Correlated Nanoscale Analysis of the Emission from Wurtzite versus Zincblende (In,Ga)As/GaAs Nanowire Core-Shell Quantum Wells.

While the properties of wurtzite GaAs have been extensively studied during the past decade, little is known about the influence of the crystal polytype on ternary (In,Ga)As quantum well structures. We address this question with a unique combination of correlated, spatially resolved measurement techniques on core-shell nanowires that contain extended segments of both the zincblende and wurtzite polytypes. Cathodoluminescence hyperspectral imaging reveals a blue-shift of the quantum well emission energy by 75 ± 15 meV in the wurtzite polytype segment. Nanoprobe X-ray diffraction and atom probe tomography enable k·p calculations for the specific sample geometry to reveal two comparable contributions to this shift. First, there is a 30% drop in In mole fraction going from the zincblende to the wurtzite segment. Second, the quantum well is under compressive strain, which has a much stronger impact on the hole ground state in the wurtzite than in the zincblende segment. Our results highlight the role of the crystal structure in tuning the emission of (In,Ga)As quantum wells and pave the way to exploit the possibilities of three-dimensional band gap engineering in core-shell nanowire heterostructures. At the same time, we have demonstrated an advanced characterization toolkit for the investigation of semiconductor nanostructures.

Excitonic Aharonov-Bohm Oscillations in Core-Shell Nanowires.

Phase coherence in nanostructures is at the heart of a wide range of quantum effects such as Josephson oscillations between exciton-polariton condensates in microcavities, conductance quantization in 1D ballistic transport, or the optical (excitonic) Aharonov-Bohm effect in semiconductor quantum rings. These effects only occur in structures of the highest perfection. The 2D semiconductor heterostructures required for the observation of Aharonov-Bohm oscillations have proved to be particularly demanding, since interface roughness or alloy fluctuations cause a loss of the spatial phase coherence of excitons, and ultimately induce exciton localization. Experimental work in this field has so far relied on either self-assembled ring structures with very limited control of shape and dimension or on lithographically defined nanorings that suffer from the detrimental effects of free surfaces. Here, it is demonstrated that nanowires are an ideal platform for studies of the Aharonov-Bohm effect of neutral and charged excitons, as they facilitate the controlled fabrication of nearly ideal quantum rings by combining all-binary radial heterostructures with axial crystal-phase quantum structures. Thanks to the atomically flat interfaces and the absence of alloy disorder, excitonic phase coherence is preserved even in rings with circumferences as large as 200 nm.

Impact of Shell Growth on Recombination Dynamics and Exciton-Phonon Interaction in CdSe-CdS Core-Shell Nanoplatelets.

We investigate the impact of shell growth on the carrier dynamics and exciton-phonon coupling in CdSe-CdS core-shell nanoplatelets with varying shell thickness. We observe that the recombination dynamics can be prolonged by more than one order of magnitude, and analyze the results in a global rate model as well as with simulations including strain and excitonic effects. We reveal that type I band alignment in the hetero platelets is maintained at least up to three monolayers of CdS, resulting in approximately constant radiative rates. Hence, observed changes of decay dynamics are not the result of an increasingly different electron and hole exciton wave function delocalization as often assumed, but an increasingly better passivation of nonradiative surface defects by the shell. Based on a global analysis of time-resolved and time-integrated data, we recover and model the temperature dependent quantum yield of these nanostructures and show that CdS shell growth leads to a strong enhancement of the photoluminescence quantum yield. Our results explain, for example, the very high lasing gain observed in CdSe-CdS nanoplatelets due to the type I band alignment that also makes them interesting as solar energy concentrators. Further, we reveal that the exciton-LO-phonon coupling is strongly tunable by the CdS shell thickness, enabling emission line width and coherence length control.

Crystal-Phase Quantum Wires: One-Dimensional Heterostructures with Atomically Flat Interfaces.

In semiconductor quantum-wire heterostructures, interface roughness leads to exciton localization and to a radiative decay rate much smaller than that expected for structures with flat interfaces. Here, we uncover the electronic and optical properties of the one-dimensional extended defects that form at the intersection between stacking faults and inversion domain boundaries in GaN nanowires. We show that they act as crystal-phase quantum wires, a novel one-dimensional quantum system with atomically flat interfaces. These quantum wires efficiently capture excitons whose radiative decay gives rise to an optical doublet at 3.36 eV at 4.2 K. The binding energy of excitons confined in crystal-phase quantum wires is measured to be more than twice larger than that of the bulk. As a result of their unprecedented interface quality, these crystal-phase quantum wires constitute a model system for the study of one-dimensional excitons.

