G. J. Brown

Wright-Patterson Air Force Base, Dayton, Ohio, United States

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Publications (134)235.59 Total impact

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    ABSTRACT: InAs/InAsSb superlattices (SLs) are being actively explored for infrared detector applications owing to their superior carrier lifetimes. However, antimony (Sb) segregation during growth can alter the properties of the grown material. In this study, using X-ray energy dispersive spectrometry, authors quantify the compositional profile of individual layers and establish epitaxial parameters for high-quality InAs/InAsSb SL materials. Epitaxial conditions are determined for a nominal 7.7 nm InAs/3.5 nm InAs0.7Sb0.3 SL structure tailored for an approximately 6 μm response at 150 K. Since the growth of mixed anion alloys is complicated by the potential reaction of As2 with Sb surfaces, authors varied the deposition temperature (Tg) in order to control As2 surface reactions on Sb surfaces. Authors find that Sb incorporation is suppressed by 21%, with the increase of Tg from 395 to 440 °C. This incorporation likely stems from Sb surface segregation during InAsSb layer growth that is driven by the As–Sb exchange mechanism, which can lead to significant compositional and dimensional deviations from the intended design.
    Full-text · Article · Oct 2015 · Journal of Crystal Growth
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    ABSTRACT: Lightly doped n-type GaSb substrates with p-type GaSb buffer layers are the preferred templates for growth of InAs/InGaSb superlattices used in infrared detector applications because of relatively high infrared transmission and a close lattice match to the superlattices. We report here temperature dependent resistivity and Hall effect measurements of bare substrates and substrate-p-type buffer layer structures grown by molecular beam epitaxy. Multicarrier analysis of the resistivity and Hall coefficient data demonstrate that high temperature transport in the substrates is due to conduction in both the high mobility zone center Γ band and the low mobility off-center L band. High overall mobility values indicate the absence of close compensation and that improved infrared and transport properties were achieved by a reduction in intrinsic acceptor concentration. Standard transport measurements of the undoped buffer layers show p-type conduction up to 300 K indicating electrical isolation of the buffer layer from the lightly n-type GaSb substrate. However, the highest temperature data indicate the early stages of the expected p to n type conversion which leads to apparent anomalously high carrier concentrations and lower than expected mobilities. Data at 77 K indicate very high quality buffer layers.
    Full-text · Article · Sep 2015 · AIP Advances
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    ABSTRACT: Minority carrier lifetimes in very long wavelength infrared (VLWIR) InAs/GaInSb superlattices (SLs) are reported using time-resolved microwave reflectance measurements. A strain-balanced ternary SL absorber layer of 47.0 Å InAs/21.5 Å Ga0.75In0.25Sb, corresponding to a bandgap of ∼50 meV, is found to have a minority carrier lifetime of 140 ± 20 ns at ∼18 K. This lifetime is extraordinarily long, when compared to lifetime values previously reported for other VLWIR SL detector materials. This enhancement is attributed to the strain-engineered ternary design, which offers a variety of epitaxial advantages and ultimately leads to a reduction of defect-mediated recombination centers.
    Full-text · Article · Sep 2015 · Applied Physics Letters
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    ABSTRACT: Gallium (Ga)-free InAs/InAsSb superlattices (SLs) are being actively explored for infrared detector applications due to the long minority carrier lifetimes observed in this material system. However, compositional and dimensional changes through antimony (Sb) segregation during InAsSb growth can significantly alter the detector properties from the original design. At the same time, precise compositional control of this mixed-anion alloy system is the most challenging aspect of Ga-free SL growth. In this study, the authors establish epitaxial conditions that can minimize Sb surface segregation during growth in order to achieve high-quality InAs/InAsSb SL materials. A nominal SL structure of 77 Å InAs/35 Å InAs 0.7 Sb 0.3 that is tailored for an approximately six-micron response at 150 K was used to optimize the epitaxial parameters. Since the growth of mixed-anion alloys is complicated by the potential reaction of As 2 with Sb surfaces, the authors varied the deposition temperature (T g) under a variety of As x flux conditions in order to control the As 2 surface reaction on a Sb surface. Experimental results reveal that, with the increase of T g from 395 to 440 °C, Sb-mole fraction x in InAs 1-x Sb x layers is reduced by 21 %, under high As flux condition and only by 14 %, under low As flux condition. Hence, the Sb incorporation efficiency is extremely sensitive to minor variations in epitaxial conditions. Since a change in the designed compositions and effective layer widths related to Sb segregation disrupts the strain balance and can significantly impact the long-wavelength threshold and carrier lifetime, further epitaxial studies are needed in order to advance the state-of-the-art of this material system.
