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In situ XPS investigations of ion beam hydrogenation of CuInSe2
Thin Solid Films 387 Ž2001. 185᎐188
In situ XPS investigations of ion beam hydrogenation of CuInSe 2 K. Otte a,U , G. Lippolda , D. Hirsch a , T. Chasse ´a , A. Schindler a , M.V. Yakushev b , R.W. Martin b , F. Bigl a a
b
Institut fur Permoserstr. 15, 04318 Leipzig, Germany ¨ Oberflachenmodifizierung, ¨ Department of Physics and Applied Physics, Strathclyde Uni¨ ersity, Glasgow, G4 ONG, UK
Abstract The long term change of the CuInSe 2 surface composition under low energy hydrogen ion beam implantation at an increased substrate temperature was studied with in situ X-ray photoelectron spectroscopy ŽXPS.. A removal of surface oxides as well as of surface contaminations due to the treatment could be observed. After storage in air, no reformation of oxides, such as SeO 2 and In 2 O 3 , was detected. Ion beam hydrogenation causes considerable post-growth changes in the defect population. Initially in p-type CuInSe 2 , the distance between the valence band maximum ŽVBM. and the Fermi level increased. A reactivation of oxygen-passivated VSe , the production of additional selenium vacancies as well as changes in the active VCu population, might explain these observations. At elevated substrate temperatures, the hydrogen ion beam produces an In-rich surface near to Cu 2 In 4 Se 7 with a slightly reduced Se-content, independent of the initial surface composition. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen; Implantation; Diffusion; CuInSe 2 ; XPS; Defects
1. Introduction CuŽIn,Ga.Se 2 is the most prominent candidate for the production of thin film solar cells. The detailed study of the complex defect chemistry and its control during and after growthrdeposition is of crucial importance for further efficiency improvements. The defect population can be modified by various post-growth treatments. For example, extensive studies exist on the air annealingroxidation process of CuŽIn,Ga.Se 2 solar cells; however, there are still contradictory results concerning its influence on the defect properties of the thin film, especially with respect to the addition of Ga and Na w1,2x. One beneficial effect of oxygen is the passivation of positively charged Se vacancies, VSe , at the surfaces. Hence, for a better understanding of the
U
Corresponding author. Tel.: q49-341-2352652; fax: q49-3412352595. E-mail address: [email protected] ŽK. Otte..
oxygen influence, one needs a tool for the controlled removal of oxygen and the possibility of influencing the electrical properties. It is known that hydrogen can be utilized to remove surface oxides from III᎐V semiconductors and CuInSe 2 ŽCIS. w3x. For CIS, this has been achieved by a new gentle method combining low energy ion beam implantation of hydrogen and the diffusion of hydrogen from the implantation zone into the bulk at an elevated substrate temperature w4x. In a previous work, a conductivity type conversion from p- to n-type, as well as the production of an In-rich phase w5x in the surface region, have been reported as effects of hydrogen implantation. This paper presents and discusses the changes in elemental compositions of CuInSe 2 after long term hydrogen exposure studied by in situ XPS. 2. Experimental setup For the ion beam experiments, we used single crys-
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 8 1 4 - 9
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K. Otte et al. r Thin Solid Films 387 (2001) 185᎐188
