Search

We report the temperature influence of the OHad and Oad electroadsorption on RuO2(110) films grown on TiO2(110) crystals in alkaline media. From the temperature effect, we evaluate the enthalpy and entropy of the OHad and Oad electroadsorption, including the adsorbate–adsorbate interactions that we analyze using the interaction parameters of the Frumkin-isotherm model. We found that the adsorbates repel each other enthalpically but attract each other entropically. Our result suggests that an entropy analysis is necessary to capture the electroadsorption behavior on RuO2 since the enthalpy–entropy competition strongly influences the electroadsorption behavior. Our observation of an entropic force is consistent with the view that water may be a mediator for adsorbate–adsorbate interactions. [ABSTRACT FROM AUTHOR]

Strain-engineering is a powerful means to tune the polar, structural, and electronic instabilities of incipient ferroelectrics. KTaO3 is near a polar instability and shows anisotropic superconductivity in electron-doped samples. Here, we demonstrate growth of high-quality KTaO3 thin films by molecular-beam epitaxy. Tantalum was provided by either a suboxide source emanating a TaO2 flux from Ta2O5 contained in a conventional effusion cell or an electron-beam-heated tantalum source. Excess potassium and a combination of ozone and oxygen (10% O3 + 90% O2) were simultaneously supplied with the TaO2 (or tantalum) molecular beams to grow the KTaO3 films. Laue fringes suggest that the films are smooth with an abrupt film/substrate interface. Cross-sectional scanning transmission electron microscopy does not show any extended defects and confirms that the films have an atomically abrupt interface with the substrate. Atomic force microscopy reveals atomic steps at the surface of the grown films. Reciprocal space mapping demonstrates that the films, when sufficiently thin, are coherently strained to the SrTiO3 (001) and GdScO3 (110) substrates. [ABSTRACT FROM AUTHOR]

We report the electrocatalysis of the chlorine evolution reaction (CER) on well-defined RuO2(110) and IrO2(110) surfaces. RuO2 and IrO2 are known for their capabilities to catalyze the CER. Until now, the CER measurements have only been reported on well-defined RuO2 surfaces and only at high Cl− concentrations. We present the CER measurement and the role of Cl− at lower concentration on single-orientation RuO2(110) and IrO2(110) films. We find that RuO2(110) is two orders of magnitude more active than IrO2(110). Moreover, we observe the correlation between the CER activity and the Oad formation potential on RuO2 and IrO2, supporting the prior suggestion that the Oad is the active site for the CER. We further use the reaction order analysis to support the Volmer-Heyrovsky mechanism of the CER, which was previously suggested from the Tafel slope analysis. Our finding highlights the importance of the surface Oad species on oxides for the CER electrocatalysis and suggests the electrochemical formation of Clad on Oad (for example, Cl− + Oad ↔ OClad + e−) as the crucial step in the CER electrocatalysis. [ABSTRACT FROM AUTHOR]

Rutile compounds have exotic functional properties that can be applied for various electronic applications; however, the limited availability of epitaxial substrates has restricted the study of rutile thin films to a limited range of lattice parameters. Here, rutile GeO2 is demonstrated as a new rutile substrate with lattice parameters of a = 4.398 Å and c = 2.863 Å. Rutile GeO2 single crystals up to 4 mm in size are grown by the flux method. X-ray diffraction reveals high crystallinity with a rocking curve having a full width half-maximum of 0.0572°. After mechanical polishing, a surface roughness of less than 0.1 nm was obtained, and reflection high-energy electron diffraction shows a crystalline surface. Finally, epitaxial growth of (110)-oriented TiO2 thin films on GeO2 substrates was demonstrated using molecular beam epitaxy. Templated by rutile GeO2 substrates, our findings open the possibility of stabilizing new rutile thin films and strain states for the tuning of physical properties. [ABSTRACT FROM AUTHOR]

Utilizing the powerful combination of molecular-beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), we produce and study the effect of different terminating layers on the electronic structure of the metallic delafossite PdCoO2. Attempts to introduce unpaired electrons and synthesize new antiferromagnetic metals akin to the isostructural compound PdCrO2 have been made by replacing cobalt with iron in PdCoO2 films grown by MBE. Using ARPES, we observe similar bulk bands in these PdCoO2 films with Pd-, CoO2-, and FeO2-termination. Nevertheless, Pd- and CoO2-terminated films show a reduced intensity of surface states. Additionally, we are able to epitaxially stabilize PdFexCo1−xO2 films that show an anomaly in the derivative of the electrical resistance with respect to temperature at 20 K, but do not display pronounced magnetic order. [ABSTRACT FROM AUTHOR]

