Transcript Folie 1 - Graz University of Technology

Surface Science and Thin Film Physics
Adolf Winkler
Institute of Solid State Physics, TU Graz
Literature: K. Oura et al., Surface Science, Springer Verlag, 2003
(ISBN 3-540-00545-5; TU Bibliothek I 190.293)
Contents:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Experimental Background – UHV Technology
Surface Analysis I: Diffraction Methods
Surface Analysis II: Electron Spectroscopy Methods
Surface Analysis III: Probing Surfaces with Ions
Surface Analysis IV: Microscopy
Atomic Structure on Clean Surfaces
Atomic Structure of Surfaces with Adsorbates
Structural Defects at Surfaces
Electronic Structure of Surfaces
Adsorption and Desorption
Surface Diffusion
Growth of Thin Films
Atomic Manipulation and Nanostructure Formation
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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1. Experimental background – UHV Technology
Experimental surface science only possible in UHV.
Reason: The surface composition should remain unchanged (clean) during the
experiment. From kinetic gas theory it follows:
Impingement rate:
I
Molecular density:
n
p
2m kT
p
kT
Mean free path:

1
2  n 2
Monolayer formation time:   n0  n0 2mkT
I
p
Some important numbers:
Pressure (Torr)
n (cm-3)
I (cm-2s-1)


760
2x1019
3x1023
700 Å
3 ns
1
3x1016
4x1020
50 μm
2μs
10-3
3x1013
4x1017
5 cm
2 ms
10-6
3x1010
4x1014
50 m
2s
10-10
3x106
4x1010
500 km
10 hours
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Some vacuum considerations:
Consider a simple model of a vacuum chamber which is evacuated by a pump via a tube
Throughput:
The effective pumping speed is:
The pumping equation:
V
Q p
dV
dt
Pumping speed: S  Q
p
Q
Conductance:
C
p
1
1
1


S S pump C
dp
 Sp  QT
dt
The base pressure: pbase 
QT
S
The gas load QT contains: a) real leaks, b) virtual leaks (e.g. diffusion),
c) degassing (i.e. desorption)
Air exposed surfaces contain thin films of water, nitrogen, oxygen etc.
Bakeout of vacuum chamber helps to increase outgassing and hence to reach
a good vacuum in shorter time (typically 150-200 °C for 24 h)
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Ultra-High-Vacuum (UHV) Technology
Material: Take only low outgassing and temperature stable materials!
Stainless steel (304), copper, aluminum, refractory metals (Ta, W, Mo)
μ-metal, glass, ceramics, teflon, viton, capton
Do not take: plastics, rubber, zinc plated steel, brass, glue,
Pumping systems:
Rotary pumps
Cryosorption pumps
Ion pumps
Turbomolecular pumps
Pressure gauges:
Thermocouple and Pirani
Ion gauge (Bayard-Alpert)
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Typical UHV pumping system and UHV hardware
Residual gas composition
a) just after pump down of tight chamber (10-6 Torr)
b) System with air leak (10-6 Torr)
c) Properly baked system (10-9 Torr)
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Preparation of atomically clean surfaces
Ex-situ: polishing, chemical etching, boiling in solvents, rinsing in de-ionized water
In-situ:
a) Cleavage: Only for brittle material, mainly for
oxides (ZnO), halides (NaCl) semiconductors
(Si, GaAs); surface is clean, but often contains
steps
b) Heating: By electrical current, electron
bombardment or laser annealing. Mostly for
metal samples, not all contaminants can be
removed, segregation may occur
c) Chemical treatment: Heating in reactive gas,
e.g. W in oxygen at 10-6 Torr and 2000 °C
removes C by CO formation and desorption
d) Ion sputtering and annealing: Bombarding the
surface by Ar ions (~ 1 kV), highly effective,
but problems are degradation of structure;
therefore subsequent annealing. Also
preferential sputtering possible
In conclusion, cleaning may by very difficult, combination of techniques necessary.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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UHV deposition technology
In some cases one wants to deposit material on surfaces, e.g by evaporation
Impingement flux: I imp 
p(T ) A
L2 2  m kT
To achieve typical deposition rates
of 1 ML/min one needs a vapour
pressure of
about 10-4 Torr
Evaporation sources:
Thermal source (W, Ta, Mo boats)
Knudsen cells
Electron beam evaporators
SAES getters (for alkalis)
Deposition monitors:
Quartz crystal thickness monitor (Eigenfrequency of quartz depends on
mass (typ. 5-10 MHz)
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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2. Surface Analysis I: Diffraction methods
In surface science very often well ordered single crystal surfaces are investigated.
This can be done by scattering of electrons (which have wave character) on the
regular array on the surface.
Electrons penetrate only a few monolayers into the sample, therefore they are
surface sensitive (for comparison X-rays are more bulk sensitive)
Low Energy Electron Diffraction (LEED)
Experimental setup:
Consists of electron gun, sample and
hemispherical grids+ fluorescence screen.
Electron energy typ. 50 – 300 V
Sample and 1st grid on earth potential
Retarding potential on 2nd grid, V-ΔV.
Therefore only elastically scattered electrons
reach the last grid and are accelerated (5 kV) to the screen.
The observed diffraction pattern is the reciprocal lattice of the geometric surface
lattice.
150
The de Broglie wavelength of electrons is given by:
 ( A) 
U (eV )
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Reciprocal lattice and diffraction pattern
The concept of reciprocal lattice (r.l.) is useful when dealing with diffraction for
structural investigations
2D r.l. is a set of points defined by:
Ghk  ha*  kb*
b n
,
Definition of reciprocal lattice vectors: a  2 
ab
(a, b: real space unit vectors)
*
b*  2 
n a
ab
From this it follows:
a) The vectors a* and b* are in the same surface plane as the real space vectors a, b.
b) a* is perpendicular to b and vice versa
2
a* 
c) The length of the rec. vectors are:
and v.v
a  sin (a, b)
a: oblique lattice
b: rectangular
c: hexagonal
d: centered rectangular
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Diffraction or elastic scattering
For elastic scattering the laws of conservation of energy and momentum have to be
fulfilled. This is true if the momentum changes by a reciprocal lattice vector:
k  k0  Ghk
and k  k0
This holds for 3D (X-ray scattering, Ghkl ) and 2D (electron scattering, Ghk)
For the 2D case the wave vector component normal to the surface is not conserved
Due to the above equations one can construct the diffraction patterns,
Ewald construction
3D:
2D:
Labeling of
LEED spots
In the 2D case the reciprocal lattice points are actually reciprocal lattice rods
normal to the surface.
Only the wave vector components parallel to the surface change by reciprocal lattice
vectors
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Interpretation of LEED patterns
a) Sharpness of LEED patterns:
Well ordered surfaces exhibit sharp bright spots and low background intensity.
The presence of surface defects and crystallographic imperfection results in
broadening and wakening of the spots and increased background
b) LEED spot geometry:
This yields information on the surface geometry, i.e. symmetry and lattice
constants. Furthermore one can deduce information on possible reconstructions or
superstructures caused by adsorbates
To produce diffraction patterns
the surface area has to be at
least the length of the coherence
length (typically several 100 Å).
Therefore sometimes
superposition of several domains
leads to new diffraction patterns
eg. 2x2 superstructure on
hexagonal lattice leads to the
same pattern as three domains of
(2x1) superstructures
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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c) LEED spot profile:
The spot profile is determined by the perfectness of the surface. Any imperfections
broaden the spot. Reducing the domain size broadens the spot too. Even for a perfect
crystal surface there is some finite spot widths due to the finite coherence length,
determined by the energy distribution and the angular spread of the electron beam.
Regularly stepped surfaces lead to split spots. In this case the diffraction conditions
are given by two regularities, the terraces and the atomic arrangement in the
terraces.
d) LEED I-V analysis:
The spot geometry gives only information on the regular arrangement on a surface. No
information can be obtained for the local arrangement of the surface atoms
(adsorbates) to the underlying array. However, due to multiple scattering the local
arrangement of the scatterer within the surface unit cell influences the scattering.
This shows up in special modulations of the spot intensities as function of the
electron beam: Therefore I-V curves have to be measured. On the other hand, I-V
curves can be calculated by assuming a special atomic arrangement.
Usually, by a trial and error method the best fit between
experimental and theoretical I-V curves yields then the most
probable atomic positions within the unit cell. A quantitative
criterion for the fit is the R-factor or the Pendry-R factor. In
many cases the results are not unambiguous.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Reflection High-Energy Electron Diffraction (RHEED)
The disadvantage of LEED is that close to normal incidence of the beam is necessary.
Therefore, one can not control the surface geometry, e.d. during film growth (epitaxy).
For this purpose RHEED is used.
High energetic electrons
(5-100 keV) impinge under grazing
angles (1-5°) on the surface. The
fluorescence screen is just a coated
viewport of the UHV chamber.
No acceleration necessary, no
background filtering necessary.
The set up:
The Ewald construction in RHEED:
In this case the radius of the Ewald sphere is much
larger than the spacing of the reciprocal rods. Due
to the gracing incidence and the finite thickness of
the rods and the sphere the diffraction spots are
noticeably streaked.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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RHEED analysis:
The spots in the pattern correspond to the
grazing intersection of reciprocal rods with the
large Ewald sphere.
RHEED is usually used to
monitor the surface
structure during epitaxial
layer growth.
RHEED also allows to
check the growth of 3D
islands
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Gracing incidence X-Ray Diffraction (GIXRD)
Typically, XRD is a bulk sensitive technique due to the small cross section (10-6 Å2
compared to 1 Å2 for LEED), but grazing incidence (<1°) makes it surface sensitive
(total reflection, because the refractive index of X-rays is slightly smaller than
unity). The refracted wave becomes an evanescent
wave traveling along the surface within a few 10 Å.
GIXRD experimental setup:
High intense and strongly collimated X-rays are produced in a synchrotron. Light
enters the UHV chamber via Be windows. High precision sample positioning
required (0.001°).
Ewald construction:
Grazing incidence, but low wavelength of X-ray
(~1.5 Å, ~8 kV)
Typically at constant incidence angle the sample
is rotated azimuthally. Only for special conditions
scattering in grazing angle appears.
Whereas the experimental procedure is quite complicated, the data analysis is
relatively simple due to the single-scattering approximation.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Surface Analysis II: Electron Spectroscopy Methods
General remarks:
If surfaces are bombarded by electrons, secondary electrons are emitted. These
electrons carry information on the electronic structure of the surface atom, i.e.
chemical surface composition can be investigated.
Surface sensitivity due to strong scattering,
i.e. low mean free path
A typical secondary electron spectrum shows
several features:
• Sharp elastic peak at primary energy Ep
• Broad featureless peak at 0-100 eV with long
tail (true secondaries)
• Small peaks in the middle range (Auger electrons)
• Small peaks close to the elastic peak (Phonon and
plasmon losses)
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Electron energy analyzers
Several analyzers can be used to measure the kinetic energy of the secondary
electrons.
•Retarding field analyzer (RFA):
RFA
Only electrons with energy E larger than a
retard potential are collected by a
detector. The retard potential is scanned.
(LEED arrangement). (High pass filter)
• Cylindrical mirror analyzer (CMA)
Band pass filter, only electrons with a particular
energy find their way through the two slits to
the detector, due to a deflection potential
• Concentric hemispherical analyzer (CHA)
Two concentric hemispheres, double focusing
after 180°.
• 127° Cylindrical sector analyzer (CSA)
Two concentric cylinder sectors, single focusing
after 127°.
CMA
CHA
CSA
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Auger Electron Spectroscopy
Is the most commonly used method to investigate surface composition.
The principle is as follows: An impinging electron (2-10 keV) creates a core hole and
both electrons leave the surface. The ionized atom relaxes by emitting either an Xray photon (X-ray fluorescence) or by ejecting another electron (Auger electron).
For lighter elements Auger emission
is favored over X-ray fluorescence.
Three electrons are involved in the
Auger process, therefore H and He
do not produce Auger electrons.
The energy of the Auger electrons
depends on the energy levels of the atom:
EKL1L2,3  EK  EL1  EL2,3  
However, this is a rough estimate, because the final state is an ion and the levels
may shift compared to the neutral atom.
Φ: Work function (energy needed to bring an electron from the Fermi level to the
vacuum level)
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AES experimental set-up
Standard equipment consists of:
Electron gun
Energy analyzer
Detector
Data processing unit
CMA
RFA
Typically, the relatively small Auger
signals N(E) are superimposed on a
large background. Therefore the
spectra are usually taken in the
derivative mode, by applying a
modulation voltage on the analyzer and
detecting with a lock-in amplifier.
dI
d 2I k 2
I ( E0  k sin t )  I 0 
k sin t  2
cos 2t
dE
dE 4
Taking the amplitude of the first
derivative () of a CMA signal, as well as
the second derivative (2) of an RFA
signal, yields dN/dE,
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AES analysis
Each atom has different electron energy levels and therefore yields different Auger
electron energies. This is used to get elemental characterization. Although chemical
shifts lead to changes in the Auger energies, AES is usually not used to get chemical
information, due to the three electrons involved.
Auger spectra for all elements are compiled in an
Auger atlas
Quantitative analysis is in principle possible but many unknown quantities involved:
I  I P (1  r )T  f (, z, ,  )
: Attenuation length
z: Escape depth
,: Azimuth and polar angle
IP: Intensity of primary beam
: Ionization cross section
: Auger transition probability
r: Backscattering factor
T: Transmission probability of analyzer
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Electron Energy Loss Spectroscopy
Inelastic scattering events might lead to well-defined energy losses, covering a wide
energy range from 104 to 10-3 eV:
Core level excitation: 100 – 104 eV (CLEELS)
Plasmon and interband excitation: 1 – 100 eV (EELS)
Phonon and adsorbate vibration excitation: 10-3 – 1 eV (HREELS)
a) Core Level Electron Energy Loss Spectroscopy (CLEELS)
The energy of the inelastically scattered electron is:
ES  EP  ( EK   C )
The loss peaks are typically much smaller than Auger
peaks, therefore one measures the second derivative.
The loss energy defines the energy levels and CLEELS
can therefore be used for elemental identification.
As the fine structure of the spectra depends on the
density of states (DOS) of the final (empty) states it
can be used to identify the unoccupied DOS.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Electron Energy Loss Spectroscopy (EELS)
This term is used generally for all ELLS but in particular for EELS in the range of a few
ten eV, i.e. for interband and plasmon excitations.
A plasmon is a collective oscillation of electron density
in the bulk and its energy is quantized:
In many cases there exists also a surface plasmon,
localized at the surface, its energy is:
1/ 2
 4ne 
 , E  
 P  
m


