Investigation of Vacuum Arc-Deposited ta-C and ta-C:N Thin Films on Silicon and Stainless-Steel Foil Substrates Using Raman Spectroscopy

Sanmitra Barman; Swarnim Neema; Anchal Rana; Arun Singh Patel; Anirban Chakraborti; Abhimanyu Singh Rana

doi:10.2320/matertrans.MT-M2022054

Abstract

In this paper, we investigated the vacuum arc-deposited tetrahedral amorphous carbon (ta-C) and nitrogen-doped ta-C (ta-C:N) thin films grown on silicon and stainless-steel foil substrates by the visible Raman Spectroscopy. We found that carbon films grown on silicon surfaces prefer more sp³ hybridized carbon compared to the stainless-steel foil, which prefer more sp² hybridized carbon even though the films are grown concurrently under the same conditions. The impact energy of plasma ions modifies the interfacial layer formation, giving a strong dependence of the sp²/sp³ ratio with the substrate bias, behaving differently for different substrates. However, nitrogen doping in amorphous carbon thin films helps to increase sp² contents uniformly on both the surfaces for a fixed substrate bias. Understanding such interfaces are of interest for many electronic, optoelectronic, and energy storage devices as these interfaces get buried and can influence the properties significantly.

Fig. 1 CVA deposited amorphous carbon thin films concurrently grown on silicon and steel foil substrates. Each graph showing the as deposited films under the base pressure (the navy-blue curve) and the films grown under nitrogen partial pressure (pink curve). The dotted and dash lines represent the Gaussian fitted D- and G-peaks, respectively.

1. Introduction

Interfaces of different allotropes of carbon are of interest for various electronic, optoelectronic and energy storage devices.¹^–³⁾ It is well known that sp, sp², and sp³ hybridization of orbitals gives an overall one-dimensional, two-dimensional, and three-dimensional character to the carbon that makes their properties unique to each allotrope.⁴^–⁶⁾ The mixture of these hybridized carbon are responsible for macroscopic properties in amorphous, micro- and nano-crystalline carbon.⁷⁾ The sp² hybridized form of carbon is more favourable thermodynamically than the sp³ bonding, so stabilizing the latter form of amorphous carbon is less energetically viable.⁸⁾ However, the high ionic energy deposition process such as cathodic vacuum arc (CVA) is known for depositing high sp³ hybridized carbon (responsible for diamond structure) content films known as tetrahedral amorphous carbon (ta-C) or diamond like carbon (DLC) films.⁹⁾ Therefore, stabilizing the sp³ content in these films are important to achieve superior mechanical properties such as high hardness, low friction and high optical transparency due to diamond character. However, sp² content plays an important role for tuning the electronic properties.¹⁰^–¹²⁾ Alternatively, doping with various elements such as N, S and O can be used to tune the desired properties of the ta-C.¹³⁾ Recently, a new phase of amorphous carbon namely “Q carbon” (or quenched carbon) has been reported by Narayan et al. by quenching from super undercooled state using high power nanosecond laser pulse.¹⁴⁾ It is proposed that that the films with higher sp³ content can act as an excellent seed layer for the subsequent microdiamond film formation.¹⁴⁾ There is also a renewed interests in amorphous carbon where the interfacial properties can play an important role for device performce.¹⁵^,¹⁶⁾ Therefore, carbon being the highly sensitive and widely used material in disordered and amorphous phase, it is of interest to understand if it would behave in the similar manner as these interfaces get buried in different electronic, optoelectronic, and energy storage devices. Moreover, diamond like carbon has been well known coating material for several biomedical applications.¹⁷⁾ However, there are not many studies that focus on the selection of the substrate influence at the interfaces of ta-C, which is usually ignored due to the lack of any epitaxy or preferred oriented growth.

In this work, we have investigated the interfacial aspect of the structural properties of amorphous carbon thin films grown on silicon and compared their results with the films grown on stainless-steel foil due to their recent interest and applicability.¹⁶^–¹⁸⁾ Interestingly, we have found that the films grown on silicon surfaces have more sp³ hybridized carbon compared to the films grown on stainless-steel foil which prefer more sp² hybridized carbon even though the films are grown concurrently under the same conditions. The impact energy of plasma ions modifies the interfacial layer formation, giving a strong dependence of sp²/sp³ bonded contents with the substrate bias. On the other hand, the nitrogen doping is found to increase sp² content on both the surfaces.

