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Low-pressure plasma processes are routinely used to grow, functionalize or etch materials, and thanks to some of its unique attributes, plasma has become a major player for some applications such as microelectronics. Plasma processes are however still at a research level when it comes to the synthesis and functionalization of nanoparticles. Yet plasma processes can offer a particularly suitable solution to produce nanoparticles having very peculiar features since they enable to: (i) reach particle with a variety of chemical compositions, (ii) tune the size and density of the particle cloud by acting on the transport dynamics of neutral or charged particles through a convenient setting of the thermal gradients or the electric field topology in the reactor chamber and (iii) manipulate nanoparticles and deposit them directly onto a substrate, or codeposit them along with a continuous film to produce nanocomposites or (iv) use them as a template to produce 1D materials. In this article, we present an experimental investigation of nanoparticles synthesis and dynamics in low-pressure microwave plasmas by combining time-resolved and in-situ laser extinction and scattering diagnostics, QCL absorption spectroscopy, mass spectrometry, optical emission spectroscopy and SEM along with a particle transport model. We showed for the first time the thermophoresis-driven dynamic of particle cloud in electrodless microwave plasmas. We showed that this effect is linked to particular fluctuations in the plasma composition and results in the formation of a void region in the bulk of the plasma surrounded by a particle cloud in the peripherical post-discharge. We also reveals and analyze the kinetics of precursor dissociation and molecular growth that result in the observed nanoparticle nucleation.

We report on the successful growth of polycrystalline GaN thin films on Si (100) substrates at room temperature (without intentional heating) using radiofrequency reactive magnetron sputtering. We use Ar and N2 as the main sputtering and N-atom precursor gas sources, respectively, and a gallium cathode as the Ga-atom source. We focus here on studying the effect of working pressure, as it is found to be the parameter that plays the most influential role on the crystalline quality of the thin films in the investigated range (20–95 mTorr). The morphology, microstructure, and composition profile of the GaN thin films are analyzed using a set of ex situ solid-state characterization techniques. This study reveals that for process pressures below 50 mTorr, the resulting films possess an amorphous nature, while for process pressures above that they become polycrystalline. Most of the crystalline films are found to be nanostructured with grain sizes typically ranging from 10 to 30 nm in size. The highest growth rate of ∼ 2.9 Å/s is obtained for the deposition carried out at 50 mTorr. At this pressure, the films exhibit the best crystallinity with a dominant wurtzite hexagonal structure. The elemental distribution of Ga and N throughout the growth profile is uniform with a sharp interface at the substrate, demonstrating one of the interests in working at low temperatures to avoid melt-back etching, a destructive reaction between gallium and silicon, that usually takes place at high temperatures.

Low-pressure plasmas, in particular magnetron sputtering discharges, are increasingly used for the deposition of wideband gap semiconductor nitrides films (e.g., GaN or AlN) considering many benefits they exhibit with respect to conventional chemical vapor deposition techniques. Plasma-based solutions enable the dissociation of N2 molecules into N-atoms under conditions that would not be possible with the thermal process. However, as the dissociation rate remains quite small due to the strong nitrogen triple bond, it is somewhat complicated to determine and correlate the N-atoms density in the gas phase with that of the grown film in low-pressure discharges. Therefore, ns-two-photon absorption laser induced fluorescence (TALIF) has been carried out to determine the absolute density of N-atoms as a function of the pressure (tens of Pa range) in a radio-frequency sputtering plasma reactor used for GaN deposition. The TALIF set-up has been optimized using a monochromator and adequate signal processing to enhance the detection limit, enabling the measurement of N-atoms density as low as 1011 cm−3 at 15 Pa. These measurements have been completed with electron density measurements performed in the same pressure range using microwave interferometry, thus providing quantitative data on both electron and N-atom densities that can be used for fundamental understanding, process optimization, and modeling of magnetron discharge intended for nitride semiconductor deposition.

Boron nitride (BN) is deposited on a Si 〈100〉 wafer (≈20 cm2) via plasma enhanced chemical vapor deposition (PECVD) using a ns-pulsed N2/Ar micro hollow cathode discharge (MHCD) as a microplasma source. Aluminum nitride (AIN) is employed as the dielectric material in the MHCD instead of more conventional Al2O3 to exclude a potential source of film contamination by atomic oxygen. For the first time, in situ diagnostics such as optical emission spectroscopy and intensified CCD imaging are used to monitor plasma properties during BN synthesis, including species emission profiles, gas temperature, and discharge morphology. Furthermore, a comprehensive ex situ multi-diagnostics approach is followed to characterize the films, including Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), x-ray photoelectron microscopy (XPS), and transmission electron microscopy coupled to energy dispersive x-ray spectroscopy (EDX). Raman spectra reveal the E2g phonon mode of BN around 1366 cm−1 (representative of hexagonal, rhombohedral, or turbostratic BN polytypes), confirming successful synthesis. Further analysis using selected area electron diffraction reveals diffraction rings, characteristic of BN films with a turbostratic structure. SEM imaging reveals an almost complete surface coverage by the film, with localized delamination though. This is probably due to an uneven resistive heating of the Si wafer, rapid post-deposition cooling (∼13 K/min), and ambient exposure. AFM analysis indicates an average thickness of about 33 nm after 90 min of deposition (∼22 nm/h deposition rate). XPS measurements reveal an average B/N atomic ratio of ∼1.5 along the wafer diameter. EDX analyses confirm the dominant presence of B- and N-atoms in the film, in fair agreement with XPS. Deviations from ideal film properties (e.g., stoichiometric unity and uniform morphology) are attributed to plasma-induced inhomogeneities (such as non-uniform species flux and temperature gradients) among other factors (e.g., oxygen impurities in the chamber and ambient exposure post-deposition), which affect nitrogen and boron incorporation and localized film properties. Despite these challenges, the MHCD-driven PECVD process demonstrates strong potential toward h-BN synthesis, with further optimization of the reactor design, plasma conditions, and gas chemistry required to grow ideal films.