D. L. Huffaker

CSU Mentor, Long Beach, California, United States

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Publications (342)767.21 Total impact

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    ABSTRACT: An ordered array of CdS nanocrystals was synthesized by vapor-solid mechanism using a template of chemically inhomogeneous pores in the form of nanocavities in a thin silicon nitride layer on a SiO2/Si wafer. The silicon oxide at the bottom of the Si3N4 cavities served for nucleation of the CdS crystals, whereas no affinity of CdS to silicon nitride was found. Tetra- and multi-pod morphology of the nanocrystals has been obtained. This morphology was attributed to a competition of nucleation rates and growth of different crystallographic planes at the Si3N4/SiO2 and Si3N4/CdS interfaces, which is different from the polymorphism growth model.
    RSC Advances 03/2015; 5:27496–27501. DOI:10.1039/C5RA01175B · 3.71 Impact Factor
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    ABSTRACT: We investigate the photoluminescence (PL) properties of a hybrid type-I InAs/GaAs and type-II GaSb/GaAs quantum dot (QD) structure grown in a GaAs matrix by molecular beam epitaxy. This hybrid QD structure exhibits more intense PL with a broader spectral range, compared with control samples that contain only InAs or GaSb QDs. This enhanced PL performance is attributed to additional electron and hole injection from the type-I InAs QDs into the adjacent type-II GaSb QDs. We confirm this mechanism using time-resolved and power-dependent PL. These hybrid QD structures show potential for high efficiency QD solar cell applications.
    Applied Physics Letters 03/2015; 106(10):103104. DOI:10.1063/1.4914895 · 3.52 Impact Factor
  • Applied Physics Letters 01/2015; 106(3):031106. · 3.52 Impact Factor
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    ABSTRACT: We use thin tensile-strained AlAs layers to manage compressive strain in stacked layers of InAs/AlAsSb quantum dots (QDs). The AlAs layers allow us to reduce residual strain in the QD stacks, suppressing strain-related defects. AlAs layers 2.4 monolayers thick are sufficient to balance the strain in the structures studied, in agreement with theory. Strain balancing improves material quality and helps increase QD uniformity by preventing strain accumulation and ensuring that each layer of InAs experiences the same strain. Stacks of 30 layers of strain-balanced QDs exhibit carrier lifetimes as long as 9.7 ns. QD uniformity is further enhanced by vertical ABAB… ordering of the dots in successive layers. Strain compensated InAs/AlAsSb QD stacks show great promise for intermediate band solar cell applications.
    Nanotechnology 10/2014; 25(44):445402. DOI:10.1088/0957-4484/25/44/445402 · 3.67 Impact Factor
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    ABSTRACT: The growth of GaAs/GaAsP axial heterostructures is demonstrated and implemented as diffusion current barriers in nanopillar light-emitting diodes at near-infrared wavelengths. The nanopillar light-emitting diodes utilize an n-GaAs/i-InGaAs/p-GaAs axial heterostructure for current injection. Axial GaAsP segments are inserted into the n- and p-GaAs portions of the nanopillars surrounding the InGaAs emitter region, acting as diffusion barriers to provide enhanced carrier confinement. Detailed characterization of growth of the GaAsP inserts and electronic band-offset measurements are used to effectively implement the GaAsP inserts as diffusion barriers. The implementation of these barriers in nanopillar light-emitting diodes provides a 5-fold increase in output intensity, making this a promising approach to high-efficiency pillar-based emitters in the near-infrared wavelength range.
    Nano Letters 10/2014; 14(11). DOI:10.1021/nl501022v · 12.94 Impact Factor
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    ABSTRACT: Strain-based band engineering in quantum dots and dashes has been predominantly limited to compressively strained systems. However, tensile strain strongly reduces the bandgaps of nanostructures, enabling nanostructures to emit light at lower energies than they could under compressive strain. We demonstrate the self-assembled growth of dislocation-free GaAs quantum dashes on an InP(111)B substrate, using a 3.8% tensile lattice-mismatch. Due to the high tensile strain, the GaAs quantum dashes luminesce at 110–240 meV below the bandgap of bulk GaAs. The emission energy is readily tuned by adjusting the size of the quantum dashes via deposition thickness. Tensile self-assembly creates new opportunities for engineering the band alignment, band structure, and optical properties of epitaxial nanostructures.
