Raman Spectroscopy of Amorphous Carbon Prepared by Pulsed Arc Discharge in Various Gas Mixtures

To meet various application requirements, it is important to enable an improvement of a-C structure and properties, such as hardness, adhesion, and wear resistance. In this study, we used the Raman spectroscopy to investigate the a-C thin films structure dependence on the different deposition parameters. The effect of nitrogen, argon, and hydrogen gas flow rate was analyzed to determine the influence on the film properties. The change in the gas type, combination, and flow had a significant influence on the D and G bands of the a-C Raman spectra. The addition of N2 into the chamber promoted the sp2 creation, while with adding hydrogen the layer contained more sp3 bonds. The depositions of a-C thin films were carried out in pulsed arc discharge vacuum installation. Micro-Raman measurements of the deposited materials were performed using an ISA Dilor-Jobin Yvon-Spex Labram confocal system with 632.8 nm radiation from a He-Ne laser using a back-scattering geometry.


Introduction
In recent years, a lot of work has been focused on the synthesis of novel carbon thin �lms materials, especially crystalline diamond, amorphous carbon (a-C), and carbon nitride (a-C:N) which offers excellent tribological, optical, electrical, and other properties with the additional advantage that carbon is a biocompatible material.e huge range of properties achievable in carbon coatings is mainly due to the ability of carbon to form different types of interatomic bonds, to take up different sites, and to adopt different structures.However, for each application, there are different requirements on the �lm properties, for example, the adhesion level achievable and coating cost.erefore, several methods to in�uence the a-C �lms structure, composition, and thus the properties, such as variation of ion energy, plasma treatment, pressure, and the use of different gases were presented [1][2][3][4][5][6].
Raman spectroscopy can be applied as a simple and accurate identi�cation method of different carbon phases within the �lm.Due to its sensitivity to variation of translation symmetry, Raman spectroscopy allows distinguishing several types of carbon such as diamond, graphite, diamondlike carbon, and carbon nitride.e Raman spectra of nanocrystalline and amorphous carbon are dominated by the D (D for disorder) and G (G for graphite) peaks with varied intensity, position, and band width [7][8][9][10][11].e G band at approximately 1540-1600 cm −1 corresponds to the symmetric E 2 mode in graphite-like materials, while the D band at 1350 cm −1 arises from the limitations in the graphite domain size, induced by grain boundaries or imperfections, such as substitutional N atoms, sp 3 carbon, or impurities.e D peak is not present in perfect single crystal graphite and becomes active only in the presence of disorder.For visible excitation, the G and D peaks are due to sp 2 sites only.e sp 2 sites have such a high cross-section that they dominate the spectra, the sp 3 sites are invisible, and the spectrum responds only to the con�guration or order of the sp 2 sites.e Raman  spectra of a-C depend on (1) clustering of the sp 2 phase; (2) bond length and bond angle disorder; (3) presence of sp 2 rings or chains; (4) the sp 2 /sp 3 ratio.e spectra directly depend on the quality or con�guration of the sp 2 phase and only indirectly on the quantity of the sp 2 phase.Most times the sp 2 con�guration varies consistently with the sp 2 fraction.However, in some cases, the sp 2 quality can be changed independently from the sp 2 /sp 3 ratio [12][13][14][15][16][17].A weak peak at 2200 cm −1 is attributed to triple C-N bonds and appears in the case of high nitrogen doping.e second-order silicon peak which appeared at 960 cm −1 can be used to measure the transparency of the �lm.e  Si / G ratio increases with the increase of N content in the �lm, revealing a reduction in the optical band gap when more N atoms are incorporated in �lms [18].In this study, we used the Raman spectroscopy to investigate a-C thin �lms structure and its dependence on the gas type and �ow.

Experimental
e depositions of amorphous carbon thin �lms were carried out in a vacuum installation type UVNIPA-1-001 described previously [19].e sputtering frequency of the arc source pulses was 5 Hz. Background pressure was 10 −4 Pa and working pressure was maintained around 1 Pa according to gas �ow.Argon and nitrogen gas �ow was varied from 0 to 100 sccm and the �ow of hydrogen from 20 to 60 sccm.e deposition temperature was kept below 100 ∘ C. Mirror polished silicon substrates were used for deposition of the coatings.Before deposition of the layer, substrates were cleaned for 10 min with Ar ions within one vacuum cycle.e e�ects of nitrogen, argon, and hydrogen gas �ow on the �lm properties were investigated by Raman spectroscopy.Micro-Raman measurements of the deposited materials were performed using an ISA Dilor-Jobin Yvon-Spex Labram confocal system with 632.8 nm radiation from a He-Ne laser using a back-scattering geometry.Microscope objective ×80 was used to focus the laser beam onto a spot of approximately 1-5 m in diameter and to collect the scattered light, which then passed through the spectrometer onto a CCD detector.e acquired Raman spectra were �tted with a Gaussian line to illustrate the G peak position and  D / G ratio.e XPS investigations were performed in an Omicron UHV system  equipped with an EA 125 hemispherical electron analyzer using monochromated AlK radiation (excitation energy ℎ = 1486.7 eV).e samples have been annealed by radiative heating from the backside at 500 ∘ C for 10 min in UHV conditions.e calibration of the energy scale was made by comparison to reference measurements on a polycrystalline silver sample and HOPG.

