Abstract:
A semiconductor waveguide based optical receiver is disclosed. An apparatus according to aspects of the present invention includes an absorption region including a first type of semiconductor region proximate to a second type of semiconductor region. The first type of semiconductor is to absorb light in a first range of wavelengths and the second type of semiconductor to absorb light in a second range of wavelengths. A multiplication region is defined proximate to and separate from the absorption region. The multiplication region includes an intrinsic semiconductor region in which there is an electric field to multiply the electrons created in the absorption region.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     Embodiments of invention relate generally to optical devices and, more specifically but not exclusively relate to photodetectors.  
         [0003]     2. Background Information  
         [0004]     The need for fast and efficient optical-based technologies is increasing as Internet data traffic growth rate is overtaking voice traffic pushing the need for fiber optical communications. Transmission of multiple optical channels over the same fiber in the dense wavelength-division multiplexing (DWDM) system provides a simple way to use the unprecedented capacity (signal bandwidth) offered by fiber optics. Commonly used optical components in the system include wavelength division multiplexed (WDM) transmitters and receivers, optical filter such as diffraction gratings, thin-film filters, fiber Bragg gratings, arrayed-waveguide gratings, optical add/drop multiplexers, lasers, optical switches and photodetectors. Photodiodes may be used as photodetectors to detect light by converting incident light into an electrical signal. An electrical circuit may be coupled to the photodetector to receive the electrical signal representing the incident light. The electrical circuit may then process the electrical signal in accordance with the desired application.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.  
         [0006]      FIG. 1A  is a diagram illustrating a cross-section view of a plurality of germanium/silicon avalanche photodetectors with separate absorption and multiplication regions in a system for an embodiment of the present invention.  
         [0007]      FIG. 1B  is a diagram illustrating a top view of a plurality of germanium/silicon avalanche photodetectors with separate absorption and multiplication regions arranged in a two-dimensional array for an embodiment of the present invention.  
         [0008]      FIG. 2  is a diagram illustrating responsivity versus wavelength relationships with respect to the silicon and germanium layers of an absorption region of an avalanche photodetector for an embodiment of the present invention.  
         [0009]      FIG. 3  is a diagram illustrating an improvement in sensitivity with the use of silicon in the multiplication region of a germanium/silicon avalanche photodetector with separate absorption and multiplication regions for an embodiment of the present invention.  
         [0010]      FIG. 4A  is a diagram illustrating a cross-section view of a germanium/silicon avalanche photodetector with a resonant cavity for an embodiment of the present invention.  
         [0011]      FIG. 4B  is another diagram illustrating a cross-section view of a germanium/silicon avalanche photodetector with a resonant cavity that shows electron-hole pairs being generated for an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0012]     Methods and apparatuses for germanium/silicon avalanche photodetectors (APDs) with separate absorption and multiplication (SAM) regions are disclosed. In the following description numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention.  
         [0013]     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. In addition, it is appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale.  
         [0014]      FIG. 1A  is a diagram illustrating a cross-section view of a system  100  including plurality of avalanche photodetectors  103 A,  103 B, . . .  103 N arranged in a grid or an array  101  having one or more dimensions for an embodiment of the present invention. Illumination  117  is incident upon one or more of the plurality of avalanche photodetectors  103 A,  103 B, . . .  103 N of the array  101 . In the illustrated example, an image of an object  116  may be focused onto the array  101  through an optical element  130  with illumination  117 . Thus, array  101  may function to sense images, similar to for example a complementary metal oxide semiconductor (CMOS) sensor array or the like.  
         [0015]     To illustrate,  FIG. 1B  shows a top view of array  101  with the plurality of avalanche photodetectors  103 A,  103 B, . . .  103 N arranged in a two dimensional grid such that each of the plurality of avalanche photodetectors  103 A,  103 B, . . .  103 N function as pixels or the like for an embodiment of the present invention. The example illustrated in  FIG. 1B  shows an image  118  of object  116  using the pixels of array  101  within illumination  117 .  
         [0016]     It is noted that although  FIGS. 1A and 1B  illustrate an example application of the avalanche photodetectors being employed in a imaging system for explanation purposes, the avalanche photodetectors may be employed in other types of applications in which for example the detection of light having any of a variety of wavelengths including visible through infrared wavelengths is realized in accordance with the teachings of the presenting invention.  
         [0017]     Referring back to  FIG. 1A , optical element  131  may be a lens or other type of refractive or diffractive optical element such that the image is focused on array  101  with illumination  117 . Illumination  117  may include visible light, infrared light and/or a combination of wavelengths across the visible through infrared spectrum for an embodiment of the present invention.  
