanium region 46 and the p-type silicon region 30.FIG. 8 also illustrates the formation of an n-type region 46 nin the germanium region 46, and the corresponding process is also shown as process 216 in the process flow 200 illustrated in FIG. 16. According to some embodiments, forming the n-type region 46 nmay include forming a patterned implantation mask (not shown) to define the implantation region and implanting an n-type impurity such as phosphorus, arsenic, etc. into an edge portion of the germanium region 46. Doping the germanium region 46 as the n-type region 46 nmay reduce the barrier height between the germanium region 46 and the n-type silicon region 32. According to some embodiments of the present disclosure, both the p-type region 46 pand the n-type region 46 nare formed. If either, not both, is to be formed to reduce manufacturing costs, then in accordance with alternative embodiments, the p-type region 46 pis formed while the n-type region 46 nis not formed, as the p-type region 46 pis more effective than the n-type region 46 nin improving device performance. In still other embodiments, neither p-type region 46 por n-type region 46 nis formed. Accordingly, dotted lines are used to mark the boundaries of the p-type region 46 pand the n-type region 46 nand indicate that they may or may not be formed.It should be appreciated that the interface 47A between the p-type germanium region 46 pand the undoped portion of the germanium region 46 may or may not be aligned with the interface 49A between the p-type silicon region 30 and the undoped portion of the silicon region 26. Accordingly, the interface 47A may be offset to the left, aligned with, or offset to the right of the interface 49A. Likewise, the interface 47B between the n-type germanium region 46 nand the undoped portion of the germanium region 46 may or may not be aligned with the interface 49B between the n-type silicon region 32 and the undoped portion of the silicon region 26. Accordingly, the interface 47B may be offset to the left, aligned with, or offset to the right of the interface 49B.The doping concentrations of the p-type germanium region 46 pand the n-type germanium region 46 nmay be less than about 5×10 19 / cm 3, and may be in the range between about 1×10 16 / cm 3 and about 5×10 19 / cm 3. According to alternative embodiments in which an avalanche photodiode is desired, the doping concentration of the p-type germanium region 46 pand the n-type germanium region 46 nmay be greater than approximately 1×10 17 / cm 3, such that the effective intrinsic germanium region 46 becomes narrow, resulting in a higher electric field in the intrinsic germanium region 46. According to some embodiments, the doping width W dope is less than about 120 nm and may be in the range between about 20 nm and about 200 nm. Other values are also within the scope of the present disclosure. It has been found that selecting the doping width W dope, to be about 100 nm can improve the device speed. When the doping width W dope is too large, such as greater than about 200 nm, the device speed deteriorates due to the scattering of carriers. On the other hand, when the doping width W dope is too small, such as less than about 20 nm, the improving effect is too small and is unable to justified the doping cost.FIG. 9A illustrates the formation of a dielectric layer 52, silicon regions 54, and contact plugs 56. The corresponding process is shown as process 220 in the process flow 200 illustrated in FIG. 16. In accordance with some embodiments, the dielectric layer 52 is formed of silicon oxide, silicon nitride, silicon oxynitride, or the like. Forming the silicon regions 54 may include etching the dielectric layers 52 and 42 to form contact openings and expose the p+ region 34 and the n+ region 36, depositing a metal layer, such as a titanium layer, extending into the contact openings, and performing a anneal process to react the metal layer with the exposed p+ region 34 and the exposed n+ region 36. Forming the contact plugs may include depositing a metal, such as tungsten, cobalt, or the like, in the contact openings and then performing a planarization process, such as a CMP process or a mechanical grinding process, to remove excess portions of the deposited metal. The germanium photodetector 60 is formed in this manner.As shown in FIG. 9A, the germanium photodetector 60 includes an intrinsic silicon region 26' which is the undoped portion of the original silicon region 26. Accordingly, the intrinsic silicon region 26' may be free of one or both of a p- and n-type impurity or lightly doped with one or both as mentioned above. The p-type region 30 and the n-type region 32 form a p-i-n photodiode with the intrinsic silicon region 26'. In operation of the germanium photodetector 60, light travels into the germanium region 46 and electrons and