O. de Melo - Academia.edu (original) (raw)

Papers by O. de Melo

2015 IEEE International Autumn Meeting on Power, Electronics and Computing (ROPEC), 2015

AIP Conference Proceedings, 1996

Low temperature photoluminescence was employed for the structural characterization of quantum wel... more Low temperature photoluminescence was employed for the structural characterization of quantum wells of Zn1−xCdxSe grown by molecular beam epitaxy. The quantum well thickness, LW, was inferred from the energy of the PL transition, and the interface roughness from the analysis of its lineshape. It is shown that predominant quantum well thickness fluctuations are of the order of one monolayer.

AIP Conference Proceedings, 1996

Using a ZnCl2 cell as doping source we have grown epitaxial layers of Cl-doped ZnSe layer onto Ga... more Using a ZnCl2 cell as doping source we have grown epitaxial layers of Cl-doped ZnSe layer onto GaAs(100) substrates by molecular beam epitaxy. The carrier concentration was controlled by the ZnCl2 source temperature. The maximum carrier concentration was 1.1×1019 cm−3, with a resistivity of 2.7×10−3 ohm-cm. Higher doses of Cl atoms tend to decrease free carrier concentration, and introduce additional

Proceedings of 1994 IEEE 1st World Conference on Photovoltaic Energy Conversion - WCPEC (A Joint Conference of PVSC, PVSEC and PSEC), 1994

Important progress has been reached in the elaboration of low cost solar cells based in CdS/CdTe ... more Important progress has been reached in the elaboration of low cost solar cells based in CdS/CdTe heterostructures. This was accomplished by using different techniques such as chemical bath deposition, close space sublimation, electrodeposition and screen printing and sintering. In all these low cost techniques, at least a relatively high temperature step is necessary during the processing for the completion of

Journal of nanoscience and nanotechnology, 2012

The growth of Anatase nanostructured films using dc reactive magnetron sputtering and post-anneal... more The growth of Anatase nanostructured films using dc reactive magnetron sputtering and post-annealing treatment is reported. TiO2 has been deposited on Porous Anodic Alumina Films used as templates which were previously grown in phosphoric acid solution and etched to modify their pore diameters. This synthesis via results in the formation of vertically aligned and spatially ordered TiO2 nanostructures replicating the underlying template. Previously, the growth optimization of TiO2 thin films deposited by dc magnetron sputtering on flat silicon substrates was done. The crystalline structure and Ti in-depth concentration profile were determined by grazing incidence X-ray diffraction and Rutherford backscattering spectrometry, respectively. The surface morphology of the samples was explored by mean of a Field Emission Gun scanning electron microscope. Optical properties of the nanostructured samples were studied by using the reflectance spectra received in the UV-visible range. In these...

Nanoscale Research Letters, 2012

Isothermal close space sublimation, a simple and low-cost physical vapour transport technique, wa... more Isothermal close space sublimation, a simple and low-cost physical vapour transport technique, was used to infiltrate ZnTe and CdSe semiconductors in porous silicon. The structure of the embedded materials was determined by X-ray diffraction analysis while Rutherford backscattering spectra allowed determining the composition profiles of the samples. In both cases, a constant composition of the II-VI semiconductors throughout the porous layer down to the substrate was found. Resonance Raman scattering of the ZnTe samples indicates that this semiconductor grows in nanostructured form inside the pores. Results presented in this paper suggest that isothermal close space sublimation is a promising technique for the conformal growth of II-VI semiconductors in porous silicon.

2014 IEEE 40th Photovoltaic Specialist Conference (PVSC), 2014

Vacuum, 1995

The band-gap energies of CdS grown by chemical-bath deposition are obtained by optical transmissi... more The band-gap energies of CdS grown by chemical-bath deposition are obtained by optical transmission and photoacoustic spectroscopy. These values and photoluminescence spectra are correlated over a range of thermal annealing temperature, in which the evolution of sample structure ...

Thin Solid Films, 2009

ZnTe films with different thickness were grown onto (001) Si substrates using isothermal closed s... more ZnTe films with different thickness were grown onto (001) Si substrates using isothermal closed space sublimation by alternated sublimation of Zn and Te sources. As a result ZnTe polycrystalline films were obtained with a strong [111] orientation as revealed by X-ray diffraction patterns. The reason for this polycrystalline nature of the samples comes from incomplete removal of SiO 2 from the Si substrate surface. A preferential adsorption of Te in the first stages of the growth was concluded from Rutherford backscattering spectra analysis.

