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Characterization of co-sputtered Cu-In alloy precursors for CuInSe2 thin films fabrication by close-spaced selenization
Solar Energy Materials and Solar Cells 55 (1998) 225—236
Characterization of co-sputtered Cu-In alloy precursors for CuInSe thin films fabrication 2 by close-spaced selenization F.O. Adurodija!, S.K. Kim!, S.D. Kim", J.S. Song!,*, K.H. Yoon!, B.T. Ahn# ! Korea Institute of Energy Research, 71-2, Jang-dong, Yusong, Taejon 305-343, South Korea " School of Material Sciences, Seoul National University, Seoul, 151-742, South Korea # Department of Material Science and Engineering, KAIST, Yusong, Taejon 305-701, South Korea Received 20 February 1998
Abstract Sputtering technique for Cu—In precursor films fabrication using different Cu and In layer sequences have been widely investigated for CuInSe production. But the CuInSe films 2 2 fabricated from these precursors using H Se or Se vapour selenization mostly exhibited poor 2 microstructural properties. The co-sputtering technique for producing Cu—In alloy films and selenization within a close-spaced graphite box resulting in quality CuInSe films was de2 veloped. All films were analysed using SEM, EDX, XRD and four-point probe measurements. Alloy films with a broad range of compositions were fabricated and XRD showed mainly In, CuIn and Cu In phases which were found to vary in intensities as the composition changes. 2 11 9 Different morphological properties were displayed as the alloy composition changes. The selenized CuInSe films exhibited different microstructural properties. Very In-rich films 2 yielded the ODC compound with small crystal sizes whilst slightly In-rich or Cu-rich alloys yielded single phase CuInSe films with dense crystals and sizes of about 5 lm. Film resistivities 2 varied from 10~2—108 ) cm. The films had compositions with Cu/In of 0.40—2.3 and Se/(Cu#In) of 0.74—1.35. All CuInSe films with the exception of very Cu-rich ones contained 2 high amount of Se ('50%). ( 1998 Elsevier Science B.V. All rights reserved. Keywords: CuInSe ; Close-spaced graphite box; Co-sputtering 2
* Corresponding author. E-mail: [email protected] 0927-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 7 - 0 2 4 8 ( 9 8 ) 0 0 1 0 2 - 0
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1. Introduction Polycrystalline CuInSe thin film and its related quartenary compounds such as 2 Cu(InGa)Se or CuIn(SSe) are among the most promising materials for photovoltaic 2 2 applications. Photovoltaic devices based on Cu(InGa)Se with efficiencies approach2 ing 18% over small areas of around 0.5 cm2 have been achieved by the sequential co-evaporation method [1]. However, the use of this technique has raised many questions about its acceptance as a commercially viable process due to associated large area uniformity problem. Investigations on more promising alternatives to co-evaporation for CuInSe production involved the deposition of Cu—In alloy 2 precursors followed by selenization using H Se or elemental Se. The Cu—In precursor 2 layers are formed using readily scaleable processes, such as thermal evaporation, sputtering or electrodeposition techniques. Different layer sequence routes to Cu—In metallic precursor formation by sputtering such as the bi-layers, multi-layers and ultra-thin multi-layers of Cu and In have been investigated for CuInSe fabrication. In this study, we present the development 2 of the co-sputtering technique for the formation of Cu—In alloy for CuInSe produc2 tion by selenization. The use of co-sputtering method for the alloy formation has the capability for achieving large area uniformity. The technique is reasonably simple, tolerant and reproducible. It involved a two stage process comprising simultaneous co-sputtering of Cu—In alloy from individual Cu and In targets followed by selenization using a partially closed graphite container. The potentials for large area compositional uniformity and control of the ratio of Cu/In by co-sputtering from Cu and In planar magnetron targets has been reported by Thorthon in a review on hybrid evaporation-sputtering process for CuInSe formation [2]. 2 Selenization of the Cu—In precursors have utilized H Se gas or Se vapour. The use 2 of H Se gas has been considered to be environmentally unfriendly due to its toxic 2 nature. In addition, there are problems relating to rapid volume expansion leading to poor adhesion of the film onto the Mo back contact, and In loss resulting from the complexity of reaction kinetics (i.e. interdiffusion of intermediate phases) leading to poor quality films. Hence, in this work, selenization was achieved using elemental Se within a newly developed close-spaced graphite container. The use of the graphite box for CuInSe fabrication using thermally evaporated Cu, In and Se layers and binary 2 compounds, sputtered Cu—In precursors under different selenization conditions has been reported in the literature [3—5]. In all the cases, significant improvements in the structural and morphological properties of the films were recorded. Recently, solar cells with efficiencies up to 9.8% has been obtained by Sato et al. using thermally evaporated In(Se)—Cu precursors followed by selenization within a sealed graphite container [6]. The use of the graphite container has also been extended to the fabrication of CuGaSe from evaporated layers by rapid thermal 2 annealing [7]. Initial investigation on the formation of CuInSe by selenization of 2 co-sputtered Cu—In alloy within a close-spaced graphite container have shown significant improvements in the film microstructural properties [8]. In this paper, we present a detailed investigation on the Cu—In precursor materials with a broad variation in composition from extremely In-rich to Cu-rich. This is expected to
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provide a comprehensive understanding necessary to fully optimize the alloy deposition conditions and the CuInSe film formation during the selenization process. To 2 our knowledge, little about this technique for precursor formation has been reported in the literature.
