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, 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)Two advantages of CL over PL (b)Two comparative advantages and disadvantages of SEM and TEM (c)The three different characterization techniques associated with SEM are ………., …….. and ………….. (d)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? (e)Why is Field Emission Gun (FEG) SEM preferred over traditional SEM? (f)How do you avoid charging issues in an insulated sample in an SEM? (g)What is a suitable method to obtain lattice spacing and chemical composition of a 10 nm diameter InN nanowire? Justify. (h)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)Calculate the De Broglie wavelengths of the electrons in SEM (30 keV) and TEM (300 keV) (b)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? (c)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.