Benedikt Niermann

Spatial dynamics of helium metastables in sheath or bulk dominated rf micro-plasma jets

Space resolved concentrations of helium He metastable atoms in an atmospheric pressure radio-frequency micro-plasma jet were measured using tunable diode laser absorption spectroscopy. The spatial profile of metastable atoms in the volume between the electrodes was deduced for various electrode gap distances. Density profiles reveal the sheath structure and reflect the plasma excitation distribution, as well as the dominance of the α-mode discharge. Gap width variations show the transition from a normal glow plasma to a pure sheath discharge. In order to analyse and verify the experimentally observed profiles of the metastable atoms, a two-dimensional simulation model was set up. Applying an appropriate He/N2/O2 chemistry model, the correlation between the metastable profiles and the underlying excitation mechanisms was obtained.

Impurity intrusion in radio-frequency micro-plasma jets operated in ambient air

Space and time resolved concentrations of helium He (3S1) metastable atoms in an atmospheric pressure radio-frequency micro-plasma jet were measured using tunable diode laser absorption spectroscopy. Spatial profiles and lifetime measurements show significant influences of air entering the discharge from the front nozzle and of impurities originating from the gas supply system. Quenching of metastables was used to deduce quantitative concentrations of intruding impurities. The impurity profile along the jet axis was determined from optical emission spectroscopy as well as their dependence on the feed gas flow through the jet.

Axial light emission and Ar metastable densities in a parallel plate dc microdischarge in the steady state and transient regimes

Axial emission profiles in a parallel plate dc microdischarge (feedgas: argon; discharge gap d = 1 mm; pressure p = 10 Torr) were studied by means of time-resolved imaging with a fast ICCD camera. Additionally, volt–ampere (V–A) characteristics were recorded and Ar* metastable densities were measured by tunable diode laser absorption spectroscopy (TDLAS). Axial emission profiles in the steady-state regime are similar to corresponding profiles in standard size discharges (d ≈ 1 cm, p ≈ 1 Torr). For some discharge conditions relaxation oscillations are present when the microdischarge switches periodically between the low current Townsend-like mode and the normal glow. At the same time the axial emission profile shows transient behavior, starting with peak distribution at the anode, which gradually moves toward the cathode during the normal glow. The development of argon metastable densities highly correlates with the oscillating discharge current. Gas temperatures in the low current Townsend-like mode (Tg = 320–400 K) and the high current glow mode (Tg = 469–526 K) were determined by the broadening of the recorded spectral profiles as a function of the discharge current.

Time-resolved characterization of a filamentary argon discharge at atmospheric pressure in a capillary using emission and absorption spectroscopy

An argon/nitrogen (0.999/0.001) filamentary pulsed discharge operated at atmospheric pressure in a quartz tube is characterized using voltage–current measurements, microphotography, optical emission spectroscopy (OES) and absorption spectroscopy. Nitrogen is applied as a sensor gas for the purpose of OES diagnostic. The density of argon metastable atoms Ar(3P2) is determined using tunable diode laser absorption spectroscopy (TDLAS). Using a plasma chemical model the measured OES data are applied for the characterization of the plasma conditions. Between intense positive pulses the discharge current oscillates with a damped amplitude. It is established that an electric current flows in this discharge not only through a thin plasma filament that is observed in the discharge image but also through the whole cross section of the quartz tube. A diffuse plasma fills the quartz tube during a time between intense current pulses. Ionization waves are propagating in this plasma between the spike and the grounded area of the tube producing thin plasma channels. The diameter of these channels increases during the pause between the propagation of ionization waves probably because of thermal expansion and diffusion. Inside the channels electron densities of ~2 × 1013 cm−3, argon metastable densities ~1014 cm−3 and a reduced electric field about 10 Td are determined.

