Comparison of Interference Colors in Diamond-like Carbon Films - Prof. 5775, Apuntes de Administración de Empresas

¡Descarga Comparison of Interference Colors in Diamond-like Carbon Films - Prof. 5775 y más Apuntes en PDF de Administración de Empresas solo en Docsity! SCIENCE CHINA Physics, Mechanics & Astronomy © Science China Press and Springer-Verlag Berlin Heidelberg 2013 phys.scichina.com www.springerlink.com *Corresponding author (email: fumingw@xmu.edu.cn) • Article • March 2013 Vol.56 No.3: 545–550 doi: 10.1007/s11433-013-5002-z Study of colors of diamond-like carbon films LI QiongYu, WANG FuMing* & ZHANG Ling Department of Physics, Xiamen University, Xiamen 361005, China Received January 13, 2012; accepted March 28, 2012 Reflective and transmissive film interference colors were calculated for DLC films on the Si and SiO2 substrates. The calcu- lated interference colors were compared with photographed colors of the prepared DLC samples. The observed film colors were found to match reasonably well with the corresponding calculated colors, indicating that DLC film colors come from in- terference instead of color center effects. Color charts for DLC films on the Si and SiO2 substrates with various optical gaps were constructed, and the relationship between interference color and film properties such as optical gap, thickness and sub- strate were investigated. Usefulness of the calculated color charts in estimating optical gap or thickness of DLC films were demonstrated. DLC, film color, interference color, color center, color chart PACS number(s): 78.66.Jg, 78.20.Bh, 77.55.-g Citation: Li Q Y, Wang F M, Zhang L. Study of colors of diamond-like carbon films. Sci China-Phys Mech Astron, 2013, 56: 545550, doi: 10.1007/s11433- 013-5002-z 1 Introduction Diamond-like carbon (DLC) is a form of amorphous carbon containing both sp3 bonds and sp2 bonds. It has been a focus of many investigations because of its extraordinary charac- teristics such as high mechanical strength, excellent tribo- logical properties, extreme chemical inertness, optical transparency, and adjustable optical gaps [1–2]. DLC films have been successfully applied to mechanical, optical and biomedical fields [3–6]. Structural imperfections such as impurities or interstitial ions, named color centers, can trap localized electrons or holes, and make transparent crystals to display various col- ors [7]. There have been several studies of color center ef- fects in CVD diamond film [8–9]. Interference effects can also make transparent thin films to present different colors and the observed color depends mainly on the refractive index and thickness of the films [10]. Many dielectric thin films display this type of color. Henrie et al. [11] has re- ported electronic color charts for dielectric films on the Si substrate. DLC films have been shown to display various colors from diverse growth methods and conditions [12]. However, no study has been carried out to verify the origin of these colors. In this paper, we calculate interference col- ors of DLC films for various optical gaps, thicknesses and substrates, and compare them with photographed colors of the prepared DLC samples. By comparing these two types of colors, we can conclude that DLC film colors come from interference effects of the thin film. We also construct sev- eral color charts for DLC films which can be used to esti- mate optical gap or thickness of DLC films according to their reflective or transmissive colors. 2 Experiment Samples of DLC films with different optical gaps and thicknesses on Si and SiO2 substrates were prepared and 546 Li Q Y, et al. Sci China-Phys Mech Astron March (2013) Vol. 56 No. 3 studied in this work. DLC films were deposited using plas- ma enhanced chemical vapor deposition (PECVD) and mul- ti-arc ion plating methods, respectively. When using mul- ti-arc ion plating system, a high purity graphite target was used as the carbon source with 20 sccm argon as the work- ing gas. A DC power supply of 30 kW and a 30 kHz pulsed substrate bias of 130 V with a duty ratio of 74% were used for sample growth. When using the PECVD system, quartz plates were thoroughly cleaned and were mounted on the powered electrode for sample growth. The system was first pumped to a vacuum of 104 Pa, and then Oxygen plasma was used to clean the sample chamber and the substrate surface for 30 min. CH4 precursor gas with flow-rates of 60–80 sccm or CH4 and H2 precursor gases with flow rates of 40 sccm and 30 sccm were used. The working pressure of the sample chamber was kept between 5 Pa and 300 Pa, the self-bias was between 40 V and 660 V, and the system power was set between 20 W and 600 W. The deposition time was 10–60 min. Optical transmission spectra of DLC films were meas- ured using a Varian Cary 5000 UV–visible spectrophotom- eter and used in calculations of optical gaps. Sample thick- ness, refractive index and extinction coefficient were meas- ured using a UVISEL FUV ellipsometer and used in calcu- lations of the interference color charts. 