holes are generated in the germanium region 46. A strong electric field is generated in the intrinsic germanium region 46, and the electrons and holes generated in the germanium region 46 are swept by the electric field to the contact plugs 56.FIG. 9B is a plan view of the photodetector 60 shown in FIG. 9A, and the cross-sectional view shown in FIG. 9A is obtained from the reference cross section 9A- 9A shown in FIG. 9B. As shown in FIG. 9B, the upper silicon region above the silicon plate 26A includes the waveguide 26C and the tapered region 26D for guiding light to the germanium photodetector 60. The waveguide 26C and the tapered region 26D also constitute the input terminal of the germanium photodetector 60. The tapered region 26D has the function of propagating light from the narrow waveguide 26C to a larger area in the silicon region 26B. According to some embodiments, the tapered region 26D is not applied, and the waveguide 26C having a uniform width is directly connected to the silicon region 26B.In some embodiments, the width W 1 of the silicon region 26B is substantially greater than the width W 2 of the germanium region 46. in some embodiments, the silicon region 26B extends laterally beyond the edges of the germanium region 46 in the +Y direction and the -Y direction by a distance S 1 that is sufficiently considerable to allow an appropriate space such that light input from the input terminal 26C / 26D propagates in the +Y direction and the -Y direction. In some embodiments, the input light 64 has a single mode referred to as a fundamental. When light 64 passes into the silicon region 26B, light is excited with higher order modes such as a first order mode, a second order mode, a third order mode, a fourth order mode, and so on, because the silicon region 26B has the large width W 1. The resultant light having the fundamental wave and the higher order modes then becomes the multimode light in the silicon region 26B. These modes interfere within region 26B and form different orders of self-images. Accordingly, the silicon region 26B will be referred to as a multimode interferometer (MMI) region below.According to some embodiments, an appropriate value is chosen for the distance S 1 in order that a desired distribution of the optical power can be achieved. The distance S 1 may not be too large or too small. If the distance S 1 is too small, for example less than 0.2 μm, the light power is not redistributed widely enough (discussed in detail in subsequent sections). Consequently, most of the light travels in the +x direction and is incident frontally on the germanium region 46. Most of the light power is therefore absorbed by the front end (the end facing the input waveguide 26C) of the germanium region 46. This results in the device's saturation power being undesirably low and the photodetector's speed also being reduced. If the distance S1 is too large, the light power is redistributed too much and spread too much, and a considerable portion of the light, as it propagates in the +X direction, passes through the distance S1 on a longer path length without encountering the germanium region 46. Light absorption is low. Consequently, the length L 2 of the germanium region may need to be increased too much to enable the appropriate absorption of the light through the germanium region 46. Accordingly, in some embodiments, the distance S 1 is in the range between about 0.4 μm and about 1.5 μm. Other values are also within the scope of the present disclosure.The multimode light, when propagating in the +Y and -Y directions, is reflected on the opposite side walls 26B-SW 1 and 26B-SW 2 of the MMI region 26B, and therefore interference is generated, forming an interference pattern. The sidewalls 26B-SW 1 and 26B-SW 2 of the MMI region 26B form interfaces with the sidewalls of the dielectric region 42 as illustrated in FIG. 9A. The power of the light is therefore redistributed in the silicon region 26. Accordingly, the front end of the germanium region 46 receives an appropriate but not excessive amount of light. The light, when reflected back and forth in the MMI region 26B upon propagation in the +X direction, is gradually absorbed by the front portion, the middle portion, and the rear portion of the germanium region 46. Therefore, by applying the MMI structure, the light is more evenly absorbed by different portions (front portions, middle portions, and back portions) of the germanium region 46. Accordingly, the saturation power of the resultant germanium photodetector 60 is increased, and the light having a higher power can be used at the photodetector 60 without causing saturation. The speed of the germanium photodetector 60 is also increased. In addition, by employing the MMI structure, a smaller portion of the light strikes the Si-Ge interface and is reflected to the input, resulting