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arXiv2026

A Comprehensive multi-species comparison of rotational temperature probes in a DC Ar/N$_2$ micro-hollow cathode discharge

Accurate gas temperature ($T_{\rm Gas}$) determination in microplasmas is critical for optimizing their applications, yet isolated diagnostic approaches may yield misleading results, especially under strong non-equilibrium conditions. Here, high resolution rotational spectra of N$_2$(C), OH(A), NH(A) and NO(A), generated in the plasma jet of a DC Ar/N$_2$ microhollow cathode discharge (MHCD), are recorded and their associated rotational temperatures ($T_{\rm rot}$) are cross compared. A detailed experimental analysis and robust fitting of the rotational spectra are performed, achieving a reliable estimation of $T_{\rm Gas}$. The dominant formation mechanisms of these species and their corresponding impact on rotational population distributions are also interrogated. Particularly, our findings indicate that the $T_{\rm rot}$of N$_2$(C) is significantly influenced by energy transfers from argon metastables (Ar$^m$) and spectral interference from NH(A). This makes it unreliable as a thermometric probe in Ar-rich MHCD, unless complex analyses are employed. In contrast, OH(A) rotational population distribution appears to be less sensitive to Ar-induced perturbations across various discharge currents and pressures, providing more straightforward results. For all molecules considered, this study reveals the conditions under which all the measured $T_{\rm rot}$ can be reliably considered to be in equilibrium with $T_{\rm Gas}$. This highlights the importance of crossvalidating multiple thermometric probes and investigating relevant excitation kinetics when measuring $T_{\rm rot}$ in reactive microplasmas.

physics.plasm-ph↗

arXiv2025

Hexagonal boron nitride thin film synthesis with a ns-pulsed MHCD: in-situ plasma diagnostics and post-growth film characterization

Hexagonal boron nitride (h-BN) is deposited on Si <100> wafer ($\approx$20 cm2) via Plasma Enhanced Chemical Vapor Deposition (PECVD) using a ns-pulsed N2/Ar Micro Hollow Cathode Discharge (MHCD) as a microplasma source. For the first time, aluminum nitride (AIN) is employed as the dielectric material in the MHCD to mitigate film contamination by atomic oxygen, an issue previously observed with conventional Al 2O3 dielectrics. A comprehensive multi-diagnostic approach is followed to characterize the deposited h-BN, including Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron microscopy (XPS). In parallel, in-situ diagnostics such as optical emission spectroscopy (OES) and intensified CCD (ICCD) imaging are used to monitor plasma properties, including emission profiles, gas temperature and discharge morphology. Raman spectra reveal the E2g phonon mode of h-BN around 1366 cm_____, confirming successful synthesis. SEM imaging reveals an almost complete surface coverage by the film, with localized delamination. This is probably due to an uneven resistive heating of the Si wafer, rapid post-deposition cooling ($\sim$13 K/min) and ambient exposure. AFM analysis indicates an average thickness of about 33 nm after 90 minutes of deposition ($\sim$22 nm/h deposition rate). XPS measurements reveal an average B/N atomic ratio of $\sim$1.5 along the wafer diameter. Deviations from ideal film properties (e.g., stoichiometric unity, uniform morphology) are attributed to plasma-induced inhomogeneities (such as non-uniform species flux and temperature gradients) among other factors (e.g., 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 for scalable h-BN synthesis, with further optimization of the reactor design, plasma conditions, and gas chemistry required to grow ideal films.

physics.plasm-ph↗