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Control of VSe− defect levels in CuInSe2 prepared by rapid thermal processing of metallic alloys
Thin Solid Films 361±362 (2000) 432±436 www.elsevier.com/locate/tsf
Control of VSe 2 defect levels in CuInSe2 prepared by rapid thermal processing of metallic alloys V. Alberts a,*, J. Bekker a, M.J. Witcomb b, J.H. SchoÈn c, E. Bucher d a b
Department of Physics, Rand Afrikaans University, PO Box 524, Auckland Park 2006, South Africa Electron Microscope Unit, University of the Witwatersrand, Private Bag 3, WITS 2006, South Africa c Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill NJ 07974-0636, USA d FakultaÈt fuÈr Physik, Postfach X916, D-78457 Konstanz, Germany
Abstract The reaction of metallic alloys to H2Se/Ar is a promising technique to produce device quality CuInSe2 thin ®lms. However, up to now the controllability of the ®lm quality has been critically in¯uenced by the segregation of secondary phases during growth. We indicate in this study that this phenomenon is strongly related to the selenization reaction temperature, and especially to the ramping procedure followed to the ®nal selenization temperature. Metallic precursors which were slowly heated to temperatures around 4008C are characterized by inhomogeneous ®lm morphologies and X-ray ¯uorescence (XRF) measurements revealed a strong segregation of In towards the Mo back contact. In contrast, rapid heating of samples in H2Se/Ar to temperatures above 4008C resulted in uniform and dense ®lms with a high degree of compositional uniformity through the thickness of the samples. Transmission electron microscopy (TEM) indicated the presence of a low density of planar defects in these optimized ®lms. Low temperature photoluminescence (PL) studies demonstrated that the optical properties of these polycrystalline thin ®lms are very sensitive to post-growth treatment in Ar/H2 and O2. q 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: CuInSe2; H2Se annealing; X-ray ¯uorescence; Photoluminescence; Post-growth treatment
1. Introduction Thin ®lm photovoltaic (PV) technologies are being developed in order to reduce the cost of PV energy conversion. The ternary chalcopyrite semiconductor CuInSe2 (CIS) is a promising material for a new generation of cost-effective, high ef®ciency polycrystalline thin ®lm solar cells. Polycrystalline thin ®lm solar cells based on these materials have already reached ef®ciencies of around 18% [1]. In order to improve the ef®ciencies of this type of solar cells further, a better understanding of the intrinsic defect structure of the material and its relation to the preparation conditions is essential. At present, there is limited knowledge available on the origin of the defects in this material which determine the electrical and optical properties and, therefore, the device performance. Furthermore, these defect properties are critically in¯uenced by the speci®c preparation techniques considered (e.g. co-evaporation or selenization processes) and, hence, need careful consideration. * Corresponding author. Tel.: 127-11-489-2844; fax: 127-11-4892339. E-mail address: [email protected] (V. Alberts)
In this contribution, we describe an experimental procedure by which device quality CuInSe2 thin ®lms with controlled stoichiometry can be obtained by the selenization of selenium free metallic precursors in H2Se/Ar. We indicate that the reported experimental dif®culties (i.e. material losses and segregation of secondary phases) can be solved to a large extent by optimization of the selenization conditions. In addition, the defect structure of the polycrystalline thin ®lms was modi®ed through annealing in an Ar/H2 or oxygen ambient under normal pressures. The optical properties of the ®lms were investigated prior to and after each annealing step in order to study the impact of these treatments on the defects and impurities of the material. 2. Experimental procedure The CuInSe2 thin ®lms studied in the present work were deposited on 7 £ 1:5 cm 2 soda-lime glass substrates. In the ®rst step of the process, thin layers (,1 mm) of molybdenum were deposited by electron-beam evaporation on the glass substrates at temperatures around 2008C. The substrate temperature was subsequently reduced to 1208C during the deposition of Cu and In layers. The availability of a single
0040-6090/00/$ - see front matter q 2000 Published by Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0081 1-1
V. Alberts et al. / Thin Solid Films 361±362 (2000) 432±436
rotatable crucible allowed for the sequential deposition of copper and indium layers in any desirable order and thickness without breaking vacuum. Various triple layer structures (Cu/In/Cu) were produced in which the total thicknesses of the Cu and In ®lms were kept constant at 200 and 440 nm, respectively. The precursor ®lms were annealed at 1508C in vacuum for periods up to three hours to ensure the proper mixing of all metals. After evaporation and vacuum annealing, the samples were placed in diffusion furnace which allowed for the rapid heating and cooling of samples. The concentration of H2Se in Ar was carefully controlled by mass ¯ow controllers and the samples were allowed to cool down in Ar at the end of each diffusion process. The reaction temperature, reaction period and ramping curves were considered as important experimental variables during this study. The surface morphologies of the