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Thin film growth of gadolinia by metal-organic chemical vapour deposition (MOCVD)
ELSEVIER
Thin Solid Films 286 (1996) 64-71
Thin film growth of gadolinia by metal-organic chemical vapour deposition (MOCVD) John McAleese •
a,., J o h n C. P l a k a t o u r a s b, B r i a n C . H . S t e e l e ~
Centrefor TechnicalCeramics, Departmentof Materials, ImperialCollege of Science, Technologyand Medicine, South Kensington. London SW7 2BP, UK Department of Chemistry. ImperialCollege of Science. Technologyand Medicine. South Kensington, London SW7 2AY, UK Received 10 July 1995; accepted 4 December 1995
Abstract We report the growth of thin films of gadolinia using metal--organic chemical vapour deposition (MOCVD) under reduced pressure. The mixed ligand complex [ {Gd(tmhd)a}2 (tetraglyme) ], where tmhd-H is 2,2,6,6-tetramethyl-3,5-heptanedione and tetraglyme is tetraethylene glycol dimethyl ether, was used as precursor. Silicon wafers with a (100) orientation were used as substrates. The work described can be considered as a preliminary stage in the eventue, manufacture of solid electrolytes with a mixed metal composition of CeogGdo.~O,.95and, as such, certain process parameters are restricted by limits associated with potential cerium precursors which are reported in a separate paper. The effects of the deposition conditions on the structure and composition of the resulting oxide films are presented. The dependence of growth t.n parametero such as the evaporation rate, moist oxygen flow rate and substrate temperature (Tsub) is discussed. Characterization of the microstmcture using scanning electron microscopy (SEM) and X-ray diffraction (XRD) confirms the first reported growth, to our knowledge, of thin, uniform layers of gadolinia by the CVD technique. Keywords: Ceramics; Chemical vapour depo.~ition(CVD); Solid electrolyte interface
1. Introduction Current solid oxide fuel cells (SOFCs) tend to operate at temperatures in the region of 1000 °C, often with their components processed at even higher temperatures. In order to reduce thermocbemieal and thermomechanical problems and to make SOFC systems more commercially attractive by lowering the cost of balance-of-plant, it is generally desirable to reduce the operating and processing temperatures. This can be attained either by developing the technology for thin film e l ~ o l y t e s that can operate efficiently at lower temperatures or developing new electrolytes with reduced interaction and lower interracial resistance. An electrolyte with the composition Ceo.gGdo.,O|.gs operating at 500 °C appears to exhibit an ionic conductivity approaching 10 -z S c m - ' , allowing current densities of 300 mA cm z to be sustained with acceptable ohmic losses if the electrolyte thickness is less than 30 I~m [ i ]. Given these criteria, a major initiative within the scientific community is currently underway to fabricate Ceo.gGdo.IOt.~s layers by a number of diverse techniques. The purpose of this work is to demonstrate the feasibility or otherwise of using the technique of metal-organic chemical * Correspondingauthor. 0040q~90t96/$15,00 © 1996Elsevier ScienceS.A, All rights reserved SSD10040-6090 (95)08513-0
vapour deposition (MOCVD) to achieve uniform, impermeable layers of this ionic conductor. Specifically, the work speculates on the suitability of tile mixed ligand complex [{Gd(tmhd)3}2(tctraglyme)] as the potential source of gadolinia. The growth and characterization of films produced using this precursor are discussed in terms of important CVD parameters.
2. Experimental details 2.,. Materials
The utilization of glymes to improve the CVD properties of several lanthanide 13-diketonate complexes has been reported previously [2]. The synthesis of several adducts of the complex Gd2(tmhd)6 ( [ {Gd(tmhd) 3 },(CHa(OCH2CH2)mOCH3) ], where n-- 1, m = 2 and n = 2, m = 3, 4, 7) using low temperature routes has been described in detail. It has been demonstrated conclusively that lengthening of the glyme chain, from monoglyme (two oxygen donors) to heptaglyme (eight oxygen donors), improves the stability of the complexes, whereas reducing oligomerization improves the volatility. Thermogravimetric analysis (TGA)
J. McAleese et al. / Thin Solid Fibns 286 (1996) 64-71
clearly indicates that the complexes are stable up to approximately 200 °(2 and then subsequently sublimate in one complete step approaching 300 °C. Quantitative sublimation was determined by a final residue of less than 2.5%. Taking the relative expense of heptaglyme into consideration, it was decided that the tetraglyme species, namely gadolinium (2,2,6,6-tetramethyl-3,5-heptanedione)6 tetraethylene glycol dimethyl ether ([{Gd(tmhd)3}2(tetraglyme)]), is sufficiently volatile for our purposes; sublimation studies indicate that this particular complex sublimates intact between 85 and 165 °C at 0.7 Pa. Silicon wafers with an orientation of (100) were used as substrates because of their compatibility with most types of film analysis and the ability to fracture or cleave easily allowing cross-sectional examination of the "as-grown" overlayer. In all cases, films were grown on the "mirror-polished" surface after pretreatment with methanol.
2.2. CVD apparatus Growth experiments were conducted within the confines of a horizontal MOCVD system. A schematic diagram of this hot wall reactor is shown in Fig. 1. The apparatus essentially consisted of a gas and precursor vapour handling system, a twin inlet mixing chamber, a reactor zone and an effluent trap. Precursor material was placed in a uniformly heated stainless steel vessel which is referred to as the "bubbler" throughout the text. Samples were maintained at fixed process temperatures by means of a band heater and standard thermoeouple/temperature controller arrangement. Vapours generated in the bubbler were delivered to a twin inlet mixing chamber using a carrier gas. Argon was used exclusively for this purpose throughout. A separate oxygen line, which included a water bubbler, was installed with the intention of using oxygen as a reactor gas to enhance pyrolysis and min_ Ar --"--"----L
imize the incorporation of impurities. The water bubbler was contained within an oil bath which allowed the transport of wet or moist oxygen into the mixing chamber depending on the oil bath temperature, usually set at 60 °C. Standard 6 mm stainless steel pipework, with Swagelok fittings, was used throughout. All gas flow rates were controlled using Nupro microtaps with separate Platon flow gauges. All pipework between the bubbler outlet and the entrance of the furnace, including the mixing chamber, was maintained at a temperature exceeding 200 °C using Simistat controlled heater tapes securely wrapped and strapped with glass tape to reduce heal loss. This precaution eliminated the possible solidification or condensation of reactants downstream. Mixing of carrier gas/ precursor vapour with reactor gas occurred immediately before entry into the hot zone. The reactor chamber itself consisted of a quartz tube adapted at both ends to accommodate a Sovirel collar at the inlet, which provided a vacuumtight seal into the mixing chamber, and a glass effluent trap at the outlet. The tube was encased within a furnace, the temperature of which was maintained in the range 200-700 °C; again a standard type K thermocouple and temperature controller were used. Waste gases and decomposition byproducts were removed from the system via the effluent trap which was also connected to a rotary vacuum pump. The backing pressure could be adjusted and monitored throughout all the experimental procedures using a "bleed" valve and vacuum dial gauge again incorporated into the trap. For the purposes of the work described in this paper, the backing pressure could be described as being "reduced" only and was kept at 267 Pa throughout.
