Melanie Jones

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Imaging Buried Monolayers at Atomic Resolution
using Electron Channeling

• Melanie Jones
• Thesis Proposal
• December 5, 2005

2
Outline

• Background
• Transistors
• STEM
• Imaging Single Impurity Atoms
• Preliminary Results
• Tasks

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Transistors

• Invented 1947 at Bell Labs
• Integrated circuits 1960s
• Moores Law number of transistors per unit area
  doubles every 18 months
• Transistors made smaller to increase speed and
  efficiency
• Insulating layer of SiO2
• SiO2 layers 6 atoms thick
• Thin SiO2 layers cause leakage current
• Minimum thickness of SiO2 0.7nm - 1.2nm (4-5 Si
  atoms)
• Location of single atoms will determine if a
  transistor works

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History of Electron Microscopy

• 1925 Louis de Broglie - electrons have wave-like
  properties (wavelengths ltlt visible light)

For 100keV electron probe

• 1927 Davisson and Germer and Thompson and Reid -
  demonstrated wave-like properties of electrons
• 1932 Knoll and Ruska obtained images from their
  electron microscope
• Commercial production of microscopes began in
  late 1930s

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Electron Microscopy

• Developed as tool to see atomic structures
• Wavelength of visible light limits resolution of
  light microscope
• Rayleigh criterion of light microscopy

d smallest distance that can be resolved, ?
wavelength, µ refractive index of viewing
medium, ? semiangle of collection of the
magnifying lens

• Green light d 300nm
• 100keV electron probe d 0.004nm

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Scanning Transmission Electron Microscopy
100 kV Incident
1 atom wide (0.2 nm) beam is scanned across the
sample to form a 2-D image
Electron Beam
(DE0.5 eV)
Thin SrTiO3 Layer on Si
y
x
SrTiO3
Si
20 A
Elastic Scattering "Z contrast"
Annular Dark Field (ADF) detector
Increasing energy loss
Electron Energy
Loss Spectrometer
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Imaging Single Impurity Atoms
-Intensity at atom proportional to probe
intensity at the atom with proportionality
constant that scales like Z1.7
Z-contrast -Differential contribution to the
STEM image by an atom at position ra and depth z
at probe position rp
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Imaging Single Impurity Atoms
-Differential contribution to the image from one
layer of atoms at a depth z
With specimen transmission function
-Final image intensity
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Electron Channeling

• Voyles, Muller and Kirkland 2004
• Electrons in the probe are attracted to the
  positive nucleus of the atoms
• Scattered electrons are attracted by the next
  atom in the column
• Voyles, Grazul and Muller 2003
• Plane-wave multislice simulation to show
  channeling
• When probe placed on column, intensity greatest
  on atom column with maximum 100? depth, minimum
  200?
• When probe placed between columns, probe
  channeled to columns

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Electron Channeling

• Occurs only in crystalline materials
• Voyles, Muller and Kirkland 2004
• Incident probe provides uniform electron
  distribution at exit surface becomes
  non-uniform in crystal, unchanged in amorphous
• Intensity determined by chance encounters with
  atoms - decrease in intensity at each atom
  balances the probes tendency to spread
• Simulation of amorphous material showed constant
  intensity, whereas crystal showed oscillations

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Preliminary Results - Channeling
Cross Section View
160A a-Si
4uc SrTiO3
Si
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Preliminary Results - Channeling

• First peak - channeling maximum 150?
• Thin layer of SrTiO3 most visible over 150? Si
  lattice

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Tasks

• Determine thickness over which SrTiO3 visible
• Microscope and simulation
• Find partial lattice that mimics imaging full
  lattice
• Shorter simulation time
• Research multislice simulation

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Schedule