Materials Science Forum Vols. 539-543

Abstract: The atomic-scale structures and properties of thin films are critically determined by the various kinetic processes activated during their atomic assembly. Molecular dynamics simulations of growth allow these kinetic processes to be realistically addressed at a timescale that is difficult to reach using ab initio calculations. The newest approaches have begun to enable the growth simulation to be applied for a wide range of materials. Embedded atom method potentials can be successfully used to simulate the growth of closely packed metal multilayers. Modified charge transfer ionic + embedded atom method potentials are transferable between metallic and ionic materials and have been used to simulate the growth of metal oxides on metals. New analytical bond order potentials are now enabling significantly improved molecular dynamics simulations of semiconductor growth. Selected simulations are used to demonstrate the insights that can be gained about growth processes at surfaces.

Abstract: The development of nanostructured materials and coatings has driven the development of indentation-based assessment techniques which aim to generate useful mechanical property information. This paper introduces an approach to determine the limits for which direct measurement of these properties are possible and highlights the importance of modelling if reliable data is to be obtained from very thin coatings (<200nm) and fine grained materials.

Abstract: We investigated InGaAs layers grown by molecular-beam epitaxy on GaAs (001) with transmission electron microscopy (TEM) and photoluminescence spectroscopy. InGaAs layers with In-concentrations of 16, 25 and 28 % and respective thicknesses of 20, 22 and 23 monolayers were deposited at 535 °C. Island formation is observed for the layer with the highest In-concentration. Inconcentration profiles were obtained from high-resolution TEM images by composition evaluation by lattice fringe analysis. The measured profiles can well be fitted applying the segregation model of Muraki et al. [Appl. Phys. Lett. 61 (1992) 557] and are in excellent quantitative agreement with the photoluminescence peak positions. From our data we conclude that island formation occurs when the amount of Indium in the In-floating layer reaches 1.1±0.2 monolayers indium.

Abstract: The molecular dynamics (MD) simulation employing the embedded atom method (EAM) has been performed to examine the phase stability of Pt nanoclusters, Ptn (n=38, 147, 309 and 561 atoms) with size and temperature. From heating and freezing curves of the nanoclusters, the clusters (Pt147, Pt309 and Pt561) larger than 1 nm in size showed an icosahedral morphology near 460 ~ 660 K during freezing, where the formation energy of the icosahedral phase is 0.051 eV/atom for Pt147, 0.056eV/atom for Pt309 and 0.067 eV/atom for Pt561. We also investigated coalescence between two Pt nanoclusters and observed that the minimum size of the coalescent one is around 1 nm at 673 K.

Abstract: Thin SiN film deposited on Si by plasma enhanced chemical vapour deposition (PECVD) is used for surface passivation of Si. During the PECVD process Hydrogen is incorporated into the SiN film, and the passivation properties of the resulting SiNx:H layers play an important role in enhancing the energy conversion efficiency of solar cells. It is believed that the Hydrogen present in SiNx:H is responsible for this enhancement, and therefore its concentration in the passivating layer is an important parameter. The Hydrogen composition and its depth profile in thin SiNx:H films of 20nm to 200nm was measured by elastic recoil detection analysis (ERDA), using a 1.7MeV He+ ion beam of (1x2)mm2, generated by a high stability 2MV Tandetron ion beam accelerator. Simultaneously, Rutherford backscattering (RBS) spectra were recorded for each sample. The results show that the Hydrogen concentration in the SiNx:H layers is dependent of the deposition conditions. Also, Hydrogen was found to be homogenously distributed across the SiNx:H layer thickness, and the SiNx:H/Si interfaces were well defined.

Abstract: When the Al/Ge/SiO2 bilayer films are annealed in-situ in a scanning electron microscope (SEM) at the temperatures lower than the crystallization temperature of amorphous Ge itself, the so-called metal-mediated-crystallization (MMC) takes place. In the course of MMC, crystalline Ge aggregates (Ge clusters) form in the bilayer films, which results in the formation and the evolution of impressive fractal patterns with branching on the free surface. In-situ SEM observations of annealed Al/Ge/SiO2 bilayer films indicate that the grain size of polycrystalline Al-layer influences the nucleation of Ge clusters and hence of fractal patterns. For the bilayer films containing larger Al grains, the nucleation rate of fractal patterns (Ge clusters) is faster and the number of patterns is larger.

Abstract: Diamond-like carbon (DLC) films were synthesized by RF plasma enhanced chemical vapor deposition using methane as carbon source. Effect of substrate on the growth of DLC films was investigated by using four different substrate materials, silicon wafer (100), glass, flat-polished and mirror-polished alumina. The carbon films were deposited at four different self-bias voltages (-157 V, -403 V, -500 V and -590 V) by changing the plasma power under fixed flow rate and working pressure. Raman analyses indicated that DLC films were deposited on silicon and glass substrates at the self-bias -403 V ~ -590 V, and polymer-like carbon films were obtained at -157 V. For the alumina substrates, different Raman results were observed for flat-polished and mirror-polished alumina substrates. The hardness of DLC films, deposited on silicon and glass substrates at the self-bias -403 V ~ -590 V, was within 16~20 GPa using nanoindentation technique.