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Ta 3 N 5 has ideal properties for the efficient water splitting under sunlight illumination, but its formation is only triggered by the incorporation of oxygen in its lattice. As a result, material properties may deviate from those of an ideal Ta 3 N 5 crystal, which can promote or impede the water splitting. This paper is to quantify variations in key properties relevant for the photoelectrolysis as a function of oxygen incorporation and to determine an optimum oxygen concentration. Thin films were prepared by direct current magnetron sputtering in an Ar/N 2 /O 2 gas mixture at two different N 2 and varying O 2 concentrations. The films were characterized by grazing-incidence x-ray diffraction (GI-XRD), total transmission and reflectance spectroscopy (TTRS) from which the band gap is deduced, photocurrent measurements on which the calculation of the efficiency-mobility-lifetime ( ημτ ) product is based and photoelectrolysis experiments. Finally, the stability of films was determined by x-ray photoelectron spectroscopy (XPS) measured before and after the photoelectrolysis. We show that material properties degrade with oxygen incorporation during the film growth. At the highest oxygen concentrations investigated, the material becomes amorphous and shows strong surface oxidation and a possible reaction with the electrolyte during the photoelectrolysis. Samples deposited at an oxygen concentration below 1%, exhibit the Ta 3 N 5 phase, a band gap below 2.0 eV, a high mobility-lifetime product, photoelectrolytic activity and a good relative stability against surface oxidation during the photoelectrolysis. [ABSTRACT FROM AUTHOR]

Electron transport across magnetic barriers is crucial in all magnetized plasmas. It governs not only the plasma parameters in the volume, but also the fluxes of charged particles towards the electrodes and walls. It is particularly important in high-power impulse magnetron sputtering (HiPIMS) reactors, influencing the quality of the deposited thin films, since this type of discharge is characterized by an increased ionization fraction of the sputtered material. Transport coefficients of electron clouds released both from the cathode and from several locations in the discharge volume are calculated for a HiPIMS discharge with pre-ionization operated in argon at 0.67 Pa and for very short pulses (few µs) using the a posteriori Monte Carlo simulation technique. For this type of discharge electron transport is characterized by strong temporal and spatial dependence. Both drift velocity and diffusion coefficient depend on the releasing position of the electron cloud. They exhibit minimum values at the centre of the race-track for the secondary electrons released from the cathode. The diffusion coefficient of the same electrons increases from 2 to 4 times when the cathode voltage is doubled, in the first 1.5 µs of the pulse. These parameters are discussed with respect to empirical Bohm diffusion. [ABSTRACT FROM AUTHOR]

Abstract: High Power Impulse Magnetron Sputtering (HiPIMS) with pre-ionization was successfully used for metal deposition at very low-pressures by feeding one cathode of a conventional dual magnetron deposition system, the other one being radio frequency (RF) supplied. By adjusting the pulse length and RF power, this hybrid RF/HiPIMS co-deposition process improved the deposition rate of niobium in a-NbSi films as well as the film homogeneity on 4in. Si substrates. Moreover, the films obtained by hybrid RF/HiPIMS showed good response for both superconducting critical temperature transition (Tc) and normal resistivity (R), which compared favorably to films obtained by Electron Beam Physical Vapor Deposition (EB PVD) and standard DC/RF MS-PVD co-sputtering deposition currently used in the micro-fabrication of cryogenic detector. [Copyright &y& Elsevier]

The oxygen dynamics in a reactive Ar/O2 high power impulse magnetron sputtering discharge has been studied using a new reactive ionization region model. The aim has been to identify the dominating physical and chemical reactions in the plasma and on the surfaces of the reactor affecting the oxygen plasma chemistry. We explore the temporal evolution of the density of the ground state oxygen molecule O2(Xµ ∑-g, the singlet metastable oxygen molecules O2 (aµΔg) and O2(bµ ∑-g, the oxygen atom in the ground state O(³P), the metastable oxygen atom O(¹D), the positive ions O+2 and O+, and the negative ion O-. We furthermore investigate the reaction rates for the gain and loss of these species. The density of atomic oxygen increases significantly as we move from the metal mode to the transition mode, and finally into the compound (poisoned) mode. The main gain rate responsible for the increase is sputtering of atomic oxygen from the oxidized target. Both in the poisoned mode and in the transition mode, sputtering makes up more than 80% of the total gain rate for atomic oxygen. We also investigate the possibility of depositing stoichiometric TiO2 in the transition mode. [ABSTRACT FROM AUTHOR]

Population densities of excited states of argon atoms in a high power impulse magnetron sputtering (HiPIMS) discharge are examined using a global discharge model and a collisional-radiative model. Here, the ionization region model (IRM) and the Orsay Boltzmann equation for electrons coupled with ionization and excited states kinetics (OBELIX) model are combined to obtain the population densities of the excited levels of the argon atom in a HiPIMS discharge. The IRM is a global plasma chemistry model based on particle and energy conservation of HiPIMS discharges. OBELIX is a collisional-radiative model where the electron energy distribution is calculated self-consistently from an isotropic Boltzmann equation. The collisional model constitutes 65 individual and effective excited levels of the argon atom. We demonstrate that the reduced population density of high-lying excited argon states scales with (p *) − 6 , where p * is the effective quantum number, indicating the presence of a multistep ladder-like excitation scheme, also called an excitation saturation. The reason for this is the dominance of electron impact processes in the population and de-population of high-lying argon states in combination with a negligible electron–ion recombination. [ABSTRACT FROM AUTHOR]

