Abstract:
A photodetector integrated within a silicon-on-insulator (SOI) structure is formed directly upon an inverse nanotaper endface coupling region to reduce polarization sensitivity at the detector&#39;s input. The photodetector may be germanium-based PN (PIN) junction photodetector, a SiGe photodetector, a metal/silicon Schottky barrier photodetector, or any other suitable silicon-based photodetector. The inverse nanotaper photodetector may also be formed as an in-line monitoring device, converting only a portion of the in-coupled optical signal and allowing for the remainder to thereafter propagate along an associated optical waveguide.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims the benefit of Provisional Application No. 60/813,936, filed Jun. 15, 2006. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention is related to a photodetector arrangement for use with silicon-based opto-electronic arrangements and, more particularly, to an SOI-based photodetector integrated with an inverse nanotaper input coupler. 
       DESCRIPTION OF THE PRIOR ART 
       [0003]    Conversion of photons to electrons is essential for the successful integration of microphotonics with microelectronics. InGaAs-based PIN photodetectors are commonly used for communication applications because of their high responsivity and speed. The majority of the InGaAs-based detectors are normal incidence detectors and, as a result, the integration of such devices on silicon surfaces is expensive and complicated. Additionally, integration of high-speed InGaAs detectors requires special optics to focus light into a small active area, which has been found to have a deleterious effect on device performance. 
         [0004]    Germanium-based area detectors are also well known in the art. Germanium detectors exhibit a higher dark current than InGaAs-based detectors, which limit their application in the communication industry. In recent years, attempts have been made to improve the performance of polycrystalline germanium-based detectors for these applications. One exemplary prior art poly-germanium detector is discussed in an article entitled “ Efficient high - speed near - infrared Ge photodetectors integrated on Si substrates ”, by L. Colace et al., appearing in Applied Physics Letters, Vol. 76, p. 1231 et seq, 2000. 
         [0005]    U.S. Pat. No. 6,897,498 issued to P. Gothoskar et al. on May 24, 2005 discloses an exemplary prior art photodetector structure for use with silicon-on-insulator (SOI)-based devices. The Gothoskar et al. reference describes a photodetector that is monolithically integrated with the relatively thin silicon surface layer (SOI layer) of the SOI structure. A poly-germanium layer is formed over a portion of the SOI layer, where the dimensions of the poly-germanium layer are used to control the amount of light that is absorbed. The bandgap of germanium (˜0.72 eV) allows for efficient absorption of near-IR wavelength light as an input optical signal propagates along an incoming SOI layer waveguide and encounters the overlying poly-germanium layer. The optical mode propagating in the silicon waveguide of the SOI structure creates electron-hole pairs when interacting with the poly-germanium region. The electron-hole pairs are collected by appropriately positioned electrodes, where the collection efficiency depends upon the separation between the electrodes and the quality of the poly-germanium material. 
         [0006]    The generation of electron-hold pairs is directly related to the absorption of light, since every absorbed photon generates one electron-hole pair. The optical generation rate g OP  is given by: 
         [0000]    
       
         
           
             
               
                 g 
                 OP 
               
               = 
               
                 ( 
                 
                   
                     α 
                      
                     
                         
                     
                      
                     
                       P 
                       
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                          
                         
                             
                         
                          
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             , 
           
         
       
     
         [0000]    where A is the illuminated area of the photodiode, P in  is the incident power, a is the absorption coefficient, h is Planck&#39;s constant, c is the velocity of light in a vacuum, and λ is the wavelength of the light signal incident on the photodetector. As an example, using a poly-germanium detector having an area A of 2 μm 2 , if an input light signal at λ=1.55 m exhibits a power of 1 μW, then the number of electron-hole pairs generated in the volume of the detector is equal to about 8×10 13  cm 3 . 
         [0007]    Assuming all electron-hole pairs generated contribute to the photocurrent, the photocurrent can be given by the following integral: 
         [0000]    
       
         
           
             
               
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                       x 
                     
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where d is the thickness of the undoped region (which depletes), q is the electron charge, and the integration is performed over the width of the depletion region. In all cases, the integral may be reduced to: 
         [0000]    
       
         
           
