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PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor
SCT-18599; No of Pages 7 Surface & Coatings Technology xxx (2013) xxx–xxx
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PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor V.S. Sulyaeva a,⁎, M.L. Kosinova a, Yu.M. Rumyantsev a, V.G. Kesler b, F.A. Kuznetsov a a b
Nikolaev Institute of Inorganic Chemistry, SB RAS 3, Acad. Lavrentiev Ave., Novosibirsk, 630090, Russia Rzhanov Institute of Semiconductor Physics, SB RAS 13, Acad. Lavrentiev Ave., Novosibirsk, 630090, Russia
a r t i c l e
i n f o
Available online xxxx Keywords: N-triethylborazine Boron carbonitride films PECVD Transparency Optical band gap energy
a b s t r a c t Thin films of boron carbonitride BCxNy were synthesized by plasma enhanced chemical vapor deposition using N-triethylborazine as a B-C-N-forming single source precursor. In this work, we investigated the effect of additional gases and temperature synthesis on the chemical composition of the BCxNy films. The plasma was activated via 40.68 MHz radio frequency. It was found by FTIR spectroscopy that the low temperature films (Tdep b 400 °C) are hydrogenated boron carbonitride BCxNy:H. It was shown by Raman spectroscopy that the films synthesized at high temperature (Tdep ≥ 600 °C) contain an additional phase of disordered carbon. The refractive index of the BCxNy films increased from 1.5 to 2.8 with increasing temperature of the synthesis. Low temperature films exhibit high transmittance values in the region from 400 to 3200 nm (transparency up to 93%). The optical band gap energy ranged from 1.2 to 4.9 eV for the films deposited under considered synthesis conditions. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Films containing boron, carbon and nitrogen (boron carbonitride, BCxNy) are of great interest because of their properties such as high hardness and resistance to heat, wear and corrosion, low-k values, high transparency and variable band gap energy. In the early stages of BCxNy film synthesis study, toxic and explosive halides and hydrides of boron were used. Recently organoboron compounds including borazine derivatives attracted increasing interest. Maya et al. used tris-(1,3,2-benzodiazaborolo) borazine for amorphous semiconductor BCN film production by means of pyrolysis at 900 °C at the end of the 80's [1–3]. Chemical analysis showed that the synthesized material had the composition BC5.2N1.8H1.9O0.45. Weber et al. used mixtures of N,N′,N″-trimethylborazine (CH3)3N3B3H3 (TMB) and argon to deposit BCN:H films by means of electron cyclotron resonance plasma enhanced chemical vapor deposition processes (PECVD) at 100–150 °C [4,5]. Amorphous BCN layers were deposited by radio frequency plasma activated enhanced chemical vapor deposition (RF-PACVD) at low temperature (95–120 °C) using a mixture of TMB and H2, N2, or Ar [6]. The composition of the layers varied in a wide range. The boron content of the films ranged from 3 to 42 at.%, the carbon content from 16 to 80 at.%, and oxygen content from 2 to 10 at.%. Stöckel et al. deposited BCN films with hexagonal turbostratic graphite like structure ⁎ Corresponding author at: Department of Functional Materials Chemistry, Nikolaev Institute of Inorganic Chemistry SB RAS, 630090, Novosibirsk, Russia. Tel.: +7 383 330 6646; fax: +7 383 330 9489. E-mail address: [email protected] (V.S. Sulyaeva).
by both isothermal chemical vapor deposition (ITCVD) under atmospheric pressure and PECVD from gaseous mixtures of trimethylborazine, toluene and ammonia at 950 °C [7,8]. Similarities between ITCVD and PECVD films showed up in the case of chemical composition and correlation between carbon content and hardness values. Considerable differences exist in regard to the microstructure, especially the texture of the films. Moreover, in ITCVD films the carbon is preferentially incorporated between the BN basal planes, whereas in PECVD films it is especially incorporated in the BN planes. BCN coatings were deposited by means of a capacitively or inductively coupled RF-PACVD using the elementalorganic compound trimethylborazine as a precursor. The influence of plasma parameters on the film properties has been discussed [9,10]. Hydrogenated BCN:H films were deposited with two different PECVD techniques by Thamm et al. [11,12]. Microwave plasma with RF-bias enhancement (MW-PECVD) and a direct current glow discharge plasma system were used with TMB and benzene as an additional carbon source. Argon and nitrogen were used as plasma gases. The substrate temperature, substrate bias and gas composition were varied. The depth profiles show a homogeneous distribution of elements B, C, N, and H over the entire layer thickness. Impurities such as oxygen or argon are detected in only small quantities (below 0.5 at.%) and the concentration does not increase towards the surface. The hydrogen content mostly depends on the substrate temperature during the coating process. If the layers are deposited on the 50 °C temperature substrate (MW-PECVD), the hydrogen content increases up to 35 at.%. If the temperature is increased up to 800 °C, only 8 at.% of hydrogen are detected, independently of the plasma gas. The variation of the MW plasma power and RF-bias has no significant effects on the layer composition. Multicomponent films were grown by PECVD from N-trimethylborazine–nitrogen mixtures at
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Please cite this article as: V.S. Sulyaeva, et al., PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor, Surface & Coatings Technology (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.06.018
