- Email: [email protected]
Morphological and structural characterization of Cu2ZnSnSe4 thin films grown by selenization of elemental precursor layers
Thin Solid Films 517 (2009) 2531–2534
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Morphological and structural characterization of Cu2ZnSnSe4 thin films grown by selenization of elemental precursor layers P.M.P. Salomé a, P.A. Fernandes a,b, A.F. da Cunha a,⁎ a b
Departamento de Física, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal Departamento de Física, Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, Rua Dr. António Bernardino de Almeida, 431, 4200-072 Porto, Portugal
a r t i c l e
i n f o
Available online 8 November 2008 Keywords: Cu2ZnSnSe4 CZTSe Sputtering Selenization Thin films Chalcogenides Chalcopyrites Kesterites
a b s t r a c t Despite the success of Cu(In,Ga)Se2 (CIGS) based PV technology now emerging in several industrial initiatives, concerns about the cost of In and Ga are often expressed. It is believed that the cost of those elements will eventually limit the cost reduction of this technology. One candidate to replace CIGS is Cu2ZnSnSe4 (CZTSe). We report the preliminary results of CZTSe thin films grown on bare and Mo coated glass through selenization of DC magnetron sputtered Cu/Zn/Sn precursor layers in an atmosphere of Se vapour. The influence of the selenization temperature on the resulting films has been studied. The resulting films were studied by SEM/EDS, XRD, and Raman scattering. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
Thin film solar cells based in a Cu(In,Ga)Se2 (CIGS) absorber layer have demonstrated in laboratory efficiencies of 19.9% [1]. Due to the increase of In prices, which can hinder the cost reduction of this type of cells, new compounds are being researched. Cu2ZnSnSe4 (CZTSe) is one of these compounds and it's a candidate to replace CIGS as the absorber layer. Friedlmeier et al. reported the growth of CZTSe by evaporation but with poor adhesion [2]. Wibobo et al. have reported CZTSe grown by PLD from a CZTSe compound target [3] and by RF sputtering from chalcogenide targets [4]. The influence of the precursors' surface morphology in the CZTS final film quality was reported by Katagiri [5]. We deposited the precursors by DC magnetron sputtering. Compact precursor films were obtained for all three elements. Given that fact, we hoped to obtain compact and uniform CZTSe films. The selenization was done with elemental Se vapour thus avoiding the use of toxic H2Se. We studied the influence of the temperature of selenization on the properties of the films. This was made in order to establish the best temperature to obtain CZTSe films. In this work we present the preliminary results of SEM/EDS, XRD, and Raman scattering of the resulting films.
2.1. Sample preparation
⁎ Corresponding author. E-mail addresses: [email protected] (P.M.P. Salomé), [email protected] (P.A. Fernandes), [email protected] (A.F. da Cunha). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.11.034
CZTSe films were grown by selenization of metallic precursors. The precursors were deposited by DC magnetron sputtering on 3 × 3 cm2 soda lime glass (SLG) in the following order: SLG/Cu/Zn/Sn. We also tested growth on Mo coated glass. The purity of the targets is N3 for Mo, N5 for Cu and N4 for Zn and Sn. Substrates were cleaned with acetone, alcohol, hot deionised water and dried using a N2 flux. The base pressure of the chamber was 5 × 10− 6 mbar. All the precursors were deposited with the same settings, namely power density of 0.16 W cm− 2, working pressure of 2 × 10− 3 mbar and a substrate to target distance of 10 cm. Thicknesses were monitored with a crystal controller Intellemetrics IL 150. To prepare CZTSe, the selenization was done in a chamber with a base pressure of 4 × 10− 6 mbar. High purity Se pellets were evaporated over the precursor from a quartz tube source at 255 °C. The distance between the substrate and the evaporator was 5 cm. The substrate temperature could be raised from room temperature up to 550 °C. All temperatures were increased at a rate of 10 °C per minute. In this work we have prepared five precursors. The thicknesses were set to the following values: Cu — 200 nm, Zn — 260 nm and Sn — 300 nm. The heat treatment was started by raising the substrate temperature from room temperature to 150 °C and held at that temperature for 30 min. After this step, the temperature of the Se source was raised to 255 °C, and held there for 30 min. Finally the substrate temperature was raised to the final value and again held there for 30 min. For the maximum substrate temperature we used: 200 °C,
