In this contribution, we report ab-initio molecular dynamics (MD) simulations of the preparation of amorphous (Si)-(B)-C-N materials formed by energetic ion assisted deposition techniques. In our liquid-quench simulations, the Kohn-Sham equations for the valence electrons are expanded in a basis of plane wave functions, while core electrons were represented using Goedecker-type pseudopotentials. We simplify the ion bombardment process by assuming that the primary impact creates a localized region of high temperature, commonly referred to as a thermal spike. Thermal spikes have a cooling time sufficiently short to be simulated using ab initio MD methods. We show formation of N₂ molecules during liquid quench simulations and investigate how density, temperature and quench rate affect the number of N₂ molecules formed in the network. We find that a higher material density and shorter cooling times lead to reduced N₂ formation and thus to a higher nitrogen incorporation into the final structure. We investigate how the presence of Ar atoms in the sample affects the liquid quench and the final structure of the material. The presence of Ar leads to the formation of silicon enriched regions in the vicinity of implanted Ar atoms, which provides new insight into the role of implanted Ar in formation of structures in amorphous materials. It can also explain the ability of Si to relieve stress generated by these implanted Ar atoms. We calculate various data affecting material properties and compare them for previously deposited materials of different Si-B-C-N compositions.

¹J.Houska, M.M.M.Bilek, O.Warschkow et al., accepted for publication in Phys. Rev. B (2005).

²J.Houska, O.Warschkow, M.M.M.Bilek et al., submitted to J. Phys. C (2005).

³J.Vlcek, S.Potocky, J.Cizek et al., J. Vac. Sci. Technol. A, in print (12/2005).

⁴J.Houska, J.Vlcek, S.Potocky et al., submitted to Diamond Relat. Mater. (2005).

GaN is a wide band gap (3.4 eV) semiconductor, which is of great interest for short wavelength optoelectronic device applications, as well as high power/high temperature electronic applications. In recent years, in addition to research work on GaN thin films and devices, there has been increasing interest in the fabrication and characterization of GaN nanostructures.

In this work, the growth of GaN nanowires from Ga and NH₃ sources in the flow of Ar carrier gas using a chemical vapor deposition (CVD) system was systematically studied. The substrates used were Si (111) and Si (100). Fabricated nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). We have investigated the influence of growth temperature, catalyst used, Ga amount, and the ratio of Ar and NH₃ flow rates on the morphology and properties of GaN nanowires. We found that the best results were obtained for growth temperature of 950 °C. Optimal catalysts were Au and metallic Ni, while the use of nickel nitrate was found to lead to formation of SiO_x nanowire bunches in addition to GaN nanowires.

For the optimal temperature and catalyst used, the influence of Ga to N ratio on the nanowire growth was studied. It was found that different types of nanostructures are observed in relatively Ga-rich and in relatively N-rich conditions. Growth mechanisms of different types of nanowires, including the stacked-cone nanowires and the microscale structures formed by lateral growth under N-rich condition are discussed.