2. Experimental method Cu—In alloy was co-sputtered from high purity Cu (99.999%) and In (99.999%) target electrodes at ambient temperature on glass or Mo coated glass substrates using radio frequency (rf ) magnetron sputtering system. The system consisted of a loadlocked chamber and can accommodate three, 4 in (10 cm) diameter planar targets. The targets were fixed at a tilt angle of 22° and adjusted to a distance of about 20 cm from the substrate. The substrate was rotated during deposition to enhance the film uniformity. An illustration of the co-sputtering system is shown in Fig. 1. A high purity (99.998%) argon gas was used to provide the plasma at a background pressure of 1.5]10~3 torr during the co-sputtering process. The deposition rate and thickness were controlled and measured using an oscillatory quartz crystal monitor located at a position close to the substrate. The overall thickness of the sputtered alloy films was about 850 nm. Alloy films with a broad range of composition from extremely In-rich to Cu-rich were produced by altering the Cu sputtering power from 15 to 55 W, while maintaining the In sputtering power at 60 W. This was necessary to determine the development in the microstructural properties of the alloy precursors as well as the selenized CuInSe films with changes in composition of Cu and In in the materials. 2
Fig. 1. An illustration of the co-sputtering system.
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The selenization system was made up of a graphite box and a covering lid with a pin hole in the middle. The box consisted of a sample holder and four pots located at both ends to accommodate the elemental Se material. The vertical spacing between the lid and the sample was about 3 mm. The sample was placed in the graphite container with some Se and loaded into a quartz tube furnace which was pumped down to a vacuum of about 5]10~3 Torr using a mechanical rotary pump. The heating was provided by quartz-halogen lamps. Selenization was accomplished using a two-step reaction temperature profile (at 250°C and 500°C). The complete details of the selenization system is reported elsewhere [8]. The first step, at 250°C was to allow sufficient Se to permeate into the alloy precursor films in order to ensure p-type materials, whilst the second step, at 500—550°C was to stimulate the film formation and recrystallization process. The total reaction and recrystallization time ranged from 45 to 70 min. The structural properties of the films were studied by X-ray diffraction (XRD) using Cu Ka (j"1.5405 A_ ) radiation whilst the film morphology and composition were investigated using scanning electron spectroscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) respectively. The electrical properties were studied using the four point probe measurement technique.
3. Results and discussion The XRD analyses of the as-deposited Cu—In alloy films exhibited only In, CuIn 2 and Cu In phases and the observed phases were found to change from the domina11 9 ting presence of elemental In to CuIn and to exclusively Cu In phase with 2 11 9 increasing Cu content in the alloy material as shown in Fig. 2a—h. The development of the structural phases from In-rich to Cu—rich with increasing concentration of Cu in the alloy films could be clearly observed. However, for the purpose of clarity in the analysis of the data, the phase development in the alloy samples, are classified into three regions based on the observed distinctive structural or composition boundaries from the XRD and EDX analyses as follows; region 1 (Samples a and b), where the alloy precursors were extremely In-rich and could be distinguished by the presence of elemental In phase and composition ratios of Cu/In)0.7. Region 2 alloy films (Samples c—e) were dominated by the formation of In-rich CuIn phase and have 2 composition ratios with 0.8)Cu/In)0.9. Region 3 alloy films (Samples f—h) were Cu-rich and had composition ratios of Cu/In*1. The Cu—In alloy structural phase formation sequence from mainly InPCuIn PCu In closely matched the Cu—In phase formation model reported 2 11 9 by Lindahl et al. in their multi-layers precursors prepared by thermal evaporation [9]. In the model a preference in the phase formation sequence from CuIn PCu In P 2 11 9 Cu InPCu In as the effective concentration of Cu increases was predicted. How2 7 3 ever, in this work, the Cu—In alloy composition boundary investigated was not extended to the formation of Cu In and Cu In Cu-rich phases. It should be noted 2 7 3 that the above order of phase formation could only be achieved for well mixed alloys and this is readily achieved by the co-sputtering process. A well mixed alloy is
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Fig. 2. XRD of the as-deposited Cu—In alloy precursor films at ambient temperature showing the structural phase development with changes in the alloy compositions.
expected to favour the film growth kinetics during CuInSe formation since many 2 complex reaction paths which could lead to poor films are avoided. In region 1, (Samples a and b in Fig. 2), elemental In phase was found to be the dominant component in the alloy films as shown by the presence of the most intense In peak in the XRD spectra, Fig. 2a. The In peak appeared to be the first phase to be formed due to its very high concentration and is thought to lie at a position below the CuIn phase formation boundary. A slight increase in the Cu content was marked 2 with a suppression of the In peaks and a subsequent increase in the CuIn peak 2 intensities, Fig. 2b. A small amount of Cu In was also detected in these films even 11 9 though the films contained excess indium. For alloy films in region 2 (Samples c—e in Fig. 2), the film composition were still In-rich, but the In content was steadily reduced. No elemental In was detected in the films in this region. The CuIn phase was found to dominate the structural constitution 2 as indicated by the strong peak intensities from the XRD, Fig. 1c—e. However, the outset of the formation and gradual development of the main Cu In peak at 2h"42.08° was 11 9 also noticeable as the effective concentration of Cu in the alloy increased. In general, the alloy film compositions suitable for solar cell fabrication could be obtained in this region based on the results of EDX analysis of selenized CuInSe films. 2 In region 3 (Samples f—h in Fig. 2), the composition of the alloy films were Cu-rich rich as evidenced from the strong Cu In peak intensity and the suppression of the 11 9
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Fig. 3. A plot of the ratio of CuIn /Cu In peak intensities (from XRD) as function of the ratio of the 2 11 9 Cu/In ratio of the alloy sputter deposition power, exhibiting a strong dependence of the phases on the changes in the Cu content in the alloy films.