2. Experimental

The cathodic vacuum arc (CVA) depositions of tetrahedral amorphous carbon (ta-C) thin film were carried out using a graphite target (75 mm diameter, ∼99.99% purity) on a thoroughly cleaned silicon and stainless-steel foil (304 grade, 0.05 mm thickness) substrates at an arc current of ∼40 A at a temperature of ∼200°C for 60 seconds at three negative bias voltages ∼0 V, −60 V and −120 V under base pressure ∼5 × 10⁻⁶ mbar without using any magnetic filter. The nitrogen doped tetrahedral amorphous carbon (ta-C:N) were deposited in the presence of background nitrogen pressures ranging from 5 × 10⁻⁵ mbar to 1 × 10⁻³ mbar using high purity (99.999%) nitrogen gas. Before deposition, the substrates were thoroughly cleaned with ultrasonication for 30 minutes by immersing them in DI water and isopropanol periodically followed by drying at 80°C for 2 h. The substrate holder was set under rotation during deposition with 20 rpm for better film uniformity. The carbon deposition rate in the cathodic arc system was ∼1 nm/second. Raman spectroscopy measurements were performed at room temperature using a green laser (532 nm) having ∼25 mW incident power by Witec Alpha 300 system. Thickness and the roughness of these thin films were measured by surface profilometer by Bruker, Dektak XT.

3. Results and Discussion

Figure 1 shows the Raman spectra of carbon thin films grown on silicon (shown on left side), and stainless-steel foil (shown on right side) substrates. Each graph compares the Raman spectrum of as grown carbon films at a base pressure ∼5 × 10⁻⁶ mbar (undoped films shown by navy blue colour) and under the background nitrogen partial pressure ∼5 × 10⁻⁵ mbar (doped films shown by pink colour) at a fixed substrate bias of −60 V.

Fig. 1

CVA deposited amorphous carbon thin films concurrently grown on silicon and steel foil substrates. Each graph showing the as deposited films under the base pressure (the navy-blue curve) and the films grown under nitrogen partial pressure (pink curve). The dotted and dash lines represent the Gaussian fitted D- and G-peaks, respectively.

The results of the other films grown at other substrate bias voltages will be discussed in Fig. 2. Interestingly, the results shown in Figs. 1 and 2 are of the ta-C and ta-C:N films grown concurrently on silicon and stainless-steel foils as both substrates were kept at the same time under the same conditions during the deposition. The carbon films grown on both silicon and stainless-steel foil clearly show a convoluted G-peak ∼1580 cm⁻¹ (also called graphitic peaks) and D-peak ∼1375–1410 cm⁻¹ (also called defects peak). These G- and D-peaks are originated from E_2g phonon modes (C-C stretching) and A_1g phonon modes, respectively. The D-peaks are produced only when there are structural defects. Visible Raman spectroscopy is more sensitive to sp² sites than sp³ sites because visible photons (2.2 eV) preferentially excite the π states.¹⁹⁾ We have found a large deviation in the D-peak value than reported values on tetrahedral amorphous carbon in the literature ∼1350 cm⁻¹. This could be due to variation in thin films properties. However, the carbon films grown on stainless-steel foil surfaces show a clear split into D-peak and G-peaks as they slightly shifted towards the lower and higher frequencies, respectively. These peaks were deconvoluted by gaussian fitting (Shown in all figures by the dotted lines for D-peaks and dash lines for G-peaks) and parameters such as the ratio of intensity (I_D/I_G), ratio of area under the curve (A_D/A_G) and full-width at half-maximum (FWHM) of D- and G-peaks were calculated and analysed. Especially, the I_D/I_G is commonly used to represent the sp²/sp³ bonding ratio in amorphous carbon thin films. Increase of I_D/I_G is related to the decrease of sp³ and increase of sp² content in the film.²⁰⁾ Interestingly, the I_D/I_G for the as grown films (undoped) on stainless steel foil is ∼1.35 compared to ∼0.4 films on silicon, both values are increased i.e., ∼1.95 for films on steel foil and ∼0.95 for films on silicon after the nitrogen doping. This clearly indicates the sp² bonded carbon atoms are preferred on stainless-steel compared to silicon surfaces in both undoped and nitrogen doped conditions, which is also expected thermodynamically as the sp² bonded carbon is more energetically preferred than sp³ bonding.¹⁰⁾ The nitrogen doping, however, favours to increase the intensity of D-peak for both the films grown on silicon and stainless-steel surfaces. Since it requires a large energy to form the sp³ bonded carbon,¹⁰⁾ the higher value of sp³ content on silicon and steel could be enhanced by the formation of silicon–carbon and carbon–carbon bonding on both the substrates.²¹⁾ This assists the growth of tetrahedral carbon at the interface and that in turn plays an important role for further nucleation and growth of thin films. As the nitrogen dopants in the tetrahedral amorphous carbon first occupy the dangling bonds of already existing sp² bonded network but the overall sp³ network is preserved, the intensity of D-peak increases with the nitrogen doping and the G-peak intensity almost remains the same.²²⁾

Fig. 2

(a)–(d) undoped (taC) and nitrogen-doped (taC:N) carbon thin films grown on silicon and steel foil surfaces under different substrate bias voltages.

