Other modes associated with SEM: EBIC In this technique, the incident electron beam is used to create electron-hole pairs that constitute the current under applied bias between two contacts The total number of electrons generated by incident probe with accelerating voltage V0 is Nehp = 1000(V0/3.6) for Si material. The generation rate of the ehps is given as G = IbNehp/qVol. The vol. is 4/3π(Re)3. For semiconductors like Si with Ldiff >> Re we can use the vol. as 4/3π(L)3. The total minority electron density is then n = Gn If the EHPs recombine quickly due to higher density of recombination center (poor material quality) then the magnitude of current drops. This gives the contrast in the image, and dislocations/defects that produce recombination centers can be identified.

Other modes: Electron Microprobe Energy dispersive X-ray spectroscopy (called EDS or EDX) is the most common part of electron probe microanalysis, and is based on the detection of the energy of the X-Rays that are generated from the electron interaction with matter. This is a widely popular technique for microanalysis since X-rays of all the energy range can be simultaneously detected. The detector consists of a reverse biased PIN or Schottky diode. The absorption of the X-ray in the material causes EHP generation that is detected. For Wavelength Dispersive Spectroscopy (WDS) the X-Ray is directed to a rotating crystal and then into a “gas proportional counter” The resultant electrons and positive charges are separated by two electrodes to get a measure of the intensity.

Other modes associated with SEM: CL Here the EHPs generated as a result of the incident electron beam recombine to give similar information as a PL spectrum. Advantage here is that materials with different bandgaps can be probed easily. Also, some depth profiling is possible. Image of a granite rock from Plano, Texas. Instrumentation: SEM FEI Quanta-FEG 400 with Gatan Chroma CL.

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 dark filed images The scattering of electrons in TEM is much less than SEM, and almost always in the forward direction due to small interaction volume

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) can be achieved FIB: Using Focused Ion beam based milling technique in modern equipments, 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

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 the 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 Dec 5, 2011,10.00 - 1.00 pm. Two page (double sided) formula sheets will be allowed The project report will be due on Dec 2, 2011 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.