J. F. Klem

Sandia National Laboratories, Albuquerque, New Mexico, United States

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Publications (244)399.55 Total impact

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    ABSTRACT: We use cross-sectional scanning tunneling microscopy (STM) to reconstruct the monolayer-by-monolayer composition profile across a representative subset of MBE-grown InAs / InAsSb superlattice layers and find that antimony segregation frustrates the intended compositional discontinuities across both antimonide-on-arsenide and arsenide-on-antimonide heterojunctions. Graded, rather than abrupt, interfaces are formed in either case. We likewise find that the incorporated antimony per superlattice period varies measurably from beginning to end of the multilayer stack. Although the intended antimony discontinuities predict significant discrepancies with respect to the experimentally observed high-resolution x-ray diffraction spectrum, dynamical simulations based on the STM-derived profiles provide an excellent quantitative match to all important aspects of the x-ray data.
    Journal of Crystal Growth 02/2015; DOI:10.1016/j.jcrysgro.2015.02.063 · 1.69 Impact Factor
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    ABSTRACT: Superconductivity in topological materials has attracted a great deal of interest in both electron physics and material sciences since the theoretical predictions that Majorana fermions can be realized in topological superconductors [1-4]. Topological superconductivity could be realized in a type II, band-inverted, InAs/GaSb quantum well if it is in proximity to a conventional superconductor. Here we report observations of the proximity effect induced giant supercurrent states in an InAs/GaSb bilayer system that is sandwiched between two superconducting tantalum electrodes to form a superconductor-InAs/GaSb-superconductor junction. Electron transport results show that the supercurrent states can be preserved in a surprisingly large temperature-magnetic field (T-H) parameter space. In addition, the evolution of differential resistance in T and H reveals an interesting superconducting gap structure.
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    ABSTRACT: Measurements of carrier recombination rates using a time-resolved pump-probe technique are reported for mid-wave infrared InAs/InAs1−xSbx type-2 superlattices (T2SLs). By engineering the layer widths and alloy compositions, a 16 K band-gap of ≃235 ± 10 meV was achieved for all five unintentionally doped T2SLs. Carrier lifetimes were determined by fitting a rate equation model to the density dependent data. Minority carrier lifetimes as long as 10 μs were measured. On the other hand, the Auger rates for all the InAs/InAsSb T2SLs were significantly larger than those previously measured for InAs/GaSb T2SLs. The minority carrier and Auger lifetimes were observed to generally increase with increasing antimony content and decreasing layer thickness.
    Applied Physics Letters 07/2014; 105(2):022107-022107-4. DOI:10.1063/1.4890578 · 3.52 Impact Factor
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    ABSTRACT: The effect of defects on the dark current characteristics of MWIR, III-V nBn detectors has been studied. Two different types of defects are compared, those produced by lattice mismatch and by proton irradiation. It is shown that the introduction of defects always elevates dark currents; however the effect on dark current is different for nBn detectors and conventional photodiodes. The dark currents of nBn detectors are found to be more tolerant of defects compared to pn-junction based devices. Defects more weakly increase dark currents, and cooling reduces the defect-produced dark currents more rapidly in nBn detectors than in conventional photodiodes.
