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TEM and EELS microanalysis of pc-Si thin film solar cells deposited by means of HW CVD
Solar Energy Materials & Solar Cells 70 (2001) 39}47
TEM and EELS microanalysis of pc-Si thin "lm solar cells deposited by means of HW CVD M. StoK ger *, P. Schattschneider , V. Schlosser, R. Schneider, H. Kirmse, W. Neumann Institute of Applied and Technical Physics, Vienna University of Technology, Wiedner Hauptstr. 8-10, A-1040 Wien, Austria Institute for Material Physics, University of Vienna, Austria Institute of Physics, Humboldt University of Berlin, Chair of Crystallography, Germany Received 28 September 1999; received in revised form 6 September 2000; accepted 29 September 2000
Abstract A p}i}n doped pc-silicon thin "lm grown by means of hot wire chemical vapour deposition (HW CVD) on a zinc oxide "lm has been investigated by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The structure of both layers, the ZnO substrate layer as much as the silicon thin "lm and the chemical composition at the interface were the subjects of our investigations. We found that a "le of pure silicon with a thickness of about 5 nm covers the substrate surface. A plausible model for getting information on the wavyness of the interface ZnO/pc-Si and the thickness of this pure Si-layer was developed. 2001 Elsevier Science B.V. All rights reserved. Keywords: Thin "lm; Transmission electron microscopy; Electron energy loss spectroscopy
1. Introduction Thin "lm crystalline silicon o!ers considerable potential for a stable and low cost solar cell technology [1,2]. With light trapping, even "lms only +10 m thick can absorb 90% of the incoming light. To realise this potential fully it will be essential not only to develop appropriate cell structures which provide the necessary optical con"nement, but to do so on low cost substrates such as glass. Although glass is the ideal substrate as far as cost and large area are concerned, its inability to withstand * Corresponding author. Tel.: #43-1-58801-13720; fax: #43-1-58801-13798. E-mail address: [email protected] (M. StoK ger). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 4 1 0 - 4
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M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
processing temperatures in excess of +5503C places severe restrictions on crystalline silicon deposition and solar cell manufacture. In the past few years polycrystalline silicon thin "lms have attracted attention for photovoltaic applications [3]. Their advantage is the combination of low temperature deposition techniques, similar to amorphous silicon, and a very low degradation, similar to crystalline silicon solar cells. For achieving higher deposition rates and stimulating crystallite formation the deposition technique is usually accompanied by providing an additional energy source in the deposition system. In this paper we report about investigations of pc-Si thin "lms produced by hot wire CVD (HW CVD), which sometimes is referred to as catalytic CVD [4]. This process allows good deposition rates and doping control. We report about the di!usion of ZnO from the electrical contact layer into the silicon thin "lm (Section 3) and a silicon enriched interlayer at the ZnO}Si interface.
2. Experimental The polycrystalline silicon thin "lm was deposited on a polycrystalline ZnO layer in a HW CVD multichamber reactor equipped with a load-lock chamber and base pressures lower than 10\ mbar. The polycrystalline ZnO layer itself was deposited on a CORNING 7059 wafer glass under low temperature conditions (about 2003C). The thicknesses and the substrate temperatures of the deposited layers in this multilayer system are given in Table 1. A gas mixture consisting of 1 and 19 sccm of silane and hydrogen, respectively, was used in the deposition chamber. The dissociation was obtained by means of a hot tungsten wire with a diameter of 1.0 mm at a temperature of about 17403C. The process pressure was 7;10\ mbar. The specimens were prepared by mechanical thinning and ion milling. Two pieces of the samples were glued together face-to-face with M-Bond 600 in order to obtain a cross-sectional preparation. After ion milling the sample was immediately put into the TEM to prevent the specimen from oxidation, which would be caused by exposure to ambient air for too long a time. The layers were investigated by transmission electron microscopy (TEM), electron energy loss spectrometry (EELS) and by energy "ltered TEM (EFTEM), which makes it possible to create one- and two-dimensional maps of the concentration of one element of interest. The three-window technique was
Table 1 Composition of the multilayer system
ZnO p-doped Si intrinsic Si n-doped Si
Dopant
Thickness (m)
Substrate temp. (3C)
* B * P
0.33 +0.1 +0.8 +0.05
* 175 215 215
M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
41
applied for EFTEM, i.e. measurements in the pre- and post-edge region were done for calculating the signal's background with the power-law "t. In order to get the pure elemental signal, this correction has to be done, because the background depends on the thickness and the matrix of the specimen. A correction corresponding to the di!erent mean free paths of the ZnO and pc-Si layers was done as suggested by Hofer [5], corrections corresponding to the chromatic aberration and the chromatic image shift were done as suggested by Schenner [6] and Schattschneider [7]. All measurements were carried out on an Hitachi H-8110 200 kV TEM accompanied with a GATAN imaging "lter (GIF) system and a scanning unit. In the STEM mode series of EEL spectra were taken by means of the GATAN digiscan.
