The determination of the thickness and optical constants of the microcrystalline silicon thin film by using envelope method

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1 OPTOELECTRONICS AND ADVANCED MATERIALS RAPID COMMUNICATIONS Vol. 3, No. 6, June 2009, p The determination of the thickness and optical constants of the microcrystalline silicon thin film by using envelope method L. LI a, J. LU a*, R. LI a,b, C. SHEN a, Y. CHEN a, S. YANG a, X. GAO a a Key Laboratory of Materials of Education, Zhengzhou University, Zhengzhou , R.P China b Henan University of Technology, Zhengzhou , R.P China The microcrystalline silicon film thin has been deposited onto glass substrates by the plasma enhanced chemical vapor deposition method. The transmitted spectrum is taken from the spectral photometer. The smooth transmitted spectrum shows that the film has an uniformity thickness. An envelope method, based on the optical reflection spectrum taken at normal incidence, has been successfully applied to the geometrical optical characterization of thin films. Such a method allows the determination of the average thickness and the optical constants such as refractive index, absorption coefficient and extinction coefficient which were determined from transmittance spectrum in the ultraviolet-visible-near infrared (UV-VIS-NIR) regions. The results show that the calculated thickness value is 574 nm, and the measured value is 609 nm with the same sample. Compared the calculated and measured values, there is a small difference. It is shown that the envelop method is useful to analyze the microcrystalline silicon film. (Received June 06, 2009; accepted June 15, 2009) Keywords: Microcrystalline silicon thin film, Envelope method, Optical constant 1. Introduction The microcrystalline silicon thin film solar cell has been focused on the photovoltaic industry due to its high conversion efficiency and high stability [1-3]. The microcrystalline silicon film is composite phase material which is made up of microcrystalline grain, the grain boundaries, cave and amorphous silicon. Compared with the amorphous silicon, the microcrystalline silicon presents high order character and improved stability. It is becoming one of the effective methods to solve the stability of the silicon-based film solar cells. Because the non-doped microcrystalline silicon material is indirect-band-gap with small absorption coefficient, it needs to increase the thickness to 1-3 micrometer for enhancing the absorption efficiency and improving the conversion efficiency of the solar cell in the near infrared region. Therefore, it needs higher deposition rate compared with the amorphous silicon. To get exact thickness of the film is necessary for calculating the deposition rate. Each kind of material has its own refractive index and scatter frequency spectrum. This property is nearly related to their electronic structure and crystal structure basing on the transmitted spectrum and reflection spectrum of the film, and the thickness and the refractive index at different region can be calculated. It has an important sense to know about the absorption character of the film. Generally speaking, there are four different kinds of methods to measure the optical characters of the film; they are luminosity method, Abbe method, microwave wave guide method and elliptical polarized light method [4]. The luminosity method is based on the transmissivity and reflectivity of the film to calculate the optical characters, and the spectral photometer is the most common instrument to measure the film. Besides the spectral photometer, the elliptic polarization instrument makes use of the variation of the polarized light phase and intensity in reflection spectrum to obtain the optical characters of the film. Therefore, it is a useful instrument to measure the refractive index and absorption coefficient of the materials, and it has been widely applied to analyze the monolayer oxidation film on the semiconductor wafer [5]. The advantage of this method is that it is sensitive to the surface of the sample; and the disadvantages are including three aspects: the first is high price; the second is that the puny stain on the film also can influence the result. However, neither the spectral photometer method nor the elliptical polarized light method has the

