Center for Quantum Devices - Most Downloaded Journal Articles

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2.	Recent progress of quantum cascade laser research from 3 to 12 μm at the Center for Quantum Devices MANIJEH RAZEGHI,* WENJIA ZHOU,STEVEN SLIVKEN,QUAN-YONG LU,DONGHAI WU, AND RYAN MCCLINTOC Applied Optics Vol. 56, No. 31 -- October 10, 2017 ...[Visit Journal] The quantum cascade laser (QCL) is becoming the leading laser source in the mid-infrared (mid-IR) range, which contains two atmospheric transmission windows and many molecular fingerprint absorption features. Since its first demonstration in 1994, the QCL has undergone tremendous development in terms of the output power, wall plug efficiency, wavelength coverage, tunability and beam quality. At the Center for Quantum Devices, we have demonstrated high-power continuous wave operation of QCLs covering a wide wavelength range from 3 to 12 μm, with power output up to 5.1 W at room temperature. Recent research has resulted in power scaling in pulsed mode with up to 203 W output, electrically tunable QCLs based on monolithic sampled grating design, heterogeneous QCLs with a broad spectral gain, broadly tunable on-chip beam-combined QCLs, QCL-based mid-IR frequency combs, and fundamental mode surface emitting quantum cascade ring lasers. The developed QCLs will be the basis for a number of next-generation spectroscopy and sensing systems. [reprint (PDF)]
2.	High Quantum Efficiency AlGaN Solar-Blind Photodetectors R. McClintock, A. Yasan, K. Mayes, D. Shiell, S.R. Darvish, P. Kung and M. Razeghi Applied Physics Letters, 84 (8)-- February 23, 2004 ...[Visit Journal] We report AlGaN-based back-illuminated solar-blind ultraviolet p-i-n photodetectors with a peak responsivity of 136 mA/W at 282 nm without bias. This corresponds to a high external quantum efficiency of 60%, which improves to a value as high as 72% under 5 V reverse bias. We attribute the high performance of these devices to the use of a very-high quality AlN and Al_0.87Ga_0.13N/AlN superlattice material and a highly conductive Si–In co-doped Al_0.5Ga_0.5N layer [reprint (PDF)]
2.	High performance focal plane array based on type-II InAs/GaSb superlattice heterostructures P.Y. Delaunay and M. Razeghi SPIE Conference, January 20-25, 2008, San Jose, CA Proceedings – Quantum Sensing and Nanophotonic Devices V, Vol. 6900, p. 69000M-1-10.-- February 1, 2008 ...[Visit Journal] Recent progress in growth techniques, structure design and processing has lifted the performances of Type-II InAs/GaSb superlattice photodetectors. A double heterostructure design, based on a low band gap (11 µm) active region and high band gap (5 µm) superlattice contacts, reduced the sensitivity of the superlattice to surface effects. The heterodiodes with an 11 µm cutoff, passivated with SiO₂, presented similar performances to unpassivated devices and a one order of magnitude increase of the resistivity of the sidewalls, even after flip-chip bonding and underfill. Thanks to this new design and to the inversion of the polarity of the devices, a high performance focal plane array with an 11 µm cutoff was demonstrated. The noise equivalent temperature difference was measured as 26 mK and 19 mK for operating temperatures of 81 K and 67 K. At an integration time of 0.08 ms, the FPA presented a quantum efficiency superior to 50%. [reprint (PDF)]