The magnetic field is a key feature that distinguishes magnetron sputtering from simple diode sputtering. It effectively increases the residence time of electrons close to the cathode surface and by that increases the energy efficiency of the discharge. This becomes apparent in high power impulse magnetron sputtering (HiPIMS) discharges, as small changes in the magnetic field can result in large variations in the discharge characteristics, notably the peak discharge current and/or the discharge voltage during a pulse. Here, we analyze the influence of the magnetic field on the electron density and temperature, how the discharge voltage is split between the cathode sheath and the ionization region, and the electron heating mechanism in a HiPIMS discharge. We relate the results to the energy efficiency of the discharge and discuss them in terms of the probability of target species ionization. The energy efficiency of the discharge is related to the fraction of pulse power absorbed by the electrons. Ohmic heating of electrons in the ionization region leads to higher energy efficiency than electron energization in the sheath. We find that the electron density and ionization probability of the sputtered species depend largely on the discharge current. The results suggest ways to adjust electron density and electron temperature using the discharge current and the magnetic field, respectively, and how they influence the ionization probability. [ABSTRACT FROM AUTHOR]

Two separate scientific communities are shown to have studied one common phenomenon, azimuthally rotating dense plasma structures, also called spokes, in pulsed-power E × B discharges, starting from quite different approaches. The first body of work is motivated by fundamental plasma science and concerns a phenomenon called the critical ionization velocity, CIV, while the other body of work is motivated by the applied plasma science of high power impulse magnetron sputtering (HiPIMS). Here we make use of this situation by applying experimental observations, and theoretical analysis, from the CIV literature to HiPIMS discharges. For a practical example, we take data from observed spokes in HiPIMS discharges and focus on their role in charged particle transport, and in electron energization. We also touch upon the closely related questions of how they channel the cross-B discharge current, how they maintain their internal potential structure and how they influence the energy spectrum of the ions? New particle-in-cell Monte Carlo collisional simulations that shed light on the azimuthal drift and expansion of the spokes are also presented. [ABSTRACT FROM AUTHOR]

Ta 3 N 5 belongs to the group of transition metal nitrides with the cation in a high oxidation state. These are typically challenging to synthesize owing to the low reactivity of nitrogen. This applies similarly to Ta 3 N 5 that crystallizes only in the presence of oxygen during synthesis. Typical preparation methods are ammonolysis of oxidized Ta or magnetron sputtering of a Ta target in an atmosphere of Ar, N 2 and O 2. However, the material typically obtained by either synthesis method is of varying degrees of crystallinity and the key parameter affecting the crystallinity remains elusive. In this study, we examine the role of oxygen for the crystallinity of Ta 3 N 5 samples by studying. Thin film samples prepared by magnetron sputtering reveal that oxygen is indeed the central driver for Ta 3 N 5 crystallinity. While little oxygen in the films yields the metallic δ-TaN phase, excess oxygen results in low crystallinity Ta 3 N 5 or Ta-O-N films. Ta 3 N 5 samples with a high degree of crystallinity are obtained by limiting the oxygen supply to the sample during the deposition. A comparison with other studies suggests a fundamental oxygen incorporation limit above which the crystallinity of Ta 3 N 5 is compromised. The most crystalline sample from this study contains 4.4 at.% of oxygen. It is grown onto a Si(100) substrate, covered with a 30 nm-thick metallic diffusion layer. For this sample, we observe Ta 3 N 5 grains between 80 and 120 nm in size. • The degree of crystallinity of Ta 3 N 5 thin films is determined by its oxygen content. • A fundamental limit of oxygen incorporation in Ta 3 N 5 crystalline films is suggested. • Ta interlayer between Si substrate and Ta 3 N 5 promotes crystallization. • Large Ta 3 N 5 grain sizes between 80 and 120 nm are obtained. [ABSTRACT FROM AUTHOR]

