ICMCTF2012 Session F1-1: Nanomaterials, Nanofabrication, and Diagnostics

The plasma inside the confinement region has been studied by several methods which are typically not quantitative due to the lack of theory and limited direct access to optical information. Nevertheless, recent fast camera studies showed the existence of drift waves in the sub-MHz region which regulate particle transport. Multi-pin Langmuir probes were used to detect lower hybrid frequency oscillations that promote anomalous transport out of the magnetic confinement. Time-resolved optical emission spectroscopy has been used to study the development of discharge chemistry from gas- to metal- dominated.

The plasma expanding towards the substrate is more accessible. Emissive probes showed high electric fields near the target, which relax as the voltage pulse is switched off resulting in strong confinement and subsequent release of plasma, including metal ions. Energy-resolved mass spectroscopy has shown the effect of gas rarefaction, especially with high sputter yield materials, which extends the region of metal-rich plasma. Plasma chemistry quantitatified by optical absorption spectroscopy is rich in metal ions, which can comprise the majority of total particle flux. Plasma composition is influenced by the peak discharge current and can be tuned precisely. In reactive plasmas, oxygen and nitrogen are dissociated to a very high degree, and affect texture in films.

Ion energy studies by E-MS show negative oxygen ions with energy of several hundred eV – corresponding to the potential on the target. Retarding field analysers show fast evolution of ion energy, reaching high values during the pulse.

The energy of metals is enhanced and conserved at high powers due to gas rarefaction. The energy of gas ions is similarly enhanced and leads to rarefaction.

Film growth is strongly influenced by the chemistry and ion energy in the HIPIMS discharge. High fluxes of dissociated nitrogen to the surface bind arriving Ti adatoms thus effectively lowering their diffusion rate and reducing outdiffusion from 200 surfaces.

Highly ionised metal flux improves coverage of high aspect ratio vias, reduces waviness in interfaces between nanolayers and affects nanocomposite grain size and misorientation.

A number of challenges to remain, mainly connected with the transient stages of the discharge, namely, diagnosing the non-plasma breakdown phase, and obtaining fast quantitative data on plasma chemistry, ion energy and electron temperatures.

The shortage crisis of ³He has created an urgent need for alternative neutron detectors based on other elements. Solid state based neutron detectors typically demand the applicability of the neutron reactive element in the form of thin films in order to optimise efficiency and usability. ¹⁰B and ¹⁵⁷Gd are among the most promising isotopes for usage in solid state neutron detectors as well as in purely absorbing, isolating layers due to their high cross section for neutron reactions. The application of these elements implies the need for new coating materials and the optimisation of a series of materials properties for each particular objective. The requirements for either metallic or semiconducting conductivity, corrosion resistance, and mechanical and thermal stability may be met by producing ceramic compounds based on the neutron detecting element, as was made for B₄C [1]. In this work we show how theoretical first-principles calculations can be used to guide this materials development with the example of an electrically conductive, oxidation resistant, neutron absorbing coating material based on TM_1-xGd_xN (TM=Ti, Zr, Hf)[2]. Results show that GdN, prone for oxidation in its pure form, readily mixes with the chemically more inert nitrides ZrN and HfN, possibly as an ordered compound. On the other hand mixing of GdN with TiN is highly unfavourable due to volume mismatch.

[1] B₄C thin films for neutron detection, C. Höglund, J. Birch, K. Andersen, T. Bigault, J.-C. Buffet, J. Correa, P. van Esch, B. Guerard, R. Hall-Wilton, J. Jensen, A. Khaplanov, F. Piscitelli, C. Vettier, W. Vollenberg, and L. Hultman, Submitted for publication.

[2] Mixing thermodynamics of TM_1−xGd_xN (TM=Ti,Zr,Hf) from first principles,

B. Alling, C. Höglund, R. Hall-Wilton, and L. Hultman, Applied Physics Letters, 98, 241911 (2011)

Phase separated Al-Si films were deposited by concurrent deposition of Al using filtered vacuum cathodic arc and Si produced by dc magnetron sputtering deposition. The resulting phase separated films were etched chemically to remove the nanocolumns of Al to produce a SiOx mesoporous structure. The resulting layer had a pore size that was dependent upon the arrival ratio of the Al:Si atoms at the substrate, substrate temperature and negative substrate bias voltage during film growth. Scanning electron microscope images of the etched AlSi films showed that the average diameter of pores could be varied from 2 nm to 11 nm with a well defined size distribution. The results are interpreted using a model to describe the pore size as a function of substrate temperature and particle impingement energy.

Silicene is an atom-thick, honeycomb sheet of silicon: Si-version of graphene. Silicene is increasingly attracting interests owing to the success of graphene. One of the major differences between these two materials is that silicene is predicted to be more stable in slightly-buckled form with neighboring Si atoms displaced out of plane while graphene is perfectly planar. Silicene may therefore form with a variety of lattice constants related to a different degree of buckling. So far, silicene only exists in the form of epitaxial layer on single-crystalline, metallic substrates, such as silver single crystals and ZrB₂ thin films. The relationship between the electronic structure and the atomistic structure of epitaxial silicenes will be discussed in detail.