Solid State Phenomena Vol. 159

Abstract: The recent attempts to design new super- and ultrahard materials concentrate predominantly on those with high elastic moduli. This approach neglects the fact that elastic moduli describe the reversible, elastic response to small strain near equilibrium, whereas hardness is related to plastic deformation, the measurement of which involves substantial plastic strain, where the electronic structure becomes strongly distorted and can often result in structural transformations to softer phases. In the superhard nanocomposites consisting of 3-4 nm size randomly oriented nanocrystals of hard transition metal nitrides joined together by about one monolayer of silicon nitride variant, which is strengthened by negative charge transfer, the nanocrystals are free of defects and therefore reach ideal strength. Because of the strengthening of the interface and of the random orientation of the nanocrystals, these nanocomposites reach hardness of more than 100 GPa as shown experimentally. We provide a simple theoretical explanation why these materials can exceed the hardness of diamond, and outline a possible way how to design new nanocomposites with even higher hardness when reduction of Friedel oscillations of the valence charge density, which weaken the strength of the transition metal nitride, can be accomplished.

Abstract: In most coating applications damage resistance is controlled by the mechanical properties of the coating, interface and substrate. For electronic and optical applications the design of coating-substrate systems has been predominantly controlled by their functional properties but more recently the mechanical response of the system has been used to enhance functional properties, as in the case of strained silicon/SiGe microelectronic devices where tensile strain has been used to enhance mobility and increase device speed. As coatings become more complex, with multilayer and graded architectures now in widespread use, it is very important to obtain the mechanical properties (such as hardness, elastic modulus, fracture toughness, etc.) of individual coating layers for use in design calculations and have failure-related design criteria which are valid for such multilayer systems. Nanoindentation testing is often the only viable approach to assess the damage mechanisms and properties of very thin coatings (<m) since it can operate at the required scale and provides fingerprint of the indentation response of the coating/substrate system. If coating properties are to be assessed, the key point is to ensure any measured value is free from the influence of the deforma-tion of the substrate or lower coating layers. Finite element analysis of indentation load displace-ment curves can be used to extract materials properties for design; as coating thicknesses decrease it is observed that the yield strength required to fit the curves increases and scale-dependent materials properties are essential for design. Since plasticity is less likely, non-linear elasticity is increasingly important as the size of a nanostructure is reduced. Similarly the assessment of fracture response of very thin coatings requires modeling of the indentation stress field and how it is modified by plas-ticity during the indentation cycle. An FE approach using a cohesive zone model has been used to assess the locus of failure and demonstrates the complexity of adhesive failure around indentations for multilayer coatings. Finally the mechanical design of a metallization stress sensor based on na-noindentation-derived materials properties, non-linear elastic and plastic behavior and the treatment of geometrical non-linearities (stress stiffening) is discussed.

Abstract: Because of strong synergy with information technology, visible light imaging, and solar cell businesses, most of the devices for medium and high voltage power electronics are based on silicon in year 2009 [1]. Still we know, for more than 50 years, that “harder” semiconductors, exhibiting higher breakdown electric field, would be preferable [2]. On the way towards the development of such new materials, the road is very narrow between so many intricate scientific and technical obstacles. After 50 years of SiC technology development, a first generation of reliable Schottky rectifiers is now available [3,4], but it will take time to turn it into a profitable business. Despite of very important progress over the past 15 years, it is not yet clear whether there will ever be any reliable high voltage switching device based on SiC MOS [5-7]. Vertical JFET have recently appeared as realistic alternative solutions [9-12]. Hetero-epitaxial GaN materials on sapphire or silicon substrate may appear as competitors to SiC. Progress on the crystal growth of Diamond, Aluminum Nitride [8] and Boron Nitrides for electronics is on the way, but there is no convincing solution identified yet for the basic doping problems. Regarding the more ionic II-VI or I-VII semiconductors, very few people still believe that they can play a role inside future device structures for power electronics.

Abstract: With the increasing requirements for microelectromechanical systems (MEMS) regarding stability, miniaturization and integration, novel materials such as wide band gap semiconductors are receiving more attention. The outstanding properties of group III-nitrides offer many more possibilities for the implementation of new functionalities and a variety of technologies are available to realize group III-nitride based MEMS. In this work we demonstrate the application of these techniques for the fabrication of full-nitride MEMS. It includes a novel actuation and sensing principle based on the piezoelectric effect and employing a two-dimensional electron gas confined in AlGaN/GaN heterostructures as integrated back electrode. Furthermore, the actuation of flexural and longitudinal vibration modes in resonator bridges are demonstrated as well as their sensing properties.

Abstract: This work presents some recent results on the 3C-SiC structural defects, studied by Transmission Electron Microscopy (TEM). The samples studied were grown in several laboratories, using different methods. Commonly used methods for growth are Sublimation Epitaxy (SE), Physical Vapour Transport (PVT), Continuous Feed Physical Vapour Transport (CF-PVT), Chemical Vapour Deposition (CVD), and Liquid Phase Epitaxy (LPE). In all these methods, for both bulk and epitaxial layer growth, substrates from other polytypes are exploited like the common hexagonal polytypes 4H- and 6H-SiC or 3C-SiC seeds both in (111) and (100) orientation.

Abstract: UNCD/a-C composite films have been deposited by microwave plasma chemical vapour deposition from methane/nitrogen mixtures with 17% CH4 in the temperature range 500-770°C on various substrates such as monocrystalline silicon wafers, polycrystalline diamond, c-BN, TiN, GaAs, and other materials of technological interest. The resulting films have been thoroughly characterized with respect to their morphology, crystallinity, composition, and bonding structure. It was found that they are composed of diamond nanocrystallites (3-5 nm in diameter) surrounded by 1-1.5 nm amorphous carbon grain boundary material; the ratio of the volume fractions of crystalline and amorphous phase is close to unity. The investigations of the application-relevant properties of the UNCD/a-C films revealed that they are attractive for a number of mechanical, tribological, structural, and biomedical applications.

Abstract: Thin hexagonal barium hexaferrite particles synthesized using the microemulsion technique were studied. A water-in-oil reverse microemulsion system with cetyltrimethylammonium bromide (CTAB) as a cationic surfactant, n-butanol as a co-surfactant, n-hexanol as a continuous oil phase, and an aqueous phase were used. The microstructural and magnetic properties were investigated. The particles obtained were mono-domain with average particle size 280 nm. The magnetic properties of the powder were investigated at 4.2 K and at room temperature. The saturation magnetization was 48.86 emu/g and the coercivity, 2.4 x 105 A/m at room temperature. The anisotropy field Ha and magneto-crystalline anisotropy K1 were 1.4 x 106 A/m and 2.37 x 105 J/m3, respectively.

Abstract: Traditionally, the term hard coatings refer to the property of high hardness in mechanical sense with good tribological properties [1]. With the development of modern technology in the areas of optical, optoelectronic, microelectronic and related defense applications, the definition of the term hard coatings can be extended. Thus, a system which operates satisfactorily, in a given environment can be said to be hard with respect to that environment [2]. Most of the hard coatings are ceramic compounds such as oxides, carbides, nitrides (AlN), ceramic alloys, cermets, metastable materials such as Diamond-Like Carbon (DLC). Their properties and environmental resistance depend on the composition, stoichiometry, impurities, microstructure, imperfections, and in the case of coatings, the preferred orientation (texture). In this paper we shall take a look at some characteristics - physicochemical and optical of AlN and DLC layers synthesized by physical vapor deposition – RF magnetron sputtering in an industrial high vacuum deposition system. The influence of the process parameters on the growth rate, morphology, topography and chemical bonding structure will be presented.