Browsing by Author "Stichtenoth, Daniel"

Gallium arsenide nanowires are grown on < 100 > GaAs substrates, adopting the epitaxial relation and thus growing with an angle around 35 degrees off the substrate surface. These straight nanowires are irradiated with different kinds of energetic ions. Depending on the ion species and energy, downwards or upwards bending of the nanowires is observed to increase with ion fluence. In the case of upwards bending, the nanowires can be aligned towards the ion beam direction at high fluences. Defect formation (vacancies and interstitials) within the implantation cascade is identified as the key mechanism for bending. Monte Carlo simulations of the implantation are presented to substantiate the results.

Details of the vapour-liquid-solid Au droplet catalysed growth of ZnS nanobelts are elucidated in this work. The inclination of the Au droplet after solidification shows that it is indeed in the liquid state during nanobelt growth. Numerous stacking faults are observed when (0001) wurtzite is the side surface of the nanobelt. Compressive stress at the droplet-nanobelt-atmosphere triple interface is the cause of the stacking faults. Sawteeth-like structures are observed on the Zn-terminated polar (0001) side surface only. These surfaces are chemically active, while S-terminated (000 (1) over bar) surfaces and non-polar surfaces are not. Oil these active surfaces, autocatalysed vapour-solid growth leads to the formation of the observed sawteeth.

Vapor-liquid-solid is a well-established process in catalyst guided growth of 1-D nanostructures, i.e., nanobelts and nanowires. The catalyst particle is generally believed to be in the liquid state during growth, and is the site for impinging molecules. The crystalline structure of the catalyst may not have any influence on the structure of the grown nanostructures. In this work, using An guided growth of ZnO, we show that the interfaces between the catalyst droplet and the nanostructure grow in well-defined mutual crystallographic relationships. The nanostructure defines the crystallographic orientation of the solidifying An droplet. Possible alloy, intermetallic, or eutectic phase formation during catalysis are elucidated with the help of a proposed ternary Au-Zn-O phase diagram.

Zinc oxide bulk crystals were doped with nitrogen by ion beam implantation. After postimplantation annealing, a luminescent transition appears at 3.230 eV. Power-dependent photoluminescence studies and time-resolved measurements at several spectral positions within this band can be described by a model for donor-acceptor-pair (DAP) transitions. By tracing the luminescence in a temperature-dependent study, a connection to phonon replicas could be excluded. Based on these results, this luminescence line could be clearly assigned to a DAP transition. In order to increase the doping efficiency, various approaches are considered and discussed. A slight increase could be obtained by high-temperature implantation without postimplantation annealing. (c) 2008 American Institute of Physics.

ZnS nanostructures of different morphologies, i.e., nanowires and nanobelts, have been ion implanted with Mn and subsequently annealed to obtain Zn1-xMnxS nanostructures. The Mn content x was adjusted to lie in the range from 4x10(-6)% to 4% corresponding to a variation of the mean Mn-Mn distance between about 200 and 2 nm, respectively. The Zn1-xMnxS nanowires have been studied by photoluminescence spectroscopy. The yellow Mn luminescence band indicates that the Mn2+ ions are incorporated on cation lattice sites replacing Zn. The temporal evolution of this internal Mn2+(3d(5)) luminescence is measured over 4 orders of magnitude in intensity. The decay behavior shows a clear dependence on the morphology of the nanostructure, in particular, on the ratio between the average Mn ion-killer center distance and the characteristic lateral size of the nanostructure. If the mean Mn-Mn distance is comparable to or smaller than the average Mn ion-killer center distance in the nanostructures, then concentration quenching of the Mn luminescence occurs similar to bulk. The nonexponential transients observed can be well described in the framework of a modified Forster model at reduced dimensionality. The photoluminescence (PL) behavior of the nanowires loses its one-dimensional character when the mean Mn ion-killer center distance becomes much smaller than the wire diameter. In contrast, the temporal PL behavior of the nanobelts is only purely two dimensional in this case and is of intermediate character between one dimensional and two dimensional otherwise.

