Evidence of structural strain in epitaxial graphene layers on 6H-SiC(0001) (0808.3605v3)

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.

• The paper demonstrates the transition from isolated C=C dimers to coherent graphene layers as indicated by specific C/Si ratios.
• The study identifies significant compressive strain originating from thermal expansion mismatch, which varies with annealing time and growth conditions.
• Depolarized Raman spectroscopy and AES are employed to resolve early submonolayer growth stages, offering actionable insights for tuning graphene's electronic properties.

Structural Strain in Epitaxial Graphene on 6H-SiC(0001)

The paper titled "Evidence of Structural Strain in Epitaxial Graphene Layers on 6H-SiC(0001)" explores the intricacies of epitaxially grown graphene on silicon carbide (SiC) substrates, focusing specifically on the Si-terminated 6H-SiC(0001) surface. Utilizing Auger electron spectroscopy (AES) and depolarized Raman spectroscopy, the authors investigate the early stages of graphene layer formation and the induced structural strains.

Investigative Approach

The experimental approach hinges on the use of depolarized Raman spectroscopy, allowing for enhanced detection of carbon-carbon bond vibrations amidst predominant signals from the SiC substrate. This method enables the identification of submonolayer graphene growth, starting with isolated C=C dimers within a diamond-like carbon matrix. The authors quantify the transition to a fully developed, delocalized electronic graphene system at critical C/Si AES ratios, indicative of carbon layer thickness.

Key Findings

1. Growth Stages and Structural Evolution: The paper resolves the progressive transformation from covalently bonded C=C dimers to full submonolayer islands characterized by delocalized electronic states and finally to a coherent epitaxial graphene layer. This process is demarcated by specific C/Si ratios detected via AES.
2. Strain Analysis: A compressive strain is identified in the epitaxial graphene at room temperature. This strain arises due to the mismatch in thermal expansion coefficients between graphene and SiC. The observed linear strain is substantially compressive and varies with growth conditions, primarily affected by annealing time.

Implications

The research illuminates critical aspects of the epitaxial growth process impacting mechanical and electronic properties of graphene films. Understanding strain in graphene has direct implications for its application in electronic devices, where mechanical stability and electron transport properties are crucial. The significant strain explored in this paper suggests potential tunability of graphene's properties through precise control of the synthesis parameters, specifically growth time and temperature.

Conjectures on Future Research Directions

Future explorations could focus on the interplay between different substrate types and graphene, which could reveal additional strain-inducing mechanisms or mitigate them. Besides, investigating how these strains impact the electronic band structures further could provide vital insights for the practical implementation of graphene in electronic and optoelectronic device technologies.

In conclusion, this work underscores the necessity of understanding substrate interactions and strain phenomena in epitaxial graphene films, contributing foundational knowledge pertinent to the advancement of graphene-based materials in semiconductor research.