Fabrication and characterization of freestanding ultrathin diamond-like carbon targets for high-intensity laser applications

Here, we report the fabrication of diamond-like carbon (DLC) thin films using pulsed laser deposition (PLD). PLD is a well-established technique for deposition of high-quality DLC thin films. Carbon tape target was ablated using a KrF (248 nm, 25 ns, 20 Hz) excimer laser to deposit DLC films on soap-coated substrates. A laser fluence between 8.5 and 14 J/cm² and a target to substrate distance of 10 cm was used. These films were then released from substrates to obtain freestanding DLC thin foils. Foil thicknesses from 20 to 200 nm were deposited using this technique to obtain freestanding targets of up to 1-inch square area. Typically, 100-nm-thick freestanding DLC films were characterized using different techniques such as AFM, XPS, and nano-indentation. AFM was used to obtain the film surface roughness of 9 nm rms of the released film. XPS was utilized to obtain 74 % sp², 23 % sp³, and 3 % C–O bond components. Nano-indentation was used to characterize the film hardness of 10 GPa and Young’s modulus of 110 GPa. Damage threshold properties of the DLC foils were studied (1,064 nm, 6 ns) and found to be 7 × 10¹⁰ W/cm² peak intensity for our best ultrathin DLC foils.     

Synthesis of carbon nitride films by direct current plasma assisted pulsed laser deposition

Carbon nitride films were deposited by pulsed laser ablation of a graphite target under a nitrogen atmosphere at room temperature. A direct current discharge apparatus was used to supply active nitrogen species during the deposition of carbon nitride films. The composition and bonding structure of carbon nitride films were determined by Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy. The incorporation of nitrogen atoms in the films is greatly improved by the using of a dc glow discharge. The ratio N/C can reach 0.34 at the discharge voltage of 400 V. Six peaks centered at 1025 cm^-1, 1226 cm^-1, 1381 cm^-1, 1534 cm^-1, 1629 cm^-1, and 2200 cm^-1 can be clearly distinguished from the FTIR spectra of the deposited films, which indicates the existence of C–N, C=N, and C≡N bonds. The fraction of sp² C, C≡N bonds, and C=N bonds in the deposited films increases with increasing discharge voltage. Deconvolution results of C 1s and N 1s spectra also indicate that nitrogen atoms in the films are chemically bonded to sp¹ C, sp² C, and sp³ C atoms. Most of the nitrogen atoms are bonded to sp² C atoms. Increasing the discharge voltage leads to a decrease of the fraction of nitrogen atoms bonded to sp² C and the fraction of amorphous carbon; however, it leads to an increase of the fraction of nitrogen atoms bonded to sp³ C and the fraction of sp² C and sp³ C atoms bonded to nitrogen atoms. Received: 7 June 2000 / Accepted: 19 February 2001 / Published online: 27 June 2001     

Deposition and characterization of ITO films produced by laser ablation at 355 nm

Indium tin oxide (ITO) films have been deposited by pulsed laser deposition (PLD) at 355 nm. Even though the absorption of laser light at the wavelength 355 nm is much smaller than that of the standard excimer lasers for PLD at 248 nm and 193 nm, high-quality films can be produced. At high fluence and at high substrate temperatures, the specific resistivity of the films, 2–3×10^-4 Ω cm, is comparable to values obtained with excimer lasers, whereas the resistivities obtained at room temperature are somewhat higher than those of films produced by excimer lasers. The transmission coefficient of visible light, about 0.9, is also comparable to values for films deposited by excimer lasers. The crystalline structure of films produced at 355 nm is similar to that of samples produced by these lasers. Received: 16 January 2001 / Accepted: 24 July 2001 / Published online: 17 October 2001     

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     

Lattice Boltzmann model for multi-component mass transfer in a solid oxide fuel cell anode with heterogeneous internal reformation and electrochemistry

