S. Logothetidis

Electronic properties of semiconducting FeSi2 films

Polycrystalline iron disilicide thin films are prepared by furnace annealing of electron‐beam deposited iron layers. As substrates we use single‐crystal silicon wafers, epitaxial silicon thin films on sapphire substrates, and low‐pressure chemical vapor deposited polycrystalline silicon thin films on oxidized silicon wafers. X‐ray diffraction indicates that orthorhombic β‐FeSi2 is obtained for growth temperatures in the range 800–900 °C. Photothermal deflection spectroscopy reveals a direct band‐gap of 0.85 eV. Optical transitions at energies above 1.4 eV are investigated by spectroscopic ellipsometry. Measurements of the subgap defect absorption, photoluminescence, conductivity, and of the Hall mobility suggest that lower growth temperatures yield material of better quality.

Optical, electronic, and transport properties of nanocrystalline titanium nitride thin films

Spectroscopic ellipsometry (SE) was employed to get insights on the optical, electronic, and transport properties of nanocrystalline titanium nitride (TiNx) films with respect to their microstructure and stoichiometry. The films’ properties can be tailored by varying the energy of bombarding ions during sputter deposition and the substrate temperature (Td). The best metallic behavior of TiNx (resistivity 40 μΩ cm and conduction density 5.5×1022 electrons/cm3) has been observed in films developed with energy above 100 eV and Td⩾400 °C. A redshift of the optical gaps has been observed for overstoichiometric films, suggesting it as a sensitive probe to investigate the TiNx stoichiometry. The energy, strength, and broadening of the interband transitions were studied with respect to the energy of ions and Td and they were explicitly correlated with the TiNx crystal cell size and grain orientation. On the other hand, the study of intraband absorption has provided the conduction electron density with respect to ...

The effect of substrate temperature and biasing on the mechanical properties and structure of sputtered titanium nitride thin films

The mechanical properties of titanium nitride (TiNx) thin films have been investigated using depth sensing nanoindentation tests. The effects of substrate temperature (Ts) and of substrate biasing (Vb) on the mechanical properties and the microstructure of the TiNx films were studied. Ts and Vb have strong effect on the films microstructural characteristics such as density, grain size and orientation. It was found that deposition at high Ts and Vb promotes the growth of (002) oriented films with density close to the bulk density of stoichiometric TiN, indicating the absence of voids and the growth of stoichiometric TiN. The film hardness and elastic modulus were measured using the continuous stiffness measurements technique. It was found that there exists a direct correlation between the films mechanical properties and microstructure. The films that exhibit the best mechanical performance are those grown along the (002) orientation and being denser and stoichiometric.

Optical performance of nanocrystalline transparent ceria films

Ceria is a transparent oxide suitable for various optical and optoelectronic devices. In this work, we tailor independently the refractive index n and fundamental gap Eg of nanocrystalline Ceria films by varying the substrate temperature or using Ar+ ion beams during growth with electron beam evaporation. Spectroscopic ellipsometry and x-ray reflectivity are employed to study n and Eg and to identify the physical parameters that affect them. We correlate n (varies from 1.65 to 2.15 in the studied films) with the film density through a universal, square law. The film composition strongly affects Eg, which varies from 2.8 to ∼2.0 eV. The optical absorption below 3 eV and the Eg shift are attributed to O-defect states and not to modifications in interband transitions.

Atomic force microscopy probing in the measurement of cell mechanics.

Atomic force microscope (AFM) has been used incrementally over the last decade in cell biology. Beyond its usefulness in high resolution imaging, AFM also has unique capabilities for probing the viscoelastic properties of living cells in culture and, even more, mapping the spatial distribution of cell mechanical properties, providing thus an indirect indicator of the structure and function of the underlying cytoskeleton and cell organelles. AFM measurements have boosted our understanding of cell mechanics in normal and diseased states and provide future potential in the study of disease pathophysiology and in the establishment of novel diagnostic and treatment options.

