- Email: [email protected]
Structural changes of CuGaSe2 films during the three-stage process observed by spectroscopic light scattering
Thin Solid Films 480–481 (2005) 367 – 372 www.elsevier.com/locate/tsf
Structural changes of CuGaSe2 films during the three-stage process observed by spectroscopic light scattering K. Sakuraia,*, S. Nakamurab, T. Babab, Y. Kimurab, A. Yamadaa, S. Ishizukaa, K. Matsubaraa, P. Fonsa, R. Scheerc, H. Nakanishib, S. Nikia a
AIST, Research Center for Photovoltaics, AIST Central #2-24412, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan b Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-0022, Japan c HMI, Division Solar Energy, Glienicker Strasse 100, 14109 Berlin, Germany Available online 10 December 2004
Abstract We have studied the three-stage deposition process of CuGaSe2 (CGS) films, with the help of spectroscopic light scattering (SLS) in situ monitoring method. During the second stage (i.e. deposition of Cu, Se), an abrupt change from a two-layered structure to a single layer, together with a significant increase in the grain size, was observed in the vicinity of the Cu/Ga=1.0 point. Shortly before this abrupt change, a characteristic peak was observed by SLS, accompanied by a definitive change in depth-oriented distribution of Cu and Na. D 2004 Published by Elsevier B.V. Keywords: Spectroscopic light scattering; CuGaSe2 films; Deposition process
1. Introduction Recently, Cu(In1 x Gax )Se2 (CIGS) solar cells have achieved a conversion efficiency of over 19%, the highest among any polycrystalline thin film solar cells, using the three-stage process [1]. Currently, the best performance is achieved at a Ga composition of x~0.3, where it gives a bandgap of E g~1.2 eV. Though the theoretical optimum is E g~1.5 eV [2] (x~0.8 [3]), the reported cell performances tend to degrade at E gN1.2 eV so far [4]. To realize the expected higher cell performances, further understanding and developments in the deposition process of CIGS film, especially at high Ga contents, are being required. For example, Hanna et al. [5] report a limitation of cell performance by the bulk defects in CIGS films with high Ga content, also showing that the deposition method can largely influence the bulk defect density. To help further analysis of the deposition process, we have previously introduced spectroscopic light scattering (SLS), which is an in situ monitoring technique useful for investigating the * Corresponding author. Tel.: +81 29 861 9176; fax: +81 29 861 5615. E-mail address: [email protected] (K. Sakurai). 0040-6090/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.tsf.2004.11.095
structural and optical properties of thin films [6,7]. We had observed distinctive changes in the SLS signal profiles influenced by the Ga content, suggesting clues for the optimization of the deposition process at high Ga (xN0.5) contents [7]. In this work, in order to obtain more clues to understand the deposition process at high Ga content, we have investigated the structural development of the CuGaSe2 (x=1, CGS) film during deposition. We have interrupted the process at several points where characteristic SLS profiles have been observed, then conducted various ex situ measurements on those interrupted samples.
2. Experimental 2.1. System We have used a molecular beam epitaxy (MBE) system for deposition of the CGS films (Fig. 1). Each source beam of Cu, Ga (7 N) or Se (6 N) was provided from a hot-lip Effusion cell, which provided a stable beam pressure throughout our relatively long deposition process (roughly 2 h per sample). The drift of the beam pressure during one
368
K. Sakurai et al. / Thin Solid Films 480–481 (2005) 367–372
was about 5 min for the interrupted samples, and about 40 min for the bfinishedQ sample (sample 9). After cooling and taking out into the air, the composition and structure of the film was studied by ex situ measurements (here, we have to be aware that some properties of the film may have changed during the cooldown process, though it was much shorter than the total deposition time (~120 min)). We have used electron probe analysis (EPMA), scanning electron microscopy (SEM), X-ray diffraction (XRD; h–2h scan), secondary ion microanalysis (SIMS), and inductively coupled plasma (ICP), and their relationships with the SLS signal were investigated. Fig. 1. Schematic diagram of the MBE system used in this work.