2532
P.M.P. Salomé et al. / Thin Solid Films 517 (2009) 2531–2534
Table 1 Composition of the films before and after selenization for samples Se200; Se255; Se300; Se350 and Se400 Before selenization Se200 Se255 Se300 Se350 Se400
After selenization
Cu (%)
Zn (%)
Sn (%)
Cu (%)
Zn (%)
Sn (%)
Se (%)
47 44 44 47 47
28 33 33 29 29
26 23 23 24 24
42 39 35 32 31
21 26 25 18 14
24 19 17 12 9
13 16 23 38 45
Table 2 Ratios of the element concentrations and thicknesses before and after selenization for samples Se200; Se255; Se300; Se350 and Se400 Before selenization
Se Se Se Se Se
200 255 300 350 400
After selenization
[Cu]/([Zn] + [Sn])
[Zn]/[Sn]
[Cu]/([Zn] + [Sn])
[Zn]/[Sn]
[Se]/[metal]
Thickness (μm)
0.87 0.78 0.78 0.87 0.87
1.09 1.42 1.42 1.19 1.19
0.92 0.86 0.82 1.05 1.51
0.90 1.32 1.45 1.43 2.22
0.15 0.20 0.30 0.61 0.82
0.9 1.0 1.0 1.5 1.6
255 °C, 300 °C, 350 °C and 400 °C. Samples were named after their maximum temperature. 2.2. Sample characterization The surface of the precursors and of CZTSe films were analysed by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometry (EDS). These analyses were used to examine film morphology and chemical composition. X-Ray Diffraction (XRD) was used to study the crystalline structures and the Cu-Kα line of 1.5406 Å was used. CZTSe films were also analysed by Raman scattering excited with an argon laser with a wavelength of 514.5 nm and 220 mW power. Film thicknesses were measured using a Dektak 150 profiler.
3. Results and discussion The control of the precursors' thicknesses was used to control the composition of the final CZTSe films. In order to determine the correct composition we measured the precursors via EDS. The results are presented in Table 1. Although the thicknesses of the precursors appeared to be similar, small differences were observed in the composition. This was probably due to sample positioning and uncertainties associated with EDS. CZTSe films were grown on SLG and on SLG/Mo. The ones on SLG peeled off, while the ones on SLG/Mo have shown poor adhesion but did not peel off. By analyzing the film's composition and element ratios, Tables 1 and 2, it is immediately perceptible that the quantity of Se in the films increases with temperature. But with the temperature increase there are also big losses in Sn and some losses in Zn. At 400 °C, 60% of the initial Sn is lost and about 30% of Zn. The loss of Sn at 400 °C is in agreement with results reported by others [2]. Just by doing this analysis, it's evident that following this procedure at even higher temperatures will not produce CZTSe due to the loss of both Sn and Zn. One problem that has been identified using the XRD analysis is that making the distinction between ZnSe and CZTSe is very difficult since their highest peaks are found at very close angles, if not the same. This means that complementary analyses are required to resolve the two phases. We have resorted to Raman spectroscopy for such purpose. In Fig. 1, it is presented the XRD spectra of the samples and in Fig. 2 the Raman scattering spectra. For low temperature samples, traces of a Cu and Sn compound, Cu6.26Sn5, are found in the XRD analysis with peaks at 30.14° and at 42.89° [6]. The amount of this compound tends to diminish with increasing selenization temperature. This indicates that although Sn evaporates from the samples at high temperatures, it starts to diffuse in the film at lower ones. For samples Se255, Se300 and Se350, the compound Cu4Zn was detected in the XRD analysis with peaks at 37.69°, 41.93° and 43.27° [6]. According to the XRD data, Cu2Se appears to be present in all samples with peaks at 26.90° and 44.69°. However, the height of those peaks is relatively small compared to the peaks of other compounds, this indicates that there is only a small amount of Cu2Se in the films. It is also quite clear from Fig. 1 that the relative
Fig. 1. XRD spectra of samples Se200; Se255; Se300; Se350 and Se400.