talline CuInSe 2 , grown by a vertical Bridgman technique. The p-type material was cut into small pieces, polished with 0.05-m Al 2 O 3 powder and annealed Ž350⬚C. in a vacuum for 30 min. After each investigation, a fresh surface was produced by the same procedure. For the ion beam etching and XPS investigations, we utilized a system of ultra high vacuum ŽUHV. chambers connected via a load lock and a transfer chamber with a base pressure greater than 5 = 10y9 mbar allowing consecutive XPS measurements after each etching step. The XPS equipment consists of an X-ray source with a MgrAl twin anode and a concentric hemispherical electrostatic energy analyzer. For most of the investigations, we utilized MgK ␣ radiation Ž1253.6 eV.. The hydrogen implantation was performed with a mass separating multi-grid system ŽJENION RAH-20.. A detailed description of this Kaufman-type ion source and its operation has been explained by Schlemm w6x. In our experiments, we adjusted the ion beam energy and the current density to 300 eV and 25 Arcm2 , respectively. At a working pressure of 8 = 10y4 mbar, the ion beam mass content was approximately q q w x Hq 1 rH 2 rH 3 s 2:3:5 7 . 3. Results In order to evaluate the changes in the composition of the sample and in the valence-band electronic structure due to the ion beam exposure, we measured the In 3d, Se 3d, Cu 3p, Cu 2p, O 1s, C 1s core-level peaks, the Cu LMM Auger peak and the valence band, respectively. We have studied various CIS samples with different bulk compositions. The surface of our Cu-poor CIS samples revealed an InrŽIn q Cu. ratio between 0.7 and 0.75, due to the formation of a composition close to CuIn 3 Se 5 , which is comparable to values from the literature. In addition, it can be seen in Fig. 1 that
Fig. 1. Total elemental composition of a CuInSe 2 sample Ž2% Se-rich. after successive hydrogen ion beam exposure steps. The sample temperature was set to 300⬚C.
the initial surface composition deviates significantly from the composition of the bulk. This observation can be explained mainly by the existence of oxides and, hence, depends on the time of storage of the sample in air. The influence of different storage times, as well as the removal of the surface oxides and contaminations Žduring the beginning of the hydrogen ion beam exposure., have been already presented in a previous publication w3x. In addition, storage of the sample in air after the hydrogen exposure for approximately 1 week did not reveal a regeneration of SeO 2 and In 2 O 3 . Fig. 1 shows one example of the long term changes of the total elemental surface composition of a CuInSe 2 Ž2% Se-rich. sample during hydrogen exposure at 300⬚C sample temperature. The elemental composition reaches an apparent equilibrium after a hydrogen exposure dose of approximately 200 = 10 16 cmy2 within the error limits of the measurement, resulting in an In-rich layer with a composition close to Cu 2 In 4 Se 7 . A simple annealing step in vacuum or in hydrogen atmo-
Fig. 2. Evaluation of the In 3d core level Žleft side. as well as the Cu LMM auger peak Žright side. for a 1% Cu-rich CuInSe 2 sample for different hydrogen exposure times. The spectra of a Cu-sample is also shown for comparison. The temperature of the sample was set to 300⬚C.
K. Otte et al. r Thin Solid Films 387 (2001) 185᎐188
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Fig. 3. The figure on the left hand side shows the total elemental composition Žincluding metallic phases. of a CuInSe 2 sample Ž2.5% Se-rich. after successive hydrogen steps. In addition, the ratio between the metallic In compound ŽIn 0 . with respect to the total In-3d content ŽIn. is given on the right axis. The graph on the right hand side illustrates the evaluation of the Cu LMM auger peak for the same sample. The hydrogen exposure has been performed at room temperature.