Influence of strain relaxation in axial [Formula: see text] nanowire heterostructures on their electronic properties.

We present a systematic theoretical study of the influence of elastic strain relaxation on the built-in electrostatic potentials and the electronic properties of axial [Formula: see text] nanowire (NW) heterostructures. Our simulations reveal that for a sufficiently large ratio between the thickness of the [Formula: see text] disk and the diameter of the NW, the elastic relaxation leads to a significant reduction of the built-in electrostatic potential in comparison to a planar system of similar layer thickness and In content. In this case, the ground state transition energies approach constant values with increasing thickness of the disk and only depend on the In content, a behavior usually associated to that of a quantum well free of built-in electrostatic potentials. We show that the structures under consideration are by no means field-free, and the built-in potentials continue to play an important role even for ultrathin NWs. In particular, strain and the resulting polarization potentials induce complex confinement features of electrons and holes, which depend on the In content, shape, and dimensions of the heterostructure.

Piezoelectric potential in axial (In,Ga)N/GaN nanowire heterostructures.

We derive analytic expressions for the built-in electrostatic potential arising from piezo- and pyroelectricity in a cylindrical axial In(x)Ga(1-x)N/GaN nanowire (NW) heterostructure. Our simulations show that, for sufficiently thin NWs, a significant reduction of the built-in potential is reached in comparison to the planar heterostructure of the same In content, thickness, and orientation. This specific feature of axial NW heterostructures makes the aspect ratio of the embedded In(x)Ga(1-x)N/GaN disks an important additional degree of freedom to control the recombination energies. We furthermore show that the magnitude of the polarization potential decreases again above a certain value of the aspect ratio and that the extrema of the potential move from the central axis of the NW towards the side facets when the thickness of the disk is increased.

Impact of Random Dopant Fluctuations on the Electronic Properties of In(x)Ga(1-x)N/GaN Axial Nanowire Heterostructures.

We study the electronic properties of axial In(x)Ga(1-x)N/GaN nanowire heterostructures with randomly placed ionized donors. Our simulations are based on an eight-band k·p model and indicate large variations of both the ground state transition energy and the spatial distribution of the electron and hole charge density. We show that these variations are intrinsic to nanostructures containing ionized donors and that the presence of donors has important consequences for all nanowire-based light-emitting devices including single-photon emitters required for quantum computing and quantum cryptography.

Coaxial multishell (In,Ga)As/GaAs nanowires for near-infrared emission on Si substrates.

Efficient infrared light emitters integrated on the mature Si technology platform could lead to on-chip optical interconnects as deemed necessary for future generations of ultrafast processors as well as to nanoanalytical functionality. Toward this goal, we demonstrate the use of GaAs-based nanowires as building blocks for the emission of light with micrometer wavelength that are monolithically integrated on Si substrates. Free-standing (In,Ga)As/GaAs coaxial multishell nanowires were grown catalyst-free on Si(111) by molecular beam epitaxy. The emission properties of single radial quantum wells were studied by cathodoluminescence spectroscopy and correlated with the growth kinetics. Controlling the surface diffusivity of In adatoms along the NW side-walls, we improved the spatial homogeneity of the chemical composition along the nanowire axis and thus obtained a narrow emission spectrum. Finally, we fabricated a light-emitting diode consisting of approximately 10(5) nanowires contacted in parallel through the Si substrate. Room-temperature electroluminescence at 985 nm was demonstrated, proving the great potential of this technology.

Luminous efficiency of axial In(x)Ga(1-x)N/GaN nanowire heterostructures: interplay of polarization and surface potentials.

Using continuum elasticity theory and an eight-band k·p formalism, we study the electronic properties of GaN nanowires with axial InxGa1-xN insertions. The three-dimensional strain distribution in these insertions and the resulting distribution of the polarization fields are fully taken into account. In addition, we consider the presence of a surface potential originating from Fermi level pinning at the sidewall surfaces of the nanowires. Our simulations reveal an in-plane spatial separation of electrons and holes in the case of weak piezoelectric potentials, which correspond to an In content and layer thickness required for emission in the blue and violet spectral range. These results explain the quenching of the photoluminescence intensity experimentally observed for short emission wavelengths. We devise and discuss strategies to overcome this problem.