CuInSe2 thin ®lms were studied by scanning electron microscopy (SEM), and the bulk composition as well as compositional uniformity with depth were determined by X-ray ¯uorescence (XRF). The defect structure of the samples was evaluated in a Philips CM200 transmission electron microscope (TEM) operating at 200 kV. Planar view TEM samples of these ®lms were prepared by grinding, dimpling and ion thinning from the substrate side. The optical quality of the ®lms was evaluated by low temperature photoluminescence (PL) measurements. PL measurements were carried out using a 40 mW krypton-ion laser at an excitation wavelength of 568.2 nm. 3. Results Selenization of metallic precursors in H2Se/Ar is considered a promising two-stage approach to produce device quality material and high ef®ciency devices. However, until now the lack of reproducibility has seriously hampered progress from laboratory to industrial scale production. In order to solve this problem, a great deal of attention has been focussed on the optimization of the material properties of the metallic precursors prior to selenization [2,3]. Various international groups, for example, have reported that different orders of metal deposition and variations in the growth parameters such as the growth temperatures and growth rates in¯uence the quality of the compound ®lms after selenization. The metallic precursors have also been prepared by various growth methods (e.g. thermal evaporation, electron-beam evaporation and sputtering) in order to determine experimental conditions for the production of CIS thin ®lms with a high degree of reproducibility. Despite these efforts, only marginal improvements have been reported, and in most cases, poor reproducibility has been attributed to material losses, resulting in the formation of secondary phases. According to the literature, relatively little attention has been given to the selenization process. It has generally been accepted that selenization at temperatures around
433
4008C (in 5±10% H2Se in Ar) for 60 min produced the best quality material. In the ®rst part of this study, attention was mainly focussed on the optimization of the selenization conditions. The main parameters considered were the reaction temperature, reaction period and ramping pro®le followed during heating to the different reaction temperatures. In our opinion, these are the most important factors controlling the material properties (structural, optical and electrical) of the semiconductor CIS thin ®lms. For the purpose of discussion, we considered 100 nm Cu/440 nm In/100 nm Cu stacked precursor layers. These speci®c layer thicknesses were selected in order to produce near-stoichiometric (Cu/In atomic ratio approximately 1.0) CIS ®lms. In these speci®c experiments, the reaction periods were maintained at 60 min, while the concentration of H2Se in Ar was kept constant at 5%. These speci®c parameters were found to be less crucial to the ®nal material quality and only in¯uenced the reaction kinetics of the process (i.e. transition rate from metallic to semiconductor properties). For a speci®c reaction temperature, an increase in H2Se concentration (or reaction period) resulted in an increase in the rate of selenium incorporation and subsequently in an increase in the reaction velocity. On the other hand, SEM and XRF studies indicated that the material quality (morphology and compositional uniformity) of selenized ®lms were critically in¯uenced by the ®nal reaction temperature and especially by the ramping procedure followed to the ®nal processing temperatures. In order to study this phenomenon in more detail, alloys were reacted to H2Se/Ar at temperatures of 350, 400, 450 and 5008C. In this study two ramping procedures were followed (i) samples were slowly (in 20 min) heated to the respective reaction temperatures and (ii) samples were rapidly heated (in two min) to the ®nal reaction temperatures. The resulting ®lms were carefully compared in terms of morphological properties and compositional changes. At reaction temperatures below 4008C, all ®lms were characterized by sub-micron grains and XRD studies revealed the presence of a high density of binary phases (e.g. Cu11In9, In6Se7 and Cu22xSe). The presence of these secondary phases was indicative of an incomplete reaction process. Comparative SEM studies, however, indicated a pronounce difference in morphological properties at reaction temperatures between 400 and 5008C when following the two different ramping procedures described above. Fig. 1a is a SEM micrograph of the surface morphology of a typical CuInSe2 thin ®lm, produced at reaction temperatures between 400 and 5008C, when the samples were slowly (in 20 min) heated to the reaction temperature. These ®lms were in general characterized by inhomogeneous ®lm morphologies with large (2±5 mm), faceted grains embedded in a smooth background of sub-micron grains. XRD studies from these speci®c layers revealed the presence of Cu-selenide secondary phases. These results were representative and were observed for all samples which were slowly heated to reaction temperatures between 400 and 5008C during the sele-
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Fig. 1. Surface morphologies of Cu/In/Cu metallic precursors selenized in 5% H2Se in Ar for 60 min. The samples were heated (a) slowly over a 20 min period to temperatures around 4008C and (b) fast in 2 min to temperatures above 4008C.