3. Results and discussion In earlier investigations [8] we found, that some ingredients of the substrate of the silicon layer like to di!use into the thin "lm even at deposition temperatures of only 2003C. Therefore, the behaviour of ZnO at the interface was of particular interest. Three di!erent methods of EELS-analysis are: 1. EELS measurements in the image mode of the TEM with high spatial resolution. 2. EELS measurements in the STEM mode. 3. Energy "ltered TEM (EFTEM): generating an elemental map by means of an energy "lter. The advantage of the "rst method is the high spatial resolution. Fig. 1 is the Zn concentration pro"le in an interval from 32 nm starting 10 nm inside the ZnO-layer. The spatial resolution in image mode depends only on the magni"cation of the microscope which was 140 000 times and the diameter of the GIF entrance slit,
Fig. 1. Zn pro"le at a linescan in an interval of 32 nm, starting 10 nm behind the interface inside the ZnO-layer.
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M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
Fig. 2. EELS-spectra taken on a linescan through the interface ZnO-pc-Si. The shadowed spectra have been taken in the interface region. The distance between two spectra is 5 nm.
which was 2 mm. The sample thickness should not vary too much in the area of interest. Table 1 clearly shows that zinc gets mixed with silicon during the process of deposition. The advantage of the STEM mode is a very rapid analysis but it demands high stability of the TEM, because the dwelltime of 0.5 s is rather long. The specimen or the probe must not move during measurements, otherwise a loss of spatial resolution is caused. The chosen spotsize was 5 nm. In Fig. 2 a line-scan through the interface with measurements for each 5 nm is shown. The shadowed spectra were taken in the interface region. It shows, that Zn gets mixed with the deposited layer at the interface. Fig. 3 is a two-dimensional map of the Zn distribution obtained in EFTEM. The spatial resolution is given by the resolution of the TEM and the inherent drift of the specimen, which is better than 0.5 nm. The inlay shows the mean concentration pro"le of the rectangular region. The interface region called w gives*as we see later on*the waviness of the substrate's surface. The surface gets reduced to metallic Zn during the process of layer growth, because atomic hydrogen is present. All measurements were done at di!erent positions on the specimen to exclude a local e!ect and to improve statistics. We found that silicon can be detected in the original ZnO region, too. This is possible, because silicon starts to cover the ZnO surface inside the little pores, which were `washed outa during the reduction to metallic Zn of the substrate's surface. In Fig. 4 a signi"cant rampart in the silicon concentration can be observed. A nearly pure layer of silicon with a mean thickness of about 5.4$1.6 nm covers the substrate. The inlay of Fig. 4 presents the mean Si concentration of the rectangle and d gives the mean thickness of the purer silicon "lm. Therefore, the interface region can be de"ned as w#d and has an extension of w#d"13.1$1.6 nm. Investigations showed, that
M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
43
Fig. 3. Zn-map of an area of 150;110 nm calculated at the Zn-L3 ionisation edge. The #oating rim at the interface can be de"ned as interface region. The scale gives the zinc concentration. w gives the roughness of the interface.
