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Structural and optical evaluation of WOxNy films deposited by reactive magnetron sputtering
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Journal of Physics and Chemistry of Solids 68 (2007) 2227–2232 www.elsevier.com/locate/jpcs
Structural and optical evaluation of WOxNy films deposited by reactive magnetron sputtering S.H. Mohameda,b,c,, Andre´ Andersa, I. Monterob, L. Gala´nd a
Lawrence Berkeley National Laboratory, 1 Cyclotron Road, California 94720, USA Instituto de Ciencia de Materiales de Madrid, Csic, Cantoblanco, Madrid 28049, Spain c Physics Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt d Departamento de Fı´sica Aplicada, Universidad Auto´noma de Madrid, Cantoblanco, 28049 Madrid, Spain b
Received 10 December 2006; received in revised form 19 May 2007; accepted 16 June 2007
Abstract Thin films of tungsten oxynitrides were deposited on substrates preheated at 300 1C from metallic tungsten target using reactive pulsed d.c. magnetron sputtering. The deposition was carried out at different nitrogen to total reactive gas partial pressures ratios. The energy dispersive analysis of X-ray showed that significant incorporation of nitrogen occurred only when the nitrogen partial pressure exceeded 74% of the total reactive gas pressure. X-ray diffraction analysis revealed that the formation of a specific crystalline phase is affected by the composition and the possibility of competitive growth of different phases. The increase of nitrogen content into the films increases the optical absorption and decreases the optical band gap. The refractive index was determined from the transmittance spectra using Swanepoel’s method. It was found that the refractive index increases with increasing nitrogen content over the entire spectral range. The values of the tungsten effective coordination number, Nc, was estimated from the analysis of the dispersion of the refractive index, and an increase in Nc with increasing nitrogen content was observed. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Thin films; B. Plasma deposition; C. X-ray diffraction; D. Optical properties
1. Introduction Tungsten oxynitrides (WOxNy) belong to the family of transition metal oxynitrides which shows a large range of behaviors and displays good mechanical and optoelectronic properties [1–4]. In particular, WOxNy have attractive potential technological applications in different fields, including coatings for decorative purposes, cutting tools, micro- and optoelectronics. WOxNy are used for heterogeneous catalysis where they behave like the noble group metals for many hydrogen transfer reactions [5]. Rare earth-WOxNy materials are promising as novel pigments [6]. Recently, WOxNy were patented as metal oxynitride capacitor barrier layer [7]. The capacitors are suited for use as memory cells and apparatus incorporating such memory cells, as well as other integrated circuits. Corresponding author.
E-mail address: [email protected] (S.H. Mohamed). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.06.005
Several methods have been used to produce WOxNy with a wide range of oxygen and nitrogen concentrations. Among these methods are reactive d.c. magnetron sputtering [3], dual reactive magnetron sputtering [4], temperature-programmed reaction of WO3 with ammonia [5] and sol–gel [6]. In this work, pulsed d.c. reactive magnetron sputtering was used because it is one of the most versatile techniques to produce new compounds with a wide range of chemical composition. By a simple control of the partial pressures of the reactive gases in the deposition chamber, it is possible to deposit coatings with a wide range of composition and, consequently, different properties. The present paper is concerned with the analysis of the composition, and structural and optical properties of WOxNy thin films prepared by reactive d.c. magnetron sputtering. Although it is possible to find in the literature some papers dealing with bulk (powder) [5,6] and thin films [3,4,7] of WOxNy, to the best of our knowledge there is no thorough determination of the refractive index, extinction
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coefficient and the dispersion parameters of WOxNy thin films. Also, in this contribution we extend our preliminary report [4] by raising the substrate temperature using pulsed d.c. sputtering. 