In-rich CuIn phase. A further increase in the Cu content resulted in a complete 2 transformation of the alloy into an exclusive Cu In phase, Fig. 2h. 11 9 Fig. 3 shows a plot of the ratio of CuIn /Cu In peak intensity against the ratio of 2 11 9 Cu/In sputter deposition power. The progression in structural characteristics of the alloy films with a change in the composition of the films as the Cu content increases is clearly noticeable. It is noteworthy to mention that only In, CuIn and Cu In 2 11 9 phases were detected in the alloy precursors in spite of the broad range of compositions investigated. Other phases or contaminants such as Cu and CuIn or In O 2 3 detected by other workers in their sputtered and evaporated Cu and In layers were not found in any of these samples [10—13]. The absence of elemental Cu even in Cu-rich precursors could be due to its high reactive state which is thought to be favoured by the broad atomic distribution of Cu and In within the entire mass of the alloy materials. These observed features are due to the enhanced mixture of the Cu and In elemental species on the substrate surface during the co-sputtering process and is attributed to the stable deposition parameters attained. Fig. 4a—d shows the SEM micrographs of the alloy films with different surface morphologies according to the three regions of the alloys groupings described above. In-rich precursor films (Sample a in region 1) possessed a smooth surface background on which are scattered large particles with sizes of about 4 lm. These particles were initially thought to be elemental In based on the strong intensities of In peaks observed from the XRD analysis. However, they where found from EDX analysis to contain slightly higher amount of Cu with Cu/In composition ratio of 0.6 when compared with the composition ratio of Cu/In"0.4 for the smooth part of the alloy film surface. Further increase in the Cu content in the films (Samples c—e, in region 2), led to a drastic change in the film morphologies, thus exhibiting uniform and less dense crystal structures with sizes of about 0.5 lm over the entire substrate area. For Cu-rich alloy films (Samples g and h in region 3), the grainy morphology started to
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Fig. 4. (a—d) SEM micrographs of the surface morphologies of the Cu—In alloy films showing the different morphological structures of the alloys with changes in the alloy compositions.
disappear, thus leading to the formation of a uniform, smooth and very dense crystal structure, Fig. 4c and d. The observed feature in Fig. 4d, was possibly due to the effect of Cu In phase. A similar morphological observations were made on Cu-rich 11 9 precursor films prepared by sputtering of ultra-thin Cu and In layers [14,15]. Fig. 5 shows a plot of the composition ratios of Cu/In of the as-deposited alloy films (measured by EDX) against the ratio of the Cu/In sputter deposition power. The near linearity shape of the curve within a wide variation of sputtering power further demonstrated the good control over the deposition parameters during the co-sputtering process. The selenized CuInSe thin films had thickness of around 2.2 lm indicating nearly 2 three-times increase in the volume of the alloy precursor materials during selenization process as determined from the thickness measurements. No adhesion problems occurred in all the films grown on glass or Mo coated glass substrates. XRD analyses of the synthesized CuInSe films is shown in Fig. 6a—h. Cu—In alloy films with very 2 In-rich compositions, i.e. Samples (a—c), exhibited the ordered defect chalcopyrite (ODC) compound with a chemical formula of CuIn Se or CuIn Se as identified by 3 5 2 3.5 the characteristic 002, 110, 202 and 114 peaks. However, Sample (a), with excessive In (Cu/In composition ratio of 0.4) was found to possess an extra peak near the 112 peak position identified as In Se from the JCPDS data files. The formation of the ODC 2 3
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Fig. 5. A plot of the Cu/In composition ratios of the Cu—In alloy films (from EDX) against the ratios of the Cu/In sputter deposition power, indicating some degree of control over the deposition parameters.
Fig. 6. (a—h) The XRD spectra of the selenized alloy films showing different compound formation such as the CuIn Se or CuIn Se (ODC) films, Samples (a—c); single-phase chalcopyrite CuInSe , Sample (e); and 3 5 2 3.5 2 Cu-rich CuInSe , Samples (f—h). 2
compound was due to the high Se over-pressure under which the films were selenized. The ODC peaks diminished gradually as the Cu content in the films increased until a single phase chalcopyrite CuInSe material was formed as shown in Fig. 6e. As the 2 film compositions shifted into the Cu-rich zone (region 3) a Cu Se secondary phase 2~x
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started to emerge and became more prominent with increasing Cu content in the films. No significant difference in crystal orientation was observed from the computation of the ratio of 112/220 CuInSe peak intensities irrespective of 2 the broad variations in composition of the Cu—In alloy precursor films investigated. Thus, the XRD results suggested the possibility of controlling the alloy composition by a simple adjustment of the sputter deposition power in order to produce single phase CuInSe films with the desired structural properties suitable for solar cell 2 manufacture. Fig. 7a—e shows typical surface and cross-sectional SEM micrographs of the CuInSe films morphologies for the different regions of alloy films’ compositions 2 investigated. The CuIn Se or CuIn Se (ODC) compounds were found to possess 3 5 2 3.5 clusters of small crystals in the form of ‘cauliflower’ and had sizes less than 0.5 lm, Fig. 7a. For films whose compositions were around the stoichiometric region (Samples in region 2), large and high denstity crystals with sizes of around 5 lm were exhibited as shown in Fig. 7b and c. Films in this region lie within the compositions suitable