Thin Solid Films, 2002

Optical and structural characterization of ZnS thin films grown by chemical bath deposition (CBD)... more Optical and structural characterization of ZnS thin films grown by chemical bath deposition (CBD), introducing for the first time a magnetic field (MF), are reported. Corning glass as well as Si and GaAs single crystals were used as substrates. The effect of H -annealing, in the range of temperature 200-300 8C, and MF on the morphology and crystalline structure of films is 2 studied in atomic force microscopy images and in X-ray diffractograms. MF does not manifest appreciable influence on the grain size (GS) when glass substrates are used. On the contrary GS diminishes noticeably when MF is applied on ZnSySi and ZnSy GaAs CBD-growths. Further, an improvement of the optical properties with post-growth annealing in H , as exhibited by optical 2 transmission, is obtained. An application of these ZnS films as antireflective coating is analyzed by reflectance spectra. ᮊ

Surface Review and Letters, 2002

Layers of 6 and 16 Cd-Te-Zn-Te periods were grown by atomic layer epitaxy (ALE) within ZnTe thin ... more Layers of 6 and 16 Cd-Te-Zn-Te periods were grown by atomic layer epitaxy (ALE) within ZnTe thin films. Different samples were grown at substrate temperatures of 260 and 290 • C. Information about the kinetics of growth and surface reconstruction during the ALE growth of CdTe and ZnTe films, and Cd-Te-Zn-Te periods was obtained by means of reflection high-energy electron diffraction (RHEED) experiments and through the analysis of the temporal behavior of the intensities of several features of the RHEED patterns. The photoluminescence of the sample grown at 260 • C presents two narrow and intense peaks corresponding to emission from quantum wells (QWs). However, the spectrum of the samples grown at 290 • C does not show any feature associated with QWs, the spectrum resembling that of a ZnTe film. Cd replacement by Zn atoms explains the absence of the CdZnTe QWs at 290 • C and a lower Cd content than expected at 260 • C. The replacement of Cd atoms by Zn atoms in the CdTe surface was clearly demonstrated by Auger experiments.

Superlattices and Microstructures, 2008

The use of the isothermal closed space sublimation technique for growing CdSe very thin films and... more The use of the isothermal closed space sublimation technique for growing CdSe very thin films and nanostructures is reported. In this technique, the samples are grown by exposing a substrate alternately to elemental sources of Cd and Se. The whole system is at the same temperature, so the only driving force for the film growth is the vapour pressure difference

Semiconductor Science and Technology, 2002

A theoretical procedure based on the model dielectric function is used to fit the room temperatur... more A theoretical procedure based on the model dielectric function is used to fit the room temperature reflectivity spectra of ZnxCd1-xTe alloys in the range 1.5-4.5 eV. This procedure allows calculation of the thickness of very thin films as well as determination of the composition dependence of the critical points. A parabolic dependence on Zn molar fraction is reported. Calculated optical data, such as refractive index and extinction coefficient, are also presented.

physica status solidi (c), 2010

physica status solidi (c), 2005

PACS 68.55.Jk, 78.55.Hx, 78.66.Li, 81.15.Gh Very thin layers of Si were grown in between silicon ... more PACS 68.55.Jk, 78.55.Hx, 78.66.Li, 81.15.Gh Very thin layers of Si were grown in between silicon nitride layers using Plasma Enhanced Chemical Vapor Deposition (PECVD) technique and SiH 2 Cl 2 /H 2 /NH 3 mixtures. Deposition conditions were selected to favor Si cluster formation. Room Temperature Photoluminescence (RT-PL) and optical transmission in different ranges were used to evaluate the optical and structural properties of the films. Scanning Electron Microscopy (SEM) of the cross section of cleaved samples allowed to observe a clear pattern of Si clusters embedded in the SiN matrix. The UV-VIS absorption spectra present two band edges. We assume that the higher band gap is due to the amorphous Si clusters. RT-PL spectra are characterized by two broad bands: one centered at 1.5 eV and the other at 2.1 eV. The broad luminescence centered at 2.1 eV could be associated with the higher band gap observed in absorption spectrum. After vacuum annealing of the samples at 400 ºC, the band at 2.1 eV disappears.

