# Presentation: 604072

## Interface Science of Functional PerovskitesSusanne Stemmer        DMR-0307914

**Interface Science of Functional Perovskites****Susanne Stemmer****        ****DMR-0307914**

***University of California, Santa Barbara***

- Thin films that *combine an electric field tunable dielectric constant* *with low dielectric losses* are of interest for novel high-frequency agile devices. Through composition control and effective use of strain, we have achieved high performance thin films of a novel dielectric, bismuth zinc niobate (BZN). Bulk BZN shows high dielectric losses at microwave frequencies due to dielectric relaxation. We had discovered that **thin film** **strain** can be used to shift the relaxation, potentially allowing for low losses.  This year, we have performed microwave frequency measurements that showed that our BZN films have orders of magnitude lower losses than BZN ceramics.  Figure 1 shows that the device quality factor Q (the inverse of the loss tangent) remains above 200 for frequencies up to 20 GHz and around 1000 up to several GHz for the smaller device. These measurements also show that BZN films allow for lower losses than conventional tunable dielectrics, such as barium strontium titanate (BST). To investigate the origins of the unusually large (for a non-ferroelectric) tunability (*n*) of BZN, *n* was measured as a function of temperature (Fig. 2). A simple model of hopping dipoles did not describe the dielectric tunability of BZN thin films.  A better description was obtained using a random-field model with hopping dipoles in a uniform distribution of random fields (Fig. 2).  The physical basis for this model is the random off-centering of ions in the BZN structure (Fig. 3).

*Fig. 1:  Extracted Q factor and capacitance C** **after accounting for pad parasitics for parallel plate capacitors made from the BZN dielectrics developed in this program.  The Q-factor is the inverse of the loss tangent. BZN ceramics have loss tangents of ~ 0.03 (or a Q of 33) at 1 GHz.*

*Fig. 2: Applied electric field to achieve a tunability n.  Markers: experimental data; solid lines: least-square fits using the model of hopping dipoles in a random field developed in this program.*

**Notes:**
Introduction: Thin films exhibiting an electric-field tunable dielectric constant are being investigated for microwave frequency agile devices, such as phase shifters, filters and tunable matching networks.  In addition to a large tunability, dielectrics with low dielectric losses ar required. Thin film ferroelectrics have large tunabilities, but suffer from high RF losses. Recently, we have shown that Bi1.5Zn1.0Nb1.5O7 (BZN) films show low losses (~ 510-4 ) and large tunabilities (55 %) at 1 MHz.  However, BZN ceramics exhibit a low temperature dielectric relaxation that is accompanied by a dielectric loss peak that shifts to room temperature at microwave frequencies.  Therefore, bulk BZN is believed to be not suitable for lwo loss GHz applications.  However, thin films may have different properties.  Furthermore, we were interested in the origin of the large electric field tunability of this material.  To data, only ferroelectrics are known for large electric field tunabilities and BZN is not a ferroelectric.  A scientific understanding of the dielectric tunability of BZN may aid in the discovery of new tunable dielectrics.
Research Results: Parallel plate capacitors with Bi1.5Zn1.0Nb1.5O7 (BZN) films were fabricated on platinized sapphire.  The total device quality (Q) factor and capacitance were analyzed at frequencies up to 20 GHz by measuring reflection coefficients with a vector network analyzer. These measurements were performed in collaboration with Prof. York’s group at UCSB. The parasitics due to the probe pads were extracted.  The total Q-factor, which included losses from the dielectric and the electrodes, was more than 200 up to 20 GHz for devices with an area of 100 m2.  No significant dispersion in the capacitance, as would be associated with a dielectric relaxation of BZN, was measured. The device Q-factors are several times higher than the best BST devices from our laboratories using similar device layouts.  At 1 GHz, the dielectric tunability is similar to that of BST devices optimized for low losses at microwave frequencies.  Optimization of the electrodes and device designs will likely further improve the performance of the microwave BZN film capacitors, including tunability and device Q-factors, which should make BZN films very attractive for applications in tunable matching networks and voltage-controlled devices in the GHz range.  [J. Park, J. W. Lu, S. Stemmer, R. A. York, Microwave dielectric properties of tunable capacitors employing bismuth zinc niobate thin films, J. Appl. Phys. 97, 084110 (2005)].
We measured the tunability as a function of temperature (Fig. 2).  The tunability was weakly temperature dependent, allowing for temperature stable devices. At temperatures below the onset of the dielectric relaxation, larger electric fields were required for high tunabilities.  A simple model of hopping, non-interacting dipoles was not suited to describe the data in Fig. 2.  The weak temperature dependence implied that additional interactions existed.  For example, the reorientation of dipoles may be affected be random fields.  A better description was obtained using hopping dipoles in a uniform distribution of random fields.  Physically reasonable values for the random fields and the dipole moments were obtained by using this model. Likely sources of random fields are the randomly off-centered ions and the chemical disorder on A- and B-sites found in the BZN structure (Fig. 3) [A.K. Tagantsev, J. W. Lu, S. Stemmer, Temperature dependence of the dielectric tunability of pyrochlore bismuth zinc niobate thin films, Applied Physics Letters 86, 032901 (2005)]

## Interface Science of Functional PerovskitesSusanne Stemmer        DMR-0307914

***University of California, Santa Barbara***

**Supplementary Information:**
- We have focused on developing a new material with a tunable dielectric constant, bismuth zinc niobate (BZN).  Although ferroelectric materials, such as barium strontium titanate, have very large tunabilities, they also show very high dielectric losses, which have prevented the wide-spread application of tunable dielectrics in microwave devices.