in reduction of back reflection from the photodetector 60. Accordingly, increasing the width W 1 of the MMI region 26B also helps reduce the reflection.The length L 1 representing the distance from the input terminal of the MMI region 26B to the germanium region 46 is referred to as the free propagation length (of the light). The free propagation length L1 may not be too small. Otherwise, there is not sufficient distance for light to propagate in the +Y and -Y directions, and higher order modes cannot be excited. In addition, there is too strong a reflection from the germanium region 46 if the free propagation length L 1 is too small. According to some embodiments of the present disclosure, the free propagation length L 1 is greater than about 1 μm to allow multimodes to be excited. In addition, as the width W 1 is larger, a larger L 1 is used. Too large a free propagation length L1 also results in increasing the size of the germanium photodetector 60 without an additional advantage. Accordingly, in some embodiments, the free propagation length L 1 is in the range between about 1 μm and about 10 μm. Other values are also within the scope of the present disclosure.When the light is redistributed into multimodes, in accordance with some embodiments, the length L 2 of the germanium region 46 may be greater than about 10 μm to substantially fully absorb the light (e.g., at an absorption rate greater than 99 percent). Too large a length L2 is also undesirable because it results in the size of the multimode germanium photodetector being increased without an additional benefit. The length L 2 may be in the range between about 10 μm and about 50 μm, in accordance with some embodiments. The width W 2 of the germanium region 46 may be in the range between about 0.3 μm and about 1.5 μm, in accordance with some embodiments. Other values are also within the scope of the present disclosure.The dimensions such as the width W 1 and the lengths L 1 and L 2 may be designed to achieve an optimal result so that the absorption of light by the germanium region 46 is more uniform over the entire length L 2. For example, W1and length L1may be selected such that the front end of germanium region 46 is located in a region of moderately high E-field of light. In addition, the length L2 may be set to have a minimum length to reduce the size of the photodetector 60, while still being long enough to substantially completely absorb the light.FIG. 9C is a cross-sectional view obtained from the reference cross-section 9C- 9C in FIG. 9B. FIG. 9C illustrates the shape of the silicon plate 26A and the free spread portion of the MMI region 26B, in accordance with some embodiments.FIGS. 10-14 show the top views of multimode germanium photodetectors 60 in accordance with alternative embodiments of the present disclosure. Unless otherwise specified, the materials and the formation processes of the components in these embodiments are substantially the same as the similar components in previous FIGS., except that the structure of the lithographic masks used in the etching processes for forming the structure shown in FIG. 2A is modified to form the structures shown in FIGS. 10-14. Accordingly, the components in FIGS. 10 to 14 are denoted by like reference numerals in the foregoing embodiments shown in FIGS. 1, 2A, 2B, 3A, 3B, 4 to 8, 9A, 9B, and 9C. The details regarding the formation processes and the materials of the components illustrated in Figs. 10 to 14 can therefore be found in the discussion of the foregoing embodiments. In addition, the cross-sectional views obtained by the embodiments shown in Figs. 10 to 14 are also the same as shown in Figs. 9A to 9C. For example, FIG. 10 schematically shows reference cross sections 9A- 9A and 9C- 9C, and the cross-sectional views obtained from these cross sections are the same as shown in FIGS. 9A and 9C, respectively.In the multimode germanium photodetectors 60, as shown in FIG. 10, the MMI region 26B has a front portion having the width W 1, and rear portions are tapered to have the width W 3. Due to the separation of the germanium region 46, two back portions 26B-B of the MMI region 26B are located on opposite sides of the germanium region 46, and a tapered portion 26B-T is provided on each side of the germanium region 46 for connecting the front portion 26B-F to the back portion 26B-B. In some embodiments, the distance S 3 of the rear portion 26B-B is less than the distance S 1 of the front portion. In some embodiments, the ratio S 3 / S 1 is in the range between about 0.1 and about 0.5. in the tapered regions 26B-T, the distances S 2 may continuously decrease from S 1 to S 3. According to some embodiments, the tapered portion distance S 2 decreases linearly, which means that the respective portions of the side wall 26B-SW 1 and 26B-SW 2 are straight in plan view. In