sphere Ž8 = 10y4 mbar. does not lead to this change of the surface composition. Thus, the observed changes are caused by the presence of atomic hydrogen, the removal of oxides and hydrogen-induced chemical reactions in the surface region, such as the production of H 2 Se. An evaluation of the core level peaks of this sample did not exhibit any chemical shift Žwith exception of the removal of surface oxides. due to the hydrogen exposure until the surface composition is in the observed apparent equilibrium. However, we observed for this, as well as all other investigated samples, that a further increase of the exposure time Žbeyond equilibrium. results in additional features in the core level as well as in the Auger peaks which can be assigned to metallic In ŽIn 0 . and Cu ŽCu0 . Žone example for a Cu-rich sample is shown in Fig. 2.. In Figs. 2 and 3, we have removed band bending effects in the core level in order to emphasize the changes in peak shapes due to the hydrogen effects. It is interesting to note that the time to establish the apparent equilibrium, as well as the time for creation of the metallic component, seems to be different for all investigated samples. This observation might be explained by two facts: Ž1. the different initial surface composition Žwhich depends strongly on the bulk composition as well as the time of storage in air.; and Ž2. the different intermissions for the XPS-measurement in between each hydrogen exposure step. However, the final surface composition at equilibrium for the sample with the same bulk composition after the hydrogen process is independent of the initial surface composition Žinfluenced by either wet etching or different storage time.. The saturated hydrogen concentration in the implanted depth range is a balance between dose and in-rout-diffusion. The generation of metallic In and Cu is the result of a high hydrogen concentration
within the implantation region. Such surface damage can occur due to chemical reactions and loss of volatile compounds, as well as a damage in the crystal lattice Žcracks, voids., if the implanted hydrogen concentration becomes too high. In order to avoid high concentrations of hydrogen, one has either to decrease the ion current density, to increase the sample temperature Žat least within a certain limit. or to use intermissions during exposure. In fact, by increasing the exposure time in-between each XPS-measurement, the generation of metallic In and Cu was observed earlier. In Fig. 3, one example of the change in total surface composition for a Se-rich sample during hydrogen exposure at room-temperature is shown. For this sample, we chose wet etching with Br᎐MeOH ŽBr s 0.5 %. w8x as a preparation step of the sample prior to insertion into the process chamber. However, as mentioned before, this modified preparation step does not influence the final surface composition. Since at room temperature the hydrogen diffusion is slow, the implanted surface region contains a very high hydrogen concentration which results in its destruction Žformation of metallic In and Cu. already after an exposure dose of 6 = 10 16 cmy2 . In order to prove the conversion of Inq3 ŽIn in CIS. to In 0 Žmetallic In., curve fitting was applied to the In 3d peak and the ratio of In 0rIn was also plotted on the left hand side of Fig. 3 Žthe energetic positions of the single peaks are shown in Fig. 2.. The generation of metallic Cu for the same processing steps can be detected from the Cu-LMM Auger peak on the right hand side of Fig. 3. The investigation of the position of the VBM with respect to the Fermi-level by a linear extrapolation of the leading edge of the valence band to the background level revealed a movement of the Fermi-level towards the conduction band for all investigated samples Žnot shown here. compared to the reference sample after
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hydrogen exposure under increased substrate temperature. The accuracy of this measurement is limited by the energy resolution of the instrument; consequently, we were only able to detect the changes qualitatively. It is known that an oxidation of the sample leads to a passivation of VSe and hence to a downward shift of the Fermi-level w1x. Due to oxide removal during the hydrogenation of the sample, this effect is probably inverted. In addition, the hydrogen exposure leads to a further reduction of the Se concentration at the surface due to the formation of H 2 Se, generating an even higher concentration of donors. As already discussed in previous papers w5,9x, there are also other possible defect reactions following hydrogenation. The hydrogen might substitute VCu , thus passivating shallow acceptors. The resulting Fermi-level shift would destabilize Cu on lattice positions, which again produces additional VCu and H substitutions of VCu , resulting in an increasing Cu deficiency in the surface range w1,2x. In this work, in fact, after hydrogenation, In-rich compositions were always found. These effects might explain the observed shift of the Fermi-level and support the suggestion of a type-conversion of the surface layer after hydrogen exposure. 