L. Morresi

Magnetic and electronic transport percolation in epitaxial Ge 1 – x Mn x films

Electronic transport and magnetic properties of Ge{sub 1-x}Mn{sub x}/Ge(100) films are investigated as a function of Mn dilution. Depending on x, characteristic temperatures separate different regimes in both properties. Resistivity exhibits an insulatorlike behavior in the whole temperature range and, below about 80 K, two distinct activation energies are observed. At a higher temperature value, T{sub R}, resistivity experiences a sudden reduction. The Hall coefficient shows a strong contribution from the anomalous Hall effect and, at T{sub R}, a sign inversion, from positive to negative, is recorded. The magnetic properties, inferred from magneto-optical Kerr effect, evidence a progressive decrease of the ferromagnetic long range order as the temperature is raised, with a Curie temperature T{sub C} not far from T{sub R}. The transport and magnetic results are qualitatively consistent with a percolation mechanism due to bound magnetic polarons in a GeMn diluted magnetic semiconductor, with localized holes [A. Kaminski and S. Das Sarma, Phys. Rev. B 68, 235210 (2003)].

Morphological and structural evolutions of diluted Ge1−xMnx epitaxial films

We investigate the structural and morphological evolutions of Ge1−xMnx films, grown by molecular beam epitaxy on Ge(100), as a function of Mn nominal concentration (x). We show that in our experimental growth conditions (growth temperature TG∼160°C), Mn atoms incorporated in the matrix increases with x up to a concentration m∼0.03. Magnetic properties of the samples are mainly related to Ge3Mn5 cluster phase, while transport properties are connected to Ge:Mn matrix.

Electrical transport properties of microcrystalline silicon grown by plasma enhanced chemical vapor deposition

The dark conductivity and Hall mobility of hydrogenated silicon films deposited varying the silane concentration f=SiH4∕(SiH4+H2) in a conventional plasma enhanced chemical vapor deposition system have been investigated as a function of temperature, taking into account their structural properties. The electrical properties have been studied in terms of a structural two-phase model. A clear transition from the electrical transport governed by a crystalline phase, in the range 1%⩽f⩽3%, to that controlled by an amorphous phase, for f>3%, has been evidenced. Some metastable effects of the dark conductivity have been noticed.

Growth and magnetic properties of MnGe films for spintronic application.

Morphological and magnetic properties of MnxGe1−x films have been investigated by scanning tunneling microscopy (STM) and magneto-optical Kerr effect (MOKE) measurements, respectively. Several MnxGe1−x alloys were grown by molecular beam epitaxy (MBE) on Ge(1 0 0) substrates, varying the growth temperature and alloy composition (x). STM analysis demonstrated island morphology with islands having a mean dimension ranging from about 100 to 130 nm, depending on the substrate temperature and Mn content in the film. Growth conditions also influence the island distribution. MOKE analysis, carried out on all the MnxGe1−x alloys, showed only a negligible hysteresis effect in the investigated temperature range from about 12 to 300 K. At low temperatures (below 70–110 K, depending on the sample), the MOKE signal tends to saturate at a magnetic field intensity less than about 0.5 T, indicating a superparamagnetic behavior. On the contrary, above that temperature the films do not show a magnetic character. The features of the MOKE curves depend on the growth parameters.

Si Nanocrystals Embedded in a Silicon Oxynitride Matrix

We investigated the morphological and structural change in silicon nanostructures embedded in the silicon oxynitride matrix. The study has been carried out on thin films thermally annealed at high temperature, after deposition at 400°C by Electron Cyclotron Resonance Plasma Enhanced Chemical Vapour Deposition (ECR- PECVD), under different deposition parameters. Our study evidenced the existence of a well defined threshold for the silicon content in the film (around 47%), to get Si nano-crystallization in the silicon oxynitride matrix. Both Si nano-crystals and Si nano-columns have been observed by TEM analysis in two samples having a similar Si content but deposited under different conditions.