Study of thin noble metal films by spectroscopic standard and ATR

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					1st Poster Session                                                                          Monday, June 11


                Study of thin noble metal films by spectroscopic standard
                                 and ATR ellipsometry

                                                                                                                  Monday, June 11
              N. Dmitruk1 , T. Mykhaylyk2 , A. Dmytruk3 , T. Barlas1 , O. Kondratenko1 ,
                                  N. Kotova1 , and V. Romanyuk1
           Institute for Physics of Semiconductors NAS of Ukraine, 41 prospect Nauki, Kyiv, 03028, Ukraine
                  Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
                         Center for Interdisciplinary Research, Tohoku University, Sendai, Japan

Thin semitransparent metal films are important elements of devices based on surface plasmon or
plasmon polariton excitation. Ellipsometry is a very sensitive and effective method for high accuracy
determination of thin film parameters. We investigated thin noble metal films with “mass” thickness
from 3 to 50 nm fabricated by two different methods: electroless chemical deposition and vacuum
thermal evaporation. The metal nanoparticles films (Au, Ag, Pt and Cu, respectively) were grown
on polished surfaces of (100) n-GaAs single crystals by electroless chemical deposition [1] from
an aqueous solutions of metal salts (AuCl3 , AgNO3 , PtCl4 and CuSO4 ). Using this method one
can fabricate isolated nanoparticles, self-organized ensembles, both ordered and disordered, and
also dense fractal aggregates. Nanoparticles fabricated in such way possess a good adhesion to the
surface. The particle size is in the range of 2 to 70 nm, it strongly depends, among other factors,
on the nature of the metal and state of semiconductor surface.
    Structure, morphology and composition of films were examined with atomic force microscopy,
scanning electron microscopy and energy dispersive x-ray analysis. Optical parameters of thin noble
metal continuous and nanoparticle films were determined from spectral ellipsometric measurements
(250-800 nm) in standard external reflectance mode as well as from multiple-angle-of-incidence
polaritonic (ATR) ellipsometric measurements at fixed wavelength (632.8 nm) in the Kretschman
    Chemically grown metal nanoparticle films demonstrate optical behavior which well described
by 3-component (metal, native oxide and voids) Bruggeman effective medium approximation where
bulk optical parameters of metals were used (even at thickness of 3-9 nm when metal film is
essentially discontinuous). Determined “optical” film thickness agrees well with microscopy data.
    Optical properties of thermally evaporated Au films agree with bulk gold optical parameters
for films with thickness larger than 15 nm, and agree poor for films thinner than 10 nm. While for
thick films so-called “sample effect” [2] takes place, it can be overpassed almost by post-fabrication
treatment. For discontinuous films agreement can be reached with taken into account nanoparticles
shape anisotropy or dimensionality effects.
    Gold films fabricated by thermal evaporation were subjected to vacuum annealing at temper-
atures from 100 ◦ C to 350 ◦ C which causes in non-monotonic modification of optical parameters
detected by ellipsometric measurements in ATR regime. Relations between optical parameters and
annealing temperatures are considered in respect to structural changes in nanostructured films.
    Results reported show that optical properties of metal nanoparticles films strongly depend on
fabrication technique and post-fabrication treatments. This allows optimizing them for application
in surface plasmon based photodetectors and optochemical sensors.
1. T.R. Barlas, N.L. Dmitruk, N.V. Kotova, O.I. Mayeva and V.R. Romanyuk, Superlatt. Microstruct. 38,
   130 (2005).
2. D.E. Aspnes, E. Kinsbron and D.D. Bacon, Phys. Rev. B 21, 3290 (1980).


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