             
               
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         [0000]    where R is the reflection at the interface between the waveguide and the detector. A near-IR wavelength light with λ=1.55 μm, P in  of about 1 μW and α=10 3 cm −1  results in 1 μA of current for a detector having a length of approximately 10 μm. 
         [0008]    Another type of photodetector useful in SOI-based structures is the Schottky barrier photodetector, which utilizes a metal-semiconductor barrier (instead of a PN junction) to convert incident light into electrical energy. In its most conventional form, a silicon-based Schottky barrier photodiode consists of a thin metallic film (such as a silicide film) disposed on a silicon layer. US Patent Publication 2005/0110108, authored by V. Patel et al. and published on May 26, 2005 describes an exemplary SOI-based Schottky barrier photodetector, where a suicide layer is disposed over a portion of the surface SOI layer of the SOI structure. An optical signal propagating laterally along the optical waveguide within the SOI layer will pass under the silicide layer, where the “tail” of the optical energy will intercept the silicide and be converted into electrical energy. 
         [0009]    As a result of this geometry, even a relatively thin silicide layer (e.g., a few monolayers thick) will absorb a large portion of the signal over a distance of a few microns. Thus, the thickness of the absorbing silicide layer can be optimized to achieve very high internal photoemission efficiency across the Schottky barrier junction. In most cases, the Schottky barrier photodetector is operated in a reversed-biased mode so as to generate a useful electrical signal output. Operation in avalanche mode is also possible. 
         [0010]    For a conventional, prior art normal incidence Schottky barrier photodetector, the responsivity of the detector can be optimized by performing a trade-off between the gain and absorption qualities, where both the gain and absorption are dependent on the wavelength at which the photodetector is operated. The gain represents the escape (emission) probability of a “hot” hole over the Schottky barrier. When a photon with an energy greater than the Schottky barrier height is absorbed into the silicide layer by the process of free carrier absorption, the hole gains energy and becomes “hot”. The term “hot” hole refers to a hole with a finite probability of emission over the Schottky barrier. The initial energy of a “hot” hole is proportional to the absorbed photon. Due to semi-elastic scattering events and reflections form the silicide boundaries, the “hot” hole loses energy and changes direction of travel. When the energy, position and direction of travel for a “hot” hole satisfies the condition of emission over the Schottky barrier, the hole is emitted over the barrier and produces a proportional photocurrent. If, during the process of a “hot” hole becoming a “cold” hole (i.e., a hole with zero probability of travel as a result of semi-elastic collisions and reflections), the condition of emission is not satisfied, then the “hot” hole will not be emitted over the barrier and no photocurrent will be generated. 
         [0011]    A side view of a prior art SOI-based photodetector is shown in  FIG. 1 , where photodetector  1  may comprise either a PN junction-type photodetector or Schottky barrier photodetector, both types as described above. As shown, photodetector  1  is integrated with an SOI structure  2  that includes a silicon substrate  3 , a buried oxide (BOX) insulating layer  4  and a relatively thin silicon surface layer  5  (hereinafter referred to as “SOI layer  5 ”). SOI layer  5  supports the propagation of a lightwave signal, as shown by the arrows in  FIG. 1 . In operation, as the lightwave signal propagates along SOI layer  5 , it will couple into detector  1 , creating electron-hole pairs and generating a photocurrent in the manner described above. 
         [0012]    While SOI-based photodetectors of the type as shown in  FIG. 1  represent a significant advance in the state of the art, problems arise when the input lightwave signal exhibits random, unknown polarization. When defining an incoming lightwave signal by the TE (transverse electric) and TM (transverse magnetic) polarization states, the TE mode of the signal propagates parallel to SOI layer  5 , while the TM mode propagates perpendicular to SOI layer  5  (see  FIG. 1 ). The optical propagation losses for the TE and TM polarized optical signals are different for a typical SOI waveguide. In a conventional, prior art waveguide-based detector configuration, the optical signal needs to propagate through an extended portion of the SOI waveguide, as shown in  FIG. 1 . As a result, the photocurrent generated in a typical waveguide-based detector exhibits strong polarization dependence. 
