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MicroRaman scattering from polycrystalline CuInS2 films: structural analysis
Thin Solid Films 361±362 (2000) 208±212 www.elsevier.com/locate/tsf
MicroRaman scattering from polycrystalline CuInS2 ®lms: structural analysis  lvarez-GarcõÂa a,*, J. Marcos-Ruzafa a, A. PeÂrez-RodrõÂguez a, A. Romano-RodrõÂguez a, J. A J.R. Morante a, R. Scheer b a
EME. Departament d'ElectroÁnica, Unitat Associada CNM-CSIC, Universitat de Barcelona, MartõÂ i FranqueÁs 1, 08028-Barcelona, Spain b Hahn-Meitner Institut, Glienicker Strasse 100, D-14109 Berlin, Germany
Abstract CuInS2 thin ®lms co-evaporated with gradual chemical composition have been characterised by MicroRaman scattering measurements. For the Cu rich region, the mode A1 at about 290 cm 21 corresponding to the chalcopyrite phase is dominant. For the Cu poor region, this mode is accompanied by a strong contribution at about 306 cm 21. Besides, the mode A1 is broadened and shifted towards higher frequencies, which suggests an inferior structural quality of the Cu poor region. Decreasing the temperature of deposition leads to a dramatic decrease of structural quality in both In and Cu rich regions. The correlation between the appearance of the 306 cm 21 mode and the spectral features of the mode A1 suggest the higher frequency mode is not related to the excess In in the layer but to structural effects as lattice disorder. Combined in-depth Auger electron spectroscopy and Raman scattering measurements have also shown the presence of a more complex structure for the Cu poor region of the layers, which presents a signi®cant CuIn5S8 secondary phase contribution in the spectra from the central region of the layers. The correlation of this contribution with the spectral features of the CuInS2 modes suggests a direct relationship between the presence of this In rich secondary phase and disorder at the CuInS2 lattice. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Raman scattering; CuInS2; Auger electron spectroscopy; Chalcopyrite; Solar cells
1. Introduction Vibrational properties of ternary chalcopyrite compounds A IB IIIC2VI have been widely studied during the last years [1,2]. These compounds show interesting properties for solar cell applications, non-linear optical devices and light emitting diodes. Among this family of materials, CuInS2 constitutes one of the most promising materials for the development of high ef®ciency solar cells, its bandgap (1.5 eV) being well suited for the solar spectrum at sea level. Environmental reasons give also interest to this material in front of other chalcopyrites such as CuInSe2. However, improvement on the detailed knowledge of the microstructural features of the CuInS2 layers as a function of their growing and processing parameters is critical for the development of a high ef®ciency solar cell technology. In this work, Raman scattering has been applied to the analysis of polycrystalline CuInS2 layers obtained by coevaporation as a function of their chemical composition and deposition temperature. Some studies of CuInS2 ®lms have been recently published [3±5] discussing the vibrational properties of these materials. However, a systematic * Corresponding author.
study is still to be done, to clarify the relationship of the observed vibrational modes with features as chemical composition, presence of secondary phases, and structural quality of the layers. With this purpose, combined in-depth Auger electron spectroscopy (AES) and MicroRaman scattering measurements have also been performed, which has allowed to correlate the in-depth chemical composition of the layers with their vibrational features.
