M. Meléndez-Lira

Effect of the substrate temperature and acidity of the spray solution on the physical properties of F-doped ZnO thin films deposited by chemical spray

F-doped ZnO thin films were prepared by using the spray pyrolysis technique. The dependence of the electrical, optical, structural and morphological properties on the substrate temperature and spray solution acidity was studied. Additionally, aging of the spray solution presents a clear effect on the resistivity of ZnO thin films. The best films obtained show a resistivity, mobility and carrier concentration of the order of 1.5 10 @2 O cm, 6 cm 2 /V s and 2 10 19 cm @3 , respectively. Wurtzite hexagonal structure, with a preferential growth along the [0 0 2] direction for all substrate temperatures and acidities used, was obtained. From scanning electron microscopy and atomic force microscopy analysis, it was determined that the grain size of the films decreases and its homogeneity increases when the acidity of the starting solution is increased. High optical transmittances, in the order of 90%, were obtained in all the cases. r 2002 Elsevier Science B.V. All rights reserved.

Optical and structural characterization of ZnSe films grown by molecular beam epitaxy on GaAs substrates with and without GaAs buffer layers

ZnSe films with thickness between 800 and 7500 A were grown on GaAs(100) by molecular beam epitaxy (MBE), and characterized by photoluminescence (PL), photoreflectance (PR), transmission electron microscopy (TEM), and high resolution x-ray diffraction. A first set of films was prepared with ZnSe directly grown on the GaAs substrate. Another set was prepared using an arsenic capped GaAs buffer layer grown on the GaAs substrate in a separated MBE system. PL studies at 18 K showed that the ZnSe films have more defects for samples grown directly on the GaAs substrate. The behavior of stacking faults and dislocations as a function of film thickness were investigated by TEM, and by the variation of the intensity of PL signals related to these defects. For both sets of samples the intensity of these signals decreased with increasing film thickness, but the decrease is steeper for films grown on GaAs buffer layers. A signal in PL spectra at ∼2.7 eV was observed only for the samples grown directly on GaAs substrates, it was associated with donor–acceptor transitions involving GaZn and VZn. The room temperature PR spectra showed, besides the GaAs and ZnSe band-gap signals, oscillations associated with the Franz–Keldysh effect due to internal electric fields. The strength of these fields was obtained by employing the asymptotic Franz–Keldysh model. A signal 22 meV below the GaAs band-gap energy was observed only in the PR spectra of the samples grown directly on GaAs substrates. This signal was associated with Zn interdiffused into the GaAs, and is correlated to the PL signal observed at 2.7 eV for the same set of samples.

Substitutional carbon in Si1−yCy alloys as measured with infrared absorption and Raman spectroscopy

We present a study of the infrared absorption and Raman scattering intensity of the local carbon mode in Si1−yCy alloys grown by direct carbon implantation followed by different recrystallization procedures. For the case of laser-induced recrystallization, the integrated infrared absorbances are found to agree with an extrapolation of the calibration curve previously determined for very low substitutional carbon concentrations in Si. We argue that this finding provides strong evidence for the achievement of nearly perfect substitutionality in laser-recrystallized films, even though their carbon concentrations are three orders of magnitude beyond the solubility limit of carbon in Si. This conclusion is found to be consistent with measurements of the intensity of defect-induced Si Raman scattering relative to the Raman intensity of the local carbon mode. The Raman intensity of the local carbon mode at 605u2009cm−1 relative to the first-order Si Raman line at 521u2009cm−1 provides an ideal spectroscopic tool for the...

Crystal structure and energy gap of CdTe thin films grown by radio frequency sputtering

We have investigated the influence of structural characteristics on the band gap of rf sputtered CdTe thin films grown at substrate temperatures in the 69–232u2009°C range. The results of scanning electron microscopy and x‐ray diffraction studies indicated that the films are a polycrystalline mixture of cubic and hexagonal phases with preferential growth of columnar type parallel to the cubic [111] direction. The band gap of the films was obtained from photoreflectance spectroscopy experiments carried out at room temperature. It was found that the films had a band gap larger than that of CdTe single crystals. This result has been correlated with the existence of lattice strain, quantum size effects, and hexagonal phase regions. By using theoretical models it was possible to estimate the contribution to the band gap shift due to strain and quantum size effects obtaining results in good agreement with the experiment. The study of annealed samples indicated that the effects of thermal treatments were to promote ...

