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Microstructure of CVD-SnO2 films
Mat. R e s . B u l l . , Vol. 25, p p . 799-806, 1990. P r i n t e d in t h e USA. 0025-5408/90 $3.00 + .00 C o p y r i g h t (c) 1990 P e r g a m o n P r e s s p l c .
MICROSTRUCTURE OF CVD-SnO2 FILMS A. Mani, N. Karuppiah and R. Mahalingam Central Electrochemical Research I n s t i t u t e Karaikudi 623 006, India ( R e c e i v e d Ap~.l 2, 1990; C o m m u n i c a t e d b y A. Wold)
ABSTRACT The photo- as well as scanning electronmicroscopic (SEM) observations of chemical vapour deposited (CVD) t i n oxide (Sn02) films are presented in this paper as part of the investigation of structure and s t r u c t u r e - r e l a t e d e l e c t r o c a t a l y t i c and transport properties of Sbdoped t i n oxide (SnO2:Sb) films. The c r y s t a l l i n e microstructures of the oxide films e x h i b i t crystallographic twins of octahedral cryst a l l i t e s of d i f f e r e n t sizes and o r i e n t a t i o n s . The deposits annealed at IO00°C for O.5h indicate "softening" of the c r y s t a l l i t e s followed by formation of "porous" nature of defects on the octahedral crystal faces. MATERIALS INDEX: t i n , oxides Introduction Stannic oxide (Sn02) is an n-type wide-band gap (3.71 eV) semiconductor (I). I t is thermally stable and has very i n t e r e s t i n g proeprties which have been harnessed for many applications (2-10). In addition to i t s potential use in ceramics and glass i n d u s t r i e s , i t has been employed as electrodes in the electro-melting of lead-glass (2) and as an e l e c t r o c a t a l y s t in origanic oxidation reactions (11,12). Although pure SnO2 has been employed as an o p t i c a l l y transparent electrode (13) f o r many electrochemical applications (14), nonstoichiometric or impurity-doped SnO2 have widely been reported as promising material in the f i e l d s of electrochemical, energy conversion and biochemical sciences. Fluorine-doped t i n oxide (SnO2:F) has been demonstrated (15) as an electrode (8 ~m thick on 5 mm glass plate) for cytochrome-c protein which is strongly adsorbed on SnO2 electrodes for the study of protein-binding mechanisms. The indium-tin-oxide (ITO) thin films have received wide i n t e r e s t due to t h e i r various technological a p p l i c a t i o n s , such as in e l e c t r o n i c display, solar c e l l s , microelectronics, etc. The SnO2 electrode, i0 to 50 ~m thick porous f i l m on a glass, quartz or titanium substrate, has been reported (14) to have many SnOH sites on i t s surface to be useful for binding reaction studies with e l e c t r o l y t e s . I t is a del i b e r a t e way to immobilise a chemical on an electrode surface so that the electrode t h e r e a f t e r displays the chemical and electrochemical properties of the immobilised molecule(s); as these confine to the electrode surface, they also 799
800
A, MANI, et al.
