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Effect of Donor, Acceptor, and DonorAcceptor Codoping onthe Electrical Properties of Ba0.6Sr0.4TiO3 Thin Films for Tunable

Device Applications

Yuanyuan Zhang, Genshui Wang, Ying Chen, Fei Cao, Lihui Yang, and Xianlin Dongw

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

We have investigated the effects of donor, acceptor, and donoracceptor codoping on both the dielectric properties and the leak-age current behavior of Ba0.6Sr0.4TiO3 thin films prepared bythe metalorganic solution deposition technique. La and Co wereselected as donor and acceptor dopants, respectively. The elec-trical properties depend strongly on the type of dopants. Com-pared with others, codoped BST films have a much lower losstangent, higher figure of merit, and lower leakage current. The

electronic conduction mechanisms of the three types of dopantsare reported.

I. Introduction

IN recent years, much attention has been paid to the develop-ment of tunable dielectric materials for tunable microwavedevice applications such as phase shifters, filters, delay lines, andso on.14 Barium strontium titanate (Ba1xSrxTiO3, BST) is theleading candidate for such applications due to its highly non-linear dielectric response to an applied electric field.57 For BSTto be used in tunable device applications, the dielectric and in-sulating properties must satisfy the following requirements: hightunability, low leakage current, moderate dielectric constant,

and low dielectric loss.6

To date, a BST thin film that simulta-neously possesses all of these electrical properties as required formany microwave device applications has not been realized. It iswell-known that dopants could significantly modify the electri-cal properties and that processing conditions also exert strongeffects on these modifications. Lower and higher valent substit-uents (which are conventionally referred to as dopants, if theconcentrations are not too high) can be accommodated on the Aor B sites of the ABO3 perovskite lattice and act as acceptorsand donors, respectively. It is well documented that small con-centrations of acceptor dopants, such as Mg21, Co21, Co31,Fe21, Fe31, Sc31, Mn21, Al31, Cr31, and Ni31, which can oc-cupy the B site of the (ABO3) perovskite structure, have beenknown to lower the dielectric loss.711 The mechanism for thisbehavior centers on the thesis that ions with a charge of less than

41 substituting for Ti41

can behave as electron acceptors andprevent the reduction of Ti41 to Ti31. Meanwhile, Lanthanum(La) cationic is recognized as a donor dopant of the A site in theBST system, which can also improve the dielectric proper-ties.6,1214 However, there is lack of investigations on the dielec-tric properties of acceptor and donor codoped BST, especially

on the tunability and leakage current properties.15,16 Here wechose La and Co as donor and acceptor dopants, respectively,and focused on the effects of acceptor, donor, and donoracceptor codoping on the structural and electrical propertiesof BST thin films.

II. Experimental Procedure

In this letter, we studied the properties of BST thin films with thecompositions of Ba0.6Sr0.4TiO3 (BST), 0.5 mol% La-dopedBa0.6Sr0.4TiO3 (LBST), 0.5 mol% Co-doped Ba0.6Sr0.4TiO3(CBST), and 0.25 mol% La plus 0.25 mol% Co codopedBa0.6Sr0.4TiO3 (LCBST). Lanthanum acetate and cobalt acetatewere used as the dopant precursors. All the films were fabricatedby the metalorganic solution deposition technique, followed bydeposition on LaNiO3/SiO2/Si substrates by spin coating at5000 rpm for 30 s. The LaNiO3 films were prepared by a solgel method, and the thickness is about 150 nm. Many more de-tails about LNO electrodes were reported elsewhere.17 Thethickness of the films was about 400 nm, measured by FE-scan-ning electron microscopy (SEM) of the cross-section. The Pt topelectrodes with a 0.2 mm diameter were deposited onto the filmsthrough a shadow mask by direct current sputtering for electri-cal testing. The samples were annealed at 4501C for 5 min in O2in order to improve the interface between the top Pt electrodeand the BST film, after the top electrode deposition.

The structures of BST thin films were characterized by X-raydiffraction (XRD) and SEM. The dielectric properties weremeasured with an Agilent 4294A precision impedance analyzer(Englewood, CO). The tunability is defined in terms of the fol-lowing equation, [C

r(0)Cr(V)]/Cr(0) 100%, where Cr(0) andCr(V) denote the capacitance without and with a direct current

(dc) bias field, respectively. Leakage current was measured atroom temperature (B151C) with a Keithley 6517A electrometer(Cleveland, OH) as the voltage source and a picoammeter. Formeasuring the true leakage current, the polarization current andthe field-induced degradation components can be effectively

avoided by this technique.

III. Results and Discussion

Figure 1 shows the XRD patterns of BST thin films. All the filmshad a typical cubic perovskite structure and were (100) oriented.The (100)-orientation parameter, a100, was calculated from therelative heights of the (100), (110), and (111) diffraction peaks,i.e., a1005 I100/(I1001I1101I111).

18 For BST, LBST, CBST, andLCBST, a100 was 0.25, 0.27, 0.34, and 0.53, respectively. It canbe seen that the dopant can dramatically influence the crystalli-nity, and the codopant is beneficial for the (100) orientation.

The surface and the cross-section microstructure of the thinfilms were investigated using SEM. The surface images indicatedthat all the films have dense and crack-free surfaces with a

fine grain structure. Among all the films, LCBST thin films show

M. W. Colecontributing editor

This work was supported partly by One Hundred Talent Project of Chinese Academy ofSciences, National Basic Research Project (No. 61363ZA09.1), Shanghai QimingxingProject (06QA14055), and Knowledge Innovation Project of Chinese Academy of Sciences(SCX200702).

wAuthor to whom correspondence should be addressed. e-mail: xldong@sunm.shcnc.

ac.cn

Manuscript No. 26155. Received April 17, 2009; approved May 27, 2009.