2. Experimental Polycrystalline ®lms of CuInS2 have been grown on Mocoated soda-lime glass substrates, by co-evaporation of Cu, In and S. Due to the separation of the indium and copper sources, a layer with a gradual composition with both Cu poor and Cu rich regions is obtained. Four different samples have been grown at different temperatures in order to study the effect of this parameter (520, 470, 420 and 3708C). Films have an average thickness of 2.5 mm, and the dimensions of the samples are 5 £ 1:25 cm 2. After the growing of the ®lms, these have been etched with a 10% KCN solution in order to remove the CuS phase, which segregates to the surface of
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0084 7-0
J. AÂlvarez-GarcõÂa et al. / Thin Solid Films 361±362 (2000) 208±212
the Cu rich regions, due to the excess of Cu. After the chemical treatment, the CuS is completely removed. MicroRaman scattering measurements have been performed at room temperature with a Jobin±Yvon T64000 spectrometer coupled with an Olympus metallographic microscope, using the green line of an Ar 1 laser (l 514 nm) as excitation light. For the objective used ( £ 100, NA 0.95), the spot size on the sample is slightly submicronic. Spectra were obtained in backscattering con®guration, collecting light from all possible polarisations. In these conditions, the depth investigated by the Raman microprobe is determined by optical absorption of light in CuInS2, which is about 1/100 nm. Excitation power of light on the sample was about 1.0 mW, for which no thermal effects in the spectra were observed. Finally, the spectra have been compared with those measured at the same conditions on single crystal stoichiometric CuInS2. For Auger measurements, a PHI 670 Scanning Auger Nanoprobe System model has been used. The electron beam energy was 10 keV, with a current of about 10 nA at the sample surface. For the ion sputtering, the energy of the Ar 1 beam was about 2 keV, and the current about 50 mA (nominal values). Raman scattering measurements performed on sputtered single crystal CuInS2 have corroborated the absence of signi®cant sputter related damage. AES measurements performed on this sample have been used as standard, to quantify the chemical composition in the analysed region. Measured area in the samples was about 10 £ 15 mm 2. In-depth Raman measurements have also been made by focusing the light spot in this region after sputtering steps of about 15 min.
3. Results and discussion 3.1. Surface analysis Raman measurements have been performed at different points from the surface of the sample co-evaporated at 5508C. The results show two different characteristic spectra corresponding to both Cu poor and Cu rich regions. For all measurements, Cu rich spectra are measured on samples previously etched in KCN. Both spectra are compared in Fig. 1. The Cu rich spectrum is characterised by the presence of the dominant A1 mode of CuInS2. For the single crystal reference sample, this mode is centred at 290.0 cm 21, with a full width at half maximum (FWHM) of 3.5 cm 21. For the Cu rich region, this mode appears at a similar position, but with a higher FWHM. The peak also shows a shoulder at higher frequencies, which can be ®tted with a lorentzian contribution at about 302 cm 21. For the Cu poor region one obtains spectra with a strong contribution at about 306 cm 21. This is accompanied by a blue shift and broadening of the A1 mode, as shown in Fig. 1. Fig. 2 represents the position and FWHM of the mode from the spectra measured at equidistant points on the
209
Fig. 1. Characteristic Cu rich and Cu poor spectra.
sample, from the Cu poor edge (x 0), up to the Cu rich edge (x 5 cm). AES surface measurements at these regions give [In]/([In] 1 [Cu]) ratio values of 0.70 and 0.55, respectively. This is related to the presence of a Cu depleted surface region trough the whole sample, as will be discussed later. As shown in the ®gure, the maximum blue shift and broadening of the mode occurs at x 0, and both parameters decrease as the measuring point approaches the Cu rich region. For this region, the spectral features of the A1 mode are closer to those from single crystal CuInS2, being the position of the mode between 289.5 cm 21 and 290.3 cm 21, with FHWM values between 4 and 5 cm 21. It is well known that the broadening of the Raman modes is closely related to the presence of structural defects or stress gradients in the scattering volume. Generally speaking this relates to a degradation of the structural quality of the lattice. Hence, the behaviour plotted in Fig. 2 shows the existence of a worse structural quality from Cu poor ®lms. This agrees with X-ray diffraction measurements [6], which show a signi®cant broadening and intensity decrease of the CuInS2 peaks in spectra from the Cu poor region of the samples. The presence of a mode at around 305±310 cm 21 has been previously reported from Cu poor samples [3±5], and
Fig. 2. Position and FWHM of the mode A1 at different points of the sample deposited at 5208C.
