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Growth behavior and microstructure of CdS thin films deposited by an electrostatic spray assisted vapor deposition (ESAVD) process
Thin Solid Films 388 Ž2001. 9᎐14
Growth behavior and microstructure of CdS thin films deposited by an electrostatic spray assisted vapor deposition Ž ESAVD. process K.L. Choy U , B. Su Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BP, UK Received 25 February 1999; received in revised form 3 December 2000; accepted 3 December 2000
Abstract Dense and adherent CdS films were successfully deposited onto glass substrates at low temperatures ŽF 450⬚C. using a novel and cost-effective electrostatic spray assisted vapor deposition process. A mixture of cadmium chloride and thiourea precursors in ethanolrwater solvent was used to deposit CdS films in an open atmosphere. The relationship between the field strength and electrostatic spray mode was established. The influences of substrate temperature and precursor flow rate on the microstructure of the films were investigated. The microstructure of the films was examined using a combination of X-ray diffraction, atomic force microscopy and scanning electron microscopy methods. At optimum process conditions, the desirable uniform CdS films with oriented columnar structure were deposited at 400 ; 420⬚C at a growth rate of 0.05 mrmin. The relationships between the growth behavior and structure of the films are discussed, and a deposition model for the process is proposed. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Cadmium sulfide; Deposition process; Growth mechanism
1. Introduction Thin films of CdS have important technological applications in heterojunction solar cells, photo-detector materials and gas sensors. CdS films have been produced using vacuum deposition techniques such as chemical vapor deposition w1x and physical vapor deposition w2x. Although these atomistic deposition methods can produce highly dense and pure CdS with well controlled microstructure, they generally involved the use of sophisticated and expensive deposition chamber andror vacuum system, which are generally too expensive for large area and mass production. Moreover, PVD is a line-of-sight process. Therefore, it has difU
Corresponding author. Tel.: q44-171-594; fax: q44-171-5946750. E-mail address: [email protected] ŽK.L. Choy..
ficulty in producing deposits with good conformal coverage. The CVD process tends to use volatile precursors which are normally toxic, flammable andror moisture sensitive. Therefore, special precursor handling system and precautions need to be considered during the CVD process. Other non-vacuum techniques and relatively low cost techniques such as spray pyrolysis w3x, electrodeposition w4x and chemical bath deposition w5x have also been investigated to deposit CdS films. The wet chemical method such as chemical bath deposition is one of the most widely reported methods for CdS thin film deposition but is not considered favorably for large-scale applications because of its poor chemical utilization and environmental concerns over the recycling of the precursors and disposal of the by-products w6x. Spray pyrolysis is another widely used method for CdS thin film deposition w7x. The advantage of this method is
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 8 9 4 - 0
10
K.L. Choy, B. Su r Thin Solid Films 388 (2001) 9᎐14
that it is relatively easy to scale up for large-area deposition. There are several methods of spraying to generate aerosols with droplet size in the micrometer to sub-micrometer range. Pressure nozzles, both hydraulic and pneumatic, are capable of producing aerosols with droplet size of approximately 10 m w8x. An ultrasonic nozzle can generate aerosols with droplet size - 5 m w9x. However, both techniques require a large volume of carrier gas Žnormally ) 1 lrmin. to deliver the aerosol onto the substrate during film deposition. The large flow of carrier gas results in turbulence near the substrate which will adversely affect the efficiency of deposition and the uniformity of the resultant films. Electrostatic spraying, on the other hand, has recently been used for film deposition, e.g. w10᎐13x. Electrostatically charged aerosol droplets have distinctive advantages during film deposition. Finer droplet size Ž- 2 m. with mono-dispersed size distribution can be achieved under proper conditions w11x. Charging can cause the spraying droplets to drift apart, whereby avoid the coalescence. Most importantly, no carrier gas is needed to transport the aerosol. The electrostatically charged droplets can be attracted towards a grounded substrate by a coulombic force. Therefore, the deposition efficiency is much higher owing to less loss of aerosols to the surrounding. Recently, we have developed a cost-effective deposition technique called electrostatic spray assisted vapor deposition process ŽESAVD. to deposit a wide range of uniform and high purity films w12,13x. The process involves spraying atomized and charged aerosol precursor across an electric field, where the aerosol undergo decomposition andror chemical reactions in the vapor phase near the vicinity of the heated substrate to produce a stable solid films with excellent adhesion. Process parameters such as electric field strength, deposition temperature, chemistry of the precursor, precursor flow rate can be varied to control microstructure, porosity, grain size and composition of the films. By varying the process parameters, a wide range of dense and porous oxide films have been deposited for a range of applications including solid oxide fuel cells, ceramic membrane for selective gas separation, oxygen generators, catalytic combustors, temperature sensing films and ferroelectric films for sensors and memory devices. This paper reports for the first time the use of an ESAVD-based method to deposit non-oxide films such as CdS. The growth of non-oxide films can be performed in a controlled atmosphere at atmospheric or reduced pressure or by careful control the chemistry of the precursors. This paper demonstrates the feasibility of depositing CdS films in an open atmosphere by careful tailoring the chemistry of the precursors and process conditions, without the need to deposit in a controlled atmosphere or expensive vacuum system.
