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Characterization of polycrystalline CuInSe2 thin films deposited by sputtering and evaporation as a function of composition
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Journal of Physics and Chemistry of Solids 69 (2008) 435–440 www.elsevier.com/locate/jpcs
Characterization of polycrystalline CuInSe2 thin films deposited by sputtering and evaporation as a function of composition L.-C. Yang, C.Y. Cheng, J.S. Fang Department of Materials Science and Engineering, National Formosa University, Huwei, Yunlin 632, Taiwan
Abstract In this study, a series of polycrystalline single-layer CulnSe2 thin films with Cu/In atomic ratios between 1.7 and 0.5 were grown by using a physical vapor deposition system Cu, In sputtering, and Se evaporation. The variation in the microstructure (surface morphology, grain size, and defects) of the CulnSe2 thin films on gradually changing the stoichiometry from Cu-rich to Cu-poor was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In general, Cu-rich thin films showed large grains with relatively low defect densities, while Cu-poor thin films exhibited highly planar defects, twins and stacking faults, with small grains. Polycrystalline bi-layer CulnSe2 thin films were also grown by depositing a Cu-poor layer on the top of a Curich layer. The bi-layer Cu-poor thin films exhibited large grains (43 mm). High-resolution TEM lattice image shows a thin coherent layer (20 nm) at the both sides of the grain boundary for a slightly Cu-poor film. TEM energy dispersive X-ray spectroscopy (EDX) analyses indicate that the grain boundary region is more Cu-poor than the intra-grain for the slightly Cu-poor film. The coherent thin layer and more Cu-poor at the grain boundary of the slightly Cu-poor film could be related to a type inverse. The surface morphologies and the grain boundaries of the polycrystalline CuInSe2 could provide an understanding of the growth behavior of CuInSe2 and the performance of CuInSe2 devices. r 2007 Published by Elsevier Ltd.
1. Introduction Considering the limitation of current energy resources and their environmental impact, greater stress has been placed on the development of renewable energy sources such as photovoltaic electric generators. Copper indium diselenide CuInSe2 (CIS) and related compounds such as Cu(InGa)Se2 and Cu(InGa)(SeS)2 are leading candidates for absorber materials in large-area photovoltaic power generation systems. Devices based on Cu(InGa)Se2 have achieved high active area efficiencies (19.2%) in small area devices [1]. One of the features of the CuInSe2 that is important to photovoltaic devices performance is the grain boundary and surface stoichiometry as a function of composition. The device-quality Cu-poor CuInSe2 films are observed to display a copper depleted surface region [2], and strong Cu-poor CuInSe2 films exhibit a defectordered surface phase with a composition typically found Corresponding author. Tel.: +886 5 632 9643; fax: +886 5 636 1981.
E-mail address: [email protected] (C.Y. Cheng). 0022-3697/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.jpcs.2007.07.027
to be Cu1In3Se5 [3]. Various electronic measurements have clarified that the free surfaces and grain boundaries of CuInSe2 films exhibit a type inversion; accordingly the surfaces or the grain boundary regions become electron rich, even though the interior are still hole rich for the Cupoor films [4,5]. Hybrid-process polycrystalline CuInSe2 has been demonstrated to be of high quality based on materials analyses comparing the resulting layers with those deposited by other methods, and by fabrication of solar cells. Devices based on hybrid-process CuInSe2 containing no Ga and with thick, heavily doped CdZnS window layers have achieved conversion efficiencies as high as 10.0% [6]. Previous studies using the hybrid method and other techniques for deposition of CuInSe2 have suggested that disorder (sphalerite structure) [7] and ordered vacancies [8] both occur on the metal sublattice of non-stoichiometric material. There is some indication that the point defect population in physical vapor deposited films may vary with deposition on the extent of disorder has not been examined systematically, particularly in sputtered films. For the
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current work, the extent of chalcopyrite ordering in CuInSe2 was examined by SEM and TEM as a function of deposition conditions. This paper describes the microstructure of surfaces and grain boundaries, and the implications of these for device performance in CuInSe2 deposited under various conditions by the hybrid process. 