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Journal of Non-Crystalline Solids 114 (1989) 813-815 North-Holland 813 RAMAN SCATTERING FROM MICROCRYSTALLINE Si FILMS: CONSIDERATIONS OF COMPOSITE STRUCTURES WITH DIFFERENT OPTICAL ABSORPTION PROPERTIES R.J. NEMANICH, E.C. BUEHLER, Y.M. LEGRICE, R.E. SHRODER,* G.N. PARSONS, C. WANG, G. LUCOVSKY Department of Physics, and Department of Materials Science and Engineering North Carolina State University Raleigh, NC 27695-8202 USA and J.B. BOYCE Xerox Palo Alto Research Center Palo Alto, CA 94304 USA Raman scattering measurements are used to characterize the amorphous and crystalline components of microcrystalline Si films. A model is described which addresses the properties of Raman scattering from composites of materials of different optical absorption. The analysis shows that the observed spectra is dependent on both the percentage of the components and on the domain size of the more highly absorbing domains. Samples of microcrystalline silicon prepared by exeimer laser exposure of hydrogenated a-Si and by magnetmn sputtering were measured, and the results were analyzed in terms of the model. The experimental results reflect the length scales of the domains and vibrational excitations. INTRODUCrlON Microcrystalline Si films are often produced by variations of the conditions for the the deposition of hydrogenated amorphous Si (a-Si:H). The films exhibit a structure which is a combination of amorphous Si and crystalline Si regions. The films often exhibit a higher electron mobility than a-Si:H, and are considered for thin film transistor applications. This study describes the application of Raman spectroscopy to characterize the films. Raman spectroscopy is particularly useful for identifying whether a film exhibits amorphous or crystalline structure. The Raman spectrum of crystalline Si is dominated by a sharp feature at 520 cm- 1 while the spectrum of a-Si:H displays features which resemble the broadened density of vibrational states of Si. There have been several detailed analysis of the evolution of the Raman spectra as a function of crystalline size. 1-3 For the particular case of Si, as the crystalline domain size decreases, the Ram_an peak broadens and shifts to lower frequency.2, 3 While Raman scattering measurements are often used to characterize the films, the optical absorption properties of the amorphous and crystalline regions are different, and these effects will influence the analysis. In this study, Raman measurements of a series of amorphous and microcrystalline Si films are presented. The spectra are decomposed into amorphous and crystalline components. The effects of optical absorption and crystal size are discussed. D r~ 7 r~ z I 100 i 300 I ! 500 FREQUENCY SHIFT (CM-I) FIGURE 1 The Ram.an spectra of excimer laser crystallized films (B-D) compared to that of amorphous Si (A). * Current address: Rockwell Int., Rocketdyne Division, Canoga Park, CA 91303 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland) j 814 R.J. Nemanich et al./ Raman scattering from microcrystalline Si films b. Z r~ z z I 100 ' 300 ' 500 100 FREQUENCY SHIFT (CM-I) FIGURE 2 The computer reconstructed Raman spectra of film B from Fig. 1. RESULTS Series of microcrystalline samples were prepared by excimer laser exposure of a-Si:H and by magnetron sputtering with high H concentrations. The Raman spectra of the series of films prepared by excimer laser exposure are shown in Fig. 1. The Raman spectra were excited with -150 mW of 514.5nm radiation from an Ar ion laser, and the scattered light was dispersed with a triple grating monochromator. Essentially similar spectra were obtained from the magnetron sputtered films. The spectrum A is of amorphous Si network vibrations while D represents predominantly crystalline regions with small domain sizes. Spectra B and C display spectral components which are attributed to both crystalline and amorphous regions in the sample. The goal of this study is to define the limits of using Raman spectroscopy to determine the relative amounts of these components. To model the crystalline and amorphous components of the samples, the spectra A and D were added together to duplicate the results shown in spectra B and C. The results are shown in Fig. 2 and 3. The general aspects of the spectra are well described by this procedure. There is a small discrepancy at ~500era-1 which could not be fit in either spectra. This aspect is discussed later. The results I 300 I I 500 FREQUENCY SHIFT (CM-1) FIGURE 3 The computer reconstructed Raman spectra of film C from Fig. 1. indicate that spectrum B can be described by 0.95 of A and 0.05 of D, and spectrum C was described by 0.78 of A and 0.22 of D. The following discussion describes the limitation of using this analysis to