- Email: [email protected]
Electrodeposition and characterisation of CuInSe2 for applications in thin film solar cells
Thin Solid Films 382 Ž2001. 158᎐163
Electrodeposition and characterisation of CuInSe 2 for applications in thin film solar cells K.T.L. De Silvaa , W.A.A. Priyanthaa , J.K.D.S. Jayanetti a , B.D. Chithrani a , W. Siripalab , K. Blake c , I.M. Dharmadasac,U a
Department of Physics, Uni¨ ersity of Colombo, Colombo 3, Sri Lanka b Department of Physics, Uni¨ ersity of Kelaniya, Kelaniya, Sri Lanka c Applied Physics Di¨ ision, Sheffield Hallam Uni¨ ersity, Sheffield S1 1WB, UK Accepted 22 June 2000
Abstract Copper indium diselenide ŽCuInSe 2 . layers have been grown at room temperature by electrochemical deposition technique in an aqueous medium. Resulting thin films have been characterised using XRD, XRF, XPS, GDOES and SEM for structural, stoichiometric and morphological properties. A considerable influence of the deposition potential on the atomic composition of In and Se present in the film was observed. Cu composition remains the same within the deposition potentials used in this investigation. The deposited layers are polycrystalline and annealing at 350⬚C for 30 min improves the crystallinity. The film quality deteriorates due to dissociation when annealed at temperatures above 350⬚C. Excessive annealing results in a surface which is depleted in Cu and rich in In and Se. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrochemistry; Selenides; Semiconductors; Solar cells
1. Introduction Copper indium diselenide ŽCuInSe 2 . thin films have been deposited using various techniques such as elemental co-evaporation w1᎐3x, sputtering w4x, pulsed laser deposition w5x and electrodeposition w6᎐8x. The thin film photovoltaic devices prepared by electrodeposited CuInSe 2 materials has reached efficiency of 7.9% w7x. Addition of Ga into the material and combination of growth techniques Želectrodeposition and co-evaporation. has produced even higher efficiencies up to 14.1% w9x. The latest news release from the National Center for Photovoltaics ŽNCPV. in US reports an efficiency of
U
Corresponding author. Tel.: q44-114-2534067; fax: q44-1142533066. E-mail address: [email protected] ŽI.M. Dharmadasa..
18.8% for vacuum evaporated CuInGaSe 2 based devices w10x. The electrodeposition method has been demonstrated as a method for producing films over a large area and could easily be scaled up to a commercial process at a lower cost. Introduction of Ga, and hence the development of CuŽIn,Ga.Se 2 material and the related devices can be easily achieved using the low-cost electrodeposition technique. This paper presents, as an initial step, the composition control of electrodeposited CuInSe 2 by applied cathode potential and by concentration of elements in the electrolyte. Bulk structural and stoichiometric properties of thin films were studied using XRD, XRF, XPS and GDOES techniques. Surface morphology was studied using SEM technique. Effects of annealing treatments have also been investigated and these results are presented and discussed in this communication.
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 1 8 5 - 8
K.T.L. De Sil¨ a et al. r Thin Solid Films 382 (2001) 158᎐163
159
2. Experimental The CuInSe 2 thin films used in this study were prepared by electrodeposition on titanium substrates. The substrates were cleaned in acetone using an ultrasonic bath and rinsed thoroughly with distilled water. The constituents of the electrolyte used for the electrodeposition were aqueous solutions of 0.005 M CuCl 2 , 0.005 M InCl 3 and 0.01 M SeO 2 . The pH of the electrolyte was adjusted to 1.5 with the addition of hydrochloric acid w6x. A carbon plate was used as the anode and was placed 3 cm away from the working electrode. The materials were deposited at room temperature with continuous stirring using a magnetic stirrer. The investigated CuInSe 2 thin films were prepared with different CurIn mixing ratios at different deposition potentials varying from y500 mV to y900 mV vs. saturated calomel electrode ŽSCE., using a PAR-363 potentiostat. Typical film thicknesses were in the order of 1 m. The composition of the samples was measured using X-ray fluorescence analysis system ŽSEIKO, SEA 2010L.. The samples were annealed in air or an argon ambient at different temperatures using CHINO Žmodel 363. specimen heating unit mounted on the X-ray diffractometer ŽSHIMADZU, model XD-D1.. The bulk crystallographic properties of the films were determined in situ as a function of annealing temperature. XPS spectra were recorded using a VG-5000 ˚ . propermicrolab for monitoring near surface Ž- 30 A ties. Glow discharge optical emission spectroscopy ŽGDOES. profiles w11᎐13x of the thin films were obtained from LECO GDS-750 QDP glow discharge spectrometer in order to monitor the uniformity of CuInSe 2 film composition with the depth. The surface morphology of the films was studied with a Topcon ABT-32 scanning electron microscope. 3. Results and discussion Fig. 1a shows the X-ray diffraction pattern of an as-deposited CuInSe 2 film on Ti substrate. The characteristic peaks of the chalcopyrite structure Ž112., Ž220. and Ž116. are present, but they are broad and weak. Fig. 1b,c show the X-ray patterns of samples annealed at 200⬚C and 350⬚C for 30 min, respectively, with air as the annealing ambient. Increase in the reflections and hence the improvement of crystallinity of the films were observed, as the annealing temperature was increased up to 350⬚C. However, if the samples were annealed at temperatures above 350⬚C, there was a drop of CuInSe 2 peak Ž112. intensity and additional peaks appeared in the XRD spectrum indicating the formation of other compounds such as In 2 O 3 and CuO. Fig. 1d shows diffraction pattern of a thin film annealed in air at 500⬚C for 30 min. This spectrum
Fig. 1. XRD spectra obtained in situ for CuInSe 2 films grown on Ti substrates as a function of annealing temperature with air as the annealing ambient: Ža. as-deposited; Žb. annealed at 200⬚C; Žc. annealed at 350⬚C; and Žd. annealed at 500⬚C, for 30 min.
