Self‐Assembled Perovskite‐Fluorite Oblique Nanostructures for Adaptive (Tunable) Electronics

Abstract

Capacitors are essential building blocks of electrical radiofrequency (rf) circuits. Reconfigurable circuits whose electrical characteristics can be altered dynamically are of interest for space and weight economy, especially in future multifunctional, multiband hand-held electronic devices. Electrically tunable capacitors, that is, varactors, are therefore in demand. Two emerging solutions compete over this market: rf-microelectromechanical systems (rf-MEMS) tunable capacitor technology and ferroelectric thin-film capacitor technology. The former has the advantages of very low loss and relatively low voltage operation, but suffers inherently from low speed and reliability problems due to the moving parts at the microsystem level.[1] The latter involves the use of a ferroelectric material as a tunable element, and is attractive for being a fast-response, low-noise, competitive-loss level, [2] robust, all-solid technology, but is limited by circuit considerations due to the high permittivity of ferroelectrics. [3,4] This communication introduces a new, rather unexpected and yet simple way to overcome the limitation of ferroelectric materials by a self-assembled oriented nanocolumnar composite approach, which lowers the permittivity of the material and simultaneously amplifies its tunable response to electric field. A ferroelectric perovskite BaTiO 3 in the form of oblique nanofibers with lateral dimensions as fine as 2-8 nm embedded in dielectric fluorite CeO 2 matrix results in an appreciable tunability with a very low relative permittivity of 50. As a result, the coefficient of dielectric nonlinearity increases up to 25 times that of conventional BaTiO 3 -based ferroelectrics. The system is attractive also in manifesting low dielectric losses and excellent temperature stability of the dielectric properties. Beyond the interest in this approach for the field of reconfigurable microelectronics, the perovskite-fluorite nanocomposite approach shown here may be of interest to other fields, such as oxide fuel microcells with fluorite electrolytes and perovskite cathodes.The high permittivity of ferroelectrics is of a structural origin. It can be tuned by an external electric field. The tunability of the ferroelectric n, under dc field E, is strongly correlated with its initial permittivity e(0), as expressed by n ¼ "ð0Þ="ðEÞ % 1 þ 3 " 0 "ð0Þ ½ 3 bE 2 , for n % 1, and 3" 0 "ð0Þb 1=3 E 2=3 , for n ) 1, where b is the coefficient of the dielectric nonlinearity and e 0 is the permittivity of vacuum. [3] Therefore, a large permittivity is necessary in order to have a large tunability. However, the circuit consideration (impedance matching) imposes the use of low permittivity materials, especially with the increase of operational frequency, and with the drive toward miniaturization, which requires the use of the material in a thin-film, parallel-plate configuration. Consequently, two conflicting demands, namely low permittivity and high tunability, are required, which hinder the use of ferroelectric materials in reconfigura...

Self-assembled ferroelectric-dielectric nanocomposite films for tunable applications

“…The mechanism of the inclined growth is discussed in Ref. [7]. Figure 4a shows the relationship between volume fraction and nominal molar fraction of CeO 2 in our BaTiO 3 -CeO 2 films on Pt(111).…”

“…Self-assembly techniques are therefore attractive. Recently, we found that, by using BaTiO 3 (i.e., BST x=1) as a ferroelectric and CeO 2 as a low-K dielectric, it is possible to self-assembly grow the oriented composite film structures [7]. In this paper, we report on the details and other features of the growth and dielectric properties of those BaTiO 3 -CeO 2 films.…”

“…Surprisingly, the inclined composite film, whose permittivity is substantially reduced to 50, shows yet an appreciable tunability. The appreciable tunability in the inclined composite film is due to the fact that dc field is efficiently applied to the high permittivity ferroelectric phase since both components are electrically connected nearly in parallel [7]. On the other hand, a separately fabricated multilayered CeO 2 /BaTiO 3 /CeO 2 film having a similar volume fraction does not show any recognizable tuning since the BaTiO 3 layer is subjected only to a small field because of the serial connectivity with the low-permittivity CeO 2 layers.…”

Defect interactions with stepped CeO2/SrTiO3 interfaces: Implications for radiation damage evolution and fast ion conduction

“…4 Finerscale thin-film composites that have a higher percentage of interfaces are more appealing for adaptive tunable electronics. 5 In these and other examples, vital properties of materials for advanced technological applications are strongly influenced or even controlled by the existence of interfaces. To optimize materials for these and other applications, it is critical to understand the role the interfaces play in modifying defect behavior within the material.…”

“…In the field of reconfigurable microelectronics, perovskite-fluorite (BaTiO 3 -CeO 2 ) nanocomposites are pivotal as two structurally different immiscible phases, one a ferroelectric and the other a low-permittivity dielectric, are needed for such applications. 5 For use as an effective threeway catalyst in automotive applications 6,7 and high temperature superconductor applications, 8 CeO 2 is often grown on SrTiO 3 (STO) substrates. On the other hand, CeO 2 can also serve as a buffer layer for epitaxial deposition of STO to study ferroelectric properties of STO.…”