Journal

J. Am. Ceram. Soc., ]] []]] 13 (2009)

DOI: 10.1111/j.1551-2916.2009.03237.x

r 2009 The American Ceramic Society

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http://i/BWUS/JACE/03237/yyzhang0416@hotmail.comhttp://i/BWUS/JACE/03237/yyzhang0416@hotmail.comhttp://i/BWUS/JACE/03237/yyzhang0416@hotmail.comhttp://i/BWUS/JACE/03237/yyzhang0416@hotmail.com

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a much more uniform surface morphology. No significantmorphological differences could be found between the foursets of BST films.

The dielectric properties of all the BST films were shown inTable I. It shows that dopants had a strong influence on thedielectric properties of the BST thin films. The dielectric con-

stants of all the doped BST thin films have a slight increase,while the loss tangents were lower. LCBST thin films (tand50.025) have the largest figure of merit (tunability/tan d,FOM). It is well demonstrated that the samples composition,grain size, and crystallinity have a strong influence on the di-electric properties of the BST-based thin films. Furthermore, theinfluence of the codoping effect on the structure, microstructure,surface morphology, and dielectric properties should be fullystudied and analyzed.

The doping effect on the leakage current characteristics forthe BST thin films is shown in Fig. 2. The figure shows an ap-preciable decrease in the leakage current for the doped BST thinfilms, especially the La and Co codoped BST thin films. Thistype of IV curve clearly exhibits the different slope region. Inthe low bias region, the slope is close to 1.0, which means that an

ohmic behavior is obtained. At the high bias region, the slopebecomes higher, up to 4.07.0, which corresponds to the ac-complishment of a trap-filling process, leading to a sharp in-crease of the leakage current. The conduction mechanism islikely to be a space-charge limited conduction, with the grainboundaries as distributed traps.1921 According to other works,the net conductivity in BST thin films is n-type, and the follow-ing is the defect equation22:

OO2VO 1=2O22e

0

(1)

We signed this is the intrinsic oxygen vacancy, and its con-centration is half of the electron concentration. La is regarded asa donor doping in the system, and the donors are compensatedby cation vacancies.2325 The incorporation reaction induced the

cation vacancy, as in the following equation:

La2O33TiO2 ! 2LaBa orSr 3TiTi 9OO V

00

Ba orSr

(2)

Based on the Schottky equilibrium, the cation vacancy cansuppress the intrinsic oxygen vacancy concentration, which can

lead to a reduction in the electron concentration through Eq. (1)and then to a decrease of the leakage current. At the same time,the smaller grain size of the films, leading to an increase in the

grain boundary that can be seen as distributed traps, also has acontribution to the low leakage current.21 On the other hand,cobalt doping is believed to be a form of acceptor doping. Thevalence state of Co (21 or 31) is not certain, but for the sake ofsimplicity we adopt 21, and the incorporation is described by

BaO orSrO CoO ! BaBa orSrSr Co00

Ti 2OO V

O

(3)

It means the cobalt doping can remarkably increase the in-duced oxygen vacancy concentration and eventually cause a re-duction in the intrinsic oxygen vacancy, which is half of theelectron concentration through Eq. (1). Kim and Park22 sug-

gested that this can increase the width of the depletion layer orthe barrier height, which will make it more difficult for an elec-tron to pass the potential barrier, finally resulting in a reductionof the leakage current. Obviously, La and Co doping can de-crease the leakage current, and there are two kinds of differentmechanisms. The LCBST thin films have both donor and ac-ceptor doping. La donor doping can induce the cation vacancy,and Co acceptor doping can increase the induced oxygen va-cancy. These all will cause the reduction of the intrinsic oxygenvacancy, which is half of the electron concentration, and thendecrease the leakage current. It means that codoping can com-bine the different mechanisms together and lower the leakagecurrent. At present, it is not clear yet as to which mechanism isdominant and how to accommodate those two mechanisms.Further studies will be carried out to clarify this.

IV. Conclusions

In summary, the dielectric and electrical properties of the BSTthin films strongly depend on the dopants. All the dopants canenhance the tunable properties, and the donoracceptor co-doped BST films have a much higher FOM. The codoped BST

Fig. 1. X-ray diffraction pattern of the films (a) BST, (b) LBST,(c) CBST, (d) LCBST.

Table I. Summary of Dielectric Properties for Undoped and Doped Ba0.6Sr0.4TiO3 Thin Films as a Frequency of 10 kHz

Samples er Tan d Tunabi lity (%) (at 12.5 k V/mm) FOM

Ba0.6Sr0.4TiO3 610 0.043 25.9 6.0Ba0.6Sr0.4TiO310.5 mol% La 636 0.034 32.7 9.6

Ba0.6Sr0.4TiO31

0.5 mol% Co 616 0.028 39.3 14.0Ba0.6Sr0.4TiO310.25 mol% La10.25 mol% Co 630 0.025 38.4 15.4

Fig. 2. Leakage current density of the films as a function of the appliedelectric fields (a) BST, (b) LBST, (c) CBST, (d) LCBST.

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also has a much lower leakage current, because both La (donor)and Co (acceptor) contribute to the reduction of the electronconcentration. There are two different mechanisms workingtogether to diminish the leakage current.

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