3. Results deposition process was smaller than 2% for Cu, Ga, and smaller than 5% for Se. The system had a separate exchange chamber so that we could exchange samples without changing the temperature of the cells, keeping the beam pressures constant all the time. Consequently, the difference of the deposition speed between consecutive deposition runs was negligible. The MBE system was equipped with liquid nitrogen cryoshrouds. The base pressure was lower than 210 7 Pa when the cells were not heated. The substrate was heated by radiation from a heater through an unpolished sapphire heat-diffuser plate, and constantly rotated in-plane for uniformity. The absolute temperature was determined by a thermocouple behind the substrate. For determining the process timings, a pyrometer was used for monitoring the relative change of the temperature [7,8]. Spatial incoherent white light from a 150 W halogen lamp was irradiated to the sample surface through a glass viewport. The scattered light in the wavelength range of 300–800 nm was monitored by a CCD spectrometer (OceanOptics USB2000) at a nonspecular location, and the signal was integrated every 5 s. The observed intensity profiles at each wavelength were recorded as the SLS signal.
3.1. SLS and pyrometer Fig. 2 shows a typical set of SLS and temperature signals during the three-stage deposition of CGS. The timings of the deposition interruption are indicated by points 1–9. During the first stage, the SLS signal was weak and below the threshold of the CCD, as most of the incident light was reflected by the smooth surface. During stage 2, after passing point 3, the SLS signal showed a rapid increase when the compositional ratio Cu/Ga reached around 0.5,
2.2. Process and evaluation Soda-lime glass (SLG) with RF-sputtered Mo was used as the substrate. Prior to use, the Mo/SLG substrate was heated in vacuum at 550 8C for 40 min, in order to adjust the Na diffusion into CIGS [9]. In our standard deposition process for CGS, the Cu/Ga ratio at the end of stage 2 was 1.3, the final Cu/Ga ratio was 0.93, and the final thickness was 1700 nm. The substrate temperature during stage 1 was 315 8C, and the maximum substrate temperature was 570 8C. To investigate the film properties during this deposition process and their relationships with the pyrometer and SLS signals, we have fabricated a series of CGS/Mo/SLG samples consecutively, with each deposition run interrupted at a different timing. After interruption of the deposition run, each sample was cooled down as rapidly as possible, under irradiation of Se until the temperature fell below 300 8C. The duration of the cooldown process from 570 to 300 8C
Fig. 2. Typical SLS and pyrometer profiles taken during our three-stage deposition process. Numbers 1–9 correspond to the deposition interruption points of samples 1–9. (Note: Only sample 9 (=finished film) was cooled down slowly as shown here; other samples were cooled down as rapidly as possible.)
K. Sakurai et al. / Thin Solid Films 480–481 (2005) 367–372
followed by distinctive peaks at higher Cu/Ga ratios. The sharp peak S S appeared simultaneously with the kink point S P of the pyrometer signal, which corresponds to the bstoichiometry pointQ, where the compositional ratio Cu/ Ga ratio is expected to become 1.0. Prior to the appearance of S S, a maximum peak ( P) was observed. This peak P was not observed for CIGS films with low (xb0.4) Ga contents under the same deposition condition [7]. The SLS signal during the cooldown process showed a difference between Cu-rich and Cu-poor samples. For the Cu-poor samples (samples 1–4 and 9), the SLS signal kept constant at all observed wavelengths during the cooldown process, indicating that the optical (and some structural) properties of the surface region of the film was preserved. On the other hand, in the case of the Cu-rich samples (samples 5–8), the SLS signal intensity changed with decreasing temperature, suggesting changes in the properties of the film.
369
3.2. SEM During stage 2, in samples 2–4, the film showed a twolayered structure as seen in Fig. 3(b)–(d). The upper layer consisted of relatively small grains. Its thickness was similar for samples 2–4, while the thickness of the lower layer increased. The grain size of the lower layer drastically increased from sample 3 to sample 4. After passing the point S S, at sample 5, the two-layered structure disappeared and the grain size increased (Fig. 3(e)), of which its interruption timing was about 5 min after sample 4 (Fig. 3(d)). The film structure observed by SEM was similar for samples 5–9, as the Cu/Ga ratio went up to 1.3 (Cu-rich) and returned to 0.93 (Cu-poor). We also note that the two-layered structure (as seen in Fig. 3(b)–(d)) was not observed for CIGS films with low (xb0.4) Ga contents, under the same deposition condition except for the In/Ga ratio.
