Microstructure dependence on substrate orientation

Figure 7.1 shows plan view SEM images of films grown on a-, r-, and c-sapphire substrates and etched with 5% HCl for 5 seconds. All films exhibit densification and grain growth when compared with films containing no liquid-phase addition, however the final microstructure varies from a relatively homogeneous grain size distribution ina-sapphire, to a distribution exhibiting a large fraction of abnormal grain growth onc-sapphire.

Using linear intercept stereology on large field of view high-resolution SEM images, greater than two hundred grains for each sample were measured. Figure 7.2 shows the cumulative

Figure 7.1: SEM plan view images of etched films grown on a) a-sapphire, b) r-sapphire, and c)c-sapphire. The average grain size, heterogeneity, and prevalence of twins increases as the

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 0 . 0 0 . 5 1 . 0

Cu

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Figure 7.2: Cumulative frequency distribution of grain sizes measured using linear intercept from SEM plan-view. As the prevalence of twins increases, the average grain size and fraction of significantly large grains increases.

frequency distribution of grain sizes for films from the three different cuts of sapphire. The average grain size increased from 130 nm on a-sapphire to 180 nm and 270 nm on r- and

c-sapphire respectively. Corresponding to the increase in average size is a large increase in the fraction of grains exhibiting abnormal growth. Ona-sapphire only 1% of grains measured were larger than 300 nm; however, this fraction increases to 10% onr-sapphire and 30% onc-sapphire.

X-ray diffraction (XRD) analysis, shown in Figure 7.3, reveals the presence of a highly oriented BaAl2O4 second phase in the c- and r-sapphire samples; on a-sapphire, the same

second phase is found, but its crystallographic orientation is close to random. Electron energy loss spectroscopy (EELS) and selected area electron diffraction confirm the presence of this Ti-free phase, and that it is restricted to the sapphire-film interface. From electron diffraction, an epitaxial relationship was identified between the BaAl2O4 and bothr- and c- sapphire, this

accounts for the relatively strong (100) and (002) family of peaks in the respective x-ray patterns for each orientation.297 In the case of ther-sapphire, BaAl2O4 (002) orients parallel to (006) of

2 0

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B a A l₂ O ₄ B a T i O ₃ a - s a p p h i r e r - s a p p h i r e

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it

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.)

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Figure 7.3: Bragg-Brentano geometry x-ray diffraction scans for samples grown on a-, r-, and c-sapphire. Unlabeled peaks are due to substrate reflections. The BaTiO3 peak intensities

decrease with decreasing grain size (from c- tor- toa-). Samples grown onc-sapphire exhibit a Lotgering factor of 0.15 for (100) reflections.

sapphire, where both are now tilted relative to the substrate surface, which for Bragg-Brentano geometry where ω= 2θ₂ , results in an intense (100) family. In the films grown ona-sapphire no preferred orientation was found between the barium aluminate and the substrate leading to the observation of only the strongest BaAl2O4 reflections.

The YBCO community previously reported a comparable epitaxial interface which results from Al out-diffusion from a sapphire substrate into the growing film.298 Burch et al.confirm this behavior in the BaTiO3-BBO-sapphire system, and show that aluminum loss leads first to

highly disordered sapphire that finally compensates through a phase transformation to the less denseγ-Al2O3 structure.297Due to the similar permittivity values of BaAl2O4 and sapphire and

the confinement to the substrate-film interface, BaAl2O4 does not lead to significantly decreased

Owing to the highly oriented nature of the BaAl2O4, XRD alone is insufficient for assessing

volume fractions of the second phase via relative intensities. However, the BaTiO3 peaks do

decrease in intensity fromc- tor- to a-, which we attribute to the decrease in grain size. As seen in Figure 7.1, the films grown on c-sapphire contain abnormally large grains that often appear to have a (100) orientation. This trend is confirmed in the XRD, where films on r- or a- sapphire exhibit no preferred orientation, but films onc-sapphire exhibit moderate increases in intensity of (100) family reflections resulting in a Lotgering factor of 0.15.

Figure 7.4 shows TEM cross-sections with BaAl2O4 grains outlined in white. Multiple image

analysis reveals that the quantity of BaAl2O4 decreases as the c-plane is tilted from parallel, to

perpendicular to the substrate surface. In the c-sapphire sample we estimate a total volume fraction below 5%. We attribute the change orientation dependence of second phase formation to anisotropic Al diffusion in sapphire. Considering that the only source of Al is the sapphire substrate, a larger quantity of BaAl2O4 requires more out-diffusion of Al, suggesting faster

egress from the (0002) sapphire surface. Unfortunately, reported values for Al self-diffusion vary by eight orders of magnitude due to difficulties in the production and use of 26_{Al as a tracer}

element.299 As such, we cannot link the present observations to an accepted set of diffusion reference data.

