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Spectromicroscopic characterisation of CuInS2 surfaces
Thin Solid Films 480–481 (2005) 291 – 294 www.elsevier.com/locate/tsf
Spectromicroscopic characterisation of CuInS2 surfaces K. Mqller*, Y. Burkov, D. Schmeiger Brandenburgisch Technische Universita¨t Cottbus, Angewandte Physik-Sensorik, 03013 Cottbus, P.O. Box 101344, Germany Available online 24 February 2005
Abstract Photoemission electron microscopy (PEEM) is used to study CuInS2 surfaces. CuInS2 (CIS) is used as polycrystalline absorber layer for thin film solar cells. A characterisation in terms of morphological information, elemental distribution, and of doping inhomogeneities at the surface is very important. We demonstrate that the method is capable for such surface studies in high lateral resolution. The use of synchrotron radiation allows the visualization of chemical inhomogeneities in single grains. By taking PEEM images around the absorption edges of Cu, In, or S, we are able to map elemental distributions, separated from morphology-dependent information in the PEEM image. Excitation with Hg illumination allows characterisation of elemental inhomogeneities. D 2004 Elsevier B.V. All rights reserved. Keywords: High-resolution PEEM; CuInS2; Solar cells; Surface characterisation
1. Introduction The method of photoemission electron microscopy (PEEM) has been known for several years. It had been shown that excitation with standard Hg lamps results in a sufficient resolution and intensity. Microscopes, based on this principle, are used for the visualization of magnetic domains [1], characterisation of adsorbates [2], mapping of doping inhomogeneities [3], surface morphology, or elemental distributions [4]. This method could be a complementary to Kelvin probe force microscopy (KPFM) measurements, which provided a direct measure of surface potential, but gives no chemical and elemental information [5]. Generally, contrast of PEEM images is a contrast of electron yield, bright areas represents regions with high electron emitting rate. The rate of electron yield, and hence the contrast, is influenced by topography (shadowing), work function (different materials or reduced work function at edges, doping level), or chemical inhomogeneities [6]. Only for smooth Si wafers, topological contrast can be excluded and a coincidence of doping level with * Corresponding author. Tel.: +49 355694067; fax: +49 355693931. E-mail address: [email protected] (K. Mqller). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.003
the rate of emitted secondary electrons was demonstrated [4]. For polycrystalline surfaces, excitation at different photon energies is useful: by taking two images with energies around the absorption edges of the elements searched for and subtracting the intensities at the related pixels, contrast of this so-called difference image should be a contrast of elemental distribution, while the topographic information is subtracted. In Fig. 1, this procedure is illustrated for the absorption edge of Cu. By subtracting micrographs, taken beneath the absorption edge (e.g., 928 eV) from those taken above (e.g., 935 eV), bright areas should represent Cu-rich regions of the sample. The contrast depends on the two selected photon energies and on the absolute value of the absorption edge. This values, as revealed by near-edge X-ray-induced absorption spectroscopy (NEXAFS) measurements of stoichiometric CuInS2 (CIS), are given in the following order of magnitude: Cu2pNS2pNIn3d [7]. In this study, we used a PEEM with both Hg lamp and synchrotron radiation (SR) for the characterisation of CuInS2 (CIS) surfaces. The lateral resolution of the PEEM was in the submicrometer range due to the brightness of the SR. This is the prerequisite for the characterisation of thin film solar cells, based on polycrystalline chalkogenides.
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the actual formation of these phases—as sandwich structure or as lateral distribution—was not visualized directly. Recently, we developed a novel technique which brings the surface composition close to the stoichiometric stability range of the CuInS2 phase. The technique used the sputtering and annealing of the sample at UHV conditions [18].
2. Experimental
Fig. 1. Absorption edge of copper, as revealed by recording the integral intensity of the micrographs as a function of the incident photon energy [near-edge X-ray-induced absorption spectroscopy (NEXAFS)]. E1 and E2 indicates typical photon energies taken for difference images of the elemental distribution of Cu (sample: Cu-rich CuInS2 surface without etching [7].
