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Preparation and structure of annealed CuInSe2 electrodeposited films
Thin Solid Films 311 Ž1997. 101–106
Preparation and structure of annealed CuInSe 2 electrodeposited films E. Tzvetkova a
a,)
, N. Stratieva a , M. Ganchev a , I. Tomov b, K. Ivanova a , K. Kochev
a
Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, Sofia, Bulgaria b Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria Received 16 September 1996; accepted 23 April 1997
Abstract Thin CuInSe 2 films were prepared by one-step electrodeposition process. The deposition was done in potentiostatic regime from an original electrolyte containing Cuq, In3q, Se 4q ions and thiocyanate as a complexing agent. The influence of deposition parameters Želectrolyte composition and concentration, temperature of the electrolyte and deposition potential. on film composition was studied. Technological parameters for preparation of films with a desired composition were found. The as-deposited films were heat treated in Ar and in Se ambient. Morphology and structure of as-deposited and of annealed films were investigated. It was established that Se treatment was more effective than the annealing in Ar in addition to crystallite size. Phase formation after Se treatment was elucidated. It was found that composition of as-deposited films mainly conditioned phase composition after the selenization. q 1997 Elsevier Science S.A. Keywords: Chalcogens; Heat treatment; Semiconductors; X-ray diffraction
1. Introduction Thin CuInSe 2 films with chalcopyrite structure are considered as the most promising material for terrestrial photovoltaic applications w1x. One of the prospective methods for large scale preparation of CuInSe 2 is electrodeposition from aqueous solution w2x. Electrodeposition of ternary compounds has many problems connected mainly with a control of electrolyte composition and a choice of suitable deposition potential Ed and pH of the electrolyte. Especially CuInSe 2 electrodeposition is rather difficult because of some peculiarities of the system such as high energy of interaction of Se with Cu and In and quite different individual deposition potentials of the elements. The individual deposition potentials can be brought closer either by using surface active substances or by introducing a complexing agent in the electrolyte w3,4x. Usually as-electrodeposited films are small grain size and poor adherent. To improve their morphology and crystallinity usually they need heat treatment in different ambient w5–7x. Of particular interest is phase formation during such annealing.
)
Corresponding author.
0040-6090r97r$17.00 q 1997 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 2 6 3 - 0
Our paper presents the results from the investigations of the processes taking place during the electrodeposition and heat treatment in Ar and Se atmosphere of CuInSe 2 thin films. 2. Preparation of CuInSe 2 films CuInSe 2 films were prepared by one-step electrodeposition process from an electrolyte containing a complexing agent. Glass plates covered with sputtered Mo Ž5–8 m Vrcm. or conductive glass of 2 cm2 area were used as substrates. The conductive glass was prepared by a spray technique based on pyrolytic decomposition of SnCl 4 P 5H 2 O alcohol solution and doping ions introduced from NH 4 F and fluorinehydrogen acid. The specific resistance of the deposits was about 50 Vrcm w8x. CuInSe 2 films were deposited in a 100-ml glass cell in unstirred bath using the potentiostatic method with a three electrode arrangement. All potentials were referred to a saturated calomel electrode ŽSCE.. A Pt foil with area five times larger than that of the substrates was used as an anode. The plating bath for codeposition was prepared dissolving calculated quantities of CuCl, In 2 ŽSO4 . 3 and SeO 2 at a total concentration of 8.5 mM Žbut in different ion ratios. in aqueous solution. The bath pH was adjusted to pH s 5 by adding 0.4 M acetate buffer. Thiocyanate of