2. Experimental details WOxNy films were prepared on microscopic glass slides and Si(1 0 0) substrates by reactive magnetron sputtering of metallic tungsten target in an argon, oxygen and nitrogen gas mixture. Sputtering was carried out from 7.5 cm diameter target at an average power of 750 W and with the substrates preheated to 300 1C. The distance between the target and the substrate holder was 8.25 cm. The substrate were ultrasonically cleaned in acetone and dried with a flow of pure nitrogen before mounting on the substrate holder. The cryogenically pumped vacuum system had a base pressure of 1.3 104 Pa at full pumping speed of 1500 l/s for air. To accommodate the relatively high pressure during sputtering, the pumping speed was throttled to about 25% of its maximum, resulting in a throttled base pressure of about 1.3 103 Pa. The total pressure during deposition was kept constant at 0.46 Pa as monitored by a Baratrons capacitance manometer. The total pressure was kept constant by adjusting the Ar flow while systematically varying the O2 and N2 partial pressures. A differentially pumped gas monitor (PPM 100 by SRS) was used to measure the partial pressures during deposition. This gas monitor was pre-calibrated via the readings of the Baratron. To prepare WOxNy films, we started with an O2 partial pressure (PO2 ) of 0.298 Pa at 40 sccm O2 flow. At this oxygen partial pressure, the W target operated in the ‘‘poisoned’’ mode (oxide layer was present). Next, the O2 flow was partially replaced by N2 flow. The N2 partial pressure (PN2 ) increased from 2.7 102 to 3.84 101 Pa (corresponding to a flow rate of 0–40 sccm N2) while the O2 partial pressure decreased from 2.98 101 to 6.18 104 Pa (by reducing the O2 flow from 40 to 0 sccm). In this way, oxynitride films of different compositions could be obtained. In the remainder of this paper, we will use G, which we define as the nitrogen partial pressure normalized by the partial pressures of both reactive gases, G PN2 =ðPN2 þ PO2 Þ. Glass slide substrates were used for X-ray diffraction (XRD) because of their amorphous structure, and for measurements of the optical properties because of their high transparency in the visible range. Si(1 0 0) substrates were used for film thickness measurements (Dektak IIA profilometer) and composition analysis (energy dispersive analysis of X-ray, EDAX). The profilometer had experimental error of about 710 nm in determining the film thickness. The crystallographic structure of the films was determined by XRD using a Siemens D-500 diffractometer with a Cu tube operated at 40 kV and 30 mA. The measurements
were carried out using Cu Ka radiation with a Ni filter to remove the Cu Kb reflections. EDAX bulk composition measurements were performed in a Philips XL 30 scanning electron microscope at 10 kV using only internal absorption values and, therefore, the O and N content could be higher than the figures obtained. The spectral transmittance (T) and reflectance (R) were measured at normal incidence using a Perkin-Elmer Lambda-19 spectrophotometer in the wavelength range l ¼ 300–2500 nm.
3. Results and discussions 3.1. Film composition and structure Fig. 1a shows the deposition rate of WOxNy thin films deposited at different G values. The deposition rate of the fully poisoned target is 0.59 nm/s. Upon increasing G, the deposition rate increases almost linearly up to G ¼ 0.86. This increase is ascribed to the incorporation of the nitrogen into the target surface layer and, thus, forming oxynitride with higher deposition rate [4]. Above G ¼ 0.86, the deposition rate decreases to reach a value of 1.03 nm/s at G ¼ 0.998. This decrease in deposition rate after certain G value has been observed for the deposition rate of titanium oxynitrides [8] and it was ascribed to the dependence of the sputtering yield on the ion energy, i.e., cathode potential.
Fig. 1. (a) Deposition rate of WOxNy and (b) N/O atomic ratio in the film as a function of the normalized nitrogen partial pressure, G ¼ PN2 = ðPN2 þ PO2 Þ.
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The change in chemical composition of WOxNy films was detected by EDAX. The peak heights in the EDAX spectra are proportional to the elements concentration. The qualitative EDAX spectra for WOxNy prepared at G ¼ 0.341, 0.956 and 0.998 are shown in Fig. 2a–c, respectively. It is seen from the spectra that in addition to O, N and W peaks there are C and Si peaks which may be ascribed to contamination by hydrocarbons and the contribution of the Si(1 0 0) substrate, respectively. The atomic concentrations of O, N and W in WOxNy films are tabulated in Table 1 and the ratio of N/O in the film is plotted as a function of G in Fig. 1b. It can be seen from Table 1 and Fig. 1b that the nitrogen content in WOxNy is very small up to G ¼ 0.742. Above G ¼ 0.742, the
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incorporation of nitrogen became significant. It is further observed that there was a small portion of oxygen even at G ¼ 0.998, i.e., when no oxygen was supplied into the deposition chamber. These are attributed to the fact that oxygen has higher reactivity than nitrogen with tungsten, and that residual water can be a source of oxygen. The crystal structure of WOxNy thin films was analyzed using XRD. Fig. 3a–h present XRD patterns of WOxNy thin films prepared at different G. The XRD pattern of the sample prepared without nitrogen (Fig. 3a) shows a sharp crystalline peak at 23.101 which corresponds to the (0 0 2) of WO3. Upon increasing the G values to 0.619, the (0 0 2)
Fig. 3. X-ray diffraction patterns of WOxNy deposited at different G: (a) G ¼ 0.009, (b) G ¼ 0.341, (c) G ¼ 0.563, (d) G ¼ 0.619, (e) G ¼ 0.742, (f) G ¼ 0.861, (g) G ¼ 0.956 and (h) G ¼ 0.998.
Fig. 2. EDAX spectra of WOxNy films deposited at different G: (a) G ¼ 0.341, (b) G ¼ 0.956 and (c) G ¼ 0.998.
