Miroslav Kolíbal

Guided Assembly of Gold Colloidal Nanoparticles on Silicon Substrates Prepatterned by Charged Particle Beams

Colloidal gold nanoparticles represent technological building blocks which are easy to fabricate while keeping full control of their shape and dimensions. Here, we report on a simple two-step maskless process to assemble gold nanoparticles from a water colloidal solution at specific sites of a silicon surface. First, the silicon substrate covered by native oxide is exposed to a charged particle beam (ions or electrons) and then immersed in a HF-modified solution of colloidal nanoparticles. The irradiation of the native oxide layer by a low-fluence charged particle beam causes changes in the type of surface-terminating groups, while the large fluences induce even more profound modification of surface composition. Hence, by a proper selection of the initial substrate termination, solution pH, and beam fluence, either positive or negative deposition of the colloidal nanoparticles can be achieved.

Low energy focused ion beam milling of silicon and germanium nanostructures

In this paper focused ion beam milling of very shallow nanostructures in silicon and germanium by low energy Ga( + ) ions is studied with respect to ion beam and scanning parameters. It has been found that, using low energy ions, many scanning artefacts can be avoided and, additionally, some physical effects (e.g. redeposition and ion channelling) are significantly suppressed. The structures milled with low energy ions suffer less subsurface ion beam damage (amorphization, formation of voids) and are thus more suitable for selected applications in nanotechnology.

In-situ observation of 〈110〉 oriented Ge nanowire growth and associated collector droplet behavior

Using in-situ microscopy, we show that germanium nanowires can be grown by a vapor-liquid-solid process in 〈110〉 directions both on Ge(100) and Ge(111) substrates if very low supersaturation in the collector droplet is ensured. This can be provided if thermal evaporation is utilized. Such a behavior is also in agreement with earlier chemical vapor deposition experiments, where 〈110〉 oriented wires were obtained for very small wire diameters only. Our conclusions are supported by in-situ observations of nanowire kinking towards 〈111〉 direction occurring more frequently at higher evaporation rates.

Real-Time Observation of Collector Droplet Oscillations during Growth of Straight Nanowires

A liquid droplet sitting on top of a pillar is crucially important for semiconductor nanowire growth via a vapor-liquid-solid (VLS) mechanism. For the growth of long and straight nanowires, it has been assumed so far that the droplet is pinned to the nanowire top and any instability in the droplet position leads to nanowire kinking. Here, using real-time in situ scanning electron microscopy during germanium nanowire growth, we show that the increase or decrease in the droplet wetting angle and subsequent droplet unpinning from the growth interface may also result in the growth of straight nanowires. Because our argumentation is based on terms and parameters common for VLS-grown nanowires, such as the geometry of the droplet and the growth interface, these conclusions are likely to be relevant to other nanowire systems.

Selective growth of Co islands on ion beam induced nucleation centers in a native SiO2 film

We present a straightforward method for fabrication of patterns of metallic nanostructures. The focused ion beam (FIB) lithography has been used to locally modify a native SiO2 layer on a silicon substrate. On the modified areas preferential nucleation of cobalt islands is observed. The cobalt islands formed upon deposition at 400–430 °C combined with an intermediate annealing at 550 °C have a uniform size distribution and their size can be controlled by the distance between the nucleation sites and the amount of deposited material. It is proposed that the island formation at patterned sites is due to reduced surface diffusion of Co atoms in the vicinity of FIB modified areas. The intermediate annealing improves the island morphology since the kinetic diffusion limits are lowered and system reconfigures toward its equilibrium state.

Controlled faceting in 〈110〉 germanium nanowire growth by switching between vapor-liquid-solid and vapor-solid-solid growth

We show that the hexagonal cross-section of germanium nanowires grown in the 〈110〉 direction by physical vapor deposition is a consequence of minimization of surface energy of the collector droplet. If the droplet is lost or solidified, two {001} sidewall facets are quickly overgrown and the nanowire exhibits a rhomboidal cross-section. This process can be controlled by switching between the liquid and solid state of the droplet, enabling the growth of nanowires with segments having different cross-sections. These experiments are supported by in-situ microscopic observations and theoretical model.

The Synergic Effect of Atomic Hydrogen Adsorption and Catalyst Spreading on Ge Nanowire Growth Orientation and Kinking

Hydride precursors are commonly used for semiconductor nanowire growth from the vapor phase and hydrogen is quite often used as a carrier gas. Here, we used in situ scanning electron microscopy and spatially resolved Auger spectroscopy to reveal the essential role of atomic hydrogen in determining the growth direction of Ge nanowires with an Au catalyst. With hydrogen passivating nanowire sidewalls the formation of inclined facets is suppressed, which stabilizes the growth in the ⟨111⟩ direction. By contrast, without hydrogen gold diffuses out of the catalyst and decorates the nanowire sidewalls, which strongly affects the surface free energy of the system and results in the ⟨110⟩ oriented growth. The experiments with intentional nanowire kinking reveal the existence of an energetic barrier, which originates from the kinetic force needed to drive the droplet out of its optimum configuration on top of a nanowire. Our results stress the role of the catalyst material and surface chemistry in determining the nanowire growth direction and provide additional insights into a kinking mechanism, thus allowing to inhibit or to intentionally initiate spontaneous kinking.

Quantitative analysis of ultra thin layer growth by time-of-flight low energy ion scattering

Time of flight low energy ion scattering is used to quantitatively characterize the growth mode of Au deposited on B. Information on the filling factor (surface area coverage) is deduced from the yield of backscattered ions and the spectrum height of scattered neutrals. From the variation of the filling factor with the nominal Au thickness, cluster growth is deduced. Information on the average cluster height is obtained from the energy distribution of scattered neutrals and the comparison with computer simulation. Finally, an estimate of the aspect ratios is obtained for the clusters from the filling factor and the cluster height.

New approaches to in-situ heating in FIB/SEM systems

Over last decades significant effort has been made on in-situ heating experiments inside SEM and FIB/SEM chambers. Traditional way is to use low vacuum environment in the entire chamber. Although this valuable approach brings various undeniable advantages, new state of the art experiments coincide with new requirements, such as rapid changes in temperature, high-vacuum operation to maximize experiment cleanliness, ultra-high resolution SEM imaging and on top of it adaptable geometry in order to investigate sample’s crystallography and composition changes using EBSD and EDS detectors. In this contribution we introduce an integration of two new modules fulfilling these requirements by allowing in-situ heating in FIB/SEM systems under high vacuum conditions. Moreover, heating in high vacuum combined with injection of selected gases was also proven capable of providing sample surface oxidation [1] or reduction (Figure 1), [2].

An ultra-low energy (30-200 eV) ion-atomic beam source for ion-beam-assisted deposition in ultrahigh vacuum.

The paper describes the design and construction of an ion-atomic beam source with an optimized generation of ions for ion-beam-assisted deposition under ultrahigh vacuum (UHV) conditions. The source combines an effusion cell and an electron impact ion source and produces ion beams with ultra-low energies in the range from 30 eV to 200 eV. Decreasing ion beam energy to hyperthermal values (≈10(1) eV) without loosing optimum ionization conditions has been mainly achieved by the incorporation of an ionization chamber with a grid transparent enough for electron and ion beams. In this way the energy and current density of nitrogen ion beams in the order of 10(1) eV and 10(1) nA/cm(2), respectively, have been achieved. The source is capable of growing ultrathin layers or nanostructures at ultra-low energies with a growth rate of several MLs/h. The ion-atomic beam source will be preferentially applied for the synthesis of GaN under UHV conditions.