2.3. Film deposition Initially, between 1.2 and 1.4 g of [{Gdttmhd)3}2(tetraglyme) l, in the form of a finely ground white crystalline r--2::7--1
x-- I
X I
Mil(tng F Chamber
\
65
,
Fig. I. Schematic diagram of chemical vapour deposition (CVD) apparatus.
66
J. McAleese et al. / Thin Solid Fihns 286 (1996) 64-71
powder, was placed in the stainless steel bubbler. Starting with a bubbler temperature (Tbub) of 100 °C, a carrier gas flow rate of 200 seem and a background pressure of 267 Pa, the temperature was carefully increased in controlled increments of 5 °C until volatilization was detected. The lowest TbubWas established at 170 °C. However, attempts to repeat the process under the same conditions with the same material proved to be unsuccessful. It was found by accurate weighing of the bubbler before and after each attempt that no weight loss had occurred. Further carry-over or evaporation could only be established after elevating Tbubup to 190 °C. Knowing that the complex is a low melting solid (m.p. 89-91 °C), the crystal structure consisting of two {Gd(tmhd)3} moieties bridged by the glyme ligand [2], these observations suggest that two different evaporation rates exist for the molten material that melts and resolidifies prior to repeated growth attempts. Aware of these differences, all precursor material was routinely melted and resolidified prior to deposition. This reduced the irregularities in the evaporation rates subsequently calculated. Pieces of silicon wafer (orientation (100)), cut using a diamond pencil, were pretreated using methanol prior to location on a stainless steel cradle within the quartz reactor tube. This cradle ensured that the substrates remained at a fixed height in the vapour stream, whilst introducing a slight tilt to help improve the film uniformity. A temperature profile was found to exist along the length of the reactor tube resulting in a position-dependent concentration. This is thought to be caused by the depletion of the reactants along the deposition zone and requires the optimum position for the cradle to be established by careful readjustment within the tube. Once this was attained, all substrates were placed at this distance measured from a fixed datum, All subsequent runs were carried out at this position yielding uniform films with areas averaging at least I cm 2. 2,4, Film thickness measurement
Early attempts were made to measure the film thickness using Talystep and Dektaking. The acute sensitivity of these techniques, combined with the apparent surface roughness of the films at the microscopic level, led to insufficiently accurate readings, deemed unsuitable for routine film thickness measurement. In the case of ellipsometry, the surface roughness tended to cause scatter of the laser beam resulting in misleading values for both the thickness and refractive index. To remove the ambiguity, the interface between the overlayer and substrate was viewed using scanning electron microscopy (SEM) and the thickness was determined by measurement of the cross-section. A standard error was calculated for the thickness by making some 30--40 measurements of what was considered to be the "roughest" film, and the error was subsequently incorporated into future growth rate calculations. For the purposes of the work described here, all SEM work was carried out on a Joel JSM T220A microscope. Samples were gold sputtered for 4 rain producing a gold film
of approximately 1000 A which appeared to optimize the resolution of the thin insulating layers. An accelerating voltage of 20 kV was used throughout. 2.5. CVD parameter limits
When discussing the CVD conditions for this work, we should remember that the gadolinium oxide growth reported here is preliminary to the eventual manufacture of a mixed metal solid solution. In other words, this precursor is only one component of a two-component system and as such must be studied under limits or conditions which may be pertinent to potential cerium precursors. The most obvious example of this is the use of moist oxygen as areactor gas, which although not required for the successful growth of gadolinia, is a prerequisite in attaining oxide films when using fluorinated 13diketonate precursors. For example, in the work of Watson et al. [3] and Duray etal. [4], the use of moist oxygen in the successful removal of fluorine from deposited layers is discussed. This is the case with the speculative cerium materials being considered in parallel with the present work where the adoption of fluorinated ligands has led to improved thermal stability. The work on these precursors is reported in greater detail elsewhere [5]; the moist oxygen stream referred to in this work was calculated to consist of 0.0023% water vapour by volume. As previously detailed, the lowest Tbubvalue allowing volatilization and growth is 190 °C. This was defined as the Tbub iower limit. Although evaporation could be achieved at 240 °C, partial sintering or annealing of the material was evident, i.e. partial solid formation or decomposition occurred at this temperature, the material forming a thick brown/black solid. This reduced the measured amoun~ of evaporation and therefore 230 °C is quoted as the upper limit. Regarding the carrier gas flow rates, upper and lower iimits of 25 and 200 seem were determined by virtue of the fact that, below the minimum, no mass transport was detected, and above the maximum either blockage in the inlet network was experienced or the residence time was so short that growth proved to be impossible. In the case of the moist oxygen flow rate, 50 and 200 seem were established as the lower and upper limits respectively. Although a T~ubrange of 250--700 °C was studied initially, later experiments were contained within the limited range of 250--400 °(2. Increments of 50 °C were chosen as these provided significant change in the surface features. Remembering that/'sub will be a common parameter at the mixing stage of the chosen cerium and gadolinium precursors, these lower and upper limits were determined by virtue of parallel results obtained from the potential cerium precursor Ce(fod)4, where fod-H is 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dione. With regard to this precursor, below the lower limit of 250 °C no growth was detected, perhaps due to an insufficient heterogeneous activation energy (EA), and above 400"C deposits resulted in powder formation and/or peeling of any layers that were formed.
J. McA leese et al. I Thin Solid l'~lms 286 (1996) 64-71
The growth of gadolinium oxide was found to increase linearly with deposition time up to a period of 240 min; the thickness was determined for layers grown under the same conditions after 30, 60. 120, 180 and 240 min. Attempts to grow films beyond this time, e.g. runs exceeding 720 min, resulted in no further increase in thickness. The material in the bubbler after such times was found to have degraded with substantial decomposition products. This information alluded to the possibility of thermal instability over extended periods of time. This is not untypical of complexes synthesized solely with organic ligands. Fortunately, the growth rate of this particular material appeared to be relatively high, with substantially thick layers being obtained after periods of only approximately 60 min. This was therefore used as the deposition time.
.~ 4 o
3
I
2
1
-
90
3. Results
67
-
I
.......
1.95
7- ..........
2.00
i
I
2.05
2.10
--I
.......