2015 IEEE International Autumn Meeting on Power, Electronics and Computing (ROPEC), 2015

AIP Conference Proceedings, 1996

Low temperature photoluminescence was employed for the structural characterization of quantum wel... more Low temperature photoluminescence was employed for the structural characterization of quantum wells of Zn1−xCdxSe grown by molecular beam epitaxy. The quantum well thickness, LW, was inferred from the energy of the PL transition, and the interface roughness from the analysis of its lineshape. It is shown that predominant quantum well thickness fluctuations are of the order of one monolayer.

AIP Conference Proceedings, 1996

Using a ZnCl2 cell as doping source we have grown epitaxial layers of Cl-doped ZnSe layer onto Ga... more Using a ZnCl2 cell as doping source we have grown epitaxial layers of Cl-doped ZnSe layer onto GaAs(100) substrates by molecular beam epitaxy. The carrier concentration was controlled by the ZnCl2 source temperature. The maximum carrier concentration was 1.1×1019 cm−3, with a resistivity of 2.7×10−3 ohm-cm. Higher doses of Cl atoms tend to decrease free carrier concentration, and introduce additional

Proceedings of 1994 IEEE 1st World Conference on Photovoltaic Energy Conversion - WCPEC (A Joint Conference of PVSC, PVSEC and PSEC), 1994

Important progress has been reached in the elaboration of low cost solar cells based in CdS/CdTe ... more Important progress has been reached in the elaboration of low cost solar cells based in CdS/CdTe heterostructures. This was accomplished by using different techniques such as chemical bath deposition, close space sublimation, electrodeposition and screen printing and sintering. In all these low cost techniques, at least a relatively high temperature step is necessary during the processing for the completion of

Journal of nanoscience and nanotechnology, 2012

The growth of Anatase nanostructured films using dc reactive magnetron sputtering and post-anneal... more The growth of Anatase nanostructured films using dc reactive magnetron sputtering and post-annealing treatment is reported. TiO2 has been deposited on Porous Anodic Alumina Films used as templates which were previously grown in phosphoric acid solution and etched to modify their pore diameters. This synthesis via results in the formation of vertically aligned and spatially ordered TiO2 nanostructures replicating the underlying template. Previously, the growth optimization of TiO2 thin films deposited by dc magnetron sputtering on flat silicon substrates was done. The crystalline structure and Ti in-depth concentration profile were determined by grazing incidence X-ray diffraction and Rutherford backscattering spectrometry, respectively. The surface morphology of the samples was explored by mean of a Field Emission Gun scanning electron microscope. Optical properties of the nanostructured samples were studied by using the reflectance spectra received in the UV-visible range. In these...

Nanoscale Research Letters, 2012

Isothermal close space sublimation, a simple and low-cost physical vapour transport technique, wa... more Isothermal close space sublimation, a simple and low-cost physical vapour transport technique, was used to infiltrate ZnTe and CdSe semiconductors in porous silicon. The structure of the embedded materials was determined by X-ray diffraction analysis while Rutherford backscattering spectra allowed determining the composition profiles of the samples. In both cases, a constant composition of the II-VI semiconductors throughout the porous layer down to the substrate was found. Resonance Raman scattering of the ZnTe samples indicates that this semiconductor grows in nanostructured form inside the pores. Results presented in this paper suggest that isothermal close space sublimation is a promising technique for the conformal growth of II-VI semiconductors in porous silicon.

2014 IEEE 40th Photovoltaic Specialist Conference (PVSC), 2014

Vacuum, 1995

The band-gap energies of CdS grown by chemical-bath deposition are obtained by optical transmissi... more The band-gap energies of CdS grown by chemical-bath deposition are obtained by optical transmission and photoacoustic spectroscopy. These values and photoluminescence spectra are correlated over a range of thermal annealing temperature, in which the evolution of sample structure ...

Thin Solid Films, 2009

ZnTe films with different thickness were grown onto (001) Si substrates using isothermal closed s... more ZnTe films with different thickness were grown onto (001) Si substrates using isothermal closed space sublimation by alternated sublimation of Zn and Te sources. As a result ZnTe polycrystalline films were obtained with a strong [111] orientation as revealed by X-ray diffraction patterns. The reason for this polycrystalline nature of the samples comes from incomplete removal of SiO 2 from the Si substrate surface. A preferential adsorption of Te in the first stages of the growth was concluded from Rutherford backscattering spectra analysis.

