Nancy Senabulya

Understanding strain-induced phase transformations in BiFeO3 thin films

Experiments demonstrate that under large epitaxial strain a coexisting striped phase emerges in BiFeO3 thin films, which comprises a tetragonal‐like (T′) and an intermediate S′ polymorph. It exhibits a relatively large piezoelectric response when switching between the coexisting phase and a uniform T′ phase. This strain‐induced phase transformation is investigated through a synergistic combination of first‐principles theory and experiments. The results show that the S′ phase is energetically very close to the T′ phase, but is structurally similar to the bulk rhombohedral (R) phase. By fully characterizing the intermediate S′ polymorph, it is demonstrated that the flat energy landscape resulting in the absence of an energy barrier between the T′ and S′ phases fosters the above‐mentioned reversible phase transformation. This ability to readily transform between the S′ and T′ polymorphs, which have very different octahedral rotation patterns and c/a ratios, is crucial to the enhanced piezoelectricity in strained BiFeO3 films. Additionally, a blueshift in the band gap when moving from R to S′ to T′ is observed. These results emphasize the importance of strain engineering for tuning electromechanical responses or, creating unique energy harvesting photonic structures, in oxide thin film architectures.

Growth of ordered and disordered ZnSnN2

A series of ZnSnN2 films has been grown by plasma assisted molecular beam epitaxy in order to investigate the possibility of controlled cation sublattice disorder as well as its effects on physical and electronic properties of the material. By varying the growth conditions, specifically either the metal to nitrogen flux ratio or the substrate temperature, the authors have confirmed the existence of both the hexagonal and orthorhombic phases of the material via synchrotron x-ray diffraction and in situ reflection high energy electron diffraction measurements. Here, the authors report the results of an initial mapping and analysis of the growth parameter space, as part of continuing efforts to improve material quality.A series of ZnSnN2 films has been grown by plasma assisted molecular beam epitaxy in order to investigate the possibility of controlled cation sublattice disorder as well as its effects on physical and electronic properties of the material. By varying the growth conditions, specifically either the metal to nitrogen flux ratio or the substrate temperature, the authors have confirmed the existence of both the hexagonal and orthorhombic phases of the material via synchrotron x-ray diffraction and in situ reflection high energy electron diffraction measurements. Here, the authors report the results of an initial mapping and analysis of the growth parameter space, as part of continuing efforts to improve material quality.

Origin of thickness dependence of structural phase transition temperatures in highly strained BiFeO3 thin films

Two structural phase transitions are investigated in highly strained BiFeO3 thin films as a function of film thickness and temperature via synchrotron x-ray diffraction. Both transition temperatures (upon heating: monoclinic MC to monoclinic MA to tetragonal) decrease as the film becomes thinner. A film-substrate interface layer, evidenced by half-order peaks, contributes to this behavior, but at larger thicknesses (above a few nanometers), the temperature dependence results from electrostatic considerations akin to size effects in ferroelectric phase transitions, but observed here for structural phase transitions within the ferroelectric phase. For ultra-thin films, the tetragonal structure is stable to low temperatures.

Strain And Symmetry-induced Structural Transitions in Ultra-thin BiFeO3 Films

As one of very few room temperature multiferroic materials, bismuth ferrite (BiFeO3: BFO) has been studied extensively in recent years. The bulk form of BFO is known to have a rhombohedrally distorted quasi-cubic perovskite structure with an (a−,a−,a−) octahedral tilt pattern, exhibiting both anti-ferrodistortive displacements and a spontaneous polarization along the <111> axes. Investigating epitaxial thin films under compressive strain, several studies have reported that the polarization direction is tilted towards the [001] out-of-plane direction, while maintaining a significant in-plane component. This effect is accompanied by a significant enhancement of the spontaneous polarization and a series of phase transitions from rhombohedral (R) for small strains to R-like monoclinic (MA) to T-like monoclinic (MC) and to tetragonal (T) for larger strains [1]. Through synchrotron-based 3-dimensional reciprocal space mapping (RSM), facilitated by using X-ray area detectors (Pilatus 100K pixel detector), we have investigated the structure of ultra-thin BFO films grown on SrTiO3 (STO), LaAlO3 (LAO), and TbScO3 (TSO) substrates with thicknesses of only several unit cells. In this thickness regime, the influence of the substrate atomic structure on the properties of the ultra-thin films is very pronounced, and the films exhibit perfect heteroepitaxy up to a critical thickness when the build up of strain energy forces the films into a relaxed structure. Both on STO [2] and LAO, the ultra-thin BFO undergoes a monoclinic to tetragonal phase transition, but with very different c/a axis ratios. On TSO, a very pronounced and well-ordered stripe domainstructure evolves where the domain sizes are strongly thicknessdependent. Argonne National Laboratorys work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract DE-AC02-06CH11357.