2.15
2.20
1000 / Tw, ( 10001 K) Fig. 2. A plot of the evaporation rate on a logarithmic scale vs. lO00/Tbu~,
3.1. Evaporation rate
Despite the careful control of process parameters, for a given set of fixed conditions, fluctuations in the evaporation rate were evident. Discrepancies may have arisen due to a number of factors. 1. The variation of the crystallinity between batches of precursor. 2. Despite rigorous purification procedures, variable amounts of residual glyme may have altered volatility. 3. The constant wrapping and rewrapping of heater tapes in order to weigh the bubbler after each run may have introduced different cold spots in the inlet feeding line. This practice was subsequently eliminated from this work. 4. Despite the autotuning of the temperature controllers, varying degrees of "overshoot" were experienced by the bubbler. Since upstream taps remained open, weight differences may have been spurious in that the amount of material lost may not have been exclusive to the actual growth process. Again this problem has now been circumvented. Such problems are perhaps responsible for the relatively large error determined at low bubbler temperatures, shown by the error bars in Fig. 2, which gives a graph of the evaporation rate plotted on a logarithmic scale against 1000/Tb,b. If we accept that the large error is associated with infrequent, unusually high, calculated weight losses and therefore consiclers only the overall general trends, the fluctuations in the evaporation rate are not of a significant order of magnitude for any given Tb,b. Again, allowing for extreme cases, the average evaporation rate with the lowest Tb,b of 190 °C was 5.53 mg min-l. Subsequent 1 °C increases in temperature resulted in a 10% improvement of this rate, e.g. when Tb,b was fixed at 210 °(2, evaporation rates averaging 12.65 mg min-~ were calculated. This is in good agreement with accepted CVD data [6]. Progressive non-linear increases in the rate were observed up to Tb,b= 240 °C; at least partial
sintering, as previously mentioned, was suspected for the reduction in the size of further increments. It was found that, for a given Tbubwithin the range 250-400 °C, the growth rate was independent of the evaporation rate, alluding to the possibility that this region is a kinetically limited regime for the process. This supposition is supported by the results obtained for the growth rate as a function of the substrate temperature, described later. 3.2. Growth rate dependence on the moist oxygen flow rate
As described previously, all films discussed in this paper were grown using moist oxygen as reactor gas. A variation in this re.actor gas flow rate did not result in a growth rate exhibiting a linear dependence at lower bubbler temperatures. Instead, the optimized growth rate, i.e. the fastest growth rate that could be attained under a particular set of fixed conditions, was thought to depend on the ratio of the precursor vapour to oxygen. As the oxygen flow rate was increased through the series 50, 100, 150 and 200 seem, the fastest growth rate associated with a particular evaporation rate, and therefore precursor concentration, appeared to depend on the attainment of a compatible oxygen flow such that all the available precursor was consumed exclusively in the growth process. In other words, if the amount of oxygen is too small, a portion of the precursor passes through the reactor undecomposed and therefore takes no part in the actual film formation process. If, on the other hand, too much oxyger, is present, the available precursor may be consumed in horaogeneous decomposition processes above the substrate :~urface. Invariably, when this latter condition occurred, powder formation was evident, substantially reducing the growth rate. Table 1 shows the calculated growth rates corresponding to each oxygen flow rate for Tb,b values of 190, 210 and 230 °(7. The carrier gas flow and T~,bwere maintained throu~;hout at 200 seem and 400 °C respectively. The corresponding plot
J. McAleese et aL / Thin Solid Films 286 (1996) 64-71
68
Table I Growth rates determined for three individual T~b values of 190. 210 and 230 °C through the oxygen flow rate series 50, 100, 150 and 200 sccm Growth rate (rim min- ~) at three bubbler temperatures (carrier gas, 200 sccm; 60 min; T~.b= 400 °C)
Moist oxygen flow rate (sccm)
50 100 150 200
190°C
210°C
230 °C
2.2 8.8 8.3 0.0
0.0 13.3 24.0 2.17
1.8 4.8 12.7 35.0
of the growth rate vs. moist oxygen flow is shown in Fig. 3. From the graph, we can see the point where oxygen begins to saturate lower evaporation rates. It appears that, at higher Tb,b, the increase is linear, suggesting that the amount of material being transported at this temperature is not sa,'urated at the oxygen parameter upper limit, namely 200 seem. Fig. 4 shows the non-powder surface of a film grown at a high growth rate (approximately 35 nm min- t ) for 60 min using Tb,b = 230 °C. The evaporation rate generated at this temperature must be sufficiently high such that the concentration of precursor can "accommodate" the 200 sccm of oxygen provided. No powder was evident, with a particle size averaging 2 I~m in contrast with the 0.2 p,m particles on films grown with the same parameters, but an oxygen flow rate of I00 sccm. If we consider that the role of oxygen in the process is to assist the pyrolysis of precursor molecules to some other reactive species, the results can be rationalized by bearing in mind the two competing processes involved at the substrate surface. On the one hand, if the amount of reactive molecular species is compatible with the diffusion rate, the species diffuse to the substrate, undergo condensation and subsequently form a film. On the other hand, if the gas phase concentration of the species is too large, the rate of particle collisions will 40 T
35
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30
_ _ _ _
/J.
2O
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e ""
t
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I
140 160 180 200 220 Moist Oxygen Flow Rate (socm ) Fig. 3, Graph of growth rate vs. moist oxygen flow rate for three separate bubbler temperatures. 40
60
80
100
120
....... : - j
~
:
Fig. 4. The surface of a gadolinium oxide film grown using Tt,,bffi230 °C and Ts,~ffi400 °C; both carder gas and oxygen flow rates were maintained at 200 sccm. Scale bars represent 10 p.m.
also increase substantially. This will lead to the formation of polymers in the gas phase, in other words, powder formation. 3.3, Growth rate dependence on the substrate wmperature
(T.,.,,I,) The growth rate as a function of the substrate temperature (Ts.h) was studied for the lowest Tb.b value of 190 °C. The carrier and oxygen flow rates were maintained at 200 and 100 sccm respectively, I00 sccm having been established as the optimum oxygen flow for the average evaporation rate associated with this specific Tb.b value. Ts.b was varied between 250 and 800 °C in increments of 50 °C. A plot of the growth rate on a logarithmic scale vs. the reciprocal T~.b value is shown in Fig. 5. The resulting curve is typical of that associated with CVD processes [7]. The first low temperature region, between 250 and 375 °C, shows a linear relationship between the growth rate and T~.b; the deposition is considered to be temperature dependent in terms of the activation energy. In this region, the deposition is perhaps controlled by the kinetics of thermal decomposition of the precursor on the hot suhstrate surface forming gadolinium oxide. Between 250 and 375 °C, the surface mobility of the ac~sorbed molecules is suspected to be low and the supersaturation of the gas phase is high; these conditions lead to high nucleation, but low crystal growth rates, and therefore to fine-grained structures. Very little structural surface detail could be ascertained from
J. McAleese er al./ Thin Solid Films 286 (1996) 64-71
87A
6-
4
2
1
1.0
I
I
I
I
1.2
1.4
1.6
1.8
2.0
1000 ! T,,~ ( 10001K )
Fig. 5. Graph of growth rate plotted on a logarithmic scale vs. 1000/T~u~. Tb,bwas maintained at 190 °C throughout.
the microscopy of films grown at low T~.b values such as these. The lack of surface features suggested that the films had a polycrystalline, granular structure with poorly defined grain boundaries and a small grain size, a fact supported by the X-ray diffraction (XRD) results discussed later. Increasing T.~,bup to 350 °C results in the development of a discernible texture. The suspected lack of surface mobility at this and lower temperatures appears to be supported by the less than continuous coverage of the surface as illustrated in Fig. 6, which shows the development of a film grown after 60 min using Tb,L,----230 °C and T~,t,= 350 °C. The acicular particle size in this case averages 0.7 p.m X 0.3 p.m. As the substrate temperature was increased, the deposition rate became independent of T~,kb.In this region, it is thought that the growth rate is limited by the mass transport of the reactive species towards the substrate surface. Within this regime, the deposition rates and surface mobility are relatively high. This, together with the low supersaturation, gives
Fig. 6. Surfacerevealing the development of texture after 60 min. Tb,b= 230 °C; Tsub-----350°C; carder gas and oxygen flow rates, 200 and 150 sccm respectively.
