Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

We present a critical review that encompasses the fundamentals and state-of-the-art knowledge of barium titanate-based piezoelectrics. First, the essential crystallography, thermodynamic relations, and concepts necessary to understand piezoelectricity and ferroelectricity in barium titanate are discussed. Strategies to optimize piezoelectric properties through microstructure control and chemical modification are also introduced. Thereafter, we systematically review the synthesis, microstructure, and phase diagrams of barium titanate-based piezoelectrics and provide a detailed compilation of their functional and mechanical properties. The most salient materials treated include the (Ba,Ca)(Zr,Ti)O3, (Ba,Ca)(Sn,Ti)O3, and (Ba,Ca)(Hf,Ti)O3 solid solution systems. The technological relevance of barium titanate-based piezoelectrics is also discussed and some potential market indicators are outlined. Finally, perspectives on productive lines of future research and promising areas for the applications of these ma...

BaTiO3-based piezoelectrics: Fundamentals, current status, and perspectives

Nonlinear Optical Crystals: A Complete Survey

The induced nonlinear electric dipole and higher moments in an atomic system, irradiated simultaneously by two or three light waves, are calculated by quantum-mechanical perturbation theory. Terms quadratic and cubic in the field amplitudes are included. An important permutation symmetry relation for the nonlinear polarizability is derived and its frequency dependence is discussed. The nonlinear microscopic properties are related to an effective macroscopic nonlinear polarization, which may be incorporated into Maxwell's equations for an infinite, homogeneous, anisotropic, nonlinear, dielectric medium. Energy and power relationships are derived for the nonlinear dielectric which correspond to the Manley-Rowe relations in the theory of parametric amplifiers. Explicit solutions are obtained for the coupled amplitude equations, which describe the interaction between a plane light wave and its second harmonic or the interaction between three plane electromagnetic waves, which satisfy the energy relationship ${\ensuremath{\omega}}_{3}={\ensuremath{\omega}}_{1}+{\ensuremath{\omega}}_{2}$, and the approximate momentum relationship ${\mathrm{k}}_{3}={\mathrm{k}}_{1}+{\mathrm{k}}_{2}+\ensuremath{\Delta}\mathrm{k}$. Third-harmonic generation and interaction between more waves is mentioned. Applications of the theory to the dc and microwave Kerr effect, light modulation, harmonic generation, and parametric conversion are discussed.

/pdf/interactions-between-light-waves-in-a-nonlinear-dielectric-2m21ug6r84.pdf

Interactions between Light Waves in a Nonlinear Dielectric

Perovskite-based photocatalysts are of significant interest in the field of photocatalysis. To date, several perovskite material systems have been developed and their applications in visible light photocatalysis studied. This article provides a review of the visible light (λ > 400 nm) active perovskite-based photocatalyst systems. The materials systems are classified by the B site cations and their crystal structure, optical properties, electronic structure, and photocatalytic performance are reviewed in detail. Titanates, tantalates, niobates, vanadates, and ferrites form important photocatalysts which show promise in visible light-driven photoreactions. Along with simple perovskite (ABO3) structures, development of double/complex perovskites that are active under visible light is also reviewed. Various strategies employed for enhancing the photocatalytic performance have been discussed, emphasizing the specific advantages and challenges offered by perovskite-based photocatalysts. This review provides a broad overview of the perovskite photocatalysts, summarizing the current state of the work and offering useful insights for their future development.

/pdf/a-review-on-visible-light-active-perovskite-based-1owjo3skuy.pdf

A review on visible light active perovskite-based photocatalysts.

Energy of the indirect and direct optical bandgap of near-stoichiometric lithium niobate (nSLN) crystals is evaluated by optical absorption measurement. The value of the indirect bandgap (3.95 eV) is consistent with the earlier reports. However, the direct bandgap energy of 4.12 eV is higher than the previously reported experimental value (3.68 eV). The direct bandgap energy obtained here is closer to the recent theoretical value estimated by Thierfelder et al. [Phys. Status Solidi C 7, 362 (2010)] in comparison to the previously predicted value by Ching et al. [Phys. Rev. B 50, 1992 (1994)]. The phonons involved in the indirect allowed transitions have energies ∼85 meV (685 cm−1) and are identified as the E(TO9) and E(LO8) optical phonon modes from the Raman scattering measurement.