P.M.P. Salomé et al. / Thin Solid Films 517 (2009) 2531–2534
Fig. 2. Raman Spectra of samples Se200; Se255; Se300; Se350 and Se400.
intensity of those peaks tends to decrease with increasing temperature. With Raman scattering analysis we can see a small peak at 263 cm− 1 assigned to Cu2Se [7] in samples Se200, Se255 and Se300. So, even at low temperatures, some Se is reaching the Cu layer and forming Cu2Se. The differences in the information obtained from XRD and the Raman scattering may be associated with different points and area of analysis. In all the films, the XRD showed Mo. At the temperature of 400 °C there are two XRD peaks at 30.66° and at 32.11° that we could not identify. Further work is underway to try to identify their origin.
2533
Regarding the detection of CZTSe/ZnSe, the only visible difference in XRD is a double peak of CZTSe at 53.39°, direction (2 3 0), and 53.45° direction (2 3 3) [6] while in cubic ZnSe there's only a peak at 53.56° which corresponds to the (3 1 1) direction. Using this double peak difference, it was possible to identify CZTSe in samples Se350 and Se400. Having identified CZTSe we can't say anything about the presence of ZnSe by looking only to XRD data. Raman scattering analysis, however, shows the presence of ZnSe with a peak at 252 cm− 1 and at 202 cm− 1 for samples Se200, Se255 and Se300 [8]. For samples Se350 and Se400 no evidence of ZnSe was found. However, in both samples, the three peaks of CZTSe identified by Altosaar et al. were found [9]: 197 cm− 1, 173 cm − 1 and 231 cm− 1. With these two analyses, it is possible to say that samples Se350 and Se400 contain CZTSe and not ZnSe. For samples Se200, Se255 and Se300 we can say that ZnSe is present, but we cannot draw any conclusion about a residual presence of CZTSe because Raman detects a broad peak at 197 cm− 1. In Table 2, it is also presented film thicknesses. What might be considered a paradox, the increase of thickness with the temperature even with less material, can be attributed to the CZTSe formation with a corresponding expansion of the film. It is known that this material has a smaller density, 5.693 g/cm3 [10], than the metallic precursors. The thickness values are the highest ones in the films that we believe that contain CZTSe namely Se350 and Se400. SEM images for samples Se255, Se300, Se350 and Se400 are displayed in Fig. 3. With the temperature increase, it seems that the films are more crystalline and more compact. 4. Conclusions The influence of the temperature of selenization of the Cu/Zn/Sn precursors was studied. We have seen that the precursors deposited on bare SLG upon selenization peeled off. However on Mo coated SLG the films withstood the selenization process. The results of the
Fig. 3. SEM surface images of samples: a) Se255; b) Se300; c) Se350 and d) Se400.
2534
P.M.P. Salomé et al. / Thin Solid Films 517 (2009) 2531–2534
characterization of the films by combining XRD and Raman spectroscopy showed that at temperatures between 350 °C and 400 °C we obtained CZTSe. We have also detected that for substrate temperatures of 400 °C there is a big loss of Sn and Zn when compared with the initial precursor composition. It also seems that even at low temperatures there is formation of binary compounds of Cu with Se, Sn and Zn. Zn seems to form compounds with Cu and with Se but not with Sn until the temperature reaches 350 °C. The results above allowed us to establish that the best temperature to prepare CZTSe films is between 350 °C and 400 °C. Other parameters can be changed in order to overcome the lack of Se incorporation at lower temperatures, such as: change working pressure by introducing an inert gas, rising the temperature of evaporation of Se and eventually the heating process itself. We have seen that XRD analysis alone is not enough to completely resolve ZnSe from CZTSe and that complementing those results with Raman scattering allows for a more thorough identification. Acknowledgement This work was partially supported by Fundação para a Ciência e Tecnologia, Portugal (FCT), through a PhD grant number SFRH/BD/ 29881/2006.
