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β-Ga2O3 is attracting considerable attention for applications in power electronics and deep ultraviolet (DUV) optoelectronics owing to the ultra-wide bandgap of 4.85 eV and amendable n-type conductivity. In this work, we report the achievement of Si-doped β-Ga2O3 (Si:β-Ga2O3) thin films grown on vicinal α-Al2O3 (0001) substrates with high electrical conductivity and DUV transparency of promising potential as transparent electrodes. The use of Al2O3 substrates with miscut angles promotes step-flow growth mode, leading to substantial improvement of crystalline quality and electrical properties of the Si:β-Ga2O3 films. A high conductivity of 37 S·cm−1 and average DUV transparency of 85% have been achieved for 0.5% Si-doped film grown on a 6° miscut substrate. High-resolution x-ray and ultraviolet photoemission spectroscopy were further used to elucidate the surface electronic properties of the grown Si:β-Ga2O3 films. An upward surface band bending was found at the surface region of Si:β-Ga2O3 films. Interestingly, all the Si:β-Ga2O3 films have a very low work function of approximately 3.3 eV, which makes Si:β-Ga2O3 suitable materials for efficient electron injection. The present Si:β-Ga2O3 films with high conductivity, DUV transparency, and low work function would be useful as the DUV transparent electrode to develop advanced DUV optoelectronic devices. [ABSTRACT FROM AUTHOR]

CuWO4 is an n-type oxide semiconductor with a bandgap of 2.2 eV which exhibits great potential for photoelectrochemical (PEC) conversion of solar energy into chemical fuels. However, the photocurrent achieved so far is limited to ∼0.3 mA cm−2 at +1.23 V vs. reversible hydrogen electrode (RHE). Possible limiting factors include slow surface reaction kinetics, poor charge carrier mobility and/or presence of surface defect states. A detailed understanding of the fundamental electronic structure and its correlation with PEC activity is of significant importance for devising strategies for further improvements. In this work, we have synthesized CuWO4 thin films showing a record photocurrent density of 0.50 mA cm−2 at +1.23 V vs. RHE. Importantly, we have used a synergistic combination of photoemission spectroscopy, X-ray absorption spectroscopy and density functional theory (DFT) to unravel the electronic structure of CuWO4. Our results show that the valence band (VB) consists of strongly hybridized states of O 2p6 and Cu 3d9, while the bottom of the conduction band (CB) is primarily composed of unoccupied Cu 3d states. The localized nature of the Cu 3d state leads to the low charge carrier mobility and the localization of the photo-excited electrons to the CB. The combined experimental and theoretical results also indicate that CuWO4 is better described as having a direct but d–d forbidden optical bandgap, leading to a low absorption coefficient for visible light. Furthermore, the implication of the electronic structure on its PEC characteristics and strategies for further improvements by adding Co3O4 as a co-catalyst or surface layer to increase the interfacial band bending to facilitate photo-carriers transport, are discussed. [ABSTRACT FROM AUTHOR]

Bismuth ferrite BiFeO3 (BFO) is an important ferroelectric material for thin-film optoelectronic sensing and potential photovoltaic applications. Its relatively large band gap, however, limits the conversion efficiency of BFO absorber-based PV devices. In this study, based on density functional theory calculations we demonstrate that with well-designed Fe-site elemental substitution, tetragonal BFO can exhibit a much lower fundamental band gap than that of conventional rhombohedral BFO without forming in-gap electronic states and unravelling the underlying mechanisms. Cation atomic size, electronegativity, and crystallographic symmetry are evidenced as critical parameters to tailor the metal 3d–oxygen 2p orbital interactions and thus intrinsically modify the electronic structure, particularly, the shape and character of the valence and conduction band edges. With the reduced band gap, improved mobility, and uncompromised ferroelectric and magnetic ground states, the present results provide a new strategy of designing high symmetry BFO for efficient optoelectronic applications. [ABSTRACT FROM AUTHOR]

We have investigated the evolution of the electronic properties of La1-xSrxCrO3 (0 ⩽ x ⩽ 1) epitaxial films deposited by molecular beam epitaxy (MBE) using x-ray diffraction, x-ray photoemission spectroscopy, Rutherford backscattering spectrometry, x-ray absorption spectroscopy, electrical transport, and ab initio modeling. LaCrO3 is an antiferromagnetic insulator, whereas SrCrO3 is a metal. Substituting Sr2+ for La3+ in LaCrO3 effectively dopes holes into the top of valence band, leading to Cr4+ (3d2) local electron configurations. Core-level and valence-band features monotonically shift to lower binding energy with increasing x, indicating downward movement of the Fermi level toward the valence band maximum. The material becomes a p-type semiconductor at lower doping levels and an insulator-to-metal transition is observed at x ⩾ 0.65, but only when the films are deposited with in-plane compression via lattice-mismatched heteroepitaxy. Valence-band x-ray photoemission spectroscopy reveals diminution of electronic state density at the Cr d t2g-derived top of the valence band, while O K-edge x-ray absorption spectroscopy shows the development of a new unoccupied state above the Fermi level as holes are doped into LaCrO3. The evolution of these bands with Sr concentration is accurately captured using density functional theory (DFT) with a Hubbard U correction of 3.0 eV (DFT + U). Resistivity data in the semiconducting regime (x ⩽ 0.50) do not fit perfectly well to either a polaron hopping or band conduction model but are best interpreted in terms of a hybrid model. The activation energies extracted from these fits are well reproduced by DFT + U. [ABSTRACT FROM AUTHOR]