    SPIE Defense + Security; 06/2014
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    ABSTRACT: Since InGaP/GaAs heterojunction bipolar transistors (HBTs) are utilized in a wide variety of RF and other applications, a great deal has been learned about their reliability. Nevertheless, this knowledge is limited by the fact that most reliability studies heavily emphasize the stress-related evolution of a single parameter, the DC current gain, beta. We have found that interrupted stressing experiments, with complete characterization of HBTs during the interruption, can give a more complete picture of the degradation that occurs during bias stressing of HBTs. We have previously correlated electrical signatures with degradation in step-stressing of packaged InGaP/GaAs HBTs [1]. In this work we will present some results of step-stress experiments which have been conducted at the wafer level for InGaP/GaAs HBTs. Wafer-level evaluation is advantageous due to the potential for greater automation and flexibility in the testing as well as the ability to identify problems at an earlier stage in the process. The HBTs are stressed at room temperature at a constant collector voltage. Stressing is initiated at a given base current, held for a fixed time, which is typically 10 s. The applied stress is stepped up to a higher level of base current and the process is iterated until a failure is recorded. Full DC parametric electrical characterization of the HBT gets carried out initially and after certain predefined stress steps. Although the stress is typically carried near room temperature, it is understood that the junction of the HBT is effectively self-heated and can reach extremely high temperatures. A certain set of degradation processes are responsible for stress-induced failures and these occur singly or in combinations. In addition, many HBTs within a wafer and among duplicate wafers have not only a number of common failure modes, but a tendency towards a consistently realized destructive power limit. The consistency of these results and the utility of electrical signature analysis lead us to conclude that wafer-level step-stressing is an excellent tool for quickly monitoring the HBT quality. We have also found many commonalities in the wafer-level and package level results for the ceramic packages used in a previous study [1]. The degradation of HBTs by these stressing methods leads eventually to a large drop in current gain. Thus, the interrupted step stressing method arrives at the same end state as continuous, single-parameter monitoring, but with much more clarity about the degradation processes that occur on the way to the end state. The interrupted step-stress process in combination with electrical signature analysis is very likely to find the weak link in an HBT process. We will discuss the utility of the technique in terms of the opportunities to ruggedize the process. It is also possible that the identified weak link is fundamental to the technology or inconsequential in terms of meeting reliability requirements. In that case, wafer-level step-stressing is still an excellent tool for early identification of quality issues that may affect reliability through monitoring the consistency (or lack thereof) in degradation observed across different wafers over different periods of time. * A.G. Baca, A.J. Scruggs, A. Gorenz, T.R. Fortune, J.F. Klem, R.D. Briggs, J.B. Clevenger, G.A. Patrizi, and C.T. Sullivan, “A Survey of Electrical Signatures Characteristic of Step-Stressed InGaP/GaAs HBTS,” ECS Transactions vol. 50 (6), pp. 273-282 (2012). Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
    225th ECS Meeting; 05/2014
  • Photonics Conference (IPC), 2014 IEEE; 01/2014
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    ABSTRACT: We present the design, realization and characterization of strong coupling between an intersubband transition and a monolithic metamaterial nanocavity in the mid-infrared spectral range. We use a ground plane in conjunction with a planar metamaterial resonator for full three-dimensional confinement of the optical mode. This reduces the mode volume by a factor of 1.9 compared to a conventional metamaterial resonator while maintaining the same Rabi frequency. The conductive ground plane is implemented using a highly doped n<sup>+</sup> layer which allows us to integrate it monolithically into the device and simplify fabrication.
    Optics Express 12/2013; 21(26):32572-81. DOI:10.1364/OE.21.032572 · 3.53 Impact Factor
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    ABSTRACT: The interaction between cavity modes and optical transitions leads to new coupled light-matter states in which the energy is periodically exchanged between the matter states and the optical mode. Here we present experimental evidence of optical strong coupling between modes of individual sub-wavelength metamaterial nanocavities and engineered optical transitions in semiconductor heterostructures. We show that this behaviour is generic by extending the results from the mid-infrared (~10 μm) to the near-infrared (~1.5 μm). Using mid-infrared structures, we demonstrate that the light-matter coupling occurs at the single resonator level and with extremely small interaction volumes. We calculate a mode volume of 4.9 × 10(-4) (λ/n)(3) from which we infer that only ~2,400 electrons per resonator participate in this energy exchange process.
    Nature Communications 11/2013; 4:2882. DOI:10.1038/ncomms3882 · 10.74 Impact Factor
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    ABSTRACT: Minority carrier lifetimes in doped and undoped mid-wave infrared InAs/InAsSb type-II superlattices (T2SLs) and InAsSb alloys were measured from 77-300 K. The temperature-dependent lifetimes were analyzed using Shockley-Read-Hall (SRH), ra- diative, and Auger recombination, allowing the contributions of the various recom- bination mechanisms to be distinguished and the dominant mechanisms identified. For the T2SLs, SRH recombination is the dominant mechanism below 200 K and Auger recombination above 200 K. The alloy lifetimes are limited by radiative and Auger recombination through the entire temperature range, with SRH not making a significant contribution. The data allowed the extraction of SRH defect level energies.
