|About the CQD|
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| Team Demonstrates Continuous Terahertz Sources at Room Temperature |
McCormick Press Release - June 4, 2014
Imagine a technology that could allow us to see through opaque surfaces without exposure to harmful x-rays, that could give us the ability to detect harmful chemicals and bio-agents from a safe distance, and that could enable us to peer so deeply into space that scientists could better understand the formation of the universe.
All of these scenarios are possible with terahertz radiation, electromagnetic waves with lengths that fall between microwaves and infrared light. However, the potential of terahertz waves has yet to be reached because they are difficult to generate and manipulate. Current terahertz sources are large, multi-component systems that require complex vacuum electronics, external pump lasers, or cryogenic cooling. Itâ€™s an expensive and cumbersome process.
Manijeh Razeghi and her team are the first to produce terahertz radiation in a simplified system, making it easier to harness the power of these elusive waves. They have developed the first room-temperature, compact, continuous terahertz radiation source, and itâ€™s six times more efficient than previous systems.
... [read more]
| Accelerating adoption of GaN substrates for LED manufacture |
Compound Semiconductor Magazine - June 1, 2014
COMMERCIALISATION of the GaN LED can be traced back to the development of p-type doping of this wide bandgap semiconductor in the early 1990s. Since then, the performance of this device has improved exponentially, enabling it to progress from use in the backlighting of mobile screens to providing a source for solid-state lighting. However, although LED lighting is now commonplace, its cost-performance profi le has a long way to go untill the incumbent vacuum-tubebased lighting technologies will cease to dominate. One of todayâ€™s key bottlenecks is the requirement to use a â€˜non-nativeâ€™ substrate. A new substrate option that has attracted a signifi cant amount of attention is ZnO, which not only has the same crystal structure as GaN, but also very similar lattice parameters and comparable thermal expansion coefficients. ... [read more]
| Razeghi Authors Technical Article in International Society for Optics and Photonics (SPIE) |
EECS Newsroom - January 10, 2014
Razeghi1-oct3 2Manijeh Razeghi, Walter P. Murphy Professor, Director, Center for Quantum Devices (CQD) wrote a featured technical article, titled, "Quantum Cascade Lasers for IR and THz Spectroscopy," that was published December 16, 2013, on the International Society for Optics and Photonics (SPIE) website for her recent research on demonstrating increased power, spectral coverage, and tunability of quantum cascade lasers.
Prof. Razeghi's recent publication, focuses on how developing quantum cascade lasers (QCLs) with the highest power, efficiency, and tunability. Unlike traditional diode lasers, the QCL has a series of quantum wells, which split the usual electron bands into subbands. The QCL emits over several intersubband transitions in this structure. Her CQD team can engineer its optical response using quantum size effects, to achieve a highly variable emitting wavelength based on nanometer-scale control of the quantum well thickness. They demonstrated room-temperature QCLs covering both the 3â€“16Î¼m and 65â€“300Î¼m wavelength ranges using an indium phosphide (InP) material system. ... [read more]
| Quantum cascade lasers for IR and THz spectroscopy |
SPIE Newsroom - December 22, 2013
Terahertz spectroscopy is widely used for chemical detection, and has applications in quality control for manufacturing, security screening, and astronomy. Most chemicals absorb in the mid-IR and THz spectral regions, which cover a wavelength range of 3â€“300 microns. During our research1 we developed quantum cascade lasers (QCLs) with the highest power, efficiency, and tunability. Unlike traditional diode lasers, the QCL has a series of quantum wells, which split the usual electron bands into subbands. The QCL emits over several intersubband transitions in this structure. We can engineer its optical response using quantum size effects, to achieve a highly variable emitting wavelength based on nanometer-scale control of the quantum well thickness. We demonstrated room-temperature QCLs covering both the 3â€“16Î¼m and 65â€“300Î¼m wavelength ranges. ... [read more]
| Researchers Develop Worldâ€™s Highest Quantum Efficiency UV Photodetectors |
McCormick Press Release - December 3, 2013
Manijeh Razeghi, Walter P. Murphy Professor of Electrical Engineering and Computer Science at McCormick, and her group have brought this AlxGa1-xN-based dream device closer to reality by developing a compact photodetector with the worldâ€™s highest quantum efficiency. This was achieved by refining the low-pressure metal-organic chemical-vapor-deposition growth as well as the UV photodetector p-i-n structure. A paper describing the results, â€œAlxGa1-xN-Based Back-Illuminated Solar-Blind Photodetectors with External Quantum Efficiency of 89%,â€ was published November 5 in the journal Applied Physics Letters.