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it has been related either to the presence of In rich secondary phases, as b -In2S3, or to localised modes related to Cu poor con®gurations. In our case, the assignment of this mode to the presence of b -In2S3 is not clear, because of the absence from the spectra of additional modes at 56 cm 21 and 70 cm 21, characteristic of this material. On the other hand, we have to remark that similar results have also been obtained for Cu poor CuInSe2 ®lms, which are also characterised by the presence of an additional mode at higher frequencies (183 cm 21) than the A1 peak (173 cm 21) [3,7]. Shirakata et al. [7] have related this additional mode to the presence of CuInSe2 with sphalerite structure, where Cu and In cations are randomly distributed in the cation sublattice. According to this, the comparison between the intensity of both A1 and higher frequency modes characterises the degree of cation disorder in the lattice. The correlation observed between the presence of this additional mode and the degradation of the spectral features of the A1 peak gives support to a disorder related origin of the mode. This interpretation is also supported by the chemical analysis of the surface of the samples by AES. These measurements show that after KCN etching there is an Cu poor surface layer through all the sample, the [In]/([In] 1 [Cu]) content surface ratio being in the range between 0.55 and 0.70. The existence of a Cu depleted surface zone has previously been reported by Scheer et al. [8], and the thickness of this region is of the order of 10±20% of the thickness of the layer, according to the in-depth AES measurements (as will be shown in the next section). This clearly rules out the possibility that the `Cu poor' mode were just related to a Cu poor composition, but to structural effects. Further evidence on the structural origin of the high frequency mode is given by Raman scattering measurements performed on samples deposited at lower temperatures. Fig. 3a,b shows series of different normalised spectra measured at equivalent Cu poor and Cu rich positions from the different samples. For the Cu poor region, similar spectra are measured for all the samples. We can see that the relative intensity of the 306 cm 21 mode increases by decreasing the co-evaporation temperature. For the Cu rich region, more dramatic changes are observed: in this case the spectra from the Cu rich region from samples grown at T 3708C and T 4208C exhibit an important contribution at about 306 cm 21, being similar to those measured at the other region of the sample (Cu poor). Samples grown at temperatures greater or equal than 4708C do not show the presence of this mode. The appearance of an increasing contribution from the 306 cm 21 mode is also accompanied by an increase of the blue shift and broadening of the A1 mode So, increasing the co-evaporation temperature increases the structural quality of the ®lm. In Fig. 4, the ratio between the integral intensity of the mode A1 and that of the contribution at higher frequencies (either the 306 cm 21 mode or the shoulder at about 302 cm 21) versus the position in the sample (from Cu poor edge (x 0) to Cu rich edge (x 5 cm)) is plotted for the
Fig. 3. Spectra from the Cu poor region (left ®gure) and Cu rich region (right ®gure) of samples grown at different temperatures.
four different samples. According to Shirakata et al. [7], this ratio characterises the degree of disorder of the cation sublattice: the higher this ratio is, the higher is the A1 contribution in the spectra, and the lower is the disorder. As already indicated, this feature correlates well with the spectral features (blue shift, broadening) of the A1 mode. According to this ®gure, the highest values of the intensity ratio are obtained for the sample grown at the highest temperature (5208C), which is related to its higher structural quality. The signi®cant increase of the intensity ratio as the measuring spot moves from the Cu poor edge towards the Cu rich region is related to the poorer structural quality of the Cu poor region. At 4708C, a similar behaviour is found, although there is an overall decrease of the relative contribution of the A1 mode through the whole sample. This contrasts with the sample deposited at the lowest temperature, 3708C, where similar spectra are obtained through the whole sample, and the low value of this intensity ratio claims for the dominant contribution of the mode at about 306 cm 21 in the spectra. The sample deposited at 4208C corresponds to an intermediate situation between these two behaviours: in this case the measurements performed close to the Cu rich edge (x 5 cm) are similar to the low
Fig. 4. Relative intensity of A1 mode with respect to the higher frequency mode.
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temperature behaviour (spectra similar to those from the sample deposited at 3708C). However, measurements performed close to the transition between both Cu rich and Cu poor regions show spectra similar to those from the samples deposited at higher temperatures. Accordingly, the transition between both high temperature and low temperature behaviours starts at the Cu rich edge, which suggests that the structural degradation of the ®lm is enhanced by the high Cu content in this region during deposition. 3.2. In-depth analysis The surface analysis presented in the above sections has been completed with the in-depth study of the layers. This has been centred in the sample deposited at the highest temperature, because of the higher ef®ciency of cell devices made on this layer, which is directly related to its higher structural quality. In-depth analysis of samples grown at lower temperatures is under progress and will be reported. Fig. 5 shows the in-depth [In]/([In] 1 [Cu]) content ratio as determined from AES spectra performed at two equivalent points from the Cu poor and Cu rich regions (in both cases, located at 0.5 cm from the edge). This plot con®rms the presence of a Cu poor layer in both regions of the sample, which extends down to about 10±20% of the layer. For deeper points, the Auger pro®les clearly show the different composition between both Cu poor and Cu rich regions. The measurements performed close to the Mo substrate layer (after about 100 min of sputtering) have a high uncertainty, due to the strong decrease in the intensity of both Cu and In signals. The whole series of Raman spectra measured after each sputter step in both Cu poor and Cu rich regions are shown in Fig. 6a,b, respectively. As previously, the intensity of the spectra is normalised. The spectra at the bottom of the ®gures correspond to the surface one, and the spectra obtained after equivalent sputter step of 12 min are vertically shifted. In both cases, the last measurement corresponds to the ®rst spectrum where Mo related peaks at
Fig. 6. (a) Cu poor and (b) Cu rich spectra from `in depth' measurements.
