EPR linewidth variation, spin relaxation times, and exchange in amorphous hydrogenated carbon (original) (raw)
Related papers
An EPR study of defects in hydrogenated amorphous carbon thin films
Diamond and related …, 1998
The defect states, with unpaired spins, in hydrogenated amorphous carbon (a-C:H) films have been studied using electron paramagnetic resonance (EPR). These EPR measurements were made at room temperature at about 9.9 GHz. The a-C:H thin films were deposited using plasma enhanced chemical vapour deposition (PECVD), firstly for various negative self-bias voltages at constant pressure, and secondly with various nitrogen contents at constant bias. Changing the self bias from -50 to -540 volts leads to a reduction in linewidth from approx. !.4 to 0.3 mTesla although the g-value of the single Lorentzian line is unchanged at g= 2.0025 +0.0002. Increasing the atomic nitrogen content from zero to 15% causes the g-value of the single line to shift from 2.0025+0.0002 to 2.0(133 +0.1)003, while the linewidth and spin populations both decrease. The effect on these defects of postannealing the films is also reported. ~:,:3 1998 Elsevier Science S.A.
Characterisation of defects in thin films of hydrogenated amorphous carbon
Diamond and Related …, 2000
Electron paramagnetic resonance (EPR) measurements have been made at X-band (f9.5 GHz) and W-band (f95 GHz) of a-C:H films on (1 0 0) silicon substrates; the sample temperature was varied in the range 5-300 K. Two types of film were examined. The first type are amorphous hydrogenated carbon (a-C:H) films grown by plasma enhanced chemical vapour deposition (PECVD) with negative self bias voltages in the approximate range 100-500 V. The second type were initially highly polymeric-like a-C:H films grown on Si placed on the earthed electrode of a PECVD system but were subsequently implanted with either 6=10 cm B or 2=10 cm B ions. At both X-and W-band and throughout the temperature range 5-300 K 15 y2 q 16 y2 q the EPR signal of the carbon unpaired electrons consists of a single symmetric line with gs2.0026"0.0002. As the temperature is lowered, several samples develop a dependence on sample orientation of the external field required for resonance. This anisotropy is explained in terms of the demagnetising fields more usually encountered in ferromagnetic resonance. ᮊ
Diamond and Related Materials, 1997
In this paper it is shown that electron spin resonance can be used as a tool for structural investigation of amorphous carbon thin films. Interesting information on the amorphous lattice is obtained from the analysis of the features of the ESR signal• Some representative examples will be discussed. For instance, it will be shown that different degrees of structural disorder exist in the various fihns, that regions having different local structure exist in sputtered amorphous carbon, and thin in soft polymeric a-C:H the unpaired spins have a very weak interaction with the lattice.
Diamond and Related Materials, 2001
Electron paramagnetic resonance (EPR) measurements have been made of defects in amorphous hydrogenated carbon (a-C:H) thin films. The films were grown on silicon substrates on the earthed electrode of an rf-powered plasma enhanced chemical vapour deposition reactor and were subsequently implanted with a range of doses of boron, carbon or nitrogen ions with energies from 20 keV to 32.5 keV. Two paramagnetic centres are observed, the carbon defect in the film with g=2.0028(1) and a silicon defect in the substrate with g=2.0058(6). The volume concentration of the carbon defect increases approximately linearly with dose, from approximately 3×1017 cm−3 for unimplanted samples to 2.7×1020 cm−3 at the highest implantation of 2×1016 B+ ions cm−2. The increase in dose over this range also causes a narrowing of the EPR line (from 0.83 mT to 0.13 mT) and a significant decrease in the spin-lattice relaxation time (from 3×10−5 s to 6×10−8 s) which approaches the spin–spin relaxation time at the highest dose. The narrowing is attributed to motional averaging produced by either exchange or hopping. We also report the effects of annealing samples implanted with a range of boron doses. The prime novelty of this paper is that it is the first EPR study of defects produced by the implantation of a range of ions into polymer-like amorphous hydrogenated carbon.
Solid state NMR study of carbon bonding in amorphous hydrogenated carbon films
Applied Magnetic Resonance, 1991
Carbon bonding environments in hydrogenated amorphous carbon films (a-C:H) deposited from ah rf-biased methane plasma onto various substrates have been quantified by application of solid state 13C NMR. A family of films were prepared by systematically varying the substrate bias voltage. Quantitative data on carbon chemistry in these films is required for modeling the impact of structure on mechanical and optical properties. A va¡ of NMR acquisition pulse sequences have been investigated to determine the conditions under which quantitative 13C NMR data can be acquired in this system. The results indicate that data acquisition from this material requires different protocols than for the study of polymeric hydrocarbon films. With proper experimental design, NMR is ah excellent technique for structural studies of these materials.