2 626 L. Li, J. Lu, R. Li, C. Shen, Y. Chen, S. Yang, X. Gao same disadvantage that it depends on the models when calculate the optical characters. Envelope method is one of the common used methods to calculate the optical characters of the semiconductor and dielectric film [6]. This method is put forward by Manifacier et.al [7] and Swanepoel [8], then it has been further optimized by Muiato [9], Chambouleyron et.al [10], Marquez et. al [9], Chiao and Macleod et. al [11]. The envelop method is also a practical method to calculate the optical characters. It can obtain the optical characters by two envelop functions. With this method, it can help to calculate the thickness of the film though seeking the extreme point and solve the optical characters in all wavelength regions. The advantage of envelop method is that the thickness of the film and the refractive index and so on can be obtained only using the transmitted spectrum [11]. Basing on the transmitted spectrum, the optical characters of the medium film has been analyzed detailedly using the envelop method [12]. The Ba(ZrTi)O 3 ferroelectric film [13], ZnO film [14] and amorphous silicon film [15] also have been analyzed. The envelop method has been improved to analyze the double deposited films [16]. The high conversion efficiency solar cell can be obtained when the non-doped transition region microcrystalline silicon film is used as the intrinsic layer between the amorphous and the microcrystalline silicon. In this paper, the microcrystalline silicon film was fabricated by PECVD (Plasma enhanced chemical vapor deposition). Basing on the transmitted spectrum, its thickness and optical constants can be obtained using the envelop method. 2. Experiment The microcrystal silicon thin film was deposited onto the glass slice in the intrinsic chamber of the Plasma enhanced chemical vapor deposition system which was made by the center of Shenyang scientific research Company Limited. The reaction gas is the mixed gas of silicone and pure hydrogen. The very high frequency power supply RFA 300Wb is made by the coaxial power systems. The distance of the polar plate is 1.0 cm. The high presser and high power method have been adopted, and the pressure is 500 Pa, the power is 110 W. The structural properties were studied by a X Pert PRO X-ray Diffractometer with CuKα (λ= Å) radiation. The average dimension of microcrystalline silicon was determined by the Scherrer method from the broadening of the diffraction peaks taking into account the instrumental broadening. The optical measurements of the crystalline thin film were carried out at room temperature using Shimadzu UV-VIS-NIR 3100 scanning spectrophotometer in the wavelength range from 190 to 3200 nm. Swanepoel s envelope method was employed to evaluate the optical constants such as the refractive index n, extinction coefficient k, and absorption coefficient α from transmittance spectra [17]. The thickness of the microcrystalline silicon thin film was determined from interference fringes of transmission data measured. 3. Results and discussion 3. 1 Structural properties of the microcrystalline silicon thin film Raman spectrum is an important way to measure the crystallization of the micro-crystalline silicon film [18]. Fig.1a shows the Raman spectrum of the microcrystalline silicon film. Based on the formula: ( I + I ) ( I + I ) X c = + (1) I520 The crystalline volume fraction can be calculated and the value is 58.9%. Intensity /a.u. Intensity /a.u Raman shift /cm -1 Si(111) (a) θ degree Si(220) Si(311) (b) Fig. 1. Raman spectrum (a) and XRD spectrum (b) of the microcrystalline silicon thin film.

3 The determination of the thickness and optical constants of the microcrystalline silicon thin film by using envelope method 627 The crystal structure and orientation of the microcrystal silicon thin film were investigated by X-ray diffraction (XRD) pattern. The X-ray diffraction spectrum for the sample shown in Fig. 1 (b), indicate that the film is microcrystalline silicon film nature. The XRD pattern consists of a (111) main peak and two (220) (311) subsidiary peaks with comparatively lower intensities. The full width half maximum (FWHM) of the (111) peak was o for the microcrystalline silicon thin film. The grain size of the crystallites was calculated using a well-known Scherrer s formula D=0.9λ/(βcosθ), where D is the grain size of the crystallite, λ (= Å) the wavelength of X-rays used, β the broadening of diffraction line measured at half its maximum intensity in radians and θ is the angle of diffraction. The value found for the grain size D(111) is 20.8nm. It can be seen that the (111) plane for microcrystalline silicon thin film implies the preferred growth The thickness and optical constants of microcrystalline silicon thin film Fig. 2 shows the model of the light shines in the film deposited on the underlay. The parameter n is the refractive index of the thin film. Fig. 2. The model of the thin film. Fig. 3 (a) shows transmittance curve for the microcrystalline silicon thin film, where the film due to interference phenomena between the wave fronts generated at the two interfaces (air and substrate) defines the sinusoidal behavior of the curves transmittance vs. wavelength of light. The microcrystalline silicon thin film showed interference fringe pattern in transmission spectrum. This revealed the smooth reflecting surfaces of the film and there was not much scattering loss at the surface. The process of the calculation can be followed in Table 1. Fig. 3 (b) shows the envelop lines of the transmission spectrum. However, T max and T min are the transmissivity envelope curve maximum and the minimum of the normal incidence transmitted spectrum. Table 1. The optical constant of the microcrystalline silicon thin film. Wavelength (nm) T max Order of interference T min Refractive Absorption Extinction index(n) coefficient 10 3 cm -1 coefficient

4 axmi628 L. Li, J. Lu, R. Li, C. Shen, Y. Chen, S. Yang, X. Gao where 1 2 N = ( 1 + s ) + 2 s ( Tmax Tmin ) ( Tmax * Tmin ) (3) 2 T(%) Wavelength (nm) (a) and s is the refractive index of the substrate (in our case s =1.52 (glass)). The refractive index of the microcrystalline silicon film was determined from transmittance measurements. Since envelope method is not valid in the strong absorption region, the calculation of the refractive index of the film in this region was calculated using the following expression: n=a+b/λ 2 (4) T(%) Wavelength(nm) (b) Fig. 3. Variation of transmittance with wavelength (a) and the envelop lines of the transmittance spectrum (b) in microcrystalline silicon thin film. The calculated steps of the envelop method can be seen Fig. 4. In the following it will be explained in detail. The index of refraction n at different wavelengths was calculated using the envelope curve for T max (TM) and T min (Tm) in the transmission spectra [18]. The expression for refractive index is given by 2 2 n = N + N s (2) TTmTnThe refractive index of the whole wave region can be extrapolated. The parameters A, B can be obtained from the least square fitting, Fig. 5 shows the refractive index of the whole wavelength region. It can be shown in the Fig. 5 that the refractive index is increasing in the visible light region when decreasing the wavelength of the incoming ray, but it is mushrooming in the ultraviolet light region. The thickness of the microcrystalline silicon thin film can be obtained from the following equation: ( ) d = λ1λ2 2 n1λ2 n2λ1 where, n1 and n2 are the refractive indices corresponding to the wavelengths λ 1 and λ 2, respectively [17]. The thickness d needs to average, because the d is different when calculating different extreme points [19]. The final thickness of the microcrystalline silicon thin film was found to be 574 nm, and the measured value is 609 nm. There is little difference between the calculated and measured values. The absorption coefficient and the extinction coefficient of the microcrystalline silicon film were determined from transmittance measurements. The extinction coefficient k and absorption coefficient α can be obtained from the experimental expressions: αλ k = (6) 4π d (5) α = ( n 1)( n s) T / T ln max min d ( n+ 1)( n+ s ) Tmax / Tmin 1 (7) Fig. 4. The calculated steps of the envelop method. The d is the thickness of the film. The optical constants such as refractive index n and extinction coefficient k were determined from a transmittance spectrum by envelope method as explained in the previous section. The values can be seen in Table 1. Fig. 6 shows the