The relationships between the growth behavior and structure of the CdS films are discussed, and a deposition model for the process is proposed. 2. Experimental There are several possible chemical precursors for cadmium and sulfide sources such as cadmium acetate, cadmium nitrate, allylthiourea and thioacetic acid, etc. However, cadmium chloride ŽCdCl 2 . and thiourea wŽNH 2 . 2 CSx were used in this study. CdCl 2 and ŽNH 2 . 2 CS with a molar ratio of 1:1 were dissolved in H 2 OrC 2 H 5 OH solvent to produce 0.005 M homogeneous solution. Non-conducting glass slides were used as substrates. The deposition apparatus used for the ESAVD process has been described in detail elsewhere w12x. The substrate temperature was varied from 200 to 500⬚C and deposition time from 10 to 60 min. The precursor flow rate was varied from 10 to 30 mlrh. The influence of substrate temperature and precursor flow rate on the growth and microstructure of the films were investigated. The electrostatic spray window was determined by varying the applied electric field potential Ž4 ; 20 kV. between the atomizer and substrate Ži.e. field strength. and the flow rate of the precursor solutions. The relationship between the field strength and electrostatic spray mode was established. The microstructure of the CdS films was examined using a combination of X-ray diffraction, scanning electron microscopy and atomic force microscopy methods. The phase and crystallinity of the films were determined using a Philips PW1710 X-ray diffraction spectrometer ŽCuK ␣ radiation.. The surface morphology and cross-section of the films were examined using a QUESANT atomic force microscopy and a JEOL 220T scanning electron microscope. 3. Results and discussion 3.1. The relationship between the field strength and electrostatic spray mode The theories and understanding of electrospraying has long been investigated since 1960s for use in industry applications such as paint spraying in automobile industry and insecticide spraying for agricultural industry. As liquid is forced to flow through a capillary which is subjected to an electric field, the coloumbic interaction of charges on the liquid and the applied field will cause charged droplets being emitted from the capillary in different spray modes owing to different electrohydroynamics mechanisms w14,15x. In the case of a continuous flow of liquid through a meniscus, the spray modes include simple-jet, single cone-jet and multi-jet as shown in Fig. 1. Dripping mode may occur when
K.L. Choy, B. Su r Thin Solid Films 388 (2001) 9᎐14
discontinuous liquid flow through the meniscus. The size and distribution of the droplet size depends upon the spray mode which consequentially is found to be strongly governed by the applied electric field strength. Although there are also other factors that will influence the spray mode, including the properties of the fluid Že.g. viscosity, electrical conductivity and dielectric constant, etc.., precursor flow rate and atmosphere. Therefore, it is vital to establish the nature of the spray mode produced during the electrostatic atomization during the ESAVD process. Fig. 2 shows the relationship between the field strength and electrostatic spray mode. As the applied field strength increases, the spray mode transforms from dripping ŽA. to simple-jet ŽB., followed by single cone-jet ŽC. and finally to multi-jet ŽD.. The single cone jet mode is preferred for the CdS thin film deposition because it can offer a stable, uniform and continuous flow pattern. The figure also
11
Fig. 2. A schematic diagram showing the spray mode changes with the applied voltage and flow rate of the precursor solution. A, dripping; B, simple jet; C, single cone-jet; D, multi-jet.
shows that the region of single cone jet mode become wider as the flow rate increases. However, according to the scaling law w16x, the size of the precursor droplet is proportional to the flow rate. Therefore, the flow rate is preferably below 25 mlrh in order to obtain fine spray. Otherwise large droplets might not decompose and react when they arrived at the heated substrate surface which caused the particle formation as shown in the following microscopy results. 3.2. Microstructure of the deposited CdS films
Fig. 1. Three different spraying modes in a capillary-plate configuration under an electric field: Ža. simple-jet; Žb. single cone-jet and Žc. multi-jet.