ZnS:Mn nanostructures with different morphology, i.e. wires and belts, have been studied by time resolved photoluminescence. The temporal evolution of the internal Mn2+(3d(5)) luminescence is measured over 4 orders of magnitude in intensity. The decay behavior shows a clear dependence on the morphology of the nanostructure, in particular, on the ratio between the average Mn ion killer center distance and the characteristic lateral size of the nanostructure. The non-exponential transients observed can be well described in the framework of a Forster-transfer at reduced dimensionality. We present an important step in resolving a longstanding controversy in the literature regarding the lifetime of the internal Mn2+ emission in ZnS nanoparticles.

To clarify the size effect in semiconductor nanowires with decreasing diameters but not yet reaching the quantum confinement region, single crystalline zinc oxide nanowires with diameters around 10 nm are synthesized. Electrical transport measurements of these thin nanowires show significant increase in conductivity accompanied by diminished gate modulation and reduced mobility. This phenomenon is a result of the enrichment of surface states owing to the increased surface-to-volume ratio. The enhanced surface effect is confirmed by the temperature dependent photoluminescence measurements and contributes to the "anomalous" blueshift. This study shows that surface states play a dominant role in the electrical and optical properties of quasi-one-dimensional materials. (c) 2007 American Institute of Physics.

ZnO nanowires with high crystalline and optical properties are characterized, showing strong effect of the surface defect states. In order to optimize the performance of devices based on these nanowires, a series of complementary metal-oxide semiconductor compatible surface passivation procedures is employed. Electrical transport measurements demonstrate significantly reduced subthreshold swing, high on/off ratio, and unprecedented field effect mobility.(c) 2006 American Institute of Physics.

The effects of doping (by ion implantation) on the electronic structure of ZnO nanowires, particularly on the defect states generation in the band gap of ZnO, are investigated using valence electron energy loss spectroscopy (VEELS) performed in a transmission electron microscope (TEM). The improved spectrum energy resolution via the introduction of a gun monochromator, together with the reduced intensity in the zero loss peak tail as realized by spectrum acquisition at non-zero momentum transfer, enable us to extract such electronic structure information from the very low loss region of the EEL spectra. We have compared the doping effects of several dopant elements, i.e., Er, Yb, and Co, and found that generation of the band tail states (similar to 2-3.3 eV) is a common consequence of the ion implantation process. On the other hand, specific mid-gap state(s) in the lower energy range are created only in the rare earth element doped ZnO nanowires, suggesting the dopant-sensitive nature of such state. (C) 2007 Elsevier Ltd. All rights reserved.

Nanostructures of zinc sulfide (ZnS), a II-VI compound semiconductor with a direct band gap of 3.66 eV in the cubic phase and 3.74 eV in the wurtzite phase, show interesting optical properties, making it a promising candidate for optoelectronic devices. Single-crystalline nanobelts and nanowires were synthesized in a computer-controlled process according to the vapor-liquid-solid-mechanism. We investigated the morphology, structure, and composition by scanning electron microscopy, transmission electron microscopy, and x-ray diffraction. The optical properties were studied by low-temperature photoluminescence (PL) and cathodoluminescence. The synthesized ZnS nanowires were implanted with nitrogen and boron as potential donor and acceptor, respectively. The implanted nanowires were investigated directly after ion implantation and showed a high quantity of defects resulting in nonluminescent material. Annealing procedures recovered the crystal structure and the luminescence, and we found emerging and varying PL lines indicating the activation of the implanted impurities. (c) 2006 American Vacuum Society.