Fig. 3. SEM images of cleaved ((Cu),Ga,Se)/Mo/SLG samples. Distinctive two-layered structures were observed in the CGS layer of samples 2–4. (Note: The white part in image (a) corresponds to the Mo layer.)
370
K. Sakurai et al. / Thin Solid Films 480–481 (2005) 367–372
3.3. XRD XRD analysis indicated that the film was comprised of GaSe and Ga2Se3 during the first stage (sample 1). For sample 3, XRD peaks of Ga2Se3 and CuGaSe2 was observed, indicating phase separation. For samples 4 and 5, only the peaks of CuGaSe2 could be observed. 3.4. EPMA
Fig. 4. EPMA results of samples 1–9. The acceleration voltage was set to 7 keV, in order to obtain relatively surface-sensitive signals. The dotted line is only an aid for eyes.
In the first half of stage 2, EPMA measurements using a relatively low energy electron beam of 7 keV (which is sensitive to the composition of the surface region) showed a Cu/Ga compositional ratio increasing faster than the expected linear increase rate (Fig. 4). The surface of the CGS film became extremely Cu-rich, as can be confirmed by the results of EPMA (Fig. 4) and SIMS depth profile (Fig. 5(c)), indicating accumulation of Cu-rich phases at the surface.
Fig. 5. SIMS depth profiles of samples 3–5.
K. Sakurai et al. / Thin Solid Films 480–481 (2005) 367–372 Table 1 Cu/Ga compositional ratios of samples 3–5 Sample no.
Expected Cu/Ga ratio
Measured Cu/Ga (by ICP)
3 4 5
0.62 0.92 1.05
0.59 0.88 1.03
bExpectedQ values are calculated from the process timings, supposing Cu/ Ga=1.0 at point S P. Compared with the ICP results, the error range of the bExpectedQ values is estimated to be around 3%, mainly due to possible pressure fluctuations of Cu.
3.5. SIMS For the samples 2 and 3, SIMS depth profiles also indicated a two-layered structure, with the Cu content higher in the upper layer of the film, as seen in Fig. 5(a). Meanwhile, the Na distribution was significantly lower in the upper layer. The Cu profile in the lower layer showed a bdipQ, while the Na distribution showed the opposite behavior. For sample 4, the Cu and Ga composition were almost uniform throughout the film, and a distinctive bhumpQ in the Na distribution around the boundary of the layered structure was observed. 3.6. ICP The average Cu/Ga ratio of the whole film at point S P was estimated to be in the range of 0.95–1.0, from the ICP measurement results on samples 4 and 5 (Table 1). This indicates that the Cu/Ga ratio estimated from the pyrometer profile was close enough to the average Cu/Ga ratio of the while film.