shows X-ray diffraction peaks corresponding to the above two native oxides, and the peaks corresponding to CuInSe 2 have completely disappeared. The peak at 38.5⬚ appears for all CuInSe 2 films and the intensity gradually increases with annealing in air. This peak corresponds to reflections from CuO.3H 2 O Ž015. planes and indicates the presence of this compound in as-deposited films and enhancement upon annealing. The peaks appearing at 23.5⬚ and 29.5⬚ after annealing at 200⬚C correspond to SeO 2 and disappear when annealed at 350⬚C. This compound appear to form at mild annealing and then disappear at high temperature annealing. The formation of CuInSe 2 is strong when annealed at 350⬚C but completely dissociates at temperatures ; 500⬚C. Sublimation and dissociation of CuInSe 2 , formation of elemental oxides ŽIn 2 O 3 , CuO and Se xO y . and their evaporation can take place at these high temperature annealing. As a result, the reflections from the titanium substrate have increased for this situation. The results obtained for CuInSe 2 films annealed in
160
K.T.L. De Sil¨ a et al. r Thin Solid Films 382 (2001) 158᎐163
argon environment are shown in Fig. 2. Relative intensities of XRD peaks of CuInSe 2 show better improvement compared to the films annealed in air. It is interesting to note the absence of two peaks arising from SeO 2 that was observed for materials annealed in air at 200⬚C Žsee Fig. 1b.. CuInSe 2 peak Ž112. can be observed even after annealing at 500⬚C in agreement with previously reported results w14x without forming native oxides as expected. The presence of CuO ⭈ 3H 2 O compound in the film is shown by the peak at 38.5⬚ and the material crystallinity improves with annealing. Strong Ti peak at high temperature annealing indicates the loss of material possibly through sublimation. The XRF measurements show that the deposition potential influences the atomic composition of In and Se present in the film. Fig. 3 shows the XRF results for deposition potentials in the range from y0.5 to y0.9 V. These results were obtained for an electrolyte containing Cu:In:Se atomic ratio of 1:5:2. Similar results
Fig. 2. XRD spectra obtained in situ for CuInSe 2 films grown on Ti substrates as a function of annealing temperature with argon as the annealing ambient: Ža. as-deposited; Žb. annealed at 200⬚C; Žc. annealed at 350⬚C; and Žd. annealed at 500⬚C, for 30 min.
Fig. 3. The trend in composition variation of CuInSe 2 films as a function of deposition voltage. Elemental ratio of the electrolyte was kept at 1:5:2 during this investigation.
