Patent Publication Number: US-2020287064-A1

Title: Photodetector with integrated reflective grating structure

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims priority to Chinese Patent Application 201910171483.4, filed on Mar. 7, 2019, the entire content of which is incorporated herein by reference. 
     FIELD OF THE DISCLOSURE 
     The present application relates to the field of optical communication technology and more particularly to a photodetector with an integrated reflective grating structure. 
     BACKGROUND 
     A high-performance photodetector is one of the core devices in high-speed optical communication. A normal incident light receiving device provides a significant advantage in spatial light or low power reception, of which optical responsivity and sensitivity are important indicators. However, as demand for bandwidth increases, the RC delay needs to be reduced, or the transit time of photo-generated carriers need to be shortened. When limited optimization of the RC delay is combined with reduction in the optical absorption layer to improve the transit time, optical responsivity suffers. 
     For optical modules and optical transceivers using silicon photonic technology, in order to solve the problem of spatial optics associated with optical module packaging and to reduce polarization dependent losses (PDL), the portion of the light receiving end is, in most cases, a normal incident germanium photodetector. Spatial light that passes through an optical fiber shines on a large-area normal incident receiver, reducing losses from optical fiber coupling significantly, and increasing the sensitivity and photodetector responsivity of the receiving end. At present, the operating bandwidths of devices are mostly 25 GHz. If the speed of a normal incident photodetector is to be increased, the transit time of the photo-generated carriers must be shortened, and the thickness of the optical absorption layer must be less than 1 μm. However, a thinner optical absorption layer will cause the photodetector responsivity problem due to insufficient optical absorption efficiency. 
     Recently, an optical resonator has been used to improve optical absorption rate, primarily by adding a high reflectivity mirror in a light receiver.  FIG. 1  is a diagram of a conventional photodetector with a distributed Bragg reflector (DBR) structure. As shown in  FIG. 1 , a plurality of silicon-on-insulator (SOI) layers are repeatedly disposed on a bottom surface of an absorption layer  20 ′ to form the DBR structure. In other words, a plurality of buried insulation layers  12 ′ and top silicon layers  13 ′ are overlaid alternately on a silicon base plate  11 ′ of a substrate  10 ′. In  FIG. 1 , two SOI layers are used to form a reflective mirror that reflects approximately 70% to 80%. If greater reflection is needed, then more SOI layers—at least 3 to 4 layers—will be needed. The cost of the manufacturing process of such a structure is extremely high. Additionally, the structure is unable to completely trap incident light in the absorption layer  20 ′, and the absorption rate cannot reach 100%. 
       FIG. 2  is a diagram of a conventional photodetector with a metal reflective mirror. As shown in  FIG. 2 , a reflective mirror  30 ′ is formed by making a hole and applying a metal coating below a buried insulation layer  12 ′ on a traditional SOI substrate  10 ′. The length of a resonant cavity of an optical resonator extends to the reflective surface of the metal reflective mirror  30 ′ to include the thickness of the SOI. Due to limitations of the fabrication technique, when making a hole on the BOX (buried insulation layer  12 ′) of the SOI layer, flatness cannot be guaranteed precisely, causing an approximately 10% error in the thickness of the BOX. In other words, the length of the resonant cavity at different locations varies, resulting in an unstable absorption spectrum. Furthermore, the structure is unable to completely trap incident light in the absorption layer  20 ′, thus the absorption rate cannot reach its optimal level. 
     SUMMARY 
     One embodiment of the present disclosure provides a photodetector with an integrated reflective grating structure. The photodetector includes a substrate, an active layer disposed on the substrate, and a grating structure disposed between the substrate and the active layer. A first doped region is formed on the substrate at a location near the grating structure. A second doped region is formed on a surface of the active layer away from the grating structure. The doping type of the second doped region is different from that of the first doped region. 