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temperatures from 100 to 700 °C and varied RF power. According to XPS and IR spectroscopy results, the major component of the films is boron nitride. The films grown at temperatures below 400 °C contain hydrogen. The high temperature films, as a rule, contain carbon [13–15] Thermolysis of N-triethylborazine (C2H5)3N3B3H3 (TEB) and N-tripropylborazine (C3H7)3N3B3H3 at 500 °C produces homogeneous, amorphous boron carbonitride phases, whose compositions depend on the borazine substituent, and corresponding structures are similar to that of icosahedral boron carbide B4C [16]. The deposition of BCN-layers by decomposing TEB was performed in a hot-filament CVD reactor and for the gas activation a carburized Ta wire was used. The substrate temperature was 890 °C. The deposition parameters filament temperature (1800–2200 °C), precursor flow rate and gas atmosphere were varied. The layer growth rates increased with the precursor flow rate and showed less influence of the substrate material. The layer morphology was a pyrolytic type. When the layers became thick, they tended to eliminate from the substrate. Due to the atomic hydrogen produced at the filament, the deposition rate decreased in case of hydrogen atmosphere compared to that for experiments in argon atmosphere. That can be explained by etching reactions of the atomic hydrogen. Due to the etching reactions of the atomic hydrogen, the IR-spectra also look different. Nevertheless, it was not possible to identify a cubic phase of boron nitride from the IR or XRD measurements [17]. So, boron carbonitride films have been obtained by different processes of chemical vapor deposition utilizing borazine derivatives. However, the production of boron carbonitrides from TEB has attracted very little attention. The aim of the present study is to explore the possibility of the synthesis of BCxNy thin films by PECVD using N-triethylborazine with ammonia or helium and to study the growth rate, refractive index, types of chemical bonds, optical properties of the films depending on the growth conditions. 2. Experimental 2.1. Synthesis of the precursor (TEB) N-triethylborazine was synthesized by a mechanochemical method [18]. The usage of solvents as a medium is excluded from mechanochemical syntheses of this kind. Volatile boron-containing compounds are isolated from the reaction medium by evaporation. Starting compounds for producing were ethylamine hydrochloride C2H5NH2 · HCl and natrium tetrahydroborate NaBH4. TEB synthesis was performed under mechanical activation of the mixture of the starting compounds in hermetic rotational and vibratory ball mills by the following solidphase reactions: C2 H5 NH2 ·HClðsÞ þ NaBH4ðsÞ →C2 H5 NH2 ·BH3ðsÞ þ NaClðsÞ þ H2ðgÞ ; next stage was performed under a step pyrolysis at temperatures up to 90 and 240 °C with a resulting gas outlet: C2 H5 NH2 ·BH3 →ðC2 H5 NHBH2 Þn →ðC2 H5 NBHÞ3 : TEB was obtained in a yield of 74%. The product was characterized by the melting point of − 48 °C, liquid density of 0.836 g/cm3 at 20 °C, refractive index n20 D = 1.4365 and saturated vapor pressure of 399.97 Pa, which correspond to the TEB literature data [19]. TEB can be regarded as a promising precursor for the preparation of BC2N films as the ratio of elements in its molecule is 1:2:1. 2.2. PECVD process Plasma enhanced chemical vapor deposition process was applied for preparing BCxNy coatings by using mixtures of N-triethylborazine and ammonia or helium as plasma gases. The processes were carried out
in a horizontal hot wall quartz reactor which has discharge and growth zones. The reactor has RF coils powered at 40 W with an RF field of 40.68 MHz. Precursor and additional gases were introduced by separate gas inlets. The hydrogen or ammonia gas passed through the discharge zone following by mixing with TEB appeared at the growth zone, which was heated up by the resistance furnace. The deposition temperature ranged from 100 to 700 °C. Residual pressure in the system was fixed at 0.67 Pa, the vapor pressure of the precursor PTEB = 2.13 Pa in all experiments. Pressure of additional gases was equal to 1.07 Pa. The substrate was placed onto a holder at the center of the furnace. Gaseous byproducts of the deposition process were removed by pumping system and collected in a liquid nitrogen cold trap.
2.3. Substrates Silicon wafers Si(100) and fused silica were used as substrates. Investigation of surface microstructure and composition was performed for coatings deposited on Si(100) substrates. Only for the evaluating layer transmission BCxNy was also deposited on fused silica substrates. The substrates were chemically treated before layer deposition. Preliminarily in all cases the substrates were cleaned in ethanol, degreased in trichlorethylene, acetone and washed in deionized water. Additionally fused silica substrates degreased in sulfuric acid with hyperoxide. Si(100) substrates were chemically etched by mixtures of NH 3 + H2O2 + H2O, washed in deionized water, HCl + H2O2 + H2O, washed in deionized water, hydrofluoric acid, washed in deionized water, and then dried in a nitrogen stream.
2.4. Analysis The BCxNy coating thickness and refractive index were determined by ellipsometry (LEF-3 M ellipsometer) at the wavelength of 632.8 nm. The measurements were carried out at seven angles. The surface microstructure and the elemental composition of the films were observed by a scanning electron microscope (SEM) JEOL JSM 6700F equipped with an EX-23000BU analyzer for element composition determination by X-ray energy dispersive spectroscopy. Fourier transform infrared (FTIR) spectra of the films were recorded using a SCIMITAR FTS 2000 spectrometer in the range of 300–4000 cm−1. Thirty-two scans and the aperture equal to 4 at achievable resolution 2 cm−1 were used during the measurements. In each case the substrate spectrum was subtracted from that of the sample. All FTIR spectra were normalized to the thickness of an appropriate film. Raman spectra were recorded on a Spex 1877 triple spectrometer excited by an argon laser. The XPS spectra were obtained by a MAC-2 (RIBER) analyzer using nonmonochromatic Al Kα radiation (1486.6 eV) with the power of 300 W, and X-ray beam diameter of about 5 mm. The energy resolution of the instrument was chosen as 0.7 eV to have sufficiently small broadening of natural core level lines at a reasonable signal-noise ratio. Under these conditions the observed full width at half maximum (FWHM) of Au 4f7/2 line was 1.31 eV. The binding energy scale was calibrated in reference to Cu 3p3/2 (75.1 eV) and Cu 2p3/2 (932.7 eV) lines, assuring the accuracy of ±0.1 eV in any peak energy position determination. Since BCxNy are dielectric films, the photoelectron energy drift, due to charging effects, was taken into account in reference to the position of C 1 s (284.6 eV) line generated by adventitious carbon on the sample surface when inserted into the vacuum chamber. The component of adventitious carbon was derived from the complex carbon peak structure by means of deconvolution. To decompose the overlapped XPS peaks, we used mixed Gaussian and Lorentzian line shape functions with the parameters of the peaks measured on h-BN and B4C standard samples. Optical transmittance of the deposited films was examined using spectrophotometry (Scanning Spectrophotometer UV-3101PC Shimadzu) in the range of 190–3200 nm (resolution of 2 nm).