P
2
S 
2
EELS spectra are recorded either as N(E) or d2N(E)/dE
EELS of Al, showing multiple losses
of bulk and surface plasmons
EELS of SiO2 layer on Si.
Use of different primary energies
(penetration depth) allows depth profiling
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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High-Resolution Electron Energy Loss Spectroscopy (HREELS)
Losses due to phonon excitation and adsorbate vibrations are very small (meV),
therefore the experimental identification is difficult.
Monochromatization of the primary
beam (typ. 10 eV with ΔE~5meV) is
necessary. Cylindrical sector analyzers
are used as monochromator and
analyser (Ibach type).
Most frequently HREELS is used to
measure adsorbate vibrations.
Identification of the adsorbate
species, the adsorption site and the
spatial orientation of the adsorbate is
possible.
In specular geometry only
vibrations normal to the
surface can be detected, in
off-specular direction also
parallel vibrations.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Photoelectron Spectroscopy (PES)
If an atom absorbs a photon, a photoelectron will be emitted. The kinetic energy is:
Ekin    Ei  
Depending on the energy (wavelength) of the photon used we distinguish between:
XPS (X-ray photoelectron spectroscopy) or ESCA (el. spectr. chem. anal.)
(E = 100 eV – 10 keV, wavelength 100 to 1 Å)
UPS (ultraviolet photoelectron spectroscopy)
(E = 10 – 50 eV, 1000 to 250 Å)
N(Ekin) reflects the density of states below
the Fermi level (valence band and high lying
core levels).
At low kinetic energy emission of inelastically
scattered electrons (secondaries) is
superimposed.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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PES Experimental set-up
a) For XPS X-ray tubes with Mg or Al anti-cathodes are used (E = 1253.6 eV and
1486.6 eV, respectively, line-widths: 0.8 eV), sometimes monochromators are
used to decrease the linewidth (0.2 eV) and suppress satellites.
b) For UPS He gas discharge lamps are used. Two intense lines can be generated
(21.2 eV (He I) and 40.8 eV (He II), depending on the discharge conditions. The
line-width is very small (3 meV for He I and 0.17 meV for He II).
c) A modern alternative is the use of synchrotron radiation. Accelerated
electrons in a ring produce a continuous radiation from a few eV to several keV.
With a monochromator one can select any required energy and tune it. The light is
of high intensity and stability, 100% linear polarized and strongly collimated.
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X-Ray Photoelectron Spectroscopy (XPS)
In XPS core levels are excited, the spectrum reflects the energy levels of the atom.
Therefore elemental characterization is possible. In addition to the photoelectrons
there is a number of additional features in the spectrum, like continuous
background, Auger peaks, plasmon losses.
Furthermore, the cross section for excitation
may be different for individual levels.
Valence band electrons are only weakly excited.
Qualitative evaluation of XPS spectra involves
the comparison of spectra in the XPS-atlas.
Quantitative evaluation can be done similarly
to that described for AES. In general this
method is more accurate for XPS, because
less electrons are involve.
Ni
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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High resolution XPS can yield a number of
additional information: In particular the fine
structure of the core levels, i.e. spin-orbit coupling
can easily be seen. This splitting increases with
binding energy.
Furthermore, slight changes in the binding energies
due to different chemical environment can be
measured (typically 1 – 10 eV): Chemical shift.
Different oxidation states will have different
chemical shifts.
The ability to investigate chemical composition is
the reason for the name: ESCA
The atomic environment on the surface normally
differs from that in the bulk. Therefore, bulk and
surface features are observed simultaneously. The
surface sensitivity can be enhanced by grazing
incidence light, and/or increasing the detection
angle.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Ultraviolet Photoelectron Spectroscopy (UPS)
This method mainly generates photoelectrons from the valence band and weakly
bonded core levels (DOS below the Fermi level).
There are two types of UPS, angle integrated and angle resolved UPS,
In the angle integrated UPS typically the retarding field analyzer is used and
yields the DOS.
In the angle resolved mode (ARUPS) one takes a hemispherical or cylinder sector
analyzer. With this technique one can determine the band structure E(k) of the
electrons in the bulk and the surface near region.
2
ex2
ex2
The measured kinetic energy can be written as:
 (k  kII )
ex
E

k is the wave vector of the emitted photoelectron
kin
2m
in the vacuum.
When the electron passes though
the solid-vacuum interface, only the parallel component
of the wave vector is preserved.
Therefore the parallel k-vector in the solid can be
determined by detection angle and the measured
electron energy.
kIIex  kIIin  Ghk
kIIin  k ex sin  
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
2m Ekin
sin 
2

