3,558 research outputs found
Molecular beam epitaxy of single crystalline GaN nanowires on a flexible Ti foil
We demonstrate the self-assembled growth of vertically aligned GaN nanowire
ensembles on a flexible Ti foil by plasma-assisted molecular beam epitaxy. The
analysis of single nanowires by transmission electron microscopy reveals that
they are single crystalline. Low-temperature photoluminescence spectroscopy
demonstrates that, in comparison to standard GaN nanowires grown on Si, the
nanowires prepared on the Ti foil exhibit a equivalent crystalline perfection,
a higher density of basal-plane stacking faults, but a reduced density of
inversion domain boundaries. The room-temperature photoluminescence spectrum of
the nanowire ensemble is not influenced or degraded by the bending of the
substrate. The present results pave the way for the fabrication of flexible
optoelectronic devices based on GaN nanowires on metal foils.Comment: 4 pages, 3 figure
Nature of excitons bound to inversion domain boundaries: Origin of the 3.45-eV luminescence lines in spontaneously formed GaN nanowires on Si(111)
We investigate the 3.45-eV luminescence band of spontaneously formed GaN
nanowires on Si(111) by photoluminescence and cathodoluminescence spectroscopy.
This band is found to be particularly prominent for samples synthesized at
comparatively low temperatures. At the same time, these samples exhibit a
peculiar morphology, namely, isolated long nanowires are interspersed within a
dense matrix of short ones. Cathodoluminescence intensity maps reveal the
3.45-eV band to originate primarily from the long nanowires. Transmission
electron microscopy shows that these long nanowires are either Ga polar and are
joined by an inversion domain boundary with their short N-polar neighbors, or
exhibit a Ga-polar core surrounded by a N-polar shell with a tubular inversion
domain boundary at the core/shell interface. For samples grown at high
temperatures, which exhibit a uniform nanowire morphology, the 3.45-eV band is
also found to originate from particular nanowires in the ensemble and thus
presumably from inversion domain boundaries stemming from the coexistence of N-
and Ga-polar nanowires. For several of the investigated samples, the 3.45-eV
band splits into a doublet. We demonstrate that the higher-energy component of
this doublet arises from the recombination of two-dimensional excitons free to
move in the plane of the inversion domain boundary. In contrast, the
lower-energy component of the doublet originates from excitons localized in the
plane of the inversion domain boundary. We propose that this in-plane
localization is due to shallow donors in the vicinity of the inversion domain
boundaries.Comment: 24 pages, 12 figures, 1 tabl
Crystal-phase quantum dots in GaN quantum wires
We study the nature of excitons bound to I1 basal plane stacking faults in
ensembles of ultrathin GaN nanowires by continuous-wave and time-resolved
photoluminescence spectroscopy. These ultrathin nanowires, obtained by the
thermal decomposition of spontaneously formed GaN nanowire ensembles, are
tapered and have tip diameters down to 6 nm. With decreasing nanowire diameter,
we observe a strong blue shift of the transition originating from the radiative
decay of stacking fault-bound excitons. Moreover, the radiative lifetime of
this transition in the ultrathin nanowires is independent of temperature up to
60 K and significantly longer than that of the corresponding transition in
as-grown nanowires. These findings reveal a zero-dimensional character of the
confined exciton state and thus demonstrate that I1 stacking faults in
ultrathin nanowires act as genuine quantum dots
Stacking faults as quantum wells in nanowires: Density of states, oscillator strength and radiative efficiency
We investigate the nature of excitons bound to I1 basal-plane stacking faults
[(I1;X)] in GaN nanowire ensembles by continuous-wave and time-resolved
photoluminescence spectroscopy. Based on the linear increase of the radiative
lifetime of these excitons with temperature, they are demonstrated to exhibit a
two-dimensional density of states, i. e., a basal-plane stacking fault acts as
a quantum well. From the slope of the linear increase, we determine the
oscillator strength of the (I1;X) and show that the value obtained reflects the
presence of large internal electrostatic fields across the stacking fault.
While the recombination of donor-bound and free excitons in the GaN nanowire
ensemble is dominated by nonradiative phenonema already at 10 K, we observe
that the (I1;X) recombines purely radiatively up to 60 K. This finding provides
important insight into the nonradiative recombination processes in GaN
nanowires. First, the radiative lifetime of about 6 ns measured at 60 K sets an
upper limit for the surface recombination velocity of 450 cm/s considering the
nanowires mean diameter of 105 nm. Second, the density of nonradiative centers
responsible for the fast decay of donor-bound and free excitons cannot be
higher than 2x10^16 cm^-3. As a consequence, the nonradiative decay of
donor-bound excitons in these GaN nanowire ensembles has to occur indirectly
via the free exciton state
Improved control over spontaneously formed GaN nanowires in molecular beam epitaxy using a two-step growth process
We investigate the influence of modified growth conditions during the
spontaneous formation of GaN nanowires on Si(111) in plasma-assisted molecular
beam epitaxy. We find that a two-step growth approach, where the substrate
temperature is increased during the nucleation stage, is an efficient method to
gain control over the area coverage, average diameter, and coalescence degree
of GaN nanowire ensembles. Furthermore, we also demonstrate that the growth
conditions employed during the incubation time that precedes nanowire
nucleation do not influence the properties of the final nanowire ensemble.
Therefore, when growing GaN nanowires at elevated temperatures or with low Ga/N
ratios, the total growth time can be reduced significantly by using more
favorable growth conditions for nanowire nucleation during the incubation time
Comparison of the luminous efficiency of Ga- and N-polar InGaN/InGaN quantum wells grown by plasma-assisted molecular beam epitaxy
We investigate the luminescence of Ga- and N-polar
InGaN/InGaN quantum wells (QWs) grown by
plasma-assisted molecular beam epitaxy on freestanding GaN as well as 6H-SiC
substrates. In striking contrast to their Ga-polar counterparts, the N-polar
QWs prepared on freestanding GaN do not exhibit any detectable
photoluminescence. Theoretical simulations of the band profiles combined with
resonant excitation of the QWs allow us to rule out carrier escape and
subsequent surface recombination as the reason for the absence of luminescence.
To explore the hypothesis of a high concentration of nonradiative defects at
the interfaces between wells and barriers, we analyze Ga- and N-polar QWs
prepared on 6H-SiC as a function of the well width. Intense luminescence is
observed for both Ga- and N polar samples. As expected, the luminescence of the
Ga-polar QWs quenches and red-shifts with increasing well width due to the
quantum confined Stark effect. In contrast, both the intensity and the energy
of the luminescence from the N-polar samples are essentially independent of
well width. Transmission electron microscopy reveals that the N-polar QWs
exhibit abrupt interfaces and homogeneous composition, excluding emission from
In-rich clusters as the reason for this anomalous behavior. The microscopic
origin of the luminescence in the N-polar QWs is elucidated using spatially
resolved cathodoluminescence spectroscopy. Regardless of well width, the
luminescence is found to not originate from the N-polar QWs, but from the
semipolar facets of v-pit defects. These results cast serious doubts on the
potential of N-polar QWs grown by plasma-assisted molecular beam epitaxy for
the development of long-wavelength light emitting diodes. What remains to be
seen is whether unconventional growth conditions may enable a significant
reduction in the concentration of nonradiative defects.Comment: 12 pages, 10 figure
Counterintuitive strain distribution in axial (In,Ga)N/GaN nanowires
We study the three-dimensional deformation field induced by an axial (In,Ga)N
segment in a GaN nanowire. Using the finite element method within the framework
of linear elasticity theory, we study the dependence of the strain field on the
ratio of segment length and nanowire radius. Contrary to intuition, the
out-of-plane-component of the elastic strain tensor is found to assume large
negative values for a length-to-radius ratio close to one. We show that this
unexpected effect is a direct consequence of the deformation of the nanowire at
the free sidewalls and the associated large shear strain components. Simulated
reciprocal space maps of a single (In,Ga)N/GaN nanowire demonstrate that
nanofocus x-ray diffraction is a suitable technique to assess this peculiar
strain state experimentally
Ga-polar (In,Ga)N/GaN quantum wells vs. N-polar (In,Ga)N quantum disks in GaN nanowires: Comparative analysis of carrier recombination, diffusion, and radiative efficiency
We investigate the radiative and nonradiative recombination processes in
planar (In,Ga)N/GaN(0001) quantum wells and (In,Ga)N quantum disks embedded in
GaN nanowires using photoluminescence spectroscopy under both
continuous-wave and pulsed excitation. The photoluminescence intensities of
these two samples quench only slightly between 10 and 300 K, which is commonly
taken as evidence for high internal quantum efficiencies. However, a
side-by-side comparison shows that the absolute intensity of the Ga-polar
quantum wells is two orders of magnitude higher than that of the N-polar
quantum disks. A similar difference is observed for the initial decay time of
photoluminescence transients obtained by time-resolved measurements, indicating
the presence of a highly efficient nonradiative decay channel for the quantum
disks. In apparent contradiction to this conjecture, the decay of both samples
is observed to slow down dramatically after the initial rapid decay.
Independent of temperature, the transients approach a power law for longer
decay times, reflecting that recombination occurs between individual electrons
and holes with varying spatial separation. Employing a coupled system of
stochastic integro-differential equations taking into account both radiative
and nonradiative Shockley-Read-Hall recombination of spatially separate
electrons and holes as well as their diffusion, we obtain simulated transients
matching the experimentally obtained ones. The results reveal that even
dominant nonradiative recombination conserves the power law decay for
(In,Ga)N/GaN{0001} quantum wells and disks
Investigating the origin of the nonradiative decay of bound excitons in GaN nanowires
We investigate the origin of the fast recombination dynamics of bound and
free excitons in GaN nanowire ensembles by temperature-dependent
photoluminescence spectroscopy using both continuous-wave and pulsed
excitation. The exciton recombination in the present GaN nanowires is dominated
by a nonradiative channel between 10 and 300 K. Furthermore, bound and free
excitons in GaN NWs are strongly coupled even at low temperatures resulting in
a common lifetime of these states. By solving the rate equations for a coupled
two-level system, we show that one cannot, in practice, distinguish whether the
nonradiative decay occurs directly via the bound or indirectly via the free
state. The nanowire surface and coalescence-induced dislocations appear to be
the most obvious candidates for nonradiative defects, and we thus compare the
exciton decay times measured for a variety of GaN nanowire ensembles with
different surface-to-volume ratio and coalescence degrees. The data are found
to exhibit no correlation with either of these parameters, i. e., the
dominating nonradiative channel in the GaN nanowires under investigation is
neither related to the nanowire surface, nor to coalescence-induced defects for
the present samples. Hence, we conclude that nonradiative point defects are the
origin of the fast recombination dynamics of excitons in GaN nanowires.Comment: 10 pages, 5 figure
An Analysis of Shareholder Agreements.
Shareholder agreements govern the relations among shareholders in privately held firms, such as joint ventures and venture capital-backed companies. We provide an economic explanation for key clauses in such agreements—namely, put and call options, tag-along and drag-along rights, demand and piggy-back rights, and catch-up clauses. In a dynamic moral hazard setting, we show that these clauses can ensure that the contract parties make efficient ex ante investments in the firm. They do so by constraining renegotiation. In the absence of the clauses, ex ante investment would be distorted by unconstrained renegotiation aimed at (i) precluding value-destroying ex post transfers, (ii) inducing value-increasing ex post investments, or (iii) precluding hold-out on value-increasing sales to a trade buyer or the IPO market.Corporate Governance; Restructuring; Investment Decision;
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