4. Discussions The most interesting point is the observation of the twolayered structure observed for samples 2–4, which was not observed for the CIGS films with low (xb0.3) Ga content. From the beginning of stage 2, the Cu composition of the upper layer was significantly higher than the lower layer, confirmed by EPMA and SIMS measurements. As our deposition process was operated relatively slowly (2 h/ sample), we can estimate that the deposition speed itself was not too fast to obstruct the Cu/Ga interdiffusion, compared to the short cooldown period after the interruption. Taking the XRD results together into account, we attribute this two-layered structure to the phase separation between Ga2Se3 and CuGaSe2. In sample 4, we could not confirm existence of Gax Sey compounds. This is reasonable, as the whole film was close to stoichiometry. However, for dissolution of the two-layered structure, it seems that a Cu/Ga ratio closer to (or exceeding) 1.0 was required. The sharp peak of SLS strongly suggest that this drastic change began from the point S S, where the Cu/Ga
371
ratio was estimated to be within the range of 0.95–1.0 by ICP. Another point of interest is the distribution of Cu and Na observed by SIMS, which evolves in opposite ways in sample 3. This SIMS profile suggests that Na (or Na compounds) had filled the Cu vacancies in some form and stabilized them, therefore slowing the Cu/Ga interdiffusion. Recently, Lundberg et al. [10] reported that the presence of Na reduces the interdiffusion speed of Cu and Ga. They also have suggested that Na reduces the concentration of metal vacancies. Their report is in correspondence with our results and assumption. Meanwhile, the bhumpQ in the Na distribution of sample 4 suggests concentration of metal vacancies at the boundary of the upper and lower layer. Other point of interest is the appearance of the peak P in the SLS signal, before the bstoichiometry pointQ S S. We have considered on the following two explanations for the peak P: (1) Early emergence of Cux Se phases at the film surface. Here, the EPMA result seems to support this hypothesis. However, as the absolute value of EPMA result can be deviated by surface roughness [11], EPMA alone is not enough to prove the existence of Cux Se phases in this case. In addition, if such Cu-rich phases have existed, they must have disappeared after the deposition interruption, as there was no indication of such phases in SEM, SIMS and XRD results. However, no such structural change was indicated by SLS after the deposition interruption at point 4. Thus we consider that this explanation is unlikely. (2) Changes in surface structure, mainly induced by the structural change of the lower layer. As seen in Fig. 3, the structure of the lower layer have changed drastically somewhere between points 3 and 4. Considering the SIMS and XRD results, we can estimate that this change was triggered by the elimination of Ga2Se3 phases at point P, followed by the enlargement of the grain size in the lower layer. Meanwhile, it is also notable that the appearance of peak P could be used for predicting the timing of S S, before actually reaching the stoichiometry.
5. Summary The structural and compositional changes during the three-stage deposition process of CGS were investigated. A series of deposition-interrupted samples were evaluated by various means including SEM, EPMA and SIMS, and their relationships with the spectroscopic light scattering (SLS) signals were discussed. A distinctive two-layered structure was observed during stage 2, and this layered structure was found to abruptly dissolve in the vicinity of Cu/Ga=1.0 point.
372
K. Sakurai et al. / Thin Solid Films 480–481 (2005) 367–372
Acknowledgements This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO). References [1] K. Ramanathan, M.A. Contreras, C.L. Perkins, S. Asher, F.S. Hasoon, J. Keane, D. Young, M. Romero, W. Metzger, R. Noufi, J. Ward, A. Duda, Prog. Photovolt. Res. Appl. 11 (2003) 225. [2] M. Green, Solar Cells, Prentice-Hall, EngleWood Cliffs, 1982. [3] C. Paorici, L. Zanotti, N. Romeo, G. Sberveglieri, L. Tarricone, Sol. Energy Mater. 1 (1979) 3. [4] S. Siebentritt, Thin Solid Films 403–404 (2002) 1.
[5] G. Hanna, A. Jasenek, U. Rau, H.W. Schock, Thin Solid Films 387 (2001) 71. [6] R. Scheer, A. Neisser, K. Sakurai, P. Fons, S. Niki, Appl. Phys. Lett. 82 (2003) 2091. [7] K. Sakurai, R. Hunger, C.A. Kaufmann, A. Yamada, Y. Baba, Y. Kimura, K. Matsubara, P. Fons, H. Nakanishi, S. Niki, Prog. Photovolt. Res. Appl. 12 (2004) 219. [8] R. Hunger, K. Sakurai, A. Yamada, P. Fons, K. Iwata, K. Matsubara, S. Niki, Thin Solid Films 431 (2003) 16. [9] K. Sakurai, A. Yamada, P. Fons, K. Matsubara, T. Kojima, T. Baba, N. Tsuchimochi, Y. Kimura, H. Nakanishi, S. Niki, J. Phys. Chem. Solids 64 (2003) 1877. [10] O. Lundberg, J. Lu, A. Rockett, E. Edoff, L. Stolt, J. Phys. Chem. Solids 64 (2003) 1499. [11] A. Yamada, P. Fons, K. Matsubara, K. Iwata, K. Sakurai, S. Niki, Thin Solid Films 431–432 (2003) 277.