Please cite this article as: V.S. Sulyaeva, et al., PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor, Surface & Coatings Technology (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.06.018
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3. Results and discussion
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BCxNy films on the Si(100) and fused silica wafers with thickness 100–300 nm were obtained. The film properties were investigated as a function of the PECVD parameters, essentially the nature of the additional gas and the temperature of deposition. Surface morphology examination by SEM study showed smooth homogeneous, not rough film surfaces. Temperature increase yields an occurrence of 20–30 nm particles in either case for BCxNy films synthesized from TEB + NH3 (Fig. 1a) and TEB + He (Fig. 1b) mixtures. 3.1. Film deposition rate The deposition rate was monitored to investigate the growth characteristics of the BCxNy coatings (Fig. 2). The substrate temperature was varied keeping the other deposition parameters constant for both mixtures of TEB with ammonia and helium. It is clear from Fig. 2 that the growth rate decreases almost linearly with rising substrate temperature. That is typical for PECVD processes. Most likely the process of polymer-like formation film occurrences from plasma induces decomposition of large fragments of the initial molecules at low temperatures [5].
b
Fig. 2. Refractive index and growth rate of BCxNy films synthesized from TEB + NH3 (a) and TEB + He (b) mixtures as function of temperature.
3.2. Film composition 3.2.1. EDX analysis X-ray energy dispersion spectroscopy was conducted for layer composition analysis. It was found by the use of this method that the basic elements of the films were boron, carbon, and nitrogen with oxygen impurities. Despite of the fact that the ratio of the elements in the molecule of TEB is B:C:N = 1:2:1, the elemental composition of the synthesized films does not meet this relation. The film compositions were in the range of 23–39, 12–39, 35–46, 0–9 at.% for B, C, N, O, respectively, depending on the synthesis conditions. Temperature significantly affects the chemical composition of the films in case of the initial mixture of TEB + NH3 (Fig. 3a). The boron, nitrogen concentrations increase and carbon concentration decreases in the films with the deposition temperature. In contrast to using ammonia at the addition of He, the elemental composition of the BCxNy films weakly depends on the synthesis temperature (Fig. 3b). Nevertheless, in the both cases high carbon concentration at low temperatures observed because of incomplete precursor decomposition and inclusion of carbon residue in the films. High carbon concentration at high temperatures caused by disordered graphite formation (as will be shown below by Raman-spectroscopy).
Fig. 1. SEM-images of BCxNy films synthesized at Tdep = 700 °C from TEB + NH3 (a) and TEB + He (b) mixtures.
3.2.2. FTIR analysis The FTIR spectra of the films synthesized at high temperature (T > 400 °C) show modes at 800 and 1380 cm−1 of h-BN due to B-N-B and B-N vibrations [20], respectively, and shoulders at 1100
Please cite this article as: V.S. Sulyaeva, et al., PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor, Surface & Coatings Technology (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.06.018
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a
a
b
b
Fig. 3. Temperature dependence of main elements atomic concentrations of BCxNy films synthesized from TEB + NH3 (a) and TEB + He (b) mixtures.
and 1600 cm−1, corresponding to ν(B-C) in B4C [21] and ν(С = N) in the amorphous CN films [22] (Fig. 4). However, FTIR-spectra of films prepared at T b 400 °C show modes at 1435 см−1 corresponding to stretching vibrations of the B-N in the ring [23] of initial TEB and the stretching and deformation vibrations of B-H, C-H, N-H, C-CH3, which allows us to consider these films as hydrogenated boron carbonitride BCxNy:H. Previously authors [16] noted that during the conversion of the precursor to a solid product, the ring system is generally preserved. 3.2.3. Raman analysis Raman spectra of BCxNy films synthesized at temperatures of 600 and 700 °C from different initial gas mixtures TEB + NH3 and TEB + He contain two overlapped strong peaks centered at 1320, 1570 cm−1 (Fig. 5a) and 1347, 1566 cm−1 (Fig. 5b), correspondingly. These spectra are similar to BCN film spectra which have D and G peaks appropriate for disordered carbon [24,25]. Because the D peak of carbon and the E2g mode of hexagonal BN (1366 cm−1) are so close, the peak at 1320 (or 1347) cm−1 is probably regarded as their overlap. The intensities of the peaks in Raman-spectra of films deposited from TEB + He mixture are higher compared to that while using TEB + NH3 mixture. 3.2.4. XPS analysis Further part of the work has been carried out to study the nature of the chemical environment of prepared BCxNy films by XPS. Fig. 6 shows the XPS core-level spectra of B 1s, C 1s and N 1s together with their deconvolution into several overlapping components for
Fig. 4. FTIR-spectra of BCxNy films synthesized from TEB + NH3 (a) and TEB + He (b) mixtures at different temperatures.
samples deposited at the substrate temperature of 400 °C and different gases, respectively. The asymmetry and broadening of the peaks imply that there is more than one type of bonding state for B, C and N atoms. A minimal number of spectral components was selected on the basis of the results for of a standard sample investigation [26], spectrometer resolution and literature data. The above deconvolution is the most realistic but not the only possible one, which may occur in the process. That is based on fact that, to receive different results at different possible input data, the existence of different environments around the boron atom has been studied earlier by near-edge x-ray absorption fine structure spectroscopy [27,28]. The B 1s spectra have specific features at 190.6, 190.1 and 191.1 eV. The first components correspond to B-N bond [29] in hexagonal boron nitride. The latter signal with the energy of 191.1 eV is in case of using the TEB with He mixture can be interpreted as B-C-N hybride bonds [16,30]. The main component of C 1s with the binding energy of 284.6 eV can be attributed to graphitic carbon due to an adventitious carbon contamination on the sample surface [31]. Other features at 281.7, 285.6 and 288.3 can be associated with C-B [32], C-N [33], C-O bonds [34]. The N 1s spectra exhibit an intensive peak at 398.4 eV that can be attributed to N-B bonds [12], which is the same as that for h-BN. The peak at 400.6 eV is observed usually ascribed to N-C bond [34]. The ratio between N 1s and B 1s peak areas is independent of carbon and oxygen contaminations and allows us to calculate the nitrogen and boron concentrations in the film. For the film synthesized by using the mixture of TEB with NH3 this ratio was equal to 1.00.