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In the experiment the dispersion curve (band structure) is restored by measuring
the photoemission spectra at different polar angles but with fixed azimuth.
surface state
surface projected
bulk states
Unfortunately, UPS is not only surface but also to some extent bulk sensitive.
Therefore, contributions of bulk and surface electronic states are observed.
There are several features which can be used to differentiate between these.
For surface states there is only one dispersion curve, independent of the photon
energy.
Surface states reside in the band gap of the bulk states.
Surface states are very sensitive to surface treatments and adsorption.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Surface Analysis III: Probing Surfaces with Ions
The most widely used techniques are the following:
• Ions Scattering Spectroscopy (ISS)
(ions elastically scattered from the surface are energy analyzed)
• Low-energy ion scattering spectroscopy (LEIS), (1-20 keV)
• Medium-energy ion scattering spectroscopy (MEIS), (20 – 200 keV)
• High energy ion scattering (HEIS) or Rutherford backscattering (RBS)
( 200 keV – 2 MeV)
• Elastic Recoil Detection Analysis (ERDA)
(target atoms or ions elastically recoiled by primary ions are energy analyzed)
• Secondary Ion Mass Spectroscopy (SIMS)
(ions sputtered from a surface by a primary beam are mass analyzed)
The major application concerns elemental composition and atomic structure of surfaces.
Structural analysis is based on real space considerations.
Mainly short range arrangements of neighboring surface atoms can be investigated
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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General Principles
a) Classical binary collisions
In a first approximation, ion scattering
can be described by elastic binary hardsphere collisions
Due to the law of conservation of
energy and momentum one obtains the following relations for the scattered
atom (E1) and the recoiled atom (E2):
 m cos   m 2  m 2 sin 2 
1
2
1
1
E1  E0  1

m1  m2





2
4m1m2 cos2 2
E2  E0
(m1  m2 )
In the case of 90° or 180° scattering detection the equation for E1 simplifies to:
90°:
m  m1
E1  E0 2
m2  m1
180°:
 m  m1 

E1  E0  2
m

m
1
 2
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The hard sphere model describes the energetics, but
ignores the particle interaction and does not describe
the true trajectories and scattering probabilities. In
fact Coulomb or Coulomb like repulsion between the
nuclei describes the physics.
The probability that an ion is scattered over a certain angle is given by the
differential cross section (Rutherford formula):
2
2