have been obtained for other ratios of 1:1:2, 1:2:2, 1:3:2 and 1:4:2 indicating the less influence of electrolyte composition on material growth within the concentration range studied in this work. The indium content gradually increases while the composition of Se gradually decreases when the negative values of the deposition potential is increased. There is no noticeable variation in the Cu concentration with the increase in deposition potential. It has been found that with annealing at 350⬚C in argon, the atomic composition of the samples move towards the stoichiometric values. The XRF data also indicates the richness of both Cu and Se in the electrodeposited layers. This suggests the possibility of formation of more than one phase within the material layer. It is more likely that the two phases are CuInSe 2 and Cu 2 Se. Both these phases give rise to very similar XRD peak positions thus making it difficult to distinguish. XPS studies carried out on CuInSe 2 layers show the chemical and stoichiometric changes occurring at the ˚ top surface layers within the probing depth of ; 30 A. Fig. 4 shows the three important regions; Cu 2p, In 3d and Se 3d for both as-deposited and annealed layers. Panels Ža., Žb. and Žc. present the emissions from three elements of the as-deposited layers and the presence of oxides of all three elements are evident as expected. The binding energy shift of In 2 O 3 Ž; 0.6 eV. is comparable with the resolution of the measurement system and therefore the oxide peak is not observed separately for the case of In 3d emission. However, the broadening of this peak indicates the presence of In 2 O 3 on the surface. Peaks labelled as A and B are shake up satellite lines due to emission from copper oxides. Annealing of the films in air at 350⬚C increases the amount of In and Se oxides and decreases the Cu and Cu-oxide emissions drastically Žsee panels a⬘, b⬘, and c⬘ of Fig. 4 and Table 1.. The reduction of Cu-oxide emission may possibly be due to formation of more stable In 2 O 3 and Se 2 O 3 and therefore due to masking
K.T.L. De Sil¨ a et al. r Thin Solid Films 382 (2001) 158᎐163
161
Fig. 4. XPS spectra of Cu 2p Ža., In 3d Žb. and Se 3d Žc. regions for as-deposited CuInSe 2 layers. Ža⬘., Žb⬘. and Žc⬘. show the corresponding spectra after annealing at 350⬚C for 30 min in air.
effect. It is also possible to have catalytic effects on the surface and one oxide may convert into its metallic form in the presence of other stable oxides. The additional peak ; 75 eV in panel c⬘ arises due to both Cu and In signals and therefore not taken into account in the analysis. Both XRD results and XPS results on the formation of surface oxides provide supporting evidence. However, it should be noted that the XRD spectra would not reveal information from amorphous oxides mostly present on as-deposited layers until they become crystalline upon annealing. Another reason for rapid decrease of Cu signal is segregation of elements
on excessive annealing due to decomposition of the material. In this case elements can redistribute in such a way that the surface is depleted in Cu and rich in In and Se. Further supporting evidence are found during the GDOES profiling experiments which are described in the following section. GDOES profiling experiments have been performed on electrodeposited CuInSe 2 films and Fig. 5a shows a typical spectrum obtained for an as-deposited film. The presence of C on the film surface and at the interface is clearly shown in the spectrum as expected. The sputtering rate is higher at the interface, most probably due to
Table 1 Comparison of XPS emissions from as-deposited and excessively annealed Žover 350⬚C. CuInSe 2 layers a Element
Cu q oxide In q oxide Se q oxide a
As-deposited CuInSe2
Excessively annealed CuInSe2
Area Žkc eVrs.
Height Žcounts.
Area Žkc eVrs.
Height Žcounts.
111 31 26
14 479 4336 5284
47 186 48
9576 24 073 3394
˚ Note the drastic reduction of Cu signal and the increase of In and Se signal from the top probing layer of ; 30 A.
162
K.T.L. De Sil¨ a et al. r Thin Solid Films 382 (2001) 158᎐163
matrix effects and the presence of C and Ti at the interface resulting in a hump in the spectrum for all three elements. Overlapping of Ti signal with Cu, In and Se signals can arise due to Ti surface roughness. Remembering the settling down of plasma at the beginning and the matrix effects, these qualitative depth profiling indicates the presence of CuInSe 2 layers containing its three constituent elements throughout its depth in a fairly even manner. Excessive annealing above 350⬚C, however, can be detrimental for the structural and therefore electrical and optical properties of the material. Fig. 5b shows a GDOES spectra obtained for an annealed sample above 350⬚C. It is very clear that the material has decomposed and copper has mainly segregated in the CuInSe 2rTi interface. There is also a small segregation of Cu on the CuInSe 2 layer and these are shown by the XPS work at a reduced level. This observation is therefore a strong indication of the necessity of annealing step but the detrimental effects it causes beyond the optimum annealing temperature. Fig. 6 shows the scanning electron micrographs obtained from as-deposited films Žpanel a. and films annealed in air at 350⬚C Žpanel b. and 600⬚C Žpanel c..
Fig. 6. Scanning electron micrographs of electrodeposited CuInSe 2 thin films: Ža. for as-deposited material; Žb. for heat treated material in air at 350⬚C; and Žc. for heat treated material in air at 600⬚C.
These experiments were devoted to attempting to observe any overall changes in topography upon annealing. The surface of the film contains cauliflower type features of various sizes up to 6 m. CuInSe 2 films annealed at 600⬚C show cracks, visible as thin lines in Fig. 6c. These may have been caused by thermal stress between the Ti metal substrate and the CuInSe 2 semiconducting film. Bhattacharya et al. w15x have reported that the SEM surface morphology corresponding to Cu 2 Se phase exhibits elongated rice grain type features whereas a cauliflower type resemblance corresponds to layers containing Cu᎐In᎐Se phases, for Cu᎐Se and Cu᎐In᎐Se films grown on Mo substrates using electrodeposition. Therefore, the SEM features shown on Fig. 6 may be an indication of a material with CuInSe 2 as the dominant phase. Fig. 5. Typical GDOES depth profiles obtained for as-deposited Ža. and excessively annealed Žb. CuInSe 2 layers deposited on Ti substrates. Note the presence of uniform semiconducting layer and the effects of Ti surface roughness for the as-deposited layers and decomposition of the material and segregation of Cu towards the interface and to the free surface after heat treatment.