Single crystal ZnO wurtzite nanowires grown along the c- axis with diameters down to 4 nm were synthesized by a catalytic vapor transport technique. Photoluminescence spectra of these wires indicate a blue shift of the free exciton by 19 meV due to confinement. This result was obtained by analyzing the line shape of the blue- shifted LO phonon replica of the free exciton. In addition, a surface- related excitonic luminescence feature centered at 3.366 eV was observed with a strongly elevated thermal activation energy.

Gallium arsenide (GaAs) nanowires with diameters of 150 nm have been grown via metal-organic vapor deposition and were subsequently implanted with (64)Zn ions. The amorphized nanowires were annealed at 800 degrees C under arsenic overpressure resulting into a full recrystallization of the nanowires as well as an activation of the implanted acceptors. Consequently, we observe a strong increase in conductivity of the GaAs:Zn nanowires, where a simple estimation of the activated acceptors matches the implantation concentration. (c) 2008 American Institute of Physics.

Silicon nanowires have been grown in a horizontal tube furnace by disproportionation of silicon monoxide in combination with the vapor-liquid-solid mechanism. We present a phase diagram of the nanowire growth, indicating different morphologies for varying growth pressure and temperature. The morphology was characterized by scanning electron microscopy and detailed structural analysis was performed by transmission electron microscopy. A variety of morphologies is found and the optimum parameter range for the growth of straight and uniform nanowires consisting of crystalline silicon cores and amorphous SiO(2) shells is identified and discussed. (C) 2010 Elsevier B.V. All rights reserved.

Zinc oxide (ZnO) nanowires were grown via thermal transport and subsequently doped with different concentrations of Tm, Yb, and Eu using ion implantation and post annealing. High ion fluences lead to morphology changes due to sputtering; however, freestanding nanowires become less damaged compared to those attached to substrates. No other phases like rare earth (RE) oxides were detected, no amorphization occurs in any sample, and homogeneous doping with the desired concentrations was achieved. Photoluminescence measurements demonstrate the optical activation of trivalent RE-elements and the emission of the characteristic intra-4f-luminescence of the respective RE atoms, which could be assigned according to the Dieke-diagram. An increasing RE concentration results into decreasing luminescence intensity caused by energy transfer mechanisms to non-radiative remaining implantation defect sites. Furthermore, low thermal quenching was observed due to the considerable wide band gap of ZnO.

We present a method which can be used for the mass-fabrication of nanowire photonic and electronic devices based on spin-on glass technology and on the photolithographic definition of independent electrical contacts to the top and the bottom of a nanowire. This method allows for the fabrication of nanowire devices in a reliable, fast, and low cost way, and it can be applied to nanowires with arbitrary cross section and doping type (p and n). We demonstrate this technique by fabricating single-nanowire p-Si(substrate)-n-ZnO(nanowire) heterojunction diodes, which show good rectification properties and, furthermore, which function as ultraviolet light-emitting diodes.

This review demonstrates that ion irradiation is a very useful tool in order to tailor the properties of semiconductor nanowires. Besides optical and electrical doping provided by adequate ion species and ion energies, one can use ion beams also for the controlled shaping of the morphology of nanostructures. Here, one utilizes the commonly as 'negative' described characteristics of ion implantation: defect formation and sputtering. We show that ion beams can be even used for an alignment of the nanowires. Furthermore, we report here on several successful experiments in order to modify the electrical and optical properties in a controlled manner of ZnO semiconductor nanowires by the use of transition metals, rare earth elements and hydrogen ions. [GRAPHICS] Schematic illustration of ion beam doping of a single contacted nanowire. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Radioactive In-111 atoms implanted into zinc oxide (ZnO) single crystals occupy substitutional Zn lattice sites after annealing to 700 degrees C. The respective photoluminescence (PL) spectra of the samples were monitored while the donor In decayed into stable and isolectronic Cd. The commonly labeled PL-I-9 line could be clearly assigned to excitons bound to the donor In. An arising luminescence band centered at 2.85 eV was observed with the characteristic lifetime of the isotope, and the origin could be identified as levels of the isoelectronic Cd impurities.