Vol. 25, No. 6
provide an opportunity to study the basis of electrochemical reactions. Another a t t r a c t i v e application is SnO2-based gas sensors. At a constant sensor temperature, measured e l e c t r i c a l resistance of SnO2 can be used to ident i f y and determine the concentration of ambient gaseous species in the environment. The main areas of SnO2-based gas sensors are ( i ) domestic inflammable gas-leakage detectors, ( i i ) instrument controls and automation, and ( i i i ) detection/control of smoke, toxic gases and organosolvent vapours. I t has a l ready been shown possible to fabricate a t i n oxide microsensor with an integrated polycrystalline Si heater produced on an Si-wafer substrate (3). In order to achieve the s e n s i t i v i t y and s e l e c t i v i t y of an SnO2-based gas sensor for a particular gaseous environment and improved c a t a l y t i c a c t i v i t i e s in case of electrode applications, a systematic approach to investigate structure and structure-related electrochemical, transport and optical properties is proposed. Since the method, condition and thickness of SnO2 f i l m deposition are essentially dependent parameters to define as well as to obtain reproducible results, and since crystal l a t t i c e and microstructures are fundamentally dependdent on these parameters, characterization of SnO2 structure and microstructures for t h e i r conducting as well as electrocatalytic properties becomes very essent i a l . As part of our investigation of SnO2 films obtained by chemical vapour deposition (CVD) method for use in various instrumentation applications, the present communication cites some microstructural observations of the CVD-SnO2 samples which have been characterized by EPMA. Experimental The SnO2 deposites on alumina ceramic substrate samples were prepared by chemical vapour deposition (CVD) with SnCI4.5H20 and 10 - 15 SbCl2 as starting materials. The chemical bath temperature was kept at a temperature range of 280° - 400°C and the vapour zone temperature at 740° - 1000°C. The samples were prepared at a few batches in an atmosphere of continuous flow of nitrogen gas. The f u l l details of the preparation and experimental set-up of the CVD process for SnO2 films could be referred elsewhere (16). The SnO2 films were deposited on 25mm¢ x 2mm alumina discs, as well as on cylindrical rods of size 5mm¢ x 25mm. The SnO2 surfaces on disc samples were uniformly rough while the deposits on the cylindrical rods, which were rotated during the deposition, were mechanically smooth. The photomicrographs of SnO2 deposits were taken by REICHERT Universal Camera Microscope with Vacu-Therm Microfurnace attachment, which was used to record the " i n - s i t u " high temperature observations in 10-s to 10-6 torr vacuum. The scanning electron microscopic (SEM) images at higher magnifications were recorded with as-deposited CVD-SnO2 and heat-treated samples after cooling down to ambient temperature in vacuum after the high temperature " i n - s i t u " microscopic observation. The EPMAwas carried out using X-ray wavelength dispersive spectrometer (WDS) available with JEOL Model SEM for the purpose of characterization of materials. Results and Discussion The physical appearance of the SnO2 f i l m samples were v i s i b l y black or deep-blue in colour. As-deposited CVD-SnO2 films were of 10 - 15 ~m thickness and thermally stable at least up to 1000°C, up to which the high temperature experiments were carried out. The black or deep-blue colour of the deposit was slowly changing to brown while direct heating (by passing a direct current
Vol. 25, No. 6
S T A N N I C OXIDE
801
through the film) or an indirect heating to a temperature of about 4000°C in the ambient atmosphere; i t comes back to its ini%ial black or deep-blue colour when cooled down to the i n i t i a l temperature or at zero-current. A systematic X-ray diffraction analysis is proposed to verify whether i t is the reported (I) reversible phase transition in pure-SnO2 or otherwise. The black or deep-blue colour is due to the evidence of rather "heavy" doping of Sb in SnO2. I t is reported (17) that low atomic percent doping of Sb in SnO2 gives rise to blue colour c r y s t a l l i t e s (17), the dark colour of the samples suggests the "heavy" Sb-content. The concentration of Sb to Sn in SnO2 samples has actually been measured as 8 - 10 at.