382 and 408 cm 21 appear. These could be related to the presence of a sulphur or sulphate compound at the interface. The spectra measured from the Cu poor region show the presence of additional modes at higher frequencies, which appear at the central region of the layer. These can be ®tted with three lorentzians, centred at about 325, 340 and 360 cm 21. These positions correspond to those reported for the three strongest vibrational modes of CuIn5S8 [9], which points out the presence of this secondary phase in the central region of the Cu poor layer. Just below this layer, a spectrum without the 306 cm 21 contribution is measured, which suggests the existence of a more ordered CuInS2 lattice. Below and above this thin region, the spectra are dominated by the disorder-like 306 cm 21 mode, and the spectral features (blue shift and FWHM) of the A1 mode correlate with the intensity of the CuIn5S8 modes. This suggests precipitation of CuIn5S8 to be related to disorder effects in the CuInS2 lattice. The non-homogeneous distribution of this phase, with a maximum in the central region, relates to the different mobility of the evaporated atoms and the accommodation of defects in the crystals during both growth and post-growth cooling processes. This contrasts with the spectra from the Cu rich region, which show a more homogeneous behaviour, in agreement with the higher structural quality of the whole layer. However, even in this case a CuIn5S8 residual contribution is still detectable through the layer. 4. Conclusions
Fig. 5. AES chemical composition through the sample for both regions (sample deposited at 5208C).
The Raman spectra from Cu poor region of polycrystalline CuInS2 ®lms are characterised by the presence of an additional mode around 306 cm 21, besides the A1 characteristic mode of the chalcopyrite structure. AES measurements have shown that the presence of this mode is not directly related to the In content in the surface of the sample. Moreover, the presence of this mode is strongly affected by the temperature of growth, and its contribution is higher for the samples grown at low temperatures (T # 4208C). For these
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samples, this mode appears even at the Cu rich region. The relative contribution of this mode in the spectra correlates with the spectral features of the mode A1, the appearance of this mode always being accompanied by a blue shift and a broadening of the mode A1. These facts suggest that the presence of the 306 cm 21 mode is not related to chemical effects, but to structural ones. The mode may be related to the existence of a different structural ordering (sphalerite) or to vacancy related vibrational modes, as already observed for other chalcopyrite compounds [7,10]. Finally, in-depth Raman measurements show the existence of a more complex structure at the Cu poor regions of the layers, observing the existence of a signi®cant CuIn5S8 secondary phase contribution in the spectra from the central region of the layers. This is also accompanied by a signi®cant blue shift and broadening of the spectral features of the A1 mode. Acknowledgements This work has been ®nancially supported by the European
Union through the JOULE contract JOR3-CT98-0297, SULFURCELL.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
J. Camassel, L. ArtuÂs, J. Pascual, Phys. Rev. B 41 (1990) 5717. J. Camassel, L. ArtuÂs, J. Pascual, Phys. Rev. B 41 (1990) 5727. K. Kondo, S. Nakamura, K. Sato, Jpn. J. Appl. Phys. 37 (1998) 5728. G. Morrel, R.S. Katiyar, S.Z. Weisz, T. Walter, W. Shocj, I. Balberg, Appl. Phys. Lett. 69 (1996) 987. R. Hunger, M. Wilhelm, K. Diesner, et al., Proc. 2nd World Conf. Photovoltaic Sol. Energy Conv., Vienna, 1998. Â lvarez-GarcõÂa, A. J. Marcos Ruzafa, A. Romano-RodrõÂguez, J. A PeÂrez-RodrõÂguez, J.R. Morante, R. Scheer, Proc. 11th Int. Conf. on Microscopy of Semiconductor Materials, Oxford, 1999. S. Shirakata, H. Kubo, C. Hamaguchi, S. Isomura, Jpn. J. Appl. Phys. 36 (1997) L1394. R. Scheer, et al., J. Vac. Sci. Technol. A 12 (1994) 51. N.M. Gasanly, S.A. El-Hamid, L.G. Gasanova, A.Z. Magomedov, Phys. Status Solidi B 169 (1992) K115. S. Nomura, S. Ouchi, S. Endo, Jpn. J. Appl. Phys. 36 (1997) L1075.
