Pyroelectricity in Highly Stressed Quasi‐Amorphous Thin Films

Abstract

Spontaneous or stress-induced polarization, signifying pyroelectricity and piezoelectricity, respectively, can appear in ionic solids solely due to a non-centrosymmetrical spatial distribution of ions in a polar crystalline structure. Although theory does not impose strict limitations on the size of a polar crystallite, [1,2] the magnitude of pyroelectric and piezoelectric effects of some ceramics, particularly BaTiO 3 , rapidly decrease as grain size diminishes to a few nanometers. [1,3,4] Determination of the minimal number of periodically arranged unit cells for which a crystal retains pyroelectric and piezoelectric properties has become increasingly important due to the rapid incorporation of these materials into nanometer-scale devices. An intriguing complication to this quest arises from the fact that pyro-and piezoelectric effects may exist in structures that lack the spatial periodicity inherent for ionic crystals, but are composed of polar molecules with directional ordering. An example of such a material is a nematic liquid crystal. Spontaneous or stress-induced dipole ordering without fine-tuned positional order is theoretically possible in ionic solids [5] as well; however, only indirect experimental evidence supporting this theory has been presented so far.[6±9] The present work reports on the preparation and pyroelectric properties of quasiamorphous BaTiO 3 thin films that represent a polar ionic solid without spatial periodicity. Pyroelectric amorphous BaTiO 3 films were obtained by passing sputtered amorphous BaTiO 3 films through a steep temperature gradient. As-deposited BaTiO 3 films were stressfree (< ± 30 MPa) with refractive indices in the range of n^= 1.97±2.02, and a very small in-plane±out-of-plane birefringence of n i ± n^= ± (0.002±0.008), confirming film isotropy. A compressive stress of r e = 2.0±2.2 GPa developed in the films passed through the temperature gradient. The refractive index of these films decreased to n^= 1.89/1.94 but the birefringence grew to n i ± n^= 0.03±0.07, corroborating the existence of high in-plane compressive stress.X-ray diffraction (XRD) spectra of the films passed through the temperature gradient were indistinguishable from those of as-deposited films (Fig. 1a). No XRD peaks apart from those of the Si substrate were observed, indicating the absence of a crystalline phase. The volume detection limit of a crystalline phase by XRD calculated from the signal to noise ratio [10] was < 0.3 %. Furthermore, thorough transmission electron microscopy (TEM) investigation only rarely detected the presence of crystalline grains (Fig. 1b) [11] and the concentration of the crystallites estimated from TEM images was far below 0.3 %. In contrast to the similarity with respect to electron and X-ray diffraction analysis, as-deposited films and the films passed through the temperature gradient have distinctly different electrical properties. The films passed through the temperature gradient show a large pyroelectric effect p measured = (1±3) 10 ±9 C cm ±2 K ±1 (5±15 % that of...

Fundamentals of flexoelectricity in solids

“…Recently, Lubomirsky and coworkers [75,[94][95][96] presented experimental data on perovskite thin films, which strongly suggest that a strain gradient can pole an amorphous material when it is thermally treated in a special way. It was shown that during such treatment the material is passing between two different amorphous states.…”

Amorphous Piezo‐ and Pyroelectric Phases of BaZrO3 and SrTiO3

“…However, the commonly accepted wisdom that inorganic solids may only be pyro-and piezoelectric in crystalline form has recently required reconsideration following the report that noncrystalline pyro-and piezoelectric thin films of BaTiO 3 could be prepared on Si substrates. [3] Subsequent investigation of this phase, called quasiamorphous, [4][5][6][7] revealed that amorphous thin films of BaTiO 3 prepared by sputtering contain a network of randomly connected TiO 6 octahedral local bonding units. These TiO 6 octahedra are slightly distorted and, therefore, polar.…”

“…SrTiO 3 differs from BaTiO 3 with respect to the stabilizing cation (Sr instead of Ba). The experimental procedure for preparation of the quasiamorphous phases of BaZrO 3 and SrTiO 3 was similar to that developed for the synthesis of the quasiamorphous phase of BaTiO 3 . [3][4][5]7] Amorphous films of SrTiO 3 or BaZrO 3 (50-200 nm thick) were deposited by radiofrequency magnetron sputtering on highly conductive (>0.01 X cm -1 ) Si substrates.…”

“…The experimental procedure for preparation of the quasiamorphous phases of BaZrO 3 and SrTiO 3 was similar to that developed for the synthesis of the quasiamorphous phase of BaTiO 3 . [3][4][5]7] Amorphous films of SrTiO 3 or BaZrO 3 (50-200 nm thick) were deposited by radiofrequency magnetron sputtering on highly conductive (>0.01 X cm -1 ) Si substrates. The as-deposited amorphous films were then pulled through a temperature gradient in order to induce a strong inhomogeneous stress [3] that could align the local bonding units.…”

“…[3][4][5]7] Amorphous films of SrTiO 3 or BaZrO 3 (50-200 nm thick) were deposited by radiofrequency magnetron sputtering on highly conductive (>0.01 X cm -1 ) Si substrates. The as-deposited amorphous films were then pulled through a temperature gradient in order to induce a strong inhomogeneous stress [3] that could align the local bonding units. The peak temperature in the hot zone was set to be slightly above the temperature of decomposition of peroxides of the stabilizing cation Sr (450-500°C [10] ) and Ba (550-600°C [3] ).…”