Structural changes of CuGaSe2 films during the three-stage process observed by spectroscopic light scattering K. Sakuraia,*, S. Nakamurab, T. Babab, Y. Kimurab, A. Yamadaa, S. Ishizukaa, K. Matsubaraa, P. Fonsa, R. Scheerc, H. Nakanishib, S. Nikia a
AIST, Research Center for Photovoltaics, AIST Central #2-24412, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan b Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-0022, Japan c HMI, Division Solar Energy, Glienicker Strasse 100, 14109 Berlin, Germany Available online 10 December 2004
Abstract We have studied the three-stage deposition process of CuGaSe2 (CGS) films, with the help of spectroscopic light scattering (SLS) in situ monitoring method. During the second stage (i.e. deposition of Cu, Se), an abrupt change from a two-layered structure to a single layer, together with a significant increase in the grain size, was observed in the vicinity of the Cu/Ga=1.0 point. Shortly before this abrupt change, a characteristic peak was observed by SLS, accompanied by a definitive change in depth-oriented distribution of Cu and Na. D 2004 Published by Elsevier B.V. Keywords: Spectroscopic light scattering; CuGaSe2 films; Deposition process
1. Introduction Recently, Cu(In1 x Gax )Se2 (CIGS) solar cells have achieved a conversion efficiency of over 19%, the highest among any polycrystalline thin film solar cells, using the three-stage process [1]. Currently, the best performance is achieved at a Ga composition of x~0.3, where it gives a bandgap of E g~1.2 eV. Though the theoretical optimum is E g~1.5 eV [2] (x~0.8 [3]), the reported cell performances tend to degrade at E gN1.2 eV so far [4]. To realize the expected higher cell performances, further understanding and developments in the deposition process of CIGS film, especially at high Ga contents, are being required. For example, Hanna et al. [5] report a limitation of cell performance by the bulk defects in CIGS films with high Ga content, also showing that the deposition method can largely influence the bulk defect density. To help further analysis of the deposition process, we have previously introduced spectroscopic light scattering (SLS), which is an in situ monitoring technique useful for investigating the * Corresponding author. Tel.: +81 29 861 9176; fax: +81 29 861 5615. E-mail address: [email protected] (K. Sakurai). 0040-6090/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.tsf.2004.11.095
structural and optical properties of thin films [6,7]. We had observed distinctive changes in the SLS signal profiles influenced by the Ga content, suggesting clues for the optimization of the deposition process at high Ga (xN0.5) contents [7]. In this work, in order to obtain more clues to understand the deposition process at high Ga content, we have investigated the structural development of the CuGaSe2 (x=1, CGS) film during deposition. We have interrupted the process at several points where characteristic SLS profiles have been observed, then conducted various ex situ measurements on those interrupted samples.
2. Experimental 2.1. System We have used a molecular beam epitaxy (MBE) system for deposition of the CGS films (Fig. 1). Each source beam of Cu, Ga (7 N) or Se (6 N) was provided from a hot-lip Effusion cell, which provided a stable beam pressure throughout our relatively long deposition process (roughly 2 h per sample). The drift of the beam pressure during one
368
K. Sakurai et al. / Thin Solid Films 480–481 (2005) 367–372
was about 5 min for the interrupted samples, and about 40 min for the bfinishedQ sample (sample 9). After cooling and taking out into the air, the composition and structure of the film was studied by ex situ measurements (here, we have to be aware that some properties of the film may have changed during the cooldown process, though it was much shorter than the total deposition time (~120 min)). We have used electron probe analysis (EPMA), scanning electron microscopy (SEM), X-ray diffraction (XRD; h–2h scan), secondary ion microanalysis (SIMS), and inductively coupled plasma (ICP), and their relationships with the SLS signal were investigated. Fig. 1. Schematic diagram of the MBE system used in this work.