MicroRaman scattering from polycrystalline CuInS2 ®lms: structural analysis  lvarez-GarcõÂa a,*, J. Marcos-Ruzafa a, A. PeÂrez-RodrõÂguez a, A. Romano-RodrõÂguez a, J. A J.R. Morante a, R. Scheer b a
EME. Departament d'ElectroÁnica, Unitat Associada CNM-CSIC, Universitat de Barcelona, MartõÂ i FranqueÁs 1, 08028-Barcelona, Spain b Hahn-Meitner Institut, Glienicker Strasse 100, D-14109 Berlin, Germany
Abstract CuInS2 thin ®lms co-evaporated with gradual chemical composition have been characterised by MicroRaman scattering measurements. For the Cu rich region, the mode A1 at about 290 cm 21 corresponding to the chalcopyrite phase is dominant. For the Cu poor region, this mode is accompanied by a strong contribution at about 306 cm 21. Besides, the mode A1 is broadened and shifted towards higher frequencies, which suggests an inferior structural quality of the Cu poor region. Decreasing the temperature of deposition leads to a dramatic decrease of structural quality in both In and Cu rich regions. The correlation between the appearance of the 306 cm 21 mode and the spectral features of the mode A1 suggest the higher frequency mode is not related to the excess In in the layer but to structural effects as lattice disorder. Combined in-depth Auger electron spectroscopy and Raman scattering measurements have also shown the presence of a more complex structure for the Cu poor region of the layers, which presents a signi®cant CuIn5S8 secondary phase contribution in the spectra from the central region of the layers. The correlation of this contribution with the spectral features of the CuInS2 modes suggests a direct relationship between the presence of this In rich secondary phase and disorder at the CuInS2 lattice. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Raman scattering; CuInS2; Auger electron spectroscopy; Chalcopyrite; Solar cells
1. Introduction Vibrational properties of ternary chalcopyrite compounds A IB IIIC2VI have been widely studied during the last years [1,2]. These compounds show interesting properties for solar cell applications, non-linear optical devices and light emitting diodes. Among this family of materials, CuInS2 constitutes one of the most promising materials for the development of high ef®ciency solar cells, its bandgap (1.5 eV) being well suited for the solar spectrum at sea level. Environmental reasons give also interest to this material in front of other chalcopyrites such as CuInSe2. However, improvement on the detailed knowledge of the microstructural features of the CuInS2 layers as a function of their growing and processing parameters is critical for the development of a high ef®ciency solar cell technology. In this work, Raman scattering has been applied to the analysis of polycrystalline CuInS2 layers obtained by coevaporation as a function of their chemical composition and deposition temperature. Some studies of CuInS2 ®lms have been recently published [3±5] discussing the vibrational properties of these materials. However, a systematic * Corresponding author.
study is still to be done, to clarify the relationship of the observed vibrational modes with features as chemical composition, presence of secondary phases, and structural quality of the layers. With this purpose, combined in-depth Auger electron spectroscopy (AES) and MicroRaman scattering measurements have also been performed, which has allowed to correlate the in-depth chemical composition of the layers with their vibrational features.