other embodiments, the sidewalls of the tapered portions may have other shapes, such as may be curved. It is understood that the distances S 1, S 2, and S 3 are also the widths of the portions of the MMI region on a side of the germanium region 46. In plan view, the portions of the sidewall 26B-SW 1 and 26B-SW 2 of the tapered portion 26B-T form an angle θ with the portions of the sidewall of the rear portion 26B-B. According to some embodiments of the present disclosure, the angle θ is less than about 10 degrees to reduce the reflection of the light back to the input end. The angle θ may be in the range between about 1 degree and about 20 degrees, in accordance with some embodiments. Other values are also within the scope of the present disclosure.The tapered portion 26B-T has a function of restricting and restricting light from a wide region (having the width W 1) toward the germanium region 46, and restricts the light to a narrower region having the pitch S 1, so that light is restricted in narrower regions having pitches S 2 and S 3. Therefore, the light intensity in the tapered portion 26B-T and the rear portion 26B-B is increased compared to when the light is not confined and condensed, and the light absorption efficiency is increased. The length L4 of the tapered portions 26B-T is preferably short so that compression of the light is terminated a short distance unless the taper causes light reflection (to the input port) to increase. In some embodiments, the length L4 is in the range between 50 percent and about 200 percent of the free propagation length L1.The length L3is related to the length L1, and it is desired that (L1+L3) be sufficiently large that at the right end of the front portion 26B-F, a major (but not excessive) portion of the light power, such as in the range between about 50 percent and about 90 percent, has been absorbed by the germanium region 46. Note that carefully adjusting the length of L 3 can advantageously increase the saturation power of the light due to the improvement of the absorption uniformity. With carefully selected lengths L 1, L 3, and L 4, it is possible to reduce the length L 5 of the rear portions 26B-B.FIG. 15 shows the rates of light leakage of the MMI region 26B as a function of the wavelength (lambda) of the light. Two lines 66 and 68 are shown, line 66 being simulated by the structure without the tapered structure (as shown in Figure 9B), and line 68 being simulated by the structure having the tapered structure (as shown in Figure 10). The results indicate that the leakage is substantially reduced for a wide range of light wavelengths (from 1.26 μm to 1.36 μm) when the tapered structure is used. The simulation results also showed that, for selected lengths L 3 and L 4, the light leakage (the remaining unabsorbed light power at the end of the MMI region 26B) is reduced by 10 dB when the length L 5 (FIG. 10 ) is 5 μm.FIGS. 11 and 12 show multimode germanium photodetectors 60 in accordance with some embodiments in which reflectors are formed to reflect the light that would otherwise exit. In Fig. 11, a reflective grating 26F is used for reflection. In Figure 12, a waveguide loop 26G is used to guide the light back through a loop into the MMI region 26B. According to some embodiments, another tapered region 26H is connected to the end of the MMI region 26B to concentrate the light first, and the concentrated light is directed into the waveguide loop 26G. The reflective grating 26F and the waveguide loop 26G may also be formed by patterning the silicon region 26 (FIG. 1 ) in the same process of forming the MMI region 26B.FIGS. 13 and 14 show multi-terminal multimode germanium photodetectors 60 in accordance with alternative embodiments of the present disclosure. Forming the wide MMI region 26B allows for the addition of multiple terminals used as input or reflection collection. For example, FIG. 13 shows a dual-terminal multimode germanium photodetector 60 having terminal 26C and terminal 26E. Light may be input through the terminal 26C or the terminal 26E or through both terminals depending on the application. In the case of simple input using the terminal 26C, the terminal 26E may be used for collecting the reflected light. According to some embodiments, a termination 70 is connected to the reflection terminal 26E, and is used to absorb the light received from the reflection terminal 26E. According to some embodiments, the termination 70 may be a silicon region having a high doping concentration of phosphorus, for example having the doping concentration higher than about 10 20 / cm 3.The position of the terminal 26C is offset from the center line ML 1 located at the center of the side wall 26B-SW 1 and the center line ML 1 of the MMI region 26B. According to some embodiments, the input terminal 26C may be located at the center of the