D. L. Huffaker

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

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Publications (179)494.71 Total impact

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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 04/2014; · 9.77 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: 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. · 3.84 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. · 3.39 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. · 1.94 Impact Factor
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    ABSTRACT: Among direct-bandgap semiconducting nanomaterials, single-walled carbon nanotubes (SWCNT) exhibit strong quasi-one-dimensional excitonic optical properties, which confer them a great potential for their integration in future photonics devices as an alternative solution to conventional inorganic semiconductors. In this paper, we will highlight SWCNT optical properties for passive as well as active applications in future optical networking. For passive applications, we directly compare the efficiency and power consumption of saturable absorbers (SAs) based on SWCNT with SA based on conventional multiple quantum wells. For active applications, exceptional photoluminescence properties of SWCNT, such as excellent light-emission stabilities with temperature and excitation power, hold these nanometer-scale materials as prime candidates for future active photonics devices with superior performances.
    Nanoscale Research Letters 06/2013; 8(1):300. · 2.52 Impact Factor
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    ABSTRACT: Cavity quantum electrodynamics advances the coherent control of a single quantum emitter with a quantized radiation field mode, typically piecewise engineered for the highest finesse and confinement in the cavity field. This enables the possibility of strong coupling for chip-scale quantum processing, but till now is limited to few research groups that can achieve the precision and deterministic requirements for these polariton states. Here we observe for the first time coherent polariton states of strong coupled single quantum dot excitons in inherently disordered one-dimensional localized modes in slow-light photonic crystals. Large vacuum Rabi splittings up to 311 μeV are observed, one of the largest avoided crossings in the solid-state. Our tight-binding models with quantum impurities detail these strong localized polaritons, spanning different disorder strengths, complementary to model-extracted pure dephasing and incoherent pumping rates. Such disorder-induced slow-light polaritons provide a platform towards coherent control, collective interactions, and quantum information processing.
    Scientific Reports 06/2013; 3:1994. · 2.93 Impact Factor
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    ABSTRACT: We propose a method of forming quantum-size emitters within a pre-defined photonic crystal in a self-aligned fashion through controlled removal of quantum well layers via selective wet-chemical etching. To demonstrate the effectiveness of our method, we take the example of a two-dimensional photonic crystal slab containing multiple quantum wells at its center. We successfully fabricate vertically stacked quantum nanostructures (or quantum dots) well aligned with respect to the photonic crystal backbone. Micro-photoluminescence measurements performed at 78 K reveal that the radiative transition energy blue-shifts when the lateral dimension reaches less than 100 nm, which is compared with a simple model based on the 'particle-in-a-box' picture. The proposed method may find a broad range of applications in photonics and quantum optics, where the coupling between an emitter and an optical mode needs to be maximized.
    Nanotechnology 06/2013; 24(26):265201. · 3.84 Impact Factor
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    ABSTRACT: Self-assembled quantum dots (SAQDs) grown under biaxial tension could enable novel devices by taking advantage of strong bandgap reduction induced by tensile strain. Tensile SAQDs with low optical transition energies could find application in the technologically important area of mid-infrared optoelectronics. In the case of Ge, biaxial tension can even cause a highly desirable crossover from an indirect- to a direct-gap band structure. However, the inability to grow tensile SAQDs without dislocations has impeded progress in these directions. In this article we demonstrate a method to grow dislocation-free, tensile SAQDs by employing the unique strain relief mechanisms of (110)-oriented surfaces. As a model system, we show that tensile GaAs SAQDs form spontaneously, controllably, and without dislocations on InAlAs(110) surfaces. The tensile strain reduces the bandgap in GaAs SAQDs by ~40%, leading to robust type-I quantum confinement and photoluminescence at energies lower than in bulk GaAs. This method can be extended to other zincblende and diamond cubic materials to form novel optoelectronic devices based on tensile SAQDs.
    ACS Nano 05/2013; 7(6):5017. · 12.06 Impact Factor
