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276.  ZnO 3D flower-like nanostructure synthesized on GaN epitaxial layer by simple route hydrothermal process
J.M. Jung, C.R. Kim, H. Ryu, M. Razeghi and W.G. Jung
Journal of Alloys and Compounds-- September 15, 2007
The 3D type, flower-like ZnO nanostructures from particle to flower-like or chestnut bur are fabricated on the GaN epitaxial layer substrate through the simple-route hydrothermal process. Structural characterization was made for the ZnO 3D nanostructures synthesized in different pH ranging from 9.5 to 11.0. The growth model was proposed and discussed regarding the fabrication mechanism and morphology of ZnO 3D flower-like nanostructure. The flower-like ZnO is composed of many thin single crystals ZnO nanorods. Bigger and thicker ZnO structure is fabricated with the increase of pH in solution. The enhanced UV emission in the PL measurement and the spectra in the Raman spectroscopy for ZnO–GaN heterojunction material were discussed. reprint
277.  Polarity inversion of Type-II InAs/GaSb superlattice photodiodes
B.M. Nguyen, D. Hoffman, P.Y. Delaunay, M. Razeghi and V. Nathan
Applied Physics Letters, Vol. 91, No. 10, p. 103503-1-- September 3, 2007
The authors demonstrated the realization of p-on-n Type-II InAs/GaSb superlattice photodiodes. Growth condition for high quality InAsSb layer lattice matched to GaSb was established for the use of an effective n-contact layer. By studying the effect of various GaSb capping layer thicknesses on the optical and electrical performances, an optimized thickness of 160 nm was determined. In comparison to as grown n-on-p superlattice photodiodes, this inverted design of p on n has shown similar quality. Finally, by analyzing Fabry-Perot interference fringes in the front side illuminated spectral measurement, the refractive index of the superlattice was determined to be approximately 3.8. reprint
278.  Passivation of type-II InAs/GaSb double heterostructure
P.Y. Delaunay, A. Hood, B.M. Nguyen, D. Hoffman, Y. Wei, and M. Razeghi
Applied Physics Letters, Vol. 91, No. 9, p. 091112-1-- August 27, 2007
Focal plane array fabrication requires a well passivated material that is resistant to aggressive processes. The authors report on the ability of type-II InAs/GaSb superlattice heterodiodes to be more resilient than homojunctions diodes in improving sidewall resistivity through the use of various passivation techniques. The heterostructure consisting of two wide band gap (5 µm) superlattice contacts and a low band gap active region (11 µm) exhibits an R0A averaging of 13·Ω cm2. The devices passivated with SiO2, Na2S and SiO2 or polyimide did not degrade compared to the unpassivated sample and the resistivity of the sidewalls increased to 47 kΩ·cm. reprint
279.  Buried heterostructure quantum cascade lasers with high continuous-wave wall plug efficiency
A. Evans, S.R. Darvish, S. Slivken, J. Nguyen, Y. Bai and M. Razeghi
Applied Physics Letters, Vol. 91, No. 7, p. 071101-1-- August 13, 2007
The authors report on the development of ~4.7 µm strain-balanced InP-based quantum cascade lasers with high wall plug efficiency and room temperature continuous-wave operation. The use of narrow-ridge buried heterostructure waveguides and thermally optimized packaging is presented. Over 9.3% wall plug efficiency is reported at room temperature from a single device producing over 0.675 W of continuous-wave output power. Wall plug efficiencies greater than 18% are also reported for devices at a temperature of 150 K, with continuous-wave output powers of more than 1 W. reprint
280.  Scaling in back-illuminated GaN avalanche photodiodes
K. Minder, J.L. Pau, R. McClintock, P. Kung, C. Bayram, M. Razeghi and D. Silversmith
Applied Physics Letters, Vol. 91, No. 7, p. 073513-1-- August 13, 2007
Avalanche p-i-n photodiodes of various mesa areas were fabricated on AlN templates for back illumination for enhanced performance through hole-initiated multiplication, and the effects of increased area on device performance were studied. Avalanche multiplication was observed in mesa sizes up to 14,063 µm^2 under linear mode operation. Uniform gain and a linear increase of the dark current with area were demonstrated. reprint
281.  Use of ZnO thin films as sacrifical templates for metal organic vapor phase epitaxy and chemical lift-off of GaN
D.J. Rogers, F. Hosseini Teherani, A. Ougazzaden, S. Gautier, L. Divay, A. Lusson, O. Durand, F. Wyczisk, G. Garry, T. Monteiro, M.R. Correira, M. Peres, A. Neves, D. McGrouther, J.N. Chapman, and M. Razeghi
