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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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AFM morphology and phase of SiC samples annealed at different temperatures. Surface morphology for the samples annealed at 1600 (a), 1700 (b) and 2000°C (c), and corresponding phase maps on the same samples ((d), (e) and (f)).
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Figure 2: AFM morphology and phase of SiC samples annealed at different temperatures. Surface morphology for the samples annealed at 1600 (a), 1700 (b) and 2000°C (c), and corresponding phase maps on the same samples ((d), (e) and (f)).

Mentions: The morphological transformation of SiC surface after annealing is described in the following. AFM images of the surface morphology for the samples annealed at 1600, 1700, and 2000°C are reported in Figure 2a,b,c, whereas the corresponding phase maps on the same samples are reported in Figure 2d,e,f. Annealed samples show wide terraces running parallel to the original steps in the virgin sample. An average terrace width of approx. 150 to 200 nm has been estimated for all the annealing temperatures, a significant increase over the small terraces observed for pristine SiC (approx. 30 nm). The estimated RMS roughness for the three samples is approx. 10 nm, which is significantly higher than on the pristine SiC substrate. Such large terraces on annealed samples are the result of the step-bunching commonly observed on off-axis SiC substrates after thermal treatments at temperatures > 1400°C. Interestingly, a network of nanometer wide linear features is superimposed to these large terraces. These features are particularly evident in the phase images of the surfaces. By accurately analyzing the morphology and phase images for the three samples, further insight can be achieved on the nature of these peculiar features. As an example, Figure 3 shows two representative linescans taken in the direction orthogonal (c) and parallel (d) to the steps obtained from on the morphology (a) and phase (b) maps for the sample annealed at 1700°C. In the direction orthogonal to the steps, it is worth noting in the height linescan, some very small steps with nm or sub-nm height are overlapped to the large terraces of the SiC substrate. Each step in the morphology corresponds to the characteristic sequence of a valley and peak in the phase linescan (Figure 3c). It is worth noting that the height of these steps is always a multiple of 0.35 nm, the height value corresponds the interlayer spacing between two stacked graphene planes in HOPG, as typically measured by AFM. As an example, an approx. 0.35-nm and an approx. 1.1-nm high step are indicated in Figure 1c. These step heights can be associated, respectively, to one and three graphene layers over the substrate or stacked over other graphene layers. As reported by other authors, graphene growth on SiC initiates at the terrace step edges of the substrate and continues over the terraces [8].


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)

AFM morphology and phase of SiC samples annealed at different temperatures. Surface morphology for the samples annealed at 1600 (a), 1700 (b) and 2000°C (c), and corresponding phase maps on the same samples ((d), (e) and (f)).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 2: AFM morphology and phase of SiC samples annealed at different temperatures. Surface morphology for the samples annealed at 1600 (a), 1700 (b) and 2000°C (c), and corresponding phase maps on the same samples ((d), (e) and (f)).
Mentions: The morphological transformation of SiC surface after annealing is described in the following. AFM images of the surface morphology for the samples annealed at 1600, 1700, and 2000°C are reported in Figure 2a,b,c, whereas the corresponding phase maps on the same samples are reported in Figure 2d,e,f. Annealed samples show wide terraces running parallel to the original steps in the virgin sample. An average terrace width of approx. 150 to 200 nm has been estimated for all the annealing temperatures, a significant increase over the small terraces observed for pristine SiC (approx. 30 nm). The estimated RMS roughness for the three samples is approx. 10 nm, which is significantly higher than on the pristine SiC substrate. Such large terraces on annealed samples are the result of the step-bunching commonly observed on off-axis SiC substrates after thermal treatments at temperatures > 1400°C. Interestingly, a network of nanometer wide linear features is superimposed to these large terraces. These features are particularly evident in the phase images of the surfaces. By accurately analyzing the morphology and phase images for the three samples, further insight can be achieved on the nature of these peculiar features. As an example, Figure 3 shows two representative linescans taken in the direction orthogonal (c) and parallel (d) to the steps obtained from on the morphology (a) and phase (b) maps for the sample annealed at 1700°C. In the direction orthogonal to the steps, it is worth noting in the height linescan, some very small steps with nm or sub-nm height are overlapped to the large terraces of the SiC substrate. Each step in the morphology corresponds to the characteristic sequence of a valley and peak in the phase linescan (Figure 3c). It is worth noting that the height of these steps is always a multiple of 0.35 nm, the height value corresponds the interlayer spacing between two stacked graphene planes in HOPG, as typically measured by AFM. As an example, an approx. 0.35-nm and an approx. 1.1-nm high step are indicated in Figure 1c. These step heights can be associated, respectively, to one and three graphene layers over the substrate or stacked over other graphene layers. As reported by other authors, graphene growth on SiC initiates at the terrace step edges of the substrate and continues over the terraces [8].

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