Universal broadband photodetector design and manufacturing method
阅读说明：本技术 通用宽带光检测器设计和制造方法 (Universal broadband photodetector design and manufacturing method ) 是由 E·Y·禅 于 2019-07-25 设计创作，主要内容包括：本申请涉及通用宽带光检测器设计和制造方法。一种宽光谱带宽光检测器,该检测器设计用于在不同的航空电子网络和传感器中使用的所有类型的光纤,以及用于制造这种光检测器的方法。提供了肖特基势垒光检测器,该检测器包括锗,其对在紫外至近红外范围(220至1600nm)内的光具有宽的光谱响应。提供具有宽光谱响应的光检测器避免了在具有不同光纤网络和传感器的航空电子平台中使用多种不同类型的光检测器和接收器。(The present application relates to a universal broadband photodetector design and fabrication method. A wide spectral bandwidth photodetector designed for all types of optical fibers used in different avionics networks and sensors, and a method for manufacturing such a photodetector. A schottky barrier photodetector is provided that includes germanium having a broad spectral response to light in the ultraviolet to near infrared range (220 to 1600 nm). Providing a photodetector with a broad spectral response avoids the use of multiple different types of photodetectors and receivers in avionics platforms with different fiber optic networks and sensors.)
1. A light detector, comprising:
a substrate made of doped germanium;
a mesa structure made of doped epitaxial germanium grown on top of the substrate, wherein a doping density of the doped epitaxial germanium is less than a doping density of the doped germanium;
a metal film made of metal deposited on top of the mesa structure;
a metal pad made of metal deposited on top of the mesa structure and in contact with the film; and
an ohmic contact layer made of metal deposited on the bottom of the substrate.
2. The photodetector of claim 1, wherein the metal film and metal pad are made of a metal selected from the group consisting of gold, silver, aluminum, copper, and indium.
3. The photodetector of claim 2, wherein the metal film and the mesa structure form a junction having a schottky barrier height in the range of 0.54 to 0.64 volts.
4. The photodetector of claim 1, further comprising a voltage source connected to the metal pad and the ohmic contact layer, wherein the metal film and the mesa structure are configured to create a depletion region in and adjacent to the metal film, the depletion region having a width that increases when a reverse bias voltage is applied by the voltage source during photons impinging on the metal film.
5. The photodetector of claim 4, wherein the metal film has a diameter in the range of 450 to 600 microns.
6. The optical detector of claims 4-5, wherein the effective Richardson constant is between 128 and 135A/cm2-°K2Within the range of (1).
7. The photodetector of claim 1, further comprising an anti-reflective coating deposited on the film.
8. The photodetector of claim 1, further comprising a dielectric passivation layer covering exposed surfaces of the mesa structures.
9. An optical fiber device, comprising:
an optical fiber having a distal end;
a hermetic metal housing comprising a base, a lens cover connected to the base, and a glass ball lens mounted in an opening in the top of the lens cover; and
a photodetector disposed inside the metal housing, the photodetector comprising a substrate made of doped germanium, a mesa structure made of low-doped epitaxial germanium grown on top of the substrate, the doped epitaxial germanium having a doping density much less than the doping density of the doped germanium, a metal film made of metal deposited on top of the mesa structure, a metal pad made of metal deposited on top of the mesa structure and in contact with the film, and an ohmic contact layer made of metal deposited on the bottom of the substrate,
wherein the optical fiber, the glass ball lens and the metal film are aligned.
10. The optical fiber device of claim 9, wherein the metal film and the metal pad are made of a metal selected from the group consisting of gold, silver, aluminum, copper, and indium,
wherein the metal film and the mesa structure form a junction having a Schottky barrier height in a range of 0.54 to 0.64 volts,
wherein the metal film and the mesa structure are configured to create a depletion region in and adjacent to the metal film, the depletion region having a width that increases when a reverse bias voltage is applied across the metal pad and ohmic contact metal layer during photons impinging on the metal film,
wherein the metal film has a diameter in the range of 450 to 600 micrometers and the glass ball lens has a diameter of 2mm,
wherein the photodetector further comprises a dielectric passivation layer covering the exposed surfaces of the mesa structures.
Technical Field
The technology disclosed herein relates generally to fiber optic networks that enable communication between electrical components. Compared with copper networks, optical fiber networks have the advantages of higher speed, lower weight and resistance to electromagnetic interference. Many models of commercial aircraft have fiber optic networks in order to reduce size, weight, and power.
Background
Optical networks using Plastic Optical Fiber (POF) have advantages over copper wire in weight, size, bandwidth, power, and immunity to electromagnetic interference. Plastic optical fibers have advantages over Glass Optical Fibers (GOFs) in terms of ease of handling, installation and maintenance. The use of POF can result in significant weight savings. Weight savings may be important for networks on vehicles, such as aircraft, where weight savings may result in reduced fuel consumption and reduced emissions.
Conventional avionics systems using GOF networks will also be used in many old and new models of commercial aircraft before the POF networks can completely replace them. The glass fiber has the lowest loss at a wavelength of 1550 nm. The glass fiber also has the lowest dispersion at 1300 nm. Short-range multimode glass fiber networks also use 850nm wavelengths because widely available, mature, and low cost VCSELs operate at 850nm wavelengths. Thus, some conventional avionic GOF networks operate at 850nm, 1300nm, and 1550 nm. These conventional GOF networks will co-exist with POF networks in many aircraft systems.
The existing solution is to use different light detectors for different fiber optic networks or sensors. For example, for low data rate POF networks (e.g., Controller Area Network (CAN) bus) using POFs made of poly (methyl methacrylate), silicon photodetectors are used for receiver operation in the visible wavelength range (450, 550, and 650 nm). For high-speed gigabit POF networks (e.g., ARINC 664, gigabit ethernet), InGaAs photodetectors are used for receiver operation in the 1200 to 1550nm wavelength range. For GOF networks, GaAs photodetectors are used for wavelengths of 850nm, and InGaAs photodetectors are used for wavelength networks of 1300nm or 1550 nm. The disadvantages of the above-described solutions are manifold.