69
rise to the possibility of highly textured, epitaxial layers. The maximum deposition rate in this region was calculated to be approximately 2 p,m h - ~. This could be increased by raising Tb,b and hence the evaporation rate or mass transport of the precursor. Although this was possible, later experiments at higher Tbub values were conducted within the kinetically controlled regime, i.e. temperatures between 250 and 400 °C. This restricted range was determined as the satisfactory limit within which a separate cerium component could be grown successfully [5] and, as such, experimental work on the gadolinium precursor was conducted within these restrictions, considering the possible mixing of these two precursors at a later juncture as previously discussed. As Ts,b was increased between 400 and 600 °C, larger grains with well-defined boundaries became more apparent. Fig. 7 shows an example of this, with a polycrystalline film grown at 600 °C. The circular domain size was typically of the order of 0.5 p,m in diameter. The absence of pores on the surface alluded to the impermeable nature of the deposited layers, which was confirmed by inspection of the cross-section. The relative smoothness of the silicon surface at the
Fig. 7. Continuous. polycrystalline film grown under similar conditions Io Fig. 6, but with TRub= 600 °C.
Fig. 8. Cross-section of a 2 ~m thick gadolinium oxide film grown after 60 rain using T ~ = 190 °C. Ts,b=400 °C and carrier gas and oxygen flow rates of 200 and 100 sccm respectively. Note the lack of any microstructural detail. Scale bar represents i0 Ixm.
J. McAleese et al, / Thin Solid Films 286 (1996) 64-71
70
Ts.b = 800 °C. The increasing extent of powder formation was reflected in the gradual reduction in the growth rate.
3.4. X-Ray diffraction
Fig. 9. Cross.sectionof a 2.3 ~m thick gadoliniumoxidefilmgrownafter 60 rain using the conditions described previouslyfor Fig. 8, but with a substrate temperatureof 600 *(2.
microscopic level allowed the interface between the overlayer and substrate to be distinguished. The microstructural detail of the cross-section exhibited a well-definedcolumnar growth when grown on substrates at high Ts.b, whereas at lower Ts.b a poorly defined structure was revealed. This is illustrated by comparison of Fig. 8 and Fig. 9, where Fig. 8 shows the cross-section of a gadolinium oxide film grown after 60 rain using Tb.b = 190 *C, T,.b = 400 °C and carrier gas and oxygen flow rates of 200 and 100 sccm respectively and Fig. 9 under similar conditions, but with a higher substrate temperature of 600 *(2. Further increases in substrate temperature from 650 °C onwards resulted in the appearance of greater amounts of powder, culminating in a completely powdered coating at
XRD was performed on a Philips PW1710 based diffractometer using Cu Kct radiation with a graphite monochromator. Samples were scanned through a 20 range of 10°-70 ° using a step size of 0.04 ° and a count time of I s. A series of films was grown with varying Ts.b and their corresponding XRD patterns were run with all other parameters kept constant. Fig. 10 shows the XRD patterns obtained. Scan (a) represents the theoretical pattern determined for cubic Gd203 as obtained from JCPDS file No. 43-1014. This is the reference to which the experimentally grown samples (b), (c), (d) and (e), representing Ts.b values of 250, 300, 350 and 400 °C respectively, were compared. As can be seen, the increase in substrate temperature results in a sharpening of the peaks, alluding to the increasing crystallinity of the samples from amorphous at low T~.b to crystalline at higher Ts.b. The overall crystallinity of the samples within this temperature range was low and no preferred orientational effects were observed. Only the one crystalline phase was observed, indicative of cubic phase gadolinia. The dominant peaks emerging at 20 = 28.59 °, 47.54 ° and 56.35 ° correspond to the (222), (440) and (622) reflections of gadolinia respectively. The sharp peak at 33.03 o present in scans (b), (c) and (d) represents the (200) reflection pertinent to silicon only. This peak becomes broadened, as observed in scan (e), when the close proximity of gadolinia (400) is superimposed upon it,
tOOO, [¢oun~:6] gO0, 800: 700: BOO~
'i
5001
400.
d)
3001 2001
(;~2a)
tOOi
a) o. o
~o
I
(440) {332) ;t___,~_~
(431)
(sii) o
[" e]
o
Fig. 10. XRDpatternsrepresentinga seriesof sampleswithvaryingTs.h.Scan (a) representsthe theoreticalpattern for cubic Gd203,whereas(b) (e). (d) and (e) representT~bvaluesof 250, 300, 350 and 400 *Crespectively.
Z McAleese et aL/Thin Solid Films 286 (1996) 64-71
4. Discussion Given that this precursor is a low temperature melting solid, its evaporation rate behaviour appears to be representative of that of a volatile liquid, the increase in rate being comparable with that of other reported materials when experimental errors are taken into account. It has been found that the growth rate of gadolinia can be manipulated by the variation of the reactor gas flow rate, i.e. the growth rate of gadolinia can be accelerated or retarded by altering the amount of oxygen. If we regard our target material to involve the doping of ceria with a small amount of gadolinia, this particular parameter may be of great importance, especially if early results from a potential cerium precursor suggest a linear increase in the growth rate with the oxygen flow rate. The morphology of the thin films depends strongly on the growth rate which, in the kinetically controlled regime p~edominantly studied here, is determined by the substrate temperature. It is a well-established principle that the surface morphology can be improved by Tsubwhich will enhance the grain growth [8]. We found that, when T~ubwas low, the growth rate was also low and films tended to have densely packed non-columnar structures with very smooth surfaces. At higher T~b values and higher growth rates, films exhibited obvious grain boundaries with columnar cross-sectional mierostructures. The results from XRD confirmed that the gadolinium oxide material deposited was cubic gadolinia or GdzO3. In addition, XRD suggested that the crystallinity of the samples improved with increasing substrate temperature.
S. Conclusions The precursor, [{Gd(tmhd)3},~(tetraglyme)], has been shown to be sufficiently volatile and thermally stable for the
71
growth of thick films of cubic phase gadolinia under reduced pressure by MOCVD. This, to our knowledge, is the first .reported example of such films being deposited by any CVD method.
Acknowledgements The authors wish to express their deepest gratitude to several colleagues for their time and expertise: Mr. Colin Robinson for his considerable effort in constructing the CVD apparatus, Mr. Bob Rudkin for assisting with the SEM work and Dr. David Waller for running samples for XRD. Last, but by no means least, the Leverhulme Trust is acknowledged for providing financial support of the project.
References [ I ] B.C.H.Steele,Ceramics bu., 19 (1993) 269. 12] I. Baxter,S.R. Drake,M.B. Hursthouse,KM.A. Malik, J. McAleese, DJ. Otwayand J.C. Plakatouras,hmrg. Chem., 34 (1995) 1384. [3] 1.M. Watson, MP. Atwood,D.A. Cardwell and T.J. Cumberbatch,J. Mater. Chem., 4 (9) (1994) 1393. [4] S.J. Duray,D.B. Buchholz,S.N. Song, D.S. Richeson,J.B. Ketterson, T.J. Marks and R.P.H.Chang,Appl. Phys. Lett., 59 ( 1991) 1503. [5 ] J. McAleese,J.C. Plakatourasand B.C.H.Steele,Thin Solid Fibns 280/ I-2 (1996) 152-159. [61 F. Sehmaderer,R. Hubar,H. Omzmannand G. Wahl,in M.L. Ititchman and N.J Archer(eds.~',Proc. 8th Et~ropean Cons on CVD, LosEditions de Physique,Glasgov,,199I, C2-539, [7] M. Becht.T. Gerfinand K,H.Dahmen,Chem. Mater., 5 (1993) 137. [8] B.S. Kwak, K. Zhang,E.P. Boydand A. Erbil,J. Appl. Phys.. 69 (2) ( 1991) 767.