Thin Solid Films, 2002

Optical and structural characterization of ZnS thin films grown by chemical bath deposition (CBD)... more Optical and structural characterization of ZnS thin films grown by chemical bath deposition (CBD), introducing for the first time a magnetic field (MF), are reported. Corning glass as well as Si and GaAs single crystals were used as substrates. The effect of H -annealing, in the range of temperature 200-300 8C, and MF on the morphology and crystalline structure of films is 2 studied in atomic force microscopy images and in X-ray diffractograms. MF does not manifest appreciable influence on the grain size (GS) when glass substrates are used. On the contrary GS diminishes noticeably when MF is applied on ZnSySi and ZnSy GaAs CBD-growths. Further, an improvement of the optical properties with post-growth annealing in H , as exhibited by optical 2 transmission, is obtained. An application of these ZnS films as antireflective coating is analyzed by reflectance spectra. ᮊ

Surface Review and Letters, 2002

Layers of 6 and 16 Cd-Te-Zn-Te periods were grown by atomic layer epitaxy (ALE) within ZnTe thin ... more Layers of 6 and 16 Cd-Te-Zn-Te periods were grown by atomic layer epitaxy (ALE) within ZnTe thin films. Different samples were grown at substrate temperatures of 260 and 290 • C. Information about the kinetics of growth and surface reconstruction during the ALE growth of CdTe and ZnTe films, and Cd-Te-Zn-Te periods was obtained by means of reflection high-energy electron diffraction (RHEED) experiments and through the analysis of the temporal behavior of the intensities of several features of the RHEED patterns. The photoluminescence of the sample grown at 260 • C presents two narrow and intense peaks corresponding to emission from quantum wells (QWs). However, the spectrum of the samples grown at 290 • C does not show any feature associated with QWs, the spectrum resembling that of a ZnTe film. Cd replacement by Zn atoms explains the absence of the CdZnTe QWs at 290 • C and a lower Cd content than expected at 260 • C. The replacement of Cd atoms by Zn atoms in the CdTe surface was clearly demonstrated by Auger experiments.

Superlattices and Microstructures, 2008

The use of the isothermal closed space sublimation technique for growing CdSe very thin films and... more The use of the isothermal closed space sublimation technique for growing CdSe very thin films and nanostructures is reported. In this technique, the samples are grown by exposing a substrate alternately to elemental sources of Cd and Se. The whole system is at the same temperature, so the only driving force for the film growth is the vapour pressure difference

Semiconductor Science and Technology, 2002

A theoretical procedure based on the model dielectric function is used to fit the room temperatur... more A theoretical procedure based on the model dielectric function is used to fit the room temperature reflectivity spectra of ZnxCd1-xTe alloys in the range 1.5-4.5 eV. This procedure allows calculation of the thickness of very thin films as well as determination of the composition dependence of the critical points. A parabolic dependence on Zn molar fraction is reported. Calculated optical data, such as refractive index and extinction coefficient, are also presented.

physica status solidi (c), 2010

physica status solidi (c), 2005

PACS 68.55.Jk, 78.55.Hx, 78.66.Li, 81.15.Gh Very thin layers of Si were grown in between silicon ... more PACS 68.55.Jk, 78.55.Hx, 78.66.Li, 81.15.Gh Very thin layers of Si were grown in between silicon nitride layers using Plasma Enhanced Chemical Vapor Deposition (PECVD) technique and SiH 2 Cl 2 /H 2 /NH 3 mixtures. Deposition conditions were selected to favor Si cluster formation. Room Temperature Photoluminescence (RT-PL) and optical transmission in different ranges were used to evaluate the optical and structural properties of the films. Scanning Electron Microscopy (SEM) of the cross section of cleaved samples allowed to observe a clear pattern of Si clusters embedded in the SiN matrix. The UV-VIS absorption spectra present two band edges. We assume that the higher band gap is due to the amorphous Si clusters. RT-PL spectra are characterized by two broad bands: one centered at 1.5 eV and the other at 2.1 eV. The broad luminescence centered at 2.1 eV could be associated with the higher band gap observed in absorption spectrum. After vacuum annealing of the samples at 400 ºC, the band at 2.1 eV disappears.