D. L. Huffaker

University of California, Los Angeles, Los Ángeles, California, United States

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Publications (348)769.31 Total impact

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    ABSTRACT: There exists a long-term need for foreign substrates on which to grow GaSb-based optoelectronic devices. We address this need by using interfacial misfit arrays to grow GaSb-based thermophotovoltaic cells directly on GaAs (001) substrates and demonstrate promising performance. We compare these cells to control devices grown on GaSb substrates to assess device properties and material quality. The room temperature dark current densities show similar characteristics for both cells on GaAs and on GaSb. Under solar simulation the cells on GaAs exhibit an open-circuit voltage of 0.121 V and a short-circuit current density of 15.5 mA/cm2. In addition, the cells on GaAs substrates maintain 10% difference in spectral response to those of the control cells over a large range of wavelengths. While the cells on GaSb substrates in general offer better performance than the cells on GaAs substrates, the cost-savings and scalability offered by GaAs substrates could potentially outweigh the reduction in performance. By further optimizing GaSb buffer growth on GaAs substrates, Sb-based compound semiconductors grown on GaAs substrates with similar performance to devices grown directly on GaSb substrates could be realized.
    Applied Physics Letters 03/2015; 106(11):111101. DOI:10.1063/1.4915258 · 3.30 Impact Factor
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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 · 3.84 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 · 3.30 Impact Factor
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    ABSTRACT: We investigate stacked structures of InAs/AlAsSb/InP quantum dots using temperature- and power-dependent photoluminescence. The band gap of InAs/AlAsSb QDs is 0.73 eV at room temperature, which is close to the ideal case for intermediate band solar cells. As the number of quantum dot layers is increased, the photoluminescence undergoes a blue-shift due to the effects of accumulated compressive strain. This PL red shift can be counteracted using thin layers of AlAs to compensate the strain. We also derive thermal activation energies for this exotic quantum dot system.
    Journal of Crystal Growth 02/2015; 425. DOI:10.1016/j.jcrysgro.2015.02.049 · 1.70 Impact Factor
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    ABSTRACT: In this paper, room temperature two-colour pump-probe spectroscopy is employed to study ultrafast carrier dynamics in type-II GaSb/GaAs quantum dots. Our results demonstrate a strong dependency of carrier capture/escape processes on applied reverse bias voltage, probing wavelength and number of injected carriers. The extracted timescales as a function of both forward and reverse bias may provide important information for the design of efficient solar cells and quantum dot memories based on this material. The first few picoseconds of the dynamics reveal a complex behaviour with an interesting feature, which does not appear in devices based on type-I materials, and hence is linked to the unique carrier capture/escape processes possible in type-II structures.
    Applied Physics Letters 01/2015; 106(3-3):031106. DOI:10.1063/1.4906106 · 3.30 Impact Factor
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    ABSTRACT: InAs quantum dots (QDs) were grown in an AlAs 0.56Sb0.44/GaAs matrix in the unintentionally doped (uid) region of an In0.52Al0.48As solar cell, establishing a variety of optical transitions both into and out of the QDs. The ultimate goal is to demonstrate sequential absorption, where one photon is absorbed, promoting an electron from the valence band into the QD, and a second photon is absorbed in order to promote the trapped electron from a QD state into the host conduction band. In this study, we directly investigate the optical properties of the solar cell using photoreflectance and evaluate the possibility of sequential absorption by measuring spectral responsivity with broadband infrared illumination.
    Applied Physics Letters 12/2014; 105(25):253903. DOI:10.1063/1.4904076 · 3.30 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 temperature. Single QD measurements reveal low FSS with a median value of 7.6 μeV, 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.
    Applied Physics Letters 12/2014; 105(25):251901. DOI:10.1063/1.4904944] · 3.30 Impact Factor
