21 research outputs found
Nanofils de GaN/AlN : nucléation, polarité et hétérostructures quantiques
Using specific conditions, GaN can be epitaxially grown on a large variety of substrates as a nanowire (NW) array. This geometry allows the subsequent growth of wire-like heterostructures likely free of extended defects, which makes them promising for increasing device controllability and performance. First, my PhD work has been devoted to the understanding of self-organized nucleation of GaN NWs on silicon substrates. For this purpose, a deep characterization of the growth mechanism of the AlN buffer deposited prior to NW nucleation has been done, emphasizing an unexpected large reactivity of Al with the substrate. The requirement of the N polarity to nucleate GaN NWs has been evidenced, although the possible existence of NWs hosting a Ga polar core has been observed as well. In these NWs, an inversion domain boundary is present and has been demonstrated to be optically active, having a photoluminescence signature at 3.45 eV. Next, GaN/AlN wire heterostructures have been grown for structural and optical characterization. It has been shown that by changing the wire diameter, different growth mode for the heterostructure could be reached.At last, thanks to the cylindrical geometry of NWs, the measurement of diffusion length for charge carriers in GaN and AlN NWs have been performed.Usant de certaines conditions, la croissance épitaxiale de GaN sur un large panel de substrats donne lieu à une assemblée de nanofils. Cette géométrie filaire peut permettre la croissance d'hétérostructures libres de tous défauts cristallins étendus, ce qui les rendent attractives pour créer des dispositifs de hautes performances. En premier lieu, mon travail de thèse a visé à clarifier le mécanisme de nucléation auto-organisé des nanofils de GaN sur substrat de silicium. Dans ce but, une étude approfondie de la couche tampon d'AlN, déposée préalablement à la nucléation des nanofils, a été réalisée, mettant en évidence une inattendue forte réactivité de l'Al avec le substrat. La nécessité de la polarité azote pour la croissance des nanofils de GaN a été mise en lumière, bien que des nanofils contenant dans leur cœur un domaine de polarité Ga ont également été observés. Dans ces nanofils, une paroi d'inversion de domaine est présente et a été démontrée être optiquement active, exhibant une photoluminescence à 3.45 eV. Ensuite des hétérostuctures filaires GaN/AlN ont été synthétisée pour des caractérisations structurales et optiques. Il a été montré que le mode de croissance de l'hétérostructure peut être changé en fonction du diamètre du nanofil. En dernier lieu, en prenant avantage de la géométrie cylindrique des nanofils, des mesures de diffusion de porteurs de charge ont été réalisées dans des nanofils de GaN et d'AlN
Cathodoluminescence of stacking fault bound excitons for local probing of the exciton diffusion length in single GaN nanowires
We perform correlated studies of individual GaN nanowires in scanning
electron microscopy combined to low temperature cathodoluminescence,
microphotoluminescence, and scanning transmission electron microscopy. We show
that some nanowires exhibit well localized regions emitting light at the energy
of a stacking fault bound exciton (3.42 eV) and are able to observe the
presence of a single stacking fault in these regions. Precise measurements of
the cathodoluminescence signal in the vicinity of the stacking fault give
access to the exciton diffusion length near this location
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
Density control of GaN nanowires at the wafer scale using self-assembled SiN patches on sputtered TiN(111)
The self-assembly of heteroepitaxial GaN nanowires using either molecular
beam epitaxy (MBE) or metal-organic vapor phase epitaxy (MOVPE) mostly results
in wafer-scale ensembles with ultrahigh ( m) or ultralow (
m) densities, respectively. A simple means to tune the density of
well-developed nanowire ensembles between these two extremes is generally
lacking. Here, we examine the self-assembly of SiN patches on TiN(111)
substrates which are eventually acting as seeds for the growth of GaN
nanowires. We first found that if prepared by reactive sputtering, the TiN
surface is characterized by \{100\} facets for which the GaN incubation time is
extremely long. Fast GaN nucleation is only obtained after deposition of a
sub-monolayer of SiN atoms prior to the GaN growth. By varying the amount
of pre-deposited SiN, the GaN nanowire density could be tuned by three
orders of magnitude with excellent uniformity over the entire wafer, bridging
the density regimes conventionally attainable by direct self-assembly with MBE
or MOVPE. The analysis of the nanowire morphology agrees with a nucleation of
the GaN nanowires on nanometric SiN patches. The photoluminescence analysis
of single freestanding GaN nanowires reveals a band edge luminescence dominated
by excitonic transitions that are broad and blue shifted compared to bulk GaN,
an effect that is related to the small nanowire diameter and to the presence of
a thick native oxide. The approach developed here can be principally used for
tuning the density of most III-V semiconductors nucleus grown on inert surfaces
like 2D materials
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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
Unraveling the strain state of GaN down to single nanowires
GaN nanowires (NWs) grown by molecular beam epitaxy are usually assumed free of strain in spite of different individual luminescence signatures. To ascertain this usual assumption, the c/a of a GaNNW assembly has been characterized using both X-ray diffraction and Raman spectroscopy, with scaling the measurement down to the single NW. Free-standing single NWs have been observed free of strain defined as [c/a-(c/a)o]/(c/a)o within the experimental accuracy mounting to 1.25 × 10-4. However, in the general case, a significant portion of the NWs is coalesced, generating an average tensile strain that can be partly released by detaching the NWs from their substrates. It is concluded that at the scale of the single NW, the free surface and the residual doping do not generate a significant strain and only coalescence does
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
Self-Assembly of Well-Separated AlN Nanowires Directly on Sputtered Metallic TiN Films
Herein, the self-assembled formation of AlN nanowires (NWs) by molecular beam epitaxy on sputtered TiN films on sapphire is demonstrated. This choice of substrate allows growth at an exceptionally high temperature of 1180 °C. In contrast to previous reports, the NWs are well separated and do not suffer from pronounced coalescence. This achievement is explained by sufficient Al adatom diffusion on the substrate and the NW sidewalls. The high crystalline quality of the NWs is evidenced by the observation of near-band-edge emission in the cathodoluminescence spectrum. The key factor for the low NW coalescence is the TiN film, which spectroscopic ellipsometry and Raman spectroscopy indicate to be stoichiometric. Its metallic nature will be beneficial for optoelectronic devices using these NWs as the basis for (Al,Ga)N/AlN heterostructures emitting in the deep ultraviolet spectral range
GaN/AlN nanowires : nucleation, polarity and quantum heterostructures
Usant de certaines conditions, la croissance épitaxiale de GaN sur un large panel de substrats donne lieu à une assemblée de nanofils. Cette géométrie filaire peut permettre la croissance d'hétérostructures libres de tous défauts cristallins étendus, ce qui les rendent attractives pour créer des dispositifs de hautes performances. En premier lieu, mon travail de thèse a visé à clarifier le mécanisme de nucléation auto-organisé des nanofils de GaN sur substrat de silicium. Dans ce but, une étude approfondie de la couche tampon d'AlN, déposée préalablement à la nucléation des nanofils, a été réalisée, mettant en évidence une inattendue forte réactivité de l'Al avec le substrat. La nécessité de la polarité azote pour la croissance des nanofils de GaN a été mise en lumière, bien que des nanofils contenant dans leur cœur un domaine de polarité Ga ont également été observés. Dans ces nanofils, une paroi d'inversion de domaine est présente et a été démontrée être optiquement active, exhibant une photoluminescence à 3.45 eV. Ensuite des hétérostuctures filaires GaN/AlN ont été synthétisée pour des caractérisations structurales et optiques. Il a été montré que le mode de croissance de l'hétérostructure peut être changé en fonction du diamètre du nanofil. En dernier lieu, en prenant avantage de la géométrie cylindrique des nanofils, des mesures de diffusion de porteurs de charge ont été réalisées dans des nanofils de GaN et d'AlN.Using specific conditions, GaN can be epitaxially grown on a large variety of substrates as a nanowire (NW) array. This geometry allows the subsequent growth of wire-like heterostructures likely free of extended defects, which makes them promising for increasing device controllability and performance. First, my PhD work has been devoted to the understanding of self-organized nucleation of GaN NWs on silicon substrates. For this purpose, a deep characterization of the growth mechanism of the AlN buffer deposited prior to NW nucleation has been done, emphasizing an unexpected large reactivity of Al with the substrate. The requirement of the N polarity to nucleate GaN NWs has been evidenced, although the possible existence of NWs hosting a Ga polar core has been observed as well. In these NWs, an inversion domain boundary is present and has been demonstrated to be optically active, having a photoluminescence signature at 3.45 eV. Next, GaN/AlN wire heterostructures have been grown for structural and optical characterization. It has been shown that by changing the wire diameter, different growth mode for the heterostructure could be reached.At last, thanks to the cylindrical geometry of NWs, the measurement of diffusion length for charge carriers in GaN and AlN NWs have been performed