GaSb Light-Emitting Diodes ... - IEEE Xplore

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IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 47, NO. 1, JANUARY 2011

Cascaded Superlattice InAs/GaSb Light-Emitting Diodes for Operation in the Long-Wave Infrared Edwin J. Koerperick, Dennis T. Norton, Jonathon T. Olesberg, Benjamin V. Olson, John P. Prineas, and Thomas F. Boggess, Senior Member, IEEE

Abstract— Superlattice InAs/GaSb light-emitting diodes with peak emission wavelength of 8.6 µm and output power approaching 190 µW at 77 K from a 120 × 120 µm2 mesa are demonstrated. Output power in excess of 600 µW was demonstrated from a 520 × 520 µm2 mesa at 1 A drive current and 50% duty cycle. Devices were grown by molecular beam epitaxy on lightly n-doped GaSb substrates and employed a 16stage cascaded active region configuration to improve current efficiency and increase optical output. Emitting regions were coupled by semi-metallic tunnel junctions consisting of a p-GaSb layer and a thickness-graded InAs/GaSb superlattice stack. Index Terms— Electroluminescence, light-emitting diodes, semiconductor growth, semiconductor superlattices.

I. I NTRODUCTION

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OLID-STATE optical sources in the mid-wave (MWIR) and long-wave infrared (LWIR) are currently of interest for numerous academic and military applications. The 3–5 and 8–12 μm spectral bands are useful windows for spectroscopy, imaging, dynamic scene projection, and chemical sensing [1]– [3]. Dynamic scene projection refers to the generation of an infrared scene by a suitable emitter array and imaging of that scene by an IR detector array (camera). Such technology is employed to calibrate IR detector arrays, as calibration of live scenes can be costly and irreproducible due to uncontrollable environmental factors. Fast switching times, high projected output powers, and the relatively narrow emission spectra of IR light-emitting diodes (LEDs) make them appealing candidates to advance the state of the art in hardware-in-theloop infrared scene generation systems [4]–[6]. While resistive emitter thermal pixel arrays have seen many milestones, such devices are still plagued by relatively slow response times and apparent temperatures well below 1000 K [7]. Alternative

Manuscript received June 3, 2010; revised August 5, 2010; accepted August 21, 2010. Date of current version December 10, 2010. This work was supported in part by the T&E/S&T Program through the University of Iowa under Contract W91ZLK-06-C-0006. E. J. Koerperick and T. F. Boggess are with the Department of Electrical and Computer Engineering, Department of Physics and Astronomy, and Optical Science and Technology Center, University of Iowa, Iowa City, IA 52242-1396 USA (e-mail: [email protected]; [email protected]). D. T. Norton, B. V. Olson, and J. P. Prineas are with the Department of Physics and Astronomy and Optical Science and Technology Center, University of Iowa, Iowa City, IA 52242-1396 USA (e-mail: [email protected]; [email protected]; [email protected]). J. T. Olesberg is with the Department of Chemistry and Optical Science and Technology Center, University of Iowa, Iowa City, IA 52242-1396 USA (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JQE.2010.2072492

methods have also been proposed for dynamic scene projection in the infrared [8]–[10]. Interband quantum cascade (IQC) LED devices based on InAs/GaInSb/AlSb multiple quantum wells have been demonstrated in the 5–8 μm band, producing output powers of nearly 1 μW at 77 K [11]. Optical output of 50 nW was also observed from IQC devices operating in the 10–15 μm spectral band. [12] Recently, Das et al. have demonstrated 30-stage IQC-based LEDs operating at a peak emission wavelength of 7 μm at 77 K [13]. Output power from a 100 × 100 μm2 mesa in a front-emitting configuration exceeded 22 μW into f/1 collection optics (approximately 104 μW full upper hemisphere power) with 150 mA drive current and 30% duty cycle.1 In a separate study, Das demonstrated an output power of 16 μW in to f/1 collection optics (76 μW full upper hemisphere power) from 100 × 100 μm2 flip-chip mounted 30-stage IQC LEDs operating at 7 μm peak emission wavelength, with 100 mA drive current at 30% duty cycle, and with substrates thinned to 200 μm [14]. In this paper, we demonstrate LWIR LEDs based on the InAs/GaSb superlattice (SL) material system in a cascaded active region configuration. We have previously demonstrated high-power LED devices based on the same material system operating in the MWIR [15], [16]. Devices with 16 active region stages were grown by molecular beam epitaxy on lightly (n ≈ 7 × 1016 cm−3 ) doped n-GaSb substrates and were designed to be flip-chip-bonded and operated in a backemission geometry. Flip-chip bonding is the standard packaging method for producing high-density arrays of emitters, as wire-bonding to individual devices is not required and arrays can be mated to very large scale integrated (VLSI) drive electronics. As optical emission was collected through the substrate in flip-chip-bonded LEDs, highly transmissive substrates were necessary. Results for small (120 × 120 μm2 ) and large (520 × 520 μm2 ) mesas are presented to show utility of the devices for both array applications and as high-power single-element emitters. There are similarities between the SL design utilized in this paper and the IQC devices [11]–[13] since both use the InAs/(Al)Ga(In)Sb material system. The primary difference is that the IQC devices utilize a single unit of InAs quantum well per stage, whereas the devices in this paper use an 11-period InAs/GaSb SL. In the LWIR, where Auger recombination is 1 The full upper hemisphere power was calculated on the basis of the authors’ description of their collection optics, this value was not explicitly stated in the manuscript.

