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326.  Negative luminescence of long-wavelength InAs/GaSb superlattice photodiodes
D. Hoffman, A. Hood, Y. Wei, A. Gin, F. Fuchs, and M. Razeghi
Applied Physics Letters 87 (20)-- November 14, 2005
The electrically pumped emission behavior of binary type-II InAs/GaSb superlattice photodiodes has been studied in the spectral range between 8 µm and 13 µm. With a radiometric calibration of the experimental setup, the internal and external quantum efficiency has been determined in the temperature range between 80 K and 300 K for both, the negative and positive luminescence. The negative luminescence efficiency approaches values as high as 35% without antireflection coating. The temperature dependence of the internal quantum efficiency near zero-bias voltage allows for the determination of the electron-hole-electron Auger recombination coefficient of Γn=1×1024 cm6 s–1. reprint
 
327.  Characterization and Analysis of Single-Mode High-Power CW Quantum-Cascade Laser
W.W. Bewley, I. Vurgaftman, C.S. Kim, J.R. Meyer, J. Nguyen, A. Evans, J.S. Yu, S.R. Darvish, S. Slivken, and M. Razeghi
Journal of Applied Physics 98-- October 15, 2005
We measured and modeled the performance characteristics of a distributed-feedback quantum-cascade laser exhibiting high-power continuous-wave (CW) operation in a single spectral mode at λ~4.8 µm and temperatures up to 333 K. The sidemode suppression ratio exceeds 25 dB, and the emission remains robustly single mode at all currents and temperatures tested. CW output powers of 99 mW at 298 K and 357 mW at 200 K are obtained at currents well below the thermal rollover point. The slope efficiency and subthreshold amplified spontaneous emission spectra are shown to be consistent with a coupling coefficient of no more than κL ~ 4–5, which is substantially lower than the estimate of 9 based on the nominal grating fabrication parameters. reprint
 
328.  On the performance and surface passivation of type-II InAs/GaSb superlattice photodiodes for the very-long- wavelength infrared
A. Hood, M. Razeghi, E. Aifer, G.J. Brown
Applied Physics Letters 87 (1)-- October 10, 2005
We demonstrate very-long-wavelength infrared Type-II InAs/GaSb superlattice photodiodes with a cutoff wavelength (λc,50%) of 17 μm. We observed a zero-bias, peak Johnson noise-limited detectivity of 7.63×109 cm·Hz½/W at 77 K with a 90%-10% cutoff width of 17 meV, and quantum efficiency of 30%. Variable area diode zero-bias resistance-area product (R0A) measurements indicated that silicon dioxide passivation increased surface resistivity by nearly a factor of 5, over unpassivated photodiodes, and increased overall R0A uniformity. The bulk R0A at 77 K was found to be 0.08 Ω·cm2, with RA increasing more than twofold at 25 mV reverse bias. reprint
 
329.  Quantum Dot Infrared Photodetectors: Comparison Experiment and Theory
H. Lim, W. Zhang, S. Tsao, T. Sills, J. Szafraniec, K. Mi, B. Movaghar, and M. Razeghi
Virtual Journal of Nanoscale Science and Technology 12 (9)-- August 29, 2005reprint
 
330.  Quantum Dot Infrared Photodetectors: Comparison Experiment and Theory
H. Lim, W. Zhang, S. Tsao, T. Sills, J. Szafraniec, K. Mi, B. Movaghar, and M. Razeghi
Physical Review B, 72-- August 17, 2005
We present data and calculations and examine the factors that determine the detectivities in self-assembled InAs and InGaAs based quantum dot infrared photodetectors (QDIPs). We investigate a class of devices that combine good wavelength selectivity with “high detectivity.” We study the factors that limit the temperature performance of quantum dot detectors. For this we develop a formalism to evaluate the optical absorption and the electron transport properties. We examine the performance limiting factors and compare theory with experimental data. We find that the notion of a phonon bottleneck does not apply to large-diameter lenslike quantum dots, which have many closely spaced energy levels. The observed strong decrease of responsivity with temperature is ultimately due to a rapid thermal cascade back into the ground states. High temperature performance is improved by engineering the excited state to be near the continuum. The good low temperature (77 K) performance in strongly bound QDIPs is shown to be due to the high gain and the low noise achievable in these micron size devices. reprint
 