First, in aircraft systems with different fiber optic networks and sensors, different receivers are required. The supply, storage and installation of components of multiple receivers increases the production costs of the aircraft avionics system.
Second, there is currently no commercial off-the-shelf (COTS) photodetector with broadband spectral characteristics that meets the wide spectral range and high quantum efficiency requirements of all fiber optic (POF and GOF) networks in future commercial aircraft avionics systems. For example, some COTS receivers have a photodetector size that is not optimized for connection with large diameter POFs. As a result, POF networks with mismatched photodetector sizes have lower receiver sensitivity, which in turn provides lower optical link margin and shorter link distance.
Third, some COTS POF receivers have a photodetector structure that causes the receiver to generate additional optical pulses below the desired optical power level. This problem is referred to herein as "arbitrary pulsing" (APP). Due to the APP problem, the dynamic range of the fiber optic network is reduced.
Disclosure of Invention
The techniques disclosed in some of the following details were designed to alleviate the above-mentioned problems. The very broad spectral bandwidth photodetector design and fabrication methods for all fiber types of different avionics networks and sensors are disclosed in particular detail below. The solution presented herein provides a schottky barrier photodetector comprising germanium (hereinafter referred to as a "germanium schottky barrier photodetector") that has a broad spectral response to light in the ultraviolet to near infrared range (220 to 1600 nm). Providing a photodetector with a broad spectral response avoids the use of multiple different types of photodetectors and receivers in avionics platforms with different fiber optic networks and sensors.
Germanium has a broad spectral response covering a wavelength range of 220 to 1600 nm. This spectral range spans all the lowest loss wavelengths of all types of POF and GOF networks or sensors, including low data rate POFs, gbpofs, single mode GOFs, and multi-mode GOF networks. In addition, the structure of the germanium schottky barrier photodetectors disclosed herein is designed to maximize the quantum efficiency of the photodetector. The schottky barrier photodetector structure according to some embodiments also includes a thick active layer and optimized photodetector size and depletion width to eliminate the APP problem created by the photodetector in some COTS receivers. The schottky barrier photodetectors disclosed herein have low fabrication costs compared to silicon and InGaAs photodetectors.
According to some embodiments, a germanium schottky barrier photodetector is designed with the following features: (a) metal layer thickness and epitaxial layer thickness optimized for different types of fiber optic networks and sensors; (b) low background doping concentration in the epitaxial layer; (c) optimizing detection areas of all avionic optical fiber networks; (d) mesa structures with dielectric passivation layers to reduce dark current, which is a major source of shot noise in fiber optic receivers. By reducing shot noise, receiver sensitivity is enhanced. Germanium schottky barrier photodetectors may also be used in non-aerospace telecommunications networks.
Although various embodiments of germanium schottky barrier photodetectors and methods of fabricating such photodetectors will be described in particular detail below, one or more of these embodiments may be characterized by one or more of the following aspects.
One aspect of the following detailed disclosed subject matter is a light detector, the detector comprising: a substrate made of doped germanium; a mesa structure made of doped epitaxial germanium grown on top of the substrate, wherein a doping density of the doped epitaxial germanium is less than a doping density of the doped germanium; a metal film made of a metal deposited on top of the mesa structure; a metal pad made of a metal deposited on top of the mesa structure and in contact with the film; an ohmic contact layer made of metal deposited on the bottom of the substrate; an anti-reflective coating deposited on the film; and a dielectric passivation layer covering the exposed surface of the mesa structure. The metal film and the metal pad are made of a metal selected from gold, silver, aluminum, copper, and indium. The metal film and the mesa structure are configured as a depletion region formed in the mesa structure and adjacent to the metal film. During the photons strike the metal film, all photons that penetrate the epitaxial layer are absorbed inside the depletion region when a suitable reverse bias voltage is applied by the voltage source.
Another aspect of the following detailed disclosed subject matter is a fiber optic device comprising: an optical fiber having a distal end; a metal case including a base, a lens cover connected to the base, and a glass ball lens mounted in an opening at the top of the lens cover; and a photodetector disposed inside the metal housing, the photodetector comprising a substrate made of doped germanium, a mesa structure made of doped epitaxial germanium grown on top of the substrate, the doped epitaxial germanium having a doping density less than that of the doped germanium substrate, a metal film made of a metal deposited on top of the mesa structure, a metal pad made of a metal deposited on top of the mesa structure and in contact with the film, and an ohmic contact layer made of a metal deposited on the bottom of the substrate. The optical fiber, the glass ball lens and the metal film are aligned.
Further aspects of the following detailed disclosed subject matter are methods for fabricating a photodetector, the method comprising: polishing and lapping (lap) the doped germanium wafer until a germanium substrate having a thickness in the range of 100 to 150 microns is formed; growing a doped germanium epitaxial layer about 15 microns thick on top of a germanium substrate; depositing an ohmic contact metal layer on the bottom of the germanium substrate; depositing a metal film and a metal pad on top of the germanium epitaxial layer such that the metal pad is in contact with the metal film; forming a mesa structure by removing some of the germanium epitaxial layer; depositing a dielectric passivation layer on the exposed surface of the mesa structure; and depositing an anti-reflective coating on top of the metal film.
Other aspects of germanium schottky barrier photodetectors and methods of fabricating such photodetectors are disclosed below.
Drawings
The features, functions, and advantages that are discussed in the foregoing sections can be achieved independently in various embodiments or may be combined in yet other embodiments. For the purpose of illustrating the above and other aspects, various embodiments will be described hereinafter with reference to the accompanying drawings. The figures described briefly in this section are not drawn to scale.
