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  Interband cascade detectorsRelated Patent Categories: Semiconductor Device Manufacturing: Process, Making Device Or Circuit Responsive To Nonelectrical Signal, Responsive To Electromagnetic RadiationThe Patent Description & Claims data below is from USPTO Patent Application 20070224721. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The present application claims priority from U.S. Provisional Application No. 60/613,554, filed Sep. 27, 2004, and U.S. Provisional Application No. 60/665,997, filed Feb. 24, 2005, both of which applications are incorporated herein by reference. TECHNICAL FIELD AND BACKGROUND ART [0003] The present invention pertains to methods and apparatus for detecting radiation, and more particularly to detection of photons in a semiconductor structure. The invention is advantageously employed in the detection of electromagnetic radiation, especially infrared and far-infrared radiation. [0004] Currently, prevalent infrared photodetection technology is based on interband absorption in bulk mecury-cadmium-teluride (HgCdTe, MCT) typically operating at cryogenic temperatures and thereby imposing attendant cost and size requirements. Moreover, material homogeneity constraints limit the applicability of MCT in the context of fabricating large focal plane array devices. [0005] Infrared detectors based on type-II superlattice structures engineered by deposition of a stack of successive semiconductor layers have shown promise for thermal imaging applications because of possible suppression of dark currents and Auger recombination. Superlattice detectors, based on optical transitions between respective electron and hole minibands in type-II superlattices of alternating III-V compounds, are theoretically superior to MCT at wavelengths long ward of 12 micrometers. Superlattice detectors, are described by Smith et al., Proposal for Strained Type II Superlattice Infrared Detectors, J. Appl. Phys., vol. 62, pp. 2545-48 (1987), which is incorporated herein by reference. Like MCT photodiodes, superlattice detectors are also typically limited to cryogenic operation. [0006] Quantum well intersubband photodetectors (QWIPs) are based on absorption between subbands as a photodetection mechanism rather than absorption between the valence and conduction bands. Due to quantum selection rules, intersubband transitions cannot be photo-excited by normal-incidence radiation (i.e., radiation polarized in the plane of the absorption layer), so grating structure is often introduced for normal incidence detection, adding cost and complication of fabrication to the device structure. QWIPs are reviewed in detail in Levine, Quantum-Well Infrared Photodetectors, J. Appl. Phys., pp. R1-R81 (1993), incorporated herein by reference. [0007] Photodetectors based on type-I intersubband quantum cascade laser (QCL) structures have also been demonstrated, as described, for example, in Hofstetter et al., Quantum-Cascade-Laser Structures as Photodetectors, Appl. Phys. Lett., vol. 81, pp. 2683-85 (2002), incorporated herein by reference. Like QWIPs, these detectors are also based on intersubband photo-excitation, and therefore, as in the case of QWIPs, quantum selection rules preclude their application to normal incidence radiation. [0008] Various applications, particularly in the field of line-of-sight communications and thermal imaging, make high-bandwidth detection of normal-incidence infrared radiation very desirable, especially if room-temperature photovoltaic operation is achieved. SUMMARY OF THE INVENTION In accordance with preferred embodiments of the present invention, detection of electromagnetic radiation is provided by [0009] a. absorbing photons in an active region of a semiconductor device, the active region having one or a plurality of photo-absorptive layers and thereby inducing an interband optical transition and generating photo-excited charge carriers; [0010] b. coupling the photo-excited electrons into a carrier transport region characterized by fast electronic processes such as intersubband relaxation; [0011] c. collecting photo-excited carriers from the carrier transport region at a conducting contact region; and [0012] d. generating a photocurrent between one conducting contact, through the active region of the device, and the other conducting contact. [0013] In various other embodiments of the invention, the semiconductor device may be maintained substantially at room temperature and may be operated in a photovoltaic mode. Alternatively, a bias potential may be applied across the semiconductor device and the device may be operated in a cooled environment to enhance performance. [0014] In accordance with a further aspect of the present invention, a radiation detector is provided that has at least one detector stage. Each detector stage has an active region having a plurality of interband absorptive layers (characterized by either a type-I or type-II quantum-well structure). Each detector stage also has a carrier transport and relaxation region coupled to the active region. The radiation detector has an electrical contact structure for coupling an external circuit to the active region and the carrier transport and relaxation region. The quantum well layers may include Ill-V compounds such as antimony compounds. [0015] In accordance with various embodiments of the invention, photon absorption may be either of photons at normal incidence to a surface of the active region or by an edge-coupled waveguide. The device may comprise a portion of a focal plane array of active regions, and the semiconductor device may also be embedded in an optical cavity for enhancing infrared absorption. BRIEF DESCRIPTION OF THE DRAWINGS [0016] The foregoing features of the invention will be more readily understood by reference to the following detailed description, taken with reference to the accompanying drawings, in which: [0017] FIG. 1 is a schematic depiction of the operation of an interband cascade detector (ICD) in accordance with preferred embodiments of the present invention; [0018] FIG. 2 shows the band edge alignment of one period of the ICD structure under zero bias for photovoltaic detection, in accordance with an exemplary embodiment of present invention; [0019] FIG. 3 shows the band edge alignment of one period of the ICD structure of FIG. 2 under reverse bias for photovoltaic detection, in accordance with an exemplary embodiment of present invention; [0020] FIG. 4 shows the band edge alignment of one period of the ICD structure of FIG. 2 under forward bias as for laser operation, in accordance with an exemplary embodiment of present invention; [0021] FIG. 5 is a perspective view of a mesa structure embodiment of the present invention for normal surface detection of radiation; [0022] FIG. 6 is a perspective view of an embodiment of the present invention in a waveguide geometry; and [0023] FIG. 7 shows typical photovoltaic spectral response of a device in accordance with an embodiment of the present invention to normal-incidence radiation at room temperature and 78 K. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS [0024] Interband cascade detectors (ICDs) provided in accordance with preferred embodiments of the present invention have three salient features. The first feature is an active region, in which photon absorption results in photoexcitation by interband transition from the valence to conduction band. The second feature is a carrier collection region where intraband processes occur at a rate typically far exceeding that of the interband transition. The third feature, in the case where more than one cascade stage is provided, is a carrier replenish region where interband tunneling recombination processes occur at a rate typically far exceeding that of the interband transition. Continue reading... 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