% (18). Microstructures of as-deposited Sn02 films The photomicrographs and SEM images of the as-deposited CVD-SnO2 films in Fig. 1 show that the SnO2 films are crystalline. The common feature of the microstructures is the well-developed octahedral crystals, indicating (i) the preferential crystal growth along the octahedral faces, and ( i i ) the common occurrence of bicrystals and twin stacking faults. This observation may be compared with the octahedral coordination (19) of the SnO2-tetragonal crystal lattice, atomic positions and the crystal orientations normal to the crystallographic c-direction [001] as shown in Fig. 2. Figure l(a)-(d) shows the micrographs of SnO2 deposited on cermaic disc substrate with two different deposition conditions. The c r y s t a l l i t e s are of similar shape but different size and orientations. In case of cylindrical rods which were continuously rotated during the depositions, due to curvature and fine particle conditions of the circumference surface of the cylindrical substrates, simultaneous nucleation at much larger sites on the surface might have taken place and a continuous collision between the surface of the samples obstruct any further growth (beyond its c r i t i c a l thickness) in a direction perpendicular to the circumferential surface. Because of these factors, the crystal size in this case has been extremely fine, about an order of magnitude finer when compared with undisturbed disc substrates as seen from Fig. 1(c)-(e). The average c r y s t a l l i t e size measured in the case of SnO2 deposits on both cylindrical and disc substrates under different conditions are I and 10 um, respectively (18). In addition to the random orientation of uniformly deposited SnO2 crystals, there appear localised clusters of crystals (Fig. 1 ( f ) ) , which are uniformly randomly oriented from the bottom layer (in the direction perpendicular to the c-axis) to the topmost layer (appering parallel to the c-axis). I t represents the local disturbance of the experimental condition during the CVD process. This kind of non-uniform situation is not desirable for a systematic achievement of electrocatalytic and transport properties although i t is a rare observation in this system. With high temperature " i n - s i t u " microscopic experiments, the size and shape of as-deposited CVD-SnO2 crystals are unaltered up to at least 700°C. However, on reaching I000°C, the SnO2 crystals begin to soften; on prolonged vacuum annealing of SnO2 at 1000°C for about O.Sh the softening of SnO2 cryst a l l i t e s becomes clear, as seen in Fig. 3(a)-(d). Following the softening of Sn02 crystals, "porous" type of defects do appear on its octahedral faces, as seen in Fig. 3(e). Similarly, some changes (18) due to " i n - s i t u " vacuum heating (700°C, lh) of Cu metal layered SnOz (CVD-SnO2:Sb/Cu) deposits (Fig. 3(f)) have also been observed as globular secondary particles dispersed in the SnO2 crystalline matrix (18).
802
A, MANI, et al.
Vol. 25, No. 6
FIG. i Microstructures of CVD-Sn02: (a) and (b) - photomicrographs (x400) of disc samples deposited for 3 and 5 minutes, respectively; (c) and (d! ~ SEM images of (a) and (b), respectively; (e) - SEM image of Sn02 c r y s t a l l i t e 5 on ceramic c y l i n d r i c a l rods; (f) - localised multilayer crystals or clusters. l
Vol. 25, No. 6
S T A N N I C OXIDE
803
A
a)
v
L I-
a =0-473727 nm
0
b)
2xO
2xO
O. 410260nm
2
0
A / \ /
.~
<....
ts ""
c)
\
I
I~,
"->
,N,.\
\
.z
I
~\
1
~
.,,/
® FIG. 2 Schematic views of SnOz-crystal l a t t i c e : (a) the octahedral coordination of an Sn cation surrounded by six ) anions shared by three tetragonal l a t t i c e s ; (b) l o c a t i o n of the ions in the octahedron when looked down the c - a x i s ; (c) o r i e n t a ~ tions of octahedral c r y s t a l s down the c - a x i s .
I
I
Vol. 25, No. 6
S T A N N I C OXIDE
805
Conclusions 1.
The CVD-SnO2 microstructures
indicate c r y s t a l l i n e nature of deposits.
2. The octahedral shape of the CVD-SnO2 c r y s t a l l i t e s e x h i b i t d i f f e r e n t sizes with d i f f e r e n t conditions of the deposits as well as with d i f f e r e n t geometry of the substrates. 3. The physical colour change of CVD-SnO2:Sb (from deep-blue or black to brown colour) is observed on i n d i r e c t or d i r e c t heating. I t is to be v e r i fied whether i t is related to the phase t r a n s i t i o n of pure-SnO 2 reported ( i ) e a r l i e r or otherwise. 4. The observed microstructures have been explained based on the (cryst a l l o g r a p h i c ) tetragonal crystal structure and i t s orientations of SnOz. 5. The heat-treated SnO2 samples at IO00°C for O.5h show "softening" of the c r y s t a l l i t e s followed by "porous" nature of defects on the octahedral SnO2 crystallites. Acknowledgments The authors are indebted to Prof. S.K. Rangarajan, Shri Y. Mahadeva lyer and Shri K.R. Ramakrishnan for t h e i r keen i n t e r e e s t and encouragement shown during the present i n v e s t i g a t i o n . References 1.
G.V. Samanov, The Oxide Handbook, IFl/Plenum Press, New York (1973).
2.
