# Room-Temperature Growth of SiC Thin Films by Dual-Ion-Beam Sputtering Deposition

Authors: C. G. Jin; X. M. Wu; L. J. Zhuge
Journal: Research Letters in Physical Chemistry
 (2008)

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## Abstract

Silicon carbide (SiC) films were prepared by single and dual-ion-beam sputtering deposition at room temperature. An assistedAr+ ion beam (ion energy Ei = 150 eV) was directed to bombard the substrate surface to be helpful for forming SiC films. The microstructure and optical properties of nonirradicated and assisted ion-beam irradicated films have been characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and Raman spectra. TEM result shows that the films are amorphous. The films exposed to a low-energy assisted ion-beam irradicated during sputtering from a-SiC target have exhibited smoother and compacter surface topography than which deposited with nonirradicated. The ion-beam irradicated improves the adhesion between film and substrate and releases the stress between film and substrate. With assisted ion-beam irradicated, the density of the Si–C bond in the film has increased. At the same time, the excess C atoms or the size of the sp2 bonded clusters reduces, and the a-Si phase decreases. These results indicate that the composition of the film is mainly Si–C bond.


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## Body

## 1. Introduction

Amorphous semiconductor alloys are of technological importance for electronic and
optoelectronic device application. Amorphous silicon carbide (a-SiC) has been recognized as a
semiconductor material with outstanding physical and chemical characteristics.
Silicon carbide exhibits a large bandgap, a higher breakdown field, a higher
thermal conductivity, and a higher saturatuin velocity, compared to widely used
silicon. On the other hand, amorphous SiC films have interest related to their
high hardness and optical properties and have potential applications as hard,
wear resistant coatings, masking material in Si micromaching technology as well
as for the formation of optical windows, filters, and color sensors [1, 2]. Recently,
plasma-assisted deposition methods such as plasma enhanced CVD [3, 4], electron
cyclotron resonance (ECR) [5, 6], the conventional physical vapour deposition methods
(magnetron sputtering [7, 8], pulsed laser deposition [9, 10]), ion implantation [11],
and molecular beam epitaxy [12] methods have been used to grow SiC films on Si
substrate. However, these methods need high grown temperature, which process
defect creation, resulting from high tensile stress generated from a
temperature dependent difference in the thermal expansion coefficient between
SiC and Si, and involve many pollutions of impurity in the films such as H
element.This letter reports the growth of SiC thin films at room temperature using dual-ion-beam
sputtering deposition (DIBSD), a method that has been used to grow many
different types of films. Recently, it has also been shown that ion-assisted
techniques can greatly improve the adhesion by minimizing
the total stress at the interface and compactness of the coatings despite
the higher optical absorption near the bandgap of the material [13, 14]. By proper adjustment of dual-ion-beam
parameters, the method permits a precise control of film composition, which is almost
independent of deposition rate. The microstructural, optical, and electrical
properties may also be tailored significantly by ion bombardment during film
deposition [15]. The energy given by the ion-beam system is high enough to
reduce the temperature needed for the formation of the SiC phase [16, 17]. In
the paper, the microstructure and morphology of the films have been measured by
Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and Raman spectra. The
effect of a low-energy assisted-ion bombardment during deposition on the film
properties has been investigated.


