In molecular beam epitaxy, a beam of molecules or atoms emanating from an evaporation or effusion source is transported in an evacuated chamber without collisions and gets deposited onto a single crystal semiconductor surface ultra high vacuum chamber. At this vacuum, the mean free path of gases is several orders of magnitude larger than the distance between source and substrate which is typically 10-20cm. At very low pressures, the mean free path is given by, L in cm = (5 x 10-3)/p where p is the pressure in Torr.