Diamond like nanocomposite of Carbon thin films Optical properties

 

Abstract:

The optical properties of silicon-incorporated diamond-like carbon (Si-DLC) nanocomposite thin films have been reported. The Si-DLC nanocomposite thin film deposited on glass and silicon substrate by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) process. Fourier transformed infrared spectroscopic analysis revealed the presence of different bonding within the deposited films and deconvolution of FTIR spectra gives the chemical composition i.e., sp3/sp2 ratio in the films. The optical band gap calculated from transmittance spectra increased from 0.98 to 2.21 eV with a variation of silicon concentration from 0 to 15.4 at. %. Due to a change in electronic structure by Si incorporation, the Si-DLC film showed broad photoluminescence (PL) peak centered at 467 nm, i.e., in the visible range, and its intensity was found to increase monotonically with at. % of Si.

INTRODUCTION:

Diamond-like carbon (DLC) has been extensively studied because of its unique properties such as high hardness, low friction coefficient, chemical inertness, and resistance against corrosion potential applications in science and technology. The DLC is a metastable form of an amorphous mixture of sp2 and sp3 hybridized carbon atoms, where the collective behavior of sp2 sites is responsible for the optical and electrical properties and the sp3 sites govern the mechanical properties. The sp2 and sp3 bonding ratio in the DLC matrix can be changed by the doping of different metals and non-metals such as Sn, Ag, Au, Pt, Cu, Mo, Ti, W, etc. In the DLC field, silicon-incorporated diamond-like carbon (Si-DLC) thin films are very important because they can solve some major drawbacks of pure DLC such as reducing residual internal stress, improved hemocompatibility properties, reduced hydrogen loss, graphitization, and improved high-temperature stability. In this present work, we reported the optical properties of silicon-incorporated diamond-like carbon (Si-DLC) nanocomposite thin films prepared by the RF-PECVD technique. Fourier transformed infrared spectroscopic analysis revealed the presence of Si-C, Si-H, and Si-H2 bonding within the deposited films, and deconvolution of FTIR spectra gives the chemical composition i.e., sp3 /sp2 ratio in the films. The transparency and the optical band gap increased with the increase in the Si atomic percentage in the films. The intensity of the photoluminescence peak in the visible range was increased with the increase of Si atomic percentage in the films. 

EXPERIMENTAL DETAILS:

                                                         Synthesis of Si-DLC thin films 

The radio frequency (13.56 MHz) plasma enhanced chemical vapor deposition (RF-PECVD) technique was used for the Si-DLC thin film deposition on glass and a silicon substrate. Silicon substrates were cleaned with 10 % HF solution in a proper way to remove the surface oxide layer. The substrates were ultrasonically cleaned for 20 min, acetone, and methanol in sequence. To carry out the deposition, the chamber was evacuated to a base pressure of 2×10-6 mbar and then acetylene (C2H2) gas was introduced into the chamber for a fixed deposition pressure of 0.5 mbar. The deposition time was varied from 12 to 15 min with RF power 180 Watt for Si-DLC thin film with thickness ~ 400 nm. Tetraethyl orthosilicate (TEOS) dissolved in methanol was used for silicon incorporation in the DLC matrix. Argon (Ar) gas was passed through the TEOS solution to form the bubble and then introduced into the deposition chamber. The at. % of Si in the deposited Si-DLC thin films varied from 0 to 15.4 by changing the TEOS percentage in the solution. 

Characterizations 

The composition of the films (Si, C) was determined by energy dispersive X-ray analysis (EDX, Oxford, model-7582). Bonding information and the composition i.e., the sp2 /sp3 ratio in the synthesized thin films were analyzed by a Fourier transform infrared spectrophotometer (FTIR, Shimadzu - 8400-S). The FTIR absorbance spectra were recorded by taking Si as the reference and subtracting the absorption due to the Si substrate. The optical transmittance measurements were done using a Perkin Elmer UV-Vis spectrometer (USA). Photoluminescence spectra have been recorded by a fluorimeter (FL 4500, Hitachi) at room temperature. The excitation wavelength was 300 nm (Xenon lamp, 100 W). 

RESULTS AND DISCUSSION 

Chemical composition analysis Compositions of the Si-DLC films (Si, C) were determined by energy dispersive X-ray (EDX) analysis. The FTIR spectra show the different vibrational modes of various bonding for Si-DLC films in figure 1(a). In the case of Si-DLC film, the peak in the range 1400 to 1600 cm-1 is assigned to vibrations due to the C = C (sp2 ) stretching mode, and the broad peak around 2950 cm-1 is assigned to the different C-Hn group stretching modes. In the FTIR spectra, the CH3 asymmetric stretching vibration was observed at around 2975 cm-1, whereas the absorption occurs at about 2930 cm-1. The CH3 symmetric vibration occurs at about 2865-2885 cm-1 and the CH2 absorption occurs at 2850 cm-1. It is clear from figure 1(a) that the absorption peaks shifted up around the wave number of 2950 cm-1 with increasing Si content in DLC films. The strong FTIR absorption of the C-H stretch was obtained in the range 2800-3150 cm-1 which reveals a high H content in the films. The baseline correction is done for the selected region of CH stretching and deconvoluted peaks were curve fitted in a Gaussian distribution. The values of the sp3 /(sp2 +sp3 ) ratio have been calculated from the deconvoluted FTIR spectra, which shows that the overall sp3 fraction increased with the increase of at. % of Si in the DLC films as shown in figure 1(b). It is clear from the above discussion that the Si-DLC films become more diamond-like in nature with increasing at. % of Si. 