Table 1 Thickness, deposition time and chemical composition of WOxNy films as a function of G G
0.009 0.341 0.563 0.619 0.742 0.861 0.956 0.998
Thickness (nm)
422 347 401 392 346 340 369 310
Deposition time (min)
12 6 6 5.5 4 3.5 4 5
WOxNy
Chemical composition atomic (%) W
O
N
23.8 24.3 23.8 24.0 24.9 25.0 26.9 35.2
76.2 68.6 67.9 68.2 66.0 58.1 37.1 7.9
0.0 7.1 8.3 7.8 9.1 17.0 35.9 56.9
W1O3.2 W1O2.82N0.29 W1O2.85N0.35 W1O2.84N0.33 W1O2.65N0.36 W1O2.32N0.68 W1O1.38N1.33 W1O0.22N1.62
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peak is shifted slightly to higher 2y angles (23.571). A possible reason for this shift of the diffraction peak is that some of the oxygen atoms are substituted by nitrogen atoms. The nitrogen atoms may occupy the oxygen atoms’ positions in the crystal lattice or stay at grain boundaries and, thus, creating some additional stresses. At 0.742, the (0 0 2) peak becomes very weak (Fig. 3e). This indicates that the crystal structure of WO3 is changed and very small crystallites are formed. The appearance of a short-range order of crystallization is associated with the formation of WOxNy. At G ¼ 0.861, a different WOxNy structure is obtained with a relatively broad peak around 43.891 indicating a reduction in the size of the crystallites. With further addition of nitrogen to G ¼ 0.956, the material assumed amorphous phase. At G ¼ 0.998, (no oxygen is supplied) crystalline W2N with preferred orientation around (1 1 1) is obtained. 3.2. Optical properties By varying the nitrogen to total reactive gas pressure ratio, the optical properties like optical transmittance, optical band gap, refractive index, etc., are modified. The optical transmittance spectra of WOxNy films are shown in Fig. 4. The transmittance of the films prepared at Go0.742 is very high. With further increase in G, the transmittance decreases, and a transition of the optical behavior from insulating WO3 compound to metallic W2N is observed. The optical band gap values, Eg, are calculated by assuming an indirect transition between the edges of the valence and the conduction bands, using the equation: ðahnÞ1=2 ¼ Aðhn E g Þ,
where a is the absorption coefficient and A is a constant. By plotting (ahn)1/2 versus hn and extrapolating the linear region of the resulting curve, Eg can be obtained. The calculated Eg values are written in Table 2. Eg decreases with the progressive substitution of O by N atoms in WOxNy films. This decrease can be attributed to the formation of well-localized N 2p states above the O 2p valence band states. Inter-relation of such energy levels in the band gap reduces the band gap and increases the visible light absorption through a charge transfer between a dopant and a conduction or valence band [9]. More discussion about the Eg narrowing upon substitution of O by N atoms can be found in Ref. [4]. The refractive index values, n, of WOxNy films prepared at different G were calculated from the spectral transmittance using the Swanepoel method [10,11]. The results are shown in Fig. 5 as a function of wavelength. The refractive index increases with increasing G. A similar increase in refractive index upon nitrogen incorporation has been previously observed in different transition metal oxynitrides [8]. This increase in refractive index may be
Table 2 Values of Eg, E0, Ed and Nc G
Eg
E0
Ed
Nc
0.009 0.341 0.563 0.619 0.742 0.861
3.07 3.07 3.05 2.88 2.29 1.85
5.75 5.69 5.32 5.22 5.11 4.85
18.14 18.53 19.34 19.48 21.91 22.90
4.36 4.45 4.65 4.68 5.27 5.50
(1)
Fig. 4. Transmittance of WOxNy films prepared at various G ¼ PN2 = ðPN2 þ PO2 Þ.
Fig. 5. Refractive index variations of WOxNy deposited at different G values as a function of wavelength. The data points denote the calculated n values while the lines are fit functions based on the two-term Cauchy dispersion relationship; n(l) ¼ (a+b)/l2.
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attributed to the increase of polarization, since metal– nitrogen bonds tend to be less polar than the corresponding metal–oxygen bonds, which leads to a higher polarizability for the metal nitrides [12]. The extinction coefficient, k, of WOxNy films prepared at different G values was determined from the equation: al , (2) 4p where a ¼ a(l) is the wavelength-dependent absorption coefficient (Fig. 6). The k values increase progressively with increasing nitrogen content in the films. The extinction coefficient is related to the creation of defect and absorption centers whose number increase with increasing nitrogen content. The refractive index dispersion of materials can be fitted by the Wemple-DiDomenico relation [13,14]: k¼
nðEÞ2 1 ¼
E0Ed , E 20 E 2
(3)
where E0 and Ed are single-oscillator fitting constants which measure the oscillator energy and strength, respectively. By plotting ðnðEÞ2 1Þ1 against E2and fitting a straight line as shown in Fig. 7, E0 and Ed can be determined directly from the slope, (E0Ed)1, and the intercept, E0/Ed, on the vertical axis. The calculated E0 and Ed values are listed in Table 2. It is observed that E0 decreases while Ed increases with increasing nitrogen content. The oscillator energy E0 is an ‘‘average’’ energy gap, and to a very good approximation it scales with the optical band gap E0E2Eg, as was found by Tanaka [15]. The oscillator strength Ed is related to other physical parameters of the material through the relation [13,14]: E d ¼ bN c Z a N e ,
(4)
Fig. 7. Plot of refractive index factor (n21)1 versus E2 for WOxNy films.
