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Strongly compensated Ga2O3 is shown to be an intrinsic (or native) p-type conductor with the largest bandgap for any reported p-type transparent semiconductor oxide which may shift the frontiers in fields such as power electronics and photonics.

The use of ultra-wide bandgap transparent conducting beta gallium oxide (-Ga 2 O 3) thin films as electrodes in ferroelectric solar cells is reported. In a new material structure for energy applications, we report a solar cell structure (a light absorber sandwiched in between two electrodes-one of them-transparent) which is not constrained by the Shockley-Queisser limit for open-circuit voltage (V oc) under typical indoor light. The solar blindness of the electrode enables a record-breaking bulk photovoltaic effect (BPE) with white light illumination (general use indoor light). This work opens up the perspective of ferroelectric photovoltaics which are not subject to the Shockley-Queisser limit by bringing into scene solar-blind conducting oxides.

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Flexoelectricity (coupling between polarization and strain gradients) is a property of all dielectric materials that has been theoretically known for decades, but only relatively recently it has begun to attract experimental attention. As a consequence, there are still entire families of materials whose flexoelectric performance is unknown. Such is the case of antiferroelectrics: materials with an antiparallel but switchable arrangement of dipoles. These materials are expected to be flexoelectrically relevant because it has been hypothesised that flexoelectricity could be linked to the origin of their antiferroelectricity. In this work, we have measured the flexoelectricity of two different antiferroelectrics (PbZrO 3 and AgNbO 3 ) as a function of temperature, up to and beyond their Curie temperature. Although their flexocoupling shows a sharp peak at the antiferroelectric phase transition, neither flexoelectricity nor the flexocoupling coefficients are anomalously high, suggesting that it is unlikely that flexoelectricity causes antiferroelectricity. Published by AIP Publishing. https://doi.org/10.1063/1.5044724 Antiferroelectricity was first proposed by Kittel in 1951 in a theory based on antiparallel dipolar displacements analogous to antiferromagnetism, 1 and it was experimentally reported at the end of the same year. 2 Compared to their ferroelectric counterparts, however, antiferroelectrics (AFEs) have been less researched, partly due to their relative rarity, and also because, not being polar, their practical applications are less obvious. So far, they have been studied mostly in the context of electrostatic energy storage 3,4 and also in electrocaloric applications, thanks to their anomalous (negative) effect, 5,6 and for high-strain actuators. 7,8 Recently, a recordbreaking photovoltaic field (6 MV/cm, the highest ever measured for any material) has also been reported in PbZrO 3 , opening a new line for antiferroelectrics in photovoltaic applications. 9 Owing to their centrosymmetric ground state, antiferroelectrics (AFEs) are not suitable for direct piezoelectric transduction (conversion of strain into voltage). They can, however, be flexoelectric (conversion of strain gradient into voltage). This effect is allowed by all crystal symmetries, 10 and it is the result of a linear coupling between a strain gradient and polarization that follows the equationMashkevich and Tolpygo 11,12 were the first ones to propose such an effect, and Kogan 13 later developed the phenomenological theory. Although it was initially predicted that flexoelectricity would be low in simple dielectrics (l % 10 À10 C/m), its proportionality to the permittivity 14,15 meant that it could reach much higher values, of the order of nC/m and even l C/m in ferroelectrics and relaxors. 16 Moreover, thanks to the barrier-layer effects, even bigger effective coefficients (mC/m) can be reached in semiconductors. 17 In addition, flexoelectricity has become a growing field in the last decade with the development of nanoscience, thanks to the i...

