National Institute Of Materials Physics - Romania

Trace amounts of substitutional Mn2+ ions and shallow donors magnetic centers were identified by electron paramagnetic resonance (EPR) in crystalline Zn(OH)(2) prepared by precipitation of a Zn-nitrate solution with NaOH. Strong changes in the Mn2+ ions spectrum, as well as a sharp increase in the concentration of the shallow donor centers were observed by EPR in the 110-140 degrees C temperature range, during pulse annealing experiments in air up to 240 degrees C. They reflect the decomposition of the crystalline Zn(OH)(2) host lattice into nanocrystalline ZnO, confirmed by X-ray diffraction and thermal analysis measurements. Accurate spin Hamiltonian parameters of the observed paramagnetic centers were determined by lineshape simulation and fitting of the EPR spectra, to be used as reference data in further studies of nanocrystalline systems involving Zn(OH)(2). (C) 2012 Elsevier B.V. All rights reserved.

The thermal stability above 300 K of the hole-trapped Fe3+ centers, formed by X-ray irradiation at 80 K in chlorinated SrCl2:Fe2+ crystals, has been investigated by Electron Paramagnetic Resonance. In the temperature range of 300 - 520 K, the trigonal Fe3+ center completely transforms into the cubic Fe3+ center, which further decays up to 700 K. The formation and transformation of the trigonal Fe3+ center, containing off-center Fe3+ ions, is associated with the presence and thermally activated movement of the interstitial Cl- ions, present in excess in chlorinated SrCl2:Fe2+ crystals.

The fast decomposition of hydrozincite [Zn-5(CO3)(2)(OH)(6)] into ZnO observed in the transmission electron microscope could be slowed down and investigated in situ by operating the microscope at very low electron beam current densities and cooling the specimen down to -160 degrees C. Thus, it was possible to observe and pursue the distinct structural steps of the disruption of the hydrozincite lattice due to the energetic emission of H2O and CO2 gases. The initially formed disordered ZnO phase was found to further crystalize resulting in a mesoporous structure of small ZnO nanocrystals.

N-type silicon single crystals with resistivity higher than 8000 Omega cm were irradiated with ions of Bi6+, of 28 MeV kinetic energy. At this energy, the ions are stopped into the wafer. Being much heavier and bigger than the host atoms, they produce major disturbances into the lattice. On the other hand, the produced collision cascade is the source of lattice defects which act as traps. We investigated them using the method of thermally stimulated currents without applied bias. The results are compared with those obtained from the analysis of silicon irradiated with I6+ ions.

This paper presents a detailed transmission electron microscopy (TEM) study of GeSiO films with Ge nanoparticles. The films with 2.5 mu m thickness were deposited by magnetron sputtering and subsequently annealed in H-2 at 2 atm and 500 degrees C for 2 h for nanostructuring. After H-2 annealing, the majority of the resulted Ge nanoparticles are amorphous, less than 5 nm in size, forming a uniform network in the film volume. Big Ge nanoparticles with sizes between 20 and 50 nm are also formed. Some of them are identified to be crystallized in the (Ge-III/ST12) tetragonal phase. The high resolution TEM observation induces the amorphysation of the Ge tetragonal phase, followed by the crystallization of the amorphous Ge phase in the cubic diamond structure (Ge-I), as an effect of electron irradiation. A secondary annealing performed in N-2 at 800 degrees C and 1 atm for 2 h induces formation of faceted cubic Ge nanoparticles distributed in the SiO2 matrix.

Quantum well solar cells with p-i-n structure are presented. The physical processes in multiple quantum well solar cells, the materials commonly used for photovoltaic applications, and technological aspects are analyzed. The quantum confinement effect produces resonant energy levels located in the valence and conduction bands of well layers. In addition, it produces energy quantum confinement levels located in the energy band gap of both well and barrier layers. The absorption on both resonant and quantum confinement levels leads to an extension of the internal quantum efficiency in near infrared domain. Several structures with different absorbers from 3-5 and 4 groups are described and discussed. Various technological and design solutions, such as multiple quantum well solar cells with graded band gap, with tandem configurations, with strain-balanced structure, and strain-balanced structure improved with nanoparticles deposited atop are analyzed. The cell parameters are discussed and related to the materials and technology.

In this article, we present and discuss how the stress influences the trapping phenomena in both the bulk Si and nanostructures based on Si. For this, we use single crystal Si irradiated with 28 MeV iodine ions of fluence (5 +/- 0.5) x 10(11) ions/cm(2) and 2D Si based nanostructures of (nc-Si/CaF2)(50) multilayers. The method of thermally stimulated discharge currents without bias was employed to investigate the trapping phenomena and the modeling of the discharge current curves was carried out. In the case of irradiated Si, we have modeled the discharge currents considering six trapping levels and a supplementary stress electric field independent of temperature superposed on the frozen-in electric field produced by the charged traps at low temperature. The stress electric field is due to the stopped iodine ions which are heavier than those of Si host. Six traps are evidenced and their parameters are calculated. In the (nc-Si/CaF2)(50) multilayer structures, we experimentally evidenced traps which produce in the discharge current maxima with two shapes. Three maxima have normal shape and the others five maxima are narrow and sharp looking like spikes. We modeled the discharge current curves taking for the concentration a power temperature dependence law, and for the cross section a Gaussian temperature dependence.

This paper analyses and discusses the effect of Ge/Si atomic ratio and annealing temperature on the conduction mechanisms governing the electrical behavior of Ge-SiO2 films containing Ge nanoparticles embedded in amorphous SiO2 matrix. For this, the experimental conductance-temperature curves are modeled in correlation with the microstructure findings for two types of films. One type of films has a lower Ge/Si ratio of 0.73 and was obtained by magnetron sputtering followed by annealing in H-2, at 500 degrees C, while the second one has a higher ratio of 1.86 and was obtained also by sputtering, but was annealed in N-2, at 800 degrees C. Both types of films show an electrical behavior with a T-1/4 conductance dependence on temperature, typical for hopping mechanism.

The presence and structure of paramagnetic point defects in C-13 and O-17 implanted ultrapure Si single-crystal material, two impurities which seem to play a major role in the detectors performance degradation and radiation resistance enhancement, respectively, are reported before and after irradiation with 6 MeV electrons. The investigation, performed by Q-band electron spin resonance spectroscopy in the 300 - 10 K temperature range, included also reference ultrapure and O-16 doped single-crystal Si-platelets. It resulted in the observation of points defects associated with lattice defects as dangling bonds and impurities.

ZnO-x%Eu2O3 (x=5 wt.%) binary oxide with straight strips morphology was successfully synthesized by hydrothermal route, using cetyltrimethylammonium bromide (CTAB) as template ZnSO4. Eu(NO3)(3)center dot 5H(2)O as inorganic precursors and ammonium hydroxide solution 28% NH3 for adjusting the pH to 11 value. The morphology, structure, photoluminescence and surface chemistry were investigated. The results highlighted that ZnO-Eu2O3 binary oxide shows straight strip morphology and crystalline framework indexed to wurtzite ZnO while Eu2O3 was detected as secondary phase. Surface chemistry revealed the presence of Eu3+. High sensor signal to 3 ppm NO2 together with its low sensitivity towards CO recommend this material as a good candidate in selective detection of NO2. (C) 2012 Elsevier B.V. All rights reserved.