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CuGaSe2 bilayer thin films by x-ray diffraction
Journal of Crystal Growth 432 (2015) 24–32
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Probing diffusion of In and Ga in CuInSe2/CuGaSe2 bilayer thin films by x-ray diffraction B. Namnuan a,b, K. Yoodee a,b, S. Chatraphorn a,b,n a b
Department of Physics, Faculty of Science, Chulalongkorn University, Phyathai Road, Bangkok 10330, Thailand Research Center in Thin Film Physics, Thailand Center of Excellence in Physics, CHE, 328 Si Ayutthaya Road, Bangkok 10400, Thailand
art ic l e i nf o
a b s t r a c t
Article history: Received 22 June 2015 Received in revised form 9 September 2015 Accepted 11 September 2015 K.H. Ploog Available online 25 September 2015
The bilayer thin films of CuInSe2 (CIS) and CuGaSe2 (CGS) are fabricated by a molecular beam deposition (MBD) technique by sequential depositions of CGS followed by CIS and vise versa on Mo-coated sodalime glass (SLG) and Mo/Al2O3-coated (Na blocking) SLG substrates. The thicknesses of CIS/CGS layers are adjusted to have the overall x ¼[Ga]/([In] þ[Ga]) at approximately 0.4 similar to what is generally used in CIGS thin film solar cells. The growth temperature and the Cu-ratio y¼ [Cu]/([In]þ [Ga]) of the CIS and CGS layers are systematically varied for Cu-rich (y 41) and Cu-poor (y o1) conditions. X-ray diffraction (XRD) technique is used to analyze the shift of diffraction peaks of the preferred orientations of the bilayers compared to those of the single-layer CIS, CGS and CIGS thin films. The XRD results show that higher Ga diffusion is observed in the Cu-rich CIS/CGS bilayer rather than others. The results suggest that the diffusion of Ga is enhanced by the excess Cu–Se phase that is dependent upon the substrate temperature. The Na from the SLG substrate is one of the important factors affecting Ga diffusion. The results show alloying CIGS patterns rather than separated CIS and CGS patterns as observed in the bilayers with Na. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Defects A1. Diffusion A1. X-ray Diffraction A3. Physical vapor deposition process B2. Semiconducting ternary compounds B3. Solar cells
1. Introduction CuIn1 xGaxSe2 (CIGS) is a chalcopyrite semiconductor that has been used as a photon absorber layer for thin film photovoltaic applications. It is a promising candidate for high efficiency energyconversion devices. Recently, lab-scale devices with more than 20% energy-conversion efficiency have been achieved [1–3]. It also has a high potential toward commercialization when compared with other existing thin film solar cells in the market. Its band gap energy can be adjusted between 1.02 and 1.66 eV by varying the atomic ratio of group III elements, i.e. x¼[Ga]/([Ga]þ[In]) in the range of 0–1. Being able to change its energy band gap is one of the advantages of the CIGS material. The amount of Ga is an important factor for adjusting the band gap energy to achieve higher performance of the photovoltaic devices. At present, the performance of the CIGS thin film solar cells can also be enhanced by Ga-grading technique [4–7]. Thus, the diffusion of Group-III elements, especially Ga, is necessary. An understanding of diffusion mechanisms or factors that drive the diffusion of In and Ga needs further investigation in order to improve the growth technique and help n Corresponding author at: Department of Physics, Faculty of Science, Chulalongkorn University, Phyathai Road, Bangkok 10330, Thailand. Tel.: þ 66 2 218 7560; fax: þ 66 2 253 1150. E-mail address: [email protected] (S. Chatraphorn).
http://dx.doi.org/10.1016/j.jcrysgro.2015.09.010 0022-0248/& 2015 Elsevier B.V. All rights reserved.
increasing the performance of the CIGS thin film solar cells. One of the commonly used deposition techniques for high efficiency CIGS thin film solar cells is the three-stage deposition process [8] which generally yields the conversion efficiency greater than 15% (also depending on the properties of other thin film layers, e.g. Mo, CdS and ZnO). However, a study of the diffusion of In and Ga during the three-stage growth process is somewhat difficult since intermixing or diffusion of both elements gradually occurs in the second and the third stage of the deposition process. In addition, the excess Cu–Se occuring when y¼[Cu]/ ([Ga]þ[In])41 is believed to be in a liquid phase [9] and plays an important role to the diffusion of In and Ga [10,11]. Thus the bilayers of CIS/CGS and CGS/CIS are used to study the diffusion of In and Ga in this work. There are some previous works related to the diffusion of In and Ga in CIGS materials suggesting different diffusion mechanisms. Walter and Schock suggested that inter-diffusion of Ga and In were arising out of the presence of the Cu2 xSe secondary phase that usually took place in Cu-rich composition (y41) [11]. On the other hand, Schroeder et al.’s work on the CIS epitaxial films showed that it did not depend on the existence of Cu2 xSe, but was closely related to the Cu-atomic composition (y) in such a way that Ga diffusion is less in the near stoichiometric CIS films (y¼1), but higher in both Cu-rich (y41) and Cu-poor (yo1) films [12]. It was suggested that the diffusion of Ga could proceed via vacant lattice sites. The difference of Cu-atomic composition between Cu-rich and
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lattice sites that Ga and In could diffuse through either Cu sites or group-III sites, while the Cu2 xSe phase has a little role for group-III diffusion, but higher diffusion was found in Na free films [15,16]. In this work, the combinations of Cu-rich and Cu-poor in the CIS/CGS and CGS/CIS bilayers have been studied systematically
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Cu-poor films leads to the different types of vacancy point defects such as Cu vacancies and group-III vacancies in the Cu-poor and Curich compositions, respectively. However, O. Lundberg et al. [13] and Bodegård et al. [14] substantiated that the diffusion of Ga and In in polycrystalline CIS and CGS bilayer structure proceeded via vacant
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using the co-evaporation technique under ultrahigh vacuum environment to observe the diffusion of In and Ga. The values of y are set at 1.2 for Cu-rich and 0.85 for Cu-poor as well as x ¼0.4 similar to the elemental compositions used in the three-stage process for solar cell fabrication. In addition, the roles substrate types, containing Na (SLG) and Na-free (Al2O3/SLG), are also investigated. In this experiment, the diffusion of In and Ga are simply observed from the shift of the 2θ peaks of the x-ray diffraction (XRD) patterns of the preferred orientations compared with the single-layer CIS, CGS and CIGS thin films.
2. Experimental details CIS/CGS and CGS/CIS bilayers as well as CIS, CGS and CIGS (x¼0.4) thin films were deposited on 3 cm 3 cm Mo-coated sodalime glass (SLG) substrates by molecular beam deposition method at Tsubstrate ¼ 600 °C (measured by a pyrometer) using EIKO model EW-100 MBE system. The 600 nm thick Mo layer was deposited on the SLG substrates by DC magnetron sputtering from a Mo metallic target using Ar gas and DC power at 6.0 10 3 mbar and 550 W, respectively. In order to prohibit the out-diffusion of Na from the SLG substrates, a 1000 nm thick Al2O3 thin film was deposited prior to the Mo layer by RF magnetron sputtering technique using Al2O3 ceramic target. The Ar gas and the RF power during the sputtering process were set at 2.0 10 3 mbar and 260 W, respectively. The target was electrically floated and the target to substrate distance was about 6 cm. The thickness of the bilayers was calculated from the flux of Cu, In and Ga for the deposition of 1000 nm with the thickness of CIS: CGS set for 600 nm: 400 nm to have x¼0.4. However, the actual thicknesses of the bilayers shown in Figs. 1–4 are in the range of 800–1190 nm resulting in the average value of x approximately 0.4070.05. The combinations of Cu-rich (y¼ 1.2) and Cu-poor (y¼0.85) in each layer as well as types of substrate, e.g. with and without Na, were investigated. The degree of Cu-rich is slightly higher than other previous works [13]. The total deposition time was approximately 40 min. The Bruker D8 Advance X-ray Diffractometer was used to investigate the shift of the (112) diffraction peak of the bilayers differentiating from those of the single-layer CIS, CGS and CIGS (x ¼0.4) thin films to indicate the diffusion of In and Ga. The compositional depth profiles of the films were verified by the glow-discharged optical emission spectroscopy (Jobin YvonGDOES) as well as series of EDS point measurement attached to the JEOL JSM-7001F FESEM.