         [0018]     In the example illustrated in  FIG. 1A , each of the plurality of avalanche photodetectors  103 A,  103 B, . . .  103 N includes semiconductor material layers,  105 ,  107 ,  109 ,  111 ,  113  and  115 . A contact  131  is coupled to layer  105  and a contact  133  is coupled to layer  115 . For one embodiment, layer  105  is a p+ doped layer of silicon having a doping concentration of for example 5e19 cm −3  and a thickness of for example 100 nanometers. For one embodiment, layer  105  has a doping concentration that provides an improved electrical coupling between a contact  131  and layer  105 . For one embodiment, layers  107  and  109  are intrinsic semiconductor material regions that form an absorption region  135  of the avalanche photodetector  103 A. Layer  107  is a layer of intrinsic silicon and layer  109  is a layer of intrinsic germanium for one embodiment. Proximate to the absorption region  135  is a separate multiplication region  137 , which includes a layer  113  of intrinsic semiconductor material such as silicon. As shown in the illustrated example, layer  113  is disposed between a layer  111  of p− doped silicon and a layer  115  of n+ doped silicon. For one embodiment, layer  111  has a thickness of for example 100 nanometers and a doping concentration of for example 1-2e17 cm −3 . For one embodiment, layer  115  has a doping concentration of for example 5e19 cm −3 . In the illustrated example, each of the plurality of avalanche photodetectors  103 A,  103 B, . . .  103 N is coupled between ground and a voltage V 1 , V 2 , . . . V n  such that each avalanche photodetector is biased resulting in an electric field between layers  105  and  115  as shown.  
         [0019]     It is appreciated of course that the specific example doping concentrations, thicknesses and materials or the like that are described in this disclosure are provided for explanation purposes and that other doping concentrations, thicknesses and materials or the like may also be utilized in accordance with the teachings of the present invention.  
         [0020]     In operation, illumination  117  is incident upon layer  105  of one or more of each of the plurality of avalanche photodetectors  103 A,  103 B, . . .  103 N. Layer  105  is relatively thin such that substantially all of illumination  117  is propagated through layer  105  to layer  107  of the absorption region  135 . For one embodiment, the intrinsic silicon of layer  107  absorbs the light having wavelengths in the range of approximately 420 nanometers to approximately ˜1100 nanometers. Most of the light having wavelengths greater than approximately ˜1100 nanometers is propagated through the intrinsic silicon layer  107  into the intrinsic germanium layer  109  of the absorption region  135 . The intrinsic germanium of layer  109  absorbs that remaining light that propagates through layer  107  up to wavelengths of approximately 1600 nanometers.  
         [0021]     To illustrate,  FIG. 2  is a diagram  201  that shows example responsivity versus wavelength relationships of silicon and germanium for an embodiment of the present invention. In particular, diagram  201  shows plot  207 , which shows the responsivity of silicon with respect to wavelength, and plot  209 , which shows the responsivity of germanium with respect to wavelength. For one embodiment, plot  207  may correspond to the responsivity of the intrinsic silicon of layer  107  and plot  209  may correspond to the responsivity of the intrinsic germanium of  FIG. 1A . As shown in plot  207 , the silicon absorbs light having wavelengths as short as approximately 420 nanometers. As the wavelengths get longer, the responsivity of silicon begins to drop off due to the lower absorption of silicon at infrared wavelengths. Indeed, as the wavelength of light increases at this point, the silicon becomes increasingly transparent as the light becomes more infrared. Thus, with respect to  FIG. 1A , the longer wavelengths of illumination  117  are not absorbed in layer  107  and are instead propagate through to layer  109 . However, plot  209  shows that the germanium absorbs the longer wavelength light in layer  109  that is propagated through layer  107  up to wavelengths of approximately 1600 nanometers for an embodiment of the present invention. The silicon in layer  107  absorbs the shorter wavelengths of light less than approximately ˜1000 nanometers, while at the same wavelength range the germanium has a much larger absorption coefficient and would otherwise not generate significant photocurrent due to surface recombination in accordance with the teachings of the present invention.  
         [0022]     Therefore, referring back to  FIG. 1A , with the combination of the intrinsic silicon of layer  107  and the intrinsic germanium of layer  109  in absorption region  135 , illumination  117  is absorbed in the absorption regions  135  of the avalanche photodetectors from visible light having a wavelength of approximately 420 nanometers all the way up to longer infrared wavelengths having wavelengths up to approximately 1600 nanometers in accordance with the teachings of the present invention. This absorption of the light of illumination  117  in semiconductor layers  107  and  109  results in the generation of photocarriers or electron-hole pairs in the absorption region  135 .  