Essential for CuInS2 based solar cells is the surface of the polycrystalline absorber layer [8]. There are many attempts for optimization of the technological steps to remove subphases [9,10] or the preparation conditions of the layer [11–13] as well as to form the p–n junction on top of it [14]. Films of CuInS2, prepared with an excess of Cu to promote the grain growth [15], need a further technological step to remove segregated phases like CuS on top of the surface. Etching with KCN was reported as a useful method to remove such phases [16]. An excess of In just after etching is well known and was revealed by XPS measurements. In rich phases like In2S3, CuIn3S5, or CuIn3S5, building up an overlayer [16] or as phase mixture on top of the absorber layer have been discussed [17], but
For our measurements, we used radiation of a standard high-pressure Hg lamp (4.9 eV) and synchrotron radiation at the BESSYII-U49/2 beam line. The PEEM optics adapted is an OMICRON FOCUS-IS-PEEM with a CCD camera and a simulated hemispherical analyzer [6]. We studied CuInS2 films, deposited on a Mo film on a soda-lime-glass substrate by sputter deposition of Cu and In. The precursor layers are sulphurised in S2 vapor at 500 8C resulting in a 3-Am-thick CuInS2 layer [19]. The films were prepared under excess of Cu to promote the grain growth [15]. An etching procedure in KCN solution, described by Weber et al. [16], was performed to remove phases like CuS on top of the absorber layer (3 min, 5% KCN). Typically, the composition of such KCN-etched surfaces was In-rich. The bulk composition, as revealed by XPS and EDX, is stoichiometric (CuInS2), and the distribution of Cu, In, and S, detected by SEM, was homogenous [20]. After mounting the samples in our UHV chamber, the Mo layer was used as resistive heater, connected with Ta clips as electrical contact. Here, an annealing procedure up to 400 8C leads to almost stoichiometric compositions of the surface, revealed by UPS and XPS measurements [20]. These measurements are integrating over the size of the
Fig. 2. CuInS2 sample after the etching procedure, but without annealing. (A) SEM picture (20 kV); (B) PEEM image, as recorded by Hg illumination (4.9 eV), compared with the SEM picture in same scale (inset).
K. Mu¨ller et al. / Thin Solid Films 480–481 (2005) 291–294
293
Fig. 3. PEEM images at different positions, taken with Hg radiation (A, B), compared with a micrograph (C) taken with synchrotron radiation (170 eV, same sample). KCN-etched CIS surface after Ar sputtering.
illuminated sample and give no direct lateral information of the surface hence.
3. Results and discussion 3.1. Hg illumination In Fig. 2, we demonstrate that contrast in PEEM is predominantly caused by surface morphology. We compare a Hg-illuminated PEEM image and an SEM micrograph of two CuInS2 samples representing the same state of the surface just after etching with KCN. The SEM image shows that the surface consists of individual grains with a rather homogeneous distribution of size with an average diameter of about 2 Am. The PEEM image shows structures similar to the SEM micrograph. Fig. 3 shows two PEEM images at different positions (Fig. 3A, B), taken with Hg radiation, compared with a PEEM micrograph taken with synchrotron radiation (Fig. 3C, 170 eV, same sample). The images represents the situation after etching and Ar sputtering. The Hg images show a distribution of bright areas, whereas synchrotron images at any position on the sample show a surface morphology without additional significant differences in intensity superimposed to the morphologic fine structure. This difference in Hg-PEEM intensities should represent
very small changes in local work function. The heterogeneities could be due to variations in doping or Fermi level or small changes in chemical composition. A charging of the surface due to illumination can be excluded: charging would give dark areas and diffuse PEEM images. The (MgKa) XPS data give an additional amount of Na (and low content of O). This could be due to the presence of Na in the glass substrate. The local variations of work function are likely the result of this Na content. The highest photon energy available by the Hg lamp is 4.9 eV. Taken an electron affinity of 4.7 and the band gap of 1.54 eV into account, illumination by Hg photons should be very sensitive to differences of work function. The work function acts as threshold for photoelectrons and differences in contrast with Hg illumination should be related to very small variations of work function. For higher photon energies, in our example, synchrotron radiation with 170 eV, this influence should be neglected and contrast is mainly due to topography. 3.2. Illumination at the BESSYII-49/II-beamline In order to achieve better lateral resolution and to record difference images, we used synchrotron radiation. We report now on the difference images with energies around the absorption edges of Cu, In, S (and O) for KCN-etched samples.