nization process. It is also important to emphasize that these dif®culties could not be solved simply by using different precursor ®lms. Similar results were also obtained in the case of rapid heating (in two min) of samples to 4008C during selenization. An important result that followed from this study though is the dramatic improvement in material quality in the case of samples rapidly heated and selenized at temperatures above 4008C. SEM studies (Fig. 1b) revealed homogeneous and dense ®lms with an average grain size around 1 mm. Even more important than these morphological features were the compositional uniformity in the depth of the selenized samples. XRF has recently been identi®ed as an excellent tool with a high degree of precision to obtain the depth distribution of the elements in chalcopyrite thin ®lms [4]. In order to conduct these studies, samples were repeatedly etched in bromine methanol, followed by measurements of the La 1- and Ka 1,2 lines after each etching step. The Ka 1,2 lines provide information about the total amount of each element and the thickness of the remaining ®lm, while the La 1-lines are more surface sensitive. Fig. 2a,b depict the etching pro®les of the samples shown in Fig. 1a,b, respectively. The initial thickness of both ®lms was around 1.2 mm and they were etched simultaneously in eight different steps to a ®nal thickness around 0.15 mm. It is also important to note that the composition of the unetched samples was almost identical with the Cu/In atomic ratio close to one. A comparison of these two pro®les clearly demonstrates the variation in compositional uniformity with depth when the two different selenization processes were employed. The ®rst class of samples which were slowly heated in 20 min to the reaction temperature between 400 and 5008C were characterized by a large variation in composition as function of sample depth. It can be seen clearly from Fig. 2a that there is a signi®cant increase in the In content towards the Mo back contact, resulting in a
sharp drop in the Cu/In atomic ratio from 1 to 0.52 when the ®lm thickness is reduced from 1.2 to 0.15 mm. This increase in In concentration corresponds to a sharp decrease in the Cu content towards the Mo back contact. In contrast, the second class of samples which were rapidly heated in 2 min to the
Fig. 2. Atomic percentage versus layer thickness of the two samples shown in Fig. 1, after various stages of etching in bromine methanol.
V. Alberts et al. / Thin Solid Films 361±362 (2000) 432±436
reaction temperature were characterized by a relatively high degree of compositional uniformity through the thickness of the sample. In the etched region between 1.2 and 0.3 mm (Fig. 2b), virtually no variation in composition was detected. A slight variation (Cu/In atomic ratio 0.97± 0.82) in composition was only detected in the region (0.3± 0.1 mm) very close to the Mo back contact. In both samples the selenium concentration, as function of depth, remained fairly constant around 50 at.%. The presence of crystalline and intrinsic defect levels in¯uences the photovoltaic properties of CuInSe2-based solar cells. Crystalline defects (e.g. stacking faults, microtwins, grain boundaries and dislocations) represent severe problems since they carry a plane of charged defects which may signi®cantly in¯uence the electrical properties of the polycrystalline thin ®lms. Intrinsic defect levels such as selenium, copper and indium vacancies (i.e. VSe, VCu and VIn) and antisites such as CuIn and InCu critically in¯uence the electro±optical properties of the material. In this study, the defect structure and dominating intrinsic defect levels in CuInSe2 were studied in detail by TEM and low temperature PL. Fig. 3a,b are representative planar view TEM images of a near-stoichiometric CIS ®lm, prepared by the rapid heating of a Cu/In/Cu alloy to 4508C during the selenization process. The morphological properties of this speci®c ®lm are depicted in Fig. 1b. These ®lms were found to consist of 1±2 mm diameter faceted grains with low defect density. No evidence of dislocation and dislocation loops could be detected in these samples. The defect structure of these near-stoichiometric ®lms was dominated by the presence of planar defects in the form of microtwins and/or stacking faults (S) which usually extended across the whole grain width. Some evidence of triangular-type defects (T) were also observed in these ®lms. The observation of these defects is associated with the presence of microvoids in these speci®c ®lms. Similar defect structures were also observed in CIS ®lms, selenized in H2Se/Ar when the temperature was slowly increased to the reaction temperature. However, in the latter case, TEM studies revealed very high densities of planar defects and hence inferior crystalline quality [5]. Fig. 4 shows the typical low temperature PL spectra at 10 K for (a) an as-grown ®lm, (b) after annealing in Ar/H2 for 1 h and (c) after annealing in O2 for 3 h. Note that plot (c) in Fig. 4 is multiplied by a factor of 30 compared to plot (b) for better resolution. The structural properties of this speci®c ®lm are demonstrated in Figs. 1b, 2b and 3, respectively. The optical properties of device quality CuInSe2 thin ®lms are known to be dominated by a donor±acceptor-pair transition at about 0.92 eV [6]. This emission process (as also depicted in Fig. 4) is ascribed to the radiative transition between an electron and hole bound to a selenium- (VSe) and a copper-vacancy (VCu) [6,7]. A comparison of Fig. 4a±c however, clearly reveals that post-annealing in Ar/H2 and O2 in¯uences the PL spectra (intensity and energy posi-
435
Fig. 3. Bright-®eld planar view transmission electron micrographs at different magni®cation for a typical near-stoichiometric CuInSe2 thin ®lm produced by rapid heating of metallic alloys in H2Se/Ar to temperatures above 4008C.