FIGURE 1. (a) FTIR spectra of Si-DLC films with different Si concentrations and (b) the variation of sp3/(sp2+sp3) ratio with Si concentration in the Si-DLC films.

Optical band gap characterization 

Figure 2(a) shows the gradual increase of the optical transmittance for Si-DLC films in the visible and near-infrared region with an increase of at. % of Si in the DLC films. The transmittance spectra revealed that the Si-DLC films have high transmittance in the visible wavelength region and sharp absorption edge in the ultraviolet region and suffer significant blue shift with the increase of the Si content. The optical band gap energy Eg of the Si-DLC calculated from the transmittance data and using Tauc expression as given below

Eq (1)

 The optical band gap energy was calculated from the intercept to the hv axis at α = 0 by extrapolating the linear portion of the Tauc plot as shown in Fig. 2(b). Fig. 3(a) shows that as increase at. % of Si in the DLC film from 0 to 15.4, the optical band gap energy increase from 0.98 to 2.21 eV respectively. The optical properties of DLC film can be changed by changing the chemical bonding structure i.e., sp2 and sp3 ratio, and by the photonic effects arising from the high-density, Si particles dispersed in the DLC matrix. The increasing Si concentration in the films induces significant compositional change with high content of sp3 structure which leads to an increased optical band gap. The sp3 content increases with the increase of the Si content and as a result, the optical gap energy decreases.

FIGURE 2. (a) The change in optical transmittance as a function of wavelength for different at. % Si content DLC films and (b)Tauc plot to calculate the optical band gap for Si-DLC films.

In the case of DLC, the inhomogeneous disorder is measured from the Urbach energy calculation. The inhomogeneous disorder or defect density can be calculated from the Urbach energy which is determined by fitting an exponential function to the slope of the absorption edge. The Urbach energy (Eu), in amorphous semiconductors, is characterized by the transitions between extended two localized states. The absorption coefficient (a) near the band edge in the energy region hv < Eg empirically follows the exponential law. The different parameters such as carrier impurity interaction, carrier-phonon interaction, structural disorder, etc. are responsible for the Urbach band tail in the case of semiconductor material. The Urbach parameter is the result of all possible defects. The Urbach parameter was calculated for different at. % of Si content DLC film. The Urbach parameter i.e., defects density increases for the Si-DLC film with an increase of Si concentration in the DLC films as shown in figure 3(b).

FIGURE 3. (a) The variation optical band gap energy and (b) and variation of Urbach energy with at. % Si content.

Photoluminescence study Figure 4 shows the room temperature photoluminescence (PL) emission spectra of Si-DLC films. The PL emission peak of the undoped DLC films was found at 467 nm and the position of the same remains unchanged with silicon incorporation but the intensity increased significantly. This indicates that the room temperature PL is in the visible range. It has been suggested that PL arises by the excitation of electrons from π to π* states in one cluster, which then recombines within the same cluster. Therefore, the PL mechanism of these films can be interpreted using the framework of DLC, which is attributed to the radiative recombination of photo-excited electron-hole pares in sp2 bonded clusters. The π - π* gap in sp2 sites is much narrower than the s-s* gap, and the latter acts as a barrier that strongly localizes the p–p* band edge states. Therefore, the electron and holes are closely correlated by coulomb interaction with a short lifetime and the corresponding PL has a strong polarization memory, which can be observed at room temperature. Urbach parameter calculation showed that the defect increases with increasing at. % of Si in the DLC films. It is shown in Fig 4(a) that the PL intensities, as well as the intensities of C-Hn(s) bonds (centered at 2950 cm-1 in the FT-IR spectra), increased with the increase of at. % of Si in the films. This suggests that the PL of Si-DLC films is dependent on C-Hn(s) bonding concentrations (where n=1,2). Since the intensity of the PL tends to increase with increasing hydrogen content in the films. Enhancement in photoluminescence properties was attributed to the alteration of the electronic structure by the incorporation of substitutional defect states and the donor activity of silicon.

FIGURE 4. Photoluminescence spectrum of Si-DLC films for different at. % of Si content.

CONCLUSION 

Silicon incorporated diamond-like carbon (Si-DLC) nanocomposite thin films synthesized on silicon and glass substrates using the RF-PECVD technique. The chemical composition i.e., sp2 /sp3 ratio of the Si-DLC films calculated from the deconvolution of FTIR spectra. The overall sp3 /sp2 ratio i.e., the chemical composition of the Si-DLC films has been changed by the incorporation of Si in the films. The transparency increases, as well as the optical band gap, increased from 0.98 to 2.21 eV as a result of an increase in the Si concentration from 0 to 15.4 at. %. The PL emission peak of the Si-DLC films has been found at 467 nm and the position of the PL emission peak remains unchanged with Si incorporation whereas the intensity increased monotonically with at. % of Si. The above study reveals that the Si-DLC films could have potential use in the fabrication of optical devices among other applications.