sidewall 26B-SW 1 and the center line ML 1. The reflection terminal 26E may be located at the center of the side wall 26B-SW 2 and the center line ML 1. According to other embodiments, the position of the input terminal 26C is set to be offset from the center of the side wall 26B-SW 1 and the center line ML. Simulation results have shown that by carefully selecting the position of the input port 26C, the E-field of the interfered multimode light may have two peaks separated from each other. When the first peak oscillates to the sidewall 26B-SW1, the second peak may fall on the germanium region 46 to absorb its power, and when the second peak oscillates to the sidewall 26B-SW2, the first peak may fall on the germanium region 46 to absorb its power. Accordingly, by selecting the positions of the input terminal 26C at the front end of the germanium region 46, the light intensity is effectively reduced by one half, and therefore the absorption of the light by the front portion of the germanium region 46 is reduced, and therefore the saturation performance of the multi-terminal multimode germanium photodetectors 60 is improved.According to some embodiments, the width W 1 and the free propagation length L 1 of the MMI region 26B may be further increased to accommodate the plurality of terminals. For example, the width W 1 of the MMI region may be increased to be in the range between about 1.5 μm and about 4 μm. The free propagation length L 1 may be increased to be in the range between about 1 μm and about 20 μm. Other values are also within the scope of the present disclosure.It will be appreciated that more terminals may be added. The total number of terminals may also be three, four, five, or more. Of these terminals, one or more may be used as the input, and the others may be used as reflection terminals for collecting the reflected light. For example, FIG. 14 shows a multimode germanium photodetector 60 having three terminals as an example. FIG. 14 shows that the terminal 26C is used as an input terminal and is located at the center of the terminals 26E used as reflection terminals, according to some embodiments. According to some embodiments, the input port may be in any other positions, such as in the position of any of the illustrated reflective ports 26E in FIG. 14. For example, the input terminal 26C may be located at the center of the MMI region 26B, or the position of the input terminal 26C may be interchanged with any of the reflection terminals 26E. According to other embodiments, both of the terminals 26E may be used as an input and 26C is used as a reflective terminal.The embodiments of the present disclosure have some advantageous features. Germanium has a high absorption coefficient in the wavelength range from 1200 nm to 1600 nm, which represents a often used range in optical applications. The high absorption coefficient may result in power saturation and speed degradation when the input power is high. Since the optical field intensity is redistributed in the multimode germanium photodetectors, according to some embodiments of the present disclosure, by appropriately disposing the germanium region, light is more evenly absorbed by the front portion, the middle portion, and the back portion of the germanium region. Accordingly, the saturation power of the germanium photodetector is increased. The speed of the photodetector is also increased, especially at high optical input. The reflection is reduced by about 10 dB compared with the single mode germanium photodetectors. In addition, the cap layer applies stress to the germanium region that increases the optical absorption coefficient of germanium in the vicinity of the C band of 1550 nm, and therefore the cap of germanium absorption at the C band is increased. The photodetector according to embodiments of the present disclosure is well suited for the bandwidth of about 1200 nm to about 1600 nm. In addition, since the MMI regions are wider and extend beyond the germanium regions a considerable distance, the process of forming the germanium photodetector has a looser process tolerance.According to some embodiments of the present disclosure, a method includes: etching a silicon layer to form a silicon plate and an upper silicon region over the silicon plate; doping the silicon plate and the upper silicon region to form a p-region, an n-region, and an intrinsic region between the p-region and the n-region; etching the p-region, the n-region, and the intrinsic region to form a trench, remaining portions of the upper silicon region forming an MMI region; performing an epitaxial process to grow a germanium region in the trench; and forming electrical connections connected to the p-region and the n-region. In an embodiment, the MMI region includes a first remaining portion of the p-type region and a second remaining portion of the