tions). The as-grown sample shows a maximum at 0.923 eV, which shifts to higher energies after a treatment in reducing atmosphere (Ar/H2). Furthermore the intensity of the radiative transition is increased. A second annealing step, now in oxidizing atmosphere (O2), transfers the maximum to lower energies and reduces the PL-intensity. These
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Fig. 4. Typical PL spectra at 10 K of (a) an as-grown CuInSe2 thin ®lm, (b) after annealing the ®lm in Ar±H2 and (c) a ®lm after annealing in O2.
effects could be reversed by a renewed Ar/H2 treatment. It is also worth to mention that samples which were selenized at temperatures above 5008C, showed a smaller effect to these treatments. Furthermore, the peak positions of the as-grown ®lms were at slightly lower energies in the case of these ®lms selenized at higher temperatures. The change of the PL spectra on oxygen and hydrogen annealing is explained by the change of the density of VSe defects. This is explained by a model in which oxygen can occupy a VSe site owing to coordinatively unsaturated In at the grain boundaries. The VSe concentration and therefore the device quality of these CuInSe2 thin ®lms can thus be controlled either by a postgrowth annealing step or during the growth process itself. A detailed discussion on the modi®cation of the polycrystalline CuInSe2 thin ®lms through post-growth treatments is presented elsewhere [8]. 4. Conclusions In this study device quality CuInSe2 thin ®lms were prepared by the reaction of selenium free metallic precursors to a controlled H2Se/Ar atmosphere. We indicate that the apparent loss of In from selenized metallic precursors is related to the segregation of In towards the Mo back contact. Identical metallic precursors were selenized in H2Se/Ar under various experimental conditions. XRF studies revealed no signi®cant change in the bulk composition of samples, before and after selenization. Although parameters such as gas concentration and reaction periods were varied,
these parameters were found to be less crucial to the ®nal ®lm quality. The most crucial parameters were found to be the reaction temperature and, in particular, the ramping procedure followed to the reaction temperature. If the samples were slowly heated in H2Se/Ar to temperatures between 400 and 5008C, the CuInSe2 thin ®lms were characterized by inhomogeneous ®lm morphologies. In addition, XRF studies revealed a strong segregation of In towards the Mo back contact in the case of these samples. It is believed that this segregation of In to the Mo back contact leads to an underestimation of the overall In-content when the composition is determined with standard EDS techniques with limited information depth. This segregation of In to a large extend was prevented when metallic precursors were rapidly heated in two min to temperatures above 4008C. These speci®c ®lms had uniform and dense surface morphologies and XRF studies as function of sample depth con®rmed a high degree of compositional uniformity. TEM studies indicated the presence of large (.1 mm), faceted grains with low defect density. The PL response from these optimized ®lms was characterized by the presence of only one broad donor±acceptor-pair transition around 0.92 eV. It was found that the optical properties of these ®lms could be modi®ed by post-treatment of samples in O2 and H2/Ar. This phenomenon is explained by a variation in the concentration of VSe defects. Acknowledgements The ®nancial support of the NRF and the University of the Witwatersrand via the Microstructural Studies Research Programme is acknowledged. The assistance of Mr M. Klenk with XRF studies is also gratefully appreciated. References [1] J.R. Tuttle, T.A. Berens, S.E. Asher, et al., Proc. 13th EC Photovoltaic Sol. Energy Conf., Barcelona, H.S. Stephens and Associates, UK, 1995, p. 2131. [2] R. Parretta, A. Rubino, Solid State Commun. 96 (1995) 767. [3] V. Alberts, R. Swanepoel, J. Mater. Sci. Mater. Electron. 7 (1996) 91. [4] M. Klenk, O. Schenker, U. Probst, E. Bucher, Sol. Energy Mater. Sol. Cells (1999). [5] V. Alberts, R. Swanepoel, M.J. Witcomb, J. Mater. Sci. 33 (1998) 2919. [6] G. MasseÂ, E.J. Redjai, Appl. Phys. 56 (1984) 1154. [7] F. Abou-Elfotouh, H. Mountinho, A. Bakry, T.J. Coutts, L.L. Kazmerski, Sol. Cells 30 (1991) 151. [8] J.H. SchoÈn, V. Alberts, E. Bucher, Semicond. Sci. Technol. 14 (1999) 657.
