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The effect of post-deposition treatments on morphology, structure and opto-electronic properties of chemically deposited CdS thin films
Thin Solid Films, 227
190
( 1993) 190- 195
The effect of post-deposition treatments on morphology, structure and opto-electronic properties of chemically deposited CdS thin films P. J. Sebastian, J. Campos and P. K. Nair Photovoltaic Systems Group, Laboratorio de Energia Solar IIM, I/NAM, (Received
September
30, 1992; accepted
December
Temisco, Morelos (Mesico)
22, 1992)
Abstract CdS thin films for opto-electronic applications were grown by chemical deposition. Characterization of early stages of growth of the films by optical and scanning electron microscopy revealed that the growth of CdS films by chemical deposition occurs as an ion-by-ion process as well as by colloidal particles of CdS adhering to the substrate. The bath parameters, such as Cd ion to ammonia, triethanolamine (TEA) and thiourea (TU) mole ratios, influenced the growth mode. The X-ray diffraction studies of the films showed the presence of some organo-metallic impurities on the film surface, which can be removed by post-deposition etching of the films in very dilute acetic acid. The film morphology depends on the bath composition and the optical transmittance of the films can be improved by etching. A bath composition comprising a Cd:TEA:NH,:TU mole ratio of about 1:3.75:14.4:1 resulted in good quality films.
1. Introduction CdS thin film is well-known for its application as an opto-electronic material in solar cells, photo-detectors etc., [ 11. It has continued to be investigated as a solarcell material [2-31 and in ternary-system photo-detectors [4, 51. A low-cost technique, namely the solution growth for the preparation of CdS polycrystalline films, was initiated by the work of Mokrushin and Tkachev [6] and Kitaev et al. [7] and followed by other researchers [8- 131. The deposition of CdS films occurs as follows: the tetraamine
complex
(Cd(NH,),)‘+
+ SC(NH2),
+ OC(NH,)
method
[9, lo];
+ 20H-
-
CdS + 4NH,
+ H,O
and the TEA complex
(1) method
(Cd(TEA))‘+
+ SC(NH,),
+ OC( NH,),
+ Hz0
[ 131;
+ 20H-
2. Experimental *
techniques
CdS + TEA (2)
The growth of CdS films by the chemical method occurs either by (i) an ion-by-ion condensation of Cd’+ and S2- on the substrate surface or (ii) by the adsorption of colloidal particles of CdS onto the substrate surface [ 121. The growth processes underlying the dominance of (i) or (ii) above have been discussed by Kaur et al. [lo], but without essentially relating the film properties such as optical transmittance, structure, morphology, and the opto-electronic characteristics.
0040-6090/93/$6.00
In the present investigation the authors try to analyze the early stages of the growth of the films in an attempt to explain the growth modes of CdS films in the chemical bath. X-ray diffraction (XRD) studies were performed on as-deposited and etched films to characterize the impurities present in the film and to study the structure and morphology. Optical measurements were made on as-deposited and etched films to study the modifications on optical transmittance, band gap etc. The surface morphological analysis of the films was performed to characterize the adsorbate nature of the film. The opto-electronic properties of the as-deposited films and those subjected to post-deposition treatments were characterized to study the influence of postdeposition treatments on the above properties.
solutions of 1M cadmium acetate Aqueous [Cd(CH,COO),], 1 M TU [SC(NH,),], 3.75 M TEA [N(CH2CH20H)], and 14.4 M ammonia were used for the preparation of the various chemical baths. The different chemical baths were set up by changing the Cd:TEA:NH,:TU mole ratio with the bath volume made up to 50 ml with deionised water. Clean glass slides of dimensions 76 mm x 25 mm x 1 mm were vertically supported on the walls of the beakers at a bath temperature of 25 or 60 “C in an air oven. The films were allowed to grow for various durations, up to 24 h
0
1993 -
Elsevier Sequoia.