or even the standard gas cooling cycle (50%). [1] First theorized in 1878 by William Thomson, [2] the high-temperature changes (ΔT = 12 K) calculated for ferroelectric (FE) thin films [3] and the discovery of an anomalous electrocaloric effect in anti-ferroelectrics (AFE) [3] have renewed its interest, with an eye put on its potential application as a solid-state cooling solution in integrated circuits.FE materials display what is regarded as the "conventional" electrocaloric effect (or positive electrocaloric effect), whereby the material increases temperature (ΔT > 0) when a voltage step is applied and decreases temperature (ΔT < 0) when it is removed. In contrast, AFE display the opposite response; they decrease temperature (ΔT < 0) when an electric field E is applied, and increase (ΔT > 0) when it is removed. The ability of anti-ferroelectrics to cool down despite electrostatic energy being pumped into them is intriguing, and different underlying mechanisms have been proposed for the negative electrocaloric effect. [4,5] Recent experimental evidence indicates that, in the archetypal anti-ferroelectric PbZrO 3 (PZO), the so-called giant negative electrocaloric effect is due to the latent heat absorbed during the adiabatic-field-induced AFE-FE transition, which is endothermic. [6] In PbZrO 3 , the direct link between the anomalous electrocaloric effect and the first-order anti-ferroelectric-ferroelectric switching implies that the field-induced nucleation and motion of the AFE-FE phase boundary will dictate the dynamics of the large negative ECE and, ultimately, the dynamics of electrocaloric devices based on first-order transitions. In ferroelectrics, the study of domain wall dynamics [7][8][9][10][11][12][13][14][15][16] has been examined in detail on account of their relevance for ferroelectric memories. In contrast, there are far fewer works regarding the dynamics of the ferroelectric-paraelectric phase boundaries in FE [17,18] or the anti-ferroelectric-ferroelectric ones in anti-ferroelectrics. [19][20][21][22][23] Yet, a priori, one cannot assume that the dynamics of domain walls will be the same as the dynamics of phase boundaries, while the former separate different domains within the same ferroelectric phase, the latter separate different phases-in antiferroelectrics, an antipolar phase from a field-induced polar one. Domain switching dynamics is defined by a nucleationpropagation process. On the one hand, nucleation refers to the appearance of "hotspots," where nanoscopic nuclei of switched domains appear and rapidly expand forward across the thickness The large electrocaloric coupling in PbZrO 3 allows using high-speed infrared imaging for visualizing anti-ferroelectric switching dynamics via the associated temperature change. It is found that in ceramic samples of homogeneous temperature and thickness, switching is fast due to the generation of multiple nucleation sites, with devices responding in the millisecond range. By introducing gradients of thickness, however, it is possible to change the dyn...

PbTiO3 (PTO) suffers from difficulty in preparing high-density robust bulk ceramics, which in turn has been a bottleneck in thin films growth with physical vapor deposition (PVD) methods. In the present work, we prepared non-doped PTO thin films by a pulsed laser deposition (PLD) method with either a single PTO target or a mosaic target consisting of PbO and TiO2 pie-shaped pieces. On the PTO single target, laser irradiation caused selective ablation of Pb, resulting in Ti-rich cone-shaped pillar structure on the surface, whereas the irradiated surface of PbO and TiO2 pieces was smoother. Epitaxial PTO films deposited on SrTiO3 (001) substrates from the pie-chart targets with PbO:TiO2 areal ratio from 3:5 to 5:3 resulted in composition, crystallinity, flatness, and ferroelectric properties almost independent of the areal ratio. The averaged composition of each film was close to stoichiometric, suggesting a compositional self-control mechanism. For growing epitaxial and high-quality non-doped PTO films, a PbO–TiO2 pie-chart target is advantageous in easiness of handling and stable surface structure.

Lateral compositionally-graded thin films are powerful media for the observation of phase boundaries as well as for high-throughput materials exploration. We herein propose a method to prepare epitaxial lateral compositionally-graded films using a dual-beam pulsed laser deposition (PLD) method with two targets separated by a partition. Tuning the ambient pressure and the partition—substrate gap makes it possible to control of the gradient length of the deposits at the small sizes (≤ 10 mm) suitable for commercial oxide single crystal substrates. A simple Monte Carlo simulation qualitatively reproduced the characteristic features of the lateral thickness distribution. To demonstrate this method, we prepared (1−x)PbTiO3—xPbZrO3 and (1−x)LaMnO3—xLa0.6Sr0.4MnO3 films with lateral composition gradient widths of 10 and 1 mm, respectively, with the partitioned dual PLD.