Reflection high energy electron diffraction (RHEED) has been the “work horse” for real-time quantitative characterization of epitaxial growth, owing to its surface sensitivity, large working distance, and the reflection geometry compatible with deposition. The grazing incident angle for the RHEED beam with respect to the surface, typically 1-2 degrees gives rise to a very small penetration depth, resulting in a diffraction pattern that is sensitive to only the outer most surface layer. Therefore, by monitoring the RHEED patterns during growth and across the sample, evolution of surface structures can be examined systematically.