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Search Publications by: Alexana Roshko (Fed)

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Displaying 1 - 25 of 88

New Constraints on Dark Photon Dark Matter with Superconducting Nanowire Detectors in an Optical Haloscope

June 10, 2022
Author(s)
Jeff Chiles, Ilya Charaev, Asimina Arvanitaki, Masha Baryakhtar, junwu huang, Robert Lasenby, Ken Van Tillburg, Alexana Roshko, George Burton, Marco Colangelo, Sae Woo Nam, Karl Berggren
Uncovering the nature of dark matter is one of the most important goals of particle physics. Light bosonic particles, such as the dark photon, are well-motivated candidates: they are generically long-lived, weakly-interacting, and naturally produced in the

Laser-assisted atom probe tomography of c-plane and m-plane InGaN test structures

April 29, 2022
Author(s)
Norman A. Sanford, Paul T. Blanchard, Alexana Roshko, Ashwin Rishinaramangalam, Daniel Feezell, Albert Davydov
Laser-assisted atom probe tomography (L-APT) was used to measure the indium mole fraction x of c-plane, MOCVD-grown, GaN/InxGa1-xN/GaN test structures and the results were compared with Rutherford backscattering analysis (RBS). Four separate sample types

Crystallographic Polarity Measurements in Two-Terminal GaN Nanowire Devices by Lateral Piezoresponse Force Microscopy

July 23, 2020
Author(s)
Matthew Brubaker, Alexana Roshko, Samuel Berweger, Paul T. Blanchard, Todd E. Harvey, Norman A. Sanford, Kris A. Bertness
Lateral piezoresponse force microscopy (L-PFM) is demonstrated as a reliable method for determining the crystallographic polarity of individual, dispersed GaN nanowires that were functional components in electrical test structures. In contrast to PFM

AlGaN/GaN core-shell heterostructures for nanowire UV LEDs

May 1, 2020
Author(s)
Matthew D. Brubaker, Bryan T. Spann, Kristen L. Genter, Alexana Roshko, Paul T. Blanchard, Todd E. Harvey, Kristine A. Bertness
Nanowire-based ultraviolet (UV) LEDs hold great promise as nanoscale light sources, potentially enabling advanced scanning microscopy probes capable of optoelectronic sensing and near-field scanning photolithography. In this work, we report on the

Eutectic Formation, V/III Ratio and Controlled Polarity Inversion in Nitrides on Silicon

November 19, 2019
Author(s)
Alexana Roshko, Matthew D. Brubaker, Paul T. Blanchard, Todd E. Harvey, Kristine A. Bertness
The crystallographic polarity of AlN grown on Si(111) by plasma assisted molecular beam epitaxy is intentionally inverted from N-polar to Al-polar at a planar boundary. The position of the inversion boundary is controlled by a two-step growth process that

The role of Si in GaN/AlN/Si(111) PAMBE Epitaxy: Polarity and Inversion

May 22, 2019
Author(s)
Alexana Roshko, Matthew D. Brubaker, Paul T. Blanchard, Todd E. Harvey, Kristine A. Bertness
The microstructure, polarity and Si distribution in AlN/GaN layers grown by PAMBE on Si(111) was assessed by STEM. Samples grown under both metal- and nitrogen-rich conditions contained defects at the AlN/Si interface which suggest formation of an Al-Si

UV LEDs Based on p-i-n Core-Shell AlGaN/GaN Nanowire Heterostructures Grown by N-polar Selective Area Epitaxy

March 20, 2019
Author(s)
Matthew Brubaker, Kristen Genter, Alexana Roshko, Paul T. Blanchard, Bryan T. Spann, Todd E. Harvey, Kris A. Bertness
Ultraviolet light-emitting diodes (UV LEDs) fabricated from N-polar AlGaN/GaN core-shell nanowires with p-i-n structure produced electroluminescence at 365 nm with 5x higher intensities than similar GaN homojunction LEDs. The improved characteristics were

Core-Shell p-i-n GaN Nanowire LEDs by N-polar Selective Area Growth

September 11, 2018
Author(s)
Matthew D. Brubaker, Kristen L. Genter, Bryan T. Spann, Alexana Roshko, Paul T. Blanchard, Todd E. Harvey, Kristine A. Bertness
GaN nanowire LEDs with radial p-i-n junctions were grown by molecular beam epitaxy using N- polar selective area growth on Si(111) substrates. The N-polar selective area growth process facilitated the growth of isolated and high-aspect-ratio n-type NW

Nano-Sized Ferrite Particles for Magnetic Resonance Imaging Thermometry

September 8, 2018
Author(s)
Janusz H. Hankiewicz, J A. Stoll, John Stroud, J Davidson, K L. Livesey, K Tvrdy, Alexana Roshko, Stephen E. Russek, Karl Stupic, P Bilski, Robert E. Camley
Recently, we reported the use of magnetic particles as temperature indicators in magnetic resonance imaging thermometry (tMRI). In this method, the brightness of the MR image changes with temperature due to a temperature-dependent magnetic field

Enhanced superconducting transition temperature in electroplated rhenium

April 30, 2018
Author(s)
David P. Pappas, Don David, Mustafa Bal, Ilke Arslan, Paul T. Blanchard, Ronald B. Goldfarb, Dustin A. Hite, Hsiang S. Ku, Russell E. Lake, Junling Long, Alexana Roshko, Lee D. Pappas, Britton L. Plourde, Jianguo Wen, Xian Wu, Corey Rae H. McRae
We show that electroplated Re films in multilayers with noble metals such as Cu, Au, and Pd have an enhanced superconducting critical temperature relative to previous methods of preparing Re. The dc resistance and magnetic susceptibility indicate a critical