4. Conclusion The effect of long term hydrogen ion beam exposure of CIS samples have been investigated by in situ XPS. Besides the observed removal of surface oxides and contamination, we found that long-time hydrogenation at an elevated substrate temperature Ž300⬚C. produces, for all investigated samples ŽCu-rich and Se-rich., inde-
pendent of the initial surface composition: Ž1. an equilibrium surface composition close to Cu 2 In 4 Se 7 ; and Ž2. metallic In and Cu after a further increase of the dose. In comparison, implantation at room temperature destroys the surface structure after a few minutes, due to the high concentration of hydrogen in the implantation region. The observed Fermi-level shift towards the conduction band can be explained by considerable post-growth changes in the defect population. High resolution XPS as well as UPS Žultraviolet photoelectron spectroscopy. measurements are under way in order to quantify the shift of the VBM. References w1x U. Rau, D. Braunger, R. Herberholz, H.W. Schock, J.-F. Guillemoles, L. Kronik, D. Cahen, J. Appl. Phys. 86 Ž1999. 497. w2x L. Kronik, U. Rau, J.-F. Guillemoles, D. Braunger, H.-W. Schock, D. Cahen, Thin Solid Films 361 Ž2000. 353. w3x K. Otte, G. Lippold, D. Hirsch, A. Schindler, M.V. Yakushev, R.W. Martin, F. Bigl, 16th European Photovoltaic Solar Energy Conference and Exhibition, 1᎐5 May 2000, Glasgow, UK, Ž2000.. w4x K. Otte, G. Lippold, A. Schindler, F. Bigl, H. Schlemm, M.V. Yakushev, R.D. Tomlinson, IOP Conf. Ser. No 152: Section E: Surfaces and Interfaces, Ž1998. 773. w5x G. Lippold, K. Otte, H. Schlemm, W. Grill, Mater. Res. Soc. Symp. Proc. 513 Ž1998. 269. w6x H. Schlemm, J. Vac. Sci. Technol. A 14 Ž1996. 223. w7x K. Otte, A. Schindler, F. Bigl, H. Schlemm, Rev. Sci. Instrum. 69 Ž1998. 1499. w8x A.J. Nelson, C.R. Schwerdtfeger, G.C. Herdt, D. King, M. Contreras, K. Ramanathan, W.L. O’Brien, Mater. Res. Soc. Symp. Proc. 426 Ž1996. 297. w9x M.V. Yakushev, H. Neumann, R.D. Tomlinson, P. Rimmer, G. Lippold, Cryst. Res. Technol. 29 Ž1994. 417.
In situ XPS investigations of ion beam hydrogenation of CuInSe 2 K. Otte a,U , G. Lippolda , D. Hirsch a , T. Chasse ´a , A. Schindler a , M.V. Yakushev b , R.W. Martin b , F. Bigl a a
b
Institut fur Permoserstr. 15, 04318 Leipzig, Germany ¨ Oberflachenmodifizierung, ¨ Department of Physics and Applied Physics, Strathclyde Uni¨ ersity, Glasgow, G4 ONG, UK
Abstract The long term change of the CuInSe 2 surface composition under low energy hydrogen ion beam implantation at an increased substrate temperature was studied with in situ X-ray photoelectron spectroscopy ŽXPS.. A removal of surface oxides as well as of surface contaminations due to the treatment could be observed. After storage in air, no reformation of oxides, such as SeO 2 and In 2 O 3 , was detected. Ion beam hydrogenation causes considerable post-growth changes in the defect population. Initially in p-type CuInSe 2 , the distance between the valence band maximum ŽVBM. and the Fermi level increased. A reactivation of oxygen-passivated VSe , the production of additional selenium vacancies as well as changes in the active VCu population, might explain these observations. At elevated substrate temperatures, the hydrogen ion beam produces an In-rich surface near to Cu 2 In 4 Se 7 with a slightly reduced Se-content, independent of the initial surface composition. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen; Implantation; Diffusion; CuInSe 2 ; XPS; Defects
1. Introduction CuŽIn,Ga.Se 2 is the most prominent candidate for the production of thin film solar cells. The detailed study of the complex defect chemistry and its control during and after growthrdeposition is of crucial importance for further efficiency improvements. The defect population can be modified by various post-growth treatments. For example, extensive studies exist on the air annealingroxidation process of CuŽIn,Ga.Se 2 solar cells; however, there are still contradictory results concerning its influence on the defect properties of the thin film, especially with respect to the addition of Ga and Na w1,2x. One beneficial effect of oxygen is the passivation of positively charged Se vacancies, VSe , at the surfaces. Hence, for a better understanding of the
U
Corresponding author. Tel.: q49-341-2352652; fax: q49-3412352595. E-mail address: [email protected] ŽK. Otte..
