National Institute Of Materials Physics - Romania

The different amount of Cu-doped Barium Strontium Titanate (BST) thick film materials have been tested for their gas-sensing performances towards NH3 and H2S under dry and 50% relative humidity (RH) background conditions. The optimum NH3 sensitivity was attained with 0.1mol% Cu-doped BST whereas the selective detection of H2S was highlighted using 5mol% Cu-doped BST material. No cross-sensitivity effects to CO, NO2, CH4 and SO2 were observed for all tested materials operated at their optimum temperature (200 degrees C) under humid conditions (50% RH). The presence of humidity clearly enhances the gas sensitivity to NH3 and H2S detection.

We propose a numerical procedure consisting of a simplified physical model and a numerical method with the aim of optimizing the performance parameters of dye-sensitized solar cells (DSSCs). We calculate the real rate of absorbed photons (in the dye spectral range) G(real)(x) by introducing a factor beta < 1 in order to simplify the light absorption and reflection on TCO electrode. We consider the electrical transport to be purely diffusive and the recombination process only to occur between electrons from the TiO2 conduction band and anions from the electrolyte. The used numerical method permits solving the system of differential equations resulting from the physical model. We apply the proposed numerical procedure on a classical DSSC based on Ruthenium dye in order to validate it. For this, we simulate the J-V characteristics and calculate the main parameters: short-circuit current density J(sc), open circuit voltage V-oc, fill factor FF, and power conversion efficiency eta. We analyze the influence of the nature of semiconductor (TiO2) and dye and also the influence of different technological parameters on the performance parameters of DSSCs. The obtained results show that the proposed numerical procedure is suitable for developing a numerical simulation platform for improving the DSSCs performance by choosing the optimal parameters.

The structure of GeSiO films resulted from deposition and annealing conditions draws their electrical behaviour. GeSiO films prepared either by magnetron sputtering or sol-gel method and subsequently annealed are formed of Ge nanocrystals and/or amorphous Ge nanoparticles embedded in amorphous SiO2 matrix. Firstly, the size effect which is the main effect in these systems produces specific quantum confinement energy levels in the enlarged forbidden energy band gap in nanocrystals. Secondly, these films are percolative systems, so that the main conduction mechanisms which govern the electrical behaviour are the tunnelling and hopping between neighbouring Ge nanocrystals or amorphous nanoparticles. Accordingly, the charge transport is strongly determined by the structure and morphology of films.

We report on the charge storage properties of trilayer structures consisting in sputtered gate HfO2/co-sputtered Ge-HfO2 layer/rapid thermal tunneling SiO2 oxide. Investigations of transmission electron microscopy and X-ray diffraction evidence the formation of HfO2 with mixed structure of monoclinic and tetragonal in the annealed structures. Capacitance-voltage (C-V) characteristics were measured on Al/HfO2/Ge-HfO2/SiO2/Si/Al metal-oxide-semiconductor capacitors based on as-deposited and annealed structures. Large C-V hysteresis is observed for the as-deposited structures and is controlled by traps present in oxide and interface. The annealing yields a C-V hysteresis with smaller memory window being due to injected charges in Ge nanocrystals.

Electron paramagnetic resonance (EPR) spectroscopy has been extensively employed to investigate the presence, localization, distribution and interaction with the host crystalline lattice of the paramagnetic point defects (intrinsic defects and transition metal ions) in semiconductors. The retrieval of such information for nanostructured semiconductors is considerably more difficult, due to the high disorder level in such systems, reflected in broad, featureless EPR spectra. We show here how, with proper adjustments of the EPR experiments and accurate numerical analysis of the resulting spectra, it was possible to obtain more accurate information regarding the localization and structure of various Mn2+ centers in ZnS and ZnO semiconductor nanoparticles (NPs). This lead to the observation and investigation of size related effects such as the presence of the extended lattice defect assisted incorporation of impurities in small (similar to 3 nm) cubic ZnS NPs, the dominant size induced lattice disorder observed for ZnO NPs, independent of the synthesis procedures, or the three steps decomposition of the epsilon-Zn(OH)(2) disordered shell of ZnS NPs with formation of new oxy-hydrated zinc compounds. These effects can be used to synthesize semiconductor nanoparticles with controlled size distribution, doping level and functionalized surfaces for specific technological applications.