    Applied Physics Letters 07/2013; 103(5):052106. DOI:10.1063/1.4817400 · 3.52 Impact Factor
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    ABSTRACT: We present ultra-strong light-matter interaction of a metamaterial mode and an intersubband transition for normal incidence radiation in the mid-infrared spectral region. The anti-crossed lines show a splitting of 15% of the central frequency.
    CLEO: QELS_Fundamental Science; 06/2013
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    ABSTRACT: Interband cascade structures are investigated for applications in the photovoltaic conversion of mid-infrared radiation. We present detailed studies of the performance characteristics and the temperature dependence of seven-stage, single-bandgap devices. These cascade structure devices were able to achieve open-circuit voltages as high as 1.68 V with a cutoff wavelength of 4.0 μm at 80 K. The total power efficiency was broken down into a product of individual efficiencies in order to determine the power loss from each of the physical mechanisms that are involved in the conversion process. We find that at low temperature, the power conversion is limited by incomplete absorption of the incident light and a fill factor below 50%. At higher temperature, the drop in open-circuit voltage limits the efficiency. We comment on how the results of this efficiency analysis will steer the direction of our future efforts toward device improvement.
    IEEE Journal of Photovoltaics 04/2013; 3(2):745-752. DOI:10.1109/JPHOTOV.2013.2239360 · 3.00 Impact Factor
  • ECS Transactions 03/2013; 50(6):273-282. DOI:10.1149/05006.0273ecst
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    ABSTRACT: We present a comprehensive study on the influence of the metamaterial geometry on ultra-strong coupling to intersubband transitions. The spatial overlap of a metamaterial cavity mode and quantum-well region shows the strongest effect.
    CLEO: Applications and Technology; 01/2013
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    ABSTRACT: Measurements of carrier recombination rates using time-resolved differential transmission are reported for an unintentionally doped mid-wave infrared InAsSb alloy and InAs/InAsSb superlattice. Measurements at 77 K yield minority carrier lifetimes of 3 μs and 9 μs for the InAsSb alloy and InAs/InAsSb superlattice, respectively. The un-optimized InAsSb-based materials also exhibit long lifetimes (>850 ns) at temperatures up to 250 K, indicating the potential use for these materials as mid-wave infrared photodetectors with improved performance over current type-II superlattice photodetectors at both cryogenic and near-ambient operating temperatures.
    Applied Physics Letters 08/2012; 101(9):092109. DOI:10.1063/1.4749842 · 3.52 Impact Factor
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    ABSTRACT: We present here our recent quantum transport results around the charge neutrality point (CNP) in a type-II InAs/GaSb field-effect transistor. At zero magnetic field, a conductance minimum close to 4e^2/h develops at the CNP and it follows semi-logarithmic temperature dependence. In quantized magnetic (B) fields and at low temperatures, well developed integer quantum Hall states are observed in the electron as well as hole regimes. Quantum transport shows noisy-like behavior around the CNP at extremely high B fields. Surprisingly, when the diagonal conductivity \sigma_{xx} is plotted against the Hall conductivity \sigma_{xy}, a circular conductivity law is discovered, suggesting a chaotic quantum transport behavior.
    Applied Physics Letters 04/2012; 102(3). DOI:10.1063/1.4789555 · 3.52 Impact Factor
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    ABSTRACT: Progress in the design and implementation of interband cascade (IC) structures for thermophotovoltaic (TPV) applications is reported. These devices were designed with enhanced electron barriers and p-type InAs/GaSb superlattice absorbers. These features have been shown to be successful in suppressing dark current in photodetectors. Our seven stage devices had cutoff wavelengths of 4.0 μm at 80 K and 5.0 μm at 300 K. Good photoresponse and dark current suppression was observed at low temperatures. Under illumination of a 1200 K blackbody, a short-circuit current of 5.46 A/cm2 and open-circuit voltage of 1.61 V were observed for a device temperature of 80 K. Above 80 K, we were able to observe high values of the open circuit voltage up to 180 K.
    Photovoltaic Specialists Conference (PVSC), 2012 38th IEEE; 01/2012
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    ABSTRACT: A series of type-II GaAs/AlAs superlattices epitaxially grown with different interrupts have been investigated using the techniques of grazing incidence X-ray scattering and diffraction. The interrupts are specifically designed to alter the interfacial roughness in the superlattices for the present study. Various structural parameters including the layer thickness, interfacial roughness, and intra-layer correlation lengths of fluctuations in the quantum-well widths have been determined. These results are compared with measurements made on the same set of samples using photoluminescence and optical imaging techniques.
