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Morphology of electrochemical vapor deposited yttria-stabilized zirconia thin films
Solid State Ionics 37 (1990) 197-202 North-Holland
MORPHOLOGY OF ELECTROCHEMICAL VAPOR DEPOSITED YTTRIA-STABILIZED ZIRCONIA THIN FILMS Michael F. C A R O L A N ~ a n d J a m e s N. M I C H A E L S Department of Chemical Engineering, Universityof California Berkeley, Berkeley, CA 94720, USA Received 18 August 1989; accepted for publication 23 August 1989
Yttria-stabilized zirconia films were deposited over alumina substrates by electrochemical vapor deposition (EVD). It was observed that the films grew with the same yttrium-to-zirconium ratio as in the reactant gases. Films deposited at temperatures of 1075°C or below have a highly faceted surface and show a preferred crystallographic orientation. Films deposited at a temperature of 1100°C show a nearly smooth surface and no crystallographic orientation. The difference in morphology between the high temperature and lower temperature films can be explained by either a change in the relative rates of film growth and surface reconstruction or, more likely, a mobile surface species becoming thermodynamically unstable at the higher temperature.
1. Introduction Yttria-stabilized zirconia ( Y S Z ) is the preferred electrolyte for high t e m p e r a t u r e solid oxide electrolyte fuel cells a n d steam electrolyzers due to its low cost, chemical stability, a n d wide electrolytic range. To m i n i m i z e entropic losses in electrochemical devices, it is desirable to m a k e the electrolyte as thin as possible; electrolyte thicknesses o f 10-50 ~tm have been described [ 1 ]. O f the thin film technologies previously investigated, only r a d i o frequency ( R . F . ) sputtering [2,3 ] a n d electrochemical v a p o r deposition ( E V D ) [4,5] are capable o f p r o d u c i n g sufficiently thin, dense, gas-tight films o f YSZ over porous substrates. The growth rates o f the R.F. sputtered films are exceedingly slow however, typically less than l ~tm/h [2,3]. The growth rates o f the E V D films are at least an o r d e r o f m a g n i t u d e greater [4]. Therefore, electrochemical v a p o r d e p o s i t i o n is the leading technology to deposit thin, gas tight layers o f solid electrolyte over porous substrates. The kinetics a n d m e c h a n i s m o f electrochemically v a p o r - d e p o s i t e d Y S Z films are discussed in a companion p a p e r [6]. In the course o f that investigaAuthor to whom all correspondence should be addressed. Present address: Air Products and Chemicals, Inc., Box 2842, Lehigh Valley, PA 18001, USA.
tion, we noted that films grew with m a r k e d l y different morphologies at different temperatures. In this paper, we explore the effect o f film c o m p o s i t i o n a n d growth t e m p e r a t u r e on film texture. The e x p e r i m e n t a l a p p a r a t u s used to deposit YSZ films on porous supports has been described elsewhere [ 7 ]. A m o d e l substrate was used in these experiments, consisting o f porous alumina, with a n o m i n a l pore d i a m e t e r o f 2 ~tm over a coarser alum i n a substrate with a n o m i n a l pore d i a m e t e r o f 30 ~tm. The fine a l u m i n a layer was 100 ~tm thick a n d the coarse substrate was 3 m m thick. Films were grown at 1000°C, 1050°C, 1075°C and l 100 ° C, total pressures o f 1.1 Torr, ZrCl4 and YCI3 partial pressures o f 0.1 Torr and 0.018 Torr respectively, a n d helium carrier gas flow rates o f 0.4 cc ( S T P ) / s . A 4 0 : 6 0 mixture o f s t e a m / h y d r o g e n was used. F i l m texture was observed with a ISI WB-6 scanning electron microscope. F i l m thickness was determ i n e d from micrographs o f cross sections o f the dep o s i t e d films. X-ray diffraction yielded i n f o r m a t i o n on the phase and the lattice p a r a m e t e r o f the electrolyte. Using the correlations o f Ingel and Lewis [ 8 ], the c o m p o s i t i o n o f the film was d e t e r m i n e d from the lattice parameter.
M.F. Carolan, J.N. Michaels / Yttria-stabilized zirconia thin films
198
2. Results and discussion
reaction probability of chloride oxidation by oxygen ions at the growing face of the YSZ film is unity.
