Evidence of structural strain in epitaxial graphene layers on 6H-SiC(0001)

TL;DR

This study investigates the early growth stages of epitaxial graphene on 6H-SiC(0001) using AES and Raman spectroscopy, revealing strain characteristics and growth dynamics at submonolayer levels.

Contribution

It provides new insights into the structural strain and growth mechanisms of epitaxial graphene on SiC, using combined spectroscopic techniques.

Findings

Detection of submonolayer growth including C=C dimers.

Identification of compressive strain in graphene layers.

Strain magnitude varies with growth time.

Abstract

The early stages of epitaxial graphene layer growth on the Si-terminated 6H-SiC(0001) are investigated by Auger electron spectroscopy (AES) and depolarized Raman spectroscopy. The selection of the depolarized component of the scattered light results in a significant increase in the C-C bond signal over the second order SiC Raman signal, which allows to resolve submonolayer growth, including individual, localized C=C dimers in a diamond-like carbon matrix for AES C/Si ratio of $\sim$3, and a strained graphene layer with delocalized electrons and Dirac single-band dispersion for AES C/Si ratio $>$6. The linear strain, measured at room temperature, is found to be compressive, which can be attributed to the large difference between the coefficients of thermal expansion of graphene and SiC. The magnitude of the compressive strain can be varied by adjusting the growth time at fixed annealing temperature.