M. Zapata-Torres

Effect of electrode type in the resistive switching behaviour of TiO2 thin films

The influence of the electrode/active layer on the electric-field-induced resistance-switching phenomena of TiO2-based metal?oxide?metal devices (MOM) is studied. TiO2 active layers were fabricated by the reactive rf-sputtering technique and devices were made by sandwiching between several metal electrodes. Three different MOM devices were made, according with the junction type formed between the electrode and the TiO2 active layer, those where Ohmic?Ohmic, Ohmic?Schottky and Schottky?Schottky. The junction type was tested by electrical I?V measurements. It was found that MOM devices made with the Ohmic?Ohmic combination did not show any resistive switching behaviour in contrast with devices made with Ohmic?Schottky and Schottky?Schottky combinations. From a detailed analysis of the I?V curves it was found that transport characteristics are Ohmic for the low-resistance state for all the contacts combinations of the MOM devices, whereas in the high-resistance state it depends on contact combinations and can be identified as Ohmic, Schottky and Poole?Frenkel type. These conduction mechanisms in the low- and high-resistance states suggest that formation and rupture of conducting filaments through the film oxide is the mechanism responsible for the resistance switching.

Physical properties of CdTe:Cu films grown at low temperature by pulsed laser deposition

CdTe:Cu films were grown by pulsed laser deposition on Corning glass slides at a substrate temperature of 300 °C. The thin films were grown using CdTe and Cu2Te powders, varying the Cu2Te concentration from 3 to 10 wt. %. The structural, compositional, optical, and electrical properties were analyzed as a function of the nominal copper concentration. X-ray diffraction shows that films have CdTe cubic phase. The compositional analysis indicates that CdTe:Cu films grown with lower Cu content have Te excess, on the other hand, films with higher Cu content have Te deficiencies. The electrical measurements showed that CdTe:Cu films grown with low Cu content present lowest resistivity.

Auger electron spectroscopy analysis of oxidation states of Te in amorphous CdTe oxide thin films

Amorphous CdTe oxide (a-CdTe:O) thin films with different concentrations of oxygen were grown by rf sputtering and analyzed by Auger electron spectroscopy. A slight change of shape in the Cd MNN peak as a function of oxygen content in a-CdTe:O is observed. This peak resembles the Cd MNN peak of CdTe, at one extreme, and that of CdTeO3, at the other. The Te MNN and the O KLL peaks have similar shapes and lower intensities for a-CdTe:O oxygen saturated films than those in CdTeO3. On the other hand, there is a large and gradual difference in shape, intensity, and energy observed in the Te MNN peak among a-CdTe:O with low, intermediate, and high concentration of oxygen. Different Te oxidation states, as Te−2 and Te+4, contribute to this change as evidenced by a simulation with combinations of CdTe and of CdTeO3 spectra, or by combinations of a-CdTe:O spectra with low and high oxygen content.

Structural and optical properties of Cu-doped CdTe films with hexagonal phase grown by pulsed laser deposition

Cu-doped CdTethin films were prepared by pulsed laser deposition on Corning glass substrates using powders as target. Films were deposited at substrate temperatures ranging from 100 to 300 °C. The X-ray diffraction shows that both the Cu-doping and the increase in the substrate temperature promote the presence of the hexagonal CdTe phase. For a substrate temperature of 300 °C a CdTe:Cu film with hexagonal phase was obtained. Raman and EDS analysis indicate that the films grew with an excess of Te, which indicates that CdTe:Cu films have p-type conductivity.

Heavily doped CdTe films grown by close‐spaced vapor transport technique combined with free evaporation

Very heavily doped n‐type polycrystalline CdTe films doped with metallic cadmium were prepared by using close‐spaced vapor transport technique combined with free evaporation and the electrical, structural, and morphological properties were investigated. Cadmium was introduced as a dopant by evaporation during films preparation. The highest dark conductivity at room temperature of the films obtained was 1.18×104 S cm−1. The dark conductivity decreased with the increase of the ambient temperature. The highest dark electron concentration obtained was 1.59×1022 cm−3 and increased with the temperature. The mobility decreased with the temperature. The film was a polycrystalline cubic phase. We show the surface topography of the film using scanning electron microscope and scanning tunneling microscopy techniques, in order to see the growth patterns. The crystallite size was very uniform of a few μm, and the growth patterns on the grain surface were in form of terraces.