d  Z1Z 2e 2 
1
m
4
1


 
1  2  sin (1 / 2) 
4

d  4 E0  sin (1 / 2) 
 m2 


for m1 « m2
This shows:
The cross-section is proportional to Z2
Forward scattering is much more probable than backward scattering
However, energy separation is higher at higher angles
So one has the choice between resolution and sensitivity.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Shadowing and blocking
When a parallel ion beam impinges on a target atom,
the trajectories are bent due to the repulsive
forces, leading to so called shadow cones. These
cones depend on the primary energy and the
electronic charge of the involved particles
There is a critical angle c above which the
scattered projectile can hit a second atom.
An additional phenomenon of shadowing is the
blocking. A blocking cone is formed behind
blocking atoms.
This blocking can be nicely seen in the experiment,
e.g. backscattering of 150 keV protons from a
W(100) crystal
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Channeling
When an ion beam is aligned along a high symmetry of a single crystal, most of the
ions can penetrate deep into the crystal (thousands of Å). This is due to the fact
that the shadow cones are small for high energetic and light ions (e.g. 1MeV He+).
During their way through the crystal electronic
interaction leads to a continuous energy loss:
electronic stopping power. For 1 MeV He+ in Si it
is about 60 eV per monolayer.
Sputtering
Impinging ions may produce a number of recoiling atoms and in form of a cascade
process some sample atoms may be ejected from the surfaces: sputtering
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Sputter yield
The number of sputtered atoms per impinging ions depends on the primary energy,
the mass of the ions and the target atoms and the angle of incidence.
The maximum yield is at about
30 keV. At higher energies ion
implantation is prevalent.
The sputter yield also
increases with increasing angle
The application of sputtering is manifold:
a)
b)
c)
d)
Detection and identification of ions in the SIMS technique
Combined sputtering and surface analysis by AES or XPS for depth profiling
Sputtering for thin film production
Sputtering for surface etching
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Ion induced electronic processes
Ions impinging on a surface may be neutralized, may ionize target atoms or may
induce electron emission. This can be due to kinetic emission or potential emission.
Kinetic emission:
Part of the kinetic ion energy can be transferred to kinetic energy of electrons.
This takes place only for high energy (MeV) ions with high probability
Potential emission:
This takes place by neutralization of low energy ions (10-100 eV). Several processes
may take place:
Resonance neutralization (RN)
Resonance ionization (RI)
Quasi-resonant neutralization (QRN)
Auger neutralization (AN)
Auger de-excitation (AD)
These processes are used in a technique
called
Ion Neutralization Spectroscopy (INS)
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Low-Energy Ion Scattering Spectroscopy (LEIS or ISS)
Extreme surface sensitive, due to large cross section and shadow cone radius (~Å).
Major application is surface composition and structure. The energy loss spectra in
LEIS give directly the composition.
Quantitative evaluation is complicated due to:
He+
Ion neutralization
Unknown scattering potential
Multiple scattering
Less neutralization takes place for alkali ions, because Ei < 2Φ.
(Alkali ISS)
Another method is to measure both ions and neutrals, with
TOF spectrometer. With this method one can switch between
ion and total yield measurement
Quantitative structural analysis is best done in the
impact-collision geometry (ICISS), (180° geometry).
Angle dependence of ion yield gives structural information.
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Rutherford Backscattering (RBS) or High-energy ion scattering (HEIS)
The basic feature of this method are:
• Small cross section and small shadow cones (<10-2 Å2)
• Low neutralization probability
• Negligible multi-scattering effects
• Simple Coulomb interaction takes place
RBS is bulk sensitive, but also surface sensitive
for highly aligned configurations. In case of an ideal
lattice an aligned beam sees only the surface atoms,
but thermal vibration increases the backscattered
flux.
Qualitatively the following information can be
gained from RBS:
The surface peak represents the surface atom density
Lateral relaxation of the first layer changes the
surface peak height.
At non normal incidence relaxation normal to the
surface can be investigated.
Adsorbates show up as new peaks in the RBS spectrum.
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Quantitative determination of surface structure
can be obtained from angular dependence of
surface and bulk peaks.
Scattering from the second layer is blocked
under special angles determined by the atom
positions. If surface relaxation occurs this angle
differs for the surface and bulk peaks,
Thin Film Analysis
Ions scattered in deeper layers have lost energy
in two forms:
Continuous energy loss (electronic stopping
power) during inward and exit path.
Discrete loss at the scattering event (nuclear
stopping power) as a function of the mass ratios.
Hence the scattering spectrum for ions from
different thin films with different masses has
special features.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Elastic Recoil Detection Analysis (ERDA)
In this case recoiled target particles are energy analyzed. The same physics and
technology as for ISS is used. Sometimes both techniques together are termed
Scattering and Recoil Spectroscopy (SARS).
The advantage of this technique is that light particles, in particular H can be
detected. Again surface composition and structure can be investigated.
As an example the adsorption geometry of hydrogen on Si(100) can be measured.
Two energy loss peaks in the spectrum, which show different angular dependence,
are caused by a direct recoil and a surface recoil process. This allows to determine
the bond angles.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Secondary Ion Mass Spectroscopy (SIMS)
Incident ions sputter particles from the surface, which are then mass analyzed.
The sputtered particles can be neutral, positively or negatively charged, or clusters.
The ratio of these individual particles strongly depends on the surface (matrix effect).
SIMS is in principle quantitative but many
parameters are not under control.
Ii  I pCi Si , ji, jT
Ip: Primary ion current
Ci: Volume concentration of species I
Si,j: Sputter yield
i,j: ion yield
T: transmission of mass spectrometer
The ion yield depends on species, primary ion and matrix. Positive ion yield is
favored for species with low ionization potential (e.g. K, Rb, Cs) and for negative ion
yield vice versa (e.g. F, Cl, O).
The ion yield can also be influenced by the primary ions: electropositive particles
(Cs) leads to enhanced negative ion yield of the surface species and electronegative
particles influence it vice versa. This is due to a change of the work function by
adsorption of these particles.
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There are two modes of SIMS:
• Static SIMS
In this case the primary ion current is very low (10-10 A/cm2). The sputter rate is only
a fraction of a monolayer per hour. This is a typical surface analytical method.
Destruction of the surface is minor
• Dynamic SIMS
In this case the primary ion current is high (10-5 A/cm2). The sputter rate is several
monolayers per second. Therefore depth profiling can be performed.
However, the depth resolution may be affected by atomic mixing, selective
sputtering, micro-roughening and uniformity of the primary beam.
Typical depth profile of a Sb modulation
doped silicon multilayer grown by molecular
beam epitaxy.
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Surface Analysis IV: Microscopy
Field Electron Microscopy (FEM)
A sharp metallic tip opposes a conducting fluorescent screen. A high voltage
between these two electrodes (1-10 keV, tip radius ~100 nm) leads to strong
electric fields (1 V/Å) at the tip and hence to electron emission. The electron
current depends on the local work function. The magnification is just given by the
ratio between tip radius and tip-screen distance. magnifications of 106 are possible.
Resolution is about 20 Å.
Close packed surfaces have
lower work function than
stepped surfaces
W(110)
FEM is limited to materials which can be
fabricated as sharp tips, cleaned in UHV, and
withstand high electric fields. It is restricted
to W, Mo, Pt, Ir. Changes of the work function
by adsorption can be studied.
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Field Ion Microscopy (FIM)
Apparatus is similar to FEM. In this case the tip is positively charged and a
working gas (He, Ne) is admitted to the chamber. The tip is usually cooled to 70 K.
The principle is the following: Gas atoms in the
vicinity of the tip are polarized and attracted by the
surface. There they are ionized and accelerated to
the screen. Therefore each ion represents one
surface atom. The resolution is about 1Å, i.e. atomic
resolution is possible.
This method is again used mainly for refractory
metals.
In addition to field ionization also field evaporation
can take place: At higher voltages the surface atoms
itself can be desorbed in the high local field (2-5
V/Å).
With FIM one can study adsorption/desorption,
surface diffusion of adatoms and clusters, adatomadatom interaction, step motion, equilibrium crystal
shape etc.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
W(110)
21 K
He-H2
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Transmission Electron Microscopy (TEM)
The principle is the same as for optical microscopy, but using electron lenses. Due to
the small de Broglie wavelength of high energetic electrons (100 keV  Δ ≈2Å) the
resolution is much higher. Due to the limited penetration depth the samples should
be very thin: about 100 - 1000Å.
In classical TEM metals were deposited on alkali
halides, covered by a thin film of carbon and
then the alkali halide substrate was removed by
dissolving in water. In this way nucleation,
growth and coalescence of metal islands can be
studied. Furthermore, the surface structure of
alkali halides can be studied by this step
decoration method.
NaCl cleavage surface decorated with Au
Another method to obtain thin samples is by
mechanical cutting, electrochemical etching and
ion milling. Cross section of hetero-structures
with atomic resolution can be studied.
Si/TbSi2/Si double heterostructure
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Low-Energy Electron Microscopy (LEEM)
In this technique low energy electrons (100 eV) which were reflected or refracted
are used to form an image of the surface. The resolution is several 10 Å
The primary beam leaves the e-gun with high energy (10 keV), it
passes several lenses and a deflection prism and is decelerated in
front of the surface. The reflected beam is again accelerated and
deflected onto a screen.
By choosing the specular (0,0) beam one gets a bright-filed image.
Images taken with any other beam lead to dark-filed images.
On Si(111) superstructures of
(1x1) and (7x7) exist. This
leads to different intensities
of the (0,0) beam
On Si(100) reconstructions of (1x2) and (2x1) exist. Using either
the (1/2,0) or the (0,1/2) beam leads to the images in b and c.
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Scanning Electron Microscopy (SEM)
A primary electron beam with 1 – 10 keV is focused to 1-10 nm and scanned over the
surface. The secondary electron yield or other quantities are used to modulate a
cathode ray tube (CRT). SEM is basically used to investigate surface topography.
SE: Using the secondary electrons (<50 eV) is the
most frequent method, it gives the topography.
BSE: Inelastic backscattered electrons depend on
the atomic number and detection of these allows
also elemental mapping.
AES: Detection of Auger peaks also allows
elemental mapping (Scanning Auger)
Sample current can also be measured as function
of the scanned primary beam. No detector
necessary.
X-rays, which are produced can also be used to
modulate the CRT. Is more bulk sensitive
Electron beam induced current (EBIC) can be
measured on semiconductors, pn-junctions.
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Scanning Tunneling Microscopy (STM)
In this case a sharp metal tip (W, Pt, etc.) is scanned over a surface in close proximity
(few Å). The front atom of the tip and the surface atoms are so close that in case of a
potential difference a tunneling current is measured. Scanning is performed via piezo
ceramics in three axes and with a feedback loop the distance between tip and surface
is regulated. This allows the determination of the surface corrugation on the atomic
scale.
The tunnel current is given by:
j
D(V )V
exp(  A1/ 2 d )
d
V: bias voltage
D(V): electron density of states
d: dip-surface distance
Φ: effective barrier height
Due to the strong d dependence of j the vertical
resolution is about 0.01 Å, the lateral resolution
is about 1 Å.
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STM is not primary sensitive to atomic positions but rather to the density of
electronic states (DOS). With positive bias voltage one probes the DOS of the
occupied states below the Fermi level, in case of negative voltage the DOS of empty
states are probed.
There are three modes of STM operation:
• Constant current mode
• Constant height mode
• Scanning tunneling spectroscopy (STS)
In the case of STS the bias voltage is modulated at
any point of the surface, or maps are made with
different bias voltage. The quantity (dI/dV)/(I/V)
corresponds to the DOS.
This technique is also referred to as current-imaging
tunneling spectroscopy (CITS).
With STS one can get local chemical information.
However, in general the evaluation is not
straightforward, due to influence of tip-DOS and
unknown tunneling transmission between different
electron orbitals.
STS on a reconstructed Si(111)(7x7) surface
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Atomic Force Microscopy (AFM)
In AFM the force between tip and sample is measured.
The interatomic force between tip and sample deflect a
cantilever which carries the tip. In this case the
morphology of conduction, semiconducting and insulating
sample surfaces can be measured.
There are several methods to measure the deflection of
the cantilever:
Use of an STM to measure the cantilever deflection
Use of optical interferometry
Deflection of a laser beam
Measurement of capacitance between cantilever and a
second electrode.
Deflections of 10-2 Å can be measured.
The AFM can be used in three modes:
• Contact mode
• Non-contact mode
• Tapping mode
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AFM contact mode:
The tip-sample distance is only few Å, i.e. in soft contact.
The spring constant of the cantilever is small so that can
bend and follow the surface contours.
A constant-height mode or a constant-force mode can be
applied.
AFM non-contact mode:
The sample-tip distance is several 10 Å. The tip is affected
by the weak attractive forces. In this case the cantilever
is vibration and the resonance frequency changes due to
the interaction with the sample. If the frequency is kept
constant by a feedback loop the tip follows constant force
gradients.
AFM tapping mode:
Non-contact AFM
Si(100)(2x1)
In this mode the tip is also vibration and closer to the
sample so that it touches the surface.
This mode is advantageous for surface with high
topographical corrugation.
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Atomic Structure of Clean Surfaces
Typically, the arrangement of surface atoms differs from that of bulk atoms because
of the absence of neighboring atoms on one side.
There exist two different types of rearrangements: relaxation and reconstruction
Relaxation:
a) Normal relaxation – modification of interlayer
spacing
b) Parallel relaxation – lateral shift of top layer
Reconstruction:
a) Conservative reconstruction – number of atoms
conserved
b) Non-conservative reconstruction – number of
atoms in the reconstructed region is changed
For metal samples mainly relaxation takes place
For semiconductors, but also for some noble metals,
reconstruction appears.
The driving force is the minimization of surface
energy. This is done by saturation of the dangling
bonds and also by charge transfer.
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Selected examples of relaxed and reconstructed surfaces
Al(110):
Only normal relaxation takes place
Pt(100):
The four-fold symmetric net plane
reconstructs on the surface to a quasi
hexagonal structure: Increase of atomic
density but mismatch with underlayer.
The result is close to a (1x5) superstructure.
Pt(110):
This surface reconstructs to a missing
row structure, forming a (2x1)
superstructure. This results in a
faceted surface with small (111) planes.
These planes have very low surface
energy.
Evaporated Pt on
Pt(110) (1x2) occupy the
troughs
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Graphite surface:
The (0001) surface of graphite preserves the
non-reconstructed bulk shape, there is also
nearly no normal relaxation.
Si(100):
This surface silicon bonding to form dimers. The
final structure shows (2x1) periodicity, Actually, the
dimers are in addition buckled by 18°, which finally
leads to a c(4x2) superstructure.
Si(111):
At room temperature this surface shows a
metastable (2x1) reconstruction, which changes
irreversibly to a (7x7) above 400 °C. This
structure is explained by the DAS (dimeradatom-stacking fault) model.
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Atomic Structure of Surfaces with Adsorbates
Depending on the interaction strength between adsorbate and substrate, adsorption is
divided into Physisorption (weak interaction) and Chemisorption (strong interaction):
Physisorption: Interactions are of Van-der-Waals type. Adsorption energies ~ 10-100
meV. Adsorbate does not influence the substrate structure. Example: noble gas
adsorption on metal surfaces at low temperature, <70 K.
Chemisorption: Adsorbate forms chemical bonds (covalent or ionic) with substrate atoms.
Binding energies in the order of 1-10 eV. Adsorbate changes chemical state and may
influence the electronic and geometric structure of the substrate. Typically for metal
atom and reactive gas adsorption on metal and semiconductor surfaces. Due to mutual
interaction the adsorbate very often forms superstructures.
Coverage of Adsorbates:
Definition of monolayer (ML): 1 ML corresponds
to 1 adsorbate atom or molecule for each 1x1 unit
cell of non-reconstructed substrate surface.
Coverage of substrate atoms:
non-reconstructed surfaces
conservatively reconstructed surfaces
non-conservatively reconstructed surfaces
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Phase diagram of adsorbates
The phase diagram shows the phase occurring regions in coverage-temperature
coordinates. Experimentally, the structures can be measured by LEED or RHEED or
STM. The corresponding coverage by quantitative AES, XPS or with a quartz crystal
monitor.
Experimentally, the coverage can be changed at constant temperature (A, C), or the
temperature can be changed at constant coverage (B).
Crossing of phase boundaries corresponds to transitions from one structure to another.
At the phase boundary coexisting domains of two phase coexist.
The boundary in the phase diagram corresponds to the adsorbate coverage where the
phases occupy about the same area fraction.
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The phase transitions may either be:
reversible transitions or irreversible transitions.
Another subdivisions includes:
order-order transitions and order-disorder transitions
Transitions are also subdivided into:
first-order transitions and second-order transitions
First-order tr.: Internal energy and density change abruptly, they are connected with a
heat of phase transition, Examples are: evaporation, melting, sublimation,
recrystallisation.
Second-order tr.: No change of internal energy or density, No heat of phase transition,
but abrupt change of specific heat, Examples are: paramagnetic-ferromagnetic
transition, liquid helium to supraliquid helium.
Some examples of phase diagrams:
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Various adsorbate structures:
3x3 structures on fcc(111) metal surfaces
There are substitutional and adatom-type
3 structures., The coverage is 1/3 ML.
Examples for substitutional: Sb on Ni(111),
Pt(111); K,Na on Al(111)
In case of adatom different sites can be
populated: on-top, fcc-hollow, hcp-hollow,
bridge site.
Examples: Cl-Ag(111), CO, S-Ni(111), H-Ni(111)
Ni(110) (2x1)-CO
At low temperature the coverage is 1 ML. The
molecules are adsorbed in bridge sites with the
C atom pointing to the surface. The molecules are
alternatively tilted by 19° leading to the (2x1)
structure. This can be seen in STM.
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(2x1), (1x1) and 3x1) phases for H/Si(100)
H2 does not dissociatively adsorb in Si. But if H atoms are dosed, the H-Si interaction
is quite strong. Three phases are formed, depending on temperature and exposure.
Si(100)(2x1)-H
At 400 °C and low exposure 1 ML forms. H forms
monohydride with the Si dimers.
Si(100)(1x1)-H
At room temperature 2 ML can adsorb. The dimers
are broken and dihydride forms with each Si atom. In
this case many defects exist, due to formation of
other phases, trihydride, gaseous silane, and
monohydride (surface etching)
Si(100)(3x1)-H
This mixed structure is formed at intermediate
temperature (110 °C). The structure consist of
alternating monohydride and dihydride species.
The coverage is 1.33 ML.
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Structural defects at surfaces
Ideal surfaces do not exist in reality, there are always defects on the surface. There
are:
• zero-dimensional or point defects: adatoms, vacancies, dislocation emergence points,
kink and step adatoms, step vacancies.
• one-dimensional or line defects: step edges, domain boundaries
Most of the defects can be illustrated in the terrace-step-kink (TSK) model
One can distinguish between kinetically stable
defect (points of dislocation emergence on the
surface, edge and screw dislocations) and
thermodynamically stable defects (adatoms
vacancies etc.).
The relative number of defects depends on
their formation energies and the temperature.
The formation energies are primarily
determined by the number of nearest
neighbors.
Site
Energy
1st
2nd
3rd
Adatom
(A)
1
4
4
Step adatom
(LA)
2
6
4
Kink atom
(K)
3
6
4
Step atom
(L)
4
6
4
Surface atom
(T)
5
8
4
Bulk atom
(B)
6
12
8
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Since the formation of defects is a thermally activated process, and the formation
energy is the difference of the bond energies of a defect and the bond energy of a kink
atom (kink atoms are special because their energy is half of a bulk atom, which is equal
to the sublimation energy, it is taken as reference).
 GA 
, with GA   A   K
kT