331.  High-power, room-temperature and continuous-wave operation of distributed-feedback quantum-cascade lasers at λ = 4.8 µm
J.S. Yu, S. Slivken, S.R. Darvish, A. Evans, B. Gokden and M. Razeghi
Virtual Journal of Nanoscale Science and Technology 12 (5)-- August 1, 2005reprint
 
332.  High-power, room-temperature and continuous-wave operation of distributed-feedback quantum-cascade lasers at λ = 4.8 µm
J.S. Yu, S. Slivken, S.R. Darvish, A. Evans, B. Gokden and M. Razeghi
Applied Physics Letters, 87 (4)-- July 25, 2005
The authors present high-power continuous-wave (cw) operation of distributed-feedback quantum-cascade lasers. Continuous-wave output powers of 56 mW at 25 °C and 15 mW at 40 °C are obtained. Single-mode emission near 7.8 µm with a side-mode suppression ratio of >=30 dB and a tuning range of 2.83 cm−1 was obtained between 15 and 40 °C. The device exhibits no beam steering with a full width at half maximum of 27.4° at 25 °C in cw mode. reprint
 
333.  Infrared detection from GaInAs/InP nanopillar arrays
A. Gin, B. Movaghar, M. Razeghi and G.J. Brown
Nanotechnology 16-- July 1, 2005
We report on the photoresponse from large arrays of 40 nm radius nanopillars with sensitivity in the long-wavelength infrared regime. Using photoluminescence techniques, a peak wavelength blue shift of approximately 5 meV was observed at 30 K from GaInAs/InP nanopillar structures, indicating carrier confinement effects. Responsivity measurements at 30 K indicated peak wavelength response at about 8 µm with responsivity of 420 mA/W at −2 V bias. We have also measured the noise and estimated the peak detectivity to be 3×108 cm·Hz½·W−1 at 1 V reverse bias and 30 K. A maximum internal quantum efficiency of 4.5% was derived from experiment. Both the photo and the dark transport have been successfully modeled as processes that involve direct and indirect field-assisted tunneling as well as thermionic emission. The best agreement with experiment was obtained when allowances were made for the non-uniformity of barrier widths and electric field heating of carriers above the lattice temperature. reprint
 
334.  Focal plane arrays based on quantum dot infrared photodetectors
Manijeh Razeghi; Wei Zhang; Ho-Chul Lim; Stanley Tsao; John Szafraniec; Maho Taguchi; Bijan Movaghar
Proc. SPIE 5838, Nanotechnology II, 125 (June 28, 2005);-- June 28, 2005
Here we report the first demonstrations of infrared focal plane array (FPA) based on GaAs and InP based quantum dot infrared photodetectors (QDIPs). QDIPs are extension of quantum well infrared photodetectors (QWIPs) and are predicted to outperform QWIPs due to their potential advantages including normally incident absorption, higher responsivity and high temperature operation. Two material systems have been studied: InGaAs/InGaP QDIPs on GaAs substrates and InAs QDIP on InP substrates. An InGaAs/InGaP QDIP has been grown on GaAs substrate by LP-MOCVD. Photoresponse was observed at temperatures up to 200 K with a peak wavelength of 4.7 μm and cutoff wavelength of 5.2 μm. A detectivity of 1.2x1011 cm·Hz1/2/W was obtained at T=77 K and bias of -0.9 V, which is the highest for QDIPs grown by MOCVD. An InAs QDIP structure has also been grown on InP substrate by LP-MOCVD. Photoresponse of normal incidence was observed at temperature up to 160K with a peak wavelength of 6.4 μm and cutoff wavelength of 6.6 μm. A detectivity of 1.0x1010 cm·Hz1/2/W was obtained at 77K at biases of -1.1 V, which is the first and highest detectivity reported for QDIP on InP substrate. 256×256 detector arrays were fabricated first time in the world for both the GaAs and InP based QDIPs. Dry etching and indium bump bonding were used to hybridize the arrays to a Litton readout integrated circuit. For the InGaAs/InGaP QDIP FPA, thermal imaging was achieved at temperatures up to 120 K. At T=77K, the noise equivalent temperature difference (NEDT) was measured as 0.509K with a 300K background and f/2.3 optics. For the InP based QDIPs, thermal imaging was achieved at 77 K. reprint
 