     Another embodiment of the present disclosure further provides a method for manufacturing a photodetector with an integrated reflective grating structure. The method includes: providing a substrate; doping the substrate to form a first doped region and producing a grating structure above the first doped region, or, producing a grating structure on the substrate and doping the substrate below the grating structure to form a first doped region; epitaxially growing an active layer on the grating structure; doping an upper portion of the active layer to form a second doped region; and forming a PIN structure from the active layer and the first and second doped regions, the first and second doped regions having different doping types. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a conventional photodetector with a distributed Bragg reflector (DBR) structure; 
         FIG. 2  is a diagram of a conventional photodetector with a metal reflective mirror; 
         FIG. 3  is a structural diagram of a chip in a photodetector of a first example embodiment of the present disclosure; 
         FIG. 4  is a reflectance curve of a photodetector with an integrated reflective grating structure of an embodiment of the present disclosure; 
         FIG. 5  is an absorption spectrum of a photodetector with an integrated reflective grating structure of an embodiment of the present disclosure; 
         FIG. 6  is a light receiving angle range of a photodetector with an integrated reflective grating structure of an embodiment of the present disclosure; 
         FIG. 7  is a diagram of a first example of a cross section of the structure of  FIG. 3  along section A-A′; 
         FIG. 8  is a diagram of a second example of a cross section of the structure of  FIG. 3  along section A-A′; 
         FIG. 9  is a diagram of a third example of a cross section of the structure of  FIG. 3  along section A-A′; 
         FIG. 10  is a diagram of a fourth example of a cross section of the structure of  FIG. 3  along section A-A′; 
         FIG. 11  is a structural diagram of the photodetector of the first example embodiment of the present disclosure; 
         FIG. 12  is a structural diagram of a chip in a photodetector of a second example embodiment of the present disclosure; 
         FIG. 13  is a structural diagram of a chip in a photodetector of a third example embodiment of the present disclosure; 
         FIG. 14  is a structural diagram of a chip in a photodetector of a fourth example embodiment of the present disclosure; 
         FIG. 15  is a structural diagram of a chip in a photodetector of a fifth example embodiment of the present disclosure; and 
         FIG. 16  is a structural diagram of a chip in a photodetector of a sixth example embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The text below provides a detailed description of the present disclosure with reference to specific embodiments illustrated in the attached drawings. However, these embodiments do not limit the present disclosure. The scope of protection for the present disclosure covers changes made to the structure, method, or function by persons having ordinary skill in the art on the basis of these embodiments. 
     In order to facilitate the presentation of the drawings in the present disclosure, the sizes of certain structures or portions have been enlarged relative to other structures or portions. Therefore, the drawings in the present disclosure are only for the purpose of illustrating the basic structure of the subject matter of the present disclosure. 
     Additionally, terms in the text indicating relative spatial position, such as “upper,” “above,” “lower,” “below,” and so forth, are used for explanatory purposes in describing the relationship between a unit or feature depicted in a drawing with another unit or feature therein. Terms indicating relative spatial position may refer to positions other than those depicted in the drawings when a device is being used or operated. For example, if a device shown in a drawing is flipped over, a unit which is described as being positioned “below” or “under” another unit or feature will be located “above” the other unit or feature. Therefore, the illustrative term “below” may include positions both above and below. A device may be oriented in other ways (rotated 90 degrees or facing another direction), and descriptive terms that appear in the text and are related to space should be interpreted accordingly. When a component or layer is said to be “above” another part or layer or “connected to” another part or layer, it may be directly above the other part or layer or directly connected to the other part or layer, or there may be an intermediate component or layer. 
     Purposes of the present disclosure include providing a photodetector with an integrated reflective grating structure, which features both high bandwidth and high responsivity at the same time. 
     The photodetector of the present disclosure integrates a reflective grating structure between a substrate and an active layer, thus achieving a nearly total reflection effect, a broader absorption spectrum, and a greater light receiving angle. The photodetector of the present disclosure increases the bandwidth and improves optical responsivity at the same time. Specifically, embodiments of the present disclosure provide a photodetector with an integrated reflective grating structure. The photodetector includes a substrate, an active layer disposed on the substrate, and a grating structure disposed between the substrate and the active layer. A first doped region is formed on the substrate at a location near the grating structure. A second doped region is formed on the active layer away from the grating structure. The first doped region and second doped region are of different doping types, thus forming a PIN junction. The specific structure is explained using examples in the following example embodiments. 
     First Example Embodiment 
       FIG. 3  is a structural diagram illustrating a photodetector that integrates a reflective grating structure, according to a first example embodiment of the present disclosure. In the first example embodiment, a grating structure is formed on a dielectric layer to constitute a grating dielectric layer that is stacked on a substrate. Specifically, the detector includes a substrate  10 , a grating dielectric layer  30 , and an active layer  20  (that is, a light absorption layer) that are stacked sequentially. In this example embodiment, the substrate  10  includes a silicon base plate  11 . The grating dielectric layer  30  is disposed on the substrate  10 . The active layer  20  extends from the bottom of grating gaps of the grating dielectric layer  30  to above the grating dielectric layer  30  and covers the grating dielectric layer  30 . A first doped region  41  is formed by doping the upper portion of the substrate  10  near the grating dielectric layer  30  to serve as a first electrode layer. A second doped region  42  is formed by doping the upper portion of the active layer  20  away from the grating dielectric layer  30  to serve as a second electrode layer. Here, the first doped region  41  may be P-type doping, and the second doped region  42  may be N-type doping. Alternatively, the first doped region  41  may be N-type doping, and the second doped region  42  may be P-type doping. 