         [0013]    The need remains in the art, therefore, for an SOI-based photodetector that is more efficient, in terms of generating an electrical output signal from a randomly polarized incoming optical signal. 
       SUMMARY OF THE INVENTION 
       [0014]    The need remaining in the prior art is addressed by the present invention, which relates to a photodetector arrangement for use with silicon-based opto-electronic arrangements and, more particularly, to an SOI-based photodetector integrated with an inverse nanotaper input coupler. 
         [0015]    In accordance with the present invention, an inverse nanotaper coupling tip is used in combination with a silicon-based photodetecting device to provide more efficient opto-electronic signal conversion. In one embodiment, the inverse nanotaper coupling tip is formed within the SOI surface layer, along (or near) an endface of the SOI structure. The photodetector device is directly formed on the top surface of the inverse nanotaper itself, within close proximity of the structure endface. In another embodiment, the photodetector itself is formed to exhibit the inverse nanotaper geometry and thus provides both input signal coupling and photo-electric conversion. 
         [0016]    The utilization of an inverse nanotaper coupling tip provides for efficient conversion of the incoming lightwave signal, where the dimensions of the tip are defined to capture the majority of the lightwave signal as it enters the SOI structure. Inasmuch as the photodetecting device is formed as an integral part of the nanotaper itself, the distance along which the signal must propagate prior to entering the photodetecting device is substantially reduced. 
         [0017]    The arrangement of the present invention may be used as a conventional photodetector, where most (if not all) of the incoming lightwave signal is converted into an electrical output (in one case, by truncating the inverse nanotaper coupler at the termination of the tapered region). Alternatively, the inverse nanotaper photodetector may be configured as an in-line device to convert any desired fraction of the incoming lightwave signal into an electrical signal, allowing the remaining optical signal to continue to propagate along an optical waveguide contiguous with the inverse nanotaper coupling waveguide. Various types of feedback and control arrangements may desire the ability to measure/monitor only a portion of the received light signal by electrical means, and this particular arrangement of the present invention allows for this monitoring to be implemented easily and efficiently. 
         [0018]    There are various types of silicon-based photodetecting devices that may be used in conjunction with the present invention, including a Ge-based (or SiGe-based) p-n junction detector, a metal-semiconductor junction-based Schottky barrier detector, or any other type of CMOS-compatible detector. 
         [0019]    Other and further arrangements and embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0020]    Referring now to the drawings, 
           [0021]      FIG. 1  is a side view of a prior art SOI-based photodetector arrangement; 
           [0022]      FIG. 2  is an isometric view of an exemplary inverse nanotaper photodetector formed in accordance with the present invention, in this embodiment including an extended waveguide portion beyond the nanotaper; 
           [0023]      FIG. 3  is a top view of the arrangement of  FIG. 2 ; 
           [0024]      FIG. 4  is a side view of the arrangement of  FIG. 2 ; 
           [0025]      FIG. 5  is an isometric view of an exemplary inverse nanotaper photodetector formed as a truncated inverse nanotaper to increase opto-electronic conversion efficiency; 
           [0026]      FIG. 6  illustrates one embodiment of an inverse nanotaper waveguide photodetector of the present invention, utilizing a Schottky barrier photodiode arrangement; 
           [0027]      FIG. 7  illustrates an alternative embodiment of the present invention, in this case comprising a germanium-based PN junction photodiode; 
           [0028]      FIG. 8  is an isometric view of another arrangement of the embodiment of  FIG. 7 , in this case including a thin dielectric layer between the SOI layer and the germanium photodetector layer; and 
           [0029]      FIG. 9  is an isometric view of yet another embodiment of the present invention, in this case comprising an inverse nanotaper coupler formed of photodetecting material (such as germanium or SiGe), so as to function as both an input lightwave coupler and an opto-electronic conversion element. 
       
