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Nanoscale structural characterization of epitaxial graphene grown on off-axis 4H-SiC (0001).

Vecchio C, Sonde S, Bongiorno C, Rambach M, Yakimova R, Raineri V, Giannazzo F - Nanoscale Res Lett (2011)

Bottom Line: Tapping mode atomic force microscopy (t-AFM) showed the formation of wrinkles with approx. 1 to 2 nm height and 10 to 20 nm width in the FLG film, as a result of the release of the compressive strain, which builds up in FLG during the sample cooling due to the thermal expansion coefficients mismatch between graphene and SiC.For each Tgr, the number of graphene layers was determined on very small sample areas by HR-XTEM and, with high statistics and on several sample positions, by measuring the depth of selectively etched trenches in FLG by t-AFM.Both the density of wrinkles and the number of graphene layers are found to increase almost linearly as a function of the growth temperature in the considered temperature range.

View Article: PubMed Central - HTML - PubMed

Affiliation: CNR-IMM, Strada VIII, 5, Catania 95121, Italy. filippo.giannazzo@imm.cnr.it.

ABSTRACT
In this work, we present a nanometer resolution structural characterization of epitaxial graphene (EG) layers grown on 4H-SiC (0001) 8° off-axis, by annealing in inert gas ambient (Ar) in a wide temperature range (Tgr from 1600 to 2000°C). For all the considered growth temperatures, few layers of graphene (FLG) conformally covering the 100 to 200-nm wide terraces of the SiC surface have been observed by high-resolution cross-sectional transmission electron microscopy (HR-XTEM). Tapping mode atomic force microscopy (t-AFM) showed the formation of wrinkles with approx. 1 to 2 nm height and 10 to 20 nm width in the FLG film, as a result of the release of the compressive strain, which builds up in FLG during the sample cooling due to the thermal expansion coefficients mismatch between graphene and SiC. While for EG grown on on-axis 4H-SiC an isotropic mesh-like network of wrinkles interconnected into nodes is commonly reported, in the present case of a vicinal SiC surface, wrinkles are preferentially oriented in the direction perpendicular to the step edges of the SiC terraces. For each Tgr, the number of graphene layers was determined on very small sample areas by HR-XTEM and, with high statistics and on several sample positions, by measuring the depth of selectively etched trenches in FLG by t-AFM. Both the density of wrinkles and the number of graphene layers are found to increase almost linearly as a function of the growth temperature in the considered temperature range.

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Optical and AFM images of O2 etched striped in graphitized 4H-SiC. Optical image on the sample annealed at 1700°C (a). AFM height profile taken on a stripe on pristine SiC (b) and on the sample annealed at 1700°C (c), respectively.
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Figure 7: Optical and AFM images of O2 etched striped in graphitized 4H-SiC. Optical image on the sample annealed at 1700°C (a). AFM height profile taken on a stripe on pristine SiC (b) and on the sample annealed at 1700°C (c), respectively.

Mentions: In the following, we will study the increase in the average number of graphene layers as a function of the growth temperature. HRXTEM images on the samples annealed at 1600, 1700 and 2000°C are reported in Figure 6a,b and 3c. From the linescans in Figure 3d,e and 3f, the interlayer spacing (~0.34 ± 0.01 nm) as well as the number of graphene layers on the surface of 4H-SiC is determined: 3, 8 and 18 layers can be estimated for the three temperatures. These cross-sectional analyses give a direct measure of the number of grown layers, but only on a very local scale. Lateral variation of the FLG thickness on different sample positions cannot be determined by such a method. To get an estimation of the number of layers at selected surface positions and with higher statistics, t-AFM was used to measure the depth of selectively etched stripes in FLG by O2 plasma. This plasma treatment is known to remove efficiently carbonaceous species through a chemical reaction leading to the formation of CO2. In Figure 7a, an optical image of the etched stripes in the sample annealed at 1700°C is reported. To obtain an accurate estimation, we checked if the SiC substrate is slightly etched by the used plasma processing. To this aim, a lithographically patterned pristine SiC substrate was simultaneously etched together with the graphitized SiC samples. Figure 7b,c show the height profile taken on a stripe on pristine SiC and on the sample annealed at 1700°C, respectively. From Figure 7b, it is clear that a thickness t0 ~2 nm of SiC is etched during the plasma treatment, due to the physical action of the plasma. This depth must be subtracted while evaluating the number of layers on graphitized SiC. Hence the number of layers can be estimated according to the relation n = (D - D0)/Dgr, being Dgr the interlayer separation between two stacked graphene layers (Dgr ≈ 0.35 nm). The average number of grown layers as a function of Tgr is reported in Figure 5b, where the error bars represent the standard deviations obtained from a large statistics on the number of layers determined at several sample positions.