The thermal decomposition of a mixture of precursors using cadmium chloride and thiourea was reported via a mechanism involving the formation and decomposition of an intermediate complex wCdŽSCN2 H 4 . 2 Cl 2 x w17,18x. A recent TG-DTA study w18x showed that there were three maximum weight loss regions at approximately 250, 370 and 660⬚C. The first weight losses starting from approximately 210⬚C was mainly attributed to the decomposition of the complex wCdŽSCN2 H 4 . 2 Cl 2 x which was formed during the precursor preparation. This complex was decomposed at approximately 250⬚C to form CdS and NH 4 CdCl 3 . The second weight loss at approximately 350⬚C was associated to the decomposition of NH 4 CdCl 3 to form NH 3 , HCl and CdCl 2 . The third weight loss, starting from approximately 500⬚C, was due to the oxidation of cadmium-containing phases to form CdO. Therefore, the deposition temperature is preferably below 500⬚C to avoid the oxidation of CdS, because oxygen was reported to have a detrimental effect on the electrical properties of the CdS films w19x. XRD results show that crystalline CdS film begins to form at deposition temperature of 250⬚C. The film is mainly consisted of hexagonal CdS and NH 4 CdCl 3 . Similar observation has been reported by Krunks et al. w18x. As the substrate temperature increases, the impu-
12
K.L. Choy, B. Su r Thin Solid Films 388 (2001) 9᎐14
Fig. 3. Crystal size vs. deposition temperature for the CdS films.
rity phases such as NH 4 CdCl 3 disappear. The crystal size estimated from Scherrer’s equation increases Žsee Fig. 3.. Fig. 4 shows the surface morphology of CdS films deposited at different temperatures. At 300⬚C, the film seems less crystallized with some pinholes and precipitates on the surface ŽFig. 4a.. As the deposition temperature increases, the film becomes smooth and well crystallized ŽFig. 4b,c.. However, when the temperature
is increased to 450⬚C, some large particles are formed on the smooth film surface ŽFig. 4d.. The above structural observations can be explained using the schematic diagram shown in Fig. 5 which describes the relationship between the growth behavior and structure of the films as a function of substrate temperature. At low substrate temperatures Že.g. below 300⬚C., the charged aerosol was eletrostatically sprayed onto the substrate, followed by the removal of the solvent through evaporation and decomposition of the precursor. The resultant films tend to be porous and amorphous Žprocess I.. At high substrate temperatures Že.g. above 450⬚C., the solvent and chemical precursors vaporized and decomposed before approaching the substrate surface which resulted in the formation of particles that subsequently deposited on the substrate surface and produced powdery films Žprocess III.. At the intermediate temperatures Že.g. 300᎐450⬚C., both processes may occur depending on the substrate temperature. At optimum substrate temperature, the solvent was evaporated closed to the heated substrate surface, and the precursor formed was subsequently volatilized near the vicinity of the substrate and adsor-
Fig. 4. AFM images of the CdS films deposited using 0.005 M precursor solution and a flow rate of 10 mlrh at different temperatures: Ža. 300⬚C; Žb. 350⬚C; Žc. 400⬚C and Žd. 450⬚C.
K.L. Choy, B. Su r Thin Solid Films 388 (2001) 9᎐14
13
Fig. 5. A schematic diagram of the proposed electrostatic spray assisted vapor deposition mechanisms under different process conditions.
