TEM charcaterization

1
TEM charcaterization

• Basic modes
• Bright field microscopy
• Dark field Microscopy
• STEM
• EDAX
• EELS

2
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.

3
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.

4
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

5
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.

6
Comparison with SEM
7
Lattice resolved TEM image

• Lattice resolved TEM image of a Nanowire section
  showing individual atoms sites (courtesy USC EM
  Center Tem facility)

8
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.

9
Final Exam Guide

• Short Questions
• 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?

10
Final Exam Guide

• Problems
• 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.