TEM charcaterization
• Basic modes
– Bright field microscopy
– Dark field Microscopy
–STEM
– EDAX
– EELS
Operating principles
• The TEM also has the electron gun and the
focusing optics like the SEM, however, it is
based on electrons transmitting through the
material for imaging
• The 3 main TEM modes are Bright field, Dark
field, and Scanning transmission electron
microscope (STEM)
• The sample is supported by small Cu grid
(few mm dimension) that is supported in
holders
• The electron energy is few hundred KeV, and
the magnification obtained could reach up to
a million times in best cases
• A part of the image can be blocked to
produce either bright or dark field images
• The scattering of electrons in TEM is much
less than SEM, and almost always in the
forward direction due to small interaction
volume. This helps in getting very high
resolution.
Sample preparation using FIB
• TEM sample preparation is
actually more involved than the
imaging technique. The sample
is usually glued in epoxy and
polished using until a very thin
cross-section (tens to hundreds
of nm) is achieved.
• FIB: Using Focused Ion beam
based milling technique, the
exact location where the image
needs to be taken can be
thinned, thus making the
imaging process much less
complicated and less time
consuming. In a FIB process
Ga+ ions are used for the milling,
and resolutions of 10 nm are
possible to obtain.
Dark field imaging
• This mode is operated by
looking at the image produced
by the “diffracted beam” with
large angular deflection. Since
the diffracted beam is usually
very weak, the direct beam is
blocked
• This image can be thought of as
some form of “phase contrast”
imaging, which are caused by
interference
• Mostly used to enhance contrast
when bright field image is not
very of high contrast
STEM
• In STEM, a thin electron beam of
diameter down to 0.1 nm is used to
raster the sample and perform the
imaging. Although the process is
similar to SEM, the spot size can be
more tightly controlled due to lower
De Broglie wavelength of the
electrons
• The operation is similar to that of an
SEM with the difference that the
beam actually passes through the
sample
• This mode is usually very useful for
elemental analysis almost on an
atom by atom basis by EELS and
EDX
• Magnification obtained is 500,000
times or more.
Comparison with SEM
Lattice resolved TEM image
• Lattice resolved TEM image of a
Nanowire section showing individual
atoms sites (courtesy: USC EM
Center Tem facility)
Final Exam
• The final exam will be on Friday, Dec 13, 2013,12.30 pm.
• 1 page (double sided) containing only formulas will be
allowed. No class notes, no worked examples, no figures.
• The project report will be due on Dec 13, 2013.
• The project report should be 10 pages, and in the format
of a journal paper; i.e. include Introduction, discussion of
major modes, conclusions, and future directions/novel
suggestions, references etc.
• With reference to the EBIC line plot,
show where the defect density is highest
and where it is lowest.
Final Exam Guide
Short Questions:
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Two advantages of CL over PL
Two comparative advantages and disadvantages of SEM and TEM
The three different characterization techniques associated with SEM are
………., …….. and …………..
Two comparative advantages and disadvantages of SIMS and RBS. Which
one would you prefer to measure background C impurity in MBE or MOCVD
growth? Why?
Why is Field Emission Gun (FEG) SEM preferred over traditional SEM?
How do you avoid charging issues in an insulated sample in an SEM?
What is a suitable method to obtain lattice spacing and chemical
composition of a 10 nm diameter InN nanowire? Justify.
A wafer of a unknown material is given. What is a simple way to determine
its chemical composition? What is a more accurate way of determining it
chemical composition? If this is a semiconductor, how will you determine its
(i) dopant density and (ii) carrier concentration?
Final Exam Guide
Problems:
(a)
(b)
(c)
Calculate the De Broglie wavelengths of the electrons in SEM (30 keV) and
TEM (300 keV)
With the help of band diagrams, (i) before electrical contact, (ii) after
electrical contact, and (iii) after application of feedback bias to the tip to
nullify electric field, explain the operation of Kelvin probe measurement.
Assume n+ doped Si probe tip (work function 4.07 eV) and Au sample
(work function 5.15 eV). What voltage needs to be applied to the probe tip
for nullifying the electrostatic force of attraction? How would the tip bias
change is the sample has a dc bias of -1 V applied to it?
Calculate the amplitude of the 17 KHz force acting on the cantilever for an
applied ac voltage of 10 V rms (frequency 17 KHz). The cantilever
dimensions are 30 and 100 microns respectively. It is held 2 microns above
the sample. The work function of the cantilever is 5.65 eV, and that of the
sample is 5.15 eV.