Fig. 4. Difference images (same position) of Cu, In, and S for a sample after etching, sputtering and annealing up to 390 8C for 10 min. (A) Cu, taken with 935.3 and 930 eV; (B) In with 455 and 430 eV; and (C) S with 175 and 157 eV.
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K. Mu¨ller et al. / Thin Solid Films 480–481 (2005) 291–294
In Fig. 4, the surface of the polycrystalline absorber layer after etching, Ar sputtering (20 min, 4.5 kV), and annealing up to a temperature of 390 8C for 10 min are shown, as recorded by difference images of Cu, In, and S. The images show structures of a size of about 2 Am, this is average grain size. An argument for elemental contrast in single grains can be extracted if we compare the difference images for Cu (Fig. 4A), In (Fig. 4B), and S (Fig. 4C). For Cu, bright and relatively homogeneous areas with dark spots are recorded. For In and S, the situation is reverse, here exactly the dark spots in the Cu image now appear brighter. We conclude that at the surface of KCN-etched CIS, In-rich sulphide seems to be incorporated in a relatively Cu-rich matrix. The surface composition of just-etched CuInS2 films is indeed known to be In-rich, and Scheer and Lewerenz [17] discussed a mixture of CuInS2 and CuIn3S5 in lateral and vertical directions, as predicted by the binary cut Cu2S– In2S3 [21]. Our XPS measurements of etched surfaces shows a ratio close to 0.75, directly positioned on this binary cut in the ternary phase diagram Cu–In–S. Sputtering and annealing lead to almost stoichiometric (CuInS2) XPS ratios of these elements [20]. This situation is reported in Fig. 4. A small amount of relatively In- and S-rich phases (for example, CuIn3S5) here seems to be embedded in a relatively Cu-rich phase (for example, CuInS2).
4. Summary Polycrystalline CuInS2 surfaces are analyzed by photoemission electron microscopy (PEEM) to characterise the elemental distributions between and around individual grains in the submicrometer range using synchrotron radiation. We are able to derive difference PEEM images and demonstrate that the method is able to separate the elemental information from contributions of the surface morphology and to characterise the elemental distribution of KCN-etched polycrystalline CuInS2 samples. We find significant variation in the Cu distribution, which is caused by submicrometer surface-enriched In–S phases, which are placed at the grains and in-between of single grains. This information is a unique feature of the PEEM characterisation, in the lateral integrating XPS spectra and in the depth integrating EDX data, these surfaces appear stoichiometric and homogeneous.
Acknowledgements We like to thank Dr. R. Scheer for providing CuInS2 samples and helpful discussions. The experimental assistance of P. Hoffmann, I. Paloumpa (KCN-etching), and G. Beuckert, as well as the help of the BESSY staff, is acknowledged. In addition, we like to thank R. Sohal for the discussion. This work was supported by DFG under grant no. GEP-SCHM 745/3.