Reduction of transition metal oxides can greatly change their physical and chemical properties. Using deposition of WO3 as a case study, we demonstrate that reflection high-energy electron diffraction (RHEED), a surface-sensitive tool widely used to monitor thin-film deposition processes, can significantly affect the cation valence and physical properties of the films through electronbeam induced sample reduction. The RHEED beam is found to increase film smoothness during epitaxial growth of WO3, as well as change the electronic properties of the film through preferential removal of surface oxygen. [ABSTRACT FROM AUTHOR]

The ability to design and fabricate electronic devices with reproducible properties using complex oxides is critically dependent on our ability to controllably synthesize these materials in thin-film form. Structure-property relationships are intimately tied to film and interface composition. Here the effect of cation stoichiometry on structural quality and defect formation in LaCrO3 heteroepitaxial films prepared using molecular beam epitaxy is reported. From first principles the regions of stability of various candidate defects, along with the predicted effects of these defects on structural parameters, are calculated as a function of Cr and O chemical potential. Epitaxial LaCrO3 films readily nucleate and remain coherently strained on SrTiO3(001) over a wide range of La-to-Cr atom ratios, but La-rich films are of considerably lower structural quality than stoichiometric and Cr-rich films. Cation imbalances are accompanied by anti-site defect formation. Cation mixing occurs at the interface for all La-to-Cr ratios investigated and is not quenched by deposition on SrTiO3(001) at ambient temperature. Indiffused La atoms occupy Sr sites. Intermixing is effectively quenched by using molecular beam epitaxy to deposit LaCrO3 at ambient temperature on defect free Si(001). However, analogous pulsed laser deposition on Si is accompanied by cation mixing. [ABSTRACT FROM AUTHOR]

Angle-resolved photoemission spectroscopy reveals the presence of a two-dimensional electron gas at the surface of In2O3(111). Quantized subband States arise within a confining potential well associated with surface electron accumulation. Coupled Poisson-Schrödinger calculations suggest that downward band bending for the conduction band must be much bigger than band bending in the valence band. Surface oxygen vacancies acting as doubly ionized shallow donors are shown to provide the free electrons within this accumulation layer. Identification of the origin of electron accumulation in transparent conducting oxides has significant implications in the realization of devices based on these compounds. [ABSTRACT FROM AUTHOR]

The Poisson ratio ν of In2O3 has been determined by measurement of the covariation of in-plane and out-of- plane lattice parameters of strained thin films grown epitaxially on (111) and (001) oriented cubic Y-stabilized ZrO2 substrates. The experimental results are in good agreement with values for ν calculated using atomistic simulation procedures. [ABSTRACT FROM AUTHOR]

The interface between polar perovskite LaCrO3 and nonpolar SrTiO3 (001) grown by molecular beam epitaxy is examined using a combination of electron microscopy, spectroscopy, and Rutherford backscattering spectrometry. The A-site cations of these ABO3 perovskites are found to diffuse across the interface to a greater extent than the B-site cations, based on high-angle annular dark-field scanning transmission electron microscopy, energy-dispersive x-ray spectroscopy, and electron-energy-loss spectroscopy (EELS). The B-site cation valences are shown to be partially reduced near the interface by analysis of EELS near-edge structures. The location and direction of these electronic modifications do not intuitively compensate the charge imbalance imposed by uneven cation interdiffusion, and yet both the film and interface are insulating. These results highlight the importance of considering both the physical and electronic structure of such complex interfaces in determining their characteristics. [ABSTRACT FROM AUTHOR]

The deposition of 1/3 of a monolayer of Pb on Ag(111) leads to the formation of PbAg(2) surface alloy with a long range ordered (√3 × √3)R30° superstructure. A detailed analysis of this structure using low-energy electron diffraction (LEED) I-V measurements together with density functional theory (DFT) calculations is presented. We find strong correlation between experimental and calculated LEED I-V data, with the fit between the two data sets having a Pendry's reliability factor of 0.21. The Pb atom is found to replace one top layer Ag atom in each unit cell, forming a substitutional PbAg(2) surface alloy, as expected, with the Pb atoms residing approximately 0.4 Å above the Ag atoms due to their size difference. DFT calculations are in good agreement with the LEED results., (© 2011 IOP Publishing Ltd)