for solar cell fabrication and it expected that the formation of large crystals would contribute to grain boundary reduction, thus enabling the production of efficient solar cell devices. Cu-rich films also exhibited large and dense crystalline structures. The hexagonal structure of the CuInSe could easily be observed from the 2 crystal shapes, Fig. 7d and e. However, these films are not suitable for device fabrication, because of the extra Cu Se phase they contained. In general, the crystal 2~x structures of the selenized CuInSe films were found to be strongly dependent on the 2 Cu/In composition ratio of the as-deposited alloy precursor films. The quality of the CuInSe films obtained in this work showed significant improvements when com2 pared with those previously reported using Se vapour or H Se gas during the 2 selenization process [10,13,14]. Fig. 8 shows the resistivity measurements of the selenized CuInSe films as a func2 tion of the ratio of Cu/In alloy sputter deposition power. The Samples (a and b) in region 1 (CuIn Se or CuIn Se compound) possessed very high resistivity as 3 5 2 3.5 expected due to the excess In they contained. The resistivity steadily decreased as the Cu content in the films increased. The values of the resistivities obtained for samples in region 2 could easily be adjusted to that suitable for device fabrication by a simple alteration of the Cu deposition power. Further increase in the Cu content in the alloy films was marked with a sharp drop in the resistivity values of the CuInSe films, 2 Samples (f—h) in region 3, which was possibly due to the formation of a Cu Se 2~x compound on the film surface as detected from XRD analysis. The EDX composition analysis of the Cu—In alloys and CuInSe films is shown in 2 Table 1. Very In-rich films produced compositions of around Cu : In : Se"1 : 3 : 5 or 1 : 2 : 3.5 (error in the EDX was less than 5%) which corresponded very well with that of the ODC compound and thus agreed with the observations made from the XRD analysis. Slightly In-rich or slightly Cu rich materials yielded compositions similar to that of single phase chalcopyrite CuInSe . A notable feature in the selenized films in 2 this region was the high amount of Se ('50%) recorded from the EDX analysis. However, films with excess Cu were found to be deficient in Se and this could be associated with the precipitation of Cu Se secondary phase on the film surface. In 2~x
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Fig. 7. Typical SEM micrographs of selenized alloy precursors showing (a) the surface of CuIn Se or 3 5 CuIn Se (ODC), Sample (b) in region 1; (b and c) surface and cross-section of near stoichiometric 2 3.5 CuInSe , Sample (e) in region 2; (d and e) surface of Cu-rich CuInSe thin films. 2 2
general, the wide range of alloy compositions investigated and the associated properties of the CuInSe films obtained has provided useful information necessary for 2 further optimization of the process steps of the co-sputtering and the graphite box selenization techniques for the fabrication of quality CuInSe thin films. The next step 2 in this study is to apply the CuInSe films in solar cells fabrication in order to 2 determine their photovoltaic properties.
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Fig. 8. A plot of the resistivities of CuInSe films obtained from the selenization of alloy films with various 2 compositions against the ratio of Cu/In sputter deposition power. Region 1 (Samples a and b) are the ODC compounds, region 2 are single phase CuInSe while region 3 are the Cu-rich CuInSe films. 2 2
Table 1 EDX composition analysis (at. %) of Cu—In alloy precursor films showing the CuIn Se or CuIn Se 2 3.5 3 5 compounds, Samples (a and b); single-phase and Cu-rich CuInSe films, Samples (c—e) and (f—h), respective2 ly Sample No.
SG1-1(a) SG13 SG59(b) SG14 SG15(c) SG16(d) SG43 SG42 SG41(e) SG7 SG12(f) SG38(g) SG50(h)
Cu—In alloy films Cu/In
0.40 0.40 0.41 0.52 0.67 0.84 0.85 0.87 0.94 0.97 1.11 1.51 1.65
Compositions of selenized CuInSe films (at. %) 2 Cu
In
Se
Cu/In
Se/(Cu#In)
13.34 13.83 14.63 15.91 20.01 21.88 23.45 23.60 24.47 26.01 28.51 36.53 39.32
29.30 32.30 30.71 31.30 27.60 26.99 25.48 25.65 25.21 21.80 23.68 20.97 17.30
57.37 53.87 54.66 52.79 52.39 51.13 51.07 50.75 50.32 52.19 47.81 42.50 43.38
0.455 0.428 0.476 0.508 0.725 0.811 0.920 0.920 0.971 1.193 1.204 1.742 2.283
1.346 1.168 1.206 1.118 1.100 1.046 1.044 1.030 1.013 1.092 0.916 0.739 0.766
4. Conclusion Detailed studies on the co-sputtered Cu—In alloys and CuInSe films obtained by 2 selenization within a close-spaced graphite container have been discussed. The XRD analysis showed a form of sequence for the structural phases formed, varying from the dominant presence of InPCuIn PCu In as the Cu content in the films increased. 2 11 9
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The morphologies of the alloy films was also found to be greatly influenced by the composition differences. The selenized CuInSe thin films exhibited different micro2 structural properties which corresponded with the change in the alloy compositions. Very In-rich films yielded the CuIn Se or CuIn Se compound which were charac3 5 2 3.5 terized by small crystal sizes. Near stoichiometric In-rich or Cu-rich films yielded single phase chalcopyrite CuInSe films with high density of crystal structures and 2 sizes of about 5 lm. The film resistivities varied from 10~2 to 108 ) cm and were found to depend on the Cu/In composition ratio of the alloy films. The films had composition ratios of Cu/In between 0.40 and 2.3, and Se/(Cu#In) between 0.74 and 1.35. All CuInSe films with the exception of very Cu-rich ones contained high 2 amount of Se content ('50%). The co-sputtering technique for Cu—In alloy fabrication and the graphite box selenization system have simple control mechanism and show strong potential for producing quality CuInSe materials. Further investiga2 tions are in progress to fully demonstrate the potential of these films in solar cell devices.
