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Electronic Materials and Applications 2013Orlando, FL
January 25, 2013
Electric Field Tunable (Ba,Sr)TiO3 Films Grown by Molecular Beam Epitaxy
Evgeny Mikheev, Adam Kajdos, Adam Hauser, and Susanne Stemmer
Materials Department, University of California, Santa Barbara
Outline
• Electric field tunable (Ba,Sr)TiO3 (BST) films
• Losses and tunability (state-of-the-art)
• Growth of highly perfect perovskite oxides by MBE
• Control of stoichiometry and point defects in SrTiO3
• High-performance BST films by MBE
• Summary
Electric Field Tunable BST FilmsFerroelectric Materials for Microwave Tunable Applications 7
losses) without and under dc field is presented and dis-cussed, first for ideal ferroelectrics, then for real mate-rials (including defects) and for multiphase materials(composites). The evaluation methods of the relevantproperties are described and the significance of the ob-tained results is assessed. Experimental work on prop-erties of modern tunable ferroelectrics is summarized,based on results reported in the literature and on resultsat the authors’ laboratory. These results are discussedin the context of the theory.
2. Applications
The applications of tunable ferroelectrics in microwavedevices are summarized below in order to estimate theperformance that is required from the materials. A de-tailed discussion of designs and applications of ferro-electric based microwave components can be found inRefs. [2, 3, 5, 7, 8].
For a microwave engineer the main attraction of fer-roelectric materials is the strong dependence of their di-electric permittivity ε on the applied bias electric fieldE0. This characteristic is commonly described by thetunability n defined as the ratio of the dielectric per-mittivity of the material at zero electric field to its per-mittivity at some non-zero electric field, as expressedby Eq. (2.1). The relative tunability (nr ) is defined byEq. (2.2).
n = ε(0)ε(E0)
(2.1)
nr = ε(0) − ε(E0)ε(0)
= 1 − 1n
(2.2)
An example of dielectric constant—electric field de-pendence for a bulk ferroelectric material ((Ba,Sr)TiO3
[9]) is presented in Fig. 1.The dielectric loss in ferroelectrics is not as small
as that of common microwave dielectric materials andthe loss tangent (tan δ) is an important characteristicof the material, which should be taken into account inthe device design. The temperature dependence of thedielectric permittivity over the operating temperatureinterval is another important issue: this is of particularconcern in the vicinity of the ferroelectric transitiontemperature, in which the material exhibits a high butstrongly temperature dependent tunability. The trend“the higher the dielectric constant, the higher the tun-ability, loss, and temperature dependence of the dielec-tric permittivity” is observed for many dielectrics [10,
Fig. 1. Typical field dependence of the dielectric permittivity of atunable ferroelectric material Ba0.6Sr0.4TiO3 [9].
11]. This trend can be traced across different materials,it is illustrated in Table 1.
The correlation between the tunability and the losstangent forces designers to choose the material with theoptimal trade-off between these two parameters for bestdevice performance. This optimal trade-off is found byan integral parameter called the Commutation QualityFactor (K ). The commutation quality factor was orig-inally introduced to characterize the switching prop-erties of semiconductor two-state microwave switches(p-i-n diodes [15] and field effect transistors). It wasshown that this parameter does not depend on the char-acteristics of the passive network that is used togetherwith the switchable element in the microwave circuitdesigns. In other words, the factor K can be consid-ered as an invariant that characterizes the tunable per-formance of the material. Later this theory was adaptedto other switchable and tunable components includingferroelectrics [16]. The Commutation Quality Factorfor ferroelectrics is given by:
K = (n − 1)2
n · tan δ(Umin) · tan δ(Umax), (2.3)
where Umax and Umin are the voltages applied in the twostates of the ferroelectric capacitor and n is the tunabil-ity of the capacitor determined as the ratio ε(Umin)/ε(Umax).
In theory, all forms of ferroelectric materials, bulksingle crystals, bulk ceramics, thin films, and thickfilms can be used in RF and microwave tunable el-ements. Each of these forms has its advantages andshortcomings.
