High-rate, room temperature plasma-enhanced deposition of aluminum-doped zinc oxide nanofilms for solar cell applications

A custom-designed inductively coupled plasma (ICP)-assisted radio-frequency magnetron sputtering deposition system has been employed to synthesize aluminium-doped zinc oxide (ZnO:Al) nanofilms on glass substrates at room temperature. The effects of film thickness and ZnO target (partially covered by Al chips) power on the structural, electrical and optical properties of the ZnO:Al nanofilms are studied. A high growth rate (∼41 nm/min), low electrical sheet resistance (as low as 30 Ω/□) and high optical transparency (>80%) over the visible spectrum has been achieved at a film thickness of ∼615 nm and ZnO target power of 150 W. The synthesis of ZnO:Al nanofilms at room temperature and with high growth rates is attributed to the unique features of the ICP-assisted radio-frequency magnetron sputtering deposition approach. The results are relevant to the development of photovoltaic thin-film solar cells and flat panel displays.

Plasma heating effects in catalyzed growth of carbon nanofibres

A theoretical model describing the plasma-assisted growth of carbon nanofibres (CNFs) that accounts for the nanostructure heating by ion and etching gas fluxes from the plasma is developed. Using the model, it is shown that fluxes from the plasma environment can substantially increase the temperature of the catalyst nanoparticle located on the top of the CNF with respect to the substrate temperature. The difference between the catalyst and the substrate temperatures depends on the substrate width, the length of the CNF, the neutral gas density and temperature as well as the densities of the ions and atoms of the etching gas. In addition to the heating of the nanostructure, the ions and etching gas atoms from the ionized gas environment also strongly affect the CNF growth rates. Due to ion bombardment, the CNF growth rates in plasma enhanced chemical vapour deposition may be much higher than the rates in similar neutral gas-based thermal processes. The CNF growth model, which accounts for the nanostructure heating by the plasma-generated species, provides the growth rates that are in better agreement with the available experimental data on CNF growth than the models in which the heating effects are ignored.

A model of a large-area planar plasma producer based on surface wave propagation in a plasma-metal structure with a dielectric sheath

A theoretical model of a large-area planar plasma producer based on surface wave (SW) propagation in a plasma-metal structure with a dielectric sheath is presented. The SW which produces and sustains the microwave gas discharge in the planar structure propagates along an external magnetic field and possesses an eigenfrequency within the range between electron cyclotron and electron plasma frequencies. The spatial distributions of the produced plasma density, electromagnetic fields, energy flow density, phase velocity and reverse skin depth of the SW are obtained analytically and numerically.

Nanostructures of various dimensionalities from plasma and neutral fluxes

A complex multi-scale model and numerical simulations are used to demonstrate, by simulating the development of patterns of nanotips, nanowalls, nanoislands and nanovoids of a characteristic size of 5-100 nm, a greater degree of determinism in the formation of various nanostructures by using the low-density, low-temperature plasma-based processes. It is shown that in the plasma, in contrast to the neutral gas-based processes, one can synthesize nanostructures of various dimensionalities and shapes with a larger surface density, desired geometrical parameters and narrower size distribution functions. This effect is mainly attributed to strong ion focusing by irregular electric fields in the nanopatterns, which effectively redistributes the influxes of plasma-generated building units and thus provides a selective control of their delivery to the growing nanostructures.

Plasma-deposited Ge nanoisland films on Si : is Stranski–Krastanow fragmentation unavoidable?

The formation of Ge quantum dot arrays by deposition from a low-temperature plasma environment is investigated by kinetic Monte Carlo numerical simulation. It is demonstrated that balancing of the Ge influx from the plasma against surface diffusion provides an effective control of the surface processes and can result in the formation of very small densely packed quantum dots. In the supply-controlled mode, a continuous layer is formed which is then followed by the usual Stranski-Krastanow fragmentation with a nanocluster size of 10 nm. In the diffusion-controlled mode, with the oversupply relative to the surface diffusion rate, nanoclusters with a characteristic size of 3 nm are formed. Higher temperatures change the mode to supply controlled and thus encourage formation of the continuous layer that then fragments into an array of large size. The use of a high rate of deposition, easily accessible using plasma techniques, changes the mode to diffusion controlled and thus encourages formation of a dense array of small nanoislands.

