Journal of the Korean Physical Society, Vol. 64, No. 6, March 2014, pp. 857∼862
Effect of Annealing Temperature on the Optical Loss and the Optical Constants of RF-magnetron Sputtered Carbon - Nickel Composite Films V. Dalouji Department of Physics, Razi University, Kermanshah, Iran
S. M. Elahi∗ Department of Physics, Razi University, Kermanshah, Iran, and Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran (Received 8 May 2013, in final form 10 October 2013) In this work, the optical properties of carbon − nickel films annealed at different temperatures (300 − 1000 ◦ C) were investigated. The films were grown on quartz substrates by radio frequency magnetron co-sputtering at room temperature with a deposition time of 600 second. The optical transmittance spectra in the wavelength range 300 − 1000 nm were used to compute the refractive index by using the Swanepoel’s method. The films annealed at 500 ◦ C showed considerable optical loss due to optical absorption by nickel atoms and to scattering caused by surface roughness. However, the film annealed at 800 ◦ C had a very small optical loss in spite of the high surface roughness. The dispersion curves of the refractive indices of the films had anomalous dispersion in the absorption region and normal dispersion in the transparent region. The dissipation rate of the electromagnetic wave at 500 ◦ C was shown to have maximum value. PACS numbers: 68.35.Dv, 68.55.Ln, 68.65.-k, 68.90.+g Keywords: Co-sputtering, Amorphous carbon matrix, Carbon-nickel films, Annealing temperatures DOI: 10.3938/jkps.64.857
I. INTRODUCTION
structural and electronic properties [11]. The high refractive index and the low absorption coefficient of the films make them suitable for optical coatings [12]. An element such as Ni with low or the affinity to the carbon atom leads to a relatively sharp interface between the carbon matrix and the metallic phase; therefore, distinct properties are produced in comparison to carbideforming elements [13,14]. Moreover, carbon-nickel composite films have been studied for their interesting properties, such as residual stress reduction [15], decreased friction coefficient [15], or improved dielectric constant [16]. In the previous reports, the dependence of these properties on the deposition times, the nickel content, the substrate temperatures and the deposition parameters were investigated. However, it less attention has been paid to the effect of the annealing temperature for films deposited by RF-magnetron sputtering. In the present work, we studied the effect of annealing temperature on the transmittance spectra and the optical dielectric constants of carbon-nickel composite films.
Recently, much attention has been given to amorphous carbon (a-c) films containing metals due to their low cost and wide variety of properties [1–3]. Because of their interesting properties, a-c: Me (Me = Au, Ag, Cu, Mn. . .) have many applications as coating materials in biomedicine, electronics, mechanics and optics [4–7]. The advantage of a-c: Me films as electronic materials is that their conductivity behavior can be changed from dielectric to metallic by a slight variation in their composition [3]. The study of optical absorption has been one of the most productive methods in understanding the band structures and energy gaps of films, and the measurement of optical absorption coefficients particularly near the fundamental absorption edges is a standard method for investigations of optical properties [8]. Knowledge of the optical constants such as the refractive index, extinction coefficient and dielectric constant of these films is important for designing new materials [9,10]. Furthermore, the change in refractive index is important for controlling the optical properties of the films because the optical properties are directly related to the ∗ E-mail:
II. EXPERIMENTAL DETAILS C − Ni composite films have been prepared onto quartz substrates by RF-magnetron co-sputtering (Vas
smohammad
[email protected]
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Fig. 1. (Color online) RBS spectra, experiment of (dotted line) and simulated (solid line), of C − Ni films annealed at different temperatures.
PFG1000-RF) using a multicomponent target (10 cm in diameter) consisting of pure graphite (99.99%) and strips of pure nickel approximately 2 cm2 attached to graphite race track, which corresponded to approximately 2.5% of the area. The angle of the incident ions relative to the target surface was 90◦ , and the substrate-target distance was 60 mm. Before loading the substrates in the deposition chamber they were ultrasonically cleaned in acetone bath for 20 min and then dried in flow hot air. The films were grown at room temperature in a deposition chamber evacuated to a base pressure 5 × 10−5 mbar; then, a constant Ar working pressure of 4 × 10−2 mbar was maintained by throttle valve. Deposition was done at a constant RF power of 400 W. The films were prepared with the same deposition times, 600 sec, and was annealed at 300, 500, 800 and 1000 ◦ C under ambient atmospheric pressure in a furnace for 2 h. The Rutherford backscattering spectrometry (RBS) spectra were obtained using incident ions (4He) with energies of 2 keV. The atomic contents of the films were obtained from the RBS data by software simulation of backscattering spectra for ion beam analysis (SIMNRA). The atomic force microscopy (AFM) analysis on non-contact mode was used to obtain surface morphology. The root mean square (RMS) roughness was obtained from the AFM data (WSxM software-2007). The transmittance spectra were obtained by a doublebeam UV-vis spectrometer in the range of 250 − 1000 nm (Jasco V-630).