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concentration 2 M was used as a complexing agent. It formed a stable complex with cuprous ions and no efficient complexing with indium ions was observed at the same time. The main distinction from previously reported CuInSe 2 electroplating processes w4,9–11x is that we have used Cu ions of the first valence and thiocyanate as a complexing agent. Several electrolytes with different composition were investigated in order to establish the dependence of film composition on electrolyte content and on deposition potential. The results presented in Fig. 1 are related to three different electrolyte compositions ŽCu:In:Se in at.%.: el1 — 32.5:36.8:30.7el2 — 18.9:34.1:46.9 and el5 — 26.6:40.3:33.1. As it is seen, the data for electrolyte composition and for the composition of the films prepared from the corresponding electrolyte lie on one and the same line. The latter is parallel to the tie lines which correspond to the constant Cu content in the deposits. This means that Cu content in the deposits does not practically depend on deposition potential Ed and approximately equals Cu percentage concentration in the solution. When cathodic potential changes from y0.7 V to y1 V the content of In gradually increases while Se content gradually decreases. At the same time the film composition approaches the composition of the electrolyte. It is obvious that for the deposition of films with preliminary desired composition the electrolyte composition and the electrode potential have to be considered. For example, two marginal possibilities are available for the preparation of stoichiometric CuInSe 2 films: Ž1. at low cathodic potential Žabout y0.7 V. and electrolyte composition 26.6 at.% Cu, 40.3 at.% In, 33.1 at.% Seand Ž2. at high cathodic Žabout y1 V. and 25.7 at.% Cu, 31.6 at.% In, 42.7 at.% Se electrolyte composition Žall other technological parameters being one and the same.. SEM photographs of stoichiometric CIS films, deposited at above
mentioned conditions are shown in Fig. 2a Žfor case 1. and Fig. 2b Žfor case 2.. The comparison of the photographs shows clearly the influence of the deposition potential on film morphology. In the first case the deposit is dendritic, the grain size being less than 0.1–0.2 m m. In the second case the films are cracked and not homogeneous and dense, although with bigger grain size. The influence of the deposition time on the film composition is presented in Fig. 3 where the changes of the three components in the film versus deposition time are shown. The curves point out that the concentration of the components practically does not change when the deposition time varies from 30 to 120 min and approaches the stoichiometric one. For times less than 30 min the samples have composition close to indium selenides with Cu deficiency. This tendency is typical for the investigated Ed range and means that the crystallization process starts from indium selenide-rich composition and than moves towards stoichiometric CuInSe 2 . To increase the deposition rate the influence of solution temperature and of concentration of electroactive species ŽCuq, In3q and SeO 32y . on film composition was investigated. As mentioned above, at room temperature Cu content in the deposits is nearly the same as that in the electrolyte regardless of deposition potential. When the electrolyte temperature is increased up to 608C the deposition rate is a 1, 5–2 times higher and the Cu content in the layers increases by 2–3%. If the total concentration of the electrolyte increases more than three times, the film morphology deteriorates. The stirring of the electrolyte causes similar detrimental effect on it. At the same time significantly bigger deviations of the In and Se contents in the films for the corresponding deposition potential are observed.
3. Film characterization
Fig. 1. The dependence of film composition on the deposition potential and electrolyte composition. Film composition vs. Ed : v –v- for el1; B–B- for el2; '–'- for el5 Ž2 M KCNS in 0.4 M acetate buffer, pH s 5, 8.5 mM total concentration of Cuq, In3q and Se 4q but in different ion ratios, 258C..
Detailed X-ray diffraction studies were carried out for phase identification and texture characterization of the deposits. They were made by Philips X-ray diffractometer. The CuK a radiation was selected by a secondary graphite monochromator. A comparison with JCPDS file cards was done for the establishing the observed peaks. The Ar heat treatment experiments were carried out in Ar gas flow. A quartz tube with the samples in it was placed in the central zone of a furnace. The annealing went on from 15 min to 120 min in the temperature range 3508–5508C in steps of 508C. The selenization was done in a two-zone furnace coupled with temperature controller, quartz tube supplied with Se source, sample holder and vacuum system. The annealing of the samples proceeded out at 2508–5808C for 30 min. The cooling to room temperature took place 2 h. Cu-rich, In-rich and stoichiometric films were studied. Their typical compositions are listed in Table 1.
E. TzÕetkoÕa et al.r Thin Solid Films 311 (1997) 101–106
103
Fig. 2. SEM images showing the surface view of CuInSe 2 samples: Ža. and Žb. unannealed stoichiometric films deposited at different conditions Žsee text.; Žc. annealed in Ar; and Žd. annealed in Se ambient.