2. Experimental The results presented below were recorded for CuInSe2 deposited from magnetron sputtered Cu and In fluxes and evaporated molecular Se, described previously [9,10]. CuInSe2 films were deposited on 25 25 1.2 mm, Corning type 7059 borosilicate or common soda-lime glass substrates. Deposition was carried out at a temperature of 350–500 1C and a deposition rate from 10 to 50 nm/min. The thickness of the grown layers was typically 2 mm. Three aspects of the film properties were analyzed: microstructure, composition, and microchemistry. Microstructural studies were carried out using transmission electron microscope (TEM; Jeol JEM 2010) equipped with a microchemical analysis energy dispersive X-ray spectroscopy (EDX; Oxford Mode 6049) system for determination of the composition of individual grains and grain boundaries of CuInSe2. Surface morphologies and compositions of the polycrystalline films were examined using a Hitachi scanning electron microscope (SEM) equipped with an EDX systems. Films examined for this work were formed either from fixed fluxes of constituents (single-layer films) or as bilayers beginning with a strongly Cu-rich film and finishing with a Cu-poor layer. In Section 3, it follows that the Cu-rich portion of the bilayer will be referred to as the ‘‘bottom’’ layer and the Cu-poor deposition as the ‘‘top’’ layer. The Cu-rich layers were deposited at substrate temperature (TS) 400 1C and had typical compositions of 30Cu, 23In, and 47Se with atomic percentage (at%) and thicknesses of 2 mm. The top Cu-poor layers had various Cu fluxes and were deposited at 400 1CoTSo500 1C. 3. Results and discussion 3.1. Surface morphologies of single-layer CuInSe2 films Figs. 1(a)–(f) are a series of SEM surface morphologies of single-layer CuInSe2 films, deposited at 350 and 450 1C, over a range from Cu-rich to Cu-poor. These SEM micrographs illustrate the dependence of film surface morphology on composition (determined by EDX over an area in excess of 60 mm 60 mm) and deposition temperature. The surfaces of Cu-rich films consist of large convex particles with size 3 mm, while stiochiometric and Cu-poor films were strongly faceted small particles with size 0.5 mm. The surface morphologies imply that the growth of Cu-rich is near isotropic, whereas Cu-poor is anisotropic growth. From interpretation of the morphology of epitaxial layers [11], it was concluded that the (1 1 2)
surface of Cu-poor CuInSe2 films is by far the lowest energy surface and that all other surfaces tend to facet to produce this face. It explains that the Cu-poor films are faceted by the most stable (1 1 2) planes. However, the Cu-rich films have relative higher energy surfaces with all the different orientations; therefore, the growth of the Curich is isotropic. TEM selected area diffraction pattern (SADP) and X-ray diffraction (XRD) results (not shown) demonstrated that the deposition temperature (TS) is also a very strong parameter on the structure of the CuInSe2 films. For equivalent compositions (either Cu-rich or Cu-poor), the films deposited at higher temperature, 450 1C, exhibited stronger chalcopyrite reflections as compared to films deposited at 350 1C. XRD pattern of a strongly Cu-poor film included the (1 0 1) and (1 0 3) chalcopyriter reflections and three additional (1 1 0), (2 2 0), and (1 1 4) reflections of a defect-ordered phase, respectively [12]. The defectordered phase is an ordered vacancy compound (OVC) with Cu vacancies (VCu) and In on Cu antisite defects (InCu) [13,14]. The intensity of the OVC reflection also increased with increasing TS, as did the chalcopyrite reflections. 3.2. Surface morphologies of bi-layer CuInSe2 films Figs. 2(a)–(f) are SEM surface morphologies of bi-layer CuInSe2 films. The bi-layers were beginning with a strongly Cu-rich film and finishing with a strong Cu-poor layer. The Cu-rich bottom layers were deposited at TS ¼ 400 1C and had typical compositions of 30 at% Cu, 23 at% In, and 47 at% Se and thicknesses of 2 mm. The Cu-poor top layers had various Cu fluxes and were deposited at 400 1C for (a) and (b), at 450 1C for (c) and (d), and at 500 1C for (e) and (f) onto Mo-coated glasses, respectively. The SEM micrographs show the dependence of film surface morphology on composition and deposition temperature. Cu-poor films were strongly faceted and the surface area decreased with increasing the deposited temperatures. The strong (1 1 2) faceted surface also implies the anisotropic growth of the Cu-poor top layers on the Cu-rich bottom layers. Previous XTEM results [15] showed that the portion of CuInSe2 films grown Cu-rich is dense, while the top layer (deposited with a net Cu-poor flux) contains significantly more voids. This increased void density near the surface of bi-layer films is most pronounced for lower TS. The increase in surface faceting and void formation can be explained by noting that the OVC compound Cu1In3Se5 segregates to the surface of Cu-poor layers. The driving force for all surface segregations is lowering of the total energy, usually by reducing the surface energy. The increase in void density in Cu-poor films is consistent with an increased tendency to facet associated with the lowered surface energy resulting from surface segregation of the OVC. At higher TS, increased surface diffusion kinetics allows larger facets to form, both decreasing the surface area and decreasing the probability that a given facet will be overgrown by
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Fig. 1. SEM surface morphologies of single-layer CuInSe2 films deposited at 450 1C for (a), (c), and (e), at 350 1C for (b), (d) and (f) onto Mo-coated glasses. Film compositions are indicated in at% Cu/In/Se. In (f) the arrows indicate the Cu-deficient (rose-like) features with compositions Cu1In2.9Se4.4 measured by (micro)-EDX (probe area: 4 m).
