U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Search Publications by: Alexana Roshko (Fed)

Search Title, Abstract, Conference, Citation, Keyword or Author
Displaying 26 - 50 of 172

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

Catalyst-free GaN Nanowires as Nanoscale Light Emitters

March 1, 2012
Author(s)
Kristine A. Bertness, Norman A. Sanford, John B. Schlager, Alexana Roshko, Todd E. Harvey, Paul T. Blanchard, Matthew D. Brubaker, Andrew M. Herrero, Aric W. Sanders
Catalyst-free growth of GaN nanowires with molecular beam epitaxy produces material of exceptionally high quality with long minority carrier lifetimes and low surface recombination velocity. The nanowires grow by thermodynamic driving forces that enhance