References [1] R. Frfmter, M. Seider, C. Schneider, Ch. Ziethen, W. Swiech, G. Schfnheise, J. Kirschner, BESSY Annual Report 1996, 482–484. [2] H. Rotermund, W. Engel, S. Jakubith, A. von Oertzen, G. Ertl, Ultramicroscopy 36 (1991) 164. [3] R. Mikalo, P. Hoffmann, T. Heller, D. Schmeiger, Solid State Phenom. 63 to 64 (1998) 317. [4] P. Hoffmann, R. Mikalo, D. Schmeiger, M. Kittler, Phys. Status Solidi B 215 (1999) 743. [5] S. Sadewasser, T. Glatzel, S. Schuler, R. Kaigawa, M.C. Lux-Steiner, Thin Solid Films 431 (2003) 257. [6] P. Hoffmann, Aufbau eines Elektronen-Energie-Analysators (AESCA) fqr PEEM, Diploma Thesis, Cottbus, 1998. [7] K. Mqller, Y. Burkov, D. Schmeiger, Thin Solid Films 413–432 (2003) 307. [8] H. Mfller, Semiconductors for Solar Cells, Artech House, Norwood, 1993. [9] B. Blachnik, A. Mqller, Thermochim. Acta 361 (2000) 31. [10] M. Aggour, H.J. Lewerenz, J. Klaer, U. Stfrkel, Electrochem. SolidState Lett. 3 (2000) 399. [11] M. Kleinfeld, H.D. Wiemhffer, Ber. Bunsenges. Phys. Chem. 90 (1986) 711. [12] R. Scheer, H.J. Lewerenz, J. Vac. Sci. Technol., A 13 (1995) 1924. [13] V. Nadenau, D. Hariskos, H.W. Schock, M. Kreijci, F. Haug, A. Tiwari, H. Zogg, G. Kostoroz, J. Appl. Phys. 85 (1999) 534. [14] M. Aggour, U. Stfrkel, C. Murrell, S.A. Campbell, H. Jungblut, P. Hoffmann, R. Mikalo, D. Schmeiger, H.J. Lewerenz, Thin Solid Films 57 (2002) 403. [15] R. Klenk, T. Walter, H.W. Schock, D. Cahen, Adv. Mater. 5 (1993) 114. [16] M. Weber, R. Scheer, H.J. Lewerenz, H. Jungblut, U. Stfrkel, J. Electrochem. Soc. 149 (1) (2002) G77. [17] R. Scheer, H.J. Lewerenz, J. Vac. Sci. Technol., A 13 (4) (1995) 1924. [18] K. Mqller, S. Milko, D. Schmeiger, Thin Solid Films 413–432 (2003) 312. [19] Lit: J. Klaer, J. Bruns, R. Henninger, K. Tfpper, R. Klenk, K. Ellmer, D. Br7uning, Proceedings of the 2nd world Conference on Photovoltaic Solar Energy Conversion, vol. 1, p. 537, European commission, 1998. [20] K. Mqller, R. Scheer, Y. Burkov, D. Schmeiger, Thin Solid Films 451–452 (2004) 120. [21] J. Binsma, L. Giling, J. Bloem, J. Cryst. Growth 50 (1980) 429.
Spectromicroscopic characterisation of CuInS2 surfaces K. Mqller*, Y. Burkov, D. Schmeiger Brandenburgisch Technische Universita¨t Cottbus, Angewandte Physik-Sensorik, 03013 Cottbus, P.O. Box 101344, Germany Available online 24 February 2005
Abstract Photoemission electron microscopy (PEEM) is used to study CuInS2 surfaces. CuInS2 (CIS) is used as polycrystalline absorber layer for thin film solar cells. A characterisation in terms of morphological information, elemental distribution, and of doping inhomogeneities at the surface is very important. We demonstrate that the method is capable for such surface studies in high lateral resolution. The use of synchrotron radiation allows the visualization of chemical inhomogeneities in single grains. By taking PEEM images around the absorption edges of Cu, In, or S, we are able to map elemental distributions, separated from morphology-dependent information in the PEEM image. Excitation with Hg illumination allows characterisation of elemental inhomogeneities. D 2004 Elsevier B.V. All rights reserved. Keywords: High-resolution PEEM; CuInS2; Solar cells; Surface characterisation
1. Introduction The method of photoemission electron microscopy (PEEM) has been known for several years. It had been shown that excitation with standard Hg lamps results in a sufficient resolution and intensity. Microscopes, based on this principle, are used for the visualization of magnetic domains [1], characterisation of adsorbates [2], mapping of doping inhomogeneities [3], surface morphology, or elemental distributions [4]. This method could be a complementary to Kelvin probe force microscopy (KPFM) measurements, which provided a direct measure of surface potential, but gives no chemical and elemental information [5]. Generally, contrast of PEEM images is a contrast of electron yield, bright areas represents regions with high electron emitting rate. The rate of electron yield, and hence the contrast, is influenced by topography (shadowing), work function (different materials or reduced work function at edges, doping level), or chemical inhomogeneities [6]. Only for smooth Si wafers, topological contrast can be excluded and a coincidence of doping level with * Corresponding author. Tel.: +49 355694067; fax: +49 355693931. E-mail address: [email protected] (K. Mqller). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.003
the rate of emitted secondary electrons was demonstrated [4]. For polycrystalline surfaces, excitation at different photon energies is useful: by taking two images with energies around the absorption edges of the elements searched for and subtracting the intensities at the related pixels, contrast of this so-called difference image should be a contrast of elemental distribution, while the topographic information is subtracted. In Fig. 1, this procedure is illustrated for the absorption edge of Cu. By subtracting micrographs, taken beneath the absorption edge (e.g., 928 eV) from those taken above (e.g., 935 eV), bright areas should represent Cu-rich regions of the sample. The contrast depends on the two selected photon energies and on the absolute value of the absorption edge. This values, as revealed by near-edge X-ray-induced absorption spectroscopy (NEXAFS) measurements of stoichiometric CuInS2 (CIS), are given in the following order of magnitude: Cu2pNS2pNIn3d [7]. In this study, we used a PEEM with both Hg lamp and synchrotron radiation (SR) for the characterisation of CuInS2 (CIS) surfaces. The lateral resolution of the PEEM was in the submicrometer range due to the brightness of the SR. This is the prerequisite for the characterisation of thin film solar cells, based on polycrystalline chalkogenides.
