-角速度
Characterization techniques:
(A) XPS (X-ray photoelectron
spectroscopy):
Hydrothermally
deposited epitaxial thin films are characterized
by XPS to retrieve useful
information
like
composition,
chemical
structure
and
local
arrangement
of
atoms
that
make
up few layers of surface of film and also the
interfacial layer between the film and
substrate.
X-ray
photoelectron
spectroscopy
(XPS)
was
developed
in
the
mid
–
1960s
by
Kai
Siegnahm
and his research group at the University of
Uppsala, Sweden.
Surface analysis by
XPS involves irradiating a solid in vacuum with
monoenergetic soft
x-rays and analyzing
the emitted electrons by energy. The spectrum is
obtained as a plot
of the number of
detected electrons per energy interval versus
their kinetic energy. The
principle on
which the XPS technique is based can explained
with the help of figure 1 as
shown
below. [27]
Figure 1. An
energy level diagram showing the physical basis of
XPS technique.
The energy carried by
an incoming X-ray photon is absorbed by the target
atom, raising
it
into
excited
state
from
which
it
relaxes
by
the
emission
of
a
photoelectron.
Mg
K
?
(1253.6eV) or Al
K
?
(1486.6 eV) x-rays are
generally used as a source of monoenergetic
soft x-rays. These photons have limited
penetrating power in a solid on the order of 1-10
micrometers.
They
interact
with
atoms
in
the
surface
region,
causing
electrons
to
be
emitted by
the photoelectric effect. The emitted electrons
have measured kinetic energies
given
by:
KE=h
?
-BE
-
?
s
Where h
?
is the
energy of the photon, BE is the binding energy of
the atomic orbital from
which
the
electron
originates
and
?
s
is
the
spectrometer
work
function.
The
binding
energy may be regarded as the energy
difference between the initial and final states
after
the photoelectron has left the
atom. Because there are a variety of possible
final states of
the ions from each type
of atom, there is corresponding variety of kinetic
energies of the
emitted
electrons.
Photoelectrons
are
emitted
from
all
energy
levels
of
the
target
atom
and hence the electron
energy spectrum
is
characteristic of the emitting
atom type and
may be thought
as its XPS fingerprint. Each element has unique
spectrum .The spectrum
from
a
mixture
of
elements
is
approximately
the
sum
of
peaks
of
the
individual
constituents.
Because
the
mean
free
path
of
electrons
in
the
solids
is
very
small,
the
detected electrons
originate from only the top few atomic layers
making XPS a unique
surface sensitive
technique for chemical analysis. Quantitative data
can be obtained from
peak heights or
peak areas and identification of chemical states
often can be made from
exact
measurement
of
peak
positions
and
separations
as
well
from
certain
spectral
features.
The
line
lengths
indicate
the
relative
probabilities
of
the
various
ionization
processes.
The
p,d
and
f
levels
split
upon
ionization
leading
to
vacancies
in
the
p1/2,p3/2,d3/2,d5/2,f5/2 and f7/ spin
orbit splitting ratio is 1:2 for p levels ,2:3 for
d
levels and 3:4 for f levels .
Because each element has a unique set
of binding energies, XPS can be used to identify
and
determine
the
concentration
of
the
elements
in
the
surface.
Variations
in
the
elemental
binding
energies
(the
chemical
shifts)
arise
from
the
differences
in
the
chemical potential and
polarizibilty of compounds. These chemical shifts
can be analyzed
to identify the
chemical state of the materials being analyzed.
The electrons leaving sample are
detected by an electron spectrometer according to
their
kinetic energy. The analyzer is
usually operated as an energy window, referred to
as pass
energy.
To
maintain
a
constant
energy
resolution,
the
pass
energy
is
fixed.
Incoming
electrons are adjusted the pass energy
before entering the energy analyzer. Scanning for
different
energies
is
accomplished
by
applying
a
variable
electrostatic
field
before
the
analyzer. This retardation voltage may
be varied from zero upto and beyond the photon
energy.
Electrons
are
detected
as
discrete
events,
and
the
number
of
electrons
for
the
given detection time.
And energy is stored and displayed.
In general, the interpretation of the
XPS spectrum is most readily accomplished first by
identifying the lines that almost
always present (specifically those of C and O),
then by
identifying major lines and
associated weaker lines.