e.g. the equilibrium number of adatoms is: nA  n0 exp 
Steps, Singular and Vicinal Surfaces, Facets
A low indexed step-free surface is a singular surface. Surfaces with a small angle  are
vicinal surfaces. Thy are built of terraces and monoatomic steps
Energetics of vicinal steps:
 ( )  ( L / a) sin    (0) cos
with (0): surface energy of terrace
L : surface energy of step
a: atomic height
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One-, two- and three dimensional plots of the surface energy for a simple cubic lattice,
taking into account only nearest neighbor bonds (-plot).
Taking also next nearest neighbors:
Wulff construction: Make planes perpendicular to each point in the -plot, The
inner envelope of the planes will yield the equilibrium shape of the crystal.
In some cases the vicinal surfaces may be unstable, then
faceted surfaces appear.
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Some examples of surface defects
Adatoms:
Ag adatoms on a Si(111)(3x3)-Ag surface.
1.7 ML of silver evaporated at 500 °C. Formation
of the superstructure with 1 ML + 0.7 ML in form
of single atoms and trimers, seen in STM at 7 K.
Vacancies:
Missing dimers on Si(100)(2x1) are the major
structural defects on Si(100). Several types can be
observed: single, double and complex dimer
vacancies. Increasing number of defects may
arrange to new superstructures
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Anti-site defects:
In compound materials anti-site defects can
occur, i.e. normally occupied sites are occupied
by the other component.
Example: GaP(110), STM image showing
the P atoms
Substitutional defects:
In on Si(111) with 1/3 ML form a (3x3)
superstructure. Some of the in atoms can be
missing (vacancies, V) or replaced by Si atoms
(substitutional, S). These two types can be
distinguished in STM by negative and positive
polarity imaging.
Dislocations:
Screw dislocations mainly in
dielectric crystals influence the
growth and etching of the crystal.
e.g. NaCl, TEM micrograph with
gold decoration Note that the
left dislocation has steps of
monoatomic height, but the right
has two-atomic height.
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Domain boundaries:
Antiphase domains: If different
domains nucleate with the same phase, but
occupy different sites in the unit cell.
Example: Si(111)(3x3) In
Orientation domain:
They form if the symmetry of the phase
is lower than that of the substrate.
Example: Chain-like Si(111)(4x1)-In
Weakly incommensurate phases:
In the case of small layer-substrate misfit
often regular superstructures appear.
(Frenkel-Kontova/Frank-van der Merve model)
Depending on b<a or b>a, heavy or light
domain walls appear.
These superstructures can be seen as
Moiré structures
Pb on Cu(111)
Ga on Ge(111)
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Steps:
Vicinal surfaces comprise a lot of steps.
In case of vicinal Si(100) monoatomic
steps appear, with alternating dimer
orientation. The step energies are
different for both cases (SA, SB).
In case of SB much more kinks appear.
For higher vicinal angle also double
steps show up.
Facetting:
Depending on the step and terrace energies
stepped surfaces may change to faceted
surfaces. This can also be induced by
adsorption. It also depends on temperature.
Example: Faceting of a Si(111) surface misoriented
by 10°.
Example: Gold induced faceting of a vicinal
Si(100) surface
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Electronic Structure of Surfaces
Breaking the 3D periodicity at the surface leads to strong modification of the
electronic structure, i.e. redistribution of the charge density and formation of new
electronic states (surface states). This influences e.g. work function and surface
conductivity.
Electronic structures can be calculated by several methods: Density functional theory
(DFT), Hartree-Fock theory etc. But many simplifications are necessary. A simple model
is the Jellium model: The ion cores are replaced by a uniform positive charge
distribution. The electron density can be calculated by DFT.
a) A spill over of the electrons appears, creating an
electrostatic dipole
b) b) The oscillation of the electron density inside are called
Friedel oscillations. There periodicity is π/kF, with kF =
(3πn)1/3 (kF: Fermi wave vector, n: mean bulk electron
density).
Similar oscillations due to defects on
surfaces can be seen in STM:
The electrostatic dipole
determines the work function.
This model can be applied to
simple metals
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Electronic surface states:
The solution of the Schrödinger equation for a 1dim. problem near a surface leads to:
bulk states, which are periodical in the bulk and
decay exponentially to the vacuum, and:
surface states, decaying exponentially both in to the
bulk and the vacuum.
The bulk states show a dispersion, whereas the
surface state have a distinct value (normal to
surface). But, surface states show a periodicity
parallel to the surface and have therefore also a
dispersion in this direction. All bulk states can be
projected onto the surface, with there k vector
components parallel to the surface (surface
projected bulk bands).
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Some frequently used terms:
Shockley state:
Arises as a solution of the Schrödinger equation in the nearly free electron model. It
derives just from the crystal termination. This approach is appropriate from normal
metals (Al, K,..) and narrow-gap semiconductors.
Tamm state:
Results from the tight-binding model, using orbital like wave functions. Applies to
localized electrons, as for transition metals and semiconductors and insulators. Is often
accompanied by surface reconstructions and dangling bonds.
True surface states:
When the energy lies in the gap of the projected bulk states
Surface resonances:
When the energy lies in the band of the projected bulk states
Intrinsic surface states:
All as described so far. They are just due to the ground state
of the electronic structure of a well ordered surface.
Example of surface dispersion
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Extrinsic surface states:
Are related to surface imperfections, like steps, defects, vacancies. There wave function
is localized to the defect and no periodicity along the surface exists.
Image potential surface states:
Electrons on a surface induce a positive image potential in the solid. If the projected
bulk states present a gap the electron cannot penetrate the bulk. It is confined
(trapped) in front of the surface. Its energy levels are described by Rydberg-like series.
Methods to investigate surface states:
Angle resolved ultra violet photoemission (ARUPS):
Probes the filled states. Measurement of photoelectron kinetic energy as function of
angle.
k-resolved inverse photoemission (KRIPS):
Probes the unfilled states, by sending electrons to the surface and measuring energy of
the emitted photons
Scanning tunneling microscopy/spectroscopy (STM/STS):
Probes either filled or unfilled states, depending on the polarity between tip and
sample. STM probes directly the DOS. Measurement of Fridel oscillations as function
of tip voltage directly yields the dispersion relation (E(kll)).
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Surface conductivity
Due to the changed electron states at the surface also the conductivity is changed.
If a surface state dispersion E(kll) crosses the Fermi level, the surface is metallic.
If the Fermi level is in the gap the surface is semiconducting, if the gap is wide is an
insulator. The presence of surface states leads to bend bending:
In addition to bend bending surface charge layers
appear, leading to a built-in potential. This influences
the surface conductivity and also the work function.
Three contributions are associated with surface
conductivity:
• surface states
• space charge layers
• bulk conductivity
With the four point probe surface conductivity can be
measured. Separation of true surface conductivity
from bulk contribution sometimes very complicated.
Scattering on surface defects also contribute to
surface resistivity.
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Work function
There are two contributions to the work function:
• Bulk contribution: The energy difference between the Fermi level and the vacuum level.
• Surface contribution: Due to electrostatic dipole barrier as a result of surface states.
Therefore the work function is different on different surfaces of the same material. As a
result macroscopic far reaching electric fields are induced outside the sample, which
compensate the different work functions if taking an electron to infinity. Therefore, the
work function is defined as the energy needed to remove an electron from the interior of a
solid to a position just outside the crystal. This means to a distance large in atomic scales
but small compared to the dimensions of the different crystal faces.
Rough surfaces have smaller work function (Smoluchowsky effect).
Examples:
Cu(110): 4.48 eV, Cu(100): 4.63 eV, Cu(111): 4.88 eV
Ir(110): 5.42 eV, Ir(100): 5.67 eV, Ir(111): 5.76 eV
Adsorption of atoms or molecules changes the dipole layer
and hence the work function:
Electronegative adsorbates (e.g. S, C, O) increase the work function
Electropositive adsorbates (e.g. K, Na, Cs) decrease the work function
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Work function of semiconductors
For semiconductors band bending also contributes
to the work function.
: electron affinity, eVs: band bending,
Work function measurements
Field emission:
A high applied voltage bends the total potential and
electrons tunnel through the barrier.
The current is given by the Fowler-Nordheim equation:
 B 3 / 2 
AF 2