4. Conclusions
XRD measurements reveal that the CuInSe 2 mate-
K.T.L. De Sil¨ a et al. r Thin Solid Films 382 (2001) 158᎐163
rial becomes more crystalline with the increase of annealing temperature up to 350⬚C. Annealing at 200⬚C in air forms crystalline SeO 2 that disappears during annealing at 350⬚C. This could be due to the sublimation of SeO 2 or conversion to a disordered form of selenium oxide. Annealing in air at temperatures over 350⬚C causes decomposition of CuInSe 2 forming native oxides such as In 2 O 3 and CuO. The XRF measurements indicate the considerable influence of deposition potential on the amount of In and Se present in the films. Increasing the deposition potential from y0.5 to y0.9 V vs. SCE leads to a gradual reduction of Se with a gradual increase of In while Cu content remained constant. XPS studies show that the presence of all ˚ . of the three native oxides on the top layer Ž; 30 A as-deposited thin films. Annealing reduces both the Cu and Cu-oxide signals and enhances the signals corresponding to In and Se and their oxides. Formation of Se-oxide is more dominant. GDOES measurements reveal the presence of all three constituent elements throughout the as-deposited CuInSe 2 films. Excessive annealing results in decomposition of CuInSe 2 and segregation of Cu towards the interface and enrichment of In and Se towards the surface layers. Acknowledgements The authors would like to thank Dr Malculm Ives, Dr Anura Samantilleke and Mr Thomas Delsol for helping in some experimental measurements. Financial assistance of the University of Colombo and the British Council, Colombo is gratefully appreciated.
163
References w1x A. Rockett, R.W. Birkmire, J. Appl. Phys. 70 Ž7. Ž1991. R81. w2x M. Nishitani, M. Ikeda, T. Negami, S. Kohoki, N. Kohara, M. Terauchi, H. Wada, T. Wada, Solar Energy Mater. Solar Cells 35 Ž1994. 203. w3x V. Alberts, S. Zweigart, H.W. Schock, Semicond. Sci. Technol. 12 Ž1997. 217. w4x T. Nakada, K. Migita, S. Niki, A. Kunioka, Jpn. J. Appl. Phys. 34 Ž1995. 4715. w5x R. Schaffler, M. Klose, M. Brieger, H. Dittrich, H.W. Schock, Mater. Sci. Forum 173r174 Ž1995. 135. w6x Y. Sudo, S. Endo, T. Irie, Jpn. J. Appl. Phys. 32 Ž1993. 1562. w7x A.M. Fernandez, P.J. Sebastian, R.N. Bhattacharya, R. Noufi, M. Contreras, A.M. Hermann, Semicond. Sci. Technol. 12 Ž1996. 994. w8x J.F. Guillemoles, P. Cowache, A. Lusson, K. Fezzaa, F. Boisivon, J. Vedel, D. Lincot, J. Appl. Phys. 79 Ž9. Ž1996. 7293. w9x R.N. Bhattacharya, W. Batchelor, H. Wiesner, F. Hasoon, J.E. Granata, K. Ramanathan, J. Alleman, J. Keane, A. Mason, R.J. Matson, R.N. Noufi, J. Electrochem. Soc. 145 Ž1998. 3435. w10x M.A. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Hasoon, R. Noufi, Prog. Photovoltaics 7 Ž1999. 311. w11x I.M. Dharmadasa, M. Ives, S.J. Brooks, G.H. France, S.J. Brown, Semicond. Sci. Technol. 10 Ž1995. 369. w12x I.M. Dharmadasa, M. Ives, C. Breen, G. Hill, C.C. Button, J.S. Roberts, M. Hopkinson, Plasma Source Mass Spectrometry; Developments and Applications, The Royal Society of Chemistry, 1997, p. 232. w13x I.M. Dharmadasa, M. Ives, S.J. Brooks, C. Breen, Glow Discharge Optical Emission Spectroscopy, John Wiley & Sons, 1997, p. 668. w14x N. Strativa, E. Tzevtkova, M. Ganchev, K. Kochev, I. Tomov, Solar Energy Mater. Solar Cells 45 Ž1997. 87. w15x R.N. Bhattacharya, A.M. Fernandez, M.A. Contreras, J. Keane, A.L. Tennant, K. Ramanathan, J.R. Tuttle, R.N. Noufi, A.M. Hermann, J. Electrochem. Soc. 143 Ž1996. 854.