2.	Development of material quality and structural design for high performance type-II InAs/GaSb superlattice photodiodes and focal plane arrays M. Razeghi, B.M. Nguyen, D. Hoffman, P.Y. Delaunay, E.K. Huang, M.Z. Tidrow and V. Nathan SPIE Porceedings, Vol. 7082, San Diego, CA 2008, p. 708204-- August 11, 2008 ...[Visit Journal] Recent progress made in the structure design, growth and processing of Type-II InAs/GaSb superlattice photo-detectors lifted both the quantum efficiency and the R0A product of the detectors. Type-II superlattice demonstrated its ability to perform imaging in the Mid-Wave Infrared (MWIR)and Long-Wave Infrared (LWIR) ranges, becoming a potential competitor for technologies such as Quantum Well Infrared Photo-detectors (QWIP) and Mercury Cadmium Telluride (MCT). Using an empirical tight-binding model, we developed superlattices designs that were nearly lattice-matched to the GaSb substrates and presented cutoff wavelengths of 5 and 11 μm. We demonstrated high quality material growth with X-ray FWHM below 30 arcsec and an AFM rms roughness of 1.5 Å over an area of 20x20 μm2. The detectors with a 5 μm cutoff, capable of operating at room temperature, showed a R0A of 1.25 106 Ω.cm2 at 77K, and a quantum efficiency of 32%. In the long wavelength infrared, we demonstrated high quantum efficiencies above 50% with high R0A products of 12 Ω.cm2 by increasing the thickness of the active region. Using the novel M-structure superlattice design, more than one order of magnitude improvement has been observed for electrical performance of the devices. Focal plane arrays in the middle and long infrared range, hybridized to an Indigo read out integrated circuit, exhibited high quality imaging. [reprint (PDF)]
2.	Very high performance LWIR and VLWIR type-II InAs/GaSb superlattice photodiodes with M-structure barrier B.M. Nguyen, D. Hoffman, P.Y. Delaunay, E.K. Huang and M. Razeghi SPIE Proceedings, Vol. 7082, San Diego, CA 2008, p. 708205-- September 3, 2008 ...[Visit Journal] LWIR and VLWIR type-II InAs/GaSb superlattice photodetectors have for long time suffered from a high dark current level and a low dynamic resistance which hampers the its emergence to the infrared detection and imaging industry. However, with the use of M-structure superlattice, a new Type-II binary InAs/GaSb/AlSb superlattice design, as an effective blocking barrier, the dark current in type-II superlattice diode has been significantly reduced. We have obtained comparable differential resistance product to the MCT technology at the cut-off wavelength of 10 and 14μm. Also, this new design is compatible with the optical optimization scheme, leading to high quantum efficiency, high special detectivity devices for photon detectors and focal plane arrays. [reprint (PDF)]
2.	III-nitride based avalanche photo detectors R. McClintock, E. Cicek, Z. Vashaei, C. Bayram, M. Razeghi and M. Ulmer Proceedings, Vol. 7780, p. 77801B, SPIE Optics and Photonics Symposium, Conference on Detectors and Imaging Devices: Infrared, Focal Plane and Single Photon, San Diego, CA -- August 4, 2010 ...[Visit Journal] Research into III-Nitride based avalanche photodiodes (APDs) is motivated by the need for high sensitivity ultraviolet (UV) detectors in numerous civilian and military applications. By designing III-Nitride photodetectors that utilize low-noise impact ionization high internal gain can be realized-GaN APDs operating in Geiger mode can achieve gains exceeding 1×10⁷. Thus with careful design, it becomes possible to count photons at the single photon level. In this paper we review the current state of the art in III-Nitride visible-blind APDs and discuss the critical design choices necessary to achieve high performance Geiger mode devices. Other major technical issues associated with the realization of visible-blind Geiger mode APDs are also discussed in detail and future prospects for improving upon the performance of these devices are outlined. The photon detection efficiency, dark count rate, and spectral response of or most recent Geiger-mode GaN APDs on free-standing GaN substrates are studied under low photon fluxes, with single photon detection capabilities being demonstrated. We also present our latest results regarding linear mode gain uniformity: the study of gain uniformity helps reveal the spatial origins of gain so that we can better understand the role of defects. [reprint (PDF)]