εr tanδ Q f (GHz)SrTiO3 314 0.0006 1667 2.883
Sr0.96Ba0.04TiO3 323 0.00068 1470 2.484Sr0.94Ba0.06TiO3 327 0.00073 1369 2.428Sr0.85Ba0.14TiO3 406 0.00065 1538 2.162Sr0.90Ba0.10TiO3 365 0.00068 1470 2.301Sr0.8Ba0.20TiO3 401 0.001 1000 2.780Sr0.70Ba0.30TiO3 570 0.0014 714 2.284Sr0.60Ba0.40TiO3 878 0.0025 400 1.887
A. Tagantsev and V. Sherman, J. Electroceram. 11, 5–66 (2003).
M. Lanagan, Ph.D. Thesis, Penn State (1987)
• (Ba,Sr)TiO3 combines low losses with high tunabilities at room temperature
• Thin films typically show much lower tunabilities and higher dielectric losses
• Films with high tunabilities show high losses, and vice versa
BST Thin Film Properties
• Tanδ no better than 0.005 (Q = 200) and tunability no better than 2:1
• Films that have a 2:1 tunability have Q ~ 20
Jackson et al., J. Mater. Sci. 44, 5288 (2009).
Loss Mechanisms
Mechanism tanδ(f) tanδ(εr)
DeviceDC Leakage (low f) ~ 1/f ~ 1/ε
DeviceHigh frequency roll-off ~ f ~ ε
Intrinsic
3-quantum ~ f ~ ε1.5
Intrinsic 4-quantum ~ f ~ ε1.5Intrinsic
Quasi-Debye ~ f ~ nr ~ 1/ε
Extrinsic (Defects)
Charged defects ~ ε
Extrinsic (Defects) Local polar regions ~ ε4.5-d (d ~ 0-2)Extrinsic (Defects)
Universal relaxation f0.1
A. Tagantsev and V. Sherman, J. Electroceram. 11, 5–66 (2003).R. A. York, in: Multifunctional Adaptive Microwave Circuits and Systems, edited by M. Steer and W. D. Palmer (SciTech Publishing, 2009).
• Most dielectric loss mechanism cause an increase in loss with higher permittivity
• Exception: Quasi-Debye (breaking of centrosymmetry results in a dependence of phonon frequencies on applied field)
Loss Mechanisms
Mechanism tanδ(f) tanδ(εr)
DeviceDC Leakage (low f) ~ 1/f ~ 1/ε
DeviceHigh frequency roll-off ~ f ~ ε
R. A. York, in: Multifunctional Adaptive Microwave Circuits and Systems, edited by M. Steer and W. D. Palmer (SciTech Publishing, 2009).
• To minimize high-frequency roll-off, need to minimize Rs (electrode resistance) and C (small capacitors)
Loss Mechanisms
Mechanism tanδ(f) tanδ(εr)
DeviceDC Leakage (low f) ~ 1/f ~ 1/ε
DeviceHigh frequency roll-off ~ f ~ ε
Intrinsic
3-quantum ~ f ~ ε1.5
Intrinsic 4-quantum ~ f ~ ε1.5Intrinsic
Quasi-Debye ~ f ~ nr ~ 1/ε
Extrinsic (Defects)
Charged defects ~ ε
Extrinsic (Defects) Local polar regions ~ ε4.5-d (d ~ 0-2)Extrinsic (Defects)
Universal relaxation f0.1
A. Tagantsev and V. Sherman, J. Electroceram. 11, 5–66 (2003).R. A. York, in: Multifunctional Adaptive Microwave Circuits and Systems, edited by M. Steer and W. D. Palmer (SciTech Publishing, 2009).
• Dielectric losses due to defects scale with dielectric constant
• Highly tunable films with high permittivity are particularly susceptible to defects
• Need to improve materials quality
Thin Film Deposition Techniques
• Most BST films have been deposited by PLD or sputtering
• Three issues:
• Impurities (H, C, transition metals, ...)
• Energetic deposition creating defects
• Poor stoichiometry control during deposition
0.0 0.1 1 10 100 Energy (eV)
RFsputtering
Direct ion beam
Laser vaporization (PLD)
MBECVD Ion beam
sputtering Ionimplantation
Regime I Regime II Regime III
Cuomo et al., J. Appl. Phys. 70 1706 (1991).
Oxide Molecular Beam Epitaxy
!
!"#$%%&'()*#+$,,
-.#$%%&'()*#+$,,
/012$*#3,"'4"#')&.+$
5$6",#).2"*(+#')&.+$!"1".789,3$.6()*#2"&2$
:,$+6.)*#2&*%).#;<::=
;$'(&7",#2"'"*",1>$.
-&?'6."6$4"*(3&,"6).
5"*(3&,"6).#%).'"43,$#6."*'%$.
;<::=-+.$$*
@.1)'A.)&7
-"43,$
<$"6$.
TiO
C
• High purity
• Low energetic deposition
• Stoichiometry control?