## 2. Experiment

SiC films were deposited by employing a DIBSD system at room temperature. A DIBSD system consists of a focused
Kaufman ion source (main ion source) and a broad-beam Kaufman ion source (assisted-ion source). The Ar gas as pure as
99.999% was used for source gas and for ion-beam generation. Main ion source of 10 cm diameter with the incident angle
of 45° served as a sputtering ion source. Normal sintered high-purity SiC target was used as a sputtering target. The
sputtering chamber was evacuated to 2×10−4 Pa
before argon was introduced through a mass flow controller. The working
pressure of argon was 4×10−2 Pa.
Before deposition, the substrates of KBr, Suprasil quatz, and p-type (100) Si wafer (resistivity, 5 Ω-cm) after standard cleaning sequence to remove organic
and inorganic surface contaminants were cleaned again with an argon ion beam
for 15 minutes using the assisted source  (200 eV energy and 10 mA beam intensity). During
deposition, the beam energy and beam current of the main ion source which was
used to sputter the SiC target were 800 eV and 40 mA, respectively. An
assisted-ion source of 10 cm diameter with the incident angle of 60° was used. The beam current of the
assisted-ion source which was used to bombardment the substrate was 10 mA, and beam energy was variable
parameter 0–150 eV. No intentional heating was applied on substrate during deposition. SiC films were
deposited for 2 hours on the substrate. All of the thicknesses of the SiC films were range of 150–260 nm,
which were measured with an ET350 surface profilometer (Kosaka Laboratory Ltd.).The films were studied by scanning electron
microscopy (SEM) using Hitachi S-4700 equipped with energy dispersive
spectroscopy (EDS), for their morphology and composition, respectively. The bonding configurations
and the structure were measured by TEM (Hitachi H600A-II), FTIR (Niolot AVATAR 360), and Raman spectra which were
obtained in the backscattering configuration between 200 cm−1 and 1800 cm−1 by JY-HR800 using an argon ion
laser at a wavelength of 514 nm. All the measurements were conducted at room temperature.