Synthesis of visible light emitting self assembled Ge nanocrystals embedded within a SiO2 matrix

As-grown light emitting self-assembled Ge nanocrystals (Ge-NCs) embedded in a SiO2 matrix were produced via a sequential deposition process of SiO2/Ge/SiO2 layers employing a reactive radio frequency sputtering technique. Obtained Ge-NCs show a crystallographic phase, the proportion, size, quality, and specific orientation of which are determined by the oxygen partial pressure. Photoluminescence (PL) spectra indicate that the size distribution of Ge-NCs is reduced and centered on about 8 nm when higher oxygen partial pressure is employed; the formation of Ge-NCs is corroborated by transmission electron microscopy measurements, and their sizes are consistent with estimates from PL measurements. Resistivity measurements are explained by a near neighbors hopping process, with specific features depending on the Ge-NCs’ size. The features of PL and resistivity measurements indicate that there is no appreciable dependence of the number of interfacial defects on the oxygen partial pressure.

Facile Synthesis of SrCO3-Sr(OH)2/PPy Nanocomposite with Enhanced Photocatalytic Activity under Visible Light

Pyrrole monomer was chemically polymerized onto SrCO3-Sr(OH)2 powders to obtain SrCO3-Sr(OH)2/polypyrrole nanocomposite to be used as a candidate for photocatalytic degradation of methylene blue dye (MB). The material was characterized by Fourier transform infrared (FTIR) spectroscopy, UV/Vis spectroscopy, and X-ray diffraction (XRD). It was observed from transmission electronic microscopy (TEM) analysis that the reported synthesis route allows the production of SrCO3-Sr(OH)2 nanoparticles with particle size below 100 nm which were embedded within a semiconducting polypyrrole matrix (PPy). The SrCO3-Sr(OH)2 and SrCO3-Sr(OH)2/PPy nanocomposites were tested in the photodegradation of MB dye under visible light irradiation. Also, the effects of MB dye initial concentration and the catalyst load on photodegradation efficiency were studied and discussed. Under the same conditions, the efficiency of photodegradation of MB employing the SrCO3-Sr(OH)2/PPy nanocomposite increases as compared with that obtained employing the SrCO3-Sr(OH)2 nanocomposite.

Structural characterization of polycrystalline Cd-Te-In films

Polycrystalline Cd–Te–In films have been grown on glass substrates by close-spaced vapor transport combined with a free evaporation technique and the stoichiometric, structural and electrical properties were investigated as functions of In2Te3 concentration added in solid solution into the CdTe structure during In incorporation. Indium was introduced by evaporation during film preparation and the incorporation was controlled by the temperature of the In source. The composition of the films was investigated by Auger electron spectroscopy, showing that, when In concentration increases the Cd concentration decreases they have a similar value (≈22 at. %) at about 750 °C In source temperature. The dark resistivity decreased monotonically four orders of magnitude with the In2Te3 concentration and reached a minimum point. From the structural characterization employed it was shown that the In atoms are incorporated in two ways: (I) for as low-In concentration, the In atoms substitute the Cd atoms, decreasing the ...

Synthesis of Light Emitting Ge Nanocrystals by Reactive RF Sputtering

In this work we report the results of the synthesis, structural and optical characterization of SiO2/Ge/SiO2 heterostructures by reactive RF sputtering. The SiO2 films were grown by reactive sputtering employing a plasma mixture of oxygen and argon. The Ge layer was grown employing an Ar atmosphere. The samples were prepared on p-type Si (1 1 1) substrates by reactive sputtering. The effect of the partial pressure of oxygen on the electronic properties of the heterostructure is reported[1]. Structural characterization was carried out by grazing angle X-ray difraction. Surface roughness was quantified by atomic force microscopy. The presence of Ge nanocrystals (Ge-NCs) was evidenced by X-ray diffraction. The vibrational properties were studied by Raman spectroscopy. The Raman spectra showed modes associated to germanium indicating the formation of low dimensionality germanium particles embedded within a SiO2 matrix. Photoluminescence emission is observed around ~1.7 eV and it is associated to the quantum confinement of carriers in Ge-NCs. Ohmic contacts were deposited using a van der Pauw geometry employing an a AuSb alloy for the contacts. Temperature dependent Hall (T-Hall) measurements were done between 35 K and 150 K, using the van der Pauw method. The results indicated low resistivity values that could be explained due to some variable range hopping conduction mechanism.

Boride coating on the surface of WC–Co-based cemented carbide

Abstract In this study, the structural properties and enhancement in the hardness of commercial WC–Co-based cemented carbide inserts are reported after the formation of a boronised layer on the surface. A boronising thermochemical treatment was given at 900 °C for 4 h. X-ray diffraction analysis verified the treatment in the presence of peaks associated with the Co2B and W2CoB2 phases. The hardness of the samples used in the boronising process increased from 1492 HV up to 2000 HV. The typical values of the thickness of the boronised layer were measured to be around 28 μm. The results of a tool-life test suggest that boride coating effectively enhances the cutting performance of the samples.