surrounding grain faces and forming in a void. XTEM micrograph of a bi-layer Cu-poor film with top layer TS ¼ 450 1C exhibited a columnar structure, a grain diameter of 4 mm and no distinct transition from the bottom Cu-rich to the top Cu-poor material. It indicates that the Cu-poor top layers grew epitaxially on the underlying grains. 3.3. Grain boundaries of CuInSe2 films Figs. 3(a) and (b) are plan-view TEM micrographs of a slightly Cu-rich and a slightly Cu-poor thin films, respectively, deposited at 450 1C. The Cu-rich thin films have relatively low defect densities, while Cu-poor thin films exhibit highly planar defects, twins and stacking faults. Besides, the boundary curvatures of the slightly Curich and the slightly Cu-poor films are different. Some boundaries are concave inwards for the Cu-rich film;
therefore, these grains will shrink and eventually disappear to reduce the total grain boundary energy resulting grain coarsening. However, the boundaries are planar and structure metastable for the slightly Cu-poor film. These results are similar with the observations of the surface morphologies above. Since (1 1 2) is the most stable for the Cu-poor films, the planar grain boundaries consists of lowest energy (1 1 2) planes with small size grains. The slightly Cu-rich films have relative higher energy planes with all the different orientations; therefore, the boundaries of the Cu-rich are concave. In order to reduce the grain boundary energy, the slightly Cu-rich film has large size grains. Figs. 4(a) and (b) are high-resolution electron microscopy lattice images taken from the grain boundaries of the films identical to Fig. 3. For the slightly Cu-rich film in Fig. 4(a), no second precipitations are observed at the grain boundary. Although the (1 0 1) and (1 0 3) spots of SADP,
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Fig. 2. SEM surface morphologies of bi-layer CuInSe2 films with bottom layers (30Cu/23In/47Se, at%) deposited at 400 1C, and top layers deposited at 400 1C for (a) and (b), at 450 1C for (c) and (d), at 500 1C for (e) and (f) onto Mo-coated glasses. Film compositions and deposition temperatures are indicated in at% Cu/In/Se and top temperature/bottom temperature, respectively. In (b) the raised (rose-like) features indicated by arrows are Cu-deficient measured by (micro)-EDX (probe area: 4 m).
superlattice reflections due to the chalcopyrite-ordering of the cation sublattice, are observed but are extremely weak for the slightly Cu-rich film deposited at 450 1C. The explanation of the observations is that the slightly Cu-rich film is disordering (sphalerite) and ordering (chalcopyrite) phases mixing. To examine the local chemical composition, EDX data were taken with a small probe size of 10 nm. Preliminary results show that the local compositions of the grain boundary (P1 spot in Fig. 4(a)) and intra-grain (P2 spot in Fig. 3(a)) are Cu-rich and near identical, e.g. the Cu/In peak ration is almost the same. For the slightly Cupoor film shown in Fig. 3(b), a thin coherent layers (20 nm) are shown at the both sides of the grain boundary. A super thin twin (10 atomic layers thick) is associated with the coherent layer. Besides the (1 0 1) and (1 0 3) spots of SADP taken from grain boundary region, three weak additional (1 1 0), (2 2 0), and (1 1 4) reflections of a defect-ordered phase were observed. EDX local
chemical compositional analysis reveals that both the grain boundary region (P1 spot in Fig. 3(b)) and the intra-grain (P2 spot in Fig. 3(b)) are Cu-poor; however, the Cu/In peak ration has a change between them. The Cu/In peak ratio is higher in the grain boundary region than in the intra-grain. This indicates that the grain boundary region is more Cu-poor than the intra-grain. The results suggest that a thin layer of more Cu-poor defect-ordered phase exists at grain boundary in the Cu-poor single-layer CuInSe2 films. It is also found the distribution of the defect-ordered phase at the grain boundary region, and the Cu difference between the grain boundary region and intra-grain may vary with film preparation. In other words, the defectordered phase and the chemical compositions at the grain boundary vary with deposition processes, such as atomic flux rates and substrate temperatures. The InCu and CuIn antisite defects which are responsible for the disordering of the chalcopyrite structure are
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Fig. 3. Plan-view TEM images and EDXs (probe size of 10 nm) spots (P1 and P2) for (a) a slightly Cu-rich films and (b) a slightly Cu-poor films.
