J. Schwan

Raman spectroscopy on amorphous carbon films

The origin and interpretation of the Raman features of amorphous (hydrogenated) carbonfilmsdeposited at room temperature in the region of 1000–1700 cm−1 is discussed in this paper. Possible interpretations of the linewidths, positions of the ‘‘G’’ graphite peak and ‘‘D’’ disordered peak, and their intensity ratios are examined using results obtained from magnetron sputtered and magnetic field enhanced plasmadepositedfilms. It is shown that even small ‘‘clusters’’ of condensed benzene rings (cluster size below 20 A) in carbonfilms can explain the observed Raman scattering. Besides the care that should be taken in the correct interpretation of Raman results, the utility of Raman scattering in obtaining an estimate of cluster sizes in amorphous (hydrogenated) carbonfilms is discussed. Carbonfilms prepared by magnetron sputtering show two additional Raman features at 1180 and 1490 cm−1 in addition to the G and D peaks. It is shown that a correlation exists between the 1180 cm−1 peak and the sp 3 content in the films.

Nitrogen modification of hydrogenated amorphous carbon films

The effect of nitrogen addition on the structural and electronic properties of hydrogenated amorphous carbon (a-C:H) films has been characterized in terms of its composition, sp3 bonding fraction, infrared and Raman spectra, optical band gap, conductivity, and paramagnetic defect. The variation of conductivity with nitrogen content suggests that N acts as a weak donor, with the conductivity first decreasing and then increasing as the Fermi level moves up in the band gap. Compensated behavior is found at about 7 at. % N, for the deposition conditions used here, where a number of properties show extreme behavior. The paramagnetic defect density and the Urbach tailwidth are each found to decrease with increasing N content. It is unusual to find alloy additions decreasing disorder in this manner.

Tetrahedral amorphous carbon films prepared by magnetron sputtering and dc ion plating

Highly tetrahedral, dense amorphous carbon (ta‐C) films have been deposited using rf sputtering of graphite by an unbalanced magnetron with intense dc Ar‐ion plating at low temperatures (<70 °C). The ratio of the argon ion flux to neutral carbon flux Φi/Φn is about 5. The film density and compressive stress are found to pass through a maximum of 2.7 g/cm3 and 16 GPa, respectively, at an ion plating energy of about 100 eV. Experiments with higher ion flux ratios of Φi/Φn=10 show that it is possible to deposit carbon films with densities up to 3.1 g/cm3 and sp3 contents up to 87%. Deposition of ta‐C in this experiment when the energetic species is Ar appears to require a minimum stress of 14 GPa to create significant sp3 bonding, which contrasts with the continuous increase in sp3 content with stress when the energetic species is C ions themselves. These results are used to discuss possible deposition mechanisms.

Microstructures and mechanical properties of amorphous hydrogenated carbon-nitrogen films

Abstract Amorphous hydrogenated carbon-nitrogen (a-C:N:H) films have been deposited on glass, silicon and aluminium substrates by r.f.-plasma-enhanced chemical vapour deposition using C2H2 and N2 gas mixtures in the plasma reactor. An enhanced nitrogen partial pressure leads to a decrease in the hydrogen content and an increase in the nitrogen content of the films as shown by combustion analysis and the IR absorption coefficients for the N-H, C-N and C-H stretching modes. The existence of nitrile groups in a-C:N:H films explains the reduced hardness and stress of the films with increasing nitrogen content. Further, decreases in the optical band gap by 0.5 eV and in the halfwidth of the electron spin resonance line down to 2.5 G are observed, as well as increases in the density of states at the Fermi level by one order of magnitude and in the d.c. conductivity by five orders of magnitude. Temperature-dependent conductivity measurements are consistent with a hopping mechanism around the Fermi level in the temperature range from 170 to 400 K and confirm that the density of states at the Fermi level is the most important parameter for d.c. conductivity. The measurements demonstrate the inefficiency of doping of a-C:H films with nitrogen.

Nitrogen doping of amorphous carbon thin films

Nitrogenated and hydrogenated amorphous carbon (a-C:H:N) films have been deposited by a plasma beam source using a gas mixture of C2H2, Ar and N2. The Ar/C2H2 ratio is kept constant at a ratio of 3, with the nitrogen flow allowed to vary. Nonnitrogenated films, with Ar/C2H2 ratios of 3 and 6 were also deposited and analyzed before attempting to identify the modifications to the microstructural properties due to nitrogen doping. The nitrogenated and hydrogenated a-C (a-C:H:N) films deposited in this study reveal interesting properties with regard to their optical gap, electrical conductivity, and mobility of the charge carriers. The optical E04 gap passes through a maximum of 2.7 eV as a function of incorporated nitrogen. The electrical conductivity, too, reaches a peak value of 10−3(Ω cm)−1 with increasing optical gap and remains constant for higher N2 flows. The electrical conductivity process is thermally activated with activation energies in the range 0.1–0.3 eV. This is discussed in terms of the mobil...

