Mater. Res. Soc. Symp. Proc. Vol. 966

1. Mater. Res. Soc. Symp. Proc. Vol. 966 © 2007 Materials Research Society 0966-T07-23
Molecular Beam Epitaxial Growth and Dielectric Characterization of Ba0.6Sr0.4TiO3 Films
P. Fisher1, M. Skowronski1, P. A. Salvador1, M. Snyder2, J. Xu2, M. Lanagan3, O. Maksimov4,
and V. D. Heydemann4
1
Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA, 15213
2
Pennsylvania State University, Engineering Science and Mechanics, University Park, PA, 16802
3
Pennsylvania State University, Materials Research Institute, University Park, PA, 16802
4
Electro-Optics Center, Pennsylvania State University, 559A Freeport Rd., Freeport, PA, 16229
ABSTRACT
Ba0.6Sr0.4TiO3 films were grown by molecular beam epitaxy on MgO(001) and
LaAlO3(001) substrates. The growth mode was determined to be two-dimensional by in-situ
reflection high-energy electron diffraction. The films were structurally and dielectrically
characterized ex-situ using X-ray diffraction, Rutherford backscattering spectrometry, and split
cavity resonance mode dielectrometry. The structural and dielectric properties of the
Ba0.6Sr0.4TiO3 film grown on MgO were determined to be inferior to the film grown on LaAlO3,
as was indicated by the broader rocking curve (0.59 deg. vs. 0.17 deg.) and higher dielectric loss
(0.29 vs. 0.12).
INTRODUCTION
The Ba1-xSrxTiO3 (BST) system is of significant technological interest owing to its non-
linear dielectric properties [1] that make it suitable for a number of microwave applications,
including filters, phased arrays, and phase shifters [2-6]. The Curie temperature of the
ferroelectric phase of this system decreases in a nearly linear fashion with x, from 400 K for
BaTiO3 to 30 K for SrTiO3 [7]. The x = 0.4 composition, or Ba0.6Sr0.4TiO3, is well-studied
because its Curie temperature is just below room temperature, allowing extensive tunability
without hysteresis under standard operating conditions [7]. Ba0.6Sr0.4TiO3 films were previously
grown by a number of techniques, including pulsed laser deposition [1,3,4,6], sputtering [5,8],
metal organic chemical vapor deposition [3,9], and sol-gel processing [7,10]. Molecular beam
epitaxy (MBE) of Ba0.6Sr0.4TiO3 has been much less studied [11].
Dielectric characterization of BST films is usually performed using capacitor structures
that have either co-planar or parallel-plate electrode configurations. In the first case, a capacitor
structure is fabricated on the surface of the deposited BST film [5]. In the second case, an
underlayer electrode is deposited onto the substrate, followed by the BST film, and then the top
electrode. Both of these techniques have significant drawbacks, particularly for measurements in
the microwave frequency range. For example, the measured capacitance of the co-planar
capacitor structure includes an additional contribution from the substrate; the measured dielectric
loss (tan ) includes contributions both from the electrodes and the substrate. For films measured
δ
in the parallel plate geometry, the crystalline quality of the BST film, and therefore the dielectric
properties, is always limited by the crystalline quality of the bottom electrode.
Split cavity resonance mode dielectrometry is a noncontacting and nondestructive
technique that does not require any sample metallization [12]. It is usually used to characterize
bulk samples. The split-cavity technique is rarely applied to measure thin films because the film
represents a small volume fraction of the overall sample under investigation. However, since the