Please cite this article as: V.S. Sulyaeva, et al., PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor, Surface & Coatings Technology (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.06.018
V.S. Sulyaeva et al. / Surface & Coatings Technology xxx (2013) xxx–xxx
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binding energies. There are small components B 1s and C 1s corresponding to B-C bonding. 3.3. Optical properties Thin BCxNy films exhibited good optical properties. Refractive index n of the BCxNy films increased from 1.5 to 2.3 with increasing temperature of synthesis from 100 to 700 °C (Fig. 2), referring to changes in the chemical composition. The value of n increasing with temperature is higher in the case of the TEB + He initial gas mixture. Fig. 7 shows the influence of deposition temperature on the optical transmission of BCxNy films deposited on fused silica substrates. It was found that the low temperature films exhibit high optical properties in the region from 400 to 3200 nm (transparency up to 93%). Transparencies of the films synthesized from TEB + NH3 mixture have a red shift with the temperature increase. For the TEB + He mixture: at synthesis temperatures above 500 °C the film transparencies sharply decreased, that was apparently due to adding disordered carbon phase. Also BCxNy film high transmission (90 %) in the UV-visible region was achieved by using TMB and (N-pyrrolidino)diethylborane as precursors and low deposition temperature as in [9]. According to the ‘non-direct transition’ model proposed by Tauc, the transmittance (%T) data and film thickness can be used to calculate the absorption coefficient by the equation [35]:
b
α ¼ − lnð%T Þ=d; where α is the absorption coefficient and d is the film thickness. The absorption coefficient values were subsequently used to calculate the optical band gap (Eg) of the amorphous BCxNy films. This is realized from the Tauc plot given as seen from the equation [35]: 1=2
ðαhνÞ Fig. 5. Raman-spectra of BCxNy films synthesized from TEB + NH3 (a) and TEB + He (b) mixtures at temperature of 700 °C.
Therefore, one can define areas in the samples which structure is similar to h-BN. This phase also dominated in the films synthesized from the mixture of TEB with He, but in this case, the nitrogen to boron concentration ratio was equal to 0.82. The use of He results in the relative increase of B 1s, N 1s and C 1s components with higher
¼ B hν−Eg ;
where B is a constant factor and hν is the photon energy. The extrapolation of the linear region of the Tauc curve to intercept the x-axis leads to Eg values for the material and the slope of this line can be used to calculate B. It can be observed from Fig. 8 that the Eg value decreases from 4.6 to 1.8 eV and from 4.9 to 1.2 for BCxNy films, synthesized from TEB + NH3 and TEB + He mixtures, correspondingly, with increasing deposition temperature from 100 to 700 °C. This can be explained by the fact that the increase of carbon content in the sample leads to the decrease of the optical band gap. As it is
Fig. 6. XPS-spectra of BCxNy films synthesized from TEB + NH3 and TEB + He mixtures at temperature of 400 °C.
Please cite this article as: V.S. Sulyaeva, et al., PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor, Surface & Coatings Technology (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.06.018
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b b
Fig. 7. Transmission spectra of BCxNy films synthesized from TEB + NH3 (a) and TEB + He (b) mixtures at different temperatures.
shown above, Raman spectroscopy confirms a disordered graphite particle formation with increasing deposition temperature. Likewise authors [36] suggest that the absorption edge shifts to the low photon energy for BCN films with increasing C incorporation. In their case the band gap varied from 5.4 to 3.4 eV for polycrystalline BCN film sensitized by PACVD under application of DC negative bias from BCl3, CH4 and N2 at 650 °C. While for the other authors the optical band gap change was in the narrow range: 1.48–2.00 eV for the RF magnetron sputtering BCN films from h-BN and graphite at 250–550 °C [37], 3.78–3.92 eV for laser ablation amorphous BCN films from B4C in nitrogen atmosphere at 500 °C [38] and 3.26–3.55 eV for CVD BCN films from dimethylammine borane at 300–400 °C [39]. 4. Conclusions BCxNy thin films were prepared by plasma enhanced chemical vapor deposition using N-triethylborazine as a single-source precursor and ammonia and helium as additional gases. Both BCxNy thin films composition and the film deposition rate were found to be dependent on the deposition temperature and initial gas mixture. Structural characterization by XPS and FTIR indicated the formation of a ternary BCxNy compound. The optical study revealed that the transmittance of the films improved with the deposition temperature decrease and with the adding of ammonia as a reactant gas but significantly changed at higher deposition temperature in case of the use of TEB + He mixture. In addition, the optical band gap (Eg) of the BCxNy films increased with the deposition temperature decrease and changed in the range from 4.9 eV to 1.2 eV. Finally, it should be highlighted that
Fig. 8. Temperature dependence of Eg of BCxNy films synthesized from TEB + NH3 (a) and TEB + He (b) mixtures.