Fig. 1 is a graph showing attenuation versus wavelength for low data rate POFs, gbpofs, and GOFs.
Fig. 2 is a graph showing absorption coefficient versus wavelength for different semiconductor materials.
Fig. 3 is a diagram showing the structure of a germanium schottky barrier photodetector according to one embodiment.
Fig. 4A to 4E are diagrams representing steps in a method of fabricating a germanium schottky barrier photodetector of the type described in fig. 3.
Fig. 5 is a diagram showing a top view of a metal film and metal pads deposited on top of an epitaxial layer, according to one embodiment.
Fig. 6 is a graph showing the principle of photocurrent generation in a germanium schottky barrier photodetector of the type depicted in fig. 3, electrons are represented by filled circles (●) and holes are represented by open circles (○).
Fig. 7 is a graph showing the principle of depletion width optimization in the germanium schottky barrier photodetector depicted in fig. 6, electrons are represented by filled circles (●) and holes are represented by open circles (○).
FIG. 8A is a graph showing the change in optical power received from an optical fiber by a typical photodetector over time.
FIG. 8B is a graph showing a series of photo-generated current pulses over time produced by a typical photo-detector if all photons are absorbed in the depletion region.
Fig. 8C is a graph showing a series of photo-generated current pulses over time produced by a typical photo-detector if a photon is absorbed outside the depletion region, in which case the response to the light pulse may have a diffuse tail.
Fig. 9 includes upper and lower graphs showing the variation in photo-generated current pulse and receiver current pulse responses (including additional pulses) over time, respectively.
Fig. 10 is a diagram showing a ge schottky barrier photodetector mounted in a hermetically sealed package with a glass ball lens using a two pin configuration.
Fig. 11 is a diagram showing a ge schottky barrier photodetector mounted in a hermetically sealed package with a glass ball lens using a three pin configuration.
Fig. 12 is a diagram showing some components and identifying some features of an optical data transmission system including a germanium schottky barrier photodetector of the type disclosed herein.
In the following, reference will be made to the drawings wherein like elements in different drawings bear the same reference numerals.
Detailed Description
Illustrative embodiments of germanium schottky barrier photodetectors are described in particular detail below. Not all features of an actual implementation are described in this specification, however. Those skilled in the art will appreciate that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.
In the future, there will be different POF fibers for commercial aircraft avionics networks and sensor systems. For low data rate networks, such as CAN bus, ARINC 629 data bus or fuel quality indicator sensor, the low data rate POF is a large diameter (1mm) fiber made from PMMA, but for high data rate networks GbPOF will be used. GbPOF is a perfluorinated polymer with a cladding of 500 microns and a core diameter of 55 microns. The low data rate POF and the high data rate GbPOF have different operating wavelengths because PMMA and perfluorinated polymers have different optical loss characteristics. Fig. 1 shows the attenuation versus wavelength for low data rate POFs, gbpofs, and GOFs. These differences in the minimum attenuation wavelength in such spectral characteristics have in the past required the use of different receivers for low data rate POF, GbPOF and GOF networks.
Conventional (legacy) GOF networks and POF networks are expected to coexist in many aircraft avionics systems. GOF networks typically have operating wavelengths of 850, 1300, and 1550 nm. Currently, there are no photodetectors with a wide spectral bandwidth for all low data rate POF, GbPOF, and GOF networks. For aircraft systems with low data rate POF, GbPOF, and GOF networks, these networks are large-scale networks that use a large number of receivers. The use of different receivers in an aircraft network can increase installation and part maintenance costs for aircraft production. A generic photodetector design with a broad spectral optical response would enable all low data rate POF, GbPOF and GOF networks and sensors to use the same receiver, greatly reducing installation and maintenance costs in aircraft production, particularly in large network-scale commercial aircraft with hundreds of network nodes.
The optical response of a fiber optic photodetector depends on the material selection of the photodetector. For GOF networks, the photodetectors are typically made of InGaAs III-V composite materials. Such materials are also feasible for fabricating optical detectors for GbPOF networks. The photodetector surface area of COTS InGaAs photodetectors for GOF networks is very small, designed to match the small diameter of single-mode or multi-mode GOFs. For example, single mode GOFs have a core diameter of about 8 microns, while multi-mode GOFs have typical diameters of 50 and 62.5 microns. Typically the GOF photodetector surface diameter does not exceed 75 microns. Although these small diameter InGaAs photodetectors may be suitable for GbPOF networks because gbpofs have diameters of about 55 or 85 microns, which is very close to the diameter of multi-mode GOFs, they are too small for low data rate POFs having a core diameter of about 1 millimeter. In addition to the diameter constraint, the low data rate POF operates in the visible wavelength range where InGaA photodetectors do not have good optical response. Commercial off-the-shelf photodetectors for low data rate POF networks are made of silicon, which does not have a good optical response for the near infrared wavelength range in which GOFs and gbpofs operate.
Fig. 2 shows the optical absorption coefficient versus wavelength for silicon (Si), germanium (Ge) and InGaAs materials. The graph shows that Si and InGaAs cannot cover the wavelength range of 0.4 microns (400nm) to 1.55 microns (1550nm) over which low data rate POF, GbPOF and GOF systems operate. Similar problems exist for indium phosphide (InP) and gallium arsenide (GaAs), both of which do not have good broadband optical absorption coefficients in the same wavelength range. But germanium has a good absorption coefficient covering wavelengths of 400 to 1550 nm.
Based on the above analysis, germanium was chosen as the semiconductor material for broadband photodetectors designed for low data rate POF, GbPOF and GOF avionics networks and sensors. More specifically, germanium schottky barrier photodetectors have optical detection diameters optimized for all fiber optic networks and sensors, including low data rate POFs, gbpofs, and GOF avionics networks and sensors.