    Full-text · Conference Paper · Sep 2015
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    Heather Haugan · G. J. Brown · S. Elhamri · L. Grazulis
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    ABSTRACT: Authors discuss how anion incorporation was controlled during the epitaxial growth process to develop InAs/GaInSb superlattice (SL) materials for very long wavelength infrared applications. A SL structure of 47.0 Å InAs/21.5 Å Ga0.75In0.25Sb was selected to create a very narrow band gap. Although a molecular beam epitaxy growth developed can produce a strain balanced ternary SL structure with a precisely controlled band gap around 50 meV, the material quality of grown SL layers is particularly sensitive to growth defects formed during an anion incorporation process. Since Group III antisites are the dominant structural defects responsible for the low radiative efficiencies, the authors focus on stabilizing III/V incorporation during SL layer growth by manipulating the growth surface condition for a specific anion cracking condition. The optimized ternary SL materials produced an overall strong photoresponse signal with a relatively sharp band edges and a high mobility of ∼10,000 cm2/V s that is important for developing infrared materials.
    Full-text · Article · Mar 2015 · Journal of Crystal Growth
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    ABSTRACT: The objective of this work is to establish molecular beam epitaxy (MBE) growth processes that can produce high quality InAs/GaInSb superlattice (SL) materials specifically tailored for very long wavelength infrared (VLWIR) detection. To accomplish this goal, several series of MBE growth optimization studies, using a SL structure of 47.0 Å InAs/21.5 Å Ga0.75In0.25Sb, were performed to refine the MBE growth process and optimize growth parameters. Experimental results demonstrated that our “slow” MBE growth process can consistently produce an energy gap near 50 meV. This is an important factor in narrow band gap SLs. However, there are other growth factors that also impact the electrical and optical properties of the SL materials. The SL layers are particularly sensitive to the anion incorporation condition formed during the surface reconstruction process. Since antisite defects are potentially responsible for the inherent residual carrier concentrations and short carrier lifetimes, the optimization of anion incorporation conditions, by manipulating anion fluxes, anion species, and deposition temperature, was systematically studied. Optimization results are reported in the context of comparative studies on the influence of the growth temperature on the crystal structural quality and surface roughness performed under a designed set of deposition conditions. The optimized SL samples produced an overall strong photoresponse signal with a relatively sharp band edge that is essential for developing VLWIR detectors. A quantitative analysis of the lattice strain, performed at the atomic scale by aberration corrected transmission electron microscopy, provided valuable information about the strain distribution at the GaInSb-on-InAs interface and in the InAs layers, which was important for optimizing the anion conditions.
    No preview · Conference Paper · Feb 2015
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    W C Mitchel · S Elhamri · H J Haugan · R Berney · Shin Mou · G J Brown
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    ABSTRACT: We show here that n-type InAs/InGaSb superlattices can be electrically isolated from lightly doped n-type GaSb substrates at much higher temperatures than from the more common undoped p-type GaSb substrates without the use of a large band gap insulating buffer layer. Temperature dependent Hall effect measurements show superlattice conduction up to near room temperature, which is significantly higher than the 20 K observed for p-type substrates. Multi-carrier analysis of magnetic field dependent transport data demonstrate the absence of a substrate related conduction channel. We argue that the isolation is due to the depletion layer at the p-n junction between the p-type buffer layer and the n-type substrate.
    Full-text · Conference Paper · Feb 2015
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    ABSTRACT: In order to develop ternary antimonide-based superlattice (SL) materials for very long wavelength infrared (VLWIR) detection, systematic growth optimization studies were performed to produce high quality ternary materials. For the studies, a SL structure of 47.0 Å InAs/21.5 Å Ga0.75In0.25Sb was selected to create a very narrow band gap. Results indicate that an epitaxial process developed can produce a precisely controlled band gap around 50 meV, but the material quality of grown SL layers is particularly sensitive to growth defects formed during the growth process. Since Group III antisites and strain-induced dislocations are the dominant structural defects responsible for the low radiative efficiencies, our optimization strategies to eliminate these defects have focused on stabilizing III/V incorporation during surface reconstruction by manipulating the growth surface temperature and balancing the residual strain of the SLs by adjusting the As/Sb flux ratio. The optimized ternary SL materials exhibited an overall strong photoresponse over a wide wavelength range up to ∼15 μm that is important for developing VLWIR detectors. A quantitative analysis of the lattice strain, performed at the atomic scale by aberration corrected transmission electron microscopy, provided valuable information about the strain distribution at the interfaces that was important for optimizing the strain balancing process during SL layer growth.
