Solar Energy Materials & Solar Cells 63 (2000) 177}184

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1 Solar Energy Materials & Solar Cells 63 (2000) 177}184 EELS microanalysis of polycrystalline silicon thin "lms for solar cells grown at low temperatures Michael StoK ger *, Michael Nelhiebel, Peter Schattschneider, Viktor Schlosser, Alexander Breymesser, Bernard Jou!rey Institute of Applied and Technical Physics, Vienna University of Technology, Wiedner Hauptstr. 8-10/1137, A-1040 Vienna, Austria Laboratoire MSSMat CNRS-URA 850, E! cole Centrale Paris, F ChaL tenay-malabry, France Institute for Material Physics, University of Vienna, A-1090 Vienna, Austria Received 5 March 1999; received in revised form 28 September 1999; accepted 28 September 1999 Abstract The aim of this work is the quantitative chemical analysis of polycrystalline silicon thin "lms grown on glass substrates at temperatures (6003C by means of transmission electron microscopy (TEM) and electron energy-loss spectrometry (EELS). Specimens produced with two di!erent methods were investigated. We found signi"cant di!erences in grain size and morphology, as well as in the distribution of oxygen. A surprisingly high amount of Ba di!usion from the subtrate was detected Published by Elsevier Science B.V. All rights reserved. Keywords: Polycrystalline silicon thin "lms; Chemical vapour deposition; Electron energy-loss spectrometry; Transmission electron microscopy 1. Introduction The microstructure and microchemistry of Si thin "lms solar cells can essentially control their properties. Therefore, transmission electron microscopy (TEM) and electron energy loss spectrometry (EELS) are highly suitable methods for such investigations. The high magni"cation of the TEM and the high contrast obtainable in bright or dark "eld mode allow an analysis of grain size, shape and crystallographic orientation on a scale of nm or less. * Corresponding author: Tel.: # ; fax: # address: stoeger@atp6000.tuwien.ac.at (M. StoK ger) /00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S ( 9 9 )

2 178 M. Sto( ger et al. / Solar Energy Materials & Solar Cells 63 (2000) 177}184 The chemical composition can be probed by EELS, a technique that uses characteristic energy losses of the incident fast electrons to indicate which elements were ionized in the specimen. The spatial resolution of EELS in the TEM depends on a number of parameters. In the present case, a resolution of a few nm can be assumed. The precision of elemental quanti"cation is of the order of 10% relative. 2. Experimental procedure Sample 1 was produced at the University Rennes 1 / France by means of low pressure chemical vapor deposition (LP CVD), Sample 2 was made at the university of Barcelona/Spain by means of hot wire CVD (HW CVD). All measurements were done at the Laboratoire MSSMat of the CNRS at ED cole Centrale Paris/France with a PHILIPS CM20 transmission electron microscope and a GATAN 666 parallel electron energy-loss spectrometer (PEELS). For the investigations cross sections of the samples were prepared. For both samples pieces were stuck together surface to surface with M-Bond 600. The M-Bond 600-layer between the two surfaces on sample 2 was thinner than observed for sample 1, which indicates that the glue di!used into the pores of the polycrystalline layer. The samples were mechanically thinned and the "nal polishing was done by means of ion-thinning. The transmission electron microscope allows to measure in two di!erent modes and di!erent magni"cations. High magni"cation was used in such a way that only a few grains and grain boundaries were contributing to the energy-loss spectrum. The analyzed area was only 13 nm in diameter. In low magni"cation, the spectrum contains the signal of many grains and boundaries. Since it was expected that oxygen is inhomogenously incorporated in the layers we can get information about its distribution when using low and high magni"cation in a comparing way. The di!erent modes of the TEM are the image mode and the di!raction mode. The advantage of the image mode is that one can make measurements with high spatial resolution but the signal intensity is very low. The di!raction mode allows to use nearly all intensity of the electron beam for the spectra and the spatial resolution is determined by the size of the probe. When measuring in image mode at high spatial resolution care was taken of the chromatic aberration of the magnetic lenses inside the transmission electron microscope and the chromatic image shift which appears by the correction of the chromatic aberration [1,2]. 3. Results and discussion 3.1. Si material prepared by low-pressure CVD Sample 1 was produced by means of low-pressure chemical vapor deposition (LP CVD) at a temperature of about 5503C by using a CORNING 1737 wafer glass substrate. This substrate contains BaO and other oxides (see Table 1). After the process of growing, the layer was heated for 10 h at a temperature of about 6003C.