n-type region on opposite sides of the germanium region, and wherein outer sidewalls of the first remaining portion and the second remaining portion are spaced apart from the germanium region by distances greater than about 0.4 μm. In an embodiment, the method further comprises: depositing a capping layer on the germanium region; and depositing a dielectric layer on a top surface and sidewalls of the capping layer. In an embodiment, the capping layer comprises silicon nitride and the dielectric layer comprises silicon oxide. In an embodiment, a sidewall of the MMI region includes: a front portion having a first distance from a respective sidewall of the germanium region; a rear portion having a second distance from the respective sidewall of the germanium region, the second distance being less than the first distance; and a tapered portion connecting the front portion to the rear portion, the tapered portion having third distances from the respective sidewall of the germanium region, and the third distances transitioning from the first distance to the second distance. In an embodiment, the method further comprises: depositing an isolation region surrounding the silicon plate and the upper silicon region, the isolation region having a first refractive index that is less than a second refractive index of the upper silicon region. In one embodiment, the etched silicon layer results in multiple terminals that can be used as input terminals and reflection terminals.According to some embodiments of the present disclosure, an apparatus includes: a P-I-N diode including a p-region, an n-region, and an intrinsic region between the p-region and the n-region; a germanium region extending into the P-IN diode; and an input terminal, wherein the P-I-N diode and the germanium region together form a multimode interferometer configured to excite a single mode from an input light input from the input terminal into a light including higher order modes. In an embodiment, the device further comprises: a first portion of the p-type region located on a first side of the germanium region and a second portion of the n-type region located on a second side of the germanium region; and the first portion and the second portion have outer edges spaced apart from respective near edges of the germanium by distances greater than about 0.4 μm. In an embodiment, the P-I-N diode includes: a silicon plate and an upper silicon region over the silicon plate, the germanium region passing through the upper silicon region, and the upper silicon region including: a front portion having a first width, a rear portion having a second width less than the first width; and a tapered portion connecting the front end to the rear end, the tapered portion having widths transitioning from the first width to the second width. In an embodiment, the device further comprises: a first dielectric layer, wherein the P-I-N diode is located in the first dielectric layer; a capping layer on the germanium region; and a second dielectric layer on a top surface and sidewalls of the capping layer. In an embodiment, the capping layer comprises silicon nitride and the first dielectric layer and the second dielectric layer comprise silicon oxide. In an embodiment, the apparatus further comprises: an input terminal, and a reflection terminal on a same side of the germanium region as the input terminal.According to some embodiments of the present disclosure, an apparatus includes: a silicon region including a silicon plate; an upper silicon region located above and associated with the silicon plate, the silicon region forming a P-I-N diode extending into both the silicon plate and the upper silicon region; and a germanium region passing through the upper silicon region, the upper silicon region including a multimode interferometer region including a first portion on a first side of the germanium region, a second portion on a second side of the germanium region opposite the first side, and an input terminal connected to the upper silicon region. In an embodiment, the multimode interferometer region further includes a free propagation region between the input terminal and the germanium region, and the free propagation region has a length greater than about 1 μm. In an embodiment, the apparatus further comprises a reflection terminal connected to the same sidewall of the multimode interferometer region. In one embodiment, the multimode interferometer region has a front portion and a rear portion that is narrower than the front portion. In an embodiment, the multimode interferometer region further includes a tapered portion connecting the front portion to the rear portion. In an embodiment, the device further comprises a capping layer over and contacting the germanium region, wherein the capping layer is configured to apply a stress to the germanium region.

