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

Abstract

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 (3x3) phase at 850°C→Si rich (6x6) clusters at 1000°C→ (6x6) rings at 1100°C. We show that this ring structure can be described by a (6x6) unit cell rather than a (63x63) R30° reconstruction. STM evidence reveals the presence of type "A" clusters (size ∼8 Å) and type "B" clusters (size ∼14.3 Å) 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 ∼3 Å), which forms the ring structure at 1100°C. This ring structure albeit less Si rich compared to the (6x6) cluster and the initial (3x3) 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 (3x3) reconstructed surface. We found that each structural phase observed can be accounted for by moving or removing at least Mx4 Si atoms (where M=number of tetraclusters). © 2006 The American Physical Society.