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    ABSTRACT: GaAs nanopillars with 150 nm - 200 nm long axial InGaAs inserts are grown by MOCVD via catalyst-free selective-area-epitaxy (SAE). The alloy composition of the InGaAs region, as determined by room-temperature photoluminescence (PL), depends critically on the pitch and diameter of the selective-area pattern geometry. The PL emission varies based on pattern geometry from 1.0 \{mu}m to 1.25 \{mu}m corresponding to a In to Ga ratio from 0.15 to > 0.3. This In enrichment is explained by a pattern dependent change in the incorporation rate for In and Ga. Capture coefficients for Ga and In adatoms are calculated for each pattern pitch. As the pitch decreases, these data reveal a contest between a synergetic effect (related to nanopillar density) that increases the growth rate and a competition for available material that limits the growth rate. Gallium is more susceptible to both of these effects, causing the observed changes in alloy composition.
    05/2013;
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    ABSTRACT: We report on InGaAs quantum disks (QDks) controllably formed on the top (001) facet of nano-patterned GaAs pyramidal platforms. The QDks exhibit pyramidal shape with special facets and varied dimensions, depending on the GaAs pyramidal buffer and the amount of InGaAs deposited. The formation of QDks is explained by the overgrowth of an InGaAs layer and thereafter coalescence of small InGaAs islands. Photoluminescence (PL) characteristics of ensemble QDks and exciton features of individual QDks together demonstrate that we may achieve a transition from zero-dimensional (0D) to two-dimensional (2D) quantum structure with increasing QDk size. This transition provides the flexibility to continuously tailor the dimensionality and subsequently the quantum confinement of semiconductor nanostructures via site-controlled self-assembled epitaxy for device applications based on single quantum structures.
    Nano Research 04/2013; 6(4):235. · 7.39 Impact Factor
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    ABSTRACT: Using temperature-dependent photoluminescence spectroscopy, we have investigated and compared intrinsic InGaAs, intrinsic GaInAsSb, and p-i-n junction GaInAsSb quantum wells (QWs) embedded in GaAs barriers. Strong carrier localization inside the intrinsic GaInAsSb/GaAs QW has been observed together with its decrease inside the p-i-n sample. This is attributed to the effect of an in-situ annealing during the top p-doped AlGaAs layer growth at an elevated temperature of 580 °C, leading to Sb-atom diffusion and even atomic redistribution. High-resolution X-ray diffraction measurements and the decrease of both maximum localization energy and full delocalization temperature in the p-i-n QW sample further corroborated this conclusion.
    Applied Physics Letters 03/2013; 102(11). · 3.79 Impact Factor
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    ABSTRACT: Semiconductor nanomaterials have recently fueled numerous photonic scientific fields. Arrays of nanopillars (NPs) have been examined by the photovoltaic (PV) community as highly efficient solar absorbers, with potential material/cost reductions compared to planar architectures. Despite modeled predictions, experimental efficiencies are limited by surface recombination and poor light management, once integrated in a practical PV device. In this Letter, we correlate optoelectronic modeling with experimental results for direct-bandgap arrays of core-multishell GaAs NPs grown by selective area, catalyst-free epitaxy and capped by epitaxial window layers, with efficiencies of 7.43%. Electrically, improved open-circuit voltages are yet partly affected by residual surface state density after epitaxial passivation. Optically, dome-shaped indium-tin-oxide (ITO) top electrode functions as a two-dimensional (2-D) periodic array of subwavelength lenses that focus the local density of optical states within the NP active volume. These devices provide a path to high-efficiency NP-based PVs by synergistically controlling the heteroepitaxy and light management of the final structure.
    Nano Letters 03/2013; · 13.03 Impact Factor
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    ABSTRACT: GaAs grows along the [111] direction much faster than other crystal axes at high growth temperature. In this work, we leverage this anisotropy, using catalyst-free selective-area epitaxy, to grow high-aspect ratio nanopillars. However, we find that the resulting crystal structure exhibits a high density of stacking faults that can have detrimental effects on the electronic and optical properties of the material. Each stacking fault is equivalent to a monolayer of wurtzite embedded in an otherwise zinc-blende lattice. The origins of stacking faults are currently under debate, with both thermodynamic equilibrium arguments and nucleation arguments proposed to explain the segments of wurtzite that appear in the primarily zinc-blende crystal. Here we present a density-functional-theory study of nucleation and island growth on the (111)B surface of GaAs that demonstrates how the smallest stable nucleus can transition and stabilize in either a wurtzite or a zinc-blende orientation.
    03/2013;
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    ABSTRACT: We report DFT calculations that study the effect of sulfur passivation ((NH4)2S and octanethiol) on GaAs surfaces. Sulfur passivation of GaAs solar cells is an area of interest, as it improves the I-V characteristics of heterojunctions by decreasing the density of surface states. We elucidate the fundamental mechanism of sulfur passivation on GaAs by showing how the sulfur species react with different reconstructed GaAs (100) and (111B) surfaces. Using state of the art hybrid functionals to calculate band structures and density of states, we find that a reconstructed GaAs surface does not have mid-gap surface states. Therefore, we show that sulfur passivation does not reduce surface states on reconstructed surfaces. We also study arsenic vacancies and adatoms on these surfaces to determine the energies of creating these imperfections. They lead to mid-gap surface states that are shown to be energetically plausible in certain GaAs surface reconstruction. We study the most energetically favorable surface reconstructions with As vacancies and show how sulfur passivation plays a role in removing surface states. These results will guide in the selection of passivating agents for GaAs solar cells and lead to a better understanding of such systems.
