Synthesis and microstructural properties of tetrahedral amorphous carbon films

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Abstract

Tetrahedral amorphous carbon (ta-C) films have been prepared using a magnetic field filtered plasma stream system. The optical and microstructural properties of these films as a function of the substrate bias voltage, Vb, have been studied using optical absorption spectroscopy, Raman spectroscopy and atomic force microscopy. The results show that ta-C films with larger fraction of sp3 bonding were formed when the substrate bias voltage was in the range from −10 to −50V. The optical band gap is about 3.0 eV. The sp3 fraction in these films was estimated to be >80% by a fitting of the Raman spectra with a single skewed Lorentzian lineshape. The surface of such ta-C films was found to be smooth and uniform from the images of atomic force microscopy. The variations of the microstructural properties and surface morphology with Vb have been discussed in relation to evolution of the D and G Raman bands and root-mean-square surface roughness.

Introduction

There has been interest in the synthesis of hydrogen-free tetrahedral amorphous carbon films with larger fraction of sp3 bonding, because of their hardness, electrical resistivity and chemical inertness [1]. There have been reports on the deposition of ta-C films and the processes governing their formation [2], [3], [4]. However, the experimental evidence for the presence of sp3 bonding has been limited by the difficulty in resolving the contributions from sp2 and sp3 sites in the experimental data. The most compelling evidence for the presence of sp3 C–C bonding has come from electron energy loss spectroscopy (EELS) [3]. However, the usefulness of EELS is also limited by the need to remove the films from the substrate, which makes EELS a destructive and time-consuming technique. Since the Raman cross section for sp2 bonded clusters is much greater than that for sp3 bonded clusters in Raman scattering with a laser source of 514 or 488 nm [5], the Raman spectrum of amorphous carbon films (a-C:H) is therefore dominated by a G band at ≈1550 cm−1 and a D band or shoulder around 1350 cm−1, both attributed to graphite sp2 bonding. The ratio of the intensities of the D and G bands increases with increasing of the sp2 content [6]. This increase is true even for the spectra of ta-C films with the smaller fraction of sp3 bonding. However, for ta-C films with a larger fraction of sp3 bonding, Prawer et al. [7] reported that the Raman spectrum can be fitted with a single skewed Lorentzian peak described by the Breit–Wigner–Fan (BWF) lineshape. The BWF coupling coefficient, Q, can be used to at least semi-quantitatively estimate the [sp3]/([sp2]+[sp3]) ratio of carbon films. The EELS results confirm that the sp3 fraction is likely to be >80% when Q is less than about −20 [7]. In our previous work [4], we reported that optical absorption spectroscopy is a simple method to compare qualitatively the sp3 fraction in the carbon films. The absorption spectra of the films were compared with those of a-Si:H and a-C:H. The spectra of the samples with larger fraction of sp3 bonding will be similar in shape to that of a-Si:H, while those of the samples with smaller sp3 fraction will be similar to that of a-C:H. In this paper, the ta-C films containing varying proportions of sp3 sites were prepared using magnetic filtered plasma stream at various substrate bias voltages, Vb. The Vb dependence of the sp3 bonding content in the films was studied using optical absorption spectroscopy, Raman spectroscopy and atomic force microscopy.

Section snippets

Experimental details

Tetrahedral amorphous carbon films were deposited using a magnetic field filtered plasma stream system. Details of the experimental set-up were described elsewhere [4]. The plasma was first formed by sputtering of a graphite cathode in an Ar atmosphere and then the plasma is passed through the curved magnetic field of a solenoid to filter out all the neutral atoms and macroparticles before deposition onto the substrate. The samples for Raman spectra and AFM image measurements were deposited

Results

The absorption coefficient, α, versus photon energy of the samples deposited with various substrate bias voltage, Vb, is shown in Fig. 1. The Raman spectra of the ta-C samples deposited at various Vb as indicated are shown in Fig. 2, where the fitting results using either the BWF lineshape or a double Lorentzian lineshape are also shown as dashed lines. The AFM images are shown in Fig. 3. The Vb dependence of the optical gap determined from the Tauc relationship [4], the ratio of maximum

Discussion

As reported in our previous work [4], the absorption spectra show a Vb dependence. For Vb⩾0 or Vb⩾−55 V, the shape of the absorption curves is similar to those of a-C:H films. The absorption increases with increasing photon energy and with a decreasing slope. These absorption spectra are consistent with the model density of states for a-C and a-C:H proposed by Dasgupta [8]. The fitted curves in Fig. 1 connecting the absorption coefficient data for the samples deposited at this Vb range are the

Conclusion

The sp3 fractions in the samples are found to depend on Vb. The samples with large or small fraction of sp3 bonding can be distinguished using optical absorption spectroscopy. The results show that ta-C films with larger sp3 fraction can be formed only in a Vb range from −10 to −50 V. The sp3 fractions in these films were estimated to be >80% according to a fitting of the Raman spectra with a single skewed Lorentzian lineshape (BWF lineshape). AFM images show that the surface of these films is

Acknowledgements

This work is supported in part by the Foundation of Key Scientific Researches of the Higher Education Bureau of Guangdong Province, People’s Republic of China.

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