Maria Losurdo

Spectroscopic ellipsometry and polarimetry for materials and systems analysis at the nanometer scale: state-of-the-art, potential, and perspectives

This paper discusses the fundamentals, applications, potential, limitations, and future perspectives of polarized light reflection techniques for the characterization of materials and related systems and devices at the nanoscale. These techniques include spectroscopic ellipsometry, polarimetry, and reflectance anisotropy. We give an overview of the various ellipsometry strategies for the measurement and analysis of nanometric films, metal nanoparticles and nanowires, semiconductor nanocrystals, and submicron periodic structures. We show that ellipsometry is capable of more than the determination of thickness and optical properties, and it can be exploited to gain information about process control, geometry factors, anisotropy, defects, and quantum confinement effects of nanostructures.

Role of sapphire nitridation temperature on GaN growth by plasma assisted molecular beam epitaxy: Part I. Impact of the nitridation chemistry on material characteristics

The impact of the nitridation temperature on sapphire/GaN interface modifications and the structural, chemical, and optical properties of GaN epitaxial thin films with N plasma radicals is investigated. Based on ex situ spectroscopic ellipsometry and x-ray photoelectron spectroscopy analysis, it is found that the sapphire nitridation chemistry, specifically AlN versus oxynitride (NO) production, depends on the surface temperature. Nitridation at 200 °C produces a very thin AlN layer with 90% coverage, while high temperature nitridation leads to a 70% coverage of AlN layer containing NO. These initial stages of growth significantly impact the characteristics of the layers following the nitridation step, specifically the low temperature buffer, annealed buffer, and the GaN epitaxial layer. The annealed buffer on a 200 °C nitridation provides a homogeneous GaN thin layer covering most of the sapphire surface. This homogeneous GaN layer after annealing produces a superior template for subsequent growth, resul...

Ellipsometry at the nanoscale

Preamble.- Preface.- A Brief History and State of the Art of Ellipsometry.-Advanced Mueller Ellipsometry Instrumentation and Data Analysis.- Data Analysis for Nanomaterials: Effective Medium Approximation, its Limits and Implementations.- Relationship between Surface Morphology and Effective Medium Roughness.- Plasmonics and Effective-Medium Theory.- Thin films of Nanostructured Plasmonic Noble Metals.- Spectroscopic Ellipsometry on Metallic Gratings.- Mueller matrix applied to nanostructures.- Spectroscopic Ellipsometry and Magneto-Optical Kerr Spectroscopy of Magnetic Garnet Thin Films Incorporating Plasmonic Nanoparticles.- Generalized Ellipsometry Characterization of Sculptured Thin Films made by Glancing Angle Deposition.- THz Generalized Ellipsometry characterization of highly-ordered 3-dimensional Nanostructures.- Infrared ellipsometric investigations of free carriers and lattice vibrations in superconducting cuprates.- Real-time Ellipsometry for Probing charge-transfer processes at the nanoscale.- Polarimetric and other Optical Probes for the Solid - Liquid Interface.- Spectroscopic Ellipsometry for functional nano-layers of flexible organic electronic devices.- Spectroscopic Ellipsometry of Nanoscale Materials for Semiconductor Device Applications.- Ellipsometry of semiconductor nanocrystals.- Spectroscopic Ellipsometry for Inline Process Control in the Semiconductor Industry.- Thin film applications in research and industry characterized by spectroscopic ellipsometry.- Ellipsometry and Correlation Measurements.- Nanotechnology: Applications and markets, present and future.

Plasma cleaning and nitridation of sapphire (α-Al2O3) surfaces: New evidence from in situ real time ellipsometry

The interaction of α-Al2O3 (0001) surfaces with H atoms and N atoms from remote rf plasmas used for the cleaning and nitridation processes, respectively, is investigated at temperatures in the range of 200–600 °C. The chemistry and kinetics of the above processes are monitored in real time by in situ spectroscopic ellipsometry. Also, the chemistry of the nitrided sapphire surfaces is verified by x-ray photoelectron spectroscopy (XPS) analysis. Specifically, H atoms treatments are effective in removing carbon contaminants from the sapphire surface at temperatures of 200–400 °C. Real time ellipsometry is suitable to detect the cleaning end point and to verify the onset of the H-atom diffusion into the sapphire substrate. Remote N2 plasma nitridation at 200 °C is found to yield homogeneous and smooth AlN layers of about 5 A, after approximately 25 min of nitridation, whereas high nitridation temperatures result in a damaged sapphire surface with AlN protrusions. Both ellipsometric and XPS data show that the ...

