Optical properties of high-density amorphous carbon films grown by nanosecond and femtosecond pulsed laser ablation

High-density tetrahedral amorphous carbon (ta-C) films have been prepared by nanosecond (17 ns) and femtosecond (150 fs) pulsed laser deposition (PLD) using fluences and repetition rates compatible with fast and homogeneous growth over large areas. Their optical properties were measured by spectroscopic ellipsometry from 1.0 to 4.7 eV and analyzed using a multi-layer Tauc-Lorentz model. In spite of very different ablation mechanisms, both PLD techniques produce high density bulk layers as revealed by a refractive index (n at 2 eV) of 2.7±0.1 for both fs-PLD and ns-PLD. Films are covered by a few nm-thick sp²-rich top layer which is denser and thicker in femtosecond PLD as compared to nanosecond PLD. The respective roles of low and high energies in the kinetic energy distribution of the incident carbon species are discussed in terms of densification and sp³↦sp² configurational relaxation predicted by the subplantation growth model. The significantly higher optical gap found in the ns-PLD films is attributed to the larger contribution of energetic species with kinetic energies E_c≥200 eV, as revealed by time-of-flight optical studies. PACS 81.40.Tv; 81.05.Uw; 81.15.Fg     

Diamond-like-carbon films produced by magnetically guided pulsed laser deposition

We have compared the quality of carbon films deposited with magnetically guided pulsed laser deposition (MGPLD) and conventional pulsed laser deposition (PLD). In MGPLD, a curved magnetic field is used to guide the plasma but not the neutral species to the substrate to deposit the films while, in conventional PLD, the film is deposited with a mixture of ions, neutral species and clusters. A KrF laser pulse (248 nm) was focused to intensities of 10 GW/cm² on a carbon source target and a magnetic field strength of 0.3 T was used to steer the plasma around a curved arc to the deposition substrate. Electron energy loss spectroscopy was used in order to measure the fraction of sp³ bonding in the films produced. It is shown that the sp³ fraction, and hence the diamond-like character of the films, increased when deposited only with the pure ion component by MGPLD compared with films produced by the conventional PLD technique. The dependence of film quality on the laser intensity is also discussed. Received: 7 December 2000 / Accepted: 20 August 2001 / Published online: 2 October 2001     

Visible photoluminescence from silicon-incorporated diamond like carbon films synthesized via direct current PECVD technique

Silicon-incorporated diamond like carbon (Si-DLC) films were deposited via DC plasma-enhanced chemical vapor deposition (PECVD) on glass and alumina substrates at a substrate temperature of 473 K. The precursor gas used was acetylene and for silicon incorporation tetraethyl orthosilicate dissolved in methanol was used. Silicon atomic percentage in the films was varied from 0% to 19.3% as measured from energy dispersive X-ray analysis. Fourier transformed infrared spectroscopy studies depicted the presence of Si-C, Si-H and Si-H₂ bonding within the films. The binding energies of C 1s, Si 2s and Si 2p were determined from X-ray photoelectron spectroscopic studies. UV-vis-NIR spectroscopic studies were used to determine the optical gaps as well as the Urbach parameters of the samples. Room temperature photoluminescence study showed a broad peak centered at around 467 nm. Also the peak intensity was found to increase monotonically with Si percentages. The results are discussed in terms of the electronic structure of a-C:H, the doping induced defect states and the enhanced carbon dangling bonds via the formation of more sp³ hybridized carbon network.     

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.     

Nano-graphene structures deposited by N-IR pulsed laser ablation of graphite on Si

Thin nano-structured carbon films have been deposited in vacuum by pulsed laser ablation, from a rotating polycrystalline graphite target, on Si 〈1 0 0〉 substrates, kept at temperatures ranging from RT to 800 °C. The laser ablation was performed by a Nd:YAG laser, operating in the near IR (λ = 1064 nm).X-ray diffraction analysis, performed at grazing incidence angle, both in-plane (ip-gid) and out-of-plane (op-gid), has shown the growth of oriented nano-sized graphene particles, characterised by high inter-planar stacking distance (d_? ∼ 0.39 nm), compared to graphite. The film structure and texturing are strongly related both to laser wavelength and substrate temperature: the low energy associated to the IR laser radiation (1.17 eV) generates activated carbon species of large dimensions that, also at low T (∼400 °C), easy evolve toward more stable sp² aromatic bonds, in the plume direction. Increasing temperature the nano-structure formation increases, causing a further aggregation of aromatic planes, voids formation, and a related density (by X-ray reflectivity) drop to very low values. SEM and STM show for these samples a strongly increased macroscopic roughness. The whole process, mainly at higher temperatures, is characterised by a fast kinetic mode, far from equilibrium and without any structural or spatial rearrangement.     