The investigated as-deposited films are characterized with small grain size and rough surface ŽFig. 2a.. The X-ray diffraction patterns of all as-deposited CuInSe 2 lay-
ers present weak and broad reflections from Ž112., Ž204, 220. and Ž116, 312. planes of CuInSe 2 ŽFig. 4a.. These three peaks correspond to most intensive peaks of both chalcopyrite and sphalerite CuInSe 2 phases. Such broad peaks are supposed to be due to pseudocrystalline CuInSe 2 phase. To make better film morphology and structural properties, the samples have been heat treated in Ar or Se ambient. The comparison of SEM photographs of Ar-treated and Se-treated films shows that the annealing results in an increase of the grain size in the both cases. More effective is the annealing in Se atmosphere ŽFig. 2d. than that in Ar
Table 1
Fig. 3. Variation of film composition with deposition time Ž Ed sy1 V SCE..
Sample
Cu Žat.%.
In Žat.%.
Se Žat.%.
Cu-rich Stoichiometric In-rich
41 25.3 27.5
21.7 25.7 31.7
37.3 49 40.8
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Fig. 4. X-ray pattern of Cu-rich samples: Ža. for as-deposited sample; Žb. for sample treated in Ar atm; Žc. treated in Se atm.
ŽFig. 2c.. The value of the grain size of Se treated films is about one order of magnitude bigger that of Ar treated ones. Of considerable significance for the crystallization is the film composition. It has been established that Cu-rich films favoured the formation of structure with bigger crystallites in a higher extent than In-rich ones. The biggest crystallite dimensions have been observed for Cu-rich films annealed in Se ambient. The average crystallite size amounts 1–3 m m. The improved film crystallinity after annealing is confirmed, also, by the comparison of X-ray diffraction patterns of heat treated samples. Fig. 4b,c shows X-ray diffraction pictures of Cu-rich films annealed in Ar and in Se, respectively. It is seen that the reflections from Ž112., Ž204, 220. and Ž116, 312. planes of Se-treated CuInSe 2 are sharper and stronger that those of Ar-treated ones. This indicates significant enlargement of the crystallite size and the volume fraction of crystalline phase in the Se-treated films. Films annealed at 3508C in Ar ambient display the mentioned above reflections ŽFig. 5a.. The intensity of X-ray diffraction lines increases with the rise of the annealing temperature ŽFig. 5b,c.. At the same time Ž112. preferred orientation is observed. Considerable variation of the composition has not been observed in the Ar treated samples. X-ray diffraction pattern of such samples exhibit peaks of CuInSe 2 only. But for samples deposited at high electronegative potentials Žbelow y1 V vs. SCE. and treated at temperatures higher than 4508C peak of In 2 O 3 at 2q s 30.88 is registered ŽFig. 5b,c.. As it might be expected the high cathodic potentials produce hydrogen evolution. The latter causes alkalization of the electrolyte near the cathodic space and formation of
indium hydroxide occurs. Probably, the annealing at temperatures above 4508C leads to its dehydration to In 2 O 3 . Quite different results were obtained for Se-treated films. Film composition varied after the selenization as Se content had increased up to 10%. At the same time, changes in the phase composition in X-ray diffraction pictures were observed. That is why detailed investigations were carried out of films heat treated in Se atmosphere varying Se pressure, temperature and duration of the annealing process. For each Se pressure, the annealing time was varied from 15 min to 120 min. X-ray patterns show that considerable changes occur for 30 min annealing. After further increase of the annealing time essential differences are not observed in the X-ray pictures. X-ray diffraction patterns of Cu-rich, stoichiometric and In-rich as-deposited films annealed at Se-pressure 2.10y1 atm are shown in Fig. 6. All pictures exhibit well crystallized CuInSe 2 . The peaks are sharp and narrow which indicates for crystalline size larger than 1 m m as the SEM photographs confirm, too Žsee Fig. 2d.. Cu-rich films are characterized by dominated presence of Cu 3 Se 2 and reflections of CuSe 2 and possibly Cu 2 Se. The predominant orientation of CuInSe 2 is Ž204, 220. ŽFig. 6a.. For stoichiometric films CuInSe 2 orientation changes from Ž204, 220. to Ž112. ŽFig. 6b. and negligible traces of a-CuSe are observed. In the both cases chalcopyrite phase of CuInSe 2 undoubtedly exists because weak reflections from Ž101., Ž103. and Ž211. planes are observed, but the sphalerite phase could, also, be present. The X-ray pattern of In-rich films exhibits ten peaks of CuInSe 2 including characteris-
Fig. 5. X-ray diffraction patterns for CuInSe 2 films deposited at y0.9 V and annealed for 120 min in Ar atm: Ža. at 3508C; Žb. at 4508C; Žc. at 5508C; ŽB. CuInSe 2 ; Ž^. SnO 2 ; Ž`. In 2 O 3 .