Claims

A method comprising: etching a silicon layer to form a silicon plate (26A) and an upper silicon region (26B) over the silicon plate (26A); doping the silicon plate (26A) and the upper silicon region (26B) to form: a p-region (30), an n-region (32), and an intrinsic region between the p-region (30) and the n-region (32); etching the p-region (30), the n-region (32), and the intrinsic region to form a trench (44), remaining portions of the upper silicon region (26B) forming a multimode interferometer, MMI, region; performing an epitaxy process to grow a germanium region (46) in the trench (44); and forming electrical connections (56, 34) connecting to the p-region (30) and the n-region (32).The method of claim 1, wherein the MMI region includes a first remaining portion of the p-type region (30) and a second remaining portion of the n-type region (32) on opposite sides of the germanium region (46), and wherein outer sidewalls of the first remaining portion and the second remaining portion are spaced apart from the germanium region (46) by distances greater than about 0.4 μm.The method of claim 1 or 2, further comprising: depositing a capping layer on the germanium region (46); and depositing a dielectric layer on a top surface and sidewalls of the capping layer.The method of claim 3, wherein the capping layer comprises silicon nitride and the dielectric layer comprises silicon oxide.The method of any preceding claim, wherein a sidewall of the MMI region comprises: a front portion having a first distance from a respective sidewall of the germanium region (46); a rear portion having a second distance from the respective sidewall of the germanium region (46), the second distance being less than the first distance; and a tapered portion connecting the front portion to the rear portion, the tapered portion having third distances from the respective sidewall of the germanium region (46), the third distances transitioning from the first distance to the second distance.The method of any preceding claim, further comprising: depositing an isolation region surrounding the silicon plate (26A) and the upper silicon region (26B), the isolation region having a first refractive index that is less than a second refractive index of the upper silicon region (26B).A method according to any preceding claim, wherein the etched silicon layer provides a plurality of terminals arranged to act as input terminals and reflection terminals to be formed.An apparatus comprising: a diode comprising: a p-type region (30); an n-type region (32); and an intrinsic region between the p-type region (30) and the n-type region (32), a germanium region (46) extending into the diode, and an input terminal (26C, 26D), wherein the diode and the germanium region (46) together form a photodetector (60) configured to convert a single mode of an input light input from the input terminal into a light having higher order modes.The device of claim 8, wherein a first portion of the p-type region (30) is located on a first side of the germanium region (46) and a second portion of the n-type region (32) is located on a second side of the germanium region (46), and the first portion and the second portion have outer edges spaced apart from respective near edges of the germanium region (46) by distances greater than about 0.4 μm.The device of claim 9, wherein the distances are between about 0.4 μm and about 1.5 μm.The method of any of the preceding claims 8 to 10, wherein the diode comprises: a silicon plate (26A); and an upper silicon region (26B) over the silicon plate (26A), wherein the germanium region (46) passes through the upper silicon region (26B), and the upper silicon region (26B) comprises: a front portion having a first width, a rear portion having a second width less than the first width, and a tapered portion connecting the front portion to the rear portion, wherein the tapered portion has widths transitioning from the first width to the second width.The device of any of claims 8 to 11, further comprising: a first dielectric layer, wherein the diode is located in the first dielectric layer; a capping layer on the germanium region (46); and a second dielectric layer on a top surface and sidewalls of the capping layer.The device of claim 12, wherein the capping layer comprises silicon nitride, and the first dielectric layer and the second dielectric layer comprise silicon oxide.The device of any of claims 8 to 13, further comprising a reflection terminal on a same side of the germanium region (46) as the input terminal.An apparatus comprising: a silicon region (26) comprising: a silicon plate (26A); an upper silicon region (26B) over and associated with the silicon plate (26A), the silicon region (26) forming a diode extending into both the silicon plate (26A) and the upper silicon region (26B); and a germanium region (46) passing through the upper silicon region (26B), the upper silicon region (26B) comprising a multimode interferometer region comprising: a first portion on a first side of the germanium region (46), a second portion on a second side of the germanium region (46) opposite the first side, and an input terminal connected to the upper silicon region (26B).The apparatus of claim 15, wherein the multimode interferometer region further comprises a free propagation region between the input terminal and the germanium region (46), and the free propagation region has a length greater than about 1 μm.The apparatus of claim 15 or 16, further comprising a reflective port connected to the same sidewall of the multimode interferometer region.The apparatus of any of claims 15 to 17, wherein the multimode interferometer region comprises: a front portion, and a rear portion narrower than the front portion.The apparatus of claim 18, wherein the multimode interferometer region further comprises a tapered portion connecting the front portion to the rear portion.The device of any of claims 15 to 19, further comprising a capping layer over and in contact with the germanium region (46), wherein the capping layer is configured to apply a stress to the germanium region (46).

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