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(a–d) show larger grains than those grown on the SLG substrates shown in Fig. 1(a–d). On the contrary, the grains become smaller when CIS is deposited as the bottom layer on the substrates without Na, as seen in Fig. 4(a–d). The XRD patterns of the bilayers on the Al2O3/SLG substrates in Figs. 3 and 4 (black lines) are also plotted in comparison with the results from the SLG substrates (gray lines). The detailed results and discussion of all bilayers are as followed. 3.1. Cu-rich CIS/Cu-rich CGS and Cu-rich CGS/Cu-rich CIS bilayers The cross-sectional images showing the grains of CIS(y¼1.2)/CGS (y¼ 1.2) and CGS(y¼1.2)/CIS(y¼1.2) bilayers along with their XRD patterns on the SLG substrates are illustrated in Figs. 1 and 2(a), whereas the corresponding bilayers on the Al2O3/SLG substrates are shown in Figs. 3 and 4(a), respectively. It is obviously seen that all Curich bilayers have relatively larger grain size. The XRD patterns indicate that the inter-diffusion of Ga and In in CIS/CGS and CGS/CIS bilayers are significantly different by the order of depositions. For the CIS/CGS bilayer on the SLG substrate, the XRD pattern plotted in Fig. 1(a) shows a single broadening peak due to the intermixing of CIS and CGS, while the XRD pattern of the CIS/CGS bilayer on the Al2O3/SLG substrate in Fig. 3(a) shows a better coalescent pattern of relatively more homogeneous CIGS with x 0.4. The sharper XRD peak is also consistent with the larger grain size observed in the bilayer on the Al2O3/SLG substrate. This result indicates that the inter-diffusion of Ga and In occurs during the deposition of the CIS with the assist of excess Cu–Se resulting in the formation of alloying CIGS [17]. On the other hand, for the CGS/CIS bilayers, the two separated peaks are clearly seen on both SLG and Al2O3/SLG substrates indicating less inter-diffusion of Ga and In. Although, the existing Cu–Se compound can enhance the elemental inter-diffusion, the CIS as a bottom layer seems to be difficult for the inter-diffusion of In and Ga. The compositional depth profile measurement shown in Fig. 5 (a) also indicates the inter-diffusion of In and Ga throughout the CGS/CIS bilayers with more Ga concentration in the upper 0.4 μm and more In concentration in the lower 0.6 μm of the bilayers. These results suggest that despite the large grain growth obtained from the deposition, it does not necessarily indicate good elemental diffusion. The sharper XRD patterns in the CIS/CGS bilayer on the substrate without Na also suggests that more vacancies are available causing higher diffusion. In this experiment, it cannot be pointed out clearly the effect of Na in the Cu-rich samples. However, the works done by Niles et al. and Wei et al. [15] suggest that Na can occupy group-III vacancies (VIn or VGa). 3.2. Cu-poor CIS/Cu-poor CGS and Cu-poor CGS/Cu-poor CIS bilayers
3. Results and discussion All bilayers were characterized for the 2θ peak of (112) orientation of the chalcopyrite structure and compared to the position of (112) orientation of CIS, CGS and CIGS (x¼0.4). The qualitative indications of In and Ga diffusions in the bilayers are determined from the deviations from the (112) orientation of CIS and CGS while the quantitative descriptions of the diffusions of In and Ga are given by their diffusion coefficients calculated from compositional depth profiles described in Section 3.5. It is also noted that the (220)/(204) orientations were also observed with significantly lower intensity and thus not shown here. The results of the bilayers on the SLG substrates with the CGS as the bottom layer and the CIS as the top layer are shown in Fig. 1(a–d) and the results of the bilayers with alternated order of depositions are shown in Fig. 2(a–d) whereas the corresponding bilayers on the substrates without Na, e.g. Al2O3/ SLG, are shown in Figs. 3 and 4(a–d), respectively. It should be noted here that the growth of the bilayers with CGS as the bottom layer and CIS as the top layer on the Al2O3/SLG substrates shown in Fig. 3
When all layers are Cu-poor (y¼ 0.85), the SEM cross-section images and the corresponding XRD patterns of these bilayers on both SLG and Al2O3/SLG substrates are shown in Figs. 1–4(b), respectively. The separation of different grain sizes of the CIS and CGS layers can visually be observed in the cross-sectional images. The XRD patterns of the bilayers on the SLG and the Al2O3/SLG substrates are nearly identical, but with lower crystal quality on the Al2O3/SLG substrates. The separated XRD peaks near CIS and CGS positions suggest that non-homogeneous alloying CIGS is formed during the bilayer depositions with the mixing of smaller and larger values of x. These results imply that the diffusion of In and Ga may proceed by the Cu vacancies [18,19], i.e. InCu and GaCu seen in other experiment by Kushiya et al. [20]. 3.3. Cu-poor CIS/Cu-rich CGS and Cu-rich CGS/Cu-poor CIS bilayers For the combinations of Cu-rich CGS and Cu-poor CIS bilayers on the SLG substrates, the grains of CIS(y¼0.85)/CGS(y¼1.2) shown in Fig. 1(c) are visibly larger than those of CGS(y¼1.2)/CIS(y¼0.85)
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on the SLG substrate shown in Fig. 5(b) indicates the diffusion of Ga from the bottom layer to about 0.3 μm below the surface of the bilayer (half of the CIS thickness) thus leaving some pure (112) CIS phase. The diffusion of In through the thickness of the bilayer with decreasing concentration toward the bottom surface results in the shifting the CGS peak toward smaller 2θ. For the alternated order of depositions (Fig. 2(c)) having the Cupoor CIS as the bottom layer, there is no Cu–Se readily available in this case. Similar diffusion of group-III occurs but the result suggests that there is more Ga diffusion causing the formation of alloying CIGS as well, but with slightly smaller value of x than that in Fig. 1 (c) and also resulting in the reduction of the intensity of CIS peak. For the CIS(y¼0.85)/CGS(y¼1.2) bilayer grown on the Al2O3/SLG substrate, the XRD pattern in Fig. 3(c) implies that the diffusions of In and Ga are much better than that on the SLG substrate as indicated by better coalescence of the CIS and CGS peaks, however, not as good as in Fig. 3(a). The shoulder of the coalescing XRD peak of the alloying CIGS in Fig. 3(c) suggests that there is some pure CIS phase left in the bilayer due its thicker layer. This also implies that In diffuses better than Ga which is consistent with the diffusion coefficient of In greater than that of Ga as shown in Table 1. On the other hand, the diffusions in the CGS(y¼1.2)/CIS(y¼ 0.85) bilayers on both SLG and Al2O3/SLG substrates are quite similar as seen in the XRD patterns in Fig. 4(c) but with lower crystal quality on the substrate without Na. The results here suggest that group-III vacancies are more preferable than Cu vacancies and the deposition order of the bilayers affects the diffusion of In and Ga. 3.4. Cu-rich CIS/Cu-poor CGS and Cu-poor CGS/Cu-rich CIS bilayers
Fig. 5. Examples of atomic compositional depth profiles obtained from GDOES measurements of (a) CGS(y¼1.2)/CIS(y¼1.2), (b) CIS(y¼ 0.85)/CGS(y¼ 1.2), (c) CIS (y¼1.2)/CGS(y¼ 0.85) bilayers on SLG substrates whose cross sectional images and XRD patterns are shown in Figs. 2(a) and 1(c and d), respectively.
