Auger Spectroscopy
Auger Electron Spectroscopy (AES)
Scanning Auger Microscopy (SAM)
Incident
Electron
Ejected
Electron
Initial State
o
o
o
o
o
Auger
Electron
Intermediate State
Physical Electronics PHI 660
UIUC
Final State
Operates like an SEM at about 20-25 kV
Auger electron energies (a few keV) determined with spectrometer (serial, electrostatic)
Requires high beam currents (~10 nA) and therefore large spot sizes, so resolution is limited to ~50 nm.
Requires ultra-high vacuum
The Auger electron characteristic energy depends upon:
•The chemical element involved
• The initial energy level of the electron which eventually becomes the Auger electron
(allows chemical state of element to be determined)
Pierre Auger c.1923-25
MSE 421/521 Structural Characterization
Scanning TEM (STEM)
o Resolution limited by probe size (1 nm in JEOL 2100)
o Annular dark field (ADF) detector allows Z contrast
• scattering proportional to Z½
• inelastic scattering from atomic columns can be
imaged
Ti
o Dedicated STEMs use FEG emitters to achieve high
resolution (small probe size)
o Require ultra-high vacuums
• enables long count times without contamination
o Excellent spatial resolution possible
o No lenses below specimen
Ge-rich
ADF
O
W
O
Si
200 nm
Semiconductor device
ASI
Si-rich
Top interface of a
Ge30Si70/Si/Ge30Si70
quantum well,
IBM
10 Å
x= 0 1 2 3 4 5 6 7
MSE 421/521 Structural Characterization
Si21-xGex
Tomography
Another way of representing 3D data
Many images are obtained at incremental tilts up to ±70°
at 60°, specimen twice as thick
at 70° nearly three times as thick
(can’t go up to ±90°)
Series of images knitted together into a tomogram
software uses either fiducial (marker) or
cross-correlation (non-marker) technique
Diesel soot,
Arizona State University,
http://7starm.asu.edu/
synthetic magnetite crystals grown by Hua Xin,
Arizona State University,
Interplanetary dust particle,
Ilke Arslan and John P. Bradley,
http://7starm.asu.edu/
http://www.gatan.com/software/tomography.php
MSE 421/521 Structural Characterization
RHEED & LEED
Reflection High-Energy Electron Diffraction (RHEED)
Low Energy Electron Diffraction (LEED)
Reduce the depth from which diffraction information is obtained.
Used to obtain extremely detailed surface structural information.
RHEED:
•impact angle typically < 5°, typically 5 – 100 kV
•characterizing thin films during growth
•sensitive to surface roughness down to monolayer
•intensity oscillations correspond to single
monolayers
MSE 421/521 Structural Characterization
LEED:
•Normal incidence, 20-200 eV
•qualitative identification of surface symmetries
and 2D periodicities
•quantitative structure determination of ordered
surfaces.
XPS
X-Ray Photoelectron Spectroscopy (XPS)
Electron Spectroscopy for Chemical Analysis (ESCA)
PHI 5000 Versaprobe
• Soft x-ray photons excite (outer-shell) photoelectrons in specimen
• E = E0 – Ebinding – φdetector, can detect Z ≥ 3 (Li)
• Energy resolution of 0.50 eV for Ag 3d5/2
• Sensitive to state of bonding
• Specific to top few atomic layers (0 < E ≤ 1500 eV)
• Poor spatial resolution (100 µm2) as x-rays cannot easily be focused
• Requires ultra-high vacuum system to avoid surface contamination
MSE 421/521 Structural Characterization
Scanning Probe Microscopy
Scanning Tunneling Microscopy (STM)
71 Å “quantum corral” of Fe atoms on Cu Gerd Binnig and
Heinrich Rohrer
won the Nobel
Prize in 1986 for
the invention of
the STM
Atomic Force Microscopy (AFM)
• Probe mounted on cantilever and kept at
constant distance from surface by
interatomic forces
• Vertical displacement (< 1 Å) of cantilever
measured by laser – Atomic resolution in 3D
laser
photodiode
IBM
• Electron quantum distributions of two
conductors brought very close together
overlap
• tunneling current possible when V applied
• Current very sensitive to spacing
• As sample height varies, tip height is
adjusted to maintain current, thus height can
be measured
• Vibrations in lab must be minimised
• STM is the highest-resolution imaging and
nanofabrication technique available
•No primary beam
•No secondary effects
•No lenses
MSE 421/521 Structural Characterization
tip
cantilever
surface
Si(111)
Magnetic Force Microscopy (MFM) is a
variant in which magnetic forces, rather than
interatomic ones, dominate (tip is coated with
a magnetic film)
Magnetic domain walls on a
BaFe12O19 single crystal