Surface morphology of epitaxial CaF₂/Si(111) and its influence on subsequent GaAs epitaxy

Atomic force microscopy (AFM) has been intensively used to investigate the surface morphology of CaF₂ films. The atomic steps of CaF₂ grown on tilted Si substrates are observed to bunch up during growth, which leads to the formation of large, atomically flat plateaus separated by polyatomic steps. Both the shape and the size of those steps strongly depend upon the misorientation direction of the Si substrate. Quantitative AFM analysis indicates that the size of the plateaus on the CaF₂ surface grown on Si substrates tilted towards the [1̄1̄2] azimuth is about eight times smaller than the size of those grown on a Si substrate tilted towards the opposite direction (the [112̄] azimuth). Smaller plateaus on the CaF₂ surface are observed to result in smaller twins which are more easily suppressed if proper growth conditions are chosen. High resolution scanning electron microscopy has also been employed to study the surface of GaAs films grown on CaF₂/Si(111) substrates with different misorientation directions. The GaAs films have a layer-by-layer growth mode when grown under optimal growth conditions.