    Full-text · Article · Sep 2014 · Infrared Physics & Technology
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    ABSTRACT: This work report new integrated high quality factor (Q) GHz magnetic transformers based on solenoid structures with FeGaB/Al2O3 multilayer films. These transformers show excellent high-frequency performance with a wide operation frequency range of 0.5-5 GHz, in which primary, secondary, and mutual inductances are flat, and the peak quality factor can reach around 14 at frequency of 1.2 GHz. High mutual coupling and low insertion loss are also demonstrated. These novel GHz transformers with high Q and mutual coupling show great promise for applications in radio frequency integrated circuits.
    Preview · Article · Apr 2014 · Journal of Applied Physics
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    ABSTRACT: We report low-temperature spin spray deposited Fe3O4/ZnO thin film microwave magnetic/piezoelectric magnetoelectric heterostructures. A voltage induced effective ferromagnetic resonance field of 14 Oe was realized in Fe3O4/ZnO magnetoelectric (ME) heterostructures. Compared with most thin film magnetoelectric heterostructures prepared by high temperature (>600 °C) deposition methods, for example, pulsed laser deposition, molecular beam epitaxy, or sputtering, Fe3O4/ZnO ME heterostructures have much lower deposition temperature (<100 °C) at a much lower cost and less energy dissipation, which can be readily integrated in different integrated circuits.
    No preview · Article · Mar 2014 · Journal of Materials Science Materials in Electronics
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    ABSTRACT: The InAs/InAsSb type-II superlattice materials studied to date for infrared detector applications have been residually n-type, but p-type absorber regions with minority carrier electrons can result in increased photodiode quantum efficiency, RoA, and detectivity. Therefore, Be-doped InAs/InAsSb superlattices were investigated to determine the p-type InAs/InAsSb superlattice material transport properties essential to developing high quality photodiode absorber materials. Hall measurements performed at 10 K revealed that the superlattice converted to p-type with Be-doping of 3 × 1016 cm-3 and the hole mobility reached 24 400 cm2/Vs. Photoresponse measurements at 10 K confirmed the 175 meV bandgap and material optical quality.
    No preview · Article · Dec 2013 · Applied Physics Letters
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    ABSTRACT: Precise quantification of the magnetoelectric coupling strength in surface charge induced magnetoelectric effect was investigated in NiFe/SrTiO3 thin film heterostructures with different ultra-thin NiFe thicknesses through voltage induced ferromagnetic resonance. The voltage induced ferromagnetic resonance field shifts in these NiFe/SrTiO3 thin films heterostructures showed a maximum value of 65 Oe at an intermediate NiFe layer thickness of ∼1.2 nm, which was interpreted based on the thin film growth model at the low-thicknesses and on the charge screening effect at large thicknesses. The precise quantification and understanding of the magnetoelectric coupling in magnetic/dielectric thin films heterostructures constitute an important step toward real applications.
    No preview · Article · Dec 2013 · Applied Physics Letters
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    F. Szmulowicz · H.J. Haugan · S. Elhamri · G.J. Brown
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    ABSTRACT: In order to limit cooling requirements, it is important to operate superlattice devices such as infrared detectors at the highest possible temperatures consistent with maintaining satisfactory figures of merit regarding signal and noise. One of the characteristics governing the device performance is vertical carrier mobility, although only horizontal mobilities are routinely measured. Recently, we calculated low-temperature vertical and horizontal mobilities, as limited by interface roughness scattering, for type-II InAs/GaSb superlattices as a function of SL dimensions and the degree of roughness. We found that the horizontal mobility was a double-valued function of the roughness correlation length, Λ. Here, we show that the indeterminacy of Λ can be overcome by comparing the temperature dependence of the calculated and measured mobilities; hence, we extend the calculation to higher temperatures. While the scattering mechanism itself is temperature independent, the band structure and the carrier distribution are temperature-dependent. As a function of temperature, we find that as a function of the correlation length, mobilities can increase, decrease, or remain constant. This behavior is explained on the basis of the physics of the problem.