III. RESULTS AND DISCUSSION Figure 1 shows RBS spectra (experiment and simulation results) for C – Ni films annealed at (a) 300 ◦ C,
Fig. 2. (Color online) Three - dimensional AFM images of C − Ni films annealed at (a) 300 ◦ C (b) 500 ◦ C (c) 800 ◦ C, and (d) 1000 ◦ C .
(b) 500 ◦ C, (c) 800 ◦ C and (d) 1000 ◦ C as a function of the incident ions (4He) energy. The results of the SIMNRA software simulation for the C − Ni annealed films for layer with a thickness 5000 (1 × 1015 atoms/cm2 ) showed that films contain 68.75, 54.16, 46.95, and 10% Ni at 300, 500, 800, and 1000 ◦ C, respectively. Because metal atoms behave as strongly absorbing media [17], with increasing temperatures from 300 to 1000 ◦ C, we expect the optical loss due to the optical absorption of Ni atoms to decrease. Figure 2 shows the three-dimensional AFM images of C − Ni films annealed at different temperatures. The surface roughness is a major factor resulting in optical loss by scattering. The RMS roughness increases with increasing annealing temperature from 1.33 nm at 300 ◦ C to 2.73 nm at 500 ◦ C and to 3.03 nm at 800 ◦ C; however, it decreases sharply with increasing annealing temperature to 0.21 nm at 1000 ◦ C. The roughness causes the optical scattering and hence, the optical loss of C − Ni annealed films [12]. Therefore, up to 800 ◦ C, the optical loss due to roughness is expected to increase and then at 1000 ◦ C decrease. Figure 3 shows the optical transmittance spectra of deposited C − Ni films annealed at 300, 500, 800 and 1000 ◦ C, respectively. The films annealed at 300 and 500 ◦ C have low transmittances with correspond to the films annealed at 800 ◦ C and 1000 ◦ C. This characteristic is mainly attributed to the presence of high concentration of Ni atoms in the films annealed at these temperatures. The films annealed at 500 ◦ C, correspond to the films annealed at 300 ◦ C, show slight decreases in transmittances, which could be related to the higher surface roughnesses of the films annealed at this tem-
Effect of Annealing Temperature on the Optical Loss· · · – V. Dalouji and S. M. Elahi
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Table 1. Values of λ1 , TM 1 , Tm1 , λ2 , TM 2 , Tm2 , n1 , n2 , M and thicknesses d for films annealed at different temperatures. Annealing temperature (◦ C) 300 500 800 1000
λ1 (nm) 451 417 440 432
TM 1 11.21 3.9 86.42 31.06
Tm1 9.64 2.85 79.37 26.88
λ2 (nm) 532 532 545 676
TM 2 12.52 4.94 95.56 49.86
Tm2 9.64 2.85 82.51 36.8
n1 1.59 1.8 1.54 1.63
n2 1.62 1.9 1.55 1.57
M 0.5 0.5 0.5 0.5
d(nm) 520.68 335.51 381.11 172.33
Fig. 3. (Color online) Optical transmittance spectra of C − Ni films annealed at different temperatures as a function of wavelength.
Fig. 4. (Color online) Absorption coefficient spectra of C − Ni films annealed at different temperatures as a function of incident photon energy.
