Edge states and ballistic transport in zigzag graphene ribbons: The role of SiC polytypes

Zigzag-edge graphene sidewall ribbons grown on 6H-SiC {11 (2) over barn} facet walls are ballistic conductors. It is assumed that graphene sidewall ribbons grown on 4H-SiC {11 (2) over barn} facets would also be ballistic. In this work, we show that SiC polytype indeed matters: ballistic sidewall graphene ribbons only grow on 6H-SiC facets. 4H and 4H-passivated sidewall graphene ribbons are diffusive conductors. Detailed photoemission and microscopy studies show that 6H-SiC sidewall zigzag ribbons are metallic with a pair of n-doped edge states associated with asymmetric edge terminations. In contrast, 4H-SiC zigzag ribbons are strongly bonded to the SiC, severely distorting the ribbon's pi bands. H-2 passivation of the 4H ribbons returns them to a metallic state but they show no evidence of edge states in their photoemission-derived band structure.