W. Germain, H.P. Wilson and V.E. Archer, in Encyclopedia of Chemical Technology, Vol. 23, 3rd ed. (Kirth-Othamer, ed.). Wiley-lnterScience, New York (1983), p. 48.
3.
S. Karpel, Tin and Its Uses, 149, i (1986).
4.
H. l i d a , T. Mishuku, A. Itu and Y. Hayashi, IEEE Trans. Electron Devices, ED-34, 271 (1987).
5.
S. Kanafusa, M. Nitta and M. Haradome, IEEE Trans. Electron Devices, ED35, 65 (1988).
6.
S . l . Ho and K. Rajeshwar, J. Electrochem. Soc., 134, 768 (1987).
7.
J. Gutierrez, F. Cebollada, C. E l v i r a , E. Millan and J. Agapito, Mat. Chem. Phys., 18, 265 (1987).
8.
D.E. Williams and P.M. Geehin, in Electrochemistry, Vol. 9. Society of Chemistry, London (1984), chap. 6, p. 280.
9.
K. Nomura, Y. U j i h i r a , 24, 937 (1989).
10.
I. Rodriguez, J. Gonzales and Z. Velasio, J. Mat. Sci. L e t t . , 6, 1319 (1987).
11.
R.W. Murray, in Electroanalytical Chemistry, Vol. 13 (A.J. Bard, ed.). March Dekker, Inc., New York (1984), p. 228.
12.
J.L. W i l l i t
13.
J.W. Strojek and T. Kuwana, J. Electroanal. Chem., 1-6, 479 (1986).
The Royal
S.S. Sarma, A. Fueda and T. Murakami, J. Mat. Sci.,
and E.F. Bowren, J. Electroanal. Chem., 16, 471 (1986).
806
14. 15. 16. 17. 18.
19.
A. MANI, et al.
Vol. 25, No. 6
T. Kuwana and N. Vinogard, in Electroanal~tical Chemistry, Vol. 17 (A.J. Bard, ed.). Marcel Dekker, New York (1974), pp. 192, 228. G. Cartoran, P.G. Orsini, P. Scardi and R. Di Maggio, J. Mat. Sci., 23, 3156 (1988). Y.M. lyer, private communication; Annual Reports of S. Viswanathan (1982). G. Cocco, S. Enzo, G. Carturan, P.G. Orsini and P. Scardi, Mat. Chem Phys. 18, 541 (1988). Y.M. lyer, K.R. Ramakrishnan, N.U. Nayak, R. Mahalingam, N. Karuppiah and A. Mani, " ' I n - s i t u ' High Temperature Microscopic Study of SnO2:Sb/M Films" communicated to J. Mat. Sci Lett. (1989). Per Kofstad, Non-Stoichiometry, Diffusion and Electrical Conductivity in Binary Metal Oxides, Wiley-lnterscience, New York (1972), p. 132.
MICROSTRUCTURE OF CVD-SnO2 FILMS A. Mani, N. Karuppiah and R. Mahalingam Central Electrochemical Research I n s t i t u t e Karaikudi 623 006, India ( R e c e i v e d Ap~.l 2, 1990; C o m m u n i c a t e d b y A. Wold)
ABSTRACT The photo- as well as scanning electronmicroscopic (SEM) observations of chemical vapour deposited (CVD) t i n oxide (Sn02) films are presented in this paper as part of the investigation of structure and s t r u c t u r e - r e l a t e d e l e c t r o c a t a l y t i c and transport properties of Sbdoped t i n oxide (SnO2:Sb) films. The c r y s t a l l i n e microstructures of the oxide films e x h i b i t crystallographic twins of octahedral cryst a l l i t e s of d i f f e r e n t sizes and o r i e n t a t i o n s . The deposits annealed at IO00°C for O.5h indicate "softening" of the c r y s t a l l i t e s followed by formation of "porous" nature of defects on the octahedral crystal faces. MATERIALS INDEX: t i n , oxides Introduction Stannic oxide (Sn02) is an n-type wide-band gap (3.71 eV) semiconductor (I). I t is thermally stable and has very i n t e r e s t i n g proeprties which have been harnessed for many applications (2-10). In addition to i t s potential use in ceramics and glass i n d u s t r i e s , i t has been employed as electrodes in the electro-melting of lead-glass (2) and as an e l e c t r o c a t a l y s t in origanic oxidation reactions (11,12). Although pure SnO2 has been employed as an o p t i c a l l y transparent electrode (13) f o r many electrochemical applications (14), nonstoichiometric or impurity-doped SnO2 have widely been reported as promising material in the f i e l d s of electrochemical, energy conversion and biochemical sciences. Fluorine-doped t i n oxide (SnO2:F) has been demonstrated (15) as an electrode (8 ~m thick on 5 mm glass plate) for cytochrome-c protein which is strongly adsorbed on SnO2 electrodes for the study of protein-binding mechanisms. The indium-tin-oxide (ITO) thin films have received wide i n t e r e s t due to t h e i r various technological a p p l i c a t i o n s , such as in e l e c t r o n i c display, solar c e l l s , microelectronics, etc. The SnO2 electrode, i0 to 50 ~m thick porous f i l m on a glass, quartz or titanium substrate, has been reported (14) to have many SnOH sites on i t s surface to be useful for binding reaction studies with e l e c t r o l y t e s . I t is a del i b e r a t e way to immobilise a chemical on an electrode surface so that the electrode t h e r e a f t e r displays the chemical and electrochemical properties of the immobilised molecule(s); as these confine to the electrode surface, they also 799