Abstract DLC based coatings are now widely used for many applications. Me-doped DLC (Me = Metal) are studied intensively in order to optimize properties such as low compressive stress, low friction coefficient and higher tribological performance. These Me-DLC coatings are often deposited by means of a PVD/PECVD hybrid process, but few studies attempt to describe the phenomena involved in these complex processes. In this work, DLC is obtained in the Ar + H 2 + C 2 H 2 mixture by RF-PECVD and the metal, which is in-situ, is added by magnetron sputtering of pure metal targets. Plasma is investigated by means of Optical Emission Spectroscopy and the main phenomena between the target and the substrate are described. The results show that atomic aluminium emission line intensity decreases continuously, moving away from the Al target whereas Ti and Nb lines are weak near electrodes and reach a maximum in the centre of the target-substrates gap. This difference is due to the low mass of Al as well as its rapid thermalisation. Addition of a metallic element (Al, Ti or Nb) in DLC seems to lead to a disorder within the carbon matrix. Carbides precipitate in Ti-DLC and Nb-DLC for high metal contents. The addition of Ti or Nb in the DLC matrix leads to open morphologies associated with high surface roughness. Like the non-doped DLC, Al-DLC coatings have a compact morphology and low surface roughness. Highlights • Magnetron sputtering and PECVD (Plasma Enhanced Chemical Vapor Deposition) are combined to deposit Me-DLC films (Me = metal). • Al emission line intensity decreases continuously from the Al target to the substrates because of its fast thermalization • Ti or Nb density is low near the electrodes' sheaths and is maximum in the center of the target-substrates gap • Additional RF power on substrates favours ionization and, as a consequence, the metal sputtering. • -Low metal contents in DLC lead to a disorder within the carbon matrix • -Above a metal content threshold, when the metal content further increases, order is favoured • -Carbides precipitate in Ti-DLC and Nb-DLC for high Ti or Nb contents. • -Addition of Ti or Nb in the DLC matrix leads to open morphologies and high surface roughness. • -Al-DLC coatings keep a compact morphology and low surface roughness. [ABSTRACT FROM AUTHOR]

Ta 3 N 5 is a promising candidate for a variety of applications, most notably as a photoactive material for solar water splitting. It is typically synthesized in a two-step process in which oxidized tantalum is annealed in NH 3 . Magnetron sputtering is an alternative synthesis method that is little explored to date, as first tries resulted in a small degree of crystallinity of the samples. In this paper, we report on the addition of an axial magnetic field to the conventional magnetron configuration which guides ionic species from the negative glow onto the growing film of Ta 3 N 5 . This ion-assisted growth is shown to result in a high degree of crystallinity, i.e. amorphous content, which is typical for conventionally sputtered films, is largely suppressed. The surface of such prepared films is nanostructured by a dense population of grains. [ABSTRACT FROM AUTHOR]

We use an ionization region model to explore the ionization processes in the high power impulse magnetron sputtering (HiPIMS) discharge in argon with a titanium target. In conventional dc magnetron sputtering (dcMS), stepwise ionization can be an important route for ionization of the argon gas. However, in the HiPIMS discharge stepwise ionization is found to be negligible during the breakdown phase of the HiPIMS pulse and becomes significant (but never dominating) only later in the pulse. For the sputtered species, Penning ionization can be a significant ionization mechanism in the dcMS discharges, while in the HiPIMS discharge Penning ionization is always negligible as compared to electron impact ionization. The main reasons for these differences are a higher plasma density in the HiPIMS discharge, and a higher electron temperature. Furthermore, we explore the ionization fraction and the ionized flux fraction of the sputtered vapor and compare with recent experimental work. [ABSTRACT FROM AUTHOR]

The lower deposition rate for high power impulse magnetron sputtering (HiPIMS) compared with direct current magnetron sputtering for the same average power is often reported as a drawback. The often invoked reason is back-attraction of ionized sputtered material to the target due to a substantial negative potential profile, sometimes called an extended presheath, from the location of ionization toward the cathode. Recent studies in HiPIMS devices, using floating-emitting and swept-Langmuir probes, show that such extended potential profiles do exist, and that the electric fields Ez directed toward the target can be strong enough to seriously reduce ion transport to the substrate. However, they also show that the potential drops involved can vary by up to an order of magnitude from case to case. There is a clear need to understand the underlying mechanisms and identify the key discharge variables that can be used for minimizing the back-attraction. We here present a combined theoretical and experimental analysis of the problem of electric fields Ez in the ionization region part of HiPIMS discharges, and their effect on the transport of ionized sputtered material. In particular, we have investigated the possibility of a 'sweet spot' in parameter space in which the back-attraction of ionized sputtered material is low. It is concluded that a sweet spot might possibly exist for some carefully optimized discharges, but probably in a rather narrow window of parameters. As a measure of how far a discharge is from such a window, a Townsend product ΠTownsend is proposed. A parametric analysis of ΠTownsend shows that the search for a sweet spot is complicated by the fact that contradictory demands appear for several of the externally controllable parameters such as high/low working gas pressure, short/long pulse length, high/low pulse power and high/low magnetic field strength. [ABSTRACT FROM AUTHOR]

This work presents the changes induced at the nanometer scale by the nitrogen incorporation into amorphous carbon nitride a-CNx films deposited by radio-frequency (r.f.) magnetron sputtering. High N content films (20 to 33 at.%) have been analysed by electron energy loss spectroscopy (EELS) and by near edge X-ray absorption fine structure (NEXAFS). EELS spectra at C K and N K edges roughly indicate the increase of the corresponding π* features, associated to N sp2 and C sp2 bonds, vs. the N content. Fine structures of the π* region of the N K edge spectra show at least three contributions, with different evolution depending on the N content. The discrepancies between the results recorded by these two techniques are only outward and they can be explained by the influence of the plasma parameters (pressure lying from 0.5 to 13 Pa and various Ar/N2 mixtures) on the deposition process. All the results are in good agreement with the XPS analysis of the N 1s data. [Copyright &y& Elsevier]