Electron diffraction identified {111} twins on c-sapphire samples, highlighted in Figure 7.4.297 SEM plan view images allow one to collect more statistically relevant data set by using of a 5% HCl etch that brings relief to twins. Both SEM and TEM reveal that the presence of twin lamellae is nearly universal in the largest observed grains, and, as seen in Figure 7.1, the prevalence of abnormally large grains decreases from c- tor- toa-. TEM cross-sections reveal that the volume fraction of BaAl2O4 also decreases fromc- to r- to a- and is correlated with a

reduction in abnormal grains and the associated twinning.

The BaAl2O4 second phase occurs at approximately 870 ◦C, which is the expected melting

point the BBO flux. This temperature is also associated with a strengthening and sharpening of BaTiO3 diffraction peaks, which we attribute to both improved crystalline quality as well as

Figure 7.4: TEM cross-sections for films grown on a)a-sapphire, b)r-sapphire, and c)c-sapphire. BaAl2O4 is outlined in white, and the relative volume fraction of second phase increases froma-

a significant amount of grain growth.297 This connection between grain growth, second phase formation, and liquid phase formation draws striking parallels to the traditional TiO2 rich

BaTiO3 growth mechanisms.217,221,222,228,230,231

Recently, groups have shown that stoichiometric BaTiO3 can exhibit abnormal grain growth

by inserting Ba6Ti17O40 seed grains that nucleate {111} twins and drive abnormal grain

growth.228Our own system mirrors this pathway for abnormal grain growth, but at significantly lower temperatures than previously observed. Specifically our system shares: 1) presence of a liquid phase, 2) formation of a second phase at flux melting point,297 3) presence of{111} twins, and 4) dependence of twinning on second phase morphology. Additionally from TEM cross- sections, twins show a tendency to intersect with the film-substrate interface where BaAl2O4 is

likely present. For these reasons we hypothesize that the BaAl2O4 is seeding twins in manner

similar to Ba6Ti17O40in the TiO2 rich system. This system demonstrates the lowest temperature

formation of twins and abnormal grain growth in polycrystalline barium titanate observed to date.

The addition of modifiers to BaTiO3 is widely reported to manipulate microstructure

evolution by modifying the critical driving force needed for grain growth.226,227,233,234 Other groups have specifically studied abnormal grain growth in Al-doped BaTiO3, reporting that

Ba4Ti10Al2O27, which is structurally similar to Ba6Ti17O40, nucleates{111}twins.226In contrast,

Fisher et al. found that higher Al2O3 concentrations inhibited abnormal grain growth below

the Ba6Ti17O40-BaTiO3 eutectic,236 but increased abnormal grain growth above the melting

temperature.235 In both cases Fisher found that Ba6Ti17O40 was associated with twin formation.

In the BaO-B2O3 fluxed BaTiO3 system, annealing of samples in hot-stage XRD reveals no

intermediate crystalline phases297 and during extensive TEM work we have found no evidence of any barium polytitanates.

In an effort to seed {111}twins, buffer layers of Ba6Ti17O40 and (002)-BaAl2O4 thin films

were prepared using PLD on c-sapphire substrates. Figure 7.5 shows the resulting BaTiO3

abnormal grain growth. In the case of the polytitanate buffer, this serves as further evidence that the twin nucleation takes place via a different pathway that in Ti-rich ceramics.

The lack of twinning in the sample grown on BaAl2O4 (a pre-grown layer) suggests the

simultaneous formation of BaAl2O4 and/or the participation of Al2O3 in the liquid phase is

critical for the observed twinning behavior. Alternatively, it is possible that patchy morphology of the in situ reacted BaAl2O4 exposes specific facets that promote twin formation.

On c-sapphire the abnormal grains tend to grow with (100) texture, visible in both SEM and in the relative intensities of XRD reflections, suggesting a crystallographic relationship between the BaAl2O4 and the nucleated twins. In ceramics, Ba6Ti17O40 particles promote (111)

texture,220 and XRD work shows that Ba6Ti17O40 tends to grow with its (001)m close packed

planes parallel to the (111)t close packed planes of BaTiO3.227 Further TEM work showed that

although the (111)t and (001)m planes are parallel on the micron scale, local faceting gives rise

to two epitaxial relationships at the nanoscale.221 It is likely that the present system has similar orientation relationships, but the simultaneous role of a liquid phase complicates our ability to identify the possible topotactic relationships.

In document Liquid-Phase Processing of Barium Titanate Thin Films. (Page 106-113)