Optical floating zone method growth and photoluminescence property of MgNb2O6 crystal

MgNb₂O₆ single crystals are grown by the optical floating zone method. The as-grown crystals are dark brown and have dimensions of Ø 4-6 mm × L 87 mm, with the largest crystal domain being Ø 5 mm × L 32 mm. After being annealed, the crystals fade to light brown. The powder X-ray diffraction analysis shows that the crystals have a columbite-type MgNb₂O₆ structure. The crystal grows along the c-axis and the cleavage plane is the (0 1 0) plane. Transmission polarized light microscopy measurements show that the crystal is free of low-angle grain boundaries and inclusions. The crystals have been characterized by Raman scattering, which reveals the change of Nb-O band in annealed and non-annealed samples. The photoluminescence spectra exhibit a broad and strong blue emission band centered at 435 nm.     

Growth, crystal structure and optical properties of layered dibarium cadmium diborate, Ba2Cd(BO3)2

A novel dibarium cadmium diborate, Ba₂Cd(BO₃)₂, has been successfully synthesized by standard solid-state reaction. Large sheet-like crystal with size up to 20 mm × 15 mm × 0.7 mm has been obtained using top-seed solution growth method. Ba₂Cd(BO₃)₂ crystallizes in the monoclinic space group C2/m with a = 9.6305(4) Å, b = 5.3626(3) Å, c = 6.5236(2) Å, β = 118.079(3)°, Z = 2. The crystal structure is composed of isolated [BO₃] triangles, [CdO₆] octahedra and [BaO₉] polyhedra. CdO₆ are vertex-connected with six BO₃ to form infinite _∞[Cd(BO₃)₂] layers extending in (0 0 1) plane, and two rows of Ba atoms closely occupy two side of _∞[Cd(BO₃)₂] layers to forming stoichiometric sheets. IR and transmittance spectrum of Ba₂Cd(BO₃)₂ were reported.     

Dielectric function and the crystal structure of MeV ion implanted LiNbO3

The behavior of the dielectric functions of MeV ion-implanted LiNbO₃ was studied using THz spectroscopy. Single crystal LiNbO₃ specimens were implanted with 3 MeV Au ions at four different levels of ion fluence (1 × 10¹², 1 × 10¹³, 1 × 10¹⁴ and 1 × 10¹⁵ cm^− 2). The optical spectra of the specimens were then measured in two ranged values: (1) reflectance THz range spectra from 40 to 700 cm^− 1 and (2) reflectance ultraviolet, visible, and near-infrared spectra from 250 to 2000 nm. The THz spectra were analyzed by fitting with the dielectric function model proposed by Brendel. The dielectric function model explained well the experimental spectra of the ion-implanted LiNbO₃. The dielectric function shows that the ion-implanted layers were amorphous phase at the fluence of 1 × 10¹⁴ and 1 × 10¹⁵ cm^− 2, whereas the layers at 1 × 10¹² and 1 × 10¹³ cm^− 2 maintained the crystal phase but significantly suppressed the phonon resonances. The volume of ion-implanted layers expanded about 8% with the fluence of 1 × 10¹⁴ and 1 × 10¹⁵ cm^− 2, i.e. after amorphization, These results show that the ion implantation significantly affected the lattice vibration mode and the structure even at very low ion fluence.     

Amorphous hafnium oxide thin films for antireflection optical coatings

Hafnium oxide (HfO₂) thin films were grown on silicon and quartz substrates by radio frequency reactive magnetron sputtering at temperature < 52 °C. X-ray diffraction of the films showed no structure, suggesting that the films grown on the substrates are amorphous. The optical properties of these films have been investigated using spectroscopic ellipsometry with wavelength range 200-1400 nm and ultraviolet-visible spectrophotometer techniques. Also, the effects of annealing temperatures on the structure and optical properties of the amorphous HfO₂ (a-HfO₂) have been investigated. The films appeared to be monoclinic structure upon high temperature (1000 °C) annealing as confirmed by X-ray diffraction. The results show that the annealing temperature has a strong effect on the optical properties of a-HfO₂ films. The optical bandgap energy of the as-deposited films is found to be about 5.8 eV and it increases to 5.99 eV after the annealing in Ar gas at 1000 °C. The further study shows that the measurement of the optical properties of the amorphous films reveals a high transmissivity (82%-99%) and very low reflectivity (< 8%) in the visible and near-infrared regions at any angle of incidence. Thus, the amorphous structure yields HfO₂ film of significantly higher transparency than the polycrystalline (68%-83%) and monoclinic (78%-89%) structures. This means that the a-HfO₂ films could be a good candidate for antireflection (AR) optical coatings.     

