Pasi Jalkanen

Magnetic Properties of Polycrystalline Bismuth Ferrite Thin Films Grown by Atomic Layer Deposition.

The atomic layer deposition (ALD) method was applied to grow thin polycrystalline BiFeO3 (BFO) films on Pt/SiO2/Si substrates. The 50 nm thick films were found to exhibit high resistivity, good morphological integrity, and homogeneity achieved by the applied ALD technique. Magnetic characterization revealed saturated magnetization of 25 emu/cm(3) with temperature-dependent coercivity varying from 5 to 530 Oe within the temperature range from 300 to 2 K. Magnetism observed in the films was found to change gradually from ferromagnetic spin ordering to pinned magnetic domain interactions mixed with weak spin-glass-like behavior of magnetically frustrated antiferromagnetic/ferromagnetic (AFM-FM) spin ordering depending on the temperature and magnitude of the applied magnetic field. Antiferromagnetic order of spin cycloids was broken in polycrystalline films by crystal sizes smaller than the cycloid length (∼60 nm). Uncompensated spincycloids and magnetic domain walls were found to be the cause of the high magnetization of the BFO films.

Composition dependence of Si{sub 1-x}Ge{sub x} sputter yield

Sputtering yields have been measured for unstrained Si{sub 1-x}Ge{sub x} (x=0-1) alloys when bombarded with Ar{sup +} ions within the linear cascade regime. Nonlinear S-shape dependence of the sputter yield as a function of the alloy composition has been revealed. The dependence is analyzed within the frameworks of the cascade theory conventionally accepted to be the most systematic to date theoretical approach in sputtering. In view of a linear composition dependence predicted for the sputter yield by the cascade theory adapted for polyatomic substrates, the nonlinearity observed in our experiments is shown to be related to the alloying effect on the surface binding energies of the alloy components. Based on this analysis, an interpretation is proposed for the experimentally observed nonlinear composition dependence of Si{sub 1-x}Ge{sub x} sputter yield. The yield is expressed by an equation derived from the cascade theory with additional terms of the composition parameter x. The form of the equation implies that for a polyatomic substrate the surface binding energy of an individual atom is determined not only by its own chemical identity but to a considerable degree by the identities of its neighbors.

Erbium-implanted silicon-germanium

Rutherford backscattering/channeling technique complemented by transmission electron microscopy has been used to study the composition dependence and the effect of carbon codoping on thermal and structural properties of unstrained Si1−xGex (x=0.1–0.8) alloys implanted with Er+ at 70keV to a fluence of the order of ∼1015cm−2. It has been shown that implantation in the low-temperature regime (350°C) led to the formation of a ternary solid solution with a peak Er concentration of around 1at.%. Implanted Er atoms were found randomly distributed in the heavily damaged host matrix. Postimplantation annealing at different temperatures up to 600°C was observed to induce solid phase epitaxial regrowth leading to the recrystallization of the damaged matrix and the simultaneous removal of almost all the implanted Er. Both rate and activation temperature of the epitaxy in Si1−xGex were found to depend strongly on the stoichiometry of the alloy. Implantation of Er in the high-temperature regime (550°C) was found to le...

Nucleation and Conformality of Iridium and Iridium Oxide Thin Films Grown by Atomic Layer Deposition

Nucleation and conformality are important issues, when depositing thin films for demanding applications. In this study, iridium and iridium dioxide (IrO2) films were deposited by atomic layer deposition (ALD), using five different processes. Different reactants, namely, O2, air, consecutive O2 and H2 (O2 + H2), and consecutive O3 and H2 (O3 + H2) pulses were used with iridium acetylacetonate [Ir(acac)3] to deposit Ir, while IrO2 was deposited using Ir(acac)3 and O3. Nucleation was studied using a combination of methods for film thickness and morphology evaluation. In conformality studies, microscopic lateral high-aspect-ratio (LHAR) test structures, specifically designed for accurate and versatile conformality testing of ALD films, were used. The order of nucleation, from the fastest to the slowest, was O2 + H2 > air ≈ O2 > O3 > O3 + H2, whereas the order of conformality, from the best to the worst, was O3 + H2 > O2 + H2 > O2 > O3. In the O3 process, a change in film composition from IrO2 to metallic Ir was seen inside the LHAR structures. Compared to the previous reports on ALD of platinum-group metals, most of the studied processes showed good to excellent results.

