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Core-shell nanowire heterostructures form the basis for many innovative devices. When compound nanowire shells are grown by directional deposition techniques, the azimuthal position of the sources for the different constituents in the growth reactor, substrate rotation, and nanowire self-shadowing inevitably lead to sequential deposition. Here, we uncover for In$_{0.15}$Ga$_{0.85}$As/GaAs shell quantum wells grown by molecular beam epitaxy a drastic impact of this sequentiality on the luminescence efficiency. The photoluminescence intensity of shell quantum wells grown with a flux sequence corresponding to migration enhanced epitaxy, i. e. when As and the group-III metals essentially do not impinge at the same time, is more than two orders of magnitude higher than for shell quantum wells prepared with substantially overlapping fluxes. Transmission electron microscopy does not reveal any extended defects explaining this difference. Our analysis of photoluminescence transients shows that co-deposition has two detrimental microscopic effects. First, a higher density of electrically active point defects leads to internal electric fields reducing the electron-hole wave function overlap. Second, more point defects form that act as nonradiative recombination centers. Our study demonstrates that the source arrangement of the growth reactor, which is of mere technical relevance for planar structures, can have drastic consequences for the materials properties of nanowire shells. We expect that this finding holds also for other alloy nanowire shells.

The realization of semiconductor structures with stable excitons at room temperature is crucial for the development of excitonics and polaritonics. Quantum confinement has commonly been employed for enhancing excitonic effects in semiconductor heterostructures. Dielectric confinement, which gives rises to much stronger enhancement, has proven to be more difficult to achieve because of the rapid nonradiative surface/interface recombination in hybrid dielectric-semiconductor structures. Here, we demonstrate intense excitonic emission from bare GaN nanowires with diameters down to 6 nm. The large dielectric mismatch between the nanowires and vacuum greatly enhances the Coulomb interaction, with the thinnest nanowires showing the strongest dielectric confinement and the highest radiative efficiency at room temperature. In situ monitoring of the fabrication of these structures allows one to accurately control the degree of dielectric enhancement. These ultrathin nanowires may constitute the basis for the fabrication of advanced low-dimensional structures with an unprecedented degree of confinement.

We study the luminescence of unintentionally doped and Si-doped In$_x$Ga$_{1-x}$N nanowires with a low In content (x, 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.5b03748

The flexibility and quasi-one-dimensional nature of nanowires offer wide-ranging possibilities for novel heterostructure design and strain engineering. In this work, we realize arrays of extremely and controllably bent nanowires comprising lattice-mismatched and highly asymmetric core-shell heterostructures. Strain sharing across the nanowire heterostructures is sufficient to bend vertical nanowires over backward to contact either neighboring nanowires or the substrate itself, presenting new possibilities for designing nanowire networks and interconnects. Photoluminescence spectroscopy on bent-nanowire heterostructures reveals that spatially varying strain fields induce charge carrier drift toward the tensile-strained outside of the nanowires, and that the polarization response of absorbed and emitted light is controlled by the bending direction. This unconventional strain field is employed for light emission by placing an active region of quantum dots at the outer side of a bent nanowire to exploit the carrier drift and tensile strain. These results demonstrate how bending in nanoheterostructures opens up new degrees of freedom for strain and device engineering.

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.

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/10.1088/0957-4484/27/45/455706

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., 10 pages, 3 figures

We study the nature of excitons bound to ${I}_{1}$ 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 blueshift 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 ${I}_{1}$ stacking faults in ultrathin nanowires act as genuine quantum dots.

A combination of far-infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline ordered GeTe-Sb2Te3 alloys (GST) epitaxially grown on Si(111). The infrared active GST mode is not observed in the Raman spectra and vice versa, indication of the fact that inversion symmetry is preserved in the metastable cubic phase in accordance with the F"Equation missing"m3 space group. For the trigonal phase, instead, a partial symmetry break due to Ge/Sb mixed anion layers is observed. By studying the crystallization process upon annealing with both the techniques, we identify temperature regions corresponding to the occurrence of different phases as well as the transition from one phase to the next. Activation energies of 0.43 eV and 0.08 eV for the electron conduction are obtained for both cubic and trigonal phases, respectively. In addition a metal-insulator transition is clearly identified to occur at the onset of the transition between the disordered and the ordered cubic phase.