oxygen influence, one needs a tool for the controlled removal of oxygen and the possibility of influencing the electrical properties. It is known that hydrogen can be utilized to remove surface oxides from III᎐V semiconductors and CuInSe 2 ŽCIS. w3x. For CIS, this has been achieved by a new gentle method combining low energy ion beam implantation of hydrogen and the diffusion of hydrogen from the implantation zone into the bulk at an elevated substrate temperature w4x. In a previous work, a conductivity type conversion from p- to n-type, as well as the production of an In-rich phase w5x in the surface region, have been reported as effects of hydrogen implantation. This paper presents and discusses the changes in elemental compositions of CuInSe 2 after long term hydrogen exposure studied by in situ XPS. 2. Experimental setup For the ion beam experiments, we used single crys-
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 8 1 4 - 9
186
K. Otte et al. r Thin Solid Films 387 (2001) 185᎐188
talline CuInSe 2 , grown by a vertical Bridgman technique. The p-type material was cut into small pieces, polished with 0.05-m Al 2 O 3 powder and annealed Ž350⬚C. in a vacuum for 30 min. After each investigation, a fresh surface was produced by the same procedure. For the ion beam etching and XPS investigations, we utilized a system of ultra high vacuum ŽUHV. chambers connected via a load lock and a transfer chamber with a base pressure greater than 5 = 10y9 mbar allowing consecutive XPS measurements after each etching step. The XPS equipment consists of an X-ray source with a MgrAl twin anode and a concentric hemispherical electrostatic energy analyzer. For most of the investigations, we utilized MgK ␣ radiation Ž1253.6 eV.. The hydrogen implantation was performed with a mass separating multi-grid system ŽJENION RAH-20.. A detailed description of this Kaufman-type ion source and its operation has been explained by Schlemm w6x. In our experiments, we adjusted the ion beam energy and the current density to 300 eV and 25 Arcm2 , respectively. At a working pressure of 8 = 10y4 mbar, the ion beam mass content was approximately q q w x Hq 1 rH 2 rH 3 s 2:3:5 7 . 3. Results In order to evaluate the changes in the composition of the sample and in the valence-band electronic structure due to the ion beam exposure, we measured the In 3d, Se 3d, Cu 3p, Cu 2p, O 1s, C 1s core-level peaks, the Cu LMM Auger peak and the valence band, respectively. We have studied various CIS samples with different bulk compositions. The surface of our Cu-poor CIS samples revealed an InrŽIn q Cu. ratio between 0.7 and 0.75, due to the formation of a composition close to CuIn 3 Se 5 , which is comparable to values from the literature. In addition, it can be seen in Fig. 1 that
Fig. 1. Total elemental composition of a CuInSe 2 sample Ž2% Se-rich. after successive hydrogen ion beam exposure steps. The sample temperature was set to 300⬚C.
the initial surface composition deviates significantly from the composition of the bulk. This observation can be explained mainly by the existence of oxides and, hence, depends on the time of storage of the sample in air. The influence of different storage times, as well as the removal of the surface oxides and contaminations Žduring the beginning of the hydrogen ion beam exposure., have been already presented in a previous publication w3x. In addition, storage of the sample in air after the hydrogen exposure for approximately 1 week did not reveal a regeneration of SeO 2 and In 2 O 3 . Fig. 1 shows one example of the long term changes of the total elemental surface composition of a CuInSe 2 Ž2% Se-rich. sample during hydrogen exposure at 300⬚C sample temperature. The elemental composition reaches an apparent equilibrium after a hydrogen exposure dose of approximately 200 = 10 16 cmy2 within the error limits of the measurement, resulting in an In-rich layer with a composition close to Cu 2 In 4 Se 7 . A simple annealing step in vacuum or in hydrogen atmo-
Fig. 2. Evaluation of the In 3d core level Žleft side. as well as the Cu LMM auger peak Žright side. for a 1% Cu-rich CuInSe 2 sample for different hydrogen exposure times. The spectra of a Cu-sample is also shown for comparison. The temperature of the sample was set to 300⬚C.