All rights
reserved
P. J. Sebastian
et al. I The effect of post-deposition
treatments
of CdS thin films
191
at room temperature or 4 h at 60 “C. The electrical measurements were made with coplanar silver electrodes printed on the film surface. The illumination was 600 W me2 (tungsten-halogen lamp) and the bias was 10 V in all cases. The optical studies were accomplished using a Shimadzu UV365 spectrophotometer, and the XRD patterns were obtained with the help of a Siemens D500 system. The film thickness was measured using an Alpha step 100 instrument. The optical micrographs of the films were obtained with the help of an Olympus AH-2 optical microscope. The scanning electron micrographs (SEMs) of the films were obtained using a JOEL scanning electron microscope. More details of the experimental techniques are given in earlier articles [ 14, 151. Fig. 2. The optical micrograph with magnification 500x for the (a) unetched and (b) etched portions of the chemically deposited film.
3. Results and discussion The chemically deposited CdS thin films were characterized by XRD and opto-electronic techniques to study the influence of the bath parameters and post-deposition treatments on their morphology, structure and opto-electronic properties. The chemically deposited CdS film was found to consist of a continuous film part relating to the ion-by-ion deposition of CdS, a particulate portion as a result of the colloidal particles of CdS adhering to the substrate, and an organo-metallic impurity part arising from the chemicals used for the film deposition. 3.1. The structure, morphology and optical transmittance of theJiIms Figure 1 shows the XRD pictures for a chemically deposited CdS film (a) before and (b) after etching in dilute acetic acid. The vertical lines in the figure correI
I
1
J b
28
ldegl
__
Fig. 1. The XRD pictures corresponding to the (a) as-deposited (b) etched (in dilute acetic acid) CdS films.
J
and
spond to the standard CdS structure. This figure indicates that there are a few additional lines, which do not belong to the CdS structure, in the XRD spectrum at low 20 values in the case of the as-deposited film. But the etched film did not exhibit these additional reflections. The broad peak at 28 = 25” indicates that the film may be relatively thin. The presence of an organometallic impurity (CdNCN?) in chemically deposited CdS films has been reported previously [ll]. In the present investigation it was observed that the presence of the above impurity phase in the film affects the optical transmittance of the film and hence hinders its application as a window layer in solar-cell structures. Figure 2 shows the optical micrograph with magnification 500 x for the boundary of the (a) unetched and (b) etched regions of the film corresponding to Fig. 1. Figure 2 clearly displays the reduction in the particle size in the etched region, which still retains the particulate nature, but Fig. 1 does not show the presence of any impurity phase in the XRD picture of the etched film. The above results confirm that those particles belong to CdS and not any impurity phase. The influence of the impurity phase on the optical transmittance of the film is displayed in Fig. 3, where (a) and (b) correspond respectively to the optical transmittance of the film before and after etching in dilute acetic acid. In this case, the film thickness is about 0.5 urn and there is no appreciable variation in film thickness after etching in dilute acid. Figure 3 shows that there is about 20”/0increase in the optical transmittance of the film after etching. An ideal non-scattering CdS film has an optical transmittance of about 80% for wavelengths above the band edge. The remaining 20% corresponds to reflectance and very little optical absorption. Since CdS is formed by an ion-by-ion deposition as well as by colloidal particles of CdS adhering to
P. J. Sebastian
192
0
1
I
I
1000
500
et al. 1 The effect of post-deposition
Fig. 3. The optical transmittance (b) etched films of CdS.
data
I
1
1500 Wavelength Inm,
2000
2500
for the (a) as-deposited
and
the substrate, about 10% of the incident light is scattered by the particulate structure of CdS film. Hence the optical transmittance of the film did not increase to the ideal level even after the removal of the organo-metallic impurities from the film by etching in dilute acid. Figure 4 displays the photocurrent response for the (a) as-deposited and (b) etched film with time. Figure 4 shows that there is no appreciable variation in the dark and photocurrent and photoresponse of the asdeposited film after etching in dilute acetic acid. Hence, it is possible to remove the organo-metallic impurity from the film by etching in dilute acetic acid without adversely affecting its opto-electronic properties.