Figure 2(a)–(d) shows the behaviour of undoped, and nitrogen-doped ta-C thin films grown on silicon and stainless-steel foil surfaces under different substrate bias voltages. Note the substrate bias voltage here represents the additional energy provided to ions in the plasma during deposition which influences the nucleation and growth process of thin films. The extracted parameters from Gaussian fitting are plotted as a function of the substrate bias voltage in Fig. 3(a)–(d) for both as undoped and nitrogen doped carbon films grown on silicon (shown in the left column) and steel surfaces (shown in the right column) respectively. Both undoped and nitrogen-doped ta-C films grown on silicon substrates show a minimum value of I_D/I_G and A_D/A_G around −60 V bias voltage. Lifshitz et al. have proposed the mechanism that increasing the energies of impact ions on the substrate promote sp³ hybridization in carbon by displacing the sp² atoms on the surface, which can explain the initial decrease of sp²/sp³ ratio with increasing the bias voltage, but if the impact energy is increased further, the bombarding ions can penetrate inside the forming layers and induce defects and disrupt the silicon–carbon interfacial layer.²³^,²⁴⁾ Interestingly, a similar change in sp³ content has also been observed by Joshi et al. in pulsed laser annealing process as function of laser energy density.¹⁶⁾ These results are also consistent with molecular-dynamic simulations studies, where a peak in the compressive stress was observed around −60 eV impact energy due to the formation of sp³ bonded atoms from the displaced sp² atoms at this energy.¹⁰⁾

Fig. 3

(a)–(h) the substrate bias dependence of I_D/I_G (on left Y-axis), A_D/A_G (on right Y-axis) and FWHM (D-peak on left Y-axis and G-peak on right Y-axis) for taC and taC:N films grown on silicon and steel substrates, the dotted lines are drawn as guides to the eye.

On the other hand, the substrate bias dependence of I_D/I_G or A_D/A_G (as shown in Fig. 3(b) and 3(d)) for both undoped and nitrogen-doped ta-C films grown on steel, are substantially different than the films grown on silicon. First, for the as grown undoped films on steel, the sp²/sp³ increases with the substrate bias voltage without having any minima as seen in the silicon case. Second, for the doped films on steel, the bias dependence is quite opposite to the silicon case (3d). The maximum value of I_D/I_G or A_D/A_G could be seen at −60 V substrate bias than other two bias values. Unlike silicon where the interface helps to grow sp³ bonded carbon, steel prefers the sp² bonded carbon. There is not a significant amount of sp³ bonded atoms presents in carbon film on the steel surface, the high impact ionic energy helps the ions to defuse inside the steel and forms the layer of sp² bonded carbon network at the interface. On the other hand, for the nitrogen doped case on steel, the intensities ratio of I_D/I_G is ∼2 at −60 V substrate bias indicating a large amount of sp² fraction, that might lead to the saturation of sp² bonded carbon atoms in a three-dimensional network. However, a more detailed study is required to further confirm the reduction of sp²/sp³ ratio with impact ionic energy on carbon films on steel surfaces. The properties observed are showing a maxima or minima at −60 V and reduces or increases in either side of this substrate bias in the present study. The properties are found to depend on the substrate bias applied,¹⁰^–¹²⁾ and it may vary from system to system.²²⁾

4. Conclusions

In summary, we highlight a significant role of the interface for the formation of sp² and sp³ carbon, which is otherwise unexpected in amorphous materials in two different substrates. We report that the carbon thin film grown on silicon prefers the sp³ bonding, but on stainless-steel foil it prefers the sp² bonding. The interfacial layer formation can be modified by the impact energy of plasma ions, giving a strong dependence of sp²/sp³ ratio on substrate bias voltage. The nitrogen doping in ta-C films grown on both surfaces changes the bonding network and enhances the sp² form of carbon. Lastly this work is particularly important for the biomedical applications where the amorphous carbon coating on flexible substrates such as medical grade polyethylene-based skeletal implants are in great demand.²⁵^,²⁶⁾

Acknowledgement

A.S.R. acknowledges the SERB Core Research Grant CRG/2021/001136. S.B acknowledges BML Munjal University (BMU) seed grant BMU/SG/SOET/APPSCI/2021-001. We acknowledge Mr. Piyush Chaturvedi for his help and support during experiments, and Prof. B.S. Satyanarayana for the discussions. We also acknowledge the Vice-Chancellor and the President of BMU for the providing the research infrastructure at CAMD.

REFERENCES

Corresponding author

Register with J-STAGE for free!