0018–9197/$26.00 © 2010 IEEE

KOERPERICK et al.: CASCADED SUPERLATTICE InAs/GaSb LIGHT-EMITTING DIODES

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InAs/GaSb 15/14 ML active region SL grade n−3 × 1018 cm−3 30 nm p-GaSb 5 × 1018 cm−3 InAs/GaSb 15/14 ML active region SL grade n−1 × 1018 cm−3 2 µm GaSb buffer n−1 × 1018 cm−3 n-GaSb substrate

Fig. 1. Growth layer structure of LWIR LEDs. Active regions were 100 nm thick each, and graded SL regions were approximately 30 nm thick. The bracket denotes one period of the device structure and the dark gray layers indicate tunnel junction components.

expected to limit the radiative efficiency, the greater emission volume per stage allows us to operate the devices at a lower carrier density, which decreases the impact of Auger recombination on the radiative efficiency. Although the 2-D density of states provided by the quantum well is very important for laser diode operation, it is much less important for LEDs. II. D EVICE D ESIGN AND G ROWTH Growth was performed in a Veeco EPI930 reactor equipped with valved cracker cells for arsenic and antimony and SUMO cells for group III materials. Beryllium and gallium telluride sources were used for p-type and n-type doping, respectively. In order to keep photon reabsorption in the substrate to a minimum, lightly n-doped substrates were used. Previous results have shown that absorption in lightly n-doped GaSb is as low as 5–10 cm−1 in the wavelength band of interest, resulting in a transmission coefficient of ∼60% for a typical 0.5-mm-thick substrate [17]. Substrates were degreased and etched in HCl prior to growth in order to thin the oxide layer and reduce roughening of the surface during oxide desorption. Annealing at 525 °C for 15 min under antimony overpressure resulted in a streaky reflection high energy electron diffraction pattern, indicating sufficient oxide removal. The growth layer sequence is shown in Fig. 1. Prior to growth of the active material, a 2-μm-thick n-GaSb buffer layer was grown with a doping level of 1 × 1018 cm−3 . Each of the cascaded active regions consisted of 11 periods of InAs/GaSb, nominally 15/14 monolayers in thickness, to give 100 nm of emitting material per stage. The GaSb buffer layer was grown at 500 °C and the SL structure was grown at 410 °C as measured by a pyrometer. Strain balancing was accomplished by encouraging InSb-like interfaces, with migration enhanced epitaxy (MEE) used on the normal (InAs on GaSb) interface and antimony soaking on the inverted interface. One monolayer of indium was deposited in the interface formed by MEE, followed by a short growth interrupt, antimony soak to help form the InSb layer, and arsenic soak

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Fig. 2. Band edge diagram of the cascaded SL structure. The gray lines indicate the bulk band edges of the InAs and GaSb layers. The colored lines show the effective band edges of the SL. Two tunnel junctions are shown separating three InAs/GaSb SL active regions.

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Fig. 3. High-resolution X-ray diffraction scan for the (0 0 4) reflection in the cascaded LED structure. SL diffraction peaks are numbered. The periodicity was determined to be 80 Å, which is 10% shorter than the period of the nominal design due to material depletion and a corresponding reduction of the growth rate.

to remove excess antimony on the growth surface [18], [19]. The tunnel junctions were comprised of a 30-nm p-doped (5 × 1018 cm−3 ) GaSb layer with a subsequent graded SL, as shown in Fig. 2. Electrons tunnel from the thick GaSb layer leftward into the n-doped SL through the semimetallic junction. The resulting hole in the GaSb layer is injected into the SL active region to the right. The SL grade to the left of the GaSb layer was designed to end with a conduction band edge coincident with the valence band edge of GaSb to provide a lowresistance semimetallic junction while providing a barrier to hole leakage. The SL grade to the right of the GaSb layer was designed to prevent tunneling between the GaSb layer and the adjacent active region. Interfaces in the graded SL region of the tunnel junctions were adjusted to compensate for the strain therein. High-resolution X-ray diffraction, shown in Fig. 3, indicated 160 arc seconds of peak separation from the substrate to the zero-order SL peak, yielding an estimated strain of 4.5 × 10−4 . First-order satellite peaks of the SL diffraction were less than 30 arc seconds in width, indicating high-quality epitaxy and high uniformity throughout the structure. The double peak appearance of the SL diffraction resulted from the additional periodicity introduced to the structure by the tunnel

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µm Fig. 4. Atomic force micrograph of the surface of the LED structure. The surface was generally step-terraced in appearance, with root mean squared (RMS) roughness over a 10 μm × 10 μm scan of 1.2 Å.