thought to have the lowest formation energies, 1.4 and 1.5 eV, respectively, of the common native point defects in CuInSe2 [16]. It can explain the existence of disordering sphalerite and ordering chalcopyrite mixing phases in Curich films and lower deposition temperatures. Firstprinciples theoretical calculations [17–20] also show that 2þ the defect pair (2V Cu þ InCu ) in the chalcopyrite CuInSe2 has extreme low formation energy (5.67 eV/pair). Because of the low formation energy of the charge–neutral defect 2þ pair 2V Cu þ InCu , actual chalcopyrite are highly nonstoichiometric, exhibiting microphase made of units of 2þ 2V Cu þ InCu and CuInSe2, resulting in Cu1In3Se5, etc. The acceptor defects Cu vacancies (VCu) exist at the polar cation (1 1 2)A surface, while the anion surface (1 1 2)B has an order-defect reconstruction involving donor defects such as anion vacancies or InCu antisites at the subsurface. The defect pair-related atomic configuration at the grain boundary region could play an important rule for the performance of the devices. Detail examination of the atomic configuration is still under investigation in our laboratory. 4. Conclusions The microstructure (surface morphology, grain size, and defects) of the CulnSe2 thin films is a function of
Fig. 4. TEM high-resolution lattice images show that the microstructures at both sides of grain boundary are similar for the slightly Cu-poor film shown in (a), while a thin coherent layers (20 nm) are shown at the both sides of the grain boundary for the slight Cu-poor film in (b).
composition and deposition temperature. The boundary curvatures of the slightly Cu-rich and the slightly Cu-poor films are different. Some boundaries are concave inwards for the Cu-rich film, while the boundaries are planar and structure metastable for the slightly Cu-poor film. It explains that the growth of Cu-rich is isotropic and results in large grains, while the Cu-poor is anisotropic growth resulting in small grains. High-resolution electron microscopy lattice image shows that no second precipitations are observed at the grain boundary for the slightly Cu-rich film, and a thin coherent layers (20 nm) are found at the both sides of the grain boundary for the slightly Cu-poor film. TEM EDX analyses show that the local compositions of the grain boundary and intra-grain are Cu-rich and near identical for the slightly Cu-rich film, on the other hand, it indicates that the grain boundary region is more Cu-poor than the intra-grain for the slightly Cu-rich film. The coherent thin layer and more Cu-poor at the grain boundary of the slightly Cu-poor film could be related to a type inverse; therefore, the grain boundary regions (similar to surface) become electron rich, even though the interior are still hole rich for the Cu-poor films.
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Acknowledgments The authors would like to thank the National Science Council of Republic of China under Contract nos. NSC 932622-E-150-037-CC3 and NSC 91-2216-E-150-002, and the National Formosa University for financial supports. References [1] K. Ramanathan, M.A. Contreras, C. Perkins, S. Asher, F. Hasoon, J. Keane, D. Young, M.J. Romero, W. Metzger, R. Noufi, J. Ward, A. Duda, Prog. Photovolt. Res. Appl. 11 (2003) 225. [2] D. Liao, A. Rockett, Appl. Phys. Lett. 82 (2003) 2829. [3] J.R. Tuttle, D.S. Albin, R. Noufi, Sol. Cells 30 (1991) 21. [4] D. Schmid, M. Ruckh, F. Grunwald, H.W. Schock, J. Appl. Phys. 73 (1993) 2902. [5] R.J. Matson, M.A. Contreras, J.R. Tuttle, A.B. Swartzlander, P.A. Parilla, R. Noufi, Mater. Res. Soc. Symp. Proc. 426 (1996) 183. [6] L.-C. Yang, G. Berry, L.J. Chou, G. Kenshole, A. Rockett, C.A. Mullan, C.J. Kiely, in: Proceeding of the 23rd IEEE Photovoltaic Specialists Conference, Louisville, KY, May 1993, p. 505. [7] C.J. Kiely, R.C. Pond, G. Kenshole, A. Reckett, Philos. Mag. A 63 (1991) 1249.
