Electrical and piezoresistive properties of ion beam deposited DLC films

In present study diamond like carbon (DLC) films were deposited by closed drift ion source from the acetylene gas. The electrical and piezoresistive properties of ion beam synthesized DLC films were investigated. Diode-like current–voltage characteristics were observed both for DLC/nSi and DLC/pSi heterostructures. This fact was explained by high density of the irradiation-induced defects at the DLC/Si interface. Ohmic conductivity was observed for DLC/nSi heterostructure and metal/DLC/metal structure at low electric fields. At higher electric fields forward current transport was explained by Schottky emission and Poole–Frenkel emission for the DLC/nSi heterostructures and by Schottky emission and/or space charge limited currents for the DLC/pSi heterostructures. Strong dependence of the diamond like carbon film resistivity on temperature has been observed. Variable range hopping current transport mechanism at low electric field was revealed. Diamond like carbon piezoresistive elements with a gauge factor in 12–19 range were fabricated.     

Influence of ion bombardment on residual stresses in diamond-like carbon films

The dependence of internal residual stresses in thin diamond-like carbon films grown by the PECVD technique on the most important growth parameters such as the power of the exciting RF discharge and the substrate bias potential is considered. The results have shown that the mechanical stresses in films reach the uppermost value of 1.9 GPa at the smallest values of power and potential. The stress decreases with the growth of both parameters and has only a slight dependence on the film thickness in the range 0.1–1 μm. The bombardment of the obtained films by argon ions with energy of 300 keV and phosphorus ions with energy of 200 keV has resulted in the reduction of compressive stress with the ion dose growth down to its inversion. AFM study of the bombarded films has revealed significant changes in their surface morphology.     

Synthesis of DLC films by PLD from liquid target and dependence of film properties on the synthesis conditions

DLC (Diamond-like carbon films) were prepared by pulsed laser ablation of a liquid target at substrate temperatures from 18 to 600°C using 248 nm KrF excimer laser. The sp³ hybridization state carbon formation was additionally promoted by gaseous H₂O₂ flow through the reaction chamber and substrate excitation by the same laser beam. Deposited DLC films were characterised by Raman scattering spectroscopy and atomic force microscopy (AFM). Comparative AFM and Raman study shows that the increase in the content of sp³ type bonding in DLC is in correlation with the increase of the surface roughness of the samples prepared.     

Diamond/graphite content and biocompatibility of DLC films fabricated by PLD

Biocompatibility and physicochemical properties of diamond-like carbon (DLC) thin layers prepared by pulsed laser deposition method were studied. The films of high and low diamond/graphite content were prepared by changing the laser energy density on the graphite target from 4 to 11 J cm⁻². The bonds and surface properties as roughness, atomic force microscopy topology, contact angle parameters, and zeta potential were measured. The cell adhesion/proliferation on DLC layers was tested using normal human fibroblasts and keratinocytes.     

Increasing adhesion of metallic films to silicon by ion bombardment during growth

Intense ion bombardment at the initial stage of film growth is used to increase the adhesion of nickel and vanadium films to silicon. The films are deposited by rf magnetron sputtering when a bias potential is applied to a substrate. The adhesion of metallic films to silicon is substantially increased due to active mixing of the contacting materials and the formation of a transition layer with a concentration gradient. A correlation between the adhesion of the films and their crystalline state is revealed.     

Effect of ion bombardment on the phase composition and mechanical properties of diamond-like carbon films

It is established that the addition of hydrogen to methane in the reaction mixture upon the fabrication of diamond-like carbon films via the plasma-enhanced chemical vapor deposition method decreases residual stresses in the obtained films and significantly reduces their growth rate. The films were investigated via atomic force microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. Irradiation of the prepared films with P⁺ and PF ₄ ⁺ ions results in strong sample swelling with increasing dose, as well as in a decrease in the compressive stress up to transition to tensile one reaching saturation. Moreover, the fraction of sp ³ bonds increases with increasing ion dose while the fraction of sp ² bonds decreases symmetrically with the processes proceeding faster upon irradiation with molecular ions. Qualitative mechanisms explaining the experimental results are proposed.     