Electrodeposition and characterisation of CuInSe 2 for applications in thin film solar cells K.T.L. De Silvaa , W.A.A. Priyanthaa , J.K.D.S. Jayanetti a , B.D. Chithrani a , W. Siripalab , K. Blake c , I.M. Dharmadasac,U a
Department of Physics, Uni¨ ersity of Colombo, Colombo 3, Sri Lanka b Department of Physics, Uni¨ ersity of Kelaniya, Kelaniya, Sri Lanka c Applied Physics Di¨ ision, Sheffield Hallam Uni¨ ersity, Sheffield S1 1WB, UK Accepted 22 June 2000
Abstract Copper indium diselenide ŽCuInSe 2 . layers have been grown at room temperature by electrochemical deposition technique in an aqueous medium. Resulting thin films have been characterised using XRD, XRF, XPS, GDOES and SEM for structural, stoichiometric and morphological properties. A considerable influence of the deposition potential on the atomic composition of In and Se present in the film was observed. Cu composition remains the same within the deposition potentials used in this investigation. The deposited layers are polycrystalline and annealing at 350⬚C for 30 min improves the crystallinity. The film quality deteriorates due to dissociation when annealed at temperatures above 350⬚C. Excessive annealing results in a surface which is depleted in Cu and rich in In and Se. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrochemistry; Selenides; Semiconductors; Solar cells
1. Introduction Copper indium diselenide ŽCuInSe 2 . thin films have been deposited using various techniques such as elemental co-evaporation w1᎐3x, sputtering w4x, pulsed laser deposition w5x and electrodeposition w6᎐8x. The thin film photovoltaic devices prepared by electrodeposited CuInSe 2 materials has reached efficiency of 7.9% w7x. Addition of Ga into the material and combination of growth techniques Želectrodeposition and co-evaporation. has produced even higher efficiencies up to 14.1% w9x. The latest news release from the National Center for Photovoltaics ŽNCPV. in US reports an efficiency of
U
Corresponding author. Tel.: q44-114-2534067; fax: q44-1142533066. E-mail address: [email protected] ŽI.M. Dharmadasa..
18.8% for vacuum evaporated CuInGaSe 2 based devices w10x. The electrodeposition method has been demonstrated as a method for producing films over a large area and could easily be scaled up to a commercial process at a lower cost. Introduction of Ga, and hence the development of CuŽIn,Ga.Se 2 material and the related devices can be easily achieved using the low-cost electrodeposition technique. This paper presents, as an initial step, the composition control of electrodeposited CuInSe 2 by applied cathode potential and by concentration of elements in the electrolyte. Bulk structural and stoichiometric properties of thin films were studied using XRD, XRF, XPS and GDOES techniques. Surface morphology was studied using SEM technique. Effects of annealing treatments have also been investigated and these results are presented and discussed in this communication.
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 1 8 5 - 8
K.T.L. De Sil¨ a et al. r Thin Solid Films 382 (2001) 158᎐163
159
2. Experimental The CuInSe 2 thin films used in this study were prepared by electrodeposition on titanium substrates. The substrates were cleaned in acetone using an ultrasonic bath and rinsed thoroughly with distilled water. The constituents of the electrolyte used for the electrodeposition were aqueous solutions of 0.005 M CuCl 2 , 0.005 M InCl 3 and 0.01 M SeO 2 . The pH of the electrolyte was adjusted to 1.5 with the addition of hydrochloric acid w6x. A carbon plate was used as the anode and was placed 3 cm away from the working electrode. The materials were deposited at room temperature with continuous stirring using a magnetic stirrer. The investigated CuInSe 2 thin films were prepared with different CurIn mixing ratios at different deposition potentials varying from y500 mV to y900 mV vs. saturated calomel electrode ŽSCE., using a PAR-363 potentiostat. Typical film thicknesses were in the order of 1 m. The composition of the samples was measured using X-ray fluorescence analysis system ŽSEIKO, SEA 2010L.. The samples were annealed in air or an argon ambient at different temperatures using CHINO Žmodel 363. specimen heating unit mounted on the X-ray diffractometer ŽSHIMADZU, model XD-D1.. The bulk crystallographic properties of the films were determined in situ as a function of annealing temperature. XPS spectra were recorded using a VG-5000 ˚ . propermicrolab for monitoring near surface Ž- 30 A ties. Glow discharge optical emission spectroscopy ŽGDOES. profiles w11᎐13x of the thin films were obtained from LECO GDS-750 QDP glow discharge spectrometer in order to monitor the uniformity of CuInSe 2 film composition with the depth. The surface morphology of the films was studied with a Topcon ABT-32 scanning electron microscope. 3. Results and discussion Fig. 1a shows the X-ray diffraction pattern of an as-deposited CuInSe 2 film on Ti substrate. The characteristic peaks of the chalcopyrite structure Ž112., Ž220. and Ž116. are present, but they are broad and weak. Fig. 1b,c show the X-ray patterns of samples annealed at 200⬚C and 350⬚C for 30 min, respectively, with air as the annealing ambient. Increase in the reflections and hence the improvement of crystallinity of the films were observed, as the annealing temperature was increased up to 350⬚C. However, if the samples were annealed at temperatures above 350⬚C, there was a drop of CuInSe 2 peak Ž112. intensity and additional peaks appeared in the XRD spectrum indicating the formation of other compounds such as In 2 O 3 and CuO. Fig. 1d shows diffraction pattern of a thin film annealed in air at 500⬚C for 30 min. This spectrum
Fig. 1. XRD spectra obtained in situ for CuInSe 2 films grown on Ti substrates as a function of annealing temperature with air as the annealing ambient: Ža. as-deposited; Žb. annealed at 200⬚C; Žc. annealed at 350⬚C; and Žd. annealed at 500⬚C, for 30 min.