    Applied Physics Letters 08/2014; 105(7):071912. DOI:10.1063/1.4893747 · 3.52 Impact Factor
  • Infrared Physics & Technology 08/2014; DOI:10.1016/j.infrared.2014.08.014 · 1.46 Impact Factor
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    ABSTRACT: A thorough study of direct InSb nanocrystal formations on patterned InAs (111)B substrates is provided. These nanostructures are created without the use of Au catalysts or initial InAs segments. Under the growth conditions generally used for selective-area, catalyst-free epitaxy, a wide range of InSb nanocrystal morphologies are observed. This is because the low-energy InSb surfaces, studied by first-principles calculations, are the {111} facets as opposed to the {110} facets. By controlling the V/III ratio during growth, different InSb nanostructures can be achieved. Using low V/III growth conditions, In droplets start to form and InSb nucleation takes place at the droplet–semiconductor interface only, resulting in vertical, self-catalyzed InSb nanopillars.
    Advanced Functional Materials 07/2014; 24(27). DOI:10.1002/adfm.201303390 · 10.44 Impact Factor
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    ABSTRACT: Semiconductor nanowires have proven to be a viable path towards nanoscale photodetectors [1], however the dramatic reduction in semiconductor absorption volume can have a negative effect on responsivity [2]. In order to overcome the reduced absorption volume, incident light must be focused within the nanopillar and surface reflections must be minimized. The ability to lithographically define the position and diameter of individual nanowires makes surface plasmon polariton (SPP) resonances an attractive option, as regular metal scattering centers can overcome the momentum mismatch between the incident wavevector and the SPP mode and scattering center size can influence optical aborption enhancement [3]. In this work we demonstrate a 3-dimensional plasmonic antenna and show enhanced spectral response within the nanopillars.
    2014 72nd Annual Device Research Conference (DRC); 06/2014
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    ABSTRACT: Interfacial misfit arrays were embedded within two avalanche photodiode (APD) structures. This allowed GaSb absorption layers to be combined with wide-bandgap multiplication regions, consisting of GaAs and Al0.8Ga0.2As, respectively. The GaAs APD represents the simplest case. The Al0.8Ga0.2As APD shows reduced dark currents of 5.07 μAcm−2 at 90% of the breakdown voltage, and values for effective below 0.2. Random-path-length modeled excess noise is compared with experimental data, for both samples. The designs could be developed further, allowing operation to be extended to longer wavelengths, using other established absorber materials which are lattice matched to GaSb. k = β / α
    Applied Physics Letters 05/2014; 104(21):213502. DOI:10.1063/1.4879848 · 3.52 Impact Factor
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    ABSTRACT: Sequential photon absorption processes in semiconductor solar cells represent a route to improving their efficiency. Fossil fuels are the most highly used sources for energy genera-tion. But as energy needs increase day by day, and fossil fuels are consumed at ever faster rates, there is a great need for alternative energy sources. Renewable sources such as wind and solar can be exploited in a wide range of geographical areas and could ef-fectively replace fossil fuels. For example, the Earth receives over 8 million quads of BTU (British thermal units) annually, mean-ing that there is enough solar energy available to fulfill all the energy requirements of the human race. However, due to the low efficiencies with which current solar cell technologies con-vert light into electricity, only a small fraction of the available solar energy can be harnessed. Deployment of solar cells will increase if their efficiency can be improved without increasing their cost. A novel concept known as the intermediate band so-lar cell (IBSC) paves the way for increasing solar cell efficiency. 1 In an IBSC, sub-bandgap photons that would be wasted in a con-ventional solar cell can be harvested effectively to create a higher photocurrent. Semiconductor quantum dots (QDs) are perhaps the best choice to create an intermediate band in a single-junction so-lar cell due to the inherent tunability of their shape, size, and quantum confinement properties. For an IBSC to work, the QD system being used must satisfy certain conditions in terms of bandgaps and band alignments. For maximum efficiency, the QD and host material bandgaps should be 0.7 and 1.93eV, respectively. There have been numerous attempts to use established QD systems for IBSCs, including indium gallium arsenide/gallium arsenide—In(Ga)As:GaAs)—gallium antimonide/gallium ar-senide (GaSb:GaAs), and indium arsenide/gallium arsenide Figure 1. Schematic of our aluminum arsenide/antimonide (AlAsSb, with the composition AlAs 0:56 Sb 0:44) p-i-n intermediate band solar cell (IBSC). This cell contains 10 layers of indium arsenide (InAs) quan-tum dots (QDs). Gallium arsenide (GaAs) and gallium arsenide/ anti-monide (GaAs 0:95 Sb 0:05) cladding layers are used below and above the QDs, respectively, for better morphology and to tune the photolumi-nescence spectra. nitride (InAs:GaAsN). 