T. Heitz

Growth and composition of dual-plasma polymer-like amorphous carbon films

Polymer-like hydrogenated amorphous carbon (a-C:H) films have been grown in a dual-plasma [radio frequency (rf)–microwave (MW)] reactor using butane as a carbon precursor and different mixtures (H2–Ar or He–Ar) in the MW plasma in order to vary the atomic hydrogen flux impinging on the growing film. Decreasing the rf power (i.e., the energy of ion bombardment on a-C:H) or increasing the H atom flux both result in a wide band gap H-rich polymer-like a-C:H network. Nuclear techniques have been combined with UV-visible ellipsometry to determine the stoichiometry, density and growth rate of a-C:H films as a function of the plasma parameters. Parametrization of UV-visible optical properties allows to monitor the changes in the optical parameters (optical gap and density of π states) attributed to the formation of structural units containing C=C double bonds. C–H bonds observed by in situ infrared ellipsometry have been used to investigate the role of ions in the growth processes (densification, cross linking) and to interpret the observed changes in optical parameters in terms of a two-phase microscopic description of polymer-like a-C:H. In order to understand the kinetic results, a phenomenological growth model is proposed including the respective roles of ion bombardment and H atom flux in the activation and deactivation of surface sites available for CxHy radical incorporation (chemisorption) to the growth zone. The activation mechanism corresponds to a chemical modification at the growth zone sites. This adlayer model includes the physisorption of both H atoms and CxHy radicals and explains why the temperature dependence of the deposition rate is found to be opposite for the limiting cases of low and high H atom fluxes. Some consequences of the model on the film stoichiometry (H/C ratio) and microstructure (sp2u2009C/sp3u2009C ratio) have also been evaluated.Polymer-like hydrogenated amorphous carbon (a-C:H) films have been grown in a dual-plasma [radio frequency (rf)–microwave (MW)] reactor using butane as a carbon precursor and different mixtures (H2–Ar or He–Ar) in the MW plasma in order to vary the atomic hydrogen flux impinging on the growing film. Decreasing the rf power (i.e., the energy of ion bombardment on a-C:H) or increasing the H atom flux both result in a wide band gap H-rich polymer-like a-C:H network. Nuclear techniques have been combined with UV-visible ellipsometry to determine the stoichiometry, density and growth rate of a-C:H films as a function of the plasma parameters. Parametrization of UV-visible optical properties allows to monitor the changes in the optical parameters (optical gap and density of π states) attributed to the formation of structural units containing C=C double bonds. C–H bonds observed by in situ infrared ellipsometry have been used to investigate the role of ions in the growth processes (densification, cross linking) ...

Photoluminescence intensity and anisotropy decays in amorphous carbon

The decays of intensity and anisotropy of UV-excited photoluminescence PL in hydrogenated amorphous carbon have been investigated in the ps-ns time range. For emission energies E between 1.8 and 3.5 eV, anisotropy decreases within em . 100 ps and reaches a plateau within 1 ns. The emission anisotropy plateau value increases from 0.02 to 0.12 and the decay . time of PL intensity decreases from 1 ns to 40 ps as E increases. The exponential increase of time-averaged anisotropy em as E increases is explained by a competition between exciton decay and randomization of polarization due to electronic em

C-H bonding of polymer-like hydrogenated amorphous carbon films investigated by in-situ infrared ellipsometry

Abstract Polymer-like hydrogenated amorphous carbon films have been deposited in a microwave-assisted radiofrequency plasma reactor. Vibrational properties are investigated in-situ by infrared ellipsometry as a function of ion bombardment. By choosing either hydrogen or helium in the microwave gas mixture, films ranging from nearly purely saturated polymer to olefinic/aromatic films can be obtained. Both absorption bands and transparent regions are studied, leading to a determination of the dielectric constant as well as the vibrational frequencies, widths and intensities as a function of macroscopic properties like density and optical gap. Hydrogen atoms attached to sp2-carbon are detected even in highly saturated films. The sensitivity of the vibrational oscillator strength to the matrix effect is used to probe the local environment of hydrogenated carbon atoms. Quantitative determination of CHn group densities in the different configurations indicates that sp3 CH groups are dominant and that the local structure evolves with atomic hydrogen concentration.

In situ infrared ellipsometry study of hydrogenated amorphous carbon/Si interface formation

The early stages of the growth of plasma-deposited hydrogenated amorphous carbon films on c-Si have been studied by in situ infrared ellipsometry. Different types of polymeric films have been obtained under soft plasma conditions. From the evolution of optical properties, a uniform growth is observed for films having the highest sp2 carbon atom content. In contrast, when the film bulk is a highly saturated polymer, where carbon configurations are mostly sp3, an interlayer of about 20 A is evidenced, correlated with the formation of sp2 CHn bonds. Moreover, infrared data tend to prove that this interlayer formation is incompatible with the presence of SiC at the interface.

