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The influence of Mg doping on the nucleation of self-induced GaN nanowires
GaN nanowires were grown without any catalyst by plasma-assisted molecular beam epitaxy. Under supply of Mg, nanowire nucleation is faster, the areal density of wires increases to a higher value, and nanowire coalescence is more pronounced than without Mg. During nanowire nucleation the Ga desorption was monitored insitu by line-of-sight quadrupolemass spectrometry for various substrate temperatures. Nucleation energies of 4.0±0.3 eV and 3.2±0.3 eV without and with Mg supply were deduced, respectively. This effect has to be taken into account for the fabrication of nanowire devices and could be employed to tune the NW areal density.
Diffraction at GaAs/Fe3Si core/shell nanowires: The formation of nanofacets
GaAs/Fe3Si core/shell nanowire structures were fabricated by molecular-beam epitaxy on oxidized Si(111) substrates and investigated by synchrotron x-ray diffraction. The surfaces of the Fe3Si shells exhibit nanofacets. These facets consist of well pronounced Fe3Si{111} planes. Density functional theory reveals that the Si-terminated Fe3Si{111} surface has the lowest energy in agreement with the experimental findings. We can analyze the x-ray diffuse scattering and diffraction of the ensemble of nanowires avoiding the signal of the substrate and poly-crystalline films located between the wires. Fe3Si nanofacets cause streaks in the x-ray reciprocal space map rotated by an azimuthal angle of 30° compared with those of bare GaAs nanowires. In the corresponding TEM micrograph the facets are revealed only if the incident electron beam is oriented along [1 1 ̄ 0] in accordance with the x-ray results. Additional maxima in the x-ray scans indicate the onset of chemical reactions between Fe3Si shells and GaAs cores occurring at increased growth temperatures.
Surface acoustic wave modulation of single photon emission from GaN/InGaN nanowire quantum dots
On-chip quantum information processing requires controllable quantum light sources that can be operated on-demand at high-speeds and with the possibility of in-situ control of the photon emission wavelength and its optical polarization properties. Here, we report on the dynamic control of the optical emission from core-shell GaN/InGaN nanowire (NW) heterostructures using radio frequency surface acoustic waves (SAWs). The SAWs are excited on the surface of a piezoelectric lithium niobate crystal equipped with a SAW delay line onto which the NWs were mechanically transferred. Luminescent quantum dot (QD)-like exciton localization centers induced by compositional fluctuations within the InGaN nanoshell were identified using stroboscopic micro-photoluminescence (micro-PL) spectroscopy. They exhibit narrow and almost fully linearly polarized emission lines in the micro-PL spectra and a pronounced anti-bunching signature of single photon emission in the photon correlation experiments. When the nanowire is perturbed by the propagating SAW, the embedded QD is periodically strained and its excitonic transitions are modulated by the acousto-mechanical coupling, giving rise to a spectral fine-tuning within a ~1.5 meV bandwidth at the acoustic frequency of ~330 MHz. This outcome can be further combined with spectral detection filtering for temporal control of the emitted photons. The effect of the SAW piezoelectric field on the QD charge population and on the optical polarization degree is also observed. The advantage of the acousto-optoelectric over other control schemes is that it allows in-situ manipulation of the optical emission properties over a wide frequency range (up to GHz frequencies).
On the electronic properties of a single dislocation
A detailed knowledge of the electronic properties of individual dislocations is necessary for next generation nanodevices. Dislocations are fundamental crystal defects controlling the growth of different nanostructures (nanowires) or appear during device processing. We present a method to record electric properties of single dislocations in thin silicon layers. Results of measurements on single screw dislocations are shown for the first time. Assuming a cross-section area of the dislocation core of about 1 nm2, the current density through a single dislocation is J = 3.8 × 1012 A/cm2 corresponding to a resistivity of ρ ≅ 1 × 10-8 Ω cm. This is about eight orders of magnitude lower than the surrounding silicon matrix. The reason of the supermetallic behavior is the high strain in the cores of the dissociated dislocations modifying the local band structure resulting in high conductive carrier channels along defect cores.