Fig. 4. Si-map of an area of 150;110 nm calculated at the Si-L2,3 edge. At the interface a higher silicon concentration can be observed. w gives the roughness of the interface and d gives the thickness of the less oxidized Si "lm.
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M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
Fig. 5. Bright "eld image of the interface of an area of 150;110 nm. w and d are shown in the inlay.
Fig. 6. Map of the oxygen distribution in an area of 150;110 nm calculated at the O}K edge. The inlay shows a line pro"le through the interface. The scale gives the oxygen concentration.
the thickness of the rampart depends on the local waviness of the interface. In Fig. 5 this model has been veri"ed in the TEM bright-"eld image. Close to the needles of the ZnO crystals, very bright areas of the interface can be seen in the image, whereas the interface close to the pores is not as bright. This is an artefact of ion thinning, because ZnO crystals are more resistent in the ion mill than Si and therefore, the interface was washed out more closer to those needles than between them. As no other elements were detected inside the pc-Si layer, the oxygen distribution can be seen as a proof of the purity of silicon at the interface with respect to the zinc
M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
45
Fig. 7. Map of the `reduceda oxygen distribution inside the silicon thin "lm in an area of 150;110 nm. The inlay shows a line pro"le through the interface. The scale gives the oxygen concentration. w represents the roughness of the interface, which now can be monitored as a delay of the increase of the oxygen signal in the pc-Si layer, d is the thickness of the less oxidized Si "lm.
oxide contamination. Fig. 6 shows the oxygen distribution measured with the GIFsystem and a line pro"le across the interface. Assuming that zinc can be found only as stoichiometric ZnO, we can take the Zn distribution equal to the oxygen distribution of ZnO. Subtracting this `ZnO-oxygena distribution from Fig. 6 we get a `reduceda oxygen distribution of the Si-layer, which is shown in Fig. 7. Therefore, we now can "nd the waviness w of the ZnO layer as a waviness of the pc-Si layer, which appears in the concentration pro"le as a delay of the increase of the oxygen signal in the pc-Si layer. In Fig. 7 one easily sees that silicon close to the interface is less contaminated by oxygen because the oxygen-signal coming from silicon oxide is rising in the "rst 15 nm of the pc-Si layer. A plausible reason is that the oxygen coming into the reactor as a contamination reacts with the metallic zinc, which has been reduced by atomic hydrogen. Atomic hydrogen is a product of the dissociation of SiH in the HW CVD reactor. Therefore, silicon can be deposited very pure onto the rough ZnO layer surface. After covering the whole surface oxygen reacts with silicon too. The waviness of the interface is too small for good light trapping, as needed in solar cells, because the ratio of height to width is only +0.3. This is not enough for e!ective light trapping. The interface region can be given as w#d"13.1$1.6 nm and the mean diameter of the ZnO needle like crystals is about 37 nm. Another topic of our investigation was the oxygen distribution throughout the whole layer. Therefore, we took an energy-loss linescan with measurements for each 130 nm. The results are shown in Fig. 8. The rise of the oxygen concentration towards the surface of the thin "lm can be interpreted as oxidation at ambient air. Since the layer is porous, oxygen is able to reach inner regions.
M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
46
Fig. 8. Concentration pro"le of oxygen with respect to the distance from the ZnO}Si interface.
Fig. 9. Dark-"eld image of the pc-Si layer. Close to the interface much more microcrystals are illuminated due to a higher density of crystals in the same orientation.
The oxygen concentration is lower at the "rst one hundred nanometers, because the layer is boron doped. As we know boron doping supports a compact and almost defect-free crystal growth. The compactness can be seen in Fig. 9, where the "rst 100 nm of the pc-Si layer close to the interface are more illuminated. This is due to a higher density of crystals in the same crystallographic orientation.