2. Experimental Polycrystalline ®lms of CuInS2 have been grown on Mocoated soda-lime glass substrates, by co-evaporation of Cu, In and S. Due to the separation of the indium and copper sources, a layer with a gradual composition with both Cu poor and Cu rich regions is obtained. Four different samples have been grown at different temperatures in order to study the effect of this parameter (520, 470, 420 and 3708C). Films have an average thickness of 2.5 mm, and the dimensions of the samples are 5 £ 1:25 cm 2. After the growing of the ®lms, these have been etched with a 10% KCN solution in order to remove the CuS phase, which segregates to the surface of
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0084 7-0
J. AÂlvarez-GarcõÂa et al. / Thin Solid Films 361±362 (2000) 208±212
the Cu rich regions, due to the excess of Cu. After the chemical treatment, the CuS is completely removed. MicroRaman scattering measurements have been performed at room temperature with a Jobin±Yvon T64000 spectrometer coupled with an Olympus metallographic microscope, using the green line of an Ar 1 laser (l 514 nm) as excitation light. For the objective used ( £ 100, NA 0.95), the spot size on the sample is slightly submicronic. Spectra were obtained in backscattering con®guration, collecting light from all possible polarisations. In these conditions, the depth investigated by the Raman microprobe is determined by optical absorption of light in CuInS2, which is about 1/100 nm. Excitation power of light on the sample was about 1.0 mW, for which no thermal effects in the spectra were observed. Finally, the spectra have been compared with those measured at the same conditions on single crystal stoichiometric CuInS2. For Auger measurements, a PHI 670 Scanning Auger Nanoprobe System model has been used. The electron beam energy was 10 keV, with a current of about 10 nA at the sample surface. For the ion sputtering, the energy of the Ar 1 beam was about 2 keV, and the current about 50 mA (nominal values). Raman scattering measurements performed on sputtered single crystal CuInS2 have corroborated the absence of signi®cant sputter related damage. AES measurements performed on this sample have been used as standard, to quantify the chemical composition in the analysed region. Measured area in the samples was about 10 £ 15 mm 2. In-depth Raman measurements have also been made by focusing the light spot in this region after sputtering steps of about 15 min.
3. Results and discussion 3.1. Surface analysis Raman measurements have been performed at different points from the surface of the sample co-evaporated at 5508C. The results show two different characteristic spectra corresponding to both Cu poor and Cu rich regions. For all measurements, Cu rich spectra are measured on samples previously etched in KCN. Both spectra are compared in Fig. 1. The Cu rich spectrum is characterised by the presence of the dominant A1 mode of CuInS2. For the single crystal reference sample, this mode is centred at 290.0 cm 21, with a full width at half maximum (FWHM) of 3.5 cm 21. For the Cu rich region, this mode appears at a similar position, but with a higher FWHM. The peak also shows a shoulder at higher frequencies, which can be ®tted with a lorentzian contribution at about 302 cm 21. For the Cu poor region one obtains spectra with a strong contribution at about 306 cm 21. This is accompanied by a blue shift and broadening of the A1 mode, as shown in Fig. 1. Fig. 2 represents the position and FWHM of the mode from the spectra measured at equidistant points on the
209
Fig. 1. Characteristic Cu rich and Cu poor spectra.
sample, from the Cu poor edge (x 0), up to the Cu rich edge (x 5 cm). AES surface measurements at these regions give [In]/([In] 1 [Cu]) ratio values of 0.70 and 0.55, respectively. This is related to the presence of a Cu depleted surface region trough the whole sample, as will be discussed later. As shown in the ®gure, the maximum blue shift and broadening of the mode occurs at x 0, and both parameters decrease as the measuring point approaches the Cu rich region. For this region, the spectral features of the A1 mode are closer to those from single crystal CuInS2, being the position of the mode between 289.5 cm 21 and 290.3 cm 21, with FHWM values between 4 and 5 cm 21. It is well known that the broadening of the Raman modes is closely related to the presence of structural defects or stress gradients in the scattering volume. Generally speaking this relates to a degradation of the structural quality of the lattice. Hence, the behaviour plotted in Fig. 2 shows the existence of a worse structural quality from Cu poor ®lms. This agrees with X-ray diffraction measurements [6], which show a signi®cant broadening and intensity decrease of the CuInS2 peaks in spectra from the Cu poor region of the samples. The presence of a mode at around 305±310 cm 21 has been previously reported from Cu poor samples [3±5], and
Fig. 2. Position and FWHM of the mode A1 at different points of the sample deposited at 5208C.