References [1] I. Repins, M.A. Contreras, Brian Egaas, Clay DeHart, John Scharf, Craig L. Perkins, B. To, R. Noufi, Prog. Photovolt. Res. Appl. 16 (2008) 235. [2] T.M. Friedlmeier, H. Dittrich, H.W. Schock, The 11th Conference on Ternary and Multinary Compounds, ICTMC-11, Salford, 8–12 September, 1997, p. 345. [3] R.A. Wibowo, E.S. Lee, B. Munir, K.H. Kim, Phys. Status Solidi, A 204 (10) (2007) 3373. [4] R.A. Wibowo, W.S. Kim, E.S. Kee, B. Munir, K.H. Kim, J. Phys. Chem. Solids 68 (2007) 1908. [5] H. Katagiri, Thin Solid Films 480–481 (2005) 426. [6] International Centre for Diffraction Data: CZTSe-04-010-6295; Cu2Se -04-006-1706; ZnSe 04-007-4741; Cu4Zn - 03-065-6066; Mo -01-071-4645; Cu6.26 Sn5 - 00-047-1575. [7] C. Xue, D. Papadimitriou, Y.S. Raptis, W. Richter, N. Esser, S. Siebentritt, M. Ch. LuxSteiner, J. Appl. Phys 96 (4) (2004) 15. [8] A. Dinger, M. Goppert, R. Becker, M. Gruun, S. Petillon, C. Klingshirn, J. Liang, V. Wagner, J. Geurts, Phys. Rev., B 64 (2001) (245310-1-8). [9] M. Altosaar, J. Raudoja, K. Timmo, M. Danilson, M. Grossberg, J. Krustok, E. Mellikov, Phys. Status Solidi, A 205 (1) (2008) 167. [10] I.D. Olekseyuk, L.D. Gulay, I.V. Dydchack, L.V. Piskach, O.V. Parasyuk, O.V. Marchuk, J. Alloys Compd. 340 (2002) 141.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Morphological and structural characterization of Cu2ZnSnSe4 thin films grown by selenization of elemental precursor layers P.M.P. Salomé a, P.A. Fernandes a,b, A.F. da Cunha a,⁎ a b
Departamento de Física, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal Departamento de Física, Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, Rua Dr. António Bernardino de Almeida, 431, 4200-072 Porto, Portugal
a r t i c l e
i n f o
Available online 8 November 2008 Keywords: Cu2ZnSnSe4 CZTSe Sputtering Selenization Thin films Chalcogenides Chalcopyrites Kesterites
a b s t r a c t Despite the success of Cu(In,Ga)Se2 (CIGS) based PV technology now emerging in several industrial initiatives, concerns about the cost of In and Ga are often expressed. It is believed that the cost of those elements will eventually limit the cost reduction of this technology. One candidate to replace CIGS is Cu2ZnSnSe4 (CZTSe). We report the preliminary results of CZTSe thin films grown on bare and Mo coated glass through selenization of DC magnetron sputtered Cu/Zn/Sn precursor layers in an atmosphere of Se vapour. The influence of the selenization temperature on the resulting films has been studied. The resulting films were studied by SEM/EDS, XRD, and Raman scattering. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
Thin film solar cells based in a Cu(In,Ga)Se2 (CIGS) absorber layer have demonstrated in laboratory efficiencies of 19.9% [1]. Due to the increase of In prices, which can hinder the cost reduction of this type of cells, new compounds are being researched. Cu2ZnSnSe4 (CZTSe) is one of these compounds and it's a candidate to replace CIGS as the absorber layer. Friedlmeier et al. reported the growth of CZTSe by evaporation but with poor adhesion [2]. Wibobo et al. have reported CZTSe grown by PLD from a CZTSe compound target [3] and by RF sputtering from chalcogenide targets [4]. The influence of the precursors' surface morphology in the CZTS final film quality was reported by Katagiri [5]. We deposited the precursors by DC magnetron sputtering. Compact precursor films were obtained for all three elements. Given that fact, we hoped to obtain compact and uniform CZTSe films. The selenization was done with elemental Se vapour thus avoiding the use of toxic H2Se. We studied the influence of the temperature of selenization on the properties of the films. This was made in order to establish the best temperature to obtain CZTSe films. In this work we present the preliminary results of SEM/EDS, XRD, and Raman scattering of the resulting films.