The structure of a germanium schottky barrier photodetector 40 according to one embodiment is shown in fig. 3. Germanium schottky barrier photodetector 40 includes a detector having n+A type-doped germanium substrate 42 and a mesa structure 44 (active layer) consisting of an epitaxial overlayer of crystalline germanium with n-type doping. As used herein, the term "mesa structure" refers toA structure having a flat top (upper surface) and a peripheral side surface intersecting the flat top along a peripheral edge having a closed contour.
Germanium schottky barrier photodetector 40 further includes a metal film 46 and a metal pad 48 deposited in respective continuous regions on the upper surface of mesa structure 44. As will be described in more detail below, the thickness and area of metal film 46 and metal pad 48 are different. An anti-reflective coating 54 is deposited on the metal film 46. A dielectric passivation layer 52 is deposited so as to cover the exposed surfaces on the periphery of mesa 44. The light detecting surface area provided by the metal film 46 is covered by the anti-reflective coating 54 and not by the dielectric passivation layer 52.
Many different metals may be used to fabricate germanium schottky barrier photodetector 40. Based on the theory of device physics, all metal contacts to germanium will make a photodiode. Gold is preferred because it has better environmental durability (not subject to corrosion from humidity, salt and fog conditions), because gold is a better material in the device fabrication process because it is easier to deposit than other metals, and is better for wire bonding in terms of packaging. However, based on theory, silver, aluminum, copper, and indium are good candidates to replace gold. All calculations discussed below assume that metal film 46 and metal pad 48 are made of gold.
Referring again to fig. 3, germanium schottky barrier photodetector 40 further includes an ohmic contact metal layer 50 deposited on the bottom of germanium substrate 42. An ohmic contact is a non-rectifying electrical junction between two conductors that theoretically has a linear current-voltage curve according to ohm's law. Low resistance ohmic contacts are used to allow charge to flow easily in both directions between two conductors without blocking due to rectification or excessive power dissipation due to voltage thresholds. In contrast, a schottky barrier is a rectifying junction or rectifying contact that does not have a linear current-voltage curve. As used herein, the term "ohmic contact" refers to an ohmic contact of a metal to a semiconductor.
Fig. 3 also shows a depletion region 56 that may be formed in the active layer below the metal film 46. When a voltage is applied across metal pad 48 and ohmic contact metal layer 50, the width of the depletion region changes. As used herein, the term "depletion region" refers to an insulating region within a doped semiconductor material where most of the charge carriers have diffused away to form a region with a high electric field.
Some features and characteristics of the germanium schottky barrier photodetector 40 depicted in fig. 3 will now be explained first with reference to the fabrication method partially described in fig. 4A-4E, and then with reference to the physical characteristics (physics) of photodetector operation partially described in fig. 6 and 7. The elimination of APP will be discussed later with reference to fig. 8A-8C and 9. Finally, the packaging of the germanium schottky barrier photodetector 40 for maximum fiber coupling efficiency will be described with reference to fig. 10 and 11.
Some steps of a manufacturing method according to one embodiment are shown in fig. 4A to 4E. As shown in FIG. 4A, the first step involves polishing and lapping highly doped (n)+Type) germanium wafer until a germanium substrate 42 having a thickness in the range of 100 to 150 microns is formed, ensuring that the top and bottom surfaces of the germanium substrate 42 are clean, smooth, and shiny.
The next step involves growing a lightly doped (n) substrate about 15 microns thick on top of the heavily doped germanium substrate 40-Type) germanium epitaxial layer 45 (as shown in fig. 4B). This can be done by vapor phase epitaxy or metal organic chemical vapor deposition methods. Germanium epitaxial layer 45 is the primary light absorbing layer of germanium schottky barrier photodetector 40. Therefore, appropriate layer thicknesses and high quality epitaxial growth processes are desirable for the photodetector designs disclosed herein. It will be understood from the foregoing use of the terms "highly doped" versus "lowly doped" that the doping density of the germanium epitaxial layer 45 is much less than the doping density of the germanium substrate 42.
Another step of the fabrication method involves depositing an ohmic contact metal layer 50 on the bottom of the germanium substrate 42 (see fig. 4B). The ohmic contact metal layer 50 is preferably made of indium or other suitable metal contact, which has a thickness in excess of 1 micron.
After the germanium epitaxial layer 45 is deposited on the germanium substrate 42, a metal film 46 and a metal pad 48 are deposited on top of the germanium epitaxial layer 45. Figure 4C is a diagram showing a side view of a metal semiconductor structure resulting from the deposition of a metal, preferably gold, on the germanium epitaxial layer 45.
As can be seen in fig. 5 (not drawn to scale), the top surface of metal film 46 has an area a, while the top surface of metal pad 48 is smaller than area a. According to one example construction of germanium schottky barrier photodetector 40 in which metal film 46 and metal pad 48 are made of gold, metal film 46 may have a diameter in the range of 450 to 600 microns and a thickness of about 50 angstroms. The metal film 46 forms a rectifying junction for the photodetector. The metal film 46 is a very thin layer to maximize coupling of light into the active region of the photodetector. Metal pad 48 is deposited by an electroplating process, which in the case of gold is about 25 microns in diameter and about 40 microns thick. As shown in fig. 5, metal pad 48 is offset from the edge of metal film 46 such that a majority of the area of metal film 46 is exposed to the optical signal from the optical fiber. Metal pad 48 is a thicker layer for wire bonding; the metal pad 48 is also in electrical contact with the metal film 46 for conducting the photo-generated current.