E. TzÕetkoÕa et al.r Thin Solid Films 311 (1997) 101–106
105
Fig. 6. X-ray patterns of CuInSe 2 samples annealed at 5808C in 2.10y1 atm Se: Ža. Cu-rich; Žb. stoichiometric; Žc. In-rich.; Ž`. InSe; Ž'. Cu 2 Se; Ž=. CuSe 2 ; ŽI. Cu 3 Se 2 ; Ž^. a-CuSe.
tic for the chalcopyrite phase Ž101., Ž103., Ž211. reflections ŽFig. 6c.. Chalcopyrite phase has been found to present, also, for evaporated In-rich films by Pern et al. w12x and by Guillemoles et al. w7x for electrodeposited ones w13x. Weak peaks of binaries Cu 2 Se, alpha CuSe and InSe are registered, too. Similar results are obtained for the same type of samples when the Se pressure is decreased to 2.10y5 atm. Cuand In-binary selenides present as the intensity of their peaks varied. The results for Se-treated films indicate that the composition of the as-deposited films strongly affects the structure in addition to phase composition and preferred orientation. Because of the availability of copper and indium binaries in the X-ray diffraction patterns shown in Fig. 6 it could be suggested that the composition of the as-de-
posited films determines the structural characteristics after annealing.
4. Results The results from investigation of the deposition process, of morphology and structure of CuInSe 2 films could be summarized in the following paragraphs. CuInSe 2 thin films could be prepared by one-step electrodeposition process from an electrolyte containing cuprous ions and thiocyanate as a complexing agent. Film composition is defined mainly by electrolyte composition and deposition potential. The variation of the deposition potential changes the ratio In:Se while Cu content in the deposits is equal to that in the electrolyte.
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Annealing in Ar or Se atmosphere improves structural characteristics of the films. Se annealing is more favourable in addition to crystallite size than heat treatment in Ar. The average crystallite dimensions enlarge to 1–3 m m after Se treatment. Cu-rich films are with bigger crystallites then In-rich films. Heat treatment in Se ambient improves significantly the crystallinity of the films regardless of their composition i.e., Cu-rich, stoichiometric or In-rich films. X-ray spectra reveal the presence of a phase corresponding to the CuInSe 2 chalcopyrite compound especially well-expressed in the diffractograms of In-rich films. Cu- and In- binaries are observed, too. Film texture depends upon the composition and upon the number of the binary phases: the less are the phases the bigger is the texture sharpness.
and by the Bulgarian Ministry of Education, Science and Technology contracts F-607.
Acknowledgements
w11x
The authors would like to thank Prof. H.W. Schock for kindly submitted Mo-coated glass substrates. This paper was partially supported by EC contract JOU2-CT92-0141
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x
w12x w13x
H.W. Schock, Solar Energy Mater. 34 Ž1994. 19. C.D. Lokhande, S.H. Pawar, Phys. Stat. Sol. Ža. 111 Ž1989. 17. C. Guillen, J. Herrero, Solar Energy Mater. 23 Ž1991. 31. L. Thouin, S. Massaccesi, S. Sanchez, J. Vedel, J. Electroanal. Chem. 374 Ž1994. 81. C.X. Qui, I. Shih, Solar Energy Mater. 15 Ž1987. 219. C. Guillen, J. Herrero, J. Electrochem. Soc. 145 Ž1994. 225. J. Guillemoles, A. Lusson, P. Cowache, S. Massaccesi, J. Vedel, D. Lincot, Adv. Mater. 6 Ž1994. 376. K. Kolentzov, L. Yurukova, A. Rachkova, Bulgarian Patent No. 35448 Ž1984.. R.N. Bhattacharya, J. Electrochem. Soc. 130 Ž1983. 2041. Y. Ueno, H. Kawai, T. Suguira, H. Minourai, Thin Solid Films 157 Ž1988. 159. S.N. Sahu, R.D.L. Kristensen, D. Haneman, Solar Energy Mater. 18 Ž1989. 385. F.J. Pern, R. Noufi, A. Mason, A. Franz, Thin Sold Films 202 Ž1991. 299. J. Guillemoles, S. Massaccesi, P. Cowache, L. Thouin, S. Sanchez, D. Lincot, J. Vedel, Proc. of 12th EPSEC, Amsterdam, 1994.