335.  Uncooled operation of Type-II InAs/GaSb superlattice photodiodes in the mid- wavelength infrared range
Y. Wei, A. Hood, H. Yau, A. Gin, M. Razeghi, M.Z. Tidrow, V. Natha
Applied Physics Letters, 86 (23)-- June 6, 2005
We report high performance uncooled midwavelength infrared photodiodes based on interface-engineered InAs/GaSb superlattice. Two distinct superlattices were designed with a cutoff wavelength around 5 µm for room temperature and 77 K. The device quantum efficiency reached more than 25% with responsivity around 1 A/W. Detectivity was measured around 109 cm·Hz½/W at room temperature and 1.5×1013 cm·Hz½/W at 77 K under zero bias. The devices were without antireflective coating. The device quantum efficiency stays at nearly the same level within this temperature range. Additionally, Wannier–Stark oscillations in the Zener tunneling current were observed up to room temperature. reprint
 
336.  Short Wavelength (λ~ 4.3 μm) High-Performance Continuous-Wave Quantum-Cascade Lasers
J.S. Yu, A. Evans, S. Slivken, S.R. Darvish, and M. Razeghi
IEEE Photonics Technology Letters, 17 (6)-- June 1, 2005
We report continuous-wave (CW) operation of a 4.3-μm quantum-cascade laser from 80 K to 313 K. For a high-reflectivity-coated 11-μm-wide and 4-mm-long laser, CW output powers of 1.34 W at 80 K and 26 mW at 313 K are achieved. At 298 K, the CW threshold current density of 1.5 kA/cm2 is observed with a CW output power of 166 mW and maximum wall-plug efficiency of 1.47%. The CW emission wavelength varies from 4.15 μm at 80 K to 4.34 μm at 298 K, corresponding to a temperature-tuning rate of 0.87 nm/K. The beam full-width at half-maximum values for the parallel and the perpendicular far-field patterns are 26° and 49° in CW mode, respectively. reprint
 
337.  Beam Steering in High-Power CW Quantum Cascade Lasers
W.W. Bewley, J.R. Lindle, C.S. Kim, I. Vurgaftman, J.R. Meyer, A.J. Evans, J.S. Yu, S. Slivken, and M. Razeghi
IEEE Journal of Quantum Electronics, 41 (6)-- June 1, 2005
We report the light-current (L-I), spectral, and far-field characteristics of quantum cascade lasers (QCLs) with seven different wavelengths in the λ=4.3 to 6.3 μm range. In continuous-wave (CW) mode, the narrow-stripe (≈13 μm) epitaxial- side-up devices operated at temperatures up to 340 K, while at 295 K the CW output power was as high as 640 mW with a wallplug efficiency of 4.5%. All devices with λ≥4.7 μm achieved room-temperature CW operation, and at T=200 K several produced powers exceeding 1 W with ≈10% wallplug efficiency. The data indicated both spectral and spatial instabilities of the optical modes. For example, minor variations of the current often produced nonmonotonic hopping between spectra with envelopes as narrow as 5-10 nm or as broad as 200-250 nm. Bistable beam steering, by far-field angles of up to ±12° from the facet normal, also occurred, although even in extreme cases the beam quality never became worse than twice the diffraction limit. The observed steering is consistent with a theory for interference and beating between the two lowest order lateral modes. We also describe simulations of a wide-stripe photonic-crystal distributed-feedback QCL, which based on the current material quality is projected to emit multiple watts of CW power into a single-mode beam at T=200 K. reprint
 