2. dielectric constant ( r) of tunable dielectric films is often several orders of magnitude larger than
ε
that of the substrate, the technique is applicable for the characterization of BST films.
In this work, Ba0.6Sr0.4TiO3 films were grown on MgO and LaAlO3 substrates by MBE
and split cavity resonance mode dielectrometry was used to measure their dielectric properties.
Although epitaxial films were achieved on both substrates, the crystalline quality of the film on
MgO was inferior to the film on LaAlO3. Furthermore, while both films had similar high
dielectric constants, the film on MgO had a significantly higher dielectric loss than the film on
LaAlO3.
EXPERIMENT
The MBE growth was performed on commercial LaAlO3(001) (using the pseudocubic
cell notation) and MgO(001) substrates. Substrates were etched in a 3:1 HCl:HNO3 solution for 2
to 3 minutes, rinsed in deionized water, and chemically degreased prior to the growth [13]. Next,
they were mounted into Inconel sample holders and loaded into the MBE system (SVT
Associates), described elsewhere [14]. They were annealed prior to the growth for one hour at
750° C under the ozone flux of 0.5 sccm (chamber pressure of 1.8x10-6 Torr). The same
conditions were also used for the Ba0.6Sr0.4TiO3 growth. Ozone was generated with a commercial
unit (Ozone Solutions) capable of producing 6% O3 in O2. It was distilled by passing the O2/O3
mixture through the liquid-nitrogen cooled dewar filled with silica gel; the O3 was adsorbed
while the remnant O2 was pumped away. After storing sufficient amount, the pure ozone stream
was generated by warming the dewar and introduced into the system through the gas injector.
The Ti flux was generated using two high-temperature Ti cells operating at 1550° C. The Sr and
Ba fluxes were produced using standard effusion cells operating at 470° C and 600° C,
respectively.
The growth mode was monitored in situ with a differentially pumped RHEED system
(Staib Instruments) operating at 12.0 kV at an incident angle of 30. The RHEED patterns were
collected using a CCD camera and processed on a computer with commercial software (Safire by
CreaTec). The films were structurally characterized ex-situ using X-ray diffraction (XRD).
Measurements were carried out on Philips X’Pert system (Philips Analytical) in -2 and
θ θ ω
modes to determine out-of-plane orientation and mosaic spread [15]. The composition was
determined using Rutherford backscattering (RBS) spectrometry and film thickness was
measured using X-ray reflectivity. The dielectric properties were studied by resonance mode
dielectrometry using a Gordon Kent (GDK) 0.8” split cavity and an HP 8510 Vector Network
Analyzer [12]. The bare substrates used for film growth were characterized for their dielectric
properties prior to the film growth to have an accurate background subtraction.
DISCUSSION
Fig. 1 shows RHEED images taken for two 90-nm thick Ba0.6Sr0.4TiO3 films grown on
LaAlO3(001) (Figs. 1a and 1b) and MgO(001) (Figs. 1c and 1d) substrates. Sharp 1x1 patterns,
that are characteristic of atomically flat, epitaxial, and highly crystalline surfaces, were clearly
evident in both cases. The absence of any 2-fold reconstructions, either along the (110) azimuths
(Figs. 1a and 1c) or the (100) azimuths (Figs. 1b and 1d), indicated that (Ba + Sr) / Ti ratio was
close to 1 [16].

3. Figure 1. RHEED images of
the Ba0.6Sr0.4TiO3 films grown
on (A and B) LaAlO3(001)
and (C and D) MgO(001)
substrates. The RHEED
images are taken along either
the 110 (A and C) or the 100
(B and D) azimuths.
Fig. 2 shows θ-2θ XRD patterns registered from the same two films. In both cases, only (00l)
Ba0.6Sr0.4TiO3 peaks, with l = 1, 2, and 3, were detected in addition to the substrates’ peaks.
Neither second phases nor alternate orientations were observed. The c-lattice parameters of the
films, calculated from the location of the (002) peaks, were 3.957 Å (2θ = 45.84°) and 3.972 Å
(2θ = 45.65°) for the films grown on MgO and LaAlO3 substrates, respectively. These values are
very close to the bulk lattice parameter of Ba0.6Sr0.4TiO3, a = 3.960 Å (see composition
discussion below). The full-width at half-maximum (FWHM) of rocking curves registered from
the films’ (002) peaks were ~ 0.59° and ~ 0.17°, for the films grown on the MgO and LaAlO3
substrates, respectively. For comparison, the FWHM of rocking curves registered from the (002)
LaAlO3 and MgO peaks were ~ 0.01°. These values are similar to the data reported for the BST
films grown on these substrates [1]. The narrower rocking curve for the film grown on the
LaAlO3 substrate is likely due to the lower lattice mismatch (f = 4.3% for LaAlO3 and f = -6.1%
for MgO) and better structural match (LaAlO3 and Ba0.6Sr0.4TiO3 are perovskites; MgO has the
rock salt structure).
LaAlO3(001) LaAlO3(002)
LaAlO3(003) Figure 2. XRD -2 patterns
θ θ
BST(001)
BST(002) registered from 90-nm thick
Ba0.6Sr0.4TiO3 films grown on
BST(003)
(A) LaAlO3(001) and (B)
Log Intensity (arb. units)
MgO(001) substrates.
MgO(002)
BST(001) BST(002)
BST(003)
20 40 60 80
θ-2θ (Degrees)
Fig. 3 shows an RBS spectrum that was registered from a Ba0.6Sr0.4TiO3 film grown on an
MgO(001) substrate under conditions identical to those used for the films described above.