shows X-ray diffraction peaks corresponding to the above two native oxides, and the peaks corresponding to CuInSe 2 have completely disappeared. The peak at 38.5⬚ appears for all CuInSe 2 films and the intensity gradually increases with annealing in air. This peak corresponds to reflections from CuO.3H 2 O Ž015. planes and indicates the presence of this compound in as-deposited films and enhancement upon annealing. The peaks appearing at 23.5⬚ and 29.5⬚ after annealing at 200⬚C correspond to SeO 2 and disappear when annealed at 350⬚C. This compound appear to form at mild annealing and then disappear at high temperature annealing. The formation of CuInSe 2 is strong when annealed at 350⬚C but completely dissociates at temperatures ; 500⬚C. Sublimation and dissociation of CuInSe 2 , formation of elemental oxides ŽIn 2 O 3 , CuO and Se xO y . and their evaporation can take place at these high temperature annealing. As a result, the reflections from the titanium substrate have increased for this situation. The results obtained for CuInSe 2 films annealed in
160
K.T.L. De Sil¨ a et al. r Thin Solid Films 382 (2001) 158᎐163
argon environment are shown in Fig. 2. Relative intensities of XRD peaks of CuInSe 2 show better improvement compared to the films annealed in air. It is interesting to note the absence of two peaks arising from SeO 2 that was observed for materials annealed in air at 200⬚C Žsee Fig. 1b.. CuInSe 2 peak Ž112. can be observed even after annealing at 500⬚C in agreement with previously reported results w14x without forming native oxides as expected. The presence of CuO ⭈ 3H 2 O compound in the film is shown by the peak at 38.5⬚ and the material crystallinity improves with annealing. Strong Ti peak at high temperature annealing indicates the loss of material possibly through sublimation. The XRF measurements show that the deposition potential influences the atomic composition of In and Se present in the film. Fig. 3 shows the XRF results for deposition potentials in the range from y0.5 to y0.9 V. These results were obtained for an electrolyte containing Cu:In:Se atomic ratio of 1:5:2. Similar results
Fig. 2. XRD spectra obtained in situ for CuInSe 2 films grown on Ti substrates as a function of annealing temperature with argon as the annealing ambient: Ža. as-deposited; Žb. annealed at 200⬚C; Žc. annealed at 350⬚C; and Žd. annealed at 500⬚C, for 30 min.
Fig. 3. The trend in composition variation of CuInSe 2 films as a function of deposition voltage. Elemental ratio of the electrolyte was kept at 1:5:2 during this investigation.
have been obtained for other ratios of 1:1:2, 1:2:2, 1:3:2 and 1:4:2 indicating the less influence of electrolyte composition on material growth within the concentration range studied in this work. The indium content gradually increases while the composition of Se gradually decreases when the negative values of the deposition potential is increased. There is no noticeable variation in the Cu concentration with the increase in deposition potential. It has been found that with annealing at 350⬚C in argon, the atomic composition of the samples move towards the stoichiometric values. The XRF data also indicates the richness of both Cu and Se in the electrodeposited layers. This suggests the possibility of formation of more than one phase within the material layer. It is more likely that the two phases are CuInSe 2 and Cu 2 Se. Both these phases give rise to very similar XRD peak positions thus making it difficult to distinguish. XPS studies carried out on CuInSe 2 layers show the chemical and stoichiometric changes occurring at the ˚ top surface layers within the probing depth of ; 30 A. Fig. 4 shows the three important regions; Cu 2p, In 3d and Se 3d for both as-deposited and annealed layers. Panels Ža., Žb. and Žc. present the emissions from three elements of the as-deposited layers and the presence of oxides of all three elements are evident as expected. The binding energy shift of In 2 O 3 Ž; 0.6 eV. is comparable with the resolution of the measurement system and therefore the oxide peak is not observed separately for the case of In 3d emission. However, the broadening of this peak indicates the presence of In 2 O 3 on the surface. Peaks labelled as A and B are shake up satellite lines due to emission from copper oxides. Annealing of the films in air at 350⬚C increases the amount of In and Se oxides and decreases the Cu and Cu-oxide emissions drastically Žsee panels a⬘, b⬘, and c⬘ of Fig. 4 and Table 1.. The reduction of Cu-oxide emission may possibly be due to formation of more stable In 2 O 3 and Se 2 O 3 and therefore due to masking
K.T.L. De Sil¨ a et al. r Thin Solid Films 382 (2001) 158᎐163
161
Fig. 4. XPS spectra of Cu 2p Ža., In 3d Žb. and Se 3d Žc. regions for as-deposited CuInSe 2 layers. Ža⬘., Žb⬘. and Žc⬘. show the corresponding spectra after annealing at 350⬚C for 30 min in air.
