A. Cremades
Universidad Complutense de Madrid, Departamento de Física de Materiales, Department Member
ABSTRACT Samples of rutile TiO2 have been prepared by sintering compacted powders under argon flow. Long (above 20 h) sintering times at 1500 degrees C led to the formation of rods with squared cross-sections in a broad range of sizes. A... more
ABSTRACT Samples of rutile TiO2 have been prepared by sintering compacted powders under argon flow. Long (above 20 h) sintering times at 1500 degrees C led to the formation of rods with squared cross-sections in a broad range of sizes. A two-step annealing treatment, at two temperatures, was found to favour the growth of low-dimensional elongated structures as well as a terraced structure on the grain surface, producing samples with high surface to volume ratio. The cathodoluminescence (CL) spectrum of the initial powder, shows an emission centred at about 2.40 eV, which can be separated into three Gaussian bands at 2.19, 2.30 and 2.55 eV. The main features of the CL spectra of sintered samples are an infrared band at 1.52 eV and a complex band in the visible range, whose peak position shifts with the annealing temperature. In samples sintered for 30 h an emission at 1.80 eV appears, while the dominant emission at 1.52 eV, due to titanium interstitials, is quenched
Research Interests:
Research Interests:
ABSTRACT A series of 100 nm thick InGaN films with Indium content up to 14% has been grown by MOVPE on SiC substrates. Cathodoluminescence (CL) and remote electron beam induced current (REBIC) in the scanning electron microscope have been... more
ABSTRACT A series of 100 nm thick InGaN films with Indium content up to 14% has been grown by MOVPE on SiC substrates. Cathodoluminescence (CL) and remote electron beam induced current (REBIC) in the scanning electron microscope have been applied to investigate with high spatial resolution the recombination of carriers at the structural defects present in the films. The observed defects are mainly pinholes formed at the surface. The density of pinholes increases with the In content in the layers, which can be explained by elastic relaxation at pinholes. CL images show the spatial distribution of the emission sites. For pinholes with diameter in the μm range we observe enhanced luminescence around the pinhole and a reduced luminescence at the apex. Pinholes are observed in REBIC images as dark spots occasionally surrounded by a bright halo. The halo spreads over an area larger than the pinhole, with a diameter of about 3–4 μm. Also a cell-like dislocation structure has been observed in some samples in the CL and REBIC images. CL spectra show, as common features of the samples, a complex emission in the blue range and a broad structured band centered around 670 nm. The influence of the inhomogeneous Indium incorporation on the luminescence of the films and of charged defects on the observed REBIC contrast is discussed.
Research Interests:
ABSTRACT Mn doped GaN films have been studied by conductive Atomic Force Microscopy (AFM), Cathodoluminescence (CL) and Electron Beam Induced Current (EBIC). AFM measurements revealed the presence of pinholes with diameters between 130... more
ABSTRACT Mn doped GaN films have been studied by conductive Atomic Force Microscopy (AFM), Cathodoluminescence (CL) and Electron Beam Induced Current (EBIC). AFM measurements revealed the presence of pinholes with diameters between 130 and 380 nm. The distribution, density and size of the pinholes depend on the Mn doping concentration. AFM Leakage Current images (LC) show a defined contrast at the pinhole planes in the sample with Mn concentration of 6.2×1020 cm−3. For the sample with an Mn concentration of 1.1×1020 cm−3, LC contrast appears around the pinholes, while no LC contrast was observed for sample with lower Mn concentration. CL measurements indicate that the samples exhibit strain related to Mn incorporation. In correlation with LC measurements, EBIC images show that pinholes are recombination sites. The combination of these techniques enabled us to analyze the Frenkel–Poole conduction in the samples and its relationship with the residual strain and the doping concentration in the films, which would exclude the mechanism of conduction through dislocations.
Research Interests:
Research Interests:
ABSTRACT Remote electron beam induced current (REBIC) and cathodoluminescence (CL) modes in the scanning electron microscope (SEM) have been used to investigate SnO2 sintered samples. The study of the electrically active boundaries... more
ABSTRACT Remote electron beam induced current (REBIC) and cathodoluminescence (CL) modes in the scanning electron microscope (SEM) have been used to investigate SnO2 sintered samples. The study of the electrically active boundaries present in the oxide shows a characteristic peak and trough (PAT) contrast after thermal treatments in oxygen. Temperature-dependent measurements of the REBIC contrast show the presence of a shallow defect level 60 meV below the conduction band. This level is asigned to oxygen species adsorbed on the defect-rich boundaries. Evolution of REBIC contrast of the grain boundaries with excitation density enabled us to perform local measurements of minority carrier diffusion length.
Research Interests:
ABSTRACT In this work, the effect of different surface orientations on the defect structure of TiO2 single crystals and the evolution of the luminescence properties under plastic deformation are investigated by cathodoluminescence (CL)... more
ABSTRACT In this work, the effect of different surface orientations on the defect structure of TiO2 single crystals and the evolution of the luminescence properties under plastic deformation are investigated by cathodoluminescence (CL) microscopy. The main features of the spectra are an infrared band at 1.53 eV, and a complex band in the visible range, whose peak position depends on the electron beam energy, and is attributed to oxygen vacancy related defects. Comparison of the intensity of these bands in the spectra recorded at low electron beam voltage indicates that the Ti3+ defects present a higher concentration at the (110) surface, whereas the (100) surface presents a more complex defect structure related to the oxygen vacancies. Competition is observed between the visible and the infrared emission centres during plastic deformation of the samples in agreement with the different diffusion mechanisms of the defects involved in the emissions.
