Jesus M. Siqueiros

Synthesis and optelectronic characterization of gallium doped zinc oxide transparent electrodes

Abstract In this work we report on pulsed laser deposition of gallium-doped zinc oxide (ZnO:Ga) transparent-conducting thin films grown on glass at different substrate temperatures. A widening in the optical bandgap and a good gallium-doping efficiency were observed in the films when the substrate temperature was raised from 150 °C to 300 °C, as determined from optical and electrical measurements. X-ray diffraction measurements revealed that the films grow preferentially oriented in the [002] crystallographic direction of the ZnO grains. The crystallinity of the films was also found to be strongly dependent on the substrate deposition temperature. The ZnO:Ga transparent films had excellent transmittance (85%) in the visible spectrum and a low electrical resistivity value (7 × 10 −4 Ω cm) in 200 nm thickness samples deposited on glass by laser ablation at 300 °C.

Physical properties of Y2O3:Eu luminescent films grown by MOCVD and laser ablation

Abstract Luminescent Y2O3:Eu3+ thin films were deposited on sapphire, polycrystalline Al2O3 and indium tin oxide coated glass or sapphire substrates by two different techniques: metallorganic chemical vapor deposition (MOCVD) and laser ablation. Microcrystalline Y2O3:Eu films were grown in a MOCVD chamber by decomposing and reacting yttrium and europium organometallic precursors in an oxygen atmosphere at low pressures (1–10 mTorr) and low substrate temperatures (500–700°C). The as-deposited films showed the characteristic red fluorescence spectrum of Y2O3:Eu with the main peak centered about 611 nm wavelength. The as-deposited films averaged 1.0 μm in particle size and 2.0 μm in thickness. Post-deposition annealing treatments in the temperature range 900–1200°C enhanced the luminescent intensity of the films. The as-deposited laser ablated oxide films were amorphous and required annealing at temperatures higher than 800°C to observe luminescence, which occurred in conjunction with crystallization. The as-deposited films averaged 500 nm in thickness and after post-annealing at 1000°C were composed of 15–200 nm grains.

Determination of the optical constants of MgF 2 and ZnS from spectrophotometric measurements and the classical oscillator method

The values of the optical constants of magnesium fluoride (MgF(2)) and zinc sulfide (ZnS) thin films are obtained using a classical oscillator model and the experimental values of their spectral transmittance. Auger electron spectroscopy was performed on the samples to determine the chemical composition of the films. These materials are important in the design of filters, mirrors, and antireflection coatings for optical instrumentation. Unfortunately their properties strongly depend on evaporation conditions. The procedure described here allows direct measurement of the dispersive refractive index of the film after deposition.

High transmittance–low resistivity ZnO:Ga films by laser ablation

High transmittance, low resistivity, and c‐axis highly oriented ZnO:Ga thin films on glass were obtained by laser ablation at different deposition temperatures. The surface morphology, crystalline structure, and optical and electrical properties were found to depend directly on substrate temperature. From optical and electrical analyses we observed that the optical transmittance, carrier concentration, and optical energy gap of the ZnO:Ga transparent conductive oxides increased when the deposition temperature is raised from 150 to 300 °C. Films grown on 300 °C substrates showed a low resistivity value of 3.6×10−4 Ω cm, a carrier concentration of 8.7×1020 cm−3, a band gap of 3.81 eV, and a visible transmission of 85%. These films were deposited with an excimer (KrF) laser beam of λ=248 nm operated under optimized conditions of 2.7 mJ/cm2 energy density and 30 Hz repetition rate.

Effect of Nb Doping on (Sr,Ba)TiO3 (BST) Ceramic Samples

The effect of doping the Sr0.3Ba0.7Ti(1−5y/4)NbyO3 ceramic with different concentration of Nb is studied by scanning electron microscopy (SEM), X-ray diffraction and thermoelectric analysis. It is observed that the grain size decreases as the Nb concentration increases. The critical temperature Tc has a linear decrease at a rate of 19°C/mol% of Nb. The temperature dependence of the dielectric permittivity presents strongly broadened curves, which suggest a non Curie-Weiss behavior near the transition temperature. The diffuse phase transition coefficient (δ) was also determined and its value leads to the conclusion that the degree of disorder in the system increases with the presence of the Nb cation.

Determination of (n,k) for absorbing thin films using reflectance measurements.