10-b
I
1
1
I
I
I 50
I 100
I
I
I
150
200
250
10-6,
. lo-'0 0
b 300
T!me IsI
Fig. 4. The photocurrent (b) etched films of CdS.
response
curves for the (a) as-deposited
and
treatments
of CdS thin films
3.2. The eflect of growth mode on film morphology It has been reported that the growth of CdS films in chemical deposition is brought about by the ion-by-ion condensation as well as colloidal particles of CdS adhering to the substrate [ 10, 121. The authors studied the different stages of growth of the films using an optical microscope to characterize the particulate nature of the film surface. Figures 5(a)-5(d) give the micrographs with magnification 500 x for the films with deposition times of 30 min, 2, 3 and 6 h respectively and at a bath temperature of 60 “C. It can be seen that the particulate nature of CdS film appears even at the early stage of growth. These particles grow in size as time passes and reach the maximum size when the film attains the terminal thickness within about 4 h at 60 “C or 18 h at 25 “C. The SEM of magnification 30 000 x taken in the plane view and cross-sectional view for a film which attained the terminal thickness is shown respectively in Figs. 6(a) and 6(b). The bottom black region in Fig. 6(b) corresponds to the glass substrate and the top lighter portion consists of the film and the particulate structure. The film thickness calculated using the Alpha Step and from the SEM is about 0.5 pm. The CdS particle size is about 10 to 15 pm. 3.3. IJl’uence of ammonia, TEA and TU concentrations on film morphology In the present investigation the effect of bath parameters on the film morphology was studied by setting up various chemical baths of different Cd ion to ammonia, TEA and TU mole ratios. In the present study it was observed that the particulate nature of chemically deposited CdS films depends on the bath composition. The effect of bath composition on the film morphology is shown in Figs. 7-9. It was seen that the abundance and size of the CdS particles in the film increased with increasing ammonia concentration in the chemical bath. Figures 7(a) and 7(b) correspond to the optical micrographs with magnification 500 x for films grown in chemical baths with Cd:NH, mole ratios of 1:17.28 and 1:57.6 respectively. In both the above cases the Cd:TEA:TU mole ratio remained at 1:3.75: 1. Figures 7(a) and 7(b) show that the film with a higher ammonia concentration is almost fully covered with the particles, whereas the other film contains a fewer number of particles. The ammonia concentration, in the above ratio, below 14.4, did not provide films due to the quick precipitation of CdS in the bath. The influence of TEA concentration in the bath on the film morphology is given in Figs. 8(a), 8(b) and 8(c) which correspond respectively to the Cd:TEA mole 1:3.75 and 1:5.625, with the ratios of 1:0.75, Cd:NH,:TU mole ratio (1:14.4:0.5) remaining the same
P. J. Sebastian
et al. 1 The effect of post-deposition
(4
treatments
of CdS thin films
193
(b)
(4 Fig. 5. The optical micrographs with magnification 500 x fort :hemic ally deposited CdS films with a deposition time of (a) 30 min,
@I 2 h (4
3 h and (d) 6 h.
(4
(b)
Fig. 6. The SEMs with magnification 30 000 x for CdS films (a) in the longitudinal and (b) in the transverse directions.
in all cases. Figures 8(a), 8(b) and 8(c) indicate that higher concentrations of TEA in the solution result in films with fewer and smaller particles. But above a certain minimum amount of TEA in the bath, very thin films result due to the high degree of complexing (TEA with Cd ions) in the chemical bath. The above results may be explained by considering the complex-
ing action of TEA upon Cd ions in the solution. A low concentration of TEA in the bath results in very little complex formation of TEA with Cd ions, which makes the number of free Cd ions in the bath abundant. Hence more colloidal particles of CdS may form and adhere to the substrate in the above situation.
P. J. Sebastiun et al. / The effect of post-deposition treatments of CdS thin films
(4 (4
(b) Fig. 7. The optical micrographs with magnification 500 x for CdS films with a Cd:NH, mole ratio of (a) l:l7.28 and (b) 1:57.6 in the chemical bath with the Cd:TEA:TU mole ratio remaining at 1:3.75:1.