     In the first example embodiment, a thickness D of the portion of the active layer  20  that is higher than the grating dielectric layer  30  is in the range of 0.1λ to 1λ, λ being a central wavelength of the absorption spectrum of the photodetector. In order to further increase the detection speed of the photodetector, that is, to shorten the transit time of photo-generated carriers, the thickness D of the portion of the active layer  20  that is higher than the grating dielectric layer  30  may be set in the range of 50 nm to 1000 nm. For example, the thickness D may be 800 nm, 600 nm, 340 nm, 280 nm, etc. In the first example embodiment, a resonant cavity is formed between the upper and lower surfaces of the active layer  20 , that is, between the grating dielectric layer  30  and the upper surface of the active layer  20 , to constitute a light absorption layer. A reflective grating structure, that is, the grating dielectric layer  30 , is integrated into the bottom of the active layer  20 , and the grating structure of the integrated grating dielectric layer  30  enhances the effect of the guided mode resonance enhancement at the bottom surface of the resonant cavity. Therefore, an optical signal that enters into the active layer  20  is trapped in the active layer  20  as it propagates, and the optical signal is then absorbed by the active layer  20 , thus achieving a near 100% reflection effect and effectively increasing light absorption efficiency. In the first example embodiment, an optical signal may enter from above the active layer  20  or below the silicon base plate  11 . 
       FIG. 4  shows a light reflectance curve of a photodetector with an integrated reflective grating structure formed according to the first example embodiment.  FIG. 5  shows an absorption spectrum of the photodetector of  FIG. 4 .  FIG. 6  shows a light receiving angle range of the photodetector of  FIG. 4 . The photodetector in these figures includes the grating dielectric layer  30  made of silicon dioxide integrated at the bottom of a 450 nm-thick germanium active layer  20 . The thickness of the germanium active layer  20  is the distance between the bottom of the grating gaps and the upper surface of the active layer  20 . As can be seen from these figures, around the communication band of 1310 nm, the grating dielectric layer  30  demonstrates a near total reflection effect, a broader absorption spectrum range, which is greater than 100 nm, and a greater light receiving angle, which is approximately between −50° and 50°. The grating dielectric layer  30  can receive not only a perpendicular incident light but also an incident light that deviates within a certain angle range. As a result, the speed and the bandwidth increase, and the optical responsivity improves at the same time. In the first example embodiment, the grating dielectric layer  30  is made of silicon dioxide. The grating has a height of 250 nm, a grating period of 680 nm, and a duty cycle of 0.54. The grating structure of the grating dielectric layer  30  may be designed according to actual needs: the grating height may be adjusted in the range of 0.1λ to 1λ, the grating period may be adjusted in the range of 0.1λ to 1λ, and the duty cycle may be adjusted in the range of 0.1 to 0.9, λ being the central wavelength of the absorption spectrum of the photodetector. 
     The active layer  20  is an epitaxial germanium layer. In other example embodiments, the active layer  20  may also be one or more of the following: a silicon layer, a germanium-silicon layer, a germanium layer, or a germanium-tin layer. Here, the germanium-silicon layer is Si x Ge 1-x , x≤10%, and the germanium-tin layer is Sn x Ge 1-x , x≤10%. Similarly, in other example embodiments, the grating dielectric layer  30  may also be made of a dielectric material, such as silicon nitride, whose refractive index is lower than the refractive index of the active layer. In the first example embodiment, the grating of the grating dielectric layer  30  is a one-dimensional grating, as shown in  FIG. 7 . In other example embodiments, the grating may be a two-dimensional grating, such as a two-dimensional grating that is axisymmetric and/or centrosymmetric, as shown in  FIGS. 8 and 9 . In a two-dimensional grating, the cross section of each grating unit may be round or polygonal in shape, or each grating unit may be a column or cone of another shape. The grating may also be formed by a plurality of centrosymmetric ring-shaped grating units, as shown in  FIG. 10 . 
     At the time of design, the thickness of the active layer is first determined, usually in the range of 50 nm to 1600 nm, in accordance with the detection speed needed and the width or narrowness of the absorption spectrum. Then parameters of the grating are adjusted to obtain optimal reflectance and absorption rate. Depending on application requirements, the detector may be designed to be a wide absorption frequency band detector with an absorption range (Δλ 1 dB ) greater than 100 nm or a narrow absorption frequency band detector with an extremely narrow absorption range. 