2.	Current status and potential of high power mid-infrared intersubband lasers S. Slivken, Y. Bai, B. Gokden, S.R. Darvish and M. Razeghi SPIE Proceedings, San Francisco, CA (January 22-28, 2010), Vol. 7608, p. 76080B-1-- January 22, 2010 ...[Visit Journal] Some of the recent advances in high power quantum cascade laser development will be reviewed in this paper. Research areas explored include short wavelength (λ <4 µm) lasers, high performance strain-balanced heterostructures, and high power long wavelength (7< λ< 16 µm) lasers. Near λ=4.5 µm, highlights include demonstration of 18% continuous wave wallplug efficiency at room temperature, 53% pulsed wallplug efficiency at 40 K, and 120 W of peak power output from a single device at room temperature. Near λ ~10 µm, up to 0.6 W of continuous output power at room temperature has also been demonstrated, with pulsed efficiencies up to 9%. [reprint (PDF)]
2.	High power InAsSb/InPAsSb/InAs mid-infrared lasers A. Rybaltowski, Y. Xiao, D. Wu, B. Lane, H. Yi, H. Feng, J. Diaz, and M. Razeghi Applied Physics Letters 71 (17)-- October 27, 1997 ...[Visit Journal] We demonstrate high-power InAsSb/InPAsSb laser bars (λ ≈ 3.2 μm) consisting of three 100 μm-wide laser stripes of 700 μm cavity length, with peak output power up to 3 W at 90 K, and far-fields for the direction perpendicular to the junction as narrow as 12° full width half maximum. Spectra and far-field patterns of the laser bars are shown to have excellent characteristics for a wide range of operating conditions, suggesting the possibility of even higher light power emission with good beam quality. Joule heating is shown to be the major factor limiting higher power operation. [reprint (PDF)]
2.	Substrate removal for high quantum efficiency back side illuminated type-II InAs/GaSb photodetectors P.Y. Delaunay, B.M. Nguyen, D. Hoffman and M. Razeghi Applied Physics Letters, Vol. 91, No. 23, p. 231106-- December 3, 2007 ...[Visit Journal] A substrate removal technique using an InAsSb etch stop layer improves by a factor of 2 the quantum efficiency of back side illuminated type-II InAs/GaSb superlattice photodetectors. After etching of the GaSb substrate with a CrO₃ based solution, the quantum efficiency of the diodes presents Fabry-Pérot oscillations averaging at 56%. Due to the confinement of the infrared light inside the devices, the quantum efficiency for certain devices reaches 75% at 8.5 µm. The implementation of this new technique to a focal plane array resulted in a decrease of the integration time from 0.23 to 0.08 ms. [reprint (PDF)]
2.	Quantum-dot infrared photodetectors and focal plane arrays M. Razeghi, H. Lim, S. Tsao, M. Taguchi, W. Zhang, and A.A. Quivy SPIE Infrared Technology and Applications Conference, April 17-21, 2006, Orlando, FL Proceedings – Infrared Technology and Applications XXXII, Vol. 6206, p. 62060I-1-- April 21, 2006 ...[Visit Journal] We report our recent results about mid-wavelength infrared quantum-dot infrared photodetectors (QDIPs) grown by low-pressure metalorganic chemical vapor deposition. A very high responsivity and a very low dark current were obtained. A high peak detectivity of the order of 3×10¹² Jones was achieved at 77 K. The temperature dependent device performance was also investigated. The improved temperature insensitivity compared to QWIPs was attributed to the properties of quantum dots. The device showed a background limited performance temperature of 220 K with a 45° field of view and 300K background. [reprint (PDF)]
2.	High Power 0.98 μm GaInAs/GaAs/GaInP Multiple Quantum Well Laser K. Mobarhan, M. Razeghi, G. Marquebielle and E. Vassilaki Journal of Applied Physics 72 (9)-- November 1, 1992 ...[Visit Journal] We report the fabrication of high quality Ga_0.8In_0.2As/GaAs/Ga_0.51In_0.49P multiple quantum well laser emitting at 0.98 μm grown by low pressure metalorganic chemical vapor deposition. Continuous wave operation with output power of 500 mW per facet was achieved at room temperature for a broad area laser with 130 μm width and 300 μm cavity length. This is an unusually high value of output power for this wavelength laser in this material system. The differential quantum efficiency exceeded 75% with excellent homogeneity and uniformity. The characteristic temperature, T₀ was in the range of 120–130 K. [reprint (PDF)]
