Polymer-like Amorphous Carbon Films Research Articles

Oxidation of Amorphous Carbon Films by Ultraviolet Light Irradiation and Thermal Annealing

Using polymer-like amorphous carbon films deposited using supermagnetron plasma, ultraviolet light irradiation and thermal annealing effects in air were studied. Upper and lower electrode rf powers (UPRF/LORF) were selected to be 5/5 W, and such films were deposited using i-C4H10, and N2 or H2 plasma chemical vapor deposition (CVD). For comparison, polymer-like amorphous carbon films were deposited using Ar magnetron sputtering. By X-ray fluorescence spectroscopy (XFS), an increase in oxygen atom concentration and a small change in nitrogen atom concentration were observed under both ultraviolet (UV) light exposure and thermal annealing.

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

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, E em, 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 E em increases. Anisotropy and intensity decays are interpreted within a dipole–dipole energy transfer (Förster) model.

Quantitative study of C—H bonding in polymerlike amorphous carbon films usingin situinfrared ellipsometry

Polymerlike hydrogenated amorphous carbon $(a\ensuremath{-}\mathrm{C}:\mathrm{H})$ films have been deposited by plasma CVD at low temperature and low pressure. Vibrational properties have been investigated by in situ infrared ellipsometry as a function of film thickness. Hydrogen distribution within the films has been changed by varying the ion energy impinging on the film surface. Vibrational properties of $\mathrm{C}\mathrm{---}\mathrm{H}$ stretching and bending modes have been analyzed as function of self-bias ${(V}_{\mathrm{bias}})$ in terms of frequency, bandwidth, and intensity. Absorption strengths are associated with effective charges that have been calculated for the different ${\mathrm{sp}}^{3}$ ${\mathrm{CH}}_{n}$ groups. In order to make a comparison with values of organic chemistry, a general review of infrared spectra including alkanes, alkenes, and aromatic hydrocarbons is presented. A dipole description taking into account the local environment of CH bonds is developed showing that methyl and methylene group effective charges are similar for polymeric $a\ensuremath{-}\mathrm{C}:\mathrm{H}$ and ${\mathrm{C}}_{x}{\mathrm{H}}_{y}$ organic compounds. Line broadening and frequency shift effects are found to depend on the type of CH groups and are explained through a model including strain and dipole-dipole interactions. The sensitivity of effective charges to the local environment and the determination of ${\mathrm{CH}}_{n}$ group densities are used to propose a description of the hydrogenated network structure.

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 (sp2 C/sp3 C ratio) have also been evaluated.

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.