• Without an MBE growth window, stoichiometry control requires precise flux control → only possible to 0.1 - 1 %
• Corresponds to defect concentrations of 1020-1021 cm-3
Molecular Beam Epitaxy
Temperature (1000/K)
Gas
Pre
ssur
e (T
orr)
GaAs(s) ⇔ Ga(l) + As(g)
4 As(s) ⇔ As4(g)
MBE Growth Window
Temperature (°C)
4As(s) ⇔ As4(g)
GaAs(s) ⇔ Ga(l) + As(g)
MBE Growth Window
C.D. Theis et al., Thin Solid Films 325, 107 (1998).J. Tsao, Materials Fundamentals of Molecular Beam Epitaxy.
Below this line, solid As will not precipitate → excess As desorbs in the chamber
Above this line, As will condense on a Ga-rich GaAs surface
A wide MBE growth window is largely
responsible for the ease and success of III-V MBE
Oxide Molecular Beam Epitaxy
MBE growth window
• TTIP desorption leads to a growth window at practical substrate temperatures and fluxes
• Stoichiometry is self-regulating within the growth window
• No need for precise flux control• Shift to higher TTIP/Sr flux ratios with
increasing temperatures shows that desorption of TTIP is responsible for growth window
SrTiO3
SrTiO3
oxygen plasma source
Sr source TTIP
B. Jalan, R. Engel-Herbert, N. J. Wright, S. Stemmer, J. Vac. Sci. and Technol. A 27, 461 (2009). B. Jalan, P. Moetakef, S. Stemmer, Appl. Phys. Lett. 95, 032906 (2009).
Oxide Molecular Beam Epitaxy
• 1:1 correspondence of La-concentration and free carrier concentration over several orders of magnitude
• Excellent control over carrier concentrations → concentration of unintentional defects below doping level
• Higher mobility than single crystals• Lower concentration of charged impurities, such as Al, Fe ..., in MBE films
MBE films
J. Son, P. Moetakef, B. Jalan, O. Bierwagen, N. J. Wright, R. Engel-Herbert, S. Stemmer,
Nature Mater. 9, 482 (2010).PLD data: Y. Kozuka et al., Appl. Phys. Lett. 97, 012107 (2010).
La:SrTiO3
SrTiO3
Outline
• Electric field tunable (Ba,Sr)TiO3 (BST) films
• Losses and tunability (state-of-the-art)
• Growth of highly perfect perovskite oxides by MBE
• Control of stoichiometry and point defects in SrTiO3
• High-performance BST films by MBE
• Summary
Parallel Plate Capacitor Structures
• Allows for high fields and lower tuning voltages• Interpretation of measured device Q in terms of film dielectric loss
straight-forward at frequencies below 1 - 10 MHz• BST films need to be grown on metal bottom electrodes
Epitaxial Pt Bottom Electrodes
(001) Pt
(001) SrTiO3
100
101
102
103
104
105
106
Inte
nsity
[C
ount
s]
24.524.023.523.022.522.0
ω (°)
13 nm Pt 39 nm Pt 65 nm Pt
002Pt 002SrTiO3
RMS = 1.087 nm6 nm 13 nm
39 nm 65 nm
1 µm
RMS = 0.283 nm
RMS = 0.371 nm RMS = 0.387 nm
10 nm
5 nm
0 nm
• Epitaxial, sputtered Pt bottom electrodes
• Strain relaxation by twinning
J. Son, J. Cagnon, S. Stemmer, J. Appl. Phys. 106, 043525 (2009).
Pt
SrTiO3 20 nm
Epitaxial Pt Bottom Electrodes(001) Pt
(001) SrTiO3
• Although Pt is closely matched with SrTiO3, (111) oriented Pt grains tend to grow upon heating up before BST growth. To minimize the problem:• Anneal Pt in O2 (RTA, 1000 oC, 30 min) prior to growth of BST• Heat platinized substrate to growth temperature under O2 plasma• Limit BST growth temperature to 750 oC
300 oC
750 oC
Pt surface RHEED
With O2 pre-anneal Without O2 pre-anneal
Molecular Beam Epitaxy of BST Films• Ba/Sr ratio determined by RBS and
x-ray diffraction• Fix Ba and Sr fluxes• Vary TTIP BEP• Tsub = 750 °C, no wide growth
window
[100]
[110]
Sr rich Ti rich
• A/B site stoichiometric films have minimum lattice parameter and 4× surface reconstruction along [110]
• Very similar to SrTiO3
(Ba0.1Sr0.9)yTi1-yO3
(Ba,Sr)TiO3
(001) SrTiO3 substrate
Molecular Beam Epitaxy of BST Films
(BaxSr1-x)TiO3
(001) (Ba1-xSrx)TiO3
(001) SrTiO3
100 nm Pt
• Predominantly (001) oriented BST• Small amounts of (111) and (110)
orientation variants present in BST and/or Pt
(110)
MBE BST Films: Dielectric Quality Factor(BaxSr1-x)TiO3
K. Bethe, Philips Research Reports 2, 1 (1970); M. T. Lanagan, Microwave Dielectric Properties of Antiferroelectric Lead Zirconate, Ph.D. Thesis, The Pennsylvania State University, 1987; H. V. Alexandru, et al., Mater. Sci. Eng. B 118, 92 (2005); Im, et al., Appl. Phys. Lett. 76, 625 (2000)’ N. K. Pervez, et al., Appl. Phys. Lett. 85, 4451 (2004); A. Vorobiev, et al., J. Appl. Phys. 96, 4642 (2004); X. Y. Zhang, et al., J. Appl. Phys. 104, 124110 (2008).