the coating microstructure and properties are clearly influenced by growth conditions, which demonstrate that the deposition temperature and gas phase composition should be optimized, in order to obtain coatings with high optical properties. Acknowledgements This work is supported by the Presidium of Russian Academy of Sciences (grant No 24-68). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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Please cite this article as: V.S. Sulyaeva, et al., PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor, Surface & Coatings Technology (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.06.018
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PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor V.S. Sulyaeva a,⁎, M.L. Kosinova a, Yu.M. Rumyantsev a, V.G. Kesler b, F.A. Kuznetsov a a b
Nikolaev Institute of Inorganic Chemistry, SB RAS 3, Acad. Lavrentiev Ave., Novosibirsk, 630090, Russia Rzhanov Institute of Semiconductor Physics, SB RAS 13, Acad. Lavrentiev Ave., Novosibirsk, 630090, Russia
a r t i c l e
i n f o
Available online xxxx Keywords: N-triethylborazine Boron carbonitride films PECVD Transparency Optical band gap energy
a b s t r a c t Thin films of boron carbonitride BCxNy were synthesized by plasma enhanced chemical vapor deposition using N-triethylborazine as a B-C-N-forming single source precursor. In this work, we investigated the effect of additional gases and temperature synthesis on the chemical composition of the BCxNy films. The plasma was activated via 40.68 MHz radio frequency. It was found by FTIR spectroscopy that the low temperature films (Tdep b 400 °C) are hydrogenated boron carbonitride BCxNy:H. It was shown by Raman spectroscopy that the films synthesized at high temperature (Tdep ≥ 600 °C) contain an additional phase of disordered carbon. The refractive index of the BCxNy films increased from 1.5 to 2.8 with increasing temperature of the synthesis. Low temperature films exhibit high transmittance values in the region from 400 to 3200 nm (transparency up to 93%). The optical band gap energy ranged from 1.2 to 4.9 eV for the films deposited under considered synthesis conditions. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Films containing boron, carbon and nitrogen (boron carbonitride, BCxNy) are of great interest because of their properties such as high hardness and resistance to heat, wear and corrosion, low-k values, high transparency and variable band gap energy. In the early stages of BCxNy film synthesis study, toxic and explosive halides and hydrides of boron were used. Recently organoboron compounds including borazine derivatives attracted increasing interest. Maya et al. used tris-(1,3,2-benzodiazaborolo) borazine for amorphous semiconductor BCN film production by means of pyrolysis at 900 °C at the end of the 80's [1–3]. Chemical analysis showed that the synthesized material had the composition BC5.2N1.8H1.9O0.45. Weber et al. used mixtures of N,N′,N″-trimethylborazine (CH3)3N3B3H3 (TMB) and argon to deposit BCN:H films by means of electron cyclotron resonance plasma enhanced chemical vapor deposition processes (PECVD) at 100–150 °C [4,5]. Amorphous BCN layers were deposited by radio frequency plasma activated enhanced chemical vapor deposition (RF-PACVD) at low temperature (95–120 °C) using a mixture of TMB and H2, N2, or Ar [6]. The composition of the layers varied in a wide range. The boron content of the films ranged from 3 to 42 at.%, the carbon content from 16 to 80 at.%, and oxygen content from 2 to 10 at.%. Stöckel et al. deposited BCN films with hexagonal turbostratic graphite like structure ⁎ Corresponding author at: Department of Functional Materials Chemistry, Nikolaev Institute of Inorganic Chemistry SB RAS, 630090, Novosibirsk, Russia. Tel.: +7 383 330 6646; fax: +7 383 330 9489. E-mail address: [email protected] (V.S. Sulyaeva).
by both isothermal chemical vapor deposition (ITCVD) under atmospheric pressure and PECVD from gaseous mixtures of trimethylborazine, toluene and ammonia at 950 °C [7,8]. Similarities between ITCVD and PECVD films showed up in the case of chemical composition and correlation between carbon content and hardness values. Considerable differences exist in regard to the microstructure, especially the texture of the films. Moreover, in ITCVD films the carbon is preferentially incorporated between the BN basal planes, whereas in PECVD films it is especially incorporated in the BN planes. BCN coatings were deposited by means of a capacitively or inductively coupled RF-PACVD using the elementalorganic compound trimethylborazine as a precursor. The influence of plasma parameters on the film properties has been discussed [9,10]. Hydrogenated BCN:H films were deposited with two different PECVD techniques by Thamm et al. [11,12]. Microwave plasma with RF-bias enhancement (MW-PECVD) and a direct current glow discharge plasma system were used with TMB and benzene as an additional carbon source. Argon and nitrogen were used as plasma gases. The substrate temperature, substrate bias and gas composition were varied. The depth profiles show a homogeneous distribution of elements B, C, N, and H over the entire layer thickness. Impurities such as oxygen or argon are detected in only small quantities (below 0.5 at.%) and the concentration does not increase towards the surface. The hydrogen content mostly depends on the substrate temperature during the coating process. If the layers are deposited on the 50 °C temperature substrate (MW-PECVD), the hydrogen content increases up to 35 at.%. If the temperature is increased up to 800 °C, only 8 at.% of hydrogen are detected, independently of the plasma gas. The variation of the MW plasma power and RF-bias has no significant effects on the layer composition. Multicomponent films were grown by PECVD from N-trimethylborazine–nitrogen mixtures at
0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.06.018
Please cite this article as: V.S. Sulyaeva, et al., PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor, Surface & Coatings Technology (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.06.018
