Thin films of amorphous diamond like carbon (a:DLC) were deposited by using a novel technique. By electrodeposition from methanol-camphor solution thin a:DLC films were deposited on non-conductive glass substrates and also on high resistive Si substrates, by using a fine wire mesh electrode, at atmospheric pressure and temperature below 350 K. Thin films of a:DLC were doped by incorporation of nitrogen (a:N-DLC) and boron (a:B-DLC) using urea and boric acid with methanol-camphor solution respectively during the electrodeposition process. From transmittance measurements in the UV-VIS-NIR region, the optical energy band gap of about 1.0 eV for undoped a:DLC film, 2.12 eV for a:N-DLC and 2.0 eV for a:B-DLC films were determined. The spectra showed high transparency in the visible and NIR region. Fourier transform infrared spectroscopy (FTIR) measurements showed the appearance of various C-H and C-C bonding in the spectrum of undoped amorphous DLC film and confirmed C-N and C=N bond formation in a:N-DLC film. From the temperature variation of d.c. conductivity studies, the activation energies were determined and found to be 0.75 eV, 0.32 eV and 0.58 eV for undoped a:DLC films, a:N-DLC and a:B-DLC films respectively. Electrical resistivity at room temperature was reduced by the doping effect, from 109 Ω-cm for undoped films to 107 Ω-cm for nitrogen doped films and 108 Ω-cm for boron doped films.
Amorphous diamond like carbon (a:DLC) films have some interesting properties such as high mechanical hardness, dielectric strength, chemical inertness, low wear and friction, optical transparency in the visible and IR range, high resistivity and low band gap. These properties are promising for a wide range of applications such as mechanical, optical, electronic and medical applications. A large number of different techniques have evolved for the deposition of amorphous diamond like carbon films during the last two decades, the majority of the techniques reported so far involved chemical vapor deposition (CVD) [1-4]. Also, various attempts have been made to dope a:DLC films with different elements such as nitrogen [5,6], iodine [
Only in recent years several attempts have been made to synthesize diamond like carbon films by electrodeposition from various organic liquids under high voltages [12,13]. Very recently, we have demonstrated the synthesis of crystalline carbon nitride by the method of electrodeposition [
The apparatus used in our experiment was a simple electrolytic cell system. The schematic diagram is shown in
For FTIR measurement, Si (400) substrate having sheet resistance of several MΩs was taken, which was cleaned ultrasonically but no etching was performed to remove surface oxide layer. The counter electrode was a graphite plate. The substrate size was 3.5 × 2.5 cm2. The thickness of the wire mesh was 0.2 mm, and the area of each small square was 1.5 × 1.5 mm2. The distance between the electrodes was approximately 7 mm. Before deposition the glass substrates were cleaned with mild soap solution, then with distilled water and also by ultrasonic cleaner in acetone and finally degreased in methanol vapor. A d.c. power supply which can be varied from 0 to 3 KV was used to apply high voltages to the cathode. During the deposition processes, voltages were kept constant, but current densities varied.
To deposit amorphous DLC film, methanol-camphor solution was used as the electrolyte. In our experiment camphor was used with the methanol to get more methyl groups in the solution which is favourable for deposition of a:DLC by electrolysis. We observed the synthesis of nano-diamond films produced from chemical vapor deposition of camphor [
It is believed that the methyl radical plays a key role in depositing diamond and DLC films, hence it is expected that organic compounds such as camphor which generate larger number of methyl groups may act as better precursor. The solution was prepared by mixing 5.0 gm. of camphor (>99.5%) per 1 liter of analytically pure methanol (99.9%). As the dielectric constant (ε) of methanol is higher (32.70) [
films increased with the increase of ε values, so methanol was used as the source solution instead of using any other solution containing ethyl groups.
For nitrogen doping in a:DLC films, methanolcamphor-urea solution was used for the electrodeposition process 5.0 gm camphor and 0.5 gm of urea (CO(NH2)2) (99.5%) were mixed per 1 liter of methanol. For boron doping methanol-camphor-boric acid solution was used as the electrolyte. The solution was prepared by mixing 5.0 gm of camphor and 0.075 gm of boric acid (H3BO3) (99.5%) per 1 liter of methanol.
A number of experiments were performed varying the distance of separation between the glass plate and the wire mesh. The films deposited with the wire mesh in contact with the substrate showed variable thickness, i.e. larger thickness in areas that are not directly under the wires than the region directly under the wire. But, keeping the wire mesh at a distance of 0.5 mm from the substrate with some suitable teflon spacer, the deposition of films with uniform thickness has been possible.
In all experiments the electrolysis voltage was kept nearly 1.0 kV. During undoped a:DLC deposition the corresponding current was varying from 240 mA to 180 mA, while for N-DLC and B-DLC corresponding variations were 400 mA to 200 mA and 600 mA to 500 mA respectively. The temperature of the solution during all the experiments was about 350 K. The deposition time was 40 minutes. The films were characterized by Fourier transform infrared spectroscopy (FTIR), UV-Visible-NIR spectrophotometry, X-ray diffraction (XRD) studies and electrical measurements.
In the deposition process the applied potential was quickly increased to 1.0 kV and held constant. The current density increased linearly with the applied potential. When the deposition started, current density decreased remarkably and then gradually saturated with time. It takes only 10 minutes to decrease the substrate current from 400 mA to 200 mA. This phenomenon may be ascribed to the increasing resistance of the deposited film on the conducting wire mesh. The deposition mechanism of a:N-DLC and a:B-DLC may be inferred as follows:
Under high voltages, the molecules of CH3OH and CO(NH2)2 were polarized, two groups became partly discharged and moved in the electric field. The alkyl group (-CH3) from methanol and (-NH2) radical from urea both with a part of positive charge moved towards the cathode, here the wire mesh electrode. Major percentage of the radicals passed through the small openings of the electrode and deposited on the substrate placed near to it.