Thin Solid Films 286 (1996) 64-71
Thin film growth of gadolinia by metal-organic chemical vapour deposition (MOCVD) John McAleese •
a,., J o h n C. P l a k a t o u r a s b, B r i a n C . H . S t e e l e ~
Centrefor TechnicalCeramics, Departmentof Materials, ImperialCollege of Science, Technologyand Medicine, South Kensington. London SW7 2BP, UK Department of Chemistry. ImperialCollege of Science. Technologyand Medicine. South Kensington, London SW7 2AY, UK Received 10 July 1995; accepted 4 December 1995
Abstract We report the growth of thin films of gadolinia using metal--organic chemical vapour deposition (MOCVD) under reduced pressure. The mixed ligand complex [ {Gd(tmhd)a}2 (tetraglyme) ], where tmhd-H is 2,2,6,6-tetramethyl-3,5-heptanedione and tetraglyme is tetraethylene glycol dimethyl ether, was used as precursor. Silicon wafers with a (100) orientation were used as substrates. The work described can be considered as a preliminary stage in the eventue, manufacture of solid electrolytes with a mixed metal composition of CeogGdo.~O,.95and, as such, certain process parameters are restricted by limits associated with potential cerium precursors which are reported in a separate paper. The effects of the deposition conditions on the structure and composition of the resulting oxide films are presented. The dependence of growth t.n parametero such as the evaporation rate, moist oxygen flow rate and substrate temperature (Tsub) is discussed. Characterization of the microstmcture using scanning electron microscopy (SEM) and X-ray diffraction (XRD) confirms the first reported growth, to our knowledge, of thin, uniform layers of gadolinia by the CVD technique. Keywords: Ceramics; Chemical vapour depo.~ition(CVD); Solid electrolyte interface
1. Introduction Current solid oxide fuel cells (SOFCs) tend to operate at temperatures in the region of 1000 °C, often with their components processed at even higher temperatures. In order to reduce thermocbemieal and thermomechanical problems and to make SOFC systems more commercially attractive by lowering the cost of balance-of-plant, it is generally desirable to reduce the operating and processing temperatures. This can be attained either by developing the technology for thin film e l ~ o l y t e s that can operate efficiently at lower temperatures or developing new electrolytes with reduced interaction and lower interracial resistance. An electrolyte with the composition Ceo.gGdo.,O|.gs operating at 500 °C appears to exhibit an ionic conductivity approaching 10 -z S c m - ' , allowing current densities of 300 mA cm z to be sustained with acceptable ohmic losses if the electrolyte thickness is less than 30 I~m [ i ]. Given these criteria, a major initiative within the scientific community is currently underway to fabricate Ceo.gGdo.IOt.~s layers by a number of diverse techniques. The purpose of this work is to demonstrate the feasibility or otherwise of using the technique of metal-organic chemical * Correspondingauthor. 0040q~90t96/$15,00 © 1996Elsevier ScienceS.A, All rights reserved SSD10040-6090 (95)08513-0
vapour deposition (MOCVD) to achieve uniform, impermeable layers of this ionic conductor. Specifically, the work speculates on the suitability of tile mixed ligand complex [{Gd(tmhd)3}2(tctraglyme)] as the potential source of gadolinia. The growth and characterization of films produced using this precursor are discussed in terms of important CVD parameters.
2. Experimental details 2.,. Materials
The utilization of glymes to improve the CVD properties of several lanthanide 13-diketonate complexes has been reported previously [2]. The synthesis of several adducts of the complex Gd2(tmhd)6 ( [ {Gd(tmhd) 3 },(CHa(OCH2CH2)mOCH3) ], where n-- 1, m = 2 and n = 2, m = 3, 4, 7) using low temperature routes has been described in detail. It has been demonstrated conclusively that lengthening of the glyme chain, from monoglyme (two oxygen donors) to heptaglyme (eight oxygen donors), improves the stability of the complexes, whereas reducing oligomerization improves the volatility. Thermogravimetric analysis (TGA)
J. McAleese et al. / Thin Solid Fibns 286 (1996) 64-71
clearly indicates that the complexes are stable up to approximately 200 °(2 and then subsequently sublimate in one complete step approaching 300 °C. Quantitative sublimation was determined by a final residue of less than 2.5%. Taking the relative expense of heptaglyme into consideration, it was decided that the tetraglyme species, namely gadolinium (2,2,6,6-tetramethyl-3,5-heptanedione)6 tetraethylene glycol dimethyl ether ([{Gd(tmhd)3}2(tetraglyme)]), is sufficiently volatile for our purposes; sublimation studies indicate that this particular complex sublimates intact between 85 and 165 °C at 0.7 Pa. Silicon wafers with an orientation of (100) were used as substrates because of their compatibility with most types of film analysis and the ability to fracture or cleave easily allowing cross-sectional examination of the "as-grown" overlayer. In all cases, films were grown on the "mirror-polished" surface after pretreatment with methanol.
2.2. CVD apparatus Growth experiments were conducted within the confines of a horizontal MOCVD system. A schematic diagram of this hot wall reactor is shown in Fig. 1. The apparatus essentially consisted of a gas and precursor vapour handling system, a twin inlet mixing chamber, a reactor zone and an effluent trap. Precursor material was placed in a uniformly heated stainless steel vessel which is referred to as the "bubbler" throughout the text. Samples were maintained at fixed process temperatures by means of a band heater and standard thermoeouple/temperature controller arrangement. Vapours generated in the bubbler were delivered to a twin inlet mixing chamber using a carrier gas. Argon was used exclusively for this purpose throughout. A separate oxygen line, which included a water bubbler, was installed with the intention of using oxygen as a reactor gas to enhance pyrolysis and min_ Ar --"--"----L
imize the incorporation of impurities. The water bubbler was contained within an oil bath which allowed the transport of wet or moist oxygen into the mixing chamber depending on the oil bath temperature, usually set at 60 °C. Standard 6 mm stainless steel pipework, with Swagelok fittings, was used throughout. All gas flow rates were controlled using Nupro microtaps with separate Platon flow gauges. All pipework between the bubbler outlet and the entrance of the furnace, including the mixing chamber, was maintained at a temperature exceeding 200 °C using Simistat controlled heater tapes securely wrapped and strapped with glass tape to reduce heal loss. This precaution eliminated the possible solidification or condensation of reactants downstream. Mixing of carrier gas/ precursor vapour with reactor gas occurred immediately before entry into the hot zone. The reactor chamber itself consisted of a quartz tube adapted at both ends to accommodate a Sovirel collar at the inlet, which provided a vacuumtight seal into the mixing chamber, and a glass effluent trap at the outlet. The tube was encased within a furnace, the temperature of which was maintained in the range 200-700 °C; again a standard type K thermocouple and temperature controller were used. Waste gases and decomposition byproducts were removed from the system via the effluent trap which was also connected to a rotary vacuum pump. The backing pressure could be adjusted and monitored throughout all the experimental procedures using a "bleed" valve and vacuum dial gauge again incorporated into the trap. For the purposes of the work described in this paper, the backing pressure could be described as being "reduced" only and was kept at 267 Pa throughout.
2.3. Film deposition Initially, between 1.2 and 1.4 g of [{Gdttmhd)3}2(tetraglyme) l, in the form of a finely ground white crystalline r--2::7--1
x-- I
X I
Mil(tng F Chamber
\
65
,
Fig. I. Schematic diagram of chemical vapour deposition (CVD) apparatus.