2.	MOCVD grown β-Ga₂O₃ metal-oxide-semiconductor field effect transistors on sapphire Ji-Hyeon Park , Ryan McClintock, Alexandre Jaud, Arash Dehzangi , Manijeh Razeghi Applied Physics Express 12, 095503-- August 28, 2019 ...[Visit Journal] We fabricated β-Ga₂O₃:Si metal-oxide field-effect transistors (MOSFETs) on c-plane sapphire substrates which typically showed maximum drain current of 100 mA·mm⁻¹. β-Ga₂O₃:Si thin films were realized on c-plane sapphire substrates through a combination of metalorganic chemical vapor deposition and post-annealing. The MOSFET device presented excellent on/off drain current ratio of ∼10¹¹ with very low gate leakage current, sharp pinch off behavior, and a breakdown voltage of 400 V at VG = −40 V. The growth and fabrication of β-Ga₂O₃:Si MOSFETs on c-plane sapphire is valuable to its demonstration of the great potential for future high-power electronic devices. [reprint (PDF)]
2.	Suppressing Spectral Crosstalk in Dual-Band LongWavelength Infrared Photodetectors With Monolithically Integrated Air-Gapped Distributed Bragg Reﬂectors Yiyun Zhang, Abbas Haddadi, Arash Dehzangi , Romain Chevallier, Manijeh Razeghi IEEE Journal of Quantum Electronics Volume: 55, Issue:1-- November 22, 2018 ...[Visit Journal] Antimonide-based type-II superlattices (T2SLs) have made possible the development of high-performance infrared cameras for use in a wide variety of thermal imaging applications, many of which could beneﬁt from dual-band imaging. The performance of this material system has not reached its limits. One of the key issues in dual-band infrared photodetection is spectral crosstalk. In this paper, air-gapped distributed Bragg reﬂectors (DBRs) have been monolithically integrated between the two channels in long-/very long-wavelength dualband InAs/InAs_1−xSb_x/AlAs_1−xSb_x-based T2SLs photodetectors to suppress the spectral crosstalk. This air-gapped DBR has achieved a signiﬁcant spectral suppression in the 4.5–7.5-µm photonic stopband while transmitting the optical wavelengths beyond 7.5 µm, which is conﬁrmed by theoretical calculations, numerical simulation, and experimental results. [reprint (PDF)]
2.	High Power, Continuous-Wave, Quantum Cascade Lasers for MWIR and LWIR Applications S. Slivken, A. Evans, J.S. Yu, S.R. Darvish and M. Razeghi SPIE Conference, San Jose, CA, Vol. 6127, pp. 612703-- January 23, 2006 ...[Visit Journal] Over the past several years, our group has endeavored to develop high power quantum cascade lasers for a variety of remote and high sensitivity infrared applications. The systematic optimization of laser performance has allowed for demonstration of high power, continuous-wave quantum cascade lasers operating above room temperature. Since 2002, the power levels for individual devices have jumped from 20 mW to 600 mW. Expanding on this development, we have able to demonstrate continuous wave operation at many wavelengths throughout the mid- and far-infrared spectral range, and have now achieved >100 mW output in the 4.0 to 9.5 µm range. [reprint (PDF)]
2.	Growth and Characterization of Type-II Non-Equilibrium Photovoltaic Detectors for Long Wavelength Infrared Range H. Mohseni, J. Wojkowski, A. Tahraoui, M. Razeghi, G. Brown and W. Mitche SPIE Conference, San Jose, CA, -- January 26, 2000 ...[Visit Journal] Growth and characterization of type-II detectors for mid-IR wavelength range is presented. The device has a p-i-n structure is designed to operate in the non-equilibrium mode with low tunneling current. The active layer is a short period InAs/GaSb superlattice. Wider bandgap p-type AlSb and n-type InAs layers are used to facilitate the extraction of both electronics and holes from the active layer for the first time. The performance of these devices were compared to the performance of devices grown at the same condition, but without the AlSb barrier layers. The processed devices with the AlSb barrier show a peak responsivity of about 1.2 A/W with Johnson noise limited detectivity of 1.1 X 10¹¹ cm·Hz^½/W at 8 μm at 80 K at zero bias. The details of the modeling, growth, and characterizations will be presented. [reprint (PDF)]