Preparation and structure of annealed CuInSe 2 electrodeposited films E. Tzvetkova a
a,)
, N. Stratieva a , M. Ganchev a , I. Tomov b, K. Ivanova a , K. Kochev
a
Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, Sofia, Bulgaria b Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria Received 16 September 1996; accepted 23 April 1997
Abstract Thin CuInSe 2 films were prepared by one-step electrodeposition process. The deposition was done in potentiostatic regime from an original electrolyte containing Cuq, In3q, Se 4q ions and thiocyanate as a complexing agent. The influence of deposition parameters Želectrolyte composition and concentration, temperature of the electrolyte and deposition potential. on film composition was studied. Technological parameters for preparation of films with a desired composition were found. The as-deposited films were heat treated in Ar and in Se ambient. Morphology and structure of as-deposited and of annealed films were investigated. It was established that Se treatment was more effective than the annealing in Ar in addition to crystallite size. Phase formation after Se treatment was elucidated. It was found that composition of as-deposited films mainly conditioned phase composition after the selenization. q 1997 Elsevier Science S.A. Keywords: Chalcogens; Heat treatment; Semiconductors; X-ray diffraction
1. Introduction Thin CuInSe 2 films with chalcopyrite structure are considered as the most promising material for terrestrial photovoltaic applications w1x. One of the prospective methods for large scale preparation of CuInSe 2 is electrodeposition from aqueous solution w2x. Electrodeposition of ternary compounds has many problems connected mainly with a control of electrolyte composition and a choice of suitable deposition potential Ed and pH of the electrolyte. Especially CuInSe 2 electrodeposition is rather difficult because of some peculiarities of the system such as high energy of interaction of Se with Cu and In and quite different individual deposition potentials of the elements. The individual deposition potentials can be brought closer either by using surface active substances or by introducing a complexing agent in the electrolyte w3,4x. Usually as-electrodeposited films are small grain size and poor adherent. To improve their morphology and crystallinity usually they need heat treatment in different ambient w5–7x. Of particular interest is phase formation during such annealing.
)
Corresponding author.
0040-6090r97r$17.00 q 1997 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 2 6 3 - 0
Our paper presents the results from the investigations of the processes taking place during the electrodeposition and heat treatment in Ar and Se atmosphere of CuInSe 2 thin films. 2. Preparation of CuInSe 2 films CuInSe 2 films were prepared by one-step electrodeposition process from an electrolyte containing a complexing agent. Glass plates covered with sputtered Mo Ž5–8 m Vrcm. or conductive glass of 2 cm2 area were used as substrates. The conductive glass was prepared by a spray technique based on pyrolytic decomposition of SnCl 4 P 5H 2 O alcohol solution and doping ions introduced from NH 4 F and fluorinehydrogen acid. The specific resistance of the deposits was about 50 Vrcm w8x. CuInSe 2 films were deposited in a 100-ml glass cell in unstirred bath using the potentiostatic method with a three electrode arrangement. All potentials were referred to a saturated calomel electrode ŽSCE.. A Pt foil with area five times larger than that of the substrates was used as an anode. The plating bath for codeposition was prepared dissolving calculated quantities of CuCl, In 2 ŽSO4 . 3 and SeO 2 at a total concentration of 8.5 mM Žbut in different ion ratios. in aqueous solution. The bath pH was adjusted to pH s 5 by adding 0.4 M acetate buffer. Thiocyanate of
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E. TzÕetkoÕa et al.r Thin Solid Films 311 (1997) 101–106