338.  Short-wavelength ultraviolet light-emitting diodes based on AlGaN
M. Razeghi; A. Yasan; R. McClintock; K. Mayes; P. Kung
2005 Conference on Lasers and Electro-Optics, CLEO. 153-155 [CMI5] (2005)-- May 22, 2005
We review our progress toward realization of highly-efficient ultraviolet light-emitting diodes (UV LEDs) based on high Al-composition AlxGa1-xN. Milliwatt level optical output powers have been measured at wavelengths as short as 247 nm.
 
339.  High Detectivity InAs Quantum-Dot Infrared Photodetectors Grown on InP by Metalorganic Chemical Vapor Deposition
W. Zhang, H. Lim, M. Taguchi, S. Tsao, B. Movaghar, and M. Razeghi
Applied Physics Letters, 86 (19)-- May 9, 2005
We report a high-detectivity InAs quantum-dot infrared photodetector. The InAs quantum dots were grown by self-assembly on InP substrates via low-pressure metal–organic chemical–vapor deposition. Highly uniform quantum dots with a density of 4×1010 cm2 were grown on a GaAs/InP matrix. Photoresponse was observed at temperatures up to 160 K with a peak of 6.4 µm and cutoff of 6.6 µm. Very low dark currents and noise currents were obtained by inserting Al0.48In0.52As current blocking layers. The background-limited performance temperature was 100 K. A detectivity of 1.0×1010 cm·Hz½/W was obtained at 77 K with a bias of –1.1 V. reprint
 
340.  High performance Type-II InAs/GaSb superlattices for mid, long, and very long wavelength infrared focal plane arrays
M. Razeghi, Y. Wei, A. Gin, A. Hood, V. Yazdanpanah, M.Z. Tidrow, and V. Nathan
SPIE Conference, Orlando, FL, Vol. 5783, pp. 86-- March 28, 2005
We present our most recent results and review our progress over the past few years regarding InAs/GaSb Type-II superlattices for photovoltaic detectors and focal plane arrays. Empirical tight binding methods have been proven to be very effective and accurate in designing superlattices for various cutoff wavelengths from 3.7 µm up to 32 µm. Excellent agreement between theoretical calculations and experimental results has been obtained. High quality material growths were performed using an Intevac modular Gen II molecular beam epitaxy system. The material quality was characterized using x-ray, atomic force microscopy, transmission electron microscope and photoluminescence, etc. Detector performance confirmed high material electrical quality. Details of the demonstration of 256×256 long wavelength infrared focal plane arrays are presented. reprint
 
341.  High-Performance Type-II InAs/GaSb Superlattice Photodiodes with Cutoff Wavelength Around 7 µm
Y. Wei, A. Hood, H. Yau, V. Yazdanpanah, M. Razeghi, M.Z. Tidrow and V. Nathan
Applied Physics Letters, 86 (9)-- February 28, 2005
We report the most recent result in the area of type-II InAs/GaSb superlattice photodiodes that have a cutoff wavelength around 7 µm at 77 K. Superlattice with a period of 40 Å lattice matched to GaSb was realized using GaxIn1–x type interface engineering technique. Compared with significantly longer period superlattices, we have reduced the dark current density under reverse bias dramatically. For a 3 µm thick structure, using sulfide-based passivation, the dark current density reached 2.6×10–5 A/cm2 at –3 V reverse bias at 77 K. At this temperature the photodiodes have R0A of 9300 Ω·cm2 and a thermally limited zero bias detectivity of 1×1012 cm·Hz½/W. The 90%–10% cutoff energy width was only 16.5 meV. The devices did not show significant dark current change at 77 K after three months storage in the atmosphere. reprint
 