     The photodetector with an integrated reflective grating structure may be formed using the method below, which includes the following steps: 
     providing a substrate; 
     doping the substrate to form a first doped region, and forming a grating structure above the first doped region; 
     epitaxially growing an active layer on the grating structure; 
     doping the upper portion of the active layer to form a second doped region; and 
     forming a PIN structure from the active layer and the first and second doped regions, which are of different doping types. 
     In the first example embodiment, the upper portion of the substrate is doped to form the first doped region. Next, a dielectric layer is made on the first doped region, and a grating structure is etched on the dielectric layer to form a grating dielectric layer. Then an active layer is epitaxially grown on the grating dielectric layer, and the upper portion of the active layer is doped to form a second doped region. 
     During the manufacturing process, the grating dielectric layer  30  may be made by epitaxially growing or depositing a dielectric layer on the substrate  10  and then etching the dielectric layer to form the grating structure. Next, epitaxial growth continues on the grating structure to form the active layer  20 . When epitaxially growing the active layer  20 , the grating dielectric layer  30  plays a role in epitaxial growth and defect blocking, and restricting lattice mismatch defects between the active layer  20  and substrate  10  to be within the grating gaps, thereby reducing dark current in the whole device and improving the quality of the active layer. The manufacturing process of the structure is compatible with semiconductor processing techniques. Wafers do not need to be customized, and the back of the wafers do not need separate processing. The process is simple, the technique is mature, and large quantities can be produced at relatively low cost. 
       FIG. 11  is a structural diagram of the photodetector of the first example embodiment of the present disclosure. As shown in  FIG. 11 , the active layer  20  of the photodetector is further covered by a cladding layer  60 . A first conductive channel  71  is disposed in the cladding layer  60  at the periphery of the active layer  20 . The first conductive channel  71  is electrically connected to the first doped region  41 . A second conductive channel  72  is disposed in the cladding layer  60  above the active layer  20 . The second conductive channel  72  is electrically connected to the second doped region  42 . The first conductive channel  71  and the second conductive channel  72  are connected to an external positive electrode and an external negative electrode, respectively. The structure illustrated in  FIG. 11  is an electrical structure of the photodetector of the present disclosure. Of course, electrical signals from the first doped region  41  and second doped region  42  may be led out by other means. For example, the conductive channels may be arranged on a side so that electrical signals from the first doped region  41  and second doped region  42  are led out from the side. Here, the cladding layer  60  may be a dielectric layer made of materials such as silicon dioxide or silicon nitride that have a refractive index lower than the refractive index of the active layer, so as to enable incident light to have better resonance between the upper surface of the active layer and the grating dielectric layer. 
     Second Example Embodiment 
       FIG. 12  is a structural diagram of a chip in a photodetector in a second example embodiment of the present application. As shown in  FIG. 12 , Example Embodiment 2 differs from Example Embodiment 1 in that the substrate  10  further includes a buried insulation layer  12  (BOX) and a top silicon layer  13  that are sequentially stacked on the silicon base plate  11 . In other words, the substrate  10  in the second example embodiment has an SOI (silicon-on-insulator) structure. A first doped region  41  is formed to serve as the first electrode layer by doping the upper portion of the top silicon layer  13  of the SOI layer. The first doped region  41  and the second doped region  42  on the upper portion of the active layer  20  form a PIN structure. In this example embodiment, since the substrate has an SOI structure, an optical signal enters from above the active layer and cannot enter from below the silicon base plate. 
     In each of the aforementioned example embodiments, a buffer layer may further be arranged between the grating dielectric layer  30  and the substrate  10 . Here, the substrate  10  may be the silicon base plate  11  as in the first example embodiment or may have an SOI structure as in the second example embodiment. The structure of the buffer layer may be as illustrated in the third and fourth example embodiments to be described below. 
     Third Example Embodiment 
       FIG. 13  is a structural diagram of a chip in a photodetector of a third example embodiment of the present disclosure. As shown in  FIG. 13 , in the third example embodiment, the buffer layer is an epitaxial silicon layer  50 . The first doped region  41  is disposed on the upper portion of the substrate  10  near the epitaxial silicon layer  50 . A third doped region  43  is formed by doping the upper portion of the epitaxial silicon layer  50  near the grating dielectric layer  30 . The doping type of the third doped region  43  is the same as that of the second doped region  42 . The second doped region  42 , third doped region  43 , and the first doped region  41  form a PIPIN structure or an NINIP structure. The doped regions are electrically connected to different conductive channels, respectively, to constitute an APD (avalanche photodiode, which is a type of photodetector) of the integrated reflective grating structure. 