Nanocrystalline vanadium oxide films synthesized by plasma-assisted reactive rf sputtering deposition

Plasma-assisted reactive rf magnetron sputtering deposition is used to fabricate vanadium oxide films on glass, silica and silicon substrates. The process conditions are optimized to synthesize phase-pure vanadium pentoxide (V2O5) featuring a nanocrystalline structure with the predominant (0 0 1) crystallographic orientation, surface morphology with rod-like nanosized grains and very uniform (the non-uniformity does not exceed 4%) coating thickness over large surface areas. The V2O5 films also show excellent and temperature-independent optical transmittance in a broad temperature range (20-95 °C). The results are relevant to the development of smart functional coatings with temperature-tunable properties. © 2007 IOP Publishing Ltd.

Plasma-aided nanofabrication: where is the cutting edge?

Plasma-aided nanofabrication is a rapidly expanding area of research spanning disciplines ranging from physics and chemistry of plasmas and gas discharges to solid state physics, materials science, surface science, nanoscience and nanotechnology and related engineering subjects. The current status of the research field is discussed and examples of superior performance and competitive advantage of plasma processes and techniques are given. These examples are selected to represent a range of applications of two major types of plasmas suitable for nanoscale synthesis and processing, namely thermally non-equilibrium and thermal plasmas. Major concepts and terminology used in the field are introduced. The paper also pinpoints the major challenges facing plasma-aided nanofabrication and identifies some emerging topics for future research. © 2007 IOP Publishing Ltd.

Deterministic surface growth of single-crystalline iron oxide nanostructures in nonequilibrium plasma

An innovative approach to precise tailoring of surface density, shapes, and sizes of single-crystalline α-Fe 2O 3 nanowires and nanobelts by controlling interactions of reactive oxygen plasma-generated species with the Fe surface is proposed. This strongly nonequilibrium, rapid, almost incubation-free, high-rate growth directly from the solid-solid interface can also be applied to other oxide materials and is based on deterministic control of the density of oxygen species and the surface conditions, which determine the nanostructure nucleation and growth.

Reactive oxygen plasma-enabled synthesis of nanostructured CdO : tailoring nanostructures through plasma–surface interactions

Plasma-assisted synthesis of nanostructures is one of the most precise and effective approaches used in nanodevice fabrication. Here we report on the innovative approach of synthesizing nanostructured cadmium oxide films on Cd substrates using a reactive oxygen plasma-based process. Under certain conditions, the surface morphology features arrays of crystalline CdO nano/micropyramids. These nanostructures grow via unconventional plasma-assisted oxidation of a cadmium foil exposed to inductively coupled plasmas with a narrow range of process parameters. The growth of the CdO pyramidal nanostructures takes place in the solid-liquid-solid phase, with the rates determined by the interaction of plasma-produced oxygen atoms and ions with the surface. It is shown that the size of the pyramidal structures can be effectively controlled by the fluxes of oxygen atoms and ions impinging on the cadmium surface. The unique role of the reactive plasma environment in the controlled synthesis of CdO nanopyramidal structures is discussed as well.

Tailoring the composition of self-assembled Si1−xCx quantum dots : simulation of plasma/ion-related controls

Precise control of composition and internal structure is essential for a variety of novel technological applications which require highly tailored binary quantum dots (QDs) with predictable optoelectronic and mechanical properties. The delicate balancing act between incoming flux and substrate temperature required for the growth of compositionally graded (Si1-xC x; x varies throughout the internal structure), core-multishell (discrete shells of Si and C or combinations thereof) and selected composition (x set) QDs on low-temperature plasma/ion-flux-exposed Si(100) surfaces is investigated via a hybrid numerical simulation. Incident Si and C ions lead to localized substrate heating and a reduction in surface diffusion activation energy. It is shown that by incorporating ions in the influx, a steady-state composition is reached more quickly (for selected composition QDs) and the composition gradient of a Si1-xCx QD may be fine tuned; additionally (with other deposition conditions remaining the same), larger QDs are obtained on average. It is suggested that ionizing a portion of the influx is another way to control the average size of the QDs, and ultimately, their internal structure. Advantages that can be gained by utilizing plasma/ion-related controls to facilitate the growth of highly tailored, compositionally controlled quantum dots are discussed as well.