bed onto the heated substrate surface, followed by decomposition andror chemical reactions to yield the desirable dense CdS film with good adhesion onto the substrate Žprocess II., resulting in a typical heterogeneous CVD reaction. Obviously, this temperature lies between 400 and 450⬚C. Fig. 6 shows the surface morphology and cross-section of a CdS film deposited at 420⬚C using a flow rate of 10 mlrh. It is clear that some particles are present on the film surface and the film exhibits the columnar growth feature. The columnar structure is desirable to enhance electrical conductivity because of less barrier effects of grain boundaries on mobility across the films w20x. The optical and electric properties of the CdS films produced using ESAVD method are similar to those using other vacuum or non-vacuum methods w21x. It should be pointed out that owing to a variation of the aerosol droplets size Že.g. 1᎐10 m. generated during the electrostatic atomization, it was difficult to ensure that all droplets, especially the large droplets would be decomposed and reacted through the heterogeneous CVD reaction route. Very large droplets might not be decomposed and reacted when they arrived at the heated substrate surface which caused the particle formation. This explained why some particles were present on the films. However, rotating the substrate
and improving the design of electrostatic atomizer might overcome such problems and ensure uniform and true CVD heterogeneous reaction. In addition, the particle size can also be decreased by reducing the precursor flow rate as illustrated in Fig. 7. 4. Conclusions Dense, uniform and adherent CdS thin films suitable for solar cell applications were successfully deposited at low temperatures ŽF 450⬚C. in an open atmosphere
Fig. 6. SEM micrograph of a CdS film deposited at 420⬚C using 0.005 M precursor solution and a flow rate of 10 mlrh.
14
K.L. Choy, B. Su r Thin Solid Films 388 (2001) 9᎐14
using a novel and cost-effective ESAVD process. The preferred single cone-jet spray mode for the ESAVD process was obtained at an electric field strength between 5 and 10 kVrcm at flow rates of 10᎐30 mlrh. The region of single cone-jet mode become wider as the flow rate increases. Uniform CdS films with the desirable columnar structure were deposited at a deposition temperature of 420⬚C using 0.005 M precursor solution containing a CdrS molar ratio of 1:1 at a flow rate of 10 mlrh. The CdS deposition rate under such a condition was 0.05 mrmin. A deposition model describing the relationship between the growth behavior and structure of the films as a function of substrate temperature was proposed. Acknowledgements The authors wish to thank Engineering Physical Science Research Council ŽGRrL73562. for financial support. References
Fig. 7. SEM micrographs of the CdS films deposited at 450⬚C at different flow rates Žmlrh.: Ža. 10; Žb. 20 and Žc. 30.
w1x K. Omura, A. Hanahusa, T. Arita et al., Renewable Energy 8 Ž1996. 405. w2x S.Y. Kim, D.S. Kim, B.T. Ahn, B.T. Im, J. Mater. Sci.: Elec. Mater. 4 Ž1993. 178. w3x Y.Y. Ma, R.H. Bube, J. Electrochem. Soc. 124 Ž1977. 1430. w4x A. Adachi, A. Kudo, T. Sakata, Bull. Chem. Soc. Jpn. 68 Ž1995. 3283. w5x P.J. Sevastian, J. Campos, P.K. Nair, Thin Solid Films 227 Ž1993. 190. w6x A.M. Hermann, Solar Energy Mater. Solar Cells 55 Ž1998. 75. w7x J.B. Mooney, S.B. Radding, Annu. Rev. Mater. Sci. 12 Ž1982. 81. w8x S.C. Zhang, G.L. Messing, W. Huebner, J. Aerosol Sci. 22 Ž1991. 585. w9x G. Bladenet, M. Court, Y. Lagarde, Thin Solid Films 77 Ž1981. 81. w10x C.H. Chen, E.M. Kelder, J. Schoonman, J. Mater. Sci. 31 Ž1996. 5437. w11x K. Tang, A. Gomez, Phys. Fluids 6 Ž1994. 2317. w12x K.L. Choy, W. Bai, British Patent 9525505.5, 1995. w13x K.L. Choy, Mater. World 6 Ž1997. 144. w14x J.M. Grace, J.C.M. Marijnissen, J. Aerosol Sci. 25 Ž1994. 1005. w15x M. Cloupeau, B. Prunet-Foch, J. Electrostatics 22 Ž1989. 135. w16x A.M. Ganan-Calvo, J. Aerosol Sci. 25 ŽSuppl. 1. Ž1994. S309. w17x R.R. Chamberlin, J.S. Skarman, J. Electrochem. Soc. 113 Ž1966. 86. w18x M. Krunks, J. Madarasz, L. Hiltunen, R. Mannonen, E. Mellikov, L. Niinisto, Acta Chem. Scand. 51 Ž1997. 294. w19x C. Wu, R.H. Bube, J. Appl. Phys. 45 Ž1974. 648. w20x F.B. Micheletti, P. Mark, Appl. Phys. Lett. 10 Ž4. Ž1967. 1368. w21x B. Su, K.L. Choy, Thin Solid Films 359 Ž2000. 160.
