Ambient Gas Atoms Research Articles

Effect of ambient gaseous environment on the properties of amorphous carbon thin films

Amorphous carbon films have been deposited by filtered cathodic jet carbon arc technique under different gaseous environments. Scanning electron microscope and atomic force microscope studies have been performed on the deposited films for the surface morphological studies. The morphology of the deposited film changes with the change in gas environment. X-ray photoelectron spectroscopic (XPS) and Raman studies have been carried out on the deposited samples for the evaluation of the chemical bonding of carbon atoms with the ambient gas atoms. The sp 3 and sp 2 contents have been evaluated from the XPS studies and found to be dependent on the gaseous environment. The film deposited under hydrogen environment has the highest value of the sp 3 content (54.6 at.%) whereas the film deposited under helium environment has the lowest value of sp 3 content (37 at.%). For the evaluation of the electrical and mechanical properties of the deposited films, the electrical conductivity and nanoindentation measurements have been performed on the deposited films. It has been observed that the film deposited under helium environment has the highest electrical conductivity and the lowest hardness (∼15 GPa) value whereas film deposited under hydrogen environment has the highest hardness (∼21 GPa) and the lowest conductivity.

Investigation of plume expansion dynamics and estimation of ablation parameters of laser ablated Fe plasma

A systematic investigation of expansion dynamics of plasma plume, generated by a Q-switched Nd–YAG laser at 532 nm with a fluence of ∼30 J cm−2 on Fe target, was done for three different ambient pressures of 2 × 10−4, 2 and 20 mbar of Ar using fast gated intensified charge coupled device imaging. The experimental observations are then fitted with the snow plow and shock wave models to estimate various laser ablation parameters such as laser energy deposited to the ablated material, mass ablated per laser shot, laser spot size and the background gas density. The estimated laser energy deposited to the ablated material is about 70% of the input energy. The density of ambient gas atoms, having interaction with the ablated species, is found to vary from 12% to 50% at 2 mbar and 24% to 80% at 20 mbar. Scanning electron microscope images confirm the effect of ambient gas pressure on the topography of the deposited thin film and size of the nanoparticles which is qualitatively explained on the basis of the changing kinetic energy of impinging plasma plume species due to change in ambient gas pressure.

Pulsed-ion-beam nitriding and smoothing of titanium surface in a vacuum

Both nitriding and smoothing of titanium have been achieved under irradiation of intense pulsed ion beam in a vacuum of 2×10−2Pa. Applying a screening method, we find that medium ion-beam intensity and multi-shot irradiation are effective for the processing, where repetitive surface melting with limited ablation favored Ti nitride formation as well as surface smoothing. The present results demonstrate that ambient gas atoms∕molecules can be efficiently incorporated in metal matrices to form compounds under the ion-beam irradiation. The finding is of great significance for extending application scope of the ion-beam technique in materials research and processing, combined with the recent success in introducing ambient gas into the processing chamber.

Pulsed-laser deposition of carbon: from DLC to cluster-assembled films

Carbon films have been synthesized on (100) Si substrates at room temperature by KrF Excimer Pulsed Laser Deposition (PLD) of highly oriented pyrolitic graphite. Changing laser power density (from 0.5 to 19 MW mm −2) and ambient conditions (vacuum; molecular nitrogen at 1 Pa; helium, from 0.6 Pa to 2 kPa), a range of structures and microstructures, from DLC to nanosized cluster assembled (CA) films were obtained. The microstructure and morphology of the films were studied by Scanning Electron Microscopy. The surface of all films deposited in vacuum and in nitrogen appears uniform, flat, featureless. In CA films, the nature-pressure of ambient gas and the energy deposited by each laser pulse determine threshold fluences of increasing value. These correspond to the sequence of dense columns, followed by nodule-like morphology and by an open dendritic structure. Atomic Force Microscopy on several CA films allowed to estimate the size distribution and relative abundancies of aggregates of carbon clusters. Film morphology is correlated with the ballistic mechanisms that occur when the plasma plume constituents interact with ambient gas atoms. For all films, local carbon coordination was studied by visible Raman spectroscopy. UV Raman spectra were collected for vacuum- and nitrogen-deposited films. The films are hydrogen-free, as shown by Fourier Transform Infra-Red spectroscopy, and result invariably structurally disordered. Vacuum-deposited films undergo a transition from mainly disordered graphitic to up to 80% tetrahedral amorphous carbon (ta-C) above a threshold power density. Transmission Electron Energy Loss spectra on selected samples confirmed ta-C formation. The same transition is absent in films deposited in N 2 atmosphere. All CA films are structurally disordered, sp 2 coordinated, and belong to the family of carbon nanoglasses. The mechanisms underlying the graphite to ta-C transition are discussed with reference to the different atmospheres.