[8] A. Rockett, A. Agarwal, L.-C. Yang, E. Banda, G. Kenshole, C.J. Kiely, H. Talieh, in: Proceedings of the 21st Institute of Electrical and Electronics Engineers Photovoltaic Specialists Conference, vol. 1, Institute of Electrical and Electronics Engineers, New York, 1990, p. 764. [9] A. Rockett, T.C. Lommasson, P. Campos, L.C. Yang, H. Taleih, Thin Solid Films 171 (1989) 109. [10] L.-C. Yang, L.J. Chou, A. Agarwal, A. Rockett, in: Proceedings of the 22nd IEEE Photovoltaic Specialists Conference, Las Vegas, NV, October 1991, p. 1185. [11] L.-C. Yang, G.S. Chen, A. Rockett, Appl. Phys. Lett. 86 (2005) 201907. [12] Joint Council on Powder Diffraction Studies (JCPDS) file No. 351349, 1991. [13] B.-H. Tseng, S.-B. Lin, G.-L. Gu, W. Chen, J. Appl. Phys. 79 (1996) 1391. [14] J.R. Tuttle, D.S. Albin, R. Noufi, Sol. Cells 30 (1991) 21. [15] L.-C. Yang, H.Z. Xiao, A. Rockett, W.N. Shafarman, R.W. Birkmire, Sol. Energy Mater. Sol. Cells 36 (1995) 445. [16] S.M. Wasim, Sol. Cells 16 (1986) 289. [17] C. Person, A. Zunger, Phys. Rev. Lett. 91 (2003) 266401. [18] S.B. Zhang, S.-H. Wei, Phys. Rev. B 65 (2002) 081402(R). [19] S.-H. Han, A.M. Hermann, F.S. Hasoon, H.A. Al-Thani, D.H. Levi, Appl. Phys. Lett. 85 (2004) 576. [20] J.E. Jaffe, A. Zunger, J. Phys. Chem. Solids 64 (2003) 1547.
Journal of Physics and Chemistry of Solids 69 (2008) 435–440 www.elsevier.com/locate/jpcs
Characterization of polycrystalline CuInSe2 thin films deposited by sputtering and evaporation as a function of composition L.-C. Yang, C.Y. Cheng, J.S. Fang Department of Materials Science and Engineering, National Formosa University, Huwei, Yunlin 632, Taiwan
Abstract In this study, a series of polycrystalline single-layer CulnSe2 thin films with Cu/In atomic ratios between 1.7 and 0.5 were grown by using a physical vapor deposition system Cu, In sputtering, and Se evaporation. The variation in the microstructure (surface morphology, grain size, and defects) of the CulnSe2 thin films on gradually changing the stoichiometry from Cu-rich to Cu-poor was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In general, Cu-rich thin films showed large grains with relatively low defect densities, while Cu-poor thin films exhibited highly planar defects, twins and stacking faults, with small grains. Polycrystalline bi-layer CulnSe2 thin films were also grown by depositing a Cu-poor layer on the top of a Curich layer. The bi-layer Cu-poor thin films exhibited large grains (43 mm). High-resolution TEM lattice image shows a thin coherent layer (20 nm) at the both sides of the grain boundary for a slightly Cu-poor film. TEM energy dispersive X-ray spectroscopy (EDX) analyses indicate that the grain boundary region is more Cu-poor than the intra-grain for the slightly Cu-poor film. The coherent thin layer and more Cu-poor at the grain boundary of the slightly Cu-poor film could be related to a type inverse. The surface morphologies and the grain boundaries of the polycrystalline CuInSe2 could provide an understanding of the growth behavior of CuInSe2 and the performance of CuInSe2 devices. r 2007 Published by Elsevier Ltd.
1. Introduction Considering the limitation of current energy resources and their environmental impact, greater stress has been placed on the development of renewable energy sources such as photovoltaic electric generators. Copper indium diselenide CuInSe2 (CIS) and related compounds such as Cu(InGa)Se2 and Cu(InGa)(SeS)2 are leading candidates for absorber materials in large-area photovoltaic power generation systems. Devices based on Cu(InGa)Se2 have achieved high active area efficiencies (19.2%) in small area devices [1]. One of the features of the CuInSe2 that is important to photovoltaic devices performance is the grain boundary and surface stoichiometry as a function of composition. The device-quality Cu-poor CuInSe2 films are observed to display a copper depleted surface region [2], and strong Cu-poor CuInSe2 films exhibit a defectordered surface phase with a composition typically found Corresponding author. Tel.: +886 5 632 9643; fax: +886 5 636 1981.
