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PIXE and RBS investigation of growth phases of ultra-thin chemical bath deposited CdS films
Nuclear Instruments and Methods in Physics Research B 190 (2002) 615–619 www.elsevier.com/locate/nimb
PIXE and RBS investigation of growth phases of ultra-thin chemical bath deposited CdS films P.C. Duncan a, S. Hinckley a
a,*
, E.A. Gluszak a, N. Dytlewski
b
School of Engineering and Mathematics, Edith Cowan University, 100 Joondalup Drive, Joondalup WA 6027, Perth, Australia b Australian Nuclear Science and Technology Organisation, 2234 Sydney, Australia
Abstract Polycrystalline CdS films, with thicknesses typically 20–180 nm, have been chemically deposited on glass substrates using an ammonia–cadmium–thiourea reaction solution. Film elemental composition, thickness and microstructure have been examined using proton-induced X-ray emission, Rutherford backscattering and atomic force microscopy. Analysis indicates that the stability of the deposition temperature plays a critical role in CdS film growth and composition. Films deposited with high temperature stability (60 0:5 °C) show a consistent 1:1 Cd:S atomic ratio for all stages of film growth, and have good substrate adhesion. Films deposited with lower temperature stability (60 4 °C) show initial high S concentrations, followed by a rapid increase in Cd concentration, until a final 1.2:1 Cd:S ratio is achieved. A mechanism is proposed to explain this difference in film composition and properties. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 81.05.Dz; 68.55.)a; 81.10.Dn; 82.80.Yc Keywords: Semiconductors; Cadmium sulphide; Chemical bath deposition; Proton-induced X-ray emission; Rutherford backscattering; Atomic force microscopy
1. Introduction Chemically-deposited CdS thin films have proven successful as window layers in high-efficiency (g > 15%) thin-film CdS/CdTe solar cells [1]. Chemical bath deposition (CBD) is a simple solution growth process for creating polycrystalline CdS thin films [2], which involves dipping a substrate into a reaction mixture for a time depending
*
Corresponding author. Fax: +61-8-9400-5811. E-mail address: [email protected] (S. Hinckley).
on the film thickness required. Films of CdS are formed through the reaction of adequately dissolved ammonia, cadmium and thiourea precursors. Under certain conditions, these precursors form thin polycrystalline CdS films on any material surface, in preference to precipitating out in solution. Film growth rates have previously been determined and modeled [3], deposited film properties being strongly dependent on temperature, relative concentrations of precursors, solution pH and stirring. However, there is still a lack of understanding of how film nucleation and subsequent crystal growth occurs and can be controlled during the deposition, and of how these
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 1 2 7 7 - 0
616
P.C. Duncan et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 615–619
conditions and processes influence the microstructure of the films. Hence, there is a need to further investigate different deposition conditions on CdS film properties using different methods. One important question that needs answering is the mechanism of film nucleation, and how film growth occurs and proceeds during the deposition process as a function of various growth conditions. For example, what is the effect of temperature stability during growth. In this study, polycrystalline ultrathin (< 0:2 lm) CdS films have been chemically deposited from an ammonia–thiourea system. The composition and thickness of these films has been studied as a function of the deposition time by proton-induced X-ray emission (PIXE), Rutherford backscattering (RBS) and atomic force microscopy (AFM).
2. Experimental The deposition bath was prepared from appropriate amounts of cadmium acetate, NH3 /NH4 OH buffer, and thiourea in de-ionized water. The CdS thin films were deposited on substrates of soda lime and borosilicate glass slides. Preparation of the slides involved hand washing with a commercial detergent and an overnight concentrated H2 SO4 solution soak. After a thorough rinse with Milli-Q water the slides are cleaned ultrasonically in Milli-Q water and then immediately placed into their reaction cell positions. With four to six slides placed in the reaction cell, the appropriate amount of ammonia solution is added and heated to 60°C with a strong enough stirring rate to remove any adhering bubbles off the substrate surface. Whilst maintaining a near constant temperature of 60°C, the cadmium acetate solution and the thiourea solution are added to obtain the final required concentrations. A timer is started and after 20 min the first substrate is removed, with subsequent removal of another substrate every 10 or 20 min until the experiment has finished. The newly created CdS thin films were washed with Milli-Q water and then ultrasonically cleaned in Milli-Q water for 10–15 min to remove non-adherent surface layers. The PIXE and RBS
analysis for qualitative and quantitative information on elemental composition was carried out at the Australian Nuclear Science and Technology Organization, Lucas Heights using their Multiple Surface Analysis Facility (SR2). Film thickness and topography were determined with a Digital Instruments Nanoscope IIIa AFM, using a series of chemical etches.