Control of VSe 2 defect levels in CuInSe2 prepared by rapid thermal processing of metallic alloys V. Alberts a,*, J. Bekker a, M.J. Witcomb b, J.H. SchoÈn c, E. Bucher d a b
Department of Physics, Rand Afrikaans University, PO Box 524, Auckland Park 2006, South Africa Electron Microscope Unit, University of the Witwatersrand, Private Bag 3, WITS 2006, South Africa c Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill NJ 07974-0636, USA d FakultaÈt fuÈr Physik, Postfach X916, D-78457 Konstanz, Germany
Abstract The reaction of metallic alloys to H2Se/Ar is a promising technique to produce device quality CuInSe2 thin ®lms. However, up to now the controllability of the ®lm quality has been critically in¯uenced by the segregation of secondary phases during growth. We indicate in this study that this phenomenon is strongly related to the selenization reaction temperature, and especially to the ramping procedure followed to the ®nal selenization temperature. Metallic precursors which were slowly heated to temperatures around 4008C are characterized by inhomogeneous ®lm morphologies and X-ray ¯uorescence (XRF) measurements revealed a strong segregation of In towards the Mo back contact. In contrast, rapid heating of samples in H2Se/Ar to temperatures above 4008C resulted in uniform and dense ®lms with a high degree of compositional uniformity through the thickness of the samples. Transmission electron microscopy (TEM) indicated the presence of a low density of planar defects in these optimized ®lms. Low temperature photoluminescence (PL) studies demonstrated that the optical properties of these polycrystalline thin ®lms are very sensitive to post-growth treatment in Ar/H2 and O2. q 2000 Published by Elsevier Science S.A. All rights reserved. Keywords: CuInSe2; H2Se annealing; X-ray ¯uorescence; Photoluminescence; Post-growth treatment
1. Introduction Thin ®lm photovoltaic (PV) technologies are being developed in order to reduce the cost of PV energy conversion. The ternary chalcopyrite semiconductor CuInSe2 (CIS) is a promising material for a new generation of cost-effective, high ef®ciency polycrystalline thin ®lm solar cells. Polycrystalline thin ®lm solar cells based on these materials have already reached ef®ciencies of around 18% [1]. In order to improve the ef®ciencies of this type of solar cells further, a better understanding of the intrinsic defect structure of the material and its relation to the preparation conditions is essential. At present, there is limited knowledge available on the origin of the defects in this material which determine the electrical and optical properties and, therefore, the device performance. Furthermore, these defect properties are critically in¯uenced by the speci®c preparation techniques considered (e.g. co-evaporation or selenization processes) and, hence, need careful consideration. * Corresponding author. Tel.: 127-11-489-2844; fax: 127-11-4892339. E-mail address: [email protected] (V. Alberts)
In this contribution, we describe an experimental procedure by which device quality CuInSe2 thin ®lms with controlled stoichiometry can be obtained by the selenization of selenium free metallic precursors in H2Se/Ar. We indicate that the reported experimental dif®culties (i.e. material losses and segregation of secondary phases) can be solved to a large extent by optimization of the selenization conditions. In addition, the defect structure of the polycrystalline thin ®lms was modi®ed through annealing in an Ar/H2 or oxygen ambient under normal pressures. The optical properties of the ®lms were investigated prior to and after each annealing step in order to study the impact of these treatments on the defects and impurities of the material. 2. Experimental procedure The CuInSe2 thin ®lms studied in the present work were deposited on 7 £ 1:5 cm 2 soda-lime glass substrates. In the ®rst step of the process, thin layers (,1 mm) of molybdenum were deposited by electron-beam evaporation on the glass substrates at temperatures around 2008C. The substrate temperature was subsequently reduced to 1208C during the deposition of Cu and In layers. The availability of a single
0040-6090/00/$ - see front matter q 2000 Published by Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0081 1-1
V. Alberts et al. / Thin Solid Films 361±362 (2000) 432±436
rotatable crucible allowed for the sequential deposition of copper and indium layers in any desirable order and thickness without breaking vacuum. Various triple layer structures (Cu/In/Cu) were produced in which the total thicknesses of the Cu and In ®lms were kept constant at 200 and 440 nm, respectively. The precursor ®lms were annealed at 1508C in vacuum for periods up to three hours to ensure the proper mixing of all metals. After evaporation and vacuum annealing, the samples were placed in diffusion furnace which allowed for the rapid heating and cooling of samples. The concentration of H2Se in Ar was carefully controlled by mass ¯ow controllers and the samples were allowed to cool down in Ar at the end of each diffusion process. The reaction temperature, reaction period and ramping curves were considered as important experimental variables during this study. The surface morphologies of the CuInSe2 thin ®lms were studied by scanning electron microscopy (SEM), and the bulk composition as well as