2.	Recent advances in antimonide-based gap-engineered Type-II superlattices material system for 2 and 3 colors infrared imagers Manijeh. Razeghi, Abbas Haddadi, Arash Dehzangi, Romain Chevallier, and Thomas Yang Proceedings of SPIE 10177, Infrared Technology and Applications XLIII, 1017705-- May 9, 2017 ...[Visit Journal] InAs/InAs_1-xSb_x/AlAs_1-xSb_x type-II superlattices (T2SLs) is a system of multi-interacting quantum wells. Since its introduction, this material system has drawn a lot of attention especially for infrared detection. In recent years, InAs/InAs_1- xSb_x/AlAs_1-xSb_x T2SL material system has experienced incredible improvements in material quality, device structure designs and device fabrication process which elevated the performances of T2SL-based photodetectors to a comparable level to the state-of-the-art material systems for infrared detection such as Mercury Cadmium Telluride (MCT). In this paper, we will present the current status of InAs/InAs_1-xSb_x/AlAs_1-xSb_x T2SL-based photodetectors for detection in different infrared regions, from short-wavelength (SWIR) to long-wavelength (LWIR) infrared, and the future outlook of this material system. [reprint (PDF)]
2.	Very Long Wavelength Infrared Type-II Detectors Operating at 80K H. Mohseni, A. Tahraoui, J. Wojkowski, M. Razeghi, G.J. Brown, W.C. Mitchel, and Y.S. Park Applied Physics Letters 77 (11)-- September 11, 2000 ...[Visit Journal] We report a demonstration of very long wavelength infrared detectors based on InAs/GaSb superlattices operating at T = 80 K. Detector structures with excellent material quality were grown on an optimized GaSb buffer layer on GaAs semi-insulating substrates. Photoconductive devices with 50% cutoff wavelength of λ_c = 17 μm showed a peak responsivity of about 100 mA/W at T = 80 K. Devices with 50% cutoff wavelengths up to λ_c = 22 μm were demonstrated at this temperature. Good uniformity was obtained over large areas even for the devices with very long cutoff wavelengths. [reprint (PDF)]
2.	High-power, continuous-operation intersubband laser for wavelengths greater than 10 micron S. Slivken, A. Evans, W. Zhang and M. Razeghi Applied Physics Letters, Vol. 90, No. 15, p. 151115-1-- April 9, 2007 ...[Visit Journal] In this letter, high-power continuous-wave emission (>100 mW) and high temperature operation (358 K) at a wavelength of 10.6 µm is demonstrated using an individual diode laser. This wavelength is advantageous for many medium-power applications previously reserved for the carbon dioxide laser. Improved performance was accomplished using industry-standard InP-based materials and by careful attention to design, growth, and fabrication limitations specific to long-wave infrared semiconductor lasers. The main problem areas are explored with regard to laser performance, and general steps are outlined to minimize their impact. [reprint (PDF)]
2.	Active and passive infrared imager based on short-wave and mid-wave type-II superlattice dual-band detectors E.K. Huang, A. Haddadi, G. Chen, A.M. Hoang, and M. Razeghi Optics Letters, Vol. 38, no. 1, p. 22-24-- January 1, 2013 ...[Visit Journal] A versatile dual-band detector capable of active and passive use is demonstrated using short-wave (SW) and midwave(MW) IR type-II superlattice photodiodes. A bilayer etch-stop scheme is introduced for back-side-illuminated detectors, which enhanced the external quantum efficiency both in the SWIR and MWIR spectral regions. Temperature-dependent dark current measurements of pixel-sized 27 μm detectors found the dark current density to be ~1 × 10^-5 A/cm² for the ∼4.2 μm cutoff MWIR channel at 140 K. This corresponded to a reasonable imager noise equivalent difference in temperature of ∼49 mK using F∕2.3 optics and a 10 ms integration time (t_int), which lowered to ∼13 mK at 110 K using tint 30 ms, illustrating the potential for high-temperature operation. The SWIR channel was found to be limited by readout noise below 150 K. Excellent imagery from the dual-band imager exemplifying pixel coincidence is shown. [reprint (PDF)]