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Characterization of co-sputtered Cu-In alloy precursors for CuInSe thin films fabrication 2 by close-spaced selenization F.O. Adurodija!, S.K. Kim!, S.D. Kim", J.S. Song!,*, K.H. Yoon!, B.T. Ahn# ! Korea Institute of Energy Research, 71-2, Jang-dong, Yusong, Taejon 305-343, South Korea " School of Material Sciences, Seoul National University, Seoul, 151-742, South Korea # Department of Material Science and Engineering, KAIST, Yusong, Taejon 305-701, South Korea Received 20 February 1998
Abstract Sputtering technique for Cu—In precursor films fabrication using different Cu and In layer sequences have been widely investigated for CuInSe production. But the CuInSe films 2 2 fabricated from these precursors using H Se or Se vapour selenization mostly exhibited poor 2 microstructural properties. The co-sputtering technique for producing Cu—In alloy films and selenization within a close-spaced graphite box resulting in quality CuInSe films was de2 veloped. All films were analysed using SEM, EDX, XRD and four-point probe measurements. Alloy films with a broad range of compositions were fabricated and XRD showed mainly In, CuIn and Cu In phases which were found to vary in intensities as the composition changes. 2 11 9 Different morphological properties were displayed as the alloy composition changes. The selenized CuInSe films exhibited different microstructural properties. Very In-rich films 2 yielded the ODC compound with small crystal sizes whilst slightly In-rich or Cu-rich alloys yielded single phase CuInSe films with dense crystals and sizes of about 5 lm. Film resistivities 2 varied from 10~2—108 ) cm. The films had compositions with Cu/In of 0.40—2.3 and Se/(Cu#In) of 0.74—1.35. All CuInSe films with the exception of very Cu-rich ones contained 2 high amount of Se ('50%). ( 1998 Elsevier Science B.V. All rights reserved. Keywords: CuInSe ; Close-spaced graphite box; Co-sputtering 2
* Corresponding author. E-mail: [email protected] 0927-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 7 - 0 2 4 8 ( 9 8 ) 0 0 1 0 2 - 0
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1. Introduction Polycrystalline CuInSe thin film and its related quartenary compounds such as 2 Cu(InGa)Se or CuIn(SSe) are among the most promising materials for photovoltaic 2 2 applications. Photovoltaic devices based on Cu(InGa)Se with efficiencies approach2 ing 18% over small areas of around 0.5 cm2 have been achieved by the sequential co-evaporation method [1]. However, the use of this technique has raised many questions about its acceptance as a commercially viable process due to associated large area uniformity problem. Investigations on more promising alternatives to co-evaporation for CuInSe production involved the deposition of Cu—In alloy 2 precursors followed by selenization using H Se or elemental Se. The Cu—In precursor 2 layers are formed using readily scaleable processes, such as thermal evaporation, sputtering or electrodeposition techniques. Different layer sequence routes to Cu—In metallic precursor formation by sputtering such as the bi-layers, multi-layers and ultra-thin multi-layers of Cu and In have been investigated for CuInSe fabrication. In this study, we present the development 2 of the co-sputtering technique for the formation of Cu—In alloy for CuInSe produc2 tion by selenization. The use of co-sputtering method for the alloy formation has the capability for achieving large area uniformity. The technique is reasonably simple, tolerant and reproducible. It involved a two stage process comprising simultaneous co-sputtering of Cu—In alloy from individual Cu and In targets followed by selenization using a partially closed graphite container. The potentials for large area compositional uniformity and control of the ratio of Cu/In by co-sputtering from Cu and In planar magnetron targets has been reported by Thorthon in a review on hybrid evaporation-sputtering process for CuInSe formation [2]. 2 Selenization of the Cu—In precursors have utilized H Se gas or Se vapour. The use 2 of H Se gas has been considered to be environmentally unfriendly due to its toxic 2 nature. In addition, there are problems relating to rapid volume expansion leading to poor adhesion of the film onto the Mo back contact, and In loss resulting from the complexity of reaction kinetics (i.e. interdiffusion of intermediate phases) leading to poor quality films. Hence, in this work, selenization was achieved using elemental Se within a newly developed close-spaced graphite container. The use of the graphite box for CuInSe fabrication using thermally evaporated Cu, In and Se layers and binary 2 compounds, sputtered Cu—In precursors under different selenization conditions has been reported in the literature [3—5]. In all the cases, significant improvements in the structural and morphological properties of the films were recorded. Recently, solar cells with efficiencies up to 9.8% has been obtained by Sato et al. using thermally evaporated In(Se)—Cu precursors followed by selenization within a sealed graphite container [6]. The use of the graphite container has also been extended to the fabrication of CuGaSe from evaporated layers by rapid thermal 2 annealing [7]. Initial investigation on the formation of CuInSe by selenization of 2 co-sputtered Cu—In alloy within a close-spaced graphite container have shown significant improvements in the film microstructural properties [8]. In this paper, we present a detailed investigation on the Cu—In precursor materials with a broad variation in composition from extremely In-rich to Cu-rich. This is expected to
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provide a comprehensive understanding necessary to fully optimize the alloy deposition conditions and the CuInSe film formation during the selenization process. To 2 our knowledge, little about this technique for precursor formation has been reported in the literature.