After depositing metal on the top surface of the germanium epitaxial layer 45, a mesa structure 44 (shown in fig. 4D) is formed by removing some of the germanium epitaxial layer 45 (shown in fig. 4C). According to one embodiment, the outer peripheral surface of mesa structure 44 has a concave curved profile, as shown in fig. 4D. For example, the geometry of the peripheral surface of the mesa 44 may be similar to a surface of rotation that would be produced by rotating an arc of a circle in three-dimensional space about a vertical axis at the center of the detector. Alternatively, the outer periphery of mesa 44 may be conical. Preferably, mesa structure 44 has a top surface with a boundary that extends beyond a boundary of a metal structure formed by metal film 46 and metal pad 48 (shown in fig. 5). For example, mesa structure 44 may have a circular top surface with a diameter D of about 600 microns on which metal film 46 and metal pad 48 are located. Mesa structure 44 may be formed by a wet chemical etch process or a plasma etch process. The purpose of the mesa 44 is to reduce reverse bias dark (or leakage) current of the photodetector.
After the mesa structures 44 are formed, an anti-reflective coating 50 is deposited on top of the metal film 46 (see fig. 4E) to reduce reflection of the optical signal from the optical fiber (not shown in fig. 4A-4E, but see the optical fiber 64 in fig. 6). The anti-reflective coating 50 may be made of silicon nitride formed by plasma deposition or high vacuum evaporation or sputtering processes. The thickness of the anti-reflective coating 50 is designed to maximize the optical coupling from the optical fiber to the active region of the photodetector. The active region is the region inside the mesa 44 directly below the metal-germanium (e.g., Au-Ge) junction.
In addition, a passivation layer 52 is deposited on the exposed surfaces of the mesa structures 44 to reduce dark (or leakage) current of the photodetector. This layer also provides physical and environmental protection for the active layer of the photodetector. Fig. 4E is a diagram representing a side view of the metal-semiconductor structure resulting from the deposition of the anti-reflective coating 50 and passivation layer 52.
The germanium schottky barrier photodetector 40 described above may be fabricated using a photolithographic process. After the fabrication method is completed, a voltage may be applied across the terminals formed by metal pad 48 and ohmic contact metal layer 50 to increase the width of depletion region 56 under the metal-germanium (e.g., Au-Ge) junction.
The physical characteristics of a germanium schottky barrier photodetector will now be briefly described with further reference to fig. 6. As shown in fig. 6, when metal film 46 is deposited on top of the epitaxial layer with mesa 44, a depletion region 56 of the photodetector is formed. The depletion region 56 is where light from the fiber 64 is absorbed and electron-hole pairs are generated. When a circuit is formed between the metal pad 48 and the ohmic contact metal layer 50, the flow directions of the electron-hole pairs are opposite to each other.
More specifically, reverse bias voltage V is applied by electrically connecting the negative terminal of voltage source 62 to metal pad 48 via electrical conductor 58 and the positive terminal of voltage source 62 to ohmic contact metal layer 50 via electrical conductor 601. As a result, a photocurrent I is generated on the external circuit formed by electrical conductors 58 and 60ph(indicated by the horizontal arrows in fig. 6).
A key technical requirement for a good photodetector is to maximize the quantum efficiency of the photocurrent generation process. Quantum efficiency is expressed as the electron generated for each incident photon and is a measure for designing a photodetector with the ability to capture incident photons with minimal loss.
As shown in fig. 6, when a metal contact (or film) is deposited on top of the Ge epilayer, a depletion region 56 is formed in the portion of mesa structure 44 directly below metal film 46. The charge carriers on both sides of the metal germanium (e.g., Au-Ge) rectifying junction create a so-called built-in electric field. The width W (measured in the thickness direction) of depletion region 56 depends on the doping concentration N of the epitaxial germanium forming mesa 44dAnd an external bias voltage V of the photodetector1. The lower doping concentration and higher reverse bias voltage increase the depletion width of the photodetector. A lower doping density means a higher purity of the epitaxial layer. Ideally, a good photodetector should be free of impurities and labeled as an intrinsic layer. (As used herein, an "intrinsic layer" is a layer of semiconductor material that is substantially pure in nature, characteristic of undoped material.)
It should be apparent from fig. 6 that the lightly doped epitaxial germanium with mesa 44 is an important reason the penetration depth of photons from the fiber 64 depends on the wavelength of the optical signal when light is coupled from the fiber 64 into the ge schottky barrier photodetector 40, the photons travel from the surface of the metal film 46 and penetrate to the lower part of the active (epitaxial) layer.
The first parameter of a high performance photodetector design is the depletion width W of the photodetector. The depletion region 56 of the ge schottky barrier photodetector 40 is also referred to as the high field region where the junction built-in electric field is highest. High performance photodetectors require absorption of all photons in depletion region 56 to achieve high speed and high quantum efficiency. This is because the photogenerated charge carriers (electrons) travel at high speed within depletion region 56. Photogenerated charge carriers (electrons) generated outside of depletion region 56 will need to enter depletion region 56 through a diffusion process and then be swept by the electric field in depletion region 56 to produce photocurrent Iph. The process isThe range reduces the response speed of the photo-detector and causes receiver APP problems, which will be explained in more detail below.
As shown in fig. 7, the high performance photodetector design should provide a large depletion width W to confine and absorb all photons from the fiber 64. The depletion width W depends on the doping density N on the photodetectordAnd a reverse bias voltage V. As shown in FIG. 7, lower doping density (or higher purity) and higher reverse bias voltage V in the active layer2(V2>V1) The depletion width W will increase. The germanium schottky barrier photodetector design parameters presented herein provide optimized depletion widths for high speed and high quantum efficiency. The depletion width W and the doping density N are shown in equations (1) to (3)dAnd bias voltage V:
W2＝qNd(3)
wherein ε is the dielectric constant of Ge ═ 16; epsilon0Is a vacuum dielectric constant of 8.85418E-14F/cm; vbiIs the built-in voltage (or potential) of the Ge schottky barrier detector; v is the bias voltage of the Ge schottky barrier detector; n is a radical ofdIs the background doping density of the active layer (in mesa 44) of the Ge schottky barrier detector; k is the boltzmann constant 1.38066E-23J/° K; t is the temperature in Kelvin degrees (° K); q is the charge of an electron 1.60218E-19C; and KT/q is the thermal voltage at 300 ° K (room temperature) ═ 0.0259V.