Applied Physics Letters, Vol. 91, No. 7, p. 071120-1-- August 13, 2007
Continued development of GaN-based light emitting diodes is being hampered by constraints imposed by current non-native substrates. ZnO is a promising alternative substrate but it decomposes under the conditions used in conventional GaN metal organic vapor phase epitaxy (MOVPE). In this work, GaN was grown on ZnO/c-Al2O3 using low temperature/pressure MOVPE with N2 as a carrier and dimethylhydrazine as a N source. Characterization confirmed the epitaxial growth of GaN. The GaN was lifted-off the c-Al2O3 by chemically etching away the ZnO underlayer. This approach opens up the way for bonding of the GaN onto a support of choice. reprint
282.  Geiger-mode operation of back-illuminated GaN avalanche photodiodes
J. L. Pau, R. McClintock, K. Minder, C. Bayram, P. Kung, M. Razeghi, E. Muñoz, and D. Silversmith
Applied Physics Letters, Vol. 91, No. 04, p. 041104 -1-- July 23, 2007
We report the Geiger-mode operation of back-illuminated GaN avalanche photodiodes fabricated on transparent AlN templates specifically for back illumination in order to enhance hole-initiated multiplication. The spectral response in Geiger-mode operation was analyzed under low photon fluxes. Single photon detection capabilities were demonstrated in devices with areas ranging from 225 to 14,063 µm2. Single photon detection efficiency of 20% and dark count rate < 10 kHz were achieved in the smallest devices. reprint
283.  High-power mid- and far- wavelength infrared lasers for free space communication
M. Razeghi; A. Evans; J. Nguyen; Y. Bai; S. Slivken; S.R. Darvish; K. Mi
Proc. SPIE 6593, Photonic Materials, Devices, and Applications II, 65931V (June 12, 2007)-- June 12, 2007
Laser-based free-space communications have been developed to serve specific roles in "last mile" high-speed data networks due to their high security, low cost, portability, and high bandwidth. Conventional free-space systems based on near infrared optical devices suffer from reliability problems due to atmospheric scattering losses and scintillation effects, such as those encountered with storms, dust, and fog. Mid-infrared wavelengths are less affected by atmospheric effects and can significantly enhance link up-time and range. This paper will discuss some of the recent advances in high-power, high temperature, high reliability mid-infrared Quantum Cascade Lasers and their potential application in highly reliable free space communication links. reprint
284.  Very high quantum efficiency in type-II InAs/GaSb superlattice photodiode with cutoff of 12 µm
B.M. Nguyen, D. Hoffman, Y. Wei, P.Y. Delaunay, A. Hood and M. Razeghi
Applied Physics Letters, Vol. 90, No. 23, p. 231108-1-- June 4, 2007
The authors report the dependence of the quantum efficiency on device thickness of Type-II InAs/GaSb superlattice photodetectors with a cutoff wavelength around 12 µm. The quantum efficiency and responsivity show a clear delineation in comparison to the device thickness. An external single-pass quantum efficiency of 54% is obtained for a 12 µm cutoff wavelength photodiodes with a -region thickness of 6.0 µm. The R0A value is kept stable for the range of structure thicknesses allowing for a specific detectivity (2.2×1011 cm·Hz½/W). reprint
285.  Near bulk-limited R0A of long-wavelength infrared type-II InAs/GaSb superlattice photodiodes with polyimide surface passivation
Andrew Hood, Pierre-Yves Delaunay, Darin Hoffman, Binh-Minh Nguyen, Yajun Wei, Manijeh Razeghi, and Vaidya Nathan
Applied Physics Letters 90, 233513-- June 4, 2007
Effective surface passivation of Type-II InAs/GaSb superlattice photodiodes with cutoff wavelengths in the long-wavelength infrared is presented. A stable passivation layer, the electrical properties of which do not change as a function of the ambient environment nor time, has been prepared by a solvent-based surface preparation, vacuum desorption, and the application of an insulating polyimide layer. Passivated photodiodes, with dimensions ranging from 400×400 to 25×25 µm2, with a cutoff wavelength of ~11 µm, exhibited near bulk-limited R0A values of ~12 Ω·cm2, surface resistivities in excess of 104 Ω·cm, and very uniform current-voltage behavior at 77 K. reprint
286.  High operating temperature 320 x 256 middle-wavelength infrared focal plane array imaging based on an InAs/InGaAs/InAlAs/InP quantum dot infrared photodetector
S. Tsao, H. Lim, W. Zhang, and M. Razeghi