2. Experimental method Cu—In alloy was co-sputtered from high purity Cu (99.999%) and In (99.999%) target electrodes at ambient temperature on glass or Mo coated glass substrates using radio frequency (rf ) magnetron sputtering system. The system consisted of a loadlocked chamber and can accommodate three, 4 in (10 cm) diameter planar targets. The targets were fixed at a tilt angle of 22° and adjusted to a distance of about 20 cm from the substrate. The substrate was rotated during deposition to enhance the film uniformity. An illustration of the co-sputtering system is shown in Fig. 1. A high purity (99.998%) argon gas was used to provide the plasma at a background pressure of 1.5]10~3 torr during the co-sputtering process. The deposition rate and thickness were controlled and measured using an oscillatory quartz crystal monitor located at a position close to the substrate. The overall thickness of the sputtered alloy films was about 850 nm. Alloy films with a broad range of composition from extremely In-rich to Cu-rich were produced by altering the Cu sputtering power from 15 to 55 W, while maintaining the In sputtering power at 60 W. This was necessary to determine the development in the microstructural properties of the alloy precursors as well as the selenized CuInSe films with changes in composition of Cu and In in the materials. 2
Fig. 1. An illustration of the co-sputtering system.
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The selenization system was made up of a graphite box and a covering lid with a pin hole in the middle. The box consisted of a sample holder and four pots located at both ends to accommodate the elemental Se material. The vertical spacing between the lid and the sample was about 3 mm. The sample was placed in the graphite container with some Se and loaded into a quartz tube furnace which was pumped down to a vacuum of about 5]10~3 Torr using a mechanical rotary pump. The heating was provided by quartz-halogen lamps. Selenization was accomplished using a two-step reaction temperature profile (at 250°C and 500°C). The complete details of the selenization system is reported elsewhere [8]. The first step, at 250°C was to allow sufficient Se to permeate into the alloy precursor films in order to ensure p-type materials, whilst the second step, at 500—550°C was to stimulate the film formation and recrystallization process. The total reaction and recrystallization time ranged from 45 to 70 min. The structural properties of the films were studied by X-ray diffraction (XRD) using Cu Ka (j"1.5405 A_ ) radiation whilst the film morphology and composition were investigated using scanning electron spectroscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) respectively. The electrical properties were studied using the four point probe measurement technique.
3. Results and discussion The XRD analyses of the as-deposited Cu—In alloy films exhibited only In, CuIn 2 and Cu In phases and the observed phases were found to change from the domina11 9 ting presence of elemental In to CuIn and to exclusively Cu In phase with 2 11 9 increasing Cu content in the alloy material as shown in Fig. 2a—h. The development of the structural phases from In-rich to Cu—rich with increasing concentration of Cu in the alloy films could be clearly observed. However, for the purpose of clarity in the analysis of the data, the phase development in the alloy samples, are classified into three regions based on the observed distinctive structural or composition boundaries from the XRD and EDX analyses as follows; region 1 (Samples a and b), where the alloy precursors were extremely In-rich and could be distinguished by the presence of elemental In phase and composition ratios of Cu/In)0.7. Region 2 alloy films (Samples c—e) were dominated by the formation of In-rich CuIn phase and have 2 composition ratios with 0.8)Cu/In)0.9. Region 3 alloy films (Samples f—h) were Cu-rich and had composition ratios of Cu/In*1. The Cu—In alloy structural phase formation sequence from mainly InPCuIn PCu In closely matched the Cu—In phase formation model reported 2 11 9 by Lindahl et al. in their multi-layers precursors prepared by thermal evaporation [9]. In the model a preference in the phase formation sequence from CuIn PCu In P 2 11 9 Cu InPCu In as the effective concentration of Cu increases was predicted. How2 7 3 ever, in this work, the Cu—In alloy composition boundary investigated was not extended to the formation of Cu In and Cu In Cu-rich phases. It should be noted 2 7 3 that the above order of phase formation could only be achieved for well mixed alloys and this is readily achieved by the co-sputtering process. A well mixed alloy is
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Fig. 2. XRD of the as-deposited Cu—In alloy precursor films at ambient temperature showing the structural phase development with changes in the alloy compositions.
expected to favour the film growth kinetics during CuInSe formation since many 2 complex reaction paths which could lead to poor films are avoided. In region 1, (Samples a and b in Fig. 2), elemental In phase was found to be the dominant component in the alloy films as shown by the presence of the most intense In peak in the XRD spectra, Fig. 2a. The In peak appeared to be the first phase to be formed due to its very high concentration and is thought to lie at a position below the CuIn phase formation boundary. A slight increase in the Cu content was marked 2 with a suppression of the In peaks and a subsequent increase in the CuIn peak 2 intensities, Fig. 2b. A small amount of Cu In was also detected in these films even 11 9 though the films contained excess indium. For alloy films in region 2 (Samples c—e in Fig. 2), the film composition were still In-rich, but the In content was steadily reduced. No elemental In was detected in the films in this region. The CuIn phase was found to dominate the structural constitution 2 as indicated by the strong peak intensities from the XRD, Fig. 1c—e. However, the outset of the formation and gradual development of the main Cu In peak at 2h"42.08° was 11 9 also noticeable as the effective concentration of Cu in the alloy increased. In general, the alloy film compositions suitable for solar cell fabrication could be obtained in this region based on the results of EDX analysis of selenized CuInSe films. 2 In region 3 (Samples f—h in Fig. 2), the composition of the alloy films were Cu-rich rich as evidenced from the strong Cu In peak intensity and the suppression of the 11 9
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Fig. 3. A plot of the ratio of CuIn /Cu In peak intensities (from XRD) as function of the ratio of the 2 11 9 Cu/In ratio of the alloy sputter deposition power, exhibiting a strong dependence of the phases on the changes in the Cu content in the alloy films.