66
J. McAleese et al. / Thin Solid Fihns 286 (1996) 64-71
powder, was placed in the stainless steel bubbler. Starting with a bubbler temperature (Tbub) of 100 °C, a carrier gas flow rate of 200 seem and a background pressure of 267 Pa, the temperature was carefully increased in controlled increments of 5 °C until volatilization was detected. The lowest TbubWas established at 170 °C. However, attempts to repeat the process under the same conditions with the same material proved to be unsuccessful. It was found by accurate weighing of the bubbler before and after each attempt that no weight loss had occurred. Further carry-over or evaporation could only be established after elevating Tbubup to 190 °C. Knowing that the complex is a low melting solid (m.p. 89-91 °C), the crystal structure consisting of two {Gd(tmhd)3} moieties bridged by the glyme ligand [2], these observations suggest that two different evaporation rates exist for the molten material that melts and resolidifies prior to repeated growth attempts. Aware of these differences, all precursor material was routinely melted and resolidified prior to deposition. This reduced the irregularities in the evaporation rates subsequently calculated. Pieces of silicon wafer (orientation (100)), cut using a diamond pencil, were pretreated using methanol prior to location on a stainless steel cradle within the quartz reactor tube. This cradle ensured that the substrates remained at a fixed height in the vapour stream, whilst introducing a slight tilt to help improve the film uniformity. A temperature profile was found to exist along the length of the reactor tube resulting in a position-dependent concentration. This is thought to be caused by the depletion of the reactants along the deposition zone and requires the optimum position for the cradle to be established by careful readjustment within the tube. Once this was attained, all substrates were placed at this distance measured from a fixed datum, All subsequent runs were carried out at this position yielding uniform films with areas averaging at least I cm 2. 2,4, Film thickness measurement
Early attempts were made to measure the film thickness using Talystep and Dektaking. The acute sensitivity of these techniques, combined with the apparent surface roughness of the films at the microscopic level, led to insufficiently accurate readings, deemed unsuitable for routine film thickness measurement. In the case of ellipsometry, the surface roughness tended to cause scatter of the laser beam resulting in misleading values for both the thickness and refractive index. To remove the ambiguity, the interface between the overlayer and substrate was viewed using scanning electron microscopy (SEM) and the thickness was determined by measurement of the cross-section. A standard error was calculated for the thickness by making some 30--40 measurements of what was considered to be the "roughest" film, and the error was subsequently incorporated into future growth rate calculations. For the purposes of the work described here, all SEM work was carried out on a Joel JSM T220A microscope. Samples were gold sputtered for 4 rain producing a gold film
of approximately 1000 A which appeared to optimize the resolution of the thin insulating layers. An accelerating voltage of 20 kV was used throughout. 2.5. CVD parameter limits
When discussing the CVD conditions for this work, we should remember that the gadolinium oxide growth reported here is preliminary to the eventual manufacture of a mixed metal solid solution. In other words, this precursor is only one component of a two-component system and as such must be studied under limits or conditions which may be pertinent to potential cerium precursors. The most obvious example of this is the use of moist oxygen as areactor gas, which although not required for the successful growth of gadolinia, is a prerequisite in attaining oxide films when using fluorinated 13diketonate precursors. For example, in the work of Watson et al. [3] and Duray etal. [4], the use of moist oxygen in the successful removal of fluorine from deposited layers is discussed. This is the case with the speculative cerium materials being considered in parallel with the present work where the adoption of fluorinated ligands has led to improved thermal stability. The work on these precursors is reported in greater detail elsewhere [5]; the moist oxygen stream referred to in this work was calculated to consist of 0.0023% water vapour by volume. As previously detailed, the lowest Tbubvalue allowing volatilization and growth is 190 °C. This was defined as the Tbub iower limit. Although evaporation could be achieved at 240 °C, partial sintering or annealing of the material was evident, i.e. partial solid formation or decomposition occurred at this temperature, the material forming a thick brown/black solid. This reduced the measured amoun~ of evaporation and therefore 230 °C is quoted as the upper limit. Regarding the carrier gas flow rates, upper and lower iimits of 25 and 200 seem were determined by virtue of the fact that, below the minimum, no mass transport was detected, and above the maximum either blockage in the inlet network was experienced or the residence time was so short that growth proved to be impossible. In the case of the moist oxygen flow rate, 50 and 200 seem were established as the lower and upper limits respectively. Although a T~ubrange of 250--700 °C was studied initially, later experiments were contained within the limited range of 250--400 °(2. Increments of 50 °C were chosen as these provided significant change in the surface features. Remembering that/'sub will be a common parameter at the mixing stage of the chosen cerium and gadolinium precursors, these lower and upper limits were determined by virtue of parallel results obtained from the potential cerium precursor Ce(fod)4, where fod-H is 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dione. With regard to this precursor, below the lower limit of 250 °C no growth was detected, perhaps due to an insufficient heterogeneous activation energy (EA), and above 400"C deposits resulted in powder formation and/or peeling of any layers that were formed.
J. McA leese et al. I Thin Solid l'~lms 286 (1996) 64-71
The growth of gadolinium oxide was found to increase linearly with deposition time up to a period of 240 min; the thickness was determined for layers grown under the same conditions after 30, 60. 120, 180 and 240 min. Attempts to grow films beyond this time, e.g. runs exceeding 720 min, resulted in no further increase in thickness. The material in the bubbler after such times was found to have degraded with substantial decomposition products. This information alluded to the possibility of thermal instability over extended periods of time. This is not untypical of complexes synthesized solely with organic ligands. Fortunately, the growth rate of this particular material appeared to be relatively high, with substantially thick layers being obtained after periods of only approximately 60 min. This was therefore used as the deposition time.
.~ 4 o
3
I
2
1
-
90
3. Results
67
-
I
.......
1.95
7- ..........
2.00
i
I
2.05
2.10
--I
.......