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Fig. 5. Electroluminescence of 120 × 120 μm2 (solid line) and 520 × 520 μm2 mesas (dashed line) at 77 K. Spectra were acquired with an Fourier transform infrared (FTIR) spectrometer used in a double-modulation configuration.

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III. R ESULTS AND D ISCUSSION As the goal of this paper was to demonstrate high-power emitters, devices were designed for operation at 77 K. Lowtemperature operation has numerous advantages for scene generation technology based on LEDs: the SL emitters generate higher power at low temperature; electromigration in VLSI drive electronics is significantly reduced; and the LED emission spectrum narrows at low temperature, potentially allowing more output to fall within the detection band [20]. Wafers were fabricated into LED devices, flip-chip bonded, and packaged in chip carriers as described elsewhere [16]. The n-GaSb substrate was not thinned, and was approximately 0.5 mm thick. Anode contact sizes for the 120 × 120 μm2 and 520 × 520 μm2 mesas were 60 × 60 μm2 and 440 × 440 μm2 , respectively. Calibrated output power measurements were performed with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector with a 2 mm × 2 mm active area. The device under test was positioned in a Dewar opposite the detector at a distance of 60 mm, and no collection optics were used. After determination of the axial power incident on the detector, the full upper hemisphere power was calculated assuming a Lambertian emission distribution. Previous studies have shown the emission distribution of devices processed in the same manner to be nearly Lambertian [16]. Emission spectra of both mesa sizes at 77 K and 140 A·cm−2 drive current are shown in Fig. 5. Spectra

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junctions, as confirmed by a fit of the data using Bede RADS 4 X-ray simulation software as well as a model of a test structure grown for lattice matching prior to the growth of the cascaded LED. Tunnel junctions were also responsible for the multiple small peaks present throughout the diffraction scan. Surface analysis by atomic force microscopy revealed submonolayer RMS roughness, as calculated from the image in Fig. 4. Interference microscopy was employed to examine surface defect characteristics and densities. The density of medium-sized (5 μm < d < 20 μm) oval defects was 3.1 × 103 cm−2 , and smaller (d < 5 μm) oval defects were present at lower density.

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Fig. 6. Current–voltage relationships of the 120 × 120 μm2 (solid line) and 520 × 520 μm2 mesas (dashed line) at 77 K. Diodes began to conduct between 1.5 and 2.5-V as expected for a 16-stage device with a bandgap of 130 meV.

were acquired with a FTIR spectrometer used in a double modulation configuration and an external MCT detector. In this configuration, the bias applied to the LEDs was sinusoidally modulated at 25 kHz, and the output of the MCT detector was processed (demodulated) by a lock-in amplifier before being returned to the FTIR instrument. This allows complete removal of the 300-K blackbody background from the measurement. The smaller mesa had a spectral peak at 8.6 μm with a cutoff wavelength of approximately 9.7 μm, while the larger mesa was peaked at 8.8 μm with a cutoff wavelength of 10.1 μm. Given the identical current density used for excitation of the devices, the cause of the slight wavelength shift and broadening of the spectrum of the larger mesa is unclear. The peak wavelength for both devices was shorter than the design value of 10 μm due to drift of the growth rates. Current–voltage characteristics (Fig. 6) show devices turning on between 1.5 and 2.5-V, indicating low resistance in the tunnel junctions given the expected per-stage voltage drop of 130 meV. Output power characteristics are shown in Fig. 7 for 77 K operation. The full upper hemisphere peak-to-peak power of the 120 × 120 μm2 mesa approached 190 μW at 200 mA

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KOERPERICK et al.: CASCADED SUPERLATTICE InAs/GaSb LIGHT-EMITTING DIODES

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Fig. 7. Light–current (L–I) characteristics showing axial and total upper hemisphere output power. Output power is given as the peak power.