E-mail address: [email protected] (C.Y. Cheng). 0022-3697/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.jpcs.2007.07.027
to be Cu1In3Se5 [3]. Various electronic measurements have clarified that the free surfaces and grain boundaries of CuInSe2 films exhibit a type inversion; accordingly the surfaces or the grain boundary regions become electron rich, even though the interior are still hole rich for the Cupoor films [4,5]. Hybrid-process polycrystalline CuInSe2 has been demonstrated to be of high quality based on materials analyses comparing the resulting layers with those deposited by other methods, and by fabrication of solar cells. Devices based on hybrid-process CuInSe2 containing no Ga and with thick, heavily doped CdZnS window layers have achieved conversion efficiencies as high as 10.0% [6]. Previous studies using the hybrid method and other techniques for deposition of CuInSe2 have suggested that disorder (sphalerite structure) [7] and ordered vacancies [8] both occur on the metal sublattice of non-stoichiometric material. There is some indication that the point defect population in physical vapor deposited films may vary with deposition on the extent of disorder has not been examined systematically, particularly in sputtered films. For the
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current work, the extent of chalcopyrite ordering in CuInSe2 was examined by SEM and TEM as a function of deposition conditions. This paper describes the microstructure of surfaces and grain boundaries, and the implications of these for device performance in CuInSe2 deposited under various conditions by the hybrid process. 2. Experimental The results presented below were recorded for CuInSe2 deposited from magnetron sputtered Cu and In fluxes and evaporated molecular Se, described previously [9,10]. CuInSe2 films were deposited on 25 25 1.2 mm, Corning type 7059 borosilicate or common soda-lime glass substrates. Deposition was carried out at a temperature of 350–500 1C and a deposition rate from 10 to 50 nm/min. The thickness of the grown layers was typically 2 mm. Three aspects of the film properties were analyzed: microstructure, composition, and microchemistry. Microstructural studies were carried out using transmission electron microscope (TEM; Jeol JEM 2010) equipped with a microchemical analysis energy dispersive X-ray spectroscopy (EDX; Oxford Mode 6049) system for determination of the composition of individual grains and grain boundaries of CuInSe2. Surface morphologies and compositions of the polycrystalline films were examined using a Hitachi scanning electron microscope (SEM) equipped with an EDX systems. Films examined for this work were formed either from fixed fluxes of constituents (single-layer films) or as bilayers beginning with a strongly Cu-rich film and finishing with a Cu-poor layer. In Section 3, it follows that the Cu-rich portion of the bilayer will be referred to as the ‘‘bottom’’ layer and the Cu-poor deposition as the ‘‘top’’ layer. The Cu-rich layers were deposited at substrate temperature (TS) 400 1C and had typical compositions of 30Cu, 23In, and 47Se with atomic percentage (at%) and thicknesses of 2 mm. The top Cu-poor layers had various Cu fluxes and were deposited at 400 1CoTSo500 1C. 3. Results and discussion 3.1. Surface morphologies of single-layer CuInSe2 films Figs. 1(a)–(f) are a series of SEM surface morphologies of single-layer CuInSe2 films, deposited at 350 and 450 1C, over a range from Cu-rich to Cu-poor. These SEM micrographs illustrate the dependence of film surface morphology on composition (determined by EDX over an area in excess of 60 mm 60 mm) and deposition temperature. The surfaces of Cu-rich films consist of large convex particles with size 3 mm, while stiochiometric and Cu-poor films were strongly faceted small particles with size 0.5 mm. The surface morphologies imply that the growth of Cu-rich is near isotropic, whereas Cu-poor is anisotropic growth. From interpretation of the morphology of epitaxial layers [11], it was concluded that the (1 1 2)
surface of Cu-poor CuInSe2 films is by far the lowest energy surface and that all other surfaces tend to facet to produce this face. It explains that the Cu-poor films are faceted by the most stable (1 1 2) planes. However, the Cu-rich films have relative higher energy surfaces with all the different orientations; therefore, the growth of the Curich is isotropic. TEM selected area diffraction pattern (SADP) and X-ray diffraction (XRD) results (not shown) demonstrated that the deposition temperature (TS) is also a very strong parameter on the structure of the CuInSe2 films. For equivalent compositions (either Cu-rich or Cu-poor), the films deposited at higher temperature, 450 1C, exhibited stronger chalcopyrite reflections as compared to films deposited at 350 1C. XRD pattern of a strongly Cu-poor film included the (1 0 1) and (1 0 3) chalcopyriter reflections and three additional (1 1 0), (2 2 0), and (1 1 4) reflections of a defect-ordered phase, respectively [12]. The defectordered phase is an ordered vacancy compound (OVC) with Cu vacancies (VCu) and In on Cu antisite defects (InCu) [13,14]. The intensity of the OVC reflection also increased with increasing TS, as did the chalcopyrite reflections. 