Critical temperature modification of low dimensional superconductors by spin doping

Ion implantation of Fe and Mn into Al thin films was used for effective modification of Al superconductive properties. The critical temperature of the transition to the superconducting state was found to decrease gradually with implanted Fe concentration. It was found that suppression by Mn implantation is much stronger compared to Fe. At low concentrations of implanted ions, suppression of the critical temperature can be described with reasonable accuracy by existing models, while at concentrations above 0.1 at.% a pronounced discrepancy between the models and experiments is observed.

Mechanical response of nanometer thick self-assembled monolayers on metallic substrates using classical nanoindentation

We report here on the mechanical response of thiolate self-assembled monolayers, a few nanometers thick, on gold and silver substrates, using ultra-low load contact probing (the limit of the nanoindentation method used) with a relatively large spherical diamond tip. Careful analysis of a large number of experimental data points enabled us to differentiate the effects stemming from the substrate, the loading rate, and the orientation and reactivity of the molecules. Three distinct patterns for the responses of the monolayers were observed, giving insight into the mechanical response of thiol modified metal surfaces.

Superconductivity suppression in Fe-implanted thin Al films

At present, ion implantation into metallic systems is given increasing attention, aiming at achieving properties and functionalities of technologically valuable materials not easily available via conventional techniques. In our experiments thin Al films were implanted with Fe ions in order to find out how the superconductive properties of the metal can be modified at will. The purpose was twofold, viz., first, to study the basic physics of superconductivity in low-dimensional metallic structures doped with impurities. The second purpose was to apply ion implantation for the suppression of undesired superconductivity in aluminum widely used for fabrication of micro- and nanodevices operated at low temperatures. In many applications transition to the superconductive state is detrimental for the device’s functionality, for instance, those based on Coulomb blockage (CB) effect (e.g., in CB thermometry).

On erbium lattice location in ion implanted Si0.75Ge0.25 alloy: Computer simulation of Rutherford backscattering/channeling

A high crystalline quality Si0.75Ge0.25 alloy layer grown by chemical vapor deposition was implanted with 70 keV Er+ ions to a fluence of 1015 cm−2 at temperature of 550 °C. The implantation was found to result in an Er depth distribution with 1 at. % maximum concentration 30 nm beneath the surface. The location of the erbium atoms in the host matrix lattice is derived through computer simulation of experimental axial channeling angular scans measured by in situ Rutherford backscattering/channeling spectrometry. Using computer code FLUX 7.7 it is shown that 60% of the implanted erbium atoms are located at ytterbium sites, 10% at tetrahedral sites, and the remainder are associated with random locations in the host matrix.A high crystalline quality Si0.75Ge0.25 alloy layer grown by chemical vapor deposition was implanted with 70 keV Er+ ions to a fluence of 1015 cm−2 at temperature of 550 °C. The implantation was found to result in an Er depth distribution with 1 at. % maximum concentration 30 nm beneath the surface. The location of the erbium atoms in the host matrix lattice is derived through computer simulation of experimental axial channeling angular scans measured by in situ Rutherford backscattering/channeling spectrometry. Using computer code FLUX 7.7 it is shown that 60% of the implanted erbium atoms are located at ytterbium sites, 10% at tetrahedral sites, and the remainder are associated with random locations in the host matrix.

Growth mode-dependent ferromagnetic properties of palladium nanoclusters

Cluster deposited Pd films exhibit ferromagnetism in the temperature range from 1.8 to 400 K. The magnetization properties are found to be dependent on the film thickness. The varying morphology of the resulting Pd film with respect to thickness suggests that cluster size, deposition energy, and substrate type are crucial for the resulting film magnetization. This is demonstrated by the characteristic ferromagnetic hysteresis with the temperature dependent saturation magnetization, remanence, and coercivity of palladium nanocluster aggregates. The temperature dependence of the saturation magnetization, remanence, and coercivity of Pd nanoclusters were measured using an ultra-high-sensitive magnetometer based on a superconducting quantum interference device, and the morphology of the samples was analyzed by tunneling electron microscopy.Cluster deposited Pd films exhibit ferromagnetism in the temperature range from 1.8 to 400 K. The magnetization properties are found to be dependent on the film thickness. The varying morphology of the resulting Pd film with respect to thickness suggests that cluster size, deposition energy, and substrate type are crucial for the resulting film magnetization. This is demonstrated by the characteristic ferromagnetic hysteresis with the temperature dependent saturation magnetization, remanence, and coercivity of palladium nanocluster aggregates. The temperature dependence of the saturation magnetization, remanence, and coercivity of Pd nanoclusters were measured using an ultra-high-sensitive magnetometer based on a superconducting quantum interference device, and the morphology of the samples was analyzed by tunneling electron microscopy.