2.1. Film composition 2.2. Morphology The ionic conductivity of YSZ is a very strong function of the yttria content, with the m a x i m u m conductivity occurring for an electrolyte composition of 9 mole% Y203- In addition, at least 9 mole% ytrria is necessary to fully stabilize zirconia in the cubic phase. Therefore, it is necessary to control precisely the composition of the EVD films. To determine how film composition varies with reactant concentration, the ratio of YCI3 to total metal chlorides in the vapor phase was varied Systematically and the resulting composition of the deposited film measured. Fig. 1 presents the observed dependence of the film composition on the gas phase metal chloride ratio. The deposition temperature was 1100°C and the total metal chloride concentration was 0.25 Torr. The line drawn represents an equal mole fraction of yttrium in the film and in the reactant gases. The data fall very near this line indicating that the film grows at the same yttrium-to-zirconium ratio as is present in the vapor phase. Similarly, films deposited at temperatures at and below 1075 °C grow at the same yttrium-to-zirconium ratio as is present in the vapor phase. Doubling the total chloride flow rate did not effect the film composition. Since film growth is limited by electron transport in the growing film [6 ], this unusual coincidence probably indicates that the 0.3 1373 K 1.1 tort
._~ .c
0.2
&
i
I °1 / 0.0 0.0
' ' 0.1 0.2 0.3 gos-phose yttrium mole froction
Fig. 1. Dependence of film composition on reactor vapor phase composition.
There is a dramatic difference in the surface texture of films grown at different temperatures. Films grown at 1075°C and below are highly faceted. This is clearly shown in fig. 2a, which is a scanning electron micrograph of a film grown at 1050°C. The film is approximately 4 ~tm thick and the facets are 1 pm or smaller on a side. In contrast, films grown at 1100°C are smooth and nearly featureless as shown by the micrograph in fig. 2b. Films grown at all temperatures are dense and show no signs of porosity. The faceting effect is independent of film compositions over the range investigated, 3 to 18 mole% Y203. It is also independent of film thicknesses from 2-27 om. The features of the smooth films grown at 1100 °C are also independent of film thickness and composition. Faceting has also been observed in EVD grown films of Sn doped In203 [ 9 ]. The faceting of these films is surprising since film growth is limited by transport through the growing films, a process which should be isotropic. As shown schematically in fig. 3, film growth occurs by counter diffusion of oxygen anions and electrons. The gradient in oxygen activity and the oxygen ion flux are largest where the film is thinnest; this implies that the films should grow fastest where they are thinnest. Hence, films grown by EVD should be self-leveling and therefore smooth and featureless. In fig. 4 the X-ray diffraction patterns of yttria stabilized zirconia powder and representative films are compared. Fig. 4a shows an X-ray pattern of 10 mole% yttria-stabilized zirconia powder, while fig. 4b shows the X-ray pattern of a 15 ~tm thick film deposited at 1100°C. The relative peak amplitudes in these two figures are equal, indicating that the film grew isotropically and exhibited no preferred orientation. In contrast, fig. 4c shows an X-ray pattern of a 27 ~m thick film deposited at 1050 ° C. The intensity of the ( 2 2 0 ) peak is enhanced, demonstrating that this film is oriented. For the C l crystal structure, the first peak in the ( 1 1 0 ) group is the ( 2 2 0 ) line. Since the ( 2 2 0 ) peak intensity is enhanced, (110) planes must be oriented parallel to the sub-
Fig. 2. Scanning electron micrographs of the surface of a film deposited at (a) 1050°C and (b) 1100°C.
200
M.F. Carolan. J.N. Michaels / Yttria-stabilized zirconia thin films ZrCI 4 / YCI a
/\ P
•
'oe Ooe H20 / H 2
Fig. 3. Schematic of counter diffusion of oxygen and electrons through a faceted film showing the longer diffusion path at the thicker sectionsof the film. (111)
(a) (22o) (331) (3u) ( 4 2 ~
I
(0)
(c)
;o
50 2e
I
30
(deg}
Fig. 4. X-ray diffraction patterns of (a) 10 mole% yttria-stabilized zireonia powder; (b) a 15 Ima film deposited at 1100°C; and (c) a 27 I~mfilm deposited at 1050°C. strate. This means that the film grew preferentially in the ( 1 1 0 ) direction. All films grown at 1075°C and below exhibit this type of orientation. Again a similar effect is observed in films of Sn doped In203 grown by the EVD method; these Trims grew preferentially in the ( 111 > direction [ 9 ]. Interestingly, vapor deposited T1203 crystals, which are isostructural with In203, also grow fastest in the ( 111 > direction [ 10 ].