3. Results deposition process was smaller than 2% for Cu, Ga, and smaller than 5% for Se. The system had a separate exchange chamber so that we could exchange samples without changing the temperature of the cells, keeping the beam pressures constant all the time. Consequently, the difference of the deposition speed between consecutive deposition runs was negligible. The MBE system was equipped with liquid nitrogen cryoshrouds. The base pressure was lower than 210 7 Pa when the cells were not heated. The substrate was heated by radiation from a heater through an unpolished sapphire heat-diffuser plate, and constantly rotated in-plane for uniformity. The absolute temperature was determined by a thermocouple behind the substrate. For determining the process timings, a pyrometer was used for monitoring the relative change of the temperature [7,8]. Spatial incoherent white light from a 150 W halogen lamp was irradiated to the sample surface through a glass viewport. The scattered light in the wavelength range of 300–800 nm was monitored by a CCD spectrometer (OceanOptics USB2000) at a nonspecular location, and the signal was integrated every 5 s. The observed intensity profiles at each wavelength were recorded as the SLS signal.
3.1. SLS and pyrometer Fig. 2 shows a typical set of SLS and temperature signals during the three-stage deposition of CGS. The timings of the deposition interruption are indicated by points 1–9. During the first stage, the SLS signal was weak and below the threshold of the CCD, as most of the incident light was reflected by the smooth surface. During stage 2, after passing point 3, the SLS signal showed a rapid increase when the compositional ratio Cu/Ga reached around 0.5,
2.2. Process and evaluation Soda-lime glass (SLG) with RF-sputtered Mo was used as the substrate. Prior to use, the Mo/SLG substrate was heated in vacuum at 550 8C for 40 min, in order to adjust the Na diffusion into CIGS [9]. In our standard deposition process for CGS, the Cu/Ga ratio at the end of stage 2 was 1.3, the final Cu/Ga ratio was 0.93, and the final thickness was 1700 nm. The substrate temperature during stage 1 was 315 8C, and the maximum substrate temperature was 570 8C. To investigate the film properties during this deposition process and their relationships with the pyrometer and SLS signals, we have fabricated a series of CGS/Mo/SLG samples consecutively, with each deposition run interrupted at a different timing. After interruption of the deposition run, each sample was cooled down as rapidly as possible, under irradiation of Se until the temperature fell below 300 8C. The duration of the cooldown process from 570 to 300 8C
Fig. 2. Typical SLS and pyrometer profiles taken during our three-stage deposition process. Numbers 1–9 correspond to the deposition interruption points of samples 1–9. (Note: Only sample 9 (=finished film) was cooled down slowly as shown here; other samples were cooled down as rapidly as possible.)
K. Sakurai et al. / Thin Solid Films 480–481 (2005) 367–372
followed by distinctive peaks at higher Cu/Ga ratios. The sharp peak S S appeared simultaneously with the kink point S P of the pyrometer signal, which corresponds to the bstoichiometry pointQ, where the compositional ratio Cu/ Ga ratio is expected to become 1.0. Prior to the appearance of S S, a maximum peak ( P) was observed. This peak P was not observed for CIGS films with low (xb0.4) Ga contents under the same deposition condition [7]. The SLS signal during the cooldown process showed a difference between Cu-rich and Cu-poor samples. For the Cu-poor samples (samples 1–4 and 9), the SLS signal kept constant at all observed wavelengths during the cooldown process, indicating that the optical (and some structural) properties of the surface region of the film was preserved. On the other hand, in the case of the Cu-rich samples (samples 5–8), the SLS signal intensity changed with decreasing temperature, suggesting changes in the properties of the film.
369
3.2. SEM During stage 2, in samples 2–4, the film showed a twolayered structure as seen in Fig. 3(b)–(d). The upper layer consisted of relatively small grains. Its thickness was similar for samples 2–4, while the thickness of the lower layer increased. The grain size of the lower layer drastically increased from sample 3 to sample 4. After passing the point S S, at sample 5, the two-layered structure disappeared and the grain size increased (Fig. 3(e)), of which its interruption timing was about 5 min after sample 4 (Fig. 3(d)). The film structure observed by SEM was similar for samples 5–9, as the Cu/Ga ratio went up to 1.3 (Cu-rich) and returned to 0.93 (Cu-poor). We also note that the two-layered structure (as seen in Fig. 3(b)–(d)) was not observed for CIGS films with low (xb0.4) Ga contents, under the same deposition condition except for the In/Ga ratio.