shown in Fig. 2(c). However, the deposition of the CIS(y¼ 0.85)/CGS (y¼ 1.2) bilayer on the Al2O3/SLG substrate exhibits relatively larger grain growth as shown in Fig. 3(c) than its counterpart on the SLG substrate shown in Fig. 1(c). For the bilayers grown on the SLG substrates, the XRD patterns show that the well-defined position of CIS is obtained while the CGS peak is shifted to form alloying CIGS toward the large x value in both bilayers. For the Cu-rich CGS as a bottom layer (Fig. 1(c)), the Cu–Se compound is readily available for the incoming CIS top layer which can support the growth of the CIS and also draws the diffusion of group-III elements. Since the CIS layer is thicker than the CGS layer, some of the pure CIS phase still remain, while the diffraction peak of CGS is shifted to smaller 2θ indicating the formation of alloying CIGS (with large x value). The compositional depth profile of the CIS(y¼0.85)/CGS(y¼ 1.2) bilayer
For the bilayers grown on the SLG substrates, it is obviously seen that when the CGS bottom layer becomes Cu-poor and the CIS top layer is Cu-rich as shown in Fig. 1(d), the grains are smaller than the bilayer with the alternated order of depositions having Cu-rich CIS bottom layer and Cu-poor CGS top layer as shown in Fig. 2(d). The results suggest that the Cu-rich bottom layer supports the larger grain growth as seen in Figs. 1(c) and 2(d). The XRD pattern of the CIS(y¼1.2)/CGS(y¼0.85) bilayer shown in Fig. 1(d) is qualitatively similar to that of CGS(y¼1.2)/CIS(y¼ 0.85) bilayer shown in Fig. 2(c). The compositional depth profile of the CIS(y¼1.2)/CGS(y¼ 0.85) bilayer shown in Fig. 5(c) indicates that Ga can diffuse from the bottom layer to about 0.1 μm below the surface thus leaving less pure CIS phase (indicated by lower XRD intensity when compared with Fig. 1(c)). The diffusion of In is observed throughout the thickness of the bilayer with less concentration toward the bottom surface similar to that in Fig. 5(b). On the other hand, the XRD pattern of CGS(y¼ 0.85)/CIS(y¼1.2) in Fig. 2(d) shows relatively less pure CIS and CGS phases indicated by the shift of CIS peak to larger 2θ and the shift of CGS peak to smaller 2θ. For the bilayers grown on the Al2O3/SLG substrates, it can be seen that the coalescing XRD pattern is also exhibited in the CIS(y¼1.2)/ CGS(y¼0.85) bilayer as shown in Fig. 3(d) with barely pure CIS left when compared with that of the CIS(y¼ 0.85)/CGS(y¼1.2) bilayer shown in Fig. 3(c). On the other hand, two separated peaks are observed in the CGS(y¼0.85)/CIS(y¼1.2) bilayer as shown in Fig. 4 (d) and is similar to that on the SLG substrate. It is worth to note here that Na does not affect the diffusion of In and Ga in the CGS/CIS bilayers as shown by the comparison of the XRD patterns in Fig. 4(a– d). The results shown in Figs. 2 and 4 where the bottom layer is CIS suggest that less inter-diffusion of In and Ga in the bilayers is due to the stronger crystal structure of CIS than that of the CGS in spite of the existence of excess Cu–Se, and the amount of Na in the films. It should be noted here that in the work by Schroeder et al. [12], they investigated and concluded that the diffusivities of In in polycrystalline bilayers and epitaxial layers were similar, thus suggesting that the diffusion in polycrystalline films was rather intragranular. In
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Table 1 Diffusion coefficients of In and Ga in the bilayers grown on the SLG and Al2O3/SLG substrates. Bilayers
Substrates Al2O3/SLG
SLG
CIS(y¼ 1.2)/CGS(y¼ 1.2) CGS(y¼ 1.2)/CIS(y¼ 1.2) CIS(y¼ 0.85)/CGS(y¼ 0.85) CGS(y¼ 0.85)/CIS(y¼ 0.85) CIS(y¼ 0.85)/CGS(y¼ 1.2) CGS(y¼ 1.2)/CIS(y¼ 0.85) CIS(y¼ 1.2)/CGS(y¼ 0.85) CGS(y¼ 0.85)/CIS(y¼ 1.2)
DIn ( 10 12 cm2/s)
DGa ( 10 12 cm2/s)
DIn ( 10 12 cm2/s)
DGa ( 10 12 cm2/s)
4.72 70.40 0.82 70.01 2.76 70.57 2.09 70.13 0.86 70.01 1.11 70.07 0.6770.01 1.65 70.07
2.92 7 0.23 0.43 7 0.03 0.777 0.08 1.017 0.06 0.707 0.01 0.737 0.05 0.82 7 0.01 1.277 0.07
5.46 7 0.22 1.28 7 0.17 2.017 0.28 1.87 7 0.19 11.6 7 0.9 2.81 7 0.31 6.137 0.41 1.60 7 0.07
21.7 76.3 0.60 70.07 0.60 70.08 2.11 70.21 2.50 70.14 3.7770.39 10.2 71.5 3.97 70.22
addition, Lundberg et al. [13] showed that there was no significant concentration gradient of diffusing elements (In or Ga) near the grain boundaries. This suggests that the grain boundary diffusion is not significantly faster than the intragranular diffusion. In our results, when comparing Fig. 1(a and c), the grain size is approximately the same but the diffusions observed from the coalescence of the XRD patterns of the two cases are significantly different. On the other hand, the grain size in Fig. 1(c) is quite larger than that in Fig. 1(d), but the coalescence of the XRD patterns in these two cases is similar suggesting that the diffusions are the same. In addition, the CIS/CGS bilayer on the substrate with Na (Fig. 1(c)) has significantly smaller grain than that of CIS/CGS bilayer on the substrate without Na (Fig. 3(c)), the merging of peaks in the XRD pattern in Fig. 3(c) is more pronounced implying more diffusion of group-III elements. Similar situation is observed in Figs. 1 and 3(d). These suggest that major diffusion is intragranular. Thus the effect of grain boundary diffusion or intergranular diffusion may be neglected in this work. The diffusions of In and Ga in the bilayers are influenced by the value of y (the richness or poorness of Cu) and the existence of Na. 3.5. Calculation of the diffusion coefficients of In and Ga As previously discussed and shown by Schroeder et al. [12] that the diffusions of In and Ga are similar in both polycrystals and expitaxial films, then the diffusion coefficient is calculated using Fick's second law based on the assumptions that (i) the diffusions of In and Ga are intragranular, (ii) the diffusion process occurs in one direction along the thickness of the films, (iii) the dominant mechanism is the lattice vacancies diffusion, (iv) the diffusion coefficient is independent of the concentration, and (v) the infinite medium is assumed for the diffusion of all species. Thus the solution to Fick's second law is given by [21,22] N z2 Cðz; tÞ ¼ pffiffiffiffiffiffiffiffiffiffiffiffie 4Dt ; 4π Dt
ð1Þ
where C is the concentration as a function of z – the distance along the diffusion direction that obtained from the compositional depth profile, t is the diffusion time, N is the number of deposited atoms obtained from the deposition flux and D is the diffusion coefficient. Thus the diffusion coefficient can be determined from the slope of the plot between ln C and z2, i.e. slope¼ 1/(4Dt). In the calculations, the diffusion coefficients of group-III elements in the bottom layer and the top layer are determined from the deposition time of the top layer only. The errors of the measurements of the compositional depth profiles are in the range of 0.5–1.0 at% in all bilayers regardless the grain size. The errors are propagated in the calculation of diffusion coefficients of In and Ga of the bilayers shown in Figs. 1–4 and are summarized in Table 1. It can be seen in most cases that the diffusion coefficients of In and Ga in the bilayers on the substrates without Na are larger than those of the substrates
with Na. This suggests that there are more vacancies in the films whose Na contribution from the SLG substrate is prohibited by the use Al2O3 layer. Among the bilayers on the substrates without Na, the diffusion coefficients of Ga are greater than those of In, while the diffusion coefficients of Ga in the bilayers on the substrate with Na are not necessarily larger than those of In. The diffusivities of In and Ga vary in the range that are comparable to those reported by Schroeder et al. [12] and Marudachalam et al. [22].
