Depletion Layers and Domain Walls in Semiconducting Ferroelectric Thin Films

Abstract

Commonly used ferroelectric perovskites are also wide-band-gap semiconductors. In such materials, the polarization and the space-charge distribution are intimately coupled, and this Letter studies them simultaneously with no a priori ansatz on either. In particular, we study the structure of domain walls and the depletion layers that form at the metal-ferroelectric interfaces. We find the coupling between polarization and space charges leads to the formation of charge double layers at the 90 domain walls, which, like the depletion layers, are also decorated by defects like oxygen vacancies. In contrast, the 180 domain walls do not interact with the defects or space charges. Implications of these results to domain switching and fatigue in ferroelectric devices are discussed. DOI: 10.1103/PhysRevLett.95.247603 PACS numbers: 77.80.Dj, 73.40.ÿc, 75.60.Ch Ferroelectric perovskites that are widely used as actuators, sensors, and memories have classically been modeled as insulators following the Devonshire-Ginsburg-Landau (DGL) theory [1]. These materials, however, are also wideband-gap semiconductors [2]. Consequently, one has bandbending and depletion layers at electrodes and charge layer formation at domain walls, which in turn affect the polarization distribution. These in turn contribute to fatigue and domain-wall pinning.Recent efforts have sought to augment the DGL theory for these effects [3]. However, they assume a priori the depletion layer and space-charge distribution and calculate the resulting polarization distribution, or vice versa. There have also been ab initio studies of the electronic band structure and atomistic studies of defects [4 -6]. However, they provide limited information at the device scale due to enormous computational effort required to handle realistic length scales and geometries. This Letter provides a comprehensive presentation of a semiconducting ferroelectric at the device scale with no a priori assumptions on the polarization or space charge.We focus on the [001]-polarized tetragonal phase of BaTiO 3 to be specific, though many of our findings are generic. This well-studied material displays both 180 and 90 domain walls. Oxygen vacancies are a common defect and they act as donors [6,7]. We focus on the parallel-plate capacitor geometry (electrode-ferroelectric-electrode), which is the building block of many devices. Our results show the formation of depletion layers and the alteration of the polarization near electrodes, reveal some essential differences between 180 and 90 domain walls, show the redistribution of oxygen vacancies to electrodes and across 90 domain walls during annealing, and provide a concrete mechanism for imprinting a 90 domain wall. These results shed new light on various experimental observations.The state of the semiconducting ferroelectric crystal is determined by the polarization density p, the strain , the space-charge density , and the defect density N d , each of which we treat as field quantities. We assume here that all defects are donors motivated...

Characterization of domain walls in BaTiO3 using simultaneous atomic force and piezo response force microscopy

“…It is accepted by now that 90°domain walls trap defects, including dopants and oxygen vacancies, and such defects widen the domain wall. 9,[15][16][17] The variations we have observed are consistent with this mechanism.…”

“…Finally, theoretical studies 13,15,17 have shown that the domain walls may bend close to free surfaces and have structures different from the bulk. Our observations, specifically the independence of observations with tip geometry and applied voltage, suggest that these do not significantly affect our conclusions.…”

Photochemistry on a polarisable semi-conductor: what do we understand today?

“…7 and 8). A depletion layer of approximately 100 nm thickness is observed near the Pt electrodes, accompanied by a voltage increase of 1.4 V across the depletion layers [40]. The space charge density is at a maximum near the electrodes and is %0 in the bulk.…”

“…In semiconducting ferroelectric thin films the coupling between polarisation and space charges leads to the formation of charge double layers at 90°domain walls, which are decorated by defects such as dopants, oxygen vacancies [40]. There is no such charge layer formation in 180°walls.…”

“…Further results have been completed to compare the electric potential, space charge density, defect density and polarisation in the surface layer to the bulk [40] of a ferroelectric material. It should be noted that there are expected significant differences in the behaviour of materials as they are reduced in thickness as the region of space layer approaches the thickness of the sample.…”