Fig. 3. SEM images of cleaved ((Cu),Ga,Se)/Mo/SLG samples. Distinctive two-layered structures were observed in the CGS layer of samples 2–4. (Note: The white part in image (a) corresponds to the Mo layer.)
370
K. Sakurai et al. / Thin Solid Films 480–481 (2005) 367–372
3.3. XRD XRD analysis indicated that the film was comprised of GaSe and Ga2Se3 during the first stage (sample 1). For sample 3, XRD peaks of Ga2Se3 and CuGaSe2 was observed, indicating phase separation. For samples 4 and 5, only the peaks of CuGaSe2 could be observed. 3.4. EPMA
Fig. 4. EPMA results of samples 1–9. The acceleration voltage was set to 7 keV, in order to obtain relatively surface-sensitive signals. The dotted line is only an aid for eyes.
In the first half of stage 2, EPMA measurements using a relatively low energy electron beam of 7 keV (which is sensitive to the composition of the surface region) showed a Cu/Ga compositional ratio increasing faster than the expected linear increase rate (Fig. 4). The surface of the CGS film became extremely Cu-rich, as can be confirmed by the results of EPMA (Fig. 4) and SIMS depth profile (Fig. 5(c)), indicating accumulation of Cu-rich phases at the surface.
Fig. 5. SIMS depth profiles of samples 3–5.
K. Sakurai et al. / Thin Solid Films 480–481 (2005) 367–372 Table 1 Cu/Ga compositional ratios of samples 3–5 Sample no.
Expected Cu/Ga ratio
Measured Cu/Ga (by ICP)
3 4 5
0.62 0.92 1.05
0.59 0.88 1.03
bExpectedQ values are calculated from the process timings, supposing Cu/ Ga=1.0 at point S P. Compared with the ICP results, the error range of the bExpectedQ values is estimated to be around 3%, mainly due to possible pressure fluctuations of Cu.
3.5. SIMS For the samples 2 and 3, SIMS depth profiles also indicated a two-layered structure, with the Cu content higher in the upper layer of the film, as seen in Fig. 5(a). Meanwhile, the Na distribution was significantly lower in the upper layer. The Cu profile in the lower layer showed a bdipQ, while the Na distribution showed the opposite behavior. For sample 4, the Cu and Ga composition were almost uniform throughout the film, and a distinctive bhumpQ in the Na distribution around the boundary of the layered structure was observed. 3.6. ICP The average Cu/Ga ratio of the whole film at point S P was estimated to be in the range of 0.95–1.0, from the ICP measurement results on samples 4 and 5 (Table 1). This indicates that the Cu/Ga ratio estimated from the pyrometer profile was close enough to the average Cu/Ga ratio of the while film.