concentration 2 M was used as a complexing agent. It formed a stable complex with cuprous ions and no efficient complexing with indium ions was observed at the same time. The main distinction from previously reported CuInSe 2 electroplating processes w4,9–11x is that we have used Cu ions of the first valence and thiocyanate as a complexing agent. Several electrolytes with different composition were investigated in order to establish the dependence of film composition on electrolyte content and on deposition potential. The results presented in Fig. 1 are related to three different electrolyte compositions ŽCu:In:Se in at.%.: el1 — 32.5:36.8:30.7el2 — 18.9:34.1:46.9 and el5 — 26.6:40.3:33.1. As it is seen, the data for electrolyte composition and for the composition of the films prepared from the corresponding electrolyte lie on one and the same line. The latter is parallel to the tie lines which correspond to the constant Cu content in the deposits. This means that Cu content in the deposits does not practically depend on deposition potential Ed and approximately equals Cu percentage concentration in the solution. When cathodic potential changes from y0.7 V to y1 V the content of In gradually increases while Se content gradually decreases. At the same time the film composition approaches the composition of the electrolyte. It is obvious that for the deposition of films with preliminary desired composition the electrolyte composition and the electrode potential have to be considered. For example, two marginal possibilities are available for the preparation of stoichiometric CuInSe 2 films: Ž1. at low cathodic potential Žabout y0.7 V. and electrolyte composition 26.6 at.% Cu, 40.3 at.% In, 33.1 at.% Seand Ž2. at high cathodic Žabout y1 V. and 25.7 at.% Cu, 31.6 at.% In, 42.7 at.% Se electrolyte composition Žall other technological parameters being one and the same.. SEM photographs of stoichiometric CIS films, deposited at above
mentioned conditions are shown in Fig. 2a Žfor case 1. and Fig. 2b Žfor case 2.. The comparison of the photographs shows clearly the influence of the deposition potential on film morphology. In the first case the deposit is dendritic, the grain size being less than 0.1–0.2 m m. In the second case the films are cracked and not homogeneous and dense, although with bigger grain size. The influence of the deposition time on the film composition is presented in Fig. 3 where the changes of the three components in the film versus deposition time are shown. The curves point out that the concentration of the components practically does not change when the deposition time varies from 30 to 120 min and approaches the stoichiometric one. For times less than 30 min the samples have composition close to indium selenides with Cu deficiency. This tendency is typical for the investigated Ed range and means that the crystallization process starts from indium selenide-rich composition and than moves towards stoichiometric CuInSe 2 . To increase the deposition rate the influence of solution temperature and of concentration of electroactive species ŽCuq, In3q and SeO 32y . on film composition was investigated. As mentioned above, at room temperature Cu content in the deposits is nearly the same as that in the electrolyte regardless of deposition potential. When the electrolyte temperature is increased up to 608C the deposition rate is a 1, 5–2 times higher and the Cu content in the layers increases by 2–3%. If the total concentration of the electrolyte increases more than three times, the film morphology deteriorates. The stirring of the electrolyte causes similar detrimental effect on it. At the same time significantly bigger deviations of the In and Se contents in the films for the corresponding deposition potential are observed.
3. Film characterization
Fig. 1. The dependence of film composition on the deposition potential and electrolyte composition. Film composition vs. Ed : v –v- for el1; B–B- for el2; '–'- for el5 Ž2 M KCNS in 0.4 M acetate buffer, pH s 5, 8.5 mM total concentration of Cuq, In3q and Se 4q but in different ion ratios, 258C..
Detailed X-ray diffraction studies were carried out for phase identification and texture characterization of the deposits. They were made by Philips X-ray diffractometer. The CuK a radiation was selected by a secondary graphite monochromator. A comparison with JCPDS file cards was done for the establishing the observed peaks. The Ar heat treatment experiments were carried out in Ar gas flow. A quartz tube with the samples in it was placed in the central zone of a furnace. The annealing went on from 15 min to 120 min in the temperature range 3508–5508C in steps of 508C. The selenization was done in a two-zone furnace coupled with temperature controller, quartz tube supplied with Se source, sample holder and vacuum system. The annealing of the samples proceeded out at 2508–5808C for 30 min. The cooling to room temperature took place 2 h. Cu-rich, In-rich and stoichiometric films were studied. Their typical compositions are listed in Table 1.