effect. It is also possible to have catalytic effects on the surface and one oxide may convert into its metallic form in the presence of other stable oxides. The additional peak ; 75 eV in panel c⬘ arises due to both Cu and In signals and therefore not taken into account in the analysis. Both XRD results and XPS results on the formation of surface oxides provide supporting evidence. However, it should be noted that the XRD spectra would not reveal information from amorphous oxides mostly present on as-deposited layers until they become crystalline upon annealing. Another reason for rapid decrease of Cu signal is segregation of elements
on excessive annealing due to decomposition of the material. In this case elements can redistribute in such a way that the surface is depleted in Cu and rich in In and Se. Further supporting evidence are found during the GDOES profiling experiments which are described in the following section. GDOES profiling experiments have been performed on electrodeposited CuInSe 2 films and Fig. 5a shows a typical spectrum obtained for an as-deposited film. The presence of C on the film surface and at the interface is clearly shown in the spectrum as expected. The sputtering rate is higher at the interface, most probably due to
Table 1 Comparison of XPS emissions from as-deposited and excessively annealed Žover 350⬚C. CuInSe 2 layers a Element
Cu q oxide In q oxide Se q oxide a
As-deposited CuInSe2
Excessively annealed CuInSe2
Area Žkc eVrs.
Height Žcounts.
Area Žkc eVrs.
Height Žcounts.
111 31 26
14 479 4336 5284
47 186 48
9576 24 073 3394
˚ Note the drastic reduction of Cu signal and the increase of In and Se signal from the top probing layer of ; 30 A.
162
K.T.L. De Sil¨ a et al. r Thin Solid Films 382 (2001) 158᎐163
matrix effects and the presence of C and Ti at the interface resulting in a hump in the spectrum for all three elements. Overlapping of Ti signal with Cu, In and Se signals can arise due to Ti surface roughness. Remembering the settling down of plasma at the beginning and the matrix effects, these qualitative depth profiling indicates the presence of CuInSe 2 layers containing its three constituent elements throughout its depth in a fairly even manner. Excessive annealing above 350⬚C, however, can be detrimental for the structural and therefore electrical and optical properties of the material. Fig. 5b shows a GDOES spectra obtained for an annealed sample above 350⬚C. It is very clear that the material has decomposed and copper has mainly segregated in the CuInSe 2rTi interface. There is also a small segregation of Cu on the CuInSe 2 layer and these are shown by the XPS work at a reduced level. This observation is therefore a strong indication of the necessity of annealing step but the detrimental effects it causes beyond the optimum annealing temperature. Fig. 6 shows the scanning electron micrographs obtained from as-deposited films Žpanel a. and films annealed in air at 350⬚C Žpanel b. and 600⬚C Žpanel c..
Fig. 6. Scanning electron micrographs of electrodeposited CuInSe 2 thin films: Ža. for as-deposited material; Žb. for heat treated material in air at 350⬚C; and Žc. for heat treated material in air at 600⬚C.
These experiments were devoted to attempting to observe any overall changes in topography upon annealing. The surface of the film contains cauliflower type features of various sizes up to 6 m. CuInSe 2 films annealed at 600⬚C show cracks, visible as thin lines in Fig. 6c. These may have been caused by thermal stress between the Ti metal substrate and the CuInSe 2 semiconducting film. Bhattacharya et al. w15x have reported that the SEM surface morphology corresponding to Cu 2 Se phase exhibits elongated rice grain type features whereas a cauliflower type resemblance corresponds to layers containing Cu᎐In᎐Se phases, for Cu᎐Se and Cu᎐In᎐Se films grown on Mo substrates using electrodeposition. Therefore, the SEM features shown on Fig. 6 may be an indication of a material with CuInSe 2 as the dominant phase. Fig. 5. Typical GDOES depth profiles obtained for as-deposited Ža. and excessively annealed Žb. CuInSe 2 layers deposited on Ti substrates. Note the presence of uniform semiconducting layer and the effects of Ti surface roughness for the as-deposited layers and decomposition of the material and segregation of Cu towards the interface and to the free surface after heat treatment.