4. Discussions The most interesting point is the observation of the twolayered structure observed for samples 2–4, which was not observed for the CIGS films with low (xb0.3) Ga content. From the beginning of stage 2, the Cu composition of the upper layer was significantly higher than the lower layer, confirmed by EPMA and SIMS measurements. As our deposition process was operated relatively slowly (2 h/ sample), we can estimate that the deposition speed itself was not too fast to obstruct the Cu/Ga interdiffusion, compared to the short cooldown period after the interruption. Taking the XRD results together into account, we attribute this two-layered structure to the phase separation between Ga2Se3 and CuGaSe2. In sample 4, we could not confirm existence of Gax Sey compounds. This is reasonable, as the whole film was close to stoichiometry. However, for dissolution of the two-layered structure, it seems that a Cu/Ga ratio closer to (or exceeding) 1.0 was required. The sharp peak of SLS strongly suggest that this drastic change began from the point S S, where the Cu/Ga
371
ratio was estimated to be within the range of 0.95–1.0 by ICP. Another point of interest is the distribution of Cu and Na observed by SIMS, which evolves in opposite ways in sample 3. This SIMS profile suggests that Na (or Na compounds) had filled the Cu vacancies in some form and stabilized them, therefore slowing the Cu/Ga interdiffusion. Recently, Lundberg et al. [10] reported that the presence of Na reduces the interdiffusion speed of Cu and Ga. They also have suggested that Na reduces the concentration of metal vacancies. Their report is in correspondence with our results and assumption. Meanwhile, the bhumpQ in the Na distribution of sample 4 suggests concentration of metal vacancies at the boundary of the upper and lower layer. Other point of interest is the appearance of the peak P in the SLS signal, before the bstoichiometry pointQ S S. We have considered on the following two explanations for the peak P: (1) Early emergence of Cux Se phases at the film surface. Here, the EPMA result seems to support this hypothesis. However, as the absolute value of EPMA result can be deviated by surface roughness [11], EPMA alone is not enough to prove the existence of Cux Se phases in this case. In addition, if such Cu-rich phases have existed, they must have disappeared after the deposition interruption, as there was no indication of such phases in SEM, SIMS and XRD results. However, no such structural change was indicated by SLS after the deposition interruption at point 4. Thus we consider that this explanation is unlikely. (2) Changes in surface structure, mainly induced by the structural change of the lower layer. As seen in Fig. 3, the structure of the lower layer have changed drastically somewhere between points 3 and 4. Considering the SIMS and XRD results, we can estimate that this change was triggered by the elimination of Ga2Se3 phases at point P, followed by the enlargement of the grain size in the lower layer. Meanwhile, it is also notable that the appearance of peak P could be used for predicting the timing of S S, before actually reaching the stoichiometry.
5. Summary The structural and compositional changes during the three-stage deposition process of CGS were investigated. A series of deposition-interrupted samples were evaluated by various means including SEM, EPMA and SIMS, and their relationships with the spectroscopic light scattering (SLS) signals were discussed. A distinctive two-layered structure was observed during stage 2, and this layered structure was found to abruptly dissolve in the vicinity of Cu/Ga=1.0 point.
372
K. Sakurai et al. / Thin Solid Films 480–481 (2005) 367–372
Acknowledgements This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO). References [1] K. Ramanathan, M.A. Contreras, C.L. Perkins, S. Asher, F.S. Hasoon, J. Keane, D. Young, M. Romero, W. Metzger, R. Noufi, J. Ward, A. Duda, Prog. Photovolt. Res. Appl. 11 (2003) 225. [2] M. Green, Solar Cells, Prentice-Hall, EngleWood Cliffs, 1982. [3] C. Paorici, L. Zanotti, N. Romeo, G. Sberveglieri, L. Tarricone, Sol. Energy Mater. 1 (1979) 3. [4] S. Siebentritt, Thin Solid Films 403–404 (2002) 1.
[5] G. Hanna, A. Jasenek, U. Rau, H.W. Schock, Thin Solid Films 387 (2001) 71. [6] R. Scheer, A. Neisser, K. Sakurai, P. Fons, S. Niki, Appl. Phys. Lett. 82 (2003) 2091. [7] K. Sakurai, R. Hunger, C.A. Kaufmann, A. Yamada, Y. Baba, Y. Kimura, K. Matsubara, P. Fons, H. Nakanishi, S. Niki, Prog. Photovolt. Res. Appl. 12 (2004) 219. [8] R. Hunger, K. Sakurai, A. Yamada, P. Fons, K. Iwata, K. Matsubara, S. Niki, Thin Solid Films 431 (2003) 16. [9] K. Sakurai, A. Yamada, P. Fons, K. Matsubara, T. Kojima, T. Baba, N. Tsuchimochi, Y. Kimura, H. Nakanishi, S. Niki, J. Phys. Chem. Solids 64 (2003) 1877. [10] O. Lundberg, J. Lu, A. Rockett, E. Edoff, L. Stolt, J. Phys. Chem. Solids 64 (2003) 1499. [11] A. Yamada, P. Fons, K. Matsubara, K. Iwata, K. Sakurai, S. Niki, Thin Solid Films 431–432 (2003) 277.