The faceting and orientation of the YSZ films indicates that a second transport process occurs in parallel with oxygen anion and electron diffusion through the film. Because faceting exposes low index planes with low surface energies in preference to high surface energy, high index planes, this process is thermodynamically driven by minimization of surface free energy. The mechanism to transform higher energy planes to lower energy planes requires a diffusional route for both anions and cations; this route is often surface diffusion [ 11 ]. It is often observed in the growth of crystals in an isotropic growth medium that certain crystalline directions will grow faster than others. For example, YSZ crystals grown by the skull melting process grow fastest in the ( 1 1 0 ) direction [ 12 ]. When the crystal nuclei are on a surface, the process of "evolutionary selection", described by van der Drift [ 13 ], may occur. Those crystals with the fastest growing direction oriented perpendicular to the substrate grow fastest. These crystals eventually overwhelm those crystals oriented with a component of the fastest growing direction parallel to the substrate. Thus, as a polycrystalline film grows thicker, the film exhibits columnar growth with the orientation of the crystals at the growing surface becoming increasingly parallel to each other. This growth mechanism should produce YSZ films oriented in the ( 110 ) direction, in agreement with films grown at 1075°C and below. Surface diffusion is necessary for evolutionary selection to occur: adsorbed species must be able to diffuse from the slower to the faster growing crystal faces. Thus, surface diffusion is required for both faceting and oriented growth to occur. The final morphology of a film deposited by EVD, therefore, is a product of two competing processes. Film growth is limited by bulk charge transport, a mechanism which favors smooth and unoriented films. Surface diffusion of reactive species occurs simultaneously and this promotes faceting and orientation. If bulk transport is very much faster than surface transport, smooth unoriented films should result. Oriented and faceted films should result when surface diffusion is the faster process. The observed transition from faceted, oriented films grown at 1075°C to smooth films grown at 1100°C indicates that the relative rates of bulk and surface diffusion must be extremely temperature sensitive.
M.F. Carolan, J.N. Michaels / Yttria-stabilized zirconia thin films
One possible explanation for the apparent temperature sensitivity of EVD film morphology can be found by considering the temperature-dependence of the film growth rate and surface diffusion rate independently. Film growth is limited by electron transport through the electrolyte and has an activation energy of 3.9 eV [6]. It can be shown that at a given time, the temperature dependence of the growth rate of the film, r, is: roc N / T e x p [ - 3"9 e V j] ,
(6)
where Tis the absolute temperature and R is the ideal gas constant [6 ]. We cannot predict the activation energy of the surface diffusional flux, but we note that this flux often decreases exponentially with increasing temperature. To show this, we start with Ficks law to describe the surface flux, Ns: (7)
N~ = - D ~ V C ~ ,
where D~ is the surface diffusivity and C~ is the surface concentration. Surface diffusion is an activated process, therefore Ds can be described by an Arrhenius expression:
o,
1
where E~ is the activation energy for diffusion and A' is a preexponential factor. On the other hand, the surface concentration of adsorbate should vary with temperature according to van't Hoff equation din [ c ~ ] / d T = R2All T ,
(9)
where ~ is the heat of adsorption and Q is the gas phase concentration of the adsorbing species. Thus, the surface flux varies with temperature according to N s o c T - I exp [,-- ( A H + E , ) ] RT A"
(10)
The activation energy for surface diffusivity is a positive number, typically on the order of a few kilocalories per mole [ 14 ]. Because the heat of adsorption is usually negative, the apparent activation energy of the surface diffusion is often negative. Therefore, the rate of surface diffusion can decrease exponentially with increasing temperature.