Low fluence heavy ions incident on high resistivity Si produce lattice defects which act as trapping centers, and produce also an important local field of strain. The strain field intensity increases with the increase of the difference in atomic size and mass between the ion and the Si atom host. We investigate the correlation between the change of trapping parameters and the strain field. The strain field produced by Bi ions in Si is two times more intense than in Si irradiated with I ions, and effects the Gaussian broadening of trapping levels and the temperature dependence of cross sections.

Here we report the extraordinary thermal stability of Mg rich bimetallic nanoparticles (NPs), which is important for hydrogen storage technology. The enhanced NP stability is accomplished because of two critical improvements: (i) no void development within NPs (nanoscale Kirkendall effect) during their formation and (ii) suppressed Mg evaporation and NP hollowing during Mg hydrogenation at elevated temperature. The mechanism leading to the improved thermal stability of Mg-based bimetallic NPs is shown to be due to MgH2 hydride formation before evaporation can take place. These findings were tested for various compositions of Mg with Ni, Cu, and Ti, which are interesting combinations of materials for hydrogen storage systems. To achieve this we first demonstrate the synthesis mechanism of Mg-Ni and Mg-Cu NPs, which is well controlled at the single particle level, in order to accomplish multi-shell, alloy and intermetallic structures of interest for hydrogen storage tests. Aberration corrected transmission electron microscopy was carried out to unravel the detailed atomic structure and composition of the bimetallic NPs after production, processing, and hydrogenation. Finally, a simple and effective methodology is proposed for tuning the composition of the Mg-based bimetallic NPs based on the temperature-dependent nucleation behavior of NPs in the gas-phase.

Multi-quantum well structures and Si wafers implanted with heavy iodine and bismuth ions are studied in order to evaluate the influence of stress on the parameters of trapping centers. The experimental method of thermostimullatedcurrents without applied bias is used, and the trapping centers are filled by illumination. By modeling the discharge curves, we found in multilayered structures the parameters of both 'normal' traps and 'stress-induced' ones, the last having a Gaussian-shaped temperature dependence of the cross section. The stress field due to the presence of stopped heavy ions implanted into Si was modeled by a permanent electric field. The increase of the strain from the neighborhood of I ions to the neighborhood of Bi ions produces the broadening of some energy levels and also a temperature dependence of the cross sections for all levels.

A study regarding the microstructure of a-C: H/W mixed materials deposited by plasma assisted sequential deposition method is presented. The deposition of layers was performed by cyclic exposing a substrate, for predefined time intervals, to two deposition processes, working in alternative sequences: Magnetron Sputtering (MS) for metal deposition and Plasma Enhanced Chemical Vapor Deposition (PECVD) for carbon deposition. During the deposition the plasma parameters were kept constant while the durations of the individual MS and PECVD steps were systematically modified. The deposited samples were investigated using both transmission and scanning electron microscopy methods. Nanocomposite structures are obtained if each metal deposition step is interrupted in the island stage of the layer formation followed by coverage with a continuous layer of carbon. Multilayer structures are obtained when the full development of a continuous metallic layer is allowed. The spatial periodicity of the multilayers can be adjusted in between tens and hundreds of nanometers by proper setting the duration of substrate exposure to plasma sources.

Oxide materials are becoming of increasing interest due to their large variety of physical properties such as dielectric, magnetism, superconductivity, conductivity, ferroelectricity, multiferroism, etc. In addition, interfacing oxides with other materials is conferring new or better device functionalities. The main physical properties of oxides interfaces and their impact on the electrical properties of interest for microelectronic applications are presented. Further on, this subchapter is also devoted to the investigation and understanding of interface effects observed in heterostructures containing linear (SiO2) and non-linear (ferroelectrics) dielectrics in combination with wide-band gap semiconductor materials (e.g. ZnO and SiC) with special emphasis on size effects, interface quality and the opportunity to control the emergent phenomena in Metal-Oxide-Semiconductor (MOS) and Metal-Ferroelectric-Semiconductor (MFS) materials systems.