2.15
2.20
1000 / Tw, ( 10001 K) Fig. 2. A plot of the evaporation rate on a logarithmic scale vs. lO00/Tbu~,
3.1. Evaporation rate
Despite the careful control of process parameters, for a given set of fixed conditions, fluctuations in the evaporation rate were evident. Discrepancies may have arisen due to a number of factors. 1. The variation of the crystallinity between batches of precursor. 2. Despite rigorous purification procedures, variable amounts of residual glyme may have altered volatility. 3. The constant wrapping and rewrapping of heater tapes in order to weigh the bubbler after each run may have introduced different cold spots in the inlet feeding line. This practice was subsequently eliminated from this work. 4. Despite the autotuning of the temperature controllers, varying degrees of "overshoot" were experienced by the bubbler. Since upstream taps remained open, weight differences may have been spurious in that the amount of material lost may not have been exclusive to the actual growth process. Again this problem has now been circumvented. Such problems are perhaps responsible for the relatively large error determined at low bubbler temperatures, shown by the error bars in Fig. 2, which gives a graph of the evaporation rate plotted on a logarithmic scale against 1000/Tb,b. If we accept that the large error is associated with infrequent, unusually high, calculated weight losses and therefore consiclers only the overall general trends, the fluctuations in the evaporation rate are not of a significant order of magnitude for any given Tb,b. Again, allowing for extreme cases, the average evaporation rate with the lowest Tb,b of 190 °C was 5.53 mg min-l. Subsequent 1 °C increases in temperature resulted in a 10% improvement of this rate, e.g. when Tb,b was fixed at 210 °(2, evaporation rates averaging 12.65 mg min-~ were calculated. This is in good agreement with accepted CVD data [6]. Progressive non-linear increases in the rate were observed up to Tb,b= 240 °C; at least partial
sintering, as previously mentioned, was suspected for the reduction in the size of further increments. It was found that, for a given Tbubwithin the range 250-400 °C, the growth rate was independent of the evaporation rate, alluding to the possibility that this region is a kinetically limited regime for the process. This supposition is supported by the results obtained for the growth rate as a function of the substrate temperature, described later. 3.2. Growth rate dependence on the moist oxygen flow rate
As described previously, all films discussed in this paper were grown using moist oxygen as reactor gas. A variation in this re.actor gas flow rate did not result in a growth rate exhibiting a linear dependence at lower bubbler temperatures. Instead, the optimized growth rate, i.e. the fastest growth rate that could be attained under a particular set of fixed conditions, was thought to depend on the ratio of the precursor vapour to oxygen. As the oxygen flow rate was increased through the series 50, 100, 150 and 200 seem, the fastest growth rate associated with a particular evaporation rate, and therefore precursor concentration, appeared to depend on the attainment of a compatible oxygen flow such that all the available precursor was consumed exclusively in the growth process. In other words, if the amount of oxygen is too small, a portion of the precursor passes through the reactor undecomposed and therefore takes no part in the actual film formation process. If, on the other hand, too much oxyger, is present, the available precursor may be consumed in horaogeneous decomposition processes above the substrate :~urface. Invariably, when this latter condition occurred, powder formation was evident, substantially reducing the growth rate. Table 1 shows the calculated growth rates corresponding to each oxygen flow rate for Tb,b values of 190, 210 and 230 °(7. The carrier gas flow and T~,bwere maintained throu~;hout at 200 seem and 400 °C respectively. The corresponding plot
J. McAleese et aL / Thin Solid Films 286 (1996) 64-71
68
Table I Growth rates determined for three individual T~b values of 190. 210 and 230 °C through the oxygen flow rate series 50, 100, 150 and 200 sccm Growth rate (rim min- ~) at three bubbler temperatures (carrier gas, 200 sccm; 60 min; T~.b= 400 °C)
Moist oxygen flow rate (sccm)
50 100 150 200
190°C
210°C
230 °C
2.2 8.8 8.3 0.0
0.0 13.3 24.0 2.17
1.8 4.8 12.7 35.0
of the growth rate vs. moist oxygen flow is shown in Fig. 3. From the graph, we can see the point where oxygen begins to saturate lower evaporation rates. It appears that, at higher Tb,b, the increase is linear, suggesting that the amount of material being transported at this temperature is not sa,'urated at the oxygen parameter upper limit, namely 200 seem. Fig. 4 shows the non-powder surface of a film grown at a high growth rate (approximately 35 nm min- t ) for 60 min using Tb,b = 230 °C. The evaporation rate generated at this temperature must be sufficiently high such that the concentration of precursor can "accommodate" the 200 sccm of oxygen provided. No powder was evident, with a particle size averaging 2 I~m in contrast with the 0.2 p,m particles on films grown with the same parameters, but an oxygen flow rate of I00 sccm. If we consider that the role of oxygen in the process is to assist the pyrolysis of precursor molecules to some other reactive species, the results can be rationalized by bearing in mind the two competing processes involved at the substrate surface. On the one hand, if the amount of reactive molecular species is compatible with the diffusion rate, the species diffuse to the substrate, undergo condensation and subsequently form a film. On the other hand, if the gas phase concentration of the species is too large, the rate of particle collisions will 40 T
35
- - 0 - lg0°C --B-- 210°O
30
_ _ _ _
/J.
2O
15 10
e ""
t
I
I
I
I
I
I
I
140 160 180 200 220 Moist Oxygen Flow Rate (socm ) Fig. 3, Graph of growth rate vs. moist oxygen flow rate for three separate bubbler temperatures. 40
60
80
100
120
....... : - j
~
:
Fig. 4. The surface of a gadolinium oxide film grown using Tt,,bffi230 °C and Ts,~ffi400 °C; both carder gas and oxygen flow rates were maintained at 200 sccm. Scale bars represent 10 p.m.
also increase substantially. This will lead to the formation of polymers in the gas phase, in other words, powder formation. 3.3, Growth rate dependence on the substrate wmperature
(T.,.,,I,) The growth rate as a function of the substrate temperature (Ts.h) was studied for the lowest Tb.b value of 190 °C. The carrier and oxygen flow rates were maintained at 200 and 100 sccm respectively, I00 sccm having been established as the optimum oxygen flow for the average evaporation rate associated with this specific Tb.b value. Ts.b was varied between 250 and 800 °C in increments of 50 °C. A plot of the growth rate on a logarithmic scale vs. the reciprocal T~.b value is shown in Fig. 5. The resulting curve is typical of that associated with CVD processes [7]. The first low temperature region, between 250 and 375 °C, shows a linear relationship between the growth rate and T~.b; the deposition is considered to be temperature dependent in terms of the activation energy. In this region, the deposition is perhaps controlled by the kinetics of thermal decomposition of the precursor on the hot suhstrate surface forming gadolinium oxide. Between 250 and 375 °C, the surface mobility of the ac~sorbed molecules is suspected to be low and the supersaturation of the gas phase is high; these conditions lead to high nucleation, but low crystal growth rates, and therefore to fine-grained structures. Very little structural surface detail could be ascertained from
J. McAleese er al./ Thin Solid Films 286 (1996) 64-71
87A
6-
4
2
1
1.0
I
I
I
I
1.2
1.4
1.6
1.8
2.0
1000 ! T,,~ ( 10001K )
Fig. 5. Graph of growth rate plotted on a logarithmic scale vs. 1000/T~u~. Tb,bwas maintained at 190 °C throughout.
the microscopy of films grown at low T~.b values such as these. The lack of surface features suggested that the films had a polycrystalline, granular structure with poorly defined grain boundaries and a small grain size, a fact supported by the X-ray diffraction (XRD) results discussed later. Increasing T.~,bup to 350 °C results in the development of a discernible texture. The suspected lack of surface mobility at this and lower temperatures appears to be supported by the less than continuous coverage of the surface as illustrated in Fig. 6, which shows the development of a film grown after 60 min using Tb,L,----230 °C and T~,t,= 350 °C. The acicular particle size in this case averages 0.7 p.m X 0.3 p.m. As the substrate temperature was increased, the deposition rate became independent of T~,kb.In this region, it is thought that the growth rate is limited by the mass transport of the reactive species towards the substrate surface. Within this regime, the deposition rates and surface mobility are relatively high. This, together with the low supersaturation, gives
Fig. 6. Surfacerevealing the development of texture after 60 min. Tb,b= 230 °C; Tsub-----350°C; carder gas and oxygen flow rates, 200 and 150 sccm respectively.