Virtual Journal of Nanoscale Science and Technology-- May 28, 2007reprint
287.  High operating temperature 320 x 256 middle-wavelength infrared focal plane array imaging based on an InAs/InGaAs/InAlAs/InP quantum dot infrared photodetector
S. Tsao, H. Lim, W. Zhang, and M. Razeghi
Applied Physics Letters, Vol. 90, No. 20, p. 201109-- May 14, 2007
This letter reports a 320×256 middle-wavelength infrared focal plane array operating at temperatures up to 200 K based on an InAs quantum dot/InGaAs quantum well/InAlAs barrier detector grown on InP substrate by low pressure metal organic chemical vapor deposition. The device's low dark current density and the persistence of the photocurrent up to room temperature enabled the high temperature imaging. The focal plane array had a peak detection wavelength of 4 µm, a responsivity of 34 mA/W, a conversion efficiency of 1.1%, and a noise equivalent temperature difference of 344 mK at an operating temperature of 120 K. reprint
288.  Current status of high performance quantum cascade lasers at the center for quantum devices
M. Razeghi; A. Evans; Y. Bai; J. Nguyen; S. Slivken; S.R. Darvish; K. Mi
Conference Proceedings - International Conference on Indium Phosphide and Related Materials. 588-593:[4266015] (2007)-- May 14, 2007
Mid-infrared laser sources are highly desired for laser-based trace chemical sensors, military countermeasures, free-space communications, as well as developing medical applications. While application development has been limited by the availability of adequate mid-infrared sources, InP-based quantum cascade lasers (QCLs) hold promise as inexpensive, miniature, portable solutions capable of producing high powers and operating at high temperatures with excellent beam quality and superior reliability. This paper discusses the most recent developments of application-ready high power (> 100 mW), continuous-wave (CW), mid-infrared QCLs operating above room temperature with lifetimes exceeding 13,000 hours. reprint
289.  Comparison of type-II superlattice and HgCdTe infrared detector technologies
Jagmohan Bajaj; Gerry Sullivan; Don Lee; Ed Aifer; Manijeh Razeghi
Proc. SPIE 6542, Infrared Technology and Applications XXXIII, 65420B (May 14, 2007)-- May 14, 2007
Performance of HgCdTe detector technology surpasses all others in the mid-wave and long-wave infrared spectrum. This technology is relatively mature with current effort focused on improving uniformity, and demonstrating increased focal plane array (FPA) functionality. Type-II superlattice (InAs-GaSb and related alloys) detector technology has seen rapid progress over the past few years. The merits of the superlattice material system rest on predictions of even higher performance than HgCdTe and of engineering advantages. While no one has demonstrated Type-II superlattice detectors with performance superior to HgCdTe detectors, the difference in performance between these two technologies is decreasing. In this paper, we review the status and highlight relative merits of both HgCdTe and Type-II superlattice based detector technologies. reprint
290.  Self-assembled semiconductor quantum dot infrared photodetector operating at room temperature and focal plane array
Ho-Chul Lim; Stanley Tsao; Wei Zhang; Manijen Razeghi
Proc. SPIE 6542, Infrared Technology and Applications XXXIII, 65420R (May 14, 2007)-- May 14, 2007
Self-assembled semiconductor quantum dots have attracted much attention because of their novel properties and thus possible practical applications including the lasers, detectors and modulators. Especially the photodetectors which have quantum dots in their active region have been developed and show promising performances such as high operation temperature due to three dimensional confinement of the carriers and normal incidence in contrast to the case of quantum well detectors which require special optical coupling schemes. Here we report our recent results for mid-wavelength infrared quantum dot infrared photodetector grown by low-pressure metalorganic chemical vapor deposition. The material system we have investigated consists of 25 period self-assembled InAs quantum dot layers on InAlAs barriers, which are lattice-matched to InP substrates, covered with InGaAs quantum well layers and InAlAs barriers. This active region was sandwiched by highly doped InP contact layers. The device operates at 4.1 μm with a peak detectivity of 2.8×1011 cm·Hz1/2/W at 120 K and a quantum efficiency of 35 %. The photoresponse can be observed even at room temperature resulting in a peak detectivity of 6×107 cm·Hz1/2/W. A 320×256 focal plane array has been fabricated in this kind of device. Its performance will also be discussed here. reprint