Urbach tail and bandgap analysis in near stoichiometric LiNbO3 crystals

Electro-caloric effect in 0.45BaZr0.2Ti0.8O3-0.55Ba0.7Ca0.3TiO3 single crystal has been investigated and electro-caloric coefficient of ∼0.38 K mm/kV has been obtained near the tetragonal-to-cubic phase transition. The value obtained is significantly higher than that for the lead-free ferroelectric materials reported so far.

Electro-caloric effect in 0.45BaZr0.2Ti0.8O3-0.55Ba0.7Ca0.3TiO3 single crystal

The photocatalytic activity of sol–gel synthesized nanostructured Bi1−xGdxFeO3 (x = 0, 0.05, 0.1, 0.15) particles on the degradation of methylene blue (MB) was demonstrated for the first time under sunlight. The X-ray diffraction (XRD) studies showed that the substitution induced structural changes in 10 and 15% Gd substituted BiFeO3 (BFO). The morphology analysis, by field emission scanning (FESEM) and high resolution transmission electron microscopy (HRTEM), presented the composition driven particle size reduction and morphology changes in BFO from irregular to spherical shape. The band gap estimation by UV-visible diffuse reflectance spectroscopy revealed that increasing concentration of Gd significantly reduced the band gap of BFO from 2.38 eV to 2.29 eV. An anomalous magnetic enhancement was observed in Bi0.90Gd0.10FeO3 nanoparticles due to the manifestation of antiferromagnetic (AFM) core–ferromagnetic (FM) shell-like structure, revealed by its M–H hysteresis curve. An increasing trend in the photocatalytic activity of BFO was observed with increasing concentration of Gd. In this case, enhanced photocatalytic activity observed in Bi0.85Gd0.15FeO3 could be due to its increased ferroelectric domains that drive the charge carriers to the catalyst surface–dye interface, leading to more effective degradation of the dye. Conversely, an anomalous photocatalytic activity was observed in Bi0.90Gd0.10FeO3 that should be attributed to its AFM–FM core–shell-like structure.

Nanostructured Bi(1−x)Gd(x)FeO3 – a multiferroic photocatalyst on its sunlight driven photocatalytic activity

We present temperature-dependent refractive index along crystallographic b[010] and a direction perpendicular to (100)-plane for monoclinic phase (β) Ga2O3 single crystal grown by the optical floating zone technique. The experimental results are consistent with the theoretical result of Litimein et al.1. Also, the Sellmeier equation for wavelengths in the range of 0.4–1.55 μm is formulated at different temperatures in the range of 30–175 °C. The thermal coefficient of refractive index in the above specified range is ∼10−5 /°C.

Temperature-dependent index of refraction of monoclinic Ga 2 O 3 single crystal

A single crystal of lithium niobate is an important optoelectronic material. It can be grown from direct melt only in a lithium deficient non-stoichiometric form as its stoichiometric composition exhibits incongruent melting. As a result it contains a number of intrinsic point defects such as Li-vacancies, Nb antisites, oxygen vacancies, as well as different types of polarons and bipolarons. All these defects adversely influence its optical and ferroelectric properties and pose a deterrent to the effective use of this material. Hence, controlling the defects in lithium niobate has been an exciting topic of research and development over the years. In this article we discuss the different methods of controlling the intrinsic defects in lithium niobate and a comparison of the effect of these methods on the crystalline quality, stoichiometry, optical absorption in the UV-vis region, electronic band-gap, and refractive index.

Indranil Bhaumik

Papers

Urbach tail and bandgap analysis in near stoichiometric LiNbO3 crystals

Electro-caloric effect in 0.45BaZr0.2Ti0.8O3-0.55Ba0.7Ca0.3TiO3 single crystal

Nanostructured Bi(1−x)Gd(x)FeO3 – a multiferroic photocatalyst on its sunlight driven photocatalytic activity

Temperature-dependent index of refraction of monoclinic Ga 2 O 3 single crystal

Control of Intrinsic Defects in Lithium Niobate Single Crystal for Optoelectronic Applications