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BMFB 3263
Materials Characterization
Scanning Probe Microscopy &
Relates Techniques
Lecture 5
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 SPM is a technique to examine materials with a solid
probe scanning the surfaces;
 It examines surfaces features whose dimensions
range form atomic spacing to a tenth of a millimeter;
 SPM considered the most powerful tool for surface
structure examination currently available because it
lets ‘see’ atom;
 SPM started with the scanning tunneling microscope
(STM):
 An STM uses tunneling current, a phenomenon of quantum
mechanics, to examine material surface;
The tunneling current flows through an atomic-scale gap
between a sharp metallic tip and conducting surface atoms;
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 SPM added a popular member, the scanning force
microscope (SFM); commonly called atomic force
microscope (AFM):
It uses near-field forces between atoms of the probe tip
apex and the surface to generate signals of surface
topography;
The AFM more widely used than the STM because it is
not restricted to electrically conducting surface;
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Instrumentation
 The main characteristic of the SPM is a sharp probe
tip that scans a sample surface;
 The tip must remain in very close proximity to the
surface because the SPM uses near-field interactions
between the tip and a sample surface for examination;
 enable to obtain a true image of surface atoms, it can
accurately measure the surface atom profiles in the
vertical and lateral directions;
 SPM must operate in a vibration-free environment
because the probe is only an atomic distance away
from the examined surface;
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 An SPM system consists of several basic components:
probe and its motion sensor, scanner, electric
controller, computer and vibration isolation;
 Please find what is application of this component; I
will ask you in test and exam…………..
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Scanning Tunneling Microscopy (STM)
 Based on quantum mechanical “tunneling” effect –
when 2 electrodes (2 metals) are brought sufficiently
close together to allow overlapping of electronic wave
functions associated with each electrode.
 Apply small bias voltage – tunneling current is formed
in direction given by the sign of applied voltage.
 Use very sharp tip (one atom’s) possible to detect
current variation as tip is scanned across surface.
 Samples must be conductive but medium can be air,
liquid or vacuum.
 Resolution up to nanoscale range.
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The reason for extreme magnification capabilities of the
STM down to atomic scale is mainly the physical
properties of the tunneling current. The tunneling current
it has very important characteristic – it exhibits an
exponentially decay with an increase of the gap d. Very
small changes in tip-sample separation induce large
changes in tunneling current!!
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Schematic of STM principle
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 Schematic of STM principle - It works by holding a very
fine needle or tip approximately a billionth of a meter
from the sample's surface. When the tip is this close,
electrons can jump the gap between it and the sample.
 This 'tunnel current' can be amplified and used to
measure the size of the gap with tremendous accuracy.
 An electronic feedback system is used to keep the
current (and hence the gap) constant as we move the tip
sideways across the surface. Because the current
detection is so sensitive the tip actually has to ride up
over the atoms of the surface in exactly the same way
that a record player's stylus tracks the groove of an LP.
 By recording the tip's vertical position at points on a
grid we can make a 3D map of the surface.
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Circular ‘quantum’ corral
48 iron atoms positioned on
copper (111) at very low T
(4K), corral 14.3 nm.
Image shows contour of
local density of electron
states or quantum state of
the corral.
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The making
of circular
corral –
positioned 48
iron atoms
into circular
ring.
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Scanning tunneling
microscopy (STM)of a
(105) facetted Ge hut
cluster formed by 5.8
monolayer (ML) of Ge
on a Si (100) surface
STM can take atomically resolved pictures of the
electron clouds surrounding surface atoms. It can tell
the difference between electrons with different
energies, and map their positions independently of each
other. Thus STM is a very powerful tool for investigating
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surfaces.
Each bright bump in the pattern
is one Si atom. The beauty of
this microscope is not just that it
can see the atoms and their
regular arrangement on the
surface, but that it can also see
defects such as the few positions
where atoms are missing.
The only other microscope that can resolve atomic detail, the electron
microscope, uses diffraction from rows of atoms to form its image and so
cannot detect a single missing atom like this.
Since the electrical conductivity of semiconductors like Si is largely
determined by the defects in the materials rather than the properties
of the pure lattice, this sort of image is very useful to people like chip
makers who want to know exactly what is going on in their devices.
Modern chips are made up of elements as small as forty of these
pictures side by side, so even single atom defects can be important. 13
Atomic Force Microscopy (AFM)
 Operates by measuring forces between sample and
probe tip. Force depends on nature of sample, probe
geo., spring constant of probe, distance between probe
& sample, and any contamination on surface.
 An atomically sharp tip is scanned over surface with
feedback mechanisms that enable the piezo-electric
scanners to maintain the tip at a constant force (to
obtain height info) or height (to obtain force info)
above the sample surface.
 Tip is brought close enough to surface to detect
repulsive force between atoms in tip and sample.
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Atomic Force Microscopy (AFM)
 Probe tip is mounted on cantilever. Interatomic forces
will induce bending and can be detected by laser
beam. Tips typically made of Si3N4 or Si .
 Surface topography of sample is tracked by
monitoring deflection of the cantilever.
 Optical detection system – diode laser is focused onto
the back of a reflective cantilever. As tip scans surface,
moving up & down with the contour, laser beam is
deflected off into a dual element photodiode.
 Measures difference in light intensities between upper
& lower photodetectors & converts to voltage.
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AFM uses forces
when 2 objects are
brought within
nano-range of each
other : probe in
contact with
surface – repulsive
force, a few nm
away – attractive
force.
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Commercial AFM tip,
and images taken by
nanotube tip compared
to normal tip
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Digital video dics surface
(10 microns).
Applications include materials evaluation (surface
roughness on implanted Si wafers, nanomechanical testing,
surface profiles), failure analysis (defect analysis of
compact disc stampers), quality control (surface profiles
of thin film & coating, surface finish of substrates for
thin film deposition).
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Monolayer prepared by self-assembly Ag-TiO2 nanoparticles
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Silica spheres
homogeneously
covered with Ag
nanoparticles
AFM showing all atoms within
hexagonal graphite unit cells,
image size 2x2 nm2
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Applications
 STM – characterization of surface structure on
atomic scale. Produce topographic map of the
surface.
 AFM – surface topography, depth profiling,
hardness at atomic level. Typically applied to studies
of phenomena such as abrasion, adhesion, cleaning,
corrosion, etching, friction, lubrication, plating &
coating, & polishing.
 Limit : STM – only for conducting materials. Image
interpretation also difficult if more than one type of
atom on surface.
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AFM compared to others :
STM – only to conducting but AFM applied to
both. AFM also more versatile. In some cases,
STM has better resolution because exponential
dependence of tunneling current on distance.
SEM – AFM provides extaordinary
topographic contrast, direct height
measurements & unobscured view of surface
features (no coating necessary).
TEM – 3D image of AFM obtained without
expensive sample preparation & yield far more
complete info than 2D profiles from crosssectioned sample.
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