Voltage tunable epitaxial Pb_xSr_(1−x)TiO₃ films on sapphire by MOCVD: Nanostructure and microwave properties

Abstract

Frequency and phase agile microwave components such as tunable filters and phase shifters will require ferroelectric thin films that exhibit a nonlinear dependence of dielectric permittivity (ɛ _r) with dc electric bias, as well as a high material (Δɛ _r/tan δ) and device (or K-factor in phase shift/dB) figure of merits (FOM). Therefore, voltage tunable (Pb_0.3Sr_0.7)TiO₃ (PST) thin films (90–150 nm) on (0001) sapphire were deposited by metalorganic chemical vapor deposition at rates of 10–15 nm/min. The as-deposited epitaxial PST films were characterized by Rutherford backscattering spectroscopy, X-ray methods, field emission scanning electron microscope, high resolution transmission electron microscopy, Raman spectroscopy, and electrical methods (7–17 GHz) using coplanar waveguide test structures. The epitaxial relationships were as follows: out-of-plane alignment of [111] PST//[0001] sapphire, and orthogonal in-plane alignments of $[1\bar 10]$ PST//$[10\bar 10]$ sapphire and $[\bar 1\bar 12]$ PST//$[1\bar 210]$ sapphire. The material FOM and device FOM (or K-factor) at 12 GHz were determined to be 632 and ∼13 degrees/dB, respectively. The results are discussed in light of the nanostructure and stress in epi-PST films. Finally, a rational basis for the selection of PST composition, substrate, and process parameters is provided for the fabrication of optimized coplanar waveguide (CPW) phase shifters with very high material and device FOMs.