342.  High-Power CW Mid-IR Quantum Cascade Lasers
J.R. Meyer, W.W. Bewley, J.R. Lindle, I. Vurgaftman, A.J. Evans, J.S. Yu, S. Slivken, and M. Razeghi
SPIE Conference, Jose, CA, -- January 22, 2005
We report the cw operation of quantum cascade lasers that do not require cryogenic cooling and emit at λ = 4.7-6.2 µm. At 200 K, more than 1 W of output power is obtained from 12-µm-wide stripes, with a wall-plug efficiency (ηwall) near 10%. Room-temperature cw operation has also been demonstrated, with a maximum output power of 640 mW (ηwall = 4.5%) at 6 µm and 260 mW (ηwall = 2.3%) at 4.8 µm. Far-field characterization indicates that whereas the beam quality remains close to the diffraction limit in all of the tested lasers, in the devices emitting at 6.2 µm the beam tends to steer by as much as 5-10° degrees in either direction with varying temperature and pump current. reprint
 
343.  Back-illuminated solar-blind photodetectors for imaging applications
R. McClintock, A. Yasan, K. Mayes, P. Kung, and M. Razeghi
SPIE Conference, Jose, CA, Vol. 5732, pp.175-- January 22, 2005
Back-illuminated solar-blind ultraviolet p-i-n photodetectors and focal plane arrays are investigated. We initially study single-pixel devices and then discuss the hybridization to a read-out integrated circuit to form focal plane arrays for solar-blind UV imaging. reprint
 
344.  AlGaN-based deep UV light emitting diodes with peak emission below 255 nm
A. Yasan, R. McClintock, K. Mayes, P. Kung, and M. Razeghi
SPIE Conference, Jose, CA, Vol. 5732, pp. 197-- January 22, 2005
We report on the growth and fabrication of AlGaN-based deep ultraviolet light-emitting diodes (LEDs) with peak emission of below 255 nm. In order to achieve such short wavelength UV LEDs, the Al mole fractions in the device layers should be greater than ~60%. This introdues serious challenges on the growth and doping of AlxGa1-xN epilayers. However, with the aid of a high-quality AlN template layer and refinement of the growth conditions we have been able to demonstrate UV LEDs emitting below 255 nm. reprint
 
345.  High performance LWIR Type-II InAs/GaSb superlattice photodetectors and infrared focal plane array
Y. Wei, A. Hood, A. Gin, V. Yazdanpanah, M. Razeghi and M. Tidrow
SPIE Conference, Jose, CA, Vol. 5732, pp. 309-- January 22, 2005
We report on the demonstration of a focal plane array based on Type-II InAs-GaSb superlattices grown on n-type GaSb substrate with a 50% cutoff wavelength at 10 μm. The surface leakage occurring after flip-chip bonding and underfill in the Type-II devices was suppressed using a double heterostructure design. The R0A of diodes passivated with SiO2 was 23 Ω·cm2 after underfill. A focal plane array hybridized to an Indigo readout integrated circuit demonstrated a noise equivalent temperature difference of 33 mK at 81 K, with an integration time of 0.23 ms. reprint
 
346.  Passivation of Type-II InAs/GaSb superlattice photodetectors
A. Hood, Y. Wei, A. Gin, M. Razeghi, M. Tidrow, and V. Nathan
SPIE Conference, Jose, CA, Vol. 5732, pp. 316-- January 22, 2005
Leakage currents limit the operation of high performance Type-II InAs/GaSb superlattice photodiode technology. Surface leakage current becomes a dominant limiting factor, especially at the scale of a focal plane array pixel (< 25 µm) and must be addressed. A reduction of the surface state density, unpinning the Fermi level at the surface, and appropriate termination of the semiconductor crystal are all aims of effective passivation. Recent work in the passivation of Type-II InAs\GaSb superlattice photodetectors with aqueous sulfur-based solutions has resulted in increased R0A products and reduced dark current densities by reducing the surface trap density. Additionally, photoluminescence of similarly passivated Type-II InAs/GaSb superlattice and InAs GaSb bulk material will be discussed. reprint
 