Surface kinetics and subplantation phenomena affecting the texture, morphology, stress, and growth evolution of titanium nitride films

We present a thorough study of the microstructure, texture, intrinsic stress, surface, and interface morphology of transition metal nitride (mainly TiN but also CrN) films grown on Si by reactive sputter deposition, with emphasis to the mechanisms of adatom migration on the surface and subplantation of energetic species. In order to study the effects of adatom mobility and the subplantation probability we vary the ion energy and growth temperature. For the experimental part of this work we used nondestructive, statistically reliable x-ray techniques (diffraction, reflectivity, scattering). The x-ray results are compared and correlated with supporting data of in situ spectroscopic ellipsometry as well as Monte Carlo simulations of the irradiation effects and surface diffusion of adatoms. We found that the texture and the surface and interface morphology are sensitive to the mechanism of dissipation of the impinging ions. If the energy is enough to overcome the subplantation threshold (∼50eV), then the film...

Insights on the deposition mechanism of sputtered amorphous carbon films

Low energy Ar+ ion bombardment (LEIB) during growth of amorphous carbon (a-C) films deposited with magnetron sputtering (MS), results to dense films, rich in sp3 C–C bonds, and exhibit high hardness and compressive stress. We present here a preliminary study of the growth mechanism of a-C films deposited with negative bias voltage (LEIB) in terms of their composition, density and mechanical properties. The experimental results showed that stress and hardness are directly related with the sp3 C–C bonding in the film and described well with the so far proposed models on the formation mechanism of tetrahedral carbon. However, the film density, that is a composite property, was found to depend not only on the sp2 and sp3 content but also on a new, denser than graphite, carbon phase when the Ar+ ion energy is above ∼130 eV.

Interface properties and structural evolution of TiN/Si and TiN/GaN heterostructures

Spectroscopic ellipsometry (SE) is employed to study the evolution of microstructure, stoichiometry, and electron-transport properties of titanium nitride (TiN) heterostructures grown on Si and gallium nitride (GaN) by reactive magnetron sputtering. In order to achieve subnanometer resolution for the SE analysis, we developed and validated the appropriate methods of interpreting the optical data. Thus, we used (a) effective medium theories describing the heterostructures in terms of their constituent materials (Si, GaN, TiN, over-stoichiometric TiNx, and voids), and (b) a combined Drude–Lorentz model describing the optical response of the conduction and valence electrons of TiN and TiNx. In the case of TiN/Si, the SE results indicate a pure Volmer–Weber type of growth with four distinct growth stages. A TiNx layer is formed before TiN due to the stress variations during growth. On the other hand, TiN/GaN exhibits a quasi two-dimensional growth character. In both cases, the TiN resistivity is very high for...

Combined electrical and mechanical properties of titanium nitride thin films as metallization materials

Titanium nitride (TiNx) thin films, ∼100 nm thick, were deposited on Si(100) substrates by dc reactive magnetron sputtering. The effects of the substrate bias voltage and deposition temperature on their optical, electrical, and mechanical properties have been studied. It was found a strong correlation between the electrical and mechanical properties of the films which are significantly improved with increasing the substrate bias voltage and the deposition temperature. The low resistivity (43 μΩ cm), combined with the high hardness and elastic modulus values, suggest the TiNx as a promising metallization material in Si technology.

Dielectric function and reflectivity of 3C–silicon carbide and the component perpendicular to the c axis of 6H–silicon carbide in the energy region 1.5–9.5 eV

The optical properties of commercially available 3C‐ and 6H‐SiC single crystals were studied in the energy region 1.5–9.5 eV with conventional and synchrotron‐radiation spectroscopic ellipsometry. The surface perfection of the materials was investigated by transmission electron and atomic force microscopies. The calculated values for the effective and static dielectric functions were higher than those found in the literature. This is consistent with the fine structure and absolute values of the corresponding reflectance spectra, which were higher than those reported in other studies. The dispersion of the refractive indices in the energy region below and above the indirect gap is determined and can be used as reference and for the design and analysis of semiconductor structures and devices.