# Patent

## Title
Germanium/silicon avalanche photodetector with separate absorption and multiplication regions

## 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.

## Background
REFERENCE TO PRIOR APPLICATION
This application is a continuation of and claims priority to U.S. application Ser. No. 11/170,556, filed Jun. 28, 2005, now U.S. Pat. No. 7,233,051.


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.



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. 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.
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.
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.
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.
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.
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
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.
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.
FIG. 1A is a diagram illustrating a cross-section view of a system 100 including plurality of avalanche photodetectors 103A, 103B, . . . 103N 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 103A, 103B, . . . 103N 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.
To illustrate, FIG. 1B shows a top view of array 101 with the plurality of avalanche photodetectors 103A, 103B, . . . 103N arranged in a two dimensional grid such that each of the plurality of avalanche photodetectors 103A, 103B, . . . 103N 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.
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.
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.
In the example illustrated in FIG. 1A, each of the plurality of avalanche photodetectors 103A, 103B, . . . 103N 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 103A. 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 103A, 103B, . . . 103N is coupled between ground and a voltage V1, V2, . . . Vn such that each avalanche photodetector is biased resulting in an electric field between layers 105 and 115 as shown.
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.
In operation, illumination 117 is incident upon layer 105 of one or more of each of the plurality of avalanche photodetectors 103A, 103B, . . . 103N. 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.
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.
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.
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.
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.
The utilization of silicon in the multiplication region 137 for an embodiment of the present invention improves sensitivity of the avalanche photodetectors 103A, 103B, . . . 103N 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 103A, 103B, . . . 103N 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:

FA(M)=kM+(1−k)(2−(1/M))  (Equation 1)

where FA is the excess noise factor and M is the gain of the avalanche photodetector.

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.
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 103A, 103B, . . . 103N 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 103A, 103B, . . . 103N 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.
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.
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.
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.
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.
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 SiO2 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.
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.
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.

## Claims

1. An apparatus, comprising:
a first region including a first intrinsic material layer different from and directly in contact with a second intrinsic material layer such that light incident on a first side of the first intrinsic material layer passes through the first side to a second side of the first intrinsic material layer opposite the fist side and enters the second intrinsic material layer, the first intrinsic material layer to absorb light in a first range of wavelengths and the second intrinsic material layer to absorb light in a second range of wavelengths, wherein the second intrinsic material layer only includes Germanium;
a second region proximate to and distinct from the first region, the second region including another intrinsic semiconductor material layer;
a first doped layer disposed between the first region and the second region;
a second doped layer disposed adjacent to the second region on an opposite side as the first doped layer;
a contact layer disposed within the first intrinsic material layer; and
doped guard rings disposed within the first intrinsic material layer and at least partially surrounding the contact layer to confine an electric field,
wherein the light enters the first region incident through the contact layer between the doped guard rings,
wherein photo-generated charge carriers that are generated in the first region and swept into the second region are multiplied in the second region when a voltage is applied across the contact layer and the second doped layer.

2. The apparatus of claim 1, wherein:
the first intrinsic material layer comprises intrinsic silicon, and
the other intrinsic semiconductor material layer comprises intrinsic silicon.

3. The apparatus of claim 2, wherein the first doped layer comprises a P doped layer and the second doped layer comprises an N doped layer.

4. The apparatus of claim 3, wherein the contact layer comprises a P doped layer through which the light passes prior to being incident upon the first intrinsic material layer.