    Full-text · Article · Jul 2013 · Infrared Physics & Technology
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    B Ullrich · J S Wang · G J Brown
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    ABSTRACT: We address erroneous statements made by Nordin et al (2012 Nanotechnology 23 275701) claiming the inadequacy of an expression we, Ullrich et al (2011 Appl. Phys. Lett. 99 081901), used to fit the temperature dependence of the photoluminescence and the absorption of PbS quantum dots. We further correct a quote by Nordin et al, who, when referring to our work, mistakenly claimed temperature invariance of the Stokes shift.
    Full-text · Article · Jun 2013 · Nanotechnology
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    ABSTRACT: We report ternary growth studies to develop a largely strained InAs/InGaSb superlattice (SL) material for very long wavelength infrared (VLWIR) detection. We select a SL structure of 47.0 Å InAs/21.5 Å In0.25Ga0.75Sb that theoretically designed for the greatest possible detectivity, and tune growth conditions for the best possible material quality. Since material quality of grown SLs is largely influenced by extrinsic defects such as nonradiative recombination centers and residual background dopings in the grown layers, we investigate the effect of growth temperature (Tg) on the spectral responses and charge carrier transports using photoconductivity and temperature-dependent Hall effect measurements. Results indicate that molecular beam epitaxy (MBE) growth process we developed produces a consistent gap near 50 meV within a range of few meV, but SL spectral sensing determined by photoresponse (PR) intensity is very sensitive to the minor changes in Tg. For the SLs grown from 390 to 470 °C, a PR signal gradually increases as Tg increases from 400 to 440 °C by reaching a maximum at 440 °C. Outside this growth window, the SL quality deteriorates very rapidly. All SLs grown for this study were n-type, but the mobility varied in a variety of range between 11,300 and 21 cm2/Vs. The mobility of the SL grown at 440 °C was approximately 10,000 V/cm2 with a sheet carrier concentration of 5 × 1011 cm-2, but the mobility precipitously dropped to 21 cm2/Vs at higher temperatures. Using the knowledge we learned from this growth set, other growth parameters for the MBE ternary SL growth should be further adjusted in order to achieve high performance of InAs/InGaSb materials suitable for VLWIR detection.
    Full-text · Article · Jun 2013 · Proceedings of SPIE - The International Society for Optical Engineering
  • F. Szmulowicz · H.J. Haugan · S. Elhamri · G.J. Brown
    [Show abstract] [Hide abstract]
    ABSTRACT: In order to limit cooling requirements, it is important to operate superlattice devices such as infrared detectors at the highest possible temperatures consistent with maintaining satisfactory figures of merit regarding signal and noise. One of the characteristics governing the device performance is vertical carrier mobility, although only horizontal mobilities are routinely measured. Recently, we calculated low-temperature vertical and horizontal mobilities, as limited by interface roughness scattering, for type-II InAs/GaSb superlattices as a function of SL dimensions and the degree of roughness. We found that the horizontal mobility was a double-valued function of the roughness correlation length, Λ. Here, we show that the indeterminacy of Λ can be overcome by comparing the temperature dependence of the calculated and measured mobilities; hence, we extend the calculation to higher temperatures. While the scattering mechanism itself is temperature independent, the band structure and the carrier distribution are temperature-dependent. As a function of temperature, we find that as a function of the correlation length, mobilities can increase, decrease, or remain constant. This behavior is explained on the basis of the physics of the problem.
    No preview · Article · Jan 2013 · Infrared Physics & Technology
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    F. Szmulowicz · G. J. Brown
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    ABSTRACT: Superlattice transport has acquired new relevance owing to the current interest in InAs/GaSb and other superlattices (SL) for third-generation infrared detector focal plane arrays. Interface-roughness scattering (IRS) is known to limit carrier mobilities at low temperatures. Whereas horizontal (in-plane) transport measurements are standard, perpendicular transport measurements (across SL layers)—the ones relevant to the operation of infrared sensors—are non-routine and seldom performed; vertical SL transport is also less well studied theoretically. Therefore, we extend our previous work on low-temperature SL transport by studying horizontal and vertical IRS-limited transport in InAs/GaSb SLs as a function of temperature, SL parameters, and the degree of roughness. Electron mobilities are calculated by solving the Boltzmann equation with temperature-dependent bands and carrier screening, and the results are discussed by analyzing the behavior of the relaxation rates and spectral mobilities, defined as mobilities as a function of carrier energy. New computational tools are devised to handle the implicit integral equation for the horizontal relaxation rates. We find that the behavior of the relaxation rates and spectral mobilities undergoes a change for energies below and above the conduction band bandwidth, which dictates the ultimate behavior of mobilities as a function of temperature. The calculated mobilities are found to display a rich variety of behaviors as a function of temperature, either increasing, decreasing, or remaining relatively constant, depending on the correlation length of interface roughness, Λ, and the conduction band bandwidth. Since the horizontal mobility is a double-valued function of Λ, the temperature dependence of mobilities can be used to eliminate this indeterminacy in order to assess the degree of interface roughness.