Thermal conductive performance of deposited amorphous carbon materials by molecular dynamics simulation

A series of diamond-like carbon (DLC) films with different microstructure were prepared by depositing carbon atoms on diamond surface with incident energy ranging from 1 to 100 eV. The thermal conductivity of the deposited films and the Kapitza resistance between the film and the diamond substrate were investigated. Results show that the average density, the average fraction of sp³ bonding and the thermal conductivity of the DLC films increase first, reaching a maximum around 20–40 eV before decreasing, while the Kapitza resistance decreases gradually with increased deposition energy. The analysis suggests that the thermal resistance of the interface layer is in the order of 10^?10 m²K/W, which is not ignorable when measuring the thermal conductivity of the deposited film especially when the thickness of the DLC film is not large enough. The fraction of sp³ bonding in the DLC film decreases gradually normal to the diamond surface. However, the thermal conductivity of the film in normal direction is not affected obviously by this kind of structural variation but depends linearly on the average fraction of sp³ bonding in the entire film. The dependence of the thermal conductivity on the fraction of sp³ bonding was analysed by the phonon theory.     

Raman light scattering in hydrogenated metal-carbon composite films

We have used Raman scattering, elemental analysis, and structural analysis to study the effect of the concentration of incorporated metals (Cu, Ni) on the ratio of sp²/sp³ carbon bonds in composite hydrogen-containing films a-C:H/Cu and a-C:H/Ni, formed by combining plasma-enhanced vapor phase deposition of carbon and sputtering of the metal, using a mixture of argon and methane or acetylene gases. We have shown that formation of a nanosized structure of metallic crystallites (2–5 nm) in the composite films leads to a significant increase in the fraction of disordered sp³-bonded carbon clusters and a decrease in the linear dimensions of the graphite-like carbon clusters. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 344–348, May–June, 2006.     

Silicon carbide synthesis with energy pulses

Polycrystalline SiC layers were synthesized through nanosecond pulse heating of thin carbon films deposited on single-crystalline silicon wafers. The samples were submitted to electron beam irradiation (25 keV, 50 ns) at various current densities in vacuum (10^–4mbar) and to XeCl excimer laser pulses (308 nm, 15ns) in air. Rutherford backscattering spectrometry (RBS) showed that in the e-beam annealed samples mixing of the elements at the interface starts at current densities of about 1200 A/cm². The mixed layer thickness increases almost linearly with current density. From the RBS spectra a composition of the intermixed layers close to the SiC compound was deduced. Transmission electron microscopy (TEM) and electron diffraction studies clearly evidenced the formation of SiC polycrystals. Using the XeCl excimer laser, intermixing of the deposited C film with the Si substrate was observed after a single 0.3 J/cm² pulse. Further analysis evidenced the formation of SiC nanocrystals, embedded in a diamond film.     

Properties of diamondlike films obtained in a barrier discharge at atmospheric pressure

Diamondlike films are synthesized from gaseous hydrocarbons in a barrier discharge at atmospheric pressure. The films were investigated using transmission electron microscopy, electron diffraction, and infrared spectroscopy. A technique for determining the quantitative characteristics of the films (hydrogen content, ratio of different types of carbon-carbon bonds and hydrocarbon groups) using standard samples is described. The highest-quality films were obtained from methane (ratio of hydrogen to carbon atoms H/C=1.04, fraction of diamondlike to graphitelike bonds sp ³: sp ²=100%: 0%) and from a mixture of acetylene and hydrogen in the ratio 1:19 (H/C=0.73, sp ³: sp ²=68%: 32%). Zh. Tekh. Fiz. 67, 100–104 (August 1997)     

Laser effects on graphene oxide irradiated in high vacuum

Graphene oxide (GO) foils were irradiated by using different fluences of an infrared nanosecond pulsed laser and characterized before and after the laser irradiation. The produced laser ablation was investigated as well as the generated plasma. Optical and AFM microscopies, mass quadrupole spectrometry, Rutherford backscattering analysis and X-ray photoelectron spectroscopy were used to analyze the irradiated GO foils. Results demonstrated that the GO loses oxygen with the laser irradiation becoming richer in sp²-hybridized carbon content.     

Raman spectroscopy of a-C:H:N films deposited using ECR-CVD with mixed gas

Ultraviolet (UV) and visible Raman spectroscopy were used to study a-C:H:N films deposited using ECR-CVD with a mixed gas of CH₄ and N₂. Small percentage of nitrogen from 0 to 15% is selected. Raman spectra show that CN bonds can be directly observed at 2220 cm⁻¹ from the spectra of visible and UV Raman. UV Raman enhances the sp¹ CN peak than visible Raman. In addition, the UV Raman spectra can reveal the presence of the sp³ sites. For a direct correlation of the Raman parameter with the N content, we introduced the G peak dispersion by combining the visible and UV Raman. The G peak dispersion is directly relative to the disorder of the sp² sites. It shows the a-C:H:N films with higher N content will induce more ordered sp² sites. In addition, upper shift of T position at 244 nm excitation with the high N content shows the increment of sp² fraction of films. That means the films with high N content will become soft and contain less internal stress. Hardness test of films also confirmed that more N content is with less hardness.     

