AVS2013 Session EL-ThP: Spectroscopic Ellipsometry Poster Session
Time Period ThP Sessions | Topic EL Sessions | Time Periods | Topics | AVS2013 Schedule
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EL-ThP-1 Uniformity of Plasma Polymer Coatings in Various Types of Capacitive Reactors
Thomas Ameringer (Australian National Nanofabrication Facility Victoria (ANFF-VIC)); Hannah Askew (Swinburne University of Technology, Australia); Jason Whittle (Mawson Institute University of South Australia); Paul Pasic (CSIRO Materials Science and Engineering, Australia); Sally M. McArthur (Australian National Nanofabrication Facility Victoria (ANFF-VIC)) Plasma polymersation is a versatile technique capable of coating almost any type of substrate. It is used in all kinds of fields from non-sticking pans to biomedical applications. Despite the growing number of applications the physical and chemical properties of the plasma and the respective coatings are complex and often ppoorly understood. These can have detrimental effects on the subsequent experiments/applications. In this study, we examined the uniformity of coating thickness and chemistry of plasma polymers in relation to the position to the electrode and in the reactor. Different types of reactors were used to coat silicon wafers with plasma polymers of allylamine, acrylic acid and octadiene. The thickness of the coatings was examined via spectroscopic ellipsometry (SE; M-2000 JA Woollam). Variations in the chemical composition of the coatings were examined with XPS and Raman spectroscopy. Spectroscopic ellipsometry showed that the coating thickness varied significantly both across a single wafer surface and between different reactors. The ‘gradient’ of coating thickness on single wafers was shown to vary up to 100 nm depending on the orientation of the substrate inside the reactor. The distance to the electrode was found to be the critical, with the coatings generally being thicker closer to the electrode. In case of metal side walls also a significant drop in coating thickness could be observed near these walls. Critically, the chemistry of the coatings varied between the different reactors, but appeared to be consistent across a single wafer. These results highlight that if a homogeneous coating thickness is essential for an application, some caution is needed on where to place the samples inside a reactor and how to tune the plasma properties. |
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EL-ThP-2 Electronic and Vibrational Properties of Nickel Oxide using Spectroscopic Ellipsometry
Cayla Nelson, Travis Willett-Gies, Lina Abdallah, Stefan Zollner (New Mexico State University) Nickel oxide (NiO) is an interesting material, because it is a Mott-Hubbard charge-transfer insulator and also displays antiferromagnetic ordering of electron spins [1]. Spectroscopic ellipsometry is able to investigate the electronic structure of NiO (from the visible and UV portions of the spectra) and also its lattice dynamics (using infrared ellipsometry). Our interest in the NiO optical constants is also of a practical nature, to model ellipsometry spectra of bulk Ni and Ni thin films with a native oxide of NiO. We measured the ellipsometric angles ψ and Δ for single-side polished bulk NiO from 0.8 to 6.5 eV with angles of incidence from 65 to 75° to determine the dielectric function. A dispersion model for the optical constants was built using two Tauc Lorentz oscillators; one with a Lorentz oscillator resonance energy at 3.96 eV and a second one with a much smaller amplitude at 6.40 eV. These peaks are in agreement with reflectance data analyzed using Kramers-Kronig transforms [2]. Our model also included a surface roughness layer with 40 Å thickness. Atomic force microscopy measurements confirmed this layer, showing an RMS roughness of 42.5 Å. We will report accurate dielectric function data for NiO from 0.8 to 6.5 eV. FTIR ellipsometry was also performed on bulk NiO from 290 to 1000 cm-1 to study the lattice vibrations. TO phonons were found at 392 cm-1 and 551 cm-1, with the corresponding LO modes at 592 cm-1 and 545 cm-1. The weak TO mode at 551 cm-1 results from the antiferromagnetic ordering of NiO, which doubles the unit cell and causes zone folding, making a zone-edge TO mode infrared-active. Previous FTIR absorption measurements of NiO [3] did not report the infrared-active zone-edge phonon. Usually, antiferromagnetic ordering is only observed using neutron scattering, not with FTIR optical methods. [1] G.A. Sawatzky and J.W. Allen, Phys. Rev. Lett. 53, 2339 (1984). [2] R.J. Powell and W.E. Spicer, Phys. Rev. B 2, 2182 (1970). [3] R. Newman and R.M. Chrenko, Phys. Rev. 114, 1507 (1959). * This work was supported by the National Science Foundation (DMR-1104934) and performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Sandia National Laboratory (Contract DE-AC04-94AL85000). |
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EL-ThP-3 Properties of Sm Doped CeO2 Thin Films Prepared by Liquid Solution Deposition
Khadijih Mitchell, Cesar Rodriguez, Travis Willett-Gies, Yuling Li, Stefan Zollner (New Mexico State University) Cerium(IV) oxide, also known as CeO2 or ceria, is a transparent (insulating) oxide of the rare earth metal cerium. It is an ionic conductor with applications in fuel cells, as a catalyst, or for photovoltaic water splitting (hydrogen production). Thin films of ceria produced by RF magnetron sputtering on sapphire at 770C have been studied extensively by Arwin's group (S. Guo et al., J. Appl. Phys. 77, 5369, 1995). They found changes in grain size, surface morphology (visible in AFM images), and optical constants varying with the film thickness. By contrast, we report analysis results for relatively thick (300-500 nm) ceria films prepared by liquid solution deposition (dip-coating) followed by annealing. We also investigate the effect of samarium doping (up to 20at.%) of ceria. The rare earth metal samarium usually forms a sesquioxide Sm2O3. Therefore, doping ceria with Sm is expected to lead to the formation of oxygen vacancies, which enhances the ionic conductivity of ceria. Our ellipsometry spectra (ellipsometric angles and depolarization) can be described very well in the transparent region (below 3 eV) using a Tauc-Lorentz dispersion model for ceria, if small amounts of surface roughness and thickness non-uniformity across the wafer are taken into account. Once these thickness parameters have been determined for our films, we obtain the optical constants of CeO2:Sm using a basis spline expansion. We find the typical dispersion expected for an insulator with a direct band gap near 3.7 eV. Samarium doping causes a significant decrease of the refractive index in the transparent region. Most likely, the films with high Sm content are less dense (have more voids, perhaps due a smaller crystallite size) than pure ceria films. An increase in disorder due to Sm doping was also found in x-ray diffraction studies of electrodeposited ceria films (Phok and Bhattacharya, phys. status solidi (a) 203, 3734, 2006). As expected from Kramers-Kronig consistency, we find a significant reduction of the height of the main absorption peak at 4 eV. The direct band gap, however, remains at 3.7 eV, independent of Sm content. There is, however, a significant decrease in the slope of the onset of absorption with increasing Sm content. In addition to ellipsometry results, we will also report AFM, XRD, Raman, and (perhaps) FTIR ellipsometry results for our Sm-doped ceria films. |