We propose a method for determination of the complex refractive index of absorbing materials either in bulk or film geometry by measuring its reflectivity when coated with a well-characterized transparent dielectric at two specific optical thicknesses: n(1)d(1) = lambda(0)/4 and n(1)d(1) = lambda(0)/2. The complex refractive index of the sample ñ = (n,k) is calculated for the monitoring wavelength lambda(0). The selected optical thicknesses of the coating allow the calculation of its geometrical thickness, therefore the variation of ñ with wavelength in the region where the reflectivity is measured can be determined.

Growth and characterization of ZnS and ZnCdS nanoclusters in mordenite zeolite host

Abstract The growth of semiconductor nanoclusters of ZnS and the ternary ZnCdS system embedded in synthetic mordenites by ionic exchange using different mixtures of CdCl 2 and ZnCl 2 as starting sources, followed by treatment in hydrogen sulphide flow, is reported. Compositional, morphological and structural studies were realized by XRD, SEM, TEM and energy dispersive spectroscopy (EDS). XRD shows well-crystallized mordenite and the absence of secondary phases; EDS gives us the chemical composition of the matrix host and the relative weight percent of Cd, Zn and S. TEM and SEM show the morphology, packing and sizes of mordenite crystals (bundled needles of 3 μm long and 0.07 μm width). Measurements of the absorption spectra by diffuse reflectance spectroscopy in the UV-visible range allow us an analysis of the optical behavior as a function of the relative concentration of Zn and Cd. Samples show blue shifts in the absorption edges when the Cd concentration decreases and correspondingly, the Zn concentration increases. The optical band gap is determined and its behavior with concentration changes is discussed. Previously, ZnCdS clusters supported on mordenite were prepared in our laboratory by hydrothermal growth. A comparative discussion of the results obtained through both synthetic processes is presented.

Magnetoelectric coupling study in multiferroic Pb(Fe0.5Nb0.5)O3 ceramics through small and large electric signal standard measurements

Multifunctional multiferroic materials such as the single phase compound Pb(Fe0.5Nb0.5)O3 (PFN), where ferroelectric and antiferromagnetic order coexist, are very promising and have great interest from the academic and technological points of view. In this work, coupling of the ferroelectric and magnetic moments is reported. For this study, a combination of the small signal response using the impedance spectroscopy technique and the electromechanical resonance method with the large signal response through standard ferroelectric hysteresis measurement, has been used with and without an applied magnetic field. The measurements to determine the electrical properties of the ceramic were performed as functions of the bias and poling electric fields. A simultaneous analysis of the complex dielectric constant ɛ, impedance Z, electric modulus M, admittance Y, and the electromechanical parameters and coupling factors is presented. The results are correlated with a previous study of structural, morphological, s...

Evidence of magnetodielectric coupling in multiferroic Pb"Fe0.5Nb0.5…O3 ceramics from ferroelectric measurements and electron paramagnetic resonance

Magnetodielectric coupling effects of single phase multiferroic Pb(Fe0.5Nb0.5)O3 ceramics have been studied using standard ferroelectric measurements and electron paramagnetic resonance (EPR). Changes in the electrical polarization behavior were observed in the paramagnetic to weakly magnetized antiferromagnetic transition near 103 K associated with a rhombohedral to monoclinic symmetry reduction. The analysis of changes in the EPR spectral parameters confirms the transition from paramagnetic to weakly magnetized antiferromagnetic and reveals noticeable anomalies in the high temperature region near the ferroelectric-paraelectric transition (383 K), which are correlated with the tetragonal-to-cubic symmetry change and the characteristic diffuse phase transition of this material.

Pulsed laser deposition of Y3Al5O12:Tb photoluminescent thin films

Transparent‐photoluminescent Y3Al5O12:Tb (YAG:Tb) thin films were deposited on single‐ crystalline sapphire at two different substrate temperatures by means of the pulsed laser deposition technique. The ablation targets were fabricated from YAG:Tb powders obtained with a new ceramic processing technique (combustion synthesis). A KrF excimer laser with an ultraviolet wavelength of 248 nm was used to evaporate the target and to grow the films. Analysis and characterization of the films were performed by scanning electron microscopy, x‐ray diffraction, Auger electron spectroscopy, and photoluminescence measurements. As‐deposited films were amorphous and deficient in oxygen as compared to the target. Additionally, these films were not luminescent. Postannealing treatments in air for at least 1 hr in the range 800–1200 °C were found to be necessary in order to induce crystallization and oxygenation in the films thus recovering the green photoemission of YAG:Tb with these treatments.