3.2. Surface morphologies of bi-layer CuInSe2 films Figs. 2(a)–(f) are SEM surface morphologies of bi-layer CuInSe2 films. The bi-layers were beginning with a strongly Cu-rich film and finishing with a strong Cu-poor layer. The Cu-rich bottom layers were deposited at TS ¼ 400 1C and had typical compositions of 30 at% Cu, 23 at% In, and 47 at% Se and thicknesses of 2 mm. The Cu-poor top layers had various Cu fluxes and were deposited at 400 1C for (a) and (b), at 450 1C for (c) and (d), and at 500 1C for (e) and (f) onto Mo-coated glasses, respectively. The SEM micrographs show the dependence of film surface morphology on composition and deposition temperature. Cu-poor films were strongly faceted and the surface area decreased with increasing the deposited temperatures. The strong (1 1 2) faceted surface also implies the anisotropic growth of the Cu-poor top layers on the Cu-rich bottom layers. Previous XTEM results [15] showed that the portion of CuInSe2 films grown Cu-rich is dense, while the top layer (deposited with a net Cu-poor flux) contains significantly more voids. This increased void density near the surface of bi-layer films is most pronounced for lower TS. The increase in surface faceting and void formation can be explained by noting that the OVC compound Cu1In3Se5 segregates to the surface of Cu-poor layers. The driving force for all surface segregations is lowering of the total energy, usually by reducing the surface energy. The increase in void density in Cu-poor films is consistent with an increased tendency to facet associated with the lowered surface energy resulting from surface segregation of the OVC. At higher TS, increased surface diffusion kinetics allows larger facets to form, both decreasing the surface area and decreasing the probability that a given facet will be overgrown by
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Fig. 1. SEM surface morphologies of single-layer CuInSe2 films deposited at 450 1C for (a), (c), and (e), at 350 1C for (b), (d) and (f) onto Mo-coated glasses. Film compositions are indicated in at% Cu/In/Se. In (f) the arrows indicate the Cu-deficient (rose-like) features with compositions Cu1In2.9Se4.4 measured by (micro)-EDX (probe area: 4 m).
surrounding grain faces and forming in a void. XTEM micrograph of a bi-layer Cu-poor film with top layer TS ¼ 450 1C exhibited a columnar structure, a grain diameter of 4 mm and no distinct transition from the bottom Cu-rich to the top Cu-poor material. It indicates that the Cu-poor top layers grew epitaxially on the underlying grains. 3.3. Grain boundaries of CuInSe2 films Figs. 3(a) and (b) are plan-view TEM micrographs of a slightly Cu-rich and a slightly Cu-poor thin films, respectively, deposited at 450 1C. The Cu-rich thin films have relatively low defect densities, while Cu-poor thin films exhibit highly planar defects, twins and stacking faults. Besides, the boundary curvatures of the slightly Curich and the slightly Cu-poor films are different. Some boundaries are concave inwards for the Cu-rich film;
therefore, these grains will shrink and eventually disappear to reduce the total grain boundary energy resulting grain coarsening. However, the boundaries are planar and structure metastable for the slightly Cu-poor film. These results are similar with the observations of the surface morphologies above. Since (1 1 2) is the most stable for the Cu-poor films, the planar grain boundaries consists of lowest energy (1 1 2) planes with small size grains. The slightly Cu-rich films have relative higher energy planes with all the different orientations; therefore, the boundaries of the Cu-rich are concave. In order to reduce the grain boundary energy, the slightly Cu-rich film has large size grains. Figs. 4(a) and (b) are high-resolution electron microscopy lattice images taken from the grain boundaries of the films identical to Fig. 3. For the slightly Cu-rich film in Fig. 4(a), no second precipitations are observed at the grain boundary. Although the (1 0 1) and (1 0 3) spots of SADP,
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Fig. 2. SEM surface morphologies of bi-layer CuInSe2 films with bottom layers (30Cu/23In/47Se, at%) deposited at 400 1C, and top layers deposited at 400 1C for (a) and (b), at 450 1C for (c) and (d), at 500 1C for (e) and (f) onto Mo-coated glasses. Film compositions and deposition temperatures are indicated in at% Cu/In/Se and top temperature/bottom temperature, respectively. In (b) the raised (rose-like) features indicated by arrows are Cu-deficient measured by (micro)-EDX (probe area: 4 m).