347.  High performance InAs quantum dot infrared photodetectors (QDIP) on InP by MOCVD
W. Zhang, H. Lim, M. Taguchi, S. Tsao, J. Szafraniec, B. Movaghar, M. Razeghi, and M. Tidrow
SPIE Conference, Jose, CA, Vol. 5732, pp. 326-- January 22, 2005
Inter-subband detectors such as quantum well infrared photodetectors (QWIP) have been widely used in infrared detection. Quantum dot infrared photodetectors (QDIPs) have been predicted to have better performance than QWIPs including higher operation temperature and normal incidence detection. Here we report our recent results of InAs QDIP grown on InP substrate by low-pressure metalorganic chemical vapor deposition (MOCVD). The device structures consist of multiple stacks of InAs quantum dots with InP barriers. High detectivities in the range of 1010cm·Hz1/2/W were obtained at 77K. The measurements at higher temperatures show better temperature dependent performance than QWIP. However, the performances of QDIPs are still far from the expected. One of the reasons is the low quantum efficiency due to the low fill factor of quantum dots layer. Resonant cavity enhanced QDIP has been studied to increase the quantum efficiency. Different schemes of mirrors using free carrier plasma and distributed Bragg reflector are discussed. reprint
 
348.  High performance InGaAs/InGaP quantum dot infrared photodetector achieved through doping level optimization
S. Tsao, K. Mi, J. Szafraniec, W. Zhang, H. Lim, B. Movaghar, and M. Razeghi
SPIE Conference, Jose, CA, Vol. 5732, pp. 334-- January 22, 2005
We report an InGaAs/InGaP/GaAs quantum dot infrared photodetector grown by metalorganic chemical vapor deposition with detectivity of 1.3x1011 cm·Hz½/W at 77K and 1.2x1010 ccm·Hz½/W at 120K. Modeling of the Quantum dot energy levels showed us that increased photoresponse could be obtained by doping the quantum dots to 4 electrons per dot instead of the usual 2 electrons per dot. This happens because the primary photocurrent transition is from the first excited state to a higher excited state. Increasing the quantum doping in our device yielded significant responsivity improvement and much higher detectivity as a result. This paper discusses the performance of this higher doping device and compares it to our previously reported device with lower doping. reprint
 
349.  GaInAs/InP nanopillar arrays for long wavelength infrared detection
A. Gin, Y. Wei, A. Hood, D. Hoffman, M. Razeghi and G.J. Brown
SPIE Conference, Jose, CA, Vol. 5732, pp. 350-- January 22, 2005
Nanopillar devices have been fabricated from GaInAs/InP QWIP material grown by MOCVD. Using electron beam lithography and reactive ion etching techniques, large, regular arrays of nanopillars with controllable diameters ranging from 150 nm to less than 40 nm have been reproducibly formed. Photoluminescence experiments demonstrate a strong peak wavelength blue shift for nanopillar structures compared to the as-grown quantum well material. Top and bottom metal contacts have been realized using a polyimide planarization and etchback procedure. I-V and noise measurements have been performed. Optical measurements indicate photoconductive response in selected nanopillar arrays. Device peak wavelength response occurs at about 8 µm with peak device responsivity of 420 mA/W. Peak detectivity of 3×108 cm·Hz½/W has been achieved at -1V bias and 30 K. reprint
 
350.  ZnO Thin Film Templates for GaN-based Devices
D.J. Rogers, F. Hosseini Teherani, A. Yasan, R. McClintock, K. Mayes, S.R. Darvish, P. Kung, M. Razeghi and G. Garry
SPIE Conference, Jose, CA, Vol. 5732, pp. 412-- January 22, 2005
GaN-based optoelectronic devices are plagued by a tendency to non-radiative transitions linked to defects in the active layers. ZnO is promising as a substrate material for GaN because it has the same wurtzite structure and a relatively small lattice mismatch (~1.8%). In this paper, we discuss use of ZnO thin films as templates for GaN based LED. reprint
 

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