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temperatures from 100 to 700 °C and varied RF power. According to XPS and IR spectroscopy results, the major component of the films is boron nitride. The films grown at temperatures below 400 °C contain hydrogen. The high temperature films, as a rule, contain carbon [13–15] Thermolysis of N-triethylborazine (C2H5)3N3B3H3 (TEB) and N-tripropylborazine (C3H7)3N3B3H3 at 500 °C produces homogeneous, amorphous boron carbonitride phases, whose compositions depend on the borazine substituent, and corresponding structures are similar to that of icosahedral boron carbide B4C [16]. The deposition of BCN-layers by decomposing TEB was performed in a hot-filament CVD reactor and for the gas activation a carburized Ta wire was used. The substrate temperature was 890 °C. The deposition parameters filament temperature (1800–2200 °C), precursor flow rate and gas atmosphere were varied. The layer growth rates increased with the precursor flow rate and showed less influence of the substrate material. The layer morphology was a pyrolytic type. When the layers became thick, they tended to eliminate from the substrate. Due to the atomic hydrogen produced at the filament, the deposition rate decreased in case of hydrogen atmosphere compared to that for experiments in argon atmosphere. That can be explained by etching reactions of the atomic hydrogen. Due to the etching reactions of the atomic hydrogen, the IR-spectra also look different. Nevertheless, it was not possible to identify a cubic phase of boron nitride from the IR or XRD measurements [17]. So, boron carbonitride films have been obtained by different processes of chemical vapor deposition utilizing borazine derivatives. However, the production of boron carbonitrides from TEB has attracted very little attention. The aim of the present study is to explore the possibility of the synthesis of BCxNy thin films by PECVD using N-triethylborazine with ammonia or helium and to study the growth rate, refractive index, types of chemical bonds, optical properties of the films depending on the growth conditions. 2. Experimental 2.1. Synthesis of the precursor (TEB) N-triethylborazine was synthesized by a mechanochemical method [18]. The usage of solvents as a medium is excluded from mechanochemical syntheses of this kind. Volatile boron-containing compounds are isolated from the reaction medium by evaporation. Starting compounds for producing were ethylamine hydrochloride C2H5NH2 · HCl and natrium tetrahydroborate NaBH4. TEB synthesis was performed under mechanical activation of the mixture of the starting compounds in hermetic rotational and vibratory ball mills by the following solidphase reactions: C2 H5 NH2 ·HClðsÞ þ NaBH4ðsÞ →C2 H5 NH2 ·BH3ðsÞ þ NaClðsÞ þ H2ðgÞ ; next stage was performed under a step pyrolysis at temperatures up to 90 and 240 °C with a resulting gas outlet: C2 H5 NH2 ·BH3 →ðC2 H5 NHBH2 Þn →ðC2 H5 NBHÞ3 : TEB was obtained in a yield of 74%. The product was characterized by the melting point of − 48 °C, liquid density of 0.836 g/cm3 at 20 °C, refractive index n20 D = 1.4365 and saturated vapor pressure of 399.97 Pa, which correspond to the TEB literature data [19]. TEB can be regarded as a promising precursor for the preparation of BC2N films as the ratio of elements in its molecule is 1:2:1. 2.2. PECVD process Plasma enhanced chemical vapor deposition process was applied for preparing BCxNy coatings by using mixtures of N-triethylborazine and ammonia or helium as plasma gases. The processes were carried out
in a horizontal hot wall quartz reactor which has discharge and growth zones. The reactor has RF coils powered at 40 W with an RF field of 40.68 MHz. Precursor and additional gases were introduced by separate gas inlets. The hydrogen or ammonia gas passed through the discharge zone following by mixing with TEB appeared at the growth zone, which was heated up by the resistance furnace. The deposition temperature ranged from 100 to 700 °C. Residual pressure in the system was fixed at 0.67 Pa, the vapor pressure of the precursor PTEB = 2.13 Pa in all experiments. Pressure of additional gases was equal to 1.07 Pa. The substrate was placed onto a holder at the center of the furnace. Gaseous byproducts of the deposition process were removed by pumping system and collected in a liquid nitrogen cold trap.
2.3. Substrates Silicon wafers Si(100) and fused silica were used as substrates. Investigation of surface microstructure and composition was performed for coatings deposited on Si(100) substrates. Only for the evaluating layer transmission BCxNy was also deposited on fused silica substrates. The substrates were chemically treated before layer deposition. Preliminarily in all cases the substrates were cleaned in ethanol, degreased in trichlorethylene, acetone and washed in deionized water. Additionally fused silica substrates degreased in sulfuric acid with hyperoxide. Si(100) substrates were chemically etched by mixtures of NH 3 + H2O2 + H2O, washed in deionized water, HCl + H2O2 + H2O, washed in deionized water, hydrofluoric acid, washed in deionized water, and then dried in a nitrogen stream.
2.4. Analysis The BCxNy coating thickness and refractive index were determined by ellipsometry (LEF-3 M ellipsometer) at the wavelength of 632.8 nm. The measurements were carried out at seven angles. The surface microstructure and the elemental composition of the films were observed by a scanning electron microscope (SEM) JEOL JSM 6700F equipped with an EX-23000BU analyzer for element composition determination by X-ray energy dispersive spectroscopy. Fourier transform infrared (FTIR) spectra of the films were recorded using a SCIMITAR FTS 2000 spectrometer in the range of 300–4000 cm−1. Thirty-two scans and the aperture equal to 4 at achievable resolution 2 cm−1 were used during the measurements. In each case the substrate spectrum was subtracted from that of the sample. All FTIR spectra were normalized to the thickness of an appropriate film. Raman spectra were recorded on a Spex 1877 triple spectrometer excited by an argon laser. The XPS spectra were obtained by a MAC-2 (RIBER) analyzer using nonmonochromatic Al Kα radiation (1486.6 eV) with the power of 300 W, and X-ray beam diameter of about 5 mm. The energy resolution of the instrument was chosen as 0.7 eV to have sufficiently small broadening of natural core level lines at a reasonable signal-noise ratio. Under these conditions the observed full width at half maximum (FWHM) of Au 4f7/2 line was 1.31 eV. The binding energy scale was calibrated in reference to Cu 3p3/2 (75.1 eV) and Cu 2p3/2 (932.7 eV) lines, assuring the accuracy of ±0.1 eV in any peak energy position determination. Since BCxNy are dielectric films, the photoelectron energy drift, due to charging effects, was taken into account in reference to the position of C 1 s (284.6 eV) line generated by adventitious carbon on the sample surface when inserted into the vacuum chamber. The component of adventitious carbon was derived from the complex carbon peak structure by means of deconvolution. To decompose the overlapped XPS peaks, we used mixed Gaussian and Lorentzian line shape functions with the parameters of the peaks measured on h-BN and B4C standard samples. Optical transmittance of the deposited films was examined using spectrophotometry (Scanning Spectrophotometer UV-3101PC Shimadzu) in the range of 190–3200 nm (resolution of 2 nm).