## 3. Results and Discussion

### 3.1. Stucture Analysis

During DIBSD, the assisted ion beam
has been utilized to significantly enhance and control the properties of the
film. In order to compare the characteristics of films prepared by DIBSD, IBSD,
which did not use assisted ion beam to irradiate the substrate, was used to
form SiC film. Figure1 shows our TEM result from which only halos are observed in the selection electron diffraction
picture, the samples regarded to be amorphous.Figure 1
The TEM micrographs of the sample with 150 eV ion-beam irradiated.At low resolution, the top SEM
micrograph of a film prepared on Si substrate nonirradicated (sample a) and irradicated
by 150 eV ion beam (sample b) is shown in Figures2(a) and 2(c), respectively.
Compared to Figure 2(a), the average grain size increases, and the surface is found to be denser
with ion-beam irradiated in Figure 2(c). The
cross-sectional SEM micrographs of sample a and sample b are shown in Figures 2(b) and 2(d), respectively. As
shown in Figure 2(b), we see that the
film/substrate interface is quite smooth and clear. Moreover, in region A, there
are many obvious cracks on both film and substrate, which might originate from
the inter-stress between film and substrate. As shown in Figure 2(d), sample b is more compact than sample a. The
film/substrate interface is roughness and dark. Nevertheless the quantity of
the cracks decreases in region C. There is a buffer layer between SiC film and
Si substrate in region B, which releases the stress caused by the large lattice
mismatch, around 20%, and indicates very good coating to substrate adhesion. It
found that the ion-beam-assisted bombarding enhanced the coating to substrate adhesion.
These results indicate that low-energy ion-assisted growth improves the adatom
mobility, which is attributed to the distribution of nonequilibrium phonon on
surface. At the same time, the ion-beam irradicated is the densification of film, increases the average of the cluster, improves the
adhesion between SiC and Si substrate, and releases the stress between SiC and
Si substrate.(a) Top-, and (b) cross-sectional SEM micrographs of
the sample prepared on Si with nonirradicated, (c) top-, and (d) cross-sectional SEM
micrographs of the sample irradiated by 150 eV ion beam.
(a)(b)(c)(d)The EDS was only used to measure the proportion of Si and C in our films grown on KBr
substrates. The composition uniformity in the film was studied by analyzing
several points (with 1 mm2 analysis area) over the
surface of the sample. EDS analyses indeed showed that all the films consist of
Si and C. The proportion in sample a is Si : C = 9 : 11 (at.% ratio), nevertheless it
decreases to 2 : 3 with ion-beam irradicated in sample b. It indicates that all the films are C excess. The main
source ion beam sputters the SiC target, then the sputtering particles deposit
on the substrate, which include Si, C, and SiC atoms. At the same time, the
assisted-source ion beam sputters the deposition, and the secondary ion sputtering rate of Si ion is higher than that of C ion in the films. The
increasing of the proportion of C atom might be attributed to resputter on
as-deposition film, which maybe due to the sputtering rate of Si ion is higher
than that of C ion.The FTIR is a powerful tool to investigate the
bonding structure ina-SiC films. The IR spectra for sample a and the sample b which are
deposited on KBr substrates are shown in Figures 3(a) and 3(b),
respectively. As can be seen, almost all the measured spectra are characterized
by the absorption peak related to the Si–C bond, which is around 800 cm−1, except the weak peaks
around 1018 cm−1, 1090 cm−1, 1260 cm−1,
and 2950 cm−1. The bands corresponding to 1018 cm−1, 1090 cm−1, and 1260 cm−1 are
due to oxygen adsorption on the film surface in air or the O atoms at the film/substrate interface [18], while the band
corresponding to 2950 cm−1 is attributed to the stretching
vibration of C–Hn groups in sp3 configurations [19],
which may be due to H2O from the atmosphere adsorbed on the film
surface [20]. The 740 cm−1 band as a broad absorption maybe related to these results, just as (a) the high
concentration of substitutional C, (b) the appearances of the C–C bonds, and (c)
the nonsubstitutional C in the Si substrate [21, 22]. Compared to Figure 3(a), the
shape of the 800 cm−1 peak progressively changes and becomes narrow in Figure 3(b). The full width at half maximum
(FWHM) of 800 cm−1 peak is smaller than that of other spectra, indicating that the stoichiometric
proportion of the SiC films after assisted ion-beam irradicated is
improved [23]. It demonstrates a
better quality of the SiC film grown with assisted ion-beam irradicated. The full width at half maximum (FWHM)
of 800 cm−1 peak is narrower than that of other spectra, indicating that the Si–C band is a majority of the film.Figure 3
The FTIR spectra of the films on KBr substrates (a)
with nonirradicated (b) with 150 eV ion-beam irradiated.Although FTIR is high efficiency of Si–C band, it is low
efficiency of Si–Si and C–C bonds. However, Raman spectra are optimum for
researching identic atomic polar bonds, such as Si–Si and C–C bands. So, we
used Raman spectra to analyze the changes of Si–Si and C–C bands. The Raman
spectra from these samples are characterized by the presence of bonds
characteristic of amorphous material, in the 200–600 cm−1 and 1300–1600 cm−1 spectral regions. This
can be seen in Figure 4(a), where
the spectra measured in these regions from the sample a. These bands are
similar to those reported for amorphous Si1−xCx films
obtained by different techniques [24–26] and have been interpreted according
to a two-mode behavior, related to the different Si–Si and C–C vibrational
modes. So, the first band are similar to the Si–Si TO mode one from amorphous Si (at about 480 cm−1). The band appearing in the
1300–1600 spectra region is related to C–C vibrational modes. This C–C
signature already observed in Si–C alloys with carbon excess [24, 27–30]
corresponds certainly to a specific structure which could be described like a
random covalent network of tetrahedral-trigonal bonding carbons with distorted
bond angles and bond lengths [31–35]. As a matter of fact, it shows that in
mixed sp2–sp3 bonded carbon layer, the overall
Raman spectrum is dominated by the G component (E2g Raman mode of
perfect graphite crystal), because the cross
section of the graphite stretching mode is much higher than that of the 1332 cm−1 diamond mode.The Raman spectra of the films on Si substrates (a)
with nonirradicated (b) with 150 eV ion-beam irradiated.
(a)(b)After assisted ion-beam irradicated, thea-Si contribution in the spectra
decreases, the first-order peak from c-Si
is restored, and the peak of C–C bond shifts toward to high-frequency while the
intensity of the peak decreases as shown in Figure 4(b). It indicates that the sp3/sp2 radio decreases while the
excess C atoms or the size of the sp2 bonded clusters reduces.The lack of C–C bands
suggests that part of excess carbon atoms are bonded to a-Si instead of forming the graphite-like structure. And that the
residual excess C can be present in solid solution.


