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Transcript of Polarity CBED
GaAs Unit cell:
How can we distinguish between 110 and 110?
Bond directions alternate between planes
Not possible with all microscopes
Not practical for all samples or checking large areas
Not to scale!
What is CBED?
α: convergence angle
θ: diffracted angle
Still not to scale!
 Williams, D. B., & Carter, C. B. (2009). Transmission Electron Microscopy.
What does CBED look like?
Simulated CBED pattern for InAs (110)
Phonons/Diffuse scattering are not considered
with diffuse scattering
More on Kikuchi lines
Not Bragg Condition
Kikuchi lines are fixed to the crystal
Conclusion: (hkl) is under Bragg condition when (-h-k-l) Kikuchi line overlaps with (000) spot
What happens in the (000) spot?
From a particle perspective, the beam is
by the Bragg reflection
line is found in the spot
From a wave perspective,
ω: Phase change after reflection
φ: Phase of structure factor
How do we calculate the structure factor phase, φ?
h: first Miller index
a: lattice constant
Polarity of GaAs
GaAs near (230) at 300 kV
InP near (230) at 120 kV
When (hkl) Kikuchi lines up with (002) spot:
Δω(5,-3,5;0,0,2) = -4°
Δω(5,-3,-3;0,0,2) = -4°
Δω(5,-3,-7;0,0,2) = 184°
Δω(5,-3,-5;0,0,2) = 184°
Δω(5,-3,-7;0,0,-2) = -4°
Δω(5,-3,5;0,0,-2) = -4°
Δω(5,-3,-5;0,0,-2) = 184°
Δω(5,-3,3;0,0,-2) = 184°
Experimental results from Jaeger
Polarity of InP
Interference is less clear
Δω for constructive is around 55°
*Stadelmann, P. A. (2012). JEMS
by Andrew Scullion
Zinc Blende Structure