j
exp

 F 
F: applied filed in (V/cm)
From plots ln(j/F2) versus 1/F one obtains the
work function
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Thermionic emission:
Increasing the temperature of a sample also leads to electron emission.
The current is given by the Richardson-Dushman equation:
j  AT 2 exp( / kT )
Again the work function is determined from plots ln(j/T2) versus 1/T.
Photoelectron emission:
Irradiation by photons also lead to electron emission (photoemission). The photocurrent
is given by the Fowler expression:
j  A(h  ) 2
All the mentioned methods are absolute
techniques, the accuracy is about 0.1 eV.
Relative measurements, as in the following,
have an accuracy of about 1 meV. In this case
only changes of  can be measured.
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Vibrating capacitor method (Kelvin probe):
The sample and a vibrating probe electrode form
a variable capacitor. Due to the contact potential
difference and an applied voltage Ucomp an AC
current can be measured:
I
dC  

 U comp 

dt  e

If one changes the applied voltage in such a way that the current goes to zero, the
compensation voltage is equal to the work function. With this method typically only
changes of work functions are followed.
Diode method:
A diode scheme is used with a cathode as reference (e.g. a
LEED gun) and the sample as anode. Variation in the work
function of the anode shifts the characteristic curve by
. In practice the work function change is monitored by
maintaining a constant current by readjustment of the
anode potential.
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Elementary processes at surfaces I: Adsorption and desorption
Adsorption kinetics
According to kinetic gas theory the impingement rate
(flux I) at a surface is:
However, not all of the impinging particles may
become adsorbed, this is defined by the sticking
coefficient or sticking probability s, hence the
adsorption rate is:
The sticking coefficient depends on the already
adsorbed particles (coverage ()), a possible activation
barrier for adsorption (Eact), and the condensation
coefficient (), which is usually unity.
I
p
2m kT
rads  sI
s  f () exp(Eact / kT )
Coverage dependence:
The simplest case is referred to as Langmuir adsorption model. It is based on the
following assumptions:
• Adsorption is limited my monolayer coverage
• All adsorption sites are equivalent
• Only one particle can reside in the adsorption site
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a) Non-dissociative Langmuir adsorption:
b) Dissociative Langmuir adsorption:
for diatomic mobile adsorbates:
f ()  1  
f ()  (1  ) 2
for diatomic immobile adsorbates
(with z nearest neighbor sites):
f () 
for molecules dissociating into
n species:
f ()  (1  ) n
z
(1  ) 2
z 
c) Precursor mediated adsorption:
Often particles first enter a weakly
bound physisorption or precursor state.
Intrinsic precursor: the precursor is located
above an empty site
Extrinsic precursor: the precursor is located
above an occupied site
If no special interaction between the adsorbate
in the precursor and the adsorbed sites exist
(non-interacting adsorbate) than the intrinsic
and extrinsic precursors are the same. The
coverage dependence is somewhat more complicated
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Temperature dependence of sticking:
It is related to the energetics of adsorption
and can be visualized by the one-dimensional
Lennard-Jones potential.
a) Non-activated adsorption without precursor:
Sticking is not temperature dependent
b) Precursor mediated activated adsorption:
c) Precursor mediated non-activated adsorption:
In both cases the sticking coefficient is given by:
 
ka
    a 
s0 
 1  d exp  d

k a  kb   a
kT


1
Therefore in case b) s0 increases with T,
in case c) s0 decreases with T
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Angular and Kinetic energy dependence
In case of activated adsorption, the kinetic energy of the impinging molecule helps to
surmount the barrier. S increases with Ekin.
In case of precursor adsorption the accommodation into the precursor state is relevant.
Accommodation typically decreases with increasing kinetic energy. Therefore in this case
S decreases with increasing Ekin.
In case of direct adsorption without activation barriers or precursors the sticking may
by independent of Ekin.
Since in many cases only the normal energy is relevant (E = E·cos2), it follows that the
angular dependence of sticking is closely related to the energy dependence.
For energy independent sticking it is also angle independent. But by convention this is
described as cosine dependent. S() = S0 cos. (Due to the geometric decrease of the
impinging flux per surface unit!).
In all other cases the angular dependence can be roughly described by a cosn function,
with
n = 1 for S(E) constant (direct unactivated adsorption)
n > 1 for S(E) increasing with E (activated adsorption)
n < 1 for S(E) decreasing with E (precursor adsorption or steering)
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Thermal Desorption
If adsorbed molecules gain energy by increasing the substrate temperature they can
escape the adsorption potential due to increased thermal vibrations.
Most frequently the desorption rate is expressed for non-interacting particles by the
Polanyi-Wigner equation:
rdes  
d
  n  n exp(  Edes / kT )
dt
Kinetic order n:
0. order:
If desorption at a given temperature is not coverage dependent, e.g. for multilayer film
desorption (evaporation) or if a 2D two-phase system exists on the surface
1. order:
If single atoms or molecules desorb directly
2. order:
If two atoms recombine during desorption
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Desorption energy:
In case of activated adsorption: Edes = Eads + Eact
In case of non-activated adsorption: Edes = Eact
In case of particle interaction (attractive or repulsive) Edes is coverage dependent
Angular and kinetic energy dependence:
Adsorption and desorption is closely related by the Principle of Detailed Balancing:
This means, e.g. for activated adsorption, if the adsorption probability is larger for high
energetic particles, also in the desorption flux high energetic particles are abundant.
This leads to a hyperhermal flux.
Also, if sticking at normal incidence is higher also the desorption flux is peaked towards
the surface normal (often expressed with cosn, with n>1)
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Thermal desorption spectroscopy:
For the determination of Edes, n and  two experimental methods are usually employed:
Isothermal desorption spectroscopy (ITDS):
First the temperature is raised rapidly to a fix value and then held constant. From the
change of the coverage and/or the desorption rate one can evaluate all the interesting
parameters.
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Temperature Programmed Desorption Spectroscopy (TPDS), or (TDS):
The most frequently used technique. In this case the temperature is linearly increased
and the pressure change in the vacuum chamber is monitored. This situation is described
by the pumping equation:
A
d V  dp S 

  p
dt kT  dt V 
a) If the pumping speed is very low, then dp/dt is proportional to d/dt
b) If S is high, the p is proportional to d/dt. This is the commonly used regime. If the
linear heating rate is , then the pressure change is given by: k n
p  n exp( Edes / kT )
The shape of the spectra depends on the order.