3 M. Sto( ger et al. / Solar Energy Materials & Solar Cells 63 (2000) 177} Table 1 Components of CORNING 1737 glass [3] and CORNING 7059 glass [4] Glass code Si (%) BaO (%) Ca (%) Al (%) B (%) Sr (%) As (%) 1737 (30 (15 (10 (10 (5 (5 ( * * * Table 2 Results of the EELS depth pro"le for oxygen and barium in sample 1 calculated with GATAN El/P software Distance from the substrate 10 nm 70 nm 120 nm 190 nm O concentration (%) 27.0$ $ $ $6.7 Ba concentration (%) 0.11$ $ $ $0.06 Fig. 1. Bright "eld image of sample 1. The bright feature is one crystal with a size of about nm, which is characteristic for this sample. During this step of annealing the layer was contaminated with Ba and oxygen (see Table 2) which were di!using out of the substrate. Since the layer is quite thin (about 250 nm) no concentration gradient of barium or oxygen can be found after ten hours. In Table 2 one can see the concentrations of oxygen and barium in the EELS spectra taken from sample 1 and calculated with consideration of the background-substraction done with the power-law background "t [5]. The layer is very compact with a density similar to pure silicon and contains grains of considerable size (Fig. 1). No other oxides of the substrate glass were detected. Possibly their concentrations are lower than the detection limit of the energy loss spectrometer.

4 180 M. Sto( ger et al. / Solar Energy Materials & Solar Cells 63 (2000) 177}184 Fig. 2. Cross section of sample 2 of the area, where the measurements were done. Fig. 3. EELS spectra (intensity versus energy loss) of the Ba M4,5 edges in the closest region to the interface, corresponding to Table Si material prepared by hot-wire CVD Sample 2 was produced by means of hot-wire CVD (HW CVD) with a substrate temperature of about 2253C and a total pressure of approximately mbar. A CORNING 7059 wafer glass was used as substrate, which contains BaO, too (see Table 1). As the process of growing was done at lower temperatures and no annealing was applied [6] the di!usion of ingredients out of the substrate is much smaller than observed in sample 1. The layer is 1200 nm thick (see Fig. 2) with needle-like crystal grains. Their mean length is about 300 nm, as TEM-dark "eld observations showed and their mean diameter about 35 nm. From AFM topography measurements we know that sample 2 is very porous. Fig. 3 shows the pro"le of the Ba M4,5 edges of EELS measurements in the "rst 25 nm next to the substrate}layer interface. A rapid decrease of the signal can be seen. The direct comparison of these four spectra is admissible, since the pre-edge background is equal in all cases without matching. This is only possible if the cross section has been prepared with nearly the same thickness at all measured points. In Table 3

5 M. Sto( ger et al. / Solar Energy Materials & Solar Cells 63 (2000) 177} Table 3 Values of the Ba distribution of sample 2 as a function of the distance from the substrate Distance from substrate 5 nm 10 nm 15 nm 20 nm 25 nm Ba/Si ratio Ba atoms/nm Fig. 4. Mean oxygen distribution of sample 2. Table 4 Oxygen concentration in sample 2 Position nbr. (Fig. 2) Distance from the substrate (nm) O concentration (%) 47.4$ $ $ $ $15.9 the Ba/Si ratio and the total amount of Ba atoms/nm projected on to the image } the area density } as a function of the distance to the substrate are listed. If BaO or only Ba di!uses was not investigated. A gradient in the concentration of oxygen was detected (Fig. 4 and Table 4). This gradient is caused by oxidation in ambient air after the production, because Si tends to get covered with an oxide layer, and by di!usion out of the substrate during layer deposition. In Table 4 the oxygen concentration as a function of the distance to