         [0023]     Due to the biasing and electric fields present in the avalanche photodetector, the holes of the electron-hole pairs generated in the absorption region  135  drift towards layer  105  and the electrons drift towards layer  115 . As the electrons drift into the multiplication region  137 , the electrons are subjected to a relatively high electric field in intrinsic silicon layer  113  resulting from the doping levels of the neighboring layers of p-doped silicon in layer  111  and n+ doped silicon in layer  115 . As a result of the high electric field in layer  113 , impact ionization occurs to the electrons that drift into the multiplication region  137  from the absorption region  135  in accordance with the teachings of the present invention. Therefore, the photocurrent created from the absorption of illumination  117  in absorption region  135  is multiplied or amplified in multiplication region  137  for an embodiment of the present invention. The photocarriers are then collected at contacts  131  and  133 . For instance holes may be collected at contact  131  and electrons are collected at contact  133 . Contacts  131  and  133  may be coupled to electrical circuitry to process the signals present at each of the contacts  131  and  133  according to embodiments of the present invention.  
         [0024]     As mentioned above, multiplication region  137  includes intrinsic silicon in layer  113  as will as silicon in neighboring p-doped and n+ doped layers  111  and  115 , respectively.  FIG. 3  is a diagram  301  illustrating an improvement in sensitivity that is realized for an embodiment of an avalanche photodetector utilizing silicon in the multiplication region  137  instead of another material, such as for example indium phosphide (InP). In particular, diagram  301  shows a relationship between a receiver sensitivity dBm versus photomultiplication gain M for various embodiments of an avalanche photodectector. In particular, plot  333  shows a receiver sensitivity versus photomultiplication gain relationship for an indium phosphide based avalanche photodetector while plot  335  shows a receiver sensitivity versus photomultiplication gain relationship for silicon based avalanche photodetector. As can be observed in  FIG. 3  by comparing plots  333  and  335 , receiver sensitivity is improved by approximately 4-5 dB by using a silicon based avalanched photodetector instead of an indium phosphide based avalanche photodetector for an embodiment of the present invention. This shows that less power is therefore needed using silicon instead of indium phosphide in multiplication region  137  to accurately detect a signal encoded in an optical signal received by an avalanche photodetector for an embodiment of the present invention.  
         [0025]     The utilization of silicon in the multiplication region  137  for an embodiment of the present invention improves sensitivity of the avalanche photodetectors  103 A,  103 B, . . .  103 N as shown in  FIGS. 1A and 1B  because of the impact ionization properties of the electrons and holes in the material. For an embodiment of the present invention, substantially only one type of carrier, in particular electrons, are able to achieve impact ionization because of the use of silicon in multiplication region  137 . This can be seen quantitatively with the k-factor, which is the ratio of impact ionization coefficients of holes to electrons. Silicon has a k-factor about one order of magnitude lower than, for example, indium phosphide. A result of the use of silicon is that substantially only electrons are selectively multiplied or amplified in multiplication region  137  instead of holes. Thus, noise and instability in the avalanche photodetectors  103 A,  103 B, . . .  103 N is reduced for an embodiment of the present invention compared to a material with a higher k-factor. An equation showing the excess noise tied to the k-factor (k) is: 
 
 F   A ( M )=k M+ (1−k)(2−(1/M))   (Equation 1) 
 
 where F A  is the excess noise factor and M is the gain of the avalanche photodetector. 
 
         [0026]     The chances of runaway resulting from the generation more than one type of carrier in multiplication region  137  is substantially reduced because substantially only electrons are able to achieve impact ionization by using silicon of multiplication region  137  for an embodiment of the present invention. To illustrate, the k-factor value of silicon for an embodiment of the present invention is less than 0.05 or approximately 0.02-0.05. In comparison, the k-factor value for other materials such as for example indium gallium arsenide (InGaAs) is approximately 0.5-0.7 while the k-factor value for germanium is approximately 0.7-1.0. Thus, the k-factor value using silicon for an embodiment of the present invention is less than other materials. Therefore, using silicon for an embodiment of an avalanche photodetector in multiplication region  137  results in improved sensitivity over avalanche photodetectors using other materials such as indium gallium arsenide or germanium or the like.  
         [0027]      FIG. 4A  is a diagram illustrating a cross-section view of a germanium/silicon avalanche photodetector  403  with a resonant cavity for an embodiment of the present invention. It is appreciated that avalanche photodetector  403  shares similarities with the examples avalanche photodetectors  103 A,  103 B, . . .  103 N shown in  FIGS. 1A and 1B  and that avalanche photodetector  403  may be used in place of any one or more of the avalanche photodetectors  103 A,  103 B, . . .  103 N in accordance with the teachings of the present invention. Referring back to the example shown in  FIG. 4A , avalanche photodetector  403  includes layers,  405 ,  407 ,  409 ,  411 ,  413  and  415 . In the example illustrated in  FIG. 4A , avalanche photodetector  403  is disposed on a silicon-on-insulator (SOI) wafer, and therefore, avalanche photodetector also includes a silicon substrate layer  419  and a reflective layer, which is illustrated in  FIG. 4A  as a buried oxide layer  425 . For one embodiment, avalanche photodetector  403  also includes guard rings  421 , which are disposed at the surface and into layer  407  on opposing sides of layer  405  at the surface of layer  407  as shown in  FIG. 4A .  
