Raman spectra of epitaxial graphene on SiC(0001)

Abstract

We present Raman spectra of epitaxial graphene layers grown on 63×63 reconstructed silicon carbide surfaces during annealing at elevated temperature. In contrast to exfoliated graphene a significant phonon hardening is observed. We ascribe that phonon hardening to a minor part to the known electron transfer from the substrate to the epitaxial layer, and mainly to mechanical strain that builds up when the sample is cooled down after annealing. Due to the larger thermal expansion coefficient of silicon carbide compared to the in-plane expansion coefficient of graphite this strain is compressive at room temperature.

Raman Topography and Strain Uniformity of Large-Area Epitaxial Graphene

“…10,12 Similarly, large deviation of the 2D peak position from that found in exfoliated graphene flakes was found previously in epitaxial graphene films. 10,11,12 The variation in the 2D peak position could be a result of graphene thickness non-uniformities. However, fitting individual spectra taken in this line scan using a single Lorentzian indicates that the film was indeed monolayer graphene as demonstrated independently by photoemission spectroscopy.…”

“…Micro-Raman spectroscopy is a rapid, highresolution optical characterization technique that yields important information on the thickness, the charge carrier density, and the strain of epitaxial graphene. 10,11,12,13 However, no studies of Raman topography, the two-dimensional mapping of Raman spectrum over large-area epitaxial graphene, have been carried out to date. …”

“…We choose to focus on the 2D peak in the Raman spectrum in graphene due to concerns regarding subtraction of the SiC background seen in, for example, the G peak. 12 For Raman topography we map a two-dimensional region of a graphene film, collecting Raman spectra with a step size of 300 nm in both x and y directions. The position of each Raman peak was identified using the center of mass of the peak via the equation To simplify the computation, we focus on a range of 2650 -2825 cm -1 at each sample location (x,y) including only the 2D peak.…”

“…12,18,19 In Figure 1a, the positions of the 2D peak in a line scan are plotted. Interestingly, its values are found to vary significantly, ranging from 2689 cm -1 to 2754 cm -1 , with many values falling below that of bulk graphite.…”

Correlating Raman Spectral Signatures with Carrier Mobility in Epitaxial Graphene: A Guide to Achieving High Mobility on the Wafer Scale

“…[15][16][17][18][19] Characterization of EG via Raman spectroscopy requires fitting the 2D Raman peak. 15,16,20 Raman spectra of EG Si fit by one or four Lorentzian functions are characteristic of monolayer or bilayer graphene, respectively. 15 Figure 1a demonstrates layer thickness evaluation for monolayer and bilayer EG Si via Lorentzian fitting of the 2D Raman spectra.…”