**Processing: **We have developed a RF sputter deposition process early in the program1. Due to loss of Zn at elevated temperatures, a process was developed where BZN was sputtered at low substrate temperatures (< 300 °C) and a post-deposition crystallization anneal is performed.  This yielded films with record low dielectric losses (tan < 5x10-4 at 1 MHz) and high dielectric tunabilities (>55%) (see Fig. 4).2
**Structure-Property Relationships**:*** (a) Dielectric losses***: Ceramic BZN of nominally the same composition is believed to be not suitable for tunable microwave applications, due to a dielectric relaxation, i.e. at time lag between the applied electric field and the polarization, as shown by the Penn State group.3  At low frequencies (MHz region) this dielectric relaxation occurs at low temperatures, but it shifts to room temperatures in the GHz region.  We also observed the dielectric relaxation at low temperatures in our films (slide 3, Fig. 5). However, we discovered4 that film strain shifted the dielectric relaxation to lower temperatures, thus possibly allowing for lower losses at higher frequencies (slide 3, Fig. 5).  This discovery opened up the possibility that low losses could be achieved at microwave frequencies.  Our most recent results confirm this.5  By fabricating parallel plate capacitors with BZN as the dielectric, and measuring the device quality (Q) factor at microwave frequencies we demonstrated very low losses of BZN films. The Q-factor is the inverse of the loss and contains contributions from the dielectric and the electrodes and requires careful analysis of the device used for characterization. For example, the observed device size dependence is not related to an intrinsic materials property. These results are presented in Fig. 1 (slide 1).

**Notes:**
References:
J. W. Lu, Z.Q. Chen, T. R. Taylor, S. Stemmer, Composition Control and Dielectric Properties of Bismuth Zinc Niobate Thin Films Synthesized by RF Magnetron Sputtering, Journal of Vacuum Science and Technology A 21, 1745-1751 (2003).
J. W. Lu, S. Stemmer, Low-loss, tunable bismuth zinc niobate films deposited by rf magnetron sputtering, Applied Physics Letters 83, 2411-2413 (2003).
J. C. Nino, M. T. Lanagan, and C. A. Randall, J. Appl. Phys. 89, 4512 (2001); J. C. Nino, M. T. Lanagan, C. A. Randall, and S. Kamba, Appl. Phys. Lett. 81, 4404 (2002).
J. W. Lu, D. O. Klenov, S. Stemmer, Influence of strain on the dielectric relaxation of pyrochlore bismuth zinc niobate thin films, Applied Physics Letters 84, 957-959 (2004).
J. Park, J. W. Lu, S. Stemmer, R. A. York, Microwave dielectric properties of tunable capacitors employing bismuth zinc niobate thin films, Journal of Applied Physics 97, art.-no. 084110 (2005).

## Interface Science of Functional PerovskitesSusanne Stemmer        DMR-0307914

***University of California, Santa Barbara***

**Structure-Property Relationships**:*** (b) Dielectric tunability:*** To date, large dielectric tunabilities have only been observed in ferroelectric materials.  BZN is not ferroelectric. To understand the origin of the tunability of BZN, which would aid in the discovery of new tunable materials, we have developed models that describe the temperature dependence of the tunability of non-ferroelectrics (Fig. 6). The dielectric tunability is believed to be related to the off-centering of ions in the BZN structure, and hopping of ions between energetically equivalent positions, which film strain would affect. We are performing detailed structure analysis of BZN thin films by Rietveld x-ray diffraction refinements (Fig. 7) and by high-angle annular dark-field (HAADF) imaging in STEM (slide 1, Fig. 3).  While the Rietveld structure refinements yielded a film structure that was undistinguishable from bulk BZN, HAADF showed intensities at the cation positions that were different from what was expected.

*Fig. 6: Under an applied field, off-centered ions can hop between energetically equivalent sites (see Fig. 3). A simple model for the electric field as function of T needed to achieve a given tunability n yields:*
*This does not describe the experimental data (Fig. 2). A better fit is obtained using a model of hopping dipoles under the influence of a random field in the structure:*
*Fits to the experimental data to this equation are shown in Fig. 2 (slide 1) and yielded physically reasonable values for the random field E**r **and the dipole moment p**0**.*

## Interface Science of Functional PerovskitesSusanne Stemmer    DMR-0307914

**Interface Science of Functional Perovskites****Susanne Stemmer****    ****DMR-0307914**

***University of California, Santa Barbara***