(b)
The effect of TU concentration in the chemical bath on the particulate nature of CdS films is displayed in Figs. 9(a) -9(c). In all the above three cases the Cd:TEA:NH, mole ratio remained at 1:3.75:14.4. Figures 9(a), 9(b) and 9(c) correspond to the Cd:TU mole ratios of 1: 1, 1:2 and 1:4 respectively. As the TU concentration in the bath increased the film surface became more particulate in nature. The film surface is almost fully covered with the CdS particles at a Cd:TU mole ratio of 1:4 in the chemical bath. Only very few particles are seen on the film surface at a Cd:TU mole ratio of 1:l in the chemical bath. High TU concentration in the chemical bath makes available more free S*ions in the bath, which favour the formation of more colloidal particles of CdS in the bath and hence the film becomes more particulate in nature. The above results show that the particulate nature of the film surface is controlled by the composition of the
Cc) Fig. 8. The optical micrographs with magnification 5()O x for CdS films with a Cd:TEA mole ratio of (a) 1:0.75, (b) l:3.75 a nd ((:) 1:.5.625 in the chemical bath with a Cd:NH,:TU mole ratio of I:14 1.4:4:0.5.
P. J. Sebastian
et al. / The effect of post-deposition
treatments
of CdS thin films
195
for opto-electronic applications by the low-cost chemical deposition method. 4. Conclusion
(4
The morphology and opto-electronic properties of chemically deposited CdS thin films were investigated with respect to the chemical bath composition and postdeposition treatments. As-deposited films contained some organo-metallic impurities on the surface, which can be removed by etching of the film surface in very dilute acetic acid. Characterization of the film surface using optical and scanning electron microscopy showed that, in addition to the ion-by-ion deposition of CdS films, colloidal particles of CdS from the chemical bath are also adhering to the substrate. The particulate nature of the CdS film surface due to the colloidal deposition may be controlled by suitably setting up the bath composition. The Cd:TEA:NH, :TU mole ratios determined the particulate nature of the film surface. A Cd:TEA:NH,:TU mole ratio of about 1:3.75: 14.4:1 was found to be suitable for getting good quality films for opto-electronic applications. Acknowledgments
(b)
The authors would like to acknowledge the assistance of Miss L. Bafios in XRD, Mr. 0. Gomez-Daza in film deposition and microscopy, Mr. A. M. Fernandez in data transfer, the III-V Materials group of CINVESTAV, and the Dept. de Materiales, Metalicos y Ceramicos, II-M UNAM for SEM studies. References
(4 Fig. 9. The optical micrographs with magnification 500 x for CdS with a Cd:TU mole ratio of (a) l:l, (b) I:2 and (c) I:4 in the the bath with the Cd:TEA:NH, mole ratio remaining at 1:3.75:14.~
chemical bath. The situation may be avoided by pro1 )erly setting up the chemical bath composition. From the results of the present study it may be recommended that a Cd:TEA:NH,:TU mole ratio of about 1:3.75:14.4.:I is suitable for getting CdS thin films devoid of colic lidal particles of CdS adhering to the film. A post-deposi tion etching of the films prepared from a bath comprisin lg of the above mole ratios provides good quality CdS films
1 A. G. Stanley, Appl. Solid State Sci., 15 ( 1975) 25 1. 2 K. Balakrishnan and A. C. Rastogi, Sol. Energy Mater., 20 (1990) 417. 3 L. P. Deshmukh, A. B. Palwe and V. S. Sawant, Sol. Energy Mater., 20(1990) 341. 4 G. Gordillo, Sol. Energy Mater., 25( 1992) 41. 5 J. Torres and G. Gordillo, Thin Solid Films, 207( 1992) 231. 6 S. G. Mokrushin and Y. D. Tkachev, Colloid. J. USSR, 23 (1961) 336. 7 G. A. Kitaev, A. A. Uritskaya and S. G. Mokrushin, Russ. J. Phys. Chem., 39(1965) 1101. 8 M. Nagao and S. Watanabe, Jpn. J. Appl. Phys., 7( 1968) 684. 9 N. R. Pavaskar, C. A. Menezes and A. P. B. Sinha, J. Electrochem. Sot., 124 (1977) 743. 10 I. Kaur, D. K. Pandya and K. L. Chopra, J. Electrochem. Sot., 127 (1980) 943. 11 R. L. Call, N. K. Jaber, K. Seshan and J. R. Whyte, Jr., Sol. Energy Mater., 2 (1980) 373. 12 W. J. Danaher, L. E. Lyons and G. C. Morris, Sol. Energy Mater., 12( 1985) 137. 13 A. Mondal, T. K. Chaudhuri and P. Pramanik, Sol. Energy Mater., 7(1983) 431. 14 P. J. Sebastian and P. K. Nair, Adun. Mater. Opt. Electron., I (1992) 211. 15 P. K. Nair, J. Campos and M. T. S. Nair, Semicond. Sci. Technol., 3 (1988) 134.