2.1. Sample preparation
⁎ Corresponding author. E-mail addresses: [email protected] (P.M.P. Salomé), [email protected] (P.A. Fernandes), [email protected] (A.F. da Cunha). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.11.034
CZTSe films were grown by selenization of metallic precursors. The precursors were deposited by DC magnetron sputtering on 3 × 3 cm2 soda lime glass (SLG) in the following order: SLG/Cu/Zn/Sn. We also tested growth on Mo coated glass. The purity of the targets is N3 for Mo, N5 for Cu and N4 for Zn and Sn. Substrates were cleaned with acetone, alcohol, hot deionised water and dried using a N2 flux. The base pressure of the chamber was 5 × 10− 6 mbar. All the precursors were deposited with the same settings, namely power density of 0.16 W cm− 2, working pressure of 2 × 10− 3 mbar and a substrate to target distance of 10 cm. Thicknesses were monitored with a crystal controller Intellemetrics IL 150. To prepare CZTSe, the selenization was done in a chamber with a base pressure of 4 × 10− 6 mbar. High purity Se pellets were evaporated over the precursor from a quartz tube source at 255 °C. The distance between the substrate and the evaporator was 5 cm. The substrate temperature could be raised from room temperature up to 550 °C. All temperatures were increased at a rate of 10 °C per minute. In this work we have prepared five precursors. The thicknesses were set to the following values: Cu — 200 nm, Zn — 260 nm and Sn — 300 nm. The heat treatment was started by raising the substrate temperature from room temperature to 150 °C and held at that temperature for 30 min. After this step, the temperature of the Se source was raised to 255 °C, and held there for 30 min. Finally the substrate temperature was raised to the final value and again held there for 30 min. For the maximum substrate temperature we used: 200 °C,
2532
P.M.P. Salomé et al. / Thin Solid Films 517 (2009) 2531–2534
Table 1 Composition of the films before and after selenization for samples Se200; Se255; Se300; Se350 and Se400 Before selenization Se200 Se255 Se300 Se350 Se400
After selenization
Cu (%)
Zn (%)
Sn (%)
Cu (%)
Zn (%)
Sn (%)
Se (%)
47 44 44 47 47
28 33 33 29 29
26 23 23 24 24
42 39 35 32 31
21 26 25 18 14
24 19 17 12 9
13 16 23 38 45
Table 2 Ratios of the element concentrations and thicknesses before and after selenization for samples Se200; Se255; Se300; Se350 and Se400 Before selenization
Se Se Se Se Se
200 255 300 350 400
After selenization
[Cu]/([Zn] + [Sn])
[Zn]/[Sn]
[Cu]/([Zn] + [Sn])
[Zn]/[Sn]
[Se]/[metal]
Thickness (μm)
0.87 0.78 0.78 0.87 0.87
1.09 1.42 1.42 1.19 1.19
0.92 0.86 0.82 1.05 1.51
0.90 1.32 1.45 1.43 2.22
0.15 0.20 0.30 0.61 0.82
0.9 1.0 1.0 1.5 1.6
255 °C, 300 °C, 350 °C and 400 °C. Samples were named after their maximum temperature. 2.2. Sample characterization The surface of the precursors and of CZTSe films were analysed by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometry (EDS). These analyses were used to examine film morphology and chemical composition. X-Ray Diffraction (XRD) was used to study the crystalline structures and the Cu-Kα line of 1.5406 Å was used. CZTSe films were also analysed by Raman scattering excited with an argon laser with a wavelength of 514.5 nm and 220 mW power. Film thicknesses were measured using a Dektak 150 profiler.