Nanoscale structural characterization of epitaxial graphene grown on off-axis 4H-SiC (0001).

Vecchio C, Sonde S, Bongiorno C, Rambach M, Yakimova R, Raineri V, Giannazzo F - Nanoscale Res Lett (2011)

Optical and AFM images of O2 etched striped in graphitized 4H-SiC. Optical image on the sample annealed at 1700°C (a). AFM height profile taken on a stripe on pristine SiC (b) and on the sample annealed at 1700°C (c), respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211332&req=5

Figure 7: Optical and AFM images of O2 etched striped in graphitized 4H-SiC. Optical image on the sample annealed at 1700°C (a). AFM height profile taken on a stripe on pristine SiC (b) and on the sample annealed at 1700°C (c), respectively.
Mentions: In the following, we will study the increase in the average number of graphene layers as a function of the growth temperature. HRXTEM images on the samples annealed at 1600, 1700 and 2000°C are reported in Figure 6a,b and 3c. From the linescans in Figure 3d,e and 3f, the interlayer spacing (~0.34 ± 0.01 nm) as well as the number of graphene layers on the surface of 4H-SiC is determined: 3, 8 and 18 layers can be estimated for the three temperatures. These cross-sectional analyses give a direct measure of the number of grown layers, but only on a very local scale. Lateral variation of the FLG thickness on different sample positions cannot be determined by such a method. To get an estimation of the number of layers at selected surface positions and with higher statistics, t-AFM was used to measure the depth of selectively etched stripes in FLG by O2 plasma. This plasma treatment is known to remove efficiently carbonaceous species through a chemical reaction leading to the formation of CO2. In Figure 7a, an optical image of the etched stripes in the sample annealed at 1700°C is reported. To obtain an accurate estimation, we checked if the SiC substrate is slightly etched by the used plasma processing. To this aim, a lithographically patterned pristine SiC substrate was simultaneously etched together with the graphitized SiC samples. Figure 7b,c show the height profile taken on a stripe on pristine SiC and on the sample annealed at 1700°C, respectively. From Figure 7b, it is clear that a thickness t0 ~2 nm of SiC is etched during the plasma treatment, due to the physical action of the plasma. This depth must be subtracted while evaluating the number of layers on graphitized SiC. Hence the number of layers can be estimated according to the relation n = (D - D0)/Dgr, being Dgr the interlayer separation between two stacked graphene layers (Dgr ≈ 0.35 nm). The average number of grown layers as a function of Tgr is reported in Figure 5b, where the error bars represent the standard deviations obtained from a large statistics on the number of layers determined at several sample positions.

Bottom Line: Tapping mode atomic force microscopy (t-AFM) showed the formation of wrinkles with approx. 1 to 2 nm height and 10 to 20 nm width in the FLG film, as a result of the release of the compressive strain, which builds up in FLG during the sample cooling due to the thermal expansion coefficients mismatch between graphene and SiC.For each Tgr, the number of graphene layers was determined on very small sample areas by HR-XTEM and, with high statistics and on several sample positions, by measuring the depth of selectively etched trenches in FLG by t-AFM.Both the density of wrinkles and the number of graphene layers are found to increase almost linearly as a function of the growth temperature in the considered temperature range.

View Article: PubMed Central - HTML - PubMed

Affiliation: CNR-IMM, Strada VIII, 5, Catania 95121, Italy. filippo.giannazzo@imm.cnr.it.

ABSTRACT
In this work, we present a nanometer resolution structural characterization of epitaxial graphene (EG) layers grown on 4H-SiC (0001) 8° off-axis, by annealing in inert gas ambient (Ar) in a wide temperature range (Tgr from 1600 to 2000°C). For all the considered growth temperatures, few layers of graphene (FLG) conformally covering the 100 to 200-nm wide terraces of the SiC surface have been observed by high-resolution cross-sectional transmission electron microscopy (HR-XTEM). Tapping mode atomic force microscopy (t-AFM) showed the formation of wrinkles with approx. 1 to 2 nm height and 10 to 20 nm width in the FLG film, as a result of the release of the compressive strain, which builds up in FLG during the sample cooling due to the thermal expansion coefficients mismatch between graphene and SiC. While for EG grown on on-axis 4H-SiC an isotropic mesh-like network of wrinkles interconnected into nodes is commonly reported, in the present case of a vicinal SiC surface, wrinkles are preferentially oriented in the direction perpendicular to the step edges of the SiC terraces. For each Tgr, the number of graphene layers was determined on very small sample areas by HR-XTEM and, with high statistics and on several sample positions, by measuring the depth of selectively etched trenches in FLG by t-AFM. Both the density of wrinkles and the number of graphene layers are found to increase almost linearly as a function of the growth temperature in the considered temperature range.

No MeSH data available.


Related in: MedlinePlus