We have deposited SiC thin films using two different deposition techniques, Pulsed Electron Deposition (PED) and Pulsed Laser Deposition (PLD). The PED technique is a relatively new technique where a pulsed electron flux with high energy directly hits the target surface producing a plasma in a very similar way to PLD, where, instead, a pulsed laser beam is used. These two techniques can give very dense thin films with stoichiometry much closer to the target one’s compared to others physical vapour deposition techniques (sputtering, e-beam). The major drawback of PED is that the thin film surface is affected by the presence of particulate, due to the impact of the high energy electron beam with the target. In the PLD system we used a magnetic field to curve the plasma path in the vacuum and we placed the substrate at 90^○ with respect to the target allowing the deposition of particulate-free samples. The characterization of the films has been made by measuring the optical reflectance vs angle of incidence in the EUV region (from 121.6 nm down to 40.7 nm), taking measurements at different time from deposition. X-ray photoemission measurements have been also carried out to show stoichiometry and the presence of contaminants. Other measurements such as X-ray diffraction, atomic force microscopy and profiling were also carried out to check crystalline domains and surface roughness.     

脉冲激光气相沉积法制备的非晶CH薄膜特性分析

采用脉冲激光气相沉积(PLD)法,研究了氢气压强对非晶CH薄膜性能的影响。原子力显微镜图和白光干涉图显示,薄膜表面平整致密,随着氢气压强增大,粗糙度变大。拉曼光谱分析表明,氢气压强增加,G峰和D峰位置都在向高波数方向移动。傅里叶变换红外光谱分析显示,薄膜中存在sp3—CH2和sp2—CH等基团。最后,采用PLD漂浮法在最优参数氢气压强为0.3 Pa下,成功制备了不同厚度(100~300 nm)、满足一定力学强度、无明显宏观缺陷的自支撑CH薄膜。     

Optical and Electrical Properties Evolution of Diamond-Like Carbon Thin Films with Deposition Temperature

Optical and electrical properties of diamond-like carbon （DLC） films deposited by pulsed laser ablation of graphite target at different substrate temperatures are reported. By varying the deposition temperature from 400 to 25℃, the film optical transparency and electrical resistivity increase severely. Most importantly, the transparency and resistivity properties of the DLC films can be tailored to approaching diamond by adjusting the deposition temperature, which is critical to many applications. DLC films deposited at low temperatures show excellent optical transmittance and high resistivity. Over the same temperature regime an increase of the spa bonded C content is observed using visible Raman spectroscopy, which is responsible for the enhanced transparency and resistivity properties.     

Vapor deposition of intact polyethylene glycol thin films

Thin films of polyethylene glycol (PEG) of average molecular weight, 1400 amu, were deposited by both matrix-assisted pulsed laser evaporation (MAPLE) and pulsed laser deposition (PLD). The deposition was carried out in vacuum (∼10^-6 Torr) with an ArF (λ=193 nm) laser at a fluence between 150 and 300 mJ/cm². Films were deposited on NaCl plates, Si(111) wafers, and glass slides. The physiochemical properties of the films are compared via Fourier transform infrared spectroscopy (FTIR), electrospray ionization (ESI) mass spectrometry, and matrix-assisted laser desorption and ionization (MALDI) time-of-flight mass spectrometry. The results show that the MAPLE films nearly identically resemble the starting material, whereas the PLD films do not. These results are discussed within the context of biomedical applications such as drug delivery coatings and in vivo applications where there is a need for transfer of polymeric coatings of PEG without significant chemical modification. Received: 2 March 2001 / Accepted: 5 March 2001 / Published online: 23 May 2001     

Critical role of laser wavelength on carbon films grown by PLD of graphite

The structure of thin films deposited by pulsed laser ablation (PLD) is strongly dependent on experimental conditions, like laser wavelength and fluence, substrate temperature and pressure. Depending on these parameters we obtained various kinds of carbon materials varying from dense, mainly tetrahedral amorphous carbon (ta-C), to less compact vertically oriented graphene nano-particles. Thin carbon films were grown by PLD on n-Si 〈100〉 substrates, at temperatures ranging from RT to 800°C, from a rotating graphite target operating in vacuum. The laser ablation of the graphite target was performed by a UV pulsed ArF excimer laser (λ=193 nm) and a pulsed Nd:YAG laser, operating in the near IR (λ=1064 nm). The film structure and texturing, characterised by X-ray diffraction analysis, performed at grazing incidence (GI-XRD), and the film density, evaluated by X-ray reflectivity measurements, are strongly affected both by laser wavelength and fluence and by substrate temperature. Micro-Raman and GI-XRD analysis established the progressive formation of aromatic clusters and cluster condensation into vertically oriented nano-sized graphene structures as a direct function of increasing laser wavelength and deposition temperature. The film density, negatively affected by substrate temperature and laser wavelength and fluence, in turn, results in a porous bulk configuration and a high macroscopic surface roughness as shown by SEM characterisation. These structural property modifications induce a relevant variation also on the emission properties of carbon nano-structures, as evidenced by field emission measurements. This work is dedicated to our friend Giorgio who passed away 20th August.     