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 300u2009°C. The thin films were grown using CdTe and Cu2Te powders, varying the Cu2Te concentration from 3 to 10u2009wt. %. 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.

Quantum Confinement and Crystalline Structure of CdSe Nanocrystalline Films

Nanocrystalline CdSe films were grown onto glass substrates by the chemical bath method. Different constant deposition temperatures (T d ) were employed in the range 4-65 °C. Average grain size (GS) increased monotonically with T d , reaching saturation at -65 °C. The GS values were in the interval 5-16 nm. At low T d values (4-15 °C), the structural phase was hexagonal wurtzite (W), for intermediate values, a wurtzite and zincblende (ZB) mixture of phases was found, and at high T d (65 °C) only cubic ZB phase was present in the layers. The variation of the bandgap as a result of the structural phase change and quantum confinement is studied.

Investigation of deep levels in ZnSe:Cl films grown by molecular beam epitaxy

Using ZnCl2 as a doping source, we have grown epitaxial layers of Cl‐doped ZnSe onto GaAs(100) substrates at a temperature of 285u2009°C by molecular beam epitaxy. The carrier concentration was controlled by the ZnCl2 source temperature. The maximum carrier concentration obtained was 1.2×1019 cm−3 with a resistivity of 2.7×10−3 Ωu2009cm. Higher doses of Cl atoms tend to decrease free carrier concentration and introduce additional defects; this effect was consistent with an increase of deep‐level photoluminescence emission. From deep‐level transient spectroscopy measurements we determined the presence of four traps with energy levels Ec around 0.50, 0.68, 0.74, and 1.20 eV. The last three traps have not been previously reported. One of them, Ec∼0.74 eV, presents a clear dependence with Cl concentration and thus can affect the performance of ZnSe:Cl‐based devices. The microscopic nature of these traps is discussed in terms of Zn vacancies (VZn) and VZn–Cl complexes.

A novel synthesis of SrCO 3 -SrTiO 3 nanocomposites with high photocatalytic activity

The results of the production and characterization of SrCO3–SrTiO3 nanocomposites as a promising candidate for efficient photocatalysts are reported. The production is based on a novelty route employing the solvothermal method with strontium chloride and titanium (IV) butoxide as the precursor solutions. The effect on the properties of the nanocomposites due to changes in the content of SrCO3 and SrTiO3 is reported. The as-prepared materials were tested in the photodegradation of methylene blue dye in aqueous solutions under the solar light. The reported route allows the production of SrCO3–SrTiO3 nanocomposites with particle sizes ranging between 18 and 29xa0nm. The SrCO3–SrTiO3 nanocomposites obtained with 19xa0% of SrCO3 phase and 81xa0% of SrTiO3 (M10) can achieve 94 and 97xa0% of dye photodegradation after 30 and 120xa0min, respectively.

On the properties of CuXC1−XTe: a novel semiconductor alloy

Abstract An investigation of the structural properties of a novel metal-semiconductor alloy based on CdTe and Cu is presented. The samples were prepared as thin films by radio frequency sputtering. On the basis of chemical analyses carried out by Auger spectroscopy and energy dispersive X-ray analysis, it was concluded that a new semiconducting alloy has been created: Cu x Cd 1− x Te. X-ray diffraction and Raman scattering experiments showed that the incorporation of Cu into the CdTe lattice did not produce significant structural changes The best crystalline properties were found for samples when x ≤0.02. The Raman scattering spectra showed up to the third-order longitudinal optic mode, which is indicative of high-quality crystalline properties. The alloy band gap was slightly reduced with respect to pure CdTe being, at most. 50 meV lower.