Topic 7: Diffraction Techniques

1. Phys 661 - Baski Diffraction Techniques Topic #7: Diffraction Techniques

2. Phys 661 - Baski Diffraction Techniques Electron Scattering: Elastic (Diffraction) & Inelastic

3. Phys 661 - Baski Diffraction Techniques Intro: Wave-like Behavior of Electrons De Broglie wavelength l for an electron is given by:

4. Phys 661 - Baski Diffraction Techniques Intro: Wave Interference

5. Phys 661 - Baski Diffraction Techniques Intro: Real vs. (Reciprocal, Diffraction, or k) Space

6. Phys 661 - Baski Diffraction Techniques LEED: History Low Energy Electron Diffraction (LEED) = e– in, e– out (elastic) 1924: Discovered accidentally by Davisson and Kunsman during study of electron emission from a Ni crystal. 1927: Davisson and Germer found diffraction maxima for: nl = D sinf where D = surface spacing, l = electron wavelength 1934: Fluorescent screen developed by Ehrenburg for data imaging. 1960: UHV technology enabled LEED of clean surfaces.

7. Phys 661 - Baski Diffraction Techniques LEED: Front-view Apparatus

8. Phys 661 - Baski Diffraction Techniques k-Space: Bragg Scattering vs. LEED Equation

9. Phys 661 - Baski Diffraction Techniques k-Space: Ewald Sphere for LEED

10. Phys 661 - Baski Diffraction Techniques k-Space: Square Lattice Reconstructions

11. Phys 661 - Baski Diffraction Techniques LEED: Si(111)7x7

12. Phys 661 - Baski Diffraction Techniques LEED: Data Analysis

13. Phys 661 - Baski Diffraction Techniques RHEED: Schematic of Technique RHEED has higher energy (keV) and lower angle (2°) vs. LEED. Real-time data acquisition possible, but diffraction pattern is distorted.

14. Phys 661 - Baski Diffraction Techniques k-Space: Ewald Sphere for RHEED

15. Phys 661 - Baski Diffraction Techniques RHEED: Si(111)7x7

16. Phys 661 - Baski Diffraction Techniques RHEED: AlN Surface periodicity given by spacing between peaks. Surface quality given by full-width at half-max of peaks.

17. Phys 661 - Baski Diffraction Techniques X-ray Diffraction (XRD) Bragg’s Law and Ewald Construction Types of Scans: Theta/2Theta (?/2?) Rocking Curve Diffraction-Space Map Philips Materials ResearchDiffractometer

18. Phys 661 - Baski Diffraction Techniques XRD: Diffraction Condition

19. Phys 661 - Baski Diffraction Techniques XRD: (?/2?) Scan or “Gonio” on MRD Vary MAGNITUDE of ?k while maintaining its orientation relative to sample normal.HOW? Usually rotate sample and detector with respect to x-ray beam. Resulting data of Intensity vs. 2q shows peaks at the detector (kf) for Dk values satisfying the diffraction condition. Detects periodicity of planes parallel to surface.

20. Phys 661 - Baski Diffraction Techniques XRD: ?/2? Example Polycrystalline sample has a number of peaks due to mixture of crystal orientations.

21. Phys 661 - Baski Diffraction Techniques XRD: “Rocking” Curve Scan Vary ORIENTATION of ?k relative to sample normal while maintaining its magnitude.How? “Rock” sample over a very small angular range. Resulting data of Intensity vs. Omega (w, sample angle) shows detailed structure of diffraction peak being investigated.

22. Phys 661 - Baski Diffraction Techniques XRD: Rocking Curve Example Rocking curve of single crystal GaN around (002) diffraction peak showing its detailed structure.

23. Phys 661 - Baski Diffraction Techniques XRD: Diffraction-Space Map Vary Orientation and Magnitude of ?k. Diffraction-Space map of GaN film on AlN buffer shows peaks of each film.

24. Phys 661 - Baski Diffraction Techniques XRD: X-ray Tube (non-monochromatic)