201
Since the reaction rate increases exponentially with temperature, while surface diffusion decreases exponentially with temperature, it is possible that different morphologies can be exhibited over a small temperature range. The magnitude of the heat of adsorption must be fairly large however. If the heat of adsorption has a magnitude of 40 kcal/mole, the ratio of the surface diffusion rate to the reaction rate, Ns/r, increases only by a factor of 1.8 between 1075°C and 1100°C. If the heat of adsorption was increased to 100 kcal/mole, this ratio only increases by a factor of 2.8 between 1075°C and 1100°C. It is probably reasonable to assume that a I 0-fold increase in the ratio between 1075°C and 1100°C is required to explain the transition from faceted to smooth films. Therefore, this explanation appears to require an unreasonably large value of the heat of adsorption. A more likely explanation is that the mobile surface species that allows faceting and oriented growth to occur might become thermodynamically unstable between 1075°C and I100°C. At the higher temperature no mechanism would be available to allow surface transport of a metal species. This phenomenon has been observed in the epitaxial chemical vapor deposition of silicon on ( 111 ) silicon substrates using SiCI3H and the etching of ( 111 ) Si substrates with gaseous HC1 [ 15 ]. At lower temperatures, the surface of the deposited or etched silicon is highly faceted with low index exposed faces. At higher temperatures, the surface is smooth. The activation energy for the transition from the faceted growth regime to the smooth growth growth regime corresponds well with the heat of formation of a surface SiCI2 species. The morphology of the lower temperature films could prove to be beneficial. The highly faceted surface has a higher surface area and a rougher surface than the smooth films. Adhesion of electrodes to such a film could be superior to adhesion to smooth films. In addition, grain boundaries in oriented films tend to be perpendicular to the surface of the film, and therefore parallel to the direction of anion conduction in electrochemical devices. The effect of impurity phases at the grain boundaries would be greatly reduced when the conduction path does not cross the grain boundaries.
202
M.F. Carolan, J.N. Michaels / Yttria-stabilized zirconia thin films
3. C o n c l u s i o n s Y t t r i a - s t a b i l i z e d z i r c o n i a films were g r o w n b y the electrochemical v a p o r d e p o s i t i o n m e t h o d at t e m p e r a t u r e s r a n g i n g f r o m 1 0 0 0 ° C to 1 1 0 0 ° C . F o r films g r o w n at l 100 ° C, the y t t r i u m - t o - z i r c o n i u m ratio i n the film e q u a l e d that in the r e a c t a n t gases. F i l m s grown b e l o w 1075 ° C were polycrystalline a n d grew preferentially in the { 1 1 0 ) direction. T h e y were also highly faceted. F i l m s g r o w n at 1 1 0 0 ° C were polycrystalline, u n o r i e n t e d a n d h a d a s m o o t h surface texture. T h i s difference in m o r p h o l o g y c a n be exp l a i n e d b y either the relative r e a c t i o n rate o f film growth to surface r e c o n s t r u c t i o n or b y a m o b i l e surface species b e c o m i n g t h e r m o d y n a m i c a l l y u n s t a b l e at the higher t e m p e r a t u r e .
Acknowledgement T h e a u t h o r s gratefully a c k n o w l e d g e the s u p p o r t o f the N a t i o n a l Science F o u n d a t i o n , grant n u m b e r CBT-8502407.
References [ 1 ] A.O. Isenberg, Solid State Ionics 3/4 ( 1981 ) 443. [2] T.L. Markin, R.J. Bones and R.M. Dell, in: Superionic conductors, eds. G. Mahan and W. Roth (Plenum Press, New York, 1976) pp. 15-35. [3]A. Negishi, K. Nozaki and T. Ozawa, Solid State Ionics 3/4 (1981) 443. [4] A.O. Isenberg, Proc. Electrochem. Soc. 77-6 (1977) 572. [ 5 ] G. Dietrich and W. Schafer, Intern. J. Hydrogen Energy, 9 (1984) 747. [6] M.F. Carolan and J.N. Michaels, Solid State Ionics 37 (1990) 189. [7] M.F. Carolan and J.N. Michaels, Solid State Ionics 25 (1987) 207. [8 ] R.P. Ingel and D. Lewis III, J. Am. Ceram. Soc. 69 (1986) 325. [ 9 ] J.H. Enloe and G.P. Wirtz, J. Electrochem. SOc. 133 (1986) 1583. [ 10] G.P. Wirtz and D.C. Seibert, J. Cryst. Growth 32 (1976) 274. [ 11 ] A.A. Chernov, Modern crystallography III, crystal growth (Springer, Berlin, 1984). [ 12] D.B. Zhang, X.M. He, J.P. Chen, J.C. Wang, Y.F. Tang and B.L. Hu, J. Crystal Growth 79 (1986) 336. [ 13] A. van der Drift, Philips Res. Repts. 22 (1967) 267. [ 14] J.M. Smith, Chemical engineering kinetics (McGraw-Hill, New York, 1981 ) p. 470. [ 15 ] J. Burmeister, J. Crystal Growth 11 ( 1971 ) 13.
