Abstract:
An avalanche photodetector is disclosed. An apparatus according to aspects of the present invention includes a mesa structure defined in a first type of semiconductor. The first type of semiconductor material includes an absorption region optically coupled to receive and absorb an optical beam. The apparatus also includes a planar region proximate to and separate from the mesa structure and defined in a second type of semiconductor material. The planar region includes a multiplication region including a p doped region adjoining an n doped region to create a high electric field in the multiplication region. The high electric field is to multiply charge carriers photo-generated in response to the absorption of the optical beam received in the mesa structure.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of invention relate generally to optical devices and, more specifically but not exclusively relate to photodetectors. 
   2. Background Information 
   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. Avalanche photodetectors provide internal electrical gain and therefore have high sensitivity suitable for very weak optical signal detection. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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. 
       FIG. 1  is a diagram illustrating an example of a cross-section view of a semi-planar avalanche photodetector with a mesa structure having an absorption region disposed over a planar region having a multiplication region in a system in accordance with the teachings of the present invention. 
       FIG. 2  is a diagram illustrating an example of a tilt view of a cross-section of a semi-planar avalanche photodetector with a mesa structure having an absorption region disposed over a planar region having a multiplication region in a system in accordance with the teachings of the present invention. 
   

   DETAILED DESCRIPTION 
   Methods and apparatuses for semi-planar avalanche photodetectors (APDs) 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. 
   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. Moreover, it is appreciated 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. 
     FIG. 1  is a diagram illustrating generally a cross-section view of a system  102  including a semi-planar avalanche photodetector (APD)  101  according to an example of the present invention. In the illustrated example, light or an optical beam  123  is directed from an optical source  139  to APD  101 . Depending on the specific application, optical beam  123  may originate from or may be reflected from optical source  139 . In one example, optical beam  123  may optionally be directed or focused from optical source  139  directly to APD  101  or may be directed through an optical element  137  to APD  101 . 
   It is appreciated that one or more APDs  101  may be used in a variety of applications and configurations. For instance, depending on the specific application, it is appreciated that APD  101  may be employed individually to for example detect a signal encoded in lower power optical beam  123  in telecommunications. In another example, APD  101  may be one of a plurality of APDs arranged in an array or grid to sense images or the like. For example, an array APD&#39;s arranged in a grid may function to sense images, similar to a complementary metal oxide semiconductor (CMOS) sensor array or the like. 
   In one example, optical element  137  may include a lens or other type of refractive or diffractive optical element such that an image is directed or focused on array of APDs  101  with illumination including optical beam  123 . Optical beam  123  may include visible light, infrared light and/or a combination of wavelengths across the visible through infrared spectrum or the like. 
   In the illustrated example, APD  101  is functionally a combination of a photodiode that converts optical signal into electrical signal and an amplifier that multiplies the detected signal with gain. As shown, APD  101  includes a mesa structure  103  including a first type of semiconductor material  111  proximate to and separated from a planar region  105  including a second type of semiconductor material  113 . As shown in the example, mesa structure  103  includes an absorption region and planar region  105  includes a separate multiplication region  109 . In the illustrated example, the first type of semiconductor material includes an intrinsic germanium region  125  and the second type of semiconductor material includes a p doped silicon region  115  adjoining an n doped silicon region  117  as shown. 
   In the example, an external bias voltage V+  135  may be applied to the APD  101  through a contact  121  coupled to the planar region  105  and a contact  122  coupled to mesa structure  103 . In one example, contact  122  is coupled to the mesa structure  103  at a p doped region of the first type of semiconductor material  127  and contact  121  is coupled to the planar region  105  at an n+ doped region of the second type of semiconductor material  119 , which help improve the ohmic contact of contacts  121  and  122  to the APD  101  in accordance with the teachings of the present invention. 
   In the example shown in  FIG. 1 , it is noted that the n+ doped region  119  is illustrated to be a region confined or centered underneath the mesa structure  103 . As will be illustrated in another example shown  FIG. 2 , it is appreciated that the n+ doped region can also be a uniform layer throughout the planar region  105 . For instance, in such an example, the n+ doped region  119  could be a highly n+ doped silicon substrate layer defined in the planar region  105  in accordance with the teachings of the present invention. 
   Referring back to the example illustrated in  FIG. 1 , it is noted that the first type of semiconductor material is shown as germanium. It is appreciated that in another example, the first type of semiconductor material may include InGaAs or another suitable type of material in accordance with the teachings of the present invention. 
   In the example shown in  FIG. 1 , APD  101  includes two regions in terms of electric field strength—one is in absorption region  107  of mesa structure  103 , in which a low electric field is created with the application of the external bias voltage V+  135  to APD  101 . The other electric field region is in the multiplication region  109  of the planar region  105 , in which a high electric field is created at the pn junction interface between the p doped silicon region  115  and the n doped silicon region  117  in accordance with the teachings of the present invention. 