Growth and spectroscopic properties of Nd-doped Na2Gd4(MoO4)7 crystal

This paper reports the growth and spectral properties of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystals. An Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal with dimensions of Ø20 × 25 mm³ has been grown by the Czochralski method. The spectroscopic properties of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal were investigated. The Judd-Ofelt theory was applied to calculate the spectral parameters. The polarized absorption cross-sections of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal are 4.25 × 10⁻²⁰ cm² with full width at half maximum (FWHM) of 14.6 nm for the π-polarization and 2.87 × 10⁻²⁰ cm² with FWHM of 16.2 nm for the σ-polarization, respectively. The emission cross-sections are 10.0 × 10⁻²⁰ cm² at 1060 nm for π-polarization and 13.6 × 10⁻²⁰ cm² at 1067 nm for σ-polarization, respectively. The fluorescence quantum efficiency has been estimated to be 90.0%. Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal may be considered as a potential laser gain medium for the diode laser pumping.     

Effects of growth temperature on structure and electrical properties of dielectric (Ba,Sr)TiO3 capacitors with transparent conducting oxide electrodes

200 nm-thick BST thin films were grown on Zr-doped In₂O₃/SrTiO₃ (1 0 0) substrates at 550-750 °C. X-ray diffraction results show that the as-deposited BST films were polycrystalline with random crystallographic orientations. X-ray diffraction patterns reveal that the BST film grown at 650 °C had the best crystalline quality of all the deposition temperatures. Atomic force microscopy and secondary ion mass spectrometry showed that the surface and interface structures of the BST films became rough as the growth temperature increased. The BST film grown at 650 °C showed the best electrical properties, with a dielectric constant of 420 at 1 MHz, dielectric tunability of 32.1%, dielectric loss of 0.015 at 300 kV/cm, and a mean optical transmittance in visible wavelength of 71.3%.     

Low-loss microwave dielectrics using rock salt oxide Li2MgTiO4

Rock-salt-structured Li₂MgTiO₄ ceramic was prepared by the conventional mixed oxide route and its microwave dielectric properties were investigated. The microstructures of the ceramics were characterized by SEM. The dielectric properties of the ceramics exhibited a significant dependence on the sintering condition and crystal structure. A new microwave dielectric material, Li₂MgTiO₄ sintered at 1360 °C has a dielectric constant (?_r) of ∼17.25, a Q × f of ∼97,300 GHz (where f = 9.86 GHz, is the resonant frequency) and a τ_f of ∼-27.2 ppm/°C. The microwave dielectric properties of the ceramic are reported for the first time.     

Effective segregation during Czochralski growth and spectral properties of Nd in GSAG crystal

GSAG and Nd:GSAG crystals were grown by standard Czochralski technique. The refractive index of GSAG in the wavelength range 500-3000 nm, effective segregation coefficient and absorption cross sections of Nd³⁺ in GSAG were determined by optical absorption method. The effective segregation coefficient was calculated to be 0.525. The spectroscopic and laser properties of Nd:GSAG crystal were studied by Judd-Ofelt analysis. The Judd-Ofelt parameters were calculated to be Ω₂ = 1.32 × 10⁻²¹ cm², Ω₄ = 2.93 × 10⁻²⁰ cm² and Ω₆ = 3.91 × 10⁻²⁰ cm². With these intensity parameters, the values of absorption and emission oscillator strengths, transition probabilities, fluorescence ratios and radiative lifetimes were obtained. The large fluorescence ratio of |[4F]3/2〉 is very favorable for laser operation at 942 nm. These results confirm that Nd:GSAG is a suitable laser material for water vapor detection by DIAL.     