compositional uniformity with depth were determined by X-ray ¯uorescence (XRF). The defect structure of the samples was evaluated in a Philips CM200 transmission electron microscope (TEM) operating at 200 kV. Planar view TEM samples of these ®lms were prepared by grinding, dimpling and ion thinning from the substrate side. The optical quality of the ®lms was evaluated by low temperature photoluminescence (PL) measurements. PL measurements were carried out using a 40 mW krypton-ion laser at an excitation wavelength of 568.2 nm. 3. Results Selenization of metallic precursors in H2Se/Ar is considered a promising two-stage approach to produce device quality material and high ef®ciency devices. However, until now the lack of reproducibility has seriously hampered progress from laboratory to industrial scale production. In order to solve this problem, a great deal of attention has been focussed on the optimization of the material properties of the metallic precursors prior to selenization [2,3]. Various international groups, for example, have reported that different orders of metal deposition and variations in the growth parameters such as the growth temperatures and growth rates in¯uence the quality of the compound ®lms after selenization. The metallic precursors have also been prepared by various growth methods (e.g. thermal evaporation, electron-beam evaporation and sputtering) in order to determine experimental conditions for the production of CIS thin ®lms with a high degree of reproducibility. Despite these efforts, only marginal improvements have been reported, and in most cases, poor reproducibility has been attributed to material losses, resulting in the formation of secondary phases. According to the literature, relatively little attention has been given to the selenization process. It has generally been accepted that selenization at temperatures around
433
4008C (in 5±10% H2Se in Ar) for 60 min produced the best quality material. In the ®rst part of this study, attention was mainly focussed on the optimization of the selenization conditions. The main parameters considered were the reaction temperature, reaction period and ramping pro®le followed during heating to the different reaction temperatures. In our opinion, these are the most important factors controlling the material properties (structural, optical and electrical) of the semiconductor CIS thin ®lms. For the purpose of discussion, we considered 100 nm Cu/440 nm In/100 nm Cu stacked precursor layers. These speci®c layer thicknesses were selected in order to produce near-stoichiometric (Cu/In atomic ratio approximately 1.0) CIS ®lms. In these speci®c experiments, the reaction periods were maintained at 60 min, while the concentration of H2Se in Ar was kept constant at 5%. These speci®c parameters were found to be less crucial to the ®nal material quality and only in¯uenced the reaction kinetics of the process (i.e. transition rate from metallic to semiconductor properties). For a speci®c reaction temperature, an increase in H2Se concentration (or reaction period) resulted in an increase in the rate of selenium incorporation and subsequently in an increase in the reaction velocity. On the other hand, SEM and XRF studies indicated that the material quality (morphology and compositional uniformity) of selenized ®lms were critically in¯uenced by the ®nal reaction temperature and especially by the ramping procedure followed to the ®nal processing temperatures. In order to study this phenomenon in more detail, alloys were reacted to H2Se/Ar at temperatures of 350, 400, 450 and 5008C. In this study two ramping procedures were followed (i) samples were slowly (in 20 min) heated to the respective reaction temperatures and (ii) samples were rapidly heated (in two min) to the ®nal reaction temperatures. The resulting ®lms were carefully compared in terms of morphological properties and compositional changes. At reaction temperatures below 4008C, all ®lms were characterized by sub-micron grains and XRD studies revealed the presence of a high density of binary phases (e.g. Cu11In9, In6Se7 and Cu22xSe). The presence of these secondary phases was indicative of an incomplete reaction process. Comparative SEM studies, however, indicated a pronounce difference in morphological properties at reaction temperatures between 400 and 5008C when following the two different ramping procedures described above. Fig. 1a is a SEM micrograph of the surface morphology of a typical CuInSe2 thin ®lm, produced at reaction temperatures between 400 and 5008C, when the samples were slowly (in 20 min) heated to the reaction temperature. These ®lms were in general characterized by inhomogeneous ®lm morphologies with large (2±5 mm), faceted grains embedded in a smooth background of sub-micron grains. XRD studies from these speci®c layers revealed the presence of Cu-selenide secondary phases. These results were representative and were observed for all samples which were slowly heated to reaction temperatures between 400 and 5008C during the sele-
434
V. Alberts et al. / Thin Solid Films 361±362 (2000) 432±436
Fig. 1. Surface morphologies of Cu/In/Cu metallic precursors selenized in 5% H2Se in Ar for 60 min. The samples were heated (a) slowly over a 20 min period to temperatures around 4008C and (b) fast in 2 min to temperatures above 4008C.