    Modern Physics Letters B 11/2011; 11(24). DOI:10.1142/S0217984997001274 · 0.69 Impact Factor
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    ABSTRACT: Interband cascade (IC) infrared (IR) photodetectors (ICIPs) are a new type of infrared detectors based on quantum-engineered InAs/GaSb/AlSb heterostructures. They combine the features of conventional interband photodiodes with the discrete nature of quantum-well IR photodetectors (QWIPs). The operation of ICIPs takes advantage of fast intersubband relaxation and interband tunneling for carrier transport, and relatively slow interband transitions (long lifetime) for photon generation. As such, ICIPs can be optimized for specific application requirements, such as higher temperature operation or lower noise. By adopting a finite type-II InAs/GaSb superlattice (SL) as the absorber, we have demonstrated mid-IR ICIPs with low noise and photovoltaic operation. In this paper, we report some of our recent efforts in the development of mid-IR ICIPs for high temperature operations. The ICIP devices with a cut-off wavelength of 3.8 mum exhibit a resistance-area product of 2.65×106 and 6.36×103Omegacm2 at 80 and 160 K, respectively.
    Proceedings of SPIE - The International Society for Optical Engineering 05/2011; DOI:10.1117/12.884216 · 0.20 Impact Factor
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    ABSTRACT: Interband-cascade infrared photodetectors (ICIPs), composed of discrete superlattice absorbers, are demonstrated at temperatures up to 350 K with a cutoff wavelength near 5 μ m at 80 K to beyond 7 μ m above room temperature. The peak responsivity exceeds 200 mA/W, higher than the values reported from early interband cascade laser structures, suggesting a significantly enhanced quantum efficiency of the superlattice absorbers. A theoretical model, originally developed for quantum well infrared photodetectors (QWIPs), is applied to ICIPs to analyze their device performance. The Johnson-limited and background-limited detectivities are extracted and indicate that background-limited performance temperatures for two ICIP structures are 126 and 105 K at 5 μ m . It is expected that optimized ICIPs will provide improved performance by combining the advantages of conventional photodiodes and the discrete nature of QWIPs and IC lasers.
    Journal of Applied Physics 04/2010; 107(5-107):054514 - 054514-6. DOI:10.1063/1.3327415 · 2.19 Impact Factor
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    ABSTRACT: A photovoltaic (PV) device based on an interband cascade (IC) structure is proposed for efficiently converting solar and thermal energy to electricity. These IC PV devices employ absorption and transport regions with characteristics that are favorable for achieving high open-circuit voltage and thus possibly improving conversion efficiency over conventional PV devices. Preliminary experiments carried out using IC infrared photodetectors (seven stages) and lasers (11 stages) showed open-circuit voltages that exceed the single-band gap voltage from these devices under infrared illumination. The observed open-circuit voltage demonstrates multiple stages operating in series and provides an initial proof of concept for IC PV devices.
    Applied Physics Letters 02/2010; 96(6-6):063504. DOI:10.1063/1.3313934 · 3.52 Impact Factor

Publication Stats

2k Citations
399.55 Total Impact Points


  • 1990–2014
    • Sandia National Laboratories
      • Semiconductor Material and Device Sciences Department
      Albuquerque, New Mexico, United States
  • 1992–1998
    • Oklahoma State University - Stillwater
      • Department of Physics
      Stillwater, Oklahoma, United States
  • 1991–1996
    • University of New Mexico
      • • Center for High Technology Materials
      • • Department of Chemical and Nuclear Engineering
      Albuquerque, NM, United States
    • Princeton University
      • Department of Electrical Engineering
      Princeton, New Jersey, United States
  • 1995
    • Oregon State University
      Corvallis, Oregon, United States
  • 1993
    • Emory University
      • Department of Physics
      Atlanta, Georgia, United States
    • Lawrence Berkeley National Laboratory
      • Materials Sciences Division
      Berkeley, CA, United States
  • 1990–1992
    • The University of Arizona
      • • Department of Electrical and Computer Engineering
      • • College of Optical Sciences
      Tucson, Arizona, United States