Van der Waals (vdW) epitaxy is an attractive method for the fabrication of vdW heterostructures. Here Sb2Te3 films grown on three different kind of graphene substrates (monolayer epitaxial graphene, quasi freestanding bilayer graphene and the SiC (6√3 × 6√3)R30° buffer layer) are used to study the vdW epitaxy between two 2-dimensionally (2D) bonded materials. It is shown that the Sb2Te3 /graphene interface is stable and that coincidence lattices are formed between the epilayers and substrate that depend on the size of the surface unit cell. This demonstrates that there is a significant, although relatively weak, interfacial interaction between the two materials. Lattice matching is thus relevant for vdW epitaxy with two 2D bonded materials and a fundamental design parameter for vdW heterostructures.

We report on coherent control of the two-photon transition between the ground state and the biexciton state on a single quantum dot level. The population of the biexciton is monitored by the cascaded spontaneous emission of the biexciton complex via the exciton states. The signal shows interference pattern at the double laser frequency and its contrast decays non-exponentially at the time scale of 10 ps. The contrast decrease is attributed to a dynamical inhomogeneous broadening of the energy levels due to long-term fluctuations in the dot environment.

The trion state is used to monitor spin processes in charged CdSe/ZnSe quantum dots. The single-dot photoluminescence from the singlet groundstate is identified by its magnetic-field pattern. The charged bi-exciton is used to uncover the fine structure of the triplet state. Clear signatures for isotropic as well as anisotropic electron-hole exchange are observed. Photoluminescence excitation measurements enable to determine the energy positions of the excited trion states. Time-resolved measurements provide a single hole spin relaxation time longer than 10 ns at low temperature. The trion emission exhibits negative circular polarization. We present single-dot polarization data that enable to identify the states responsible for this effect.

We have studied the energy spectrum of negatively charged single CdSe quantum dots using two-beam photoluminescence excitation spectroscopy. We observe bleaching of the linear trion spectrum and appearance of induced absorption due to excitation of charged biexciton states. The data embody clear signatures for Pauli blocking, many-particle Coulomb interactions as well as electron–hole exchange fine structure.

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. Hence we conclude that nonradiative point defects are the origin of the fast recombination dynamics of excitons in GaN nanowires.

GaN nanowires form spontaneously on a wide variety of substrates without suffering from extended defects. However, their quasi-one-dimensional nature causes these structures to have an extended free surface, resulting in a surface-to-volume ratio orders of magnitude larger than that of a planar layer. Additionally, the high nucleation density of spontaneously formed GaN nanowire ensembles results in an unintentional, but inevitable coalescence between individual nanowires. In this work, we investigate the impact of both the surface and the coalescence of nanowires on the recombination dynamics of excitons in GaN nanowire ensembles. Using simple models to simulate the change in recombination dynamics of bound excitons in GaN NWs with varying diameter and coalescence degree, we show that the comparatively short decay times at low temperatures are not generally caused by either of these mechanisms. Furthermore, we demonstrate that the biexponential decay for the donor-bound exciton is also not related to a coexistence of nonradiative and radiative recombination channels, but originates from a coupling of the donor- and acceptor-bound exciton states in the GaN NWs.

We study the anisotropic in-plane strain in M -plane GaN films by photoreflectance, photoluminescence, Raman, and time-resolved pump-and-probe spectroscopy. We find that a highly strained film partially relaxes, if it is pumped by an intense optical pulse. The strain relaxation can be observed by a shift of the E2-Raman line to lower energies and by a shift of the fundamental interband transition energies in the photoreflectance spectra. The photoluminescence intensity of the exposed areas is significantly reduced as compared to the one for areas, which have not been exposed to the intense optical pulse. This suggests that the strain relaxation is connected to the introduction of defects, which can act as non-radiative recombination centers. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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., 5 pages, 4 figures

We investigate a strongly anisotropically strained $M$-plane GaN film by photoluminescence spectroscopy. The spectra are dominated by the lowest-energy free-exciton transition, but the second lowest one is also detectable even at low temperatures. Rotating the linear polarization of the excitation results in a systematic change of the intensity ratio between these two transitions. We analyze this observation with the help of $6\ifmmode\times\else\texttimes\fi{}6$ $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ calculations and associated rate equation models. This analysis shows that the pronounced excitation polarization anisotropy for this $M$-plane GaN film originates from a competition between hole relaxation and exciton recombination.

The emission from the charged biexciton is used to monitor the energy structure of the trion triplet state in a negatively charged CdSe/ZnSe quantum dot. The isotropic part of the electron–hole exchange interaction regroups the otherwise sixfold degenerated state in three Kramers doublets. The energy separation between the radiative pairs with total spin projection Fz=±3/2 and ±1/2 is 1.6 meV. The anisotropic part mixes states with ΔFz=±2 resulting in a partly linearly polarized transition dipole. Application of magnetic field lifts the degeneracy of the Kramers doublets. The g factors also manifest the anisotropic state mixing.

