Publications

2014 APL Materials

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GaN“Faceting control in core-shell GaN micropillars using selective epitaxy“, S. Krylyuk, R. Debnath, H. P. Yoon, M. R. King, J. Ha, B. Wen, A. Motayed, and A. V. Davydov, APL Materials 2, 106104, 2014.

1. Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
2. Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742
3. N5 Sensors, Inc., Rockville, Maryland 20852
4. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
5. Maryland Nanocenter, University of Maryland, College Park, Maryland 20742
6. Northrop Grumman ES, Linthicum, Maryland 21090

ABSTRACT. We report on the fabrication of large-area, vertically aligned GaN epitaxial core-shell micropillar arrays. The two-step process consists of inductively coupled plasma (ICP) etching of lithographically patterned GaN-on-Si substrate to produce an array of micropillars followed by selective growth of GaN shells over these pillars using Hydride Vapor Phase Epitaxy (HVPE). The most significant aspect of the study is the demonstration of the sidewall facet control in the shells, ranging from {1 1̄ 01} semi-polar to {1 1̄ 00} non-polar planes, by employing a post-ICP chemical etch and by tuning the HVPE growth temperature. Room temperature photoluminescence, cathodoluminescence, and Raman scattering measurements reveal substantial reduction of parasitic yellow luminescence as well as strain-relaxation in the core-shell structures. In addition, X-ray diffraction indicates improved crystal quality after the shell formation. This study demonstrates the feasibility of selective epitaxy on micro-/nano- engineered templates for realizing high-quality GaN-on-Si devices.

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2014 Microscopy and Microanalysis

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MandM“Effects of Focused-Ion-Beam Processing on Local Electrical Measurements of Inorganic Solar Cells“, H. P. Yoon, P. M. Haney, J. Schumacher, K. Siebein, Y. Yoon, and N. B. Zhitenev, Microscopy and Microanalysis 20 (S3), 544-545, 2014.

1.Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
2. Maryland Nanocenter, University of Maryland, College Park, MD 20742, USA

ABSTRACT. Quantitative determination of electronic properties at high spatial resolution is crucial for the development of high-efficiency solar cells. Electron beam induced current (EBIC) is a powerful technique in which electron-hole pairs are created in proximity to an exposed surface, and the carrier collection efficiency is measured as a function of excitation position. Cross-sections of device are often created by focused ion beams (FIB) due to the flexibility of the patterning and milling processes. However, the irradiating Ga ions of the FIB fabrication may introduce unintended artifacts, affecting local electronic properties. In this study, we investigate the impact of the FIB process observed in EBIC measurements and two-dimensional finite element simulations.

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2014 Progress in Photovoltaics

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pip_epi_EBIC“Comparison of thin epitaxial film silicon photovoltaics fabricated on monocrystalline and polycrystalline seed layers on glass”, C. W. Teplin, S. Grover, A. Chitu, A. Limanov, M. Chalal, J. Im, D. Amkreutz, S. Gall, H. P. Yoon, V. Lasalvia, H. M. Branz, P. Stradins, K. M. Jones, A. G. Norman, D. L. Young, B. Lee, Progress in Photovoltaics, in press (DOI: 10.1002/pip.2505), 2014.

1. National Renewable Energy Laboratory, Golden, CO, USA
2. Columbia University, New York, NY, USA
3. Helmholtz Zentrum Berlin für Materialien und Energie, Berlin, Germany
4. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

ABSTRACT:  We fabricate thin epitaxial crystal silicon solar cells on display glass and fused silica substrates overcoated with a silicon seed layer. To confirm the quality of hot-wire chemical vapor deposition epitaxy, we grow a 2-μm-thick absorber on a (100) monocrystalline Si layer transfer seed on display glass and achieve 6.5% efficiency with an open circuit voltage (V_OC) of 586mV without light-trapping features. This device enables the evaluation of seed layers on display glass. Using polycrystalline seeds formed from amorphous silicon by laser-induced mixed phase solidification (MPS) and electron beam crystallization, we demonstrate 2.9%, 476mV (MPS) and 4.1%, 551mV (electron beam crystallization) solar cells. Grain boundaries likely limit the solar cell grown on the MPS seed layer, and we establish an upper bound for the grain boundary recombination velocity (S_GB) of 1.6×104 cm/s.