190
( 1993) 190- 195
The effect of post-deposition treatments on morphology, structure and opto-electronic properties of chemically deposited CdS thin films P. J. Sebastian, J. Campos and P. K. Nair Photovoltaic Systems Group, Laboratorio de Energia Solar IIM, I/NAM, (Received
September
30, 1992; accepted
December
Temisco, Morelos (Mesico)
22, 1992)
Abstract CdS thin films for opto-electronic applications were grown by chemical deposition. Characterization of early stages of growth of the films by optical and scanning electron microscopy revealed that the growth of CdS films by chemical deposition occurs as an ion-by-ion process as well as by colloidal particles of CdS adhering to the substrate. The bath parameters, such as Cd ion to ammonia, triethanolamine (TEA) and thiourea (TU) mole ratios, influenced the growth mode. The X-ray diffraction studies of the films showed the presence of some organo-metallic impurities on the film surface, which can be removed by post-deposition etching of the films in very dilute acetic acid. The film morphology depends on the bath composition and the optical transmittance of the films can be improved by etching. A bath composition comprising a Cd:TEA:NH,:TU mole ratio of about 1:3.75:14.4:1 resulted in good quality films.
1. Introduction CdS thin film is well-known for its application as an opto-electronic material in solar cells, photo-detectors etc., [ 11. It has continued to be investigated as a solarcell material [2-31 and in ternary-system photo-detectors [4, 51. A low-cost technique, namely the solution growth for the preparation of CdS polycrystalline films, was initiated by the work of Mokrushin and Tkachev [6] and Kitaev et al. [7] and followed by other researchers [8- 131. The deposition of CdS films occurs as follows: the tetraamine
complex
(Cd(NH,),)‘+
+ SC(NH2),
+ OC(NH,)
method
[9, lo];
+ 20H-
-
CdS + 4NH,
+ H,O
and the TEA complex
(1) method
(Cd(TEA))‘+
+ SC(NH,),
+ OC( NH,),
+ Hz0
[ 131;
+ 20H-
2. Experimental *
techniques
CdS + TEA (2)
The growth of CdS films by the chemical method occurs either by (i) an ion-by-ion condensation of Cd’+ and S2- on the substrate surface or (ii) by the adsorption of colloidal particles of CdS onto the substrate surface [ 121. The growth processes underlying the dominance of (i) or (ii) above have been discussed by Kaur et al. [lo], but without essentially relating the film properties such as optical transmittance, structure, morphology, and the opto-electronic characteristics.