Structural, optical and electrical properties of In-doped Cd2SnO4 thin films by spray pyrolysis method

Preparation of highly conducting and transparent In-doped Cd₂SnO₄ thin film by spray pyrolysis method at a substrate temperature of 525 °C is reported. In-doping concentration is varied between 1 and 5 wt.%. The effect of In-doping on structural, optical and electrical properties was investigated using different techniques such as X-ray diffraction, atomic force microscopy, optical transmittance and Hall measurement. X-ray diffraction studies revealed that the films are polycrystalline with cubic crystal structure. The undoped and In-doped Cd₂SnO₄ films exhibit excellent optical transparency. The average optical transmittance is ∼87% in the visible range for 3 wt.% In-doping. Further In-doping widens the optical band gap from 2.98 ± 0.1 eV to 3.04 ± 0.1 eV. A minimum resistivity of 1.76 ± 0.2 × 10⁻³ Ω cm and maximum carrier concentration of 9.812 ± 0.4 × 10¹⁹ cm⁻³ have been achieved for 1 wt.% In-doping in Cd₂SnO₄ thin films.     

Electrical and optical properties of tantalum oxide thin films prepared by reactive magnetron sputtering

Tantalum oxide thin films were prepared by using reactive dc magnetron sputtering in the mixed atmosphere of Ar and O₂ with various flow ratios. The structure and O/Ta atom ratio of the thin films were analyzed by X-ray diffraction and X-ray photoelectron spectroscopy (XPS). The optical and dielectric properties of the Ta₂O₅ thin films were investigated by using ultraviolet-visible spectra, spectral ellipsometry and dielectric spectra. The results reveal that the structure of the samples changes from the amorphous phase to the β-Ta₂O₅ phase after annealing at 900 °C. The XPS analysis showed that the atomic ratio of O and Ta atom is a stoichiometric ratio of 2.50 for the sample deposited at Ar:O₂ = 4:1. The refractive index of the thin films is 2.11 within the wavelength range 300-1000 nm. The dielectric constants and loss tangents of the Ta₂O₅ thin films decrease with the increase of measurement frequency. The leakage current density of the Ta₂O₅ thin films decreases and the breakdown strength increases with the increase of Ar:O₂ flow ratios during deposition.     

Absorption-edge anisotropy of Cu2ZnSiQ4 (Q = S, Se) quaternary compound semiconductors

Polarization-dependent absorption characterization of Cu₂ZnSiQ₄ (Q = S, Se) quaternary single crystal compound semiconductors were carried out in the temperature range of 10-300 K. The absorption measurements were performed on the as grown basal plane with the normal along [2 1 0] and the axis c parallel to the long edge of the crystal platelet. A significant shift towards lower energy was observed in the absorption spectra of E⊥c polarization with respect to those corresponding to E∥c polarization. A comprehensive analysis of the absorption spectra revealed that the absorption edges of the studied crystals are indirect allowed transitions. A schematic representation of the plausible assignments for the observed near band edge optical transitions for Cu₂ZnSiQ₄ was proposed. The variation of the indirect transition energies with temperature were analyzed by Varshni and Bose-Einstein expressions. The parameters that describe the temperature dependence of the indirect transition energies with different polarizations were evaluated and discussed.     

First-principles study of the ferroelectric and optical properties of BaTeMo2O9

Electronic structure, spontaneous polarization and optical properties of single crystal BaTeMo₂O₉ have been investigated based on density-functional theory. It is found that BaTeMo₂O₉ has a direct-band-gap of 2.78 eV and there exists obvious hybridization between O 2p-Te 5p and O 2p-Mo 4d states. The results show that this compound is a good ferroelectric with large spontaneous polarization, which mainly arises from the strong Te-O and Mo-O hybridization. The interband contributions to the peaks of the optical spectra are discussed in details. Furthermore, the nonlinear optical properties are calculated by using 2n + 1 theorem applied to an electric-field dependent energy functional. The large NLO susceptibilities reveal that BaTeMo₂O₉ is a high-performance NLO crystal.     