Comparison of CBED and ABF Atomic Imaging for GaN Polarity Determination

November 8, 2016
Author(s)
Alexana Roshko, Matthew D. Brubaker, Paul T. Blanchard, Kristine A. Bertness, Todd E. Harvey, Igor Levin, R.H. Geiss
A comparison of two electron microscopy techniques used to determine the polarity of GaN nanowires is presented. The techniques are convergent beam electron diffraction (CBED) in TEM mode and annular bright field (ABF) imaging in aberration corrected STEM

Electron Enhanced Growth of Crystalline Gallium Nitride Thin Films at Room Temperature and 100 C Using Sequential Surface Reactions

July 1, 2016
Author(s)
Alexana Roshko, Jaclyn Sprenger, Andrew S. Cavanagh, Steven George, Huaxing Sun, Kathryn J. Wahl
Low energy electrons may provide mechanisms to enhance thin film growth at low temperatures. In this work, gallium nitride (GaN) films were deposited over areas of ~5 cm2 at room temperature and 100 C using electrons with a low energy of 50 eV from an

Structural and optical nanoscale analysis of GaN core-shell microrod arrays fabricated by combined top-down and bottom-up process on Si (111)

May 15, 2016
Author(s)
Sergiy Krylyuk, Marcus Muller, Gordon Schmidt, Sebastian Metzner, Peter Veit, Frank Bertram, Ratan K. Debnath, Jong Yoon Ha, Baomei Wen, Paul T. Blanchard, Alexana Roshko, Abhishek Motayed, Matthew R. King, Albert Davydov, Jurgen Christen
Large arrays of GaN core-shell microrods were fabricated on Si(111) substrates applying a combined bottom-up and top-down approach which includes inductively coupled plasma (ICP) etching of patterned GaN films grown by metal-organic vapor phase epitaxy

Spontaneous growth of GaN nanowire nuclei on N- and Al-polar AlN: A piezoresponse force microscopy study of crystallographic polarity

March 2, 2016
Author(s)
Matthew D. Brubaker, Alexana Roshko, Paul T. Blanchard, Todd E. Harvey, Norman A. Sanford, Kristine A. Bertness
The polarity of gallium nitride (GaN) nanowire nuclei grown on AlN layers was studied by piezoresponse force microscopy (PFM). N- or Al-polar AlN layers were grown by molecular beam epitaxy (MBE) on Si (111) substrates by use of Al- or N-rich growth

Polarity-Controlled GaN/AlN Nucleation Layers for Selective-Area Growth of GaN Nanowire Arrays on Si(111) Substrates by Molecular Beam Epitaxy

December 18, 2015
Author(s)
Matthew D. Brubaker, Shannon M. Duff, Todd E. Harvey, Paul T. Blanchard, Alexana Roshko, Aric W. Sanders, Norman A. Sanford, Kristine A. Bertness
We have demonstrated dramatic improvement in the quality of selective-area GaN nanowire growth by controlling the polarity of the underlying nucleation layers. In particular, we find that N- polarity is beneficial for the growth of large ordered nanowire

Selective Area Growth of Ga- and N-polar GaN Nanowire Arrays on Non-Polar Si (111) Substrates

September 7, 2014
Author(s)
Matthew D. Brubaker, Shannon M. Duff, Todd E. Harvey, Paul T. Blanchard, Alexana Roshko, Aric W. Sanders, Norman A. Sanford
This study presents a technique for obtaining Ga- and N-polar Gallium Nitride nanowire (GaN NW) arrays on non-polar Si (111) substrates by use of polarity-controlled AlN/GaN buffer layers. AlN films are demonstrated to adopt Al-/N-polarity for N-/Al-rich

Characterization of InGaN quantum disks in GaN nanowires

March 4, 2014
Author(s)
Alexana Roshko, Roy H. Geiss, John B. Schlager, Matthew D. Brubaker, Kristine A. Bertness, Norman A. Sanford, Todd E. Harvey
Catalyst-free GaN nanowires with InGaN quantum disks (QDs) were characterized by scanning/transmission elec-tron microscopy (S/TEM) and photoluminescence. A va-riety of structures, from QDs with large strain fields to apparently strain free QDs were

Characterization of InGaN quantum disks in GaN nanowires

February 27, 2014
Author(s)
Alexana Roshko, Roy H. Geiss, John B. Schlager, Matthew D. Brubaker, Kristine A. Bertness, Norman A. Sanford, Todd E. Harvey
Catalyst-free GaN nanowires with InGaN quantum disks (QDs) were characterized by scanning/transmission electron microscopy (S/TEM) and photoluminescence. A variety of structures, from QDs with large strain fields to apparently strain free QDs were observed

Gallium Nitride Nanowires for On-Chip Optical Interconnects on Ex-Situ Substrates

January 16, 2013
Author(s)
Matthew D. Brubaker, Paul T. Blanchard, John B. Schlager, Aric W. Sanders, Alexana Roshko, Shannon M. Duff, Jason Gray, Victor M. Bright, Norman A. Sanford, Kristine A. Bertness
In this letter we report on the fabrication, device characteristics, and optical coupling of a two-nanowire device comprising light-emitting diode and photoconductive GaN nanowires. Axial p-n junction GaN nanowires were grown by molecular beam epitaxy

Microstructure evolution and development of annealed Ni/Au contacts to GaN nanowires

August 21, 2012
Author(s)
Andrew M. Herrero, Paul T. Blanchard, Aric W. Sanders, Matthew D. Brubaker, Norman A. Sanford, Alexana Roshko, Kristine A. Bertness
The development of Ni/Au contacts to Mg-doped GaN nanowires (NWs) is examined. Current-voltage (I-V) measurements of these Mg-doped nanowire devices frequently exhibit a strong degradation after annealing in N 2/O 2. This degradation originates from the