     Fourth Example Embodiment 
       FIG. 14  is a structural diagram of a chip in a photodetector of the fourth example embodiment of the present disclosure. As shown in  FIG. 14 , the buffer layer in the fourth example embodiment is a dielectric layer  80  that is crystallized or monocrystalline. The first doped region  41  includes the dielectric layer  80 , or the first doped region  41  is disposed on the upper portion of this dielectric layer  80  near the grating dielectric layer  30 . In other words, the first doped region  41  is formed on the dielectric layer  80  at a location near the grating dielectric layer  30 . Depending on the thickness of the dielectric layer  80 , the doping depth of the first doped region  41  may reach just the upper portion of the dielectric layer  80 , may be the same as the thickness of the entire dielectric layer  80 , or may penetrate down to the upper portion of the substrate  10 . In the fourth example embodiment, the dielectric layer  80  that serve as the buffer layer may be made of a crystallized or monocrystalline material such as silicon, germanium-silicon, or germanium. It can play the role of adjusting the optical refractive index or, as an epitaxial buffer layer, play the role of improving the epitaxial growth quality. 
     Fifth Example Embodiment 
       FIG. 15  is a structural diagram of a chip in a photodetector of a fifth example embodiment of the present disclosure. As shown in  FIG. 15 , the fifth example embodiment differs from all of the aforementioned example embodiments in that the photodetector with an integrated reflective grating structure includes a substrate  10  and an active layer  20  disposed on the substrate  10 , wherein the substrate  10  includes at least one silicon layer. In this example embodiment, the silicon layer is the silicon base plate  11 , and a grating structure  31  is formed on the surface of this silicon base plate  11  near the active layer  20 . The first doped region  41  is formed on the silicon base plate  11  below this grating structure  31 , and the second doped region  42  is formed on the upper portion of the active layer  20  away from the grating structure  31 . The second doped region  42  and the first doped region  41  have different doping types and form a PIN structure. The mechanism of operation of the photodetector in the fifth example embodiment is the same as that in the first example embodiment. Therefore, no details are repeated here. 
     In the fifth example embodiment, a dielectric material, such as silicon oxide, whose refractive index is lower than that of the silicon base plate, is disposed in grating gaps  32  of the grating structure  31 . In the manufacturing process, the grating structure  31  may be formed by etching on the silicon base plate  11 . Next, a dielectric material whose refractive index is lower than that of silicon is filled or deposited in the grating gaps  32  using a semiconductor fabrication technique such as STI (shallow trench isolation). Then, the active layer  20  is epitaxially grown on the grating structure  31 . The first doped region  41  may be formed by doping the silicon base plate  11  using a method such as ion implantation before the etching of the grating structure  31 . The first doped region  41  may also be formed by doping the silicon base plate  11  after etching the grating structure  31 . 
     Sixth Example Embodiment 
       FIG. 16  is a structural diagram of a chip in a photodetector of a sixth example embodiment of the present disclosure. The photodetector shown in  FIG. 16  differs from that of the fifth example embodiment in that no dielectric material is added to the grating gaps  32  of the grating structure  31 . Instead, the active layer  20  extend from the bottom of the grating gaps  32  of the grating structure  31  to above the grating structure  31 , thereby removing the step of filling or depositing a dielectric material and simplifying the production process. 
     In the fifth and sixth example embodiments, a low doped region may further be formed on the upper portion of the silicon layer that forms the grating structure. The doping concentration of this low doped region is lower than or equal to that of the low doped region of the aforementioned first doped region, and the doping type of this low doped region is the same as that of the first doped region. Here, the doping concentration of the low doped region is in the range of 1×10 15  cm −3  to 1×10 19  cm −3 . 
     In each of the aforementioned example embodiments, an antireflection film may further be coated on the incidence plane of the active layer to reduce optical insertion loss and further improve optical responsivity. 
     Embodiments of the present disclosure provide the following benefits. The photodetector of the present disclosure integrates a reflective grating structure between a substrate and an active layer, thus achieving a nearly total reflection effect, a broader absorption spectrum, and a greater light receiving angle. The photodetector of the present disclosure increases the bandwidth and improves optical responsivity at the same time. 
     The series of detailed descriptions above is only intended to provide specific descriptions of feasible embodiments of the present disclosure. They are not to be construed as limiting the scope of protection for the present disclosure. All equivalent embodiments or changes that are not detached from the techniques of the present disclosure in essence should fall under the scope of protection of the present claims.