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The bulk photovoltaic effect (BPE) has drawn considerable attention due to its ability to generate photovoltages above the bandgap and reports of highly enhanced photovoltaic current when using nanoscale absorbers or nanoscale electrodes, which, however, do not lend themselves to practical, scalable implementation. Herein, it is shown that a strikingly high BPE photoresponse can be achieved in an ordinary thin-film configuration merely by tuning fundamental ferroelectric properties. Nonmonotonic dependence of the responsivity (RSC) on the ferroelectric polarization is observed and at the optimal value of the film polarization, a more than three orders of magnitude increase in the RSC from the bulk BaTiO3 value is obtained, reaching RSC close to 10−2 A W−1, the highest value reported to date for the archetypical ferroelectric BaTiO3 films. Results challenge the applicability of standard first-principles-based descriptions of BPE to thin films and the inherent weakness of BPE in ferroelectric thin films.

Ordering of ferroelectric polarization1 and its trajectory in response to an electric field2 are essential for the operation of non-volatile memories3, transducers4 and electro-optic devices5. However, for voltage control of capacitance and frequency agility in telecommunication devices, domain walls have long been thought to be a hindrance because they lead to high dielectric loss and hysteresis in the device response to an applied electric field6. To avoid these effects, tunable dielectrics are often operated under piezoelectric resonance conditions, relying on operation well above the ferroelectric Curie temperature7, where tunability is compromised. Therefore, there is an unavoidable trade-off between the requirements of high tunability and low loss in tunable dielectric devices, which leads to severe limitations on their figure of merit. Here we show that domain structure can in fact be exploited to obtain ultralow loss and exceptional frequency selectivity without piezoelectric resonance. We use intrinsically tunable materials with properties that are defined not only by their chemical composition, but also by the proximity and accessibility of thermodynamically predicted strain-induced, ferroelectric domain-wall variants8. The resulting gigahertz microwave tunability and dielectric loss are better than those of the best film devices by one to two orders of magnitude and comparable to those of bulk single crystals. The measured quality factors exceed the theoretically predicted zero-field intrinsic limit owing to domain-wall fluctuations, rather than field-induced piezoelectric oscillations, which are usually associated with resonance. Resonant frequency tuning across the entire L, S and C microwave bands (1-8 gigahertz) is achieved in an individual device-a range about 100 times larger than that of the best intrinsically tunable material. These results point to a rich phase space of possible nanometre-scale domain structures that can be used to surmount current limitations, and demonstrate a promising strategy for obtaining ultrahigh frequency agility and low-loss microwave devices.

An understanding of the growth of (110) quantum wells (QWs) is of great importance to spin systems due to the observed long spin relaxation times. In this article, we report on the metamorphic growth and characterization of high mobility undoped InAs (110) QWs on GaAs (110) substrates. A low-temperature nucleation layer reduces dislocation density, results in tilting of the subsequent buffer layer and increases the electron mobility of the QW structure. The mobility varies widely and systematically (4000-16 000 cm2/Vs at room temperature) with deposition temperature and layer thicknesses. Low-temperature transport measurements exhibit Shubnikov de-Haas oscillations and quantized plateaus in the quantum Hall regime. [ABSTRACT FROM AUTHOR]

We have successfully synthesized the colossal dielectric constant oxide La2-xSrxNiO4 in thin film form by reactive cosputtering from metallic targets and careful annealing protocols. Composition and phase purity was determined through energy dispersive spectra and x-ray diffraction, respectively. The dielectric constant exceeds values of over 20 000 up to 1 kHz and the activation energy for the frequency-independent conductivity plateau was extracted to be approximately 155 meV from 300 to 473 K, both in agreement with measurements conducted on bulk single crystals. However, unlike in single crystals, we observe early onset of relaxation in thin films indicating the crucial role of grain boundaries in influencing the dielectric response. ac conductivity at varying temperatures is analyzed within the framework of the universal dielectric law leading to an exponent of approximately 0.3, dependent on the electrode material. Impedance spectroscopy with electrodes of different work function (Pt, Pd, and Ag) was further carried out as a function of temperature and applied bias to provide mechanistic insights into the nature of the dielectric response. [ABSTRACT FROM AUTHOR]

The electrical characteristics of reactive metal–oxide interfaces are largely dependent on the thermodynamics and kinetics of interfacial reactions that may occur during synthesis and further processing conditions. Such reactions may lead to undesirable properties such as increased loss tangent, frequency dispersion, or asymmetric capacitance. Metals such as Ni are being explored as electrodes in various electronic packaging as well as on-chip decoupling capacitors and this presents a challenge to advance processing routes that render utilization of high-dielectric constant oxide thin films. In this work, we demonstrate that photon-assisted oxidation enables a unique low temperature processing route to synthesize reduced loss (Ba,Sr)TiO3 thin film based capacitors with Ni bottom electrodes. Our results show a decrease in the dielectric loss tangent in photon-assisted oxidized films compared to conventionally re-oxidized at an applied field of 0.33 MV/cm, tan δ of 0.08 versus 0.63, respectively. Leakage currents showed a significant decrease, 7.8 A/cm2 for conventional re-oxidation versus 0.046 A/cm2 for photon assisted, at an applied field of 0.33 MV/cm. The results are of relevance to processing reactive materials wherein athermal routes are necessary to avoid interfacial reactions. [ABSTRACT FROM AUTHOR]