Optical and luminescence properties of polymer-like a-C:H films deposited in a dual-mode PECVD reactor

Abstract Butane has been decomposed in the post-discharge of a helium-argon microwave plasma, assisted by a radiofrequency (rf) plasma. The ion bombardment on the substrate, monitored by the rf power, is found to control the optical and luminescence properties of hydrogenated amorphous carbon (a-C:H) thin films: the extinction coefficient of a-C:H increases with rf power while the photoluminescence efficiency decreases by three orders of magnitude. The parametrization of the dielectric function, using a three-phase effective medium approximation, is discussed in relation to the expected size effects on the dielectric function of the π-bonded clusters.

Correlation between luminescence properties and microstructure of hydrogenated amorphous carbon films investigated by X-ray diffraction and infrared ellipsometry

Abstract Plasma deposited polymer-like amorphous carbon (PLC) films show a strong photoluminescence (PL) in the visible range at room temperature. However, the origin of the luminescence in terms of microstructure is poorly understood. In this study, by varying the plasma conditions, hydrogenated carbon (a-C:H) films, ranging from soft and transparent to denser and more absorbent materials, have been grown. Steady-state PL measurements reveal a steep quenching of the luminescence intensity when film density is higher than 1.3xa0gxa0cm−3. Using in situ infrared ellipsometry, C–H bonding analysis shows that this quenching corresponds to a strong decrease of the sp3 CHn=2,3 group concentrations, consistent with a network crosslinking and to an enrichment of the methyl group environment in π bonds. In correlation, the evolution of the carbon skeleton has been studied using X-ray diffraction measurements. Structure factors and pair correlation functions evidence clear differences in the network structure, in particular a shortening of the carbon–carbon bond when density increases.

X-ray diffraction investigation of polymer-like hydrogenated amorphous carbon films

Abstract Structural properties of polymer-like amorphous carbon (a-C:H) films grown in a dual-plasma (microwave (MW) assisted radio frequency (RF)) reactor have been investigated. An interesting transition appears as the film density increases due to increasing ion energy: (1) in situ infrared ellipsometry shows a transition in the C–H bonding modes corresponding to carbon skeleton cross-linking, (2) a quenching of the photoluminescence (PL) efficiency is observed when film density is >1.3 g cm −3 . To analyze the origin of such an evolution, X-ray diffraction investigations have been performed on a-C:H powders with densities increasing from 0.9 to 1.7 g cm −3 . This paper reports on X-ray measurements performed in transmission geometry using the high photon flux at LURE facilities. The data show differences between the samples in the structure factor, S ( K ), and in the pair correlation function, G ( r ). X-ray spectra have pre-peaks in the vicinity of 1 A −1 , which change in position and shape with the density of the powders. The contraction of the first distances, C–C, is observed as the density increases, from 0.152±0.002 nm for transparent and soft films to 0.142±0.002 nm for the denser one. The evolution of C-skeleton structure is described in terms of local co-ordination, in relation with the C–H bonding.

Photoluminescence of polymer-like amorphous carbon films grown in different plasma reactors

The visible photoluminescence (PL) properties (emission and excitation spectra) of hydrogenated amorphous carbon (a-C:H) have been investigated for polymer-like thin films grown at low substrate temperature and small ion energies. For a large number of a-C:H films grown in three reactors (dual-mode plasma, radio frequency plasma and electron cyclotron resonance plasma) the emission spectra appear as a sum of three peaks with energies (a) 2.28 eV, (b) 2.65 eV and (c) 2.95 eV, being independent of the excitation energy (3.54 eV or 4.13 eV). Their relative intensities depend on the growth parameters while their positions are almost independent. Preliminary picosecond time-resolved PL data indicate a decrease of the decay time for increasing emission energies. A two-phase model of a-C:H is proposed to explain the PL excitation spectra.

Early stages of the growth of hydrogenated amorphous carbon investigated by in situ infrared ellipsometry

Abstract The early stages of the growth of hydrogenated amorphous carbon deposited from plasmas onto crystalline silicon have been studied by in situ infrared spectroscopic ellipsometry (IRSE). Under soft plasma conditions (ion energy less than 10 eV), two types of a-C:H films have been obtained using several gas mixtures. For films having the largest sp2-configurated carbon content, the growth is uniform whereas a 2 nm interlayer is observed for highly saturated polymer films. Analysis of CH vibrations shows that this interlayer is rich in sp2 CHn bonds and that their concentration decreases from the c-Si surface to the film interior. This interlayer formation may be explained by interfacial stress relaxation effects leading to sp2 CHn bond creation.

Time-resolved study of photoluminescence polarization in a-C:H films

Abstract The decays of intensity and polarization anisotropy of the photoluminescence (PL) of polymer-like amorphous carbon films have been measured at 300 and 80 K using 4.13 and 2.15 eV excitation. For emission energies, Eem, between 1.8 and 3.5 eV, PL polarization anisotropy decreases within 100 ps and reaches a plateau within 1 ns. The anisotropy plateau increases from 0.02 to 0.12 and the intensity decay time decreases from 1 ns to 40 ps as Eem increases. Anisotropy and intensity decays are interpreted within a dipole–dipole energy transfer (Forster) model.