Role of Si clusters in the phase transformation and formation of (6x6)-ring structures on 6H-SiC(0001) as a function of temperature: An STM and XPS study

We report the formation of the 6H-SiC(0001)/(6x6) ring structure using scanning tunneling microscopy (STM) and x-ray photoemission spectroscopy. This reconstruction is obtained from the evolution of surface structures with progressive annealing; starting from a (3 x 3) phase at 850 degrees C -> Si rich (6 x 6) clusters at 1000 degrees C ->(6 x 6) rings at 1100 degrees C. We show that this ring structure can be described by a (6x6) unit cell rather than a (6 root 3 x 6 root 3)R30 degrees reconstruction. STM evidence reveals the presence of type A clusters (size similar to 8 angstrom) and type B clusters (size similar to 14.3 angstrom) on the surface during this structural transformation. The density of these clusters decreases with progressive annealing, eventually leaving behind only the type C Si adatoms (size similar to 3 angstrom), which forms the ring structure at 1100 degrees C. This ring structure albeit less Si rich compared to the (6 x 6) cluster and the initial (3 x 3) surface is not graphitic. We propose a ball and stick model taking into consideration the role of Si as tetraclusters to describe the structural transformation and formation of the ring structure from the initial (3 x 3) reconstructed surface. We found that each structural phase observed can be accounted for by moving or removing at least M x 4 Si atoms (where M = number of tetraclusters).