3. Results and discussion The results of CBD CdS thin film growth experiments are compared for two different groups deposited under slightly different environments. Each experiment produces a set of glass slides with increasing CdS film thickness, representing successive phases of the chemical deposition process. With CBD CdS growth, the film morphology consists of a continuous phase (described by monolayer growth) and a particulate phase (described by nucleation growth) [3]. The continuous phase results from an ion-by-ion condensation of the impinging species, [Cd2þ ] and [S2 ], while the particulate phase is from colloidal particles of CdS adhering to the substrate surface. These colloidal particles grow in size with time and form the particulate part of the film [3]. Both groups were deposited using the same experimental procedure and initial conditions, except that the reaction cell temperature for group A films was held to 60 0:5 °C during the experiments, while the group B film reaction cell temperature was 60 4:0 °C. The atomic Cd:S ratios obtained using PIXE are plotted in Fig. 1 as a function of deposition time. Corresponding film thicknesses, calculated assuming constant and uniform film density, are shown in Fig. 2. Group A films show a consistent Cd:S atomic ratio of approximately 1:1 throughout the entire deposition process, even at the initial stages of growth. For the group B films, an initial dominance of sulphur is evident, the Cd:S atomic ratio increases rapidly and stabilizes at approximately 1.2:1 after 20–30 min of growth. Calculated thicknesses for the CdS films, as shown in Fig. 2, indicate that group A films have a consistent
P.C. Duncan et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 615–619
Fig. 1. Cadmium to sulphur atomic ratios determined by PIXE for group A and group B films. All experiments used near identical solution compositions of [NH3 ] 1.8 M, [Cd2þ ] 2–3 mM, with [S2 ] concentrations indicated.
Fig. 2. Film thicknesses calculated from data in Fig. 1, assuming constant and uniform CdS film density. All experiments used near identical solution compositions of [NH3 ] 1.8 M, [Cd2þ ] 2–3 mM, with [S2 ] concentrations indicated.
growth rate over the entire period of deposition, whereas group B films show an initial rapid growth rate, followed by a reduced (but still greater than group A) growth rate, once compound formation begins. RBS analysis is shown in Fig. 3 for films from both groups, at the end of the deposition process; that is, for deposition times of 70–80 min. The results show that there is not a great difference in the final composition of both types of film. The
617
Fig. 3. RBS spectra for group A and B films at the end of deposition.
Fig. 4. RBS spectra for group B films deposited on borosilicate and soda-lime glass substrates.
RBS results also showed that there was a problem with uniformity of glass substrate type (see Fig. 4). The glass slides were all supposed to be borosilicate, but the RBS results indicated that there were some soda-lime glass slides mixed into the batches. AFM micrographs of group A and group B films are shown in Fig. 5(a) and (b), respectively, for a deposition time of 20 min. The group A micrograph shows numerous small nucleation sites with an average crystal size of approximately 150 nm. The group A films achieved a high level of
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P.C. Duncan et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 615–619
Fig. 5. AFM image of (a) type A film and (b) type B film, after 20 min of growth.
continuous phase growth. For group B films, the AFM image of Fig. 5(a) shows a much higher degree of larger particulates adhering to the substrate surface. The CdS bonding of group A was particularly strong and showed good resistance to scratching, whereas the group B films did not adhere strongly to the substrate surface and could easily be scratched off with moderate force. A possible cause of the initial dominance of sulphur in group B films is the larger error in temperature control during deposition. Elevated temperatures increase the rate of deposition [3] and at the same time cause the particulate stage to dominate the deposition process. Also, temperature fluctuations cause irreversible changes to the thin film deposition process by encouraging the particulate CdS formation process that inevitably upsets the solubility equilibrium of the CdS precursor ions and results in non-uniform film growth. A possible explanation for this film breakdown is provided as follows: the rate of thiourea decomposition to form S2 ions increases at higher temperatures, but the rate of Cd2þ production hardly increases at all due to the presence of excess OH from the ammonia solution. This creates an excess of S2 ions which attach themselves to the decomplexed Cd2þ ions. The CdS particulates grow quickly and stay in solution even though the temperature eventually decreases. When the tem-
perature increases again these initial CdS particulates grow larger, resulting in a cyclic process. The excess S2 ions not used in creation of the CdS particles are evenly dispersed throughout the solution and appear to attach weakly to the substrate surface forming nucleated sites, which draw [Cd2þ ] species away from existing CdS lattice sites, resulting in a breakdown in the film uniformity on the substrate (as illustrated in the AFM micrographs), in addition to the formation of CdS colloids in solution.