The second photodetector parameter for the optimized photodetector is low capacitance. The capacitance of the photodetector increases with the photodetector surface area a and decreases with decreasing doping density. For general purpose fiber optic photodetectors that require coupling to optical fibers having diameters ranging from 8 microns to 1mm, a photodetector surface area having a diameter of 550 microns is presented herein, as with this diameter, a 1mm POF can be coupled with a lens package, as will be described in detail below. The photodetector capacitance design is shown in equation (4):
where a is the area of the Ge schottky barrier detector.
A third parameter of the optimized photodetector design is the dark current of the photodetector. The dark current of the photodetector is a major source of shot noise for the receiver circuit that interfaces with the photodetector. The dark current of the photodetector is proportional to the light detection area a of the photodetector, the material of the photodetector, and the type of electrical junction of the photodetector. The germanium schottky barrier photodetector 40 disclosed herein has a diameter that minimizes dark current and provides a surface area that maximizes photon absorption for different sized optical fibers used in avionics applications. The mesa structure 44 made of n-doped epitaxial germanium eliminates additional dark current due to surface leakage that may occur in a planar photodetector structure. The dark current is shown in equation (5) as a function of the light detection area a and the photodetector junction parameters:
wherein a is an effective richardson constant; and
A fourth parameter of high performance photodetector design is quantum efficiency. The quantum efficiency versus photo-generated current is shown in equation (6):
η thereinQEIs the quantum efficiency of the Ge schottky barrier detector; piIs the incident power on the Ge schottky barrier detector; and λ is the wavelength (in microns) of the incident photons on the Ge schottky barrier detector.
Higher quantum efficiency results in higher photo-generated current, which in turn results in higher receiver sensitivity. The photodetector area and package design disclosed herein provides maximum quantum efficiency for various fiber sizes from 8 μm to 1mm diameter.
Design equations and parameters for Schottky barrier detectors can be found in Physics of Semiconductor Device, Chapter 5, Metal-Semiconductor contacts, by S.M.Sze, pp.245-311, Press John Wiley and Son, 1981.
The detector diameter determines the size of the light detector. It also affects the optical response of the detector. Thus, the diameter provides an optimized dark current, capacitance and light response of the detector. Of these three features, capacitance and dark current are better at smaller detector diameters, but the optical response of the detector requires a large diameter detector. The detector diameter can be calculated that optimizes these three characteristics. Suitable detector diameters range from 450 to 600 microns.
The above-described optimum design also attempts to solve the APP problem in some COTS POF photodetectors that results from too small a depletion width within the active region of the photodetector. As shown in fig. 6, if the depletion width is too small, a large number of photons will be absorbed outside the depletion region 56. The photogenerated charge carriers (electrons) need to undergo a diffusion process before reaching the high field depletion region. This process produces a "diffuse tail" of the optical response pulse of the photodetector when the receiver is designed with Automatic Gain Control (AGC) circuitry that produces an optical power at a certain level and an electrical pulse within a certain response period after the occurrence of the first optical pulse from the photodetector. The purpose of AGC is to increase the dynamic range of the receiver. If the input optical signal is above a certain power level, the AGC will automatically reduce the gain of the receiver. The AGC increases the gain of the receiver if the input optical signal is below a specified optical power level. For example, according to one pulsed mode optical receiver design, the AGC is required to detect a signal 30dB below the initial optical pulse within a specified time period after the optical pulse occurs. If the optical response of the photodetector has a large "diffuse tail", it may cause the receiver to produce an unintended optical response pulse. These unintended optical pulses may cause errors in the receiver. The physical problem with APP is shown in FIGS. 8A-8C and 9. The design of the Ge schottky barrier photodetector disclosed herein eliminates this APP problem in avionics applications.
Fig. 8A shows the change in optical power received from an optical fiber by a typical photodetector over time. Figure 8B shows the time-course of a series of photo-generated current pulses produced by a typical photo-detector if all photons are absorbed in the depletion region. Fig. 8C shows the time-dependent variation of a series of photo-generated current pulses produced by a typical photo-detector if a photon is absorbed outside the depletion region, in which case the response to the light pulse may have a diffuse tail.
Figure 9 includes upper and lower graphs showing the variation over time of the detector photo-generated current pulses and receiver impulse response (including additional pulses), respectively. The receiver AGC described above is designed to trigger a response at a certain level (indicated by the horizontal line in the upper diagram of fig. 9) below the starting optical signal pulse. A long "diffusion tail" is detected by the AGC at the time of AGC triggering and causes the receiver to output an additional response pulse (input voltage), such as the response pulse depicted in the lower graph of fig. 9.
The upper graph in fig. 9 shows the same photo-generated current pulses as depicted in fig. 8C, but the lower graph shows the variation over time of a series of receiver response pulses (caused by additional current pulses of the detector) that can be generated by the COTS receiver in response to the photo-generated current (from the detector) with a diffuse tail depicted in fig. 8C, with respect to various parameters of the automatic gain control circuit incorporated in some COTS receivers (i.e., the AGC response time period and AGC response magnitude).
