Conventional TEM STEM N o r t h w e s t e r n U n i v e r s i t y - M a t e r i a l s S c i e n c e
Reciprocity 1 1 C CCCCCCCC(2ππππππ. rr) CCCCCC(2ππππππ. rr)
Reciprocity 2 1 C+D CC(gg)CCCCCC(2ππππππ. rr) DD(gg)CCCCCC( 2ππππππ. rr) CCCCCC(2ππππππ. rr) eeeeee( 2ππππππ. rr)
STEM v TEM STEM = TEM, but with the optics reversed (if you ignore inelastic scattering) BF, DF, HREM can be done in a STEM almost the same as a TEM However: The imaging is serial as against parallel, so in general has more noise and lower signals Scan coils and electronics involved, which leads to noise Some types of imaging can be done easier in a STEM than a TEM (they have all been done in both) STEM needs a small source (demagnified further) Some manufacturers have decided to reinvent the wheel
Comparison TEM Condensor Aperture Objective aperture after sample No analogue Selected Area Aperture STEM Detector Objective aperture before sample Virtual objective aperture (condensor) No analogue
Environmental Sensitivity Building door opens twin 20 Å Building door closes D. A. Muller JEM 50, 219 (2001) 1 Pa change in air pressure 1 Å stage deflection P. M. Voyles, 5/3/12 7
Effect of Cs on Ronchigram Increasing Objective lens strength UIC Electron Microscopy Service
Stigmating using the Ronchigram a b c Ronchigram from amorphous area. Select Scan mode Spot 1 and a magnification above 100Kx. a) underfocus astigmatic Red circle marks unaberrated part of Ronchigram that should be selected by Objective aperture b) underfocus stigmated c) Gausian focus stigmated (almost!) UIC Electron Microscopy Service
STEM - the important parts
Real Life FEG STEM at UIC
More detectors are available!
JEOL HADF Detector
Bright Field & Dark Field Pick different parts of scattering space with different detectors. probe Caveat: do not blindly use Zcontrast as a term without thinking, common error sample unscattered beam Bragg beams diffuse & HOLZ BF ADF HAADF P. M. Voyles, 5/3/12 15
Partitioning Signal in Scattering Space Scattering from one atom is proportional to scattering factor f e (q). 12 10 8 low-angle scattering (CTEM) C, Z=6 Si, Z=14 Cu, Z=29 Ge, Z=32 Sn, Z=50 Pb, Z=82 10 1 high-angle scattering ADF-STEM C, Z=6 Si, Z=14 Cu, Z=29 Ge, Z=32 Sn, Z=50 Pb, Z=82 f e (q) 6 4 f e (q) 0.1 2 0.01 0 0.0 0.5 1.0 q (Å -1 ) 1.5 2.0 0 2 4 q (Å -1 ) 6 8 10 BF CTEM involves mostly lowangle scattering: Cu & Si cross. ADF-STEM involves highangle scattering only. P. M. Voyles, 5/3/12 16
Partitioning Signal in Scattering Space q = 0-0.2 Å -1 q = 2.0-10.0 Å -1 low-angle cross section high angle cross section 20 40 60 atomic number (Z) 80 20 40 60 atomic number (Z) 80 sensitive to electron shell filling, complicated (also diffraction & channelling, later) monotonic, proportional to Z 2 Rutherford billiard ball scattering P. M. Voyles, 5/3/12 17
Z-contrast Scattering scales as ~Z 1.7 for common scattering angles Images are readily interpretable Images contain some chemical information Many problems with this simple picture (good enough for Govt work) E. J. Kirkland, Advanced Computing in Electron Microscopy (Plenum, 1998). P. M. Voyles, 5/3/12 18
Caveat Consider via Fermi s Golden Rule Signal = ρ(r) <f V i> 2 Z-contrast only corresponds to the <f V i> 2 contribution (HOLZ, phonons) But ρ(r) comes from dynamical diffraction True Z-contrast occurs iff ρ(r) has a simple form, which it does in special cases
Comparison between High-Angle and Low- Angle Annular Dark-Field Image Cross sectional sample through InGaAs quantum dots and associated quantum well grown on GaAs and capped with GaAs. Dots are arrowed. 20 nm 20 nm 20 nm Low-angle annular dark field showing strain contast ASU Winter School 2013 High-angle annular dark field showing mostly Z contrast
Contrast Reversals in Thick Samples at 200kV ADF-STEM (φ c >45 mr) ADF-STEM (φ c >75 mr) No more diffraction contrast Signal in W plug not monotonic, could be mistaken for voids Effect reduced by increasing the collector angle David Muller 2006 21
Strain Contrast at Si/SiO 2 Interfaces (JEOL 2010F, 200 kv, C s =1mm) ADF Inner angle: 50 mrad 25 mrad a-si SiO 2 c-si Strain Fields cause dechanneling (and scattering to small angles) David Z. Yu, Muller D. A. 2001 Muller, and J. Silcox, J. Appl. Phys. 95, 3362 (2004).
Strain Contrast vs. Sample Thickness Contrast at a c-si/-asi is strongly depends on sample thickness 100 kv, 45 mrad ADF inner angle 130 Å thick 340 Å thick Strain Contrast effects at the interface: for 130 Å thick sample, ~0%; for 340 Å thick sample, 15%. Z. Yu, D. A. Muller, and J. Silcox, J. Appl. Phys. 95, 3362 (2004).
Examples: Low Resolution oxidized Co nanoparticle Co Co x O y Co / AlO x / Co tunnel junction M. J. Plisch et al. APL 79, 391 (2001). P. M. Voyles, 5/3/12 24
Bi-Implanted Si A shows low-res BF Can t really see implant Can see damage B is Z-contrast image Bi lights up like a Christmas Tree The damage layer is not so visible. No phase contrast Examples borrowed from Williams & Carter
Examples: High Resolution Sb-doped Si 10 Å As δ-doped layer in Si D. A. Muller PRL 83, 3234 (1999) undoped Si 1 nm Grain boundary in MgO Y. Yan PRL 81, 3675 (1998) Individual Sb atoms and defect nanoclusters in Si P. M. Voyles, Nature 416, 826 (2002) P. M. Voyles, 5/3/12 26
SiO 2 on Ge on Si Amorphous region is visible in hi-res Oxide is dark, Ge is light, Si in between - can see lattice
HAADF-STEM High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy Au13 Huiping Xu, Ray Twesten A representative STEM- HAADF image (HB 501, 100kv, inner angle: 96mgrad) of sample Au 13 (PPh 3 ) 4 (SC12) 4
Examples Growth of Er clusters in SiC on annealing. U. Kaiser, Nature Materials 1, 102 (2002) P. M. Voyles, 5/3/12 29
Examples STO LTO SrTiO 3 / LaTiO 3 multilayers A. Ohtomo Nature 419, 378 (2002) P. M. Voyles, 5/3/12 30
I = ß Z n Z O = 8 Z Fe = 26 Z Pb = 82 Abakumov et al. Angewandte Chemie (2006)
Summary: STEM Similar signals to TEM, often easier to obtain some types Parallel collection of different signals, but serial detection Easier for EELS/EDS mapping and similar Annular dark-field is not as simple as often thought, serious misinterpretations exist in the current literature Strain/diffraction contrast effects in ADF Approaches Z-contrast in HAADF Sometimes not Z-contrast