US 20080006816 A1
A photodetector for use at wavelengths of 2 μm and longer has an intersubband absorption region to provide absorption at wavelengths beyond 2 μm, integrated with an avalanche multiplier region to provide low-rise gain. In one particular design, the intersubband absorption region is a quantum-confined absorption region (e.g., based on quantum wells and/or quantum dots).
1. A photodetector comprising: an intersubband absorption region for absorbing optical radiation at a wavelength of 2 .mu.m or greater to generate carriers based on an intersubband transition; and an avalanche multiplier region electrically coupled to receive and multiply the carriers generated in the intersubband absorption region.
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This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 60/510,986, “Intersubband Quantum Dot Detectors with Avalanche Photodiodes,” filed Oct. 14, 2003. The subject matter of the foregoing is incorporated herein by reference in its entirety.
1. Field of the Invention
This invention relates to photodetectors and, more particularly, to photodetectors where absorption is based on an intersubband transition and gain is provided by an avalanche multiplier region.
2. Description of the Related Art
With the recent increased interest in mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) optoelectronic devices and applications, much attention has been directed to semiconductor optoelectronic devices, such as lasers, light emitting diodes (LEDs), photodetectors and the like. Particular concern has been directed to the area of detectors that operate at wavelengths between approximately 2 μm and 30 μm. Such devices are an important component in optical systems that can be used for applications including remote sensing, LADAR, detection of chemical warfare agents, intelligence surveillance and reconnaissance (ISR), enemy missile tracking and infrared countermeasures (IRCM).
Currently, high-performance photonic detectors in this wavelength range typically must be cooled to cryogenic temperatures (4-100K) to overcome deleterious effects arising due to thermionic emission. The cooling system itself can be complicated, requiring multi-stage Sterling coolers, and can comprise up to 60% of the total cost of an infrared camera based upon infrared photodetectors. These cameras have a variety of applications ranging from thermal imaging and night vision systems to effluent detection and medical diagnostics. If the operating temperature of a detector could be increased from cryogenic temperatures to temperatures achievable by the relatively inexpensive Peltier coolers (150-250K), this would lead to a significant reduction in the cost and complexity of infrared sensors and imaging systems.
State of the art MWIR and LWIR detectors are usually based on narrow bandgap mercury cadmium telluride (MCT) material, which generally offers the highest single pixel performance at a given temperature. However, non-uniformity issues associated with native defects have limited the progress of MCT-based focal plane arrays. Presently, high performance LWIR cameras used for military applications are grown on CdZnTe wafers that are expensive, can exhibit high levels of defects that subsequently degrade device performance, and are incompatible with the electronic circuitry.
One alternative to cryogenically cooled photonic detectors is bolometer-based detectors. However, this is still an emerging technology that suffers from poor performance relative to cooled detectors.
Another alternative that can be used to detect light in the >2 μm region is a quantum dot infrared photodetector (QDIP), whose operation is based on intersubband transitions of electrons. QDIPs offer many advantages. They can be operated in normal incidence. They can be based on mature GaAs-based technology. The multi-color response can be tailored from 3-30 μm. They typically have low dark current. They can also have large quantum confined Stark effect, which can be exploited to realize hyperspectral sensors. However, one of the problems facing QDIPs is their low quantum efficiency, which leads to a lower detectivity and responsivity. This, in turn, typically limits their operating temperature to about 70-80K.
In addition, the infrared wavelength region beyond approximately 2 μm is a rich area of spectroscopic research, allowing the detection of complex molecules, based on absorption arising from vibrational and rotational modes of the molecules. However, studies in this region are hampered by the absence of sufficiently sensitive detectors. Photon-counting systems are regarded as the ultimate in photon-sensing techniques from a sensitivity perspective, and have applications for sensing ultralow-level images and signals in many scientific and engineering fields stretching from microscopy and medical imaging to astronomy and astrophysics, where the photon flux is very limited. Presently, no single photon detectors are available for wavelengths beyond 2 μm.
Thus, there is a need for MWIR and longer wavelength infrared detectors that have good performance with only Peltier cooling or less. There is also a need for photon-counting and other ultra sensitive detectors at these wavelengths.
