Reflection High Energy Electron Diffraction Wei-Li Chen 11/15/2007

RHEED Reflection High Energy Electron Diffraction, RHEED, is an important real time analytical tool to monitor growth front condition. The small incident angle makes it sensitive to the structure of top monolayers.  Substrate thermal cleaning monitoring  Controlling initial growth stage  Monitoring surface structure and growth dynamics  Growth rate measurement

Bravais Lattice A Bravais lattice is an infinite arrays of discrete points with an arrangement and orientation that appears exactly the same, from whichever of the points the array is viewed. A Bravais lattice consists of all points with position vectors R of the form

Crystal Structure The atom group, which builds complete crystal structure by translational operations is called crystallographic unit cell Crystal structure = Lattice + Basis In General: a ≠ b ≠ c and α ≠ ß ≠ γ

Reciprocal Lattice

Miller Indices Plane Vector in Bravais lattice Crystal symmetry equivalent plane sets Crystal symmetry equivalent vector sets

Bragg’s and von Laue’s Formula

The Edwald Construction

The Geometry of RHEED The geometry of RHEED is quite simple, Fig. 1. An accelerated electron beam (5 – 100 keV) is incident on the surface with a glancing angle (< 3 deg) and is reflected. The high energy of the electrons would result in high penetration depth. However, because of the glancing angle of incidence, a few atomic layers are only probed. This is the reason of the high surface sensitivity of RHEED. Upon reflection, electrons diffract, forming a diffraction pattern that depends on the structure and the morphology of the probed surface.

“Molecular Beam Epitaxy “ edited by R. F. C. Farrow

Fourier Transform

Ideal smooth surface real smooth surface 3D clusters polycrystal

Powder or Polycrystal diffraction spots  rings Rotation of crystal

Growth Calibration by RHEED Oscillation The reflection intensity of the specular point is related to the roughness of the surface, which changes periodically with the accumulation of film thickness. “Molecular Beam Epitaxy “ edited by R. F. C. Farrow

GaN RHEED Oscillation Growth rate reduction due to thermal decomposition

Surface Reconstruction In order to minimize the energy of the near-surface region of the crystal, the atoms rearrange themselves in a regular fashion which exhibits long range order. Each ordered arrangement of the near surface region is known as a surface reconstruction. Surface reconstruction reflects the stoichiometry of the growth process and influences the growth mechanism. RHEED is used to monitor the surface reconstruction of the growth front since it is sensitive to atomic layers near the surface. Usually surface reconstruction is affected by substrate temperature, impinging fluxes, III/V flux ratio, and the existence of surfacants.

“Molecular Beam Epitaxy” edited by R. F. C. Farrow “Molecular Beam Epitaxy “ edited by R. F. C. Farrow

Polarity of GaN c-plane surface

GaN Surface Reconstruction 2-fold 3-fold

As as a Surfacant