Structural and optical properties of Zn(Mg,Cd)O alloy films grown by remote-plasma-enhanced MOCVD

We report the structural and optical properties of wurtzite-structure Zn(Mg,Cd)O ternary alloys. Wurtzite (0 0 0 1) Zn_1−xCd_xO and Mg_yZn_1−yO films were grown on (11–20) sapphire substrates using remote-plasma-enhanced metalorganic chemical vapor deposition. The large bowing parameters of Zn_1−xCd_xO and Mg_yZn_1−yO ternary alloys are 3.0 and 3.5, respectively, which reflects the large difference of each binary’s electronegativity. We have analyzed the broadening of photoluminescence (PL) in Zn(Mg,Cd)O alloys on alloy content by taking into account the statistical alloy fluctuation and the localization of the exciton, and have clarified that the localization of the exciton strongly affects to PL full-width at half-maximum (FWHM) in Zn(Mg,Cd)O alloys. The alloy broadenings in steady-state PL of Zn(Mg,Cd)O alloys are in good agreement with the calculated tendency by the theoretical model based on the statistical alloy fluctuation, while PL FWHM of Zn_1−xCd_xO is three times larger than the calculated results. Moreover, as another way to confirm alloy broadening, we also have done time-resolved PL measurements and derived the localized depth of the exciton in ZnO-based system, indicating a good agreement with the tendency of PL FWHM broadening.     

Electrical and optical properties of Cd1−xZnxS (0x0.18) grown by chemical bath deposition

Cd_1−xZn_xS films with 0x0.18 were grown by chemical bath deposition technique on glass substrates from an aqueous solution containing cadmium and zinc sulfate, ammonia and thiourea. Microstructural features, obtained from X-ray diffraction and scanning electron microscopy (SEM) measurements, reveal a predominance of Wurtzite structure and an homogenous microstructure formed by densely microcrystallines for all the samples studied. Cd_1−xZn_xS semiconductor was found to be resistive and of n-type. Also, the electron density decreases with increased x and the mobility reaches a maximum around x=0.12. Which means that the Cd_1−xZn_xS films at this composition are of high crystalline quality, i.e. having reduced intrinsic defect concentrations.     

Structural and optical properties of single-crystal ZnMgO thin films grown on sapphire and ZnO substrates by RF magnetron sputtering

High-quality ZnMgO films were grown by the radio frequency (RF) magnetron sputtering technique in pure oxygen ambient. Single-crystal films were obtained, when the Mg concentration was Zn_0.87Mg_0.13O or lower in the case of ZnMgO/Al₂O₃ and when it was Zn_0.65Mg_0.35O or lower in the case of ZnMgO/ZnO. Polycrystalline films were obtained when the growth temperature was lower than 500 °C, regardless of the Mg concentration. Position of the photoluminescence (PL) ultraviolet (UV) peak of the ZnMgO film shifted with the addition of Mg, from 3.33 eV (ZnO) to 3.51 eV (Zn_0.87Mg_0.13O) and 3.70 eV (Zn_0.65Mg_0.35O). It was also observed that growth of the ZnMgO films at higher temperature resulted in higher band-gap energy. It was proposed that this phenomenon is because concentration of the substitutional Mg atoms occupying Zn site is increased as the growth temperature increases.     

Growth and property of Zn-doped near-stoichiometric LiTaO3 crystal

Near-stoichiometric LiTaO₃ (SLT) and Zn-doped near-stoichiometric LiTaO₃ (Zn:SLT) crystals with 10–15 mm in diameter and 10 mm in length were grown by using TSSG technique with K₂O as the flux. The effect of adding amount of K₂O was discussed in the growing process. The crystals were characterized by inductively coupled plasma-optical emission (ICP-OES), X-ray diffraction (XRD) and differential thermal analysis (DTA). The lattice constants of Zn:SLT were smaller than those of SLT and Curie temperature was higher than that of SLT. It was found that Zn doping is an efficient way to improve the optical damage resistance ability of SLT crystal. Compared with SLT crystal, Zn:SLT exhibited a much higher optical damage threshold, more than 500 MW/cm², which was attributed to Zn self-compensated effect that formed the charge compensated complexes, (Zn_Ta)³⁻–3(Zn_Li)⁺ in SLT crystal.     