drive current, while output from the 520 × 520 μm2 mesa exceeded 600 μW at 1 A drive current. Taking the operating current of the devices to be the current at which the optical output is 80% of the maximum value, the 120 × 120 μm2 mesa had a wallplug efficiency of 0.036% and an external quantum efficiency of 1.1% at 100 mA drive current. The 520 × 520 μm2 mesa had a wallplug efficiency of 0.033% and a quantum efficiency of 0.81% at 470 mA drive current. In order to evaluate the 120 × 120 μm2 mesa for potential emitter array applications, it was of interest to calculate the effective apparent temperature generated by the device. Calculation of the apparent temperature may be performed by equating the integrated blackbody intensity to the radiance of the LED and solving for the temperature λ2 Paxial I (λ, T )dλ = A λ1 where I (λ, T ) is the blackbody spectral radiance and Paxial /A is the LED radiance. Taking the integration limits to be 7 and 11 μm and using the maximum output power of the 120 × 120 μm2 mesa, the single-element (no fill-factor included) apparent temperature was calculated to be T = 1400 K. IV. C ONCLUSION In summary, cascaded active region LWIR LEDs based on the InAs/GaSb SL material system and operating at ∼8.6 μm have been demonstrated. Output power exceeding 600 μW was produced from a 520 × 520 μm2 mesa, and a 120 × 120 μm2 mesa was shown to emit 190 μW of power which translates to an apparent temperature of 1400 K under the conditions described. This level of performance demonstrates the potential for LWIR devices based on this material system to provide a means of advancing high apparent temperature scene generator technology. ACKNOWLEDGMENT The authors would like to thank the Test Resource Management Center, Arlington, VA, Test and Evaluation/Science and Technology Program for support. They also would like to

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thank the staff at the University of Iowa Central Microscopy Research Facility, University of Iowa, Iowa City, and Office for the Vice President of Research.

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Benjamin Olson received the B.A. degree in physics from Gustavus Adolphus College, St. Peter, MN, in 2007. He is currently pursuing the Ph.D. degree in physics at the University of Iowa, Iowa City. His current research interests include ultrafast infrared spectroscopy of semiconductor heterostructures and fabrication of semiconductor devices.

Edwin Koerperick received the B.S.E. degree in electrical and computer engineering and the B.S. degree in applied physics from the University of Iowa, Iowa City, in 2005. He received the Ph.D. degree in electrical and computer engineering from the same university in 2009 for work on developing infrared LEDs based on type-II InAs/GaSb superlattices. His current research interests include molecular beam epitaxial growth of III/V semiconductor heterostructures, with emphasis on development of high-power LED arrays for infrared scene generation, semiconductor device processing and fabrication, and ultrafast infrared spectroscopy of quantum semiconductor structures.

John Prineas received the Ph.D. degree in physics from the University of Arizona, Tucson, in 2000, for study of polaritons in periodic resonant and microcavity semiconductor heterostructures. He has been with the Department of Physics and Astronomy, University of Iowa, Iowa City, since 2001, where he is now an Associate Professor of physics, and runs a molecular beam epitaxy facility and an ultrafast optics laboratory. He studied nonlinear pulse propagation in resonant photonic bandgap structures for a year as a Humbolt Post-Doctoral Fellow at the Max Planck Institute, Stuttgart, Germany. His current research interests include the development of antimonide mid- to long-wave infrared optoelectronics, resonant periodic semiconductor heterostructures for slowing, trapping, and switching light.

Dennis Norton received the B.S. degree in physics and mathematics from Western Illinois University, Macomb, in 2007. He is currently pursuing the Ph.D. degree in physics at the University of Iowa, Iowa City. His current research interests include the development of high-power infrared LED arrays for scene generation and the fabrication and processing of GaSb/InAs semiconductor material systems.

Jonathon Olesberg received the Ph.D. degree in physics from the University of Iowa, Iowa City, in 1999. His current research interests include the design and characterization of mid-infrared laser diodes, especially incorporating large type-II band offsets, and developing applications of mid-infrared optoelectronics in noninvasive biomedical spectroscopy.

Thomas F. Boggess (S’81–M’82–SM’90) received the B.S. degree in physics from Lamar University, Beaumont, TX, in 1978, and the M.S. and Ph.D. degrees in physics from the University of North Texas, Denton, in 1980 and 1982, respectively. He worked as a member of the Technical Staff at Hughes Research Laboratories, Malibu, CA, where he was responsible for developing and applying ultrafast optical technology in support of Hughes Aircraft Company interests. In 1989, he joined the faculty at the University of Iowa, Iowa City, where he is now a Professor in the Department of Physics and Astronomy and the Department of Electrical and Computer Engineering. His current research interests include ultrafast spectroscopy of semiconductors, semiconductor spintronics, and the development of infrared optoelectronics based on 6.1-Å semiconductor heterostructures. Dr. Boggess is a member of the American Physical Society, the Optical Society of America, and the American Association for the Advancement of Science.