MORPHOLOGY OF ELECTROCHEMICAL VAPOR DEPOSITED YTTRIA-STABILIZED ZIRCONIA THIN FILMS Michael F. C A R O L A N ~ a n d J a m e s N. M I C H A E L S Department of Chemical Engineering, Universityof California Berkeley, Berkeley, CA 94720, USA Received 18 August 1989; accepted for publication 23 August 1989
Yttria-stabilized zirconia films were deposited over alumina substrates by electrochemical vapor deposition (EVD). It was observed that the films grew with the same yttrium-to-zirconium ratio as in the reactant gases. Films deposited at temperatures of 1075°C or below have a highly faceted surface and show a preferred crystallographic orientation. Films deposited at a temperature of 1100°C show a nearly smooth surface and no crystallographic orientation. The difference in morphology between the high temperature and lower temperature films can be explained by either a change in the relative rates of film growth and surface reconstruction or, more likely, a mobile surface species becoming thermodynamically unstable at the higher temperature.
1. Introduction Yttria-stabilized zirconia ( Y S Z ) is the preferred electrolyte for high t e m p e r a t u r e solid oxide electrolyte fuel cells a n d steam electrolyzers due to its low cost, chemical stability, a n d wide electrolytic range. To m i n i m i z e entropic losses in electrochemical devices, it is desirable to m a k e the electrolyte as thin as possible; electrolyte thicknesses o f 10-50 ~tm have been described [ 1 ]. O f the thin film technologies previously investigated, only r a d i o frequency ( R . F . ) sputtering [2,3 ] a n d electrochemical v a p o r deposition ( E V D ) [4,5] are capable o f p r o d u c i n g sufficiently thin, dense, gas-tight films o f YSZ over porous substrates. The growth rates o f the R.F. sputtered films are exceedingly slow however, typically less than l ~tm/h [2,3]. The growth rates o f the E V D films are at least an o r d e r o f m a g n i t u d e greater [4]. Therefore, electrochemical v a p o r d e p o s i t i o n is the leading technology to deposit thin, gas tight layers o f solid electrolyte over porous substrates. The kinetics a n d m e c h a n i s m o f electrochemically v a p o r - d e p o s i t e d Y S Z films are discussed in a companion p a p e r [6]. In the course o f that investigaAuthor to whom all correspondence should be addressed. Present address: Air Products and Chemicals, Inc., Box 2842, Lehigh Valley, PA 18001, USA.
tion, we noted that films grew with m a r k e d l y different morphologies at different temperatures. In this paper, we explore the effect o f film c o m p o s i t i o n a n d growth t e m p e r a t u r e on film texture. The e x p e r i m e n t a l a p p a r a t u s used to deposit YSZ films on porous supports has been described elsewhere [ 7 ]. A m o d e l substrate was used in these experiments, consisting o f porous alumina, with a n o m i n a l pore d i a m e t e r o f 2 ~tm over a coarser alum i n a substrate with a n o m i n a l pore d i a m e t e r o f 30 ~tm. The fine a l u m i n a layer was 100 ~tm thick a n d the coarse substrate was 3 m m thick. Films were grown at 1000°C, 1050°C, 1075°C and l 100 ° C, total pressures o f 1.1 Torr, ZrCl4 and YCI3 partial pressures o f 0.1 Torr and 0.018 Torr respectively, a n d helium carrier gas flow rates o f 0.4 cc ( S T P ) / s . A 4 0 : 6 0 mixture o f s t e a m / h y d r o g e n was used. F i l m texture was observed with a ISI WB-6 scanning electron microscope. F i l m thickness was determ i n e d from micrographs o f cross sections o f the dep o s i t e d films. X-ray diffraction yielded i n f o r m a t i o n on the phase and the lattice p a r a m e t e r o f the electrolyte. Using the correlations o f Ingel and Lewis [ 8 ], the c o m p o s i t i o n o f the film was d e t e r m i n e d from the lattice parameter.
M.F. Carolan, J.N. Michaels / Yttria-stabilized zirconia thin films
198
2. Results and discussion
reaction probability of chloride oxidation by oxygen ions at the growing face of the YSZ film is unity.
