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Search Publications by: Kris A. Bertness (Fed)

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Displaying 1 - 25 of 178

Semiconductor thermal and electrical properties decoupled by localized phonon resonances

May 10, 2023
Bryan Spann, Joel Weber, Matthew Brubaker, Todd E. Harvey, LINA YANG, Hossein Honarvar, Chia-Nien Tsai, Andrew Treglia, MINHYEA LEE, MAHMOUD HUSSEIN, Kris A. Bertness
Thermoelectric materials convert heat into electricity through thermally driven charge transport in solids or vice versa for cooling. To compete with conventional energy-conversion technologies, a thermoelectric material must possess the properties of both

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

July 23, 2020
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
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
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
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
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

Axisymmetric scalable magneto-gravitational trap for diamagnetic particle levitation

December 11, 2018
Kristine A. Bertness, John P. Houlton, Max L. Chen, Matthew D. Brubaker, Charles T. Rogers
We report an axisymmetric magnetic trap design for levitating diamagnetic particles. The magnetic traps each consist of two iron pole pieces passively driven by a neodymium iron boron (NdFeB) permanent magnet. The magnetic field configuration between the

An optical Bragg scattering readout for simultaneous detection of all low-order mechanical modes of gallium nitride nanowires in nanowire arrays

September 19, 2018
Kristine A. Bertness, John P. Houlton, Matthew D. Brubaker, Charles T. Rogers
We report the use of optical Bragg scattering and homodyne interferometry to simultaneously measure all the first order cantilever-mode mechanical resonance frequencies and quality factors (Q) of gallium nitride nanowires (GaN NWs) in 100 NW periodic

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

September 11, 2018
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

Raman spectroscopy for dopant optimization in GaN nanowire light-emitting diodes

May 1, 2018
Kristine A. Bertness, Bryan T. Spann, Matthew D. Brubaker, Todd E. Harvey, Paul T. Blanchard
We apply Raman spectroscopy to optimize both n-type and p-type doping in GaN nanowire light- emitting diodes (LEDs) grown with selective epitaxy on Si(111) with molecular beam epitaxy. N-type doping with Si is characterized using the peak shift in the LO

Spectral tuning of localized surface phonon-polariton modes in selective area epitaxy GaN nanowire arrays

May 1, 2018
Bryan T. Spann, J. Ryan Nolen, Matthew D. Brubaker, Thomas G. Folland, Chase T. Ellis, Joseph G. Tischler, Todd E. Harvey, Joshua D. Caldwell, Kristine A. Bertness
Polar semiconductor materials, such as GaN, InP, SiC et al., offer a basis to manufacture innovative long-wavelength photonic devices.1 Such semiconductors can support propagating and localized surface phonon-polariton (SPhP) resonances that provide highly

GaN Nanowire MOSFET with Near-Ideal Subthreshold Slope

December 21, 2017
Kristine A. Bertness, Wenjun Li, Matthew D. Brubaker, Bryan T. Spann, Patrick Fay
Top-gated GaN nanowire MOSFETs using Al2O3 as gate oxide have been experimentally demonstrated. The fabricated devices exhibit a minimum subthreshold slope of 60 mV/dec, an average subthreshold slope of 68 mV/dec over three decades of drain current, drain

Dimensional Measurement of Nanostructures with Scanning Electron Microscopy

September 20, 2017
Kristine A. Bertness
Scanning electron microscopy (SEM) is widely used for the measurement of dimensions of nanostructures. This document describes the calibration of SEM magnification using the ASTM E766-14 practice with NIST Reference Material (RM) 8820 and the calculation

Lithographic sonication patterning of large area nanopillar forests

July 27, 2017
Joel C. Weber, Matthew D. Brubaker, Thomas M. Wallis, Kristine A. Bertness
This paper demonstrates a highly-scalable, material-independent method for patterning nanopillar forests known as lithographic sonication patterning. Through contact lithography, patterns with dimensions down to 3 υm were written across a 3-inch silicon

Comparison of CBED and ABF Atomic Imaging for GaN Polarity Determination

November 8, 2016
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

Raman spectroscopy based measurements of carrier concentration in n-type GaN nanowires grown by plasma-assisted molecular beam epitaxy

September 30, 2016
Lawrence H. Robins, Elizabeth Horneber, Norman A. Sanford, Kristine A. Bertness, John B. Schlager
The carrier concentration in ensembles of n-type GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy was determined by curve-fitting analysis of Raman spectra, based on modeling of the carrier concentration dependence of the longitudinal

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

March 2, 2016
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

Near-field control and imaging of free charge carrier variations in GaN nanowires

February 15, 2016
Samuel Berweger, Paul T. Blanchard, Matthew Brubaker, Kevin J. Coakley, Norman A. Sanford, Thomas Mitchell (Mitch) Wallis, Kris A. Bertness, Pavel Kabos
Despite their uniform crystallinity, the shape and faceting of semiconducting nanowires (NWs) can give rise to variations in structure and associated electronic properties. Here we investigate local variations in electronic structure across individual n

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
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

Periodic dynamics, localization metastability, and elastic interaction of colloidal particles with confining surfaces and helicoidal structure of cholesteric liquid crystals

December 4, 2014
Michael C. Varney, Qiaoxuan Zhang, Mykola Tasinkevych, Nuno M. Silvestre, Kristine A. Bertness, Ivan I. Smalyukh
Although nematic and cholesteric liquid crystals are three-dimensional fluids, they also possess long- range orientational ordering of their constituent molecules, supporting both topological defects and chiral superstructures. Implications of this