In-rich CuIn phase. A further increase in the Cu content resulted in a complete 2 transformation of the alloy into an exclusive Cu In phase, Fig. 2h. 11 9 Fig. 3 shows a plot of the ratio of CuIn /Cu In peak intensity against the ratio of 2 11 9 Cu/In sputter deposition power. The progression in structural characteristics of the alloy films with a change in the composition of the films as the Cu content increases is clearly noticeable. It is noteworthy to mention that only In, CuIn and Cu In 2 11 9 phases were detected in the alloy precursors in spite of the broad range of compositions investigated. Other phases or contaminants such as Cu and CuIn or In O 2 3 detected by other workers in their sputtered and evaporated Cu and In layers were not found in any of these samples [10—13]. The absence of elemental Cu even in Cu-rich precursors could be due to its high reactive state which is thought to be favoured by the broad atomic distribution of Cu and In within the entire mass of the alloy materials. These observed features are due to the enhanced mixture of the Cu and In elemental species on the substrate surface during the co-sputtering process and is attributed to the stable deposition parameters attained. Fig. 4a—d shows the SEM micrographs of the alloy films with different surface morphologies according to the three regions of the alloys groupings described above. In-rich precursor films (Sample a in region 1) possessed a smooth surface background on which are scattered large particles with sizes of about 4 lm. These particles were initially thought to be elemental In based on the strong intensities of In peaks observed from the XRD analysis. However, they where found from EDX analysis to contain slightly higher amount of Cu with Cu/In composition ratio of 0.6 when compared with the composition ratio of Cu/In"0.4 for the smooth part of the alloy film surface. Further increase in the Cu content in the films (Samples c—e, in region 2), led to a drastic change in the film morphologies, thus exhibiting uniform and less dense crystal structures with sizes of about 0.5 lm over the entire substrate area. For Cu-rich alloy films (Samples g and h in region 3), the grainy morphology started to
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Fig. 4. (a—d) SEM micrographs of the surface morphologies of the Cu—In alloy films showing the different morphological structures of the alloys with changes in the alloy compositions.
disappear, thus leading to the formation of a uniform, smooth and very dense crystal structure, Fig. 4c and d. The observed feature in Fig. 4d, was possibly due to the effect of Cu In phase. A similar morphological observations were made on Cu-rich 11 9 precursor films prepared by sputtering of ultra-thin Cu and In layers [14,15]. Fig. 5 shows a plot of the composition ratios of Cu/In of the as-deposited alloy films (measured by EDX) against the ratio of the Cu/In sputter deposition power. The near linearity shape of the curve within a wide variation of sputtering power further demonstrated the good control over the deposition parameters during the co-sputtering process. The selenized CuInSe thin films had thickness of around 2.2 lm indicating nearly 2 three-times increase in the volume of the alloy precursor materials during selenization process as determined from the thickness measurements. No adhesion problems occurred in all the films grown on glass or Mo coated glass substrates. XRD analyses of the synthesized CuInSe films is shown in Fig. 6a—h. Cu—In alloy films with very 2 In-rich compositions, i.e. Samples (a—c), exhibited the ordered defect chalcopyrite (ODC) compound with a chemical formula of CuIn Se or CuIn Se as identified by 3 5 2 3.5 the characteristic 002, 110, 202 and 114 peaks. However, Sample (a), with excessive In (Cu/In composition ratio of 0.4) was found to possess an extra peak near the 112 peak position identified as In Se from the JCPDS data files. The formation of the ODC 2 3
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Fig. 5. A plot of the Cu/In composition ratios of the Cu—In alloy films (from EDX) against the ratios of the Cu/In sputter deposition power, indicating some degree of control over the deposition parameters.
Fig. 6. (a—h) The XRD spectra of the selenized alloy films showing different compound formation such as the CuIn Se or CuIn Se (ODC) films, Samples (a—c); single-phase chalcopyrite CuInSe , Sample (e); and 3 5 2 3.5 2 Cu-rich CuInSe , Samples (f—h). 2
compound was due to the high Se over-pressure under which the films were selenized. The ODC peaks diminished gradually as the Cu content in the films increased until a single phase chalcopyrite CuInSe material was formed as shown in Fig. 6e. As the 2 film compositions shifted into the Cu-rich zone (region 3) a Cu Se secondary phase 2~x
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started to emerge and became more prominent with increasing Cu content in the films. No significant difference in crystal orientation was observed from the computation of the ratio of 112/220 CuInSe peak intensities irrespective of 2 the broad variations in composition of the Cu—In alloy precursor films investigated. Thus, the XRD results suggested the possibility of controlling the alloy composition by a simple adjustment of the sputter deposition power in order to produce single phase CuInSe films with the desired structural properties suitable for solar cell 2 manufacture. Fig. 7a—e shows typical surface and cross-sectional SEM micrographs of the CuInSe films morphologies for the different regions of alloy films’ compositions 2 investigated. The CuIn Se or CuIn Se (ODC) compounds were found to possess 3 5 2 3.5 clusters of small crystals in the form of ‘cauliflower’ and had sizes less than 0.5 lm, Fig. 7a. For films whose compositions were around the stoichiometric region (Samples in region 2), large and high denstity crystals with sizes of around 5 lm were exhibited as shown in Fig. 7b and c. Films in this region lie within the compositions suitable for solar cell fabrication