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