nization process. It is also important to emphasize that these dif®culties could not be solved simply by using different precursor ®lms. Similar results were also obtained in the case of rapid heating (in two min) of samples to 4008C during selenization. An important result that followed from this study though is the dramatic improvement in material quality in the case of samples rapidly heated and selenized at temperatures above 4008C. SEM studies (Fig. 1b) revealed homogeneous and dense ®lms with an average grain size around 1 mm. Even more important than these morphological features were the compositional uniformity in the depth of the selenized samples. XRF has recently been identi®ed as an excellent tool with a high degree of precision to obtain the depth distribution of the elements in chalcopyrite thin ®lms [4]. In order to conduct these studies, samples were repeatedly etched in bromine methanol, followed by measurements of the La 1- and Ka 1,2 lines after each etching step. The Ka 1,2 lines provide information about the total amount of each element and the thickness of the remaining ®lm, while the La 1-lines are more surface sensitive. Fig. 2a,b depict the etching pro®les of the samples shown in Fig. 1a,b, respectively. The initial thickness of both ®lms was around 1.2 mm and they were etched simultaneously in eight different steps to a ®nal thickness around 0.15 mm. It is also important to note that the composition of the unetched samples was almost identical with the Cu/In atomic ratio close to one. A comparison of these two pro®les clearly demonstrates the variation in compositional uniformity with depth when the two different selenization processes were employed. The ®rst class of samples which were slowly heated in 20 min to the reaction temperature between 400 and 5008C were characterized by a large variation in composition as function of sample depth. It can be seen clearly from Fig. 2a that there is a signi®cant increase in the In content towards the Mo back contact, resulting in a
sharp drop in the Cu/In atomic ratio from 1 to 0.52 when the ®lm thickness is reduced from 1.2 to 0.15 mm. This increase in In concentration corresponds to a sharp decrease in the Cu content towards the Mo back contact. In contrast, the second class of samples which were rapidly heated in 2 min to the
Fig. 2. Atomic percentage versus layer thickness of the two samples shown in Fig. 1, after various stages of etching in bromine methanol.
V. Alberts et al. / Thin Solid Films 361±362 (2000) 432±436
reaction temperature were characterized by a relatively high degree of compositional uniformity through the thickness of the sample. In the etched region between 1.2 and 0.3 mm (Fig. 2b), virtually no variation in composition was detected. A slight variation (Cu/In atomic ratio 0.97± 0.82) in composition was only detected in the region (0.3± 0.1 mm) very close to the Mo back contact. In both samples the selenium concentration, as function of depth, remained fairly constant around 50 at.%. The presence of crystalline and intrinsic defect levels in¯uences the photovoltaic properties of CuInSe2-based solar cells. Crystalline defects (e.g. stacking faults, microtwins, grain boundaries and dislocations) represent severe problems since they carry a plane of charged defects which may signi®cantly in¯uence the electrical properties of the polycrystalline thin ®lms. Intrinsic defect levels such as selenium, copper and indium vacancies (i.e. VSe, VCu and VIn) and antisites such as CuIn and InCu critically in¯uence the electro±optical properties of the material. In this study, the defect structure and dominating intrinsic defect levels in CuInSe2 were studied in detail by TEM and low temperature PL. Fig. 3a,b are representative planar view TEM images of a near-stoichiometric CIS ®lm, prepared by the rapid heating of a Cu/In/Cu alloy to 4508C during the selenization process. The morphological properties of this speci®c ®lm are depicted in Fig. 1b. These ®lms were found to consist of 1±2 mm diameter faceted grains with low defect density. No evidence of dislocation and dislocation loops could be detected in these samples. The defect structure of these near-stoichiometric ®lms was dominated by the presence of planar defects in the form of microtwins and/or stacking faults (S) which usually extended across the whole grain width. Some evidence of triangular-type defects (T) were also observed in these ®lms. The observation of these defects is associated with the presence of microvoids in these speci®c ®lms. Similar defect structures were also observed in CIS ®lms, selenized in H2Se/Ar when the temperature was slowly increased to the reaction temperature. However, in the latter case, TEM studies revealed very high densities of planar defects and hence inferior crystalline quality [5]. Fig. 4 shows the typical low temperature PL spectra at 10 K for (a) an as-grown ®lm, (b) after annealing in Ar/H2 for 1 h and (c) after annealing in O2 for 3 h. Note that plot (c) in Fig. 4 is multiplied by a factor of 30 compared to plot (b) for better resolution. The structural properties of this speci®c ®lm are demonstrated in Figs. 1b, 2b and 3, respectively. The optical properties of device quality CuInSe2 thin ®lms are known to be dominated by a donor±acceptor-pair transition at about 0.92 eV [6]. This emission process (as also depicted in Fig. 4) is ascribed to the radiative transition between an electron and hole bound to a selenium- (VSe) and a copper-vacancy (VCu) [6,7]. A comparison of Fig. 4a±c however, clearly reveals that post-annealing in Ar/H2 and O2 in¯uences the PL spectra (intensity and energy posi-
435
Fig. 3. Bright-®eld planar view transmission electron micrographs at different magni®cation for a typical near-stoichiometric CuInSe2 thin ®lm produced by rapid heating of metallic alloys in H2Se/Ar to temperatures above 4008C.