Ultrawide-band-gap (UWBG) semiconductors are promising for fast, compact, and energy-efficient power-electronics devices. Their wider band gaps result in higher breakdown electric fields that enable high-power switching with a lower energy loss. Yet, the leading UWBG semiconductors suffer from intrinsic materials' limitations with regard to their doping asymmetry that impedes their adoption in CMOS technology. Improvements in the ambipolar doping of UWBG materials will enable a wider range of applications in power electronics as well as deep-UV optoelectronics. These advances can be accomplished through theoretical insights on the limitations of current UWBG materials coupled with the computational prediction and experimental demonstration of alternative UWBG semiconductor materials with improved doping and transport properties. As an example, we discuss the case of rutile GeO2 (r-GeO2), a water-insoluble GeO2 polytype, which is theoretically predicted to combine an ultra-wide gap with ambipolar dopability, high carrier mobilities, and a higher thermal conductivity than β-Ga2O3. The subsequent realization of single-crystalline r-GeO2 thin films by molecular beam epitaxy provides the opportunity to realize r-GeO2 for electronic applications. Future efforts toward the predictive discovery and design of new UWBG semiconductors include advances in first-principles theory and high-performance computing software, as well as the demonstration of controlled doping in high-quality thin films with lower dislocation densities and optimized film properties. [ABSTRACT FROM AUTHOR]

We have studied the growth of epitaxial films of stannate pyrochlores with a general formula A2Sn2O7 (A = La and Y) and find that it is possible to incorporate ∼25% excess of the A-site constituent; in contrast, any tin excess is expelled. We unravel the defect chemistry, allowing for the incorporation of excess A-site species and the mechanism behind the tin expulsion. An A-site surplus is manifested by a shift in the film diffraction peaks, and the expulsion of tin is apparent from the surface morphology of the film. In an attempt to increase La2Sn2O7 conductivity through n-type doping, substantial quantities of tin have been substituted by antimony while maintaining good film quality. The sample remained insulating as explained by first-principles computations, showing that both the oxygen vacancy and antimony-on-tin substitutional defects are deep. Similar conclusions are drawn on Y2Sn2O7. An alternative n-type dopant, fluorine on oxygen, is shallow according to computations and more likely to lead to electrical conductivity. The bandgaps of stoichiometric La2Sn2O7 and Y2Sn2O7 films were determined by spectroscopic ellipsometry to be 4.2 eV and 4.48 eV, respectively. [ABSTRACT FROM AUTHOR]

This paper introduces a growth method—suboxide molecular-beam epitaxy (S-MBE)—which enables a drastic enhancement in the growth rates of Ga2O3 and related materials to over 1 μ m h−1 in an adsorption-controlled regime, combined with excellent crystallinity. Using a Ga + Ga2O3 mixture with an oxygen mole fraction of x(O) = 0.4 as an MBE source, we overcome kinetic limits that had previously hampered the adsorption-controlled growth of Ga2O3 by MBE. We present growth rates up to 1.6 μ m h−1 and 1.5 μ m h−1 for Ga2O3/Al2O3 and Ga2O3/Ga2O3 structures, respectively, with very high crystalline quality at unparalleled low growth temperature for this level of perfection. We combine thermodynamic knowledge of how to create molecular beams of targeted suboxides with a kinetic model developed for the S-MBE of III–VI compounds to identify appropriate growth conditions. Using S-MBE, we demonstrate the growth of phase-pure, smooth, and high-purity homoepitaxial Ga2O3 films that are thicker than 4.5 μ m. With the high growth rate of S-MBE, we anticipate a significant improvement to vertical Ga2O3-based devices. We describe and demonstrate how this growth method can be applied to a wide range of oxides. With respect to growth rates and crystalline quality, S-MBE rivals leading synthesis methods currently used for the production of Ga2O3-based devices. [ABSTRACT FROM AUTHOR]

Hexagonal LuFeO3 exhibiting ferroelectricity and weak ferromagnetism is grown on metal-polar GaN by molecular-beam epitaxy. The oxide films exhibit smooth surface morphologies and are found to be single crystalline with an epitaxial relationship related by a 30° in-plane rotation relative to the GaN crystal structure. The LuFeO3 layers grown on GaN exhibit room-temperature ferroelectricity and low-temperature magnetic ordering. This epitaxial integration creates a heterostructure platform to explore and exploit the coupling of the ferroelectricity and magnetism of oxides with the strong spontaneous and piezoelectric polarization and the unique electronic and photonic properties of nitride semiconductors. [ABSTRACT FROM AUTHOR]

