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Search Publications by: Alexana Roshko (Fed)

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Displaying 26 - 50 of 88

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

Controlled nucleation of GaN nanowires grown with molecular beam epitaxy

July 13, 2010
Author(s)
Kristine A. Bertness, Aric W. Sanders, Devin M. Rourke, Todd E. Harvey, Alexana Roshko, Norman A. Sanford
The location of GaN nanowires was controlled with essentially perfect selectivity using patterned SiNx prior to molecular beam epitaxy growth. Growth was uniform within mask openings and absent on the mask surface for over 95 % of the usable area of a 76

Steady-state and transient photoconductivity in c-axis GaN nanowires grown by nitrogen-plasma-assisted molecular beam epitaxy

February 12, 2010
Author(s)
Norman A. Sanford, Paul T. Blanchard, Kristine A. Bertness, Lorelle Mansfield, John B. Schlager, Aric W. Sanders, Alexana Roshko, Beau Burton, Steven George
Analysis of steady-state and transient photoconductivity measurements at room temperature performed on c-axis oriented GaN nanowires yielded estimates of free carrier concentration, drift, mobility, surface band bending, and surface capture coefficient for

Lateral variations in self-assembled InGaAs quantum dot distributions

June 26, 2009
Author(s)
Alexana Roshko, Todd E. Harvey, Brittany L. Hyland, Lehman Y. Susan, Keith D. Cobry
The lateral uniformity of self-assembled InGaAs quantum dots grown by molecular beam epitaxy(MBE) was assessed as a function of growth conditions. Variations in the dot density and height were determined from atomic force micrographs. Growth rate had a

Topography of epitaxial GaAs surfaces for growth

April 30, 2009
Author(s)
Susan Y. Lehman, Alexana Roshko, Richard Mirin, Kristine A. Bertness, Todd E. Harvey, Keith D. Cobry
The topography and surface roughness of (100) GaAs substrates and buffers after different preparation procedures were determined from atomic force microscopy measurements. The influence of substrate aging and chemical etching on buffers grown by molecular

Steady-state and time-resolved photoluminescence from relaxed and strained GaN nanowires grown by catalyst-free molecular-beam epitaxy

June 24, 2008
Author(s)
John B. Schlager, Kristine A. Bertness, Paul T. Blanchard, Lawrence H. Robins, Alexana Roshko, Norman A. Sanford
We report steady-state and time-resolved photoluminescence (TRPL) measurements on individual GaN nanowires (6-20 micrometers in length, 30-940 nm in diameter) grown by nitrogen-plasma-assisted, catalyst-free MBE on Si(111) and dispersed onto fused quartz

Mechanism for spontaneous growth of GaN nanowires with molecular beam epitaxy

April 1, 2008
Author(s)
Kristine A. Bertness, Alexana Roshko, Lorelle Mansfield, Todd E. Harvey, Norman A. Sanford
Although most semiconductor nanowires are grown via the vapor-liquid-solid mechanism, we present evidence that GaN nanowires form because of thermodynamically driven variations in surface sticking coefficients on different crystallographic planes under

Nucleation and propagation mechanisms for catalyst-free growth of GaN nanowires

September 16, 2007
Author(s)
Kristine A. Bertness, Devin M. Rourke, Alexana Roshko, Lorelle Mansfield, Aric W. Sanders, Todd E. Harvey, Norman A. Sanford
Nucleation of GaN nanowires without catalysts in molecular beam epitaxy is shown to be a distinct process from nanowire propagation. Nanowire growth is relatively insensitive to starting conditions once the nanowire morphology is established, and is driven

Persistent photoconductivity studies of c-axis GaN nanowires grown by MBE

September 16, 2007
Author(s)
Norman Sanford, Paul T. Blanchard, Kristine A. Bertness, Alexana Roshko, Beau Burton, Lorelle Mansfield, John B. Schlager, Steven George, Aric Sanders
Photoconductivity (PC) and persistent photoconductivity (PPC) were measured on c-axis GaN nanowires (NWs) grown by catalyst-free nitrogen plasma assisted MBE on Si(111) substrates. Unintentionally doped n-type NWs from two separate growth batches were

Nucleation conditions for catalyst-free GaN nanowires

December 8, 2006
Author(s)
Kristine A. Bertness, Alexana Roshko, Lorelle Mansfield, Todd E. Harvey, Norman Sanford
We have examined the initial steps for catalyst-free growth of GaN using molecular beam epitaxy on Si (111) substrates using AlN buffer layers. These wires form spontaneously under high N-to-Ga ratios for a growth temperature range of about 810 to 830 °C