## 3.1. Stucture Analysis

During DIBSD, the assisted ion beam
has been utilized to significantly enhance and control the properties of the
film. In order to compare the characteristics of films prepared by DIBSD, IBSD,
which did not use assisted ion beam to irradiate the substrate, was used to
form SiC film. Figure1 shows our TEM result from which only halos are observed in the selection electron diffraction
picture, the samples regarded to be amorphous.Figure 1
The TEM micrographs of the sample with 150 eV ion-beam irradiated.At low resolution, the top SEM
micrograph of a film prepared on Si substrate nonirradicated (sample a) and irradicated
by 150 eV ion beam (sample b) is shown in Figures2(a) and 2(c), respectively.
Compared to Figure 2(a), the average grain size increases, and the surface is found to be denser
with ion-beam irradiated in Figure 2(c). The
cross-sectional SEM micrographs of sample a and sample b are shown in Figures 2(b) and 2(d), respectively. As
shown in Figure 2(b), we see that the
film/substrate interface is quite smooth and clear. Moreover, in region A, there
are many obvious cracks on both film and substrate, which might originate from
the inter-stress between film and substrate. As shown in Figure 2(d), sample b is more compact than sample a. The
film/substrate interface is roughness and dark. Nevertheless the quantity of
the cracks decreases in region C. There is a buffer layer between SiC film and
Si substrate in region B, which releases the stress caused by the large lattice
mismatch, around 20%, and indicates very good coating to substrate adhesion. It
found that the ion-beam-assisted bombarding enhanced the coating to substrate adhesion.
These results indicate that low-energy ion-assisted growth improves the adatom
mobility, which is attributed to the distribution of nonequilibrium phonon on
surface. At the same time, the ion-beam irradicated is the densification of film, increases the average of the cluster, improves the
adhesion between SiC and Si substrate, and releases the stress between SiC and
Si substrate.(a) Top-, and (b) cross-sectional SEM micrographs of
the sample prepared on Si with nonirradicated, (c) top-, and (d) cross-sectional SEM
micrographs of the sample irradiated by 150 eV ion beam.
(a)(b)(c)(d)The EDS was only used to measure the proportion of Si and C in our films grown on KBr
substrates. The composition uniformity in the film was studied by analyzing
several points (with 1 mm2 analysis area) over the
surface of the sample. EDS analyses indeed showed that all the films consist of
Si and C. The proportion in sample a is Si : C = 9 : 11 (at.% ratio), nevertheless it
decreases to 2 : 3 with ion-beam irradicated in sample b. It indicates that all the films are C excess. The main
source ion beam sputters the SiC target, then the sputtering particles deposit
on the substrate, which include Si, C, and SiC atoms. At the same time, the
assisted-source ion beam sputters the deposition, and the secondary ion sputtering rate of Si ion is higher than that of C ion in the films. The
increasing of the proportion of C atom might be attributed to resputter on
as-deposition film, which maybe due to the sputtering rate of Si ion is higher
than that of C ion.The FTIR is a powerful tool to investigate the
bonding structure ina-SiC films. The IR spectra for sample a and the sample b which are
deposited on KBr substrates are shown in Figures 3(a) and 3(b),
respectively. As can be seen, almost all the measured spectra are characterized
by the absorption peak related to the Si–C bond, which is around 800 cm−1, except the weak peaks
around 1018 cm−1, 1090 cm−1, 1260 cm−1,
and 2950 cm−1. The bands corresponding to 1018 cm−1, 1090 cm−1, and 1260 cm−1 are
due to oxygen adsorption on the film surface in air or the O atoms at the film/substrate interface [18], while the band
corresponding to 2950 cm−1 is attributed to the stretching
vibration of C–Hn groups in sp3 configurations [19],
which may be due to H2O from the atmosphere adsorbed on the film
surface [20]. The 740 cm−1 band as a broad absorption maybe related to these results, just as (a) the high