210
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it has been related either to the presence of In rich secondary phases, as b -In2S3, or to localised modes related to Cu poor con®gurations. In our case, the assignment of this mode to the presence of b -In2S3 is not clear, because of the absence from the spectra of additional modes at 56 cm 21 and 70 cm 21, characteristic of this material. On the other hand, we have to remark that similar results have also been obtained for Cu poor CuInSe2 ®lms, which are also characterised by the presence of an additional mode at higher frequencies (183 cm 21) than the A1 peak (173 cm 21) [3,7]. Shirakata et al. [7] have related this additional mode to the presence of CuInSe2 with sphalerite structure, where Cu and In cations are randomly distributed in the cation sublattice. According to this, the comparison between the intensity of both A1 and higher frequency modes characterises the degree of cation disorder in the lattice. The correlation observed between the presence of this additional mode and the degradation of the spectral features of the A1 peak gives support to a disorder related origin of the mode. This interpretation is also supported by the chemical analysis of the surface of the samples by AES. These measurements show that after KCN etching there is an Cu poor surface layer through all the sample, the [In]/([In] 1 [Cu]) content surface ratio being in the range between 0.55 and 0.70. The existence of a Cu depleted surface zone has previously been reported by Scheer et al. [8], and the thickness of this region is of the order of 10±20% of the thickness of the layer, according to the in-depth AES measurements (as will be shown in the next section). This clearly rules out the possibility that the `Cu poor' mode were just related to a Cu poor composition, but to structural effects. Further evidence on the structural origin of the high frequency mode is given by Raman scattering measurements performed on samples deposited at lower temperatures. Fig. 3a,b shows series of different normalised spectra measured at equivalent Cu poor and Cu rich positions from the different samples. For the Cu poor region, similar spectra are measured for all the samples. We can see that the relative intensity of the 306 cm 21 mode increases by decreasing the co-evaporation temperature. For the Cu rich region, more dramatic changes are observed: in this case the spectra from the Cu rich region from samples grown at T 3708C and T 4208C exhibit an important contribution at about 306 cm 21, being similar to those measured at the other region of the sample (Cu poor). Samples grown at temperatures greater or equal than 4708C do not show the presence of this mode. The appearance of an increasing contribution from the 306 cm 21 mode is also accompanied by an increase of the blue shift and broadening of the A1 mode So, increasing the co-evaporation temperature increases the structural quality of the ®lm. In Fig. 4, the ratio between the integral intensity of the mode A1 and that of the contribution at higher frequencies (either the 306 cm 21 mode or the shoulder at about 302 cm 21) versus the position in the sample (from Cu poor edge (x 0) to Cu rich edge (x 5 cm)) is plotted for the
Fig. 3. Spectra from the Cu poor region (left ®gure) and Cu rich region (right ®gure) of samples grown at different temperatures.
four different samples. According to Shirakata et al. [7], this ratio characterises the degree of disorder of the cation sublattice: the higher this ratio is, the higher is the A1 contribution in the spectra, and the lower is the disorder. As already indicated, this feature correlates well with the spectral features (blue shift, broadening) of the A1 mode. According to this ®gure, the highest values of the intensity ratio are obtained for the sample grown at the highest temperature (5208C), which is related to its higher structural quality. The signi®cant increase of the intensity ratio as the measuring spot moves from the Cu poor edge towards the Cu rich region is related to the poorer structural quality of the Cu poor region. At 4708C, a similar behaviour is found, although there is an overall decrease of the relative contribution of the A1 mode through the whole sample. This contrasts with the sample deposited at the lowest temperature, 3708C, where similar spectra are obtained through the whole sample, and the low value of this intensity ratio claims for the dominant contribution of the mode at about 306 cm 21 in the spectra. The sample deposited at 4208C corresponds to an intermediate situation between these two behaviours: in this case the measurements performed close to the Cu rich edge (x 5 cm) are similar to the low
Fig. 4. Relative intensity of A1 mode with respect to the higher frequency mode.