Nonresonant power transfer in plasma-surface interactions via two-surface wave decay

The excitation of pairs of electron surface waves via nonresonant decay of plasma waves incident onto a solid surface is studied in the context of controlling the interaction of pulsed electromagnetic radiation with plasma-exposed solid surfaces. The role of the plasma-exposed surfaces in nonlinear heating of the plasma edge and related power transfer is discussed. It is shown that the maximum efficiency of the power transfer at solid surfaces with dielectric permittivity εd <3 corresponds to the resonant two-surface wave decay. On the other hand, for solids with εd >3 the maximum power transfer efficiency is achieved through nonresonant excitation of the quasistatic surface waves. In this case the plasma waves generated by external radiation dissipate their energy into the plasma periphery most effectively.

Influence of hydrogen dilution on the growth of nanocrystalline silicon carbide films by low-frequency inductively coupled plasma chemical vapor deposition

Nanocrystalline silicon carbide (nc-SiC) films are prepared by low-frequency inductively coupled plasma chemical vapor deposition from feedstock gases silane and methane diluted with hydrogen at a substrate temperature of 500 °C. The effect of different hydrogen dilution ratios X [hydrogen flow (sccm) / silane + methane flow (sccm)] on the growth of nc-SiC films is investigated by X-ray diffraction, scanning electron microscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). At a low hydrogen dilution ratio X, cubic silicon carbide is the main crystal phase; whereas at a high hydrogen dilution ratio X, hexagonal silicon carbide is the main crystal phase. The SiC crystal phase transformation may be explained by the different surface mobility of reactive Si-based and C-based radicals deposited at different hydrogen dilution ratios X. The FTIR and XPS analyses show that the Si-C bonds are the main bonds in the films and elemental composition of SiC is nearly stoichiometric with almost equal share of silicon and carbon atoms.

Plasma-assembled carbon nanotubes : electric field–related effects

The paper presents results of comparative investigation of carbon nanotubes growth processes in dense low-temperature plasma and on substrate surface. Hybrid/Monte-Carlo numerical simulations were used to demonstrate the differences in the ion fluxes, growth rates and kinetics of adsorbed atoms re-distribution on substrate and nanotubes surfaces. We show that the plasma parameters significantly affect the nanotubes growth kinetics. We demonstrate that the growth rates of the nanotubes in plasma and on surface can differ by three orders, and the specific fluxes to the nanotube in the plasma can exceed the flux to surface-grown nanotube by six orders. We also show that the metal catalyst used for the nanotubes production on surface and in arc is a subject to very different conditions and this may be a key factor for the nanotube growth mode. The obtained dependencies for the ion fluxes to the nanotubes and nanotubes growth rates on the plasma parameters may be useful for selection of the production methods.

Self-organized nanoarrays : plasma-related controls

The paper presents an investigation of self-organizational and -assembly processes of nanostructure growth on surfaces exposed to low-temperature plasmas. We have considered three main growth stages-initial, or sub-monolayer growth stage, separate nanostructure growth stage, and array growth stages with the characteristic sizes of several nm, several tens of nm, and several hundreds of nm, respectively, and have demonstrated, by the experimental data and hybrid multiscale numerical simulations, that the plasma parameters can strongly influence the surface processes and hence the kinetics of self-organization and -assembly. Our results show that plasma-controlled self-organization is a promising way to assemble large regular arrays of nanostructures. © 2008 IUPAC.

Self-assembled low-dimensional nanomaterials via low-temperature plasma processing

Examples of successful fabrication of low-dimensional semiconducting nanomaterials in the Integrated Plasma-Aided Nanofabrication Facility are shown. Self-assembled size-uniform ZnO nanoparticles, ultra-high-aspect ratio Si nanowires, vertically aligned cadmium sulfide nanostructures, and quarternary semiconducting SiCAlN nanomaterial have been synthesized using inductively coupled plasma-assisted RF magnetron sputtering deposition. The observed increase in crystallinity and growth rates of the nanostructures are explained by using a model of plasma-enhanced adatom surface diffusion under conditions of local energy exchange between the ion flux and the growth surface. Issues related to plasma-based growth of low-dimensional semiconducting nanomaterials are discussed as well. © 2007 Elsevier B.V. All rights reserved.