Optical properties of single semiconductor nanowires and nanowire ensembles – probing surface physics by photoluminescence spectroscopy
This thesis presents a detailed investigation of the optical properties of semiconductor nanowires (NWs) in general and single GaN NWs and GaN NW ensembles in particular by photoluminescence (PL) spectroscopy. NWs are often considered as potential building blocks for future nanometer-scaled devices. This vision is based on several attractive features that are generally ascribed to NWs. In the first part of the thesis, some of these features are examined using semiconductor NWs of different materials. On the basis of the temperature-dependent PL of Au- and self-assisted GaAs/(Al,Ga)As core-shell NWs, the influence of foreign catalyst particles on the optical properties of NWs is investigated. The effect of the substrate choice is studied by comparing the PL of ZnO NWs grown on Si, Sapphire, and ZnO substrates. The major part of this thesis discusses the optical properties of GaN NWs. The investigation of the PL of single GaN NWs and GaN NW ensembles reveals the significance of their large surface-to-volume ratio and that each NW exhibits its own individual recombination behavior. An unexpected broadening of the donor-bound exciton transition is explained by the abundant presence of surface donors in NWs. The existence and statistical relevance of these surface donors is confirmed by PL experiments of single GaN NWs which are either dispersed or free-standing. Furthermore, the influence of electric fields on the optical properties of GaN NWs is investigated and the coupling of light with GaN NWs is studied by reflectance and Raman measurements. The central results of this thesis motivate the introduction of a model that explains the typically observed nonexponential recombination dynamics in NW ensembles. It is based on a distribution of recombination rates. Preliminary simulations using this model describe the nonexponential decay of GaN NW ensembles satisfactorily and allow for an estimation of their internal quantum efficiency.
Molecular beam epitaxy of GaAs nanowires and their suitability for optoelectronic applications – comparing Au- and self-assisted growth methods
In this work the synthesis of GaAs nanowires by molecular beam epitaxy (MBE) using the vapour-liquid-solid (VLS) mechanism is investigated. A comparison between Au- and self-assisted VLS growth is at the centre of this thesis. While the Au-assisted method is established as a versatile tool for nanowire growth, the recently developed self-assisted variation results from the exchange of Au by Ga droplets and thus eliminates any possibility of Au incorporation. By both methods, we achieve nanowires with epitaxial alignment to the Si(111) substrates. Caused by differences during nanowire nucleation, a parasitic planar layer grows between the nanowires by the Au-assisted method, but can be avoided by the self-assisted method. Au-assisted nanowires grow predominantly in the metastable wurtzite crystal structure, while their self-assisted counterparts have the zincblende structure. All GaAs nanowires are fully relaxed and the strain arising from the lattice mismatch between GaAs and Si of 4.1\% is accommodated by misfit dislocations at the interface. Self-assisted GaAs nanowires are generally found to have vertical and non-polar side facets, while tilted and polar nanofacets were described for Au-assisted GaAs nanowires. We employ VLS nucleation theory to understand the effect of the droplet material on the lateral facets. Optoelectronic applications require long minority carrier lifetimes at room temperature. We fabricate GaAs/(Al,Ga)As core-shell nanowires and analyse them by transient photoluminescence (PL) spectroscopy. The results are 2.5 ns for the self-assisted nanowires as well as 9 ps for the Au-assisted nanowires. By temperature-dependent PL measurements we find a characteristic activation energy of 77 meV that is present only in the Au-assisted nanowires. We conclude that most likely Au is incorporated from the droplets into the GaAs nanowires and acts as a deep, non-radiative recombination centre.