291.  Type II superlattice infrared detectors and focal plane arrays
Vaidya Nathan; Manijeh Razeghi
Proc. SPIE 6542, Infrared Technology and Applications XXXIII, 654209 (May 14, 2007)-- May 14, 2007
Type II superlattce photodetectors have recently experienced significant improvements in both theoretical structure design and experimental realization. Empirical Tight Binding Method is initiated and developed for Type II superlattice. Growth characteristics such as group V segregation and incorporation phenomena are taken into account in the model and shown higher precision. A new Type II structure, called M-structure, is introduced and theoretically demonstrated high R0A, high quantum efficiency. Device design is optimized to improve the performance. As a result, 55% quantum efficiency and 10 Ohm·cm² R0A are achieved for an 11.7 μm cut-off photodetector at 77K. FPA imaging at longwavelength is demonstrated with a capability of imaging up to 171K. At 81K, the noise equivalent temperature difference presented a peak at 0.33K. reprint
292.  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
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
293.  320x256 infrared focal plane array based on type-II InAs/GaSb superlattice with a 12 μm cutoff wavelength
P.Y. Delaunay, B.M. Nguyen, D. Hoffman, and M. Razeghi
SPIE Porceedings, Vol. 6542, Orlando, FL 2007, p. 654204-- April 9, 2007
In the past few years, significant progress has been made in the structure design, growth and processing of Type-II InAs/GaSb superlattice photodetectors. Type-II superlattice demonstrated its ability to perform imaging in the middle and long infra-red range, becoming a potential competitor for technologies such as QWIP and HgCdTe. Using an empirical tight-binding model, we developed a superlattice design that matches the lattice parameter of GaSb substrates and presents a cutoff wavelength of 12 μm. Electrical and optical measurements performed on single element detectors at 77 K showed an R0A averaging 13 Ω·cm² and a quantum efficiency as high as 54%. 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. A 320x256 array of 25x25μm² pixels, hybridized to an Indigo Read Out Integrated Circuit, performed thermal imaging up to 185 K with an operability close to 97%. The noise equivalent temperature difference at 81 K presented a peak at 270 mK, corresponding to a mean value of 340 mK. reprint
294.  Hole-initiated multiplication in back-illuminated GaN avalanche photodiodes
R. McClintock, J.L. Pau, K. Minder, C. Bayram, P. Kung and M. Razeghi
Applied Physics Letters, Vol. 90 No. 14, p. 141112-1-- April 2, 2007
Avalanche p-i-n photodiodes were fabricated on AlN templates for back illumination. Structures with different intrinsic layer thicknesses were tested. A critical electric field of 2.73 MV/cm was estimated from the variation of the breakdown voltage with thickness. From the device response under back and front illumination and the consequent selective injection of holes and electrons in the junction, ionization coefficients were obtained for GaN. The hole ionization coefficient was found to be higher than the electron ionization coefficient as predicted by theory. Excess multiplication noise factors were also calculated for back and front illumination, and indicated a higher noise contribution for electron injection. reprint
295.  Materials characterization of n-ZnO/p-GaN:Mg/c-Al(2)O(3) UV LEDs grown by pulsed laser deposition and metal-organic chemical vapor deposition
D. Rogers, F.H. Teherani, P. Kung, K. Minder, and M. Razeghi
Superlattices and Microstructures-- April 1, 2007
n-ZnO/p-GaN:Mg hybrid heterojunctions grown on c-Al2O3 substrates showed 375 nm room temperature electroluminescence. It was suggested that the high materials and interface quality obtained using pulsed laser deposition for the n-ZnO growth and metal–organic chemical vapor deposition for the p-GaN:Mg were key factors enabling the injection of holes and the radiative near band edge recombination in the ZnO. In this paper we present the materials characterization of this structure using x-ray diffraction, scanning electron microscopy and atomic force microscopy. reprint
296.  High-performance InAs quantum-dot infrared photodetectors grown on InP substrate operating at room temperature
H. Lim, S. Tsao, W. Zhang, and M. Razeghi
