Folie 1

1
(No Transcript)
2
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
Mean free path
Impingement rate
Monolayer formation time
Molecular density
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 2 s
10-10 3x106 4x1010 500 km 10 hours
3
Some vacuum considerations Consider a simple
model of a vacuum chamber which is evacuated by a
pump via a tube
Throughput
Pumping speed
Conductance
The effective pumping speed is
The pumping equation
The base pressure
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)
4
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
Ion gauge (Bayard-Alpert)
Thermocouple and Pirani
5
Typical UHV pumping system and UHV hardware
Residual gas composition

1. just after pump down of tight chamber (10-6 Torr)
2. System with air leak (10-6 Torr)
3. Properly baked system (10-9 Torr)

6
Preparation of atomically clean
surfaces Ex-situ polishing, chemical etching,
boiling in solvents, rinsing in de-ionized
water In-situ

1. Cleavage Only for brittle material, mainly for
   oxides (ZnO), halides (NaCl) semiconductors (Si,
   GaAs) surface is clean, but often contains steps
2. Heating By electrical current, electron
   bombardment or laser annealing. Mostly for metal
   samples, not all contaminants can be removed,
   segregation may occur
3. 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
4. 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.
7
UHV deposition technology In some cases one
wants to deposit material on surfaces, e.g by
evaporation Impingement flux
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) Depositio
n monitors Quartz crystal thickness monitor
(Eigenfrequency of quartz depends on mass (typ.
5-10 MHz)
8
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-?V. 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. The de Broglie
wavelength of electrons is given by
9

• 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
• Definition of reciprocal lattice vectors
• (a, b real space unit vectors)
• From this it follows
• The vectors a and b are in the same surface
  plane as the real space vectors a, b.
• a is perpendicular to b and vice versa
• The length of the rec. vectors are
  and v.v

a oblique lattice b rectangular c hexagonal d
centered rectangular
10
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
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
2D
3D
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
11
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 weakening 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
12
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.
13
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. The set up
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 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.
14
RHEED is usually used to monitor the surface
structure during epitaxial layer growth.
RHEED analysis The spots in the pattern
correspond to the grazing intersection of
reciprocal rods with the large Ewald sphere.
RHEED also allows to check the growth of 3D
islands
15
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 (lt1) 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.
16
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)

17
Electron energy analyzers Several analyzers can
be used to measure the kinetic energy of the
secondary electrons.

• Retarding field analyzer (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.

RFA
CMA
CHA
CSA
18
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 X-ray 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
However, this is a rough estimate, because the
final state is an ion and the levels may shift
compared to the neutral atom. F Work function
(energy needed to bring an electron from the
Fermi level to the vacuum level)
19
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.
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,
20
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
IP Intensity of primary beam ? Ionization cross
section ? Auger transition probability r
Backscattering factor T Transmission probability
of analyzer
? Attenuation length z Escape depth ?,?
Azimuth and polar angle
21

• 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)
• Core Level Electron Energy Loss Spectroscopy
  (CLEELS)
• The energy of the inelastically scattered
  electron is

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.
22
b) 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
EELS spectra are recorded either as N(E) or
d2N(E)/dE2
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
23
c) 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 ?E5meV) 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.
24
Photoelectron Spectroscopy (PES) If an atom
absorbs a photon, a photoelectron will be
emitted. The kinetic energy is
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.
25
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.
26
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 involved.
Ni
27
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.
28
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. The measured
kinetic energy can be written as kex is the wave
vector of the emitted photoelectron 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 the detection
angle and the measured electron energy.
29
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.
30

• 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

31

• General Principles
• Classical binary collisions
• In a first approximation, ion scattering can be
  described by elastic binary hard-sphere 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)
In the case of 90 or 180 scattering detection
the equation for E1 simplifies to 90

180
32
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)
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.
33
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
34
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
35
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
• 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

36
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)
37
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 Ion neutralization Unknown
scattering potential Multiple scattering
He
Less neutralization takes place for alkali ions,
because Ei lt 2F. (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.
38

• Rutherford Backscattering (RBS) or High-energy
  ion scattering (HEIS)
• The basic feature of this method are
• Small cross section and small shadow cones
  (lt10-2 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.
39
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.
40
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.
41
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.
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) lead 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.
42

• 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.
43
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 higher work function
than stepped surfaces
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.
W(110)
44
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, adatom-adatom
interaction, step motion, equilibrium crystal
shape etc.
W(110) 21 K He-H2
45
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 ? ? 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. 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.
NaCl cleavage surface decorated with Au
Si/TbSi2/Si double heterostructure
46
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-field image. Images taken with any other
beam lead to dark-field 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.
47
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 (lt50 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.
48
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
V bias voltage D(V) electron density of
states d dip-surface distance F 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.
49

• 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
50

• 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 conducting,
  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

51
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 it
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 in 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 In
this mode the tip is also in vibration and closer
to the sample so that it touches the
surface. This mode is advantageous for surfaces
with high topographical corrugation.
Non-contact AFM Si(100)(2x1)
52
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
• Normal relaxation modification of interlayer
  spacing
• Parallel relaxation lateral shift of top layer

• Reconstruction
• Conservative reconstruction number of atoms
  conserved
• 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.

53
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
54
Graphite surface The (0001) surface of graphite
preserves the non-reconstructed bulk shape, there
is also nearly no normal relaxation.
Si(100) This surface reconstructs by forming
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
(dimer-adatom-stacking fault) model.
55
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, lt70 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
56
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.
57
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
58
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.
59
(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.
60

• 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
61
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). e.g. the
equilibrium number of adatoms is
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
with ?(0) surface energy of terrace ?L
surface energy of step a atomic height
62
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
the vector 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.
63
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
64
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) superstruct
ure. 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.
65
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 blta or bgta, heavy or
light domain walls appear. These superstructures
can be seen as Moiré structures
Pb on Cu(111)
Ga on Ge(111)
66
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
67
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.

1. A spill over of the electrons appears, creating
   an electrostatic dipole
2. b) The oscillation of the electron density inside
   are called Friedel oscillations. There
   periodicity is p/kF, with kF (3p2?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
68
Electronic surface states The solution of the
Schrödinger equation for a 1-dim. 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 their k-vector components parallel
to the surface (surface projected bulk bands).
69
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 for simple 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
70
Extrinsic surface states Are related to surface
imperfections, like steps, defects, vacancies.
Their 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)).
71
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 it 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.

72

• 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

73
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
F applied field in (V/cm) From plots ln(j/F2)
versus 1/F one obtains the work function
74
Thermionic emission Increasing the temperature
of a sample also leads to electron emission. The
current is given by the Richardson-Dushman
equation
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
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.
75
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
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 difference.
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.
76
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.

• 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

77

• a) Non-dissociative Langmuir adsorption
• b) Dissociative Langmuir adsorption
• for diatomic mobile adsorbates
• for diatomic immobile adsorbates
• (with z nearest neighbor sites)
• for molecules dissociating into
• n species
• 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

78

• Temperature dependence of sticking
• It is related to the energetics of adsorption
• and can be visualized by the one-dimensional
• Lennard-Jones potential.
• 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

Therefore in case b) s0 increases with T, in case
c) s0 decreases with T
79
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?
Ecos2?), 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 cosn-1?. (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 gt
1 for S(E) increasing with E (activated
adsorption) n lt 1 for S(E) decreasing with E
(precursor adsorption or steering)
80
Thermal Desorption If adsorbed molecules gain
energy by increasing the substrate temperature
they can escape the adsorption potential due to
incre