Research Interests:
Nano- and microstructures of SnO(2), In(2)O(3) and ZnO have been grown during thermal treatment of compacted powders under argon flow. Indium-doped SnO(2) tube-shaped structures with rectangular cross-section are obtained by adding a... more
Nano- and microstructures of SnO(2), In(2)O(3) and ZnO have been grown during thermal treatment of compacted powders under argon flow. Indium-doped SnO(2) tube-shaped structures with rectangular cross-section are obtained by adding a fraction of In(2)O(3) to the starting SnO(2) powder. In-rich nanoislands were found to grow on some edges of the tubes. ZnO nanostructures doped with Sn or Eu were grown by adding SnO(2) and Eu(2)O(3) powder, respectively, to the ZnO precursor powder. All the samples have been characterized by the emissive and cathodoluminescence (CL) modes of scanning electron microscopy. CL images from SnO(2):In and In(2)O(3):Sn tubes and islands show a higher emission from the Sn-rich structures related to oxygen deficiency. CL of doped ZnO enables to detect the presence of dopant in specific regions or structures. CL appears to be a useful technique to study optical and electronic properties of semiconductor oxide nanostructures.
Research Interests:
Research Interests:
Research Interests:
ABSTRACT
Research Interests:
... C. Johnson, JY Lin, HX Jiang, M. Asif Khan, and CJ Sun, Metastability and persistent photoconductivity in Mg-doped p-type GaN, Appl ... G. Mohs, B. Fluegel, H. Giessen, H. Tajalli, N. Peyghambarian, P.-C. Chin, B.-S. Philips, and M.... more
... C. Johnson, JY Lin, HX Jiang, M. Asif Khan, and CJ Sun, Metastability and persistent photoconductivity in Mg-doped p-type GaN, Appl ... G. Mohs, B. Fluegel, H. Giessen, H. Tajalli, N. Peyghambarian, P.-C. Chin, B.-S. Philips, and M. Osinski, Photoluminescence decay dynamics ...
Research Interests:
ABSTRACT
Research Interests:
ABSTRACT
Research Interests:
ABSTRACT
Research Interests: Engineering and Materials
Electron beam-induced current (EBIC) is applied to the characterization of chemical vapor-deposited (CVD) diamond films. EBIC contrast shows a strong dependence on the orientation of grains relative to the incident electron beam. This is... more
Electron beam-induced current (EBIC) is applied to the characterization of chemical vapor-deposited (CVD) diamond films. EBIC contrast shows a strong dependence on the orientation of grains relative to the incident electron beam. This is due to the correspondence between certain faces of the diamond grains and enhanced recombination. Cathodoluminescence (CL) images enable identification of the (100) faces of the epitaxial
Research Interests:
The properties of microelectrical conduction in microwave plasma assisted chemical vapour deposition (MPCVD) diamond films were investigated using an atomic force microscopy probe, giving a morphological map of the electrical conduction... more
The properties of microelectrical conduction in microwave plasma assisted chemical vapour deposition (MPCVD) diamond films were investigated using an atomic force microscopy probe, giving a morphological map of the electrical conduction with a spatial resolution ...
Research Interests:
Research Interests:
Research Interests:
ABSTRACT In2O3 nanorods have been grown by a catalyst free evaporation-deposition method. Transmission electron microscopy (TEM) investigations reveal that the rods contain tubular cavities, herein referred to as... more
ABSTRACT In2O3 nanorods have been grown by a catalyst free evaporation-deposition method. Transmission electron microscopy (TEM) investigations reveal that the rods contain tubular cavities, herein referred to as "nanopipes", along the total length of the nanorods. The nanopipe diameters are constant along the nanorod axis and appear to be independent of the nanorod thickness. In most of the investigated In2O3 nanorods, these nanopipes are centered within the nanorod. An average nanopipe diameter of (18.5 +/- 0.7) nm has been determined from the TEM observations. Extended thermal treatments lead to nanorods with more complex morphologies, roughened interfaces, and formation of voids. The possibility that the nanopipes are related with a growth mechanism involving a dislocation along the growth axis is discussed.
Research Interests:
Research Interests:
Indium oxide elongated micro- and nanostructures have been grown by thermal treatment of InN powder. Chains of nanopyramids connected by nanowires, forming a necklace-like structure, as well as cubes and arrow-like structures consisting... more
Indium oxide elongated micro- and nanostructures have been grown by thermal treatment of InN powder. Chains of nanopyramids connected by nanowires, forming a necklace-like structure, as well as cubes and arrow-like structures consisting of a long rod with a micron size pyramid on the top, grow at temperatures in the range 600-700 °C in a catalyst free process. The structures have been characterized by scanning electron microscopy and cathodoluminescence.