A study on structural formation and optical property of wide band-gap Be0.2Zn0.8O layers grown by RF magnetron co-sputtering system

Wide band-gap BeZnO layers were grown on Al₂O₃ (0 0 0 1) substrate using radio-frequency magnetron co-sputtering. The rate of Be_xZn_1−xO crystallized as a hexagonal structure was x=0.2. From the X-ray photoelectron spectroscopy measurement, the O–Zn bonds relating the crystal structure and the Be–O bonds related to the deviation of the stoichiometry in the BeZnO layer were caught at 530.4 and 531.7 eV in the O 1s spectrum, respectively. Thus, the observance on the Be 1s peak of 113.2 eV associated with the bonding Be–O indicates that the sputtered Be atoms are substituted for the host-lattice site in ZnO. This Be–O bonding shows a relatively low intense and broadening spectrum caused by large fluctuation of Be content in the BeZnO layer. From the photoluminescence and transmittance measurement, the free exciton and the neutral donor-bound exciton (D⁰, X) emissions were observed at 3.7692 and 3.7313 eV, respectively, and an average transmittance rate over 95% was achieved in a wide ultraviolet (UV)–visible region. Also, the binding energy for the (D⁰, X) emission was extracted to be 37.9 meV. Through the wide band-gap material BeZnO, we may open some possibilities for fabricating a ZnO-based UV light-emitting diode to be utilized as a barrier layer comprised of the ZnO/BeZnO quantum well structure and/or an UV light emitting material itself.     

Influence of some foreign metal ions on crystal growth kinetics of brushite (CaHPO4·2H2O)

Brushite, CaHPO₄·2H₂O, has been precipitated at 25 °C in the presence of Mg²⁺, Ba²⁺ or Cu²⁺ at concentrations up to 0.5 mM. When initial pH is sufficiently low to exclude nanocrystalline apatite as the initial solid phase, overall crystal growth rate may be determined from simple mass crystallization by recording pH as function of time. A combination of surface nucleation (birth-and-spread) and spiral (BCF) growth was found. Edge free energy was determined from the former contribution and was found to be a linear function of chemical potential of the additive, indicating constant adsorption over a wide range of additive concentrations. Average distances between adsorbed additive ions as calculated from slopes of plots are compatible with lattice parameters of brushite: 0.54 nm for Mg²⁺, 0.43 nm for Ba²⁺ and 0.86 nm for Cu²⁺. With the latter a sharp decrease in growth rate occurred early in the crystallization process, followed by an equally sharp increase to the previous level. When interpreted in terms of the Cabrera–Vermilyea theory of crystal growth inhibition, the results are consistent with an average distance between Cu ions of 0.88 nm, in perfect agreement with the above value.     

Growth and shape control of orthorhombic Fe5(PO4)4(OH)3·2H2O single crystalline dendrites

Orthorhombic Fe₅(PO₄)₄(OH)₃·2H₂O single crystalline dendritic nanostructures have been synthesized by a facile and reproducible hydrothermal method without the aid of any surfactants. The influences of synthetic parameters, such as reaction time, temperature, the amount of H₂O₂ solution, pH values, and types of iron precursors, on the crystal structures and morphologies of the resulting products have been investigated. The formation process of Fe₅(PO₄)₄(OH)₃·2H₂O dendritic nanostructures is time dependent: amorphous FePO₄·nH₂O nanoparticles are formed firstly, and then Fe₅(PO₄)₄(OH)₃·2H₂O dendrites are assembled via a crystallization-orientation attachment process, accompanying a color change from yellow to green. The shapes and sizes of Fe₅(PO₄)₄(OH)₃·2H₂O products can be controlled by adjusting the amount of H₂O₂ solution, pH values, and types of iron precursors in the reaction system.     