2.	Band edge tunability of M-structure for heterojunction design in Sb based Type-II superlattice photodiodes B.M. Nguyen, D. Hoffman, P.Y. Delaunay, E.K. Huang, M. Razeghi, and J. Pellegrino Applied Physics Letters, Vol. 93, No. 16, p. 163502-1-- October 20, 2008 ...[Visit Journal] We present theoretically and experimentally the effect of the band discontinuity in Type-II misaligned InAs/GaSb superlattice heterodiodes. Calculations using the empirical tight binding method have shown the great flexibility in tuning the energy levels of the band edge in M-structure superlattice as compared to the standard InAs/GaSb superlattice. Through the experimental realization of several p-pi-M-n photodiodes, the band discontinuity alignment between the standard binary-binary superlattice and the M-structured superlattice was investigated via optical characterization. The agreement between the theoretical predictions and the experimental measurement confirms the capability of controlling the M-structure band edges and suggests a way to exploit this advantage for the realization of heterostructures containing an M-structured superlattice without bias dependent operation. [reprint (PDF)]
2.	Inductively coupled plasma etching and processing techniques for type-II InAs/GaSb superlattices infrared detectors toward high fill factor focal plane arrays E.K. Huang, B.M. Nguyen, D. Hoffman, P.Y. Delaunay and M. Razeghi SPIE Proceedings, San Jose, CA Volume 7222-0Z-- January 26, 2009 ...[Visit Journal] A challenge for Type-II InAs/GaSb superlattice (T2SL) photodetectors is to achieve high fill factor, high aspect ratio etching for third generation focal plane arrays (FPAs). Initially, we compare the morphological and electrical results of single element T2SL photodiodes after BCl₃/Ar inductively coupled plasma (ICP) and electron cyclotron resonance (ECR) dry etching. Using a Si₃N₄ hard mask, ICP-etched structures exemplify greater sidewall verticality and smoothness, which are essential toward the realization of high fill factor FPAs. ICP-etched single element devices with SiO₂ passivation that are 9.3 µm in cutoff wavelength achieved vertical sidewalls of 7.7 µm in depth with a resistance area product at zero bias of greater than 1,000 Ω·cm² and maximum differential resistance in excess of 10,000 Ω·cm² at 77 K. By only modifying the etching technique in the fabrication steps, the ICP-etched photodiodes showed an order of magnitude decrease in their dark current densities in comparison to the ECR-etched devices. Finally, high aspect ratio etching is demonstrated on mutli-element arrays with 3 µm-wide trenches that are 11 µm deep. [reprint (PDF)]
2.	Molecular Beam Epitaxial Growth of High Quality InSb for p-i-n Photodetectors G. Singh, E. Michel, C. Jelen, S. Slivken, J. Xu, P. Bove, I. Ferguson, and M. Razeghi Journal of Vacuum Science and Technology B, 13 (2)-- March 1, 1995 ...[Visit Journal] The InSb infrared photodetectors grown heteroepitaxially on Si substrates by molecular beam epitaxy (MBE) are reported. Excellent InSb material quality is obtained on 3-inch Si substrates (with a GaAs predeposition) as confirmed by structural, optical, and electrical analysis. InSb infrared photodetectors on Si substrates that can operate from 77 K to room temperature have been demonstrated. The peak voltage-responsitivity at 4 μm is about 1.0×10³ V/W and the corresponding Johnson-noise-limited detectivity is calculated to be 2.8×10¹⁰ cm·Hz^½/W. This is the first important stage in developing InSb detector arrays or monolithic focal plane arrays (FPAs) on silicon. The development of this technology could provide a challenge to traditional hybrid FPA's in the future. [reprint (PDF)]