Applied Physics Letters, Vol. 90, No. 13, p. 131112-1-- March 26, 2007
The authors report a room temperature operating InAs quantum-dot infrared photodetector grown on InP substrate. The self-assembled InAs quantum dots and the device structure were grown by low-pressure metal-organic chemical vapor deposition. The detectivity was 2.8×1011 cm·Hz1/2/W at 120 K and a bias of −5 V with a peak detection wavelength around 4.1 μm and a quantum efficiency of 35%. Due to the low dark current and high responsivity, a clear photoresponse has been observed at room temperature, which gives a detectivity of 6.7×107 cm·Hz1/2/W. reprint
297.  Quantum cascade laser: A tool for trace chemical detection
Allan J. Evans; Manijeh Razeghi
-- March 26, 2007
Laser-based trace chemical sensors are highly desired to enhance pollution filtering, health and safety monitoring, and filter efficiency monitoring for industrial processes. Limitations of current monitoring and sensing techniques are discussed and the benefits of mid-infrared spectroscopy using novel Quantum Cascade semiconductor Lasers (QCLs) are presented. These new techniques promise inexpensive, miniaturized sensors, capable of remote detection of trace chemicals in liquids, solids, and gasses with levels less than 1 part-per-billion. Applications of these techniques are discussed and the most recent developments of application-ready high power (> 100 mW), continuous-wave, mid-infrared QCLs operating above room temperature with lifetimes exceeding 12,000 hours are presented.
298.  Etching of ZnO Towards the Development of ZnO Homostructure LEDs
K. Minder, F.H. Teherani, D. Rogers, C. Bayram, R. McClintock, P. Kung, and M. Razeghi
SPIE Conference, January 25-29, 2007, San Jose, CA Proceedings – Zinc Oxide Materials and Devices II, Vol. 6474, p. 64740Q-1-6-- January 29, 2007
Although ZnO has recently gained much interest as an alternative to the III-Nitride material system, the development of ZnO based optoelectonic devices is still in its infancy. Significant material breakthroughs in p-type doping of ZnO thin films and improvements in crystal growth techniques have recently been achieved, making the development of optoelectonic devices possible. First, a survey of current ZnO processing methods is presented, followed by the results of our processing research. reprint
299.  Type-II InAs/GaSb Superlattice Focal Plane Arrays for High-Performance Third Generation Infrared Imaging and Free-Space Communication
M. Razeghi, A. Hood and A. Evans
SPIE Conference, January 25-29, 2007, San Jose, CA Proceedings – Optoelectronic Integrated Circuits IX, Vol. 6476, p. 64760Q-1-9-- January 29, 2007
Free-space optical communications has recently been touted as a solution to the "last mile" bottleneck of high speed data networks providing highly secure, short to long range, and high bandwidth connections. However, commercial near infrared systems experience atmospheric scattering losses and scintillation effects which can adversely affect a link's uptime. By moving the operating wavelength into the mid or long wavelength infrared enhanced link uptimes and increased range can be achieved due to less susceptibility atmospheric affects. The combination of room temperature, continuous wave' high power quantum cascade lasers and high operating temperature Type-II superlattice photodetectors offers the benefits of mid and long wavelength infrared systems as well as practical operating conditions. reprint
300.  III-Nitride Avalanche Photodiodes
P. Kung, R. McClintock, J. Pau Vizcaino, K. Minder, C. Bayram and M. Razeghi
SPIE Conference, January 25-29, 2007, San Jose, CA Proceedings – Quantum Sensing and Nanophotonic Devices IV, Vol. 6479, p. 64791J-1-12-- January 29, 2007
Wide bandgap III-Nitride semiconductors are a promising material system for the development of ultraviolet avalanche photodiodes (APDs) that could be a viable alternative to photomultiplier tubes. In this paper, we report the epitaxial growth and physical properties of device quality GaN layers on high quality AlN templates for the first backilluminated GaN p-i-n APD structures on transparent sapphire substrates. Under low bias and linear mode avalanche operation where they exhibited gains near 1500 after undergoing avalanche breakdown. The breakdown electric field in GaN was determined to be 2.73 MV/cm. The hole impact ionization coefficients were shown to be greater than those of electrons. reprint

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