4. Conclusion The diffusion of In and Ga in the polycrystalline CIS/CGS and CGS/CIS bilayer thin films grown by molecular beam deposition technique was investigated by observing the shift of the XRD peaks of CIS and CGS as well as the depth profiles of In and Ga in the bilayers. The richness and poorness denoted by y¼ [Cu]/ ([Ga]þ [In]) was set for y¼ 1.2 and 0.85, respectively. The results can be summarized as: (1) When both layers are Cu-rich, the inter-mixing of In and Ga is relatively higher in CIS/CGS than in CGS/CIS. Stronger coalescence is observed in CIS/CGS on the substrate without Na. The diffusions of In and Ga in CGS/CIS on the substrates with and without Na are similar and less than that in CIS/CGS. It is implied that the diffusion is proceeded by the vacancies of group-III elements, e.g. existence of excess Cu–Se. However, the existence of Cu–Se does not necessarily promote the inter-diffusion of In and Ga as seen when the bottom layer is CIS. This suggests that the order of depositions affect the inter-diffusion of In and Ga. (2) Among the bilayers with the Cu-rich CGS as a bottom layer, the inter-diffusion of In and Ga is higher in the Na-free films than in the films containing Na. (3) The diffusion of In and Ga observed in CIS/CGS and CGS/CIS when both are Cu-poor should be proceeded by the Cu vacancies. However the strength of diffusion in this case is relatively less than in the case when both CIS and CGS are Cu-rich. (4) When the bottom layers are CIS, the diffusion of In and Ga are qualitatively similar in all bilayers both with and without Na, in spite of the overall richness or poorness of Cu in the bilayers. (5) Despite some variations in [Ga]/([In]þ[Ga]), approximately 10%, the diffusions of In and Ga are influenced by [Cu]/([In]þ[Ga]). Lastly, it is worth to note here that despite the diffusion mechanisms of In and Ga, the optical properties are crucial for the photovoltaic applications of the CIGS materials which needs further investigation. However, some of optical measurements, for example, by Thiru et al. [23] showed that the inter-mixing of In and Ga in the bilayers or trilayers occurred at higher substrate temperature which resulted in the increase of transition energy or
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energy gap of the alloying CIS and CGS layers due to Ga intermixing. This should have a direct effect on the performance of the photovoltaic devices made from these materials, i.e. the increase of open-circuit voltage (Voc) of such devices.
Acknowledgments This work was supported in part by the Asahi Glass Foundation, Ratchadaphiseksomphot Endowment Fund of Chulalongkorn University (RES560530181-AM), the Special Task Force for Activating Research (STAR), Ratchadaphiseksomphot Endowment Fund, Chulalongkorn University through the Energy Materials Physics Research Group and Research Funds from the Faculty of Science, Chulalongkorn University. B.N. also received support from the 90th Anniversary of Chulalongkorn University Fund (Ratchadaphiseksomphot Endowment Fund). The authors would also like to thank P. Panchawirat (ThEP Center, Thailand) for helping with SEM and EDS measurements, and the Department of Geology, Faculty of Science, Chulalongkorn University for the access to the XRD facility.
References [1] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Prog. Photovolt. 19 (2011) 894. [2] A. Chirilă, P. Reinhard, F. Pianezzi, P. Bloesch, A.R. Uhl, C. Fella, L. Kranz, D. Keller, C. Gretener, H. Hagendorfer, D. Jaeger, R. Erni, S. Nishiwaki, S. Buecheler, A.N. Tiwari, Nat. Mater. 12 (2013) 1107. [3] P. Jackson, D. Hariskos, R. Wuerz, W. Wischmann, M. Powalla, Phys. Status Solidi RRL 8 (3) (2014) 219. [4] J.R. Tuttle, D.S. Albin, A. Tennant, A.M. Gabor, M. Contreras, R. Noufi, Sol. Energy Mater. Sol. Cells 35 (1994) 193.
[5] T. Dullweber, G. Hanna, W. Shams-Kolahi, A. Schwartzlander, M.A. Contreras, R. Noufi, H.W. Schock, Thin Solid Films 361–362 (2000) 478. [6] O. Lunberg, M. Bodegård, J. Malmstrom, L. Stolt, Prog. Photovolt.: Res. Appl. 11 (2003) 77. [7] O. Lunberg, M. Edoff, L. Stolt, Thin Solid Films 480–481 (2005) 520. [8] R. Sakdanuphab, C. Chityuttakan, A. Pankiew, N. Somwang, K. Yoodee, S. Chatraphorn, J. Cryst. Growth 319 (2011) 44. [9] B. Walker, R. Agrawal, in: Proceedings of the IEEE 38th Photovoltaic Specialists Conference, 2012, p. 002654. [10] B. Arthibenyakul, C. Chityuttakan, S. Chatraphorn, J. Cryst. Growth 316 (2011) 97. [11] T. Walter, H.W. Schock, Thin Solid Films 224 (1993) 74. [12] D.J. Schroeder, G.D. Berry, A. Rockett, Appl. Phys. Lett. 69 (1996) 4068. [13] O. Lundberg, J. Lu, A. Rockett, M. Edoff, L. Stolt, J. Phys. Chem. Solids 64 (2003) 1499. [14] M. Bodegård, O. Lundberg, J. Lu, L. Stolt, Thin Solid Films 431–432 (2003) 46. [15] (a) S.H. Wei, S.B. Zhang, A. Zunger, J. Appl. Phys. 85 (1999) 7214; (b) D.W. Niles, K. Ramanathan, F. Hasoon, R. Noufi, B.J. Tielsch, J.E. Fulghum, J. Vaccum Sci. Technol. A 15 (1997) 3044; (c) D.W. Niles, M. Al-Jassim, K. Ramanathan, J. Vaccum Sci. Technol. A 17 (1999) 291. [16] D. Rudmann, M. Kaelin, F.J. Haug, F. Kurdesau, H. Zogg, A.N. Tiwari, in: Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion, 2003, p. 376. [17] S. Yoon, S. Kim, V. Cracium, W.K. Kim, R. Kaczynki, R. Acher, T.J. Anderson, O. D. Crisalle, S.S. Li, J. Cryst. Growth 281 (2005) 209. [18] S. Niki, A. Yamada, R. Hunger, P.J. Fons, K. Iwata, K. Matsubara, A. Nishio, H. Nakanishi, J. Cryst. Growth 237–239 (2002) 1993. [19] S. Niki, Y. Makita, A. Yamada, O. Hellman, P.J. Fons, A. Obara, Y. Okada, R. Shioda, H. Oyanagi, T. Kurafuji, S. Chichibu, H. Nakanishi, J. Cryst. Growth 150 (1995) 1201. [20] K. Kushiya, Y. Ohtake, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 33 (1994) 6599. [21] J. Crank, The Mathematics of Diffusion, 2nd ed., Oxford University Press, Oxford, 1975 (Chap. 2–3). [22] M. Marudachalam, H. Hichiri, R.W. Birkmire, J.M. Schultz, in: Proceedings of the IEEE 25th Photovoltaic Specialist Conference, 1996, p. 805. [23] S. Thiru, M. Asakawa, K. Honda, A. Kawaharazuka, A. Tackeuchi, T. Makimoto, Y. Horikoshi, AIP Adv. 5 (2015) 027120.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Probing diffusion of In and Ga in CuInSe2/CuGaSe2 bilayer thin films by x-ray diffraction B. Namnuan a,b, K. Yoodee a,b, S. Chatraphorn a,b,n a b
Department of Physics, Faculty of Science, Chulalongkorn University, Phyathai Road, Bangkok 10330, Thailand Research Center in Thin Film Physics, Thailand Center of Excellence in Physics, CHE, 328 Si Ayutthaya Road, Bangkok 10400, Thailand
art ic l e i nf o
a b s t r a c t
Article history: Received 22 June 2015 Received in revised form 9 September 2015 Accepted 11 September 2015 K.H. Ploog Available online 25 September 2015