800
A, MANI, et al.
Vol. 25, No. 6
provide an opportunity to study the basis of electrochemical reactions. Another a t t r a c t i v e application is SnO2-based gas sensors. At a constant sensor temperature, measured e l e c t r i c a l resistance of SnO2 can be used to ident i f y and determine the concentration of ambient gaseous species in the environment. The main areas of SnO2-based gas sensors are ( i ) domestic inflammable gas-leakage detectors, ( i i ) instrument controls and automation, and ( i i i ) detection/control of smoke, toxic gases and organosolvent vapours. I t has a l ready been shown possible to fabricate a t i n oxide microsensor with an integrated polycrystalline Si heater produced on an Si-wafer substrate (3). In order to achieve the s e n s i t i v i t y and s e l e c t i v i t y of an SnO2-based gas sensor for a particular gaseous environment and improved c a t a l y t i c a c t i v i t i e s in case of electrode applications, a systematic approach to investigate structure and structure-related electrochemical, transport and optical properties is proposed. Since the method, condition and thickness of SnO2 f i l m deposition are essentially dependent parameters to define as well as to obtain reproducible results, and since crystal l a t t i c e and microstructures are fundamentally dependdent on these parameters, characterization of SnO2 structure and microstructures for t h e i r conducting as well as electrocatalytic properties becomes very essent i a l . As part of our investigation of SnO2 films obtained by chemical vapour deposition (CVD) method for use in various instrumentation applications, the present communication cites some microstructural observations of the CVD-SnO2 samples which have been characterized by EPMA. Experimental The SnO2 deposites on alumina ceramic substrate samples were prepared by chemical vapour deposition (CVD) with SnCI4.5H20 and 10 - 15 SbCl2 as starting materials. The chemical bath temperature was kept at a temperature range of 280° - 400°C and the vapour zone temperature at 740° - 1000°C. The samples were prepared at a few batches in an atmosphere of continuous flow of nitrogen gas. The f u l l details of the preparation and experimental set-up of the CVD process for SnO2 films could be referred elsewhere (16). The SnO2 films were deposited on 25mm¢ x 2mm alumina discs, as well as on cylindrical rods of size 5mm¢ x 25mm. The SnO2 surfaces on disc samples were uniformly rough while the deposits on the cylindrical rods, which were rotated during the deposition, were mechanically smooth. The photomicrographs of SnO2 deposits were taken by REICHERT Universal Camera Microscope with Vacu-Therm Microfurnace attachment, which was used to record the " i n - s i t u " high temperature observations in 10-s to 10-6 torr vacuum. The scanning electron microscopic (SEM) images at higher magnifications were recorded with as-deposited CVD-SnO2 and heat-treated samples after cooling down to ambient temperature in vacuum after the high temperature " i n - s i t u " microscopic observation. The EPMAwas carried out using X-ray wavelength dispersive spectrometer (WDS) available with JEOL Model SEM for the purpose of characterization of materials. Results and Discussion The physical appearance of the SnO2 f i l m samples were v i s i b l y black or deep-blue in colour. As-deposited CVD-SnO2 films were of 10 - 15 ~m thickness and thermally stable at least up to 1000°C, up to which the high temperature experiments were carried out. The black or deep-blue colour of the deposit was slowly changing to brown while direct heating (by passing a direct current