0040-6090/93/$6.00
In the present investigation the authors try to analyze the early stages of the growth of the films in an attempt to explain the growth modes of CdS films in the chemical bath. X-ray diffraction (XRD) studies were performed on as-deposited and etched films to characterize the impurities present in the film and to study the structure and morphology. Optical measurements were made on as-deposited and etched films to study the modifications on optical transmittance, band gap etc. The surface morphological analysis of the films was performed to characterize the adsorbate nature of the film. The opto-electronic properties of the as-deposited films and those subjected to post-deposition treatments were characterized to study the influence of postdeposition treatments on the above properties.
solutions of 1M cadmium acetate Aqueous [Cd(CH,COO),], 1 M TU [SC(NH,),], 3.75 M TEA [N(CH2CH20H)], and 14.4 M ammonia were used for the preparation of the various chemical baths. The different chemical baths were set up by changing the Cd:TEA:NH,:TU mole ratio with the bath volume made up to 50 ml with deionised water. Clean glass slides of dimensions 76 mm x 25 mm x 1 mm were vertically supported on the walls of the beakers at a bath temperature of 25 or 60 “C in an air oven. The films were allowed to grow for various durations, up to 24 h
0
1993 -
Elsevier Sequoia.
All rights
reserved
P. J. Sebastian
et al. I The effect of post-deposition
treatments
of CdS thin films
191
at room temperature or 4 h at 60 “C. The electrical measurements were made with coplanar silver electrodes printed on the film surface. The illumination was 600 W me2 (tungsten-halogen lamp) and the bias was 10 V in all cases. The optical studies were accomplished using a Shimadzu UV365 spectrophotometer, and the XRD patterns were obtained with the help of a Siemens D500 system. The film thickness was measured using an Alpha step 100 instrument. The optical micrographs of the films were obtained with the help of an Olympus AH-2 optical microscope. The scanning electron micrographs (SEMs) of the films were obtained using a JOEL scanning electron microscope. More details of the experimental techniques are given in earlier articles [ 14, 151. Fig. 2. The optical micrograph with magnification 500x for the (a) unetched and (b) etched portions of the chemically deposited film.
3. Results and discussion The chemically deposited CdS thin films were characterized by XRD and opto-electronic techniques to study the influence of the bath parameters and post-deposition treatments on their morphology, structure and opto-electronic properties. The chemically deposited CdS film was found to consist of a continuous film part relating to the ion-by-ion deposition of CdS, a particulate portion as a result of the colloidal particles of CdS adhering to the substrate, and an organo-metallic impurity part arising from the chemicals used for the film deposition. 3.1. The structure, morphology and optical transmittance of theJiIms Figure 1 shows the XRD pictures for a chemically deposited CdS film (a) before and (b) after etching in dilute acetic acid. The vertical lines in the figure correI
I
1
J b
28
ldegl
__
Fig. 1. The XRD pictures corresponding to the (a) as-deposited (b) etched (in dilute acetic acid) CdS films.
J
and
spond to the standard CdS structure. This figure indicates that there are a few additional lines, which do not belong to the CdS structure, in the XRD spectrum at low 20 values in the case of the as-deposited film. But the etched film did not exhibit these additional reflections. The broad peak at 28 = 25” indicates that the film may be relatively thin. The presence of an organometallic impurity (CdNCN?) in chemically deposited CdS films has been reported previously [ll]. In the present investigation it was observed that the presence of the above impurity phase in the film affects the optical transmittance of the film and hence hinders its application as a window layer in solar-cell structures. Figure 2 shows the optical micrograph with magnification 500 x for the boundary of the (a) unetched and (b) etched regions of the film corresponding to Fig. 1. Figure 2 clearly displays the reduction in the particle size in the etched region, which still retains the particulate nature, but Fig. 1 does not show the presence of any impurity phase in the XRD picture of the etched film. The above results confirm that those particles belong to CdS and not any impurity phase. The influence of the impurity phase on the optical transmittance of the film is displayed in Fig. 3, where (a) and (b) correspond respectively to the optical transmittance of the film before and after etching in dilute acetic acid. In this case, the film thickness is about 0.5 urn and there is no appreciable variation in film thickness after etching in dilute acid. Figure 3 shows that there is about 20”/0increase in the optical transmittance of the film after etching. An ideal non-scattering CdS film has an optical transmittance of about 80% for wavelengths above the band edge. The remaining 20% corresponds to reflectance and very little optical absorption. Since CdS is formed by an ion-by-ion deposition as well as by colloidal particles of CdS adhering to
P. J. Sebastian
192
0
1
I
I
1000
500
et al. 1 The effect of post-deposition
Fig. 3. The optical transmittance (b) etched films of CdS.