3. Results and discussion The control of the precursors' thicknesses was used to control the composition of the final CZTSe films. In order to determine the correct composition we measured the precursors via EDS. The results are presented in Table 1. Although the thicknesses of the precursors appeared to be similar, small differences were observed in the composition. This was probably due to sample positioning and uncertainties associated with EDS. CZTSe films were grown on SLG and on SLG/Mo. The ones on SLG peeled off, while the ones on SLG/Mo have shown poor adhesion but did not peel off. By analyzing the film's composition and element ratios, Tables 1 and 2, it is immediately perceptible that the quantity of Se in the films increases with temperature. But with the temperature increase there are also big losses in Sn and some losses in Zn. At 400 °C, 60% of the initial Sn is lost and about 30% of Zn. The loss of Sn at 400 °C is in agreement with results reported by others [2]. Just by doing this analysis, it's evident that following this procedure at even higher temperatures will not produce CZTSe due to the loss of both Sn and Zn. One problem that has been identified using the XRD analysis is that making the distinction between ZnSe and CZTSe is very difficult since their highest peaks are found at very close angles, if not the same. This means that complementary analyses are required to resolve the two phases. We have resorted to Raman spectroscopy for such purpose. In Fig. 1, it is presented the XRD spectra of the samples and in Fig. 2 the Raman scattering spectra. For low temperature samples, traces of a Cu and Sn compound, Cu6.26Sn5, are found in the XRD analysis with peaks at 30.14° and at 42.89° [6]. The amount of this compound tends to diminish with increasing selenization temperature. This indicates that although Sn evaporates from the samples at high temperatures, it starts to diffuse in the film at lower ones. For samples Se255, Se300 and Se350, the compound Cu4Zn was detected in the XRD analysis with peaks at 37.69°, 41.93° and 43.27° [6]. According to the XRD data, Cu2Se appears to be present in all samples with peaks at 26.90° and 44.69°. However, the height of those peaks is relatively small compared to the peaks of other compounds, this indicates that there is only a small amount of Cu2Se in the films. It is also quite clear from Fig. 1 that the relative
Fig. 1. XRD spectra of samples Se200; Se255; Se300; Se350 and Se400.
P.M.P. Salomé et al. / Thin Solid Films 517 (2009) 2531–2534
Fig. 2. Raman Spectra of samples Se200; Se255; Se300; Se350 and Se400.
intensity of those peaks tends to decrease with increasing temperature. With Raman scattering analysis we can see a small peak at 263 cm− 1 assigned to Cu2Se [7] in samples Se200, Se255 and Se300. So, even at low temperatures, some Se is reaching the Cu layer and forming Cu2Se. The differences in the information obtained from XRD and the Raman scattering may be associated with different points and area of analysis. In all the films, the XRD showed Mo. At the temperature of 400 °C there are two XRD peaks at 30.66° and at 32.11° that we could not identify. Further work is underway to try to identify their origin.
2533
Regarding the detection of CZTSe/ZnSe, the only visible difference in XRD is a double peak of CZTSe at 53.39°, direction (2 3 0), and 53.45° direction (2 3 3) [6] while in cubic ZnSe there's only a peak at 53.56° which corresponds to the (3 1 1) direction. Using this double peak difference, it was possible to identify CZTSe in samples Se350 and Se400. Having identified CZTSe we can't say anything about the presence of ZnSe by looking only to XRD data. Raman scattering analysis, however, shows the presence of ZnSe with a peak at 252 cm− 1 and at 202 cm− 1 for samples Se200, Se255 and Se300 [8]. For samples Se350 and Se400 no evidence of ZnSe was found. However, in both samples, the three peaks of CZTSe identified by Altosaar et al. were found [9]: 197 cm− 1, 173 cm − 1 and 231 cm− 1. With these two analyses, it is possible to say that samples Se350 and Se400 contain CZTSe and not ZnSe. For samples Se200, Se255 and Se300 we can say that ZnSe is present, but we cannot draw any conclusion about a residual presence of CZTSe because Raman detects a broad peak at 197 cm− 1. In Table 2, it is also presented film thicknesses. What might be considered a paradox, the increase of thickness with the temperature even with less material, can be attributed to the CZTSe formation with a corresponding expansion of the film. It is known that this material has a smaller density, 5.693 g/cm3 [10], than the metallic precursors. The thickness values are the highest ones in the films that we believe that contain CZTSe namely Se350 and Se400. SEM images for samples Se255, Se300, Se350 and Se400 are displayed in Fig. 3. With the temperature increase, it seems that the films are more crystalline and more compact. 4. Conclusions The influence of the temperature of selenization of the Cu/Zn/Sn precursors was studied. We have seen that the precursors deposited on bare SLG upon selenization peeled off. However on Mo coated SLG the films withstood the selenization process. The results of the
Fig. 3. SEM surface images of samples: a) Se255; b) Se300; c) Se350 and d) Se400.