Role of growth temperature on nanostructure and field emission properties of PLD thin carbon films

Thin carbon films have been deposited in vacuum (∼10⁻⁴ Pa) on Si substrates by pulsed laser ablation of a graphite target using a Nd:YAG laser operating in the near infrared region (λ=1064 nm). The samples have been deposited at different substrate temperatures (T _sub) ranging from room temperature (RT) to 800°C. X-ray diffraction analysis established the progressive formation of nanosized graphene structures as T _sub increased. In fact, film structure evolves from almost amorphous to nanostructured phase characterized by graphene layers oriented perpendicularly to the film plane. The film density, evaluated by X-ray reflectivity measurements, is strongly affected by T _sub. At RT the film density is similar to the graphite one, while it decreases at higher T _sub. The electrical properties of the samples have been characterized by field emission measurements. The parameters describing the emitter properties (threshold field E _th and field enhancement factor β) have been evaluated using variable anode-to-cathode distance method. Samples deposited at low T _sub have shown the best emission properties, presenting lower E _th and larger β values than those deposited at higher T _sub. This is mainly attributed to the sensible density variation, which is in competition with the slighter augment of mean nanoparticle size.     

Field emission properties of DLC and phosphorus-doped DLC films prepared by electrochemical deposition process

Field emission behavior of diamond-like carbon (DLC) and phosphorus-doped DLC (p-DLC) films prepared by electrochemical deposition process was comparatively investigated. It was shown phosphorus incorporation in the DLC film could lower the turn on field from 12 to 9.5 V/μm and increase the current density from 12.6 to 45.7 μA/mm² under high electric field. And better field emission performance of p-DLC films would be mainly attributed to the influence of the surface morphology and the changes of microstructure due to the phosphorus incorporation.     

Resistance changes in thin metallic films under ion bombardment

Resistance changes in thin films of copper, aluminium and bismuth have been studied under the bombardment of nitrogen, carbon and argon ions. Variations in resistance with implantation dose have been observed upto doses of ∼ 3 × 10¹⁷ ions/cm² for ion energies in the range 40 to 120 keV. The results are discussed in terms of desorption of gases from the film and a composite action of sputter removal of the film and its structural changes upon ion bombardment. A simple theoretical model is discussed which can qualitatively explain the experimental observations.     

DLC coating of textile blood vessels using PLD

Textile blood vessels with a length of 30 cm were coated with amorphous diamond-like carbon (DLC) layers with thicknesses up to 200 nm. The layers were created by pulsed laser deposition in vacuum or argon ambient. The percentage of sp³ carbon was evaluated using X-ray photoelectron spectroscopy, X-ray excited Auger electron spectroscopy and Raman spectroscopy. Depending on the deposition conditions the sp³ content varied from ∼40% to 60%. The adhesion of the DLC layers to the textile vessels was checked. The preliminary biocompatibility results from in vivo tests with sheep are also given.     

Morphological and optical properties of silicon thin films by PLD

Silicon thin films have been prepared on sapphire substrates by pulsed laser deposition (PLD) technique. The films were deposited in vacuum from a silicon target at a base pressure of 10⁻⁶ mbar in the temperature range from 400 to 800 °C. A Q-switched Nd:YAG laser (1064 nm, 5 ns duration, 10 Hz) at a constant energy density of 2 J × cm⁻² has been used. The influence of the substrate temperature on the structural, morphological and optical properties of the Si thin films was investigated.Spectral ellipsometry and atomic force microscopy (AFM) were used to study the thickness and the surface roughness of the deposited films. Surface roughness values measured by AFM and ellipsometry show the same tendency of increasing roughness with increased deposition temperature.     

The deformation of thin aluminium films under helium ion bombardment

ABSTRACT : Aluminium films 2000 Å thick have been bombarded with various flu-ences of 5 keV He⁺ ions and the resulting bubble distribution observed by Transmission Electron Microscopy. Scanning Electron Microscopy of the films indicates an areal increase in the unsupported regions of the films, which, if it results from three dimensional swelling of the films, is two orders of magnitude larger than would be calculated from the total bubble volume as observed by T. E. M. In addition features closely resembling blisters observed in bulk materials, form on the supported regions of the film. These blisters like features are thought to result from lateral stresses in the films.     

Effect of low-energy ion bombardment on the crystal structure and superconductivity of niobium films

The effect of ion bombardment on the growth of Nb films and their crystal structure is investigated. Epitaxial niobium films with the (001) orientation are grown on (01i2) Al₂O₃ substrates heated to 600°C and biased at ?20 V. Niobium films with pronounced axial texture in the [110] direction are grown on water-cooled Sitall (devitrified glass such as Pyroceram) substrates. In Nb films biased at ?50 V, which are in the superconducting state, the motion of individual magnetic vortices is observed with a magnetooptic indicator.     