    Full-text · Article · Jan 2013 · Journal of Applied Physics
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    B. Ullrich · J. S. Wang · G. J. Brown
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    ABSTRACT: The effect of a laterally applied electric field (<= 10 kV/cm) on the photoluminescence of colloidal PbS quantum dots (diameter of 2.7 nm) on glass was studied. The field provoked a blueshift of the emission peak, a reduction of the luminescent intensity, and caused an increase in the full width at half maximum of the emission spectrum. Upon comparison with the photoluminescence of p-type GaAs exhibits the uniqueness of quantum dot based electric emission control with respect to bulk materials. Copyright 2012 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4766329]
    Full-text · Article · Nov 2012 · AIP Advances
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    ABSTRACT: We explore the optimum growth space for a 47.0 Å InAs/21.5 Å Ga0.75In0.25Sb superlattices (SLs) designed for the maximum Auger suppression for a very long wavelength infrared gap. Our growth process produces a consistent gap of 50 ± 5 meV. However, SL quality is sensitive to the growth temperature (Tg). For the SLs grown at 390−470 °C, a photoresponse signal gradually increases as Tg increases from 400 to 440 °C. Outside this temperature window, the SL quality deteriorates very rapidly. All SLs were n-type with mobility of ∼10 000 V/cm2 and 300 K recombination lifetime of ∼70 ns for an optimized SL.
    Full-text · Article · Oct 2012 · Applied Physics Letters
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    ABSTRACT: Annealing effect on the quality of long wavelength infrared (LWIR) InAs/GaSb superlattices (SLs) has been investigated using atomic force microscopy (AFM), photoconductivity, temperature dependent Hall, and time-resolved differential transmission measurements using an electronically delayed pump-probe technique. Quarters of a single SL wafer were annealed at 440, 480, and 515 °C, respectively for 30 minutes under a Sb-over pressure. Morphological qualities of the SL surface observed by AFM did not show any indication of improvement with annealing. However, the spectral intensity measured by photoconductivity showed an approximately 25 % improvement, while the band gap energy remained at ~107 meV for each anneal, The electron mobility was nearly unaffected by the 440 and 480 °C anneals, however showed the improvement with the 515 °C anneal, where the mobility increased from ~4500 to 6300 cm 2 /Vs. The minority carrier lifetime measured at 77 K also showed the improvement with annealing, increasing from 12.0 to 15.4 nanoseconds. In addition to the longer lifetimes, the annealed samples had a larger radiative decay component than that of unannealed sample. Both the longer measured lifetime and the larger radiative decay component are consistent with the modest improvement in the quality of the annealed SL sample. Overall the qualities of LWIR SL materials can be benefit from a post growth annealing technique we applied.
    Full-text · Conference Paper · Oct 2012

Publication Stats

1k Citations
235.59 Total Impact Points

Institutions

  • 1983-2015
    • Wright-Patterson Air Force Base
      Dayton, Ohio, United States
  • 2001-2014
    • Air Force Research Laboratory
      Washington, Washington, D.C., United States
  • 2011
    • Rice University
      • Department of Electrical and Computer Engineering
      Houston, TX, United States
  • 1995-2011
    • Northwestern University
      • Center for Quantum Devices (CQD)
      Evanston, Illinois, United States
  • 2006
    • Universal Technology Corporation
      Dayton, Ohio, United States
  • 2002
    • United States Air Force
      • Air Force Research Laboratory
      New York, New York, United States
  • 1998
    • National Research Council Canada
      • Institute for Microstructural Sciences (IMS)
      Ottawa, Ontario, Canada