The incorporation of Au during vapor−liquid−solid nanowire growth might inherently limit the performance of nanowire-based devices. Here, we assess the material quality of Au-assisted and Au-free grown GaAs/(Al,Ga)As core−shell nanowires using photoluminescence spectroscopy. We show that at room temperature, the internal quantum efficiency is systematically much lower for the Au-assisted nanowires than for the Au-free ones. In contrast, the optoelectronic material quality of the latter is comparable to that of state-of-the-art planar double heterostructures.

International audience; We observe a significant increase in the photoluminescence intensity of GaN nanowires under continuous ultraviolet irradiation on a time scale of minutes. Experiments carried out under different ambient conditions demonstrate that this increase is caused by the photoinduced desorption of oxygen from the nanowire side-walls. The slow, highly nonexponential temporal evolution of the photoluminescence signal is modeled by a random-walk approach. The model reveals that already desorbed oxygen molecules are likely to be readsorbed at adjacent nanowires. Time-resolved photoluminescence measurements are performed to unravel the correlation between the oxygen desorption and the increase in the photoluminescence intensity. We find that the oxygen desorption unpins the Fermi level, which in turn leads to an increase in quantum efficiency by enhancing the radiative decay of excitons.

We report on two-photon coherent control of the biexciton state in single Stranski-Krastanov CdSe quantum dots. Clear interference patterns are observed at twice the optical frequency. The decay of the interference contrast is nonexponential and caused by a dynamical inhomogeneous broadening of the energy levels due to long-term fluctuations in the dot environment.

Using the photoluminescence of the trion feature as a monitor, we study experimentally the spin relaxation of the hole in self-assembled CdSe/ZnSe quantum dots. We observe two kinds of depolarization. The first one is due to an imperfect spin imprint of the circular photon polarization. The second transient depolarization is caused by the hole spin-flip in the Kramers degenerate trion doublet. At low temperature, the spin relaxation exhibits a slow component with a time constant longer than 10 ns. A considerable speedup takes place at higher temperature, reaching the sub nanosecond range at about 70 K. The activation energy is consistent with LO phonons.

Basal-plane stacking faults are an important class of optically active structural defects in wurtzite semiconductors. The local deviation from the 2H stacking of the wurtzite matrix to a 3C zinc-blende stacking induces a bound state in the gap of the host crystal, resulting in the localization of excitons. Due to the two-dimensional nature of these planar defects, stacking faults act as quantum wells, giving rise to radiative transitions of excitons with characteristic energies. Luminescence spectroscopy is thus capable of detecting even a single stacking fault in an otherwise perfect wurtzite crystal. This review draws a comprehensive picture of the luminescence properties related to stacking faults in GaN. The emission energies associated with different types of stacking faults as well as factors that can shift these energies are discussed. In this context, the importance of the quantum-confined Stark effect in these zinc-blende/wurtzite heterostructures, which results from the spontaneous polarization of wurtzite GaN, is underlined. This discussion is extended to zinc-blende segments in a wurtzite matrix. Furthermore, other factors affecting the emission energy and linewidth of stacking fault-related peaks as well as results obtained at room temperature are addressed. The considerations presented in this article should be transferable also to other wurtzite semiconductors., 16 pages, 14 figures

Single-photon interference is observed on the ultranarrow long-term stable exciton resonance of an individual semiconductor quantum dot. This interference is related to the fine-structure splitting and allows direct conclusions about the coherence properties of the exciton. When selectively addressing a particular dot by quasiresonant phonon-assisted excitation, despite a rapid orientation relaxation on a 1-ps time scale, coherence is partly maintained. No significant further decoherence occurs when the ground state is reached until the exciton recombines radiatively $(\ensuremath{\approx}300\mathrm{ps})$.

Single, free-standing GaN nanowires grown by plasma-assisted molecular-beam epitaxy have been investigated with low temperature micro-photoluminescence. The quantitative analysis of the luminescence spectra of around 100 nanowires revealed that each nanowire exhibits its own individual spectrum. A significant fraction of nanowires exclusively emits at energies corresponding to either surface-donor-bound or free excitons, demonstrating that optical properties of individual nanowires are determined by a few impurity atoms alone. The number of impurities per nanowire and their location within the nanowires varies according to Poissonian statistics.