The above problems and others are at least partially solved and the above purposes and others realized by providing a photodetector having an intersubband absorption region (e.g., an absorption region based on quantum dots) to provide absorption at wavelengths beyond 2 μm, integrated with an avalanche multiplier region to provide low-noise gain. In one particular design, the intersubband absorption region is a quantum-confined absorption region (e.g., based on quantum wells and/or quantum dots).
In another aspect, a photodetector includes an n-i-n structure integrated with a p-i-n structure. The n-i-n structure includes the intersubband absorption region and the p-i-n structure includes the avalanche multiplier region. Incident light generates photocarriers in the absorption region, which are swept towards the p-i-n structure by an applied bias. The carriers tunnel their way into the avalanche multiplier region, where the carriers are multiplied in an avalanche process. Electrical contacts are used to apply the correct biases across both the absorption region and the avalanche multiplier region.
In a specific design, the photodetector includes a GaAs substrate and the following regions in order away from the substrate: an avalanche multiplier region, a highly doped p-type region, a first n-type contact region, a quantum-confined absorption region, and a second n-type contact region. Most, if not all, of the regions are based on GaAs. One example of a quantum-confined absorption region is a dot-in-well (DWELL) design, for example InAs dots within InyGa1-yAs wells. Electrical contacts to the substrate and the two contact regions allow for the application of bias voltages across the absorption region and the avalanche multiplier region.
The foregoing and further and more specific objects and advantages of the invention will become readily apparent to those skilled in the art from the following detailed description taken in conjunction with the drawings in which:
In the example embodiment of
An avalanche multiplier region 104 is positioned on substrate 102 and a highly doped contact region 106 is positioned on avalanche multiplier region 104. The contact region 106 includes doped material with a different conductivity type to substrate 102. In the example of
Highly doped contact region 106, avalanche multiplier region 104, and doped substrate 102 form a p-i-n structure 105. In alternate designs, a region other than substrate 102 may form the n part of the p-i-n structure. For example, an additional n-type contact layer may be formed between the substrate 102 and avalanche multiplier region 104, thus forming a p-i-n structure with regions 106 and 104.
A first contact region 108 is positioned on highly doped region 106. This contact region 108 includes doped material with a different conductivity type to highly doped region 106. An intersubband absorption region 110 is positioned on contact region 110 and a second contact region 112 is positioned on that. The two contact regions 108 and 112 are doped with the same conductivity type. In the example of
The quantum dot absorption region 110 includes a self-assembled array of quantum dots embedded in a material with a larger bandgap than the quantum dots, capable of absorbing an infrared wavelength >2 μm.
In the preferred embodiment, a mesa 101 is formed by etching through into first contact region 108, as shown. Mesa 101 can be formed using conventional techniques, such as wet etching, dry etching, or the like. Bottom, middle and top ohmic contact regions 114, 116, and 118 are formed by standard semiconductor processing techniques.
A first barrier region 210 is positioned on contact region 108. The first barrier region 210 provides a barrier to electron motion for an absorption region 110 subsequently grown thereon. In his example, first barrier region 210 is GaAs, when grown on a GaAs substrate. Other suitable barrier materials can also provide a conduction band offset, including but not limited to AlxGa1-xAs, where x may vary from 0% to 100%, and GaAsP, when grown on a GaAs substrate. Other materials are suitable for other substrate types.
The absorption region 110 is then formed by sequential deposition of a first well region 212, a quantum-dot region 214 and a second well region 216. Absorption region 110 is illustrated as consisting of a single set of regions 212,114 and 216 for simplicity and ease of discussion, and is not intended to limit the scope of the invention. In a preferred embodiment, the absorbing region 110 can include many more regions than are shown in
First well region 212 is formed using a material with a lower conduction band edge than barrier region 210. In the example embodiment, first well region 212 is InxGa1-xAs, where x is approximately equal to 15%. However, other compositions and other suitable materials may be chosen, depending on the starting substrate and desired band offset.