0. order
1. order
2. order
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From the TDS one can obtain immediately a lot of information:
•
•
•
•
•
The shape of the spectra yields the desorption order
The desorption peak temperature gives roughly the desorption energy
The number of desorption peaks gives the number of different desorption states
The area under the desorption curves yields the coverage
The relative change of the TDS with increasing exposure gives information on the
sticking coefficient and of the saturation coverage
Example: Au desorption from W(110):
• Two peaks indicate two different states
• The higher is of first order with saturation at 1 ML
• The lower is of zero-order and does not saturate
(This means a chemisorbed monolayer is followed by a
multilayer condensation/evaporation)
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Adsorption isotherms
In the case of thermal equilibrium rads = rdes. Taking the equations for adsorption and
desorption one obtains the following general relationship between coverage  and the
equilibrium pressure p:

p
1 f ()
K f ()
Depending on the special physics, i.e. coverage dependence of adsorption and desorption,
one bets different adsorption isotherms:
a) Henry´s law: If f() = 1 and f*() =   (p) = Kp (holds for small coverages)
b) Langmuir Isotherm: If f() = 1- and f*() =  then ( p)  Kp
1  Kp
(holds for most adsorbates in the monolayer regime
if no lateral interaction exists)
c) 2D condensation (Hill-DeBoer equation):
If attractive interaction exists, this leads
to island formation within a lattice gas
(2-D condensation)
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d) Multilayer adsorption (BET-Isotherme, after Brunauer, Emmett, Teller)
If adsorption still proceeds after saturation of 1st monolayer (condensation)
Non-thermal desorption:
There are also other methods to desorb particles without temperature:
• Electron-stimulated desorption (electronic excitation)
• Photodesorption (electronic excitation)
• Ion impact desorption (momentum transfer)
• Field desorption
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Elementary processes at Surfaces II: Surface Diffusion
Random walk motion:
Atoms on a surface can hop from site to site. The
mean square displacement is:
2
2
r
 a t
The diffusion coefficient (or diffusivity) is:
D  a 2 / z
(with z neighbor sites)
Diffusion is an activated process, the hopping
frequency is:
  0 exp(Ediff / kT )
Typically, Ediff is much smaller than Edes. (5-20%)
Fick´s laws:
In the presence of an atom concentration gradient the random walk motion results in a
net diffusion to the region of lower concentration:
c
J  D
x
(1st Fick law)
c
 2c
D 2
t
x
(2nd Fick law)
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Solutions of the Fick´s law for three different examples:
a) Diffusion from a source of constant concentration
Initial conditions: c(0,t) = c0, c(x,0) = 0 for x > 0
 x 
c( x, t )  c0erfc

 2 Dt 
b) Diffusion from a semi-infinite source of infinite extent
Initial conditions: c(x,0) = c0 for x < 0, c(x,0) = 0 for x ≥ 0
c( x, t ) 
c0
 x 
erfc

2
2
Dt


c) Diffusion from a finite source of limited extent
initial conditions: c(x,0) = c0 for lxl < h and c(x,0) = 0 for lxl ≥ h
c( x, t ) 
c0
2

 hx 
 h  x 
erfc

erfc





2
Dt
2
Dt





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Some expressions:
Self-diffusion: Diffusing particles are the same as substrate
Hetero-diffusion: Diffusing particles are different from substrate
Tracer diffusion: Diffusion of single, non-interacting atoms at very low concentration
Chemical diffusion: For large coverages and interaction between particles
Intrinsic diffusion: Motion of particles across a surface with uniform potential, no
sources or traps
Mass transfer diffusion: If sources and traps are available (steps, kinks, vacancies,..)
Anisotropy of Surface diffusion:
Orientational anisotropy: Different diffusion at different crystal planes
Directional anisotropy: Anisotropic behavior at a given plane
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Atomic mechanisms of surface diffusion
a)Hopping mechanism
Hopping or jumping mechanism is the simplest form,
although actually it might not be that simple. Also
distortion of substrate atoms and existence of
saddle points. STM can reveal the details
b) Atom exchange mechanism
In this case an adatom replaces a substrate atom
which moves to an adatom site. This is often the
case for self diffusion. In the case of
heterodiffusion this leads to surface alloying.
c) Tunneling mechanism
This takes place predominantly for hydrogen on
metals at low temperature.
d) Vacancy mechanism
Takes place at a nearly complete surface layer
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
Ge vacancy
produced by
STM
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Surface diffusion of clusters
Sometimes whole clusters (islands) can diffuse at
once. Generally this becomes more difficult the
larger the cluster is (the activation barrier
increases with cluster size). Interestingly,
especially shaped clusters may be more stable.
The following individual or concerted mechanisms can take place:
•
•
•
•
•
•
•
•
Sequential displacement
Edge diffusion
2D Evaporation-condensation
Leapfrog mechanism
Gliding
Shearing
Reptation
Dislocation mechanism
STM and FIM are experimental techniques to reveal
the atomistic processes for diffusion. In many cases
the actual transition states cannot be observed.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Surface diffusion is a complicated matter and is also influenced, e.g. by phase formations
on the surface or by the existence of electric fields (electromigration)
Experimental study of surface diffusion
a)Direct observation of diffusing atoms
FIM and STM allow a direct observation of self and heterodiffusion. Typically one uses
the image-annealing-image method with FIM. With STM also image-while-hot
measurements are possible. Time resolution can be enhanced by the atom-tracking
method.
b) Profile evolution method
The smearing of an initially sharp concentration profile, e.g. after evaporation through a
mask, is monitored, e.g. by AES, SIMS, XPS, SEM, work-function measurements, etc.
c) Capillarity technique
Self diffusion can be studied by e.g. scratching or roughening a surface and following the
reflectivity change (due to smoothing) by optical methods
d) Island growth technique
Island growth and surface diffusion is closely related. This can be used to measure
diffusion coefficients by measuring island densities versus temperature and deposition
rate.
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Growth of Thin Films
When an adsorbate coverage exceed 1 monolayer one speaks of thin film growth.
Oriented growth is referred to as epitaxy. It is subdivided into homo-epitaxy and
hetero-epitaxy.
Thin film growth is controlled by the interplay of thermodynamics and kinetics.
Growth modes:
• Layer-by-layer, or Franck van der Merve (FM) growth
• Island, or Vollmer-Weber (VW) growth
• Layer plus island, or Stranski-Krastanov (SK) growth
The occurrence of the individual growth modes is governed by the bond
strength between the atoms in the layer and the atom-substrate bonds.
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The occurrence of different growth modes can be understood qualitatively in terms of
the surface tension,  (surface energy). It is defined as energy per area of force per
length. The wetting angle  of a drop-like island is determined by the surface tensions
of substrate (S), film (F) and film-substrate interface (FS).
 S   FS   F cos
In the case of FM growth is  = 0, and the
following condition holds:
In the case of VW growth,  > 0 and:
 S   FS   F
 S   FS   F
Experimentally, the growth modes can be identified by Auger signal changes of film
and substrate.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Island number density
The following elementary processes take part at layer growth: Adsorption, surface
diffusion, re-evaporation, capturing by defects and combination with other adatoms to
form clusters (nucleation).
Small clusters are metastable but they
become stable at a critical island size,
determined by the energy gain for
condensation and the energy cost to form
new surfaces.
The atomic processes can be described by rate equations. An example for subcritical
islands:
nj: cluster density with j atoms
n1: monomer density
D: diffusion coefficient
: capture probability; : decay probability
A set of rate equations can be formulated for
the change of adatom density, subcritical
cluster density and stable cluster density.
Integration of these rate equations finally
yield the time evolution of adatoms and islands.
Calculated n1 and nx values for the case i=1 (i.e.
2 atoms are already stable islands)
L: low-coverage
nucleation
I: Intermediatecoverage
A: Aggregation
regime
C: Coalescence and
Percolation regime
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Equilibrium density of 2D islands (nx)
 4R
nx
  (, i )
n0
  0 n0


