Igor Levin

H2 evolution at Si-based metal–insulator–semiconductor photoelectrodes enhanced by inversion channel charge collection and H spillover

Photoelectrochemical (PEC) water splitting represents a promising route for renewable production of hydrogen, but trade-offs between photoelectrode stability and efficiency have greatly limited the performance of PEC devices. In this work, we employ a metal-insulator-semiconductor (MIS) photoelectrode architecture that allows for stable and efficient water splitting using narrow bandgap semiconductors. Substantial improvement in the performance of Si-based MIS photocathodes is demonstrated through a combination of a high-quality thermal SiO2 layer and the use of bilayer metal catalysts. Scanning probe techniques were used to simultaneously map the photovoltaic and catalytic properties of the MIS surface and reveal the spillover-assisted evolution of hydrogen off the SiO2 surface and lateral photovoltage driven minority carrier transport over distances that can exceed 2 cm. The latter finding is explained by the photo- and electrolyte-induced formation of an inversion channel immediately beneath the SiO2/Si interface. These findings have important implications for further development of MIS photoelectrodes and offer the possibility of highly efficient PEC water splitting.

Phase Diagram of the Perovskite System (1−x)BiScO3-xPbTiO3

The perovskite (1−x)BiScO3-xPbTiO3 (BS-PT) system exhibits a large piezoelectric coefficient (d33>460pC∕N in polycrystalline form) and a high Curie temperature TC=450°C in the vicinity of the morphotropic phase boundary (x=0.64mol fraction PbTiO3), which separates the rhombohedral and tetragonal phases. The present contribution reports on revisions to the BiScO3-PbTiO3 phase diagram specifically, (i) the compositional dependence of the octahedral tilt-transition temperature (x<0.62mol fraction PT), as determined using variable-temperature transmission electron microscopy, (ii) high-temperature curvature of the rhombohedral-tetragonal morphotropic phase boundary determined using dielectric measurements on single crystals, and (iii) Curie temperatures in the tetragonal phase field, which exceed that of the PbTiO3 end member.

Nd-doped BiFeO3 ceramics with antipolar order

Bi1−xNdxFeO3 (0≤x≤0.2) ceramics have been investigated using x-ray diffraction (XRD) and electron diffraction (ED). XRD patterns for x≤0.1 were consistent with rhombohedral BiFeO3, whereas those for x=0.15 and x=0.2 exhibited peak splitting and superlattice reflections representative of orthorhombic, antiferroelectric PbZrO3. ED for the latter samples confirmed the presence of 14(hk0) superlattice reflections typical of a PbZrO3-like structure but additional superlattice reflections were observed at 14(00l) yielding a √2a,2√2a,4a (where a is the pseudocubic lattice parameter) unit cell. The transition from rhombohedral BiFeO3 to the PbZrO3-like structure implies a modification from polar to antipolar behavior.

Transition layers at the SiO2/SiC interface

The electrical performance of SiC-based microelectronic devices is strongly affected by the densities of interfacial traps introduced by the chemical and structural changes at the SiO2∕SiC interface during processing. We analyzed the structure and chemistry of this interface for the thermally grown SiO2∕4H-SiC heterostructure using high-resolution transmission electron microscopy (TEM), Z-contrast scanning TEM, and spatially resolved electron energy-loss spectroscopy. The analyses revealed the presence of distinct layers, several nanometers thick, on each side of the interface; additionally, partial amorphization of the top SiC surface was observed. These interfacial layers were attributed to the formation of a ternary Si–C–O phase during thermal oxidation.

Phase transitions and microwave dielectric properties in the perovskite-like Ca(Al0.5Nb0.5)O3−CaTiO3 system

Phase transitions and microwave dielectric properties in the (1−x)Ca(Al0.5Nb0.5)O3–xCaTiO3 system were analyzed using x-ray and neutron powder diffraction, transmission electron microscopy, Raman spectroscopy, and dielectric measurements at microwave frequencies (2–8 GHz). Rietveld structural refinements demonstrated that both end compounds exhibit similar octahedral tilted frameworks, while in Ca(Al0.5Nb0.5)O3, tilting is superimposed onto NaCl-type ordering of Al and Nb on the B sites. Accordingly, the room-temperature structures of CaTiO3 and Ca(Al0.5Nb0.5)O3 are described by orthorhombic Pbnm and monoclinic P21/n symmetries, respectively, with similar lattice parameters, √2ac×√2ac×2ac (where ac is the lattice parameter of cubic perovskite). The (1−x)Ca(Al0.5Nb0.5)O3–xCaTiO3 system features both cation ordering and octahedral tilting phase transitions. The Ca(Al0.5Nb0.5)O3 structure remains ordered at least up to 1625 °C. However, the temperature of the order/disorder transition decreases rapidly with ...