4. Conclusion Our investigations show that a mixture of both materials is present at the interface between the ZnO layer, which is used as an electric contact material, and the pc-Si thin "lm, which acts as the absorber of the light. It seems to be likely, that the presence of
M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
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atomic hydrogen during the process of layer growth reduces the surface of the ZnO layer to metallic zinc. As shown in Fig. 5 the interface gets rough, little pores are enlarged at the grain boundaries. Afterwards oxidation prefers elemental Zn to silicon. So we can assume that within the "rst few nanometers the silicon is deposited nearly without oxidation until the whole layer surface is covered with silicon. Bindings between oxygen and silicon are very rare but oxidized Zn is still present. The roughness of the interface (w) and the thickness of the purer Si layer (d ) can be detected in the concentration pro"les of Figs. 3 and 4. Our interface region therefore is about 13 nm thick. Of course some kind of di!usion into the other layer of both, zinc and silicon, cannot be neglected.
Acknowledgements This work has been supported by the JOULE project `CRYSTALa, contract no. JOR-CT47-0126 of the European Commission. The authors would like to thank Prof. Dr. J. Andreu from the Department of Physics and Electronics of the University of Barcelona for providing the samples.
References [1] [2] [3] [4] [5] [6] [7] [8]
J.H. Werner, R. Bergmann, R. Brendel, Advances in Solid State Tech. 34 (1994) 115. Z. Shi, S.R. Wenham, Prog. Photovoltaics 3 (1994) 153. S.K. Deb, Curr. Opin. Solide State Mater. Sci. 3 (1998) 51. R. Iiduka, A.R. Heya, H. Matsumura, Sol. Energy Mater. Sol. Cells 48 (1997) 279. F. Hofer, W. Grogger, G. Kothleitner, P. Warbichler, Ultramicroscopy 67 (1996) 83. M. Schenner, P. Schattschneider, Ultramicroscopy 55 (1994) 31. P. Schattschneider, B. Jou!rey, Ch. Tischler, H. Bangert, Ultramicroscopy 53 (1994) 181. M. StoK ger, M. Nelhiebel, P. Schattschneider, V. Schlosser, A. Breymesser, B. Jou!rey, Sol. Energy Mater. Sol. Cells 63 (2000) 177.
TEM and EELS microanalysis of pc-Si thin "lm solar cells deposited by means of HW CVD M. StoK ger *, P. Schattschneider , V. Schlosser, R. Schneider, H. Kirmse, W. Neumann Institute of Applied and Technical Physics, Vienna University of Technology, Wiedner Hauptstr. 8-10, A-1040 Wien, Austria Institute for Material Physics, University of Vienna, Austria Institute of Physics, Humboldt University of Berlin, Chair of Crystallography, Germany Received 28 September 1999; received in revised form 6 September 2000; accepted 29 September 2000
Abstract A p}i}n doped pc-silicon thin "lm grown by means of hot wire chemical vapour deposition (HW CVD) on a zinc oxide "lm has been investigated by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The structure of both layers, the ZnO substrate layer as much as the silicon thin "lm and the chemical composition at the interface were the subjects of our investigations. We found that a "le of pure silicon with a thickness of about 5 nm covers the substrate surface. A plausible model for getting information on the wavyness of the interface ZnO/pc-Si and the thickness of this pure Si-layer was developed. 2001 Elsevier Science B.V. All rights reserved. Keywords: Thin "lm; Transmission electron microscopy; Electron energy loss spectroscopy
1. Introduction Thin "lm crystalline silicon o!ers considerable potential for a stable and low cost solar cell technology [1,2]. With light trapping, even "lms only +10 m thick can absorb 90% of the incoming light. To realise this potential fully it will be essential not only to develop appropriate cell structures which provide the necessary optical con"nement, but to do so on low cost substrates such as glass. Although glass is the ideal substrate as far as cost and large area are concerned, its inability to withstand * Corresponding author. Tel.: #43-1-58801-13720; fax: #43-1-58801-13798. E-mail address: [email protected] (M. StoK ger). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 4 1 0 - 4
40
M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
processing temperatures in excess of +5503C places severe restrictions on crystalline silicon deposition and solar cell manufacture. In the past few years polycrystalline silicon thin "lms have attracted attention for photovoltaic applications [3]. Their advantage is the combination of low temperature deposition techniques, similar to amorphous silicon, and a very low degradation, similar to crystalline silicon solar cells. For achieving higher deposition rates and stimulating crystallite formation the deposition technique is usually accompanied by providing an additional energy source in the deposition system. In this paper we report about investigations of pc-Si thin "lms produced by hot wire CVD (HW CVD), which sometimes is referred to as catalytic CVD [4]. This process allows good deposition rates and doping control. We report about the di!usion of ZnO from the electrical contact layer into the silicon thin "lm (Section 3) and a silicon enriched interlayer at the ZnO}Si interface.