Please cite this article as: V.S. Sulyaeva, et al., PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor, Surface & Coatings Technology (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.06.018
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3. Results and discussion
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a
BCxNy films on the Si(100) and fused silica wafers with thickness 100–300 nm were obtained. The film properties were investigated as a function of the PECVD parameters, essentially the nature of the additional gas and the temperature of deposition. Surface morphology examination by SEM study showed smooth homogeneous, not rough film surfaces. Temperature increase yields an occurrence of 20–30 nm particles in either case for BCxNy films synthesized from TEB + NH3 (Fig. 1a) and TEB + He (Fig. 1b) mixtures. 3.1. Film deposition rate The deposition rate was monitored to investigate the growth characteristics of the BCxNy coatings (Fig. 2). The substrate temperature was varied keeping the other deposition parameters constant for both mixtures of TEB with ammonia and helium. It is clear from Fig. 2 that the growth rate decreases almost linearly with rising substrate temperature. That is typical for PECVD processes. Most likely the process of polymer-like formation film occurrences from plasma induces decomposition of large fragments of the initial molecules at low temperatures [5].
b
Fig. 2. Refractive index and growth rate of BCxNy films synthesized from TEB + NH3 (a) and TEB + He (b) mixtures as function of temperature.
3.2. Film composition 3.2.1. EDX analysis X-ray energy dispersion spectroscopy was conducted for layer composition analysis. It was found by the use of this method that the basic elements of the films were boron, carbon, and nitrogen with oxygen impurities. Despite of the fact that the ratio of the elements in the molecule of TEB is B:C:N = 1:2:1, the elemental composition of the synthesized films does not meet this relation. The film compositions were in the range of 23–39, 12–39, 35–46, 0–9 at.% for B, C, N, O, respectively, depending on the synthesis conditions. Temperature significantly affects the chemical composition of the films in case of the initial mixture of TEB + NH3 (Fig. 3a). The boron, nitrogen concentrations increase and carbon concentration decreases in the films with the deposition temperature. In contrast to using ammonia at the addition of He, the elemental composition of the BCxNy films weakly depends on the synthesis temperature (Fig. 3b). Nevertheless, in the both cases high carbon concentration at low temperatures observed because of incomplete precursor decomposition and inclusion of carbon residue in the films. High carbon concentration at high temperatures caused by disordered graphite formation (as will be shown below by Raman-spectroscopy).
Fig. 1. SEM-images of BCxNy films synthesized at Tdep = 700 °C from TEB + NH3 (a) and TEB + He (b) mixtures.
3.2.2. FTIR analysis The FTIR spectra of the films synthesized at high temperature (T > 400 °C) show modes at 800 and 1380 cm−1 of h-BN due to B-N-B and B-N vibrations [20], respectively, and shoulders at 1100
Please cite this article as: V.S. Sulyaeva, et al., PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor, Surface & Coatings Technology (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.06.018
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a
b
b
Fig. 3. Temperature dependence of main elements atomic concentrations of BCxNy films synthesized from TEB + NH3 (a) and TEB + He (b) mixtures.
and 1600 cm−1, corresponding to ν(B-C) in B4C [21] and ν(С = N) in the amorphous CN films [22] (Fig. 4). However, FTIR-spectra of films prepared at T b 400 °C show modes at 1435 см−1 corresponding to stretching vibrations of the B-N in the ring [23] of initial TEB and the stretching and deformation vibrations of B-H, C-H, N-H, C-CH3, which allows us to consider these films as hydrogenated boron carbonitride BCxNy:H. Previously authors [16] noted that during the conversion of the precursor to a solid product, the ring system is generally preserved. 3.2.3. Raman analysis Raman spectra of BCxNy films synthesized at temperatures of 600 and 700 °C from different initial gas mixtures TEB + NH3 and TEB + He contain two overlapped strong peaks centered at 1320, 1570 cm−1 (Fig. 5a) and 1347, 1566 cm−1 (Fig. 5b), correspondingly. These spectra are similar to BCN film spectra which have D and G peaks appropriate for disordered carbon [24,25]. Because the D peak of carbon and the E2g mode of hexagonal BN (1366 cm−1) are so close, the peak at 1320 (or 1347) cm−1 is probably regarded as their overlap. The intensities of the peaks in Raman-spectra of films deposited from TEB + He mixture are higher compared to that while using TEB + NH3 mixture. 3.2.4. XPS analysis Further part of the work has been carried out to study the nature of the chemical environment of prepared BCxNy films by XPS. Fig. 6 shows the XPS core-level spectra of B 1s, C 1s and N 1s together with their deconvolution into several overlapping components for
Fig. 4. FTIR-spectra of BCxNy films synthesized from TEB + NH3 (a) and TEB + He (b) mixtures at different temperatures.
samples deposited at the substrate temperature of 400 °C and different gases, respectively. The asymmetry and broadening of the peaks imply that there is more than one type of bonding state for B, C and N atoms. A minimal number of spectral components was selected on the basis of the results for of a standard sample investigation [26], spectrometer resolution and literature data. The above deconvolution is the most realistic but not the only possible one, which may occur in the process. That is based on fact that, to receive different results at different possible input data, the existence of different environments around the boron atom has been studied earlier by near-edge x-ray absorption fine structure spectroscopy [27,28]. The B 1s spectra have specific features at 190.6, 190.1 and 191.1 eV. The first components correspond to B-N bond [29] in hexagonal boron nitride. The latter signal with the energy of 191.1 eV is in case of using the TEB with He mixture can be interpreted as B-C-N hybride bonds [16,30]. The main component of C 1s with the binding energy of 284.6 eV can be attributed to graphitic carbon due to an adventitious carbon contamination on the sample surface [31]. Other features at 281.7, 285.6 and 288.3 can be associated with C-B [32], C-N [33], C-O bonds [34]. The N 1s spectra exhibit an intensive peak at 398.4 eV that can be attributed to N-B bonds [12], which is the same as that for h-BN. The peak at 400.6 eV is observed usually ascribed to N-C bond [34]. The ratio between N 1s and B 1s peak areas is independent of carbon and oxygen contaminations and allows us to calculate the nitrogen and boron concentrations in the film. For the film synthesized by using the mixture of TEB with NH3 this ratio was equal to 1.00.