where Nc is the effective coordination number of the cation (nearest neighbor to the anion), Za the formal chemical valency of the anion, Ne the effective number of valence electrons per anion and b is a two-valued constant with either an ionic or a covalent value (bi ¼ 0.2670.03 eV and bc ¼ 0.3770.04 eV, respectively). Taking into account Eq. (4), and assuming that the parameters Ne ¼ 8 and Za ¼ 2 [13] retain approximately the same values along the analyzed composition values, it would seem reasonable to ascribe the trend observed in the values of Ed to an increase in the effective cation coordination number, Nc. On the other hand, the possible influence of the parameter b on the increase observed for the oscillator strength should also be mentioned. According to Pauling’s electronegativities, ionicity in a single bond increases with the difference in values of electronegativity between two elements forming the bond. The electronegativity of oxygen (3.5) is greater than that of nitrogen (3.0), which indicates that the metal–O bond involves a larger charge transfer than the metal–N bond. Assuming that metal–O and metal–N bonds coexist in the films, thus, the nature of the chemical bonding could change towards being less ionic with increasing nitrogen content. 4. Conclusions
Fig. 6. Extinction coefficient variations of WOxNy deposited at different G values as a function of wavelength.
Tungsten oxynitride films were prepared by reactive magnetron sputtering on glass and Si(1 0 0) substrates at different oxygen and nitrogen partial pressures. A range of materials with different N/O ratio were processed and checked by EDAX. The different WOxNy phases were affected by compositions. The optical properties of the WOxNy films are thus controlled by changing the
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deposition conditions. It was found that the refractive index increases with increasing nitrogen content over the entire spectral range (300–2500 nm) studied. The dispersion parameter, Ed, increases from 18.14 to 22.90 with increasing nitrogen content from G ¼ 0.009 to 0.861. The increase of Ed points towards an increase in the interactions between the structural layers, hence leads to an increase of the tungsten effective coordination number, Nc. Nevertheless, a decrease in the ionic character of the chemical bonding with increasing nitrogen content, and its possible influence on the value of the parameter b, cannot be excluded either. A decrease in the oscillator energy E0 and the optical band gap Eg were obtained. Acknowledgments The authors are grateful for technical support by Sakon Sansongsiri, Michael Dickinson and Ainhoa Pardo. One of us (S. H. M.) would like to express his sincere gratitude to the Fulbright and the TEMPUS Commissions for fellowships to carry out this research work at Lawrence Berkeley National Laboratory, CA, USA and ICMM-CSIC, Cantoblanco, Madrid, Spain, respectively. A.A. acknowledges support by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Technology, of
the US Department of Energy under Contract no. DEAC02-05CH11231. References [1] O. Banakh, P.-A. Steinmann, L. Dumitrescu-Buforn, Thin Solid Films 513 (2006) 136. [2] M.-S. Wong, H.P. Chou, T.-S. Yang, Thin Solid Films 494 (2006) 244. [3] N.M.G. Parreira, N.J.M. Carvalho, F. Vaz, A. Cavaleiro, Surf. Coat. Technol. 200 (2006) 6511. [4] S.H. Mohamed, A. Anders, Surf. Coat. Technol. 201 (2006) 2977. [5] D.-H. Cho, T.-S. Chang, C.-H. Shin, Catal. Lett. 67 (2000) 163. [6] N. Diot, O. Larcher, R. Marchand, J.Y. Kempf, P. Macaudiere, J. Alloys Compd. 323–324 (2001) 45. [7] S. Yang, A.K. Vishnu, US Patent 7002202, 2006. [8] S.H. Mohamed, O. Kappertz, J.M. Ngaruiya, T. Niemeier, R. Drese, R. Detemple, M.M. Wakkad, M. Wuttig, Phys. Status Solidi (a) 201 (2004) 90. [9] C. Di Valentin, G. Pacchioni, A. Selloni, Phys. Rev. B 70 (2004) 085116. [10] R. Swanepoel, J. Phys. E: Sci. Instrum. 16 (1983) 1214. [11] E.R. Shaaban, N. El-Kabnay, A.M. Abou-sehly, N. Afify, Phys. B: Condens. Matter 381 (2006) 24. [12] M. Ohring, The Material Science of Thin Films, Academic Press, San Diego, 1991. [13] S.H. Wemple, W. DiDomenico, Phys. Rev. B 3 (1971) 1338. [14] S.H. Wemple, Phys. Rev. B 7 (1973) 3767. [15] K. Tanaka, Thin Solid Films 66 (1980) 271.