3 Calculation of reflective and transmissive spectra Figure 1 illustrates the light incidence from air into the sys- tem of DLC film and substrate. Pr() and Pt() are the spec- tra of the reflected and transmitted lights, and can be calcu- lated by the following equations: r i t i ( ) ( ) ( ), ( ) ( ) ( ), P R P P T P         (1) Where R() and T() are the reflectance and transmittance of the DLC film, and Pi() is the spectrum of the incident light source. Figure 2 shows the spectrum of the standard illuminant D65, which is used as incident light source in our calculations [13]. The standard illuminant D65 was chosen to get a more accurate comparison with the photographs of our DLC samples, which were taken under daylight. Figure 1 Optical path of the DLC film and substrate system. Figure 2 Spectrum of the standard illuminant D65. The reflectance R() and transmittance T() are calcu- lated as a function of incident wavelength  using the standard thin film matrix approach [14]. The reflectance and transmittance are respectively given by 22 s c f c s f 2 s c f c s f ( ) cos i( )sin ( ) , ( ) cos i( )sin R                         (2) and c f s 2 2 c f s c f s 4 Re( ) ( ) , (( ) cos i( )sin ) T                 (3) where parameters c, f and s are the impedance of the cover, film and substrate, respectively. Under normal inci- dent condition, impedance  is given by ,aN  (4) where 0 0/a   is the impedance of free-space, and N=nik is the complex index of refraction. The phase delay delta for normal incident lights is given by 0 f ,k N d  (5) where k0=2/ is the wave number of the free-space, Nf is the complex index of refraction of the DLC film, and d is the thickness of the DLC film. Complex index of refraction N of the film and substrate can be measured or looked up. Figure 3 shows the index of refraction n and extinction co- efficient k for silicon dioxide, silicon and a DLC film sam- ple of 2.0 eV optical gap and 50 nm thickness [15]. 4 Calculation of interference color charts The general procedure for calculating interference color charts from the reflected or the transmitted spectra obtained in sect. 3 can be separated into two steps. First, we calculate the CIE XYZ color parameters, and then we convert the XYZ parameters into RGB color parameters that can be displayed or printed. The CIE color system is a color standard based on the Li Q Y, et al. Sci China-Phys Mech Astron March (2013) Vol. 56 No. 3 549 Figure 9 Reflective interference colors of DLC samples on Si (a) and the corresponding calculated color (b). Figure 10 Reflective and transmissive interference colors of DLC samples on SiO2 (a) and the corresponding calculated color (b). Labels with letter T indicate transmissive interference colors, while labels with letter R indicate reflective colors. come from errors of thickness and optical gap measure- ments. In Figure 10(a), we have photographed the reflective and the transmissive interference colors of DLC films on- SiO2 substrates under similar conditions. The calculated interference colors with corresponding optical gaps and thicknesses are shown in Figure 10(b). Again, we observe reasonable correspondence between the photographed and calculated colors, and we believe that the slight differences come from color distortions and errors in thickness and op- tical gap measurements. Comparing Figures 9 and 10, we can see that both the observed and calculated interference colors of DLC film on the SiO2 substrates are lighter than that on the Si substrates, which is in agreement with the earlier discussions. Based on these observations, we con- clude that interference color can reasonably explain the ob- served colors of diverse DLC films, and should be the origin of DLC film colors. 6 Conclusions Reflective and transmissive film interference colors were calculated for DLC films on the Si and SiO2 substrates and compared with photographed colors of DLC samples. The observed film colors were found to match reasonably well with the corresponding calculated colors, indicating that DLC film colors come from interference instead of color center effects. Color charts for DLC films on Si and SiO2 substrates with various optical gaps were constructed. The color charts indicate that the reflective interference color of DLC films have a strong periodic oscillation with the in- creasing of the film thickness, and the oscillation period increases with the optical gaps. The transmissive interfer- ence colors have observable periodic oscillations as well, and the interference color becomes darker with the increas- ing thickness but becomes lighter and finally transparent with the increasing optical gaps. These color charts can be used to estimate optical gap or thickness of DLC films ac- cording to their reflective or transmissive colors. This work was supported in part by the National Basic Research Program of China (Grant No. 2012CB933502). 1 Robertson J. Diamond-like amorphous carbon. Mater Sci Eng R-Rep, 2002, 37: 129–281 2 Kumar A, Ekanayake U, Kapat J S. Characterization of pulsed la- ser-deposited diamond-like carbon films. Surf Coat Technol, 1998, 102: 113–118 3 Lin C R, Liu S H, Liou W J, et al. 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