Although sapphire is the most common choice for back-illuminated devices, researchers also developed alternative low-cost UV photodetectors grown on silicon substrate. Razeghiâ€™s group used a novel maskless Lateral Epitaxial Overgrowth (LEO) technique for the growth of a high-quality aluminum nitride (AlN) template layer on silicon substrate. Following the template growth, a p-i-n structure is grown and processed. This low-cost approach eventually led to the worldâ€™s first successful implementation of UV-PD structure grown on a silicon substrate. A paper describing the findings, â€œAlxGa1-xN-Based Solar-Blind Photodetector Based on Lateral Epitaxial Overgrowth of AlN on Si Substrate,â€ was published October 30 in the journal Applied Physics Letters. ... [read more]
| CQD Research Highlighted on the Cover |
Physica Status Solidi C - October 14, 2013
Self-forming, vertically-aligned, arrays of black-body-like ZnO moth-eye nanostructures were grown on Si(111), c-Al2O3, ZnO and high manganese austenitic steel substrates using Pulsed Laser Deposition. X-ray diffraction (XRD) revealed the nanostructures to be well-crystallised wurtzite ZnO with strong preferential c-axis crystallographic orientation along the growth direction for all the substrates. Cathodoluminescence (CL) studies revealed emission characteristic of the ZnO near band edge for all substrates. Such moth-eye nanostructures have a graded effective refractive index and exhibit black-body characteristics. Coatings with these features may offer improvements in photovoltaic and LED performance. Moreover, since ZnO nanostructures can be grown readily on a wide range of substrates it is suggested that such an approach could facilitate growth of GaN-based devices on mismatched and/or technologically important substrates, which may have been inaccessible till present. ... [read more]
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| SOLID-STATE DEEP UV EMITTERS/DETECTORS: Zinc oxide moves further into the ultraviolet |
Laser Focus World - October 10, 2013
Zinc oxide (ZnO) is a remarkable, multifunctional semiconducting material with a direct, wide bandgap energy (Eg ~ 3.4 eV), intrinsically high transparency over the whole visible range, and a resistivity that can be tuned from semi-insulating right through to semi-metallic by doping. In photovoltaics, ZnO is currently displacing indium tin oxide for use as a transparent conducting electrical contact due to recent improvements in conductivity obtainable with aluminium-doped ZnO, combined with processing, cost, and toxicity advantages. Alloys of ZnO with magnesium (MgxZn1-xO) have been explored as an alternative to (Al)GaN for UV LED applications. For fabricating (Mg)ZnO-based solar-blind photodetectors, Mg content should be at least 45%, but many studies have found that phase segregation of rock-salt phase MgO appears when Mg content is over 36 at %. Recently, however, single-phase wurtzite layers with Mg concentrations over 49 at % were prepared through strain engineering at the substrate interface by means of buffer layers (ZnO and strontium tin oxideâ€”SrTiO3â€”or bulk ZnO substrates). ... [read more]
| Northwestern Researchers Develop Compact, High-Power Terahertz Source at Room Temperature |
News from McCormick - October 3, 2013
Terahertz (THz) radiation â€” radiation in the wavelength range of 30 to 300 microns â€” is gaining attention due to its applications in security screening, medical and industrial imaging, agricultural inspection, astronomical research, and other areas. Traditional methods of generating terahertz radiation, however, usually involve large and expensive instruments, some of which also require cryogenic cooling. A compact terahertz source â€” similar to the laser diode found in a DVD player â€”operating at room temperature with high power has been a dream device in the terahertz community for decades. ... [read more]
| Stress-reduced solar-blind AlGaN-based FPA is crack-free |
Laser Focus World, Vol. 49, No. 4, p. 14 - April 30, 2013
Focal-plane arrays (FOAs) that operate in the solar-blind spectral region (created by the absorption of light below 290 nm by atmospheric ozone)have uses in covert non-line-of-sight other forms of free space communications as well as UV spectroscopy, flame detection,and many other applications. With their extremely high rejection of the non-solar-blind spectrum , aluminum gallium nitride (AlGaN) -based structures are potentially excellent photodetecting candidates for solar blind FPAs. However this type of structures is difficult to fabricate without the formation of cracks. ... [read more]
| Imager combines SWIR and MWIR sensitivity |
Laser Focus World, Vol. 49, No. 2, p. 24-25 - February 27, 2013
Manijeh Razeghi and her group at Northwestern University have created High performance infrared (IR) image that combines detection in both the shortwave and midwave IR(SWIR and MWIR) in one device-a quality particularly valuable for tracking and reconnaissance. The device is based on a III-V semiconductor-based type-II indium arsenide/gallium antimonide (InAs/GaSb) superviously had only been implemented for MWIR and longwave IR (LWIR). Each pixel is made of an MWIR single heterodiode grown on top of a p-i-n SWIR homodiode, all fabricated on a GaSb substrate, which is subsequently removed for backside illumination (meaning the SWIR layer is on top). Crucially, the residual InAsSb etch-stop layer is also removed, using a citric-acid-based solution that etches away a specially inserted GaSb layer, eliminating the problem of the 4 Âµm InAsSb absorption-band edge. The prototype focal-plane array (FPA) has 320 256 pixels. ... [read more]