Crystal growth, characterization and spectroscopic study of europium-doped NaY(PO3)4

Europium-doped NaY(PO₃)₄ single crystals have been synthesized by the flux method with sizes around 1 mm³. The unit cell parameters at room temperature refined by X-ray powder diffraction are a=7.1510(4) Å; b=13.0070(8) Å; c=9.6973(2) Å; β=90.606(3)°, Z=4 with the space group P2₁/n in monoclinic system. The present single crystals have a needle shape, they are elongated along the a crystallographic direction, and their size is in the 500 μm–1 mm range. The linear thermal expansion tensor parameters were determined, being the maximum value along the b direction, 16.1×10⁻⁶ K⁻¹ and the minimum along the a direction being 11.7×10⁻⁶ K⁻¹. The IR vibration modes attributed to the group P–O are consistent with the crystallographic data concerning the chain aspect of the phosphate anion. This material melts incongruently at 1141 K. Intense visible emissions attributed to Eu^{3+ 5}D₀→⁷F₁, ⁵D₀→⁷F₂ and ⁵D₀→⁷F₄, electronic transitions have been observed after pumping at 355 nm at room temperature.     

Large-size and high-quality Zn2TiO4 single crystal grown by the optical floating zone method

Crack-free and transparent Zn₂TiO₄ single crystals of 4–6 mm in diameter and 30 mm in length have been grown by the optical floating zone method. The powder X-ray diffraction (XRD) results show that the as-grown crystals have the spinel-type Zn₂TiO₄ structure. XRD² measurements on Zn₂TiO₄ wafers cut perpendicular to the growth direction display only one peak at 42.7°, which indicates that the Zn₂TiO₄ single crystals grow along the 〈4 0 0〉 direction (a-axis). The formation of bubble inclusions was effectively suppressed by lowering rotation rate. Transmission polarized-light microscopy results showed that as-grown crystals were free of low angle grain boundaries.     

Nano-crystallization in LaF3–Na2O–Al2O3–SiO2 glass

Aiming at tailoring optical properties, the precipitation of LaF₃ nano-crystals in LaF₃–Na₂O–Al₂O₃–SiO₂ glass-ceramics is studied thoroughly on the nano-scale using advanced transmission electron microscopic techniques. Nano-sized phase-separation droplets enriched in lanthanum and silicon are formed already in the base glass. Within these less than 20 nm large droplets, LaF₃ crystallizes upon heat treatment. The nano-crystallization mechanism revealed is self-limited since growth is restricted by the size of the droplets. An average crystallite size of around 12 nm is achieved with a narrow size distribution since the phase-separation droplets also contain silicon not incorporated into the growing crystal. Instead, excess silicon relocated to the periphery of the pre-existing phase-separation droplets forms a diffusion barrier around the LaF₃ nano-crystals preventing further crystal growth and/or ripening.     

Structure and optical properties of Mg‐doped ZnO nanoparticles by polyacrylamide method

Mg‐doped ZnO (Mg_xZn_1‐xO) nanoparticles with precise stoichiometry are synthesized through polyacrylamide polymer method. Calcination of the polymer precursor at 650 °C gives particles of the homogeneous solid solution of the (Mg_xZn_1‐xO) system in the composition range (x < 0.15). ZnO doping with Mg causes shrinkage of lattice parameter c. The synthesized Mg_xZn_1‐xO nanoparticles are typically with the diameter of 70–85 nm. Blue shift of band gap with the Mg‐content is demonstrated, and photoluminescence (PL) from ZnO has been found to be tunable in a wide range from green to blue through Mg doping. The blue‐related PL therefore appeared to be caused by energetic shifts of the valence band and/or the conduction band of ZnO. Mg_xZn_1‐xO nanoparticles synthesized by polyacrylamide‐gel method after modified by polyethylene glycol surfactant have a remarkable improvement of stability in the ethanol solvent, indicating that these MZO nanoparticles could be considered as the candidate for the application of solution–processed technologies for optoelectronics at ambient temperature conditions.     

