US 20070224721 A1
A device for detecting radiation, typically in the infrared. Photons are absorbed in an active region of a semiconductor device such that the absorption induces an interband electronic transition and generates photo-excited charge carriers. The charge carriers are coupled into a carrier transport region having multiple quantum wells and characterized by intersubband relaxation that provides rapid charge carrier collection. The photo-excited carriers are collected from the carrier transport region at a conducting contact region. Another carrier transport region characterized by interband tunneling for multiple stages draws charge carriers from another conducting contact and replenishes the charge carriers to the active region for photo-excitation. A photocurrent is generated between the conducting contacts through the active region of the device.
1. A method for detecting electromagnetic radiation, the method comprising:
a. absorbing photons in an active region of a semiconductor device, the active region having a plurality of photo-absorptive type-II quantum well layers and thereby inducing an interband electronic transition and generating photo-excited charge carriers or electron-hole pairs;
b. coupling the photo-excited charge carriers into at least one carrier transport region-characterized by carrier relaxation, transport, and tunneling processes significantly faster than recombination processes in the active region;
c. collecting photo-excited carriers from the carrier transport region at a conducting contact region; and
d. generating a photocurrent between the conducting contact regions through the active region of the device, the photocurrent characterizing the number of photons absorbed in the interband electronic transition.
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11. A radiation detector comprising:
a. at least one detector stage, each detector stage comprising:
i. an active region having a plurality of photo-absorptive layers, the active region characterized by a structure enabling interband optical transition;
ii. a carrier transport and relaxation region coupled to the active region and containing multiple type-II quantum wells;
b. an electrical contact structure for coupling an external circuit to one active region and one carrier transport and relaxation region; and
c. a current loop for continuous flow of photocurrent through the device and the external circuit upon absorption of photons in the active region.
12. A radiation detector in accordance with
13. A radiation detector in accordance with
14. A radiation detector in accordance with
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.
The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. § 202) in which the Contractor has elected to retain title. This invention was developed with Government support under Contract Number DAAD19-01-1-0591 awarded by the United States Army Research Office. The Government has certain rights in the invention.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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:
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.
Exemplary embodiments of the present invention are discussed in a paper entitled Interband Cascade Detectors with Room Temperature Photovoltaic Operation, which is appended hereto and incorporated herein by reference.
The ICD may have one or more stages, each stage having an active region where photons are absorbed and charge carriers created, a multiple-QW region forming an energy ladder enabling efficient transport of electrons via intraband (or so-called intersubband) relaxation after photon excitations, and a carrier transport region enabling efficient replenishment of carriers into the active region. Operation of a multiple-stage device is described with reference to
The next region to the left of absorption region 10 in
Multiple QW region 16 is designed to provide an energy ladder of subbands 18, 19, 20, enabling fast and efficient transport of electrons that have been photo-excited in the active region. More particularly, the energy ladder is formed in the region, with multiple energy levels 18, 19, 20, having energy separations typically close to or somewhat larger than an optical phonon energy hυop (˜30 meV in InAs), however all ranges of energy separation among the energy sublevels are within the scope of the present invention as described herein.
Barrier layers in the multiple QW region 16 are thin enough to allow significant wavefunction overlaps between adjacent quantum wells in the energy ladder similar to that of type-I QC structures, enabling fast and efficient transport of electrons via intraband (or so-called intersubband) relaxation after photon excitations.
The intraband (or intersubband) relaxation typically occurs on a time scale (˜picoseconds) that is much shorter than the time scale (˜nanoseconds) for interband recombination in active region 10. Thus extremely high-speed (broadband, on the order of many GHz) operation of the ICD may advantageously be achieved.
As described, multiple stages of active region/relaxation region zones may be provided, with a second active region 22 shown to the far left in
It is to be understood that the scope of the invention is not limited to the discretely (digital-) graded band gaps discussed herein nor to operation in a photovoltaic mode, and it is further to be understood that the invention also encompasses continuously graded band-gap layers or the application of a bias potential such that long-wavelength infrared detection may be achieved in accordance with particular embodiments of the invention.
Referring now to
Under reverse bias for photovoltaic detection, the band edge alignment for the structure of
The band edge alignment of one period of the ICD structure under forward bias is shown in
Due to the photovoltaic nature of the detector operation enabled by the present invention, the ICD may operate limited only by Johnson noise, in 2=4 kT/R0, per unit bandwidth, where k is the Boltzmann constant, T is temperature, and R0 is the differential resistance. The Johnson noise in the devices fabricated in accordance with the invention is, itself, very low due to the large differential resistance (typically 104-105 Ω) of the devices of typical area (100,000 μm2).
An embodiment of the invention fabricated as a mesa geometry photodetector 50 is shown in
An embodiment of the invention fabricated as a waveguide photodetector 60 is shown in
The embodiments of the invention heretofore described are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. In particular, while the invention has been described in the context of infrared photon detection, it is to be understood that the invention may advantageously be applied to the detection of other radiation exciting charge carriers in the semiconductor structure as described. All such variations and modifications are intended to be within the scope of the present invention as defined in any appended claims.