(B) Auger electron spectroscopy:
Auger electron spectroscopy
is a very useful technique in elemental
characterization of
thin films. In the
current project this technique has been utilized
not only for elemental
compositional
analysis but also for understanding nucleation and
growth mechanism.
Auger electron effect
is named after the French physicist Pierre Auger
who described the
process involved in
is process is bit more complicated than the XPS
process.
The
Auger
process
occurs
in
three
stages.
First
one
being
atomic
ionization.
Second
being
electron
emission
(Auger
emission)
and
third
being
analysis
of
emitted
auger
electrons .The source of radiation used
is electrons that strike in the range of 2 to 10
kev.
The
interatomic
process
resulting
in
the
production
of
an
Auger
electron
is
shown
in
figure 2 below.
Figure 2 showing the interatomic
process resulting in production of the Auger
electrons.
One electron
falls a higher level to fill an initial core hole
in the k-shell and the energy
liberated
in this process is given to second electron
,fraction of this energy is retained by
auger
electron
as
kinetic
energy.X-ray
nomenclature
is
used
for
the
energy
levels
involved
and
the
auger
electron
is
described
as
originating
from
for
example
,an
ABC
auger transition where A
is the level of the original core hole,B is the
level from which
core hole was filled
and C is the level from which auger electron was
emitted. In above
figure 2 shown above
the auger transition is described as L
3
M
1
M
2
,
3
.
The
calculation of energies of the lines in the Auger
electron spectrum is complicated by
the
fact
that
emission
occurs
from
an
atom
in
an
excited
state
and
consequently
the
energies of the levels involved are
difficult to define precisely.
Each
element in a sample being studied gives rise to
characteristic spectrum of peaks at
various kinetic energies. Area
generally scanned is 1
mm
2
.To understand the
variation in
the concentration with the
distance from the surface depth profiling can also
be carried
out. For depth profiling the
surface has to be etched away by using argon beam.
The principle advantage that AES hold
over XPS is that the source of excitation in case
of AES is electrons which allows it to
take a spectra from micro-regions as small as 100
nm diameters or less instead of
averaging over the whole of the surface of the
sample as
is done generally in XPS.
(C) Atomic
force Microscope:
Atomic
Force
Microscope
(AFM
)
is
being
used
to
solve
processing
and
materials
problems in a wide range of
technologies affecting the electronics,
telecommunications,
biological,
chemical, automotive, aerospace, and energy
industries. The materials being
investigating
include
thin
and
thick
film
coatings,
ceramics,
composites,
glasses,
synthetic and
biological membranes, metals, polymers, and
semiconductors.
In the current work AFM
was used to understand the nucleation and growth
mechanism
of
the
epitaxial
thin
films
and
to
understand
the
surface
morphology
of
totally
grown
films in terms of surface coverage and
surface roughness.
In
the
fall
of
1985
Gerd
Binnig
and
Christoph
Gerber
used
the
cantilever
to
examine
insulating
surfaces.
A
small
hook
at
the
end
of
the
cantilever
was
pressed
against
the
surface while the sample
was scanned beneath the tip. The force between tip
and sample
was measured by tracking the
deflection of the cantilever. This was done by
monitoring
the tunneling current to a
second tip positioned above the cantilever. They
were able to
delineate
lateral
features
as
small
as
300
?.
This
is
the
way
force
microscope
was
developed.
Albrecht,
a
fresh
graduate
student,
who
fabricated
the
first
silicon
microcantilever
and
measured
the
atomic
structure
of
boron
nitride.
The
tip-cantilever
assembly typically is microfabricated
from Si or
Si
3
N
4
.
The force between the tip and the
sample surface is very small, usually
less than 10
-9
N.
According to the interaction of the tip
and the sample surface, the AFM is classified as
repulsive
or
Contact
mode
and
attractive
or
Noncontact
mode.
In
contact
mode
the
topography is measured
by sliding the probe tip across the sample
surface. In noncontact
mode, topography
is measured by sensing Van de Waals forces between
the surface and
probe
tip.
Held
above
the
surface.
The
tapping
mode
which
has
now
become
more
popular measures topography by tapping
the surface with an oscillating probe tip which
eliminates shear forces which can
damage soft samples and reduce image resolution.