Electrical conditioning of diamond-like carbon films for the formation of coated field emission cathodes

Diamond-like carbon (DLC) films deposited on different substrates by plasma enhanced chemical vapour deposition were investigated. Bonding states and film quality were characterized by FT-IR spectroscopy. The influence of the power of plasma and the deposition time on the sp²/sp³ ratio as well as the concentration of CH_n bonds was studied. The influence of sp²/sp³ ratio on the formation process of conducting channels in diamond-like carbon films as a result of electrical breakdown was determined. Reproducible increase of diamond-like carbon film conductivity, with initial sp²/sp³ ratio larger than 0.16, was observed after electrical breakdown.     

Effect of temperature on pulsed laser deposition of HgCdTe films

HgCdTe thin films have been deposited on Si(1 1 1) substrates at different substrate temperatures by pulsed laser deposition (PLD). An Nd:YAG pulsed laser with a wavelength of 1064 nm was used as laser source. The influences of the substrate temperature on the crystalline quality, surface morphology and composition of HgCdTe thin films were characterized by X-ray diffraction (XRD), selected area electron diffraction (SAED), atomic force microscopy (AFM) and energy dispersive X-ray spectroscopy (EDS). The results show that in our experimental conditions, the HgCdTe thin films deposited at 200 °C have the best quality. When the substrate temperature is over 250 °C, the HgCdTe film becomes thermodynamically unstable and the quality of the film is degraded.     

Fabrication of DLC films by pulsed ion beam ablation in a dense plasma focus device

The pulsed intense ion beam, emitted from a dense plasma focus (DPF) discharges performed with hydrogen gas, has been used to ablate the graphite target depositing diamond-like carbon (DLC) films on Si substrates. The substrates were mounted on a holder, which allowed for deposition at positions between normal and 20° off-normal to the target. The samples were removed for analysis after 10 and 20 shots. Nano-particles were observed in the films by a field-emission scanning electron microscope. Raman spectra indicate that sample deposited at 20° off-normal with 20 shots possesses the highest sp³ content among the samples. The film deposited at this position was also found has the highest hardness.     

Effect of nickel on the structure and bond type in amorphous diamond-like carbon films

Experimental data are presented from studies of the structure and bond type of carbon atoms in amorphous carbon-nickel films deposited from pulsed vacuum-arc discharge plasma sources. X-ray photoelectron spectroscopy was used. The characteristics of the plasmon loss spectra depend significantly on the deposition parameters. Carbon exists in a mixed sp²+sp³ hybridized state in the carbon–nickel films. The ratio of sp³/sp² carbon bonds increases when the nickel content is reduced (from 5.5 to 1.0 atomic %) and the deposition angle is increased. The structure closest to that of diamond was with a substrate bias voltage of –80 to –100 V and a deposition angle of 90°.     

Pulsed laser deposition and its current research status in preparing hydroxyapatite thin films

Pulsed laser deposition (PLD) is a conceptually and experimentally simple yet highly versatile tool for thin films and multi-layer film research. The mechanisms, advantages and disadvantages of pulsed laser deposition were reviewed. The process and some methods to resolve the drawbacks of PLD were discussed. Pulsed laser deposition of hydroxyapatite thin films was reviewed. Simple adjustment of PLD parameters can deposit hydroxyapatite in situ in crystalline form, amorphous films or HA with other calcium phosphate phases. Compared with plasma sprayed HA coatings the pulsed laser deposition HA thin films have higher coating/substrate adhesion and have minor undesirable phases under optimal conditions. Finally, we suggested some new researches should be done.     