Photon irradiation-induced structural and interfacial phenomena in pure and alio-valently doped zirconia thin films

We report on the effect of ultra-violet (UV) irradiation on structural and interfacial phenomena in pure and doped zirconia thin film grown by physical vapour deposition. Interfacial layer formation by substrate oxidation and resultant densification of zirconia layer was found in yttria-doped zirconia (YDZ) films grown on Si, while no change was observed in identical films grown on Ge. A comparison of un-doped zirconia and YDZ films indicates yttria-doping significantly assists structural changes during UV irradiation. Interestingly, the effect of UV photons becomes minimal at ～300°C in films grown on Si, while the effect of UV becomes more pronounced in YDZ films grown on Ge. An interfacial layer was formed between the YDZ and Ge substrate at 300°C in the presence of UV irradiation, in contrast to the sharp interface maintained, even after annealing at 300°C, without UV. The results suggest that photon irradiation may be an elegant approach to tailor structural and interfacial properties at near-atomic length scales.     

Pulsed laser deposition of polymers doped with fluorescent molecular sensors

Pulsed laser deposition (PLD) has been used to obtain thin films of poly(methyl-methacrylate), PMMA, and polystyrene, PS, doped with fluorescent probes based in the amino aromatic compounds S6, DMA-2,4, Dans and Acrid-acryl. These compounds, both in solution and inserted in polymeric films, show solvatochromic emission band shifts upon changes of pH, polarity and viscosity in their micro-environment and, prepared in thin films, could prove advantageous for use as sensors of the presence of contaminating environmental agents. Deposits, obtained by irradiating targets consisting in films of doped PMMA and PS with a Ti:sapphire laser (800 nm, 120 fs pulse), were analyzed by optical and environmental scanning electron microscopy, fluorescence microscopy, laser-induced fluorescence, micro-Raman spectroscopy and flow injection analysis-mass spectrometry. Results show the effective transfer of the polymer and the probe to the substrate and the high dependence on film structure on the polymer used in the targets. Irradiation with a KrF laser (248 nm, 20 ns) of Dans-doped targets only produced deposits when using PMMA for target preparation. Results are discussed in terms of the participation of multiphoton processes in the fs IR irradiation, the influence of the linear absorption coefficient of the targets and of the different contributions of thermal and chemical effects in the IR femtosecond and UV nanosecond domains. PACS 81.15.Fg; 61.82.Pv     

Room temperature deposition of amorphous SiC thin films using low energy ion bombardment

Silicon carbide (SiC) films are prepared by single- and dual-ion beam sputtering deposition at room temperature, respectively. An assisting argon ion beam (ion energy Ei=150 eV) bombards directly the substrate surface to modify the SiC film surface. The thin films are characterized by the Fourier transform infrared spectroscopy (FTIR) and the Raman spectra. With assisting ion beam bombardment, the density of the Si–C bond in the film increases. Meanwhile, the excess carbon or the size of the sp² bonded clusters and the amorphous Si (a-Si) phase decrease. These results indicate that the composition of the films is mainly Si–C bond. UV-vis transmission shows that the E_opt increases steadily from 1.85 eV for the amorphous SiC (a-SiC) films without bombardment to about 2.29 eV for those with assisting ion beam bombardment.     

Dependence of the sp3 bond fraction on the laser wavelength in thin carbon films prepared by pulsed laser deposition

3 bonds in the carbon films prepared by pulsed laser deposition of carbon obtained from graphite was investigated by electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS). The fraction of sp³ bonds increased with a decreasing laser wavelength. Energetic C⁺ ion species were effectively produced by using a short-wavelength laser. The sp³ bond fraction increased with an increasing amount of energetic C⁺ ion species. The fractions of sp³ bonds in the carbon film were 80%, 42%, 26% and 15% at wavelengths of 193, 248, 532 and 1064 nm, respectively. Received: 28 October 1997/Accepted:29 October 1997     

Correlation between substrate bias, growth process and structural properties of phosphorus incorporated tetrahedral amorphous carbon films

