Margit Zacharias

Multilevel charge storage in silicon nanocrystal multilayers

The feasibility of multilevel charges in layered arranged Si nanocrystals in a metal-oxide-semiconductor structure is investigated. The structures are created with up to three layers of size-controlled Si nanocrystals having a size of around 3.9nm (±0.4nm). Using a suitable write bias, the apparent states of charge storage are evident in the series of capacitance-voltage (C‐V) curves. These memory effects are due to the successive charging of a varied number of Si nanocrystal layers in the floating gate. The widths of the memory windows were estimated by a modified charge equation for the multilayered nanocrystal samples, i.e., each memory window is quantified by charging a different number of Si nanocrystal layers. The static current-voltage (I‐V) curve can be fitted by Fowler–Nordheim tunneling which represents the dominant tunneling behavior through the Si nanocrystal multilayers separated by thin silicon dioxide. Time retention investigations demonstrate the stability of the programming states. The re...

Arrays of vertically aligned and hexagonally arranged ZnO nanowires: a new template-directed approach

A new template-directed method for large-scale fabrication of hexagonally patterned and vertically aligned ZnO nanowires is demonstrated. The process involves a novel type of metal membrane, gold catalyst templates produced using the membrane as the deposition mask, and catalyst-guided vapour-phase growth of ZnO nanowires. The metal membranes, composed of hexagonal nanotube arrays, are electrochemically replicated from ordered porous alumina. The obtained ZnO nanowires are uniformly aligned perpendicular to the GaN surface and have a distribution according to the pattern defined by the nanotube membrane. We also demonstrate that by modifying the electrochemical parameters and growth conditions, the diameter of the nanowires can be varied in the range 30?110?nm.

Fracture strength and Young's modulus of ZnO nanowires

The fracture strength of ZnO nanowires vertically grown on sapphire substrates was measured in tensile and bending experiments. Nanowires with diameters between 60 and 310 nm and a typical length of 2 μm were manipulated with an atomic force microscopy tip mounted on a nanomanipulator inside a scanning electron microscope. The fracture strain of (7.7 ± 0.8)% measured in the bending test was found to be close to the theoretical limit of 10% and revealed a strength about twice as high as in the tensile test. From the tensile experiments, the Youngs modulus could be measured to be within 30% of that of bulk ZnO, contrary to the lower values found in the literature.

Two-dimensional dendritic ZnO nanowires from oxidation of Zn microcrystals

Spontaneous formation of ZnO dendritic nanowires has been achieved on the faceted surfaces of polyhedral Zn microcrystals by oxidizing the latter at 600°C. Electron microscopy investigations reveal that all the dendritic branches are elongated in ⟨11–20⟩ directions within the ±(0001) primary planes, forming two-dimensional web-like structure. Homoepitaxial interconnections are observed at the branch-to-arm and branch-to-branch regions, and the whole dendrites are wurtzite single crystals. The growth process of the dendritic nanowires is discussed, which is proposed to be a combination of “self-catalytic liquid–solid” and vapor–solid process.

Vapour-transport-deposition growth of ZnO nanostructures: switch between c-axial wires and a-axial belts by indium doping

Zn On anowires and nanobelts are two representatives of one-dimensional semiconductor nanomaterials possessing potential applications as optoelectronic and sensor devices. In this study, we applied a vapour-transport-deposition method to synthesize both types of nanostructures using relatively low temperatures (860 ◦ C) by controlling the source materials. We found that the resulting product under similar growth conditions can be switched between [0001]-axial nanowires and � 11 ¯ 20� -axial nanobelts simply by adding indium to the source. The former appear as ordered vertical arrays of pure ZnO while the latter are belts without spatial ordering. Both represent defect-free single crystals grown via the vapour–liquid–solid mechanism using nanosphere lithography-fabricated catalyst Au templates. Examination of the early growth stage suggests that the dissolution of In into Au influences the nucleation of ZnO at the solid–liquid interface, and subsequently defines the structure and crystallographic orientation of the nanobelts. The optical properties of both nanostructures are studied by photoluminescence and resonant Raman scattering, which indicate consistently that the doped nanobelts have a higher carrier concentration than the nanowires. (Some figures in this article are in colour only in the electronic version)