   In operation, free charge carriers or electron-hole pairs are initially photo-generated in the absorption region  107  in mesa structure  103  by the incident photons of optical beam  123  if the photon energy is equal to or higher than the band gap energy of the semiconductor material (e.g. germanium or InGaAs) inside low electric field absorption region  107 . These photo-generated charge carriers are illustrated in  FIG. 1  as holes  131  and electrons  133 . 
   With the application of the external bias voltage V+  135  to APD  101  resulting in the low electric field in mesa structure  103 , the holes  131  are accelerated towards contact  122  coupled to the mesa structure  103  while the electrons  133  are accelerated towards contact  121  out from the mesa structure  103  into the planar region  105  in accordance with the teachings of the present invention. It is noted that the speed performance of APD  101  is improved by having mesa structure  103  localize the low electric field in the absorption region  107  in accordance with the teachings of the present invention. 
   Electrons  133  are separated from holes  131  as they injected as a result of the low electric field in the absorption region  107  into the high electric field in multiplication region  109  as a result of the pn junction interface between the p and n doped silicon region  115  and  117 . Impact ionization occurs as electrons  133  gain enough kinetic energy and collide with other electrons in the semiconductor material in multiplication region  109  resulting in at least a fraction of the electrons  133  becoming part of a photocurrent. A chain of such impact ionizations leads to carrier multiplication in accordance with the teachings of the present invention. Avalanche multiplication continues to occur until the electrons  133  move out of the active area of the APD  101  to contact  121 . 
   Therefore, with the low electric field absorption region  107  part of the APD  101  included in a mesa structure  103  and with the high electric field multiplication region  109  included in a planar region  105  as shown, a “semi-planar” APD  101  is realized in accordance with the teachings of the present invention. In other words, with the combination of a planar structure for planar region  105  for the silicon portion of APD  101 , and a mesa structure  103  for the germanium portion of APD  101 , a semi-planar APD  101  is realized. 
   In the illustrated example, with the combination of a planar structure of the silicon portion and a mesa structure for the germanium or InGaAs portion of APD  101 , benefits of having both planar and mesa structures may be realized in accordance with the teachings of the present invention. For example, by having the planar region  105  for the silicon, APD  101  has low dark current, increased reliability and uniform avalanche gain in accordance with the teachings of the present invention. In addition, by having the mesa structure  103  for the germanium or InGaAs, APD  101  has high speed and low crosstalk between any neighboring pixels in arrays of APDs since the low electric field is confined in the mesa structure  103  in accordance with the teachings of the present invention. 
   In addition, with a semi-planar APD  101 , where one material, such as silicon, is included in the multiplication region  109  and another material, such as germanium or InGaAs, is included in the absorption region  107  allows different processing and design techniques that can be optimized for each specific region and/or material in accordance with the teachings of the present invention. 
   For instance, in one example, germanium may be epitaxially grown using selective growth germanium on tope of the silicon of planar region  105 . Mesa structure  103  can then be etched with the etching being stopped at the silicon of planar region  105 . By etching the mesa structure  103  and stopping the etching at the silicon, a mesa structure  103  including the absorption region  107  is provided while maintaining planar region  105  with a multiplication region  109  including silicon in accordance with the teachings of the present invention. 
   Thus, in the specific example illustrated in  FIG. 1 , a germanium on silicon, or Ge—Si, APD  101  is illustrated where the germanium mesa structure  103  includes the absorption region  107 , which has low electric field; while silicon is in the multiplication agent in which high electric field is concentrated under the central p doped region  115 . In one example, due to the curvature of the central p doped region  115 , the high electric field peaks along the edge of the center p doped region  115 . 
     FIG. 1  also illustrates an optional guard ring structure  129  that may included in APD  101 , which in the example is shown as a floating guard ring having a p doped silicon region disposed in the silicon of planar region  105 .  FIG. 2  is another diagram illustrating an example of a tilt view of the cross-section of the semi-planar APD  101  shown in  FIG. 1  with mesa structure  103  having absorption region  107  disposed over planar region  105  having multiplication region  109  in accordance with the teachings of the present invention. In the example illustrated in  FIG. 2 , it is noted that the n+ doped region  119  is a uniform highly doped silicon layer throughout the planar region  105 , as mentioned previously. As shown the example illustrated in  FIG. 2 , guard ring structure  129  is a floating guard ring including p doped silicon disposed in the silicon of planar region  105  surrounding the mesa structure  103  in accordance with the teachings of the present invention. Thus, in the example, the guard ring structure  129  is at or proximate to the interface between the absorption region  107  and the multiplication region  109  of APD  101  in accordance with the teachings with the present invention. In the illustrated example, guard ring structure  129  provides the structure to help reduce or prevent premature breakdown in the multiplication region  109  at the device periphery. In one example, guard ring structure  129  may be included using ion implantation, diffusion or another suitable technique. 
   It is appreciated that a “sandwiched” guard ring structure as illustrated is made possible the semi-planar structure of the APD  101  as such a structure would not be possible with a mesa only device. In addition, it is noted that by having multiplication region  109  in a planar region  105 , sensitivity to side walls passivation, which can cause undesired leakage current due to the high electric field in the multiplication region  109  is eliminated in accordance with the teachings of the present invention. 
   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 any 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. 
   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.