69
rise to the possibility of highly textured, epitaxial layers. The maximum deposition rate in this region was calculated to be approximately 2 p,m h - ~. This could be increased by raising Tb,b and hence the evaporation rate or mass transport of the precursor. Although this was possible, later experiments at higher Tbub values were conducted within the kinetically controlled regime, i.e. temperatures between 250 and 400 °C. This restricted range was determined as the satisfactory limit within which a separate cerium component could be grown successfully [5] and, as such, experimental work on the gadolinium precursor was conducted within these restrictions, considering the possible mixing of these two precursors at a later juncture as previously discussed. As Ts,b was increased between 400 and 600 °C, larger grains with well-defined boundaries became more apparent. Fig. 7 shows an example of this, with a polycrystalline film grown at 600 °C. The circular domain size was typically of the order of 0.5 p,m in diameter. The absence of pores on the surface alluded to the impermeable nature of the deposited layers, which was confirmed by inspection of the cross-section. The relative smoothness of the silicon surface at the
Fig. 7. Continuous. polycrystalline film grown under similar conditions Io Fig. 6, but with TRub= 600 °C.
Fig. 8. Cross-section of a 2 ~m thick gadolinium oxide film grown after 60 rain using T ~ = 190 °C. Ts,b=400 °C and carrier gas and oxygen flow rates of 200 and 100 sccm respectively. Note the lack of any microstructural detail. Scale bar represents i0 Ixm.
J. McAleese et al, / Thin Solid Films 286 (1996) 64-71
70
Ts.b = 800 °C. The increasing extent of powder formation was reflected in the gradual reduction in the growth rate.
3.4. X-Ray diffraction
Fig. 9. Cross.sectionof a 2.3 ~m thick gadoliniumoxidefilmgrownafter 60 rain using the conditions described previouslyfor Fig. 8, but with a substrate temperatureof 600 *(2.
microscopic level allowed the interface between the overlayer and substrate to be distinguished. The microstructural detail of the cross-section exhibited a well-definedcolumnar growth when grown on substrates at high Ts.b, whereas at lower Ts.b a poorly defined structure was revealed. This is illustrated by comparison of Fig. 8 and Fig. 9, where Fig. 8 shows the cross-section of a gadolinium oxide film grown after 60 rain using Tb.b = 190 *C, T,.b = 400 °C and carrier gas and oxygen flow rates of 200 and 100 sccm respectively and Fig. 9 under similar conditions, but with a higher substrate temperature of 600 *(2. Further increases in substrate temperature from 650 °C onwards resulted in the appearance of greater amounts of powder, culminating in a completely powdered coating at
XRD was performed on a Philips PW1710 based diffractometer using Cu Kct radiation with a graphite monochromator. Samples were scanned through a 20 range of 10°-70 ° using a step size of 0.04 ° and a count time of I s. A series of films was grown with varying Ts.b and their corresponding XRD patterns were run with all other parameters kept constant. Fig. 10 shows the XRD patterns obtained. Scan (a) represents the theoretical pattern determined for cubic Gd203 as obtained from JCPDS file No. 43-1014. This is the reference to which the experimentally grown samples (b), (c), (d) and (e), representing Ts.b values of 250, 300, 350 and 400 °C respectively, were compared. As can be seen, the increase in substrate temperature results in a sharpening of the peaks, alluding to the increasing crystallinity of the samples from amorphous at low T~.b to crystalline at higher Ts.b. The overall crystallinity of the samples within this temperature range was low and no preferred orientational effects were observed. Only the one crystalline phase was observed, indicative of cubic phase gadolinia. The dominant peaks emerging at 20 = 28.59 °, 47.54 ° and 56.35 ° correspond to the (222), (440) and (622) reflections of gadolinia respectively. The sharp peak at 33.03 o present in scans (b), (c) and (d) represents the (200) reflection pertinent to silicon only. This peak becomes broadened, as observed in scan (e), when the close proximity of gadolinia (400) is superimposed upon it,
tOOO, [¢oun~:6] gO0, 800: 700: BOO~
'i
5001
400.
d)
3001 2001
(;~2a)
tOOi
a) o. o
~o
I
(440) {332) ;t___,~_~
(431)
(sii) o
[" e]
o
Fig. 10. XRDpatternsrepresentinga seriesof sampleswithvaryingTs.h.Scan (a) representsthe theoreticalpattern for cubic Gd203,whereas(b) (e). (d) and (e) representT~bvaluesof 250, 300, 350 and 400 *Crespectively.
Z McAleese et aL/Thin Solid Films 286 (1996) 64-71
4. Discussion Given that this precursor is a low temperature melting solid, its evaporation rate behaviour appears to be representative of that of a volatile liquid, the increase in rate being comparable with that of other reported materials when experimental errors are taken into account. It has been found that the growth rate of gadolinia can be manipulated by the variation of the reactor gas flow rate, i.e. the growth rate of gadolinia can be accelerated or retarded by altering the amount of oxygen. If we regard our target material to involve the doping of ceria with a small amount of gadolinia, this particular parameter may be of great importance, especially if early results from a potential cerium precursor suggest a linear increase in the growth rate with the oxygen flow rate. The morphology of the thin films depends strongly on the growth rate which, in the kinetically controlled regime p~edominantly studied here, is determined by the substrate temperature. It is a well-established principle that the surface morphology can be improved by Tsubwhich will enhance the grain growth [8]. We found that, when T~ubwas low, the growth rate was also low and films tended to have densely packed non-columnar structures with very smooth surfaces. At higher T~b values and higher growth rates, films exhibited obvious grain boundaries with columnar cross-sectional mierostructures. The results from XRD confirmed that the gadolinium oxide material deposited was cubic gadolinia or GdzO3. In addition, XRD suggested that the crystallinity of the samples improved with increasing substrate temperature.
S. Conclusions The precursor, [{Gd(tmhd)3},~(tetraglyme)], has been shown to be sufficiently volatile and thermally stable for the
71
growth of thick films of cubic phase gadolinia under reduced pressure by MOCVD. This, to our knowledge, is the first .reported example of such films being deposited by any CVD method.
Acknowledgements The authors wish to express their deepest gratitude to several colleagues for their time and expertise: Mr. Colin Robinson for his considerable effort in constructing the CVD apparatus, Mr. Bob Rudkin for assisting with the SEM work and Dr. David Waller for running samples for XRD. Last, but by no means least, the Leverhulme Trust is acknowledged for providing financial support of the project.
References [ I ] B.C.H.Steele,Ceramics bu., 19 (1993) 269. 12] I. Baxter,S.R. Drake,M.B. Hursthouse,KM.A. Malik, J. McAleese, DJ. Otwayand J.C. Plakatouras,hmrg. Chem., 34 (1995) 1384. [3] 1.M. Watson, MP. Atwood,D.A. Cardwell and T.J. Cumberbatch,J. Mater. Chem., 4 (9) (1994) 1393. [4] S.J. Duray,D.B. Buchholz,S.N. Song, D.S. Richeson,J.B. Ketterson, T.J. Marks and R.P.H.Chang,Appl. Phys. Lett., 59 ( 1991) 1503. [5 ] J. McAleese,J.C. Plakatourasand B.C.H.Steele,Thin Solid Fibns 280/ I-2 (1996) 152-159. [61 F. Sehmaderer,R. Hubar,H. Omzmannand G. Wahl,in M.L. Ititchman and N.J Archer(eds.~',Proc. 8th Et~ropean Cons on CVD, LosEditions de Physique,Glasgov,,199I, C2-539, [7] M. Becht.T. Gerfinand K,H.Dahmen,Chem. Mater., 5 (1993) 137. [8] B.S. Kwak, K. Zhang,E.P. Boydand A. Erbil,J. Appl. Phys.. 69 (2) ( 1991) 767.