• Very high quality factors > 1000• Decrease in Q with permittivity (Ba
content)
• Universal, inverse relationship between Q and permittivity (Ba content)
• MBE films have higher Q than even bulk BST materials
E. Mikheev, A. P. Kajdos, A. J. Hauser, and S. Stemmer, Appl. Phys. Lett. 101, 252906 (2012).
MBE BST Films: Dielectric Tunability(BaxSr1-x)TiO3 • Films with higher Ba content
are more tunable• Greater than 1:5 tunabilities can
be obtained• Films with lowest Ba content
still show 1:3 tunability
E. Mikheev, A. P. Kajdos, A. J. Hauser, and S. Stemmer, Appl. Phys. Lett. 101, 252906 (2012).
• Q decreases with electric field• Show figure of merit as a
function of field, rather than the commonly used n(E)Q(0)
MBE BST Films: Figure of Merit(BaxSr1-x)TiO3
Use commutation quality factor as a function of electric field as a figure of merit:*
CQF E( ) =n(E)!1( )
2
n E( )Q E( )Q 0( )
* I. B. Vendik, O. G. Vendik, and E. L. Kollberg, IEEE Trans. Microw. Theory Tech. 48,
802 (2000).
Jackson et al., J. Mater. Sci. 44, 5288 (2009).
• Combination of large tunability and low losses results in CQFs that are much higher than in literature
• Note the values here are at 1 MHz, but BST itself is not expected to degrade into the few-GHz regime
MBE BST Films: Dielectric Quality Factor
MBE (BaxSr1-x)TiO3 Typical sputtered BST
Pervez et al., Appl. Phys. Lett. 85, 4451 (2004)
Q decreases with field (at low Ba content)
Q initially increases with field, due to suppression of the dielectric constant
• Intrinsic, quasi-Debye loss shows a decrease in Q with field (breaking of the symmetry*
• Not found in films for which extrinsic (defect) losses dominate• High-quality BST films allow for investigation of intrinsic losses
* K. F. Astafiev et al., Appl. Phys. Lett. 84, 2385 (2004).
MBE BST Films: Permittivity
• Bulk-like dielectric constant is surprising, because BST thin films usually suffer from a “dielectric deadlayer” that reduces the apparent permittivity*
• Strain effects that offset permittivity reduction?• More investigations needed
M. T. Lanagan, Microwave Dielectric Properties of Antiferroelectric Lead Zirconate, Ph.D. Thesis, The Pennsylvania State University, 1987; H. V. Alexandru, et al., Mater. Sci. Eng. B 118, 92 (2005); Im, et al., Appl. Phys. Lett. 76, 625 (2000)’ N. K. Pervez, et al., Appl. Phys. Lett. 85, 4451 (2004); X. Y. Zhang, et al., J. Appl. Phys. 104, 124110 (2008).
*S. K. Streiffer et al., J. Appl. Phys. 86, 4565 (1999);D. S. Boesch, et al., Appl. Phys. Expr. 1, 091602 (2008).
Substrate
Pt BST
Pt Deadlayers
Summary
• MBE allows for BST films with losses that are lower than those of the best bulk materials, while maintaing a high dielectric tunability
• The quality of the Pt bottom electrode was found to be crucial for achieving low leakage and high breakdown strength
• Stoichiometry control is the major factor allowing for low losses, while extended defects (grain boundaries) appear to be less detrimental
• High-quality MBE films allow for low-loss microwave devices
• Future studies should focus on minimizing device-related loss mechanisms