Wafer-level uniformity of atomic-layer-deposited niobium nitride thin films for quantum devices
Superconducting niobium nitride thin films are used for a variety of photon detectors, quantum devices, and superconducting electronics. Most of these applications require highly uniform films, for instance, when moving from single-pixel detectors to arrays with a large active area. Plasma-enhanced atomic layer deposition (ALD) of superconducting niobium nitride is a feasible option to produce high-quality, conformal thin films and has been demonstrated as a film deposition method to fabricate superconducting nanowire single-photon detectors before. Here, we explore the property spread of ALD-NbN across a 6-in. wafer area. Over the equivalent area of a 2-in. wafer, we measure a maximum deviation of 1% in critical temperature and 12% in switching current. Toward larger areas, structural characterizations indicate that changes in the crystal structure seem to be the limiting factor rather than film composition or impurities. The results show that ALD is suited to fabricate NbN thin films as a material for large-area detector arrays and for new detector designs and devices requiring uniform superconducting thin films with precise thickness control.
Transition to the quantum hall regime in InAs nanowire cross-junctions
We present a low-temperature electrical transport study on four-terminal ballistic InAs nanowire cross-junctions in magnetic fields aligned perpendicular to the cross-plane. Two-terminal longitudinal conductance measurements between opposing contact terminals reveal typical 1D conductance quantization at zero magnetic field. As the magnetic field is applied, the 1D bands evolve into hybrid magneto-electric sub-levels that eventually transform into Landau levels for the widest nanowire devices investigated (width = 100 nm). Hall measurements in a four-terminal configuration on these devices show plateaus in the transverse Hall resistance at high magnetic fields that scale with (ve 2 /h) -1 . e is the elementary charge, h denotes Planck's constant and v is an integer that coincides with the Landau level index determined from the longitudinal conductance measurements. While the 1D conductance quantization in zero magnetic field is fragile against disorder at the NW surface, the plateaus in the Hall resistance at high fields remain robust as expected for a topologically protected Quantum Hall phase. © 2019 IOP Publishing Ltd.
Scanning single quantum emitter fluorescence lifetime imaging: Quantitative analysis of the local density of photonic states
Their intrinsic properties render single quantum systems as ideal tools for quantum enhanced sensing and microscopy. As an additional benefit, their size is typically on an atomic scale that enables sensing with very high spatial resolution. Here, we report on utilizing a single nitrogen vacancy center in nanodiamond for performing three-dimensional scanning-probe fluorescence lifetime imaging microscopy. By measuring changes of the single emitter's lifetime, information on the local density of optical states is acquired at the nanoscale. Three-dimensional ab initio discontinuous Galerkin time-domain simulations are used in order to verify the results and to obtain additional insights. This combination of experiment and simulations to gather quantitative information on the local density of optical states is of direct relevance for the understanding of fundamental quantum optical processes as well as for the engineering of novel photonic and plasmonic devices.
Influence of Growth Polarity Switching on the Optical and Electrical Properties of GaN/AlGaN Nanowire LEDs
For the development and application of GaN-based nanowire structures, it is crucial to understand their fundamental properties. In this work, we provide the nano-scale correlation of the morphological, electrical, and optical properties of GaN/AlGaN nanowire light emitting diodes (LEDs), observed using a combination of spatially and spectrally resolved cathodoluminescence spectroscopy and imaging, electron beam-induced current microscopy, the nano-probe technique, and scanning electron microscopy. To complement the results, the photo- and electro-luminescence were also studied. The interpretation of the experimental data was supported by the results of numerical simulations of the electronic band structure. We characterized two types of nanowire LEDs grown in one process, which exhibit top facets of different shapes and, as we proved, have opposite growth polarities. We show that switching the polarity of nanowires (NWs) from the N- to Ga-face has a significant impact on their optical and electrical properties. In particular, cathodoluminescence studies revealed quantum wells emissions at about 3.5 eV, which were much brighter in Ga-polar NWs than in N-polar NWs. Moreover, the electron beam-induced current mapping proved that the p–n junctions were not active in N-polar NWs. Our results clearly indicate that intentional polarity inversion between the n- and p-type parts of NWs is a potential path towards the development of efficient nanoLED NW structures.