2.1. Film composition 2.2. Morphology The ionic conductivity of YSZ is a very strong function of the yttria content, with the m a x i m u m conductivity occurring for an electrolyte composition of 9 mole% Y203- In addition, at least 9 mole% ytrria is necessary to fully stabilize zirconia in the cubic phase. Therefore, it is necessary to control precisely the composition of the EVD films. To determine how film composition varies with reactant concentration, the ratio of YCI3 to total metal chlorides in the vapor phase was varied Systematically and the resulting composition of the deposited film measured. Fig. 1 presents the observed dependence of the film composition on the gas phase metal chloride ratio. The deposition temperature was 1100°C and the total metal chloride concentration was 0.25 Torr. The line drawn represents an equal mole fraction of yttrium in the film and in the reactant gases. The data fall very near this line indicating that the film grows at the same yttrium-to-zirconium ratio as is present in the vapor phase. Similarly, films deposited at temperatures at and below 1075 °C grow at the same yttrium-to-zirconium ratio as is present in the vapor phase. Doubling the total chloride flow rate did not effect the film composition. Since film growth is limited by electron transport in the growing film [6 ], this unusual coincidence probably indicates that the 0.3 1373 K 1.1 tort
._~ .c
0.2
&
i
I °1 / 0.0 0.0
' ' 0.1 0.2 0.3 gos-phose yttrium mole froction
Fig. 1. Dependence of film composition on reactor vapor phase composition.
There is a dramatic difference in the surface texture of films grown at different temperatures. Films grown at 1075°C and below are highly faceted. This is clearly shown in fig. 2a, which is a scanning electron micrograph of a film grown at 1050°C. The film is approximately 4 ~tm thick and the facets are 1 pm or smaller on a side. In contrast, films grown at 1100°C are smooth and nearly featureless as shown by the micrograph in fig. 2b. Films grown at all temperatures are dense and show no signs of porosity. The faceting effect is independent of film compositions over the range investigated, 3 to 18 mole% Y203. It is also independent of film thicknesses from 2-27 om. The features of the smooth films grown at 1100 °C are also independent of film thickness and composition. Faceting has also been observed in EVD grown films of Sn doped In203 [ 9 ]. The faceting of these films is surprising since film growth is limited by transport through the growing films, a process which should be isotropic. As shown schematically in fig. 3, film growth occurs by counter diffusion of oxygen anions and electrons. The gradient in oxygen activity and the oxygen ion flux are largest where the film is thinnest; this implies that the films should grow fastest where they are thinnest. Hence, films grown by EVD should be self-leveling and therefore smooth and featureless. In fig. 4 the X-ray diffraction patterns of yttria stabilized zirconia powder and representative films are compared. Fig. 4a shows an X-ray pattern of 10 mole% yttria-stabilized zirconia powder, while fig. 4b shows the X-ray pattern of a 15 ~tm thick film deposited at 1100°C. The relative peak amplitudes in these two figures are equal, indicating that the film grew isotropically and exhibited no preferred orientation. In contrast, fig. 4c shows an X-ray pattern of a 27 ~m thick film deposited at 1050 ° C. The intensity of the ( 2 2 0 ) peak is enhanced, demonstrating that this film is oriented. For the C l crystal structure, the first peak in the ( 1 1 0 ) group is the ( 2 2 0 ) line. Since the ( 2 2 0 ) peak intensity is enhanced, (110) planes must be oriented parallel to the sub-
Fig. 2. Scanning electron micrographs of the surface of a film deposited at (a) 1050°C and (b) 1100°C.
200
M.F. Carolan. J.N. Michaels / Yttria-stabilized zirconia thin films ZrCI 4 / YCI a
/\ P
•
'oe Ooe H20 / H 2
Fig. 3. Schematic of counter diffusion of oxygen and electrons through a faceted film showing the longer diffusion path at the thicker sectionsof the film. (111)
(a) (22o) (331) (3u) ( 4 2 ~
I
(0)
(c)
;o
50 2e
I
30
(deg}
Fig. 4. X-ray diffraction patterns of (a) 10 mole% yttria-stabilized zireonia powder; (b) a 15 Ima film deposited at 1100°C; and (c) a 27 I~mfilm deposited at 1050°C. strate. This means that the film grew preferentially in the ( 1 1 0 ) direction. All films grown at 1075°C and below exhibit this type of orientation. Again a similar effect is observed in films of Sn doped In203 grown by the EVD method; these Trims grew preferentially in the ( 111 > direction [ 9 ]. Interestingly, vapor deposited T1203 crystals, which are isostructural with In203, also grow fastest in the ( 111 > direction [ 10 ].
