Interface Science of Functional Perovskites Susanne Stemmer DMR-0307914

1. Short Top Contact (Au) SiO2 cross-over A A' BZN film Open Substrate DUT Device area: 50 ~ 900 m2 Bottom electrode (Pt) De-embedded Data 100 m2 225 m2 225 m2 100 m2 1 nm Interface Science of Functional PerovskitesSusanne StemmerDMR-0307914 University of California, Santa Barbara Thin films that combine an electric field tunable dielectric constantwith 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 filmstrain 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 Cafter 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. 3: (Left:) BZN structure with random off-centering of ions (yellow: O, green: Bi). Hopping between the off-centered positions is the physical basis of the model used to describe the tunability n (Fig. 2). (Right:) HAADF image acquired in STEM of a BZN film. The variation in intensity is due to random substitution of Zn on the Bi-sites and one possible origin of random fields incorporated in the model used in Fig. 2. 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.

2. Fig. 4: Applied bias field dependence of the permittivity and dielectric loss of a BZN film on Al2O3/Pt at 1 MHz Interface Science of Functional PerovskitesSusanne StemmerDMR-0307914 University of California, Santa Barbara 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). 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.

3. Top Electrode Bottom Electrode BZN Film SiO2 Film Fig. 5: Influence of film strain on the dielectric relaxation of BZN films. The film strain was controlled by deposition on different substrates with different thermal mismatch. The amorphous SiO2 interlayer served to obtain the same microstructures on all substrates. The film on vycor is under large tensile stress and did not show a dielectric relaxation (loss peak, drop in permittivity) in the measured temperature range. Substrate Fig. 7: Rietveld x-ray powder diffraction refinement of a BZN film on vycor. The refinement showed two phases, zirconolite and pyrochlore. The off-centering of ions was similar to that found in the bulk material. Interface Science of Functional PerovskitesSusanne StemmerDMR-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 Er and the dipole moment p0.

4. Education: Outreach: Professor Stemmer gave several talks at outreach symposia. In September 2005, she will participate in a workshop “Future Trends in Materials Science” organized by the NSF funded UCSB International Center for Materials Research. She gave a tutorial overview on tunable dielectrics at the International Microwave Symposium in June 2005. The notes from from this tutorial are posted on her webpage. In the summer of 2004 she participated in a mini-conference with students and faculty from minority serving institutions (organized by the NSF-funded UC-HBCU AGEP partnership). The program supports one graduate student (Mr. Jiwei Lu). The program has also supported several undergraduate research interns, most recently Ms. Lia Bregante (who will be attending the UCSB Materials program as a graduate student in the Fall of 2005) and Mr. Stephen Vampola (Spring 2005), who is also planning to attend graduate school. Prof. Stemmer has also developed several new courses at UCSB. Interface Science of Functional PerovskitesSusanne StemmerDMR-0307914 University of California, Santa Barbara