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temperature behaviour (spectra similar to those from the sample deposited at 3708C). However, measurements performed close to the transition between both Cu rich and Cu poor regions show spectra similar to those from the samples deposited at higher temperatures. Accordingly, the transition between both high temperature and low temperature behaviours starts at the Cu rich edge, which suggests that the structural degradation of the ®lm is enhanced by the high Cu content in this region during deposition. 3.2. In-depth analysis The surface analysis presented in the above sections has been completed with the in-depth study of the layers. This has been centred in the sample deposited at the highest temperature, because of the higher ef®ciency of cell devices made on this layer, which is directly related to its higher structural quality. In-depth analysis of samples grown at lower temperatures is under progress and will be reported. Fig. 5 shows the in-depth [In]/([In] 1 [Cu]) content ratio as determined from AES spectra performed at two equivalent points from the Cu poor and Cu rich regions (in both cases, located at 0.5 cm from the edge). This plot con®rms the presence of a Cu poor layer in both regions of the sample, which extends down to about 10±20% of the layer. For deeper points, the Auger pro®les clearly show the different composition between both Cu poor and Cu rich regions. The measurements performed close to the Mo substrate layer (after about 100 min of sputtering) have a high uncertainty, due to the strong decrease in the intensity of both Cu and In signals. The whole series of Raman spectra measured after each sputter step in both Cu poor and Cu rich regions are shown in Fig. 6a,b, respectively. As previously, the intensity of the spectra is normalised. The spectra at the bottom of the ®gures correspond to the surface one, and the spectra obtained after equivalent sputter step of 12 min are vertically shifted. In both cases, the last measurement corresponds to the ®rst spectrum where Mo related peaks at
Fig. 6. (a) Cu poor and (b) Cu rich spectra from `in depth' measurements.
382 and 408 cm 21 appear. These could be related to the presence of a sulphur or sulphate compound at the interface. The spectra measured from the Cu poor region show the presence of additional modes at higher frequencies, which appear at the central region of the layer. These can be ®tted with three lorentzians, centred at about 325, 340 and 360 cm 21. These positions correspond to those reported for the three strongest vibrational modes of CuIn5S8 [9], which points out the presence of this secondary phase in the central region of the Cu poor layer. Just below this layer, a spectrum without the 306 cm 21 contribution is measured, which suggests the existence of a more ordered CuInS2 lattice. Below and above this thin region, the spectra are dominated by the disorder-like 306 cm 21 mode, and the spectral features (blue shift and FWHM) of the A1 mode correlate with the intensity of the CuIn5S8 modes. This suggests precipitation of CuIn5S8 to be related to disorder effects in the CuInS2 lattice. The non-homogeneous distribution of this phase, with a maximum in the central region, relates to the different mobility of the evaporated atoms and the accommodation of defects in the crystals during both growth and post-growth cooling processes. This contrasts with the spectra from the Cu rich region, which show a more homogeneous behaviour, in agreement with the higher structural quality of the whole layer. However, even in this case a CuIn5S8 residual contribution is still detectable through the layer. 4. Conclusions
Fig. 5. AES chemical composition through the sample for both regions (sample deposited at 5208C).
The Raman spectra from Cu poor region of polycrystalline CuInS2 ®lms are characterised by the presence of an additional mode around 306 cm 21, besides the A1 characteristic mode of the chalcopyrite structure. AES measurements have shown that the presence of this mode is not directly related to the In content in the surface of the sample. Moreover, the presence of this mode is strongly affected by the temperature of growth, and its contribution is higher for the samples grown at low temperatures (T # 4208C). For these
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samples, this mode appears even at the Cu rich region. The relative contribution of this mode in the spectra correlates with the spectral features of the mode A1, the appearance of this mode always being accompanied by a blue shift and a broadening of the mode A1. These facts suggest that the presence of the 306 cm 21 mode is not related to chemical effects, but to structural ones. The mode may be related to the existence of a different structural ordering (sphalerite) or to vacancy related vibrational modes, as already observed for other chalcopyrite compounds [7,10]. Finally, in-depth Raman measurements show the existence of a more complex structure at the Cu poor regions of the layers, observing the existence of a signi®cant CuIn5S8 secondary phase contribution in the spectra from the central region of the layers. This is also accompanied by a signi®cant blue shift and broadening of the spectral features of the A1 mode. Acknowledgements This work has been ®nancially supported by the European
Union through the JOULE contract JOR3-CT98-0297, SULFURCELL.
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