The faceting and orientation of the YSZ films indicates that a second transport process occurs in parallel with oxygen anion and electron diffusion through the film. Because faceting exposes low index planes with low surface energies in preference to high surface energy, high index planes, this process is thermodynamically driven by minimization of surface free energy. The mechanism to transform higher energy planes to lower energy planes requires a diffusional route for both anions and cations; this route is often surface diffusion [ 11 ]. It is often observed in the growth of crystals in an isotropic growth medium that certain crystalline directions will grow faster than others. For example, YSZ crystals grown by the skull melting process grow fastest in the ( 1 1 0 ) direction [ 12 ]. When the crystal nuclei are on a surface, the process of "evolutionary selection", described by van der Drift [ 13 ], may occur. Those crystals with the fastest growing direction oriented perpendicular to the substrate grow fastest. These crystals eventually overwhelm those crystals oriented with a component of the fastest growing direction parallel to the substrate. Thus, as a polycrystalline film grows thicker, the film exhibits columnar growth with the orientation of the crystals at the growing surface becoming increasingly parallel to each other. This growth mechanism should produce YSZ films oriented in the ( 110 ) direction, in agreement with films grown at 1075°C and below. Surface diffusion is necessary for evolutionary selection to occur: adsorbed species must be able to diffuse from the slower to the faster growing crystal faces. Thus, surface diffusion is required for both faceting and oriented growth to occur. The final morphology of a film deposited by EVD, therefore, is a product of two competing processes. Film growth is limited by bulk charge transport, a mechanism which favors smooth and unoriented films. Surface diffusion of reactive species occurs simultaneously and this promotes faceting and orientation. If bulk transport is very much faster than surface transport, smooth unoriented films should result. Oriented and faceted films should result when surface diffusion is the faster process. The observed transition from faceted, oriented films grown at 1075°C to smooth films grown at 1100°C indicates that the relative rates of bulk and surface diffusion must be extremely temperature sensitive.
M.F. Carolan, J.N. Michaels / Yttria-stabilized zirconia thin films
One possible explanation for the apparent temperature sensitivity of EVD film morphology can be found by considering the temperature-dependence of the film growth rate and surface diffusion rate independently. Film growth is limited by electron transport through the electrolyte and has an activation energy of 3.9 eV [6]. It can be shown that at a given time, the temperature dependence of the growth rate of the film, r, is: roc N / T e x p [ - 3"9 e V j] ,
(6)
where Tis the absolute temperature and R is the ideal gas constant [6 ]. We cannot predict the activation energy of the surface diffusional flux, but we note that this flux often decreases exponentially with increasing temperature. To show this, we start with Ficks law to describe the surface flux, Ns: (7)
N~ = - D ~ V C ~ ,
where D~ is the surface diffusivity and C~ is the surface concentration. Surface diffusion is an activated process, therefore Ds can be described by an Arrhenius expression:
o,
1
where E~ is the activation energy for diffusion and A' is a preexponential factor. On the other hand, the surface concentration of adsorbate should vary with temperature according to van't Hoff equation din [ c ~ ] / d T = R2All T ,
(9)
where ~ is the heat of adsorption and Q is the gas phase concentration of the adsorbing species. Thus, the surface flux varies with temperature according to N s o c T - I exp [,-- ( A H + E , ) ] RT A"
(10)
The activation energy for surface diffusivity is a positive number, typically on the order of a few kilocalories per mole [ 14 ]. Because the heat of adsorption is usually negative, the apparent activation energy of the surface diffusion is often negative. Therefore, the rate of surface diffusion can decrease exponentially with increasing temperature.