4. Conclusions Ultra-thin CdS polycrystalline films, deposited by chemical bath deposition, show high sensitivity to variations in deposition temperature. The Cd:S atomic ratio of 1:1 is uniform with time when the temperature of the deposition process is constant. When the temperature is allowed to fluctuate, this Cd:S atomic ratio becomes unbalanced and can result in an initial dominance of sulphur on the glass substrate. This sulphur dominance reduces as the chemical reaction progresses and at later stages, a ratio of approximately 1.2:1 for cadmium and sulphur atoms is observed. This initial dominance of sulphur on the substrate appears to be the direct result of a cyclic change in chemical reaction
P.C. Duncan et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 615–619
rates during deposition due to a cyclic temperature variation of only a few degrees. Acknowledgements The authors wish to thank Dr. M. Reyhani (Curtin University) for the use of the AFM. This work was supported by an Australian Institute of Nuclear Science and Engineering (AINSE) Research Grant.
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References [1] S.N. Oju, W.W. Smith, I. Shih, J. Appl. Surf. Sci. 113 (1997) 1147. [2] E.A. Gluszak, S. Hinckley, Proceedings of the Conference on Optoelectonic and Microelectronic Materials and Devices, Perth, 1998, IEEE Press, 1999, p. 426. [3] E.A. Gluszak, S. Hinckley, Proceedings of the Conference on Optoelectonic and Microelectronic Materials and Devices, Melbourne, 2000, IEEE Press, in press.
PIXE and RBS investigation of growth phases of ultra-thin chemical bath deposited CdS films P.C. Duncan a, S. Hinckley a
a,*
, E.A. Gluszak a, N. Dytlewski
b
School of Engineering and Mathematics, Edith Cowan University, 100 Joondalup Drive, Joondalup WA 6027, Perth, Australia b Australian Nuclear Science and Technology Organisation, 2234 Sydney, Australia
Abstract Polycrystalline CdS films, with thicknesses typically 20–180 nm, have been chemically deposited on glass substrates using an ammonia–cadmium–thiourea reaction solution. Film elemental composition, thickness and microstructure have been examined using proton-induced X-ray emission, Rutherford backscattering and atomic force microscopy. Analysis indicates that the stability of the deposition temperature plays a critical role in CdS film growth and composition. Films deposited with high temperature stability (60 0:5 °C) show a consistent 1:1 Cd:S atomic ratio for all stages of film growth, and have good substrate adhesion. Films deposited with lower temperature stability (60 4 °C) show initial high S concentrations, followed by a rapid increase in Cd concentration, until a final 1.2:1 Cd:S ratio is achieved. A mechanism is proposed to explain this difference in film composition and properties. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 81.05.Dz; 68.55.)a; 81.10.Dn; 82.80.Yc Keywords: Semiconductors; Cadmium sulphide; Chemical bath deposition; Proton-induced X-ray emission; Rutherford backscattering; Atomic force microscopy
1. Introduction Chemically-deposited CdS thin films have proven successful as window layers in high-efficiency (g > 15%) thin-film CdS/CdTe solar cells [1]. Chemical bath deposition (CBD) is a simple solution growth process for creating polycrystalline CdS thin films [2], which involves dipping a substrate into a reaction mixture for a time depending
*
Corresponding author. Fax: +61-8-9400-5811. E-mail address: [email protected] (S. Hinckley).