Journal of Applied Physics, 2012
An automated spin-assisted approach for molecular layer-by-layer assembly of crosslinked polymer thin films Rev. Sci. Instrum. 83, 114102 (2012) The improved imprint effect in poly(vinylidene fluoride-trifluoroethylene) copolymer ultrathin films Appl. Phys. Lett. 101, 183504 (2012) Effect of film thickness on structural, morphology, dielectric and electrical properties of parylene C films J. Appl. Phys. 112, 064103 (2012) Polymer film deposition on agar using a dielectric barrier discharge jet and its bacterial growth inhibition Hydrogen (H) stability in hydrogenated amorphous carbon (a-C:H) films with different structures grown by (biased) electron-cyclotron-resonance chemical vapor deposition has been studied against thermal annealing and swift-ion impact (2 MeV He þ ). For this purpose, a-C:H films with either polymer-like (PLCH) or diamond-like (DLCH) character grown on grounded or biased (À200 V) substrates, respectively, were annealed up to 450 C. The local-order structural evolution around C sites was analyzed by x-ray absorption near-edge spectroscopy (XANES) and the H content and radiation-induced release were determined by successive elastic recoil detection analysis (ERDA) acquisitions. A relatively high H content is measured for both asgrown PLCH ($45 at. %) and DLCH films ($33 at. %). Upon annealing, PLCH films suffer thermal-induced surface decomposition resulting in a thickness reduction and only above 350 C the H content in the film matrix decreases. PLCH films also display a pronounced H loss rate during ERDA measurements, whereas H is stable in DLCH. These results indicate that H bonding differs in both structures (i.e., weaker C-H bonds in PLCH). XANES shows that upon annealing both structures suffer H loss at the near surface region together with a graphitization process, although the impact is more pronounced in PLCH. XANES fine-structure reveals that aromatic clusters are formed upon annealing in PLCH due to H loss, whereas this process is partially inhibited in DLCH due to the thermal stability of the C-H bonds. V C 2012 American Institute of Physics. [http://dx.
Effects of ion implantation on electron centers in hydrogenated amorphous carbon films
Journal of Applied Physics, 2003
Electron spin resonance (ESR) and Raman spectra measurements are carried out on a-C:H and a-C:H:N films both as grown and implanted with W and Ni ions with doses ranged from 0.5×1015 to 1.2×1016 cm-2. The as-grown films have small concentration of paramagnetic centers with a spin density Ns of 1017 cm-3. Upon implantation a significant increase in Ns of (0.5-22)×1019 cm-3 centers with g(Si)=2.0055 and g(C)=2.0025 was observed. These defects are ascribed to dangling bonds in the silicon substrate and in the carbon film, respectively. The correlation between variation of Ns value with implantation dose and behavior of D and G band position and their intensity ratio in the visible Raman spectra is observed. The effects are attributed to changes in the sp2-sp3 systems and hydrogen loss due to ion induced annealing of the carbon films at high ion doses. The temperature and concentration dependencies of the ESR line shape and linewidth are explained using the mechanism of motional narrowing over the temperature range 4.2-300 K. Low temperature anisotropy of the g value is found in the ESR spectra and is explained as arising from the dipole-dipole interaction in the infinitely thin films.
Paramagnetic defects in hydrogenated amorphous carbon powders
2003
Hydrogenated amorphous carbon materials typically contain high concentrations of paramagnetic defects, the density of which can be quantified by electron paramagnetic resonance (EPR). In this work EPR measurements near 9.5, 94, and 189 GHz have been performed on polymeric and diamond-like hydrogenated amorphous carbon (a-C:H) powder samples. A similar single resonance line was observed at all frequencies for the two forms of a-C:H studied. No contributions to the spectrum from centres with resolved anisotropic g-values as reported earlier were detected. An increase in linewidth with microwave frequency was observed. Possible contributions to this frequency dependence are discussed.
Physical Review B, 2010
Three paramagnetic defects were revealed in amorphous hydrogenated carbon-rich silicon-carbon alloy films ͑a-Si 0.3 C 0.7 :H͒. Two of them were attributed to silicon ͑Si͒ dangling bonds ͑Si DBs͒ and carbon-related defects ͑CRDs͒. The third defect, based on its g-value and linewidth, was tentatively attributed to a bulk Si DB defect bonded with nitrogen atoms in Si-N 2 Si configuration. The effect of thermal vacuum annealing on the properties of the a-Si 0.3 C 0.7 : H films was studied in the temperature range of T ann = 400-950°C. A strong increase in CRD concentration was observed in high temperature annealed a-Si 0.3 C 0.7 : H films, which was explained by hydrogen effusion process occurred at T ann above 400°C. The increase in the concentration of the CRDs is accompanied by the exchange narrowing of its electron paramagnetic resonance ͑EPR͒ linewidth due to the formation of carbon clusters having ferromagnetic ordering. The temperature dependent g-factor anisotropies observed at Q-band and D-band frequencies for the CRD signal in the samples annealed at high temperature ͑950°C͒ were explained by the presence of graphitelike sp 2 -coordinated carbon clusters and demagnetization field ͑shape-dependent anisotropy term͒. The demagnetizing field B dem =−4M s , where M s is the sample magnetization, was found to be equal to 0.44 mT at 37 GHz and 1.1 mT at 140 GHz. Analysis of the temperature dependences of the integral intensities of the SiDB and CRD EPR signals has shown that they do not obey the Curie-Weiss law, and their spin systems exhibit superparamagnetic and ferromagnetic properties, respectively.
Journal of Non-Crystalline Solids, 2000
Hydrogenated amorphous carbon (a-C:H) ®lms were prepared by low-pressure PECVD of CH 4 in a dual electron cyclotron resonance (ECR)±radio frequency (RF) discharge by applying an independently controlled RF substrate bias voltage from 0 to )200 V. The ®lms were analyzed with core-level and valence band (VB) X-ray photoemission spectroscopy (XPS) as well as spectroscopic ellipsometry. A progressive change from soft transparent polymer-like to hard absorbing diamond-like was observed with increasing bias voltage in this range. The C 1s core-level spectra were ®tted by using a Doniach±Sunjic photoemission line-shape. The C 1s spectra showed the enhanced asymmetric behavior with increasing bias voltage. In addition, the VB spectra exhibited distinct p density of states according to the bias. Such XPS features were analyzed and interpreted in terms of the p 3 p à electron transition model on amorphous carbon through spectroscopic ellipsometry. Ó