    03/2013;
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    ABSTRACT: The localization energies, capture cross sections, and storage times of holes in GaSb quantum dots (QDs) are measured for three GaSb/GaAs QD ensembles with different QD sizes. The structural properties, such as height and diameter, are determined by atomic force microscopy, while the electronic properties are measured using deep-level transient spectroscopy. The various QDs exhibit varying hole localization energies corresponding to their size. The maximum localization energy of 800 (±50) meV is achieved by using additional Al0.3Ga0.7As barriers. Based on an extrapolation, alternative material systems are proposed to further increase the localization energy and carrier storage time of QDs.
    Applied Physics Letters 02/2013; 102(5). · 3.79 Impact Factor
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    ABSTRACT: We investigate the effects of in-situ passivation on the electrical transport of InAs nanopillars (NPs) grown on InAs (111)B substrates via selective-area epitaxy. Before passivation, the transport properties of InAs NPs, studied by single-NP field-effect transistors, are highly dependent on NP dimensions. With diameters ranging from 70 nm to 200 nm, we find significant differences in resistivity and extracted field-effect mobility (μeff). Growing a 6 nm InP shell for in-situ passivation significantly enhances these transport properties of InAs channel with diameter-independent μeff as high as 6900 cm2/V s. Such heterostructures have the potential as future high electron mobility transistors.
    Applied Physics Letters 02/2013; 102(5). · 3.79 Impact Factor
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    ABSTRACT: We characterize the electro-optical and lasing properties of a hybrid material consisting of multiple InAs quantum dot (QD) layers together with an InGaAs quantum well (QW) grown on a GaAs substrate. Over 40 nm Stark shift of the InGaAs QW leading to 9 dB extinction ratio was demonstrated. Lasing operation at the QD first excited state transition of 1070 nm was achieved and together with < 10 ps absorption recovery the system shows promise for high-speed mode-locked lasers and electro-modulated lasers.
    Applied Physics Letters 02/2013; 102(5). · 3.79 Impact Factor
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    ABSTRACT: We report on an InAs quantum dot (QD) hybrid structure with a top surface QD layer coupled to two buried QD layers that is highly sensitive to surface passivation. After 180 min of passivation, the photoluminescence (PL) peak of the surface QDs shifts from 1545 to 1275 nm while its intensity decreases by one order of magnitude. Time-resolved PL reveals a significant decrease of carrier tunneling between the QD layers because of the surface state modification by chemical treatment. A simple model with rate equations is used to explain the observed optical performance. Our results show that the optical performance of this hybrid structure is very sensitive to the surface environment, making it a potential candidate for sensing applications.
    Nanotechnology 01/2013; 24(7):075701. · 3.84 Impact Factor
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    ABSTRACT: The structural and optical properties of InAs self-assembled quantum dots buried in AlAs0.56Sb0.44 barriers can be controlled using GaAs1−xSbx cladding layers. These cladding layers allow us to manage the amount of Sb immediately underneath and above the InAs quantum dots. The optimal cladding scheme has a GaAs layer beneath the InAs, and a GaAs0.95Sb0.05 layer above. This scheme results in improved dot morphology and significantly increased photoluminescence (PL) intensity. Both power-dependent and time-resolved photoluminescence confirm that the quantum dots have type-II band alignment. Enhanced carrier lifetimes in this quantum dot system show great potential for application in intermediate band solar cells.
    Applied Physics Letters 01/2013; 102(2). · 3.79 Impact Factor

Publication Stats

917 Citations
494.71 Total Impact Points

Institutions

  • 2008–2014
    • University of California, Los Angeles
      • • Department of Electrical Engineering
      • • Department of Chemical and Biomolecular Engineering
      Los Angeles, California, United States
  • 2013
    • Columbia University
      New York City, New York, United States
  • 2010–2012
    • University of Southern California
      • Department of Electrical Engineering
      Los Angeles, CA, United States
    • Sandia National Laboratories
      Albuquerque, New Mexico, United States
  • 2002–2011
    • University of New Mexico
      • • Department of Earth & Planetary Sciences
      • • Center for High Technology Materials
      Albuquerque, NM, United States
  • 2009
    • Tyndall National Institute
      • Photonic Device Dynamics Group
      Corcaigh, Munster, Ireland
  • 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