The bilayer thin films of CuInSe2 (CIS) and CuGaSe2 (CGS) are fabricated by a molecular beam deposition (MBD) technique by sequential depositions of CGS followed by CIS and vise versa on Mo-coated sodalime glass (SLG) and Mo/Al2O3-coated (Na blocking) SLG substrates. The thicknesses of CIS/CGS layers are adjusted to have the overall x ¼[Ga]/([In] þ[Ga]) at approximately 0.4 similar to what is generally used in CIGS thin film solar cells. The growth temperature and the Cu-ratio y¼ [Cu]/([In]þ [Ga]) of the CIS and CGS layers are systematically varied for Cu-rich (y 41) and Cu-poor (y o1) conditions. X-ray diffraction (XRD) technique is used to analyze the shift of diffraction peaks of the preferred orientations of the bilayers compared to those of the single-layer CIS, CGS and CIGS thin films. The XRD results show that higher Ga diffusion is observed in the Cu-rich CIS/CGS bilayer rather than others. The results suggest that the diffusion of Ga is enhanced by the excess Cu–Se phase that is dependent upon the substrate temperature. The Na from the SLG substrate is one of the important factors affecting Ga diffusion. The results show alloying CIGS patterns rather than separated CIS and CGS patterns as observed in the bilayers with Na. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Defects A1. Diffusion A1. X-ray Diffraction A3. Physical vapor deposition process B2. Semiconducting ternary compounds B3. Solar cells
1. Introduction CuIn1 xGaxSe2 (CIGS) is a chalcopyrite semiconductor that has been used as a photon absorber layer for thin film photovoltaic applications. It is a promising candidate for high efficiency energyconversion devices. Recently, lab-scale devices with more than 20% energy-conversion efficiency have been achieved [1–3]. It also has a high potential toward commercialization when compared with other existing thin film solar cells in the market. Its band gap energy can be adjusted between 1.02 and 1.66 eV by varying the atomic ratio of group III elements, i.e. x¼[Ga]/([Ga]þ[In]) in the range of 0–1. Being able to change its energy band gap is one of the advantages of the CIGS material. The amount of Ga is an important factor for adjusting the band gap energy to achieve higher performance of the photovoltaic devices. At present, the performance of the CIGS thin film solar cells can also be enhanced by Ga-grading technique [4–7]. Thus, the diffusion of Group-III elements, especially Ga, is necessary. An understanding of diffusion mechanisms or factors that drive the diffusion of In and Ga needs further investigation in order to improve the growth technique and help n Corresponding author at: Department of Physics, Faculty of Science, Chulalongkorn University, Phyathai Road, Bangkok 10330, Thailand. Tel.: þ 66 2 218 7560; fax: þ 66 2 253 1150. E-mail address: [email protected] (S. Chatraphorn).
http://dx.doi.org/10.1016/j.jcrysgro.2015.09.010 0022-0248/& 2015 Elsevier B.V. All rights reserved.
increasing the performance of the CIGS thin film solar cells. One of the commonly used deposition techniques for high efficiency CIGS thin film solar cells is the three-stage deposition process [8] which generally yields the conversion efficiency greater than 15% (also depending on the properties of other thin film layers, e.g. Mo, CdS and ZnO). However, a study of the diffusion of In and Ga during the three-stage growth process is somewhat difficult since intermixing or diffusion of both elements gradually occurs in the second and the third stage of the deposition process. In addition, the excess Cu–Se occuring when y¼[Cu]/ ([Ga]þ[In])41 is believed to be in a liquid phase [9] and plays an important role to the diffusion of In and Ga [10,11]. Thus the bilayers of CIS/CGS and CGS/CIS are used to study the diffusion of In and Ga in this work. There are some previous works related to the diffusion of In and Ga in CIGS materials suggesting different diffusion mechanisms. Walter and Schock suggested that inter-diffusion of Ga and In were arising out of the presence of the Cu2 xSe secondary phase that usually took place in Cu-rich composition (y41) [11]. On the other hand, Schroeder et al.’s work on the CIS epitaxial films showed that it did not depend on the existence of Cu2 xSe, but was closely related to the Cu-atomic composition (y) in such a way that Ga diffusion is less in the near stoichiometric CIS films (y¼1), but higher in both Cu-rich (y41) and Cu-poor (yo1) films [12]. It was suggested that the diffusion of Ga could proceed via vacant lattice sites. The difference of Cu-atomic composition between Cu-rich and
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2 (Degree) Fig. 1. Cross-sectional images and XRD patterns of (112) CIS/CGS bilayers on SLG substrates.
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lattice sites that Ga and In could diffuse through either Cu sites or group-III sites, while the Cu2 xSe phase has a little role for group-III diffusion, but higher diffusion was found in Na free films [15,16]. In this work, the combinations of Cu-rich and Cu-poor in the CIS/CGS and CGS/CIS bilayers have been studied systematically
Log of Intensity (a.u.)
Cu-poor films leads to the different types of vacancy point defects such as Cu vacancies and group-III vacancies in the Cu-poor and Curich compositions, respectively. However, O. Lundberg et al. [13] and Bodegård et al. [14] substantiated that the diffusion of Ga and In in polycrystalline CIS and CGS bilayer structure proceeded via vacant
Cu-rich CIS (112)
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using the co-evaporation technique under ultrahigh vacuum environment to observe the diffusion of In and Ga. The values of y are set at 1.2 for Cu-rich and 0.85 for Cu-poor as well as x ¼0.4 similar to the elemental compositions used in the three-stage process for solar cell fabrication. In addition, the roles substrate types, containing Na (SLG) and Na-free (Al2O3/SLG), are also investigated. In this experiment, the diffusion of In and Ga are simply observed from the shift of the 2θ peaks of the x-ray diffraction (XRD) patterns of the preferred orientations compared with the single-layer CIS, CGS and CIGS thin films.
2. Experimental details CIS/CGS and CGS/CIS bilayers as well as CIS, CGS and CIGS (x¼0.4) thin films were deposited on 3 cm 3 cm Mo-coated sodalime glass (SLG) substrates by molecular beam deposition method at Tsubstrate ¼ 600 °C (measured by a pyrometer) using EIKO model EW-100 MBE system. The 600 nm thick Mo layer was deposited on the SLG substrates by DC magnetron sputtering from a Mo metallic target using Ar gas and DC power at 6.0 10 3 mbar and 550 W, respectively. In order to prohibit the out-diffusion of Na from the SLG substrates, a 1000 nm thick Al2O3 thin film was deposited prior to the Mo layer by RF magnetron sputtering technique using Al2O3 ceramic target. The Ar gas and the RF power during the sputtering process were set at 2.0 10 3 mbar and 260 W, respectively. The target was electrically floated and the target to substrate distance was about 6 cm. The thickness of the bilayers was calculated from the flux of Cu, In and Ga for the deposition of 1000 nm with the thickness of CIS: CGS set for 600 nm: 400 nm to have x¼0.4. However, the actual thicknesses of the bilayers shown in Figs. 1–4 are in the range of 800–1190 nm resulting in the average value of x approximately 0.4070.05. The combinations of Cu-rich (y¼ 1.2) and Cu-poor (y¼0.85) in each layer as well as types of substrate, e.g. with and without Na, were investigated. The degree of Cu-rich is slightly higher than other previous works [13]. The total deposition time was approximately 40 min. The Bruker D8 Advance X-ray Diffractometer was used to investigate the shift of the (112) diffraction peak of the bilayers differentiating from those of the single-layer CIS, CGS and CIGS (x ¼0.4) thin films to indicate the diffusion of In and Ga. The compositional depth profiles of the films were verified by the glow-discharged optical emission spectroscopy (Jobin YvonGDOES) as well as series of EDS point measurement attached to the JEOL JSM-7001F FESEM.
