D. Muller

Structural, optical, and electrical properties of Yb-doped ZnO thin films prepared by spray pyrolysis method

Yb-doped ZnO thin films were prepared on glass substrates by spray pyrolysis technique in order to investigate the insertion of Yb ions in the ZnO matrix and the related optical properties of the films. The molar ratio of Yb in the spray solution was varied in the range of 0–5 at. %. X-ray diffraction patterns showed that the undoped and Yb-doped ZnO films exhibit the hexagonal wurtzite crystal structure with a preferential orientation along [002] direction. No secondary phase is observed in Yb-doped ZnO films. All films exhibit a transmittance between 75 and 90% in the visible range with a sharp absorption onset about 375 nm corresponding to the fundamental absorption edge at 3.3 eV. The photoluminescence measurements show a clear luminescence band at 980 nm that is characteristic of Yb3+ transition between the electronic levels F25/2 and F27/2. This is an experimental evidence for an efficient energy transfer from ZnO matrix to Yb3+. Hall effect measurements showed low resistivities and high carrier mob...

Stimulated emission in blue-emitting Si+-implanted SiO2 films?

We investigate the blue photoluminescence of Si+-implanted SiO2 films under picosecond UV excitation. The emission intensity exhibits a nonlinear increase with increasing excitation intensities, accompanied by pulse shortening. The photoluminescence decays nonmonoexponentially in time. However, the nonlinearities are not associated with significant spectral narrowing. To explain the results, we propose and numerically investigate a kinetic model based on competition between radiative (both spontaneous and stimulated) and nonradiative recombination in isolated luminescence centers in the SiO2 matrix. Good agreement between theoretical and experimental data seems to confirm the existence of stimulated emission in the films, however, under extremely high excitation densities only (approximately 100 MW/cm2).

Magnetic patterning using ion irradiation for highly ordered CoPt alloys with perpendicular anisotropy

We used a combination of ion irradiation and e-beam lithography to magnetically pattern an ordered CoPt alloy with strong perpendicular magnetic anisotropy. Ion irradiation disorders the alloy and strongly reduces the magnetic anisotropy. Magnetic force microscopy showed a regular array of 1μm2 square dots with perpendicular anisotropy separated by 1 μm large ranges with in-plane anisotropy. This is further confirmed by magnetic measurements, which showed that arrays protected by a 200 nm Pt layer present the same coercive field and the same perpendicular anisotropy as before irradiation. This is promising for applications in magnetic recording technologies.

Control of silicon nanoparticle size embedded in silicon oxynitride dielectric matrix

In this study, silicon rich silicon oxynitride layers containing more than 15% nitrogen were deposited by electron cyclotron resonance assisted plasma enhanced vapor deposition in order to form silicon nanoparticles after a high temperature thermal annealing. The effect of the flows of the precursor gases on the composition and the structural properties of the layers was assessed by Rutherford backscattering spectroscopy, elastic recoil detection analysis, and infrared spectroscopic measurements. The morphological and crystallinity properties were investigated by energy filtered transmission electron microscopy and Raman spectroscopy. We show that the excess of silicon in the silicon oxynitride layer controls the silicon nanoparticles size. On the other hand, the crystalline fraction of particles is found to be strongly correlated to the nanoparticle size. Finally, the photoluminescence measurements show that it is also possible to tune the photoluminescence peak position between 400 and 800 nm and its intensity by changing the silicon excess in the silicon rich silicon oxynitride matrix.

Thermal stability of spin valve sensors using artificial CoFe/Ir based ferrimagnets

Hard–soft spin valve structures have been grown by molecular beam epitaxy on MgO(001) substrates. The hard magnetic layer consists of (Co50Fe50)/Ir/(Co50Fe50) artificial ferrimagnetic (AFi) system, while a Fe/Co bilayer integrated in the buffer, has been used as a soft detection layer. The Fe has been grown at 500 °C giving rise to a monocrystalline layer with a body centered cubic structure. The spin valve structure presents a progressive evolution after successive annealing steps up to 350 °C. The total giant magnetoresistance (GMR) reaches its maximum (5.3%) after annealing at 250 °C, together with a good rigidity of the hard layer and a sharp switch of the magnetic moments. Such characteristics are reduced, but still interesting, after annealing at 300 °C. For annealing at higher temperature (350 °C) the total GMR signal and the coercive field of the AFi decrease dramatically and all the stack behaves like a single magnetic layer. Rutherford backscattering measurements were performed in order to inves...