represent the relative concentrations of the amorphous and crystalline components. DISCUSSION The optical absorption differences of the amorphous and microcrystalline regions and spectral changes can effect the straightforward analysis described above. The aspects of optical absorption were identified in recent studies of Raman characterization of CVD diamond films and diamondgraphite powder composites.4 In these studies it was noted that the Raman intensity from the highly absorbing regions of the sample depended on the crystalline domain size. The ratio of the Raman intensities of the amorphous and crystalline regions can be given by Ia Aa Na Va l~t A ~ Np. Vgt (I) R.J. Nemanich et al. / Raman scattering from microcrystalline Si films where I, A, N and V are the Raman intensity, Raman crosssection, atomic density, and illuminated volume respectively, and the subscripts a and Ix refer to the amorphous and microcrystalline components. The illuminated volume will depend on the fraction of amorphous or microcrystalline regions and on the absorption constant and size of the domains. For the case of microcrystalline Si we can assume that a - 1 of the crystalline regions is larger than the domain size, thus it follows that the crystalline regions are fully illuminated. In contrast, the a of the amorphous regions is - 1 0 times greater than that of crystalline Si and cannot be neglected. Eq 1. thus becomes Ia AaNaPa [ 1] ii - A: N~ (-T~_pa)L~a~a/j (2) Where I is the size of the domains, and Pa is the fraction of a-Si. This equation is valid for c~-a~<l. The dependence of the observed Raman intensities for several different domain sizes are shown in Fig. 4. For the calculation the ratio of the Raman cross-sections was determined from the peak intensities of the materials, c¢ for a-Si of 2 xl05 cm-1, and the densities were assumed to be the same. From the figure it is clear that the observed Raman ratio cannot be directly related to the amorphous/microcrystalline ratio unless the domain size is known. Below ~500A domain size, the model will break down, and in the limit of very small domains, the term, 1/Ctal will go to 1. (In that limit, the straightforward analysis will apply.) The second aspect of the analysis that must be considered is that the spectral shape and scattering efficiency of the amorphous and crystalline regions can vary as a function of the crystalline domain size. It has been demonsWated that the linewidth and peak frequency changes as a function of the crystalline domain size for Si.2,3 These changes are most important in the range of 30 to 500A domain sizes. It has been estimated that vibrational modes in amorphous materials are localized to regions -50 A, thus the Raman spectra of the amorphous regions should not change unless the domain size approaches this limit. The most evident spectral change is the broadening of the 520cm -1 Raman feature for microcrystalline domains. Inspection of the spectra shown in Fig. 1 indicates that the crystalline Si feature has a different linewidth for the different samples. The sample with the smallest amorphous component exhibits the narrowest linewidth. This aspect can account to large part for the deviation in the reconstructed spectra shown in Fig. 2 and 3. 815 0.3" ~A 0.2" 3000 0.l" 0.0 0.0 • , . 0.2 FRACTION , 0.4 OF . , 0.6 AMORPHOUS . , 0.8 1.0 SILICON FIGURE 4 The theoretical ratio of the Raman signal from the am.orphous and crystalline regions vs. fraction of amorphous regmn. The calculations are for different domain sizes. CONCLUDING REMARKS The results described here show that the Raman spectra of microcrystalline Si films can be described as a combination of the spectra of a-Si and crystalline Si with small domains. Because of the large difference in the optical absorption of the crystalline and amorphous domains, the relative fraction of the two regions cannot be deduced unless the domain size has been determined. In addition, it has been established that the lineshape of the spectra of microcrystalline regions changes, and this leads to deviations in the fitting procedure. In principle, these changes can also be accounted for by fitting the lineshape of the crystalline component. One aspect that is yet to be determined is the dependence of the absolute intensity vs. crystalline domain size. ACKNOWI .EDGEMENTS We acknowledge Shannon Wells for her assistance on the data analysis. This work was partially supported by the North Carolina Board of Science and Technology. REFERENCES 1. R.J. Nemanich, S.A. Solin, and R.M. Martin, Phys. Rev. B23 (1981) 6348. 2. Z. Iqbal and S. Veprek, J. Phys. C15 (1982) 377. 3. P.M. Fauchet and I.H. Campbell, CRC Critical Reviews in Solid State and Materials Sciences 14 (1988) $79. 4. R.E. Shroder, R.J. Nemanich, and J.T. Glass, (to be published).