The effect of annealing on the structure and dielectric properties of Ba x Sr1x TiO3ferroelectric thin filmsL. A. Knauss, J. M. Pond, J. S. Horwitz, D. B. Chrisey, C. H. Mueller, and Randolph Treece

Citation: Applied Physics Letters 69, 25 (1996); doi: 10.1063/1.118106 View online: http://dx.doi.org/10.1063/1.118106 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/69/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Microstructure dependence of electrical properties of (Ba0.5Sr0.5)TiO3 thin films deposited on Pt/SiO2/Si J. Appl. Phys. 80, 5891 (1996); 10.1063/1.363583 Structural and optical properties of laser deposited ferroelectric (Sr0.2Ba0.8)TiO3 thin films J. Appl. Phys. 79, 7965 (1996); 10.1063/1.362346 A systematic study on structural and dielectric properties of lead zirconate titanate/(Pb,La)(Zr(1x)Ti(x))O3 thinfilms deposited by metalloorganic decomposition technology J. Appl. Phys. 79, 4283 (1996); 10.1063/1.361797 The preparation of superconducting thin films of YBa2Cu3O7x by COevaporation with electronbeam/thermalsources AIP Conf. Proc. 165, 28 (1988); 10.1063/1.37110 Growth of YBa2Cu3O70x thin films by sputtering AIP Conf. Proc. 165, 50 (1988); 10.1063/1.37080

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The effect of annealing on the structure and dielectric propertiesof Ba xSr12xTiO3 ferroelectric thin films

L. A. Knauss,a) J. M. Pond, J. S. Horwitz, and D. B. ChriseyNaval Research Laboratory, Code 6670, 4555 Overlook Avenue, SW, Washington, D.C. 20375

C. H. Mueller and Randolph TreeceSCT, 720 Corporate Circle, Golden, Colorado 80401

~Received 9 February 1996; accepted for publication 26 April 1996!

The effect of a postdeposition anneal on the structure and dielectric properties of epitaxiaBaxSr12xTiO3 ~BST! thin films ~x50.35–0.65! have been measured. The films were grown bypulsed laser deposition on LaAlO3 ~001! substrates. The films were single phase and~001! orientedwith a lattice parameter larger than the bulk. The dielectric properties of thex50.35 film exhibiteda broad temperature dependence and a peak at 168 K, which is 36 K below the peak observedbulk BST ~x50.35!. Annealing films for 8 h in flowing oxygen at 900 °C caused the latticeparameter to decrease and dielectric properties to become more like the bulk. Annealing alsresulted in an increased electric field dependent dielectric tuning without increased dielectric loss© 1996 American Institute of Physics.@S0003-6951~96!00127-1#

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Ferroelectrics are a class of nonlinear dielectrics thathibit an electric field dependent dielectric constant. Thematerials are currently being used to develop active micwave electronics such as phase shifters, frequency agileters, and tunable highQ resonators.1,2 Thin film ferroelectricsoffer several advantages over bulk ferroelectrics for thapplications. Large electric fields~0–200 kV/cm! can beachieved in thin films~;0.5 mm! using low bias voltages~0–10 V!.2 BaxSr12xTiO 3 ~BST! is currently the material ofchoice for microwave applications due to its low loss acomposition dependent Curie temperature,Tc . The Tc ofbulk BST ranges from 30 to 400 K forx50 to x51,respectively.3 The ability to control the dielectric propertiein a simple way will allow device structures to be easoptimized for maximum tunability and minimum loss at thdesired frequency and operating temperature. We reporinvestigation of the effects of a postdeposition anneal onstructural and dielectric properties of BST thin films.

Thin films of BST on ~001! LaAlO3 were grown bypulsed laser deposition~PLD! for compositionsx50.35,0.50, and 0.65. The PLD system used to grow these Bfilms has been described previously in more detail.4,5 Thefilms were grown on LaAlO3 at 750 °C in an oxygen ambienpressure of 350 mTorr to a total film thickness of 0.6mm.6

The films were then cooled to room temperature in oxygen;10 °C/min.

Films were annealed in flowing oxygen at 900 °C for 8in a quartz tube furnace. Structural measurements were mbefore and after annealing the films by x-ray diffraction uing u/2u scans andv scans. Measurements of the dielectrproperties were made on as-deposited and annealed filming 1500 Å Cr/Au interdigital electrodes on the surfacethe BST films. The interdigital electrodes had a gap spacof 10mm, a finger width of 7.5mm, and a finger length of 75mm. The capacitance and relative dissipation factor wmeasured as a function of temperature~30–375 K! invacuum and with dc bias voltages~0–40 V! at 1 MHz using

a!NRL/NRC Cooperative Research Associate.

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a HP4285ALCR meter. In the coplanar geometry, the ca-pacitance measures the dielectric properties of vacuum, thfilm, and the LaAlO3 substrate. A reference measurementwas made on a bare LaAlO3 substrate, which showed a neg-ligible temperature and dc bias field dependence. Thuschanges in the capacitance as a function of temperature werdue to changes in the dielectric susceptibility of the film.