Structure and mechanical properties of low stress tetrahedral amorphous carbon films prepared by pulsed laser deposition

Tetrahedral amorphous carbon films have been produced by pulsed laser deposition, at a wavelength of 248 nm, ablating highly oriented pyrolytic graphite at room temperature, in a 10^-2 Pa vacuum, at fluences ranging between 0.5 and 35 Jcm^-2. Both (100) Si wafers and wafers covered with a SiC polycrystalline interlayer were used as substrates. Film structure was investigated by Raman spectroscopy at different excitation wavelength from 633 nm to 229 nm and by transmission Electron Energy Loss Spectroscopy. The films, which are hydrogen-free, as shown by Fourier Transform Infrared Spectroscopy, undergo a transition from mainly disordered graphitic to up to 80% tetrahedral amorphous carbon (ta-C) above a threshold laser fluence of 5 J cm^-2. By X-ray reflectivity roughness, density and cross-sectional layering of selected samples were studied. Film hardness as high as 70 GPa was obtained by nanoindentation on films deposited with the SiC interlayer. By scratch test film adhesion and friction coefficients between 0.06 and 0.11 were measured. By profilometry we obtained residual stress values not higher than 2 GPa in as-deposited 80% sp³ ta-C films. Received 25 June 2001     

Preparation of super-hard coatings by pulsed laser deposition

Amorphous diamond-like carbon (DLC) films and nanocrystalline cubic boron nitride (c-BN) films were prepared by pulsed laser deposition. DLC films with 80 to 85% sp³ bonds prepared at a laser fluence above 6 J/cm² and a substrate temperature below 100 °C show high compressive stresses in the range of 8 to 10 GPa. Those stresses can be completely removed by means of pulsed laser annealing, allowing the preparation of DLC films with several-micrometre thickness. c-BN films were prepared with additional ion-beam bombardment at a substrate temperature of 250 °C. The properties of DLC and c-BN films deposited at high growth rates up to 100 nm/min are presented . PACS 81.15.Fg; 68.60.Bs: 62.40.+i     

高脉冲功率能量PLD法制备MgZnO薄膜中的沉积机理

用PLD法成功制备了一系列高质量的MgZnO薄膜。实验中发现高脉冲能量沉积薄膜的结构和发光特性随基片温度的变化规律与低脉冲能量下的结果不一样:基片在室温时高脉冲能量制备薄膜的XRD峰的半峰全宽比高基片温度时的结果相对更小;AFM显示其颗粒变大,柱状生长突出;PL谱紫峰与绿峰强度比最大,结晶质量反而提高。另一方面,与低脉冲能量时相反,增大氧气压强后高脉冲能量沉积的薄膜XRD半峰全宽变窄。结合实验现象和表征,合理解释了高脉冲能量沉积的机理。室温制备高质量MgZnO薄膜的PLD沉积机理对于以后在柔性衬底上沉积薄膜的研究有重要的参考价值。     

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.     

Microstructure and tribological behavior of pulsed laser deposited a-CNx films

The a-CN_x films were deposited onto high-speed steel substrate by pulsed laser deposition at different nitrogen pressures. The tribological properties of the films in humid air and in vacuum were investigated using a ball-on-disk tribometer under various loads. The composition, microstructure and morphology of the films, wear tracks and paired balls were characterized by energy dispersive X-ray analysis (EDXA), X-ray photoelectron spectrum (XPS), Raman spectroscopy and scanning electron microscopy (SEM). With increasing the deposition pressure, the fraction of sp³ C bond reduces, the fraction of trapped nitrogen increases and the friction coefficient of the films declines both in humid air and vacuum. The friction coefficient of a-CN_x film decreases with increasing normal load. The tribological performances of the films in humid air are better than those of in vacuum. A transferred graphite-like tribo-layer is observed from a-CN_x film to the paired ball for both environments.     

Effect of the structure of a graphite target on the parameters of carbon films obtained by the laser plasma method

We have used Raman light scattering and electron paramagnetic resonance methods to study carbon films obtained by laser plasma deposition, using different types of graphite targets. We have established that the films deposited in this way have a diamond-like structure and are a nanostructured composite containing clusters of both sp² and sp³-hybridized carbon. We have shown that an increase in structural perfection of the graphite target causes an improvement in the structure of the carbon films obtained from it and an increase in the content of sp³-hybridized carbon in it. Thermal stimulation of the substrate during application of a coating leads to the same effect. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 4, pp. 539–546, July–August, 2008.