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(a–d) show larger grains than those grown on the SLG substrates shown in Fig. 1(a–d). On the contrary, the grains become smaller when CIS is deposited as the bottom layer on the substrates without Na, as seen in Fig. 4(a–d). The XRD patterns of the bilayers on the Al2O3/SLG substrates in Figs. 3 and 4 (black lines) are also plotted in comparison with the results from the SLG substrates (gray lines). The detailed results and discussion of all bilayers are as followed. 3.1. Cu-rich CIS/Cu-rich CGS and Cu-rich CGS/Cu-rich CIS bilayers The cross-sectional images showing the grains of CIS(y¼1.2)/CGS (y¼ 1.2) and CGS(y¼1.2)/CIS(y¼1.2) bilayers along with their XRD patterns on the SLG substrates are illustrated in Figs. 1 and 2(a), whereas the corresponding bilayers on the Al2O3/SLG substrates are shown in Figs. 3 and 4(a), respectively. It is obviously seen that all Curich bilayers have relatively larger grain size. The XRD patterns indicate that the inter-diffusion of Ga and In in CIS/CGS and CGS/CIS bilayers are significantly different by the order of depositions. For the CIS/CGS bilayer on the SLG substrate, the XRD pattern plotted in Fig. 1(a) shows a single broadening peak due to the intermixing of CIS and CGS, while the XRD pattern of the CIS/CGS bilayer on the Al2O3/SLG substrate in Fig. 3(a) shows a better coalescent pattern of relatively more homogeneous CIGS with x 0.4. The sharper XRD peak is also consistent with the larger grain size observed in the bilayer on the Al2O3/SLG substrate. This result indicates that the inter-diffusion of Ga and In occurs during the deposition of the CIS with the assist of excess Cu–Se resulting in the formation of alloying CIGS [17]. On the other hand, for the CGS/CIS bilayers, the two separated peaks are clearly seen on both SLG and Al2O3/SLG substrates indicating less inter-diffusion of Ga and In. Although, the existing Cu–Se compound can enhance the elemental inter-diffusion, the CIS as a bottom layer seems to be difficult for the inter-diffusion of In and Ga. The compositional depth profile measurement shown in Fig. 5 (a) also indicates the inter-diffusion of In and Ga throughout the CGS/CIS bilayers with more Ga concentration in the upper 0.4 μm and more In concentration in the lower 0.6 μm of the bilayers. These results suggest that despite the large grain growth obtained from the deposition, it does not necessarily indicate good elemental diffusion. The sharper XRD patterns in the CIS/CGS bilayer on the substrate without Na also suggests that more vacancies are available causing higher diffusion. In this experiment, it cannot be pointed out clearly the effect of Na in the Cu-rich samples. However, the works done by Niles et al. and Wei et al. [15] suggest that Na can occupy group-III vacancies (VIn or VGa). 3.2. Cu-poor CIS/Cu-poor CGS and Cu-poor CGS/Cu-poor CIS bilayers
3. Results and discussion All bilayers were characterized for the 2θ peak of (112) orientation of the chalcopyrite structure and compared to the position of (112) orientation of CIS, CGS and CIGS (x¼0.4). The qualitative indications of In and Ga diffusions in the bilayers are determined from the deviations from the (112) orientation of CIS and CGS while the quantitative descriptions of the diffusions of In and Ga are given by their diffusion coefficients calculated from compositional depth profiles described in Section 3.5. It is also noted that the (220)/(204) orientations were also observed with significantly lower intensity and thus not shown here. The results of the bilayers on the SLG substrates with the CGS as the bottom layer and the CIS as the top layer are shown in Fig. 1(a–d) and the results of the bilayers with alternated order of depositions are shown in Fig. 2(a–d) whereas the corresponding bilayers on the substrates without Na, e.g. Al2O3/ SLG, are shown in Figs. 3 and 4(a–d), respectively. It should be noted here that the growth of the bilayers with CGS as the bottom layer and CIS as the top layer on the Al2O3/SLG substrates shown in Fig. 3
When all layers are Cu-poor (y¼ 0.85), the SEM cross-section images and the corresponding XRD patterns of these bilayers on both SLG and Al2O3/SLG substrates are shown in Figs. 1–4(b), respectively. The separation of different grain sizes of the CIS and CGS layers can visually be observed in the cross-sectional images. The XRD patterns of the bilayers on the SLG and the Al2O3/SLG substrates are nearly identical, but with lower crystal quality on the Al2O3/SLG substrates. The separated XRD peaks near CIS and CGS positions suggest that non-homogeneous alloying CIGS is formed during the bilayer depositions with the mixing of smaller and larger values of x. These results imply that the diffusion of In and Ga may proceed by the Cu vacancies [18,19], i.e. InCu and GaCu seen in other experiment by Kushiya et al. [20]. 3.3. Cu-poor CIS/Cu-rich CGS and Cu-rich CGS/Cu-poor CIS bilayers For the combinations of Cu-rich CGS and Cu-poor CIS bilayers on the SLG substrates, the grains of CIS(y¼0.85)/CGS(y¼1.2) shown in Fig. 1(c) are visibly larger than those of CGS(y¼1.2)/CIS(y¼0.85)
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on the SLG substrate shown in Fig. 5(b) indicates the diffusion of Ga from the bottom layer to about 0.3 μm below the surface of the bilayer (half of the CIS thickness) thus leaving some pure (112) CIS phase. The diffusion of In through the thickness of the bilayer with decreasing concentration toward the bottom surface results in the shifting the CGS peak toward smaller 2θ. For the alternated order of depositions (Fig. 2(c)) having the Cupoor CIS as the bottom layer, there is no Cu–Se readily available in this case. Similar diffusion of group-III occurs but the result suggests that there is more Ga diffusion causing the formation of alloying CIGS as well, but with slightly smaller value of x than that in Fig. 1 (c) and also resulting in the reduction of the intensity of CIS peak. For the CIS(y¼0.85)/CGS(y¼1.2) bilayer grown on the Al2O3/SLG substrate, the XRD pattern in Fig. 3(c) implies that the diffusions of In and Ga are much better than that on the SLG substrate as indicated by better coalescence of the CIS and CGS peaks, however, not as good as in Fig. 3(a). The shoulder of the coalescing XRD peak of the alloying CIGS in Fig. 3(c) suggests that there is some pure CIS phase left in the bilayer due its thicker layer. This also implies that In diffuses better than Ga which is consistent with the diffusion coefficient of In greater than that of Ga as shown in Table 1. On the other hand, the diffusions in the CGS(y¼1.2)/CIS(y¼ 0.85) bilayers on both SLG and Al2O3/SLG substrates are quite similar as seen in the XRD patterns in Fig. 4(c) but with lower crystal quality on the substrate without Na. The results here suggest that group-III vacancies are more preferable than Cu vacancies and the deposition order of the bilayers affects the diffusion of In and Ga. 3.4. Cu-rich CIS/Cu-poor CGS and Cu-poor CGS/Cu-rich CIS bilayers
Fig. 5. Examples of atomic compositional depth profiles obtained from GDOES measurements of (a) CGS(y¼1.2)/CIS(y¼1.2), (b) CIS(y¼ 0.85)/CGS(y¼ 1.2), (c) CIS (y¼1.2)/CGS(y¼ 0.85) bilayers on SLG substrates whose cross sectional images and XRD patterns are shown in Figs. 2(a) and 1(c and d), respectively.