superlattice reflections due to the chalcopyrite-ordering of the cation sublattice, are observed but are extremely weak for the slightly Cu-rich film deposited at 450 1C. The explanation of the observations is that the slightly Cu-rich film is disordering (sphalerite) and ordering (chalcopyrite) phases mixing. To examine the local chemical composition, EDX data were taken with a small probe size of 10 nm. Preliminary results show that the local compositions of the grain boundary (P1 spot in Fig. 4(a)) and intra-grain (P2 spot in Fig. 3(a)) are Cu-rich and near identical, e.g. the Cu/In peak ration is almost the same. For the slightly Cupoor film shown in Fig. 3(b), a thin coherent layers (20 nm) are shown at the both sides of the grain boundary. A super thin twin (10 atomic layers thick) is associated with the coherent layer. Besides the (1 0 1) and (1 0 3) spots of SADP taken from grain boundary region, three weak additional (1 1 0), (2 2 0), and (1 1 4) reflections of a defect-ordered phase were observed. EDX local
chemical compositional analysis reveals that both the grain boundary region (P1 spot in Fig. 3(b)) and the intra-grain (P2 spot in Fig. 3(b)) are Cu-poor; however, the Cu/In peak ration has a change between them. The Cu/In peak ratio is higher in the grain boundary region than in the intra-grain. This indicates that the grain boundary region is more Cu-poor than the intra-grain. The results suggest that a thin layer of more Cu-poor defect-ordered phase exists at grain boundary in the Cu-poor single-layer CuInSe2 films. It is also found the distribution of the defect-ordered phase at the grain boundary region, and the Cu difference between the grain boundary region and intra-grain may vary with film preparation. In other words, the defectordered phase and the chemical compositions at the grain boundary vary with deposition processes, such as atomic flux rates and substrate temperatures. The InCu and CuIn antisite defects which are responsible for the disordering of the chalcopyrite structure are
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Fig. 3. Plan-view TEM images and EDXs (probe size of 10 nm) spots (P1 and P2) for (a) a slightly Cu-rich films and (b) a slightly Cu-poor films.
thought to have the lowest formation energies, 1.4 and 1.5 eV, respectively, of the common native point defects in CuInSe2 [16]. It can explain the existence of disordering sphalerite and ordering chalcopyrite mixing phases in Curich films and lower deposition temperatures. Firstprinciples theoretical calculations [17–20] also show that 2þ the defect pair (2V Cu þ InCu ) in the chalcopyrite CuInSe2 has extreme low formation energy (5.67 eV/pair). Because of the low formation energy of the charge–neutral defect 2þ pair 2V Cu þ InCu , actual chalcopyrite are highly nonstoichiometric, exhibiting microphase made of units of 2þ 2V Cu þ InCu and CuInSe2, resulting in Cu1In3Se5, etc. The acceptor defects Cu vacancies (VCu) exist at the polar cation (1 1 2)A surface, while the anion surface (1 1 2)B has an order-defect reconstruction involving donor defects such as anion vacancies or InCu antisites at the subsurface. The defect pair-related atomic configuration at the grain boundary region could play an important rule for the performance of the devices. Detail examination of the atomic configuration is still under investigation in our laboratory. 4. Conclusions The microstructure (surface morphology, grain size, and defects) of the CulnSe2 thin films is a function of
Fig. 4. TEM high-resolution lattice images show that the microstructures at both sides of grain boundary are similar for the slightly Cu-poor film shown in (a), while a thin coherent layers (20 nm) are shown at the both sides of the grain boundary for the slight Cu-poor film in (b).
composition and deposition temperature. The boundary curvatures of the slightly Cu-rich and the slightly Cu-poor films are different. Some boundaries are concave inwards for the Cu-rich film, while the boundaries are planar and structure metastable for the slightly Cu-poor film. It explains that the growth of Cu-rich is isotropic and results in large grains, while the Cu-poor is anisotropic growth resulting in small grains. High-resolution electron microscopy lattice image shows that no second precipitations are observed at the grain boundary for the slightly Cu-rich film, and a thin coherent layers (20 nm) are found at the both sides of the grain boundary for the slightly Cu-poor film. TEM EDX analyses show that the local compositions of the grain boundary and intra-grain are Cu-rich and near identical for the slightly Cu-rich film, on the other hand, it indicates that the grain boundary region is more Cu-poor than the intra-grain for the slightly Cu-rich film. The coherent thin layer and more Cu-poor at the grain boundary of the slightly Cu-poor film could be related to a type inverse; therefore, the grain boundary regions (similar to surface) become electron rich, even though the interior are still hole rich for the Cu-poor films.
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