    
    
     DETAILED DESCRIPTION  
       [0030]      FIG. 2  is an isometric view of an exemplary inverse nanotaper photodetector  10  formed in accordance with the present invention, where  FIG. 3  is a top view and  FIG. 4  is a side view of the same embodiment. As shown, inverse nanotaper photodetector  10  is disposed on an SOI structure  12 , where structure  12  includes a silicon substrate  14 , buried oxide layer  16  and a silicon surface layer  18  (hereinafter referred to as “SOI layer  18 ”). The isometric view of  FIG. 2  is a view along an endface  20  of SOI structure  12 , with the incoming lightwave signal approaching detector  10  in the manner shown. 
         [0031]    Detector  10  comprises an inverse nanotaper coupling waveguide  22 , which is formed as an etched portion of SOI layer  18 , beginning at endface  20 . Waveguide  22  is formed to exhibit a predetermined height h (which may simply be the thickness of SOI layer  18 , in most cases less than one micron) and a relatively narrow tip width w t  at waveguide endface  24  (also referred to as “tip  24 ”). Waveguide  22  includes a pair of adiabatically-tapered sidewalls  26 ,  28  that extend along a length l t  from waveguide tip  24  into a conventional rectangular waveguide portion  30 , also formed within SOI layer  18 . Waveguide portion  30  is shown as having a predetermined, conventional waveguide width w w . 
         [0032]    In accordance with this embodiment of the present invention, a photosensitive layer  32  is disposed over inverse nanotaper coupling waveguide  22  and utilized to collect the propagating optical signal and convert this signal into an electrical representation. As shown, layer  32  is formed to essentially follow the surface topology of the inverse nanotaper, expanding adiabatically from waveguide tip  24  toward rectangular waveguide  30 . The thickness and shape of layer  32  are controlled to achieve the desired low dark current, high absorption and high speed characteristics of photodetector  10 . As described above, the length l t  and associated area of layer  32  are configured to provide for a maximum amount of optical signal to be collected and converted into an output photocurrent. 
         [0033]    Moreover, it has been found that increased coupling efficiency between an incoming free space optical signal and inverse nanotaper coupling waveguide  22  can be achieved by recessing waveguide tip  24  by a few microns from endface  20  of SOI structure  12 . The views of  FIGS. 3 and 4  best illustrates this aspect, with waveguide tip  24  recessed from endface  20  a distance of x, on the order of 1-5 μm, for example. 
         [0034]    It is further possible to utilize the inverse nanotaper photodetector of the present invention as either a conventional detector where the incoming lightwave signal is essentially completely converted into an electrical signal, or as an in-line monitoring device, where only a portion of the incoming signal is converted into an electrical representation and the remaining signal propagates along a contiguous waveguide (such as waveguide  30  of  FIGS. 2-4 ).  FIG. 5  illustrates an exemplary embodiment of the present invention where a complete conversion of the incoming optical signal into an electrical form is desired. In this case, inverse nanotaper waveguide  22  is truncated at the end of the tapered region (at length l t , defining a rear vertical endface  25  of waveguide  22 . Thus, any optical signal remaining within waveguide  22  at endface  25  will be reflected and again pass under photodetecting layer  32 . 
         [0035]    As mentioned above, the inverse nanotaper photodetector of the present invention may comprise either a Schottky barrier type of photodetector, or a germanium (including SiGe) pn junction photodiode.  FIG. 6  is an isometric view of one particular embodiment of the present invention, in this case incorporating a metal-semiconductor Schottky-barrier detector  40  with inverse nanotaper coupling waveguide  22 . In this particular embodiment, a layer  42  of a silicide material (such as, for example, tungsten silicide or tantalum silicide) is formed on top surface  23  of inverse nanotaper coupling waveguide  22  so as to extend nearly to waveguide tip  24 . 
         [0036]    In practice, any one of the silicides that forms an appropriate Schottky barrier for the wavelength to be detected may be used in the formation of this structure, since the ability to form a silicide on a silicon surface (that is, on the portion of SOI layer  18  forming inverse nanotaper  22 ) is generally understood in the planar CMOS processing industry. In a typical silicide formation process, a thin metal layer is deposited on a pre-cleaned silicon surface and then reacted with the silicon at a specific temperature(s) under controlled ambient conditions to form a silicide with specific electrical properties (e.g., resistance) and physical properties (e.g., crystal structure, grain size). The unreacted metal layer is then removed using an etch process, leaving only the silicide material on the silicon surface. 