In the solution of methanol (CH3OH) and boric acid (H3BO3), they formed a compound, methyl borate (B(OCH3)3) which dissociated easily in B3+ and ions. The methyl group (-CH3) from methanol, which was partly positively charged, and B3+ ion both moved towards cathode and deposited on the substrate placed very near to it.
The spectra were measured in a FTIR spectrometer (Nicolet Magna-750) from 400 to 4000 cm–1 by subtracting the absorption due to the Si substrate. The spectra showed different vibrational modes of various bonding. In the IR spectrum of undoped a:DLC film, the 2958 cm–1 peak is assigned to an asymmetric mode of a saturated sp3 -CH3 group. The band at 2932 cm–1 corresponds to asymmetric -CH2 stretch and the 2850 cm–1 peak is assigned to a symmetric mode of a sp3 bonded -CH3 group. The peaks in the range of 1375 - 1600 cm–1 are assigned to vibrations due to sp2 carbon bonding. The broad band around 3400 cm–1 can be attributed to -OH stretching vibrations.
In the IR spectrum of a:N-DLC film, the band at 2923 cm–1 corresponds to the asymmetric -CH2 stretch for a typical hydrocarbon. It can also be observed that there are asymmetric -CH3 stretch mode at 2957 cm–1 and symmetric stretch mode at 2857 cm–1. All these vibrational frequencies are typical of tetrahedrally bonded carbon (sp3 bonding) in hydrocarbons. In this spectrum apart from the different C-H vibrational bands, few new bands appeared due to nitrogen incorporation. The absorption at 3200 - 3500 cm–1 suggests the existence of NHX (x = 1, 2) bonds. The strong absorption at 1400 - 1700 cm–1 can be attributed to the presence of C=N bonds. The absorption bands around 1100 - 1300 cm–1 can be assigned with C-N stretching vibration. The appearance of these peaks confirms the presence of carbon-nitrogen bond formation in the lattice. Thus nitrogen incorporation in the DLC matrix was confirmed. It was observed from IR spectrum of a:B-DLC film (not shown) that boron was also incorporated in the DLC film.
The spectra were recorded in a spectrophotometer (Hitachi-U 3410) by subtracting the transmittance of the glass substrate taking as reference. The optical transmission spectra indicated that the films showed high
transparency in visible and NIR region. The thickness values of the films were determined from SEM crosssectional images to be nearly 1.0 μm. From transmittance data using Manifacier model [
As the films were amorphous in nature as evident from XRD spectra (not shown here), Tauc relation [
where A is a constant, Eg is the band gap of the material. The (αhν)1/2 vs. hν plots of undoped a:DLC, a:N-DLC and a:B-DLC films are shown in the
The temperature dependence of d.c. conductivity (σ) of undoped and doped amorphous DLC films were measured by standard four probe technique using Kiethley electrometer (Model 6514) in the temperature range of 300 - 500 K. The experimental lnσ vs. 1/T plots for a:DLC, a:N-DLC and a:B-DLC films are shown in the
The conductivity in a:DLC films, which increased with increasing temperature at T > 300 K, suggested a thermally activated conduction mechanism. For extended state conduction the conductivity expression is of the form [
where, Ea is the activation energy, σ0 is a constant.
The values of σ were measured and it was found that nitrogen and boron doping caused an increase in the value of room temperature conductivity by two orders of magnitude for nitrogen doped films and one order of magnitude for boron doped films in our described deposition conditions. We have not yet optimized the doping concentration to obtain the lowest possible resistivity. Variation of electrical and optical properties with doping concentration for the electrolysis deposited DLC films will be a topic of future publication. Activation energies were calculated from the slope of lnσ vs. 1/T plots. Nitrogen doping increased n-type conductivity of a:DLC films, and the nature of n-type conduction was confirmed by hot probe measurement. Due to nitrogen substitution, all the excess electrons condensed into empty gap states
above the Fermi level (EF) and thus displaced the Fermi level towards extended state conduction band (EC). Activation energy (Ea)n can be identified with (EC − EF)0 i.e. the energy difference between extended states and Fermi level extrapolated to T = 0. Similarly, boron doping increased p-type conductivity shifting the Fermi level towards the extended state valance band (EV). Activation energy (Ea)p in this case can be expressed as (EF − EV)0. The values of activation energies were found to be 0.75 eV for undoped a:DLC film, 0.32 eV for a:N-DLC film and 0.58 eV for a:B-DLC films. At higher temperatures conductivity in a:N-DLC and a:B-DLC films decreased in comparison with undoped films, which could be observed from
Amorphous diamond like carbon films were deposited successfully on non-conductive glass as well as Si substrates by the electrolysis of methanol-camphor solution with the help of wire mesh electrode. Nitrogen and boron doping of the a:DLC films were achieved by using urea and boric acid with the precursor solution. FTIR spectrum of undoped a:DLC film showed the typical vibrations for tetrahedrally bonded carbon. The IR spectra of a:N-DLC and a:B-DLC films suggested incorporation of nitrogen and boron in DLC matrix. Optical measurements in UV-VIS-NIR region showed an increase in the optical energy band gap in a:N-DLC film, from 1.0 eV in the undoped film to 2.12 eV and in a:B-DLC film to 2.0 eV. The films showed high transparency in visible region. Room temperature conductivity was increased by two orders of magnitude in a:N-DLC films and one order of magnitude in a:B-DLC films. Incorporation of nitrogen and boron reduced the electrical activation energy of the films from 0.75 eV for a:DLC to 0.32 eV and 0.58 eV respectively for doped films.