Quantum Electronic Effects on Growth and Structure of Thin Films

1
Quantum Electronic Effects on Growth and
Structure of Thin Films P. Czoschke, Hawoong
Hong, L. Basile, C.-M. Wei, M. Y. Chou, M. Holt,
Z. Wu, H. Chen and T.-C. Chiang
Introduction When the dimensions of a nano-device
approach the de Broglie wavelength of the
devices conduction electrons, quantum effects
become increasingly important to the physical
properties of the system. We have performed
surface x-ray diffraction studies with
high-intensity synchrotron radiation to probe the
growth behavior and structure of ultrathin films
of lead on silicon. We observe an unusual
magic height effect that results in alternating
island and layer-by-layer growth as well as
significant quasi-bilayer layer relaxations in
the structure of the films. Both effects have
been linked to the electronic properties of the
system.
Quantum Oscillations in Film Structure

• Using the layer-by-layer growth mode, flat films
  with near atomic uniformity are grown and
  studied.
• Confinement of conduction electrons in the film
  leads to Friedel-like oscillations in the charge
  density.
• If present, atomic displacements should be
  approximately proportional to the first
  derivative of the charge density variations.
• The charge density variations in the metal film
  have a wavelength approximately equal to half the
  Fermi wavelength, as with Friedel oscillations.
• In Pb/Si(111), this is about 1.8 atomic layer
  spacings, so layers should relax with a
  quasi-bilayer periodicity.

Growth Behavior and Morphology

• 1st stage - a wetting monolayer (ML) forms
• 2nd stage - uniform-height (5 ML) magic islands
  form on the wetting layer and grow laterally to
  fill the surface
• 3rd stage - layer-by-layer growth beyond 6 ML

A schematic of the quantum well formed by a thin
metal film and the Friedel oscillations in the
charge density that result.
Extracted Profiles of the Pb(10L) Rod
Extended x-ray reflectivity data show multiple
interference fringes due to the Pb overlayers
for (a) a 10 layer thick film and (b) an 8 layer
thick film. The features pointed out with arrows
are halfway between the Pb Bragg peaks (at l
3.3 and 6.6) and are thus an indication of a
quasi-bilayer periodicity in the interlayer
spacings of the film. The solid curves are fits
to a model following the Friedel oscillations of
the charge density and the dashed lines are fits
assuming all the Pb overlayers have the same
thickness. The model accurately reproduces the
features of the data.
(a) Beyond 6 ML, smooth evolution of multilayer
satellite peaks implies layer-by-layer growth
(b) 1.5 6 ML, constant profile islands of
height 5?1 ML growing laterally to fill surface
Layer relaxations resulting from the model fits
to the experimental data (points). The origin
corresponds to the substrate side of the quantum
well. The solid lines are derived from the first
derivative of the charge density.
(c) Sharp onset of Pb Bragg peaks at 350s (1.5
ML) formation of Pb islands
Physical Review Letters, 91, 226801
First-principles electronic structure
calculations

• Summary
• The global energy landscape, rather then the
  local stability as emphasized in previous
  studies, is key to understanding this type of
  film growth involving alternating layer and
  magic-island formation.
• It has been shown that quantum confinement can
  have significant consequences on the structure
  and growth behavior of thin metal films.
• A free-electron model in which Friedel-like
  oscillations are present in the charge density
  can be used to quantitatively understand the
  structural relaxations in Pb/Si.
• These effects have significant implications and
  must be taken into consideration in the
  development of nanostructures that approach the
  atomic scale.

• Both freestanding and supported films show
  oscillations with period ? 1.8 ML (1/2 of the
  Fermi wave length), which is very close to 2 ML.
• Beating period ?/(2-?) 9 ML for the envelope
  function (dotted curves)
• Phase of beating pattern depends on boundary
  conditions
• A phase change for Pb on Si leads to two deep
  minima at N 1 and 6.
• Energy minimization results in spontaneous phase
  separation into a wetting layer and magic
  islands.
• At higher coverages, smooth layer growth takes
  place to minimize step and kink energies
• Physical Review Letters, 90, 76104 (2003)

Acknowledgements The UNICAT facility at the
Advanced Photon Source (APS) is supported by the
University of Illinois at Urbana-Champaign,
Materials Research Laboratory (U.S. DOE, the
State of Illinois-IBHE-HECA, and the NSF), the
Oak Ridge National Laboratory (U.S. DOE under
contract with UT-Battelle LLC), the National
Institute of Standards and Technology (U.S.
Department of Commerce) and UOP LLC. The APS is
supported by the U.S. DOE, Basic Energy Sciences,
Office of Science under contract No.
W-31-109-ENG-38.