         [0037]    In accordance with the present invention, silicides based on cobalt, nickel, molybdenum, tantalum, tungsten and titanium are the most desired silicide layers for communication applications. The typical thickness for a silicide in the arrangement of the present invention is on the order of, for example, 5-30 Å. Silicide layer  42  may be formed as a single crystal (which is possible with some silicides), or as a polycrystalline material. For a polycrystalline silicide layer, scattering from grain boundaries plays a role (in association with the strip thickness) in determining the gain factor of detector  40 . In this case, the processing conditions can be controlled, using well-known means, to optimize the grain formation in the silicide. 
         [0038]    Referring back to  FIG. 6 , a first metal contact  44  is formed on silicide layer  42  and a second metal contact  46  is formed on rectangular waveguide portion  30  of SOI layer  18 . As an optical signal is coupled into tip  24  of inverse nanotaper coupling waveguide  22 , it will encounter the Schottky barrier created by the combination of SOI layer  18  and silicide layer  42 , creating an electrical output signal between contacts  44  and  46 . It is to be understood that detector  40  as shown in  FIG. 6  may be formed with or without waveguide portion  30 . That is, detector  40  may be terminated at endface  25  of waveguide  22  to allow for essentially all of the incoming lightwave signal to be converted to an electrical output signal. Alternatively, detector  40  may comprise an in-line arrangement, where only a portion of the incoming lightwave signal has the opportunity to be converted by silicide layer  42 , the remainder of the lightwave signal thereafter propagating along waveguide portion  30 . 
         [0039]      FIG. 7  is an isometric view of an alternative embodiment of the present invention, in this case utilizing a germanium-based PN junction photodetector  50 . In this case, a germanium layer  52  (which may be either single crystal or polycrystalline germanium) is disposed over inverse nanotaper coupling waveguide  22 . As with the embodiment described above, the shape and placement of layer  52  is selected to achieve low dark current, high absorption and high speed. Indeed, germanium layer  52  may be grown in a self-aligned arrangement with respect to the edge of inverse nanotaper waveguide  22 . Germanium layer  52  is doped with suitable materials to create the PN structure as shown, where boron may be used to create a p-doped region  54  and phosphorus used to create an n-doped region  56 . A p-contact  58  is formed on portion  54  and an n-contact  60  is formed on portion  56 . In this example, an incoming lightwave signal will create electron-hole pairs when interacting with the germanium region, the pairs thereafter collected by contacts  54  and  60  to create the output electrical signal. In place of utilizing pure germanium, it is to be understood that a detector utilizing SiGe may also be formed in accordance with the present invention. Although not specifically shown, the PN junction arrangement may be replaced by a PIN structure, with an “intrinsic” area formed between the P and N separate regions. 
         [0040]    Many of the conventional processes utilized to form SOI-based opto-electronic devices include the step of forming a relatively thin dielectric layer over the top surface of the SOI layer. A germanium-based photodetector of the present invention may include this relatively thin dielectric layer, as shown in the embodiment of  FIG. 8 . It is clear from this view that a relatively thin dielectric layer  70  is disposed between germanium layer  52  and nanotaper waveguide  22 . The presence of layer  70  allows for a simplification in the fabrication process (which would otherwise necessitate the removal of dielectric layer  70  from the photodetector area). 
         [0041]    In another variation of this embodiment of the present invention, inverse nanotaper waveguide  22  may be replaced with a germanium (or SiGe) inverse nanotaper element  80 , as shown in  FIG. 9 , to simultaneously provide coupling and opto-electronic conversion. In this embodiment, the incoming lightwave signal will couple into a tip portion  82  of inverse nanotaper  80  and generate a photocurrent output in the manner as described above. Again, the embodiment of  FIG. 9  may comprise only inverse nanotaper  80  (in cases where it is desired to convert essentially all of the incoming lightwave signal into an electrical output signal), or may include a waveguiding region  84  (shown in phantom in  FIG. 9 ) formed from SOI layer  18  disposed at the termination of germanium inverse nanotaper  80 . 
         [0042]    It is to be understood that the above-described embodiments of the present invention are considered to be exemplary only and should not be considered to define or limit the scope of the present invention, as defined by the claims appended hereto: