52 research outputs found
Individual electron and hole localization in submonolayer InN quantum sheets embedded in GaN
We investigate sub-monolayer InN quantum sheets embedded in GaN(0001) by
temperature-dependent photoluminescence spectroscopy under both continuous-wave
and pulsed excitation. Both the peak energy and the linewidth of the emission
band associated with the quantum sheets exhibit an anomalous dependence on
temperature indicative of carrier localization. Photoluminescence transients
reveal a power law decay at low temperatures reflecting that the recombining
electrons and holes occupy spatially separate, individual potential minima
reminiscent of conventional (In,Ga)N(0001) quantum wells exhibiting the
characteristic disorder of a random alloy. At elevated temperatures, carrier
delocalization sets in and is accompanied by a thermally activated quenching of
the emission. We ascribe the strong nonradiative recombination to extended
states in the GaN barriers and confirm our assumption by a simple rate-equation
model.Comment: 10 pages, 3 figure
Exciton dynamics in GaAs/(Al,Ga)As core-shell nanowires with shell quantum dots
We study the dynamics of excitons in GaAs/(Al,Ga)As core-shell nanowires by
continuous-wave and time-resolved photoluminescence and photoluminescence
excitation spectroscopy. Strong Al segregation in the shell of the nanowires
leads to the formation of Ga-rich inclusions acting as quantum dots. At 10 K,
intense light emission associated with these shell quantum dots is observed.
The average radiative lifetime of excitons confined in the shell quantum dots
is 1.7 ns. We show that excitons may tunnel toward adjacent shell quantum dots
and nonradiative point defects. We investigate the changes in the dynamics of
charge carriers in the shell with increasing temperature, with particular
emphasis on the transfer of carriers from the shell to the core of the
nanowires. We finally discuss the implications of carrier localization in the
(Al,Ga)As shell for fundamental studies and optoelectronic applications based
on core-shell III-As nanowires
Quenching of the luminescence intensity of GaN nanowires under electron beam exposure: Impact of C adsorption on the exciton lifetime
Electron irradiation of GaN nanowires in a scanning electron microscope
strongly reduces their luminous efficiency as shown by cathodoluminescence
imaging and spectroscopy. We demonstrate that this luminescence quenching
originates from a combination of charge trapping at already existing surface
states and the formation of new surface states induced by the adsorption of C
on the nanowire sidewalls. The interplay of these effects leads to a complex
temporal evolution of the quenching, which strongly depends on the incident
electron dose per area. Time-resolved photoluminescence measurements on
electron-irradiated samples reveal that the carbonaceous adlayer affects both
the nonradiative and the radiative recombination dynamics.Comment: This is an author-created, un-copyedited version of an article
accepted for publication/published in Nanotechnology. IOP Publishing Ltd is
not responsible for any errors or omissions in this version of the manuscript
or any version derived from it. The Version of Record is available online at
http://dx.doi.org/10.1088/0957-4484/27/45/45570
Coupling of exciton states as the origin of their biexponential decay dynamics in GaN nanowires
Using time-resolved photoluminescence spectroscopy, we explore the transient
behavior of bound and free excitons in GaN nanowire ensembles. We investigate
samples with distinct diameter distributions and show that the pronounced
biexponential decay of the donor-bound exciton observed in each case is not
caused by the nanowire surface. At long times, the individual exciton
transitions decay with a common lifetime, which suggests a strong coupling
between the corresponding exciton states. A system of non-linear rate-equations
taking into account this coupling directly reproduces the experimentally
observed biexponential decay.Comment: 5 pages, 4 figure
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
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
Coexistence of quantum-confined Stark effect and localized states in an (In,Ga)N/GaN nanowire heterostructure
We analyze the emission of single GaN nanowires with (In,Ga)N insertions
using both micro-photoluminescence and cathodoluminescence spectroscopy. The
emission spectra are dominated by a green luminescence band that is strongly
blueshifted with increasing excitation density. In conjunction with
finite-element simulations of the structure to obtain the piezoelectric
polarization, these results demonstrate that our (In,Ga)N/GaN nanowire
heterostructures are subject to the quantum-confined Stark effect. Additional
sharp peaks in the spectra, which do not shift with excitation density, are
attributed to emission from localized states created by compositional
fluctuations in the ternary (In,Ga)N alloy.Comment: 6 pages, 8 figures; accepted for publication in Phys. Rev.
Radial Stark effect in (In,Ga)N nanowires
We study the luminescence of unintentionally doped and Si-doped
InGaN nanowires with a low In content (x<0.2) grown by molecular
beam epitaxy on Si substrates. The emission band observed at 300 K from the
unintentionally doped samples is centered at much lower energies (800 meV) than
expected from the In content measured by x-ray diffractometry and energy
dispersive x-ray spectroscopy. This discrepancy arises from the pinning of the
Fermi level at the sidewalls of the nanowires, which gives rise to strong
radial built-in electric fields. The combination of the built-in electric
fields with the compositional fluctuations inherent to (In,Ga)N alloys induces
a competition between spatially direct and indirect recombination channels. At
elevated temperatures, electrons at the core of the nanowire recombine with
holes close to the surface, and the emission from unintentionally doped
nanowires exhibits a Stark shift of several hundreds of meV. The competition
between spatially direct and indirect transitions is analyzed as a function of
temperature for samples with various Si concentrations. We propose that the
radial Stark effect is responsible for the broadband absorption of (In,Ga)N
nanowires across the entire visible range, which makes these nanostructures a
promising platform for solar energy applications.Comment: This document is the unedited Author's version of a Submitted Work
that was subsequently accepted for publication in Nano Letters (2016),
copyright (C) American Chemical Society after peer review. To access the
final edited and published work see
http://dx.doi.org/10.1021/acs.nanolett.5b0374
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
Luminescent N-polar (In,Ga)N/GaN quantum wells grown by plasma-assisted molecular beam epitaxy at high temperature
N-polar (In,Ga)N/GaN quantum wells prepared on freestanding GaN substrates by
plasma-assisted molecular beam epitaxy at conventional growth temperatures of
about 650 {\deg}C do not exhibit any detectable luminescence even at 10 K. In
the present work, we investigate (In,Ga)N/GaN quantum wells grown on Ga- and
N-polar GaN substrates at a constant temperature of 730 {\deg}C. This
exceptionally high temperature results in a vanishing In incorporation for the
Ga-polar sample. In contrast, quantum wells with an In content of 20% and
abrupt interfaces are formed on N-polar GaN. Moreover, these quantum wells
exhibit a spatially homogeneous green luminescence band up to room temperature,
but the intensity of this band is observed to strongly quench with temperature.
Temperature-dependent photoluminescence transients show that this thermal
quenching is related to a high density of nonradiative Shockley-Read-Hall
centers with large capture coefficients for electrons and holes.Comment: 10 pages, 2 figures, 1 tabl
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