Imaging with Diffraction Contrast

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Jerome Daniel
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1 Imaging with Diffraction Contrast Duncan Alexander EPFL-CIME 1 Introduction When you study crystalline samples TEM image contrast is dominated by diffraction contrast. An objective aperture to select either the direct beam for a bright-field image, or diffracted beam(s) for a dark-field image. The TEM image effectively maps spatially the intensity in this beam. Changes in intensity are given by: - general crystal orientation relative to the electron beam, and crystal phase - local changes in crystal orientation from crystal defects and bending - sample thickness changes. These effects combine to give the diffraction contrast that are your TEM data. Inherent connection to diffraction pattern of your specimen. 2

$Diffraction contrast?$

2 Diffraction contrast? Possible interactions of beam with specimen Only some lead to contrast in TEM image Auger electrons Backscattered electrons BSE Incident beam secondary electrons SE Characteristic X-rays visible light nm absorbed electrons Specimen electron-hole pairs elastically scattered electrons direct beam Bremsstrahlung X-rays inelastically scattered electrons Diffraction contrast? Biological specimen with heavy metal stain: majority of contrast from absorption of electrons. Sample of interest biologically, but physics of scattering are boring. Backscattered electrons BSE Incident beam nm absorbed electrons Specimen elastically scattered electrons direct beam inelastically scattered electrons

$Diffraction contrast?$ Elastic scatter

$electrons (i.e. diffraction) by crystal is major contribution to contrast.$ $grains at defects change the diffraction condition and hence the$

3 Diffraction contrast? Materials science specimen: CVD-grown ZnO thin film. Elastic scattering of the electrons (i.e. diffraction) by crystal is major contribution to contrast. Changes in crystal orientation from grain to grain or within grains at defects change the diffraction condition and hence the contrast. Backscattered electrons BSE Incident beam nm absorbed electrons Specimen elastically scattered electrons direct beam inelastically scattered electrons TEM imaging/diffraction modes recap Diffraction mode Image mode 6

$aperture Dark-field image = select diffracted beam with objective aperture 7 Nanocrystalline$ $sample image/diffraction Bright field image setup - select direct beam with objective aperture$

4 Bright-field image vs dark-field image Bright-field image = select direct beam with objective aperture Dark-field image = select diffracted beam with objective aperture 7 Nanocrystalline sample image/diffraction Bright field image setup - select direct beam with objective aperture Diffraction mode Image mode Contrast from different crystals according to diffraction condition 8

$Nanocrystalline sample image/diffraction Dark field image setup - select some transmitted beams with objective aperture Diffraction mode Image mode Only crystals diffracting strongly into objective$

5 Nanocrystalline sample image/diffraction Dark field image setup - select some transmitted beams with objective aperture Diffraction mode Image mode Only crystals diffracting strongly into objective aperture give bright contrast in image 9 Nanocrystalline sample image/diffraction Dark field image setup - select some transmitted beams with objective aperture Diffraction mode Image mode Only crystals diffracting strongly into objective aperture give bright contrast in image 10

6 Nanocrystalline sample image/diffraction DF A BF DF B DF images allow us to pick out individual grains 11 Diffraction contrast on/off zone axis In bright-field imaging, zone axis condition => more scattering to diffracted beams Therefore intensity in direct beam goes down and bright-field image has strong contrast 12

$Diffraction contrast on/off zone axis In$ $more scattering to diffracted beams Therefore$ bright-field image has strong contrast

axis CBED SADP BF On zone axis 13 BF off

7 Diffraction contrast on/off zone axis In bright-field imaging, zone axis condition => more scattering to diffracted beams Therefore intensity in direct beam goes down and bright-field image has strong contrast Example: GaN nanowire CBED SADP BF Off zone axis CBED SADP BF On zone axis 13 Diffraction contrast on/off zone axis In bright-field imaging, zone axis condition => more scattering to diffracted beams Therefore intensity in direct beam goes down and bright-field image has strong contrast BF off zone axis Example: GaN nanowire BF on zone axis 14

$Image delocalization TEM image with no objective aperture. Image formed from direct beam and diffracted beams.$ Gives shadow images that move with objective focus (mostly result from Cs).

Gives shadow images that move with objective focus (mostly result from Cs).