shown in Fig. 2(c). However, the deposition of the CIS(y¼ 0.85)/CGS (y¼ 1.2) bilayer on the Al2O3/SLG substrate exhibits relatively larger grain growth as shown in Fig. 3(c) than its counterpart on the SLG substrate shown in Fig. 1(c). For the bilayers grown on the SLG substrates, the XRD patterns show that the well-defined position of CIS is obtained while the CGS peak is shifted to form alloying CIGS toward the large x value in both bilayers. For the Cu-rich CGS as a bottom layer (Fig. 1(c)), the Cu–Se compound is readily available for the incoming CIS top layer which can support the growth of the CIS and also draws the diffusion of group-III elements. Since the CIS layer is thicker than the CGS layer, some of the pure CIS phase still remain, while the diffraction peak of CGS is shifted to smaller 2θ indicating the formation of alloying CIGS (with large x value). The compositional depth profile of the CIS(y¼0.85)/CGS(y¼ 1.2) bilayer
For the bilayers grown on the SLG substrates, it is obviously seen that when the CGS bottom layer becomes Cu-poor and the CIS top layer is Cu-rich as shown in Fig. 1(d), the grains are smaller than the bilayer with the alternated order of depositions having Cu-rich CIS bottom layer and Cu-poor CGS top layer as shown in Fig. 2(d). The results suggest that the Cu-rich bottom layer supports the larger grain growth as seen in Figs. 1(c) and 2(d). The XRD pattern of the CIS(y¼1.2)/CGS(y¼0.85) bilayer shown in Fig. 1(d) is qualitatively similar to that of CGS(y¼1.2)/CIS(y¼ 0.85) bilayer shown in Fig. 2(c). The compositional depth profile of the CIS(y¼1.2)/CGS(y¼ 0.85) bilayer shown in Fig. 5(c) indicates that Ga can diffuse from the bottom layer to about 0.1 μm below the surface thus leaving less pure CIS phase (indicated by lower XRD intensity when compared with Fig. 1(c)). The diffusion of In is observed throughout the thickness of the bilayer with less concentration toward the bottom surface similar to that in Fig. 5(b). On the other hand, the XRD pattern of CGS(y¼ 0.85)/CIS(y¼1.2) in Fig. 2(d) shows relatively less pure CIS and CGS phases indicated by the shift of CIS peak to larger 2θ and the shift of CGS peak to smaller 2θ. For the bilayers grown on the Al2O3/SLG substrates, it can be seen that the coalescing XRD pattern is also exhibited in the CIS(y¼1.2)/ CGS(y¼0.85) bilayer as shown in Fig. 3(d) with barely pure CIS left when compared with that of the CIS(y¼ 0.85)/CGS(y¼1.2) bilayer shown in Fig. 3(c). On the other hand, two separated peaks are observed in the CGS(y¼0.85)/CIS(y¼1.2) bilayer as shown in Fig. 4 (d) and is similar to that on the SLG substrate. It is worth to note here that Na does not affect the diffusion of In and Ga in the CGS/CIS bilayers as shown by the comparison of the XRD patterns in Fig. 4(a– d). The results shown in Figs. 2 and 4 where the bottom layer is CIS suggest that less inter-diffusion of In and Ga in the bilayers is due to the stronger crystal structure of CIS than that of the CGS in spite of the existence of excess Cu–Se, and the amount of Na in the films. It should be noted here that in the work by Schroeder et al. [12], they investigated and concluded that the diffusivities of In in polycrystalline bilayers and epitaxial layers were similar, thus suggesting that the diffusion in polycrystalline films was rather intragranular. In
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Table 1 Diffusion coefficients of In and Ga in the bilayers grown on the SLG and Al2O3/SLG substrates. Bilayers
Substrates Al2O3/SLG
SLG
CIS(y¼ 1.2)/CGS(y¼ 1.2) CGS(y¼ 1.2)/CIS(y¼ 1.2) CIS(y¼ 0.85)/CGS(y¼ 0.85) CGS(y¼ 0.85)/CIS(y¼ 0.85) CIS(y¼ 0.85)/CGS(y¼ 1.2) CGS(y¼ 1.2)/CIS(y¼ 0.85) CIS(y¼ 1.2)/CGS(y¼ 0.85) CGS(y¼ 0.85)/CIS(y¼ 1.2)
DIn ( 10 12 cm2/s)
DGa ( 10 12 cm2/s)
DIn ( 10 12 cm2/s)
DGa ( 10 12 cm2/s)
4.72 70.40 0.82 70.01 2.76 70.57 2.09 70.13 0.86 70.01 1.11 70.07 0.6770.01 1.65 70.07
2.92 7 0.23 0.43 7 0.03 0.777 0.08 1.017 0.06 0.707 0.01 0.737 0.05 0.82 7 0.01 1.277 0.07
5.46 7 0.22 1.28 7 0.17 2.017 0.28 1.87 7 0.19 11.6 7 0.9 2.81 7 0.31 6.137 0.41 1.60 7 0.07
21.7 76.3 0.60 70.07 0.60 70.08 2.11 70.21 2.50 70.14 3.7770.39 10.2 71.5 3.97 70.22
addition, Lundberg et al. [13] showed that there was no significant concentration gradient of diffusing elements (In or Ga) near the grain boundaries. This suggests that the grain boundary diffusion is not significantly faster than the intragranular diffusion. In our results, when comparing Fig. 1(a and c), the grain size is approximately the same but the diffusions observed from the coalescence of the XRD patterns of the two cases are significantly different. On the other hand, the grain size in Fig. 1(c) is quite larger than that in Fig. 1(d), but the coalescence of the XRD patterns in these two cases is similar suggesting that the diffusions are the same. In addition, the CIS/CGS bilayer on the substrate with Na (Fig. 1(c)) has significantly smaller grain than that of CIS/CGS bilayer on the substrate without Na (Fig. 3(c)), the merging of peaks in the XRD pattern in Fig. 3(c) is more pronounced implying more diffusion of group-III elements. Similar situation is observed in Figs. 1 and 3(d). These suggest that major diffusion is intragranular. Thus the effect of grain boundary diffusion or intergranular diffusion may be neglected in this work. The diffusions of In and Ga in the bilayers are influenced by the value of y (the richness or poorness of Cu) and the existence of Na. 3.5. Calculation of the diffusion coefficients of In and Ga As previously discussed and shown by Schroeder et al. [12] that the diffusions of In and Ga are similar in both polycrystals and expitaxial films, then the diffusion coefficient is calculated using Fick's second law based on the assumptions that (i) the diffusions of In and Ga are intragranular, (ii) the diffusion process occurs in one direction along the thickness of the films, (iii) the dominant mechanism is the lattice vacancies diffusion, (iv) the diffusion coefficient is independent of the concentration, and (v) the infinite medium is assumed for the diffusion of all species. Thus the solution to Fick's second law is given by [21,22] N z2 Cðz; tÞ ¼ pffiffiffiffiffiffiffiffiffiffiffiffie 4Dt ; 4π Dt
ð1Þ
where C is the concentration as a function of z – the distance along the diffusion direction that obtained from the compositional depth profile, t is the diffusion time, N is the number of deposited atoms obtained from the deposition flux and D is the diffusion coefficient. Thus the diffusion coefficient can be determined from the slope of the plot between ln C and z2, i.e. slope¼ 1/(4Dt). In the calculations, the diffusion coefficients of group-III elements in the bottom layer and the top layer are determined from the deposition time of the top layer only. The errors of the measurements of the compositional depth profiles are in the range of 0.5–1.0 at% in all bilayers regardless the grain size. The errors are propagated in the calculation of diffusion coefficients of In and Ga of the bilayers shown in Figs. 1–4 and are summarized in Table 1. It can be seen in most cases that the diffusion coefficients of In and Ga in the bilayers on the substrates without Na are larger than those of the substrates
with Na. This suggests that there are more vacancies in the films whose Na contribution from the SLG substrate is prohibited by the use Al2O3 layer. Among the bilayers on the substrates without Na, the diffusion coefficients of Ga are greater than those of In, while the diffusion coefficients of Ga in the bilayers on the substrate with Na are not necessarily larger than those of In. The diffusivities of In and Ga vary in the range that are comparable to those reported by Schroeder et al. [12] and Marudachalam et al. [22].