Growth behavior and microstructure of CdS thin films deposited by an electrostatic spray assisted vapor deposition Ž ESAVD. process K.L. Choy U , B. Su Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BP, UK Received 25 February 1999; received in revised form 3 December 2000; accepted 3 December 2000
Abstract Dense and adherent CdS films were successfully deposited onto glass substrates at low temperatures ŽF 450⬚C. using a novel and cost-effective electrostatic spray assisted vapor deposition process. A mixture of cadmium chloride and thiourea precursors in ethanolrwater solvent was used to deposit CdS films in an open atmosphere. The relationship between the field strength and electrostatic spray mode was established. The influences of substrate temperature and precursor flow rate on the microstructure of the films were investigated. The microstructure of the films was examined using a combination of X-ray diffraction, atomic force microscopy and scanning electron microscopy methods. At optimum process conditions, the desirable uniform CdS films with oriented columnar structure were deposited at 400 ; 420⬚C at a growth rate of 0.05 mrmin. The relationships between the growth behavior and structure of the films are discussed, and a deposition model for the process is proposed. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Cadmium sulfide; Deposition process; Growth mechanism
1. Introduction Thin films of CdS have important technological applications in heterojunction solar cells, photo-detector materials and gas sensors. CdS films have been produced using vacuum deposition techniques such as chemical vapor deposition w1x and physical vapor deposition w2x. Although these atomistic deposition methods can produce highly dense and pure CdS with well controlled microstructure, they generally involved the use of sophisticated and expensive deposition chamber andror vacuum system, which are generally too expensive for large area and mass production. Moreover, PVD is a line-of-sight process. Therefore, it has difU
Corresponding author. Tel.: q44-171-594; fax: q44-171-5946750. E-mail address: [email protected] ŽK.L. Choy..
ficulty in producing deposits with good conformal coverage. The CVD process tends to use volatile precursors which are normally toxic, flammable andror moisture sensitive. Therefore, special precursor handling system and precautions need to be considered during the CVD process. Other non-vacuum techniques and relatively low cost techniques such as spray pyrolysis w3x, electrodeposition w4x and chemical bath deposition w5x have also been investigated to deposit CdS films. The wet chemical method such as chemical bath deposition is one of the most widely reported methods for CdS thin film deposition but is not considered favorably for large-scale applications because of its poor chemical utilization and environmental concerns over the recycling of the precursors and disposal of the by-products w6x. Spray pyrolysis is another widely used method for CdS thin film deposition w7x. The advantage of this method is
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 8 9 4 - 0
10
K.L. Choy, B. Su r Thin Solid Films 388 (2001) 9᎐14
that it is relatively easy to scale up for large-area deposition. There are several methods of spraying to generate aerosols with droplet size in the micrometer to sub-micrometer range. Pressure nozzles, both hydraulic and pneumatic, are capable of producing aerosols with droplet size of approximately 10 m w8x. An ultrasonic nozzle can generate aerosols with droplet size - 5 m w9x. However, both techniques require a large volume of carrier gas Žnormally ) 1 lrmin. to deliver the aerosol onto the substrate during film deposition. The large flow of carrier gas results in turbulence near the substrate which will adversely affect the efficiency of deposition and the uniformity of the resultant films. Electrostatic spraying, on the other hand, has recently been used for film deposition, e.g. w10᎐13x. Electrostatically charged aerosol droplets have distinctive advantages during film deposition. Finer droplet size Ž- 2 m. with mono-dispersed size distribution can be achieved under proper conditions w11x. Charging can cause the spraying droplets to drift apart, whereby avoid the coalescence. Most importantly, no carrier gas is needed to transport the aerosol. The electrostatically charged droplets can be attracted towards a grounded substrate by a coulombic force. Therefore, the deposition efficiency is much higher owing to less loss of aerosols to the surrounding. Recently, we have developed a cost-effective deposition technique called electrostatic spray assisted vapor deposition process ŽESAVD. to deposit a wide range of uniform and high purity films w12,13x. The process involves spraying atomized and charged aerosol precursor across an electric field, where the aerosol undergo decomposition andror chemical reactions in the vapor phase near the vicinity of the heated substrate to produce a stable solid films with excellent adhesion. Process parameters such as electric field strength, deposition temperature, chemistry of the precursor, precursor flow rate can be varied to control microstructure, porosity, grain size and composition of the films. By varying the process parameters, a wide range of dense and porous oxide films have been deposited for a range of applications including solid oxide fuel cells, ceramic membrane for selective gas separation, oxygen generators, catalytic combustors, temperature sensing films and ferroelectric films for sensors and memory devices. This paper reports for the first time the use of an ESAVD-based method to deposit non-oxide films such as CdS. The growth of non-oxide films can be performed in a controlled atmosphere at atmospheric or reduced pressure or by careful control the chemistry of the precursors. This paper demonstrates the feasibility of depositing CdS films in an open atmosphere by careful tailoring the chemistry of the precursors and process conditions, without the need to deposit in a controlled atmosphere or expensive vacuum system.