Ternary system Li2O–K2O–Nb2O5: Re-examination of the 30 mol% K2O isopleth and growth of fully stoichiometric potassium lithium niobate single crystals by the micro-pulling down technique

Potassium lithium niobate (KLN) single crystals have attractive properties for non-linear optical applications based on frequency conversion of laser diodes in the blue range. Especially, fully stoichiometric K₃Li₂Nb₅O₁₅ crystals would be capable of doubling a laser light in the near UV range. Using powder X-ray diffraction and DSC experiments, we have re-investigated the 30 mol% K₂O isopleth of the ternary system Li₂O–K₂O–Nb₂O₅ in order to explore the possibility of a limited existence field for this phase. From our results, it was shown that the stoichiometric KLN phase exists between 970 and 1040 °C, temperature at which it undergoes a non-congruent melting. From this conclusion, compositionally homogeneous a-axis oriented single crystals fibers of stoichiometric K₃Li₂Nb₅O₁₅ were successfully grown by the micro-pulling down technique with pulling rates in the range 0.3–0.7 mm min⁻¹. The crystal length was between 10 and 120 mm for an apparent diameter near 500 μm. The fibers, characterized by optical microscopy, X-ray diffraction and Raman spectroscopy, appeared free of macro-defects and of good quality and their stoichiometric composition was also confirmed.     

Bulk crystal growth of congruent MgO-doped LiNbO3 crystal with stoichiometric structure and its second-harmonic-generation properties

A new LiNbO₃ bulk crystal has been grown by doping with MgO (cs-MgO:LN; Li₂O:Nb₂O₅:MgO=45.30:50.00:4.70, (Li_0.906Mg_0.047V^Li_0.047)NbO₃), which successfully has the congruent point coinciding with the stoichiometric point. Its second-harmonic-generation (SHG) properties were evaluated. It was found that cs-MgO:LN has a much more homogeneous composition leading to uniform in-plane distribution of the non-critical phase-matching wavelength than the conventional LiNbO₃ crystals such as congruent LiNbO₃ (c-LN), stoichiometric LiNbO₃ (s-LN), and MgO-doped congruent LiNbO₃ (5MgO:LN). This homogeneity arose from the observation that none of the solute components including ionic species were segregated at the interface during growth. The SHG conversion efficiency of cs-MgO:LN is comparable to those of s-LN and 5MgO:LN.     

Crystal growth and spectroscopic properties of erbium doped Lu2SiO5

A high optical quality erbium doped Lu₂SiO₅ single crystal has been grown by the Czochralski method. The distribution coefficient of Er³⁺ was measured to be ∼0.926. The absorption and emission spectra as well as the fluorescence decay curve of the excited state ⁴I_13/2 were measured at room temperature. The spectroscopic parameters were calculated using the Judd–Ofelt theory, and the J–O parameters Ω₂, Ω₄ and Ω₆ were found to be 4.451×10^-20, 1.614×10^-20 and 1.158×10^-20 cm², respectively. The room-temperature fluorescence lifetime of the Er³⁺⁴I_13/2→⁴I_15/2 transition was measured to be 7.74 ms. The absorption and emission cross-section as well as the gain cross-section in the eye-safe regime of 1400–1700 nm were also determined and discussed.     

Microstructural changes in epitaxial YBa2Cu3O7−δ thin films due to creation of O vacancies

Structural and transport properties of YBa₂Cu₃O_7−δ thin films subjected to different heat treatments and, consequently, having different O vacancy contents are investigated. It is observed that lattice parameters of the films follow the trend of the bulk material, indicating that either the additional stresses created in c-axis oriented films by O vacancies are negligibly small or they are relaxed at elevated temperatures. In addition, heat treatment under relatively low O₂ partial pressures, used to create O vacancies, leads to considerable reduction in local variations of lattice spacings in YBa₂Cu₃O_7−δ thin films.     

Thermal-induced phase transition and assembly of hexagonal metastable In2O3 nanocrystals: A new approach to In2O3 functional materials

This paper reports on the thermal-induced performance of hexagonal metastable In₂O₃ nanocrystals involving in phase transition and assembly, with particular emphasis on the assembly for the preparation of functional materials. For In₂O₃ nanocrystals, the metastable phase was found to be thermally unstable and transform to cubic phase when temperature was higher than 600 °C, accompanied by assembly as well as evolution of optical properties, but the two polymorphs coexisted at the temperature ranging from 600 to 900 °C, during which the content of product phase and crystal size gradually increased upon increasing temperature. The assembly of In₂O₃ nanocrystals can be developed to fabricate In₂O₃ functional materials, such as various ceramic materials, or even desired nano- or micro-structures, by using metastable In₂O₃ nanocrystals as precursors or building blocks. The electrical resistivity of In₂O₃ conductive film fabricated by a hot-pressing route was as low as 3.72×10⁻³ Ω cm, close to that of In₂O₃ single crystal, which is important for In₂O₃ that is always used as conductive materials. The findings should be of importance for both the wide applications of In₂O₃ in optical and electronic devices and theoretical investigations on crystal structures.     