2.	Molecular beam epitaxial growth of InSb p-i-n photodetectors on GaAs and Si E. Michel, R. Peters, S. Slivken, C. Jelen, P. Bove, J. Xu, I. Ferguson, and M. Razeghi Optoelectronic Integrated Circuit Materials, Physics and Devices, SPIE Conference, San Jose, CA; Proceedings, Vol. 2397-- February 6, 1995 ...[Visit Journal] High quality InSb has been grown by Molecular Beam Epitaxy and optimized using Reflection High Energy Electron Diffraction. A 4.8 micrometers InSb layer grown on GaAs at a growth temperature of 395 degree(s)C and a III/V incorporation ratio of 1:1.2 had an X-ray rocking curve FWHM of 158 arcsec and a Hall mobility of 92300 cm2V-1s-1 at 77 K, the best reported to date for InSb nucleated directly onto GaAs. InSb p-i-n structures of 5.8 micrometers grown under the same conditions demonstrated a X-ray Full Width at Half Maximum of 101 arcsec and 131 arcsec for GaAs and Si substrates, respectively, and exhibited excellent uniformity of +/- 3 arcsec over a 3' substrate. Prototype InSb p-i-n detectors on Si have been fabricated and have demonstrated photovoltaic response at 6.5 micrometers up to 200 K. These p-i-n detectors have also exhibited the highest D* for a device grown onto Si. [reprint (PDF)]
2.	Monolithically, widely tunable quantum cascade lasers based on a heterogeneous active region design Wenjia Zhou, Neelanjan Bandyopadhyay, Donghai Wu, Ryan McClintock & Manijeh Razeghi Nature Scientific Reports 6, Article number: 25213 -- June 8, 2016 ...[Visit Journal] Quantum cascade lasers (QCLs) have become important laser sources for accessing the mid-infrared (mid-IR) spectral range, achieving watt-level continuous wave operation in a compact package at room temperature. However, up to now, wavelength tuning, which is desirable for most applications, has relied on external cavity feedback or exhibited a limited monolithic tuning range. Here we demonstrate a widely tunable QCL source over the 6.2 to 9.1 μm wavelength range with a single emitting aperture by integrating an eight-laser sampled grating distributed feedback laser array with an on-chip beam combiner. The laser gain medium is based on a five-core heterogeneous QCL wafer. A compact tunable laser system was built to drive the individual lasers within the array and produce any desired wavelength within the available spectral range. A rapid, broadband spectral measurement (520 cm⁻¹) of methane using the tunable laser source shows excellent agreement to a measurement made using a standard low-speed infrared spectrometer. This monolithic, widely tunable laser technology is compact, with no moving parts, and will open new opportunities for MIR spectroscopy and chemical sensing. [reprint (PDF)]
2.	Room temperature neagtive differential resistance characteristics of polar III-nitride resonant tunneling diodes C. Bayram, Z. Vashaei, and M. Razeghi Applied Physics Letters, Vol. 97, No. 9, p. 092104-1-- August 30, 2010 ...[Visit Journal] III-nitride resonant tunneling diodes (RTDs), consisting Al_0.2Ga_0.8N/GaN double-barrier (DB) active layers, were grown on c-plane lateral epitaxial overgrowth (LEO) GaN/sapphire and c-plane freestanding (FS) GaN. RTDs on both templates, fabricated into mesa diameters ranging from 5 to 35 μm, showed negative differential resistance (NDR) at room temperature. NDR characteristics (voltage and current density at NDR onset and current-peak-to-valley ratio) were analyzed and reported as a function of device size and substrate choice. Our results show that LEO RTDs perform as well as FS ones and DB active layer design and quality have been the bottlenecks in III-nitride RTDs. [reprint (PDF)]

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