2. Experimental The polycrystalline silicon thin "lm was deposited on a polycrystalline ZnO layer in a HW CVD multichamber reactor equipped with a load-lock chamber and base pressures lower than 10\ mbar. The polycrystalline ZnO layer itself was deposited on a CORNING 7059 wafer glass under low temperature conditions (about 2003C). The thicknesses and the substrate temperatures of the deposited layers in this multilayer system are given in Table 1. A gas mixture consisting of 1 and 19 sccm of silane and hydrogen, respectively, was used in the deposition chamber. The dissociation was obtained by means of a hot tungsten wire with a diameter of 1.0 mm at a temperature of about 17403C. The process pressure was 7;10\ mbar. The specimens were prepared by mechanical thinning and ion milling. Two pieces of the samples were glued together face-to-face with M-Bond 600 in order to obtain a cross-sectional preparation. After ion milling the sample was immediately put into the TEM to prevent the specimen from oxidation, which would be caused by exposure to ambient air for too long a time. The layers were investigated by transmission electron microscopy (TEM), electron energy loss spectrometry (EELS) and by energy "ltered TEM (EFTEM), which makes it possible to create one- and two-dimensional maps of the concentration of one element of interest. The three-window technique was
Table 1 Composition of the multilayer system
ZnO p-doped Si intrinsic Si n-doped Si
Dopant
Thickness (m)
Substrate temp. (3C)
* B * P
0.33 +0.1 +0.8 +0.05
* 175 215 215
M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
41
applied for EFTEM, i.e. measurements in the pre- and post-edge region were done for calculating the signal's background with the power-law "t. In order to get the pure elemental signal, this correction has to be done, because the background depends on the thickness and the matrix of the specimen. A correction corresponding to the di!erent mean free paths of the ZnO and pc-Si layers was done as suggested by Hofer [5], corrections corresponding to the chromatic aberration and the chromatic image shift were done as suggested by Schenner [6] and Schattschneider [7]. All measurements were carried out on an Hitachi H-8110 200 kV TEM accompanied with a GATAN imaging "lter (GIF) system and a scanning unit. In the STEM mode series of EEL spectra were taken by means of the GATAN digiscan.
3. Results and discussion In earlier investigations [8] we found, that some ingredients of the substrate of the silicon layer like to di!use into the thin "lm even at deposition temperatures of only 2003C. Therefore, the behaviour of ZnO at the interface was of particular interest. Three di!erent methods of EELS-analysis are: 1. EELS measurements in the image mode of the TEM with high spatial resolution. 2. EELS measurements in the STEM mode. 3. Energy "ltered TEM (EFTEM): generating an elemental map by means of an energy "lter. The advantage of the "rst method is the high spatial resolution. Fig. 1 is the Zn concentration pro"le in an interval from 32 nm starting 10 nm inside the ZnO-layer. The spatial resolution in image mode depends only on the magni"cation of the microscope which was 140 000 times and the diameter of the GIF entrance slit,
Fig. 1. Zn pro"le at a linescan in an interval of 32 nm, starting 10 nm behind the interface inside the ZnO-layer.