A major challenge for ferroelectric devices is the depolarization field, which competes with and often destroys long-range polar order in the limit of ultrathin films. Recent theoretical predictions suggest a new class of materials, termed hyperferroelectics, that should be robust against the depolarization field and enable ferroelectricity down to the monolayer limit. Here, the authors demonstrate the epitaxial growth of hexagonal LiZnSb, one of the hyperferroelectric candidate materials, by molecular-beam epitaxy on GaSb (111)B substrates. Due to the high volatility of all three atomic species, they find that LiZnSb can be grown in an adsorption-controlled window, using an excess zinc flux. Within this window, the desired polar hexagonal phase is stabilized with respect to a competing cubic polymorph, as revealed by x-ray diffraction and transmission electron microscopy measurements. First-principles calculations show that for moderate amounts of epitaxial strain and moderate concentrations of Li vacancies, the cubic LiZnSb phase is lower in formation energy than the hexagonal phase, but only by a few millielectronvolts per formula unit. Therefore, they suggest that kinetics plays a role in stabilizing the desired hexagonal phase at low temperatures. Their results provide a path toward experimentally demonstrating ferroelectricity and hyperferroelectricity in a new class of ternary intermetallic compounds. [ABSTRACT FROM AUTHOR]

We report a fully transparent thin-film transistor utilizing a La-doped BaSnO3 channel layer that provides a drain current of 0.468 mA/μm and an on-off ratio of 1.5 × 108. The La-doped BaSnO3 channel is grown on a 100–150 nm thick unintentionally doped BaSnO3 buffer layer on a (001) MgO substrate by molecular-beam epitaxy. Unpatterned channel layers show mobilities of 127–184 cm2 V−1 s−1 at carrier concentrations in the low to mid 1019 cm−3 range. The BaSnO3 is patterned by reactive ion etching under conditions preserving the high mobility and conductivity. Using this patterning method, a sub-micron-scale thin film transistor exhibiting complete depletion at room temperature is achieved. [ABSTRACT FROM AUTHOR]

We report the in situ, direct epitaxial synthesis of (0001)-oriented PdCoO2 thin films on c-plane sapphire using ozone-assisted molecular-beam epitaxy. The resulting films have smoothness, structural perfection, and electrical characteristics that rival the best in situ grown PdCoO2 thin films in the literature. Metallic conductivity is observed in PdCoO2 films as thin as ∼2.0 nm. The PdCoO2 films contain 180° in-plane rotation twins. Scanning transmission electron microscopy reveals that the growth of PdCoO2 on the (0001) surface of Al2O3 begins with the CoO2 layer. [ABSTRACT FROM AUTHOR]

La-doped BaSnO3 has been epitaxially integrated with (001) Si using an SrTiO3 buffer layer via molecular-beam epitaxy (MBE). A 254 nm thick undoped BaSnO3 buffer layer was grown to enhance the mobility of the overlying La-doped BaSnO3 layer. The x-ray diffraction rocking curve of the BaSnO3 002 peak has a full width at half maximum of 0.02°. At room temperature, the resistivity of the La-doped BaSnO3 film is 3.6 × 10−4 Ω cm and the mobility is 128 cm2 V−1 s−1 at a carrier concentration of 1.4 × 1020 cm−3. These values compare favorably to those of La-doped BaSnO3 films grown by all techniques other than MBE on single-crystal oxide substrates. Our work opens an exciting arena for integrating hyper-functional oxide electronics that make use of high-mobility oxide films with the workhorse of the semiconductor industry, silicon. [ABSTRACT FROM AUTHOR]

(1-x)(Bi0.9La0.1)FeO3–xPb(Fe0.5Nb0.5)O3 (BLFO-PFNO) (x=0.2, 0.7, and 0.8), are investigated as an A-site substituted counterpart multiferroic solid solution of previous (Bi0.9La0.1)FeO3–Ba(Fe0.5Nb0.5)O3 (BLFO-BFNO). The unique BLFO-PFNO solid solution system enables soft ferromagnetism at room temperature and macroscopic polarization with the ferroelectric domain evolution due to nanoscale ordered single grains at high composition of relaxor-type ferroelectric PFNO. [ABSTRACT FROM AUTHOR]

LaxBa1-xSnO3 is a promising transparent conducting oxide whose high mobility facilitates potential applications in transparent electronics, oxide electronics, and power electronics. Here, we report quantitative comparisons between angle-resolved photoemission and density functional theory, demonstrating a close agreement between calculations and the measured bulk electronic structure. Further measurements reveal upward band bending at the film-vacuum interface, while ultraviolet (UV) exposure is found to increase the surface electron density, similar to other oxides. These results elucidate the LaxBa1-xSnO3 (LBSO) interfacial electronic structure and offer a route for UV carrier density control, critical steps towards realizing LBSO-based electronic devices. [ABSTRACT FROM AUTHOR]