perature. However, the films annealed at 800 ◦ C show a significant increase in the transmittance visible light, which is due to the low densification. With increasing annealing temperature from 800 to 1000 ◦ C the film morphology changed from a porous structure to a more compact structure because of increasing thermal diffusion of atoms [18]. Therefore, the effect of roughness on the increase in the optical loss at temperatures from 300 to 500 ◦ C is greater than the effect of Ni content. However, at temperatures from 500 to 800 ◦ C, the effect of Ni content on the decrease in the optical loss is greater than the effect of roughness on the increase in the optical loss; that is; the roughness may causes light to in trapped. At 1000 ◦ C the transmittance is decreased, which might be due to an increase in the number of localized states, and hence optical loss might be increased. The refractive index of a film can be calculated from the transmittance spectrum by using Swanepoel’s method [19–21]. According to this method, the value of the refractive index at a determined wavelength can 1 1 be calculated using the expression =[N +(N 2 − S 2 ) 2 ] 2 , 2 S +1 m and s = 1.55 is the refracwhere N = 2s TTMM−T Tm + 2 tive index of quartz; TM and Tm are the transmittance maximum and the corresponding minimum at a certain wavelength λ. The films thicknesses can be obtained
from the refractive index corresponding to adjacent extreme values n1 = n (λ1 ) and n2 = n (λ2 ) through the λ2 , with M = 1 following expression: d = M 2(λ1 nλ21−λ 2 n1 ) for two adjacent maxima (or minima) and M = 1/2 for two adjacent unlike extremes. The calculated values of d, λ1 , λ2 , n1 , n2 and TM and Tm corresponding to λ1 and λ2 of the studied films are listed in Table 1. Figure 4 shows the optical absorption coefficients of the films annealed at 300, 500, 800 and 1000 ◦ C as functions of the incident photon energy. The absorption coefficients of films for photon energies greater than 1.1 eV are non zero, and with increasing annealing temperature up to 500 ◦ C due to the increase in the density of localized states, they increase and at temperatures from 500 to 800 ◦ C due to the decrease in the density of localized states, they decrease. The film annealed at 1000 ◦ C has higher density of localized states than the film annealed at 800 ◦ C; therefore, it has a higher absorption coefficient. The increases in the absorption coefficient at about 2.6 eV and 3.5 eV can be due to gap excitations which is consistent with increasing optical conductivity (Fig. 6) at short wavelengths (about 400 nm) [22]. Figure 5 shows the optical constants of the films annealed at different temperatures from 300 to 1000 ◦ C
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Fig. 5. (Color online) Optical parameters n, k, ε1 , and ε2 of C − Ni films annealed at different temperatures as functions of wavelength.
as functions of wavelength. When an electromagnetic wave propagates through a loss medium, it experiences attenuation such as the generation of phonons, photon generation, free carrier absorption, and scattering [23]. In such materials, the refractive index becomes a complex function of the wavelength of the light wave [23]. The complex refractive index n∗ = n+ik and the dielectric function ε∗ = ε1 + iε2 characterize the optical properties of any solid material. The real and the imaginary parts of the complex dielectric constants are expressed as ε1 = n2 - k2 and ε2 = 2nk, where n and k are the real part of the refractive index and the extinction coefficient, respectively. As shown in Figs. 5 (a) and (c), the behavior of ε1 is similar to refractive index because of the smaller value of k2 in comparison to n2 , while ε2 mainly depends on the value of k (Figs. 5 (b) and (d)), which is related to the variation of absorption coefficient [24]. The extinction coefficient k is given by λα (λ)/4π, where α (λ) = − ln(T)/d is the absorption coefficient of the thin films. The films annealed at 300, 500 and 800 ◦ C attain a peak at about ∼ 550 nm and show partial increases at about ∼ 750 nm due to their being in the absorption region
[25]. However, the film annealed at 1000 ◦ C has nearly normal dispersion up to about ∼ 750 nm where a partial increase occurs. On the other hand, for the entire wavelength range the refractive indices of the films annealed at 300 ◦ C and 500 ◦ C are higher than refractive indices of the films annealed at 800 ◦ C and 1000 ◦ C, which could be related to the high concentration of Ni in these films. Interesting refractive index of films annealed at 500 ◦ C is greater than that of films annealed at 300 ◦ C; this increase may be attributed to a higher packing density and to a change in the crystalline structure of the films [26]. Figure 5(b) shows the extinction coefficient of films as a function of wavelength. At temperature from 300 to 500 ◦ C, the optical loss increases; hence, the extinction coefficient increases. At temperature however at 800 ◦ C, it decreases. Due to the low thickness of the film annealed at 1000 ◦ C correspond to other films, the optical loss increases; hence, the extinction coefficient increases. Figures 5 (c) and (d) show the real part and the imaginary parts of the complex dielectric function of the films as functions of wavelength. The real parts of the dielectric constants of the films are higher than the imaginary