42
M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
Fig. 2. EELS-spectra taken on a linescan through the interface ZnO-pc-Si. The shadowed spectra have been taken in the interface region. The distance between two spectra is 5 nm.
which was 2 mm. The sample thickness should not vary too much in the area of interest. Table 1 clearly shows that zinc gets mixed with silicon during the process of deposition. The advantage of the STEM mode is a very rapid analysis but it demands high stability of the TEM, because the dwelltime of 0.5 s is rather long. The specimen or the probe must not move during measurements, otherwise a loss of spatial resolution is caused. The chosen spotsize was 5 nm. In Fig. 2 a line-scan through the interface with measurements for each 5 nm is shown. The shadowed spectra were taken in the interface region. It shows, that Zn gets mixed with the deposited layer at the interface. Fig. 3 is a two-dimensional map of the Zn distribution obtained in EFTEM. The spatial resolution is given by the resolution of the TEM and the inherent drift of the specimen, which is better than 0.5 nm. The inlay shows the mean concentration pro"le of the rectangular region. The interface region called w gives*as we see later on*the waviness of the substrate's surface. The surface gets reduced to metallic Zn during the process of layer growth, because atomic hydrogen is present. All measurements were done at di!erent positions on the specimen to exclude a local e!ect and to improve statistics. We found that silicon can be detected in the original ZnO region, too. This is possible, because silicon starts to cover the ZnO surface inside the little pores, which were `washed outa during the reduction to metallic Zn of the substrate's surface. In Fig. 4 a signi"cant rampart in the silicon concentration can be observed. A nearly pure layer of silicon with a mean thickness of about 5.4$1.6 nm covers the substrate. The inlay of Fig. 4 presents the mean Si concentration of the rectangle and d gives the mean thickness of the purer silicon "lm. Therefore, the interface region can be de"ned as w#d and has an extension of w#d"13.1$1.6 nm. Investigations showed, that
M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
43
Fig. 3. Zn-map of an area of 150;110 nm calculated at the Zn-L3 ionisation edge. The #oating rim at the interface can be de"ned as interface region. The scale gives the zinc concentration. w gives the roughness of the interface.
Fig. 4. Si-map of an area of 150;110 nm calculated at the Si-L2,3 edge. At the interface a higher silicon concentration can be observed. w gives the roughness of the interface and d gives the thickness of the less oxidized Si "lm.
44
M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
Fig. 5. Bright "eld image of the interface of an area of 150;110 nm. w and d are shown in the inlay.
Fig. 6. Map of the oxygen distribution in an area of 150;110 nm calculated at the O}K edge. The inlay shows a line pro"le through the interface. The scale gives the oxygen concentration.
the thickness of the rampart depends on the local waviness of the interface. In Fig. 5 this model has been veri"ed in the TEM bright-"eld image. Close to the needles of the ZnO crystals, very bright areas of the interface can be seen in the image, whereas the interface close to the pores is not as bright. This is an artefact of ion thinning, because ZnO crystals are more resistent in the ion mill than Si and therefore, the interface was washed out more closer to those needles than between them. As no other elements were detected inside the pc-Si layer, the oxygen distribution can be seen as a proof of the purity of silicon at the interface with respect to the zinc
M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
45
Fig. 7. Map of the `reduceda oxygen distribution inside the silicon thin "lm in an area of 150;110 nm. The inlay shows a line pro"le through the interface. The scale gives the oxygen concentration. w represents the roughness of the interface, which now can be monitored as a delay of the increase of the oxygen signal in the pc-Si layer, d is the thickness of the less oxidized Si "lm.