The relationships between the growth behavior and structure of the CdS films are discussed, and a deposition model for the process is proposed. 2. Experimental There are several possible chemical precursors for cadmium and sulfide sources such as cadmium acetate, cadmium nitrate, allylthiourea and thioacetic acid, etc. However, cadmium chloride ŽCdCl 2 . and thiourea wŽNH 2 . 2 CSx were used in this study. CdCl 2 and ŽNH 2 . 2 CS with a molar ratio of 1:1 were dissolved in H 2 OrC 2 H 5 OH solvent to produce 0.005 M homogeneous solution. Non-conducting glass slides were used as substrates. The deposition apparatus used for the ESAVD process has been described in detail elsewhere w12x. The substrate temperature was varied from 200 to 500⬚C and deposition time from 10 to 60 min. The precursor flow rate was varied from 10 to 30 mlrh. The influence of substrate temperature and precursor flow rate on the growth and microstructure of the films were investigated. The electrostatic spray window was determined by varying the applied electric field potential Ž4 ; 20 kV. between the atomizer and substrate Ži.e. field strength. and the flow rate of the precursor solutions. The relationship between the field strength and electrostatic spray mode was established. The microstructure of the CdS films was examined using a combination of X-ray diffraction, scanning electron microscopy and atomic force microscopy methods. The phase and crystallinity of the films were determined using a Philips PW1710 X-ray diffraction spectrometer ŽCuK ␣ radiation.. The surface morphology and cross-section of the films were examined using a QUESANT atomic force microscopy and a JEOL 220T scanning electron microscope. 3. Results and discussion 3.1. The relationship between the field strength and electrostatic spray mode The theories and understanding of electrospraying has long been investigated since 1960s for use in industry applications such as paint spraying in automobile industry and insecticide spraying for agricultural industry. As liquid is forced to flow through a capillary which is subjected to an electric field, the coloumbic interaction of charges on the liquid and the applied field will cause charged droplets being emitted from the capillary in different spray modes owing to different electrohydroynamics mechanisms w14,15x. In the case of a continuous flow of liquid through a meniscus, the spray modes include simple-jet, single cone-jet and multi-jet as shown in Fig. 1. Dripping mode may occur when
K.L. Choy, B. Su r Thin Solid Films 388 (2001) 9᎐14
discontinuous liquid flow through the meniscus. The size and distribution of the droplet size depends upon the spray mode which consequentially is found to be strongly governed by the applied electric field strength. Although there are also other factors that will influence the spray mode, including the properties of the fluid Že.g. viscosity, electrical conductivity and dielectric constant, etc.., precursor flow rate and atmosphere. Therefore, it is vital to establish the nature of the spray mode produced during the electrostatic atomization during the ESAVD process. Fig. 2 shows the relationship between the field strength and electrostatic spray mode. As the applied field strength increases, the spray mode transforms from dripping ŽA. to simple-jet ŽB., followed by single cone-jet ŽC. and finally to multi-jet ŽD.. The single cone jet mode is preferred for the CdS thin film deposition because it can offer a stable, uniform and continuous flow pattern. The figure also
11
Fig. 2. A schematic diagram showing the spray mode changes with the applied voltage and flow rate of the precursor solution. A, dripping; B, simple jet; C, single cone-jet; D, multi-jet.
shows that the region of single cone jet mode become wider as the flow rate increases. However, according to the scaling law w16x, the size of the precursor droplet is proportional to the flow rate. Therefore, the flow rate is preferably below 25 mlrh in order to obtain fine spray. Otherwise large droplets might not decompose and react when they arrived at the heated substrate surface which caused the particle formation as shown in the following microscopy results. 3.2. Microstructure of the deposited CdS films
Fig. 1. Three different spraying modes in a capillary-plate configuration under an electric field: Ža. simple-jet; Žb. single cone-jet and Žc. multi-jet.