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K. Mu¨ller et al. / Thin Solid Films 480–481 (2005) 291–294
the actual formation of these phases—as sandwich structure or as lateral distribution—was not visualized directly. Recently, we developed a novel technique which brings the surface composition close to the stoichiometric stability range of the CuInS2 phase. The technique used the sputtering and annealing of the sample at UHV conditions [18].
2. Experimental
Fig. 1. Absorption edge of copper, as revealed by recording the integral intensity of the micrographs as a function of the incident photon energy [near-edge X-ray-induced absorption spectroscopy (NEXAFS)]. E1 and E2 indicates typical photon energies taken for difference images of the elemental distribution of Cu (sample: Cu-rich CuInS2 surface without etching [7].
Essential for CuInS2 based solar cells is the surface of the polycrystalline absorber layer [8]. There are many attempts for optimization of the technological steps to remove subphases [9,10] or the preparation conditions of the layer [11–13] as well as to form the p–n junction on top of it [14]. Films of CuInS2, prepared with an excess of Cu to promote the grain growth [15], need a further technological step to remove segregated phases like CuS on top of the surface. Etching with KCN was reported as a useful method to remove such phases [16]. An excess of In just after etching is well known and was revealed by XPS measurements. In rich phases like In2S3, CuIn3S5, or CuIn3S5, building up an overlayer [16] or as phase mixture on top of the absorber layer have been discussed [17], but
For our measurements, we used radiation of a standard high-pressure Hg lamp (4.9 eV) and synchrotron radiation at the BESSYII-U49/2 beam line. The PEEM optics adapted is an OMICRON FOCUS-IS-PEEM with a CCD camera and a simulated hemispherical analyzer [6]. We studied CuInS2 films, deposited on a Mo film on a soda-lime-glass substrate by sputter deposition of Cu and In. The precursor layers are sulphurised in S2 vapor at 500 8C resulting in a 3-Am-thick CuInS2 layer [19]. The films were prepared under excess of Cu to promote the grain growth [15]. An etching procedure in KCN solution, described by Weber et al. [16], was performed to remove phases like CuS on top of the absorber layer (3 min, 5% KCN). Typically, the composition of such KCN-etched surfaces was In-rich. The bulk composition, as revealed by XPS and EDX, is stoichiometric (CuInS2), and the distribution of Cu, In, and S, detected by SEM, was homogenous [20]. After mounting the samples in our UHV chamber, the Mo layer was used as resistive heater, connected with Ta clips as electrical contact. Here, an annealing procedure up to 400 8C leads to almost stoichiometric compositions of the surface, revealed by UPS and XPS measurements [20]. These measurements are integrating over the size of the
Fig. 2. CuInS2 sample after the etching procedure, but without annealing. (A) SEM picture (20 kV); (B) PEEM image, as recorded by Hg illumination (4.9 eV), compared with the SEM picture in same scale (inset).