Atomic resonance behavior in laser-induced Rb atom desorption—the effect of ambient gas atoms and molecules

Abstract We report an observation that in a typical optical pumping cell containing Rb metal and 150 Torr N 2 gas, a cw laser beam of a few tens of mW and beam size 5.5 mm 2 can desorb Rb films on cell surfaces when the laser is tuned to the Rb D 1 line. The frequency dependence of the Rb desorption rate displays atomic resonance behavior. The desorption is suggested to be mediated by the ambient gas atoms (Rb) and molecules (N 2 ). The phenomenon was used to provide evidence for the existence of thin Rb films on seemingly clear cell surfaces.

Structural reordering and electrical activation of ion-implanted GaAs and InP due to laser annealing in a controlled atmosphere

The effects of the ambient atmosphere in the annealing chamber on the electrical and structural characteristics of Zn-implanted III-V compound semiconductors, processed by low-power pulsed-laser annealing are presented. The samples were analyzed using several complementary experimental techniques: Reflection high-energy electron diffraction, Rutherford backscattering spectroscopy, Raman spectroscopy, secondary ion mass spectroscopy, and electrical measurements. During the laser beam irradiation in the presence of gas inlet into the annealing chamber the ambient gas atoms diffused well into the target changing the stoichiometry and the electrical parameters. Redistribution of the implanted impurity was also observed. By varying the type of gas used and its pressure, it was possible to achieve electrical activation of up to 80%. It seems all structure and electrical parameters achieve their best values at the same ambient atmosphere and density of the deposited laser power P, e.g., 1.5 atm of ${\mathrm{N}}_{2}$ and $P=6.5 {\mathrm{M}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ for InP.

Dynamics of laser ablation plume penetration through low pressure background gases

The dynamics of laser-ablated yttrium plume propagation through background argon have been investigated with fast time- and spatially-resolved plasma diagnostics in order to characterize a general phenomenon believed to be important to film growth by pulsed laser deposition (PLD). During expansion into low-pressure background gases, the ion flux in the laser ablation plasma plume is observed to split into fast and slow components over a limited range of distances including those typically utilized for PLD. Optical absorption and emission spectroscopy are employed to simultaneously identify populations of both excited and ground states of Y and Y+. These are correlated with intensified-CCD (ICCD) photographs of visible plume luminescence and ion fluxes recorded with fast ion probes. These measurements indicate that plume-splitting in background gases is consistent with scattering of target constituents by ambient gas atoms. The momentum transfer from these collisions produces a transition from the initial, ‘‘vacuum’’ velocity distribution into a velocity distribution which is significantly slowed in accordance with shock or drag propagation models.

Enhancement of the vacuum ultraviolet emission from excimer laser-generated plasmas by ambient gas atoms

We have measured the axial (z) and radial (x) distribution of the vacuum ultraviolet emission from excimer laser generated aluminum plasmas in vacuum and in 300 mTorr of argon. The ratio of the radiated line intensities (emission in a gas versus vacuum) on the z axis (i.e., x=0) increased exponentially with distance from the target surface for plasmas generated in a 300 mTorr argon ambient. The absolute line intensities increased linearly with the argon pressure and approximately linearly with the ambient gas atomic cross section when other rare gases were substituted. The line intensity radial distribution was broader for plasmas in argon than in vacuum and the magnitude of the effect increased monotonically with z. The spectral data obtained from plasmas in a gas ambient are discussed in terms of the diffusion of plasma electrons in an ionized gas.

The adiabatic mechanism of the collisional broadening of Rydberg states via a loosely bound resonance state of ambient gas atoms

The adiabatical mechanism of the oscillations of the broadened cross sections produced by collisions of Rydberg alkali atoms with ground-state atoms of an ambient alkali gas is presented. The dependence of the oscillations of the cross sections on the principal quantum number n arises due to the oscillations of the mixing parameter for the pseudocrossing between the 3P ion state energy curve of the ambient gas atom and the Rydberg state dependent on n. The latter oscillations are generated by the oscillations of the Rydberg electron wavefunction in the coordinate space. The numerical results obtained are in good agreement with the experimental data for different colliding pairs. The matching of the adiabatical picture and the conventional impulse approximation is discussed.

Physical aspects of ion beam assisted deposition

Progress toward obtaining an understanding of the physical processes that are active during ion beam assisted deposition (IBAD) is reviewed. A model is presented that includes the effects of sputtering, reflection of ions, multiple species beam. charge exchange neutralization, and incorporation of ambient gas atoms. Good agreement is found with data for the composition of silicon nitride. boron nitride and titanium nitride films as a function of the arrival ratio of nitrogen ions to evaporant atoms. Calculations based upon collision cascade simulation are reviewed which predict the energy dependence of the critical arrival ratio to achieve low intrinsic stress in deposited films. Factors which influence the choice of IBAD system geometry, the choice of ion beam energy, and applications of the films are also discussed.