on the film thickness required. Films of CdS are formed through the reaction of adequately dissolved ammonia, cadmium and thiourea precursors. Under certain conditions, these precursors form thin polycrystalline CdS films on any material surface, in preference to precipitating out in solution. Film growth rates have previously been determined and modeled [3], deposited film properties being strongly dependent on temperature, relative concentrations of precursors, solution pH and stirring. However, there is still a lack of understanding of how film nucleation and subsequent crystal growth occurs and can be controlled during the deposition, and of how these
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 1 2 7 7 - 0
616
P.C. Duncan et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 615–619
conditions and processes influence the microstructure of the films. Hence, there is a need to further investigate different deposition conditions on CdS film properties using different methods. One important question that needs answering is the mechanism of film nucleation, and how film growth occurs and proceeds during the deposition process as a function of various growth conditions. For example, what is the effect of temperature stability during growth. In this study, polycrystalline ultrathin (< 0:2 lm) CdS films have been chemically deposited from an ammonia–thiourea system. The composition and thickness of these films has been studied as a function of the deposition time by proton-induced X-ray emission (PIXE), Rutherford backscattering (RBS) and atomic force microscopy (AFM).
2. Experimental The deposition bath was prepared from appropriate amounts of cadmium acetate, NH3 /NH4 OH buffer, and thiourea in de-ionized water. The CdS thin films were deposited on substrates of soda lime and borosilicate glass slides. Preparation of the slides involved hand washing with a commercial detergent and an overnight concentrated H2 SO4 solution soak. After a thorough rinse with Milli-Q water the slides are cleaned ultrasonically in Milli-Q water and then immediately placed into their reaction cell positions. With four to six slides placed in the reaction cell, the appropriate amount of ammonia solution is added and heated to 60°C with a strong enough stirring rate to remove any adhering bubbles off the substrate surface. Whilst maintaining a near constant temperature of 60°C, the cadmium acetate solution and the thiourea solution are added to obtain the final required concentrations. A timer is started and after 20 min the first substrate is removed, with subsequent removal of another substrate every 10 or 20 min until the experiment has finished. The newly created CdS thin films were washed with Milli-Q water and then ultrasonically cleaned in Milli-Q water for 10–15 min to remove non-adherent surface layers. The PIXE and RBS
analysis for qualitative and quantitative information on elemental composition was carried out at the Australian Nuclear Science and Technology Organization, Lucas Heights using their Multiple Surface Analysis Facility (SR2). Film thickness and topography were determined with a Digital Instruments Nanoscope IIIa AFM, using a series of chemical etches.
3. Results and discussion The results of CBD CdS thin film growth experiments are compared for two different groups deposited under slightly different environments. Each experiment produces a set of glass slides with increasing CdS film thickness, representing successive phases of the chemical deposition process. With CBD CdS growth, the film morphology consists of a continuous phase (described by monolayer growth) and a particulate phase (described by nucleation growth) [3]. The continuous phase results from an ion-by-ion condensation of the impinging species, [Cd2þ ] and [S2 ], while the particulate phase is from colloidal particles of CdS adhering to the substrate surface. These colloidal particles grow in size with time and form the particulate part of the film [3]. Both groups were deposited using the same experimental procedure and initial conditions, except that the reaction cell temperature for group A films was held to 60 0:5 °C during the experiments, while the group B film reaction cell temperature was 60 4:0 °C. The atomic Cd:S ratios obtained using PIXE are plotted in Fig. 1 as a function of deposition time. Corresponding film thicknesses, calculated assuming constant and uniform film density, are shown in Fig. 2. Group A films show a consistent Cd:S atomic ratio of approximately 1:1 throughout the entire deposition process, even at the initial stages of growth. For the group B films, an initial dominance of sulphur is evident, the Cd:S atomic ratio increases rapidly and stabilizes at approximately 1.2:1 after 20–30 min of growth. Calculated thicknesses for the CdS films, as shown in Fig. 2, indicate that group A films have a consistent
P.C. Duncan et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 615–619
Fig. 1. Cadmium to sulphur atomic ratios determined by PIXE for group A and group B films. All experiments used near identical solution compositions of [NH3 ] 1.8 M, [Cd2þ ] 2–3 mM, with [S2 ] concentrations indicated.