8 Image delocalization TEM image with no objective aperture. Image formed from direct beam and diffracted beams. Dark-field images from diffracted beams delocalize from bright-field image of direct beam. Gives shadow images that move with objective focus (mostly result from Cs). 15 Image delocalization Image of same nanowires but with objective aperture to make bright-field image. No diffracted beams => no shadow images. This is how you should take your TEM data! 16

9 Image delocalization TEM (multi-beam) image Bright-field image Direction of delocalization depends on direction of diffracted beam in diffraction pattern 17 Dark-field imaging: displaced aperture vs beam tilt 18

10 Bend contours From Williams & Carter Transmission Electron Microscopy 19 Bend contours Example: electropolished Ni3Al superalloy Sample at lower magnification. 20

11 Bend contours Example: electropolished Ni3Al superalloy Bright-field images. Same region at different sample tilts. See bend contours interacting with dislocations. 21 Bend contours Example: mechanically polished and ion beam milled silicon 22

=> extinction at different thickness compared to substrate 23 SADP simple cubic γ Phase contrast Example:

12 Thickness fringes Dynamical scattering in the dark-field image => Intensity zero for thicknesses t = nξg (integer n) See effect as dark thickness fringes on wedge-shaped sample: Composition changes in quantum wells => extinction at different thickness compared to substrate 23 SADP simple cubic γ Phase contrast Example: electropolished Ni3Al superalloy Simple cubic γ precipitates in FCC γ matrix SADP FCC γ

25 Phase contrast Example: electropolished Ni3Al superalloy Simple cubic γ

13 SADP simple cubic γ Phase contrast Example: electropolished Ni3Al superalloy Simple cubic γ precipitates in FCC γ matrix Bright-field image Phase contrast Example: electropolished Ni3Al superalloy Simple cubic γ precipitates in FCC γ matrix SADP simple cubic γ Dark-field image g =

image g = 2 0 0 0 0 0 1 0 0 2 0 0 27 Crystal defects: dislocations

14 Phase contrast Example: electropolished Ni3Al superalloy Simple cubic γ precipitates in FCC γ matrix SADP simple cubic γ Dark-field image g = Crystal defects: dislocations Burgers vector for edge (top right) and screw (bottom right) dislocations 28

$Crystal defects: dislocations Local bending of crystal planes around the dislocation change their diffraction condition This produces a contrast in the image =>$

15 Crystal defects: dislocations Local bending of crystal planes around the dislocation change their diffraction condition This produces a contrast in the image => g.b analysis for Burgers vector From Williams & Carter Transmission Electron Microscopy 29 Crystal defects: dislocations - g.b analysis Invisibility criterion! 30

visibility in for images a) to c) and near invisibility

Similarly to Burgers vector analysis, stacking faults

r = 0 Example: analysis of basal plane stacking faults in

16 Crystal defects: dislocations - g.b analysis Example: threading dislocations in GaN From visibility in for images a) to c) and near invisibility in d), dislocation B found to have: b = ±1/3[ ] or b = ±1/3[ ] Sakai et al. APL 71 (1997) Planar defects stacking faults Similarly to Burgers vector analysis, stacking faults with displacement vector R are invisible for g.r = 0 Example: analysis of basal plane stacking faults in ZnO SADP on [1 1 00] zone axis Bright-field g = Dark-field g = g g g parallel to R: stacking faults visible 32

Planar defects stacking faults Similarly to Burgers vector analysis, stacking faults with displacement vector

r = 0 Example: analysis of basal plane stacking faults in ZnO SADP on [1 1 00] zone axis Bright-field g = 0 0

$Or low diffraction contrast?$

17 Planar defects stacking faults Similarly to Burgers vector analysis, stacking faults with displacement vector R are invisible for g.r = 0 Example: analysis of basal plane stacking faults in ZnO SADP on [1 1 00] zone axis Bright-field g = Dark-field g = g g g perpendicular to R: stacking faults invisible 33 Amorphous? Or low diffraction contrast? Bright-field image of Al-Cr-N-O thin film Some regions show clear diffraction contrast, so obviously crystalline. Other regions did not. Combined with contrast from probable amorphous surface layer, looked amorphous. However CBED proved crystalline, just in low diffraction contrast condition. 34

$diffraction condition and hence intensity and contrast in the resulting image.$

18 Phase contrast, grain boundaries, bend contours, dislocations in one image 35 Summary When imaging crystalline samples in the TEM, any change in the crystal will change the elastic scattering diffraction condition and hence intensity and contrast in the resulting image. This can be from: - crystal orientation relative to the electron beam and deviations from bending - defects in the crystal - changes in phase (e.g. precipitates) - changes in thickness (dynamical scattering). Using an objective aperture to select the direct beam (bright-field image) or diffracted beam (darkfield image) combined with carefully selected diffraction conditions gives many possibilities for analysing and understanding your specimen. Even without pursuing this you still have to learn to live with diffraction contrast and how it can unhelpfully affect your TEM data (e.g. bend contours). In this way TEM is not like other types of microscopy! No objective aperture => ill-defined diffraction condition and image delocalization! 36

TEM imaging and diffraction examples

$TEM imaging and diffraction examples$ TEM imaging and diffraction examples Duncan Alexander EPFL-CIME 1 Diffraction examples Kikuchi diffraction Epitaxial relationships Polycrystalline samples Amorphous materials Contents Convergent beam electron