2–6 However, these QD systems have had only limited success because their band alignments do not meet the requirements. In contrast, a novel QD system consisting of InAs(Sb) QDs within aluminum arsenide/antimonide barriers (with the composition AlAs 0:56 Sb 0:44) on indium phosphide (InP) substrates was identified by Levy and colleagues as be-ing well suited to IBSCs. 7 Nearly ideal bandgaps are available for these QD and host materials. Furthermore, InAs(Sb)/AlAsSb QDs have type II band alignment, where one of the carriers is delocalized. This offers strong electron confinement, while the valence band (VB) offset at the InAs(Sb)/AlAsSb interface is small (zero for certain As and Sb compositions). These properties are essential for high-efficiency IBSCs. To our knowledge there have been no previous reports of growth of InAs(Sb) QDs on AlAsSb.
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    ABSTRACT: This paper presents the device design, modeling, materials growth, and device fabrication results of wafer scale monolithically integrated modules (MIMs) of series interconnected GaSb thermo-photovoltaic (TPV) cells grown on 50 mm diameter semi-insulating (SI) GaAs substrates. The feasibility of using GaSb epi-layers grown on SI GaAs for fabricating modules of photovoltaic (PV) cells connected in series for the conversion of low temperature heat radiating sources into electrical energy has been demonstrated. Device modeling shows that assuming an Shockley-Read-Hall recombination lifetime of 100 ns, in addition to intrinsic radiative and Auger recombination in GaSb, it is possible to design PV cells that when placed at sub-micron distance from a 900 °C radiating source are able to convert the heat into electrical energy at a power density of 1.5 to 3 W/cm2 using GaSb epi-layers grown on SI GaAs. The advantage of using SI GaAs is that it is possible to produce MIM modules of PV cells that can have output voltages of 6 V to 10 V decreasing the internal resistance of the PV cell. The device design and fabrication process presented here can be used for large area device arrays high efficiency solar photovoltaic cells employing other semiconductor materials for terrestrial and space applications with back-side illumination architecture.
    Journal of Renewable and Sustainable Energy 01/2014; 6(1):011207. DOI:10.1063/1.4828368 · 0.93 Impact Factor
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    ABSTRACT: Symmetric quantum dots (QDs) on (111)-oriented surfaces are promising candidates for generating polarization-entangled photons due to their low excitonic fine structure splitting (FSS). However, (111) QDs are difficult to grow. The conventional use of compressive strain to drive QD self-assembly fails to form 3D nanostructures on (111) surfaces. Instead, we demonstrate that (111) QDs self-assemble under tensile strain by growing GaAs QDs on an InP(111)A substrate. Tensile GaAs self-assembly produces a low density of QDs with a symmetric triangular morphology. Coherent, tensile QDs are observed without dislocations, and the QDs luminescence at room tem-perature. Single QD measurements reveal low FSS with a median value of 7.6 leV, due to the high symmetry of the (111) QDs. Tensile self-assembly thus offers a simple route to symmetric (111) QDs for entangled photon emitters. V C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4904944] Quantum dots (QDs) have great potential for entangled photon sources in quantum information, 1 since they can generate pairs of polarization-entangled photons via a biexciton-exciton cascade. 2,3 The entangled two-photon states are energetically indistinguishable, provided that the exciton levels have negligible fine structure splitting (FSS). 4 However, non-zero FSS often occurs in QDs due to asymmetric QD con-finement potentials. This asymmetry typically arises for QDs grown on (001) surfaces due to their in-plane piezoelectric field and their commonly asymmetric shape. 4 Low FSS has been achieved in (001) QDs, but it typically requires post-growth adjustments, such as careful selection of low FSS QDs from a wide size distribution, or external manipulation using magnetic fields. 4,5 In contrast, QDs grown on (111) surfaces have been proposed as ideal sources of entangled photon pairs, since their piezoelectric field naturally lies perpendicular to the surface, preserving the electronic symmetry. 6,7