2534
P.M.P. Salomé et al. / Thin Solid Films 517 (2009) 2531–2534
characterization of the films by combining XRD and Raman spectroscopy showed that at temperatures between 350 °C and 400 °C we obtained CZTSe. We have also detected that for substrate temperatures of 400 °C there is a big loss of Sn and Zn when compared with the initial precursor composition. It also seems that even at low temperatures there is formation of binary compounds of Cu with Se, Sn and Zn. Zn seems to form compounds with Cu and with Se but not with Sn until the temperature reaches 350 °C. The results above allowed us to establish that the best temperature to prepare CZTSe films is between 350 °C and 400 °C. Other parameters can be changed in order to overcome the lack of Se incorporation at lower temperatures, such as: change working pressure by introducing an inert gas, rising the temperature of evaporation of Se and eventually the heating process itself. We have seen that XRD analysis alone is not enough to completely resolve ZnSe from CZTSe and that complementing those results with Raman scattering allows for a more thorough identification. Acknowledgement This work was partially supported by Fundação para a Ciência e Tecnologia, Portugal (FCT), through a PhD grant number SFRH/BD/ 29881/2006.
References [1] I. Repins, M.A. Contreras, Brian Egaas, Clay DeHart, John Scharf, Craig L. Perkins, B. To, R. Noufi, Prog. Photovolt. Res. Appl. 16 (2008) 235. [2] T.M. Friedlmeier, H. Dittrich, H.W. Schock, The 11th Conference on Ternary and Multinary Compounds, ICTMC-11, Salford, 8–12 September, 1997, p. 345. [3] R.A. Wibowo, E.S. Lee, B. Munir, K.H. Kim, Phys. Status Solidi, A 204 (10) (2007) 3373. [4] R.A. Wibowo, W.S. Kim, E.S. Kee, B. Munir, K.H. Kim, J. Phys. Chem. Solids 68 (2007) 1908. [5] H. Katagiri, Thin Solid Films 480–481 (2005) 426. [6] International Centre for Diffraction Data: CZTSe-04-010-6295; Cu2Se -04-006-1706; ZnSe 04-007-4741; Cu4Zn - 03-065-6066; Mo -01-071-4645; Cu6.26 Sn5 - 00-047-1575. [7] C. Xue, D. Papadimitriou, Y.S. Raptis, W. Richter, N. Esser, S. Siebentritt, M. Ch. LuxSteiner, J. Appl. Phys 96 (4) (2004) 15. [8] A. Dinger, M. Goppert, R. Becker, M. Gruun, S. Petillon, C. Klingshirn, J. Liang, V. Wagner, J. Geurts, Phys. Rev., B 64 (2001) (245310-1-8). [9] M. Altosaar, J. Raudoja, K. Timmo, M. Danilson, M. Grossberg, J. Krustok, E. Mellikov, Phys. Status Solidi, A 205 (1) (2008) 167. [10] I.D. Olekseyuk, L.D. Gulay, I.V. Dydchack, L.V. Piskach, O.V. Parasyuk, O.V. Marchuk, J. Alloys Compd. 340 (2002) 141.