4. Conclusion The diffusion of In and Ga in the polycrystalline CIS/CGS and CGS/CIS bilayer thin films grown by molecular beam deposition technique was investigated by observing the shift of the XRD peaks of CIS and CGS as well as the depth profiles of In and Ga in the bilayers. The richness and poorness denoted by y¼ [Cu]/ ([Ga]þ [In]) was set for y¼ 1.2 and 0.85, respectively. The results can be summarized as: (1) When both layers are Cu-rich, the inter-mixing of In and Ga is relatively higher in CIS/CGS than in CGS/CIS. Stronger coalescence is observed in CIS/CGS on the substrate without Na. The diffusions of In and Ga in CGS/CIS on the substrates with and without Na are similar and less than that in CIS/CGS. It is implied that the diffusion is proceeded by the vacancies of group-III elements, e.g. existence of excess Cu–Se. However, the existence of Cu–Se does not necessarily promote the inter-diffusion of In and Ga as seen when the bottom layer is CIS. This suggests that the order of depositions affect the inter-diffusion of In and Ga. (2) Among the bilayers with the Cu-rich CGS as a bottom layer, the inter-diffusion of In and Ga is higher in the Na-free films than in the films containing Na. (3) The diffusion of In and Ga observed in CIS/CGS and CGS/CIS when both are Cu-poor should be proceeded by the Cu vacancies. However the strength of diffusion in this case is relatively less than in the case when both CIS and CGS are Cu-rich. (4) When the bottom layers are CIS, the diffusion of In and Ga are qualitatively similar in all bilayers both with and without Na, in spite of the overall richness or poorness of Cu in the bilayers. (5) Despite some variations in [Ga]/([In]þ[Ga]), approximately 10%, the diffusions of In and Ga are influenced by [Cu]/([In]þ[Ga]). Lastly, it is worth to note here that despite the diffusion mechanisms of In and Ga, the optical properties are crucial for the photovoltaic applications of the CIGS materials which needs further investigation. However, some of optical measurements, for example, by Thiru et al. [23] showed that the inter-mixing of In and Ga in the bilayers or trilayers occurred at higher substrate temperature which resulted in the increase of transition energy or
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energy gap of the alloying CIS and CGS layers due to Ga intermixing. This should have a direct effect on the performance of the photovoltaic devices made from these materials, i.e. the increase of open-circuit voltage (Voc) of such devices.
Acknowledgments This work was supported in part by the Asahi Glass Foundation, Ratchadaphiseksomphot Endowment Fund of Chulalongkorn University (RES560530181-AM), the Special Task Force for Activating Research (STAR), Ratchadaphiseksomphot Endowment Fund, Chulalongkorn University through the Energy Materials Physics Research Group and Research Funds from the Faculty of Science, Chulalongkorn University. B.N. also received support from the 90th Anniversary of Chulalongkorn University Fund (Ratchadaphiseksomphot Endowment Fund). The authors would also like to thank P. Panchawirat (ThEP Center, Thailand) for helping with SEM and EDS measurements, and the Department of Geology, Faculty of Science, Chulalongkorn University for the access to the XRD facility.
References [1] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Prog. Photovolt. 19 (2011) 894. [2] A. Chirilă, P. Reinhard, F. Pianezzi, P. Bloesch, A.R. Uhl, C. Fella, L. Kranz, D. Keller, C. Gretener, H. Hagendorfer, D. Jaeger, R. Erni, S. Nishiwaki, S. Buecheler, A.N. Tiwari, Nat. Mater. 12 (2013) 1107. [3] P. Jackson, D. Hariskos, R. Wuerz, W. Wischmann, M. Powalla, Phys. Status Solidi RRL 8 (3) (2014) 219. [4] J.R. Tuttle, D.S. Albin, A. Tennant, A.M. Gabor, M. Contreras, R. Noufi, Sol. Energy Mater. Sol. Cells 35 (1994) 193.
[5] T. Dullweber, G. Hanna, W. Shams-Kolahi, A. Schwartzlander, M.A. Contreras, R. Noufi, H.W. Schock, Thin Solid Films 361–362 (2000) 478. [6] O. Lunberg, M. Bodegård, J. Malmstrom, L. Stolt, Prog. Photovolt.: Res. Appl. 11 (2003) 77. [7] O. Lunberg, M. Edoff, L. Stolt, Thin Solid Films 480–481 (2005) 520. [8] R. Sakdanuphab, C. Chityuttakan, A. Pankiew, N. Somwang, K. Yoodee, S. Chatraphorn, J. Cryst. Growth 319 (2011) 44. [9] B. Walker, R. Agrawal, in: Proceedings of the IEEE 38th Photovoltaic Specialists Conference, 2012, p. 002654. [10] B. Arthibenyakul, C. Chityuttakan, S. Chatraphorn, J. Cryst. Growth 316 (2011) 97. [11] T. Walter, H.W. Schock, Thin Solid Films 224 (1993) 74. [12] D.J. Schroeder, G.D. Berry, A. Rockett, Appl. Phys. Lett. 69 (1996) 4068. [13] O. Lundberg, J. Lu, A. Rockett, M. Edoff, L. Stolt, J. Phys. Chem. Solids 64 (2003) 1499. [14] M. Bodegård, O. Lundberg, J. Lu, L. Stolt, Thin Solid Films 431–432 (2003) 46. [15] (a) S.H. Wei, S.B. Zhang, A. Zunger, J. Appl. Phys. 85 (1999) 7214; (b) D.W. Niles, K. Ramanathan, F. Hasoon, R. Noufi, B.J. Tielsch, J.E. Fulghum, J. Vaccum Sci. Technol. A 15 (1997) 3044; (c) D.W. Niles, M. Al-Jassim, K. Ramanathan, J. Vaccum Sci. Technol. A 17 (1999) 291. [16] D. Rudmann, M. Kaelin, F.J. Haug, F. Kurdesau, H. Zogg, A.N. Tiwari, in: Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion, 2003, p. 376. [17] S. Yoon, S. Kim, V. Cracium, W.K. Kim, R. Kaczynki, R. Acher, T.J. Anderson, O. D. Crisalle, S.S. Li, J. Cryst. Growth 281 (2005) 209. [18] S. Niki, A. Yamada, R. Hunger, P.J. Fons, K. Iwata, K. Matsubara, A. Nishio, H. Nakanishi, J. Cryst. Growth 237–239 (2002) 1993. [19] S. Niki, Y. Makita, A. Yamada, O. Hellman, P.J. Fons, A. Obara, Y. Okada, R. Shioda, H. Oyanagi, T. Kurafuji, S. Chichibu, H. Nakanishi, J. Cryst. Growth 150 (1995) 1201. [20] K. Kushiya, Y. Ohtake, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 33 (1994) 6599. [21] J. Crank, The Mathematics of Diffusion, 2nd ed., Oxford University Press, Oxford, 1975 (Chap. 2–3). [22] M. Marudachalam, H. Hichiri, R.W. Birkmire, J.M. Schultz, in: Proceedings of the IEEE 25th Photovoltaic Specialist Conference, 1996, p. 805. [23] S. Thiru, M. Asakawa, K. Honda, A. Kawaharazuka, A. Tackeuchi, T. Makimoto, Y. Horikoshi, AIP Adv. 5 (2015) 027120.