K. Mu¨ller et al. / Thin Solid Films 480–481 (2005) 291–294
293
Fig. 3. PEEM images at different positions, taken with Hg radiation (A, B), compared with a micrograph (C) taken with synchrotron radiation (170 eV, same sample). KCN-etched CIS surface after Ar sputtering.
illuminated sample and give no direct lateral information of the surface hence.
3. Results and discussion 3.1. Hg illumination In Fig. 2, we demonstrate that contrast in PEEM is predominantly caused by surface morphology. We compare a Hg-illuminated PEEM image and an SEM micrograph of two CuInS2 samples representing the same state of the surface just after etching with KCN. The SEM image shows that the surface consists of individual grains with a rather homogeneous distribution of size with an average diameter of about 2 Am. The PEEM image shows structures similar to the SEM micrograph. Fig. 3 shows two PEEM images at different positions (Fig. 3A, B), taken with Hg radiation, compared with a PEEM micrograph taken with synchrotron radiation (Fig. 3C, 170 eV, same sample). The images represents the situation after etching and Ar sputtering. The Hg images show a distribution of bright areas, whereas synchrotron images at any position on the sample show a surface morphology without additional significant differences in intensity superimposed to the morphologic fine structure. This difference in Hg-PEEM intensities should represent
very small changes in local work function. The heterogeneities could be due to variations in doping or Fermi level or small changes in chemical composition. A charging of the surface due to illumination can be excluded: charging would give dark areas and diffuse PEEM images. The (MgKa) XPS data give an additional amount of Na (and low content of O). This could be due to the presence of Na in the glass substrate. The local variations of work function are likely the result of this Na content. The highest photon energy available by the Hg lamp is 4.9 eV. Taken an electron affinity of 4.7 and the band gap of 1.54 eV into account, illumination by Hg photons should be very sensitive to differences of work function. The work function acts as threshold for photoelectrons and differences in contrast with Hg illumination should be related to very small variations of work function. For higher photon energies, in our example, synchrotron radiation with 170 eV, this influence should be neglected and contrast is mainly due to topography. 3.2. Illumination at the BESSYII-49/II-beamline In order to achieve better lateral resolution and to record difference images, we used synchrotron radiation. We report now on the difference images with energies around the absorption edges of Cu, In, S (and O) for KCN-etched samples.
Fig. 4. Difference images (same position) of Cu, In, and S for a sample after etching, sputtering and annealing up to 390 8C for 10 min. (A) Cu, taken with 935.3 and 930 eV; (B) In with 455 and 430 eV; and (C) S with 175 and 157 eV.
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K. Mu¨ller et al. / Thin Solid Films 480–481 (2005) 291–294
In Fig. 4, the surface of the polycrystalline absorber layer after etching, Ar sputtering (20 min, 4.5 kV), and annealing up to a temperature of 390 8C for 10 min are shown, as recorded by difference images of Cu, In, and S. The images show structures of a size of about 2 Am, this is average grain size. An argument for elemental contrast in single grains can be extracted if we compare the difference images for Cu (Fig. 4A), In (Fig. 4B), and S (Fig. 4C). For Cu, bright and relatively homogeneous areas with dark spots are recorded. For In and S, the situation is reverse, here exactly the dark spots in the Cu image now appear brighter. We conclude that at the surface of KCN-etched CIS, In-rich sulphide seems to be incorporated in a relatively Cu-rich matrix. The surface composition of just-etched CuInS2 films is indeed known to be In-rich, and Scheer and Lewerenz [17] discussed a mixture of CuInS2 and CuIn3S5 in lateral and vertical directions, as predicted by the binary cut Cu2S– In2S3 [21]. Our XPS measurements of etched surfaces shows a ratio close to 0.75, directly positioned on this binary cut in the ternary phase diagram Cu–In–S. Sputtering and annealing lead to almost stoichiometric (CuInS2) XPS ratios of these elements [20]. This situation is reported in Fig. 4. A small amount of relatively In- and S-rich phases (for example, CuIn3S5) here seems to be embedded in a relatively Cu-rich phase (for example, CuInS2).
