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RHEED (Reflection high-energy electron diffraction) spot size is 60um to 1mm, so it reveals that the surface property of an area about 60um to 1mm. However, literature usually use RHEED pattern as an identification of sample quality. Why is that?

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Because the incident angle of electron beam in RHEED is very small, typically 1-2degree. Even though the spot size is small, the area where the electron beam cast on can be very large. For example, if spot size is 60um, and incident angle is 1degree, the area in its length should be 3437um(3.4mm).

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RHEED as a surface analysis tool provides feedback over features or lack thereof at length scales, which are significantly larger than transmission electron microscopy (TEM) images, but on the other hand much shorter than what you could resolve with optical microscopy. I'm aware that I'm mixing in-situ and ex-situ techniques here, but for the sake of what you can see with different tool, I guess this comparison is fine.

A typical RHEED system operates at acceleration voltages somewhere between 5 and 15 kV, which translates into de Broglie wavelengths in the range of 0.01-0.02 nm. This length scale is below the lattice constant of relevant materials and therefore in principle allows to resolve information at the order of single bonds or crystal unit cells.

RHEED can be used to determine surface reconstruction patterns, which are a few unit cells in length, but due to their periodicity give nice diffraction spots on a RHEED screen. In principle, one can monitor effects like Stranski-Krastanov formation of quantum dots, but such features are typically very faint and only visible in somewhat ideal cases. On the other hand, RHEED can also be used to determine the diffusion length on terraced surfaces. Broadening of individual diffraction spots can also give you an estimate on the crystal quality at larger length scales or the presence of an amorphous layer (e.g. the native oxide on semiconductor wafers).

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