Results and Discussion
�aman spectra of the �lms deposited with various argon or nitrogen �ow during the deposition process are shown in Figure 1.We can see that, with changing both Ar and N 2 �ow the intensity maximum of all three depicted bands (Si, D, G band) changes signi�cantly.e maximum intensity in Si band falls with increasing the process gas �ow from 0 to 100 sccm.is indicates lowered transparency of the �lms due to the growth of a thicker layer and also due to more graphitic character of the �lms deposited with higher Ar or N 2 �ow [18].Figure 2 shows the  D / G ratio and G peak position dependence of the �lms from Figure 1.For the layers deposited in nitrogen atmosphere both  D / G and G peak position increases as the amount of N 2 in the chamber during deposition rises.e  D / G ratio is 0.25 for pure a-C �lm and 1.35 for �lm deposited with 100 sccm of N 2 �ow.e G peak position rises from 1518 cm −1 for a-C to 1574 cm −1 for a-C:N with 100 sccm of nitrogen �ow.e simultaneous growth of both  D / G ratio and G peak position with increasing of nitrogen �ow is a reliable indicator of sp 3 /sp 2 ratio decrease and corresponds to another research [20].From the spectra, we can assume that, nitrogen creates rather sp 2 than sp 3 bonds and the �lms become more graphitic when the N 2 �ow rises, which is similar to other studies [11,14,18].e  D / G ratio and G peak position of the �lms deposited with various Ar �ow shows slight grow from 0 to 30 sccm, drop around 60 sccm and sharp increase for �lms deposited in atmosphere with 100 sccm of Ar �ow.e �lms around 60 sccm Ar �ow have the lowest  D / G ratio and it seems such a moderate Ar �ow is optimal for deposition of the �lms with higher sp 3 /sp 2 ratio.Further increasing of Ar �ow caused more graphitic bonds within the �lms.
�aman spectra of the a-C:N �lms deposited with combined Ar/N 2 �ow are shown in Figure 3. e Ar �ow was set constant to the values with best results from previous depositions (40 and 60 sccm) while the N 2 �ow was changed from 40 to 100 sccm to estimate the in�uence of nitrogen in the combined gas �ow.From  D / G ratio and G peak  Binding energy (eV)   position dependence shown in the Figure 4 it is obvious that, for the �lms deposited with 40 and 60 sccm argon �ows, the maximum of G peak position is around 80 and 60 sccm of nitrogen �ow, respectively.e values of  D / G ratio and G peak position for all gas combined depositions are higher than for layers deposited with only one gas type.erefore, we can assume that, the further Ar/N 2 admixture into the chamber during deposition process promotes the creation of sp 2 bonding in the thin �lm.Besides the main D and G broad peaks, with increasing the N 2 �ow we can observe also a small local maximum at approximately 2200 cm −1 attributed to triple C-N bonds.is band is characteristic with very small intensity in the Raman spectra made using visible laser excitation [21,22].
Figure 5 shows the Raman spectra and data abstracted from �tting of the �lms deposited with constant Ar/N 2 �ow while adding H 2 into the gas mixture.As we can see, the  D / G ratio decreased from 1.6 for nonhydrogenated a-C:N to 1.1 due to the presence of hydrogen in the gas mixture.e G peak position is changing only slightly towards higher values.e creation of sp 2 phase is due to the hydrogen addition lower.is may be due to hydrogen affecting as an etchant of graphitic bonds during growth of the �lm causing the higher sp 3 content.
For comparison, XPS analysis of the a-C:N samples deposited with 30, 50, and 70 sccm of N 2 �ow was made (Figure 6).e resulting sp 3 /sp 2 ratio decreased with nitrogen �ow increasing.e results correspond to the Raman measurements made on these samples and are in agreement with other studies [13,15,18].

Conclusions
We deposited continuous, homogeneous, and adhesive a-C, a-C:N and a-C:N:H thin �lms on Si substrates using pulsed vacuum arc discharge technique.Following the Raman spectra we found that, with adding different gases with various concentrations the �lm structure and properties change signi�cantly.e addition of N 2 and/or Ar caused the simultaneous growth of both  D / G ratio and G peak position which is a reliable indicator of sp 3 /sp 2 ratio decrease and increase of graphitic contents in the �lms.e �lms with only a moderate Ar �ow had lower  D / G ratio and this was found to be optimal for deposition of the �lms with high sp 3 /sp 2 ratio.e addition of hydrogen into the gas mixture caused the decrease in  D / G ratio and thus we can expect lower amount of graphitic sp 2 bonding.We found that, the maximum intensity of Si band falls with increasing of the process gas �ow.is indicates lowered transparency of the �lms due to the growth of a thicker layer and also due to the more graphitic character of �lms deposited with higher Ar and/or N 2 �ow.

1 )F 2 :
D / G and G peak position dependence of the �lms deposited with various N 2 and Ar �ows.

F 3 :
�aman spectra of a-C:N �lms deposited with constant Ar �ow and various N 2 �ows.

F 4 :
D / G and G peak position dependence of the �lms deposited with constant Ar �ow and various N 2 �ows.

F 5 :
Raman spectra of a-C:N:H grown with constant Ar/N 2 �ow (a)�  D / G an� G pea� position �epen�ence of the �lms (b).