    Applied Physics Letters 01/2014; 105:251901. DOI:10.1063/1.4904944] · 3.52 Impact Factor
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    ABSTRACT: Impressive opto-electronic devices and transistors have recently been fabricated from GaAs nanopillars grown by catalyst-free selective-area epitaxy, but this growth technique has always resulted in high densities of stacking faults. A stacking fault occurs when atoms on the growing (111) surface occupy the sites of a hexagonal-close-pack (hcp) lattice instead of the normal face-centered-cubic (fcc) lattice sites. When stacking faults occur consecutively, the crystal structure is locally wurtzite instead of zinc-blende, and the resulting band offsets are known to negatively impact device performance. Here we present experimental and theoretical evidence that indicate stacking fault formation is related to the size of the critical nucleus, which is temperature dependent. The difference in energy between the hcp and fcc orientation of small nuclei is computed using density-function theory. The minimum energy difference of 0.22 eV is calculated for a nucleus with 21 atoms, so the population of nuclei in the hcp orientation is expected to decrease as the nucleus grows larger. The experiment shows that stacking fault occurrence is dramatically reduced from 22% to 3% by raising the growth temperature from 730 to 790 ° C. These data are interpreted using classical nucleation theory which dictates a larger critical nucleus at higher growth temperature.
    Nanotechnology 11/2013; 24(47):475601. DOI:10.1088/0957-4484/24/47/475601 · 3.67 Impact Factor
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    ABSTRACT: The surface passivation of semiconductors on different surface orientations results in vastly disparate effects. Experiments of GaAs/poly(3,4-ethylenedioxythiophene/indium tin oxide solar cells show that sulfur passivation results in threefold conversion efficiency improvements for the GaAs (100) surface. In contrast, no improvements are observed after passivation of the GaAs (111B) surface, which achieves 4% conversion efficiency. This is explained by density-functional theory calculations, which find a surprisingly stable (100) surface reconstruction with As defects that contains midgap surface states. Band structure calculations with hybrid functionals of the defect surface show a surface state on the undimerized As atoms and its disappearance after passivation.
    Applied Physics Letters 10/2013; 103(17):173902-173902-5. DOI:10.1063/1.4826480 · 3.52 Impact Factor
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    ABSTRACT: We investigated the anisotropic electro-optic (EO) effect on InGaAs quantum dot (QD) chain modulators. The linear EO coefficients were determined as 24.3 pm/V (33.8 pm/V) along the [011] direction and 30.6 pm/V (40.3 pm/V) along the [011¯] direction at 1.55 μm (1.32 μm) operational wavelength. The corresponding half-wave voltages (Vπs) were measured to be 5.35 V (4.35 V) and 4.65 V (3.86 V) at 1.55 μm (1.32 μm) wavelength. This is the first report on the anisotropic EO effect on QD chain structures. These modulators have 3 dB bandwidths larger than 10 GHz.
    Optics Letters 10/2013; 38(20):4262-4. DOI:10.1364/OL.38.004262 · 3.18 Impact Factor
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    ABSTRACT: The difference in the growth conditions, crystalline structure and morphology of CdS nanowire crystals synthesized via the vapor–solid (VS) mechanism on bare substrates and via the vapor–liquid–solid (VLS) mechanism using gold catalyst, respectively, has been studied. It has been found that the growth of CdS nanowire crystals by the VS mechanism depends on the substrate material used and that the roughness of the substrate surface is also an important parameter influencing nucleation of CdS nanowires. The crystals obtained by VLS demonstrated perfect crystal plane orientation and the wurtzite crystallographic symmetry, whereas the crystals obtained by VS mechanism showed transition regions with more rich morphology resulting in changes of the crystalline plane orientation of the growing nanowire, as well as inhomogeneous stoichiometry along the crystal. The rich morphology was attributed to the competition of nucleation rates and growth rates among different crystallographic planes which may be affected by changes in temperature regimes of the growing zone.