201
Since the reaction rate increases exponentially with temperature, while surface diffusion decreases exponentially with temperature, it is possible that different morphologies can be exhibited over a small temperature range. The magnitude of the heat of adsorption must be fairly large however. If the heat of adsorption has a magnitude of 40 kcal/mole, the ratio of the surface diffusion rate to the reaction rate, Ns/r, increases only by a factor of 1.8 between 1075°C and 1100°C. If the heat of adsorption was increased to 100 kcal/mole, this ratio only increases by a factor of 2.8 between 1075°C and 1100°C. It is probably reasonable to assume that a I 0-fold increase in the ratio between 1075°C and 1100°C is required to explain the transition from faceted to smooth films. Therefore, this explanation appears to require an unreasonably large value of the heat of adsorption. A more likely explanation is that the mobile surface species that allows faceting and oriented growth to occur might become thermodynamically unstable between 1075°C and I100°C. At the higher temperature no mechanism would be available to allow surface transport of a metal species. This phenomenon has been observed in the epitaxial chemical vapor deposition of silicon on ( 111 ) silicon substrates using SiCI3H and the etching of ( 111 ) Si substrates with gaseous HC1 [ 15 ]. At lower temperatures, the surface of the deposited or etched silicon is highly faceted with low index exposed faces. At higher temperatures, the surface is smooth. The activation energy for the transition from the faceted growth regime to the smooth growth growth regime corresponds well with the heat of formation of a surface SiCI2 species. The morphology of the lower temperature films could prove to be beneficial. The highly faceted surface has a higher surface area and a rougher surface than the smooth films. Adhesion of electrodes to such a film could be superior to adhesion to smooth films. In addition, grain boundaries in oriented films tend to be perpendicular to the surface of the film, and therefore parallel to the direction of anion conduction in electrochemical devices. The effect of impurity phases at the grain boundaries would be greatly reduced when the conduction path does not cross the grain boundaries.
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M.F. Carolan, J.N. Michaels / Yttria-stabilized zirconia thin films
3. C o n c l u s i o n s Y t t r i a - s t a b i l i z e d z i r c o n i a films were g r o w n b y the electrochemical v a p o r d e p o s i t i o n m e t h o d at t e m p e r a t u r e s r a n g i n g f r o m 1 0 0 0 ° C to 1 1 0 0 ° C . F o r films g r o w n at l 100 ° C, the y t t r i u m - t o - z i r c o n i u m ratio i n the film e q u a l e d that in the r e a c t a n t gases. F i l m s grown b e l o w 1075 ° C were polycrystalline a n d grew preferentially in the { 1 1 0 ) direction. T h e y were also highly faceted. F i l m s g r o w n at 1 1 0 0 ° C were polycrystalline, u n o r i e n t e d a n d h a d a s m o o t h surface texture. T h i s difference in m o r p h o l o g y c a n be exp l a i n e d b y either the relative r e a c t i o n rate o f film growth to surface r e c o n s t r u c t i o n or b y a m o b i l e surface species b e c o m i n g t h e r m o d y n a m i c a l l y u n s t a b l e at the higher t e m p e r a t u r e .
Acknowledgement T h e a u t h o r s gratefully a c k n o w l e d g e the s u p p o r t o f the N a t i o n a l Science F o u n d a t i o n , grant n u m b e r CBT-8502407.
References [ 1 ] A.O. Isenberg, Solid State Ionics 3/4 ( 1981 ) 443. [2] T.L. Markin, R.J. Bones and R.M. Dell, in: Superionic conductors, eds. G. Mahan and W. Roth (Plenum Press, New York, 1976) pp. 15-35. [3]A. Negishi, K. Nozaki and T. Ozawa, Solid State Ionics 3/4 (1981) 443. [4] A.O. Isenberg, Proc. Electrochem. Soc. 77-6 (1977) 572. [ 5 ] G. Dietrich and W. Schafer, Intern. J. Hydrogen Energy, 9 (1984) 747. [6] M.F. Carolan and J.N. Michaels, Solid State Ionics 37 (1990) 189. [7] M.F. Carolan and J.N. Michaels, Solid State Ionics 25 (1987) 207. [8 ] R.P. Ingel and D. Lewis III, J. Am. Ceram. Soc. 69 (1986) 325. [ 9 ] J.H. Enloe and G.P. Wirtz, J. Electrochem. SOc. 133 (1986) 1583. [ 10] G.P. Wirtz and D.C. Seibert, J. Cryst. Growth 32 (1976) 274. [ 11 ] A.A. Chernov, Modern crystallography III, crystal growth (Springer, Berlin, 1984). [ 12] D.B. Zhang, X.M. He, J.P. Chen, J.C. Wang, Y.F. Tang and B.L. Hu, J. Crystal Growth 79 (1986) 336. [ 13] A. van der Drift, Philips Res. Repts. 22 (1967) 267. [ 14] J.M. Smith, Chemical engineering kinetics (McGraw-Hill, New York, 1981 ) p. 470. [ 15 ] J. Burmeister, J. Crystal Growth 11 ( 1971 ) 13.