K. Otte et al. r Thin Solid Films 387 (2001) 185᎐188
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Fig. 3. The figure on the left hand side shows the total elemental composition Žincluding metallic phases. of a CuInSe 2 sample Ž2.5% Se-rich. after successive hydrogen steps. In addition, the ratio between the metallic In compound ŽIn 0 . with respect to the total In-3d content ŽIn. is given on the right axis. The graph on the right hand side illustrates the evaluation of the Cu LMM auger peak for the same sample. The hydrogen exposure has been performed at room temperature.
sphere Ž8 = 10y4 mbar. does not lead to this change of the surface composition. Thus, the observed changes are caused by the presence of atomic hydrogen, the removal of oxides and hydrogen-induced chemical reactions in the surface region, such as the production of H 2 Se. An evaluation of the core level peaks of this sample did not exhibit any chemical shift Žwith exception of the removal of surface oxides. due to the hydrogen exposure until the surface composition is in the observed apparent equilibrium. However, we observed for this, as well as all other investigated samples, that a further increase of the exposure time Žbeyond equilibrium. results in additional features in the core level as well as in the Auger peaks which can be assigned to metallic In ŽIn 0 . and Cu ŽCu0 . Žone example for a Cu-rich sample is shown in Fig. 2.. In Figs. 2 and 3, we have removed band bending effects in the core level in order to emphasize the changes in peak shapes due to the hydrogen effects. It is interesting to note that the time to establish the apparent equilibrium, as well as the time for creation of the metallic component, seems to be different for all investigated samples. This observation might be explained by two facts: Ž1. the different initial surface composition Žwhich depends strongly on the bulk composition as well as the time of storage in air.; and Ž2. the different intermissions for the XPS-measurement in between each hydrogen exposure step. However, the final surface composition at equilibrium for the sample with the same bulk composition after the hydrogen process is independent of the initial surface composition Žinfluenced by either wet etching or different storage time.. The saturated hydrogen concentration in the implanted depth range is a balance between dose and in-rout-diffusion. The generation of metallic In and Cu is the result of a high hydrogen concentration
within the implantation region. Such surface damage can occur due to chemical reactions and loss of volatile compounds, as well as a damage in the crystal lattice Žcracks, voids., if the implanted hydrogen concentration becomes too high. In order to avoid high concentrations of hydrogen, one has either to decrease the ion current density, to increase the sample temperature Žat least within a certain limit. or to use intermissions during exposure. In fact, by increasing the exposure time in-between each XPS-measurement, the generation of metallic In and Cu was observed earlier. In Fig. 3, one example of the change in total surface composition for a Se-rich sample during hydrogen exposure at room-temperature is shown. For this sample, we chose wet etching with Br᎐MeOH ŽBr s 0.5 %. w8x as a preparation step of the sample prior to insertion into the process chamber. However, as mentioned before, this modified preparation step does not influence the final surface composition. Since at room temperature the hydrogen diffusion is slow, the implanted surface region contains a very high hydrogen concentration which results in its destruction Žformation of metallic In and Cu. already after an exposure dose of 6 = 10 16 cmy2 . In order to prove the conversion of Inq3 ŽIn in CIS. to In 0 Žmetallic In., curve fitting was applied to the In 3d peak and the ratio of In 0rIn was also plotted on the left hand side of Fig. 3 Žthe energetic positions of the single peaks are shown in Fig. 2.. The generation of metallic Cu for the same processing steps can be detected from the Cu-LMM Auger peak on the right hand side of Fig. 3. The investigation of the position of the VBM with respect to the Fermi-level by a linear extrapolation of the leading edge of the valence band to the background level revealed a movement of the Fermi-level towards the conduction band for all investigated samples Žnot shown here. compared to the reference sample after