BST films were found, by x-ray diffraction, to be singlephase and exclusively~001! oriented with a~002! v-scan fullwidth at half-maximum between 0.16° and 0.50°. The filmlattice parameters normal to the substrate were preciselymeasured by x-ray diffraction before and after annealing forBST ~x50.35, 0.50, and 0.65! using a technique describedelsewhere.6,7 The lattice parameters for the as-deposited andannealed thin films and bulk BST are plotted in Fig. 1 as afunction of % Ba composition. The as-deposited films hadlarger lattice parameters than bulk BST samples of the samcomposition. After annealing in oxygen, the lattice param-eters were smaller and closer to the bulk lattice parametersThe change in the lattice parameter may be caused by a

FIG. 1. Lattice parameters normal to the film surface vs % Ba composition,determined by x-ray diffraction, for as-deposited and annealed thin films on~001! LaAlO3. The composition dependence of the bulk BST lattice param-eter has been determined by Vegard’s law using Slater-type orbitals andBST ~x50.5! as references. Lattice parameter error is60.0001 Å.

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increase in oxygen stoichiometry for the film or by a redution in strain, which is caused by the 3% lattice mismatbetween the film and substrate.

Capacitance and dissipation factor measurements aMHz were made as a function of temperature for severalbias electric fields for a BST thin film of compositionx50.35 ~Fig. 2!. Field biases were applied from 0 to 4kV/cm ~40 V applied for a gap spacing of 10mm!. Thecapacitance is suppressed by as much as 20% with increadc bias, and the peak in the capacitance as a functiontemperature shifts to higher temperatures with increasingbias. This behavior is typical of a ferroelectric, but the temperature dependence is significantly different from that of tbulk material. The capacitance vs temperature curve forfilm is much broader than that of the bulk with a peak at 1K. This is about 36 K lower than observed in the bulk.similar peak shift has been observed previously in thin filmof x50.35 and 0.50 composition.2,8 The dissipation factor~tan d! is presented in Fig. 3 for the as-depositedx50.35composition. The lowest value of the measured dissipatfactor is 731023 at 375 K and increases to 231022 at thepeak. By applying a 40 kV/cm dc bias electric field, thdissipation factor can be reduced by 29% at the peak. Th

FIG. 2. Capacitance vs temperature measured at 1 MHz for dc bias elefields from 0 to 40 kV/cm~0–40 V! in 5 kV/cm steps for as-deposited BST~x50.35!.

FIG. 3. Dissipation factor~tan d! as a function of temperature measured1 MHz for dc bias electric fields from 0 to 40 kV/cm~0–40 V! in 5 kV/cmsteps for as-deposited BST~x50.35!.

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is a second peak in the dissipation factor between 50 andK that is coincident with a shoulder in the capacitance. Thiis attributed to a second phase transition from tetragonalorthorhombic with decreasing temperature.3

After annealing the films for 8 h at 900 °C inoxygen,significant changes were observed in the dielectric propeties. Figure 4 shows the capacitance and dissipation factora function of temperature for the as-deposited and annealfilms of BST ~x50.35!. The measurements were made at 1MHz without a dc bias electric field. The peak in the tem-perature dependence of the annealed film capacitancecreased in temperature by 26 K, which is only 10 K belowthe bulk transition temperature. Also, the breadth of the temperature dependence of the capacitance and dissipation facis narrower in the annealed film. When cooling the film from375 to 30 K, the annealed film had a lower dissipation factoat high temperature that did not increase until the temperture was closer to the transition temperature. This is in contrast to the as-deposited film for which the dissipation factoincreased immediately with decreasing temperature. Thtemperature at which the dissipation factor was maximizealso shifted closer to the capacitance peak after annealing

The postdeposition anneal also provided greater tunabity with reduced dielectric losses. Figure 5 shows the tunabiity as a function of dc bias. From these results, the maximumtunability, for a 40 kV/cm field, is found to be 20% for theas-deposited films and 32% for the annealed films. Thmaximum tunability occurs at the capacitance peak, whichalso the temperature at which the dissipation factor is largesA better comparison for device applications would be in aregion where the losses are minimal. The lowest loss for thas-deposited film is tand 5 731023 where the tunability isonly 1%, but for the same loss tangent value in the annealefilm there is 8% tuning. Furthermore, at the highest temperature, 374 K, the annealed film has a loss tangent as low431023 where a tuning of 2.4% is still available. By chang-ing the film composition, the temperature of the most desirable film properties could be shifted to the operating temperature of a particular device.

In summary, we have shown that high quality thin filmsof BST ~x50.35–0.65! suitable for use in active microwave

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FIG. 4. Capacitance and dissipation factor vs temperature for BST~x50.35!as-deposited and annealed films. Measurements were made at 1 MHz wout a dc bias field.

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circuits have been grown by PLD on~001! LaAlO3 sub-strates. Deposited films were annealed at 900 °C for 8 h. Tlattice parameter for all compositions of the annealed filmare closer to the bulk lattice parameters. The capacitancethe as-deposited film as a function of temperature is browith a maximum at a lower temperature than the bulk. Tcapacitance and dissipation factor of the annealed filmshibit a narrower temperature dependence and peaks that

FIG. 5. Capacitance tuning„@C~0!2C~v!#/C~0!3100… vs dc bias for BST~x50.35! at 1 MHz.

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closer to the bulk transition temperature. Also, the dielectrilosses in the annealed films are lower than the as-depositfilms in the high temperature range. The annealed film hagreater tunability with reduced loss. In particular, for a lossof 731023, the as-deposited film had a tunability of 1%,while the annealed film had a tunability of 8%. The changin dielectric properties as a function of oxygen annealingmay be due to filling of oxygen vacancies or a reduction infilm strain.

Support of this research has been provided by the Officof Naval Research and by Superconducting Core Technolgies NCRADA-NRL-94-027.

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