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    ABSTRACT: Interfacial charge transfer is ubiquitous in many chemical and physical processes and can occur on ultrafast time scales of femtoseconds to picoseconds. Probing dynamics on such time scales necessitates the use of ultrafast laser spectroscopies, but signatures of interfacial charge transfer can be overwhelmed by the signal from bulk materials. This problem may be alleviated in second-harmonic generation, which can be specifically sensitive to interfacial charge transfer if other bulk and interfacial contributions to the measured second-harmonic signal can be resolved. We report the development of a femtosecond spectral interferometry technique for second-harmonic generation with time, energy, and phase resolution. Using the model systems of a passivated GaAs(100) surface and copper phthalocyanine/GaAs(100) interface, we demonstrate the application of this technique in unveiling the rich dynamics of band renormalization, charge carrier motion, and interfacial charge transfer, all induced by across-bandgap optical excitation of the semiconductor.
    The Journal of Physical Chemistry C 12/2014; 118(48):27981-27988. DOI:10.1021/jp5094614 · 4.77 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 · 3.82 Impact Factor
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    Adam C Scofield · Andrew Lin · Michael Haddad · Diana L Huffaker
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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 · 13.59 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 · 3.30 Impact Factor
  • A.R.J. Marshall · A.P. Craig · C.J. Reyner · D.L. Huffaker
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    ABSTRACT: Interfacial misfit (IMF) arrays were used to create two APD structures, allowing GaSb absorption layers to be combined with wide-gap multiplication regions, grown using GaAs and Al0.8Ga0.2As, respectively. The GaAs APD represents a proof-of-principle, which is developed in the Al0.8Ga0.2As APD to achieve reduced dark currents, of 5.07 μA cm−2 at 90% of the breakdown voltage, and values for effective k = β/α below 0.2. A random-path-length (RPL) simulation was used to model the excess noise in both structures, taking into account the effects of dead space. It is envisaged that the GaSb absorption regions could be replaced with other materials from the 6.1 Å family, allowing for long-wavelength APDs with reduced dark currents and excess noise.
    Infrared Physics & Technology 08/2014; 70. DOI:10.1016/j.infrared.2014.08.014 · 1.55 Impact Factor
  • Andrew Lin · Joshua N. Shapiro · Holger Eisele · Diana L. Huffaker
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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 · 11.81 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
  • A. P. Craig · C. J. Reyner · A. R. J. Marshall · D. L. Huffaker
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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 · 3.30 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.
  • Giacomo Mariani · Yue Wang · Richard B. Kaner · Diana L. Huffaker
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    ABSTRACT: Photovoltaic technologies could play a pivotal role in tackling future fossil fuel energy shortages, while significantly reducing our carbon dioxide footprint. Crystalline silicon is pervasively used in single junction solar cells, taking up ~80 % of the photovoltaic market. Semiconductor-based inorganic solar cells deliver relatively high conversion efficiencies at the price of high material and manufacturing costs. A great amount of research has been conducted to develop low-cost photovoltaic solutions by incorporating organic materials. Organic semiconductors are conjugated hydrocarbon-based materials that are advantageous because of their low material and processing costs and a nearly unlimited supply. Their mechanical flexibility and tunable electronic properties are among other attractions that their inorganic counterparts lack. Recently, collaborations in nanotechnology research have combined inorganic with organic semiconductors in a “hybrid” effort to provide high conversion efficiencies at low cost. Successful integration of these two classes of materials requires a profound understanding of the material properties and an exquisite control of the morphology, surface properties, ligands, and passivation techniques to ensure an optimal charge carrier generation across the hybrid device. In this chapter, we provide background information of this novel, emerging field, detailing the various approaches for obtaining inorganic nanostructures and organic polymers, introducing a multitude of methods.
    High-Efficiency Solar Cells, 01/2014: pages 357-387;
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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 · 0.90 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 · 3.82 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 · 3.30 Impact Factor

Publication Stats

5k Citations
769.31 Total Impact Points


  • 2008–2015
    • University of California, Los Angeles
      • Department of Electrical Engineering
      Los Ángeles, California, United States
  • 2012–2014
    • CSU Mentor
      Long Beach, 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
  • 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