data
I
1
1500 Wavelength Inm,
2000
2500
for the (a) as-deposited
and
the substrate, about 10% of the incident light is scattered by the particulate structure of CdS film. Hence the optical transmittance of the film did not increase to the ideal level even after the removal of the organo-metallic impurities from the film by etching in dilute acid. Figure 4 displays the photocurrent response for the (a) as-deposited and (b) etched film with time. Figure 4 shows that there is no appreciable variation in the dark and photocurrent and photoresponse of the asdeposited film after etching in dilute acetic acid. Hence, it is possible to remove the organo-metallic impurity from the film by etching in dilute acetic acid without adversely affecting its opto-electronic properties.
10-b
I
1
1
I
I
I 50
I 100
I
I
I
150
200
250
10-6,
. lo-'0 0
b 300
T!me IsI
Fig. 4. The photocurrent (b) etched films of CdS.
response
curves for the (a) as-deposited
and
treatments
of CdS thin films
3.2. The eflect of growth mode on film morphology It has been reported that the growth of CdS films in chemical deposition is brought about by the ion-by-ion condensation as well as colloidal particles of CdS adhering to the substrate [ 10, 121. The authors studied the different stages of growth of the films using an optical microscope to characterize the particulate nature of the film surface. Figures 5(a)-5(d) give the micrographs with magnification 500 x for the films with deposition times of 30 min, 2, 3 and 6 h respectively and at a bath temperature of 60 “C. It can be seen that the particulate nature of CdS film appears even at the early stage of growth. These particles grow in size as time passes and reach the maximum size when the film attains the terminal thickness within about 4 h at 60 “C or 18 h at 25 “C. The SEM of magnification 30 000 x taken in the plane view and cross-sectional view for a film which attained the terminal thickness is shown respectively in Figs. 6(a) and 6(b). The bottom black region in Fig. 6(b) corresponds to the glass substrate and the top lighter portion consists of the film and the particulate structure. The film thickness calculated using the Alpha Step and from the SEM is about 0.5 pm. The CdS particle size is about 10 to 15 pm. 3.3. IJl’uence of ammonia, TEA and TU concentrations on film morphology In the present investigation the effect of bath parameters on the film morphology was studied by setting up various chemical baths of different Cd ion to ammonia, TEA and TU mole ratios. In the present study it was observed that the particulate nature of chemically deposited CdS films depends on the bath composition. The effect of bath composition on the film morphology is shown in Figs. 7-9. It was seen that the abundance and size of the CdS particles in the film increased with increasing ammonia concentration in the chemical bath. Figures 7(a) and 7(b) correspond to the optical micrographs with magnification 500 x for films grown in chemical baths with Cd:NH, mole ratios of 1:17.28 and 1:57.6 respectively. In both the above cases the Cd:TEA:TU mole ratio remained at 1:3.75: 1. Figures 7(a) and 7(b) show that the film with a higher ammonia concentration is almost fully covered with the particles, whereas the other film contains a fewer number of particles. The ammonia concentration, in the above ratio, below 14.4, did not provide films due to the quick precipitation of CdS in the bath. The influence of TEA concentration in the bath on the film morphology is given in Figs. 8(a), 8(b) and 8(c) which correspond respectively to the Cd:TEA mole 1:3.75 and 1:5.625, with the ratios of 1:0.75, Cd:NH,:TU mole ratio (1:14.4:0.5) remaining the same
P. J. Sebastian
et al. 1 The effect of post-deposition
(4
treatments
of CdS thin films
193
(b)
(4 Fig. 5. The optical micrographs with magnification 500 x fort :hemic ally deposited CdS films with a deposition time of (a) 30 min,
@I 2 h (4
3 h and (d) 6 h.