LaxBa1-xSnO3 is a promising transparent conducting oxide whose high mobility facilitates potential applications in transparent electronics, oxide electronics, and power electronics. Here, we report quantitative comparisons between angle-resolved photoemission and density functional theory, demonstrating a close agreement between calculations and the measured bulk electronic structure. Further measurements reveal upward band bending at the film-vacuum interface, while ultraviolet (UV) exposure is found to increase the surface electron density, similar to other oxides. These results elucidate the LaxBa1-xSnO3 (LBSO) interfacial electronic structure and offer a route for UV carrier density control, critical steps towards realizing LBSO-based electronic devices. [ABSTRACT FROM AUTHOR]

Erratum: "Adsorption-controlled growth of Ga2O3 by suboxide molecular-beam epitaxy", [APL. REFERENCE 1 P. Vogt, F. V. E. Hensling, K. Azizie, C. S. Chang, D. Turner, J. Park, J. P. McCandless, H. Paik, B. J. Bocklund, G. Hoffman, O. Bierwagen, D. Jena, H. G. Xing, S. Mou, D. A. Muller, S.-L. Liu, and D. G. Schlom, APL Mater. [Extracted from the article]

The crystal structure and multiferroic properties of single-phase (Bi0.9La0.1)FeO3–Ba(Fe0.5Nb0.5)O3 ceramics were investigated. The rhombohedral-type structure of 1.0(Bi0.9La0.1)FeO3) was transformed to a monoclinic-type structure for the 0.5(Bi0.9La0.1)FeO3–0.5Ba(Fe0.5Nb0.5)O3) composition. It shows the coexistence of ferroelectricity and magnetism at room temperature, represented by a high dielectric constant with ferroelectric hysteresis and clear magnetic hysteresis behavior. The authors propose that the solid solution of A and B site cations according to their ionic radii and valence state stabilized the spontaneous polarization and magnetization at room temperature. [ABSTRACT FROM AUTHOR]

The structural evolution of tensile strained vanadium dioxide thin films was examined across the electrically driven insulator-to-metal transition by nanoscale hard X-ray diffraction. A metallic filament with rutile (R) structure was found to be the dominant conduction pathway for an electrically driven transition, while the majority of the channel area remained in the monoclinic M1 phase. The filament dimensions were estimated using simultaneous electrical probing and nanoscale X-ray diffraction. Analysis revealed that the width of the conducting channel can be tuned externally using resistive loads in series, enabling the M1/R phase ratio in the phase coexistence regime to be tuned. [ABSTRACT FROM AUTHOR]

A correction to the article "Intrinsic electronic switching time in ultrathin epitaxial vanadium dioxide thin film" that was published in the September 16, 2013 issue is presented.

This letter investigates the intrinsic electronic switching time associated with the insulator-to-metal phase transition in epitaxial single crystal vanadium dioxide (VO2) thin films using impedance spectroscopy and ac conductivity measurements. The existence of insulating and metallic phase coexistence, intrinsic to the epitaxial (001) oriented VO2 thin film grown on a (001) rutile TiO2 substrate, results in a finite capacitance being associated with the VO2 films in their insulating phase that limits the electronic switching speed. Insights into the switching characteristics and their correlation to the transport mechanism in the light of phase coexistence are obtained by performing a detailed scaling study on VO2 two-terminal devices. [ABSTRACT FROM AUTHOR]

We report the temperature influence of the OH ad and O ad electroadsorption on RuO 2 (110) films grown on TiO 2 (110) crystals in alkaline media. From the temperature effect, we evaluate the enthalpy and entropy of the OH ad and O ad electroadsorption, including the adsorbate-adsorbate interactions that we analyze using the interaction parameters of the Frumkin-isotherm model. We found that the adsorbates repel each other enthalpically but attract each other entropically. Our result suggests that an entropy analysis is necessary to capture the electroadsorption behavior on RuO 2 since the enthalpy-entropy competition strongly influences the electroadsorption behavior. Our observation of an entropic force is consistent with the view that water may be a mediator for adsorbate-adsorbate interactions.

We report the electrocatalysis of the chlorine evolution reaction (CER) on well-defined RuO 2 (110) and IrO 2 (110) surfaces. RuO 2 and IrO 2 are known for their capabilities to catalyze the CER. Until now, the CER measurements have only been reported on well-defined RuO 2 surfaces and only at high Cl - concentrations. We present the CER measurement and the role of Cl - at lower concentration on single-orientation RuO 2 (110) and IrO 2 (110) films. We find that RuO 2 (110) is two orders of magnitude more active than IrO 2 (110). Moreover, we observe the correlation between the CER activity and the O ad formation potential on RuO 2 and IrO 2 , supporting the prior suggestion that the O ad is the active site for the CER. We further use the reaction order analysis to support the Volmer-Heyrovsky mechanism of the CER, which was previously suggested from the Tafel slope analysis. Our finding highlights the importance of the surface O ad species on oxides for the CER electrocatalysis and suggests the electrochemical formation of Cl ad on O ad (for example, Cl - + O ad ↔ OCl ad + e - ) as the crucial step in the CER electrocatalysis.