Nucleation-growth process of scale electrodeposition – influence of the magnesium ions

Electrodeposition of CaCO₃ was studied under the influence of magnesium ions present in a carbonically pure water. The investigation was performed using electrochemical, gravimetric and optical methods. The chronoamperometric measurements confirmed the inhibiting property of Mg²⁺ when its concentration is higher than 120 mg L⁻¹ in a solution containing 160 mg L⁻¹ of Ca²⁺. The optical technique led the nucleation-growth process to be accessed by means of an optical microscope positioned behind a transparent electrode. The increase of Mg²⁺ concentration changed drastically the calcite morphology. At 360 mg L⁻¹ of Mg²⁺, the calcite morphology was optically amorphous but the Raman spectrum confirmed its structure. The crystal growth was recorded in situ and the image analysis software characterised the nucleation process as well as the growth rate of the crystals. It allowed the influence of the Mg²⁺ ions on the crystallisation process of CaCO₃ to be quantified.     

Local structural,magnetic, and optical properties of Zn1−xFexO thin films

High quality Zn_1−xFe_xO thin films were deposited on α-sapphire$\alpha \text{-sapphire}$ substrates by RF magnetron sputtering. X-ray absorption fine structure measurements showed that the chemical valence of Fe ions in the films was a mixture of 2+ and 3+ states, and Fe ions substituted mainly for the Zn sites in the films. DC-magnetization measurements revealed ferromagnetic properties from 5 to 300 K. The photoluminescence measurements at 15 K showed a sharp main transition peak at 3.35 eV along with a broad impurity peak at 2.45 eV. The structural and magnetization analyses of the Zn_1−xFe_xO films strongly suggested that the ferromagnetism was the intrinsic properties of the films.     

Ca3N2 as a flux for crystallization of GaN

In this work Ca₃N₂ was investigated as a potential flux for crystallization of GaN. Melting temperature of the potential flux at high N₂ pressure evaluated by thermal analysis as 1380 °C is in good agreement with the theoretical prediction. It is shown that Ca₃N₂ present in the liquid gallium in small amount (1 at%) dramatically accelerates synthesis of GaN from its constituents. On the other hand, it does not influence significantly the rate of GaN crystallization from solution in gallium in temperature gradient for both unseeded and seeded configurations. However the habit and color of the spontaneously grown GaN crystals change drastically. For 10 mol% Ca₃N₂ content in the liquid Ga it was found that the GaN thick layer and GaN crystals (identified by micro-Raman scattering measurements) were grown on the substrate. For growth from molten Ca₃N₂ (100%) with GaN source, the most important observations were (i) GaN source material was completely dissolved in the molten Ca₃N₂ flux and (ii) after experiment, GaN crystals were found on the sapphire substrate.     

The effect of solvent ratio and water on the growth of C60 nanowhiskers

The growth of C₆₀ nanowhiskers (C₆₀NWs) prepared by a modified liquid–liquid interfacial precipitation method is investigated, focusing on the effect of solvent ratio and water content in the C₆₀–toluene–isopropyl alcohol (IPA) solution system. The precipitation of C₆₀NWs was markedly influenced by the solvent ratio of toluene to IPA, and the C₆₀NWs were found to grow longer above a critical diameter (D_c), which depends on the solvent ratio. The addition of a small amount of H₂O to the C₆₀–toluene–IPA solution promoted the growth of C₆₀NWs. This catalytic effect of water on the growth of C₆₀NWs was confirmed also by the experiment using heavy water (D₂O) and by the decrease of growth activation energy of C₆₀NWs with increase of H₂O content in the C₆₀–toluene–IPA solution.