(4
(b)
Fig. 6. The SEMs with magnification 30 000 x for CdS films (a) in the longitudinal and (b) in the transverse directions.
in all cases. Figures 8(a), 8(b) and 8(c) indicate that higher concentrations of TEA in the solution result in films with fewer and smaller particles. But above a certain minimum amount of TEA in the bath, very thin films result due to the high degree of complexing (TEA with Cd ions) in the chemical bath. The above results may be explained by considering the complex-
ing action of TEA upon Cd ions in the solution. A low concentration of TEA in the bath results in very little complex formation of TEA with Cd ions, which makes the number of free Cd ions in the bath abundant. Hence more colloidal particles of CdS may form and adhere to the substrate in the above situation.
P. J. Sebastiun et al. / The effect of post-deposition treatments of CdS thin films
(4 (4
(b) Fig. 7. The optical micrographs with magnification 500 x for CdS films with a Cd:NH, mole ratio of (a) l:l7.28 and (b) 1:57.6 in the chemical bath with the Cd:TEA:TU mole ratio remaining at 1:3.75:1.
(b)
The effect of TU concentration in the chemical bath on the particulate nature of CdS films is displayed in Figs. 9(a) -9(c). In all the above three cases the Cd:TEA:NH, mole ratio remained at 1:3.75:14.4. Figures 9(a), 9(b) and 9(c) correspond to the Cd:TU mole ratios of 1: 1, 1:2 and 1:4 respectively. As the TU concentration in the bath increased the film surface became more particulate in nature. The film surface is almost fully covered with the CdS particles at a Cd:TU mole ratio of 1:4 in the chemical bath. Only very few particles are seen on the film surface at a Cd:TU mole ratio of 1:l in the chemical bath. High TU concentration in the chemical bath makes available more free S*ions in the bath, which favour the formation of more colloidal particles of CdS in the bath and hence the film becomes more particulate in nature. The above results show that the particulate nature of the film surface is controlled by the composition of the
Cc) Fig. 8. The optical micrographs with magnification 5()O x for CdS films with a Cd:TEA mole ratio of (a) 1:0.75, (b) l:3.75 a nd ((:) 1:.5.625 in the chemical bath with a Cd:NH,:TU mole ratio of I:14 1.4:4:0.5.
P. J. Sebastian
et al. / The effect of post-deposition
treatments
of CdS thin films
195
for opto-electronic applications by the low-cost chemical deposition method. 4. Conclusion
(4
The morphology and opto-electronic properties of chemically deposited CdS thin films were investigated with respect to the chemical bath composition and postdeposition treatments. As-deposited films contained some organo-metallic impurities on the surface, which can be removed by etching of the film surface in very dilute acetic acid. Characterization of the film surface using optical and scanning electron microscopy showed that, in addition to the ion-by-ion deposition of CdS films, colloidal particles of CdS from the chemical bath are also adhering to the substrate. The particulate nature of the CdS film surface due to the colloidal deposition may be controlled by suitably setting up the bath composition. The Cd:TEA:NH, :TU mole ratios determined the particulate nature of the film surface. A Cd:TEA:NH,:TU mole ratio of about 1:3.75: 14.4:1 was found to be suitable for getting good quality films for opto-electronic applications. Acknowledgments
(b)
The authors would like to acknowledge the assistance of Miss L. Bafios in XRD, Mr. 0. Gomez-Daza in film deposition and microscopy, Mr. A. M. Fernandez in data transfer, the III-V Materials group of CINVESTAV, and the Dept. de Materiales, Metalicos y Ceramicos, II-M UNAM for SEM studies. References
(4 Fig. 9. The optical micrographs with magnification 500 x for CdS with a Cd:TU mole ratio of (a) l:l, (b) I:2 and (c) I:4 in the the bath with the Cd:TEA:NH, mole ratio remaining at 1:3.75:14.~
chemical bath. The situation may be avoided by pro1 )erly setting up the chemical bath composition. From the results of the present study it may be recommended that a Cd:TEA:NH,:TU mole ratio of about 1:3.75:14.4.:I is suitable for getting CdS thin films devoid of colic lidal particles of CdS adhering to the film. A post-deposi tion etching of the films prepared from a bath comprisin lg of the above mole ratios provides good quality CdS films
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