We investigate the growth process and structural properties of phosphorus incorporated tetrahedral amorphous carbon (ta-C:P) films which are deposited at different substrate biases by filtered cathodic vacuum arc technique with PH₃ as the dopant source. The films are characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy, Raman spectroscopy, residual stress measurement, UV/VIS/NIR absorption spectroscopy and temperature-dependent conductivity measurement. The atomic fraction of phosphorus in the films as a function of substrate bias is obtained by XPS analysis. The optimum bias for phosphorus incorporation is about −80 V. Raman spectra show that the amorphous structures of all samples with atomic-scaled smooth surface are not remarkably changed when PH₃ is implanted, but some small graphitic crystallites are formed. Moreover, phosphorus impurities and higher-energetic impinging ions are favorable for the clustering of sp² sites dispersed in sp³ skeleton and increase the level of structural ordering for ta-C:P films, which further releases the compressive stress and enhances the conductivity of the films. Our analysis establishes an interrelationship between microstructure, stress state, electrical properties, and substrate bias, which helps to understand the deposition mechanism of ta-C:P films.     

用高能H+束辐照类金刚石碳膜的研究

用能量(112,89keV)和剂量(1×10¹⁷,5×10¹⁶个/cm²)配比不同的H⁺束对双离子束溅射淀积的类金刚石碳(DLC)膜进行辐照,用Raman光谱、红外透射光谱和膜层电阻率测量、粘着力测定等多种手段对辐照前后的DLC膜进行,表征和分析,结果表明,高能H⁺束辐照效应跟高能重离子辐照效应是不同的,H⁺束辐照使膜层sp³C—H键相对减少,sp关键词：     

Laser-assisted synthesis of semiconductor chromium disilicide films

Different photo-assisted techniques were employed for chromium disilicide (CrSi₂) semiconductor film fabrication. Flash evaporation of CrSi₂ powder on the Si substrate heated to ∼740 K leads to the formation (according to XRD study) of amorphous films. Post-annealing at 920 K leads to the formation of polycrystalline CrSi₂ phase. Crystallization is improved by further annealing with 1500 Q-Switched Nd:YAG laser pulses. Optical properties of the as deposited and annealed CrSi₂ films have been investigated in the 240-1100 nm spectral range by using spectroscopic ellipsometry. The formation of CrSi₂ semiconductor phase was additionally confirmed by the temperature dependence of electrical resistance of the films treated by Q-switched Nd:YAG laser. The band gap for intrinsic conductivity results E_g ≅ 0.2 eV. Backward laser-induced film transfer (LIFT) was also used for CrSi₂ film deposition from bulk material on Si substrates. Pulsed CO₂ laser was employed for this purpose, because of transparency of silicon at the 10.6 μm wavelength. Measurements of the electrical resistance of the deposited films as a function of temperature showed their semiconductor behavior (E_g = 6 × 10⁻⁴ eV). Chromium disilicide films were also deposited by congruent pulsed laser ablation deposition on Si substrates either at room temperature or heated to about 740 K. In this last case the deposit exhibits semiconducting properties with E_g ≅ 0.18 eV.     

Role of carbon in the formation of hard Ge1−xCx thin films by reactive magnetron sputtering

We have deposited germanium carbide (Ge_1−xC_x) films on Si(1 0 0) substrate via radio-frequency (RF) reactive magnetron sputtering in a CH₄/Ar mixture discharge, and explored the effects of carbon content (x) on the chemical bonding and hardness for the obtained films. We find that x significantly influences the chemical bonding, which leads to a pronounced change in the hardness of the film. To reveal the relationship between the chemical bonding and hardness, first-principles calculations have been carried out. It is shown that as x increases from 0 to 0.33, the fraction of sp³ C-Ge bonds in the film increases at the expense of Ge-Ge bonds, which promotes formation of a strong covalently bonded network, and thus enhances the hardness of the film. However, as x further increases from 0.33 to 0.59, the fraction of sp³ C-Ge bonds in the film gradually reduces, while that of sp³ C-H and graphite-like sp² C-C bonds increases, which damages the compact network structure, resulting in a sharp decrease in the hardness. This investigation suggests that the medium x (0.17<x<0.40) is most favorable to the preparation of hard Ge_1−xC_x films due to the formation of dominant sp³ C-Ge bonds.     

Structural modification of fullerene thin films by highly charged iron ions

In this paper, the results of structural modification of fullerene thin films bombarded by highly charged iron ions (Fe⁶⁺) are presented. The properties of as-deposited and irradiated fullerene thin films have been investigated by Raman spectroscopy, UV/Vis spectrophotometry and atomic force microscopy (AFM). The results of Raman spectroscopy have indicated structural changes of irradiated thin films depending on fluences. It was found that iron doped fullerene films are dominated by sp³ rather than sp² after bombardment which might be due to formation of nanodiamond structures. AFM analysis showed that the ion beam had destroyed the surface ordering. The optical band gap was found to be in the range of 0.6 to 1.4 eV for irradiated films by Fe⁶⁺ ions at the highest fluences. PACS 61.48.+c; 78.30.-j; 79.20.Rf