5 The determination of the thickness and optical constants of the microcrystalline silicon thin film by using envelope method 629 variations of α and k with wavelength in the region 200 nm-800 nm. It can be seen that the values of α and k decrease with the increasing of the wavelength. The optical absorption gap was analyzed by the following equation [20]: (αhυ) 0.5 =B(hυ Eg) (8) where B is a constant. The variation of (αhυ) 1/2 with photon energy hυ for the microcrystalline silicon thin film is shown in Fig. 7. It has been observed that the plots of (αhυ) 0.5 versus hυ are linear over a wide range of photon energies. The intercepts (extrapolations) of these plots (straight lines) on the energy axis give the energy band gaps. From this drawing, the band gap, E g =1.65 ev is deduced. Refractive index n Wavelength ( nm) Fig. 5. Variation of refractive index with wavelength in the microcrystalline silicon thin film. Absorption coefficient Absorption Coefficient Extinction Coefficient Wavelength (nm) Fig. 6. Variations of absorption coefficient and extinction coefficient with wavelength in the microcrystalline silicon thin film (αhυ) 1/2 ( 103cm-1) h υ ( ev) Fig. 7. Variation of (αhυ) 1/2 with hυ in microcrystalline silicon thin film Extinction coefficient 4. Conclusions The microcrystalline silicon film has been deposited onto glass substrates by Plasma Enhanced Chemical Vapor Deposition method at 220 o C substrate temperature. The crystallization of the microcrystalline silicon film can be obtained from the Raman spectrum. The crystal structure and orientation of the microcrystalline silicon thin film were investigated by XRD pattern. The XRD pattern shows the preferred growth orientation of the microcrystalline silicon thin film is along (111) orientation. Optical constants such as refractive index n and extinction coefficient k were determined from transmittance spectrum in the UV-VIS-NIR regions using envelope method. The thickness of the film d was calculated from interference of transmittance spectra. Also, E g energy band gap value has been calculated. In conclusion, the envelop method is a good method to analyze the microcrystalline silicon film which was deposited by Plasma Enhanced Chemical Vapor Deposition method. Acknowledgments This Project was supported by the State Key Development Program for Basic Research of China Grant No.2006CB References [1] Q. C. Guo, X. H. Geng, J. Sun, C. C. Wei, X. Y. Han, X. D. Zhang, Y. Zhao, Acta Phys. Sin. 56(5), 2790 (2007). [2] M. Kondo, T. Matsui, Y. Nasuno, H. Sonobe, S. Shimizu, Thin Solid Films, 501(1-2), 243 (2006). [3] X. D. Zhang, Y. Zhao, Y. T. Gao, F. Zhu, C. C. Wei, J. Sun, Y. Wang, X. H. Geng, S. Z. Xiong, Acta Phys. Sin. 54(04), 1899 (2005). [4] P. F. Gu, Film Technology, Zhejiang University publishing company, [5] D. Poelman, P. F. Smet, J. Phys. D: Appl. Phys. 36(15), 1850 (2003). [6] D. P. Arndt, R. M. A. Azzam, J. M. Bennett, J. P. Borgogno, C. K. Carniglia, W. E. Case, J. A. Dobrowolski, U. J. Gibson, T. Tuttle Hart, F. C. Ho, V. A. Hodgkin, W. P. Klapp, H. A. Macleod, E. Pelletier, M. K. Purvis, D. M. Quinn, D. H. Strome, R. Swenson, P. A. Temple, T. F. Thonn, Appl. Optic. 23(20), 3571 (1984). [7] J. C. Manifacier, J. Gasiot, J. P. Fillard. J. Phys. E: Sci. Instrum. 9(11) (1976). [8] R. Swanepoel, J. Phys. E: Sci. Instrum. 16 (12) (1983).

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