Journal of Physics and Chemistry of Solids 68 (2007) 2227–2232 www.elsevier.com/locate/jpcs
Structural and optical evaluation of WOxNy films deposited by reactive magnetron sputtering S.H. Mohameda,b,c,, Andre´ Andersa, I. Monterob, L. Gala´nd a
Lawrence Berkeley National Laboratory, 1 Cyclotron Road, California 94720, USA Instituto de Ciencia de Materiales de Madrid, Csic, Cantoblanco, Madrid 28049, Spain c Physics Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt d Departamento de Fı´sica Aplicada, Universidad Auto´noma de Madrid, Cantoblanco, 28049 Madrid, Spain b
Received 10 December 2006; received in revised form 19 May 2007; accepted 16 June 2007
Abstract Thin films of tungsten oxynitrides were deposited on substrates preheated at 300 1C from metallic tungsten target using reactive pulsed d.c. magnetron sputtering. The deposition was carried out at different nitrogen to total reactive gas partial pressures ratios. The energy dispersive analysis of X-ray showed that significant incorporation of nitrogen occurred only when the nitrogen partial pressure exceeded 74% of the total reactive gas pressure. X-ray diffraction analysis revealed that the formation of a specific crystalline phase is affected by the composition and the possibility of competitive growth of different phases. The increase of nitrogen content into the films increases the optical absorption and decreases the optical band gap. The refractive index was determined from the transmittance spectra using Swanepoel’s method. It was found that the refractive index increases with increasing nitrogen content over the entire spectral range. The values of the tungsten effective coordination number, Nc, was estimated from the analysis of the dispersion of the refractive index, and an increase in Nc with increasing nitrogen content was observed. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Thin films; B. Plasma deposition; C. X-ray diffraction; D. Optical properties
1. Introduction Tungsten oxynitrides (WOxNy) belong to the family of transition metal oxynitrides which shows a large range of behaviors and displays good mechanical and optoelectronic properties [1–4]. In particular, WOxNy have attractive potential technological applications in different fields, including coatings for decorative purposes, cutting tools, micro- and optoelectronics. WOxNy are used for heterogeneous catalysis where they behave like the noble group metals for many hydrogen transfer reactions [5]. Rare earth-WOxNy materials are promising as novel pigments [6]. Recently, WOxNy were patented as metal oxynitride capacitor barrier layer [7]. The capacitors are suited for use as memory cells and apparatus incorporating such memory cells, as well as other integrated circuits. Corresponding author.
E-mail address: [email protected] (S.H. Mohamed). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.06.005
Several methods have been used to produce WOxNy with a wide range of oxygen and nitrogen concentrations. Among these methods are reactive d.c. magnetron sputtering [3], dual reactive magnetron sputtering [4], temperature-programmed reaction of WO3 with ammonia [5] and sol–gel [6]. In this work, pulsed d.c. reactive magnetron sputtering was used because it is one of the most versatile techniques to produce new compounds with a wide range of chemical composition. By a simple control of the partial pressures of the reactive gases in the deposition chamber, it is possible to deposit coatings with a wide range of composition and, consequently, different properties. The present paper is concerned with the analysis of the composition, and structural and optical properties of WOxNy thin films prepared by reactive d.c. magnetron sputtering. Although it is possible to find in the literature some papers dealing with bulk (powder) [5,6] and thin films [3,4,7] of WOxNy, to the best of our knowledge there is no thorough determination of the refractive index, extinction
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coefficient and the dispersion parameters of WOxNy thin films. Also, in this contribution we extend our preliminary report [4] by raising the substrate temperature using pulsed d.c. sputtering. 2. Experimental details WOxNy films were prepared on microscopic glass slides and Si(1 0 0) substrates by reactive magnetron sputtering of metallic tungsten target in an argon, oxygen and nitrogen gas mixture. Sputtering was carried out from 7.5 cm diameter target at an average power of 750 W and with the substrates preheated to 300 1C. The distance between the target and the substrate holder was 8.25 cm. The substrate were ultrasonically cleaned in acetone and dried with a flow of pure nitrogen before mounting on the substrate holder. The cryogenically pumped vacuum system had a base pressure of 1.3 104 Pa at full pumping speed of 1500 l/s for air. To accommodate the relatively high pressure during sputtering, the pumping speed was throttled to about 25% of its maximum, resulting in a throttled base pressure of about 1.3 103 