The germanium schottky barrier photodetector 40 disclosed herein may be packaged in various ways. Fig. 10 shows a light detector packaged in a hermetically sealed metal can 66 (e.g., a transistor-can (TO) package such as TO-18 or TO-46), the hermetically sealed metal can 66 having a base 80, a lens housing 70 (also referred TO as a "lid") supported by the base 80, and a 2mm diameter glass ball lens 72 (e.g., made of BK7 glass), the glass ball lens 72 being mounted in an opening on the top of the lens housing 70. As can be seen in fig. 10, the ends of the optical fibers 64, the glass ball lens 72 and the metal film 46 are aligned with one another. Ideally, if the end faces of the optical fiber 64 and the metal film 46 are rounded and the glass ball lens 72 is spherical, then the centers of the circle and the sphere would be collinear. Photons exiting the fiber 64 pass through the glass ball lens 72 and then impinge on the metal film 46, resulting in maximizing the photo-generated current of the detector. According to one proposed implementation, the metal film 46 is located at the back focal length of the glass ball lens 72.
Two electrical leads for the photodetector package are required. One lead (detector front contact pin 84 insulated from package base 80) is electrically connected to metal pad 48 on the surface of germanium schottky barrier photodetector 40 by bonding one end of wire 74 to metal pad 48 and the other end of wire 74 to detector front contact pin 84. The other lead (detector rear contact pin 82) is connected to the base 80 of the metal housing 66 for grounding. The photodetector back ohmic contact metal layer 50 is bonded to the base 80 of the metal housing 66. Thus, the detector rear contact pins 82 also make contact with the back of the light detector.
If the application requires the photodetector TO be completely isolated from the package, a three lead TO package may be used. Fig. 11 shows a germanium schottky barrier photodetector 40 packaged in a hermetically sealed metal housing 68 using a three pin configuration. This is accomplished by placing an insulating layer 88 (layer made of an insulating material) and a metal layer 78 (layer made of metal) under the back of the photodetector. The metal layer 78 is in contact with the ohmic contact metal layer 50 of the photodetector. Insulating layer 88 electrically isolates metal layer 78 from submount 80. The metal layer 78 is then electrically connected to the detector back contact pins 82 (insulated from the package base 80) by bonding the opposite ends of the wires 76. The ground pin 86 is in contact with the base 80 of the metal housing 68 for grounding purposes.
Because the germanium schottky barrier photodetector 40 disclosed herein is designed for optical fibers ranging in diameter from 8 microns to 1mm, the optimized diameter is calculated to be about 550 microns. Thus, using a 2mm diameter glass ball lens 72 on the lens housing 70, the photodetector is positioned at a distance equal to the back focal length of the glass ball lens 72, coupling fibers having an optimized diameter range from 8 microns to 1 mm.
As mentioned previously, germanium schottky barrier photodetectors of the type disclosed herein may be used in fiber optic networks on board aircraft. For purposes of illustration, one embodiment of such a fiber optic network for enabling optical communication between line replaceable units on an aircraft will now be described in more detail below. However, the use of germanium schottky barrier photodetectors of the type disclosed herein is not limited solely to aircraft environments, but may be used in fiber optic networks on other types of vehicles or in other types of fiber optic networks (e.g., long-haul terrestrial applications, data center applications, internet of things applications, and fiber to the home/office applications).
Fig. 12 is a diagram identifying some features of a bi-directional, full-duplex data transmission system 30, the bi-directional, full-duplex data transmission system 30 including a pair of dual-fiber (dual-fiber) bi-directional transceivers 2a and 2b, each of the pair of dual-fiber bi-directional transceivers 2a and 2b transmitting and receiving light at the same wavelength, each of the single-wavelength dual-fiber bi-directional transceivers 2a and 2b including a laser 4 and a photodetector 8. In this example, each photodetector 8 is a germanium schottky barrier photodetector of the type depicted in fig. 3. By passingThe laser driver and transmission circuitry 6 drives the laser 4 in response to a differential transmission signal Tx received from an associated line replaceable unit (not shown) via transmission electrical signal lines 12a and 12b, respectively+And Tx-Emission wavelength of lambda1Of (2) is detected. The laser driver and transmission circuitry 6 includes circuitry that converts these electrical differential signals into electrical digital signals that represent the data transmitted by the laser 4. In contrast, the photodetector 8 receives a signal having a wavelength λ1And converts the detected light into an electrical digital signal that is provided to a detector amplifier and receive circuit 10. The detector amplifier and receiving circuit 10 in turn comprises a conversion of those electrical digital signals into electrical differential receiving signals Rx+And Rx-The electrical differential reception signal Rx+And Rx-Representing the received data. Electrical differential receive signal Rx+And Rx-To other circuits in a line replaceable unit (not shown) via receive electrical signal lines 14a and 14b, respectively. The single wavelength dual fiber bi-directional transceiver 2 receives a signal having a voltage V via a transceiver power line 16ccThe power supply of (1).
In the example depicted in fig. 12, the lasers 4 of the single wavelength dual fiber bi-directional transceiver 2a are optically coupled to emit light toward the photodetectors 8 of the single wavelength dual fiber bi-directional transceiver 2b via an optical cable 32, the optical cable 32 including a fiber jumper 18a connected in series, a connector 22a mounted at a patch panel 23a, a plastic or glass fiber 24a, a connector 22b mounted at a patch panel 23b, and a fiber jumper 18 c. Similarly, the lasers 4 of the single wavelength dual fiber bi-directional transceiver 2b are optically coupled to emit light towards the photodetectors 8 of the single wavelength dual fiber bi-directional transceiver 2a via an optical cable 34, the optical cable 34 including a series connected optical fiber jumper 18d, a connector 22c mounted at a patch panel 23b, a plastic or glass optical fiber 24b, a connector 22d mounted at the patch panel 23a, and the optical fiber jumper 18 b. Both single wavelength dual fiber bi-directional transceivers 2a and 2b transmit and receive signals having a wavelength λ1Of (2) is detected. The construction of the cables 32 and 34 may be identical. Optionally, fibers 24a and 24b may be GbPOF to achieve high data rates: (>1 gigabit/second) bidirectional full-duplex (full-duplex) data transmission。
In addition to the application shown in fig. 12, the universal broadband fiber optic photodetectors disclosed herein have other avionics applications. The disclosed optical detector eliminates the need for multiple optical detectors and receivers in a single or multiple avionics platforms and simplifies the parts management and inventory process for aircraft, military systems, and sensor system applications. In addition, the design of the photodetector has superior operating characteristics and eliminates the APP problem in the COTS receiver.