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2014 The Society for Information Display

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sid2“Cathodoluminescence quantum efficiency of quantum dot thin films”, H. P. Yoon, C. D. Bohn, Y. Lee, S. Ko, J. S. Steckel, S. Coe-Sullivan, N. B. Zhitenev, The Society for Information Display Technical Digests 45, 71–74, 2014.

1. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, United States
2. Maryland Nanocenter, University of Maryland, College Park, MD 20742, United States
3. QD Vision Inc., 29 Hartwell Ave., Lexington, MA 02421, United States

ABSTRACT. A thin film of quantum dots (QD) was used to visualize the local photo-response of polycrystalline CdTe solar cells by downconverting an electron beam of high energy to photons of visible light. The efficient photon generation in the QD film is compared to cathodoluminescence of the high-purity bulk semiconductors and phosphor.

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2014 7th Annual FIB SEM Workshop

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usermeeting
“Effects of Focused-Ion-Beam Processing on Local Measurements of Semiconductor Solar Cells”, H. P. Yoon, P. M. Haney, J. Schumacher, K. Siebein, Y. Yoon, and N. B. Zhitenev, presented by H. Yoon at the 7th Annual FIB SEM Workshop in Laurel, MD, February 2014.

1. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland, 20899, USA. 2. Maryland Nanocenter, University of Maryland, College Park, Maryland, 20742, USA

ABSTRACT. Quantitative determination of electronic properties at high spatial resolution is crucial for the development of high-efficiency solar cells. Electron beam induced current (EBIC) is a powerful technique in which electron-hole pairs are created in proximity to an exposed surface, and the carrier collection efficiency is measured as a function of excitation position. Cross-sections of device are often created by focused ion beams (FIB) due to the flexibility of the patterning and milling processes. However, the irradiating Ga ions of the FIB fabrication may introduce unintended artifacts, affecting local electronic properties. In this talk, we discuss the impact of the FIB process observed in EBIC measurements and two-dimensional finite element simulations. A series of EBIC data was obtained on a single crystalline solar cell at different electron beam voltages and beam currents to examine the depth and carrier injection level dependence inside the depletion region and away from the p-n junction. Quantitative analysis shows that the EBIC efficiency of the FIB sample is much lower (< 40 %) than that of the cleaved sample (100%) at low beam voltages (<10 keV). We discuss the effects of FIB processing of other types of photovoltaic devices including CdTe and CIGS solar cells.

LINK (NDR contents are not presented)

2014 Science

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mof“Tunable electrical conductivity in metal-organic framework thin film devices”, A. A. Talin, A. Centrone, M. E. Foster, V. Stavila, P. M. Haney, R. A. Kinney, V. A. Szalai, F. E. Gabaly, H. P. Yoon, F. Léonard, and M. D. Allendorf, Science 343, 66-69, 2014.

1. Sandia National Laboratories, Livermore, CA 94551, USA.
2. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
3. Maryland Nanocenter, University of Maryland, College Park, MD 20742, USA.

ABSTRACT. We report a strategy for realizing tunable electrical conductivity in metal-organic frameworks (MOFs) in which the nanopores are infiltrated with redox-active, conjugated guest molecules. This approach is demonstrated using thin-film devices of the MOF Cu3(BTC)2 (also known as HKUST-1; BTC, benzene-1,3,5-tricarboxylic acid) infiltrated with the molecule 7,7,8,8-tetracyanoquinododimethane (TCNQ). Tunable, air-stable electrical conductivity over six orders of magnitude is achieved, with values as high as 7 siemens per meter. Spectroscopic data and first-principles modeling suggest that the conductivity arises from TCNQ guest molecules bridging the binuclear copper paddlewheels in the framework, leading to strong electronic coupling between the dimeric Cu subunits. These ohmically conducting porous MOFs could have applications in conformal electronic devices, reconfigurable electronics, and sensors.

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2013 Solar Energy Materials and Solar Cells

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solmat_CdTe“Local electrical characterization of cadmium telluride solar cells using low-energy electron beam”, H. P. Yoon, P. M. Haney, D. Ruzmetov, H. Xu, B. H. Hamadani, A. A. Talin, and N. B. Zhitenev, Solar Energy Materials and Solar Cells 117, 499-504, 2013.

1. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899.
2. Energy and Environment Division, National Institute of Standards and Technology, Gaithersburg, Maryland, 20899, USA.
3. Maryland Nanocenter, University of Maryland, College Park, MD 20742, USA

ABSTRACT. We investigate local electronic properties of cadmium telluride solar cells using electron beam induced current (EBIC) measurements with patterned contacts. EBIC measurements are performed with a spatial resolution as high as ≈20 nm both on the top surface and throughout the cross-section of the device, revealing an enhanced carrier collection in the vicinity of grain boundaries. Furthermore, we measure local current-voltage characteristics using contacts with dimension both larger (≈5 µm × 10 µm) and smaller (≈1 µm × 1 µm) than the device thickness (≈4 µm), finding that the value of local open-circuit voltage is also larger near grain boundaries.

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2013 AIP Advances

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qebic“High-resolution photocurrent microscopy using near-field cathodoluminescence of quantum dots”, H. P. Yoon, Y. Lee, C. D. Bohn, S. Ko, A. G. Gianfrancesco, J. S. Steckel, S. Coe-Sullivan, A. A. Talin, and N. B. Zhitenev, AIP Advances 3, 062112, 2013.

1. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
2. Maryland Nanocenter, University of Maryland, College Park, MD 20742, USA
3. Department of physics, Worcester Polytechnic Institute, Worcester, MA 01602, USA
4. QD Vision Inc., 29 Hartwell Ave., Lexington, MA 02421, USA
5. Sandia National Laboratories, Livermore, CA 94550, USA

ABSTRACT. We report a fast, versatile photocurrent imaging technique to visualize the local photo response of solar energy devices and optoelectronics using near-field cathodoluminescence (CL) from a homogeneous quantum dot layer. This approach is quantitatively compared with direct measurements of high-resolution Electron Beam Induced Current (EBIC) using a thin film solar cell (n-CdS / p-CdTe). Qualitatively, the observed image contrast is similar, showing strong enhancement of the carrier collection efficiency at the p-n junction and near the grain boundaries. The spatial resolution of the new technique, termed Q-EBIC (EBIC using quantum dots), is determined by the absorption depth of photons. The results demonstrate a new method for highresolution, sub-wavelength photocurrent imaging measurement relevant for a wide range of applications.

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2013 Chemical Communications

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ChemComm“Quantum dot-DNA origami binding: A single particle, 3D, real-time tracking study”, K. Du, S. Ko, G. M. Gallatin, H. P. Yoon, J. A. Liddle, and A. J. Berglund, Chemical Communications 49, 909-909, 2013

1. Center for Nanoscale Science and Technology, National Institute of Standards and  Technology, Gaithersburg, MD 20899, USA.
2. Maryland Nanocenter, University of Maryland, College Park, MD 20742, USA

ABSTRACT. The binding process of quantum dots and DNA origami was monitored using a 3D, real-time, single-particle tracking system. Single-molecule binding events were directly observed and precise measurements of the diffusion coefficient and second-order photon correlation function, g2(s), were combined to distinguish free quantum dots from different conjugates of nQdot-origami.

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2012 IEEE Photovoltaic Specialists Conference

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pvsc_ebic“High-resolution local-current measurement of CdTe solar cells”, H. P. Yoon, D. Ruzmetov, P. M. Haney, B. H. Hamadani, A. A. Talin, N. B. Zhitenev, 38th IEEE Photovoltaic Specialists Conference, pp. 3217-3219, 2012.

1. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland, 20899, USA.
2. Energy and Environment Division, National Institute of Standards and Technology, Gaithersburg, Maryland, 20899, USA
3. Maryland Nanocenter, University of Maryland, College Park, Maryland, 20742, USA

ABSTRACT. We investigate local electronic properties of CdTe solar cells using electron beam to excite electron-hole pairs and evaluate spatially resolved photocurrent characteristics. Standard semiconductor processes were used to fabricate Ohmic metal contacts on the surface of p-type CdTe / n-type CdS device extracted from a commercial solar panel. An ion milling process was used to prepare cross-sections of the devices. Local injection of carriers was controlled by an acceleration voltage of electron beam (1 kV to 30 kV) in a scanning electron microscope, and the
results were correlated with the local morphology, microstructure, and chemical composition of the devices.

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