tions). The as-grown sample shows a maximum at 0.923 eV, which shifts to higher energies after a treatment in reducing atmosphere (Ar/H2). Furthermore the intensity of the radiative transition is increased. A second annealing step, now in oxidizing atmosphere (O2), transfers the maximum to lower energies and reduces the PL-intensity. These
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V. Alberts et al. / Thin Solid Films 361±362 (2000) 432±436
Fig. 4. Typical PL spectra at 10 K of (a) an as-grown CuInSe2 thin ®lm, (b) after annealing the ®lm in Ar±H2 and (c) a ®lm after annealing in O2.
effects could be reversed by a renewed Ar/H2 treatment. It is also worth to mention that samples which were selenized at temperatures above 5008C, showed a smaller effect to these treatments. Furthermore, the peak positions of the as-grown ®lms were at slightly lower energies in the case of these ®lms selenized at higher temperatures. The change of the PL spectra on oxygen and hydrogen annealing is explained by the change of the density of VSe defects. This is explained by a model in which oxygen can occupy a VSe site owing to coordinatively unsaturated In at the grain boundaries. The VSe concentration and therefore the device quality of these CuInSe2 thin ®lms can thus be controlled either by a postgrowth annealing step or during the growth process itself. A detailed discussion on the modi®cation of the polycrystalline CuInSe2 thin ®lms through post-growth treatments is presented elsewhere [8]. 4. Conclusions In this study device quality CuInSe2 thin ®lms were prepared by the reaction of selenium free metallic precursors to a controlled H2Se/Ar atmosphere. We indicate that the apparent loss of In from selenized metallic precursors is related to the segregation of In towards the Mo back contact. Identical metallic precursors were selenized in H2Se/Ar under various experimental conditions. XRF studies revealed no signi®cant change in the bulk composition of samples, before and after selenization. Although parameters such as gas concentration and reaction periods were varied,
these parameters were found to be less crucial to the ®nal ®lm quality. The most crucial parameters were found to be the reaction temperature and, in particular, the ramping procedure followed to the reaction temperature. If the samples were slowly heated in H2Se/Ar to temperatures between 400 and 5008C, the CuInSe2 thin ®lms were characterized by inhomogeneous ®lm morphologies. In addition, XRF studies revealed a strong segregation of In towards the Mo back contact in the case of these samples. It is believed that this segregation of In to the Mo back contact leads to an underestimation of the overall In-content when the composition is determined with standard EDS techniques with limited information depth. This segregation of In to a large extend was prevented when metallic precursors were rapidly heated in two min to temperatures above 4008C. These speci®c ®lms had uniform and dense surface morphologies and XRF studies as function of sample depth con®rmed a high degree of compositional uniformity. TEM studies indicated the presence of large (.1 mm), faceted grains with low defect density. The PL response from these optimized ®lms was characterized by the presence of only one broad donor±acceptor-pair transition around 0.92 eV. It was found that the optical properties of these ®lms could be modi®ed by post-treatment of samples in O2 and H2/Ar. This phenomenon is explained by a variation in the concentration of VSe defects. Acknowledgements The ®nancial support of the NRF and the University of the Witwatersrand via the Microstructural Studies Research Programme is acknowledged. The assistance of Mr M. Klenk with XRF studies is also gratefully appreciated. References [1] J.R. Tuttle, T.A. Berens, S.E. Asher, et al., Proc. 13th EC Photovoltaic Sol. Energy Conf., Barcelona, H.S. Stephens and Associates, UK, 1995, p. 2131. [2] R. Parretta, A. Rubino, Solid State Commun. 96 (1995) 767. [3] V. Alberts, R. Swanepoel, J. Mater. Sci. Mater. Electron. 7 (1996) 91. [4] M. Klenk, O. Schenker, U. Probst, E. Bucher, Sol. Energy Mater. Sol. Cells (1999). [5] V. Alberts, R. Swanepoel, M.J. Witcomb, J. Mater. Sci. 33 (1998) 2919. [6] G. MasseÂ, E.J. Redjai, Appl. Phys. 56 (1984) 1154. [7] F. Abou-Elfotouh, H. Mountinho, A. Bakry, T.J. Coutts, L.L. Kazmerski, Sol. Cells 30 (1991) 151. [8] J.H. SchoÈn, V. Alberts, E. Bucher, Semicond. Sci. Technol. 14 (1999) 657.