oxide contamination. Fig. 6 shows the oxygen distribution measured with the GIFsystem and a line pro"le across the interface. Assuming that zinc can be found only as stoichiometric ZnO, we can take the Zn distribution equal to the oxygen distribution of ZnO. Subtracting this `ZnO-oxygena distribution from Fig. 6 we get a `reduceda oxygen distribution of the Si-layer, which is shown in Fig. 7. Therefore, we now can "nd the waviness w of the ZnO layer as a waviness of the pc-Si layer, which appears in the concentration pro"le as a delay of the increase of the oxygen signal in the pc-Si layer. In Fig. 7 one easily sees that silicon close to the interface is less contaminated by oxygen because the oxygen-signal coming from silicon oxide is rising in the "rst 15 nm of the pc-Si layer. A plausible reason is that the oxygen coming into the reactor as a contamination reacts with the metallic zinc, which has been reduced by atomic hydrogen. Atomic hydrogen is a product of the dissociation of SiH in the HW CVD reactor. Therefore, silicon can be deposited very pure onto the rough ZnO layer surface. After covering the whole surface oxygen reacts with silicon too. The waviness of the interface is too small for good light trapping, as needed in solar cells, because the ratio of height to width is only +0.3. This is not enough for e!ective light trapping. The interface region can be given as w#d"13.1$1.6 nm and the mean diameter of the ZnO needle like crystals is about 37 nm. Another topic of our investigation was the oxygen distribution throughout the whole layer. Therefore, we took an energy-loss linescan with measurements for each 130 nm. The results are shown in Fig. 8. The rise of the oxygen concentration towards the surface of the thin "lm can be interpreted as oxidation at ambient air. Since the layer is porous, oxygen is able to reach inner regions.
M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
46
Fig. 8. Concentration pro"le of oxygen with respect to the distance from the ZnO}Si interface.
Fig. 9. Dark-"eld image of the pc-Si layer. Close to the interface much more microcrystals are illuminated due to a higher density of crystals in the same orientation.
The oxygen concentration is lower at the "rst one hundred nanometers, because the layer is boron doped. As we know boron doping supports a compact and almost defect-free crystal growth. The compactness can be seen in Fig. 9, where the "rst 100 nm of the pc-Si layer close to the interface are more illuminated. This is due to a higher density of crystals in the same crystallographic orientation.
4. Conclusion Our investigations show that a mixture of both materials is present at the interface between the ZnO layer, which is used as an electric contact material, and the pc-Si thin "lm, which acts as the absorber of the light. It seems to be likely, that the presence of
M. Sto( ger et al. / Solar Energy Materials & Solar Cells 70 (2001) 39}47
47
atomic hydrogen during the process of layer growth reduces the surface of the ZnO layer to metallic zinc. As shown in Fig. 5 the interface gets rough, little pores are enlarged at the grain boundaries. Afterwards oxidation prefers elemental Zn to silicon. So we can assume that within the "rst few nanometers the silicon is deposited nearly without oxidation until the whole layer surface is covered with silicon. Bindings between oxygen and silicon are very rare but oxidized Zn is still present. The roughness of the interface (w) and the thickness of the purer Si layer (d ) can be detected in the concentration pro"les of Figs. 3 and 4. Our interface region therefore is about 13 nm thick. Of course some kind of di!usion into the other layer of both, zinc and silicon, cannot be neglected.
Acknowledgements This work has been supported by the JOULE project `CRYSTALa, contract no. JOR-CT47-0126 of the European Commission. The authors would like to thank Prof. Dr. J. Andreu from the Department of Physics and Electronics of the University of Barcelona for providing the samples.
References [1] [2] [3] [4] [5] [6] [7] [8]
J.H. Werner, R. Bergmann, R. Brendel, Advances in Solid State Tech. 34 (1994) 115. Z. Shi, S.R. Wenham, Prog. Photovoltaics 3 (1994) 153. S.K. Deb, Curr. Opin. Solide State Mater. Sci. 3 (1998) 51. R. Iiduka, A.R. Heya, H. Matsumura, Sol. Energy Mater. Sol. Cells 48 (1997) 279. F. Hofer, W. Grogger, G. Kothleitner, P. Warbichler, Ultramicroscopy 67 (1996) 83. M. Schenner, P. Schattschneider, Ultramicroscopy 55 (1994) 31. P. Schattschneider, B. Jou!rey, Ch. Tischler, H. Bangert, Ultramicroscopy 53 (1994) 181. M. StoK ger, M. Nelhiebel, P. Schattschneider, V. Schlosser, A. Breymesser, B. Jou!rey, Sol. Energy Mater. Sol. Cells 63 (2000) 177.