Crystal structure and microwave dielectric properties of ZnTi(Nb1−xTax)2O8 ceramics

The crystal structure and microwave dielectric properties of ZnTi(Nb_1−xTa_x)₂O₈ (0 ≤ x ≤ 1) ceramics sintered at 1200 °C for 2 h were investigated. For x < 0.5, solid solution phases with the α-PbO₂ structure, typical of ZnTiNb₂O₈, were obtained, whereas for 0.5 ≤ x < 1, mixtures of two solid solutions each respectively based on the α-PbO₂ structure and a trirutile structure, were obtained. The relative amount of the trirutile-structured phases increased as the Ta content increased in a given region, and the end member ZnTiTa₂O₈ formed a single phase with the trirutile structure. The microwave dielectric properties were closely related to the crystal structures. A material with a near zero temperature coefficient of resonant frequency could be obtained for x = 0.8, and its dielectric constant and quality factor (Q × f) were 40.5 and 41,000 GHz, respectively.     

Spatial stress distribution and optical properties of GaN films grown on convex shape-patterned sapphire substrate by metalorganic chemical vapor deposition

μ-Raman and μ-photoluminescence methods have been employed to investigate microscopic spatial stress distribution and optical properties of GaN films grown on the convex shape-patterned sapphire substrate (CSPSS). By comparison of the μ-Raman and μ-PL spectra, we found that significantly large difference, Δσ_xx ∼0.46 GPa, in biaxial compressive stress between the flat trench and convex regions in the side facet of the GaN film, around ∼2 μm below the surface whereas on the GaN surface, little difference with large residual stress was observed in both regions compared to those from the side facet. Temperature dependent and time-resolved photoluminescence spectra have shown that the GaN film grown on the CSPSS has improved crystal purity through the reduction of intrinsic point defects.     

Structural, optical and electrical properties of CuIn5S8 thin films grown by thermal evaporation method

Stoichiometric compound of copper indium sulfur (CuIn₅S₈) was synthesized by direct reaction of high purity elemental copper, indium and sulfur in an evacuated quartz tube. The phase structure of the synthesized material revealed the cubic spinel structure. The lattice parameter (a) of single crystals was calculated to be 10.667 Å. Thin films of CuIn₅S₈ were deposited onto glass substrates under the pressure of 10⁻⁶ Torr using thermal evaporation technique. CuIn₅S₈ thin films were then thermally annealed in air from 100 to 300 °C for 2 h. The effects of thermal annealing on their physico-chemical properties were investigated using X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDX), scanning electron microscope (SEM), optical transmission and hot probe method. XRD studies of CuIn₅S₈ thin films showed that as-deposited films were amorphous in nature and transformed into polycrystalline spinel structure with strong preferred orientation along the (3 1 1) plane after the annealing at 200 °C. The composition is greatly affected by thermal treatment. From the optical transmission and reflection, an important absorption coefficient exceeds 10⁴ cm⁻¹ was found. As increasing the annealing temperature, the optical energy band gap decreases from 1.83 eV for the as-deposited films to 1.43 eV for the annealed films at 300 °C. It was found that CuIn₅S₈ thin film is an n-type semiconductor at 300 °C.     

Structural, optical and charge transport study of rutile TiO2 nanocrystals at two calcination temperatures

In this study the influence of two different calcination temperatures 80 °C and 450 °C on the structural, optical and charge transport properties of rutile TiO₂ nanocrystals has been investigated. TiO₂ nanocrystals have been prepared at low temperature by a simple hydrolysis method using titanium tetrachloride as starting precursor. The results of X-ray diffraction (XRD) showed that the prepared nanocrystals have a rutile tetragonal crystalline structure. Specific surface area of 80 °C and 450 °C calcinated rutile TiO₂ nanocrystals are 25.38 × 10⁵ cm²/g and 7.61 × 10⁵ cm²/g respectively, which has been calculated by X-ray diffraction data. Williamson-Hall plot results indicate the presence of compressive strain at 80 °C and tensile strain at 450 °C. Ultraviolet-visible (UV-vis) absorption spectroscopy is used to calculate the band gap of the material and the shift in absorption edge and it has been observed that the absorption spectra are strongly modified by the calcination temperature. The red-shift in photoluminescence (PL) is attributed to the change in strain from compressive to tensile. Photoconductivity (PC) measurements showed that capture cross-section of 80 °C (R1) and 450 °C (R2) calcinated rutile nanocrystals are 55.10 × 10⁻¹⁰ and 39.50 × 10⁻¹⁰ cm² respectively. High value of electron life-time, low value of radiative recombination and a four order increase in photogenerated charge carriers have been reported for the rutile TiO₂ nanocrystals calcinated at 450 °C.     