E. TzÕetkoÕa et al.r Thin Solid Films 311 (1997) 101–106
103
Fig. 2. SEM images showing the surface view of CuInSe 2 samples: Ža. and Žb. unannealed stoichiometric films deposited at different conditions Žsee text.; Žc. annealed in Ar; and Žd. annealed in Se ambient.
The investigated as-deposited films are characterized with small grain size and rough surface ŽFig. 2a.. The X-ray diffraction patterns of all as-deposited CuInSe 2 lay-
ers present weak and broad reflections from Ž112., Ž204, 220. and Ž116, 312. planes of CuInSe 2 ŽFig. 4a.. These three peaks correspond to most intensive peaks of both chalcopyrite and sphalerite CuInSe 2 phases. Such broad peaks are supposed to be due to pseudocrystalline CuInSe 2 phase. To make better film morphology and structural properties, the samples have been heat treated in Ar or Se ambient. The comparison of SEM photographs of Ar-treated and Se-treated films shows that the annealing results in an increase of the grain size in the both cases. More effective is the annealing in Se atmosphere ŽFig. 2d. than that in Ar
Table 1
Fig. 3. Variation of film composition with deposition time Ž Ed sy1 V SCE..
Sample
Cu Žat.%.
In Žat.%.
Se Žat.%.
Cu-rich Stoichiometric In-rich
41 25.3 27.5
21.7 25.7 31.7
37.3 49 40.8
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Fig. 4. X-ray pattern of Cu-rich samples: Ža. for as-deposited sample; Žb. for sample treated in Ar atm; Žc. treated in Se atm.
ŽFig. 2c.. The value of the grain size of Se treated films is about one order of magnitude bigger that of Ar treated ones. Of considerable significance for the crystallization is the film composition. It has been established that Cu-rich films favoured the formation of structure with bigger crystallites in a higher extent than In-rich ones. The biggest crystallite dimensions have been observed for Cu-rich films annealed in Se ambient. The average crystallite size amounts 1–3 m m. The improved film crystallinity after annealing is confirmed, also, by the comparison of X-ray diffraction patterns of heat treated samples. Fig. 4b,c shows X-ray diffraction pictures of Cu-rich films annealed in Ar and in Se, respectively. It is seen that the reflections from Ž112., Ž204, 220. and Ž116, 312. planes of Se-treated CuInSe 2 are sharper and stronger that those of Ar-treated ones. This indicates significant enlargement of the crystallite size and the volume fraction of crystalline phase in the Se-treated films. Films annealed at 3508C in Ar ambient display the mentioned above reflections ŽFig. 5a.. The intensity of X-ray diffraction lines increases with the rise of the annealing temperature ŽFig. 5b,c.. At the same time Ž112. preferred orientation is observed. Considerable variation of the composition has not been observed in the Ar treated samples. X-ray diffraction pattern of such samples exhibit peaks of CuInSe 2 only. But for samples deposited at high electronegative potentials Žbelow y1 V vs. SCE. and treated at temperatures higher than 4508C peak of In 2 O 3 at 2q s 30.88 is registered ŽFig. 5b,c.. As it might be expected the high cathodic potentials produce hydrogen evolution. The latter causes alkalization of the electrolyte near the cathodic space and formation of
indium hydroxide occurs. Probably, the annealing at temperatures above 4508C leads to its dehydration to In 2 O 3 . Quite different results were obtained for Se-treated films. Film composition varied after the selenization as Se content had increased up to 10%. At the same time, changes in the phase composition in X-ray diffraction pictures were observed. That is why detailed investigations were carried out of films heat treated in Se atmosphere varying Se pressure, temperature and duration of the annealing process. For each Se pressure, the annealing time was varied from 15 min to 120 min. X-ray patterns show that considerable changes occur for 30 min annealing. After further increase of the annealing time essential differences are not observed in the X-ray pictures. X-ray diffraction patterns of Cu-rich, stoichiometric and In-rich