Pa. The total pressure during deposition was kept constant at 0.46 Pa as monitored by a Baratrons capacitance manometer. The total pressure was kept constant by adjusting the Ar flow while systematically varying the O2 and N2 partial pressures. A differentially pumped gas monitor (PPM 100 by SRS) was used to measure the partial pressures during deposition. This gas monitor was pre-calibrated via the readings of the Baratron. To prepare WOxNy films, we started with an O2 partial pressure (PO2 ) of 0.298 Pa at 40 sccm O2 flow. At this oxygen partial pressure, the W target operated in the ‘‘poisoned’’ mode (oxide layer was present). Next, the O2 flow was partially replaced by N2 flow. The N2 partial pressure (PN2 ) increased from 2.7 102 to 3.84 101 Pa (corresponding to a flow rate of 0–40 sccm N2) while the O2 partial pressure decreased from 2.98 101 to 6.18 104 Pa (by reducing the O2 flow from 40 to 0 sccm). In this way, oxynitride films of different compositions could be obtained. In the remainder of this paper, we will use G, which we define as the nitrogen partial pressure normalized by the partial pressures of both reactive gases, G PN2 =ðPN2 þ PO2 Þ. Glass slide substrates were used for X-ray diffraction (XRD) because of their amorphous structure, and for measurements of the optical properties because of their high transparency in the visible range. Si(1 0 0) substrates were used for film thickness measurements (Dektak IIA profilometer) and composition analysis (energy dispersive analysis of X-ray, EDAX). The profilometer had experimental error of about 710 nm in determining the film thickness. The crystallographic structure of the films was determined by XRD using a Siemens D-500 diffractometer with a Cu tube operated at 40 kV and 30 mA. The measurements
were carried out using Cu Ka radiation with a Ni filter to remove the Cu Kb reflections. EDAX bulk composition measurements were performed in a Philips XL 30 scanning electron microscope at 10 kV using only internal absorption values and, therefore, the O and N content could be higher than the figures obtained. The spectral transmittance (T) and reflectance (R) were measured at normal incidence using a Perkin-Elmer Lambda-19 spectrophotometer in the wavelength range l ¼ 300–2500 nm.
3. Results and discussions 3.1. Film composition and structure Fig. 1a shows the deposition rate of WOxNy thin films deposited at different G values. The deposition rate of the fully poisoned target is 0.59 nm/s. Upon increasing G, the deposition rate increases almost linearly up to G ¼ 0.86. This increase is ascribed to the incorporation of the nitrogen into the target surface layer and, thus, forming oxynitride with higher deposition rate [4]. Above G ¼ 0.86, the deposition rate decreases to reach a value of 1.03 nm/s at G ¼ 0.998. This decrease in deposition rate after certain G value has been observed for the deposition rate of titanium oxynitrides [8] and it was ascribed to the dependence of the sputtering yield on the ion energy, i.e., cathode potential.
Fig. 1. (a) Deposition rate of WOxNy and (b) N/O atomic ratio in the film as a function of the normalized nitrogen partial pressure, G ¼ PN2 = ðPN2 þ PO2 Þ.
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The change in chemical composition of WOxNy films was detected by EDAX. The peak heights in the EDAX spectra are proportional to the elements concentration. The qualitative EDAX spectra for WOxNy prepared at G ¼ 0.341, 0.956 and 0.998 are shown in Fig. 2a–c, respectively. It is seen from the spectra that in addition to O, N and W peaks there are C and Si peaks which may be ascribed to contamination by hydrocarbons and the contribution of the Si(1 0 0) substrate, respectively. The atomic concentrations of O, N and W in WOxNy films are tabulated in Table 1 and the ratio of N/O in the film is plotted as a function of G in Fig. 1b. It can be seen from Table 1 and Fig. 1b that the nitrogen content in WOxNy is very small up to G ¼ 0.742. Above G ¼ 0.742, the
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incorporation of nitrogen became significant. It is further observed that there was a small portion of oxygen even at G ¼ 0.998, i.e., when no oxygen was supplied into the deposition chamber. These are attributed to the fact that oxygen has higher reactivity than nitrogen with tungsten, and that residual water can be a source of oxygen. The crystal structure of WOxNy thin films was analyzed using XRD. Fig. 3a–h present XRD patterns of WOxNy thin films prepared at different G. The XRD pattern of the sample prepared without nitrogen (Fig. 3a) shows a sharp crystalline peak at 23.101 which corresponds to the (0 0 2) of WO3. Upon increasing the G values to 0.619, the (0 0 2)
Fig. 3. X-ray diffraction patterns of WOxNy deposited at different G: (a) G ¼ 0.009, (b) G ¼ 0.341, (c) G ¼ 0.563, (d) G ¼ 0.619, (e) G ¼ 0.742, (f) G ¼ 0.861, (g) G ¼ 0.956 and (h) G ¼ 0.998.
Fig. 2. EDAX spectra of WOxNy films deposited at different G: (a) G ¼ 0.341, (b) G ¼ 0.956 and (c) G ¼ 0.998.