4. Summary Polycrystalline CuInS2 surfaces are analyzed by photoemission electron microscopy (PEEM) to characterise the elemental distributions between and around individual grains in the submicrometer range using synchrotron radiation. We are able to derive difference PEEM images and demonstrate that the method is able to separate the elemental information from contributions of the surface morphology and to characterise the elemental distribution of KCN-etched polycrystalline CuInS2 samples. We find significant variation in the Cu distribution, which is caused by submicrometer surface-enriched In–S phases, which are placed at the grains and in-between of single grains. This information is a unique feature of the PEEM characterisation, in the lateral integrating XPS spectra and in the depth integrating EDX data, these surfaces appear stoichiometric and homogeneous.
Acknowledgements We like to thank Dr. R. Scheer for providing CuInS2 samples and helpful discussions. The experimental assistance of P. Hoffmann, I. Paloumpa (KCN-etching), and G. Beuckert, as well as the help of the BESSY staff, is acknowledged. In addition, we like to thank R. Sohal for the discussion. This work was supported by DFG under grant no. GEP-SCHM 745/3.
References [1] R. Frfmter, M. Seider, C. Schneider, Ch. Ziethen, W. Swiech, G. Schfnheise, J. Kirschner, BESSY Annual Report 1996, 482–484. [2] H. Rotermund, W. Engel, S. Jakubith, A. von Oertzen, G. Ertl, Ultramicroscopy 36 (1991) 164. [3] R. Mikalo, P. Hoffmann, T. Heller, D. Schmeiger, Solid State Phenom. 63 to 64 (1998) 317. [4] P. Hoffmann, R. Mikalo, D. Schmeiger, M. Kittler, Phys. Status Solidi B 215 (1999) 743. [5] S. Sadewasser, T. Glatzel, S. Schuler, R. Kaigawa, M.C. Lux-Steiner, Thin Solid Films 431 (2003) 257. [6] P. Hoffmann, Aufbau eines Elektronen-Energie-Analysators (AESCA) fqr PEEM, Diploma Thesis, Cottbus, 1998. [7] K. Mqller, Y. Burkov, D. Schmeiger, Thin Solid Films 413–432 (2003) 307. [8] H. Mfller, Semiconductors for Solar Cells, Artech House, Norwood, 1993. [9] B. Blachnik, A. Mqller, Thermochim. Acta 361 (2000) 31. [10] M. Aggour, H.J. Lewerenz, J. Klaer, U. Stfrkel, Electrochem. SolidState Lett. 3 (2000) 399. [11] M. Kleinfeld, H.D. Wiemhffer, Ber. Bunsenges. Phys. Chem. 90 (1986) 711. [12] R. Scheer, H.J. Lewerenz, J. Vac. Sci. Technol., A 13 (1995) 1924. [13] V. Nadenau, D. Hariskos, H.W. Schock, M. Kreijci, F. Haug, A. Tiwari, H. Zogg, G. Kostoroz, J. Appl. Phys. 85 (1999) 534. [14] M. Aggour, U. Stfrkel, C. Murrell, S.A. Campbell, H. Jungblut, P. Hoffmann, R. Mikalo, D. Schmeiger, H.J. Lewerenz, Thin Solid Films 57 (2002) 403. [15] R. Klenk, T. Walter, H.W. Schock, D. Cahen, Adv. Mater. 5 (1993) 114. [16] M. Weber, R. Scheer, H.J. Lewerenz, H. Jungblut, U. Stfrkel, J. Electrochem. Soc. 149 (1) (2002) G77. [17] R. Scheer, H.J. Lewerenz, J. Vac. Sci. Technol., A 13 (4) (1995) 1924. [18] K. Mqller, S. Milko, D. Schmeiger, Thin Solid Films 413–432 (2003) 312. [19] Lit: J. Klaer, J. Bruns, R. Henninger, K. Tfpper, R. Klenk, K. Ellmer, D. Br7uning, Proceedings of the 2nd world Conference on Photovoltaic Solar Energy Conversion, vol. 1, p. 537, European commission, 1998. [20] K. Mqller, R. Scheer, Y. Burkov, D. Schmeiger, Thin Solid Films 451–452 (2004) 120. [21] J. Binsma, L. Giling, J. Bloem, J. Cryst. Growth 50 (1980) 429.