 iE  Ei 
 exp diff

 (i  2)kT 

Example: Cu epitaxy on Ni(100)
STM study counting island densities
Different slopes indicate different
critical island size as function of
temperature
n0: available adsorption sites
i: critical cluster size
: scaling exponent,  = i/(i+2)
Ei: binding energy of critical cluster
Ediff: diffusion energy of adatoms
R: evaporation rate
0: frequency factor
: pre-exponential factor (~0.1-1)
Double logarithmic plots of the island
density as function of evaporation rate
yields the critical cluster size
Finally from the equation also Ediff and Ei can be determined
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Island shapes
Depending on the growth conditions one can get: Compact islands or ramified islands
The compactness depends on the diffusion probability along the island edges. This
depends on the surface temperature. In the extreme case of
hit-and-stick the diffusion limited aggregation (DLA) model
holds. It predicts fractal islands.
With STM the island shape can be observed.
Example: Pt growth on Pt(111)
T = 300 K
T = 400 K
annealed at 700 K
The equilibrium shape are hexagons. The different length depend on the different
step energies.
According to the Wulff theorem: The border distances from the crystal center are
proportional to their free energy per unit length.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Island size distribution
The size distribution is affected by: critical island size,
coverage, substrate temperature.
According to scaling theory one can derive a scaling
relation for the size distribution which only depends
on the critical size.
Experimental example: Fe on Fe(100), STM observations
Below ~ 250 °C the critical size is i =1,
whereas at higher temperature i = 3.
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Coarsening phenomena
The process of increasing the mean island size to the expense of decreasing the number of
islands.
a) Static coalescence: When neighboring immobile islands coalesce during adsorption
b) Dynamic coalescence or Smoluchowski coalescence:
If mobile islands encounter and coalesce.
When the coverage increases percolation growth
takes place (interconnected structures), e.g surface
conductivity begins.
Ripening or Ostwald ripening:
In this case larger islands grow on the expense of smaller islands, because the vapor
pressure of the smaller islands is larger. The chemical potential  of circular islands is:
 (r )  

r
: area occupied per atom
: step line tension
r: island radius
Magic islands: When due to
their specific structure only
selected island sizes are
stable
Vacancy
islands:
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Kinetic effects in homoepitaxial growth
The actual growth is not only determined by thermodynamics but also by kinetics.
In particular the mass transport on the surface determines the growth:
Intralayer mass transport (diffusion on a flat terrace)
Interlayer mass transport (diffusion across a step edge)
An atom approaching a lower step site will stick.
Coming from the high step site there may also be a
barrier (Schwöbel-Ehrlich barrier, EES), because at the
step the coordination is reduced.
The interlayer mass transport probability is given
by:
 E 
s  exp  ES 
 kT 
Depending on the relative rate of intra and interlayer mass transport different growth
modes exist:
step-flow growth
layer-by-layer growth
multilayer growth
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Strain effects in heteroepitaxy
If the crystal structure (lattice constants) is different for substrate and film,
strain appears at the interface. It depends on the misfit:  = (b-a)/a, with a and b
the lattice constants for substrate and film.
The elastic strain can be accommodated either by a pseudomorphic growth or by
misfit dislocations.
Which growth mode actually occurs depends on the relationship between the free
energy associated with only strain (E) and that with disclocations (ED).
Transition between
strained pseudomorphous
growth and relaxed
dislocated growth for
GexSi1-x on Si(100).
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Thin film growth techniques
a) Molecular beam epitaxy:
Molecules or atoms are evaporated onto a surface
under UHV conditions. Sample at elevated
temperature. Small flux. High quality layers.
Monitoring of film structure by RHEED.
b) Solid phase epitaxy:
First deposition of amorphous layer at medium
temperature, then annealing
c) Chemical beam epitaxy:
In this case material is supplied to the surface by
gaseous compounds at rather high pressure.
Substrate at high temperature, compounds
dissociate (Chemical vapor deposition (CVD)). e.g.
SiH4, AsH3, Trimethyl gallium).
If growth is performed at UHV conditions, then it
is called metal-organic MBE (MOMBE) or chemical
beam epitaxy (CBE)
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Surfactant mediated growth
Certain impurities on a surface may change the growth modes. These impurities are
called surfactants. The classical definition is, that a surfactant lowers surface tension
and thereby increases the spreading and wetting of the film. Actually, it turns out that
rather a modification of the kinetics takes place
rather then a change of the energetics. Fort example
the adatom diffusivity on the terraces might be
influenced, the number of nucleation centers can be
changed, the Schwöbel barrier may be changed etc.
The classical example is the growth of Ge on Si(111).
On the bare surface there is a Stranski-Krastanov
growth. With 1 ML Sb as a surfactant the film grows
in a layer-by-layer fashion. Another example is the
influence of H on Si(111) on the growth of Ag.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Atomic Manipulation and Nanostructure Formation
Typically nanostructures have dimensions of 1-10 (100) nm. They can be produced either
by atomic manipulation or self-organization.
nanostructures are characterized by surface dominated effects and quantum size
effects. A typical surface dominated effect is the decrease of the melting temperature
with decreasing cluster size (T ~ 1/L).
A typical quantum size effect is the discretisation of the energy levels (DOS).
• Quantum films are confined in one spatial direction
• Quantum wires are confined in two dimensions
• Quantum dots are confined in three dimensions
Short overview of the density of electronic states (DOS) for 3D, 2D, 1D and 0D free
electron gases:
For 3D: All the possible states are the solution of the Schrödinger equation: standing
waves in form of:  (r )  exp(ikr) with components of k: k x  2n , k y  2n , k z  2n ,
The energy of this state is:
L

 2k 2  2 2
En 

k x  k y2  k z2
2m 2m
L
L

Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Each allowed state occupies a volume in k-space of (2π/L)3, The volume of a spherical
shell with radius k and thickness dk is 4πk2dk. Then the number of states in this volume
are (taking into account two valus of spin):
4k 2 dk k 2 L3dk
dN  2

(2 / L)3
2
With
(2m E)1/ 2
k

it follows the DOS per unit volume and energy E :
dN
1  2m 
D3D ( E )  3  2  2 
L dE 2   
3/ 2
E1/ 2
For 2D:
The procedure is similar, now each state occupies an area of (2π/L)2, and the area of an
annulus is 2πkdk. This leads to a DOS for the ground state:
D2 D ( E ) 
dN
m

L2 dE  2
The energy accompanied with the k-vector in the third direction is quantized.
The total DOS is the sum of all DOS for each discrete energy levels due to normal
confinement.
with the Heaviside function H
D2 D ( E ) 
m
 2
 H (E  E )
n
n
H = 0 for E < En and H = 1 for E ≥ En
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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For 1D: The DOS takes the following form
g n H ( E  En )
(2m)1/ 2
D1D ( E ) 

 n ( E  En )1/ 2
For 0D finally all energy states exist only as discrete levels (like for an atom)
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Atomic manipulation with STM
Lateral atomic displacements, surface atom extraction
and single atom deposition can be performed with STM
Lateral atomic displacement:
Can be induced by interatomic forces (atom sliding) or electrostatic fields (field
assisted diffusion).
Xe on Ni(110)
Fe on Cu(111)
Different examples of atom arrangements by STM manipulation
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Field assisted diffusion:
In the case of adsorbed particles they may form a
static dipole and even an induced dipole in the
inhomogeneous field E(r) around an STM tip. The total
dipole moment is:
p = p0 +E; the potential energy is U(r) = - p0E(r) E2(r)/2.
Static dipoles may be attracted or repulsed by the
STM field, induced dipoles are always attracted.
Atomic extraction:
Atoms can be removed from the surface and transferred to the tip by:
strong interatomic interactions (touching the surface),
filed evaporation (by high fields, several V),
electron-stimulated desorption (due to electron current between tip and sample).
Atom deposition:
Single atoms can be deposited by the same
method (touching). Small clusters can also be
deposited, with the z-pulse method or the V
pulse method.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Self-Organization of Nanostructures
In some cases evaporated material arranges in special lateral periodicities. This is
typically a result of lattice mismatch. Also reconstructed and strained surfaces can be
used as templates to form especially shaped overlayers.
Some examples:
Ge growth on Si(100) forms island with shapes
of huts, pyramides, domes and superdomes. But
the regularity is not high. However, with multilayers
of Si and Ge evaporation the regularity becomes
increasingly higher.
Hydrogen interaction with metal/silicon surface phases:
Al on Si(100) forms a continuous layer. But after hydrogen
adsorption, nanoclusters are formed.
Superlattice of nanoclusters on Si(111) 7x7:
Evaporation of Al leads to adsorbates only on
special sites. leading to a regular array of Al nanodots
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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Fullerenes and Carbon nanotubes
Besides the well known forms of carbon (graphite, diamond)
also spheroidal molecules with special numbers of C atoms
and carbon hollow fibers can be formed. The best known
molecules is C60, also called Buckminster fullerene.
These fullerenes can be doped by alkali atoms, which show
interesting features (superconducting). Fullerenes
adsorbed on surfaces form dense packed hexagonal
layers. The condensed form of fullerenes is solid
fullerite. It evaporates at a few hundred °C.
Carbon nanotubes can be thought of a rolled up sheet of
graphene. Depending on the special bond orientation there
are zigzag, armchair and chiral nanotubes.
Besides single wall nanotubes there exist also multi-wall
nanotubes.
Adolf Winkler, Universitätslehrgang „Nanotechnologie und Nanoanalytik“ , TU Graz
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