Identification of Ternary Phases in TiBC/a-C Nanocomposite Thin Films: Influence on the Electrical and Optical Properties

The local structure of TiBC and amorphous carbon (a-C) nanocomposite films (TiBC/a-C) was correlated with their optical and electrical properties. TiBC/a-C films with increasing C content were deposited by magnetron co-sputtering from TiC:TiB2 (60: 40) and graphite targets. Chemical composition is determined by electron energy-loss spectroscopy. Grazing incidence X-ray diffraction reveals that the microstructure of the films is amorphous with small nanocrystallites emerging by increasing the C content that could be attributed to the formation of ternary (TiBxCy) or mixed binary (TiB2 and TiC) phases. Further information was then obtained by studying the chemical bonding by measuring the near-edge fine structure (NES) by electron energy-loss (B K-, C K-, and Ti L-edges) and X-ray absorption (B K-and Ti L-edges) spectroscopies. The NES analysis indicates the formation of a nanocrystalline ternary TiBxCy compound concomitant with the segregation of an a-C phase as the carbon content is increased. The optical properties were studied by spectroscopic ellipsometry and the electrical resistivity was measured by the Van der Pauw method between 20 and 300 K. The films continuously lose their metallic character in terms of optical constants and resistivity with increasing carbon content. Theoretical fitting of the electrical properties using the grain-boundary scattering model supported the formation of a nanocomposite structure based on a ternary TiBxCy phase embedded in a matrix of a-C. The electron transport properties are mainly limited by the high density of point defects, grain size, and transmission probability.

Electrical and optical properties of Ta-Si-N thin films deposited by reactive magnetron sputtering

David Oezer, Rosendo Sanjines

The electrical and optical properties of TaxSiyNz thin films deposited by reactive magnetron sputtering from individual Ta and Si targets were studied in order to investigate the effects of nitrogen and silicon contents on both properties and their correlation to the film microstructure. Three sets of fcc-TaxSiyNz thin films were prepared: sub-stoichiometric TaxSiyN0.44, nearly stoichiometric TaxSiyN0.5, and over-stoichiometric TaxSiyN0.56. The optical properties were investigated by near-normal-incidence reflectivity and ellipsometric measurements in the optical energy range from 0.375 eV to 6.8 eV, while the d.c. electrical resistivity was measured in the van der Pauw configuration from 20 K to 300 K. The optical and electrical measurements were interpreted using the standard Drude-Lorentz model and the so-called grain boundary scattering model, respectively. The electronic properties were closely correlated with the compositional and structural modifications of the TaxSiyNz films due to variations in the stoichiometry of the fcc-TaNz system and the addition of Si atoms. According to the nitrogen and silicon contents, fcc-TaxSiyNz films can exhibit room temperature resistivity values ranging from 10(2) mu Omega cm to about 6 x 10(4) mu Omega cm. The interpretation of the experimental temperature-dependent resistivity data within the Grain Boundary Scattering model, combined with the results from optical investigations, showed that the mean electron transmission probability G and the free carriers concentration, N, are the main parameters that control the transport properties of these films. The results indicated that the correlation between electrical and optical measurements with the chemical composition and the nanostructure of the TaxSiyNz thin films provides a pertinent and consistent description of the evolution of the Ta-Si-N system from a solid solution to a nanocomposite material due to the addition of Si atoms. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4766904]

Amer Inst Physics2012

A unique approach to reveal the nanocomposite nc-MN/SiN-layer architecture of thin films via electrical measurements

Scott Harada, Silviu Cosmin Sandu, Rosendo Sanjines

By addition of Si to a binary transition metal nitride MN (e.g. TiN, ZrN, NbN, CrN), the hardness, thermal stability and chemical inertness of films have been considerably improved. The formation of a ternary M–Si–N ternary phase is possible under specific conditions such as low temperature, high deposition rate and low nitrogen pressure. The formation of nanocomposite materials (e.g. crystallites of MN + a thin layer of SiNx) is also possible under a wide range of deposition conditions. In such nanocomposite thin films the crystallite sizes are on the order of a few nanometers. The grain surfaces and boundaries have an important effect on the physical properties. The arrangement and chemical composition of the so-called “amorphous” minority phase (SiNx) are crucial for electrical and mechanical properties. The location, composition and thickness of the amorphous phase must therefore be known precisely. Their experimental determination is challenging due to the small concentration and in particular the geometry of the “amorphous” phase: approximately one monolayer either completely or partially covering the MN nanocrystallites. TEM investigations on such composites are known to have their limitations. It will be shown that the electrical resistivity, measured as a function of temperature, provides an experimental means for following the thickness evolution of the SiNx coverage layer, in such nanocomposite films.

2010

Electrical and optical properties of Ta-Si-N thin films deposited by reactive magnetron sputtering

David Oezer, Rosendo Sanjines

Amer Inst Physics2012