Optical gain in monodispersed silicon nanocrystals

Stimulated emission from silicon-nanocrystal planar waveguides grown via phase separation and thermal crystallization of SiO∕SiO2 superlattices is presented. Under high power pulsed excitation, positive optical gain can be observed once a good optical confinement in the waveguide is achieved and the silicon nanocrystals have proper size. A critical tradeoff between Auger nonradiative recombination processes and stimulated emission is observed. The measured large gain values are explained by the small size dispersion in these silicon nanocrystals.

Ferroelectric nanotubes fabricated using nanowires as positive templates

The authors report on fabrication and electrical characterization of ferroelectric nanotubes and metal-ferroelectric-metal composite nanotubes using silicon and ZnO nanowires as positive templates. Nanotubes of high aspect ratio with a minimum inner diameter of about 100nm and a length ranging from 0.5μm to a few microns have been obtained by magnetron sputtering and/or pulsed laser deposition. Metal-ferroelectric one-dimensional structures were characterized by piezoelectric scanning probe microscopy, showing piezoelectric hysteresis loops and ferroelectric switching. The presented fabrication approach can be used to fabricate three-dimensional capacitors for ferroelectric nonvolatile memories as well as nanosize piezoelectric scanners and actuators.

Si nanocrystal based memories: Effect of the nanocrystal density

Si nanocrystals designed for memory applications were prepared in a layered arrangement by using a SiOx∕SiO2 multilayer structure with a variation of the stoichiometry parameter x from 0.9 to 1.63. The stoichiometry of the SiOx layers is controlled by adjusting the oxygen pressure during the growth which influences the resulting area density of the Si nanocrystals after high temperature annealing from around (2.8–0.93)×1012∕cm2. The tuning of the Si nanocrystal area density in the layers is demonstrated by transmission electron microscopy as well as by comparison of capacitance-voltage and photoluminescence measurements. The influence of the nanocrystal density on the charge behavior is demonstrated and discussed. Our method realizes a simple way to control the area density by maintaining equally sized nanocrystals, that gives unique possibilities to study the influence of the nanocrystal density on the electrical properties.

Multitwinned Spinel Nanowires by Assembly of Nanobricks via Oriented Attachment: A Case Study of Zn2TiO4

Nanowires with twinned morphology have been observed in many cubic-phase materials including spinel. We study systematically the formation of multitwinned Zn(2)TiO(4) nanowires based on a solid-solid reaction of ZnO nanowires with a conformal shell of TiO(2), which is deposited by atomic layer deposition (ALD). By varying the solid-state reaction temperature, reaction time, and TiO(2) shell thickness, the formation process is carefully analyzed with the help of transmission electron microscopy. It is found that the multitwins develop through an oriented attachment of initially separated spinel nanobricks and a simultaneous Ostwald ripening process. The oriented assembly of the individual bricks is strongly dependent on annealing conditions, which is required to favor the motion and interaction of the bricks. This mechanism differs dramatically from those proposed for twinned nanowires grown with the presence of metal catalysts. Our result provides new insights on controlling the morphology and crystallinity of designed 1-D nanostructures based on a solid-state reaction route.

Electrical properties of nominally undoped silicon nanowires grown by molecular-beam epitaxy

Single undoped Si nanowires were electrically characterized. The nanowires were grown by molecular-beam epitaxy on n+ silicon substrates and were contacted by platinum/iridium tips. I-V curves were measured and electron beam induced current investigations were performed on single nanowires. It was found that the nanowires have an apparent resistivity of 0.85Ωcm, which is much smaller than expected for undoped Si nanowires. The conductance is explained by hopping conductivity at the Si–SiO2 interface of the nanowire surface.