Phase equilibria, crystal structures, and dielectric anomaly in the BaZrO3-CaZrO3 system

Abstract Phase equilibria in the (1−x)BaZrO3–xCaZrO3 system were analyzed using a combination of X-ray and neutron powder diffraction, and transmission electron microscopy. The proposed phase diagram features two extended two-phase fields containing mixtures of a Ba-rich cubic phase and a tetragonal, or orthorhombic Ca-rich phase, all having perovskite-related structures. The symmetry differences in the Ca-rich phases are caused by different tilting patterns of the [ZrO6] octahedra. In specimens quenched from 1650°C, CaZrO3 dissolves only a few percent of Ba, whereas the solubility of Ca in BaZrO3 is approximately 30 at % . The BaZrO3–CaZrO3 system features at least two tilting phase transitions, Pm3m→I4/mcm and I4/mcm→Pbnm. Rietveld refinements of the Ba0.8Ca0.2ZrO3 structure using variable-temperature neutron powder diffraction data confirmed that the Pm3m→I4/mcm transition corresponds to a rotation of octahedra about one of the cubic axes; successive octahedra along this axis rotate in opposite directions. In situ variable-temperature electron diffraction studies indicated that the transition temperature increases with increasing Ca-substitution on the A-sites, from approximately −120°C at 5 at % Ca to 225°C at 20 at % Ca. Dielectric measurements revealed that the permittivity increases monotonically from 36 for BaZrO3 to 53 for Ba0.9Ca0.1ZrO3, and then decreases to 50 for Ba0.8Ca0.2ZrO3. This later specimen was the Ca-richest composition for which pellets could be quenched from the single-phase cubic field with presently available equipment. Strongly non-monotonic behavior was also observed for the temperature coefficient of resonant frequency; however, in this case, the maximum occurred at a lower Ca concentration, 0.05⩽x⩽0.1. The non-linear behavior of the dielectric properties was attributed to two competing structural effects: a positive effect associated with substitution of relatively small Ca cations on the A-sites, resulting in stretched Ca–O bonds, and a negative effect, related to the distortion of the A-site environment (bond strain relaxation) upon octahedral tilting.

Coupling between octahedral tilting and ferroelectric order in tetragonal tungsten bronze-structured dielectrics

Strong coupling between local polar displacements and a commensurate octahedral tilting is proposed to explain the onset of classic ferroelectric behavior in tetragonal tungsten bronzelike dielectrics Ba2LaxNd1−xNb3Ti2O15. The ferroelectric phase transition is associated with a discontinuous non-lock-in transformation of an incommensurate tilted structure to a commensurate superstructure. In a manner reminiscent of perovskitelike oxides, the driving force for commensurate tilting increases as the average ionic radius of the rare-earth ion decreases; no classical ferroelectric transition is observed for compositions with x>0.75, which remain incommensurate and exhibit only relaxor behavior below room temperature.

Self-assembled multiferroic nanostructures in the CoFe2O4-PbTiO3 system

The effect of substrate orientation on the morphologies of epitaxial self-assembled nanostructures was demonstrated using multiferroic 0.67PbTiO3-0.33CoFe2O4 thin films. The two-phase composite films were grown by pulsed laser deposition on single crystal SrTiO3 substrates having (001) and (110) orientations. The nanostructures of both orientations consisted of vertical rod- or platelet-like columns of CoFe2O4 dispersed in a PbTiO3 matrix. For the (001) orientation the platelet habits were parallel to the {110} planes, whereas for the (110) orientation the platelets were parallel to the {111} planes. The differences were explained using a thermodynamic theory of heterophase structures.

Dielectric and structural studies of Ba2MTi2Nb3O15 (BMTNO15, M=Bi3+,La3+,Nd3+,Sm3+,Gd3+) tetragonal tungsten bronze-structured ceramics

The structure and dielectric properties of a new family of tetragonal tungsten bronze (TTB) ceramics with the general formula, Ba2MTi2Nb3O15 where M=Bi3+,La3+,Nd3+,Sm3+,Gd3+, have been investigated. Hereafter the compositions will be referred to by the abbreviation BMTNO15 where M=B(Bi3+), L(La3+), N(Nd3+), S(Sm3+) or G(Gd3+). BLTNO15 [permittivity maximum (Tm)∼−80°C] and BBTNO15 (Tm∼−100°C) exhibited relaxorlike dielectric behavior. In contrast, BNTNO15 (Tm∼165°C), BSTNO15 (Tm∼250°C), and BGTNO15 (Tm∼320°C) were classic ferroelectrics. Tm increased with decreasing radius of the M ion. Room temperature x-ray powder diffraction (XRD) patterns of all the compounds indexed on a prototype P4∕mbm (or P4bm) space group with lattice parameters a≈12.4A and c≈4A. However, electron diffraction revealed that the relaxor phases, BLTNO15 and BBTNO15, exhibited an incommensurate modulation, whereas the classic ferroelectric BNTNO15, BSTNO15, and BGTNO15 featured an orthorhombic superstructure with lattice parameters a≈...

Fabrication of GaN-based nanoscale device structures utilizing focused ion beam induced Pt deposition

In this work we have demonstrated nanoscale GaN device structures made from individual GaN nanowires and electrical contacts utilizing focused ion beam (FIB) induced Pt deposition. These GaN nanowires were grown by direct reaction of Ga vapor with NH3 and had diameters ranging from 100nmto250nm and lengths up to 200μm. As-grown nanowires were dispersed on SiO2 coated p++ Si substrate. A 30keV Ga+ ion beam was used to dissociate (trimethyl)methylcyclopentadienyl-platinum precursor for depositing Pt contacts to GaN nanowires. FIB-deposited Pt contacts to GaN nanowires showed nonlinear I-V characteristics, which turned linear after annealing at 500°C for 30s in argon. Resistivity of a GaN nanowire measured using a four terminal contact geometry fabricated by depositing Pt with a FIB was in the range of 5×10−3Ωcm. Temperature dependent resistivity measurement of the GaN nanowire revealed semiconducting behavior with a weak temperature dependence of the resistivity. In this study both Ohmic and Schottky contac...