23 research outputs found
Top-down fabrication of ordered arrays of GaN nanowires by selective area sublimation
We demonstrate the top-down fabrication of ordered arrays of GaN nanowires by
selective area sublimation of pre-patterned GaN(0001) layers grown by hydride
vapor phase epitaxy on AlO. Arrays with nanowire diameters and
spacings ranging from 50 to 90 nm and 0.1 to 0.7 m, respectively, are
simultaneously produced under identical conditions. The sublimation process,
carried out under high vacuum conditions, is analyzed \emph{in situ} by
reflection high-energy electron diffraction and line-of-sight quadrupole mass
spectromety. During the sublimation process, the GaN(0001) surface vanishes,
giving way to the formation of semi-polar facets
which decompose congruently following an Arrhenius temperature dependence with
an activation energy of () eV and an exponential prefactor of
atoms cm s. The analysis of the samples by
low-temperature cathodoluminescence spectroscopy reveals that, in contrast to
dry etching, the sublimation process does not introduce nonradiative
recombination centers at the nanowire sidewalls. This technique is suitable for
the top-down fabrication of a variety of ordered nanostructures, and could
possibly be extended to other material systems with similar crystallographic
properties such as ZnO.Comment: This is the accepted manuscript version of an article that appeared
in Nanoscale Advances. The CC BY-NC 3.0 license applies, see
http://creativecommons.org/licenses/by-nc/3.0
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Top-down fabrication of ordered arrays of GaN nanowires by selective area sublimation
We demonstrate the top-down fabrication of ordered arrays of GaN nanowires by selective area sublimation of pre-patterned GaN(0001) layers grown by hydride vapor phase epitaxy on Al2O3. Arrays with nanowire diameters and spacings ranging from 50 to 90 nm and 0.1 to 0.7 µm, respectively, are simultaneously produced under identical conditions. The sublimation process, carried out under high vacuum conditions, is analyzed in situ by reflection high-energy electron diffraction and line-of-sight quadrupole mass spectrometry. During the sublimation process, the GaN(0001) surface vanishes, giving way to the formation of semi-polar {1103} facets which decompose congruently following an Arrhenius temperature dependence with an activation energy of (3.54 ± 0.07) eV and an exponential prefactor of 1.58 × 1031 atoms per cm2 per s. The analysis of the samples by lowerature cathodoluminescence spectroscopy reveals that, in contrast to dry etching, the sublimation process does not introduce nonradiative recombination centers at the nanowire sidewalls. This technique is suitable for the top-down fabrication of a variety of ordered nanostructures, and could possibly be extended to other material systems with similar crystallographic properties such as ZnO. © 2019 The Royal Society of Chemistry
A route for the top-down fabrication of ordered ultrathin GaN nanowires
Ultrathin GaN nanowires (NWs) are attractive to maximize surface effects and
as building block in high-frequency transistors. Here, we introduce a facile
route for the top-down fabrication of ordered arrays of GaN NWs with aspect
ratios exceeding and diameters below nm. Highly uniform thin GaN NWs
are first obtained by using electron beam lithography to pattern a Ni/SiN
hard mask, followed by dry etching and wet etching in hot KOH. The SiN is
found to work as an etch stop during wet etching in hot KOH. Arrays with NW
diameters down to nm can be achieved with a yield exceeding
. Further reduction of the NW diameter down to nm is obtained by
applying digital etching which consists in plasma oxidation followed by wet
etching in hot KOH. The NW radial etching depth is tuned by varying the RF
power during plasma oxidation. NW breaking or bundling is observed for
diameters below nm, an effect that is associated to capillary
forces acting on the NWs during sample drying in air. This effect can be
principally mitigated using critical point dryers. Interestingly, this
mechanical instability of the NWs is found to occur at much smaller aspect
ratios than what is predicted for models dealing with macroscopic elastic rods.
Explicit calculations of buckling states show an improved agreement when
considering an inclined water surface, as can be expected if water assembles
into droplets. The proposed fabrication route can be principally applied to any
GaN/SiN nanostructures and allows regrowth after removal of the SiN
mask
Electroluminescence and current-voltage measurements of single (In,Ga)N/GaN nanowire light-emitting diodes in the nanowire ensemble
We present the combined analysis of the electroluminescence (EL) as well as
the current-voltage (I-V) behavior of single, freestanding (In,Ga)N/GaN
nanowire (NW) light-emitting diodes (LEDs) in an unprocessed, self-assembled
ensemble grown by molecular beam epitaxy. The data were acquired in a scanning
electron microscope equipped with a micromanipulator and a luminescence
detection system. Single NW spectra consist of emission lines originating from
different quantum wells, and the width of the spectra increases with decreasing
peak emission energy. The corresponding I-V characteristics are described well
by the modified Shockley equation. The key advantage of this measurement
approach is the possibility to correlate the EL intensity of a single NW LED
with the actual current density in this NW. This way, the external quantum
efficiency (EQE) can be investigated as a function of the current in a single
NW LED. The comparison of the EQE characteristic of single NWs and the ensemble
device allows a quite accurate determination of the actual number of emitting
NWs in the working ensemble LED and the respective current densities in its
individual NWs. This information is decisive for a meaningful and comprehensive
characterization of a NW ensemble device, rendering the measurement approach
employed here a very powerful analysis tool