Stress-induced formation of high-density amorphous carbon thin films

Amorphous carbon films with high sp3 content were deposited by magnetron sputtering and intense argon ion plating. Above a compressive stress of 13 GPa a strong increase of the density of the carbon films is observed. We explain the increase of density by a stress-induced phase transition of sp2 configured carbon to sp3 configured carbon. Preferential sputtering of the sp2 component in the carbon films plays a minor role compared to the sp2⇒sp3 transition at high compressive stress formed during the deposition process. Transmission electron microscopy shows evidence of graphitic regions in the magnetron sputtered/Ar plated amorphous carbon thin films. Differences in the microstructure of the tetrahedral amorphous carbon (ta–C) films deposited by filtered arc and mass selected ion beam; and those films deposited using magnetron sputtering combined with intense ion plating can be used to explain the different electronic and optical properties of both kinds of ta–C films.

Structure and luminescence properties of an amorphous hydrogenated carbon

Abstract A low hydrogen content form of amorphous hydrogenated carbon (a-C: H) was deposited from a magnetically confined plasma, for a range of self bias voltages. The structure was characterized in terms of its density, sp3 bonding fraction, H content, and H bonding. The Tauc optical gap was found to vary from 2 to 3 eV as the negative bias voltage decreased. The density of paramagnetic centres ranged from below 1018cm∼3 to above 1020cm″3. The films are strongly photoluminescent (PL) at room temperature. Their PL properties were measured and correlated with the optical gap and defect density. The PL and the PL efficiency was found to increase strongly with optical gap.

Preparation of cubic boron nitride films by radio frequency magnetron sputtering and radio frequency ion plating

Cubic boron nitride (c‐BN) thin films have been deposited by unbalanced rf (13.56 MHz) magnetron sputtering of a hexagonal boron nitride target in a pure argon discharge. Deposition parameters have been 300 W rf target power, 8×10−4 mbar argon pressure, 3.5 cm target substrate distance, and 800 K substrate temperature. Under these conditions the ion current density is 2.25 mA/cm2 and the growth rate is ∼1.1 A/s. By applying a rf substrate bias the ion plating energy is varied from plasma potential of 37 eV up to 127 eV. The films have been characterized by infrared (IR) and Auger electron spectroscopy (AES), x‐ray diffraction (XRD), x‐ray reflectivity, elastic recoil detection (ERD), Rutherford backscattering (RBS), nuclear resonance analysis (NRA), and stress measurements. The subplantation model proposed by Lifshitz and Robertson can be applied to the c‐BN formation. An energy of about 85±5 eV is found where the stress (25 GPa, 200 nm film thickness) and the c‐BN content (≳90%) have a maximum. The grain...

Radio frequency ion plating-induced phase transition from h-BN to nanocrystalline c-BN

Boron nitride thin films were deposited by unbalanced r.f. magnetron sputtering from a hexagonal boron nitride (h-BN) target and by r.f. ion plating at low gas pressures. The content of cubic boron nitride (c-BN) is determined by IR spectroscopy to be higher than 90%. The grain size derived from X-ray diffraction (XRD) is in the region of 5 nm. Furthermore, the films are characterized by Auger electron spectroscopy (AES) and stress measurements. The deposition of the cubic phase is only possible in a small range of boron to nitrogen ratio (0.9 < BN < 1.1). The film-forming particles (flux Φn) are mainly neutral boron and nitrogen atoms with typical sputtering energies of a few electronvolts, and the plating particles (flux gFi) are argon and krypton ions with a well defined energy up to 140 eV. The current density is more than 2 mA cm−2, as determined from energy and mass analysis. The arrival ratio ΦiΦn is approximately 20. The high ion current density causes a substrate temperature of about 510 K, and external substrate heating allows us to raise the temperature to 800 K. The c-BN formation depends mostly on the r.f. substrate bias and slightly on the substrate temperature and the rare gas masses. The results are compared with trim calculations. The film properties are discussed on the basis of the subplantation model.

Nitrogenated amorphous carbon as a semiconductor

Abstract At present, hydrogenated amorphous carbon (a-C:H) is a poor electronic material primarily due to the excessive density of defect states in the band gap which act as trapping centres. The ability of nitrogen to improve the semiconducting properties of a-C:H is examined. A reduction in the activation energy for electronic conduction in nitrogenated a-C:H (a-C:H:N) films and the approximately constant optical band gap with increasing N content suggest that N influences the bulk electronic properties of a-C:H. Electron spin resonance shows a reduction of the density of gap states in a-C:H:N with increasing N content. Electron energy loss spectroscopy shows the films to be predominantly sp2 bonded with band edge properties which change significantly as a function of the N content. The C:H:N contents of the films were determined by elastic recoil detection analysis and Rutherford backscattering.