Application of a mode-locked fiber laser for highly time resolved broadband absorption spectroscopy and laser-assisted breakdown on micro-plasmas

Absorption spectroscopy is known to be a powerful tool to gain spatially and temporally resolved information on excited and reactive species in a plasma discharge. Furthermore, the interaction of the discharge with short intense laser pulses can trigger the ignition and the transition into other transient states of the plasma. In this context laser-assisted ‘pump-probe’ experiments involving simultaneously generated supercontinuum radiation yield highly temporally resolved and spatially well-defined information on the transient phenomena. In this paper we demonstrate the possibility for ‘pump–probe’ experiments by initiating breakdown on a picosecond time scale (‘pump’) with a high-power beam and measuring the broadband absorption with the simultaneously provided supercontinuum (‘probe’). Since both pulses are generated from the same mode-locked master oscillator, they have a strong level of synchronization. (Some figures may appear in colour only in the online journal)

MOCVD of Gallium Oxide Thin Films Using Homoleptic Gallium Complexes: Precursor Evaluation and Thin Film Characterisation

Gallium oxide (Ga2O3) is a wide band gap (~5 eV) material possessing good chemical and thermal stabilities and therefore hosting a range of potential applications, from gas sensors to optoelectronic devices. Among the various thin film deposition techniques, metalorganic chemical vapor deposition (MOCVD) has intrinsic advantages such as good step coverage and uniformity. However, compared to other oxide materials, only limited reports are available on the MOCVD growth of Ga2O3, mainly for the lack of suitable precursors. In fact, despite the use of chlorides, alkyls and β-diketonates has been reported, the design of novel gallium complexes with improved properties still represents an open challenge. The goal should be the coordinative saturation and stabilization of the Lewis acidic metal centre, which is essential to prevent oligomerisation and consequent volatility losses. In this context, we have employed different strategies to develop novel and improved precursors in terms of volatility and handling. For example, the use malonic diester anions as chelating ligands led to stable monomeric gallium complexes [1]. In this paper, the thermal characterization and the MOCVD of Ga2O3 thin films of the most promising homoleptic gallium malonate precursor candidate, Ga(dEtml)3 1 (dEtml =diethylmalonate) [1], are compared with those of the commercially available Ga β-diketonate complex Ga(acac)3 2 (acac = 2,4-pentanedionate) and the cyclic dimethylamide complex DMG [GaNMe2)3]2 3. Among the various β-diketonate ligands, acetylacetonate probably represents the most investigated one for the development of MOCVD oxide precursors. Nevertheless, there are only a few reports using 2 for the deposition of gallium oxide [2]. DMG, which is very sensitive to air and moisture, was primarily used as a staring material for the synthesis of gallium alkoxides by ligand exchange [3,4]. The volatility and decomposition characteristics of the three different precursors (1-3) were investigated by thermogravimetric (TG) studies. The obtained results revealed that in call cases the temperature window between sublimation and decomposition was sufficiently large for MOCVD applications. Compound 3 exhibited the lowest temperature for the volatilization onset (3>1>2), which was also consistent with the respective melting points measured in sealed capillaries {90°C (3) 180°C (1) > 125°C (3)}. Beside the investigation on the compound thermal properties, our primary target was to investigate the application of compounds 13 in the MOCVD of Ga2O3 thin films. The ultimate goal of this research is the comparative study of film crystallinity, surface morphology and composition as a function of the adopted CVD process parameters.

Plasma enhanced chemical vapor deposition of wear resistant gradual a-Si1-x: Cx: H coatings on nickel-titanium for biomedical applications

Plasma enhanced chemical vapor deposition has been used to deposit thin films with gradual transitions from silicon to carbon on Cu, Ni, stainless steel, and NiTi. Thus show low stress, elasticity, and wear resistance with excellent adhesion on all metals under investigation. Already at low Si concentrations of 10 at. % the intrinsic stress is considerably reduced compared to pure diamondlike carbon (DLC) films. The deposition process is controlled by optical emission spectroscopy. This technique has been applied to monitor the growth precursors and to correlate them with the film composition. The compositions of the films were determined by Rutherford backscattering spectroscopy and XPS measurements. Due to the elastic properties of the gradual transition and the excellent biocompatibility of DLC, the described film systems present a useful coating for biomedical applications.