Table 1 Thickness, deposition time and chemical composition of WOxNy films as a function of G G
0.009 0.341 0.563 0.619 0.742 0.861 0.956 0.998
Thickness (nm)
422 347 401 392 346 340 369 310
Deposition time (min)
12 6 6 5.5 4 3.5 4 5
WOxNy
Chemical composition atomic (%) W
O
N
23.8 24.3 23.8 24.0 24.9 25.0 26.9 35.2
76.2 68.6 67.9 68.2 66.0 58.1 37.1 7.9
0.0 7.1 8.3 7.8 9.1 17.0 35.9 56.9
W1O3.2 W1O2.82N0.29 W1O2.85N0.35 W1O2.84N0.33 W1O2.65N0.36 W1O2.32N0.68 W1O1.38N1.33 W1O0.22N1.62
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peak is shifted slightly to higher 2y angles (23.571). A possible reason for this shift of the diffraction peak is that some of the oxygen atoms are substituted by nitrogen atoms. The nitrogen atoms may occupy the oxygen atoms’ positions in the crystal lattice or stay at grain boundaries and, thus, creating some additional stresses. At 0.742, the (0 0 2) peak becomes very weak (Fig. 3e). This indicates that the crystal structure of WO3 is changed and very small crystallites are formed. The appearance of a short-range order of crystallization is associated with the formation of WOxNy. At G ¼ 0.861, a different WOxNy structure is obtained with a relatively broad peak around 43.891 indicating a reduction in the size of the crystallites. With further addition of nitrogen to G ¼ 0.956, the material assumed amorphous phase. At G ¼ 0.998, (no oxygen is supplied) crystalline W2N with preferred orientation around (1 1 1) is obtained. 3.2. Optical properties By varying the nitrogen to total reactive gas pressure ratio, the optical properties like optical transmittance, optical band gap, refractive index, etc., are modified. The optical transmittance spectra of WOxNy films are shown in Fig. 4. The transmittance of the films prepared at Go0.742 is very high. With further increase in G, the transmittance decreases, and a transition of the optical behavior from insulating WO3 compound to metallic W2N is observed. The optical band gap values, Eg, are calculated by assuming an indirect transition between the edges of the valence and the conduction bands, using the equation: ðahnÞ1=2 ¼ Aðhn E g Þ,
where a is the absorption coefficient and A is a constant. By plotting (ahn)1/2 versus hn and extrapolating the linear region of the resulting curve, Eg can be obtained. The calculated Eg values are written in Table 2. Eg decreases with the progressive substitution of O by N atoms in WOxNy films. This decrease can be attributed to the formation of well-localized N 2p states above the O 2p valence band states. Inter-relation of such energy levels in the band gap reduces the band gap and increases the visible light absorption through a charge transfer between a dopant and a conduction or valence band [9]. More discussion about the Eg narrowing upon substitution of O by N atoms can be found in Ref. [4]. The refractive index values, n, of WOxNy films prepared at different G were calculated from the spectral transmittance using the Swanepoel method [10,11]. The results are shown in Fig. 5 as a function of wavelength. The refractive index increases with increasing G. A similar increase in refractive index upon nitrogen incorporation has been previously observed in different transition metal oxynitrides [8]. This increase in refractive index may be
Table 2 Values of Eg, E0, Ed and Nc G
Eg
E0
Ed
Nc
0.009 0.341 0.563 0.619 0.742 0.861
3.07 3.07 3.05 2.88 2.29 1.85
5.75 5.69 5.32 5.22 5.11 4.85
18.14 18.53 19.34 19.48 21.91 22.90
4.36 4.45 4.65 4.68 5.27 5.50
(1)
Fig. 4. Transmittance of WOxNy films prepared at various G ¼ PN2 = ðPN2 þ PO2 Þ.
Fig. 5. Refractive index variations of WOxNy deposited at different G values as a function of wavelength. The data points denote the calculated n values while the lines are fit functions based on the two-term Cauchy dispersion relationship; n(l) ¼ (a+b)/l2.
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attributed to the increase of polarization, since metal– nitrogen bonds tend to be less polar than the corresponding metal–oxygen bonds, which leads to a higher polarizability for the metal nitrides [12]. The extinction coefficient, k, of WOxNy films prepared at different G values was determined from the equation: al , (2) 4p where a ¼ a(l) is the wavelength-dependent absorption coefficient (Fig. 6). The k values increase progressively with increasing nitrogen content in the films. The extinction coefficient is related to the creation of defect and absorption centers whose number increase with increasing nitrogen content. The refractive index dispersion of materials can be fitted by the Wemple-DiDomenico relation [13,14]: k¼
nðEÞ2 1 ¼
E0Ed , E 20 E 2
(3)
where E0 and Ed are single-oscillator fitting constants which measure the oscillator energy and strength, respectively. By plotting ðnðEÞ2 1Þ1 against E2and fitting a straight line as shown in Fig. 7, E0 and Ed can be determined directly from the slope, (E0Ed)1, and the intercept, E0/Ed, on the vertical axis. The calculated E0 and Ed values are listed in Table 2. It is observed that E0 decreases while Ed increases with increasing nitrogen content. The oscillator energy E0 is an ‘‘average’’ energy gap, and to a very good approximation it scales with the optical band gap E0E2Eg, as was found by Tanaka [15]. The oscillator strength Ed is related to other physical parameters of the material through the relation [13,14]: E d ¼ bN c Z a N e ,
(4)
Fig. 7. Plot of refractive index factor (n21)1 versus E2 for WOxNy films.