4. Conclusions
XRD measurements reveal that the CuInSe 2 mate-
K.T.L. De Sil¨ a et al. r Thin Solid Films 382 (2001) 158᎐163
rial becomes more crystalline with the increase of annealing temperature up to 350⬚C. Annealing at 200⬚C in air forms crystalline SeO 2 that disappears during annealing at 350⬚C. This could be due to the sublimation of SeO 2 or conversion to a disordered form of selenium oxide. Annealing in air at temperatures over 350⬚C causes decomposition of CuInSe 2 forming native oxides such as In 2 O 3 and CuO. The XRF measurements indicate the considerable influence of deposition potential on the amount of In and Se present in the films. Increasing the deposition potential from y0.5 to y0.9 V vs. SCE leads to a gradual reduction of Se with a gradual increase of In while Cu content remained constant. XPS studies show that the presence of all ˚ . of the three native oxides on the top layer Ž; 30 A as-deposited thin films. Annealing reduces both the Cu and Cu-oxide signals and enhances the signals corresponding to In and Se and their oxides. Formation of Se-oxide is more dominant. GDOES measurements reveal the presence of all three constituent elements throughout the as-deposited CuInSe 2 films. Excessive annealing results in decomposition of CuInSe 2 and segregation of Cu towards the interface and enrichment of In and Se towards the surface layers. Acknowledgements The authors would like to thank Dr Malculm Ives, Dr Anura Samantilleke and Mr Thomas Delsol for helping in some experimental measurements. Financial assistance of the University of Colombo and the British Council, Colombo is gratefully appreciated.
163
References w1x A. Rockett, R.W. Birkmire, J. Appl. Phys. 70 Ž7. Ž1991. R81. w2x M. Nishitani, M. Ikeda, T. Negami, S. Kohoki, N. Kohara, M. Terauchi, H. Wada, T. Wada, Solar Energy Mater. Solar Cells 35 Ž1994. 203. w3x V. Alberts, S. Zweigart, H.W. Schock, Semicond. Sci. Technol. 12 Ž1997. 217. w4x T. Nakada, K. Migita, S. Niki, A. Kunioka, Jpn. J. Appl. Phys. 34 Ž1995. 4715. w5x R. Schaffler, M. Klose, M. Brieger, H. Dittrich, H.W. Schock, Mater. Sci. Forum 173r174 Ž1995. 135. w6x Y. Sudo, S. Endo, T. Irie, Jpn. J. Appl. Phys. 32 Ž1993. 1562. w7x A.M. Fernandez, P.J. Sebastian, R.N. Bhattacharya, R. Noufi, M. Contreras, A.M. Hermann, Semicond. Sci. Technol. 12 Ž1996. 994. w8x J.F. Guillemoles, P. Cowache, A. Lusson, K. Fezzaa, F. Boisivon, J. Vedel, D. Lincot, J. Appl. Phys. 79 Ž9. Ž1996. 7293. w9x R.N. Bhattacharya, W. Batchelor, H. Wiesner, F. Hasoon, J.E. Granata, K. Ramanathan, J. Alleman, J. Keane, A. Mason, R.J. Matson, R.N. Noufi, J. Electrochem. Soc. 145 Ž1998. 3435. w10x M.A. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Hasoon, R. Noufi, Prog. Photovoltaics 7 Ž1999. 311. w11x I.M. Dharmadasa, M. Ives, S.J. Brooks, G.H. France, S.J. Brown, Semicond. Sci. Technol. 10 Ž1995. 369. w12x I.M. Dharmadasa, M. Ives, C. Breen, G. Hill, C.C. Button, J.S. Roberts, M. Hopkinson, Plasma Source Mass Spectrometry; Developments and Applications, The Royal Society of Chemistry, 1997, p. 232. w13x I.M. Dharmadasa, M. Ives, S.J. Brooks, C. Breen, Glow Discharge Optical Emission Spectroscopy, John Wiley & Sons, 1997, p. 668. w14x N. Strativa, E. Tzevtkova, M. Ganchev, K. Kochev, I. Tomov, Solar Energy Mater. Solar Cells 45 Ž1997. 87. w15x R.N. Bhattacharya, A.M. Fernandez, M.A. Contreras, J. Keane, A.L. Tennant, K. Ramanathan, J.R. Tuttle, R.N. Noufi, A.M. Hermann, J. Electrochem. Soc. 143 Ž1996. 854.