Temperature-dependence of the phase-coherence length in InN nanowires

We report on low-temperature magnetotransport measurements on InN nanowires, grown by plasma-assisted molecular beam epitaxy. The characteristic fluctuation pattern observed in the conductance was employed to obtain information on phase-coherent transport. By analyzing the root-mean-square and the correlation field of the conductance fluctuations at various temperatures the phase-coherence length was determined.Comment: 4 pages, 4 figure

Ultrafast Carrier Relaxation in InN Nanowires Grown by Reactive Vapor Transport

We have studied femtosecond carrier dynamics in InN nanowires grown by reactive vapor transport. Transient differential absorption measurements have been employed to investigate the relaxation dynamics of photogenerated carriers near and above the optical absorption edge of InN NWs where an interplay of state filling, photoinduced absorption, and band-gap renormalization have been observed. The interface between states filled by free carriers intrinsic to the InN NWs and empty states has been determined to be at 1.35 eV using CW optical transmission measurements. Transient absorption measurements determined the absorption edge at higher energy due to the additional injected photogenerated carriers following femtosecond pulse excitation. The non-degenerate white light pump-probe measurements revealed that relaxation of the photogenerated carriers occurs on a single picosecond timescale which appears to be carrier density dependent. This fast relaxation is attributed to the capture of the photogenerated carriers by defect/surface related states. Furthermore, intensity dependent measurements revealed fast energy transfer from the hot photogenerated carriers to the lattice with the onset of increased temperature occurring at approximately 2 ps after pulse excitation

Electronic Transport in Narrow-Gap Semiconductor Nanowires

Throughout this work the electronic transport properties of InAs, InN, and GaAs/InAs core/shell nanowires have been analyzed. This includes the analysis of specific resistivity at room temperature and low temperatures as well as the breakdown of resistivity by a contribution of mobility and carrier concentration using gate measurements. While the InN nanowires showed homogeneous transport properties, there was a large statistical spread in the properties of InAs nanowires. Differing crystal structures and the surface conditions are identified to be the main reasons for the statistical spread. Both quantities of influence have been pointed out by comparing the transport parameters before and after a surface treatment (electron irradiation and long time ambient air exposure), and by comparing the transport parameters of wires grown by different growth methods which exhibit different kinds of crystal structure. In particular, the temperature dependence of the conductivity revealed different activation energies in nanowires with differing crystal structures. An explanation has been suggested in terms of stacking fault induced potential barriers. A field-effect measurement setup has been utilized to determine the nanowire mobility and carrier concentration. Even though this method is widely used for nanowires, it is subject to a serious disadvantage concerning the influence of surface and interface states on the measurements. As an alternative method which does not suffer from this drawback, Hall measurements have been successfully performed on InAs nanowires for the first time. These measurements became possible because of the utilization of a new electron beam lithographic procedure with an alignment accuracy in the 5 nm range. Carrier concentration values could be determined and compared to the ones obtained from conventional field-effect measurements. The results of the Hall measurements revealed a methodical overestimation of the carrier concentrations obtained from the field-effect measurements due to the influence of surface states. The homogeneity in transport characteristics of the InN nanowires allowed for an accurate analysis of the diameter dependence of the nanowire resistivity. The effect of donor deactivation has been found to increase the resistivity of InN nanowires with small diameters. Furthermore, a quantum confinement effect has been observed in GaAs/InAs core/shell nanowires. For very low shell thicknesses below 10 nm a drastic resistivity increase has been found. Simulations with a self consistent Schrödinger-Poisson solver confirmed the interpretation in terms of quantum confinement. A further major topic of this work has been the analysis of phase coherent transport at low temperatures. In particular, universal conductance fluctuations have been analyzed and a consistent method to determine the phase coherence length quantitatively has been developed. In addition, transport measurements on GaAs/InAs core/shell nanowires with a magnetic field applied parallel to the wire axis demonstrated Aharonov-Bohm-type conductance oscillations. An explanation in terms of coherent angular momentum quantum states in the conductive InAs shell has been developed to interpret these oscillations. To conclude, both room temperature and low temperature measurements allowed gaining insights into basic classical as well as quantum transport properties of nanowires. In the face of a future application of nanowires in quantum information processing or their use in so-called phase-based switching devices, valuable information is provided within this work. Furthermore, the room temperature results show that for application of nanowires in electronic devices, both the crystal structure and the surface conditions have to be controlled. Here, it will be inevitable for future progress to achieve a controlled passivation of the wire surfaces for defined and stable surface conditions. Furthermore, a more detailed investigation of the correlation between the crystal structure and the transport properties is needed

Aerodynamische Rotorblattauslegung einer Multirotor- Kleinstwindenergieanlage

Die vorliegende Bachelorthesis befasst sich mit der Auslegung und Optimierung eines Windrotors mit hori-zontaler Drehachse. Es werden physikalische Zusammenhänge und Berechnungsmethoden, welche der Auslegung dienen, erläutert. Der ausgelegte Rotor wird anschließend mittels eines CAD Programms kon-struiert. Die Konstruktion dient zu weiteren Forschungszwecken an der HAW.This report contains strategies for the design and the optimization of a horizontal-axis wind turbine. Physical relations and calculation methods for the purpose of designing a wind turbine will be explained. Further, the wind turbine will be designed using CAD software tools and used in future research projects at the HAW

Hall effect measurements on InAs nanowires

We have processed Hall contacts on InAs nanowires grown by molecular beam epitaxy using an electron beam lithography process with an extremely high alignment accuracy. The carrier concentrations determined from the Hall effect measurements on these nanowires are lower by a factor of about 4 in comparison with those measured by the common field-effect technique. The results are used to evaluate quantitatively the charging effect of the interface and surface states

Phase coherent transport in InSb nanowires

Comprehensive electrical transport studies are performed on InSb nanowires by varying temperature, gate voltage, and magnetic field. The 3-dimensional bulk conduction is found to dominate in the nanowire channel after investigating a large number of nanowires with different diameters, which show approximately a linear relation between the conductance normalized to the length and the wire cross section. At low temperatures, universal conductance fluctuations are observed. From the amplitude and the correlation voltage of the conductance fluctuations, the phase-coherence length in InSb nanowires is determined at various temperatures. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4747200

Flux periodic magnetoconductance oscillations in GaAs/InAs core/shell nanowires

Magnetotransport experiments on epitaxial GaAs/InAs core/shell nanowires are performed in which the InAs shell forms a tube-like conductive channel around the highly resistive GaAs core. The core/shell nanowires are grown by molecular beam epitaxy. It is found that the nanowire conductance oscillates with the magnetic field oriented parallel to its axis, with a period of the magnetic flux quantum ϕ0=h/e. Related to that, it is shown that the electronic transport is mediated by closed loop quantum states encircling the wire axis rather than by electron interference of partial waves. By means of a gate voltage the conductance at zero magnetic field can be changed between an oscillation minimum and maximum. The experimental findings are supported by numerical calculations