Please cite this article as: V.S. Sulyaeva, et al., PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor, Surface & Coatings Technology (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.06.018
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binding energies. There are small components B 1s and C 1s corresponding to B-C bonding. 3.3. Optical properties Thin BCxNy films exhibited good optical properties. Refractive index n of the BCxNy films increased from 1.5 to 2.3 with increasing temperature of synthesis from 100 to 700 °C (Fig. 2), referring to changes in the chemical composition. The value of n increasing with temperature is higher in the case of the TEB + He initial gas mixture. Fig. 7 shows the influence of deposition temperature on the optical transmission of BCxNy films deposited on fused silica substrates. It was found that the low temperature films exhibit high optical properties in the region from 400 to 3200 nm (transparency up to 93%). Transparencies of the films synthesized from TEB + NH3 mixture have a red shift with the temperature increase. For the TEB + He mixture: at synthesis temperatures above 500 °C the film transparencies sharply decreased, that was apparently due to adding disordered carbon phase. Also BCxNy film high transmission (90 %) in the UV-visible region was achieved by using TMB and (N-pyrrolidino)diethylborane as precursors and low deposition temperature as in [9]. According to the ‘non-direct transition’ model proposed by Tauc, the transmittance (%T) data and film thickness can be used to calculate the absorption coefficient by the equation [35]:
b
α ¼ − lnð%T Þ=d; where α is the absorption coefficient and d is the film thickness. The absorption coefficient values were subsequently used to calculate the optical band gap (Eg) of the amorphous BCxNy films. This is realized from the Tauc plot given as seen from the equation [35]: 1=2
ðαhνÞ Fig. 5. Raman-spectra of BCxNy films synthesized from TEB + NH3 (a) and TEB + He (b) mixtures at temperature of 700 °C.
Therefore, one can define areas in the samples which structure is similar to h-BN. This phase also dominated in the films synthesized from the mixture of TEB with He, but in this case, the nitrogen to boron concentration ratio was equal to 0.82. The use of He results in the relative increase of B 1s, N 1s and C 1s components with higher
¼ B hν−Eg ;
where B is a constant factor and hν is the photon energy. The extrapolation of the linear region of the Tauc curve to intercept the x-axis leads to Eg values for the material and the slope of this line can be used to calculate B. It can be observed from Fig. 8 that the Eg value decreases from 4.6 to 1.8 eV and from 4.9 to 1.2 for BCxNy films, synthesized from TEB + NH3 and TEB + He mixtures, correspondingly, with increasing deposition temperature from 100 to 700 °C. This can be explained by the fact that the increase of carbon content in the sample leads to the decrease of the optical band gap. As it is
Fig. 6. XPS-spectra of BCxNy films synthesized from TEB + NH3 and TEB + He mixtures at temperature of 400 °C.
Please cite this article as: V.S. Sulyaeva, et al., PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor, Surface & Coatings Technology (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.06.018
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b b
Fig. 7. Transmission spectra of BCxNy films synthesized from TEB + NH3 (a) and TEB + He (b) mixtures at different temperatures.
shown above, Raman spectroscopy confirms a disordered graphite particle formation with increasing deposition temperature. Likewise authors [36] suggest that the absorption edge shifts to the low photon energy for BCN films with increasing C incorporation. In their case the band gap varied from 5.4 to 3.4 eV for polycrystalline BCN film sensitized by PACVD under application of DC negative bias from BCl3, CH4 and N2 at 650 °C. While for the other authors the optical band gap change was in the narrow range: 1.48–2.00 eV for the RF magnetron sputtering BCN films from h-BN and graphite at 250–550 °C [37], 3.78–3.92 eV for laser ablation amorphous BCN films from B4C in nitrogen atmosphere at 500 °C [38] and 3.26–3.55 eV for CVD BCN films from dimethylammine borane at 300–400 °C [39]. 4. Conclusions BCxNy thin films were prepared by plasma enhanced chemical vapor deposition using N-triethylborazine as a single-source precursor and ammonia and helium as additional gases. Both BCxNy thin films composition and the film deposition rate were found to be dependent on the deposition temperature and initial gas mixture. Structural characterization by XPS and FTIR indicated the formation of a ternary BCxNy compound. The optical study revealed that the transmittance of the films improved with the deposition temperature decrease and with the adding of ammonia as a reactant gas but significantly changed at higher deposition temperature in case of the use of TEB + He mixture. In addition, the optical band gap (Eg) of the BCxNy films increased with the deposition temperature decrease and changed in the range from 4.9 eV to 1.2 eV. Finally, it should be highlighted that
Fig. 8. Temperature dependence of Eg of BCxNy films synthesized from TEB + NH3 (a) and TEB + He (b) mixtures.
the coating microstructure and properties are clearly influenced by growth conditions, which demonstrate that the deposition temperature and gas phase composition should be optimized, in order to obtain coatings with high optical properties. Acknowledgements This work is supported by the Presidium of Russian Academy of Sciences (grant No 24-68). References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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Please cite this article as: V.S. Sulyaeva, et al., PECVD synthesis and optical properties of BCXNY films obtained from N-triethylborazine as a single-source precursor, Surface & Coatings Technology (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.06.018