Synthesis, structural and microwave dielectric properties of Al2W3−xMoxO12 (x = 0-3) ceramics

Low dielectric ceramics in the Al₂W_3−xMo_xO₁₂ (x = 0-3) system have been prepared through solid state ceramic route. The phase purity of the ceramic compositions has been studied using powder X-ray diffraction (XRD) studies. The microstructure of the sintered ceramics was evaluated by Scanning Electron Microscopy (SEM). The crystal structure of the ceramic compositions as a result of Mo substitution has been studied using Laser Raman spectroscopy. The microwave dielectric properties of the ceramics were studied by Hakki and Coleman post resonator and cavity perturbation techniques. Al₂Mo_xW_3−xO₁₂ (x = 0-3) ceramics exhibited low dielectric constant and relatively high unloaded quality factor. The temperature coefficient of resonant frequency of the compositions is found to be in the range −41 to −72 ppm/°C.     

Influence of oxygen partial pressure on electrical and optical properties of Zn0.93Mn0.07O thin films

The Zn_1−xMn_xO (x = 0.07) thin films were grown on glass substrates by direct current reactive magnetron cosputtering. The influence of oxygen partial pressure on the structural, electrical and optical properties of the films has been studied. X-ray-diffraction measurement revealed that all the films were single phase and had wurtzite structure with c-axis orientation. The experimental results indicated that there was an optimum oxygen partial pressure where the film shows relative stronger texture, better nano-crystallite and lower surface roughness. As the oxygen partial pressure increases, the carrier concentration systematically decreases and photoluminescence peaks related to zinc interstitials gradually diminish. The minimal resistivity of 70.48 Ω cm with the highest Hall mobility of 1.36 cm² V⁻¹ s⁻¹ and the carrier density of 6.52 × 10¹⁶ cm⁻³ were obtained when oxygen partial pressure is 0.4. All films exhibit a transmittance higher than 80% in the visible region, while the deposited films showed a lower transmittance when oxygen partial pressure is 0.4. With the increasing of oxygen partial pressure, the peak of near-band-edge emission has firstly a blueshift and then redshift, which shows a similar trend to the band gap in the optical transmittance measurement.     

Epitaxial 0.65PbMg1/3Nb2/3O3-0.35PbTiO3 (PMN-PT) thin films grown on LaNiO3/CeO2/YSZ buffered Si substrates

Ferroelectric PMN-PT thin films with a thickness of 600 nm were epitaxially grown on buffered Si (0 0 1) substrates at a substrate temperature that ranged from 550 to 700 °C using pulsed laser deposition (PLD). LaNiO₃ (LNO) electrode thin films with a resistivity of ∼1900 μΩ cm were epitaxially grown on CeO₂/YSZ buffered Si (0 0 1) substrates. The PMN-PT thin films grown at 600 °C on LNO/CeO₂/YSZ/Si substrates had a pure perovskite and epitaxial structure. The PMN-PT films exhibited a high dielectric constant of about 1818 and a low dissipation factor of 0.04 at a frequency of 10 kHz. Polarization-electric-field (P-E) hysteresis characteristics, with a remnant polarization of 11.1 μC/cm² and a coercive field of 43 kV/cm, were obtained in the epitaxial PMN-PT films.     

Optical spectra and excited state relaxation dynamics of Nd in CaF2 single crystal

Nd-doped CaF₂ crystal with high optical quality was obtained by a temperature gradient technique (TGT). Energies of the crystal field levels of Nd³⁺ multiplets relevant to laser operation were determined based on optical spectra recorded at T = 10 K. Room temperature absorption spectra were analyzed in the framework of the Judd-Ofelt theory to calculate radiative transition rates and luminescence branching ratios for the ⁴F_3/2 level. The ⁴F_3/2 radiative lifetime was calculated to be 1295 μs whereas a monotonous decrease of the ⁴F_3/2 luminescence lifetime value from 1.45 ms to 0.9 ms was observed when the temperature increased from 10 K to 300 K. The stimulated emission cross-section of 1.48 × 10⁻²⁰ cm² at 1061 nm was determined using the Fuchtbauer-Ladenburg relation. All the results showed that Nd:CaF₂ would be a promising gain media in solid-state lasers.