6 182 M. Sto( ger et al. / Solar Energy Materials & Solar Cells 63 (2000) 177}184 Table 5 Local values of di!erent points of measurement in sample 2. Column A is a single crystal A B C D E O/Si ratio 0.03$ $ $ $ $0.093 SiO/Si ratio SiO2/Si ratio F G H I J O/Si ratio 0.25$ $ $ $ $0.092 SiO/Si ratio SiO /Si ratio the substrate is shown. The position numbers are with respect to the numbers in the photograph (Fig. 2). This oxygen pro"le was measured with an astigmatic electron beam with a size of about nm. Therefore, the values of Table 4 can be considered to be mean values. The local values obtained with measurements with a focussed beam deviate from those mean values quite a lot. In general one can say, that the porous phase between the grains contains up to 2.15 times more oxygen than the grains. The local values of the atomic ratios of O/Si vary from 0.03 to 0.28 (see Table 5). This is because there is far more oxygen on the inner surface of the pores than there could be in a grain boundary. Therefore, the value of the ratio depends on the measured point. A di!raction pattern of point A (Table 5) showed that it was a single crystal. The concentrations of silicon and oxygen are corresponding to a thickness of 37 As Si and 560 As Si. It is plausible that nearly all of the O-signal comes from the surface contamination potentially caused by native oxidation during exposure to air. A similar thickness of native silicon oxide has been reported [7]. The last two rows in Table 5 show the volume ratio of pure silicon/sio and of Si/Si which we assumed to be the minimum and maximum values. The true ratio will be somewhere in between but far closer to the Si/Si ratio, because only the "rst monolayer of silicon oxide is Si with x3(0, 2) and all the rest is Si [7]. We calculated the volume ratio of the oxide phase on the assumption that it consists of either SiO or Si. With ρ [nm ]"ρ[g cm ]Z[g ]10, (1) where ρ is the mass density, Z is the number of molecules/g in crystalline SiO and Si and ρ is the atomic density, and D " I σ I (2) with D as the area density of the atoms of interest, I is the signal intensity due to the atoms of interest, σ is the cross section of the electron transition of interest and I is the whole intensity of the spectrum.

7 M. Sto( ger et al. / Solar Energy Materials & Solar Cells 63 (2000) 177} Fig. 5. EELS spectra (intensity versus energy loss) of the carbon pro"le (see Table 6). Table 6 Total counts in the carbon K-edge Distance from surface (nm) Counts in the C K-edge The value represents the carbon contamination caused by the electron beam in the microscope. The volume ratio of the Si and SiO/Si phases (Table 5) can now be calculated from Eqs. (1) and (2) with < "ad /ρ, where a is the illuminated area contributing to the spectrum. Since M-Bond 600 di!used into the layer, measurements of the carbon-edge close to the layers surface were done. And indeed, up to a depth of 300 nm carbon could be detected (Fig. 5 and Table 6). 4. Conclusion It was shown that even low production temperatures cannot suppress di!usion of constituents from the glass substrate into the Si thin "lm. The microstructure, crystallinity and impurity distribution depend on the production process and on the temperature. Sample 1 which was annealed for 10 h at 6003C has impurities throughout the whole layer. In sample 2, which was in the reactor much shorter at temperatures of 2253C, only the "rst few nanometers close to the substrate are contaminated by di!usion of glass constituents. Due to the fact that sample 2 was not annealed, a recrystallisation did not take place, and the oxygen can be found at the porous grain boundaries. The di!erent local values of the atomic O/Si ratio and the volume ratios SiO/Si and Si /Si can be taken as a measure of porosity.

8 184 M. Sto( ger et al. / Solar Energy Materials & Solar Cells 63 (2000) 177}184 Acknowledgements This work was supported by the JOULE project `CRYSTALa, Contract no. JOR3-CT of the European Commission. The authors would like to thank Prof. Dr. J. Andreu from the Departament de FmH sica Aplicada i Electrònica of the Universitat de Barcelona and Prof. Dr. T. Mohammed-Brahim of the Groupe de MicroeH lectronique et Visualisation of the UniversiteH Rennes 1 for providing the samples. References [1] M. Schenner, P. Schattschneider, Ultramicroscopy 55 (1994) 31}41. [2] P. Schattschneider, B. Jou!rey, Ch. Tischler, H. Bangert, Ultramicroscopy 53 (1994) 181}190. [3] Corning Incorp. Glass Code 1737 material data sheet. [4] ULLMANN's Encyclopedia of Industrial Chemistry, A12, p 382 f. [5] R.F. Egerton, Electron Energy-Loss Spectroscopy in the Electron Microscope, 2nd Edition, 1996, Plenum Press, NY, pp. 269}277. [6] A. Breymesser, V. Plunger, M. Ramadori, V. Schlosser, M. Nelhiebel, P. Schattschneider, D. Peiro, C. Voz, J. Bertomeu, J. Andreu, Proceedings of the 2nd World Conference and Exhibition on Photovoltaic Solar Energy, [7] H. Kobayishi, Y. Yamashita, T. Mori, Y. Nakato, T. Komeda, Y. Nishioka, Jpn. J. Appl. Phys. 34 (1995) 959}964.