H. Ehrhardt

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.

HIGHLY TETRAHEDRAL, DIAMOND-LIKE AMORPHOUS HYDROGENATED CARBON PREPARED FROM A PLASMA BEAM SOURCE

A highly tetrahedral, diamond‐like form of hydrogenated amorphous carbon (a‐C:H) with density of 2.9 g cm−3, sp3 fraction of 0.75, and hardness of 61 GPa, the highest values so far attained from a hydrocarbon source gas, has been deposited from acetylene in a novel plasma beam source. The ion energy for highest sp3 fraction is measured to be about 100 eV per C atom, similar to that for a‐C, indicating that the subplantation deposition model of a‐C also describes a‐C:H.

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.

MECHANISM OF BIAS-ENHANCED NUCLEATION OF DIAMOND ON SI

The nucleation of diamond on Si is enhanced for negative substrate bias of 200–250 V. We show that the ion flux is the critical factor causing the enhanced nucleation. The ion energy distribution has a maximum at about 80 eV, the optimum to subplant C ions into a‐C. We propose that subplantation causes deposition of nanocrystalline graphitic C, and that diamond nucleates where the graphitic planes are locally oriented perpendicular to the surface. An atomic model is proposed that allows a matching of the diamond, graphite, and Si lattice.

Effects of deposition temperature on the properties of hydrogenated tetrahedral amorphous carbon

The properties of hydrogenated carbon films deposited from a highly ionized hydrocarbon plasma beam are studied as a function of deposition temperature. At low temperatures, the films have high sp3 bonding, density, and compressive stress and are very smooth. Two transition temperatures are observed, a lower transition T1 around 250 °C, dependent on ion energy, due to graphitization of C–C bonds, and a higher one T2 at about 450 °C due to the loss of hydrogen. The roughness rises at T1 and falls above T2. These transitions are used to understand the subplantation deposition mechanism. The optical gap varies differently, decreasing gradually across T1 due to ordering of sp2 sites. We also report the temperature dependence of the x-ray diffraction, Raman spectrum, elastic modulus, hardness, substrate adhesion, friction coefficient, refractive index, and paramagnetic defect density. The friction coefficient of ta-C:H is low (0.05–0.1), and is maintained at ambient humidities, unlike for a-C:H. The friction m...

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.

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...

Experimental characterisation of bias-enhanced nucleation of diamond on Si

Abstract This paper reports an extensive characterisation of the bias process used to increase the nucleation density of diamond on Si. The nucleation density has been measured as a function of bias voltage, methane gas flow ratio and temperature. The nucleation density is found to be increased above 650 °C and reach a maximum at around −250 V. The nucleation density increases rapidly with time, up to a saturation value of about 10 10 cm −2 . The ion energy distribution is measured by a retarding field probe and has a maximum at ≈ 70–90 eV. This is close to the optimum energy for ion subplantation, responsible for sp 3 bonding in diamond-like carbon, which suggests that bias aids nucleation by some form of subplantation process.