Fig. 2. Film thicknesses calculated from data in Fig. 1, assuming constant and uniform CdS film density. All experiments used near identical solution compositions of [NH3 ] 1.8 M, [Cd2þ ] 2–3 mM, with [S2 ] concentrations indicated.
growth rate over the entire period of deposition, whereas group B films show an initial rapid growth rate, followed by a reduced (but still greater than group A) growth rate, once compound formation begins. RBS analysis is shown in Fig. 3 for films from both groups, at the end of the deposition process; that is, for deposition times of 70–80 min. The results show that there is not a great difference in the final composition of both types of film. The
617
Fig. 3. RBS spectra for group A and B films at the end of deposition.
Fig. 4. RBS spectra for group B films deposited on borosilicate and soda-lime glass substrates.
RBS results also showed that there was a problem with uniformity of glass substrate type (see Fig. 4). The glass slides were all supposed to be borosilicate, but the RBS results indicated that there were some soda-lime glass slides mixed into the batches. AFM micrographs of group A and group B films are shown in Fig. 5(a) and (b), respectively, for a deposition time of 20 min. The group A micrograph shows numerous small nucleation sites with an average crystal size of approximately 150 nm. The group A films achieved a high level of
618
P.C. Duncan et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 615–619
Fig. 5. AFM image of (a) type A film and (b) type B film, after 20 min of growth.
continuous phase growth. For group B films, the AFM image of Fig. 5(a) shows a much higher degree of larger particulates adhering to the substrate surface. The CdS bonding of group A was particularly strong and showed good resistance to scratching, whereas the group B films did not adhere strongly to the substrate surface and could easily be scratched off with moderate force. A possible cause of the initial dominance of sulphur in group B films is the larger error in temperature control during deposition. Elevated temperatures increase the rate of deposition [3] and at the same time cause the particulate stage to dominate the deposition process. Also, temperature fluctuations cause irreversible changes to the thin film deposition process by encouraging the particulate CdS formation process that inevitably upsets the solubility equilibrium of the CdS precursor ions and results in non-uniform film growth. A possible explanation for this film breakdown is provided as follows: the rate of thiourea decomposition to form S2 ions increases at higher temperatures, but the rate of Cd2þ production hardly increases at all due to the presence of excess OH from the ammonia solution. This creates an excess of S2 ions which attach themselves to the decomplexed Cd2þ ions. The CdS particulates grow quickly and stay in solution even though the temperature eventually decreases. When the tem-
perature increases again these initial CdS particulates grow larger, resulting in a cyclic process. The excess S2 ions not used in creation of the CdS particles are evenly dispersed throughout the solution and appear to attach weakly to the substrate surface forming nucleated sites, which draw [Cd2þ ] species away from existing CdS lattice sites, resulting in a breakdown in the film uniformity on the substrate (as illustrated in the AFM micrographs), in addition to the formation of CdS colloids in solution.
4. Conclusions Ultra-thin CdS polycrystalline films, deposited by chemical bath deposition, show high sensitivity to variations in deposition temperature. The Cd:S atomic ratio of 1:1 is uniform with time when the temperature of the deposition process is constant. When the temperature is allowed to fluctuate, this Cd:S atomic ratio becomes unbalanced and can result in an initial dominance of sulphur on the glass substrate. This sulphur dominance reduces as the chemical reaction progresses and at later stages, a ratio of approximately 1.2:1 for cadmium and sulphur atoms is observed. This initial dominance of sulphur on the substrate appears to be the direct result of a cyclic change in chemical reaction
P.C. Duncan et al. / Nucl. Instr. and Meth. in Phys. Res. B 190 (2002) 615–619
rates during deposition due to a cyclic temperature variation of only a few degrees. Acknowledgements The authors wish to thank Dr. M. Reyhani (Curtin University) for the use of the AFM. This work was supported by an Australian Institute of Nuclear Science and Engineering (AINSE) Research Grant.
619
References [1] S.N. Oju, W.W. Smith, I. Shih, J. Appl. Surf. Sci. 113 (1997) 1147. [2] E.A. Gluszak, S. Hinckley, Proceedings of the Conference on Optoelectonic and Microelectronic Materials and Devices, Perth, 1998, IEEE Press, 1999, p. 426. [3] E.A. Gluszak, S. Hinckley, Proceedings of the Conference on Optoelectonic and Microelectronic Materials and Devices, Melbourne, 2000, IEEE Press, in press.