as-deposited films annealed at Se-pressure 2.10y1 atm are shown in Fig. 6. All pictures exhibit well crystallized CuInSe 2 . The peaks are sharp and narrow which indicates for crystalline size larger than 1 m m as the SEM photographs confirm, too Žsee Fig. 2d.. Cu-rich films are characterized by dominated presence of Cu 3 Se 2 and reflections of CuSe 2 and possibly Cu 2 Se. The predominant orientation of CuInSe 2 is Ž204, 220. ŽFig. 6a.. For stoichiometric films CuInSe 2 orientation changes from Ž204, 220. to Ž112. ŽFig. 6b. and negligible traces of a-CuSe are observed. In the both cases chalcopyrite phase of CuInSe 2 undoubtedly exists because weak reflections from Ž101., Ž103. and Ž211. planes are observed, but the sphalerite phase could, also, be present. The X-ray pattern of In-rich films exhibits ten peaks of CuInSe 2 including characteris-
Fig. 5. X-ray diffraction patterns for CuInSe 2 films deposited at y0.9 V and annealed for 120 min in Ar atm: Ža. at 3508C; Žb. at 4508C; Žc. at 5508C; ŽB. CuInSe 2 ; Ž^. SnO 2 ; Ž`. In 2 O 3 .
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105
Fig. 6. X-ray patterns of CuInSe 2 samples annealed at 5808C in 2.10y1 atm Se: Ža. Cu-rich; Žb. stoichiometric; Žc. In-rich.; Ž`. InSe; Ž'. Cu 2 Se; Ž=. CuSe 2 ; ŽI. Cu 3 Se 2 ; Ž^. a-CuSe.
tic for the chalcopyrite phase Ž101., Ž103., Ž211. reflections ŽFig. 6c.. Chalcopyrite phase has been found to present, also, for evaporated In-rich films by Pern et al. w12x and by Guillemoles et al. w7x for electrodeposited ones w13x. Weak peaks of binaries Cu 2 Se, alpha CuSe and InSe are registered, too. Similar results are obtained for the same type of samples when the Se pressure is decreased to 2.10y5 atm. Cuand In-binary selenides present as the intensity of their peaks varied. The results for Se-treated films indicate that the composition of the as-deposited films strongly affects the structure in addition to phase composition and preferred orientation. Because of the availability of copper and indium binaries in the X-ray diffraction patterns shown in Fig. 6 it could be suggested that the composition of the as-de-
posited films determines the structural characteristics after annealing.
4. Results The results from investigation of the deposition process, of morphology and structure of CuInSe 2 films could be summarized in the following paragraphs. CuInSe 2 thin films could be prepared by one-step electrodeposition process from an electrolyte containing cuprous ions and thiocyanate as a complexing agent. Film composition is defined mainly by electrolyte composition and deposition potential. The variation of the deposition potential changes the ratio In:Se while Cu content in the deposits is equal to that in the electrolyte.
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E. TzÕetkoÕa et al.r Thin Solid Films 311 (1997) 101–106
Annealing in Ar or Se atmosphere improves structural characteristics of the films. Se annealing is more favourable in addition to crystallite size than heat treatment in Ar. The average crystallite dimensions enlarge to 1–3 m m after Se treatment. Cu-rich films are with bigger crystallites then In-rich films. Heat treatment in Se ambient improves significantly the crystallinity of the films regardless of their composition i.e., Cu-rich, stoichiometric or In-rich films. X-ray spectra reveal the presence of a phase corresponding to the CuInSe 2 chalcopyrite compound especially well-expressed in the diffractograms of In-rich films. Cu- and In- binaries are observed, too. Film texture depends upon the composition and upon the number of the binary phases: the less are the phases the bigger is the texture sharpness.
and by the Bulgarian Ministry of Education, Science and Technology contracts F-607.
Acknowledgements
w11x
The authors would like to thank Prof. H.W. Schock for kindly submitted Mo-coated glass substrates. This paper was partially supported by EC contract JOU2-CT92-0141
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x
w12x w13x
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