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  Intersubband detector with avalanche multiplier regionUSPTO Application #: 20080006816Title: Intersubband detector with avalanche multiplier region Abstract: 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). (end of abstract) Agent: Fenwick & West LLP - Mountain View, CA, US Inventors: Sanjay Krishna, John P. R. David, Majeed M. Hayat USPTO Applicaton #: 20080006816 - Class: 257014000 (USPTO) Related Patent Categories: Active Solid-state Devices (e.g., Transistors, Solid-state Diodes), Thin Active Physical Layer Which Is (1) An Active Potential Well Layer Thin Enough To Establish Discrete Quantum Energy Levels Or (2) An Active Barrier Layer Thin Enough To Permit Quantum Mechanical Tunneling Or (3) An Active Layer Thin Enough To Permit Carrier Transmission With Substantially No Scattering (e.g., Superlattice Quantum Well, Or Ballistic Transport Device), Heterojunction, Quantum Well The Patent Description & Claims data below is from USPTO Patent Application 20080006816. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATION [0001] This application claims priority under 35 U.S.C. .sctn.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. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] 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. [0004] 2. Description of the Related Art [0005] 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 .mu.m and 30 .mu.m. 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). [0006] 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. [0007] 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. [0008] 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. [0009] Another alternative that can be used to detect light in the >2 .mu.m 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 .mu.m. 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. [0010] In addition, the infrared wavelength region beyond approximately 2 .mu.m 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 .mu.m. [0011] 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. SUMMARY OF THE INVENTION [0012] 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 .mu.m, 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). [0013] 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. [0014] 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 In.sub.yGa.sub.1-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. BRIEF DESCRIPTION OF THE DRAWINGS [0015] 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: [0016] FIG. 1 is a sectional view of a quantum dot avalanche photodetector (QDAP) according to the invention. [0017] FIG. 2 is a sectional view of a dot-in-well (DWELL) absorption region for the QDAP of FIG. 1. [0018] FIG. 3 is a schematic of a band-edge diagram of the QDAP of FIG. 1. [0019] FIG. 4 is a graph of a simulation of a band structure of a QDAP. [0020] FIG. 5 is a graph showing a variation of a barrier height of a highly doped region as a function of doping level, for a fixed width of 10 nm in a QDAP. Continue reading... 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