RHEED oscillations provide a very fast and accurate method of growth rate determination for 2D materials. The principle involves variation of the electron scattering which can be monitored by integrating the primary RHEED spot intensity. The idea being a smooth surface provides an intense, coherent primary spot, whilst a rough surface provides a weak, incoherent primary spot. The degree of roughness corresponds to each fraction of a ML growth with a maximum roughness and hence low intensity for 0.5ML deposited and a maximum smoothness and hence high intensity for the smooth surface after 1 full ML. This is shown diagrammatically in figure 1 (below).
There is actually a lot more going on than meets the eye. To begin with we can determine the growth rate of GaAs and AlAs on GaAs(100). To start you will want to anneal the surface under a low As flux (~0.3ML/s at 600°C) to ensure you have a very flat starting surface. Then you simply set your optimum As growth flux and open the respective Ga or Al shutter and monitor the intensity. Typically a frame grabber card and appropriate software is used, but even the naked eye can discern the first few intensity oscillations.
It is a simple case of counting the number of oscillations (1 oscillation = 1ML) and averaging them over time (in seconds) to determine the growth rate in ML/s.
The oscillations will eventually dampen out, it depends on how smooth the starting surface was and how well you balanced the III:V ratio. 30+ oscillations are good. The reason for the damping is due to the fact that the 2nd ML starts on the wide islands before the 1st ML is fully formed and so on for the 3rd and 4th MLs; hence the system is moving toward some equilibrium surface roughness.
The oscillations may also not be equally spaced. The first few may have a larger or small period than the last 30. This is actually informing you about the growth rate perturbation caused by the shutter transient. This can actually be significant ±20% has been observed on poorly designed or orientated sources. The time and magnitude of the perturbation can have serious consequences, especially when growing thin QWs or SLs where the growth only comprises the shutter transient. The WEZ-type sources with integral shutters from MBE Komponenten utilised on our system have excellent stability and virtually no shutter transients.
 J.H. Neave, B.A. Joyce, P.J. Dobson and N. Norton, Appl. Phys. A 1983 31(1):1