The thermal decomposition of a mixture of precursors using cadmium chloride and thiourea was reported via a mechanism involving the formation and decomposition of an intermediate complex wCdŽSCN2 H 4 . 2 Cl 2 x w17,18x. A recent TG-DTA study w18x showed that there were three maximum weight loss regions at approximately 250, 370 and 660⬚C. The first weight losses starting from approximately 210⬚C was mainly attributed to the decomposition of the complex wCdŽSCN2 H 4 . 2 Cl 2 x which was formed during the precursor preparation. This complex was decomposed at approximately 250⬚C to form CdS and NH 4 CdCl 3 . The second weight loss at approximately 350⬚C was associated to the decomposition of NH 4 CdCl 3 to form NH 3 , HCl and CdCl 2 . The third weight loss, starting from approximately 500⬚C, was due to the oxidation of cadmium-containing phases to form CdO. Therefore, the deposition temperature is preferably below 500⬚C to avoid the oxidation of CdS, because oxygen was reported to have a detrimental effect on the electrical properties of the CdS films w19x. XRD results show that crystalline CdS film begins to form at deposition temperature of 250⬚C. The film is mainly consisted of hexagonal CdS and NH 4 CdCl 3 . Similar observation has been reported by Krunks et al. w18x. As the substrate temperature increases, the impu-
12
K.L. Choy, B. Su r Thin Solid Films 388 (2001) 9᎐14
Fig. 3. Crystal size vs. deposition temperature for the CdS films.
rity phases such as NH 4 CdCl 3 disappear. The crystal size estimated from Scherrer’s equation increases Žsee Fig. 3.. Fig. 4 shows the surface morphology of CdS films deposited at different temperatures. At 300⬚C, the film seems less crystallized with some pinholes and precipitates on the surface ŽFig. 4a.. As the deposition temperature increases, the film becomes smooth and well crystallized ŽFig. 4b,c.. However, when the temperature
is increased to 450⬚C, some large particles are formed on the smooth film surface ŽFig. 4d.. The above structural observations can be explained using the schematic diagram shown in Fig. 5 which describes the relationship between the growth behavior and structure of the films as a function of substrate temperature. At low substrate temperatures Že.g. below 300⬚C., the charged aerosol was eletrostatically sprayed onto the substrate, followed by the removal of the solvent through evaporation and decomposition of the precursor. The resultant films tend to be porous and amorphous Žprocess I.. At high substrate temperatures Že.g. above 450⬚C., the solvent and chemical precursors vaporized and decomposed before approaching the substrate surface which resulted in the formation of particles that subsequently deposited on the substrate surface and produced powdery films Žprocess III.. At the intermediate temperatures Že.g. 300᎐450⬚C., both processes may occur depending on the substrate temperature. At optimum substrate temperature, the solvent was evaporated closed to the heated substrate surface, and the precursor formed was subsequently volatilized near the vicinity of the substrate and adsor-
Fig. 4. AFM images of the CdS films deposited using 0.005 M precursor solution and a flow rate of 10 mlrh at different temperatures: Ža. 300⬚C; Žb. 350⬚C; Žc. 400⬚C and Žd. 450⬚C.
K.L. Choy, B. Su r Thin Solid Films 388 (2001) 9᎐14
13
Fig. 5. A schematic diagram of the proposed electrostatic spray assisted vapor deposition mechanisms under different process conditions.
