Ferroelectric thin films with polarization gradients normal to the growth surface

Abstract

Thin film ferroelectrics with polarization gradients normal to the growth surface readily form when gradients in temperature, strain, or composition are coupled to the polarization vector in ferroelectric materials. This letter describes the formation of thin films of potassium tantalum niobate with graded polarizations obtained by grading the tantalum to niobium ratio of the ferroelectric phase. Unlike a simple structure consisting of laminated layers of ferroelectric material, the polarization gradient which forms breaks the natural symmetry of the ferroelectric material at any given plane, resulting in a self-poling of the structure subsequent to excitation by an oscillatory electric field. Once poled, the devices reveal a measurable potential across the structure which varies with temperature.

Large built-in electric fields due to flexoelectricity in compositionally graded ferroelectric thin films

“…Such macroscopic inversion symmetry breaking and the associated novel physical properties have attracted widespread experimental and theoretical attention in the past decade. In contrast to single-component ferroelectrics, the degeneracy between the two polarization states in these materials is broken by a built-in electric field, which has been shown to result in self-poling, 1 shifted hysteresis loops, 2,3 enhanced susceptibilities, [4][5][6][7] and signatures of geometric frustration. 8 Such a built-in bias within the material is typically generated by an inhomogeneous strain through lattice engineering (via multicomponent superlattices) 9,10 or a global composition gradient.…”

“…8 Such a built-in bias within the material is typically generated by an inhomogeneous strain through lattice engineering (via multicomponent superlattices) 9,10 or a global composition gradient. [1][2][3][4][5][6] Since the built-in fields can result in enhanced susceptibilities and directly affect the polarization switching characteristics, understanding the origin of the observed behavior is key to utilizing it in practical applications such as nonvolatile memories, piezoelectric sensors, and thermal imaging systems. In spite of extensive theoretical 4,11-13 and experimental [1][2][3][4][5][6][7] studies on compositionally graded ferroelectric heterostructures over the last two decades, the physical mechanism behind the built-in fields has been difficult to identify due to a plethora of intrinsic and extrinsic factors that have been proposed to give rise to the observed behavior.…”

Understanding order in compositionally graded ferroelectrics: Flexoelectricity, gradient, and depolarization field effects

“…Particularly fruitful has been the combination of advanced GLD models with recent developments in the growth of ferroelectric thin films which has enabled unprecedented study of model versions and understanding of the fundamental physics of these materials [22][23][24][25]. This also includes the ability to produce deterministically controlled gradients of phys-* lwmartin@berkeley.edu ical quantities such as strain and composition in ferroelectric films [1,6,8,[26][27][28]. In order to account for the resulting spatial asymmetries that form in these graded films, researchers have suggested the possible inclusion of a number of additional energy terms to adapt such phenomenological models, including (1) flexoelectric, (2) gradient, and (3) depolarization energies [3,10,29,30].…”

“…In contrast to their homogeneous counterparts, compositionally graded ferroelectrics exhibit self-poling [1], built-in potentials [2], asymmetric or shifted hysteresis loop [3], and the potential for geometric frustration [4]. Their distinctive electric-field, thermal, and stress susceptibilities make such systems potentially important for a range of devices [2].…”

Performance Enhanced Complex Oxide Thin Films for Temperature Stable Tunable Device Applications: A Materials Design and Process Science Prospective

“…For the BST thin film, not only is the dielectric constant much lower, it also does not have a sharp peak as a function of temperature. This broad dielectric anomaly, indicative of a diffuse phase transition has been attributed to the finer grain sizes, residual strains, composition heterogeneities inherent to synthesis (Kim et al, 2000, Zhang et al, 2010, Mantese et al, 1995. This observed flattening of the dielectrictemperature peak in thin film BST with respect to that of bulk ceramic BST has led many to incorrectly conclude that BST in thin film form is temperature stable.…”