Vol. 25, No. 6
S T A N N I C OXIDE
801
through the film) or an indirect heating to a temperature of about 4000°C in the ambient atmosphere; i t comes back to its ini%ial black or deep-blue colour when cooled down to the i n i t i a l temperature or at zero-current. A systematic X-ray diffraction analysis is proposed to verify whether i t is the reported (I) reversible phase transition in pure-SnO2 or otherwise. The black or deep-blue colour is due to the evidence of rather "heavy" doping of Sb in SnO2. I t is reported (17) that low atomic percent doping of Sb in SnO2 gives rise to blue colour c r y s t a l l i t e s (17), the dark colour of the samples suggests the "heavy" Sb-content. The concentration of Sb to Sn in SnO2 samples has actually been measured as 8 - 10 at.% (18). Microstructures of as-deposited Sn02 films The photomicrographs and SEM images of the as-deposited CVD-SnO2 films in Fig. 1 show that the SnO2 films are crystalline. The common feature of the microstructures is the well-developed octahedral crystals, indicating (i) the preferential crystal growth along the octahedral faces, and ( i i ) the common occurrence of bicrystals and twin stacking faults. This observation may be compared with the octahedral coordination (19) of the SnO2-tetragonal crystal lattice, atomic positions and the crystal orientations normal to the crystallographic c-direction [001] as shown in Fig. 2. Figure l(a)-(d) shows the micrographs of SnO2 deposited on cermaic disc substrate with two different deposition conditions. The c r y s t a l l i t e s are of similar shape but different size and orientations. In case of cylindrical rods which were continuously rotated during the depositions, due to curvature and fine particle conditions of the circumference surface of the cylindrical substrates, simultaneous nucleation at much larger sites on the surface might have taken place and a continuous collision between the surface of the samples obstruct any further growth (beyond its c r i t i c a l thickness) in a direction perpendicular to the circumferential surface. Because of these factors, the crystal size in this case has been extremely fine, about an order of magnitude finer when compared with undisturbed disc substrates as seen from Fig. 1(c)-(e). The average c r y s t a l l i t e size measured in the case of SnO2 deposits on both cylindrical and disc substrates under different conditions are I and 10 um, respectively (18). In addition to the random orientation of uniformly deposited SnO2 crystals, there appear localised clusters of crystals (Fig. 1 ( f ) ) , which are uniformly randomly oriented from the bottom layer (in the direction perpendicular to the c-axis) to the topmost layer (appering parallel to the c-axis). I t represents the local disturbance of the experimental condition during the CVD process. This kind of non-uniform situation is not desirable for a systematic achievement of electrocatalytic and transport properties although i t is a rare observation in this system. With high temperature " i n - s i t u " microscopic experiments, the size and shape of as-deposited CVD-SnO2 crystals are unaltered up to at least 700°C. However, on reaching I000°C, the SnO2 crystals begin to soften; on prolonged vacuum annealing of SnO2 at 1000°C for about O.Sh the softening of SnO2 cryst a l l i t e s becomes clear, as seen in Fig. 3(a)-(d). Following the softening of Sn02 crystals, "porous" type of defects do appear on its octahedral faces, as seen in Fig. 3(e). Similarly, some changes (18) due to " i n - s i t u " vacuum heating (700°C, lh) of Cu metal layered SnOz (CVD-SnO2:Sb/Cu) deposits (Fig. 3(f)) have also been observed as globular secondary particles dispersed in the SnO2 crystalline matrix (18).
802
A, MANI, et al.