Simulation of fractal cluster growth on a substrate under ion bombardment

Ion-stimulated growth of a fractal cluster on the substrate is simulated in regimes that are intermediate between diffusion- and reaction-limited aggregations. It is shown that the fractal dimension of a growing cluster and the type of formed epitaxial structure are due to the sequence of ion-stimulated processes with different fractal dimensions. The algorithm for revealing the ion stimulation mechanism of processes based on fractal laws is proposed.     

Stability of subatomic carbon films on metal surfaces against low-energy gas ion bombardment

The earlier developed original experimental technique for measuring and analyzing the parameters of low-frequency fluctuations of the field-emission current in metal film systems is used to measure the sputtering yield Y _f of carbon films (with a coverage Θ ranging from 1 to 4) applied on Fe, Nb, Ta, and U substrates. The value of Y _f is calculated by an expression derived within a theoretical model developed. The sputtering ratios were measured for the case when the carbon films are sputtered by H⁺ and He⁺ ions with an energy E _i between 2 and 10 keV. With Θ fixed, the energy dependences of Y _f are obtained for each of the ions. In addition, for each of the ions, the Θ dependences of Y _f are found for several values of E _i. In all the cases, the measured values of Y _f far exceed those for pure carbon. With another original technique that combines field-ion microscopy (FIM) and precise measurement of current and/or luminous properties of local regions in FIM images, the energy thresholds E _th of sputtering carbon films applied on the metal surfaces are found. The energy distributions of Y _f in the near-threshold energy range for various Θ are obtained.     

Atomic layer growth on Al(111) by ion bombardment

We study equilibrium crystal shapes (ECS) near facet ridge end points (FRE) by means of a numerical study of a body-centered solid-on-solid model on a square lattice with an enhanced uniaxial interaction range. This tests the stability of the so-called stochastic FRE point where the model maps exactly onto one dimensional Kardar-Parisi-Zhang-type growth and where the local ECS is simple. We find that the generic shapes are more complex. They contain first-order faceted to rough boundaries terminating in Pokrovsky-Talapov-type end points, and first-order ridges inside the rounded part of the ECS where two rough surface orientations coexist.     

The effect of low-energy ion bombardment on the density and crystal structure of thin films

The effect of low-energy ion bombardment on the growth and properties of thin films deposited by rf plasma sputtering at low substrate temperatures is studied. The dependences of the film thickness, density, crystal structure, and conductivity on the bias voltage applied to the substrate are obtained. At biases ranging from 0 to −30 V, nickel films are polycrystalline; at higher biases, they exhibit axial (111) texture. At the bias −60 V, the density of the Ni films is close to that of the bulk metal and the crystal structure of the films is the most ordered. With a further increase in the bias, the density of the films drops because of gas (argon and residual gases) atoms incorporated into the films. The same bias dependence of the density is observed for amorphous films of binary alloys of d and f metals. In this case, the films deposited at the substrate bias −40 V have the highest density.     

Submonolayer films on a Si(111) surface under low-energy ion bombardment

The kinetics of deposition for monomolecular submonolayer films on a Si(111) surface is studied via low-energy electron diffraction with measurements of the intensities of diffraction reflection and the elastic background. The degree of structural perfection in growing films is estimated for alkali-metal silicides and silicon from low-energy beams. The optimum energy and dose intervals of silicide film formation are determined.     

Influence of ion bombardment on the photoluminescence response of embedded CdS nanoparticles

Semiconductor nanoparticles (CdS) were fabricated by an inexpensive chemical route using polyvinyl alcohol (PVA) as the dielectric host matrix. Nano-CdS in PVA were subjected to ion irradiation (using oxygen, chlorine and gold) in the medium energy range (80–100 MeV) and under fluence variation of 10¹¹–10¹³ ions/cm². The nature of light emission was found to be drastically different in each of the three cases. Photoluminescence spectra of oxygen irradiated samples exhibit band edge emission (2.8 eV) as well as trap related emission (1.76 eV) whereas band edge emission is found to be bleached out for chlorine ion irradiated nano-CdS. The intense broad PL peaks, noticeable in the case of gold ion irradiated samples suggest superposition of the two peaks — namely, band edge emission and trap related emission. Furthermore, in the case of gold ion irradiated nano-CdS, energy shift in the PL spectra reveals variation in size distribution caused by the extra pressure effect of heavy gold ion beams. The mechanism of such a difference as a result of ion irradiation-type and ion-fluence is discussed in detail.