where Nc is the effective coordination number of the cation (nearest neighbor to the anion), Za the formal chemical valency of the anion, Ne the effective number of valence electrons per anion and b is a two-valued constant with either an ionic or a covalent value (bi ¼ 0.2670.03 eV and bc ¼ 0.3770.04 eV, respectively). Taking into account Eq. (4), and assuming that the parameters Ne ¼ 8 and Za ¼ 2 [13] retain approximately the same values along the analyzed composition values, it would seem reasonable to ascribe the trend observed in the values of Ed to an increase in the effective cation coordination number, Nc. On the other hand, the possible influence of the parameter b on the increase observed for the oscillator strength should also be mentioned. According to Pauling’s electronegativities, ionicity in a single bond increases with the difference in values of electronegativity between two elements forming the bond. The electronegativity of oxygen (3.5) is greater than that of nitrogen (3.0), which indicates that the metal–O bond involves a larger charge transfer than the metal–N bond. Assuming that metal–O and metal–N bonds coexist in the films, thus, the nature of the chemical bonding could change towards being less ionic with increasing nitrogen content. 4. Conclusions
Fig. 6. Extinction coefficient variations of WOxNy deposited at different G values as a function of wavelength.
Tungsten oxynitride films were prepared by reactive magnetron sputtering on glass and Si(1 0 0) substrates at different oxygen and nitrogen partial pressures. A range of materials with different N/O ratio were processed and checked by EDAX. The different WOxNy phases were affected by compositions. The optical properties of the WOxNy films are thus controlled by changing the
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deposition conditions. It was found that the refractive index increases with increasing nitrogen content over the entire spectral range (300–2500 nm) studied. The dispersion parameter, Ed, increases from 18.14 to 22.90 with increasing nitrogen content from G ¼ 0.009 to 0.861. The increase of Ed points towards an increase in the interactions between the structural layers, hence leads to an increase of the tungsten effective coordination number, Nc. Nevertheless, a decrease in the ionic character of the chemical bonding with increasing nitrogen content, and its possible influence on the value of the parameter b, cannot be excluded either. A decrease in the oscillator energy E0 and the optical band gap Eg were obtained. Acknowledgments The authors are grateful for technical support by Sakon Sansongsiri, Michael Dickinson and Ainhoa Pardo. One of us (S. H. M.) would like to express his sincere gratitude to the Fulbright and the TEMPUS Commissions for fellowships to carry out this research work at Lawrence Berkeley National Laboratory, CA, USA and ICMM-CSIC, Cantoblanco, Madrid, Spain, respectively. A.A. acknowledges support by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Technology, of
the US Department of Energy under Contract no. DEAC02-05CH11231. References [1] O. Banakh, P.-A. Steinmann, L. Dumitrescu-Buforn, Thin Solid Films 513 (2006) 136. [2] M.-S. Wong, H.P. Chou, T.-S. Yang, Thin Solid Films 494 (2006) 244. [3] N.M.G. Parreira, N.J.M. Carvalho, F. Vaz, A. Cavaleiro, Surf. Coat. Technol. 200 (2006) 6511. [4] S.H. Mohamed, A. Anders, Surf. Coat. Technol. 201 (2006) 2977. [5] D.-H. Cho, T.-S. Chang, C.-H. Shin, Catal. Lett. 67 (2000) 163. [6] N. Diot, O. Larcher, R. Marchand, J.Y. Kempf, P. Macaudiere, J. Alloys Compd. 323–324 (2001) 45. [7] S. Yang, A.K. Vishnu, US Patent 7002202, 2006. [8] S.H. Mohamed, O. Kappertz, J.M. Ngaruiya, T. Niemeier, R. Drese, R. Detemple, M.M. Wakkad, M. Wuttig, Phys. Status Solidi (a) 201 (2004) 90. [9] C. Di Valentin, G. Pacchioni, A. Selloni, Phys. Rev. B 70 (2004) 085116. [10] R. Swanepoel, J. Phys. E: Sci. Instrum. 16 (1983) 1214. [11] E.R. Shaaban, N. El-Kabnay, A.M. Abou-sehly, N. Afify, Phys. B: Condens. Matter 381 (2006) 24. [12] M. Ohring, The Material Science of Thin Films, Academic Press, San Diego, 1991. [13] S.H. Wemple, W. DiDomenico, Phys. Rev. B 3 (1971) 1338. [14] S.H. Wemple, Phys. Rev. B 7 (1973) 3767. [15] K. Tanaka, Thin Solid Films 66 (1980) 271.