and it expected that the formation of large crystals would contribute to grain boundary reduction, thus enabling the production of efficient solar cell devices. Cu-rich films also exhibited large and dense crystalline structures. The hexagonal structure of the CuInSe could easily be observed from the 2 crystal shapes, Fig. 7d and e. However, these films are not suitable for device fabrication, because of the extra Cu Se phase they contained. In general, the crystal 2~x structures of the selenized CuInSe films were found to be strongly dependent on the 2 Cu/In composition ratio of the as-deposited alloy precursor films. The quality of the CuInSe films obtained in this work showed significant improvements when com2 pared with those previously reported using Se vapour or H Se gas during the 2 selenization process [10,13,14]. Fig. 8 shows the resistivity measurements of the selenized CuInSe films as a func2 tion of the ratio of Cu/In alloy sputter deposition power. The Samples (a and b) in region 1 (CuIn Se or CuIn Se compound) possessed very high resistivity as 3 5 2 3.5 expected due to the excess In they contained. The resistivity steadily decreased as the Cu content in the films increased. The values of the resistivities obtained for samples in region 2 could easily be adjusted to that suitable for device fabrication by a simple alteration of the Cu deposition power. Further increase in the Cu content in the alloy films was marked with a sharp drop in the resistivity values of the CuInSe films, 2 Samples (f—h) in region 3, which was possibly due to the formation of a Cu Se 2~x compound on the film surface as detected from XRD analysis. The EDX composition analysis of the Cu—In alloys and CuInSe films is shown in 2 Table 1. Very In-rich films produced compositions of around Cu : In : Se"1 : 3 : 5 or 1 : 2 : 3.5 (error in the EDX was less than 5%) which corresponded very well with that of the ODC compound and thus agreed with the observations made from the XRD analysis. Slightly In-rich or slightly Cu rich materials yielded compositions similar to that of single phase chalcopyrite CuInSe . A notable feature in the selenized films in 2 this region was the high amount of Se ('50%) recorded from the EDX analysis. However, films with excess Cu were found to be deficient in Se and this could be associated with the precipitation of Cu Se secondary phase on the film surface. In 2~x
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Fig. 7. Typical SEM micrographs of selenized alloy precursors showing (a) the surface of CuIn Se or 3 5 CuIn Se (ODC), Sample (b) in region 1; (b and c) surface and cross-section of near stoichiometric 2 3.5 CuInSe , Sample (e) in region 2; (d and e) surface of Cu-rich CuInSe thin films. 2 2
general, the wide range of alloy compositions investigated and the associated properties of the CuInSe films obtained has provided useful information necessary for 2 further optimization of the process steps of the co-sputtering and the graphite box selenization techniques for the fabrication of quality CuInSe thin films. The next step 2 in this study is to apply the CuInSe films in solar cells fabrication in order to 2 determine their photovoltaic properties.
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Fig. 8. A plot of the resistivities of CuInSe films obtained from the selenization of alloy films with various 2 compositions against the ratio of Cu/In sputter deposition power. Region 1 (Samples a and b) are the ODC compounds, region 2 are single phase CuInSe while region 3 are the Cu-rich CuInSe films. 2 2
Table 1 EDX composition analysis (at. %) of Cu—In alloy precursor films showing the CuIn Se or CuIn Se 2 3.5 3 5 compounds, Samples (a and b); single-phase and Cu-rich CuInSe films, Samples (c—e) and (f—h), respective2 ly Sample No.
SG1-1(a) SG13 SG59(b) SG14 SG15(c) SG16(d) SG43 SG42 SG41(e) SG7 SG12(f) SG38(g) SG50(h)
Cu—In alloy films Cu/In
0.40 0.40 0.41 0.52 0.67 0.84 0.85 0.87 0.94 0.97 1.11 1.51 1.65
Compositions of selenized CuInSe films (at. %) 2 Cu
In
Se
Cu/In
Se/(Cu#In)
13.34 13.83 14.63 15.91 20.01 21.88 23.45 23.60 24.47 26.01 28.51 36.53 39.32
29.30 32.30 30.71 31.30 27.60 26.99 25.48 25.65 25.21 21.80 23.68 20.97 17.30
57.37 53.87 54.66 52.79 52.39 51.13 51.07 50.75 50.32 52.19 47.81 42.50 43.38
0.455 0.428 0.476 0.508 0.725 0.811 0.920 0.920 0.971 1.193 1.204 1.742 2.283
1.346 1.168 1.206 1.118 1.100 1.046 1.044 1.030 1.013 1.092 0.916 0.739 0.766
4. Conclusion Detailed studies on the co-sputtered Cu—In alloys and CuInSe films obtained by 2 selenization within a close-spaced graphite container have been discussed. The XRD analysis showed a form of sequence for the structural phases formed, varying from the dominant presence of InPCuIn PCu In as the Cu content in the films increased. 2 11 9
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The morphologies of the alloy films was also found to be greatly influenced by the composition differences. The selenized CuInSe thin films exhibited different micro2 structural properties which corresponded with the change in the alloy compositions. Very In-rich films yielded the CuIn Se or CuIn Se compound which were charac3 5 2 3.5 terized by small crystal sizes. Near stoichiometric In-rich or Cu-rich films yielded single phase chalcopyrite CuInSe films with high density of crystal structures and 2 sizes of about 5 lm. The film resistivities varied from 10~2 to 108 ) cm and were found to depend on the Cu/In composition ratio of the alloy films. The films had composition ratios of Cu/In between 0.40 and 2.3, and Se/(Cu#In) between 0.74 and 1.35. All CuInSe films with the exception of very Cu-rich ones contained high 2 amount of Se content ('50%). The co-sputtering technique for Cu—In alloy fabrication and the graphite box selenization system have simple control mechanism and show strong potential for producing quality CuInSe materials. Further investiga2 tions are in progress to fully demonstrate the potential of these films in solar cell devices.
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