concentration of substitutional C, (b) the appearances of the C–C bonds, and (c)
the nonsubstitutional C in the Si substrate [21, 22]. Compared to Figure 3(a), the
shape of the 800 cm−1 peak progressively changes and becomes narrow in Figure 3(b). The full width at half maximum
(FWHM) of 800 cm−1 peak is smaller than that of other spectra, indicating that the stoichiometric
proportion of the SiC films after assisted ion-beam irradicated is
improved [23]. It demonstrates a
better quality of the SiC film grown with assisted ion-beam irradicated. The full width at half maximum (FWHM)
of 800 cm−1 peak is narrower than that of other spectra, indicating that the Si–C band is a majority of the film.Figure 3
The FTIR spectra of the films on KBr substrates (a)
with nonirradicated (b) with 150 eV ion-beam irradiated.Although FTIR is high efficiency of Si–C band, it is low
efficiency of Si–Si and C–C bonds. However, Raman spectra are optimum for
researching identic atomic polar bonds, such as Si–Si and C–C bands. So, we
used Raman spectra to analyze the changes of Si–Si and C–C bands. The Raman
spectra from these samples are characterized by the presence of bonds
characteristic of amorphous material, in the 200–600 cm−1 and 1300–1600 cm−1 spectral regions. This
can be seen in Figure 4(a), where
the spectra measured in these regions from the sample a. These bands are
similar to those reported for amorphous Si1−xCx films
obtained by different techniques [24–26] and have been interpreted according
to a two-mode behavior, related to the different Si–Si and C–C vibrational
modes. So, the first band are similar to the Si–Si TO mode one from amorphous Si (at about 480 cm−1). The band appearing in the
1300–1600 spectra region is related to C–C vibrational modes. This C–C
signature already observed in Si–C alloys with carbon excess [24, 27–30]
corresponds certainly to a specific structure which could be described like a
random covalent network of tetrahedral-trigonal bonding carbons with distorted
bond angles and bond lengths [31–35]. As a matter of fact, it shows that in
mixed sp2–sp3 bonded carbon layer, the overall
Raman spectrum is dominated by the G component (E2g Raman mode of
perfect graphite crystal), because the cross
section of the graphite stretching mode is much higher than that of the 1332 cm−1 diamond mode.The Raman spectra of the films on Si substrates (a)
with nonirradicated (b) with 150 eV ion-beam irradiated.
(a)(b)After assisted ion-beam irradicated, thea-Si contribution in the spectra
decreases, the first-order peak from c-Si
is restored, and the peak of C–C bond shifts toward to high-frequency while the
intensity of the peak decreases as shown in Figure 4(b). It indicates that the sp3/sp2 radio decreases while the
excess C atoms or the size of the sp2 bonded clusters reduces.The lack of C–C bands
suggests that part of excess carbon atoms are bonded to a-Si instead of forming the graphite-like structure. And that the
residual excess C can be present in solid solution.


## 4. Conclusion

SiC films were prepared by single and dual-ion-beam sputtering deposition at room
temperature. The microstructure and optical properties of nonirradicated and assisted ion-beam irradicated films
have been characterized by TEM, EDS, SEM, FTIR, and Raman spectra. The films exposed to a low-energy
assisted ion-beam irradicated during sputtering from a SiC target
have exhibited smoother and compacter surface topography and different
optical behavior than the films deposited with nonirradicated.
With assisted ion-beam irradicated, the density of the Si–C bonding in the film
has increased. At the same time, the excess C atoms or the size of thesp2 bonded clusters reduces,
and the a-Si phase decreases. These results
indicate that the composition of the film is mainly Si–C bond. These date
suggest the high stability of a-SiC to be related to the absence of a
complete chemical order, which can be attributed to the ion-assisted irradicated at 150 eV during deposition.



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*Source: 760650-2008-04-03.xml*