188
K. Otte et al. r Thin Solid Films 387 (2001) 185᎐188
hydrogen exposure under increased substrate temperature. The accuracy of this measurement is limited by the energy resolution of the instrument; consequently, we were only able to detect the changes qualitatively. It is known that an oxidation of the sample leads to a passivation of VSe and hence to a downward shift of the Fermi-level w1x. Due to oxide removal during the hydrogenation of the sample, this effect is probably inverted. In addition, the hydrogen exposure leads to a further reduction of the Se concentration at the surface due to the formation of H 2 Se, generating an even higher concentration of donors. As already discussed in previous papers w5,9x, there are also other possible defect reactions following hydrogenation. The hydrogen might substitute VCu , thus passivating shallow acceptors. The resulting Fermi-level shift would destabilize Cu on lattice positions, which again produces additional VCu and H substitutions of VCu , resulting in an increasing Cu deficiency in the surface range w1,2x. In this work, in fact, after hydrogenation, In-rich compositions were always found. These effects might explain the observed shift of the Fermi-level and support the suggestion of a type-conversion of the surface layer after hydrogen exposure. 4. Conclusion The effect of long term hydrogen ion beam exposure of CIS samples have been investigated by in situ XPS. Besides the observed removal of surface oxides and contamination, we found that long-time hydrogenation at an elevated substrate temperature Ž300⬚C. produces, for all investigated samples ŽCu-rich and Se-rich., inde-
pendent of the initial surface composition: Ž1. an equilibrium surface composition close to Cu 2 In 4 Se 7 ; and Ž2. metallic In and Cu after a further increase of the dose. In comparison, implantation at room temperature destroys the surface structure after a few minutes, due to the high concentration of hydrogen in the implantation region. The observed Fermi-level shift towards the conduction band can be explained by considerable post-growth changes in the defect population. High resolution XPS as well as UPS Žultraviolet photoelectron spectroscopy. measurements are under way in order to quantify the shift of the VBM. References w1x U. Rau, D. Braunger, R. Herberholz, H.W. Schock, J.-F. Guillemoles, L. Kronik, D. Cahen, J. Appl. Phys. 86 Ž1999. 497. w2x L. Kronik, U. Rau, J.-F. Guillemoles, D. Braunger, H.-W. Schock, D. Cahen, Thin Solid Films 361 Ž2000. 353. w3x K. Otte, G. Lippold, D. Hirsch, A. Schindler, M.V. Yakushev, R.W. Martin, F. Bigl, 16th European Photovoltaic Solar Energy Conference and Exhibition, 1᎐5 May 2000, Glasgow, UK, Ž2000.. w4x K. Otte, G. Lippold, A. Schindler, F. Bigl, H. Schlemm, M.V. Yakushev, R.D. Tomlinson, IOP Conf. Ser. No 152: Section E: Surfaces and Interfaces, Ž1998. 773. w5x G. Lippold, K. Otte, H. Schlemm, W. Grill, Mater. Res. Soc. Symp. Proc. 513 Ž1998. 269. w6x H. Schlemm, J. Vac. Sci. Technol. A 14 Ž1996. 223. w7x K. Otte, A. Schindler, F. Bigl, H. Schlemm, Rev. Sci. Instrum. 69 Ž1998. 1499. w8x A.J. Nelson, C.R. Schwerdtfeger, G.C. Herdt, D. King, M. Contreras, K. Ramanathan, W.L. O’Brien, Mater. Res. Soc. Symp. Proc. 426 Ž1996. 297. w9x M.V. Yakushev, H. Neumann, R.D. Tomlinson, P. Rimmer, G. Lippold, Cryst. Res. Technol. 29 Ž1994. 417.