Quantum dot region 214 is formed using a self-assembled arrangement of quantum dots that is lightly doped with Si to provide 1-2 electrons per dot. The doping is optional. In the design of
Second well region 216 is formed using a material with a lower conduction band edge than barrier region 210 and barrier region 218. In the
Barrier region 218 is positioned on second well region 216. Barrier region 218 includes a material capable of providing a barrier to electron motion for absorption region 110. In the
Well regions 212 and 216, surrounded by barrier regions 210 and 218 provide a quantum well. Quantum dot region 214 sits in this quantum well. Absorbing region 110 combined with barrier region 210 provides a quantum potential system wherein the allowed states for electrons in the dots and wells are determined by the compositions and thicknesses of the wells and barriers, together with the composition and geometry of the dots. Intersubband transitions between energy levels in dots, wells and barriers define an infrared absorption spectrum for the absorption region 110. A contact region 112 is positioned on barrier region 218.
The absorption region 110 can be fabricated using conventional means. In a preferred approach, the various regions are deposited using Molecular Beam Epitaxy, Metalorganic Chemical Vapor Deposition, or similar conventional techniques. The conditions used to grow quantum dot region 214 can be used to vary the size, shape and density of the quantum dots using well-known principles. Using conventional techniques, large number of individual devices and/or arrays of devices can be simultaneously fabricated on a substrate. Other materials systems can also be used. For example, an alternate embodiment uses quantum dots made with InxGayAlzAs, quantum wells made with InxGayAlzAs and barriers using a different composition of InxGayAlzAs.
Referring now to the avalanche multiplier region 104 of
The thickness and composition of highly doped region 106 is chosen to allow photoexcited electrons to tunnel into the avalanche multiplier region 104. A reverse bias applied to p-i-n structure 105, preferably close to the reverse breakdown voltage, results in gain due to an avalanche multiplication process, resulting in an amplified photocurrent. Electrons that tunnel into avalanche multiplier region 104 are accelerated by the high applied field, resulting in ionizing collisions with the semiconductor lattice. Secondary carriers are produced, thus increasing current. This process is known as impact ionization and leads to carrier multiplication and hence gain. Excess noise in the avalanche multiplier region 104 is significantly lower than for a conventional APD, since only electrons are injected into the avalanche multiplier region 104 and holes play no part in the avalanche process.
Note that the quantum dot absorption region 110 is located within an n-i-n structure 103, whereas the avalanche multiplier region 104 is located within a p-i-n structure 105 that is operated under a reverse bias close to a breakdown voltage. The design of the device addresses the interaction between the n-type contact region 108 and the p-type contact region 106. This interface between n-i--n region 103 and p-i-n region 105 is important in determining the operation of QDAP 100. Doping, concentration and a composition of highly doped region 106 is chosen to enable tunneling.
Transport properties of QDAP 100 were studied by simulating a current-voltage (or I-V) characteristic of QDAP 100.
The performance of QDAP 100 can be further improved by use of heterostructures. For example, in a preferred embodiment, highly doped region 106 is GaAs. However, it is also possible to use a material such as AlxGa1-xAs (0≦x≦1). The use of heterostructures provides several advantages. It provides the ability to control a barrier by varying the composition of the AlGaAs layer while keeping the doping fixed. In addition, a barrier such as AlGaAs or the lice serves to improve both the excess-noise and breakdown characteristics of the avalanche multiplier region. In particular, for a Geiger-mode design, used in photon counting applications, a thickness of an intrinsic region of a diode (such as avalanche multiplier region 104) can be decreased, thereby leading to a decrease in a value of an operating voltage, for example. The use of a heterostructure, e.g. by including InxAlyGa1-x-yAs, can also be used to control a band-offset of the structure.
Various changes and modifications to one or more of the embodiments herein chosen for purposes of illustration will readily occur to those skilled in the art. For example, the absorption region 110 can be based on quantum wells or other quantum-confined structures rather than quantum dots. Alternately, it can be based on non-quantum structures, for example impurity level detectors or other absorption regions based on intersubband transitions (i.e., the transition energy is less than the bandgap between the conduction and valence bands). These intersubband transitions generally lead to absorption at wavelengths at or longer than 2 μm. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof, which is assessed only by a fair interpretation of the following claims.