Ferroelectric oxides have been demonstrated to be promising in developing emerging photovoltaic technologies because of the various mechanisms that allow above‐band‐gap photovoltages and higher efficiencies. However, the wide band gaps of conventional ferroelectric oxides (2.7‐4 eV) limit their utilization of the solar spectrum. Here the phase stability, absorption properties, ferroelectric, and photovoltaic responses of Ni‐ and Ni‐Nb‐substituted BaTiO3 were explored to evaluate their potential as visible‐light‐absorbing photovoltaic materials. Although the acceptor substitution of Ni2+ stabilized a hexagonal 6H polymorph in both systems, post‐annealing treatments allowed restoration of a tetragonal 3C phase for Ba(Ti0.99Ni0.01)O2.99 and (0.9)BaTiO3‐(0.1)Ba(Ni1/2Nb1/2)O2.75. The oxygen vacancies accompanying the Ni and Ni‐Nb substitutions significantly lower the optical band gap of BaTiO3 to ~1.5 eV and the visible light absorption can be systematically tuned between 380 and 1000 nm by varying the Ni:Nb ratio. Room temperature ferroelectricity was observed in Ba(Ti0.99Ni0.01)O2.99 with a saturation polarization = 18 μC/cm2 and remnant polarization = 1 μC/cm2. The Ni‐Nb substituted composition (0.9)BaTiO3‐(0.1)Ba(Ni1/2Nb1/2)O2.75 shows a ferroelectric response with a remnant polarization of 5 μC/cm2 at 77 K, which gradually decreases as temperature increases. Both compositions exhibit ferroelectrically switchable photoresponses under an AM 1.5 G sunlight simulator; the highest switchable steady‐state current of 8 nA/cm2 observed for (0.9)BaTiO3‐(0.1)Ba(Ni1/2Nb1/2)O2.75 exceeds those reported in previous studies of BaTiO3 ceramic samples (J Solid State Chem, 1975; 12: 193; Jpn J Appl Phys, 2013; 52: 09KF03). [ABSTRACT FROM AUTHOR]

InxGay Al1−x−y Sb alloys have been grown by molecular beam epitaxy for use as a buffer layer for growing semiconductors on GaAs substrates with lattice constants beyond that of AlSb. This is an extension of the use of AlxGa1−xSb alloys to accommodate for the lattice mismatch with semi-insulating GaAs substrates. The growth of In0.21Ga0.19Al0.6Sb with a 6.2 Å lattice constant on semi-insulating GaAs substrates is the focus of this work. Several measures of the quality of a 1μm-thick In0.21Ga0.19Al0.6Sb layer improved when the growth temperature was increased from 460 to 600°C. Atomic force microscopy root-mean-square values decreased from 2.9 to 1.8nm and the peak-to-valley values decreased from 17.7 to 9.7nm. In addition, double crystal x-ray diffraction ω-2Θ spectra linewidths decreased from 568 to 482arc sec. At the lower growth temperatures, several photoluminescence (PL) peaks associated with radiative recombination from regions with different alloy compositions were found. However, on increasing the growth temperature a single PL line was observed, strongly suggesting a more uniform alloy composition. [ABSTRACT FROM AUTHOR]

pn junctions formed with complex oxides may be of interest in emerging fields of electronics and energy conversion owing in part to their rich electronic structure. We report on the successful synthesis of epitaxial La1.875Sr0.125NiO4 (LSNO) films on Nb-doped SrTiO3 (Nb:STO) single crystals and a detailed study of their electrical properties. The junctions measured displayed highly rectifying current-voltage characteristic from 298 to 373 K. Mechanism for rectification is likely due to the formation of a pn junction between heavily doped n-type Nb:STO and semiconducting p-type LSNO. Capacitance-voltage (C-V) measurements as a function of temperature and frequency were used to determine the admittance of the pn junction. We investigate the thickness dependence of capacitance and highlight the role of defects in influencing the electronic properties of complex oxide pn junctions. [ABSTRACT FROM AUTHOR]

The effect of ultraviolet (UV) photon assisted annealing on the structural, dielectric, and insulating properties of barium strontium titanate (BST) thin films was investigated. Isothermal-temporal annealing experiments revealed that the films annealed utilizing UV assist possessed enhanced crystallization and lattice parameters closer to bulk values with respect to conventional thermal annealed films. The leakage current density and dielectric loss of the films was significantly reduced by employing UV-assisted annealing without degradation of film tunability. Mitigation of oxygen vacancies, associated with the UV-assisted annealing, is deemed responsible for the improvement of the dielectric and electrical properties of the BST thin films. [ABSTRACT FROM AUTHOR]

Epitaxial thin films of La1.875Sr0.125NiO4 (lanthanum strontium nickelate, LSNO) have been synthesized by sputtering onto single crystal oxide substrates and their structural and dielectric properties are reported. High dielectric constants on the order of 107 have been measured up to 1 MHz in interdigitated capacitors with a frequency dependence that correlates with substrate imposed strain and texture. The observation of a high dielectric constant albeit with moderate loss tangent at high frequencies motivates further explorations of charge ordering phenomena in such complex oxides and serves to examine size effects on dielectric response by comparison with studies on bulk single crystal LSNO. [ABSTRACT FROM AUTHOR]

We show how finite-size scaling of a bulk photovoltaic effect-generated electric field in epitaxial ferroelectric insulating BaTiO3(001) films and a photo-Hall response involving the bulk photovoltaic current reveal a large room-temperature mean free path of photogenerated nonthermalized electrons. Experimental determination of mesoscopic ballistic optically generated carrier transport opens a new paradigm for hot electron-based solar energy conversion, and for facile control of ballistic transport distinct from existing low-dimensional semiconductor interfaces, surfaces, layers, or other structures. [ABSTRACT FROM AUTHOR]