Further, the present disclosure includes embodiments according to the following clauses:
clause 1. a light detector, comprising:
a substrate made of doped germanium;
a mesa structure made of doped epitaxial germanium grown on top of the substrate, wherein a doping density of the doped epitaxial germanium is less than a doping density of the doped germanium;
a metal film made of metal deposited on top of the mesa structure;
a metal pad made of metal deposited on top of the mesa structure and in contact with the film; and
and an ohmic contact layer made of metal deposited on the bottom of the substrate.
Clause 2. the photodetector of clause 1, wherein the metal film and the metal pad are made of a metal selected from the group consisting of gold, silver, aluminum, copper, and indium.
Clause 3. the photodetector of clause 2, wherein the metal film and the mesa structure form a junction having a schottky barrier height in a range of 0.54 to 0.64 volts.
Clause 4. the photodetector of clause 1, further comprising a voltage source connected to the metal pad and the ohmic contact layer, wherein the metal film and the mesa structure are configured to produce a depletion region in and adjacent to the metal film, the depletion region having a width that increases when a reverse bias voltage is applied by the voltage source during photons impinging on the metal film.
Clause 5. the photodetector of clause 4, wherein the metal film has a diameter in the range of 450 to 600 microns.
Clause 6. the photodetector of clause 4, wherein the effective Richardson constant is from 128 to 135A/cm2-°K2Within the range of (1).
Clause 7. the photodetector of clause 1, further comprising an anti-reflective coating deposited on the film.
Clause 8. the photodetector of clause 1, further comprising a dielectric passivation layer covering the exposed surface of the mesa structure.
Clause 9. an optical fiber device, comprising:
an optical fiber having a distal end;
a hermetic metal case including a base, a lens cover connected to the base, and a glass ball lens mounted in an opening at the top of the lens cover; and
a photodetector disposed inside the metal housing, the photodetector comprising a substrate made of doped germanium, a mesa structure made of lowly doped epitaxial germanium grown on top of the substrate, the doped epitaxial germanium having a doping density much less than the doping density of the doped germanium, a metal film made of metal deposited on top of the mesa structure, a metal pad made of metal deposited on top of the mesa structure and in contact with the film, and an ohmic contact layer made of metal deposited on the bottom of the substrate,
wherein the optical fiber, the glass ball lens and the metal film are aligned.
Clause 10. the optical fiber device of clause 9, wherein the metal film and the metal pad are made of a metal selected from the group consisting of gold, silver, aluminum, copper, and indium.
Clause 11. the fiber optic device of clause 9, wherein the metal film and the mesa structure form a junction having a schottky barrier height in the range of 0.54 to 0.64 volts.
Clause 12. the fiber device of clause 9, wherein the metal film and the mesa structure are configured to produce a depletion region in and adjacent to the metal film, the depletion region having a width that increases when a reverse bias voltage is applied across the metal pad and the ohmic contact metal layer during photons striking the metal film.
Clause 13. the optical fiber device of clause 9, wherein the metal film has a diameter in the range of 450 to 600 microns and the glass ball lens has a diameter of 2 mm.
Clause 14. the optical fiber device of clause 13, wherein the metal film is located at a back focal length of the glass sphere lens.
Clause 15. the fiber optic device of clause 9, further comprising:
a metal layer in contact with the ohmic contact metal layer; and
an insulating layer electrically isolating the metal layer from the substrate.
Clause 16. the fiber optic device of clause 9, wherein the photodetector further comprises a dielectric passivation layer covering the exposed surface of the mesa structure.
Clause 17. a method for manufacturing a photodetector, the method comprising:
polishing and lapping the doped germanium wafer until a germanium substrate having a thickness in a range of 100 to 150 microns is formed;
growing a doped germanium epitaxial layer about 15 microns thick on top of a germanium substrate;
depositing an ohmic contact metal layer on the bottom of the germanium substrate;
depositing a metal film and a metal pad on top of the germanium epitaxial layer such that the metal pad is in contact with the metal film;
forming a mesa structure by removing some of the germanium epitaxial layer; and
a dielectric passivation layer is deposited on the exposed surfaces of the mesa structures.
Clause 18. the method of clause 17, further comprising depositing an antireflective coating on top of the metal film.
Clause 19. the method of clause 17, wherein the metal film and the metal pad are made of a metal selected from the group consisting of gold, silver, aluminum, copper, and indium.
Clause 20. the method of clause 17, wherein the top surface of the mesa structure has a diameter in the range of 450 to 600 microns.
While a germanium schottky barrier photodetector and a method for fabricating such a photodetector have been described with reference to various embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the teachings herein. In addition, many modifications may be made to adapt the teachings herein to a particular situation without departing from its scope. Therefore, it is intended that the claims not be limited to the particular embodiments disclosed herein.
The method claims set forth below should not be construed as requiring that steps described therein be performed in alphabetical order (any alphabetical order in the claims is used solely for the purpose of referring to the previously described steps) or in the order in which they are recited, unless the claim language explicitly specifies or states a condition indicating a specific order of performing some or all of the steps. The method claims should not be construed to exclude any portion of two or more steps being performed simultaneously or in alternation, unless the claim language expressly states a condition that excludes such interpretation.