Vol. 25, No. 6
FIG. i Microstructures of CVD-Sn02: (a) and (b) - photomicrographs (x400) of disc samples deposited for 3 and 5 minutes, respectively; (c) and (d! ~ SEM images of (a) and (b), respectively; (e) - SEM image of Sn02 c r y s t a l l i t e 5 on ceramic c y l i n d r i c a l rods; (f) - localised multilayer crystals or clusters. l
Vol. 25, No. 6
S T A N N I C OXIDE
803
A
a)
v
L I-
a =0-473727 nm
0
b)
2xO
2xO
O. 410260nm
2
0
A / \ /
.~
<....
ts ""
c)
\
I
I~,
"->
,N,.\
\
.z
I
~\
1
~
.,,/
® FIG. 2 Schematic views of SnOz-crystal l a t t i c e : (a) the octahedral coordination of an Sn cation surrounded by six ) anions shared by three tetragonal l a t t i c e s ; (b) l o c a t i o n of the ions in the octahedron when looked down the c - a x i s ; (c) o r i e n t a ~ tions of octahedral c r y s t a l s down the c - a x i s .
I
I
Vol. 25, No. 6
S T A N N I C OXIDE
805
Conclusions 1.
The CVD-SnO2 microstructures
indicate c r y s t a l l i n e nature of deposits.
2. The octahedral shape of the CVD-SnO2 c r y s t a l l i t e s e x h i b i t d i f f e r e n t sizes with d i f f e r e n t conditions of the deposits as well as with d i f f e r e n t geometry of the substrates. 3. The physical colour change of CVD-SnO2:Sb (from deep-blue or black to brown colour) is observed on i n d i r e c t or d i r e c t heating. I t is to be v e r i fied whether i t is related to the phase t r a n s i t i o n of pure-SnO 2 reported ( i ) e a r l i e r or otherwise. 4. The observed microstructures have been explained based on the (cryst a l l o g r a p h i c ) tetragonal crystal structure and i t s orientations of SnOz. 5. The heat-treated SnO2 samples at IO00°C for O.5h show "softening" of the c r y s t a l l i t e s followed by "porous" nature of defects on the octahedral SnO2 crystallites. Acknowledgments The authors are indebted to Prof. S.K. Rangarajan, Shri Y. Mahadeva lyer and Shri K.R. Ramakrishnan for t h e i r keen i n t e r e e s t and encouragement shown during the present i n v e s t i g a t i o n . References 1.
G.V. Samanov, The Oxide Handbook, IFl/Plenum Press, New York (1973).
2.
W. Germain, H.P. Wilson and V.E. Archer, in Encyclopedia of Chemical Technology, Vol. 23, 3rd ed. (Kirth-Othamer, ed.). Wiley-lnterScience, New York (1983), p. 48.
3.
S. Karpel, Tin and Its Uses, 149, i (1986).
4.
H. l i d a , T. Mishuku, A. Itu and Y. Hayashi, IEEE Trans. Electron Devices, ED-34, 271 (1987).
5.
S. Kanafusa, M. Nitta and M. Haradome, IEEE Trans. Electron Devices, ED35, 65 (1988).
6.
S . l . Ho and K. Rajeshwar, J. Electrochem. Soc., 134, 768 (1987).
7.
J. Gutierrez, F. Cebollada, C. E l v i r a , E. Millan and J. Agapito, Mat. Chem. Phys., 18, 265 (1987).
8.
D.E. Williams and P.M. Geehin, in Electrochemistry, Vol. 9. Society of Chemistry, London (1984), chap. 6, p. 280.
9.
K. Nomura, Y. U j i h i r a , 24, 937 (1989).
10.
I. Rodriguez, J. Gonzales and Z. Velasio, J. Mat. Sci. L e t t . , 6, 1319 (1987).
11.
R.W. Murray, in Electroanalytical Chemistry, Vol. 13 (A.J. Bard, ed.). March Dekker, Inc., New York (1984), p. 228.
12.
J.L. W i l l i t
13.
J.W. Strojek and T. Kuwana, J. Electroanal. Chem., 1-6, 479 (1986).
The Royal
S.S. Sarma, A. Fueda and T. Murakami, J. Mat. Sci.,
and E.F. Bowren, J. Electroanal. Chem., 16, 471 (1986).
806
14. 15. 16. 17. 18.
19.
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