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Table 2. of RMS roughness, Ni concentration, thickness, and annealing temperatures on the optical parameters for films annealed in the temperature ranges 300 − 500 ◦ C, 500 − 800 ◦ C and 1000 ◦ C. Optical 300 − 500 ◦ C 500 − 800 ◦ C 1000 ◦ C parameters RMS % Ni Thickness Annealing RMS % Ni Thickness Annealing RMS % Ni Thickness Annealing (nm) (nm) (◦ C) (nm) (nm) (◦ C) (nm) (nm) (◦ C) T High Low Low Low Low High Low Low Low Low High High n Low Low Low High Low High Low Low Low Low High High k High Low Low Low Low High Low Low Low Low High High Low Low Low High Low High Low Low Low Low High High ε1 High Low Low Low Low High Low Low Low Low High High ε2 σ Low Low Low High Low High Low Low Low Low High High
parts, which is related to dispersion, whereas the dissipative rate of the electromagnetic wave in the dielectric medium is provided by the imaginary part. Moreover, the imaginary part of the complex dielectric function is directly related to the density of states within the band gaps of semiconductors. The dissipation rate of the electromagnetic wave at 500 ◦ C is higher than that of the other films. The width of the tail of localized states in the band gap region is calculated by using Urhach’s rule α(v) = αexp(hv − Eu ), where v is the frequency of the radiation, α is a constant, h is Plank’s constant, and Eu is Urbach’s energy, which is interpreted as the widths of the tails of localized states in the band gap [22]. The tail width (Eu ) in the energy gap was obtained by plotting Ln α as a function of hv, and the value of Eu was calculated from the reciprocal slope of the linear part. The energy Eu for films annealed at 300, 500, 800 and 1000 ◦ C are 0.45, 0.83, 0.2 and 0.35 eV, respectively. These data show that with increasing annealing temperature up to 500 ◦ C, the density of states within the band gap increases; however, at temperatures from 500 to 800 ◦ C, it decreases. Then, it increases from 800 to 1000 ◦ C. Figure 6 shows the real part of the optical conductivity of films annealed at 300, 500, 800 and 1000 ◦ C as functions of wavelength. The real part of the optical conductivity σ of the film is given by σ = αnc/4π(sec−1 ), where α is the absorption coefficient, n is the refractive index and c is the velocity of light [27]. With increasing of wavelength, the optical conductivity decreases. For the entire wavelength range, with increasing of annealing temperatures up to 500 ◦ C the optical conductivity increases, due to the increase of in the density of localized states in the gap [20]. However, at 800 ◦ C, the optical conductivity decreases then, at 1000 ◦ C, due to the increase in the density of localized states, it increases. The sudden increase in the real part of the optical conductivity at short wavelengths for films annealed at 500, 800, and 1000 ◦ C is consistent with the increase in the absorption coefficient at about 2.6 eV, which may be due to the gap excitation. The high value of the absorp-
Fig. 6. (Color online) Real part of the optical conductivity σ of C − Ni films annealed at different temperatures as functions of wavelength.
tion of coefficient at 500 ◦ C may be due to the large tail width of localized states in band gap being larger than the width for the other films. In summary, the Table 2 reveals three different regions that show the effects of the RMS roughness, Ni concentration, thickness, and annealing temperature on the following optical parameters: the transmittance spectra T, the refractive indices n, the extinction coefficient k, the real and imaginary parts of the dielectric constants ε1 and ε2 , and the real part of the optical conductivity σ.
IV. CONCLUSION The optical properties of C − Ni films annealed at different temperatures (300 − 1000 ◦ C) have been investigated. The Ni content and roughness have been observed to have important influences on the optical properties of the C − Ni annealed films. We noticed that the optical
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loss at 500 ◦ C had a maximum value and that the optical loss at 800 ◦ C had a minimum value. The refractive index, the absorption coefficient, the extinction coefficient, the real and imaginary parts of the dielectric constant, and the optical conductivity were shown to be strongly affected by these characteristics. The dispersion curves of the refractive indices of the films have anomalous dispersion in the absorption region and normal dispersion in the transparent region. The effect of the roughness in increasing the optical loss at temperatures from 300 to 500 ◦ C has been shown to be greater than the effect of Ni content. At 800 ◦ C, in addition to the effect of the Ni content in decreasing the optical loss, the roughness mainly traps light, so the films have low optical loss and, hence, intense transmittance spectra.
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