1. Laser
2.
Mirror
3. Photo detector
4. Amplifier
5.
Register
6. Sample
7.
Probe
8. Cantilever
Figure 3 showing a
schematic diagram of the principle of AFM.
Compared with Optical
Interferometric Microscope (optical profiles), the
AFM provides
unambiguous measurement
of step heights, independent of reflectivity
differences
between materials.
Compared with Scanning Electron Microscope, AFM
provides
extraordinary topographic
contrast direct height measurements and unobscured
views of
surface features (no coating
is necessary). One of the advantages of the
technique being
that it can be applied
to insulating samples as well. Compared with
Transmission
Electron Microscopes,
three dimensional AFM images are obtained without
expensive
sample preparation and yield
far more complete information than the two
dimensional
profiles available from
cross-sectioned samples.
(D) Fourier Transform
Infrared Spectroscopy:
Infrared
spectroscopy
is
widely
used
chemical
analysis
tool
which
in
addition
to
providing information on chemical
structures also can give quantitative information
such
as concentration of molecules in a
sample.
The development in FTIR
started with use of Michelson interferometer an
optical
device invented in 1880 by
Albert Abraham Michelson. After many years of
difficulties
in working out with time
consuming calculations required for conversion
intereferogram
into
spectrum,
the
first
FTIR
was
manufactured
by
the
Digilab
in
Cambridge
Massachusetts
in
1960s .These
FTIR
machines
stared
using
computers
for
calculating
fourier
transforms faster.
The set up consists
of a source, a sample and a detector and it is
possible to send all the
source energy
through an interferometer and onto the sample. In
every scan, all source
radiation
gets
to
the
sample.
The
interferometer
is
a
fundamentally
different
piece
of
equipment than a
monochromater. The light passes through a
beamsplitter, which sends
the light in
two directions at right angles. One beam goes to
a stationary mirror then back
to the
beamsplitter. The other goes to a moving mirror.
The motion of the mirror makes
the
total path length variable versus that taken by
the stationary-mirror beam. When the
two meet up again at the beamsplitter,
they recombine, but the difference in path lengths
creates constructive and destructive
interference: an interferogram:
The
recombined beam passes through the sample. The
sample absorbs all the different
wavelengths characteristic of its
spectrum, and this subtracts specific wavelengths
from
the
interferogram.
The
detector
reports
variation
in
energy
versus
time
for
all
wavelengths simultaneously. A laser
beam is superimposed to provide a reference for
the
instrument operation.
Energy
versus
time
was
an
odd
way
to
record
a
spectrum,
until
the
point
it
was
recognized
that
there
is
reciprocal
relationship
between
time
and
frequency.
A
Fourier
transform allows to
convert an intensity-vs.-time spectrum into an
intensity-vs.-frequency
spectrum.
The advantages of FTIR are that all of
the source energy gets to the sample, improving
the
inherent
signal-to-noise
ratio.
Resolution
is
limited
by
the
design
of
the
interferometer. The longer the path of
the moving mirror, the higher the resolution.
One minor drawback is that the FT
instrument is inherently a single-beam instrument
and
the
result
is
that
IR-active
atmospheric
components
(CO
2
,
H
2
O)
appear
in
the
spectrum.
Usually, a
is
run, and then
automatically subtracted
from every
spectrum.
(E) Scanning
Electron Microscopy:
Scanning electron microscopy is one the
most versatile characterization techniques that
can
give
detailed
information
interms
of
topography,
morphology,
composition
and
crystallography. This
has made it widely useful in thin film
characterization.
The scanning
electron microscope is similar to its optical
counterparts except that it uses
focused
beam
of
electrons
instead
of
light
to
image
the
specimen
to
gain
information
about the
structure and composition.
A
stream
electron
is
accelerated
towards
positive
electrical
potential.
This
stream
is
confined
and
focused
using
metal
apertures
and
magnetic
lenses
into
a
thin,
focused,
monochromatic
beam.
This
beam
is
focused
onto
the
sample
using
a
magnetic
lens.
Interactions
occur
inside
the
irradiated
sample,
affecting
the
electron
beam.
These
interactions and
effects are detected and transformed into an
image. The electron detector
collects
the electrons and then image is created. Scanning
with SEM is accomplished by