         [0028]     For one embodiment, layer  405  and guard rings  421  are p+ doped silicon having a doping concentration that provides an improved electrical coupling between a contact coupled to layer  405  and layer  407 . For one embodiment, guard rings  421  are disposed proximate to layer  405  as shown in  FIG. 4A  to help prevent or reduce electric field from extending to or past the edges of avalanche photodetector  403 . By helping to isolate or confine the electric field within the structure of avalanche photodetector  403 , guard rings  431  help to reduce leakage current from the avalanche photodetector  403  structure in accordance with the teachings of the present invention.  
         [0029]     For one embodiment, layers  407  and  409  form an absorption region  435  of the avalanche photodetector  403 . Layer  407  is a layer of intrinsic silicon and layer  409  is a layer of intrinsic germanium for one embodiment. Proximate to the absorption region  435  is a separate multiplication region  437 , which includes a layer  413  of intrinsic silicon. As shown in the depicted example, layer  413  is disposed between a layer  411  of p− doped silicon and a layer  415  of n+ doped silicon. For one embodiment, layers  411  and  415  having doping concentrations that result in a high electric field in layer  413  of multiplication region  437 . For example, layer  411  has doping concentration of for example 1-2e17 cm −3  and layer  415  has a doping concentration of for example 5e19 cm −3  for one embodiment. In addition, a lower electric field is also present between layer  405  and layer  415  for an embodiment of the present invention.  
         [0030]     In operation, as shown in  FIG. 4A , illumination  417  is directed to avalanche photodetector  403  and is incident upon a surface of avalanche photodetector  403 . In the example illustrated in  FIG. 4A , illumination  417  is directed through free space and is incident upon a surface of layer  405 . The light from illumination  417  is absorbed in absorption region  435  and electrons from the photocurrent or electron-hole pairs generated in absorption region  435  are multiplied in multiplication region  437  as a result of impact ionization in accordance with the teachings of the present invention. For one embodiment, a resonant cavity is also defined in avalanche photodetector  403  between buried oxide layer  425  and the surface of avalanche photodetector  403  on which the light of illumination  417  is incident. As a result, the light illumination  417  circulates in the resonant cavity between buried oxide layer  425  and the surface of the avalanche photodetector as shown in  FIG. 4A  as shown.  
         [0031]      FIG. 4B  is another diagram illustrating increased detail of a cross-section view of avalanche photodetector  403  with a resonant cavity that shows electron-hole pairs being generated for an embodiment of the present invention. In particular,  FIG. 4B  shows illumination  417  incident on the surface of layer  405  of avalanche photodetector  403 . As illumination propagates through layers  407  and  409  of the absorption region  435 , the light is absorbed, which generates photocurrent or electron-hole pairs including electron  427  and hole  429 . With the electric field between p+ doped layer  405  and n+ doped layer  415 , electrons  427  drift from absorption region  435  into multiplication region  437 . With the high electric field present in layer  413  of multiplication region  437 , impact ionization occurs with the electrons  427 , which generates additional electron-hole pairs and therefore results in the multiplication or amplification of the photocurrent generated in absorption region  435 . The holes  429  and electrons  427  are then collected by contacts that are coupled to layers  405  and  415  for an embodiment of the present invention.  
         [0032]     As further illustrated, light from illumination  417  that is not absorbed in the first pass through avalanche photodetector  403  is reflected from buried oxide layer  425 , illustrated as SiO 2  in  FIG. 4B , and is recirculated back and forth through avalanche photodetector  403  as shown. As a result, the light from illumination  417  is recycled within the absorption region  435  and multiplication region  437 , thereby increasing the probability of absorption of illumination  417  and improving the performance of avalanche photodetector  403  in accordance with the teachings of the present invention.  
         [0033]     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to be limitation to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent refinements and modifications are possible, as those skilled in the relevant art will recognize. Indeed, it is appreciated that the specific wavelengths, dimensions, materials, times, voltages, power range values, etc., are provided for explanation purposes and that other values may also be employed in other embodiments in accordance with the teachings of the present invention.  
         [0034]     These modifications can be made to embodiments of the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.