bed onto the heated substrate surface, followed by decomposition andror chemical reactions to yield the desirable dense CdS film with good adhesion onto the substrate Žprocess II., resulting in a typical heterogeneous CVD reaction. Obviously, this temperature lies between 400 and 450⬚C. Fig. 6 shows the surface morphology and cross-section of a CdS film deposited at 420⬚C using a flow rate of 10 mlrh. It is clear that some particles are present on the film surface and the film exhibits the columnar growth feature. The columnar structure is desirable to enhance electrical conductivity because of less barrier effects of grain boundaries on mobility across the films w20x. The optical and electric properties of the CdS films produced using ESAVD method are similar to those using other vacuum or non-vacuum methods w21x. It should be pointed out that owing to a variation of the aerosol droplets size Že.g. 1᎐10 m. generated during the electrostatic atomization, it was difficult to ensure that all droplets, especially the large droplets would be decomposed and reacted through the heterogeneous CVD reaction route. Very large droplets might not be decomposed and reacted when they arrived at the heated substrate surface which caused the particle formation. This explained why some particles were present on the films. However, rotating the substrate
and improving the design of electrostatic atomizer might overcome such problems and ensure uniform and true CVD heterogeneous reaction. In addition, the particle size can also be decreased by reducing the precursor flow rate as illustrated in Fig. 7. 4. Conclusions Dense, uniform and adherent CdS thin films suitable for solar cell applications were successfully deposited at low temperatures ŽF 450⬚C. in an open atmosphere
Fig. 6. SEM micrograph of a CdS film deposited at 420⬚C using 0.005 M precursor solution and a flow rate of 10 mlrh.
14
K.L. Choy, B. Su r Thin Solid Films 388 (2001) 9᎐14
using a novel and cost-effective ESAVD process. The preferred single cone-jet spray mode for the ESAVD process was obtained at an electric field strength between 5 and 10 kVrcm at flow rates of 10᎐30 mlrh. The region of single cone-jet mode become wider as the flow rate increases. Uniform CdS films with the desirable columnar structure were deposited at a deposition temperature of 420⬚C using 0.005 M precursor solution containing a CdrS molar ratio of 1:1 at a flow rate of 10 mlrh. The CdS deposition rate under such a condition was 0.05 mrmin. A deposition model describing the relationship between the growth behavior and structure of the films as a function of substrate temperature was proposed. Acknowledgements The authors wish to thank Engineering Physical Science Research Council ŽGRrL73562. for financial support. References
Fig. 7. SEM micrographs of the CdS films deposited at 450⬚C at different flow rates Žmlrh.: Ža. 10; Žb. 20 and Žc. 30.
w1x K. Omura, A. Hanahusa, T. Arita et al., Renewable Energy 8 Ž1996. 405. w2x S.Y. Kim, D.S. Kim, B.T. Ahn, B.T. Im, J. Mater. Sci.: Elec. Mater. 4 Ž1993. 178. w3x Y.Y. Ma, R.H. Bube, J. Electrochem. Soc. 124 Ž1977. 1430. w4x A. Adachi, A. Kudo, T. Sakata, Bull. Chem. Soc. Jpn. 68 Ž1995. 3283. w5x P.J. Sevastian, J. Campos, P.K. Nair, Thin Solid Films 227 Ž1993. 190. w6x A.M. Hermann, Solar Energy Mater. Solar Cells 55 Ž1998. 75. w7x J.B. Mooney, S.B. Radding, Annu. Rev. Mater. Sci. 12 Ž1982. 81. w8x S.C. Zhang, G.L. Messing, W. Huebner, J. Aerosol Sci. 22 Ž1991. 585. w9x G. Bladenet, M. Court, Y. Lagarde, Thin Solid Films 77 Ž1981. 81. w10x C.H. Chen, E.M. Kelder, J. Schoonman, J. Mater. Sci. 31 Ž1996. 5437. w11x K. Tang, A. Gomez, Phys. Fluids 6 Ž1994. 2317. w12x K.L. Choy, W. Bai, British Patent 9525505.5, 1995. w13x K.L. Choy, Mater. World 6 Ž1997. 144. w14x J.M. Grace, J.C.M. Marijnissen, J. Aerosol Sci. 25 Ž1994. 1005. w15x M. Cloupeau, B. Prunet-Foch, J. Electrostatics 22 Ž1989. 135. w16x A.M. Ganan-Calvo, J. Aerosol Sci. 25 ŽSuppl. 1. Ž1994. S309. w17x R.R. Chamberlin, J.S. Skarman, J. Electrochem. Soc. 113 Ž1966. 86. w18x M. Krunks, J. Madarasz, L. Hiltunen, R. Mannonen, E. Mellikov, L. Niinisto, Acta Chem. Scand. 51 Ž1997. 294. w19x C. Wu, R.H. Bube, J. Appl. Phys. 45 Ž1974. 648. w20x F.B. Micheletti, P. Mark, Appl. Phys. Lett. 10 Ž4. Ž1967. 1368. w21x B. Su, K.L. Choy, Thin Solid Films 359 Ž2000. 160.