    Surface and Coatings Technology 09/2013; 230:234–238. · 2.20 Impact Factor
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    ABSTRACT: The difference in the growth conditions, crystalline structure and morphology of CdS nanowire crystals synthesized via the vapor-solid (VS) mechanism on bare substrates and via the vapor-liquid-solid (VLS) mechanism using gold catalyst, respectively, has been studied. It has been found that the growth of CdS nanowire crystals by the VS mechanism depends on the substrate material used and that the roughness of the substrate surface is also an important parameter influencing nucleation of CdS nanowires. The crystals obtained by VLS demonstrated perfect crystal plane orientation and the wurtzite crystallographic symmetry, whereas the crystals obtained by VS mechanism showed transition regions with more rich morphology resulting in changes of the crystalline plane orientation of the growing nanowire, as well as inhomogeneous stoichiometiy along the crystal. The rich morphology was attributed to the competition of nucleation rates and growth rates among different crystallographic planes which may be affected by changes in temperature regimes of the growing zone.
    Surface and Coatings Technology 09/2013; 230:234-238. DOI:10.1016/j.surfcoat.2013.06.058 · 2.20 Impact Factor
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    ABSTRACT: Arrays of III-V direct-bandgap semiconductor nanopillars represent promising photovoltaic candidates due to their inherent high optical absorption coefficients and minimized reflection arising from light trapping, efficient charge collection in the radial direction and the ability to synthesize them on low-cost platforms. However, the increased surface area results in surface states that hamper the power conversion efficiency. Here, we report the first demonstration of GaAs nanopillar-array photovoltaics employing epitaxial passivation with air mass 1.5 global power conversion efficiencies of 6.63%. High-bandgap epitaxial InGaP shells are grown in situ and cap the radial p-n junctions to alleviate surface-state effects. Under light, the photovoltaic devices exhibit open-circuit voltages of 0.44 V, short-circuit current densities of 24.3 mA cm(-2) and fill factors of 62% with high external quantum efficiencies >70% across the spectral regime of interest. A novel titanium/indium tin oxide annealed alloy is exploited as transparent ohmic anode.
    Nature Communications 07/2013; 4:1497. DOI:10.1038/ncomms2509 · 10.74 Impact Factor
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    ABSTRACT: GaSb/InGaAs quantum dot–well (QDW) hybrid active regions with type-II band alignment are explored for increasing the infrared absorption in GaAs solar cells. Analyzed GaAs p–i–n structures comprise five layers of either GaSb quantum dot (QD), InGaAs quantum well (QW) or GaSb/InGaAs QDW layers in the i-region. It is found that the QDW solar cells outperform the QW and QD solar cells beyond GaAs band edge. In QDW solar cells an increase in efficiency is observed over QD solar cells due to additional QW absorption. An analysis of bulk response degradation in QDW solar cell is also presented. Improved photoresponse in QDW solar cells over QW and QD solar cells proves the potential for QDW hybrid structures in achieving high efficiency intermediate band solar cells.
    Solar Energy Materials and Solar Cells 07/2013; 114:165-171. DOI:10.1016/j.solmat.2013.02.027 · 5.03 Impact Factor

Publication Stats

5k Citations
767.21 Total Impact Points


  • 2012–2014
    • CSU Mentor
      Long Beach, California, United States
  • 2008–2014
    • University of California, Los Angeles
      • Department of Electrical Engineering
      Los Angeles, California, United States
  • 1992–2011
    • University of Texas at Austin
      • • Department of Electrical & Computer Engineering
      • • Center for Microelectronics Research
      Austin, Texas, United States
  • 2002–2009
    • University of New Mexico
      • Center for High Technology Materials
      Albuquerque, New Mexico, United States
  • 2006
    • Technische Universität Berlin
      • Department of solid state Physics
      Berlin, Land Berlin, Germany
    • University of Michigan
      • Department of Electrical Engineering and Computer Science (EECS)
      Ann Arbor, MI, United States
  • 2000–2001
    • California Institute of Technology
      • Department of Electrical Engineering
      Pasadena, CA, United States
  • 1997
    • Cornell University
      • Department of Electrical and Computer Engineering
      Ithaca, NY, United States
  • 1995
    • Martin Marietta Laboratories
      Baltimore, Maryland, United States