Real-time plasmon resonance tuning of liquid Ga nanoparticles by in situ spectroscopic ellipsometry

Liquid Ga nanoparticles have been deposited on sapphire substrates at room temperature. The optical evolution of Ga nanoparticle surface plasmon resonance during deposition has been characterized by in situ real-time spectroscopic ellipsometry to control and tune the plasmon resonance photon energy. The existence of both longitudinal and transverse modes for spheroidal Ga nanoparticles supported on a sapphire substrate is demonstrated and the dependence of the longitudinal and transverse plasmon energies on particle size is discussed. Stability of the Ga surface plasmon resonance to air exposure and high temperature is also demonstrated.

Dielectric function of nanocrystalline silicon with few nanometers (<3 nm) grain size

The dielectric function of nanocrystalline silicon (nc-Si) with crystallite size in the range of 1 to 3 nm has been determined by spectroscopic ellipsometry in the range of 1.5 to 5.5 eV. A Tauc–Lorentz parameterization is used to model the nc-Si optical properties. The nc-Si dielectric function can be used to analyze nondestructively nc-Si thin films where nanocrystallites cannot be detected by x-ray diffraction and Raman spectroscopy.

Demonstration of surface-enhanced Raman scattering by tunable, plasmonic gallium nanoparticles

Size-controlled gallium nanoparticles deposited on sapphire were explored as alternative substrates to enhance Raman spectral signatures. Galliums resilience following oxidation is inherently advantageous in comparison with silver for practical ex vacuo nonsolution applications. Ga nanoparticles were grown using a simple molecular beam epitaxy-based fabrication protocol, and monitoring their corresponding surface plasmon resonance energy through in situ spectroscopic ellipsometry allowed the nanoparticles to be easily controlled for size. The Raman spectra obtained from cresyl fast violet (CFV) deposited on substrates with differing mean nanoparticle sizes represent the first demonstration of enhanced Raman signals from reproducibly tunable self-assembled Ga nanoparticles. Nonoptimized aggregate enhancement factors of approximately 80 were observed from the substrate with the smallest Ga nanoparticles for CFV dye solutions down to a dilution of 10 ppm.

Role of sapphire nitridation temperature on GaN growth by plasma assisted molecular beam epitaxy: Part II. Interplay between chemistry and structure of layers

The effect of sapphire nitridation temperature on the chemistry and microstructure of the sapphire substrate/GaN interface, nucleation layer, and of the GaN epilayers grown by rf plasma assisted molecular beam epitaxy is investigated. It is found that a sapphire nitridation temperature as low as 200 °C improves the structural and optical quality of GaN epilayers. This result can be explained by the chemistry of the sapphire nitridation process, which is discussed in the framework of a model considering the competitive formation of AlN and oxynitride (NO). In particular, at 200 °C, NO desorbs from the sapphire surface, yielding an homogeneous 6 A AlN layer upon N2 plasma nitridation. This low temperature AlN template favors the nucleation of hexagonal GaN nuclei which coalesce completely resulting in a hexagonal GaN buffer layer that homogeneously covers the sapphire substrate. This condition promotes the growth of a high quality GaN epilayer. In contrast, high nitridation temperatures result in a mixed Al...

Influence of process parameters on the morphology of Au∕SiO2 nanocomposites synthesized by radio-frequency sputtering

Metal nanoparticles on oxide matrices have gained a markedly increasing consideration with regard to both scientific and applicative purposes, thanks to the possibility of tailoring the system characteristics by a proper choice of the preparation route and the processing conditions. In the present work, Au∕SiO2 nanocomposites were prepared by radio-frequency (rf) sputtering of gold from Ar plasmas on amorphous silica substrates. Particular attention was devoted to the influence of the synthesis parameters on the chemicophysical properties of the final nanosystems. To this regard, both in situ and ex situ characterization techniques were adopted. In particular, laser reflection interferometry was employed for an in situ monitoring of growth processes, while ex situ analyses were specifically dedicated to the investigation of Au∕SiO2 nanostructure, chemical composition, optical properties, and surface morphology (glancing-incidence x-ray diffraction, transmission electron microscopy, spectroscopic ellipsome...

N2–H2 remote plasma nitridation for GaAs surface passivation

A remote N2–H2 (a mixture of 97% N2–3% H2) rf plasma nitridation procedure has been developed to form a very thin (∼5A) GaN layer successful in the electronic and chemical passivation of GaAs (100) surfaces. The interaction of the plasma with the GaAs surface has been controlled in situ and in real time by spectroscopic ellipsometry. The stability of the chemical and electronic passivation is demonstrated by the nonoxidation and by the nondecaying behavior of the photoluminescence efficiency of the GaAs passivated surface over months of air exposure.