4. Based on the RBS data, the composition of the film, given as ratios of the atomic percent of the
three cations, was Ti:Sr:Ba 49.9:17.1:33.0. Therefore, while the (Ba + Sr) / Ti ratio was close
≈
to 1 ( 50.1 / 49.9), the Ba to Sr ratio was slightly off the designated 60/40 ( 66/34). Ba excess
≈ ≈
is expected to cause a small increase in the lattice parameter (a = 3.964 Å for Ba0.66Sr0.34TiO3)
and Curie temperature (15-20 K) [7]. Assuming that the relaxed lattice parameter of these films
is ~ 3.964 Å, we calculated a 0.2% expansion of the out-of-plane lattice parameter for film on
LaAlO3 and a 0.1% contraction of the out-of-plane lattice parameter for film on MgO. Both of
which, while very small, are in the expected directions based on the mismatch between the film
and substrates lattices.
6000
Composition Ba Figure 3. RBS spectrum registered
5000
Ti Sr Ba from a Ba0.6Sr0.4TiO3 film grown on
49.9% 17.1% 33.0% an MgO substrate. The composition
determined from this data is given in
4000 the figure.
Yield
3000
Ti Sr
2000
1000 1100 1200 1300
Channel
Split cavity resonance mode measurements were carried out on both films described
above. In Table 1 we present the measured dielectric properties ( r and tan ) for the bare
ε δ
LaAlO3(001) and MgO(001) substrates, as well as for the film/substrate composites. The
(relative) dielectric constant and loss for LaAlO3 (measured at 10.9 GHz) were 23.648 and 4.9 x
10-5, respectively. The values for MgO (measured at 15.5 GHz) were 9.621 and 5.3 x 10-5. These
values are comparable to the data provided by the substrate vendors (MTI Corporation, CrysTec
GmbH) and reported in the literature [17]. More importantly, significant measurable differences
were observed, on both substrates, between the bare substrates and composites. These results
indicate that the low dielectric constants and losses of the MgO and LaAlO3 substrates allow for
the use of the split cavity resonance mode technique in the measurement of films having high
dielectric constants and losses.
The thin film dielectric constants ( r2) were calculated using the following equation:
ε
εeff − V 1εr1
εr 2 = (1)
V2
where eff is the effective dielectric constant of the composite, and V1 and V2 are the volume
ε
fractions of the substrate and film respectively. Equation (2) defines the volume fraction, Vi (i =
1 or 2), of a given phase with respect the its thickness ti:
ti
Vi = (2)
teff
with teff being the total thickness of the composite structure. Once the thin film dielectric
constant was determined, Equation (3) was used to relate the dielectric loss of the thin film (tan

5. δ2) to the known value of the substrate dielectric loss (tan 1) and the measured value of the
δ
overall composite loss (tan eff):
δ
tan δeff (V 1εr1 + V 2εr 2) − V 1εr1 tan δ 1
tan δ 2 = (3).
V 2εr 2
Using these equations, the dielectric constants and losses were extracted from the values
measured for the substrates and composites. The relatively high dielectric constants of 1367 and
1323 were calculated for the Ba0.6Sr0.4TiO3 films on LaAlO3 and MgO substrates, respectively.
Because the dielectric properties of BST films strongly depend on a number of parameters,
including stoichiometry, strain state, and crystalline quality, reported dielectric constants vary
widely (even for the same composition), from under 400 [9] to over 6000 [18]. However, values
between 800 and 2000 are much more common [4,6]. Dielectric losses of 0.125 and 0.295 were
also calculated for the films on LaAlO3 and MgO substrates, respectively. For comparison, the
dielectric losses between 0.01 and 0.2 are generally reported for BST thin films [2]. Although
our values are rather high, they are within the expectation range. Further improvement of the
dielectric properties can be achieved through the optimization of growth conditions.
The loss of the film on MgO was significantly higher, more than double, than that of the
film on LaAlO3. Since the strain states and dielectric constants of the two films were very
similar, we attribute the increase in loss to the lower crystalline quality of the film grown on
MgO. As was previously mentioned, the FWHM of the rocking curve of the film on MgO was
three times larger than of the film on LaAlO3. We should also note that tunability, the relevant
property for device application, has been found to be proportional to dielectric constant. For
example, films with dielectric constants of ~1300 are often observed to have tunabilities of
around 50% [6].
Table 1. Dielectric properties of Ba0.6Sr0.4TiO3 films grown on LaAlO3 and MgO substrates.
Sample Frequency εr tan δ
substrate composite film substrate composite film
BST/LaAlO3 10.9 GHz 23.648 23.875 1367 4.9x10-5 1.235x10-3 0.125
BST/MgO 15.5 GHz 9.621 9.876 1323 5.3x10-5 7.670x10-3 0.295
The accuracy of the split cavity measurements depend on a number of factors. It
increases with increasing film dielectric constant, decreasing substrate dielectric constant, and
increasing film thickness. Here, the theoretical accuracy given this film thickness, film dielectric
constant, and substrate dielectric constant is about 0.5%. However, the actual limit on the
accuracy of these results is likely to arise from film thickness variation across the wafer and/or
the accuracy limits of film thickness measurements. Therefore, the actual error in dielectric
measurements is probably on the order of a few percent.
CONCLUSIONS
In summary, Ba0.6Sr0.4TiO3 thin films were grown by MBE on MgO(001) and
LaAlO3(001) substrates. Structural and dielectric properties of the films were assessed using

6. XRD and split cavity resonance mode dielectrometry. The Ba0.6Sr0.4TiO3 films grew epitaxially
on both substrates, but the rocking curve of the Ba0.6Sr0.4TiO3 film on MgO was broader than
that on LaAlO3 substrate. Although both films had similar dielectric constants in excess of 1300,
the dielectric loss of the film on MgO (0.29) was significantly higher than the dielectric loss of
the film on LaAlO3 (0.12) substrate. We ascribe this difference in the loss values to the lower
crystalline quality of the film grown on MgO substrate.
ACKNOWLEDGMENTS
This work was supported by the Office of Naval Research under grants N00014-05-1-
0238 and N00014-06-1-1018. Any opinions, findings, and conclusions or recommendations
expressed in this material are those of the authors and do not necessarily reflect the views of
the Office of Naval Research.
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