Catalyst-Free Growth of GaN Nanowires

January 1, 2006
Author(s)
Kristine A. Bertness, Norman Sanford, Joy Barker, John B. Schlager, Alexana Roshko, Albert Davydov, Igor Levin
We have grown GaN and AlGaN nanowires on Si (111) substrates with gassource molecular beam epitaxy (MBE). No metal catalysts were used. The nanowires displayed a number of interesting materials properties, including room-temperature luminescence intensity

High Degree of Crystalline Perfection in Spontaneously Grown GaN Nanowires

January 1, 2006
Author(s)
Kristine A. Bertness, Alexana Roshko, Albert Davydov, Igor Levin, Mark D. Vaudin, Joy Barker, John B. Schlager, Norman Sanford, Larry Robins
We have grown a variety of isolated GaN nanowires using gas-source molecular beam epitaxy (MBE) and characterized their structural and optical properties. The nanowires have demonstrated a number of promising materials characteristics, including low defect

Spontaneously grown GaN and AlGaN nanowires

January 1, 2006
Author(s)
Kristine A. Bertness, Alexana Roshko, Norman A. Sanford, Joy Barker, Albert Davydov
We have identified crystal growth conditions in gas-source molecular beam epitaxy (MBE) that lead to spontaneous formation of GaN nanowires with high aspect ratio on Si (1 1 1) substrates. The nanowires were oriented along the GaN c-axis and normal to the

GaAs Buffer Layer Morphology and Lateral Distributions of InGaAs Quantum Dots

May 1, 2005
Author(s)
Alexana Roshko, Todd E. Harvey, Susan Y. Lehman, Richard Mirin, Kristine A. Bertness, Brittany Hyland
Atomic force microscopy was used to study the morphology of GaAs buffer layers and the density and height distributions of self-assembled InGaAs quantum dots (QDs) grown on these buffers by molecular beam epitaxy. The surface roughness and terrace size of

Storage Conditions for High-Accuracy Composition Standards of AlGaAs

May 1, 2005
Author(s)
Kristine A. Bertness, Alexana Roshko, S. E. Asher, Craig L. Perkins
AlGaAs epitaxial films were stored under different environmental conditions and the resulting surface oxidation and contamination variations measured using several analytical techniques. Auger depth profiles confirmed that oxidation and carbon

Fabrication and Analysis of GaN Nanorods grown by MBE

January 1, 2005
Author(s)
Norman Sanford, Larry Robins, Matthew H. Gray, J E. Van Nostrand, C Stutz, R Cortez, Albert Davydov, Alexander J. Shapiro, Igor Levin, Alexana Roshko
GaN nanorods were grown on c-plane sapphire substrates under N-rich conditions by plasma-assisted molecular-beam epitaxy. Scanning electron microsopy revealed densely packed nanorods of hexagonal cross section with diameters ranging from roughly 40 to 100

EBSD Measurement of Strains in GaAs due to Oxidation of Buried AlGaAs Layers

February 27, 2004
Author(s)
T Keller, Alexana Roshko, R.H. Geiss, Kristine A. Bertness, T.P. Quinn
We have characterized elastic strain fields associated with the wet-thermal oxidation of buried Al xGa 1-xAs (x 0.98) layers of thickness 80 nm, situated between GaAs layers of thickness 200 nm, on a GaAs substrate. The compressive strains accompanying

Electron Backscatter Diffraction for Studies of Localized Deformation

July 1, 2003
Author(s)
Roy H. Geiss, Alexana Roshko, Kristine A. Bertness, R R. Keller
Electron backscatter diffraction (EBSD) was used to study localized deformation in two types ofconstrained-volume materials. We present a study of deformation in narrow aluminum interconnects afterlow frequency, AC cycling at high current density. Joule

Electron Backscatter Diffraction for Studies of Localized Deformation

March 1, 2003
Author(s)
R.H. Geiss, Alexana Roshko, Kristine A. Bertness, T Keller
Electron backscatter diffraction (EBSD) was used to study localized deformation in two types of constrained-volume materials. We present a study of deformation in narrow aluminum interconnects after low frequency, AC cycling at high current density. Joule

Biaxial Stress Dependence of the GaAs-Like and AlAs-Like Raman Lines in Al x Ga 1-x As

January 1, 2003
Author(s)
Grady S. White, Albert J. Paul, Kristine A. Bertness, Alexana Roshko
Raman peak shifts of Al xGa 1-xAs have been measured as a function of both x and applied biaxial stress in a backscatter geometry. Within the precision of the Raman measurements, the stress-induced peak shifts of the Raman singlet GaAs-like and AlAs-like

Interlaboratory comparison of InGaAsP EX-SITU characterization

January 1, 2003
Author(s)
Alexana Roshko, Kristine A. Bertness
A study to improve the accuracy of ex-situ characterization of InGaAsP materials for optoelectronics is underway. Six InGaAsP thin film specimens, with nominal photoluminescence wavelengths of 1.1, 1.3 and 1.5 mm, have been measured, with X-ray diffraction